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Effect of isoelectronic substitution of Bi on the photoelectrical properties in amorphous Sn–Sb–Se films
Thin Solid Films 517 (2009) 5965–5968
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of isoelectronic substitution of Bi on the photoelectrical properties in amorphous Sn–Sb–Se films Muneer Ahmad, P. Kumar, R. Thangaraj ⁎ Semiconductors Laboratory, Department of Applied Physics, Guru Nanak Dev University, Amritsar, Punjab-143005, India
a r t i c l e
i n f o
Article history: Received 2 July 2008 Received in revised form 13 March 2009 Accepted 16 March 2009 Available online 25 March 2009 PACS: 71.20.Nr 72.20.-I 78.66.Jg
a b s t r a c t Amorphous thin films of Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) system have been prepared by thermal evaporation technique. The optical gap and dc activation energy first increases with the addition of Bi (x = 2) and then decreases sharply with further addition. The photocurrent initially increases with time and then saturates to a constant value for all the samples. The decay portion of photocurrent has two components, fast one followed by a slow decay. Photocurrent (Iph) versus light intensity (F) follows the power law Iph ∝ Fγ and the value of the exponent (γ) decreases from 0.76 to 0.53 as the Bi concentration varied from x = 0 to 6 in the present system. The photosensitivity of these samples varies from 1.27 to 1.13 as Bi content increases. © 2009 Elsevier B.V. All rights reserved.
Keywords: A. Semiconductors A. Thin films D. Optical properties X-ray diffraction
1. Introduction Chalcogenide glasses exhibit a variety of light induced effects. Photoconductivity studies are of considerable interest because they yield information about the defects and metastable structural states in amorphous solids [1]. The structure and bonding configuration of chalcogenide glasses can be changed by thermal treatment or light soaking which can be sometimes reversible. The reversible photoinduced change in chalcogenides is favored by the rapid localization of the photo-excited carriers. As the photoconductivity behavior is controlled by carrier localization and delocalization processes, transient photoconductivity measurements are expected to give information about the localized states in these materials. Transient photoconductivity measurements on different chalcogenide glasses have been reported earlier by various workers [2–7]. The electrical properties of chalcogenide glasses are in general not affected appreciably by the addition of impurities because the random networks of atoms can accommodate an impurity by adjusting its nearest neighbor environment, causing negligible effect on the electrical properties [8]. Also, the high density of localized states present in the forbidden gap effectively pin the Fermi level. However, experimental results reported by several researchers [9–12] have shown that the addition of certain selected impurities brings about noticeable changes in the electrical properties of chalcogenide glasses. In par⁎ Corresponding author. E-mail address: [email protected] (R. Thangaraj). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.130
ticular, the effect of incorporation of Bi as an impurity in chalcogenide glasses has been of great interest ever since the synthesis of such glassy systems because of its larger polarizability which leads to partially ionic Bi-chalcogen bonds more likely and the small band gap of Bi-doped glasses make a shift in Fermi level possible [13]. For example, in the case of an amorphous Ge–Se system, the addition of a critical quantity of Bi brings about a carrier type reversal from p-type to n-type [14–16]. The study of Sn–Sb–Se glasses has been of recent interest due to their small glass-forming region [17] as compared to other alloys from the IV–V–VI ternary system, where light mass elements were used. The present work reports on photoconductivity measurements in Bi doped Sn–Sb–Se glassy systems. The study of rise and fall of photocurrent has been carried out on thermally evaporated thin films of Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) system. The effect of composition on the photoconduction has also been investigated. 2. Experimental details Bulk samples of Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) were prepared by conventional melt quenching technique. High-purity (99.999%) elements with appropriate atomic percentage were sealed in a quartz ampoule (length ~ 10 cm and internal diameter ~ 6 mm), in a vacuum of ~10− 2 Pa. The ampoules were kept in a vertical furnace for 48 h. The temperature was raised to 1123 K, at a rate of 4–5 K/min. The ampoule was inverted at regular intervals (~1 h) to ensure the homogenous mixing of the constituents, before quenching in an ice bath. The
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M. Ahmad et al. / Thin Solid Films 517 (2009) 5965–5968
