Mitsushi Suzuno

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.

Fabrication of n+-BaSi2/p+-Si Tunnel Junction on Si(111) Surface by Molecular Beam Epitaxy for Photovoltaic Applications

n+-BaSi2/p+-Si tunnel junctions with different BaSi2 template layer thicknesses were grown by molecular beam epitaxy. The template was found to be indispensable for growing epitaxial n+-BaSi2, but the resistance of the junctions increased with template thickness. However, both epitaxial growth and low resistance were achieved for a template thickness of 1 nm. A current density of 21.9 A/cm2 was achieved at 0.5 V. The photoresponsivity of 360-nm-thick undoped BaSi2 grown on the tunnel junction increased with bias voltage and reached 74 mA/W at 2.3 eV under a reverse bias of 4 V, the highest value ever reported for semiconducting silicides.

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.

Photoluminescence decay time and electroluminescence of p-Si∕β-FeSi2 particles∕n-Si and p-Si∕β-FeSi2 film∕n-Si double-heterostructures light-emitting diodes grown by molecular-beam epitaxy

We have epitaxially grown Si∕β-FeSi2∕Si (SFS) structures with β-FeSi2 particles on Si(001), and SFS structures with β-FeSi2 continuous films on both Si(001) and Si(111) substrates by molecular-beam epitaxy. All the samples exhibited the same photoluminescence (PL) peak wavelength of approximately 1.54 μm at low temperatures. However, the PL decay times for the 1.54 μm emission were different, showing that the luminescence originated from different sources. The decay curves of the SFS structures with β-FeSi2 continuous films were fitted assuming a two-component model, with a short decay time (τ∼10 ns) and a long decay time (τ∼100 ns), regardless of substrate surface orientation. The short decay time was comparable to that obtained in the SFS structure with β-FeSi2 particles. The short decay time was due to carrier recombination in β-FeSi2, whereas the long decay time was probably due to a defect-related D1 line in Si. We obtained 1.6 μm electroluminescence (EL) at a low current density of 2 A∕cm2 up to aro...

p-Si/β-FeSi2/n-Si double-heterostructure light-emitting diodes achieving 1.6 μm electroluminescence of 0.4 mW at room temperature

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. This emission power is obtained at room temperature under current injection of 460 mA, corresponding to an external quantum efficiency of approximately 0.1%. Photoluminescence and time-resolved photoluminescence measurements for devices with different thicknesses of β-FeSi2 indicate that radiative recombination rate increased as the thickness of the β-FeSi2 active layer is increased.

Effect of using a high-purity Fe source on the transport properties of p-type β-FeSi2 grown by molecular-beam epitaxy

Intentionally undoped p-type β-FeSi2 thin films were grown on Si(111) substrates by molecular-beam epitaxy using low-purity (4N) and high-purity (5N) Fe sources to investigate the effect of using a high-purity Fe source on the electrical properties of β-FeSi2. The hole mobility increased and the hole density decreased greatly as the annealing temperature and time were increased, particularly for the β-FeSi2 films produced using 5N-Fe. The observed temperature dependence of the hole mobility was reproduced well by considering various carrier scattering mechanisms due to acoustic-phonon, polar-optical phonon, nonpolar-optical phonon, and ionized impurities.

Improved Room-Temperature 1.6 µm Electroluminescence from p-Si/β-FeSi2/n-Si Double Heterostructures Light-Emitting Diodes

We have epitaxially grown p-Si/β-FeSi2/n-Si double heterostructures light-emitting diodes (LEDs) on Si(111) substrates by molecular-beam epitaxy. The 1.6 µm electroluminescence intensity measured at room temperature (RT) was improved significantly for LEDs constructed using a thick β-FeSi2 active layer (190 nm) embedded in heavily-doped Si p–n diodes formed on floating-zone Si(111) substrates. The external quantum efficiency was increased up to approximately 0.02% at RT.

Structural Study of BF2 Ion Implantation and Post Annealing of BaSi2 Epitaxial Films

We have investigated the effects of BF2 ion implantation and subsequent annealing on the structure of epitaxial BaSi2 thin films with the aim of the fabrication of a p-type B-doped BaSi2 film. After 10 min of annealing at 600 °C and above, BaSi2 is lost at least partly accompanied by appearance of Si as evidenced by X-ray diffraction and Raman spectroscopy. Element mapping by energy dispersive X-ray spectroscopy revealed that a barium oxide is formed on the surface, which indicates that BaSi2 is oxidized into a barium oxide and Si during annealing. Such oxidation was found to be suppressed by employing rapid thermal annealing for 30 s even when the annealing temperatures of 700 and 800 °C were chosen. Analysis of the full width at half maximum of the Raman peak showed that the inhomogeneous stress in the film produced by ion implantation can be decreased to the as-grown level by rapid thermal annealing at 700 and 800 °C for 30 s. At the same time, the red shift of the Raman peak is shown, based on which the possibility of B substitution for Si is discussed.

Enhanced Room-Temperature 1.6 µm Electroluminescence from Si-Based Double-Heterostructure Light-Emitting Diodes Using Iron Disilicide

We have fabricated Si/β-FeSi2/Si (SFS) double-heterostructure (DH) light-emitting diodes (LEDs) on Si(111) substrates with β-FeSi2 thickness ranging from 80 nm to 1 µm, and Si0.7Ge0.3/β-FeSi2/Si0.7Ge0.3(SGFSG) DH LEDs with a 200-nm-thick β-FeSi2 layer using lattice-matched Si0.7Ge0.3 layers by molecular-beam epitaxy. The electroluminescence (EL) peaked at an emission wavelength of approximately 1.6 µm at room temperature. As the thickness of the β-FeSi2 layer was increased in the SFS DH LEDs, the emission power of EL increased for a given current density J. EL with an emission power of over 0.4 mW and an external quantum efficiency of approximately 0.1% was achieved for the SFS DH LED with a 1-µm-thick β-FeSi2 layer. The smallest J value necessary for EL output, which is approximately 1 A/cm2, was achieved for the SGFSG DH LEDs.