Incorporation of silicon during MBE growth of GaAs on (111)A substrates

Journal of Crystal Growth 127 (1993) 871—876 North-Holland

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CRYSTAL GROWTH

Incorporation of silicon during MBE growth of GaAs on (111)A substrates M.R. Fahy, J.H. Neave, M.J. Ashwin, R. Murray, R.C. Newman, B.A. Joyce IRC Semiconductor Materials, The Blacketi Laboratory, Imperial College, Prince Consort Road, London SW7 2AZ, UK

Y. Kadoya and H. Sakaki The Quantum Wave Project, JRDC, Keyaki House 302, 4-3-24, Komaba, Meguro-ku, Tokyo 153, Japan

Si-doped GaAs has been grown on (111)A and (111)A vicinal GaAs substrates and carrier concentrations measured for a range of Si fluxes and growth temperatures. The use of As 2 as opposed to As4 has been examined. These results are discussed with respect to the growth mechanisms. Photoluminescence measurements have been made and compared with growth on an (001) substrate. The nature of the lattice site of incorporated Si is confirmed using local vibrational mode measurements.

1. Introduction Although silicon is the most widely used n-type dopant in the growth of gallium arsenide by molecular beam epitaxy (MBE). it is a group IV atom and as such has the potential to act as either a donor or an acceptor according to which sub-lattice site it occupies. This site occupancy has been shown to be strongly dependent on the substrate orientation; for growth on (001) surfaces it is predominately a donor, at for 3, least whereas concentrations up to 5 x 1018 cm on (111)A it behaves overwhelmingly as an acceptor up to 5 x 1019 cm3 [1,21. These orientation effects are becoming increasingly important for growth on patterned substrates, a technique now being used for fabrication of low-dimensional structures [3] in which several different orientations are exposed simultaneously. However, Si incorporation mechanisms on (111)A substrates in particular are not well understood. In this paper we report the results of an investigation of the effects of growth temperature, arsenic species (As 2 or As4), silicon concentration and substrate misorientation on the silicon incorporation behaviour on GaAs (111)A substrates using a combination of temperature de‘~

0022-0248/93/$06.00 © 1993

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pendent Hall measurements, photoluminescence (PL) and local vibrational mode (LVM) spectroscopy [1]. Direct comparison is made with (001) oriented material.

2. Experimental procedure Samples were grown in three separate systems: two commercial (VG V80 and one purpose built machine [4].and TheAnelva) same substrate preparation and growth practices were used in each system and the results from all three showed remarkable consistency. Semi-insulating GaAs substrates (Sumitomo Electric Company) with appropriate orientations were mounted side by side on a Mo block using indium solder. Ga and As fluxes were calibrated using RHEED oscillations on a (001) substrate. Temperatures were measured using a pyrometer which was calibrated using the oxide desorption temperature (600°C) and the (2 x 4) to c(4 x 4) transition (530°C)on the (001) substrate. Due to the strong dependeuce on the growth conditions all cells and substrates were given 2—3 h to reach a stable condition before growth was begun.

Elsevier Science Publishers B.V. All rights reserved

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/ Incorporation of Si during MBE growth of GaAs on

Hall measurements were made at room ternperature using a Polaron Hall measurement system. PL was carried out at 11 K using a tunable Ti: sapphire laser pumped with an argon ion laser and the luminescence detected by a photomultiplier mounted at the exit slits of a 0.85 m double grating monochromator. The LVM spectroscopy was carried out at 4.2 K using a Bruker IFS12O interferometer. RHEED patterns gave no indication of facet formation, but Nomarski interference microscopy showed a degree of surface roughness, particularly at higher As4 fluxes or lower growth temperatures.

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3. Results

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3.1. Electrical measurements The carrier concentrations, from room temper-

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ature Hall measurements, for samples grown on (001) and (111)A substrates, with intended Si concentrations between 10 15 and 10 20 cm 3 grown at 580°Cwith an As: Ga flux ratio of 2, are given in fig. 1. For (111)A the layer is p-type, but is n-type for (001). Under the same growth conditions, results on a vicinal (111)A substrate ented by 2°towards the (001) plane were misoriidentical to the singular (111)A surface, while a singular (111)B substrate behaved similarly to the (001). The free electron concentration saturates at —~ 6 x 1018 cm3 for (001) layers as the result of Si occupying As sites, forming clusters or complexes at high concentrations [1]. In the (111)A case, a much higher carrier (hole) concentration is achievable, appearing to saturate at 6 X 1019 cm3, in agreement with Okano et al. [2]. The effect of changing the growth temperature on the carrier concentration is shown in fig. 2 and suggests that there is an optimum temperature at which to obtain the maximum acceptor doping level for a given silicon flux. At a Si concentration of 1018 cm3, lowering the growth temperature to 540°Chas very little effect but at lower temperatures there is increased compensation. It has been claimed [5] that lower temperatures lead to a higher As surface population which favours Si occupying Ga sites. The situation is different at

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Smcon Concentration (cm~) Fig. 1. The carrier concentrations for growth of Si doped GaAs on (111)A and (001) substrates for a range of silicon concentrations grown at 580°Cwith a flux ratio of 2.

