Electrical and galvanomagnetic properties of cadmium telluride films synthesized under highly nonequilibrium conditions

The results of experimental studies of electrical and galvanomagnetic properties of CdTe films synthesized under highly nonequilibrium conditions via vapor condensation on a substrate cooled with liquid nitrogen are reported. The temperature dependences of dark conductivity, current-voltage characteristics with and without illumination, temperature dependences of the Hall coefficient R _H and effective Hall mobility μ_H in the planar geometry, and dark current-voltage characteristics in the sandwich geometry are reported. Anisotropy of conductivity is revealed. It is shown that the electrical and galvanomagnetic properties of the films are consistently described by a percolation model of charge transport, according to which, at high temperatures, the charge transport takes place over the percolation level of the valence band, and at low temperatures, over the percolation level of the impurity band.     

Mechanism of nucleation of oriented cadmium telluride films forming under highly nonequilibrium conditions

The results of electron microscopy and sensor studies of nucleation of oriented CdTe films synthesized under highly nonequilibrium conditions on the substrate cooled to T _s = 228 K are reported. The photomicrographs of the surface, the distribution functions of the islands of a new phase at the initial stage, and the kinetic curves of the sensor analysis obtained during the synthesis are presented. It is shown that the experimental results can be noncontradictorily accounted for by the spinodal evolution character of the population of adatoms.     

Photoelectric properties of a sandwich structure composed of films synthesized under highly nonequilibrium conditions

Results of an experimental study of the photoelectric properties of a ZnSe/CdTe/(ZnTe)_{1 ? x} (In₂Te₃) _x sandwich structure synthesized under highly nonequilibrium conditions by vapor condensation onto a substrate cooled with liquid nitrogen are reported. Dark current-voltage characteristics and dependences of the “dark” and “light” currents on the synthesis mode, annealing time, and solid-solution layer composition are presented. The properties of structures synthesized under highly nonequilibrium and quasi-equilibrium conditions are compared.     

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. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 11, 2003, pp. 1298–1302. Original Russian Text Copyright ? 2003 by Ushakov, Klevkov.     

Effect of highly nonequilibrium conditions on the stoichiometry of cadmium telluride layer obtained by vapor-phase condensation

The results of comparison studies of the stoichiometry of cadmium telluride epitaxial films synthesized from the vapor phase in equilibrium and highly nonequilibrium (on cooled substrates) conditions are reported. The data of X-ray microanalysis and atomic-absorption analysis of the films and the vapor phase are given. The stoichiometry of the films is found to increase with lowering the substrate temperature. It is shown that the films obtained in highly nonequilibrium conditions have the best crystal structure.     

Structure of cadmium selen-telluride alloy films grown by the thermal-screen method under highly nonequilibrium conditions

The results of technological experiments and structural investigations of films of CdSe _x Te_{1 ? x} alloys synthesized by the thermal-screen method on heated and cooled substrates (under highly nonequilibrium conditions) are presented. It is shown that the synthesis of the entire range of compositions of alloy films with the structure from epitaxial to amorphous is possible from the mechanical mixture of CdSe and CdTe powders of the same composition under highly nonequilibrium conditions. The electron diffraction patterns and the microphotographs of film surfaces are reported.     

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.     

Electrical properties of cadmium telluride films synthesized in a thermal field with a temperature gradient

The correlation between the electrical properties of CdTe films synthesized in a thermal field with a temperature gradient and their crystal structure, the ambient pressure, and the film thickness was investigated. Films of different structure, from polycrystalline to block, were studied. The conductivity of the films increases with improving their crystal structure, increasing the ambient pressure, and decreasing the film thickness. The experimental results can be satisfactorily explained in the context of a model of an inhomogeneous semiconductor with intercrystalline barriers.     

Effect of annealing in liquid cadmium upon photoluminescence in polycrystalline cadmium telluride grown under nonequilibrium conditions

The effect of annealing in liquid Cd at a temperature of 600°C on the photoluminescence spectra of polycrystalline CdTe produced under conditions of rapid crystallization is demonstrated. It is shown that the annealing-induced redistribution of point defects completely suppresses the emission attributed to nonstandard acceptors with activation energies of ～48, ～98, and ～120 meV and observed in the luminescence spectra of as-prepared crystals and radically modifies the emission structure in the range 1.2–1.35 eV, which corresponds to extended defects caused by twinning. In the photoluminescence spectra of the annealed poly-crystals, the emission related to exciton-impurity complexes involving hydrogen-like donors and CuCd acceptors is dominant. A correlation between the concentration of extended defects and the intensity of long-wavelength emission (in the range 0.8–1.2 eV) is established.     

