Influence of microwave treatment on the electrophysical characteristics of technically important semiconductors and surface-barrier structures

An investigation is made of changes in the electrophysical parameters of narrow-gap (Cd_xHg_1−x Te x=0.22–0.24) and wide-gap (gallium arsenide, indium and gallium phosphides) semiconductor materials and Schottky-barrier diode structures based on these materials, stimulated by microwave electromagnetic radiation. It is shown that the parameters of materials and device structures may be improved by defect gettering. An analysis is made of possible mechanisms for the interaction between microwave radiation and the objects being studied. Zh. Tekh. Fiz. 68, 49–53 (December 1998)     

A spectroscopic study of CNx formation by the keV N2 irradiation of highly oriented pyrolytic graphite surfaces

Amorphous carbon nitride (CN_x) thin layer, formed by the keV N₂⁺ irradiation of highly oriented pyrolytic graphite, has been investigated using X-ray photoelectron and raman spectroscopies, and time-of-flight secondary ion mass spectrometry. C1s X-ray photoelectron spectroscopy (XPS) peak separations indicate that C-N bonds form over and above the graphite fragmentation previously obtained on Ar⁺ irradiation. N1s XPS peak separations indicate three components. Their attributions, and the resultant CN_x structure, are confirmed by angle-resolved XPS and TOF-SIMS analyses.     

The soft mode and phase transition in Pb1-xSnxTe

The ferroelectric phase transition critical temperature T_c and static lattice dielectric constant ?₀ have been calculated within the isotropic two-band model for a degenerate narrow-gap semiconductor. The carrier concentration dependence of T_c has been discussed and results compared with the experiments on Pb_1-xSn_xTe.     

Formation and decomposition of nitrides under ion bombardment

The chemical processes of formation and decomposition of narrow-gap nitrides InN and GaAs_{1 ? x} N _x under ion bombardment have been investigated by Auger electron spectroscopy. It is shown that, due to chemical instability, a large fraction of InN decomposes with formation of metal clusters under ion bombardment. It is established that bombardment of GaAs with a beam of N ₂ ⁺ and Ar⁺ ions makes it possible to obtain a chemically homogeneous GaAs_{1 ? x} N _x solid solution with a high nitrogen content (x = 0.1), whereas implantation of only N ₂ ⁺ ions leads to the formation of a mixture of GaN, GaAsN, and GaAs phases. It is concluded that secondary ion cascades, induced by heavy ions, stimulate nitridation reaction, homogenize the spatial atomic distribution, and shift the dynamic equilibrium to the formation of a single-phase solution.     

Catalyst-free and controllable growth of SiCxNy nanorods

Non vapor–liquid–solid (VLS) method of growing high-purity silicon carbon nitride (SiC_xN_y) nanorods with rod widths ranging from 10 to 60 nm and lengths of microns is reported. Unlike the case for the ordinary VLS or catalyst-mediated growth, the two-stage process presented here is a catalyst-free approach since it does not involve any catalyst during the growth of the nanorods. The first stage involves formation of a buffer layer containing various densities of SiC_xN_y nanocrystals by electron cyclotron resonance plasma enhanced chemical vapor deposition (PECVD); whereas the second stage involves a high growth rate along a preferred orientation to produce high-aspect-ratio nanorods using microwave PECVD. Moreover, the number density and the diameter of the nanorods can be controlled by the number density and the size of the nanocrystals in the buffer layer. Production of quasi-aligned SiC_xN_y nanorods with a number density of the rods as high as 10¹⁰ cm⁻² has been achieved. The SiC_xN_y nanorods thus produced exhibit good field emission characteristics with stable operation over 8 h. The approach presented here provides a new advance to synthesize nanorod materials in a controllable manner.     

