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Title: GaN radiation detectors for particle physics and synchrotron applications
Author: Grant, James Paul
ISNI:       0000 0001 3511 3660
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2007
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In this thesis the work will focus on the development of wide band gap radiation detectors for radiation hard, biological and monitoring applications. Gallium nitride (GaN) was investigated as a radiation hard particle detector and as an UV light detector while the properties of single crystal diamond as a soft x-ray beam position monitor were assessed. Photolithographic processes were used to produce Schottky pad detectors of 1 mm diameter on three epitaxial GaN wafers grown on a sapphire substrate. Two of the wafers were obtained from Tokushima University, Japan and had an epitaxial thickness of 2.5 mum while the third GaN wafer was grown by Lumilog, France and had an epitaxial thickness of 12 mum. Devices were irradiated with 24 GeV/c protons and neutrons (1 MeV equivalent) to fluences of 10[14], 10[15], 2x10[15], 5x10[15] and 10[16] particles cm[-2] and the macroscopic properties characterised through current-voltage (I-V), capacitance-voltage (C-V) and charge collection efficiency measurements using alpha particles. The leakage currents of the irradiated GaN detectors were in some cases orders of magnitude smaller than the unirradiated devices. This phenomenon has also been observed in other irradiated wide band gap semiconductors, SiC and diamond. The maximum CCE of the thin epitaxial GaN detector was 97% while the thicker epitaxial GaN detector exhibited a maximum CCE of 53%. Irradiation with protons and neutrons led to a dramatic reduction in the CCE of the GaN detectors. For example, the CCE of one of the thin epitaxial GaN detectors dropped from 97% pre-irradiation to 40% after irradiation to 10[16] neutrons cm[-2] and 13% after irradiation to 10[16] protons cm[-2]. The drop in CCE of the thicker epitaxial material was less pronounced however the devices irradiated to the highest fluences, 1016 neutrons cm[-2] and 10[16] protons cm-2 exhibited CCEs of only 17% and 25% respectively. Attempts were made at identifying and understanding the microscopic as-grown and radiation-induced defects that determine the macroscopic characteristics of the GaN detectors. The microscopic properties of unirradiated and irradiated GaN detectors were evaluated using photoluminescence (PL), contact photoconductivity (CPC) and thermally stimulated current (TSC) techniques. Both PL and CPC measurements of the irradiated devices revealed a substantial increase in non-radiative recombination. In particular the intensity of the yellow band PL peak is significantly reduced after irradiation to 10[16]particles cm[-2]. TSC measurements of the GaN detectors revealed several competing complicated transport mechanisms. Thermal activation energies of 0.16-0.2, 0.27-0.32, 0.36-0.45 and 0.73-0.74 eV were extracted from neutron irradiated thin epitaxial GaN detectors. Dry etching of various GaN materials was done in a inductively coupled plasma (ICP) machine. The GaN samples were etched in order to produce ohmic contacts to the n-GaN buffer layers and to realise a parallel plate capacitor detector geometry. From the current-voltage and capacitance- voltage characteristics of the etched devices the ideality factor, Schottky barrier height and carrier concentration were extracted. The parallel plate capacitor geometry of the etched devices resulted in an increase in charge collection efficiency compared to the unetched devices. This is attributed to better definition of the electric field within the etched devices resulting in significantly improved charge transport.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available