In situ growth and ab initio optical characterizations of amorphous Ga3Se4 thin film: A new chalcogenide compound semiconductor thin film

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Scripta Materialia 69 (2013) 381–384 www.elsevier.com/locate/scriptamat

In situ growth and ab initio optical characterizations of amorphous Ga3Se4 thin film: A new chalcogenide compound semiconductor thin film M.M. Abdullah,a,b,c,⇑ Preeti Singh,a Mohd Hasmuddin,a G. Bhagavannarayanab and M.A. Wahaba a

Crystal Growth and XRD Lab, Department of Physics, Jamia Millia Islamia, New Delhi 110025, India CGC Section, Materials Characterization Division, National Physical Laboratory, New Delhi 110012, India c Promising Centre for Sensors and Electronic Devices (PCSED), Department of Physics, Faculty of Science and Arts, Najran University, PO Box 1988, Najran 11001, Saudi Arabia b

Received 19 March 2013; revised 13 May 2013; accepted 13 May 2013 Available online 18 May 2013

This paper reports the first systematic study of the structural and optical characterization of Ga3Se4 thin film. Powder X-ray diffractometery analysis revealed that the films are amorphous. Optical reflection and transmittance measurements have been done by UV–visible–near-infrared spectrophotometry, and the values of various optical constants, such as the lower cut-off wavelength (550 nm), optical band gap (2.14 eV), dispersive energy (6.75 eV), oscillator energy (2.86 eV), static refractive index (1.83) and static dielectric constant (3.35), have been estimated. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Semiconductor; Thin films; Thermal heating; X-ray diffraction (XRD); Optical properties

There has been a renaissance of interest in group III/VI (13/16) semiconductor materials because of their potential applications in various electronic and optoelectronic devices. In general, these compounds are used as precursors and contain direct bonds between metal and chalcogen [1]. The solid phases of these compounds have a broad diversity of structural types. A number of diverse stoichiometries has been found for the binary combinations of a group 13 metal (Ga, In, Tl) and a group 16 chalcogen (S, Se, Te). Several structural types have been found for these compounds, including a defect wurzite structure, a defect spinel and layered structures [2,3], while, in most of the cases, the corresponding thin films have been observed to be amorphous in nature. For example, singlecrystal and polycrystalline GaSe has a hexagonal structure [4,5], whereas the corresponding thin film was found to be amorphous [6]. This is possibly because of the disintegration of the compound semiconductor chalcogenide during the process of thermal evaporation. Thin films of III/VI materials have been prepared by various growth

⇑ Corresponding

author at: Promising Centre for Sensors and Electronic Devices (PCSED), Department of Physics, Faculty of Science and Arts, Najran University, PO Box 1988, Najran 11001, Saudi Arabia. Tel.: +966 551973452; e-mail: [email protected]

techniques and are widely used not only for optoelectronic and photovoltaic devices, but also as passivating layers for III–V devices [7]. Developments in the deposition of these materials have resulted in the invention of new and/or novel phases. These materials are known to exhibit a variety of electronic properties, including photoconduction, semimetallic behaviour and even low-temperature superconduction. The majority of III–VI materials are mid- to wide-bandgap semiconductors. The structural and optical properties of the thin films of most of the compounds of gallium and selenium are already well studied and reported [8,9]; however, the structural and optical properties of Ga3Se4 thin films have not yet been explored. Therefore, a systematic study of the structural and optical characterization of these films has been performed and is presented in this paper. Thin films of Ga3Se4 were deposited onto wellcleaned glass substrates by a thermal heating method. At ambient temperature, substrates were placed 15 cm above the source, with the surface being perpendicular to the vapor flux. Polycrystalline Ga3Se4 powder kept in a molybdenum boat was used as the thermal evaporation source material. The synthesis and characterization of polycrystalline Ga3Se4 semiconducting chalcogenide

