<Emphasis Type="Italic">Y</Emphasis> and <Emphasis Type="Italic">Z</Emphasis> luminescence of polycrystalline cadmium telluride

The properties of polycrystalline CdTe with a grain size of 5–30 μm have been investigated using the microphotoluminescence methods of spectral analysis and topography. This material has been prepared by direct synthesis in a vapor flow of components at a low temperature. The dominance of the Y and Z bands in the spectra reflects a nonequilibrium character of the crystallization processes. The superlinear dependences of the luminescence intensity on the level of the band-to-band excitation indicate the exciton nature of the corresponding transitions. The activation energies for temperature quenching of luminescence in the temperature range T = 100–150 K are found to be 120 meV for the Y luminescence and 180 meV for the Z luminescence, which correspond to the dissociation of excitons bound to defects with the transition of charge carriers to the conduction and valence bands. The monochromatic topography data indicate that Y and Z defects have different material bases.     

Nonequilibrium phonon propagation in high-purity CdTe

Propagation of nonequilibrium acoustic phonons in samples of high-purity CdTe (impurity content ～10¹⁶ cm^?3) was studied using the heat pulse technique under pulsed photoexcitation. An analysis of nonequilibrium phonon propagation made by comparing the experimental response with Monte Carlo calculations assuming samples to be without twins provided an estimate for the spontaneous anharmonic phonon decay constant A_L=2×10^?52 s^?1 Hz^?5. The probability of free phonon transit through a twin boundary in a sample with twin structure was estimated as A_C=0.96.     

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.     

The role of extended defects in the formation of the spectrum of electronic states in high-purity p-CdTe polycrystals with stoichiometric composition

The spectra of the low-temperature photoluminescence in the cases of band-to-band and subgap laser excitation and the luminescence-excitation spectra of high-purity [111]-textured p-CdTe polycrystals with stoichiometric composition were measured. The spectrum of electronic states in the 1.3–1.6 eV energy range and the changes in this spectrum as a result of annealing of the samples in cadmium vapor were studied. It is shown that the electron levels located in the CdTe band gap that are responsible for the Z and Y luminescence bands (?ω=1.36 and 1.47 eV) are formed as a result of interaction of extended defects with background impurities.     

Deep-level optical spectroscopy of ZnTe

The variation of the deep-level spectrum of stoichiometric ZnTe in the various stages of its purification and annealing in saturated Zn vapor has been studied by low-temperature photoluminescence and IR Fourier spectroscopy. The relation between the concentration of the main residual impurities with complex formation probability is analyzed. We have succeeded in observing for the first time Z center emission in ZnTe, as previously in CdTe and ZnSe. This center is shown to be a multicharged impurity in ZnTe. The activation energies of these levels have been determined. A comparison of the data obtained by chemical analysis with optical spectra has led to a conclusion that this center is associated with isolated oxygen present on the metalloid sublattice. While this emission exhibits the same specific features in a number of II–VI compounds (a high recombination rate, narrow emission lines, extremely weak electron-phonon coupling), the positions of the levels in the band gap and the characteristic charge state distinguish ZnTe from CdTe and ZnSe. As a rule, the Z center forms in a material in decomposition of various complexes (for instance, of the complex responsible for the 1.65-eV emission in ZnTe) and disappears when the material is doped leading to formation of the same complexes. An assumption is put forward that this center creates the main compensating deep levels and is an essential component of easily forming complexes with impurities. Its position in the lower half of the ZnTe band gap (in contrast to ZnSe and CdTe) makes preparation of the n material difficult. Fiz. Tverd. Tela (St. Petersburg) 40, 1010–1017 (June 1998)     

Photoluminescence of ZnTe crystals grown under deviation from thermodynamic equilibrium

Crystalline textured and columnar structures, as well as needlelike ZnTe single crystals, were grown from the vapor phase and in a tellurium melt under deviation of the growth conditions from equilibrium. Low-temperature photoluminescence and x-ray structural studies showed the samples thus grown to exhibit high structural perfection, a uniform impurity distribution, and weak interaction between impurities and defects of the crystal structure. Polariton scattering from neutral donors was detected in structures having a noticeable residual concentration of donors in the substituted state. It is shown that the spectrum of samples grown under nonequilibrium conditions exhibits transitions that are not typical of equilibrium crystals. Measurements of the luminescence spectra as a function of temperature, excitation level, and annealing conditions made it possible to draw tentative conclusions about the nature of these transitions.     

Changes in the photoluminescence spectrum near twin boundaries in ZnTe crystals produced by rapid crystallization

{111} ZnTe crystals with various densities of twin boundaries in the growth direction were produced at ～670°C by the chemical vapor deposition method with the vapor environment offset toward an excess of Zn. Defects are formed in conical crystallites (up to 5 mm in height and with lateral dimensions of 10–500 μm at the bottom and up to 2 mm at the top) due to instabilities in the crystallization front, which arise because of convection-type heat and mass exchange in the oversaturated vapor medium. The influence of twin boundaries on the distribution of chemical impurities and the electronic spectrum of ZnTe was studied using x-ray diffractometry, scanning electron microscopy, and low-temperature photoluminescence (PL). It is found that rapid low-temperature growth of [111] ZnTe polycrystals from the vapor phase with an excessive Zn content favors the intensive formation of rotation and reflection twins. The incoherent [111] boundary of reflection twins is conductive to the separation and accumulation of impurities. In the regions of a crystal with a high density of reflection twins, exciton-impurity complexes (I _C , I _X ) and a Y strip, which is usually related to extended defects (dislocations, twins, crystallite boundaries), are found in the low-temperature PL spectra. Additional studies show that I _X is related to excitons trapped by neutral isoelectronic or charged defects and that I _C is probably due to an impurity of group IV of the Periodic Table.     

Propagation of nonequilibrium acoustic phonons in high-purity coarse-grained ZnTe

Propagation of nonequilibrium acoustic phonons in coarse-grained ZnTe obtained through vacuum sublimation was studied using the heat pulse method under both optical and metallic-heater phonon generation. The phonon mean free paths in the samples studied were shown to be 14 µm and to be dominated by scattering from twin boundaries.     

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 (Ga_Cd and Cl_Te 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 Ga_Cd donor.     

Laser Nanostructuring and Three-Dimensional Ink-Jet Printing of Nanocoatings

High-production laser ablation is used to prepare nano-ink that is colored colloidal solutions of nanoparticles in water, characterized by a variety of physicochemical properties. Using the nano-ink, singleand multi-layered nanocoatings are printed by a jet technique and studied in detail.