Ritwik Nag

Date of Award

Spring 2022

Document Type

Open Access Thesis

Department

Electrical Engineering

First Advisor

Krishna C. Mandal

Abstract

In this research work Cd0.9Zn0.1Te0.03Se0.97 (CZTS) single crystals were grown using in-house zone refined 7N (99.99999%) purity elements through vertical Bridgman (VBM) method and vertical gradient freeze (VGF) method. The morphological, compositional, and structural characteristics were then performed on the grown CZTS crystals. The x-ray diffraction (XRD) analysis on the grown crystals showed sharp diffraction peaks indicating a highly crystalline nature, with a lattice constant of ~6.45 Å. The elemental and stoichiometric ratio of the grown CZTS crystals were examined by using energy-dispersive x-ray analysis (EDX), which confirmed the formation of the quaternary compound in its desired stoichiometry. Scanning electron microscopy (SEM) on the surface of the processed (etched and polished) crystals displayed a smooth surface with no noticeable defects or cracks. The oxidation states of the precursor elements were analyzed using x-ray photoelectric spectroscopy (XPS), which was found to be relevant to the states found in the precursor compounds such as Cd0.9Zn0.1Te (CZT) and CdTe. Current-voltage characterizations were performed on the fabricated devices with 11 × 11 × 3 mm3 dimension wafer cut out from the grown ingots. The leakage current was low for crystals grown using the vertical Bridgman method (VBM) as well as for the vertical gradient freeze method (VGF). The bulk resistivity was in the range of 109 to 1010 Ω-cm and the log J – log V plots revealed a trap space charge limited current (SCLC) mechanism at moderate bias. Alpha spectroscopy using 241Am (americium radio isotope as a radiation source) was carried out to calculate the charge transport properties in the fabricated devices. The mobility-lifetime product was found to be ~1.5×10-3 cm2/V while the drift mobility was calculated to be ~710 cm2/V-s. The observed electron-transport properties were in good agreement with the reported values for the best CZTS crystals. The single pass Bridgman technique and the vertical gradient technique, used to grow the crystals can help reduce the crystal growth duration and the production cost related to large volume growth of CZTS semiconductor crystal boules, without diminishing any of the charge transport and radiation detection properties found in CZTS crystals.

Rights

© 2022, Ritwik Nag

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