Kazushige Ueda

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.

Frontier of transparent oxide semiconductors

Recent advancements of transparent oxide semiconductors (TOS) toward new frontiers of “oxide electronics” are reviewed based on our efforts, categorized as “novel functional materials”, “heteroepitaxial growth techniques”, and “device fabrications”. Topics focused in this paper are: (1) highly conductive ITO thin film with atomically flat surface, (2) p-type TOS material ZnRh2O4, (3) deep-ultraviolet (DUV) transparent conductive oxide β-Ga2O3 thin film, (4) electrochromic oxyfuolide NbO2F, (5) single-crystalline films of InGaO3(ZnO)m grown by reactive solid-phase epitaxy, (6) p-type semiconductor LaCuOS/Se epitaxial films capable of emitting UV- and purple-light, (7) p–n homojunction based on bipolar CuInO2, (8) transparent FET based on single-crystalline InGaO3(ZnO)5 films, and (9) UV-light emitting diode based on p–n heterojunction.

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 1000u200a°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.

Electrical conductivity control in transparent p-type (LaO)CuS thin films prepared by rf sputtering

Thin films of wide-gap p-type (LaO)CuS were prepared by rf sputtering followed by postannealing. Undoped (LaO)CuS films are electrically insulating and emit ultraviolet (UV) light arising from excitons at room temperature upon photoexcitation. The hole carrier concentration in the film was controlled in the range ∼1015 to ∼1020 cm−3 by doping Sr2+ ions to replace La3+. The hole concentration reached a maximum of 2.7×1020 cm−3 at a doping level of 3 at.u200a% Sr2+ ions, and then the carrier generation efficiency was ∼50%. These results demonstrate that the (La1−xSrxO)CuS films are potentially applicable to optoelectronic devices operating at short wavelengths because a transparent p electrode, as well as UV-emitting layers, may be fabricated using this material by controlling the doping level.

Intrinsic excitonic photoluminescence and band-gap engineering of wide-gap p-type oxychalcogenide epitaxial films of LnCuOCh (Ln=La, Pr, and Nd; Ch=S or Se) semiconductor alloys

The optical spectroscopic properties of layered oxychalcogenide semiconductors LnCuOCh (Ln=La, Pr, and Nd; Ch=S or Se) on epitaxial films were thoroughly investigated near the fundamental energy band edges. Free exciton emissions were observed for all the films between 300 and ∼30u2009K. In addition, a sharp emission line, which was attributed to bound excitons, appeared below ∼80u2009K. The free exciton energy showed a nonmonotonic relationship with lattice constant and was dependent on lanthanide and chalcogen ion substitutions. These results imply that the exciton was confined to the (Cu2Ch2)2− layer. Anionic and cationic substitutions tune the emission energy at 300 K from 3.21 to 2.89 eV and provide a way to engineer the electronic structure in light-emitting devices.

Preparation and crystal structure analysis of CeCuOS

Abstract A single phase of CeCuOS was prepared by sulfurization of starting materials, CeO 2 +Cu 2 S, and subsequent post-annealing in evacuated silica tubes. The crystal structure of the CeCuOS single phase was examined by the Rietveld analysis. The shrinkage of the unit-cell volume for CeCuOS was obviously observed in the series of Ln CuOS ( Ln =La–Nd) oxysulfides. The examination of interatomic distances revealed that Ce–S distances in CeCuOS are shorter than those predicted from the lanthanide contraction and this short Ce–S bond length is responsible for the unit-cell-volume shrinkage of CeCuOS. The origin of the shrinkage is discussed in the viewpoints of the Ce valence and interatomic distances along with the results of magnetic measurements.

Wide gap p-type degenerate semiconductor: Mg-doped LaCuOSe

Abstract Hole transport and optical properties were investigated on undoped and Mg-doped LaCuOS 1− x Se x ( x =0–1) epitaxial films. Both electrical conductivity and Hall mobility were found to increase monotonously with increasing Se content in the films. The increase in Hall mobility is considered to be associated with the increase in valence band dispersion. Mg ion doping increased hole concentrations in the undoped films by an order of magnitude to ∼2×10 20 cm −3 , while Mg doping reduced mobility to merely half that of undoped films. The results suggest that hole scattering due to Mg impurity ions is suppressed by natural modulation doping originating from the layered structure of LaCuOS 1− x Se x . Hole concentrations showed no temperature dependence, indicating degenerate conduction. The largest value for conductivity, 140 Sxa0cm −1 , was obtained with Mg-doped LaCuOSe epitaxial film. Accompanying characteristics included moderately high optical transparency in the visible region and blue photoluminescence.

EPITAXIAL GROWTH OF TRANSPARENT CONDUCTIVE OXIDES

Transparent conductive oxides of ITO, ZnO, β-Ga2O3, and CuGaO2 and SrCu2O2 were grown on single crystal substrates of α-Al2O3 and YSZ by pulsed-laser deposition, and their crystallinity was evaluated by using high-resolution X-ray diffraction and electron microscope. Heteroepitaxial growth was observed in spite of relatively large lattice mismatches between film and substrate. A few disordered layers were generated automatically at film/substrate boundary, played buffer layer to suppress propagation of edge dislocations.

Transparent Conductive Oxides

Transparent conducting oxides (TCO s) such as doped ZnO, In2O3 and SnO2 play important roles as transparent electrodes in commercial applications such as display and lighting devices. Although transparency and electrical conductivity are inherently conflicting, TCOs possess both properties simultaneously. To understand the fundamentals of TCOs, the essetials of the transparency and electrical conductivity are reviewed simply. Comprehension of the essentials enables us to develop novel TCOs following the principles of the materials design that carrier conduction paths should be constructed in wide-gap oxides. Because the electronic structure of oxides is considerably different between the conduction band and the valence band, procedures to form the conduction paths for electrons are in contrast to those for holes. In n-type TCOs, only isotropically spread ns0 orbitals of heavy cations such as Zn2+, In3+ and Sn4+ are necessary to form conduction paths for electrons at the bottom of the conduction band. In p-type TCOs, however, hybridization of orbitals between oxygen 2p6 orbitals and other orbitals such as Cu 3d10, Sn 5s2 and S 3p6 orbitals is essential to shape conduction paths for holes at the top of the valence band.