Effect of Gamma Radiation on Electrical and Optical Properties of (Teo2)0·9 (In2o3)0·1 Thin ﬁlms

Bull. Mater. Sci., Vol. 34, No. 1, February 2011, pp. 61–69. c Indian Academy of Sciences.

Effect of gamma radiation on electrical and optical properties of (TeO2)0·9 (In2O3)0·1 thin ﬁlms

S L SHARMA∗ and T K MAITY Department of Physics & Meteorology, Indian Institute of Technology, Kharagpur 721 302, India

MS received 13 March 2010

Abstract. We have studied in detail the gamma radiation induced changes in the electrical properties of the (TeO2)0·9 (In2O3)0·1 thin films of different thicknesses, prepared by thermal evaporation in vacuum. The currentÐ voltage characteristics for the as-deposited and exposed thin films were analysed to obtain current versus dose plots at different applied voltages. These plots clearly show that the current increases quite linearly with the radiation dose over a wide range and that the range of doses is higher for the thicker films. Beyond certain dose (a quantity dependent on the film thickness), however, the current has been observed to decrease. In order to understand the dose dependence of the current, we analysed the optical absorption spectra for the as-deposited and exposed thin films to obtain the dose dependences of the optical bandgap and energy width of band tails of the localized states. The increase of the current with the gamma radiation dose may be attributed partly to the healing effect and partly to the lowering of the optical bandgap. Attempts are on to understand the decrease in the current at higher doses. Employing dose dependence of the current, some real-time gamma radiation dosimeters have been prepared, which have been found to possess sensitivity in the range 5Ð55 μGy/μA/cm2. These values are far superior to any presently available real-time gamma radiation dosimeter.

Keywords. Thin ﬁlms; tellurium dioxide; indium oxide; dosimeter; sensitivity.

1. Introduction due to its large size. Naturally, availability of a simple hand- held real-time radiation dosimeter will bring improvement in The trend of radiation awareness has greatly improved over the situation drastically. the last two decades or so. This has become so because of The exposure of every solid material to ionizing radi- the fact that ionizing radiations (such as X-rays, gamma rays, ations produces changes in the microstructural properties beta particles, alpha particles, fission fragments, etc) are find- of the material, which in turn affects the optical, electrical ing more and more applications in several fields including and other physical properties of the material (Ibrahim and industry, medicine, military, research, nuclear power produc- Soliman 1998; Atanassova et al 2001; Clough 2001). Efforts tion, etc. Unfortunately, almost everywhere radiation expo- are on world over to investigate the influence of gamma radi- sure may occur and strict monitoring of radiation levels is ation on thin films and thin film structures of different metal very essential to ensure that the surrounding environment oxides and polymers, in order to find out the suitability of remains harmless for the workers involved. To ease monitor- using thin films and thin film structures of different metal ing of the radiation levels, it is important to develop novel oxides and polymers as post-exposure as well as real-time cost-effective radiation sensors with real-time response that gamma radiation dosimeters (Colby et al 2002; Arshak et al would initiate an immediate alert signal in case of emergency. 2004, 2006; Arshak and Korostynska 2006; Kumar et al Gamma radiation dosimeters can be of a variety of types 2006; Maity and Sharma 2008; Sharma et al 2008). Natu- according to their physical detection principles and materi- rally, a deep understanding of the physical properties of these als used. A simple hand-held real-time radiation dosimeter thin films and thin film structures under the influence of is usually not available, though it is highly needed. Gene- gamma radiation is quite important from the viewpoint of rally, policemen take response to an emergency situation but design of novel real-time gamma radiation dosimeters poss- they may not be aware that the situation might be involving essing high sensitivity (i.e. large change in the output signal some radioactive materials and they will not even think about for small change in the radiation dose). The effect of gamma bringing a Geiger–Mueller counter based radiation dosimeter radiation on the microstructural, optical and electrical properties of thin films of tellurium dioxide (TeO2) as well as thin films of the mixture of tellurium dioxide and indium oxide (TeO2–In2O3) has been the subject of seve- ∗Author for correspondence ([email protected]) ral experimental investigations in the recent past (Arshak and

