Gallium-doped Zinc Oxide Research Articles

Influence of sputtering power on the properties of thin layers of GZO for photovoltaic applications

In the present work, we have deposited gallium doped zinc oxide thin films by magnetron sputtering technique based nanocrystalline particles elaborated by sol–gel method. In the first step, the nanoparticles were synthesized by sol–gel method using supercritical drying in ethyl alcohol. The structural studied by X-ray diffractometry indicates that GZO has a polycrystalline hexagonal wurzite structure with a grain size of about 30 nm. Transmission electron microscopy measurements have shown that very small particles of GZO are present in the aerogel powder. Then, thin films were deposited onto glass substrates by rf-magnetron sputtering at ambient temperature. The influence of RF sputtering power on structural, morphological, electrical, and optical properties were investigated. It has been found that all of the films deposited were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (002) crystallographic direction. The film deposited at 60 W showed the lowest electrical resistivity of 3.5 × 10−3 Ω cm achieved through the highest Hall mobility of 9.30 cm2 V−1 s−1. All the GZO films in this study showed the optical transmittances higher than 80 %.

Ultrabroadband terahertz conductivity of highly doped ZnO and ITO

The broadband complex conductivities of transparent conducting oxides (TCO), namely aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO) and tin-doped indium oxide (ITO), were investigated by terahertz time domain spectroscopy (THz-TDS) in the frequency range from 0.5 to 18 THz using air plasma techniques, supplemented by the photoconductive antenna (PCA) method. The complex conductivities were accurately calculated using a thin film extraction algorithm and analyzed in terms of the Drude conductivity model. All the measured TCOs have a scattering time below 15 fs. We find that a phonon response must be included in the description of the broadband properties of AZO and GZO for an accurate extraction of the scattering time in these materials, which is strongly influenced by the zinc oxide phonon resonance tail even in the low frequency part of the spectrum. The conductivity of AZO is found to be more thickness dependent than GZO and ITO, indicating high importance of the surface states for electron dynamics in AZO. Finally, we measure the transmittance of the TCO films from 10 to 200 THz with Fourier transform infrared spectroscopy (FTIR) measurements, thus closing the gap between THz-TDS measurements (0.5-18 THz) and ellipsometry measurements (200-1000 THz).

Magnetoresistance of thin films due to weak localization under the variation of the dimensionality induced by the magnetic field and temperature

The magnetoresistance due to the weak localization of electrons for films with thickness comparable with the electron diffusion length during the phase relaxation time of the electron wavefunction or with the magnetic length has been analyzed theoretically and studied experimentally. The expression for the magnetoresistance of thin films at the arbitrary relation between the film thickness and the electron diffusion length during the phase relaxation time of the electron wavefunction has been derived. It has been shown that the derived expression describes well the magnetoresistance of gallium-doped zinc oxide films under conditions of the variation of their effective dimension with respect to weak localization induced by the magnetic field and temperature.

Highly reliable switching via phase transition using hydrogen peroxide in homogeneous and multi-layered GaZnO(x)-based resistive random access memory devices.

Here, we propose an effective method for improving the resistive switching characteristics of solution-processed gallium-doped zinc oxide (GaZnO(x)) resistive random access memory (RRAM) devices using hydrogen peroxide. Our results imply that solution processed GaZnO(x) RRAM devices could be one of the candidates for the development of low cost RRAM.

Solvothermal synthesis of gallium-doped zinc oxide nanoparticles with tunable infrared absorption

The doping of ZnO nanoparticles (NPs) has been attracting a lot of attention both for fundamental studies and potential applications. In this manuscript, we report the preparation of gallium doped zinc oxide (GZO) NPs through the solvothermal method. In order to obtain the effective Ga doping in the ZnO crystalline lattice, we identified the optimal reaction conditions in terms of different Zn precursors, temperature, and heating rate. The results show that GZO NPs with tunable infrared absorption can be received using different molar ratios of Ga(NO3)3 and zinc stearate (Zn[CH3(CH2)16COO]2, ZnSt2) kept in the sealed autoclaves at 160 °C for 8 h. Furthermore, the growth of the GZO NPs was investigated by monitoring the optical absorption spectral and the corresponding chemical composition of aliquots extracted at different reaction time intervals.

