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

Journal of Crystal Growth 217 (2000) 355}359

Evolution from point defects to arsenic clusters inlow-temperature grown GaAs/AlGaAs multiple quantum wells

M.H. Zhang!,*, Y.J. Han!, Y.H. Zhang", Q. Huang!, C.L. Bao!, W.X. Wang!,J.M. Zhou!, L.W. Lu"

!Center for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080,People's Republic of China

"Laboratory for Semiconductor Materials, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083,People's Republic of China

Received 18 February 2000; accepted 16 May 2000Communicated by A.Y. Cho

Abstract

Optical transient current spectroscopy (OTCS), photoluminescence (PL) spectroscopy and excitonic electroabsorptionspectroscopy have been used to investigate the evolution of defects in the low-temperature grown GaAs/AlGaAs multiplequantum well structures during the postgrowth rapid thermal annealing. The sample was grown at 3503C by molecularbeam epitaxy on miscut (3.43 o! (0 0 1) towards (1 1 1) A) (0 0 1) GaAs substrate. After growth, the sample was subjected to30 s rapid thermal annealing in the range of 500}8003C. It is found that the integrated PL intensity "rst decreases with theannealing temperature, then gets a minimum at 6003C and "nally recovers at higher temperatures. OTCS measurementshows that besides As

G!antisites and arsenic clusters, there are several relatively shallower deep levels with excitation

energies less than 0.3 eV in the as-grown and 5003C-annealed samples. Above 6003C, OTCS signals from AsG!

antisitesand shallower deep levels become weaker, indicating the decrease of these defects. It is argued that the excess arsenicatoms group together to form arsenic clusters during annealing. ( 2000 Elsevier Science B.V. All rights reserved.

Keywords: LT-GaAs; LT MQWs; Defect; Photoluminescence; Electroabsorption

1. Introduction

Recently, the low-temperature (LT) growth ofphotorefractive GaAs/AlGaAs multiple quantumwell (MQW) structures has been demonstrated[1}3]. The photorefractive devices fabricated bythese MQW structures combine the advantages of

large excitonic electroabsorption with ultrafast life-times. LT growth can incorporate a great numberof defects (due to excess arsenic) into materials,leading to very short carrier lifetimes [4}5]. Thus,the control of concentrations and types of defects inLT grown materials is very important for the per-formance of photorefractive MQW devices.

The properties of defects in LT-GaAs grownat 200}4003C have been studied extensively. Thedominant defects are As

G!antisites, as has been

veri"ed by deep-level transient spectroscopy and

0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 5 2 9 - 7

Fig. 1. The integrated PL intensity at n"1 heavy-hole excitontransition and the resistivity in LT MQWs samples annealed atdi!erent temperatures.

other measurements [6]. After annealing, excessarsenic atoms group together to form arsenic clus-ters [7]. A great number of As

G!antisites or clus-

ters can change the electrical and optical propertiesof GaAs strongly, for example, high resistivity andultrafast carrier lifetimes in LT-GaAs [8}10]. Al-though very promising application of the LT grownphotorefractive GaAs/AlGaAs MQW structureshas been demonstrated, the fundamental propertiesconcerning the evolution of As

G!antisites and clus-

ters are still not completely clear. Our former workshows that after postgrowth rapid thermal anneal-ing, the point defects in LT grown GaAs/AlGaAsMQW structure become arsenic clusters at an an-nealing temperature of about 6003C with theweakest PL intensity [11]. In this paper, we furtherinvestigate this evolution of defects using opticaltransient current spectroscopy (OTCS), photo-luminescence (PL) spectroscopy and excitonic elec-troabsorption spectroscopy.

2. Experimental results and discussion

The wafer was grown on a miscut (3.43 o! (0 0 1)towards (1 1 1) A) (0 0 1) GaAs substrate ina VG80H MBE system. The substrate temperaturewas measured by a thermocouple, calibrated by theoxide-desorption temperature of 5803C on (0 0 1)GaAs. The growth sequence consisted of 1 lmGaAs bu!er layer, 500 As AlAs sacri"cial layer,2000As Al

0.3Ga

0.7As cladding layer, and a 100-

period Al0.3

Ga0.7

As/GaAs MQW structure, andanother 2000As Al

0.3Ga

0.7As cladding layer. The

AlGaAs barrier and GaAs well in the MQW struc-ture are 40 and 78As in thickness, respectively. TheAlAs sacri"cial layer was used for the lift-o! pro-cess. The GaAs bu!er and AlAs sacri"cial layerswere grown at normal growth temperatures, whilethe two Al

0.3Ga

0.7As cladding layers and the

MQW structure were grown at 3503C. The arsenicpressure was chosen as 1.1]10~7Torr in compari-son with the normal one of 3.0]10~7Torr. Aftergrowth, the wafer was cleaved into several smallpieces, and each piece was subjected to rapid ther-mal annealing at temperatures ranging from 500 to8003C for 30 s. In order to perform OTCS measure-ment, two indium contacts were made on the

surface of some pieces with a spacing of 2 cm andannealed at 3003C for 15min under a #ow of hy-drogen. In order to measure excitonic electroab-sorption spectra and resistivity, the epi-layers ofother pieces were "rst removed by lift-o! in dilutehydro#uoric acid, and then Van der Waals bondedto a glass slide. Finally, two gold contacts witha spacing of 1 mm were deposited on the epi-layersurface by evaporation.

