Takashi Suemasu

Room Temperature 1.6 µm Electroluminescence from a Si-Based Light Emitting Diode with β-FeSi2 Active Region

Room temperature electroluminescence (EL) was obtained for the first time from a Si-based light emitting diode with semiconducting silicide (β-FeSi2) active region. The peak wavelength was 1.6 µm and it is from β-FeSi2 balls embedded in a Si p-n junction. An a-axis oriented β-FeSi2 layer was grown on n+-(001) Si by reaction deposition epitaxy (RDE), then p- and p+-Si layers were grown by molecular beam epitaxy (MBE). The β-FeSi2 aggregated into balls with about 100 nm diameter in this process. The EL intensity increased superlinearly with increase of the injected current density, and reasonable EL was obtained at current density above 10 A/cm2, though the photoluminescence (PL) was difficult to be detected at room temperature.

Epitaxial Growth of Semiconducting BaSi2 Films on Si(111) Substrates by Molecular Beam Epitaxy

We have grown [100]-oriented BaSi2 multidomain epilayers on Si(111) substrates by molecular beam epitaxy (MBE) using a BaSi2 epitaxial template formed by reactive deposition epitaxy (RDE). The [100]-oriented BaSi2 films were obtained over a wide temperature range from 450 to 700°C: The optimum growth temperature was about 600°C at which the integrated intensity of X-ray diffraction peak was maximum and the full width at half maximum (FWHM) was minimum. X-ray pole figure measurements revealed that there were three epitaxial variants of [100]-oriented BaSi2 due to the three-fold symmetry of the Si(111) surface.

Investigation of the energy band structure of orthorhombic BaSi2 by optical and electrical measurements and theoretical calculations

Optical and electrical properties of polycrystalline orthorhombic BaSi2 prepared by arc melting in Ar atmosphere were investigated. The optical absorption spectra measured at room temperature showed that indirect and direct absorption edges were 1.15 and 1.25 eV, respectively. The activation energy estimated from temperature dependence of the resistivity was 1.10 eV. These results agreed well with a calculated band structure of the orthorhombic BaSi2 by first principles calculation using density functional theory.

Epitaxial Growth of Semiconducting BaSi2 Thin Films on Si(111) Substrates by Reactive Deposition Epitaxy

The optimum growth temperature is determined for the epitaxial growth of semiconducting orthorhombic BaSi2 films on Si(111) substrates by reactive deposition epitaxy (RDE). X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) analyses confirmed that smooth, [100]-oriented epitaxial BaSi2 films could be obtained by RDE at a substrate temperature of 600–650°C when the Ba deposition rate was 10 nm/min.

Formation of β-FeSi 2 Layers on Si(001) Substrates

The crystal quality of β-FeSi2 formed on Si(001) by both the thermal reaction method and reactive deposition epitaxy (RDE) was investigated under various growth conditions. Compared with the thermal reaction method, the crystal quality of β-FeSi2 formed by RDE was improved. In the RDE method, the Fe deposition rate as well as the growth temperature influenced the crystal quality of β-FeSi2. Among various growth temperatures and deposition rates of Fe, highly (100)-oriented epitaxial β-FeSi2 was formed by RDE at 470°C and 0.1 A/s, respectively. Measurements of the absorption coefficient at room temperature (RT) indicate that β-FeSi2 has a direct band gap of about 0.83 eV.

Highly (111)-oriented Ge thin films on insulators formed by Al-induced crystallization

(111)-oriented Ge thin films on insulators are essential for advanced electronics and photovoltaic applications. We investigate Al-induced crystallization of amorphous-Ge films (50-nm thickness) on insulators focusing on the annealing temperature and the diffusion controlling process between Ge and Al. The (111)-orientation fraction of the grown Ge layer reaches as high as 99% by combining the low-temperature annealing (325 °C) and the native-oxidized Al (AlOx) diffusion-control layer. Moreover, the transmission electron microscopy reveals the absence of defects on the Ge surface. This (111)-oriented Ge on insulators promises to be the high-quality epitaxial template for various functional materials to achieve next-generation devices.

Influence of Si growth temperature for embedding β-FeSi2 and resultant strain in β-FeSi2 on light emission from p-Si/β-FeSi2 particles/n-Si light-emitting diodes

We have fabricated Si/β-FeSi2 particles/Si structures by reactive deposition epitaxy for β-FeSi2 and molecular-beam epitaxy (MBE) for Si. It was found that the photoluminescence (PL) intensity of β-FeSi2 strongly depended on MBE-Si growth temperature for embedding β-FeSi2 in Si. Room temperature 1.6 μm electroluminescence was realized from a Si-based light-emitting diode by embedding the β-FeSi2 active region with Si grown at 500 °C. Observation with transmission electron microscopy revealed that the a axis of β-FeSi2 from which PL was observed was about 9% longer than that of β-FeSi2 without PL or of bulk β-FeSi2.

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.

Low-temperature (180 °C) formation of large-grained Ge (111) thin film on insulator using accelerated metal-induced crystallization

The Al-induced crystallization (AIC) yields a large-grained (111)-oriented Ge thin film on an insulator at temperatures as low as 180 °C. We accelerated the AIC of an amorphous Ge layer (50-nm thickness) by initially doping Ge in Al and by facilitating Ge diffusion into Al. The electron backscatter diffraction measurement demonstrated the simultaneous achievement of large grains over 10 μm and a high (111) orientation fraction of 90% in the polycrystalline Ge layer formed at 180 °C. This result opens up the possibility for developing Ge-based electronic and optical devices fabricated on inexpensive flexible substrates.

Investigation of the recombination mechanism of excess carriers in undoped BaSi2 films on silicon

Excess-carrier recombination mechanisms in undoped BaSi2 epitaxial films grown by molecular beam epitaxy on n-type silicon substrates have been studied by the microwave-detected photoconductivity decay measurement. The measured excess-carrier decay is multiexponential, and we divided it into three parts in terms of the decay rate. Measurement with various excitation laser intensities indicates that initial rapid decay is due to Auger recombination, while the second decay mode with approximately constant decay to Shockley-Read-Hall recombination. Slow decay of the third decay mode is attributed to the carrier trapping effect. To analyze Shockley-Read-Hall recombination, the formulae are developed to calculate the effective lifetime (time constant of decay) from average carrier concentration. The measurement on the films with the thickness of 50–600 nm shows that the decay due to Shockley-Read-Hall recombination is the slower in the thicker films, which is consistent with the formulae. By fitting the calcul...