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Electronic transport in amorphous CdAs2
Journal of Non-Crystalline Solids 35 Q North-HollandPubllshing Company
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36 (1980) 427-432
ELECTRONIC TRANSPORT IN AMORPHOUSCdAs2 P. Nagels, R. Colson and L. Van Gool
Materials Science Department S.C.K./C.E.N. B-2400 Mol Belgium
D.c. conductivity o and thermopower S measurements have been carried out on a-CdAs2 as a function of temperature. The a.c. conductivity o(m) has also been investigated between 5-500 KHz and at temperatures down to 90 K. Below 320 K conduction is due to holes hopping between localized states in the band t a i l . At higher temperatures transport in extended states takes over. However, in order to explain a more rapid decrease of the thermopower than that expected for unipolar transport by holes, an electron contribution has to be taken into account. Below about 180 K the a.c. conductivity obeys the law o(m) ~ ms with s = 1.1. This indicates that variable-range hopping in localized states at the Fermi level is the dominating process between 90-180 K. INTRODUCTION In contrast to other tetrahedrally coordinated amorphous materials, such as Si, Ge and the III-V compounds, glasses based on CdAs2 can be prepared in bulk form by quenching from the melt. Several studies dealing with structural, optical and electrical properties of CdAs2 glasses to which various amounts of Ge are added were reported in the last decade. In particular, the structure of alloys in the ternary CdGexAs2 system has been the subject of detailed investigation, mainly by X-ray diffraction measurements. I t is well known that, in general, the short-range crder in the amorphous state closely resembles that of the crystal, but in the case of CdGexAs2 with high amounts of Ge i t was not clear whether this amorphous material should have the basic atomic arrangement of crystalline CdAs2 or of crystalline CdGeAs2. High resolution RDF's by Popescu et al. [I] have sho~n that stoichiometric a-CaCeAs2 possesses the local structure of the corresponding crystalline compound. Much of the interest in the tetrahedrally coordinated glasses based on CdAs2 arose from the quite unusual behaviour of the thermopower and the Hall coefficlent 12]. The thermopower of CdGe~As2 glasses was found to change in sign from positive at low Ge content to negatlve at high Ge content. On the other hand, the Hall coefficient was negative for all compositions except in the case of x = 1.0 for which i t was positive.
This paper reports measurements of d.c. and a.c. conductivity, the~mopower and Hall mobility carried out on the binary CdAs2 glass. According to Cervinka et al. [3] this material is expected to have the local atomic arrangement of crystalline CdAs2. The electrical properties of this tetrahedrally coordinated glass were f i r s ~ investigated by ChUb et al. [4]. The authors explained their d.c. conductiv i t y ~ n d thermopower results, obtained in the temperature range between 200 and 300 K, by conduction in extended states. In this paper an alternative interpretatioe based on Mott's model of the band structure for amorphous semiconductors w i l l be presented. I t w i l l be shown that the three types of conduction mechanisms predicted by this model, i.e. variable-range hopping, conduction in localized band t a i l ~states and conduction in extended states, have to be taken into accodnt in order to get a consistent picture of the conductivity and thermopower data. 427
&
36 (1980) 427-432
ELECTRONIC TRANSPORT IN AMORPHOUSCdAs2 P. Nagels, R. Colson and L. Van Gool
Materials Science Department S.C.K./C.E.N. B-2400 Mol Belgium
D.c. conductivity o and thermopower S measurements have been carried out on a-CdAs2 as a function of temperature. The a.c. conductivity o(m) has also been investigated between 5-500 KHz and at temperatures down to 90 K. Below 320 K conduction is due to holes hopping between localized states in the band t a i l . At higher temperatures transport in extended states takes over. However, in order to explain a more rapid decrease of the thermopower than that expected for unipolar transport by holes, an electron contribution has to be taken into account. Below about 180 K the a.c. conductivity obeys the law o(m) ~ ms with s = 1.1. This indicates that variable-range hopping in localized states at the Fermi level is the dominating process between 90-180 K. INTRODUCTION In contrast to other tetrahedrally coordinated amorphous materials, such as Si, Ge and the III-V compounds, glasses based on CdAs2 can be prepared in bulk form by quenching from the melt. Several studies dealing with structural, optical and electrical properties of CdAs2 glasses to which various amounts of Ge are added were reported in the last decade. In particular, the structure of alloys in the ternary CdGexAs2 system has been the subject of detailed investigation, mainly by X-ray diffraction measurements. I t is well known that, in general, the short-range crder in the amorphous state closely resembles that of the crystal, but in the case of CdGexAs2 with high amounts of Ge i t was not clear whether this amorphous material should have the basic atomic arrangement of crystalline CdAs2 or of crystalline CdGeAs2. High resolution RDF's by Popescu et al. [I] have sho~n that stoichiometric a-CaCeAs2 possesses the local structure of the corresponding crystalline compound. Much of the interest in the tetrahedrally coordinated glasses based on CdAs2 arose from the quite unusual behaviour of the thermopower and the Hall coefficlent 12]. The thermopower of CdGe~As2 glasses was found to change in sign from positive at low Ge content to negatlve at high Ge content. On the other hand, the Hall coefficient was negative for all compositions except in the case of x = 1.0 for which i t was positive.
This paper reports measurements of d.c. and a.c. conductivity, the~mopower and Hall mobility carried out on the binary CdAs2 glass. According to Cervinka et al. [3] this material is expected to have the local atomic arrangement of crystalline CdAs2. The electrical properties of this tetrahedrally coordinated glass were f i r s ~ investigated by ChUb et al. [4]. The authors explained their d.c. conductiv i t y ~ n d thermopower results, obtained in the temperature range between 200 and 300 K, by conduction in extended states. In this paper an alternative interpretatioe based on Mott's model of the band structure for amorphous semiconductors w i l l be presented. I t w i l l be shown that the three types of conduction mechanisms predicted by this model, i.e. variable-range hopping, conduction in localized band t a i l ~states and conduction in extended states, have to be taken into accodnt in order to get a consistent picture of the conductivity and thermopower data. 427