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Optical and structural characterizations for optimized growth of In0.52Al0.48As on InP substrates by molecular beam epitaxy
I,I A T I m l A L S Ir4~IIEIIME & ENIIINEEININ| ELSEVIER
Materials Science and Engineering B35 (1995) 109-116
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Optical and structural characterizations for optimized growth of Ino.52Alo.48As on InP substrates by molecular beam epitaxy S.F. Yoon*, Y.B. Miao, K. Radhakrishnan, S. Swaminathan Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 2263, Singapore
Abstract
Growth of In0.52A10.4sAs epilayers on InP(100) substrates by molecular beam epitaxy at a wide range of substrate temperatures (470-550 °C) and at high arsenic beam equivalent pressures is carried out. Analysis performed using low-temperature photoluminescence (PL) and double axis X-ray diffraction (XRD) showed a strong dependence of the PL and XRD linewidths and lattice-mismatch on the substrate temperature, with minimum linewidths and lattice-mismatch occurring between approximately 500 and 520 °C. The XRD intensity ratio (Intepi/Intsub) varied in opposition to the lattice-mismatch, with higher intensity ratios Corresponding to lower lattice-mismatches. From the X-ray diffraction curves of samples grown at low temperatures, it was observed that the main peak associated with the InA1As epilayer is comprised of smaller peaks, which strongly indicates disordering owing to the presence of alloy clustering. PL spectrum taken at increasing temperatures showed the quenching of the main emission peak followed by the evolution of a distinct peak at lower energy, possibly associated with carrier localization due to the presence of lattice disorder. In addition to the InAs-like and AlAs-like longitudinal-optic (LO) phonon modes at 234 cm- 1 and 370 cm- l, respectively, Raman scattering measurements also showed an additional higher energy mode at 273 cm- ~ in the samples grown at lower temperatures approaching 470 °C. Withinthe range of V/III flux ratios investigated (32-266), the lowest PL linewidth of 14 meV was recorded for the samples grown at a V/III ratio of 160 at a substrate temperature of 510 °C. The lattice-mismatch between the epilayer and the substrate for these samples was also found to be relatively insensitive to changes in the V/III flux ratios.
Keywords: Molecular beam epitaxy; Photoluminescence; X-ray diffraction; Raman scattering; Indium aluminum arsenide
1. Introduction
InxA1]_ xAs grown lattice-matched on InP substrates has emerged as a heterostructure material system presently attracting a high level of attention owing to its potential applications in optoelectronic devices such as quantum-well optical modulators [1] and in highspeed electronic devices such as high electron mobility transistors [2]. While near perfect heterointerfaces have been reported in the AIGaAs/GaAs material system [3,4], growth of InAIAs on InP substrates has been reported to be difficult [5,6]. The problems are associated with the difficulty in achieving good quality layers
* Corresponding author. 0921-5107/95/$09.50 © 1995 - - Elsevier Science S.A. All rights reserved
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arising from the large bond strength difference between I n - A s and AI-As, thus making it difficult to control the cation migration rates during, growth. This in turn results in difficulty in controlling its layer quality. However, despite the problems associated with its epitaxial growth, the demand for good quality InA1As material continues to be high because of its potential applications in advanced devices. This calls for continuous effort to improve its epitaxial growth process, one of which is by molecular beam epitaxy (MBE). Ohno et al. [7] first reported 4 K photoluminescence (PL) linewidth of 25 meV for this material. Since then numerous reports have appeared on experimental [5,6,810] and theoretical investigations [1 l] on the electrical and optical properties as well as the epitaxial growth aspects of InA1As.
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As expected, the MBE growth conditions for InA1As are different from those for GaAs and A1GaAs where a large cation surface mobility is required for low defect density and high quality heterojunction interfaces. For good quality growth of these two materials, the cation mobilities are usually enhanced by increasing the substrate temperature and/or decreasing the V/Ill flux ratio during growth. However, in the case of InA1As, the high volatility of the InAs component in this alloy system requires the use of low substrate temperatures and high arsenic overpressures [12] (V/III flux ratio). However, substrate temperatures which are overly low may adversely affect the cation surface migration rates to such an extent that deviation from random growth occurs. Besides using a reasonable growth rate, careful control of the substrate temperature and arsenic overpressure during MBE growth should be crucial in determining the ultimate optical and structural qualities of the InA1As material. In line with these criteria, an elaborate set of experiments involving the MBE growth of In0.52A10.a8As on InP substrates at a wide range of substrate temperatures and at V/Ill ratios which are higher than previously reported [8] were carried out. The results from those experiments are reported in this paper. Using low temperature PL and double axis X-ray diffraction (XRD), we have characterized the PL and XRD linewidths, XRD intensity ratio (Intepi/Intsub) and lattice-matching of the InAIAs/InP samples as a function of substrate temperature. In all the cases, there is evidence of good correlation between the PL, X-ray diffraction and Raman scattering data. PL spectra taken at increasing temperatures suggest that the luminescence is a convolution of more than one transition mechanism. The dependence of the PL linewidth on the flux ratio variations at high arsenic beam equivalent pressure will be discussed. The PL and XRD linewidth variations with substrate temperature will be explained by evidence of increasingly poorer lattice-matching at higher temperatures due to indium desorption and at lower temperatures due to alloy clustering.
