Date of Award

1-1-2013

Document Type

Open Access Thesis

Department

Electrical Engineering

First Advisor

Krishna C Mandal

Second Advisor

Bin Zhang

Abstract

The need of room-temperature, compact, and high resolution radiation detectors has opened the path for the development of new materials with suitable properties. Silicon carbide (SiC) and CdZnTe (CZT) are semiconductor materials with wide band gap that can operate efficiently at room temperatures (RT) and above. CZT has a high average atomic number (Z) which offers a high photoelectric absorption coefficient for x-/gamma rays. SiC allows high energy heavy ion detection due to its high radiation hardness even at temperatures much higher than room temperatures (RT). This thesis focuses on the fabrication and characterization of semiconductor based radiation detectors for alpha particles and gamma rays and development of various software codes (based on Matlab) required to derive important device parameters from the experimental results in order to improve the device performance further. Characterization includes electrical (I-V and C-V) and spectroscopic (alpha particles and gamma rays) techniques.

Silicon carbide is a semiconductor material suitable for radiation detection at room-temperature and above, and in high radiation field environment due to its wide band gap (3.27 eV at 300 K), high breakdown field, and extremely high radiation hardness. In the present work, Schottky barrier detectors have been fabricated on high quality n-type 4H-SiC epitaxial layer grown on n-type 4H-SiC bulk substrate. The application of epitaxial layer allows to obtain high crystallinity, which is desirable for better charge transport properties. The detectors have been fabricated on 8x8 mm24H-SiC 4° off-cut samples by depositing 10 nm thick nickel contacts (~11 mm2of area). Current-voltage (I-V) and capacitance-voltage (C-V) are measured to investigate the Schottky barrier properties. The I-V characteristics revealed an extremely low leakage current of the order of few pA at high operating reverse biases. Using a thermionic emission model, large Schottky barrier heights of the order of 1.6 eV were calculated. Alpha spectroscopic measurements were conducted using a241Am radiation source (5.48 MeV) to investigate the charge collection efficiency (CCE) and energy resolution. The spectroscopic measurements revealed very high energy resolution (&sim0.37% at 5.48 MeV) and minority carrier diffusion length of &sim13.2 &mum were calculated using a Matlab program developed on the basis of a drift-diffusion model. Electronic noise analyses in terms of equivalent noise charge (ENC) were performed to determine the effect of different noise components that contribute to the total electronic noise in the detection system. A least square fitting algorithm was implemented in Matlab in order to execute these analyses.

For gamma ray detection application, there are certain requirements that the material should fulfill in order to fabricate an efficient detector. Cd0.9Zn0.1Te (CZT) is a high Z compound semiconductor material with convenient properties for gamma ray detection. CZT presents a very high gamma ray absorption coefficient, very low leakage current due to high resistivity, wide band gap (&ge1.5 eV at 300K), and high density. The fabrication of the detector involved different steps of polishing and passivation of the surfaces. Electrical contacts were fabricated with gold deposition by DC sputtering, which produced good Ohmic contact. Current-voltage (I-V) characterization was conducted to study the behavior of the contact and to calculate the resistivity of the samples. Gamma ray spectroscopic measurements were performed by using137Cs radiation source of 662 keV of energy. Digital data were collected and analyzed offline in order to generate biparametric plots. In general, the performance of CZT detectors is limited by the poor hole transport properties and the presence of various macroscopic and microstructural defects within the crystal, leading to charge trapping or loss. Methods have been developed to compensate for charge trapping by applying offline correction schemes to digitally obtained pulse-height spectra. Biparametric correlation based correction is one such scheme developed in this thesis in which the pulse-heights from the same energy events are correlated to their corresponding depth of interaction and any discrepancies are taken care of by applying suitable correction factor. A Matlab based software was developed in order to apply a correction scheme to the biparametric plots and to improve the results acquired from the detector.

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