GZO Films Research Articles

Thin Ga-doped ZnO Film with Enhanced Dual Visible Lines Emission

Introduction: In this study, a simple and facile route was employed to prepare a highly transparent and luminescent ultra-thin gallium doped ZnO film (GZO). Methods: The thin GZO film has been deposited using the simultaneously ultrasonic vibration and sol-gel spin-spray coating technique. The structural and optical properties of pure and doped thin films were investigated by various methods, such as X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), scanning electron microscopy (SEM), UV-Vis, and PL spectroscopy. Results: XRD results indicated that both pure and doped ZnO films had a hexagonal wurtzite structure with (101) preferred orientation. XPS and EDX studies confirmed the incorporation and presence of Ga ions into the ZnO lattice structure. The doped sample showed nearly 90% of transparency, and a strong blue-green emission in the visible region. Conclusion: The obtained results proved that the prepared thin film could be a novel candidate for optoelectronic applications.

Crystallinity-Driven Formation of Highly Arrayed ZnO Nanorods Via Low-Temperature Chemical Bath Deposition on Transparent Conductive Oxide Substrates

The wide-band gap semiconductor zinc oxide (ZnO) nanostructures have attracted much interest in extensive applications such as gas sensors, photoelectrodes in dye sensitized solar cells and catalysts, etc. Among the various synthesis methods of ZnO nanostructures, chemical bath deposition (CBD) method is a low cost and simple technique to synthesize nanostructures. However, there are still remaining issues on growth of well- arrayed ZnO nanostructures with high uniformity and high crystallinity. In this research, we conducted a comprehensive investigation into various parameters, including seed layers, seed-layer thickness, precursor concentration, and reaction time during CBD process. Three kinds of transparent conductive oxide (TCO) films, specifically AZO, GZO, and ITO films, each with a thickness of 300nm, were individually deposited on glass substrates using radio frequency magnetron sputtering and DC magnetron sputtering. These obtained TCO films served as seed layers for the fabrication of ZnO nanorods in CBD process. The solution was prepared with Zn(NO3)2•6H2O and hexathylenetetramine (HMTA) diluted into deionized water, which was heated to 95 °C during CBD process. As the results: 1) Effect of different seed layers: ZnO nanorods exhibited vertically aligned hexagonal structures with uniform morphology when grown on AZO and GZO thin films. The alignment was found to depend on the crystal growth direction of the underlying TCO seed layers. Higher crystallinity of the seed layer contributed to superior alignment, with the crystal mismatch ratio between ZnO nanorods and GZO/AZO films being significantly lower than that with ITO film. 2) The thickness effect investigation was using AZO films varied from 100nm to 300nm. It was found that the thickness influenced the morphology of ZnO nanorods, correlating with the grain sizes of the seed layer thin films. Increasing film thickness led to larger average grain sizes, contributing to the diameter increase of ZnO nanorods in the CBD process, resulting in the improvement of crystallinity of ZnO nanorods. 3) The precursor ratio between concentration ratio of Zn(NO3)2•6H2O and HMTA was set as 2:0.25, 2:0.5, 2:1 and 2:2 individually for ZnO nanorods growth on AZO film. Increasing HMTA concentration initially led to an increase in diameter and length of ZnO nanorods, followed by a decrease at higher ratio of 2:2. The highest crystallinity of ZnO nanorods was obtained at the optimized ratio at 2:1. 4) The growth reaction time was investigated on ZnO nanorods grown on AZO film with concentration ratio of Zn(NO3)2•6H2O and HMTA at 2:1 with varied reacting time from 1 to 5 hours in CBD process. It was found that the reaction time significantly influenced on the growth and crystallinity of well-aligned ZnO nanorods on the as-deposited AZO seed layer. The crystallinity of ZnO nanorods in (0001) growth orientation was improved when the reaction time was increased from 1 hour to 5 hours during the CBD growth. The highest crystallinity was obtained from the ZnO nanorods grown with 5 hours reaction time. When the growth reaction time was increased, more Zn2+ ions were generated and strongly bonded with oxygen. The formed ZnO could be stacked along the c-axis (0001) growth direction and resulted in the length increasing during the CBD process. All fabricated ZnO nanorods showed a high transmittance of over 70% in the visible region. The ZnO nanorods synthesized on AZO substrates with optimized fabrication conditions will be in high potential for optoelectronic applications.

