Kaoru Toko

Giant Ge-on-Insulator Formation by Si--Ge Mixing-Triggered Liquid-Phase Epitaxy

Formation of giant single-crystalline Ge-on-insulator (GOI) with 400 µm length is demonstrated by using seeding lateral liquid-phase epitaxy (L-LPE). High quality (100), (110), and (111) oriented single-crystals are obtained on Si substrates covered with SiO2. A mechanism based on the solidification temperature gradient due to Si–Ge mixing and the thermal gradient due to latent heat at the growth front is proposed. An additional experiment on quartz substrates well supports the importance of Si–Ge mixing as a trigger for giant L-LPE of Ge. This process opens up the possibility of Ge-based fully-depleted field-effect transistors with high electron and hole mobilities.

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.

Temperature dependent metal-induced lateral crystallization of amorphous SiGe on insulating substrate

Metal-induced lateral crystallization (MILC) of amorphous SiGe films on SiO2 has been investigated as a function of Ge fraction (0%–100%) and annealing temperature (320–550°C). High temperature annealing (>500°C) caused spontaneous nucleation in amorphous SiGe with a high Ge fraction (>70%). This suppressed the progress of MILC. Spontaneous nucleation was significantly suppressed by lowering the annealing temperature ( 20μm) were observed around Ni patterns even for high Ge fractions (>70%). In this way, MILC of amorphous SiGe was achieved for samples with whole Ge fractions (0%–100%).

High-quality single-crystal Ge stripes on quartz substrate by rapid-melting-growth

Single-crystal Ge on a transparent insulating substrate is desired to achieve advanced thin-film transistors (TFTs) with high speed operation. We have developed the rapid-melting-growth process of amorphous Ge by using polycrystalline Si islands as the growth seed. High-quality and dominantly (100)-oriented single-crystal Ge stripes with 400 μm length are demonstrated on quartz substrates. The temperature dependence of the electrical conductivity shows a high hole mobility of 1040 cm2/V s. This method opens up a possibility of Ge-channel TFT with the high carrier mobility.

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.

Ni-imprint induced solid-phase crystallization in Si1−xGex (x: 0–1) on insulator

Position control of solid-phase crystallization in the amorphous Si1−xGex (x: 0–1) films on insulating substrates was investigated by using Ni-imprint technique. Crystal nucleation at the imprinted positions proceeded approximately 2–20 times, depending on Ge fraction, faster than the conventional solid-phase crystallization, which was due to the catalytic effect of Ni. As a result, large SiGe crystal regions (∼2μm) were obtained at controlled positions. On the other hand, the growth velocity did not changed, which suggested that grown regions contained few residual Ni atoms.

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...

Determination of Bulk Minority-Carrier Lifetime in BaSi2 Earth-Abundant Absorber Films by Utilizing a Drastic Enhancement of Carrier Lifetime by Post-Growth Annealing

We have successfully determined the bulk minority-carrier lifetime in BaSi2 epitaxial films by utilizing a drastic enhancement of lifetime by post-growth annealing at 800 °C, which is attributed to strain relaxation. From the film-thickness dependence of lifetime, we reveal that the bulk lifetime is 14 µs, which is long enough for thin-film solar cell applications. In addition, the sum of surface and interface recombination velocities is found to be as low as 8.3 cm/s presumably due to the ionic nature of BaSi2. This confirms that BaSi2 is promising as an absorption-layer material for earth-abundant thin-film solar cells.

In-situ heavily p-type doping of over 1020 cm−3 in semiconducting BaSi2 thin films for solar cells applications

B-doped p-BaSi2 layer growth by molecular beam epitaxy and the influence of rapid thermal annealing (RTA) on hole concentrations were presented. The hole concentration was controlled in the range between 1017 and 1020 cm−3 at room temperature by changing the temperature of the B Knudsen cell crucible. The acceptor level of the B atoms was estimated to be approximately 23 meV. High hole concentrations exceeding 1 × 1020 cm−3 were achieved via dopant activation using RTA at 800 °C in Ar. The activation efficiency was increased up to 10%.

Improved photoresponsivity of semiconducting BaSi2 epitaxial films grown on a tunnel junction for thin-film solar cells

The highest photoresponsivity and an internal quantum efficiency exceeding 70% at 1.55 eV were achieved for 400 nm thick undoped n-type BaSi2 epitaxial layers formed on a n+-BaSi2/p+-Si tunnel junction (TJ) on Si(111). The diffusion of Sb atoms was effectively suppressed by an intermediate polycrystalline Si layer grown by solid phase epitaxy, located between the TJ and undoped BaSi2 layers.