Novel n-type conducting amorphous chalcogenide CdS·In2Sx: an extension of working hypothesis for conducting amorphous oxides

doi:10.1016/S0022-3093(98)00169-0

Journal of Non-Crystalline Solids

Volumes 227–230, Part 2, May 1998, Pages 804-809

https://doi.org/10.1016/S0022-3093(98)00169-0 Get rights and content

Abstract

Novel n-type conducting amorphous Cd–In–S chalcogenide have been prepared by extending a working hypothesis to explore transparent conducting amorphous oxide to chalcogenides. The dc conductivity of the as-deposited samples is ∼10⁻⁴ S cm⁻¹ at 300 K and can be further increased to ∼10⁻² S cm⁻¹ by heat treatment at temperature less than crystallization temperature. The activation energy of dark conductivity was ∼0.3 eV in the as-deposited samples and ∼0.13 eV for the annealed samples. Both activation energies are much less than the half of the Tauc optical band gap (∼2.2 eV). Variable-range hopping was observed in the annealed samples at temperatures below 40 K. The signs of Seebeck and Hall coefficients were negative, indicating that conduction is n-type and no sign anomaly was observed in the Hall voltage. We suggest that the Fermi energy is controllable by doping of carrier electrons via thermal formation of the anion vacancy as has been observed in amorphous ionic oxides such as Cd₂PbO₄.

Introduction

Amorphous chalcogenides have several characteristic transport properties. First, they exhibit band-type electric conduction as evidenced by the activation energy of dark conduction being approximately half of the optical band gap. Second, the electric conduction is p-type except for a few systems such as Bi–Ge–S 1, 2. Finally, amorphous chalcogenides almost invariably exhibit p-type thermopower, but n-type Hall coefficients 3, 4.

Here we report on the preparation of n-type conducting amorphous chalcogenides. These materials have several transport properties which differ from those in conventional amorphous chalcogenides, i.e., they give normal sign Hall voltage sign and variable-range-hopping is observed at low temperatures.

We explored optically transparent and electroconducting amorphous oxides following a working hypothesis which was set up on the basis of simple considerations concerning chemical bonding. This hypothesis [5]predicts that amorphous oxides composed of heavy metal cations with an electronic configuration of (n−1)d¹⁰ ns⁰ (n⩾5) may be converted from insulators into conductors when carriers are successfully doped.

Section snippets

Working hypothesis

Since the mobility is proportional to the width of the conduction bands, a large overlap between relevant basis orbitals is required for high electron mobility. In addition, the magnitude of the overlap must be insensitive to the structural randomness which is intrinsic to the amorphous state. Therefore, we thought that ionic metal oxides composed of heavy metal cations (HMCs) with an electronic configuration (n−1)d¹⁰ ns⁰ (n⩾5) could satisfy these requirements. The bottom part of the conduction

Experimental

Amorphous thin films of Cd–In–S were prepared on Pyrex or silica glass plates by vacuum evaporation. Polycrystalline CdIn₂S₄ powders were synthesized by heating a mixture of 4 N (purity) CdS, In and S in an evacuated silica ampoule in a rotary furnace at 1050°C for 24 h. These polycrystalline powders were then used as an evaporation source for the preparation of amorphous thin films. The films were formed in a vacuum of ∼7×10⁻⁴ Pa and the deposition rate was 5 to 10 nm s⁻¹. The film thickness

Results

Fig. 2 shows the DTA trace, glancing angle XRD and ED patterns of the as-deposited specimen. An exothermic peak with a shoulder is seen at ∼300°C. It was evident from the glancing angle XRD patterns on the as-deposited specimen and the specimen after annealing at 240°C, which is lower than the onset (∼280°C) of the exothermic peak, that the exothermic peak originates from crystallization and the specimens remain in an amorphous state at temperatures below ∼280°C. The latter result is further

Discussion

Table 1 summarizes electrical properties of the annealed amorphous thin films along with the Tauc gap. The conduction is n-type and the activation energy at temperature >240 K is ∼0.13 eV, which is approximately 9 times smaller than half of the Tauc gap (∼2.2 eV). We therefore assume that electron donor levels are located at an energy level of ∼0.13 eV. Since the activation energy of dc conduction is ∼0.3 eV in the as-deposited specimen, the location of donor levels approaches the mobility gap

Summary

(1) Novel n-type conducting amorphous chalcogenides were found in the system Cd–In–S. DC conductivity in the as-evaporated thin films was ∼10⁻⁴ S cm⁻¹ and the activation energy of conduction was ∼0.3 eV, which is much smaller than the half of the Tauc gap (∼2.2 eV).

(2) Upon annealing in vacuum at 200°C, a temperature below the crystallization temperature (∼300°C), the activation energy of dc conduction was reduced to ∼0.13 eV and the conductivity at 300 K was increased to ∼10⁻² S cm⁻¹.

Acknowledgements

The authors thank Professor K. Shimizu of Keio University for a help of TEM observation. This work was supported in part by a grant-in-aid for scientific researches from the Japanese Ministry of Education, Science, Sports and Culture, and by a grant from the Nissan Science Foundation.

References (12)

H. Hosono et al.
J. Non-Cryst. Sol.
(1996)
H. Hosono et al.
J. Non-Cryst. Sol.
(1996)
J.H.W. de Wit et al.
J. Phys. Chem. Solids
(1977)
S. Endo et al.
J. Phys. Chem. Solids
(1976)
N. Tohge et al.
Appl. Phys. Lett.
(1979)
I. Watanabe et al.
Jpn. J. Appl. Phys.
(1985)

There are more references available in the full text version of this article.

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Section 9. Chalcogenides: transport and electronic propertiesNovel n-type conducting amorphous chalcogenide CdS·In2Sx: an extension of working hypothesis for conducting amorphous oxides

Abstract

Introduction

Section snippets

Working hypothesis

Experimental

Results

Discussion

Summary

Acknowledgements

J. Non-Cryst. Sol.

J. Non-Cryst. Sol.

J. Phys. Chem. Solids

J. Phys. Chem. Solids

Appl. Phys. Lett.

Jpn. J. Appl. Phys.

Section 9. Chalcogenides: transport and electronic properties
Novel n-type conducting amorphous chalcogenide CdS·In₂S_x: an extension of working hypothesis for conducting amorphous oxides