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Properties of InAsSb films grown on GaSb by metal-organic chemical vapor deposition
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Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
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www.elsevier.com/locate/procedia
Procedia Engineering 215 (2017) 24–30
9th International International Conference Conference on on Materials Materials for for Advanced Advanced Technologies Technologies(ICMAT (ICMAT2017) 2017) 9th
Properties of InAsSb films grown on GaSb by metal-organic chemical vapor deposition Pei-Nan Nia, Jin-Chao Tonga, Zheng-Ji Xua, Xiao-Hong Tanga, Dao-Hua Zhanga* OPTIMUS, School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore.
Abstract We report high quality InAsSb films grown on GaSb substrates using metal-organic chemical vapor deposition (MOCVD) technique through interface engineering. This work shows that the interface quality of the hetero-epitaxial InAsSb layer strongly depends on the V/III ratio during the growth and it plays a crucial role in the surface morphology, composition, and electrical and optical properties of InAsSb films. Furthermore, by properly engineering the interface conditions, high optical quality InAsSb films which show obvious mid-infrared emissions even at room temperature have been obtained. Therefore, this work provides a simple and feasible method for the growth of high quality InAsSb film which can be used for room temperature mid-infrared optoelectronics. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. under responsibility of the scientific committee Symposium 2017 ICMAT. Selection Selection and/or and/or peer-review peer-review under responsibility of the scientific committee of of Symposium 2017 ICMAT. Keywords: Semiconductor; infrared; InAsSb; MOCVD
1. Introduction Infrared optoelectronic devices which can operate at room temperature are in high demand due to their wide applications. Currently, the popular materials for the infrared devices are usually based on compound semiconductors.[1-8] InAsSb alloy has long been recognized as an attractive material for applications in infrared optoelectronic devices, especially in photodetetors, due to its tunable band-gap energy, which can cover the range from 0.1 eV to 0.35 eV, corresponding to the cut-off wavelength range from 12.4 μm to 3.54 μm.[9-12] This broad wavelength range nearly covers the two most important atmospheric infrared windows, i.e., 3-5 and 8-12 μm. Photodetectors that can work within these wavelength ranges are highly desired, because they have variety of
* Corresponding author. Tel.: +65 6790 4841 E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. 10.1016/j.proeng.2017.09.828
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applications in gas analysis for pollutant monitoring (i.e., CO2, CH4, N2O, O3, and CO gas), medical diagnostics, thermal imaging, and so on.[13-16] Compared with the HgCdTe alloy, which is currently the most widely used semiconductor material for infrared photodetectors, InAsSb material offers a number of advantages, such as better uniformity and stability over large area, higher electron and hole mobility.[17,18] In particular, InAsSb has a much smaller Auger coefficient than the HgCdTe alloy, which makes this material a promising candidate for the development of middle infrared devices operating at room temperature.[19,20] So far, great efforts have been paid to the growth of InAsSb alloy using variety of techniques, such as liquid phase epitaxy (LPE), molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD).[21-24] Among those different epitaxy growth techniques, MOCVD offers many advantages, such as short downtimes, high throughput and availability to large scale growth. Furthermore, the process safety of the MOCVD growth of InAsSb films can be improved by using organo-arsine, such as tertiarybutylarsine (TBAs) for replacing the highly toxic hydride arsine (AsH 3) in the MOCVD growth. As a member of the 6.1 Å family, GaSb is a promising substrate for the growth of InAsSb as it has a closer lattice constant. However, the quality of the currently commercial available GaSb substrates still cannot meet the technical requirements for the “epi-ready” substrates, due to the non-optimized oxide layer as well as the large amount of macroscopic defects present on the surface. In this work, we show that by adopting appropriate substrate etching process and pretreatment as well as carefully controlling the interface conditions, mirror-like InAsSb films can been grown on GaSb substrates via MOCVD technique using TBAs as the As-precursor. The effects of interface conditions on the properties of the InAsSb films have been studied in details. It is found that the Ⅴ/Ⅲ ratio is a very important parameter in the MOCVD growth of InAsSb, as it will strongly affect the interface quality of heteroepitaxial InAsSb layer. 