Optical Studies of Indium Gallium Nitride Nanostructures

InGaN;nanostructures;exciton-phonon interaction;LED;Physics;Science;Physics

Katcher, AdamSih, Vanessa ;

University of Michigan

关键词: InGaN;    nanostructures;    exciton-phonon interaction;    LED;    Physics;    Science;    Physics;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/147624/akatcher_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Indium gallium nitride (InGaN) is a semiconductor material that is in widespread use in blue light emitting diodes (LEDs) and blue laser diodes and is being used in solid-state lighting, displays, and scientific applications. The scientific understanding of the physical mechanisms responsible for the performance of these devices is still developing; this includes the description of localization of carriers in this material – a fundamental issue which is believed to be responsible for the origin of luminesce in blue LEDs – as well as that of performance limitations of existing devices, including the so-called ;;green gap” and ;;efficiency droop,” which are related in part to the exciton-phonon interaction. This thesis studies top-down etched InGaN quantum disks(QDs) embedded in GaN nanopillars, focusing on the effect of localization and of the exciton-phonon interaction. The exciton-phonon interaction is studied with two experiments which examine the effect of quantum dot size and shape on the strength of the optical phonon replica in the PL. First, we study the effect of asymmetrical strain on the exciton-phonon coupling by examining the optical phonon replica in the PL of nineteen individual elliptical QDs with dimensions of 22nm x 36nm. We show that the effect of strain on the phonon coupling strength should be observable by a reduction in the degree of polarization (DOP) of the optical phonon replica. Measurements confirm that there is a reduction in the DOP of the optical phonon replica, with reasonable agreement with theory for the high DOP dots.Lower DOP dots, which arise to due irregularities in the shape and size of the fabricated nanopillars, also show a reduction in DOP of the phonon replica but are more sensitive to the effect of asymmetrical phonon coupling and warrant further study. Second, we examine the effect of nanopillar diameter on exciton-phonon coupling strength in InGaN quantum disks. We observe an enhancement of the phonon replica as the nanopillar diameter is reduced from 1000nm to 60nm. This effect is explained by a reduction in the lateral Bohr radius of the exciton which accompanies the decrease in vertical electron-hole separation in smaller nanopillar diameters. To quantify this effect, a simple model is used to infer that, based on the measured phonon coupling strengths, the Bohr radius reduces from approximately 2.5nm to 2nm as the diameter is reduced over the observed range.In order to study the effect of localization, we measure the Stokes shift, which is the energy difference between emission and absorption. By measuring this quantity as a function of nanopillar diameter, we demonstrate the ability to separately determine the contributions of the strain-induced quantum confined Stark effect and of localization to the observed Stokes shift. In our case, we find that the two effects have approximately equal contributions for the range of nanopillar diameters studied here. Furthermore, the site control of InGaN/GaN quantum disks using this top-down fabrication method assists the integration of advanced device structures with individual nanopillars. We demonstrate the enhancement of light collimation by a factor of 1.8x from single nanopillar LEDs using an integrated nanolens. Additionally, we report measurements of enhanced QD brightness and radiative emission rate using an open-top plasmonic cavity; this demonstration is tailored for applications in quantum technologies such as quantum cryptography.