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

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)

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

High photoresponsivity in a GaAs film synthesized on glass using a pseudo-single-crystal Ge seed layer

Research to synthesize a high-quality GaAs film on an inexpensive substrate has been continuing for decades in the quest to develop a solar cell that achieves both high efficiency and low-cost. Here, we applied a large-grained Ge layer on glass, formed by Al-induced layer exchange, to an epitaxial template for a GaAs film. The GaAs film, grown epitaxially from the Ge seed layer at 520 °C, became a pseudosingle crystal (grain size > 100 μm) with high (111) orientation. Reflecting the large grain size, the internal quantum efficiency reached 70% under a bias voltage of 1.0 V. This value approaches that of a simultaneously formed GaAs film on a single-crystal Ge wafer and is the highest for a GaAs film synthesized on glass at a low temperature. The application of a Ge seed layer formed by layer exchange offers excellent potential to develop high-efficiency thin-film solar cells with III–V compound semiconductors based on low-cost glass substrates.This work was supported financially by the JSPS KAKENHI (No. 17H04918). The authors are grateful to Dr. Y. Tominaga (Hiroshima University) for helpful discussions and Professor N. Usami (Nagoya University) for assistance with the microwave photoconductivity decay measurement. Some experiments were conducted at the International Center for Young Scientists at NIMS and the Nanotechnology Platform at the University of Tsukuba

Spin and orbital magnetic moments of molecular beam epitaxy γ′-Fe4N films on LaAlO3(001) and MgO(001) substrates by x-ray magnetic circular dichroism

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. The lattice mismatch of these substrates to γ′-Fe4N is 0% and 11%, respectively. 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, 2.44±0.06 μB and 2.47±0.06 μB, respectively. These values are very close to those predicted theoretically, and distinctively larger than that for α-Fe

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

Mechanisms of carrier lifetime enhancement and conductivity-type switching on hydrogen-incorporated arsenicdoped BaSi2

A comparative experimental and theoretical study of the role of H incorporation in As-doped BaSi2 films has been carried out based on the experimental results that an optimal time of H treatment for the increase in photoresponsivity and carrier lifetime was in the range of 1 – 20 min. Adequate theoretical representation of the decay curves in the framework of the model for non-radiative processes accounted for various trap-related recombination mechanisms to estimate the trap concentration to be in the range of 1.9 × 1013 to 1.7 × 1014 cm-3. Additionally, the extended theoretical ab initio quantum-chemical simulation of the electronic structure of the studied systems was performed. It was revealed that interstitial As atoms can mostly provide trap states in the gap while H atoms neutralize such traps. The experimentally observed unexpected switching in conductivity from n-type to p-type and vice versa in As-doped BaSi2 with H incorporation was explained to specific configurations of point defects (an As impurity with a H atom in different positions and various interatomic As-H distances) which affect the position of states in the gap