are used to calculate the crystallization kinetic parameters K and n of ah AVRAMI equation in the form, a(t) = 1 -- Exp (--Ktn). The activation energy of the ...
Acta Physica Acadcmiae Scientiarum Hungaricae, 52 (1), pp. 3--13 (1982)
ELECTRICAL TRANSPORT AND STRUCTURAL PROPERTIES
OF Se-Te
SEMICONDUCTORS*
M. F. KOTKATA** INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS, TRIESTE, ITALY
A.A. EL-ELA, E. A. MAHMOUD and M.K. EL-MOUSLY PHYSICS DEPARTMENT, FACULTY OF SCIENECE, AIN SHAMS UNIVERSITY, CAIRO, EGYPT
(Received 10. III. 1981) dc conductivity of different TeSe x semiconducting al|oys, with x ~ 50, 30, 20, 15, 10 and 5, are studied in both the liquid and the crystallized phases. The composition dependence of e0 (temperature independent pre-exponent) and E a (activation energy of conduction) in liquid Se-Te mixtures has been correlated with the electronic features and ability of the Te atoms to dissoeiate the Se chains. The conductivity dependence on time (t) in supercooled liquid Te-Se alloys is investigated in the temperature range of 90 180 ~ The data obtained are used to calculate the crystallization kinetic parameters K and n of ah AVRAMIequation in the form, a(t) = 1 -- Exp (--Ktn). The activation energy of the crystallization process shows a certain compositional dependence with a minimum value 0.37 eV at the composition of TeSea0.
I. Introduction According to the phase diagram of the selenium--tellurium system, solid solutions a t e f o r m e d f r o m t h e t w o e l e m e n t s in all a t o m i c p r o p o r t i o n s a n d t h e v a r i a t i o n o f t h e l a t t i c e p a r a m e t e r s w i t h c o n c e n t r a t i o n shows little d e v i a t i o n f r o m VECARV'S law [ 1 - - 3 ] . T h e liquid T e - - S e a l l o y s y s t e m spans a wide r a n g e o f electronic b e h a v i o u r b e t w e e n m e t a l s a n d i n s u l a t o r s . L i q u i d Se is a f a i r l y w i d e gap s e m i c o n d u c t o r w h i c h d i s p l a y s a c t i v a t e d electrical cond u c t i v i t y [4]. I n liquid Te, e x p e r i m e n t a l e v i d e n c e s u g g e s t s t h a t t h e F e r m i e n e r g y E v i s w i t h i n t h e v a l e n c e b a n d [5] a n d t h a t t h e d e n s i t y of s t a t e s at t h e F e r m i e n e r g y N ( E e ) is t e m p e r a t u r e - d e p e n d e n t . X - r a y a n d n e u t r o n d i f f r a c t i o n e x p e r i m e n t s [ 6 - - 1 0 ] i n d i c a t e t h a t t h e c o - o r d i n a t i o n n u m b e r o f liquid Te is a b o u t t h r e e at t h e m e l t i n g p o i n t TM a n d increases r a p i d l y w i t h t e m p e r a t u r e , while t h a t o f l i q u i d Se is a b o u t t w o a n d w e a k l y t e m p e r a t u r e - d e p e n d e n t . I n liquid S e - - T e m i x t u r e s t h e a v e r a g e c o - o r d i n a t i o n n u m b e r r e m a i n s n e a r l y two, i r r e s p e c t i v e o f c o m p o s i t i o n in t h e Se-rich r e g i o n a n d it increases w i t h t h e Te c o n c e n t r a t i o n in t h e T e - r i c h region. These e v i d e n c e s led t o suggest t h a t a c h a n g e f r o m Se-like to Te-like s t r u c t u r e m a y o c c u r at some i n t e r m e d i ate c o n c e n t r a t i o n region in Se---Te m i x t u r e s [11]. * Published in ICTP Preprints, Miramare-Trieste, IC/80/70, 1980. ** Permanent address: Physics Department, Faculty of Science, Ain Shams University, Cairo, Egypt. 1"
Acta Physica Academiae Scien~iarum Hungaricae 52, 1982
4
1~. F. K O T K A T A et al.
Recently, KOTKATA et al [3] have eonstrueted ah equilibrium phase diagram of the S e - T e system from DTA thermograms considering both the endothermic effect on heating and the exothermic effect on eooling. Ah eutectic point has been proved to exist at the composition of TeSe30 and 180 ~ during the supercooling processes. This paper presents results of de conductivity measurements on some Se-Te alloys, including TeSea0, in both liquid and crystallized states. Also, the crystallizing kinetic parameters of supercooled liquid Se--Te alloys are studied in detail. This has been done by eonsidering the dc conductivity at any intermediate point during the liquid--crystalline transition as a struetural eharacteristic q u a n t i t y f o r a material containing two phases, liquid and crystalline [12].
