GZO Films Research Articles

Mechanical and Electrical Properties of Ga-Doped ZnO Films Deposited on Flexible Substrate by DC Magnetron Sputtering

Ga-doped ZnO films were deposited on polyethylene terephthalate (PET) substrate by dc magnetron sputtering using a high density GZO target (doped with 6.65 wt% Ga2O3) without substrate heating. We investigated electrical, structural, and mechanical properties of GZO films deposited under various total gas pressures (Ptot). GZO films deposited at Ptot of 2.0 Pa showed the lowest resistivity (2.91 x 10-2 7cm), which could be attributed to higher crystallinity of the film. Also, this GZO film showed the lowest change in resistance (8 R/R0 = 0.3) for the dynamic bending test.

Comparison of the optical properties of undoped and Ga-doped ZnO thin films deposited using RF magnetron sputtering at room temperature

The optical properties of undoped zinc oxide (ZnO) thin films of various thicknesses were compared with those of Ga-doped (GZO) thin films. Transparent, high-quality undoped ZnO and GZO films were deposited successfully using radio-frequency (RF) sputtering at room temperature. The films were polycrystalline with a hexagonal structure and a strongly preferred orientation along the c-axis. The films had an average optical transmission >85% in the visible part of the electromagnetic spectrum. The undoped ZnO thin films were more transparent than the GZO thin films. In the photoluminescence (PL) spectrum, ZnO film has higher quality than GZO as a result of decrease in the green emission intensity.

Dependences of structural and electrical properties on thickness of polycrystalline Ga-doped ZnO thin films prepared by reactive plasma deposition

Polycrystalline Ga-doped (Ga content: 4 wt%) ZnO (GZO) thin films were deposited on glass substrates at 200 ∘C by a reactive plasma deposition with DC arc discharge technique. The dependences of structural and electrical properties of GZO films on thickness, ranging from 30 to 560 nm, were investigated. Carrier concentration, n, and Hall mobility, μ, increases with increasing film thickness below 100 nm, and then the n remains nearly constant and the μ gradually increases until the thickness reaches 560 nm. The resistivity obtained of the order of 10−4 Ω cm for these films decreases with increasing film thickness: The highest resistivity achieved is 4.4×10−4 Ω cm with n of 7.6×1020 cm−3 and μ of 18.5 cm2/V s for GZO films with a thickness of 30 nm and the lowest one is 1.8×10−4 Ω cm with n of 1.1×1021 cm−3 and μ of 31.7 cm2/V s for the GZO film with a thickness of 560 nm. X-ray diffraction pattern for all the films shows a hexagonal wurtzite structure with its strongly preferred orientation along the c-axis. Full width at half maximum of the (002) preferred orientation diffraction peak of the films decreases with increasing film thickness below 100 nm.

Effects of substrate temperature on crystallinity and electrical properties of Ga-doped ZnO films prepared on glass substrate by ion-plating method using DC arc discharge

The effects of substrate temperature, T s, on the crystallinity and the electrical properties of Ga-doped ZnO films (GZO) with a thickness of 200 nm were investigated. GZO films were prepared on glass substrates at various T s in the range from 150 to 400 °C by ion-plating method with DC arc discharge. X-ray diffraction analysis reveals that GZO film prepared at 250 °C shows the preferential orientation of c-axis and the highest crystallinity. Williamson–Hall analysis indicates that the crystallite size of GZO films remains nearly constant with increasing T s up to 300 °C, and then, with further increasing T s, decreases gradually. The Ga concentration in the films, estimated by secondary ion mass spectroscopy and X-ray fluorescence analyses, increases monotonically with increasing T s above 250 °C. Hall effect measurements show that resistivity decreases slightly with increasing T s up to 250 °C, leading to the lowest resistivity of 2.1 × 10 − 4 Ω cm at 250 °C, and then exhibits a gradual increase with further increasing T s.

