N-type Oxide Semiconductor Research Articles

Photochronocoulometric Measurement of the Coverage of Surface-Bound Dyes on Titanium Dioxide Crystal Surfaces

Atomically flat terraced single-crystal anatase and rutile surfaces can be prepared allowing for the reproducible adsorption of covalently attached sensitizing dyes. Once reproducible surfaces and dye coverages are achieved, a photochronocoulometric technique is developed to measure the surface coverage of the dyes, an important parameter in determining the efficiency of sensitization. The surface-bound dyes are irreversibly oxidized by exposure to a light pulse with the n-type oxide semiconductor electrode held in depletion. A double-exponential decay of the subsequent photocurrent is then measured, where the integration of the faster decay is associated with the adsorbed dye coverage and the second much slower decay is attributed to trace regenerators, including water, in the nonaqueous electrolyte. The ruthenium-based N3 dye shows the expected linear dependence of the rate constant on light intensity whereas a dicarboxylated thiacyanine dye shows a square root dependence of its photooxidation rate on light intensity. The sublinear response of the thiacyanine dye is discussed in terms of the more complex surface chemistry that is known for this family of sensitizing dyes.

Growth of n-type ZnO thin films by using mixture gas of hydrogen and argon

High-quality oxide semiconductor ZnO thin films were prepared on single-crystal sapphire and LaAlO3 substrates by pulsed laser deposition (PLD) in the mixture gas of hydrogen and argon. Low resistivity n-type ZnO thin films with smoother surface were achieved by deposition at 600°C in 1Pa of the mixture gas. In addition, ferromagnetism was observed in Co-doped ZnO thin films and rectification I−V curves were found in p-GaN/n-ZnO and p-CdTe/n-ZnO heterostructure junctions. The results indicated that using mixture gas of hydrogen and argon in PLD technique was a flexible method for depositing high-quality n-type oxide semiconductor films, especially for the multilayer thin film devices.

Large thermoelectric performance of heavily Nb-doped SrTiO3 epitaxial film at high temperature

Carrier concentration dependence of the thermoelectric figure of merit, ZT of SrTiO3 at high-temperature (1000 K) is clarified using heavily Nb-doped SrTiO3 epitaxial films, which were grown on insulating (100)-oriented LaAlO3 single-crystalline substrates by a pulsed-laser deposition method. Carrier concentration, Hall mobility, Seebeck coefficient, and thermal conductivity of Nb-doped SrTiO3 epitaxial films were experimentally evaluated at 1000 K with an aid of theoretical analysis. ZT of Nb-doped SrTiO3 increases with Nb concentration and it reaches ∼0.37 (20% Nb doped), which is the largest value among n-type oxide semiconductors ever reported.

Effect of corner sharing of TiO6 octahedra for carrier transport mechanism of Ti-containing n-type oxide semiconductor

Effect of corner sharing of TiO6 octahedra for carrier transport mechanism of Ti-containing n-type oxide semiconductor

All oxide transparent MISFET using high- k dielectrics gates

Metal–insulator-semiconductor field-effect transistors (MISFETs) were fabricated using a n-type transparent oxide semiconductor, InGaO 3(ZnO) 5, single-crystalline layer as n-channel. High- k dielectric materials, amorphous Al 2O x and HfO x , were examined for gate insulators. The deposition and post-annealing conditions of the gate insulators were optimized for leakage current and MISFET performances. The MISFETs exhibited good performances such as normally off characteristics, an on/off current ratio as large as 10 5 and insensitivity to visible light. A field-effect mobility for oxide MISFETs (∼80 cm 2 V −1 s −1) was obtained when amorphous HfO x was used for gate insulator.

A Novel Theoretical Model for Semiconductor Oxide Gas Sensor

ABSTRACTA new constitutive equation for the gas sensitivity of n-type semiconductor oxide thin film gas sensor has been proposed here based on a single-crystal model. The derived constitutive equation shows the dependence of the gas sensitivity on various critical parameters such as nanocrystallite size, space-charge-layer thickness, reducing gas concentration, bulk charge-carrier-concentration, surface-density of states, oxygen-ion-vacancy concentration, operating temperature, and film thickness. The present theoretical model is applicable to all n-type semiconductor oxides gas sensors.

Carrier transport of extended and localized states in InGaO3(ZnO)5

ABSTRACTCarrier transport properties and electronic structure of an n-type transparent oxide semiconductor, InGaO3(ZnO)5, were investigated using single-crystalline thin films. Room-temperature Hall mobility strongly depends on carrier concentration, and rapidly increased from ∼ 2 cm2(Vs)-1 to > 10 cm2(Vs)-1 around the carrier concentration (Nth ∼3 × 1018 cm−3. This change is associatedwith insulator-metal transition. These results are explained by a model similar to Anderson localization, in which shallow semi-localized states are formed originating from random distribution of Ga3+ and Zn2+ ions in the intrinsic crystal structure of InGaO3(ZnO)5. The present conclusion suggests that electron densities larger than Nth are necessary to attain high performances in drift carrier devices fabricated using InGaO3(ZnO)5. It was demonstrated that transparent filed-effect transistors exhibited good performances such as a “normally-offcharacteristics”, an on/off current ratios as large as 105 and a field-effect mobility ∼80 cm2(Vs)-1when high-k material, amorphous HfOx, was used as a gate insulator.

