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Effect of composition and structure on electrical conduction of Se(100−x)Te(x) films
Solid State Communications,
Pergamon
Vol. 95, No. 5, p. 335-339, 1995 E&&r Science Ltd Printed in Great Britain. 0038-1098/95 $9.50 + .oo
003th1098(94)00913-9
EFFECT OF COMPOSITION
AND STRUCTURE ON ELECTRICAL OF Se(iWXJTe(,) FILMS
CONDUCTION
A. El-Korashy Assiut University, Faculty of Science, Phys. Dept., Assiut, Egypt and H. El-Zahed, H.A. Zayed and M.A. Kenawy University Girls College for Art, Science and Education, Ain Shams University, Phys. Dept., Helioples, Cairo, Egypt (Received 27 April 1994; accepted in revisedform
8 December 1994 by S.G. Louie)
Among several physical properties measured about mordenite and trigonal Se(iWxjTe(,), we choose the electrical Se(tw,)Te(, properties f’or our study in this article. The electrical properties of Se iWx)Te(,) (x = 0, 20, 40, 60, 80, 100) films of thickness about 106 nm, deposited by thermal evaporation onto glass substrate have been measured as a function of tellurium content. Electrical resistivity is observed to decrease with increased temperature and tellurium content. It is found that the activation energy varies from 0.87 eV to about 0.15 eV with increased concentration of tellurium. Sharp decrease of activation energy (AE) and increase of carrier concentration (n) for x > 50 has been ascribed to the phase transition from chain like structure (M Se-Te system) to trigonal structure (t Se-Te system), i.e. we clarify that the characteristic features of (t Se-Te) reflect in the concentration (x) dependence of AE.
1. INTRODUCTION INVESTIGATION of the properties of semiconducting materials in the form of thin films have drawn world attention. Selenium and tellurium belong to the VIb group elements, which may prove to be an efficient material for thin film circuits [l], fabrication of semiconductor devices, transistors [2, 31, detectors [4] and elsewhere [5-71. Trigonal selenium (t-Se) and trigonal tellurium (t-Te) are semiconductors. A trigonal crystal is composed of helical chains arranged parallel to each other. In a trigonal crystal, each atom has two nearest neighbor atoms at a distance (r) on the same chain and four second neighbor atoms at a distance R(> r) on the adjacent chains. The ratio of R/r of t-Se (R/r = 1.45)is greater than that of t-Te (R/r = 1.23),i.e., the degree of chain like nature of t-Se is larger than that of t-Te. The difference of R/r is expected to influence their physical properties. Recently these elements, Se and Te, are
constructed in the form of isolated chains by confining in the narrow channels of mordenite (M Se-Te) [8-111. Yamaguchi et al. [12] construct these isolated chains to study the physical properties of a onedimensional (1D) system in which the interchain interactions are absent and compare them with those of a three dimensional (3D) system, where the interchains are present. Generally various investigators have found that the electrical resistivity decreases with increase thickness of Te fihns. Some of them have investigated the temperature dependence of the electrical properties [13]. Unusual variation of electrical resistivity with thickness has been found by Chaudhury [14] in Te films and Sharma et al. [ 151 in Se films. They reported that electrical resistivity first increases with thickness and then decreases. Goswami and Ojha [16] give extensive results of electrical resistivity (p), activation energy (A&‘), carrier concentration (n), Hall coeffrcient (RH), TCR, etc. in the temperature range from
335
336
ELECTRICAL
CONDUCTION
Vol. 95, No. 5
OF Se(rocrxjTe(,) FILMS
78 K to 450 K for Te fihns. Mzakaria et al. [ 171 have studied the changes in electrical resistivity, Hall coefficient and Hall mobility at room temperature of vacuum deposited tellurium films as a function of thickness (180-800 nm thick) and magnetic field (O.O-15kG). In the present study, an effort is made to understand the compositional dependence of the electrical resistivity (p) and the activation energy (AE) of Se(locrx~Te(,) films and correlate it with our previous results [ 181 of optical energy gap, in which the phase transition from chain like structure (M Se(lo-,_x,Te~x$ to trigonal (t-Se(l~x~Te(,)) occur according to the change of Te concentration. 2. EXPERIMENTAL 1
