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Pressure and temperature dependence of the electrical resistivity of bulk Al23 Te77 glass
Journal of Non-Crystalline Solids 72 (1985) 73-79 North-Holland, Amsterdam
73
PRESSURE AND TEMPERATURE DEPENDENCE OF THE ELECTRICAL RESISTIVITY OF BULK AI23 Te77 GLASS (3. PARTHASARATHY *, E.S.R. GOPAL and S.T. LAKSHMI K U M A R ** Department of Physics, Indian Institute of Science, Bangalore 560 012, India Received 2 July 1984 Revised manuscript received 10 September 1984
Electrical resistivity of bulk amorphous Al23Te77 samples has been studied as a function of pressure (up to 80 kbar) and temperature (down to 77 K). At atmospheric pressure the temperature dependence of resistivity obeys the relation p = #0 exp(A E / R T ) with two activation energies. In the temperature range 300 K >/T > 234 K the activation energy is 0.58 eV and for 234 > T >/185 K the value is A E = 0.30 eV. The activation energy has been measured as a function of pressure. The electrical resistivity decreases exponentially with the increase of pressure and at 70 kbar pressure the electrical behaviour of the sample shows a metallic nature with a positive temperature coefficient. The high pressure phase of the sample is found to be a crystalline hexagonal phase.
I. Introduction
There have been very few studies of the pressure induced semiconductorto-metal transition in amorphous semiconductors [1-5], with detailed studies on amorphous Se [2,5], amorphous As [4], amorphous As2Te 3 [3] and tetrahedrally bonded systems [1]. Usually amorphous chalcogenides under pressure become metallic with a continuous decrease in resistivity and they retain their molecular structure [2,3]. In this paper we report a pressure induced semiconductor to metal transition in bulk amorphous A123Te77 samples. Measurements of electrical resistivity have already been performed on Al25Te75 samples at atmospheric pressure [6]. Some work has also been done on electrical switching [7]. The alloy A123Te77 w a s chosen because the glass forming region in A1xTel_ x system is limited to 0.15 < x < 0.30 [8], i.e. the glass forming ability is much higher near the eutectic. Moreover the high pressure behaviour of this system has not yet been studied, though there is a little experimental work on electrical switching and crystallization [7,9]. * Now at Instrumentation and Services Unit, l.l.Sc., Bangalore, India. ** Now at the National Physical Laboratory, Hill Side Road, New Delhi 110 012, India. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
74
G. Parthasarathy et al. / Electrical resistioity of bulk Ale ~Te77 glass
2. Experimental studies The high-pressure apparatus for the measurements of the electrical resistance of an amorphous m123Te77 sample as a function of pressure and temperature consists of a pair of tungsten carbide anvils with 4 mm face diameter and a hydraulic press with compression and tension members, made of stainless steel to facilitate the pressurization in a low temperature cryostat. The pressure are calibrated at room temperature, using as fixed points the bismuth transitions at 25.4, 27 and 77 kbar, thallium transition at 35.7 kbar and ytterbium transition at 40 kbar. The pressures at low temperatures are also calibrated by studying the behaviour of Bi, T1, Yb and Pb. The details of the instrumentation and calibration will be discussed elsewhere [10]. The resistances of the samples were measured by using a four probe method. A Keithley constant current source (model No. 225), a Keithley Digital multimeter (model No. 177) and a Keithley electrometer (model No. 616) were used as measuring instruments. For measuring the temperature, copper-constantan thermocouples were used. The temperature was controlled within + 0.3 K. Bulk samples of A123Te27 were prepared by melting together appropriate amounts of the elements (99.999% purity) in sealed, evacuated quartz tubes in a rotary furnace at a temperature of - 8 6 0 ° C for 18 h and subsequent quenching in an ice-water mixture. The bulk solids were confirmed to be amorphous by studies of X-ray diffraction, electron microscopy and differential scanning calorimetry. The resistivity of the sample was calculated by measuring the dimensions of the sample at normal atmospheric pressure. The change in the dimensions of the sample under pressure was not taken into account; the effect of this on the resistivity was generally small and can be calculated only if the compressibility of the material is known as a function of pressure. For recovering the pressure-quenched samples, sodium chloride was used as a pressure transmitting medium instead of steatite. After the application of the pressure, the sample was recovered by dissolving the pressure transmitting media along with the sample in double distilled water. The recovered sample was examined under an electron microscope and X-ray diffractometer. The method is useful for the cases where the material undergoes irreversible transformations.
