Ultrathin SrTiO3 Films Research Articles

Tunneling Hot Spots in Ferroelectric SrTiO3.

Strontium titanate (SrTiO3) is the "silicon" in the emerging field of oxide electronics. While bulk properties of this material have been studied for decades, new unexpected phenomena have recently been discovered at the nanoscale, when SrTiO3 forms an ultrathin film or an atomically sharp interface with other materials. One of the striking discoveries is room-temperature ferroelectricity in strain-free ultrathin films of SrTiO3 driven by the TiSr antisite defects, which generate a local dipole moment polarizing the surrounding nanoregion. Here, we demonstrate that these polar defects are not only responsible for ferroelectricity, but also propel the appearance of highly conductive channels, "hot spots", in the ultrathin SrTiO3 films. Using a combination of scanning probe microscopy experimental studies and theoretical modeling, we show that the hot spots emerge due to resonant tunneling through localized electronic states created by the polar defects and that the tunneling conductance of the hot spots is controlled by ferroelectric polarization. Our finding of the polarization-controlled defect-assisted tunneling reveals a new mechanism of resistive switching in oxide heterostructures and may have technological implications for ferroelectric tunnel junctions. It is also shown that the conductivity of the hot spots can be modulated by mechanical stress, opening a possibility for development of conceptually new electronic devices with mechanically tunable resistive states.

Heteroepitaxial growth and characterization of BiFeO3 thin films on GaAs

The paper deals with the integration of well-known bismuth ferrite (BiFeO3) multiferroic oxide with GaAs semiconductor. First 5 nm ultrathin SrTiO3 films were grown on GaAs (001) substrates as an intermediate buffer layer by molecular beam epitaxy. Then, room temperature multiferroic BiFeO3 (BFO) thin films were deposited by pulsed laser deposition. X-ray diffraction measurement showed high quality epitaxial BFO films with pure (00l) orientation. The dielectric loss has been effectively suppressed and the saturated polarization–voltage (P–V) hysteresis loops were obtained. The ferroelectric domains switching was affirmed by piezo-response force microscopic studies. A large remnant polarization Pr (∼80 μC cm−2) combined with the enhanced magnetization (72 emu cm–3) at 300 K was achieved for the optimal growth conditions. The optical properties were measured using the ellipsometry technique for the BFO thin films. The thickness and optical constants of the BFO films were obtained by taking into consideration the dielectric parameters as described by the Tauc-Lorentz model. Finally, direct bandgap was estimated at 2.70 eV which is highly comparable to BFO films grown on different substrates.

Initial growth of SrTiO3 on Sr/Si(001) studied by scanning tunneling microscope

SrTiO3 ultra-thin film was deposited on the Sr/Si(001) surface using pulsed laser deposition (PLD) at room temperature and studied using scanning tunneling microscopy (STM). After annealing at 660 ℃ for about 60 minutes in ultrahigh vacuum (UHV), nanosize islands were formed on the Sr/Si(001) surface. High resolution STM images and dI/dV mapping of islands on Sr/Si(001) were obtained. The islands can be attributed to TiSi2 islands with C49 and C54 structures. The existence of Sr on Si is not sufficient to prevent the reaction between Si and Ti in preparation of ultra-thin SrTiO3 films.

Valence electron energy-loss spectroscopy of ultrathin SrTiO3 films grown on silicon (100) single crystal

Valence electron energy-loss spectroscopy is used to investigate the plasmon excitations of ultrathin SrTiO3 sandwiched between amorphous Si and crystalline Si. Two plasmon excitations were observed, one at 15.8 eV and the other at 28.7 eV. Our calculations, based on dielectric-function theory, suggest that the former peak originates from the coupling of the Si layers and is related to the geometry of the structure, and the latter peak results from the SrTiO3 bulk plasmon after a redshift. Our findings demonstrate the value of valence electron energy-loss spectroscopy in detecting a local change in the effective electron mass.

Ultrathin SrTiO3 films prepared by chemical vapor deposition on Nb-doped SrTiO3 substrates

The SrTiO3 ultrathin film capacitors were realized on Nb-doped single-crystal SrTiO3 substrates by chemical vapor deposition. The leakage current density of 10.4 nm thick SrTiO3 thin-film capacitor was below 1×10−8 A/cm2 in the applied voltage range of −1.8 to +0.45 V, and its SiO2 equivalent thickness was 0.48 nm. The relative dielectric constant was over 160 for SrTiO3 thickness above 20 nm, but it decreased for SrTiO3 thickness below 20 nm. Dependence of leakage current on SrTiO3 film thickness was slight. These results could be explained by the existence of applied electric field concentration near the SrTiO3/electrode interface.