Enhancement of efficiency in dye-sensitized solar cell using spray deposited Ga doped ZnO thin films

Gallium doped Zinc Oxide thin films have been deposited by means of spray pyrolysis technique onto preheated glass substrates using Gallium (III) acetyl acetonate as precursor for Ga ions. The effect of Ga doping on the structural, morphological and optical properties of sprayed Ga doped ZnO thin films were investigated using X-ray diffraction, Scanning electron microscopy, and Optical absorption techniques. The films are crystalline with hexagonal (wurtzite) crystal structure with preferential orientation along (002) plane. From SEM studies it is confirmed that the grains were distributed randomly. The average transparency in the visible range was found to be 70 %. The Gallium doped ZnO thin film has been deposited above the Indium Tin Oxide (ITO) coated glass substrate. Bromophenol and platinum coated ITO substrate were used as the photo sensitizer and counter electrode, respectively. The efficiency of the obtained DSSC measured by sensitizing for 12 h was found to be η = 0.44, 0.50 and 0.60 % for every increase in doping of Gallium concentrations which can be utilized for the application of Dye-Sensitized Solar Cell.

Scattering matrix analysis for evaluating the photocurrent in hydrogenated-amorphous-silicon-based thin film solar cells.

A scattering matrix (S-matrix) analysis method was developed for evaluating hydrogenated amorphous silicon (a-Si:H)-based thin film solar cells. In this approach, light wave vectors A and B represent the incoming and outgoing behaviors of the incident solar light, respectively, in terms of coherent wave and incoherent intensity components. The S-matrix determines the relation between A and B according to optical effects such as reflection and transmission, as described by the Fresnel equations, scattering at the boundary surfaces, or scattering within the propagation medium, as described by the Beer-Lambert law and the change in the phase of the propagating light wave. This matrix can be used to evaluate the behavior of angle-incident coherent and incoherent light simultaneously, and takes into account not only the light scattering process at material boundaries (haze effects) but also nonlinear optical processes within the material. The optical parameters in the S-matrix were determined by modeling both a 2%-gallium-doped zinc oxide transparent conducting oxide and germanium-compounded a-Si:H (a-SiGe:H). Using the S-matrix equations, the photocurrent for an a-Si:H/a-SiGe:H tandem cell and the optical loss in semitransparent a-Si:H solar cells for use in building-integrated photovoltaic applications were analyzed. The developed S-matrix method can also be used as a general analysis tool for various thin film solar cells.

Taguchi method-optimized sputter deposition of hydrogenated gallium-doped zinc oxide films in argon/hydrogen ambient

Transparent conductive films of hydrogenated gallium-doped zinc oxide (HGZO) were deposited on glass under various deposition conditions (ratio of H2 to Ar, RF power, sputtering pressure and time) by RF magnetron sputtering in Ar+H2 ambient at room temperature (RT). In this work, the Taguchi method was used to find optimal deposition conditions and it was found that sputtering time and ratio of H2 to Ar were significantly influencing parameters on figure of merit of HGZO films. For the HGZO film grown under the optimal condition, the highest figure of merit of 33.94×10−3Ω−1, i.e. the lowest sheet resistance of 10.62Ω/sq (ρ=3.40×10−4Ωcm) and high transmittance of 90.03% were obtained. In this case, hall mobility and carrier concentration are 8.87cm2V−1s−1 and 1.177×1021cm−3, respectively.

Comparative study of highly dense aluminium- and gallium-doped zinc oxide transparent conducting sol-gel thin films

Transparent conducting aluminium- and gallium-doped zinc oxide (AZO and GZO) layers have been deposited by spin coating on glass substrates. The coatings have been sintered in air at 450 °C for 30 min and then post-annealed at 350 °C in a reducing atmosphere for 30 min. The electrical, optical and morphological properties of both coatings have been studied and compared. The conventional sols lead to very thin coating, typically 24 nm for a single layer of AZO and 32 nm of GZO with electrical resistivity of 0.72 and 0.35 Ω cm, respectively. The value however, drastically decreases down to a minimum of 2.6 × 10−2 Ω cm for AZO and 1.76 × 10−2 Ω cm for GZO, when five multilayer coatings are made. The origin of these differences is due to the different morphology of the coatings showing different electron scattering process. The GZO sol leads to denser smoother structure (porosity of 5%) layers with an average roughness of 2.76 Å, while the AZO coating is formed by a more porous assembly (porosity of 20%) with an average roughness of 3.46 Å. Both coatings exhibit high transparency (T > 85%) in the visible spectrum range with a slight shift of the absorption energy gap.

