Deconvolution of the light- and heavy-hole free exciton fine structure in AlxGa1−xAsGaAs multi quantum wells using photoluminescence excitation spectroscopy

Superlattices

and Microstructures,

723

Vol. 4. No. 6, 1988

LICONVOLUTION OF THE LlGHT- AND HEAVY-HULE FKEE EXCITUN FINE STKUCTUKE 11~AlxGal_xAs-GaAs MULTI QUtiTUM WELLS USIN PHOTOLtiMINESCENCE W[CITATlON SPECTKOSCOPY D. C. Keynoids, K. K. BajaJ, G. Peters, and C. Leak Air Force Wright herouautical LaboratorIes (AFWAL/AADR) Avionics LLlboratory, Wright-Patterson AFB, Uhio 45433 W. M. Theis and P. W. Yu University Kesedrch Center, Wright State University, Dayton, Ohio 45435 H. Morkoc Department of Electrical Englneerlng and Coordinated Science Laboratory University of Illinois, Urbana, Illinois blbO1 h. Alavi, C. Colvard. ano I. Shidlovsky Siemens Research and Technology Laboratories, Princeton, LUJ 08540 (Received 24 June 1988)

exciton (LHFE) and heavy-hole bhary-line structure associated with borh the light-hole tree free exclton (HHFE) transitions has been observed in mulri-quantum-well (MQW) structures of iour weii sines in photolumlnescence (PL) and reilection spectra. These spectra have been deconvoluteu using photoluminescence excitation spectroscopy (PLE:). The LHFE and lihFE sharpline structure is associated with interface structure composed of growth islands ar the interface between the barriers and the wells. Estimates of the average interface island sizes for the tour uifferent MQW strucfures are made based on theoretical modelling. A correlation is established between particular l&FE rine structure componenrs and specific HHFE fine structure components. A model is developed to account for the LBFL and HHFE rine structure based on a non-random distribution of the interface sfructure. The physlcal location of the excitons is uetiunstrated to be ~1 regions of the wells wirh essentially identical interfacial microstructure. Evidence Ior aitrusion of excit~us from effectively narrow well regions to wider wrii regions is presruted.

1.

Intrcducrion

The quality of the interface in quantum heil structures is knuwn to affect the character 10f optical transifions. This effect has been studied by a variety of methods, both thepg'i$. in recent years Cal and experimental, Much of this work haa dealt with the influence of interrace structure on the energy and width of optically observed excitonic transifiuns. In references (l-9) the interlace structure associdted with growth islands, resulted in monolayer lluctuations in the well size. The current paper and references (10-12) deal primarily with z.I,terfacr structure thht effective produces sub-monolayer flucLuations in well size. The geurral conclusion was thar sharp transition lines are indicaLlve o fl~oOfhhow;;~iacer. It that in has been pointed out structures grown by mole&r beam epitaxy (F!BE) one does noL LO from a pure 6aAs iayel to a pure Al Ga i_xA~ layer over one cation layer; under opfimum growth conditions the besL that can be done ib LCI make the trausirion over two cation

0749-6036/88/060723+06

$02.00/O

layers.

Thus, any interface has an inherent surface roughness oi at ieast one monolayer. lor multi-quantum well (MQW) samples grown by MBE, it was therelore predicted that. high quality Interfaces could result in sharp line fine structure (alscrete peaks) associated with the excitonic transitions. he previously reported the observation of this rine structure associated with both the heavy-Lole free exciton (HHFEj and the light-hole fret exciton (l&FE) transirions for two GaAs-Al Ga~_xh~lZ~~sBo:; phoroiuminescence (PL) an= leffec iw 01 these samples (well siztb i35A and 180A) showed very sharp fine structure wlrh total energy sprrau corresponding to a two monolayer fluctuaeion 11; well size. In this current study, we report.results gn two additioudl MQWs of well sizes 96A and 2258. both or thebe samples showed tine structure for the LHFE and l:hhr;rransitions, agalu with an energy spread corresponding LU a two monolayer rluctuation jn wtiil size, as observed for rhe previous two samples. All four of the MQW s~mplrs display very narruw linewidths for the

0 1988

Academic

Press Limited

Superlattices

724

and Microstructures,

Vol. 4, No. 6, 1988

individual fine structure components, with the narrowest lines of each being at the minimum predicted by the theory. In the following we first describe the experimental apparatus and the structure of the samples in more detail. Next the experimental 0 0 data for the 90A and 225A MQW samples are presented, followed by an explanation or the Finally, the origins of the observed features. data for all four MQW's are examined in light of the theory to extract information about the interfaces. 2.

