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Effect of Ag addition on the photoconductivity of amorphous Se–Sb thin films
Solid State Communications 144 (2007) 83–87 www.elsevier.com/locate/ssc
Effect of Ag addition on the photoconductivity of amorphous Se–Sb thin films K.S. Bindra, N. Suri, P. Kumar, R. Thangaraj ∗ Semiconductors Laboratory, Department of Applied Physics, Guru Nanak Dev University, Amritsar, 143005, India Received 30 April 2007; accepted 15 May 2007 by A.H. MacDonald Available online 21 May 2007
Abstract Amorphous thin films of Se80−x Sb20 Agx (0 ≤ x ≤ 16) have been prepared by a thermal evaporation technique. The dark conductivity and the photoconductivity of Se80−x Sb20 Agx (0 ≤ x ≤ 16) thin films have been determined. Dark conductivity increases with the increase in the silver content. The photoconductivity of these films has been studied at different illumination intensities. With illumination the photocurrent increases initially with time and then saturates at a constant value for low intensities of light (<400 lx), however, with higher intensities (>600 lx) the photocurrent increases initially, attains a maximum, and then decreases with time. The results have been explained on the basis of the Dember voltage and the interaction between the photogenerated holes and the trapped electrons on the surface. The variation of photocurrent with illumination intensity indicates that the recombination mechanism is bimolecular. The photosensitivity of these samples lies between 2.06 and 3.50. c 2007 Elsevier Ltd. All rights reserved.
PACS: 71.20.Nr; 72.20.-I; 78.66.Jg Keywords: A. Amorphous thin films; D. Dark conductivity; D. Photoconductivity; D. Dember voltage
1. Introduction Chalcogenide glasses are a very important class of semiconductor materials because of their technological applications in infrared optical fibers, xerography, switching and memory devices, etc. [1–4]. Chalcogenide glasses containing Ag, generally, exhibit single glass transition and single crystallization temperatures, which are important conditions for rewritable disks. Thin films of chalcogenide glasses containing Ag have found application in erasable phase change optical recording devices [5–9]. Ag-doped chalcogenide glasses are preferred because of their ionic nature, i.e. the electrical conductivity is governed by Ag+ ionic conduction since the hole conduction is substantially smaller and electron conductivity is not detected [10–12]. There have been several reports on the structural and ionic conductivity of these ionconducting chalcogenide glasses [13–20], however, only a few
∗ Corresponding author. Tel.: +91 183 2258802 3165; fax: +91 183 2258819.
E-mail address: [email protected] (R. Thangaraj). c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.05.014
studies have been done on the photoelectrical properties of silver doped chalcogenide thin films [21–24]. The present work reports on photoconductivity measurements in Ag doped Se–Sb glassy systems. The rise of photoconductivity has been carried out in thermally evaporated thin films of amorphous Se80−x Sb20 Agx (0 ≤ x ≤ 16). The effect of different illumination intensities of light on the photoconduction has also been investigated. 2. Experimental details Bulk samples of Se80−x Sb20 Agx (0 ≤ x ≤ 16) were prepared by a conventional melt quenching technique. Highpurity (99.99%) elements with appropriate atomic percentage were sealed in a quartz ampoule (length ∼ 10 cm and internal diameter ∼ 6 mm), in a vacuum of ∼10−4 mbar. The ampoules were kept in a vertical furnace for 48 h. The temperature was raised to 1273 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 material was separated from the quartz ampoule by dissolving
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the ampoule into a solution of HF + H2 O2 for approximately 48 h. Using this as source material, thin films were deposited onto well-cleaned glass substrates by a thermal evaporation technique in a vacuum better then 10−4 mbar using a Hind High Vacuum coating unit (model 12A4D). The thickness of the films was measured by the Tolansky interference method. The amorphous nature of the thin films was confirmed by the absence of sharp peaks in the X-ray diffractograms. The surface microstructure was examined using a scanning electron microscope (SEM). The composition of the thin films was analyzed by electron probe microanalysis (EPMA) using a JEOL JXA 8600 M superprobe with accelerator voltage 15 keV, with probe diameter 5 µm and probe current 50 nA. The optical transmission spectrum was recorded at room temperature for all samples using an UV–visible spectrophotometer (UV-160A Shimadzu, Japan) in the wavelength range 300–1100 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 a widely accepted procedure. The conductivity measurements were carried out in the temperature range 273–333 K in a running vacuum of 10−3 mbar. Electrical contacts with an electrode gap of ∼2 mm in a coplanar geometry were made using silver paint. The ohmic nature of the contact was verified by a straight line passing through the origin of a voltage versus current plot. For photoconductivity measurements the sample was mounted inside a metallic cryostat with a transparent window. All the measurements were made in a vacuum of ∼10−3 mbar. A tungsten halogen lamp (Halonix, India) of 500 W was used for illumination. The infrared (IR) part of the light was cut off using IR filters. 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 discussions 3.1. Characterization
Fig. 1. Scanning electron microscope image of Se80 Sb20 film showing Sb2 Se3 nanophase (white spot).