Switching effect in Si-CdS heterojunctions synthesized in highly nonequilibrium conditions

The results of the investigation of electrical properties and processes of the formation of heterojunctions on crystalline Si substrate cooled to negative centigrade temperatures is reported. The data of technology, electron diffraction analysis, and electrical investigations are given. The effect of conductivity switching is revealed in heterojunctions based on two-phase CdS films, and conditions for the formation of amorphous CdS films and amorphous CdS films with crystalline inclusions are determined. It is demonstrated that the results are in agreement with the soliton model.     

Evolution of the density of states during phase transitions in films of cadmium sulfotellurides synthesized under profoundly nonequilibrium conditions

The results of structural, electrical, and photoelectric investigations of phase transitions in layers of cadmium-sulfide and cadmium-telluride solid solutions synthesized under profoundly nonequilibrium conditions are reported. As a model process, the thermally activated decomposition of a CdS_xTe_1?x single-phase solution containing the α₁ (x ₁=0.4–0.6) phase was chosen. Increasing the temperature was shown to result in the formation a heterophase system with two phases α₂ (x ₂=0.1–0.2) and α₃ (x ₃=0.78–0.99). The electron diffraction patterns, the photoelectric-current spectrum, and the temperature dependences of the dark electrical conductivity in the course of and after phase transitions are reported. The results of electrical and photoelectric investigations are compared with the results of structural investigations. Changes in the dark conductivity in the course of phase transitions are shown to simultaneously account for the evolution of the density of states near the Fermi level.     

Microphotoluminescence of undoped cadmium telluride grown by nonequilibrium direct synthesis from the flow of components’ vapors

Properties of undoped CdTe obtained by nonequilibrium direct synthesis from the flow of components’ vapors are studied by the method of microphotoluminescent spectral analysis and imaging. In spite of an appreciable increase in the crystallization rate, the high intensity of the edge band with resolution of spectral features within the contour of this band at temperatures ～ 100 K is indicative of the high quality of the crystalline material with the sizes of single-crystal grains as large as 0.5 mm. The form of luminescence maps of the samples under study resembled that of the crystals grown by quasi-equilibrium methods. However, the mechanism of segregation of impurities at the intergrain boundaries in the crystal structures under study was not related to diffusion; rather, it had another nature.     

The effect of laser radiation on the formation of oriented cadmium sulfide layers under highly nonequilibrium conditions

The effect of low-power laser radiation on the formation of oriented cadmium sulfide layers from a vapor phase on a substrate cooled with liquid nitrogen (highly nonequilibrium conditions) is studied. The results of technological experiments, the results of a study of electron diffraction (electron diffraction patterns), and condensation diagram data are reported. It is found that, depending on the substrate temperature, laser radiation can both improve and worsen the crystal quality of films. It is shown that a condensation diagram of cadmium sulfide layers formed on a substrate exposed to low-power laser radiation is shifted to higher temperatures relative to a condensation diagram obtained without laser radiation. The experimental results are interpreted in the context of a solitonic heteroepitaxy model.     

Influence of cadmium concentration on the optical and structural properties of cadmium selenide thin films

Thin films of cadmium selenide (CdSe) and Cd_xSe_100−x (x=54, 34) have been deposited by vacuum evaporation onto ultraclean glass substrates at room temperature from as-prepared powders. Fabricated samples were characterized using X-ray diffraction (XRD), UV–vis and Fourier transform infrared (FTIR) spectroscopy. XRD indicated the formation of polycrystalline Cd_xSe_100−x thin films polycrystalline in nature with the preferred orientation along the (002) plane. The crystallite sizes of thin films calculated by the Scherer formula were found to be in the range of 29–82 nm. The crystallinity of thin films degraded on increasing the Cd concentration in the Cd–Se system. The band gaps of thin films were obtained from their optical absorption spectra, which were found in the range of 1.69–2.20 eV. The band gap of the Cd₃₄Se₆₆ thin films was found to be very high because of the decrease of their crystallites sizes in comparison to the CdSe and Cd₅₄Se₄₆ thin films. From the FTIR spectra it was revealed that the Cd–Se peaks shifted to lower wavelengths with increase in selenium concentration. The SEM measurements for CdSe, Cd₅₄Se₄₆ and Cd₃₄Se₆₆ thin films reveal that the particle size and the crystalline nature decreased when Se content increased in the system.     

