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Abstract
This report classifies emission inhomogeneities that manifest in InGaN quantum well blue light-emitting diodes grown by plasma-assisted molecular beam epitaxy on free-standing GaN substrates. By a combination of spatially resolved electroluminescence and cathodoluminescence measurements, atomic force microscopy, scanning electron microscopy and hot wet potassium hydroxide etching, the identified inhomogeneities are found to fall in four categories. Labeled here as type I through IV, they are distinguishable by their size, density, energy, intensity, radiative and electronic characteristics and chemical etch pits which correlates them with dislocations. Type I exhibits a blueshift of about 120 meV for the InGaN quantum well emission attributed to a perturbation of the active region, which is related to indium droplets that form on the surface in the metal-rich InGaN growth condition. Specifically, we attribute the blueshift to a decreased growth rate of and indium incorporation in the InGaN quantum wells underneath the droplet which is postulated to be the result of reduced incorporated N species due to increased N2 formation. The location of droplets are correlated with mixed type dislocations for type I defects. Types II through IV are due to screw dislocations, edge dislocations, and dislocation bunching, respectively, and form dark spots due to leakage current and nonradiative recombination.
Highlights
Where EInN is the bandgap of InN (0.65 eV at room temperature) and EGaN is the bandgap of GaN (3.4 eV at room temperature), and b is the bowing parameter for wurtzite InGaN
The energy band offset is larger for the conduction band than for the valence band and we make the approximation that the ratio between those offsets is 7:3
By equation 2 and using b ≈ 2.9 eV, the sole effect of a reduction of indium incorporation leads to ∆x ≈ −0.021, reducing the quantum well indium content from ≈ 15% to ≈ 12.9%
Summary
Where EInN is the bandgap of InN (0.65 eV at room temperature) and EGaN is the bandgap of GaN (3.4 eV at room temperature), and b is the bowing parameter for wurtzite InGaN. The perturbation to the quantum well transition energy in the inhomogeneous region with reduced indium incorporation ∆x compared to the rest of the (homogeneous) LED, is approximately equal to:
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