Mechanism of the current flow in Pd-(heavily doped p-AlxGa1−x N) ohmic contact

The physical mechanism of the current flow in Pd-(heavily doped p-Al_xGa_1?x N) ohmic contact is studied. Chloride-hydride epitaxy was used to grow the p-Al_0.06Ga_0.94N solid solution with uncompensated acceptor concentration N _a –N _d ranging from 3×10¹⁸ up to 10¹⁹ cm^?3. Thermal vacuum deposition and subsequent thermal treatment were used to form an ohmic Pd contact. It is shown that, after the thermal treatment, the Pd-p-Al_0.06Ga_0.94N barrier contact with a potential barrier height of about 2.3 V becomes ohmic and the barrier height decreases to approximately 0.05 V. For uncompensated acceptor concentration N _a –N _d =3×10¹⁸ cm^?3, thermionic emission is found to be the main mechanism of the current through the Pd-p-Al_0.06Ga_0.94N ohmic contact. An increase in N _a –N _d to approximately 10¹⁹ cm^?3 in the solid solution leads to a transition from thermionic emission (at high temperatures) to tunneling (at low temperatures).     

Interaction of infrared radiation with free carriers in mesoporous silicon

Features of absorption and reflection of infrared radiation in the range 500–6000 cm^?1 are investigated; these features are associated with free carriers in the layers of mesoporous Si (porosity, 60–70%) formed in single-crystal p-Si(100) wafers with a hole concentration of N _p ≈10²⁰ cm^?3. It is found that the contribution of free holes to the optical parameters of the samples decreases as the porosity of the material increases and further falls when the samples are naturally oxidized in air. The experimental results are explained in the context of a model based on the Bruggeman effective medium approximation and the Drude classical theory with a correction for additional carrier scattering in silicon residues (nanocrystals). A comparison between the calculated and experimental dependences yields a hole concentration in nanocrystals of N _p ≈10¹⁹ cm^?3 for as-prepared layers and shows a reduction of N _p when they are naturally oxidized.     

Method for optimizing the parameters of heterojunction photovoltaic cells based on crystalline silicon

An approach is proposed to calculate the optimal parameters of silicon-based heterojunction solar cells whose key feature is a low rate of recombination processes in comparison with direct-gap semiconductors. It is shown that at relatively low majority-carrier concentrations (N_d ～ 10¹⁵ cm^–3), the excess carrier concentration can be comparable to or higher than N_d. In this case, the efficiency η is independent of N_d. At higher N_d, the dependence η(N_d) is defined by two opposite trends. One of them promotes an increase in η with N_d, and the other associated with Auger recombination leads to a decrease in η. The optimum value N_d ≈ 2 × 10¹⁶ cm^–3 at which η of such a cell is maximum is determined. It is shown that maximum η is 1.5–2% higher than η at 10¹⁵ cm^–3.     

Conductivity of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>–GaAs Heterojunctions

The effect of annealing in argon at temperatures of T_an = 700–900°C on the I–V characteristics of metal–Ga₂O₃–GaAs structures is investigated. Samples are prepared by the thermal deposition of Ga₂O₃ powder onto GaAs wafers with a donor concentration of N _d = 2 × 10¹⁶ cm^–3. To measure theI–V characteristics, V/Ni metal electrodes are deposited: the upper electrode (gate) is formed on the Ga₂O₃ film through masks with an area of S _k = 1.04 × 10^–2 cm² and the lower electrode in the form of a continuous metallic film is deposited onto GaAs. After annealing in argon at T_an ≥ 700°C, the Ga₂O₃-n-GaAs structures acquire the properties of isotype n-heterojunctions. It is demonstrated that the conductivity of the structures at positive gate potentials is determined by the thermionic emission from GaAs to Ga₂O₃. Under negative biases, current growth with an increase in the voltage and temperature is caused by field-assisted thermal emission in gallium arsenide. In the range of high electric fields, electron phonon-assisted tunneling through the top of the potential barrier is dominant. High-temperature annealing does not change the electron density in the oxide film, but affects the energy density of surface states at the GaAs–Ga₂O₃ interface.     

