Abstract
Among the III-nitride semiconductors, InxGa1-xN 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 InxGa1-xN layer. When InxGa1-xN-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 InxGa1-xN quantum well light-emitting layers. Herein we map the synchrotron radiation microbeam X-ray fluorescence of InxGa1-xN 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 InxGa1-xN 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.
Highlights
InxGa1-xN is an alloy composed of InN and GaN, and a key material for visible light-emitting or detecting devices because the bandgap can be adjusted between 3.4 eV (365 nm wavelength) and 0.6 eV (2.1 μm) by changing the In composition x1
One important factor that determines the optical properties of InxGa1-xN quantum well (QW) light emitters is the immiscibility between InN and GaN caused by the large lattice mismatch[4]
We succeeded in evaluating small In variations of blue light-emitting diodes (LEDs) epitaxial layers using the microbeam X-ray fluorescence (XRF) technique of the current world’s highest performance synchrotron radiation facility
Summary
To analyze small variations in the XRF signals emitted from a small volume defined by the QW width and X-ray microbeam size, a SR facility should have a high brightness and a low divergence light source. It was assumed that the averaged XRF intensity over all measurement points directly corresponds to the In composition revealed by the macroscopic XRD measurements, which are ~13% for Sample A and ~11% for Sample B This assumption seems reasonable because the low-energy X-ray (9 keV) can excite the In L (~3.3 keV), Al K (~1.5 keV), and Ga L (~1.2 keV) lines, but re-excitation of the In atoms by the Al K or Ga L lines does not occur. With this experimental configuration, the signals from the inclined {1122} planes can be detected. Scatterplot on the XRF map designates the intensity averaged over all measured points along the [11 ̄00] direction
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