Through variable-energy electron beam excitation (0.5 keV-2.0 keV), we observe depth dependent differences in the cathodoluminescence spectra of 6H-SiC (0001) Si-terminated surfaces. The etched SiC exhibits three deep level defects in the near-surface region, including a defect peak observed at 0.92 eV, known to be associated with vanadium. In-situ annealing produces a dramatic relative decrease in the luminescence from vanadium impurities near the surface after annealing to 500 °C, and then the subsequent re-emergence of vanadium in the nearsurface regime after annealing to 810 °C. This temperature-dependent redistribution suggests either diffusion or segregation of vanadium from the bulk up toward the near-surface region.

The closed-form analytic solutions for the breakdown voltage of 6H-SiC RTD, reachthrough diode, having the structure of p+-n-n+, are successfully derived by solving the impact ionization integral using effective ionization coefficient in the reachthrough condition. In the region of the lowly doped epitaxial layer, the breakdown voltages of 6H-SiC RTD nearly constant with the increased doping concentration. Also the breakdown voltages of 6H-SiC RTD decrease, in the region of the highly doped epitaxial layer, which coincides with Baliga'seq. [1].

Reverse bias breakdown behaviour of high quality 4H silicon carbide p-n diodes was investigated, using optical and electrical measurement techniques. Most of the sample diodes suffered from early breakdown phenomena in the form of microplasmas at about 80% of the calculated parallel plane breakdown voltage for the diodes, as evident from measured I-V curves. A group of these microplasmas could be correlated to micropipes, identified by optical microscopy, while a large number of microplasmas were caused by other defects and inhomogenities in the space charge region under reverse bias. The same spots that revealed early breakdown phenomena under reverse bias also showed a different electroluminescence (EL) behaviour under low forward current densities compared to those areas with a homogeneous breakdown behaviour. However, even diodes containing one or more micropipes in the region of the junction showed good rectifying behaviour up to two third of the parallel plane breakdown voltage, where the turn-on of a microplasma was observed.

The optical beam induced current (OBIC) technique allows a direct imaging of high voltage PN junctions at a microscopic level under reverse operating conditions by measuring the local variation of the photocurrent. In this paper we focus on the application of the UV-OBIC technique for failure analysis of 4H SiC high voltage P+N diodes.

4H SiC P+N diodes with a 2-zone junction termination extension were used. The diodes were characterized in terms of reverse leakage current and breakdown voltage. Various devices were chosen for failure analysis on the base of early breakdown and/or excessive leakage current for OBIC measurements to study extrinsic failures. As a reference we selected diodes that blocked more than 2 kV with a leakage current density of typically less than 0.1 μA/cm2.

OBIC measurements have been used to detect failures in devices that manifest themselves as peaks or “hot spots” in the photocurrent distribution. Early breakdown of diodes could be attributed to formation of hot spots in the periphery of the diodes. The appearance of a hot spot preceded any noticeable increase in reverse leakage current and is thus a very sensitive tool to identify defective diodes already at low voltage levels.

The photocurrent generated by illumination of hot spots has been measured as a function of reverse bias voltage and the current multiplication factor has been determined.

Rutherford backscattering, Raman spectroscopy as well as photoluminescence, resistivity and Hall measurements have been used to investigate the doping behaviour of Scandium and Gallium ions implanted into Silicon Carbide respectively. The recovery of the crystal lattice after implantation at room temperature followed by rapid thermal annealing is shown to be less effective in the case of Scandium compared with Gallium. Scandium implanted SiC exhibited a high resistivity in comparison to Gallium implanted crystals.

Highly perfect 3C-SiC thin films, on 6H-SiC deposited by the chemical vapor deposition at low temperature with various Cl/Si, H/Si and C/Si ratios were characterized by x-ray high resolution triple crystal diffractometry and double crystal topographic methods. The films were epitaxial with a low defect density present (mostly in the range of 107/cm2). X-ray topography revealed stacking faults, low angle grain boundaries, dislocations and inversion double positioning boundaries present in the film and substrate.

The barrier heights of various Schottky diodes in n-type 4H-SiC and 6H-SiC is estimated from published data at Si-face and C-face, respectively, employing the LSM (least square method). It is found that the barrier height in SiC Schottky diode is a linear function of metal work function as φB. =a φm + b. The a is about 0.63 ∼ 0.72. The already established analytic expression in [9] is compared with the estimated linear expression and revised by employing the empirical factor, α between the upper and lower boundary of interface state density, DIT. The values of a lie in 1.65 ∼ 32.1.

