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Some physical properties of thin films containing Se–Ge–Tl
Vacuum 57 (2000) 365 } 376
Some physical properties of thin "lms containing Se}Ge}Tl E. Abd El-Wahabb*, M.M. Abd El-Aziz, M. Fadel Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt accepted 23 February 2000
Abstract Thin "lms from the Se Ge Tl system have been prepared by thermal evaporation. The current}voltage characteristics in the temperature range 290}373 K and the thickness range 98}400 nm are ohmic in the lower "eld regime followed by non-ohmic behaviour in the higher voltage regime which has been satisfactorily explained by the anomalous Poole}Frenkel e!ect. The e!ects of temperature on the dielectric constant have been studied. The dependence of the electrical conductivity, on exposure to di!erent durations of light and on temperature has been studied. The activation energy was found to decrease with increasing periods of light exposure. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Thin "lm; Amorphous semiconductor
1. Introduction Normally, control of the properties and expansion of the scope of application of two-component systems is achieved through the introduction of a third component. Thus, the addition of Tl in the binary system Ge}Se is expected to a!ect some of its physical properties Thallium-containing chalcogenide glasses have attracted much attention because of their technological application to acousto-optical devices [1]. Addition of Tl is accompanied by a marked change in the physical properties of the chalcogenide glasses of binary systems, indicating organic incorporation of Tl into the network of the glasses, together with a change in its structure. Glass-formation in the Se Ge Tl system has been discussed by many authors [2}4]. However, these studies have looked at the glass-formation region in the Se Ge Tl system using an air-quenching technique. The present work reports studies of (a) The voltage and temperature dependence of current in Se Ge Tl structures, produced by vacuum deposition, to determine the dominating * Corresponding author. 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 1 4 5 - 7
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mechanism of charge transport. (b) The DC electrical conductivity of amorphous Se Ge Tl has been determined during and after light exposure and at di!erent temperatures. 2. Experimental procedure The bulk glass Se Ge Tl was prepared in the conventional way [5,6] by melting a mixture of "ve nine-purity elements in an evacuated silica ampoule (total batch 10 gm) at 10003C for 6 h with continuous rotation to ensure the homogenization of the composition. The ampoule was then quenched in icy water. X-ray di!raction characterization of powder and thin "lms was carried out using "ltered CuK , a radiation (Phillips PM 8203) operated at 40 kV and 25 mA. Also, the amorphization of the quenched ingots was examined by di!erential thermal analysis (DTA). A micro-DTA apparatus of a Shimadzu DT-30 model was used for the DTA investigation to detect the temperature ranges of investigation. The chemical composition of both bulk and thin-"lm material was monitored by carrying out quantitative analysis energy-dispersive spectroscopy, (EDX) on a JEOL 6400 scanning electron microscope with a Link-eXL EDS detector. The data was processed through a ZAF correction program contained within the Link-eXL package. These measurements were used to obtain the correct composition. Samples in the form of thin "lms were prepared by evaporating the synthesized material thermally at a base pressure of 2;10\ Torr onto precleaned corning glass 7059 substrate using an Edwards E-306A coating unit. For the electrical measurements, Al electrodes were evaporated onto the corning substrates at an interval of 2 mm width and 3 mm length before evaporating the Se Ge Tl sample. The "lm resistance was measured using a digital electrometer (Keithley type E616A) and the corresponding conductivity was then deduced. The "lm thickness was measured by an interferometric method. The excitation light was provided by a tungsten lamp (luminous intensity K56 lumen/Solid angle) operated by a controlled timer. Thin "lms of the investigated composition were prepared by a thermal evaporation technique using an Edwards 306A high-vacuum plant, with Al electrodes of suitable dimensions. Samples of sandwich con"guration with di!erent thickness were prepared under the same evaporation conditions. Film thickness (ranging from 90 to 400 nm) was measured by multiple beam interferometry (Fiseau Fringes). The I}< characteristics were measured throughout the thickness range 90}400 nm and in the temperature range 290}373 K below the glass transition temperature using an electrometer (Keithley type E616A) for the potential drop measurements and a microdigital multimeter (TE 924) for the current measurements. The temperature of the sample was monitored using a chromel}alumel thermocouple. 3. Results and discussion The absence of crystallinity is con"rmed by the XRD pattern for the as-quenched material (curve a) and the evaporated thin "lms (curve b) given in Fig. 1. Typical DTA thermograms of the
E. Abd El-Wahabb et al. / Vacuum 57 (2000) 365}376
Fig. 1. X-ray di!raction patterns of Se
367
Ge Tl in (a) powder and in (b) thin-"lm form analysis.