Influence of oxygen partial pressure on the strain behaviour of reactively co-sputtered Ga doped ZnO thin films

Gallium doped ZnO films (GZO) were deposited by reactive co-sputtering of Zn and GaAs at different partial pressures of oxygen in Ar-O2 mixture. The GZO films were deposited at 375 °C, with GaAs area coverage of 0 - 3 % and 4 - 10 % O2 in sputtering atmosphere. The composition of GZO films was studied by X-ray photoelectron spectroscopy, which showed that the films deposited at 4 % and 5 % O2 display Ga/Zn ratio of ∼ 0.1 and contain oxygen vacancies, while those deposited at 6 % and 7 % O2 display Ga/Zn ratio of ∼ 0.01 and contain chemisorbed/interstitial oxygen. X-ray diffraction measurements supported by Raman studies revealed that undoped ZnO films are strongly c-axis oriented and display decrease of crystallite size along c-axis, along with substantial increase of hydrostatic tensile strain with increase of O2 percentage, which are attributed to the presence of interstitial oxygen. In sharp contrast, GZO films deposited at 4 % and 5 % O2 under limited oxygen availability, display smaller and mis-oriented crystallites along c-axis and large compressive strain, which is also hydrostatic, owing to the incorporation of Ga. The GZO films deposited at 6 % and 7 % O2 display improved crystallinity and decrease of compressive strain, which are attributed to reduced Ga content of the films and sufficient availability of oxygen during deposition. This study reveals the critical influence of the partial pressure of oxygen on the stoichiometry, doping concentration, crystallinity and the nature of strain in reactively sputtered GZO films.

Optical and electrical properties of GaZnO films deposited by co-sputtering method on two types of substrates

ABSTRACT Gallium zinc oxide GaZnO (GZO) thin films are deposited on silica glass and quartz substrates using DC power and RF power on ZnO and GaO targets, respectively. Structural characteristics are investigated using X-ray diffraction XRD, optical parameters such as refractive index n, optical energy gap Eop and dispersion parameters in addition to the electrical property are investigated for all considered films. The optical properties reveal that GZO films have high transmittance (70–98%) and a low absorption coefficient in the visible range of the spectra. The optical band gap was found to increase from 3.28 to 3.44 eV with increasing the RF power on GaO target. The dielectric constant of GZO was found to be larger than the dielectric loss and has high optical conductivity (≈1015 S−1), The VELF and SELF values of GZO films are calculated and investigated.

Excellent properties of Ga‐doped ZnO film as an alternative transparent electrode for thin‐film solar cells

AbstractOptical and electrical properties of transparent electrode are directly related to the photoelectric conversion efficiency of thin‐film solar cells. For this reason, Ga and Al‐doped ZnO (GZO and AZO) transparent conducting films were fabricated on float glass through the magnetic sputtering technique. Compared with AZO films, GZO films show a higher figure of merit (FoM) value indicating their outstanding optical and electrical properties. The smaller difference of ionic radius between Ga3+ and Zn2+ than Al3+ and Zn2+ contributes to high carrier concentration and electron mobility of GZO films. In addition, it has been shown that GZO films can be stable at high substrate temperatures. After being annealed at 550°C in N2 atmosphere, the FoM value of GZO films is twice as much as that of FTO films, indicating that GZO can be applied as the front contact material not only in CIGS thin‐film solar cells, but also in CdTe thin‐film solar cells.