2. Experimental procedure Samples consisting of undoped Ino.52Alo.48As epitaxial layers 1 pm thick were grown in a Riber MBE32P system at substrate temperatures of 470, 490, 510, 520, 530 and 550 °C keeping a constant V/III flux ratio of 106, which corresponds to an arsenic beam equivalent pressure of 3 x 10-5 Torr. Substrate temperatures and beam equivalent pressures were measured using an IRCON infrared pyrometer of a suitable wavelength sensitivity and an ion gauge, respectively. The growth rate of the Ino.52A10.a8As as measured from the reflection high energy electron diffraction (RHEED) inten-
sity oscillations was 4600 A h 1. The actual alloy compositions were measured using X-ray diffraction and calculated separately from the PL peak energies, A separate set of In0.52A10.48As samples 1 pm thick were grown at a substrate temperature of 510 °C, but at varying V/III flux ratios from 32 to 266, corresponding to an arsenic beam equivalent pressure ranging from 9 x 10 -6 Torr to 7.5 x, 10 -5 Torr. The X-ray diffractometer used was a Bede Scientific model-200 system. Working in the double axis configuration, a beam conditioner consisting of a Si(220) channel-cut collimator was used in conjunction with a Si(111) monochromator on the first axis, with the sample mounted on the second axis. The (400) reflection from the Cu Kcq radiation detected from our samples of InA1As grown on InP(100) substrates was used for analysis of the rocking curve Bragg peak separations from which information on epilayer-to-substrate latticemismatch, film composition, XRD linewidth and intensity ratio could be deduced. For low temperature (4 K) PL measurements, the sample was mounted in a closed-cycle helium cryostat and excited at near normal incidence to the plane of the sample using a 514 nm argon laser. The PL was collected in the reflection direction by a 0.75 m grating spectrometer and detected using a Peltier-effect-cooled GaAs photomultiplier or a liquid-nitrogen-cooled germanium detector used in association with a conventional lock-in technique. The Raman spectra were taken at room temperature under 514 nm argon laser excitation and the back-scattered signals were collected by a high-resolution double-pass spectrometer and detected using a cooled GaAs photomultiplier detector.
3. Results and discussions Fig. 1 shows the dependence of the PL and XRD linewidths on the substrate temperature. All the samples were grown at a flux ratio of 106, which corresponds to an arsenic beam equivalent pressure of 3 x 10 -5 Torr. Sharp increases in both the PL and XRD linewidths could be seen from the samples grown at lower temperatures (approaching 470 °C) and higher temperatures (approaching 550 °C). From the fitted curves, it can be seen that the lowest PL linewidth was achieved at almost the same substrate temperature range (500-520 °C) as the lowest XRD linewidth. In this temperature range the lowest PL linewidth of 15 meV was measured from the sample grown at a substrate temperature of 520 °C. The XRD and PL linewidths can be used as general indications of the structural and optical quality of the material, respectively. Hence, from Fig. 1 it is clear that the best substrate temperature range to achieve InA1As with good structural and optical quality is 500-520 °C.
S.F. Yoon et al./ Materials Science and Engineering B35 (1995) 109-116
Fig. 2 shows a comparison of the lattice-mismatch and XRD intensity ratio as a function of the substrate temperature. The XRD intensity ratio is the ratio of the maximum epilayer peak intensity to the maximum substrate peak intensity (Inte~,i/Intsub) as measured from the X-ray diffraction curve. In a trend similar to the linewidth vs. substrate temperature variation (Fig. 1), a minimum in the lattice-mismatch of 4 x 10 -4 was observed at a substrate temperature of 520 °C. The XRD intensity ratio varies in opposition to the lattice-mismatch and generally shows higher intensity ratios at regions of lower lattice-mismatches, consistent with the linewidth vs. substrate temperature variation in Fig. 1. In the case of the InA1As material, because of the large changes in the surface migration rates of the indium and aluminum cations as the growth temperature is varied, and a difference in the indium and aluminum bond-related energies, the diffusion lengths of the aluminum and indium cations are insufficient for them to reach energetically favorable incorporation sites at low growth tempe,ratures. This results in clustering into AlAs- and InAs..rich regions. These alloy clustering effects are manifesl:ed by a reduction in the XRD intensity ratio (Fig. 2) and a broadening of the XRD linewidth and low temperature PL linewidth (Fig. 1) due to degradation in the structural quality of the material and additional optical scattering effects result-
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ing from the clustering. Because the clustering can result in the creation of localized strained regions at the heterojunction interface, this also accounts for an increase in the lattice-mismatch between the epilayer and the substrate, as shown in Fig. 2. The PL linewidth broadening at lower substrate temperatures is consistent with the previous reports [5,6,810] which suggest clustering as the main effect contributing to an increase in the linewidth as the growth temperature is lowered. In particular, two reports by Brown et al. [9] and.Oh et al. [10] have shown that the clustering effects become most significant at substrate temperatures around 400 °C. This increased clustering at lower growth temperatures was also found to be accompanied by surface morphology features elongated along the (110) direction [9]. Besides PL linewidth broadening, changes in the electrical parameters such as a change in the carrier concentration and a reduction in the mobility of the charge carriers have also been reported [5,6,9]. At higher substrate temperatures (approaching 520 °C), it is expected that entropy will dominate the internal energy gained by the clustering and a statistically more random growth process [9], with a decreased tendency towards phase separation [10] is favored. These effects, together with the e n -
S.F. Yoon et al./ Materials Science and Engineering B35 (1995) 109-116
112
