New semiconductor detectors for nuclear radiation utilizing Si1-x Ge x alloy

Abstract

Single-crystal solid solutions of Si t _xGex are of interest as a new material for producing lithium-drifted detectors for providing the possibility of raising the recording efficiency for a, 0, and 3' radiation [1]. It is well known that the detectivity of the material is proportional to Z 5/2 (the nuclear charge of the atoms) and to the degree of homogeneity of the semiconductor material. The temperature operating range of a detector is lowered with a reduction in the width of the forbidden band of the semiconductor used to make it. Well-known analogs operating at room temperature have been made using silicon with Z = 14. Germanium detectors, which possess a higher sensitivity (Z = 32), require cooling to liquid nitrogen temperature (around -200~ Detectors utilizing Sil_xGe x with a small germanium content on the one hand retain the temperature operating range of silicon analogs and on the other hand are more sensitive on account of the presence of germanium atoms located in the semiconductor lattice in a substitution position. However, a solid solution compensated with lithium (as a material suitable for making a p-i-n detector) should then possess a high degree of compensation and a long carrier lifetime. These quantities are determined both by the. homogeneity of composition and distribution of impurities in the original solid solution and by the technology of compensating the material with lithium.In order to make a high-efficiency detector utilizing this material one must provide a compensated high-resistance sensitive region, a -1 eV high potential barrier, low reverse currents in the structure, and consequently a minority carrier lifetime of shorter than 10 -6 s. We previously showed that detectors utilizing Si z_xGex have a 0-particle count rate which is a factor of three higher than that for their silicon analog [1].In the present work we investigate the barrier height and the current-voltage characteristics of p-i-n detectors utilizing single crystals of Si 1 _xGex solid solutions with gold and aluminum contacts, and the influence of the material on the electrophysical properties of a lithium-compensated/-region with an atomic germanium concentration in the alloy of up to 10%. Lithium was drifted into original p-type alloy samples of resistivity 50-100 from, obtained by electron-beam crucible-free zone melting in a vacuum, using a technology developed for p-Si at 60~ in a field of 10 4 V/cm, the thickness of the i region being 400-600/~m.As is well known, the photoelectric method is direct and one of the most accurate for determining the barrier height since one can fred the height El, of the energy barrier by extrapolating the linear dependence of the square root of the photoresponse onto the energy axis [2]. As can be seen from Fig. 1, El, is independent of the germanium content and is equal to 0.95-I eV. At the same time, it is well known that the maximum Au-p-Si barrier (and our material has similar properties to silicon) is 0.75 eV.The barrier height 'r in two limiting cases is as follows: if D...