Atomic layer deposited metal oxides for semiconductors used in aqueous solutions [electronic resource]
Keywords: Materials Science and Engineering

In recent years, atomic layer deposition (ALD) has become a popular technique to deposit ultra-thin films with superior conformality and thickness control. Because of its unique surface adsorption-limited mechanism and the resulting capability of deposition at low temperatures and moderate pressures, ALD has found industrial applications in field effect transistor fabrication and coating of multilayer interconnection metallization. In this work, I have explored the potential of ALD-grown metal oxide layers in applications beyond typical electronics technologies. In particular, this research has focused on using ALD-grown metal oxides to enhance the performance and stability in aqueous solutions of biomolecular sensors and semiconducting anodes for photoelectrochemical fuel synthesis. In the biosensing application, we have replaced the SiO2 gate dielectric material typically used in high sensitivity bio-field-effect-transistors (bioFET) with high dielectric constant HfO2. The SiO2 bioFET gate dielectric suffers from poor stability and non-ideal dielectric response at the very small physical thicknesses required to achieve high sensitivity. ALD-grown HfO2, on the other hand, is capable of providing high capacitance density with a physically thicker dielectric layer, thanks to its large dielectric constant. With the ALD-HfO2 gate dielectric, biosensor switching behavior was demonstrated using capacitance-voltage measurements in water, while at the same time maintaining the desired high capacitance. In addition, we have verified bio-functionalization of the HfO2 film surface with biotin molecules via photoelectron spectroscopy, and detected streptavidin and avidin binding with capacitance-voltage analysis and molecular AFM imaging methods respectively. For the solar fuel synthesis, we have studied the behavior of ALD-TiO2 tunnel oxides that can protect heretofore unstable semiconductors, such as Si, used as photoanodes in water splitting. For several decades, intense research effort has been devoted to identifying an efficient photoelectrochemical cell for oxidizing water under solar illumination. The resulting hydrogen and oxygen can be used to store energy from the intermittent terrestrial solar resource renewably, using water as a feedstock. However, photoanode materials choices have always been limited because the water oxidation half reaction at the anode surface is highly corrosive and requires large overpotentials. As a result, only oxidation-stable wide bandgap semiconductors such as TiO2 and Fe2O3 have been used as the photoanode. These photoanodes exhibit poor efficiency, however, because of their large bandgaps. Lower bandgap semiconductors, such as Si, are capable of absorbing solar light much more efficiently, but are easily corroded during water oxidation. In this work, a silicon photoanode was passivated by a thin and pinhole-free layer of ALD-TiO2 such that efficient light absorption in the Si and the chemical stability of the TiO2 can be exploited at the same time. This ALD-grown nanocomposite photoanode has been demonstrated to perform water oxidation with low overpotentials, while at the same time maintaining good stability with hours of continuous operation. The tunneling of electronic carriers through the thin ALD-TiO2, required to sustain high oxidation rates, has also been investigated by varying the TiO2 thickness. The annealing temperature and ambient have also been investigated.