Ga-doped Zinc Oxide Thin Films Research Articles

Enhanced photovoltaic performance of silicon-based solar cell through optimization of Ga-doped ZnO layer

In the present study, the impact of deposition pressure and substrate temperature of Ga-doped Zinc Oxide (GZO) thin film and the photovoltaic performance of this structure as a transparent conductive oxide (TCE) layer in silicon-based solar cell were investigated. Implementing a single target of GZO, the structural, optical, and electrical properties of 350 nm thick GZO thin films with various deposition pressure (5 mTorr, 10 mTorr, 15 mTorr and 20 mTorr) at room temperature (RT) and substrate temperature (RT, 150 °C, 200 °C, 250 °C) at 15 mTorr deposition pressure were fabricated using RF magnetron sputtering technique. The aim here was to find out the GZO films with the optimum pressure and substrate temperature to incorporate them into solar cell as a TCE layer. The X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques were used to determine the structural properties of all samples. The optical transmission measurements were performed using spectroscopic Ellipsometer and the band gap values were calculated by Tauc plot using optical transmission data. In addition, the electrical characterization of the GZO samples were analyzed by the Van der Pauw method and Hall measurements. Finally, the most promising GZO thin film was determined based on the structural and optoelectrical characterization. The findings indicated that the XRD pattern of all the prepared films was dominated by (002) preferential orientation irrespective of the deposition pressure and substrate temperature. The AFM measurements showed that all the samples had a dense surface morphology regardless of the deposition pressures, but the surface morphology of the samples was clearly changed upon increasing substrate temperatures. The transmission values of the film did not significantly alter (∼82%) when the deposition pressures except for the substrate temperature of 200 °C (86%) were changed. The band gap values were calculated between 3.30 eV and 3.36 eV, which can be associated with enhancement of crystalline quality of the films. The lowest resistivity and the highest carrier concentration values belonged to the film fabricated at 15 mTorr@200 °C by 2.0 × 10−3 Ω.cm and 1.6 × 1020 cm−3, respectively. Both increasing the deposition pressure (up to 15 mTorr) and substrate temperature (up to 200 °C) contributes to improving the crystallite size, widening the optical band gap, lowering the resistivity, and increasing the carrier concentration. In order to evaluate and compare the effect of both deposition pressure and substrate temperature, Silicon-based solar cells were fabricated using the most promising layers (15 mTorr@RT, 15 mTorr@200 °C). The cell performance with the GZO thin film as a TCE layer showed that varying both the pressure and substrate temperature of the GZO film contributed to enhancing the solar cell parameters. Thus, the conversion efficiency increased from 9.24% to 12.6% with the sequential optimization of pressure and temperature. It can be concluded that the pressure applied during the deposition and substrate temperature had a significant impact on the properties of GZO thin films and its photovoltaic performance of solar cell used as TCE layer.

Annealing Effects on Gas Sensing Response of Ga-Doped ZnO Thin Films.

The high thermal conductivity, high electron mobility, the direct wide band gap, and large exciton binding energy of zinc oxide (ZnO) make it appropriate for a wide range of device applications like light-emitting diodes, photodetectors, laser diodes, transparent thin-film transistors, and so forth. Among the semiconductor metal oxides, zinc oxide (ZnO) is one of the most commonly used gas-sensing materials. The gas sensor made of nanocomposite ZnO and Ga-doped ZnO (ZnO:Ga) thin films was developed by the sol–gel spin coating method. The gas sensitivity of gallium-doped ZnO thin films annealed at 400, 700, and 900 °C was studied for ethanol and acetone gases. The variation of electrical resistance of gallium-doped ZnO thin films with exposure of ethanol and acetone vapors at different concentrations was estimated. Ga:ZnO thin films annealed at 700 °C show the highest sensitivity and shortest response and recovery time for both ethanol and acetone gases. This study reveals that the 5 at. % Ga-doped ZnO thin film annealed at 700 °C has the best sensing property in comparison to the film annealed at 400 and 900 °C. The sensing response of ZnO:Ga thin films was found higher for ethanol gas in comparison to acetone gas.

