RESEARCH INTERESTS

Over the past nine years I have been active in the field of complex perovskite oxides.1) Perovskite oxides offer unprecedented possibilities in the emerging area of oxide electronics due to their exotic properties such as high temperature superconductivity, colossal magneto-resistance, large ferroelectric polarizations, and photovoltaic behavior.

My focus has been on understanding the physics behind these exciting properties through the growth and analysis of thin films. Improvements in film growth techniques over the past decades allow the creation of thin films with atomic precision, which has led to the discovery of many new materials as a result of different stacking of atoms.

Interfaces between materials can now be made atomically sharp and analyzed on an atomic scale with various imaging techniques. The physics at such interfaces may be very different from the bulk components that form it and thus allowing new physics and states of matter to be discovered. The epitaxial relationship between substrate and film offers the possibility of inducing strain in materials beyond what is possible in bulk. Strain can alter the properties of materials dramatically and offers an additional degree of freedom in the search for new materials or material properties.

I have divided my research on complex oxides into four areas: interface physics, correlated electron systems, epitaxial growth and stabilization, and materials for energy. Herewith is a brief overview of my contributions to each of these areas.

Interface physics
In 2004 it was discovered2) that a conducting electron layer exists at this interface with a sheet carrier density of ~1017 electrons/cm2 and a mobility of 104 cm2 V-1 s-1; each of these numbers is strikingly large. The origin of the conductivity is still not clear, but is suggestive of a very interesting charge transfer system due to the polar nature of these materials.3) As a graduate student at Stanford University I studied the LaAlO3/SrTiO3 interface to determine the origin of the conducting layer.

I demonstrated that the magnitude of the sheet density and the mobility of the electrons are sensitive functions of the deposition conditions in ways that suggest that the origin of this large sheet charge density is oxygen vacancies (donating electrons to the SrTiO3 substrate).4) These vacancies are introduced by the pulsed laser deposition (PLD) process, which is used to create the interface. Ultraviolet photoelectron spectroscopy (UPS) spectra showed states at the Fermi level, which indicated a conducting interface. The number of these states is lowered when the sample is oxidized, insinuating oxygen vacancies play an essential role in supplying the charge carriers.5)

The location of the charge carriers changes dramatically as a function of temperature. I calculated the carrier density in the SrTiO3 to determine where the electrons are located as a function of distance from the interface and concluded that the electrons move into the pristine SrTiO3 over large distances mainly due to the high dielectric constant of SrTiO3 at low temperatures. I then used this effect to create devices in which the charge carrier modulation is accomplished through a mismatch in dielectric permittivities.6)

Correlated electron systems
It is widely agreed that electron-electron correlation plays an important role in many complex oxide systems. Models exist that predict behavior based on on-site Coulomb interaction and electron bandwidth. However, changing correlation experimentally is not a straightforward task and usually involves changing the compound (changing one cation for another). I found that by changing the Ru stoichiometry slightly in SrRuO37),8) the electron correlation could be changed by an amount that would normally require a change from SrRuO3 to CaRuO3.9)

I analyzed magnetism in ultrathin films of SrRuO3 and showed that itinerancy and ferromagnetism disappear abruptly at a critical film thickness between 3 and 4 monolayers.10) I believe this metal-insulator transition is correlation driven and more experiments under different strain states are in progress to prove this.

Epitaxial stabilization
Applying strain through epitaxy is a powerful tool to influence material properties and even create new structural phases of materials. For example, I have studied the effect of epitaxial strain in CuO. CuO is a candidate compound to study the influence of correlation on the electronic structure of transition metal compounds, in particular high temperature cuprate superconductors.

I grew CuO with a higher degree of symmetry by using epitaxial stabilization on a SrTiO3 substrate.11) I found that this large compressive strain makes the CuO undergo a phase transition. A new phase is formed in which the out-of-plane lattice parameter is over 30% larger than the in-plane lattice parameter, resulting in a highly tetragonal unit cell with rock-salt planes. Recent theoretical studies have shown that the magnetic properties of this new phase are unusual in that the Néel temperature is expected to be three times higher than for bulk CuO. Experiments at the Advanced Light Source at Lawrence Berkeley National Laboratory are ongoing to determine the magnetic properties.

