Session W12: Electronic Transport in Novel Materials and Nanostructures

Random Telegraph Noise in Silicon Nanowire BioFETs

Noise spectroscopy is important for nanostructures because it represents a highly sensitive and non-destructive means of studying surface states/defects. In this study we characterize the low frequency noise of top-down fabricated silicon nanowire FETs with exposed channels at low temperature. For some devices, we observe a change in the noise spectra as temperature is lowered, from 1/f to Lorentzian. This indicates the presence of random telegraph signals (RTS) due to an interface trap which we confirm from the time-domain measurements. By making measurements at different temperatures, we can probe into the dynamic properties of the trap. In this way, the activation energies for the emission and capture of electrons are determined. The nature and position of the trap is deduced from the gate voltage dependence of the emission and capture time constants. We also observe an increase in the relative RTS noise amplitude as temperature is decreased and report on very large ($>$100{\%}) relative noise amplitudes for measurements carried out at low temperature.   

Distinct microwave photoresistivity peak in a high-mobility quantum Hall system

We report on a distinct resistivity peak in a microwave-irradiated high-mobility two-dimensional electron system at low temperatures. This peak appears in the regime of well separated Landau levels near the second harmonic of the cyclotron resonance and is in addition to microwave-induced resistance oscillations. This talk will focus on the generic characteristics of this peak, such as its dependence on microwave power and temperature.   

Theory of nonlinear transport in separated Landau levels of two-dimensional electron systems

Recent experiments have shown that the differential magnetoresistivity of a high mobility two-dimensional electron system (2DES) is strongly suppressed under applied dc bias. This phenomenon is most pronounced when the Landau level width becomes smaller than the cyclotron energy. Using the quantum kinetics approach we calculate the characteristic current responsible for the suppression and compare the results to the experimental data obtained on a high mobility 2DES at low temperatures.   

Terahertz Spectroscopy of Semiconductor Materials and Nanostructures

Terahertz (THz) time-domain spectroscopy is an attractive method to obtain the electronic transport properties in a variety of semiconductor materials and nanostructures. Unlike traditional techniques, THz spectroscopy does not require the realization of electrical contacts or even direct contact to the material probed. Here, we report the use of THz time-domain spectroscopy to determine the dielectric constant of a variety of semiconductor materials in the THz spectral range, and extract the refractive index, absorption coefficient and electrical conductivity. We also present a comparison of the results obtained from other techniques, including four-point probe resistivity measurements.   

Statistics of excitations in the electron glass model

We study the statistics of elementary excitations in the classical electron glass model of localized electrons interacting via the unscreened Coulomb interaction in the presence of disorder. We reconsider the long-standing puzzle of the exponential suppression of the single-particle density of states near the Fermi level, by measuring accurately the density of states of charged and electron-hole pair excitations via finite temperature Monte Carlo simulation and zero-temperature relaxation. We also investigate the statistics of large charge rearrangements after a perturbation of the system, which may shed some light on the slow relaxation and glassy phenomena recently observed in a variety of Anderson insulators. In collaboration with Martin Goethe.   

Kondo Effect in a mesoscopic system

At low temperatures, transport and thermodynamic properties of Coulomb blockade systems are characterized by the energy scale $T_K$ (the Kondo temperature). We show that the Kondo temperature is subject to strong mesoscopic fluctuations. In a quantum dot system with many single-particle energy levels, the Kondo temperature acquires a log-normal distribution.   

Two-Dimensional Electrostatic Lattices for Excitons

We report on a method for the realization of two-dimensional electrostatic lattices for excitons in quantum well structures. The lattice structure is set by an electrode pattern and the amplitude of the lattice potential is controlled by applied voltages. We demonstrate square, hexagonal, and honeycomb lattices created by this method.   

Non-Markovian effects in the quantum noise of interacting nanostructures

We present a theory of finite-frequency noise in non-equilibrium conductors, and in particular, interacting nanostructures. We employ a quantum master equation approach and treat correlations between the system and the reservoirs in a nonMarkovian fashion. These correlations are pivotal in properly describing current fluctuations in situations where the measuring frequency is larger than both the applied voltage and the temperature. We explicitly show the importance of nonMarkovian effects in different contexts, including the finite-frequency current noise through a double quantum dot charge qubit and the short-time counting statistics of quantum dots.   

