Nanoscale surface finishing studies and characterizations of cadmium zinc telluride crystals

Kabukcuoğlu, Merve Pınar
Cadmium Zinc Telluride (Cd1-xZnxTe, CdZnTe) crystals are used in two different applications depending on Zinc (Zn) concentrations. CdZnTe crystals are one of the most promising materials for X-ray and gamma-ray detector applications due to unique material properties such as high atomic number and high resistivity. Wide band gap and high stopping potential of CdZnTe crystals allow operation at room temperature for high performance detectors with several applications including medical imaging, astronomy, and homeland security. CdZnTe crystals are also known to be substrate material for epitaxial growth of Mercury Cadmium Telluride (HgCdTe, MCT) high performance infrared detectors by Molecular Beam Epitaxy (MBE) since lattice constants of HgCdTe and Cd0.96Zn0.04Te are perfectly matched. Producing high quality epi-ready substrates with low defect density on large size wafers (i.e. 2 cm x 2 cm) is highly critical for the MCT based infrared detectors. High quality MBE requires uniform zinc concentration and low-defect density. This thesis focuses on surface treatments and characterization of finished Cd1-xZnxTe single crystals with surface quality factors such as surface roughness and orientation accuracy for {111} and {211} orientations. These two application desire ultra-flat wafers vi with desired thickness variation. The main aim is to have sub-nanometer surface roughness and ultra-flat surfaces on the large wafer areas. Within the scope of surface finishing studies, I focused on the optimization of lapping and polishing processes with respect to material removal rate (MRR) and its relation to surface roughness. Nano-mechanical behaviors of various crystallographic orientations and different zinc concentrations were to be understood for machining damage and MRR. Surface and subsurface damages prevent growth of high quality MCT epi-layers on CZT substrates. Microcracks, plastic deformations, and dislocations induced by machining processes are the main issues required to be investigated. The surface topography and morphology after each process was observed by surface profilometer, atomic force microscopy (AFM), and scanning electron microscopy (SEM) techniques. Surface and subsurface damage were observed by SEM to understand induced defects after each surface process. Finished CdZnTe single crystals have been characterized in terms of structural, compositional, and optical. X-ray Diffraction (XRD), Electron Dispersive X-ray (EDX) Spectroscopy, and Fourier Transform Infrared (FTIR) methods were employed after each surface treatment to understand the effect of surface preparations. Moreover, Te-inclusions that appears during mechanical polishing analyzed after polishing and chemical etching with Nakagawa and Everson etchants. (211) and (111) oriented CdZnTe crystals having sizes up to 20 x 30 mm2 were sliced with high orientation accuracy based on XRD measurements. Sub-nanometer surface roughness rms down to 0.4 nm was achieved on larger surface area. Zinc distribution throughout finished samples were uniform to be around 10% for Cd0.90Zn0.10Te and 4% for Cd0.96Zn0.04Te crystals. CdZnTe crystals are stoichiometric after final chemical polishing. IR transmittance of mirror-like polished samples were near 66%, which is the theoretical limit.