-A179 466 MNOLITHIC THIN FILM SAW (SURFACE ACOUSTIC WAVE) 7DEVICES IMPLANT-ISOLATED (U) PURDUE UNIV LAFAYETTE INSCHOOL OF ELECTRICAL ENGINEERING R L GUNSHOR ET AL

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SECURITY CLASSIFICATION OF THIS PAGE (When De nitered)

REPORT DOCUMENTATION PAGE BEFORE COMPLETING FORM

1. REP*I . 12. GOVT ACCESSION No. 3. RECIPIENT'S CATALOG NUMBER%

8 87-04961 ITT TL E VOV_4 TITLE (and Subtitle) 5. TYPE OF REPORT & PERIOD COVERED

Interim Scientific ReportMonolithic Thin Film SAW Stro&-twr " 1 July 84 to 31 August 85 "

it\ )lC~ w -t,.,, (,, , L',- 6 PERFORMING OG. REPORT NUMBER

AUTHOR(*) a. CONTRACT OR GRANT NUMBER(s)

R. L. Gunshot, S. Datta, and N. Otsuka* AFOSR-83-0237

PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT, PROJECT. TASK ,.AREA II WORK UNIT NUMBERS

School of Electrical Engineering (*School of A WR I M

Purdue University Materials Eng)

West Lafayette, IN 47907 2306/B2I. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

Air Force Office of Scientific Research 1 September 1985Building 410 13. NUMBER OF PAGESBolling AFB, D.C. 20332 -

4. MONITORING AGENCY NAME & ADDRESS(I dilferent from Controlling Office) 15. SECURITY CLASS. (of this report)

SAME UNCLASSIFIED I,®R15. DECL ASSI FICATION/DOWNGRADING , ,,,k

SCHEDULE

%i16. DISTRIBUTION STATEMENT (of this Report) N

Approved for public release; distribution unlimited.

17. DISTRIBUTION STATEMENT (of the ab,,.ct entered In Block 20, If different from Report) l1o.

SAME IELECTE .q APR 2 3 18

,,. SUPPLEMENTARY NOTES :.-.

N/A/ ' .. .... , 1

19. KEY WORDS (Conllnue on reverse aide II neceaery and Identify by block number)

Surface acoustic waves; ZnO, storage correlators, resonators, X-ray sources,'Molecular Beam Epitaxy; II-VI Semiconductors; Multiple Quantum Wells,Superlattices.

/ --

20. ABSTRACT (Continue on reverse side it necessary mid tdentity by block number)----A new surface acoustic wave resonator provides for greatly relaxed fabrica-

tional tolerances. The first multiple quantum well and superlatticestructures are reported for the wide bandgap semiconductor system (Zn,Mn)Se.

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rRESEARCH OBJECTIVES

Introduction

The Research reviewed in this report reflects the fact that the period reviewed .+

was one of transition between the past emphasis on monolithic (ZnO-on-silicon) sur-

face acoustic wave devices, to the present activity focused on the development of new-'r device concepts based on the use of II-VI semiconductor materials in configurations

fabricated using molecular beam epitaxy (MBE). The new emphasis is on the widegap II-VI semiconductors, and includes the growth and evaluation of superlattice and .3quantum well structures in the ZnSe/Znl-MnxSe materials system.

Specific Results

A. Implant-Isolated Storage Correlator

The past year saw the completioti of our work on monolithic devices used to per-form real-time nonlinear signal processing in the VHF/UHF portion of the rf spec-

A:f. trum. Details of the fabrication and operating characteristics of this particular dev-.r. ice, together with a brief review of the progress of the field, were included in an

invited paper for a special issue of the IEEE Transactions on Sonics and Ultrasonicsdevoted to surface acoustic wave convolvers and correlators (Appendix A).

B. Mode Conversion Resonators

A new surface acoustic wave resonator configuration was developed which hasthe advantage of eliminating the critical positioning requirement for the input/outputinterdigiial transducers, a characteristic feature of conventional SAW resonators.The key factor leading to this advantage is the resonator concept (developed under

.4 AFOSR support) utilizing Rayleigh-Sezawa mode conversion. In this concept thereflectors are replaced by periodic gratings which provide for a conversion betweenthe Rayleigh and Sezawa wave models. This appears to be a novel concept, as weknow of no other instance where a mode conversion, occurring with such highefficiency, is employed to implement a useful device function. Details of these resona-tors are found in the publication reprinted in Appendix B.

C. Novel Applications of Multilayered Structures and Superlattices

During this period three ideas for novel applications of ultrathin layered struc-tures were pursued theoretically, though no experimental work was done due to prac-

tical limitations. First, we considered the possibility of a new kind of space charge

wave in a pair of quantum wells that could be useful for signal processing in theinfrared. Seco-id, we discussed the possibility of generation of visible and UV

4

1-. . . . . . . . . . . . . . . . ....

2-

radiation by electron beams traversing a superlattice. This work has been pursuedfurther by one of our colleagues (A. E. Kaplan). Third, we explored the possibility ofnew guided acoustic waves in layered structures that could lead to the development ofintegrated acoustics similar to integrated optics.

D. Molecular Beam Epitaxy of Wide-gap il-VI Semiconductors

During the past year, considerable progress was made in developing techniquesfor the fabrication of superlattice and quantum well structures based on ZnSe. Hay-ing a direct bandgap in the blue portion of the visible spectrum, structures based on

ZnSe hold promise for new light emitting devices. Quantum well structures wereformed using the bandgap modulation provided by incorporation of Mn (substituting

on Zn sites) into ZnSe. By means of collaborations with several other groups, a widerange of techniques were used for the evaluation of the (Zn,Mn)Se structures. Duringthe reporting period, two publications were prepared which describe the growth,

microstructural evaluation, and optical properties of these structures. (Appendix Cand D).

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PUBLICATIONS

S. W. Schwartz, R. L. Gunshor, and R. F. Pierret, "Implant-Isolated SAW StorageCorrelator," INVITED, IEEE Transactions on Sonics and Ultrasonics (Special Issue onSAW Convolvers and Correlators SU-32, 707 (1985).

S. S. Schwartz, S. J. Martin, R. L. Gunshor, S. Datta, and R. F. Pierret, "SAW Reso-nators Insensitive to IDT Position," Proceedings 1984 IEEE Ultrasonics Symposium, p.229 (1984).

A. E. Kaplan and S. Datta, "Extreme-Ultraviolet and X-ray emission andamplification by nonrelativistic electron beams traversing a superlattice," Appl. Phys.Lett. 44, 661 (1984).

L. A. Kolodziejski, R. L. Gunshor, T. C. Bonsett, R. Venkatasubramanian, S. Datta,

R. B. Bylsma, and W. M. Becker, "Wide Gap II-VI Superlattices of ZnSe-ZnMnSe,Appl. Phys. Lett. 47, 169 (1985).

Y. Hefetz, J. Nakahara, N. V. Nurmikko, L. A. Kolodziejski, R. L. Gunshor, and S.Datta, "Optical Properties of ZnSe/(Zn,Mn)Se Multiquantum Wells," Appl. Phys.Lett. 47, 989 (1985).

R. L. Gunshor, N. Otsuka, M. Yamanishi, L. A. Kolodziejski, T. C. Bonsett, R. B.Bylsma, S. Datta, "Diluted Magnetic Semiconductor Superlattices," Invited paper

presented at 2nd International Conference on II-VI Compounds, Aussois (France),March 1985.

PERSONNEL

Robert L. Gunshor, Professor of Electrical EngineeringSupriyo Datta, Associate Professor of Electrical EngineeringNobuo Otsuka, Assistant Professor of Materials EngineeringStephen S. Schwartz, Graduate Research AssistantBiswajit Das, Graduate Research AssistantTimothy J. Miller, Technician

DEGREES AWARDED

Stephen S. Schwartz, Ph.D., "Zinc Oxide-on-silicon Surface Acoustic Wave Devices,"May 1985. -"

J..

APPENDIX A

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS. VOL. SU-32. NO S. SEPTEMB.R 19113 AFOSR -T . 8 7 -0 44 6

*Implant-Isolated SAW Storage CorrelatorSTEPHEN S. SCHWARTZ, ROBERT L. GUNSHOR, SENIOR MEMBER, IEEE, AND

ROBERT F. PIERRET, SENIOR MEMBER, IEEE

Approved for public release;i d istribut ion unl imited.

* 'Abstmact-The de -lopment of surface acoustIc wo (SAW) ctod

.3' yalvers end correlators is reviewed. This Is followed by the introductionof a new type of monolithic metal-zinc oxide-silcon diottide-sillicon _JU . . _ JLUstorage correlator. Fabricalion and operation of the ImplanIsolated - N

storage correlator, which relies on ion implatatlion for coninement of TRBMSDU:tV GATE

storage regions, is detailed. A capacitance-time measurement proce- PIEZOEL.C...C

dure for evaluation or the charge storage capability of the device is de-scribed. and correlation output inrormalion is used to estimate the ef--_Al________

fective recombination rate of the inversion layer charges. Finally, SO I

operational characteristics are examined and the new bias stable device S,

is shown to exhibit a 3-dR storage time in excess of 0. s. The cited ALUMUMstorage lime exceedq reported storage limes of other structures fabri- Fig. I. Schematic of a typical monolithic SAW correlator.caled in the ZnO-SiO-Si layered medium configuration.

ciencies at wide bandwidth compared to other configura-1. INTRODUCTION lions-even though substantial progress has been made to-

URFACE acoustic wave (SAW) convolvers and mem- ward improving these efficiencies 12]. The development ofbory correlators are devices that utilize the nonlinear layered medium monolithic SAW devices has emphasizedinteraction of the electric field associated with acoustic the use of ZnO and AIN as piezoelectric films. Althoughwaves in a piezoelectric medium to perform real time mul- ZnO was the first thin film having high performance in thetiplication of two signals. A representative monolithic SAW context 131, and hence has received the most atten-metal-piezoelectric film-SiO2-Si convolver is shown in tion 141, 15]. AIN (61-[81 has some attractive features re-Fig. I. The piezoelectric layer enables one to excite sur- lated to ruggedness and the potential for low dispersion.face acoustic waves in the silicon by applying RF signals Both materials can be utilized to achieve a temperatureto interdigitated transducers on the surface of the sub- stable configuration.

strate. The electric field pattern produced by fingers of It should be noted that the configuration of Fig. I is butalternating polarity induce periodic time and spatial one of several ways to realize the piezoelectric-semicon-stresses in the piezoelectric layer, thereby exciting a sur- ductor interaction. For example, a separated-mediumface acoustic wave. Associated with each propagating sur- technique has been employed where a semiconductor (Si)face wave is an accompanying electric field. It is the non- and a bulk piezoelectric crystal (usually LiNbO3) are

linear interaction within the semiconductor between the mounted in close proximity 191-114i. The separated me-electric field patterns of two contra-propagating waves that dium structure, although mechanically more complex thanproduces a signal proportional to the product of the two a monolithic configuration, has a wide bandwidth capa-acoustic signals at every point in the device. The function bility 1151 as a result of the high electromechanical cou-of the third electrode, known as the gate, is to sum the pling coefficient of LiNbO3. To increase the bandwidthproduct signal over the length of the device. When two capability of devices fabricated in the monolithic ZnO-signals are applied simultaneously to the two transducers, SiO 2-Si system-thereby making them more attractive asthe output at the gate is proportional to the time-coi- practical signal processing devices-higher electrome-presse onolutio f th e osoinal. tothchanical coupling was sought by employing a higher orderpressed convolution of the two signals. "'

The family of monolithic SAW devices includes struc- Rayleigh mode. It has been shown that the second-ordertures fabricated on GaAs substrates, which have demon- or Sezawa mode of the layered ZnO-SiO2-Si configura-strafed outstanding time-bandwidth products l . Be- tion can be efficiently excited in high-quality films 116].cause of the relatively weak piezoelectric nature of the The electromechanical coupling coefficient available from

% GaAs, however, these devices, when implemented without this mode exceeds that for Rayleigh waves in bulk ZnO,a piezoelectric film overlayer, have lower correlation effi- and is close to the value for LiNbO. Several convolvers

and diode storage correlators which exploit the bandwidthadvantage of the Sezawa mode have been reported 1171-

Manuscript received September I. 1994 revised January 22. 1985. This 120).work was supported by the Air Force Office of Scientific Research under Convolvers, it should be pointed out, can be used toGrant AFOSR.3-0237.

