Ellipsometry Study of CdSe Thin Films Deposited by PLD on ITO Coated Glass Substrates.

Flavia P N Inbanathan,Jixin Chen,Wojciech M Jadwisienczak,Ram S Katiyar,Pawan Kumar,Kiran Dasari

doi:10.3390/ma14123307

Abstract

Cadmium selenide (CdSe) thin films were deposited on indium tin oxide (ITO) coated glass substrates using pulsed laser deposition (PLD) technique under different growth temperatures. Samples were investigated for their structural, morphological, and optical properties through X-ray diffraction (XRD), atomic force microscopy (AFM), and UV-Vis-NIR spectroscopy. AFM analysis revealed that the surface roughness of the as-grown CdSe thin films increased with the increase in deposition temperature. The optical constants and film thickness were obtained from spectroscopic ellipsometry analysis and are discussed in detail. The optical band gap of the as-grown CdSe thin films, calculated from the Tauc plot analysis, matched with the ellipsometry measurements, with a band gap of ~1.71 eV for a growth temperature range of 150 °C to 400 °C. The CdSe thin films were found to have a refractive index of ~3.0 and extinction coefficient of ~1.0, making it a suitable candidate for photovoltaics.

Highlights

The X-ray diffraction (XRD) spectra showed the presence of monocrystalline hexagonal Cadmium selenide (CdSe), whereas the thin films grain size strongly was affected by the grain boundaries and was found to increase non-monotonically with the rise in deposition temperatures
The CdSe optical band gap of ~1.71 eV, calculated using the Tauc plot method, matches with the spectroscopic ellipsometry results; for CdSe samples deposited at higher temperatures, a red shift of 0.1 eV was observed in the optical absorption edge
We observed that the maximum refractive index and low extinction coefficient for the CdSe samples occurred in the near band edge energy region due to the modification of the excitonic emission peaks at the absorption edge, and strongly affected by the CdSe thin film morphology

Summary

Introduction

The spectroscopic ellipsometry technique is a useful tool for revealing the electronic structure and complex optical properties of semiconductors by measuring the optical responses of a flat substrate to polarized light. It is mainly used to measure film thickness and the optical constants. For novel photoelectronic device design, knowing the optical constants such as the refractive index and dielectric constant is essential for studying the pattern of light being guided through the thin films [1]. Group II-VI compound semiconductors and their alloys, with a band gap energy ranging from 1.20 eV to 3.91 eV, have been explored for their electronic and photonic applications [2,3,4]. To date, different band gap energies for binary and more complex alloys have been achieved via proper band gap engineering approaches.

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