Fig. 2. Plot of (αhν)1/2 versus hν for Se80−x Sb20 Agx (0 ≤ x ≤ 16), Se80 Sb20 , Se78 Sb20 Ag2 , N Se76 Sb20 Ag4 , Se72 Sb20 Ag8 , ♦ Se68 Sb20 Ag12 , 4 Se64 Sb20 Ag16 .
3.2. Optical energy gap and dark conductivity The X-ray diffraction shows the presence of only broad features and the absence of any sharp peaks in the diffractograms. These broad features indicate that the samples are amorphous in nature. The surface morphology of the samples was examined using SEM. The scanning electron micrograph of Se80 Sb20 film is shown in Fig. 1. The SEM picture shows the presence of nano particles embedded in an amorphous background. The presence of this nanophase in films with x = 0 indicates that the nanophase may be due to Sb2 Se3 . The addition of Ag has been found to segregate Ag2 Se nanophase [25,26]. As a result, Ag-rich and Sb-rich nanophases may both coexist in the present Se–Sb–Ag system. EPMA studies have shown that the actual composition of the elements in thin films differ from that of the bulk glass by about 2%, 5% and 1% for silver, antimony and selenium respectively.
Fig. 2 shows a plot of (αhν)1/2 versus hν for the asdeposited Se80−x Sb20 Agx (0 ≤ x ≤ 16) thin films. The opt optical energy gap (E g ) is calculated from the intercept by extrapolating the straight portions of the lines to the energy axis. The optical gap varies from 1.51 to 1.29 eV as x varies from 0 to 16. The decrease in the optical gap with increase in Ag content may be due to an increase in disorder and defects. The dark conductivity of Se80−x Sb20 Agx (0 ≤ x ≤ 16), measured as a function of temperature, is shown in Fig. 3. It is observed that the dark conductivity increases with an increase in temperature, is found to be activated over the entire temperature range and follows the Arrhenius equation: σ = σo e−∆ Ea /kT
(1)
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opt
Fig. 4. Variations of optical energy gap (E g ) and activation energy (∆E a ) with Ag concentration. (x = 2 and 4 show two activation energies). Fig. 3. Variations of dark conductivity with temperature for Se80−x Sb20 Agx (0 ≤ x ≤ 16) films. Se80 Sb20 , Se78 Sb20 Ag2 , N Se76 Sb20 Ag4 , Se72 Sb20 Ag8 , ♦ Se68 Sb20 Ag12 , 4 Se64 Sb20 Ag16 .