Fig. 2. Di!erential thermal analysis (DTA) for Se
Ge
Tl bulk material.
studied glass, in powder form, are illustrated in Fig. 2. As it is clear from Fig. 2, the basic Se Ge Tl glass shows a glass transition temperature ¹ "3703C and a single exothermic E peak with crystallization temperature ¹ "4333C. These values are in good agreement with the data of a previous study [4,7]. From this, "gure, ¹ (all the measurements below ¹ ) can be found. 3.1. I}V characteristics Fig. 3(a) and (b) present the current}voltage (I}<) characteristics of Se Ge Tl at di!erent temperatures ranging from 290 to 373 K for di!erent thickness from 98.11 to 412.46 nm. It can be seen that the curves are linear for lower voltages indicating ohmic conduction and become non-linear at higher voltages. It was found that, with increasing temperature, the linear regions extend to higher values of the applied "eld. The ohmic region is believed to be controlled by electron hopping. Similar results have been reported for Te}As}Ge}Si thin "lms [8]. The I}< characteristics exhibited a transition from an
368
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Fig. 3. (a and b) Current}voltage characteristic at various temperature Se Ge Tl thin "lms at di!erent thicknesses t ranging from 98.01 to 412.46 nm.
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369
Fig. 4. Log I vs. E at various temperatures for a Se Ge Tl thin "lm (thickness 234.26 nm).
ohmic region at low applied voltage to an over-linear region at higher voltages up to a breakdown voltage. Fig. 4 shows the plots of log I vs. E for the data of Fig. 3 and "lm thickness 234.26 nm as a representative example. The linear portion of the graphs in the non-ohmic region has been taken as evidence of either the Schottky [9] or the Poole}Frenkel e!ect [10]. However, the temperaturedependence measurements at various voltages suggest the Poole}Frenkel e!ect to be operative. This is supported by the fact that the various parameters evaluated lie in an acceptable range only when the Poole}Frenkel e!ect is assume to be operative. In the Poole}Frenkel e!ect, electrons are thermally emitted from the randomly distributed traps to the conduction band by the lowering of the coulumbic potential barrier by an external electric "eld. The current density in a thin "lm containing shallow traps is given by [11] I"I exp(B E /k¹),
(1)
where E is the applied electric "eld, I is the current at very high (in"nite) temperature, k is the Boltzmann constant, ¹ is the absolute temperature and B is the Poole}Frenkel "eld-lowering coe$cient which is given by B "(e/Pm m) ,
(2)
where e is the electronic charge, m is the permittivity of free space and m is the dielectric constant of the material forming the "lm.
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Table 1 Values of b and m with temperature for Se Ge Tl thin "lms Temperature
Log I vs. E ;10 (Fig. 4)
Log I E vs. E ;10 (Fig. 5)
K
b;10\
m
b;10\
m
290 299 303 313 323 334 344 353 363 373
1.190 * * * 0.508 0.753 1.070 1.145 1.053 1.202
40.54 * * * 222.90 101.58 50.31 43.90 51.86 39.78
0.735 0.745 0.725 0.814 0.867 0.896 0.854 0.992 1.150 1.010
77.85 103.87 104.23 87.00 76.69 71.81 79.05 52.56 43.59 56.07
Fig. 5. Log IE vs. E at various temperatures for a Se Ge Tl thin "lm (thickness 412.64 nm).
The values of B were obtained from the slope of the linear part of the curves in Fig. 4 and, by using these values in Eq. (2), to calculate the values of m at di!erent temperatures. The experimentally obtained values of B and the estimated m are given together in Table 1. Fig. 5 depicts the relation between log IE vs. E for 412.46 nm as a representative example. Values of B and m derived from this graph are also illustrated in Table 1.
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3.2. I}T characteristics The temperature dependence of current was measured for several constantly applied voltages because of the importance of these characteristics for the proper choice of charge transfer model. The typical In I vs. 1/¹ curve for Se Ge Tl "lms at constant values of applied voltage is shown in Fig. 6. It is clear that all curves are straight lines indicating that the conduction, in these thin "lms, is through an activated process having a single activation energy in the temperature range 294}345 K. The activation energy *E was found from I"I exp!*E/k¹,
(3)
where k is Boltzmann's constant and I is max current at in"nite ¹. Values of *E were calculated from the slopes of the curves in Fig. 6 and are listed in Table 2. The slopes of the curves decrease
Fig. 6. Log I vs. 1000 ¹\ for a Se Ge Tl thin "lm at various applied biases (thickness 98.01 nm).
