Conductive Oxide Thin Films Research Articles

Control of ZnO Nanorod Defects to Enhance Carrier Transportation in p-Cu₂O/i-ZnO Nanorods/n-IGZO Heterojunction.

The p-Cu₂O/i-ZnO nanorods/n-IGZO heterojunctions were fabricated by electrochemical and sputtering method. ZnO nanorods were grown on conductive indium gallium zinc oxide (IGZO) thin film and then p-Cu₂O layer was deposited on ZnO nanorods to form the heterojunction. ZnO nanorods play an important role in carrier transport mechanisms and performance of the junction. The changing of defects in ZnO nanorods by annealing samples in air and vacuum have studied. The XRD, photoluminescence (PL) spectroscopy, and FTIR were used to study about structure, and defects in ZnO nanorods. The SEM, i–V characteristics methods were also used to define structure, electrical properties of the heterojunctions layers. The results show that the defects in ZnO nanorods affected remarkably on performance of heterojunctions of solar cells.

Transparent conducting oxide thin films of Si-doped ZnO prepared by aerosol assisted CVD

For the first time, aerosol assisted chemical vapour deposition (AACVD) was used to deposit Si-doped ZnO thin films on glass.

Development of a highly transparent, low-resistance lithium-doped nickel oxide triple-layer film deposited by magnetron sputtering.

This research presents a triple-layer transparent conductive oxide thin film, with a lithium-doped nickel oxide/silver/lithium-doped nickel oxide (L-NiO/Ag/L-NiO) structure using radio-frequency (RF) magnetron sputtering on glass substrates. The high transmittance L-NiO thin films were deposited using the sputtering method with Ar/H2 as the reaction gases. The triple-layer structure, L-NiO/Ag/L-NiO, showed impressive electrical conductivity. The figure of merit (FOM) results indicated that the L-NiO/Ag/L-NiO triple-layer thin films with Ag deposition times of 2 min possessed satisfactory optical and electrical properties for potential applications.

Plasmonic nanomesh sandwiches for ultrathin film silicon solar cells

We theoretically investigate the strategy of integrating metal nanoparticles (NPs)/nanomeshes (NMs) into the top and/or bottom of crystalline silicon (c-Si) thin film solar cells for light trapping and enhanced carrier collection. C-Si thin films exhibit absorption resonances corresponding to Fabry–Pérot modes. Frontside metal NPs enhance absorption by additionally coupling light into localized surface plasmon resonances and c-Si waveguide modes, while a frontside metal NM increases absorption by coupling light into surface plasmon polaritons and c-Si waveguide modes. The frontside metal NM also functions as a flexible top electrode, which may replace conventional brittle transparent conductive oxide thin films. The backside metal NM exhibits enhanced absorption due to the coupling of light into c-Si waveguide modes and the cavity modes within the holes of metal NM. We illustrate how the optimal metal NM sandwich, consisting of NMs on both sides of a 300 nm thick c-Si with an appropriate antireflection coating (ARC), achieves a 72.9% enhancement in short-circuit current density compared with that of a 300 nm thick c-Si thin film solar cell with 100 nm thick Si3N4 ARC and 300 nm thick Ag back reflector. The current generation in the metal NM sandwich is more in the center of the thin film such that there should be less surface recombination. The uniform current generation throughout the film results in less overall recombination.

Atomic oxygen irradiation resistance of transparent conductive oxide thin films

One set of tungsten-doped indium oxide thin films (In2O3:W, IWO) and another set of tin-doped indium oxide thin films (In2O3:Sn, ITO) were prepared on glass substrates by radio frequency (RF) reactive magnetron sputtering method. The as-deposited IWO and ITO films have resistivity values at a level of 10−4Ω•cm. Average transmittance values of these films are above 85% in visible-light region as well as in near-infrared region. All these films were irradiated by atomic oxygen (AO) with different amount of flux in a ground-based simulation system close to the environment of low Earth orbit. Changes in characteristics including surface morphology, mechanical properties, optical and electrical properties were compared between IWO and ITO films after AO irradiation. The effects of AO irradiation on transparent conductive oxide thin films were analyzed. As a result, AO has influences on ITO and IWO thin films through the way of oxidation and erosion. Both ITO and IWO films possess suitable anti-AO properties. IWO films are more appropriate as AO protective coatings due to their compact microstructure, the coexistence of W4+ and W6+ ions in their chemical systems, and incremental WO3 protective layer under AO oxidation.

