Electrical and thermal transport properties of binary chalcogenide indium polytelluride crystals

Transcription

1 Life Science Journal 01;9(4) Electrical and thermal transort roerties of binary chalcogenide indium olytelluride crystals Nagat A.T. 1, S.A.Al-gahtani 1, F.S. Shokr 1, S.E. AlGarni 1,S.R. Al-Harbi 1 and. A.Quhim 1- Physics Deartment, sciences of Faculty for Girls, ing AbdulAziz University, Jeddah, SA -Physics Deartment college of Education, for girls in Namas, ing halid University sr_alharbi@yahoo.com Abstract: In the resent work, single crystals of In Te 5 were grown by the modified Bridge man technique. An investigation has been carried out on the influence of temerature on the electrical conductivity, Hall effect and thermoelectric ower. The energy ga calculated to be 0.88 ev, the ionization energy of accetor was determined to be 0.14 ev. The conductivity throughout the entire temerature range was found to be of P-tye. The electrical conductivity, Hall coefficient, carrier concentration ration at room temerature were estimated to be 1.47 x10 - ( -1 cm -1 ), 4.6 x10 4 cm /C and 1. x10 14 cm -5 resectively, the electron and hole mobility are found to be 8.5 x10 cm /v.sec and 6.78x10 cm /v.sec resectively. The effective masses of charge carriers are 1.59 x10-9 kg and.4 x10-8 g for electrons and holes resectively. The diffusion coefficient for both majority and minority carriers was estimated to be cm /sec and 1. cm /sec resectively. The diffusion length as well as the relaxation times of holes and electrons are found to be L= 4.9x10-7 cm, L n = 1.68x10-7 cm, = 1.6 x sec and n = 8.4 x10-17 sec resectively. In addition to these ronounced arameters, the efficiency of the thermoelectric element (figure of merit) was checked, which leads to better alication in many fields keywords; crystal growth, In Te 5, electrical conductivity, Hall effect, thermoelectric ower. [Nagat A.T., S.A.Al-gahtani, F.S..Shokr, S.E. AlGarni,S.R. Al-Harbi and. A.Quhim. Electrical and thermal transort roerties of binary chalcogenide indium olytelluride crystals. Life Sci J 01;9(4): ] (ISSN: ).. 7 ey words: In Te 5, Hall effect, TEP, electrical conductivity 1. Introduction Research on binary semiconducting comound formed by the elements from grous (III) and (VI) of the eriodic table as a collective grou of materials have been and are still the subject of much intensive investigation. In the last few years widesread attention has been aid to the semiconductors of the (A III B VI ) grou. This interest has been driven by their ossible device alication. (1) Great attention has been aid () to Ga, In and Te chalcogenides. In articular the III VI III VI A B A B5 A III B VI study of the, and 4 comounds is quite attractive during the last few decades (). Indium tellurides are useful as materials for electronics and for otical recording. The hase diagram of the In Te system studied by many authors (4-7), and according to the last study it is obvious that four comounds in the In-Te system, InTe, In Te, IaTe 5 and In 4 T 5 are stable. In Te 5 is a member of this family, has interesting roerties. Among A III B VI comounds not much attention has been aid to investigations of indium telluride, there are some works devoted to InTe and In Te, (8-19), but only very few works devoted to structure and some hysical roerties of In Te 5 (0-5). The search for new semiconducting materials has led us to the resent investigation on In Te 5, As far as we know from the ublished literature u to now, there is still insufficient data to throw a clear light uon the actual behavior of this comound, also some of the 1495 results given in the literatures show some discreancies. In view of this, the resent work aims to reare In Te 5 in single crystal form and investigate the main transort roerties of this comound. This study is a timely one in view of the recent interest in this comound. - Exerimental.1. Prearation of samle In Te 5 samles were reared using a modified Bridgman technique for growing crystal from melt. Indium olytelluride monocrystals was reared from high urity indium (6N) reresenting 6.467% and tellurium (5N) reresenting 7.58%, stoichiometeric of the elements was used as starting material in the growth exeriments. At the beginning of the growth run, the amoule with its charge was held in the hot zone of the furnace at 78k for 10 h for melt homogenization the charge was shaken during heating several times to accelerate the diffusion of contaminates through each other. Then the amoule was moved into the middle zone of the furnace with a temerature of 740k corresonding to the crystallization temerature according to the hase diagram (4), Afterwards, the amoule was cooled down slowly in the third zone of the furnace, then the furnace was switched. Details of the exerimental equiment for crystal growth and rearation rocedures are