High haze Ga and Zr co-doped zinc oxide transparent electrodes for photovoltaic applications

Creating textured front electrodes to improve optical path length is promising to enhance the power conversion efficiency of solar cells. Deposition of transparent conductive oxides with textured surfaces usually requires additional processing steps, such as etching and coating nanoparticles. However, this makes the process complicated and inefficient. We demonstrate a one-step fabrication process to deposit high haze gallium and zirconium co-doped zinc oxides (GZO:Zr) prepared by atmospheric pressure plasma jets. GZO:Zr (2 at%) films achieve a low resistivity (7.88 × 10–4 Ω cm), a high haze (34.8%), and a great FoM (8.22 × 10−3 Ω−1). The haze factor increases from 7.19% to 34.8% (+384%) when 2% Zr is doped into GZO films. Such an enhancement of haze is attributed to the increased surface roughness and deterioration of crystallinity. AFM results show that roughness increases from 18.4 to 122 nm after 2% Zr is doped. SEM images show that spherical particles appeared on the film surface when Zr was doped into GZO films. Unlike conventional methods, our method produces hazy transparent electrodes in one step without changing any operational parameters and is suitable for industrial-scale mass production. Our findings pave the way for new applications of co-doping in transparent conductive oxides.

Optical and photoluminescence studies of precursor stabilized gallium zinc oxide thin films

Precursor stabilized Gallium doped ZnO thin films were grown using spray pyrolysis on glass substrates. UV–vis spectrophotometry and an X-ray diffractometer were used to characterise the GZO thin films. Gallium is doped with a concentration ranging from 1 at% to 4 at% and its effects on structural and optical features were studied. XRD studies found the thin films to be polycrystalline with a hexagonal wurtzite structure. In the visible range of wavelength (400–800 nm), all GZO films exhibit average transmittance above 90%, with a pronounced absorption edge in the UV area. The PL emission spectra show the Near-band-edge emissions at about 389 nm and strong deep-level emissions from oxygen defect states at about 545 nm are observed in all the films. With increase in concentration of Ga, the intensity of these two bands decreased.

Signature of strong localization and crossover conduction processes in doped ZnO thin films: synergetic effect of doping fraction and dense electronic excitations

Ga x Zn1−x O thin films with varying Ga fraction within the solubility limit were irradiated with high-energy heavy ions to induce electronic excitations. The films show good transmittance in the visible region and a reduction of about 20% in transmittance was observed for irradiated films at higher ion fluences. The Urbach energy was estimated and showed an augmenting response upon increase in doping fraction and ion irradiation, this divulges an enhancement of localized states in the bandgap or disorder in the films. The evolution of such localized states plays a vital role in charge transport and thus the temperature dependent electrical conductivity of irradiated thin films was studied to elucidate the dominant conduction mechanisms. The detailed analysis unfolds that in the high-temperature regime (180 K < T < 300 K), the charge conduction was dominated by thermally activated band conduction followed by the nearest neighbor hopping (NNH) mechanism. Whereas in the lower temperature regime (25 K < T < 170 K), the conduction mechanism was governed by Mott-VRH (variable range hopping) followed by Efros–Shklovskii (ES)-VRH. A sudden and steep rise in resistivity below 30 K was observed for GZO films with higher doping fraction at higher ion fluence and proclaims the presence of strong localization of carriers.

High thermoelectric performance of high‐mobility Ga‐doped ZnO films via homogenous interface design

AbstractGa‐doped ZnO (GZO) thin films grown on sapphire substrates have been widely investigated as a promising transparent thermoelectric (TE) material. However, due to the large lattice mismatch and thermal expansion between the sapphire substrate and GZO film, strain‐induced lattice distortion impedes the transport of electrons, leading to low carrier mobility. In this study, ZnO homo‐buffer layers with different thicknesses were inserted between sapphire substrates and GZO films, and their effect on the TE properties was investigated. A thin ZnO interlayer (10 nm) effectively reduced the lattice mismatch of the GZO film and improved the carrier mobility, which contributed to the large enhancement in the electrical conductivity. Simultaneously, energy filtering occurred at the interface between GZO and ZnO, resulting in a relatively high density of states (DOS) effective mass and maintaining a high Seebeck coefficient compared to that of the unbuffered GZO films. Consequently, the GZO film with a 10 nm thick ZnO buffer layer possessed a high power factor value of 449 μW m−1 K−2 at 623 K. This study provides a facile and effective method for optimizing the TE performance of oxide thin films by synergistically improving their carrier mobility and enhancing their effective mass.

