Koji Kobashi

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.

Smooth and high-rate reactive ion etching of diamond

Diamond surfaces with patterned Al masks were etched by a reactive ion etching (RIE) system under conditions that the RF power was 100-280 W, the CF 4 /O 2 ratio was 0-12.5% and the gas pressure 2-40 Pa. It was found that the roughness of the etched diamond surface decreased with an increase in the CF 4 /O 2 ratio, although this reduced the selective etching ratio of diamond against Al. The gas pressure also affected the surface roughness and the etching anisotropy. The etching rate of diamond considerably increased upon a small addition of CF 4 in O 2 . Based on these results, we were successful in an anisotropic etching of diamond at a very high rate (∼9.5 μm/h) with a smooth etched surface (R a < 0.4 nm), a high selective etching ratio of diamond vs. Al, and a high aspect ratio (the height/diameter was ∼8 for array structures and ∼25 for exceptional cases) by choosing appropriate etching conditions.

On the ‘‘band‐A’’ emission and boron related luminescence in diamond

Cathodoluminescence spectra of undoped and boron‐doped diamond films are reported and are compared with each other. The dominant spectra feature of these samples is the broad luminescence bands centered around 2.83 and 2.32 eV in undoped and relatively heavily doped samples, respectively. Our result indicates that the commonly observed 2.83 eV band (often called by the name ‘‘band‐A’’) is neither related to boron nor to donor‐acceptor pairs in diamond. From all of the available data, we conclude that the 2.32 eV band is due to boron related centers and the 2.83 eV band is due to dislocations in diamond.

Selected‐area deposition of diamond films

Selected‐area deposition of diamond film has been accomplished on Si substrates prepared by two different methods: reactive‐ion etching (RIE) and amorphous‐Si masking (ASM). In the RIE method, a Si substrate polished by a diamond paste was patterned with a photoresist mask, and the unprotected areas were etched by RIE, followed by a complete removal of the photoresist films. The diamond deposition was done by electron‐assisted chemical‐vapor deposition (CVD), and diamond films grew only in the areas once covered with the photoresist films and not etched by RIE. In the ASM method, a polished Si substrate was also photolithographically masked with photoresist, followed by a uniform deposition of a hydrogenated amorphous silicon (a‐Si:H) film. The photoresist film was then lifted off together with the overlay of deposited a‐Si:H, leaving the polished Si surface patterned with an a‐Si:H mask. In this case, the diamond deposition was done by microwave plasma CVD, and diamond films grew only in the areas not co...

Heteroepitaxial diamond growth on platinum(111) by the Shintani process

Abstract Diamond films with (111) facets were grown heteroepitaxially on Pt(111) using microwave plasma enhanced chemical vapor deposition. It was observed that many of the neighboring facets coalesced with each other. X-Ray diffraction analyses revealed that the (111) planes of diamond films were parallel to the substrate surface and azimuthally well oriented. The films were confirmed as diamond using Raman spectroscopy.

The analysis of defect structures and substrate/film interfaces of diamond thin films

Diamond is an excellent candidate material for use in selected electronic and wear resistant coating applications due to its superior hardness, strength and thermal conductivity as well as its high electron drift velocity, chemical and thermal stability, radiation hardness and optical transmission. Electronic devices of particular interest include those having high-power, -frequency and -temperature applications, as well as those for chemically harsh and/or high radiation flux environments. The recent development of techniques for growth of crystalline diamond films at low pressures using common hydrocarbon and H2 gases has created the potential for growing thin films for such devices or wear resistant coatings and a host of related applications. In this research, diamond thin films grown from a low pressure methane-hydrogen gas mixture by microwave plasma enhanced chemical vapor deposition (CVD) have been examined by various transmission electron microscopy (TEM) techniques including bright and dark field, high resolution (HREM), selected area diffraction (SAD) and electron energy loss spectroscopy (EELS). Columnar growth of polycrystalline grain structure, twins, stacking faults, dislocations and intermediate layers were characteristic of the diamond films. No sp2 bonding character in the grains, defects or grain boundaries was detected by EELS.

Metal‐intrinsic semiconductor‐semiconductor structures using polycrystalline diamond films

The electrical characteristics of a metal‐intrinsic semiconductor‐semiconductor structure formed by Al‐undoped polycrystalline diamond‐B‐doped polycrystalline diamond were investigated. Boron‐doped diamond films containing B‐to‐C ratios of 400 and 4000 ppm in gas phase were deposited on (111)‐oriented B‐doped Si substrates. Subsequently, undoped diamond layers were deposited on the B‐doped diamond films for 60 min. The existence of a bilayer structure in terms of the atomic B concentration was confirmed by a secondary‐ion mass spectroscopy. Significant improvements in the rectifying characteristics could be obtained with the introduction of an undoped diamond layer.

Fabrication of diamond thin-film thermistors for high-temperature applications

Abstract CVD diamond thin-film thermistors have been fabricated in various geometries and at different doping levels in an effort to achieve practical resistance- vs. -temperature characteristics. Typical activation energy values reported for polycrystalline films were combined with a targeted resistance range to plot an idealized relationship between resistance and temperature for CVD diamond thin films. The optimum device geometry and boron concentration were subsequently approximated from these ideal plots. Fabricated devices were electrically characterized in air at temperatures ranging from 25°C to over 500°C. Repeatability was demonstrated over two temperature cycles and stability was maintained at 500°C for 9 h. Effects of varying thermistor geometry and boron dopant concentration to achieve useful resistance-temperature relationships will be discussed.

Morphology of heavily B-doped diamond films

B-doped diamond films were synthesized by microwave plasma chemical vapor deposition using a mixture of methane (0.5% or 1.2%) and diborane (B 2 H 6 ) below 50 ppm on either Si substrates or undoped diamond films that had been synthesized using 0.5% or 1.2% methane. The surface morphologies of the synthesized films were observed by Secondary Electron Microscopy, and the infrared absorption and Raman spectra were measured. It was found that when diborane concentration was low, B-doped films preferred (111) facets. On the other hand, high diborane concentrations resulted in a deposition of needle-like material that was identified as graphite by x-ray diffraction.

AZIMUTHAL ROTATION OF DIAMOND CRYSTALS EPITAXIALLY NUCLEATED ON SILICON 001

Highly oriented diamond films with {001} facets were grown on Si{001} using microwave plasma enhanced chemical vapor deposition. The tilt and rotation of the diamond crystals were measured by polar x‐ray diffraction. The full widths at half‐maximum of {004} and {220} diffraction peaks were 5° and 10°, respectively. It was found that the {220} diffraction poles split into two peaks by approximately 5°. This result indicated that there were two possible azimuthal rotations about the surface normal of the substrate for the epitaxially nucleated diamond grains.