Date of Award
Open Access Dissertation
Krishna C. Mandal
Schottky barrier radiation detectors were fabricated on n-type 4H-SiC epitaxial layers (12 – 50 μm) grown by hot wall CVD process on highly nitrogen doped 4H-SiC (0001) substrates with 4-8º off-cut towards the ̅ direction. Ni/4H-SiC Schottky barrier radiation detectors, a very low leakage current of 0.18 nA at 250 V bias, revealing low thermal noise, was observed in current-voltage (I-V) measurements. Using a thermionic emission model, junction properties such as barrier height of ≥1.10 eV and an ideality factor of ≤1.29 were determined. An effective carrier concentration of 1.03×1015 cm-3 was calculated by capacitance-voltage (C-V) measurement. Deep level transient spectroscopy (DLTS) was used to investigate electrically active defects in epilayer. Defect parameters such as activation energy, capture cross-section, and density of defects were calculated from Arrhenius plots. DLTS revealed the presence of shallow level defects related to titanium impurities, electrically active lifetime killer Z1/2 defect, and deep level defects assigned as EH6/7 which are related to carbon and carbon-silicon vacancies. The density of Z1/2 defect, the most detrimental to detector performance, was 1.6×1012 cm-3, orders of magnitude lower compared to other 4H-SiC detectors.
Detector performances were evaluated in terms of the energy resolution at full-width at half-maximum (FWHM) using pulse height spectroscopy (PHS) measurements with 0.1 μCi 241Am source. Charge collection efficiency was investigated using a drift-diffusion charge transport model. The energy resolution for 5.486 MeV alpha particles was 166 keV with charge collection efficiency of 22.6%. Electronic noise analysis of front-end readout system was carried out in terms of equivalent noise charge (ENC) in order to study the contribution of white series noise, pink noise ( parallel and ⁄ series) and white parallel noise to the total electronic noise in the detection system.
New edge termination was developed using surface passivating layers of silicon dioxide (SiO2) and silicon nitride (Si3N4) in order to improve detector performance. With edge termination, reverse leakage current of Ni/4H-SiC epilayer detector was improved significantly (nA to pA) leading to an increased signal-to-noise ratio. Improved Schottky properties such as barrier height of ~1.7 eV and diode ideality factor of ~1.07 were observed indicating a better surface uniformity that enhanced charge collection efficiency. C-V measurement confirmed a doping concentration of 2.4 x 1014 cm-3 ensuring a fully depleted (~20 μm) detector at bias voltages as low as ~70 V. DLTS analysis showed a decreased concentration of performance limiting Z1/2 defect level and absence of EH6/7 deep-levels with edge termination, ensuing a more complete charge collection. Alpha spectroscopy measurements revealed an improved detector energy resolution from ~0.7% to ~0.4% for 5.48 MeV alpha particles with edge termination.
4H-SiC epitaxial detector with ruthenium (Ru) Schottky barrier contact (in addition to Ni being used in above studies) was investigated for operation in harsh environments with high temperature and high radiation. Ru/4H-SiC Schottky detectors exhibited excellent rectification and improved junction properties, even without edge termination. However, inhomogeneity of the Schottky barrier heights was observed due to interfacial defects resulting from a solid-state reaction involving Ru, Si, and C. As a result, pulse-height spectra with 241Am source were broad, and the three characteristic alpha peaks were not resolved. The energy resolution was calculated to be ~ 0.75% at 180 V reverse bias at room temperature.
V. Nguyen, K.(2017). Improved N-Type 4h-Sic Epitaxial Layer Radiation Detectors and Noise Analysis of Front-End Readout Electronics. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/4430