*Corresponding author. Fax: #86-10-82649531.E-mail address: mhzhang@aphy.iphy.ac.cn (M.H. Zhang)

Journal of Crystal Growth 209 (2000) 37}42

Photoluminescence characterization of Si-dopedlow-temperature grown GaAs and GaAs/AlGaAs

multiple quantum wells

M.H. Zhang*, Y.F. Zhang, Q. Huang, C.L. Bao, J.M. Sun, J.M. Zhou

Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, People's Republic of China

Received 13 April 1999; accepted 3 September 1999Communicated by T.F. Kuech

Abstract

The Si-doped low-temperature (LT) grown GaAs and GaAs/AlGaAs multiple quantum well (MQW) structures havebeen studied by photoluminescence (PL) spectroscopy. The samples were grown at 3203C with di!erent As pressures,then subjected to rapid thermal annealing from 500 to 9003C. Besides the band-edge PL feature from the band}bandrecombination, a defect-related PL feature has also been observed in both GaAs and GaAs/AlGaAs MQW structures.Deep-level transient spectroscopy measurement shows that the As antisite-like defects exist in the as-grown and6003C-annealed Si-doped LT-grown GaAs. The di!erent annealing temperature dependence of the band-edge PL featurein two kinds of materials is observed and discussed. The defect-related PL feature may be related to the defect complexconsisting of Si and As atoms. ( 2000 Elsevier Science B.V. All rights reserved.

PACS: 78.55.Cr; 61.72.Cc; 71.55.Eq; 61.72.Vv

Keywords: LT-GaAs; Si-doping; Defect; Photoluminescence

1. Introduction

In recent years, novel device applications havebeen exploited to utilize the unique high resistivityof low-temperature (LT) grown GaAs or GaAs/AlGaAs multiple quantum wells (hereafter referredto as LT-GaAs and LT-GaAs/AlGaAs MQWs, re-spectively) [1}3]. It is generally believed that excessAs is incorporated into epilayer during LT growth[4]. LT-GaAs in the early work was grown by

molecular beam epitaxy (MBE) around 2003C [5].Due to the large concentration of deep defects, it isconductive while growing, but becomes semi-insu-lating after annealing above 6003C. During pos-tgrowth annealing, excess As atoms precipitate toform As clusters. Later LT growth technique wasextended to medium temperature of around 3003C.As temperature increases, the amount of excess Asatoms decreases, and as-grown LT-GaAs becomessemi-insulating. LT-GaAs/AlGaAs MQW struc-tures grown around 3003C have the coexistence ofultrafast photocarrier lifetimes and sharp excitonicabsorption, and are very attractive for photo-refractive device applications [6,7], electro-optic

0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 4 9 5 - 9

sampling [8], and low photocurrent modulators[9].

It has been shown that AsG!

(antisite) and VG!

(avacancy on the Ga site) are the dominant defects inAs-rich LT-GaAs [10]. V

G!can enhance As

G!dif-

fusion and thus accelerate the formation of Asclusters during annealing [11]. In As-rich material,As

G!may exist as double positively charged, posit-

ively charged and neutral, denoted as As`2G!

, As`G!

,As0

G!, respectively [12]. V

G!may be in triply nega-

tively charged vacancy, double negatively chargedvacancy, negatively charged vacancy and neutralvacancy, denoted as V~3

G!, V~2

G!, V~1

G!, and V0

G!,

respectively. Because the doping in#uences theconcentration of charged defects, the total concen-trations of As

G!and V

G!can be changed through

di!erent doping (Fermi-level e!ect). According tothis consideration, Chang et al. [13] have studiedthe behavior of As precipitates in LT-GaAs underdi!erent types of doping. They found that in com-parison with intrinsic and p-type LT-GaAs, theleast As precipitates could be observed in n-typeLT-GaAs due to higher concentration of V

G!-re-

lated defects and lower concentration of AsG!

