Lawrence Grazulis

Voltage‐Impulse‐Induced Non‐Volatile Ferroelastic Switching of Ferromagnetic Resonance for Reconfigurable Magnetoelectric Microwave Devices

A critical challenge in realizing magnetoelectrics based on reconfigurable microwave devices, which is the ability to switch between distinct ferromagnetic resonances (FMR) in a stable, reversible and energy efficient manner, has been addressed. In particular, a voltage-impulse-induced two-step ferroelastic switching pathway can be used to in situ manipulate the magnetic anisotropy and enable non-volatile FMR tuning in FeCoB/PMN-PT (011) multiferroic heterostructures.

Epitaxial Graphene Growth by Carbon Molecular Beam Epitaxy (CMBE)

A novel growth method (carbon molecular beam epitaxy (CMBE)) has been developed to produce high-quality and large-area epitaxial graphene. This method demonstrates significantly improved controllability of the graphene growth. CMBE with C(60) produces AB stacked graphene, while growth with the graphite filament results in non-Bernal stacked graphene layers with a Dirac-like electronic structure, which is similar to graphene grown by thermal decomposition on SiC (000-1).

Growth of short-period InAs∕GaSb superlattices

The purpose of this work is to explore materials for midinfrared detectors that can operate at room temperature. Shorter-period InAs∕GaSb superlattices (SLs) have larger intervalance band separations, which are beneficial for reducing Auger recombination and tunneling current, thus making room temperature operation possible. To test these possibilities, several short-period SLs ranging from 50to11A were grown and their morphological properties were carefully monitored by transmission electron microscopy. The effect of structural degradation caused by the period reduction on the optical properties was studied using low-temperature photoluminescence (PL). The samples with larger periods (50–32A) showed excellent structural qualities and produced narrow full width at half maximum (FWHM) of the PL peak (5meV). As the period approached 24A, slight layer thickness undulations within the SLs were observed and these undulations intensified as the period further reduced to 17A. These structural degradations strong...

Effect of interfacial formation on the properties of very long wavelength infrared InAs/GaSb superlattices

In InAs/GaSb superlattices (SLs) designed for infrared detection, the interfacial layers comprise approximately 10%–15% of the heterostructure. As interdiffusion into the InAs and GaSb layers is considered, this percentage is expected to be even higher. Although the primary goal for engineering these transient layers is to balance the SL strain to the GaSb substrate, the interfacial quality can impact the performance of the SL in other ways as well. Many believe that the majority of nonradiative defects that shorten carrier lifetime can be generated from the SL interfaces or regions near them due to the poor interface engineering. Because the degree of lattice mismatch tends to be higher in very long wavelength infrared InAs/GaSb designs, the approach tuning growth parameters to optimize the strain balancing process is different from that for midinfrared SLs. To investigate this optimization, a systematic approach was applied to achieve strain compensated 16 monolayers (MLs) InAs/7 MLs GaSb SLs aimed for ...

Observation of the intrinsic bandgap behaviour in as-grown epitaxial twisted graphene.

Twisted graphene is of particular interest due to several intriguing characteristics, such as its the Fermi velocity, van Hove singularities and electronic localization. Theoretical studies recently suggested the possible bandgap opening and tuning. Here, we report a novel approach to producing epitaxial twisted graphene on SiC (0001) and the observation of its intrinsic bandgap behaviour. The direct deposition of C60 on pre-grown graphene layers results in few-layer twisted graphene confirmed by angular resolved photoemission spectroscopy and Raman analysis. The strong enhanced G band in Raman and sp(3) bonding characteristic in X-ray photoemission spectroscopy suggests the existence of interlayer interaction between adjacent graphene layers. The interlayer spacing between graphene layers measured by transmission electron microscopy is 0.352 ± 0.012 nm. Thermal activation behaviour and nonlinear current-voltage characteristics conclude that an intrinsic bandgap is opened in twisted graphene. Low sheet resistance (~ 160 Ω □(-1) at 10 K) and high mobility (~2,000 cm(2) V(-1) s(-1) at 10 K) are observed.

Optical and electrical quality improvements of undoped InAs∕GaSb superlattices

The performance and operating temperature of infrared (IR) detectors are largely limited by thermal generation and noise processes in the active region of the device. Particularly, excess background charge carriers enhance dark currents and depress the detector figures of merit. Therefore, reducing the overall defects and background carriers in the undoped region of p-i-n diodes is an important issue for developing high-operating temperature IR detectors. In this article, the authors discuss how several postgrowth annealing conditions and interface shutter sequences are optimized to reduce the density of nonradiative defect trap centers and background carriers and studied their relevance to the photoluminescence (PL) emission qualities of typical mid-IR InAs∕GaSb superlattices (SLs). Among the several in situ postgrowth annealing temperatures investigated, the SLs annealed at 450°C had the highest carrier density and the lowest PL intensity, while the SLs annealed at 475°C had the lowest carrier density a...