Se(tWxjTe(,) [x = 0, 20, 40, 60, 80, 1001 bulk glasses were prepared by the usual melt quenching technique. The constituent elements Se and Te (99.999% pure) were weighed and mixed in the appropriate stoichiometric proportion and sealed in an evacuated (at 10-6Torr) quartz ampoule which was placed in a furnace and heated stepwise at 700°C to allow Se and Te to react completely. The ampoule was frequently rocked for 10 h at the maximum temperature to make the melt homogeneous. The quenching was performed in ice water, then the alloy was taken out by cutting the ampoule. The initial vitreous alloys were powdered and separated according to sizes. Thin lihns of different compositions of Se(lWx)Te(x) were prepared by thermal evaporation method onto cleaned glass substrates at a pressure of lo-’ Torr with the help of LEYBOLD coating unit (constructed in Dr Sorour Res. Lab.). Molybdenum boats were used for evaporation. The glass substrates were first cleaned with acetone and alcohol and then washed in an ultrasonic bath with de-ionized water. Finally the glass substrates were dried in a dust free atmosphere. Two different types of masks were prepared. One for the sample Se(tocrx)Te(,) and the other for electrode (Ag). Growth conditions of all fihns were practically the same. After deposition, the fihns were allowed to stay about 24 h in vacuum. It was observed that without settle up time in vacuum, the lihns show some irregular behavior, which indicates that the structural changes continue for a considerable time. This time is shorter than the time required for the attainment of the thermodynamic equilibrium in SeTe alloy [19]. The tlhn thickness was measured0 using Multiple Fizeau fringes [20] to an accuracy of 30 A. The electrode contacts were of an ohmic nature as determined. by I-V characteristic study. An electrometer of the type Keithly 610C was used for measurements of
Fig. 1. Reciprocal temperature dependence of the dark electrical resistivity (p) of Se(locrxJTe(,) 8lms (x = 0, 20,40, 60, 80, 100). resistivity in the temperature range (300-5OOK). Temperature of the fihus have been measured by copper constantan thermocouple placed on the same side of the iihn. For all measurements, sufhcient time has been devoted to secure stability before taking readings. 3. RESULTS AND DISCUSSION Properties of thin film materials are different from those of bulk, due to the present of several additional factors, such as discontinuities, structural defects, grain growth, phase changes, etc. induced by the deposition parameters. The variation of log resistivity of Se(n,,-,+Te(,) fihns with reciprocal of temperature are shown in
0.1) B 20.6 w -a4
WI .NT \ \ \
Fig. 2. (A) Variation of activation energy (AE) with different compositions of Se(iW, Te(,) flhns. (B) Variation of optical energy gap (B ) with different compositions of Se(iocrxJTe (x. films [f8].
Vol. 95, No. 5
ELECTRICAL
CONDUCTION
OF Se(tocrx)Te(,) FILMS
337
Fig. 1. It is seen that, with rising temperature, the resistivity decreases at first slowly and later quite rapidly. One can observe a knee in the temperature range from 391 K to 410 K. These knees occur around the crystallization temperature T,,and are in good agreement with that reported [21] using differential thermal analysis (DSC). In general, the electrical resistivity obeys the relation: p = po exp(-AE/kT), where (po) and AE are the pre exponential factor and activation energy respectively. The activation energies for conduction (AE) calculated from Fig. 1 as a function of composition are shown in Fig. 2(A). The activation energy AE was obtained by a least square fit to experimental data. It is clear from this figure that the activation energy decreases with increasing Te content. An abrupt decrease in AE is observed for x > 50. It is worth comparing this result with our previous work [18], as shown in Fig. 2(B). The results depicted in Fig. 2(A) are in a good agreement with our previous results in which optical energy gap G+l/2 activation energy (E,, g 1/2AE). We may assume the structural features of Se-Te system to understand the observed results. It has been established that Se-Te system exhibits two structures. The chain like behavior dominates at high Se contents. Such behavior is modtied to trigonal structure by increasing Te content. From this point, as shown in Fig. 2(A), the activation energy of chain like structure (high Se content) is greater than that of trigonal structure (high Te content), that is to say the increase of Te content weakens the chain like character of a trigonal crystal. We identify the activation energy AE as the difference between the lower edge of the conduction band or the upper edge of valence band and Fermi level. In the case of a chain structure, AE decreases little in region of x < 50 while in the case of a trigonal crystal AE shows a sharp decrease as x increases. The difference in increase of x dependence of AE is explained by the difference of dimensionality. The degree of the band width is small in the case of one dimensional chains, but large in the case of three dimension mixtures. A trigonal crystal has wide bands because of the existence of interchain interaction. Thus, the composition tends to transfer from semiconductor to metallic behaviour as Te content increases. The resistivity of this compositions at room temperature changes from lo6 to 10’ as Te content increases. Figure 3 shows the variation of resistivity of Se-Te system with Te content at room temperature. Also one can observe a knee in the range of
-0 I
I
I
20
40
60
I
I
80 100 te%
Fig. 3. Plot of tellurium content (Te%) vs resistivity for different compositions of Se(tmXjTe(,) tilms.
