Ga Co-doped ZnO Thin Films Research Articles

Single Crystalline Transparent Conducting F, Al, and Ga Co-Doped ZnO Thin Films with High Photoelectrical Performance.

Transparent conductive film (TCF) is a material that integrates electrical conductivity and optical transparency. It is widely used as an electrode material in thin-film solar cells. However, considerable progress is needed to facilitate its high performance and low-cost preparation. In this study, a preparation scheme for AlF3 and GaF3 co-doped ZnO (FAGZO) thin films was designed and implemented by magnetron sputtering (MS). The mutual restraint between the electrical properties and the wide-spectrum transmission performance of ZnO films was resolved. First-principles calculations showed that the doped ZnO system had n-type conductivity and that the most stable structure was the FO-AlZn-GaZn system. The experimental results verified the theoretical predictions. Single crystalline ZnO transparent conducting films (ZnO-TCFs) of high quality were achieved by MS. After rapid thermal annealing (RTA) treatment, the mobility reached 49.6 cm2/V s, and the resistivity decreased to 3.82 × 10-4 Ω cm. The AT was 90% between 380 and 1200 nm. Furthermore, the application of the prepared FAGZO film in perovskite solar cells (PSCs) has been verified. Compared to the reference indium tin oxide film, the PSCs using the FAGZO film showed higher JSC and power conversion efficiency. These results demonstrate that MS combined with anion and cation co-doping provides an effective means of exploring high-quality and high-performance ZnO-TCFs.

Synthesis and characterization of Co and Ga co-doped ZnO thin films as an electrode for dye sensitized solar cells

Using a time efficient and cost effective approach so called sol‒gel dip coating route, we deposited thin films of ZnO and 1% Co & (0%, 0.5%, 1%, and 1.5%) Ga co-doped ZnO on the fluorine doped tin oxide substrates. All the films exhibited hexagonal wurtzite structure. X-rays study revealed that crystallite size increased with increase of doping. Optical parameters like absorbance, transmittance, refractive index, band gap, extinction coefficient and dielectric constants were measured using UV–Vis spectroscopy and it has been noticed that doping resulted in reduction of band gap. It has also been observed that, the films prepared with 1% Co & 1% Ga co-doped ZnO have comparatively smaller band gap and thus have high refractive index and high transmittance in visible region. The calculation of different electrically important factors for instance, high current density, small open circuit voltage, efficiency and fill factor revealed that co-doped ZnO with 1% Co and 1% Ga has relatively high efficiency of 2.43% and thus shows the potential of this composition as an electrode for solar cell devices.

Improved electrical properties of laser annealed In and Ga co-doped ZnO thin films for transparent conducting oxide applications

In this research, dopants with same stoichiometric composition of In and Ga were varied in the range of 1–4 mol% to increase the electrical conductance and optical transmittance. The total sum of 2 mol% of In and Ga co-doped ZnO thin films showed the lowest sheet resistance of 15.9 kΩ/sq. and high transmittance of more than 90% at 380 nm after rapid thermal annealing (RTA) and CO2 laser annealing. Based on scanning electron microscopy images and calculations using the Scherrer formula, the grain sizes were measured and were compared for the increase of In and Ga dopants. The RTA and CO2 laser annealed ZnO thin films co-doped with 2 mol% of In and Ga exhibited a higher energy band gap of 3.29 eV than those of other specimens. The root mean square (RMS) values of the surface roughness were extracted from atomic force microscopy (AFM) images, and the lowest RMS value of 22 nm was obtained for the RTA and CO2 laser annealed ZnO thin films co-doped with 2 mol% of In and Ga. We believe that this increase in the energy band gap is related to the blue shift in the dopant ionization process of In and Ga dopants. And these RTA and CO2 laser annealing can improve the ionization of In and Ga dopants, which is related to the increased energy band gap.

Transparent heater based on Al,Ga co-doped ZnO thin films

Al,Ga co-doped ZnO thin films were deposited on to glass substrates by aerosol assisted chemical transport (AACT) and were studied for application as transparent thin film heaters. The film thickness was around 400 nm after 60 min of AACT deposition; this gave a sheet resistance of 142.5 Ω/sq, which corresponded to a resistivity of 5.7 × 10−3 Ω·cm. The thin films exhibited a maximum transmittance of 90% in the visible region. A mean temperature of 132.3 °C was reached after applying a voltage of 18 V for 10 min, giving a power consumption of 2.11 W. These results could provide a possibility to use Al,Ga co-doped ZnO thin films as transparent heaters to replace the more expensive indium tin oxide.

