Ga-doped polycrystalline ZnO films on glass substrates were prepared by sputtering the targets, which had been prepared by sintering discs consisting of ZnO powder and various amounts of $Ga_2O_3$, to investigate the effect of gallium doping and sputtering conditions on electrical and optical properties. The density of sintered targets was ~80% of the theoretical value and was independent of the amount of $Ga_2O_3$ added.
The films were deposited onto glass substrates placed parallel to the target and the distance between the target and the substrate was fixed to 5cm.
Ohmic contacts were provided by coating with In-Ag paste and annealing at 200°C for 10 min in nitrogen. Electrical resistivity, carrier concentration and mobility were measured by the van der Pauw method. The optical transmission spectra was measured in the wavelength range from 300 to 800 nm with a spectrophotometer (Varian Super Scan). Scanning electron microscope (SEM), X-ray diffractometer and Auger electron spectroscopy (AES) were used to characterize the crystal structure, microstructure and uniformity of the films.
The microstructure of RF sputtered ZnO films showed uniform and smooth surfaces, and the grain size increased from ~500A° in undoped specimen to ~1000A° in Ga-doped one.
In X-ray diffraction analysis, a strong peak intensity at 2θ = 33.5° is observed in each pattern, and the width of the peak decreases with Ga-doping. For the specimens that was doped with 15 wt% $Ga_2O_3$, extra intensity beyond 33.5° increases while a broad intensity is observed 2θ = 20°~30° for the undoped specimen. These facts imply that the grains are strongly oriented (c-axis of the hexagonal ZnO grains normal to substrate surface) and grain size increases with Ga-doping and that some secondary phase exists when Ga content exceeds approximately 7.5 wt%.
For a quantitative analysis, it can be seen that the normalized Ga peak-to-peak intensities of the films coincide very closely with Ga wt% in the targets indicating that the composition of the sputter deposited films is almost identical to that of the target. The composition of film specimens was uniform in thickness direction.
The deposition rate increases linearly with the RF power intensity. The electrical resistivity shows a minimum value at the power density of 0.84 W/$cm^2$. The resistivity decreases sharply with increasing Ga content up to 5 wt% and then increases with further increasing in the Ga content.
The electron concentration of undoped ZnO film is $7.5×10^{17}/cm^3$ and increases with increasing Ga concentration up to 5wt% $Ga_2O_3$ and then stays about the same value of $~10^{21}/cm^3$ with further increase in the amount of $Ga_2O_3$. The electron mobility, on the other hand, decreases continuously with increasing amount of $Ga_2O_3$. The density of states in conduction band(Nc) at room temperature was calculated to be $4×10^{18}/cm^3$. Thus the effect of trap density at grain boundary on the carrier concentration should be negligible for the Ga-doped ZnO films since their electron concentrations are larger than $10^{20}/cm^3$ and the films are degenerated semiconductors. If we assume that the Ga doping is substitutional, the solubility of Ga in ZnO at 200°C, at which the electrode contacts were annealed, is an order of $10^{21}/cm^3$ since the electron concentration of 4wt% of Ga-doped film is $~10^{21}/cm^3$ and stays constant with further increase in Ga content. The 5wt% of $Ga_2O_3$ in the ZoO target is equivalent to Ga concentration of $~1.5×10^{21}/cm^3$ in ZnO. If the above assumption is correct, then it implies the possibility of Ga segregation at grain boundary when the Ga content exceeds ~5wt%.
There was some evidence that secondary phase exists in the specimen that contained more than 7.5wt% Ga in the X-ray diffraction patterns.
The electron mobility decreases rather sharply from $~30cm^2$/V.s in undoped specimen to $~7cm^2$/V.s in 2wt% Ga-doped film and decreases further with increasing Ga content. It appears that the decrease in the mobility is mainly caused by ionized impurity scattering for the films doped with Ga up to 5wt% $Ga_2O_3$. The further decreases in electron mobility as the amount of $Ga_2O_3$ exceeds 7.5wt% may have been caused additional scattering due to gallium segregation at grain boundaries.
The value of energy band gap determined by measuring optical transmission spectra came out to be 3.28eV and 3.59eV for undoped and 5wt% Ga-doped film respectively. The increase in the optical band gap with an increase in electron concentration is related with the increase in the Fermi level in conduction band in degenerated semiconductor.
In calculation of $E_g$ as a function of the electron concentration, it is seen that our data extent well the data reported by Roth et al. for the ZnO films deposited by CVD method. The deviation from the theory line is not quite clear, but Roth et al. explained it an terms of band gap narrowing due to the overlap of wave function of donor states as the donor density increases.
