Haruki Ryoken

Structural and magnetic properties of Mn-ion implanted ZnO films

Zinc oxide films doped with Mn (Mn:ZnO) were prepared by implanting Mn+ ions into ZnO films deposited by pulsed laser deposition, and their structure and magnetic properties were studied. The Raman spectra of the films indicated that Mn ions occupied the Zn site of ZnO after annealing, while the as-implanted films were amorphous like the ones with very low crystallinity. Magnetic measurements revealed that neither as-implanted nor annealed Mn:ZnO films showed ferromagnetism. The Mn:ZnO films demonstrated paramagnetism that was likely due to Mn2+ ions at the substitutional Zn site.

Non-equilibrium defects in aluminum-doped zinc oxide thin films grown with a pulsed laser deposition method

Zinc oxide (ZnO) films doped with aluminum (Al) were deposited with a pulsed laser deposition technique to characterize the charge compensation phenomena in ZnO. In particular, oxygen radical (O*) irradiation during film deposition was used to modify the oxygen stoichiometry. Irradiation with O* decreased electron concentration in Al-doped ZnO. The lattice parameter of the resultant films also varied with the growth conditions. However, no obvious correlation between electron concentration and lattice parameter was found. The self-diffusion coefficients indicated the presence of non-equilibrium defects. The properties of the films are discussed from the viewpoint of non-equilibrium compensated defects detected in the diffusion measurements.

Study of Optical Property in ZnO Thin Film Implanted with Eu by Combinatorial Ion Implantation Techniques

Combinatorial ion implantation techniques with mask and digital scanning systems were applied to optimize conditions of the optimal Eu dose for the orange luminescence originating from Eu phosphor in ZnO thin films. Eu ions accelerated to 150 keV were implanted continuously in the range of 1×1016 to 2×1017 ions/cm2. The sample was annealed at 600°C for 10 min in air, in order to observe the orange luminescence at 618 nm. Optimal Eu dose was determined to be about 1×1017 ions/cm2. The luminescence intensity related to Eu3+ was decreased by the annealing at 800°C. The intensity of orange luminescence is related to the degree of recovery from radiation damage.

Hard x-ray photoemission spectroscopy in wurtzite-type zinc magnesium oxide solid-solution films grown by pulsed-laser deposition

The electronic structure of Zn1−xMgxO alloy thin films was determined by hard x-ray photoemission spectroscopy (HX-PES) and optical transmittance measurements. HX-PES measurements revealed that the binding energies of valence band, Zn 2p, and O 1s levels increased with increasing MgO fraction. The energy shift correlated with a widening of the band gap and change in ionicity of chemical bonds by alloying. The Fermi level in the alloy compounds was found to be close to the bottom of conduction band regardless of the MgO fraction.

Formation of compensated defects in zinc magnesium oxides assignable from diffusion coefficients and hard x-ray photoemission

We studied metastable (Zn1−xMgx)O alloy films having a high MgO fraction, e.g., x=0.47, in excess of its solubility limit (x≈0.15). Residual electron concentrations in the metastable films were close to those of the stable films having a smaller MgO fraction (x=0.07). In contrast to the electric conductivity, diffusivities of both cation and anion in the metastable films were surprisingly higher than those in the stable films, indicating that a high concentration of compensated defects were generated in the metastable alloy with a high MgO fraction. Photoemission spectroscopy confirmed the presence of ionized acceptors for charge compensation in the metastable (Zn1−xMgx)O.

Interfacial Structure of GaN and InN Thin Films Grown on ZnO Substrates

Gallium nitride (GaN) and indium nitride (InN) films were grown on a zinc oxide (ZnO) single crystalline substrate with a (0001) orientation using molecular beam epitaxy. The interfacial structure and relaxation mechanism of the lattice mismatch at the nitride/oxide interface were investigated, particularly the effects of an (In,Ga)N alloy buffer layer on the interfacial structure of the GaN films. This layer significantly improved the crystallinity of the GaN films by gradually relaxing the lattice mismatch between the GaN and ZnO. In spite of the large lattice mismatch between InN and ZnO, InN films with high crystallinity were grown without an (In,Ga)N buffer layer. Structural analysis revealed that an InN layer with low crystallinity formed spontaneously during the initial growth stage, and this amorphous-like layer likely contributed to relaxation of the interfacial stress caused by the lattice mismatch.

Optimization of Annealing Time and Cu Concentration for Study of Luminescence Properties of Cu-Implanted ZnO Thin Films

New ion implantation techniques were applied to optimize conditions such as annealing time, Cu dose, and Cu concentration for the green emission originating from Cu phosphor in ZnO thin films. Copper ions accelerated to 150 keV were implanted at room temperature. In the ZnO thin films implanted with Cu, the suitable annealing time was 90 min at 800°C. The optimal Cu dose was determined to be 4.5 ×1014 ions/cm2. The ZnO thin films optimized for effective green emission had a Cu concentration of 9 ×1018 ions/cm3.

Virtual structure in luminescence profile of zinc oxide films causing discrepancy in peak identification

The luminescence properties of zinc oxide (ZnO) films prepared by pulsed laser deposition were investigated by photoluminescence spectroscopy. Films of different thicknesses were deposited on silicon, glass, and sapphire substrates. The visible luminescence spectrum is dependent on film thickness: both the number of peaks and their peak positions varied systematically as a function of film thickness. However, the variations in the luminescence spectra were caused by optical interference in the film/substrate systems. Any effort to determine the mechanisms responsible for the visible luminescence must properly account for these interference effects. Luminescence spectra free of distortion can be obtained from ZnO films thinner than 70nm or much thicker than 1.5μm.

Basic Examination for Nodulation-Doped (Zn,Mg,Al)O/ZnO

(Zn,Mg,Al)O films, expected to work as a carrier injection laye r of a modulation-doped multi-layered structure of [(Zn,Mg,Al)O/ZnO] n, have been deposited on (11 20) sapphire substrate at a temperature of 700 C in 1-5×10 torr of O2 by pulsed laser deposition technique using fourth harmonic generation of Nd:YAG laser, and the physical properties of t he films were investigated. The band gap energy of the films lineally increased with increas ing Mg content, and the carrier concentrations increased by doping with Al regardless of Mg content. T hus, (Zn,Mg,Al)O is a possible material for carrier injection layer of modulation-doped ZnO films.

Defect Structure in (Zn,Mg)O Films Prepared on YSZ Substrate

The defect structure of undoped ZnO and (Zn1-x,Mgx)O solid-solution films were deposited on YSZ substrate with pulsed laser deposition (PLD) to investigate defect equilibria in those films. In particular, the effects of thermal treatment on the structures and prosperities of (Zn1-x,Mgx)O solid-solution films were examined. The films with high MgO concentration (x>0.12) decomposed to the wurtzite-type and rock-salt-type phase after thermal treatment, indicating that the solubility limit of Mg was about x=0.12 and the wurtzite-type (Zn,Mg)O films with x>0.12 were indicated to be non-equilibrium ones. The subsequent analyses of oxygen diffusivity in those films revealed that the films under non-equilibrium state, i.e., wurtzite-type (Zn1-x,Mgx)O with x>0.12, contained significantly high concentration of anion defects.