The authors are with the School of Electrical Engineering. Purdue Uni- perform correlation between two signals. A disadvantageversity. West Lafayette. IN 47907. USA. of correlating two signals using a convolver is that the ref-

0018-9537/85/0900-070701.00 © 1985 IEEE

% ,

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS. VOL. SU-.32. NO 5. SEPTEMBER 1995

concentration) of free electrons at the Si-SiO, interface."J" J ' Subsequently, a portion of the electrons are trapped at the ".

interface thereby producing a spatially varying chargepattern. The storage time of these devices corresponds tothe detrapping time, which is a function of the propertiesof the interface. Surface state storage was subsequentlyreplaced by more easily controlled and repeatable diodestorage arrays. More recently, reference-signal storage ininduced junctions was reported [32]. In both diode and

so ,' CORM"am,e induced junction-array processes, the principle of chargeFig. 2. Correlationi and convolution using* a storag c . storage is essentially the same. Recombination of minor-

ity carriers injected out of a p+ region (or inversion layerin the case of the induced junction device) by an applied

erence signal must be time reversed. An electronic time- signal causes an alteration of the diode (induced junction)reversal technique t21 e has been demonstrated in a con- depletion width.e o t i n d ivolver system [221, but at a sacrifice of high insertion loss The monolithic metal-ZnO-SiO 2-Si (MZOS) [331, [34)which leads to a degraded dynamic range. Additionally, memory correlator introduced in this paper utilizes deple-an incoming signal arriving at an unknown time necessi- tion regions in the silicon to store a reference signal, but ittates repeated application of the reference signal to counter in a metal-oxide semiconductor (MOS) region of n-typethe uncertainty of the arrival time which in turn leads to a silicon only, that is, no diodes or surface states are em-smaller effective time-bandwidth product. ployed for storage. Furthermore, the uncontrollable and

The storage correlator is a device that performs both often undesirable charge-injcction process [31], [351-[371convolution and correlation without the need for time re- which was used constructively by the induced junctionversal and without concern for the uncertainty of the ar- storage corrclator is successfully ignored in the implant-rival time of the signal to be interrogated. Hence, two of isolated storage device.the major drawbacks of performing correlation with the In the following section we present a detailed descrip-convolver system are eliminated by utilizing a memory tion of the implant-isolated storage correlator fabricationcorrelator and operation as well as experimental verification of the

A simplified description of the storage correlation pro- existence of induced junction storage regions. Section IIIcess can be given with the aid of Fig. 2, which depicts a consists of an in-depth discussion of charge storage in thetypical memory correlator. Under normal operation, one implant-isolated device. An approximation for the effec-introduces a signal at one of the transducers such that a tive recombination lifetime of minority carriers is alsosampled version of this reference signal S(O) is stored be- presented. Experimental results for the implant-isolatedneath the gate of the device by one of several possible writ- storage correlator are given in Section IV followed by con- ..- 'ing procedures. At some later instant, but still within the clusions in Section V. I>

storage time of the reference signal, a second signal R(t)is applied to the gate of the correlator. The introduction !!. THE IMPLANT-ISOLATED STRUCTUREof R(1) at the gate electrode excites two contra-propagat- Device Structureing signals representing the product of R(t) and S(t), which The implant isolated storage correlator, pictured in Fig.subsequently appear at the two output transducers; it can 3, consists of an n-type 10-fl-cm (100)-cut silicon sub-be shown that the correlation appears at one transducer, strate, which is ion implanted with phosphorus in a grat-while the convolution output is available at the other. ing pattern at the sample surface. Charge storage, it should

To date, several memory-array configurations have been be noted, occurs in the nonimplanted regions. Subsequentemployed in the fabrication of storage correlators. to the grating region implantation, a wet oxidation is per-Schemes for signal storage by means of surface states [231, formed at 9000C for 40 min. This oxidation, yielding a[24), p-n diodes [181, 125)-1281, and Schottky diodes 1000-A-thick insulating layer, simultaneously passivates[291-1311 have been examined. More recently, a new type the silicon surface and activates the implant. Prior to de-of junction storage correlator has been introduced [321, in positing the 1.7-pm ZnO layer by RF sputtering. 1000- A- I,. J

which the spatial variation of inversion layer charge at the thick aluminum shorting planes are evaporated on the sub-SiO 2-Si interface of the ZnO monolithic configuration has strate in the regions below the transducers to enhance thebeen utilized for signal storage. With the exception of this electromechanical coupling. The top aluminum metalli-"induced junction" storage correlator, the various storage zation pattern consists of two pairs of single comb trans-schemes have been employed in both monolithic and sep- ducers 1381 and a split gate electrode arranged so as totated medium configurations. form a dual-track structure J39). The transducers. used to

The first memory devices utilized semiconductor sur- excite Rayleigh waves, have equal 17.75-pm finger widthsface states for signal storage 123), [24). In a surface state and gaps. the center frequency of the transducer responsememory correlator, the electric field pattern associated is 128 MHz. The separate comb dual-track structure iswilh an applied signal causes a bunching (periodic spatial employed because this technique has been shown to sup-

• .,. ~~~.........,....,-....... .-

SCHWARTZ eg at.: IMPLANT-ISOLATED SAW STORAGE CORRELATOR 709

Ii I IIIII

ITUUUUUIU

(b i /r nn Fig. 5. Electrical m odel for the gate of the im plant-isolated storage cor -lator. The implanted and nonimplanted regions are denoted by C1. and

'- .. C rspectively. R., is the bulk silicon resistance and C, is the com binedIrwi nto oxid capacitance.

ic, cond , ion a range of gate bias voltage would exi. t whereFig. I Sclefmt c diagran o .an imp un,' -isolated itorage correlator. (a) Top the higher doped device would be depleted and the lowerview of dual-rate %tructure and sir i Ic-comb tr. nsducers. (h) hl Impln- theh device would be dnepleentao lofItaior lo .ern (c) Side view ofco 'r.red d ev,:e , do le, device would be inverted. In tfae implementation of

the i-.-iplant-isolated storage correlator, higher doped de-pletion regions are used to isolate the lower doped storageo r in ',ers io n re g io n s .

To achieve lateral variations in doping density one can'" em ploy either ion im plaitation or diffusion: ion im plan-

- - tatior was chosen because it offered advantages in ease offabrication. Phosphorus was implanted into the unoxi-

. dized silcon wafer through a photoresist mask at a con-centr.,tion ,evel of 8.0 x l0' 2/cm2 using an implant energy

S ,of 25 keV. The implant concentration varies of course asa function of depth into the semiconductor. An estimate

- [of the doping profile derived from the C-V profiling tech-nique, however, indicates that the semiconductor may beapproximately modeled as uniformly doped (- 1017/cm)

" "... .. .in the bias range of interest. As a result of the ion im-Fig d. N~rnat~ed C-I/curves fortypical MOS capacitors. A and Bdenote plantation. a periodic grating pattern of alternating highly

the highly doped (1.25 x 0 '"icm') n-type Si and moderately doped doped and lowly doped semiconductor is established be-(5.0 iO"'cm') n-type Si. respectively. neath the gate of the correlator. The periodicity of the

grating corresponds to 5-pum-wide storage regions sepa-press the self-convolution caused by reflections from the rated by 5-pm-wide implanted regions. A simple electricaltransducers. By slanting the gate metallization pattern ad- model for the correlator gate appears in Fig. 5. while Fig.jacent to the transducers one minimizes the undesirable 6 shows experimental C-V curves derived from an unim-output observed at the transducers caused by waves planted wafer, a wafer implanted over its entire surfacelaunched at the ends of the gate. area, and a grating structure wafer. (The rise in the ca-

pacitance of the grating structure near -8.0 V is causedConcept of Implant-Isolation by lateral effects.) It is apparent from the curves in Fig.

Insight into the operation of the implant-isolated storage 6. that one can select a bias voltage between -1.0 andcorrelator can best be achieved by understanding the - 10.0 V such that the inverted storage (nonimplanted) re-method in which charge is stored in the device. We present gions will be electrically isolated from one another by de-a qualitative explanation based on typical n-type MOS ca- pletion regions in the implanted areas. This is the desiredpacitance versus voltage characteristics. To first order the result as illustrated schematically in Fig. 7. The implant-ZnO layer can be modeled as a constant capacitance in isolated storage regions. however, in contrast to thoseseries with the constant SiO 2 layer capacitance: the com- formed by charge injection and trapping at the ZnO /SiO2bination can in turn be modeled as a single effective in- interface [321, are established by a repeatable, well-con-sulating layer. The C-V curves for two structurally iden- trolled fabrication process and simple selection of thetical hut differently doped metal-insulator-semiconductor proper operating gate bias.(MIS) capacitors are displayed in Fig, 4. Note that for bothdevices, accumulation occurs over the same bias range. Verification of Storage-Region IsolationHowever. the onset of inversion occurs for different values Subsequent to ion implantation, implant activation dur-of capacitor bias. Suppose the two differently doped de- ing thermal oxidation, and deposition of the shorting padvices were side-by-side as part of the same substrate and metallization. special gate-pattern test structures werecovered by the same insulator and metal gate. Under this formed directly on the oxidized silicon surface. The test",'"

_WRYrrr, 11 qV04411 %,

710 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS. VOL. SU-32. NO. S. SEPTEMBER 1983

C.C

c C

(00j rip MCI

Fig. 8. Capacitance versus time (C-t transient) results for applied pulses

-s -" [ 0 1 1 is described in Section I.

G..C aiM 4' depletion width in the implanted region. Once the gate isFig. 6. Experimentally determined C-V characteristics derived from a (A) pulsed from accumulation to a bias that tends to invert

totally implanted (B) combined (grating) (C) non-implanted wafer. (VA, both the implanted and nonimplanted regions, the totalcapacitance always relaxes to the same minimum value asexemplified by curves E and F. This is true even thougha larger negative pulse depletes the capacitors farther andfor a longer time. The existence of a biasing range where

the device relaxes to a decreasing final capacitance, fol-,.Kr LETzO REG:o1 E .E lowed by a biasing range where the final capacitance is

always the same independent of bias, is direct verificationFig. 7. Schematic of charge storage regions (inducedjnctions) established of the underlying concept of the device: implant isolation -.

beneath the gate of the correlator. Plus signs denote inversion layer of the storage regions. The biasing range over which thechae, device relaxes to a decreasing final capacitance corre-

sponds, of course, to the set of biases used in the normalstructures were subjected to a post-metallization anneal at operation of the storage correlator.4800 C for 5 min to minimize the SiO2-Si interface stateconcentration. Capacitance-versus-time (C-t) transientmeasurements were then performed on the Si0 2-Si sys- Ill. THE WRITING PRocESStern in order to determine the storage capability of the Several methods for writing a reference signal into theU grating, and to examine the effectiveness of the implant in storage regions of a device have been demonstrated in-isolating the storage regions. cluding the flash mode [29] and various RF writing tech-

The C-t transient results can be explained readily with niques [40]; we limit ourselves to the RF gate-acousticthe aid of Figs. 6 and 8. Fig. 8 shows a series of capaci- writing mode for the implant-isolated storage correlator.tance-versus-time characteristics of the grating region The RF gate-acoustic writing method involves the appli-when pulsed from an equilibrium state at VG = V. (ac- cation of an RF signal of short duration to the gate of thecumulation) to various voltages (VA. 1 8, - - , VF) labeled correlator while the reference signal to be stored is prop-in Fig. 6. Trace A shows the result of pulsing the capacitor agating beneath the gate. The resultant electrical potentialfrom the accumulation reference bias V., to an applied in the gate region during the writing process has compo-bias which is still in accumulation. As expected, there is nents of potential due to the acoustic reference signal,no change in capacitance with time. Similarly for curve V.(t) cos (wst - kz), (launched by a transducer) and theB, when both the storage and isolation regions are de- writing signal, V,(f) cos w , applied to the gate. Both sig-pleted, the t > 0 capacitance is time independent, but is nals contribute to the total gate potential associated with

IN lower because of the increased depletion widths of the two the writing process, which is given byregions. Once the device is pulsed such that the nonim-planted region beneath the gate is inversion biased (curveC), the sudden pulse causes a deep depletion condition in wherethe nonimplanted region with an accompanying finite re-laxation time back to equilibrium. The difference in the v. sin kzC-t transient of curves C and D is that a larger negative = tan-U + v. cos " (2)pulse causes a greater degree of depletion in the implantedregion and also a greater deep depletion width in the non- Here v. and v. are the acoustic signal and gate signal po-implanted regions. After relaxation, the nonimplanted de- tentials referenced to the gate [411. As will be describedpletion widths remain at their maximum values. However, below, Vs(z, t) determines the surface charge density dur-the total equilibrium capacitance is lower due to the larger ing a write sequence for any (z, t).