Observation of helicon-excited electron paramagnetic resonance in a high-mobility semiconductor

Helicon waves can be exploited in EPR studies of localized magnetic moments in high-conductivity semiconductors when the free carriers have small effective mass and high mobility. In an electron plasma helicons have precisely the polarization required to elicit EPR, and the plasma background actually enhances the resonance intensity. Microwave transmission experiments on Hg_1-xMn_xTe, a narrow-gap semiconductor with localized Mn⁺⁺ magnetic moments, are described to illustrate this effect.     

Oxidation of vanadium nitride and titanium nitride coatings

The oxidation of vanadium nitride (VN) and titanium nitride (TiN) coatings in ultra-high vacuum has been investigated in situ by X-ray photoelectron spectroscopy. On the VN coatings mixed oxide layers containing V³⁺ and V⁴⁺ species form at elevated temperatures (?600°C) and at high oxygen exposures, which cover completely the VN surface. Under similar oxidation conditions the TiN surface oxidises partially to a mixture of TiO₂ and Ti oxynitride (TiO_xN_y) phases. This oxidation behaviour has been correlated to the tribological properties of the VN and TiN coatings investigated recently.     

Interface property of silicon nitride films grown by inductively coupled plasma chemical vapor deposition and plasma enhanced chemical vapor deposition on In0.82Al0.18As

Au/silicon nitride/In_0.82Al_0.18As metal insulating semiconductor (MIS) capacitors were fabricated and then investigated by capacitance voltage (C–V) test at variable frequencies and temperatures. Two different technologies silicon nitride (SiN_x) films deposited by inductively coupled plasma chemical vapor deposition (“ICPCVD”) and plasma enhanced chemical vapor deposition (“PECVD”) were applied to the MIS capacitors. Fixed charges (N_f), fast (D_it) and slow (N_si) interface states were calculated and analyzed for the different films deposition MIS capacitors. The D_it was calculated to be 4.16 × 10¹³ cm⁻² eV⁻¹ for “ICPCVD” SiN_x MIS capacitors, which was almost the same to that of “PECVD” SiN_x MIS capacitors. The D_it value is obviously higher for the extended wavelength In_xGa_1−xAs (x > 0.53) epitaxial material as a result of lattice mismatch with substrate. Compared to the results of “PECVD” SiN_x MIS capacitors, the N_si was significantly lower and the N_f was slightly lower for “ICPCVD” SiN_x MIS capacitors. X-ray photoelectron spectroscopy (XPS) analysis shows good quality of the “ICPCVD” grown SiN_x. The low temperature deposited SiN_x films grown by “ICPCVD” show better effect on decreasing the dark current of In_xGa_1−xAs photodiodes.     

Laser synthesis of nitrides on the surface of refractory metals immersed in liquid nitrogen

In this paper we describe an approach for the formation of composite layers on the surface of refractory metals. We show that laser radiation on refractory metals (Ti, V, Zr, Mo, Hf, Ta, and W) immersed in liquid nitrogen can provide a chemical synthesis of nitride phases on the surface of metals. The metals were subjected to pulsed laser radiation with a wavelength of 1.06 μm. The power density ranged from 10⁴ to 10⁹ W cm⁻². The synthesis of nitrides began with the formation of Me_xN_y (x > y) phases with low contents of nitrogen. When the melting point was reached at the metal surface, the quantity of MeN phases increased sharply. Study of the melting zone showed that it contained a non-uniform distribution of nitride phases. The quantity of nitrides was a maximum on the surface and decreased with the increase of the depth of melting zone. Due to the high-cooling rates, titanium nitride crystallized in the form of columns. Maximum microhardness in the Ti surface layer was up to 20,000 MPa.     

Effect of p-μc-Si1−xOx:H layer on performance of hetero-junction microcrystalline silicon solar cells under light concentration

Preparation of p-type hydrogenated microcrystalline silicon oxide thin films (p-μc-Si_1−xO_x:H) by 13.56 MHz RF-PECVD method for use as a p-layer of hetero-junction μc-Si:H solar cells is presented. We investigated effects of wide-gap p-μc-Si_1−xO_x:H layer on the performance of hetero-junction μc-Si:H solar cells under various light intensity. We observed that a wide-gap p-μc-Si_1−xO_x:H was effective in improving the open-circuit voltage (V_oc) of the solar cells. We also confirmed that the V_oc logarithmically increased with increasing light intensity, and the enhancement of V_oc became larger with increasing band gap of p-layer. These results indicate that wide-gap p-μc-Si_1−xO_x:H is a promising material for use as window layer in hetero-junction μc-Si:H solar cells.     

