We are engaged in studies of interfaces that form between deposited metal oxides and various high-quality covalent semiconductor crystals, including Si, Ge, InGaAs and GaN. Most of this work is motivated by the continued dimensional scaling of field effect transistors, which prompts interest in new high permittivity dielectric materials and new semiconductors for the transistor channel. Making interfaces with low areal densities of electronic carrier traps is essential for efficient operation of such devices. Our group studies the role of deposition conditions, process chemistry, post-deposition annealing on the stability of these oxide/semiconductor interfaces, and the defects they form. We also investigate methods for protecting semiconductor surfaces during metal oxide deposition and for post-deposition passivation of interface defects.
Atomic layer deposition (ALD), a method of surface adsorption-limited chemical vapor deposition, is renowned for its ability to deposit ultra-thin and pin-hole free films on a wide variety of substrates. We are investigating applications of ALD in solid oxide fuel cell membranes (e.g. Y2O3-ZrO2 alloy films), ultra-high permittivity dielectrics (e.g. SrTiO3 multi-cation oxides), and in protection of semiconductors from harsh electrochemical environments (e.g. TiO2 on n-Si photoanodes for solar water splitting). An important theme in this work is to exploit and investigate possible changes in the functional properties of these oxide layers as their thicknesses reach nanoscopic dimensions.
We are engaged in studies of nanoscale crystal growth, with particular attention given to Group IV nanowires (NWs), such as Ge NWs, Ge-core/Si-shell NWs, and GeSn NWs. These unique, molecular-scale structures exhibit fascinating electronic and photonic properties, and can be synthesized under conditions that are compatible with silicon device fabrication. Our research focuses on deep sub-eutectic vapor-liquid-solid (VLS) growth of Ge nanowires, the mechanisms and inhibition of misfit strain relaxation in large-mismatch Ge-core/Si shell nanowires, and kinking during VLS growth of NWs. In collaboration with others, we use photoluminescence and time-resolved reflectivity measurements to probe the effects of nanowire diameter, strain and surface defect passivation on electronic structure.