Gallium-doped ZnO Thin Films Research Articles

Realizing bulk and surface attributes driven third order nonlinear optical traits of ZnO layers for optical limiting operation; A thickness and impurity dependent investigation

The intrinsic, aluminium doped and gallium doped ZnO thin films with one, three and five layers of gradience have been successfully deposited using sol-gel spin coating method. Optical investigations were performed to analyze the linear as well as the nonlinear characteristics of the material along with its optical limiting caliber. The structural and morphological evaluations were also carried out to examine the bulk and surface properties of the samples. It is found that both dopant type and thickness have an independent effect on the structure, morphology, and optical features of the thin films. The surface traits have turned out to be significant along with the bulk properties to make the nonlinear absorption coefficient of the five-layered gallium doped ZnO thin film enhanced. Whereas, the improved crystallinity has been found to lower the optical limiting threshold of the specimens. The five-layered gallium doped ZnO has been evidenced as an optimized material for optical limiting applications with nonlinear absorption coefficient of 1.10 x 10–8 cm/W and an optical limiting threshold of 7.11 x 1012 W/m2.

Gallium-Modified Zinc Oxide Thin Films Prepared by Chemical Solution Deposition

Gallium-doped ZnO (GZO) thin films on glass, which can be used as transparent electrodes, were prepared using a spin coating technique. Thermal analysis and Fourier-transform infrared spectroscopy of the dried precursor solution of Zn acetate and Ga nitrate dissolved in ethanol with diethanolamine confirmed the decomposition of the organic components upon heating and the formation of ZnO at 450 °C. The thin films fired at 600 °C in oxygen and air, and the films annealed at 400 °C in Ar/H2, were polycrystalline, 140 nm thick, and exhibited a homogeneous microstructure with 50 nm grains and a smooth surface, as shown by X-ray powder diffraction and scanning electron and atomic force microscopy. The sheet resistance Rs measured using the 4-probe technique showed a change in Rs within 80 days for all samples. The Rs of the GZO thin films annealed in oxygen and air with values of MΩ/sq decreased over time. Rs values of 150 kΩ/sq were obtained for GZO thin films annealed in Ar/H2, but the Rs increased over time. We suggest that the degradation of Rs is related to the adsorption of water on GZO and that the responses depend on the nature of the defects in the GZO lattice.

Gas sensing response of ion beam irradiated Ga-doped ZnO thin films

The ion beam induced modified gallium doped ZnO thin films are studied for their gas sensing applications. The Ag9+ and Si6+ irradiated gallium doped zinc oxide thin films were exposed to various concentrations of ethanol and acetone gas for gas sensing applications. The Ag9+ ion irradiated Ga-doped ZnO thin was optimized at different operating temperature. It was observed that gas sensing response for both ethanol and acetone gas increases with increasing Ag9+ ion fluence. This indicates that the swift heavy ions have improved the sensitivity of Ga-doled ZnO thin film by reducing the particle size. The Si6+ ion irradiated Ga-doped ZnO thin films were also exposed to ethanol and acetone gas for gas sensing applications. In comparison to Ag9+ ion irradiated thin film, the film irradiated with Si6+ ion beam exhibits a greater sensing response to both ethanol and acetone gas.

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.

Comparing Thermal Durability and Effects of Annealing Temperature on Characteristics of Hydrogen-Doped ZnO, AZO, and GZO Thin Films

In this work, undoped, aluminum-, and gallium-doped ZnO thin films (ZnO-H, AZO-H, and GZO-H, respectively) deposited on soda-lime glass substrates by magnetron sputtering method in a gas mixture of hydrogen and argon are annealed at various temperatures in the range of 200–500°C in air to evaluate the durability of those films under annealing temperature. From photoluminescence spectra, formation of point defects, especially oxygen vacancies, when hydrogen diffuses out of the films at high annealing temperature is exhibited via a significant increase of visible emissions. We find out that carrier concentration and Hall mobility of AZO-H and ZnO-H films dramatically decrease, while those of GZO-H film are still stable as the annealing temperature increased from 200°C to 300°C. We proposed a model for interpreting the thermal durability of GZO-H film that, at an annealing temperature of 300°C, Ga3+ ions located at adjacent Zn sites can push hydrogen atoms, which are broken out of the antibonding sites which are perpendicular to the c -axis (AB┴), into bond center sites paralleled to the c -axis (BC//). The movement of hydrogen from AB┴ to BC// site also gives rise to the durability of electrical properties of GZO-H films at the high annealing temperature.

