Paul L. Bergstrom

Room temperature operational single electron transistor fabricated by focused ion beam deposition

We present the fabrication and room temperature operation of single electron transistors using 8nm tungsten islands deposited by focused ion beam deposition technique. The tunnel junctions are fabricated using oxidation of tungsten in peracetic acid. Clear Coulomb oscillations, showing charging and discharging of the nanoislands, are seen at room temperature. The device consists of an array of tunnel junctions; the tunnel resistance of individual tunnel junction of the device is calculated to be as high as 25.13GΩ. The effective capacitance of the array of tunnel junctions was found to be 0.499aF, giving a charging energy of 160.6meV.

Multi-resonant silver nano-disk patterned thin film hydrogenated amorphous silicon solar cells for Staebler-Wronski effect compensation

We study polarization independent improved light trapping in commercial thin film hydrogenated amorphous silicon (a-Si:H) solar photovoltaic cells using a three-dimensional silver array of multi-resonant nano-disk structures embedded in a silicon nitride anti-reflection coating to enhance optical absorption in the intrinsic layer (i-a-Si:H) for the visible spectrum for any polarization angle. Predicted total optical enhancement (OE) in absorption in the i-a-Si:H for AM-1.5 solar spectrum is 18.51% as compared to the reference, and producing a 19.65% improvement in short-circuit current density (JSC) over 11.7 mA/cm2 for a reference cell. The JSC in the nano-disk patterned solar cell (NDPSC) was found to be higher than the commercial reference structure for any incident angle. The NDPSC has a multi-resonant optical response for the visible spectrum and the associated mechanism for OE in i-a-Si:H layer is excitation of Fabry-Perot resonance facilitated by surface plasmon resonances. The detrimental Staebler...

Sensor Network Localization in Constrained 3-D Spaces

Localization of sensor nodes is important for many sensor network applications such as distance-based routing and target tracking. This paper presents a localization approach for sensor networks in constrained 3-D space. By assuming that all the original beacon nodes are on the bottom plane, the localization procedure for the entire network is primarily from the bottom to the top and at the same time the localization process is carried out in all directions from the regions where the original beacon nodes are clustered. Simulated results showed that efficiency is improved for network structures whose profiles are cuboid or cone shaped, and whose node distributions are layered or random. The effects of the number of original beacon nodes as well as the node density on the localizing errors and/or the localizing successful rate are explored. Finally, a large scale sensor network is analyzed to verify the propagating trend of localization

Simulation of charge transport in multi-island tunneling devices: Application to disordered one-dimensional systems at low and high biases

Although devices have been fabricated displaying interesting single-electron transport characteristics, there has been limited progress in the development of tools that can simulate such devices based on their physical geometry over a range of bias conditions up to a few volts per junction. In this work, we present the development of a multi-island transport simulator, MITS, a simulator of tunneling transport in multi-island devices that takes into account geometrical and material parameters, and can span low and high source-drain biases. First, the capabilities of MITS are demonstrated by modeling experimental devices described in the literature, and showing that the simulated device characteristics agree well with the experimental observations. Then, the results of studies of charge transport through a long one-dimensional (1D) chain of gold nano-islands on an insulating substrate are presented. Current-voltage (IV) characteristics are investigated as a function of the overall chain-length and temperature. Under high bias conditions, where temperature has a minimal effect, the IV characteristics are non-Ohmic, and do not exhibit any Coulomb staircase (CS) structures. The overall resistance of the device also increases non-linearly with increasing chain-length. For small biases, IV characteristics show clear CS structures that are more pronounced for larger chain-lengths. The Coulomb blockade and the threshold voltage (Vth ) required for device switching increase linearly with the increase in chain length. With increasing temperature, the blockade effects are diminished as the abrupt increase in current at Vth is washed out and the apparent blockade decreases. Microscopic investigations demonstrate that the overall IV characteristics are a result of a complex interplay among those factors that affect the tunneling rates that are fixed a priori (island sizes, island separations, temperature, etc.), and the evolving charge state of the system, which changes as the applied source-drain bias (VSD ) is changed. In a system of nano-islands with a broad distribution of sizes and inter-island spacings, the applied bias is divided across the junctions as one would expect of a voltage divider, with larger potential drops across the wider junctions and smaller drops across the narrower junctions. As a result, the tunneling resistances across these wider junctions decrease dramatically, relative to the other junctions, at high VSD thereby increasing their electron tunneling rates. IV behavior at high VSD follows a power-law scaling behavior with the exponent dependent on the length of the chain and the degree of disorder in the system.