Effect of Thermal Annealing on Ga-Doped ZnO Films Prepared by Magnetron Sputtering

In this study, Ga-doped (GZO) films deposited on a sapphire utilizing magnetron cosputtering method using and targets were demonstrated. The results revealed that the resistivities of the GZO films reduced by at least two orders of magnitude after the thermal annealing. The reduction in resistivity could be attributed not only to the activation of Ga dopants and to the increase of electron mobility, but also to the enlargement of the grain size that occurred as a result of thermal annealing. The resistivity of GZO films reduced from to after the films had undergone thermal annealing in nitrogen ambience with temperature ranging from 400 to . Additionally, the GZO films exhibited semiconducting conductivity and metallic conductivity in as-deposited and annealed samples , respectively.

Substitution of transparent conducting oxide thin films for indium tin oxide transparent electrode applications

The present status and prospects for further development of reduced or indium-free transparent conducting oxide (TCO) materials for use in practical thin-film transparent electrode applications such as liquid crystal displays are presented in this paper: reduced-indium TCO materials such as ZnO-In2O3, In2O3-SnO2 and Zn-In-Sn-O multicomponent oxides and indium-free materials such as Al- and Ga-doped ZnO (AZO and GZO). In particular, AZO thin films, with source materials that are inexpensive and non-toxic, are the best candidates. The current problems associated with substituting AZO or GZO for ITO, besides their stability in oxidizing environments as well as the non-uniform distribution of resistivity resulting from dc magnetron sputtering deposition, can be resolved. Current developments associated with overcoming the remaining problems are also presented: newly developed AZO thin-film deposition techniques that reduce resistivity as well as improve the resistivity distribution uniformity using high-rate dc magnetron sputtering depositions incorporating radio frequency power. In addition, stability tests of resistivity in TCO thin films evaluated in air at 90% relative humidity and 60 °C have demonstrated that sufficiently moisture-resistant AZO thin films can be produced at a substrate temperature below 200 °C when the film thickness was approximately 200 nm. However, improving the stability of AZO and GZO films with a thickness below 100 nm remains a problem.

Effects of oxygen-gas flow rate on lattice dynamics and microstructure for Ga-doped ZnO thin films prepared by reactive plasma deposition

The dependences of lattice constant, crystallite size and internal strain in the (100) direction for Ga-doped ZnO (GZO; Ga content, 3 wt%) films on O 2 gas flow rate (0–20 sccm) during deposition were investigated. GZO films have been prepared by reactive plasma deposition (RPD). A series of GZO thin films with a thickness of 200 nm were deposited on glass substrate at 200 ∘C. In the range of O 2 gas flow rate from 0 to 10 sccm, from out-of-plane and in-plane X-ray diffraction (XRD) measurements, we find a large lattice constant of the c -axis and small changes in the lattice constant of the a -axis compared with those of undoped ZnO. This is in good agreement with the theoretical results for GZO crystals based on a model that the dominant defect is Ga at Zn sites (Ga Zn). Excess O 2 gas flow increases both crystallite size and internal strain in the (100) direction and reduces the lattice constant of the c -axis for GZO films. This is caused by the formation of complex defects including n -type killers associated with vacant defects as determined by secondary-ion mass spectrometry (SIMS) and Raman spectroscopy.

Transparent conducting oxide semiconductors for transparent electrodes

The present status and prospects for further development of polycrystalline or amorphous transparent conducting oxide (TCO) semiconductors used for practical thin-film transparent electrode applications are presented in this paper. The important TCO semiconductors are impurity-doped ZnO, In2O3 and SnO2 as well as multicomponent oxides consisting of combinations of ZnO, In2O3 and SnO2, including some ternary compounds existing in their systems. Development of these and other TCO semiconductors is important because the expanding need for transparent electrodes for optoelectronic device applications is jeopardizing the availability of indium-tin-oxide (ITO), whose main constituent, indium, is a very expensive and scarce material. Al- and Ga-doped ZnO (AZO and GZO) semiconductors are promising as alternatives to ITO for thin-film transparent electrode applications. In particular, AZO thin films, with a low resistivity of the order of 10−5 Ω cm and source materials that are inexpensive and non-toxic, are the best candidates. However, further development of the deposition techniques, such as magnetron sputtering or vacuum arc plasma evaporation, as well as of the targets is required to enable the preparation of AZO and GZO films on large area substrates with a high deposition rate.