material was separated from the quartz ampoule by dissolving the ampoule into a solution of HF + H2O2 for approximately 48 h. Using this as source material, thin films were deposited onto well-cleaned glass substrates by thermal evaporation technique in a vacuum better than 10− 3 Pa using a HINDHIVAC Vacuum coating unit (model 12A4D). The thickness was measured by KLA Tencor P15 surface profiler and thickness values are summarized in Table 1. The amorphous nature of the thin films was confirmed by the absence of sharp peaks in the X-ray diffractograms. The optical transmission spectrum was recorded at room temperature for all the samples using UV– visible spectrophotometer (VARIAN Cary500 UV–VIS–NIR) in the wavelength range of 400–3000 nm. The optical energy gap was obtained from a plot of (αhν)1/2 vs hν and taking the intercept on the energy axis, where α is the absorption coefficient, which is widely accepted procedure. The conductivity measurements were carried out in the temperature range 233–343 K in a running vacuum of 10− 2 Pa. Electrical contacts with an electrode gap of ~2 mm in a coplanar geometry were made by pre-depositing aluminum. For photoconductivity measurements the sample was mounted inside a metallic cryostat with a transparent window. All the measurements were made in a vacuum of ~10− 2 Pa. A tungsten halogen lamp (Halonix, India) of 500 W was used for illumination. The heating of the samples by infrared (IR) part of the light was avoided by using water filter. The intensity of light was measured using a digital lux meter (LX-101, Taiwan). The current was measured using a digital picoammeter (DPM-111 Scientific Equipments, Roorkee). During photoconductivity studies the electrodes were well-covered using aluminum foil to avoid photo diffusion. 3. Results and discussion
Fig. 1. X-ray diffraction patterns for the Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) as-prepared thin films.
considerable decrease in activation energy (Ed) is observed. Increase in dark conductivity and decrease in activation energy as seen from Fig. 2 and Table 1 respectively are found to be associated with the shift in Fermi level in impurity-doped chalcogenide glasses [19–23]. Fig. 3 shows a plot of (αhν)1/2 versus (hν) for different Bi concentrations. The absorption coefficient α was computed from the experimental measured values of R and T according to the following expression 2
T = ð1−RÞ expð−αdÞ Where d is the thickness of investigated films. In the high absorption region (α ≥ 104 cm− 1), photon energy dependence of absorption coefficient can be described according to a model proposed by Tauc [24]. The model is
3.1. Temperature-dependent dark conductivity and optical band gap studies Fig. 1 shows the X-ray diffraction patterns for the as-prepared Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) thin films. The absence of sharp peaks for the samples with x = 0, and 2 show that they are amorphous in nature. However, samples with x = 4 and 6 exhibit a small peak around 2θ = 28.61° which can be attributed to a small fraction of Bi3Se2 crystalline phase as identified from the JCPDS Card No. 40-0935 [18]. The dark conductivity of the Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) samples, measured as a function of temperature, is shown in Fig. 2. The conductivity of the samples increases with increase in temperature, as observed from the Fig. 2.The dc conductivity can be expressed by the relation −ΔEd σ = σ 0 exp kT
opt 2 ðαhvÞ = B hv −Eg Where B is a constant which depends upon the transition proopt bability and Eopt g is the optical band gap. The optical band gap (Eg ) is calculated by taking the intercept on the energy axis and the values for different Bi concentrations are listed in Table 1. From this table it can be seen that the optical band gap first increases for x = 2 and then decreases with increase in bismuth concentration. Optical absorption depends upon both the short-range order in the amorphous state and the defects associated with it. The change in the optical band gap in our system may be partly due to the
where σo is the pre-exponential factor, ΔEd is the dc activation energy and is calculated from the slope of ln(σ) versus 1000/T plots, k is the Boltzmann constant and T is the absolute temperature. The Table 1 shows the variation of Ed with bismuth concentration. It is seen from Table 1 that at low Bi concentration there is a little change in activation energy where as at higher concentrations a
Table 1 Calculated values of thickness (d), optical band gap (Eopt g ), activation energy in the dark (Ed) and light (Eph), photosensitivity (S) and recombination parameter (γ) for different values of x. x (Bi at.%)
d (nm)
Eopt (eV) g
Ed (eV)
Eph (eV)
S
γ
0 2 4 6
1273 1079 1250 1250
0.92 0.95 0.62 0.60
0.45 0.47 0.29 0.24
0.27 0.28 0.23 0.19
1.27 1.21 1.22 1.13
0.76 0.69 0.57 0.53
Fig. 2. Temperature variation of dark conductivity ln(σ) for Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) thin films.
