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_.....RT DATE I.REPORT TYPE AND DATES COVEREDAug 93 1 Technical

4. TITLE AND SUBTITLE S. FUNDING NUMBERS

MOCVD-Gaawn InGaAsP Double Heterostructure Diode Lasers

C> 6. AUTHOR(S)

M. Razeghi, et. al. 0 H ' o i 'OTIG ______

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(I 1 PERFORMING ORGANIZATION•sf Northwestern University ELIECTE REPORT NUMBER

Center for Quantum Devices OCT 2 01993633 Clark Street UEvanston, IL 60208A

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADORESS(ES) r- 10. SPONSORING / MONITORINGU.S. Army Research Office AGENCY REPORT NUMBER

P. 0. Box 12211Research Triangle Park, NC 27709-2211 3uot". I-PH

11. SUPPLEMENTARY NOTESThe view, opinions and/or findings contained in this report ire those of theauthor(s) and should not be construed as an official Department of the Armyposition, policy, or decision, unless so designated by other documentation.12a. DISTRIBUTION / AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Approved for public release; distribution unlimited.

• 13. ABSTRACT (Maximum 200 words)

InGaAsP/InGaP/GaAs compound has been recently proposed to replace AlGaAs/GaAs asa= advantageous material for high-power laser applications. In this work we report

successful fabrication and study of broad-area double-heterostructureInGaAsP/InGaP laser diodes reproducibly grown by MOCVD. The diodes demonstrated

M) near-100% efficiency of spontaneous radiative recombination in the active region.=A Threshold current densities of 700A/cm2 obtained for 900um-long cavities were*4 close to the theoretically predicted low limit and lower than respective values

experimentally obtained for similar AlGaAs/GaAs diodes. Narrow far-fielddistribution with FWHM of 280 in the direction transverse to p-n junction planemay be of practical interest. Saturation of spontaneous emission above the thres-hold as well as a flat near field pattern confirm the uniformity of lasing inten-sity across a 100um-wide stripe and the homogeneity of the structures studied.Relatively high internal losses of approximately 40cm-I and low differentialquantum efficiency of 25% were shown to be inherent for this laser structure designand should be overcome by a combination of the advantages of separate-confinementheterostructures with this laser-quality material.

14. SUBJECT TERMS GaInAsP-GaAs, High-Power Laser, Heterostructure 15. NUMBER OF PAGES18

16. PRICE COOE

17. SECURITY CLASSIFICATION 1B. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACTOF REPORTI OF THIS PAGEI OF ABSTRACT

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED ULNSN 7540-01-280-S500 StarOdafd For- 298 1Qev 2-89)

P Ds ooo AN' I3 _'3 4. 129S. 102

Acce'.;'on For

NTIS CV: LI& LDTIC- TA;' [

J .. . . . .,-. . ..-

By ___B ..... ...........................

Principal Investigator: Prof. M. Razeghi A,..Avoi;, J!:( / :r

Dist Li__Material growth

and characterization: Staff Students

Dr. E. Kolev X. He

J. Gwilliam J. Hoff

C. Jelen

S. Slivken

Laser processing

and measurements: Staff Students

Dr. D. Garbuzov J. Diaz

Dr. L. Wang I. Eliashevich

Dr. W. Carvalho K. Mobarhan

Theoretical calculations: X. He

H.-J. Yi

MOCVD-grown InGaAsP double heterostructurediode lasers

Center for Quantum Devices, Department of Electrical Engineeringand Computer Science, Northwestern University, Evanston, Illinois60208

In the previous works [1-4] where MOCVD technology was used, ithas been demonstrated that aluminum-free InGaP/GaAs system hasmany advantages in comparison with conventional AIGaAs/GaAssystem for the optoelectronic device fabrication including strained0.98pm laser diodes [5-6]. However, till now, Al-free InGaAsP/GaAs0.8pm lasers that are of great practical importance, have beenprepared only with liquid phase epitaxy method [7-8] which does notprovide the possibility for full realization of all advantages of Al-free system for high power laser diode fabrication as well as fordevelopment of their mass production capability. In this report wedemonstrate the first MOCVD grown InGaAsP/InGaP/GaAs laseremitting near 0.8pm.

