Naomichi Sakamoto

Optical Bond Gap and Tauc Gap in a-SiOx:H and a-SiNx:H Films.

Changes in the joint density of state (JDOS) of a-SiOx:H and a-SiNx:H films with x were investigated by optical measurements. The optical bond gap E 0 which corresponds to the bonding-antibonding splitting energy was deduced from the dispersion of the refractive indices based on a single-oscillator model. The energy of the Si-Si bond, E 0 Si-Si, was estimated from E 0. The Tauc gap energy E t and the energy width of the linear tail were determined from optical transmission spectra. The correlation among these characteristic energies was investigated to estimate the JDOS in these alloys. It was found that E 0 Si-Si increases with x, while the bandwidth of JDOS derived from the Si-Si bond is kept constant for both a-SiOx:H and a-SiNx:H systems. This suggests that the increase in the Tauc gap energy is due to the increase in Si-Si bond energy rather than the change in the energy width of the Si-Si-bond-derived JDOS.

Effect of structure on radiative recombination processes in amorphous silicon suboxide prepared by rf sputtering

We studied the relationship between the structure and photoluminescence (PL) mechanism of amorphous silicon suboxide (a-SiOx) thin films prepared by the cosputtering method. The microscopic structure of the film was estimated by x-ray photoemission spectroscopy and infrared absorption spectroscopy. Electronic states were investigated by optical absorption and photothermal deflection spectroscopy. The results indicate that the film is separated into two types of regions: Si-rich cluster regions and amorphous SiO2-rich regions. The size of the Si-rich cluster is estimated, according to the quantum confinement model in which no other effects are assumed to exist, to be less than several nm, when the oxygen fraction x is larger than 1.0. The PL peak energy increased monotonically with the x value, whereas it showed different temperature dependences between the larger x value samples and the smaller ones. The PL characteristics observed can consistently be explained by assuming that there are two origins for P...

Orientation selective epitaxial growth of CeO2(100) and CeO2(110) layers on Si(100) substrates

It is found that epitaxial CeO2 layers with (100) or (110) orientation can be selectively grown on Si(100) substrates by controlling substrate bias in reactive dc magnetron sputtering. Adopting a two step growth method; ultrathin metallic Ce layer deposition at room temperature followed by a silicidation process at 800 °C, and subsequent reactive sputtering in an Ar/O2 mixture environment, the CeO2(100) layer is grown on practical Si(100) surfaces prepared by the usual wet cleaning method.

Photoluminescence properties of amorphous silicon-based oxygen and hydrogen alloys

We prepared hydrogen and oxygen alloys of a-Si by the method of rf sputtering and found similarities in their photoluminescence (PL) features. Depression of PL intensity at high temperature is reduced by alloying and results in a larger PL intensity at room temperature. Nonradiative recombination processes in the alloys are discussed in terms of the temperature dependence of the PL intensity. It is concluded that band gap fluctuations induced by alloying reduce transitions to nonradiative defect centers in these systems.

Origin of crystalline quality deterioration in epitaxial growth of CeO2 layers on Si substrates

CeO2 layers with various degrees of crystallinity are characterized to get insight into the origin of the deterioration of epitaxial layers on Si substrates. The surface morphology of CeO2 layers is studied using atomic force microscopy and is correlated with the crystallinity of the layer determined by reflection high-energy electron diffraction. It is clearly observed that the surface morphology changes according to the crystallinity of the CeO2 layers. Single crystalline CeO2(110) layers with good crystallinity have a nanometer-scale periodically corrugated surface structure, which consists of (111) facets. As the crystalline quality of the CeO2 layers becomes worse, the number of irregularly shaped hillocks and tetrahedral hillocks increases and corresponds to distorted (110) grains and (111)-oriented polycrystalline grains, respectively. It is found that the crystalline quality is not uniform; different regions of crystallinity are distributed from place to place and the ratio of their population cha...

A comparative study on structural and electronic properties of PECVD a-SiOx with a-SiNx

Abstract Structural and electronic properties of plasma-enhanced chemical vapor deposition (PECVD) a-SiO x thin films were investigated and compared with those of PECVD a-SiN x thin films. The hydrogen content of a-SiO x is much smaller than that of a-SiN x in a composition range of large x values. Discontinuous chemical shifts with x were found in the IR absorption peak of the SiH stretching mode and also in the XPS spectral peak of the Si 2p core level. These chemical shifts were continuous in a-SiN x . The results indicate that the random bonding model, which is applicable to a-SiN x , is not strictly applicable to a-SiO x . The network formation by PECVD is supply-controlled in a-SiN x but somewhat reaction-controlled in a-SiO x .

Influence of compressive load on microstructure of micro-scaled DLC structures produced by FIB-CVD

Influence of compressive load on microstructure of micro-scaled diamond-like carbon (DLC) structures was investigated by high-resolution electron transmission microscopy (HRTEM) analyses. The micro-scaled structures were produced by the focused ion-beam chemical vapor deposition method. The compressive loads were applied by a dynamic ultra micro hardness tester equipped with a flat-tip diamond indenter. Microgrid images in HRTEM observation and selected area diffraction patterns were varied by the compressive loads. The variations revealed that constituent atoms arranged disorderly in DLC were clustered by compressive loads. Electron energy loss spectroscopy measurements indicated that sp3 fraction in DLC was decreased by the compressive load. Consequently, it was suggested that graphitic clusters were formed in the microstructure of DLC by the compressive loads.

Post hydrogenation of sputtered amorphous silicon by pressurized water boiling

In this paper, we demonstrate that sputtered amorphous silicon films were hydrogenated by means of boiling in water at high pressure. For as-deposited films, infrared (IR) absorption measurements show that hydrogen atoms were evolved by boiling. On the other hand, hydrogen atoms were incorporated for annealed films in which hydrogen was once effused completely.