Epitaxial growth of SiGe films by annealing Al-Ge alloyed pastes on Si substrate.

A simple, low-cost, and non-vacuum epitaxial growth method to realize large-area semiconductors on crystalline silicon will become the game-changer for various applications. For example, we can expect the disruptive effect on the cost of large-scale III-V multi-junction solar cells if we could replace the high-cost germanium substrate with silicon-germanium (SiGe) on Si. For SiGe epitaxial growth, we attempted to develop a process using original Al-Ge pastes for screen printing and subsequent annealing. We compare two pastes including Al-Ge alloyed pastes with compositional uniformity in each particle and Al-Ge mixed pastes. We revealed that Al-Ge alloyed paste could form flatter SiGe film with much less residual pastes, supported by in-situ observations. The uniform and sufficient dissolution of the alloyed paste is responsible for these and led to higher average Ge-composition by annealing at 500 °C. The composition in SiGe was vertically graded up to ~ 90% at the topmost surface. These results show that printing and firing of Al-Ge alloyed paste on Si is the desirable, simple, and high-speed process for epitaxial growth of SiGe, which could be potentially used as the lattice-matched virtual substrate with III-V semiconductors.

Heteroepitaxial Growth of Colloidal Crystals: Dependence of the Growth Mode on the Interparticle Interactions and Lattice Spacing.

Epitaxial growth is one of the most important techniques for the control of crystal growth, especially for growing thin-film semiconductor crystals. Similarly, colloidal epitaxy, a template-assisted self-assembly method, is a powerful technique for controlling the structure of colloidal crystals. In this study, heteroepitaxial growth, which differs from homoepitaxial growth of conventional colloidal epitaxy, using foreign colloidal crystals as a substrate, was used to grow single-component colloidal crystal films. The Frank-van der Merwe (FM), Stranski-Krastanov (SK), and Volmer-Weber (VW) modes were observed, and the mode varied with the lattice-misfit ratio and interparticle interactions between the substrate and epitaxial phase. The transition of the growth mode (from SK to VW) and the coexistence of different growth modes (FM and VW) were observed, and their processes were revealed by in situ observation. Colloidal heteroepitaxy was confirmed to be useful for controlling structure, which will enable exploration of novel colloidal self-assembly structures.

Impurity partitioning during colloidal crystallization.

We have found that an impurity partitioning takes place during growth of colloidal crystals, which was recognized by the fact that the impurity concentration in the solid (CS) was different from that in the initial solution (C0). The effective partition coefficient k(eff) (=CS/C0) was investigated for pure polystyrene and polystyrene dyed with fluorescent particles by changing the ratio of particle diameters d(imp)/d(cryst) and growth rate V. At each size ratio for the polystyrene impurity, k(eff) was less than unity and increased to unity with increasing V, whereas at a given growth rate, k(eff) increased to unity as d(imp)/d(cryst) approached unity. These results were consistent with the solute behavior analyzed using the Burton, Prim, and Slichter (BPS) model. The obtained k0, equilibrium partition coefficient, from a BPS plot increased as d(imp)/d(cryst) approached unity. In contrast, while the fluorescent particles also followed the BPS model, they showed higher k0 values than those of the same size of polystyrene particles. A k0 value greater than unity was obtained for impurities that were similar in size to the host particle. This behavior is attributed to the positive free energy of fusion associated with the incorporation of the fluorescent particles into the host matrix. Such positive free energy of fusion implies the presence of the enthalpy associated with interaction between particles.

Wave-dispersive x-ray spectrometer for simultaneous acquisition of several characteristic lines based on strongly and accurately shaped Ge crystal.

Si and Ge are widely used as analyzing crystals for x-rays. Drastic and accurate shaping of Si or Ge gives significant advance in the x-ray field, although covalently bonded Si or Ge crystals have long been believed to be not deformable to various shapes. Recently, we developed a deformation technique for obtaining strongly and accurately shaped Si or Ge wafers of high crystal quality, and the use of the deformed wafer made it possible to produce fine-focused x-rays. In the present study, we prepared a cylindrical Ge wafer with a radius of curvature of 50 mm, and acquired fluorescent x-rays simultaneously from four elements by combining the cylindrical Ge wafer with a position-sensitive detector. The energy resolution of the x-ray fluorescence spectrum was as good as that obtained using a flat single crystal, and its gain was over 100. The demonstration of the simultaneous acquisition of high-resolution x-ray fluorescence spectra indicated various possibilities of x-ray spectrometry, such as one-shot x-ray spectroscopy and highly efficient wave-dispersive x-ray spectrometers.

Point-focusing monochromator crystal realized by hot plastic deformation of a Ge wafer.

Pre-polished Ge(111) single-crystal wafers were deformed just below the melting temperature to prepare point-focusing Johansson monochromator crystals. The (111) lattice plane had curvature 2R in the focusing plane and R perpendicular to it, with a hemispherical inner surface with a radius of R = 600 mm. By using Cu Kalpha radiation, the diverging X-ray beam was focused onto a small spot.