Session C54: Ballistic Transport in Semiconductor Devices

Ballistic Hot Electron Transport in Heteroepitaxial SrRuO$_{3}$ Metal-Base Transistors

Perovskite oxide heterostructures is a rapidly emerging field significant for interface-induced electronic and magnetic reconstructions, resulting in novel phases distinct from those found in the bulk counterparts. Notably, utilizing device structures is an effective way to probe these interface-induced phases. One of the most prevalent device structures that has been adopted so far is a three-terminal field-effect geometry, used to probe in-plane electronic transport properties. However, the out-of-plane three-terminal device geometry, though less studied due to its complexity, is also useful in many aspects. In the metal-base transistor (MBT), for instance, ballistic transport of hot electrons injected across a Schottky diode emitter can be used to probe hot electron properties of the metal-base, providing information on inelastic scattering mechanisms, electron confinement effects, and intervalley transfer. One promising model system for the metal-base is SrRuO$_{3}$ (SRO), characterized by intermediate electron correlations with unusual transport properties. Here we present an all-perovskite oxide heteroepitaxial MBT using SRO as a metal-base layer. Successful MBT operation for various metal-base layer thicknesses was achieved, from which the hot electron attenuation length of SRO was deduced. These results form a foundation on which to examine the properties of hot electrons in strongly correlated systems using the out-of-plane three-terminal device geometry.   

Nondiffusive thermal transport increases temperature rise in RRAM filaments

Resistive-switching memory (RRAM) offers benefits to nonvolatile memory systems due to scalability, fast switching, and easy fabrication. In RRAM, electrical stimulation switches the resistance of a metal-insulator-metal memory cell. A low-resistance state is achieved during the set process, when a conductive filament (CF) is formed by dielectric breakdown. During the reset process, disruption of the CF restores the device to a high-resistance state. Studies suggest that dissolution of the CF during the reset process occurs when the CF reaches a critical temperature due to Joule heating. Typically, the heat diffusion equation with bulk thermal properties is used to model the thermal processes both within the CF and the surrounding oxide. It is well known, however, that heat transport is nondiffusive when experimental length scales are comparable to energy carrier mean free paths (MFPs). We suggest that heat transport in RRAM is nondiffusive by determining the phonon MFP spectrum in TiO$_{\mathrm{2}}$ (i.e., a promising material for RRAM) and showing that MFPs that contribute significantly to heat transport are comparable to the diameter of the CF. Thus, we approximate the CF as an infinitely long cylinder embedded in crystalline rutile TiO$_{\mathrm{2}}$ and develop an approximate analytical solution to the BTE in the TiO$_{\mathrm{2}}$. We find that the surface temperature of the CF predicted by the BTE is larger than that predicted by the heat diffusion equation. If the heat diffusion equation is used to model thermal transport in RRAM, a reduced effective TiO$_{\mathrm{2}}$ thermal conductivity should be used.   

AC Josephson effect without superconductivity, and other effects of radio frequency quantum nanoelectronics

With single coherent electron sources and electronic interferometers now~available in the lab, the time resolved dynamics of electrons can now be probed directly. I will discuss how a fast raise of voltage propagates inside an electronic interferometer and leads to an oscillating current of well controled frequency. This phenomena is the normal counterpart to the AC josephson effect. I will also briefly advertize our software for computing quantum transport properties, Kwant (\underline {http://kwant-project.org}) and its time-dependent extension T-Kwant.   

Orbital magnetoresistance of two-dimensional electron systems in the hydrodynamic regime

We develop a theory of magnetoresistance of two-dimensional electron systems in the hydrodynamic regime. It applies to two-dimensional semiconductor structures with strongly correlated carriers when the electron-electron scattering length is sufficiently short. We find that the magnetoresistance is positive quadratic at weak fields. Although the resistivity is governed by both viscosity and thermal conductivity of the electron fluid, the temperature dependence of magnetoresistance depends on the viscosity only. This enables extration of viscosity of the electron liquid from magnetotranspot measurements.   

