Determination and analysis of optical constants for Ga₂Se₃ films near absorption edge

Abstract

Ga₂Se₃ has been prepared in bulk and thin film forms. The composition of films has been checked using energy dispersive X-ray spectroscopy technique. X-ray diffraction measurements have shown that Ga₂Se₃ films evaporated at room temperature substrates were amorphous. The transmittance (T) of Ga₂Se₃ thin films has been measured over the wavelength range (400–$\text{900}\text{nm}$). The optical constants, the refractive index (n) and the absorption index (k) have been determined from the analysis of the transmittance data. Analysis of the refractive index (n) yields the values of the long wavelength dielectric constant (ε_∞), the average oscillator wavelength (λ_o), average oscillator strength (S_o), average oscillator energy E_o, the refractive index dispersion parameter (E_o/S_o) and the dispersion energy (E_d). Analysis of absorption index (k) yields both direct and indirect allowed transitions with optical energy gaps of 2.65 and $\text{2.056}\text{eV}$, respectively. The effect of annealing at different temperatures on optical constants is also investigated.

Introduction

Studies of M₂^IIIX₃^VI (M=Ga, In, Tl and X=S, Se, Te) compounds are attracting wide attention because of their importance as good photovoltaic materials. Moreover, these compounds have attractive properties for applications in electrothermal devices [1] such as solid solution electrodes. Many papers have dealt with the ordering of these compounds and it has been found that annealing affects the degree of ordering [2], [3], [4]. Ga₂Se₃ is a member of the above family and has become a matter of great interest, because it serves as a good candidate for optoelectronic applications [5], [6]. It has three phases (α, β, and γ). Two of them, named α and γ phases, have a disordered sphalerite-type structure, the first being tetragonal with slight difference between the a and c parameters and the second is cubic. In the third phase, named β phase, a certain ordering of vacancies in the metallic sublattice takes place leading to a superstructure formation. Mikkelsen [7] showed by differential thermal analysis (DTA) that α phase was obtained by isothermal crystal growth of the melt above $\text{1023}\text{K}$. γ Phase was obtained by quenching the melt to room temperature. Below $\text{953}\text{K}$ the β phase is obtained either by fast or slow cooling of the material. Several authors investigated the electrical and optical properties of Ga₂Se₃ in bulk form [5], [8], [9], [10], [11], [12], whilst few authors investigated these properties for Ga₂Se₃ thin films [13], [14].

In this paper, optical properties were studied at room temperature for Ga₂Se₃ films deposited on glass substrates with different thicknesses (360–$\text{631}\text{nm}$) at room temperature using optical transmittance data in the spectral region (400–$\text{900}\text{nm}$). The data of transmittance were analyzed to determine the optical constants (refractive index n, absorption index k and absorption coefficient α). An analysis of the refractive index has been carried out to obtain the long wavelength dielectric constant and other related parameters. Analysis of the absorption coefficient has been carried out to obtain the optical band gap and determine the nature of transitions involved. Films were annealed for $\text{3}\text{h}$ at different temperatures. Our results are compared with those obtained previously.