Investigation of optoelectronic performance in In, Ga co-doped ZnO thin films with various In and Ga levels

In, Ga co-doped ZnO (IGZO) thin films were deposited by radio frequency sputtering at room temperature in this study. The film's composition was adjusted by varying the sputtering power of ZnO, Ga2O3 and In targets. Compared with pure ZnO film, the crystallinity of IGZO films is obviously reduced, and the secondary phases such as In2O3 and ZnGa2O4 are detected. The influence of In, Ga, and Zn content on the optoelectronic properties of IGZO films is then investigated. Results show that the carrier mobility is sensitive to the decrement in Zn content, while the carrier concentration can be suppressed by increasing Ga content. In regards to the film's transmittance, In content plays a most important role. From the viewpoint of Haacke's figure of merit (FOM), IGZO films with high optoelectronic performance can be realized when In content is no more than 25at.% and Ga content is limited to less than 10at.%. In the end of this study, the electrical stability of IGZO film under different temperature and time conditions is analyzed. It reveals that IGZO film can withstand a temperature up to 450°C; above this temperature, the film's conductivity significantly declines. Besides this, IGZO film is very stable in long-term operation under normal atmospheric conditions.

Effect of annealing temperature on the optoelectronic characteristic of Al and Ga co-doping ZnO thin films

An investigation is performed into the optical and electronic properties of Al and Ga co-doped ZnO thin films deposited on glass substrates using radio frequency magnetron sputtering (RF-sputtering) and then annealed at temperatures of 200, 300 and 400 °C. The XRD analysis results show that the films have a (002) preferential orientation. However, the intensity of the (002) peak increases with an increasing annealing temperature. Moreover, the optical transmittance increases and the electrical resistivity decreases as the annealing temperature increases.

The effects of substrate temperature on properties of B and Ga co-doped ZnO thin films grown by RF magnetron sputtering

Abstract In this work, B and Ga co-doped ZnO (BGZO) films were fabricated by RF magnetron sputtering method. The effects of substrate temperature on the electrical, optical, structural and morphological properties of B and Ga co-doped ZnO films were investigated. XRD results revealed that deposited films were textured along c-axis and maintain wurtzite crystalline symmetry. As substrate temperature increased up to 200 °C, the film crystallinity was improved and the crystallite sizes became larger. At the substrate temperature of 200 °C, the films showed lower resistivity, higher Hall mobility and higher optical band gap. It was also observed that all films showed high transparency in the visible range (400–800 nm), which did not change obviously with the substrate temperature.

Characteristics of Quaternary Flexible Mg and Ga Co-Doped ZnO Thin Films Fabricated Using RF Magnetron Sputtering

Characteristics of Quaternary Flexible Mg and Ga Co-Doped ZnO Thin Films Fabricated Using RF Magnetron Sputtering

Structural characterization and optoelectrical properties of Ti–Ga co-doped ZnO thin films prepared by magnetron sputtering

The Ti–Ga co-doped zinc oxide (TGZO) thin films were deposited on glass substrates by radio frequency magnetron sputtering technique in an argon atmosphere. The effect of working pressure on the crystallinity, microstructure, morphology and optoelectrical properties of thin films was investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, four-point probe and spectrophotometer. It is observed that all the deposited films are polycrystalline with a hexagonal wurtzite structure and highly textured along the c-axis perpendicular to the substrate surface. As the working pressure increases, the dislocation density, stress and resistivity decrease initially and then increase, while the crystallite size, average visible transmittance and figure of merit exhibit the reverse variation trend. The TGZO thin film deposited at the working pressure of 0.4 Pa possesses the best optoelectronic properties, with the largest crystallite size of 85.66 nm, the lowest dislocation density of 1.363 × 1014 lines m−2, the minimum compressive stress of 0.232 GPa, the lowest resistivity of 6.715 × 10−4 Ω cm, the highest average visible transmittance of 88.24 % and the maximum figure of merit of 1.314 × 103 Ω−1 cm−1. The optical energy-gaps of thin films were estimated by Tauc’s law and observed to be in the range of 3.515–3.548 eV which are hardly affected by the working pressure. Furthermore, the optical parameters such as refractive index, extinction coefficient, dielectric constant and nonlinear optical susceptibility were determined by the method of optical spectrum fitting, and the dispersion behavior of refractive index was studied in terms of the Sellmeier’s dispersion model. The results show that the microstructure and optoelectrical properties of the TGZO thin films are subjected to the working pressure.