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.

Preparation of Transparent Metal Films, Gallium-Doped Zinc-Oxide (Ga2O3)x (ZnO)100-x Films by Using Facing Target Sputtering System

(Ga2O3)x(ZnO)100-x, x = 3, 5, 7, 9 wt.%, (GZO) films were prepared at room temperature by using a conventional rf-magnetron sputtering method. Their electrical resistivity was investigated as a function of the Ga2O3 content. The GZO film with x = 7 wt.%, shows the lowest resistivity of 1.5×10−3Ω. cm. This GZO films were also prepared at various substrate temperatures from room temperature to 400°C, Their electrical resistivity was found to be improved as the substrate temperature was increased, A very low resistivity of 4.5×10−4Ω.cm was obtained in the film prepared at the substrate temperature of 300°C. In addition, we found that the GZO films prepared by using facing target sputtering (FTS) system showed a dramatically improved conductivity as compared with that of the film prepared by the conventional sputtering system. In particular, we also found that the lowest resistivity of 2.8×10−4Ω.cm that is almost comparable with that of ITO film was obtained in the GZO films prepared at the substrate temperature of 300°C by using the facing target sputtering (FTS) system.

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.

Trajectory effect on the properties of large area ZnO thin films deposited by atmospheric pressure plasma jet

Abstract Large area (117 mm × 185 mm) gallium-doped zinc oxide (GZO) films are prepared on glass substrates by atmospheric pressure plasma jet (APPJ) technique. The uniformity of material properties, in particular the electrical resistivity, of the deposited film is of great importance in reducing design complexity of the electron devices. We investigate the effects of scanning trajectory recipe (speed, pitch and number of passes) on structural and electrical properties of GZO thin films. We find that the trajectory has significant effects on the magnitude and uniformity of sheet resistance over the glass substrates. For single pass, the resistance appears higher at the starting part of spray, whereas, for cases of multiple passes, the highest resistance appears in the central part of the substrate. XRD, SEM, Hall measurement and residual stress are used to study the film properties and identify root causes of the nonuniform distribution of sheet resistance. We conclude that annealing time is the dominant root cause of the nonuniform resistance distribution, and other factors such as residual stress and structural characteristics may also have contributions.

Improving the optoelectronic properties of gallium ZnO transparent conductive thin films through titanium doping

The optoelectronic characteristics of gallium-doped ZnO ternary alloy transparent conductive oxide were improved through titanium (Ti) doping, and showed enhanced optoelectronic properties, and crystallite quality by radio frequency magnetron sputtering. The room-temperature photoluminescence measurement showed improved near band edge emission for ZnO peaks, and suppressed deep defect level emission at the green light band. The study results show that the lowest thin film resistivity was 4.95×10−4Ω-cm, with a mobility and carrier concentration of 4.8cm2/V-s and 2.64×1021cm−3, respectively. The superior carrier concentration of Ti-doped GZO alloys (>1021cm−3) with high figure of merit (35.3×10−3Ω−1), demonstrating the pronounced contribution made by Ti doping, and is highly suitable for the optoelectronic device applications.

Transparent conducting ZnO-Based thin films deposited by magnetron sputtering of a ZnO:Ga-C composite target

We have studied the structure, electrical, and optical properties of thin transparent conducting films based on gallium-doped zinc oxide (ZnO:Ga), which were deposited by magnetron sputtering of a ZnO:Ga-C composite target. The ion-bombardment-induced interaction of ZnO with carbon in a thin surface layer of the target leads to an increase in the excess zinc content in the reagent flow. The formation of transparent conducting ZnO:Ga films at a substrate temperature above 100°C in the presence of excess zinc near the growth surface leads to improvement in the structure and conductivity of films without decreasing their transmission in the visible spectral range.

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.