Fig. 4. Tellurium content dependence of carrier concentration (n) for different compositions of Se(lmX)Te(,) films.
338
ELECTRICAL
CONDUCTION
OF Se(i~X~Te(,) FILMS
Vol. 95, No. 5
Table 1. The activation energies (AE), carrier concentration (n), resistivities (p) at room temperature, optical band gap (Es) and crystallization temperature T, of Se(t,,,+Te(,)) Comp Se-Te
$)
Se SesTe2 SehTed Se4Te6 SezTes Te
0.87 0.68 0.61 0.51 0.18 0.15
Carr. Cont. tn)
Elect. Resis.
opt. Gap Es
Cryst. temp.
7-c Present
(PI
(ev)
Tc 0-Q
WI
6.13 x 10” 2.82 x lOI 8.89 x lOI 6.03 x lOI 3.3 x 1o16 5.9 x lOi
7.5 x IO5 1.78 x lo5 3.98 x lo4 8.5 x lo3 2.09 x lo2 0.87 x lo2
1.3 I .28 1.06 1.06 -
392 406.6 409.9 -
391 395 400 407 410 400
40 < x < 60. This may be again due to the presence of a mixture of both chain and modified trigonal structures. The carrier concentration (n) was calculated from the relation [19] nl = 2((27rm*kT)/Z~~}~/~exp(AE/2kT). Figure 4 represents the variation in carrier concentration (n) with Te content. The sharp increases in carrier concentration of Se(iml)Te(,) films at
x > 50 may be understood as a structural transition from chain like structure to triagonal. The sudden increase in carrier concentration, again, at x > 50 supports the transformation to metallic behavior as Te content increases. The carrier concentration increases from 10” to lOI at x < 50 with a sharp increases from 1013to lOI6 at x > 50. Temperature coefficient of resistance (TCR) was determined for various films at different temperature regions using the relation TCR = (l/R)(dR/dt).
l
Generally it increases in magnitude with both increasing temperature and Te content as shown in Fig. 5.
Se
4. SUMMARY We study the dependence of the activation energy AE on Te content with connection to phase transition of structure. One can conclude that AE has a sharp decreases at x > 50 where the structural transition occurred. We also show that the dimensions of a system influence the way in which AE changes as a function of x. We also report the dependence of both n and TCR as a function of x, which gives an idea of such structural transformation. The summary of these results is tabulated in Table 1. REFERENCES 1. 2. 3. I
3lO
I
320
Fig. 5. Variation of temperature tivity of Se(iocrx)Te(,) films.
I
330
I
T(K)
coefficient of resis-
4. 5.
P.K. Weimer, H. Borkan, G. Sadasiva, L. Merray-Horvath & F.V. Shallcross, Proc. IEEE 52, 1479 (1968). P.K. Weimer, Proc. IEEE (USA) 52,608 (1964). H.L. Wilson & W.A. Gutierrotz, Proc. IEEE (USA) 52,415 (1967). J. Grosvalet & C. Jund, IEEE Trans. Electron Device (USA) 14, 777 (1967). R.W. Dutton & R.S. Muller, Proc. IEEE 59, 1511 (1971).
ELECTRICAL
Vol. 95, No. 5
CONDUCTION
6.
P.M. Heyman & G.H. Heilmeier, Proc. IEEE
7.
J. Dresner
8. 9.
10.
54, 842 (1966).
15.
State
16.