A sandwich structure assisted by defect engineering for higher thermoelectric performance in ZnO‐based films

AbstractGallium (Ga) doping together with low dimensionality has been a promising approach to improve thermoelectric performance of zinc oxide (ZnO) materials, due to the increase of carrier concentration and suppression of phonon transport. So far, the highest power factor of Ga‐doped ZnO (GZO) thin films has reached 280 μW m−1 K−2, which is still limited for practical applications. In this work, we have simultaneously optimized the electrical conductivity and Seebeck coefficient of GZO thin films using the combination of oxygen defects and sandwich structure (GZO‐ZnO‐GZO). Benefiting from energy filtering effect at the interface between GZO and ZnO layers and high oxygen vacancy concentration, the density of states (DOS) effective mass has been increased together with a relatively high carrier concentration. As a result, an improved power factor value of 434 μW m−1 K−2 at 623 K has been achieved, which is comparable to the best values reported for ZnO‐based films. This method of combining defect engineering and sandwich structure design shows great potential in enhancing the thermoelectric performance of ZnO‐based thin films or other oxide materials.

Monitoring the characteristic properties of Ga-doped ZnO by Raman spectroscopy and atomic scale calculations

We experimentally and theoretically study how the structural and vibrational properties of zinc oxide (ZnO) are modified upon Gallium (Ga) doping. The characteristics of Ga-doped ZnO thin films which are synthesized by sol-gel spin coating method on glass substrates are monitored by using X-ray diffraction (XRD) and Raman scattering measurements. For atomic-level understanding of the experimental findings state-of-the-art density functional theory (DFT) based calculations are also performed. DFT calculations reveal that both the substitution and adsorption of Ga atoms in ZnO are energetically possible and substitutional doping in ZnO is the most favourable scenario. XRD measurements show that all the films are in wurtzite structure and the crystallite size of the films decreases with increasing Ga doping. In addition, Raman analysis show that strong vibrational modes at about 100 and 441 cm−1 are associated with E2low and E2high phonon branches of ZnO, respectively. While the frequency of the E2low mode downshifts with increasing Ga concentration, the E2high phonon mode is not affected by the Ga doping. Furthermore, EGa phonon branch, stemming from the substituted Ga atoms, emerges at low frequencies. It is also seen that the Raman intensity of the EGa peak linearly increases with increasing Ga concentration. Experimental results on the vibrational properties are in good agreement with the ab initio phonon calculations.

First principle calculations and opto-electric enhancement in laser ablated GZO thin films

Optical and electrical properties of Ga-doped Zinc Oxide (GZO) has been studied in the present article. Density functional theory and Hubbard U (DFT + Ud + Up) based first principle calculations were employed for theoretical estimation whereas Laser ablation method has been used to fabricate GZO thin films on p-GaN, Al2O3 and p-Si substrate for experimental analysis. Single crystal growth of GZO thin films with (002) preferred crystallographic orientation has been shown in X-ray diffraction graphs. The elemental composition of all samples has been studied via EDS (energy dispersive X-ray) spectroscopy, no other unwanted impurity related peaks were found which indicates the impurity-free growth of GZO. Noodle, seed and granular-like structures of GZO/GaN, GZO/Al2O3, and GZO/Si have been revealed via Field-emission scanning electron microscopy micrographs respectively. The morphological analysis suggested GaN substrate as the best candidate for uniform and better quality GZO thin films. Highest carrier mobility (53 cm2/V s) with higher carrier concentration (> 1020 cm−3) has been found with low (1800) laser shots. Fivefold photoluminescence enhancement in the noodle-like structure of GZO/GaN with compared to GZO/Al2O3 and GZO/Si has been recorded. As the noodle-like structure supposed to be a more favorable structure for enhanced optical properties of GZO which points toward shape-driven optical properties. Theoretical (3.539 eV) and experimental (3.43, 3.6 eV) values of band-gap were found quite comparable. Moreover, lowest resistivity (3.5 × 10−4 Ω cm) with 80% transmittance is evident for GZO as a successful alternate of ITO.

