Yu. V. Klevkov

Optical and electrophysical properties of defects in high-purity CdTe

The electronic spectrum of defects formed in low-temperature synthesis and growth of high-purity CdTe[111]from the vapor phase of the starting components has been studied by the photoluminescence and photoconductivity methods, as well as using the analysis of the behavior with temperature of the conductivity. The studies have revealed in the crystals, in addition to the comparatively shallow centers involving primarily donors and acceptors contained in residual substitutional impurities, deep acceptor states with activation energies of 0.25, 0.60, and 0.86 eV, which differ in the character and magnitude of the localizing potential. While the deep centers at 0.60 and 0.86 eV display strong localization of electronic states, the center with the 0.25-eV activation energy is associated with defects for which the major part of the localizing potential extends uniformly in space over several unit cells. Such centers are assumed to originate from twinning-induced extended defects.

Compensation effect in undoped polycrystalline CdTe synthesized under nonequilibrium conditions

The compensation effect has been revealed in undoped polycrystalline CdTe synthesized during rapid crystallization. The revealed effect leads to an increase in the electrical resistivity to 108–1010 Ω cm at a background impurity concentration of ∼1015 cm−3 (GaCd and ClTe donors, unidentified acceptors). For some samples, this effect is accompanied by the appearance of persistent photoconductivity, which disappears at a temperature of ∼200 K. It has been shown that all the polycrystals studied are characterized by a three-level compensation mechanism in which the fundamental properties of the material are determined by deep donors and/or acceptors with a concentration of 1012 cm−3. Depending on the specific growth conditions, the electrical resistivity at room temperature is determined by deep centers with activation energies of 0.59 ± 0.10 and 0.71 ± 0.10 eV, which are supposedly related to intrinsic point defects, and deep centers with activation energies of 0.4 ± 0.1 eV, which belong to the DX center formed by the GaCd donor.

Evolution of photoluminescence spectra of stoichiometric CdTe: Dependence on the purity of starting components

A new sublimation method of material purification in a vacuum based on the crystal-vapor-crystal phase transformation is outlined. A basic design of growth reactor and the main parameters of CdTe fine purification are given. With the use of low-temperature photoluminescence (PL), the purification dynamics are revealed. This process is characterized by a complete decomposition of various complexes and by a sharp decrease in the concentration of both shallow-and deep-level residual impurity. At the final stage of purification, a polycrystalline CdTe with stoichiometric composition is obtained, and the impurity-related emission is entirely absent in PL spectra that involve only excitonic lines. The necessity of the fine purification for starting components is experimentally confirmed. Similar results are obtained for ZnTe and ZnSe.

Photoluminescence of CdTe grown in conditions considerably deviating from thermodynamic equilibrium

Undoped cadmium telluride produced in different conditions of fast crystallization of the vapor phase at temperatures of 420–600°C is studied by the low-temperature photoluminescence technique and electrical measurements. It is shown that, despite the relatively high rate of growth (∼1 μm s−1), the basic parameters of the lattice and band structure of the material are close to the corresponding parameters reported in publications for single-crystal CdTe. The conductivity type of the crystals grown in the study is controlled by hydrogen-like donors and acceptors associated with background impurities. It is found that, along with background impurities in n-type textures, there exist compensating acceptors with the nonstandard activation energies 48.5 ± 0.5, 98.0 ± 0.5, and 119.5 ± 0.5 meV. It is shown that the lowered temperature of growth, together with the excess tellurium content, yields a sharp increase in the concentration of deep electron states and isoelectronic defects with lowered symmetry.