-re-lated defects. Their study was exclusively based onthe TEM image analysis. However, the change pro-cess of defect con"guration during annealing couldnot be studied. Photoluminescence (PL) is verysensitive to the change of the defect con"gurationand very suitable to study this problem. In thispaper we provide an investigation of PL spectrain Si-doped LT-GaAs and GaAs/AlGaAs MQWstructures. Because high concentration of excess Asatoms could quench PL, we grew samples at lowerAs pressures and a medium substrate temperatureof 3203C to decrease the concentration of deepdefects. These growth conditions are suitable togrow high-quality MQW photorefractive material[6,7]. Thus we also have the opportunity to studythe in#uence of Si-doping on photorefractive prop-erties of LT-MQW materials.

All samples explored in this study were grown ina VG80H MBE system. The substrates were semi-insulating GaAs(0 0 1) wafers. Two LT-GaAs sam-ples (referred to as Samples A and B) had the samestructure: 1 lm GaAs bu!er layer grown at 5803C,then 1 lm LT-GaAs layer grown at 3203C. Thedoping level in GaAs bu!er and LT-GaAs is

1]1018 and 7]1017 cm~3, respectively. Two LT-GaAs/AlGaAs MQW samples (referred to as Sam-ples C and D) were grown with the same sequence:1 lm GaAs bu!er layer at 5803C, then a LT MQWstructure grown at 3203C. The LT MQW structureconsisted of 250 nm Al

0.3Ga

0.7As cladding layer,

70 periods of 7 nm GaAs quantum well and 10 nmAl

0.3Ga

0.7As barrier, "nally another cladding

layer of 100 nm Al0.3

Ga0.7

As. All GaAs quantumwells were Si-doped with the level of 7]1017 cm~3.The As pressure during growing LT layers for Sam-ples A and C was 8]10~8 Torr, for Samples B andD was 1.0]10~7 Torr, respectively. After growth,the samples were cleaved into small pieces, andeach piece was subjected to 30 s rapid thermalannealing (RTA) in the range between 500 and9003C. To prevent arsenic loss, RTA was performedunder the #ow of nitrogen gas and the sample wascapped by a piece of GaAs substrate. For SamplesA and B, mesas were also fabricated on some piecesby photolithography, with Ohmic contact to Si-doped GaAs bu!er layer by AuGeNi and Schottkycontact to Si-doped LT-GaAs layer by Au. Thendeep-level transient spectroscopy (DLTS) was mea-sured in the temperature range from 77 to 340 K.The photoluminescence from samples was disper-sed by a SPEX 1403 double grating spectrometerand detected by a photomultiplier tube coupled toa photon counter. All measurements were per-formed at 77 K. The Ar` laser power of 50 and4 mW was used during PL measurement of twoLT-GaAs samples and two LT-GaAs/AlGaAsMQW structures, respectively.

2. Experimental results and discussion

PL spectra of Samples A and B annealed atdi!erent temperatures are shown in Figs. 1 and 2,respectively. It can be seen that for the as-grownSample A, besides the band-edge PL feature peak-ing at 818 nm, there is another weak PL featurepeaking around 852 nm, which we refer to as thedefect-related PL feature. Increasing the annealingtemperature, the intensities of both PL featuresincrease up to 8003C, then suddenly decrease afterannealing at 9003C. For Sample B, similar phenom-enon could be observed. However, there are also

38 M.H. Zhang et al. / Journal of Crystal Growth 209 (2000) 37}42

Fig. 1. PL spectra of Sample A after annealing at di!erenttemperatures. The curve etched refers to PL spectra of the8003C-annealed piece after annealing the LT-GaAs layer.

Fig. 3. The Arrhenius plot of deep-levels signals in the as-grownSamples A and B. Here ¹ is the peak-temperature of DLTSsignals in a single-rate window scan, and q is the correspondingrate window. Four rate windows 1.656, 16.56, 165.6 and 1656 mswere used during measurement.

