Transparent conducting oxide (TCO)

Transparent conducting oxide semiconductors for transparent electrodes

For most optoelectronic devices such as flat panel displays, it is essential to use a transparent electrode consisting of a thin film of a transparent conducting oxide (TCO) semiconductor. Although tin-doped indium oxide (commonly called indiumtin-oxide, or ITO) thin films deposited bymagnetron sputtering (MSP) have been in practical use for most transparent electrode applications, there are many reports on other TCO semiconductors as well as deposition methods. A stable supply of ITO may be difficult to achieve for the recently expanding market for optoelectronic devices because of the cost and scarcity of indium, the principal material of ITO. In addition, recent developments in optoelectronic devices have frequently required thin-film transparent electrodes with specialized properties. Recent research concerning thin-film transparent electrodes using TCO semiconductors has focused on resolving these problems. For example, we have proposed the use of impurity-doped zinc oxide (ZnO) as an alternative to ITO.

Semiconductor Science and Technology 20 (2005) S35–S44, ScienceDirect


SURFACE CHEMISTRY OF Ga2O3 NANORIBBONS





The structure and surface chemistry of crystalline β-Ga2O3 nanoribbons (NRs), deposited in a thin layer on various metallic and dielectric substrates (mainly on Au), have been characterized using vibrational spectroscopy. The results have been analyzed with the aid of a previous ab initio theoretical model for the β-Ga2O3 surface structure. Raman spectra and normal-incidence infrared (IR) transmission data show little if any difference from corresponding results for bulk single crystals.

For a layer formed on a metallic substrate, IR reflection-absorption spectroscopy (IRRAS) shows longitudinal-optic (LO) modes that are red-shifted by 37 cm-1 relative to those of a bulk crystal. Evidence is also seen for a bonding interaction at the Ga2O3/Au interface following heating in room air. Polarization-modulated IRRAS has been used to study the adsorption of pyridine under steady-state conditions in ambient pressures as high as 5 Torr. The characteristic V19b and V8a modes of adsorbed pyridine exhibit little or no shift from the corresponding gas-phase values. This indicates that the surface is only weakly acidic, consistent with the theoretical prediction that singly unsaturated octahedral Ga sites are the only reactive cation sites on the NR surface. However, evidence for adsorption at defect sites is seen in the form of more strongly shifted modes that saturate in intensity at low pyridine coverage.

The surface properties of Ga2O3, particularly in the most stable monoclinic (β) form, are important in several areas of technology. This material can be made to be n-type semiconducting, either by doping or through the introduction of oxygen vacancies, and sensors capable of operation at elevated temperature have been
developed for a variety of chemical species on the basis of the conductivity changes that result from adsorption. Because of its wide band gap (~4.5-4.9 eV), semiconducting β-Ga2O3 is of interest as a transparent conducting oxide to replace indium doped tin oxide (ITO) in electro-optic devices. β-Ga2O3 has superior optical transmission in the near-ultraviolet and is easier than ITO to fabricate reproducibly. The oxide surface properties are important in determining the stability and electronic characteristics of the interface with the active material, e.g., an organic light-emitting diode.

...In bulk Ga2O3, depending on the crystal form, gallium occupies either octahedral [Ga(Oh)] sites or both octahedral and tetrahedral [Ga(Td)] sites. Different inequivalent oxygen sites are also involved. The High Surface Area HSA powders can exhibit surface planes with varying degrees of coordinative unsaturation of both Ga and O sites and also with varying concentrations of defects such as O vacancies and OH sites. Thus, the distribution of reactive sites can depend on both the bulk crystallographic structure and the method of surface preparation and can differ from that expected for an ideally terminated bulk structure. By analogy with Al2O3 HSA powders, the Ga(Td) site is thought to be a stronger Lewis acid than the Ga(Oh) one. This is supported by IR data,8 and the relative concentration and coordinative unsaturation of the two types of sites are important factors in the surface chemistry. Surface acidity is traditionally studied using the adsorption of
a molecular charge donor such as pyridine.

The use of nanoribbons (NRs), which are true single crystals with well-defined surface planes, offers advantages in studies of β-Ga2O3 surface chemistry. Nanoribbons have a higher surface to-volume ratio (SVR) than a thin film, meaning that a larger fraction of the atoms is on or near the surfaces. Hence, NRs are
potentially useful as catalyst supports, an area traditionally dominated by HSA powders. The unique structural properties of NRs open the possibility of studying surface reactions on a substrate with well-defined surfaces that is, nevertheless, high in surface to volume ration SVR. The wide face of the NR is the (100) plane, which is also the preferred cleavage plane of the bulk single crystal.

The high SVR also makes NRs potentially useful for highsensitivity sensors, an application for which thin films are currently used. The use of NRs permits the monitoring of chemisorptioninduced changes in electrical conductivity and, conversely, the effects on catalytic activity of changes in the NR electron density.
This is important in sensor applications and also as a potential method for achieving electronic control of surface chemical reactions.


Previous studies of the IR transmission and Raman spectra of β-Ga2O3 nanostructures disagree as to the direction and magnitude of the shift between corresponding nanocrystalline and bulk-crystalline modes, and in some cases substantial shifts
(30 cm-1 or more) are observed. The shift may depend on the nanostructure growth direction, and various explanations have been proposed, including mode confinement, symmetry breaking, and/or strain. For GaP nanowires, surface phonon excitation has
also been shown to produce red-shifts in the Raman spectrum that depend on the dielectric constant of the surrounding medium. In any case, the shifts observed in the present TO and Raman data are much smaller than in some previous studies.