Dopant-driven enhancements in the optoelectronic properties of laser ablated ZnO: Ga thin films

Theoretically and experimentally evaluated optoelectronic properties of GZO (Ga-doped zinc oxide) were correlated in the present article. Density functional theory and Hubbard U (DFT + Ud + Up) first-principle calculations were used for the theoretical study. The pulsed laser deposition technique was used to fabricate GZO thin films on p-GaN, Al2O3, and p-Si substrates. X-ray diffraction graphs show single crystal growth of GZO thin films with (002) preferred crystallographic orientation. The chemical composition was studied via energy dispersive X-ray spectroscopy, and no other unwanted impurity-related peaks were found, which indicated the impurity-free thin film growth of GZO. Field emission scanning electron microscopic micrographs revealed noodle-, seed-, and granular-like structures of GZO/GaN, GZO/Al2O3, and GZO/Si, respectively. Uniform growth of GZO/GaN was found due to fewer mismatches between ZnO and GaN (0.09%). Hall effect measurements in the van der Pauw configuration were used to check electrical properties. The highest mobility (53 cm2/Vs) with a high carrier concentration was found with low laser shots (1800). A 5-fold photoluminescence enhancement in the noodle-like structure of GZO/GaN compared with GZO/Al2O3 and GZO/Si was detected. This points toward shape-driven optical properties because the noodle-like structure is more favorable for optical enhancements in GZO thin films. Theoretical (3.539 eV) and experimental (3.54 eV) values of the band-gap were also found to be comparable. Moreover, the lowest resistivity (3.5 × 10−4 Ωcm) with 80% transmittance is evidence that GZO is a successful alternate of ITO.

Physical properties of Al-doped ZnO and Ga-doped ZnO thin films prepared by direct current sputtering at room temperature

Al-doped zinc oxide (AZO) and Ga-doped zinc oxide (GZO) thin films with the same doping concentration (3.6 at%) were deposited on glass substrates at room temperature by direct current (DC) magnetron sputtering. Consequently, we comparatively studied the doped thin films on the basis of their structural, morphological, electrical, and optical properties for optoelectronic applications. Both thin films exhibited excellent optical properties with more than 85% transmission in the visible range. The GZO thin film had better crystallinity and smoother surface morphology than the AZO thin film. The conductivity of the GZO thin film was improved compared to that of the AZO thin film: the resistivity decreased from 1.01 × 10-3 to 3.5 × 10-4 Ω cm, which was mostly due to the increase of the carrier concentration from 6.5 ×1020 to 1.46 × 1021 cm-3. These results revealed that the GZO thin film had higher quality than the AZO thin film with the same doping concentration for optoelectronic applications.

Influences of the RF power ratio on the optical and electrical properties of GZO thin films by DC coupled RF magnetron sputtering at room temperature

Ga-doped zinc oxide (GZO) thin films were deposited by closed field unbalanced DC coupled RF magnetron sputtering system at room temperature. The RF sputtering power ratio was adjusted from 0% to 100%. The crystal structure, surface morphology, transmittance and electrical resistivity of GZO films mainly influenced by RF sputtering power ratio were investigated by X-ray diffractometer, scanning electronic microscope, ultraviolet-visible spectrophotometer and Hall effect measurement. The research results indicate that the increasing RF power ratio can effectively reduce the discharge voltage of system and increase the ionizing rate of particles. Meanwhile, the higher RF power ratio can increase the carrier mobility in GZO thin film and improve the optical and electrical properties of GZO thin film significantly. Within the optimal discharge voltage window, the film deposits at 80% RF power ratio exhibits the lowest resistivity of 2.6×10−4Ωcm. We obtain the GZO film with the best average optical transmittance is approximately 84% in the visible wavelength. With the increasing RF power ratio, the densification of GZO film is enhanced. The densification of GZO film is decrease when the RF power ratio is 100%.

