Structural, electrical and optical properties of Ga-doped ZnO films on cyclo-olefin polymer substrates

Ga-doped ZnO (GZO) films with a thickness of 100 nm were prepared on cyclo-olefin polymer (COP) and glass substrates at various temperatures below 100 °C by ion plating with direct-current arc discharge. The dependences of the characteristics of GZO films on the substrate temperature Ts were investigated. All the polycrystalline GZO films, which exhibited a high average visible transmittance of greater than 86%, were crystallized with a wurtzite structure oriented along the c-axis. The lowest resistivities of the GZO films were 5.3 × 10^− 4 Ωcm on the glass substrate and 5.9 × 10^− 4 Ωcm on the COP substrate.     

Structural, electrical and moisture resistance properties of Ga-doped ZnO films

The structural, electrical and moisture resistance properties of Ga-doped ZnO (GZO) films with 200 nm thickness in terms of their dependence on oxygen gas flow rate (fO₂) during deposition were studied. GZO films are deposited on glass substrates by ion plating with DC arc discharge. After a reliability test at a temperature of 60 °C and a relative humidity of 95% for 500 h, the percentage of resistivity change of GZO films decreased from 16–20% to 3–11% with increasing fO₂ from 6–12 to 14–25 sccm. The minimum percentage of the resistivity change was observed in the GZO films deposited at fO₂ of 21 sccm and the resistivity after the reliability test was 3.5 × 10^{− 4} Ω cm. The effects of the intrinsic defects on the percentage of resistivity change are discussed on the basis of electrical and optical characteristics of GZO films.     

Thermally stable transparent conducting and highly infrared reflective Ga-doped ZnO thin films by metal organic chemical vapor deposition

Highly transparent conductive Ga-doped ZnO (GZO) thin films have been prepared on glass substrates by metal organic chemical vapor deposition. The effect of Ga doping on the structural, electrical and optical properties of GZO films has been systematically investigated. Under the optimum Ga doping concentration (∼4.9 at.%), c-axis textured GZO film with the lowest resistivity of 3.6 × 10⁻⁴ Ω cm and high visible transmittance of 90% has been achieved. The film also exhibits low transmittance (<1% at 2500 nm) and high reflectance (>70% at 2500 nm) to the infrared radiation. Furthermore, our developed GZO thin film can well retain the highly transparent conductive performance in oxidation ambient at elevated temperature (up to 500 °C).     

Influence of thermal annealing on electrical and optical properties of Ga-doped ZnO thin films

Influence of thermal annealing on electrical properties of GZO films has been studied by means of Hall effect measurements and optical characterization based on Drude model analysis for transmission and reflection spectra. Electrical resistivity increased with increasing annealing temperature. Changes of electrical properties were compared between air and N₂ gas atmosphere. Thermal stability in the air was worse compared to the N₂ gas atmosphere. Annealing at rather high temperature caused decrease in the Hall mobility and increase in optical mobility. The difference between the Hall mobility and the optical mobility was attributed to carrier scattering at grain boundaries. Three kinds of deposition method, ion plating using DC arc discharge, DC magnetron sputtering, and RF power superimposed DC magnetron sputtering were compared in terms of the thermal stability.     

Thermal stability of carrier centers in various types of transparent conductive ZnO and Ga-doped ZnO films

We investigated the thermal stability of the transparent conductive properties of undoped ZnO and Ga-doped ZnO (GZO) films when they were annealed in a high vacuum with stepwise increasing temperature. The ZnO samples included V_O-rich and Zn-rich ZnO films; the primary donors were respectively oxygen vacancies (V_O) or Zn atoms highly unsaturated with oxygen atoms. V_O-rich ZnO was the most unstable against annealing; resistivity initially within the 10⁻³ Ω cm range diverged higher than 10 Ω cm when a critical temperature was exceeded. The critical temperature between 350 and 450 °C depended on the film thickness, which indicated that V_O's were diminished through recombination with migrating interstitial oxygen atoms. In contrast, Zn-rich ZnO films remained highly conductive up to 550 °C. They became more and more transparent and their crystallinity improved at higher annealing temperatures, which was the consequence of metallic-like Zn atoms being removed through desorption from the surface or being accommodated into the crystalline lattice. Comparatively, GZO films were more robust against annealing with their resistivities remaining unchanged up to 350 °C.     

