Sn-doped In2O3 Films Research Articles

Probing Driving Force and Electron Accepting State Density Dependent Interfacial Electron Transfer Dynamics: Suppressed Fluorescence Blinking of Single Molecules on Indium Tin Oxide Semiconductor.

Photoinduced, interfacial electron transfer (ET) dynamics between m-ZnTCPP and Sn-doped In2O3 (ITO) film has been studied using single-molecule photon-stamping spectroscopy. The observed ET dynamics of single m-ZnTCPP adsorbed on ITO was compared with that of m-ZnTCPP adsorbed on TiO2 NPs with and without applied electric potential. Compared to m-ZnTCPP on the TiO2 NP surface, m-ZnTCPP on the ITO surface shows a reduced lifetime as well as suppressed blinking and a quasi-continuous distribution of fluorescence intensities, presumably due to higher electron density in ITO. The higher electron density leads to the occupancy of CB acceptor states/trap states, which supports a higher backward electron transfer (BET) rate that results in a quasi-continuous distribution of fluorescence intensities. The dependence of BET rate on electron density and charge trapping is consistent with our previous observations of quasi-continuous distribution of fluorescence intensities of m-ZnTCPP on TiO2 NPs with applied negative potential across the dye-TiO2 interface. The quasi-continuous distribution of fluorescence intensities in both cases of m-ZnTCPP on the ITO surface and m-ZnTCPP on TiO2 NPs with applied negative potential indicates that the electron density in the semiconductor plays a dominant role in dictating the changes in rates of charge transfer, rather than the relative energetics between electrons in the semiconductor and the oxidized sensitizer.

Effect of hybrid nanoinks on solution-based Sn-doped In2O3 films under low-temperature microwave annealing

Solution-based Sn-doped In2O3 (ITO) films were fabricated using well-dispersed hybrid ITO nanoinks, which consisted of high-crystalline ITO nanoparticles and ITO sols, by combining the methods of spin-coating and low-temperature microwave annealing. To investigate the optimum conditions for the hybrid ITO nanoinks, we used four different weight ratios of ITO nanoparticles to ITO sols in the hybrid nanoinks: 0 (sample A), 0.06 (sample B), 0.12 (sample C), and 0.24 (sample D). Sample D exhibited superior sheet resistance (~131.3Ω/□), , good transmittance (~87.2%), and excellent figure of merit (~19.3×10−4Ω−1) when compared to the other samples. This improvement performance is due to the improved densification of the solution-based ITO films, where the optimum amount of ITO sols in the films served as a smooth medium that linked the high-crystalline ITO nanoparticles. Therefore, our study can offer a novel strategy that can be applied to flexible transparent conducting films (TCFs) as well as to the low-cost fabrication of high-performance TCFs in various optoelectronic applications.

Thermophysical properties of SnO2-based transparent conductive films: Effect of dopant species and structure compared with In2O3-, ZnO-, and TiO2-based films

Abstract

Highly transparent Ag doped In2O3 electrodes with high work function for organic solar cells

We report highly transparent Ag-doped In2O3 (IAO) films with high work function for use as transparent anodes in organic solar cells (OSCs). The electrical, optical, structural, and morphological properties of IAO films and their work function were investigated as a function of the rapid thermal annealing (RTA) temperature. At an RTA temperature of 600 °C, the IAO film showed a sheet resistance of 23.12 Ohm/square, an optical transmittance of 79.28%, and a work function of 5.21 eV, similar to conventional Sn-doped In2O3 (ITO) films. The low resistivity of the IAO film was closely related to oxygen vacancies caused by Ag suboxide formation in the In2O3 matrix. A bulk-heterojunction OSC with the optimized IAO anode showed performance comparable to that of an OSC with a reference ITO anode, indicating that the IAO films is a promising anode material for use in OSCs.

