High In-Content InGaN Platelets as Underlayer for Light-Emitting Diodes Toward Long Wavelength Application

Abstract

It was both theoretically and experimentally proved that the growth of In Ga 4-xN quantum wells (QWs) on In Ga,_ N underlayer (x > y) would significantly improve the light emitting diodes (LEDs) performance, attributing to the reduction of non-radiative recombination centers, the decline of the internal electrostatic field, and the improvement of indium composition uniformity in QWs. Moreover, the reduced lattice mismatch between QWs and underlayer greatly suppresses composition pulling effect, which improves indium incorporation rate grown at the same temperature, resulting in In-rich InGaN with highly crystalline. However, to serve as templates for red emission QWs, a thicker InGaN pseudo-substrate with closer indium mole fraction to QWs grown beyond the critical layer thickness is desired. In this work, we present an approach to achieve a sub-micron size InGaN underlayer with high to 30 % indium composition, while the thickness is around 90 nm, far exceeding the critical layer thickness of such In-rich InGaN layer on planar c-plane GaN (reported less than 5 nm). The InGaN platelet is demonstrated by selective area growth (SAG) on arrays of GaN seed from circle openings with a diameter of 400 nm ina SiN, mask. To our best knowledge, SAG of InGaN results in random nucleation and uneven shape due to the low diffusion length of gallium precursor at relatively low growth temperature. It was reported that the selectivity of InGaN can be enhanced by introducing pyramid shaped GaN seed first, resulting in InGaN pyramid. However, the local indium composition fluctuation on the semipolar facet of the InGaN pyramid hinders pure red emission of QWs with small full width at half maximum (FWHM). In this work, planar hexagonal shape of GaN seed was introduced first before SAG of InGaN, leading to the formation of uniform InGaN platelets with c-plane domain morphology. The optical properties of the InGaN platelets were characterized by photoluminescence (PL, excited by a He-Cd laser of 325 nm, 15 mW, room-temperature), showing PL peaks around 530 nm with FWHM of 141 meV, suggesting around 30 % indium incorporated. AFM scan of the top c-plane surface of the InGaN platelet indicates an atomically flat surface without v-pits or trench defects while the thickness is over 90 nm. Cross-sectional TEM characterization of InGaN platelet suggests homogenous crystalline and shape interface between InGaN and GaN seed. According to localized EDX line measurements recorded with the same area, the InGaN platelet shows indium composition almost linearly increased from InGaN/GaN interface to c-plane surface. Detailed line profiles show indium composition of 25 % to 30 % at the top surface and 15 % to 20 % at InNGaN/GaN interface. Such high indium-content InGaN platelets are competitive candidates for the development of high-quality yellow-to-red emission InGaN micro-LED, by increasing the indium incorporation rate and reducing the lattice mismatch for later Inrich quantum wells.