Phase transformations of Fe3N-Fe4N iron nitrides at pressures up to 30 GPa studied by in situ X-ray diffractometry

The parameters of the equation of state of the stable ?-Fe₃N _x (where x = 0.8) nitride in the Fe-N system have been determined at pressures up to 30 GPa and temperatures up to 1273 K. The parameters V ₀ = 81.48(2) Å^?3, K _T = 162(3) GPa, K _T = 4.0 = 1.66(2), γ₀ = 555 K, and q = 1 have been determined for ?-Fe₃N_0.8 by the approximation of the P-V-T data with the Vinet equation of state and the thermal parameters within the Mie-Grüneisen-Debye formalism. No anomalous change in the volume of the cell owing to possible magnetic transitions has been revealed. The instability of Fe₄N at high pressures and Fe₃N in the presence of a deficit of nitrogen in the system has been established. The stable phase in the temperature range of 300–673 K and the pressure range of 20–30 GPa is ?-Fe₃N rather than ?-Fe₃N_0.8.     

Structuring effect of heteroepitaxial CdHgTe/CdZnTe systems under irradiation with silver ions

The characteristics of a damaged layer of p-Cd _x Hg_{1 ? x} Te/CdZnTe (x ～ 0.223) heterostructures after implantation by 100-keV silver ions with the implantation dose Q = 3.0 × 10¹³ cm^?2 have been obtained using X-ray diffraction, atomic force microscopy, and electron microscopy. It has been found that, as a result of the ion implantation and subsequent annealing (75°C), a uniform array of nanostructures is formed on the surface of Hg(Cd)Te/Zn(Cd)Te samples. The X-ray diffraction patterns of the structurized Hg(Cd)Te/Zn(Cd)Te sample indicate the formation of polycrystalline Hg(Cd)Te phases of cubic structure with a composition x ～ 0.20 and also oxide Ag₂O in the subsurface (<100 nm) region of the host material. The observed effects of transformation of the defect-impurity system and structuring of the surface of the heteroepitaxial film of the low-energy-gap semiconductor have been explained using a deformation model.     

Influence of SiNx:H film properties according to gas mixture ratios for crystalline silicon solar cells

The Hydrogenated silicon nitride (SiN_x:H) using plasma enhanced chemical vapor deposition is widely used in photovoltaic industry as an antireflection coating and passivation layer. In the high temperature firing process, the SiN_x:H film should not change the properties for its use as high quality surface layer in crystalline silicon solar cells. For optimizing surface layer in crystalline silicon solar cells, by varying gas mixture ratios (SiH₄ + NH₃ + N₂, SiH₄ + NH₃, SiH₄ + N₂), the hydrogenated silicon nitride films were analyzed for its antireflection and surface passivation (electrical and chemical) properties. The film deposited with the gas mixture of SiH₄ + NH₃ + N₂ showed the best properties in before and after firing process conditions.The single crystalline silicon solar cells fabricated according to optimized gas mixture condition (SiH₄ + NH₃ + N₂) on large area substrate of size 156 mm × 156 mm (Pseudo square) was found to have the conversion efficiency as high as 17.2%. The reason for the high efficiency using SiH₄ + NH₃ + N₂ is because of the good optical transmittance and passivation properties. Optimized hydrogenated silicon nitride surface layer and high efficiency crystalline silicon solar cells fabrication sequence has also been explained in this study.     

