- Email: [email protected]
Optical band-gap of InSb0.97Bi0.03 thin films
Vacuum 154 (2018) 49–51
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Optical band-gap of InSb0.97Bi0.03 thin films
T
∗
Ketan Chaudhari, P.H. Soni , Ashwini Mahadik Department of Physics, Faculty of Science, The MaharajaSayajirao University of Baroda, Vadodara, 390002, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Absorbance Band-gap Film thickness Size effect
Nowadays, there has been extensive interest shown in infrared detectors operating in the 8–12 μm wavelength region wherein minimum atmospheric absorption is present. The more developed III-V materials are preferred to II-VI compounds. InSb, among the III-V semiconductors, has sufficiently narrow band-gap suitable in this regard. InSb crystallizes into the zinc-blende structure. However, the band-gap of InSb is considerably wide for Low Wavelength IR applications. The band-gap can be reduced by incorporation of Bi atoms into host InSb. Thin films of InSb0.97Bi0.03 were grown at room temperature on (001) NaCl crystal substrates under a pressure of 10−5 Pa using the thermal evaporation technique. The films obtained were characterized using FTIR absorption spectra. From the absorption data analysis, the band-gap is determined. It has been observed that Bi doping reduces the band-gap of InSb by a fair degree and, with film thickness the band-gap is found to change too.
1. Introduction
2. Experimental details
Narrow band-gap semiconductors play a major role in development of detector technology. Long wavelength photodetectors are useful for many military and civilian applications. For infrared detectors operating in the 8–12 μm wavelength range, the group II-VI material HgCdTe has been the first choice but it has some drawbacks like thermal instability and poor compositional uniformity over a large area [1–5]. The significant difficulties in growing HgCdTe are due to the high vapour pressure of Hg. This also results into stoichiometric fluctuation of Hg in the grown crystals or films [6]. This needs to be replaced by some new thermally stable material with good compositional uniformity. In this regard, group III-V semiconductors exhibit very useful physical properties. Sb based semiconductors are very much investigated by researchers. InSb, among the III-V semiconductors, has sufficiently narrow band-gap suitable for the said application. It crystallizes into the zinc-blende structure. It has a band-gap of 0.18 eV and its melting point is ≈ 527 °C. Doping InSb with Bi, the band-gap can be reduced by a fair margin. Bismuth (Bi) is the heaviest and the largest group V element. It has solubility up to 2.6 at% in InSb with a favourable phase diagram [7,8]. It has isoelectronic nature which induces strong interactions with the host material. This interaction results in a large reduction of the band-gap [9–12]. Thin films of InSb0.97Bi0.03 can be used for various applications. Hence the band-gap of InSb0.97Bi0.03 thin films was taken up for this study. In this paper we discuss film thickness dependence of band-gap of InSb0.97Bi0.03 thin films.
The material synthesis was carried out using mixtures of the respective elements (5 N purity) in stoichiometric proportion which was vacuum-sealed under residual air pressure, ∼10−5Pa, in a quartz ampoule. The ampoule was inserted in a furnace that provides rotation and rocking of the ampoule-ingot at 577 °C, which is well above the melting point. The molten charge was gradually cooled to room temperature over a period of two days after 48 h of thorough alloy mixing. Thin films, of various thicknesses, were deposited over (001) oriented NaCl substrates using thermal evaporation under a pressure of 10−5 Pa at room temperature, i.e., 30 °C. The vacuum coating equipment used was Model 12A4 supplied by Hind Hivac, Bangalore. Film thickness was measured using the inbuilt digital thickness monitor. IR spectra were obtained using FTIR spectrophotometer of JASCO (Japan) make, Model-4100 while the X-ray diffraction data were obtained using Bruker D8 Advance X-ray Diffractometer using the as-deposited film samples on the NaCl substrates.
∗
3. Results and discussion The XRD plot obtained is shown in Fig. 1. The peaks observed at 23.76°, 63.12° and 71.90°correspond to (111), (331) and (422) InSb reflections, respectively, (ASTM-JCPDS card no. 73–1985). The peak at 31.84° corresponds to (020) InSb reflection (ASTM - JCPDS card no. 71–1309), whereas the 48.54° peak corresponds to (113) Bi reflection (ASTM - JCPDS card no. 02–0592).
