A study of amorphous and crystalline phases in In2O3-10 wt.% ZnO thin films deposited by DC magnetron sputtering

We report on the processing, phase stability, and electronic transport properties of indium oxide (In₂O₃) doped with 10 wt.% zinc oxide (ZnO) deposited to a thickness of 100 nm using DC magnetron sputter deposition at room temperature and 350 °C. We compare the optimum oxygen content in the sputter gas for pure In₂O₃ and doped with (i) 10 wt.% ZnO and (ii) 9.8 wt.% SnO₂. Amorphous IZO films were annealed at 200 °C in air and N₂/H₂ and resistivity, Hall mobility, and carrier density along with molar volume change were monitored simultaneously as a function of time at temperature. We report that annealing the amorphous oxide in air at 200 °C does not lead to crystallization but does result in a 0.5% decrease in the amorphous phase molar volume and an associated drop in carrier density. Annealing in forming gas leads to an increase in carrier density and a small decrease in molar volume. We also report that when annealed in air at 500 °C, the amorphous IZO phase may crystallize either in the cubic bixbyite or in a recently observed rhombohedral phase.     

Temperature dependence of the microstructure and resistivity of indium zinc oxide films deposited by direct current magnetron reactive sputtering

Indium zinc oxide (IZO) films were deposited as a function of the deposition temperature using a sintered indium zinc oxide target (In₂O₃:ZnO = 90:10 wt.%) by direct current (DC) magnetron reactive sputtering method. The influence of the substrate temperature on the microstructure, surface roughness and electrical properties was studied. With increasing the temperature up to 200 °C, the characteristic properties of amorphous IZO films were improved and the specific resistivity was about 3.4 × 10^− 4 Ω cm. Change of structural properties according to the deposition temperature was also observed with X-ray diffraction patterns, transmission electron microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. IZO films deposited above 300 °C showed polycrystalline phases evolved on the amorphous IZO layer. Very flat surface roughness could be obtained at lower than 200 °C of the substrate temperature, while surface roughness of the films was increased due to the formation of grains over 300 °C. Consequently, high quality IZO films could be prepared by DC magnetron sputtering with O₂/Ar of 0.03 and deposition temperature in range of 150-200 °C; a specific resistivity of 3.4 × 10^− 4 Ω cm, and the values of peak to valley roughness and root-mean-square roughness are less than 4 nm and 0.5 nm, respectively.     

Fabrication of transparent conducting amorphous Zn-Sn-In-O thin films by direct current magnetron sputtering

Amorphous ZnO-SnO₂-In₂O₃ films were grown by direct current magnetron sputtering from vacuum hot pressed ceramic oxide targets of Zn:In:Sn cation ratios 1:2:1 and 1:2:1.5 onto glass substrates. X-ray diffraction analysis showed that the microstructure remained amorphous during annealing at 200 °C for up to 5 hours. By monitoring the electrical resistivity, oxygen content and substrate temperature were optimized during deposition. The optimal films were characterized by Hall Effect, work function and optical spectroscopy measurements. Films of 1:2:1 composition showed the lowest resistivity (7.6 × 10^− 4 Ω-cm), when deposited onto substrates preheated to 300 °C. Transmissivity of all films exceeded 80% in the visible spectral region. The energy gap was 3.52-3.74 eV, and the work function ranged 5.08-5.22 eV, suitable for cathode applications in organic light emitting diodes. Overall, the film characteristics were comparable or superior to those of amorphous tin-doped indium oxide and zinc-doped indium oxide films and may serve as viable, lower-cost alternatives.     

Advantages of using amorphous indium zinc oxide films for window layer in Cu(In,Ga)Se2 solar cells

The advantages of using indium zinc oxide (IZO) films instead of conventional Ga-doped zinc oxide (ZnO:Ga) films for Cu(In,Ga)Se₂ (CIGS) solar cells are described. The electrical properties of IZO are independent of film thickness. IZO films have higher mobility (30-40 cm²/Vs) and lower resistivity (4-5 × 10^− 4 Ω cm) compared to ZnO:Ga films deposited without intentional heating, because the number of grain boundaries in amorphous IZO films is small. The properties of a CIGS solar cell using IZO at the window layer were better than those obtained using a conventional ZnO:Ga at the window layer; moreover, the properties tended to be independent of thickness. These results indicate that use of IZO as a transparent conducting oxide layer is expected to increase the efficiency of CIGS solar cells.     