Effect of thermal annealing on electrical properties of transparent conductive Ga-doped ZnO films prepared by ion-plating using direct-current arc discharge

The effect of thermal annealing on the electrical properties of highly transparent conductive Ga-doped ZnO (GZO) films deposited on glass substrates at 200 °C by an ion-plating deposition was investigated. GZO films were annealed in the temperature range from 200 to 600 °C for 30 min under the atmospheric pressure of high-purity N₂ gas. Up to 300 °C, GZO films were electrically very stable, and there was little change in resistivity. When the annealing temperature exceeded 400 °C, resistivity increased rapidly, originating from an abrupt decrease in carrier concentration. It was suggested to be due to both desorption of Zn from GZO films and grain boundary segregation of Ga dopants.     

Microstructural and optical properties of Ga-doped ZnO semiconductor thin films prepared by sol-gel process

Transparent thin films of Ga-doped ZnO (GZO), with Ga dopant levels that varied from 0 to 7 at.%, were deposited onto alkali-free glass substrates by a sol-gel process. Each spin-coated film was preheated at 300 °C for 10 min, and then annealed at 500 °C for 1 h under air ambiance. The effects of Ga dopant concentrations on crystallinity levels, microstructures, optical properties, and electrical resistivities of these ZnO thin films were systematically investigated. Photoluminescence spectra of GZO thin films were examined at room temperature. XRD results revealed that the undoped ZnO thin films exhibited a preferred orientation along the (002) plane and that the ZnO thin films doped with Ga showed degraded crystallinity. Experimental results also showed that Ga doping of ZnO thin films could markedly decrease surface roughness, improve transparency in the visible range, and produce finer microstructures than those of undoped ZnO thin films. The most promising films for transparent thin film transistor (TTFT) application produced in this study, were the 3 and 5 at.% Ga-doped ZnO thin films, both of which exhibited an average transmittance of 90.6% and an RMS roughness value of about 2.0 nm.     

Anode material properties of Ga-doped ZnO thin films by pulsed DC magnetron sputtering method for organic light emitting diodes

This study examined the anode material properties of Ga-doped zinc oxide (GZO) thin films deposited by pulsed DC magnetron sputtering along with the device performance of organic light emitting diodes (OLEDs) using GZO as the anode. The structure and electrical properties of the deposited films were examined as a function of the substrate temperature. The electrical properties of the GZO film deposited at 200 °C showed the best properties, such as a low resistivity, high mobility and high work function of 5.3 × 10^− 4Ω cm, 9.9 cm²/Vs and 4.37 eV, respectively. The OLED characteristics with the GZO film deposited under the optimum conditions showed good brightness > 10,000 cd/m². These results suggest that GZO films can be used as the anode in OLEDs, and a lower deposition temperature of 200 °C is suitable for flexible devices.     

Electrical and optical properties of Al-doped ZnO films deposited by hollow cathode gas flow sputtering

Al-doped ZnO (AZO) films were deposited on glass by hollow cathode gas flow sputtering using Zn-Al alloy targets. Sputtering power for all the depositions was fixed at 1500 W. Resistivities of 0.81-1.1 × 10^− 3 Ω cm were obtained for AZO films deposited at room temperature with an O₂ flow from 38 to 50 standard cubic centimetre/minute (SCCM), while static deposition rates were almost constant at 270-300 nm/min. On the other hand, lower resistivities of 5.2-6.4 × 10^− 4 Ω cm were obtained for AZO films deposited at 200 °C with an O₂ flow from 25 to 50 SCCM, while the static deposition rates were almost constant at 200-220 nm/min. Average transmittances in the visible light region were above 80% for both sets of films.     

Highly transparent conductive and near-infrared reflective ZnO:Al thin films

Al-doped ZnO (AZO) thin films have been prepared on glass substrates by pulsed laser deposition. The structural, optical, and electrical properties were strongly dependent on the growth temperatures. The lowest resistivity of 4.5 × 10⁻⁴ Ωcm was obtained at an optimized temperature of 350 °C. The AZO films deposited at 350 °C also had the high optical transmittance above 87% in the visible range and the low transmittance (<15% at 1500 nm) and high reflectance (∼50% at 2000 nm) in the near-IR region. The good IR-reflective properties of ZnO:Al films show that they are promising for near-IR reflecting mirrors and heat reflectors.     

