SGUYEN V A N DoNa e t al.: Transport Properties of CdCel -,Sb,Asl 295

phys. stat. sol. (a) S3, 296 (1976)

Subject claseification: 2 and 14.3; 15; 22.8

Labordoire dc Phyaiqvc dca Mdiriauz, Service d’Electroniqw Phyaiquc (a) and Service dc Chimu Physique ( b ) ,

Centre d’Etudea NuclCairea dc SACLA Y , Qi/ aur Yvette

Transport Properties of Glassy CdGe,-Sb=As, BY

NGUYEN VANDONG (a), %AN Hvu DANH (a), R. A v a c i ~ (b), and A. DEVBESNE (b)

A variety of bulk glasses in the system CdGel. =Sb,As, with z ranging from 0 to 0.5 are prepared by waterquenohing of the melt. The glass transition temperature T , and orystallisa- tion temperature T , are determined by differential thermal analysis. It is found that samples containing Sb exhibit two glass transition temperatures one of which depends upon com- position. The electrical conductivity u and thermopower 8 of the glasses are measured as a function of composition and absolute temperature between 273 and 600 K. Thermopower measurements indicate n-type conduction in all samples. An analysis of the experimental results shows that the activation energy E, calculated from data thermopower is consider- ably more sensitive to composition than the activation energy E, obtained from conductivity data. For samples containing Sb. the slight change in the slope of u versus 1/T observed near 330 K suggests that below this temperature the conduction should be due to electrons hopping between the localized states a t the conduction band edge.

Une variktk de verres massifs du systkme CdCel -,Sb,As, avec z variant entre 0 et 0,5 ont dtd prdparbs par trempe rapido des composes fondue dans I’eau froide. La tempbrature de transition vitreuse T , e t la tempkrature de cristalliaation T , ont btb ddterminws par l’analyse thermique diffdrentielle. Nous avons trouvd que lee Bchantillons contenant Sb posskdent deux tempdratures de transition vitreuae, dont I’uno ddpend de la composition. La conductivitb blectrique u et le pouvoir thermodlectrique 8 dea vcrres dtudids ont dti. mesurks en fonotion do la composition et de la temperature abaolue entre 273 et 600 K. Lea mesures du poiivoir thermoblectrique indiquent une conduction de type n pour toua lea 6chantillons. L’analyse des rbsultats expkrimentaux montre que l’dnorgie #activation A!, calculde B partir des donnbe du pouvoir thermoklectrique eat beaucoup plus sensible B la composition que l’dnergie Pa obtenue L partir des donnkea de la conductivite dlectrique. Pour lee bohantillons contenant Sb, le Idger changement de pente des courbes u = / ( l / T ) au voiainage de 330 K sugghre qu’au desaous de cette tempkrature, la conduction serait due B des dlectrons se dkplapant par sauts entre lea ktata localisbs B I’extrkmitd de la bande de conduction.

1. Introduction It is known that the compounds of the system CdX,Aa, with the element X

being Ge, Si, T1, In, Sb, Mg, and Ga can be prepared in the vitreous state by rapid quenching from the melt. The compound on which the physical proper- ties have been mostly investigated is CdGe,As, [ 1,2].

This paper is a preliminary report on o u r study of the transport properties of bulk CdCel-,Sb,As, glasses as a function of temperature with x ranging from x = 0 to 0.5. The purpose of our work is to investigate how the electronic struc- ture and the conduction mechanism in the samples change by replacing pro- gressively Ge by the same amount of Sb.

296 NOUYEN VAN DONO, "RAN Hun DANH, B. AUGUIN. and A. D E B R ~ N I E

6-

T 1 5 -

P & 63

= 1 4

1

2. Sample Preparation and DTA Obeervations CdGel-,$b,As, glasses of different compositions (z = 0, 0.1, 0.3, 0.5) were

prepared from ultra pure elements. The elements were weighed and put in quartz ampoulee of 5 mm diameter which were evacuated and sealed. The ampoules were slowly heated up to 850 "C (about 200 "C above the melting-point of the compound). To assure mixing of the melt, the ampoules were rotated in the furnace for 15 h. Afterwards, the melts were quenched in cold water and the samples were annealed for 12 h at. 200 "C in order to remove internal strains. Small portions were cut from the samples for DTA measurements.

Fig. 1 shows the thermal analysis of different compounds between room tem- perature and 500 "C for a heating rate of about 8"C/min. For CdGeAs,, the DTA trace exhibits a broad exotherm a t about 370 "C and a prominent exo- thermic peak a t 430 "C, which are ascribed to the glass transition and recrystalli- sation, respectively. These values agree roughly with those previously reported for thie material [3]. However, samples containing Sb show two glass transition temperatures: the first T,1 which is sensitive to composition decreases as the Sb content increases, whereas the second T12, a t about 160 "C, is approximately the same for these compounds. The observation of two glass transitions may indi- cate a phase separation in compounds containing Sb. The crystallisation peak depends also on composition and it is shifted towards lower temperatures with increasing Sb content.

1' ture, the conductivity curves exhibit a slight change of slope. The temperature dependence of the thermopower S is shown in Fig. 3. S varies linearly with 1/T within experimen- tal error. The slope of S is more sensitive to vomposition than that of u.