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Table 2 Activation energy of Se Ge Tl thin "lms of thickness 98.01 nm at various applied biases Applied bias, <
Activation energy, *E, eV
30 40 50 60 80 90
0.761 0.745 0.736 0.722 0.686 0.673
Fig. 7. The dependence of dark and illuminated electrical conductivity on temperature.
slowly with applied voltage. Values of the activation energy agree well with many results quoted elsewhere [4}7]. 3.3. Ewect of temperature on electrical conductivity The plot of DC electrical conductivity, p , as a function of temperature for Se Ge Tl amorphous "lm with a thickness of 98.01 nm is given in Fig. 7 (curve a). As showing the "gure, the plot of p vs. 1/¹ gives a straight line over the temperature region above ¹"603C and it can
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373
therefore be described by the Arrhenius relation [12]. p"p exp(!*E/k¹), (4) where the symbols have their usual meaning. Values of p (the conductivity measured at 303C), E and p are respectively, 1.8;10\ )\ cm\, 0.883 eV and 1.05;10 )\ cm\. These high values of both *E and p suggest that electrical conduction takes place between extended states beyond the mobility edges [12,13]. Below ¹"603C, the plot deviates from linearity, thus re#ecting the beginning of a second conducting mechanism due to the onset of hopping conduction between localized states. These states may arise from the lack of long-range order in the amorphous network. 3.4. The ewect of light on electrical conductivity Fig. 7 shows the temperature dependence of electrical conductivity in the dark, p (curve a) and under illumination with white light p (curve b) as a function of 1000/T. Photoconductivity *p was calculated by subtracting the illuminated conductivity p from the dark conductivity p and is given in Fig. 8 for the composition investigated. It is clear from this "gure that this dependence is in the considered temperature range, which indicates that the photoconductivity is an activated process. The values of activation energy of illuminated sample E and the pre-exponential factor p are 0.697 eV and 3.95;10 )\ cm\ for curve b from Fig. 7. But the values of *E and *p for photoconductivity values from the slopes of the obtained line of Fig. 8 according to Eq. (3) are 0.674 eV and 8.23;10 )\ cm\.
Fig. 8. The di!erence between the illuminated and dark conductivity (*p ) vs. 1000/¹.
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Fig. 9. The dependence of dark electrical conductivity of a Se Ge Tl thin "lm (thickness 98.01 nm) on the period of light exposure at di!erent temperatures. Table 3 Values of Time of illumination, activation energy and electrical conductivity at room temperature Time of illumination (min)
*E, eV
p , () cm)\ 02
0 10 20 30
0.884 0.749 0.723 0.709
1.05;10 1.84;10 8.46;10 6.11;10
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It is obvious from these results that *E is less than E and E for the system under test which may be explained by the presence of charge carrier trapping levels in the pseudo-gap of the glass studied [14]. The general behaviour for temperature dependence of photoconductivity obtained for all the investigated thicknesses is shown in Fig. 9 as a representative example, for samples with thickness 98.01 nm exposed to light for di!erent durations. From Fig. 9, it is observed that the conductivity increases exponentially with increasing temperature according to the relation of Eq. (3) and also the illumination decreases the conduction activation energy *E (see Table 3). The decrease in the value of *E due to illumination may be due to the introduction of Tl, and is accompanied by a decrease in the average size of the clusters, leading to an increase in the chemical disorder of the material [3]. It has been mentioned [15] that the width of clusters varies from an average of ten chains in GeSe to only two chains in Ge}Sn}Se. A decrease in the cluster width results in a decrease in the vibrational mode of frequency and in the glass transition temperature (¹ ). For Ge}Se , the Fermi level is approximately pinned at the centre of the mobility gap since the charges defect centres C> and C\ are equal in concentration. When Tl is added to the Ge}Se system, it induces structural changes in the host network Ge}Se which leads to a readjustment in the local environment. This might disturb the balance of the characteristic charged defects, (C\ & C>) in a chalcogenide semiconductor which can change the electronic conduction. In such a situation, the distribution and density of localized (charged) states are modi"ed and even some new trap states can appear in the gap of the semiconductor. Also, the decrease of *E with duration of light exposure may occur because of the fact that the addition of Tl Se to GeSe leads to the presence of non-bridging selenium atoms in the same way as non-bridging oxygen in modi"ed silicate glasses [16] as proposed by Sanghera [17] using X-ray photoelectron spectroscopy (XPS). Also, the percentage of non-bridging Se atoms has a pronounced tendency to shift the electronic binding energies of the Ge 3d and the bridging Se 3d level to lower values. This has been attributed, by these authors, to a non-localized interaction between the non-bridging Se and the vitreous network.
