Masahiro Orita

Current injection emission from a transparent p–n junction composed of p-SrCu2O2/n-ZnO

An ultraviolet light-emitting diode (LED) operating at room temperature was realized using a p–n heterojunction composed of transparent conductive oxides, p-SrCu2O2 and n-ZnO. Multilayered films prepared by a pulsed-laser deposition technique were processed by conventional photolithography with the aid of reactive ion etching to fabricate the LED device. A rather sharp emission band centered at 382 nm was generated when a forward bias voltage exceeding the turn-on voltage of 3 V was applied to the junction. The emission may be attributed to a transition associated with the electron–hole plasma of ZnO.

Deep-ultraviolet transparent conductive β-Ga2O3 thin films

Thin films of β-Ga2O3 with an energy band gap of 4.9 eV were prepared on silica glass substrates by a pulsed-laser deposition method. N-type conductivity up to ∼1 S cm−1 was obtained by Sn-ion doping and deposition under low O2 partial pressure (∼10−5 Pa) at substrate temperatures above 800 °C. The resulting internal transmittance at the wavelength (248 nm) of the KrF excimer laser exceeded 50% for the 100-nm-thick film, making this the most ultraviolet-transparent conductive oxide thin film to date and opening up prospects for applications such as ultraviolet transparent antistatic electric films in ultraviolet lithography.

Epitaxial growth of transparent p-type conducting CuGaO2 thin films on sapphire (001) substrates by pulsed laser deposition

Transparent p-type conducting CuGaO2 thin films were prepared on α-Al2O3 (001) single-crystal substrates by pulsed laser deposition. The films were grown epitaxially on the substrates in an as-deposited state. X-ray pole figure analysis revealed that the films were composed of two types of epitaxial grains, both with c axes oriented perpendicular to the surface and a axes rotated 60° with respect to each other around the c axis. Observation of the CuGaO2 thin films by atomic force microscopy and high-resolution transmission electron microscopy substantiated this conclusion. The films have high optical transparency (∼80%) in the visible region, and the energy gap of CuGaO2 for direct allowed transition was estimated to be 3.6 eV. p-type conductivity was confirmed by Seebeck and Hall measurements. The electrical conductivity, carrier (positive hole) density, and Hall mobility of the films at room temperature were 6.3×10−2 S cm−1, 1.7×1018 cm−3, and 0.23 cm2 V−1 s−1, respectively.

Highly electrically conductive indium–tin–oxide thin films epitaxially grown on yttria-stabilized zirconia (100) by pulsed-laser deposition

Highly electrically conductive indium–tin–oxide thin films were epitaxially grown on an extremely flat (100) surface of yttria-stabilized zirconia single-crystal substrates at a substrate temperature of 600 °C by a pulsed-laser deposition technique. A resistivity down to 7.7×10−5 Ω cm was reproducibly obtained, maintaining optical transmission exceeding 85% at wavelengths from 340 to 780 nm. The carrier densities of the films were enhanced up to 1.9×1021 cm−3, while the Hall mobility showed a slight, almost linear, decrease from 55 to 40 cm2 V−1 s−1 with increasing SnO2 concentration. The low resistivity is most likely the result of the good crystal quality of the films.

Preparation of highly conductive, deep ultraviolet transparent β-Ga2O3 thin film at low deposition temperatures

Abstract β-Ga 2 O 3 is a unique oxide that exhibits deep ultraviolet transparency as well as good electric conductivity when dopants are introduced. With the purpose of realizing highly conductive films using this material, tin doped Ga 2 O 3 films were deposited on Al 2 O 3 (0001) substrates by the pulsed laser deposition method. (201) oriented β-phase films were grown at substrate temperatures of between 380 °C and 435 °C, and deep ultraviolet transparent films with a conductivity as high as 8.2 S cm −1 were obtained at 380 °C under laser ablation with a low repetition rate of 1 Hz. With a further increase in the deposition temperature, the films underwent a crystalline phase transition, accompanied by an abrupt decrease in conductivity.

