,~pplied Surface Science 60/61 tl9921553-558 North-Holland B,O~ied

surface Sciertce

Raman scattering in GaP/AlP short-period superlattices grown by gas source molecular beam epitaxy

R.K. Son i ~, H. Asah i , S. Ernura , T. W a t a n a b e , K. A s a m i and S. G o n d a The Institl~te of Scientific mid blltusrrial Re,~earch, Osaka Onber~ity, 8-1 Mihoglttlka+ Iharaki, Osaka 567, Japan

Received Ig November ltJ91; accepted fur publicalilm 3 December 1991

We pre~ent Raman scallering characteri~athm ~[ sh~rt-period (GaP),tAIP~,,, superhal;ees grown on GaAs (fl()l)~ubstrale by gas source MBE. Confined optical vibrations are obseceed in GaP and AlP la~ers which sh~r.~ SlrOng sensitivily to the Ihiekness, The measured confined frequencies are explained by considering an intermedlale alk~y layer in the interface plane due Io sfiorl-range roughness. The influence of interface ruughness on the behavior of confined vibrations is '.~eak in thicker layer in, m _~ 5) but strong ~nd a~yrametric~J m thin layer ~n, m = 3) supt+rlattlccs.

L Introduction

Modern growth techniques have made it possi- ble to grow ultra-thin superlattices (SL) in which optical phonon confinement is clearly seen. These phonons are sensitive to structural details, such as layer thickness and departure from the ideal compositional profile of the grown material, and have proved to be of great practical ase for characterization [I]. In particular, the sensitivity of confined phonons to atomic scale roughness at the interface, which occurs naturally as a conse- quence of the growth procedure, makes these phonons an ideal tool for interface characteriza- tion.

Much of the experimental and theoretical work on the confined optical vibrations have been de- voted to the G a A s / A I A s system [2-6]. Ab-initio calculations [3] of the phonon dispersion in bulk GaAs and AlAs make it possible to analyze whole $L Raman spectra quantitatively. The discrep- ancy between theory and experiment, particularly for thin layer samples, has been interpreted as evidence of roughness in the interface plane.

i On leave frum Laser Technology Research Programme, Indian tnstgul¢ of Teclmology. New Delhi I 1[~]16. India.

Microroughness or short-range structure at the interface has been treated as an intermediate alloy layer and its effect on the confined frequen- cies has heen calculated by models of various levels of sophistication [3,4,6]. The spatial charac- teristics of the interface, i.e., its amplitude along the growth axis and its distribution parallel to the layer still remain unclear. More recently, Gam- mon et al. [71 using photoluminescence in con- junction with Raman scattering on a single G a A s / A I A s QW have shown that the interface toughness has a bimodal character meaning that thc roughness spectrum has both short-range and long-range distribution. In a high-quality sample, it is generally accepted that the interf~¢ce is one monolayer thick. "Fuis is interred from the split- ting of the exciton recombination peaks corre- sponding to one monolayer. Existence of short- range roughness has been demonstrated even in such high-quality sample with atomically smooth interfaces and is believed to he related to het- erointerface growth characteristics [7].

The G a P / A l P so0erlattices form a new class of material which offers the interesting possibility of a direct band-gap ,aaterial from indirect gap constituents. Theoretically, it has been shown [8] that the band gap and oscillator strength of

0169-4332/92/S05.00 ~ 1992 - Elsevier Science Publishers B,V. All righls r~servcd

R.I(. Sent ¢1 ill / Ritn;a~t .wattering hi ( l aP /A lP sh,wt-pcrl~l ~l¢p¢'rlattites

(GaP),~(AIP),,, superlattices critically depend ,'n the band alignments of GaP and AlP at the interface as well as on the number of monolayers n and m. We have recently reported [9] growth of G a P / A l P superlattices on GaP and GaAs sub- strates for the first time by gas source MBE. Low-temperature photolamineseence results indi- cate that the superlaUic¢ has a type [I band alignment [10,11].

In this paper, we report Raman scattering characterization of G a P / A l P supcrlaltices with varied layer thickness using folded acoustic and confined optical phooons as structural probes.

