Goro Shimaoka

Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation

The dependence of the structural, photoluminescent and cathodoluminescent properties of ZnO thin films deposited by electron beam evaporation on the preparation conditions has been investigated. Both as-deposited and annealed thin films deposited at substrate temperatures higher than 200°C showed c-axis orientation, and their crystallinity was improved with increasing annealing temperature. The films showed the emission with a peak at around 510 nm in photoluminescence (PL) and cathodoluminescence (CL) except for the film annealed at 800°C in air. The emission seems to be well-known blue-green emission due to ZnO:Zn phosphor. The strong green emission with a peak at around 540 nm was obtained from the film annealed at 800°C in air. The origin of the emission is not understood. The film showed CL luminance of about 60 cd/m2 under excitation of 2 kV, 400 μA/cm2. Moreover, it showed CL under excitation even at 250 V without charging-up.

Low voltage cathodoluminescent properties of phosphors coated with In2O3 by sol-gel method

Abstract The phosphors used in usual CRTs are difficult to be used in a lower voltage than 500 V, because of charging-up on their surfaces. ZnS:Ag, Cl phosphor was coated with In2O3 using indium-iso-propoxide by the sol-gel method. It was found that a very thin In2O3 conductive layer could be coated uniformly on the phosphor surface. The low voltage cathodoluminescent properties were studied and compared with those without In2O3. The luminance of the phosphor excited with an electron beam of lower voltage than 500 V was considerably improved. Moreover, it was found that the formation of the thin layer is also quite effective for the improvement of the aging characteristics of cathodoluminescence.

Properties of ZnTe-ZnSe and -ZnS superlattices prepared by hot wall epitaxy

Abstract ZnTe-ZnSe and -ZnS superlattices were prepared on GaAs(100) substrates by hot wall epitaxy. Their high-energy electron and θ-2θ X-ray diffraction and photoluminescence at 77 and 300 K were measured. Though the lattice mismatches between the materials are 7% (ZnTe-ZnSe) and 13% (ZnTe-ZnS), strong photoluminescence associated with band edges was observed, i.e. a superlattice consisting of a thin ZnTe layer and a thick ZnSe layer showed strong luminescence associated with band edges and weak luminescence associated with deep level defects. The ZnTe (20 A)-ZnS (50 A) superlattice showed strong but rather broad luminescence near 2.6 eV. Variations of the luminescence photon energy of the ZnTe-ZnSe superlattice with the thickness of the ZnTe or the ZnSe layer can be explained qualitatively by strain effects due to the lattice mismatch on the band gap of ZnTe and on the band offsets of the superlattice. The experimental results show the superlattices are of type II, where the conduction and valence band edges of ZnTe are above the corresponding edges of ZnSe or ZnS.

Preparation and luminescent properties of SrSe:Ce thin films

Abstract SrSe thin films doped with Ce which are expected to be effective for blue-emitting thin film electroluminescent (TFEL) devices with good chromaticity have been prepared by a multi-source deposition technique, and their luminescent properties have been investigated. The structural properties and quality of the films are considerably influenced by the ratio of Sr atoms to Se arriving at the substrate. The properties are also sensitive to the substrate temperature. When the ratio is kept at 1 : 3, the SrSe films prepared at substrate temperature of 400°C and at temperatures higher than 500°C show [100] and rather random orientations, respectively. On the other hand, the crystallinity is improved with increasing substrate temperature. It is found that the luminescence of SrSe:Ce films changes from blue to green when increasing the contents of Ce, therefore, the control of the contents of Ce is important in order to develop the blue-emitting TFEL devices.

Crystal growth of ZnO by chemical transport

Abstract Crystal growth of ZnO has been carried out by chemical transport in a closed tube using ammonium halides (NH 4 X), Zn, ZnCl 2 , etc. as transport agents. Prismatic crystals of ZnO up to 1.5 mm in diameter and 8 mm in length were grown when NH 4 Cl was used. The transport rate depended nearly proportionally on the undercooling, but not markedly on the amount of NH 4 Cl added. When Zn and ZnCl 2 were used, prismatic crystals were grown as well. The crystals grown were colorless or light-brown colored, and showed blue-green or yellow-green photoluminescence under UV excitation. When NH 4 Br or NH 4 I was used, however, only few crystals were obtained.