61 62 S L Sharma and T K Maity

Korostynska 2003a, 2006; Arshak et al 2006; Kumar et al pared. The current versus applied voltage plots for the as- 2006; Maity and Sharma 2008; Sharma et al 2008). Also, deposited thin films as well as for the thin films of the three Arshak and Korostynska (2003b) have recently reported that thicknesses mentioned above exposed to different levels of In2O3 plays an important role in regard to increasing the free gamma radiation dose were then recorded. Subsequently, for carrier concentration whenever it is doped into metal oxides each of the three film thicknesses, these plots were analy- such as TiO2,TeO2, NiO, etc. Naturally, there is a strong sed to obtain variations of the current density with gamma necessity of further studying the effect of gamma radiation radiation dose at different voltages in the voltage range 2– on different physical properties of thin films of TeO2–In2O3 5 V applied to the thin film structure. For each of the cur- mixture having varying compositions. rent density versus gamma dose plots, so obtained, the slope The effect of ionizing radiations depends on both the radia- (representing change in the current density per unit change tion dose and the parameters of the films including film thick- in the gamma radiation dose) has been estimated for the line- ness. It is already an established fact that the degradation is ar or near linear portion of the plot. Subsequently, the sen- much more severe for the higher radiation doses and thinner sitivity for each of the three film thicknesses at each voltage films (Arshak and Korostynska 2006). Present work aims to applied to the thin film structure has been estimated. In order study in detail the gamma radiation induced changes in the to understand these electrical measurements, we have also electrical properties of thin films of (TeO2)0·9 (In2O3)0·1 of recorded optical absorption spectra for the as-deposited thin different thicknesses for a much wider range of gamma radia- films as well as for the thin films of (TeO2)0·9 (In2O3)0·1 tion doses than done here-to-fore. The work has been carried of thickness 220 nm exposed to different levels of gamma out in order to understand the dose dependence of the current radiation dose using Perkin-Elmer Lambda 45 UV-Visible for the thin films of different thicknesses and then to deter- Spectrometer for the wavelengths in the range 270–770 nm. mine the thickness dependence of the range of gamma radi- Subsequently, these optical absorption spectra have been ation doses over which the current versus dose plot remains analysed to obtain dose dependence of the optical bandgap linear or near linear. Further, work has been carried out with and that of the energy width of band tails of the localized the idea of determining sensitivity (in terms of the radia- states. tion dose required to produce unit change in the output current) for each of the film thicknesses studied here. In order to understand these variations, we have also studied the effect 3. Results and discussion of gamma radiation on the optical properties of thin films of (TeO2)0·9 (In2O3)0·1 for a much wider range of gamma The typical plots of the current versus applied voltage for radiation doses. These measurements are expected to play a thethinfilmsof(TeO2)0·9 (In2O3)0·1 of thickness 300 nm, very important role in the design of real-time gamma radia- recorded for the as-deposited thin films as well as for the thin tion dosimeters of different sensitivities which can be used films exposed to different levels of gamma radiation dose, to monitor ionizing radiations under a variety of practical are shown in figure 1. Subsequently, these plots were analy- situations. sed to obtain variations of the current density with gamma

2500 2. Experimental Thickness = 300 nm 80 Gy

Employing thermal evaporation in a vacuum, thin films of 2000 100 Gy (TeO2)0·9 (In2O3)0·1 of thicknesses 300, 450 and 600 nm were prepared on suitably prepared glass substrates. During 60 Gy film deposition, the vacuum inside the chamber was of the 1500 40 Gy −5 60 order of 10 mbar. At room temperature, a disc-type Co 20 Gy gamma radiation source was used to expose these thin films to various levels of gamma radiation dose. Thin films, hav- Current ( A) 1000 ing planar configuration as described earlier (Kumar et al As-deposited 2006), were prepared for electrical measurements. Two rect- angular layers of aluminium, each of thickness, 150 nm and 500 separated by a linear gap of 3 mm, were deposited on each of the suitably prepared glass substrates to act as electri- 0 cal contacts. A tungsten filament in the form of a coil was 0.0 1.1 2.2 3.3 4.4 5.5 used for the evaporation of aluminium contacts. On the top Applied Voltage (Volts) of each pair of aluminium contacts, a thin film of (TeO2)0·9 ) (In2O3 0·1 of thickness 300 or 450 or 600 nm was deposited Figure 1. Typical current vs applied voltage plots for the from a tungsten boat. In this manner, several samples of (TeO2)0·9 (In2O3)0·1 thin films of thickness, 300 nm, exposed to each of the three thicknesses mentioned above were pre- different levels of gamma radiation dose. Effect of gamma radiation on electrical and optical properties of (TeO2)0·9 (In2O3)0·1 thin films 63 radiation dose at different voltages applied to the thin film 6.5 Thickness = 300 nm structure. These variations of the current density with the gamma radiation dose at different voltages applied to the (g) 5.2 thin film structure in the voltage range 2–5 V are shown ) (f) in figure 2. Clearly, at different applied voltages, the cur- -2 rent density has been found to increase quite linearly with 3.9 (e) the gamma radiation dose over a dose range of 0–80 Gy. Beyond the gamma radiation dose of 80 Gy, however, the (d) current density has been observed to show a reverse trend 2.6 (c) at all applied voltages. For each of the current density ver-

