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III flux ratio variation
Superlattices and Microstructures, Vol. 23, No. 2, 1998
Growth of In0.52 Al0.48 As on InP substrates by molecular beam epitaxy: some effects of V/III flux ratio variation S. F. Yoon School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Republic of Singapore (Received 18 December 1996) Growth of In0.52 Al0.48 As epitaxial layers on InP(100) substrates by molecular beam epitaxy at a wide range of arsenic overpressure (V/III flux ratio from 30 to 300) is carried out. Analysis performed using low-temperature photoluminescence (PL) and double-axis X-ray diffraction (XRD) showed a strong and prominent dependence of the PL and XRD linewidths on the V/III flux ratio. Under our growth conditions, both the PL and XRD linewidths exhibit a minimum point at a V/III flux ratio of 150 which corresponds to a maximum in the PL intensity and XRD intensity ratio. Flux ratios exceeding 150 results in an increase in both the PL and XRD linewidths corresponding to a reduction in their associated intensities. Room-temperature Raman scattering measurements show a narrowing in the InAs-like and AlAs-like longitudinal-optic (LO) phonon linewidths which broaden at high flux ratios, while the LO phonon frequencies exhibit a gradual reduction as the flux ratio is increased. PL spectrum taken at increasing temperature show a quenching of the main emission peak followed by the evolution of a broad lower energy emission which is possibly associated with deep-lying centres. This effect is more prominent in samples grown at lower V/III flux ratios. Hall effect measurements showed a gradual reduction in the mobility in correspondence to an increase in the electron concentration as the flux ratio is increased. c 1998 Academic Press Limited
1. Introduction The Inx Al1−x As material grown lattice-matched onto InP substrates by molecular beam epitaxy (MBE) is presently attracting much interest due to its potential applications in high-frequency electronic devices such as the high electron mobility transistor (HEMT) [1] and in optoelectronic devices such as quantum-well modulators [2]. While high-quality growth of the AlGaAs/GaAs material system is now routinely achieved [3, 4], various difficulties associated with the growth of InAlAs on InP substrates have been reported [5, 6]. The main problems are usually those which arise from the large bond strength difference between In–As and Al–As. This problem creates difficulty in controlling the cation migration rates during MBE growth which, in turn, affects the layer quality. Since the early report by Ohno et al. [7] of a 4 K photoluminescence (PL) linewidth of 25 meV for this material, a number of reports have been published on experimental [5,6,8–10] as well as theoretical aspects [11] of this material with emphasis on its electrical and optical properties related to its potential device applications. In the case of GaAs and AlGaAs growth, the cation mobilities are usually enhanced by increasing the .
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substrate temperature and/or decreasing the V/III flux ratio during growth in order to achieve low defect density and high-quality interfaces. In InAlAs, however, the high volatility of the InAs component in this material system necessitates the use of lower substrate temperatures and higher arsenic overpressures (V/III flux ratios). A number of previous studies [8, 12, 13] have established the optimum MBE growth temperature for InAlAs lattice-matched onto InP to be in the region of 515 to 525 ◦ C. Hence careful control and regulation of the V/III flux ratio during growth is crucial for determining good structural and optical properties in this material system. In this paper, a series of experiments involving the MBE growth of In0.52 Al0.48 As on InP substrates were carried out at a wide range of arsenic overpressure (V/III flux ratio) to determine its effect on the optical and structural characteristics of this material system which have been analysed using low-temperature PL, double-axis X-ray diffraction (XRD) and Raman scattering measurements. The dependence of the PL and XRD linewidths, XRD intensity ratio (I ntepi /I ntsub ) and PL intensity on the V/III flux ratio will be discussed in conjunction with the results from Raman scattering measurements which suggest an increase in the material disorder at high flux ratios. The evolution of the PL spectrum at different temperatures for samples grown at different flux ratios will also be discussed in association with electron mobility and concentration results from Hall effect measurements. .