1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2013.05.019

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Figure 1. (a) X-ray diffraction pattern and (b) typical FESEM images of a-Ga3Se4 thin film deposited at room temperature.

material has been reported in our earlier paper [10]. The pressure inside the chamber was maintained at 105 torr with the help of a high-vacuum system. The deposited films were found to be uniform and demonstrate very good adhesion to the substrate surface. The film stoichiometry was investigated by energy-dispersive X-ray analysis (Shimadzu EDX-GP). The film composition was found to be 42.957 at.% Ga and 57.043 at.% Se, showing that the sample is slightly Se2 deficient. This is plausible because Ga3Se4 is a compound semiconductor and therefore disintegrates slightly during the thermal evaporation process. Structural analysis of the as-deposited films was carried out by powder X-ray diffraction (XRD), using a PANalytical PW 1830 apparatus with monochromated ˚ ) at 35 kV and 30 mA, Cu Ka radiation (k = 1.54056 A scanned with a step size of 0.02° (2h s–1) for the angular 2h range 5°–70°. The obtained XRD spectrum (Fig. 1a) only shows a hump and does not show any sharp peak, which in turn confirms the amorphous nature of the sample. Similar results have also been observed for thin films of GaSe [6,11] and Ga2Se3 [12]. Figure 1b shows field emission scanning electron microscopy (FESEM) images of as-grown thin film. At low magnification (Fig. 1b, inset), the well-cleaned smooth surfaces of the film can be clearly seen, whereas at high magnification (Fig. 1b), the image exhibits particles of irregular shapes and sizes. The optical absorption measurements of thin films were recorded at room temperature in the wavelength region 400–900 nm using a JASCO V-570 UV–visible near-infrared spectrophotometer. The substrate absorption was corrected by introducing uncoated cleaned glass substrate into the reference beam. From the spectrum (Fig. 2a), it is observed that the lower cut-off wavelength for the as-deposited thin film is around 570 nm. Also, the spectrum does not show any absorption peak in the wavelength range 600–900 nm. Hence, the investigated compound may be useful for the fabrication of non-lin-

ear optical (NLO) devices in this wavelength region, as the absence of absorption peaks is the main requirement for the materials to show NLO properties [13,14]. Figure 2b shows the absorption coefficient (a) as a function of the photon energy (E = hm). According to Tauc [6,15], the absorption spectrum of amorphous semiconductors can be divided into three distinct regions: (1) a weak absorption tail, which originates from defects and impurities; (2) an exponential edge region, which is strongly related to the structural randomness of the system; and (3) a high absorption region, which is used to find the optical energy band gap. It is clear from Figure 2b that the absorption coefficient of amorphous Ga3Se4 thin films increases with increasing photon energy in the region 2.25–3.1 eV, while below 2.25 eV, it shows a long tail with two extra peaks located at 1.6 and 2.1 eV. The presence of the long tail and its associated peaks may be ascribed to the amorphous nature of the films and/or randomly distributed impurities in the film [6,16]. The spectrum shows the exponential edge region (as indicated in Fig. 2b with dotted lines) in the incident photon energy range of 2.25–2.5 eV. In this region, the value of absorption coefficient (a) increases from 6.0  103 to 1.6  104 cm1 and follows the relation [6,17]: E a ¼ a0 expð Þ ð1Þ Ee where ao is a constant, E (= hm) is the energy of incident photons and Ee is the width of the band tails of the localized states that describes the slope of the exponential edge region. To find the value of Ee, the semilogarithmic plot of “a” as a function of energy (E = hm) in the exponent photon energy region is plotted (Fig. 2b, inset). The slope of this straight-line plot provideds the value of Ee = 0.25 eV. In the high absorption region (2.5–3.1 eV), the value of a increases from 1.6  104 to 6.5  104 cm1 and can be represented by the relation [18]: n

aE ¼ AðE  Eg Þ

ð2Þ

where A is a constant which depends on the transition probability, Eg is the optical energy band gap and n is an index that differentiates the process of optical absorption. The index value of n corresponds to 2, 1/2, 3 or 3/2 for indirect allowed, direct allowed, indirect forbidden and direct forbidden transitions, respectively. The optical band gap (Eg) of the material is determined by extrapolating the plotted graph of (aE)1/n vs. E to the E-axis. In the present investigation, four graphs (Fig. 3a–d) have been plotted for all the four corresponding values of n