Ga-Doped ZnO Coating—A Suitable Tool for Tuning the Electrode Properties in the Solar Cells with CdS/ZnS Core-Shell Quantum Dots

Two layer system from sputtered indium tin oxide (ITO) and gallium doped zinc oxide (Ga:ZnO, GZO) were studied for transparency in the visible electromagnetic range, reflectivity in the near infrared range, conductivity and valent band for a solar cells with quantum dots. The bi-layer coatings produced at optimized oxygen partial pressure, films thickness and surface roughness exhibit improved optical properties without worsening the electrical parameters, even if additional oxygen introduction during the reactive sputtering of the GZO. With an average optical transmittance of 91.3% in the visible range, average reflection and resistivity lower than 0.4 × 10−2 Ω.cm, these coatings are suitable for top electrode in the solar cells. The obtained results reveal that multilayered stacks of transparent ITO/Ga-doped ZnO coatings possess relatively low surface roughness (7–9 nm) and appropriate refractive index. The additional oxidation of GZO films induces modification of the film thickness and respectively of their optical performances.

Dopants and induced residual stress-controlled thermoelectric properties of ZnO thin films

We investigate the structure, optical properties, and thermoelectric power in terms of residual stresses induced in the undoped ZnOand X-doped ZnO (X = Al, Ga) films. The results show that the different native seed layers self-grown during the initial growth cause compressive stress for ZnO and Ga-doped ZnO films and tensile stress for Al-doped ZnO films. The band-gap of the investigated films follows the Burstein–Moss (BM) effect. It is noted that the ZnO film with the lowest compressive stress has a better PF value below 275 °C. The GZO film is more stable at higher temperatures; therefore, it has a better PF value above 275 °C. The residual stress induced in the investigated films is demonstrated through the formation of a self-grown seed layer under the bombardment of high-energy sputtered Ga, Al, Zn, and O species onto the substrate and during film growth.

Charge-Transfer Effect of GZO Film on Photochemical Water Splitting of Transparent ZnO@GZO Films by RF Magnetron Sputtering

With the purpose of enhancing the photochemical water splitting performance, GZO film has been used to increase the charge transfer of the ZnO@GZO films. The characterization of ZnO film, GZO film and ZnO@GZO films was carried out by X-ray diffraction (XRD), atomic force microscope, DRUV-vis spectra (DRUV-vis), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and photochemical water splitting. All samples show a related highly c-axis peak (002) from XRD patterns. DRUV-vis results show that the formed GZO film with wide band gap but low resistivity, would not affect the band gap of the ZnO@GZO films. PL results reveal that the decreased recombination of electron and hole, via the increased charge transfer by GZO film. XPS results hint that the obvious change of chemisorbed oxygen species on the surface of ZnO and GZO, and Ga atoms mainly substitute into Zn sites in the ZnO matrix. Compared with that of ZnO film, the ZnO@GZO films even with lower specific surface area, could efficiently enhance the photochemical water splitting performance via the charge-transfer effect of the GZO film.

Abrupt conversion of the conductivity and band-gap in the sputter grown Ga-doped ZnO films by a change in growth ambient: Effects of oxygen partial pressure

GZO films were sputter grown under the different O2/Ar + O2 ratios and the systematic investigation has been tried in an effort to get a better understanding on the correlations between the involved defects and the film properties. The abrupt conversion of the resistivity was observed from the highly conductive, 2.52 ⅹ10−4 Ω cm, at pure Ar ambient to resistive, 0.295 Ω cm, with an oxygen addition of 1% in the ambient, accompanying the significant reduction in the electron density and mobility. Hall measurement at 10 K–300 K and in-depth XPS analysis suggested that the higher Hall mobility and carrier density in the film grown in pure Ar ambient may be due to a lower compensation ratio and the compensation via Gazn-Vzn complexes may play a major role in the sudden reduction in the electron density and the reduced density of the Gazn, substitutional donors induced by oxygen addition in growth ambient.