sus gamma dose plots, so obtained, the slope (representing Current Density (Acm (b) current density per unit gamma radiation dose) has been esti- 1.3 mated for the linear portion of the plot. The slopes for the (a) linear portions of these plots have been found to be in the 0.0 2 range 42–210 mA/cm /Gy. The corresponding sensitivities 0 22 44 66 88 110 of the film at different applied voltages have been found Gamma Radiation Dose (Gy) to be in the range 4·7–23·8 μGy/μA/cm2. For the sake of comparison, the variations of the current density with the Figure 3. Plots of the current density vs gamma radiation dose gamma radiation dose for the TeO2 thin films of thickness for the TeO2 thin films of thickness, 300 nm, for applied voltages 300 nm at different voltages applied to the thin film struc- (V)of(a)2·4, (b) 2·8, (c) 3·2, (d) 3·6, (e) 4·0, (f) 4·4and(g)4·8. ture in the voltage range 2–5 V are shown in figure 3, a figure obtained for the recently published results of Maity and 1.2 Sharma (2008). Clearly, for the TeO2 thin films of thickness, 300 nm, also at different applied voltages, the current den- Thickness = 300 nm sity has been found to increase quite linearly with the gamma 1.0 radiation dose over a dose range of 0–80 Gy. Beyond the gamma radiation dose of 80 Gy, however, the current density has been observed to show a reverse trend at all applied 0.8 voltages. The slopes for the linear portions of these plots have been found to be in the range 8·5–37·0 mA/cm2/Gy and 0.6 the corresponding sensitivities of the TeO2 film of thickness, 300 nm, at different applied voltages have been found to be

0.4 Normalized Current 40 Thickness = 300 nm (g) 0.2 35

30 0.0

) (f) 0 30 60 90 120 2 25 Gamma Radiation Dose (Gy) (e) 20 Figure 4. Typical variation of the normalized current with gamma ) ) (d) radiation dose for the (TeO2 0·9 (In2O3 0·1 thin ﬁlms of thickness, 15 · (c) 300 nm, at an applied voltage of 3 5V.

10 (b) Current Density (A/cm · · 2 5 (a) in the range 27 0–117 6 μGy/μA/cm . Clearly, the sensitivity of (TeO2)0·9 (In2O3)0·1 thin films of thickness, 300 nm, 0 is 5–6 times better than that of TeO2 thin films of the same 0 20 40 60 80 100 120 thickness, 300 nm. Gamma Radiation Dose (Gy) In order to understand relative change in the current due to gamma radiation exposure of different levels, a quantity Figure 2. Plots of the current density vs gamma radiation dose for called normalized current, IN, is defined as the (TeO ) · (In O ) · thin films of thickness, 300 nm, for applied 2 0 9 2 3 0 1 − voltages (V) of (a) 2·4, (b) 2·8, (c) 3·2, (d) 3·6, (e) 4·0, (f) 4·4and I I0 IN = , (1) (g) 4·8. I0 64 S L Sharma and T K Maity where I denotes the current for the thin films exposed to a 40 Thickness = 450 nm (g) certain level of radiation dose and I0 the current for the as- deposited thin films. Figure 4 shows typical variation of the 35

normalized current with the gamma radiation dose at a volt- ) 2 30 age of 3·5 V applied to the thin film structure of (TeO2)0·9 (f) (In2O3)0·1 of thickness, 300 nm. Clearly, the normalized cur- 25 rent increases quite linearly with the gamma radiation dose up to a dose of 80 Gy. Beyond the dose of 80 Gy, however, the 20 (e) normalized current has also been observed to show a reverse (d) trend. A comparison of figure 4 with figure 6 of Maity and 15