Investigation on structural, electrical and optical properties of tungsten-doped tin oxide thin films

Tungsten-doped tin oxide (SnO₂:W) transparent conductive films were prepared on quartz substrates by pulsed plasma deposition method with a post-annealing. The structure, chemical states, electrical and optical properties of the films have been investigated with tungsten-doping content and annealing temperature. The lowest resistivity of 6.67 × 10^− 4 Ω cm was obtained, with carrier mobility of 65 cm² V^− 1 s^− 1 and carrier concentration of 1.44 × 10²⁰ cm^− 3 in 3 wt.% tungsten-doping films annealed at 800 °C in air. The average optical transmittance achieves 86% in the visible region, and approximately 85% in near-infrared region, with the optical band gap ranging from 4.05 eV to 4.22 eV.     

Effect of sublayer surface treatments on ZnO transparent conductive oxides using dc magnetron sputtering

Novel sublayer surface treatments were investigated to improve the conductivity of aluminum-doped zinc oxide (ZnO:Al) fabricated by using dc magnetron sputtering on a glass substrate. Introducing artificial minute flaws on the surface of glass substrates enhanced the crystallinity of ZnO:Al films and decreased the resistivity accompanying the increase of electron mobility. Combination of the surface treatment and sputter beam control, i.e., interruption of high-energy oxygen with shadow masks, further reduced the resistivity of the film to 3.7 × 10^− 4 Ω cm (thickness 70 nm).     

Effect of target properties on transparent conducting impurity-doped ZnO thin films deposited by DC magnetron sputtering

For the purpose of using transparent conducting impurity-doped ZnO thin films in liquid crystal display (LCD) applications, the relationship between the properties of dc magnetron sputtering (dc-MS) deposited thin films and the properties of the oxide targets used to produce them is investigated. Both Al-doped and Ga-doped ZnO (AZO and GZO) thin films were deposited on glass substrates using a dc-MS apparatus with various high-density sintered AZO or GZO disk targets (diameter of about 150 mm); the target and substrate were both fixed during the depositions. Using targets with a lower resistivity results in attaining more highly stable dc-MS depositions with higher deposition rates and lower arcing. In addition, dc-MS depositions using targets with a lower resistivity produced improvements in resistivity distribution on the substrate surface. It was found that the oxygen content in deposited thin films decreased as the oxygen content of the target used in the deposition was decreased. As a result, the dc-MS deposition of transparent conducting impurity-doped ZnO thin films suitable for LCD applications requires the preparation of significantly reduced AZO and GZO targets with low oxygen content.     

Effect of the substrate temperature on the properties of Ga-doped ZnO films for photovoltaic cell applications deposited by a pulsed DC magnetron sputtering with a rotating cylindrical target

Effect of substrate temperature on the properties of Ga-doped ZnO (GZO) films was investigated by pulsed DC magnetron sputtering with a rotating cylindrical target with an aim to establish suitable process conditions for their photovoltaic (PV) cell applications. Without formation of undesirable secondary oxide phases such as Ga₂O₃ and ZnGa₂O₄, the GZO film having mixed orientation at lower deposition temperature evolved into the c-axis oriented one with increasing deposition temperature to 230 °C, which accompanied morphological evolution to vertically oriented dense columnar structure and improved doping efficiency. Correlated with this, crater-like surface texturing was possible only on the sample deposited at 230 °C. Electrical resistivity and diffuse surface reflectance over the spectral range of 200-1200 nm of this GZO film after surface texturing were 8.73 × 10⁻⁴ Ω cm and 3.32%, respectively, indicating that the film has application potential as anti-reflection coating and front electrode of PV cells. Morphological features, surface texturing behavior, electrical and optical properties of the GZO films in this study suggest that this novel technique would be applicable to the fabrication of anti-reflection coating and front electrode of PV cells only when substrate temperature is sufficiently high.     

Preparation and electrical properties of Ga-doped ZnO nanoparticles by a polymer pyrolysis method

In this article, preparation of Ga-doped zinc oxide (GZO) nanoparticles by a polymer pyrolysis method is reported. The pyrolysis behaviors of the polymer precursors prepared via the in situ polymerization of metal salts and acrylic acid are analyzed using thermalgravity-differential scanning calorimetry (TG-DSC) techniques. Then, the structural characteristics of the products are studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is revealed by the results that the GZO nanoparticles calcined at 600 °C show good crystallinity with the wurtzite structure. GZO nanoparticles doped with 4 mol% Ga have a mean particle size of 26 nm with spherical-like morphology. Electrical resistivity measurement shows that, before and after high temperature annealing under H₂ atmosphere, the resistivity of the GZO nanoparticles is decreased by one and four orders in magnitude, respectively, compared with the pure ZnO nanoparticles. In addition, due to its versatility, this in situ polymer pyrolysis method can be easily extended to synthesis of other n-type doped semiconductor, such as In and Al doped ZnO or Sb doped SnO₂.     