Features of the energy spectrum and hole-scattering mechanisms in PbSb2Te4

The kinetic Hall R _ikl , electrical-conduction σ _kk , thermopower α _kk coefficients, and their anisotropy are investigated in the temperature range of 77–450 K for a series of copper-doped PbSb₂Te₄ crystals with various hole concentrations of (1.6–3.2) × 10²⁰ cm^?3. The thermopower anisotropy observed in all investigated crystals is indicative of a mixed hole-scattering mechanism. The main scattering mechanisms are scattering at acoustic phonons and at the Coulomb potential of impurities and defects. The experimental effective scattering parameters and partial mobilities are determined in the cleavage plane and along the trigonal axis. The theoretical estimates of mobility are in satisfactory agreement with the experiment. It is established that the character of the temperature dependence for the ratio α _kk /T = f(T) depends on the hole concentration. It is shown that the physical properties can be described within the framework of the one-band model for a crystal with p _min = 1.6 × 10²⁰ cm^?3; the value of the effective mass density of the hole state in the main-valence band extremum is estimated as m _d ≈ 0.5m ₀. The energy gap between the valence-band extrema is estimated as ΔE _v ≈ 0.23 eV.     

Synthesis,characterization and photocatalytic behavior of mesoporous ZnS nanoparticles prepared by hybrid salt extraction and structure directing agent method

Mesoporous ZnS nanoparticles have been synthesized using hybrid salt extraction and surfactant assisted approach. Synthesis procedures were conducted in ethanol media under ultrasonic irradiation. As a by-product of the synthesis reaction, NaNO₃, has a very low solubility in ethanol and acts as a template for porosity formation in the final solid structure. In addition, various structure directing agents, namely, CTAB, DDA, F127 and SDS have been used as soft templates for further porosity formation. Characterization of the synthesized specimens was performed using Transmission Electron Microscopy (TEM), X-ray diffraction (XRD), N₂-physisorption, Furrier Transformation Infrared Spectroscopy (FTIR) and Diffuse Reflectance Spectroscopy (DRS). Results show that the by-product has the major contribution on the specific surface area of the obtained samples. It was found that MP-ZnS synthesized by DDA surfactant has the highest values of surface area (261 m² g⁻¹), pore volume (0.43 cm³ g⁻¹) and average pore diameter amongst all of the samples under study. Studies on adsorption efficiency and photocatalytic behavior of the synthesized specimens were conducted for methylene blue removal from aqueous solution. Results revealed that the MP-ZnS sample synthesized using CTAB surfactant has the best total degradation efficiency amongst the samples under study. Findings were explained using optical band gap energy values (deduced from DRS spectra), the intensity of C-H bond vibrations in FTIR spectra, water droplet contact angle, average pore volume and diameter and also specific surface area from N₂-physisorption data. At the end a simple mechanism was proposed in order to explain the main phenomena leading to photocatalytic removal of MB from aqueous solutions.     

High-temperature nuclear-detector arrays based on 4 <Emphasis Type="Italic">H</Emphasis>-SiC ion-implantation-doped <Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-<Emphasis Type="Italic">n</Emphasis> junctions

Results obtained in a study of spectrometric characteristics of arrays of four detectors based on 4H-SiC ion-implantation-doped p ⁺-n junctions in the temperature range 25–140 °C are reported for the first time. The junctions were fabricated by ion implantation of aluminum into epitaxial 4H-SiC layers of thickness ≤45 μm, grown by chemical vapor deposition with uncompensated donor concentration N _d ? N _a = (4–6) × 10¹⁴ cm^?3. The structural features of the ion-implantation-doped p ⁺-layers were studied by secondary-ion mass spectrometry, transmission electron microscopy, and Rutherford backscattering spectroscopy in the channeling mode. Parameters of the diode arrays were determined by testing in air with natural-decay alpha particles with an energy of 3.76 MeV. The previously obtained data for similar single detectors were experimentally confirmed: the basic characteristics of the detector arrays, the charge collection efficiency and energy resolution, are improved as the working temperature increases.     

