Gallium-doped Zinc Oxide Films Research Articles

Gallium and indium co-doping of epitaxial zinc oxide thin films grown in water at 90 °C

Zinc oxide (ZnO) thin films were intentionally co-doped with group III elements (gallium) in order to investigate and understand the effects of co-doping on the morphological, electrical and optical properties of gallium-doped ZnO (GZO) films. The co-doped films were grown on MgAl2O4 spinel substrates using a low-temperature solution-phase method known as hydrothermal synthesis. Gallium with indium co-doped ZnO (GIZO) films displayed a dramatic improvement in surface morphology as compared with the Ga-doped ZnO (GZO) films due to the compensation effect of gallium and indium doping which reduced the lattice strain. The 0.0033M gallium with 3.3 × 10−4M indium co-doped film exhibited an electron concentration of 3.14 × 1020 cm−3 and resistivity of 7.4 × 10−4 Ω cm which were both enhancements of 1.5 times over the GZO film. These films were comparable to the films fabricated by more expensive and complicated vapour-phase methods. The figure of merit for this film was determined to be 1.63 × 10−2 sq/Ω which was very close to the indium tin oxide conducting films currently used commercially. Finally, the GIZO film was hydrothermally grown on a p-GaN film to form an n-ZnO/p-GaN heterojunction light-emitting diode (LED). This LED showed diode I–V characteristics and exhibited strong cool-white light emission which signified the prospect of using GIZO as an effective and low-cost n-type layer in LEDs.

A Photochemically Induced Molecular Pathway to Smooth and Transparent Gallium-Doped Zinc Oxide Ceramic

The photochemical conversion of the zinc oximato complex of di-aqua-bis[2-(methoxyimino)propanoatolzinc 1 into zinc oxide thin films is introduced as an alternative route compared with the traditional thermal process. Irradiation of 1 by UV light produces smoother films with random orientation of crystalline grains, compared with the thermal route, which results in films with a high degree of (002) orientation (texture) toward the substrate. By addition of Ga(NO 3 ) 3 , gallium-doped zinc oxide (GZO) thin films can be obtained after further sintering, e.g., in air. The GZO films exhibit a low surface roughness and a high optical transparency. Electrical resistivities as low as 7.4 x 10 -2 Ω · cm were observed, making it a candidate for application in the field of transparent conduction oxides.

Atomic Layer Deposition of Gallium-Doped Zinc Oxide Transparent Conducting Oxide films

ABSTRACTThin transparent conducting oxide (TCO) films of gallium-doped zinc oxide have been deposited on glass substrates by atomic layer deposition (ALD) using diethyl zinc, triethyl gallium and water vapour as precursors. The gallium-doped zinc oxide films were deposited over the temperature range 100-350°C. Transmission electron microscopy reveals that the as-deposited films are polycrystalline in character. The electrical resistivity of the gallium-doped zinc oxide films was evaluated using four-point probe and contactless measurement methods as a function of film thickness. The lowest sheet resistance of 16 Ω/☐ was measured from a film thickness of 400nm and a gallium content of 5 atomic percent. The electron Hall mobility of this film was 12.3 cm2/Vs. The visible transmittance of the films was 78% with a haze of 0.2%.

Structure and properties of GZO thin films grown on ZnO buffer layers

Thin gallium-doped zinc oxide (in GZO the Ga 2O 3 contents are approximately 3 wt%) films having different ZnO buffer layers were deposited using radio frequency (rf) magnetron sputtering. The use of a grey-based Taguchi method to determine the processing parameters of ZnO buffer layer deposition has been studied by considering multiple performance characteristics. A Taguchi method with an L 9 orthogonal array, signal-to-noise (S/N) ratio, and analysis of variance (ANOVA) is employed to investigate the performance characteristics in the deposition operations. The effect and optimization of ZnO buffer deposition parameters (rf power, sputtering pressure, thickness, and annealing) on the structure, morphology, electrical resistivity, and optical transmittance of the GZO films are studied. Annealing treatment and reduction in thickness resulted in a decrease in root-mean-square (RMS) surface roughness of the ZnO buffer layer. Using the optimal ZnO buffer layer obtained by the application of the grey-based Taguchi method, the electrical resistivity of GZO films was decreased from 2.94×10 −3 to 9.44×10 −4 Ω cm and the optical transmittance in the visible region was slightly increased from 84.81% to 85.82%.