Deposition of low-resistivity gallium-doped zinc oxide films by low-temperature radio-frequency magnetron sputtering

The transparent and conductive gallium-doped zinc oxide (GZO) film was deposited on 1737F Corning glass using the radio-frequency (RF) magnetron sputtering system with a GZO ceramic target. (The Ga₂O₃ contents are approximately 5 wt. %). In this study, the effect of the sputtering pressure on the structural, optical and electrical properties of GZO films upon the glass or polyester film (PET) substrate was investigated and discussed in detail. The GZO film was grown under a steady RF power of 400 W and a lower substrate temperature from room temperature up to 200 °C. The crystal structure and orientation of GZO thin films were examined by X-ray diffraction. All of the GZO films under various sputtering pressures had strong c-axis (002)-preferred orientation. Optical transparency was high (> 80%) over a wide spectral range from 380 nm to 900 nm. According to the experimental data, the resistivity of a single-layered GZO film was optimized at  8.3 × 10^− 4 Ω cm and significantly influenced by the sputtering pressure. In further research, the sandwich structure of the GZO film/Au metal/GZO film was demonstrated to improve the electrical properties of the single-layered GZO film. The resistivity of the sandwich-structured GZO film was around 2.8 × 10^− 4 Ω cm.     

Comparison of ZnO:Al,ITO and carbon nanotube transparent conductive layers in flexible solar cells applications

In the paper the mechanical, optical and electrical parameters of transparent conductive layers (TCLs) made of carbon nanotubes and metal conductive oxides are explored and compared. All investigated materials are deposited on transparent, flexible polymer foils used for solar cell applications. Obtained results are compared with available parameters of rigid transparent conductive oxides (TCOs) as well as literature reports about Indium–Tin Oxide (ITO) on flexible substrates. Presented paper is a report from the preliminary stage of a new flexible solar cell construction.     

Effect of annealing treatment on structural, electrical, and optical properties of Ga-doped ZnO thin films deposited by RF magnetron sputtering

The effect of annealing on structural, electrical, and optical properties of Ga-doped ZnO (GZO) films prepared by RF magnetron sputtering was investigated in air and nitrogen. GZO films are polycrystalline with a preferred 002 orientation. The resistivities of annealed films are larger than the as-deposited. The transmittance in the near IR region increases greatly and the optical band gap decreases after annealing. The photoluminescence spectra is composed of a near band edge emission and several deep level emissions (DLE) which are dominated by a blue emission. After annealing, these DLEs are enhanced evidently.     

Study on the structural, electrical, optical, adhesive properties and stability of Ga-doped ZnO transparent conductive films deposited on polymer substrates at room temperature

Flexible optoelectronic devices are attractive because of light weight, small volume, flexibility and easy transport. Transparent conductive oxide thin films deposited on polymer substrates could satisfy the flexibility for optoelectronic devices. Ga-doped ZnO (GZO) films have been prepared on polycarbonate substrates by radio frequency magnetron sputtering at room temperature. The dependence of the structural, electrical, optical and adhesive properties for films on the sputtering powers was investigated. We also investigated the stability of the electrical property through doing Hall-effect measurements 18 months later. The lowest sheet resistance was 5.8 Ω/sq. After 18 months, the lowest sheet resistance was 6.5 Ω/sq. The stability of the electrical property is excellent. The average transmittance in the visible region of all the films was as high as 85 %, using air as reference. The good transparency-conducting property, excellent stability and room-temperature deposition on polymeric substrates enable GZO films to be widely used in optoelectronic devices.     

Thermally stable, highly conductive, and transparent Ga-doped ZnO thin films

Highly conductive and transparent films of Ga-doped ZnO (GZO) have been prepared by pulsed laser deposition using a ZnO target with Ga₂O₃ dopant of 3 wt.% in content added. Films with resistivity as low as 3.3 × 10^− 4 Ω cm and transmittance above 80% at the wavelength between 400 and 800 nm can be produced on glass substrate at room temperature. It is shown that a stable resistivity for use in oxidation ambient at high temperature can be attained for the films. The electrical and optical properties, as well as the thermal stability of resistivity, of GZO films were comparable to those of undoped ZnO films.