Nanosecond pulse laser scribing using Bessel beam for single shot removal of transparent conductive oxide thin film

Nanosecond laser Bessel beam scribing on the TCO thin film was investigated to improve processing precision and robustness of optical system. Fundamental wave (1064nm) of Nd:YAG laser was shaped into high-quality Bessel beam by using novel optical system consisting of axicons and convex lens. Spatial FWHM of the beam was only 1.5μm in the present context, and significantly precise scribing with minimum width of 2.3μm was achieved on 600–700nm-thick FTO film with electrical isolation. Furthermore, due to the critically deep focal length of millimeters-order, robustness on sample positioning was greatly improved. Additionally, experimental results showed that single shot removal of entire film can be achieved using film side irradiation unlike conventional Gaussian beam. Temperature distribution during the process was calculated by a numerical model in which we have taken into account beam propagation inside the film to give comparison with a Gaussian beam irradiation. The calculation results showed that only Bessel beam is self-reconstructed behind plasma shielding so that entire film can be removed by single shot. Our findings suggest that Bessel beam can be used for efficient IR scribing with significantly high quality without selecting substrate material.

Aerosol assisted chemical vapour deposition of transparent conductive aluminum-doped zinc oxide thin films from a zinc triflate precursor

The use of zinc triflate (trifluoromethanesulfonate), [Zn(OTf)2] as a precursor in the aerosol assisted chemical vapour deposition of zinc oxide thin films is described. Aluminum doped zinc oxide (AZO) thin films are also shown to be deposited when aluminum acetylacetonate [Al(acac)3] was introduced into the precursor solution, illustrating the versatility of this system. Film characterization techniques include glancing angle X-ray powder diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and optical and electrical measurements. AZO films with an Al content of 7at.% were found to have favourable transparent conducting oxide properties with simultaneous high transparency (>80%) in the visible light region and a low electrical resistivity (1.96×10−3Ωcm).

Inductive Loops in Impedance Spectra of Polarized SrTiO3 Thin Films: A Trace of Oxide Ion Motion Rather than an Experimental Artefact

Several reasons may cause inductive loops in impedance spectra of solid state electrochemical systems. Some of them are simply artefacts due to instrumental effects [1] or due to an unfavorable sample set up [2]. In this contribution, we discuss inductive loops which represent true sample properties of oxide thin films. A detailed analysis of the loops even introduces a novel method for measuring the ionic conductivity of oxides with very low ionic transference number. Slightly Fe-doped SrTiO3 (Fe:STO) thin films on a conducting Nb-doped SrTiO3 substrate were investigated by means of impedance spectroscopy under DC voltages in the +/- 500 mV range. For all temperatures (ca. 300°C - 700°C) the applied bias voltage causes either an unusual “inductive loop” in the low frequency range of impedance spectra (see Fig. 1) or an additional semicircle. Moreover, current-voltage (I-V) curves strongly vary with changing scan rate and the shape of quickly measured I-V curves depends on the starting voltage. All findings can be understood in terms of bias induced ion motion in the SrTiO3 thin films: Upon a DC voltage, stoichiometry polarization takes place in the SrTiO3 thin films which means that the oxygen vacancy concentration changes until an inhomogeneous steady state distribution is reached. For slowly measured current voltage (I-V) curves, each measurement point thus corresponds to a different oxygen vacancy profile in the film. I-V curves measured with fast scan rates, however, reflect the steady state established prior to the measurement. Accordingly, fast and slow I-V curves exhibit different slopes at one and the same voltage. This difference is again found in the impedance spectra since those probe the slopes of I-V curves. If slopes of fast curves are smaller than those of slowly measured curves, the low frequency resistance is smaller than that at medium frequencies and an “inductive loop” has to result. The frequency range of the loop indicates the time required for oxygen vacancies to redistribute after a voltage change. Accordingly, the inductive loops (or additional arcs) measured upon bias are caused by ion motion in the films, even though the ionic current is very small compared to the electronic one. An equivalent circuit model is derived, which not only fits the spectra but can also explain the main features of all deduced parameters. Based on an estimate of the interfacial capacitance at the ion blocking electrode, such measurements can be used as a novel method for obtaining information on the ionic conductivity of mixed conducting thin films with very small ionic transference number. Similar effects are expected in many mixed conducting oxide thin films, provided time constants of stoichiometry polarization get into the frequency range of impedance measurements. Details are given in Ref. [3]. [1] G. Fafilek, Soid State Ionics 176 (2003) 223 [2] J. Fleig, J. Jamnik, J. Maier, and J. Ludvig, J. Electrochem. Soc. 143 (1996) 3636-3641[3] S. Taibl, G. Fafilek, J. Fleig, Nanoscale 2016 (in press). Figure 1

