Katsuaki Toh

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...

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.

Epitaxy of Orthorhombic BaSi2 with Preferential In-Plane Crystal Orientation on Si(001): Effects of Vicinal Substrate and Annealing Temperature

We attempted to grow orthorhombic BaSi2 epitaxial films having preferential in-plane crystallographic orientation on both exact and vicinal Si(001) substrates with a miscut angle of 2° toward Si[10] by reactive deposition epitaxy (RDE) and subsequent molecular beam epitaxy (MBE). On the vicinal Si(001) substrate, the initial BaSi2 nuclei formed by RDE tended to grow unidirectionally with the [010] direction parallel to Si[110] when the annealing temperature of the Si substrate before the growth was increased from 830 to 1000 °C. Transmission electron microscopy showed that the grain size of the BaSi2 films grown by MBE increased up to approximately 9 µm on the vicinal Si(001) substrate when the substrate annealing temperature was 1000 °C. This is the largest grain size ever obtained for BaSi2. Even in the case of the exact Si(001) substrate, the MBE-grown BaSi2 grains preferentially grew with the [010] direction along Si[110] when the annealing temperature was 1000 °C.

Realization of Large-Domain Barium Disilicide Epitaxial Thin Film by Introduction of Miscut to Si(111) Substrate

The domain structure of BaSi2 epitaxial films grown on vicinal Si(111) substrates has been studied in order to fabricate high-quality BaSi2 crystals with large domains. The X-ray pole figure measurement shows that the BaSi2 films grown on vicinal substrates as well as the on-axis substrate consist of three epitaxial variants which are equivalent in terms of 60° in-plane rotations, and that one of the variants is dominant in the film grown on the 2°-inclined substrate. The orientation maps produced by electron backscatter diffraction show that the domains with the b axis parallel to the miscut direction are larger than the others in the film grown on the 2°-inclined substrate, while the domain sizes of three variants are found similar in the films grown on the on-axis and 4°-inclined substrates. The possible origin of the large domain formation is discussed with the focus on the initial growth stage observed by atomic force microscopy. Nucleation from the step edge is proposed as the mechanism of the large-domain formation considering the lattice matching to the step edge, while nucleation is suggested to occur at the terrace edges on the 4°-inclined substrate.

Investigation of the carrier recombination process in undoped barium disilicide epitaxial films

Excess-carrier recombination in undoped BaSi2 epitaxial films grown by molecular beam epitaxy on n-type silicon has been studied by the microwave-detected photoconductivity decay measurement. The measurement with various excitation laser intensities shows that dominant recombination processes change from Auger recombination to Shockley-Read-Hall recombination, and to carrier trapping with time. The numerical analysis of the Shockley-Read-Hall recombination lifetime and the X-ray rocking curve measurement of the films of 50-130 nm in thickness show that carrier capture probability has a positive correlation with the full-width at half-maximum of X-ray rocking curves, suggesting that dislocations act as recombination centers.

Improved internal quantum efficiency in high-quality BaSi 2 films grown by molecular beam epitaxy

A new method has been developed for the formation of undoped BaSi₂ absorption layers on an Sb-doped n⁺-BaSi₂/p⁺-Si tunnel junction (TJ) formed on Si(111). The diffusion of Sb atoms from the n⁺-BaSi₂ layer was effectively suppressed by inserting a thin Si layer between the n⁺-BaSi₂ and 400-nm-thick undoped BaSi₂ layers. The inserted Si layer was firstly deposited at room temperature, followed by annealing at 650°C by solid phase epitaxy (SPE). Both X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) patterns indicated the good crystalline quality of undoped BaSi₂ overlayers; the I-V characteristics revealed the excellent tunneling effect of the TJ. The photoresponsivity has been greatly improved by the new growth method and reached a maximum of 0.37 A/W at 1.55 eV under a bias voltage of 2 V; the corresponding external quantum efficiency (EQE) and internal quantum efficiency (IQE) increased up to 60% and 70%, respectively, which is the highest value ever reported for semiconducting silicides.