Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S11: Group IV- and III-V-Based Low-Dimensional Semiconductor Heterostructures |
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Sponsoring Units: DCMP Chair: John Colton, Brigham Young University Room: BCEC 152 |
Thursday, March 7, 2019 11:15AM - 11:27AM |
S11.00001: Band engineering of semiconductor artificial graphene and exploration of flat band physics(*) Lingjie Du, Loren Pfeiffer, Kenneth West, Saeed Fallahi, Geoffrey Gardner, Michael Manfra, Vittorio Pellegrini, Shalom Wind, Aron Pinczuk Semiconductor artificial graphene (AG) has been realized on 2D electron systems in GaAs quantum wells subjected to a lateral potential modulation with honeycomb symmetry [1, 2]. Tunability of the AG band structure is a key property of the artificial systems. Here, we present effective band engineering of semiconductor AG with multiple control knobs that offer flexible and effective exclusive tunability of electron properties. Using the cutting-edge fabrication technology, we fabricated small-period triangular antidot lattices with different honeycomb lattice periods (35 nm to 45 nm) and various AG potentials. The M-point gap, which reveals key characteristics of the AG band structure, was measured by intersubband excitations using resonant inelastic light scattering (RILS) at low temperature. We found that the M-point gap could be tuned from 0.5 meV to 1.2 meV, with the band dispersion near the M point evolving from the Dirac-like case to flat band. Emerging flat-band physics in such tunable system will be discussed. [1] S. Wang, et al. Nature Nanotech. 13, 29 (2018). [2] L. Du, et al., Nature Commun. 9, 3299 (2018). |
Thursday, March 7, 2019 11:27AM - 11:39AM |
S11.00002: Fabricating shallow 2D systems in GaAs/AlGaAs heterostructures towards the creation of Artificial Graphene Yonatan Ashlea Alava, Oleh Klochan, Qingwen WANG, Andreas D. Wieck, Arne Ludwig, Julian Ritzmann, Alex R Hamilton Patterning ordinary GaAs/AlGaAs heterostructures with a triangular nano-array of quantum anti-dots has been shown to yield Artificial Graphene. The lattice spacing of the patterned quantum anti-dot array must be in the order of 100 nm to enable the observation of Dirac physics. This implies that the GaAs/AlGaAs interface has to be considerably shallower than 100 nm to enable strong modulation of the 2D carriers with surface gates gates. Fabricating low disorder shallow 2D systems is experimentally challenging due to the close proximity to the surface and the thin barrier to the surface gate, determined by the AlGaAs layer. This thinner barrier increases leakage between the gate and the 2D system, and increases scattering at the interface, damaging the mobility of the 2D carriers. |
Thursday, March 7, 2019 11:39AM - 11:51AM |
S11.00003: A novel route to achieve two dimensional (2D) carrier confinement in a wedge-shaped polar semiconductor structure Swarup Deb, Subhabrata Dhar A novel route to achieve two dimensional (2D) carrier confinement in a wedge-shaped wall structure made of a polar semiconductor has been demonstrated theoretically. Tapering of the wall along the direction of the spontaneous polarization leads to the development of charges of equal polarity on the two inclined facades of the wall. Negative/positive polarization charges on the facades can push the electrons/holes inward for an n-/p-type material leading to the formation of 2D electron/hole gas at the central plane and depletion regions at the outer edges of the wall. Schrodinger and Poisson equations are solved self-consistently to obtain the potential and charge density distribution in n-type GaN nanowalls, tapered along c-axis by different angles. Carrier mobility in this 2D carrier gas is estimated to be significantly higher than that of bulk. A recent experimental finding indeed shows a very high electron mobility in wedge-shaped GaN nanowall networks. Properties of high mobility and the vertical orientation of the 2D confinement can be exploited in fabricating highly conducting transparent interconnects and high mobility field effect transistors, which can lead to a large-scale integration of 2D devices in future. |
Thursday, March 7, 2019 11:51AM - 12:03PM |
