Akihisa Yoshida

Atomic level smoothing of CVD diamond films by gas cluster ion beam etching

Abstract Chemical vapor deposited diamond films on silicon substrates were etched by a gas cluster ion beam. We found that a gas cluster ion beam of 10 17 ions/cm 2 would be effective to smooth the surface of the CVD diamond films. It was confirmed that atomic level smooth surfaces ( R a = 1.9 nm by AFM measurements) were formed by Ar gas cluster ion beam (Ar 3000 + ) etching. We believe that the gas cluster ion beam etching technique will be a key technology for diamond device fabrication.

Homoepitaxial growth of ZnSe on dry‐etched substrates

High quality ZnSe layers have been grown by molecular beam epitaxy on dry‐etched ZnSe substrates. Surface damage caused by cutting and polishing of the ZnSe substrate was removed by dry etching using BCl3 gas to 10 μm depth. The dry‐etched ZnSe substrates exhibited smooth surface morphology and showed excitonic emissions stronger than that from as‐polished substrates in photoluminescence (PL) measurements at 11 K. The low‐temperature PL spectra obtained from homoepitaxial ZnSe layers grown on the substrates dry etched at the optimum condition showed a strong free‐exciton emission at 2.804 eV and a dominant donor‐bound exciton emission at 2.798 eV. Since each excitonic emission shows a single peak, the homoepitaxial layers appear to be free from strain.

Phosphorus doping for hydrogenated amorphous silicon films by a low‐energy ion doping technique

A heated film of hydrogenated amorphous silicon was doped with phosphorus and hydrogen by a 6.0‐kV diffused and accelerated beam of ions from rf discharge in a magnetic field, which produced a plasma from phosphine gas containing hydrogen. This doping technique achieved a dark conductivity of 8.7×10−4 (Ω cm−1) at room temperature. The conductivity activation energy was 0.17 eV.

Plasma ion-doping technique with 20 kHz biased electron cyclotron resonance discharge

Boron ions have been successfully doped into crystalline silicon substrates using AC(20 kHz)-biased microwave electron cyclotron resonance plasma, as functions of microwave power and AC bias power. Diborane gas diluted with hydrogen was used as a gas source. A sheet resistance of 10100 kΩ/\Box, junction depth of 0.1 µm and boron concentration at the silicon surface of 1018 atoms/cm3 were attained using this technique.

β-SiC Formation by Low-Energy Ion-Doping Technique

β-SiC was prepared on silicon (Si) substrates by a low-energy ion-doping technique without mass separation. Carbon (C) and hydrogen (H) ions obtained from a discharge of diluted methane gas with hydrogen gas were implanted into Si substrates at the DC acceleration voltage of 3.0 kV. The dependence of carbon dose, dilution ratio and annealing temperature (Ta) on the Si-C bond formation was investigated. Simultaneous implantation of H ions as well as C ions was effective for tight Si-C bond formation at low Ta( 800°C), the β-SiC network was formed in the implanted layer. With increasing carbon dose, the amount of the β-SiC increased and was saturated at the dose of ~4×1015 ions/cm2.

Fabrication of a-Si:H thin film transistors on 4-inch glass substrates by a large area ion doping technique

We developed a large area ion doping apparatus with an rf ion source of 50 cm in diameter. Charge accumulation during ion doping was suppressed by using a double grid. We fabricated a-Si:H TFTs on a 4-inch glass substrate by using the doping apparatus. By optimizing the thickness of the passivation film of TFTs, a field effect mobility, threshold voltage and an ON/OFF current ratio of 0.6 cm2/Vs, 4.9 V and 107, respectively, were obtained. We confirmed the uniformity of electrical properties for TFTs on 4-in. glass substrates to be about ±2%. These characteristics of TFTs are suitable for switching elements in a large area LCD.

Large Area Doping Technique Using an Ion Source of rf Discharge with Magnetic Field

A low energy broad ion beam doping technique has been developed. This technique enables the doping of impurities into semiconductor materials over a large area (370 mm ) without mass separation and beam scanning. Diborane diluted in hydrogen was discharged by rf (13.56 MHz) power and magnetic fields. Ions from discharged gas were accelerated by an acceleration voltage of 1.5 kV and were implanted into silicon wafers. We confirmed that resistance uniformity in five pieces of 3in.-Si wafers simultaneously doped by this technique was ±3.44 percent.

Low-Temperature and High-Rate Deposition of SrTiO3 Thin Films by RF Magnetron Sputtering

SrTiO3 thin films have been prepared by RF magnetron sputtering at the a substrate temperature of 200°C and a high deposition rate of 35 nm/min on Al/glass substrates. The deposition rate of the SrTiO3 thin film was controlled by RF power during the deposition. The orientation of the films changed and the grain boundaries became sparse with increasing RF power. Furthermore, the leakage current of the capacitors also increased with increasing RF power. As the RF power increased, the signal intensity ratios of O2+/O and Ti/Sr in the optical emission spectroscopy decreased. The application of optimum working pressure resulted in recovery of the signal intensity ratios of O2+/O in the optical emission spectroscopy and improvement in the properties of the SrTiO3 film deposited at a high RF power. The SrTiO3 film of 300 nm thickness exhibited a relative dielectric constant of 64 and a leakage current of 4×10-10 A/cm2 at the voltage of +3 V.

Improvement of Properties of SrTiO3 Thin Films Deposited at Low Temperature and High Rate by Sputtering Gas.

SrTiO3 thin films have been prepared by RF magnetron sputtering at a low substrate temperature of 200°C and a high deposition rate of 35 nm/min on Al/glass substrates. X-ray diffraction data indicated that deposited SrTiO3 thin films were partially polycrystalline and/or amorphous. The properties of SrTiO3 thin films were improved by sputtering gas. As the mass of the sputtering gas was light, the signal intensity ratios of Ti/Sr in the optical emission spectra increased, and surface protrusions were suppressed. In particular, in the presence of N2 gas, the properties of SrTiO3 films were improved, and the leakage current density and tan δ markedly decreased. SrTiO3 film of 300 nm thickness exhibited the dielectric properties with a relative dielectric constant of 62 and a leakage current of 4×10-10 A/cm2 at a voltage of +5 V, and tan δ of 0.6% at 1 kHz.

Hydrogen, fluorine ion implantation effects on polycrystalline silicon grain boundaries

Abstract The influences of hydrogen and fluorine ion implantation on polycrystalline silicon (poly-Si) grain boundaries have been investigated. Effective passivation of poly-Si grain boundaries was achieved by hydrogen ion implantation at 300–400°C. From fluorine implantation experiments, we confirmed that fluorine atoms in poly-Si were redistributed with a diffusion tail after annealing at above 600°C. A diffusion tail of fluorine redistribution was not observed in single crystalline silicon. We think that it is possible to passivate poly-Si grain boundaries by controlling the diffusion of fluorine.