Laura Biedermann

Electrical current leakage and open-core threading dislocations in AlGaN-based deep ultraviolet light-emitting diodes

Electrical current transport through leakage paths in AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) and their effect on LED performance are investigated. Open-core threading dislocations, or nanopipes, are found to conduct current through nominally insulating Al0.7Ga0.3N layers and limit the performance of DUV-LEDs. A defect-sensitive phosphoric acid etch reveals these open-core threading dislocations in the form of large, micron-scale hexagonal etch pits visible with optical microscopy, while closed-core screw-, edge-, and mixed-type threading dislocations are represented by smaller and more numerous nanometer-scale pits visible by atomic-force microscopy. The electrical and optical performances of DUV-LEDs fabricated on similar Si-doped Al0.7Ga0.3N templates are found to have a strong correlation to the density of these nanopipes, despite their small fraction (<0.1% in this study) of the total density of threading dislocations.

Electrostatic transfer of patterned epitaxial graphene from SiC(0001) to glass

We report on a scalable electrostatic process to transfer epitaxial graphene onto alkali-containing glass substrates. Multilayer epitaxial graphene (MEG) was grown by heating silicon carbide () to high temperatures (1650?1700??C) in an argon-mediated environment. Optical lithography was used to define patterned graphene regions, typically 20?20??m2, which were then transferred to Pyrex substrates. For the electrostatic transfer, a large electric potential (1.2?kV) was applied between the donor MEG sample (anode) and the heated acceptor glass substrate (cathode). Atomic force microscopy scans of the transferred graphene showed that the morphology of the transferred multilayer graphene resembles that of the donor MEG. Raman spectroscopy analysis confirmed that the graphene can be transferred without inducing defects. The sheet resistance of the transferred graphene was as low as 150??/. The transfer of small (1?2??m wide) and large (~70?70??m2) graphene patterns to Zerodur demonstrates the versatility of this transfer technique.

Dielectric-directed surface flashover under atmospheric conditions

Summary form only given. High-voltage arc formation near a dielectric material is a complex process by which surface charging, secondary electron emission, and photoelectron emission modify the local electric field to determine the arc path and breakdown threshold. Strong electric field enhancement at the triple-point junction of dielectric, metal, and atmosphere may act to generate initiating electrons to seed prompt formation of streamers. This study investigates the fundamental role of dielectrics in influencing voltage breakdown threshold, statistical time lag, and reproducibility under high voltage conditions with and without external ultraviolet stimulation. We are investigating whether field emission at triple points can sufficiently minimize variance in atmospheric breakdown. The experimental data presented here are from the Advanced Component Development laboratory at Sandia National Laboratories. The experiments were performed in dry air at 600-Torr using a low-inductance test-stand. Dielectric granules were placed on a planar brass electrode, offset from a rounded brass rod electrode which defined the 1-mm gap. 200 μm dielectric granules minimally affect electric field in the majority of the gap, but set up high field cathode triple points on the ground plane. We will discuss how dielectric material properties impact surface charging, electron emission, and material conversion, thereby directing the flashover path. Concurrent continuum modelling of surface flashover examines the mechanisms of electron absorption and emission at the surface (c.f. H. Hjalmarson et al.) Furthermore, these flashover experiments are to be used in validation efforts of our PIC-DSMC code (c.f. “Development and Validation of PIC-DSMC Air Breakdown Model in the Presence of Dielectric Particles”).

Development of PIC-DSMC air breakdown model in the presence of a dielectric: Breakdown time sensitivity to self-absorption and photoemission

Summary form only given. Electrical breakdown between electrodes in the presence of a dielectric cylinder is simulated using an electrostatic particle-in-cell (PIC) code that models particle-particle collisions using the direct simulation Monte Carlo (DSMC) method. In this talk we will present recent work on sensitivity of the breakdown time delay to the dielectric photoemission yield and to inclusion of self-absorption of photons by the background gas. Validation of the simulation model is being performed against prior experimental data on breakdown across a 13mm rod-to-plane gap with a 10mm dielectric cylinder in the middle1. The dielectric cylinder provides both an electron source by photoemission from low energy photons and enhances the reduced field (thus changing the plasma radiation spectrum)2.The model includes electron-neutral elastic, excitation, ionization, and attachment collision chemistry; ion and photon induced electron emission from surfaces; ion-neutral collisions; and self-absorption, photoionization, and photodissociation. The model tracks excited state neutrals which can be quenched through collisions with the background gas and surfaces or spontaneously emit a photon (isotropically) and transition to a lower state. Each simulated photon from an emission event is given a wavelength based on the transition that includes natural and Doppler broadening3. Emitted photons have an energy dependent probability of causing photoemission from the dielectric or electrode surfaces, as pre-computed by a separate electron Monte Carlo transport code4.

Development of PIC-DSMC model for laser-trigged vacuum switch

Laser-triggered vacuum switches (LTVS) are used for low-jitter, low-inductance, synchronized high current pulsed-power switching. Single-pulse laser irradiation (UV, visible, or IR) heats a target cathode material, ablating the cathode surface and creating a plasma within the gap. UV irradiation may additionally seed the gap with photoemitted electrons. Reported LTVS have laser energies 10s μJ-10s mJ; cathode materials include KCl, Ti, W, and graphite1.

DESALINATION AND WATER TREATMENT RESEARCH AT SANDIA NATIONAL LABORATORIES.

Water is the backbone of our economy – safe and adequate supplies of water are vital for agriculture, industry, recreation, and human consumption. While our supply of water today is largely safe and adequate, we as a nation face increasing water supply challenges in the form of extended droughts, demand growth due to population increase, more stringent health-based regulation, and competing demands from a variety of users. To meet these challenges in the coming decades, water treatment technologies, including desalination, will contribute substantially to ensuring a safe, sustainable, affordable, and adequate water supply for the United States. This overview documents Sandia National Laboratories’ (SNL, or Sandia) Water Treatment Program which focused on the development and demonstration of advanced water purification technologies as part of the larger Sandia Water Initiative. Projects under the Water Treatment Program include: (1) the development of desalination research roadmaps (2) our efforts to accelerate the commercialization of new desalination and water treatment technologies (known as the ‘Jump-Start Program),’ (3) long range (high risk, early stage) desalination research (known as the ‘Long Range Research Program’), (4) treatment research projects under the Joint Water Reuse & Desalination Task Force, (5) the Arsenic Water Technology Partnership Program, (6) water treatment projects funded under the New Mexico Small Business Administration, (7) water treatment projects for the National Energy Technology Laboratory (NETL) and the National Renewable Energy Laboratory (NREL), (8) Sandiadeveloped contaminant-selective treatment technologies, and finally (9) current Laboratory Directed Research and Development (LDRD) funded desalination projects. Desalination and Water Treatment Research at Sandia National Laboratories November 11, 2016

Enabling Graphene Nanoelectronics

Recent work has shown that graphene, a 2D electronic material amenable to the planar semiconductor fabrication processing, possesses tunable electronic material properties potentially far superior to metals and other standard semiconductors. Despite its phenomenal electronic properties, focused research is still required to develop techniques for depositing and synthesizing graphene over large areas, thereby enabling the reproducible mass-fabrication of graphene-based devices. To address these issues, we combined an array of growth approaches and characterization resources to investigate several innovative and synergistic approaches for the synthesis of high quality graphene films on technologically relevant substrate (SiC and metals). Our work focused on developing the fundamental scientific understanding necessary to generate large-area graphene films that exhibit highly uniform electronic properties and record carrier mobility, as well as developing techniques to transfer graphene onto other substrates.