4. Conclusion The I}< characteristics of thermally evaporated Se Ge Tl thin "lms were obtained in the temperature range from 290 to 373 K for di!erent thicknesses from 98.11 to 412.46 nm. The characteristics exhibited a transition from an ohmic region at low applied voltages to the above-linear region at higher voltage up to breakdown voltage. The dependence of ohmic current on temperature corresponds to a thermally activated process. The values of the activation energy for conduction (*E) and dielectric constant (m) were investigated. The decrease in the value of *E due to illumination may be due to introduction of T1, which is accompanied by an increase in the chemical disorder of the material. The position of the Fermi level was changed and the e!ect of doping, temperature and optical exposure plays an important role in the electronic conduction processes.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Feichtner JD, Gottlieb M, Conroy JJ. IEEE J Quantum Electron QE-2 1976:660. Dembovskii SA. Zh Neorg Khim 1968;13:1721. Borisova ZU, Kornienko LP, Obraztsou E, Turkina Yu. Izv Akad Nauk SSR Neorg Mater 1976;12:592. Parthasarathy G, Naik GM, Asokam S. J Mater Sci Lett 1987;6:214. Kotkata MF, EL-Shair HT, A"" MA, Abdel-Aziz MM. J Phys D 1994;27:623. Kotkata MF, A"" MA, Labib HH, Hegab NA, Abdel-Aziz MM. Thin Solid Films 1994;240:143. Naik GM, Parthasarathy G, Gopal ESR, Narayanan PS. J Mater Sci Lett 1985;4:1017. Elsamanoudy MM, Hegab NA, Fadel M. Vacuum 1995;46:701. Schottky W. Z Phys 1914;15:872. Frenkel J. Phys Rev 1938;54:647. Mead CA. Phys Rev 1962;128:2088. Mott NF, Davis EA. In: Electronic processes in non-cryst materials. 2nd ed. Oxford: Clarendon Press, 1979. p. 206. Fadel M, Sedeek K, Abd El Salam F. Thin Solid Films 1996;283:239. Guriin NT, Semak DG, Fedak VV. Sov Phys Semicond 1975;9(4):499. Mikrut JM, McNeil LE. J Non-Cryst Solids 1989;109:237. Bruckner R, Chun H, Goretzki H. Glastech Ber 1976;49:211. Sanghera JS, Heo J, Mackenzie JD. J Non-Cryst Solids 1988;101:8.
