非刻意掺杂4H-SiC同质外延中的深能级缺陷

Unintentionally doped 4H-SiC homoepitaxial layers grown by hot-wall chemical vapor deposition （HWCVD） have been studied using photoluminescence （PL） technique in the temperature range of 10 to 240 K. A broadband green luminescence has been observed. Vacancies of carbon （Vc） are revealed by electron spin resonance （ESR） technique at 110 K. The results strongly suggest that the green band luminescence, as shallow donor-deep accepter emission, is attributed to the vacancies of C and the extended defects. The broadband green luminescence spectrum can be fitted by the two Gauss-type spectra using nonlinear optimization technique. It shows that the broad-band green luminescence originates from the combination of two independent radiative transitions. The centers of two energy levels are located 2.378 and 2.130 eV below the conduction band, respectively, and the ends of two energy levels are expanded and superimposed each other.     

Crystal growth study using combination of molecular dynamics and Monte Carlo methods

Molecular dynamics method although provides details of energies of the system as a function of time, is not suited to simulate the processes involving activation processes. Therefore, we attempted to combine the molecular dynamics and Monte Carlo methods. Using molecular dynamics, the energies of the system were calculated which were subsequently combined with Monte Carlo method using random numbers, epitaxial growth of (111) plane of copper, silver, and gold. While surface adsorption and surface diffusion for copper, silver, and gold were simulated by use of molecular dynamics method, the relation between the growth rate of thin films and the packing density of atoms were obtained using Monte Carlo simulation. Thus, by combining the results of the molecular dynamics method and the Monte Carlo method the growth process of thin films at elevated temperatures were obtained, which is too tedious to be calculated by molecular dynamics alone.     

Electrical properties of MgO/p-diamond junction fabricated on homoepitaxial single-crystalline diamond

Sufficiently flat and hillock-free insulating homoepitaxial diamond films were successfully grown on high-pressure–high-temperature-synthesized diamond using a 5-kW microwave-plasma chemical-vapor-deposition system with a 20-ppm-nitrogen-included source gas of 4% CH₄ diluted with H₂. Then, layered MgO/boron-doped (p-type) diamond structures were fabricated on the homoepitaxial insulating diamond. Current (I)–voltage (V) characteristics of these device structures showed strong nonlinear behaviors for both current directions, or those for two electrically-parallel, reversed diodes, with conduction limited mainly by sheet resistance of the p-diamond layer in a temperature range from 300 to 600 K. This suggests that the carrier transport occurred through different current passes in the junction region at both biases. Low-frequency capacitances measured were mainly dominated by the depletion capacitance which was influenced by the bias voltage. At relatively high frequencies, however, the total capacitance measured (C) was determined not only by the depletion capacitance but also by the series resistance and the dispersion capacitance. Equivalent circuits of the MgO/p-diamond structure were deduced to explain the measured I–V and C–V results. A possible conduction mechanism is proposed in relation to the electronic structure of the MgO/p-diamond junction.     

Growth of single unit-cell superconducting La2−xSrxCuO4 films

We have developed an approach to grow high quality ultra-thin films of La_2−xSr_xCuO₄ with molecular beam epitaxy, by adding a homoepitaxial buffer layer in order to minimize the degradation of the film structure at the interface. The advantage of this method is to enable a further reduction of the minimal thickness of a superconducting La_1.9Sr_0.1CuO₄ film. The main result of our work is that a single unit cell (only two copper oxide planes) grown on a SrLaAlO₄ substrate exhibits a superconducting transition at 12.5 K (zero resistance) and an in-plane magnetic penetration depth λ_ab(0)=535 nm.     

MOCVD-grown homoepitaxial YBa2Cu3Ox thin films and its multilayers

YBa₂Cu₃O _x (YBCO) films, Zn-doped YBCO (YBCO : Zn) films, and their bilayers have been epitaxially grown on SrTiO₃(100) and single-crystal YBCO(001) substrates by metalorganic chemical vapour deposition. The YBCO(001) films homoepitaxially grown on YBCO(001) substrates have flat surfaces on an atomic scale, and interfaces free from crystalline defects. We can systematically reduce the superconducting transition temperature (T _c) of YBCO : Zn films from 90 K to 37 K by increasing Zn concentration. The bilayers have a sharp distribution of Zn as evaluated fromT _c measurements of the upper YBCO films and depth profiles of secondary ion mass spectrometer, suggesting the possibility to form the homoepitaxial SNS (S, superconductor; N, normal metal) junction operatable between 40 K and 90 K.     

Homoepitaxial growth and characterization of phosphorus-doped diamond using tertiarybutylphosphine as a doping source

Phosphorus-doped diamond was grown by plasma-enhanced chemical vapor deposition (CVD) using tertiarybutylphosphine (TBP) as doping gas. It was confirmed that phosphorus was successfully doped into homoepitaxial diamond films. The films exhibited a negative Hall coefficient throughout the temperature range, from room temperature to 673 K, indicating n-type conductivity. The best sample showed a mobility of 250 cm²/V s at room temperature and the activation energy of the carrier conduction was 0.6 eV. From observations of the bound-exciton emission in cathodoluminescence measurements, it was also shown that phosphorus atoms are located in substitutional sites of the diamond lattice as donor atoms.     

High-quality homoepitaxial diamond (100) films grown under high-rate growth condition

We present advantages of high-power microwave plasma chemical vapor deposition (MPCVD) in homoepitaxial diamond film deposition. Diamond films grown at comparatively high growth rate of 3.5 μm/h showed intense free-exciton recombination emission at room temperature. The free-exciton decay time of the diamond film at room temperature, 22 ns, was much longer than that of type-IIa single crystal, indicating electronically high quality of the homoepitaxial films. Dislocation-related emissions were locally observed, a part of which created by mechanical polishing process was successfully removed by surface etching process using oxygen plasma. Another advantage of the high-power MPCVD is effective impurity doping; boron-doped diamond films with high carrier mobility and high carrier concentration were reproducibly deposited. An ultraviolet photodetector fabricated using the high-quality undoped diamond film showed lower noise equivalent power as well as higher photoresponsivity for ultraviolet light with better visible-blind property, compared to those of standard Si-based photodetectors. The high-power MPCVD is, thus, indispensable technique for depositing high quality diamond films for electronic devices.     

Growth process observation of homoepitaxial ZnO thin films using optical emission spectra during pulsed laser deposition

Homoepitaxial ZnO thin films were prepared on the Zn-polar or O-polar ZnO substrates by pulsed laser deposition method. Optical emission spectroscopy of the plume was carried out to estimate O/Zn flux ratio under the various deposition conditions such as oxygen pressure, laser fluence, and the distance between target and substrate. It is revealed that the O/Zn flux ratio could be controlled by laser fluence, oxygen pressure, and target-substrate distance. Zn-rich O/Zn flux promotes pit formation and O-rich flux yields the three-dimensional growth. The difference of the growth process on Zn-polar or O-polar substrates is also discussed.     

Device advantage of the dislocation-free pressure grown GaN substrates

The process of GaN crystallization from solutions under high pressure will be described. The formation of point defects and their distribution in the crystals will be discussed. The results of the use of the pressure grown crystals for quantum epitaxial structures and blue laser devices will be used to demonstrate the advantage of dislocation-free substrates.