Early Stages of Sn-Doped In2O3 Film Growth on Amorphous SiO2 Surfaces Observed by Atomic Force Microscopy and Transmission Electron Microscopy

The early stages of Sn-doped In2O3 (ITO) film growth on amorphous SiO2 (a-SiO2) surfaces were observed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). AFM measurements clearly showed that the surface morphologies of the ITO films (3–6 nm in thickness) possessed an extended hill-and-valley structure. TEM analysis revealed that small ITO islands on the a-SiO2 surface could aggregate, resulting in the formation of larger ITO islands approximately 15–30 nm in diameter. The combined results indicate that the ITO film growth on a-SiO2 surfaces is similar to that on glass surfaces.

Effects of Ar ion beam treatment on properties of roll-to-roll sputtered Sn-doped In2O3 films on colorless polyimide substrate

We report on the effects of Ar ion beam treatment on the electrical, optical, and mechanical properties of flexible Sn-doped In2O3 (ITO) films grown on colorless polyimide (CPI) by roll-to-roll (R2R) sputtering. Effective Ar ion beam treatment on ITO films led to the reduction of sheet resistance without degradation of optical transmittance of ITO films even though they were prepared at room temperature. In addition, Ar ion beam treatment resulted in crystallization of the R2R-sputtered ITO films without intentional substrate heating. At optimized conditions, the Ar ion beam treated ITO film showed a sheet resistance of 54.6Ω/square and an optical transmittance of 80.1%. A flexible organic solar cell (FOSC) fabricated with the Ar ion beam treated ITO film exhibited a power conversion efficiency (PCE) of 3.137%, which was higher than that of a FOSC made with untreated ITO film. This indicates that the in-situ Ar ion treatment is a promising method for fabrication of crystalline ITO films on flexible substrates without intentional substrate heating in a R2R sputtering process.

Transparent indium oxide films doped with high Lewis acid strength Ge dopant for phosphorescent organic light-emitting diodes

We report on Ge-doped In2O3(IGO) films prepared by co-sputtering GeO2 and In2O3 targets for anode of phosphorescent organic light-emitting diodes (POLEDs). Under optimized annealing conditions, the IGO film exhibited a low sheet resistance of 14.0 Ω/square, a high optical transmittance of 86.9% and a work function of 5.2 eV, comparable to conventional Sn-doped In2O3 (ITO) films. Due to the higher Lewis acid strength of the Ge4+ ion (3.06) than that of Sn3+(1.62), the IGO film showed higher transparency in the near infrared and higher carrier mobility of 39.16 cm2 V−1 s−1 than the ITO films. In addition, the strongly preferred (2 2 2) orientation of the IGO grains, caused by Zone II grain growth during rapid thermal annealing, increased the carrier mobility and improved the surface morphology of the IGO film. POLEDs fabricated on IGO anodes showed identical current density–voltage–luminance curves and efficiencies to POLEDs with ITO electrodes due to the low sheet resistance and high transmittance of the IGO anode.

High-rate deposition of high-quality Sn-doped In2O3 films by reactive magnetron sputtering using alloy targets

Sn-doped In2O3 (ITO) films were deposited on heated (200°C) fused silica glass substrates by reactive DC sputtering with mid-frequency pulsing (50kHz) and a plasma control unit combined with a feedback system of the optical emission intensity for the atomic O* line at 777nm. A planar In–Sn alloy target was connected to the switching unit, which was operated in the unipolar pulse mode. The power density on the target was maintained at 4.4Wcm−2 during deposition. The feedback system precisely controlled the oxidation of the target surface in “the transition region.” The ITO film with lowest resistivity (3.1×10−4Ω cm) was obtained with a deposition rate of 310nmmin−1 and transmittance in the visible region of approximately 80%. The deposition rate was about 6 times higher than that of ITO films deposited by conventional sputtering using an oxide target.

Evaluation of indium tin oxide films grown at room temperature by pulsed electron deposition

Good quality Sn-doped In2O3 films with thickness of 30 nm were deposited using a vapor deposition technique known as pulsed electron deposition. The films were deposited on (100) Si substrates, at room temperature from a ceramic target of indium tin oxide (90/10). A pulsed electron beam was used for ablation of the target. Voltage of the electron source was maintained at 18 kV with frequency of pulses at 3 Hz. Distance between source and target was maintained around 6 mm, and the substrate to target distance was ∼7 cm. Oxygen pressure in the chamber during growth was varied from 3.1 to 20 mTorr. To evaluate the quality of grown films, various characterization techniques were employed. The effect of oxygen chamber pressure on resistivity (ρ), carrier concentration (N), mobility (μ), and optical constants (n and k) was carried out. Optical transparency and electrical conductivity of the films were seen to improve with increasing oxygen pressure. Details about the film preparation and evaluation of properties are reported.

