Nuclear Engineering ETDs

Erin Vaughan

Publication Date

7-1-2016

Abstract

Theoretical predictions for AlSb material properties have not been realized using bulk growth methods. This research was motivated by advances in molecular beam epitaxial (MBE) growth technology to produce high-quality thin-film AlSb for the purpose of evaluating transport properties and suitability for radiation detection. Simulations using MCNP5 were performed to benchmark an existing silicon surface barrier detector and to predict ideal AlSb detector behavior, with the finding that AlSb should have improved detection efficiency due to the larger atomic number of Sb compared with Si. GaSb diodes were fabricated by both homoepitaxial MBE and ion implantation methods in order to determine the effect on the radiation detection performance. It was found that the radiation response for the MBE grown GaSb diodes was very uniform, whereas the ion-implanted GaSb diodes exhibited highly variable spectral behavior. Two sets of AlSb heterostructures were fabricated by MBE methods; one for a Hall doping study and the other for a radiation response study. The samples were characterized for material quality using transmission electron microscopy (TEM), Nomarski imaging, atomic force microscopy (AFM), x-ray diffraction (XRD), I-V curve analysis, and Hall effect measurements. The Hall study samples were grown on semi-insulating (SI) GaAs substrates and contained a thin GaAs layer on top to protect the AlSb from oxygen. Doping for the AlSb layer was achieved using GaTe and Be for n- and p-type conductivity, respectively, with intended doping densities ranging from 1015 to 1017 cm-3. Results for net carrier concentration ranged 2×109 to 1×1017 cm-3, 60 to 3000 cm2/Vs for mobility, and 2 to 106 Ω-cm for resistivity, with the undoped AlSb samples presenting the best values. The radiation detector samples were designed to be PIN diodes, with undoped AlSb sandwiched between n-type GaAs substrate and p-type GaSb as a conductive oxygen-protective layer. Energy spectra were measured from 241Am, 252Cf, and 239Pu sealed sources, with good peak resolution and signal to noise response. Both GaSb PN diodes and AlSb PIN diodes exhibited larger pulses for smaller surface area samples, in good agreement with voltage-capacitance relationships for junctions. Microwave photoconductive decay (MW-PCD) measurements were performed on the Hall samples to determine the effect of doping on the minority carrier lifetime. Contrary to expectations, more heavily doped samples presented with longer decay times, some as large as hundreds of microseconds. There also appeared to be multiple exponential decay curves, potentially associated with different decay mechanisms. Collectively, the studies presented here reinforce the predicted nature of AlSb with respect to radiation detection.

Keywords

Semiconductor detectors, radiation detection, transport properties, Hall effect, MW-PCD, AlSb, thin film

Document Type

Dissertation

Language

English

Degree Name

Nuclear Engineering

Level of Degree

Doctoral

Department Name

Nuclear Engineering

First Committee Member (Chair)

Balakrishnan, Ganesh

Second Committee Member

de Oliviera, Cassiano

Third Committee Member

Sharma, Ashwani

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