Photoluminescence of ZnO:Ga Thin Films Fabricated by Pulsed Laser Deposition Technique

Highly c-axis oriented Ga-doped ZnO films (GZO) have been grown on sapphire (0001) substrates by pulsed laser deposition (PLD) method. Photoluminescence (PL) spectra indicate that Ga atoms have a large effect on the luminescent properties of ZnO films. PL spectra of GZO films show near band edge (NBE) emissions and broad orange deep-level emissions. The NBE emission shifts to higher energy region and the intensity decreases with the increase of Ga concentration. The blue shift of NBE emission results from Burstein-Moss effect. The quenching of NBE emission is ascribed to the noradiative recombination. The orange emission is related to the oxygen vacancies.

パルスレーザー堆積法でプルームに磁場を印加して作製した酸化亜鉛系透明導電膜のドーパント依存性

ZnO doped with 1.5 wt.% Al2O3 (AZO) and 5 wt.% Ga2O3 (GZO) films were deposited on glass substrates by pulsed laser deposition method. In all experiments, repetition rates of 10 Hz, the energy density of 2 J/cm2, and an ablation time of 20 min irrespective of magnetic field. For AZO films deposited in magnetic field, the diffraction peak of ZnO (0002) facets grew large and a crystallinity improved. On the other hand, for GZO films fabricated with or without magnetic field, the crystallinity did not change. It was observed that for AZO films fabricated with magnetic field, the transmittance increased in the visible wavelength range and decreased in the infrared range being reflective of the reduction of carrier concentration.

Doped ZnO thin films as anode materials for organic light-emitting diodes

Zinc oxide thin films, doped with aluminum (AZO), gallium (GZO) and zirconium (ZZO), have been deposited on glass substrates by pulsed laser deposition. The electrical and optical properties of these films were investigated as a function of oxygen deposition pressure. These doped zinc oxide films were used as an anode contact to fabricate organic light-emitting diodes. Luminance efficiencies of 0.1 lm/W, 0.2 lm/W and 0.9 lm/W were measured at the light output of 100 cd/m 2 for the AZO, GZO and ZZO films, respectively. Doped zinc oxides are attractive alternative materials as transparent conducting electrode because they are nontoxic and inexpensive compared with indium tin oxide (ITO).

酸化亜鉛系ホモｐ‐ｉ‐ｎ接合の作製と光起電力特性

Transparent p-i-n junction based on ZnO was fabricated by radio-frequency magnetron sputtering and pulsed laser deposition methods, and its photovoltaic effect was examined. The p-type and i-type layers consist of CZO film (ZnO : CuO, optical energy gap Egopt= 3.0 eV) and undoped ZnO film (Egopt= 3.3 eV) fabricated using sputtering method, respectively. The n layer is GZO film (ZnO : Ga2O3, Egopt =3.6 eV) prepared by laser ablation method. For CZO films, an average transmittance of 63% in the visible region, the lowest resistivity of 26 Ω-cm and the maximum Seebeck coefficient of +200 μV/K were obtained. The photovoltaic effect was observed in a see-through structure of CZO/undoped ZnO/GZO/glass substrate by illumination of AM-1 light from both faces of the structure. It was recognized that in the series-connected-structure of CZO/ZnO/GZO + Si solar cell, the photovoltage was effectively caused with two peaks in the wavelength near 350 nm and 700 nm.