K. Tamura, S. Hoso Kawa, H. Endo, S. Yamasaki & H. Oyanagi, J. Phys. Sot. Jpn 55, 528
17.
M. Inui, M. Yao & H. Endo, J. Phys. Sot. Jpn
18.
H. Sakai, M. Yao, M. Inui, K. Maruyama, K. Tamura, K. Takimoto & H. Endo, J. Phys. Sot.
19.
& F.V.
Electron 5,205
Shallcross,
(1962).
Solid
OF Se(twX)Tet,) FILMS
(1986).
57, 553 (1988).
12.
Sot. Jpn 61, 1240 (1992). A. Devos & D. Van Dhelsen, Revue De Physique
20.
13. 14.
A.K. Chaudhuri, Indian J. Pure Appl. Phys. 12,
21.
Appliquee 14, 815 (1979). 399 (1974).
A.K. Sharma & B. Singh, Indian J. Pure Appl. Phys. 21,420 (1983). A. Goswami & S.M. Ojha, Thin Solid Films 16, 187 (1973).
Jpn 57, 3587 (1988). Y. Katayama, M. Yao, Y. Ajiro, M. Inui & H. Endo, J. Phys. Sot. Jpn 58, 1811 (1989). T. Yamaguchi & Fumiko Yonozawa, J. Phys.
11.
339
A.K. Mzakaria, M.A. Quasem, M. Imamuddin, S.M.P. Hasan, N. Aktar & M.A. Subhan, Indian
J. Pure Appl. Phys. 58 (199 1).
H. El-Zahed, M.A. Khaled, A. El-Korashy, SM. Youssef & M. El-Ocker, Solid State Commun. 89, 1013.
M. Abkowitz, G.M.T. Foley, J.M. Markovics & A.C. Pulumbo, Optical Eflects in Amorphous Semiconductors (Edited by P.C. Taylor & S.G. Bishop). AIP Conf. Proceedings 120, American Institute for Physics, pp. 117-124. S. Tolansky, Multiple Beam Interferometry of Surfaces and Films, p. 147. Oxford University, London and New York. A.H. Abou Elela M.K. Ehnously & K.S. Abdu, J. Mater. Sci. 15, 871 (1980).
Pergamon
Vol. 95, No. 5, p. 335-339, 1995 E&&r Science Ltd Printed in Great Britain. 0038-1098/95 $9.50 + .oo
003th1098(94)00913-9
EFFECT OF COMPOSITION
AND STRUCTURE ON ELECTRICAL OF Se(iWXJTe(,) FILMS
CONDUCTION
A. El-Korashy Assiut University, Faculty of Science, Phys. Dept., Assiut, Egypt and H. El-Zahed, H.A. Zayed and M.A. Kenawy University Girls College for Art, Science and Education, Ain Shams University, Phys. Dept., Helioples, Cairo, Egypt (Received 27 April 1994; accepted in revisedform
8 December 1994 by S.G. Louie)
Among several physical properties measured about mordenite and trigonal Se(iWxjTe(,), we choose the electrical Se(tw,)Te(, properties f’or our study in this article. The electrical properties of Se iWx)Te(,) (x = 0, 20, 40, 60, 80, 100) films of thickness about 106 nm, deposited by thermal evaporation onto glass substrate have been measured as a function of tellurium content. Electrical resistivity is observed to decrease with increased temperature and tellurium content. It is found that the activation energy varies from 0.87 eV to about 0.15 eV with increased concentration of tellurium. Sharp decrease of activation energy (AE) and increase of carrier concentration (n) for x > 50 has been ascribed to the phase transition from chain like structure (M Se-Te system) to trigonal structure (t Se-Te system), i.e. we clarify that the characteristic features of (t Se-Te) reflect in the concentration (x) dependence of AE.