Coloration of SnSbO thin films

Abstract Coloration of spray-pyrolysis-deposited thin films of Sn Sb O as a function of Sb content as well as changes in resistivity and structure have been investigated. It is well known that SnO 2 films doped with a few % of Sb exhibit low resistivity and high optical transmittance. However, the resistivity of these films increases with increasing Sb content. Moreover, the films become darker with an additional bluish coloration with increasing Sb content up to [Sb][Sn]≈0.3, but this coloration is noticed to disappear at [Sb][Sn]> 1. To understand this phenomenon, the atomic ratio of the films and the oxidation states of the constituent elements were analyzed by XPS measurements. In the case of low-resistive and high-transmittance samples, the Sb 3+ component, as compared to the Sb 3+ component, is dominant in Sb 3d spectra. The Sb 3+ component increases with increasing Sb content. It was found from X-ray diffraction measurements that the films consisted of SnO 2 with partial [100] orientation at [Sb][Sn]⩽ 1, and were nearly amorphous at [Sb][Sn]⩾4. The degree of [100] orientation decreased with increasing [Sb][Sn]. It is concluded from these results that the darkening is due to optical scattering at the boundaries between SnO 2 and Sb 2 O x (x=3or5) micro-grains, and that the broad absorption is based on a charge transfer between Sb 3+ and Sb 5+ .

Crystal growth of ZnO by vapor transport in a closed tube using Zn and ZnCl2 as transport agents

Abstract Zinc oxide single crystals were grown by vapor transport in a closed tube. When only Zn or ZnCl 2 was used as a transport agent, polycrystalline ZnO or small needle-like ZnO crystals were obtained. However, using both Zn and ZnCl 2 as transport agents, prismatic and almost colorless single crystals of ZnO were obtained (e.g. 1.1 mm in diameter and 4.5 mm in length after a 100 h transport). Thermodynamic considerations of the system and evaluation of the transport rate suggested that a small amount of H 2 O adsorbed on the source material contributed to the transport reaction. Under UV excitation bright green luminescence was observed, where it was implied that for the ZnO crystals the deviation from stoichiometry was strongly related to the luminescence.

Crystal growth of ZnS and ZnSe by chemical transport using NH4Cl as a transport agent

Abstract Crystal growth of ZnS and ZnSe has been carried out by chemical transport using NH4Cl as a transport agent. Single crystals of both compounds with cubic zinc-blende structure have been obtained. The transport rate depended proportionally on the undercooling, but not markedly on the amount of NH4C1 added. The electrical resistivity of the as-grown crystals was of the order of 108–109 ohm cm. Crystal growth of solid solutions, Zn(SxSe1−x), has also been carried out using mixed powders of ZnS and ZnSe as source materials. Cubic zinc-blende-type single crystals have been obtained over the entire composition range. The compositions of these crystals were in good agreement with those of the mixing ratio of the source powder. These crystals showed self-activated luminescence under ultraviolet excitation, and the color of the luminescence varied from blue (ZnS) to red (ZnSe), depending on the composition.

Properties of CdS-ZnS superlattices prepared by hot wall epitaxy

Abstract CdS-ZnS superlattices (SLs) have for the first time been prepared on GaAs (100) substrates by hot wall epitaxy, using a flip-flop (growth-interruption) process. Single crystalline (cubic) patterns were observed in reflected high energy electron diffraction measurements for SLs consisting of thin layers of less than 30 A. Their SL structures were ascertained by the observation of satellites in the X-ray diffraction spectra. Electronic properties of the SLs have been studied through photoluminescence (PL) measurements in the temperature range of 10–300 K. Very strong PLs associated with the band gap of the SLs have been observed in the photon energy region higher than 2.5 eV (the energy gap of CdS, green), even though the lattice mismatch between CdS and ZnS is 7.5%. The peak photon energy was analyzed using the Kronig-Penney model. The results indicate that the SL is type I: Both electrons and holes are confined in the CdS layer which has the smaller energy gap.

Some properties of Al-doped ZnO transparent conducting films prepared by RF reactive sputtering

Abstract Aluminum-doped zinc oxide films were studied as a transparent electrode material. The films were prepared by RF reactive sputtering using a composite target consisting of a Zn disc and Al wires. X-ray diffraction studies showed that in order to prepare Al 2 O 3 crystallite-free ZnO:Al film, films had to be deposited at an oxygen pressure p 0 which is very low but sufficient to give rise to transparency, or should be deposited at substrate temperatures higher than 400 °C, or should be annealed at high temperature in vacuum after deposition. By sputtering a target with ≌ 7% Al at p 0 of 0.25%, we could deposit a transparent (about 90%) ZnO: Al film with a resistivity of ≌ 10 -3 Ω cm on a glass substrate heated to 100°C. If the deposition is followed by in-situ annealing at 400°C in vacuum for 1 h, the resistivity of the film could be reduced to ≌ 10 -4 Ω cm.