hanced surface migration rates of the cations at higher substrate temperatures, leading to better incorporation into the lattice will lead to lower alloy clustering and a sharp reduction in the PL and XRD linewidths and lattice-mismatch to minimum values, as presently observed. Fig. 1 also shows that the PL and XRD linewidths broaden significantly at substrate temperatures higher than 530 °C. Houdr6 et al. [13] have previously reported significant indium desorption from InA1As grown on InP substrates above a temperature of 545550 °C, from measurements using high resolution X-ray diffraction and wedge transmission electron microscopy. Consistent with their observations, our measurements of the low temperature PL peak energies as a function of the substrate temperature (Fig. 3) also show a significant shift in the peak energy to higher energies in samples grown at substrate temperatures approaching 550 °C. It is possible that the process of indium desorption, if severe enough, can lead to compositional changes in the InAIAs material, which can subsequently affect its stoichiometry. This can degrade the latticematching between the epilayer and the substrate, and result in PL and XRD linewidth broadening as well as a reduction in the XRD intensity ratio in samples grown at higher substrate temperatures. However, other mechanisms associated with material growth at high substrate temperatures that have been reported [8],
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include the desorption of arsenic from the InA1As surface leading to enhanced incorporation of arsenicvacancies-related defects and resulting in the degradation of the optical quality of the layer. While it is possible that the direct effects of these mechanisms can also cause broadening of the PL linewidth, their overall effect on the lattice-mismatch would be small, and could not have been the main reason for an increase in the lattice-mismatch in samples grown at higher substrate temperature. It is worthwhile to note in Fig. 3 that the PL peak energies from samples grown at substrate temperatures from 490 to 520 °C compare favorably with the 4 K binding-energy-corrected latticematched InA1As bandgap of 1.516 eV reported by Houdr6 et al. [13], and confirms that this temperature range should be the most suitable to achieve good lattice-matching in the InAIAs/InP material system. Our results in Figs. 1-3 are substantiated by a comparison of the X-ray diffraction curves for samples grown at 470, 520 and 550 °C, as shown in Fig. 4. Besides the strong InP substrate peaks that are clearly evident, significant differences in linewidth and struc-
S.F. Yoon et al. / Materials Science and Engineering B35 (1995) 109-116
ture are observed in the peaks associated with the InAIAs epilayers grown at these three different substrate temperatures. The peak associated with the InAlAs layer, which was grown at 470 °C, is comprised of what appear to be three smaller peaks, which strongly indicate the presence of some structural disorder, most likely the result of non-:random growth due to alloy clustering. On the other h,and, the peak associated with the InA1As layer, which was grown at 520 °C, is well defined and has significa~atly narrower linewidth, indicating improved structural quality when growth is carried out in this temperature regime. At a substrate temperature of 550 °C, ill can be seen that the InA1As epilayer peak becomes, significantly weaker and broader. The integrated peak intensity ratio, which is the ratio of the area under the epilayer peak to that under the substrate peak, was also found to be significantly lower compared with that of the sample grown at 520 °C. The integrated intensity ratio is a measure of the relative thickness of the epilayer [14]. This ratio, being lower at 550 °C, could well indicate a significant reduction in the epilayer thickness at this high substrate temperature, owing probably to indium desorption. Using the bandgap vs. composition relation due to Wakefield et al. [15], the energy bandgap Eg(XRD) of the different samples was estimated from the compositions derived from X-ray measurements. This energy bandgap is compared with the low temperature PL peak energy E(PL), and the results are plotted in Fig. 5 for the different substrate temperatures. It can be seen that the energy difference, AE = Eg(XRD)- E(PL), is strongly dependent on the substrate temperature. Within the range of substrate temperatures investigated, this discrepancy between Eg(XRD) and E(PL) is greatest at 470 °C and de:creases with an increase in the temperature. In the case of the PL measurements, the near-bandgap energy of the clusters with the lowest bandgap is predominantly measured [10]. Hence, this can account for the smaller value of E(PL) compared with Eg(XRD) at all the substrate temperatures investigated. Therefore, the higher AE at 470 °C should indicate the presence of a higher level of alloy clustering compared with other substrate temperatures. In this regard, within the range of the substrate temperature investigated, Fig. 5 shows that the lowest value of AE was measured in the samples grown at around 520 °C, consistent with the resul~:s discussed previously. Figs. 6(a)-(c) show the temperature dependence of the PL emission peak for the samples grown at three different substrate temperatures, 550, 510 and 470 °C, respectively. The behavior of the PL emission with increasing measurement temperature strongly suggests that it results from a convolution of more than one transition mechanism. Unlike the low temperature emission at 4 K of the sample grown at 510 °C, the 4 K emissions of the samples grown at 470 and 550 °C have
113
a Gaussian-like lineshape asymmetrically broadened on the low energy side. Between about 30 and 50 K, the low temperature emission at 4 K has almost completely quenched and another distinct emission, which appeared to have arisen from the low energy shoulder of the 4 K emissions of the samples grown at 470 and 550 °C, dominates the spectrum at lower energies. In the case of the sample grown at 510 °C, the 4 K emission is not asymmetrically broadened and its quenching at between 30 and 50 K is followed by the evolution of a similar lower energy emission peak. This anomalous temperature variation in the energy peak position of InAIAs has also been observed by Ferguson et al. [16] and Olsthoorn et al. [17], and has been attributed to the localization of the carriers, but the specific localization mechanism is at this moment uncertain. Carrier localization can occur in the presence of lattice disorder. In the present case, the effect of carrier localization is more significant in the samples grown at 470 and 550 °C as a result of alloy clustering due to non-random distribution of the cations, and changes in the alloy composition due to indium desorption, respectively. This is clearly evident from the asymmetric broadening
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S.F. Yoon et aLI Materials Science and Engineering B35 (1995) 109 116
