H. Makino

Band alignment at a ZnO/GaN (0001) heterointerface

We report the experimental results of the valence band offset at a ZnO/GaN (0001) heterointerface. The ZnO/GaN (0001) heterointerface is prepared by growing a ZnO layer on (0001) GaN/Al2O3, in which the ZnO layer is epitaxially deposited by plasma-assisted molecular-beam epitaxy, while the GaN template is prepared by metalorganic chemical-vapor deposition. Ex situ ultraviolet and x-ray photoelectron spectroscopy have been used to measure the valence band offset ΔEV. The photoelectron spectroscopy measurements are done before and after Ar+ ion cleaning of the surfaces. Type-II band alignments with band offsets of ΔEV=1.0 eV (before cleaning) and 0.8 eV (after cleaning) with the valence band maximum of GaN being placed above that of ZnO are obtained.

Si-based omnidirectional reflector and transmission filter optimized at a wavelength of 1.55 μm

Omnidirectional (omni-) reflector and transmission filter optimized at a wavelength of 1.55 μm have been realized using Si/SiO2 one-dimensional photonic crystals (1D PCs). Photonic band structures in the PCs with and without a defect layer have been also analyzed. The 1D PCs fabricated have very large omnidirectional photonic band gaps (omni-PBGs), and their measured reflectance spectra are in very good agreement with the calculated results. The omni-PBG in a six-pair Si/SiO2 1D PC (no defect layer, a filling factor η=0.406) exists in a normalized frequency range of 0.223–0.297. Introducing a defect layer generates a defect state in the PBG, and it behaves like electronic impurity levels in the gap with a change in thickness of the defect layer (dD). A six-pair Si/SiO2 1D PC with the structure parameters of η=4.060, dH (Si thickness)=95.3 nm, dL (SiO2 thickness)=234.9 nm, and dD=2.336⋅dH could act as a 1.55 μm-transmission filter for normal incidence light.

Structural and magnetic properties of Mn3O4 films grown on MgO(001) substrates by plasma-assisted MBE

Mn3O4 thin films with distorted spinel structure are grown on MgO(0 0 1) substrates by plasma-assisted molecular beam epitaxy (MBE) The films are (0 0 1) oriented and the lattice parameters are a = b approximate to 5.72 Angstrom, and c approximate to 9.5 Angstrom, with c/a = 1.66 slightly larger than the ratio of 1.64 for bulk single-crystal samples, if a body-center tetragonal unit cell is adopted for Mn3O4. It is found that the Curie temperature T-CF of the Mn3O4 film is 46 K higher than (T-CB = 42 K) for bulk single-crystal samples. The spontaneous magnetization reaches 1.73 mu(B)/molecule which is in between the reported results of 1.56 mu(B)/molecule for polycrystalline and 1.85 mu(B)/molecule for single-crystal sample. The temperature dependence of the inverse magnetic susceptibility in the paramagnetic range agrees well with the Curie-Weiss formula. The induced effective magnetic moment for Mn ion is about 3.59 mu(B), which is small compared with the 5.24 mu(B)/magnetic atom calculated based on electronic spins only

Electron-trap centers in ZnO layers grown by molecular-beam epitaxy

We have investigated electron-trap centers in ZnO layers grown under different Zn∕O flux ratios by molecular-beam epitaxy. Frequency-dependent capacitance measurements show that ZnO layers grown under Zn-rich and stoichiometric flux conditions suffer from larger dispersion than a ZnO layer grown under an O-rich flux condition. Temperature-dependent capacitance measurements reveal that all the ZnO layers have shallow electron-trap centers ET1 and deep electron-trap centers ET2, while the Zn-rich ZnO layer has another shallow electron-trap center ET3 besides ET1 and ET2: the thermal activation energies of ET1, ET2, and ET3 are estimated to be 0.033–0.046, 0.12–0.15, and 0.065 eV, respectively. Moreover, it is exhibited that the trap density of ET2 is larger than those of ET1 or ET3 in all the cases and increases as the Zn∕O flux ratio increases. Consequently, it is suggested that the large dispersion effect observed in the Zn-rich and stoichiometric ZnO layers is ascribed to the large density of deep electron-trap center ET2.We have investigated electron-trap centers in ZnO layers grown under different Zn∕O flux ratios by molecular-beam epitaxy. Frequency-dependent capacitance measurements show that ZnO layers grown under Zn-rich and stoichiometric flux conditions suffer from larger dispersion than a ZnO layer grown under an O-rich flux condition. Temperature-dependent capacitance measurements reveal that all the ZnO layers have shallow electron-trap centers ET1 and deep electron-trap centers ET2, while the Zn-rich ZnO layer has another shallow electron-trap center ET3 besides ET1 and ET2: the thermal activation energies of ET1, ET2, and ET3 are estimated to be 0.033–0.046, 0.12–0.15, and 0.065 eV, respectively. Moreover, it is exhibited that the trap density of ET2 is larger than those of ET1 or ET3 in all the cases and increases as the Zn∕O flux ratio increases. Consequently, it is suggested that the large dispersion effect observed in the Zn-rich and stoichiometric ZnO layers is ascribed to the large density of deep electr...

