Bissell Distinguished Professor,Adjunct Professor, Department of Physics and Optical Science
UNC Charlotte, NC, USA
Title: The roles of point and extended defects in semiconductors, and the interplay of point defects, extended defects, and carrier localization in the quantum efficiency droop in InGaN QW LEDs
Authors: Ying Liu, Simiao Niu, Qing Yang, Benjamin Klein, Yu Sheng Zhou, Zhong Lin Wang
We perform spatially resolved photoluminescence near an isolated dislocation, a commonly seen extended defect, using a confocal microscope system. Individual dislocations are shown to be effective sensors for probing the carrier dynamics. High quality GaAs epilayer is used as a model system to derive the general understanding for the interplay of point and extended defects in determining the carrier diffusion and recombination in a semiconductor. The carrier diffusion length is found to vary with carrier density in a non-monotonic manner. Furthermore, local laser induced heating leads to defect mutation, changing from a hole trap to an electron trap, thus a greatly enhanced impact range of the same defect.
The general understanding derived from the above model system is used to gain insight into the long-standing issue of the external quantum efficiency (EQE) droop in InGaN QW LEDs. Spatially-resolved electroluminescence (as a function of injection current) and spatially-resolved photoluminescence (as a function of excitation power) are used to investigate the EQE droop with increasing current. The spatially-resolved EL mapping allows us to study two most commonly observed but distinctly different droop behaviors on a single device, minimizing the ambiguity trying to compare independently fabricated devices. We conclude that the interplay of three effects, nonradiative recombination through point defects, carrier localization due to either In composition or well width fluctuation, and nonradiative recombination through the extended defects, plays an important role in the device performance, which is in fact common to various optoelectronic devices. By comparing EL and PL mapping data, we show that individual singular sites exhibiting either particularly strong or weak emission in PL do not usually play any significant and direct role in the EQE droop. We introduce a two-level model that can capture the basic physical processes that dictate the EQE ~ current dependence and describe the whole operating range of the device from 0.01 - 100 A/cm2. We further observe that carrier localization in fact occurs in different length scales due to the material inhomogeneity.
 Gfroerer et al., APL 102,012114(2013).  Lin et al., APL 101,252103(2012).  Lin et al., JAP 115,023103(2014).
Yong Zhang received B.S. and M.S. in Physics from Xiamen University, and Ph.D. in Physics from Dartmouth College. He was with National Renewable Energy Laboratory (NREL) from 1994-2009. In 2009, he joined the ECE Dept of UNC-Charlotte as Bissell Distinguished Professor (adjunct Professor of Physics Dept). His current research activities include alternative materials and structures for photovoltaics (e.g., ZnO/ZnSe nano-heterostructures, CZTSSe, organic-inorganic hybrid perovskite), IR detector materials (e.g., InAs/GaSb SLs and MCT), 2-D materials (e.g., graphene, MoS2, WS2, phosphorene), semicondcutor heterostrutures (e.g., InGaN and AlGaN LED materials), and fundamental sciences in solid state physics. He has published more than 170 papers and book chapters.