Photoluminescence of electron-hole plasma in semi-insulating undoped GaAs

The dependence of the photoluminescence spectrum of electron-hole plasma in semi-insulating undoped GaAs on the concentration of the background carbon impurity N_C(3×10¹⁵ cm^?3≤N_C≤4×10¹⁶ cm^?3) is studied at 77 K. It is established that the density of the electron-hole plasma, which is equal to n _e?h ≈1.1×10¹⁶ cm^?3 in crystals with the lowest impurity concentration at an excitation intensity of 6×10²² photons/(cm² s), decreases considerably as the value of N_C increases in the range mentioned above. A decrease in the density of the electron-hole plasma with increasing N_C is attributed to the effect of fluctuations in the carbon concentration N_C, which give rise to a nonuniform distribution of interacting charge carriers and to localization of holes in the tails of the density of states of the valence band.     

Analysis of Carrier Transport in <Emphasis Type="Italic">n</Emphasis>-Type Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te with Ultra-Low Doping Concentration

Mercury cadmium telluride (HgCdTe, or MCT) with low n-type indium doping concentration offers a means for obtaining high performance infrared detectors. Characterizing carrier transport in materials with ultra low doping (N_D?=?10¹⁴ cm^?3 and lower), and multi-layer material structures designed for infrared detector devices, is particularly challenging using traditional methods. In this work, Hall effect measurements with a swept B-field were used in conjunction with a multi-carrier fitting procedure and Fourier-domain mobility spectrum analysis to analyze multi-layered MCT samples. Low temperature measurements (77 K) were able to identify multiple carrier species, including an epitaxial layer (x?=?0.2195) with n-type carrier concentration of n?=?1?×?10¹⁴ cm^?3 and electron mobility of μ?=?280000 cm²/Vs. The extracted electron mobility matches or exceeds prior empirical models for MCT, illustrating the outstanding material quality achievable using current epitaxial growth methods, and motivating further study to revisit previously published material parameters for MCT carrier transport. The high material quality is further demonstrated via observation of the quantum Hall effect at low temperature (5 K and below).     

A 4<Emphasis Type="Italic">H</Emphasis>-SiC<Emphasis Type="Italic">p-i-n</Emphasis> diode fabricated by a combination of sublimation epitaxy and CVD

The possibility of fabricating heavily doped (N _a ?N _d ≥ 1 × 10¹⁹ cm_?3) p⁺-4H-SiC layers on CVD-grown lightly doped n-4H-SiC layers by sublimation epitaxy has been demonstrated. It is shown that a Au/Pd/Ti/Pd contact, which combines a low specific contact resistance (～2 × 10^?5 Ω cm²) with high thermal stability (up to 700°C), is the optimal contact to p-4H-SiC. The p-n structures obtained are used to fabricate packaged diodes with a breakdown voltage of up to 1400 V.     

Dependence of the mechanism of current flow in the in-<Emphasis Type="Italic">n</Emphasis>-GaN alloyed ohmic contact on the majority carrier concentration

Based on the study of the temperature dependence of resistance of the In-n-GaN alloyed ohmic contacts, it is found that the mechanism of current flow in them substantially depends on the concentration N of uncompensated donors in GaN. At N = 5 × 10¹⁶ ? 1 × 10¹⁸ cm^?3, current mainly flows along the metallic shunts, and at N ? 8 × 10¹⁸ cm^?3 it flows by tunneling.     

Effective electron mass in a Mn<Subscript>x</Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te system

The reflection spectra of n-Mn_xHg_1?xTe single crystals and epitaxial layers were measured at 300 K. The effective electron mass was determined for the samples with x=0.06–0.10 and an electron concentration N>6×10¹⁶ cm^?3. The calculated values of effective electron mass are close to experimental values.     

Electrical properties of Si:H/<Emphasis Type="Italic">p</Emphasis>-Si structures fabricated by hydrogen implantation

Hydrogenated silicon (Si:H) layers and Si:H/p-Si heterostructures were produced by multiple-energy (3–24 keV) high-dose (5×10¹⁶–3×10¹⁷ cm^?2) hydrogen implantation into p-Si wafers. After implantation, current transport across the structures is controlled by the Poole-Frenkel mechanism, with the energy of the dominating emission center equal to E _c ?0.89 eV. The maximum photosensitivity is observed for structures implanted with 3.2×10¹⁷ cm^?2 of hydrogen and annealed in the temperature range of 250–300°C. The band gap of the Si:H layer E _g ≈2.4 eV, and the dielectric constant ?≈3.2. The density of states near the Fermi level is (1–2)×10¹⁷ cm^?3 eV^?1.     

