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Abstract: Step-by-step change of growth temperature was investigated to control GaN nanorod diameter in the molecular-beam epitaxy. It was found that when the growth temperature is decreased, the GaN nanorod diameter gradually increases with the progress of lower temperature growth. On the contrary, opposite change of the diameter was observed when the growth temperature is increased during nanorod growth. The Ga adatom density on the GaN surface and the dissociation in the GaN nanorods play an important role on the shape variation of nanorods and we can control the diameter of GaN nanorods by regulating the growth temperature. [Copyright &y& Elsevier]

Effect of Si doping on low-temperature grown GaCrN films has been investigated by positron annihilation spectroscopy. In undoped GaCrN films grown at 540 °C, vacancy clusters with sizes of V6-V12 were found to be responsible for positron trapping. Such vacancy clusters were considerably suppressed in Si-doped GaCrN films grown at 540 °C, although divacancies (VGaVN) still survived. The Si-doping may be one possible way to suppress vacancy aggregation during low temperature crystal growth, and the further methods to remove divacancies are required. [ABSTRACT FROM AUTHOR]

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 1.25 V/μm at a current density of 0.1 μA/cm2 and a field emission current density as high as 2.5 mA/cm2 at an applied field of 2.5 V/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. [ABSTRACT FROM AUTHOR]

X-ray photoemission spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED) have been used to investigate the initial stages of InP growth on H-terminated Si and native Si oxide surfaces. By annealing at 800°C, the H-terminated Si surface exhibits a (2×1) RHEED pattern while the native oxide surface just shows a halo RHEED pattern. After exposing them to P2 flux, the Si(2×1) clean surface exhibits a (2×1) RHEED pattern while the native Si oxide layer always shows the halo RHEED pattern. XPS measurement reveals that while P atoms adsorb on the Si(2×1) surface, the adsorption of P atoms does not take place on the native Si oxide surfaces. In consequence of the difference in the adsorption of P atoms between the clean Si and Si oxide surfaces, InP does not grow on the Si oxides at temperatures above 450°C while InP grows on the clean Si substrates. Based on the findings, we will discuss the mechanism of the selective area growth of InP on patterned Si substrates with Si oxide mask in terms of desorption of In and P. [ABSTRACT FROM AUTHOR]

Abstract: The growth parameter dependence of structural, electrical and magnetic properties in Gd-doped GaN layers grown by plasma-assisted molecular beam epitaxy has been investigated. The structural and magnetic properties of Gd-doped GaN films grown on GaN templates strongly depend on the MBE growth condition. While Gd-doped GaN grown under relatively high Ga fluxes consist of wurtzite GaGdN layers without Gd-related precipitates, Gd-incorporated GaN films grown under low Ga fluxes contain a lot of nanoparticles ranging from several nm to several tens nm in size. The samples with Gd-related nanoparticles exhibit hysteresis in the magnetization–magnetic field curves at 10K. The separation between the field-cooled and zero-field-cooled magnetization–temperature curves is observed at around 30K. This behavior is understood in terms of super-paramagnetism originating from the ferromagnetic nanoparticles observed in the cross-sectional transmission electron microscopy images. [Copyright &y& Elsevier]

Abstract: We have investigated optical property of GaGdN grown on GaN template by radio-frequency plasma-assisted molecular beam epitaxy by varying the concentration of Gd. All the samples exhibit photoluminescence emission at the near band edge of 356nm, a blue luminescence (BL) band around 420nm and a green luminescence (GL) band around 500nm, instead of yellow luminescence (YL) band around 550nm frequently observed in n-GaN layer. The band edge emission is independent of the Gd concentration. This shows that the values of band gap for the GaGdN layers are the same as that of GaN. The temperature dependencies of the GL band intensity and peak wavelength are very similar to those of the YL band observed in the GaN epitaxial layers on the GaN templates. On the other hand, with increasing the Gd concentration, the BL bands observed in the GaGdN layers are red-shifted and their intensities decrease. The BL band intensity remains unchanged in the temperature range of . It rapidly decreases at temperature above 100K, and disappears at 200K. The BL band is red-shifted in the temperature range of and its value is about twice larger than that of the GL band. [Copyright &y& Elsevier]

Abstract: GaDyN/GaN double-barrier magnetic tunneling junctions (DB-MTJs) structures are grown on GaN templates by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). X-ray diffraction (XRD) θ/2θ scan curves exhibited a clear peak of GaDyN at low angle side of the GaN (0002) diffraction peak. From analyzing extended x-ray absorption fine structure (EXAFS), majority of Dy atoms are found to be incorporated into substituting the Ga sites in GaN with wurtzite structure, and GaDyN has longer bond length than that of GaN. From XRD and EXAFS results, the lattice constant of the GaDyN is larger than that of GaN due to large radius of Dy ion. Photoluminescence from GaDyN DB-MTJs structure was found, which is related to the Dy ions. Ferromagnetism is confirmed in the magnetization versus magnetic field curves at room temperature for the all samples. It is found that the magnetization per unit volume and the coercivity show change with increasing the thickness of the middle GaDyN layer. This implies that the interlayer interaction exists between the GaDyN layers. [Copyright &y& Elsevier]