2. Experimental details All the InAsSb samples were grown using a horizontal low pressure MOCVD system (Aixtron, AIX200). The metal-organic sources used in the growth were trimethylindium (TMIn), tertiarybutylarsine (TBAs), and trimethylantimony (TMSb). High purity (99.999%) hydrogen gas was used as the carrier gas. Te-doped GaSb (001) substrates were used. Before loaded into the MOCVD reactor, the GaSb substrate was etched in 37% HCl to remove the oxidation layer onto its surface. Before starting the growth of InAsSb, the GaSb substrate was firstly annealed inside the MOCVD reactor under the H2 environment at 600 oC for 10 min. TMSb sources were introduced into the reactor in order to protect the GaSb substrates from decomposing when the temperature is larger than 300 oC. Then the substrate temperature was lowered to 500 oC and then a 600 nm thick GaSb buffer layer was grown on GaSb substrates. After that, 800 nm thick InAsSb films with different Ⅴ/Ⅲ ratios were grown at 500 oC. During the growth, the pressure of the MOCVD reactor was kept at 100 mBar. The total H2 flow during the whole process was kept at 3 slm. The surface morphologies of InAsSb films were studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The composition of the InAsSb films was determined by the X-ray diffraction (XRD) measurements. The electrical properties of the InAsSb films were obtained by Hall measurements using the Van der Pauw method at 300 K. The photoluminescence (PL) measurements were carried out under the excitation of an Nd: YAG laser with peak wavelength at 1064 nm. A compressor with continuous helium flow is used to realize low temperatures. A liquid-nitrogen cooled MCT detector and lock-in amplifier were used to detect the PL signal. 3. Results and Discussion It has been widely recognized that the use of a high Ⅴ/Ⅲ ratio is usually favorable for better surface morphology during the MOCVD growth of Ⅲ-Ⅴ semiconductor materials.[25,26] However, for the case of MOCVD growth of InAsSb alloy, the Ⅴ/Ⅲ ratio needs to be carefully optimized and a large Ⅴ/Ⅲ ratio may instead lead to poor surface morphology. This is because the interface quality between the InAsSb layer and the GaSb substrate strongly depends on the V/III ratio. In order to study the influence of the V/III ratio on the interface quality, InAsSb films were grown under different Ⅴ/Ⅲ ratios. Fig. 1 shows the SEM images of the InAsSb films grown under different Ⅴ/Ⅲ ratios. It can be seen that the Ⅴ/Ⅲ ratios strongly effect on the surface morphology of the InAsSb films. Under
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a relative small Ⅴ/Ⅲ ratio of 6, there are many large hillocks on the surface of InAsSb film. When the Ⅴ/Ⅲ ratio is increased to 7.5, mirror-like surface can be obtained. However, it is found that the surface morphology of the InAsSb films will deteriorate when further increasing the Ⅴ/Ⅲ ratio to larger than 7.5 during our experiments.
Fig. 1. SEM images of the InAsSb films grown at Ⅴ/Ⅲratios of 6, 7.5, 9 and 11.
Fig. 2. AFM images of the InAsSb films grown at different Ⅴ/Ⅲratios.
The roughness of the InAsSb films with different Ⅴ/Ⅲ ratios can be evaluated by AFM images (scan area 2×2 μm2), as shown in Fig. 2. The root mean square (RMS) roughness of the InAsSb films grown under the Ⅴ/Ⅲ ratio of 6 is 8.741 nm. The InAsSb film with the Ⅴ/Ⅲ ratio of 7.5 has the smallest RMS roughness (4.423 nm), and the RMS of the InAsSb film will increase to 5.796 nm under the Ⅴ/Ⅲ ratio of 9. Further increase the Ⅴ/Ⅲ ratio to 11, the surface of the InAsSb film will become even rougher with the RMS of 7.449 nm. The strong dependence of the roughness on the V/III ratio further confirms the importance of interface to grow high quality InAsSb films.
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The reasons why the interface conditions of the InAsSb films are so sensitive to the Ⅴ/Ⅲ ratio can be understood as follows: during the MOCVD growth of InAsSb films, arsenic is preferentially incorporated into the film due to the facts that the reaction equilibrium constant of InAs (KInAs) is larger than that of InSb (KInSb).[27,28] Therefore, if the Ⅴ/Ⅲ ratio is too high, the antimony will be excess and rejected from the solid. Since the vapor pressure of the antimony is very low, any excess antimony will concentrate and float on the surface of InAsSb films, deteriorating the interface quality greatly.[27] Therefore, the surface morphology of the InAsSb films became poor when the Ⅴ/Ⅲ ratio was larger than 7.5 in our experiments due to the excess antimony during the growth under such Ⅴ/Ⅲ ratio. In order to confirm this speculation, XRD measurements have been performed onto the InAsSb films to determine the composition of the antimony in the as-grown InAsSb films, as shown in Fig. 3. It can be seen that the position of the diffraction peak (004) of the InAsSb film shifts towards the small-angle side when increase the Ⅴ/Ⅲ ratio from 6 to 7.5. However, when further increasing the Ⅴ/Ⅲ ratio, the diffraction peak (004) of the InAsSb films slightly moves towards the large-angle side, which indicates the composition of InAs in the InAsSb films increases when increasing the Ⅴ/Ⅲ ratio to larger than 7.5. The compositions of antimony in the InAsSb films determined by the XRD results are 4.53%, 5.54% , 4.33%, and 3.18% for the Ⅴ/Ⅲ ratio of 6, 7.5, 9, and 11, respectively. As discussed above, since the arsenic is much easier to be incorporated into the InAsSb film than the antimony due to the different reaction equilibrium constants of InAs and InSb, the composition of the InAs in the InAsSb films will increase if theⅤ/Ⅲ ratio is too high. Consequently, less antimony atoms could enter into the crystal lattice of InAsSb films, leading to a lower composition of antimony in the InAsSb films when the Ⅴ/Ⅲ ratio is larger than 7.5. Meanwhile, the excess antimony will condense and float on the surface of InAsSb film due to the low vapor pressure of antimony. As a result, the interface quality between the InAsSb and the GaSb substrate became poor when the Ⅴ/Ⅲ ratio is larger than 7.5 during our experiments, which in turn deteriorates the surface morphology of the grown InAsSb films dramatically.