Hall effect in the Fe<Subscript>x</Subscript>Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S magnetic semiconductors

The electrical properties and the Hall effect in the Fe_xMn_1?xS magnetic semiconductors (0<x≤0.5) have been experimentally studied in the range 77–300 K in magnetic fields of up to 15 kOe. The cation-substituted sulfides with 0.25≤x≤0.3 possessing colossal magnetoresistance (CMR) were established to be narrow-gap semiconductors with carrier concentrations n ～ 10¹¹–10¹⁵ cm^?3 and high carrier mobilities μ ～ 10²–10⁴ cm² V^?1 s^?1. It is believed that the CMR effect in these sulfides can be explained in terms of the model of magnetic and electron phase separation, which is analogous to the percolation theory in the case of heavily doped semiconductors.     

Structural and magnetic inhomogeneities,phase transitions, 55Mn nuclear magnetic resonance,and magnetoresistive properties of La0.6 − x Nd x Sr0.3Mn1.1O3-δ ceramics

The structure, lattice imperfection, and properties of ceramic samples La_{0.6 ? x} Nd _x Sr_0.3Mn_1.1O_3-δ (x = 0–0.4) have been investigated using the X-ray diffraction, resistive, magnetic (χ_ac, ⁵⁵Mn NMR), magnetoresistive and microscopic methods. It has been shown that there is a satisfactory agreement between the concentration decrease in the lattice parameters a of the rhombohedral (x = 0, 0.1, 0.2) and cubic (x = 0.3, 0.4) perovskite structures and the average ionic radii $\bar R$ for the lattice containing anion vacancies, cation vacancies, and nanostructured clusters with Mn²⁺ ions in A-positions. With an increase in the neodymium concentration x, the vacancy-type imperfection increases, the cluster-type imperfection decreases, the temperatures of metal-semiconductor phase transition T _ms and ferromagnetic-paramagnetic phase transition T _C decrease, and the content of the ferromagnetic phase decreases. The anomalous hysteresis is associated with the appearance of unidirectional exchange anisotropy induced in a clustered perovskite structure consisting of a ferromagnetic matrix and a planar antiferromagnetic cluster coherently coupled with it. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra has revealed a high-frequency electronic double exchange (Mn³⁺-O^2?-Mn⁴⁺) ? (Mn⁴⁺-O^2?-Mn³⁺) and an inhomogeneity of the magnetic and charge states of manganese due to the heterogeneous environment of the manganese ions by other ions and defects. The observed changes in the resonant frequency and width of the resonance curve are caused by changes in the ratio Mn³⁺/Mn⁴⁺ and magnetic inhomogeneity. An increase in the neodymium concentration x leads to a decrease in the ferromagnetic phase content determined from the dependences 4πNχ_ac(T) and the ⁵⁵Mn NMR curves. The phase diagram characterizes an interrelation between the composition, the imperfection of the structure, and the transport, magnetic, and magnetoresistive properties of lanthanum neodymium manganite perovskites. It has been found that there is a correlation between the imperfection, magnetic inhomogeneity, coercive force, and magnetoresistance effect exhibited by the perovskite structure.     

Luminescence properties of solid solutions of borates doped with rare-earth ions

The structural and luminescence properties of Lu _x Y_{1 ? x} BO₃ solid solutions doped with Ce³⁺ or Eu⁺³ have been investigated. It has been found that the solid solutions crystallize in the vaterite phase with a lutetium concentration x < 0.5. For a higher lutetium concentration x, the solid solutions contain an additional calcite phase with a content less than 5 wt %. The luminescence spectra are characterized by intensive impurity emission under excitation with the synchrotron radiation in the X-ray and ultraviolet spectral ranges. It has been shown that, as the lutetium concentration x in the Lu _x Y_{1 ? x} BO₃: Ce³⁺ solid solutions increases, the emission intensity smoothly decreases, which is associated with a gradual shift of the Ce³⁺ 5d(1) level toward the bottom of the conduction band, as well as with a decrease in the band gap. It has been established that, in the Lu _x Y_{1 ? x} BO₃: Eu³⁺ solid solutions with intermediate concentrations x, the efficiency of energy transfer to luminescence centers increases. This effect is explained by the limited spatial separation of electrons and holes in the solid solutions. It has been demonstrated that the calcite phase adversely affects the luminescence properties of the solid solutions.     