Experimental Method

The 9Oi MQW used in this investigation was grown on (100) oriented U-doped GaAs substrates by MBE at 6OO'C. as previously desTPIei~~'4r""~~~~v~O~~~~~~ superlattice consisting or 20A Al0 25Gap 75A~ and 20A GaAs was grown on the substrate fo lowed by a 0.5 micron thick GaAs buffer layer, and completed with the MQW structure. The 225A MQW was also grown at 6OO'C, however, it was grown on a 2.g micron GaAs buffer which was grown direct15 on a semi-insulating GaAs substrate. The 2258 hQW consisted of 30 cycles yith 120A barriers ot Al 35Ga 65As, while the 90A MQW had 30 cycles of @As with lOUA A1.25Ga barriers. The 90A MQW structura was grown .lzA: Kiber machine whereas the 225A structure was grown in a Varian Gen II. and reflection The high-resolution PL measurements reported in this investigation were made at 2'K with the sample immersed in liquid He. In the igtrinsic region of GaAs a dispersion of 0.54 A/mm was achieved using a 4-meter dual recording (electronic and photographic) spectrometer. PL and reflection data were collected on Kodak-type I-N SpectrogFaphic plates. The PL was excited with the 6471A line of a Krypton ion laser while the reflection measurements used the broad spectrum of a Zr-lamp. The selective excitation spectroscopy was accomplished with a tunable dye laser using PLE styryl 9 dye pumped by an argon ion laser. data were collected from a kCA C31034A photomultiplier tube. 3.

Fig. 1.

Keflection spectra tar a 225; MQW showing fine structure ior both the LHFE and the HHFE transitions.

Fig. 2.

Emission spectra for the MQW in Fig. 1 showing the fine btructure for the HHFE Seven lines are seen over transition. an energy spread OL -0.5 meV corresponding to a wall width fluctuatiou of two monolayers.

Fig. 3.

Keflection spectra for a 9OA MQW shoving sine structure tar both the LHFE and the hliFL transitions.

Kxperimental Kesults

The HHFh is observed In PL and reflection for both of the MQWs, while the LHFE is only observed in reflection. The reflection~pectrum showing the LHFE and HHFK for the 225A MQW 1s shown in Fig. 1. The emission spectrum for the HHFE is shown in Fig. 2. Seven emission lines are observed over an energy spread of 0.5 meV, with the narrowest of the lines having a full width at halt maximum (FWHM) of 0.02 meV. This energy spread of the exciton fine structure for either the LHFE or the BhFE corresponds to a well size fluctuation of two monolayers. The reflection spectrum fur both the LHFE and HHFE transitions in the 9OA MQW is shown In Fig. 3. The emission spectrum for the hHFE exciton is

ENERGY

1s”) 0

Superlattices

and Microstructures,

725

Vol. 4, No. 6, 1988

growth. The Al flux is shut off during the tormaLion of the well and is aliowed to impinge on the growing surrace during the growth or the A perfect interface would be one in barrier. which the cation layer I is pure Ga and the layer i-1 or i+l is AlxGal_x where x is &he Al concentration in the barrier region. Lincertainties involved with the shutter Lontrol make it shutter impossible to open or close tht precisely when the surface layer is exacrly full. Even if this did occur in one reglun of the substrate, because L;I statistical ;,;;;;;f~sa;~th~;e

Fig. 4.