The simplest version of a laser structure that is a doubleheterostructure was prepared and investigated in these firstexperiments. The band diagram is schematically shown in Fig. 1.InGaP/InGaAsP/InGaP epitaxial layers were grown on (100) orientedSi-doped GaAs substrates with doping concentration of lx101 8 cm-3.The laser structure consists of (i) 0.7pm Si-doped InGaP(Nd=4.7X101 7 cm-3) as the confinement and cladding layer; (ii)0.1/pm undoped InGaAsP active layer with background dopingconcentration lower than 101 7 cm-3; (iii) 1.51pm InGa? Zn doped(Na=5.6X101 7 cm- 3 ) as the second confinement and cladding layer;(iv) 0.1Pm GaAs contact layer (Na=1X10 1 9 cm-3). The distribution ofimpurity concentration obtained by electrochemical capacitance-voltage profiling across the structure is shown in Fig. 2. The detailsof the growth and material characterization are presentedelsewhere (9].

2

Though broad-area lasers are of small practical importancebecause of high threshold currents, the data obtained for them areoften used as reference values or for the purpose of comparison. Theinvestigated broad area contact laser diodes with stripe width of100 and 200pm were fabricated of two wafers (#36 and #43).100pm - wide stripe diodes were manufactured of both structures atthe Center for Quantum Devices. For comparison, 200pm - widestripe lasers with Ti/Pt/Au p-contact were made of wafer #43 byAmoco Technology Co. 1 O0pm stripe lasers had annealedAu/AuZn/Au p-contact, metallization and p-GaAs between thestripes had been removed, thus providing an opportunity to observespontaneous emission through p-lnGaP cladding layer in thedirection normal to the stripe. Both types of diodes had AuGe/Ni/Aun-contact. Diode chips with cavity length varying from 110 to 910pm were cleaved and tested with needle probe under pulse operation.Several 100pm and 200pm - wide stripe diodes were In-bonded oncopper heatsinks.

Fig. 3 shows I-V characteristics for two bonded diodes with stripewidths 100 and 200pm having the same contact area _10- 3 cm 2 . Ascan be seen, the characteristics shown are very close to each other,cutoff voltage exceeding Eg of the active region by 0.4-0.5 V. Sincethis feature was observed for the diodes processed independently intwo different ways, we suppose it to be attributed to the intrinsicproperties of the laser structure rather than to the fabricationprocedure. For both types of diodes series resistance decreases athigher current densities and at J over 1 kA/cm2 is close to 1 Ohm.This value is several times higher than that for the best AIGaAsdiodes and calculation shows that a substantial part of thisresistance is introduced by p-lnGaP cladding layer and may beeliminated with higher doping of this layer.

Spectra of lasing for two diodes with cavity length of 500pmprepared from wafers #36 and #43 are given in Fig. 4. The closelasing peak positions for the diodes fabricated in different growthruns imply a good reproducibility of the technology developed.

The threshold condition for lasing to occur is that gain should behigh enough to compensate for all losses through the length of thecavity during one period of oscillation. A pumping current necessaryto provide this value of gain is the threshold current. It is

3

convenient to describe the quality of planar laser structure in termsof threshold current density rather than absolute threshold current.Having a laser stripe long and broad enough to neglect all mirror andlateral effects, threshold current per unit area depends exclusivelyon specific quality of the wafer. Minimal threshold current densityis highly desirable. It results in low Joule heating of laser deviceand therefore is important for high-power applications. Thresholdcurrent density depends on various laser structure and materialparameters (d-active layer thickness, qi -internal quantumefficiency, Jo-transparency current density, r-optical confinementfactor, 1-fitting parameter, ai-internal losses, L-cavity length, R-reflectivity of the laser mirrors) and is given by:

Jth--d/qi (Jo+ 1/rF (ai+(1/L)ln(l /R))) .............................................................. (1)

Physical meaning of those parameters will be discussed later.

A linear gain-current relation is assumed here:

gm aterial = 03(J- Jo) .................................................................................... (la)

Fig. 5 shows the threshold current density versus reciprocal cavitylength for 1 OOpm - wide diodes* fabricated of the structures #36and 43. The results obtained for the bonded diodes are shown in thesame graph. No substantial difference in Jth was observed forbonded and unbonded diodes with cavity lengths shorter than 500 Aim.However, for the chips longer than 500 pm, bonded diodes exhibitedlower threshold current densities than under testing needle.Minimal values of threshold current densities obtained are 700-800A/cm2 , that is better than for similar InGaAsP/GaAs doubleheterostructure lasers grown by liquid phase epitaxy [10]. Thoseresults are also superior than well-known data of 1 kA/cm 2 forAIGaAs diodes with the same bandgap difference between activeregion and cladding layers and with a similar width of the activeregion (d-O.lpm) (11]. The quoted data for AIGaAs are close totheoretical predictions [12]. We also calculated threshold currentsfor long-cavity lasers, assuming refractive index and its dispersionfor InGaAsP and InGaP corresponding to the known values for AIGaAscompounds with the same bandgap [13]. For double heterostructure,which is essentially a three-layer waveguide, optical confinement

* 200um - stripe laser diodes demonstrated the same Jth.

