M Oshima

Growth and characterization of low-temperature grown GaN with high Fe doping

We succeeded in growing highly Fe-doped GaN films by solid-source molecular beam epitaxy using an electron-cyclotron-resonance microwave nitrogen plasma. The substrate temperature was in the range of 380–520u200a°C. The samples were analyzed by x-ray diffraction and transmission electron microscopy, and showed hexagonal (wurtzite) or cubic (zincblende) structure or a mixture of both phases. The Fe concentration was on the order of 1019u200acm−3 and extended x-ray absorption fine structure data show that the Fe is substituting the Ga in GaN. The magnetization measurements as a function of temperature reveal ferromagnetic properties below 100 K for the sample grown at the lowest temperature.

Epitaxial growth of AlN on (La,Sr)(Al,Ta)O3 substrate by laser MBE

We have grown aluminum nitride (AlN) on LSAT substrates for the first time using laser MBE and investigated its growth mechanism. It has turned out that surfaces of LSAT substrates become atomically flat by annealing in a UHV chamber. Reflection high energy electron diffraction observations and AFM measurements have shown that the growth of the AlN film starts with two-dimensional amorphous growth followed by three-dimensional epitaxial island growth. The epitaxial relationship between AlN and LSAT substrates is AlN(0xa00xa00xa01)||LSAT(1xa01xa01) and AlN||LSAT, which is rotated from the expected lattice matched alignment by 30° along AlN[0xa00xa00xa01]. X-ray diffraction measurements have revealed that the crystalline quality of the AlN film grown on LSAT is superior to that on a conventional Al2O3 substrate.

Nanoscale InGaAs concave disks fabricated by heterogeneous droplet epitaxy

The detailed cross-sectional structure of InGaAs quantum dots fabricated by a heterogeneous droplet epitaxy method was investigated by means of cross-sectional transmission electron microscopy observation. It was confirmed that concave disks without any dislocations or wetting layer were formed at the upper part of the flat surface. This result was consistent with the change of photoluminescence intensity and peak position. The sizes of the disks were estimated to be 30 and 12 nm in lateral and vertical directions, respectively. From this estimation, the occurrence of a phase-separation effect is suggested.

Organometallic synthesis and magnetic properties of ferromagnetic Sm-Co nanoclusters

We have successfully prepared Sm–Co composite nanoclusters using liquid-phase organometallic synthesis. The chemical composition was determined by quantitative x-ray fluorescence analysis and it is found that the composition of synthesized Sm–Co nanoclusters was Sm poor while the Sm–Co nanoclusters with required composition could be obtained in starting from the excess Sm(acac)3. From the transmission electron microscopy measurements, the Sm–Co nanoclusters have the uniform size with the diameter of 9 nm. The crystalline structure was fcc which is different from that of bulk SmCo alloy with the same Sm and Co content. The magnetic property was observed by superconducting quantum interference magnetometer and shows the ferromagnetic characteristics.

RHEED and XPS study of GaN on Si(111) grown by pulsed laser deposition

Abstract We have investigated growth mechanisms of GaN on Si (1xa01xa01) by pulsed laser deposition (PLD) using reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). We have found that the use of the AlN buffer layer dramatically improves the crystal quality of GaN. RHEED observations have shown that the epitaxial growth of the buffer layer starts with a two-dimensional mode followed by island formation. XPS measurements have revealed that the AlN/Si hetero-interface has no SixNy interfacial layer. We have also found that the Si atoms in N sites are segregated at the surface of the AlN buffer layer.

Epitaxial growth of semiconductors on SrTiO3 substrates

Abstract We have grown III–V compound semiconductors, such as GaAs, InAs, AlN, and GaN, on SrTiO 3 (STO) substrates. Annealing in a UHV chamber at around 700°C effectively reduces surface roughness of STO (1xa01xa01) and (1xa00xa00) substrates. We have succeeded in the epitaxial growth of the III–V compound semiconductors on STO substrates using this UHV annealing technique. We found that (1xa01xa01) is the dominant growth plane for zincblende semiconductors regardless of the crystal symmetry matching between the films and the STO substrates.

Molecular dynamics simulation of III–V compound semiconductor growth with MBE

Abstract We have developed a new potential function for classical molecular dynamics (MD) calculations of III–V compound semiconductors. The key idea for the new potential function is incorporation of the ionic nature of the bonds into the conventional Tersoff potential. It has been found that this potential can be used for the simulation of the MBE epitaxial growth for the III–V compound semiconductors. Using this potential, we have compared the initial growth stages of GaAs, AlAs, and InAs on GaAs substrates and found that the lattice mismatch causes serious distortion in the arrangement of the atoms even for the first layer.

Room-temperature photoinduced magnetoresistance effect in GaAs including MnSb nanomagnets

We show a photoinduced positive magnetoresistance (MR) effect (about 20%) under a low magnetic field (less than 0.1 T) at room temperature. The photoinduced MR effect has been observed in GaAs including nanoscale MnSb islands, when photons with the energy above the band gap of GaAs irradiated the sample. The photoinduced phenomena are due to an enhancement of tunneling probability between MnSb islands by photogenerated carriers in the GaAs matrix.

Crystallographic and magneto-optical studies of nanoscaled MnSb dots grown on GaAs

MnSb ultrathin films with the nominal thickness of 0–1.40 nm were grown on sulfur passivated GaAs substrates by molecular beam epitaxy. Atomic force microscopy analysis showed that MnSb formed nanosize clusters on the substrate, and the coalescence of the clusters occurred at the nominal thickness between 0.70 and 1.05 nm. The intensity of the polar magnetic circular dichroism of MnSb clusters suddenly increased when the nominal thickness reached the critical value of 1.05 nm. The coalescence among the dots can be correlated with the sharp increase of the magnetic circular dichroism intensity.

Mechanism for low temperature activation of Mg-doped GaN with Ni catalysts

The activation mechanism of Mg-doped GaN with Ni catalysts has been investigated by thermal desorption spectroscopy. It has been revealed that Ni deposited on Mg-doped GaN enhances the hydrogen recombination reaction at temperatures below 200u200a°C with the activation energy of 1.3 eV. The hydrogen desorbed at this temperature can be attributed to a part of the passivating hydrogen in GaN with a weak binding energy. The enhancement of the hydrogen recombination reaction on the GaN surface is essential to decreasing hydrogen concentration efficiently at low temperatures.