Emission Intensity Improvement of InGaN Ultraviolet Light-Emitting Diodes Grown on Wet-Etched Sapphire Substrates

摘要

High-brightness and high-efficiency GaN-based light-emitting diodes (LEDs) have attracted great attention because of their vital roles in full-color display and solid-state lighting. For the latter application, ultra-violet LED (UV LED), which pumps red-green-blue phosphors to generate white light, is expected to give good color rendering and power conversion efficiency. However, high threading dislocation density, on the order of 10~9 cm~(-2), in LEDs grown on sapphire substrates degrades the internal quantum efficiency and lifetime significantly [1]. In particular for UV LEDs which have less carrier confinement quantum structure, their emission efficiency is more sensitive to dislocation density than the blue and green ones. Reducing dislocation density in the active layer is thus an important task for developing high-brightness UV LEDs. There are many growth techniques, such as epitaxy lateral overgrowth (ELOG) [2], pendeo epitaxy [3], facet-controlled epitaxial lateral overgrowth (FACELO) [4], and lateral epitaxial on patterned substrates (LEPS) [5], have been proposed to reduce the dislocation density in GaN epilayers. Although the overgrowth technique can dramatically improve crystalline quality, the requirement of two-step growth procedure is time-consuming and yield-killing. Maskless patterned sapphire substrate (PSS) fabricated by dry etching process can alleviate the aforementioned issues to some extent. However, the dry etching process is slow and causes surface damage. In this work, a wet etching process with high etching rate is used to fabricate PSSs. The optical properties of 400 nm UV LEDs prepared on the PSSs with different etching depth are investigated. Figurel shows the etching pits density and room temperature photoluminescence (PL) integrated intensity of GaN epilayer grown on the stripe-PSS with different etching depths. The etch pits density decreases and the PL intensity increases with the etching depth of stripe-PSS. It indicates that the material quality of GaN epilayer grown on deeper stripe-PSS is better. Transmission electron microscopy (TEM) is employed for detailed investigation on the dislocation distribution in the sample. Fig. 2 is the cross-section TEM image of a GaN epilayer grown on a 0.9 μm-deep stripe-PSS. Three regions denoted by A, B and C in this figure are noteworthy. In region A, misfit dislocations are generated at the GaN/sapphire interface as usual. In region B, dislocations bend during the vertical growth stage as they meet the inclined {11-22} facet. In region C, threading dislocations are generated due to the coalescence of the two growth fronts from the etched and un-etched regions. Accompanied with this growth process is the formation of voids at both sides of the ridge. Since the slope of the two sides is different, the size of the voids is different as well. Show in Fig. 3 is the cathodoluminescence (CL) mapping image of a GaN epilayer grown on a 0.9 μm-deep stripe-PSS by integrating the 364 nm GaN band edge emissions. The bright and dark regions are periodically distributed in accordance with the B and C regions in Fig. 2, respectively. Fig. 4 is the output power of the 400 nm UV LED grown on stripe-PSSs with different etching depths. The output power of the 400 nm UV LED can be greatly improved from 21% to 87% as the etching depth is increased from 0.2 to 0.9 μm. Fig. 5 is the normalized far field emission patterns of 400 nm LEDs grown on 0.9 μm-deep stripe-PSS and planar sapphire substrate. It is believed that more light is scattered toward the vertical direction by the stripe-PSS. Top-emitting image of 400 nm UV LED grown on 0.9 μm-deep stripe-PSS is inserted in the Fig. 5.