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2. Experimental procedure InAlAs samples of 1 µm thickness were grown on InP (100) oriented semi-insulating substrates by molecular beam epitaxy in a Riber MBE32 system with computer-controlled process monitoring. Standard substrate preparation process was used. Surface oxides were desorbed by heating the substrate to approximately 510 ◦ C in an excess arsenic flux. Substrate temperatures and beam equivalent pressures (BEPs) were measured using an IRCON infrared pyrometer of a suitable wavelength sensitivity and an ion gauge, respectively. Reflection high-energy electron diffraction (RHEED) measurements were used to monitor the surface crystallinity and the ˚ h−1 ). The alloy composition was determined intensity oscillations used to determine the growth rate (4600 A using X-ray diffraction and calculated separately from the PL peak energies. All samples were uniformly doped with silicon at a concentration of 1 × 1017 cm−3 and grown at a substrate temperature of 520 ◦ C which is the optimum growth temperature for this material system. Samples were grown at a range of V/III flux ratios from 30 to 300, corresponding to an arsenic BEP of 1.4 × 10−5 Torr to 8.4 × 10−5 Torr. The BEP of In and Al was 2.1 × 10−7 Torr and 7.2 × 10−8 Torr, respectively. PL measurements were conducted in a variable-temperature closed-cycle helium cryostat and the sample was excited at near normal incidence 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 detector or a liquid-nitrogen-cooled germanium detector used in association with a conventional lock-in technique. X-ray diffraction measurements were taken using a Bede Scientific model-200 system working in doubleaxis 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 Kα1 radiation detected from the samples was used for the analysis of the rocking curve Bragg peak separations from which the lattice mismatch, XRD linewidth and intensity ratio can be deduced. The Raman spectra were taken at room temperature with the samples under 514 nm argon laser excitation and the backscattered signals were collected by a high-resolution spectrometer and detected using a cooled GaAs photomultiplier detector. The Hall effect measurements were conducted on suitable samples prepared with cloverleaf patterns using standard photolithographic techniques. The measurements were carried out on the Bio-Rad HL5500 Hall effect system at 77 K.
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Fig. 1. Plot of PL linewidth (5 K) and intensity as a function of V/III flux ratio showing a minimum linewidth and maximum intensity at a V/III ratio of 150.
3. Results and discussion Figure 1 shows the dependence of the PL linewidth (at 5 K) and the associated intensity as a function of V/III flux ratio. It can be seen that the linewidth decreases significantly to a minimum value of 18 meV at a V/III flux ratio of 150 (corresponding to BEPAs of 4.2 × 10−5 Torr) and increases thereafter. The linewidth reduction with increase in the V/III flux ratio is consistent with a previous report by Welch et al. [8]. High arsenic overpressure during the growth of InAlAs should have a beneficial effect on the crystalline quality as it helps to suppress the process of surface indium desorption during growth [14, 15]. From an analysis of the longitudinal-optic (LO) and transverse-optic (TO) phonon modes in the Raman spectra, effects associated with a reduction in the arsenic-vacancy related defects in growths carried out at high V/III flux ratios have also been reported [8]. Also Choi and Fonstad [12] have observed narrower PL linewidths in InAlAs samples grown at higher flux ratios, an effect which has been attributed to a reduction in the alloy clustering arising from the resulting lower surface adatom mobilities. The clustering effect can be reduced through a reduction in the thermodynamically favoured phase separation between InAs and AlAs. A combination of these factors should have a beneficial effect on the quality of the InAlAs as seen from the reduction in the linewidth with increase in the flux ratio. However, it is also possible that an excessively high arsenic overpressure may have an adverse effect on the surface mobilities of the Al and In cations to such an extent that clustering into Inand Al-rich regions can occur. This can lead to a degradation in the film quality as seen from the increase in the PL linewidth at flux ratios exceeding 150. Within the range of flux ratio investigated, the lattice mismatch in the samples did not exhibit any specific trend of variation. The maximum and minimum values of the lattice mismatch recorded was 1.10 × 10−3 and 9.34 × 10−4 , respectively. The fact that the lattice mismatch was relatively insensitive to changes in the flux ratio suggests that the possible clustering effect arising from the high arsenic overpressures did not lead to the formation of localized strained regions at the heterointerface. The PL linewidth variation with flux ratio is supported by a corresponding, but opposite variation in the PL intensity over the same variation of the flux ratio, with a maximum intensity occurring at a flux ratio of 150. Further increase in the flux ratio beyond 150 causes a reduction in the intensity as observed. The dependence of the XRD linewidth and intensity ratio on the V/III flux ratio variation is shown in Fig. 2. The XRD intensity ratio is the ratio of the maximum epilayer peak intensity to the maximum substrate peak .
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Fig. 2. Plot of XRD linewidth and intensity ratio as a function of V/III flux ratio showing a minimum linewidth and maximum intensity ratio at a function of 150.