Fig. 2. (a) Optical absorption spectra for Ga3Se4 thin film. (b) Variation of absorption coefficient as a function of incident photon energy. The inset illustrates ln(a)–hm dependence.

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Fig. 3. Plots of (aE)1/n as a function of E (for n = 2, 1/2, 3 and 3/2) for Ga3Se4 thin films.

Fig. 4. (a) Optical transmission spectra of Ga3Se4 thin films. (b) Plot of refractive index (n) vs. wavelength (k). (c) Plots of (n2  1)1 vs. E2.

(2, ½, 3 and 3/2). Among these four graphs, Figure 3a demonstrates the best fit as it encompasses the widest range of experimental data. This confirms the dominance of indirect allowed transitions in amorphous Ga3Se4 thin films. In addition, the optical band gap of the Ga3Se4 was found to be about 2.14 eV by extrapolation of the linear part of the spectrum (ahm)1/2 = f(hm) to the hm-axis. Thus, Ga3Se4 is a semiconducting material with indirect optical band gap of 2.14 eV. Figure 4a shows the transmittance (T) spectrum of asdeposited amorphous Ga3Se4 thin film in the wavelength range 300–900 nm. The method suggested by Swanepoel [19] and Manifacier [20] was used to calculate the refractive index (n) of the film using the result obtained from the interference pattern. According to this method, an upper and lower envelope of the transmission spectrum beyond the absorption edge is created to ascertain the refractive index in the region where a approaches zero. In accordance with this method, the refractive index of the film can be explored by using the following equations: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n ¼ N þ N 2  S2 ð3Þ where N ¼ 2Sð

Tmax  Tmin S2 þ 1 Þþ 2 Tmax  Tmin

ð4Þ

where S is the refractive index of the glass substrate and is equal to 1.51. Tmax and Tmin are the corresponding values of transmittance for the upper and lower envelope (represented by the dotted line in Fig. 4a), respectively. Thus Tmax and Tmin are continuous functions of k and therefore, for any k, Tmax has a corresponding Tmin value. For example, for wavelength k = 700 nm, the corresponding values of Tmax and Tmin are 0.84 and 0.66, respectively (Fig. 4a). Thus, by knowing the corresponding values of Tmax and Tmin for different k, the values of refractive index were calculated using Eqs. (3) and (4). The calculated values of the refractive index in the wavelength region 625–900 nm are plotted in Figure 4b. We found that the refractive index decreases with increasing wavelength down to 800 nm and then remains constant. To evaluate the data of dispersive refractive index, n(k), the corresponding Eq. (5) for the effective-oscillator model [21,22] can be used: Ed E0 n2 ðEÞ ¼ 1 þ 2 ð5Þ E0  E2 where Ed is the oscillator strength or dispersion energy and Eo is the oscillator energy. The oscillator parameters can be estimated by linear fitting of the plot of (n2  1)1 vs. E2 as shown in Figure 4c. The values of Ed and Eo thus calculated are 6.75 and 2.86 eV, respectively. The value of the static refractive index was found to be equal to 1.83 using the obtained Ed and Eo values in the following equation: Ed n2 ð0Þ ¼ 1 þ ð6Þ E0 By knowing the value of the static refractive index, the value of the static dielectric constant was calculated as 3.35 using the well-known equation es = n2(0). The source material was crystalline in nature whereas the deposited film is amorphous. The crystalline or amorphous nature of the deposited film in particular depends upon the growth method used, the characteristics of the material, the nature of the substrate, and the film thickness. In the present study, the film was deposited by thermal evaporation technique. Selenium sublimates at comparatively low temperature and therefore it breaks the crystalline structure of the source material. Furthermore, the molecules evaporated from the boat precipitate randomly on the surface of glass substrate and the subsequently condensing molecules adhere randomly, leading to disorder in films with increased thickness [8]. Moreover, the glass substrate is itself amorphous in nature, and therefore it is not possible to match the lattices to develop a particular orientation of the growth plane even if the molten material tries to recrystallize during cooling. In addition, the loss of adequate kinetic energy for the

