Yuta Matsumoto

Control of Electron and Hole Concentrations in Semiconducting Silicide BaSi2 with Impurities Grown by Molecular Beam Epitaxy

Highly a-axis-oriented n- and p-type BaSi2 films were grown on Si(111) substrates by molecular beam epitaxy using Sb and In doping atoms, respectively. The hole concentration of In-doped BaSi2 was controlled in the range between 1016 and 1017 cm-3 at room temperature by changing the temperature of the In Knudsen cell crucible. In contrast, the electron concentration of Sb-doped BaSi2 was controlled in the range between 1016 and 1020 cm-3 by the substrate temperature. The electron and hole mobilities decreased with increasing electron and hole density, respectively.

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).

Photoresponse Properties of Semiconducting BaSi2 Epitaxial Films Grown on Si(111) Substrates by Molecular Beam Epitaxy

n-Type BaSi2 epitaxial films with 900 nm thickness were grown on Si(111) by molecular beam epitaxy, and striped Au electrodes were formed on the surface. Photocurrents were clearly observed for photons with energies greater than 1.25 eV under bias voltages applied between the electrodes, and this increased sharply with increasing photon energy to attain a maximum at approximately 1.70 eV. The external quantum efficiency increased with the bias voltage and reached approximately 7% at 1.70 eV for a bias voltage of 7 V. This value is 100 times larger than the highest value ever reported for semiconducting silicide films.

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.

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.

Photoresponse properties of BaSi 2 films on N + -BaSi 2 /P + -Si tunnel junction for high efficiency thin film solar cells

We have grown n⁺-BaSi<inf>2</inf>/p⁺-Si tunnel junctions with different BaSi<inf>2</inf> template thicknesses on Si(111) by molecular beam epitaxy. Both the epitaxial growth and low resistance as small as 0.05 Ω·cm² were achieved for a bias voltage of 0.1 V when the template layer thickness was 1 nm. The photoresponse spectra were measured at room temperature for a 360-nm-thick undoped BaSi<inf>2</inf> film grown on the tunnel junction. The photoresponsivity reached 74 mA/W at 2.3 eV under a reverse bias of 4 V, the highest value ever reported for semiconducting silicides.