Recently, a convenient wet-chemical treatment method to terminate silicon surface with hydrogen atoms has been developed. It is attractive from the viewpoint of surface science and device technology. However, since the atomic structure and surface microroughness of the surfaces treated by the wet-chemical process is strongly affected by the condition, we should optimize the treatment condition in order to make atomicallyflat and well-ordered hydrogen-terminated surface. Here we discuss the optimization of wetchemical treatment of the hydrogen-terminated Si(111) and (001) surfaces.
The static and dynamic features of Si(001)/H(1×1) surface are studied with the tight-binding molecular-dynamics method. The repulsive interactions between the adjacent dihydrides are found to play a crucial role on their features. First, we present the surface structure distorted by the interaction between the H atoms. Although the dihydrides are known to have the canted structure on a single domain, we have found the structures at the domain boundaries. As for the vibrational spectra the H-H interactions are found to bear a novel peak above the stretching frequency of the Si-H bonds of the isolated dihydrides. The occurrence of the new peaks are predicted even on small terraces. Finally, the dissipation of the vibrational energy on the surface is discussed. The vibrational energy is observed to be quickly transferred to the other dihydrides via the H-H interactions, which result in the shortest relaxation time among the hydrogenterminated Si(001) surfaces.
A brief review is given on density functional calculations performed for group-IV adatom diffusion on hydrogenated Si(100) surfaces. It is shown that the diffusion is a complex atomic process in which hydrogen capture and emission as well as adatom-exchange with subsurface atoms are involved. Step structures are determined and the modulation of the diffusion process near the steps is clarified. Relations between microscopic atomic processes and macroscopic thin-film morphology are also discussed.
The surface composition of Ge/Si(001)2×1 surfaces after atomic hydrogen (H) irradiation was investigated using IR reflection spectroscopy in UHV. It was observed that H irradiation at room temperature causes etching of the surface Ge layer. However, when H is irradiated at elevated temperatures, the etching reaction does not occur. Instead, Ge at the surface moves into the subsurface region and Si atoms tend to exist on the topmost surface as monohydrides of mixed Ge-Si and pure Si-Si dimer structures. This behavior is remarkably different from that of Ge/Si(001) surfaces in absence of hydrogen, where Ge is segregated at the surface and forms Ge-Ge pure dimers. First-principles total energy calculations indicates that the presence of hydrogen reverses the stability of the surface composition from Ge to Si, in good agreement with the experimental results.
Initial oxidation processes and local bonding structures of hydrogen-terminated Si(100)-2×1 and H2O-terminated Si(100)-2×1 surfaces have been examined by high-resolution energy loss spectroscopy (HREELS). The hydrogen adsorption on Si(100) surfaces suppress the oxidation of dimer bond sites, and oxygen atoms preferentially adsorb on one of the two back-bond sites of surface Si atoms. On the other hand, oxidation proceeds randomly up to an oxygen coverage of 3 ML on H-terminated Si(111) surfaces. The Si-O-Si bonds formed on H-terminated Si(100) surfaces are more relaxed than those on clean Si(100) surfaces, which is considered to originate from the change in bond angles of Si-O-Si bonds. The H2O-terminated Si(100)-2×1 surfaces are also stable for the adsorption of oxygen molecules. However, the uptake of oxygen atoms of Si-OH species into back-bond sites occurs even at room temperature by the reaction of H2O-terminated Si(100) surfaces with atomic hydrogen.
The oxidation reaction of an unpaired dangling bond (DB) and a DB atomic wire on a hydrogen-terminated Si(100) 2×1 surface was studied using an ultrahigh-vacuum scanning tunneling microscopy. We found that O2 molecules dissociated into atomic oxygen after reacting with DBs. The atomic oxygen oxidized also the bonds near DBs. The oxidation reaction was turned to be considerably enhanced at the DB atomic wire, compared with that at an unpaired DB. This suggests that the electron transfer to anti-bonding π orbital of O2 molecule depends on the local density of states at an unpaired DB and at a DB atomic wire.
Scanning tunneling microscope (STM) induced light emission from nano structures comprising silicon dangling bonds on deuterium terminated Si(001) surfaces has been mapped spatially and analyzed spectroscopically in the visible spectral range. The light emission is based on a novel mechanism involving optical transitions between a tip state and localized states on the sample surface. The wavelength of the photons can be varied by the bias voltage of the STM. The spatial resolution of the photon maps is as accurate as that of STM topographic images and the photons are emitted from a quasi-point source with a spatial extension similar to the size of a dangling bond.
Technical surfaces are composed of numbers of protrusions and grooves microscopically, however the appearances of the surfaces seem smooth. Surface roughness may be characterized by their intervals, periodicity, gradients and so on. We measured topographs of Fe-42 Ni alloys by atomic force microscopy. Surface roughness was evaluated in terms of power spectral density (PSD) which provides information on periodicity, and of slope histogram. It is found that a gradient of PSD is proportional to 1/f2, and the slope histogram is well fitted with the Gaussian distribution. The data of PSD show that the surface is naturally random with a strong correlation between neighborhood. It is found that our findings are explained by a simulated surface based on a random walk.
Stable structures and electronic states of Si(100)/SiO2 interface are investigated using the first-principles molecular dynamics method. Quartz, tridymite, and pseudo beta-cristobalite are employed as the initial structures of the SiO2 at the interface to find the stable ones by the structural optimization. It is found that the optimized tridymite-type SiO2 structure on Si is the most stable for thin (about 7Å) SiO2 layer. For the thicker (about 15Å) layer, however, this structure becomes less stable and the optimized quartz-type SiO2 structure is the most stable. The band gap variation along the direction perpendicular to the interface is also investigated for the optimized structures. In the SiO2 region within 1Å from the structural interface, the band gap remains as narrow as that of silicon. The drastic change of the band gap takes place in the SiO2 region between 1 and 4Å.
Recently, much attention has been paid to the microwave processing in the preparation of inorganic materials. The advantages of microwave processing are uniformity of heat treatment and saving of energy and time, which are similar to those of microwave cooking. In this report, our recent research of the synthesis of High-Tc superconductors using a domestic microwave oven is described. We have succeeded in obtaining single-phase samples of the Y-123, Bi-2201 and Bi-2212 phases for several ten min without any post-heat-treatment using an electric furnace. In addition, several reports on the synthesis of other inorganic materials using a domestic microwave oven are introduced.