REVISTA MEXICANA DE FíSICA 44 SUI'LEMENTO 3. 154-157 DICIEMBRE 1998

Lasing and electronic properties oCa single InAs monolayer embeddedin bulklike GaAs

A.R. Goñi, M. Slroh, C. Thomsen, E lIeioriehsdorff. V. Türek, A. KroSl, aod D. BimbergInstitutfür Festkorperphysik, Technische Universitlit Berlin

10623 Ber/in. Germnny

Recibido el 10 de enero de 1998: aceptado el 14 de enero de 1998

Wc have sludied the sponlaneous and stimulated emission charactcristics of a single 1.5-monoJayers-thick InAs ¡ayer in bulklike GaAs.Lasing action is obtaincd by continuous-wave-optical pumping always al the wavelength of the loAs thin-laycr excitonic luminescence(_ 8iO om) independent on pump power density. Gaio rncasurcrncnls yield a very high material gaío uf 1.0(5)xlO"'cm-1 fUf the InAslayer whcn pumped with 12 kWlcm2 allow tcmperatures. This value is twicc as high as that oblained fUf rclated InGaAsl InGaAsP multiplequantum wells using much larger pump power dcnsities. bUIcomparable (O the gains measured recently in InAs/GaAs quantum dot lascrs.The large material gain and the absence of bandgap renormalization effects with increasing pump level speak fm an excitonic mechanism ofpopulation inversion charactcristic of low-dimensionallasing nanostructures. As c1early shown by cathodoluminescence measurements. ex-citon localilation effects due to layer-width fluctuations playa fundamental role in detcrmining the lasing mechanism for the InAs monolayer

as compared to wider quantum wells.

Key",u,.ds: Semiconductor lascr; low-dimensional structures; stimulated emission; excitons

En este trabajo se han estudiado las características de la emisión espont:inea y estimulada de luz de una capa delgada de InAs con un espesorefectivo de 1.5 monocapas intercalada en GaAs. Mediante el bombeo óptico de onda continua se consigue la emisión laser siempre a lamisma longitud de onda correspondiente al máximo de la fotoluminiscencia (PL). la cual proviene de procesos de recombinación excitónicaen la monocapa de InAs. ~lediciones de la ganancia material de esta estructura arrojan valores extremadamente altos de hasta 1.0(5) x 10

4

c01-1 con 12 kW/cm2 de potencia de bombeo y a bajas temperaturas. Esta ganancia es dos veces mayor que la obtenida en pow'\ cuánticosmúltiples de InGaAsJlnGaAsP para densidades de potencia de bombeo mucho más elevadas. resultando más bien comparable a los valoresde ganancia recientemente alcanzados con láseres de puntos cuánticos de InAslGaAs. La alta ganancia material y la ausencia de efectosde rcnormalización de la brecha de energía al incrementar el nivel de hOl11heoindican que. tal como es característico en estructuras dehaja Jímcnsionalidad. el mecanismo suhyaccnte a la emisión estimulada es excitónico. Por otro lado. mediciones de catodoluminiscenciamuestran daramente la localización de los e",citones en el plano de la monocapa de InAs debido a las fluctuaciones en el espesor Je la misma.Este hecho cumple un rol determinante para el mecanismo de lasco de la monocapa proporcionándole características propias de sistemasO-dimensionales y lleva a que ésta se diferencie en sus propiedades ápticas de polOS cuánticos más anchos.

Descriptore.c Laser de semiconductores: cstnlcturas de baja dimensionalidad; emisión estimulada: excitones

PACS: 73.20.Dx; 78.45.+h; 78.55.Cr

lo Introduction

lo reeenl years InAs/GaAs quanlum weIl (QW) slruetureswilh wcll widths ranging in the monolayer (ML) regimehavc attracted much interest due to thcir unusual clectronicand aplical properties {l-7l. Al low tcmperatures thcscslructurcs exhihit, for instance, a very strong but narrow(F\VHM '" 8 meV) phololumineseenee (PL) line [1,5.8]and lasing aelioo in lhin InAs/GaAs ML's has been alreadyobserved [2,9]. Anolher peeuliarily of lhe loAs IGaAs sys-lem is lhe high built-in s/rain (~ i%) whieh enables an Iso-morphic cpitaxial growth up lO a critical lhickncss 01' twoML's [1,3, 4J. Beyood Ihis limil lhe growlh proceeds lhreedimensional bUI self.organized lcading lO the formation ofquamum dOIS [10-13J. As a maller of fael. every quanlumdot sample grown by this method contains a ML-thick wet-ting layer. Thus, for a betlcr understanding of cmission pro-cesses in quanlum dots systems it is important to study the

single layer case tirst. Furthcrmore, for thin-Iayer structurcsolle would expect that thcir opLical propertics will resernblelhal ()f quanlulll dOlSsyslems 11.11 beeause of Ihe 10eaIization01' e.xcitons in the plane of the monolayer duc to tluctuationsin its width. In spitc of this, an invcstigation of the lasingpcrformances nI' InAs/GaAs ML structures in connection toan exciLonic kind of mcchanism for population invcrsion andils OD characteristics is still laking.