Some physical properties of thin "lms containing Se}Ge}Tl E. Abd El-Wahabb*, M.M. Abd El-Aziz, M. Fadel Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt accepted 23 February 2000
Abstract Thin "lms from the Se Ge Tl system have been prepared by thermal evaporation. The current}voltage characteristics in the temperature range 290}373 K and the thickness range 98}400 nm are ohmic in the lower "eld regime followed by non-ohmic behaviour in the higher voltage regime which has been satisfactorily explained by the anomalous Poole}Frenkel e!ect. The e!ects of temperature on the dielectric constant have been studied. The dependence of the electrical conductivity, on exposure to di!erent durations of light and on temperature has been studied. The activation energy was found to decrease with increasing periods of light exposure. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Thin "lm; Amorphous semiconductor
1. Introduction Normally, control of the properties and expansion of the scope of application of two-component systems is achieved through the introduction of a third component. Thus, the addition of Tl in the binary system Ge}Se is expected to a!ect some of its physical properties Thallium-containing chalcogenide glasses have attracted much attention because of their technological application to acousto-optical devices [1]. Addition of Tl is accompanied by a marked change in the physical properties of the chalcogenide glasses of binary systems, indicating organic incorporation of Tl into the network of the glasses, together with a change in its structure. Glass-formation in the Se Ge Tl system has been discussed by many authors [2}4]. However, these studies have looked at the glass-formation region in the Se Ge Tl system using an air-quenching technique. The present work reports studies of (a) The voltage and temperature dependence of current in Se Ge Tl structures, produced by vacuum deposition, to determine the dominating * Corresponding author. 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 1 4 5 - 7
366
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mechanism of charge transport. (b) The DC electrical conductivity of amorphous Se Ge Tl has been determined during and after light exposure and at di!erent temperatures. 2. Experimental procedure The bulk glass Se Ge Tl was prepared in the conventional way [5,6] by melting a mixture of "ve nine-purity elements in an evacuated silica ampoule (total batch 10 gm) at 10003C for 6 h with continuous rotation to ensure the homogenization of the composition. The ampoule was then quenched in icy water. X-ray di!raction characterization of powder and thin "lms was carried out using "ltered CuK , a radiation (Phillips PM 8203) operated at 40 kV and 25 mA. Also, the amorphization of the quenched ingots was examined by di!erential thermal analysis (DTA). A micro-DTA apparatus of a Shimadzu DT-30 model was used for the DTA investigation to detect the temperature ranges of investigation. The chemical composition of both bulk and thin-"lm material was monitored by carrying out quantitative analysis energy-dispersive spectroscopy, (EDX) on a JEOL 6400 scanning electron microscope with a Link-eXL EDS detector. The data was processed through a ZAF correction program contained within the Link-eXL package. These measurements were used to obtain the correct composition. Samples in the form of thin "lms were prepared by evaporating the synthesized material thermally at a base pressure of 2;10\ Torr onto precleaned corning glass 7059 substrate using an Edwards E-306A coating unit. For the electrical measurements, Al electrodes were evaporated onto the corning substrates at an interval of 2 mm width and 3 mm length before evaporating the Se Ge Tl sample. The "lm resistance was measured using a digital electrometer (Keithley type E616A) and the corresponding conductivity was then deduced. The "lm thickness was measured by an interferometric method. The excitation light was provided by a tungsten lamp (luminous intensity K56 lumen/Solid angle) operated by a controlled timer. Thin "lms of the investigated composition were prepared by a thermal evaporation technique using an Edwards 306A high-vacuum plant, with Al electrodes of suitable dimensions. Samples of sandwich con"guration with di!erent thickness were prepared under the same evaporation conditions. Film thickness (ranging from 90 to 400 nm) was measured by multiple beam interferometry (Fiseau Fringes). The I}< characteristics were measured throughout the thickness range 90}400 nm and in the temperature range 290}373 K below the glass transition temperature using an electrometer (Keithley type E616A) for the potential drop measurements and a microdigital multimeter (TE 924) for the current measurements. The temperature of the sample was monitored using a chromel}alumel thermocouple. 3. Results and discussion The absence of crystallinity is con"rmed by the XRD pattern for the as-quenched material (curve a) and the evaporated thin "lms (curve b) given in Fig. 1. Typical DTA thermograms of the
E. Abd El-Wahabb et al. / Vacuum 57 (2000) 365}376
Fig. 1. X-ray di!raction patterns of Se
367
Ge Tl in (a) powder and in (b) thin-"lm form analysis.
Fig. 2. Di!erential thermal analysis (DTA) for Se
Ge
Tl bulk material.
studied glass, in powder form, are illustrated in Fig. 2. As it is clear from Fig. 2, the basic Se Ge Tl glass shows a glass transition temperature ¹ "3703C and a single exothermic E peak with crystallization temperature ¹ "4333C. These values are in good agreement with the data of a previous study [4,7]. From this, "gure, ¹ (all the measurements below ¹ ) can be found. 3.1. I}V characteristics Fig. 3(a) and (b) present the current}voltage (I}<) characteristics of Se Ge Tl at di!erent temperatures ranging from 290 to 373 K for di!erent thickness from 98.11 to 412.46 nm. It can be seen that the curves are linear for lower voltages indicating ohmic conduction and become non-linear at higher voltages. It was found that, with increasing temperature, the linear regions extend to higher values of the applied "eld. The ohmic region is believed to be controlled by electron hopping. Similar results have been reported for Te}As}Ge}Si thin "lms [8]. The I}< characteristics exhibited a transition from an
368
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Fig. 3. (a and b) Current}voltage characteristic at various temperature Se Ge Tl thin "lms at di!erent thicknesses t ranging from 98.01 to 412.46 nm.