Fig. 3. XRD spectra of the InAsSb films grown at the Ⅴ/Ⅲratio of 6, 7.5, 9, and 11.
Furthermore, Hall measurements have been performed on the InAsSb films which have been grown on semiinsulating GaAs (001) substrates. The effect of the Ⅴ/Ⅲ ratios on the electrical properties of the InAsSb films can be examined through the Hall measurements, as shown in Fig. 4. It can be seen that the as-grown InAsSb films in our
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experiments show n-type conductivity. Moreover, both the electron concentration and the electron mobility of the InAsSb films exhibit strong dependence onto the Ⅴ/Ⅲ ratios during the MOCVD growth. InAsSb films with higher electron mobility and lower electron concentration can be obtained when the Ⅴ/Ⅲ ratio is close to 7.5. It is well known that the mobility of the free carriers in the epitaxial layer has a strong relation with its crystalline quality as well as the interface quality due to the facts that the crystalline defects (for example, point defects and the residual misfit dislocations) and the interface disorders might act as scattering centers and reduce the carrier mobility.[28,29] Therefore, the better electrical qualities of the InAsSb film grown when the Ⅴ/Ⅲ ratio is close to 7.5 indicates the InAsSb films grown under this condition have better crystalline and interface qualities. This can be understood as follow: under a higher V/III ratio, some of the antimony atoms will be rejected from the solid during the beginning stages of the growth. Meanwhile, due to the very low vapor pressure of the antimony, any rejected antimony atoms will concentrate and float on the surface of InAsSb films, deteriorating the crystalline and interface qualities greatly.[30] The above electrical measurement further confirm the importance of optimization of V/III ratio in order to obtain high quality InAsSb films.
Fig. 4. Electron concentration and mobility of the InAsSb films under different Ⅴ/Ⅲratios.
Fig. 5. (a) The low temperature (10K) PL spectra of the InAsSb films grown under different V/III ratios (b) the room temperature PL spectra of the InAsSb films grown under the V/III ratio of 7.5.
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PL measurements both at low temperature and room temperature have been carried out on the InAsSb films grown under different V/III ratios to investigate the effect of the interface conditions on the optical quality. It is found that the InAsSb film grown under the V/III ratio of 7.5 has much stronger emissions around 3300 nm, which can be attributed to the band-gap emissions of the InAsSb film, than the other InAsSb films. Meanwhile, the full width at half maximum (FWHM) of the emissions is much smaller from the InAsSb film grown under the V/III ratio of 7.5, as shown in Fig. 5. Furthermore, obvious emissions can be observed from the InAsSb film grown under the V/III ratio of 7.5 even at room temperature. The above facts show that the InAsSb film grown under the V/III ratio of 7.5 has the best optical quality, indicating that good interface quality is favorable for improving the optical quality. 4. Conclusions The influence of the interface conditions on the properties of the hetero-epitaxial InAsSb layer has been studied in this work. Our results show that the interface quality strongly depends on the V/III ratio during the MOCVD growth and it directly affects the surface morphology, composition, and electrical and optical properties of InAsSb films. By properly engineering the interface condition, high quality InAsSb films which show obvious mid-infrared emissions even at room temperature have been obtained. This work provides a simple and feasible strategy for the growth of high quality InAsSb film which can be used for room temperature mid-infrared optoelectronics. Acknowledgements This work is supported by the Economic Development Board (NRF2013SAS-SRP001-019), the Ministry of Education (RG86/13), A*Star (1220703063), Singapore and Asian Office of Aerospace Research and Development (FA2386-17-1-0039). References [1] A. Rogalski, Recent progress in Quantum Electronics, Infrared Physics & Technology 54 (2011) 136-154. [2] W. Shi, D.H. Zhang, H.Q. Zheng, S.F. Yoon, C.H. Kam, A. Raman, Effects of arsenic beam equivalent pressure on InGaAsP grown by solid source molecular beam epitaxy with continuous white phosphorous production, J. Cryst. Growth 197 (1999) 89-94. [3] D.H. Zhang, W. Liu, L. Sun, W.J. Fan, S.F. Yoon, S.Z. Wang, H.C. Liu, Transverse electric dominant intersubband absorption in Si-doped GaInAsN∕GaAsGaInAsN∕GaAs