II. Samples, preparation and experimental details Six alloys of the general formula TeSex, with x ~- 50, 30, 20, 15, 10 a~d 5, were prepared from high purity (99.999~) elemental selenium and tellurium. The exact proportions required to prepare 6 g of each composition (see Table I) were enclosed in a vacuum-sealed (~-~10-2 Pa) quartz ampoule (0.012 m in diameter). The tubes were heated at 800 ~ for eight hours in an electric oyen and shaken several times during the course of synthesis to ensure complete mixing. The molten materials were then quenehed in ley water. Tllis method of preparation leads to the formation of the considered compositions in the amorphous state as confirmed by the presence of halo patterns in the X-ray diffraction [13] as well as strong exothermic peaks in the DTA thermograms [3]. I n order to apply electrieal measurements, about 2 g from the bulk amorphous sample (6 g), which had been obtained by the quenching technique, was sealed (~vl0 -2 Pa) in a pyrex ampoule (0.01 m in diameter) provided with two tungsten electrodes. Measurements of dc conductivity were carried out
Table I Summary of de conductivity measurements of liquid and crystalline Se-Te alLoys Liquid samples Sample
TES%0 TeSe30 TeSe20 TeSe~~ TeSel0 TeSes
Te content
(%)
1.96 3.23 4.76 6.25 9.09 16.67
E~r (eV)
In ~o(D -1 cm -1)
1.60 1.44 1.28 1.72 1.90 2.20
Acta Physica Academiae Scientiarum Hungvtricae 52, 1982
0.8 --0.1 --0.3 1.9 2.8 4.1
Samples as.erystallized at 130 oC Eff(eV)
- - I n o'2o(.Q-1 cm -1)
1.23 0.90 1.05
11.0 7.9 8.4
0.73 0.40
6.1 5.4
ELECTRICAL TRANSPORT
5
using ah electrometer of the type VA J-52 (error less than 2 . 5 ~ ) and applying low stable voltage ( < 5 V). Three sets of measurements were considered: i) Study of the function ~ =- f(T) in the liquid state, i.e. at temperatures T higher t h a n TM which had been previously determined [3]. ii) Study of the function a -=--f(t) during the crystal growth in the supercooled liquid phases. This had been done by heating the as-prepared quenched sample for one hour at 300 ~ then rapidly transferring ir into a preheated oyen adjusted to a required temperature in the range of 90-- 180 ~ The latter was kept constant during the disorder--order transition period using ah automatic control system with temperature variation less than 0.5 ~ iii) Study of the function a =-f(T) for samples crystallized at different isothermal temperatures, i.e. after the end of each transitional step in set ii).
III. Results
The results of dc conductivity measurements are given in Figs. 1 to 3. Fig. 1 shows the plot o f l n ~ versus Iq for the investigated liquid alloys, where the relationship is linear in the measured temperature range. Table I gives E~, the activation energy of eonduetion a s a function of composition, calculated from the slopes of the curves In o" versus l/T, and In o"0, the intercept of the ordinate for Iq = 0. Values of E~ vary from 1.28 to 2.20 eV, while those of --In a0 lie in the region of +0.3 to --4.1 ( a i s measured in Ohm -1 cm-i), and norte of the two funetions varŸ in a monotonic way. The respective values for the composition of TeSe30 are 1.44 eV and 0.1. The change of the electrical conductivity ~ during the disorder--order phase transitions of the supercooled liquid Se--Te alloys has been recorded continuously a s a function of time t. In Figs. 2a and 2b, the variations of In /, f e d
"~"
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210-91
Fig. 1. Temperature-dependence of electrical conductivity of TeSex liquid alloys; x = 50 (a), 30 (b), 20 (c), 15 (d), 10 (e) and 5 (f). .4eta Physica Academiae Scientiarum Httngttricae 52, 1982
6
M . F . KOTKATA et al.
