Shigehiko Hasegawa

Electron field emission from GaN nanorod films grown on Si substrates with native silicon oxides

GaN nanorod films have been grown on Si(001) substrates with native silicon oxides by radio-frequency plasma-enhanced molecular beam epitaxy. GaN nanorod films are made up of single-crystalline nanorods with a so-called (0001) fiber-like texture. Each nanorod is elongated along c axis in perpendicular to the substrate surface and has no preferential axis in film plane. Excellent electron field emission characteristics were observed for the fabricated GaN nanorod films with a field emission threshold as low as 1.25V∕μm at a current density of 0.1μA∕cm2 and a field emission current density as high as 2.5mA∕cm2 at an applied field of 2.5V∕μm. These excellent characteristics are attributed to the geometrical configuration of nanorods and their good crystalline quality as well as the low electron affinity of GaN.

Formation and photoluminescence of quantum wire structures on vicinal (110) GaAs substrates by MBE

Abstract Quantum wire structures are formed by the molecular beam epitaxial growth of AlAs-AlGaAs-AlAs quantum well on the large-growth-step structures of superlattices on vicinal (110) GaAs substrates. From the PL spectra, Al compositions in the wire are estimated to be much smaller than those in the well layers. The PL is polarized parallel to the wire direction. The observed large polarization supports the carrier confinement to the quantum wire structures.

Large magnetization in high Gd concentration GaGdN and Si-doped GaGdN grown at low temperatures

GaGdN layers were grown at temperatures below 300°C by radio-frequency plasma-assisted molecular-beam epitaxy on sapphire substrates. GaGdN samples with high Gd concentration as high as 12.5% were obtained. X-ray diffraction results showed no obvious secondary phase, which means that the phase separation can be suppressed by the growth at low temperatures. Local structure around the Gd atom was investigated by extended x-ray absorption fine structure measurement using GdLIII edge. It was shown that Gd atoms were mainly incorporated into the Ga sites in the GaGdN layers. All the samples grown at low temperatures exhibited ferromagnetic characteristics. It is considerable that electrons coming from defects, especially, nitrogen vacancy, stabilize ferromagnetism, and that the carrier-induced ferromagnetism occurs in the low-temperature-grown GaGdN. Furthermore, Si was doped into GaGdN layers during growth in order to increase carrier density, and larger magnetization was observed.

Growth and characterization of GaAs films on porous Si

Abstract The initial growth of GaAs films on porous Si substrates has been investigated using reflection high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS). XPS measurements show that at a considerably low temperature, clean porous Si surfaces are easily obtained. To obtain atomically flat GaAs films, thicker buffer layers of GaAs (2000–4000 A) are needed. From RBS measurements, a minimum channeling yield χ min of 7% is obtained, smaller than the results reported previously. The results show that porous Si is a promising candidate for obtaining GaAs films with good crystalline quality on Si substrates.

Hydrogen terminated Si(100) surfaces studied by scanning tunneling microscopy, x‐ray photon spectroscopy, and Auger electron spectroscopy

Si(100) surfaces were terminated by hydrogen atoms by means of several chemical methods: (1) Conventional dipping into hydrofluoric acid (HF) followed by immersion into several HF solutions of various concentrations diluted by ethanol (C2H5OH), and (2) hydrogenated porous silicon film anodically formed in an HF and C2H5OH solution. Quite clear STM images with atomic steps were obtained for HF‐dipped surfaces during the first several hours in a dry N2 ambient. The observable time was longer for surfaces dipped into less dilute HF. Atomic steps could be observed on the anodically produced porous silicon surfaces even after ten days exposure to air. These phenomena were consistent with the initial oxidation behaviors of the surfaces studied by x‐ray photon spectroscopy (XPS) and Auger electron spectroscopy (AES).

New model for Si(111)-(3×1)Li through determination of its surface Si atom density with the use of scanning tunneling microscopy

Abstract We report on surface Si atom density in a Si(111)-(3×1)Li structure determined from scanning tunneling microscopy observation. During a Li adsorption process on Si(111) surfaces, the imbalance of surface Si atom density between the clean (7×7) and Li-induced (3×1) structures causes the nucleation of Si islands. From measurements of the coverage of nucleated islands, we determine surface Si atom density in the Si(111)-(3×1)Li as four atoms per (3×1) unit cell and propose a structural model accounting for the present results.

Diffuse scattering in the high-temperature (1×1) state of Si(111)

Diffuse spots slightly displaced from the (\(\sqrt{3} \times \sqrt{3}\)) positions were observed in the high-temperature (1×1) state of Si (111) by low-energy electron diffraction. The spots showed threefold 3m symmetry and rotated around the \(\sqrt{3}\) positions with changes in primary energy. These are reproduced by kinematic calculations for a surface configuration with adatoms distributed randomly over both the threefold-hollow and threefold atop sites of a (1×1) substrate with the exclusion of forming bonds with a common substrate atoms.

STACKED GAAS MULTI-QUANTUM WIRES GROWN ON VICINAL GAAS(110) SURFACES BY MOLECULAR BEAM EPITAXY

Stacked GaAs quantum wires (QWRs) are grown on the surfaces with giant steps which are naturally formed on vicinal GaAs(110) substrates by molecular beam epitaxy. Transmission electron microscopy observation clearly shows stacked structures of coherently aligned quantum wires which are induced by GaAs layer thickness modulation at the step edges. Photoluminescence peak shifts with the thickness of the AlGaAs barrier layers are explained as due to the coupling between the QWRs.

Observation of photoluminescence emission in ferromagnetic semiconductor GaCrN

Optical properties of GaN-based diluted magnetic semiconductor GaCrN were studied. The GaCrN layers were grown by electron-cyclotron-resonance plasma-assisted molecular-beam epitaxy. They exhibited ferromagnetic behavior at room temperature and strong photoluminescence (PL) emission at 3.29 eV (10 K). The PL emission peak energy of the GaCrN decreases with increasing temperature in accordance with the Varshini formula similar to the GaN excitonic transition peak. The 3.29 eV PL emission was assigned to be a band-to-band transition in GaCrN from the temperature- and the excitation-power-density dependences of the PL.

Scanning tunneling microscopy study of the hydrogen-terminated n- and p-type Si(001) surfaces

Abstract We have investigated hydrogen-passivated Si(001) surfaces with various doping conditions using X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). It is found that Si 2 p peak energies of XPS for the hydrogen-passivated surfaces depend on the doping conditions while Si 2 p XPS peaks for reconstructed surfaces after annealing them at 700°C always have the same binding energies without depending on the doping conditions. This suggests that the surface Fermi level on the hydrogen-passivated Si(001) surface is unpinned. STM/STS measurements reveal that a shoulder originating from a dopant level exists in the current–voltage ( I – V ) curve obtained by STS. We discuss the possibility to obtain bulk electronic characteristics of Si through its hydrogen-passivated surface with the use of STM/STS.