Optical characterization of polycrystalline ZnSe_1−xTe_x thin films using variable angle spectroscopic ellipsometry and spectrophotmetery techniques

Abstract

Polycrystalline ZnSe_1−xTe_x (0.0≤x≤1.0) thin films were deposited by the electron beam deposition technique on corning glass substrate. Two different techniques, Variable Angle Spectroscopic Ellipsometry (VASE) and spectrophotometry, have been applied and compared for characterization of the optical properties of ZnSe_1−xTe_x films. The film thickness and optical constants (refractive index (n) and extinction coefficient (k)) of polycrystalline ZnSe_1−xTe_x films were obtained by fitting the spectroscopic ellipsometric data (ψ, ∆) using a three-layer model system in the wavelength range from 400 to 1100 nm. Nevertheless, the optical band gap $E_g^{opt}$ determined from k values indicating a direct allowed transition. The optical studies of the polycrystalline ZnSe_1−xTe_x films showed that the refractive index increases and the $E_g^{opt}$ decreases. It is worth noting that the obtained values of the optical band gap of the different Te doped films remain in between the reported optical energy gap values of the two ends of the solid solution,ZnSe and ZnTe thin films,. Furthermore, the transmittances spectra of the ZnSe_1−xTe_x films are obtained experimentally from spectrophotometry measurements and theoretically calculated using Murmann's exact equation. Comparing the results yielding a fully agreement between experimental and fitted transmittance data.

Introduction

II–VI wide band gap semiconductors are of great interest as blue and green light emitters [1], [2], [3]. For these, as well as for many other devices, good bipolar conductivity is required. Among many of the II–VI semiconductors compounds is the ternary alloy ZnSe_1−xTe_x which is obtained from the combination of the ZnSe (E_g=2.69 eV) and ZnTe (E_g=2.25 eV) binary compounds. Previously, different spectroscopic techniques like Photoluminescence (PL) and photoconductivity (PC) were used to investigate the different optical transitions and the absorption process associated with these transitions in ZnSe_1−xTe_x compound [4], [5]. Recently, the bulk sample of ZnSe_xTe_1−x has been synthesized using conventional solid state reaction method, in which stoichiometric amount of the binary constituents of ZnSe and ZnTe powders were placed together and heated in a sealed silica tube at 1370 K for 24 h [6]. The optical constants (n, k) and thickness of ZnSe_xTe_1−x thin films have been obtained in litratures from reflectance (T) and transmittance (R) measurements using the spectrophotometery technique [7], [8], [9], [10], [11], [12], [13] and from psi (ψ) and delta (Δ) using the spectroscopic ellipsometry (SE) technique [14], [15], [16], [17], [18], [19]. These methods represent powerful techniques to investigate the optical response of materials. Spectroscopic ellipsometry measurements of isotropic materials are typically performed at oblique angles near the Brewster condition. This provides the highest sensitivity to material properties, such as refractive index and film thickness. The Brewster condition is different for each sample and also changes over the spectral range [16]. Thus, variable angle of incidence allows the measurement to be optimized for each data point. In the present work, we employed spectroscopic ellipsometry technique in order to determine the optical constants and optical band gap of ZnSe_1−xTe_x (0.0≤x≤1.0) thin films. Furthermore, the experimental transmittances spectra obtained from spectrophotometry technique are compared to the theoretically transmittances spectra calculated in terms of Murmann's exact equation using the fitted thickness and optical constants from SE model.