Corresponding author. E-mail address: [email protected] (P.H. Soni).
https://doi.org/10.1016/j.vacuum.2018.04.051 Received 12 December 2017; Received in revised form 13 April 2018; Accepted 28 April 2018 Available online 30 April 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
Vacuum 154 (2018) 49–51
K. Chaudhari et al.
Fig. 1. X-ray diffractogram of thin film of InSb097Bi0.03. Fig. 3. Plot of Ez versus 1/t2: InSb0.97Bi0.03 thin film.
parameters, the typical example being the de Broglie wavelength of the charge carriers [14–16]. With film thickness falling in such a domain, the transverse quasi-momentum component of the carrier gets quantized and the electron/hole states assume quasi-discrete energy values. This results in corresponding increase in valence-conduction band separation by an amount Ez as per the equation quoted above. The value of effective mass as obtained using Eg versus 1/t2 plots in Fig. 3 (assuming electrons to be heavy) was found to be 18.402 × 10−4 mo, on average, mo being the electron rest mass. The de Broglie wavelength of holes, calculated using the Fermi energy to be half of the average bandgap, turns out to be about 107 nm, giving rise to quantum size effect in the films with thicknesses in the range used, viz., 500 Å to 1503 Å, particularly so for the smaller film thicknesses in the range. In the case of films obtained at different substrate temperatures, from room temperature to 413 K, the band-gap did not show any noticeable variation from the average value of 0.14 eV, the average being over the films of different thicknesses.
Fig. 2. Plot of (αhυ)2 versus hυ: InSb0.97Bi0.03 thin film (1.503 Å).
4. Conclusions
For the band-gap evaluation, FTIR spectra of the samples were taken in the wave number range 400 cm−1 to 4000 cm−1 with a resolution of 4 cm−1. The absorption coefficient α obtained as a function of photon energy hν was used to obtain plots of (αhν)2→hν for various film thicknesses. A typical plot for films of thickness of 1503 Å is shown in Fig. 2. The plot is linear in the region of strong absorption near the fundamental absorption edge, implying absorption through direct interband transition. The linear extrapolation of the absorption edge to the zero of the ordinate gives the corresponding band-gap. The average band-gap obtained has been found to be 0.143 eV. The error in the band-gap determination based on the straight line fitting to the absorption edge data is estimated to be at the most ± 10%. The band-gap variation with film thickness is found to follow the relation [13]:
Ez =
The band-gap of InSb0.97Bi0.03 thin film is on an average 0.143 eV (direct type). At smaller thicknesses the band-gaps are larger than the bulk values. The band-gap of InSb0.97Bi0.03 depends on film thickness. The film thickness dependence of the band-gap of InSb0.97Bi0.03 indicates the optical transition is governed by quantum size effect within the film thickness range. There is no significant effect, if any, of the deposition temperature on the band-gap. Acknowledgement The authors are grateful to the UGC, N. Delhi, for the support through UGC-SAP grants and the facilities created thereby.
ℏ2π 2 1 2m∗ t 2
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.vacuum.2018.04.051.
where m* is the effective mass of the charge carrier, t is the thickness of the film and Ez is the kinetic energy contribution due to carrier motion normal to the film plane. The band-gap variation with film thickness is explained to be a result of the quantum size effect that prevails in the size domain of around 100 nm and less. This effect is exhibited when certain physical properties of a solid shows dependence on its characteristic geometric dimensions when these dimensions become comparable to certain length
References [1] A. Rogalski, History of infrared detectors, Opto-Electron. Rev. 20 (3) (2012) 279. [2] M. Razeghia, Overview of antimonide based III-V semiconductor epitaxial layers and their applications at the center for quantum devices, Eur. Phys. J. Appl. Phys.