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Fig. 3. Plot of Raman linewidth for both AlAs-like and InAs-like phonon modes as a function of V/III flux ratio showing minimum linewidths at a V/III ratio of 150.
intensity (I ntepi /I ntsub ) as measured from the X-ray diffraction curve. The XRD linewidth varies in a similar trend as the PL linewidth in Fig. 1 and clearly shows an initial improvement in the film quality until a flux ratio of 150, beyond which degradation in the film quality occurs. This is strongly supported by the observed variation in the XRD intensity ratio which shows a maximum value corresponding to a minimum in the XRD linewidth. Choi and Fonstad [12] have earlier shown a reduction in the XRD linewidth in InAlAs samples grown at 525 ◦ C when the arsenic BEP was increased from 1.0 × 10−5 Torr to 1.5 × 10−5 Torr, but no XRD data beyond this BEP were reported. Our results clearly show the presence of an optimum arsenic BEP in .
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Fig. 4. Plot of Raman frequency for both AlAs-like and InAs-like phonon modes as a function of V/III flux ratio showing a gradual reduction in the frequency as the flux ratio is increased.
InAlAs growth, the exact value of which would probably depend on growth conditions such as the substrate temperature and film composition. The Raman spectra of the samples show clear InAs-like and AlAs-like LO phonon modes at ∼234 cm−1 and ∼370 cm−1 , respectively. No symmetry forbidden phonon modes were detected. As shown in Fig. 3, the linewidths of the InAs-like and AlAs-like phonon modes show a similar trend of variation as the PL and XRD linewidth data with both the linewidths reaching minimum values of 14 cm−1 and 18 cm−1 , respectively at a flux ratio of 150. Further increase in the flux ratio beyond 150 causes a significant increase in the linewidths suggesting a degradation in the structural quality of the films, consistent with the PL and XRD results discussed earlier. Increase in the V/III flux ratio has a tendency to cause a reduction in the Raman frequency of the InAs-like and AlAs-like phonon modes as shown in Fig. 4. In both the phonon modes, the shift to lower frequencies was gradual initially, but became more prominent at flux ratios exceeding 150, in coincidence with the broadening of the phonon lineshapes as seen in Fig. 3. Tiong et al. [16] have previously reported a significant shift to lower frequencies coupled with an asymmetric broadening of the LO phonon modes in GaAs samples which were subjected to heavy As+ implantation. This effect was attributed to the generation of disordered regions in the material resulting from the implant. A similar reduction in the frequencies of the LO phonon modes in silicon-doped InAlAs samples was observed at increasing silicon concentration [17], an effect attributed to the presence of doping-induced disorder in the material. Applying this observations qualitatively to our results suggest that the reduction in the Raman frequencies of the LO phonon modes can possibly be due to the presence of disorder in the material, an effect which could have arisen from the clustering due to the overly high arsenic overpressure during growth. However, the actual atomistic nature of this disorder is presently unknown and warrants further investigation. Figures 5, 6 and 7 show the variation of the PL spectrum as a function of temperature for InAlAs samples grown at three different V/III flux ratios (V/III = 50, 210 and 280). The behaviour of the PL emission with increase in temperature suggests that it arises from a combination of more than one transition mechanism. In all the three samples, the PL spectrum at 5 K has a Gaussian-like lineshape which is asymmetrically broadened at the low-energy side. A low-energy shoulder was clearly evident in the 5 K spectrum of the samples grown at V/III ratio of 210 and 280, respectively. In the sample grown at V/III ratio = 280, the intensity of this .
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Superlattices and Microstructures, Vol. 23, No. 2, 1998 1600 V/III = 50
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Fig. 5. PL spectrum at different temperatures for sample grown at V/III flux ratio of 50 showing the evolution of a broad low-energy emission when the temperature is increased.
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Fig. 6. PL spectrum at different temperatures for sample grown at V/III flux ratio of 210.
low-energy shoulder is almost as high as the main emission peak. Apart from this low-energy shoulder, the low-temperature PL emission seems to be dominated by exciton recombination and band-to-band transitions. As the temperature is increased, the main emission peak started to quench and a broad low-energy emission started to evolve in all the three samples, although this effect is most pronounced in the sample grown at V/III ratio of 50. It is possible that growth at low V/III flux ratios incorporates a greater number of arsenic (group V) vacancies. Hence at higher temperatures, the impurities become ionized and the evolution of the PL spectrum suggests that the emission could be dominated by free-to-bound or bound-to-bound donor-to-acceptor-like transitions. This is supported by the fact that samples prepared at low V/III flux ratios were observed to have a lower electron concentration as seen in Fig. 8. The electron concentration increases as the V/III ratio is
Superlattices and Microstructures, Vol. 23, No. 2, 1998 5K 35 K (× 3)
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Fig. 7. PL spectrum at different temperatures for sample grown at V/III flux ratio of 280.