3. Results Fig. 1 shows the variation of electrical resistivity as a function of pressure for bulk Ak23Te77 glass at room temperature. At room temperature and pressure, the resistivity of our sample is found to be 5.55 x 105 $2 cm, which is in good agreement with the reported value [6]. With an increase of pressure, the resistivity decreases continuously and becomes of the order of 10 -4 I2 cm
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Fig. 2. Semi log plot o f electrical resistivity versus ( 1 0 0 0 / T ) for AI23Te77 glass at different pressures: curve (A) for P : 1 bar: (B) P : 10 kbar; (C) P = 20 kbar; (D) P = 30 kbar; (E) P = 49 kbar; (F) P = 50 kbar.
Fig. 1. Variation of electrical resistivity of AI23Te77 glass as a f u n c t i o n of pressure.
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at 80 kbar. The resistivity of the sample at 80 kbar is found to be less than the Mott's maximum metallic resistivity [11]. Fig. 2 shows the temperature dependence of electrical resistivity for bulk A123Te77 glass at different pressures. We find that the resistivity variation obeys the relation p = P0 exp(AE/kT) where p0 = pre-exponential factor, AE = activation energy for electronic conduction, k = Boltzmann's constant and T = temperature. At zero pressure there are two activation energies: 0.58 eV for 234 K < T~< 300 K and 0.30 eV for 185 K ~< T < 234 K. Because of the high resistance of the sample the temperature range of investigation is limited to 185 K. At 10 kbar pressure the value of the first activation energy decreased to 0.49 eV; on the other hand the value of the second activation energy was increased to 0.41 eV. For a pressure of 20 kbar and above the conduction process is still thermally activated but with a single activation energy over the temperature range of investigation. Fig. 3 shows the variation of electrical resistivity with temperature at pressures of 60 kbar and 70 kbar. At 60 kbar pressure, the temperature coefficient of resistivity is found to be zero for 250 T < T ~< 300 K and for temperature below 250 K the samples show the semiconducting behaviour with an activation energy 0.05 eV. At 70 kbar pressure the temperature coefficient of electrical resistivity is found to be positive and is equal to 2.03 s × 10 -7 9cm/K. Fig. 4 shows the variation of the activation energy with pressure. For 0 ~ P < 40 kbar the pressure coefficient of activation energy is (dE*/dP)=
G. Parthasarathy et a L / Electrical resistivity of bulk A le, Te77 glass
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Fig. 4. Variation of the activation energy AE with pressure for AI23Te77 glasses. Closed circle AE measured from p = Po e x p ( A E / k T ) ; open circle AE derived from the p ( P ) curve. Fig. 5. X-ray diffraction (powder) pattern for the pressure quenched AI23Te77 sample from a pressure of 70 kbar.
-0.0103 eV/kbar, and for 40 < P ~< 70 kbar (dE*/dP) = - 0 . 0 0 6 eV kbar. In the same figure E* values obtained from the expression (dE*/dP)= kT(d In p/dP) are also plotted [5]. Both values are found to be in good agreement. The values of the activation energy at different pressures and the corresponding pre-exponential factors are given in table 1. The X-ray diffraction pattern for pressure quenched ( P = 70 kbar) AI23Tevv glass is shown in fig. 5. It is found that the high pressure phase is a crystalline hexagonal structure with c/a = 1.7. Table 1 Activation energies and the pre-exponential factors for electric conduction in A123Te77 glass at different values of pressure Pressure (kbar) 0 10 20 30 40 50 60
Temperature range (K)
Activation energy E (eV)
Pre-exponential factor (/2 cm)
300-234 234-185 300-242 242-170 300-133 300-96 300-77 247-77 250-77
0.58 0.30 0.49 0.41 0.37 0.26 0.18 0.11 0.05
1.16 x 10 -4 99.6 1.02 x 10 4 1.85 x 10 -4 1.29 x 10-4 2.53 x 10 -4 9.98 x 10- 5 0.98 X 1 0 - s 1.00 X 10- 4
20
78
G. Parthasarathy et al. / Electrical resistivity of bulk AI 23Te77 glass