SCHWARTZ eq al.: IMPLANT-ISOLATED SAW STORAGE CORRELATOR 711

In this section a simple model is presented to describe c Ithe information storage process. To gain physical insightinto the operation of a storage array, we first examine the ,"response of the inversion layer charge and depletion ca-pacitance of a single element of the storage region to anarrow (much shorter than one RF period) pulse appliedto the gate. After analyzing the effect of a narrow pulse,each period of the RF signal of (1) is treated as a discre-tized sequence of narrow pulses. Using this discretized ! -representation. the charge storage in response to any timevarying writing signal can be estimated. -

Charge Storage I U, U, ,...GATE DIAS (U)

Consider the effect upon a single storage region result- Fig. 9. C-V characteristics of a typical MIS capacitor (solid curve) and an %ing from the application of a narrow positive pulse at the MZOS capacitor exhibiting charge injection into the ZnO.

gate of a device that is biased for normal operation. Underequilibrium inversion conditions there is a maximum de- state conditions a virtual gate is formed at the ZnO-SiO 2pletion width in the silicon. Applying a positive pulse of interface [37]. The ZnO layer, therefore, can be neglectedshort duration to the gate while it is under equilibrium when calculating the equilibrium minority carrier inver- '...inversion bias will cause the storage-region depletion width sion layer charge density, in which the magnitude is givento narrow by some amount, depending on the pulse am- byplitude and duration. At the same time, minority carrierholes in the inversion layer will be injected into the semi- C 1conductor depletion region, where some will .recombine. Q2q = Co T UK, (3)Termination of this pulse causes the depletion width to [ q C Nq Jinstantaneously assume a new value equal to its equilib- where Co is the capacitance (per unit area) of the SiO 2rium width plus an increment dependent upon the amount layer, N O is the background doping level of the n-type sil-of minority carriers which recombined during the pulse. icon substrate, and UF is the substrate doping parameter.The change in depletion width changes the capacitance of The dielectric constant of the silicon is represented by K,,the storage region, thereby contributing to signal storage the temperature by T, eo is the permittivity of free space,in much the same way as in a p-n diode memory corre- and k is Bohzmann's constant. The constant UI is givenlator

In a quantitative description of the charge storage pro- by

cess, some assumptions have been made regarding the re- NDcombination and generation of minority carriers in the sil- U = -InnN- (4)

icon. Here it is assumed that the recombination rate of -injected holes, crucial to information storage, can be where n is the intrinsic carrier concentration.modeled by an effective recombination lifetime Tf. More- Due to the localization of injected charge at the ZnO-over. the generation of carriers during a narrow negative SiO 2 interface, the equilibrium inversion layer charge un-pulse (part of the negative going portion of the writing der dc bias is independent of the ZnO layer capacitance.cycle) can be neglected because, as is obvious from the However, this is not true of the inversion-layer charge den-lengthy storage times, the return to equilibrium is very sity when there are rapid changes in the gate bias. For RFslow compared to the writing recombination rate. Surface signals in the frequency range of interest, voltage changesstates, traps in the semiconductor and ZnO, and charges are too fast for the gate-injected electrons to follow thein the oxides will also be neglected because they compli- applied signal. Hence the capacitance of the ZnO layercate the analysis without offering any insight into the writ- must be included in any calculation of the redistributeding process. With the stated assumptions, a quantitative surface charge. The condition just described can be vis-description of the charge storage process has been per- ualized more easily with the aid of Fig. 9. In this figure,formed for a single storage region. the solid curve represents a normal C-V characteristic for

As discussed in Section II, proper operation of this de- an n-type semiconductor, and VG is the bias point. In avice is dependent upon the selection of an appropriate op- normal MOS capacitor one would have to pulse the gateerating point gate bias. Assuming that a bias of VG has to some bias greater t han VT to move out of inversion andbeen applied to the gate, one can, using the 6-depletion into depletion. In the case of a correlator with an injectingapproximation (421. calculate the inversion layer charge ZnO film, however, the gate must be pulsed to VT,, (on theat the silicon surface. Because the ZnO layer is only semi- dashed curve) to be at the edge of inversion. For compar-insulating, the dc charge applied to the gate is readily in- ison to the redistributed charge it is therefore more con-jected into the ZnO film and is subsequently stored in traps venient to rewrite the equilibrium inversion layer chargeadjacent to the ZnO-SiO2 interface; i.e., under steady- density (3) for a given gate bias VG in terms of VT,. and

-.-. "- -.. -'

712 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS. VOL. SU-32. NO. 5. SEPTEMBER 19S5

C, (the series combination of the ZnO and Si0 2 capaci- As mentioned earlier, the change in surface chargetances). The resulting equivalent expression for Qseq is caused by recombination during a positive pulse of dura-given by tion tp results in an increase in the depletion width of the

storage regions; the new depletion width is given by2kT

Q q C V G + V,. W (V , VA, tp)

C, q VA

w h e re + + (V 1)

whereC s (6) K, ,qN.

--iO + Cz.O' (12)The analysis of the charge storage process deals with theredistribution of this surface charge in response to an ap- The modified depletion width corresponds to a decreasedplied signal. capacitance given by

The application to the gate of a narrow positive pulse of:• amplitude VA will cause a portion of the equilibrium sur- C(Vo, VA, tp)

face charge to be injected into the semiconductor, where* some of it will recombine. The amount of charge which - C,

recombines per pulse is dependent upon the pulse ampli- F / VA, ) 21tude. pulse duration, the minority carrier recombination +C, kI (, V I rate, and the inversion layer charge available prior to each + + + /pulse. Repeated applications of such a pulse within an in-

-. terval that is short compared to the device storage time (13)I(- s) eventually leads to a steady-state deep depletion

condition beneath the gate where the new inversion layer Thus far we have described a method for determiningsurface charge density QA saturates at the change in the depletion capacitance of a single-storage

region in response to a narrow pulse applied to the device2kT gate. To elaborate on the writing process, it is necessary

.'-., QA =C VG - VA + Vr.f - - UF to determine the effect of a time varying signal on the"""q several hundred storage regions. That is, for a complete

4K 1I description of the writing function, one must define thec, 4 K ,I •UF (7) depletion capacitance (stored charge) as a function of timeC, qND q and position. The treatment of a time varying signal is

given in the Appendix for a signal represented by a se-For a given VA, the maximum charge storage by one stor- quence of pulses of finite duration.age region in the implant-isolated correlator is therefore

Effective Recombination LifetimeAQ = Qq - Q4 = CVA. (8) As described in the previous section, the amount of

Assuming an effective recombination lifetime, the ap- charge stored in the gate region of the device is dependentplication of a dc pulse of amplitude VA and aibitrary du- upon the amplitude and duration of the write signal, theration t, changes the inversion layer charge by an amount effective recombination lifetime, and many parameters

which are determined by the materials used. In this sec-AQ, = Ct-VA(i (9) tion an experimental method is presented for determining

The expression for the inversion layer surface charge as a the effective recombination lifetime for the correlator.function of gate bias, applied pulse magnitude, and ap- To obtain the desired correlation output, one applies theplied pulse duration is given by RF signal to be correlated at the gate electrode (gate-to-

= Q - acoustic reading mode). This process is analogous to the": Q,(V 0 , VA, tp) f q- CVA(1 I e-). (10) readingprocessinap-ndiodememorycorrelator[401. Due

If one were to pulse the device again immediately after to the spatially varying depletion capacitance, electricthe application of the first pulse, the starting inversion layer fields are established in the ZnO film which excite contra-• " minority carrier density would be Q,(VG, VA, t,,) instead propagating acoustic signals representing the convolution "

of Qq. The subsequent surface charge after additional and correlation of the stored reference signal with the ap-pulses would be dependent upon the pulse durations as plied reading signal. Furthermore, it can be shown thatwell as the pulse amplitudes, the correlation output voltage is directly proportional to

. i

SCHWARTZ t al.: IMPLANT-ISOLATED SAW STORAGE CORRELATOR 713

V."", .U- ".-'

1.00

AI"

1 6

, 1

000 1I

,, ,0 1 0 20 3 b) 40 t te (seco ns ) • '

EFFECTrV WRITING TIME (/o) Fig. 11. Correlation output versus storagec time. .e.Fig. 10. Normalized correlation output voltage versus effective writing

time. Solid curve corresponds to an effective minority carrier lifetime of,," - 0.67 its,

i~i I the amount of stored charge in the device. Therefore onei '"ii %; 0 "''

can determine the effective recombination lifetime by fit- ,ting the predicted stored-charge dependence of (AM) to the .% c.actual correlation output versus writing time. A %to e m

To observe the variation of correlation output with write *time, an experiment was performed using the gate-acous-

0%

tic writing mode and the gate-to-acoustic reading mode. 0. at?

The reference and reading signals were of 1.0-As durationand the writing signal duration was 0.4 /us. To vary the ,writing time, multiple writing sequences were performed ",in rapid succession prior to each reading sequence. Thenumber of successive writes determined the effective writ- , -. .... . ,. ..-ing time (400-ns writing pulse times the number of writes Gat I.,$ (U)

prior to each read). The writeg were performed 20 f$ apart Fig. 12. Correlation output versus gate bias for different read-write power

so any storage region relaxation between write pulses can level combinations, where A denotes Pw = 40.1 dBm. and PR = 36.7dBm; B denots Pw - 37.1 dBm and P - 33.7 dBm; and C denotes

be ignored. The experimental variation of correlation out- Ps, - 34.1 dBm and P, - 30.7 dBm.put versus write time is seen in Fig. 10. All values arenormalized to the maximum value of correlation output. signal. Fig. II displays the correlation output versus stor-(It should be noted that in this experiment the time be- age time; the data points fall along the dashed curve whichtween readouts was several seconds to ensure sufficient corresponds to a 3-dB storage time of 0.56 s. Althoughdecay of the stored signal before initiating a new write this is certainly a long storage time compared to other pre-sequence.) The solid curve represents the predicted de- vious MZOS correlator configurations, with advanced

. pendence of the correlation output voltage for an effective fabrication procedures it is not unreasonable to expectrecombin'tion lifetime of Tr = 0.67 us as computed for storage times of at least several seconds.i" Ithe discretized sinusoidal approximation. A plot of normalized correlation output versus gate bias

is shown in Fig. 12 for three different values of read-writeIV. OPERATIONAL CHARACTERISTICS power level combinations. It is seen that for gate biases

In this section we present experimental results for the less than a certain value, the correlation output always re-correlation of two 1.0-ps RF bursts in order to examine cor- mains within 85 percent of its maximum. This indicates arelator storage time, dynamic range and efficiency. All very wide bias range over which one may operate the de-correlation measurements were performed using the RF vice without significant degradation of the output signal.gate-acoustic writing mode and the gate-to-acoustic read- Based on theoretical considerations, one might expect thating mode. It should be pointed out that the implant-iso- once the dc gate bias was sufficiently large to invert notlated storage correlator presented in this paper is bias-sta- only the storage regions, but also the ion implanted sep- .-

ble; that is, results of all correlation measurements are aration (and lateral) regions, then the lateral flow of in-totally repeatable at any time without any special precau- version layer minority carriers from implanted regionstions, irrespective of the previous gate bias. surrounding the storage areas would eliminate the stored

A key feature of the implant isolated correlator device signal. The experimental results presented, however, in-is the time the device is capable of storing a reference dicate that the implanted region is of insufficient area to

,....

714 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS. VOL. SU-32. NO. 5. SEPTEMBER 1995

% 3f do. 1merit into the range of previously reported monolithic-" '063. memory correlators. 'P:..9d.

• " ".Iv. CoNCLUSIoN.

• A monolithic MZOS surface acoustic wave memory- *" correlator that utilizes ion implantation to confine inducedI " • • junction storage regions has been presented. The storage

*V- . ,. *time capability of the device before fabrication is com-A pleted has been demonstrated by means of a pulsed ca-

- pacitance-versus-time experiment. A simple model of theA ~writing process was detailed which allows one to deter-L

",". . --. a mine an effective minority carrier lifetime for the inver-I.sion layer charge. It has been demonstrated that 3 dB stor-

SW -W SWCU*age times in excess of 0.5 s are achievable with the prom-WwW..a.. ,,, 3.olitidw, (U p-P) ise of signal storage up to several seconds. Finally, this

(a) device exhibits bias stability, making the implant isolatedstorage correlator an attractive device for use as an on-

,wrr •305 63 dchip circuit element.•P., 40q 0 111 •

S.- * P *P 30 3 d. 5 5

, APPENDIX

For a single pulse applied to a storage region one has.," nafter a rectangular pulse of amplitude VI and duration At,,

a modified surface charge density of Q, given by

694 " Q, = Qq - CV,(l - e - '' ") (Al)

If ore applies another pulse of amplitude V2 and durationo // .A t2 at some instant immediately after turning off pulse VI,

• then the surface charge density is given by

I__._-" .,,_Q2 = Q, - C, V2'a 600 Ito ISC otC oC U I US l

.a, Sial (U ,-, + [Q, - (Qq - CV 2)J eCarl(b)

Fig. 13. (a) Correlation output versus reference signal amplitude. (b) Cor- = Q - CV 21l -e - ar21TR

)

relation output versus read signal amplitude.- CV eI a ft (1 - eAi"). (A2)

' supply the signal-eliminating flow of minority carriers. Similarly, with the application of numerous narrow pulsesThe top surface of the device after sawing and mounting of amplitudes V,, one can replicate any time varying sig-extends approximately 1 mm to either side of the 1-mm- nal. If one discretizes the desired signal into uniform in-wide gate. This area is subject to lateral effects and should crements of duration At, one can determine the surfacecontribute to the overall relaxation, but it is apparently too charge density for the application of a signal NAt long bysmall to degrade the output significantly.