Recently, I have studied the structural and magnetic properties of a tetragonal form of BiFeO3. BiFeO3 undergoes a phase transition similar to CuO under large applied stress. I found that the structural and magnetic transition temperatures are suppressed in the tetragonal-like BiFeO3, compared to the bulk rhombohedral phase.12),13)

Materials for energy
At the University of California at Berkeley I studied materials for energy applications, specifically thermoelectrics. Oxide materials for thermoelectrics can be divided into two groups: correlated electron systems and uncorrelated electron systems, both are interesting for thermoelectrics.

I used SrTiO3 doped with oxygen vacancies and La as a model band insulator.14) This double doping mechanism enabled me to study a wide range of carrier densities in the material15) to study how the thermoelectric properties vary with carrier density. The maximum figure of merit (ZT) reached for these materials at high temperature is about 0.2, still lower than state-of-the-art values of more than 1 for conventional semiconductors, but encouraging because oxides are stable at high operating temperatures.

Through the many doping experiments on SrTiO3 I discovered a region of doping where the SrTiO3 will be a transparent conducting oxide (TCO).16) The advantage of this TCO is that it is crystalline and can be used as an epitaxial template for other oxide materials. The crystallinity, together with the high growth temperature, makes SrTiO3 an interesting option as a TCO in all-oxide epitaxial photovoltaic devices.

My preliminary work on a correlated electron system, namely Bi2Sr2Co2Oy, has shown that the thermopower is unusually robust against surface scattering. By changing the thickness systematically from 100 to 5 nm the thermopower is unaffected unlike the carrier mobility, which goes down.

References
    Ramesh, R. & Schlom, D.G. Whither Oxide Electronics? Mrs Bull 33, 1006–1014 (2008.
    Ohtomo, A. & Hwang, H.Y. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. Nature 427, 423–426 (2004.
    Singh-Bhalla, G. et al. Built-in and induced polarization across LaAlO3/SrTiO3 heterojunctions. Nat Phys 7, 80–86 (2011).
    Siemons, W. et al. Origin of Charge Density at LaAlO3 on SrTiO3 Heterointerfaces: Possibility of Intrinsic Doping. Phys. Rev. Lett. 98, 196802 (2007).
    Siemons, W. et al. Experimental investigation of electronic properties of buried heterointerfaces of LaAlO3 on SrTiO3. Phys. Rev. B 76, 155111 (2007).
    Siemons, W. et al. Dielectric-permittivity-driven charge carrier modulation at oxide interfaces. Phys. Rev. B 81, 241308 (2010).
    Vailionis, A., Siemons, W. & Koster, G. Room temperature epitaxial stabilization of a tetragonal phase in ARuO3 (A=Ca and Sr) thin films. Appl. Phys. Lett. 93, 1909 (2008).
    Vailionis, A., Siemons, W. & Koster, G. Strain-induced single-domain growth of epitaxial SrRuO3 layers on SrTiO3: A high-temperature x-ray diffraction study. Appl. Phys. Lett. 91, 1907 (2007).
    Siemons, W. et al. Dependence of the electronic structure of SrRuO3 and its degree of correlation on cation off-stoichiometry. Phys. Rev. B 76, 75126 (2007).
    Xia, J., Siemons, W., Koster, G., Beasley, M.R. & Kapitulnik, A. Critical thickness for itinerant ferromagnetism in ultrathin films of SrRuO3. Phys. Rev. B 79, 140407 (2009).
    Siemons, W. et al. Tetragonal CuO: End member of the 3d transition metal monoxides. Phys. Rev. B 79, 195122 (2009).
    Siemons, W., Biegalski, M.D., Nam, J.H. & Christen, H.M. Temperature-Driven Structural Phase Transition in Tetragonal-Like BiFeO3. Applied Physics Express 4, 5801 (2011).
    MacDougall, G.J. et al. Antiferromagnetic transitions in T-like BiFeO3. arXiv cond-mat.mtrl-sci, 2975 (2011).
    Ravichandran, J. et al. High-temperature thermoelectric response of double-doped SrTiO3 epitaxial films. Phys. Rev. B 82, 165126 (2010).
    Ravichandran, J. et al. Tuning the electronic effective mass in double-doped SrTiO3. Phys. Rev. B 83, 035101 (2011).
    Ravichandran, J. et al. An Epitaxial Transparent Conducting Perovskite Oxide: Double-Doped SrTiO3. Chem Mater 22, 3983–3987 (2010).