Electrical and optical properties of amorphous indium zinc oxide films

Valence electron control and electron transport mechanisms on the amorphous indium zinc oxide (IZO) films were investigated. The amorphous IZO films were deposited by dc magnetron sputtering using an oxide ceramic IZO target (89.3 wt.% In₂O₃ and 10.7 wt.% ZnO). N-type impurity dopings, such as Sn, Al or F, could not lead to the increase in carrier density in the IZO. Whereas, H₂ introduction into the IZO deposition process was confirmed to be effective to increase carrier density. By 30% H₂ introduction into the deposition process, carrier density increased from 3.08 × 10²⁰ to 7.65 × 10²⁰ cm^− 3, which must be originated in generations of oxygen vacancies or interstitial Zn²⁺ ions. Decrease in the transmittance in the near infrared region and increase in the optical band gap were observed with the H₂ introduction, which corresponded to the increase in carrier density. The lowest resistivity of 3.39 × 10^− 4 Ω cm was obtained by 10% H₂ introduction without substrate heating during the deposition.     

Electrical and optical properties of indium tin oxide films prepared on plastic substrates by radio frequency magnetron sputtering

The influence of deposition power, thickness and oxygen gas flow rate on electrical and optical properties of indium tin oxide (ITO) films deposited on flexible, transparent substrates, such as polycarbonate (PC) and metallocene cyclo-olefin copolymers (mCOC), at room temperature was studied. The ITO films were prepared by radio frequency magnetron sputtering with the target made by sintering a mixture of 90 wt.% of indium oxide (In₂O₃) and 10 wt.% of tin oxide (SnO₂). The results show that (1) average transmission in the visible range (400-700 nm) was about 85%-90%, and (2) ITO films deposited on glass, PC and mCOC at 100 W without supplying additional oxygen gas had optimum resistivity of 6.35 × 10⁻⁴ Ω-cm, 5.86 × 10⁻⁴ Ω-cm and 6.72 × 10⁻⁴ Ω-cm, respectively. In terms of both electrical and optical properties of indium tin oxide films, the optimum thickness was observed to be 150-300 nm.     

The effect of annealing on Al-doped ZnO films deposited by RF magnetron sputtering method for transparent electrodes

In this study, transparent conducting Al-doped zinc oxide (AZO) films with a thickness of 150 nm were prepared on Corning glass substrates by the RF magnetron sputtering with using a ZnO:Al (Al₂O₃: 2 wt.%) target at room temperature. This study investigated the effects of the post-annealing temperature and the annealing ambient on the structural, electrical and optical properties of the AZO films. The films were annealed at temperatures ranging from 300 to 500 °C in steps of 100 °C by using rapid thermal annealing equipment in oxygen. The thicknesses of the films were observed by field emission scanning electron microscopy (FE-SEM); their grain size was calculated from the X-ray diffraction (XRD) spectra using the Scherrer equation. XRD measurements showed the AZO films to be crystallized with strong (002) orientation as substrate temperature increases over 300 °C. Their electrical properties were investigated by using the Hall measurement and their transmittance was measured by UV-vis spectrometry. The AZO film annealed at the 500 °C in oxygen showed an electrical resistivity of 2.24 × 10^− 3 Ω cm and a very high transmittance of 93.5% which were decreased about one order and increased about 9.4%, respectively, compared with as-deposited AZO film.     

Transparent Ga and Zn co-doped In2O3 electrode prepared by co-sputtering of Ga:In2O3 and Zn:In2O3 targets at room temperature

This study examined the characteristics of Ga:In₂O₃ (IGO) co-sputtered Zn:In₂O₃ (IZO) films prepared by dual target direct current (DC) magnetron sputtering at room temperature in a pure Ar atmosphere for transparent electrodes in IGZO-based TFTs. Electrical, optical, structural and surface properties of Ga and Zn co-doped In₂O₃ (IGZO) electrodes were investigated as a function of IGO and IZO target DC power during the co-sputtering process. Unlike semiconducting InGaZnO₄ films, which were widely used as a channel layer in the oxide TFTs, the co-sputtered IGZO films showed a high transmittance (91.84%) and low resistivity (4.1 × 10^− 4 Ω cm) at optimized DC power of the IGO and IZO targets, due to low atomic percent of Ga and Zn elements. Furthermore, the IGO co-sputtered IZO films showed a very smooth and featureless surface and an amorphous structure regardless of the IGO and IZO DC power due to the room temperature sputtering process. This indicates that co-sputtered IGZO films are a promising S/D electrode in the IGZO-based TFTs due to their low resistivity, high transmittance and same elements with channel InGaZnO₄ layer.     

