Session H18: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Nanowires and Superlattices

Energy Transfer of Excitons and Electron-Hole Plasmas between Quasi-two-dimensional Semiconductor Layers

We study the energy transfer mechanisms of excitons and electron-hole plasmas between two quasi-two dimensional (2D) semiconductor quantum wells. It is shown that dipolar (i.e., Foerster) energy transition (ET) mechanism dominates at a short distance while photon-mediated ET mechanism plays a more important role for transfer over a long distance. The magnitude and the dependence of the transfer rates of plasmas and excitons on the temperature, the well-to-well distance, and the density are compared for both ET mechanisms. The spatial dependence of the 2D-2D transfer rates will be compared with that of the ET rate between two quantum dots.   

Charge transport physics of individual PbSe Nanowire field effect transistors

We report the charge transport properties of individual PbSe nanowire (NW) field-effect transistors (FETs) fabricated from single crystalline, PbSe NWs 10 nm in diameter synthesized by wet-chemical methods. PbSe is a particularly interesting semiconductor to study in one-dimension as the diameter of the NWs is smaller than the electron, hole and exciton Bohr radii, allowing study of strongly quantum confined NWs. We investigate the temperature dependent charge transport properties of ambipolar and reversibly, surface, p-doped PbSe NW FETs. We demonstrate that PbSe NW FETs behave as Schottky Barrier (SB) FETs, in which the off current is limited by the SB height and decreases as temperature decreases, while the on current is achieved by gate thinning and increases as temperature decreases. We calculate the SB heights for electron and hole injection in ambipolar and hole injection in p-type PbSe NW FETs. The hole mobility in surface-doped, p-type FETs is temperature dependent, rising monotonically from 200 cm2/Vs at room temperature to 2000 cm2/Vs at 4.5K, without signatures of impurity scattering which commonly limits carrier mobilities at low temperatures in substitutionally doped NWs.   

Electronic transport in individual, vertical, catalyst free GaN/AlN nanowires

In the recent years, the advances in the THz technology, for example the development of the quantum cascade laser, promotes the possibility to use resonant tunneling diodes (RTD) to enhance such technologies. Coaxial m-plane AlN/GaN nanowire based resonant tunneling diode structures were formed by plasma assisted molecular beam epitaxy (MBE) using a two-step growth method that allows for control of vertical and lateral growth [1]. They are spontaneously formed MBE nanowires and they are integrated in Si (111). We discuss ongoing work on the electronic transport in these individual, vertical nanowires using two nanoprobes to contact the top of the nanowire and the substrate. The IV characteristics reveal a clear negative differential resistance (NDR) at room temperature (RT). The NDR was observed $\sim $ 4V with a peak-to-valley ratio as high as 10. [1] S.D. Carnevale \textit{et al}., \textit{Nano Letters, }11, (2), 2011.   

Correlated study of individual nanowires with electronic transport and scanning tunneling microscopy

The electronic conductance in quantum wires is often dictated by quantum instabilities and strong localization at the atomic scale. We present a novel nano-transport technique which combines local nano-contacts and four-probe STM. The approach allows for correlated study of electron transport and scanning tunneling spectroscopy in individual nanowires. We first apply it to the GdSi2 quantum wires, which show that isolated nanowires exhibit a metal-insulator transition upon cooling, driven by the defect-induced localizations, while wire bundles maintain a robust metallic state, stabilized by interwire electronic coupling. We then demonstrate applications of this transport technique with cabon nanotubes and copper wires in situ. The method bridges the gap between the transport and the local electronic and structural properties down to the atomic scale.   

The influence of excitonic effects on charge separation at hybrid nanoscale interfaces

Efficient charge separation is critical in nanostructured photovoltaic systems, but the accurate prediction of charge separation rates remains an extremely difficult task. Although considerable progress has been made, reliable theoretical schemes to describe the charge-separation mechanisms and to compute the interfacial charge transfer dynamics have not been fully developed. In this work, we embrace the excitonic effects that are of critical importance in nanoscale geometries to derive a criterion for charge separation at hybrid nanoscale interfaces beyond the traditional quasiparticle energy-level alignment. The approach utilizes calculations from many-body perturbation theory with Green functions to accurately account for self-energy and electron-hole interactions. Four representative interfaces between Si quantum dots and small molecules are considered using this approach, in order to demonstrate that both excitonic and Coulomb stabilization effects are essential for correctly predicting charge separation at nanostructured interfaces. Interestingly, our calculations suggest that preemptory exciton transfer across interfaces only suppresses the subsequent charge separation. We also compute charge separation rates using the Marcus theory and other methods, and their accuracy an   

QM/EM simulation of Junctionless FinFET

We present here the simulations of a junctionless transistor. Its source, channel and drain are embeded in a piece of uniformly doped silicon nanowire. In the earlier stage, it has been designed to be a normally ``ON'' device. Quite reversely, the first experimentally presented junctionless transistor is in the ``OFF" state when the applied gate voltage is absent. Simulations show that the depletion occurs between the nanowire and hetero-doped gate. A ``P-N junction'' is formed in the junctionless transistor, whose direction is perpendicular to the direction of the current flowing. Our simulation considers the depletion effect in the Qauntum mechanical calculation. I-V curves of transistors with the gate doped by the same and different type of dopants have been obtained. The results match the experiments.   