quantum wells, J. Appl. Phys. 99 (2006) 043514. [4] X.Z. Chen, D.H. Zhang, W. Liu, Y. Wang, J.H. Li, A.T.S. Wee, A. Ramam, InSbN based p-n junctions for infrared photodetection, Electronics Letters 46 (2010) 787-788. [5] D.H. Zhang, X.Z. Wang, H.Q. Zheng, S.F. Yoon, C.H. Kam, GaInAsP grown on GaAs substrate by solid source molecular beam epitaxy with a valve phosphorous cracker cell, J. Vac. Sci. & Technol. B 18 (2000) 2274-2278. [6] D.H. Zhang, W. Liu, Y. Wang, X.Z. Chen, J.H. Li, Z.M. Huang, S. Zhang, InSbN alloys prepared by two-step ion implantation for infrared photodetection, Appl. Phys. Lett. 93 (2008) 131107. [7] Y. Wang, D.H. Zhang, X.Z. Chen, Y.J. Jin, J.H. Li, C.J. Liu, A.T.S. Wee, S. Zhang, A. Ramam, Bonding and diffusion of nitrogen in the InSbN alloys fabricated by two-step ion implantation, Appl. Phys. Lett. 101 (2012) 021905. [8] D.H. Zhang, K. Radhakrishnan, S.F. Yoon, Characterization of beryllium doped molecular beam epitaxial grown GaAs by photoluminescence, J. Crystal Growth 148 (1995) 35-40. [9] S. Kim, M. Erdtmann, D. Wu, E. Kass, H. Yi, J. Diaz, M. Razeghi, Photoluminescence study of InAsSb/InAsSbP heterostructures grown by low-pressure metalorganic chemical vapor deposition, Appl. Phys. Lett. 69 (1996) 1614-1616. [10] W. Du, X. Yang, H. Pan, X. Wang, H. Ji, S. Luo, X. Ji, Z. Wang, T. Yang, Two Different Growth Mechanisms for Au Free InAsSb Nanowires Growth on Si Substrate, Cryst. Growth Des. 15 (2015) 2413-2418. [11] X. Marcadet, A. Rakovska, I. Prevot, G. Glastre, B. Vinter, V. Berger, MBE growth of room temperature InAsSb mid-infrared detectors, J. Crystal Growth 227-228 (2011) 609-613. [12] J.C. Tong, Y.Y. Xie, Z.J. Xu, S.P. Qiu, P.N. Ni, L.Y.M. Tobing, D.H. Zhang, Study of dual color infrared photodetection from n-GaSb/nInAsSb heterostructures, AIP Advances 6 (2016) 025120. [13] B.M. Borg, K.A. Dick, J. Eymery, L. Wernersson, Enhanced Sb incorporation in InAsSb nanowires grown by metalorganic vapor phase epitaxy, Appl. Phys. Lett. 98 (2011) 113104. [14] M. Carras, J.L. Reverchon, G. Marre, C. Renard, B. Vinter, X. Marcadet, V. Berger, Interface band gap engineering in InAsSb photodiodes, Appl. Phys. Lett. 87 (2010) 102103. [15] E.A. Anyebe, A.M. Sanchez, S. Hindmarsh, X. Chen, J. Shao, M.K. Rajpalke, T.D. Veal, B.J. Robinson, O. Kolosov, F. Anderson, R. Sundaram, Z.M. Wang, V. Falko, Q. Zhuang, Realization of Vertically Aligned, Ultrahigh Aspect Ratio InAsSb Nanowires on Graphite, Nano Lett. 15 (2015) 4348-4355.
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[16] Y.Z. Gao, H. Kan, F.S. Gao, X.Y. Gong, T. Yamaguchi, Improved purity of long-wavelength InAsSb epilayers grown by melt epitaxy in fused silica boats, J. Crystal Growth 234 (2002) 85-90. [17] E. Plis, S. Myers, M.N. Kutty, J. Mailfert, E.P. Smith, S. Johnson, S. Krishna, Lateral diffusion of minority carriers in InAsSb-based nBn detectors, Appl. Phys. Lett. 97 (2010) 123503. [18] J.D. Kim, S. Kim, D. Wu, J. Wojkowski, J. Xu, J. Piotrowski, E. Bigan, M. Razeghi, 8-13 μm InAsSb heterojunction photodiode operating at near room temperature, Appl. Phys. Lett. 67 (1995) 2645-2647. [19] H. Shao, W. Li, A. Torfi, D. Moscicka, W.I. Wang, Room temperature InAsSb photovoltaic detectors for mid-infrared applications, IEEE Photonics Technology Letters 18 (2006) 1756-1758. [20] A. Khoshakhlagh, S. Myers, E. Plis, M.N. Kutty, B. Klein, N. Gautam, H. Kim, E.P.G. Smith, D. Rhiger, S.M. Johnson, S. Krishna, Midwavelength InAsSb detectors based on nBn design, Proc. Of SPIE 7660 (2000) 76602Z. [21] A.S. Popov, A.M. Koinova, S.L. Tzeneva, The In-As-Sb phase diagram and LPE growth of InAsSb layers on InAs at extremely low temperatures, J. Crystal Growth 186 (1998) 338-343. [22] S. Elies, A. Krier, I.R. Cleverley, K. Singer, Photoluminescence of MBE-grown InAs1-xSbx lattice matched to GaSb, J. Phys. D: Appl. Phys. 26 (1993) 159-162. [23] A. Rakovska, V. Berger, X. Marcadet, B. Vinter, G. Glastre, T. Oksenhendler, D. Kaplan, Room temperature InAsSb photovoltaic midinfrared detector, Appl. Phys. Lett. 77 (2000) 397-399. [24] J.D. Kim, D. Wu, J. Wojkowski, J. Piotrowski, J. Xu, M. Razeghi, Long wavelength InAsSb photoconductors operated at near room temperatures (200-300 K), Appl. Phys. Lett. 68 (1996) 99-101. [25] W.T. Liao, J.R. Gong, S.W. Lin, C.L. Wang, T.Y. Lin, K.C. Chen, Y.C.Cheng, W.J. Lin, Growth of AlGaN and GaN films on (11-20) Al2O3 substrates and the influence of V/III ratio on the properties of GaN films, J. Crystal Growth 286 (2006) 28-31. [26] M.T. Borgström, J. Wallentin, J. Trägårdh, P. Ramvall, M. Ek, L.R. Wallenberg, L. Samuelson, K. Deppert, In