4. Discussions The effect of pressure on the electrical and optical properties of amorphous semiconductors has been reviewed by Minomura et al. [2]. In tetrahedrally bonded glasses the pressure induced transitions are discontinuous, accompanied by changes in structure to high co-ordination. On the other hand most of the chalcogenide glasses becomes metallic under pressure with a continuous decrease in optical gap and electrical resistivity even as they retain their molecular structure. The pressure coefficient of the optical gap in the case of chalcogenide glasses lies in between - 1 . 0 × 10 2 and - 2 × 10 -2 eV/kbar. The activation energy E* (or) A E, derived from the temperature dependence of resistivity and the optical gap E 0, are related by E* = AE = ½E0 indicating that the Fermi level should linear the center of the gap [3]. So the value of the pressure coefficient of the electrical activation energy should lie between - 0 . 5 x 10 .2 and - 1 . 0 X l 0 -2 eV/kbar. In our experiment the value of - 1 . 0 3 x 10 2 e V / k b a r of the pressure coefficient of the electrical activation energy agrees with the above expected value. This indicates that the decreases in the electrical resistivity under pressure arises from a gradual decrease in the gap. But the pressure dependence of the resistivity is more complicated because the resistivity depends not only upon the gap but also on the mobility and the position of the Fermi level [13]. Therefore an independent experiment on the pressure dependence of mobility is required. From fig. 4 and table 1 it is seen that the activation energy decreases with the increase of pressure. This implies that at room temperature, the fundamental gap decreases with the increase of pressure and becames zero at P - 70 kbar. The pre-exponential factor O0 in the resistivity expression O = O0 exp(AE/kT) has a value of the order of 10 -4 ~2 cm. From the analysis of Mort [11], we feel that the observed value of the pre-exponential factor represents that the carriers are excited across the mobility edge into the extended states. The approximate constancy of the pre-exponential factor with the increase of pressure implies that the mobility gap decreases with the increase of pressure. At normal or zero pressure the conduction at high temperature is through the extended states and in fact the pre-exponential factor in the higher temperature region is 1.16 × 10 .4 $2 cm. It is found that the conduction in the low temperature region is probably through the tail of the localized states via thermally activated hopping since its pre-exponential factor is 99.6 12 cm which is six orders of magnitude more than that for extended state conduction [14]. With the application of pressure up to 10 kbar the width of the localized states near the mobility edges is found to increase with the pressure. But beyond 10 kbar pressure the conduction via thermally activated tunneling in the localized states is found to be suppressed, at least in the temperature range studied. It is likely that the pressure induced distortions cause changes in the density of states, particularly in the inter-band gap, which could decrease the mobility gap. Such a decrease in the mobility gap can reduce the extent of localization
G. Parthasarathy et al. / Electrical resistivity of bulk AI 2 ~Te77 glass
79
due to enhancement of the dielectric constant, and in turn decrease the gap further [3]. Thus the gap m a y close up as a function of pressure and metallic conductivity will emerge at high pressures. Fig. 5 shows the X-ray diffraction pattern for pressure quenched Alz3TeTv glass. The high pressure phase seems to have hexagonal structure with c / a = 1.7. The structure of A123Te77 glass has been considered to be a complicated three dimensional network of A1 atoms tetrahedrally co-ordinated with Te [15]. The excess Te atoms form chains which are linked together with the distorted tetrahedral. By the application of heat the excess Te crystallizes first and the remaining a m o r p h o u s matrix of A12Te3 is crystallized at a higher temperature, which leads to a double stage crystallization in this glass [9]. On the other hand we get a single hexagonal phase of this material, with c / a = 1.7, by the application of pressure. The similar pressure induced crystallization in a m o r p h o u s As2Te 3 has been reported [2], where the high pressure phase of a m o r p h o u s As2T % and the normal crystalline phase of AszTe 3 are found to be the same. F r o m this study, we can conclude that for chalcogenic glasses, the molecular structure can be retained under the application of high pressure. In A123Te77 glass, the high pressure pressure crystalline phase is more stable than the temperature induced crystalline phase. The details of the temperature induced effects will be published elsewhere [16]. The authors would like to thank the D S T and C S I R schemes, G o v e r n m e n t of India, for financial support.