Resistive switching and synaptic properties modifications in gallium-doped zinc oxide memristive devices

The massively parallel computing capabilities of the human brain can be mimicked with the help of neuromorphic computing approach and this can be achieved by developing the electronic synaptic device. In the present work, we have synthesized gallium-doped ZnO thin films using a cost-effective hydrothermal method and characterized the thin films using field-emission scanning electron microscopy and energy-dispersive X-ray spectroscopy. Furthermore, gallium-doped ZnO memristive devices were developed using standard procedure and electrically characterized for the neuromorphic application. In particular, resistive switching and synaptic properties of gallium-doped ZnO thin films were investigated. The bipolar resistive switching with an analog memory like behavior was observed in the developed memristive devices. In the present case, good synaptic properties, endurance, and retention characteristics were observed for 0.5% Ga doped memristive device. Our results suggested that the synaptic weight, potentiation-depression, and symmetric Hebbian learning can be tuned with properly engineering the ZnO memristive device with appropriate gallium doping. The detailed analysis of I-V results suggested that resistive switching is occurred due to Ohmic and Schottky conduction mechanisms.

The study of transparent conducting gallium doped ZnO thin films in order to use in solar cells

In this study, transparent conducting Ga-doped ZnO thin films were deposited on glass substrate using chemical spray pyrolysis technique. The effect of Ga-doping concentration (0, 1, 2 and 3 at.%) on microstructural, optical and electrical characteristics of layers have been investigated. The studies of X-ray diffraction and optical transmission spectra show these films have a hexagonal wurtzite structure with (002) preferred growth direction, also a high transmission of 85-95% in visible range. Data analysis show that the band gap energies in these films are varying in the range of 3.27-3.33 eV, consistent with the Burstein-Moss shift effect, with Urbach tail widths between 114-160 meV. The 2 wt% Ga sample showed the maximum figure of merit (3×10-2Ω-1), with an electron concentration and sheet resistance of ~1.42×1019 cm-3 and 13 kΩ/square, respectively. Â

High energy electron irradiation effects on Ga-doped ZnO thin films for optoelectronic space applications

Gallium-doped ZnO (GZO) thin films of thickness 394 nm were prepared by a simple, cost-effective sol–gel spin coating method. The effect of 8 MeV electron beam irradiation with different irradiation doses ranging from 0 to 10 kGy on the structural, optical and electrical properties was investigated. Electron irradiation influences the changes in the structural properties and surface morphology of GZO thin films. X-ray diffraction analysis showed that the polycrystalline nature of the GZO films is unaffected by the high energy electron irradiation. The grain size and the surface roughness were found maximum for the GZO film irradiated with 10 kGy electron dosage. The average transmittance of GZO thin films decreased after electron irradiation. The optical band gap of Ga-doped ZnO films was decreased with the increase in the electron dosage. The electrical resistivity of GZO films decreased from 4.83 × 10−3 to 8.725 × 10−4 Ω cm, when the electron dosage was increased from 0 to 10 kGy. The variation in the optical and electrical properties in the Ga-doped ZnO thin films due to electron beam irradiation in the present study is useful in deciding their compatibility in optoelectronic device applications in electron radiation environment.

Aluminium- and gallium-doped homoepitaxial ZnO thin films: Strain-engineering and electrical performance

Aluminium- and gallium-doped ZnO thin films with nominal source target compositions of and were grown homoepitaxially and in-plane lattice matched on c-plane ZnO single crystalline substrates. In dependence on dopant concentration and oxygen partial pressure during growth we found compressive or tensile out-of-plane epitaxial strain up to . Electrical investigations showed that the conductivity of the substrates alters transport properties at room temperature. Correct interpretation requires variable temperature Hall effect measurements. These reveal, that besides the contributions of the substrate and the deposited thin film an additional conduction channel, likely located at the surface of the samples must be considered to explain and model the temperature dependence of the free electron concentration and their mobility.

Ultraviolet random lasing from Mg0.12Zn0.88O:N/ZnO:Ga single-heterostructure diode

A heterostructure device consisting of nitrogen-doped Mg0.12Zn0.88O and gallium-doped ZnO thin films was grown on c-plane sapphire substrate using RF plasma-assisted molecular beam epitaxy. Current–voltage and photocurrent characteristics indicate the formation of a p–n junction. Random lasing behavior with lasing modes centered at 356 nm was observed. A low-threshold current of 6 mA was determined, and an output power of 34 nW was measured at an injection current of 8 mA. The film contains columnar structures with much air gaps, which assist in light scattering to achieve necessary gain for random lasing.

