Electrical Properties of <Emphasis Type="Italic">p</Emphasis>-NiO/<Emphasis Type="Italic">n</Emphasis>-Si Heterostructures Based on Nanostructured Silicon

Silicon nanowires are formed on n-Si substrates by chemical etching. p-NiO/n-Si heterostructures are fabricated by reactive magnetron sputtering. The energy diagram of anisotype p-NiO/n-Si heterostructures is constructed according to the Anderson model. The current–voltage and capacitance–voltage characteristics are measured and analyzed. The main current-transport mechanisms through the p-NiO/n-Si heterojunction under forward and reverse biases are established.     

Electrical Characteristics of <Emphasis Type="Italic">n</Emphasis>-ZnO/<Emphasis Type="Italic">p</Emphasis>-Si Heterojunction Diodes Grown by Pulsed Laser Deposition at Different Oxygen Pressures

Heterojunction diodes of n-type ZnO/p-type silicon (100) were fabricated by pulsed laser deposition of ZnO films on p-Si substrates in oxygen ambient at different pressures. These heterojunctions were found to be rectifying with a maximum forward-to-reverse current ratio of about 1,000 in the applied voltage range of −5 V to +5 V. The turn-on voltage of the heterojunctions was found to depend on the ambient oxygen pressure during the growth of the ZnO film. The current density–voltage characteristics and the variation of the series resistance of the n-ZnO/p-Si heterojunctions were found to be in line with the Anderson model and Burstein-Moss (BM) shift.     

Specific features of intervalley scattering of charge carriers in <Emphasis Type="Italic">n</Emphasis>-Si at high temperatures

In n-Si, intervalley scattering of electrons can be of two types, f scattering and g scattering. With the purpose of establishing the contributions of f- and g-type transitions to intervalley scattering, the piezoresistance of n-Si crystals is studied in the temperature range T = 295–363 K. The initial concentration of charge carriers in the n-Si samples is 1.1 × 10¹⁴ cm⁻³, and the resistivity at 300 K is ρ = 30 Ω cm. As the temperature is increased, the region of leveling-off of the piezoresistance shifts to lower voltages. The characteristic feature of the dependence ρ = ρ(T) plotted in the double logarithmic coordinates (logρ = f(logT)) is the transition from the slope 1.68 to the slope 1.83 at T > 330 K. This is attributed to the substantial contribution of g transitions to intervalley scattering in the high-temperature region. For verification of the interpretation of the dependence ρ = ρ(T), the dependence is calculated on the basis of the theory of anisotropic scattering with consideration for intervalley transitions.     

Study of Current Flow Mechanisms in a CdS/<Emphasis Type="Italic">por</Emphasis>-Si/<Emphasis Type="Italic">p</Emphasis>-Si Heterostructure

The temperature dependence of the forward and reverse portions of the current–voltage characteristic and the photovoltage spectrum of a CdS/por-Si/p-Si semiconductor heterostructure are studied. It is found that the current-flow mechanisms are controlled by generation–recombination processes in the spacecharge region of the por-Si/p-Si heterojunction, carrier tunneling in the por-Si film, and the model of space-charge-limited currents. A simplified version of the energy-band diagram of the heterostructure under study is proposed.     

Anomalous scattering of electrons in <Emphasis Type="Italic">n</Emphasis>-Si crystals irradiated with protons

The aim of this study was to gain insight into the effect of irradiation with 25-MeV protons on the Hall mobility of electrons in n-Si crystals. Irradiated crystals with an initial electron concentration 6 × 10¹³ cm^-3 were studied using the Hall method in the range of temperatures 77–300 K. The studies showed that, in crystals irradiated with the proton dose Φ = 8.1 × 10¹² cm^-2, the effective mobility of conduction electrons μ_eff increases drastically. This effect is direct evidence that inclusions with relatively high conductivity and with nonrectifying junction at interfaces with semiconductor matrix are predominantly formed in n-Si crystals under these conditions. Agglomerations of interstitial atoms or their associations can represent such inclusions.     