Hcrc we report on the spontaneous and stimulalcd cmis-sion from eXCilUllS in a single InAs monolayer in bulk.likc GaAs. Laser cmission occurs at constant cnergy, inde.penuent uf the pump powcr, at about 100 I11CVbelow theGaAs band gap involving radiative recombination processesfrom ground-state cxcilons bound 10 Lhe InAs layer {8].Cathodoluminesccncc measurements show that these cxci-tons are localized in the InAs plane by layer-width fluctua-tions. The thin-Iaycr samples exhibit a strikingly large ma-terial gain which is twice as high as the maximum gaio oh.

LASING AND ELECTRONIC PROPERTlES OF A SINGLE InA, MONOLAYER EMBEDDED IN BULKLlKE GaA, 155

1.42

2K514"m

, ,411.40

'1.0 1.10'••• ENERGY (eV) ¡, ,. ,. ,. ,

~'"Z resolu1ionw

~¡j

1,39

1.5 ML InAslGaAs

o

:i! CBrF"" M" r --12 loA trIAs~>- VB~r>-¡¡; G", GwZW>-~

"J\"~"-

XlO

I..J1.40 1.45 1,50 1.55

1.5 MllnAs/GaAs5K

FIGURE 1. PL spectrum ofthe 1.5 ML InAs/GaAs sample al lowexcitation power and al 2 K. A sketch of the r -point band edgeprofilcs of the structure is shown in the insel. Arrow indicates lheobserved optlcal transitiol1.

ENERGY (eV)

2. Expel"imental

lained for InGaAs/lnGaAsP mullip!e quanlum wells (QW)using much largcr pump powcr densities [15 J, bUI compara-ble lo the valucs measurcd rcccntly in InAs/GaAs quantumdOllasers [16].

The sample was grown by convenlional MOCVD on a semi-insulating (001) GaAs subslrate as descrihed elsewhere [17J.The laser structurc consists 01'a single loAs layer (effcclivcIhickness ahout 1.5 ML) sandwiched between 300 nm IhickGaAs layers. A waveguide is formed by two c1adding layersofundoped AI0.5Gao.5As. Short cavities from250 lo 400 11min lcngth wilh mirrorlike surfaccs perpendicular lo the plancof the InAs MI. were oblained hy cleaving Ihe sample.

Standard photoluminescence (PL) was exciled in hack-scaltering with (he 514 om lioc of an Ar+ -ion laser. whcrcasfor phololuminescence excilalion (PLE) a lunable Ti: sap-phirc laser was uscd. Gaio mcasurements WeTe pcrformcLl in90o-scattering gcornctry al low tcmpcralurcs hClwecn 2 and120 K. Light cmission wa~ excilcd in continuous-wavc (cw)mode wilh Ihe Ti: sapphire laser luned lo 750 nm. The pumplaser was inciden! on (he (00 1) growlh surface and was fo-cused to a 40 I"TI wide slripe of variable length (as shownschematically in the insel lo Fig. 3). Light cmittcd from oneof the cleavagc mirrors was analyzed by a triple-grating spec-tromeler and detecled with a charge-coupled-device camera.Calhodolumincsccnce (eL) mcasurcmcnts wcre performcd allow tempcraturcs hetween 5 and 50 K and lIsing moderale ex-cilation (voHagc 7 kV and heam currenl 2.5 nA). The lumi-ncscencc is dClccted with a 512-channel infrared-intensificddiode array. Thc spectral rcsolution was ahollt 0.27 mev'

3. Results and Discussion

Figure I shows a low-tempcrature PL speclrum 01' lheInAs/GaAs MI. sample measured in backscanering from the(0.0, 1) growlh surl'ace al luw excitation power. The "pticalemission is dominaled by a single intense and narrow line(F\VH~I "" lO meV), which is redshined hy 93 meV fmm{he GaAs gap cncrgy. The much weaker struclures at around1.48-1.50 eV correspond lo lransitions involving carbon illl-purities and bouml excilons in hulk GaAs. The energy levelscheme is also shown in Fig. 1.

Tighl-binding calculations for stricUy one ML includingstmin effects indicate thal only a single eleclron state andtwo praclically degenerale hole levels are hound lo lhe InAslayer [5]. For our sample wilh 1.5ML width a lhird hole levelalso become slighlly bound [1,6,7]. We lherefore assign lhemain PL peak lO optical transilions belween heavy-hole ex-citon slates bound to lhe thin InAs layer. Oplical lransitionscorresponding lo Ihe recombinalion oflhe 2nd haevy-hole ex-cilons have becn observed in PLE spcctra at about 65 mcVhigher in energy [18].