Effects of substrate temperature on the structural, morphological, electrical and optical properties of Al and Ga co-doped ZnO thin films grown by DC magnetron sputtering

In this work, Al and Ga co-doped zinc oxide (AGZO) thin films were deposited on glass substrates by DC magnetron sputtering under different substrate temperatures. Evolutions of the structural, morphological, electrical and optical properties of the AGZO thin films as a function of substrate temperature were analyzed. Results showed that the average transmittance in the visible range (400–800nm) for all the thin films was over 82%, which did not change obviously with the substrate temperature. The average grain size increased from 20.6nm to 51.4nm and the RMS surface roughness decreased from 21.1nm to 4.0nm with substrate temperatures ranging from 150°C up to 450°C. The carrier concentration, Hall mobility of the thin films increased when the substrate temperature was increased from 150°C to 350°C, and then decreased with a further increase of substrate temperature. The film deposited at 350°C exhibited a lowest resistivity of 3.0×10−4Ωcm with the highest carrier concentration of 5.0×1020cm−3 and Hall mobility of 42cm2V−1s−1.

Transparent conductive Mg and Ga co-doped ZnO thin films for solar cells grown by magnetron sputtering: H2 induced changes

Transparent conductive hydrogenated Mg and Ga co-doped ZnO (HMGZO) thin films for solar cells were deposited via pulsed direct current (DC) magnetron sputtering on glass substrates at a substrate temperature of 553K. The structural, morphological, electrical, and optical properties of HMGZO thin films were investigated with various H2 flow rates. The experiment results showed that the HMGZO thin films are polycrystalline with a hexagonal wurtzite structure exhibiting a preferred (002) crystal plane orientation and typical HMGZO thin films present cone-like texture surface. The carrier concentration rapidly increases from 7.57×1019cm−3 to 5.25×1020cm−3 with increasing the H2 flow rate from 0sccm to 4.0sccm. Optical measurements indicated that the optical band-gap (Eg) of HMGZO thin films varies from 3.43eV to 3.66eV with adjusting H2 flow rate from 0sccm to 4.0sccm. The glass/HMGZO thin film deposited at the H2 flow rate of 4.0sccm exhibits the lowest resistivity of 6.24×10−4Ωcm (sheet resistance Rs~8.68Ω) and an average transmittance (Ta) of 78.8% in the wavelength range from 340nm to 1100nm. Burstein–Moss band-filling, band gap renormalization effects determined by carrier concentrations and the incorporation of Mg atoms together contribute to an Eg widening phenomenon. The electron mobility of HMGZO thin films can be effectively improved through thermal annealing process. Finally, the HMGZO thin film was preliminarily applied in pin a-Si:H thin film solar cell with an efficiency of 8.20% (Voc=0.904V, Jsc=12.906mA/cm2 and FF=0.702).

Sol–gel derived Al and Ga co-doped ZnO thin films: An optoelectronic study

Al and Ga co-doped ZnO (AGZO) thin films with different doping contents of 0.5–4at.% were synthesized via sol–gel route using dip coating method and the results were compared to the single doped specimens Al:ZnO (AZO) and Ga:ZnO (GZO). All samples were highly transparent in visible region (T>85%) with band gap values around 3.3eV. Introduction of Al and Ga to the ZnO crystal structure decreased the crystallinity and reduced the particle size of the films. Electrical resistivity was investigated and engineered in this study as the main parameter. Single doped samples showed reduction of resistivity compared to the un-doped ZnO. In this regard, Ga was more efficient than Al in decreasing the electrical resistivity. Furthermore, samples with 1at.% Al and 1at.% Ga showed the minimum amount of electrical resistivity. Co-doping was performed with two different approaches including variable doping content (Al+Ga≠cte) and constant doping content (Al+Ga=0.5, 1, and 2at.%) for the sake of the comparison with single doped samples. Samples with Al=1at.% and Ga=1at.% showed the lowest electrical resistivity in AGZO samples of former approach. However, in latter approach the lowest resistivity was obtained in Al+Ga=2at.% sample. The results proved the capability of co-doped samples in optoelectronic industry regarding partially substitution of expensive Ga with Al and obtaining co-doped AGZO transparent conductive thin films with lower resistivity compared to conventional AZO thin films and also achieving commercial advantages compared to costly GZO thin films.

