Impact of microscopic In fluctuations on the optical properties of InxGa1-xN blue light-emitting diodes assessed by low-energy X-ray fluorescence mapping using synchrotron radiation

Abstract

Among the III-nitride semiconductors, In x Ga 1- x N is a key material for visible optical devices such as light-emitting diodes (LEDs), laser diodes, and solar cells. Light emission is achieved via electron-hole recombination within the In x Ga 1- x N layer. When In x Ga 1- x N-based blue LEDs were first commercialized, the high probability of electron-hole radiative recombination despite the presence of numerous threading dislocations was a mystery. Extensive studies have proposed that carrier localization in nanoscopic potential fluctuations due, for example, to the immiscibility between InN and GaN or random alloy fluctuations is a key mechanism for the high emission efficiency. In actual LED devices, not only nanoscopic potential fluctuations but also microscopic ones exist within the In x Ga 1- x N quantum well light-emitting layers. Herein we map the synchrotron radiation microbeam X-ray fluorescence of In x Ga 1- x N blue LEDs at a sub-micron level. To acquire weak signals of In, Ar, which is in the air and has a fluorescent X-ray energy similar to that of In, is evacuated from the sample chamber by He purge. As a result, we successfully visualize the spatial In distribution of In x Ga 1- x N layer nondestructively and present good agreement with optical properties. Additionally, we demonstrate that unlike nanoscopic fluctuations, microscopic In compositional fluctuations do not necessarily have positive effects on device performance. Appropriately controlling both nanoscopic and microscopic fluctuations at the same time is necessary to achieve supreme device performance.

Correlation of Optical, Structural, and Compositional Properties with V-Pit Distribution in InGaN/GaN Multiquantum Wells

“…Likewise, these inhomogeneities will influence the local optoelectronic activity in a complex way, which has been studied intensively. Structural homogeneity of the atomic order has been investigated by extended X-ray absorption fine structure, whereas atom probe tomography and transmission electron microscopy were applied to investigate the short-range fluctuations on a nanoscale. − However, long-range structural fluctuations on a microscale, as indicated by micro-photoluminescence (PL) mappings, − have not been studied intensively, for example, by synchrotron-based X-ray fluorescence …”

“…30−33 However, long-range structural fluctuations on a microscale, as indicated by micro-photoluminescence (PL) mappings, 34−37 have not been studied intensively, for example, by synchrotronbased X-ray fluorescence. 38 Modern synchrotrons are at the very heart of fundamental and applied research because of their superior brilliance and the improvement in focusing X-ray optics. The scanning X-ray diffraction (XRD) microscopy technique quicK-mapping (Kmap), developed at the European Synchrotron Radiation Facility (ESRF) is ideally suited to nondestructively image with submicron resolution structural inhomogeneities even in buried heterostructures.…”

Dislocation and indium droplet related emission inhomogeneities in InGaN LEDs

“…Both can also be explained by compositional fluctuations as well as quantum well monolayer thickness variations. The nanoscopic fluctuations have been held responsible for the high efficiency achieved in quantum well based InGaN blue LEDs, whereas the microscopic fluctuations are in general not desirable [29,30].…”

Dislocation and Indium Droplet Related Emission Inhomogeneities in InGaN LEDs

“…Both can also be explained by compositional fluctuations as well as quantum well monolayer thickness variations. The nanoscopic fluctuations have been held responsible for the high efficiency achieved in quantum well based In-GaN blue LEDs, whereas the microscopic fluctuations are in general not desirable [30,31].…”