ENERGY (eV)

FIGURE 2. lIigh-resolution eL spectrum al a given spot 011lhc 1.5ML InAs / GaAs sample al 5 K. The inset shows the low energytail of the spcc[rum in more detall. The 6-like peaks correspond lOemission from indlviduallocalizcd 00 excilons.

A clcar evidence of excitan localization effecls is ob-tained from speclrally and spalially resolved calhodolumi-nescence measurements. Figure 2 shows a represcl1lative elspeclrum laken from a single spot 01' Ihe InAs/GaAs MI.sample. The Iinewidlh of lhe main cmission is mainly duelo inhomogeneous broadening due to flucluations in ¡ayerwidlh. In facl. a closer look at lhe low-energy tail 01' lheCL peak c1early indicates lhal it consisls of an ensembleof J-funetion like lines, whose linewidlh is determined bythe spectral resolution of lhe experimental setup. As demon-SIraled hy el. scans 01' Ihe sample al fixed photon energy,cach linc can be attributed to the recombination of an indi-viduallocalized ODexcilon [19J.

Re" Mex. Fú. 44 S3 (1998) 154-157

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1.3 kW/cm2

••••

1.415 1.420 1.425

ENERGY (.V)1.43C

O 50 lOO 150 200 250STRlPE LENGTH I (11m)

FH,URE 3. Stimulatcd emission spectra 01'lhe 1.5 t\1L InA!\/GaAssample for diffcrenr stripc length~ al a pump powcr densily of1..3kW/cm2 aoove lasing threshold (2 K).

Figure 3 shows TE-polarizcd cmission spectra fmm lhec1caved edge ITlcasurcd in 90° geornctry fOf two diffcrcnl spnllengths al 2 K and using a pump power dcnsity of 1.3 kW/cm'l(above lasing Ihreshold). The speclra exhibil lhe characleris-tic Fabry-Pérot oscillations corrco;ponding to lhe longitudi-nal modes al' Ihe cavity. With ¡ncrcasing cxcitation powcrlevel lhe cmission bccomes sharpcr with nnly a few longi-tudinal modes gaining exponentially in intcnsity. An impor-lanl poin! is that foc all the cw power densities used in theseexpcrimcnls {he slimulated emission occurs al lhe encrgy oflhe PL peak wilhoul showing any redshift or broadening. Asimilar lasing behavior was rcccntly found in self.organizcdInAs/ GaAs quanlum dOls [201. This speaks againsl band gaprcnormalizalion eJTects due 10 huild-up of phutocxcitcd freecarricrs. and ,"'C thercfore eonelude that the lasing mcehanismis of excilunic nature [21].

For the delermination of the oet optieal gaio (or modalgain) 01"the t\.lL struc{urc we pcrformed gain measurelllentsby lhe slripe excilalion melhod [22J. The sample is oplieallycxcited from the (00 1) surface with a stripclikc focus of vari.able length ,. whilc the stimulated cmission from one 01'thecleaved edges is deleeled (see insel to Fig. 3). The depen-dente of the maximum lasing inlcnsily on s{ripc length 1 isdisplayed in Fig, 4 for a sample wilh a resonator lenglh ofL = 250 /1m. A careful analysis of Ihe finesse of Ihe Fabry-Pérol oscillations shows that lhe number 01' passcs for am-plified lighl in lhe resonator rapidly grows wilh Ihe slripelength resulling in lhe abrupt incrcase in laser intensily ahoyeI ~ 220 ¡tln, For stripe lenglhs shortcr than 170 ¡un, howcvcr,light oscillates hack and forth only unes in the cavily, and lhec,-;'littedinlensity I"romthe edge can be then expressed as

I(",) =(1 - r) Isp (eG(~)1 - 1)G(",)

FIGURE 4. Dcpcndence of lhe maximum lasing intensily on thelength uf slripc focus al fixcd pump power densily. The solid linereprcscnts lhe resuh of fittingEq (1) 10 (he data poinls (see text fordetails).

where G(w) is lhe modal gain (or nel oplical gain) al givenfrequency, o: is lhe coefficient of internal absorplion losses,Isp is the spontaneous cmission rate and r = 0.3 is the rc-tlcctivity 01' the GaAs-aie interface.

