Makoto Sawamura

Effect of Plasma Power on Structure of Hydrogenated Nanocrystalline Cubic Silicon Carbide Films Deposited by Very High Frequency Plasma-Enhanced Chemical Vapor Deposition at a Low Substrate Temperature

The effect of plasma power on the structural properties of hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) films deposited by very high frequency plasma-enhanced chemical vapor deposition was investigated. The film structure was strongly influenced by the plasma power due to the change in atomic hydrogen density in the vapor phase. A high plasma power of above 170 W (2.17 W/cm2) is required for depositing nc-3C-SiC:H films.

Effects of Hydrogen Dilution Ratio on Properties of Hydrogenated Nanocrystalline Cubic Silicon Carbide Films Deposited by Very High-Frequency Plasma-Enhanced Chemical Vapor Deposition

We have successfully deposited nanocrystalline cubic silicon carbide (nc-3C-SiC:H) films at a low substrate temperature of 360 °C by very high-frequency plasma-enhanced chemical vapor deposition using monomethylsilane and hydrogen. Spectroscopic ellipsometry revealed that the crystalline volume fraction of the films increased from 69 to 92% with increasing hydrogen dilution ratio from 100 to 500. We found that the dark conductivity of the films was strongly affected by the crystalline volume fraction. A high deposition rate of 0.15 nm/s was achieved under a low hydrogen dilution ratio of 100.

Scanning Tunneling Spectroscopy of c(2*2) Reconstructed Fe Thin-Film Surfaces.

Fe thin films with flat surfaces are obtained on a MgO(001) substrate at a growth temperature of 550 K. The surfaces with atomically flat and wide terraces exhibit a c(2×2) reconstructed structure. To evaluate the effect of impurity atoms at the surface on the surface structures, scanning tunneling spectroscopy (STS), reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS) studies were performed. The differential conductivity (dI/dV) spectrum of the c(2×2) Fe(001) thin-film surfaces indicates an intense peak at the sample bias voltage of 0.4 V. Since there is no clear evidence of impurity adatoms forming such a surface structure, we expect that the topmost atoms are Fe, and that the observed peak originates from surface states.

Molecular Orbital Theory of Field Evaporation

We present a theory based on nonempirical molecular orbital calculations for field evaporation. To investigate desorption under a high electric field, a cluster model is constructed for a silicon surface with a hydrogen atom absorbed on top of it. The peripheral dangling bonds of the silicon surface are terminated by hydrogen atoms. We obtained the potential energy surfaces for an absorbed hydrogen atom on a silicon surface under both positive and negative biases. In case the field strength is 20.0 V/nm, the activation energy decreases to 0.6 eV under a negative bias and to 1.5 eV under a positive bias, while it is 11.7 eV for the nonbiased surface. Electron population analysis reveals that the desorbed particles are a proton under a positive bias and a negative hydrogen ion under a negative bias. We employed unrestricted Hartree-Fock calculations with an STO-3G basis set.

Atomic Scale Observation of Domain Boundaries on c(2×2) Fe(001) Thin Film Surfaces

Scanning tunneling microscopy (STM) measurements reveal that Fe thin films epitaxially grown on a MgO(001) substrate under appropriate conditions have an atomic step-terrace surface with a c(2×2) surface reconstruction, and that this surface is often accompanied by line-like patterns along the Fe direction. We have investigated the electronic properties of the patterns in detail by atomically resolved STM. We found that the pattern is a domain phase boundary of the c(2×2) domains, and that the domain boundaries consist of atoms exhibiting a (1×1) structure. The STM image shows remarkable bias-voltage dependency as indicated by the contrast inversion between the patterns and c(2×2) domains as the polarity of the applied bias-voltage is changed, and the contrast achieved its maximum value at +0.2 V. This bias-voltage dependency of the patterns can be explained by the existence of surface states which are observed at around 0.2 eV above the Fermi level on the Fe(001) (1×1) surface and 0.4 eV on the c(2×2) domains.

Low Temperature Deposition of Hydrogenated Nanocrystalline Cubic Silicon Carbide Thin Films by HWCVD and VHF-PECVD

Hydrogenated nanocrystalline cubic silicon carbide (nc-SiC:H) films were successfully deposited on glass substrates at low substrate temperatures below 300 degC by hot wire chemical vapor deposition (HWCVD) and very high frequency plasma chemical vapor deposition (VHF-PECVD). We investigated structural properties of the films by spectroscopic ellipsometry and TEM observations. Photo-sensitivity of the films deposited by VHF-PECVD was higher than that of the films deposited by HWCVD. The secondary ion mass spectroscopy measurement revealed that the low photo-sensitivity of the films deposited by HWCVD was due to the metal contamination from the hot wires

Spin-Dependent Surface Characteristics of an Absorbed Hydrogen Atom under a Scanning Tunneling Microscope Environment : Atom Manipulation by Magnetic Field

We investigate the electronic states of a scanning tunneling microscope environment using first principles molecular orbital calculations, explicitly including spin states. We employ a cluster model comprised of a silicon surface and an absorbed hydrogen atom under a gold probe tip. We find that spin multiplicity of the system drastically changes the potential energy surfaces of the absorbed atom between the surface and the probe. Under the gold probe tip, it is observed that the desorption energy for a hydrogen atom from the silicon surface decreases from 4.09 eV to 2.84 eV when an external electric field, biased sample-negative, is applied along the cluster axis with the value of 0.3 V/A at the singlet spin state. At the triplet spin state, however, the barrier of the potential well completely disappears under the electric field, sample-negative bias, with the value of 0.1 V/A, which induces atom transfer from the surface to the probe. We assume that the tip-sample distance is 6.0 A.