AlAs single quantum wells

Journal of Crystal Growth 127 (1993) 788—792 North-Holland

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CRYSTAL GROWTH

Surface relaxation kinetics and growth interruption effects in GaAs/AlAs single quantum wells A. Yoshinaga

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P. Mookherjee, R. Murray, J.H. Neave and B.A. Joyce

Interdisciplinary Research Centre for Semiconductor Materials, Imperial College, Prince Consort Road, London SW7 2BZ, UK

A systematic investigation of growth interruption in molecular beam epitaxial growth has been carried out. From an analysis of the recovery of the reflection high energy electron diffraction (RHEED) specular beam, the growth rate is found to affect both the absolute value of the time constant of the recovery and the activation energy which is derived from this time constant at different temperatures. Using optimized growth conditions we have performed a systematic optical investigation of GaAs/AlAs single quantum wells with and without growth interruption and conclude that growth interruption of the inverted interface (GaAs on AlAs) is the most important factor in the formation of high quality single quantum wells.

1. Introduction Growth interruption (GI) has been widely applied to obtain atomically abrupt surfaces during growth by molecular beam epitaxy (MBE). Two questions must be addressed concerning GI: (1) what happens during GI, and (2) does GI result in “atomically” smooth surfaces? An answer to the first question may be provided from studies of the recovery of the specular beam in the reflection high energy electron diffraction (RHEED) pattern during GI. The recovery curve of the RHEED intensity satisfies the empirical equation [11 1(t) =A +A* exp(—t/r 0

1

1

) +A~’exp(—t/r ) 2

(1) where A. are constants and

T

1 and r2 are time constants which correspond to fast and slow recovery processes, respectively. We reported previously that activation energies which are derived from Ti depend critically on the point of growth termination [2]. In this paper, we extend these results to include effects of the growth conditions

1

Permanent address: Chichibu Works, Showa Denko, 1505 Shimokagemori, Chichibu City, Saitama 369-18, Japan.

0022-0248/93/$06.00 © 1993

—

on the activation energy for the fast recovery. An answer to the second question concerning the nature of the interfaces grown with GI can only be obtained after growth. This is usually done optically, using excitons to probe the interfaces. The technique is extremely sensitive but can give conflicting results concerning the nature of the interface morphology. In particular, some authors observe multiple excitonic emission peaks, whose energy separations vary at different locations over the sample [3,41 from which they deduce that GI leads to pseudo-smooth rather than truly smooth interfaces. Others suggest that GI results in a narrowing of the exciton linewidth rather than the creation of multiple peaks [7]. In this paper, we examine the influence of GI on the inverted and normal interfaces of single quantum wells (SQWs) using photoluminescence (PL) and PL excitation (PLE) results from points taken along a single direction on each SQW investigated. .

.

2. Experimental details The measurement of the specular RHEED beam recovery was performed in a specially designed MBE system. The GaAs/AlAs SOW samples for optical measurements were grown in a

Elsevier Science Publishers B.V. All rights reserved

A. Yoshinaga et al.

/ Surface relaxation kinetics and growth

interruption in GaAs /AIAs

789

Table 1

Sample 1 Sample 2

Normal interface (AlAs on GaAs) interrupted

Inverted interface (GaAs on AlAs) interrupted

Yes Yes

Yes No

a

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Ea:1.5eV

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3MLs 1 1 As=1.OMLs 4

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Eal3eV = .

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Sample3 No Yes Sample4 No No _____________________________________________

Ga=O.18MLs As =O.6OMLi

a

10

a

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,.~

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Vacuum Generators V8OH machine. Tetrameric arsenic (As The diffraction conditions were chosen so that a 4) was in all oscillation the experiments. maximum of the used RHEED corresponded to a ML increment [1] and growth was terminated after 3 ML deposition. Each recovery curve was recorded digitally and then fitted to eq. (1). The structure of the SQWs is shown in the inset of fig. 3. The first buffer layer was grown at a substrate temperature of 580°C,all subsequent layers were grown at 630°C.The growth rate was 1 ML s for both GaAs and AlAs. The period of GI was 120 s and applied to the samples as indicated in table 1. The optical measurements were performed with the samples held at 12 K in a variable temperature closed cycle cryostat. The samples were excited by light from a tunable titanium sapphire laser pumped by an argon ion laser and the luminescence detected by a photomultiplier mounted at the exit slits of a SPEX 1404, 0.85 m, double grating monochromator. -

:

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As4 =1.3MLs1 1 Ea=1.leV

—

Ga=O.57MLsT

As 1 4=1.9MLs Ea=O.9eV

1

______________________________ 1.12

1.14

1.16

1.18

1.20

1.22

1.24

Substrate Temperature 1 03/T (K I) Fig. 1. Temperature dependence of the fast time constant (r 1) of recovery. The flux ratio is fixed such that Ga: As4 of 3: 10.

tatively shows the same behaviour as that shown in figs. 1 and 2 and indicates that a smaller T1 value leads to a smaller activation energy. We

Ga=O.3MLi

a 1 0

•

1

Ea 11eV

a a

3. Results and discussion

A~

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.

