Transparent conducting Ga:ZnO (GZO) and Al:ZnO (AZO) layers have been deposited by spin coating on glass substrates using crystalline nanoparticles redispersed in 1-propanol. The coatings have been sintered in air at 600°C for 15 min and then postannealed in a reducing atmosphere at 400°C for 90 min. The effect of Ga and Al doping on the structural, morphological, optical, and electrical properties of the obtained thin films was investigated. Both films were found to be crystalline with a hexagonal structure. A single step spin coated layer 52–56 nm thick is obtained. To increase the thickness and lower the obtained sheet resistance multilayers coatings have been used. The visible transmission of both layers is high (
Transparent conducting oxide (TCO) films have been intensively investigated for optical and electrical applications, such as flat-panel displays, liquid crystal displays, organic light-emitting diodes, thin-film transistors, and thin-film solar cells. Indium doped tin oxide In2O3:Sn (ITO) is the most popular TCO. Unfortunately, the application of ITO conducting coating is limited due to high price of indium, thermal instability, lack of corrosion resistance, and poor adhesion of the coating under various environments. Zinc oxide (ZnO) thin films are emerging as the most attractive alternate to ITO because they are inexpensive and nontoxic and have high thermal and chemical stability and a wide energy band gap (~3.3 eV). The substitution of Zn2+ ions with group III ions (B3+, Al3+, Ga3+, and In3+) generates extra electrons and improves ZnO optical, electrical, thermal, and magnetic properties. The most common dopant is Aluminum, where Al:ZnO (AZO) films exhibit high transparency and low resistivity [
Such materials are suitable for fabricating transparent electrodes in solar cells, gas sensors, optical waveguides, and micromachined actuators [
In this study, gallium and aluminum doped zinc oxide nanopowders (GZO) and (AZO) were synthesized using a hydrothermal process with a different doping concentration. The obtained nanopowders were characterized and dispersed in an organic solvent to produce transparent conducting layers on glass substrates. The structural, electrical, and optical properties of the obtained thin films have been studied as a function of doping concentration, thickness, and sintering temperature using X-ray diffraction, scanning electron microscopy, and UV-Visible spectrophotometry measurement.
The AZO and GZO nanopowder were synthesized hydrothermally using the precipitation method. The precursor solution for GZO was prepared by using zinc nitrate hexahydrate reagent grade Zn(NO3)2·6H2O dissolved in absolute ethanol under stirring at 50°C till clear solution was obtained. The used doping precursor is Gallium(III) nitrate hydrate crystalline Ga(NO3)3·
The Brunauer-Emmett-Teller (BET) gas adsorption measurement technique was used to measure the surface area of the powders. The powder degassing was achieved by using Autosorb Degasser station, and the measurements were conducted at liquid nitrogen saturation vapor pressure using Autosorb-6B from Quantachrome.
The dried material for both GZO and AZO powders is grinded to reduce the size of the agglomerates until a fine powder is obtained (size < 0.2 mm). The fine powder was wetted using a small amount of polyethylene glycol (PEG600) and 3,6,9-trioxadecanoic acid (TODs) as a dispersing agent. The wetted GZO and AZO powder were dispersed mechanically using a mortar for 15 min. This process breaks the agglomerated powder and produces a homogenous paste. The paste was then dissolved in 1-propanol as a solvent. The obtained suspension was centrifuged at 4000 rpm for 20 min to remove the remaining large agglomerates.
A spin coating technique was used to deposit transparent conductive coatings (spin coater model 1001 CPSII from CONVAC). The substrates, low iron borosilicate glass (C-glass), were cleaned in a washing machine using bidistilled water and then heated in an oven at 500°C for one hour. Single and multilayer coatings were spun at a speed of 1000 rpm for 15 s. The layers were first dried in air for a few minutes and then sintered for 15 min at 600°C. When the effect of sintering temperatures was studied, fused quartz substrates were used in a temperature range of 500 to 700°C. The thickness and refractive index of the layers were measured by the ellipsometry technique (Spectroscopic Ellipsometer M-2000, J.A. Woollam Co., Inc.).
X-ray diffraction (XRD) patterns of the layers were collected on XPERT-PRO-MPD (PANalytical) diffractometer unit, using Cu anode material operating at 40 kV and 30 mA with wavelength (K
In order to better understand the thermal behavior of the as-synthesized coating and to ensure the removal of the organic species of the paste, a differential thermal analysis and thermal gravimetry (DTA/TG) spectra were obtained for the GZO (a) and AZO (b) paste doping ratio of 1 mol.% prepared by wetting nanoparticles in PEG and TODs and then dissolving them in 1-propanol (see Figure
TG and DTA for GZO (a) and AZO (b) pastes with doping ratio of 1 mol.% prepared by wetting nanoparticles in PEG and TODs then dissolving in 1-propanol.