Epitaxial Growth and Device Design Optimization of Full-Vertical GaN <Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">i</Emphasis>-<Emphasis Type="Italic">n</Emphasis> Rectifiers

The optimization of growth parameters, epitaxial structure, and device design for full-vertical gallium nitride (GaN) p-i-n rectifiers grown on n-type 6H-SiC substrates employing AlGaN:Si conducting buffer layers have been studied. The Al _x Ga_1−x N:Si (x = ~0.1) nucleation layer is calibrated to be capable of acting as a good buffer layer for subsequent GaN growth as well as to provide excellent electrical properties. Two types of full-vertical devices were fabricated and compared: one without any current guiding and the other with the current guiding in the p-layer. The reverse breakdown voltage for rectifiers with a relatively thin 2.5-μm-thick i-region without p-current guiding was found to be over −330 V, while one with p-current guiding was measured to be over −400 V. Devices with p-current guiding structures exhibit reduced reverse leakage current by an order of magnitude >4 at −100 V.     

Fabrication and photoelectric properties of <Emphasis Type="Italic">n</Emphasis>-ZnO:Al/Pd<Emphasis Type="Italic">Pc/p</Emphasis>-Si structures

Photosensitive n-ZnO:Al/PdPc/p-Si structures were fabricated by vacuum sublimation of palladium phthalocyanine with subsequent magnetron sputtering of ZnO:Al films on p-Si substrates. The current transport mechanisms and the photosensitivity of the structures obtained were investigated. It is shown that structures based on PdPc films are promising for photosensitive devices based on contacts between organic and inorganic semiconductors.     

Photovoltaic poperties of <Emphasis Type="Italic">n</Emphasis>-ZnO:Al/PbPc/<Emphasis Type="Italic">p</Emphasis>-Si structures

n-ZnO:Al/PbPc/p-Si photosensitive structures are fabricated for the first time. The steady-state current-voltage characteristics and spectral dependences of the relative quantum efficiency of the photoconversion of these structures are studied, and the mechanisms of charge transport and the photosensitivity processes are discussed. It is concluded that they are promising for application as multiband photoconverters of natural light.     

Au-Doped Indium Tin Oxide Ohmic Contacts to <Emphasis Type="Italic">p</Emphasis>-Type GaN

Indium tin oxide (ITO) thin films doped with Au, Ni, or Pt (3.5 at.% to 10.5 at.%) were deposited on p-GaN epilayers (Mg ~4 × 10¹⁹ cm⁻³) using direct-current (DC) sputter codeposition. It was found that undoped ITO con- tacts to p-GaN exhibited leaky Schottky behavior, whereas the incorporation of a small amount of Au (3.5 at.% to 10.5 at.%) significantly improved their ohmic characteristics. Compared with standard Ni/ITO contacts, the Au-doped ITO contacts had a similar specific contact resistance in the low 10⁻² Ω cm⁻² range, but were more stable above 600°C and more transparent at blue wavelengths. These results provide support for the use of Au-doped ITO ohmic contact to p-type GaN in high-brightness blue light-emitting diodes.     

HgCdTe <Emphasis Type="Italic">p</Emphasis>-on-<Emphasis Type="Italic">n</Emphasis> Focal-Plane Array Fabrication Using Arsenic Incorporation During MBE Growth

Extrinsic p-type doping during molecular-beam epitaxy (MBE) growth represents an essential generic toolbox for advanced heterostructures based on the HgCdTe material system: PiN diodes, mesa avalanche photodiodes (APD) or third-generation multispectral focal-plane arrays. Today, arsenic appears to be the best candidate to fulfill this role and our group is actively working on its incorporation during MBE growth, using an original radio frequency (RF) plasma source for arsenic. Such a cell is supposed to deliver a monatomic As flux, and as expected we observed high As electrical activation rates after annealing short-wave (SW), mid-wave (MW), and long-wave (LW) layers. At last, a couple of technological runs have been carried out in the MW range in order to validate the approach on practical devices. p-on-n focal-plane arrays (FPA) have been fabricated using a mesa delineated technology on an As-on-In doped metallurgical heterojunction layer grown on a lattice-matched CdZnTe layer (320 × 256, 30 μm pitch, 5 μm cutoff at 77 K). Observed diodes exhibit very interesting electro-optical characteristics: large shunt impedance, high quantum efficiency, and no noticeable excess noise. The resulting focal-plane arrays were observed to be very uniform, leading to high operabilities. Noise equivalent temperature difference (NETD) distributions are very similar to those observed with the As ion-implanted p-on-n technology, fabricated in our laboratory as well. In our opinion, those excellent results demonstrate the feasibility of our MBE in situ arsenic doping process. Good electrical activation rates and high-quality layers can be obtained. We believe that such an approach allows precise control of the p-doping profile in the HgCdTe layer, which is necessary for advanced structure designs.     