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of the 4 K emissions of samples grown at these two temperatures compared with that of the sample grown at 510 °C in which no asymmetric broadening was observed. The Raman spectrum of the four samples grown at 470, 490, 520 and 550 °C are shown in Fig. 7. Besides the clear InAs-like and AlAs-like longitudinal-optic (LO) phonon modes at a Raman shift of 234 and 370 c m - t , respectively, an additional mode at a Raman shift of 273 c m - t, as a likely result of clustering, was observed in the spectrum of the sample grown at 470 °C. A similar conclusion was reported by Emura et al. [18], who in an experiment which involved a series of InA1As/InP samples grown at various indium compositions, have observed the evolution of a prominent high energy shoulder in the InAs-like .phonon mode spectrum in samples that were considered to be highly disordered and have the highest lattice-mismatch to the InP substrate. The high energy shoulder was attributed to disorder-induced scattering by acoustical vibrations due to the presence of defects and macro clusters in these films. Fig. 8 shows the PL linewidth and lattice-mismatch dependence on the V/III flux ratios, with the substrate temperature kept at 510 °C. A range of flux ratios from 32 to 266 corresponding to an arsenic beam equivalent pressure range from 9 x 10 6 Torr to 7.5 x 10 ~ Torr was investigated. The arsenic beam equivalent pressure range is higher than that reported by Welch et al. [8], and unlike their observations, no minimum dip in the linewidth was observed in our case, although the linewidths tended to be lower at V/III flux ratios
greater than 150. Being an alloy with high volatility in the InAs component, growth of InA1As at high arsenic overpressures should be beneficial in suppressing the process of arsenic and indium desorption from the surface, as reported by Turco et al. [19] and Nakagawa et al. [20]. However, since all the samples were grown at a substrate temperature of 510 °C in this experiment, the effect of indium desorption was expected to be insignificant, and other mechanisms associated with high arsenic overpressures, such as a reduction in arsenic-vacancies-related defects could possibly be dominant. This has been reported by Welch et al. [8] from an analysis of the peaks associated 'with the longitudinal-optical (LO) and transverse-optical (TO) phonons in the Raman spectra of InA1As samples. These factors should have a beneficial effect on the quality of the InA1As, as can be seen from the lowest linewidth of 14 meV which was achieved at a V/III flux ratio of 160. This corresponds to an arsenic beam equivalent pressure of 4.5 x l0 -5 Torr. Hence, high optical quality InA1As can be achieved at a substrate temperature of 510 °C and at high arsenic overpressure conditions. However, an excessively high arsenic overpressure condition may adversely affect the surface mobilities of the aluminum and indium cations by reducing the surface diffusion length to such an extent that clustering into A1- and In-rich regions occurs, leading to localized strained regions at the heterointerface. This effect, however, is not evident in our samples as the lattice-mismatch was relatively insensitive to V/III flux ratio variations, as shown in Fig. 8.
S.F. Yoon et al. / Materials Science and Engineering B35 (1995) 109-116 4. Conclusions
ArsenicBeam Pressure
This paper reports an analysis, using low temperature PL and XRD, of Ino.5::A10.4sAs grown on InP(100) substrates at a wide range of substrate temperatures and at high arsenic overpressures using MBE. Significant PL and XRD linewidth broadening occur in samples grown at lower and higher substrate temperatures approaching 470 and 550 °C, respectively. The broadening of the PL and XRD linewidths in samples grown at lower substrate temperatures is due to the presence of alloy clustering as a result of reduced surface migration rates of the aluminum and indium cations. This causes an increase in the lattice-mismatch between the epilayer and the substrate. Besides resulting in broadening of the linewidths, this effect also causes a reduction in the XRD intensity ratio. The structural disorder resulting from the clustering effect is clearly evident from the X-ray diffraction peaks of the InA1As epilayers grown at low substrate temperatures, showing a group of smaller layer peaks, and from Raman scattering measurements, which showed an additional prominent mode at a Raman shift of ~ 40 c m - 1 higher than that of the InAs-like LO mode. Further supportive evidence came from the increasingly
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large discrepancy between the PL peak energy and the bandgap calculated from the compositions derived from X-ray measurements in samples grown at low temperatures. Our results show that the effect of indium desorption can become prominent in samples grown at higher substrate temperatures approaching 550 °C. This can lead to compositional changes in the InA1As, resulting in increased lattice-mismatch and broadening of the PL and XRD linewidths. Within the range of the V/III flux ratios investigated, narrow linewidths of less than 20 meV were measured and the lowest value of 14 meV was achieved at a flux ratio of 160 in the sample grown at a substrate temperature of 510 °C. It is possible that the high arsenic overpressures used in our experiments have the beneficial effect of suppressing crystalline defects related to arsenic vacancies. Our results also suggest that the high arsenic overpressures used have had no significant adverse effect on the quality of the heterointerface as the lattice-mismatch was relatively insensitive to flux ratio variations within the range investigated.
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S.F. Yoon et al. / Materials Science and Engineering B35 (1995) 109-116
Acknowledgments The a u t h o r s are grateful to the N a n y a n g Technological University a n d the Microelectronics Centre of the School of Electrical a n d Electronics E n g i n e e r i n g for the f u n d i n g of the M B E research project, a n d to T.H. F o o a n d M. Shamsul for technical support. The work o f Y . B . M i a o is s u p p o r t e d by a N a n y a n g Technological University P o s t g r a d u a t e Research Scholarship.
References [1] E. Bigan, J.C. Harrnand, M. Allovon, M. Carr6, A. Carenco and P. Voisin, 1EEE Photon. Technol. Lett., 3 (1991) 1107. [2] T. Akazaki, K. Arai and T. Enoki, 1EEE Electron. Devices Lett., 13 (1992) 325. [3] D.C. Reynolds, K.K. Bajaj, C.W. Litton, P.W. Yu, W.T. Masselink, R. Rischer and H. Morkoq, Phys. Rev., B29, (1984) 7038. [4] F.Y. Juang, Y. Nashimoto and P.K. Bhattacharya, J. Appl. Phys., 58 (1985) 1986. [5] W.P. Hong, A. Chin, N. Debbar, J. Hinckley, P.K. Bhattacharya and J. Singh, J. Vac. Sci. Technol., B5 (1987) 800. [6] W.P. Hong, P.K. Bhattacharya and J. Singh, Appl. Phys. Lett., 50 (1987) 618.