Sharma (2008) clearly shows that the dose dependence of the Current Density (A/cm (c) 10 normalized current for the thin films of (TeO2)0·9 (In2O3)0·1 (b) of thickness, 300 nm, is quite similar to that for the TeO2 thin 5 (a) films of the same thickness, 300 nm. Figure 5 shows typical plots of the current versus 0 applied voltage for the thin films of (TeO2)0·9 (In2O3)0·1 0 20 40 60 80 100 120 140 160 of thickness, 450 nm, recorded for the as-deposited thin Gamma Radiation Dose (Gy) films as well as for the thin films exposed to different levels of gamma radiation dose. The corresponding Figure 6. Plots of the current density vs gamma radiation dose for ) ) variations of the current density with the gamma radiation the (TeO2 0·9 (In2O3 0·1 thin films of thickness, 450 nm, for applied · · · · · · dose at different voltages applied to the thin film structure voltages (V) of (a) 2 4, (b) 2 8, (c) 3 2, (d) 3 6, (e) 4 0, (f) 4 4and (g) 4·8. in the voltage range 2–5 V are shown in figure 6. Clearly, at different applied voltages, the current density has been found to increase quite linearly with the gamma radiation dose over a dose range of 0–120 Gy. Beyond the gamma radia- 6 tion dose of 120 Gy, however, the current density has been Thickness = 450 nm observed to show a reverse trend at all applied voltages. The 5 slopes for the linear portions of these plots have been found (g) 2 to be in the range 39–208 mA/cm /Gy. The corresponding ) sensitivities of the film at different applied voltages have -2 4 (f) been found to be in the range 4·8–25·6 μGy/μA/cm2.For (e) 3

(d) 4000 2 (c)

Thickness = 450 nm Current Density (Acm (b) 1 3200 120 Gy 140 Gy (a) 100 Gy 0 80 Gy 2400 0 30 60 90 120 150 Gamma Radiation Dose (Gy) 60 Gy

40 Gy Figure 7. Plots of the current density vs gamma radiation dose

Current ( A) 1600 20 Gy for the TeO2 thin ﬁlms of thickness, 450 nm, for applied voltages (V)of(a)2·4, (b) 2·8, (c) 3·2, (d) 3·6, (e) 4·0, (f) 4·4and(g)4·8.

800 As-deposited the sake of comparison, the variations of the current density with gamma radiation dose for the TeO2 thin films of thick- 0 0.0 1.1 2.2 3.3 4.4 5.5 ness, 450 nm, at different voltages applied to the thin film Applied Voltage (Volts) structure in the voltage range, 2–5 V, are shown in figure 7, a figure obtained for the recently published results of Maity Figure 5. Typical current vs applied voltage plots for the and Sharma (2008). Clearly, for the TeO2 thin films of (TeO2)0·9 (In2O3)0·1 thin films of thickness, 450 nm, exposed to thickness, 450 nm, also at different applied voltages, the different levels of gamma radiation dose. current density has been found to increase quite linearly Effect of gamma radiation on electrical and optical properties of (TeO2)0·9 (In2O3)0·1 thin films 65 with the gamma radiation dose over a dose range of 0– 5500 Thickness = 600 nm 120 Gy. Beyond the gamma radiation dose of 120 Gy, 160 Gy however, the current density has been observed to show a reverse trend at all applied voltages. The slopes for 4400 200 Gy the linear portions of these plots have been found to be 120 Gy in the range 8·0–25·5 mA/cm2/Gy and the correspond- 80 Gy ing sensitivities of the TeO2 film of thickness, 450 nm, A) 3300 at different applied voltages have been found to be in · · 2 the range 39 2–125 0 μGy/μA/cm . Clearly, the sensiti- 40 Gy ) ) 2200 vity of (TeO2 0·9 (In2O3 0·1 thin films of thickness, 450 nm, Current ( is 5–8 times better than that of TeO2 thin films of the same thickness, 450 nm. Figure 8 shows typical variation of the normalized current 1100 with gamma radiation dose at a voltage of 3·5 V applied to As-deposited thethinfilmsof(TeO2)0·9 (In2O3)0·1 of thickness, 450 nm. Clearly, the normalized current increases quite linearly with 0 gamma radiation dose up to a gamma radiation dose of 0123456 120 Gy. Beyond 120 Gy, however, the normalized current has Applied Voltage (Volts) also been observed to show a reverse trend. A comparison of figure 8 with figure 8 of Maity and Sharma (2008) clearly Figure 9. Typical current vs applied voltage plots for the ) ) shows that the dose dependence of the normalized current for (TeO2 0·9 (In2O3 0·1 thin films of thickness, 600 nm, exposed to different levels of gamma radiation dose. thethinfilmsof(TeO2)0·9 (In2O3)0·1 of thickness, 450 nm, is quite similar to that for the TeO2 thin films of the same thickness, 450 nm. The typical plots of the electrical current vs applied volta- 50 ) ) ge for the thin films of (TeO2 0·9 (In2O3 0·1 of thickness, Thickness = 600 nm 600 nm, recorded for the as-deposited thin films as well as (g) for the thin films exposed to various levels of gamma radi- 40 ation dose, are shown in figure 9. The corresponding varia- ) tions of the current density with the gamma radiation dose 2 at different voltages applied to the thin film structure in the 30 (f)