The effect of split gate dimensions on the electrostatic potential and 0.7 anomaly within one-dimensional quantum wires on a modulation doped GaAs/AlGaAs heterostructure

We use a multiplexing scheme to measure the conductance properties of 95 split gates of 7 different gate dimensions fabricated on a GaAs/AlGaAs chip, in a single cool down [1]. The number of devices for which conductance is accurately quantized reduces as the gate length increases. However, even the devices for which conductance is accurately quantized in units of $2e^2/h$ show no correlation between the length of electrostatic potential barrier in the channel and the gate length, using a saddle point model to estimate the barrier length. Further, the strength of coupling between the gates and the 1D channel does not increase with gate length beyond 0.7 $\mu$m. The background electrostatic profile appears as significant as the gate dimension in determining device behavior. We find a clear correlation between the curvature of the electrostatic barrier along the channel and the strength of the ``0.7 anomaly'' which identifies the electrostatic length of the channel as the principal factor governing the conductance of the 0.7 anomaly. \newline $^{*}$ Present address: Wisconsin Institute for Quantum Information, University of Wisconsin-Madison, Madison, WI. \newline [1] L. W. Smith \emph{et al.}, arXiv:1508.03085   

Electron Energy Levels in the 1D-2D Transition

Using GaAs-AlGaAs heterostructures we have investigated the behaviour of electron energy levels with relaxation of the potential confining a 2D electron gas into a 1D configuration. In the ballistic regime of transport, when the conductance shows quantized plateaux, different types of behaviour are found according to the spins of interacting levels, whether a magnetic field is applied and lifting of the momentum degeneracy with a source-drain voltage. We have observed both crossing and anti-crossing of levels and have investigated the manner in which they can be mutually converted. In the presence of a magnetic field levels can cross and lock together as the confinement is altered in a way which is characteristic of parallel channels. The overall behaviour is discussed in terms of electron interactions and the wavefunction flexibility allowed by the increasing two dimensionality of the electron distribution as the confinement is weakened.   

Single-electron devices fabricated using double-angle deposition and plasma oxidation

We report on development of plasma oxidized, single-electron transistors (SETs) where we seek low-capacitance and small-area Al/AlOx/Al tunnel junctions with small charge offset drift. Performance of metal-based SET quantum devices and superconducting devices has suffered from long-term charge offset drift, high defect densities and charge noise. We use plasma oxidation to lower defect densities of the oxide layer, and adjustable deposition angles to control the overlapping areas for Al/AlOx/Al tunnel junctions. Current-voltage and charge offset drift measurements are planned for cryogenic temperatures. Other electrical properties will be measured at room temperature. We hope to see Coulomb blockade oscillations on these devices and better charge offset stability than typical Al/AlOx/Al SETs.   

Quantum point contacts on two-dimensional electron gases with a strong spin-orbit coupling

Studies of electrical transport in one-dimensional semiconductors in a presence of a strong spin-orbit interaction are crucial not only for exploring the emergent phenomena, such as topological superconductivity, but also for potential spintronic applications by controlling of the electron spins. We investigate the electrical transport properties of one-dimensional confinement defined by electrostatic potentials on large area two-dimensional electron gases of InAs and InSb, which have a strong spin-orbit coupling. The high-quality InAs and InSb quantum wells are grown on antimonide buffers by molecular beam epitaxy, and the gate-tunable regions are created using Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ or HfO$_{\mathrm{2}}$ gate dielectrics by atomic layer deposition. We will discuss the modulation of spin-orbit coupling in the two-dimensional electron gases and the spin-orbit-induced spin splitting by the split-gate quantum point contacts.   

Quenching Plasma Waves in Two Dimensional Electron Gas by a Femtosecond Laser Pulse

Plasmonic detectors of terahertz (THz) radiation using the plasma wave excitation in 2D electron gas are capable of detecting ultra short THz pulses. To study the plasma wave propagation and decay, we used femtosecond laser pulses to quench the plasma waves excited by a short THz pulse. The femtosecond laser pulse generates a large concentration of the electron-hole pairs effectively shorting the 2D electron gas channel and dramatically increasing the channel conductance. Immediately after the application of the femtosecond laser pulse, the equivalent circuit of the device reduces to the source and drain contact resistances connected by a short. The total response charge is equal to the integral of the current induced by the THz pulse from the moment of the THz pulse application to the moment of the femtosecond laser pulse application. This current is determined by the plasma wave rectification. Registering the charge as a function of the time delay between the THz and laser pulses allowed us to follow the plasmonic wave decay. We observed the decaying oscillations in a sample with a partially gated channel. The decay depends on the gate bias and reflects the interplay between the gated and ungated plasmons in the device channel.   