Tuning the hole mobility in InP semiconductor nanowires

Transport properties of holes in InP nanowires were calculated considering the effect of temperature and the presence of realistic strain fields. The mobility of holes is obtained analytically by considering electron-phonon interaction via deformation potential through longitudinal optical (LO) phonons. Using molecular dynamics with realistic force potentials, we simulate nanowire structures and the associated phonon density of states; the structures show effects of LO phonon energy renormalization due to the reduced dimensionality and variation of the phonon lifetimes important for carrier mobility. Our mobility calculations include heavy and light hole subbands in a Luttinger Hamiltonian formalism and consider how the valence band ground state changes between light- and heavy-hole character, as both the strain field configuration and the nanowire size are changed. Depending on the dimensions and characteristics of the nanowire, we find interesting sudden changes in the mobility, which arise with the onset of a resonance between the LO phonon frequencies and the subband separation between the ground and first hole state. We will present the effect of strains and temperature on this resonant behavior and discuss the consequences for carrier mobility in these systems.   

First-principles study on metal-TaO$_x$-metal heterostructures: response to applied bias voltages

Metal-TaO$_x$-metal heterostuructures are promising as a novel nonvolatile memory device [1]. The formation of conduction paths in the TaO$_x$ layer is responsible for the low resistance state and the switching mechanism is understood as the electrochemical redox reaction involved with the bias-voltage application. However, microscopic details of the conduction path and switching mechanism have not been clarified yet. We examine electronic states, and electron and ion transport in Cu-TaO$_x$ (x$\sim$2.5)-Pt(or Cu) heterostructures from first principles, focusing on the response to applied bias voltages. We show that Cu interstitials in crystalline Ta$_2$O$_5$ enhances electronic conduction considerably [2], while does not in amorphous one. We also show that the potential change due to the bias application is sensitive to the structure of TaO$_x$ layer and/or metal-TaO$_x$ interface: in some case, the potential change may be very small near the Cu/TaO$_x$ interface in the TaO$_x$ layer so that the bias application hardly change the mobility of Cu ions in this region. These results will be discussed in terms of electronic states. [1] T. Sakamoto et al., APL 91 (2007) 092110; [2] T. K. Gu et al., ACS Nano 4 (2010) 6477.   

Theory of spatially inhomogneous Bloch oscillations in semiconductor superlattices

In a semiconductor superlattice with long scattering times, damping of Bloch oscillations due to scattering is so small that nonlinearities may compensate it and Bloch oscillations persist even in the hydrodynamic regime. To demonstrate this, we propose a Boltzmann-Poisson transport model of miniband superlattices with inelastic collisions and derive hydrodynamic modulation equations for the electron density, the electric field and the complex amplitude of the Bloch oscillations. For appropriate parameter ranges, we solve numerically these equations and show that there are solutions having the form of stable Bloch oscillations with spatially inhomogeneous field, charge, current density and energy density profiles. These Bloch oscillations disappear as scattering times become sufficiently short. For sufficiently low lattice temperatures, Bloch and Gunn type oscillations mediated by electric field, current and energy domains coexist for a range of voltages. For larger lattice temperatures (300 K), there are only Bloch oscillations with stationary amplitude and electric field profiles.   

Stochastic current switching in semiconductor superlattices: observation of non-exponential kinetics

We report the experimental measurement of first-passage-time distributions associated with noise-induced current switching in doped, weakly-coupled GaAs/AlAs superlattices, in a regime of nonlinear electronic transport where the static current-voltage ($I - V$) curves exhibit multiple branches and bistability. For applied voltages near the end of each branch, internal shot noise induces switching of measured current to the next branch with a stochastically varying switching time. Switching time distributions are constructed by carrying out up to $10^5$ measurements under identical initial conditions. We have implemented a novel, high bandwidth technique that permits measurement of switching times over very large dynamic range of approximately $10^9$, with measured times ranging from $4$ ns to $10$ s. For relatively small times ($<$ 10$\mu$s), the switching time distributions show exponential tails, as expected for activated escape from an initial metastable state. However, at larger times ($>$ 10 $\mu$s), the distributions exhibit approximate power law tails that extend over several decades of time, with additional fine structure. A rate equation model indicates the possible role of multiple, nearly degenerate metastable states in producing the long tail behavior.   

Pseudospin Transfer Torques in Semiconductor Electron Bilayers

We use self-consistent quantum transport theory to investigate the influence of interactions on interlayer transport in semiconductor electron bilayers in the absence of an external magnetic field. We conclude that even though spontaneous pseudospin ferromagnetism does not occur at zero field, interaction-enhanced quasiparticle tunneling does alter the resultant interlayer I-V curves. We find that the system exhibits a critical bias voltage that is similar to that of the pseudospin ferromagnetic system, but whose properties depend heavily on the charge imbalance between the two layers and their relative spatial separation. When the bias voltage exceeds the critical value, interlayer current is gradually droped due to the charge imbalance between the layers until the transport current no longer reaches steady state values.   

