Ion Beam Mixing and Thermoelectric Characteristics of Bi-Sb Thin Film Alloys

Abstract

<p>This thesis is concerned with investigating the production of thin film Bi-Sb alloys by ion beam mixing. Samples were composed of a thin Sb film sandwiched between two Bi layers; the thicknesses of the films were varied to produce composition range from 0 to ~45% Sb. The overall thickness was about 55 nm.</p> <p>Analysis of the films by Rutherford Backscattering Spectrometry (RBS) and Auger Electron Spectroscopy (AES) indicated that ion beam mixing can produce uniform Bi-Sb alloys with doses of about 2.8X10¹⁵ cmˉ² of 80 KeV Ar⁺ at room temperature. The results of Electron Microscopy showed a grain growth due to ion bombardment.</p> <p>The mixing was characterized by the increase in the full width at half maximum (FWHM) of the Sb signal in the RBS spectra. It was measured as a function of ion dose, dose rate, mass, and energy. The dependence of mixing on the temperature was also investigated. A square-root dependence of mixing on the dose confirmed a diffusion like mixing process. The rate of mixing per ion dose was found to increase with the elastic energy deposited at the Bi/Sb interface. The mixing is temperature independent in the range of ~40 to ~266 K, i.e., collision cascade mixing dominates. An effective diffusion coefficient, D*, of about 4.9X10ˉ²⁸cm⁴/ion was determined. At temperatures higher than ~266 K, the mixing increases with the temperature. This suggests that a radiation enhanced diffusion mechanism is the dominant mixing process. An effective diffusion coefficient of ~1.7X10ˉ²⁷cm⁴/ion at room temperature was calculated: an activation energy of ~0.15 eV was found.</p> <p>Thermoelectric characteristics of the alloys were investigated by measuring the thermal voltage, V₃, as a function of the temperature, T, in the range 300-500 K, for alloys with various composition. The thermal voltage was found to have a linear dependence on temperature. The thermoelectric power, dV₃/dT, reaches a maximum of about 70 μV/deg, for Bi₈₇Sb₁₃. The dependence of the thermoelectric power on Sb concentration agrees with the model put forward to explain the transport properties of bulk Bi-Sb alloys. An empirical model has been proposed to relate the state of mixing with the measured thermoelectric power.</p>