High resolution radiation detectors based on 4H-SiC N-type epitaxial layers and pixilated CDZNTE single crystal devices

Abstract

Silicon Carbide (SiC) is an indirect wide bandgap semiconductor with high thermal conductivity, high breakdown electric field, high carrier saturation drift velocity, and large displacement energy making it a suitable candidate for replacing conventional radiation detectors based on Si, Ge, CdTe, and CdZnTe (CZT). In this dissertation, fabrication and characterization of high-resolution Schottky barrier detectors for alpha particles using 20 µm thick n-type 4H-SiC epitaxial layers are reported. Schottky barriers were obtained by depositing circular nickel contacts of ~10 mm2 area.

Room temperature current-voltage (I-V) measurements revealed Schottky barrier heights of the order of 1.7 eV, ideality factor of ~1.1, and leakage currents as low as 1 nA at an operating reverse bias of -170 V. Deep level transient spectroscopy (DLTS) revealed the presence of shallow defects at Ec – (0.14 ± 0.01) eV and Ec – (0.18 ± 0.01) eV corresponding to titanium (Ti) substitution in silicon (Si) lattice, and at Ec – (0.62 ± 0.02) eV corresponding to Z1/2 defects caused by carbon vacancies. Deep level defects have been found at Ec – (1.42 ± 0.04) eV, and Ec – (1.52 ± 0.03) eV respectively that are related to C-C or C-Si di-vacancies.

A 0.1 μCi 241Am radiation source was used to assess the detector performance by pulse height spectroscopy, and an energy resolution of ~ 0.38% full-width half maxima (FWHM) was observed for alpha particles at ~ 5447 keV. The average diffusion length (Ld) of holes (minority carriers) were calculated to be ~ 13.6 µm using a drift-diffusion model and MATLAB code. A noise analysis in terms of equivalent noise charge revealed that the white series noise due to the detector capacitance has substantial effect on their spectroscopic performance.

A new edge termination technique was developed by depositing thin Si3N4 passivating layer on 4H-SiC epitaxial layer surrounding nickel (Ni) contact in order to improve detector performance. The 4H-SiC detector with Si3N4 edge termination showed a higher barrier height with improved rectifying characteristics and a leakage current in pA range, which was two orders of magnitude lower compared to conventional detector fabricated from the same parent wafer. DLTS measurements revealed a reduction in life-time killing defects of detectors with Si3N4 edge termination which could be correlated to the observed improvements in energy resolution.

In addition to SiC alpha detector, Cd0.9Zn0.1Te (CZT) based pixelated detectors were fabricated and characterized for gamma ray detection. Large area CZT single crystals has been grown using a tellurium (Te) solvent method. A 3×3 guarded pixilated detector has been fabricated on a ~ 20×20×5 mm3 crystal cut out from the grown ingot. A guard ring was used to reduce inter-pixel/inter-electrode leakage. I-V measurements revealed a leakage current of ~ 5 nA at a bias voltage of 1000 V and a resistivity of ~ 1011 Ωcm. The mobility-lifetime product (μτ) was calculated to be 6 × 10-3 cm2/V using alpha spectroscopic method. Using time of flight measurements, electron mobility was determined to be ~ 1192 cm2.V-1.s-1. Gamma spectroscopy using a 137Cs source on the pixelated structure showed fully resolved 662 keV gamma peaks for all the pixels, with a resolution (FWHM) of ~ 1.51%, which exhibited a significantly improved resolution.