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Fig. 8. Plot of Hall mobility and electron concentration at 77 K as a function of V/III flux ratio.
increased suggesting that samples prepared at high flux ratios have a higher number of group III vacancies. Figure 8 also shows a corresponding decrease in the electron mobility as the flux ratio is increased. The rapid quenching of the main PL emission peak and the evolution of a broad low-energy emission as the temperature is increased could also suggest the presence of deep-lying centres, but their exact nature is uncertain at this moment.
4. Conclusions In conclusion, the growth of In0.52 Al0.48 As on InP at a wide range of arsenic overpressures (V/III ratio
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Superlattices and Microstructures, Vol. 23, No. 2, 1998
from 30 to 300) is reported in this paper. Samples grown under our growth conditions exhibit a minimum point in both the PL and XRD linewidths and in the InAs-like and AlAs-like LO phonon linewidths at a V/III ratio of 150. This minimum point corresponds to a maximum in the PL intensity and XRD intensity ratio. The LO phonon frequencies also exhibit a gradual reduction as the flux ratio is increased, an effect due probably to the presence of disorder in the material. The results suggest a degradation in the quality of films grown at excessively high arsenic overpressure due possibly to clustering into In- and Al-rich regions. This, however, has been observed to have no effect on the lattice mismatch between the epilayer and the substrate. PL spectrum from samples taken at different temperatures show a rapid quenching of the main emission peak followed by an evolution of a broad low-energy emission at higher temperature suggesting the presence of deep-lying centres possibly related to donor-to-acceptor-like transitions. This effect is more pronounced in samples grown at low V/III flux ratio. Hall effect measurements showed an increase in the electron concentration and a corresponding decrease in the mobility in samples grown at increasing V/III flux ratio.
References [1] }T. Akazaki, K. Arai, and T. Enoki, IEEE Electron. Devices Lett. EDL-13, 325 (1992). [2] }E. Bigan, J.C.Harmand, M. Allovon, M. Carre, A. Carenco and P. Voisin, IEEE Photonics Technol. Lett. PTL-3, 1107 (1991). [3] }D. C. Reynolds, K. K. Bajaj, C. W. Litton, P. W. Yu, W. T. Maselink, R. Rischer, and H. Morkoc, Phys. Rev. B29, 7038 (1984). [4] }F. Y. Juang, Y. Nashimoto, and P. K. Bhattacharya, J. Appl. Phys. 58, 1986 (1985). [5] }W. P. Hong, A. Chin, N. Debbar, J. Hinckley, P. K. Bhattacharya, and J. Singh, J. Vac. Sci. Technol. B5, 800 (1987). [6] }W. P. Hong, P. K. Bhattacharya, and J. Singh, Appl. Phys. Lett. 50, 618 (1987). [7] }H. Ohno, C. E. C. Wood, L. Rathbun, D. V. Morgan, G. W. Wicks, and L. F. Eastman, J.Appl. Phys. 52, 4033 (1981). [8] }D. F. Welch, G. W. Wicks, and L. F. Esatman, Appl. Phys. Lett. 46, 169 (1985). [9] }A. S. Brown, M. J. Delaney, and J. Singh, J. Vac. Sci. Technol. B7, 384 (1989). [10] }J. E. Oh, P. K. Bhattacharya, Y. C. Chen, O. Aina, and M. Mattingly, J. Electron. Mater. 19, 435 (1990). [11] }J. Singh, S. Dudley, B. Davies, and K. K. Bajaj, J. Appl. Phys. 60, 3167 (1986). [12] }W. Y. Choi and C. G. Fonstad, J. Vac. Sci. Technol. B12, 1013 (1994). [13] }J. P. Reithmaier, S. Hausser, H. P. Meier, and W. Walter, J. Cryst. Growth 127, 755 (1993). [14] }F. Turco, J. C. Guillaume, and J. Massies, J. Cryst. Growth 88, 282 (1988). [15] }T. Nakagawa, S. Gonda, and S. Emura, J. Cryst. Growth 87, 276 (1988). [16] }K. K. Tiong, P. M. Amirtharaj, and F. H. Pollak, Appl. Phys. Lett. 44, 122 (1984). [17] }S. F. Yoon, Y. B. Miao, and K. Radhakrishnan, J. Appl. Phys. 78, 126 (1995). .