An RF writing signal of 140 ns (18 cycles) was found to _ ea,,) .'" -"QN = Q, CI(l - e- ENVe -( - ) ' .

produce the highest correlation output, and this writingpulse duration was used when performing the correlationmeasurements shown in Fig. 13. Fig. 13(a) shows the cor- Vi > 0. (A3)relation output versus the reference signal amplitude and Furthermore, in addition to the constraint that V, must bethe result is linear over a 25-dB range in correlator output positive, as the storage depletion width expands, only

* - power. The correlation output versus read pulse power canb...oer seeni Forrlatig.13(b)tHersi thedamc rcan opulses sufficiently large to contribute to further chargeir -:-:,be seen in Fig. 13(b). Here again the dynamic range of soaeaeue ntecmuaino

the device is approximately 25 dB. In all measurementsthe correlation efficiency is between - 100 and - 110 dBm.The efficiency and dynamic range of this device are lower ACKNOWLEDGMENT

than expected. ZnO film quality during this particular The authors wish to thank M. Young and T. Miller forsputter produced a delay line insertion loss some 14 dB the ion implantation of the devices as well as G. MaGee,more than in previous devices with the same dimensions. S. Phillips, and T. Corbin of the Naval Avionics Center in(The same was true Icir nonimplanted test structures.) A Indianapolis, IN, for their assistance in making photo-better piezoelectric film should upgrade both figures of masks.

.4

% f-

SCHWARTZ ef at.: IMPLANT-ISOLATED SAW STORAGE CORRELATOR 715

REFERENCES 127) P. G. Borden, R. Joly, and G. S. Kino, "Correlation With the StorageConvolver," IEEE UIrson. Symp. Proc., pp. 348-351. 1976. -,

II] M. R. Melloch and R. S. Wagers, "Wide-band monolithic GaAs con- (281 F. C. Lo. R. L. Gunshor. R. F. Pierret. and J. K. Elliott. "Separation

volver and memory correlator." Appl. Phys. Lett., vol. 43. no. I, pp. of storage effects in monolithic p-n diode correlators." AppI. Phys.

48-50. July I. 1984. Lett.. vol. 33. no. II. pp. 903-905. Dec. 1. 1978.

121 - . "Controlled diode profiling forGaAs strip-coupledcorrelators." 1291 K. A. Ingcbrigtsen, R. A. Cohen. and R. W. Mountain. "A Schottky- -.

IEEE Elec. Dev. Ler.. vol. EDL-5. no. 5. pp. 136-138, May 1984. diode acoustic memory and correlator." Appl. Phys. Lea.. vol. 26.

13] F. S. Hickernell, "DC triode sputtered zinc oxide surface elastic wave no. I. pp. 596-598. June 1975.transducers,." J. App!. Phys.. vol. 44, no. 3, pp. 1061-1071, Mar. 1973. 130] K. A. Ingehrigtsen, "The Schottky diode acoustoelectric memory and

[41 R. L. Gunshor, S. J. Martin, and R. F. Pierret. "Surface acoustic correlator: A novel programmable signal processor.*" Proc. IEEE. vol.wave devices on silicon." in Proc. 1982 Int. Conf, Solid State De- 64, no. 5. pp. 764-769. May 1976.vices. 1982. Also see Jap. J. Appl. Phys.. vol. 22. no. I, pp. 37-41, 1311 W. C. Wang, "Physical processes of SAW Schottky-diode memory

1983. correlator," Electron. Lett.. vol, 12, pp. 105-106. 1976.

151 G. S. Kino, "Zinc oxide on silicon acoustoelectric devices," in Proc. 1321 K. C. K. Weng, It. L. Gunshor. and R. F. Pierret. "Induced junction1979 IEEE Ultrason. Symp. Proc.. 1979, pp. 900-910. monolithic zinc oxide-on-silicon storage correlator." Appl. Phys. Ltt..

6 . M. Lakin. 1. Liu, and K. Wang, "Aluminum nitride on sapphire," vol. 40, pp. 71-73. 1982.in Proc. 1983 IEEE Ultrason. Symp. Proc., 1974. pp. 302-306. (331 J. K. Elliott. R, L. Gunshot. R. F. Pierret. and K. L. Davis. "Char-

[71 L. G. Pearce. R. L. Gunshot. and R. F. Pierret. "Aluminum nitride acteristics of zinc-oxide-silicon surface wave convolvers for optical

on silicon surface acoustic wave devices," Appl. Phys. Lear.. vol. 39, imaging and memory," in Proc. IEEE Ultrson. Symp.. 1975. pp. 141-

no. II. pp. 878-879. 1981. 143.

181 K. Tsubouchi and N. Mikoshiba. "Zero temperature coefficient SAW [341 K. L. Davis, "Properties of the MZOS surface wave convolver con-

delay line on AIN epitaxial films." in Proc. 1983 IEEE Ultroson. figuration." IEEE Trans. Electron Devices, vol. 23. no. 6. pp. 554-

Symp., 1983. pp. 299-310. 559. June 1976.(91 C. W. Lee and R. L. Gunshor, "Nonlinear interaction of acoustic sur- 135] L. A. Coldren. "Effect of bias field in a zinc-oxide-on-silicon acoustic "-

face waves from coupling to charge carriers," J. Appl. Phys., vol. 44. convolver." Appl. Phys. Lett., vol. 25, no. 9, pp. 473-475. Nov. 1974.

no. II, pp. 4807-4812, Nov. 1973. 1361 J. K. Elliott, R. L. Gunshor. R. F. Pierret, and K. L. Davis. "Zinc-[10] W. C. Wang. "Convolution of surface waves in a structure of semi- oxide-silicon monolithic acoustic surface wave optical image scan-

conductor on LiNbO," Appl. Phys. Lett.. vol. 20, no. 10, pp. 389- ncr," Appl. Phys. Lett.. vol. 27. no. 4, pp. 179-182. Aug. 15. 1975.

392, May 15. 1972. (371 R. F. Pierret, R. L. Gunshor, and M. E. Cornell. "Charge injection

[II] P. Das. M. N. Araghi, and W. C. Wang. "Convolution of signals using in metal-ZnO-SiO 2 -Si structures," J. App!. Phys.. vol. 50, no. 12. pp.surface wave delay line." App!. Phys. Let., vol. 21.-no. 4, pp. 152- 8112-8124, Dec. 1979.1254. Aug. 2972. [38] M. R. Melloch, R. L. Gunshor, and R. F. Pierret. "Single phase and

1221 M. Yamanishi. T. Kawamura. and Y. Nakayama. "Acoustic Surface balanced separate comb transducer configurations in a ZnOISi SAWWave Convolver Using Nonlinear Interaction in a Coupled PZT-Si structure," IEEE Trans. Sonics Ultrason., vol. 28. no. I, pp. 55-58.System." Apply Phys. Lett.. vol. 21, no. 4, pp. 146-148. Aug. 1972. 1982.

113] G. S. Kino. W. R. Shreve. and H. R. Gautier, "Parametric interac- 1391 1. Yao. "High-performance elastic convolver with parabolic horns."tions, of rayleigh waves," in Proc. 1972 IEEE Ultrason. Syrtp.. 1972, in Proc. 1980 IEEE Ultrason. Symp.. 1980.pp. 285-287. 140] Ph. Defranould, H. Gautier, C. Maerfeld. and P. Tournois. "p-n diode

1-4 1R. L. Gunshor. "The interaction between semiconductors and acous- memory correlator." in Proc. 1976 IEEE Ultrason. Symp.. pp. 336-tic surface waves-A review." Solid-State Electron.. vol. 18, pp. 1089- 347, 1976.1093. 1975. 1411 H. C. Tuan, J. E. Bowers, and G. S. Kino, "Theoretical and experi- •

[151 R. W. Ralston, J. H. Cafarella. S. A. Reible, and E. Stern, "Improved mental results for monolithic SAW memory correlators." IEEE Trans. a

acoustoelectric schottky-diode/LiNbOl memory correlator," in Proc. Sonics Ultrason., vol. 27. no. 6. pp. 360-369. Nov. 1980. '

1977 IEEE Ultrason. Symp.. 1977. pp. 472-477. 1421 R. F. Pierret. Modular Series on Solid State Devices. (Field Effect1163 T Shiosaki. T. Yamamoto, and A. Kawahata "Higher order mode Devire.r, vol, 4). New York: Addison-Wesley. 1983. pp. 32-36.

rayleigh waves propagating on ZnO/substrate structures." in Proc.

1977 IEEE Ultrson. Syrp., 1977, pp. 914-819. [Stephen Schwartz was born in Boston, Massa-117] ). K. Elliott, R. L. Gunshor. R. F. Pierret. and A. R. Day, "A wide- chusetts on October 24. 1959. He received the

band SAW convolver utilizing Sezawa waves in the Metal-ZnO-SiO 2 - B.S.. MS.. and Ph.D. degrees in electrical engi-Si configuration." App. Phvs. LIe., vol. 32, no. 9, pp. 515-516. May nering from Purdue University. West Lafayette.

2978.n frmPru nvriy etLfyte1978. IN. in 1981. 1982. and 1985. respectively.zaa1 F. C. Lo. R. L. Gunshot and R. F. Pierret, "Monolithic (Z O) Se- He is currently involved in postdoctoral re-zawa-mode p-n diode-array memory correlator." Appl. Phys. Lett., search at Purdue University. His research interestsvol 34. no. II. pp. 725-726, June 1979. include zinc oxide-on-silicon surface acoustic wave

a,, 1193 S. Minigawa. T. Okamoto. R. Asai. and Y. Sato, "Efficient monolithic resonators, correlators. and filters.

ZnO/Si Sezawa wave convolver." in Proc. 1982 IEEE Ultraso. Syrup., Dr. Schwartz is a member of Tau Beta Pi, Eta

1982. Kappa Nu. and Sigma Xi.(201 J. E. Bowers, B. T. Khuri-Yakub. and G. S. Kino. "Monolithic Se-

zawa wave storage correlators and convolvers," in Proc. 1980 IEEEUltrasonics Symposium Proceedmngs, 1990, pp. 98-103. Robert L. Gunsher was born in New York in

1211 R. L. Gunshor and C. W. Lee. "Generation of a surface acoustic wave 1935. He received the B.E.E. degree from Newcorrelation echo from coupling-to-charge carriers," Appl. Phys. Lett., York University. the M.S.E degree from Unionvol. 21, no. I, pp. 11-12. July 1972. College. Schenectady, NY, and the Ph.D degree

1223 M. Luukkala and G. S. Kino. "Convolution and time inversion using from Rensselaer Polytechnic Institute, Rensselaer.parametric interactions of acoustic surface waves, " Appl. Phys. Let(.. NY, all in electrical engineering.vol. 18. no. 9. pp. 393-394. May 1971. He is Professor of Electrical Engineering at

(231 A. Bets and 3. H. Cafarella. "Surface state memory in surface scous- Purdue University. He has worked on microwavetoelectric correlator." Appl. Phys. Let.. vol. 25. no. 3, pp. 133-135. tubes, plasma waves, and Gunn devices. His cur-August 1974. rent interests include layered thin-film SAW de-

1241 H. Hayakawa and G. S. Kino. "Storage of acoustic signals in surface vices and the growth and evaluation of the Ii-VIstates in silicon."* AppI. Pys. Lett., vol. 25, pp. 178-180, 2974. semiconductor quantum well structures by molecular beam epitaxy.

251] C. Maerfeld, Ph. Defranould. and P. Tourois. "Acoustic storage and Dr. Gunshor is a member of the American Physical Society, Sigma Xi.processing device using p-n diodes." Appl. Phys. Lett., vol. 27. no. and is an associate editor of the IEEE TRANSAC'rIONS ON SONICs AND ULTRA.II, pp. 577-578. Dec. 1975. soNs.