FABRICATION AND CHARACTERIZATION OF ULTRAVIOLET-EMITTING DIODES COMPOSED OF TRANSPARENT P-N HETEROJUNCTION, P-SRCU2O2 AND N-ZNO

An ultraviolet light-emitting diode (UV-LED) was realized using a p-n heterojunction composed of the transparent oxide semiconductors p-SrCu2O2 and n-ZnO. A Ni/SrCu2O2/ZnO/ITO multilayered film was epitaxially grown on an extremely flat YSZ (111) surface by a pulsed-laser deposition technique. SrCu2O2 (112) was preferentially grown on ZnO (0001) at 350°C, while the preferential plane was changed into the (100) when the temperature was increased to 600 °C. The grown films were processed by conventional photolithography followed by reactive ion etching to fabricate heterojunction diodes. The resulting devices exhibited rectifying I-V characteristics inherent to p-n junctions. A relatively sharp electroluminescence band centered at 382 nm, attributed to transitions associated with exciton-exciton collision or electron-hole plasma in ZnO, was generated by applying a forward bias voltage greater than the turn-on voltage of 3 V. UV-LED performance characteristics such as threshold current and conversion efficie...

Frontier of transparent conductive oxide thin films

The recent advancement of transparent conductive oxide thin films is reviewed using our strategy. Topics focused are the lowest resistive indium tin oxide films, deep-ultraviolet (deep-UV) transparent conductive oxides (TCO) thin film, p–n homo-junction based on bipolar CuInO2, and UV-light emitting diode based on p–n hetero-junction. A frontier of transparent oxide semiconductors is opening as a consequence of discovery of p-type TCO.

Heteroepitaxial growth of a wide-gap p-type semiconductor, LaCuOS

High-quality epitaxial films of LaCuOS, a wide-gap p-type semiconductor, are grown on yittria-stabilized-zirconia (001) or MgO (001) single-crystal substrates by a unique method. Postannealing of a bilayer film composed of an extremely thin metallic copper layer and an amorphous LaCuOS layer at 1000 °C results in an epitaxially grown LaCuOS thin film. This thin copper layer with high orientation, which likely acts as an epitaxial initiator, is essential for epitaxial growth. The resulting epitaxial films exhibit relatively intense ultraviolet emission associated with excitons at room temperature, confirming the high crystal quality of the films.

Surface morphology and crystal quality of low resistive indium tin oxide grown on yittria-stabilized zirconia

Highly electrically conductive (resistivity: 1×10−4 Ω cm) indium tin oxide (ITO) thin films were grown heteroepitaxially on (111) and (100) surfaces of yttria-stabilized zirconia (YSZ) at a growth temperature of 900 °C using a pulsed-laser deposition technique, and their crystal quality and surface morphology were evaluated by using high-resolution x-ray diffraction and an atomic force microscope. Higher growth temperature, the use of a cleaved surface as a substrate, and an increase in oxygen pressure are essential to fabricate an ITO thin film with an atomically flat surface. The full width at half maximum of out-of-plane x-ray rocking curves of ITO (222) grown on YSZ (111) and of ITO (400) on YSZ (100) were 54 and 720 arcseconds, respectively, which indicates that (111)-oriented ITO films exhibited higher crystal quality than (100)-oriented ITO films. The ITO film was grown on YSZ (111) by the three-dimensional spiral growth mode, with flat terraces and steps corresponding to (222) plane spacing of 0.2...

Decomposition of water by a CaTiO3 photocatalyst under UV light irradiation

Platinized CaTiO{sub 3} powder (band gap 3.5 eV) had a high photocatalytic activity of decomposing water into H{sub 2} and O{sub 2} under the irradiation of UV light, especially with photon energies above 3.8 eV. The existence of this photocatalytic activity is further supported on electrochemical grounds in that the photoinduced current spectrum measured between a CaTiO{sub 3} single crystal and a Pt electrode without applied voltage in water showed a maximum near 4.1 eV. This result indicates that the direct measurement of the spectrum corresponding to the efficiency of water decomposition is an effective method to survey photocatalytic activities of materials.