Fig. 1 shows the dependence of resistivity andintegrated PL intensity at n"1 heavy-hole excitontransition versus annealing temperature. The resis-tivity is measured at room temperature and the PLspectra at 77K with an Ar` laser as an excitationsource. It can be seen that the integrated PL inten-sity "rst decreases with the annealing temperature,then reaches a minimum at 6003C and "nally re-covers at higher temperatures. As a comparison,the resistivity "rst decreases, then has a smoothchange between 500 and 7003C and increases againat the annealing temperature of 8003C. Similar be-havior of the dependence of PL intensity on anneal-ing temperature has been reported in our formerwork, where the appearance of the weakest PLintensity was thought to be indicative of the forma-tion of arsenic clusters. However, no direct evidencewas presented.

For photorefractive applications, it is requiredthat the LT GaAs/AlGaAs MQWs structureshould have strong excitonic electroabsorption.

356 M.H. Zhang et al. / Journal of Crystal Growth 217 (2000) 355}359

Fig. 2. Excitonic electroabsorption of LT MQWs samples an-nealed at di!erent temperatures in a DC electric "eld of0.9 kV/cm under Frantz}Keldysh geometry.

Fig. 3. Deep levels in LT MQWs samples annealed at di!erenttemperatures measured by OTCS under a rate window of100ms. The data attached to deep-level signals are the corre-sponding excitation energies in the unit of eV.

Fig. 2 presents the relative change of the transmis-sion of samples under a DC electric "eld of0.9 kV/cm. Since the electric "eld is in the plane ofMQWs, the observed excitonic electroabsorption iscaused by the Frantz}Keldysh e!ect. The peakvalue of the transmission change is about 37% inthe as-grown and 5003C-annealed samples, and de-creases to about 25% in samples annealed at tem-peratures higher than 5003C. The present resultshows that although weak annealing does not in#u-ence excitonic electroabsorption greatly, the strongone does. It is also in good agreement with the resultof Lahiri et al. [12]. These authors have investigatedthe excitonic electroabsorption in LT GaAs/Al-GaAs MQWs structure under Stark geometry.

In order to investigate the underlying reason forthe dependence of the integrated PL intensity andthe excitonic electroabsorption on annealing tem-perature, the evolution of deep levels has beenmeasured using OTCS. During the measurement,a high bright red light-emitting diode (LED) wasused as the excitation source. Fig. 3 shows thedeep-level spectra under a rate window of 100ms.Using the model of point defect, the excitationenergies for all deep levels are calculated and pre-sented in the unit of eV. In the as-grown and5003C-annealed samples there are three types ofdeep levels: two shallower ones appearing below200K, an As

G!antisite related one around 240 K

and an arsenic cluster related one around 330K.The reason for the latter two assignments will be

given later. In samples annealed at temperaturesabove 5003C, OTCS signals from shallower deeplevels become weak, indicating the decrease of theconcentrations of these shallower deep levels. Inaddition, the types and the distribution of shal-lower deep levels have also been changed duringannealing, as can be seen from the change of excita-tion energies for these defects in di!erent samples.For defects appearing around 240 K, a similar phe-nomenon can be observed. From the change of theexcitation energies of defects, one can conclude thatAs

G!antisites remain stable till the annealing tem-

perature of 6003C and new types of defects aregenerated in samples annealed at higher temper-atures. The trend of defect evolution is that withincreasing annealing temperature, excess arsenicatoms group together to form arsenic clusters. Thevalley-shape change of the integrated PL intensityversus annealing temperature can be explained bythe evolution of all the three types of defects. FromFig. 3, it is reasonable to think that As

G!antisites

have the largest carrier capture cross-section, ar-senic clusters follow next and shallower deep levelsare at the other extreme. The "rst decrease of PLintensity is the result of the defect evolution fromshallower deep levels to arsenic clusters, and itsrecovery at annealing temperatures above 6003C isdriven by the transition from As

G!antisites to

arsenic clusters. The decrease of excitonic elec-troabsorption in samples annealed at temperaturesabove 5003C may be caused by the increasing

M.H. Zhang et al. / Journal of Crystal Growth 217 (2000) 355}359 357

Fig. 4. Deep-level signals in the as-grown LT-GaAs samplemeasured by OTCS under di!erent rate windows (a) and thecorresponding Arrhenius plot for the determination of excita-tion energies (b). This LT-GaAs sample is used as a reference toidentify di!erent defects in the LT MQWs sample.

scattering rate due to a large concentration of ar-senic clusters. The present observation that the PLintensity increases with annealing temperatureabove 5003C correlates well with the result of Loch-tefeld et al. [13], who found the increase in thecarrier lifetime in LT-GaAs annealed between 600and 8003C.