[7] H. Ohno, C.E.C. Wood, L. Rathbun, D.V. Morgan, G.W. Wicks and L.F. Eastman, J. Appl. Phys., 52 (1981) 4033. [8] D.F. Welch, G.W. Wicks and L.F. Eastman, Appl. Phys. Lett., 46 (1985) 169. 19] A.S. Brown, M.J. Delaney and J. Singh, J. Vac. Sci. Technol., B7 (1989) 384. [10] J.E. Oh, P.K. Bhattacharya, Y.C. Chen, O. Aina and M. Mattingly, J. Electron. Mater., 19 (1990) 435. [111 J. Singh, S. Dudley, B. Davies and K.K. Bajaj, J. Appl. Phys., 60 (1986) 3167. [12] S. Chika, H. Kato, M. Nakayama and N. Sano, Jpn. J. Appl. Phys., 25 (1986) 1441. [13] R. Houdr6, F. Gueissaz, M. Gailhanou, J.D. Gani+re, A. Rudra and M. Ilegems, J. Cryst. Growth, 111 (1991) 456. [14] M.A.G. Halliwell, M.H. Lyons, B.K. Tanner and P. Ilczyszyn, J. Cryst. Growth, 65 (1983) 672. [15] B. Wakefield, M.A.G. Halliwell, T. Kerr, D.A. Andrews, G.J. Davies and D.R. Wood, Appl. Phys. Lett., 44 (1984) 341. [16] I.T. Ferguson, T.S. Cheng, C.M. Sotomayor Torres and R. Murray, J. Vac. Sci. Technol., BI2 (1994) 1319. [17] S.M. Olsthoorn, F.A. Driessen, A.P. Eijkelenboom and L.J. Giling, J. Appl. Phys., 73 (1993) 7798. [18] S. Emura, T. Nakagawa and S. Gonda, J. Appl. Phys., 62(1987) 4632. [19] F. Turco, J.C. Guillaume and J. Massies, J. Cryst. Growth, 88 (1988) 282. [20] T. Nakagawa, S. Gonda and S. Emura, J. Cryst. Growth, 87 (1988) 276.
Materials Science and Engineering B35 (1995) 109-116
B
Optical and structural characterizations for optimized growth of Ino.52Alo.48As on InP substrates by molecular beam epitaxy S.F. Yoon*, Y.B. Miao, K. Radhakrishnan, S. Swaminathan Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 2263, Singapore
Abstract
Growth of In0.52A10.4sAs epilayers on InP(100) substrates by molecular beam epitaxy at a wide range of substrate temperatures (470-550 °C) and at high arsenic beam equivalent pressures is carried out. Analysis performed using low-temperature photoluminescence (PL) and double axis X-ray diffraction (XRD) showed a strong dependence of the PL and XRD linewidths and lattice-mismatch on the substrate temperature, with minimum linewidths and lattice-mismatch occurring between approximately 500 and 520 °C. The XRD intensity ratio (Intepi/Intsub) varied in opposition to the lattice-mismatch, with higher intensity ratios Corresponding to lower lattice-mismatches. From the X-ray diffraction curves of samples grown at low temperatures, it was observed that the main peak associated with the InA1As epilayer is comprised of smaller peaks, which strongly indicates disordering owing to the presence of alloy clustering. PL spectrum taken at increasing temperatures showed the quenching of the main emission peak followed by the evolution of a distinct peak at lower energy, possibly associated with carrier localization due to the presence of lattice disorder. In addition to the InAs-like and AlAs-like longitudinal-optic (LO) phonon modes at 234 cm- 1 and 370 cm- l, respectively, Raman scattering measurements also showed an additional higher energy mode at 273 cm- ~ in the samples grown at lower temperatures approaching 470 °C. Withinthe range of V/III flux ratios investigated (32-266), the lowest PL linewidth of 14 meV was recorded for the samples grown at a V/III ratio of 160 at a substrate temperature of 510 °C. The lattice-mismatch between the epilayer and the substrate for these samples was also found to be relatively insensitive to changes in the V/III flux ratios.
Keywords: Molecular beam epitaxy; Photoluminescence; X-ray diffraction; Raman scattering; Indium aluminum arsenide
1. Introduction
InxA1]_ xAs grown lattice-matched on InP substrates has emerged as a heterostructure material system presently attracting a high level of attention owing to its potential applications in optoelectronic devices such as quantum-well optical modulators [1] and in highspeed electronic devices such as high electron mobility transistors [2]. While near perfect heterointerfaces have been reported in the AIGaAs/GaAs material system [3,4], growth of InAIAs on InP substrates has been reported to be difficult [5,6]. The problems are associated with the difficulty in achieving good quality layers
* Corresponding author. 0921-5107/95/$09.50 © 1995 - - Elsevier Science S.A. All rights reserved
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arising from the large bond strength difference between I n - A s and AI-As, thus making it difficult to control the cation migration rates during, growth. This in turn results in difficulty in controlling its layer quality. However, despite the problems associated with its epitaxial growth, the demand for good quality InA1As material continues to be high because of its potential applications in advanced devices. This calls for continuous effort to improve its epitaxial growth process, one of which is by molecular beam epitaxy (MBE). Ohno et al. [7] first reported 4 K photoluminescence (PL) linewidth of 25 meV for this material. Since then numerous reports have appeared on experimental [5,6,810] and theoretical investigations [1 l] on the electrical and optical properties as well as the epitaxial growth aspects of InA1As.
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As expected, the MBE growth conditions for InA1As are different from those for GaAs and A1GaAs where a large cation surface mobility is required for low defect density and high quality heterojunction interfaces. For good quality growth of these two materials, the cation mobilities are usually enhanced by increasing the substrate temperature and/or decreasing the V/Ill flux ratio during growth. However, in the case of InA1As, the high volatility of the InAs component in this alloy system requires the use of low substrate temperatures and high arsenic overpressures [12] (V/III flux ratio). However, substrate temperatures which are overly low may adversely affect the cation surface migration rates to such an extent that deviation from random growth occurs. Besides using a reasonable growth rate, careful control of the substrate temperature and arsenic overpressure during MBE growth should be crucial in determining the ultimate optical and structural qualities of the InA1As material. In line with these criteria, an elaborate set of experiments involving the MBE growth of In0.52A10.a8As on InP substrates at a wide range of substrate temperatures and at V/Ill ratios which are higher than previously reported [8] were carried out. The results from those experiments are reported in this paper. Using low temperature PL and double axis X-ray diffraction (XRD), we have characterized the PL and XRD linewidths, XRD intensity ratio (Intepi/Intsub) and lattice-matching of the InAIAs/InP samples as a function of substrate temperature. In all the cases, there is evidence of good correlation between the PL, X-ray diffraction and Raman scattering data. PL spectra taken at increasing temperatures suggest that the luminescence is a convolution of more than one transition mechanism. The dependence of the PL linewidth on the flux ratio variations at high arsenic beam equivalent pressure will be discussed. The PL and XRD linewidth variations with substrate temperature will be explained by evidence of increasingly poorer lattice-matching at higher temperatures due to indium desorption and at lower temperatures due to alloy clustering.