Electrical and gas sensing properties of <Emphasis Type="Italic">por</Emphasis>-Si/SnO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanocomposite layers

The electrical characteristics and chemical reactant sensitivity of layers of heterogeneous nanocomposites based on porous silicon and nonstoichiometric tin oxide por-Si/SnO _x , fabricated by the magnetron sputtering of tin with subsequent oxidation, are studied. It is shown that, in the nanocomposite layers, a system of distributed heterojunctions (Si/SnO _x nanocrystals) forms, which determine the electrical characteristics of such structures. The sensitivity of test sensor structures based on por-Si/SnO _x nanocomposites to NO₂ is determined. A mechanism for the effect of the adsorption of NO₂ molecules on the current-voltage characteristics of the por-Si(p)/SnO _x (n) heterojunctions is suggested.     

Practical Surface Treatments and Surface Chemistry of <Emphasis Type="Italic">n</Emphasis>-Type and <Emphasis Type="Italic">p</Emphasis>-Type GaN

X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) were used to study the electronic structure of n-GaN and p-GaN(0001) surfaces after three ex situ surface treatments: aq-HCl, annealing in NH₃, and annealing in HCl vapor. The combination of in situ vacuum annealing and re-exposure of samples to air revealed that the adsorption of ambient contaminants can reduce the surface state density and subsequent band bending on both n-GaN and p-GaN surfaces. Insights derived from first-principles calculations of the adsorption of relevant species on the Ga-terminated GaN(0001) surface are used to explain these experimental observations qualitatively.     

A DLTS study of 4<Emphasis Type="Italic">H</Emphasis>-SiC-based <Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">n</Emphasis> junctions fabricated by boron implantation

Deep-level transient spectroscopy (DLTS) has been used to study p-n junctions fabricated by implantation of boron into epitaxial 4H-SiC films with n-type conductivity and the donor concentration (8–9) × 10¹⁴ cm⁻³. A DLTS signal anomalous in sign is observed; this signal is related to recharging of deep compensating boron-involved centers in the n-type region near the metallurgical boundary of the p-n junction.     

Effect of a high-energy proton-irradiation dose on the electron mobility in <Emphasis Type="Italic">n</Emphasis>-Si crystals

n-Si single crystals produced by the floating zone method are studied. The concentration of electrons in the crystals is 6 × 10¹³ cm⁻³. The samples are irradiated with 25-MeV protons at 300 K. The irradiation dose is varied in the range (1.8–8.1) × 10¹² cm⁻². The measurements are carried out by means of the Hall technique in the range of temperatures T = 77−300 K. In samples irradiated with different proton doses, a sharp increase in the experimental effective Hall mobility μ_eff or a deep minimum in the dependence μ_eff(T) in the region of phonon scattering of electrons is observed immediately after irradiation or after aging of the samples, respectively. The observed effect is attributed to the formation of high-conductivity (metal-like) inclusions in the irradiated samples and to changes in the degree of screening of the inclusions by impurity-defect shells in relation to the irradiation dose, the time of natural aging, and the temperature of measurements. The impurity-defect shells are formed around metal-like inclusions during isochronal annealing or natural aging of the irradiated samples. It is suggested that metal-like inclusions formed in the n-Si crystals on irradiation with protons with the energy 25 MeV are atomic nanoclusters with an 80-nm radius.     

Power Factor Anisotropy of <Emphasis Type="Italic">p</Emphasis>-Type and <Emphasis Type="Italic">n</Emphasis>-Type Conductive Thermoelectric Bi-Sb-Te Thin Films

