D.P. Pivin

Flexible polyimide-based intracortical electrode arrays with bioactive capability

The promise of advanced neuroprosthetic systems to significantly improve the quality of life for a segment of the deaf, blind, or paralyzed population hinges on the development of an efficacious, and safe, multichannel neural interface for the central nervous system. The candidate implantable device that is to provide such an interface must exceed a host of exacting design parameters. The authors present a thin-film, polyimide-based, multichannel intracortical Bio-MEMS interface manufactured with standard planar photo-lithographic CMOS-compatible techniques on 4-in silicon wafers. The use of polyimide provides a mechanically flexible substrate which can be manipulated into unique three-dimensional designs. Polyimide also provides an ideal surface for the selective attachment of various important bioactive species onto the device in order to encourage favorable long-term reactions at the tissue-electrode interface. Structures have an integrated polyimide cable providing efficient contact points for a high-density connector. This report details in vivo and in vitro device characterization of the biological, electrical and mechanical properties of these arrays. Results suggest that these arrays could be a candidate device for long-term neural implants.

Quantum transport in open mesoscopic cavities

Abstract In this review we describe the results of magneto-transport studies in open quantum dots, in which electronic motion is expected to be predominantly ballistic in nature. The devices themselves are realized in different semiconductor materials, using quite distinct fabrication techniques. Electron interference is an important process in determining the electrical properties of the devices at low temperatures and is manifested through the observation of periodic magneto-conductance fluctuations. These are found to result from selective excitation of discrete cavity eigenstates by incoming electrons, which are directed into a collimated beam by the input point contact. Under conditions of such restricted injection, quantum mechanical simulations reveal highly characteristic wavefunction scarring, associated with the remnants of a few classical orbits. The scarring is built up by interference between electrons, confined within the cavities over very long time scales, suggesting the underlying orbits are highly stable in nature. This characteristic is also confirmed by the results of experiment, which reveal the discrete components dominating the interference to be insensitive to changes in lead opening or temperature. The fluctuations decay with increasing temperature, although they can nonetheless still be resolved at a few degrees kelvin. This characteristic is confirmed by independent studies of devices, fabricated using very different techniques, further demonstrating the universal nature of the behavior we discuss here. These results therefore demonstrate that the correct description of electron interference in open quantum cavities, is one in which only a few discrete orbits are excited by the collimating action of the input lead, giving rise to striking wavefunction scarring with measurable magneto-transport results.

Carrier Transport in Nanodevices

Future VLSI scaling realization of gate lengths is expected to 70 nm and below. While we do not know all the underlying physics, we are beginning to understand some limiting factors, which include quantum transport, in these structures. The discrete nature of impurities, the fact that devices have critical lengths comparable to their coherence lengths, and size quantization will all be important in these structures. These phenomena will lead to pockets of charge, which will appear as coupled quantum dots in the device transport. We review some of the physics of these dots.

Silicon Quantum Dot in a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) Structure

We have fabricated a 200 nm quantum dot in a silicon Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) structure. Confining gates situated on top of a 10 nm oxide deplete the electron gas created by an inversion gate 70 nm away from the Si-SiO2 interface. Measurements indicate that the size of the dot can be tuned by the gates. Furthermore, we observe conductance fluctuations in the gate characteristic which are indicative of single-electron behavior.

Quantum Transport in Single and Multiple Quantum Dots

Ballistic quantum dots have been used in a wide variety of studies ranging from single-electron charging to chaotic systems. However, in open, ballistic quantum dots, the behavior is significantly different. Here, we discuss (1) the observation of regular, periodic fluctuations arising from the existence of stable orbits, (2) the regular and chaotic behavior of coupled dots, and (3) the theory of such dots. The regular orbit properties of these dots are their most stable, generic property, and are clearly reflected in the magnetoresistance. These give rise to periodic fluctuations, which are the result of a very few, periodic orbits within the dot that give rise to scarred wave functions and harmonically related frequencies in the Fourier spectrum. The orbits arise from the role of regular trajectories in the oscillatory density of states and the crucial collimation effects of the quantum point contacts.

Size-dependent effects on the magnetotransport fluctuations of square quantum dots

Recently, experimental studies of magnetotransport in nominally square quantum dots have revealed a high degree of periodicity in the conductance fluctuations, with only a few apparently harmonically related frequencies dominating the power spectra. Analysis of dots of different sizes has revealed that there are unresolved issues regarding the scaling of the dominant frequency of the fluctuations. The experimental data appear to suggest that the dominant frequency scales with , where A is the area of the dot. On the other hand, a semiclassical analysis of periodic orbits suggests that the scaling should be with area. In this paper, we attempt to resolve this issue by simulating dots of many different sizes using both quantum mechanical and classical approaches.

Single electron effects in silicon quantum dots in a MOSFET structure

We have fabricated silicon quantum dot devices based on a dual gate technique. Two lateral gates deplete the inversion layer which is induced by a top gate, thus forming a quantum dot located between the source and drain of a long channel MOSFET. Lithographic dimensions of the dots ranged from 40 nm to 200 nm. Measurements at low temperatures indicate that electrostatic confinement reduces the dot size to 15 nm. We observe evidence of quantum effects as we step the fermi energy by the top gate as well as magnetic field.

Magnetotransport spectroscopy of a quantum dot: Effects of lead opening and phase coherence

Abstract We compare open quantum dot magnetoconductance spectra from experiment and theory in the presence of environmental coupling and attributed broadening. Estimates of the phase-breaking time in experiment, and effective broadening in simulation, are determined independently. In a larger, more open dot, with a significantly shorter phase-breaking time, the observed spectrum is broadened, most noticeably about B=0. The required broadening in simulation is characterized by effective temperatures higher than estimates from experiment; however, without accounting for disorder, which will further broaden the spectrum, the agreement is reasonable.

The effect of mode coupling on conductance fluctuations in ballistic quantum dots

We study the effect of mode coupling on the magnetoconductance fluctuations of a square ballistic quantum dot fabricated in AlGaAs/GaAs heterostructure material and defined by Schottky gates. Structures prepared allow for the independent variation of the number of propagating input and output modes and their coupling to the main cavity. While adjusting the opening of the input and output point contacts changes the number of modes allowed to propagate into the dot, the excitation from a quantum wire creates an additional selection rule. The autocorrelation and Fourier analyses of the fluctuations demonstrate that the formation of the quantum wire has a strong effect, not only on the average conductance of the dot but also on the excitation of the semiclassical orbits within it.

Quantum transport in ballistic quantum dots

Carriers in small 3D quantum boxes take us from unintentional qquantum dots in MOSFETs (arising from the doping fluctuations) tto single-electron quantum dots in semiconductor hheterostructures. In between these two extremes are the realm of oopen, ballistic quantum dots, in which the transport can be quite regular. Several issues must be considered in treating the transport in these dots, among which are: (1) phase coherence within the dot; (2) the transition between semi-classical and fully quantum transport, (3) the role of the contacts, vis-a-vis the fabricated boundaries, and (4) the actual versus internal boundaries. In this paper, we discuss these issues, including the primary observables in experiment, the intrinsic nature of oscillatory behavior in magnetic field and dot size, and the connection to semi-classical transport emphasizing the importance of the filtering by the input (and output) quantum point contacts.