where σo is the pre-exponential factor, ∆E a is the activation energy, k is the Boltzmann constant and T is the absolute temperature. The activation energy for films with x = 0 is for the electronic conduction. With the addition of Ag, the conduction becomes mixed. The two activation energies (for x = 2, 4) may be due to the different conduction mechanisms predominating in the relevant temperature regions. In films with higher Ag contents (for x > 4) the predominant conduction may be due to Ag+ ions. Similar results have been reported in Ag–Ge–Sb–Se glasses by Bychkov et al. [27]. Fig. 4 shows the variation of activation energy and optical energy gap with opt x. It can be observed that both ∆E a and E g follow the same trend. As a consequence of silver addition to Se80 Sb20 system, a significant decrease of the activation energy from 0.61 to 0.49 eV is observed. It seems with the addition of Ag, an Ag-rich phase separation occurs in the Se-rich matrix [25,26]. This macroscopic phase separation of the Ag2 Se amorphous phase probably softens the network elastically and leads to a decrease in activation energy. It can be seen from Fig. 4 that the activation energy is roughly half of the optical gap, which is a general feature in chalcogenides. This is generally valid for electronic conduction in chalcogenide glasses. The presence of this feature in the present case shows that it may be a coincidence that the activation energy for ionic conduction and hole conduction are nearly the same as in the Ag–As–S [12]. 3.3. Steady state and transient photoconductivity The variation of the photocurrent with light intensity is shown in Fig. 5. It is observed that this variation obeys the power law: Iph = F γ
(2)
Fig. 5. Variations of photocurrent with light intensity for Se80−x Sb20 Agx (0 ≤ x ≤ 16). Se80 Sb20 , Se78 Sb20 Ag2 , N Se76 Sb20 Ag4 , Se72 Sb20 Ag8 , ♦ Se68 Sb20 Ag12 , 4 Se64 Sb20 Ag16 .
where Iph is the photocurrent (total current minus dark current), F is the intensity of the light and γ is the exponent, which depends on the recombination mechanism. The value of γ ∼ 0.5 indicates a bimolecular recombination process, whereas γ ∼ 1.0 indicates a monomolecular recombination mechanism [28]. The value of the exponent lies between 0.5 and 1.0 for continuous distribution of traps [29]. In the present work the photocurrent was found to be square-rootdependent on the illuminated light intensity, i.e. γ ∼ 0.5 for all the samples. This indicates the existence of bimolecular recombination in Se80−x Sb20 Agx (0 ≤ x ≤ 16) films. Tanaka et al. have reported similar variation of photocurrent with light intensity for measurements at 1 Hz for the As–S–Ag
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Fig. 6. Rise in photocurrent with time for Se80−x Sb20 Agx (x = 16) films at different illumination intensities of light. 200 lx, 400 lx, 4 600 lx, 800 lx, ♦ 1000 lx, N 1400 lx.
system [23]. The transient photoconductivity was determined by exposing all the samples to visible light from a tungsten halogen lamp and simultaneously recording the current. Fig. 6 shows the rise of the photocurrent in Se64 Sb20 Ag16 with time at room temperature at different illumination levels. For these measurements light was shone on the sample for 100 s. It is clear from the Fig. 6 that the photocurrent rises monotonically to the steady state value at lower intensities (<400 lx). Such a rise in photocurrent is common in chalcogenide glasses [21,30– 33]. However, at higher intensities (>600 lx), the photocurrent decreases after passing through a maximum value. This type of behavior is not common in chalcogenide glasses. This type of behavior has only been reported in a few other systems (As2 Se3, Ge22 Se78 , AgAs(Ge)S, Se75 Te20 Ag5 ), [34,35]. When Ag is added into the Se-based glasses, Ag–Se ionic bonds are formed. In Ag-containing chalcogenide glasses, + µh > µ + Ag and µh µe , where µh , µAg and µe are mobilities of the hole, Ag+ ion and electron respectively [24]. When light is incident on the sample with applied bias, the photoexcited holes may flow from the positive electrode to the negative electrode. Ag+ ions may not influence the hole flow if the distribution of Ag+ ions is uniform. The photoexcited electrons will not contribute to the photocurrent, since their mobility is negligibly small. Therefore the photocurrent may be considered as only due to the hole flow. In the present case, the photocurrent initially rises and becomes constant with time for lower intensities. The rise of photocurrent is due to the increase of photo-generated holes and it becomes constant once the steady state is reached. At higher intensities of light, the photocurrent rises to a maximum and then slightly decreases to a lower value. The decrease may be due to the appearance of a negative Dember voltage [23,24] and the interaction between the holes and the trapped electrons on the surface. In Agcontaining chalcogenide glasses, upon illumination, a negative Dember photovoltage has been reported [23]. The cause of
Fig. 7. Variations of photosensitivity (Iph /Id ) with Ag concentration.
