Investigation of charge transport processes in amorphous GaSe films

Abstract

Results concerning the static current-voltage characteristic and the electrical conduction of a-GaSe films in a wide range of electrical fields (10−4 × 10 5 V cm -1) and temperatures (97–373 K) are presented in this paper. It was shown that at low and medium electric fields and for temperatures T < 300 K the main contribution to the conduction is that due to the hopping of charge carriers in localized states within the forbidden gap. Values for the localization radius (a ≈ 10 Å) and the density of localized states near the Fermi level ( N F = 5 × 10 19−2.5 × 10 21 eV -1 cm -3) are calculated on the basis of the hopping mechanism for conduction. The existence of a space-charge-limited conduction regime at medium and high temperatures with F < 10 5 V cm -1 was confirmed. In strong electric fields ( F > 10 5 V cm -1) the conduction in GaSe films is due to thermal field ionization as described by Frankel. The high frequency permittivity (ε ≈ 11), the shape of the potential well of the ionized centres and the mean free path of the charge carriers ( λ = 3.5 × 10 -7 cm) are determined on the basis of Frankel's theory and using the experimental data on the electrical conductivity. Relaxation currents were found in the a-GaSe films and these were investigated. Experimental results for the relaxation currents correspond well to the relay mechanism of charge transport in deep trap levels due to carrier injection from the contacts.