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Title: Design, growth, fabrication and characterization of white LEDs by monolithic on-chip epitaxial integration on (11-22) semi-polar GaN
Author: Poyiatzis, Nicolas
ISNI:       0000 0004 9356 4218
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2020
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Ultimate lighting sources for general illumination are monolithic on-chip white light emitting diodes (LEDs) containing multiple colour emissions, either red-green-blue or blue-yellow, but without involving any yellow phosphors. It is highly likely that current white LEDs fabricated by using a “blue LED+ yellow phosphors” approach will be eventually replaced by monolithic on-chip white LEDs. One of the direct routes for the fabrication of monolithic white LEDs is to utilize InGaN quantum wells (QWs) with different emission wavelengths as an active region, which will involve a number of fundamental issues, such as the design of an active region, carrier transport, etc. So far, these fundamental issues have not been understood. In this work, a systematic simulation study on these challenging issues has been carried out, achieving a full understanding of these issues and thus leading to the design of optimized white LED structures on (11-22) semi-polar substrates by taking the major advantages of semi-polar LEDs in comparison with their c-plane counterparts. Finally, the monolithic on-chip white LED epiwafer based on these designs have been successfully grown on our well-established (11-22) GaN templates with a step-change in crystal quality. Detailed device characterization has been performed on these LEDs, validating these approaches and designs. The design of dual-colour (11-22) semi-polar LEDs aiming at white LEDs and their carrier transport issues have been systemically studied by using one-dimensional drift-diffusion simulations. Due to the much heavier effective mass of holes than that of electrons and also the much larger activation energy of p-GaN than n-GaN, the distribution of injected carrier (mainly holes) is extremely uneven during LED operation. Furthermore, the residual polarization of semi-polar LEDs makes the case even more complicated. Based on a systematic study, carrier transport issues for (11-22) semi-polar white LEDs and their c-plane counterparts have been fully understood, demonstrating their major differences. In addition a novel structure utilizing an extra thin GaN spacer prior to the growth of blue InGaN quantum well, has been design to effectively improve hole transportation and a dual-colour emission LED has been achieved. A tri-chromatic emission has been subsequently designed by further optimizing two key factors, indium content in InGaN quantum wells and barrier thicknesses. In order to validate our simulation results dual-color emission LEDs have been grown on our high quality (11-22) semi-polar GaN templates. Simulations have agreed very well with experimental results demonstrating that both the growth order of the yellow and blue InGaN quantum wells and the growth of a thin GaN spacer are of vital importance. A different approach has been developed, leading to the growth and then the fabrication of monolithically integrated white light LEDs on (11-22) semi-polar GaN template. In this approach, an electrically injected semi-polar blue LED is firstly grown, followed by a yellow multiple quantum well structure as a down conversion layer. This forms a white LED. For the first time, a systematic and comprehensive study on optical polarization properties has been conducted on (11-22) semi-polar LEDs with a wide spectral region from blue to yellow as a function of indium concentration and injection current. Fundamental understanding of the polarization properties and the emission mechanisms of (11-22) semi-polar LEDs has been achieved. Detailed polarization dependent electroluminescence measurements have demonstrated that both indium content and current injection play crucial roles in the optical polarization properties of (11-22) semi-polar LEDs.
Supervisor: Wang, Tao ; Smith, Richard Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available