3 Si concentration. At 580°Cthere is the 1020 cmactivation of the silicon, but by lowerincomplete ing the temperature to 540°C this effect is reduced or eliminated. This suggests that at very high Si concentrations some of the Si atoms can either be forced onto Ga sites or can migrate to form clusters or complexes, as in the (001) case. Lowering the temperature will reduce both of these possibilities. Lowering the growth temperature below 540°Cproduces a similar reduction in carrier concentration to that obtained for lower Si concentrations. The temperature dependence of the carrier concentration for this series of samples is given in fig. 3. The straight line part of the curves at the lower Si concentrations have a slope corresponding to an activation energy of about 32 meV, in good agreement with the value from PL (section 3.2) for the major acceptor energy level, which is an indication that the material is only slightly compensated [6]. This low level of compensation could be the reason for the kink at low tempera-

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Fig. 2. The effect of growth temperature on carrier concentration for growth on a (111)A substrate with high silicon concentrations.

Fig. 3. The temperature dependence of the carrier concentralion, from Hall measurements for the (111)A samples given in fig. 1.

ture in the 3 doped sample. Once the doping level 10i5 has cm reached about 1018 cm3, the

relevant temperature regime the dissociation pathway is not via As 2 [91. As shown previously by Okano et al. [10], misorienting a (111)A substrate towards the (001) has a large effect on the silicon incorporation. The vicinal samples grown at 480°C are fully compensated using As2 oforthe As4. Okano et al. [10] explained thiseither in terms growth from

wavefunctions for the acceptor band and the valence band begin to overlap. The v shape at this concentration is then explained by there being two parallel conduction pathways, one through the ionized acceptor level 3 andcarriers one through there isthe a valence band. By 1019 cm single band and the material is metallic. It has been suggested that part of the reason Si can compete effectively with As for lattice sites on the (111)A surface is the difficulty that a single dangling bond Ga site has in decomposing As 4 molecules to As2 [7,8]. To examine this hypothesis we have grown 1 j~mthick samples at a range of temperatures using equivalent molecular fluxes of As2 As4cm3. and intended silicon concentration of 5and X 1017 The results are given in table 1 and although the differences are small, they indicate that As 4 alwaysinproduces slightly higher hole concentration, contrasta to that expected if As 4 decomposition is important [7,8]. It should be pointed out, however, that in the

Table 1 The effect of As species on silicon incorporation; the (001) samples are n-type and the (111)A samples are p-type, unless otherwise stated

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linesand which are exciton attributed to free exciton (1.515 eV) bound (a shoulder to low energy of the previous peak) recombination and free-tobound emission involving neutral acceptor levels lying 26 and 33 meV above the valence band. These lines are assigned to CAS and SiAS accep-

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tors, respectively. As the increases doping level is increased, the Si(e, A°)peak in strength relative to the C(e, At)) peak, while the bound I

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Fig. 4. PL spectra for Si-doped GaAs samples grown on (Ill )A substrates with silicon concentrations between 1011 and 1020 cm3.

the “(001)-like sites” (at the step edge) which will incorporate Si onto Ga sites, versus the number of (111)A like sites which will incorporate Si onto As sites. The vicinal substrates always have a better surface morphology than the singular ones and this is taken to mean that growth is mainly occurring from the step edge. We observed, additionally, that the choice of As species was significant, with As 4 giving a smoother morphology on vicinal surfaces but As2 giving better results on singular surfaces. 3.2. Photoluminescence We have made a systematic study of the PL obtained from a series of Si doped GaAs epitaxial layers, covering a range of doping levels, grown on (001), (111)B and (111)A oriented substrates. Fig. 4 shows the emission peaks obtained from the (11 1)A samples. The most lightly doped sampie (i0’~ cm3) exhibits several luminescence

The luminescence obtained from the samples grown on (001) and (111)B oriented substrates occurs mainly at the band edge, at least up to a doping level of 1018 cm and the peaks are typical of n-type degenerate GaAs [12]. However, ~,

the most heavily doped samples (i0’~ and 1020 crn3) show only luminescence involving deep levels at 1.2 eV and 1 eV, which may be attributed to the autocompensation which is known to occur in heavily doped (001) GaAs [1]. 3.3. Local mode spectroscopy The inference that the p-type material is due to Si occupying As lattice sites has not been verified directly and hence we have carried out LVM measurements. The sample studied consisted of an epilayer 2 ~m thick grown on a (111)A substrate at 580°Cwith intended Si 3. Hallanmeasurements concentration of to 1019 showed the layer be cm p-type with a hole concentration of 9.6 X lO~~ cm3. The absorbance spectrum is shown in fig. 5a along with a corresponding spectrum showing the intrinsic two photon features of GaAs (fig. Sb). The expanded (xlO) difference spectrum (spectrum (a) spectrum (b)) shows an asymmetric dip at around 395 cm’, but no detectable absorption from 28S~Gaat 384 cm~ —

M.R. Fahy ci a!.

