Modulating Extreme Electron Densities in Complex Oxides

Strong electron-electron interactions in transition metal oxides (TMOs) give birth to many exciting and potentially useful phenomena, such as metal-insulator transition, high-Tc superconductivity, colossal magnetoresistance etc. For practical applications, controlling these phenomena reversibly by a field effect like in a Silicon metal-oxide-semiconductor or MOS transistor would be very useful. However, modulating large carrier densities ~1014 cm-2 and greater to see drastic changes in TMO properties is challenging. Recently, a two-dimensional electron gas (2DEG) with extremely high density of ~3 x 1014 cm-2 has been discovered at the heterointerface of SrTiO3/RTiO3 (R:Gd, Sm). In addition to being interesting in its own right for high power electronic and tunable plasmonic devices, this 2DEG can also act as a test bed for modulating extremely large charges and to explore potential challenges in this endeavor and possible solutions. Such an effort on modulating large electron concentrations in SrTiO3 and in SrTiO3/RTiO3 (R:Gd, Sm) systems is the central focus of this work.

GdTiO3 is a Mott-Hubbard insulator whose bandgap is not well known. We performed photoluminescence spectroscopy measurements of GdTiO3 thin films and discovered room temperature emission at red wavelengths. These measurements suggest that the bandgap of GdTiO3 is much larger than previously suspected.

SrTiO3/GdTiO3 heterostructures have a type-II energy band lineup, and the 2DEG resides in SrTiO3. To develop an understanding of the electron transport in SrTiO3 we modeled Hall-effect mobility data measured in SrTiO3 thin films by considering various electron scattering mechanisms. We quantitatively explain the mobility values from 2-300K. We propose that in intermediate temperatures of ~10-200K, the electron scattering is dominated by a ~6meV transverse optical phonon present in SrTiO3, thus resolving a long-standing problem in the field.

With its high room temperature dielectric constant of ~300, SrTiO3 is attractive as a gate dielectric for modulating extreme electron concentrations. To realize field-effect devices, we have developed dry etching, Ali/Au ohmic contacts, and Pt & Au based Schottky contacts for SrTiO3. Using temperature-dependent capacitance-voltage measurements on SrTiO3 Schottky diodes, we have discovered a ferroelectric transition in compressively strained SrTiO3 grown on LSAT substrates. Using Au Schottky contacts, we demonstrate ~ 1.6 x 1014 cm-2 charge modulation in SrTiO3 MESFETs. We also demonstrate SrTiO3/GdTiO3 Schottky diodes and HEMTs on LSAT substrate modulating ~1014 cm-2 electron density, and SrTiO3/GdTiO3 Schottky diodes on (100) Si substrates modulating greater than 1.1 x 1014 cm-2 electron density. Using capacitance enhancement in Fin structures, we demonstrate record electron density modulation of ~2.5 x 1014 cm-2 in SrTiO3/SmTiO3 FinFETs.