Fig. 2. PL spectra of Sample B after annealing at di!erenttemperatures. The curve etched refers to PL spectra of the8003C-annealed piece after annealing the LT-GaAs layer.

some di!erences. In Sample A, the peak positions ofboth band-edge and defect-related PL features al-most do not change with annealing. In Sample B,the weak defect-related PL feature peaks at 852 nmin both the as-grown and the 5003C-annealedpieces. After annealing above 5003C, the defect-related PL feature has a blue shift and peaks at838 nm, whereas the band-edge PL feature hasa red shift and is a little quenched. In order to makesure that the defect-related PL feature indeed com-es from LT-GaAs layer, we have measured PL

spectra of the 8003C-annealed pieces of both Sam-ples A and B after etching the LT-GaAs layers. Theresults are also presented in Figs. 1 and 2 for a com-parison. It is clearly seen that there are only band-edge PL features in two etched pieces. So thedefect-related PL feature indeed comes fromLT-GaAs layer.

Fig. 3 shows the Arrhenius plot of deep levels inthe as-grown Samples A and B. The measurementwas performed in the range of 77}340 K. Duringmeasurement the amplitudes of the reverse bias andthe forward-bias pulse are 1.0 and 0.5 V, respective-ly. Four rate windows of 1.656, 16.56, 165.6 and1656 ms were used. The same deep level atE#!0.66 eV was observed in both samples. Its

concentrations were 9.6]1015 and 1.3]1016 cm~3

in the as-grown Samples A and B, respectively. Themeasured activation energy is very close to re-ported value for As

G![14]. In 6003C-annealed

pieces of Samples A and B, the deep level atE#!0.66 eV still exists, however, its concentra-

tions decreased to 9.3]1015 and 1.12]1016 cm~3,respectively. For pieces annealed above 6003C,DLTS signals were very di$cult to detect becauseof large leak current. The shallow-level defectresponsible for the defect-related PL featurewas not observed due to high temperature duringmeasurement. This measurement indicates a higherAs pressure leads to a higher concentration of As

G!.

M.H. Zhang et al. / Journal of Crystal Growth 209 (2000) 37}42 39

Fig. 4. PL spectra of Sample C after annealing at di!erenttemperatures.

Fig. 5. PL spectra of Sample D after annealing at di!erenttemperatures.

The defect-related PL feature has also been ob-served in Si-doped LT-GaAs/AlGaAs MQW struc-tures grown at 3203C. Two samples (Samples C andD) were grown at di!erent As pressures. Figs. 4 and5 show their PL spectra measured at 77 K. A de-fect-related PL feature peaking around 800 nm canbe seen clearly from both "gures, its intensity muchweaker than that of the band-edge PL feature re-lated to the e1}hh1 transition in quantum wells. Atthe annealing temperature of 9003C, the defect-related PL feature disappears. The intensity of theband-edge PL feature versus annealing temper-ature shows a valley-shaped change in two samples.Upon annealing, its intensity "rst decreases and

reaches a minimum at 7003C, then recovers withincreasing annealing temperature. Such a behaviorof the band-edge PL feature in MQW samples isvery di!erent from that in LT-GaAs. The apparentblue shift of the band-edge PL feature in SampleC with annealing temperature also can be seen,which is induced by Al}Ga interdi!usion duringannealing.

