Evaluation of epitaxial n-GaAs for nuclear radiation detection

Abstract

The high purity of n-GaAs grown by the liquid phase epitaxial process makes it suitable for the evaluation of this compound semiconductor in nuclear radiation detection. The wide band gap and high atomic number make it possible to operate surface barrier diodes at room temperature as high resolution detectors. The resolutions (fwhm) at 300 K have been found to be 21 keV for 5.49 MeV α-particles and 2.5 keV for 140 keV γ-rays. The best resolutions observed were 640 eV (130 K) for 59.54 keV γ-rays, 1.3 keV (200 K) for 84 keV conversion electrons and 16 keV (205 K) for 8.785 MeV α-particles. The thickness of the epitaxial region was ≈60 μm with contact diameter up to 3 mm. The energy per electron hole pair for α-particles is 4.27 eV ± 0.05 at 300 K with a linear variation with band gap of 2.7 over the temperature range 87 K to 340 K. The Fano factor for γ-rays is ≤ 0.18 ± 0.04 ( T ≈ 125 K). Variations of capacitance and pulse height with detector bias, as well as etch patterns, indicated a discontinuity in the impurity density profile in the region of the interface between the substrate and the epitaxial layer. Consideration of the theory of excess carrier recombination in direct band gap semiconductors shows that the charge collection efficiency is not expected to be significantly reduced by direct radiative recombination, even in the case of fission fragments. This is confirmed by the experimental observations that 252Cf fission fragment energy deficits of 14.4 and 6.3 MeV are not much larger than have been observed with Si detectors. The advantages of room temperature operation open up a wide field of application for GaAs detectors in both the physical and biomedical fields. For their full exploitation it will be necessary: (a) to grow thicker layers of high purity GaAs by the epitaxial or some other process; (b) to overcome the problem of signal attenuation at the epitaxial-substrate interface; (c) to develop a more rugged form of front contact and of detector encapsulation.