Fabrication and characterization of high-performance thermoelectiric n-type oxide semiconductor;Nb-doped SrTiO3 ceramics

Fabrication and characterization of high-performance thermoelectiric n-type oxide semiconductor;Nb-doped SrTiO3 ceramics

Structure engineering of WO 3 nanoparticles for porous film applications by advanced reactive gas deposition

Fine-grained crystalline or amorphous thin films are normally produced by atomistic deposition of species under conditions giving a low adaton mobility. Here we consider an alternative high-temperature route employing advanced reactive gas deposition for structure engineering of WO 3 nanoparticles in making nanocrystalline films, especially, for gas sensing applications. For instance, structure engineering may be necessary in the case of very fine-grained n-type semiconductor oxide films in order to increase the electrical conductivity of the films. The crystal structure of WO 3 together with its different phases makes possible structure engineering of its nanoparticles by high-temperature processes such as the reactive gas deposition. Produced nanocrystalline WO 3 films in metastable tetragonal crystal structure were found to be very sensitive to H 2S in air already at room temperature.

Fabrication of MISFET exhibiting normally-off characteristics using a single-crystalline InGaO3(ZnO)5 thin film

ABSTRACTTransparent metal-insulator-semiconductor field-effect transistors (MISFETs) were fabricated using a single-crystalline thin film of an n-type transparent oxide semiconductor, a homologous compound InGaO3(ZnO)5, grown by a reactive solid phase epitaxy method. The transparent MISFET exhibited good performances with “normally-off characteristics”, “an on/off current ratio as large as 105” and “insensitivity to visible light”. Field-effect mobility was about 2 cm2(Vs)-1, which is larger than those reported previously for MISFETs fabricated in transparent oxide semiconductors. These improved performance is thought to result from the low defect density and intrinsic-level carrier concentration of the single-crystalline InGaO3(ZnO)5 film.

Effect of antimony on the semiconducting properties of the anodic plumbous oxide film formed in sulfuric acid solution I. Studies with alternating-current (a.c.) impedance

The semiconducting properties of the anodic plumbous oxide films formed on lead and leadantimony alloys in 4.5 M H 2SO 4 (20 °C) at 0.9 V (versus Hg/Hg 2SO 4) for 2 h have been studied using the a.c. method. From the Mott-Schottky plots, the films are demonstrated to be n-type semiconductors. The flat-band potentials of the films on Pb, Pb—1at.%Sb, Pb—3at.%Sb and Pb—9at.%Sb are −0.95, −1.0, −0.69 and −0.70 V (versus Hg/Hg 2SO 4), respectively; while the corresponding donor densities are 0.82×10 16, 1.2×10 17, 5.5×10 17 and 6.3×10 17 cm −3. The shift of the flat-band potential to more positive values with increase in the antimony content is probably due to the occlusion of a Sb 2O 3 phase in the film. The effect of antimony on the donor density and the lattice-defect density of the n-type semiconductor oxide both conform to the Hauffe Rules.

A p-type semiconductor thick film gas sensor

Semiconducting lanthanide—transition metal mixed oxides (p-type) of the perovskite structure are described as combustible gas-sensing thick film materials. Lanthanum nickelate (LaNiO 3) has the most favorable combustible sensing electronic properties at gas concentrations larger than 2000 ppm from about 250° to ⩽ 350 °C in oxidizing atmospheres. This response becomes excellent under overall reducing conditions and near the stoichiometric value of oxygen—fuel gas mixtures over a wider temperature range. The conditions of the above favorable gas-sensor response are not covered by the currently available n-type semiconductor oxide gas-sensing materials. Two prime, wide market applications for a LaNiO 3 thick film combustible gas sensor are cited: (a) the high pressure, reducing, coal gasification product stream at ⩾ 600 °C; (b) the control of combustion in near stoichiometry air/fuel mixtures of car engines that use the three-way catalytic converters necessary for the simultaneous reduction of the pollutants CO, NO x and hydrocarbons.

Photovoltage Response to Temperature Change at Oxide Semiconductor Electrodes

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Exciton-enhanced conductivity and photoresistivity in tin oxide–dye films

An enhancement of conductivity was observed in an n-type tin oxide semiconductor when coated with a highly polarizable insulating dye. The enhanced conductivity is thought to be a result of conduction electron–exciton interactions. The relationship between the observed photoresistivity and the excited states of the dye molecules is discussed.

The electrochemistry of the tin oxide semiconductor—I. the establishment of mechanisms at polarized n-type tin oxide

The mechanisms of electrode potential re-establishment of the n-type unstoichiometrical tin oxide semiconductor electrode after cathodic and anodic polarization was investigated. The tin oxide semiconductor was polarized cathodically and anodically without evolution of hydrogen or oxygen and the potential/time curves at open circuit were observed. After cathodic polarization, the potential/time curve had one wave. The potential of this wave depended on the concentration and nature of the electrolytes, while its life-time depended on the imposed electrode charge. After anodic polarization, the potential/time curves did not show a similar wave.This behaviour is interpreted as follows: Sn—O—H bonds are formed on a SnO2 lattice during cathodic polarization by the reduction of adsorbed H+ ions and H2O. The Sn—O—H formed on the semiconductor surface decomposes through an electron transfer process at open circuit between the SnO2 lattice and the OH on the surface.The limiting cd below which hydrogen was not observed was 3·052 × 10−4 A/cm2. After about 1 s, the polarization potential at this cd had a constant value dependent upon the logarithm of the electrolyte concentration and the nature of the electrolyte. This influence is interpreted as the adsorption of ions on the SnO2 lattice and the change of the concentration of the Sn—O—H, and, consequently, the change of the specific charge of the electrode surface.