Michitoshi Takeishi

Photoresponse Properties of Polycrystalline BaSi2 Films Grown on SiO2 Substrates Using (111)-Oriented Si Layers by an Aluminum-Induced Crystallization Method

Polycrystalline BaSi2 layers with 300 nm thickness were grown by molecular beam epitaxy on (111)-oriented 100-nm-thick polycrystalline Si layers fabricated by an aluminum-induced crystallization method on SiO2. Photocurrents were clearly observed for photons with energies greater than 1.25 eV when bias voltage was applied between the 1.5-mm-spacing striped Al electrodes formed on the surface. The photoresponsivity increased sharply with increasing photon energy, attaining a maximum at approximately 1.60 eV. The external quantum efficiency increased with the bias voltage and reached approximately 8% at 5 V. This value is larger than that obtained for BaSi2 epitaxial films on Si(111).

Fabrication of n+-BaSi2/p+-Si Tunnel Junction on Si(111) Surface by Molecular Beam Epitaxy for Photovoltaic Applications

n+-BaSi2/p+-Si tunnel junctions with different BaSi2 template layer thicknesses were grown by molecular beam epitaxy. The template was found to be indispensable for growing epitaxial n+-BaSi2, but the resistance of the junctions increased with template thickness. However, both epitaxial growth and low resistance were achieved for a template thickness of 1 nm. A current density of 21.9 A/cm2 was achieved at 0.5 V. The photoresponsivity of 360-nm-thick undoped BaSi2 grown on the tunnel junction increased with bias voltage and reached 74 mA/W at 2.3 eV under a reverse bias of 4 V, the highest value ever reported for semiconducting silicides.

Impact of Thin Island-Like BaSi2 Template on the Formation of n+-BaSi2/p+-Si Tunnel Junction on Si(111) Surface by Molecular Beam Epitaxy

We have grown n+-BaSi2/p+-Si tunnel junctions with different BaSi2 template layer thicknesses by molecular beam epitaxy. Even when the template layer was 1 nm in thickness, which was not actually a continuous film but small islands, they act as seed crystals for the initiation of overlayer growth. The electrical resistance of the junctions increased with template thickness. Both epitaxial growth and low resistance were achieved for thin island-like BaSi2 templates.

Epitaxial Growth and Photoresponse Properties of BaSi2 Layers toward Si-Based High-Efficiency Solar Cells

We have grown BaSi2 epitaxial films and polycrystalline films by molecular-beam epitaxy on Si(111) and on (111)-oriented 100-nm-thick polycrystalline Si layers fabricated by an aluminum-induced crystallization method on SiO2, respectively. Electron backscatter diffraction analysis was performed on the 200-nm-thick BaSi2 epitaxial film and the grain size of the film was found to be approximately 3–10 µm. Photocurrents were clearly observed for the BaSi2 (900 nm)/Si and BaSi2 (300 nm)/SiO2 samples for photons with energies greater than 1.25 eV at room temperature when bias voltage was applied between the 1.5-mm-spacing striped electrodes formed on the surface. The photoresponsivity of the samples increased sharply with increasing photon energy and attained its maximum at approximately 1.60 eV. From the temperature dependence of the photoresponsivity, the activation energies of the BaSi2 epitaxial films and polycrystalline films were estimated to be approximately 6 and 52 meV, respectively.

Molecular Beam Epitaxy of Cu-Doped BaSi2 Films on Si(111) Substrate and Evaluation & Qualification of Depth Profiles of Cu Atoms for the Formation of Efficient Solar Cells

The doping of Cu in the BaSi2 films grown by molecular beam epitaxy (MBE) with various Cu concentrations for the suitability of the solar cells was studied in this paper. The main objective of the present work is to investigate and compare the carrier concentration of Cu-doped BaSi2 films grown with different Cu Knudsen cell temperatures and qualify as a potential candidate for more efficient solar cells. The reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) measurements and secondary ion mass spectroscopy (SIMS), were used to determine the structure, depth profile and composition of the grown samples. The electrical properties like resistivity as well as carrier concentration were measured by using a four point probe method and Van der Pauw technique, respectively. During the MBE growth, different temperatures for Cu Knudsen cell ranging from 800 to 1200 °C were chosen and the optimum growth condition for both heavily doped n-type as well as p-type in the MBE was investigated. In our previous work, the Al, Sb doped BaSi2 were used as a potential candidate for the formation of pn-junction for solar cells, but the result was not encouraging one due to diffusion and segregation problems in the surface and BaSi2/Si interface regions. In the present work n-type BaSi2 layers with their dopant atoms uniformly distributed in the grown layers for the formation of high-quality of BaSi2 pn-junction with single crystal nature were successfully developed. The realizations to develop cost effective and more efficient solar cells are inevitable for both terrestrial as well as space applications.

Wet Chemical Etching and X-ray Photoelectron Spectroscopy Analysis of BaSi2 Epitaxial Films Grown by Molecular Beam Epitaxy

Approximately 250-nm-thick undoped n-type BaSi2 epitaxial films were grown on n-Si(111) substrates by molecular beam epitaxy, and 1-mm-diameter mesa-isolated n-BaSi2/n-Si diode structures were successfully produced by wet chemical etching. Etching of BaSi2 layers was carried out using diluted hydrochloric acid solution, followed by diluted hydrofluoric acid solution. X-ray photoelectron spectroscopy measurements revealed that the surface of BaSi2 was changed to Si oxides by treatment with the hydrochloric solution, and these oxides were then etched away by the hydrofluoric solution. The surface morphology of samples was significantly dependent on the ratio of H2O included in the etching solutions. Lower ratios of H2O resulted in rougher sample surfaces. Wet chemical etching of BaSi2 layers was successfully carried out by first using HCl:H2O = 1:199 and then HF:H2O = 1:49 solutions at 0 °C.

Polymer Stabilized Liquid Crystal Grating Consisting of Periodic Reverse Twist Domains

We attempt to form periodic reverse twist domains using a microscale rubbing pattern. Conditions for the injection of a liquid crystal (LC) material into an empty cell are discussed. The fabricated LC cells with periodic reverse twist domains are applied to electrically controllable LC gratings. The stability of the periodic reverse twist domains, when a high voltage is applied and the LC cell is heated, is discussed. To stabilize the periodic reverse twist domains, a polymer stabilization technique using a UV-curable LC material is employed. The influence of the density of the UV-curable LC material on the diffraction properties of LC gratings is investigated, experimentally.