(Invited) Chemical Stability and Electronic Behavior of Atomic Layer Deposited Metal Oxide Thin Films

Modification of semiconductor surfaces via the growth of metal oxide overlayers, particularly by atomic layer deposition (ALD), has become a topic of recent interest for surface passivation and chemical protection in semiconductor/liquid junctions.  For example, in photoelectrochemical water splitting, semiconductor surfaces typically must be protected with insulating or conducting layers that are stable in the electrochemical environment.  Further, ALD thin films have been used as overlayers in a variety of other environments including those relevant to batteries, corrosion protection of metals, and for water diffusion barriers.  Although thermodynamic stability of a solid phase in various environments can be readily assessed, more complex phenomena such as film hydration and dynamic dissolution/precipitation can occur, dramatically impacting the efficacy of thin protective ALD films. 

            Motivated by this widespread interest in ALD protective layers, we have examined the chemical stability of the two of the most common ALD materials, alumina and titania, in a variety of environments.  Films were deposited using thermal ALD using trimethylaluminum and tetrakis dimethylamido titanium for alumina and titania, respectively.  Film thickness was then monitored after immersion in various liquid media, which included acidic, neutral and basic aqueous solutions and mercury.  Surface morphology was investigated using atomic force microscopy.  Our results indicate a drastic increase in stability of the ALD films after post-deposition thermal treatment. Further, for alumina films in pure water, a dramatic restructuring occurs changing film density, thickness, and morphology.  Mercury contacts induce a slow increase in film thickness that is attributed to mercury oxide deposition.  Finally, we report how the effective surface recombination velocities at these interfaces evolve over the course of thermal annealing and various solution treatments.  These results provide important guidelines for the applicability of ALD oxides as protective/passivating films on semiconductor electrodes for applications in sensing and photoelectrochemical water splitting.