M. Ahmad et al. / Thin Solid Films 517 (2009) 5965–5968
Fig. 3. Plots of (αhα)1/2 vs hm for the Sn10Sb20−xBixSe70 (0 ≤ x ≤ 6) thin films.
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Fig. 5. The rise and decay of photocurrent with time at room temperature for Sn10 Sb20 − xBi xSe70 (0 ≤ x ≤ 6) thin films at 1400 lx.
change of bonding from covalent to partially ionic and partly due to the increase of disorder [19,25].
3.2. Temperature and intensity dependent photoconductivity The temperature dependence of steady state photoconductivity has been studied for a-Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) system in the temperature range 233–343 K. The temperature dependence of photoconductivity at 1400 lx for different samples is shown in Fig. 4. The variation of photoconductivity with temperature is similar to the variation of the dark conductivity. The enhancement in conductivity with increase of temperature can be interpreted as due to the enhancement in carrier density and/or mobility with doping. Increase in carrier density leads to an increase in the density of states in the gap region [26]. The values of the activation energy in the light (Eph) have been calculated from the slopes of photoconductivity versus 1000/T plots and are given in Table 1. It is clear from the Table 1 that the activation energy in the light is less than the activation energy in the dark. This is in accord with a model of Simmons and Taylor with distributed set of defect states [27]. In this model, there is a continuous distribution of localized states present in the mobility gap of the material. This model provides the information for the energy location of discrete sets of localized states (D+ and D−) in the mobility gap. These centers D+ and D− act as discrete traps for the photo generated electrons and holes, thereby giving rise to neutral (D0) sites which, due to polaronic lattice deformation, produce energy levels roughly midway between
Fig. 4. The variation of photoconductivity (lnσph) with the inverse temperature for Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) thin films at 1400 lx.
the band edges and the Fermi level. The position of these localized states depends on the composition and light intensity. Also, the Fermi level splits into electron and hole quasi Fermi levels, and moves towards the valence band for holes (EFp), and towards the conduction band for electrons (EFn), with increasing light intensity. The increase in the light intensity further facilitates the shift due to the increase in the number of photo generated carriers or carrier mobility, results in the decrease in Eph value. 3.3. Photosensitivity The time dependence of photocurrent due to white light 1400 lx is shown in Fig. 5. The photocurrent is found to reach saturation and the rise is found to be slow with the increase in exposure time. The photo degradation has not been observed in the present system, as with narrow gap chalcogenide films [28]. All the samples show the same trend. After switching off the light, the photocurrent decays to a constant value. It has two components, fast one in the beginning and decays slowly after long time as observed in Ge–Se–Bi glasses [29]. The photocurrent remains greater than the dark current value, known as persistent photocurrent, increases with the increase in Bi content. This behavior is attributed to the increase in the concentration of trapping centers with the increase in Bi content, which is understandable in chalcogenide glasses due to the continuous distribution of localized states in these materials. An important parameter in the
Fig. 6. Dependence of photocurrent on light intensity for Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) thin films at room temperature.