Let us "rst discuss the annealing temperaturedependence of the intensity of the band-edge PLfeature in LT-GaAs and LT-GaAs/AlGaAs MQWstructures. In LT-GaAs, the intensity of the band-edge PL feature increases with annealing temper-ature up to 8003C, and then suddenly decreasesafter annealing at 9003C. In LT-GaAs/AlGaAsMQW structure, it shows a valley-shaped changewith annealing temperature. Previous studies showthat carrier lifetimes strongly depend on the sizeand spacing of As clusters [15]. In LT-GaAs/Al-GaAs MQW structures, the valley-shaped changeof PL intensity also has been observed in our for-mer investigation [16], where we found the fastestcarrier capture rate of As clusters around the bot-tom of the valley. The present results show that Asclusters in the 9003C-annealed LT-GaAs and the7003C-annealed LT-MQW structures have the fas-test carrier capture rate relative to those piecesannealed at other temperatures. Because of thesimilar growth conditions, we consider that compa-rable size and spacing of As clusters in LT-GaAsand LT-GaAs/AlGaAs MQW structures annealedat the same temperature. However, the situations ofthe carrier con"nement in two kinds of materialsare di!erent, which would be the reason for di!er-ent annealing temperature dependence of theband-edge PL feature. It can also be seen that at theannealing temperature of 8003C, the defect-relatedPL feature still can be observed in Sample D, butnot in Sample C. Such a di!erence may be causedby the di!erent As pressures during the growth oftwo samples. There is a higher concentration ofAs

G!defects in Sample D due to higher As pressure.

Further TEM structure analysis and carrier lifetimemeasurements would be helpful to explain the pres-ent results.

Next, we turn to discuss the origin of the defect-related PL feature. The di!erent peak positionsin two LT-GaAs samples imply that there are

40 M.H. Zhang et al. / Journal of Crystal Growth 209 (2000) 37}42

di!erent structures of the shallow-level defect re-sponsible for the defect-related PL feature. As sucha defect-related PL feature could not observed inundoped LT-GaAs or LT-GaAs/AlGaAs MQWstructures under present growth conditions, Siatom may take part in the shallow-level defect. Onthe other hand, As

G!and V

G!are the dominant

point defects in Si-doped LT-GaAs; both may alsobe involved in the shallow-level defect. Previousstudies show that upon annealing, As

G!and V

G!group together to form As-clusters and vacancyclusters, respectively [17]. The valley-shapedchange of the intensity of the band-edge PL featurecould also be observed in undoped LT-MQW sam-ples [16], which have less V

G!concentration than

the Si-doped samples [13]. It implies that theformation process of As clusters is irrespective ofthe presence of V

G!, which, however, can enhance

the speed of As precipitation. Figs. 4 and 5 showthat the defect-related PL feature disappears inannealed LT-MQW samples after increasing theannealing temperature through the bottom of theintensity valley of the band-edge PL feature. Ac-cording to these observations, we suggest that theatoms of excess As instead of V

G!are also involved

in the shallow-level defect, the disappearance of thedefect-related PL feature at high annealing temper-ature being the result of the increasing size of Asclusters. In the LT grown p}i}n photorefractivedevices, postgrowth annealing is usually needed toadjust the carrier lifetime [18]. During annealing Siatoms may di!use into intrinsic region consisting ofLT-GaAs/AlGaAs MQWs with both resistivityand excitonic electroabsorption decreasing. There-fore, the photorefractive e!ect also becomes weak.Such a result should be avoided. The present obser-vation of the defect-related PL feature can be usedto identify this phenomenon directly.

3. Conclusions

In summary, we have investigated the behaviorof As precipitates in Si-doped LT-GaAs andGaAs/AlGaAs MQW structures by PL spectro-scopy. Besides the band-edge PL feature from theband}band recombination, a defect-related PL fea-ture has also been observed in two kinds of mater-

ials. DLTS measurement shows that the AsG!

-likedefects exist in the as-grown and 6003C-annealedSi-doped LT-GaAs. The di!erent annealing tem-perature dependence of the band-edge PL featurein two kinds of materials is observed and discussed.Based on PL spectra, we propose that the defect-related PL features come from a defect complexconsisting of Si atom and the atom of excess As.The present observation shows that not only car-riers, but also Si atoms themselves in#uence theformation of As precipitates in LT-GaAs and LT-GaAs/AlGaAs MQWs.

Acknowledgements

Authors thank W. Li for helping in fabricatingsamples. This work is supported by Chinese Na-tional Foundation of Natural Sciences under con-tract No. 69896260.

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