Lithium-drift detectors based on the solid solutions Si1−x-Gex

Abstract

The silicon-germanium solid solutions Si l_x-Ge x are of interest in connection with investigations of promising materials for semiconductor detectors (SCDs) of ionizing radiation [1]. The positive qualities of both silicon and germanium are combined in this material: silicon as the wider-gap and higher-temperature material and germanium as the more efficient material with a quite high cross section of interaction with ionizing radiation.In the present paper we present the results of an investigation of the effect of the germanium content in single crystals of the alloy Sit_x-Ge x on the electrophysical, radio-and spectrometric characteristics of lithium-drift detectors based on them. Ingots of p-type Si l_x-Gex single crystals with resistivity of 10-100 fFcm, 5-10 mm in diameter, and with (I 11) crystallographic orientation were obtained by the method of crucibleless zone melting in vacuum. Semiconductor detectors based on Si l_x-Gex are 0.5-1.5 mm thick and they have a sensitive area of 0.1-0.2 cm2; they have gold metallization on the entrance window and an aluminum contact on the backside [2].The degree of structural uniformity of the volume of the base semiconductor Sit_x-Ge x was investigated as a function of the germanium content by the phase-frequency method [3].The degree of uniformity of the surface and volume of p-i-n structures was determined by measuring the current, capacitance, noise, spectrometric (R-V t) and radiometric (N-V t) parameters of the characteristics of the SCDs and the C-V characteristics of test MIS structures with pyrolytic silicon dioxide SiO2p, synthesized by a low-temperature method in an ultrapure medium [4, 5]. The energy resolution (R-V t) and the counting rate (N#-Vt) for 3 particles from 2~ on the E 3 -I MeV line as a function of the external bias voltage was determined at 300 K by the method of [6]. Differential analysis of the characteristics, taking into account the C-V characteristics of the test MIS structures, made it possible to estimate not only the density of surface and volume electronic states, but also the contribution of surface and volume defects to the volume and surface generation and excess components of the reverse current, as well as to the components of the intrinsic noise and energy resolution of the SCDs [7, 8]. The contribution of the indicated parameters of the SCDs to the radiometric characteristics of Sil_x-Ge x p-i-n detectors was determined by establishing a correlation between the electric and spectro-and radiometric parameters, Figure 1 displays the counting rate N 3 for 3 particles versus V t of Si l_x-Ge x SCDs with different germanium content in the base semiconductor. A characteristic feature of the (N~-V t) characteristics is that the counting rate depends strongly on the germanium concentration in Sil_x-Ge x SCD with x = 0,1-2% of the atomic fraction and almost total absence with x = 2-6% of the atomic fraction. The maximum increase of a factor of 3-4 in the counting rate was observed when the germanium concentration was increased from 0.5 ...