A

3.1.RHEED Fig. 1 shows the temperature dependence of Ti with increasing Ga flux for a fixed flux ratio Ga: As4 of 3: 10. The absolute value of ~1 and the activation energies derived from each line tend to decrease with increasing Ga flux, i.e. the growth rate. The same tendency can be found in fig. 2, where the As flux was fixed. We have also investigated of theover As4the flux at a from constant growth the rateeffect (Ga flux) range 2 ~ which quali6.24 x 1014 to 1.19 x 10i5 cm

—

a A

C

— £

E

i

Ga=l.OMLs Ea=O.6eV

1

Ga=O.57MLs Ea=O.9eV

1.12 1.14 ~ 1i8 1.20 3/T 1.22 (K l>1.24 Substrate Temperature 1 0 time constant (r Fig. 2. Temperature dependence of the fast 1) of recovery. The As 4 flux is fixed at 1.9 ML s~.

790

A. Yoshinaga et a!.

/ Surface relaxation

kinetics and growth interruption in GaAs /A!As

GaAs cap AlAs GaAs sow AlAs GaAs 6ML~

23ML 22ML

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~j~j}

GaAs

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~trate 5~

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1.600 1.610 1.620 Energy (eV) Fig. 3. PLE spectra showing the le—lhh transitions at points along a single direction on the wafer. The spectra are displaced for clarity and the well widths are calculated using the parameters contained in ref. [71. The inset shows the structure grown for optical investigations.

observe all of the events within the coherent area of the RHEED measurement, we must consider the distribution of coordination numbers of the surface atoms. If the dendricity of the surface is have athen high, smalltheN population is large andofthe theaverage atoms of which the activation energy within the observation area is small. The measured activation energies simply represent some average kinetic barrier and do not have absolute values, but are sensitive functions of growth conditions. and incremental MLs. Given the flux variation across the substrate, even with rotation, it is clear that different parts of the layer will terminate at different ML increments, having been grown at different rates with different flux ratios. Consequently there will be a position-dependent range of recovery time constants and activation energies. It is therefore of great importance to map systematically the optical behaviour over the wafer.

1.590

have not yet, however, fully evaluated the influence of arsenic on growth morphology. This trend concerning the absolute value of Ti can be explained as follows: the higher the Ga flux, the rougher the surface becomes. As we have shown previously [21, during the first fast stage, the surface dendricity is reduced by means of the movement of atoms from the topmost layer to the second layer. The rate of this process is proportional to the product of the population of atoms, which are at step edges of the upper layer, and the availability of sites in the lower layer. When the surface is rough, these populations become high and the absolute value of T1 becomes small. By considering the surroundings of the atoms of the topmost layer, the influence of the growth rate on the activation energy can be explained. We introduce a coordination number (N) of surface atoms within the layer and if N is small, the activation energy is small. Because we

3.2. Optical measurements PL and PLE measurements were taken at vanous points along a single direction on each of the samples listed in table 1 to determine the position(s) and the full width half maxima (FWHM) of the emission and absorption peaks across the wafer. We observed narrow peaks (FWHM 1 meV) associated with those structure(s) in which GI is applied to both interfaces, fig. 3. Verification of the abruptness of the interfaces associated with sample 1 is indicated by the negligible variation in the peak positions from point to point on the sample and the observation that the separation between the peaks correlates to a thickness variation of 1 ML, within experimental error [7]. For sample 2, in which only the normal interface was interrupted, two PLE peaks were observed. However, unlike sample 1 the energy p0sitions of the peaks varied by up to 3.3 meV across the sample. The FWHM of the peaks were also slightly larger (FWHM 2 3 meV) than those measured for sample 1. Sample 3, where only the inverted interface was interrupted, exhibited three PLE peaks with linewidths similar to those of sample 2, but with peak positions that were constant from point to point on the wafer. —