Metal β-diketoiminate precursor use in aerosol assisted chemical vapour deposition of gallium- and aluminium-doped zinc oxide

Aerosol assisted chemical vapour deposition (AACVD) has been used to deposit thin films of ZnO from the single-source precursor [Zn(OC(Me)CHC(Me)N(iPr))2] (1) affording highly transparent (>80%) and conductive films (sheet resistance ∼70 KΩ/sq). Extension of this AACVD method whereby related precursors of the type, [R2M(OC(Me)CHC(Me)N(iPr))] (R = Et, M = Al (2); R = Me, M = Ga (3)), isolated as oils, were added to the precursor solution allowed for the deposition of aluminium- and gallium-doped ZnO (AZO and GZO) films, respectively. Complexes 1–3 were characterised by elemental analysis, NMR and mass spectrometry. Films were deposited in under 30 min at 400 °C, from CH2Cl2/toluene solutions with a N2 carrier gas. Herein we report the bulk resistivity, ρ, of AZO (0.252 Ω cm) and GZO (0.756 Ω cm) films deposited from this novel approach. All the films transparency exceeded 80% in the visible, X-ray diffraction (XRD) showed all films to crystallise in the wurtzite phase whilst X-ray photoemission spectroscopy (XPS) confirmed the presence of the Al and Ga dopants in the films, and highlighted the low C-contamination (<5%) this route offers. Investigation of a mechanism analogous to the Kirkendall effect confirmed that heating of GZO films at 1000 °C produced the spinel structure GaZn2O4.

Quantum confinement effect and size-dependent photoluminescence in laser ablated ultra-thin GZO films

The structural, morphological, and optical characteristics of GZO ultra-thin films were investigated using XRD (X-ray diffraction), FE-SEM (field emission electron microscopy), in situ EDS (Energy Dispersive X-ray) spectroscopy, UV–VIS-IR spectroscopy and photoluminescence spectroscopy (PL) respectively. Morphological analysis reveals the noodle, seed and particle like structure of GZO for GaN, sapphire and Si substrates respectively with average grain size ranging from 5 to 20nm. An effective mass model (EM-Model) for particle in a cylindrical wave function of e–h pair was correlated with experimental results. The reduction in FWHM value (from 31nm to 13nm) of NBE (near-band-edge) emission peak and enhanced NBE intensity have been achieved with small grain size. Blue shift in optical band gap is explained in term of grain radius by EM-model. Improved optical and structural properties were found in relation with quantum confinement effect. The current study states that grain size plays vital role in order to tailor optical properties of GZO thin films.

Ga-doped ZnO films by magnetron sputtering at ultralow discharge voltages: Effects of defect annihilation

Preparation of high quality transparent conductive oxide (TCO) films by sputter deposition involves an intricate balance of defect generation by the highly energetic negative oxygen ions (depending on the discharge voltage) and the concomitant annihilation of these defects during film growth. Ga-doped ZnO films with a low Ga content (1.7at.%) were deposited to investigate the effects of defect annihilation on the microstructure evolutions as well as the optical and electrical properties. To achieve this aim, we prepared the GZO films by magnetron sputtering at ultralow discharge voltages (~70V) to minimize the defect generation, and varied the substrate temperature (from 130°C to 380°C) to adjust the annihilation rates. The microstructure was systematically characterized by X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Raman Spectroscopy. The electrical and optical properties were obtained by a Hall-effect measurement system and Spectroscopic Ellipsometry (SE), respectively. It was found that (i) even under the condition of highly controlled defect generation from the bombardment of negative oxygen ions, a sufficient annihilation of the defects cannot be realized without externally heating the substrate; and (ii) there existed a threshold temperature, above which both the structural quality and the electrical properties were improved with the increased temperature over the temperature range we examined. These results reveal that the growth temperature during the GZO film deposition has played an important role in effective annihilation of the irradiation-induced structural defects, which may be due to the higher diffusion barrier of Ga atoms in our GZO films.

Characterization of transparent conductive ZnO and Ga-doped ZnO films on polyethylene naphthalate sheets aged for six years in ambient atmospheric

