Single Interface Structure Research Articles

Numerical surface-corrected nonlocal electrodynamic model for nanophotonic structures

Recent work has shown the significance of a nonlocal dielectric response and surface correction for nanometer-length-scale plasmonic structures. In this paper, we propose a new surface hydrodynamic model which incorporates such nonlocality and surface correction. Our approach, which is based on the hydrodynamic model, uses a numerical approach based on full-wave numerical computation and thus enables calculation of surface plasmon polariton (SPP) fields. The model also introduces a discontinuity in the normal component of the electric displacement at the interface by taking into account the change in the electron distribution at the interface. The method makes use of the Feibelman d parameter approach for the surface correction by equating the corrected Fresnel reflection of a p-polarized incident field across a planar interface. To demonstrate our method, we first examine numerical calculation of SPP propagation at a single interface structure; this work is then followed by a demonstration for a set of more complex thin-film structures. The latter demonstrates that our method is suitable for use in numerical modeling of complex nanophotonic structures.

Photoluminescence and the gallium problem for highest-mobility GaAs/AlGaAs-based 2d electron gases

The quest for extremely high mobilities of 2d electron gases in MBE-grown heterostructures is hampered by the available purity of the starting materials, particularly of the gallium. Here we compare the role of different Ga lots having nominally the highest possible quality on the mobility and the photoluminescence (PL) of modulation doped single interface structures and find significant differences. A weak exciton PL reveals that the purity of the Ga is insufficient. No high mobility can be reached with such a lot with a reasonable effort. On the other hand, a strong exciton PL indicates a high initial Ga purity, allowing to reach mobilities of 15 million (single interface) or 28millioncm2/Vs (doped quantum wells) in our MBE systems. We discuss possible origins of the inconsistent Ga quality. Furthermore, we compare samples grown in different MBE systems over a period of several years and find that mobility and PL are correlated if similar structures and growth procedures are used.

CoFeB Thickness Dependence of Damping Constants for Single and Double CoFeB-MgO Interface Structures

We investigate the CoFeB thickness dependence of the damping constant α of single and double CoFeB-MgO interface structures by vector-network-analyzer ferromagnetic resonance (VNA-FMR). The damping constant increases with decreasing the CoFeB thickness for the single interface structure, and the damping constant for the double-interface structure takes a similar value to that for the single-interface structures with the same CoFeB thickness.

Transverse shifts of a light beam reflected from a uniaxially anisotropic chiral slab

We study for the first time the transverse shifts of a Gaussian beam reflected from a uniaxially anisotropic chiral (UAC) slab, where the chirality appears only in one direction and the host medium is a uniaxial crystal or an electric plasma. The results indicate that the transverse shifts are closely related to the propagation behaviors of the eigenwaves in the slab. Specifically, when one or both of the eigenwaves are totally reflected at the second interface of the slab, the spatial transverse shift becomes resonances but is not enhanced; when one eigenwave is totally reflected at the first interface and the other is transmitted at the second interface, the larger and negative transverse shifts can be obtained. The propagation behaviors of the eigenwaves in the UAC slab provide more abundant information about the transverse shifts than in a single interface structure.

Robustness of topological superconductivity in proximity-coupled topological insulator nanoribbons

We study the low-energy physics of topological insulator (TI) nanoribbons proximity-coupled to s-wave superconductors (SCs) by explicitly incorporating the proximity effects that emerge at the TI-SC interface. We construct a low-energy effective theory that incorporates the proximity effect through an interface contribution containing both normal and anomalous terms and an energy-renormalization matrix. We show that the strength of the proximity-induced gap is determined by the transparency of the interface and the amplitude of the low-energy TI states at the interface. Consequently, the induced gap is strongly band-dependent and collapses for bands containing states with low amplitude at the interface. We find that states with energies within the bulk TI gap have surface-type character and, in the presence of proximity-induced or applied bias potentials, have most of their weight near either the top or the bottom surface of the nanoribbon. As a result, single interface TI-SC structures are susceptible to experiencing a collapse of the induced gap whenever the chemical potential is far enough from the value corresponding to the bulk TI Dirac point and crosses weakly coupled bands. We also find that changing the chemical potential in single-interface structures using gate potentials may be ineffective, as it does not result in a significant increase of the induced gap. On the other hand, we find that symmetric structures, such as a TI nanowire sandwiched between two superconductors, are capable of realizing the full potential of TI-based structures to harbor robust topological superconducting phases.