2. Experimental

The superlattice samples were grown by gas source molecular beam epitaxy (GSMBE) on GaAs (00l) oriented substrates held at 6000C. Details of the growth procedure are given else- where [9]. Individual layer thicknesses were con- trolled from the (bulk) growth rates measured in the test growth of GaP and AlP layers before the superlattiee growth using R H E E D intensity oscil- lations which were in good agreement with the superlattice period deduced from the satellite peak positions in the X-ray diffraction measure- ments. A typical sample contained over 200 peri- ods with total grown layer thickness ~ 1 p-m. In order to obtain an atomically smooth heterointer- face,'~he supply of Oa and AI was interrupted for l0 s at each interface. The growth interruption time was estimated from the recovcl~ of R H E E D specular beam intensity after the interruption of the Group 111 supply. A GaP buffer layer was used in all the samples to accommodate lattice mismatch (1.9%) between the substrate and the superlatticc constituents. The ultra-thln layer su- perlattices tn = m = 3, 5) were grown with addi- tio~lal AIGaP buffer layers.

The Raman experiments were carried out in a standard backseattering geometry at room tem- perature using the 488 nm line of an argon-ion laser and a photon counting detection system. The G a P / a d P superiat,.ice ~rown along the (00l) direction has the point-group symmetr:' D_o, ~2:e incident light couples 132 ( s = o d d ) phonons

(GAP) 9 (,klPt s SL

too

40 60 80 100 120

RAMAN SHIFT (cm - I ) Fig. l. Raman speelrum in Ihe acoustic phontm [lequenc¥ range from tile (GaP)~jtAIP), supeHattlce. Insel sho~. calcu- lalud folded acoustic phonon frequency from an elaslic con-

linuum model.

through deformation potential interaction in the Z ( X Y ) Z geometq, and A I ( s - e v e n ) phonons through Fr6hlich interaction in the Z(X,k ' )Z ge- ometry.

3. Results and discussion

In the low-frequency region (40-120 c m - t ) the Raman spectrum from the G a P / A l P exhibits folded acoustic phonons and provides two useful probes for the structural characterization, i.e. pe- riodicity and inner siructure of the superlattiee period. Fig. I shows the Raman spectrum from a representative (GaPI,,(AIP)~ sample. The folded acoustic phonons appear as doublets, two folded orders are observed in this sample. These dou- blets correspond to scattering from acoustic phonons with wave vector q +_2rrs/d [l]. The inset in fig. 1 shows the calculated dispersion curve using an elastic continuum model (ECM) [1]. The good agreement between experiment and theory confirms the period "d'" deduced from growth rates as well as from X-ray diffraction measurements. Large acoustic modulation in

R.bL Stud el aL / Ruman acuttering in GaP ~AlP short-pt, ritM superlaltice~

O a F / A l P superlattiee, due to a large sound ve- locity of AlP, the gap opening ( ~ 5 cm - t ) in the dispersion curve at the mini-zone center and edge is accessible to our experimental set-up. The gap opening in samples with constant period d ex- hibits sensitivity to AlP layer thickness, in qualita- tive agreement with the prediction of the ECM [t].

The optical vibrations of GaP and AlP have large mismatch between them [12,13]; the optical phonous are therefore confined to individual lay- ers and decay within a monolayer in the neigh- boring layer and their frequencies reflect bound- ary conditions at each interface. Furthermore, due to the large dispersion of the LO phonon in the bulk GaP, the confined frequencies are ex- tremely sensitive to the effective thickness of the GaP layer. This is shown in fig. 2. The strongest line, labelled as LO, at 402 cm- i arises from the GaP buffer and cap layers. Odd-indexed LO phonons (LO t, LO 3, LO~) are B 2 phonons from the GaP layers in the superlattice. These phonons show a systematic confinement shift with the GaP monolaycrs ill the sample. The LO I frequency, for instance, lies closer to the bulk frequency in the (9, 9) sample and moves towards lower fre- quency side with decreasing layer thickness. Simi- lar shift in the frequencies is also seen for higher-indexed modes but they are more sensitive to the thickness variation than LO r In fig. 3a wc have plotted these frequencies as a function of GaP monolayers.