2 Life Science Journal 01;9(4) described elsewhere (7). The resulting ingots had a late-like habit with metallic bright color. The roducing ingot showed good agreement with the obtained data reorted early (1). The single crystallinity of the comound was checked using X-ray diffraction technique. From the X-ray studies it was evident that the crystal has a high degree of crystallinity with the required hase without any secondary hase... Exerimental arrangement Measurements of the electrical conductivity and Hall effect were done with the hel of a Pyrex glass cryostat, which was designed (8) for this urose. The cryostat is used as a holder evacuated container for liquid nitrogen (for low-temerature measurement) and suort to the electric heater (for high-temerature measurements. Coer-constantan thermocoule was used for measuring the temerature of the samle. Silver aste was used for the ohmic contact. Tyical dimension for rectangular samle were ( ) mm. The Hall measurements were made in a magnetic field of 0.5 tesla and were erformed using the conventional DC otentiometer method. For thermoelectric ower (TEP) measurements, an evacuated working chamber was used to rotect the samle from oxidation and water vaor condensation at high and low temerature resectively. The outer heater discharge its heat slowly to the secimen environment. The inner heater was attached to lower end of the crystal in order to control the temerature and its gradient along the secimen. More details about the aaratus and technique of measuring have been ublished. (9,0) - Results and discussion:.1.temerature deendence of electrical conductivity and Hall effect for In Te 5 Fig.1 shows the variation of electrical conductivity versus inverse temerature for In Te 5 single crystal. The electrical conductivity and Hall coefficient measurements were erformed in the temerature rang extend from 198 u to 558. The comlete temerature range can be subdivided into three regions, below the transition, the transition region and above the transition. These regions are quite clearly shown in fig.1. With increase of temerature the electrical conductivity at first increases gradually, then it reaches the transition region at 7k, then the ln vs 1/ T curve assed through an intermediate region (7-47k) in which the carrier concentration is not actually constant, and in the third region, rises raidly. This attern of changes in the electrical conductivity is due to the aeance of imurity and intrinsic conductivity, resectively, and to the variation of the carrier mobility and concentration with temerature. In the intermediate region where the carrier density (N A -N D ) = constant, until the intrinsic region is reached. The decrease of the value of in this region may be due to the increase of intensity of lattice vibration which leads to decrease in the carrier mobility. This discussion is accetable, since the conductivity decrease in this region and this extended to full ionization of imurity at the end of imurity at the end of exhaustion region. At temeratures above the transition oint the conductivity increases raidly (47-558). The temerature deendence exhibits a transition from a region of lower sloe to one of higher sloe. The sloes of the curve increase with increasing temerature, and are higher at higher temerature because of the carriers being excited from the extended state of the valance band into the conduction band. The width of the forbidden ga as calculated from the sloe of the curve in the high- temerature region is found to be E g =0.88 ev. Also the ionization energy as deduced in the imurity region was evaluated to be E a = 0.14 ev. The room temerature conductivity of In Te 5 single crystal equal to 1.47x10 ( cm) -1. The Hall coefficient (R H ) variation with temerature and a ositive sign of (R H ) indicated the major contribution to the conductivity by holes. Fig.. Shows the temerature deendence of (R H T / ), it is the usual tye for semiconductors. Assessment of the forbidden band width from this grah and found have a value close to that determined from electrical conductivity and that reorted early. (6). It was found also the deth of the accetor centre is 0.14 ev. The variation of the Hall mobility with temerature is shown in fig. It was found that the exonent(n) in the relation ~ n below 7k is equal to 1.11, while in the high temerature range (T>7) the mobility decreases according to the low ~ -1.9 from this relation, it seems that the value of n close to imurity and lattice scattering mechanism, in the low and high temerature resectively. The room temerature mobility was found to be xl0 cm /V.sec. The variation of carrier concentration with temerature is shown in fig 4. As the, In Te 5 samle exhibiting intrinsic behavior above 47 the exected value for the intrinsic concentration will be given as / /4 / Eg m m T ex. k ni n h kt One can see that the carrier concentration varies sharly with increasing temerature. The room temerature concentration is 1.x10 14 cm -.. Temerature deendence of TEP for In Te 5 single crystal. 1496