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precipitated molecules makes them unable to orient themselves to produce the chain structure required for the crystalline structure. At the same time, the internal stresses generated in the layers of the film due to the continuous deposition of hot molecules on the cold predeposited layers increase both the disorder and the degree of randomness, which in turn leads to amorphous films [12]. In semiconductors, structural randomness is produced due to a change in orbital hybridization which greatly degrades the magnitude of bond overlap [23]. Residual stresses are the stresses generated by a number of factors such as temperature gradients, different thermal expansion coefficients, lattice mismatches, structural changes, inelastic deformation, etc., that remain intact within the material and at the interface even after the elimination of these factors. In the present work, the generation of residual stress is possibly due to temperature gradients, thermal expansion coefficients and lattice mismatches in particular. The stress present in the specimen can be explored using the obtained XRD results. Any change in peak position (i.e. change in d-spacing) from the corresponding stress-free XRD data demonstrates a development of stress in the specimen. A decrease or increase in the value of the Bragg diffraction angle shows the generation of tensile or compressive stress, respectively. A corresponding increase in the full width at half maximum of the diffraction peak indicates the development of non-uniform stress in the specimen [24]. The XRD pattern of the film shows a hump at a diffraction angle of around 2h = 25o, which might indicate a peak, whereas the strongest peak in the corresponding crystalline Ga3Se4 is at around 2h = 28o [10]. Thus the decreasing value of diffraction angle and broadening of the peak corresponding to the strongest peak indicates the possible presence of non-uniform tensile stress in the specimen. This affects crystalline quality as well as optical and electrical properties [25]. A tensile stress will result in a decrease in the energy band gap, while a compressive strain causes an increase in the band gap [26]. It has been identified that both unsaturated and saturated bonds are formed in an amorphous film because of an insufficient number of atoms being deposited in the amorphous films [27]. As per the model of Mott and Davis [28], the unsaturated bonds in amorphous films are responsible for the formation of some defects in the material, and the width of localized states near the mobility edge depends on the degree of disorder and defects present in the amorphous structure [29]. The presence of a high concentration of localized states gives the films a low optical band gap. The obtained atomic percentage of Ga and Se in Ga3Se4 shows that the sample is slightly Se2 deficient. Therefore this deficiency of Se2 increases the concentration of localized states in the material, leading to a decrease in the band gap. In addition to it, the electronegativity of Ga (1.8) is less than that of Se (2.55), and therefore the deficiency of Se2 may increase the energy of some lone-pair states and hence give rise to two additional absorption peaks located at 1.6 and 2.1 eV, respectively [30]. In conclusion, Ga3Se4 film deposited by means of thermal evaporation was found to amorphous in nature, and the result obtained from absortion coefficient plots and transmittance interference patterns proved effective for exploring the important optical constants of Ga3Se4.