Emissiou spectra showing a seven line fine structure for the BHFE transitions for the hQk' shown in Fig. 3.

shown in Fig. 4. A six line fine structure is observed with the lines having vary narrow half widths for a well of this size. This MQW also shows an energy spread of the fine structure corresponding to a well size fluctuation of two monolayers for both the LHFE and HHFE transitions. The magnitudes of the energy spread or the fine structure for both the LHFE and the HHFE transitions for all four MQWs, along with the calculated energy differences ior a two monolayer fluctuation in well size are given in Table 1. The tine structure in these hQWs is related to the microscopic quality of the interfaces. The formation of the interfaces is directly related to the growth process. The heterostructure is grown by aliowing the Ga ana arsenic (As2 and/or As4) beams to impinge continuously during 011 the growing surface

r;g;;;14f~ux9tiu~

zy

TABLE 1 TWO MONCiLAYEK FLUCTUATION

IN

WELL

SIZE

HEAVY HOLE EXCllON

TWO MONULAYEli WELL SIZE; FLlJCILiATlONIN WELL 5iZE

LIGHT tiIOLEUCiTON

ELFRGY SPREAl! OF F.XClTON FINL STRUCTURE

'ikiOMON~JLAYER FLUClUaTION IN IJELL 5IZE

ENERGY Sl'h&U OF EXCLTON FlNk STRUCTURE

4.0 meV

3.9 meV

5.2 meV

4.5 meV

135:

2.32 meV

2.5 me\

2.82 mev

3.01 meV

225;

0.4& meV

U.5L me\:

0.66 meV

0.55 meV

90;

t;;,"

inherent uncertainty, the surface layer i wili have a coverage 6,(<1) when the shutter is opened or closed to form the interface. It is therefore clear that even irt the most uptimized growth conditions, the Al composition at the interface will not go from x to 0% over one cation layer, but m&y change from x to x. to 0% over two cation layers. This results'in an interrace roughness which extends over one monolayer ac each interface. The MQk' structure consists ot two dominant well sires separated in energy corresponding to one monolayer clrilngein well size. Emission from both the wide and narrow wells bhows sharp exciton lines, reflecting the interface structure associated with each This st+ucture is shown clearly in well size. Fig. 5 ior the 2Z5A MQW where PLL 1s used to detail the structure. For simplicity, all of The the structure is shown at one interface. model does not allow one to distinguish the isiand structure at each Interface, but rather gives the combined ellect of both intertaces. the The experirlent is done by pusitloning spectrometer ~1 the line labeled 1 or rbe HHFE. The dye laber is then scanned over the higher energy excited states oi the hhkE. hs well as the It is seen that when rhe entire LHFE stkuccure. 1, the bpectrometer is positiol‘ed on line

726

Superlattices

and Microstructures,

Vol. 4, No. 6, 1988

i

I 3

1

Pig.

5.

Emission ior the various HHFI: iins structure corpcrents listed In the insgt versus pump energy tar tl!t 1258 ?‘lQW. Features thiir appear ir. these spectra are uue to various The components of the LHFE struLture. u=2 excited states 01 the HHFE and the CA-U excitons for both rioainant well bet text for blocs are also observed. additional details.

energy complll,aUt oi LhC t&FE fine highest tbac rhe Lighest energy componeuL uf structure, the LHFO LJ.I~~ structure is dominantly pumping that particular 11~. The spectrcneter 1s rhen positioned sequentially GI. the remainder of the sharp lines 111 chr tl;FE fine structure ana the 1i11rs primarily responsible 1or pumping each of the HHFE 11~ structure lines arc shown lu Pig. A Lurrriation is observed between a particu5. lar componeilt of tl,t rjne structure assoclbced with the LHFE spectra and a sprciilc component of the tine structure associated with rt.r bltFE SpecLra. These spectra (lahriled 1-7 the corresponding to ulorntoriug emission intensity of the sevti~; compol,rnts of the HHF1: as *l&ted in the inset) cieazlr demonstrat.e that a specilic LHFE peak dtimnates the pumping of i: particular HIiF& peak which establishes this It is bellckeo chat this pair of GUI relation. transitions occurs in the same wtiii or regitin of Ll;C same the kit211 with interiacial lJ11 the oclier band, the mixing mlcrticrructure. ot these wave funccltins 111 ditferent wel.t 1s <.xpected to bc negllglbit so that the emission tit a tlHFL in one well pu~+rd by a LIFE in a d;iferent we1.L Is upobseivable. Since the L25A P&l: of Fig 5 consists ot thirty cycirs, a hlinimum of th;rLy lines wculd be expected tu Lc erdtted frol~ this sExucture if each qusl,rum weii ha< a differrl;r etlectivc well width. Furrlermore, it is possible that the cffectivc well widtli Lf the indivlibal well ~tiy “cry latelt;ly over the extent 01 tlie spot size This t.eing excited produciug addition& ilnes.