Highly Ordered Mesoporous Silicon Carbide Ceramics with Large Surface Areas and High Stability

Highly ordered mesoporous silicon carbide ceramics have been successfully synthesized with yields higher than 75 % via a one‐step nanocasting process using commercial polycarbosilane (PCS) as a precursor and mesoporous silica as hard templates. Mesoporous SiC nanowires in two‐dimensional (2D) hexagonal arrays (p6m) can be easily replicated from a mesoporous silica SBA‐15 template. Small‐angle X‐ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images show that the SiC nanowires have long‐range regularity over large areas because of the interwire pillar connections. A three‐dimensional (3D) bicontinuous cubic mesoporous SiC structure (Ia3d) can be fabricated using mesoporous silica KIT‐6 as the mother template. The structure shows higher thermal stability than the 2D hexagonal mesoporous SiC, mostly because of the 3D network connections. The major constituent of the products is SiC, with 12 % excess carbon and 14 % oxygen measured by elemental analysis. The obtained mesoporous SiC ceramics are amorphous below 1200 °C and are mainly composed of randomly oriented β‐SiC crystallites after treatment at 1400 °C. N₂‐sorption isotherms reveal that these ordered mesoporous SiC ceramics have high Brunauer–Emmett–Teller (BET) specific surface areas (up to 720 m² g^–1), large pore volumes (～ 0.8 cm³ g^–1), and narrow pore‐size distributions (mean values of 2.0–3.7 nm), even upon calcination at temperatures as high as 1400 °C. The rough surface and high order of the nanowire arrays result from the strong interconnections of the SiC products and are the main reasons for such high surface areas. XRD, N₂‐sorption, and TEM measurements show that the mesoporous SiC ceramics have ultrahigh stability even after re‐treatment at 1400 °C under a N₂ atmosphere. Compared with 2D hexagonal SiC nanowire arrays, 3D cubic mesoporous SiC shows superior thermal stability, as well as higher surface areas (590 m² g^–1) and larger pore volumes (～ 0.71 cm³ g^–1).     

Electrical and thermoelectric properties of ZnO under atmospheric and hydrostatic pressure

Temperature (for T = 77–400 K) and pressure (for P ≤ 8 GPa) dependences of conductivity σ(T,P). Hall coefficient R _H(T, P), and Seebeck coefficient Q(T) were studied in single-crystal n-ZnO samples with the impurity concentration N _i = 10¹⁷ ? 10¹⁸ cm^?3 and free-electron concentration n = 10¹³?10¹⁷ cm^?3. Single crystals were grown by the hydrothermal method. Dependence of the ionization energy of a shallow donor level on the impurity concentration E _d1(N _d) is determined, along with the pressure coefficients for the ionization energy ?E _d1/?P and static dielectric constant ?x/?P. A deep defect level with the energy E _d2 = 0.3 eV below the bottom of the conduction band is found. The electron effective mass is calculated from the obtained data on the kinetic coefficients R _H(T) and Q(T).     

Characterization of TiOxNy nanoparticles embedded in HfOxNy as charge trapping nodes for nonvolatile memory device applications

Silicon-oxide–nitride-oxide–silicon devices with nanoparticles (NPs) as charge trapping nodes (CTNs) are important to provide enhanced performance for nonvolatile memory devices. To study these topics, the TiO_xN_y metal oxide NPs embedded in the HfO_xN_y high-k dielectric as CTNs of the nonvolatile memory devices were investigated via the thermal synthesis using Ti thin-film oxidized in the mixed O₂/N₂ ambient. Well-isolated TiO_xN_y NPs with a diameter of 5–20 nm, a surface density of ～3 × 10¹¹ cm^?2, and a charge trap density of around 2.33 × 10¹² cm^?2 were demonstrated. The writing characteristic measurements illustrate that the memory effect is mainly due to the hole trapping.     

Gallium-oxide films obtained by thermal evaporation

The current-voltage (I–V), capacitance-voltage (C–V), and conductance-voltage (G–V) characteristics of metal/Ga _x O _y /GaAs/metal structures are investigated. Gallium-oxide films with a thickness of 150–170 nm are deposited by the thermal evaporation of Ga₂O₃ powder onto n-type GaAs substrates with the donor concentration N _d = 2 × 10¹⁶ cm^?3. Treatment of the Ga _x O _y films in oxygen plasma causes a decrease in both the forward and reverse currents and a shift of the C–V and G–V curves to higher positive voltages. The Fermi level at the insulator/semiconductor interface in the structures under study is unpinned. The density of states at the Ga _x O _y /GaAs interface is N _t = (2–6) × 10¹² eV^?1 cm^?2.     