Nonlinear Insulator in Complex Oxides

For complex oxides, the very concept of an insulator must be re-examined as they behave differently from conventional insulators such as SiO$_{2}$ due to the presence of multiple defect levels within bandgap. As the semiconductor industry is moving to such oxides for high-$k$ materials, we need to truly understand the insulating properties of them under various electrical excitations. We report a class of material which we coin as nonlinear insulators that exhibit reversible electric-field-induced metal-insulator transitions (MIT). We show this behaviour for an insulating LaAlO$_{3 }$thin film with a large bandgap of $\sim $5.6 eV in a metal/LaAlO$_{3}$/Nb-SrTiO$_{3}$ heterostructure. The reversible MIT is attributed to the formation of a quasi-conduction band (QCB) in the defect states of LaAlO$_{3}$ that forms a continuum state with the conduction band of the Nb-SrTiO$_{3}$. An opposing voltage is required to deplete the charges from the QCB. The implications of these nonlinear insulators are far-reaching. For example, the use of multi-component oxides as insulators in devices (e.g., high-$k$ dielectrics in silicon CMOS devices) must be exercised with caution.   

Tailoring the Crystal Structure of Individual Silicon Nanowires by Polarized Laser Annealing

We study the effect of polarized laser annealing on the crystalline structure of individual amorphous and nano-crystalline silicon nanowires (Si NWs) using Raman spectroscopy. The crystalline fraction of annealed NWs increases dramatically from 0 to 0.93 with increasing incident laser power. We observe Raman line shape narrowing and frequency hardening upon laser annealing due to the increase in crystal grain size. The Raman anti-Stokes:Stokes intensity ratio is used to determine the local heating temperature caused by the intense focused laser spot, which shows a strong polarization dependence on both single crystal bulk Si and nano-crystalline Si NWs. This method provides a new approach to control the crystal structure rather than by simply adjusting the laser power. Furthermore, strain induced linewidth broadening and frequency softening was also observed in bent nano-crystalline Si NWs, and the deformation stress can be released via laser annealing.   

A novel method for measuring electrical conductance in thin solid films that is insensitive to contact effects

The synthesis of novel materials has been a primary driver in the emerging fields of organic-based electronics and nanoelectronics. One major obstacle to the research and development of novel materials is the ability to electrically characterize the material without introducing a significant contact resistance or damaging the material. In organic materials, the contact material has been found to alter the morphology of and to penetrate into the organic molecules, to form Schottky barriers, and to be thermally and mechanically unstable. Throughout nanoelectronics, unstable contacts and large contact resistances arise because of the reduced contact area as devices shrink in size. I will present a novel method for measuring electrical conductance in thin solid films that is insensitive to contact effects. In place of standard current measurements, a nanoscale metal-oxide-semiconductor field-effect transistor (MOSFET) is used to sense charge diffusion in a thin film of amorphous germanium. The contact resistance between the amorphous germanium and a pair of gold electrodes can be modulated \textit{in situ} without affecting the conductance measurement. Moreover, our technique enables the measurement of conductance as low as 10$^{-19}$ S with application of only 1 V to the film. This method can be used to electrically characterize any thin film which is sensitive to contact effects or where the resistance is too high to measure with conventional methods.   

Spectrometry of electron pumping by surface acoustic waves

Surface acoustic waves (SAWs) generate an electrostatic potential wave when applied to a GaAs/AlGaAs heterostructure. Electrons may be captured in a SAW minimum, creating a dynamic quantum dot (QD). SAW-defined QDs may be useful for certain quantum computing schemes, since, for example, they provide reliable single-electron transport and reduce the need for fast gate switching. Surface gates above a 2D electron gas (2DEG) are used to define a quasi-1D channel (Q1DC) at a potential far above the Fermi level. A SAW pulse captures electrons from the 2DEG and pumps a number of them controllably through the Q1DC. As a SAW minimum rises up the potential slope at the channel entrance, the QD is squeezed and some electrons are ejected back into the 2DEG with energies above the Fermi level. In our experiment, we probe the range of energies at which the electrons are emitted using a narrow potential barrier as an energy spectrometer. We can also measure electrons that have been pumped through the channel. We compare these results with a model for the SAW capture/pumping process.