Date of Award

2015

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Krishna C. Mandal

Abstract

Silicon carbide (SiC) is one of the key materials for high power opto-electronic devices due to its superior material properties over conventional semiconductors (e.g., Si, Ge, GaAs, etc). SiC is also very stable and a highly suitable material for radiation detection at room temperature and above. The availability of detector grade single crystalline bulk SiC is limited by the existing crystal growth techniques which introduce extended and microscopic crystallographic defects during the growth process. Recently, SiC based high-resolution semiconductor detectors for ionizing radiation have attracted world-wide attention due to the availability of high resistive, highly crystalline epitaxial layers with very low micropipe defect content (< 1 cm-2). SiC Schottky barrier radiation detectors on epitaxial layers can be operated with a high signal-to-noise ratio even above room temperature due to its wide band-gap. However, significant amount of intrinsic defects and impurities still exist in the grown SiC epilayer which may act as traps or recombination/generation centers and can lead to increased leakage current, poor carrier lifetime, and reduced carrier mobility. Unfortunately, the nature of these electrically active deep levels and their behavior are not well understood. Therefore, it is extremely important to identify these electrically active defects present in the grown epitaxial layers and to understand how they affect the detector performance in terms of leakage current and energy resolution.

In this work, Schottky barrier radiation detectors were fabricated on high quality n-type 4H-SiC epitaxial layers. The epitaxial layers were grown on nitrogen doped n-type 4H-SiC (0001) substrates by a hot wall chemical vapor deposition (CVD) process. The epitaxial growth was carried out with 8o off-cut towards the [112̅0] direction. The Schottky barriers were formed on the epitaxial layers (Si-face) by depositing thin (~10 nm) circular Ni contact (area ~10 mm2) which acts as the detector window. The thickness of the detector window was decided such that there was minimal alpha energy attenuation while maintaining a reliable electrical contact. For the back contact, ~100 nm thick square (~40 mm2) Ni contact was deposited on the C-face of the 4H-SiC substrate.

The junction properties of the fabricated Schottky barrier radiation detectors were characterized through current-voltage (I-V) and capacitance-voltage (C-V) measurements. From the fabricated devices, those with high barrier height (~ 1.6 eV) and extremely low leakage current (few pA at a reverse bias of ~ -100 V) were selected for alpha spectroscopic measurements. Alpha pulse-height spectra was obtained from the charge pulses produced by the detector irradiated with a standard 0.1 μCi 241Am source. The charge transport and collection efficiency results, obtained from the alpha particle pulse-height spectroscopy, were interpreted using a drift-diffusion charge transport model. The detector performances were evaluated in terms of the energy resolution. From alpha spectroscopy measurements the FWHM (full width at half maxima) of the fabricated Schottky barrier detectors were in the range of 0.29% - 1.8% for the main alpha peak of 241Am (5.486 MeV).

Deep level transient spectroscopy (DLTS) studies were conducted in the temperature range of 80 K - 800 K to identify and characterize the electrically active defects present in the epitaxial layers. Deep level defect parameters (i.e. activation energy, capture cross-section, and density) were calculated from the Arrhenius plots which were obtained from the DLTS spectra at different rate windows. The observed defects in various epitaxial layers were identified and compared with the literature. In the 50 μm epitaxial layer, a new defect level located at Ec - 2.4 eV was observed for the first time. The differences in the performance of different detectors were correlated on the basis of the barrier properties and the deep level defect types, concentrations, and capture cross-sections. It was found that detectors, fabricated on similar wafers, can perform in a substantially different manner depending on the defect types. For 20 μm epitaxial layer Schottky barrier radiation detectors, deep levels Z1/2 (located at ~ Ec - 0.6 eV) and EH6/7 (located at ~ Ec - 1.6 eV) are related to carbon vacancies and their complexes which mostly affect the detector resolution. For 50 μm epitaxial layer Schottky barrier radiation detectors, Z1/2, EH5, and the newly identified defect located at Ec - 2.4 eV mostly affect the detector resolution.

The annealing behavior of deep level defects was thoroughly investigated by systematic C-DLTS measurements before and after isochronal annealing in the temperature range of 100 ˚C - 800 ˚C. Defect parameters were calculated after each isochronal annealing. Capture cross-sections and densities for all the defects were investigated to analyze the impact of annealing. The capture cross-sections of the defects Ti (c) (located at Ec ˗ 0.17 eV) and EH5 (located at Ec ˗ 1.03 eV) were observed to decrease with annealing temperature while the densities did not change significantly. Deep level defects Z1/2 and EH6/7 were found to be stable up to the annealing temperature of 800 ˚C.

Rights

© 2015, Mohammad Abdul Mannan

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