2.5 20 (e) Thickness = 450 nm Current Density (A/cm (d) 2.0 10 (c) (b) (a) 1.5 0 0 30 60 90 120 150 180 210 Gamma Radiation Dose (Gy)

1.0 Figure 10. Plots of the current density vs gamma radiation dose

Normalized Current for the (TeO2)0·9 (In2O3)0·1 thin ﬁlms of thickness, 600 nm, for applied voltages (V) of (a) 2·4, (b) 2·8, (c) 3·2, (d) 3·6, (e) 4·0, 0.5 (f) 4·4and(g)4·8.

voltage range 2–5 V are shown in figure 10. Clearly, at di- 0.0 0 30 60 90 120 150 fferent applied voltages, the current density has been found Gamma Radiation Dose (Gy) to increase quite linearly with the gamma radiation dose over a dose range of 0–160 Gy. Beyond the gamma radiation dose Figure 8. Typical variation of the normalized current with gamma of 160 Gy, however, the current density has been observed radiation dose for the (TeO2)0·9 (In2O3)0·1 thin films of thickness, to show a reverse trend at all applied voltages. The slopes 450 nm, at an applied voltage of 3·5V. for the linear portions of these plots have been found to be 66 S L Sharma and T K Maity in the range 18–191 mA/cm2/Gy. The corresponding sensi- 7.5 tivities of the film at different applied voltages have been Thickness = 600 nm found to be in the range 5·2–55·5 μGy/μA/cm2.Forthe sake of comparison, variations of the current density with 6.0 gamma radiation dose for the TeO2 thin films of thickness, 600 nm, at different voltages applied to the thin film structure in the voltage range 2–5 V are shown in figure 11, a fig- 4.5 ure obtained for the recently published results of Maity and Sharma (2008). Clearly, for the TeO2 thin films of thickness, 600 nm, also at different applied voltages, the current den- 3.0