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Growth of In0.52 Al0.48 As on InP substrates by molecular beam epitaxy: some effects of V/III flux ratio variation S. F. Yoon School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Republic of Singapore (Received 18 December 1996) Growth of In0.52 Al0.48 As epitaxial layers on InP(100) substrates by molecular beam epitaxy at a wide range of arsenic overpressure (V/III flux ratio from 30 to 300) is carried out. Analysis performed using low-temperature photoluminescence (PL) and double-axis X-ray diffraction (XRD) showed a strong and prominent dependence of the PL and XRD linewidths on the V/III flux ratio. Under our growth conditions, both the PL and XRD linewidths exhibit a minimum point at a V/III flux ratio of 150 which corresponds to a maximum in the PL intensity and XRD intensity ratio. Flux ratios exceeding 150 results in an increase in both the PL and XRD linewidths corresponding to a reduction in their associated intensities. Room-temperature Raman scattering measurements show a narrowing in the InAs-like and AlAs-like longitudinal-optic (LO) phonon linewidths which broaden at high flux ratios, while the LO phonon frequencies exhibit a gradual reduction as the flux ratio is increased. PL spectrum taken at increasing temperature show a quenching of the main emission peak followed by the evolution of a broad lower energy emission which is possibly associated with deep-lying centres. This effect is more prominent in samples grown at lower V/III flux ratios. Hall effect measurements showed a gradual reduction in the mobility in correspondence to an increase in the electron concentration as the flux ratio is increased. c 1998 Academic Press Limited
1. Introduction The Inx Al1−x As material grown lattice-matched onto InP substrates by molecular beam epitaxy (MBE) is presently attracting much interest due to its potential applications in high-frequency electronic devices such as the high electron mobility transistor (HEMT) [1] and in optoelectronic devices such as quantum-well modulators [2]. While high-quality growth of the AlGaAs/GaAs material system is now routinely achieved [3, 4], various difficulties associated with the growth of InAlAs on InP substrates have been reported [5, 6]. The main problems are usually those which arise from the large bond strength difference between In–As and Al–As. This problem creates difficulty in controlling the cation migration rates during MBE growth which, in turn, affects the layer quality. Since the early report by Ohno et al. [7] of a 4 K photoluminescence (PL) linewidth of 25 meV for this material, a number of reports have been published on experimental [5,6,8–10] as well as theoretical aspects [11] of this material with emphasis on its electrical and optical properties related to its potential device applications. In the case of GaAs and AlGaAs growth, the cation mobilities are usually enhanced by increasing the .
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substrate temperature and/or decreasing the V/III flux ratio during growth in order to achieve low defect density and high-quality interfaces. In InAlAs, however, the high volatility of the InAs component in this material system necessitates the use of lower substrate temperatures and higher arsenic overpressures (V/III flux ratios). A number of previous studies [8, 12, 13] have established the optimum MBE growth temperature for InAlAs lattice-matched onto InP to be in the region of 515 to 525 ◦ C. Hence careful control and regulation of the V/III flux ratio during growth is crucial for determining good structural and optical properties in this material system. In this paper, a series of experiments involving the MBE growth of In0.52 Al0.48 As on InP substrates were carried out at a wide range of arsenic overpressure (V/III flux ratio) to determine its effect on the optical and structural characteristics of this material system which have been analysed using low-temperature PL, double-axis X-ray diffraction (XRD) and Raman scattering measurements. The dependence of the PL and XRD linewidths, XRD intensity ratio (I ntepi /I ntsub ) and PL intensity on the V/III flux ratio will be discussed in conjunction with the results from Raman scattering measurements which suggest an increase in the material disorder at high flux ratios. The evolution of the PL spectrum at different temperatures for samples grown at different flux ratios will also be discussed in association with electron mobility and concentration results from Hall effect measurements. .
