arXiv subjects

M. P. Lilly

Publications and source records attributed to M. P. Lilly.

At least 37 records · Page 2Linked to original sources

arXiv2014

1D-1D Coulomb Drag Signature of a Luttinger Liquid

We report Coulomb drag measurements between vertically-integrated quantum wires separated by a barrier only 15 nm wide. The temperature dependence of the drag resistance is measured in the true one-dimensional (1D) regime where both wires have less than one 1D subband occupied. As a function of temperature, an upturn in the drag resistance is observed in three distinct devices at a temperature $T^* \sim 1.6$ K. This crossover in Coulomb drag behaviour is consistent with Tomonaga-Luttinger liquid models for the 1D-1D drag between quantum wires.

cond-mat.mes-hall↗

arXiv2012

Coherent Propagation of Spin Helices in a Quantum-Well Confined Electron Gas

We use phase-resolved transient grating spectroscopy to measure the propagation of spin helices in a high mobility $n$-GaAs/AlGaAs quantum well with an applied in-plane electric field. At relatively low fields helical modes crossover from overdamped excitations where the spin-precession period exceeds the spin lifetime, to a regime of coherent propagation where several spin-precession periods can be observed. We demonstrate that the envelope of a spin polarization packet reaches a current-driven velocity of 10$^7$ cm s$^{-1}$ in an applied field of 70 V cm$^{-1}$.

cond-mat.mes-hall↗

arXiv2012

Positive and negative Coulomb drag in vertically integrated one-dimensional quantum wires

Electron interactions in and between wires become increasingly complex and important as circuits are scaled to nanometre sizes, or employ reduced-dimensional conductors like carbon nanotubes, nanowires and gated high mobility 2D electron systems. This is because the screening of the long-range Coulomb potential of individual carriers is weakened in these systems, which can lead to phenomenon such as Coulomb drag: a current in one wire induces a voltage in a second wire through Coulomb interactions alone. Previous experiments have observed electron drag in wires separated by a soft electrostatic barrier $\gtrsim$ 80 nm. Here, we measure both positive and negative drag between adjacent vertical quantum wires that are separated by $\sim$ 15 nm and have independent contacts, which allows their electron densities to be tuned independently. We map out the drag signal versus the number of electron subbands occupied in each wire, and interpret the results in terms of momentum-transfer and charge-fluctuation induced transport models. For wires of significantly different subband occupancies, the positive drag effect can be as large as 25%.

cond-mat.mes-hall↗

arXiv2012

Doppler velocimetry of spin propagation in a two-dimensional electron gas

Controlling the flow of electrons by manipulation of their spin is a key to the development of spin-based electronics. While recent demonstrations of electrical-gate control in spin-transistor configurations show great promise, operation at room temperature remains elusive. Further progress requires a deeper understanding of the propagation of spin polarization, particularly in the high mobility semiconductors used for devices. Here we report the application of Doppler velocimetry to resolve the motion of spin-polarized electrons in GaAs quantum wells driven by a drifting Fermi sea. We find that the spin mobility tracks the high electron mobility precisely as a function of T. However, we also observe that the coherent precession of spins driven by spin-orbit interaction, which is essential for the operation of a broad class of spin logic devices, breaks down at temperatures above 150 K for reasons that are not understood theoretically.

cond-mat.mes-hall↗

arXiv2011

Probing Band-Tail States in Silicon MOS Heterostructures with Electron Spin Resonance

We present an electron spin resonance (ESR) approach to characterize shallow electron trapping in band-tail states at Si/SiO2 interfaces in metal-oxide-semiconductor (MOS) devices and demonstrate it on two MOS devices fabricated at different laboratories. Despite displaying similar low temperature (4.2 K) peak mobilities, our ESR data reveal a significant difference in the Si/SiO2 interface quality of these two devices, specifically an order of magnitude difference in the number of shallow trapped charges at the Si/SiO2 interfaces. Thus, our ESR method allows a quantitative evaluation of the Si/SiO2 interface quality at low electron densities, where conventional mobility measurements are not possible.

cond-mat.mtrl-sci↗

arXiv2011

Enhancement-mode buried strained silicon channel quantum dot with tunable lateral geometry

We propose and demonstrate a relaxed-SiGe/strained-Si (SiGe/s-Si) enhancement-mode gate stack for quantum dots. The enhancement-mode SiGe/s-Si structure is pursued because it spaces the quantum dot away from charge and spin defect rich dielectric interfaces and minimizes background dopants. A mobility of 1.6\times10^5 cm^2/Vs at 5.8\times10^{11}/cm^2 is measured in Hall bars that witness the same device process flow as the quantum dot. Periodic Coulomb blockade (CB) is measured in a double-top-gated lateral quantum dot nanostructure. The CB terminates with open diamonds up to \pm 10 mV of DC voltage across the device. The devices were fabricated within a 150 mm Si foundry setting that uses implanted ohmics and chemical-vapor-deposited dielectrics, in contrast to previously demonstrated enhancement-mode SiGe/s-Si structures made with AuSb alloyed ohmics and atomic-layer-deposited dielectric. A modified implant, polysilicon formation and annealing conditions were utilized to minimize the thermal budget so that the buried s-Si layer would not be washed out by Ge/Si interdiffusion.