Electrical properties and luminescence spectra of light-emitting diodes based on InGaN/GaN heterostructures with modulation-doped quantum wells

The distribution of charged centers N(w), quantum efficiency, and electroluminescence spectra of blue and green light-emitting diodes (LED) based on InGaN/AlGaN/GaN p-n heterostructures were investigated. Multiple InGaN/GaN quantum wells (QW) were modulation-doped with Si donors in GaN barriers. Acceptor and donor concentrations near the p-n junction were determined by the heterodyne method of dynamic capacitance to be about N _A ≥ 1 × 10¹⁹ cm^?3 ? N _D ≥ 1 × 10¹⁸ cm^?3. The N(w) functions exhibited maxima and minima with a period of 11–18 (±2–3 nm) nm. The energy diagram of the structures has been constructed. The shifts of spectral peaks with variation of current (J=10^?6–3×10^?2 A) are smaller (13–12 meV for blue and 20–50 meV for green LEDs) than the corresponding values for the diodes with undoped barriers (up to 150 meV). This effect is due to the screening of piezoelectric fields in QWs by electrons. The dependence of quantum efficiency on current correlates with the charge distribution and specific features in the current-voltage characteristics.     

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.     

Raman and infrared spectroscopy of GaN nanocrystals grown by chloride-hydride vapor-phase epitaxy on oxidized silicon

Raman and infrared spectroscopy were applied to study nanocrystalline GaN films grown by chloride-hydride vapor-phase epitaxy on SiO₂/Si(111) substrates at T=520°C. It was ascertained that GaN nanocrystals are formed on the oxidized silicon surface at a rate of 10^?2 nm/s. It was shown that the peaks in the Raman spectra E₂(high)=566 cm^?1 and A₁(LO)=730 cm^?1 correspond to the elastically strained GaN wurtzite structure. It was detected that a peak related to E₁(TO)=558 cm^?1 arises in the infrared spectra, which shows that elastic stresses in the nanocrystals are insignificant.     

High-voltage (900 V) 4 <Emphasis Type="Italic">H</Emphasis>-SiC Schottky diodes with a boron-implanted guard <Emphasis Type="Italic">p-n</Emphasis> junction

High-voltage (900 V) 4H-SiC Schottky diodes terminated with a guard p-n junction were fabricated and studied. The guard p-n junction was formed by room-temperature boron implantation with subsequent high-temperature annealing. Due to transient enhanced boron diffusion during annealing, the depth of the guard p-n junction was equal to about 1.7 μm, which is larger by approximately 1 μm than the projected range of 11 B ions in 4H-SiC. The maximum reverse voltage of fabricated 4H-SiC Schottky diodes is found to be limited by avalanche breakdown of the planar p-n junction; the value of the breakdown voltage (910 V) is close to theoretical estimate in the case of the impurity concentration N = 2.5 × 10¹⁵ cm^?3 in the n-type layer, thickness of the n-type layer d = 12.5 μm, and depth of the p-n junction r _j = 1.7 μm. The on-state diode resistance (3.7 mΩ cm²) is controlled by the resistance of the epitaxial n-type layer. The recovery charge of about 1.3 nC is equal to the charge of majority charge carriers that are swept out of an epitaxial n-type layer under the effect of a reverse voltage.     

Diffusion of chromium into epitaxial gallium arsenide

Diffusion of Cr into epitaxial GaAs in an open system in the temperature range of 750–850°C was studied. Temperature dependences of the diffusion coefficient and solubility of Cr in GaAs were determined. Temperature dependences of the diffusion coefficient and solubility of Cr are described by the Arrhenius equation with the parameters D ₀ = 1.9 × 10⁹ cm²/s and E = 4.1 ± 0.2 eV for the diffusion coefficient and N ₀ = 2.3 × 10²⁴ cm^?3 and E ₀ = 1.9 ± 0.4 eV for solubility. The effect of protective SiO₂ filmon the Cr diffusion coefficient and morphology of the GaAs surface after diffusion was studied.     

The Improvement of Electrical Characteristics of Pt/Ti Ohmic Contacts to Ga-Doped ZnO by Homogenized KrF Pulsed Excimer Laser Treatment

We investigated the effect of KrF excimer laser surface treatment on Pt/Ti ohmic contacts to Ga-doped n-ZnO (N_d = 4.3 × 10¹⁷ cm^?3). The treatment of the n-ZnO surfaces by laser irradiation greatly improved the electrical characteristics of the metal contacts. The Pt/Ti ohmic layer on the laser-irradiated n-ZnO showed specific contact resistances of 2.5 × 10^?4 ～ 4.8 × 10^?4 Ω cm² depending on the laser energy density and gas ambient, which were about two orders of magnitude lower than that of the as-grown sample, 8.4 × 10^?2 Ω cm². X-ray photoelectron spectroscopy and photoluminescence measurements showed that the KrF excimer laser treatments increased the electron concentration near the surface region of the Ga-doped n-ZnO due to the preferential evaporation of oxygen atoms from the ZnO surface by the laser-induced dissociation of Zn-O bonds.     

