Sensors and Actuators

Transport Imaging of  Semiconductor Nanowires

(Supported by the National Science Foundation)

Nancy Haegel - Roya Maboudian, UC Berkeley

 Overview:

        The goal of this work is to image the motion and recombination of charge in luminescent nanowires and other nanostructures.   Direct information can be extracted on the diffusion and related transport properties with high spatial resolution.

Project Description:

        In this project, we work to develop a unique nanoscale imaging technique to study the relationship between individual component synthesis and the contact/transport behavior that will be critical to device level integration.  Transport imaging combines the scanning resolution of near field optics with the charge generation beam control of a scanning electron microscope (SEM).  The goal is to "image transport" by monitoring the motion of charge via the recombination emission of light.  Although the technique builds upon standard cathodoluminescence (CL), it is significantly different, since it maintains the spatial information of the emitted light.  This allows for direct measure of minority carrier diffusion length, mobility-lifetime products under applied fields and contact resistance - important parameters that are difficult to measure directly on a single device.

        The work described here takes this imaging to the ~ 50-100 nm scale with the use of a scanning collection tip in AFM mode, operating inside the SEM.  By decoupling the e-beam excitation from the optical collection, one can image transport of charge over distances from ~ 100 nm to microns along the wire length.  A Nanonics MultiView 2000 system is used, which allows for tip scanning, while the e-beam excitation spot remains fixed at various points of interest along the wire.  

         Nanowire synthesis is performed by co-PI Prof. Roya Maboudian, using the VLS technique.  This technique has been successfully demonstrated for the growth of solid whiskers and nanowires; bridged growth has been demonstrated within silicon microtrenches.  We also study GaN nanowires in collaboration with Sandia National Laboratories and NIST Boulder, as well as ZnO structures grown by the group of Z. L Wang at Georgia Tech.

        Impact:  The broader impact of the work lies in its impact on understanding of transport and contact behavior in nanowires, with contact behavior being a critical area for future progress in device fabrication.  The work would demonstrate a novel tool to interrogate carrier transport in-situ and would provide a means for direct measurement of contact resistance that could be applied on an industrial scale.

For more information, contact Nancy Haegel at nmhaegel@nps.edu

High Z Materials for Nuclear Detection: Synergy of Growth, Characterization and Defect Physics for Room Temperature Devices

(supported by the Department of Homeland Defense, DNDO)                      

Nancy M. Haegel - Eugene E. Haller, UC Berkeley

 Overview:

       Transport imaging is used in this project to study the diffusion behavior of excess carriers, leading to determination of the mobility-lifetime (??) product that is critical to the performance of nuclear radiation detector materials.  The example above shows how the luminescent images can lead to distributions reflecting material-dependent diffusion behavior.

 Project Description:

              Homeland security requires development of cost-effective nuclear detection capability to distinguish threats from non-threats.  High atomic number (Z) semiconductor devices with high efficiency, sufficient energy resolution and room temperature operation offer the potential to meet this objective rapidly, reliably and inexpensively, but have been challenging to realize, despite significant efforts spanning 30 years.  To achieve this important goal, there is strong consensus that fundamental limitations on charge collection in high Z materials must be understood, material quality must be improved, and large scale techniques for production of novel detector material films must be developed.

        A synergistic coupling of crystal growth, characterization and device performance is applied to evaluate new, high Z thin film materials for nuclear radiation detectors.  The intellectual merit of the work would be development of a fundamental framework for understanding the current limitations of compound semiconductor devices, identification of candidate materials for a next generation of devices and development of new tools based on miniaturization of electronics and high throughput characterization to support future transfer of technologies.  The team, based at UC Berkeley and the Naval Postgraduate School, combines expertise from materials science, physics and electrical engineering.  
  
       The specific objectives are summarized as follows:  
• Perform state-of-the-art characterization of charge collection behaviors that currently limit widespread use of high Z compound semiconductors.  This will lead to the identification and removal of charge trapping centers that limit the applications of novel materials.
• Explore complex oxides with embedded high Z elements (e.g., Bi and Pb) as radiation sensing elements.    Complex oxides provide the ability to systematically tune the band gap (from 0.2 eV to 6 eV) and insert high Z elements.  
• Apply a new non-contact optical technique for spatially resolved transport measurements to allow for rapid materials and device assessment.
• Develop electronics capable of resolving charge pulses with amplitude below the noise floor and picosecond resolution.  Time-domain network analysis is ideally suited for this purpose but requires specialized rack-size equipment without a path to later deployment in portable applications.  We will exploit advances in CMOS transimpedance amplifiers for optical receivers to reduce this function to an integrated circuit for in-the-field use.
• Include outreach to provide quality materials for outreach to homeland security policy makers and first responders through collaboration with the NPS Center for Homeland Defense and Security.

For more information, contact Nancy Haegel at nmhaegel@nps.edu

Remotely Triggered Vehicle Mounted IFF (VMIFF)

(supported by the Rapid Reaction Technology Office of OSD and the Office of Naval Research, Code 30)

Nancy M Haegel

 Project Description:

       The Vehi cle Mounted Identification Friend or Foe (VMIFF) technology is a self contained fratricide mitigation device intended to provide an additional means of identifying friendly personnel and equipment to supporting aircraft. The system is intended to interrupt friendly aircraft from targeting U.S. and allied ground forces by identifying the targeted position to the pilot before a laser weapons release occurs.  Currently, friendly forces use active Infrared (IR) strobe lights or thermal panels to mark their locations to supporting aircraft. In the case of IR strobes, this allows enemy forces possessing  Night Vision Goggles (NVGs) the ability to locate our forces during night operations.  Thermal panels have limited range and effectiveness.  The VMIFF device does not constantly emit IR or MWIR energy and remains covert until activated by a targeting laser system, at which point it emits a distinct strobe emission for a limited time.   Therefore, the VMIFF approach can be simultaneously more covert and more effective than existing approaches.

For more information, contact Nancy Haegel at nmhaegel@nps.edu