the negative Dember voltage is the non-uniform generation of carriers due to non-uniform absorption. More electron–hole pairs are generated at the surface due to higher absorption of photons there. The carriers diffuse away from the surface due to the concentration gradient. The holes, being more mobile in chalcogenide, diffuse faster as compared to electrons thereby generating a negative Dember voltage. The electrons at the surface recombine with the photogenerated holes decreasing the photocurrent. But this negative Dember voltage is short lived, since the positive mobile silver ions move towards the surface in response to the above voltage. Hence the Dember voltage decays with a time constant that depends on the ionic motion [23]. In conventional semiconductors the surface acquires a positive polarity due to the higher mobility of electrons [36]. The increased concentration of surface electrons causes the increase in recombination of holes and thereby a decrease in the photocurrent. The photosensitivity, calculated as Iph /Id for all samples at 1400 lx, is shown in Fig. 7. Photosensitivity is found to increase with increasing Ag content in the Se–Sb matrix. The photosensitivity lies between 2.06 and 3.50. 4. Conclusion Thin films of Se80−x Sb20 Agx (0 ≤ x ≤ 16) have been prepared by thermal evaporation. The dark conductivity increases with increasing Ag content and it is found to be activated over the entire temperature range. The activation energy is roughly half of the optical gap, which is a general feature in chalcogenides. The photocurrent is found to be square-root-dependent on illumination intensity, which indicates bimolecular recombination kinetics. The photoconductivity increases initially and saturates thereafter for low intensities (<400 lx) of light but for higher intensities (>600 lx) the photocurrent increases initially, attains a maximum, and then decreases with time. The results have been explained on the basis of the Dember voltage and interaction
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between the photoexcited holes and the trapped electrons on the surface. The photosensitivity lies between 2.06 and 3.50 for Se80−x Sb20 Agx (0 ≤ x ≤ 16). References [1] Z.L. Mao, H. Chen, A.I. Jung, J. Appl. Phys. 78 (1995) 2338. [2] K. Hayashi, K. Daisaka, K. Shimakawa, J. Non-Cryst. Solids 198–200 (1996) 696. [3] M. Mitkova, T. Petkova, P. Markovski, V. Mattev, J. Non-Cryst. Solids 164–166 (1993) 1203. [4] K. Morigaki, Physics of Amorphous Semiconductors, Imperial College Press, London, 1999. [5] Y.Y. Chang, L.H. Chou, Japan J. Appl. Phys. Part 2 39 (2000) L 294. [6] G.F. Zhou, Mater. Sci. Eng. A 73 (2001) A304. [7] L.H. Chou, Y.Y. Chang, Y.C. Chai, S.Y. Wang, J. Appl. Phys. Part 1 40 (2001) 4924. [8] J. Li, L. Hou, H. Raun, Q. Xie, F. Gan, Proceedings SPIE - Int. Soc. Opt. Eng. 125 (2001) 4085. [9] T. Wagner, M. Frumar, S.O. Kasap, Mir. Vlcek, Mil. Vlcek, J. Optoelectron. Adv. Mater. 2 (2001) 227. [10] Z.U. Borisova, Glassy Semiconductors, Plenum Press, New York, 1981. [11] K. Tanaka, J. Non-Cryst. Solids 164–166 (1993) 1179. [12] M. Ohto, M. Itho, K. Tanaka, J. Appl. Phys. 77 (1995) 1034. [13] M. Mitkova, M.N. Kozicki, H.C. Kim, T.L. Alford, J. Non-Cryst. Solids 338–340 (2004) 552.
