Takuma Sato

Impact of Ba to Si deposition rate ratios during molecular beam epitaxy on carrier concentration and spectral response of BaSi2 epitaxial films

Undoped 0.5-μm-thick BaSi2 epitaxial films were grown on Si(111) substrates with various ratios of the Ba deposition rate to the Si deposition rate (RBa/RSi) ranging from 1.0 to 5.1, and their electrical and optical properties were characterized. The photoresponse spectra drastically changed as a function of RBa/RSi, and the quantum efficiency reached a maximum at RBa/RSiu2009=u20092.2. Hall measurements and capacitance versus voltage measurements revealed that the electron concentration drastically decreased as RBa/RSi approached 2.2, and the BaSi2 films with RBa/RSi=u20092.0, 2.2, and 2.6 exhibited p-type conductivity. The lowest hole concentration of approximately 1u2009×u20091015u2009cm−3 was obtained for the BaSi2 grown with RBa/RSiu2009=u20092.2, which is the lowest value ever reported. First-principles calculations suggest that Si vacancies give rise to localized states within the bandgap of BaSi2 and therefore degrade the minority-carrier properties.

Temperature dependence of magnetic hysteresis scaling for cubic Fe3O4 nanoparticles

We have examined magnetic hysteresis scaling of minor loops for cubic Fe3O4 nanoparticles with particle diameter of 265 nm in order to seek a possible application of the method for characterization of magnetic nanoparticles. While the conventional Steinmetz law fails, a power-law scaling with an exponent of 1.3±0.1 was found to universally hold true between the remanence and hysteresis loss of minor loops below and above the Verwey temperature of ∼ 100 K. A minor-loop coefficient obtained from the power law exhibits a sudden increase with an onset of Verwey transition, followed by a steep increase with decreasing temperature. The increase of the coefficient at Verwey transition is by 390%, being much higher than 260% for major-loop coercivity. There observations demonstrate that a magnetic hysteresis scaling using symmetrical minor loops can be a possible technique for characterizing magnetic nanoparticles due to its sensitivity to materials intrinsic properties and low measurement fields below 1 kOe.

Detection of local vibrational modes induced by intrinsic defects in undoped BaSi2 light absorber layers using Raman spectroscopy

We fabricate BaSi2 epitaxial films on Si(111) substrates by molecular beam epitaxy and investigate point defects inside the films using Raman spectroscopy with the help of first-principles calculation. Point defects such as Ba substituted for Si antisites, Si vacancies, and Si interstitials are considered as candidates for native point defects in BaSi2. Vibration analysis based on first-principles calculation suggests that local vibrational modes caused by these point defects appear at around 430, 480, and 560u2009cm−1, respectively, and are in good agreement with Raman peak positions. Comparing calculations with Raman spectra of the films formed with different Ba to Si deposition rate ratios RBa/RSi from 1.0 to 5.1, we conclude that the density of point defects reaches a minimum at RBa/RSi = 2.2. Furthermore, the position of Raman peaks at approximately 490u2009cm−1 shifts to a lower wavenumber, depending on RBa/RSi and thereby the density of point defects.We fabricate BaSi2 epitaxial films on Si(111) substrates by molecular beam epitaxy and investigate point defects inside the films using Raman spectroscopy with the help of first-principles calculation. Point defects such as Ba substituted for Si antisites, Si vacancies, and Si interstitials are considered as candidates for native point defects in BaSi2. Vibration analysis based on first-principles calculation suggests that local vibrational modes caused by these point defects appear at around 430, 480, and 560u2009cm−1, respectively, and are in good agreement with Raman peak positions. Comparing calculations with Raman spectra of the films formed with different Ba to Si deposition rate ratios RBa/RSi from 1.0 to 5.1, we conclude that the density of point defects reaches a minimum at RBa/RSi = 2.2. Furthermore, the position of Raman peaks at approximately 490u2009cm−1 shifts to a lower wavenumber, depending on RBa/RSi and thereby the density of point defects.

Improving the photoresponse spectra of BaSi2 layers by capping with hydrogenated amorphous Si layers prepared by radio-frequency hydrogen plasma

We studied the surface passivation effect of hydrogenated amorphous silicon (a-Si:H) layers on BaSi2 films. a-Si:H was formed by an electron-beam evaporation of Si, and a supply of atomic hydrogen using radio-frequency plasma. Surface passivation effect was first investigated on a conventional n-Si(111) substrate by capping with 20 nm-thick a-Si:H layers, and next on a 0.5 μm-thick BaSi2 film on Si(111) by molecular beam epitaxy. The internal quantum efficiency distinctly increased by 4 times in a wide wavelength range for sample capped in situ with a 3 nm-thick a-Si:H layer compared to those capped with a pure a-Si layer.We studied the surface passivation effect of hydrogenated amorphous silicon (a-Si:H) layers on BaSi2 films. a-Si:H was formed by an electron-beam evaporation of Si, and a supply of atomic hydrogen using radio-frequency plasma. Surface passivation effect was first investigated on a conventional n-Si(111) substrate by capping with 20 nm-thick a-Si:H layers, and next on a 0.5 μm-thick BaSi2 film on Si(111) by molecular beam epitaxy. The internal quantum efficiency distinctly increased by 4 times in a wide wavelength range for sample capped in situ with a 3 nm-thick a-Si:H layer compared to those capped with a pure a-Si layer.

Magnetization reversal mechanism for Co nanoparticles revealed by a magnetic hysteresis scaling technique

We report results of magnetic hysteresis scaling of minor loops for cobalt nanoparticles with variable mean particle size of 53 and 95 nm. A power-law scaling with an exponent of 1.40±0.05 was found to hold true between minor-loop remanence and hysteresis loss in the wide temperature range of 10 − 300 K, irrespective of particle size and cooling field. A coefficient deduced from the scaling law steeply increases with decreasing temperature and exhibits a cooling field dependence below T ∼ 150 K. The value obtained after field cooling at 5 T was lower than that after zero-field cooling, being opposite to a behavior of major-loop coercivity. These observations were explained from the viewpoint of the exchange coupling between ferromagnetic Co core and antiferromagnetic CoO shell, which becomes effective below T ∼ 150 K.