Transparent Conducting ZnO:Ga Epitaxial Films Prepared on Epi-GaN/Al<sub>2</sub>O<sub>3</sub> (0001) by MOCVD

High-quality Gallium-doped zinc oxide (ZnO:Ga) films have been prepared on epi-GaN/sapphire (0001) substrates by the metalorganic chemical vapour deposition (MOCVD) method. The relative amount of gallium doping was varied from 0 to 8% (atomic ratio). The structural, electrical and optical properties of the ZnO:Ga films have been investigated in detail, as a function of Ga content. All the prepared samples have the wurtzite structure of pure ZnO with a strong (0002) preferred orientation. The microstructure for the surface of films was markedly influenced by the amount of Ga doping. The resistivity decreases continuously with adding Ga content and reaches to the value of 8.4×10-3 Ω•cm at 8%. The average transmittance for the deposited ZnO:Ga samples in the visible range was over 75%.

X-ray images obtained from cold cathodes using carbon nanotubes coated with gallium-doped zinc oxide thin films

X-ray imaging data obtained from cold cathodes using gallium-doped zinc oxide (GZO)-coated CNT emitters are presented. Multi-walled CNTs were directly grown on conical-type (250 μm-diameter) tungsten-tip substrates at 700 °C via inductively coupled plasma-chemical vapor deposition (ICP-CVD). GZO films were deposited on the grown CNTs at room temperature using a pulsed laser deposition (PLD) technique. Field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) were used to monitor the variations in the morphology and microstructure of the CNTs before and after GZO coating. The formation of the GZO layers on the CNTs was confirmed using energy-dispersive X-ray spectroscopy (EDX). The CNT-emitter that was coated with a 10-nm-thick GZO film displayed an excellent performance, such as a maximum emission current of 258 μA (at an applied field of 4 V/μm) and a threshold field of 2.20 V/μm (at an emission current of 1.0 μA). The electric-field emission characteristics of the GZO-coated CNT emitter and of the pristine (i.e., non-coated) CNT emitter were compared, and the images from an X-ray system were obtained by using the GZO-coated CNT emitter as the cold cathode for X-ray generation.

Solution epitaxy of gallium-doped ZnO on p-GaN for heterojunction light-emitting diodes

We report white light emission from a Ga-doped ZnO/p-GaN heterojunction light-emitting diode which was fabricated by growing gallium-doped ZnO film on the p-GaN in water at 90°C. As determined from Ga-doped ZnO films grown on (111) oriented MgAl2O4 spinel single crystal substrates, thermal treatment at 600°C in nitrogen ambient leads to a carrier concentration of 3.1×1020 cm−3 (and carrier mobility of 28 cm2/Vs) which is two orders of magnitude higher than that of the undoped films. Electroluminescence emissions at wavelengths of 393 nm (3.155 eV) and 529.5 nm (2.4 eV) were observed under forward bias in the heterojunction diode and white light could be visibly observed. The high concentration of electrons supplied from the Ga-doped ZnO films helped to enhance the carrier recombination and increase the light-emitting efficiency of the heterojunction diode.