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.     

Structural defects and electrical conductivity in nanocrystalline SiC:H films doped with boron and grown by photostimulated chemical-vapor deposition

The paramagnetic DB defects and dark conductivity σ_d in films of nanocrystalline hydrogenated silicon doped with boron and carbon (nc-SiC:H) and grown by photostimulated chemical vapor deposition are studied. It is shown that an increase in the doping level leads to a phase transition from the crystalline structure to an amorphous structure. The electrical conductivity increases as the doping level increases and attains the value of σ_d = 5.5 × 10^?2 Ω^?1 cm^?1; however, the conductivity decreases once the phase transition has occurred. The concentration of DB defects decreases steadily as the doping level increases and varies from 10¹⁹ cm^?3 (in the crystalline structure) to 9×10¹⁷ cm^?3 (in the amorphous structure).     

Variable-Field Hall Effect Analysis of HgCdTe Epilayers with Very Low Doping Density

Ultra-low-doped mercury cadmium telluride (HgCdTe, or MCT) is of significant interest for infrared detectors designed to suppress Auger recombination. Measurement of low doping levels in multi-layered structures is difficult with traditional 4-point Hall effect measurements. Multi-layered Hg_.79Cd_.21Te samples were analyzed using variable magnetic field Hall effect measurements and a multi-carrier fitting procedure. The measurements resolve two distinct carrier species corresponding to surface and/or buffer layer conduction and conduction through the primary low-doped material. High-quality electronic transport is achieved, including the demonstration of an epitaxial layer (x = 0.2195) with n = 1.09 × 10¹⁴ cm^?3 and μ = 275,000 cm²/Vs at 77 K. This technique shows promise as a way to analyze layers with significantly lower doping, and a starting point to understand and advance the development of HgCdTe epilayers with very low doping concentration.     

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.     

Electrical properties of <Emphasis Type="Italic">n-</Emphasis>GaN/<Emphasis Type="Italic">p</Emphasis>-SiC heterojunctions

Epitaxial GaN layers were grown by hydride vapor phase epitaxy (HVPE) on commercial (CREE Inc., USA) p⁺-6H-SiC substrates (Na ? Nd ≈ 7.8 × 10¹⁷ cm^s?3) and n⁺-6H-SiC Lely substrates with a predeposited p⁺-6H-SiC layer. A study of the electrical properties of the n-GaN/p-SiC heterostructures obtained confirmed their fairly good quality and demonstrated that the given combination of growth techniques is promising for fabrication of bipolar and FET transistors based on the n-GaN/p-SiC heterojunctions.     

Dielectric waveguide for middle and far infrared radiation

The possibility of using the normal skin effect in dielectric waveguides for long-wavelength radiation is analyzed. A design of a waveguide integrated with a heterolaser is suggested, in which an undoped layer of GaAs is clad between heavily-doped n- and p-Al _x Ga_{1 ? x} As alloy layers, reflecting radiation because of the normal skin effect. It is shown that an efficient waveguide can be formed using n-Al _x Ga_{1 ? x} As layers with x < 0.45 and the electron concentration N > 5 × 10¹⁸ cm^?3 and p-Al _x Ga_{1 ? x} As layers of any composition with the hole concentration P ≥ 3 × 1019 cm^?3.     

Low-temperature (77 K) impurity breakdown in <Emphasis Type="Italic">p</Emphasis>-type 4<Emphasis Type="Italic">H</Emphasis>-SiC

The impact ionization of acceptors in aluminum-doped 4H-SiC epitaxial films (Al concentration 2 × 10¹⁵ cm^?3) at a temperature of 77 K is studied. It is found that the impact-ionization coefficient exponentially depends on the reverse electric field: α _p = α*_pexp(?F*/F). The largest ionization coefficient is α* _p = 7.1 × 10⁶ cm^?3 s^?1, and the threshold field is F* = 2.9 × 10⁴ V/cm.     

Specific features of electron spin resonance in 4<Emphasis Type="Italic">H</Emphasis>-SiC in the vicinity of the insulator-metal phase transition: II. Analysis of the width and shape of lines

The variation in the width and shape of the ESR line of nitrogen in 4H-SiC in the concentration range corresponding to the insulator-metal phase transition was investigated. It is shown that the spin relaxation in the region of hopping and metal conduction occurs at electrical multipoles (clusters) whose sizes decrease from rather large to small (characteristic of interimpurity distances) as the concentration of impurity centers increases. Analysis of the temperature dependences of the resistance made it possible to estimate the critical concentration for the insulator-metal phase transition (N _D -N _A )_c≈1.5×10¹⁹ cm^?3. The values of other characteristic concentrations that determine the effects of electron-electron interaction in the system under study were also found.