Evgeny Mikheev

Resistive switching and its suppression in Pt/Nb:SrTiO3 junctions

Oxide-based resistive switching devices are promising candidates for new memory and computing technologies. Poor understanding of the defect-based mechanisms that give rise to resistive switching is a major impediment for engineering reliable and reproducible devices. Here we identify an unintentional interface layer as the origin of resistive switching in Pt/Nb:SrTiO3 junctions. We clarify the microscopic mechanisms by which the interface layer controls the resistive switching. We show that appropriate interface processing can eliminate this contribution. These findings are an important step towards engineering more reliable resistive switching devices.

Electric field-tunable BaxSr1−xTiO3 films with high figures of merit grown by molecular beam epitaxy

We report on the dielectric properties of BaxSr1−xTiO3 (BST) films grown by molecular beam epitaxy on epitaxial Pt bottom electrodes. Paraelectric films (x ≲ 0.5) exhibit dielectric losses that are similar to those of BST single crystals and ceramics. Films with device quality factors greater than 1000 and electric field tunabilities exceeding 1:5 are demonstrated. The results provide evidence for the importance of stoichiometry control and the use of a non-energetic deposition technique for achieving high figures of merit of tunable devices with BST thin films.

Correlation between stoichiometry, strain, and metal-insulator transitions of NdNiO3 films

The interplay of film stoichiometry and strain on the metal-insulator transition (MIT) and Hall coefficient of NdNiO3 films grown under different conditions is investigated. Unstrained lattice parameters and lattice mismatch strains are evaluated for films grown under a range of growth pressures and on different substrates. It is shown that both the temperature of the MIT and the Hall coefficient in the metallic phase are highly sensitive to film strain. In films grown with lower oxygen/total growth pressures, very large compressive in-plane strains can be obtained, which can act to suppress the MIT. Both the Hall coefficient and the temperature of the MIT are relatively insensitive to growth pressure, provided that films under the same strain are compared. The results support an itinerant picture of the transition that is controlled by the Ni eg bands, and that is relatively insensitive to changes in film stoichiometry.

Tuning bad metal and non-Fermi liquid behavior in a Mott material: Rare-earth nickelate thin films.

This work elucidates unconventional metallic behavior and metal-insulator transitions in a strongly correlated materials system. Resistances that exceed the Mott-Ioffe-Regel limit (known as bad metal behavior) and non-Fermi liquid behavior are ubiquitous features of the normal state of many strongly correlated materials. We establish the conditions that lead to bad metal and non-Fermi liquid phases in NdNiO3, which exhibits a prototype bandwidth-controlled metal-insulator transition. We show that resistance saturation is determined by the magnitude of Ni eg orbital splitting, which can be tuned by strain in epitaxial films, causing the appearance of bad metal behavior under certain conditions. The results shed light on the nature of a crossover to a non-Fermi liquid metal phase and provide a predictive criterion for Anderson localization. They elucidate a seemingly complex phase behavior as a function of film strain and confinement and provide guidelines for orbital engineering and novel devices.

Limitations to the room temperature mobility of two- and three-dimensional electron liquids in SrTiO3

We analyze and compare the temperature dependence of the electron mobility of two- and three-dimensional electron liquids in SrTiO3. The contributions of electron-electron scattering must be taken into account to accurately describe the mobility in both cases. For uniformly doped, three-dimensional electron liquids, the room temperature mobility crosses over from longitudinal optical (LO) phonon-scattering-limited to electron-electron-scattering-limited as a function of carrier density. In high-density, two-dimensional electron liquids, LO phonon scattering is completely screened and the mobility is dominated by electron-electron scattering up to room temperature. The possible origins of the observed behavior and the consequences for approaches to improve the mobility are discussed.