cond-mat.mes-hall↗

arXiv2011

Measurement of electron-hole friction in an n-doped GaAs/AlGaAs quantum well using optical transient grating spectroscopy

We use phase-resolved transient grating spectroscopy to measure the drift and diffusion of electron-hole density waves in a semiconductor quantum well. The unique aspects of this optical probe allow us to determine the frictional force between a two-dimensional Fermi liquid of electrons and a dilute gas of holes. Knowledge of electron-hole friction enables prediction of ambipolar dynamics in high-mobility electron systems.

cond-mat.mes-hall↗

arXiv2010

Double quantum dot with tunable coupling in an enhancement-mode silicon metal-oxide semiconductor device with lateral geometry

We present transport measurements of a tunable silicon metal-oxide-semiconductor double quantum dot device with lateral geometry. Experimentally extracted gate-to-dot capacitances show that the device is largely symmetric under the gate voltages applied. Intriguingly, these gate voltages themselves are not symmetric. Comparison with numerical simulations indicates that the applied gate voltages serve to offset an intrinsic asymmetry in the physical device. We also show a transition from a large single dot to two well isolated coupled dots, where the central gate of the device is used to controllably tune the interdot coupling.

cond-mat.mes-hall↗

arXiv2010

Scattering Mechanism in Modulation-Doped Shallow Two-Dimensional Electron Gases

We report on a systematic investigation of the dominant scattering mechanism in shallow two-dimensional electron gases (2DEGs) formed in modulation-doped GaAs/Al_{x}Ga_{1-x}As heterostructures. The power-law exponent of the electron mobility versus density, mu \propto n^{alpha}, is extracted as a function of the 2DEG's depth. When shallower than 130 nm from the surface, the power-law exponent of the 2DEG, as well as the mobility, drops from alpha \simeq 1.65 (130 nm deep) to alpha \simeq 1.3 (60 nm deep). Our results for shallow 2DEGs are consistent with theoretical expectations for scattering by remote dopants, in contrast to the mobility-limiting background charged impurities of deeper heterostructures.

cond-mat.mes-hall↗

arXiv2009

Charge sensing in enhancement mode double-top-gated metal-oxide-semiconductor quantum dots

Laterally coupled charge sensing of quantum dots is highly desirable, because it enables measurement even when conduction through the quantum dot itself is suppressed. In this work, we demonstrate such charge sensing in a double top gated MOS system. The current through a point contact constriction integrated near a quantum dot shows sharp 2% changes corresponding to charge transitions between the dot and a nearby lead. We extract the coupling capacitance between the charge sensor and the quantum dot, and we show that it agrees well with a 3D capacitance model of the integrated sensor and quantum dot system.

cond-mat.mes-hall↗

arXiv2009

Enhancement mode double top gated MOS nanostructures with tunable lateral geometry

We present measurements of silicon (Si) metal-oxide-semiconductor (MOS) nanostructures that are fabricated using a process that facilitates essentially arbitrary gate geometries. Stable Coulomb blockade behavior free from the effects of parasitic dot formation is exhibited in several MOS quantum dots with an open lateral quantum dot geometry. Decreases in mobility and increases in charge defect densities (i.e. interface traps and fixed oxide charge) are measured for critical process steps, and we correlate low disorder behavior with a quantitative defect density. This work provides quantitative guidance that has not been previously established about defect densities for which Si quantum dots do not exhibit parasitic dot formation. These devices make use of a double-layer gate stack in which many regions, including the critical gate oxide, were fabricated in a fully-qualified CMOS facility.

cond-mat.mes-hall↗

arXiv2009

The Observation of Percolation-Induced 2D Metal-Insulator Transition in a Si MOSFET

By analyzing the temperature ($T$) and density ($n$) dependence of the measured conductivity ($σ$) of 2D electrons in the low density ($\sim10^{11}$cm$^{-2}$) and temperature (0.02 - 10 K) regime of high-mobility (1.0 and 1.5 $\times 10^4$ cm$^2$/Vs) Si MOSFETs, we establish that the putative 2D metal-insulator transition is a density-inhomogeneity driven percolation transition where the density-dependent conductivity vanishes as $σ(n) \propto (n - n_p)^p$, with the exponent $p \sim 1.2$ being consistent with a percolation transition. The `metallic' behavior of $σ(T)$ for $n > n_p$ is shown to be well-described by a semi-classical Boltzmann theory, and we observe the standard weak localization-induced negative magnetoresistance behavior, as expected in a normal Fermi liquid, in the metallic phase.

