Kyung-Ah Son

Alligator clips to molecular dimensions

Techniques for fabricating nanospaced electrodes suitable for studying electron tunneling through metal-molecule-metal junctions are described. In one approach, top contacts are deposited/placed on a self-assembled monolayer or Langmuir-Blodgett film resting on a conducting substrate, the bottom contact. The molecular component serves as a permanent spacer that controls and limits the electrode separations. The top contact can be a thermally deposited metal film, liquid mercury drop, scanning probe tip, metallic wire or particle. Introduction of the top contact can greatly affect the electrical conductance of the intervening molecular film by chemical reaction, exerting pressure, or simply migrating through the organic layer. Alternatively, vacant nanogaps can be fabricated and the molecular component subsequently inserted. Strategies for constructing vacant nanogaps include mechanical break junction, electromigration, shadow mask lithography, focused ion beam deposition, chemical and electrochemical plating techniques, electron-beam lithography, and molecular and atomic rulers. The size of the nanogaps must be small enough to allow the molecule to connect both leads and large enough to keep the molecules in a relaxed and undistorted state. A significant advantage of using vacant nanogaps in the construction of metal-molecule-metal devices is that the junction can be characterized with and without the molecule in place. Any electrical artifacts introduced by the electrode fabrication process are more easily deconvoluted from the intrinsic properties of the molecule.

Effect of hydrostatic pressure on the current-voltage characteristics of GaN∕AlGaN∕GaN heterostructure devices

The current-voltage characteristics of n-GaN∕u-AlGaN∕n-GaN heterostructure devices are investigated for potential pressure sensor applications. Model calculations suggest that the current decreases with pressure as a result of the piezoelectric effect, and this effect becomes more significant with thicker AlGaN layers and increasing AlN composition. The change in current with pressure is shown to be highly sensitive to the change in interfacial polarization charge densities. The concept is verified by measuring the current versus voltage characteristics of an n-GaN∕u-Al0.2Ga0.8N∕n-GaN device under hydrostatic pressure over the range of 0–5kbars. The measured current is found to decrease approximately linearly with applied pressure in agreement with the model results. A gauge factor, which is defined as the relative change in current divided by the in-plane strain, approaching 500 is extracted from the data, demonstrating the considerable potential of these devices for pressure sensing applications.

GaN-based high temperature and radiation-hard electronics for harsh environments

We develop novel GaN-based high temperature and radiation-hard electronics to realize data acquisition electronics and transmitters suitable for operations in harsh planetary environments. In this paper, we discuss our research on AlGaN/GaN metal-oxide-semiconductor (MOS) transistors that are targeted for 500 °C operation and >2 Mrad radiation hardness. For the target device performance, we develop Schottky-free AlGaN/GaN MOS transistors, where a gate electrode is processed in a MOS layout using an Al2O3 gate dielectric layer. The AlGaN/GaN MOS transistors fabricated with the wide-bandgap gate oxide layer enable Schottky-free gate electrodes, resulting in a much reduced gate leakage current and an improved sub-threshold current than the current AlGaN/GaN field effect transistors. In this study, characterization of our AlGaN/GaN MOS transistors is carried out over the temperature range of 25°C to 500°C. The Ids- Vgs and Ids-Vds curves measured as a function of temperature show an excellent pinch-off behavior up to 450°C. Off-state degradation is not observed up to 400 °C, but it becomes measurable at 450 °C. The off-state current is increased at 500 °C due to the gate leakage current, and the AlGaN/GaN MOS HEMT does not get pinched-off completely. Radiation hardness testing of the AlGaN/GaN MOS transistors is performed using a 50 MeV 60Co gamma source to explore effects of TID (total ion dose). Excellent Ids-Vgs and Ids-Vds characteristics are measured even after exposures to a TID of 2Mrad. A slight decrease of saturation current (ΔIdss~3 mA/mm) is observed due to the 2Mrad irradiation.

Development of GaN-based micro chemical sensor nodes

Sensors based on III-N technology are gaining significant interest due to their potential for monolithic integration of RF transceivers and light sources and the capability of high temperature operations. We are developing a GaN-based micro chemical sensor node for remote detection of chemical toxins, and present electrical responses of AlGaN/GaN HEMT (high electron mobility transistor) sensors to chemical toxins as well as other common gases. Upon exposure to a chemical toxin, the sensor showed immediate increase in source-drain current (Ids). The electrical response of the sensor was clear, reproducible and characteristic of the concentration of the analyte. This is the first time that electrical responses of chemical toxins are measured with a GaN-based microsensor. Detailed analysis on response time, sensitivity and temperature dependence will be discussed

GaN-based micro chemical sensor nodes for early warning chemical agents

We are developing micro chemical sensor nodes that can be used for real time, remote detection and early warning of chemical agent threats. The chemical sensors in our sensor nodes utilize GaN HEMTs (High Electron Mobility Transistors) fabricated with catalytically active transition metal gate electrodes. The GaN HEMT chemical sensors exhibit high sensitivity and selectivity toward chemical agent simulants such as DECNP (Diethyl cyano phosphonate), and this is the first time that chemical agent simulants have been detected with GaN micro sensors. Response time of the GaN HEMT sensor to a chemical species is within a second, and the maximum electronic response speed of the sensor is ~3 GHz. A prototype micro chemical sensor node has been constructed with the GaN sensor, a micro controller, and an RF link. The RF sensor node is operated with a single 3V Li battery, dissipating 15 mW during the RF transmission with 5 dBm output power. The microcontroller allows the operation of the RF sensor nodes with a duty cycle down to 1 %, extending lifetime of the RF sensor nodes over 47 days. Designed to transmit RF signals only at the exposures to chemical agents and produce collective responses to a chemical agent via a sensorweb, the GaN micro chemical sensor nodes seem to be promising for chemical agent beacons.