kudel tar ~11e LliFE and the 2111YEflnr structure components obsrrvtu 111 the MQW samples. variation ol The spatial clzectlve well width ill the presence of lnterlhcial microstructure (labelled i-7 for the specillc example shown In Eig. 5) al16 two monolayer ~slanas (1,. 1%) is represcl,trd schematically.

be rxpccted to be over the pubsible which kould be expected aIteCLive Well Widths, The fact zirhL to be a continuobb tariarion. Lt,iti i.s not the exprrlmel~t;ily observed sltuarion suggests that m&ny physical regions of ~-lie interface structure comparable have NW resulting, ids an efiecLive well widtld that SIOXLS a L the distinct energy with the line width ?he same nieasureu tor spectrum 1, for exampie. is true il,ulvldually for the ren;lllder ot the spectra 1aLtiled 2 through i indichiing a total of seven dibLlncc acrostructure ctintlgurations Lt is beiieved possible 111 this particular k&W. lines are that 11ot: dll of the completeiy SiK,CfZ t&vhy comparable wells rtsolved, structure emit iines having 1nterLICP r;lll The tot_;1 comparsbie ourrg;) and line wiatll. energy spread of the excitor: line structure tar both. the ~liFb anu Lhe kHFE corrcsyonds CC. d well size iluituation of two monolayers. Thus, this can be vlt*(;d ds two domil,auc weii sizes, i 1: width lil t 11 separated by one mcnolayer, interface structure creating changes III cIlect1vt: wtll width uf Less than one monolayer. The excitor will see ;he dominant well width some w1tb cln added correction res~ltlug ir, well d”rrhbt widtt, cue to c he lhterface The energy structure. C).‘1COU then is ‘liiLz,c: determirlrd by tin:, i:verage well width. restiltb are readily illustzhted by the model in Fig. h where rhe spLtia1 effective weil size js y. two k.chemat~chll shoviil In this rlgure, well dom11.ant SiZf2S (iabellrd A and B) arc septrrated in wid:li Lj OUS uluhulayer (size In additiun, sever&l difference labelieti C). clangrs in both dominanr ktll sizes (labelled l-7) of le:>s than ON monolayer due LU the seitv microsrructure configurcrions are qualitativclq The IiarrowesL shown. sub-well is assoclateu large nuber statistical.l>

of

lines tilstributed

Would

Superlattices

and Microstructures,

727

Vol. 4, No. 6, 1988

with the highest-energy fine structure componcut Since it is alid the HHFE. oi the LhFb energetically possible for the exciton from the narrower sub-wells to lose energy and diffuse to sub-wells before collapsing, the the wider appearance of other light-hole tine structure may be seeu in addition to the componentn dominant one, when the excitation of the lower energy heavy-hole flue structure componrurs are One can estimate the number mouitored by PLE. or tiue structure iinrs that will be observed in In this case with the the dominant weil &se. well size being 2258, a one monolayer change in weil size corresponds to tin energy change of 11 the average line width 16 6.06 0.24 meV. meV, the maximum number of lines that could be observed would be four, which would result in eight lines for a two monolayer change in weii size. This is consistent wlrb the structure of Fig. 5. When the spectrometer is changed from heavy-hole emibsiou of the monitoring the component 4 to componeut 5 of the inset, the spectrum changes over from the narrower dominant well size to the larger dominant well size (a monoiayer change in wall size). It is seen that the fine structure components of the HHFE lines I through 4, are situated in the narrower of the The line labelled two dominant well sizes. Cl-H2 in Fig. 5 is associated with the exciton from sub-band one of the conduction band and sub-band two of the heavy hole valence band. The line labrlled n=2 is the excited state (2s) The extent or the wave function of the hHFE. for the n=2 state or the hhFb is much larger than that of the ground state. The ortails 01 the interface structure detected by the exciton :;,.li;;R!?(y Ylr;epJirt;ry,'n tzhitshecaz,e r;