2. Experimental procedure Samples consisting of undoped Ino.52Alo.48As epitaxial layers 1 pm thick were grown in a Riber MBE32P system at substrate temperatures of 470, 490, 510, 520, 530 and 550 °C keeping a constant V/III flux ratio of 106, which corresponds to an arsenic beam equivalent pressure of 3 x 10-5 Torr. Substrate temperatures and beam equivalent pressures were measured using an IRCON infrared pyrometer of a suitable wavelength sensitivity and an ion gauge, respectively. The growth rate of the Ino.52A10.a8As as measured from the reflection high energy electron diffraction (RHEED) inten-
sity oscillations was 4600 A h 1. The actual alloy compositions were measured using X-ray diffraction and calculated separately from the PL peak energies, A separate set of In0.52A10.48As samples 1 pm thick were grown at a substrate temperature of 510 °C, but at varying V/III flux ratios from 32 to 266, corresponding to an arsenic beam equivalent pressure ranging from 9 x 10 -6 Torr to 7.5 x, 10 -5 Torr. The X-ray diffractometer used was a Bede Scientific model-200 system. Working in the double axis configuration, a beam conditioner consisting of a Si(220) channel-cut collimator was used in conjunction with a Si(111) monochromator on the first axis, with the sample mounted on the second axis. The (400) reflection from the Cu Kcq radiation detected from our samples of InA1As grown on InP(100) substrates was used for analysis of the rocking curve Bragg peak separations from which information on epilayer-to-substrate latticemismatch, film composition, XRD linewidth and intensity ratio could be deduced. For low temperature (4 K) PL measurements, the sample was mounted in a closed-cycle helium cryostat and excited at near normal incidence to the plane of the sample using a 514 nm argon laser. The PL was collected in the reflection direction by a 0.75 m grating spectrometer and detected using a Peltier-effect-cooled GaAs photomultiplier or a liquid-nitrogen-cooled germanium detector used in association with a conventional lock-in technique. The Raman spectra were taken at room temperature under 514 nm argon laser excitation and the back-scattered signals were collected by a high-resolution double-pass spectrometer and detected using a cooled GaAs photomultiplier detector.
3. Results and discussions Fig. 1 shows the dependence of the PL and XRD linewidths on the substrate temperature. All the samples were grown at a flux ratio of 106, which corresponds to an arsenic beam equivalent pressure of 3 x 10 -5 Torr. Sharp increases in both the PL and XRD linewidths could be seen from the samples grown at lower temperatures (approaching 470 °C) and higher temperatures (approaching 550 °C). From the fitted curves, it can be seen that the lowest PL linewidth was achieved at almost the same substrate temperature range (500-520 °C) as the lowest XRD linewidth. In this temperature range the lowest PL linewidth of 15 meV was measured from the sample grown at a substrate temperature of 520 °C. The XRD and PL linewidths can be used as general indications of the structural and optical quality of the material, respectively. Hence, from Fig. 1 it is clear that the best substrate temperature range to achieve InA1As with good structural and optical quality is 500-520 °C.
S.F. Yoon et al./ Materials Science and Engineering B35 (1995) 109-116
Fig. 2 shows a comparison of the lattice-mismatch and XRD intensity ratio as a function of the substrate temperature. The XRD intensity ratio is the ratio of the maximum epilayer peak intensity to the maximum substrate peak intensity (Inte~,i/Intsub) as measured from the X-ray diffraction curve. In a trend similar to the linewidth vs. substrate temperature variation (Fig. 1), a minimum in the lattice-mismatch of 4 x 10 -4 was observed at a substrate temperature of 520 °C. The XRD intensity ratio varies in opposition to the lattice-mismatch and generally shows higher intensity ratios at regions of lower lattice-mismatches, consistent with the linewidth vs. substrate temperature variation in Fig. 1. In the case of the InA1As material, because of the large changes in the surface migration rates of the indium and aluminum cations as the growth temperature is varied, and a difference in the indium and aluminum bond-related energies, the diffusion lengths of the aluminum and indium cations are insufficient for them to reach energetically favorable incorporation sites at low growth tempe,ratures. This results in clustering into AlAs- and InAs..rich regions. These alloy clustering effects are manifesl:ed by a reduction in the XRD intensity ratio (Fig. 2) and a broadening of the XRD linewidth and low temperature PL linewidth (Fig. 1) due to degradation in the structural quality of the material and additional optical scattering effects result-
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ing from the clustering. Because the clustering can result in the creation of localized strained regions at the heterojunction interface, this also accounts for an increase in the lattice-mismatch between the epilayer and the substrate, as shown in Fig. 2. The PL linewidth broadening at lower substrate temperatures is consistent with the previous reports [5,6,810] which suggest clustering as the main effect contributing to an increase in the linewidth as the growth temperature is lowered. In particular, two reports by Brown et al. [9] and.Oh et al. [10] have shown that the clustering effects become most significant at substrate temperatures around 400 °C. This increased clustering at lower growth temperatures was also found to be accompanied by surface morphology features elongated along the (110) direction [9]. Besides PL linewidth broadening, changes in the electrical parameters such as a change in the carrier concentration and a reduction in the mobility of the charge carriers have also been reported [5,6,9]. At higher substrate temperatures (approaching 520 °C), it is expected that entropy will dominate the internal energy gained by the clustering and a statistically more random growth process [9], with a decreased tendency towards phase separation [10] is favored. These effects, together with the e n -
S.F. Yoon et al./ Materials Science and Engineering B35 (1995) 109-116
112