Effect of Ar ion irradiation on structural and optical properties of e-beam evaporated cadmium telluride thin films

CdTe thin films were prepared using e-beam evaporation technique. The prepared films were irradiated by Ar⁺ ions at different fluencies using multipurpose aluminum (Al) probe as in-situ. This could also be used in ion bombardment for cleaning the substrate prior to coating. The as grown and Ar⁺ ion irradiated films were confirmed to be of polycrystalline nature with X-ray technique. Ar⁺ ion irradiation enhances the growth of (1 1 1) oriented CdTe crystals and the Cd enrichment on the surface of CdTe thin films. Higher Ar⁺ ion flux helps to grow (2 2 0) oriented CdTe thin film. A considerable change in structural parameters like crystallite size, lattice parameter, internal strain, etc. could be observed as a result of high Ar⁺ ion flux. The applied in-plan stress in both as grown and irradiated film was identified to be of tensile nature. The applied stress was observed between 0.016 and 0.067 GPa for all Ar⁺ ion irradiated samples. As a result of the Ar⁺ ion irradiation, the in-plan stress varies between 1.38×10⁹ and 5.58×10⁹ dyn/cm². The observed bad gap was increased for higher Ar⁺ ion flux. It shows the effect of Ar⁺ ion irradiation on the modifications of optical properties. The observed results were encouraging on the use of simple multipurpose Al probe for Ar⁺ ion irradiation process as in-situ.     

Formation of an interphase boundary under highly nonequilibrium conditions

The results of comparison studies of the CdTe-CdS interphase boundary in Au/CdTe/CdS sandwich structures synthesized on a substrate of artificial fluorophlogopite mica in highly nonequilibrium conditions (with a substrate temperature T _s = 125 K) and in quasi-equilibrium conditions (T _s > 720 K) are reported. The X-ray diffraction patterns and a capacitance-voltage characteristic are also reported. It is shown that highly nonequilibrium conditions allow synthesis of structures with excellent crystalline quality and with an interphase boundary that is no worse than in the structures grown under equilibrium conditions.     

Self-diffusion of cadmium in cadmium telluride annealed in tellurium-saturated atmosphere

Diffusion behavior of Cd in volume and along dislocations in high-purity CdTe annealed in Te-saturated atmosphere has been studied by the radioactive tracer method with a serial ion-beam sputter-microsectioning technique. The temperature dependence of volume diffusion coefficients shows a bend around 773 K, whereas that of the self-diffusion along dislocations shows a straight line. This suggests that the defect induced by impurities enhances the volume diffusion but does not affect the diffusion along dislocations.     

Influence of pH of spray solution on optoelectronic properties of cadmium oxide thin films

Highly conducting transparent cadmium oxide thin films were prepared by the conventional spray pyrolysis technique. The pH of the spray solution is varied by adding ammonia/hydrochloric acid. The effect of pH on the morphology, crystallinity and optoelectronic properties of these films is studied. The structural analysis showed all the films in the cubic phase. For the films with pH < 7(acidic condition), the preferred orientation is along the(111) direction and for those with pH >7(alkaline condition), the preferred orientation is along the(200) direction.A lowest resistivity of 9.9104 cm(with carrier concentration D5.11020cm3, mobility D12.4 cm2/(V s))is observed for pH12. The resistivity is tuned almost by three orders of magnitude by controlling the bath pH with optical transmittance more than 70%. Thus, the electrical conductivity of CdO films could be easily tuned by simply varying the pH of the spray solution without compromising the optical transparency.     

Transport properties of heteroepitaxial films based on bismuth telluride in strong magnetic fields

The temperature and magnetic-field dependences of the galvanomagnetic properties of n-Bi₂Te₃ heteroepitaxial films in magnetic fields to 14 T at low temperatures are studied. It is shown that steps in the magnetic-field dependences of the quantum Hall effect and the plateau in the temperature dependences of the magnetoresistance of films are caused by topological surface states of Dirac fermions.     

Electrical and morphological properties of CdTe films synthesized by the method of molecular deposition

Films of cadmium telluride are synthesized by molecular deposition on the substrates made of graphite, mica, and Si. Homogeneous photosensitive layers with the area 65 cm² and thickness from 0.5 to 5 μm and hole concentration of 6.3 × 10¹⁶ cm^?3 (300 K) are obtained.