# InGaN Resonant-Cavity Light-Emitting Diodes with Porous and Dielectric Reflectors
## 3 Results  
In Figure 1a, the bottom porous-GaN/n-GaN DBR with blue light image and the laser scribing lines were observed by using optical microscope. The porous-DBR stacked structure consisted of a 55 nm thick, porous-GaN layer and a 45 nm thick GaN layer approximately in the cross-sectional SEM micrograph as shown in Figure 1b. The etching interface of the porous-GaN/n-GaN stack structure was observed clearly after the doping-selective electrochemical wet etching process. The 148 nm-thick LED structure with the InGaN MQW layers was shown in Figure 1c as the epitaxial cavity structure above the porous DBR structure. In our previous reports [19], the InGaN-based LED consisted of a large cavity length and an embedded nanoporous DBR structure without capping the top dielectric DBR structure. In this study, the 30 nm-thick Ta\({}_{2}\)O\({}_{5}\) spacer layer and 8.5-period Ta\({}_{2}\)O\({}_{5}\)/SiO\({}_{2}\) dielectric DBR were deposited on the LED epitaxial layer as shown in Figure 1d. The resonant cavity consisted of the epitaxial InGaN active layer and the deposited Ta\({}_{2}\)O\({}_{5}\) spacer layer between top dielectric DBR and bottom porous DBR structures. In Figure 2, the reflectance spectra of the top Ta\({}_{2}\)O\({}_{5}\)/SiO\({}_{2}\) dielectric DBR and the bottom porous-GaN/n-GaN DBR structure were measured by using Xe lamp as a reference light source. The central wavelength, reflectivity, and the stopband width of the reflectance spectra were measured at 421.3 nm/98.1%/67 nm for the top dielectric DBR and at 434.3 nm/98.5%/55 nm for the porous DBR, respectively, listed in Table 1. The stopband width of the reflector is defined as the wavelength region that the reflectivity is higher than 90%. High reflectance and wide stopband width were observed on the top dielectric DBR and the bottom porous DBR structures. As per Figure 3, the photoluminescence (PL) spectra were measured through angle-resolved measurement with a 405 nm laser source. When the incident 405 nm laser illu  
Figure 1: **(a)** The OM image of the InGaN LED with bottom porous-GaN DBR structure was observed with a blue reflected light image. **(b)** The cross-sectional SEM micrograph of the porous- distributed Bragg reflector (DBR) structure was observed. **(c)** A 148 nm thick InGaN LED structure was shown above the porous reflector. **(d)** The top reflector consisted of a 30 nm thick Ta\({}_{2}\)O\({}_{5}\) spacer layer and a top Ta\({}_{2}\)O\({}_{5}\)/SiO\({}_{2}\) DBR structure. Figure 2: Reflectance spectra of top dielectric DBR and bottom porous-GaN DBR were measured at room temperature. \begin{table}
\begin{tabular}{c c c c} \hline  & **Wavelength** & **Reflectivity** & **Stopband Width** \\ \hline Top dielectric DBR & 421.3 nm & 98.1\% & 67 nm \\ \hline Bottom porous DBR & 434.3 nm & 98.5\% & 55 nm \\ \hline \end{tabular}
\end{table} 
Table 1: Top and bottom DBR structures. minated from the backside of the LED samples, the laser light will not be absorbed by the sapphire substrate, the n-type GaN:Si layer, and the p-type GaN:Mg layer at a normal direction. Only the InGaN active layers were excited and emitted the photoluminescence for the angle-resolved PL measurement. Then, the PL emission spectra were measured from the front-side of the LED chips. In Figure 3a, the Fabry-Perot (FP) interference line patterns of the non-treated LED (ST-LED) structure were observed with multiple curves due to the light reflected at the flat top GaN:Mg/Air interface and the bottom GaN/Al2O3 interface. The thickness of the epitaxial layers was calculated as the value of 6.1 \(\upmu\)m for the ST-LED, which was similar to the actual epitaxial thickness grown on sapphire substrate. In non-matched RC-LED structure, 8.5-period Ta2O5/SiO2 dielectric DBR layers were deposited on the LED structure directly without a spacer layer. The peak intensities of the far-field pattern measured at 72\({}^{\circ}\) and 108\({}^{\circ}\) detected angles that were higher than it at normal direction (90\({}^{\circ}\)) as shown in Figure 3b.