Details of the electronic bandstructure in pseudomorphic Gal-xInxN/GaN single heterostructures (0 < x < 0.22) are studied. In photocarrier modulated reflectance strong modulation of the density of states (Franz-Keldysh oscillations) is found due to a piezoelectric field of about 0.6 MV/cm in the strained layer. No excitons are expected to form in the presence of this field. Studying the composition dependence we determine a piezoelectric coefficient ∂|P|/∂εzz = 0.46 C/m2 and extrapolate a spontaneous polarization in GaN |Peq| = 3.9 mC/m2. Photoreflection indicates the presence of localized tail states 50 – 100 meV below the bandgap which are well explained by the Franz-Keldysh effect involving k non-conserving transitions in the large electric field. Luminescence is found to originate in these electric field induced states. The derived bandgap energies can be approximated by an interpolation yielding bowing parameters b = 2.6 eV (photoreflection) and b = 3.2 eV (luminescence) for pseudomorphic films with 0.07 ≤ x ≤ 0. 22. These findings may affect interpretation of device performance.

A photoluminescence from a multiquantum GaN/InxGa1−xN/GaN well structure (x varies between 0.1 to 0.4) was investigated at various temperatures, pumping powers. While the temperature dependence of the peak position indicates normal band to band character of radiative recombination, the large pumping power induced “blue”shift of the peak position (up to 200 meV) can be observed. This kind of shift cannot be easily explained by the band tailing effect but is most likely the result of the screening of the strain-induced piezoelectric field. By evaluating the theoretical values of the piezoelectric filelds in the quantum well, we can show that in order to account for the experimental results we have to assume the partial relaxation of the strain.

Spatially, spectrally, and temporally resolved cathodoluminescence (CL) techniques have been employed to examine the optical properties and kinetics of carrier relaxation in InGaN/GaN heterostructure and single quantum well (QW) samples. CL images of the QW sample revealed a spotty cellular pattern indicative of local In compositional fluctuations on a scale of < 100 nm. The compositional variations induce local potential fluctuations, resulting in a strong lateral excitonic localization at InN-rich regions in the InGaN QW layer. Time-resolved CL measurements revealed a lateral spatial variation in the luminescence decay time which correlates with the spatial variation in the luminescence efficiency. A reduced lifetime is observed at boundary regions between centers of excitonic localization. A detailed time-resolved CL study shows that carriers generated in the boundary regions will diffuse toward and recombine at the InN-rich centers. An electron beam induced modification of the emission spectra was observed for InGaN/GaN heterostructure samples. Exposure to the e-beam resulted in a shift in the near-band gap emission to higher energies with a simultaneous increase in the emission intensity. These result are interpreted as a modification of the surface passivation through e-beam exposure and carbidization of the surface.

We present the results of an experimental study on stimulated emission (SE) in optically pumped InGaN/GaN multi-quantum well (MQW) structures in the temperature range of 175 K to 575 K. Samples used in this work consisted of 12 QWs and the GaN barriers were intentionally doped with different Si concentrations. The effect of doping on the SE thresholds of the MQWs were investigated. We observed that the SE spectra were comprised of many narrow peaks of less than I Å full width at half maximum (FWHM). No broadening of the FWHMs of the peaks occurred as the temperature was raised from 175 to 575 K. The SE threshold was measured as a function of temperature and compared with that of a thin GaN film. Low SE thresholds were attributed to high quantum efficiency of the MQWs, possibly associated with large carrier localization. A characteristic temperature of 162 K was derived from the temperature dependence of the SE threshold. The integrated emission intensity versus pumping density was examined for different temperatures. This study shows that InGaN/GaN MQWs are suitable for the development of laser diodes that can operate well above room temperature.

We argue that the nanopipes recently observed in device-quality GaN grown on sapphire play an important role in degradation of nitride-based devices requiring high driving voltage, such as diode lasers or high-power electronics. The nanopipes offer a preferential path for the top (p-side) contact metal to migrate down towards the p-n junction under high-voltage operation, eventually causing a short and device failure. The metal migration process is enhanced by high voltage (tens of volts) required to drive high-current pulses through the device, and its elimination is of critical importance for achieving reliable GaN-based power devices and diode lasers.

The turn-on process and the steady state current-voltage characteristics have been investigated in 4H-SiC thyristors with a forward breakover voltage close to 700 V and pulse switch current density of 16 000 A/cm2. The rise current constant, τr, has been found to decrease monotonically with temperature: τr, ≈ 10.5 nsec at T = 300 K and τr ≈ 1.2 nsec at T = 500 K at high bias Uo ≥ 300 V. The value of τr ≈ 1.2 nsec is the lowest observed value for SiC thyristors. At very high current density, j ≥ 5 103 A/cm2, the residual voltage drop for 4H-SiC thyristors is lower than for identically rated Si thyristors. The voltage drop Udo decreases monotonically with temperature.