S11.00004: Classical elements in the magnetoconductance of two-dimensional Lorentz gases Beate Horn-Cosfeld, Jakob Schluck, Nima Hamidi Siboni, Jürgen Horbach, Thomas Heinzel Electrons moving through an array of identically shaped obstacles at random positions form a Lorentz gas. [1] In two-dimensional systems, strong deviations of the magnetoresistance from the Boltzmann model are observed experimentally and in numerical simulations, which have a classical origin. At low magnetic fields, memory effects [2] cause subdiffusive transport, which generates a strong positive magnetoconductivity, while weak localization is absent. At intermediate magnetic fields, the transport develops a superdiffusive character, which is visible as a pronounced maximum in the magnetoconductance at sufficiently large obstacle densities. [3] In dilute two-dimensional Lorentz gases, the interplay between the Lorentz obstacles and the residual background disorder causes strong corrections to the behavior of a clean Lorentz gas. |
Thursday, March 7, 2019 12:03PM - 12:15PM |
S11.00005: Temperature dependence of transverse magnetic focusing in high-mobility GaAs/AlGaAs Adbhut Gupta, Jean Heremans, Saeed Fallahi, Michael Manfra The temperature dependence of the ballistic and mesoscopic transport phenomenon of transverse magnetic focusing (TMF) is investigated in a two-dimensional electron system in a high-mobility GaAs/AlGaAs heterostructure at low temperatures 0.4 K < T < 20 K (electron mean free path ~ 80 μm at 4.2 K). Measurements use in-line configurations with distance between the injector and collector of L = 7 μm, and bent TMF configurations with L = 5 μm horizontal and 2 μm vertical. In both configurations, the TMF amplitude (non-local resistance Rf) shows a monotonic decrease as T is increased from 4 K to 20 K, according to Rf ∼1/T2. The dependence on T is analyzed by fitting Rf according to different scattering mechanisms, with Rf ∼1/T2 indicating a dominant role for inelastic electron-electron interactions. In the particular temperature range, electron-electron interactions point to the importance of momentum exchange between the ballistic beam and the surrounding carrier fluid, and hence to the existence of a hydrodynamic transport regime, which has recently received increasing attention. The experiments suggest that in addition to ballistic aspects, TMF presents hydrodynamic aspects as well. |
Thursday, March 7, 2019 12:15PM - 12:27PM |
S11.00006: WITHDRAWN ABSTRACT
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Thursday, March 7, 2019 12:27PM - 12:39PM |
S11.00007: Carrier Lifetime Analysis in a Transistor Laser by Using Non-Equilibrium Green’s Function Method with Effective Bond-Orbital Model Fu-Chen Hsiao, Yia-Chung Chang, John Dallesasse Carrier injection and recombination in a single quantum well AlGaAs/InGaP/GaAs/InGaAs transistor laser (TL) is examined theoretically with the non-equilibrium Green’s function (NEGF) method. The multi-band effects are incorporated in the NEGF method by applying effective bond-orbital model (EBOM). Carrier-phonon scattering are taken into account within the deformation-potential approximation. The squared momentum matrix elements, scattering rate, and the density of states for the quantum well in a TL are presented. In addition, the simulated I-V characteristics as well as the frequency response of the TL is shown and compared with the experimental data. In the end, the carrier recombination in the quantum well as well as the base region are discussed based on the results calculated by the present model. |
Thursday, March 7, 2019 12:39PM - 12:51PM |
S11.00008: Acoustic Nanostructures for Charge Carrier Confinement in GaAs/AlGaAs Multiple Quantum Wells Kevin Vallejo, Christopher F Schuck, Kathryn E Sautter, Trent Garrett, Ariel E Weltner, Baolai Liang, Zilong Hua, David H Hurley, Paul J Simmonds Quantum confinement of charge carriers in semiconductors is at the heart of next generation technologies for energy conversion, encryption and computation. We use picosecond-duration surface acoustic phonon pulses to produce lateral 2D and 3D carrier confinement in polar III-V semiconductor quantum wells. Shear strain and dilatation generated by the phonon pulses vary with depth below the sample surface, locally deforming the valence and conduction bands to produce lateral confinement in the plane of a quantum well that is externally