Fabrication of High-Resolution Nuclear Detectors Using 4H-SiC n-type Epitaxial Layers
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Fabrication of High-Resolution Nuclear Detectors Using 4H-SiC n-type Epitaxial Layers Kelvin J. Zavalla, Sandeep K. Chaudhuri, and Krishna C. Mandal* Department of Electrical Engineering, 301 Main Street, University of South Carolina, Columbia, SC 29208, U.S.A. ABSTRACT High resolution Schottky barrier detectors for alpha particles have been fabricated on 20 m n-type 4H-SiC epitaxial layers. Schottky barrier contact structure was accomplished by deposition of 10 nm nickel on the Si face of the epilayers. The detectors were characterized for structural, electrical, and spectroscopic properties. Scanning electron microscopy and Nomarski optical microscopy revealed a micropipe density lower than 1 cm-2. The current-voltage (I-V) characteristics of the device exhibited very low leakage current of the order of 6.5 pA at an operating bias of 90 V. C-V measurements revealed a typical effective doping concentrations of 2.4 × 1014 cm-3 in these epilayers. The detectors were evaluated for alpha particles detection using a 241Am source. An energy resolution of ~0.98% for 5.48 MeV alpha particles was observed. The separate contribution of charge carrier drift and diffusion to the total charge collection efficiency has been calculated in these detectors following a drift-diffusion model. Detailed electronic noise analysis in terms of equivalent noise charge (ENC) was carried out to study the effect of various noise components that contribute to the total electronic noise in the detection system. Effect of shaping time, presence of source and bias on the ENC has been studied in details. INTRODUCTION The development of growth techniques for high quality epitaxial layers has increased the availability of 4H-SiC epilayers with favorable properties for fabrication of radiation detectors which are truly compact in size, operable at room or elevated temperature, physically rugged and radiation hard [1-3]. Due to its wide band-gap (3.27 eV at 300 K), and better crystallinity compared to its bulk counterpart, 4H-SiC epitaxial layers have become a very suitable material for fabrication of high-resolution alpha particle detectors [1]. And hence they can be considered as compact neutron detectors when coupled with neutron-alpha conversion layers [4]. Due to the large hole diffusion lengths [5], and built-in potential of the order of ~1.5 eV [3] in 4H-SiC epitaxial Schottky barrier detectors, they can also be configured for bias-less operation [6]. Biasless operation is a much sought-after quality for field deployment of stand-off detection systems for homeland security purposes. For high-resolution and high-efficiency detection systems the following factors need to be accomplished; (1) minimum energy scattering of the incident radiation at the detector entrance window, (2) efficient stopping of the ionizing particle within the effective volume of the detector (depletion region in the case of Schottky barrier detectors), (3) high charge carrier mobility and lifetime, (4) minimum detector leakage current, and (5) minimum noise associated wit
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