Effect of deposition distance and temperature on electrical, optical and structural properties of radio-frequency magnetron-sputtered gallium-doped zinc oxide

Films of gallium-doped zinc oxide (GZO) were deposited on glass substrates by radio-frequency magnetron sputtering using a ceramic target of Ga:ZnO (4 at.% Ga vs. Zn). Both the substrate temperature ( T s) and the target–substrate distance ( d ts) were varied and the effect on electrical, optical and structural properties of the resulting films were measured. The highest conductivity of 3200 S/cm was obtained at a deposition temperature of 250 °C, at a d ts of 51 mm. This sample had the highest carrier concentration in this study, 9.6 × 10 20/cm 3. Optical transmittance of all films was <90% in the visible range. The grain size of the film grown at d ts = 51 mm was smaller than the grain size for films grown with a shorter d ts; moreover, the films with d ts = 51 mm exhibited the smoothest surface, with a root mean square surface roughness of 2.7 nm. Changes in T s have a more pronounced effect on conductivity compared to changes in d ts; however, variations in structure do not appear to be well-correlated with conductivity for samples in the 2000–3200 S/cm range. These results suggest that incorporation and activation of Ga is of key importance when attempting to obtain GZO films with conductivities greater than 2000 S/cm.

Effect of substrate temperature on the structural, electrical and optical properties of ZnO:Ga thin films prepared by RF magnetron sputtering

Gallium doped zinc oxide (GZO) thin films were prepared by RF magnetron sputtering on glass substrates. The influence of substrate temperature ( T s) on structure, surface morphologies, chemical atomic compositions, electrical and optical properties was investigated by XRD, FE-SEM, XPS, Hall measurement and double-beam UV spectrophotometer, respectively. It is found that with increasing T s, film crystallinity is improved, stress in the GZO films is obviously relaxed and concentrations of Ga atoms and oxygen vacancies increase. The lowest resistivity (1.41×10 −3 Ω cm), and the highest figure of merit (1.94×10 −2 sq Ω −1) were obtained at 300 °C. The relationship between electrical properties and T s was clarified by analyzing the chemical element compositions and the chemical states on the surface of GZO films.

Highly efficient blue organic light-emitting devices with indium-free transparent anode on flexible substrates

Indium-free transparent conducting oxides (TCOs) may provide a lower cost solution for the transparent anode in large area displays and solid-state lighting. Low temperature deposition processes are essential for manufacturing compatible with flexible substrate materials. We report herein a near room temperature sputtering process for generating an indium-free TCO coating on a flexible substrate. Specifically, we deposited gallium-doped zinc oxide (GZO) uniformly over a 12″ diameter area at nominally room temperature on polyethylene terephthalate (PET), without any noticeable damage to the PET substrate. The GZO films exhibit excellent physical, optical and electrical properties: roughness ∼7 nm, transmittance >85% and resistivity ∼10 −3 ohm cm. Phosphorescent blue organic light-emitting devices (OLEDs) were fabricated on these substrates with comparable performance (16% external quantum efficiency and 33 lm/W power efficiency at 1 mA/cm 2) to that of devices fabricated on glass with GZO or indium tin oxide (ITO) as the anodes. These results demonstrate the utility of using GZO instead of the higher cost material ITO on PET for flexible displays and solid-state lighting.

Optical and Electrical Properties of Ga-Doped ZnO Single Crystalline Films Grown on MgAl2O4(111) by Low Temperature Hydrothermal Synthesis

Single crystalline gallium-doped ZnO films have been hydrothermally grown at using a ZnO seed layer on (111) substrates. High resolution X-ray diffraction showed an epitaxial relationship between the ZnO single crystalline film and the spinel substrate with an out-of-plane orientation of and an in-plane orientation of and . The effects of the doping concentration as well as the postannealing treatments on the electrical and optical properties were examined. After thermal treatment, the carrier concentration in the 2.48% Ga-doped ZnO films was 2 orders of magnitude higher at with a carrier mobility of . The blueshift of the optical bandgap was also observed from the photoluminescence and optical absorption measurements as the doping concentration increased. This can be explained in the framework of the induced stress and Burstein–Moss effects at high carrier concentration.