Surface and optical properties of indium tin oxide layer deposition by RF magnetron sputtering in argon atmosphere

This study focused on the characterization and properties of transparent and conductive indium tin oxide (ITO) thin films deposited in argon atmosphere. ITO thin films were coated onto glass substrates by radio frequency (RF) magnetron sputtering technique at 75 and 100 W RF powers. Structural characteristics of producing films were investigated through X-ray diffraction analysis. UV–Vis spectrophotometer and interferometer were used to determine transmittance, absorbance and reflectance values of samples. The surface morphology of the films was characterized by atomic force microscope. The calculated band gaps were 3.8 and 4.1 eV for the films at 75 and 100 W, respectively. The effect of RF power on crystallinity of prepared films was explored using mentioned analysis methods. The high RF power caused higher poly crystallinity in the produced samples. The thickness and refractive index values for all samples increased respect to an increment of RF power and were calculated as 20, 50 nm and 1.71, 1.86 for samples at 75 and 100 W, respectively. Finally, the estimated grain sizes for all prepared films decreased with increasing of 2θ degrees, and the number of crystallite per unit volume was calculated. It was found that nearly all properties including sheet resistance and resistivity depend on the RF power.

Atomic step-and-terrace surface of polyimide sheet for advanced polymer substrate engineering

Typical thermostable and flexible polyimide polymers exhibit many excellent properties such as strong mechanical and chemical resistance. However, in contrast to single-crystal substrates like silicon or sapphire, polymers mostly display disordered and rough surfaces, which may result in instability and degradation of the interfaces between thin films and polymer substrates. As a step toward the development of next-generation polymer substrates, we here report single-atom-layer imprinting onto the polyimide sheets, resulting in an ultrasmooth 0.3 nm high atomic step-and-terrace surface on the polyimides. The ultrasmooth polymer substrates are expected to be applied to the fabrication of nanostructures such as superlattices, nanowires, or quantum dots in nanoscale-controlled electronic devices. We fabricate smooth and atomically stepped indium tin oxide transparent conducting oxide thin films on the imprinted polyimide sheets for future use in organic-based optoelectronic devices processed with nanoscale precision. Furthermore, toward 2D polymer substrate nanoengineering, we demonstrate nanoscale letter writing on the atomic step-and-terrace polyimide surface via atomic force microscopy probe scratching.

Preparation and characterization of highly transparent and conductive indium-zinc oxide thin films deposited by pyrolysis spray technique

Ternary compound films of ZnO–In2O3 system were prepared by spray pyrolysis method. The films were deposited on heated glass substrates by pyrolytic decomposition of zinc chloride and indium chloride in distilled water with different ratio x of [Zn] to [Zn + In] (x = [Zn]/[Zn + In]). The phase change, chemical composition, electrical and optical properties of the Zn–In oxides were investigated over wide range of ratio x. Quasi-amorphous sub-oxide films with thickness of 200–400 nm were first obtained, the films were then annealed in Argon atmosphere at 550 °C for 1 h. To optimize the physical properties of sprayed ZnO–In2O3 thin films, we carry out on a systematic study of their microstructural characterization and transport properties. Continuous structural variation from In2O3 to ZnO through Zn2In2O5 and Zn3In2O6 was observed on the deposited films with increasing the ratio x. Maximum of conductivity of 1.47 × 103 Ω−1 cm−1 was reached for x = 0.5 (at.%). We also concluded that the transport properties of the films are greatly governed by their microstructure.

