Comparison of the Performance of Lateral and Vertical InGaN/GaN-Based Light-Emitting Diodes with GaN and AlN Nucleation Layers

Abstract

We investigated the effects of in situ GaN and sputtered AlN nucleation layers on the output power of GaN-based blue (445 nm) LEDs on patterned sapphire substrate (PSS) and planar sapphire substrates. Both the PSS LEDs and planar LEDs showed the same operation voltages, but the LEDs with the AlN layer had much lower reverse leakage current at −15 V. The LEDs with the AlN layer showed higher external quantum efficiency (EQE) at low current regions than those with the GaN nucleation layer. However, the AlN nucleation layer LEDs experienced a little more severe efficiency droop than did the GaN layer LEDs. It was shown that the AlN nucleation layer samples contained fewer defects than the GaN nucleation layer samples. The Raman spectral results showed that the AlN nucleation layer samples experienced higher compressive stress than the GaN nucleation layer samples. Simulation results also showed that both the AlN nucleation layer and air voids in the GaN nucleation layer samples hardly affected the light extraction efficiency. Based on the electrical, X-ray diffraction (XRD), cathodoluminescence (CL), and Raman results, the electrical behavior of the PSS and planar LEDs are described and discussed. InGaN/GaN-based light-emitting diodes (LEDs) are technologically important because of their use in a variety of emitting device applications, which cover wavelengths from green to ultraviolet (UV) spectra.1-4 Currently, high-brightness InGaN/GaN-based LEDs are commercially available. On the one hand, in order to realize general illumination application, the output performance of LEDs should be improved further. One of the major barriers to the enhancement of the light output efficiency of GaN-based LEDs is a high density of threading dislocations (TDs) (in the range of 10 8 -10 10 cm -2 ), 5 which are generated during GaN growth because of the large lattice mismatch and thermal expansion coefficient difference between GaN and sapphire substrate.6,7 Thus, it is crucial to develop ways of reducing TD density to enhance the light output performance. To minimize the generation of TDs, different methods have been proposed, including epitaxial lateral overgrowth (ELOG), [8][9][10][11] SiO 2 patterned mask, 12,13 and ELOG on patterned sapphire substrate (PSS). 14-19 Among them, ELOG on PSS are most commonly used to grow high-quality GaNbased epitaxial layers. For example, Kissinger et al., 15 investigating the output performance of blue LEDs fabricated on lens PSS (LPSS), reported that LPSS-LEDs exhibited 250% higher luminous intensity at an injection current of 20 mA than conventional LEDs and 117% higher emission angle than conventional LEDs. Wuu et al., 18 investigating the effect of hole-PSS on the output performance of near-UV GaN-based LEDs, reported that typical lamp-form PSS-LEDs had an external quantum efficiency of 14.1%, which is higher than that of conventional LEDs. The improvement was attributed to the decrease in TD density and the increase in light extraction efficiency. It is well known that the nucleation layer...

Insights into the Silver Reflection Layer of a Vertical LED for Light Emission Optimization

“…In this regard, gallium nitride (GaN)-based vertical light-emitting diode (VLED) has been developed, which possesses superior light emission property compared with the lateral LED (LLED) architecture, including large light-emitting area by using one top electrode (n-side up and p-side down), mitigation of current-crowding problem, small series resistance (vertical current injection), better heat dissipation via a metal-based substrate or bendable substrateless techniques, higher light-extraction efficiency by roughening the top n-GaN surface, and better reflection of downward-absorbing light by placing a mirror layer on the top of the substrate. − Despite these progresses, VLED still needs improvement, particularly on the luminous efficacy and admittedly external quantum efficiency (EQE). The EQE (η EQE ) is the product of the injection efficiency η inj , the internal quantum efficiency (IQE) η IQE , and the light extraction efficiency (LEE) η extr , that is, η EQE = η inj ·η IQE ·η extr .…”