Tailoring resistive switching in Pt/SrTiO3 junctions by stoichiometry control

Resistive switching effects in transition metal oxide-based devices offer new opportunities for information storage and computing technologies. Although it is known that resistive switching is a defect-driven phenomenon, the precise mechanisms are still poorly understood owing to the difficulty of systematically controlling specific point defects. As a result, obtaining reliable and reproducible devices remains a major challenge for this technology. Here, we demonstrate control of resistive switching based on intentional manipulation of native point defects. Oxide molecular beam epitaxy is used to systematically investigate the effect of Ti/Sr stoichiometry on resistive switching in high-quality Pt/SrTiO3 junctions. We demonstrate resistive switching with improved state retention through the introduction of Ti- and Sr-excess into the near-interface region. More broadly, the results demonstrate the utility of high quality metal/oxide interfaces and explicit control over structural defects to improve control, uniformity, and reproducibility of resistive switching processes. Unintentional interfacial contamination layers, which are present if Schottky contacts are processed at low temperature, can easily dominate the resistive switching characteristics and complicate the interpretation if nonstoichiometry is also present.

Extremely scaled high-k/In0.53Ga0.47As gate stacks with low leakage and low interface trap densities

Highly scaled gate dielectric stacks with low leakage and low interface trap densities are required for complementary metal-oxide-semiconductor technology with III-V semiconductor channels. Here, we show that a novel pre-deposition technique, consisting of alternating cycles of nitrogen plasma and tetrakis(dimethylamino)titanium, allows for HfO2 and ZrO2 gate stacks with extremely high accumulation capacitance densities of more than 5 μF/cm2 at 1 MHz, low leakage current, low frequency dispersion, and low midgap interface trap densities (1012 cm−2 eV−1 range). Using x-ray photoelectron spectroscopy, we show that the interface contains TiO2 and small quantities of In2O3, but no detectable Ga- or As-oxides, or As-As bonding. The results allow for insights into the microscopic mechanisms that control leakage and frequency dispersion in high-k/III-V gate stacks.

Temperature-dependence of the Hall coefficient of NdNiO3 thin films

The Hall coefficient of epitaxial NdNiO3 films is evaluated in a wide range of temperatures, from the metallic into the insulating phase. It is shown that for temperatures for which metallic and insulating regions co-exist, the Hall coefficient must be corrected for the time-dependence in the longitudinal resistance, which is due to a slow evolution of metallic and insulating domains. The positive Hall and negative Seebeck coefficients, respectively, in the metallic phase are characteristic for two bands participating in the transport. The change in the sign of the Hall coefficient to negative values in the insulating phase is consistent with the suppression of the contribution from the large hole-like Fermi surface, i.e., the formation of a (pseudo)gap due to charge ordering.

Electric field effect near the metal-insulator transition of a two-dimensional electron system in SrTiO3

SmTiO3/SrTiO3 interfaces exhibit a two-dimensional electron system with carrier densities in the order of 3 × 1014 cm−2 due to the polar discontinuity at the interface. Here, electric field effect is used to investigate an electron system at this interface whose carrier density has been depleted substantially by the gate metal and by reducing the thickness of the SmTiO3. At zero applied gate voltage, the sheet resistance exceeds the quantum resistance, h/e2, by more than an order of magnitude, and the SrTiO3 channel is in the hopping transport regime. The electric field modulates the carrier density in the channel, which approaches the transition to a metal at positive gate bias. The channel resistances are found to scale by a single parameter that depends on the gate voltage, similar to two-dimensional electron systems in high-quality semiconductors.

Carrier density independent scattering rate in SrTiO3-based electron liquids.

We examine the carrier density dependence of the scattering rate in two- and three-dimensional electron liquids in SrTiO3 in the regime where it scales with Tn (T is the temperature and n ≤ 2) in the cases when it is varied by electrostatic control and chemical doping, respectively. It is shown that the scattering rate is independent of the carrier density. This is contrary to the expectations from Landau Fermi liquid theory, where the scattering rate scales inversely with the Fermi energy (EF). We discuss that the behavior is very similar to systems traditionally identified as non-Fermi liquids (n < 2). This includes the cuprates and other transition metal oxide perovskites, where strikingly similar density-independent scattering rates have been observed. The results indicate that the applicability of Fermi liquid theory should be questioned for a much broader range of correlated materials and point to the need for a unified theory.