1. INTRODUCTION INVESTIGATION of the properties of semiconducting materials in the form of thin films have drawn world attention. Selenium and tellurium belong to the VIb group elements, which may prove to be an efficient material for thin film circuits [l], fabrication of semiconductor devices, transistors [2, 31, detectors [4] and elsewhere [5-71. Trigonal selenium (t-Se) and trigonal tellurium (t-Te) are semiconductors. A trigonal crystal is composed of helical chains arranged parallel to each other. In a trigonal crystal, each atom has two nearest neighbor atoms at a distance (r) on the same chain and four second neighbor atoms at a distance R(> r) on the adjacent chains. The ratio of R/r of t-Se (R/r = 1.45)is greater than that of t-Te (R/r = 1.23),i.e., the degree of chain like nature of t-Se is larger than that of t-Te. The difference of R/r is expected to influence their physical properties. Recently these elements, Se and Te, are
constructed in the form of isolated chains by confining in the narrow channels of mordenite (M Se-Te) [8-111. Yamaguchi et al. [12] construct these isolated chains to study the physical properties of a onedimensional (1D) system in which the interchain interactions are absent and compare them with those of a three dimensional (3D) system, where the interchains are present. Generally various investigators have found that the electrical resistivity decreases with increase thickness of Te fihns. Some of them have investigated the temperature dependence of the electrical properties [13]. Unusual variation of electrical resistivity with thickness has been found by Chaudhury [14] in Te films and Sharma et al. [ 151 in Se films. They reported that electrical resistivity first increases with thickness and then decreases. Goswami and Ojha [16] give extensive results of electrical resistivity (p), activation energy (A&‘), carrier concentration (n), Hall coeffrcient (RH), TCR, etc. in the temperature range from
335
336
ELECTRICAL
CONDUCTION
Vol. 95, No. 5
OF Se(rocrxjTe(,) FILMS
78 K to 450 K for Te fihns. Mzakaria et al. [ 171 have studied the changes in electrical resistivity, Hall coefficient and Hall mobility at room temperature of vacuum deposited tellurium films as a function of thickness (180-800 nm thick) and magnetic field (O.O-15kG). In the present study, an effort is made to understand the compositional dependence of the electrical resistivity (p) and the activation energy (AE) of Se(locrx~Te(,) films and correlate it with our previous results [ 181 of optical energy gap, in which the phase transition from chain like structure (M Se(lo-,_x,Te~x$ to trigonal (t-Se(l~x~Te(,)) occur according to the change of Te concentration. 2. EXPERIMENTAL 1
Se(tWxjTe(,) [x = 0, 20, 40, 60, 80, 1001 bulk glasses were prepared by the usual melt quenching technique. The constituent elements Se and Te (99.999% pure) were weighed and mixed in the appropriate stoichiometric proportion and sealed in an evacuated (at 10-6Torr) quartz ampoule which was placed in a furnace and heated stepwise at 700°C to allow Se and Te to react completely. The ampoule was frequently rocked for 10 h at the maximum temperature to make the melt homogeneous. The quenching was performed in ice water, then the alloy was taken out by cutting the ampoule. The initial vitreous alloys were powdered and separated according to sizes. Thin lihns of different compositions of Se(lWx)Te(x) were prepared by thermal evaporation method onto cleaned glass substrates at a pressure of lo-’ Torr with the help of LEYBOLD coating unit (constructed in Dr Sorour Res. Lab.). Molybdenum boats were used for evaporation. The glass substrates were first cleaned with acetone and alcohol and then washed in an ultrasonic bath with de-ionized water. Finally the glass substrates were dried in a dust free atmosphere. Two different types of masks were prepared. One for the sample Se(tocrx)Te(,) and the other for electrode (Ag). Growth conditions of all fihns were practically the same. After deposition, the fihns were allowed to stay about 24 h in vacuum. It was observed that without settle up time in vacuum, the lihns show some irregular behavior, which indicates that the structural changes continue for a considerable time. This time is shorter than the time required for the attainment of the thermodynamic equilibrium in SeTe alloy [19]. The tlhn thickness was measured0 using Multiple Fizeau fringes [20] to an accuracy of 30 A. The electrode contacts were of an ohmic nature as determined. by I-V characteristic study. An electrometer of the type Keithly 610C was used for measurements of
Fig. 1. Reciprocal temperature dependence of the dark electrical resistivity (p) of Se(locrxJTe(,) 8lms (x = 0, 20,40, 60, 80, 100). resistivity in the temperature range (300-5OOK). Temperature of the fihus have been measured by copper constantan thermocouple placed on the same side of the iihn. For all measurements, sufhcient time has been devoted to secure stability before taking readings. 3. RESULTS AND DISCUSSION Properties of thin film materials are different from those of bulk, due to the present of several additional factors, such as discontinuities, structural defects, grain growth, phase changes, etc. induced by the deposition parameters. The variation of log resistivity of Se(n,,-,+Te(,) fihns with reciprocal of temperature are shown in
0.1) B 20.6 w -a4
WI .NT \ \ \
Fig. 2. (A) Variation of activation energy (AE) with different compositions of Se(iW, Te(,) flhns. (B) Variation of optical energy gap (B ) with different compositions of Se(iocrxJTe (x. films [f8].