Authors are grateful to the Department of Physics, Jamia Millia Islamia and the Director, NPL for his constant encouragement for these studies. M.M. Abdullah acknowledges the Director, NPL for giving him an opportunity to work at the NPL under the CAP-24 program and the UGC for providing a research fellowship. [1] Lazell Mike, O’Brien Paul, J. Otway David, Park Jin-Ho, J. Chem. Soc., Dalton Trans. (2000) 4479. [2] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, 1984, Chap. 7. [3] L.I. Man, R.M. Imanov, S.A. Semiletov, Sov. Phys. Crystallogr. 21 (1976) 355. [4] M.M. Abdullah, G. Bhagavannarayana, M.A. Wahab, J. Cryst. Growth 312 (2010) 1534. [5] M.M. Abdullah, G. Bhagavannarayana, M.A. Wahab, J. Mater. Sci. 45 (2010) 4088. [6] A.F. Qasrawi, Cryst. Res. Technol. 40 (2005) 610. [7] A.R. Barron, CVD of Non-Metals, in: W.J. Rees (Ed.), VCH, Weinheim, 1997, p. 261. [8] T. Siciliano, M. Tepore, A. Genga, G. Micocci, M. Siciliano, A. Tepore, Vacuum 92 (2013) 65. [9] Ibrahim H. Mutlu, Maharram Z. Zarbaliyev, J. Sol-Gel. Sci. Technol. 50 (2009) 271. [10] M.M. Abdullah, Preethi Singh, D.P. Singh, M.A. Wahab, G. Bhagavannarayana, Optik 8 (2013) 9, doi: http:// dx.doi.org/10.1016/j.ijleo.2012.12.002. [11] M. Thamilselvan, K. Preemnazeer, D. Mangalaraj, Sa K. Narayandass, Yi Junsin, Cryst. Res. Technol. 39 (2004) 137. [12] M.A. Afifi, A.E. Bekheet, H.T. El-Shair, I.T. Zedan, Physica B 325 (2003) 308. [13] Amirdha Sher Gill, S. Kalainathan, G. Bhagavannarayana, Arch. Appl. Sci. Res. 2 (5) (2010) 199. [14] X.M. Duan, S. Okada, H. Oikawa, H. Matsuda, H. Nakanishi, Jpn. J. Appl. Phys. 33 (1994) 1559. [15] J. Tauc, Amorphous and liquid semiconductors, Plenum, New York, 1974, Chap 4. [16] V.N. Chernayaev, V.F. Korzo, Thin Solid Films 37 (1976) L61. [17] F. Urbach, Phys. Rev. 92 (1953) 1324. [18] Vanessa M. Huxter, Tihana Mirkovik, P.S. Sreekumari Nair, Gregory D. Scholes, Adv. Mater. 20 (2008) 2439. [19] R. Swanepoel, J. Phys. E. 16 (1983) 1214. [20] J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E. 9 (1976) 1002. [21] S.H. Wemple, M. Didomenico, Phys. Rev. B. 3 (1971) 1338. [22] S.H. Wemple, Phys. Rev. B. 7 (1973) 3767. [23] Kenji Nomura, Hirmoichi Ohta, Akihiro Takagi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono, Nature 432 (2004) 488. [24] Pankaj Tyagi, A.G. Vedeshwar, Phys. Rev. B 66 (2002) 075422. [25] Y.F. Li, B. Yao, Y.M. Lu, C.X. Cong, Z.Z. Zhang, Y.Q. Gai, C.J. Zheng, B.H. Li, Z.P. Wei, D.Z. Shen, X.W. Fan, L. Xiao, S.C. Xu, Y. Liu, Appl. Phys. Lett. 91 (2007) 021915. [26] D.G. Zhao, S.J. Xu, M.H. Xie, S.Y. Tong, Hui Yang, Appl. Phys. Lett. 83 (2003) 677. [27] M.L. Theye, Proc. Vth Int. Confer. “Amorphous Liquid Semi-conductors” 1 (1973) 479. [28] N.F. Mott, E.A. Davis, Electronic Process in Noncrystalline Materials, Clarendon, Oxford, 1971. [29] H. Khan Zishan, A. Khan Shamshad, Salah Numan, S.M. Sami Habib, A.A. Al-Ghamdi Abdallah El-Hamidy, Nanoscale Res. Lett. 5 (2010) 1512–1517. [30] M. Sushil Kumar, A. Majeed Khan, Chalcogenide Lett. 9 (2012) 145–149.