Spectroscopic and electrical analysis of vacuum co-evaporated Cd_xZn_1-xTe thin films

Highlights

• Demonstrates successful growth of Cd_xZn_1-xTe films with desired stoichiometry by.

• Vigard's plot established the relation between optical band gap and composition.

• p-type to n-type switching observed for films with x = 0.4 and above.

• Optimized composition of x = 0.8 suits for solar absorber layer application.

Abstract

Cd_xZn_1-xTe (CZT) films with different compositions were grown by thermal co-evaporation method, in which cadmium telluride (CdTe) and zinc telluride (ZnTe) were used as the source materials. The structural, morphological and optical properties of the films were characterized by x-ray diffraction (XRD), UV–Visible spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), photoluminescence (PL) and Raman spectroscopy. Detailed analysis has shown that films were having cubic phase with orientation along (111) plane and the optical band gap reduced from 2.17 to 1.53 eV when x was varied from 0 to 1. SEM micrographs confirmed the homogeneity of the films. Depending on the composition, Raman peaks of CZT samples were mixture of the fundamental modes of the CdTe and ZnTe and indicate “trigonal” lattice of Tellurium (Te). Analysis of PL spectra suggested tellurium as isoelectronic exciton traps, recombination of carriers in the surface traps and donor-acceptor pair present in the samples. Under electrical properties, p to n-type conversion was witnessed for x = 0.4 and above. Optical bad gap of about 1.60 eV with minimum Urbach energy (28.88 meV) and favourable electrical properties has led to conclude that CZT films with x = 0.8 may be suitable for solar cell absorber.

Introduction

Polycrystalline Cd_xZn_1-xTe (CZT) is a versatile ternary chalcogenide, which is well suited for optoelectronic device applications. Apart from tunability of the optical band gap, CZT preserves some of the promising properties like high absorption coefficient [1], high binding energy [2], high average atomic number [3], good electron-transport properties and higher quantum efficiency compared to CdTe [4]. Due to these properties, CZT ternary chalcogenide thin films are widely used in opto-electronic devices such as solar cells [5], photo-detectors [6], gamma and X-ray detectors [7] etc. In most of the CdTe based solar cells, p-type CdTe is used as absorbing layer with different back contact methods, which suffer from long-term instabilities due to the in-diffusion of contaminants or impurities. To overcome this drawback, as an alternative, p-type window layer with n-type CdTe absorber can be used. Reports on n-type CdTe absorber layer used in graphitic carbon Schottky - type heterojunction [8], reduced grapheme oxide (rGO) solar cells [9] and diamond/CdTe inverted heterojunction solar cells [10] are available in literature. CZT films with n-type conductivity can improve the performance of these type of solar cells due to their favourable optoelectronic properties mentioned in the beginning.

Several physical and chemical methods were reported on the preparation of CZT films such as molecular beam epitaxy [11], pulsed laser deposition [12], sputtering [13], thermal vacuum evaporation [14], chemical bath deposition [15], closed space sublimation method [16] etc. Substrate temperature [17], thickness of the films [18], types of substrates [19] and post deposition heat treatment [11] are the important factors that influence the structural and optical properties of CZT thin films. Current research in renewable energy is focussed on low cost, excellent stability, high efficiency in solar cells. Solar cell with high efficiency can be achieved by using wide band gap window and tandem solar cell in which structure consists of top cell and bottom cell which are connected in series. Applications like radiation detectors and solar cell require high quality, cost effective CZT films as absorber layer with large grains, low defect concentration, high lattice ordering and uniform distribution of the elements.

In the present work, thermal co-evaporation technique is adopted for synthesis of the CZT films with CdTe and ZnTe as the source materials. Because of advantages low cost, like high deposition rate, uniform deposition, less material consumption and most productive over other methods, thermal co-evaporation method is preferred over other techniques. The objective of this work is to study the structural, morphological, optical and electrical properties; analyse defects states present in the compositionally varied CZT films.