Vol. 95, No. 5
ELECTRICAL
CONDUCTION
OF Se(tocrx)Te(,) FILMS
337
Fig. 1. It is seen that, with rising temperature, the resistivity decreases at first slowly and later quite rapidly. One can observe a knee in the temperature range from 391 K to 410 K. These knees occur around the crystallization temperature T,,and are in good agreement with that reported [21] using differential thermal analysis (DSC). In general, the electrical resistivity obeys the relation: p = po exp(-AE/kT), where (po) and AE are the pre exponential factor and activation energy respectively. The activation energies for conduction (AE) calculated from Fig. 1 as a function of composition are shown in Fig. 2(A). The activation energy AE was obtained by a least square fit to experimental data. It is clear from this figure that the activation energy decreases with increasing Te content. An abrupt decrease in AE is observed for x > 50. It is worth comparing this result with our previous work [18], as shown in Fig. 2(B). The results depicted in Fig. 2(A) are in a good agreement with our previous results in which optical energy gap G+l/2 activation energy (E,, g 1/2AE). We may assume the structural features of Se-Te system to understand the observed results. It has been established that Se-Te system exhibits two structures. The chain like behavior dominates at high Se contents. Such behavior is modtied to trigonal structure by increasing Te content. From this point, as shown in Fig. 2(A), the activation energy of chain like structure (high Se content) is greater than that of trigonal structure (high Te content), that is to say the increase of Te content weakens the chain like character of a trigonal crystal. We identify the activation energy AE as the difference between the lower edge of the conduction band or the upper edge of valence band and Fermi level. In the case of a chain structure, AE decreases little in region of x < 50 while in the case of a trigonal crystal AE shows a sharp decrease as x increases. The difference in increase of x dependence of AE is explained by the difference of dimensionality. The degree of the band width is small in the case of one dimensional chains, but large in the case of three dimension mixtures. A trigonal crystal has wide bands because of the existence of interchain interaction. Thus, the composition tends to transfer from semiconductor to metallic behaviour as Te content increases. The resistivity of this compositions at room temperature changes from lo6 to 10’ as Te content increases. Figure 3 shows the variation of resistivity of Se-Te system with Te content at room temperature. Also one can observe a knee in the range of
-0 I
I
I
20
40
60
I
I
80 100 te%
Fig. 3. Plot of tellurium content (Te%) vs resistivity for different compositions of Se(tmXjTe(,) tilms.
Fig. 4. Tellurium content dependence of carrier concentration (n) for different compositions of Se(lmX)Te(,) films.
338
ELECTRICAL
CONDUCTION
OF Se(i~X~Te(,) FILMS
Vol. 95, No. 5
Table 1. The activation energies (AE), carrier concentration (n), resistivities (p) at room temperature, optical band gap (Es) and crystallization temperature T, of Se(t,,,+Te(,)) Comp Se-Te
$)
Se SesTe2 SehTed Se4Te6 SezTes Te
0.87 0.68 0.61 0.51 0.18 0.15
Carr. Cont. tn)
Elect. Resis.
opt. Gap Es
Cryst. temp.
7-c Present
(PI
(ev)
Tc 0-Q
WI
6.13 x 10” 2.82 x lOI 8.89 x lOI 6.03 x lOI 3.3 x 1o16 5.9 x lOi
7.5 x IO5 1.78 x lo5 3.98 x lo4 8.5 x lo3 2.09 x lo2 0.87 x lo2
1.3 I .28 1.06 1.06 -
392 406.6 409.9 -
391 395 400 407 410 400
40 < x < 60. This may be again due to the presence of a mixture of both chain and modified trigonal structures. The carrier concentration (n) was calculated from the relation [19] nl = 2((27rm*kT)/Z~~}~/~exp(AE/2kT). Figure 4 represents the variation in carrier concentration (n) with Te content. The sharp increases in carrier concentration of Se(iml)Te(,) films at
x > 50 may be understood as a structural transition from chain like structure to triagonal. The sudden increase in carrier concentration, again, at x > 50 supports the transformation to metallic behavior as Te content increases. The carrier concentration increases from 10” to lOI at x < 50 with a sharp increases from 1013to lOI6 at x > 50. Temperature coefficient of resistance (TCR) was determined for various films at different temperature regions using the relation TCR = (l/R)(dR/dt).