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[14] J.K. Olson, H. Li, P.C. Taylor, J. Ovonic Res. 1 (2005) 1. [15] K. Tanaka, J. Non-Cryst. Solids 164–166 (1993) 1179. [16] M. Mirandou, M. Fontona, B. Arcondo, J. Mater. Process. Technol. 143 (2003) 420. [17] M. Ohto, Phys. Status Solidi A 159 (1997) 461. [18] Y. Kawamoto, M. Nishida, J. Non-Cryst. Solids 20 (1976) 393. [19] C.J. Benmore, P.S. Salmon, Phys. Rev. Lett. 73 (1994) 264. [20] A. Pradel, M. Ribes, J. Solid State Chem. 96 (1992) 247. [21] M.S. Kamboj, G. Kaur, R. Thangaraj, Thin Solid Films 350 (2002) 420. [22] T. Kawaguchi, S. Maruna, K. Tanaka, J. Non-Cryst. Solids 164 (1993) 1231. [23] K. Tanaka, M. Itho, M. Yoshida, M. Ohto, J. Appl. Phys. 78 (1995) 3895. [24] D. Sharma, R.K. Shukla, A. Kumar, Thin Solid Films 357 (1999) 214. [25] M. Mitkova, Yu Wang, P. Boolchand, Phys. Rev. Lett. 83 (1999) 3848. [26] P. Boolchand, W.J. Bresser, Nature 410 (2001) 1070. [27] E. Brchkov, V. Tsegelnik, Yu. Vlasov, A. Pradel, M. Ribes, J. Non-Cryst. Solids 208 (1996) 1. [28] R.H. Bube, Photoconductivity of Solids, Wiley, New York, 1960. [29] A. Rose, Concepts in Photoconductivity, Interscience, New York, 1960. [30] R. Mathur, A. Kumar, Solid State Commun. 59 (1986) 163. [31] R. Mathur, A. Kumar, Solid State Commun. 61 (1987) 785. [32] E.A. Fagen, H. Fritzache, J. Non-Cryst. Solids 2 (1970) 180. [33] S.K. Tripathi, A. Kumar, Thin Solid Films 165 (1988) 99. [34] T. Kawaguchi, S. Maruna, J. Appl. Phys. 79 (1996) 9096. [35] S. Goel, A. Kumar, Solid State Commun. 64 (1987) 371. [36] P.S. Kireev, Semiconductor Physics, Mir Publishers, Moscow, 1978.
Effect of Ag addition on the photoconductivity of amorphous Se–Sb thin films K.S. Bindra, N. Suri, P. Kumar, R. Thangaraj ∗ Semiconductors Laboratory, Department of Applied Physics, Guru Nanak Dev University, Amritsar, 143005, India Received 30 April 2007; accepted 15 May 2007 by A.H. MacDonald Available online 21 May 2007
Abstract Amorphous thin films of Se80−x Sb20 Agx (0 ≤ x ≤ 16) have been prepared by a thermal evaporation technique. The dark conductivity and the photoconductivity of Se80−x Sb20 Agx (0 ≤ x ≤ 16) thin films have been determined. Dark conductivity increases with the increase in the silver content. The photoconductivity of these films has been studied at different illumination intensities. With illumination the photocurrent increases initially with time and then saturates at a constant value for low intensities of light (<400 lx), however, with higher intensities (>600 lx) the photocurrent increases initially, attains a maximum, and then decreases with time. The results have been explained on the basis of the Dember voltage and the interaction between the photogenerated holes and the trapped electrons on the surface. The variation of photocurrent with illumination intensity indicates that the recombination mechanism is bimolecular. The photosensitivity of these samples lies between 2.06 and 3.50. c 2007 Elsevier Ltd. All rights reserved.