The confined phonons are commonly modeled [1] as a particle in an infinite square well with a width of N +1 monolayers which leads to an effective wave vec|or ~ = s T r / ( N + I)d~, s = 1, 2 . . . . . N (N is the number of monolayers) for these phonons. The extra monolaycr in the well width is included to account for the common P atom at each interface. The measured frequen- cies correspond to the bulk LO frequencies at the wave vector k , and they can be used to map the bulk GaP LO dispersion along (001) direction. A!ternatively, a linear-chain model {LCM) [1] with force constants that reproduces the bulk GaP dispersion curve can provide a good description of the confined phonons in tfie suFerlattice. Re- sults from both these simple models have been

E

y-

555

340 360 380 400 420

RAi ' IAN SHIFT ( c m - I )

Fig. 2. Raman spectra of GaP uptlcal phonons in (Ga Pt,,tAIP),,, superlaltice~ with vaDin~ layer thickness. The arrows indicate u GaP-like LO phtmon from file buffer GaAIP alloy layer in

the thin samples.

shown [1] to match those obtained from more sophisticated calculations for G a A s / A I A s system with perfect interfaces. The solid lines in fig. 3a are the results of linear-chain model calculations for LO~, LO~, LO~ phonous. The obscrvcd con- fined frequencies generally fall below the calcu- lated curves; the mismatch is more prono~mred for samples with thin GaP layers (n = 3, 5). The stronger confinement shift compared to those calculated lor perfect interfaces indicates reduc- tion of ~he GaP effective thickness caused by short-range interface roughness. As different confined phonons probe a different lateral region within the layer they have varying layer thickness

556 R.I~ Soni et aL / R n a .~catter g G P / A / P sllorz.peril~l snperlattit'(~

dependence , as shown in fig. 3a. In the presence of interface roughness, 'he higher-indexed modes are the ones most slrongly affected even in the thick-layer sa:-~ples For atomically fine short- range roughness, we assume ihat tile interface layer is an alloy of GaP and AlP with width and relative concentration of AI as adjustable param- eters. The dashed lines in fig. 3a are calculated

curves, neglecting concentration variation within the alloy layer, for one or two monolayers of AIt~.sGa,.sP alloy at the interface. The disordered interface layer moves the frequencies (ff LO e up, as it approaches the vibrational frequency of the alloy (to,.l~,,y::380 emi l ) , rather than shifting down with decreasing GaP layer. Close to toa,oy tile confined vibration,~ penetrate into the inter-

. , . . . . . , . , m

~ " 5 0 0

B

/¢ / / 4 ,90 ! •

i o

,;80 ~_. a, p ,p u . • •

{ b )

4 7(3 , , , , , i , I ' 1 IO 2 4 6 a 12

Mono laye rs m

410 ~ - ~ , • , . , -

37(J I t I I , . - ~ ,

2 4 6 8 10 12

Nono laye rs n

Fig. 3. Me~sured and caictLlalcd confined LO-phonon frequencies as a function of (u) GaP Ia~cr thick'~csP ~md. (b) AlP layer thickness in the superlatlice. 121e solid curves are for a perfect intcl~ce, dashed and dc~t dashed curves for one and two

mol,olayers, respectively, of alloy at the interface. Indexes I. 3 and 5 d~llote LO I, LO~ and LO 5 phamms, respecdvcly.

~r-

R.K- Sop i el at / Ranlan scattering in GaP ~AlP ¢hort-penod supedatrtces

i : / ~ 1 and t b 'ved. ln fig . 4 a t h e Raman . . . . . . . . . . (G~p)n(AiP~r ~ (O) spectra from two samples having constant AlP

layer (tit = 3) and varying P a P layer are shown, z ix~l~ Similar spectra for samples having cgnstant GaP

layer (n = 9) and varying AlP layer are displayed TO ] ill fig. 4b, We see that the LO I frequency is

r¢9 ( i 1,3)

~20 460 500 540

RAI"IAN SHIFT ( c m -~) Fig. 4. Raman spect r:l tlf AlP tlptical plall~!ln:, iu (GaP)n(AIP), P superlanice samples: {~U wilh et)nManl AlP layer {m = 3) and (bl with constant GaP layer Oi = q). "fhc nrrow indlcales an AlP-like LO phonon from the buffer C, aAIP alloy la!,er in tile

thin 13.3), ~mole.

face layer resulting in an increase in the confine- ment length. A substantial upward shirr and alloy scattering, i.e. broad line width is e'~pected for confined phonons whose frequencies fall below ~;,II,,S r (fig. 2).