l
Generally it increases in magnitude with both increasing temperature and Te content as shown in Fig. 5.
Se
4. SUMMARY We study the dependence of the activation energy AE on Te content with connection to phase transition of structure. One can conclude that AE has a sharp decreases at x > 50 where the structural transition occurred. We also show that the dimensions of a system influence the way in which AE changes as a function of x. We also report the dependence of both n and TCR as a function of x, which gives an idea of such structural transformation. The summary of these results is tabulated in Table 1. REFERENCES 1. 2. 3. I
3lO
I
320
Fig. 5. Variation of temperature tivity of Se(iocrx)Te(,) films.
I
330
I
T(K)
coefficient of resis-
4. 5.
P.K. Weimer, H. Borkan, G. Sadasiva, L. Merray-Horvath & F.V. Shallcross, Proc. IEEE 52, 1479 (1968). P.K. Weimer, Proc. IEEE (USA) 52,608 (1964). H.L. Wilson & W.A. Gutierrotz, Proc. IEEE (USA) 52,415 (1967). J. Grosvalet & C. Jund, IEEE Trans. Electron Device (USA) 14, 777 (1967). R.W. Dutton & R.S. Muller, Proc. IEEE 59, 1511 (1971).
ELECTRICAL
Vol. 95, No. 5
CONDUCTION
6.
P.M. Heyman & G.H. Heilmeier, Proc. IEEE
7.
J. Dresner
8. 9.
10.
54, 842 (1966).
15.
State
16.
K. Tamura, S. Hoso Kawa, H. Endo, S. Yamasaki & H. Oyanagi, J. Phys. Sot. Jpn 55, 528
17.
M. Inui, M. Yao & H. Endo, J. Phys. Sot. Jpn
18.
H. Sakai, M. Yao, M. Inui, K. Maruyama, K. Tamura, K. Takimoto & H. Endo, J. Phys. Sot.
19.
& F.V.
Electron 5,205
Shallcross,
(1962).
Solid
OF Se(twX)Tet,) FILMS
(1986).
57, 553 (1988).
12.
Sot. Jpn 61, 1240 (1992). A. Devos & D. Van Dhelsen, Revue De Physique
20.
13. 14.
A.K. Chaudhuri, Indian J. Pure Appl. Phys. 12,
21.
Appliquee 14, 815 (1979). 399 (1974).
A.K. Sharma & B. Singh, Indian J. Pure Appl. Phys. 21,420 (1983). A. Goswami & S.M. Ojha, Thin Solid Films 16, 187 (1973).
Jpn 57, 3587 (1988). Y. Katayama, M. Yao, Y. Ajiro, M. Inui & H. Endo, J. Phys. Sot. Jpn 58, 1811 (1989). T. Yamaguchi & Fumiko Yonozawa, J. Phys.
11.
339
A.K. Mzakaria, M.A. Quasem, M. Imamuddin, S.M.P. Hasan, N. Aktar & M.A. Subhan, Indian
J. Pure Appl. Phys. 58 (199 1).
H. El-Zahed, M.A. Khaled, A. El-Korashy, SM. Youssef & M. El-Ocker, Solid State Commun. 89, 1013.
M. Abkowitz, G.M.T. Foley, J.M. Markovics & A.C. Pulumbo, Optical Eflects in Amorphous Semiconductors (Edited by P.C. Taylor & S.G. Bishop). AIP Conf. Proceedings 120, American Institute for Physics, pp. 117-124. S. Tolansky, Multiple Beam Interferometry of Surfaces and Films, p. 147. Oxford University, London and New York. A.H. Abou Elela M.K. Ehnously & K.S. Abdu, J. Mater. Sci. 15, 871 (1980).