Magnetoresistance characteristics of Fe3Si/CaF2/Fe3Si heterostructures grown on Si(111) by molecular beam epitaxy

AbstractFe3Si/CaF2/Fe3Si magnetic tunnel junctions (MTJs) have been investigated to demonstrate the tunnel magnetoresistance effects. We fabricated Fe3Si(20 nm)/CaF2(2 nm)/Fe3Si(15 nm) heterostructures epitaxially on a Si(111) substrate by molecular beam epitaxy. The current-voltage characteristics for the MTJs measured at room temperature (RT) were well fitted to Simmons’ equation. The fitting yields the barrier height φ=2.5 eV and the barrier thickness d=1.26 nm. The magnetoresistance ratio for the MTJs were approximately 0.28% under a bias voltage of 20 mV at RT

Low temperature synthesis of highly oriented p-type Si1-xGex (x: 0–1) on an insulator by Al-induced layer exchange

A composition tunable Si1-xGex alloy has a wide range of applications, including in electronic and photonic devices. We investigate the Al-induced layer exchange (ALILE) growth of amorphous Si1-xGex on an insulator. The ALILE allowed Si1-xGex to be large grained (> 50 μm) and highly (111)-oriented (> 95%) over the whole composition range by controlling the growth temperature (≤ 400 °C). From a comparison with conventional solid-phase crystallization, we determined that such characteristics of the ALILE arose from the low activation energy of nucleation and the high frequency factor of lateral growth. The Si1-xGex layers were highly p-type doped, whereas the process temperatures were low, thanks to the electrically activated Al atoms with the amount of solid solubility limit. The electrical conductivities approached those of bulk single crystals within one order of magnitude. The resulting Si1-xGex layer on an insulator is useful not only for advanced SiGe-based devices but also for virtual substrates, allowing other materials to be integrated on three-dimensional integrated circuits, glass, and even a plastic substrate

Transport properties of n- and p-type polycrystalline BaSi2

Electron and hole mobilities versus temperature in semiconducting barium disilicide (BaSi2) have been systematically studied both experimentally and theoretically. The experiments were performed with undoped 250 nm-thick BaSi2 polycrystalline films grown by molecular beam epitaxy. The grain size of films ranged from 0.2 to 5 μm with the electron concentration of 5.0 × 1015 cm−3. To investigate the hole mobility, B-doped p-BaSi2 films with various dopant concentrations were fabricated and studied. The experimental temperature dependence of the electron mobility in the range of 160–300 K was found to have a maximum of 1230 cm2/V∙s at 218 K, while at room temperature (RT) it dropped down to 816 cm2/V∙s. We demonstrate that the temperature dependence of the electron mobility cannot be adequately reproduced by involving standard scattering mechanisms. A modified approach accounting for the grained nature of the films has been proposed for the correct description of the mobility behavior. The highest hole mobility in p-BaSi2 films reaching ~ 80 or 200 cm2/V∙s (for the films grown on (111) or (001) Si substrates, respectively) at RT is about an order or four times of magnitude smaller than that in n-BaSi2 films. Such a great difference we ascribe to the specific features of electron-phonon and hole-phonon coupling in semiconducting BaSi2

Fabrication of SrGe2 thin films on Ge (100), (110), and (111) substrates

Semiconductor strontium digermanide (SrGe2) has a large absorption coefficient in the near-infrared light region and is expected to be useful for multijunction solar cells. This study firstly demonstrates the formation of SrGe2 thin films via a reactive deposition epitaxy on Ge substrates. The growth morphology of SrGe2 dramatically changed depending on the growth temperature (300−700 °C) and the crystal orientation of the Ge substrate. We succeeded in obtaining single-oriented SrGe2 using a Ge (110) substrate at 500 °C. Development on Si or glass substrates will lead to the application of SrGe2 to high-efficiency thin-film solar cells

Recent Progress Toward Realization of High-Efficiency BaSi2 Solar Cells: Thin-Film Deposition Techniques and Passivation of Defects

Safe, stable, and earth-abundant materials for solar cell applications are of particular importance to realize a decarbonized society. Semiconducting barium disilicide (BaSi2), which is composed of nontoxic and earth-abundant elements, is an emerging material to meet this requirement. BaSi2 has a bandgap of 1.3 eV that is suitable for single-junction solar cells, a large absorption coefficient exceeding that of chalcopyrite, and inactive grain boundaries. This review is started by describing the recent progress of BaSi2 thin-film deposition techniques using radio-frequency sputtering and discuss the high photoresponsivity of BaSi2 thin films. Special attention is paid to passivation of the defects in BaSi2 films by hydrogen or carbon doping. Ab initio studies based on density-functional theory are then used to calculate the positions of the localized defective states and the Fermi level to discuss the experimentally obtained passivation effects. Finally, the issues that need to be resolved toward realization of high-efficiency BaSi2 solar cells are addressed

Large-Grained Polycrystalline (111) Ge Films on Insulators by Thickness-Controlled Al-Induced Crystallization

Low-temperature (350°C) crystallization of amorphous Ge films on SiO2 was investigated using Al-induced layer exchange (ALILE) process. Thicknesses of Ge and catalytic Al layers were varied in the range of 30–300 nm, which strongly influenced the ALILE growth morphology. Based on the study, the Ge thickness was adjusted to 40 nm while the Al thickness was adjusted 50 nm. This sample satisfied both of the surface coverage of polycrystalline-Ge and the annihilation of randomly oriented Ge regions. Moreover, the enhancement of the heterogeneous Ge nucleation improved the (111) orientation and the grain size. As a result, the area fraction of the (111)-orientation reached as high as 97% and the average grain size as large as 70-μm diameters. This (111)-oriented Ge layer with large-grains promises to be the high-quality epitaxial template for various functional materials to achieve next-generation devices

Polycrystalline thin-film transistors fabricated on high-mobility solid-phase-crystallized Ge on glass

Low-temperature formation of Ge thin-film transistors (TFTs) on insulators has been widely investigated to improve the performance of Si large-scale integrated circuits and mobile terminals. Here, we studied the relationship between the electrical properties of polycrystalline Ge and its TFT performance using high-mobility Ge formed on glass using our recently developed solid-phase crystallization technique. The field-effect mobility μFE and on/off currents of the accumulation-mode TFTs directly reflected the Hall hole mobility μHall, hole concentration, and film thickness of Ge. By thinning the 100-nm thick Ge layer with a large grain size (3.7 μm), we achieved a high μHall (190 cm2/Vs) in a 55-nm thick film that was almost thin enough to fully deplete the channel. The TFT using this Ge layer exhibited both high μFE (170 cm2/Vs) and on/off current ratios (∼102). This is the highest μFE among low-temperature (<500 °C) polycrystalline Ge TFTs without minimizing the channel region (<1 μm)

Growth and characterization of Si-based light-emitting diode with beta-FeSi2-particles/Si multilayered active region by molecular beam epitaxy

We fabricated single-, double- and triple-layered beta-FeSi2-particles structure on Si(001) substrates by reactive deposition epitaxy (RDE) for beta-FeSi2 and by molecular beam epitaxy (MBE) for Si, and realized electroluminescerice (EL) at 310K. Photoluminescence (PL) measurements revealed that the 77K PL intensity of beta-FeSi2 increased almost proportionally with the number of beta-FeSi2-particles/Si layers. It was also found that the multilayered structure enhanced the EL intensity of beta-FeSi2 particularly at low temperatures