Satoshi Fujii

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 260u2009°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.

Diamond-based surface acoustic wave devices

Research and development have been carried out to apply the CVD diamond film to surface acoustic wave (SAW) devices. Several kinds of layered structures including a diamond layer have been investigated by the calculations and experiments, and it has been found that the diamond SAW has great advantages for the application of high-frequency SAW devices with high SAW velocity, small temperature coefficient and high power durability. Practical SAW devices have been successfully fabricated with ZnO/diamond/Si and SiO2/ZnO/diamond/Si structures whose characteristics are superior to those with conventional SAW materials.

Low-Loss Diamond Surface Acoustic Wave Devices Using Small-Grain Poly-Crystalline Diamond

We have developed a poly-crystalline diamond film whose grain size is about 0.5 µm, which is smaller than that of our previously studied diamond films applied in surface acoustic wave (SAW) devices. Using the small-grain diamond, three kinds of narrow-band SAW filters were fabricated with the SiO2/interdigital-transducer (IDT)/ZnO/diamond/Si structure at the center frequencies from 2.488 to 5.0 GHz. The insertion losses of these filters were lower by 1.3 to 3.2 dB and the Q values were 200 higher than those obtained using the standard diamond. The Q value is defined as the center frequency divided by the bandwidth at a loss level 3 dB lower than the minimum insertion loss. Standard diamond refers to our previously studied diamond. The propagation loss, electro-mechanical coupling coefficient (K2), phase velocity and temperature coefficient of frequency (TCF) were investigated on the basis of the measured characteristics of these filters. It was found that the propagation loss was about two thirds of that obtained using the standard diamond film while TCF, K2 and phase velocity were almost the same as those obtained using the standard diamond.

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.

Annealing behaviours of defects in electron-irradiated diamond probed by positron annihilation

Annealing behaviours of defects in electron-irradiated diamond were studied using the positron annihilation technique. For a type IIa specimen after 3 MeV electron irradiation with a dose of 1 × 1018 cm-2, the major species of vacancy-type defects was determined to be neutral monovacancies, V0. The trapping rate of positrons by V0 decreased above 600 °C annealing, but the annihilation mode of positrons trapped by vacancy-type defects was observed even after 900 °C annealing. For a type Ib specimen after the irradiation, the major species of vacancy-type defects was determined to be negative monovacancies, and the formation of nitrogen-monovacancy pairs, N-V, was observed after 650 °C annealing. The annihilation probability between positrons and electrons with a broad momentum distribution was found to be increased by the trapping of positrons by N-V.

Defects in synthesized and natural diamond probed by positron annihilation

Defects in synthesized and natural diamond were studied using the positron annihilation technique. For a synthesized type IIa specimen, the lifetime of positrons annihilating from the free state was determined to be 98.7 ps. For a synthesized type Ib specimen, the effects of the annihilation of positrons trapped by open spaces introduced by substitutional nitrogen atoms on the positron parameters were discussed. After electron irradiation, the species of the major vacancy-type defects was identified to be a neutral and/or negatively charged monovacancy. For natural type IIa and IIb specimens, the annihilation mode of positrons trapped by vacancy clusters was observed. For the natural type IIb specimen, the temperature dependence of the trapping rate of the vacancy clusters was explained assuming that these clusters act as compensators for acceptor impurities (boron).

Characterization of diamond single crystals by means of double-crystal X-ray diffraction and positron annihilation

The crystallinity of synthesized and natural crystals of diamond was characterized by double-crystal X-ray diffraction and positron annihilation. The two-dimensional angular correlation of annihilation radiation and positron lifetime measurements revealed that in natural crystals positroniums are formed in a high fraction. The synthesized crystal Ib showed both an extremely small width for the diffraction and a positron lifetime spectrum with a single component of the lifetime of 115 ps. In contrast, the natural diamonds contain a long-lived component of lifetime longer than 2 ns. The diffusion length of positrons was also measured by a variable-energy positron beam. In the synthesized crystal IIa, a diffusion length of about 100.8 nm was observed.

The study of native defects in as-grown GaAs by positron annihilation

A mono-energetic positron beam was applied for a study of native defects in GaAs. From measurements of Doppler broadening profiles and lifetime spectra of positrons, it was found that Ga-vacancies were introduced by Si-doping. On the other hand, for p-type GaAs, interstitial clusters were found to be introduced by Zn-doping. The observed species of defects are in agreement with those expected from the Fermi level.

Characterization of vacancy‐type defects in Se‐implanted GaAs by means of a slow positron beam

Variable‐energy (0–30 keV) positron beam studies have been carried out on 200 keV Se‐implanted GaAs specimens before and after annealing for the electrical activation. From the measurements of Doppler broadened profiles of the positron annihilation as a function of the incident positron energy, it was found that vacancy clusters with high concentration were introduced especially in the annealed specimens after Se implantation. From the parallel measurement of electric characteristics, the higher activation efficiency was found to be obtained for the higher concentration of vacancy clusters. This fact implies that electrons supplied by the activation of Se also convert the charge state of As vacancies from positive to negative.