hanced surface migration rates of the cations at higher substrate temperatures, leading to better incorporation into the lattice will lead to lower alloy clustering and a sharp reduction in the PL and XRD linewidths and lattice-mismatch to minimum values, as presently observed. Fig. 1 also shows that the PL and XRD linewidths broaden significantly at substrate temperatures higher than 530 °C. Houdr6 et al. [13] have previously reported significant indium desorption from InA1As grown on InP substrates above a temperature of 545550 °C, from measurements using high resolution X-ray diffraction and wedge transmission electron microscopy. Consistent with their observations, our measurements of the low temperature PL peak energies as a function of the substrate temperature (Fig. 3) also show a significant shift in the peak energy to higher energies in samples grown at substrate temperatures approaching 550 °C. It is possible that the process of indium desorption, if severe enough, can lead to compositional changes in the InAIAs material, which can subsequently affect its stoichiometry. This can degrade the latticematching between the epilayer and the substrate, and result in PL and XRD linewidth broadening as well as a reduction in the XRD intensity ratio in samples grown at higher substrate temperatures. However, other mechanisms associated with material growth at high substrate temperatures that have been reported [8],
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include the desorption of arsenic from the InA1As surface leading to enhanced incorporation of arsenicvacancies-related defects and resulting in the degradation of the optical quality of the layer. While it is possible that the direct effects of these mechanisms can also cause broadening of the PL linewidth, their overall effect on the lattice-mismatch would be small, and could not have been the main reason for an increase in the lattice-mismatch in samples grown at higher substrate temperature. It is worthwhile to note in Fig. 3 that the PL peak energies from samples grown at substrate temperatures from 490 to 520 °C compare favorably with the 4 K binding-energy-corrected latticematched InA1As bandgap of 1.516 eV reported by Houdr6 et al. [13], and confirms that this temperature range should be the most suitable to achieve good lattice-matching in the InAIAs/InP material system. Our results in Figs. 1-3 are substantiated by a comparison of the X-ray diffraction curves for samples grown at 470, 520 and 550 °C, as shown in Fig. 4. Besides the strong InP substrate peaks that are clearly evident, significant differences in linewidth and struc-
S.F. Yoon et al. / Materials Science and Engineering B35 (1995) 109-116
ture are observed in the peaks associated with the InAIAs epilayers grown at these three different substrate temperatures. The peak associated with the InAlAs layer, which was grown at 470 °C, is comprised of what appear to be three smaller peaks, which strongly indicate the presence of some structural disorder, most likely the result of non-:random growth due to alloy clustering. On the other h,and, the peak associated with the InA1As layer, which was grown at 520 °C, is well defined and has significa~atly narrower linewidth, indicating improved structural quality when growth is carried out in this temperature regime. At a substrate temperature of 550 °C, ill can be seen that the InA1As epilayer peak becomes, significantly weaker and broader. The integrated peak intensity ratio, which is the ratio of the area under the epilayer peak to that under the substrate peak, was also found to be significantly lower compared with that of the sample grown at 520 °C. The integrated intensity ratio is a measure of the relative thickness of the epilayer [14]. This ratio, being lower at 550 °C, could well indicate a significant reduction in the epilayer thickness at this high substrate temperature, owing probably to indium desorption. Using the bandgap vs. composition relation due to Wakefield et al. [15], the energy bandgap Eg(XRD) of the different samples was estimated from the compositions derived from X-ray measurements. This energy bandgap is compared with the low temperature PL peak energy E(PL), and the results are plotted in Fig. 5 for the different substrate temperatures. It can be seen that the energy difference, AE = Eg(XRD)- E(PL), is strongly dependent on the substrate temperature. Within the range of substrate temperatures investigated, this discrepancy between Eg(XRD) and E(PL) is greatest at 470 °C and de:creases with an increase in the temperature. In the case of the PL measurements, the near-bandgap energy of the clusters with the lowest bandgap is predominantly measured [10]. Hence, this can account for the smaller value of E(PL) compared with Eg(XRD) at all the substrate temperatures investigated. Therefore, the higher AE at 470 °C should indicate the presence of a higher level of alloy clustering compared with other substrate temperatures. In this regard, within the range of the substrate temperature investigated, Fig. 5 shows that the lowest value of AE was measured in the samples grown at around 520 °C, consistent with the resul~:s discussed previously. Figs. 6(a)-(c) show the temperature dependence of the PL emission peak for the samples grown at three different substrate temperatures, 550, 510 and 470 °C, respectively. The behavior of the PL emission with increasing measurement temperature strongly suggests that it results from a convolution of more than one transition mechanism. Unlike the low temperature emission at 4 K of the sample grown at 510 °C, the 4 K emissions of the samples grown at 470 and 550 °C have
113
a Gaussian-like lineshape asymmetrically broadened on the low energy side. Between about 30 and 50 K, the low temperature emission at 4 K has almost completely quenched and another distinct emission, which appeared to have arisen from the low energy shoulder of the 4 K emissions of the samples grown at 470 and 550 °C, dominates the spectrum at lower energies. In the case of the sample grown at 510 °C, the 4 K emission is not asymmetrically broadened and its quenching at between 30 and 50 K is followed by the evolution of a similar lower energy emission peak. This anomalous temperature variation in the energy peak position of InAIAs has also been observed by Ferguson et al. [16] and Olsthoorn et al. [17], and has been attributed to the localization of the carriers, but the specific localization mechanism is at this moment uncertain. Carrier localization can occur in the presence of lattice disorder. In the present case, the effect of carrier localization is more significant in the samples grown at 470 and 550 °C as a result of alloy clustering due to non-random distribution of the cations, and changes in the alloy composition due to indium desorption, respectively. This is clearly evident from the asymmetric broadening