Figure
Crystallite size and lattice dimensions of GZO and AZO layers.
Coating | Phase name | Crystallite size (nm) |
|
|
(101)/(002) | |||
---|---|---|---|---|---|---|---|---|
AZO | Zincite | 28 | 36 | 26 | 30 | 3.250360 | 5.209163 | 1.65 |
GZO | Zincite | 38 | 57 | 40 | 45 | 3.248412 | 5.205542 | 2.67 |
XRD pattern of spin coated GZO and AZO layers (doping ratio of 1 mol.%) deposited on borosilicate glass and sintered in air at 600°C.
The intensity ratio of (101)/(002) for both layers is higher than unity, but its value for the GZO layer (2.67) is higher than that of the AZO (1.65). This shows that both layers do not have a preferred growth along the (002) plane (
The morphological and structural features of GZO (a) and AZO (b) coatings with a doping ratio of 1 mol.% sintered in air at 600°C are shown in Figure
SEM images of the surface morphology of spin coated GZO (a) and AZO (b) sintered at 600°C. Both layers have doping ratio of 1 mol.%.
Many structures have been reported for ZnO based layers. ZnO whiskers and acicular particles were prepared by hydrothermal method [
The electrical conductivity of gallium and aluminum-doped zinc oxide (GZO and AZO) films was characterized as a function of doping ratio, thickness of the coatings, and sintering temperature. Figure
The electrical resistivity,
The crystallite size was calculated from the XRD spectrum of the GZO and AZO nanopowder having different doping ratios (see Figure
Particles sizes of GZO and AZO powder synthesized hydrothermally with different doping ratio.
Further increase of the doping ratio above 1 mol.% seems to promote the crystallite growth of AZO nanoparticles where the particle size increased to 47 nm at a doping ratio of 2 mol.%. This seems to be caused by exceeding the thermodynamic limitation of solubility of Al in ZnO [
The lower resistivity of the GZO layer compared to the AZO might refer to the high scattering at the boundaries in the AZO layer where XRD pattern of nanoparticles showed that it has smaller grains. It is well known that the scattering at grain boundaries has a great effect on the conductivity and the charge mobility. The same result was reported in the comparative study made by [
Aegerter et al. [
Sheet resistance and optical transmission at 550 nm, of spin coated GZO (a) and AZO (b) multilayers, sintered in air at 600°C and further postannealed in forming gas at 400°C, as a function of layer thickness. Each single layer is approximately 52–55 nm thick.
As seen in Figure
Fortunato et al. [
The optical properties of GZO and AZO layers were studied by UV-Vis spectrophotometer. Figure
The optical transmittance in the wavelength range 300–3000 nm (a),
The influence of the heat treatment temperature on electrical conductivity of GZO and AZO layers prepared from colloidal solution of GZO and AZO nanoparticles (doping ratio 1 mol.%) in 1-propanol as a solvent was tested in the temperature range 500–700°C. For the GZO layer, no significant changes have been noticed by increasing the temperature between 500°C and 600°C where the electrical resistivity decreased from
Resistivity of GZO and AZO 5 layers with doping ratio of 1 mol.%, deposited on quartz substrate and sintered in air at different temperatures and then postannealed in reducing gas for 90 min at 400°C.
Aluminum and gallium doped ZnO TCO thin films were deposited by spin coating on glass substrates using crystalline nanoparticles redispersed in 1-propanol. All films have a hexagonal wurtzite crystal structure. The particle size and electrical and optical properties for both films are found to be dependent on the doping ratio. A minimum sheet resistance was obtained for GZO and AZO films at a doping ratio of 1 mol.%. The heat treatment has a great effect on the obtained layers where no phase changes were observed at temperature higher than 500°C, while the electrical conductivity is found to decrease by heating the films at temperature higher than 600°C. The transmittance of the AZO and GZO thin films is higher than 80% in the visible region. The optical band gap of the AZO is higher than that of the GZO, and both of them are higher than that of bulk ZnO in accordance with the Burstein-Moss effect. In conclusion, GZO films showed better properties than AZO ones when used as transparent conducting coatings.
The author declares that there is no conflict of interests regarding the publication of this paper.