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2. Experimental procedure InAlAs samples of 1 µm thickness were grown on InP (100) oriented semi-insulating substrates by molecular beam epitaxy in a Riber MBE32 system with computer-controlled process monitoring. Standard substrate preparation process was used. Surface oxides were desorbed by heating the substrate to approximately 510 ◦ C in an excess arsenic flux. Substrate temperatures and beam equivalent pressures (BEPs) were measured using an IRCON infrared pyrometer of a suitable wavelength sensitivity and an ion gauge, respectively. Reflection high-energy electron diffraction (RHEED) measurements were used to monitor the surface crystallinity and the ˚ h−1 ). The alloy composition was determined intensity oscillations used to determine the growth rate (4600 A using X-ray diffraction and calculated separately from the PL peak energies. All samples were uniformly doped with silicon at a concentration of 1 × 1017 cm−3 and grown at a substrate temperature of 520 ◦ C which is the optimum growth temperature for this material system. Samples were grown at a range of V/III flux ratios from 30 to 300, corresponding to an arsenic BEP of 1.4 × 10−5 Torr to 8.4 × 10−5 Torr. The BEP of In and Al was 2.1 × 10−7 Torr and 7.2 × 10−8 Torr, respectively. PL measurements were conducted in a variable-temperature closed-cycle helium cryostat and the sample was excited at near normal incidence 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 detector or a liquid-nitrogen-cooled germanium detector used in association with a conventional lock-in technique. X-ray diffraction measurements were taken using a Bede Scientific model-200 system working in doubleaxis 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 Kα1 radiation detected from the samples was used for the analysis of the rocking curve Bragg peak separations from which the lattice mismatch, XRD linewidth and intensity ratio can be deduced. The Raman spectra were taken at room temperature with the samples under 514 nm argon laser excitation and the backscattered signals were collected by a high-resolution spectrometer and detected using a cooled GaAs photomultiplier detector. The Hall effect measurements were conducted on suitable samples prepared with cloverleaf patterns using standard photolithographic techniques. The measurements were carried out on the Bio-Rad HL5500 Hall effect system at 77 K.
Superlattices and Microstructures, Vol. 23, No. 2, 1998
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Arsenic Beam Equivalent Pressure (× 10–5 Torr) 5.6 7.0 1.4 2.8 4.2
100
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Fig. 1. Plot of PL linewidth (5 K) and intensity as a function of V/III flux ratio showing a minimum linewidth and maximum intensity at a V/III ratio of 150.
3. Results and discussion Figure 1 shows the dependence of the PL linewidth (at 5 K) and the associated intensity as a function of V/III flux ratio. It can be seen that the linewidth decreases significantly to a minimum value of 18 meV at a V/III flux ratio of 150 (corresponding to BEPAs of 4.2 × 10−5 Torr) and increases thereafter. The linewidth reduction with increase in the V/III flux ratio is consistent with a previous report by Welch et al. [8]. High arsenic overpressure during the growth of InAlAs should have a beneficial effect on the crystalline quality as it helps to suppress the process of surface indium desorption during growth [14, 15]. From an analysis of the longitudinal-optic (LO) and transverse-optic (TO) phonon modes in the Raman spectra, effects associated with a reduction in the arsenic-vacancy related defects in growths carried out at high V/III flux ratios have also been reported [8]. Also Choi and Fonstad [12] have observed narrower PL linewidths in InAlAs samples grown at higher flux ratios, an effect which has been attributed to a reduction in the alloy clustering arising from the resulting lower surface adatom mobilities. The clustering effect can be reduced through a reduction in the thermodynamically favoured phase separation between InAs and AlAs. A combination of these factors should have a beneficial effect on the quality of the InAlAs as seen from the reduction in the linewidth with increase in the flux ratio. However, it is also possible that an excessively high arsenic overpressure may have an adverse effect on the surface mobilities of the Al and In cations to such an extent that clustering into Inand Al-rich regions can occur. This can lead to a degradation in the film quality as seen from the increase in the PL linewidth at flux ratios exceeding 150. Within the range of flux ratio investigated, the lattice mismatch in the samples did not exhibit any specific trend of variation. The maximum and minimum values of the lattice mismatch recorded was 1.10 × 10−3 and 9.34 × 10−4 , respectively. The fact that the lattice mismatch was relatively insensitive to changes in the flux ratio suggests that the possible clustering effect arising from the high arsenic overpressures did not lead to the formation of localized strained regions at the heterointerface. The PL linewidth variation with flux ratio is supported by a corresponding, but opposite variation in the PL intensity over the same variation of the flux ratio, with a maximum intensity occurring at a flux ratio of 150. Further increase in the flux ratio beyond 150 causes a reduction in the intensity as observed. The dependence of the XRD linewidth and intensity ratio on the V/III flux ratio variation is shown in Fig. 2. The XRD intensity ratio is the ratio of the maximum epilayer peak intensity to the maximum substrate peak .