i";

state of the exciton only detects monolayer It is seen in fluctuations in the well size. Fig. 5 that the n=i state does not exhibit line structure, but rather detects the rirot. tour rine structure line6 in the narrower of the two dominant well sizes and lines five to seven in the wider of the two dominant well sizes. it is noted that the 11-2 excitons in the narrower of the two aominaut well sizes cau also pump the ground state HHFE in the wider or the two dominant well sizes since they can diffuse from the narrower part to the wider part or the well. It is noted that the Cl-H2 exciton also does not This is partly because its shuw fine structure. wave function is mixoa with that of the n=2 lt is clear that the state or the HHFE. narrower wells coutribute to emission from the wider wells since the excitons can dirfuse from Diffusion of the narrower to the wider wells. excitors in the opposite direction does not occur. The solid lines depicting the sub-wells in Pig. 6 would result in very sharp transitions separated in energy by the amount determiuea by however, the observed linethe step heights. widths are comparabir to their separations which can be incorporated into the model by grading the sub-well widths as shown by the dashed liues One can visualize drawn on the step heights.

In fact, if many variations of this structure. the interface structures were completely random, one would observe a single liue whose linewidth to a two monolayer well width corresponded The structure in Fig. 6 is drawn to variation. qualitatively explain that the distribution of microstructure is not ranuom aud produces the number of lines and the approximate linewidths for the sample shown in Fig. 5. The interfacial microstructure can have various configurations which can occur along the same interface, from one region of the substrate to auuther, as well as from one lnterfacf to mentioned a MQW structure. AS e;;;;E:(lil; , the above interface rmcrcntructure has definite implications concerning the optical linewidths as weil as the peali energy puuitions When the of the oprlcal transitions in MQWs. two dimensional island size is appreciably larger than the exclton size, the exciruns see smooth wells and, the excittin transitions are sharp. A theory for the linewidth for the c&be when the island sizes tire comparable to, or excitori bize has been reportec ;~;:~erht~n$Si . This theory is based on the realization that t.Le optical probg, i.e., the exciton, has a finlte extent (c250A in CaAs, ior exasplej ana the energy of tl;t emitted radiation reflects the averabe well width or the region seen by the rxciton. The exteut of the exciton will vary with well size. Assuming the excitons are created randomly in the wel,, the liur shape is determined by rhe probability distribution of rindlug iiuctuations in the well size extending over the erciton sire. For example, 102‘ a IOOA quantum weli, ;r the average islana sire is comparabir to the exciton size wi.cL an i&and the linewidth ir height of one mOnOlayer, about 3 meV; howeves, If the average island blue is dpproximatrly iCA the linewidth is rctiuced to oniy 0.15 meV. The haii-wldih of PL lines as a runction of well siL.r for varying island sizeb and oue monolayer height, as determined from the above theory, is sikwn in Fig. i. Also shown are the experimental values for the uarroweet anu broadest linewidths observed ior the four MQWs that were investigated. The exyerimer‘rJ linewidths show that the laterai extent of the lntelisce islands, of which the smallest misrostructurr is composed, is approximately The largest is>nds in the interface 20A. structure arti about -gOA in lateral extent, for a one monolayer island height.

4.