A. Yoshinaga et a!.

/ Surface relaxation kinetics and growth

This suggests that the increased migration of Al adatoms on the AlAs surface during GI has an important effect on the formation of large islands, and probably results in a reduction in the step density [5], which ultimately leads to atomically smooth interfaces. The observation of three PLE peaks for GI of either the inverted interface or both interfaces and only two when the normal interface is interrupted has been confirmed for other samples with similar structures during different growth runs. A comparatively broader single peak (FWHM 4 meV) was found for sample 4, which was grown without GI (fig. 4a). However, a nominally identical sample exhibited two PLE peaks as indicated in fig. 4b. Two PLE peaks have also been reported in SQWs grown wi ou A Stokes shift of 2 meV was measured for the sample grown without GI, which is comparable with similar structures reported in the literature [9]. The Stoke shifts for sample 1 was below our measurements accuracy, implying that delocalized excitons are present which are able to migrate to other regions in the quantum well provided the exciton diffusion length is greater than the lateral extent of the islands. Stokes shifts associated with samples 2 and 3 are small (—~1 meV) compared with their linewidths and small compared with the Stokes shift associated with sample 4. These observations indicate that GI at the normal or the inverted interface results in the formation of large interface islands which are not microrough [4]. GI for periods <120 s resulted in PLE spectra which did not show the three narrow peaks shown in fig. 4. Details of this investigation will be contained in a future publication, but it is evident that the major smoothing of the interfaces occurs during the second (slow) stage of the surface relaxation process. This is entirely consistent with our model [1,21of the fast stage kinetics in which single atoms in low coordination positions move to highly coordinated sites, but smooth areas comparable to an exciton diameter are not formed. The GI period for either interface must be long enough to incorporate the slow stage of the surface relaxation process. The fast stage has ~.

interruption in GaAs /AL4s

a

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(b)

5-2-3meV

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A..4meV

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1 ~

1 62 .

1 63 .

Energy (eV)

1 64 .

1.59

1.60

1.61

Energy (ev)

1.62

Fig. 4. (a) and (b) show PLE spectra that have been obtained from nominally identical samples that have been grown without GI (sample 4).

only a limited effect on surface morphology on the scale of the exciton diameter.

4. Conclusions A systematic investigation has been performed through an analysis of the intensity recovery of the RHEED specular beam, together with PL and PLE measurements of SQWs with and without GI. The RHEED intensity study showed that when the growth rate is increased the fast time constant of the recovery curve and the activation energy, which is derived from this time constant, both decrease. We attribute this to differences in the coordination number of surface atoms under different growth conditions. GI at both interfaces results in narrow PLE peaks with energy positions that are constant from point to point on the wafer. In addition, there is a negligible Stokes shift between PL and PLE peaks. These properties are indicative, we believe, of truly smooth interfaces. GI of the inverted interface also results in narrow peaks at constant energy positions. However, a small Stokes shift is observed in this case due to the microrough (Di>> lateral extent of the interface defects) nature of the normal interface.

792

A. Yoshinaga et a!.

/ Surface relaxation kinetics and growth

We interpret variations in the energies of the PLE peaks in samples where only the normal interface is interrupted as arising from local fluctuations in the roughness (Dex lateral extent of the interface defects) of the inverted interface. The presence of two peaks in the PLE spectrum indicates that the normal interface is now atomically smooth. Samples without growth interruption also show a variation in the PLE peak position, as expected if the inverted interface is rough. The sample which showed only one peak also exhibited the largest Stokes shift, which may mdicate greater localization of the excitons by an increased concentration of interface defects. Nevertheless, growth interruption of the inverted rather than normal interface has a more significant effect in the formation of high quality SQWs.

Acknowledgements The support of Imperial College and the Research Development Corporation of Japan under the auspices of the “Atomic Arrangement: De sign and Control for New Materials” Joint Re-

interruption in GaAs /AL4s

search Program is gratefully acknowledged. One of us (P.M.) would like to acknowledge the support of the SERC.

References [11J.

Neave, BA. Joyce, P.J. Dobson and N. Norton, Appi.

Phys. A 31(1983)1. [2] A. Yoshinaga, M. Fahy, S. Dosanjh, J. Zhang, J.H. Neave and BA. Joyce, Surface Sci. Letters 264 (1992) L157. [31C.A. Warwick and R.F. Kopf, AppI. Phys. Letters 60 (1992) 386. [4] R.F. Kopf, E.F. Schubert, T.D. Harris and R.S. Becker, Appi. Phys. Letters 58 (1991) 631. [5] J. Zhang, P. Dawson, J.H. Neave, K.J. Hugill, I. Galbraith, P.N. Fawcett and BA. Joyce, J. Appl. Phys. 68 (1990) [6] L. Goldstein, Y. Horikoshi, S. Tarucha and H. Okamoto, Japan. J. App!. Phys. 22 (1983) 1489. [7] Parameters used in our finite square potential well model are: GaAs: me /m 0 = 0.067, mh /m0 = 0.34, Eg = 1.5190 eV. AlAs: m~/m0 0.15, mh /m0 = 0.5, Eg = 3.1130 eV. The conduction band offset was taken to be 0.67 iE5. [81 B. Deveaud, A. Regreny, J.-Y. Emery and A. Chomette, J. AppI. Phys. 59 (1986) 1633. [9] R.C. Miller, C.W. Tu, 5K. Sputz and R.K. Kopf, AppI. Phys. Letters 49 (1986) 1246.