Noriyuki Urakami

Growth and luminescence characterization of dilute InPN alloys grown by molecular beam epitaxy

The authors have investigated the growth and luminescence properties of InPN alloys grown by solid-source molecular-beam epitaxy (MBE). The N composition increases with decreasing growth rate, P2∕In flux ratio, and growth temperature. In this work, the highest N composition obtained is 0.56% for the InPN sample. The appropriate growth temperature is around 400°C. However, the growth-temperature window of the InPN alloys having a smooth surface is very narrow. In order to obtain photoluminescence (PL) emission from the InPN samples grown by solid-source MBE, InPN alloys must be grown under the condition of lower-plasma power since the grown-in point defects induced by N plasma are reduced. Thermal treatment is effective to improve the luminescence efficiency of InPN alloys, and the appropriate annealing temperature is around 700°C. However, the S-shape behavior is observed only for the annealed InPN samples by atomic rearrangements during thermal treatment, which is attributed to the weaker bond strength o...

Epitaxial growth of ReS2(001) thin film via deposited-Re sulfurization

In this paper, we present the formation of large-size rhenium disulfide (ReS2) films via the sulfurization of Re films deposited on sapphire substrates. The effects of sulfurization temperature and pressure on the crystal quality were investigated. A [001]-oriented single crystal of ReS2 films with 6 × 10 mm2 area was realized. By sulfurizing Re films at 1100 °C, ReS2 films with well-defined sharp interfaces to c-plane sapphire substrates could be formed. Below and above the sulfurization temperature of 1100 °C, incomplete sulfurization and film degradation were observed. The twofold symmetry of the monocrystalline in-plane structure composed of Re–Re bonds along with Re–S bonds pointed to a distorted 1T structure, indicating that this structure is the most stable atomic arrangement for ReS2. For a S/Re compositional ratio equal to or slightly lower than 2.0, characteristic Raman vibrational modes with the narrowest line widths were observed. The typical absorption peak of ReS2 can be detected at 1.5 eV.

Formation of graphitic carbon nitride and boron carbon nitride film on sapphire substrate

As a novel production method of boron carbon nitride (BCN) films, in this paper, we present the incorporation of B into graphitic carbon nitride (g-C3N4). First, we investigated the formation of g-C3N4 films via chemical vapor deposition (CVD) using melamine powder as the precursor. The formation of g-C3N4 films on a c-plane sapphire substrate was confirmed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy measurements. The deposition temperature of g-C3N4 films was found to be suitable between 550 and 600 °C since the degradation and desorption of hexagonal C–N bonds should be suppressed. As for BCN films, we prepared BCN films via two-zone extended CVD using ammonia borane as the B precursor. Several XPS signals from B, C, and N core levels were detected from B-incorporated g-C3N4 films. While the N composition was almost constant, the marked tendencies for increasing B composition and decreasing C composition were achieved with the increase in the B incorporation, indicating the incorporation of B atoms by the substitution for C atoms. Optical absorptions were shifted to the high-energy side by B incorporation, which indicates the successful formation of BCN films using melamine and ammonia borane powders as precursors.

III–V-N compounds for multi-junction solar cells on Si

We proposed a GaPN/GaAsPN/Si multi-junction solar cell in which lattice constants for all layers matched to Si bottom cell. Initial growth of GaP layer on Si is an important role to suppress the anti-phase domain, stacking faults, threading dislocations and melt-back etching. According to theoretical estimation considering the strain effect, band-gap energy about 1.65 eV can be obtained by GaAs0.2P0.74N0.06 which satisfies the lattice-matching condition to Si. In order to clarify the possibility of this material system, we grew GaPN and GaAsPN on the structural defect-free GaP/Si template by using rf-MBE. We also investigated the pinning state at the GaP(N)/Si heterointerface to design a low-loss tunneling junction.