Si-DopedMBE(111)A

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of Si during MBE growth of GaAs on (111)A substrates

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Fano dip and the appearance of the normal LVM line from SiAS at 399 cm’. More extended irradiations are required to effect a full interpretation. I

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380 390 I 1) 400 410 Wavenumber (cm Fig. 5. LVM spectra for (a) 1 /.Lm, lx iO’~cm3 Si-doped, GaAs grown on (111 )A substrate, (b) undoped GaAs and (c) difference spectrum (x 10).

We have shown that, at lower concentrations, silicon is incorporated as a well-behaved acceptor with an ionization energy of about 33 meV on (111)A substrates. Using As2 instead of As4 has only a the small effect on studied. the site The distribution Si under conditions acceptorofsite has been positively identified as silicon on an As site from LVM spectroscopy, but the incorporation mechanism remains unclear.

[1]. The latter observation indicates that there is negligible compensation from Si donors and it follows that, for this sample, all Si is present as acceptors, in full agreement with the Hall measurements. The asymmetric dip is interpreted as a Fano profile [13,14], resulting from a strong electron—phonon interaction between the electronic hole continuum of states and the LVM from 5’As, which appears as a line with symmetrical shape in compensated GaAs [1].The Fano profile makes it impossible to make a quantitative estimate of the concentration of 51As using previously derived calibrations. The present interpretation is also consistent with the observations of LVM Fano resonances reported by us for C,~[14] and Beoa [13] in GaAs and for B acceptors in Si [13] although the shapes of the profiles vary with the impurity and host crystal. In other p-type MBE samples we have observed 5~Ga donors, indicating incomplete compensation. It should be noted that, from work on growth on (001) oriented substrates, there is a deep acceptor centre labelled Si—X, believed to involve S~AS~VGacomplexes, which is observed at high doping levels for growth temperatures > 500°C[1]. This should be considered when compensation ratios are equated with [~~cia] and [SiAS]. Preliminary electron beam irradiated studies of the sample show that the partial compensation achieved leads to a reduction in the depth of the

Acknowledgements The support of Imperial College and the Research Development Corporation of Japan under the auspices of the “Atomic Arrangements; Design and Control for New Materials” Joint Research Program is gratefully acknowledged. One of us (M.F.) would like the thank The British Council and ERATO for support during his stay in Japan. We would also like to thank Christine Roberts for substrate preparation.

References [11 R. Murray, R.C. Newman, M.J. Sangster, R. Beall, J.J. Harris, P.J. Wright, J. Wagner and M. Ramsteiner, J. AppI. Phys. 66 (1989) 2589. [2] Y. Okano, H. Seto, H. Katahama, S. Nishine, I. Fujimoto and T. Suzuki, Japan. J. AppI. Phys. 28 (1989) L151. [3] H. Sakaki, Japan. J. Appl. Phys. 19 (1980) 94. [4] J. Zhang, J.H. Neave, B.A. Joyce, P.J. Dobson and P.N. Fawcett, Surface Sci. 231 (1990) 379. [51Y. Kadoya, A. Sato, H. Kano and H. Sakaki, J. Crystal Growth 111 (1991) 280. [61 K. Seeger, Semiconductor Physics: An Introduction, 5th ed. (Springer, Berlin, 1991). [7] S. Bose, B. Lee, Kim,743. G. Stillman and W. Wang, J. Appl. Phys. 63 (3)M. (1988) [81Y. Okano, M. Shigeta, H. Seto, H. Katahama, S. Nishine and I. Fujimoto, Japan. J. AppI. Phys. 29 (1990) L1357. 191 C.T. Foxon and B.A. Joyce, Surface Sci. 50 (1975) 434.

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/ Incorporation of Si during MBE growth of GaAs on

[10] M. Shigeta, Y. Okano, H.Seto, H. Katahama, S. Nishine, K. Kobayshi and 1. Fujimoto, J. Crystal Growth 111 (1991) 284. [11] R.A. Abram, G.J. Rees and B.H. Wilson, Advan. Phys. 27 (1978) 799. [12J Jiang De-Sheng, Y. Makita, K. Ploog and H.J. Queisser, J. AppI. Phys. 53 (1982) 999.

(1]I)A substrates

[13] R. Murray, R.C. Newman, R.S. Leigh, R.B. Beall, J.J. Harris, M.R. Brozel, A. Mohades-Kassai and M. Goulding, Semicond. Sci. Technol. 4 (1989) 423. [14] K.T. Woodhouse, R.C. Newman, T.J. de Lyon, J.M. Woodhall, G.J. Silla and F. Cardone, Semicond. Sci. Technol. 6 (1991) 330.