Research Challenge 1: Nanowires
Composite CL image of InGaN/GaN core-shell nanowire luminescence.
This Research Challenge explores the synthesis and properties of GaN/InGaN nanowires as a materials architecture for visible light-emission. If nanowire devices could span the entire visible spectrum, the RYG-gap technology challenges would be overcome. In addition, such full-spectrum emission would enable chromaticity-tunable light, which could also impact so-called smart (or higher functionality) lighting, another technology challenge.
Among the many advantages of InGaN nanowires over presently-employed planar InGaN films are: the potential for high-quality material, without detrimental line defects; the ability to accommodate lattice strain and therefore a wider range of alloy compositions and bandgaps; manipulability of growth geometries so as to expose surface orientations with tailored light emission properties; and compatibility with novel nanoscale intrawire and interwire energy transfer processes. That said, InGaN nanowires are a relatively immature materials architecture. Thus, this Research Challenge is wide-ranging in scope, encompassing materials synthesis and processing of both photo- and electro-luminescent structures, and optical, electrical, mechanical and structural characterization.
Cross-section STEM image of a nanowire LED showing the formation of both nonpolar and semipolar InGaN/GaN MQWs.
Our initial work in this Research Challenge focused on a bottom-up fabrication method based on highly anisotropic metal-catalyzed epitaxy. We had also initiated exploratory work on a top-down fabrication method, based on a sequence of planar epitaxy, anisotropic dry and wet etching, and subsequent radial and vertical epitaxy. This top-down method has shown sufficient promise for us to shift our focus entirely to it. The top-down method enables higher optoelectronic material quality, an ability to create both axial and radial heterostructures, and increased control over nanowire periodicity and uniformity – all critical for individual and arrayed electroluminescent light-emitting nanowire devices.As we go forward, we anticipate our focus to be on the use of this top-down method to fabricate nanowires with electroluminescence in the visible. We will emphasize the longer green-yellow-red wavelengths that are difficult to achieve in planar architectures, and the use of nanowires as a cross-cutting materials architecture for the other Research Challenges focusing on light-emission phenomena in our EFRC.
Indeed, we note here that progress in nanowire research in our laboratory and others has reached the point of enabling what we believe will be a wide range of nanowire architectures tailored to exhibit phenomena of importance to solid-state lighting and beyond.
(a) Ordered GaN nanowire array (~2 µm height) fabricated using our new top-down process; (b) top-down fabricated flashlight-shaped axial InGaN/GaN single nanowire LED; (c) yellow-red electroluminescence from a vertically integrated array of radial InGaN/GaN nanowire LEDs.
- Dr. George Wang (SNL) – Principal Investigator – determines research goals and priorities, directs/coordinates team, oversees work, and interfaces with other Research Challenges.
- Dr. Andrew Armstrong (SNL) – Deep level optical spectroscopy (DLOS) of nanowires.
- Dr. Igal Brener, Dr. Willie Luk and Jeremy Wright (Ph.D student) (SNL/UNM) – Study and control of nanowire lasing properties using optical spectroscopy and modeling.
- Prof. Lincoln Lauhon and Jim Riley (Ph.D student) (Northwestern) –3D atom probe tomography (APT), and Raman spectroscopy analysis of nanoscale variations in structure and composition.
- Dr. Francois Leonard (SNL) – Theoretical modeling of nanowire electronic structure.
- Dr. Rohit Prasankumar (LANL) – Ultrafast optical studies of nanowire carrier dynamics.
Research Challenge Publications
Li, Qiming; Westlake, Karl R.; Crawford, Mary H.; Lee, Stephen R. ; Koleske, Daniel D.; Figiel, Jeffrey J. ; Cross, Karen C.; Fathololoumi, Saeed; Mi, Zetian; and Wang, George T. Optical performance of top-down fabricated InGaN/GaN nanorod light emitting diode arrays, Optics Express, 19, 25528 (2011). [10.1364/OE.19.00A982]
Wong, Bryon M.; Leonard, Francois; Li, Qiming; and Wang, George T. Nanoscale Effects on Heterojunction Electron Gases in GaN/AlGaN Core/Shell Nanowires, Nano Letters, 11(8), 3074-3079 (2011). [10.1021/nl200981x]
Baird, Lee; Ong, Cp. P.; Cole, R. Adam; Haegel, Nancy M.; Talin, A. Alec; Li, Qiming; and Wang, George T. Transport imaging for contact-free measurements of minority carrier diffusion in GaN, GaN/AlGaN, and GaN/InGaN core-shell nanowires, Applied Physics Letters, 98, 132104 (2011). [10.1063/1.3573832]
Huang, Jianyu; Zheng, He; Mao, S X; Li, Qiming; and Wang, George T. In Situ Nanomechanics of GaN Nanowires, Nano Letters, 11, 1618-1622 (2011). [10.1021/nl200002x]
Li, Qiming; and Wang, George T. Strain influenced indium composition distribution in GaN/InGaN core-shell nanowires, Applied Physics Letters, 97, 181107 (2010). [10.1063/1.3513345]