Takeshi Ohgaki

Growth condition dependence of morphology and electric properties of ZnO films on sapphire substrates prepared by molecular beam epitaxy

Zinc oxide (ZnO) films were grown on sapphire (1120) substrates by molecular beam epitaxy under oxygen radical irradiation. The effect of the growth conditions, including the Zn/O ratio supplied to the film surface, on the electrical properties of ZnO films was studied in relation to the film morphology. We found that the growth rate strongly depended on the Zn flux from the Knudsen cell and the optimum condition for high growth rate was very narrow. The grain size in the lateral direction increased with increasing growth rate in the thickness direction. The increase in growth rate, especially in the lateral direction, resulted in the carrier mobility increasing up to 42 cm2 V−1 s−1. The carrier concentration N was sensitive to the substrate temperature, while the value of N was not sensitive to the source supplying ratio Zn/O. We discuss the decrease of the carrier concentration with increasing substrate temperature in regard to the formation of nonequilibrium defects.

Isothermal capacitance transient spectroscopy for deep levels in co- and Mn-doped ZnO single crystals

Deep donor levels in ZnO single crystals doped with transition metal (TM; Co or Mn) were characterized by isothermal capacitance transient spectroscopy (ICTS) applied to ZnO-based Schottky junctions, Au/ZnO (0001) or Ag/ZnO (0001). The barrier height at the junction and donor concentration was not influenced by TM. A deep donor level at 0.28 eV was detected by ICTS; however, its energy dispersion and concentration was composition independent. The effect of doping with TM was found in the magnitude of leakage current; in other words, the leakage current at the Au/ZnO:Mn junction was lower than the other junctions on undoped or Co-doped crystals.

Electric Properties of Zinc Oxide Epitaxial Films Grown by Ion-Beam Sputtering with Oxygen-Radical Irradiation

Undoped and aluminum-doped ZnO epitaxial films were grown on (001) sapphire substrates by an ion-beam sputtering method with or without the irradiation of oxygen radicals. The effect of oxygen-radical irradiation was notable in the undoped ZnO films when the growth temperature was relatively low. The irradiation improved the crystallinity and decreased the oxygen-vacancy concentration, while it induced internal stress into the films. The carrier concentration of the undoped ZnO films was decreased by the oxygen-radical irradiation, which was attributable to a decrease in the oxygen-vacancy concentration. The Hall mobility of the undoped ZnO films was as low as 1–3 cm2 V-1 s-1. The low mobility was explained by carrier scattering due to the potential barriers at the grain boundaries. The height of the potential barriers at the grain boundaries decreased with increasing carrier concentration. This behavior was well explained by a simple model assuming a single defect state at grain boundaries.

Band-edge emission of undoped and doped ZnO single crystals at room temperature

Band-edge emission of ZnO at around room temperature was investigated by measuring the temperature dependence of cathodoluminescence spectra at 20–300 K. Undoped crystals grown by a vapor transport method and Al-doped crystals by flux method were employed to elucidate the effect of doping on luminescence properties. For the Al-doped crystals, the free-exciton emission was weak through out the temperature range T<300 K. The most intense emission peak of the Al-doped crystal was energetically close to bound exciton annihilation emission. On the other hand, for undoped crystals, it was found that the most intense emission peak at room temperature was at E≈Eg−60 meV and this peak was not assignable to free-exciton annihilation emission. It was also found that this peak is not a reason for the reduction in emission efficiency.

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.

SYNTHESIS OF ZNO BICRYSTALS DOPED WITH CO OR MN AND THEIR ELECTRICAL PROPERTIES

Bicrystals of ZnO without intergranular additives were synthesized to elucidate the effect of Co and Mn doping on the varistor properties of ZnO ceramics. Nonlinear current-voltage (I–V) characteristics due to the formation of an interfacial barrier were observed in bicrystals doped with Co or Mn, while the nondoped bicrystals exhibited linear I–V relationship. Since the charge transport properties of bulk ZnO crystals were not changed, it is indicated that the energy dispersion and/or density of the interfacial states were changed by the Co and Mn doping.

Synthesis of zinc oxide varistors with a breakdown voltage of three volts using an intergranular glass phase in the bismuth–boron–oxide system

Zinc oxide (ZnO) crystals joined by using an intergranular glass phase were investigated in order to develop surge filters for low-voltage applications and to clarify the current transport mechanism through the junction. The junctions having a ZnO/glass/ZnO sandwich structure were synthesized by using an interfacial glass phase of the Bi–B–O system. A highly nonlinear current–voltage curve was obtained for the ZnO/glass/ZnO junction with Co-doped ZnO single crystals, and the current through the junction at the breakdown voltage was proportional to the 30th power of the bias voltage. Dielectric measurements revealed that a double-sided depletion layer was formed at the interface, and the observed high nonlinearity was ascribed to the corresponding potential barrier formed at the interface.

Optical Properties of Heavily Aluminum-Doped Zinc Oxide Thin Films Prepared by Molecular Beam Epitaxy

Aluminum (Al) doped ZnO (AZO) films were grown on sapphire substrate via oxygen radical assisted molecular beam epitaxy (MBE) technique. The results of XRD measurement and temperature dependence Hall measurement confirmed that the AZO films were typically highly degenerate semiconductor with good crystallinity. The optical properties of these films were investigated by photoluminescence (PL) spectra at RT and absorption spectra at 5 K. Strong band-edge emission was observed in the AZO films in spite of high carrier concentration more than 10 cm. A sift of absorption edge to higher energy side and a gradual increase of the absorption was observed for the AZO film.

Role of Crystalline Polarity in Interfacial Properties of Zinc Oxide Varistors

The role of the crystalline polarity of zinc oxide (ZnO) in the interfacial properties was investigated with semiconductor–insulator–semiconductor structures prepared from ZnO single crystals and an oxide glass insulator. The insulator/ZnO(0001) interface showed highly nonlinear current–voltage (I–V) relationships, characteristic of ZnO varistors, with clear breakdown behavior at a bias voltage of 3 V. Whereas, the insulator/ZnO(0001) interface was characterized by a relatively large leakage current in the pre-breakdown region and very slow decay behavior in the I–V relationships. The dielectric properties of the insulator/ZnO interface also exhibited crystalline polarity dependence.

Eu-Doped CaAl2Si2O8 Nanocrystalline Phosphors Crystallized from the CaO – Al2O3 – SiO2 Glass System

A new process to synthesis Eu-doped CaAl 2 Si 2 O 8 (CaAl 2 Si 2 O 8 :Eu) nanocrystals coated with SiO 2 is proposed. In this process, CaAl 2 Si 2 O 8 :Eu nanocrystals were precipitated in a SiO 2 -rich matrix through phase separation and crystallization. Eu-doped CaO-Al 2 O 3 -SiO 2 system glasses, with a composition of (x - 1)SiO 2 -(100 - x)CaAl 2 O 4 -1Si mol % in the immiscibility region, were prepared by a conventional melt-quenching process. They were heat-treated for phase separation and crystallization to precipitate the CaAl 2 Si 2 O 8 :Eu crystals into the phase separated glass. We succeeded in obtaining CaAl 2 Si 2 O 8 :Eu crystals in a SiO 2 -rich matrix, composed of glass and α-cristobalite phase. These glasses and glass ceramics show blue luminous properties due to the 4f 7 -4f 6 5d 1 transition of the Eu 2+ ions. The blue-emission intensity of the glass ceramics containing CaAl 2 Si 2 O 8 :Eu significantly depended on the heat-treatment time for phase separation and crystallization.