GaN-based micro pressure sensor for extreme environments

n-GaN/Al_xGa_1-xN/n-GaN (n-I-n) heterostructure devices are investigated for potential applications as pressure sensors in extreme environments. Theoretical modeling of n-In sensors performed with various compositions (x equiv 0.1, 0.15, & 0.2) and thicknesses (10 nm and 20 nm) of Al_xGa_1-xN suggests that electrical current will decrease with increasing pressure and this effect becomes more significant with higher AlN compositions in the Al_xGa_1-xN layer and thicker Al_xGa_1-xN layer. The effects of hydrostatic pressure on the electrical properties of n-GaN/Al _0.15Ga_0.85N/n-GaN structures were also measured over the range of 0-6 kbar. The current was found to decrease linearly and reversibly with increasing pressure. The normalized change in current with pressure is consistent with our modeling studies. The linearity and reversibility in pressure response suggest that these newly investigated n-GaN/Al_xGa_1-xN/n-GaN devices are promising candidates for high-pressure sensor applications

MEMS-Based Micro Instruments for In-Situ Planetary Exploration

NASAs planetary exploration strategy is primarily targeted to the detection of extant or extinct signs of life. Thus, the agency is moving towards more in-situ landed missions as evidenced by the recent, successful demonstration of twin Mars Exploration Rovers. Also, future robotic exploration platforms are expected to evolve towards sophisticated analytical laboratories composed of multi-instrument suites. MEMS technology is very attractive for in-situ planetary exploration because of the promise of a diverse and capable set of advanced, low mass and low-power devices and instruments. At JPL, we are exploiting this diversity of MEMS for the development of a new class of miniaturized instruments for planetary exploration. In particular, two examples of this approach are the development of an Electron Luminescence X-ray Spectrometer (ELXS), and a Force-Detected Nuclear Magnetic Resonance (FDNMR) Spectrometer. The ELXS is a compact (< 1 kg) electron-beam based microinstrument that can determine the chemical composition of samples in air via electron-excited x-ray fluorescence and cathodoluminescence. The enabling technology is a 200-nm-thick, MEMS-fabricated silicon nitride membrane that encapsulates the evacuated electron column while yet being thin enough to allow electron transmission into the ambient atmosphere. The MEMS FDNMR spectrometer, at 2-mm diameter, will be the smallest NMR spectrometer in the world. The significant innovation in this technology is the ability to immerse the sample in a homogenous, uniform magnetic field required for high-resolution NMR spectroscopy. The NMR signal is detected using the principle of modulated dipole-dipole interaction between the samples nuclear magnetic moment and a 60-micron-diameter detector magnet. Finally, the future development path for both of these technologies, culminating ultimately in infusion into space missions, is discussed.

Nanocrossbar arrays as molecular sensors

Electron tunneling between nanospaced electrodes provides a mechanism for directly transducing the presence of molecular analytes into electrical signals. Crossbar junctions with vertical separations on the order of a few nanometers were fabricated using a combination of electron-beam lithography and selective chemical etching. The current-voltage properties of the nanojunctions are highly sensitive to the chemical environment. The tunneling currents increase over one order of magnitude in response to water and organic vapors diluted with a background of pure nitrogen. The resistance of the junctions is also dependent on the concentration of the analyte. These results demonstrate that tunneling can be used to detect changes in the chemical environment.

Investigation of current-voltage characteristics of n-GaN/i-AlxGa1-xN/n-GaN structures

Although standard GaN device structures used for FETs, light emitters, and detectors have been investigated reasonably extensively, the device structures relying on the particulars of current transport over barriers in this material system have not received as much attention, to a large extend due to the insufficient quality of the layers. Unless special measures are taken, the defects present in the barrier material induce current conduction paths that preclude any possibility of observing the fundamental current conduction mechanisms. To overcome this impediment, high quality GaN layers, followed by the vertical single barrier heterostructures, have been grown on sapphire substrates using epitaxial lateral epitaxy in a metal organic chemical vapor deposition system with the aid of an in-situ deposited SiNx nanonet. Structural and optical properties of the films indicate their superior nature. With these templates in hand, n-GaN/i-AlxGa1-xN/n- GaN structures with varying barrier width and height have been prepared and tested for their IV characteristics. The rectification observed is consistent with the barrier design. Because the band bending is affected by polarization charge, which is dependent on pressure, current vs. voltage measurements under pressure have also been recorded. In this presentation, the details of the measurements and analyses, as well as the pertinent aspects of growth related issues will be discussed.