1261 H. C. Tluan, B. T. Khuri-Yakub. and G. S. Kino, "A new zinc-oxide-on-Silicon Monolithic Storage Correlator." in Proc. 1977 IEEE UI- Robert F. Plerret (M'72-SM'84). photograph and biography not available

trison. Symp.. pp. 496-499. 1977. at the time of publication.

I qN.

APPENDIX B

SAW RESONATORS INSENSITIVE TO IDT POSITION

S. S. Schwartz, S.J.Martin*, R. L. Gunshor, S. Datta, and R. F. Pierret

School of Electrical Engineering, Purdue UniversityWest Lafayette, Indiana 47007

Abstract REFLECTOR ARRAY/TRANSDUCER

A theoretical analysis of the ZnO-on-Si surface [p7acoustic wave mode conversion resonator indicates that 77/optimum performance can be attained irrespective oftransducer location between reflector arrays. That is, a hsingle transducer of Rayleigh or Sezawa periodicity can

be placed anywhere between properly spaced mode ZnO

conversion gratings and it will couple optimally to a SI h

standing wave of the same periodicity. An extension of SIthe theory allows one to fabricate both one-port andtwo-port resonators with non-critical spacing between ALUMINUMthe transducers and the gratings. Experimentalverification of the position independent behavior ispresented for one-port and two-port devices.

I. Introduction 0Since the introduction of the first surface acoustic -

wave (SAW) resonator by Ashl, this class of devices (b) 0-49-has been useful in performing many signal processing 0 -r0tasks. Among other applications, these rugged, high Qelements have been used as accelerometers 2, pressuresensors 3, vapor sensors 4, narrowband filters 5, ,', , and s 0 O 115 125 136frequency control elements for oscillators , 30. Although FREQUENCY (MH-)most SAW resonators constructed to date have beenfabricated on single crystal substrates such as quartz

.. and lithium niobate, a great deal of progress has been- achieved in implementing SAW resonators in the

"-:- ZnO/SiOSSi layered medium configuration 3 . Like thesingle crystal resonators, these devices have demon-strated low insertion los, high Q values 12, favorableaging characteristics13 , and temperature stability com- 0I. parable to ST-quartsz t . To exemplify some of these Z

properties, the two-port transmission response of a (20SAW resonator in the ZnO/SiO,/Si configuration isshown in Fig. 1. The device, whose response is shown, .40was constructed in the temperature stable configurationwith apodized transducers to eliminate undesirabletransverse modes. The insertion loss of 4 dB is an indi- 10 10 110cation of the high performance capability of SAW reso. FREQUENCY (3415)nators in the layered configuration.

* Fig. 1. (a) Schematic of a typical two-portZnO/SiO,/Si SAW resonator. (b) Two-port tra,, - Ision response for a ZaO-o-Si SAW resonator. (c)

• Present addrem: Sandia National Laboratoies, Two-port transmission vespoun without transverseAlbuquerque, New Meaieo modes.

0090-560784=000229 $1.00 1984 IEEE 1964 ULTRASONICS SYMPOSIUM - 229 *"i '"

.At

, '..

SCHWARTZ, et &I

Recently, a new concept in ZnO-on-Si SAW reso. periodicity of the reflector array, d, is determined bynators was reported16 involving mode conversion the mode conversion condition 17, 18

between Rayleigh and Sezawa waves rather than single ks + k = r (1)mode Bragg reflection for confinement of SAW energy. S + dThe first of these mode conversion devices were fabri-cated with the promise of high out-of-band rejection where kR and ks are the Rayleigh and Sezawadue to the relatively low cross coupling level between wavenumbers respectively.interdigital transducers (IDT's) of different periodicity. As previously described 16 , the reflector array N% 6%

In this paper we present yet another advantage of this periodicity is chosen to satisfy Eq. I such that a wavetype of SAW resonator - that of a relaxed spacing of Rayleigh or Sezawa type incident upon a reflectorrequirement between the transducers and the gratings. will be back-scattered as a wave of the other type atWe demonstrate that the location of the standing the same frequency. That is, a wave launched by thewaves in the device are determined by the position of Rayleigh transducer which impinges on a grating will -.

the transducers and that regardless of transducer loca- be reflected as a Sezawa wave and, after traversing thetion, one will couple optimally to the resonant cavity, cavity as a Sezawa wave, will be re-reflected by the ""

Although a detailed description of the theory of opposite grating as a Rayleigh wave. (The same holdsoperation of the mode conversion resonator will be pub- true for Sezawa waves converted to Rayleigh waves.)lished at a later time, we use this paper to present Because bi-directional transducers are used, the process .experimental verification of our predictions. Section 1H occurs in both directions; when the total round tripprovides a description of the basic device structure and phase shift of the multiply reflected waves is a multipleoperation, followed by a brief outline of the device of 2r, standing Rayleigh and Sezsawa wave patternstheory in Section M. We present experimental results result and resonance is achieved. It is this 2r phasein Section IV for resonators fabricated in both one-port shift requirement which imposes a restriction on theand two-port configurations. separation between reflector arrays. More precisely, for

resonance to occur, the separation, L., between inneredges of the mode converting arrays must beii. Device Structure "'-

The mode conversion resonator developed by Mar- L, = md , (2) --fin et. &116. consists of the same basic features as aconventional single mode resonator (i.e. transducers and where m is any integer and d is the array periodicity.

reflector arrays); it is dimensionally, however, very When this condition has been satisfied one will coupledifferen .. A schematic of a two-port version of the optimally to the resonant cavity when each of themode conversion resonator is shown in Fig. 2. The transducers is positioned such that its fingers are atdevice consists of two interface transducers 6 between locations of standing wave maxima of a wave with theion beam etched reflector arrays onto which a ZnO same periodicity.film, of sufficient thickness to support the Sesawa We will show that for mode conversion resonators,mode, has been deposited by rf diode sputtering. One it is the placement of the transducers (between properlytransducer is of Rayleigh wave type with periodicity spaced gratings) which dictates the location of theAR, and the other has Sesawa wave periodicity Xs . The standing wave pattern, irrespective of where the tras-

ducer is located between mode converting arrays.Furthermore, a transducer of Rayleigh or Sezawa type,placed randomly in the cavity will always coupleoptimally to the resonant modes. This is in marked

W .. ;;Tcontrast to a conventional SAW resonator where thegrating periodicity and separation between gratings -7determine the location of the standing wave pattern -*and hence the placement of the transducers for optimal.".

Amp performance.

I. Operation

To examine the position independence of the modeAt conversion device we consider the resonant cavity in

'the absence of transducers. The two modes, A and B,depicted in Fig. 3 represent independent normal modesof the resonator. When the spacing between reflectorsis such that the total round-trip phase shift for each

Fig. 2. Schematic of a two-port mode conversion reso- mode (as Rayleigh wave in one direction ad anator using both Rayleigh and Sezawa transducers. Sezawa wave in the other direction) is a multiple of 2w,

V.

230 - 1964 ULTRASONICS SYMPOSIUM

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SCHWARTZ, et &I

/ MODE CONVEING RZVLB17OR5 To fabricate a two-port resonator and still be ableto exploit the positional independence of this structure,one must use transducers of the same periodicity whichare spaced an integer number of half wavelengths fromeach other. As described above, placement of the first

... ... IDT guarantees the standing wave pattern location;Ithis ensures maximum coupling to the same standing

wave at the other transduer. As long as both tru_-ducers are spaced properly with respect to each other,tducer ardepedt prtorly, ithsect to acih oTr

R the two-port device using identical IDT's will be posi-tion independent. Additionally, either a Rayleigh IIT

I I A pair or aSezawa IDT pair may be used for this resona-I J tor.S I

IV. Experimental Results

R To demonstrate the positional independence of theresonator response on IDT position, we fabricated bothone-port and two-port mode conversion devices with

B different spacings between transducers and reflectorS arrays. Both types of devices were fabricated on (100)

cut (<100> propagating) silicon substrates. The only

Fig. 3. Schematic of the mode conversion resonator changes between devices were the positions of the

cavity illustrating two independent resonant modes. transducers between reflector arrays. All transducersused were of Sezawa type and had a periodicity of

each of these independent modes is a resonant mode of 355pm. The transducers for the one-port devices had

% the cavity. Unlike a conventional resonator whose nine finger-pairs while the transducers used in the two-

resonant mode is a standing wave pattern, each of port devices had five finger-pairs each. For both struc-

these resonant modes consists of travelling Rayleigh tures, the reflector arrays consisted of 400 ion beam

and Sezawa waves. The introduction of a transducer of etched grooves 1300 A deep with a periodicity of 12.7

either Rayleigh or Sezawa type serves to couple these pm and a beamwidth of 1.5 mm. For the devices used

modes together by fixing a finite phase relationship in this experiment, the transducers and reflector arrays

between them. It is this coupling of the resonant were defined on the top surface of the ZnO rather than

. modes which ensures that the fingers of the transducer at the ZnO/Sin a interface (Fig. 2) because equivalentwill be located at the positions of standing wave max- coupling to both wave types was no longer necessary' 5 .

ima of the wave with the same periodicity as the IDT. The array separation used in ali devices was 0.762 mmAny shift in the position of the IDT will cause the which corresponds to 60 grating periods.standing wave of the same periodicity to move along In the one-port mode conversion resonator wewith the transducer irrespective of where it is posi- fabricated a series of devices with the transducer posi-tioned between reflectors. It is important to note that tion varied by fractions of a Sezawa wavelength froma standing wave of the other type will exist simultane- the center of the cavity. The devices fabricated hadously, but a shift in position of the IDT will cause a tarSdisproportionate shift in the location of the other transducers placed F6 from the center of the

standing wave. In a one-port resonator the location of reflectors with n = 0,1,2,...,?. Results of this experi-the other standing wave need not be considered since ment can be seen in Fig. 4 which shows the measured

*, the transducer couples automatically to only one stand- radiation conductace, G., for a number of transducering wave type. However, when coupling to both stan.-4 positions for the mode conversion resonator togethering wave types in a two-port configuration, ,ne must with computed G. values for conventional one-portknow the relative positions of both standing waves. -esonators with the same spacings.

To couple optimally to this resonator through As is expected, in a conventional one-port strue-transducers of differing periodicity one can place a ture with the same spacing, the resonant coupling eon-transducer of Rayleigh or Sezaws type anywhere dition greatly depends on the transducer placementbetween reflectors but must place the transducer of the between reflectors. In the mode conversion resonatorother periodicity at one of a limited number of allowed structure, however, resonance as demonstrated by thepositions which are determined by the location of the enhanced radiation conductance, can be observed forfirst IDT with respect to the reflector arrays. That is, any location of the IDT.in a two-port mode conversion resonator with transduc- A similar experiment was performed for two-porters of different periodicity, the placement of one of the mode conversion resonators with a fixed spacing oftwo IDT's is critical. II 5 between centers of identical Sesawa transducers.

1984 ULTRASONICS SYMPOSIUM - 231

II... ... ... .... ... ... .. .. . .. . .. . . .. . .. . .. . . .. . .. . .. . .

SCHWARTZ, et al

(b). (d)• Ii

I - I ,

i 4e vs .frequeacyd experimta fr on I(a) a.. (b) (c) s (d)

L'I,

S. .. -

ve.i

(e) .. () ... (g) ...

KFig. 4. Radiation conductance vs. frequency determined experimentally for one-port mode cover- .

sion resonators (right) and computed for conventional one-port resonators (left). In both result, the

transducer was displaced from the cavity center an amount -H for (a) nO (b) n (c) n2 (d)

cn=3 (n= (e) n= n-4=7 18' for (a =1() n=2

lop, 1 UT ::NC SYMPOSIUM(a)~ ~ ~ ~~~ ... ().. (),,

5

-a

( e n 3 ( d ) 3 5 , ,e ' " " ' ( e) 7 .-- 1' "6- = .. ..'

232 - 164 ULTRASONICS SYMPOSIUM 4'".

; " ". " , " " ". .' " .'-" """ " ".'" " '"" " ". " "" .'.'""" ." . ."-" " ."- ',"." ""." . -" ". .. -'a'

SCHWARTZ, et al

The midpoint of the pair of transducers was varied Referencesfrom the cavity center by the same fraction as in theone-port experiment; the results are shown in Fig. E.Here again, the calculated response for a conventional Resonatr.s, IEEE Sympouium Microwave Theordevice with the same dimensions was computed and and Techniques, Newport Beach, Calif., May 11-plotted alongside the experimental results. It is evidentthat as the location of the transducer pair is moved 14, 1070.within the cavity, there are drastic changes in theresponse of a conventional device (i.e. resonance and 2. S. J. Martin, R. L. Gunshor, R. F. Pierret, and G.antiresonance) but the results for a two-port mode Gorodetsky, "Uniaxially Strained ZnO/SiOS/Siconversion resonator are almost invariant. SAW Resonators," Electronice Letters, vol. 18, no.