Pulsed laser deposited indium tin oxides as alternatives to noble metals in the near-infrared region

Transparent conductive indium tin oxide thin films with thickness around 200 nm were deposited on glass substrates by pulsed laser deposition technology. The microstructure and the electrical and optical properties of the ITO films deposited under different oxygen pressures and substrate temperatures were systematically investigated. Distinct different x-ray diffraction patterns revealed that the crystallinity of ITO films was highly influenced by deposition conditions. The highest carrier concentration of the ITO films was obtained as 1.34 × 1021 cm−3 with the lowest corresponding resistivity of 2.41 × 10−4 Ω cm. Spectroscopic ellipsometry was applied to retrieve the dielectric permittivity of the ITO films to estimate their potential as plasmonic materials in the near-infrared region. The crossover wavelength (the wavelength where the real part of the permittivity changes from positive to negative) of the ITO films exhibited high dependence on the deposition conditions and was optimized to as low as 1270 nm. Compared with noble metals (silver or gold etc), the lower imaginary part of the permittivity (<3) of ITO films suggests the potential application of ITO in the near-infrared range.

Paramagnetic Meissner Effect in Electrochemically Doped Indium-Tin Oxide Films

Transparent conductive indium-tin oxide (ITO) thin films, electrochemically intercalated with alkali (Li+, Na+, K+, Rb+, Cs+), alkali earth (Mg+2, Ca+2), or complex NH\(_{4}^{+}\) ions, show tunable superconducting transitions with dome-shaped behavior of Tc versus electron density around the maximum at ∼ 5 K. On field cooling, the transition into the superconducting state is accompanied by a paramagnetic response, i.e., an increase of magnetization, rather than the usual diamagnetic Meissner response. We provide an extensive study of this so-called paramagnetic Meissner effect (PME), using DC SQUID, transport measurements and a variety of sample sizes and growth conditions. We show that the PME in electrochemically doped ITO films results from a higher Tc at the sample edges than at the center of disk-shaped samples, causing flux to be expelled towards the center of the disk, following the flux-compression theory of Koshelev and Larkin. Changing to the opposite spatial Tc profile largely removes the paramagnetic response. The paramagnetic magnetization is strongly influenced by sample geometry and flux pinning conditions. The reduction of pinning defects by thermal annealing removes the paramagnetic response. An alternation of the external magnetic field restores the usual Meissner diamagnetism.

Temperature dependence of electrical transport properties of La4BaCu5−xCoxO13+δ conducting oxide thin films

We fabricated Co-doped La4BaCu5O13+δ (La4BaCu5−xCoxO13+δ) epitaxial thin films with various amount of Co-substitution on SrTiO3 single-crystal substrates by a pulsed laser deposition method, and measured the resistivity from 10 to 1000 K. The resistivity below 700 K became large with increasing amount of Co-substitution, and that above 700 K was independent of Co-substitution. These measurement results suggested that the dominant component of carrier scattering changed from the impurity scattering of Co to phonon scattering at 700 K. As a result, the temperature dependence of the resistivity became weak systematically with increasing amount of Co-substitution. The resistivity measurement in N2-based 1% H2 clarified that the Co-substitution has no effect on the oxygen retention in the reduction atmosphere.

Facile Preparation of Highly Conductive Metal Oxides by Self-Combustion for Solution-Processed Thermoelectric Generators.