Study of the driving force for the self-assembly of heterojunction quantum dots (zero D molecules) using finite element analysis

The formation and control of quantum dots have been the focus of considerable research. This research is largely motivated by the unique properties zero dimensional structure possess. During heteroepitaxy, quantum dots form through a self-assembly process in order to reduce the strain energy of the heteroepitaxial material while increasing the surface area. Using this procedure, quantum dots composed of many different materials have been formed on various substrates or buffer layers. These structures have been classified into two basic groups (type I and type II) based on how the dot band structure lines up relative to the surrounding barrier band structure. In the case of a type I quantum dot the dot band gap is contained within the energy range of the band gap of the barrier material. This arrangement is typical for InxGa1−xAs on GaAs and InP. The other type of quantum dot, referred to as type II, indicates that the quantum dot band gap straddles the conduction band (or valence band) of the barrier laye...

Nanofabricated quantum dot array formation through annealing of nano-patterned planar InAs

Quantum dots (QDs) are typically formed using a self-assembly process that results in random placement and size distributions, thus limiting their applicability for many devices. In this work, we report a process which uses nano-patterned planar InAs and subsequent annealing under As stabilized conditions to produce QDs with uniform placement and size distribution. The authors demonstrate the ability to form ordered QD arrays with a density of 3u2009×u20091010 dots/cm2 and QD base widths of <30u2009nm. The authors achieved photoluminescence from the patterned area at a temperature below 100u2009K.

Epitaxial titanium nitride on sapphire: Effects of substrate temperature on microstructure and optical properties

Titanium nitride (TiN) is a mechanically robust, high-temperature stable, metallic material receiving considerable attention for resilient plasmonics. In this work, the authors fabricated six heteroepitaxial TiN films on sapphire using controllably unbalanced reactive magnetron sputtering. They examined the effect of substrate growth temperature on the plasmonic and crystalline quality of the film. Optical properties of all films were obtained from spectroscopic ellipsometry; plasmonic quality factors were determined from the real and imaginary parts of the dielectric function. The authors determined crystallinity using x-ray diffraction and surface morphology using atomic force microscopy. X-ray diffraction showed (111) TiN peaks with Pendellosung fringes indicating consistent heteroepitaxy. Atomic force microscopy showed smooth surfaces with root mean square surface roughness ranging from 0.2 to 2.6u2009nm. Based on this characterization, the authors determined that the substrate deposition temperature of 550u2009°C yielded (111)-oriented heteroepitaxial TiN with minimal surface roughness. The authors found that 550u2009°C also gave highest plasmonic quality factors for all wavelengths, approaching the values of todays best plasmonic materials (such as Au and Ag). Further, the Q-factors at wavelength 1550u2009nm inversely correlated with calculated lattice constants. Their results indicate that the plasmonic response of TiN is directly linked with structural quality of the film.Titanium nitride (TiN) is a mechanically robust, high-temperature stable, metallic material receiving considerable attention for resilient plasmonics. In this work, the authors fabricated six heteroepitaxial TiN films on sapphire using controllably unbalanced reactive magnetron sputtering. They examined the effect of substrate growth temperature on the plasmonic and crystalline quality of the film. Optical properties of all films were obtained from spectroscopic ellipsometry; plasmonic quality factors were determined from the real and imaginary parts of the dielectric function. The authors determined crystallinity using x-ray diffraction and surface morphology using atomic force microscopy. X-ray diffraction showed (111) TiN peaks with Pendellosung fringes indicating consistent heteroepitaxy. Atomic force microscopy showed smooth surfaces with root mean square surface roughness ranging from 0.2 to 2.6u2009nm. Based on this characterization, the authors determined that the substrate deposition temperature of 5...

Novel TEM Specimen Preparation Using Multi-Source Focused Ion Beams for Real-Time Electrostatic Biasing Studies

As Transmission Electron Microscopy (TEM) technology improves, and cutting-edge, in-situ experiments are developed, the need for ultra-pristine, extremely high-quality TEM specimens continues to grow. One such novel and rapidly growing field within the community involves electrostatic biasing experiments performed real-time in the TEM. Two main approaches have been taken to accomplish this; one involves conventional TEM specimen preparation with a conductive probe tip being brought into contact with the surface, another requires a FIB-fabricated Microelectromechanical (MEMs) device (see Figure 1). We have applied both techniques to investigate the microscopic origins of enhanced straindriven magnetoelectric coupling (i.e. voltage control of magnetism) in thin magnetic layers (Ni-Fe and FeCoB alloy layers) grown on ferroelectric PMN-PT ([Pb(Mg1/3Nb2/3)O3]‐[PbTiO3]) substrates[1].