Carbon-doped high-mobility hole gases on (0 0 1) and (1 1 0) GaAs

Since Stormer and Tsang have introduced the first two-dimensional hole gas (2DHG) in the GaAs/AlGaAs heterosystem, the choice of suitable dopants was limited to beryllium and silicon over the last 20 years. Both acceptor atoms have significant disadvantages, i.e. either high-diffusion rates or a limitation to specific growth directions. Utilizing a carbon filament-doping source, we prepared high-quality 2DHGs in the (0 0 1) and the nonpolar (1 1 0) crystal plane with carrier mobilies beyond 10 6 cm 2/Vs in quantum well and single interface structures. Low-temperature magnetoresistance measurements recover a large number of fractional QHE states and show a pronounced beating pattern from which the Rashba induced spin-splitting has been determined. In addition, 2DHGs have been grown on cleaved edges of (1 1 0) and (0 0 1) wafers with transport features in qualitative agreement to our findings on (1 1 0) substrates.

Carbon-doped high-mobility hole gases on (001) and (110) GaAs

Since Stormer and Tsang have introduced the two-dimensional hole gas (2DHG) in the GaAs∕AlGaAs heterosystem, the choice of suitable dopants was limited to beryllium and silicon over the last 20years⁠. Both acceptor atoms have significant disadvantages, i.e., either high diffusion rates or a limitation to specific growth directions. Utilizing a carbon filament doping source we prepared high-quality 2DHGs in the (001) and the nonpolar (110) crystal plane with carrier mobilies beyond 106cm2∕Vs in quantum-well and single-interface structures. Low temperature magnetoresistance measurements recover a large number of fractional quantum Hall effect states and show a pronounced beating pattern from which the Rashba-induced spin splitting has been determined. In addition, 2DHGs have been grown on cleaved edges of (110) and (001) wafers with transport features in qualitative agreement with our findings on (110) substrates.

Thermal stability of Si/GexSi1−x/Si heterostructures

The thermal stability of Si/GexSi1−x/Si (100) heterostructures is studied by in situ electron microscope observations of the thermal relaxation process. Structures are observed to relax by misfit dislocation propagation at both strained interfaces, and are stable to significantly higher temperatures than equivalent strained GexSi1−x layers at a free surface. This observed difference in thermal stability between double and single interface structures is due at least in part to much lower dislocation velocities in the double interface structures, with a measured glide activation energy approximately twice as high as single interface structures. It is argued that this difference in glide activation energies may be due to a smaller kink nucleation activation energy in the single interface structure.

Electron mobilities exceeding 107 cm2/V s in modulation-doped GaAs

A modulation-doped Al0.35Ga0.65As/GaAs single interface structure with a 700 Å undoped setback grown by solid-source molecular beam epitaxy (MBE) shows a Hall mobility of 11.7×106 cm2/V s at a carrier density of 2.4×1011 electrons/cm2 measured in van der Pauw geometry after exposure to light at 0.35 K. This is the highest carrier mobility ever measured in a semiconductor. Similar Al0.32Ga0.68As/GaAs structures with 1000–2000 Å setbacks show Hall mobilities in the dark at 0.35 K as high as 4.9×106 cm2 /V s for carrier densities of 5.4×1010 electrons/cm2 and lower.

Modulation Doped GaAs with Electron Mobilities Exceeding 107cm2/V sec

AbstractA modulation doped Al.35Ga.65As/GaAs single interface structure using Si delta doping setback from the two dimensional electron gas (2 DEG) by 700Å, was measured in van der Pauw geometry at 0.35K and showed an electron mobility of 1.17× 107 cm2 /V see at a carrier density of 2.2× 1011electrons/cm2. An Al.30Ga.70As/GaAs single quantum-well 250Å wide of similar structure with a 500Å setback yielded a 2 DEG mobility of 5.1× 106 cm2 /Vsec at 3.0×1011 electrons/cm2under similar measurement conditions. These mobilities exceed those previously published by more than a factor of two for the single interface structure, and by nearly an order of magnitude for the quantum-well. The samples were grown by solid source MBE in a Varian Gen II modified with He closed-cycle cryogenic vacuum pumping, and in other ways, to achieve a base pressure of 1.5× 10−12 torr, as measured on a extractor-type ionization gauge that is not subject to the usual x-ray limited reading.

Electronic properties of In0.53Ga0.47As-InP single quantum wells grown by chemical beam epitaxy

In0.53Ga0.47As-InP single quantum well (SQW) structures grown by chemical beam epitaxy (CBE) were studied using low-field magnetotransport, the quantum Hall effect, and far-infrared cyclotron resonance measurements at 4.2 K. We compare results on the two-dimensional electron gas (2DEG) in the SQW’s and the single interface heterojunctions. These results confirm the high quality of the material grown by CBE and demonstrate conclusively that the use of the SQW’s for 2DEG confinement can effectively eliminate the parallel conduction often present in single interface structures.