4

factor r is roughly proportional to d2 for d < SOOA. Generally, r maybe calculated as

4 /Z 2

02

In our case r for d-0.1 pm was determined to be about 0.3. Utilizingthis value of r it was previously showed that for the diodes withcavity length -I mm the value of Ith = 700 A/cm2 is close to theminimal values of threshold corresponding to the 100% internalefficiency of radiative recombination of carriers in the activeregion[10]. qi is the internal quantum efficiency given by the ratioof radiative recombination rate in the active region to thc totalrecombination rate. This important parameter is the number ofphotons emitted in the active medium per one electron injected.

Additional confirmations of the assumption of qi~100% were givenby our studies of external efficiency of spontaneous emission. Thisparameter integrates radiative recombination efficiency with lossesin laser structure giving a number of photons emitted per injectedelectron. It can be measured experimentally as

ne = ePout / hvl ............................................................................................... (3)

and is linearly related to qi as

qe = y ni .................................................................................................................... (3a),

where y is current-independent coupling factor depending upon thegeometry and absorption losses.

Spontaneous emission intensity observed in the direction normal tothe stripe increased linearly with current up to the value of about1 kA/cm2 and demonstrated a weak temperature dependence (adecrease of no more than 25% for the temperature increase from 300to 380K at 400 A/cm2 ), as shown in Fig.6. Both of those factsindicate that major part of the injected carriers recombineradiatively in the active region. However, those experiments did notprovide for estimating the absolute values of external efficiency,

5

since major part of the radiation emitted towards the surface wasabsorbed by stripe contact metallization. In order to determine theexternal efficiency, we fabricated and bonded very short diodes withcavity length of about 100 pm. The current dependence of theintensity of spontaneous emission observed in continuous waveoperation through the mirror of one of those short diodes is shown inFig.7. External quantum efficiency, determined according to eq. (3)from the linear part of this curve, where effects of heating arenegligible, is -0.4%. This value seems quite consistent withqi~100%, since our estimates show the maximum value of ne for theradiation propagating through a non-absorbing waveguide to themirror cannot exceed 1.5%. This value was previously obtained forshort cavity SCH-SQW LPE-grown InGaAsP/GaAs laser diodes. In ourcase only long-wavelength part of spontaneous emission can beefficiently coupled out through the mirror even for the diodes withL-1OOpm due to a strong self-absorption inside of their 0.1pm -thick active layer and ne~0. 4 % indicates a very high value of ni.

A value of To is used to characterize the threshold current behaviorunder changing lattice temperature. With temperature growingcurrent leakage over the barriers and nonradiative recombinationrate are likely to increase, resulting in higher values of thresholdcurrent. This dependence is observed in the form of experimentalrelationship:

I(T) = IoeT-To/To .............................................................................................. (4)

Fig.8 shows temperature dependence of threshold current for asample with L = 710pm with TO-90 0 C. This value is somewhatsmaller than that typical of AIGaAs lasers with higher barriers andconsequently better carrier confinement.

Fig.9 presents a far-field pattern in the direction transverse to theplane of p-n junction for one of the 200pm - wide diodes. Arelatively small half-width of the angular distribution (-280)exactly corresponds to the calculated value of r = 0.3 which we usedfor threshold current calculations. Narrow far-field distribution isof practical interest since high laser beam divergence may preventefficient coupling of emission into optical fiber. At the same timedeep penetration of lasing mode to the cladding layers may have anegative effect on some other parameters of the laser diodes. For

6

example, in these first experiments we were not able to obtain highdifferential efficiencies typical for contemporary lasers withquantum well active regions. Differential quantum efficiency is afraction of electrons that is converted into light output if injectedby increasing the driving current:

qd = eAPOut / hvAl ...................................................................................... (5 ).

At threshold 9d is related to the internal quantum efficiency ofstimulated emission as

1 /id = (1/q i)(1 + ai L Iln (I/R)) ........................................................................ (6).