Composition, structural and electrical properties of thin films prepared by laser ablation of neodymium-doped potassium gadolinium tungstate

Thin films were grown on (001) SiO₂, SiO₂/(100) Si or (100) MgO substrates by laser ablation of neodymium-doped potassium gadolinium tungstate (Nd:KGW) single crystal target. The films were deposited at temperatures between room temperature and 750 °C and pressures between 1 × 10^− 4 Pa and 50 Pa of oxygen ambient. The influence of the deposition conditions on the composition, structure, morphology and electrical properties of the films was investigated. Special attention was paid to the films deposited in vacuum (1 × 10^− 4 Pa) or at very low oxygen pressures. Under such conditions, the potassium (K), gadolinium (Gd) and oxygen (O) content decreased strongly as the temperature was increased. At room temperature, the films were K and O stoichiometric, in contrast with Gd, which showed a concentration twice higher. The films were polycrystalline, with the exception of those deposited at temperatures below 500 °C, which were amorphous. However, all were smooth and dense. The films grown in vacuum and at temperatures between 500 and 700 °C consist mainly of “â-tungsten” - tungsten oxide (W₃O) phase. The films grown on SiO₂/Si possessed the best surface quality with nano-size relief. The resistivity measurements as a function of the temperature showed that the films produced in vacuum and at temperatures below 500 °C were highly insulating, whereas at 600 °C they exhibited semiconducting behavior or a metallic one at 700 °C. This behavior can be attributed to the existence of various valence states for tungsten below W⁶⁺ in the films and to their crystal structure.     

Properties of Ce-doped ITO films deposited on polymer substrate by DC magnetron sputtering

Ce-doped indium tin oxide (ITO:Ce) films were deposited on flexible polyimide substrates by DC magnetron sputtering using ITO targets containing various CeO₂ contents (CeO₂ : 0, 0.5, 3.0, 4.0, 6.0 wt.%) at room temperature and post-annealed at 200 °C. The crystallinity of the ITO films decreased with increasing Ce content, and it led to a decrease in surface roughness. In addition, a relatively small change in resistance in dynamic stress mode was obtained for ITO:Ce films even after the annealing at high temperature (200 °C). The minimum resistivity of the amorphous ITO:Ce films was 3.96 × 10^− 4 Ωcm, which was deposited using a 3.0 wt.% CeO₂ doped ITO target. The amorphous ITO:Ce films not only have comparable electrical properties to the polycrystalline films but also have a crystallization temperature > 200 °C. In addition, the amorphous ITO:Ce film showed stable mechanical properties in the bended state.     

Growth and vacuum post-annealing effect on the properties of the new absorber CuSbS2 thin films

Post-growth treatments in vacuum atmosphere were performed on CuSbS₂ films prepared by the single-source thermal evaporation method on glass substrates. The films were annealed in vacuum atmosphere for 2 h in temperature range 130-200 °C. The effect of this thermal treatment on the structural, optical and electrical properties of the films was studied. X-ray diffraction (XRD) patterns indicated that the films exhibited an amorphous structure for annealing temperature below 200 °C and a polycrystalline structure with CuSbS₂ principal phase. For the films annealed at temperatures below 200 °C one direct optical transition in range 1.8-2 eV was found. For the films annealed at 200 °C, two optical direct transitions emerged at 1.3 and 1.79 eV corresponding to the CuSbS₂ and Sb₂S₃ values respectively. The electrical measurements showed a conversion from low resistivities (3.10^− 2-9.10^− 2) Ω cm for the samples annealed at temperatures below 200 °C to relatively high resistivities (2 Ω cm) for the samples annealed at 200 °C. In all cases the samples exhibited p-Ztype conductivity.     

Effect of crystallographic orientations on electrical properties of sputter-deposited nickel oxide thin films

Nickel oxide thin films of various preferred orientations were deposited by radio-frequency (RF) magnetron sputtering process in different gas ratios of oxygen atmosphere at RF power 200 W on unheated and heated for (673 K) substrates. The relationships among substrate temperature, preferred orientation and electrical properties of the NiO films were investigated. The resulting films were analyzed by grazing-incidence X-ray diffraction, high-resolution transmission electron microscopy (HR-TEM), and ultrahigh resolution scanning electron microscopy (HR-SEM). The electrical properties were measured using four probe and Hall effects measurements. The results show that films deposited at room temperature with the ratio of oxygen varying from 0 to 100% develop a (111) preferred orientation. At temperature of 673 K, while the (111)-orientated film was obtained under a low ratio of oxygen (<50% O₂), a (200) preferred orientation was developed under 100% oxygen. The lowest sheet resistance 0.01 MΩ/□, resistivity 0.83 Ω-cm and higher carrier density 7.35 × 10¹⁸ cm⁻³ could be obtained on (111) preferred orientation samples prepared on unheated substrates in pure oxygen atmosphere. The relationship between preferred orientation and electrical properties was proposed in this paper.     