In urdcr 10 he able to obtain the maximum Illodal gaioG(w",) from Eq. (1) we need lo determine lhe inlernal ab-sorplion eocfticient Q uf the nonilluminated resonalor regioo,l.e. whcrc thcrc is no active medium. For thal purpose wehave monitored the uecay of the emiued inlcnsily when apoint focus for excitation in 900 geometry is moved awayfrom lhe clcaved edge uf a sample w¡th a vcry long resona{or(1.5 mm). In Fig. 5 we have plotted several speelra of lhe edgecmission roe differcnt distances 01' the poinL focus from thecica ved surfare. Thc cmitteu Iight intcnsity decreases with¡ncrcasing distance bcing ilS cxponcntial decay strongly de-pcndcnl un photon encrgy. As 'In cxample. the ¡nsel to Fig. 5shows this distance dependcnce of lhe intensily for a givenenergy whieh corresponds roughly lo lhal of lhe PL maxi-IllUIl1.Tite solid linc rcprcsents a fit using an cxponential-Jcca)' function; Ihe corrcsponding absorplion cucfficicnt be~ing in this C:.l~c45 cm-l.

In this way. we have determincd the modal gajn by fittingEq. (1) 10 Ihe inlensily dala poinLs in lhe range of 1 < 170/lmusing ¡SI' and G as adjustablc parameters (solid curve inFig.4). Mcasurements pcrformcd undcr a cw excitalion of10 kW/cm' al 2 K yield a very high maximum modal gain01' G = 130(20) cm-l Thc large modal gain enables single-mode opcration in samples wilh short c3vilY rcsonalOr. forwhieh lhe lighl oscillale, baek and forth in lhe cavily per-forming up lo 10 passes. As indiealed by Ihe slimulaled emis-sion specl •." ()f Fig. 3, we achicved highly efficienllasing fora few longitudinal mudes (3 lo 5) with a 250 Itm long res-onator.

Re>,.Mex. Fis. 44 S3 (\99~) 154--157

LASING AND ELECTRONIC PROPERTIES OF A SINGLE InAs MONOLAYER EMBEDDED IN BULKLlKE GaAs 157

1.40 142 1,44 1.48 1.48

ENERGY (eV)

FIGURE 5. Spcctra of (he edge emission ofthe 1.5 ML InAsl GaAssample for differenl distanccs of Ihe point focus from Ihe c1eavedsurface. The ¡nset shows the decay of the emiued intensily at agiven pho(on energy as a function 01'the point-focus distance fromthe sample edge.

4. Summary

In summary, we demonstrated lhal large malerial gain val-ues were achieved for a structure consisting of a single InAsML in bulklike GaAs as eompared to lhicker QW's emiltingal similar wavelengths. The very high gain values ohlaincdfor lhe lhin InAs layer and lhe insensitivily of ils emissionenergy upon pump power levels taken logelher wilh lhe di-rcct proof of exciton localization from cathodoluminescencegive strong cvidence for an excitonic mechanism of stimu-laled emission similar to thaI observed in quantum dots.

We are graleful to O. Slier for helpful diseussions. Parls ofthis work are funded by Deulsehe Forsehungsgemeinsehaftin lhe framework of Sfb 296.

Acknowledgments

oblained for lhe 1.5 ML-lhiek InAs layer a malerial gain of9 = 1.0(5) X lO' em-1 al low lemperatures. This valueis lwiee as large as for InGaAslInGaAsP MQW slrueluresemitting al similar wavelenglhs bul when pumped with 10times larger power densilies 115). Aelualiy. our gains beeomeeloserto lhal reported reeenlly (- 10' em-l) for InAs/GaAsquanlum dOl injeelion lasers [13.16].

We interpret such increase in material gain for a mono-layer as eompared lo quanlum welis as due lo the differenllasing mechanism being active in each case. For lhe InAs MLthe widlh fiuelualions lead lO localization of exeilons withinlhe x. y-plane of lhe InAs layer wilh lhe eonsequenl quanti-zation of their center-of-mass motion as in zero dimensions.Thus. the Coulomb inleraetion between eleetron and hole 10-ealized al lhe same site is enhaneed and the ML exeilonieemission aquires OD eharaeler similarly as for quantum dOlS.which is retlected in the device performance.

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SPOTPOSITION (llm)

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The malerial gain 9 = (G + a;)/r is related lo themodal gain lhrough lhe confinement factor r. whieh givesIhe fraelion of lhe radiation field that overlaps with lhe ex-cilon slales in lhe InAs layer, and thc internal powcr losseoeffieienl ai of Ihe la,er eavily. By solving the differen-lial equalions of transverse eavity modes [231 for our par-ticular waveguiding stripc geometry and by laking into ac-counl lhe laleral spread of the eleclron and hole wavefunc-lions in lhe direelion perpendicular to Ihe plane of lhe InAslayer (- 5 nm from lighl-binding ealeulalions 15]). lhe eon-finemenl faelor is ealculaled lo he r = O.O~1. We assumedlhe eoeffieienl a. lo be Iess lhan 10 cm-l. as lypiealiy foundfor similar InAs/GaAs nanoslruelures [lOJ. In lhis way we

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