3 Life Science Journal 01;9(4) The thermoelectric ower (TEP) measurements were erformed in a wide, temerature range (15-450k). Fig5 illustrates the general mode of variation of TEP with temerature. This was done by lotting the relation between and ln T in the low temerature range. Fig (5) shows a straight line relation in this region of temerature. In the imurity region the following formula could be alied (1). ln Ph / e ( m T ) Thus the effective mass of holes is evaluated to be.4x10-8 kg. Some features of these results may be ointed out (1) Our samle shows P-tye conductivity within the temerature range of investigation, which is in quantitative agreement our revious data of the Hall coefficient and another ublished data (6). () The room temerature thermoelectric ower value for In Te 5 mounted to be 7.47 µv/. () The figure shows that the value of decreases as the temerature rises. This may be due to the resence of some crystal defects or traing centers in the direction of the carrier flow. (4) With further rise of temerature decreases i.e in the whole extrinsic range of temerature decreases with T. As follows from the exression for TEP of a semiconductor in the intrinsic region (). k b1 Eg 1 ln 1 mn Where e b T m k is the Boltzmann constant, b is the ratio of the electron to hole motilities, E g is the ga width and m n, m are the effective masses of electrons and holes resectively. Fig 6 shows the relation between the thermoelectric ower and the inverse of temerature. This relation show that a lot of in the intrinsic range, as a function of recirocal of absolute temerature is a straight line. The measured thermoelectric ower in conjunction with the reviously obtained Hall effect data are used to calculate electron to hole mobility ratio and also the ratio of effective masses of both electrons and holes. The sloes of the curve are used to estimate the ratio of the electron and hole mobilities. Taking E g =0.88 ev, the ratio b= / is found to be 1.45, n / since =6.86x10 cm /v.sec, then we can evaluate n =8.5x10 cm /v.sec. Another imortant arameter can be deduced with the aid of the obtained values of n and using Einstein relation, that is the diffusion coefficient for both majority and minority carriers at room temerature can be evaluated to be and 1. cm /sec resectively. The ratio between the effective masses of both electrons and holes can be estimated from the intersection of the curve. We evaluate this ratio as ( m n / m 0.065) Combining the value of affective mass of holes with that obtained for the ratio m n / m, one obtains an effective mass of minority carriers of the -9 value m n = 1.57x10 kg. By using the effective mass values of electrons and holes, the relaxation time for both current carriers can be determined. Its value for holes comes to be 1.08 x10-15 sec, while for electrons it is equal to 8.4x10 17 sec. Using the values of diffusion coefficient and relaxation time, the diffusion length for both charge carriers can be determined. The values of the diffusion length for electrons and holes are found to be L n =1.6x10-7 cm and L = 4.9x10-7 cm, resectively. Our results are in good agreement with each other, since the mobility 0f holes is smaller comared with that of electrons,and its effective mass is larger than that of electrons. Its relaxation time will be larger than that of electrons. Fig.7 reresent the deendence of on carrier concentration for a given In samle, as we have seen increases sharly and Te5 linearly with the decrease of carrier density. ln ( ohm.cm ) / T ( ) -1 Fig.1:The temerature deendence of electrical conductivity for In Te 5 single crystal. 1497