For both one-port and two-port device structures 24, pp. 1030-1031, 25 November 1982.

the resonant Q values were in the range 00-1500 but itshould be noted that no attempt was made to optimize 3. P. Ds, C. Lanzi, and H. F. Tiersten, "A Pressurethese resonators but rather to demonstrate the posi- Sensing Acoustic Surface Wave Resonator," IEEEtional independence of the transducer placement. it Ultrasonics Synposium Proceedings, pp. 306-308,should also be pointed out that in both one-port and 1976.two-port devices, slight variations in the deviceresponses for different spacings are due to the fact that 4. S. J. Martin, K. S. Schweizer, S. S. Schwartz, andthe standing Rayleigh wave pattern is not invisible to R. L. Gunshor, "Vapor Sensing by Means of athe Sezawa IDT's and some coupling will exist which ZnO-on-Si Surface Acoustic Wave Resonator,"will be different for each spacing chosen. This variation IEEE Ultrasonics Symposium Proceeding., To becan be minimized by increasing the number of fingers published in this issue.per transducer.

5. R. Stevens, P. D. White, and R. F. Mitchell,"Stopband Level of 2-Port SAW ResonatorV. Conclusions Filters," IEEE Ultrasonics Symposium Proceed-

We have defined a technique whereby one can ings, pp. 905-908, 1977.fabricate SAW resonators whose response is indepen-dent of IDT positioning. Results have been presented 6. R. V. Schmidt and P. S. Cross, "Externally Cou-for devices fabricated in both one-port and two-port pled Resonator-Filter (ECRF)," IEEE Trans. onconfigurations and comparisons have been made with Sonic. and Ultrasonics, vol. SU-26, no. 2, pp. 88-conventional SAW resonators. Furthermore, the 92, March 1979.relaxed spacing requirement we have demonstratedallows one to more easily incorporate these devices intoa standard monolithic IC fabrication process. The 7. R. L. Rosenberg and L. A. Coldren, "Scatteringeased fabrication stems from the fact that a grating can Analysis and Design of SAW Resonator Filters,"be defined in the Si0 2 during an already existing etch- IEEE Trani. on Sonic. and Ultrasonics, vol. SU-ing step and the transducers can be defined during the 26, no. 3, pp. 205-229, May 1979.final metallization step in a completely different planewith no need for a critical alignment. 8. G. L. Matthaei, E. B. Savage, and F. Barman,

"Synthesis of Acoustic-Surface-Wave-ResonatorFilters Using Any of Various Coupling Mechan-

Acknowledgements isms," IEEE Trans. on Sonics and Ultrasonics, vol.The authors wish to thank S. Phillips, T. Corbin, SU-25, no. 2, pp. 72-84, March 1978.

and G. MaGee of the Naval Avionics Facility at Indi-anapolis for the photoplates used in these experiments 9. T. O'Shea, V. Sullivan, and R. Kindel, Precisionas well as T. Miller and M. Young for their assistance L-bond SAW Oacillator for Satellite Application.in the fabrication of the devices.

This work was sponsored by the Air Force Office 10. R. D. Colvin, "UHF Aeoustie Oscillaton,"

of Scientific Research under Grant No. AFOSR-a&- Microwave Journal, pp. 22-33, November, 1980.0237.

1914 ULTRASONICS SYMPOSIUM - 233

W..

SCHWARTZ, et al

11. S. J. Martin, S. S. Schwartz, R. L. Gunshor, and 15. S. J. Martin, R. L. Gunshor, M. R. Meliocb, S.R. F. Pierret, "Surface Acoustic Wave Resonators Datta, and R. F. Pierret, "Surface Acoustic Waveon a ZnO-on-Si Layered Medium," J. Appl. Phy., Mode Conversion Resonator," Appl. PApy. Lett.,vol. 54, no. 2, pp. 581-589, February 1983. vol. 43, no. 3, pp. 238240, August 1983.

12. S. S. Schwartz, S. J, Martin, R. L. Gunshor, S. 16. M. R. Melloch, R. L. Gunshor, C. L. Liu, and R.Datta, and R. F. Pierret, "SAW Resonators on Sil- F. Pierret, "Interface Transduction in theicon with ZnO Limited to IDT Regions," IEEE ZnO-SiO2 -Si Surface Acoustic Wave DeviceUltrasonics Symposium Proceedings, pp. 161-163, Configuration," Appl. PAg.. Lett., vol. 37, no. 2,1983. pp. 147-150, 15 July 1980.

13. S. J. Martin, R. L. Gunshor, M. R. Melloch, S. 17. M. R. Melloch, R. L. Gunsbor, and R. F. Pierret, "Datta, and R. F. Pierret, "Surface Acoustic Wave "Conversion of the Sezawa to RayleiKh Waves inMode Conversion Resonator," LEEE Ultraonics the ZnO-SiO2-Si Configuration," IEEE Ultrs-Symposium Proceedings, 1982. sonic$ Symposium Proceedings, 1981.

14. S. J. Martin, R. L. Gunshor, and R. F. Pierret, 18. M. R. Melloch, R. L. Gunshor, and R. F. Pierret,"High Q, Temperature Stable ZnO-on-Silicon "Sezawa to Rayleigh Mode Conversion in theSAW Resonators," IEEE Ultruoni:. Symposium ZnO-on-Si Surface Acoustic Wave deviceProceedings, 1981. Configuration," AppI. Phys. Lel., vol. 39, so. 6,

pp. 476-477, September 1981.

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APPENDIX C

Wide gap ll-Vl superlattices of ZnSe-Zn, _,Mn SeL. A. Kolodziejski, R. L. Gunshor, T. C. Bonset, R. Venkatasubramanian, and S. DattaSchool of Electrical Engineering. Purdue University. West Lafayette Indiana 47907

R. B. Bylsma and W. M. BeckerPhysics Department. Purdue University. West Lafayette. Indiana 47907

N. OtsukaSchool of Materials Engineering. Purdue University West Lafayette Indiana 47907

(Received 21 March 1985; accepted for publication 29 April 1985)

In this letter we report the first growth of wide gap Il-VI semiconductor superlattices ofZn, , Mn, Se (0 < (0.51). The superlattices are grown by molecular beam epitaxy. Bulkcrystals of Zn, , Mn, Se (0 <x <0.57) grown in the past have shown a predominance of thezincblende phase only up to x = 0.3; above this mole fraction a predominance of the hexagonal

0,r phase is observed. For the superlattices and epilayers reported here, only the zincblende phase(100) is present over the entire composition range investigated. Transmission electron microscopyshows clear evidence of the superlattice structure. Photoluminescence measurements of ZnSe '

epilayers show a dominant free-exciton feature while the Zn, Mn, Se epilayers exhibit twodistinct photoluminescence peaks. The relative intensities of band-to-band transitions and Mn-related transitions are somewhat comparable for the epilayers. However, the superlattices havingZnSe in the wells show virtually no Mn-related emission regardless of the mole fraction of Mn in

a4 the barrier layers indicating significant carrier confinement. Thc intensity of the band-to-bandemission is two orders of magnitude more intense for the superlattices than the ZnSe epilayer at ...r6.5 K. The superlattices show a red shift of the photoluminescence peak which can be explained interms of shifts in the band gaps due to the presence of strains.

Recently the growth of epilayers and superlattices of 1I- vestigated; no hexagonal phases were observed.' The dimen-VI compounds and their alloys by molecular beam epitaxy sions ofthe superlattice structure areconfirmed by transmis- .-.

IMBEI has attracted much attention. Two alloy systems sion electron microscope (TEM) investigations. Electronhave been reported so far, the narrow band-gap diffraction shows satellite spots characteristic of the super- - -

Hg, .Cd,Te system and the Cd, , Mn,Te system which lattice structure; bright-field and dark-field imaging revealshas a band gap slightly larger than that of GaAs.' - This the occurrence of very abrupt interfaces between each super-letter reports the first growth of superlattices based on a new lattice layer. Photoluminesence (PL) studies indicate a con-alloy system Zn, _,Mn, Se. Because of their wide band gap cave upward bowing of the band-gap variation as a function(-2.7 eV at 300 K). these materials may prove useful in the of Mn concentration. The dominant PL feature of the ZnSedevelopment of blue light emitters and display devices. Also epilayer is a free exciton having a full width at half-maxi-the large exciton binding energies in these materials are of mum (FWHM) of 1.5 meV. The near-band-edge emission forinterest both from a basic as well as an applied point ofview. the superlattices shows a continual red shift as the barrier

In this letter we report the growth of wide gap semicon- height is increased, dominating the expected blue shift origi-ductor superlattices of Zn, - Mn, Se/ZnSe; this is also the nating from quantum confinement of the carriers. For epi-first growth of films of the alloy material system layers of ZnMnSe a broad Mn emission near 2.1 eV becomesZn, Mn,Se (0<x <0.51). The various superlattice struc- dominant as the Mn mole fraction increases, with the near-tures and epilayers have been grown by molecular beam epi- band-gap luminescence peak becoming less intense. In con-taxy. In all superlattices ZnSe was used for the well material trast, the superlattices show virtually no Mn emission for allwith well dimensions ranging from 60 to 90 A. The compositions of the ZnMnSe barrier layers. The PL intensi-Zn, , Mn, Se barrier material consisted of various Mn mole ty for the superlattices is 100 times more intense than ZnSefractions (0.23 <x < 0.51)with dimensionsof 100-150 A. All and ZnMnSe thin films at 6.5 K.superlattice structures consisted of 67 periods grown on a The ZnMnSe superlattices and epilayers were grown us- ...ZnSe buffer layer of 0.57-0.93 pm. In order to characterize ing a Perkin-Elmer model 400 MBE system. The MBE sys-the barrier material, ZnMnSe epilayers were grown on a tern and the various in situ analytical facilities have beenZnSe buffer layer of 0.25-0.44/pm and were typically 2-3 described previously.' Briefly however, a movable nude ionpm thick. Th-! Mn mole fraction reported here was deter- gauge is used to measure the individual fluxes and reflectionmined by energy dispersive analysis of x rays (EDAX). The of high-energy electron diffraction (RHEED) is used to ob-concentrations are believed to be accurate to within 10%. serve the growth behavior. The base pressure of the growthAlthough Zn, Mn5 Se (0.0<.< .571 has been grown in chamber is typically I x 10 0 Tor.

bulk crystals, a predominance of the zincblende phase is only Three individual effusion cells containing elemental Zn,observed up to x = 0.3, whereas above this mole fraction a Se, and Mn were used for sources. These elements were ob-predominance ofthe hexagonal phase exists.' For the super- tained commercially at a purity of 6N, 6N, and 4N, respec- ,..lattices and epilayers reported here, only the zincblende tively. Each source material was vacuum distilled prior to 4%

phase 100) is present over the entire composition range in- loading into the MBE system, which provided a final no-

1" Apio Lett 47(2),15 July 1965 0003.6951185/140 69-03$01 00 M 8 INS AmerW lngttutgeofPhya 169

,t)t .j ]

FIG. 1RHEED patterns during growth or Zn Mn, Se x .0. 33)/ZnSesuperlattice: (a) ZnSe well layer; (b) Zn, ,MnSe (x = 0.33) barrier layer.

minal purity of 6N for the Mn, while also enhancing the FIG. 2. Dark-field image of the Zn, M n. Se (x =0. 33)/ZnSe superlattice

purity of the Zn and Se." The temperatures of the Zn and Se with the (200) diffraction spot (inset) showing satellite spots.

ovens were typically 325 and 200 C, respectively. The Mnoven temperature, however, ranged from 820 to 950 "C to planes were also found in the superlattices and buffers. Theyprovide for various Mn mole fractions. are often accompanied by dislocations, suggesting that these

GaAs substrates are used for their technological impor- microtwins provide nucleation sites for the generation of dis-tance and because of the close lattice match with ZnSe locations during film growth. Figure 2 is a dark-field image(0.25% mismatch). Cr-doped, semi-insulating GaAs (100) of a Zn, - .Mn.Se (x = 0.23)/ZnSe superlattice. Shown insubstrates oriented 2 off axis were used. The substrates were the inset of the figure is the (200) diffraction spot which wasprepared by standard chemical etching procedures.6 The used for the dark-field image. These films were found to besubstrate temperature was calibrated for each film growth highly susceptible to the radiation damage induced by theby observing a Au/Ge eutectic phase change. The calibrated ion beam during sample preparation. Although a very lowsubstrate temperature for the superlattices and the ZnMnSe voltage was used at the final stage of ion thinning, fine struc-epilayers was 400 "C, while growth rates ranged from 2 to 5 tures due to the radiation damage are still seen in the image.A/s. From the image, thicknesses of the ZnSe and ZnMnSe layers