Silicon light-emitting diodes with strong near-band-edge luminescence

Electroluminescence (EL) in the region of interband transitions from silicon light-emitting diodes (LEDs) fabricated by cutting a solar cell with an area of 21 cm² and external quantum efficiency η_ext of EL up to 0.85% has been studied at room temperature. Despite the considerable decrease in η_ext because of the cutting and Auger recombination, record-breaking values of the total power emitted by a diode (up to W = 8 mW) and emitted power per unit area (up to P ₀ = 65 mW/cm²) were achieved at pulse currents of up to 10 A and structure areas in the range S = 0.1–0.9 cm². The EL decay kinetics was measured for LEDs with different areas. The emission pattern of a Si LED with a textured surface and the emission intensity distribution along different directions in the plane of the emitting area of the LED were measured.     

Effect of series resistance on the electrical characteristics and interface state energy distributions of Sn/p-Si (MS) Schottky diodes

In this study, electrical characteristics of the Sn/p-type Si (MS) Schottky diodes have been investigated by current-voltage (I-V) and capacitance-voltage (C-V) measurements at room temperature. The barrier height obtained from C-V measurement is higher than obtained from I-V measurement and this discrepancy can be explained by introducing a spatial distribution of barrier heights due to barrier height inhomogeneities, which are available at the nanostructure Sn/p-Si interface. A modified Norde’s function combined with conventional forward I-V method was used to extract the parameters including barrier height (Φ_b) and the series resistance (R_S). The barrier height and series resistance obtained from Norde’s function was compared with those from Cheung functions. In addition, the interface-state density (N_SS) as a function of energy distribution (E_SS-E_V) was extracted from the forward-bias I-V measurements by taking into account the bias dependence of the effective barrier height (Φ_b) and series resistance (R_S) for the Schottky diodes. While the interface-state density (N_SS) calculated without taking into account series resistance (R_S) has increased exponentially with bias from 4.235 × 10¹² cm⁻²eV⁻¹ in (E_SS - 0.62) eV to 2.371 × 10¹³ cm⁻²eV⁻¹ in (E_SS - 0.39) eV of p-Si, the N_SS obtained taking into account the series resistance has increased exponentially with bias from of 4.235 × 10¹² to 1.671 × 10¹³ cm⁻²eV⁻¹ in the same interval. This behaviour is attributed to the passivation of the p-doped Si surface with the presence of thin interfacial insulator layer between the metal and semiconductor.     

Optical and electrical properties of 4H-SiC irradiated with Xe ions

Structures with aluminum-ion-implanted p ⁺-n junctions formed in 26-μm-thick chemicalvapor-deposited-epitaxial 4H-SiC layers with an uncompensated donor concentration N _d ?N _a = (1–3) × 10¹⁵ cm^?3 are irradiated with 167-MeV Xe ions at fluences of 4 × 10⁹ to 1 × 10¹¹ cm^?2 and temperatures of 25 and 500°C. Then as-grown and irradiated structures are thermally annealed at a temperature of 500°C for 30 min. The as-grown, irradiated, and annealed samples are analyzed by means of cathodoluminescence, including the cross-sectional local cathodoluminescence technique, and electrical methods. According to the experimental data, radiation defects penetrate to a depth in excess of several tens of times the range of Xe ions. Irradiation of the structures at 500°C is accompanied by “dynamic annealing” of some low-temperature radiation defects, which increases the radiation resource of 4H-SiC devices operating at elevated temperatures.     

Violation of neutrality and occurrence of S-shaped current-voltage characteristic for doped semiconductors under double injection

It is shown that violation of quasi-neutrality and its subsequent recovery (with an increase in the current density) may occur in doped n layers of p ⁺-n-n ⁺ structures under double injection at a high injection level and for a certain combination of electrical parameters. The violation of quasi-neutrality leads to a significant increase in the voltage across the base and subsequent recovery of neutrality gives rise to sharp decrease in voltage drop, as a result of which an S-shaped current-voltage characteristic is formed. The characteristic threshold current density for this effect is proportional to the base doping level N _d .     