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S.F. Yoon et aLI Materials Science and Engineering B35 (1995) 109 116
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of the 4 K emissions of samples grown at these two temperatures compared with that of the sample grown at 510 °C in which no asymmetric broadening was observed. The Raman spectrum of the four samples grown at 470, 490, 520 and 550 °C are shown in Fig. 7. Besides the clear InAs-like and AlAs-like longitudinal-optic (LO) phonon modes at a Raman shift of 234 and 370 c m - t , respectively, an additional mode at a Raman shift of 273 c m - t, as a likely result of clustering, was observed in the spectrum of the sample grown at 470 °C. A similar conclusion was reported by Emura et al. [18], who in an experiment which involved a series of InA1As/InP samples grown at various indium compositions, have observed the evolution of a prominent high energy shoulder in the InAs-like .phonon mode spectrum in samples that were considered to be highly disordered and have the highest lattice-mismatch to the InP substrate. The high energy shoulder was attributed to disorder-induced scattering by acoustical vibrations due to the presence of defects and macro clusters in these films. Fig. 8 shows the PL linewidth and lattice-mismatch dependence on the V/III flux ratios, with the substrate temperature kept at 510 °C. A range of flux ratios from 32 to 266 corresponding to an arsenic beam equivalent pressure range from 9 x 10 6 Torr to 7.5 x 10 ~ Torr was investigated. The arsenic beam equivalent pressure range is higher than that reported by Welch et al. [8], and unlike their observations, no minimum dip in the linewidth was observed in our case, although the linewidths tended to be lower at V/III flux ratios
greater than 150. Being an alloy with high volatility in the InAs component, growth of InA1As at high arsenic overpressures should be beneficial in suppressing the process of arsenic and indium desorption from the surface, as reported by Turco et al. [19] and Nakagawa et al. [20]. However, since all the samples were grown at a substrate temperature of 510 °C in this experiment, the effect of indium desorption was expected to be insignificant, and other mechanisms associated with high arsenic overpressures, such as a reduction in arsenic-vacancies-related defects could possibly be dominant. This has been reported by Welch et al. [8] from an analysis of the peaks associated 'with the longitudinal-optical (LO) and transverse-optical (TO) phonons in the Raman spectra of InA1As samples. These factors should have a beneficial effect on the quality of the InA1As, as can be seen from the lowest linewidth of 14 meV which was achieved at a V/III flux ratio of 160. This corresponds to an arsenic beam equivalent pressure of 4.5 x l0 -5 Torr. Hence, high optical quality InA1As can be achieved at a substrate temperature of 510 °C and at high arsenic overpressure conditions. However, an excessively high arsenic overpressure condition may adversely affect the surface mobilities of the aluminum and indium cations by reducing the surface diffusion length to such an extent that clustering into A1- and In-rich regions occurs, leading to localized strained regions at the heterointerface. This effect, however, is not evident in our samples as the lattice-mismatch was relatively insensitive to V/III flux ratio variations, as shown in Fig. 8.
S.F. Yoon et al. / Materials Science and Engineering B35 (1995) 109-116 4. Conclusions
ArsenicBeam Pressure
This paper reports an analysis, using low temperature PL and XRD, of Ino.5::A10.4sAs grown on InP(100) substrates at a wide range of substrate temperatures and at high arsenic overpressures using MBE. Significant PL and XRD linewidth broadening occur in samples grown at lower and higher substrate temperatures approaching 470 and 550 °C, respectively. The broadening of the PL and XRD linewidths in samples grown at lower substrate temperatures is due to the presence of alloy clustering as a result of reduced surface migration rates of the aluminum and indium cations. This causes an increase in the lattice-mismatch between the epilayer and the substrate. Besides resulting in broadening of the linewidths, this effect also causes a reduction in the XRD intensity ratio. The structural disorder resulting from the clustering effect is clearly evident from the X-ray diffraction peaks of the InA1As epilayers grown at low substrate temperatures, showing a group of smaller layer peaks, and from Raman scattering measurements, which showed an additional prominent mode at a Raman shift of ~ 40 c m - 1 higher than that of the InAs-like LO mode. Further supportive evidence came from the increasingly
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large discrepancy between the PL peak energy and the bandgap calculated from the compositions derived from X-ray measurements in samples grown at low temperatures. Our results show that the effect of indium desorption can become prominent in samples grown at higher substrate temperatures approaching 550 °C. This can lead to compositional changes in the InA1As, resulting in increased lattice-mismatch and broadening of the PL and XRD linewidths. Within the range of the V/III flux ratios investigated, narrow linewidths of less than 20 meV were measured and the lowest value of 14 meV was achieved at a flux ratio of 160 in the sample grown at a substrate temperature of 510 °C. It is possible that the high arsenic overpressures used in our experiments have the beneficial effect of suppressing crystalline defects related to arsenic vacancies. Our results also suggest that the high arsenic overpressures used have had no significant adverse effect on the quality of the heterointerface as the lattice-mismatch was relatively insensitive to flux ratio variations within the range investigated.
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S.F. Yoon et al. / Materials Science and Engineering B35 (1995) 109-116
Acknowledgments The a u t h o r s are grateful to the N a n y a n g Technological University a n d the Microelectronics Centre of the School of Electrical a n d Electronics E n g i n e e r i n g for the f u n d i n g of the M B E research project, a n d to T.H. F o o a n d M. Shamsul for technical support. The work o f Y . B . M i a o is s u p p o r t e d by a N a n y a n g Technological University P o s t g r a d u a t e Research Scholarship.
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