Submicron-thick microcavity InGaN light emitting diodes

ABSTRACT We report on submicron-thick microcavity light emitting diodes (MCLEDs) emitting at the wavelengths of 415 nm ~ 460 nm. These devices were fabricated by flip-chip-bonding, laser lift-off, and thinning processes. Growth of a high-quality Al x Ga 1-x N interlayer and etch selec tivity between N-face GaN and Al x Ga 1-x N allowed high-precision control of microcavity thickness, resulting in controlled microcavity effects. Single Fabry-Perot modes confined in 2 ~ 2.5 -thick MCLEDs gave rise to characteristic angular emission, in contrast to a Lambertian emission. High current operation (~100 mA) showed robustness of these thin devices with prom ising the possibility of high-brightness application. We will discuss design and processing issues regarding photonic-crystal integration towards higher improvements in light extraction efficiency. Keywords: GaN, LED, microcavity, photonic crystal. 1. INTRODUCTION Light-emitting diodes (LEDs) based on GaN-based material systems hold much promise in energy-efficient solid-state lighting, display and projection applications, optical interconnects, and optical communication through plastic optical fibers. A typical GaN-based LED is a planar st ructure with a large lateral dimension (~300 P m or larger) and a thin (~5 P m) GaN layer on the insulating sapphire substrate. For many applications, a direct vertical light emission through the large top surface is desirable, as it can maximize the output power and the out coupling efficiency. A vertical light extraction scheme can eliminate the need for additional optics to redirect the light and the optical loss originating from such components. Furthermore, the optical path for the vertical emission is much shorter than that for the lateral emission through the cleavage, minimizing the optical loss by the constituting LED materials and metal contacts. However, the vertical light emission is significantly limited by the total internal reflection at the air/material interface, which leads to strong guiding effect in the thin GaN between the air superstrate and the sapphire substrate. Indeed, the vertical emission of conventional planar LEDs bears some attributes of a weak microcavity due to two low-reflectivity mirrors, i.e., a top GaN/air and a bottom GaN/ sapphire interfaces. This planar LED structure supports Fabry-Perot modes for the generated light impinging the Ga N/air and GaN/sapphire interfaces with the angles smaller than the Brewster angles. However, the cavity effect is mediocre due to low cavity finesse and narrow mode spacing. Therefore, the overall emission profile resembles the emission profile from a point source without any strong cavity signature. Considering that the Brewster angles are very small, a larger portion of the generated light should be trapped in the planar LED structure. The planar LED structure serves as a planar waveguide, holding the trapped light in the form of multiple guided modes. These guided modes should trespass a long distance to emit through cleavages. In doing so, the guided modes undergo serious absorption loss, which leads to a side effect such as heating. The light extraction efficiency can be substantially improved through the formation of a microcavity LED (MCLED) or resonant cavity LEDs [1-7]. In particular, MCLEDs supporting a single Fabry-Perot mode can provide unique control of microcavity effects, such as the control of spontaneous emission, spectral purity, and emission profile. Recently submicron-thick InGaN MCLEDs have been realized by flip-chip bonding, laser lift-off (LLO), nonselective dry etching,