References [1] O. Shimomura, S. Minomura, N. Sakai, K. Asaumi, K. Tamura, J. Fukushima and H. Endo, Phil. Mag. 29 (1974) 547. [2] S. Minomura, Amorphous Semiconductors Technologies and Devices, ed., Y. Hamakawa (North-Holland, Amsterdam, 1982) p. 245. [3] N. Sakai and H. Frizache, Phys. Rev. B15 (1977) 973. [4] S.R. Elliot, E.A. Davis and G.D. Pitt, Solid St. Commun. 22 (1977) 481. [5] W. Fuhs, P. Schlotter and J. Stuke, Phys. Stat. Sol. (b)57 (1973) 587. [6] A.A. Andreev, M.S. Ablova, B.T. Meleck, F. Nasredinov, P.P. Seregin and E. Turaev, in: Amorphous and Liquid Semiconductors ed., W.E. Spear (Institute of Physics, Bristol, 1978) p. 44. [7] A.A. Andreev, M.S. Ablova, V.P. Podkhalyuzin, Z.V. Maslova and B.t. Melelch, Sov. J. Glass Phys. and Chem. (USA) 5 (1979) 334. [8] J. Cornet, Proc. Sixth Int. Conf. on Amorphous and liquid Semiconductors, Leningrad, USSR (1975) p. 72. [9] J. Colmenero and J.M. Barandiaran, J. Non-Crystalline Solids 30 (1979) 263. [10] G. Parthasarathy, A.K. Bandyopadyay, S.T. Lakshmikumar and E.S.R. Gopal, J. Inst. Soc. India 14 (1984) 169. [11] N.F. Mott, Phil. Mag. 22 (1970) 7. [12] R.P. Elliot, Constitution of Binary Alloys: First Supplement (McGraw-Hill, New York, 1965) p. 57. [13] W. Paul and D.M. Warschauer, Solids under Pressure (McGraw-Hill, New York, 1963) p. 179. [14] E.A. Davis and N.F. Mott, Phil. Mag. 22 (1970) 903. [15] A. D'Anjou and P. Sanz, J. Non-Crystalline Solids 28 (1978) 319. [16] G. Parthasarathy and E.S.R. Gopal, to be published.
73
PRESSURE AND TEMPERATURE DEPENDENCE OF THE ELECTRICAL RESISTIVITY OF BULK AI23 Te77 GLASS (3. PARTHASARATHY *, E.S.R. GOPAL and S.T. LAKSHMI K U M A R ** Department of Physics, Indian Institute of Science, Bangalore 560 012, India Received 2 July 1984 Revised manuscript received 10 September 1984
Electrical resistivity of bulk amorphous Al23Te77 samples has been studied as a function of pressure (up to 80 kbar) and temperature (down to 77 K). At atmospheric pressure the temperature dependence of resistivity obeys the relation p = #0 exp(A E / R T ) with two activation energies. In the temperature range 300 K >/T > 234 K the activation energy is 0.58 eV and for 234 > T >/185 K the value is A E = 0.30 eV. The activation energy has been measured as a function of pressure. The electrical resistivity decreases exponentially with the increase of pressure and at 70 kbar pressure the electrical behaviour of the sample shows a metallic nature with a positive temperature coefficient. The high pressure phase of the sample is found to be a crystalline hexagonal phase.
