Kenshiro Nakashima

Electrical and Optical Studies in Gallium Antimonide

Hall effect, photoluminescence, and photoconductivity measurements on residual acceptors in GaSb grown by the Bridgman method, are presented and discussed with emphasis on the determination of energy level positions. The effect of thermal annealing or Li-diffusion on the behavior of the energy levels of residual acceptors is investigated to ascertain the nature of doubly-ionizable acceptors. Photoluminescence data on the Au-GaSb Schottky barrier give additional results confirming that the rediual acceptor is doubly ionizable. Direct evidence for the defect configuration is also presented.

Optical and Thermal Ionization of Iron-Related Defects in Silicon

The optical and thermal ionization energies of several Fe-related defects in Fe-diffused Si are determined simultaneously using the photostimulated capacitance transient method together with deep-level transient spectroscopy. Two defect states with thermal electron-ionization energies of 0.36 and 0.56 eV to the conduction band, respectively, are confirmed to originate from the Fe-related defects, in addition to the donor level due to interstitial Fe. The origins of these defects are discussed on the basis of their behaviors in n- and p-type Si.

The Effects of Neutron Irradiation in p-Type Silicon

Radiation damages in silicon single crystals irradiated by pile neutrons were investigated by using Hall effect and the carrier lifetime measurements. In p -type silicon donor-like centers were found to be introduced, the energy level of which located at about E V +0.3 eV. Its introduction rate did not depend on the kind of impurities, carrier concentrations and crystal growth methods. At the small irradiation dose level the minority carrier lifetime was controlled by the recombination through the defect centers. With the increase of dose the trapping effect became remarkable even at room temperature in conjunction with the recombination effect. At 2.6×10 12 nvt irradiation dose the trapping level at E C -1.16 eV was found to be predominantly introduced. The annealing studies of 2.6×10 12 nvt irradiated p -type silicon indicated that trap centers annealed at around 200°C, subsequently recombination centers annealed at 430∼480°C. Recombination centers annealed over the broad temperature range and did not s...

FORMATION OF AN OHMIC ELECTRODE IN SIC USING A PULSED LASER IRRADIATION METHOD

Abstract We have succeeded in the formation of a Ni electrode on n-type silicon carbide (6HSiC) at room temperature using the pulsed laser doping (PLD) method. Characteristics of the PLD method for the formation of an ohmic electrode in SiC are shown to be as follows; (1) The formation of an ohmic electrode in SiC at room temperature was achieved. (2) Metal dopants are doped to a depth of about 150 nm from the surface. (3) The interface between the laser doped region and the SiC substrate was found to be an abrupt junction. Furthermore, we have attempted to form the ohmic contact by metal atoms doping with laser irradiation into a bulk SiC through a mask during thermal evaporation of the metal. This technique is successful for the formation of dot electrodes 0.8 mm in diameter on the SiC substrate.

Defect Levels in Thermally-Quenched Silicon Crystals

The energy levels due to quenched-in defects in thermally-treated p-type silicon are analyzed by capacitance transient spectroscopy (DLTS). The quenching rate is found to affect the introduction of several kinds of hole trap, but neither the oxygen concentration nor the dislocation density in the starting materials influences the formation of defects. A hole trap at Ev+0.32 eV, which is not correlated with transition metal impurities such as iron or copper, is found to be introduced efficiently in the rapidly-quenched crystals.

Formation of a p‐n junction in silicon carbide by aluminum doping at room temperature using a pulsed laser doping method

A p‐n junction has been formed in n‐type silicon carbide (6H–SiC) by aluminum (Al) doping at room temperature using a pulsed laser doping method. A 6H–SiC substrate in [H2+(CH3)3Al] atmosphere was irradiated by an KrF excimer laser of 20 ns full width at half‐maximum. The depth profile of Al determined by secondary ion mass spectroscopy showed a rectangularlike shape with a maximum concentration of 1×1022 Al/cm3. Furthermore, Al atoms were doped to a depth of about 0.05 μm from the surface. The current–voltage characteristics of the p‐n junction clearly showed the rectifying property with low leakage current.