...Exposure to pyridine vapour under steady-state conditions results in IR reflection-absorption spectroscopy IRRAS data that indicate weak adsorption, consistent with a surface comprising only weakly acidic Ga(Oh) sites. A small coverage of pyridine remains after evacuation of the vapour. The IRRAS data for this species indicate a stronger interaction, possibly due to defect sites (i.e., O vacancies).

Infrared Spectroscopy and Surface Chemistry of β-Ga2O3 Nanoribbons, Langmuir 2007, 23, 12566-12576







Transparent conductive oxide thin films of CdTe-doped indium oxide prepared by pulsed-laser deposition

Transparent-conducting oxide (TCO) thin film coatings are important in a number of optoelectronics devices including photovoltaic cells and large area displays [1] and [2]. In solar cells, conducting oxide films are used as contact materials, due to the fact that some transparent oxide films such as indium oxide (In2O3) and indium tin oxide (ITO) are not only transparent to visible light but also have very high electrical conductivity. Thus they can serve as good contact materials that also allow light to pass on to the active layers of the solar cells. Several workers have investigated the properties of doped and undoped In2O3 films, both regarding their optical transparency and electrical conductivity. In2O3 has a complex cubic bixbyite structure originating from an array of unoccupied tetrahedral oxygen anion sites [3]. Stoichiometric (pure) In2O3 single crystal can exhibit ρ<10 85="" a="" above="" an="" and="" another="" application="" as="" average="" be="" been="" better="" between="" br="" by="" cadmium="" cation="" cd3teo6="" cds.="" cdte="" cell="" cm="" combination="" compound="" conducting="" conductor="" contains="" could="" coupling="" deposition="" done="" dopant="" doped="" doping="" ds="" dte2o5="" electro-optical="" fabricated="" family="" fluor="" formation="" future="" has="" have="" higher="" however="" in2o3="" in="" include="" indium="" is="" it="" ito="" low-resistivity="" low="" materials="" molybdenum="" much="" n2o3="" new="" obtains="" of="" offering="" on="" or="" others="" possible="" precursors="" prepared="" properties="" propose="" proposed="" pulsed-laser="" recently="" report="" resistivity="" same="" semiconductor="" sn:in2o3="" sn="" solar="" tco="" telluride.="" that="" the="" these="" titanium="" to="" transmittance="" transparent="" tungsten="" used="" using="" valency="" we="" when="" which="" with="" work="" x="">







PLD has emerged as one of the simplest and most versatile methods for the deposition of thin films of a wide variety of materials [16]. A major advantage of PLD is that the stoichiometry of the target can be retained in the deposited films. In particular, PLD in oxidizing gases is one of the most promising methods of preparing multicomponent oxide films and has been applied to the deposition of TCO films including various high-quality In2O3 and ITO thin films with low resistivity [16], [17] and [18]. In all the techniques used for the fabrication of ITO or In2O3 films, the electrical properties of the films were found to depend critically on the oxygen partial pressures (PO2) and the substrate temperature (Ts) [19], [20], [21], [22] and [23] Ch.Y. Wang, V. Cimalla, H. Romanus, Th. Kups, G. Ecke and Th. Stauden et al., Phase selective growth and properties of rhombohedral and cubic indium oxide, Appl Phys Lett 89 (2006), p. 11904.[23]. In this work, we report on the electrical, optical and structural properties of In2O3 films n-type prepared as a function of PO2 by PLD applying doping of an appropriate impurity of CdTe and tuning the deposition conditions.

....In general, conduction electrons in the n-type oxide semiconductors are generated from interstitial or substituted cation and/or vacancies [36]. At the oxygen pressure of 4 Pa, energetic atoms and ions of various species combine to produce the film with minimum ρ. This can be explained as follow: first the Te+4, Te+6, and Cd+2 ions fill In+3 vacancies in In2O3 crystallites substitutionally at 4 Pa. After the In+3 vacancies are completely compensated by Te+4, Te+6, and Cd+2 ions, then O−2 ions can be doped interstitially into the films when the PO2 increase between 6.7 and 29.3 Pa. A direct experimental confirmation of the existence of oxygen interstitials in ITO has been reported [37]. The interstitial O−2 ions disturb the crystal periodicity, and as a result the electron transport is limited by the impurity scattering and the lattice scattering increasing the ρ of the In2O3 films

Science Direct; Optic & Laser Technology; (will be published) Volume 40, Issue 7, October 2008, Pages 895-900




"Front Drive" Display Structure for Color Electronic Paper Using Fully Transparent Amorphous Oxide TFT Array


Recently, transparent oxide semiconductors have been attracted tremendous attention as a novel candidate for thin film transistor (TFT) materials. Various kinds of oxide semiconductors have been reported so far, such as ZnO, MgxZn1−xO, Zn-Sn-O, Zn-In-O, SnO2, Ga2O3, In-Ga-O, In2O3, TiO2, single crystalline InGaO3(ZnO)5 superlattice and amorphous In-Ga-Zn-O (a-InGaZnO). Among these recently explored oxide semiconductors, a-InGaZnO is superior in its high mobility (∼10 cm2/Vs), high on/off current ratios (∼106), low process temperature and long-term chemical stability.

We demonstrate a novel display structure for color electronic paper for the first time. Fully transparent amorphous oxide TFT array is directly deposited onto color filter array and combined with E Ink Imaging Film. Taking advantage of the transparent property of the oxide TFT, the color filter and TFT array are positioned at the viewing side of the display. This novel "Front Drive" display structure facilitates the alignment of the color filter and TFT dramatically.

IEICE TRANS. ELECTRON., VOL.E90–C, NO.11 NOVEMBER 2007; Oxfordjournals.org