Brandon Isaac

Variable-angle high-angle annular dark-field imaging: application to three-dimensional dopant atom profiling

Variable-angle high-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy is developed for precise and accurate determination of three-dimensional (3D) dopant atom configurations. Gd-doped SrTiO3 films containing Sr columns containing zero, one, or two Gd dopant atoms are imaged in HAADF mode using two different collection angles. Variable-angle HAADF significantly increases both the precision and accuracy of 3D dopant profiling. Using image simulations, it is shown that the combined information from the two detectors reduces the uncertainty in the dopant depth position measurement and can uniquely identify certain atomic configurations that are indistinguishable with a single detector setting. Additional advances and applications are discussed.

Interface-Driven Ferromagnetism within the Quantum Wells of a Rare Earth Titanate Superlattice

Here we present polarized neutron reflectometry measurements exploring thin film heterostructures composed of a strongly correlated Mott state, GdTiO_{3}, embedded with SrTiO_{3} quantum wells. Our results reveal that the net ferromagnetism inherent to the Mott GdTiO_{3} matrix propagates into the nominally nonmagnetic SrTiO_{3} quantum wells and tracks the magnetic order parameter of the host Mott insulating matrix. Beyond a well thickness of 5 SrO layers, the magnetic moment within the wells is dramatically suppressed, suggesting that quenched well magnetism comprises the likely origin of quantum critical magnetotransport in this thin film architecture. Our data demonstrate that the interplay between proximate exchange fields and polarity-induced carrier densities can stabilize extended magnetic states within SrTiO_{3} quantum wells.

Ring Resonator Delay Elements for Integrated Optical Beamforming Networks: Group Delay Ripple Analysis

The group delay ripple of cascaded optical ring resonator (ORR) delays is quantitatively studied with a genetic algorithm. Initial measurement results are also reported for a fabricated optical beam forming network chip utilizing cascaded ORRs.

Octahedral tilt independent magnetism in confined GdTiO3 films

Low temperature polarized neutron reflectometry measurements are presented, exploring the evolution of ferrimagnetism in thin GdTiO3 films embedded within a SrTiO3 matrix. In GdTiO3 films thinner than ∼4 nm, the TiO6 octahedral tilts endemic to GdTiO3 coherently relax toward the undistorted, cubic phase of SrTiO3. Our measurements indicate that the ferrimagnetic state within the GdTiO3 layers survives as these TiO6 octahedral tilts are suppressed. Furthermore, our data suggest that layers of suppressed magnetization (i.e., magnetic dead layers) develop within the GdTiO3 layer at each GdTiO3/SrTiO3 interface and explain the apparent magnetization suppression observed in thin GdTiO3 films when using volume-averaged techniques. Our data show that the low temperature magnetic moment inherent to the core GdTiO3 layers is only weakly impacted as the octahedral tilt angles are suppressed by more than 50% and the t2g bandwidth is dramatically renormalized.Low temperature polarized neutron reflectometry measurements are presented, exploring the evolution of ferrimagnetism in thin GdTiO3 films embedded within a SrTiO3 matrix. In GdTiO3 films thinner than ∼4 nm, the TiO6 octahedral tilts endemic to GdTiO3 coherently relax toward the undistorted, cubic phase of SrTiO3. Our measurements indicate that the ferrimagnetic state within the GdTiO3 layers survives as these TiO6 octahedral tilts are suppressed. Furthermore, our data suggest that layers of suppressed magnetization (i.e., magnetic dead layers) develop within the GdTiO3 layer at each GdTiO3/SrTiO3 interface and explain the apparent magnetization suppression observed in thin GdTiO3 films when using volume-averaged techniques. Our data show that the low temperature magnetic moment inherent to the core GdTiO3 layers is only weakly impacted as the octahedral tilt angles are suppressed by more than 50% and the t2g bandwidth is dramatically renormalized.

Ring resonator based integrated optical beam forming network with true time delay for mmW communications

An optical ring resonator (ORR) based integrated optical beamforming network (OBFN) for a W-band millimeter wave phased array antenna is reported. The delay response of a 3-ORR delay line is optimized and dynamic tuning ranges of 208.7 ps and 172.4 ps for the true time delay bandwidths of 6.3 GHz and 8.7 GHz are achieved. Moreover, all of the delay paths are successfully tuned with 4. 2 ps delay difference from the neighboring paths. Eye diagrams of a 3 Gbps NRZ OOK modulated signal are measured to show that no noticeable signal deterioration is induced by the OBFN chip.