Fig. 10 shows light-current characteristic of a bonded 910pm - longdiode. The laser was tested up to pulse power of 150mW anddemonstrated nd-l 8% per two facets, that is typical of these long-cavity diodes. Maximum value of rd-25% was obtained for thediodes with L = 450-SOOpm. An inhomogeneous lasing intensitydistribution across the stripe area (lasing in "filaments") may beone of the reasons for the low value of qd for broad-area lasers [11 ].However, in this case spontaneous emission intensity should beexpected to increase above the threshold, while in our case itexhibits a rapid saturation with current, typical of high-qualityMOCVD-grown AlGaAs/GaAs lasers. Taking into account this resultas well as the above mentioned high efficiency of spontaneousemission, we have to explain the low differential efficiency byinternal losses only. The main sources of internal losses in doubleheterostructures are light absorption by free carriers in the activeregion and cladding layers, optical scattering loss due toirregularities at the heterointerfaces and coupling loss when theoptical field spreads beyond the cladding layers. The experimentaldata for the lasers with cavity lengths of 450-91 Opm can beexplained with the assumption of internal losses ai of about 40cr- 1which is higher than the known values for AIGaAs structures andcannot be ascribed solely to the free carrier absorption. Low valueof [-0.3 and the deep penetration of the lasing mode to the claddinglayers may be the reason for additional distributed losses, such aslight scattering at heterointerf aces and coupling losses in thecontact layer. We expect this source of losses to be eliminated withthe transition to separate confinement heterostructure lasershaving the lasing mode confined stronger to the waveguide region.

7

Separate-confinement heterostructure (SCH) laser, as proposed in1973 by Thompson ano Kirkby, is basically a four- or five-layerdielectric waveguide where first composition step gives an energyband discontinuity necessary to confine injected carriers within theactive layer, while second step in the refractive index between thewaveguide and cladding layers provides light confinement within theoptical cavity. Separate optical and electrical confinement ensuresmoderate beam divergence and low optical power density on thelaser mirror while preserving fundamental transverse modeoperation and low threshold current density for the lasers with thinactive layers. Separate confinement is critical for modern quantumwell lasers being the only structure design providing strong overlapof lasing mode with thin active layer. Also, since optical wave ispropagating mostly through undoped waveguide layers rather thandoped cladding layers, as in double heterostructure lasers, externalefficiency may be made higher due to lower free carrier losses.

As has been mentioned earlier, decreasing cavity length from 910 toabout 400pm leads to some increase in differential externalefficiency. However, further decreasing the cavity length results inthe rapid growth of the threshold current densities without anyimprovement of rd. We consider the reason for this kind ofanomalous behaviour of short diodes to be the relatively lowconfinement barrier height. Under this condition, the increase ofexcess carrier concentration in the active region can lead to a sharpincrease in electron current leakage from the active region to the p-type cladding layer. As shown in [14], this leakage not only resultsin higher threshold current density, but also reduces the differentialefficien.;. The current leakage enhancement under current densityincreasing up to 2-3kA/cm 2 manifests itself by the correspondingenhancement of temperature dependence of spontaneous emissionefficiency for short-cavity non-lasing diodes (a decrease of ne ismore than 50% for temperature increase from 300 to 380K at3kA/cm2 ). The effect of leakage should also be less pronounced inSCH-SQW lasers with lower threshold current densities of a fewhundreds A/cm2 .

In conclusion, we report the following results obtained in our work:

1. InGaAsP/GaAs laser diodes emitting in the 0.8pm region wereprepared by MOCVD for the first time. The investigation of thelasers fabricated showed that at the pumping current densities

8

below 1 kA/cm2 almost all the injected carriers recombineradiatively in the quaternary 0.1pm - thick InGaAsP active region(ni~100%). This provides the threshold current densities for long-cavity double heterostructure laser diodes to be as low as 700A/cm2 which is better than the values for similar AIGaAs DH lasersand approaches the theoretical limit.

2. The results reported allow to suggest the fabrication of separateconfinement heterostructure lasers based on the same MOCVD-grownhigh-quality lnGaAsP material to eliminate the inherent drawbacksof DH lasers studied and to obtain laser diodes with the parameterssuperior than the best results for SCH SQW AlGaAs lasers.

9

References:

[1] M. Razeghi, F. Omnes, Ph. Maurel, Y. J. Chan and D. Pavlidis,Semicond, Sci Technol. 6, 103 (1991).

[2] Y.J. Chan, D. Pavlidis, M. Razeghi, F. Omnes, IEEE J. ElectronDevices 37, 2141 (1990).

(3] M. Razeghi, F. Omnes, M. Defour, Ph. Maurel, J. Hu, E. Wolk and D.Pavlidis, Semicond. Scl. Technol. 5, 278 (1990).

[4] MI. Razeghi, F. Omnes, M. Detour, Ph. Maurel, Ph. Bone, Y. J. Chen,and D. Pavlidis, Semicon. Sci. Technol. 5, 274 (1990).