Corlclusions

5harp line structure has been observed iu both the LHFE and HHFE cransitious in four MQW Structures ol differrut well sizes. From PLL it was shown that the MQW structure consisted of by one tU@ Clomillal~L well sizes separated well width, with interface monolayer in structure creating changes in errective well Lmissiou irom width of iess than one monolayer. both the uarrower and wider well sizes showed

Superlattices

and Microstructures,

Vol. 4, No

6. 7988

size suggest Lhnt all four Flqha were gr"w~~ dt near "pc~mum growth conditions. The optical inrormation coucernrng the spectra provide mlcroscoplc structure or the rntrrfaces as well ‘is inkormation pertinent to the heterostructurr growth process.

References

a 1.

2.

2.

5. w, Fig. 7.

Ii)

Halr-width or photoluminescence Lines ( ")ab a iunction of well si2e ror The "i one monolayer. islana heights solid curves a and b are theoretical curves f"r,two dimensional islanas "i The experi2Uh and bUx rtcpectively. mental points represent the narrowest and broadest emission lines observed for each airferent well size.

sharp line structure reflecting interface microstructure for each well size. The exclton rine structure had an energy spread or one monolayer for each dominant well size giving a total The narrowest energy spread or two monolayers. and the broadest linewidths (EWIJI) for the fine structure for each MQW were measured. BY comparing these linewidths with their theoretically calculated vaiues, it was possible to determine the island sizes that make up the interracial microstructure ror the four MUWs investigated. The two-dimensioual island size was as small as 20& for the narrowest exciton transitious and 80A for the broadest exciton cransitrons. The narrow linewidths coupled with the minimum two monolayer fluctuations ln well

6. 7.

6. 9.

1U.

li.

12.

13.

14.

15.

C. W. Weisbucb, R. C. Miller, K. IJingie, A. C. Gossard, and k. Wiegmann, Solid State Commun.. 2,709 (1981). C. Weishuch, K. Diugle, P. M. Prtroir, A. C. Uossard, and W. Wiegmann, nppl. Phys. Left. 2,840 (1981). L. Goldstein, Y. horikushi, S. Tarucha, and H. Ukamat", Jpn. J. Appl. Phys. 2.1489 (1983). 4. b. Deveaud. J. Y. Emery, A. Chomette, b. Lambert and 11. Baudet, Appi. Phys. Lett. 45,1078 (1984). h. Deveaud, A. Pegreny, J. Y. Emery and A. Chomette, J. Appl. Phys. 2,1633, (19b6j. h. Sakaki, M. Tanaka, aud J. Yoshino, Jpn. J. xppl. Phys. 24,L417 (1485). T. hayakawa, T.%yama, K. Takahashi, !I. Kondo, S. Pamamoto, S. Yan", and 1. Hiijikata, Apple. Phys. lett. 47,952 (1965J. k. C. Miller, C. W. Tu, S. K. Sputz, and K. F. kept, Appl. Phys. iett. 44,124s (198tj. D. Bimberg, II. Mars, J. N. Miller, K. baurr, =nu D. Uertel, J. Vat. Sci. Technol. B4,1014 (15&b;. U. C. Reynolds, K. K. bajaj, C. W. Littun, P. W. Yu, S. Yingh, W. T. Masselink, K. Fischer, and h. Piorkoc, Appl. Phys. Lect. 46,51 (1985). IJ. C. Reynolds, h. K. Bajaj, (. W. Litton, J. Singh. P. W. Yu, P. Pearah, .I. Klem, an" H. Morkoc, Phys. Rev., =,5931 (1986). IJ. L. keynolds, K. K. bajaj, L. I'eters, C. Leak, W. M. Theis, P. W. Yu and H. MorkoL. Phys. kev. E, 3117 (19&b). J. Singh, K. k. Bajaj, lJ. C. Reynolds, L. W. Litton, P. k. Yu. W. 'i. Mdsselink, K. Fischer, and h. korkoc, J. V&C. Sci. Technol. B3(4), lob1 (1985). 'ia Li Sun, W. 'I. Masseliuh, k. Fischer, k. V. Klein, H. Morkoc, ana K. K. bajaj, J. xppl. Phys. 55,3554 (1984). J. blngh, K. K. Baja~, dnd S. Lbdudhuri, Appl. Phys. Lett ‘+/+,&I5(1984).