Effects of oxygen gas flow rate and ion beam plasma conditions on the opto-electronic properties of indium molybdenum oxide films fabricated by ion beam-assisted evaporation

The purpose of the present work is to experimentally study the effects of the oxygen gas flow rate and ion beam plasma conditions on the properties of indium molybdenum oxide (IMO) films deposited onto the polyethersulfone (PES) substrate. Crystal structure, surface morphology, and optoelectronic properties of IMO films are examined as a function of oxygen gas flow rate and ion beam discharge voltage.Experimental results show that the IMO films consist of a cubic bixbyite B-In₂O₃ single phase with its crystal preferred orientation alone B(222). Mo⁶⁺ ions are therefore considered to partially substitute In³⁺ sites in the deposit. Under-controlled ion bombardment during deposition enhances the reaction among those arriving oxygen and metal ion species to condense into IMO film and facilitates a decreased surface roughness of IMO film. The film with ultimate crystallinity and the lowest surface roughness is obtained when the oxygen flow rate of 3 sccm and the discharge voltage of 110 V are employed. This results in the lowest electrical resistivity due mainly to the increased Hall mobility and irrelevant to carrier concentration. The lowest electrical resistivity of 8.63 × 10^− 4 ohm-cm with a 84.63% transmittance at a wavelength of 550 nm can be obtained, which satisfies the requirement of a flexible transparent conductive polymer substrate.     

Dielectric properties of DC reactive magnetron sputtered Al2O3 thin films

Alumina (Al₂O₃) thin films were sputter deposited over well-cleaned glass and Si < 100 > substrates by DC reactive magnetron sputtering under various oxygen gas pressures and sputtering powers. The composition of the films was analyzed by X-ray photoelectron spectroscopy and an optimal O/Al atomic ratio of 1.59 was obtained at a reactive gas pressure of 0.03 Pa and sputtering power of 70 W. X-ray diffraction results revealed that the films were amorphous until 550 °C. The surface morphology of the films was studied using scanning electron microscopy and the as-deposited films were found to be smooth. The topography of the as-deposited and annealed films was analyzed by atomic force microscopy and a progressive increase in the rms roughness of the films from 3.2 nm to 4.53 nm was also observed with increase in the annealing temperature. Al-Al₂O₃-Al thin film capacitors were then fabricated on glass substrates to study the effect of temperature and frequency on the dielectric property of the films. Temperature coefficient of capacitance, AC conductivity and activation energy were determined and the results are discussed.     

Electrical, optical, and structural properties of InZnSnO electrode films grown by unbalanced radio frequency magnetron sputtering

We have investigated the electrical, optical, structural, and annealing properties of indium zinc tin oxide (IZTO) films prepared by an unbalanced radio frequency (RF) magnetron sputtering at room temperature, in a pure Ar ambient environment. It was found that the electrical and optical properties of unbalanced RF sputter grown IZTO films at room temperature were influenced by RF power and working pressure. At optimized growth condition, we could obtain the IZTO film with the low resistivity of 3.77 × 10^− 4 Ω cm, high transparency of ~ 87% and figure of merit value of 21.2 × 10^− 3Ω^− 1, without the post annealing process, even though it was completely an amorphous structure due to low substrate temperature. In addition, the field emission scanning electron microscope analysis results showed that all IZTO films are amorphous structures with very smooth surfaces regardless of the RF power and working pressure. However, the rapid thermal annealing process above the temperature of 400 °C lead to an abrupt increase in resistivity and sheet resistance due to the transition of film structure from amorphous to crystalline, which was confirmed by X-ray diffraction examination.     

Polycrystalline films of tungsten-doped indium oxide prepared by d.c. magnetron sputtering

Electrical and optical properties of polycrystalline films of W-doped indium oxide (IWO) were investigated. These films were deposited on glass substrate at 300 °C by d.c. magnetron sputtering using ceramic targets. The W-doping in the sputter-deposited indium oxide film effectively increased the carrier density and the mobility and decreased the resistivity. A minimum resistivity of 1.8 × 10^− 4 Ω cm was obtained at 3.3 at.% W-doping using the In₂O₃ ceramic targets containing 7.0 wt.% WO₃. The 2.2 at.% W-doped films obtained from the targets containing 5.0 wt.% WO₃, showed the high Hall mobility of 73 cm² V^− 1 s^− 1 and relatively low carrier density of 2.9 × 10²⁰ cm^− 3. Such properties resulted in novel characteristics of both low resistivity (3.0 × 10^− 4 Ω cm) and high transmittance in the near-infrared region.     

