Quantum confinement in amorphous TiO2 films studied via atomic layer deposition

Despite the significant recent increase in quantum-based optoelectronics device research, few deposition techniques can reliably create the required functional nanoscale systems. Atomic layer deposition (ALD) was used here to study the quantum effects attainable through the use of this angstrom-level controlled growth process. Size-dependent quantum confinement has been demonstrated using TiO2 layers of nanoscale thickness applied to the surfaces of silicon wafers. TiO2 films were deposited at 100 degrees C using TiCl4 and H2O2 in a viscous flow ALD reactor, at a rate of 0.61 angstrom/cycle. The low-temperature process was utilized to guarantee the amorphous deposition of TiO2 layers and post- deposition thermal annealing was employed to promote crystallite-size modification. Hydrogen peroxide significantly reduced the residual chlorine that remained from a typical TiCl4-H2O ALD process at this temperature, down to 1.6%. Spectroscopic ellipsometry was used to quantify the optical properties both below and above the bandgap energy. A central composite design was employed to map the surface response of the film thickness-dependent bandgap shift for the as- deposited case and up to a thermal annealing temperature of 550 degrees C. The Brus model was used to develop a correlation between the amorphous TiO2 film thickness and the quantum length to promote equivalent bandgap shifts.