I. Introduction
There have been very few studies of the pressure induced semiconductorto-metal transition in amorphous semiconductors [1-5], with detailed studies on amorphous Se [2,5], amorphous As [4], amorphous As2Te 3 [3] and tetrahedrally bonded systems [1]. Usually amorphous chalcogenides under pressure become metallic with a continuous decrease in resistivity and they retain their molecular structure [2,3]. In this paper we report a pressure induced semiconductor to metal transition in bulk amorphous A123Te77 samples. Measurements of electrical resistivity have already been performed on Al25Te75 samples at atmospheric pressure [6]. Some work has also been done on electrical switching [7]. The alloy A123Te77 w a s chosen because the glass forming region in A1xTel_ x system is limited to 0.15 < x < 0.30 [8], i.e. the glass forming ability is much higher near the eutectic. Moreover the high pressure behaviour of this system has not yet been studied, though there is a little experimental work on electrical switching and crystallization [7,9]. * Now at Instrumentation and Services Unit, l.l.Sc., Bangalore, India. ** Now at the National Physical Laboratory, Hill Side Road, New Delhi 110 012, India. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
74
G. Parthasarathy et al. / Electrical resistioity of bulk Ale ~Te77 glass
2. Experimental studies The high-pressure apparatus for the measurements of the electrical resistance of an amorphous m123Te77 sample as a function of pressure and temperature consists of a pair of tungsten carbide anvils with 4 mm face diameter and a hydraulic press with compression and tension members, made of stainless steel to facilitate the pressurization in a low temperature cryostat. The pressure are calibrated at room temperature, using as fixed points the bismuth transitions at 25.4, 27 and 77 kbar, thallium transition at 35.7 kbar and ytterbium transition at 40 kbar. The pressures at low temperatures are also calibrated by studying the behaviour of Bi, T1, Yb and Pb. The details of the instrumentation and calibration will be discussed elsewhere [10]. The resistances of the samples were measured by using a four probe method. A Keithley constant current source (model No. 225), a Keithley Digital multimeter (model No. 177) and a Keithley electrometer (model No. 616) were used as measuring instruments. For measuring the temperature, copper-constantan thermocouples were used. The temperature was controlled within + 0.3 K. Bulk samples of A123Te27 were prepared by melting together appropriate amounts of the elements (99.999% purity) in sealed, evacuated quartz tubes in a rotary furnace at a temperature of - 8 6 0 ° C for 18 h and subsequent quenching in an ice-water mixture. The bulk solids were confirmed to be amorphous by studies of X-ray diffraction, electron microscopy and differential scanning calorimetry. The resistivity of the sample was calculated by measuring the dimensions of the sample at normal atmospheric pressure. The change in the dimensions of the sample under pressure was not taken into account; the effect of this on the resistivity was generally small and can be calculated only if the compressibility of the material is known as a function of pressure. For recovering the pressure-quenched samples, sodium chloride was used as a pressure transmitting medium instead of steatite. After the application of the pressure, the sample was recovered by dissolving the pressure transmitting media along with the sample in double distilled water. The recovered sample was examined under an electron microscope and X-ray diffractometer. The method is useful for the cases where the material undergoes irreversible transformations.
3. Results Fig. 1 shows the variation of electrical resistivity as a function of pressure for bulk Ak23Te77 glass at room temperature. At room temperature and pressure, the resistivity of our sample is found to be 5.55 x 105 $2 cm, which is in good agreement with the reported value [6]. With an increase of pressure, the resistivity decreases continuously and becomes of the order of 10 -4 I2 cm
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Fig. 2. Semi log plot o f electrical resistivity versus ( 1 0 0 0 / T ) for AI23Te77 glass at different pressures: curve (A) for P : 1 bar: (B) P : 10 kbar; (C) P = 20 kbar; (D) P = 30 kbar; (E) P = 49 kbar; (F) P = 50 kbar.
Fig. 1. Variation of electrical resistivity of AI23Te77 glass as a f u n c t i o n of pressure.
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G. Parthasarathy et al. / Electrical resistivity of bulk A12 3Te 77 glass
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at 80 kbar. The resistivity of the sample at 80 kbar is found to be less than the Mott's maximum metallic resistivity [11]. Fig. 2 shows the temperature dependence of electrical resistivity for bulk A123Te77 glass at different pressures. We find that the resistivity variation obeys the relation p = P0 exp(AE/kT) where p0 = pre-exponential factor, AE = activation energy for electronic conduction, k = Boltzmann's constant and T = temperature. At zero pressure there are two activation energies: 0.58 eV for 234 K < T~< 300 K and 0.30 eV for 185 K ~< T < 234 K. Because of the high resistance of the sample the temperature range of investigation is limited to 185 K. At 10 kbar pressure the value of the first activation energy decreased to 0.49 eV; on the other hand the value of the second activation energy was increased to 0.41 eV. For a pressure of 20 kbar and above the conduction process is still thermally activated but with a single activation energy over the temperature range of investigation. Fig. 3 shows the variation of electrical resistivity with temperature at pressures of 60 kbar and 70 kbar. At 60 kbar pressure, the temperature coefficient of resistivity is found to be zero for 250 T < T ~< 300 K and for temperature below 250 K the samples show the semiconducting behaviour with an activation energy 0.05 eV. At 70 kbar pressure the temperature coefficient of electrical resistivity is found to be positive and is equal to 2.03 s × 10 -7 9cm/K. Fig. 4 shows the variation of the activation energy with pressure. For 0 ~ P < 40 kbar the pressure coefficient of activation energy is (dE*/dP)=
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Fig. 4. Variation of the activation energy AE with pressure for AI23Te77 glasses. Closed circle AE measured from p = Po e x p ( A E / k T ) ; open circle AE derived from the p ( P ) curve. Fig. 5. X-ray diffraction (powder) pattern for the pressure quenched AI23Te77 sample from a pressure of 70 kbar.