50
Vacuum 154 (2018) 49–51
K. Chaudhari et al.
[10] D.S. Maske, M. Joshi, D.B. Gadkari, Microhardness and electrical properties a bulk crystal grown from mixture of two different compositions of InSbBi bulk crystals, Int. J. Sci. and Res. Publ. 3 (2) (2013) ISSN 2250–3153. [11] P. Mohan, S.M. Babu, P. Santhanaraghavan, P. Ramasamy, Vertical Bridgman growth of InSb1-xBix crystals for LWIR applications, J. Mater. Sci. letters 20 (2001) 241–244. [12] M.K. Rajpalke, W.M. Linhart, K.M. Yu, M. Birkett, J. Alaria, J.J. Bomphrey, S. Sallis, L.F.J. Piper, T.S. Jones, M.J. Ashwins, T.D. Veal, Bi-induced band gap reduction in epitaxial InSbBi alloys, Appl. Phys. Lett. 105 (2014) 212101. [13] P.H. Soni, S.R. Bhavsar, C.F. Desai, Optical band gap of Sb0.2Bi1.8Te3 thin films, J. Mater. Sci. 38 (2003) 1931. [14] V. Damodardas, D. Karunakaran, Semiconducting behaviour of Ag2Te thin films and the dependence of band gap on thickness, J. Appl. Phys. 54 (1983) 5252. [15] K.L. Chopra, Thin Film Phenomena, McGraw Hill, New York (, 1969. [16] M. Singh, K.C. Bhahada, Y.K. Vijay, Variation of optical band-gap in obliquely deposited selenium thin films, Indian J. Pure Appl. Phys. 43 (2005) 129.
23 (2003) 149. [3] D.A. Miller, S.D. Smith, A. Johnston, Optical bistability and signal amplification in a semiconductor crystal: applications of new low-power nonlinear effects in InSb, App. Phy. Latters 35 (1976) 658. [4] S.G. Pandya, M. Kordesch, Characterization of InSb nanoparticles synthesized using inert gas condensation, Nano Res. Latter 10 (2015) 258. [5] A.A. Ebnalwaled, Evolution of growth and enhancement in power factor of InSb bulk crystal, J. Cryst. Growth 311 (2009) 4385. [6] P. Norton, HgCdTe infrared detectors, Opto-electron. Rev. 10 (3) (2002) 159. [7] Yuxin Song, Yi Gu, Jun Shao, Shumin Wang, “Dilute bismides for mid-IR application”, bismuth containing compounds, in: Handong Li, M. Zhiming (Eds.), Wang Springer Series in Materials Science, vol.186, 2013, p. 143. [8] S. Premila Mohan, P. Moorthy Babu, P. Santhanaraghavan, Ramasamy, Growth, phase analysis and mechanical properties of InSb1−xBix crystals, Mater. Chem. Phys. 66 (2000) 17. [9] J.J. Lee, J.D. Kim, M. Razeghi, Growth and characterization of InSbBi for long wavelength infrared photodetectors, App. Phy. Latters 70 (1997) 3266.
51
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Optical band-gap of InSb0.97Bi0.03 thin films
T
∗
Ketan Chaudhari, P.H. Soni , Ashwini Mahadik Department of Physics, Faculty of Science, The MaharajaSayajirao University of Baroda, Vadodara, 390002, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Absorbance Band-gap Film thickness Size effect
Nowadays, there has been extensive interest shown in infrared detectors operating in the 8–12 μm wavelength region wherein minimum atmospheric absorption is present. The more developed III-V materials are preferred to II-VI compounds. InSb, among the III-V semiconductors, has sufficiently narrow band-gap suitable in this regard. InSb crystallizes into the zinc-blende structure. However, the band-gap of InSb is considerably wide for Low Wavelength IR applications. The band-gap can be reduced by incorporation of Bi atoms into host InSb. Thin films of InSb0.97Bi0.03 were grown at room temperature on (001) NaCl crystal substrates under a pressure of 10−5 Pa using the thermal evaporation technique. The films obtained were characterized using FTIR absorption spectra. From the absorption data analysis, the band-gap is determined. It has been observed that Bi doping reduces the band-gap of InSb by a fair degree and, with film thickness the band-gap is found to change too.
1. Introduction
2. Experimental details
Narrow band-gap semiconductors play a major role in development of detector technology. Long wavelength photodetectors are useful for many military and civilian applications. For infrared detectors operating in the 8–12 μm wavelength range, the group II-VI material HgCdTe has been the first choice but it has some drawbacks like thermal instability and poor compositional uniformity over a large area [1–5]. The significant difficulties in growing HgCdTe are due to the high vapour pressure of Hg. This also results into stoichiometric fluctuation of Hg in the grown crystals or films [6]. This needs to be replaced by some new thermally stable material with good compositional uniformity. In this regard, group III-V semiconductors exhibit very useful physical properties. Sb based semiconductors are very much investigated by researchers. InSb, among the III-V semiconductors, has sufficiently narrow band-gap suitable for the said application. It crystallizes into the zinc-blende structure. It has a band-gap of 0.18 eV and its melting point is ≈ 527 °C. Doping InSb with Bi, the band-gap can be reduced by a fair margin. Bismuth (Bi) is the heaviest and the largest group V element. It has solubility up to 2.6 at% in InSb with a favourable phase diagram [7,8]. It has isoelectronic nature which induces strong interactions with the host material. This interaction results in a large reduction of the band-gap [9–12]. Thin films of InSb0.97Bi0.03 can be used for various applications. Hence the band-gap of InSb0.97Bi0.03 thin films was taken up for this study. In this paper we discuss film thickness dependence of band-gap of InSb0.97Bi0.03 thin films.