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Superlattices and Microstructures, Vol. 23, No. 2, 1998 Arsenic Beam Equivalent Pressure (× 10–5 Torr) 5.6 7.0 1.4 2.8 4.2
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40
150
30
100
20
50
10
0
0
50
100
150 200 V/III Flux Ratio
250
300
XRD Intensity Ratio
XRD Linewidth (meV)
300
0
Fig. 2. Plot of XRD linewidth and intensity ratio as a function of V/III flux ratio showing a minimum linewidth and maximum intensity ratio at a function of 150.
Arsenic Beam Equivalent Pressure (× 10–5 Torr) 1.4 2.8 4.2 5.6 7.0
8.4
Raman Linewidth (cm–1)
45 40 AlAs-Like
35 30 25 20
InAs-Like 15 10
0
50
100
150 200 V/III Flux Ratio
250
300
Fig. 3. Plot of Raman linewidth for both AlAs-like and InAs-like phonon modes as a function of V/III flux ratio showing minimum linewidths at a V/III ratio of 150.
intensity (I ntepi /I ntsub ) as measured from the X-ray diffraction curve. The XRD linewidth varies in a similar trend as the PL linewidth in Fig. 1 and clearly shows an initial improvement in the film quality until a flux ratio of 150, beyond which degradation in the film quality occurs. This is strongly supported by the observed variation in the XRD intensity ratio which shows a maximum value corresponding to a minimum in the XRD linewidth. Choi and Fonstad [12] have earlier shown a reduction in the XRD linewidth in InAlAs samples grown at 525 ◦ C when the arsenic BEP was increased from 1.0 × 10−5 Torr to 1.5 × 10−5 Torr, but no XRD data beyond this BEP were reported. Our results clearly show the presence of an optimum arsenic BEP in .
Superlattices and Microstructures, Vol. 23, No. 2, 1998 Arsenic Beam Equivalent Pressure (× 10–5 Torr) 1.4 2.8 4.2 5.6 7.0
400
Raman Frequency (cm–1)
539
350
8.4
AlAs-Like
300
InAs-Like
250
200
0
50
100
150 200 V/III Flux Ratio
250
300
Fig. 4. Plot of Raman frequency for both AlAs-like and InAs-like phonon modes as a function of V/III flux ratio showing a gradual reduction in the frequency as the flux ratio is increased.
InAlAs growth, the exact value of which would probably depend on growth conditions such as the substrate temperature and film composition. The Raman spectra of the samples show clear InAs-like and AlAs-like LO phonon modes at ∼234 cm−1 and ∼370 cm−1 , respectively. No symmetry forbidden phonon modes were detected. As shown in Fig. 3, the linewidths of the InAs-like and AlAs-like phonon modes show a similar trend of variation as the PL and XRD linewidth data with both the linewidths reaching minimum values of 14 cm−1 and 18 cm−1 , respectively at a flux ratio of 150. Further increase in the flux ratio beyond 150 causes a significant increase in the linewidths suggesting a degradation in the structural quality of the films, consistent with the PL and XRD results discussed earlier. Increase in the V/III flux ratio has a tendency to cause a reduction in the Raman frequency of the InAs-like and AlAs-like phonon modes as shown in Fig. 4. In both the phonon modes, the shift to lower frequencies was gradual initially, but became more prominent at flux ratios exceeding 150, in coincidence with the broadening of the phonon lineshapes as seen in Fig. 3. Tiong et al. [16] have previously reported a significant shift to lower frequencies coupled with an asymmetric broadening of the LO phonon modes in GaAs samples which were subjected to heavy As+ implantation. This effect was attributed to the generation of disordered regions in the material resulting from the implant. A similar reduction in the frequencies of the LO phonon modes in silicon-doped InAlAs samples was observed at increasing silicon concentration [17], an effect attributed to the presence of doping-induced disorder in the material. Applying this observations qualitatively to our results suggest that the reduction in the Raman frequencies of the LO phonon modes can possibly be due to the presence of disorder in the material, an effect which could have arisen from the clustering due to the overly high arsenic overpressure during growth. However, the actual atomistic nature of this disorder is presently unknown and warrants further investigation. Figures 5, 6 and 7 show the variation of the PL spectrum as a function of temperature for InAlAs samples grown at three different V/III flux ratios (V/III = 50, 210 and 280). The behaviour of the PL emission with increase in temperature suggests that it arises from a combination of more than one transition mechanism. In all the three samples, the PL spectrum at 5 K has a Gaussian-like lineshape which is asymmetrically broadened at the low-energy side. A low-energy shoulder was clearly evident in the 5 K spectrum of the samples grown at V/III ratio of 210 and 280, respectively. In the sample grown at V/III ratio = 280, the intensity of this .
