Kazuyuki Ikuta

Substrate dependence of initial growth of microcrystalline silicon in plasma‐enhanced chemical vapor deposition

Very thin microcrystalline silicon films have been deposited by plasma‐enhanced chemical vapor deposition (CVD) from hydrogen diluted SiH4 gas in order to study its initial growth mechanism. The dependence of the crystallinity and the Si‐H bonding configuration on electrical and chemical properties of substrates have been studied using a high sensitivity Raman spectrometer. It is found that conductive and oxygen free substrates provide good crystallinity in the initial stage. The origin of the substrate dependence is explained in terms of the ion bombardment from the plasma and the chemical nature of the substrate surface.

Growth of amorphous-layer-free microcrystalline silicon on insulating glass substrates by plasma-enhanced chemical vapor deposition

Using a triode plasma-enhanced chemical vapor deposition (PECVD) system and high H2 dilution of SiH4 (down to a SiH4/H2 gas flow ratio of 0.33/99), amorphous-layer-free μc-Si:H has been successfully grown on insulating glass substrates in the continuous PECVD growth mode. It is demonstrated that an ultrathin layer of such μc-Si:H can serve as a seed layer to facilitate an epitaxial-like growth of μc-Si:H (seeded growth).

Control of crystallinity of microcrystalline silicon film grown on insulating glass substrates

The amorphous incubation layer commonly found in microcrystalline silicon (μc-Si:H) grown on insulating glass substrates by plasma-enhanced chemical vapor deposition has been successfully eliminated simply by dilution of the SiH 4 to a SiH 4 /H 2 flow ratio < 1/99. Using an amorphous layer free μc-Si:H as a seed an epitaxial-like growth of μc-Si:H was obtained. It is shown that, with the seeded growth scheme, one can grow amorphous layer free thin μc-Si:H films and control the crystallinity of the films with relative ease.

SELECTIVE-AREA FORMATION OF SI MICROSTRUCTURES USING ULTRATHIN SIO2 MASK LAYERS

We have developed a technique to form Si microstructures at preassigned positions on Si substrates. The key element of this technique is resistless patterning of ultrathin SiO2 mask layers by direct electron-beam exposure. Selective-area growth of Si was accomplished by two different chemistries: flow-modulated plasma-enhanced chemical vapor deposition (CVD) at 473 K or ultra-high-vacuum CVD at 853 K. Epitaxial deposition was achieved by the latter growth method when a mask layer with minimum thickness for deposition selectivity (approximately 0.2 nm) was employed.

STM Observation on the Initial growth of Amorphous and Microcrystalline Silicon Films

Direct nanoscale observation on the nucleation and growth of hydrogenated amorphous and microcrystalline silicon on graphite substrates was made using scanning tunneling microscopy, atomic force microscopy, and Raman scattering spectroscopy. Nucleation of hydrogenated silicon clusters is initiated through the nucleation sites created by reactive hydrogen species coming from the source gas plasma. The difference in spatial distribution of nucleated clusters at the initial stage of deposition between a-Si:H and {micro}c-Si:H is ascribed to the difference in the number density of nucleation sites which results in difference in the diffusion length of a SiH{sub 3} radical at the initial stage of deposition on the graphite substrate. The RMS roughness of {micro}c-Si:H films is larger than that of a-Si:H when the film thickness is larger than 10 {angstrom}, which is opposite to the behavior at the initial nucleation stage on the graphite substrate.

Nucleation and coalescence in hydrogenated amorphous silicon studied by scanning tunneling microscopy

Direct subnanometer‐scale observation was made on an ultrathin film of hydrogenated amorphous silicon deposited on a highly oriented pyrolytic graphite substrate, using a ultrahigh vacuum scanning tunneling microscopy. Subnanostructures with a size of 5–10 A were observed on the top surface independent of the film thickness below 400 A, which are speculated to be SiH3. It is demonstrated that coalescence between nuclei (clusters) is enhanced by a surface diffusion of SiH3 precursors.

Comparative Evaluation of Ultrathin Mask Layers of SiO2, SiOxNy and SiNxfor Selective Area Growth of Si

Ultrathin SiO2 and SiNx layers on Si are potential mask materials for nanoscale selective-area chemical vapor deposition (CVD) in reduced dimension. This study investigates the mask-material dependence of Si nucleation processes on these ultrathin layers. Thin layers of SiO2, SiNx and SiOxNy were formed by plasma oxidation and nitridation. They were subjected to CVD processing without exposure to air, which prevented the results from being affected by surface contamination. Incubation time for nucleation was checked by both in-line Auger electron spectroscopy (AES) and atomic force microscopy (AFM). In the case of ultrahigh vacuum CVD (UHV-CVD) at 853 K, the incubation time for nucleation decreased in the order of SiO2, SiOxNy and SiNx. In the case of flow-modulated plasma-enhanced CVD (FM-PECVD) at 473 K, Si growth did not depend on the kind of the mask material. Direct electron-beam patterning of this mask layer is briefly reported. Based on the results presented, a novel bilayer mask structure is proposed.

Pulsed ESR Study of the Conduction Electron Spin Center in μc-Si:H

The spin-lattice relaxation times (T 1 ) of conduction electron (CE) and dangling bond (DB) centers in μc-Si:H have been directly measured using the 3-pulse inversion recovery method. For both CE and DB, the inversion recovery curve follows a stretched exponential form. T 1 of DB is about twice the T 1 of CE, however the temperature dependence of T 1 seems to be the same for both CE and DB and can be approximated by T −4 While the DB echo decay is modulated by both 29 Si and 1 H nuclei, we found no modulation of the CE echo decay by the H nucleus, indicating that CE centers are located in H-depleted phases in μc-Si:H. The modulation results are direct evidence that CE centers are located in the crystalline grains and DB centers in the amorphous phases.

Ultra Thin SiO 2 Mask Layer for Nano-Scale Selective-Area Pecvd of Si

We discuss the applicability of ultrathin SiO 2 layers as a mask for low-temperature selective-area deposition of Si. Thin oxide layers with estimated thickness ranging from 4 to 20 A were formed by oxidizing H-terminated Si(100) surfaces by a remote plasma exposure at room temperature. Low-temperature selective-area deposition was carried out using two different techniques: flow-modulated plasma-enhanced chemical vapor deposition (FM-PECVD) using SiH 4 and H 2 , and very low pressure CVD (VLPCVD) using Si 2 H 4 . We show that the ultra-thin plasma oxide layers exhibit good properties for a use as a passivating mask layer, and that the oxide layer can be patterned directly by E-beam irradiation. These results open up a possibility to realize Si-nanostructures formation by selective-area processing. Degradation of the oxide layer by plasma processing is also discussed.

SELECTIVE AREA GROWTH OF SI ON THIN INSULATING LAYERS FOR NANOSTRUCTURE FABRICATION

A new method of fabricating Si microstructures on thin insulating films is described. It combines selective-area growth of Si with electron-beam direct patterning of a SiO2/SiNx bilayer mask. The chemical composition of the top SiO2 layer of the mask can be modified locally by electron-beam irradiation. The different chemical properties between irradiated and nonirradiated surfaces make it possible to deposit Si only on the irradiated surface. The bottom SiNx layer is stable against electron-beam irradiation, and thus insulates electrically the deposited Si microstructure from the Si substrate. Selective-area growth of Si was performed by ultrahigh-vacuum chemical vapor deposition (UHVCVD).