Situ Etching for Total Control Over Axial and Radial Nanowire Growth, Nano Res 3 (2010) 264-270. [27] H. Seke, A. Koukitu, Solid composition of alloy semiconductors grown by MOVPE, MBE, VPE hand ALE, J. Crystal Growth 98 (1989) 118-126. [28] R.M. Biefeld, The metal-organic chemical vapor deposition and properties of Ⅲ-Ⅴ antimony-based semiconductor materials, Materials Science and Engineering 36 (2002) 105-142. [29] R.J. Egan, V.W.L. Chin, T.L. Tansley, Dislocation scattering effects on electron mobility in InAsSb, J. Appl. Phys. 75 (1994) 2473-2476. [30] H.Y. Deng, X.K. Hong, W.Z. Fang, N. Dai, Microstructure characterization of InAs0.93Sb0.07 films grown by ramp-cooled liquid phase epitaxy, Mater. Charac. 58 (2007) 307-311.
ScienceDirect
Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Procedia Engineering 215 (2017) 24–30
9th International International Conference Conference on on Materials Materials for for Advanced Advanced Technologies Technologies(ICMAT (ICMAT2017) 2017) 9th
Properties of InAsSb films grown on GaSb by metal-organic chemical vapor deposition Pei-Nan Nia, Jin-Chao Tonga, Zheng-Ji Xua, Xiao-Hong Tanga, Dao-Hua Zhanga* OPTIMUS, School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore.
Abstract We report high quality InAsSb films grown on GaSb substrates using metal-organic chemical vapor deposition (MOCVD) technique through interface engineering. This work shows that the interface quality of the hetero-epitaxial InAsSb layer strongly depends on the V/III ratio during the growth and it plays a crucial role in the surface morphology, composition, and electrical and optical properties of InAsSb films. Furthermore, by properly engineering the interface conditions, high optical quality InAsSb films which show obvious mid-infrared emissions even at room temperature have been obtained. Therefore, this work provides a simple and feasible method for the growth of high quality InAsSb film which can be used for room temperature mid-infrared optoelectronics. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. under responsibility of the scientific committee Symposium 2017 ICMAT. Selection Selection and/or and/or peer-review peer-review under responsibility of the scientific committee of of Symposium 2017 ICMAT. Keywords: Semiconductor; infrared; InAsSb; MOCVD
1. Introduction Infrared optoelectronic devices which can operate at room temperature are in high demand due to their wide applications. Currently, the popular materials for the infrared devices are usually based on compound semiconductors.[1-8] InAsSb alloy has long been recognized as an attractive material for applications in infrared optoelectronic devices, especially in photodetetors, due to its tunable band-gap energy, which can cover the range from 0.1 eV to 0.35 eV, corresponding to the cut-off wavelength range from 12.4 μm to 3.54 μm.[9-12] This broad wavelength range nearly covers the two most important atmospheric infrared windows, i.e., 3-5 and 8-12 μm. Photodetectors that can work within these wavelength ranges are highly desired, because they have variety of
* Corresponding author. Tel.: +65 6790 4841 E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. 10.1016/j.proeng.2017.09.828
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applications in gas analysis for pollutant monitoring (i.e., CO2, CH4, N2O, O3, and CO gas), medical diagnostics, thermal imaging, and so on.[13-16] Compared with the HgCdTe alloy, which is currently the most widely used semiconductor material for infrared photodetectors, InAsSb material offers a number of advantages, such as better uniformity and stability over large area, higher electron and hole mobility.[17,18] In particular, InAsSb has a much smaller Auger coefficient than the HgCdTe alloy, which makes this material a promising candidate for the development of middle infrared devices operating at room temperature.[19,20] So far, great efforts have been paid to the growth of InAsSb alloy using variety of techniques, such as liquid phase epitaxy (LPE), molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD).