Unique deep levels in spectroscopic CdZnTe: Compensation, trapping, and polarization

As yet, the role of the main native defects in the compensation, trapping, and polarization of x-ray and gamma-ray room-temperature detectors based on semi-insulated cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) is indeterminate. To better quantify it, we assessed the ionization energy, i.e., the binding energy for the hole of the second (2-/1-) acceptor level of Cd vacancies in Cd_1-xZn_xTe (x ≈ 0.1), and that of the deep donor levels located at E_C-0.4 eV and E_C-0.7 eV. We characterized the defects in several ways, including measuring the photoconductivity at below-bandgap excitation, and photoconductivity quenching by comparing their positions in the bandgap with that of the native energy-levels in CdTe quantum dots (QDs) and other II-VI semiconductors. In this way, we determined unambiguously that a deep acceptor, Cd vacancy, behaves as a doubly charged acceptor, and the second ionization level is located at ~ EV+(0.5±0.05) eV, i.e., far from the mid-gap. This configuration may determine the lifetime of holes, but it does not stabilize precisely the compensation condition, and it is not responsible for electron trapping and polarization. We demonstrated that a self-consistent model of compensation, electron trapping, and polarization should be based on a doubly charged donor (D) with two electrical states D(2+/1+) and D(1+/0), one of which is located close to the mid-gap and is separated from the second by a potential barrier that prevents fast trapping of the photoelectrons from the conduction band, but can be responsible for polarization.