J. H. Ngai

Active Silicon Integrated Nanophotonics: Ferroelectric BaTiO3 Devices

The integration of complex oxides on silicon presents opportunities to extend and enhance silicon technology with novel electronic, magnetic, and photonic properties. Among these materials, barium titanate (BaTiO3) is a particularly strong ferroelectric perovskite oxide with attractive dielectric and electro-optic properties. Here we demonstrate nanophotonic circuits incorporating ferroelectric BaTiO3 thin films on the ubiquitous silicon-on-insulator (SOI) platform. We grow epitaxial, single-crystalline BaTiO3 directly on SOI and engineer integrated waveguide structures that simultaneously confine light and an RF electric field in the BaTiO3 layer. Using on-chip photonic interferometers, we extract a large effective Pockels coefficient of 213 ± 49 pm/V, a value more than six times larger than found in commercial optical modulators based on lithium niobate. The monolithically integrated BaTiO3 optical modulators show modulation bandwidth in the gigahertz regime, which is promising for broadband applications.

Tuning the Structure of Nickelates to Achieve Two-Dimensional Electron Conduction

Metallic electronic transport in nickelate heterostructures can be induced and confined to two dimensions (2D) by controlling the structural parameters of the nickel-oxygen planes.

Achieving A‐Site Termination on La0.18Sr0.82Al0.59Ta0.41O3 Substrates

Smooth substrate surfaces terminated with a single atomic plane are essential for perfecting the epitaxial growth of transition metal oxide fi lms. SrTiO 3 (STO), which is closely lattice matched to many transition metal oxides, is extensively used in part because TiO 2 -terminated surfaces exhibiting singleunit-cell steps can be achieved through etching and annealing techniques. [ 1–5 ] In addition to improving the epitaxial growth of fi lms, substrates terminated by a single atomic plane enable interfaces between transition metal oxides to be studied. For example, TiO 2 -terminated substrates have enabled the interface between LaAlO 3 and STO to be probed where metallicity and superconductivity have been unexpectedly observed. [ 6 , 7 ]

A high density two-dimensional electron gas in an oxide heterostructure on Si (001)

We present the growth and characterization of layered heterostructures comprised of LaTiO3 and SrTiO3 epitaxially grown on Si (001). Magnetotransport measurements show that the sheet carrier densities of the heterostructures scale with the number of LaTiO3/SrTiO3 interfaces, consistent with the presence of an interfacial 2-dimensional electron gas (2DEG) at each interface. Sheet carrier densities of 8.9 × 1014 cm−2 per interface are observed. Integration of such high density oxide 2DEGs on silicon provides a bridge between the exceptional properties and functionalities of oxide 2DEGs and microelectronic technologies.

Scanning SQUID susceptometry of a paramagnetic superconductor

Scanning SQUID susceptometry images the local magnetization and susceptibility of a sample. By accurately modeling the SQUID signal we can determine the physical properties such as the penetration depth and permeability of superconducting samples. We calculate the scanning SQUID susceptometry signal for a superconducting slab of arbitrary thickness with isotropic London penetration depth, on a non-superconducting substrate, where both slab and substrate can have a paramagnetic response that is linear in the applied field. We derive analytical approximations to our general expression in a number of limits. Using our results, we fit experimental susceptibility data as a function of the sample-sensor spacing for three samples: 1) delta-doped SrTiO3, which has a predominantly diamagnetic response, 2) a thin film of LaNiO3, which has a predominantly paramagnetic response, and 3) a two-dimensional electron layer (2-DEL) at a SrTiO3/AlAlO3 interface, which exhibits both types of response. These formulas will allow the determination of the concentrations of paramagnetic spins and superconducting carriers from fits to scanning SQUID susceptibility measurements.

Phase diagram of compressively strained nickelate thin films

The complex phase diagrams of strongly correlated oxides arise from the coupling between physical and electronic structure. This can lead to a renormalization of the phase boundaries when considering thin films rather than bulk crystals due to reduced dimensionality and epitaxial strain. The well-established bulk RNiO3 phase diagram shows a systematic dependence between the metal-insulator transition and the perovskite A-site rare-earth ion, R. Here, we explore the equivalent phase diagram for nickelate thin films under compressive epitaxial strain. We determine the metal-insulator phase diagram for the solid solution of Nd1-yLayNiO3 thin films within the range 0 ≤ y ≤ 1. We find qualitative similarity between the films and their bulk analogs, but with an overall renormalization in the metal-insulator transition to lower temperature. A combination of x-ray diffraction measurements and soft x-ray absorption spectroscopy indicates that the renormalization is due to increased Ni–O bond hybridization for cohe...

Interfacial structure in epitaxial perovskite oxides on (001) Ge crystal

We investigated the interfacial structure of hetero-epitaxial SrZr0.68Ti0.32O3 thin film deposited on (001) Ge single crystal via transmission electron microscopy (TEM). The results from high-resolution scanning TEM and electron energy-loss spectroscopy show an atomically abrupt interface without secondary phase. We found misfit dislocations with Burgers vector of 1/2a 〈111〉 and threading dislocations with Burgers vector of a 〈100〉. Furthermore, we observed the coupling between dislocation half-loop and anti-phase boundary induced by the lattice terrace of Ge along 〈100〉 direction and their decoupling after annealing. We proposed models based on half-loop theory to interpret the coupling and the dislocation reactions.

Hysteretic electrical transport in BaTiO3/Ba1−xSrxTiO3/Ge heterostructures

We present electrical transport measurements of heterostructures comprised of BaTiO3 and Ba1−xSrxTiO3 epitaxially grown on Ge. Sr alloying imparts compressive strain to the BaTiO3, which enables the thermal expansion mismatch between BaTiO3 and Ge to be overcome to achieve c-axis oriented growth. The conduction bands of BaTiO3 and Ba1−xSrxTiO3 are nearly aligned with the conduction band of Ge, which facilitates electron transport. Electrical transport measurements through the dielectric stack exhibit rectifying behavior and hysteresis, where the latter is consistent with ferroelectric switching.

Scanning tunneling spectroscopy under pulsed spin injection

An experimental technique combining cryogenic scanning tunneling spectroscopy (STS) and pulsed quasiparticle spin injection has been developed. The spin injection is intended to perturb a superconducting thin film from spin equilibrium, while the STS monitors its steady-state quasiparticle spectrum. A pulsed injection circuit was designed to minimize Joule heating while being both synchronized with and decoupled from the STS circuitry. A detailed description of the technique is presented, along with its application to spin-injection heterostructures comprising the half-metallic ferromagnet La0.7Ca0.3MnO3 and the high-Tc superconductor YBa2Cu3O7−δ.

Tunneling Spectroscopy of c -Axis Y 1 − x Ca x Ba 2 Cu 3 O 7 − δ Thin-Film Superconductors

Scanning tunneling spectroscopy was performed on c-axis Y{1-x}Ca{x}Ba2Cu3O{7-delta} thin films for x= 0, 0.05, 0.15 and 0.20 at 4.2K. The measured spectra show main-gap, sub-gap and satellite features which scale similarly in energy versus Ca-doping, suggesting that they are associated with a single pairing energy. The data is analyzed with a multiband tunneling model which attributes the sub-gap features to the chain band and the satellite and main-gap features to the plane band for d+s pairing symmetry. These results suggest that the superconductivity in Y{1-x}Ca{x}Ba2Cu3O{7-delta} involves multiple bands.