sity has been found to increase quite linearly with the gamma Normalized Current radiation dose over a dose range of 0–160 Gy. Beyond the gamma radiation dose of 160 Gy, however, the current den- 1.5 sity has been observed to show a reverse trend at all applied voltages. The slopes for the linear portions of these plots have been found to be in the range 4·2–24·5 mA/cm2/Gy and 0.0 0 40 80 120 160 200 240 the corresponding sensitivities of the TeO2 film of thickness, 600 nm, at different applied voltages have been found to be Gamma Radiation Dose (Gy) in the range 40·8–238·0 μGy/μA/cm2. Clearly, the sensiti- Figure 12. Typical variation of the normalized current with vity of (TeO ) · (In O ) · thin films of thickness, 600 nm, 2 0 9 2 3 0 1 ) ) is 5–8 times better than that of TeO thin films for the same gamma radiation dose for the (TeO2 0·9 (In2O3 0·1 thin films of 2 thickness, 600 nm, at an applied voltage of 3·5V. thickness, 600 nm. Figure 12 shows typical variation of the normalized current with the gamma radiation dose at a voltage of 3·5V applied to the thin films of (TeO2)0·9 (In2O3)0·1 of thickness, In order to understand the above results on the electrical 600 nm. Clearly, the normalized current increases quite lin- properties, we have analysed the optical absorption spectra early with the gamma radiation dose up to a gamma radiation recorded for the as-deposited thin films as well as for the thin dose of 160 Gy. Beyond the dose of 160 Gy, however, the films of (TeO2)0·9 (In2O3)0·1 of thickness, 220 nm, exposed normalized current has also been observed to show a reverse to different levels of gamma radiation dose and have obtained trend. A comparison of figure 12 with figure 10 of Maity and the dose dependence of the optical bandgap as well as that of Sharma (2008) clearly shows that the dose dependence of the the energy width of the band tails of localized states. The said normalized current for the thin films of (TeO2)0·9 (In2O3)0·1 analysis has been carried out in the framework of Maity and of thickness, 600 nm, is quite similar to that for the TeO2 thin Sharma (2008), a framework based on the Urbach’s empiri- films of the same thickness, 600 nm. cal formula for the absorption coefficient near the band edge (Urbach 1953), Mott’s concept in regard to the creation of localized energy states in the normally forbidden energy gap (Mott 1969) and Mott–Davis model for the optical bandgap 7.5 of thin films (Mott and Davis 1979). From the absorption Thickness = 600 nm coefficient versus photon energy plots for the as-deposited (g) thin films as well as for the films exposed to different levels 6.0 ) E -2 of gamma radiation dose, the values of (the energy width

(f) of the band tails of localized states) were calculated from 4.5 the slopes of the straight-line portions (Urbach 1953). With E0 as the energy width of the band tails of localized states (e) for the as-deposited thin films, the variation of the normali- 3.0 zed energy width EN (= (E − E0)/E0) of the band (d) tails of localized states with the dose is shown in figure 13. Current Density (Acm The energy width of the band tails of localized states has 1.5 (c) (b) been found to increase with the gamma radiation dose up to a (a) dose of about 75 Gy. This increase in the energy width of the 0.0 0 55 110 165 220 band tails of localized states can be attributed to the induced Gamma Radiation Dose (Gy) defects due to the exposure to gamma radiation. As the as-deposited thin films are generally amorphous in nature, Figure 11. Plots of the current density vs gamma radiation dose the atomic arrangement in terms of nearest neighbours for the TeO2 thin films of thickness, 600 nm, for applied voltage (V) departs only slightly from the ideal crystalline state. Further- of (a) 2·4, (b) 2·8, (c) 3·2, (d) 3·6, (e) 4·0, (f) 4·4and(g)4·8. more, since the amorphous thin films contain relatively rigid Effect of gamma radiation on electrical and optical properties of (TeO2)0·9 (In2O3)0·1 thin films 67

