Optimal width of barrier region in X/γ-ray Schottky diode detectors based on CdTe and CdZnTe

The spectral distribution of quantum detection efficiency of X- and γ-ray Schottky diodes based on semi-insulating CdTe or Cd_0.9Zn_0.1Te crystals is substantiated and obtained in analytical form. It is shown that the width of the space charge region (SCR) of 6–40 μm at zero bias in CdTe (Cd_0.9Zn_0.1Te) Schottky diode is optimal for detecting radiation in the photon energy range above 5–10 keV. Based on the Poisson equation, the relationship between the SCR width and the composition of impurities and the degree of their compensation are investigated. It is shown that the presence of deep levels in the bandgap leads to a considerable increase in space charge density and electric field strength near the crystal surface. However, this effect contributes a small error in the determination of the SCR width using the standard formula for the Schottky diode. It is also shown that the concentration of uncompensated impurities in CdTe and Cd_0.9Zn_0.1Te crystals within the 4 × 10¹¹–10¹³ cm^–3 range is optimal for the detection efficiency of X- and γ-rays in the photon high-energy range. The record-high values of energy resolution have been obtained in the spectra of ²⁴¹Am, ⁵⁷Co, ¹³³Ba and ¹³⁷Cs isotopes measured using CdTe crystals with Schottky diodes because the concentration of uncompensated donors in the CdTe crystals (1–2) × 10¹² cm^–3 falls on an interval of maximum detection efficiency. In the spectrum of ⁵⁷Co isotope, the limiting energy resolution has been achieved.