Development of flexible Mg and Ga co-doped ZnO thin films with wide band gap energy and transparent conductive characteristics

• Mg and Ga doped ZnO films with transparent conducting characteristics were prepared on flexible substrate. • MGZO thin films had a larger grain size with increasing thickness. • The optical and electrical properties of MGZO thin films were improved with increasing thickness. The transparent conductive and flexible Mg and Ga co-doped ZnO (MGZO) thin films were prepared on poly-ethylene telepthalate (PET) by the RF magnetron sputtering technique at room temperature. The effects of different thicknesses on the structural, chemical, morphological, optical and electrical properties of MGZO thin films were investigated. X-ray diffraction studies showed that MGZO thin films were grown as a polycrystalline hexagonal wurtzite phase without a secondary phase. The peak intensities for the (0002) plane of MGZO thin films were enhanced with increasing thickness. A typical survey spectrum of MGZO thin films confirmed the presence of Mg, Ga, Zn and O resulting from MGZO films regardless of thickness. The MGZO thin films had a larger grain size with increasing thickness. The MGZO thin films showed the widest optical band gap energy of 3.91 eV (50 nm) and lowest electrical resistivity of 5.76 × 10 −3 Ω cm (400 nm).

Effect of Sputtering Powers on Mg and Ga Co-Doped ZnO Thin Films with Transparent Conducting Characteristics

ZnO thin films co-doped with Mg and Ga (MxGyZzO, x + y + z = 1, x = 0.05, y = 0.02 and z = 0.93) were prepared on glass substrates by RF magnetron sputtering with different sputtering powers ranging from 100W to 200W at a substrate temperature of <TEX>$350^{\circ}C$</TEX>. The effects of the sputtering power on the structural, morphological, electrical, and optical properties of MGZO thin films were investigated. The X-ray diffraction patterns showed that all the MGZO thin films were grown as a hexagonal wurtzite phase with the preferred orientation on the c-axis without secondary phases such as MgO, <TEX>$Ga_2O_3$</TEX>, or <TEX>$ZnGa_2O_4$</TEX>. The intensity of the diffraction peak from the (0002) plane of the MGZO thin films was enhanced as the sputtering power increased. The (0002) peak positions of the MGZO thin films was shifted toward, a high diffraction angle as the sputtering power increased. Cross-sectional field emission scanning electron microscopy images of the MGZO thin films showed that all of these films had a columnar structure and their thickness increased with an increase in the sputtering power. MGZO thin film deposited at the sputtering power of 200W showed the best electrical characteristics in terms of the carrier concentration (<TEX>$4.71{\times}10^{20}cm^{-3}$</TEX>), charge carrier mobility (<TEX>$10.2cm^2V^{-1}s^{-1}$</TEX>) and a minimum resistivity (<TEX>$1.3{\times}10^{-3}{\Omega}cm$</TEX>). A UV-visible spectroscopy assessment showed that the MGZO thin films had high transmittance of more than 80 % in the visible region and that the absorption edges of MGZO thin films were very sharp and shifted toward the higher wavelength side, from 270 nm to 340 nm, with an increase in the sputtering power. The band-gap energy of MGZO thin films was widened from 3.74 eV to 3.92 eV with the change in the sputtering power.

Development of transparent conductive Mg and Ga co-doped ZnO thin films: Effect of Mg concentration

Transparent conductive Mg and Ga co-doped ZnO (MGZO) thin films were prepared on glass substrates by RF magnetron sputtering technique. The effect of Mg concentration from 0 to 15at% on the structural, chemical, morphological, optical and electrical properties of MGZO thin films was investigated. X-ray diffraction studies showed that the pure ZnO, Mg-doped ZnO (MZO) and MGZO thin films were grown as a polycrystalline hexagonal wurtzite phase without secondary phase. The 2θ angle position of the (0002) peak of the MZO and MGZO thin films was shifted towards a higher angle with increasing Mg concentration. A typical survey XPS spectrum of the MGZO thin films confirmed the presence of Mg, Ga, Zn and O in the MGZO films. The MGZO thin films had a smoother surface morphology than those of ZnO and MZO. The MGZO thin film deposited at Mg concentration of 5at% showed the widest optical band gap energy of 3.75eV and the lowest electrical resistivity of 6.89×10−4Ωcm. However, the electrical and optical characteristics of the MGZO thin films deposited over 5 at% Mg concentration were deteriorated with increasing Mg concentration.