0.15 optical bandgap with the radiation dose is shown in figure 14. Clearly, the optical bandgap decreases from 2·7to2·5eV for the gamma radiation dose of 75 Gy. This decrease in the 0.12 optical bandgap is basically due to the increase in the energy width of the band tails of localized states. During gamma 0.09 irradiation, the defects are created within the thin film. At the same time, the defects also get annihilated even under the N E normal room temperature conditions (Deng et al 1999). This 0.06 creation and annihilation of defects coexist together and at higher doses of gamma radiation, the number of defects created due to irradiation becomes much more than the number 0.03 of defects annihilated. Arshak and Korostynska (2002) have also reported a similar decrease in the optical bandgap with the increase of gamma radiation dose up to a certain dose 0.00 0 40 80 120 160 for the TeO2 thin films. They, however, did not report for Gamma Radiation Dose (Gy) the doses higher than certain dose. The optical bandgap for the thin films of (TeO2)0·9 (In2O3)0·1 of thickness, 220 nm, Figure 13. Variation of the normalized energy width of band in the present work has, however, been found to increase tails of the localized states with the gamma radiation dose for the with further increase in the gamma radiation dose beyond (TeO2)0·9 (In2O3)0·1 thin films of thickness, 220 nm. 75 Gy. In a recent publication, Arshak et al (2006) have also reported a similar observation for thin films of mixture of MnO/TeO2. The nature of optical properties of thin films of (TeO2)0·9 (In2O3)0·1, described above, is quite similar to the dose- response of most materials used in thermoluminescence chemical bonds, the deviation from the crystalline state dosimetry (Horowitz 2001). These materials usually show a is small resulting in the requirement of excessive energy linear, then super-linear, followed by saturating response and to distort this rigidity. This excessive energy leads to further increase in radiation dose results in the high structural the formation of short-range order (Mott 1969) or cre- disorder. The high structural disorder at very high radiation ation of localized energy states in the normally forbidden doses reduces the sensitivity of material parameters to further energy gap. Whenever gamma radiation interacts with the exposure of radiation. The increase in optical bandgap for thin film, induced defects will be formed and the den- the (TeO2)0·9 (In2O3)0·1 thin films of thickness, 220 nm, for sity of the localized states increases resulting in increase the gamma radiation doses beyond 75 Gy may be attributed in the energy width of the band tails of localized states. Arshak and Korostynska (2002) have also reported that the energy width of the band tails of localized states for the TeO2 thin films of thickness, 50 nm, increased 2.8 with the increase of radiation dose up to a certain value. They, however, did not report for higher doses of gamma radiation. The energy width of the band tails of locali- 2.7 zed states for the thin films of (TeO2)0·9 (In2O3)0·1 of thickness, 220 nm, in the present study has, however, been found to decrease with further increase in the gamma radiation dose beyond 75 Gy. 2.6 The values of the optical bandgap, Eopt,fortheas- deposited thin films as well as for the thin films exposed to different levels of gamma radiation dose have been estimated

2 Optical Bandgap (eV) from the (αν hν) versus photon energy, hν plots using the 2.5 Mott and Davis (1979) model for the direct allowed transition using the equation: α ν = ν − 1/2 , 2.4 ν h B h Eopt (2) 0 40 80 120 160 Gamma Radiation Dose (Gy) where hν denotes the energy of the incident photon and B a constant. The optical bandgap for the as-deposited thin ﬁlms Figure 14. Variation of the optical bandgap with gamma radiation has been found to be about 2·7 eV and the variation of the dose for the thin ﬁlms of (TeO2)0·9 (In2O3)0·1 of thickness, 220 nm. 68 S L Sharma and T K Maity

2 Table 1. Sensitivity (in μGy/μA/cm )fortheTeO2 and (TeO2)0·9 (In2O3)0·1 thin ﬁlms.

Applied Sensitivity for TeO2 thin ﬁlms of thickness Sensitivity for (TeO2)0·9 (In2O3)0·1 thin ﬁlms of thickness voltage 300 nm 450 nm 600 nm 300 nm 450 nm 600 nm

2·4 V 117·6 125·0 238·023·825·655·5 2·8V 83·382·6 135·116·117·827·0 3·2V 62·566·6 102·012·513·320·4 3·6V 40·055·573·58·09·113·5 4·0V 33·345·447·17·68·29·1 4·4V 29·441·644·66·76·86·9 4·8V 27·039·240·84·74·85·2