The material synthesis was carried out using mixtures of the respective elements (5 N purity) in stoichiometric proportion which was vacuum-sealed under residual air pressure, ∼10−5Pa, in a quartz ampoule. The ampoule was inserted in a furnace that provides rotation and rocking of the ampoule-ingot at 577 °C, which is well above the melting point. The molten charge was gradually cooled to room temperature over a period of two days after 48 h of thorough alloy mixing. Thin films, of various thicknesses, were deposited over (001) oriented NaCl substrates using thermal evaporation under a pressure of 10−5 Pa at room temperature, i.e., 30 °C. The vacuum coating equipment used was Model 12A4 supplied by Hind Hivac, Bangalore. Film thickness was measured using the inbuilt digital thickness monitor. IR spectra were obtained using FTIR spectrophotometer of JASCO (Japan) make, Model-4100 while the X-ray diffraction data were obtained using Bruker D8 Advance X-ray Diffractometer using the as-deposited film samples on the NaCl substrates.
∗
3. Results and discussion The XRD plot obtained is shown in Fig. 1. The peaks observed at 23.76°, 63.12° and 71.90°correspond to (111), (331) and (422) InSb reflections, respectively, (ASTM-JCPDS card no. 73–1985). The peak at 31.84° corresponds to (020) InSb reflection (ASTM - JCPDS card no. 71–1309), whereas the 48.54° peak corresponds to (113) Bi reflection (ASTM - JCPDS card no. 02–0592).
Corresponding author. E-mail address: [email protected] (P.H. Soni).
https://doi.org/10.1016/j.vacuum.2018.04.051 Received 12 December 2017; Received in revised form 13 April 2018; Accepted 28 April 2018 Available online 30 April 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
Vacuum 154 (2018) 49–51
K. Chaudhari et al.
Fig. 1. X-ray diffractogram of thin film of InSb097Bi0.03. Fig. 3. Plot of Ez versus 1/t2: InSb0.97Bi0.03 thin film.
parameters, the typical example being the de Broglie wavelength of the charge carriers [14–16]. With film thickness falling in such a domain, the transverse quasi-momentum component of the carrier gets quantized and the electron/hole states assume quasi-discrete energy values. This results in corresponding increase in valence-conduction band separation by an amount Ez as per the equation quoted above. The value of effective mass as obtained using Eg versus 1/t2 plots in Fig. 3 (assuming electrons to be heavy) was found to be 18.402 × 10−4 mo, on average, mo being the electron rest mass. The de Broglie wavelength of holes, calculated using the Fermi energy to be half of the average bandgap, turns out to be about 107 nm, giving rise to quantum size effect in the films with thicknesses in the range used, viz., 500 Å to 1503 Å, particularly so for the smaller film thicknesses in the range. In the case of films obtained at different substrate temperatures, from room temperature to 413 K, the band-gap did not show any noticeable variation from the average value of 0.14 eV, the average being over the films of different thicknesses.
Fig. 2. Plot of (αhυ)2 versus hυ: InSb0.97Bi0.03 thin film (1.503 Å).
4. Conclusions
For the band-gap evaluation, FTIR spectra of the samples were taken in the wave number range 400 cm−1 to 4000 cm−1 with a resolution of 4 cm−1. The absorption coefficient α obtained as a function of photon energy hν was used to obtain plots of (αhν)2→hν for various film thicknesses. A typical plot for films of thickness of 1503 Å is shown in Fig. 2. The plot is linear in the region of strong absorption near the fundamental absorption edge, implying absorption through direct interband transition. The linear extrapolation of the absorption edge to the zero of the ordinate gives the corresponding band-gap. The average band-gap obtained has been found to be 0.143 eV. The error in the band-gap determination based on the straight line fitting to the absorption edge data is estimated to be at the most ± 10%. The band-gap variation with film thickness is found to follow the relation [13]:
Ez =
The band-gap of InSb0.97Bi0.03 thin film is on an average 0.143 eV (direct type). At smaller thicknesses the band-gaps are larger than the bulk values. The band-gap of InSb0.97Bi0.03 depends on film thickness. The film thickness dependence of the band-gap of InSb0.97Bi0.03 indicates the optical transition is governed by quantum size effect within the film thickness range. There is no significant effect, if any, of the deposition temperature on the band-gap. Acknowledgement The authors are grateful to the UGC, N. Delhi, for the support through UGC-SAP grants and the facilities created thereby.