Design and Growth of Quaternary Mg and Ga Codoped ZnO Thin Films with Transparent Conductive Characteristics

This study reports the design growth and characterization of quaternary Mg and Ga codoped ZnO (MGZO) thin films with transparent conductive characteristics deposited on glass substrates by RF magnetron sputtering. The effects of the Ga concentration (from 0 to 2 at %) on the structural, chemical, morphological, optical, and electrical properties of MGZO thin films were investigated. X-ray diffraction study showed that all the MGZO thin films were grown as a polycrystalline hexagonal wurtzite phase with a c-axis preferred orientation and random in-plane orientation. The 2θ value of the (0002) peak of MGZO thin films decreased with increasing Ga concentration. X-ray photoelectron spectroscopy confirmed the Mg and Ga binding energy peaks from the MGZO thin films. The MGZO thin films had a smoother surface morphology. The optical study showed that the band gap energy of MGZO thin films systematically increased from 3.25 to 3.75 eV with increase Ga concentration. The electrical resistivity of the MGZO thin fil...

Investigation of electronic and optical properties in Al−Ga codoped ZnO thin films

Electronic and optical properties of Al−Ga codoped ZnO thin films were investigated by post-annealing. The lowest resistivity of the Al–Ga codoped ZnO films was observed from the 450 °C-annealed sample. The Fermi-level shift of the Al−Ga codoped ZnO film was ∼0.6 eV from x-ray photoelectron spectroscopy, and the widening of optical-bandgap in the Al−Ga codoped ZnO film was ∼0.3 eV. The correlations of optical-bandgap with Fermi-level shift and conduction band filling were suggested by schematic band diagrams.

Properties of Ni doped and Ni–Ga co-doped ZnO thin films prepared by pulsed laser deposition

ZnNiO and Zn(Ni,Ga)O thin films were prepared on glass substrates by pulsed laser deposition. The obtained films are of good crystal quality and have smooth surfaces, which have a hexagonal wurtzite ZnO structure with a highly c-axis orientation without any Ga or Ni related phases. Hall-effect measurements showed that the ZnNiO film is n-type, in which the carrier concentration would be greatly enhanced by the addition of Ga. Room temperature ferromagnetism is observed for the ZnNiO and Zn(Ni,Ga)O films. The addition of Ga into the ZnNiO films increases the electron concentration but weakens the room temperature ferromagnetism.

Enhanced Optical Output Power of Tunnel Junction GaN-Based Light Emitting Diodes with Transparent Conducting Al and Ga-Codoped ZnO Thin Films

High quality Al and Ga-codoped ZnO (AGZO) thin films were successfully deposited both on sapphire substrates and GaN-based light emitting diodes (LED) with a tunnel junction (TJ) layer by using the radio frequency magnetron sputtering technique at room temperature. The AGZO thin films grown on sapphire substrate showed high transparency (96.3% at 460 nm) and low resistivity (6.8 ×10-4 Ω cm). The AGZO thin films deposited on the GaN-based LED with a TJ layer exhibited weak ohmic behavior although it improved slightly with the annealing. The optical output power of the TJ GaN-based LED with an AGZO transparent conducting layer was about 12.6 mW at 20 mA, and the external quantum efficiency was 23.0%. These values are approximately 1.7 times larger than that of the TJ GaN-based LED with a conventional Ni/Au layer.

Electrical resistivity and transmittance properties of Al and Ga-codoped ZnO thin films

Films were deposited on glass substrates by sputtering gallium-doped zinc oxide (GZO) (Ga2O3: 90 wt%, ZnO: 10 wt%) and aluminum-doped zinc oxide (AZO) (Al2O3: 3 wt%, ZnO: 97 wt%) targets simultaneously using a direct-current (DC)/radio-frequency (RF) magnetron cosputtering system. The concentration of gallium (Ga) in the film was varied by using different RF powers for sputtering the GZO target with the DC power for sputtering the AZO target fixed. A minimum resistivitywas obtained at an RF sputtering power of 200 W for the GZO target when the DC sputtering power for the AZO target was fixed at 60 W. It was found that the resistivity of AZO thin films (6.40 × 10−3 Ω-cm for 0 W) could be lowered by more than one order by cosputtering AZO and GZO targets to make AGZO thin films (2.14 × 10−3Ω-cm for 200 W) without lowering their transmittance at all. In addition, the origin of the enhancement in the electrical property of the AZO by additional Ga doping is discussed.