Junji Shirafuji

Electrical properties of diamond surfaces

Abstract An attempt is made to explain the presence of a low-resistivity layer on the surface of as-grown undoped CVD films and the unusual nature of the surface sensitivity to various surface treatments in terms of band bending. The experimental results of contact potential difference measurement by Kelvin probe and C 1s core level emission spectrum by X-ray photoelectron spectroscopy analysis are correlated reasonably with the band-bending scheme in combination with the Fermi-level position in the bulk and with the pinned surface Fermi level (Ev + 1.7 eV) in oxygenated samples. The energy band of the as-grown surfaces bends upwards to form an accumulation layer for holes owing to the existence of acceptor-type surface states well below the bulk Fermi level. On the other hand, on oxygen-terminated surfaces produced by oxygen-ambient annealing or oxygen-plasma treatment, there is a depletion layer for holes because of donor-type surface states existing about 1.7 eV above the valence band. The origin of these surface states is not known at present.

Electrical conduction in undoped diamond films prepared by chemical vapor deposition

Electrical conduction along conducting layers between grains in chemical vapor deposited polycrystalline diamond films has been studied. The dc current‐voltage characteristic consists of an ohmic region at low voltages and a highly nonlinear region at high voltages. As‐grown films are dominated by the ohmic conduction with a small activation energy through disordered graphitic (disordered sp2 bonded carbon) regions between grains. When the as‐grown film is annealed at 670 K, the conductivity of the ohmic region becomes governed by levels at 0.93 eV leading to a drastic decrease by several orders of magnitude, and the nonlinear conduction dominates. Hydrogenation of the annealed sample causes an increase in the conductivity around room temperature with little change in the high‐temperature conductivity governed by 0.93 eV levels.

Characterization and Field Emission of Sulfur-Doped Boron Nitride Synthesized by Plasma-Assisted Chemical Vapor Deposition

Polycrystalline boron nitride (BN) films are synthesized using BCl3 and N2 as source gases, by plasma-assisted chemical vapor deposition. BN films consist of nanocrystalline grains of 3 nm in size. The energy gap is estimated to be 6.0 eV from ultraviolet-visible optical transmission measurement. The electrical resistivity is also estimated to be 2×1011 and 4.9×102 Ω cm for undoped and sulfur (S)-doped BN films, respectively. Electron field emission is observed from S-doped BN film deposited on the Si substrate. The emission current is detected at electric fields higher than 9 V/µ m. The emission current of 10 µ A is obtained at 21 V/µ m. This field emission characteristic is compared with that of polycrystalline diamond films treated with H2 plasma.

Electron emission from boron nitride coated Si field emitters

Electron emission characteristics of sulfur (S)-doped boron nitride (BN) films synthesized by plasma-assisted chemical vapor deposition (PACVD) are investigated. The BN film consists of hexagonal grains of 3 nm in size. The energy gap is estimated to be as wide as 6.0 eV from ultraviolet-visible optical transmission measurement. The electrical resistivity is reduced to 4.9×102 Ω cm. Si tip field emitters coated with the BN film are fabricated. The electron emission occurs at an electric field as low as 6 V/μm, while a high electric field of 20 V/μm is needed to emit electrons from the Si tip array without BN coating. It is deduced that the tunneling barrier height of 0.1 eV exists at the surface of the BN film.

Raman and Infrared Studies on Vibrational Properties of Ge–Se Glasses

Raman scattering and infrared absorption measurements have been performed on amorphous Ge x Se 1- x with 0≦ x ≦0.4 and crystalline GeSe 2 . Molecular model is applied to explain the vibrational spectra of amorphous GeSe 2 . Variation of Raman spectra in Ge x Se 1- x glasses with Ge composition shows that local order is not largely changed with the glass composition. The vibrational spectra, particularly infrared absorption spectra, of amorphous and crystalline GeSe 2 are very similar; suggesting that amorphous GeSe 2 may have the same local order as crystalline GeSe 2 .

Optimization of single crystal preparation of Bi2Sr2CaCu2Ox superconductor by the travelling solvent floating zone method

Optimization of single crystal preparation of the Bi2.2(Sr, Ca)2.8Cu2.0Oy superconductor by the travelling solvent floating zone method has been performed with the intention of obtaining large single crystals and improving the superconductive transition temperature, Tc. The composition ratio, Sr/Ca, and the oxygen partial pressure of growth atmosphere, PO2 , were varied in the crystal growth. A single crystal typically obtained was a thin plate of the dimensions of 5×50×2 mm3. The thickness of the crystal was the largest in the crystal growth at Sr/Ca=1.4, while no distinct change was observed in the resulting crystals in the range of 22~100 kPa. Electric and magnetic properties, the formal charge of copper ions and existence of ion defects are discussed.

Edge and Donor-Acceptor Pair Emissions in Cadmium Telluride

The luminescent properties of the edge and 1.42 eV emission bands in CdTe single crystals have been investigated in the temperature range from 140 to 4.2 K. Two distinct edge emission spectra (1.545 and 1.528 eV bands) are found in as-grown p-type crystals depending on crystal growth conditions. The time-resolved spectra and the dependence on the excitation intensity of the photoluminescence confirm that the 1.42 eV band which is usually observed in both n- and p-type CdTe is associated with transitions between donors and acceptors. The values of the ionization energy of donors and acceptors and some parameters involved in the pair emission are estimated.

A New Type of Tunnel-Effect Transistor Employing Internal Field Emission of Schottky Barrier Junction

A new type of tunnel-effect transistor, which has nearly the same structure as conventional metal-oxide-silicon field-effect transistors (MOSFETs) except for a Schottky barrier source contact and a low resistivity channel layer, has been proposed. The structure has the advantage of an easy fabrication process and is capable of submicron channel length without the short channel effect. In the proposed device the drain current is controlled by the gate bias through the tunnel injection of electrons at the Schottky barrier source contact. A 2-D device simulation has shown that this device can have a high transconductance of 138 mS/mm at a drain voltage of 2 V.

Photoluminescence Studies in Irradiated Si-Doped Gallium Arsenide

In order to reveal the origin of the 1.40–1.42 eV band which is peculiar to Si-doped GaAs, elaborate investigations of photoluminescence spectra in irradiated and heat-treated Si-doped samples were carried out mainly at 77K. All experimental evidences supported the assignment that the band was caused from a complex of Si atom on As site with As vacancy. Unidentified emission bands at 1.305 and 1.44 eV were observed respectively in (Te + Si)-doped samples subject to electron irradiation and Si-doped samples fired in a dissociation As pressure.

X-ray photoelectron spectroscopy analysis of plasma-treated surfaces of diamond films

Abstract The surface band bending of polycrystalline diamond films treated with O 2 or H 2 plasma has been investigated by means of X-ray photoelectron spectroscopy. A shift in the binding energy in the carbon 1s core level spectrum is observed depending on the plasma treatment. The surface Fermi level moves away from the valence band edge by O 2 plasma treatment, while it comes close to the valence band edge by H 2 plasma treatment. The energy difference between the surface Fermi level positions of the O 2 - and H 2 -plasma-treated samples is estimated to be about 0.7 eV. It is also observed that the surface Fermi level of the Ar-sputtered sample is located closer to the valence band edge than that of the H 2 -plasma treated sample.