V. Babentsov

Defect structure of high resistive CdTe:In prepared by vertical gradient freeze method

High resistive and photosensitive CdTe doped with In aimed for fabrication of X- and gamma-ray detectors was produced by vertical gradient freeze method. A complex investigation of defects and compensation by a number of optical and photoelectrical mapping methods was performed. A model of energy levels dominating the recombination processes in the material was elaborated, where the role of In, and related complexes as well as native defects (Cd vacancy and its competes) is discussed.

Compensation and Photosensitivity in CdTe Doped With Indium

To better our knowledge of the characteristics of semi-insulated cadmium telluride (CdTe) doped with indium (In), we explored the role of deep levels in compensation and trapping. We assessed the defects and their distribution across a wafer in several ways; by measuring dark resistivity and photosensitivity maps, photoluminescence, Photo-Induced Current Transient Spectroscopy (PICTS), and Thermoelectric Effect Spectroscopy (TEES). We determined that electron trapping to a near midgap level in CdTe:In begins when the Fermi-level lies above this level. We demonstrated first that a small movement (ap 1divide2 kT) of the Fermi-level downward significantly increases electron trapping. PICTS and TEES measurements confirmed the presence of a positively charge electron trap at E C-0.65 eV (plusmn 0.05 eV) with a high capture cross-section. This level transforms into a neutral one when the Fermi-level moves above it. Photoluminescence measurements detected this energy level that, when positively charged, was responsible for a 0.68-eV emission, while in a neutral state, it was accountable for an emission peak located at 0.87 eV. We discuss the nature of the deep donors, considering the latest ldquoab initiordquo calculations: also, the Te anti-site is compared to complex defects, such as H-OCd.

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 Cd1−xZnxTe (x ≈ 0.1), and that of the deep donor levels located at EC-0.4 eV and EC-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.

Semiconductor-polymer composite with strong nonlinear optical properties

Photoluminescence (PL) characteristics of CdTe nanocrystals (NCs) adsorbed into polymer films are presented compared to the PL of bulk CdTe. Using the dependence of PL on excitation power we determined the carrier lifetime at 80K which indicates that carrier trapping in surface and polymer states was the main decay mechanism of excitation. The occurrence of at least two deep defect induced luminescence bands in CdTe NCs was demonstrated for the first time. Our investigation also revealed changes in PL properties induced by the visible light irradiation or by the preparation conditions. These results were explained by an increase of nonradiative recombination in the surface states of NCs and in the polymer.