Jason Lapano

Transport properties of ultra-thin VO2 films on (001) TiO2 grown by reactive molecular-beam epitaxy

We report the growth of (001)-oriented VO2 films as thin as 1.5 nm with abrupt and reproducible metal-insulator transitions (MIT) without a capping layer. Limitations to the growth of thinner films with sharp MITs are discussed, including the Volmer-Weber type growth mode due to the high energy of the (001) VO2 surface. Another key limitation is interdiffusion with the (001) TiO2 substrate, which we quantify using low angle annular dark field scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy. We find that controlling island coalescence on the (001) surface and minimization of cation interdiffusion by using a low growth temperature followed by a brief anneal at higher temperature are crucial for realizing ultrathin VO2 films with abrupt MIT behavior.

High-quality LaVO3 films as solar energy conversion material

Mott insulating oxides and their heterostructures have recently been identified as potential photovoltaic materials with favorable absorption properties and an intrinsic built-in electric field that can efficiently separate excited electron-hole pairs. At the same time, they are predicted to overcome the Shockley-Queisser limit due to strong electron-electron interaction present. Despite these premises a high concentration of defects commonly observed in Mott insulating films acting as recombination centers can derogate the photovoltaic conversion efficiency. With use of the self-regulated growth kinetics in hybrid molecular beam epitaxy, this obstacle can be overcome. High-quality, stoichiometric LaVO3 films were grown with defect densities of in-gap states up to 2 orders of magnitude lower compared to the films in the literature, and a factor of 3 lower than LaVO3 bulk single crystals. Photoconductivity measurements revealed a significant photoresponsivity increase as high as tenfold of stoichiometric LaVO3 films compared to their nonstoichiometric counterparts. This work marks a critical step toward the realization of high-performance Mott insulator solar cells beyond conventional semiconductors.

Self-regulated growth of CaVO3 by hybrid molecular beam epitaxy

The authors report on the growth of stoichiometric CaVO3 thin films on LaSrAlO4 (001) using hybrid molecular beam epitaxy approach, whereby the metalorganic vanadium oxytriisopropoxide (VTIP) and Ca was cosupplied from a gas injector and a conventional effusion cell, respectively. Films were grown using a fixed Ca flux while varying the VTIP flux. Reflection high energy electron diffraction, x-ray diffraction, atomic force microscopy, energy-dispersive x-ray spectroscopy, and high resolution transmission electron microscopy were employed to relate film quality to growth conditions. A wide growth window was discovered in which the films were stoichiometric and film lattice parameter was found independent of the Ca/VTIP flux ratio, allowing more than 10% unintentional deviation in the Ca flux while maintaining stoichiometric growth conditions. Films grown within the growth window showed atomically smooth surfaces with stepped terrace morphology and narrow rocking curves in x-ray diffraction with a full widt...

Continuously Tuning Epitaxial Strains by Thermal Mismatch

Strain engineering of thin films is a conventionally employed approach to enhance material properties and to energetically prefer ground states that would otherwise not be attainable. Controlling strain states in perovskite oxide thin films is usually accomplished through coherent epitaxy by using lattice-mismatched substrates with similar crystal structures. However, the limited choice of suitable oxide substrates makes certain strain states experimentally inaccessible and a continuous tuning impossible. Here, we report a strategy to continuously tune epitaxial strains in perovskite films grown on Si(001) by utilizing the large difference of thermal expansion coefficients between the film and the substrate. By establishing an adsorption-controlled growth window for SrTiO3 thin films on Si using hybrid molecular beam epitaxy, the magnitude of strain can be solely attributed to thermal expansion mismatch, which only depends on the difference between growth and room temperature. Second-harmonic generation measurements revealed that structure properties of SrTiO3 films could be tuned by this method using films with different strain states. Our work provides a strategy to generate continuous strain states in oxide/semiconductor pseudomorphic buffer structures that could help achieve desired material functionalities.

Native oxide removal from Ge surfaces by hydrogen plasma

The mechanisms to remove the native oxide layer on Ge(001) surfaces by an in situ hydrogen plasma inside an atomic layer deposition (ALD) reactor has been studied. A strong dependence of the reaction mechanism in the temperature range commonly employed by ALD has been identified through the combined analysis of atomic force microscopy, x-ray photoelectron and Raman spectroscopy. At low temperatures (e.g., 110 °C), the hydrogen plasma removed both Ge and O species from the native GeO2 layer, but also induced surface damage to Ge substrate. At high temperatures (e.g., 330 °C), only O species were removed from the native oxide leaving a nanocrystalline Ge overlayer behind. The thermodynamically unstable nature of hydrogen passivation on Ge resulted in a Ge surface with a high density of dangling bonds. The transition temperature between the two reaction mechanisms was determined to be about 270 °C, allowing to compromise between removing a native oxide layer entirely and hydrogenating the underlying Ge surfa...

Temperature-dependent growth window of CaTiO3 films grown by hybrid molecular beam epitaxy

The authors report the effects of growth temperature on the self-regulated growth window of CaTiO3 thin films grown by hybrid molecular beam epitaxy (hMBE). Films were grown on (001) (La0.3Sr0.7)(Al0.65Ta0.35)O3 at temperatures between 700 and 950 °C. Calcium was supplied by a standard thermal effusion cell, while the metalorganic precursor titanium tetra-isopropoxide (TTIP) was used as the titanium source. The stoichiometric growth conditions were mapped using a combination of x-ray diffraction, reflection high energy electron diffraction, and atomic force microscopy. It is found that the growth window widened and shifted to higher TTIP fluxes with increasing temperature. Further, the shift of the growth window edge to Ti-rich conditions is three times larger than the growth window edge to Ca-rich conditions, which is discussed in general terms of the kinetic processes involved in hMBE.