Research Areas of Interest

The following are a few brief areas of research that our lab is interested in and is currently pursuing.

Integration of complex oxides with semiconductors

The combination of the rich functionality and emergent phenomena in complex oxide materials and their heterostructures is very attractive for making new types of device applications. However, it is important that one is able to deposit these functional oxides on semiconductor substrates, particularly silicon, in order for these materials to be technologically useful. We are working on developing processes for combining functional oxides directly on semiconductors for various applications. We work with ferroelectric BaTiO₃, photocatalytic TiO₂, high-k LaAlO₃, and ferromagnetic LaCoO₃ and Sr(Co,Ti)O₃ and integrating them on both silicon and germanium substrates, using a combination of both MBE and ALD growth methods.

Controlling the interface between dissimilar materials using Zintl phases

The ability to grow SrTiO₃ on Si hinges on the use of a one-half monolayer Sr template that forms a Zintl phase transition layer allowing for a smooth chemical bonding transition between SrTiO₃ and Si. The detailed mechanism for how this works is still not fully settled to date and we are using a combination of ARPES, RHEED and STM analysis in conjunction with first principles theory to study the mechanism of this process, as well as the equivalent process on Ge. We are also developing new types of Zintl phase layers based on aluminides that may allow for the opposite sequence of growing a single crystalline semiconductor layer on an oxide surface. This opposite sequence has enormous consequences for being able to obtain alternative channel materials more cheaply, as well as opening up new avenues for achieving even more complex heterostructures.

All-oxide quantum wells and superlattices

The growth of oxide superlattices is now at a level approaching those of the III-V semiconductors. Such quantum wells based on oxides can enable similar optical and resonant tunneling effects that have been observed in semiconductors but with potentially larger/stronger effects. We have recently started looking into the design of all-oxide artificial superlattice tailored to produce specific optical/transport effects.

Strain tuning of electronic properties

By growing highly correlated electron materials on carefully chosen substrates or pseudosubstrates, one can tune the strain in these materials and control the ground state. We have shown that LaCoO₃ can be induced to become ferromagnetic when biaxially tensile strained and that there is a spin state disproportionation in the system. In addition to the cobaltates, we are also looking into strain tuning of the magnetic and metal-insulator transition in (La,Eu)NiO₃ where it is expected that a similar disproportionation of an electronic degree of freedom will also occur. We are also investigating the effect of strain on the f-electron ferromagnetic semiconductor EuO.

Oxidation and growth mechanisms of oxides on semiconductors

Our ability to have submonolayer control of the deposition process using both physical (MBE) and chemical (ALD) processes, and our ability to dose oxygen with high precision, in combination with in situ RHEED and ARXPS allows us to follow the growth of an oxide, particularly on semiconductor substrates, from initial deposition to post-annealing (with or without oxygen). We can follow the evolution of the chemical state of the various elements and the extent of oxygen diffusion across different layers as the growth or annealing process proceeds. This information can then be used to optimize the growth of specific oxide systems for certain applications, for example by tuning band offsets between oxides.

Wetting of metals on oxide surfaces

Metals on oxides are utilized for electrical contacts, which require continuous and smooth films, or for catalysis, which require nanocrystals. Using a combination of STM, RHEED, and density functional theory, we study the wetting behavior of various metals on different oxide surfaces. We also study the submonolayer behavior of metals on oxide surfaces, which have completely different behavior from thicker films.