Electronic Characteristics of Bacteriorhodopsin

To effectively integrate bacteriorhodopsin (bR) with micro electromechanical systems (MEMS) and nano electromechanical systems (NEMS) devices, it is critical to know the electrical properties of the material and to understand how it will affect the functionality of the device. Inductance, capacitance and resistance (LCR) measurements can be used to determine material characteristics, such as permittivity, film thickness, and capacitance. In this paper, LCR measurements of dried films of oriented and unoriented bR with both light-on and light-off conditions are presented. Tests were performed on dried films of bR to determine if there is a relationship between LCR measurements and orientation, light-on/off, frequency, and time. The results indicated that the LCR measurements depended on the thickness and area of the film, but not on the orientation, as with other biological materials such as muscle. However, there was a transient LCR response for both oriented and unoriented bR which depends on light intensity. The possible effect of integrating this material with a single electron transistor (SET) is also briefly discussed.

Diamond scribing and breaking of silicon for MEMS die separation

We describe a post-release die separation process for 1 0 0 silicon wafers with polysilicon surface micromachines using a combination of diamond scribing and mechanical breaking. The theoretical basis of scribing is reviewed and the theoretical relationship between scribe force and median crack depth was experimentally verified. Also the relationship between the scribe angle and the median crack depth as well as the stress to fracture is described. Scribe speed and scribe wear were also investigated. The results from our experiments were used to develop a process that had yields above 80% for two types of electrostatic MEMS actuators.

Light Sensor Platform Based on the Integration of Bacteriorhodopsin with a Single Electron Transistor

This paper reports on the integration of an optical protein with single electron transistors to form a nano-bio-hybrid device for sensing. Bacteriorhodopsin (bR) is an optoelectric protein that translocates a proton across a distance of several nanometers in response to an absorbed photon of incident light. This charge gradient results in a measurable voltage in the dried state. Single electron transistors (SETs) have active regions consisting of one or more quantum islands with a size typically 10 nanometers or less. Integrating bacteriorhodopsin with the gate of a SET provides a device capable of a modulated electrical output in response to optical modulation at the device gate. Modulation of the optoelectric activity of the bR by chemical binding with a targeted environmental antigen can form a direct chemical-to-electrical sensor reducing the size and complexity of fluorescence-based systems. The work resulted in electrical resistance and capacitance characterization of purple membrane containing bR under variable illumination to ensure minimal impact on SET operation. Purple membrane containing bacteriorhodopsin was electrodeposited on the SET gates, and current throughput was well correlated with variable and cyclic illumination. It was confirmed that bR optoelectric activity is capable of driving SETs.

New Flexible Channels for Room Temperature Tunneling Field Effect Transistors

Tunneling field effect transistors (TFETs) have been proposed to overcome the fundamental issues of Si based transistors, such as short channel effect, finite leakage current, and high contact resistance. Unfortunately, most if not all TFETs are operational only at cryogenic temperatures. Here we report that iron (Fe) quantum dots functionalized boron nitride nanotubes (QDs-BNNTs) can be used as the flexible tunneling channels of TFETs at room temperatures. The electrical insulating BNNTs are used as the one-dimensional (1D) substrates to confine the uniform formation of Fe QDs on their surface as the flexible tunneling channel. Consistent semiconductor-like transport behaviors under various bending conditions are detected by scanning tunneling spectroscopy in a transmission electron microscopy system (in-situ STM-TEM). As suggested by computer simulation, the uniform distribution of Fe QDs enable an averaging effect on the possible electron tunneling pathways, which is responsible for the consistent transport properties that are not sensitive to bending.

Single Electron Transistor Fabrication using Focused Ion Beam direct write technique

We report on the fabrication of single electron transistors using focused ion beam (FIB) etching technology. Single electron transistors (SETs) are comprised of small conducting islands called the Coulomb blockade islands and tunnel junctions, allowing quantum mechanical tunneling of electrons onto and off of the islands. The typical random deposition of islands makes it difficult to fabricate SETs with the same parameters, because the position of the island strongly impacts the tunnel capacitance and resistance. We report the use of maskless FIB direct write technology to fabricate SETs, producing quantum islands less than 50nm in diameter. The FIB direct writing technique allows the exact placement of islands at a desired location in order to better control the device parameters. The initial characteristics of the devices, at room temperature, show that the Coulomb oscillations are smeared out, as expected for this condition. The conductance displays an asymptotic behavior, which is attributed to the operation of the SET in the strong tunnel regime and to its operation at room temperature

Implementation of porous silicon technology for flow-through sensing using electro-osmotic phenomenon

A demonstration of pH sensing for natural waters using nanoscaled porous media and electro-osmotic flow sampling has been performed. This paper presents an innovative integration of sensing and nano-scaled fluidic actuation in the combination of pH sensitive optical dye immobilization with the electro-osmotic phenomena in polar solvents like water. Through wafer nano-scaled porous silicon and porous silica membranes have been demonstrated as porous media and template for high density fluorescence measurements on optical dyes.