to a similar high structural disorder at very high radiation results in lowering of the optical bandgap. In turn, the fill- doses. ing of the hole-states causes the current to increase with the Present studies have clearly shown that the gamma radia- increase in the gamma radiation dose up to a certain dose. tion dose up to which the current keeps increasing near lin- The current is, however, observed to decrease with increase early depends upon the thickness of thin films of (TeO2)0·9 in the radiation dose above a certain dose level which may be (In2O3)0·1 and that it is 80 Gy for thin films of thickness, partly attributed to the increase in the optical bandgap (see 300 nm, 120 Gy for thin films of thickness, 450 nm and figure 14). 160 Gy for thin films of thickness, 600 nm. For the thin films The near linear variation of the current with of (TeO2)0·9 (In2O3)0·1 of thicknesses 300, 450 and 600 nm, the gamma radiation dose up to a certain criti- however, the current has been observed to decrease beyond cal dose (a quantity that depends upon the film thickness) the gamma radiation doses of 80, 120 and 160 Gy, respec- can be considered as a working region for real-time gamma tively. Recently, Arshak et al (2004) have also reported for radiation dosimetry. For understanding the importance of thin films of some metal oxides that the sensitivity of thin the present work, table 1 lists the values of the sensitivity films for low radiation doses is much higher than that of the for the (TeO2)0·9 (In2O3)0·1 thin films of thicknesses, 300, thick films and that the thick films sustain up to much higher 450 and 600 nm, along with the values of the sensitivity for radiation doses than the thin films. the TeO2 thin films of thicknesses, 300, 450 and 600 nm. The current for the thin films of (TeO2)0·9 (In2O3)0·1 has Accordingly, (i) a real-time gamma radiation dosimeter pre- been observed to increase near linearly with dose up to a pared from the (TeO2)0·9 (In2O3)0·1 thin films of thickness, certain gamma radiation dose (critical dose) and the value 300 nm, at a typical voltage of 3·6 V will have a sensiti- of this dose is higher for the thicker films. This increase in vity of 8·0 μGy/μA/cm2 which is nearly five times better the current may be attributed to the healing effect (Zaykin than that prepared from the TeO2 thin films of thickness, and Aliyev 2002). During the process of film deposition, 300 nm, (ii) a real-time gamma radiation dosimeter pre- some intrinsic defects are always formed. The interaction of pared from the (TeO2)0·9 (In2O3)0·1 thin films of thickness, gamma radiation induces defects during its passage through 450 nm, at a typical voltage of 3·6 V will have a sensiti- the thin film resulting into disorder in the microstructure of vity of 9·1 μGy/μA/cm2 which is nearly six times better the film. At small doses, these thin films have fine homo- than that prepared from the TeO2 thin films of thickness, geneous grain structure without any big pores and the num- 450 nm and (iii) a real-time gamma radiation dosimeter pre- ber of defects (induced plus residual intrinsic) is smaller pared from the (TeO2)0·9 (In2O3)0·1 thin films of thickness, than the number of intrinsic defects due to the recombina- 600 nm, at a typical voltage of 3·6 V will have a sensiti- tion of defects. The recombination of defects reduces the vity of 13·5 μGy/μA/cm2 which is nearly six times better resistivity of thin film, giving rise to an increase in the cur- than that prepared from the TeO2 thin films of thickness, rent. The critical dose dependence on the film thickness can 600 nm. also be understood in terms of the healing effect (Zaykin and Aliyev 2002). Accordingly, thicker films require much higher radiation doses for healing in comparison to thinner 4. Conclusions films. The near linear increase of current with the gamma radia- The current–voltage characteristics for the (TeO2)0·9 tion dose can also be understood in terms of a simple charge (In2O3)0·1 thin films clearly show that the current increases transfer model (Arshak and Korostynska 2006), according to quite linearly with gamma radiation dose up to a certain criti- which the Fermi level shifts upward in the exposed thin films cal dose (a quantity dependent upon the film thickness) and as compared to that in the as-deposited thin films which then that the current decreases for the doses above this value. The Effect of gamma radiation on electrical and optical properties of (TeO2)0·9 (In2O3)0·1 thin films 69 increase in the current may be attributed partly to the heal- Arshak K, Korostynska O, Molly J and Harris J 2006 IEEE Sens. J. ing effect (Zaykin and Aliyev 2002) and partly to the low- 6 656 ering of the optical bandgap. Attempts are on to understand Atanassova E, Pasksleva A, Konakova R, Spassov D and Mitin V F the decrease in the current at higher doses. Employing the 2001 Microelectron. J. 32 553 dose dependence of the current, some real-time gamma radi- Clough R L 2001 Nucl. Instrum. Meth. B185 8 Colby E, Lum G, Plettner T and Spencer J 2002 IEEE Trans. Nucl. ation dosimeters based on the (TeO ) · (In O ) · thin films 2 0 9 2 3 0 1 Sci. NS-49 2857 of different thicknesses have been prepared, which have been 2 Deng Q, Yin Z and Zhu R 1999 Nucl. Instrum. Meth. A438 found to possess sensitivity in the range 5–55 μGy/μA/cm . 415 Clearly, these values are far superior to any presently avai- Horowitz Y S 2001 Nucl. Instrum. Meth. B184 68 lable real-time gamma radiation dosimeter. Further work Ibrahim A M and Soliman L I 1998 Rad. Phys. Chem. 53 is in progress for developing real-time gamma radiation 469 dosimeters possessing still better sensitivities. 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