Abstract: Cubic GaCrN layers are grown on MgO(001) substrates at 350–700°C by plasma-assisted molecular beam epitaxy. Substrate temperature dependences of their structural and magnetic properties were systematically studied. It is found that the solubility limit of Cr atoms in cubic GaCrN is dramatically improved by the low temperature (350°C) growth, though crystalline quality becomes poorer. It is also observed that the magnetic ordering increases with Cr content in the low Cr content region, but after showing highest ordering it decreases in the high Cr content region. The Cr content range showing ferromagnetic behavior increases with lowering substrate temperature. However, the magnetization vs. magnetic field curve shows “emaciated” hysteresis for the low temperature grown samples. [Copyright &y& Elsevier]

Abstract: Epitaxial growth of transition-metal Cr-doped cubic-phase GaN (GaCrN) layers on GaAs (001) substrates by plasma-assisted molecular-beam epitaxy and the observation of ferromagnetism above room temperature are reported for the first time. Influence of V/III ratio and Cr-cell temperature on the physical properties of the grown layers is studied extensively. X-ray diffraction studies and transmission electron microscopy observations confirm the predominantly cubic-nature of GaN and GaCrN for the epitaxial layers grown under Ga-rich condition. Cr dopant atoms exhibit surfactant nature and improve the crystalline quality and also enhance the growth of cubic-phase of GaN and GaCrN. Strong and broad blue-luminescent band (centered at 2.85eV) is induced by the Cr doping in GaN. The saturation magnetization of the grown layers initially increases with Cr concentration and then decreases for further increase of Cr concentration. [Copyright &y& Elsevier]

Abstract: TlInGaAs/TlInP/InP separate confinement heterostructures (SCHs) were grown by gas-source molecular-beam epitaxy and metal stripe laser diodes (LDs) were fabricated. Temperature variation of electroluminescence (EL) peak wavelength was as small as 0.01nm/K. Pulsed laser operation was achieved at 77–302K. Threshold current density for the TlInGaAs/TlInP/InP SCH LD (0.6kA/cm2 at 77K) was smaller than that for the TlInGaAs/InP double heterostructure LD (0.8kA/cm2 at 77K). This is due to the increased refractive index of TlInP and the improved optical confinement. Temperature variation of main peak wavelength in the lasing spectra was as small as 0.06nm/K. This is smaller than that of InGaAsP/InP DFB LDs. [Copyright &y& Elsevier]

Abstract: TlInGaAsN/GaAs double quantum well (DQW) structures were grown on GaAs (100) substrates by gas source molecular-beam epitaxy. It has been found that high Tl flux is needed for the incorporation of Tl into the films. Reduction in the temperature variation of electroluminescence (EL) peak energy has been observed by the addition of Tl into quantum well (QW) layers; −0.62meV/K for the InGaAsN/GaAs DQW light emitting diodes (LEDs) and −0.53meV/K for the TlInGaAsN/GaAs DQW LEDs. By replacing GaAs barrier layers with TlGaAs barrier layers, further reduction could be obtained; −0.35meV/K for TlInGaAsN/TlGaAs DQW LEDs. SIMS measurements indicated that this improvement is caused by the increased incorporation of Tl into the QW layers. [Copyright &y& Elsevier]

Abstract: We have grown polycrystalline GaN on quartz, refractory metal (W, Mo, Ta and Nb) and Si substrates by using plasma-assisted molecular beam epitaxy. It has been found that polycrystalline GaN grown on quartz and refractory metal substrates shows a strong band-edge emission without yellow emission. GaN growth on Si with native oxide produces well c-orientated nanorods exhibiting a low field emission threshold and high emission current density. We will review the growth of polycrystalline GaN films and the evaluation of their optical properties and electron field emission characteristics. [Copyright &y& Elsevier]

Abstract: Local structural change around Cr atoms in GaN-based diluted magnetic semiconductor GaCrN grown by radio frequency plasma-assisted molecular-beam epitaxy was studied with fluorescent X-ray absorption fine structure (XAFS) and X-ray diffraction measurements. The lattice constant of Ga1-xCrxN depends linearly on x up to and it approaches to that of non-doped GaN above x of 4.7%. It was found from our XAFS study that the majority of Cr atoms substitute the Ga lattice sites of GaN and that the critical Cr concentration of solid solution is around . At the critical Cr concentration, onset of the formation of NaCl-type CrN-like structure occurs, and above the critical concentration, the local structure around Cr atoms transforms to a completely CrN-like structure. Our X-ray absorption near-edge structure studies also indicate a drastic change in local electronic structure of Ga1-xCrxN at x between 4% and 4.7%. This is ascribed to the fact that the atomic configuration of Cr and N undergoes a drastic transition from the substitutionally incorporated GaCrN into the CrN-like atomic configuration. The local structural change into NaCl-type CrN was also supported by the XAFS pre-edge structures and the polarized XANES spectra. [Copyright &y& Elsevier]

To realize the temperature-stable threshold-current and lasing-wavelength 1.3–1.5 μm laser diodes, TlInGaAsN/AlGaAs heterostructures were proposed. As a first step, TlInGaAs was successfully grown, for the first time, on GaAs substrate by molecular beam epitaxy. The incorporation of Tl was confirmed with reflection high-energy electron diffraction intensity oscillation measurement. Photoluminescence (PL) measurements were conducted on the TlInGaAs/GaAs and InGaAs/GaAs heterostructure samples and the red shift of the PL peak energy was observed for the TlInGaAs/GaAs samples, as expected. [Copyright &y& Elsevier]