Highly conductive indium zinc oxide (IZO) thin films were successfully fabricated via a self-combustion reaction for application in solution-processed thermoelectric devices. Self-combustion efficiently facilitates the conversion of soluble precursors into metal oxides by lowering the required annealing temperature of oxide films, which leads to considerable enhancement of the electrical conductivity of IZO thin films. Such enhanced electrical conductivity induced by exothermic heat from a combustion reaction consequently yields high performance IZO thermoelectric films. In addition, the effect of the composition ratio of In to Zn precursors on the electrical and thermoelectric properties of the IZO thin films was investigated. IZO thin films with a composition ratio of In:Zn = 6:2 at the low annealing temperature of 350 °C showed an enhanced electrical conductivity, Seebeck coefficient, and power factor of 327 S cm(-1), 50.6 μV K(-1), and 83.8 μW m(-1) K(-2), respectively. Moreover, the IZO thin film prepared at an even lower temperature of 300 °C retained a large power factor of 78.7 μW m(-1) K(-2) with an electrical conductivity of 168 S cm(-1). Using the combustive IZO precursor, a thermoelectric generator consisting of 15 legs was fabricated by a printing process. The thermoelectric array generated a thermoelectric voltage of 4.95 mV at a low temperature difference (5 °C). We suggest that the highly conductive IZO thin films by self-combustion may be utilized for fabricating n-type flexible printed thermoelectric devices.

Annealing-induced modifications in sol–gel spin-coated Ga:ZnO thin films

Transparent conductive gallium-doped zinc oxide (Ga:ZnO) thin films were prepared by the optimized sol–gel spin-coating technique on glass substrate. After deposition, the films were annealed at temperatures ranging from 200 to 500 °C in the air. The effect of post-annealing temperature on structural, optical and electrical behaviors of Ga:ZnO (GZO) thin films was investigated. Experimental results showed the polycrystalline nature of the films with a hexagonal (wurtzite) crystal structure and a preferred orientation with c-axis, perpendicular to the substrate. The average optical transmittance of all the annealed films is over 89 %. The grain size and optical transmittance of the GZO films increased with increasing annealing temperatures. The photoluminescence spectra of GZO films showed the near-band-edge (NBE) emissions at 390 nm and blue emissions at 456 nm. The NBE emissions might originate from excitonic recombination, and the blue emission peak can be attributed to the zinc interstitial and oxygen vacancies in the ZnO lattice. The lowest resistivity of 2.23 × 10−3 Ω cm and the highest figure of merit are achieved for a GZO film annealed at 500 °C. The new results obtained from this study indicate that the structural, optical and electrical properties of GZO thin films can all be improved through an appropriate post-annealing operation. The obtained results confirm that the thermal annealing has considerable effects on the properties of GZO thin films deposited by sol–gel spin-coating method and can be used as a promising TCO for optoelectronic applications. AFM surface morphology of GZO thin films annealed at (a) 200 °C, (b) 300 °C, (c) 400 °C, (d) 500 °C

Doping Ga effect on ZnO radio frequency sputtered films from a powder target

Undoped ZnO and ZnO:Ga films have been radio frequency (RF) sputtered on glass substrates using powder targets. X-ray diffraction studies reveal that the films are polycrystalline with a hexagonal wurtzite structure and a preferential orientation along the c-axis. The (002) peak shift to lower 2θ values indicates that the films suffered compressive stress with Ga incorporation. The grain sizes are found in the range of 20–23nm showing a nanostructure. All the films are highly transparent (>85%) in the visible region and the optical band gap shows a blue shift with Ga incorporation up to 3wt.% then a red shift with further Ga content from 3.56 to 3.47eV due to band tail. Urbach energy increases from 0.06 to 0.15eV with Ga content revealing disorder increase in the lattice.Besides the near band emission peak, which shows the same trend for the optical band gap variation, Photoluminescence measurements show deep level emissions in the violet, blue and green regions.The electrical parameters were obtained with both optical reflectance based on the Drude free-electron model and the Hall method and similar tendencies were observed within the two methods. The highest figure of merit observed in this study was 3.57×10−3Ω−1 showing that the 3wt.% ZnO:Ga films are potentially attractive for application as transparent conductive oxide thin films in solar cells. RF sputtering from a powder target was found to be a simple and cost effective technique for the synthesis of ZnO thin films for potential applications in the renewable energy devices.

