Kimihisa Matsumoto

Oxidation processes of surface hydrogenated silicon nanocrystallites prepared by pulsed laser ablation and their effects on the photoluminescence wavelength

Natural oxidation processes of surface hydrogenated silicon nanocrystallites prepared by pulsed laser ablation under various hydrogen gas pressures are discussed by measuring the vibrational frequency of Si–Hn units on the surface and intensity of Si–O–Si stretching vibration. The surfaces of nanocrystallites are predominantly composed of Si–H bonds and oxidation starts from backbonds of these bonds. The deposited nanocrystal films have a porous secondary structure which depends on the background gas pressure. The oxidation rate observed by infrared absorption measurements depended on this porous secondary structure. The oxidation process is discussed by the correlation between oxidation rate and porous structure of nanocrystal film. We found that Si–O bond density increases with covering the surface of the nanocrystallites during the diffusion of oxygen-related molecules through the void spaces in the porous structure. The surface oxidation of each nanocrystallite is not homogeneous; after the coverage o...

Correlation between Natural Oxidation Process and Photoluminescence Properties of Hydrogenated Si Nanocrystallites Prepared by Pulsed Laser Ablation

Natural oxidation processes of hydrogenated Si nanocrystallites were investigated to clarify effects of surface oxidation on photoluminescence wavelength. Hydrogenated Si nanocrystallites were prepared by pulsed laser ablation in hydrogen gas ambient. The Si–H bonds on the surface of the nanocrystallites enable us to estimate the local configuration of Si–O bonds using infrared frequency shifts. The natural oxidation process was investigated by measuring the density and local configuration of Si–O bonds. The oxidation process can be classified into first and second stages. The first stage is due to the diffusion of oxygen molecules in the nanocrystallites through voids in the porous structure, and the second stage is due to the oxidation of each nanocrystallite from the top surface to the sub-surface. The configurations of Si–O bonds in the first and second stages are silicon-rich and oxygen-rich compositions, respectively. The photoluminescence wavelength was blue-shifted with increasing Si–O bond density. This PL peak shift was not continuous, but three PL peak regions at 800, 600–700, and 400–500 nm were observed. This result indicates that the origin of this PL peak shift is not due to quantum confinement because of decreased diameter of Si nanocrystallites, but is due to the existence of surface oxide. A photoluminescence peak at 800 nm was observed in fresh specimens, and those at 600–700 and 400–500 nm were observed from the first and second stages of oxidation, respectively.

Marked increase in photoluminescence from porous Si aged in ethanol solution

Photoluminescence properties of porous Si aged in ethanol solution and porous Si films aged in ambient air were studied. It was shown that the photoluminescence properties of porous Si strongly depend on aging time and environmental conditions. The photoluminescence intensity of porous Si dispersed in ethanol solution increases 50-fold with aging for 7 d. This increase is accompanied by increases in Si–O bond density and lifetime, indicating that the electron–hole pair is strongly confined by the formation of surface oxide thin films. The photoluminescence intensity of porous Si films decreases with aging in ambient air, suggesting that nonradiative recombination centers are formed by natural oxidation.

Nano-oxidation of an amorphous silicon surface with an atomic force microscope

Abstract A surface anodization technique was applied to an amorphous silicon (a-Si) surface by means of an atomic force microscope (AFM). The oxide line height increased with increasing applied voltage. The height of these lines on the un-hydrogenated amorphous silicon (a-Si:H) film was greater than that on crystalline silicon (c-Si) due to the large defect density in the former. Although the height of the oxide lines did not depend on film thickness, their width increased with decreasing film thickness. The photoluminescence (PL) intensity of the nano-scale lattice pattern drawn on a-Si:H was measured by micro-scale PL equipment. The PL intensity from the area having narrow oxide lines was smaller than that from the non-oxidized area. This indicates that anodization affects not only the area observed by the AFM, but its effects also spread over a sub-micron region.

Study of the wurtzite zinc-blende mixed-structured GaAs nanocrystals grown on Si (111) substrates

The structure and growth mechanism of GaAs nanocrystals grown on Si (111) substrates by using the molecular beam epitaxy method have been studied using transmission electron microscopy. The isolated nanocrystals had hexangular shapes, with aspect ratio ∼1 and high symmetry. The crystal structure of the GaAs nanocrystals contains a mixture of a stable state of zinc-blende and a metastable state of wurtzite. A number of thin wurtzite layers parallel to the Si (111) plane are introduced into the zinc-blende GaAs nanocrystals as stacking faults. Formation of partial dislocations near the GaAs/Si interface and the small difference in the Gibbs free energy between the zinc-blende and wurtzite structures could cause formation of wurtzite as stacking faults in the zinc-blende structure

Luminescence stability of porous Si terminated by hydrophilic organic molecules

The effects of the surface termination of a porous Si surface by propionic acid and by undecylenic acid on their hydrophilicity and luminescence stability were studied. In the measurements of the contact angle of water droplets on porous Si films, the hydrophilicity of porous Si is improved by the surface termination each types of organic molecule. The PL intensity of as-prepared porous Si decreased with increasing aging time in ambient air. As PL quenching involves PL blue shift and increasing Si–O bonds density, nonradiative recombination centers are formed in the surface oxide. After the hydrosilylation process of propionic acid and undecylenic acid, PL intensity decreased and became 30% that of as-prepared porous Si film. However, the PL intensity was stable and exceeded that of the as-prepared film after 1000 min of aging in the ambient air. The PL stabilities are contributed to the termination by organic molecules that inhibits surface oxidation.

Structural Stabilities in GaAs Nanocrystals Grown on Si (111) Surface

Structural stabilities in GaAs nanocrystals grown on the Si (111) substrate have been studied by transmission electron microscopy in order to see the structure and growth mechanism. The GaAs nanocrystals grown epitaxially on the Si (111) surface kept at 573 K have thin shapes consisting of a flat surface which is parallel to the Si (111) surface. The crystalline structure of the initial growth layer, below approximately 5 nm in thickness is zincblend, but with increasing thickness the structure changes to the wurtzite structure by formation of orderly-arranged stacking faults. The small difference in the driving force between wurtzite and the zincblende structure could lead to a situation where the kinetic rate of nucleus formation is higher for the wurtzite structure than for the zincblende structure. It would highly increase the probability that the wurtzite structure is formed as a non-equilibrium state.

Correlation between PL emission band and growth of oxide layer on surface of silicon nanocrystallites

A systematic study of the correlation between surface oxidation and PL properties is important to clarify the nature of silicon nanocrystal. We prepared hydrogenated silicon nanocrystallites (nc‐Si:H) by pulsed laser ablation, and observed IR absorption and PL spectra. Analysis of the Si–H bond gives us information on the local configuration of Si–O bond. The PL peak wavelength shifted from 800 nm to 400 nm with increasing the Si–O bond density. The frequency resolved PL spectra indicate that the PL peak is composed of at least three frequency regions. We found that PL peak is composed of 400, 700 and 800 nm bands by comparing frequency region and PL bands. The PL peak wavelength depends not on the composition but on the thickness of surface oxide layer.

Nanoscale anodization of an amorphous silicon surface with an atomic force microscope

Nanoscale anodization was performed on the surface of amorphous silicon thin films by means of an atomic force microscope. The anodization mechanism was different from that previously reported on metal thin films. We found that the anodization was a function of defect density and current through the sample. The optical properties of the anodized area were measured by means of micro-photoluminescence, in which photoluminescence intensity decreases with oxidation. We concluded that both defect reaction and creation processes are important during the nanoscale anodization of amorphous material.