Paper No P20: Effects of the Channel Thickness on Characteristics of Ga‐Doped Zinc Oxide Thin‐Film Transistors Fabricated on Glass

AbstractIn this paper, high‐performance fully transparent bottom‐gate type Ga‐doped zinc oxide thin‐film transistors (GZO TFTs) have been successfully fabricated on glass substrate. The effects of channel thickness on the electrical performances of GZO TFTs are researched. We study to improve the performance of bottom gate Ga‐doped Zinc Oxide Thin Film Transistors by optimizing the active layer process. Optimized TFTs exhibit excellent electrical properties, with high Ion/Ioff current ratio of 1.3 × 109, saturation mobility (μsat) of 99 cm2/Vs, subthreshold swing (SS) of 77mV/decade and threshold voltage (Vth) of 2.2 V. The variation trend of Ion/Ioff, μsat, SS, and Vth against channel thickness is analyzed in detail. The experimental results suggest that the performance of GZO TFT can be improved effectively by optimum channel thickness.

Fully transparent flexible dual‐layer channel Ga‐doped ZnO thin‐film transistors on plastic substrates

Fully transparent dual-layer Ga-doped zinc oxide (GZO) thin-film transistors (TFTs) were fabricated on flexible plastic substrate by room temperature processes. The GZO thin films are deposited by radio-frequency sputtering according to the variation of the depositing time in order to optimise the performance of GZO TFTs. The results show that dual-layer GZO TFTs exhibit excellent electrical properties, mechanical flexibility and optical transparency. A saturation mobility μ s of 350 cm2/V.s, a linear field effect mobility of 281 cm2/V.s, a threshold voltage V TH of 3.2 V, a steep subthreshold swing of 268 mV/decade, a low off-state current value (I OFF) of 1.5 × 10−11 A and a high on/off ratio of about 108 were extracted. The TFTs also have admirable transparency with an average visible transmittance of 87.2%. These results indicate that GZO material is suitable for the next generation flexible displays.

Effect of RF power on the optical, electrical, mechanical and structural properties of sputtering Ga-doped ZnO thin films

We present the influences of radio-frequency (RF) power on the optical, electrical, mechanical, and structural properties of Ga-doped zinc oxide (GZO) thin films by RF magnetron sputtering at room temperature. GZO thin films were grown on unheated glass and silicon substrates using radio-frequency (RF) magnetron sputtering method with different RF powers (from 60W to 160W). The optical properties of the GZO thin film were determined by a UV–vis spectrophotometer. The residual stress in GZO films were measured by a home-made Twyman–Green interferometer with the fast Fourier transform (FFT) method. The surface roughness of GZO films were measured by a microscopic interferometry. The microstructure, composition and crystal orientation of the GZO films were determined by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). This paper revealed that the optical, electrical, mechanical, and structural properties of GZO thin film are subject to the RF power. For the optical spectrum measurement, an average optical transmittance in the visible region of the spectra of 85% was obtained. For the characteristic measurements, all the GZO thin films deposited by RF magnetron sputtering have compressive stress at different RF powers. A minimum residual stress of 0.24GPa is found at the RF power of 140W. A four-point probe method was used to measure the resistivity of the GZO thin films with different powers, the results indicate that the resistivity increases with increasing of RF power. In addition, the root-mean-square (RMS) surface roughness of GZO thin films slightly increases as the RF power is increasing. We have also compared the results with the relevant literatures.

Annealing-dependent electrical properties of Ga-doped ZnO film on silicon carbide.

The effect of substrate temperature and thermal annealing on the structural, electrical, and optical properties of Ga-doped zinc oxide (GZO) thin films were examined for use as transparent electrodes on silicon carbide (SiC). The variation of these properties is thought to be related to the presence and stability of Ga3+ ions substituted into Zn2+ sites and adsorbed oxygen ions in GZO thin films, as well as the interfacial thickness of GZO/SiC. GZO thin film deposited at 400 degrees C without annealing has the lowest resistivity of - 1.96 x 10(-4) Ω x cm, which increased after annealing. The photosensitivity of GZO/SiC was increased with increased substrate temperature (from 1.51 to 6.87%) and after annealing (from 2.78 to 8.67%). These results were clarified by comparatively analyzing the chemical composition ratio of oxygen in the GZO thin films and the interfacial thickness between GZO/SiC.