パルスレーザー堆積法でプルームに垂直な磁場を印加して作製したＺｎＯ：Ｇａ透明導電膜

It is urgent to establish a new pulsed laser deposition technique to prepare the high-quality transparent conducting film with large area, which is essential to put into practice. We have attempted to fabricate the GZO (ZnO : Ga2O3) films by applying a magnetic field perpendicular to the plume generated between the target and the substrate. When the films were fabricated by applying Nd : YAG laser (λ =532 nm) and the magnetic field at the same time, it was found that the carrier concentration drastically increased and the resistivity remarkably decreased and also the surface morphology was improved. By applying an ArF excimer laser (λ=193 nm) and the magnetic field at the same time, the films with rsistivities of the order of 10-4Ω-cm were obtained at the substrate temperature between room temperature and 350°C. It was proved to be a very useful method for the fabricating of the high-quality transparent conducting films to apply the magnetic field perpendicular to the plume.

Performance improvement of CIGS-based modules by depositing high-quality Ga-doped ZnO windows with magnetron sputtering

Improvement of the performance of Cu(InGa)Se 2 (CIGS)-based thin film submodules by depositing high-quality ZnO:Ga (GZO) window layers with sputtering method is performed to aim for establishing deposition technology of GZO windows of CIGS submodules. In order to reduce damage onto CIGS absorber/Zn(O,S,OH) x buffer interface due to the bombardment of high-energy particles during DC sputtering process of GZO window layers, growth of multilayered GZO window layers is developed. By using RF/DC/DC sputtered GZO window layers instead of the conventional DC sputtered GZO window layers, fill factor (FF) and conversion efficiency are increased over 10%. Increasing short-circuit current density ( J sc) of CIGS submodules is also investigated by improving the transparency of GZO window layers. Furthermore, damp heat test of the sputtered GZO films is carried out, and it is found that the GZO films have good stability of electrical properties.

パルスレーザー堆積法により作製した凹凸構造を有するＺｎＯ系透明導電膜

Thin films of ZnO : Ga (GZO) have been deposited on glass substrates by a pulsed laser deposition technique using SHG of Nd : YAG laser (λ = 532 nm). The size of droplets was lessened by a reduction of the thermal ablation effect in the evaporation processes. Therefore, electrical and optical properties were improved : (1) when prepared at a laser output of 0.2 W, the lowest resistivity of 2.77 × 10-4 Ω·cm and an optical transmittance of more than 95% in the visible region of the spectrum were obtained for 100-150 nm thick GZO (4 wt%) films deposited at substrate temperature of 300°C and on oxygen flow rate of 2 sccm, (2) when prepared at a laser output of 1 W, the lowest resistivity of 6.90 × 10-4 Ω·cm and an optical transmittance of more than 85% and a haze ratio of 33% (λ =550 nm) were obtained at substrate temperature of 200°C and on oxygen flow rate of 2 sccm. Thus, the texture structures with the confinement effect of incident light were observed for 400-500 nm thick GZO (4 wt%) films because the concavo-convex surfaces due to droplets generated from start of deposition.

Surface Flatness of Transparent Conducting ZnO:Ga Thin Films Grown by Pulsed Laser Deposition

ZnO films doped with 4, 7 and 13 wt% Ga2O3 (GZO films) have been deposited on glass and quartz substrates using a pulsed laser deposition technique with an ArF laser (λ=193 nm). In all experiments, a repetition rate of 10 Hz, an energy density of 1 J/cm2, and an irradiation time of 20-30 min (12000-18000 shots) were used. An optical transmittance of more than 90% was obtained in the visible region of the spectrum for 150-200-nm-thick GZO (7 wt%) films deposited at substrate temperatures of 200-300° C. The lowest resistivity of 2.08×10-4 Ω⋅cm and the lowest sheet resistance of 14.5 Ω/sq were obtained for GZO (7 wt%) films grown at a substrate temperature of 200° C. It was found from AFM images that there were minute irregularities (50-70 nm in size, average roughness 0.8 nm) on the surfaces of GZO (7 wt%) films grown at substrate temperatures of 25-300° C.