with t h e annealing time during transitions carried out at different annealing t e m p e r a t u r e s are given for the compositions of TeSes0 aad TeSel0, respectively. T h e small difference b e t w e e n the initial values of a, the points A in Fig. 2a or Fig. 2b, is related with the m o m e n t at which the measurements s t a r t e d to record. Fig. 2c shows the v a r i a t i o n of In a versus t for different compositions crystallized at 130 ~ I n Fig. 2, ir is obvious to see t h a t two processes combine
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F i g . 2b .Acta Physica Academiae Scientiarum Hungaricae 52, 1982
9
l
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ELECTRICAL TRANSPORT
7
at each annealing temperature. The first causes a gradual decrease of the conductivity ~ due to the normal cooling of the liquid sample from 300 ~ (path AB). This process termiitates w h e n the temperature of the sample drops to the temperature of the oven (point B). The second process could be identified with the structural ordering occurring during the isothermal heat treatment and, as a consequence, the growth of the new phase in the supercooled liquid matrix (path BC), X - r a y analysis has proved that the crystallization process in the supercooled samples does not start before the point B [13]. C j..,- ~ 91 I
c
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.
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of (a) TeSes0, (b) TeSel0 and (c) for different compositions crystallized at 130 ~
6
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Fig. 3. Temperature-dependence of electrical conductivity of TeSex alloys as-crystallized at
130 ~
x = 50 (a), 30 (b), 20 (c), 10 (e), 5 2 eV. T h i s is l i k e l y t o be d u e t o t h e d i f f e r e n c e in v i s c o s i t i e s of t h e s u p e r cooled l i q u i d a n d o f t h e v i t r e o u s s t a t e . S u c h a d e c r e a s e m i g h t m a k e t h e diffus i o n , s e l f - d i f f u s i o n , a n d s w i t c h i n g o f t h e c h e m i c a l b o n d s easier.
Acknowledgement 0he of us (M.F.K.) would like to thaak Professor ABDUS SA.L&M, the Internati ona Centre for Theoretieal Physics, Trieste, for hospitality.
REFERENCES 1. E. GmSON, J. Chem. Phys., 19, 1109, 1951. 2. P. M. H.~lUSENand K. ANDERKO, Construction of Binary Alloys, 2nd Edition, McGrawHill, New York, 1958, pp. 1188--1189. 3. M. F. KOTKATA, E. A. MAHMOUDand M. K. ~EL-MOUSLY, Acta Phys. Hung., 50, 61, 1981. 4. H. GOBRECHT,D. GAWLIKand E. MArlJURI, Phys. Condens. Mater., 13, 156, 1971. 5. M. CUTLE1%Phil. Mag., 33, 559, 1976. 6. G. TOURAND, B. CABANE and M. BREUIL, J. Non-Cryst. Solids, 8--10, 676, 1972. 7. G. TOURAND, J. Phys. (France), 34, 937, 1973. 8. W. HoYER, E. TttOMXS and M. WOBST, Z. ~aturforsch., 30a, 1633, 1975. 9. M. MISAWAand K. SuzuKI, Trans. JIM., 18, 427, 1977. 10. t{. BELLISSENT and G. ToulaAl~V, J. 31on-Cryst. Solids, 36, 1221, 1980. 11. M. YAO, M. MlSONOU, K. TA~tURA, K. ISmDX and J. ENDO, J. Phys., Soc. (Japan) 48, 109, 1980. 12. M. K. EL-MOUSLY, M. 17'. KOTKATAand S. A. SALAI~,ff. Phys. C., 11, 1077, 1978. 13. M. F. KOTKATA, E. A. MAHMOUD and M. K. EL-I~OUSLY, to be published. 14. H. FmTZSCH~, Amorphous and Liquid Semiconductors, Ed. J. Tauc, Plenum Press, London, 1974, Chap. 5. 15. R. S. ALLGAm, J. Vac. Sci. Technol., 8, 113, 1971. 16. P. K. BHAT, K. L. BHATIAand S. C. KATYAL,J. l~on-Cryst. Solids, 27, 399, 1978. 17. D. SH. ABDII~OV,V. R. NAMAZOVand G. M. ALIEV, Inorgan. Mater., 10, 1960, 1974. 18. M. AVl~MI, J. Chem. Phys., 8, 212, 1940. 19. U. B. ]~VANS, Trans. Faraday Soc., 41, 365, 1945. 20. N. HAY, Br. Polym. J., 3, 74, 1971. 21. L. N. SUVOROVAand E. V. SHKOLI~IKOV,Inorgan. Mater., 12, 610, 1976. 22. M. K. EL-MOvsLY and M. M. EL-ZAIDIA, J. 1Non-Cryst. Solids, 27, 265, 1978. 23. M. F. KOTKATAand E. A. MAHMOUD,Mat. Sci. and Engng., 54, 165, 1982.
Acta Physi•a Academiae Scientiarum Hungaricae 52, 1982