Low-loss silicon nitride integrated optical beamforming network for wideband communication

Integrated optical beamforming networks (OBFNs) are critical for photonics-assisted wideband microwave phased array antennas. Here, a 1x4 integrated OBFN based on low loss silicon nitride waveguide technology was demonstrated for millimeter wave (mmW) communications. Three cascaded optical ring resonators serve as tunable true time delays (TTDs) for each channel, which overcomes the beam squint issue associated with wideband communication. The tuning of rings was carefully calibrated using the lossless ring delay response theoretical model, and experiments were performed verifying the tuning accuracy. Theoretical simulations were performed to optimize the delay response of the OBFN using a genetic algorithm, which revealed tradeoffs among the delay response flatness, absolute delay value, and TTD bandwidth. Two topologies of the OBFN were theoretically and experimentally compared. A lookup table of the optimized ring parameters was generated, based on which single-delay channels with dynamic tuning ranges of 208.7 ps and 172.4 ps for TTD bandwidths of 6.3 GHz and 8.7 GHz were achieved, which correspond to phase shifts of 37.5π and 31π for a 90-GHz signal, respectively. Moreover, all four channels were tuned to a delay distribution with a differential delay around 4.2 ps, which is equivalent to a 49° beamsteering angle for a 90 GHz half-wavelength dipole antenna array. Using heterodyne upconversion and a single delay path, a 41 GHz mmW signal with 3-Gbps NRZ OOK data modulation was generated and delayed. Future work will focus on higher frequencies into the W-band and on beamsteering experiments.

Strain-Compensated InGaAsP Superlattices for Defect Reduction of InP Grown on Exact-Oriented (001) Patterned Si Substrates by Metal Organic Chemical Vapor Deposition

We report on the use of InGaAsP strain-compensated superlattices (SC-SLs) as a technique to reduce the defect density of Indium Phosphide (InP) grown on silicon (InP-on-Si) by Metal Organic Chemical Vapor Deposition (MOCVD). Initially, a 2 μm thick gallium arsenide (GaAs) layer was grown with very high uniformity on exact oriented (001) 300 mm Si wafers; which had been patterned in 90 nm V-grooved trenches separated by silicon dioxide (SiO2) stripes and oriented along the [110] direction. Undercut at the Si/SiO2 interface was used to reduce the propagation of defects into the III–V layers. Following wafer dicing; 2.6 μm of indium phosphide (InP) was grown on such GaAs-on-Si templates. InGaAsP SC-SLs and thermal annealing were used to achieve a high-quality and smooth InP pseudo-substrate with a reduced defect density. Both the GaAs-on-Si and the subsequently grown InP layers were characterized using a variety of techniques including X-ray diffraction (XRD); atomic force microscopy (AFM); transmission electron microscopy (TEM); and electron channeling contrast imaging (ECCI); which indicate high-quality of the epitaxial films. The threading dislocation density and RMS surface roughness of the final InP layer were 5 × 108/cm2 and 1.2 nm; respectively and 7.8 × 107/cm2 and 10.8 nm for the GaAs-on-Si layer.

Integrated optical beamforming network for millimeter wave communications

An optical ring resonator (ORR) based 1 × 4 integrated optical beamforming network (OBFN) targeting mmW is reported. The optimized true time delay response of a 3-ORR delay line with a bandwidth of 8.7 GHz was achieved over tuning range of 172.4 ps, which corresponds to a 31π phase shift for a 90 GHz signal. A 3 Gbps OOK NRZ data signal was transmitted through the delay line to verify the overall performance. A 49° beamsteering angle equivalent OBFN delay response was achieved for a 90 GHz half-wavelength dipole antenna array. Using heterodyne upconversion technology and a single delay path, 41 GHz mmW signal with 3 Gbps OOK NRZ data modulation was generated and delayed.

Ring Resonator True Time Delay Arrays with Sharing Optimization for Wideband Applications

The delay response and sharing topology of a 3-optical-ring-resonator based optical beamforming network were experimentally studied and optimized. Eye diagrams were measured to evaluate the signal degradation imposed by the chip.

Potential Fluctuations at Low Temperatures in Mesoscopic-Scale SmTiO3/SrTiO3/SmTiO3 Quantum Well Structures

Heterointerfaces of SrTiO3 with other transition metal oxides make up an intriguing family of systems with a bounty of coexisting and competing physical orders. Some examples, such as LaAlO3/SrTiO3, support a high carrier density electron gas at the interface whose electronic properties are determined by a combination of lattice distortions, spin-orbit coupling, defects, and various regimes of magnetic and charge ordering. Here, we study electronic transport in mesoscale devices made with heterostructures of SrTiO3 sandwiched between layers of SmTiO3, in which the transport properties can be tuned from a regime of Fermi-liquid like resistivity (ρ ∝ T2) to a non-Fermi liquid (ρ ∝ T5/3) by controlling the SrTiO3 thickness. In mesoscale devices at low temperatures, we find unexpected voltage fluctuations that grow in magnitude as T is decreased below 20 K, are suppressed with increasing contact electrode size, and are independent of the drive current and contact spacing distance. Magnetoresistance fluctuations are also observed, which are reminiscent of universal conductance fluctuations but not entirely consistent with their conventional properties. Candidate explanations are considered, and a mechanism is suggested based on mesoscopic temporal fluctuations of the Seebeck coefficient. An improved understanding of charge transport in these model systems, especially their quantum coherent properties, may lead to insights into the nature of transport in strongly correlated materials that deviate from Fermi liquid theory.