PACS: 71.20.Nr; 72.20.-I; 78.66.Jg Keywords: A. Amorphous thin films; D. Dark conductivity; D. Photoconductivity; D. Dember voltage
1. Introduction Chalcogenide glasses are a very important class of semiconductor materials because of their technological applications in infrared optical fibers, xerography, switching and memory devices, etc. [1–4]. Chalcogenide glasses containing Ag, generally, exhibit single glass transition and single crystallization temperatures, which are important conditions for rewritable disks. Thin films of chalcogenide glasses containing Ag have found application in erasable phase change optical recording devices [5–9]. Ag-doped chalcogenide glasses are preferred because of their ionic nature, i.e. the electrical conductivity is governed by Ag+ ionic conduction since the hole conduction is substantially smaller and electron conductivity is not detected [10–12]. There have been several reports on the structural and ionic conductivity of these ionconducting chalcogenide glasses [13–20], however, only a few
∗ Corresponding author. Tel.: +91 183 2258802 3165; fax: +91 183 2258819.
E-mail address: [email protected] (R. Thangaraj). c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.05.014
studies have been done on the photoelectrical properties of silver doped chalcogenide thin films [21–24]. The present work reports on photoconductivity measurements in Ag doped Se–Sb glassy systems. The rise of photoconductivity has been carried out in thermally evaporated thin films of amorphous Se80−x Sb20 Agx (0 ≤ x ≤ 16). The effect of different illumination intensities of light on the photoconduction has also been investigated. 2. Experimental details Bulk samples of Se80−x Sb20 Agx (0 ≤ x ≤ 16) were prepared by a conventional melt quenching technique. Highpurity (99.99%) elements with appropriate atomic percentage were sealed in a quartz ampoule (length ∼ 10 cm and internal diameter ∼ 6 mm), in a vacuum of ∼10−4 mbar. The ampoules were kept in a vertical furnace for 48 h. The temperature was raised to 1273 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 material was separated from the quartz ampoule by dissolving
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the ampoule into a solution of HF + H2 O2 for approximately 48 h. Using this as source material, thin films were deposited onto well-cleaned glass substrates by a thermal evaporation technique in a vacuum better then 10−4 mbar using a Hind High Vacuum coating unit (model 12A4D). The thickness of the films was measured by the Tolansky interference method. The amorphous nature of the thin films was confirmed by the absence of sharp peaks in the X-ray diffractograms. The surface microstructure was examined using a scanning electron microscope (SEM). The composition of the thin films was analyzed by electron probe microanalysis (EPMA) using a JEOL JXA 8600 M superprobe with accelerator voltage 15 keV, with probe diameter 5 µm and probe current 50 nA. The optical transmission spectrum was recorded at room temperature for all samples using an UV–visible spectrophotometer (UV-160A Shimadzu, Japan) in the wavelength range 300–1100 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 a widely accepted procedure. The conductivity measurements were carried out in the temperature range 273–333 K in a running vacuum of 10−3 mbar. Electrical contacts with an electrode gap of ∼2 mm in a coplanar geometry were made using silver paint. The ohmic nature of the contact was verified by a straight line passing through the origin of a voltage versus current plot. For photoconductivity measurements the sample was mounted inside a metallic cryostat with a transparent window. All the measurements were made in a vacuum of ∼10−3 mbar. A tungsten halogen lamp (Halonix, India) of 500 W was used for illumination. The infrared (IR) part of the light was cut off using IR filters. 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 discussions 3.1. Characterization
Fig. 1. Scanning electron microscope image of Se80 Sb20 film showing Sb2 Se3 nanophase (white spot).
Fig. 2. Plot of (αhν)1/2 versus hν for Se80−x Sb20 Agx (0 ≤ x ≤ 16), Se80 Sb20 , Se78 Sb20 Ag2 , N Se76 Sb20 Ag4 , Se72 Sb20 Ag8 , ♦ Se68 Sb20 Ag12 , 4 Se64 Sb20 Ag16 .
3.2. Optical energy gap and dark conductivity The X-ray diffraction shows the presence of only broad features and the absence of any sharp peaks in the diffractograms. These broad features indicate that the samples are amorphous in nature. The surface morphology of the samples was examined using SEM. The scanning electron micrograph of Se80 Sb20 film is shown in Fig. 1. The SEM picture shows the presence of nano particles embedded in an amorphous background. The presence of this nanophase in films with x = 0 indicates that the nanophase may be due to Sb2 Se3 . The addition of Ag has been found to segregate Ag2 Se nanophase [25,26]. As a result, Ag-rich and Sb-rich nanophases may both coexist in the present Se–Sb–Ag system. EPMA studies have shown that the actual composition of the elements in thin films differ from that of the bulk glass by about 2%, 5% and 1% for silver, antimony and selenium respectively.