Transparent conductive ZnO and Ga-doped (GZO) films that had been deposited on polyethylene naphthalate (PEN) sheet substrates were aged in ambient atmosphere conditions for six years. The films then evaluated to characterize the degradation in their sheet resistance over the substrate surface due to moisture permeation. The conductive properties of the ZnO and GZO films deposited on PEN substrates were more degraded compared to those for films deposited on glass substrates, exhibiting a nonuniform distribution of resistance. While ZnO and GZO films thinner than 100 nm were particularly sensitive to moisture infiltration, the increase in sheet resistance relative to the initial value became less obvious as thickness increased. This observation indicates that moisture permeation is limited to a certain depth from the surface. While GZO films thicker than 100 nm were free from signs of aging at Ga concentrations corresponding to standard transparent electrodes, regions with Ga concentrations more than 10% suffered from aging. This phenomenon has been observed in GZO films on glass substrates. The signs of aging were more serious for the ZnO films compared to the GZO films, as the oxygen vacancies in the ZnO films acted as reaction centers with H2O molecules. Furthermore, ZnO films thicker than 200 nm cracked because of misfit strain between the film and the PEN substrate, and cracks were widened and deeper for the thicker films. The cracked films experienced far greater deterioration than the films without cracks because moisture could easily enter the cracks and reach deep regions of the films.

Influence of RF power on the opto-electrical and structural properties of gallium-doped zinc oxide thin films

GZO thin films were succesfully deposited onto n-Si and glass substrates by RF magnetron sputtering at room temperature. The structural, morphological and opto-electrical properties of the films were investigated in terms of RF power, using various methods such as XRD, AFM, SEM, EDX, XPS, SIMS, UV–Vis–NIR spectroscopy and Hall effect measurements. The achieved results revealed that the all films have highly c-axis (002) oriented polycrystalline structure with high transmittance in Vis and high reflectance in NIR region as well as good conductivity. Meanwhile, surface of the films was uniform, compact and crack-free. With incerasing of RF power, it was seen that crystallinity of the films improved and the grain size became larger. It was also observed that optical band gap of the films was increased to the order of 0.15 eV as well as decreasing the resistivity to the order of 6.38 Ω cm with increasing RF power from 100 to 200 W. Deposited film at 200 W, which can be optimum sputtering power for coating GZO films, having high concentration of free electrons and lowest resistivity exhibited the highest IR reflectivity (~55%) in NIR region. In addition, deposited GZO films at this power have larger particle size and highly optical transmittance (~87%) in visible region. Obtained both of optical and electrical results suggested that the deposited GZO films can be used in low thermal emissivity coating for energy efficient glass and the UV-blocking layer as well as transparent conductive oxide electrode for flexible opto-electronic devices.

Enhancement of Ga-doped zinc oxide film properties and deposition rate by multiple deposition using atmosphere pressure plasma jet

Multilayer film with an interfacial buffer layer made of different material has been show to improve the film properties. However, it is not well understood if similar benefits may be achieved by multiple deposition of GZO without the addition of new interfacial materials. Here we prepared single-, double-, and triple-deposited GZO films on glass substrate (preheated to 180 °C) by atmosphere pressure plasma jet (APPJ). We found that the triple-deposited film exhibits overall enhancement over the single-deposited film in deposition rate (110 nm/min) and the key properties, such as resistivity (9.4 × 10−4 Ω cm), crystallite size (16 nm), carrier concentration, mobility, and band gap. In particular, the resistivity and deposition rate are significantly improved by 33% and 35%, respectively. The results are attributed to the mitigation of lattice-mismatch between the film and the glass substrate by the bottom thin GZO film. The findings may also be applied to other deposition methods with moving nozzle or moving stage such as spray pyrolysis.

Growth and characterization of high transmittance GZO films prepared by sol-gel method

High transmittance Ga-doped ZnO thin films were deposited on glass substrates using sol-gel spin coating technique. Crystallinity levels, microstructures, optical and electrical properties of the thin films were systematically investigated by scanning electron microscope, X-ray diffractometer, UV–visible spectrophotometer, PL spectrometer and four-point probe method, respectively. All GZO thin films exhibited polycrystalline with a hexagonal wurtzite structure and slight (0 0 2) preferred orientation growth, which show excellent transmittance (>95%) in the 380–780nm wavelength range. The PL spectra of the GZO films revealed a strong ultraviolet emission peaks at around 393nm and a weak blue emission peak at around 468nm. When the Ga doping level was 4at.%, the minimum resistivity of 1.12×10−2Ω·cm with the highest transmittance was reached. The optical band gaps (Eg) of the films were increased from 3.311eV to 3.329eV with the increase of Ga dopant concentration.