Gallium-doped zinc oxide films deposited using an unbalanced magnetron sputtering system

Gallium-doped zinc oxide films are deposited using an unbalanced magnetron sputtering system. The films are deposited by dc sputtering of a conducting ceramic target in an argon atmosphere. The substrate temperature is 150°C. The film surface morphology is studied by scanning electron microscopy and atomic force microscopy. As the degree of magnetron unbalance increases, the electrophysical properties of the films deposited along the system axis are shown to improve, and the distribution of the film electrical resistivity over the substrate surface becomes more uniform.

Room-temperature growth and optoelectronic properties of GZO/ZnO bilayer films on polycarbonate substrates by magnetron sputtering

Gallium-doped zinc oxide (GZO) films have been grown on polycarbonate (PC) substrates with and without ZnO buffer layers by radio frequency (r.f.) magnetron sputtering at room temperature. The optimization of growth parameters (sputtering power, sputtering pressure, sputtering time, oxygen partial pressure ratio) on ZnO buffer layers with multiple qualities based on the orthogonal array has been studied. The optimal parameter of the ZnO buffer layer could be obtained according to the range analysis method. The lowest electrical resistivity of GZO/ZnO/PC films is about 5.21×10 -4 Ωcm, and the transmittance in the visible range is about 80%, using air as reference. The good transparency-conducting property and the room-temperature depositon on polymeric substrates enable GZO films to be widely used in flexible optoelectronic devices such as thin film solar cells.

Optimization of laser patterning of textured gallium-doped zinc oxide for amorphous silicon photovoltaics

Laser scribing process of in-house textured gallium-doped zinc oxide (GZO) is optimized, aiming to improve the performance of amorphous silicon (a-Si:H) photovoltaic (PV) modules. The reasons for different scribing quality of textured GZO and SnO 2:F scribed at 1064 nm with pulse duration of 40 ns were analyzed. Apart from separation resistance, quality of the scribed lines was evaluated by laser scan microscopy from three-dimensional images. Other types of lasers, such as laser with shorter pulse duration, laser at 355 nm and laser with Gaussian-to-tophat converter, were used to smooth the edges and flatten the bottoms of the scribed lines. The proper laser scribing realizes the advantages of textured GZO films used as front contacts in PV modules. A short-circuit current density of 14.3 mA/cm 2 and an initial aperture area efficiency of 8.8% were obtained on 16 cm × 16 cm textured GZO coated glass scribed at 355 nm with pulse duration of 40 ns.

Effect of rf-power density on the resistivity of Ga-doped ZnO film deposited by rf-magnetron sputter deposition technique

Gallium-doped zinc oxide (GZO) films were studied using rf-magnetron sputter deposition technique for the use of transparent conducting oxide film. The effects of deposition parameters such as pressure, rf-power, and film thickness on electrical and optical characteristics of GZO films were evaluated to obtain transparent conducting film with high transmittance and low resistivity for a-Si:H thin film solar cells. The resistivity and the structural properties of GZO film strongly depended on rf-power density and film thickness. The 1200 nm-thick GZO film deposited at 15 mTorr and 150 °C with rf-power density of 4.46 W/cm 2 showed the resistivity as low as 6.2 × 10 −4 Ω cm and the average transmittance of 86.5% in visible light wavelength region of 400–800 nm.

GZO透明導電膜へのNd:YAGレーザによるアニーリング効果

In order to cope with the depletion of iudium, a strong interest has been recently turned to transparent conducting oxide (TCO) films based on a zinc oxide system. Focusing to this theme, it was tried to fabricate Gallium-doped zinc oxide (GZO) TCO films. But, it was difficult to make good films of GZO at low temperature. Therefore, it was tried to use laser-annealing-method for improved characteristics of GZO films. Approximately 200-nm-thick 5.0 wt.% Ga-doped zinc oxide (GZO: 5.0 wt.% Ga2O3) films have been deposited on glass substrates at Room Temperature by pulsed laser deposition (PLD) using the fourth harmonic generation (FHG) of Nd: YAG laser (λ=266 nm). In order to reduce resistivity of as-deposited films, annealing process was carried out by Nd:YAG (FHG) laser with laser energy density of 40 mJ/cm2. As a result, the resisivity was improved from 8.16×10-4 to 6.27×10-4 Ω·cm (improvement effect of 23.2 %). An average transmittance of more than 85% in the visible region was obtained for the fabricated.