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369
Fig. 4. Log I vs. E at various temperatures for a Se Ge Tl thin "lm (thickness 234.26 nm).
ohmic region at low applied voltage to an over-linear region at higher voltages up to a breakdown voltage. Fig. 4 shows the plots of log I vs. E for the data of Fig. 3 and "lm thickness 234.26 nm as a representative example. The linear portion of the graphs in the non-ohmic region has been taken as evidence of either the Schottky [9] or the Poole}Frenkel e!ect [10]. However, the temperaturedependence measurements at various voltages suggest the Poole}Frenkel e!ect to be operative. This is supported by the fact that the various parameters evaluated lie in an acceptable range only when the Poole}Frenkel e!ect is assume to be operative. In the Poole}Frenkel e!ect, electrons are thermally emitted from the randomly distributed traps to the conduction band by the lowering of the coulumbic potential barrier by an external electric "eld. The current density in a thin "lm containing shallow traps is given by [11] I"I exp(B E /k¹),
(1)
where E is the applied electric "eld, I is the current at very high (in"nite) temperature, k is the Boltzmann constant, ¹ is the absolute temperature and B is the Poole}Frenkel "eld-lowering coe$cient which is given by B "(e/Pm m) ,
(2)
where e is the electronic charge, m is the permittivity of free space and m is the dielectric constant of the material forming the "lm.
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Table 1 Values of b and m with temperature for Se Ge Tl thin "lms Temperature
Log I vs. E ;10 (Fig. 4)
Log I E vs. E ;10 (Fig. 5)
K
b;10\
m
b;10\
m
290 299 303 313 323 334 344 353 363 373
1.190 * * * 0.508 0.753 1.070 1.145 1.053 1.202
40.54 * * * 222.90 101.58 50.31 43.90 51.86 39.78
0.735 0.745 0.725 0.814 0.867 0.896 0.854 0.992 1.150 1.010
77.85 103.87 104.23 87.00 76.69 71.81 79.05 52.56 43.59 56.07
Fig. 5. Log IE vs. E at various temperatures for a Se Ge Tl thin "lm (thickness 412.64 nm).
The values of B were obtained from the slope of the linear part of the curves in Fig. 4 and, by using these values in Eq. (2), to calculate the values of m at di!erent temperatures. The experimentally obtained values of B and the estimated m are given together in Table 1. Fig. 5 depicts the relation between log IE vs. E for 412.46 nm as a representative example. Values of B and m derived from this graph are also illustrated in Table 1.
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371
3.2. I}T characteristics The temperature dependence of current was measured for several constantly applied voltages because of the importance of these characteristics for the proper choice of charge transfer model. The typical In I vs. 1/¹ curve for Se Ge Tl "lms at constant values of applied voltage is shown in Fig. 6. It is clear that all curves are straight lines indicating that the conduction, in these thin "lms, is through an activated process having a single activation energy in the temperature range 294}345 K. The activation energy *E was found from I"I exp!*E/k¹,
(3)
where k is Boltzmann's constant and I is max current at in"nite ¹. Values of *E were calculated from the slopes of the curves in Fig. 6 and are listed in Table 2. The slopes of the curves decrease
Fig. 6. Log I vs. 1000 ¹\ for a Se Ge Tl thin "lm at various applied biases (thickness 98.01 nm).
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Table 2 Activation energy of Se Ge Tl thin "lms of thickness 98.01 nm at various applied biases Applied bias, <
Activation energy, *E, eV
30 40 50 60 80 90
0.761 0.745 0.736 0.722 0.686 0.673
Fig. 7. The dependence of dark and illuminated electrical conductivity on temperature.
