Date of Award

2016

Document Type

Open Access Dissertation

Department

Electrical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Krishna C. Mandal

Abstract

Radioactive materials, as they decay, generate different high-frequency electromagnetic radiation such as alpha particles, beta particles, x-rays, gamma-rays, and neutrons. Nuclear detectors could stop these high-energy ionizing radiations, collect and transport the charges generated to an external circuit, and produce an electrical signal which is amplified by readout electronics to measure the energy of nuclear interaction. Thus, nuclear detectors are important tools for accounting of radioactive materials and have widespread applications in nuclear power plants, nuclear waste management, in national security, in medical imaging such as x-ray mammography, digital chest radiography, CT scan, and in high energy astronomy for NASA space exploration.

In this dissertation three different types of wide bandgap (WBG) radiation detectors were studied: (1) amorphous selenium (a-Se), (2) cadmium zinc telluride (CZT), and (3) silicon carbide (SiC). All three semiconductors have attractive electrical properties such as wide bandgap (≥ 1.5 eV) facilitating room temperature operation, high resistivity (≥ 1010 Ω-cm) contributing to low thermal noise for high-resolution, and high charge carrier mobility-lifetime product offering high charge collection efficiency.

However, these semiconductors have distinct characteristics that set them apart from one another. For example, high Z (atomic numbers of constituent elements, Cd=48, Zn=30, Te=52) and high density of CZT offering high stopping power to absorb high energy x- and gamma-rays so CZT could be used for x- and gamma ray spectrometers at room temperature. On the other hand, SiC has low Z value (Si = 14, C = 6) appropriate for detection of alpha particles and low energy x-rays and gamma rays (<60 keV) regime. Furthermore, high bandgap energy (~3.27 eV at 300 K) of 4H-SiC allows detector operation well above room temperature (~773 K) as required for nuclear fuel processing environment in nuclear power plants. Amorphous Se alloy with enriched boron (10B) has high thermal neutron cross-section (3840 barns, 10-24 cm2) due to boron and high radiation tolerance due to its amorphous structure, making it a favorable candidate for solid-state thermal neutron detectors.

In this study, semiconductors were grown from in-house zone-refined ultra-pure precursor materials using specialized growth furnaces, which were modified, re-coded and optimized to obtain high quality detector materials. Different metal-semiconductor contacts with metals of various work functions and metal-semiconductor-metal (MSM) devices with planar, guard-ring, and large area thin-film structures have been studied to ensure good charge transport properties and opto-electronic device performances. A series of characterization were carried out including scanning electron microscopy (SEM), x-ray diffraction (XRD), glow discharge mass spectroscopy (GDMS), optical absorption study, thermally stimulated current (TSC), deep-level transient spectroscopy (DLTS), and current-voltage (I-V) measurements. These extensive characterizations provided information on stoichiometry, morphology, purity, bandgap energy, resistivity, leakage current and presence of any performance-limiting electrical defect levels. Finally, to determine detection specificity, sensitivity and energy resolution, fabricated detector devices were evaluated with radiation sources, such as 241Am (5.5 MeV) for alpha, 137Cs (662 keV) for gamma, and 252Cf for moderated thermal neutron source.

High quality boron (10B) doped a-Se (As, Cl) alloys were synthesized in a specially designed alloying reactor. Alloy films were deposited using thermal evaporation, a low-cost technique which can be scaled up for large area detector production. The films used for detector fabrication had smooth, defect-free amorphous structure as determined by SEM and XRD. The bandgap and resistivity of 10B-doped a-Se (As, Cl) alloy was determined to be 2.21 eV and ≥1012 Ω-cm, respectively, at 300K. Single layer planer MSM (4″ x 4″) detectors were fabricated on ITO glass and oxidized aluminum substrates. Current-voltage (I-V) characteristics showed very low leakage (~-10 nA at -1000V); by using Al2O3 as blocking layer, leakage current was reduced to pA to a fraction of nA at -1000V. Nuclear testing with high energy alpha particles (241Am) and neutron (252Cf) sources showed specific signature of thermal neutron detection. The data confirms that 10B-doped a-Se (As, Cl) alloy films can be used to construct high performance compact neutron detectors.

The CZT crystals were grown at a stoichiometry of Cd0.9Zn0.1Te from zone refined ultra-pure precursor materials with 50% excess Te using modified multi-pass vertical furnace. The bandgap energy was determined to be 1.56 eV. The electrical resistivity was estimated to be ~ 6 × 1010 Ω-cm, which is high enough to fabricate a functional CZT radiation detector. The CZT detectors showed very low leakage current at a high bias (below 5 nA at –1000V) due to their high resistivity, which are beneficial for high resolution detectors. The drift mobility and mobility-lifetime product of electrons were estimated to be 1186 cm2/V.s and 5.9×10-3 cm2/V, respectively. An energy resolution of 6.2% was obtained for CZT planar detector when irradiated with 60 keV low-energy gamma radiations (241Am). The peaks were sharper and better resolution was observed for the CZT detector with guard ring geometry. An energy resolution of 2.6% was observed for detector with guard-ring structure irradiated with high energy 662 keV gamma radiations using 137Cs radiation source.

Schottky barrier detectors in planar configuration have been fabricated on 50 ��m n-type 4H-SiC epitaxial layers grown on 360 ��m SiC substrates by depositing ~10 nm nickel (Ni) Schottky contact. Current-voltage (I-V), capacitance-voltage (C-V), and alpha ray spectroscopic measurements were carried out to evaluate the Schottky barrier detector properties. Room temperature I-V measurement revealed a very low leakage current of ~ 0.78 nA at 250 V reverse bias. The barrier height for Ni/4H-SiC Schottky contact was found to be ~1.4 eV and the diode ideality factor was measured to be 1.4, which is slightly higher than unity showing the presence of deep levels as traps and/or recombination centers. Capacitance mode deep level transient spectroscopy (DLTS) revealed the presence of the deep levels along with two shallow level defects related to titanium impurities (Ti(h) and Ti(c)) and an unidentified deep electron trap located at 2.42 eV below the conduction band minimum which is being reported for the first time. The concentration of the lifetime killer Z1/2 defects was found to be 1.6× 1013 cm-3. The detectors’ performances were evaluated for alpha particle detection using a 241Am alpha source. An energy resolution of ~ 2.58 % was obtained with a reverse bias of 100 V for 5.48 MeV alpha particles. The measured charge collection efficiency (CCE) was seen to vary as a function of bias voltage. With increased reverse bias, the detector active volume increases with the increase in depletion layer width accommodating more number threading type dislocations at the epilayer/substrate interface resulting in higher FWHM values as observed experimentally.

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© 2016, Rahmi Orhon Pak

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