Surface structure of Sn-doped In2O3 (111) thin films by STM

High-quality Sn-doped In2O3 (ITO) films were grown epitaxially on yttria stabilized zirconia (111) with oxygen-plasma assisted molecular beam epitaxy (MBE). The 12 nm thick films, containing 2–6% Sn, are fully oxidized. Angle-resolved x-ray photoelectron spectroscopy (ARXPS) confirms that the Sn dopant substitutes In atoms in the bixbyite lattice. From XPS peak shape analysis and spectroscopic ellipsometry measurements it is estimated that, in a film with 6 at.% Sn, ∼1/3 of the Sn atoms are electrically active. Reflection high energy electron diffraction (RHEED) shows a flat surface morphology and scanning tunneling microscopy (STM) shows terraces several hundred nanometers in width. The terraces consist of 10 nm wide orientational domains, which are attributed to the initial nucleation of the film. Low energy electron diffraction (LEED) and STM results show a bulk-terminated (1 × 1) surface, which is supported by first-principles density functional theory (DFT) calculations. Atomically resolved STM images are consistent with Tersoff–Hamann calculations that show that surface In atoms are imaged bright or dark, depending on the configuration of their O neighbors. The coordination of surface atoms on the In2O3(111)–1×1 surface is analyzed in terms of their possible role in surface chemical reactions.

Characterization of the Sn Doped In2O3 Film Prepared by DC Magnetron Sputter Type Negative Metal Ion Beam Deposition

Transparent conducting indium tin oxide (ITO) films were prepared on a glass substrate by DC magnetron sputter type negative ion source from a target of a mixture of In2O3 (90%) and SnO2 (10%). In order to investigate the influence of cesium (Cs) partial pressure (PCs) on the optoelectronic property of ITO films, the PCs was varied from 1×10-3 to 2.2×10-3 Pa. The O2/Ar flow rate, working pressure, power density, and substrate temperature were kept constant at 3.6%, 9×10-2 Pa, 2.5 Wcm-2 and 70°C, respectively. By optimizing the PCs at 1.7×10-3 Pa, transparent (transmission ∼81% at 550 nm) and conducting (resistivity ∼4.3×10-4 Ωcm) ITO films were prepared. Atomic force microscopy (AFM) images showed that the surface morphology also varied significantly with PCs and that the lowest surface rms roughness was 0.6 nm measured at PCs=1.7×10-3 Pa. XPS measurement showed that the Cs concentration in the ITO films was less than 0.2 at%.

Characterization of Sn-doped In2O3 Film on Roll-to-Roll Flexible Plastic Substrate Prepared by DC Magnetron Sputtering

Electrical and crystallographic properties of Sn-doped In2O3 (ITO) films prepared on polyethylene terephthalate (PET) webs as the roll-to-roll flexible substrates by DC magnetron sputtering were examined before and after the annealing under air. The ITO films were deposited at the substrate temperature of 100°C under several O2/Ar ratios in the range between 0 to 0.42. Before the deposition, repeated passing of the PET web over the heated drum in the vacuum chamber, i.e. degassing process, was carried out to control the contained H2O molecules in the PET substrate. When the number of degassing repetitions was insufficient, the deposited ITO films were more affected by H2O because more H2O remained in the substrate. In the case of insufficient degassing, crystallization of the ITO films by the annealing was suppressed, and the ionization of Sn atoms into donors was prevented. Such properties caused by insufficient degassing were estimated to be due to termination of the In–O network with the –OH radicals. On the other hand, the ITO films deposited on sufficiently degassed PET substrate were well crystallized and Sn atoms were well ionized.