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photoconductivity measurements is the photosensitivity (S = Iph / Id), which reveals the effect of composition, light intensity and temperature on the defect state density in the gap. The Table 1 summarizes the calculated values of S at room temperature for all the samples. The photosensitivity varies from 1.27 to 1.13 as the amount of Bi content increases from 0 to 6. The value of S depends on the lifetime of the excess carriers, which in turn depends on the density of localized states in a particular material. The higher the density of defect states, the lower will be the life time, as these defect states act as recombination centers in the presence of light [30,31]. Therefore the change in the photosensitivity could be due to the change in the defect state density with the composition. The effect of light intensity (F) on the photoconductivity at room temperature (306 K) was studied for all the samples. Fig. 6 shows the plots of lnIph versus lnF for all the samples. These plots indicate that the photocurrent (Iph) increases with intensity following the power law: γ
Iph ~F ; where γ, is the exponent and the value of it is determined by the recombination mechanism [27]. The value of γ≈0.5 indicates a bimolecular recombination process, whereas γ≈1.0 indicates a monomolecular mechanism. If the value of the exponent lies between 0.5≤γ≤1.0, then there is continuous distribution of traps [31]. The exponent γ has been calculated for all the samples and summarized in Table 1. The value of the exponent decreases with the increase in Bi concentration from x=0 to 6, indicating a change in the recombination process with the substitution of Bi in the parent system. 4. Conclusion Photoconductivity measurements have been performed on thin films of Sn–Sb–Bi–Se system as a function of temperature, time and
illumination intensity. The change in the activation energy for dark conductivity as well as for photoconductivity is observed with the change in Bi content. The recombination mechanism is approaching bimolecular and the photosensitivity decreases for higher Bi concentrations. References [1] A.E. Owen, A.P. Firth, P.J.S. Ewen, Phila. Mag. B52 (1985) 347. [2] E.A. Fagen, H. Fritzsche, J. Non-Cryst. Solids 4 (1970) 480. [3] R.H. Bube, H.E. Mahan, R.T.S. Shiah, H.A. Van Der Plas, Appl. Phys. Lett. 25 (1974) 419. [4] K. Shimakawa, A. Yoshida, T. Arizumi, J. Non-Cryst. Solids 16 (1974) 258. [5] J.M. Chamberlain, A.J. Moseley, Jpn. J. Appl. Phys. 21 (1982) 13. [6] M. Igalson, R. Trykozko, Solid State Commun. 40 (1981) 99. [7] A.S. Maan, A.K. Sharma, D.R. Goyal, A. Kumar, J. Non-Cryst. Solids 109 (1988) 273. [8] N.F. Mott, Adv. Phys. 16 (1967) 49. [9] E.A. Davis, N.F. Mott, Phila. Mag. 22 (1970) 903. [10] B.T. Kolomiets, A. Lebedev, N.A. Rogachev, Fiz. Tekh. Poluprovodn. 8 (1974) 545. [11] S. Okano, M. Suzuki, T. Imura, N. Fukud, A. Hiraki, J. Non-Cryst. Solids 969 (1983) 59. [12] R. Misra, S. Goel, A.K. Agnihotri, A. Kumar, J. Mater. Sci. Lett. 11 (1992) 212. [13] S.R. Elliot, A.T. Steel, Phys. Rev. Lett. 57 (1986) 1316. [14] N. Toghe, H. Matsuo, T. Minami, J. Non-Cryst. Solids 809 (1987) 95. [15] N. Toghe, T. Minami, Y. Yamamoto, K. Tanaka, J. Appl. Phys. 51 (1980) 1048. [16] J.C. Phillips, Phys. Rev. B 36 (1987) 4265. [17] P. Kumar, J. Kumar, M. Ahmad, R. Thangaraj, Appl. Phys. A 90 (2008) 469. [18] JCPDS-International Centre for Diffraction Data Card No. 40-0935 (1997). [19] Z.H. Khan, M. Zulfequar, M. Hussian, J. Mod. Opt. 44 (1997) 55. [20] B.T. Kolomiets, E.A. Lebedex, N.A. Rogachov, Fiz. Tekh. Poluprovodn. 8 (1974) 545. [21] S. Koichi, Y. Akaira, A. Tatsuya, J. Non-Cryst. Solids 16 (1974) 258. [22] E.A. Davis, N.F. Mott, Phila. Mag. 22 (1970) 903. [23] R.A. Street, Phys. Rev. Lett. 49 (1982) 1187. [24] J. Tauc, Amorphous and Liquid Semiconductors, Plenum Press, New York, 1974. [25] S.R. Elliott, A.T. Steel, J. Phys.C: Solid State Phys. 20 (1987) 4335. [26] A. Onozuka, O. Oda, I. Tsuboya, Thin Solid Films 149 (1987) 9. [27] J.G. Simmons, G.W. Taylor, J. Phys. C: Solid State Phys. 7 (1974) 3051. [28] K. Hayashi, Y. Hikida, K. Shimakawa, S.R. Elliott, Philos. Mag. Lett. 76 (1997) 233. [29] R. Mathur, A. Kumar, Solid State Commun. 59 (1986) 163. [30] K. Shimakawa, S. Inami, S.R. Elliott, Phys. Rev. B 42 (1990) 118. [31] K. Shimakawa, A.V. Kolobov, S.R. Elliott, Adv. Phys. 44 (1995) 475.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of isoelectronic substitution of Bi on the photoelectrical properties in amorphous Sn–Sb–Se films Muneer Ahmad, P. Kumar, R. Thangaraj ⁎ Semiconductors Laboratory, Department of Applied Physics, Guru Nanak Dev University, Amritsar, Punjab-143005, India
a r t i c l e
i n f o
Article history: Received 2 July 2008 Received in revised form 13 March 2009 Accepted 16 March 2009 Available online 25 March 2009 PACS: 71.20.Nr 72.20.-I 78.66.Jg
a b s t r a c t Amorphous thin films of Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) system have been prepared by thermal evaporation technique. The optical gap and dc activation energy first increases with the addition of Bi (x = 2) and then decreases sharply with further addition. The photocurrent initially increases with time and then saturates to a constant value for all the samples. The decay portion of photocurrent has two components, fast one followed by a slow decay. Photocurrent (Iph) versus light intensity (F) follows the power law Iph ∝ Fγ and the value of the exponent (γ) decreases from 0.76 to 0.53 as the Bi concentration varied from x = 0 to 6 in the present system. The photosensitivity of these samples varies from 1.27 to 1.13 as Bi content increases. © 2009 Elsevier B.V. All rights reserved.
