Pattern-Coverage Effect on Light Extraction Efficiency of GaN LED on Patterned-Sapphire Substrate

Abstract

GaN-based LEDs were fabricated on patterned-sapphire substrates with various pyramidal-pattern coverage percentages (31-81%). Three distinguish stages are observed in the behavior of the light out-put power against coverage percentage. By etching-pit density measurement, dislocation density is found to be independent with coverage percentage. The light extraction efficiency (LEE) should be the main mechanism for the enhancement in the light out-put power. LEE would be enhanced, when the pattern coverage is less than 39% or over 67%. The light extraction efficiency remains similar, when the pattern coverage in between 39% ∼ 67%.The sapphire substrate patterning technique has been widely used to improve the external quantum efficiency (EQE) of GaN-based LEDs. 1-10 The improvement obtained using the patterned-sapphire substrate (PSS) technique includes the enhancements of both the internal quantum efficiency (IQE) and light extraction efficiency (LEE). The LEE is enhanced because of the patterns on the PSS act to destroy the TIR (Total Internal Reflection) effect at the GaN/sapphire interface. 1, 3, 4 Several published literatures have pointed out that the light extraction efficiency could be increased by using periodic or non-periodic micron-sized patterning on sapphire substrate. 11-16 On the other hand, the patterns on the PSS can reduce the threading dislocations in the GaN epitaxial layers owing to the possible lateral growth of the GaN epi-layer. 2,5,6,8,17 Some recent studies indicated that the use of nano-scale patterned sapphire substrate leads to increase in IQE, which is resulted from the significant reduction in the threading dislocation density (two-orders of magnitude reduction) and reduction in the fraction of the screw dislocation density. 18-21 Besides, some researchers also indicate that IQE can be improved by QW designing (for examples, non-polar QW and large overlap QW). [22][23][24][25][26][27][28][29] Generally, it seems that the greater the pattern coverage created on the PSS, the higher the LEE would be. 30 However, there so far been no any experimental study or systematically investigation of the effect of the pattern coverage of the PSS on the and LEE. Another key issue that is not yet well understood is how does the pattern coverage (or c-plane coverage) influence the IQE of a GaN-based LED on the PSS. In this study, a mask-free wet-etching process is used to produce a so-called naturally patterned-sapphire substrate (n-pss). 31 By varying the etching time and temperature, different amount of pyramidal pattern coverage can be created on the n-pss wafers. The effect of the pyramidal pattern coverage on the IQE and LEE of the GaN epi-layers can be studied.The mask-free wet-etching process used to produce the pyramidal n-pss wafers is described in detail below. Before the wet-etching process, a PECVD (Plasma Enhanced Chemical Vapor Deposition) SiO 2 layer was deposited on the back-side of the c-plane sapphire wafers to prevent the back-side of sapphire wafers from being etched. The flat back-...

Improved performance of InGaN/GaN flip-chip light-emitting diodes through the use of robust Ni/Ag/TiW mirror contacts

“…The enhancement of IQE has been generally realized by tailoring the epitaxial structure, [3][4][5][6][7] or incorporation of plasmonic nanostructures. [8][9][10] Meanwhile, LEE can be improved by patterning the substrate, 11,12 using epitaxial structure 13,14 or current spreading layers, 15,16 fabricating different LED configurations (conventional, flip-chip or vertical), or including photonic crystals. [17][18][19] Flip-chip LEDs (FCLEDs) possess a suitable configuration for high-power devices because of their effectiveness at increasing LEE and heat dissipation.…”

Enhanced Light Output Power and Growth Mechanism of GaN-Based Light-Emitting Diodes Grown on Cone-Shaped ${\hbox{SiO}}_{2}$ Patterned Template

“…Among all of the above technologies, PSS has become more and more popular in the current LED industry due to the threading dislocation density in the GaN epilayer can be effectively reduced by ELOG and the micron-scaled patterns act as scattering centers for the light trapped inside the LED structures [19]- [21]. Moreover, the pattern density of micron-scaled PSS had been shown to have strong effect on the LEE in GaN-based LEDs [22]. In addition, it is noteworthy that some recent works indicate using nano-scaled PSS can also improve the IQE and the EQE of an InGaN-based LED [23]- [28].…”

Investigation and Comparison of the GaN-Based Light-Emitting Diodes Grown on High Aspect Ratio Nano-Cone and General Micro-Cone Patterned Sapphire Substrate

“…The surface patterning based on photonic crystals [19], [20] and self-assembled microlens arrays [21], [22] had been reported for achieving large increase in light extraction efficiency (LEE) in III-Nitride LEDs. Recently, the effect of pattern density in PSS had been also shown to affect the light extraction in GaN-based LEDs [23]. Although the technology of NPSS has been reported to play an important role for improving LEDs efficiency, there are few reports on the relation between the efficiency and the NPSS geometry.…”