Estimation of Chemical States and Carrier Density of Sn-doped In2O3 (ITO) by Mössbauer Spectrometry

Chemical states of Sn in Sn-doped In2O3 (ITO) powders and films were studied by Mossbauer spectrometry. From the deconvolution analysis of the spectra it was found that there were 7- and 8-fold coordinated, electrically inactive Sn4+ species in the host lattice in addition to two electrically active Sn4+ species. The concentration of the electrically inactive dopants and the carrier densities could be estimated by Mossbauer spectra. The estimated carrier densities for the powders and films were in good agreement with the measured values.

Thin film phosphor prepared by physical vapor deposition for field emission display application

We fabricated very thin ZnO:Zn film using rf magnetron sputtering and ZnS:Pr,Ce layer by electron-beam evaporation for color field emission display application. Also, in order to investigate the influence of surface conductive layer onto the phosphor films upon the emission characteristics, we prepared the layered structures having an additional conducting layer of 2–6-nm-thick Sn-doped In2O3 film. Cathodoluminescence spectrum of sputtered ZnO:Zn (250 nm) film shows two broad peaks centered around 540 and 600 nm, respectively. The spectra characteristics of ZnS:Pr,Ce,F films show white spectrum that covered a whole visible wavelength. One of the effects of an additional surface layer of indium tin oxide on ZnO and ZnS phosphor is the shift of spectrum toward shorter wavelength as well as low voltage emission, as compared to our phosphor film without the surface conductive layer.

Effects of Tin Concentrations on Structural Characteristics and Electrooptical Properties of Tin-Doped Indium Oxide Films Prepared by RF Magnetron Sputtering

Structural characteristics and electrooptical properties of Sn-doped In2O3 (ITO) films were investigated in terms of Sn concentrations from 5.34 to 8.99 (Sn/In at.%) with changing oxygen partial pressure and substrate temperature during deposition, in spite of using an ITO target with the same Sn concentration ( 7.50 SnO2 wt%, 7.17 Sn/In at.%). The resistivity of the films deposited at 200 and 300° C had a clear tendency to decrease with decrease of the total Sn content. Sn atoms incorporated in the ITO films were classified into two types, i.e., electrically active substitutional Sn atoms contributing to carrier density and electrically nonactive impurities forming nonreducible tin-oxide complexes, which were revealed by precise lattice constant measurement. The change in the Sn concentration was found to be associated with the preferred orientation of the crystal grains, which was dominated by the deposition conditions and should reflect the crystal growth processes.

Doping mechanisms of tin-doped indium oxide films

The polycrystalline Sn-doped In2O3 (ITO) films with different Sn concentrations of 0–6.7 wt% (SnO2 wt%) were prepared by postannealing the amorphous ITO films deposited by highly dense plasma-assisted EB evaporation at the low substrate temperature. These films were confirmed to show the high crystallinity and homogeneous Sn distribution by x-ray diffraction (XRD), scanning electron microscopy, and electron spectroscopy for chemical analysis. The detail XRD pattern of 1.9 wt% Sn concentration film showed the doublet peaks, indicating that the film structure was the mixture of two different lattice parameters (LP); one was +0.18% larger and another was −0.13% smaller than the In2O3 LP, which could be attributed to the interstitial and the substitutional Sn atoms, respectively.

Influence of Preferred Orientation in Indium Tin Oxide.

ABSTRACTWe investigated the electrical and optical properties, the chemical composition, the surface morphology and the crystallinity of sputtered Sn-doped In2O3 films deposited on different substrate positions. Both the electric conductivity and the optical transparency are related to the free carrier density and moreover the preferred crystal orientation. The former relation is attributed to the Burstein-Moss effect and the latter suggests the Sn doping mechanism.