Keywords: A. Semiconductors A. Thin films D. Optical properties X-ray diffraction
1. Introduction Chalcogenide glasses exhibit a variety of light induced effects. Photoconductivity studies are of considerable interest because they yield information about the defects and metastable structural states in amorphous solids [1]. The structure and bonding configuration of chalcogenide glasses can be changed by thermal treatment or light soaking which can be sometimes reversible. The reversible photoinduced change in chalcogenides is favored by the rapid localization of the photo-excited carriers. As the photoconductivity behavior is controlled by carrier localization and delocalization processes, transient photoconductivity measurements are expected to give information about the localized states in these materials. Transient photoconductivity measurements on different chalcogenide glasses have been reported earlier by various workers [2–7]. The electrical properties of chalcogenide glasses are in general not affected appreciably by the addition of impurities because the random networks of atoms can accommodate an impurity by adjusting its nearest neighbor environment, causing negligible effect on the electrical properties [8]. Also, the high density of localized states present in the forbidden gap effectively pin the Fermi level. However, experimental results reported by several researchers [9–12] have shown that the addition of certain selected impurities brings about noticeable changes in the electrical properties of chalcogenide glasses. In par⁎ Corresponding author. E-mail address: [email protected] (R. Thangaraj). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.130
ticular, the effect of incorporation of Bi as an impurity in chalcogenide glasses has been of great interest ever since the synthesis of such glassy systems because of its larger polarizability which leads to partially ionic Bi-chalcogen bonds more likely and the small band gap of Bi-doped glasses make a shift in Fermi level possible [13]. For example, in the case of an amorphous Ge–Se system, the addition of a critical quantity of Bi brings about a carrier type reversal from p-type to n-type [14–16]. The study of Sn–Sb–Se glasses has been of recent interest due to their small glass-forming region [17] as compared to other alloys from the IV–V–VI ternary system, where light mass elements were used. The present work reports on photoconductivity measurements in Bi doped Sn–Sb–Se glassy systems. The study of rise and fall of photocurrent has been carried out on thermally evaporated thin films of Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) system. The effect of composition on the photoconduction has also been investigated. 2. Experimental details Bulk samples of Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) were prepared by conventional melt quenching technique. High-purity (99.999%) elements with appropriate atomic percentage were sealed in a quartz ampoule (length ~ 10 cm and internal diameter ~ 6 mm), in a vacuum of ~10− 2 Pa. The ampoules were kept in a vertical furnace for 48 h. The temperature was raised to 1123 K, at a rate of 4–5 K/min. The ampoule was inverted at regular intervals (~1 h) to ensure the homogenous mixing of the constituents, before quenching in an ice bath. The
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M. Ahmad et al. / Thin Solid Films 517 (2009) 5965–5968
material was separated from the quartz ampoule by dissolving the ampoule into a solution of HF + H2O2 for approximately 48 h. Using this as source material, thin films were deposited onto well-cleaned glass substrates by thermal evaporation technique in a vacuum better than 10− 3 Pa using a HINDHIVAC Vacuum coating unit (model 12A4D). The thickness was measured by KLA Tencor P15 surface profiler and thickness values are summarized in Table 1. The amorphous nature of the thin films was confirmed by the absence of sharp peaks in the X-ray diffractograms. The optical transmission spectrum was recorded at room temperature for all the samples using UV– visible spectrophotometer (VARIAN Cary500 UV–VIS–NIR) in the wavelength range of 400–3000 nm. The optical energy gap was obtained from a plot of (αhν)1/2 vs hν and taking the intercept on the energy axis, where α is the absorption coefficient, which is widely accepted procedure. The conductivity measurements were carried out in the temperature range 233–343 K in a running vacuum of 10− 2 Pa. Electrical contacts with an electrode gap of ~2 mm in a coplanar geometry were made by pre-depositing aluminum. For photoconductivity measurements the sample was mounted inside a metallic cryostat with a transparent window. All the measurements were made in a vacuum of ~10− 2 Pa. A tungsten halogen lamp (Halonix, India) of 500 W was used for illumination. The heating of the samples by infrared (IR) part of the light was avoided by using water filter. The intensity of light was measured using a digital lux meter (LX-101, Taiwan). The current was measured using a digital picoammeter (DPM-111 Scientific Equipments, Roorkee). During photoconductivity studies the electrodes were well-covered using aluminum foil to avoid photo diffusion. 3. Results and discussion
Fig. 1. X-ray diffraction patterns for the Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) as-prepared thin films.