-0.0103 eV/kbar, and for 40 < P ~< 70 kbar (dE*/dP) = - 0 . 0 0 6 eV kbar. In the same figure E* values obtained from the expression (dE*/dP)= kT(d In p/dP) are also plotted [5]. Both values are found to be in good agreement. The values of the activation energy at different pressures and the corresponding pre-exponential factors are given in table 1. The X-ray diffraction pattern for pressure quenched ( P = 70 kbar) AI23Tevv glass is shown in fig. 5. It is found that the high pressure phase is a crystalline hexagonal structure with c/a = 1.7. Table 1 Activation energies and the pre-exponential factors for electric conduction in A123Te77 glass at different values of pressure Pressure (kbar) 0 10 20 30 40 50 60
Temperature range (K)
Activation energy E (eV)
Pre-exponential factor (/2 cm)
300-234 234-185 300-242 242-170 300-133 300-96 300-77 247-77 250-77
0.58 0.30 0.49 0.41 0.37 0.26 0.18 0.11 0.05
1.16 x 10 -4 99.6 1.02 x 10 4 1.85 x 10 -4 1.29 x 10-4 2.53 x 10 -4 9.98 x 10- 5 0.98 X 1 0 - s 1.00 X 10- 4
20
78
G. Parthasarathy et al. / Electrical resistivity of bulk AI 23Te77 glass
4. Discussions The effect of pressure on the electrical and optical properties of amorphous semiconductors has been reviewed by Minomura et al. [2]. In tetrahedrally bonded glasses the pressure induced transitions are discontinuous, accompanied by changes in structure to high co-ordination. On the other hand most of the chalcogenide glasses becomes metallic under pressure with a continuous decrease in optical gap and electrical resistivity even as they retain their molecular structure. The pressure coefficient of the optical gap in the case of chalcogenide glasses lies in between - 1 . 0 × 10 2 and - 2 × 10 -2 eV/kbar. The activation energy E* (or) A E, derived from the temperature dependence of resistivity and the optical gap E 0, are related by E* = AE = ½E0 indicating that the Fermi level should linear the center of the gap [3]. So the value of the pressure coefficient of the electrical activation energy should lie between - 0 . 5 x 10 .2 and - 1 . 0 X l 0 -2 eV/kbar. In our experiment the value of - 1 . 0 3 x 10 2 e V / k b a r of the pressure coefficient of the electrical activation energy agrees with the above expected value. This indicates that the decreases in the electrical resistivity under pressure arises from a gradual decrease in the gap. But the pressure dependence of the resistivity is more complicated because the resistivity depends not only upon the gap but also on the mobility and the position of the Fermi level [13]. Therefore an independent experiment on the pressure dependence of mobility is required. From fig. 4 and table 1 it is seen that the activation energy decreases with the increase of pressure. This implies that at room temperature, the fundamental gap decreases with the increase of pressure and becames zero at P - 70 kbar. The pre-exponential factor O0 in the resistivity expression O = O0 exp(AE/kT) has a value of the order of 10 -4 ~2 cm. From the analysis of Mort [11], we feel that the observed value of the pre-exponential factor represents that the carriers are excited across the mobility edge into the extended states. The approximate constancy of the pre-exponential factor with the increase of pressure implies that the mobility gap decreases with the increase of pressure. At normal or zero pressure the conduction at high temperature is through the extended states and in fact the pre-exponential factor in the higher temperature region is 1.16 × 10 .4 $2 cm. It is found that the conduction in the low temperature region is probably through the tail of the localized states via thermally activated hopping since its pre-exponential factor is 99.6 12 cm which is six orders of magnitude more than that for extended state conduction [14]. With the application of pressure up to 10 kbar the width of the localized states near the mobility edges is found to increase with the pressure. But beyond 10 kbar pressure the conduction via thermally activated tunneling in the localized states is found to be suppressed, at least in the temperature range studied. It is likely that the pressure induced distortions cause changes in the density of states, particularly in the inter-band gap, which could decrease the mobility gap. Such a decrease in the mobility gap can reduce the extent of localization
