(Invited) GaN Optoelectronic Devices Based on Local Strain Engineering

Abstract

Controlling strain is an important topic for the design of modern optoelectronic devices based on compound semiconductors. Excessive strain leads to defects and lowers device performance. But an adequate amount of strain can profoundly modify optoelectronic properties, resulting in new device capabilities. In this work, the strain in gallium nitride based heterostructures and its effects on optoelectronic properties are systematically studied. Specifically, strain is locally controlled and isolated using nanostructures. The effects of strain on the optical properties were obtained including emission wavelength, radiative efficiency, absorption, and polarization. Applications resulting from local strain engineering are proposed and demonstrated including single photon sources, microdisplays, spectrometers, and tactile sensing.This study focuses on GaN disk-in-wire nanopillars fabricated using a top-down process although the results are expected to be applicable to similar structures obtained by bottom-up fabrication. The strain profile is modified due to the relaxation of strain around the perimeter of the nanopillar. A radial confinement potential ~hsec(r) is formed in a compressively strained layer, such as InGaN sandwiched by GaN. This radial confinement potential can help confine the electrons and holes toward the center of the nanopillar and reduce the impact of nonradiative recombinations on the surface. Moreover, the radial confinement potential enhances quantum confinement, enabling single photon emission to be observed from a nanopillar with a diameter as large as 37nm. In comparison, the exciton Bohr radius is only 3nm.Top-down fabrication of GaN disk-in-wire nanopillars is enabled by a two-stage dry/wet etching process. A vertical sidewall with a high aspect ratio ~10 can be obtained. The top-down fabrication strategy has the unique advantage of allowing the cross-sectional shape of the nanopillar to be lithographically defined. Specifically, elliptically-shaped nanopillars were extensively studied. The asymmetric shape changes the light polarization properties, as shown by both theoretical and experimental studies.Device applications were also studied. Because controlling the strain changes the emission wavelength, microdisplay devices comprising of multiple GaN light-emitting diodes of different colors can be monolithically integrated. In addition, similar multi-wavelength absorption properties can be achieved. In this talk, several device applications will be discussed including single photon sources, microdisplays, spectrometers, and tactile sensors.