Sep 6 (Fri) @ 1:00pm: "Strain Relaxation Engineering towards Efficient III-Nitride based Laser Diodes," Hsun-Ming Chang, ECE PhD Defense

Zoom Meeting: https://ucsb.zoom.us/j/83224086106

Abstract

III-Nitride based laser diodes (LDs) have brought far-reaching impacts on multiple fields of industry and research; however, the high misfit strain between these semiconductors has posed great challenges in both material epitaxy and improving the efficiency. For visible-wavelength LDs, the high misfit compressive strain of InGaN active region to GaN substrate not only generates defects but also hinders the incorporation of indium towards longer wavelengths. For c-plane orientation, the strain causes quantum-confined Stark effect (QCSE), which significantly degrades the optical gain of LDs. Therefore, being able to relax the misfit strain and tailor the strain relaxation becomes important and can open new epitaxial design pathways to improve efficiency of LDs.

In this work, strain engineering for InGaN based LDs towards higher efficiency on c-plane GaN are presented. Based on the previously demonstrated strain relaxed template (SRT), we demonstrated blue (468 nm) LDs with a 61.9% strain-relaxed InGaN buffer. Furthermore, a patterned SRT method was developed, which greatly reduces the threading dislocation density (TDD) on SRT while maintaining the partially relaxed InGaN layers. LDs with a 15% relaxed InGaN buffer showed a threshold current density (Jth) as low as 7.4 kA/cm2, which is much lower than the Jth over 30 kA/cm2 from previous SRT. Moreover, an enhanced optical gain as well as differential gain than conventional c-plane LDs was demonstrated. 

Polarization dependent micro-photoluminescence (μ-PL) showed a degree of linear polarization within local areas for patterned SRT LDs, indicating the presence of local anisotropic strain relaxation. We attribute such local anisotropic strain to be the main mechanism of enhanced optical gain. Finally, by improving the strain relaxation of InGaN buffer to 52%, blue LDs (443.8 nm) with a Jth of 4.7 kA/cm2 were demonstrated, which is over 35% lower than LDs with the same epi on conventional c-plane, and a higher wall-plug efficiency was exhibited. This threshold is close to the best reported value for strain-relaxed InGaN LDs in the literature. 

Bio

Hsun-Ming Chang received his B.S. in Material Science Engineering and his M.S. in Graduate Institute of Photonics and Optoelectronics in National Taiwan University. He is currently pursuing his Ph.D degree in the Department of Electrical and Computer Engineering at University of California, Santa Barbara. His research focuses on III-Nitride based laser diodes and light-emitting diodes, and strain engineering of III-Nitrides for efficiency improvement.

Hosted by: Professor Shuji Nakamura

Submitted by: Hsun-Ming Chang <hsun-ming@ucsb.edu>