The best films for thermoelectric applications near room temperature are based on the compounds Bi₂Te₃, Sb₂Te₃, and Bi₂Se₃, which as single crystals have distinct anisotropy in their electrical conductivity σ regarding the trigonal c-axis, whereas the Seebeck coefficient S is nearly isotropic. For p- and n-type alloys, P _⊥c > P _||c, and the power factors P _⊥c of single crystals are always higher compared with polycrystalline films, where the power factor is defined as P = S ² σ, ⊥c and ||c are the direction perpendicular and parallel to the c-axis, respectively. For the first time in sputter-deposited p-type (Bi_0.15Sb_0.85)₂Te₃ and n-type Bi₂(Te_0.9Se_0.1)₃ thin films, the anisotropy of the electrical conductivity has been measured directly as it depends on the angle φ between the electrical current and the preferential orientation of the polycrystals (texture) using a standard four-probe method. The graphs of σ(φ) show the expected behavior, which can be described by a weighted mixture of σ _⊥c and σ _||c contributions. Because (σ _⊥c/σ _||c) _p  < (σ _⊥c/σ _||c) _n , the n-type films have stronger anisotropy than the p-type films. For this reason, the angular weighted contributions of P _||c lead to a larger drop in the power factor of polycrystalline n-type films compared with p-type films.     

Light-Harvesting in <Emphasis Type="Italic">n</Emphasis>-ZnO/<Emphasis Type="Italic">p</Emphasis>-Silicon Heterojunctions

Zinc oxide (ZnO) films were deposited onto Si to form n-ZnO/p-Si heterojunctions. Under the illumination of by both ultraviolet (UV) light and sunlight, obvious photovoltaic behavior was observed. It was found that the conversion efficiency of the heterojunctions increased significantly with increasing thickness of the ZnO film, and the mechanism for light-harvesting in the heterojunctions is discussed. The results suggest that ZnO films may be helpful to increasing the harvesting of UV photons, thus decreasing the thermalization loss of UV energy in Si-based solar cells.     

Formation of Low-Resistance Ohmic Contact by Damage-Proof Selective-Area Growth of Single-Crystal <Emphasis Type="Italic">n</Emphasis><Superscript>+</Superscript>-GaN Using Plasma-Assisted Molecular Beam Epitaxy

To achieve very low ohmic contact resistance, an n ⁺-GaN layer was selectively deposited using plasma-assisted molecular beam epitaxy (PAMBE). During this process polycrystalline GaN grew on the patterned SiO₂ region, which was subsequently removed by a heated KOH solution, resulting in damage to the n ⁺-GaN surface. To prevent this damage, an additional SiO₂ layer was selectively deposited only on the n ⁺-GaN region. To optimize the fabrication process the KOH etching time and n ⁺-GaN layer thickness were adjusted. This damage-proof scheme resulted in a specific contact resistance of 4.6 × 10⁻⁷ Ω cm². In comparison, the resistance with the KOH etching damage was 4.9 × 10⁻⁶ Ω cm² to 24 × 10⁻⁶ Ω cm². The KOH etching produced a large number of pits (4.1 × 10⁸ cm⁻²) and degraded the current transport. X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectrometry (SIMS) analysis indicated that KOH etching was very effective in removing the oxide from the GaN surface and that the O-H bonding at the GaN surface was likely responsible for the degraded contact performance. The optimum n ⁺-GaN thickness was found to be 54 nm.     

Optical storage on the basis of an <Emphasis Type="Italic">n</Emphasis>-InSb-SiO<Subscript>2</Subscript>-<Emphasis Type="Italic">p</Emphasis>-Si heterostructure

The presence of potential barriers and deep traps in n-InSb/SiO₂/p-Si heterostructures makes it possible to realize the optical memory function on the basis of this structure. The maximal memory coefficient measured on the forward current voltage characteristic is as large as ～10⁴. This heterostructure can be used as an optoelectronic memory cell, which provides a means not only for the storage of signals but also for their summation.     

Optical properties of thin GaSe/<Emphasis Type="Italic">n</Emphasis>-Si(111) films

Morphological and optical studies (ellipsometry and reflectance spectroscopy in the ranges 400–750 nm and 1.4–25 μm) of thin GaSe films fabricated by thermal evaporation on the n-Si (111) single-crystal substrates are reported. The film thickness was 15–60 nm. It is established that, in the initial stage of growth, the growth of GaSe on the n-Si (111) substrates occurs via formation of islands (three-dimensional growth). It is shown that, as the thickness increases, the physical parameters of the film change and the films approach single crystals in crystalline and energy band structure. For films with a thickness of 60 nm, the reflectance band peak is attributed to indirect optical transitions enhanced by reflection from the film-substrate interface. From the results of optical studies, quantum effects in the surface region of the thin films are conjectured.