Structural, morphological, optical and compositional characterization of spray deposited Ga doped ZnO thin film for Dye-Sensitized Solar Cell application

Zinc Oxide and Gallium doped Zinc Oxide films have been deposited by the Spray Pyrolysis method onto preheated glass substrates using Zinc acetate and Gallium (III) acetyl acetonate as precursors for Zn and Ga ions, respectively. The effect of ZnO and Ga doping on the structural, morphological, optical and chemical properties of sprayed ZnO and Gallium doped ZnO thin films were investigated. XRD studies reveal that the films are crystalline with hexagonal (wurtzite) crystal structure. The average transparency in the visible range was around 75% for the thin film deposited using Gallium doping. X-ray Photoelectron Spectroscopy (XPS) was utilized to view the changes in the oxidation state of ZnO and Gallium doped ZnO thin films. The ZnO and Gallium doped ZnO thin film has been deposited above the Indium Tin Oxide (ITO) coated glass substrate. The efficiency of the obtained DSSC measured for 0.1M ZnO thin film by sensitizing for 12h was, ŋ=2.5%. Similarly for Gallium doped Zinc Oxide, the efficiency ŋ is found to be 3.9%, 4.1% and 4.3% for every increase in doping of Gallium concentrations which is utilized for the application of Dye-Sensitized Solar Cell.

Comparative study of thermal stability of aluminium- and gallium-doped ZnO thin films prepared by sol–gel method

To study the thermal stability of Al- and Ga-doped ZnO (AZO and GZO) films, these films with a doping concentration from 0·5 to 3 at-% have been prepared by sol–gel method on glass substrate followed by annealing in air. It has been found out that the AZO and GZO films have similar thermal stability, and Ga dopant cannot improve the stability. Moreover, the doping concentration has no prominent effect on the thermal stability. The resistivity of the films is stable when the annealing temperature is lower than 250°C and increases rapidly after that, while the transmittance increases with annealing temperature. The variations of the resistivity and the transmittance are closely related to the microstructure, and vacancy and electron concentration affected by the annealing treatment.

Sol-Gel processed GZO thin film from low concentration solution and investigating GZO/Cs2CO3 bilayer

Highly transparent Gallium doped ZnO (GZO) thin films with different ratios of Gallium were synthesized by sol-gel process and dip coating technique. Concentration of the used solutions for dip coating was 0.09 M. Morphological investigations were done on GZO thin films with 0, 0.5, 1 and 2 at. % gallium content. Influences of doping ratio and post-annealing temperature on optical and electrical properties of the films were also studied. It was found that the film prepared from the solution containing 2 at. % Ga had improved structural properties, less electrical resistivity and biggest optical transmittance. The thickness dependence of the band gap of 2 at. % GZO thin films was also examined. Effect of Cs2CO3 nanolayer and ZnO thin film deposited on 2 at. % GZO thin film was studied. SEM images exhibited island features on the surface of GZO/Cs2CO3 bilayer.

Ga-doped ZnO conducting antireflection coatings for crystalline silicon solar cells

Transparent, conductive gallium-doped ZnO thin films are evaluated for application as conducting antireflection coatings (ARC) for crystalline silicon solar cells as a means to enhance efficiency by reducing the overall resistivity of the photovoltaic circuit. All Ga-doped ZnO thin films in this study were deposited using pulsed laser deposition. Synthesis conditions were first optimized for maximum electrical resistivity and minimal visible light absorption. The ideal combination contained 1 mol. % Ga doping and exhibited ∼90% transmission, with resistivity in the 1 × 10−3 ohm-cm range. Optimized films were prepared on reference flat silicon wafers with known dopant densities and on commercially obtained solar cell emitters without ARCs. Circular transmission line method measurements were used to measure specific contact resistivity (ρc). For n-type doped solar cell emitters, contact resistivity values of 0.1 mΩ cm2 were observed repeatedly. These values are consistent with, or lower than, contact resistivities associated with conventional silver paste metallization.

Optimization of gallium-doped ZnO thin films grown using Grey–Taguchi technique

In this paper, optimization of the process parameters considering multiple performance characteristics to prepare the transparent conducting gallium-doped zinc oxide thin films with radio frequency (RF) magnetron sputtering was investigated. Experiments based on the Grey---Taguchi technique were conducted to examine the effect of deposition parameters including RF power, process pressure, substrates temperature and process time, aiming to obtain highly transparent and conductive films. Comparing with the optimal parameter set selected from orthogonal array by Taguchi method, the optimal grey theory prediction design (GTPD) can receive an improvement of 5.75 % in electrical resistivity and 1.47 % in optical transmittance. Further refinements respectively to RF power and process pressure with fixing other parameters level in GTPD were explored. The results show the alteration on RF power and process pressure in the GTPD can receive over 31 % and 51 % of improvement in electrical resistivity, respectively, with keeping the visible range optical transmittance over 85 %.