Evidence for a New Intermediate Phase in a Strongly Correlated 2D System near Wigner Crystallization

How the two dimensional (2D) quantum Wigner crystal (WC) transforms into the metallic liquid phase remains an outstanding problem in physics. In theories considering the 2D WC to liquid transition in the clean limit, it was suggested that a number of intermediate phases might exist. We have studied the transformation between the metallic fluid phase and the low magnetic field reentrant insulating phase (RIP) which was interpreted as due to the WC [Qiu et al, PRL 108, 106404 (2012)], in a strongly correlated 2D hole system in GaAs quantum well with large interaction parameter $r_s$ ($\sim$20-30) and high mobility. Instead of a sharp transition, we found that increasing density (or lowering $r_s$) drives the RIP into a state where the incipient RIP coexists with Fermi liquid. This apparent mixture phase intermediate between Fermi liquid and WC also exhibits a non-trivial temperature dependent resistivity behavior which can be qualitatively understood by the reversed melting of WC in the mixture, in analogy to the Pomeranchuk effect in the solid-liquid mixture of Helium-3. Reference: R. Qiu et al, arXiv:1509.07463.   

Electron bilayers in an undoped Si/SiGe double-quantum-well heterostructure

We report the design, fabrication, and the magneto-transport study of an undoped Si/SiGe double quantum well heterostructure. We show that employing asymmetric quantum wells for our single-side-gated devices allows us to observe a cross-over from single-layer-like to bi-layer-llike behavior in the mobility-density dependence. We also observe an integer quantum Hall state at filling factor $\nu =$2, which is expected to arise from inter-layer effects for Si electrons. This state could be due to either inter-layer coherence, or the symmetric-antisymmetric tunneling gap. This work has been supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy (DOE). Sandia National Laboratories is a multi program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. DOE's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

What carries heat in novel 2D semiconductors?

When materials are scaled down to the microscopic scale, or when dimensionality is reduced, thermal transport exhibits new intriguing behaviors that are not present in conventional bulk crystals. While phonons are typically considered to be the excitations responsible for carrying heat through a crystal, as dimensionality is reduced, the motion of phonons driven by a temperature perturbation becomes correlated, and collective excitations of many phonons arise [1]. This leads to a wealth of complex phenomena, such as very high thermal conductivity (the highest known conductivities are indeed found in 2D materials), or wave-like heat diffusion, with second sound, hitherto found only in a few exotic materials at cryogenic temperatures, routinely present at room temperature [2]. In this contribution, we show that heat transport in crystals can be described exactly with the kinetic theory of a gas of collective phonon excitations, termed relaxons. In this way, it is possible to recover a microscopic interpretation based on mean free paths and relaxation times without any simplification of the linearised phonon Boltzmann equation. [1] G. Fugallo, A. Cepellotti, et al., Nano Lett. 14, 6109 (2014) [2] A. Cepellotti, et al., Nat. Commun. 6, 6400 (2015)   

Coherent dynamics of Landau-Levels in modulation doped GaAs quantum wells at high magnetic fields

By using two-dimensional Fourier transform spectroscopy, we investigate the dynamics of Landau-Levels formed in modulation doped GaAs/AlGaAs quantum wells of 18 nm thickness at high magnetic fields and low temperature. The measurements show interesting dephasing dynamics and linewidth dependency as a function of the magnetic field. The work at USF and UAB was supported by the National Science Foundation under grant number DMR-1409473. The work at NHMFL, FSU was supported by the National Science Foundation under grant numbers DMR-1157490 and DMR-1229217. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.   

Probing Carrier Transport across Patterned Interfaces with Ballistic Electron Emission Microscopy

Electron scattering from sidewalls within aggressively scaled metallic interconnects dramatically increases the resistance, since the mean free path (\textasciitilde 40 nm) is larger than the dimensions of the structure. One method to study hot-electron scattering in nm-thick metallic films is Ballistic Electron Emission Microscopy (BEEM), which is an STM based technique. In this work, we perform BEEM imaging and scattering measurements on lithographically patterned nanoscale oxide ``fin'' structures with a Schottky diode interface to determine its ability to measure sidewall scattering. This is accomplished by acquiring data from BEEM images and spectra on a regularly spaced grid and fitting the results to determine both the Schottky barrier height and the amplitude of the spectra. The amplitude of the spectra is related to the scattering in the film and interface. The position of fin structures is then determined by mapping both the Schottky height and amplitude over a square micron to observe scattering at the interface caused by the patterned structures. The fabrication of the patterned 50-nm-pitched sidewall structures and the preliminary BEEM imaging measurements on these structures will be presented.