Effects of Fermion Flavor on Excitonic Condensation in Double Layer Systems

We perform fermionic path integral quantum Monte Carlo (PIMC) simulations to study the physical properties of dipolar exciton condensates in symmetric double layer systems. Recently, the role of screening of additional fermion flavors has been a source of contention in exciton condensates. A room temperature superfluid state has been predicted in bilayer graphene assuming that the condensate screens out additional fermion flavors.\footnote{H. K. Min, R. Bistritzer, J. J. Su and A. H. MacDonald.\emph{Phys. Rev.} B \textbf{78}, 121401 (2008)} On the other hand, large-N calculations have resulted in much weaker screening of fermion flavors and have placed the transition temperature far lower, around one millikelvin.\footnote{M. Y. Kharitonov and K. B. Efetov, \emph{Semicond. Sci. Technol.} \textbf{25}, 034004 (2010)} We demonstrate the effect of added fermion flavor on the Kosterlitz-Thouless transition temperature ($T_{KT}$) in symmetric electron-hole bilayers by collecting static and dynamic response functions.\footnote{J. Shumway and M. J. Gilbert, \emph{arXiv:1108.6107} (2011)} We find that the addition of fermion flavors decreases $T_{KT}$, however, due to strong exciton binding, the decrease we observe is not as drastic as is predicted in the earlier large-N calculations.   

Magnetic-Field Modulated Josephson Oscillations in a Planar Semiconductor Microcavity

An exciton-polariton Josephson junction in a planar semiconductor microcavity is studied by considering an external magnetic field applied normal to its plane [1]. Theoretical results show that there is a competition between the Zeeman energy and the interactions of exciton polaritons, and a critical magnetic field can be determined. Below the critical magnetic field, there are time-independent extrinsic and intrinsic Josephson currents which manifest the dc Josephson effect; while above the critical magnetic field, the ac Josephson effect occurs with the oscillating extrinsic and intrinsic Josephson currents. The oscillation frequency and oscillation amplitude of the Josephson currents are modulated by the magnetic field, and also the spontaneous polarization separation and the macroscopic quantum self-trapping of condensate can be realized under an appropriate magnetic field. The physical origin behind is exposed and the analogy with the Josephson effect in a conventional superconducting Josephson junction is discussed. It is suggested that magnetic fields can be used to facilitate the experimental investigations of the exciton polaritons Josephson effect in semiconductor microcavities. \\[4pt] [1] Chuanyi Zhang and Guojun Jin, Phys. Rev. B 84, 115324 (2011).   

Lagrange formalism of memory circuit elements: classical and quantum formulations

The general Lagrange-Euler formalism for the three memory circuit elements, namely the memristor, memcapacitor, and meminductor [1,2] is introduced for circuits with voltage or current sources. In addition, \textit{mutual meminductance}, i.e., mutual inductance with a state depending on the past evolution of the system, is introduced. The Lagrange-Euler formalism for a general circuit network and the corresponding work-energy theorem are also obtained. We provide examples of this formalism applied to specific circuits both in the classical and quantum regimes showing under which conditions the quantum excitations of the memory degrees of freedom can be observed experimentally. Work was supported in part by NSF.\\[4pt] [1] M. Di Ventra, Y. V. Pershin, and L. O. Chua, \textit{Proc. IEEE} \textbf{97}, 1717 (2009).\\[0pt] [2] Y.V. Pershin and M. Di Ventra, \textit{Advances in Physics} \textbf{60}, 145-227 (2011).   

Electrolyte Gated Transistors based on Solution Processed Mesoporous Tungsten Trioxide Thin Films

Tungsten trioxide (WO3) is an important material for electrochromic displays, gas sensors, and photoelectrochemical cells. Despite intensive research efforts, the charge transport properties of nanostructured WO3 films, as well as of other metal oxide films, are still largely undiscovered. Electrolyte gating provides a powerful platform to study the charge transport properties of nanostructured WO3 films permitting to achieve high charge density regimes. In turn, this opens the possibility to improve the film transport properties for a wide range of applications. Here we report on electrolyte gated transistors making use of WO3 films as the semiconductor and H2SO4(aq) 1M as the gate dielectric. WO3 films, prepared by sol-gel method, were deposited on source and drain patterned ITO substrates. The liquid electrolyte was confined using a PDMS well. Atomic force microscopy and scanning electron microscopy images show a mesoporous film structure where the electrolyte can easily penetrate. The mesoporous structure permits an efficient electrolyte gating compared to bulk WO3 films because of the higher surface available for electrical double layers, which are the underpinning of the electrolyte gating. Upon application of gate bias in the 0-1 V range, with an applied drain voltage ranging between 0-1 V, we were able to tune the conductivity in the WO3 transistor channel: electrolyte gating of the films led to clear transistor behaviour. Electrolyte gating of WO3 electrochromism is presently under investigation.