Optical, electrical and mechanical properties of Ga-doped ZnO thin films under different sputtering powers

We present the optical, electrical and mechanical properties of Ga-doped zinc oxide (GZO) thin films prepared by radio-frequency (RF) magnetron sputtering at room temperature under different RF powers (80–180W). The thickness, electron concentration, and electron mobility of the GZO thin film were determined by fitting the visible-to-near-infrared transmittance spectrum of GZO film/glass using the transfer matrix method. The bending force per unit width was measured by a home-made Twyman–Green interferometer with the fast Fourier transform method. The obtained results show that the optical, electrical and mechanical properties of GZO thin film are subject to the RF power. At an RF power of 140W, the local minimum of bending force per unit width corresponds to the highest electron mobility in GZO thin film. This study demonstrates that the optical, electrical and mechanical properties of GZO thin film can be fully resolved by non-contact optical methods.

High-quality, conductive, and transparent Ga-doped ZnO films grown by atmospheric-pressure chemical-vapor deposition

Ga-doped zinc oxide (GZO) thin films were prepared by atmospheric-pressure chemical-vapor deposition (APCVD) using Ga injection rates over the range 70–150sccm. When Ga atoms substituted the Zn atoms in the ZnO matrix, the electrical resistance of GZO thin films decreased, because the dopant atom generated one extra free electron. When the injection rate of Ga was 100sccm, a very low resistivity of 2.03×10−4Ω·cm was realized; however, the resistivity was higher when the injection rate of Ga was over 100sccm because of the agglomeration of Ga atoms in the ZnO matrix. All GZO thin films showed an optical transmittance of over 85%; in particular, when the Ga injection rate was 100sccm, a superior figure-of-merit (FOM) of 7.58×10−2Ω−1 was found. GZO thin film prepared by APCVD could be a suitable transparent conductive oxide (TCO) material for variable optoelectronic devices.

Effect of energetic electron beam treatment on Ga-doped ZnO thin films

Transparent conductive zinc oxide (ZnO) thin films were synthesized by a sol–gel spin coating method with the addition of Ga(NO3)3 in a Zn(CH3COO)2 solution and exposed to electron beam treatment. The UV–Vis spectra demonstrated that all of the films had transmittances of over 85% in the visible region. When Ga(NO3)3 was added to the ZnO precursor solution, the resistivity of the ZnO thin film decreased and the carrier concentration increased significantly. After electron beam treatment was performed on the 0.4 at.% Ga-doped ZnO film, the optical band gap increased and the resistivity significantly decreased resulting from the increases of the carrier concentration and mobility. By combining Ga doping and electron beam treatment, the resistivity of the ZnO thin film was reduced by a factor of nine hundred.

Comparative studies of Al-doped ZnO and Ga-doped ZnO transparent conducting oxide thin films

We have investigated the influences of aluminum and gallium dopants (0 to 2.0 mol%) on zinc oxide (ZnO) thin films regarding crystallization and electrical and optical properties for application in transparent conducting oxide devices. Al- and Ga-doped ZnO thin films were deposited on glass substrates (corning 1737) by sol–gel spin-coating process. As a starting material, AlCl3⋅6H2O, Ga(NO3)2, and Zn(CH3COO)2⋅2H2O were used. A lowest sheet resistance of 3.3 × 103 Ω/□ was obtained for the GZO thin film doped with 1.5 mol% of Ga after post-annealing at 650°C for 60 min in air. All the films showed more than 85% transparency in the visible region. We have studied the structural and microstructural properties as a function of Al and Ga concentrations through X-ray diffraction and scanning electron microscopy analysis. In addition, the optical bandgap and photoluminescence were estimated.

