Materials composed of abundant chemical elements are ordinary non-toxic and friendly to biological organs. I think that this is because we are unavoidably exposed to these elements from the very beginning of the birth of life. We should also note that these abundant chemical elements are widely absorbed in our body and they work as indispensable ones. Our goal is to fabricate new funtional photonics and spintronics devices using those materials. We have three main research projects, that is, thin-film crystalline solar cells using Si-based semiconductor BaSi2, Arsen-free infrared photodetectors using semiconducting β-FeSi2, and spin sources using ferromagnetic Fe3Si and γ'-Fe4N. These projects include a wide range of subjects ranging from thin-film growth by molecular-beam epitaxy, sputtering and organometallic vapor phase epitaxy, to evaluation of basic physical properties of grown films, device fabrication using photolithography, and first-principles calculations. So please join us.

Thin-film crystalline solar cells using Ba1-xMxSi2(M=Sr, Mg,....) on glass


Novel Si-based materials have potential interest for high-efficiency thin-film solar cells. With respect to such materials, we have focused on orthorhombic barium disilicide (BaSi2). Optical absorption measurements indicated that the band gap of BaSi2 can be increased up to approximately 1.4eV by replacing half of the Ba atoms with isoelectric Sr atoms (JJAP 45 (2006) L390). In addition, the optical absorption coefficient reaches approximately 105 cm-1 at 1.5 eV (TSF 508 (2006) 363), almost the same as those of CIGS. Large absorption coefficient in BaSi2 comes from its specific electronic band strucutre of a direct transition a few kBT above the indirect band edge. Indirect bandgaps tend to reduce radiative recombination considerably and hense results in larger minority-carrier diffusion length. In conventional semiconductors, either absorption coefficient or minority-carrier diffusion length is large,and the other one tends to be small. However, we can utilize both large absorption coefficient and large minority-carrier diffusion length at the same time in BaSi2. Control of conduction type and carrier density have been achieved very recently by impurity doping (APEX 1 (2008) 051403). Recent reports on the photoresponse properties of BaSi2 epilayers on Si(111) and polycrystalline BaSi2 layers on (111)-oriented Si films deposited on SiO2 using an Al-induced crystallization method have shown that BaSi2 is an interesting and useful alternative material for solar cell applications (APEX 2 (2009) 05160, APEX 3 (2010) 021301).
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IV group semiconductor thin films on glass or plastic substrates


IV group semiconductors, including Si, are widely used as materials for electronic devices. If these materials are formed on low cost substrates, such as glass or plastics, we can fabricate high-efficiency, inexpensive gthin film tandem solar cellsh or multi-functional, light gsheet computersh. Our laboratory have achieved high-quality Ge thin films on glass and plastic substrates using metal-induced crystallization (MIC). We have expanded the application of the MIC technique: an inorganic semiconductor thin film (GeSn) was crystallized at a record low temperature (70‹C); aligned nanowires were directly synthesized on a plastic substrate for the first time. We are now investigating the crystal quality and device performance aiming at solar cell applications.

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As-free infrared photodetector using β-FeSi2 on Si


We have been paying special attention to semiconducting β-FeSi2 as a promising material for use in silicon-based light emitters and photodetectors operating at a wavelength of approximately 1.5μm (JJAP 39(2000) L1013, APL 79 (2001)1804). Electroluminescence at an emission power of over 0.4 mW is achieved at an emission wavelength of 1.6 μm using a p-Si/β-FeSi2/n-Si double-heterostructure light-emitting diode (APL 94 (2009) 213509). This emission power is obtained at room temperature under current injection of 460 mA, corresponding to an external quantum efficiency of approximately 0.1%. As for photodetectors, photoresponsivity beyond 100 mA/W for 1.31μm light obtained for single-crystal n-type β-FeSi2 bulk has renewed significant interest in this material (APL 91 (2007) 142114, APL 92 (2008) 192114, APL 92 (2008) 042117). We aim to realize much higher photoresponsivity for β-FeSi2 films on Si substrates.
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Spin source using ferromagnetic γ'-(Fe4N) and (Fe3Si) on Si


We aim to realize spin emitters using ferromagnetic resonant-tunneling diodes. Ferromagnetic resonant tunneling diodes (RTDs), where the quantum levels in a ferromagnetic quantum well split depending on the electron spin, are considered to be an alternative way to obtain spin-dependent transport. Such devices are expected to operate as a very sharp spin filter as well as an energy filter. As for γ'-Fe4N, recently, we have confirmed, from point-contact Andreev reflection measurements, that spin polarization in γ'-Fe4N thin films grown on MgO(001) substrates by molecular beam epitaxy MBE is larger than that in α-Fe (APL 94 (2009) 202502). In addition, 10-nm-thick γ'-Fe4N films were grown epitaxially on LaAlO3(001) and MgO(001) substrates by molecular beam epitaxy using solid Fe and a radio-frequency NH3 plasma, and spin and orbital magnetic moments of these γ'-Fe4N epitaxial films were deduced by x-ray magnetic circular dichroism measurements at 300 K. The total magnetic moments are almost the same for the two substrates, that is, approximately 2.44μB and 2.47μB, respectively. These values are very close to those predicted theoretically, and distinctively larger than that for α-Fe (APL 98 (2011)102507).
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Double-barrier resonant-tunneling diodes were fabricated from Fe3Si (10 nm)/CaF2 (5 nm)/Fe3Si (4 nm)/CaF2 (5 nm) on n-Si(111) substrates by molecular beam epitaxy. Negative differential resistance was observed in the current - voltage characteristics at room temperature (RT) (APEX 2 (2009) 063006). The peak-to-valley current ratio was found to reach a value as large as approximately 1000 at RT. We further fabricated 0.2μm-diameter RTD using CaF2/Fe3Si/CaF2 structure by selected-area MBE, and showed that approximately 40% of the RTDs showed clear NDR in the RT current - voltage characteristics, where the forward bias was applied to the Fe3Si upper layers with respect to the n-Si substrate (JJAP 49 (2010) 060212). These results show that quantized levels exist in the namometers-thick Fe3Si quantum well.
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