controllable. We grew a GaAs/AlGaAs heterostructure containing three quantum wells, 5, 7, and 10 nm wide, at depths of 14, 49, and 112 nm beneath the sample surface respectively, to coincide with different piezoelectric field strengths. We verify carrier confinement and transport using metallic gratings on the surface of the sample to generate and detect surface acoustic phonons. These gratings modulate light absorption on length scales below the optical diffraction limit. Our approach enables a spectrally selective scheme for generating surface acoustic phonons 20 – 200 nm in wavelength. Length scales at the lower end of this range are associated with quantum confinement. |
Thursday, March 7, 2019 12:51PM - 1:03PM |
S11.00009: The Effect of Nanostructure on Near-Infrared Absorption in M-Plane AlGaN/(In)GaN Heterostructures Trang Nguyen, MohammadAli Shirazi-Hosseini-Dokht, Yang Cao, Alexander Senichev, Brandon Dzuba, Rosa Diaz, Geoffrey Gardner, Oana Malis, Michael Manfra Near infrared intersubband (ISB) absorption of nonpolar AlxGa1-xN/(In)GaN is studied due to its broad potential applications into optoelectronic devices. Above 60% Al-composition, m-plane AlGaN alloy becomes kinetically unstable during the plasma-assisted molecular beam epitaxy growth generating unique nanostructures. The nanostructures are imaged with energy dispersive x-ray spectroscopy and scanning transmission electron microscopy. They consist of platforms of higher than expected Al-composition and islands of lower Al-composition AlGaN. We discuss the consequences of these nanostructures on ISB absorption energy and linewidth. We are also investigating strain-balanced AlxGa1-xN/InGaN (x<0.6) as an alternative to AlxGa1-xN/GaN (x>0.6). |
Thursday, March 7, 2019 1:03PM - 1:15PM |
S11.00010: Influence of QD Morphology on Photoluminescence in GaSb/GaAs Multilayers Christian Greenhill, Eric Zech, Alexander Chang, Stephen Clark, Ganesh Balakrishnan, Rachel Goldman Due to the predicted composition and strain dependence of type I versus type II band offsets, GaSb/GaAs quantum dots (QDs) are promising for a variety of optoelectronic applications. For GaSb/GaAs multilayers, atomic structures ranging from QDs, quantum rings and clusters have been observed, with photoluminescence (PL) energies ranging from 0.9 to 1.3eV. However, the association of these emission energies with specific nanostructure morphologies remains elusive. We investigate the structural and optical properties of GaSb/GaAs multilayers, with and without 3D nanostructures, using cross-sectional transmission electron microscopy, atom probe tomography (APT), and PL. In both cases, we find PL energies at 1.33eV and 1.49eV, which are attributed to 2D GaSb(As) layers and the GaAs matrix, respectively. For the case with 3D nanostructures, both individual QDs and circular arrangements of QDs, termed QD-rings, are observed. For both cases, Sb-rich cores are apparent, with xSb up to 0.40 (individual QDs) and 0.25 (QD-rings). Thus, we attribute 1.08 eV and 1.2 eV emissions to the individual QDs and QD-rings, respectively. Local measurements of the electronic states using scanning tunneling spectroscopy will also be presented. |
Thursday, March 7, 2019 1:15PM - 1:27PM |
S11.00011: Lattice-matched In0.17Al0.83N/GaN heterostructures for near-infrared intersubband absorption Alexander Senichev, Trang Nguyen, Rosa Diaz, Brandon Dzuba, MohammadAli Shirazi-Hosseini-Dokht, Yang Cao, Michael Manfra, Oana Malis Lattice-matched In0.17Al0.83N/GaN heterostructures are promising platforms for near-infrared optoelectronic devices based on intersubband transitions. Previously, strong near-infrared intersubband absorption (ISBA) was demonstrated in InAlN/GaN superlattices. But, reducing compositional inhomogeneity of InAlN barriers and effective doping of these heterostructures that affect ISBA properties presented open challenges. Here, we demonstrate that InAlN/GaN structures with saturated indium composition of 0.17 can be grown by plasma-assisted molecular beam epitaxy over a wide range of growth conditions and constructed the growth diagram for lattice-matched InAlN layers. Our results indicate that the studied growth conditions alone are not sufficient to eliminate the indium segregation and additional mechanisms are involved. The impact of an indium adlayer and a thin AlN nucleation layer on compositional inhomogeneity of InAlN barriers is discussed. We report the impact of different Si doping profiles and doping levels on structural quality and hence ISBA properties of lattice-matched InAlN/GaN superlattices. |