considerable decrease in activation energy (Ed) is observed. Increase in dark conductivity and decrease in activation energy as seen from Fig. 2 and Table 1 respectively are found to be associated with the shift in Fermi level in impurity-doped chalcogenide glasses [19–23]. Fig. 3 shows a plot of (αhν)1/2 versus (hν) for different Bi concentrations. The absorption coefficient α was computed from the experimental measured values of R and T according to the following expression 2
T = ð1−RÞ expð−αdÞ Where d is the thickness of investigated films. In the high absorption region (α ≥ 104 cm− 1), photon energy dependence of absorption coefficient can be described according to a model proposed by Tauc [24]. The model is
3.1. Temperature-dependent dark conductivity and optical band gap studies Fig. 1 shows the X-ray diffraction patterns for the as-prepared Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) thin films. The absence of sharp peaks for the samples with x = 0, and 2 show that they are amorphous in nature. However, samples with x = 4 and 6 exhibit a small peak around 2θ = 28.61° which can be attributed to a small fraction of Bi3Se2 crystalline phase as identified from the JCPDS Card No. 40-0935 [18]. The dark conductivity of the Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) samples, measured as a function of temperature, is shown in Fig. 2. The conductivity of the samples increases with increase in temperature, as observed from the Fig. 2.The dc conductivity can be expressed by the relation −ΔEd σ = σ 0 exp kT
opt 2 ðαhvÞ = B hv −Eg Where B is a constant which depends upon the transition proopt bability and Eopt g is the optical band gap. The optical band gap (Eg ) is calculated by taking the intercept on the energy axis and the values for different Bi concentrations are listed in Table 1. From this table it can be seen that the optical band gap first increases for x = 2 and then decreases with increase in bismuth concentration. Optical absorption depends upon both the short-range order in the amorphous state and the defects associated with it. The change in the optical band gap in our system may be partly due to the
where σo is the pre-exponential factor, ΔEd is the dc activation energy and is calculated from the slope of ln(σ) versus 1000/T plots, k is the Boltzmann constant and T is the absolute temperature. The Table 1 shows the variation of Ed with bismuth concentration. It is seen from Table 1 that at low Bi concentration there is a little change in activation energy where as at higher concentrations a
Table 1 Calculated values of thickness (d), optical band gap (Eopt g ), activation energy in the dark (Ed) and light (Eph), photosensitivity (S) and recombination parameter (γ) for different values of x. x (Bi at.%)
d (nm)
Eopt (eV) g
Ed (eV)
Eph (eV)
S
γ
0 2 4 6
1273 1079 1250 1250
0.92 0.95 0.62 0.60
0.45 0.47 0.29 0.24
0.27 0.28 0.23 0.19
1.27 1.21 1.22 1.13
0.76 0.69 0.57 0.53
Fig. 2. Temperature variation of dark conductivity ln(σ) for Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) thin films.
M. Ahmad et al. / Thin Solid Films 517 (2009) 5965–5968
Fig. 3. Plots of (αhα)1/2 vs hm for the Sn10Sb20−xBixSe70 (0 ≤ x ≤ 6) thin films.