In order to obtain electrically stabilized MIS interface, a diamond metal-insulator- semiconductor field-effect transistor (MISFET) was prepared by means of reduced-oxygen process including ultrahigh-vacuum (UHV) process, and its electrical properties were closely investigated. According to the results, observed effective mobility (μeff) was 400 cm2/Vs at room temperature, which is the highest value obtained until now in the diamond FET at room temperature. The transconductance (gm) and surface state density (Nss) of the device operation region was 5mS/mm and ∼ 1010/cm2 eV, respectively, which is also comparable with conventional Si MOSFETs with the same gate length (Lg = 30μm).

In this paper the effect of NO nitridation on the electrical characteristics of the oxides grown on p-type 4H SiC has been investigated. The results show that NO nitrided oxide has a lower interfacial trap density and a lower net oxide charge compared to N2 annealed oxides.

The use of purified hydrazine cyanurate as a solid source of hydrazine in the low temperature nitridation of GaAs (100) and (111) and sapphire (0001) is demonstrated. Thenitridated surfaces were analyzed by X-ray Photoelectron Spectroscopy (XPS) for chemical composition and Atomic Force Microscopy for surface morphology. The GaAs surfaces were composed primarily of GaN, GaAs, and Ga2O3, and were as smooth as unprocessed standards. The nitridated sapphire surfaces were composed of A1NxO1-x and exhibited three-dimensional growth for long nitridation times.

The thermal behavior of CHx(x=2–3) radicals and H atoms adsorbed on Si(111) surface was investigated using high-resolution electron-energy-loss spectroscopy (HREELS), quadrupole mass spectrometry (QMS), and low-energy electron diffraction (LEED). We found that CHx(x=2–3) radicals in the top adlayers of saturate adsorption were not stable, and the loose structures among a few top adlayers broke up completely till 550K. TDMS(thermal desorption mass spectrometry) analysis showed significant desorption of atomic hydrogen, CH2 and CH3. In the region of 550K-750K, only H2 was detected by TDMS, and we speculate that the CHx(x=2–3) species catenate each other, and form a network of hydrocarbon, which is possibly the initial stage of diamond nucleation on Si. This formed network was not stable above 800K, C2 hydrocarbon species were observed after the thermal desorption temperature was over 800K. HREELS measurement demonstrated that no H atoms existed when substrate temperature reached 980K. Compared with the result reported by S.-Tong Lee for diamond(Ref. 12), we conclude that low stability of hydrocarbon species on Si is the dominant reason that results in the difficulty of diamond nucleation on perfect Si surface.

Deep levels in high resistivity layers of AlGaN grown by MOCVD on sapphire were studied by means of dark current and photocurrent versus temperature measurements and by photoinduced current relaxation spectroscopy (PICTS). Strong temperature quenching of photocurrent was observed and explained by the presence of hole traps with energies 0.2 eV, 0.3 eV and 0.35 eV for films with correspondingly 5%, 15% and 25% of Al. Two hole traps with activation energies of 0.27–0.35 eV and 0.85–1.05 eV depending on composition were detected in PICTS spectra. It is suggested that the former traps are the same as observed in temperature quenching of photocurrent while the latter traps are related to the yellow luminescence band in AlGaN. The photocurrent decay is shown to have a complex form with a rapid decrease followed by a long tail associated with detrapping of holes. It is argued that, at room temperature, the ∼1 eV hole traps are mainly responsible for the amplitude of the tail.

The intermetallic compound RuAl2 with Nowotny chimney-ladder structure is synthesized using arc melting technique. The electrical resistity and thermo electric power measurements were carried out in the temperature range 300–1000K. The resistivity increases with increasing temperature and reaches a maximum value at about 700K. Thermo electric power (TEP) of the sample is negative and the value is about -80 µV/K at RT. The value increases with increasing temperature reaching a maximum value of -140 µV/K at about 600K. The compound exhibits temperature independent power factor in the temperature range 300–550K The calculated figure of merit 1.3 × K-1 is comparable to 7 × 10-4 K-1 of Si-Ge alloys which are used as high temperature thermoelectric materials.

The solid solutions (Y1-xYbx)2Mo2O7 were prepared and the systematic changes in the electrical resistivity (ρ=l/σ), thermopower (S) and power factor (S2σ) have been studied in the temperature range 300–900 K. The lattice parameters ‘a’ and ‘c’ are smaller for higher Yb3+ content phases due to smaller Yb3+ radius and a small tetragonality is observed for all the phases. Semiconducting behaviour is seen for all compositions with systematic increase in activation energy with increasing Yb content. All compositions show negative thermopower indicating electrons are the majority charge carriers in the temperature range of measurements. The calculated power factor values S2σ increase with increasing temperature in the low temperature region and a maximum power factor of ∼0.76×10−7 Wcm−1K−2 is observed at 650K.