Following oxide desorption, the GaAs substrate was were estimated to be 63 and 104 A, respectively. No misfitmonitored with RHEED to observe the initial growth be- dislocations are observed at interfaces between superlatticehavior. Since all superlattices and epilayers were grown on a layers as is excepted from the size of the lattice mismatchZnSe buffer laye7, the initial growth behavior for ZnSe will (0.95%) and the layer thicknesses. The lattice mismatch isbe described for a substrate temperature of 400 *C. Immedi- totally accommodated by elastic strain giving rise to strainedately after the shutters are opened, the streaked RHEED layer superlattices. Both dark-field images and diffractionpattern of the substrate changes to a less intense spot pattern. patterns show the highly regular structure of the superlat-The presence of the spot pattern suggests that the initial nu- tice. No sign of disordering at the interfaces is seen in thecleation occurs by a three-dimensional growth mechanism. image. The number of visible satellite spots around the (200)After approximately one minute, the spots elongate into spot is comparable to those of typical AIGaAs-GaAs super-streaks of uniform intensity. After three minutes a twofold lattices. Sixth and seventh order satellite spots are clearlyreconstruction pattern appears which remains for the entire seen in negative films. This suggests that the structural quali-period of growth for the ZnSe buffer layer. At the onset of ty of these superlattices is similar to those of well-developedfilm growth for the ZnMnSe layer (for either the superlattice III-V compound superlattices. The greater intensity of theor the epilayer), the reconstruction streaks become less in- satellite spots at the higher angle side compared to the lowertense intially, but the intensity increases as the film growth angle side is due to the built-in strain caused by the latticecontinues. Figure 1(a) shows the RHEED pattern of the mismatch.ZnSe well layer in a superlattice and Fig. l(b) shows the Photoluminescence measurements were made using UVZnMnSe (x = 0.33) barrier layer. The high crystalline quali- lines at 3336-3638 A from a cw argon ion laser. The detec-ty of both ZnSe and ZnMnSe is illustrated by the presence of tion system consisted of a 1/4-m double grating monochro-Kikuchi lines, and very defined, sharp streaks arising from meter in conjunction with a cooled RCA 31034 photomulti-the bulk ofthe film in addition to intense surface reconstruc- plier tube connected to photon counting electronics.tion streaks. These RHEED patterns persist until the end of Samples were either immersed in LN2 or suspended in athe film growth for all superlattices. helium gas at about 6.5 K. The 10-mW beam was focused to

Transmission electron microscope observations of a spot 100pum in diameter.cros-sectional specimens were performed with a JEM Photoluminescence measurements were carried out on200CX electron microscope. These observations have shown all epilayers and superlattices at 77 and 6.5 K. Data of bandthat typical dislocation densities in superlattices, buffers, gap versus Mn mole fraction for the epilayers were obtained.and interfacial regions between buffers and substrates are The initial variation of the band gap exhibits an upward con-l0", 10' , and 10' cm-', respectively. Many dislocations cave bowing. Such a bowing has also been observed in bulkwhich propagate from the interfacial regions are either ter- crystals by Twardowski et al." At 6.5 K the ZnSe epilayerminated or reflected by boundaries between superlattices (Fig. 3) exhibited a free-exciton dominant peak at 2.79Q eVand buffers. A small number of microtwins parallel to I I I) having a full width at half-maximum of 1.5 meV.' The two

170 AppI Pt ys Left, Vol 47, No 2, I5Jury 1985 Kolodezeiski ea 170

A'.

q..Z%...v

2.1, FIG. 4. PL spectra for the-- PWHrM 1 .6 _V. X~e 1lOO

S4_ -- 4 .l 62 (x = 0.331/ZnSe super-

. *.., lattice and (blZn, _MnSe )x = 0.33)epilayer illustrating rela.

439 440 441 442 443 444 446 444 447 .448 Z , us4

tve PL intensities of the.. e. ,., band edge and Mn emis. ..

FI.3 Pl setrr of the Zn~eepilayer exhibiting the dominant free cxci- siovt~ns.deadMnei-*~ton.

free-exciton features at 2.799 and 2.804 eV are attributed tostrain splitting of the valence band" (the origin of the strain isthe small but finite lattice mismatch between film and sub- O *"° b ..,

strate). The small feature at 2.796 eV represents an impuritybound exciton. In addition to the near-band-edge emission, a confinement of the carriers, these wide gap semiconductor

broad deep level emission centered at about 2.4 eV is ob- superlattices show a continual red shift in energy as the bar-

served with a peak intensity of 0.02 times the dominant free- rier height is increased. Using the Kronig-Penny model with

exciton feature. finite barrier height, one predicts a blue shift for the superlat-Texto netue ia sxiit a tice of 15-22 meV. However, these strained-layer superlat-., ~~The ZnMnSe epilayers exhibit two peaks which are..-.1

identified as a near-band-edge emission and a Mn emission tices contain expansive hydrostatic and compressive uniax-

corresponding to a Mn + + transition. The relative intensities ial strains which result in a lowering of the conduction-band

of these two PL peaks vary with Mn mole fraction. As the minimum and a splitting of the valence-band maxima givingMn fraction increases, the PL intensity of the Mn emission rise to the observed red shift. The material system InAsSbincreases rapidly with the PL intensity of the band-edge peak described by Osbourn has shown similar behavior.'0 The

decreasing. A broadening of the FWHM is also observed for magnitude of the strain-induced shift can be varied by eitherthe band-edge PL peak as the Mn fraction increases, and is changing the Mn mole fraction or the relative thicknesses of

attributed to alloy broadening. the well and barrier. The various superlattices show a red

When comparing the PL spectra of the superlattices shift from the PL free-exciton energy of the ZnSe epilayer of

with the various epilayers grown under identical conditions 3-52 meV.

as the material in the barrier, several very interesting differ- The authors are very grateful to U. Debska for provid-ences become apparent. Figure 4 shows a quantitative com- ing the high-purity vacuum distilled source material, J. K.parison of a Zn, _ Mn5 Se (x = 0.33)/ZnSe superlattice Furdyna and D. Yoder-Short for many discussions related * -

with a Zn, - Mn, Se (x = 0.33) epilayer mounted on the to bulk ZnMnSe, M. Udo for help in the film growth, and R.

same header to provide identical conditions of excitation. A Frohne for working through the strain calculations. The

typical PL spectrum of the epilayer is obtained exhibiting the authors also thank T. Miller and M. Young for their help indominant Mn emission. In this case the ratio of the intensity operation ofthe MBE. L. A. K. would like to thank IBM foroftheMnemissiontotheintensityoftheband-edgeemission fellowship support. This work was supported by Office of

(R ) is 180. In contrast, the superlattice shows an R value of Naval Research contract 014-82-K0563, the Air Force Of-1.6X 10- . Similar behavior is seen in all the superlattices fice of Scientific Research grant 83-0237, and NSF-MRL

grown regardless of the barrier height present (i.e., mole grant DMR-83-16999. I ,

fraction of Mn in the barrier layer) indicating significant car- 'J. P. Faurie. A. Million, and J. Piaquet, AppI. Phys. Lett. 41,713 (1982).rier confinement. 'L. A. Kolodziejski, T. C. Bonsett, R. L. Gunshor, S. Datta, R. B. Bylsma,

W. M. Becker, and N. Otsuka, Appl. Phys. Lett. 48, 440 (1984).For all samples (again with several samples mounted on 'R. N. Bicknell, R. W. Yanks, N. C. Giles-Taylor, D. K. Blanks. E. L.

the same header) the superlattices were much brighter than Buckland, and J. F. Schetzina. Appl. Phys. Let. 45. 92 (1984). :,-

the epilayers of ZnSe and ZnMnSe as determined by the 'D. Yoder.Shori and J. Furdyna (private communication).

relative PL intensities of the band-edge related emission. L. A. Kolodziejki. M. K. Udo, T. C. Bonsett, R. L. Gunshor, S. Data. R.B. Bylsma, W. M. Becker, and N. Otsuka, paper presented at the General

Specifically, at 6.5 K the PL intensity of the superlattices is Meeting of the American Physical Society, Baltimore, MD. March 1985.100 times greater than the brightest ZnSe epilayer described 'L. A. Kolliieski. T. Sakamoto, R. L. Gunshor. and S. Data. Appiearlier. The FWHM of the superlattice ranges from 4 to 9 Phys. Lett. 44. 799 (1984).

'R. L. Gunshor. N. Otsuka, M. Yamanishi, L. A. Kolodziejski. T. C. Bon-meV. At 77 K the superlattices show a PL band-edge emis- Sett, R. B. Bylsma, S. Datta, W. M. Becker, and J. Furdyna, presented at .. ',

sion which is 20 times greater than the ZnSe epilayer. For the International ll-VI Conference. Aussois, France. March 1985.this temperature the FWHM ranges from 8 to 12 meV. All "A. Twardowski, T. Dietl. and M. Demianiuk. Solid State Commun. 48.superlattices showed comparable FL intensities when com- 845 (19831.

*T. Yao. paper presented at the il-VI Internation Conference. Ausois.pared to each other. France. March 1985.

Although a blue shift might be expected due to quantum '"G. C. Ohourn. J. Vac. Sci. Technol. B 2, 176 (1984).

171 AppI. Phys. Lel., Vol. 47, No. 2, 15 July 1985 Kolodzijiski etaf 171

%.~a.-P ~ .% -PPI

irw % sk .1': eL t-.L

APPENDIX D

Optical properties of ZnSe/(Zn,Mn)Se multiquantum wellsY. Hefetz, J. Nakahara. and A. V. NurmikkoDivision of Engineering. Brown University, Providence. Rhode Island 02912

L. A. Kolodziejski, R. L. Gunshor, and S. DattaSchool of Electrical Engineering, Purdue University, West Lafayette, Indiana 47907

(Received 20 June 1985; accepted for publication 9 August 1985)

Luminescence, excitation, and reflectance spectroscopy have been employed to study the excitonground state in ZnSe/(Zn,MnSe) multiquantum wells. We find that the ground state showssplittings which are attributed to the superlattice layer strain. The measured large magneto-optical shifts and splittings indicate that the valence-band discontinuity is likely to be quite smallin these structures.

The recent successful growth ofZnSe/(Zn,MnlSe multi copy in which radiation from the He-Cd laser was used toquantum wells (MQW) has made another addition to the inject a modulating component and the induced incrementalnew family of II-VI compound semiconductor superlat- cb ,ge% in reflectance were phase-lock detected (In such atices.i The molecular beam epitaxial (MBE) growth of single p, ,tomodulAtion technique exciton-exciton and exciton-crystal films of the "semimagnetic semiconductor" frt carrwr interaction modulated the interband dielectric(Zn,Mn)Se has also yielded for the first time material which constant 'i The magnetic field experiments were carried outremains in the cubic phase at Mn-ion concentrations in ex- in a superconducting magnet dewar up to B = 4.0 T, with .- .

cess of x = 0.5, much higher than those in conventionally the field orientation either parallel or perpendicular to the " ."

grown bulk material (where transition to a mixed cubic-hex- superlattice z axis and with circular or linear optical polar- -'-

agonal structure occurs at about x = 0. 1). The interfacial ization.quality of the ZnSe/(Zn,Mn)Se MQW's is high as evidence Figure I shows spectral line shape profiles near the exci-by the narrow linewidths observed in the very bright low- ton ground state for the MQW sample described above, ob- ....

temperature photoluminescence and by transmission elec- Waned by the different techniques at 2 K. The excitationtron diffraction patterns which show interracial abruptness spectra show the existence of two primary transitions in thisform the presence of up to seventh order satellites. In this spectral region near the exciton ground state (the higher en-letter we present results of optical measurements of the exci- ergy resonance had further structure which became particu-ton ground state in this novel structure, obtained from pho- larly evident with the application of an external magnetictoluminescence, excitation, and reflectance spectroscopy at field). Somewhat unexpectedly, the amplitudes of the reflec- -"-

low tempertures. The main early conclusion which emerges tance signals were substantially larger for the higher energyfrom our work is that the valence-band offsets are likely to bevery small in this MQW structure and that the exciton statesare strongly influenced by the superlattice strain The pres- T: 2Kence of the magnetically active ion Mn in the superlattice Lummakes the external magnetic fields particularly useful probesfor the spatial extent of the exciton wave function2 and have _provided us with direct evidence for a relatively weakly con- V

fined hole state. The results presented below represent a par- " "tial summary of work; other details including temperature tdependence and lifetime of exciton luminescence will be dis-cussed elsewhere.' 3

The optical studies were carried out on several MQW Rsamples, grown in the [100] direction of GaAs substrates "following the initial deposition of a ZnSe buffer layer.' The -

Mn-ion concentrations ranged from x =0.23 tox =0.51. Inthis letter we show results for a sample with x = 0.23, con- "sisting of 67 layer pairs of approximately 67-A-thick ZnSe 1. Rwells and 1 10-A-thick (Zn,Mn)Se layers. Comparison with 0

optical properties of a (Zn,Mn)Se single crystal film(x = 0.23) shows a total band-gap difference of approximate- Ily 10 meV for the unstrained heterojunction. Luminescence 2.77 2.81 2.85was excited at 0.325um from a low power cw He-Cd laser ENERGY (eV)FIG, 1. Illustration of spectral line shapes near the exciton pround state for "-

and collected parallel to the superlattice growth axis (z). Ex- A ,. l aion of ael thinss und 6" ate,""* ~ZnSe/Zn.,,Mn(2,Se MQW sampte of well thickness 1. m 67 A siTw.2citation spectra were obtained by using a pulsed dye laser, K. obtained (from top) by luminescence, excitation (at RAs - 2.783 eV),pumped by an excimer laser source. Standard reflectance modulated reflectance (low-amplitude resolution), and reflectance spectres-measurements were supplemented by differential spectros- copy. The arrows indicate the primary resonances discussed in text.

