Characterization of molecular beam epitaxially crown CdTe layers over GaAs by spectroscopic ellipsometry

Abstract

The infrared detectors consist of two main parts that are optical elements and sensing elements. The sensor component is generally formed by semiconductor materials that can detect Infrared (IR) light which cannot be seen by human eye. Mercury Cadmium Telluride (MCT, HgCdTe) is widely used as a sensor material for this purpose. The adjustable bandgap (0-1.5 eV) which corresponds to energies of IR light can be obtained by changing the composition x of cadmium (Cd) in the ternary alloy Hg1-xCdxTe. HgCdTe has very high quantum efficiency for the detectible IR wavelengths in the atmospheric windows. HgCdTe which has a great importance in defense industry as an IR detecting material should be grown with high crystallinity in order to obtain high resolution images even under bad weather conditions. In addition, HgCdTe must be grown uniformly over a large area in order to have large format and high operability focal plane arrays. The defect density of HgCdTe strongly depends on the lattice mismatch between substrate and HgCdTe. In order to reduce the lattice mismatch which causes dislocations in HgCdTe the best suitable option is to grow Cadmium Telluride (CdTe) buffer layer on a substrate before growing HgCdTe. Studies have been focusing on semiconductors which are Gallium Arsenide (GaAs), Silicon (Si) and Germanium (Ge) as alternative substrates for CdTe growth. In this study, the CdTe films grown on (211) oriented GaAs wafers by molecular beam epitaxy (MBE) were characterized by ex-situ spectroscopic ellipsometry (SE). The properties of CdTe films such as thickness, surface roughness and optical constants were characterized by comparison with the growth conditions. It was also investigated that how these properties vary over the film surface. Characterization results were compared to those obtained by atomic force microscopy (AFM), Nomarski microscopy, Fourier transformation infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The temperature dependencies of the optical properties of the material obtained by SE were also investigated.