Low temperature amorphous growth of semiconducting Y-Ba-Cu-O oxide thin films in view of infrared bolometric detection

YBa₂Cu₃O_6 + x (YBCO) compounds are well known to exhibit superconducting properties for x > 0.5 and semiconducting properties for lower oxygen content. In this work, YBCO oxide thin films of the semiconducting phase were deposited by direct-current (DC) hollow cathode sputtering at low temperature in the 100 to 400 °C range. Structural, electrical and optical properties are investigated and discussed in relation with the envisaged bolometric detection application. Structural characterizations show that films are amorphous, with a granular structure of low roughness (3 nm rms). DC electrical measurements both reveal that films grown at 100 °C exhibit a high temperature coefficient of resistance (TCR ~ − 3% K^− 1 to − 4% K^− 1 at 300 K) and an optimized low resistivity value of 345 Ω·cm at 300 K. Consequently, this material is suitable for uncooled infrared bolometer application and can be deposited at 100 °C in a complementary metal-oxide-semiconductor compatible technological process for co-integration with readout circuitry. In addition, optical measurements performed in the 0.5 to 2.2 μm wavelength range on films grown at 100 °C highlight optical conductivity values in line with those expected for YBCO material, as well as the presence of two optical band gaps that are discussed with respect to the film nanostructure.     

Structural, optical and electrical properties of In doped CdO thin films for optoelectronic applications

Thin films of indium doped cadmium oxide were deposited on quartz substrate using pulsed laser deposition technique. The effect of growth temperature and partial oxygen pressure on structural, optical and electrical properties was studied. We find that the optical transparency of the films largely depends on the growth temperature, while partial oxygen pressure has virtually no effect on the transparency of the films. Electrical properties are found to be sensitive to both the growth temperature and oxygen pressure. It is observed that conductivity and carrier concentration decreases with temperature. The film grown at 200 °C under an oxygen pressure of 5.0 × 10^− 4 mbar shows high mobility (155 cm²/V s), high carrier concentration (1.41 × 10²¹ cm³), and low resistivity (2.86 × 10^− 5 Ω cm).     

Thin film fabrication of nano-porous 12CaO·7Al2O3 crystal and its conversion into transparent conductive films by light illumination

Thin film fabrication of crystalline 12CaO·7Al₂O₃ (C12A7) with zeolitic structure was examined, and their electrical and optical properties were measured. Polycrystalline thin films were prepared by post-annealing of amorphous films in oxygen atmosphere at temperatures above 800 °C. Choice of substrates was crucial for obtaining single-phase thin films. Although various oxide substrates (single crystals of Al₂O₃, Y-stabilized ZrO₂, MgO and silica glass) were examined, single-phase films were obtained only for MgO substrates and the other substrates reacted with the CaO component in the films during post-annealing. The optical band gap of C12A7 was evaluated to be 5.9 eV. Hydride ions were incorporated into the film by a thermal treatment in a hydrogen atmosphere at 1200 °C. The resulting transparent thin films were converted into transparent persistent electronic conductors exhibiting an electrical conductivity 6.2×10⁻¹ S cm⁻¹ at 300 K by ultraviolet light illumination. This is the first example of transparent conductive thin film in which conductive areas can be patterned directly by light.     

Low-resistance and highly transparent Ag/IZO ohmic contact to p-type GaN

The electrical, structural, and optical characteristics of Ag/ZnO-doped In₂O₃ (IZO) ohmic contacts to p-type GaN:Mg (2.5 × 10¹⁷ cm^− 3) were investigated. The Ag and IZO (10 nm/50 nm) layers were prepared by thermal evaporation and linear facing target sputtering, respectively. Although the as-deposited and 400 °C annealed samples showed rectifying behavior, the 500 and 600 °C annealed samples showed linear I-V characteristics indicative of the formation of an ohmic contact. The annealing of the contact at 600 °C for 3 min in a vacuum (~ 10^− 3 Torr) resulted in the lowest specific contact resistivity of 1.8 × 10^− 4 Ω·cm² and high transparency of 78% at a wavelength of 470 nm. Using Auger electron spectroscopy, depth profiling and synchrotron X-ray scattering analysis, we suggested a possible mechanism to explain the annealing dependence of the electrical properties of the Ag/IZO contacts.