Yuichi Hiroyama

Effect of Very Thin SiC Layer on Heteroepitaxial Growth of Cubic GaN on Si (001)

We have investigated the effect of very thin SiC layer formation on Si (001) for cubic GaN growth by RF plasma-assisted molecular beam epitaxy. It is found that a cubic GaN film can be epitaxially grown on Si (001) covered with an approximately 2.5-nm-thick cubic SiC layer, while GaN grown on Si (001) without such an SiC layer results in the polycrystal growth of a predominantly hexagonal phase. In the latter case, an approximately 1-nm-thick amorphous Si layer is formed at the interface between GaN and Si by the irradiation of nitrogen plasma.

Raman Spectroscopy and Positron Lifetime Studies of Structural Relaxation and Defect Evolution in Amorphous Silicon

Correlations between structural relaxation and defect evolution in amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) have been investigated by Raman scattering spectroscopy and positron lifetime measurements. Various a-Si films were prepared by ion implantation, evaporation, and electron-cyclotron resonance plasma-enhanced chemical vapor deposition (ECR CVD). The bond angle deviation, Δθ, of nonhydrogenated a-Si decreased during 450° C isothermal annealing, while Δθ of a-Si:H increased due to dehydrogenation. Ion implanted a-Si contained only small vacancies of which size was the same as that of the divacancy in crystalline Si, which were dissociated and annihilated during annealing. On the other hand, both evaporated a-Si and ECR CVD a-Si:H contained larger vacancies, and nanometer-size vacancy clusters were formed during the annealing processes, more significantly in ECR CVD a-Si:H. Based on these experimental results, we concluded that large vacancies tend to form vacancy clusters or microvoids which suppress structural relaxation in a-Si.

Amorphization processes and structural relaxation in ion implanted Si

Amorphization processes in Si + , P + , Ge + , and As + ion implanted Si as well as structural relaxation of amorphous Si (a-Si) during low-temperature (200-450 o C) annealing have been investigated using Raman spectroscopy and depth-resolved positron annihilation spectroscopy. Based on the analysis of bond angle deviation Δθ derived from the a-Si Raman TO peak width and line-shape parameter, the S prameter, obtained by the positron annihilation measurements, we have proposed that (1) amorphization is controlled by ion-beam-induced divacancy concentration; and (2) there exist microvoids in the a-Si layers and during isothermal annealing, both of Δθ and the S parameter do not decrease monotonically but show an increase after the initial decrease due to dissociation of the microvoids

Role of Vacancy-Type Defects during Structural Relaxation of Amorphous Si

Structural relaxation processes in amorphous Si (a-Si) have been investigated using Raman spectroscopy and positron lifetime measurements. Bond angle deviation Δθ and sizes of vacancy-type defects were estimated for ion-beam-induced a-Si (IBa-Si) and ECR plasma CVD a-Si (ECRa-Si). During 450° C annealing, Δθ decreased appreciably and only small vacancies with a size less than that of the divacancy were observed in IBa-Si, while in ECRa-Si large vacancy clusters were formed and Δθ was approximately constant indicating that relaxation processes are suppressed in ECRa-Si. These results suggest that large vacancy-type defects can stabilize amorphous structures.

Structural relaxation in amorphous silicon prepared by ion implantation

Abstract Structural relaxation processes in amorphous silicon (a-Si) prepared by Si+, Ge+ and P+ implantation into Si at room temperature were investigated by Raman spectroscopy, electron spin resonance (ESR), and positron annihilation. Raman measurements indicated that the average bond angle deviation Δθ in a-Si networks first decreased with low temperature (200–450°C) annealing, but slightly increased with the annealing time. Based upon ESR and positron annihilation measurements, the density of dangling bonds and the free volume (e.g. microvoids) in a-Si layers were found to behave in a similar manner as Δθ during thermal treatments. We propose the following model for the results described above. Microvoids stored in the a-Si layer are annihilated and/or dissociated, which results in an increase in Δθ or dangling bond densities.

Defect in amorphous silicon prepared by ion implantation

Amorphization and structural relaxation in Si‐implanted Si has been studied using slow positrons and Raman spectroscopy. The S parameter in the defect region decreases slightly with the annealing temperature. However, no significant change of S parameter was observed with 200 °C and 450 °C isothermal annealing. One interesting point is that S parameter holds lower value in the amorphized region, but such a behavior cannot be observed by the W parameter. The change of TO peak half width in the Raman spectra (480 cm−1) was found to correlate with the change of S parameter. The defect configuration is expected to alter during isothermal annealing.