Effect of growth temperature on properties of transparent conducting gallium-doped ZnO films

Transparent conducting gallium-doped ZnO films are deposited on glass substrates by magnetron sputtering of conducting ceramic targets. The dependences of structural, electric, and optical characteristics of ZnO:Ga films on the substrate temperature are investigated during the deposition. Stability of resistivity of films is considered during annealing in air. It is found that the films deposited at the substrate temperature of 250°C have the lowest resistivity of 3.8 × 10−4 Ω cm, while those deposited at 200°C have the highest thermal stability.

Characterizations of Ga-doped ZnO films on Si (111) prepared by atmospheric pressure metal-organic chemical vapor deposition

Gallium-doped ZnO films were grown on p-Si(111) substrates by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) using diethylzinc and water as reactant gases and triethyl gallium (TEG) as a n-type dopant gas. The structural, electrical, and optical properties of ZnO:Ga films obtained by varying the flow rate of TEG from 0.56to3.35μmol∕min were examined. X-ray diffraction patterns and scanning electron microscopy images indicated that Ga doping plays a role in forming microstructures in ZnO films. A flat surface with a predominant orientation (101) was obtained for the ZnO:Ga film fabricated at a flow rate of TEG=2.79μmol∕min. This film also revealed a lowest resistivity of 4.54×10−4Ωcm, as measured using the van der Pauw method. Moreover, low temperature photoluminescence (PL) emission recorded at 12K demonstrated the Burstein Moss shift of PL line from 3.365to3.403eV and a line broadening from 100to165meV as the TEG flow rate varied from 0.56to2.79μmol∕min. This blueshift behavior of PL spectra from ZnO:Ga films features the degeneracy of semiconductor, which helps to recognize the enhancing of transparency and conductivity of ZnO films fabricated by AP-MOCVD using Ga-doping technique.

Effects of substrate temperature on the properties of heavy Ga-doped ZnO transparent conductive film by RF magnetron sputtering

Gallium-doped zinc oxide (GZO) transparent conductive thin films were deposited on glass substrate by RF magnetron sputtering using a ceramic ZnO target with 10 wt. % Ga2O3 doped. The effects of substrate temperature on structural, electrical and optical properties of the films were investigated. X-ray diffraction (XRD) measurements indicated that GZO films were polycrystalline and strongly c-axis oriented. The surface roughness evaluated in terms of RRMS measured by atomic force microscope (AFM) was 1.8 nm for the smoothest surface obtained at 250 °C. The electrical resistivity decreases with increasing substrate temperature and the lowest resistivity obtained was 5.8×10-4 Ω·cm at 350 °C. The average transmittance in the visible region was over 90%. The carrier concentration increased with substrate temperature increasing, and the highest mobility value was 22.1 cm2v-1·s-1.

기판 바이어스 전압을 이용한 태양전지용 GZO 박막의 전기적, 광학적 특성

In this paper we report upon an investigation into the effect of DC bias voltage on the electrical and optical properties of Gallium doped zinc oxide (GZO) film. GZO films were deposited on glass substrate without substrate temperature by RF magnetron sputtering from a ZnO target mixed with 5 wt% <TEX>$Ga_{2}O_{3}$</TEX>. we investigated sample properties of bias voltage change in 0 to -60 V. We were able to achieve as low as <TEX>$5.89{\times}10^{-4}{\Omega}cm$</TEX> and transmittance over 88 %. without substrate heating.