ApoA. Phys. Left. 47 (9). 1 November t95 00036951/5/210909-03 i.00 ( t 985 Amerlcan Instfluta of Phy9s 99

components. A weak contribution in luminescence was also "seen in this region, becoming relatively larger with increas- o -.ing temperature.' Such a split resonance character in the 2.12 .exciton ground state was observed also in the other MQWsamples studied, with the mean separation of energies equal . "to approximately 19 meV for the case in Fig. I. More de- -" ° "tailed work on the excitation spectroscopy is also being per-formed by Bylsma and Becker' in their study of these super- K "lattice structures. For example, for MQW samples with - 8 -x = 0.33 (I,/Ib = 60 A/150A)and x = 0.51 (/,,/Ib = 90,/ D130 A) the corresponding separation was approximately 40 - -- -

and 50 meV, respectively. The bright low-temperature lumi- .nescence peak showed evidence of internal structure, as il- "lustrated by the presence of the distinct low-energy shoulder. ccThe radiative efficiency from all the MQW samples de-creased precipitously with increasing temperature, typically 2.78F T 2 Kshowing a reduction by up to two orders in magnitude at 77K. I__

The application of an external magnetic field leads to 0 2 4 1 _shifts and additional splitting of the exciton ground state in M ,(; N ET IC F I EI) ( 1) Tthese superlattice structures. These effects originate from FIG. 2 Summary of data for the sphlt exciton ground state resonance for the

the "giant" spin splitting which are due to the spin exchange ZnSe/Zn, ,,Mn,, Se MQW sample at T = 2 K as a function of magneticinteraction from the overlap of the exciton wave function field (Faraday geometry) The triangles denote data obtained from lumine-'

with Mn-ion magnetic moments, completely dwarfing the cence. circles by excitation spectroscopy. squares by reflectance, and dia-effects. 2 The results, which combine the use monds from modulated reflectance Open and closed symbols refer to oppo-

normal Zeeman e in the circular polarization as labeled i the figure The line% are to

of all of the optical techniques listed above, are'partly sum- guide the eye

marized in Fig. 2. where the main resonances near the exci- -.ton ground state are graphed as a function of the magnetic components. while the reverse occurs in the (Zn.Mn)Se lay-field in the Faraday geometry, and the dominant polariza- ers. The uniaxial components split the Im I = 3 /2, 1/2 holetion of the optical signals is labeled. The reflectance spectra orbital degeneracy at k = 0 so that for the compressive ele-(Fig. I i show the presence of Fabry-Perot fringes; folh,<',ing ment in ZnSe, the In, I = 1/2 should push up to higher ener-their subtraction the actual resonances were defined by not- gy in bulk material while the im, I = 3/2 state is lowered.'ing the coincidence with peaks in the excitation spectra. Ad- Assuming that the elastic constants are equal in both materi- . -

ditional spectral structure was also evident, particularly in als, using the available hydrostatic and deformation poten-the photomodulated reflectance, but we focus our attention tial constants for ZnSe, and taking all of the band offset tohere to discuss of the main resonances. occur in the conduction band (with n = I electron confine-

At zero magnetic field (T = 2 K) the lowest resonance in ment energy of about 20 meV), the observed energies andFig 2 (at about 2.796 eV), measured, e.g., through excitation their primary zero field splittings in Fig. 2 (and the trendspectroscopy, was 3-4 meV above the low-energy shoulder toward lower photon energies of exciton emission with in-of the luminescence. The shoulder grew in amplitude with creasing x in other MQW samples) are in good numericalthe magnetic field, becoming the dominant feature at agreement with estimates.' The additional doublet structureB = 1.5 T. Furthermore, a splitting is present at the higher in these exciton ground state resonances may also be strainenergy resonance and easily seen, e.g., in the excitation spec- related (e.g., from the strain splitting of the valence band in -,

trum (peaks at 2.810 and 2.816eV), particularly when extra- the (ZnMn)Se "barrier" layers, which translated to split-polating magnetic field dependent shifts to B = 0 T. tings in the effective quantum well potential). However, the

We now consider the physical origin of these splittings full details of this effect need to take into account properof the exciton ground state and their behavior in an external matching of the wave functions at the interfaces, the effectivefield. The appearance of the two main resonances in zero mass anisotropies, and the (unknown) details of the excitonfield excitation and reflection spectra of the ground state envelope function.(and in part their further splitting) is expected from consider- The magnetic field induced changes in the exciton ener- ,.-ing the influence of strain on the valence bands in these zinc- gy add support to our assignment for strain split valence-blende semiconductors. The rather narrow linewidth (aboui band ordering and also yield an idea about the hole confine-5 meV) of the main luminescence peak suggests that such ment in the ZnSe/(Zn,Mn)Se quantum wells. In bulkstrains are reasonably homogeneous across the MQW struc- (Zn,Mn)Se, the spin exchange coefficients a and # for theture. Since the lattice constant of (ZnMn)Se increases with conduction- and valence-band edges have been measured atthe Mn concentration x, elastic accommodation of the lat- low Mn concentrations (x < 0.10) and show that -,61tice mismatch in the ZnSe/(Zn,Mn)Se structure (1.05% for a = 4.2.' Assuming that the conduction electron is well con-x = 0.23) implies that the strain in the ZnSe layers has dila- fined in the ZnSe layers of the MQW structure and ignoring -.-

tional hydrostatic and compressional uniaxial (in z direction) the exciton influence, we then attribute most of the measured ,]

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magneto-optical effects to the valence band. The magnetic 20 meV. As shown above, the strain splittings of the valence-field lifts the remaining Kramers degeneracy (m, = + 3/2, band top are of comparable magnitude for the sample dis-- 3/2 and + 1/2, - 1/2, respectively). The field induced cussed here and increase for structures with higher Mn-ion

splitting and shifts have contributions through the exchange concentrations in the "barriers." This may affect significant-mechanism from the penetration of the hole wave function ly the degree of confinement for the "heavy" and "light"into the (Zn,Mn)Se layers. Furthermore, modulation (spin holes; in principle it would be possible for one of them to besplitting) of the valence-band top of the (Zn,Mn)Se layers by in confined particle states and the other in the (delocalized)the magnetic field also changes the effective "barrier" height continuum. We particularly note the puzzle concerning thewhich influences the penetration depth for a confined parti- relative amplitudes of the excitation and reflectance spectracle hole. In a self-consistent calculation, these effects need to in Fig. I, where the reflectance amplitudes are much larger " -

be included appropriately. The field splitting for the upper for the upper strain split resonance; this behavior could beresonance in Fig. 2 at a given field is substantially larger than related to such differences in the valence-band potentials.that for the lower resonance. The opposing senses of circular Generally, for weak confinement of the holes the exciton -0polarization measured for the two transitions are consistent problem becomes considerably more complicated than that

, with expected angular momentum selection rules. We also allowed for by presently available calculations [constructedmeasured the magnetic field induced shifts in low-tempera- mainly for the GaAs/(Ga,AI)As quantum well case]. At theture exciton luminescence in a single crystal film of same time, it is unlikely that the ZnSe/(Zn,MnlSe quantum(Zn,Mn)Se with x = 0.23. (In bulk material the lumines- well system is significantly of type II since the magnitudes of.._,a -cence is dominated by the mj = - 1/2 to m, = - 3/2 con- the observed magneto-optical shifts are not large enough toduction- to valence-band transition.) At a field B = 2 T, the support this conclusion. Other complications arise from theratio of the splittings in the MQW sample to that determined strain induced anisotropies which lead to anomalous effec- ...from the bulk films is approximately AEq,,/AEb = 0.18 and tive mass changes at the heterointerfaces and which mightAEq,,/AE, = 0.41 for the lower and upper strain split exci- considerably modify the spatial extent of the exciton waveton resonances, respectively. From these values we conclude function (possibly enhancing its amplitude near the inter-that the assignment of the strain split ordering for the va- faces).lence band with the lm,I = 1/2 state at the higher energy is We wish to thank J. Torreao for expert help in thesecorrect Ithe exchange effect being proportional to m, in bulk experiments. The work at Brown University was supportedsemimagnetic material). The two ratios do not differ by the by the Office of Naval Research contract N00014-83-K0638factor of 3 which would occur in bulk (Zn,Mn)Se; this devi- and a Department of Energy grant DE-FG02-84-ER13235.ation is not surprising considering that the details of the exci- The work at Purdue University was supported by the Officeton envelope functions for the two transitions are likely to be of Navel Research contract N00014-82-K0563, the Airdifferent The magnitudes of the observed splittings Force Office of Scientific Research grant 83-0237, and NSF-A. (B), however, show directly that a large portion of the MRL grant DMR-83-16988. L. A. K. would like to thankhole envelope function resides in the (Zn,Mn)Se layers, im- IBM for fellowship support.plying that the valence-band offset is small For the Im, I= 1/2 holes state as much as half of the wave function is

est-mated to reside outside the ZnSe layers if we assume an 'L A. Kolodziejski. R. L Gunshor. T. C. Bonsett, R. Venkatasubraman-tan, S Datia. R B Bylsma. W M. Becker, and N Otsuka. Appl Phys.infini*e Mn-concentration gradient (i.e., no diffusion to the Lett 47. 169119851.

ZnSe layer,) When rotating the magnetic field to direction 2X.-C Zhang. S-K. Chang. A V. Nurmikko, L. A. Kolodziejski. R L.perpendicular to the superlattice axis )z). we have seen only Gunshor. and S Datta, Phys Rev B 31. 4056 (1985)'X -C Zhang, Y Hefetz. S K. Chang. J. Nakahara, A V. Nurmikko. L Asmall changes in AE. B. This is in contrast with behavior Kolodziejski. and R. L Gunshor. Proceedings ofConference of Modulat-observed in the CdTe/(Cd.Mn)Te MQW's 2 and suggests ed Semiconductor Structures. Kyoto. Japan 1985 Ito be publi-hedlthat the hole (and the excton Iin the ZnSe/(Zn,Mn)Se system "Y Hefetz. X -C Zhang, and A V Nurmikko, Phys Rev. B 31. 5371 9are relatively thret.dimensional for the 67-A well width in 119851

'Data also corroborated by measurements by T. C. Bonsetl and R. Bthe case considered here Ibulk exciton Bohr radius in ZnSe is Bylsma lpnvate communication)approximately 28 A) *R B Bvisma and W M. Becker (unpublished).

While these results need to be better correlated with 'See. e g. F H Pollak and M Cardona. Phys. Rev. 172.1 6(1068).R Frohne and S Datta (unpublished).more precise calculations, we estimate from the magnetic 'A Twardowski, T Dietl, and M Demanuk, Solid State Commun 4. U4'

field studies that the average valence-band offset in the t9831,D Heiman, Y. Shapira. and S. Foner, Solid StateCommun S1.6 c,ZnSe/(ZnMn)Se quantum wells is likely to be less than some (19841

991Q Api Phyl Lol Vol 47. No 9. Nove'nber 196 tSfetzeto 991

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