Thursday, March 7, 2019 1:27PM - 1:39PM |
S11.00012: Integrated III-V/Si Visible and IR Nanowire Photodetectors Arjun Shetty, Eduardo Barrera, Francois Sfigakis, Mitchell Robson, Nebile Isik, Curtis Goosney, Ray LaPierre, Jonathan D Baugh Self-assembled nanowires can support optical resonant modes and act as very effective waveguides that concentrate and absorb light over only a few microns of nanowire length, enabling highly efficient photodetection. The resonant absorption shows wavelength selectivity that can be tuned continuously across the visible and IR wavelengths by adjusting the nanowire diameter during molecular beam epitaxy growth on silicon wafers1. Thus, ordered arrays of III-V semiconductor nanowires, integrated with standard Si technology, could provide low-cost, high-performance multi-spectral photodetectors. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S11.00013: White-light Emitted InGaN Nanorods Grown on Pyramided Si Substrate Chun-Yeh Lin, Chung-Lin Wu, Shu-Ju Tsai, Sheng-Shong Wong, Nien-Ting Tsai In this study, white-light emitted InGaN nanorods were grown on pyramided Si substrate by plasma-assisted molecular beam epitaxy system (PA-MBE) with fixed In/Ga flux ratio. Due to the textured morphology of pyramided Si substrate, using energy-dispersive X-ray spectroscopy (EDS), we demonstrate that the impingement flux ratios of Ga/In and N-plasma varied with different facets of Si pyramids, which results in different light emission ranges on different facets and mixed into white light emission. By conducting spatial resolved catholuminescence spectroscopy (CL), we have resolved that each facet emits different CL spectrum. This finding could assist the design of nanorods-embedded light emitted devices that can be grown more efficient and compact in structure. |
Thursday, March 7, 2019 1:51PM - 2:03PM |
S11.00014: Tensile-Strained Ge Quantum Dots on (111) and (110) Surfaces Kathryn Sautter, Christopher Schuck, Trent Garrett, Ariel E Weltner, Kevin Vallejo, Paul J Simmonds Si and Ge are ubiquitous in electronics, but their indirect bandgaps make them unsuitable for light emitting devices. Theory shows that placing Ge under tensile strain will allow us to engineer its electronic band structure. Tensile strains of 2–4% should transform Ge either into a direct band gap semiconductor or a semimetal, with applications from infrared emitters to low-loss tunnel junctions. Researchers have therefore tried various ingenious methods to create tensile-strained Ge, but these attempts typically generate strain-induced defects and do not result in viable optoelectronic materials. Our approach to this problem is to create Ge quantum dots (QDs) that self-assemble as a result of biaxial tensile strains on low-index surfaces. We have previously demonstrated defect-free, tensile GaAs(111) QDs. Given the similar lattice constants of GaAs and Ge, we follow the same approach to produce self-assembled Ge QDs at 3.7% tensile strain, which we anticipate should lead to optically active Ge with a reduced bandgap. We will discuss our control of Ge QD properties with growth parameters, and discuss possible reasons for some of the structural and optical phenomena we observed. |
Thursday, March 7, 2019 2:03PM - 2:15PM |
S11.00015: Light Emission from Direct Bandgap Hexagonal Silicon Germanium Elham Fadaly, Alain Dijkstra, Jens Renè Suckert, Marcel Verheijen, Sebastian Koelling, Jonathan Finley, Silvana Botti, Jos Haverkort, Erik P. A. M. Bakkers Efficient light emission from Si and Ge has been a holy grail due to their indirect bandgap nature. Recently, Ge- rich alloys, with a hexagonal structure have been theoretically predicted to exhibit a direct band gap nature. Density functional theory (DFT) calculations predict a 0.3 eV bandgap for hex- Ge, which can be tuned up to 0.9 eV by alloying with Si1. Yet, the fundamental bottleneck is that Ge and its alloys crystallize naturally in the cubic structure which is optically inactive due to its indirect bandgap nature. |
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