Masaki Okamoto

Formation of Cubic Boron Nitride Film on Si with Boron Buffer Layers

The c-BN (cubic BN)/BNX/B/Si structure was formed using plasma CVD techniques. In contrast with the direct growth of c-BN films, the adherence of the structure on Si was entirely satisfactory. The characterization of buffer layers (BN(X=0.6), BN(X=0.3), B) by X-ray photoelectron, infrared absorption spectroscopies and internal stress measurements show that the c-BN/BNX/B/Si structure is useful as a mechanically stable passivation film.

Effects of a negative self-bias on the growth of cubic boron nitride prepared by plasma chemical vapor deposition

Microcrystalline cubic BN films have been prepared on a Si substrate by plasma chemical vapor deposition. The formation of the cubic BN phase in the films necessitated an appropriate negative self-bias on the substrate electrode. Effects of the negative self-bias on the growth of the cubic phase were studied. The dependence of the deposition rate and etching rate of the films on a negative self-bias shows that the cubic phase is generated by the precursors of radical or ion species induced by an ion current in a plasma, and the deposition rate is determined by the generation rate due to these precursors and the etching rate of the cubic phase. In contrast with this, the generation rate of the hexagonal BN phase is independent of the ion current in a plasma.

Formation of cubic boron nitride films on diamond by plasma CVD technique

Cubic boron nitride (c-BN) thin films could be deposited on diamond using the plasma chemical vapor deposition (CVD) technique at low pressure. The deposited films are characterized by infrared absorption spectroscopy and reflection high-energy electron diffraction (RHEED). The c-BN phase on diamond can be synthesized at the appropriate self-bias and microwave power. In contrast to the Si substrate, the peeling of the c-BN phase from the diamond substrate has not been observed for six months. The RHEED pattern of c-BN film on diamond shows that the c-BN film consists of microcrystals.

Optical and mechanical properties of hard hydrogenated amorphous carbon films deposited by plasma CVD

Hydrogenated amorphous carbon (a-C:H) films were deposited by plasma chemical vapor deposition from pure CH4 at a low pressure of the order of 10-4 Torr under a magnetic field to confine the plasma on negatively self-biased electrode. The reflectance spectra were measured in a wide range of 0.5–25 eV by using synchrotron radiation and a usual light source. The dielectric constants were determined by applying the Kramers-Kronig relation. Using an effective medium approximation, volume fractions of diamondlike, graphitelike and polymeric (polyethylene) components were derived. These results were compared with Raman scattering, hydrogen contents and Vickers hardness of the asdeposited and annealed films.

Imaginary Part of the Dielectric Function of Sintered and Microcrystalline Cubic Boron Nitride

The optical reflectance of cubic boron nitride (c-BN) was measured for sintered and microcrystalline samples at room temperature in the photon energy range 5–25 eV. Kramers-Kronig analysis was performed to determine the real e1 and imaginary e2 parts of the dielectric function. The spectrum of e2 for sintered c-BN shows peaks at 9.05 eV and 11.7 eV and shoulders at 10 eV, 13.2 eV and 16.7 eV. This spectrum was compared with the one theoretically obtained by Tsay et al. and assignment of the structure was made. The e2 for microcrystalline c-BN has a broad structure with a peak at 11.7 eV.

Photoacoustic Spectra of Heavily Co-Doped ZnO Powders

Photoacoustic (PA) spectra were measured for heavily Co-doped ZnO powders using a microphone at room temperature. In PA spectra, the PA signal peaks of Co2+ and Co3+ can be observed with the ZnO band-gap signals. The two PA peaks related to Co ions show the Co valency change induced by the sintering, and depend on the sintering temperature and the Co concentration. For the heavily Co-doped samples, the Co valency conversion occurred at a low sintering temperature. PA spectroscopy has potential for estimating the inner ion state change during the sintering process.

Superimposed Emissions on Enhanced Green Emission from ZnO:Pr Powders by Evacuated Sealed Silica Tube Method

The origin of the superimposed emissions in the green photoluminescence (PL) spectra of ZnO:Pr powders is discussed by comparing the PL excitation and photoacoustic (PA) spectra of ZnO:RE (Pr, Nd, and Ho) powders prepared by the evacuated sealed silica glass method. Assuming that the superimposed emissions are related to the shorter-wavelength-side light absorption of Pr3+, the absorption of Pr3+ is caused by the nonradiative recombination of excitons localized and delocalized at Pr3+ in the vicinity of the grain boudary and ZnO matrix.

Nonradiative Transition Processes Observed from Photoacoustic Spectra of ZnO Thin Films Fabricated by Pulsed Laser Deposition

Photoacoustic (PA) spectroscopy is a powerful tool for evaluating nonradiative transition processes in semiconductors. By this technique, we evaluated the nonradiative transition process (NRTP) in ZnO thin films that were fabricated by pulsed laser deposition on R-sapphire and C-sapphire substrates in vacuum and in oxygen ambient. The samples each exhibited a specific PA spectrum reflecting the characteristics based on different crystalline orientations and qualities. The relationships between a NRTP and a radiative transition process (RTP) were studied using photoluminescence and PA spectroscopy to investigate the effect of oxygen vacancy (VO) compensation in films. (1120) ZnO/R-sapphire indicated that the NRTP in the UV region and the RTP in the green-yellow region dominate strongly over transition processes. On the other hand, (0001) ZnO/C-sapphire had opposite domination relationships in the RTP and the NRTP to those of (1120) ZnO/R-sapphire. We concluded that the decrease of the NRTP that originated from the compensation of VO strongly affected the RTP in (0001) ZnO/C-sapphire and the NRTP in (1120) ZnO/R-sapphire.

Estimation of Schottky Contacts to Porous Si by Photoacoustic Spectroscopy

The photoacoustic (PA) spectra of porous Si (PS) were measured using a microphone under various surface treatments such as vibration during anodisation, vibration during rinsing, and acid surface treatment before contact deposition. The Fourier transform infrared (FTIR) spectra were also compared for the estimation of surface conditions. To obtain good contacts for electroluminescence (EL) devices, where a large injection current during operation is required, the vibration during rinsing in distilled (DI) water and hydrochloric (HCl) acid surface treatment are important. The PA signals increase with such vibration because of the enhancement of the pressure effects in pores of PS. Moreover, the decrease in PA signals caused by the surface change of pores can be also observed with the surface treatments. As the good contacts are obtained both by the vibration and surface treatments, the surface states in the pores seem to be important for the contacts to PS. This process can be monitored by PA spectroscopy in a noncontact manner because the PA signals from the PS are enhanced in the pores by the pressure effects, and PA spectroscopy can be sensitive to the change in surface of pores of PS.

Photoacoustic and Photoluminescence Spectra of Annealed 3,4,9,10-Perylenetetracarboxylic Dianhydride Films

Annealing effects on 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) films deposited on glass substrates were studied by photoacoustic (PA) spectroscopy, photoluminescence (PL) and photoluminescence excitation (PLE) measurements. PA spectra had large nonradiative peaks in a short-wavelength region, and they steeply decreased in a long-wavelength region. Differences were observed in a long-wavelength region in the PA spectra by annealing, and they depended on the annealing temperature. In PL spectra, the luminescent peak was observed at the same wavelength, but the PLE spectra changed depending on the temperature. The films were damaged by annealing, and the structural changes of film surfaces were observed. Thus, the changes of PA spectra and PLE spectra occurred related to the film morphology.