Let us now turn to the assignment of di!erentdefects shown in Fig. 3. To accomplish this, we havegrown one LT-GaAs sample as a reference ata relatively higher substrate temperature of 4003Cand a smaller arsenic pressure of 9.0]108Torr.Under this latter condition, the incorporation ofexcess amount of arsenic is greatly reduced, andthus the concentration of arsenic clusters is ex-pected to be very small. The thickness of totalLT-GaAs layer is 1.5lm. Before annealing, theabove LT-GaAs sample is found to be semi-insulat-ing in comparison with those samples grownaround 2003C, which are conductive due to a largeconcentration of arsenic-related defects. Figs. 4(a)and (b) show the measured deep-level signals underdi!erent rate windows and the corresponding Ar-rhenius plot for the determination of excitationenergies. In the scanned temperature range, thereare three deep levels with the excitation energies of0.21, 0.24 and 0.65 eV. As in this LT-GaAs and theabove LT MQWs sample the defects appearingaround 240K have similar excitation energies, theycan be considered as the same type. On the otherhand, the excitation energy of 0.65 eV measuredhere is in good agreement with the previously re-ported values for As

G!antisites. Thus this defect

can be identi"ed as an AsG!

antisite. The other twodefects with excitation energies of 0.21 and 0.24 eVare shallower deep levels. Although no deep-levelsignals appear around 330K in rate windows largerthan 20ms, a negative decay of optical transientcurrent relative to the averaged background is ob-served in smaller rate windows. The negative decayhas the meaning that just after the turn-o! of theLED, the optical transient current has a large dipand then recovers with time. In comparison, thenormally positive decay is that the optical transientcurrent decreases monotonously with time if theLED is turned o!. We have observed a strongnegative decay of optical transient current in an-nealed LT-GaAs grown at about 2003C, which is

expected to contain a large concentration of arsenicclusters. Thus we reach the conclusion that, there isa small concentration of arsenic clusters in ourabove LT-GaAs grown at 4003C and the negativedeep-level signals come from arsenic clusters. De-tails of defect evolution and the negative decay ofOTC in LT-GaAs materials will be given in Ref.[14]. In the above LT MQWs sample, the defectappearing around 330K has an excitation energyof about 0.55 eV, and thus, should contributea mid-gap state. Since this defect appears in thesame range of temperature as the arsenic cluster inour reference LT-GaAs, we identify it as an arseniccluster. However, there is another point whichshould be noticed. No apparent negative deep-levelsignals have been observed for this defect in theabove LT MQWs sample, which, we think, may be

358 M.H. Zhang et al. / Journal of Crystal Growth 217 (2000) 355}359

caused by the di!erent structures for LT-GaAs andLT MQWs sample. Thus, we have direct supportfor the defect assignments in Fig. 3. Although ar-senic clusters have been clearly observed in an-nealed LT-GaAs using transmission electronmicroscopy and scanning tunneling microscope[7,15] as far as we know, they have not yet beenclearly identi"ed in deep-level transient spectro-scopy and transport measurements [16,17]. Thepresent investigation provides another corroborat-ing identi"cation of its existence and evolution dur-ing annealing and will be very signi"cant for thepractical application of LT-GaAs materials.

3. Conclusions

In summary, we have used optical OTCS, PLspectroscopy and excitonic electroabsorption spec-troscopy to investigate the evolution of defects inthe low-temperature grown GaAs/AlGaAs mul-tiple quantum well structures after postgrowthrapid thermal annealing. It is found that the integ-rated PL intensity "rst decreases with the annealingtemperature, then attains a minimum at 6003C and"nally recovers at higher temperatures. UsingOTCS we have measured the deep levels in theannealed samples. Three types of deep levels havebeen found. By comparison with the defects in anLT-GaAs sample grown at a higher substrate tem-perature and a lower arsenic pressure, we are ableto identify shallower deep levels, As

G!antisites and

arsenic clusters directly in the LT MQWs sample.After annealing, the concentrations of both shal-lower deep levels and As

G!antisites decrease. We

argue that the excess arsenic atoms from thesedefects group together to form arsenic clusters dur-ing annealing.

Acknowledgements

This work was supported by the NaturalSciences Foundation of China under Contract No.69896260.

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