Microphotoluminescence Spectra of Cadmium Telluride Grown under Nonequilibrium Conditions

Microphotoluminescence spectroscopy and imaging were used to study impurities and defects in CdTe crystals grown by nonequilibrium techniques. The growth procedure includes low-temperature synthesis and purification of the material via congruent sublimation, with subsequent deposition under the conditions of gas-dynamic vapor flow and high-rate low-temperature condensation. Although the growth conditions are highly nonequilibrium, the obtained polycrystalline material with a grain size of 1–2 mm exhibits strong low-temperature exciton luminescence, whose intensity is nearly uniform over the bulk of the ingots. At the same time, it is found that residual impurities and defects have a tendency to accumulate to form clusters within certain areas which are a hundred micrometers in size; the density of some impurities in these clusters is sufficiently high.

Effect of passivation of the p-CdTe surface in (NH4)2Sx on the current-voltage characteristics of contacts

The current-voltage (I–V) characteristics of contacts to CdTe are studied at 300 and 77 K. The contacts were obtained by chemical deposition of Au on the (111) surface of polycrystalline p-CdTe after etching in bromine methanol and after passivation in (NH4)2Sx. It is shown that passivation of the surface noticeably linearizes the I–V characteristics at 300 K and only slightly at 77 K. The measurements of the low-temperature photoconductivity of the samples showed that the passivation of the surface in (NH4)2Sx decreases the surface-state density at the metal-semiconductor interface.

Electrical properties of fine-grained polycrystalline CdTe

The temperature dependence of conductivity, the photoconductivity, and the hole mobility in fine-grained polycrystalline CdTe have been studied. The results obtained agree well with the concept of charged grain boundaries. The potential barriers at grain boundaries and the way in which these barriers affect the free carrier mobilities have been determined.

Effect of grain boundaries on the properties of cadmium telluride grown under nonequilibrium conditions

Microphotoluminescence spectroscopy and imaging were used to study the effect of grain boundaries on the properties of textured CdTe polycrystals with a single-crystal grain size of 1–2 mm grown under nonequilibrium conditions. The technological procedure included low-temperature synthesis and purification of the material via congruent sublimation, with subsequent deposition under conditions of gas-dynamic vapor flow and high-rate low-temperature condensation. Microluminescence probing revealed that most of the grain boundaries are decorated with local centers emitting in the 1.4-eV band of donor-acceptor recombination involving shallow donors and A-center acceptors. The boundary regions are, to some extent, free of nonradiative recombination centers active at room temperature. Gettering activity of the grain boundaries could be detected at the distances up to 100 µm, which reflects the specific features of the nonequilibrium crystallization conditions of the material under study.

A DLTS study of deep levels in the band gap of textured stoichiometric p-CdTe polycrystals

Deep-level transient spectroscopy (DLTS) was used to identify a set of deep electronic states in the band gap of textured p-CdTe polycrystals whose composition was almost stoichiometric. Four hole traps and two electron traps were observed. It is shown that the deepest hole trap with a level at Ev+0.86 eV corresponds to a prevalent defect in this material. Special features of the line shape in the DLTS spectrum and the logarithmic dependence of population of this level on the duration of the filling pulse correspond to an extended defect related most probably to dislocations at the grain boundaries.

Electronic spectrum of non-tetrahedral acceptors in CdTe:Cl and CdTe:Bi,Cl single crystals

The electronic spectra of complex acceptors in compensated CdTe:Cl, CdTe:Ag,Cl, and CdTe:Bi,Cl single crystals are studied using low-temperature photoluminescence (PL) measurements under both nonresonant and resonant excitation of distant donor–acceptor pairs (DAP). The wavelength modulation of the excitation source combined with the analysis of the differential PL signal is used to enhance narrow spectral features obscured because of inhomogeneous line broadening and/or excitation transfer for selectively excited DAPs. For the well-known tetrahedral (TD) AgCd acceptor, the energies of four excited states are measured, and the values obtained are shown to be in perfect agreement with the previous data. Moreover, splitting between the 2P3/2 (Г8) and 2S3/2 (Г8) states is clearly observed for AgCd centers located at a short distance (5–7 nm) from a hydrogen-like donor (ClTe). This splitting results from the reduction of the TD symmetry taking place when the acceptor is a member of a donor–acceptor pair. For ...