[21-24] Among those different epitaxy growth techniques, MOCVD offers many advantages, such as short downtimes, high throughput and availability to large scale growth. Furthermore, the process safety of the MOCVD growth of InAsSb films can be improved by using organo-arsine, such as tertiarybutylarsine (TBAs) for replacing the highly toxic hydride arsine (AsH 3) in the MOCVD growth. As a member of the 6.1 Å family, GaSb is a promising substrate for the growth of InAsSb as it has a closer lattice constant. However, the quality of the currently commercial available GaSb substrates still cannot meet the technical requirements for the “epi-ready” substrates, due to the non-optimized oxide layer as well as the large amount of macroscopic defects present on the surface. In this work, we show that by adopting appropriate substrate etching process and pretreatment as well as carefully controlling the interface conditions, mirror-like InAsSb films can been grown on GaSb substrates via MOCVD technique using TBAs as the As-precursor. The effects of interface conditions on the properties of the InAsSb films have been studied in details. It is found that the Ⅴ/Ⅲ ratio is a very important parameter in the MOCVD growth of InAsSb, as it will strongly affect the interface quality of heteroepitaxial InAsSb layer. 2. Experimental details All the InAsSb samples were grown using a horizontal low pressure MOCVD system (Aixtron, AIX200). The metal-organic sources used in the growth were trimethylindium (TMIn), tertiarybutylarsine (TBAs), and trimethylantimony (TMSb). High purity (99.999%) hydrogen gas was used as the carrier gas. Te-doped GaSb (001) substrates were used. Before loaded into the MOCVD reactor, the GaSb substrate was etched in 37% HCl to remove the oxidation layer onto its surface. Before starting the growth of InAsSb, the GaSb substrate was firstly annealed inside the MOCVD reactor under the H2 environment at 600 oC for 10 min. TMSb sources were introduced into the reactor in order to protect the GaSb substrates from decomposing when the temperature is larger than 300 oC. Then the substrate temperature was lowered to 500 oC and then a 600 nm thick GaSb buffer layer was grown on GaSb substrates. After that, 800 nm thick InAsSb films with different Ⅴ/Ⅲ ratios were grown at 500 oC. During the growth, the pressure of the MOCVD reactor was kept at 100 mBar. The total H2 flow during the whole process was kept at 3 slm. The surface morphologies of InAsSb films were studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The composition of the InAsSb films was determined by the X-ray diffraction (XRD) measurements. The electrical properties of the InAsSb films were obtained by Hall measurements using the Van der Pauw method at 300 K. The photoluminescence (PL) measurements were carried out under the excitation of an Nd: YAG laser with peak wavelength at 1064 nm. A compressor with continuous helium flow is used to realize low temperatures. A liquid-nitrogen cooled MCT detector and lock-in amplifier were used to detect the PL signal. 3. Results and Discussion It has been widely recognized that the use of a high Ⅴ/Ⅲ ratio is usually favorable for better surface morphology during the MOCVD growth of Ⅲ-Ⅴ semiconductor materials.[25,26] However, for the case of MOCVD growth of InAsSb alloy, the Ⅴ/Ⅲ ratio needs to be carefully optimized and a large Ⅴ/Ⅲ ratio may instead lead to poor surface morphology. This is because the interface quality between the InAsSb layer and the GaSb substrate strongly depends on the V/III ratio. In order to study the influence of the V/III ratio on the interface quality, InAsSb films were grown under different Ⅴ/Ⅲ ratios. Fig. 1 shows the SEM images of the InAsSb films grown under different Ⅴ/Ⅲ ratios. It can be seen that the Ⅴ/Ⅲ ratios strongly effect on the surface morphology of the InAsSb films. Under
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a relative small Ⅴ/Ⅲ ratio of 6, there are many large hillocks on the surface of InAsSb film. When the Ⅴ/Ⅲ ratio is increased to 7.5, mirror-like surface can be obtained. However, it is found that the surface morphology of the InAsSb films will deteriorate when further increasing the Ⅴ/Ⅲ ratio to larger than 7.5 during our experiments.
Fig. 1. SEM images of the InAsSb films grown at Ⅴ/Ⅲratios of 6, 7.5, 9 and 11.
Fig. 2. AFM images of the InAsSb films grown at different Ⅴ/Ⅲratios.
The roughness of the InAsSb films with different Ⅴ/Ⅲ ratios can be evaluated by AFM images (scan area 2×2 μm2), as shown in Fig. 2. The root mean square (RMS) roughness of the InAsSb films grown under the Ⅴ/Ⅲ ratio of 6 is 8.741 nm. The InAsSb film with the Ⅴ/Ⅲ ratio of 7.5 has the smallest RMS roughness (4.423 nm), and the RMS of the InAsSb film will increase to 5.796 nm under the Ⅴ/Ⅲ ratio of 9. Further increase the Ⅴ/Ⅲ ratio to 11, the surface of the InAsSb film will become even rougher with the RMS of 7.449 nm. The strong dependence of the roughness on the V/III ratio further confirms the importance of interface to grow high quality InAsSb films.