I1 --

Fig. 1. Differential thermal analyeie diagram8 of 0- CdGel -,Sb,An, glaeeea. Heating rate 8 'C/min;

1 3 weight of each sample 100mg. (1) CdGeh, ; (2) CdCeo.oSbo.&,; (3) CdCe0.7Sbo.s~~;

2:

Transport Properties of Glassy CdCel -+Sb,An, 297

Fig. 2

7

Fig. 3 Fig. 2. Temperature dependence of electrical conductivity for differont U G e l -=Yb,A8,

gleesee. 0 CdGeAe,; o CdGeo.oSbo.lAs,; A CdGeo..~Sbo.~As,; c CdCq1.5Sbo.& Fig. 3. Temperature dependence of thermoelectric power for CdCel -,Sb,ke, glasses.

0 CdGeAs,; 0 CdCeo.oSbo.i&; A CdCe0.7Sbo.sb; 0 CdGQ.aSbo.sb

4. Discueeion We try to interpret electrical data obtained on the CdGel -,Sb,As, glasses in

terms of a model for electronic conduction and density of states proposed by Davis and Mott [4].

The dc electrical cofiductivity is written as u = C exp (-E,lkT) (1)

with C = exp ( y / k ) .

Jn these expressions, E, is the activation energy and 7 the temperature coeffi- cient of the band gap. If conduction is due to electrons excited beyond the mobility shoulder E , into

the extended states, E, is equal to the half of the band gap extrapolated to zero temperature

If current is carried by electrons excited into localized states at the band edge E , where they move by hopping, the activation energy is

E , = ( E , - EF)O -

E, = E, . - Ey + AW , where A W is the activation energy for hopping.

The value of C for this conduction mechanism is estimated to be a factor of 1W to 105 smaller compared with the case of conduction in extended states.

For one-carrier conduction, the thermopower is given by

298 Nou Y E N VAN h N O , %AN Hun DANE, B. Anonr~, and A. DEWESNE

If conduction takes place by electrons excited into the extended states near E,, E, is equal to E , provided the mobility p ( E , ) is unactivated.

With the same assumption, the thcrmopower for two-carrier intrinsic conduc- tion can be expreseed as

a = - (3)

where b is the ratio of electron mobility to hole mobility. In the high temperaturerange, the values of the activation energy E, obtained

from conductivity data and those E, determined from thermopower data are shown in Table 1. We have found that E, is considerably more sensitive to com- position than 6,.

For CdGeAs,, the value of E, and E, are comparable to those reported by other workers [ 1,2]. They are approximately equal within experimental error.

For compounds containing Sb, the slopes E, of eS versus 1/T are about 0.2 eV less than the conductivity activation energies E,.

Table 1

compounda I E, (ev) I E , ( ~ v ) I &a - E,(ev) . -

I CdGe Ae, I 0.60 0.66 I 0.05 CdGeo.pSbo.lAe, 0.68 0.40 0.18 CdGeo..rSbo.sAe, ' 0.65 , 0.30 0.25 CdGeo.sSbo.sAs, I 0.50 , 0.25 I 0.25

This fact could be attributed to an activation energy for electron mobility. However, the high values of the conductivity pre-exponential term C, 10' to 2 x lo4 R-l ern-', obtained from the straight lines of u curves extrapolated to 1/T = 0, suggest a conduction by electrons excited in the extended states near the mobility edge.

The difference between the activation energy deducod from conductivity data and that given by thermopower may also result from a two-carrier intrinsic contribution without thermally activated mobility. In this case, it is necessary to suppose that the electron mobility i s larger than the hole mobility. Relation (3) should give a ratio of mobilities ranging from 5 to 3.

Near room temperature, the compounds containing Sb exhibit a slight change in the slope of u versus 1/T. The change of slope is about 0.10 eV and the inter- cept on the u-axis gives a value of C which is a factor of 1W smaller than that found at high temperature. We believe that near room temperature and below, conduction in these compounds is dominated by carriers hopping between loca- lized states a t the conduction band edge. If the hopping energy is 0.2 eV, the range of localized states AEe is estimated to be about 0.3 eV. Measurements of electrical conductivity and thermopower a t low temperature which are under- way should give more information about this conduction mechanism.

Acknowledgement

The authors would like to express their thanks to J. Y. Leny for technical assistance.

Transport Properties of Gleesy CdCe, -#bZAst 299

Relerencee 111 L. CERYINU, R. HOSEMANN, and VOOKL, J. non-crystall. Solids 3, 294 (1970). [2] R. CUAERTS, M. DENAYER, F. H. HASHMI. and P. NAGUS, Disc. Faraday SOC. 60,

27 (1971). [3] N. A. GORYUNOVA, G . S. KUZMENKO, and E. 0. OSMANOV, Mater. Sci. Engng. 7, 64

(1971). F. J. DI SALVO, B. G. BAGLEY, J. TAUC, and J. V. WASZEZAK, Proc. V. Internet. Conf. Amorphous and Liquid Semiconductors Vol. 2, Garmiach-Partenkirchen 1973 (p. 1043).

[4] E. A. DAVIS and N. F. Mom. Phil. Mag. $32, 903 (1970).

(Received November 7, 1975)