Zn&lt;SUB&gt;1–&lt;I&gt;x&lt;/I&gt;&lt;/SUB&gt;Al&lt;SUB&gt;&lt;I&gt;x&lt;/I&gt;&lt;/SUB&gt;O:NiO Composite Transparent Conducting Oxide Thin Film/&lt;I&gt;p&lt;/I&gt;-Type Silicon Photodiodes

Zn1-xAlxO:NiO composite transparent conducting oxide thin films were prepared by sol gel method to prepare Zn1-xAlxO:NiO/p-type silicon photodiodes. The structural properties of the films were analyzed by AFM measurements. The diodes were coated on p-type silicon having ohmic contact by sol gel spin coating method. The ideality factor and barrier height values of the diodes were obtained. The photoresponse properties of the diodes were performed under solar light illuminations. It was found that the photocurrent of the diodes is higher than the dark current. This indicates that the diodes exhibited a photodiode behavior. The obtained results indicate that the photoresponse properties of Zn1-xAlxO:NiO composite transparent conducting oxide thin film/p-type silicon photodiodes are controlled by NiO doping

Thermal study on the synthesis of the doped ZnO to be used in TCO films

ZnO, Li doped, and Li, Ni co-doped ZnO powders to be later used as transparent conductive oxide thin film were prepared by heat treatment of gels obtained from alcoholic Zn(CH3COO)2·2H2O, LiNO3·nH2O and NiSO4·6H2O solutions with (CH3CH2OH)3N as chelating agent. The properties of the powders and their thermal treatment were studied by thermogravimetric and differential thermal analysis (TG/DTA), differential scanning calorimetry, evolved gas analysis coupled with mass spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy coupled with energy-dispersive X-ray. The as-prepared gels consisted of submicron platelet-like particles and contained zinc acetate dihydrate and hydrozincite in different amount and with different preferred orientations. During annealing of the gels, zinc acetate decomposed between 110 and 350 °C with the release of CH3COOH, acetone and CO2. The N content of the chelating agent was responsible for NH3 and NO evolution. The thermal behavior of the doped gels was similar, but there were also differences in the mass losses, amount of released gases. Based on TG/DTA data, ZnO powders were obtained from the gels by annealing both at 275 and 500 °C. After heating at 275 °C, the obtained powders consisted of spherical 1–2 micron grains of wurtzite. The inclusion of the dopants was successful according to EDX and cell parameter data. Thermal study of the powder annealed at 275 °C confirmed that they still contain some zinc acetate. In the case of the doped samples, the mass loss was smaller, since the Li and Ni dopants catalyzed the composition of zinc acetate during the previous annealing at 275 °C. After annealing the gels at 500 °C, stable undoped ZnO or doped ZnO particles were obtained.

Optical Properties and Electrochemical Performance of LiFePO4 Thin Films Deposited on Transparent Current Collectors.

LiFePO4 thin film cathodes are deposited on various transparent conducting oxide thin films on glass, which are used as cathode current collectors. The XRD patterns show that the thin films have the phase of LiFePO4 with an ordered olivine structure indexed to the orthorhombic Pmna space group. LiFePO4 thin film deposited on various TCO glass substrates exhibits transmittance of about 53%. The initial specific discharge capacities of LiFePO4 thin films are 25.0 μAh/cm2 x μm on FTO, 33.0 μAh/cm2 x μm on ITO, and 13.0 μAh/cm2 x μm on AZO coated glass substrates. Interestingly, the retention capacities of LiFePO4 thin films are 76.0% on FTO, 31.2% on ITO, and 37.7% on AZO coated glass substrates at 20th cycle. The initial specific discharge capacity of the LiFePO4/FTO electrode is slightly lower, but the discharge capacities of the LiFePO4/FTO electrode relatively decrease less than those of the others such as LiFePO4/ITO and LiFePO4/AZO with cycling. The results reported here provide the high transparency of LiFePO4 thin films cathode materials and the good candidate as FTO current collector of the LiFePO4 thin film cathode of transparent thin film rechargeable batteries due to its high transparency and cyclic retention.