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The reasons why the interface conditions of the InAsSb films are so sensitive to the Ⅴ/Ⅲ ratio can be understood as follows: during the MOCVD growth of InAsSb films, arsenic is preferentially incorporated into the film due to the facts that the reaction equilibrium constant of InAs (KInAs) is larger than that of InSb (KInSb).[27,28] Therefore, if the Ⅴ/Ⅲ ratio is too high, the antimony will be excess and rejected from the solid. Since the vapor pressure of the antimony is very low, any excess antimony will concentrate and float on the surface of InAsSb films, deteriorating the interface quality greatly.[27] Therefore, the surface morphology of the InAsSb films became poor when the Ⅴ/Ⅲ ratio was larger than 7.5 in our experiments due to the excess antimony during the growth under such Ⅴ/Ⅲ ratio. In order to confirm this speculation, XRD measurements have been performed onto the InAsSb films to determine the composition of the antimony in the as-grown InAsSb films, as shown in Fig. 3. It can be seen that the position of the diffraction peak (004) of the InAsSb film shifts towards the small-angle side when increase the Ⅴ/Ⅲ ratio from 6 to 7.5. However, when further increasing the Ⅴ/Ⅲ ratio, the diffraction peak (004) of the InAsSb films slightly moves towards the large-angle side, which indicates the composition of InAs in the InAsSb films increases when increasing the Ⅴ/Ⅲ ratio to larger than 7.5. The compositions of antimony in the InAsSb films determined by the XRD results are 4.53%, 5.54% , 4.33%, and 3.18% for the Ⅴ/Ⅲ ratio of 6, 7.5, 9, and 11, respectively. As discussed above, since the arsenic is much easier to be incorporated into the InAsSb film than the antimony due to the different reaction equilibrium constants of InAs and InSb, the composition of the InAs in the InAsSb films will increase if theⅤ/Ⅲ ratio is too high. Consequently, less antimony atoms could enter into the crystal lattice of InAsSb films, leading to a lower composition of antimony in the InAsSb films when the Ⅴ/Ⅲ ratio is larger than 7.5. Meanwhile, the excess antimony will condense and float on the surface of InAsSb film due to the low vapor pressure of antimony. As a result, the interface quality between the InAsSb and the GaSb substrate became poor when the Ⅴ/Ⅲ ratio is larger than 7.5 during our experiments, which in turn deteriorates the surface morphology of the grown InAsSb films dramatically.
Fig. 3. XRD spectra of the InAsSb films grown at the Ⅴ/Ⅲratio of 6, 7.5, 9, and 11.
Furthermore, Hall measurements have been performed on the InAsSb films which have been grown on semiinsulating GaAs (001) substrates. The effect of the Ⅴ/Ⅲ ratios on the electrical properties of the InAsSb films can be examined through the Hall measurements, as shown in Fig. 4. It can be seen that the as-grown InAsSb films in our
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experiments show n-type conductivity. Moreover, both the electron concentration and the electron mobility of the InAsSb films exhibit strong dependence onto the Ⅴ/Ⅲ ratios during the MOCVD growth. InAsSb films with higher electron mobility and lower electron concentration can be obtained when the Ⅴ/Ⅲ ratio is close to 7.5. It is well known that the mobility of the free carriers in the epitaxial layer has a strong relation with its crystalline quality as well as the interface quality due to the facts that the crystalline defects (for example, point defects and the residual misfit dislocations) and the interface disorders might act as scattering centers and reduce the carrier mobility.[28,29] Therefore, the better electrical qualities of the InAsSb film grown when the Ⅴ/Ⅲ ratio is close to 7.5 indicates the InAsSb films grown under this condition have better crystalline and interface qualities. This can be understood as follow: under a higher V/III ratio, some of the antimony atoms will be rejected from the solid during the beginning stages of the growth. Meanwhile, due to the very low vapor pressure of the antimony, any rejected antimony atoms will concentrate and float on the surface of InAsSb films, deteriorating the crystalline and interface qualities greatly.[30] The above electrical measurement further confirm the importance of optimization of V/III ratio in order to obtain high quality InAsSb films.
Fig. 4. Electron concentration and mobility of the InAsSb films under different Ⅴ/Ⅲratios.
Fig. 5. (a) The low temperature (10K) PL spectra of the InAsSb films grown under different V/III ratios (b) the room temperature PL spectra of the InAsSb films grown under the V/III ratio of 7.5.
