Toshio Kawahara

SiO2-based nonplanar structures fabricated using femtosecond laser lithography

SiO2-based hybrid diffractive-refractive microlenses were fabricated by femtosecond laser lithography-assisted micromachining, which is a combined process of nonlinear lithography and plasma etching. The high-aspect-ratio patterns of resist were formed by laser exposure without translating the laser spot. By scanning this rod three-dimensionally, micro-Fresnel lens patterns were written directly inside resists on the convex lenses. Then, the resist patterns were transferred to the underlying lenses by CHF(3) plasma. We obtained SiO2 nonplanar structures with smooth surfaces. This hybridization shifted the focal length of the lens by 216 microm, which was consistent with theoretical value.

Photoacoustic spectra of Sb-doped ZnSe

Photoacoustic (PA) spectra were made on Sb-doped ZnSe samples grown by metalorganic vapor phase epitaxy (MOVPE). The samples deposited at a low temperature under irradiation show the PA spectrum with a sharp edge near the band gap and with three distinct peaks. From the Sb flow rate dependence of PA peaks, two of them seem to be related to Sb impurities. Non-doped sample shows only one peak, which is tentatively ascribed to the deep level associated with Se defects.

Coordination number constraint models for hydrogenated amorphous Si deposited by catalytic chemical vapour deposition

We measured structure factors of hydrogenated amorphous Si by x-ray diffraction and analysed the obtained structures using a reverse Monte Carlo (RMC) technique. A small shoulder in the measured structure factor S(Q) was observed on the larger Q side of the first peak. The RMC results with an unconstrained model did not clearly show the small shoulder. Adding constraints for coordination numbers 2 and 3, the small shoulder was reproduced and the agreement with the experimental data became better. The ratio of the constrained coordination numbers was consistent with the ratio of Si–H and Si–H2 bonds which was estimated by the Fourier transformed infrared spectra of the same sample. This shoulder and the oscillation of the corresponding pair distribution function g(r) at large r seem to be related to the low randomness of cat-CVD deposited a-Si:H.

Structural Analysis of Hydrogenated a-Si Films by Reverse Monte Carlo Simulation

Reverse Monte Carlo (RMC) simulation is one of the most useful tools for determining the three-dimensional (3D) atomic structure of non hydrogenated amorphous silicon (a-Si) or a-Si containing only a small atomic percentage of hydrogen. In this paper, the applicability of RMC simulation to conventional hydrogenated a-Si (a-Si:H) with a hydrogen content of 10% or more, which is prepared by plasma enhanced chemical vapor deposition (PECVD), is extensively studied. The coordination constraint method, in which coordination numbers are specified in advance as boundary conditions of the RMC simulation, is introduced using information on the concentration of Si–H and Si–H2 bonds from infrared (IR) absorption measurements. The consistency of the RMC structural model with other experimental results obtained by Raman spectroscopy and small angle X-ray scattering is checked to confirm the validity of the RMC analysis itself. It is found that the RMC simulation with coordination constraints can be adapted even for the PECVD a-Si:H containing 10% hydrogen, and that the method is applicable to know the 3D atomic structure of various types of a-Si film.

Possibility of reverse Monte Carlo modelling for hydrogenated amorphous Si deposited on reactive ion etched Si substrate

We examined the x-ray diffraction (XRD) patterns of hydrogenated amorphous Si (a-Si:H) and of crystalline Si (c-Si) substrate for high-Q measurements. A structural analysis of thin films on substrates is important for the development of real devices. A transmission geometry with high-energy x-rays was used for this investigation, together with very thin substrates, in an effort to reduce substrate signals. A small area of the substrate was etched using the reactive ion etching (RIE) plasma process to maintain free-standing structures, and a-Si was deposited using catalytic chemical vapour deposition techniques. The x-ray beam was focused on the processed area and a-Si diffraction using a thin Si layer was measured. Unlike a-Si:H films on substrates without etching, we succeeded in detecting amorphous signals from samples deposited on the processed substrate. Application of reverse Monte Carlo (RMC) modelling using these data and subtracting Si substrate peaks was investigated. Direct subtraction and MCGR program (Pusztai and McGreevyxa01997xa0Physicaxa0Bxa0234-236xa0357-8) normalization for the ratio estimation between c-Si and a-Si:H structure factors was employed. MCGR normalization was found to improve subtraction of the c-Si peaks and the first peak at r = 2.3 in the pair distribution function g(r) could be calculated.