Annealing of a Vacancy-Type Defect and Diffusion of Implanted Boron in 6H-SiC

We have investigated the annealing of vacancy-type defects and B diffusion in SiC using positron annihilation spectroscopy (PAS) and secondary ion mass spectroscopy (SIMS). Boron (B) and carbon (C) ions were implanted into 6H-SiC substrates, and the samples were annealed up to 1650 C in Ar. In the PAS study, defective layer to a depth of 200 nm from the surface is observed after annealing between 800 and 1200 C. This region disappears after annealing above 1500 C. No significant difference in the annealing behavior of the vacancy-type defects is observed for only B-and C/B-implanted samples. In the SIMS study, the out-diffusion of B in 6H-SiC is observed after annealing above 1400 C and a dip in the B concentration is created close to the surface. This outdiffusion of B is suppressed by C/B-implantation. This suppression tends to be enhanced by the temperature of the implantation and the concentration of the implanted C. Introduction Selective doping of silicon carbide (SiC) is an important issue due to the potential of SiC in high-power and high-frequency applications. Since the thermal diffusion coefficients of impurities such as aluminum (Al), phosphorus (P), and nitrogen are very low in SiC, ion implantation is considered to be the most suitable method for selective doping in SiC. To electrically activate implanted species and to reduce residual defects, high temperature annealing above 1600 C is necessary. After high temperature annealing, no significant change in the depth profile of the above-mentioned species is observed. However, Laube et al. [1] reported transient-enhanced diffusion of B after high temperature annealing. They also showed that the B diffusion was suppressed either by co-implantation of C and B or annealing at 900 C prior to high temperature annealing [1]. This result suggests that B diffusion in implanted SiC at high temperature is related to residual defects. Bracht et al. [2] explained that the transient-enhancement of B diffusion occurred via the kick-out mechanism [3], which is related to silicon interstitials, Sii. However, no evidence was provided. In this study, we investigated the annealing behavior of vacancy-type defects in 6H-SiC implanted with B using PAS, and the diffusion of implanted B using SIMS. We will show that the diffusion of B is mediated by interstitials rather than vacancy-type defects.

n-Type Doping Characteristics of O-Implanted AlGaN

We have systematically investigated the n-type doping characteristics of O-implanted GaN and AlGaN from the viewpoint of annealing temperature. The n-type regions were produced in undoped materials by O + implantation and subsequent annealing with an SiO 2 encapsulation layer at temperatures between 1000 and 1300°C. From room-temperature Hall-effect measurements, both materials showed an increase in sheet carrier concentration with the rising of annealing temperature. The effective activation efficiency tended to decrease from 1.3 to 0.5% with increasing Al content in Al x Ga 1-x N up to 10%, even after a 1300°C anneal. Furthermore, O-implanted AlGaN displayed a large increase of electron mobility with increasing annealing temperature, which was clearly different from the situation of O-implanted GaN. These results indicate that the high-temperature annealing for O-implanted AlGaN probably enables an improvement of crystallinity in addition to an increase of O-doping characteristics.

Er-Doping in Silicon by Pulsed Laser Irradiation

Erbium ions were successfully doped in silicon by pulsed laser irradiation above the threshold laser energy density. Photoluminescence peaks at 1.54, 1.59 and 1.64 µm from Er-optical centers were observed after annealing of Er-doped samples. The intensity of the 1.54 µm Er-emission band increased upon increase in the laser energy density, and then gradually decreased after reaching the maximum, due to the laser sputtering of the silicon substrate. Oxygen atoms, which were unintentionally codoped with Er-ions, were found to be distributed in the same region as in Er-ions, and were suggested to play roles to activate Er-optical centers. The maximum concentration of Er-ions doped in the solid state regime were estimated to be the order of 10 18 cm −3 by the Rutherford backscattering measurements.

Studies of Recombination Centers in Gamma-Irradiated p-Type Sillicon

Recombination centers produced by Co 60 γ-ray irradiation at room temperature were investigated by measuring the temperature dependence and the annealing behaviors of the minority carrier lifetime. Both isochronal and isothermal annealing of the lifetime at a fixed measurement-temperature were observed. The measurement at 150°K show that A-center at E C –0.17 eV is the most effective recombination center about the region of this temperature in all p -type specimens. The first order annealing process with an activation energy of 1.27 eV was found in agreement with the values reported earlier. The capture cross section for electrons and holes were estimated to be 1.2×10 -15 cm 2 and 1.8×10 -14 cm 2 , respectively. The introduction rate of inverse lifetime measured at 150°K seems to show that A center is introduced at nearly the same rate in both floating zone and pulled crystals. At and above room temperature the recombination center at ∼ E V +0.3 eV was found to control the lifetime. The annealing activati...