[5] K. Mobarhan, M. Razeghi, G. Marquebielle, and E. Vassilaki, J.Appi. Phys. 72, 9 (1992).

[6] K. Mobarhan, M. Razeghi, and R. Blondeau, Electronics Lett. 28,16 (1992).

[7] D. Z. Garbuzov, N. Yu. Antonishkis, S. N. Zhigulin, N. D. Il'inskaya,A. V. Kochergin, D. A. Lifshitz, E. V. Rafailov, and M. V. Fuksman,Appi. Phys. Lett. 62,10 (1993).

(8] D. Z. Garbuzov, N. Yu. Antonishkis, A. D. Bondarev, A. B. Gulakov,S. N. Zhigulin, N. 1. Katsavets, A. V. Kochergin, and E. V.Rafailov, IEEE J. Quantum Electron. 27, 6 (1991).

(9] X. He, E. Kolev, and M. Razeghi, to be published.

[10] Zh. 1. Alferov, 1. N. Arsent'ev, D. Z. Garbuzov, E. Tulashvili, Soy.Phys. Semicond. 25, 99 (1983).

[11] J. C. Dyment, F. R. Nash, C. J. Hwang, G. A. Rozgonyi, R. L.Hartman, H. M. Marcos, and S. E. Haszko, Appi. Phys. Lett. 24,481 (1974).

10

[12] J. Thompson, "Physics of Semiconductor Laser Devices", John

Wiley & Sons Ltd. (1980).

[13] S. Adachi, J. Appl. Phys. 53, 5863 (1982).

[14] D. Z. Garbuzov, V. B. Khalfin, In: "Quantum Well Lasers*, ed. P.

Zory, Academic Press (1993).

11

Eg (eV)

In0.49Ga 0.51P: Si In0 .4 9 Gao.,1P: Zn10.5

1.8

1.7 -- 0.7 pm O -- 1.5 pm --

1.6

1.5 n - GaAs InGaAsP p - GaAssubstrate

1.4 0.1 pm0.1 "m

growth direction

Fig. 1 Schematic band diagram of the double heterostructureInGaAsP/lnGaP/GaAs 0.8pim laser structure.

S.- -- _- n "O43-19.

-L.. 0

n -t-ype.. .. -- p-Np

-- 17 -

I...... 1 .5 2 2.S 3.. . .

Fig.2 Electrochemical doping concentration profile of the laserstructure #43.

12

A-AiUwcC),r COD,!

01-

0 '. 1 .5 0. U,v

Fig.3 Typical current - voltage characteristics of two diodesof wafer #43 with the same stripe area prepared by AmocoTechnology Co. and Center for Quantum Devices.

SPEC Pk: 0.81825p NO.B7070n AVG: 1/.

io.OnW

o. OnW , . ... jO. 7680/m O. 8180#m iO. OOnm/D 0. 86 )im

SPEC Pk:0.817742Pm 730. 76 _,W AVG: /2S.OnW

0.S160

O.OnW --

O.7680pp O.818OM W0. iOOnm/D -O.8680Pm

Fig.4 Lasing spectra of two diodes prepared from wafers #43and #36.

13

ILI

OIIT INC

J+141~Acr~ 14

_ _ _ _ _ _ _ 7

-7

0 0~

all,

-- jj 77

-7 7:.: -0

- -- ----..-------- -im7m

uL~0u C---- CL

- -....... o

'IT~~ ~~ Mcz- C

77 17.4~Ar :77I

15

P10 -6-D I

Fig.7 Spontaneous emission output through one of the mirrorsfor a diode with cavity length 11 Op .__________

___-~ --- =---

Fig -- Temperature --- deedneO-- hfrbne lsrdoeo

wafer-- #43* wihcaiy eg 1-gm

_____ - ~ 1 --- 4 - - __16

Vertical Far-Field Plot

0.9

0.0 8 .-- SM I Xu.M.

0.9 ~. ... .. ..

0.. 4S

~0 3

~0.2

0.2......... ......

0?.O5.-550-45,4035-3025-2015-105 0 5 10'1'52'025 30'3'540 455055Theta (Degrees)

Fig.9 Far-field pattern in the direction perpendicular to thejunction plane of 2OOgrm - wide stripe bonded laser diode ofwafer #43.

17

I A7

r-- -- - --

67 /

13.A __ _ _ __ _ __ __-_ _ _

o1 A 2 2.5

Fig. 10 Light-current characteristic of a bonded diode of wafer#36 with cavity length 91 Og m.

- 18