Enhancement in light extraction efficiency of organic light emitting diodes using double-layered transparent conducting oxide structure

We examined double-layered transparent conducting oxide (TCO) anode structures consisted of zinc-doped indium oxide (IZO) over the gallium-doped zinc oxide (GZO), and IZO over the GZO with electrochemical treatment. In bottom type OLEDs, power efficiency and current efficiency were enhanced by a factor of 1.50 and 1.14 at a current density of 10mA/cm2 in IZO/GZO anode structure, compared to the only IZO anode structure. Due to the reduced sheet resistance of the IZO/GZO TCO surface, the operating voltage of the OLED with IZO/GZO anode structure was lowered, leading to mostly enhance power efficiency. More enhanced in power efficiency and current efficiency by a factor of 1.21 and 1.25 at a current density of 10mA/cm2 were achieved in IZO/GZO anode structure with electrochemical treatment, compared to the IZO/GZO anode structure due to the change of the surface morphology of the GZO and the existence of the nanoporous layer beneath the GZO surface by an electrochemical treatment. In total, double-layered IZO/GZO anode structure with electrochemical treatment was revealed at an enhancement factor of 1.80 in power efficiency and 1.42 in current efficiency, compared to the only IZO anode structure.

Gallium-doped zinc oxide targets fabricated by sintering: Impact of target quality on sputtered thin film properties

This research reports the sintering mechanism of Ga doped ZnO (GZO) ceramic targets. The GZO powders with different Ga doping concentrations were synthesized by chemical co-precipitation method and compacted to form green compacts. The green compacts underwent a normal sintering process under different temperatures. Then a series of GZO targets sintered at 1300°C were used for depositing thin films by DC magnetron sputtering method. The influences of sintering temperature and doping concentration on the densification, structural and electrical properties of GZO targets were investigated. Results showed that Ga doping effectively promoted the qualities of GZO targets. When sintering temperature surpassed 1300°C, the relative densities for all the targets were higher than 97%. The lowest resistivity of 2.25×10−3Ωcm was attained on the 0.5at% Ga doped ZnO sintered at 1300°C. In addition, the performances of GZO thin films were greatly influenced by the qualities of their sputtering targets.

The sintering behavior, microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets

Abstract The properties of sputtering targets have recently been found to affect the performances of sputtered films and the sputtering process. To develop high-quality GZO ceramic targets, the influences of Ga 2 O 3 content and sintering temperature on the sintering behavior, microstructure, and electrical properties of GZO ceramic targets were studied. The results showed that the increase in Ga 2 O 3 content from 3 wt% (GZO-3Ga) and 5 wt% (GZO-5Ga) not only inhibited the densification but retarded grain growth. During sintering, ZnGa 2 O 4 phase formed before 800 °C, and Zn 9 Ga 2 O 12 phase was found after sintering at 1000 °C. Moreover, after sintering at 1200 °C, the number of Zn 9 Ga 2 O 12 precipitates increased at the expense of ZnGa 2 O 4 and ZnGa 2 O 4 disappearing completely. The relative density, grain size, and resistivity of GZO-3Ga sintered at 1400 °C in air were 99.3%, 3.3 μm, and 2.8 × 10 −3 Ω cm, respectively. These properties of GZO ceramics are comparable to properties reported in the literature for AZO sintered in air.

Ga-Doped ZnO Films Deposited by Atmospheric Pressure Plasma

Deposition of gallium-doped zinc oxide (GZO) thin films using atmospheric pressure plasma jet (APPJ) system is presented in this work. High quality GZO films were demonstrated: The resistivity of as-deposited film achieves up to ~7×10-4 ohm-cm, which is comparable to that deposited using vacuum process. Further, the optical transmission with optimized thickness is > 89 % at wavelength of 550 nm. The Hall mobility increased as GZO deposition thickness increase to 300 nm. In order to study thermal stability of GZO thin films, the effect of thermal annealing on the optical and electrical properties was studied. Samples annealed in vacuum and in air showed opposite characteristics: resistivity decrease for vacuum annealing samples and increase for air annealing samples. Carrier reduction mainly attributed to the resistivity increase in air annealing. Mobility increases but carrier concentration decreases when samples were annealed in vacuum: The combined effects resulted in resistivity decrease to half of the prior-annealing values after 500 °C vacuum annealing. The GZO thin films used in capacitive touch sensors were also evaluated. We demonstrated that APPJ-deposited GZO thin films can be successfully applied to touch sensors in our work. These results indicate that our APPJ system can deposit good quality TCO films, which have potential to be applied in optoelectronics field.