4 Life Science Journal 01;9(4) ln R H T / ( cm. / / C ) V/ / T / T º ( ) - 1 Fig.:The deendence of (R H T / ) against the temerature for In Te 5 single crystal. Fig 6: Plot of against 10 /T for In Te 5 crystal V/ ln (cm /V.s ) ln P ( c m ) - Fig.7 Reresent the deendence of TEP on carrier density Fig: behavior of the Hall mobility with temerature. ln P(cm - ) / T º ( ) - 1 Fig4: Variation of carrier concentration with temerature From this behavior we realize the effect of the charge carriers is a strong factor governing the variation of.the same behavior was observed when we lotted vs Ln for In Te5 samle as in Fig.8 This figure shows the deendence of thermoelectric ower coefficient on the natural logarithm of electrical conductivity according to the ublished formula () lin m T A e h em lin e 4 V/ V/ ln (.c m ) ln T Fig5: Relation between TEP for In Te 5 and ln T. Fig. 8 The deendence of TEP on the natural logarithm of for In Te 5 It is seen from the curve that the thermoelectric ower decrease raidly as the electrical conductivity increased. From figures 7 and 8, we can deduced that 1498

5 Life Science Journal 01;9(4) the variation of with the environmental temerature is not a mobility effect, but is deendent on the variation of concentration. The choice of material for thermoelectric generators and refrigerators is based on the efficiency arameter Z defined by the relation; Z where is the thermal conductivity of semiconductor and is the electrical conductivity. value of In Te was determined reviously. This 5 arameter was deduced and its value was found to be The roosed treatment of the exerimental data sheds newlight on the main hysical arameters in In Te single crystal.however those ronounced 5 arameters are found to be sufficient to give comlete information about the hysical behavior of our best comound. This gives the chance of ractical alication in different fields. 4-Conclusion Measurement of electrical conductivity, Hall coefficient and TEP of as grown In Te 5 single crystals are erformed over wide range of temerature. The conductivity was found to be P-tye. The energy ga was calculated to be 0.88 ev, while the deth of the accetor level is 0.14 ev. The exerimental data gives us the chance to determine the following ronounced arameters, carrier mobility, effective masses, diffusive coefficient, diffusive length as well as the relaxation time of both tyes of charge carriers.also the efficiency of the material as thermo-element was checked. This mode of investigation (crystal growth, electrical conductivity, Hall effect and thermoelectric roerties study) is an ideal was for finding out the ossibility of making alication for this semiconductor comound esecially in the field of energy conversion, semiconductor devices and electronic engineering. References 1. Aydogan S.,T. aracah and, J. crysr. growth 79 (005):110.. Hussein S. A. and.t. Nagat, hys. stat. sol. (a)114(1989) 05.. Emziane- M.,.J. C. Bernede, J. 0uerfelli, H. Essaidi and A. Barreau,, Material chem.and hys.61(1999) Zlomanov V. P., M. S. Sheiman, V. N. Demin and B. Legendre J. Phase equilibria (001)1. (4) 5. Okamoto H. and C.E.T. White, Series Binary alloy hase digrams 9(1991). 6. Oh C.S. and D. N. Lee calhad 17(199) Eutelus Y. F and B. Legendre, Thermochim acta 14(1998)7. 8. Bose D.N., S. De urkayastha, Mat. Res. Bull. 16(1981) Belal A. E., S.A. Hussein, H. Madkour and H.A. EL Shaikh, Indian J. Pure and Ale. hys 1(199) Balevicius S., A. Cesnys and A. Daksnys,hys Stat. Sol.(a)5(1976) Nassary M.M.,S.A. Hassein, A.E. Belal and H.A.El- Shaikh, Phys.Stat,Sol.(a)145(1994) Zaslavskun A.I.,N.F.arteko,Z.A. arachentsevac Soviet hy.solid.stat 1(197) Jung A.L.,H. Situ,Y.W.,Lu.Z.T.Wang, Z.O.He.J.Non.Sol. 114(1989) Finkman E.,.J.Tauc,hysical Review Letters 14(1975) Finkman, E.,J.Tauc.R.ershow and. A.Wold. hysical Review (B)10(1975) oshkin V.M.,,L.B.Galchinetsku,V.N.ulik,B.I Minkov and U.A.Ulmanis,Sol.Stat.Comm.B (197) Bleris G.L.,T.arakostas, J.Stoemenos and N.A. Economou, hys.stat.sol.(a)4(1976) S.Sen,and D.N.Bose,Sol.Stat.Comm.50 (1984) N.A.Hegab,A.E.Bekheet and A.E.EL-Zhazly Al. hys.(a)66 (1998) G.A.Belyshevo,E.V.Anokhina,X.Yue,V.N.Demi and V. P. Zlomanov, Inorganic materials (1997)5. 1. H.J.Deiseroth,D.Chem,P.Amann and H.Than,.Zeitschrift für anorg- aninche und allgemeine chemie 6(1996) O.Modelung,U.Rossler and M.Schulz, hys.cond. Matt.41 (1998)1.. H.Okamoto,Binary alloy hase diagrams the material information society (1996). 4. D.Forkel, W.Engel, M.Iwatschenko,R.eitel and W. Witthuhn 16 (198) E.G.Lavuk,N.V.Chelovskaya,G.A.Q.Paukar and V.N. Demin and V.P.Zlomanov J.Chem.Thermody.9 (1997) M.M.Nassary,M.Dongal,M..Gerges and M.A.Sebug hys. Stat.Sol.(a)199(00) S.A.Hussein and A.T.Nagat,Cryst.Res.Tech.4(1989)8. 8. S.A.Hussein,Cryst.Res.Tech. 4(1998) S.A.Hussein and A.T.Nagat, Y.H.Gameel and A.A Belal, Egyt. J.Sol.10(1988) A.T.Nagat, S.A.Hussein and Y.H.Gameel and A.A Belal, Egyt. J.Sol.11(1988) A.H.Wilson,the theory of metals ed. Cambridge; Cambridge University ress L.Johnson and.horovitz,phys.rev., (195)6.. P.H.E Schmid and E.Mooser, Phys.acta. 45(197) N.h.Abrikosov, V.F.Bankina, I.V.Poretsk, E.V. Skundnova and S.N. Chizhevskaya (1975) Semiconducting chalcogenides and alloys based on them (in Russian) Nauka. nd 9//