Turning to the phonons in the AlP layer, the spectrum in the AlP phonon frequency range (420-520 cm l) shows much less structure as illustrated in fig. 4. A single well resolved phonon lint:, labelled LOt is oLserved in all the samples studied and exhibits downward confinement shift in frequency from thc bulk LO phonon at 501 cm t the higher-indexed phonons are very weak

557

independent of the growth sequence and, as ex- pected of the confined vibration, depend strongly on the AlP layer thickness. Fig. 3b shows a plot of the measured frequeucies along with Ihe theoreti- cal curve based on a linear chain model for a perfect interface and for a disordered interface leading to one or two monolayerx of AI0.sGatL~P alloy in the interface plane. Not much is experi- mentally known about the phonon dispersion in AlP because of difficulty in preparing bulk crystal due to the high hygroscopic nature and therefore instability in the air. We therefore used GaP dispersion to describe tile LO phonon in AlP and shifted rigidly to match the zone center LO at 501 cm- t [14]. The only reported theoretical curve [13] and few available experimental data [14,15] for the bulk material suggest that the LO phonon dispersion is ~:arly fiat in the fir.~t half of the Brillouin zone nnd has a band width (17 em-I) , approximately half that of GaP. The confinement shift of AlP phonons is therefore expected to be much smaller than calculated here.

With two alloy monolayers at each interface, the confined frequencies are very close to the measured values as shown in figs. 3a and 3b. The experimental downward shift of LO~ in both GaP and AlP layers is almost identical except for the thin la~er sample ( n , m = 3). Taking into account weak dispersion of AlP compared to GaP, the observed asymmetry becomes larger. Clearly, the AlP layer is more strongly perturbed by the inter- face roughness than the GaP layer and related to the asymmetric nature of the two interfaces. It is interesting to note that the RHEED intensity recovery during the growth interruption is smaller for the AlP surface than for the GaP surface. A similar asymmetry has also been reported [5] in ultra-thin GaAs/AIAs and was attributed to atomic intermixing in the inverse interface (i.e. GaAs on AIAs) plane. The confined frequencies in GaAs/,"dAs superla'.ticcs grown at high tem- perature (6O0°C) show larger deviation from the

558 R,I(. Split el aL / Ram~m s¢~ztering in GaP~AlP short-period supt'rlattices

ca lcu la ted f requenc ies than t hose grown at lower 1empetatures, The short-range roughness, it ap- pears, is sensitive to growth temperature. It has been suggested [5] that the high growth tempera- ture leads to improved long-range order and con- s e q u e n t l y long terrace size but interior short- range interface quality especially at the GaAs- on-AlAs interface. The complex nature of the roughness and its dependence on growth eondi- lion is not very well understooci even in the widely studied GaAs/.adAs system,

The above description of interface roughness, tho~lgh qualitatively explaining the behavior of the confined vibrations, is fairly simple and unre- alistic under typical M B E growth conditions. A superlatticc sample is not llke!y to have one or two homogeneous monolayers of Alo sOa , sP al- loy at each interface but would most likely have an interface layer with component of disorder both along the growth axis and parallel to the interface layer, Furthermore, the migration lengths of AI and Ga are unequal, fhe AI length being smaller, and the two interfaces (i.e GaP on AlP and AlP on GaP) are expected to have different roughness configurations. The simple one.dimensional LCM is inadequate for a quanti- tative estimation of such interface roughness.

In conclusion, we have studied the off-reso- nance phonon Raman scattering in shert-period G a P / A l P superlattices, The supcrlattiee period~ determined from folded acoustic frequencies are in good agreement with those estimated from X-ray measurements and superlattice growth rate. We have shown that the optical vibrations in these structures, confined to the GaP and the AlP layers, displayed strong sensitivity to the layer thickness as well as to tile presence of atomic- scale roughness at the interface. Using simple one-dimensional LCM we have explained the qualitative b~havior of the confined vibrations. Full miero~opic calculations are necessary for a quantitative estimate of interface morphology, such calculations are now becoming available [4] for the GaAs/A1As system.

A c k . u w l e d g e m e n t

One of us (R.K,S.) would like to thank the Japan Society for th~ Promotion of Science for the financial support.

R~|'errencos

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