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PL measurements both at low temperature and room temperature have been carried out on the InAsSb films grown under different V/III ratios to investigate the effect of the interface conditions on the optical quality. It is found that the InAsSb film grown under the V/III ratio of 7.5 has much stronger emissions around 3300 nm, which can be attributed to the band-gap emissions of the InAsSb film, than the other InAsSb films. Meanwhile, the full width at half maximum (FWHM) of the emissions is much smaller from the InAsSb film grown under the V/III ratio of 7.5, as shown in Fig. 5. Furthermore, obvious emissions can be observed from the InAsSb film grown under the V/III ratio of 7.5 even at room temperature. The above facts show that the InAsSb film grown under the V/III ratio of 7.5 has the best optical quality, indicating that good interface quality is favorable for improving the optical quality. 4. Conclusions The influence of the interface conditions on the properties of the hetero-epitaxial InAsSb layer has been studied in this work. Our results show that the interface quality strongly depends on the V/III ratio during the MOCVD growth and it directly affects the surface morphology, composition, and electrical and optical properties of InAsSb films. By properly engineering the interface condition, high quality InAsSb films which show obvious mid-infrared emissions even at room temperature have been obtained. This work provides a simple and feasible strategy for the growth of high quality InAsSb film which can be used for room temperature mid-infrared optoelectronics. Acknowledgements This work is supported by the Economic Development Board (NRF2013SAS-SRP001-019), the Ministry of Education (RG86/13), A*Star (1220703063), Singapore and Asian Office of Aerospace Research and Development (FA2386-17-1-0039). References [1] A. Rogalski, Recent progress in Quantum Electronics, Infrared Physics & Technology 54 (2011) 136-154. [2] W. Shi, D.H. Zhang, H.Q. Zheng, S.F. Yoon, C.H. Kam, A. Raman, Effects of arsenic beam equivalent pressure on InGaAsP grown by solid source molecular beam epitaxy with continuous white phosphorous production, J. Cryst. Growth 197 (1999) 89-94. [3] D.H. Zhang, W. Liu, L. Sun, W.J. Fan, S.F. Yoon, S.Z. Wang, H.C. Liu, Transverse electric dominant intersubband absorption in Si-doped GaInAsN∕GaAsGaInAsN∕GaAs quantum wells, J. Appl. Phys. 99 (2006) 043514. [4] X.Z. Chen, D.H. Zhang, W. Liu, Y. Wang, J.H. Li, A.T.S. Wee, A. Ramam, InSbN based p-n junctions for infrared photodetection, Electronics Letters 46 (2010) 787-788. [5] D.H. Zhang, X.Z. Wang, H.Q. Zheng, S.F. Yoon, C.H. Kam, GaInAsP grown on GaAs substrate by solid source molecular beam epitaxy with a valve phosphorous cracker cell, J. Vac. Sci. & Technol. B 18 (2000) 2274-2278. [6] D.H. Zhang, W. Liu, Y. Wang, X.Z. Chen, J.H. Li, Z.M. Huang, S. Zhang, InSbN alloys prepared by two-step ion implantation for infrared photodetection, Appl. Phys. Lett. 93 (2008) 131107. [7] Y. Wang, D.H. Zhang, X.Z. Chen, Y.J. Jin, J.H. Li, C.J. Liu, A.T.S. Wee, S. Zhang, A. Ramam, Bonding and diffusion of nitrogen in the InSbN alloys fabricated by two-step ion implantation, Appl. Phys. Lett. 101 (2012) 021905. [8] D.H. Zhang, K. Radhakrishnan, S.F. Yoon, Characterization of beryllium doped molecular beam epitaxial grown GaAs by photoluminescence, J. Crystal Growth 148 (1995) 35-40. [9] S. Kim, M. Erdtmann, D. Wu, E. Kass, H. Yi, J. Diaz, M. Razeghi, Photoluminescence study of InAsSb/InAsSbP heterostructures grown by low-pressure metalorganic chemical vapor deposition, Appl. Phys. Lett. 69 (1996) 1614-1616. [10] W. Du, X. Yang, H. Pan, X. Wang, H. Ji, S. Luo, X. Ji, Z. Wang, T. Yang, Two Different Growth Mechanisms for Au Free InAsSb Nanowires Growth on Si Substrate, Cryst. Growth Des. 15 (2015) 2413-2418. [11] X. Marcadet, A. Rakovska, I. Prevot, G. Glastre, B. Vinter, V. Berger, MBE growth of room temperature InAsSb mid-infrared detectors, J. Crystal Growth 227-228 (2011) 609-613. [12] J.C. Tong, Y.Y. Xie, Z.J. Xu, S.P. Qiu, P.N. Ni, L.Y.M. Tobing, D.H. Zhang, Study of dual color infrared photodetection from n-GaSb/nInAsSb heterostructures, AIP Advances 6 (2016) 025120. [13] B.M. Borg, K.A. Dick, J. Eymery, L. Wernersson, Enhanced Sb incorporation in InAsSb nanowires grown by metalorganic vapor phase epitaxy, Appl. Phys. Lett. 98 (2011) 113104. [14] M. Carras, J.L. Reverchon, G. Marre, C. Renard, B. Vinter, X. Marcadet, V. Berger, Interface band gap engineering in InAsSb photodiodes, Appl. Phys. Lett. 87 (2010) 102103. [15] E.A. Anyebe, A.M. Sanchez, S. Hindmarsh, X. Chen, J. Shao, M.K. Rajpalke, T.D. Veal, B.J. Robinson, O. Kolosov, F. Anderson, R. Sundaram, Z.M. Wang, V. Falko, Q. Zhuang, Realization of Vertically Aligned, Ultrahigh Aspect Ratio InAsSb Nanowires on Graphite, Nano Lett. 15 (2015) 4348-4355.
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