Evaporated Sn-doped In2O3 films: Basic optical properties and applications to energy-efficient windows

We review work on In2O3:Sn films prepared by reactive e-beam evaporation of In2O3 with up to 9 mol % SnO2 onto heated glass. These films have excellent spectrally selective properties when the deposition rate is ∼0.2 nm/s, the substrate temperature is ≳150 °C, and the oxygen pressure is ∼5×10−4 Torr. Optimized coatings have crystallite dimensions ≳50 nm and a C-type rare-earth oxide structure. We cover electromagnetic properties as recorded by spectrophotometry in the 0.2–50-μm range, by X-band microwave reflectance, and by dc electrical measurements. Hall-effect data are included. An increase of the Sn content is shown to have several important effects: the semiconductor band gap is shifted towards the ultraviolet, the luminous transmittance remains high, the infrared reflectance increases to a high value beyond a certain wavelength which shifts towards the visible, phonon-induced infrared absorption bands vanish, the microwave reflectance goes up, and the dc resisitivity drops to ∼2×10−4 Ω cm. The corresponding mobility is ∼30 cm2/V s. The complex dielectric function ε is reported. These data were obtained from carefully selected combinations of spectrophotometric transmittance and reflectance data. It is found that ε can be reconciled with the Drude theory only by assuming a strongly frequency-dependent relaxation energy between the plasma energy and the band gap. We review a recently formulated quantitative theoretical model for the optical properties which explicitly includes the additive contributions to ε from valence electrons, free electrons, and phonons. The theory embodies an effective-mass model for n-doped semiconductors well above the Mott critical density. Because of the high doping, the Sn impurities are singly ionized and the associated electrons occupy the bottom of the conduction band in the form of an electron gas. The Sn ions behave approximately as point scatterers, which is consistent with pseudopotential arguments. Screening of the ions is described by the random phase approximation. This latter theory works well as a consequence of the small effective electron radii. Exchange and correlation in the electron gas are represented by the Hubbard and Singwi–Sjölander schemes. Phonon effects are included by three empirically determined damped Lorentz oscillators. Free-electron properties are found to govern the optical performance in the main spectral range. An analysis of the complex dynamic resistivity (directly related to ε) shows unambiguously that Sn ions are the most important scatterers, although grain-boundary scattering can play some role in the midvisible range. As a result of this analysis one concludes that the optical properties of the best films approach the theoretical limit. Band-gap shifts can be understood as the net result of two competing mechanisms: a widening due to the Burstein–Moss effect, and a narrowing due to electron-electron and electron-ion scattering. The transition width—including an Urbach tail—seems to be consistent with these notions. Window applications are treated theoretically from detailed computations of integrated luminous, solar, and thermal properties. It is found that In2O3:Sn films on glass can yield∼78% normal solar transmittance and ∼20% hemispherical thermal emittance. Substrate emission is found to be insignificant. Antireflection with evaporated MgF2 or high-rate sputtered aluminum oxyfluoride can give ∼95% normal luminous transmittance, ∼5% normal luminous reflectance, little perceived color and little increase in emittance. A color purity <1% in normal transmission and <10% in normal reflection is achievable for a daylight illuminant within extended ranges of film thickness.

Electrical properties of Sn-doped In2O3 prepared by reactive evaporation

Sn-doped In2O3 films (ITO) have been prepared by coevaporation of In and Sn on to heated glass substrates at 10-3 Torr O2 and the electrical properties have been investigated. Tin-doping suppresses the oxidation of In under evaporation, and higher substrate temperatures, above 400 degrees C, are necessary to obtain a visible transparent film (80% at 550 nm) than in the case of undoped In2O3 films. By tin-doping, an ITO film with the lowest resistivity rho =6*10-4 Omega cm has been obtained at tin-doping of Sn/In approximately=0.05 atomic ratio, where the carrier concentration is 7*1020 cm-3 and the mobility is 15 cm2 V-1 s-1. All the films show n-type conductivity with complete degeneracy.

Preparation of Sn-doped In2O3 (ITO) films at low deposition temperatures by ion-beam sputtering

High-quality films of Sn-doped In2O3 (ITO) have been prepared by ion-beam sputtering at deposition temperatures below 100 °C. As in the case of rf-sputtered films, the electrical and optical properties of these films depend strongly on the O2 partial pressure during deposition. As-deposited films with low electrical resistivity (∼5.5×10−4 Ω cm), high visible transmission (≳80%), and high infrared reflectivity at 10 μm (∼84%) have been obtained at O2 pressures of (2–3) ×10−5 Torr by deposition on both glass and Mylar substrates.