Shin-Ichi Shikata

Diamond(001) single‐domain 2×1 surface grown by chemical vapor deposition

Diamond homoepitaxial films were grown on an off‐angle (001) substrate with a misorientation of 4.3° toward the [110] direction by microwave plasma‐assisted chemical vapor deposition from a methane‐hydrogen gas mixture. The single domain 2×1 surface was observed by low‐energy electron diffraction (LEED) and scanning tunneling microscopy, after growth with methane concentration of 2%. With a methane concentration of 6%, the LEED superstructure spot intensity from another domain increased, suggesting a higher rate of two‐dimensional nucleation.

HETEROEPITAXIAL GROWTH OF ZNO FILMS ON DIAMOND (111) PLANE BY MAGNETRON SPUTTERING

ZnO thin film has been epitaxially grown on the (111) plane of the diamond substrate by rf magnetron sputtering at substrate temperature as low as 260 °C. The crystallinity was examined by x‐ray diffraction and reflection high‐energy electron diffraction. It was found that the smallest standard deviation angle estimated from the x‐ray rocking curve of the ZnO(0002) peak was 0.27° whereas the mismatch of the lattice parameter between film and substrate is 28.8%. The epitaxial relationship between ZnO film and the diamond is determined as [1120] ZnO//[101] diamond.

THERMALLY STABLE PD/GE OHMIC CONTACTS TO N-TYPE GAAS

The thermally stable Pd/Ge ohmic contacts to n‐type GaAs are obtained by a rapid thermal annealing method. Compared to the conventional AuGe based ohmic metals, the surface morphology is smooth with high uniformity of constituent. A small deterioration from 1.01.0× 10−5 Ω cm2 to 1.2 × 10−5 Ω cm2 is observed at 300 °C for 103 h. The activation energy was 1.9 eV and the lifetime of this contact, defined as the time of 10% deterioration, is expected to be over 109 years at 70 °C. The mechanism of the deterioration for Pd/Ge ohmic contacts is also discussed, utilizing Auger electron spectroscopy and x‐ray photoemission spectroscopy analysis.

High frequency bandpass filter using polycrystalline diamond

Abstract Because of its high Youngs modulus, diamond has the highest acoustic velocity of all materials, and thus for surface acoustic wave (SAW) devices diamond can be expected to be one of the candidates for passive component applications. Polycrystalline diamond 40 μm thick was deposited on a silicon substrate by hot-filament chemical vapour deposition. After the diamond surface had been polished, 100 nm of Al was deposited and fabricated into interdigital transducers, utilizing conventional photolithography and etching processes. This was followed by the deposition of a ZnO thin film, which served as a piezoelectric material to generate SAWs. Finally, an SAW device using a ZnO/diamond system was fabricated for the first time. With the high SAW velocity of the ZnO/diamond system being up to 8600 m s, −1 a 1.07 GHz bandpass filter was successfully demonstrated using 2 μm line and space interdigital transducers; additionally −25 dB insertion loss was observed. This preliminary result suggests that, by utilizing diamond, high frequency (2–5 GHz) bandpass filters can be made available, and broad applications in the optical and personal communication systems can be expected.

Diamond homoepitaxial growth on (111) substrate investigated by scanning tunneling microscope

Epitaxial films were grown on diamond(111) substrates by microwave‐plasma‐assisted chemical vapor deposition, and their surfaces were studied by scanning tunneling microscope (STM). STM images with atomic order resolution were obtained, and showed 1×1 periodicity, which was also observed by low‐energy electron diffraction. 〈112〉 single bilayer steps, 〈112〉 single bilayer steps, and 〈112〉 double bilayer steps were observed, and the majority of the steps were found to be of the first type. Steps were found to be very straight in the atomic scale, and there were suggestions of the incorporation of carbon atoms at the kinks. Even a small island with a nanometer order size showed a triangular shape surrounded by three 〈112〉 single bilayer steps.

High frequency surface acoustic wave filter using ZnO/diamond/Si structure

The surface acoustic wave (SAW) characteristics in a ZnO/diamond/Si structure are studied. Using theoretical calculations, effective film thicknesses of ZnO and diamond providing high phase velocity and K2 are found. Based on these data, a SAW filter of a ZnO/diamond/Si structure is fabricated. It is shown that large velocities can be obtained as expected from theoretical calculation and that a high-frequency SAW filter can be fabricated with much larger scale interdigital transducers (IDTs) than those in conventional material systems. The results indicate that this material system will find broad application in high-frequency SAW filters and resonators.<<ETX>>

Synthesis and Surface Acoustic Wave Property of Aluminum Nitride Thin Films Fabricated on Silicon and Diamond Substrates Using the Sputtering Method

C-axis oriented aluminum nitride (AlN) thin films with a thickness of 1 µm were prepared by reactive DC magnetron sputtering on polycrystalline diamond substrates at a substrate temperature of 623 K. The average surface roughness (Ra) of the AlN thin films was less than 2 nm obtained by locating the diamond substrates at a position of 100 mm from the aluminum target. The full width at half maximum (FWHM) of the rocking curve for the AlN(002) peak determined by X-ray diffraction analysis was about 0.2°. The surface acoustic wave (SAW) structures were completed by the deposition of aluminum electrodes on the as-deposited AlN surfaces. The SAW characteristics of an interdigital transducer (IDT)/AlN/diamond structure were investigated. The phase velocity and coupling coefficient were 10,120 m/s and 0.3%, respectively.

Vacancy-type defects in ion-implanted diamonds probed by monoenergetic positron beams

Vacancy-type defects introduced by 180-keV B+-, C+- and N+-ion implantation in synthesized diamonds (type Ib) were probed by the positron annihilation technique. For an unimplanted specimen, the diffusion length of positrons was shorter and the lifetime of positrons was longer than those for a type IIa specimen. These facts were attributed to the trapping or the scattering of positrons by nitrogen-related defects. For ion-implanted specimens, the depth distributions of vacancy-type defects were determined from measurements of Doppler broadening profiles of the annihilation radiation as a function of incident positron energy. The obtained profiles of defects were in agreement with those of an energy loss of ions calculated by the Monte Carlo method. From measurements of lifetime spectra of positrons, the dominant defect species introduced by the B+- or N+-ion implantation were identified as complexes of vacancy clusters and these impurities.

Study of various types of diamonds by measurements of double crystal x‐ray diffraction and positron annihilation

Annihilation characteristics of positrons in various types of diamonds were studied by measurements of two‐dimensional angular correlation of positron annihilation, those of Doppler Broadening profiles and those of lifetime spectra. From the measurements, it was found that there is a small amount of defects in type Ib synthesized diamond and the positron lifetime is 115 ps. On the other hand, it was found that positronium is formed in natural diamonds. Results of the double x‐ray measurements suggested that IIa, IIb, and Ia type natural diamonds also may have empty space between crystallites or grain boundaries.

Suppression of the emitter size effect on the current gain of AlGaAs/GaAs heterojunction bipolar transistor by utilizing (NH4)2Sx treatment

The (NH4)2Sx surface treatment was applied to the AlGaAs/GaAs heterojunction bipolar transistor. The suppression of the emitter size effect on the current gain was observed for up to 4×4 μm2 emitter size devices, and 2.5 times high current gain was obtained with the improvement of the ideality factor. These results indicate that the surface recombination at emitter‐base junction area are largely reduced by this treatment. The (NH4)2Sx treatment also proved to be applicable to the conventional device fabrication processes and highly reliable for the heat treatment.