EXAFS of the superconducting oxide BaPb1−xBixO3

The two-valence state Bi³⁺ and Bi⁵⁺ of Bi in the semiconductor BaBiO₃ and in the BaPb_1?xBi_xO₃ (x = 0, 0.25, 0.6 and 1) system has been determined from the EXAFS spectra above the L_III-edges of Ba, Pb and Bi. Peaks in the radial distribution function up to 5 Å from the absorber have been identified through a model calculation using theoretical amplitudes and phase shifts and the interatomic distances from neutron diffraction measurements. We found indication of local disorder in the Pb-Bi sublattice for the mixed compound.     

Structural and magnetic studies of the alloy system GdInxAg1-x

The structural and magnetic studies below room temperature of the alloy system GdIn_xAg_1-x(0?x?1) are reported. It has been found that alloys with x?0.5 crystallize in CsCl type cubic structure, but the distortion of the unit cell starts for alloys with x#62;0.5 and for these alloys the unit cell becomes tetragonal. No phase change is evident from the low temperature (295-8 K) structural studies. However, a break appears in the X^-1_m vs. T linear plot of each alloy of this system at a specific temperature (designated as break temperature T_B). The variation of T_B with x is similar to the variation of phase transition temperature with x reported for LaIn_xAg_1-x. Close agreement has been found in the values of effective magneton number (p), magnetic ordering [Néel (T_N) or Curie (T_C)] and paramagnetic Curie (θ_p) temperature for materials studied by us and earlier workers. The variation of magnetization with applied field strength (2.5-65)×10⁵ Am^-1) at 4.2 K has also been reported for ferromagnetic of this system. It has been concluded that alloys with 0.4?x?0.6 are simple ferromagnets with parallel alignment of magnetic moment in the ground state. The angular arrangement of the magnetic moment starts appearing in the ground state for alloys with x?0.4 for x#62;0.6 and continues till x becomes closure to 0.17 or 0.84. The alloys with x=0.17 or x=0.84 have θ_p and T_C equal to zero and appear paramagnetic. Angular arrangement in spins again appear for alloys with x?0.17 or x#62;0.84, however all materials with 0?x?0.17 or 0.84?x?1 remain antiferromagnetic.     

Effect of pressure and anionic substitution on the electrical properties of HgTeS crystals

The resistivity ρ and the Hall constant R for the HgTe_1?x S_x (0.04≤x≤0.6) crystals have been investigated in the temperature range 4.2–350 K in the magnetic fields B up to 14 T. The pressure dependences of the resistivity ρ have been measured at the pressures P as high as 1 GPa at temperature T=77–300 K and magnetic field B=0–2 T. It is found that the samples with x≤0.20 exhibit a decreasing dependence ρ(T) typical of zero-gap semiconductors, whereas the samples with x≥0.27 show the dependence ρ(T) characteristic of semimetals. For the semiconducting crystals with x≈0.20 and x≈0.14, the temperature coefficient of ρ(T) changes sign at T=265 and T>300 K, respectively. Under a pressure of ≈1 GPa, the temperature of the sign inversion decreases by ≈30 K. An increase in the magnetic field B and a rise in the temperature T lead to a change in the sign of the Hall constant R for the semiconducting samples, but do not affect the electronic sign of R for the semimetallic samples. The behavior of R and ρ correlates with the thermoemf data obtained at the quasi-hydro-static pressure P up to 3 GPa. It is demonstrated that the substitution of sulfur atoms for tellurium atoms brings about an increase in the concentration of electrons and a decrease in their mobility. The transition to the wide-gap semiconductor phase is observed at P>1–1.5 GPa. The conclusion is drawn that the semimetallic crystals HgTe_1?x S_x with x≥0.27 and HgSe are similar in properties.