Structural, optoelectronic, luminescence and thermal properties of Ga-doped zinc oxide thin films

Ga-doped ZnO thin films are synthesized by chemical spray pyrolysis onto corning glass substrates in aqueous media. The influence of gallium doping on to the photoelectrochemical, structural, Raman, XPS, morphological, optical, electrical, photoluminescence and thermal properties have been investigated in order to achieve good quality films. X-ray diffraction study depicts the films are polycrystalline and fit well with hexagonal (wurtzite) crystal structure with strong orientations along the (002) and (101) planes. Presence of E2high mode in Raman spectra indicates that the gallium doping does not change the wurtzite structure. The coupling strength between electron and LO phonon has experimentally estimated. In order to understand the chemical bonding structure and electronic states of the Ga-doped ZnO thin films XPS analysis have been studied. SEM images shows the films are adherent, compact, densely packed with hexagonal flakes and spherical grains. Optical transmittance and reflectance measurements have been carried out. Room temperature PL spectra depict violet, blue and green emission in deposited films. The specific heat and thermal conductivity study shows the phonon conduction behavior is dominant in these polycrystalline films.

Nanocrystalline GaZnO Films for Transparent Electrode to Silicon Carbide

Nanocrystalline 2% Ga doped zinc oxide (GaZnO) thin films were epitaxially deposited on n-type 4H―SiC (0001) by a pulsed laser deposition (PLD) at different substrate temperatures of 250, 400, and 550 ° C, respectively. Structural and electrical properties of nanocrystalline GaZnO thin film on 4H―SiC were investigated by using X-ray diffraction, atomic force microscopy (AFM), Hall effect measurement, transmission line method (TLM), and Auger electron spectroscopy (AES). The nanocrystalline GaZnO film deposited at 400 °C show the lowest resistivity of 3.3 x 10 ―4 Ω cm, and highly c-axis oriented crystalline quality with being sharper and higher diffraction angle, which result in. The specific contact resistance (ρ c ), measured from the Au/Ti/GaZnO/SiC of ∼0.05 Ω cm 2 . The relative amount of activated Ga 3+ ions was 2.02% in GaZnO film by AES measurement.

GaZnO as a Transparent Electrode to Silicon Carbide

The characteristics of Ga-doped zinc oxide (GaZnO) thin films deposited at different substrate temperatures (TS~250 to 550oC) on 4H-SiC have been investigated. Structural and electrical properties of GaZnO thin film on n-type 4H-SiC (100)were investigated by using x-ray diffraction, atomic force microscopy (AFM), Hall effect measurement, and Auger electron spectroscopy (AES). Hall mobility is found to increase as the substrate temperature increase from 250 to 550 oC, whereas the lowest resistivity (~3.3 x 10-4 Ωcm) and highest carrier concentration (~1.33x1021cm-3) values are observed for the GaZnO films deposited at 400 oC. It has been found that the c-axis oriented crystalline quality as well as the relative amount of activated Ga3+ Introduction ions may affect the electrical properties of GaZnO films on SiC.

Electrical and optical studies of Ga-doped ZnO thin films

Nanostructure Ga-doped zinc oxide (GZO) thin films with highly (0 0 2) preferred orientation were fabricated on glass substrates, using radio frequency magnetron sputtering with an GZO ceramic target (The Ga2O3 contents was about 3 wt%) and different deposition conditions. The structural features, surface morphology and electrical and optical properties of the GZO thin films were studied, in terms of the deposition parameters. A Grey-based Taguchi method was used to determine the optimal deposition parameters for GZO thin films by considering multiple performance characteristics. The response graph and table for each level of the deposition parameters forms the Grey relational grade and the optimal levels of the deposition parameters were chosen. The experimental results show that the process pressure and the thickness make the most significant contribution to the overall performance. In the confirmation runs, Grey relational analysis showed that the improvement in deposition rate is 14.2 %, the improvement in electrical resistivity 38.1 % and the improvement in optical transmittance is 1.2 %. Annealing in a vacuum further improved the crystalline quality and optoelectronic performances of the GZO thin films.