Effect of Dislocation-related Deep Levels in Heteroepitaxial InGaAs/GaAs and GaAsSb/GaAs <Emphasis Type="Italic">p</Emphasis>–<Emphasis Type="Italic">i</Emphasis>–<Emphasis Type="Italic">n</Emphasis> Structures on the Relaxation time of Nonequilibrium Carriers

The results of an experimental study of the capacitance–voltage (C–V) characteristics and deep-level transient spectroscopy (DLTS) spectra of p⁺–p⁰–i–n⁰ homostructures based on undoped dislocationfree GaAs layers and InGaAs/GaAs and GaAsSb/GaAs heterostructures with homogeneous networks of misfit dislocations, all grown by liquid-phase epitaxy (LPE), are presented. Deep-level acceptor defects identified as HL2 and HL5 are found in the epitaxial p⁰ and n⁰ layers of the GaAs-based structure. The electron and hole dislocation-related deep levels, designated as, respectively, ED1 and HD3, are detected in InGaAs/GaAs and GaAsSb/GaAs heterostructures. The following hole trap parameters: thermal activation energies (E _t ), capture cross sections (σ _p ), and concentrations (N _t ) are calculated from the Arrhenius dependences to be E _t = 845 meV, σ _p = 1.33 × 10^–12 cm², N _t = 3.80 × 10¹⁴ cm^–3 for InGaAs/GaAs and E _t = 848 meV, σ _p = 2.73 × 10^–12 cm², N _t = 2.40 × 10¹⁴ cm^–3 for GaAsSb/GaAs heterostructures. The concentration relaxation times of nonequilibrium carriers are estimated for the case in which dislocation-related deep acceptor traps are involved in this process. These are 2 × 10^–10 s and 1.5 × 10^–10 s for, respectively, the InGaAs/GaAs and GaAsSb/GaAs heterostructures and 1.6 × 10^–6 s for the GaAs homostructures.     

Properties of inversion layers for MIS/IL solar cells studied on low-temperature-processed MNOS transistors

For Si-nitride/thin Si-oxide/p-Si structures as used for MIS/IL solar cells, the inversion layer (IL) sheet resistance R_□ and the effective electron mobility μ_eff were determined as a function of the positive nitride charge density Q_N/q up to values > 10¹³ cm^?2. Special low-temperature-processed MNOS (metal-nitride-oxide-silicon) transistors with MIS contacts for source and drain were used for the measurements. The μ_eff values were ranging from 100 to 400 cm²/Vs, depending on Q_N and surface preparation. Despite the decrease of μ_eff with increasing nitride charge density, values of R_□ as low as $\text{2.2 kΩ/□}$ could be obtained. By oxidation of the Si surface at temperatures ≤600°C(d_ox ～ 1.8 nm) prior to the nitride deposition, the μ_eff values could be improved by 50% compared to those when the native oxide was present. Grain boundaries in polycrystalline Si (Silso) did not affect R_□. It could be shown that Coulomb scattering plays a minor role at high values of Q_N.     

Modification of Photovoltaic Laser-Power (λ = 808 nm) Converters Grown by LPE

Laser-power converters for the wavelength λ = 808 nm are fabricated by liquid-phase epitaxy (LPE) on the basis of n-Al_0.07GaAs–p-Al_0.07GaAs–p-Al_0.25GaAs single-junction heterostructures. The converters are tested with uniform (pulse simulator) and partly nonuniform (laser beam) illumination distribution over the photoreceiving surface. In the former case, a monochromatic efficiency of η = 53.1% is achieved for samples with an area of S = 4 cm² at a power of 1.2 W. At S = 10.2 mm² the efficiency is 58.3% at a laser power of 0.7 W.     

Investigation of the ZnS-CdHgTe interface

The ZnS-Cd_xHg_1?xTe interface was investigated using the capacitance-voltage characteristics of MIS structures in experimental samples. During fabrication of the n⁺-p junctions based on p-Cd_xHg_1?xTe, the density of states within the range N _ss =(1–6)×10¹¹ cm^?2 eV^?1 at T=78 K was obtained. The experiments showed that the conditions in which n⁺-p junctions are fabricated only slightly affect the state of the ZnS-CdHgTe interface. The negative voltages of the at bands V _FB , even if immediately after deposition of the ZnS films V _FB >0, point to the enrichment of the ZnS-p-CdHgTe near-surface layer with majority carriers, specifically, holes. This led to a decrease in the leakage current over the surface. During long-term storage (as long as ～15 years) in air at room temperature, no degradation of differential resistance R _d , current sensitivity S _i , and detectivity D* of such n⁺-p junctions with a ZnS protection film was observed.