Fig. 2 shows a plot of (αhν)1/2 versus hν for the asdeposited Se80−x Sb20 Agx (0 ≤ x ≤ 16) thin films. The opt optical energy gap (E g ) is calculated from the intercept by extrapolating the straight portions of the lines to the energy axis. The optical gap varies from 1.51 to 1.29 eV as x varies from 0 to 16. The decrease in the optical gap with increase in Ag content may be due to an increase in disorder and defects. The dark conductivity of Se80−x Sb20 Agx (0 ≤ x ≤ 16), measured as a function of temperature, is shown in Fig. 3. It is observed that the dark conductivity increases with an increase in temperature, is found to be activated over the entire temperature range and follows the Arrhenius equation: σ = σo e−∆ Ea /kT
(1)
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opt
Fig. 4. Variations of optical energy gap (E g ) and activation energy (∆E a ) with Ag concentration. (x = 2 and 4 show two activation energies). Fig. 3. Variations of dark conductivity with temperature for Se80−x Sb20 Agx (0 ≤ x ≤ 16) films. Se80 Sb20 , Se78 Sb20 Ag2 , N Se76 Sb20 Ag4 , Se72 Sb20 Ag8 , ♦ Se68 Sb20 Ag12 , 4 Se64 Sb20 Ag16 .
where σo is the pre-exponential factor, ∆E a is the activation energy, k is the Boltzmann constant and T is the absolute temperature. The activation energy for films with x = 0 is for the electronic conduction. With the addition of Ag, the conduction becomes mixed. The two activation energies (for x = 2, 4) may be due to the different conduction mechanisms predominating in the relevant temperature regions. In films with higher Ag contents (for x > 4) the predominant conduction may be due to Ag+ ions. Similar results have been reported in Ag–Ge–Sb–Se glasses by Bychkov et al. [27]. Fig. 4 shows the variation of activation energy and optical energy gap with opt x. It can be observed that both ∆E a and E g follow the same trend. As a consequence of silver addition to Se80 Sb20 system, a significant decrease of the activation energy from 0.61 to 0.49 eV is observed. It seems with the addition of Ag, an Ag-rich phase separation occurs in the Se-rich matrix [25,26]. This macroscopic phase separation of the Ag2 Se amorphous phase probably softens the network elastically and leads to a decrease in activation energy. It can be seen from Fig. 4 that the activation energy is roughly half of the optical gap, which is a general feature in chalcogenides. This is generally valid for electronic conduction in chalcogenide glasses. The presence of this feature in the present case shows that it may be a coincidence that the activation energy for ionic conduction and hole conduction are nearly the same as in the Ag–As–S [12]. 3.3. Steady state and transient photoconductivity The variation of the photocurrent with light intensity is shown in Fig. 5. It is observed that this variation obeys the power law: Iph = F γ
(2)
Fig. 5. Variations of photocurrent with light intensity for Se80−x Sb20 Agx (0 ≤ x ≤ 16). Se80 Sb20 , Se78 Sb20 Ag2 , N Se76 Sb20 Ag4 , Se72 Sb20 Ag8 , ♦ Se68 Sb20 Ag12 , 4 Se64 Sb20 Ag16 .
where Iph is the photocurrent (total current minus dark current), F is the intensity of the light and γ is the exponent, which depends on the recombination mechanism. The value of γ ∼ 0.5 indicates a bimolecular recombination process, whereas γ ∼ 1.0 indicates a monomolecular recombination mechanism [28]. The value of the exponent lies between 0.5 and 1.0 for continuous distribution of traps [29]. In the present work the photocurrent was found to be square-rootdependent on the illuminated light intensity, i.e. γ ∼ 0.5 for all the samples. This indicates the existence of bimolecular recombination in Se80−x Sb20 Agx (0 ≤ x ≤ 16) films. Tanaka et al. have reported similar variation of photocurrent with light intensity for measurements at 1 Hz for the As–S–Ag
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Fig. 6. Rise in photocurrent with time for Se80−x Sb20 Agx (x = 16) films at different illumination intensities of light. 200 lx, 400 lx, 4 600 lx, 800 lx, ♦ 1000 lx, N 1400 lx.