Nanocrystalline Zn2SiO4:Mn2+ grown in oxidized porous silicon

Zn2SiO4:Mn2+ nanocrystals were grown in an oxidized porous silicon layer using a chemical impregnation method. Apparently two classes of samples have been obtained. One is characterized by the formation of α-phase zinc silicate crystalline particles, which show green luminescence, and the other one is characterized by β-phase particles, showing yellow luminescence. It was found that in general prolonged annealing, as well as a high degree of impregnation leads to the formation of green-luminescent samples. The decay time of both yellow and green luminescence decreases with the concentration of Mn activator, while generally the decay time of yellow luminescence is considerably larger than that of green luminescence.

Characteristics of Schottky contacts to ZnO:N layers grown by molecular-beam epitaxy

We have investigated the characteristics of Au Schottky contacts to ZnO:N layers grown on (0001) GaN/Al2O3 substrates by plasma-assisted molecular-beam epitaxy. It is found that the Schottky characteristics are dependent on the growth temperature and polar direction of ZnO:N layers. The Schottky barrier height for the Au contact to a ZnO:N layer (300 °C, Zn-polar) is estimated to be 0.66 and 0.69 eV by current–voltage measurements and capacitance–voltage measurements, respectively. It is found that the Schottky barrier height is proportional to the resistivity and incorporated N concentration of ZnO:N layers. Consequently, we believe that the low growth temperature and Zn-polar direction are favored for N incorporation in the growth of ZnO:N layers, which contributes to the increased resistivity in ZnO:N layers and results in good Schottky characteristics.

Multiple-wavelength-transmission filters based on Si-SiO2 one-dimensional photonic crystals

The Si/SiO2 one-dimensional photonic crystals of heterostructural multilayers with two periods, ΛA and ΛB, have great potential for multiple-wavelength-transmission filters. These structures were prepared by inserting N pairs of ΛB (as the defect region) in the middle of two sets of two pairs of ΛA, so that the structure becomes air→[(2∙ΛA)→(N∙ΛB)→(2∙ΛA)]→substrate. N means the number of ΛB pairs in the defect region. The complex refractive indices of Si and SiO2 are assumed to be 3.7+i0 and 1.5+i0 in the transfer matrix calculation. The number of transmission channels or defect branches m is given by 2N, that is, m=2N. For large N(>10), the photonic band gap exists in a normalized frequency range ω of 0.0846–0.3838, which corresponds to the wavelength range of 0.84–6.67 μm. The defect branches are placed on a branch band between two symmetric flat bands. For a filling factor η=0.406, a matching condition of optical length in two alternating layers, the branches at the center of the branch band are divide...

Growth of luminescent Zn2SiO4:Mn2+ particles inside oxidized porous silicon: emergence of yellow luminescence

Porous silicon layers were activated by the addition cof Zn 2 SiO 4 :Mn 2+ which resulted in green or yellow luminescent samples depending on the treatment conditions. The yellow peak occurs at 575 nm and has a considerably longer decay time than the green peak, which occurs at 525 nm. It is demonstrated that the yellow luminescence collies from small particles, typically 30 nm in size, which have been crystallized in the β-phase while the green luminescence comes from the usual α-phase, which occurs with larger particles. β-Phase appears to be the dominant phase for small particles and any condition that results in the formation of small particles gives rise to yellow emission.

Intrinsic Valence Band Study of Molecular-Beam-Epitaxy-Grown GaAs and GaN by High-Resolution Hard X-ray Photoemission Spectroscopy

The electronic structures of molecular beam epitaxy (MBE)-grown GaAs and GaN have been studied by means of a technique using a newly developed surface-insensitive probe, namely, high-resolution hard X-ray (HX) synchrotron radiation (hν= 5.95 keV) photoemission spectroscopy (PES). The obtained valence band spectra and shallow core electronic states are compared with those calculated by the full-potential local density approximation (LDA) calculations explicitly including the Ga 3d core state. The experimental valence band spectra show a very good match with the calculations, simulated with linear combinations of the partial density of states. The Ga 3d core on d core states in GaN indicates a set of fine structures which are attributed to the Ga 3d-N 2s hybridization effect. The present experiments indicate that HX-PES provides an indispensable probe for investigating valence band electronic structures of materials, which has so far been impossible due to the limitations of proper surface preparation methods.

Time Dependence of the Growth Morphology of GaN Single Crystals Prepared in a Na?Ga Melt

The yields of GaN prepared in a Na–Ga melt at 700–800°C and 1–5 MPa of N2 for 200 h were measured. The morphology of the GaN crystals changed from pyramidal (yields 6–13%) to prismatic (yields 19–100%), and finally to thin platelets (yields 61–100%) with increasing temperature and N2 pressure. A time dependence of the morphology was observed for the sample prepared at 750°C and 5 MPa of N2. The morphology changed from pyramidal, prismatic to thick platelets with heating times up to 50 h. The yield of GaN increased linearly during this period. The formation rate of GaN increased after 50 h, and the crystal growth perpendicular to the c axis was enhanced. The crystal growth was completed within 200 h, and thin platelet single crystals with a size of 1–2 mm were formed. Microphotoluminescence spectra were measured at the cross section of a thin platelet GaN crystal. A large broad luminescence peak at 3.26 eV, probably associated with Mg or Si acceptors, was observed in the spectra obtained from the regions near the (0001) Ga polar plane.