Light Extraction Enhancement of Gallium Nitride Epilayers with Stripe Pattern Transferred from Patterned Sapphire Substrate

Abstract

In this study, the epilayers of gallium nitride light-emitting diode grown by metallorganic chemical vapor deposition on the stripe-patterned sapphire substrate prepared by chemical wet etching were characterized. The stripe pattern on vertical GaN/Si surface transferred by wafer bonding and laser lift-off techniques from the patterned sapphire substrate contributes much stronger photoluminescence intensity.Gallium nitride ͑GaN͒ blue light-emitting diodes ͑LEDs͒ have been commercially available and widely used as the light sources for backlighting, traffic signals, outdoor displays, and head lamps of automobiles. It is well known that GaN grown on sapphire has high dislocation density due to 16% lattice mismatch, which will degrade GaN characteristics, such as electron mobility, optical quantum efficiency, and carrier and device lifetimes. Therefore, the reduction of dislocation density is an important issue for the improvement of GaN optical and electrical qualities. Many approaches were proposed to reduce the dislocation density of GaN. 1,2 The lateral epitaxial overgrowth with SiN x or SiO 2 stripe-type mask is a common technique and can decrease the dislocation density to the range of 10 7 cm −2 . 3,4 Meanwhile, the GaN epitaxial layer grown directly on the stripe-patterned sapphire substrate ͑GaN/PSS͒ along ͑11-20͒ 5 or ͑1-100͒ directions 6 can have lower dislocation density. By this technique, the contamination from the mask can be prevented and the epilayer growth process is simplified.The poor power dissipation of GaN/sapphire LEDs due to the poor thermal conductivity of sapphire substrate leads to higher junction temperature and degrades its performance. By using wafer bonding and laser liftoff ͑LLO͒ techniques ͑LLO GaN/Si͒, the GaN/sapphire LED can be transformed into a vertical-type GaN/silicon LED. This structure can provide higher performance due to higher thermal and electrical conductivities of silicon substrate. In addition, the larger light-emitting area of the vertical GaN/Si LED can increase the optical output power. [7][8][9] Meanwhile, the texture surface was used on GaAs LEDs to improve the light extraction efficiency. 10 The texture structure was also used on p-GaN 11,12 or n-GaN surfaces 13 of GaN/sapphire LED to improve the light extraction efficiency. However, an additional etching process is needed to form the texture structure and the reproducibility is also an issue. In this study, the stripe-patterned PSS substrate was prepared by chemical wet etching. The LLO technique was used to transform GaN/sapphire into a vertical-type GaN/Si ͑LLO-PSS GaN/Si͒. The stripe-patterned texture structure was obtained simultaneously. Higher optical quality from the stripepatterned texture structure on the surface of LLO-PSS GaN/Si was expected. ExperimentalThe GaN epilayers of a simple LED structure were used in this experiment. Figure 1 shows the fabrication of the GaN/Si structure prepared by sapphire chemical wet etching, GaN epilayer growth, wafer bonding, and LLO techniques. A 2 in., 430 m thick ...

Improving InGaN-LED performance by optimizing the patterned sapphire substrate shape

“…Currently, GaNbased LEDs grown on patterned sapphire substrates (PSSes) have been studied by more and more researchers. [9,10] The PSS, which is a mask-free and therefore contamination-free method, reduces the threading dislocation density in the GaN epilayers and enhances the light extraction efficiency of the LED. The TD density reduces significantly due to the introduction of the ELOG and its combination with the selective area growth (SAG) in the epitaxial processes on the PSS.…”

Laser Lift-Off Mechanisms of GaN Epi-Layer Grown on Pattern Sapphire Substrate

“…5,6,9 It is obvious that a combination of the advantages of PSS-LED with the thin-GaN LED process will improve the LED performance. [10][11][12][13][14][15] However, compared with conventional flat thin-GaN LED (FT-LED), the critical laser energy of PT-LED was high, 14 the yield rate was very low, and the leakage current was high. 15 In this study, the requirement of high laser energy and the root causes of low yield rate and high leakage current were investigated.…”

Surface Modification on Wet-Etched Patterned Sapphire Substrates Using Plasma Treatments for Improved GaN Crystal Quality and LED Performance

“…Patterned sapphire substrates (PSSs) fabricated using dry-or wet-etching methods are another method of obtaining high quality GaN films. [6][7][8][9][10][11][12] PSSs can reduce TDs via a single growth process without an interruption layer. In addition the light extraction efficiency (LEE) of LEDs grown on PSSs is increased by enhanced light scattering off the PSS.…”