| Description |
The indium-gallium-nitride on gallium-nitride (InGaN/GaN) materials system is a promising candidate for providing a high intensity, high efficiency solution to the yet unsolved problem of solid state lighting in the range of 550 to 590 nm, a.k.a the Green Gap. The bandgap of InGaN/GaN spans the visual spectrum, making it tunable for emission at any wavelength. The lattice constant mismatch and resulting strain at the heterojunction induces a miscibility gap that enables spontaneous self-assembly of indium-rich nm-scale islands during Stranski-Krastanov growth of the InGaN active layer of the Light Emitting Diode (LED). These islands serve as quantum discs, confining excitons, and increasing internal quantum efficiency. InGaN/GaN/Al2O3 samples from Cao Group Inc. were interrupted after the InGaN growth step of the Organometallic Vapor-Phase Epitaxy (OMVPE) process and analyzed using Atomic Force Microscopy (AFM), High Resolution Scanning Electron Microscopy (HRSEM), Transmission Electron Microscopy (TEM), Raman, and Photoluminescence (PL) spectroscopies. Results show inhomogeneities in light emission and structure as well as conductivity.Coherence, strain, and the presence of indium were all confirmed through TEM and Energy Dispersive X-ray spectroscopy (EDX). Islands (3-10nm in height) and pits (1-4nm deep) cluster in patches with a marbled pattern. Islands tend to be found most predominantly at the edges or the bottom of pits, suggesting indium-rich island growth is preferred along the edges of structures of high surface area, where strain from lattice mismatch is most compensated by surface area. Despite coherence, Fast Fourier Transform (FFT) reciprocal space patterns show lattice spacing and possibly the crystal structure type itself differ between island and subisland areas. The difference between island emission energy and the energy required to excite carriers is greater than expected. PL peak energy and Raman shift frequency test results were used to estimate island indium composition. Polarization of emission was observed, though was unexpected due to the InGaN having been grown on c-plane GaN which should result in isotropic biaxial strain and a valence band configuration that does not lend to bifurcating probabilities of polarization. Near-Field PL testing was performed, also with unexpected results. |