G. Parthasarathy et al. / Electrical resistivity of bulk AI 2 ~Te77 glass
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due to enhancement of the dielectric constant, and in turn decrease the gap further [3]. Thus the gap m a y close up as a function of pressure and metallic conductivity will emerge at high pressures. Fig. 5 shows the X-ray diffraction pattern for pressure quenched Alz3TeTv glass. The high pressure phase seems to have hexagonal structure with c / a = 1.7. The structure of A123Te77 glass has been considered to be a complicated three dimensional network of A1 atoms tetrahedrally co-ordinated with Te [15]. The excess Te atoms form chains which are linked together with the distorted tetrahedral. By the application of heat the excess Te crystallizes first and the remaining a m o r p h o u s matrix of A12Te3 is crystallized at a higher temperature, which leads to a double stage crystallization in this glass [9]. On the other hand we get a single hexagonal phase of this material, with c / a = 1.7, by the application of pressure. The similar pressure induced crystallization in a m o r p h o u s As2Te 3 has been reported [2], where the high pressure phase of a m o r p h o u s As2T % and the normal crystalline phase of AszTe 3 are found to be the same. F r o m this study, we can conclude that for chalcogenic glasses, the molecular structure can be retained under the application of high pressure. In A123Te77 glass, the high pressure pressure crystalline phase is more stable than the temperature induced crystalline phase. The details of the temperature induced effects will be published elsewhere [16]. The authors would like to thank the D S T and C S I R schemes, G o v e r n m e n t of India, for financial support.
References [1] O. Shimomura, S. Minomura, N. Sakai, K. Asaumi, K. Tamura, J. Fukushima and H. Endo, Phil. Mag. 29 (1974) 547. [2] S. Minomura, Amorphous Semiconductors Technologies and Devices, ed., Y. Hamakawa (North-Holland, Amsterdam, 1982) p. 245. [3] N. Sakai and H. Frizache, Phys. Rev. B15 (1977) 973. [4] S.R. Elliot, E.A. Davis and G.D. Pitt, Solid St. Commun. 22 (1977) 481. [5] W. Fuhs, P. Schlotter and J. Stuke, Phys. Stat. Sol. (b)57 (1973) 587. [6] A.A. Andreev, M.S. Ablova, B.T. Meleck, F. Nasredinov, P.P. Seregin and E. Turaev, in: Amorphous and Liquid Semiconductors ed., W.E. Spear (Institute of Physics, Bristol, 1978) p. 44. [7] A.A. Andreev, M.S. Ablova, V.P. Podkhalyuzin, Z.V. Maslova and B.t. Melelch, Sov. J. Glass Phys. and Chem. (USA) 5 (1979) 334. [8] J. Cornet, Proc. Sixth Int. Conf. on Amorphous and liquid Semiconductors, Leningrad, USSR (1975) p. 72. [9] J. Colmenero and J.M. Barandiaran, J. Non-Crystalline Solids 30 (1979) 263. [10] G. Parthasarathy, A.K. Bandyopadyay, S.T. Lakshmikumar and E.S.R. Gopal, J. Inst. Soc. India 14 (1984) 169. [11] N.F. Mott, Phil. Mag. 22 (1970) 7. [12] R.P. Elliot, Constitution of Binary Alloys: First Supplement (McGraw-Hill, New York, 1965) p. 57. [13] W. Paul and D.M. Warschauer, Solids under Pressure (McGraw-Hill, New York, 1963) p. 179. [14] E.A. Davis and N.F. Mott, Phil. Mag. 22 (1970) 903. [15] A. D'Anjou and P. Sanz, J. Non-Crystalline Solids 28 (1978) 319. [16] G. Parthasarathy and E.S.R. Gopal, to be published.