5967
Fig. 5. The rise and decay of photocurrent with time at room temperature for Sn10 Sb20 − xBi xSe70 (0 ≤ x ≤ 6) thin films at 1400 lx.
change of bonding from covalent to partially ionic and partly due to the increase of disorder [19,25].
3.2. Temperature and intensity dependent photoconductivity The temperature dependence of steady state photoconductivity has been studied for a-Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) system in the temperature range 233–343 K. The temperature dependence of photoconductivity at 1400 lx for different samples is shown in Fig. 4. The variation of photoconductivity with temperature is similar to the variation of the dark conductivity. The enhancement in conductivity with increase of temperature can be interpreted as due to the enhancement in carrier density and/or mobility with doping. Increase in carrier density leads to an increase in the density of states in the gap region [26]. The values of the activation energy in the light (Eph) have been calculated from the slopes of photoconductivity versus 1000/T plots and are given in Table 1. It is clear from the Table 1 that the activation energy in the light is less than the activation energy in the dark. This is in accord with a model of Simmons and Taylor with distributed set of defect states [27]. In this model, there is a continuous distribution of localized states present in the mobility gap of the material. This model provides the information for the energy location of discrete sets of localized states (D+ and D−) in the mobility gap. These centers D+ and D− act as discrete traps for the photo generated electrons and holes, thereby giving rise to neutral (D0) sites which, due to polaronic lattice deformation, produce energy levels roughly midway between
Fig. 4. The variation of photoconductivity (lnσph) with the inverse temperature for Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) thin films at 1400 lx.
the band edges and the Fermi level. The position of these localized states depends on the composition and light intensity. Also, the Fermi level splits into electron and hole quasi Fermi levels, and moves towards the valence band for holes (EFp), and towards the conduction band for electrons (EFn), with increasing light intensity. The increase in the light intensity further facilitates the shift due to the increase in the number of photo generated carriers or carrier mobility, results in the decrease in Eph value. 3.3. Photosensitivity The time dependence of photocurrent due to white light 1400 lx is shown in Fig. 5. The photocurrent is found to reach saturation and the rise is found to be slow with the increase in exposure time. The photo degradation has not been observed in the present system, as with narrow gap chalcogenide films [28]. All the samples show the same trend. After switching off the light, the photocurrent decays to a constant value. It has two components, fast one in the beginning and decays slowly after long time as observed in Ge–Se–Bi glasses [29]. The photocurrent remains greater than the dark current value, known as persistent photocurrent, increases with the increase in Bi content. This behavior is attributed to the increase in the concentration of trapping centers with the increase in Bi content, which is understandable in chalcogenide glasses due to the continuous distribution of localized states in these materials. An important parameter in the
Fig. 6. Dependence of photocurrent on light intensity for Sn10Sb20 − xBixSe70 (0 ≤ x ≤ 6) thin films at room temperature.
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photoconductivity measurements is the photosensitivity (S = Iph / Id), which reveals the effect of composition, light intensity and temperature on the defect state density in the gap. The Table 1 summarizes the calculated values of S at room temperature for all the samples. The photosensitivity varies from 1.27 to 1.13 as the amount of Bi content increases from 0 to 6. The value of S depends on the lifetime of the excess carriers, which in turn depends on the density of localized states in a particular material. The higher the density of defect states, the lower will be the life time, as these defect states act as recombination centers in the presence of light [30,31]. Therefore the change in the photosensitivity could be due to the change in the defect state density with the composition. The effect of light intensity (F) on the photoconductivity at room temperature (306 K) was studied for all the samples. Fig. 6 shows the plots of lnIph versus lnF for all the samples. These plots indicate that the photocurrent (Iph) increases with intensity following the power law: γ
Iph ~F ; where γ, is the exponent and the value of it is determined by the recombination mechanism [27]. The value of γ≈0.5 indicates a bimolecular recombination process, whereas γ≈1.0 indicates a monomolecular mechanism. If the value of the exponent lies between 0.5≤γ≤1.0, then there is continuous distribution of traps [31]. The exponent γ has been calculated for all the samples and summarized in Table 1. The value of the exponent decreases with the increase in Bi concentration from x=0 to 6, indicating a change in the recombination process with the substitution of Bi in the parent system. 4. Conclusion Photoconductivity measurements have been performed on thin films of Sn–Sb–Bi–Se system as a function of temperature, time and
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