.
540
Superlattices and Microstructures, Vol. 23, No. 2, 1998 1600 V/III = 50
5K 35 K (× 3) 55 K (× 6) 75 K (× 8) 95 K (× 12) 135 K (× 19) 155 K (× 25)
PL Intensity (a.u.)
1300 1000 700 400 100 0
1.40
1.45
1.50 1.55 Photon Energy (eV)
1.60
1.65
Fig. 5. PL spectrum at different temperatures for sample grown at V/III flux ratio of 50 showing the evolution of a broad low-energy emission when the temperature is increased.
V/III = 210
5K 35 K (× 4) 75 K (× 10) 95 K (× 13) 135 K (× 20) 155 K (× 26)
PL Intensity (a.u.)
2000
1400
800
0 1.40
1.45
1.50 1.55 Photon Energy (eV)
1.60
1.65
Fig. 6. PL spectrum at different temperatures for sample grown at V/III flux ratio of 210.
low-energy shoulder is almost as high as the main emission peak. Apart from this low-energy shoulder, the low-temperature PL emission seems to be dominated by exciton recombination and band-to-band transitions. As the temperature is increased, the main emission peak started to quench and a broad low-energy emission started to evolve in all the three samples, although this effect is most pronounced in the sample grown at V/III ratio of 50. It is possible that growth at low V/III flux ratios incorporates a greater number of arsenic (group V) vacancies. Hence at higher temperatures, the impurities become ionized and the evolution of the PL spectrum suggests that the emission could be dominated by free-to-bound or bound-to-bound donor-to-acceptor-like transitions. This is supported by the fact that samples prepared at low V/III flux ratios were observed to have a lower electron concentration as seen in Fig. 8. The electron concentration increases as the V/III ratio is
Superlattices and Microstructures, Vol. 23, No. 2, 1998 5K 35 K (× 3)
V/III = 280
PL Intensity (a.u.)
541
1600
55 K (× 7) 75 K (× 9) 95 K (× 11)
1100
115 K (× 17) 150 K (× 27)
600
0 1.40
1.45
1.50 1.55 Photon Energy (eV)
1.60
Fig. 7. PL spectrum at different temperatures for sample grown at V/III flux ratio of 280.
Arsenic Beam Equivalent Pressure (× 10–5 Torr) 5.6 7.0 1.4 2.8 4.2
1200
25
20 800 15
600 400
10 200 0
0
50
100
150 200 V/III Flux Ratio
250
300
Electron Concentration (× 1016 cm–3)
T=77 K
1000 Hall Mobility (cm2/V.s)
8.4
5
Fig. 8. Plot of Hall mobility and electron concentration at 77 K as a function of V/III flux ratio.
increased suggesting that samples prepared at high flux ratios have a higher number of group III vacancies. Figure 8 also shows a corresponding decrease in the electron mobility as the flux ratio is increased. The rapid quenching of the main PL emission peak and the evolution of a broad low-energy emission as the temperature is increased could also suggest the presence of deep-lying centres, but their exact nature is uncertain at this moment.
4. Conclusions In conclusion, the growth of In0.52 Al0.48 As on InP at a wide range of arsenic overpressures (V/III ratio
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Superlattices and Microstructures, Vol. 23, No. 2, 1998
from 30 to 300) is reported in this paper. Samples grown under our growth conditions exhibit a minimum point in both the PL and XRD linewidths and in the InAs-like and AlAs-like LO phonon linewidths at a V/III ratio of 150. This minimum point corresponds to a maximum in the PL intensity and XRD intensity ratio. The LO phonon frequencies also exhibit a gradual reduction as the flux ratio is increased, an effect due probably to the presence of disorder in the material. The results suggest a degradation in the quality of films grown at excessively high arsenic overpressure due possibly to clustering into In- and Al-rich regions. This, however, has been observed to have no effect on the lattice mismatch between the epilayer and the substrate. PL spectrum from samples taken at different temperatures show a rapid quenching of the main emission peak followed by an evolution of a broad low-energy emission at higher temperature suggesting the presence of deep-lying centres possibly related to donor-to-acceptor-like transitions. This effect is more pronounced in samples grown at low V/III flux ratio. Hall effect measurements showed an increase in the electron concentration and a corresponding decrease in the mobility in samples grown at increasing V/III flux ratio.
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