system [23]. The transient photoconductivity was determined by exposing all the samples to visible light from a tungsten halogen lamp and simultaneously recording the current. Fig. 6 shows the rise of the photocurrent in Se64 Sb20 Ag16 with time at room temperature at different illumination levels. For these measurements light was shone on the sample for 100 s. It is clear from the Fig. 6 that the photocurrent rises monotonically to the steady state value at lower intensities (<400 lx). Such a rise in photocurrent is common in chalcogenide glasses [21,30– 33]. However, at higher intensities (>600 lx), the photocurrent decreases after passing through a maximum value. This type of behavior is not common in chalcogenide glasses. This type of behavior has only been reported in a few other systems (As2 Se3, Ge22 Se78 , AgAs(Ge)S, Se75 Te20 Ag5 ), [34,35]. When Ag is added into the Se-based glasses, Ag–Se ionic bonds are formed. In Ag-containing chalcogenide glasses, + µh > µ + Ag and µh µe , where µh , µAg and µe are mobilities of the hole, Ag+ ion and electron respectively [24]. When light is incident on the sample with applied bias, the photoexcited holes may flow from the positive electrode to the negative electrode. Ag+ ions may not influence the hole flow if the distribution of Ag+ ions is uniform. The photoexcited electrons will not contribute to the photocurrent, since their mobility is negligibly small. Therefore the photocurrent may be considered as only due to the hole flow. In the present case, the photocurrent initially rises and becomes constant with time for lower intensities. The rise of photocurrent is due to the increase of photo-generated holes and it becomes constant once the steady state is reached. At higher intensities of light, the photocurrent rises to a maximum and then slightly decreases to a lower value. The decrease may be due to the appearance of a negative Dember voltage [23,24] and the interaction between the holes and the trapped electrons on the surface. In Agcontaining chalcogenide glasses, upon illumination, a negative Dember photovoltage has been reported [23]. The cause of
Fig. 7. Variations of photosensitivity (Iph /Id ) with Ag concentration.
the negative Dember voltage is the non-uniform generation of carriers due to non-uniform absorption. More electron–hole pairs are generated at the surface due to higher absorption of photons there. The carriers diffuse away from the surface due to the concentration gradient. The holes, being more mobile in chalcogenide, diffuse faster as compared to electrons thereby generating a negative Dember voltage. The electrons at the surface recombine with the photogenerated holes decreasing the photocurrent. But this negative Dember voltage is short lived, since the positive mobile silver ions move towards the surface in response to the above voltage. Hence the Dember voltage decays with a time constant that depends on the ionic motion [23]. In conventional semiconductors the surface acquires a positive polarity due to the higher mobility of electrons [36]. The increased concentration of surface electrons causes the increase in recombination of holes and thereby a decrease in the photocurrent. The photosensitivity, calculated as Iph /Id for all samples at 1400 lx, is shown in Fig. 7. Photosensitivity is found to increase with increasing Ag content in the Se–Sb matrix. The photosensitivity lies between 2.06 and 3.50. 4. Conclusion Thin films of Se80−x Sb20 Agx (0 ≤ x ≤ 16) have been prepared by thermal evaporation. The dark conductivity increases with increasing Ag content and it is found to be activated over the entire temperature range. The activation energy is roughly half of the optical gap, which is a general feature in chalcogenides. The photocurrent is found to be square-root-dependent on illumination intensity, which indicates bimolecular recombination kinetics. The photoconductivity increases initially and saturates thereafter for low intensities (<400 lx) of light but for higher intensities (>600 lx) the photocurrent increases initially, attains a maximum, and then decreases with time. The results have been explained on the basis of the Dember voltage and interaction
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