Basic pmperties of molten silicon are examined experimentally. The temperature dependence of the density was determined in the range from solidification point of about 1,415℃ to 1,650℃ using an improved Archimedian method. An anomalous value in the thermal volume expansion coefficient of about 8×10^<-4>℃^<-1> was observed just above the solidification point. The time dependence of the density has also been recognized. The detailed temperature dependence of the surface tension of molten silicon was measured using an accurate ring method. The surface tension data showed an approximately linear temperature dependence from 1,430℃ to 1,650℃. The temperature coefficient of the surface tesion becomes positive at about 1,425℃ and returns to a negative value just above the solidification point. The viscosity of molten silicon was measured in the temperature range from the solidification point to about 1,600℃ using an oscillating cup made of SiC. The activation energy estimated from the data was 0.20eV. An anoomalous increase in viscosity with decreasing temperature was observed for temperatures lower than about 1,430℃. The temperature dependence of the electrical resistivity of molten silicon was measured based on the dirct-current four-probe method in the range from the solidification point to 1,630℃. The temperature coefficient of the resistivity varied slowly with temperature, being negative near the solidification point and positive above 1,500℃. The resistivity of molten silicon was calculated based on Ziman's formula. The temperature dependence of the measured resistivity was not reproduced when the structure factor S (Q) calculated by a simple hard-sphere model was substituted into Ziman's formula, but was reproduced by using the experimental data of S(Q) measured by Waseda which shows the first peak of asymmetric shape. These results indicate that the anomalous behavior of different properties occurs similtaneously, reflecting a kind of essential variation in the melt. The specific melt structure of molten silicon may have a significant effect on the anomaly in the temperature range near the solidncation point.
Surface morphologies of GaAs grown by metalorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are compared. Since a stepflow growth mode is dominant for MOVPE, regularly-arrayed wide terraces are formed. 0n the other hand, two-dimensional growth mode, which is dominant for MBE, generates many sizes of islands on the surface. The wide terrace is supposed to be formed by the enhanced surface migration of column-III adatoms possibly caused by the hydrogen termination on the top surface As. A nanometer-scale surface and interface roughness are controllable by the suface adatom concentration. Formation of two-dimensional islands, rough steps and bunched steps, which are factors degrading roughness, can be controlled by growth temperature and substrate misorientation angle for MOVPE growth. Moreover, the monolayer terrace width can be controlled in the range from 16 nm to 1μm. These sophisticated interfaces are expected to improve quantum devices.
We have discovered and developed new nonlinear optical crystal CsLiB_60_<10> (CLBO). In this review, we show the basic concept of the borate crystals and how we could discovered CLBO. The crysal structure, phase diagram, growth method, mechanical properties and optical properties of CLBO are briefly summarized. The superlarge CLBO crystal with demensions of 14×11×11cm^3 was obtained by the top-speeded solution growth method. Fouth harmonic and fifth harmonic generations of the 1,064μm Nd: YAG laser radiation with type-I phase matching can be realized in CLBO crystal.
Carbon forms a variety of allotropes among diamond, graphite, fullerene and one-dimensional carbon. The stuctural consideration for high pressure phases is presented for discussing possible high pressure allotropes. The electronic structures of liquid carbon and amorphous carbon are discussed in relation to the disordered structures with the mixing of 3- and 4-coordinated carbon atoms.
Well-faceted diamond films have been fabricated at 200℃ by the magneto-active microwave plasma chemical vapor deposition (CVD) method. The silicon substrate for deposition was seeded with nanocrystal diamond instead of the conventional scratching procedure with diamond powder. The commercially available nanocrystal diamond was synthesized by an implosion process and 5 nm in diameter. For the successful fabrication of rather high quality diamond films, it was neccssary to purify and disperse the nanocrystal powder. The nanocrystal seeding brought about the improvement in quality of the resulting films and cutting down the nucleation time specially for the low temperature growth.
A Highly oriented diamond film was synthesized on an Si(100) substrate by bias-enhanced microwave plasma chemical vapor deposition (MPCVD). The substrate was carburized prior to the biasing. This treatment did not change the surface morphology of the substrate but was indispensable for oriented nucleation of diamond. The MPCVD with a negative bias rapidly changed the flat substrate surface to a mesh structure that was strongly sensitive to plasma and bias conditions. RHEED and cross-sectional TEM examinations revealed that β-SiC was fromed epitaxially on the Si surface. 0riented diamond particles were deposited on the top of the mesh structure. The mesh structure was connected to formation of anti-phase domain boundary in the epitaxial β-SiC layer. Further improvement of orientation and quality of epitaxial SiC layer will lead to formatiion of extreamly oriented diamond film.
Smooth and continuous diamonds films have been heteroepitaxially grown on β-SiC(001). The epitaxial growth is composed of three steps; (i) Bias enhanced nucleation on β-SiC(001) grown on Silicon (001), (ii) <001> growth mode for the selection of epitaxially oriented particles, and (iii)<111> fast growth mode for the smoothing (001) surface. The optimization of the bias enhanced nucleation, the selection of oriented particles and the surface adjustment are important to form the smooth diamond films.