cond-mat.mes-hall↗

arXiv2008

Coulomb Drag in the Exciton Regime in Electron-Hole Bilayers

We report electrical transport measurements on GaAs/AlGaAs based electron-hole bilayers. These systems are expected to make a transition from a pair of weakly coupled two-dimensional systems to a strongly coupled exciton system as the barrier between the layers is reduced. Once excitons form, phenomena such as Bose-Einstein condensation of excitons could be observed. In our devices, electrons and holes are confined in double quantum wells, and carriers in the devices are induced with top and bottom gates leading to variable density in each layer. Separate contact to each layer allows Coulomb drag transport measurements where current is driven in one layer while voltage is measured in the other. Coulomb drag is sensitive to interlayer coupling and has been predicted to provide a strong signature of exciton condensation. Drag measurement on EHBLs with a 30 nm barrier are consistent with drag between two weakly coupled 2D Fermi systems where the drag decreases as the temperature is reduced. When the barrier is reduced to 20 nm, we observe a consistent increase in the drag resistance as the temperature is reduced. These results indicate the onset of a much stronger coupling between the electrons and holes which leads to exciton formation and possibly phenomena related to exciton condensation.

cond-mat.mes-hall↗

arXiv2008

Layer interdependence of transport in an undoped electron-hole bilayer

The layer interdependence of transport in an undoped electron-hole bilayer (uEHBL) device was studied as a function of carrier density, interlayer electric field, and temperature. The uEHBL device consisted of a density tunable, independently contacted two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) induced via field effect in distinct GaAs quantum wells separated by a 30 nm Al$_{0.9}$Ga$_{0.1}$As barrier. Transport measurements were made simultaneously on each layer using the van der Pauw method. An increase in 2DHG mobility with increasing 2DEG density was observed, while the 2DEG mobility showed negligible dependence on the 2DHG density. Decreasing the interlayer electric-field and thereby increasing interlayer separation also increased the 2DHG mobility with negligible effects on the 2DEG mobility. The change in interlayer separation as interlayer electric-field changed was estimated using 2DHG Coulomb drag measurements. The results were consistent with mobility of each layer being only indirectly dependent on the adjacent layer density and dominated by background impurity scattering. Temperature dependencies were also determined for the resistivity of each layer.

cond-mat.mes-hall↗

arXiv2006

Undoped Electron-Hole Bilayers in a GaAs/AlGaAs Double Quantum Well

We present the fabrication details of completely undoped electron-hole bilayer devices in a GaAs/AlGaAs double quantum well heterostructure with a 30 nm barrier. These devices have independently tunable densities of the two-dimensional electron gas and two-dimensional hole gas. We report four-terminal transport measurements of the independently contacted electron and hole layers with balanced densities from $1.2 \times 10^{11}$cm$^{-2}$ down to $4 \times 10^{10}$ cm$^{-2}$ at $T = 300 mK$. The mobilities can exceed $1 \times 10^{6}$ cm$^{2}$ V$^{-1}$ s$^{-1}$ for electrons and $4 \times 10^{5}$ cm$^{2}$ V$^{-1}$ s$^{-1}$ for holes.

cond-mat.mes-hall↗

arXiv2005

Surface Acoustic Wave Propagation and Inhomogeneities in Low Density Two-Dimensional Electron Systems Near the Metal-Insulator Transition

We have measured the surface acoustic wave velocity shift in a GaAs/AlGaAs heterostructure containing a two-dimensional electron system (2DES) in a low-density regime ($<$ $10^{10}$ cm$^{-2}$) at zero magnetic field. The interaction of the surface acoustic wave with the 2DES is not well described by a simple model using low-frequency conductivity measurements. We speculate that this conflict is a result of inhomogeneities in the 2DES which become very important at low density. This has implications for the putative metal-insulator transition in two dimensions.

cond-mat.mes-hall↗

arXiv2004

Tunneling and nonlinear transport in a vertically coupled GaAs/AlGaAs double quantum wire system

We report low-dimensional tunneling in an independently contacted vertically coupled quantum wire system. This nanostructure is fabricated in a high quality GaAs/AlGaAs parallel double quantum well heterostructure. Using a novel flip chip technique to align top and bottom split gates to form low-dimensional constrictions in each of the independently contacted quantum wells we explicitly control the subband occupation of the individual wires. In addition to the expected 2D-2D tunneling results, we have found additional tunneling features that are related to the 1D quantum wires.

cond-mat.mes-hall↗

arXiv2004

2D Metal-Insulator transition as a percolation transition

By carefully analyzing the low temperature density dependence of 2D conductivity in undoped high mobility n-GaAs heterostructures, we conclude that the 2D metal-insulator transition in this system is a density inhomogeneity driven percolation transition due to the breakdown of screening in the random charged impurity disorder background. In particular, our measured conductivity exponent of $\sim 1.4$ approaches the 2D percolation exponent value of 4/3 at low temperatures and our experimental data are inconsistent with there being a zero-temperature quantum critical point in our system.

cond-mat.mes-hall↗