slowly with applied voltage. Values of the activation energy agree well with many results quoted elsewhere [4}7]. 3.3. Ewect of temperature on electrical conductivity The plot of DC electrical conductivity, p , as a function of temperature for Se Ge Tl amorphous "lm with a thickness of 98.01 nm is given in Fig. 7 (curve a). As showing the "gure, the plot of p vs. 1/¹ gives a straight line over the temperature region above ¹"603C and it can
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373
therefore be described by the Arrhenius relation [12]. p"p exp(!*E/k¹), (4) where the symbols have their usual meaning. Values of p (the conductivity measured at 303C), E and p are respectively, 1.8;10\ )\ cm\, 0.883 eV and 1.05;10 )\ cm\. These high values of both *E and p suggest that electrical conduction takes place between extended states beyond the mobility edges [12,13]. Below ¹"603C, the plot deviates from linearity, thus re#ecting the beginning of a second conducting mechanism due to the onset of hopping conduction between localized states. These states may arise from the lack of long-range order in the amorphous network. 3.4. The ewect of light on electrical conductivity Fig. 7 shows the temperature dependence of electrical conductivity in the dark, p (curve a) and under illumination with white light p (curve b) as a function of 1000/T. Photoconductivity *p was calculated by subtracting the illuminated conductivity p from the dark conductivity p and is given in Fig. 8 for the composition investigated. It is clear from this "gure that this dependence is in the considered temperature range, which indicates that the photoconductivity is an activated process. The values of activation energy of illuminated sample E and the pre-exponential factor p are 0.697 eV and 3.95;10 )\ cm\ for curve b from Fig. 7. But the values of *E and *p for photoconductivity values from the slopes of the obtained line of Fig. 8 according to Eq. (3) are 0.674 eV and 8.23;10 )\ cm\.
Fig. 8. The di!erence between the illuminated and dark conductivity (*p ) vs. 1000/¹.
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Fig. 9. The dependence of dark electrical conductivity of a Se Ge Tl thin "lm (thickness 98.01 nm) on the period of light exposure at di!erent temperatures. Table 3 Values of Time of illumination, activation energy and electrical conductivity at room temperature Time of illumination (min)
*E, eV
p , () cm)\ 02
0 10 20 30
0.884 0.749 0.723 0.709
1.05;10 1.84;10 8.46;10 6.11;10
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It is obvious from these results that *E is less than E and E for the system under test which may be explained by the presence of charge carrier trapping levels in the pseudo-gap of the glass studied [14]. The general behaviour for temperature dependence of photoconductivity obtained for all the investigated thicknesses is shown in Fig. 9 as a representative example, for samples with thickness 98.01 nm exposed to light for di!erent durations. From Fig. 9, it is observed that the conductivity increases exponentially with increasing temperature according to the relation of Eq. (3) and also the illumination decreases the conduction activation energy *E (see Table 3). The decrease in the value of *E due to illumination may be due to the introduction of Tl, and is accompanied by a decrease in the average size of the clusters, leading to an increase in the chemical disorder of the material [3]. It has been mentioned [15] that the width of clusters varies from an average of ten chains in GeSe to only two chains in Ge}Sn}Se. A decrease in the cluster width results in a decrease in the vibrational mode of frequency and in the glass transition temperature (¹ ). For Ge}Se , the Fermi level is approximately pinned at the centre of the mobility gap since the charges defect centres C> and C\ are equal in concentration. When Tl is added to the Ge}Se system, it induces structural changes in the host network Ge}Se which leads to a readjustment in the local environment. This might disturb the balance of the characteristic charged defects, (C\ & C>) in a chalcogenide semiconductor which can change the electronic conduction. In such a situation, the distribution and density of localized (charged) states are modi"ed and even some new trap states can appear in the gap of the semiconductor. Also, the decrease of *E with duration of light exposure may occur because of the fact that the addition of Tl Se to GeSe leads to the presence of non-bridging selenium atoms in the same way as non-bridging oxygen in modi"ed silicate glasses [16] as proposed by Sanghera [17] using X-ray photoelectron spectroscopy (XPS). Also, the percentage of non-bridging Se atoms has a pronounced tendency to shift the electronic binding energies of the Ge 3d and the bridging Se 3d level to lower values. This has been attributed, by these authors, to a non-localized interaction between the non-bridging Se and the vitreous network.
4. Conclusion The I}< characteristics of thermally evaporated Se Ge Tl thin "lms were obtained in the temperature range from 290 to 373 K for di!erent thicknesses from 98.11 to 412.46 nm. The characteristics exhibited a transition from an ohmic region at low applied voltages to the above-linear region at higher voltage up to breakdown voltage. The dependence of ohmic current on temperature corresponds to a thermally activated process. The values of the activation energy for conduction (*E) and dielectric constant (m) were investigated. The decrease in the value of *E due to illumination may be due to introduction of T1, which is accompanied by an increase in the chemical disorder of the material. The position of the Fermi level was changed and the e!ect of doping, temperature and optical exposure plays an important role in the electronic conduction processes.
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