ℏ2π 2 1 2m∗ t 2
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.vacuum.2018.04.051.
where m* is the effective mass of the charge carrier, t is the thickness of the film and Ez is the kinetic energy contribution due to carrier motion normal to the film plane. The band-gap variation with film thickness is explained to be a result of the quantum size effect that prevails in the size domain of around 100 nm and less. This effect is exhibited when certain physical properties of a solid shows dependence on its characteristic geometric dimensions when these dimensions become comparable to certain length
References [1] A. Rogalski, History of infrared detectors, Opto-Electron. Rev. 20 (3) (2012) 279. [2] M. Razeghia, Overview of antimonide based III-V semiconductor epitaxial layers and their applications at the center for quantum devices, Eur. Phys. J. Appl. Phys.
50
Vacuum 154 (2018) 49–51
K. Chaudhari et al.
[10] D.S. Maske, M. Joshi, D.B. Gadkari, Microhardness and electrical properties a bulk crystal grown from mixture of two different compositions of InSbBi bulk crystals, Int. J. Sci. and Res. Publ. 3 (2) (2013) ISSN 2250–3153. [11] P. Mohan, S.M. Babu, P. Santhanaraghavan, P. Ramasamy, Vertical Bridgman growth of InSb1-xBix crystals for LWIR applications, J. Mater. Sci. letters 20 (2001) 241–244. [12] M.K. Rajpalke, W.M. Linhart, K.M. Yu, M. Birkett, J. Alaria, J.J. Bomphrey, S. Sallis, L.F.J. Piper, T.S. Jones, M.J. Ashwins, T.D. Veal, Bi-induced band gap reduction in epitaxial InSbBi alloys, Appl. Phys. Lett. 105 (2014) 212101. [13] P.H. Soni, S.R. Bhavsar, C.F. Desai, Optical band gap of Sb0.2Bi1.8Te3 thin films, J. Mater. Sci. 38 (2003) 1931. [14] V. Damodardas, D. Karunakaran, Semiconducting behaviour of Ag2Te thin films and the dependence of band gap on thickness, J. Appl. Phys. 54 (1983) 5252. [15] K.L. Chopra, Thin Film Phenomena, McGraw Hill, New York (, 1969. [16] M. Singh, K.C. Bhahada, Y.K. Vijay, Variation of optical band-gap in obliquely deposited selenium thin films, Indian J. Pure Appl. Phys. 43 (2005) 129.
23 (2003) 149. [3] D.A. Miller, S.D. Smith, A. Johnston, Optical bistability and signal amplification in a semiconductor crystal: applications of new low-power nonlinear effects in InSb, App. Phy. Latters 35 (1976) 658. [4] S.G. Pandya, M. Kordesch, Characterization of InSb nanoparticles synthesized using inert gas condensation, Nano Res. Latter 10 (2015) 258. [5] A.A. Ebnalwaled, Evolution of growth and enhancement in power factor of InSb bulk crystal, J. Cryst. Growth 311 (2009) 4385. [6] P. Norton, HgCdTe infrared detectors, Opto-electron. Rev. 10 (3) (2002) 159. [7] Yuxin Song, Yi Gu, Jun Shao, Shumin Wang, “Dilute bismides for mid-IR application”, bismuth containing compounds, in: Handong Li, M. Zhiming (Eds.), Wang Springer Series in Materials Science, vol.186, 2013, p. 143. [8] S. Premila Mohan, P. Moorthy Babu, P. Santhanaraghavan, Ramasamy, Growth, phase analysis and mechanical properties of InSb1−xBix crystals, Mater. Chem. Phys. 66 (2000) 17. [9] J.J. Lee, J.D. Kim, M. Razeghi, Growth and characterization of InSbBi for long wavelength infrared photodetectors, App. Phy. Latters 70 (1997) 3266.
51