Transparent conductive oxides (TCOs) are wide bandgap semiconductors characterized by high conductivity and high transparency in the visible and near infrared spectrum of light. TCOs find applications in displays, thin film transistors, solar cells, etc., due to these distinctive properties. Hydrogen-doped indium oxide (IO:H), a TCO first developed in 2007, is emerging as a material of interest because of its high mobility (>100 cm²/V·s) and high transparency (>90 % in the visible region of the spectrum) as compared to other TCOs such as tin doped indium oxide (ITO) and aluminium doped zinc oxide (AZO). Recently, IO:H has been developed using sputtering and temporal atomic layer deposition. However, each has its disadvantages such as the development of pinholes in TCO films, long purge steps in ALD process, etc. For this reason, a new deposition method has been investigated.
In this thesis, IO:H has been deposited using atmospheric-pressure plasma-enhanced spatial atomic layer deposition. In the first part of this project, the use of N₂ plasma along with water and trimethylindium (TMI) vapour as the prime reactants for the deposition of IO:H films with thicknesses typically around 140 nm on alumino-boro-silicate glass substrates was studied. The process parameters such as flow rate of water, exposure time of the substrate to the reactants, etc. were optimized, and the ALD process temperature window was found to be between 150 °C and 225 °C. In the next part of the project, H₂/N₂
plasma was employed to further reduce the total process time and to enable deposition of a high-quality TCO at a lower temperature. The use of H₂/N₂ plasma almost doubled the rotation speed of the substrate (i.e., substrate exposure time reduced from typically 300 ms to 150 ms). This opens ways for faster net deposition rates that typically correspond to 0.02 nm/s.
The electrical, optical and structural properties of the thin films deposited using both the plasmas were studied. It is demonstrated that using H₂/N₂ plasma, it is possible to deposit high-quality thin films with carrier mobility values as high as 85 cm²/V·s at 150 °C and 110 cm²/V·s at 175 °C with resistivity values of only ∼ 2.5 mΩ·cm. When N₂ plasma was used similar values were recorded at 200 °C and 225 °C. A transparency of ∼ 90 % in the visible region of the spectrum and ∼ 85 % on an average was observed in the spectral range of 300-1800 nm in both the cases and the absorbance was found to be below 10 %. XRD and SEM analysis of the thin films showed that the layers deposited are polycrystalline. The average size of the crystallite increases in the direction parallel to the surface of the substrate with an increase in the deposition temperature. The average size
increases from ∼ 50 nm to ∼ 100 nm when N₂ plasma is used and from ∼ 35 nm to ∼ 70 nm when plasma is used.
Finally, a preliminary optical analysis of the thin film on a CuInSe (CIS) solar cell was performed and compared with an ITO-based CIS cell stack. From the simulation results, it was seen that no parasitic absorption occurs in IO:H in the near infrared region of the spectrum. The advantage of high transparency of IO:H, as compared to ITO, is somewhat lost as the thickness of IO:H thin film has to be increased to compensate for the relatively higher resistivity of the material as compared to ITO.
The knowledge gained from the spatial ALD process developed in this thesis and the properties of the thin films deposited as such will propel advancements to further improve the deposition technique for low temperature and large area applications in commercial roll-to-roll and sheet-to-sheet production of thin films on polymer substrates.