Radio-frequency superimposed direct current magnetron sputtered Ga:ZnO transparent conducting thin films

The utilization of radio-frequency (RF) superimposed direct-current (DC) magnetron sputtering deposition on the properties of gallium doped ZnO (GZO) based transparent conducting oxides has been examined. The GZO films were deposited using 76.2 mm diameter ZnO:Ga2O3 (5 at. % Ga vs. Zn) ceramic oxide target on heated non-alkaline glass substrates by varying total power from 60 W to 120 W in steps of 20 W and at various power ratios of RF to DC changing from 0 to 1 in steps of 0.25. The GZO thin films grown with pure DC, mixed approach, and pure RF resulted in conductivities of 2200 ± 200 S/cm, 3920 ± 600 S/cm, and 3610 ± 400 S/cm, respectively. X-ray diffraction showed all films have wurtzite ZnO structure with the c-axis oriented perpendicular to the substrate. The films grown with increasing RF portion of the total power resulted in the improvement of crystallographic texture with smaller full-width half maximum in χ and broadening of optical gap with increased carrier concentration via more efficient doping. Independent of the total sputtering power, all films grown with 50% or higher RF power portion resulted in high mobility (∼28 ± 1 cm2/Vs), consistent with observed improvements in crystallographic texture. All films showed optical transmittance of ∼90% in the visible range.

Resistivity Stability of Ga Doped ZnO Thin Films with Heat Treatment in Air and Oxygen Atmospheres

The effect of annealing in air and oxygen on structural, electrical and optical properties of gallium doped ZnO thin films was investigated. The X-ray diffraction patterns showed that the films were highly preferentially oriented along (002) plane. After the heat treatment in air and oxygen environments, the intensity of (002) peak was apparently improved. It was found that heat treatment in air atmospheres lead to increase in surface roughness of the film. The GZO films annealed in oxygen at 673 K exhibited low resistivity of 4.21 × 10–3 Ω.cm, while the resistivity of film annealed in air showed a slightly higher value of 7.14 × 10–3 Ω.cm. In addition to this, all films have good optical transmittance about 80% in the visible region. It is found from the photoluminescence studies that the broad visible emissions in GZO films originated from the intrinsic shallow traps (VZn) and deep level vacancies (ZZi, OZn and Vo)

Influence of the grain boundary barrier height on the electrical properties of Gallium doped ZnO thin films

The pulsed laser deposition (PLD) technique is used to deposit Gallium doped zinc oxide (GZO) thin films on glass substrates at 250 with different Gallium (Ga) doping concentration of 0, 1.0, 3.0 and 5.0%. The influence of Ga doping concentration on structure, chemical atomic compositions, electrical and optical properties was investigated by XRD, XPS, Hall measurement and UV spectrophotometer, respectively. The relationship between electrical properties and Ga doping concentration was clarified by analyzing the chemical element compositions and the chemical states on the GZO films. It is found that the carrier concentrations and oxygen vacancies in the GZO films increase with increasing Ga doping concentration. The lowest resistivity (3.63 × 10 −4 Ω cm) and barrier height of grain boundaries (14 mV) were obtained with 3% Ga doping. In particular, we suppose the band gap of 5% Ga doping sample larger than that of 3% Ga doping sample is due to the quantum size effect from the amorphous structure rather than Moss–Burstein shift.

Growth of Cu2O on Ga-doped ZnO and their interface energy alignment for thin film solar cells

Cu 2 O thin films are deposited by direct current reactive magnetron sputtering on borofloat glass and indium tin oxide (ITO) coated glass at room temperature. The effect of oxygen partial pressure on the structures and properties of Cu2O thin films are investigated. We show that oxygen partial pressure is a crucial parameter in achieving pure phases of CuO and Cu2O. Based on this finding, we fabricate heterojunctions of p-type Cu2O with n-type gallium doped ZnO (GZO) on ITO coated glass substrates by pulsed laser deposition for GZO thin films. The energy band alignment for thin films of Cu2O/GZO on ITO glass is characterized using high-resolution x-ray photoelectron spectroscopy. The energy band alignment for the Cu2O/GZO heterojunctions is determined to be type II with a valence band offset of 2.82 eV and shows negligible effects of variation with gallium doping. The higher conduction band of the Cu2O relative to that of GZO in the obtained band alignment shows that the heterojunctions are suitable for solar cell application based on energy levels consideration.

Characteristics of Gallium Doped ZnO Thin Films Deposited by Sol Gel Dip Coating

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