# Distributed Bragg Reflectors for GaN-Based Vertical-Cavity Surface-Emitting Lasers
## 5 Airgap and Porous DBRs--A New Approach to Epitaxial DBR
### Nanoporous GaN/GaN DBRs  
Recently the development of nanoporous (NP) GaN has received much attention. NP GaN can be considered as a nano-composite of perfect single crystalline GaN filled with nanoscale, inter-connected tubes that produce an overall foam or sponge-like texture. A vast amount of literature exists on the formation mechanisms of anodic aluminum oxide [105; 106; 107], porous silicon [108; 109; 110; 111], and silicon carbide [112; 113; 114]. The morphology of porous textures and patterns is determined by the highly inhomogeneous etching process, which consists of successive steps including carrier transport in the space-charge (SC) layer, oxidation at the semiconductor surface, transport of ions in the oxide layer (OL), field-enhanced dissolution of oxide at the oxide-electrolyte interface, and ionic transport within the interfacial double layer (DL) [115]. Figure 12a shows a cross-sectional scanning electron microscopy (SEM) image of a porous GaN DBR near the center of the cross-shaped alignment mark. A close-up SEM image of the same structure is shown in Figure 12b with the NP-GaN having a porosity of ~70% and an average pore size of 30 nm. The electrochemistry is highly selective based on the conductivity of and etching bias applied to GaN [116]. The non-porous layer corresponds to lightly n-doped GaN, and the porous layers correspond to n\({}^{++}\)-GaN before etching. The NP GaN region serves as a  
Figure 11: (**a**) Schematic image (view from inside); (**b**) SEM image of the fabricated nitride air-gap DBR micro cavity (MC) (3 top DBRs/MC/4 bottom DBRs); (**c**) close SEM view of the edge [104]. low-index medium for the producing of \(\lambda\)/4 reflector. Highly reflective (>99.5%) DBR mirrors in the blue (440 nm), green (520 nm), and red (600 nm) wavelength range have been demonstrated by changing the layer thicknesses (Figure 12c). The epitaxy of n-GaN/n\({}^{++}\)-GaN structures are straightforward when compared with the challenges in growing Al(GaN) or AlInN DBRs. Nanoporous GaN DBR mirrors with good uniformity are shown under room-light illumination in Figure 12d. Unique advantages of using n-type doping profile to create nanostructured GaN layers include (i) complete lattice matching to conventional GaN structures with negligible degradation in microstructure or morphology, (ii) a high tunability in the index of refractive index, and (iii) the feasibility of electrical injection through n-type NP GaN layers. Using NP GaN, new freedom is introduced in optical engineering microcavity design without the constraints in epitaxy or complex fabrication. VCSELs with NP GaN based DBR mirrors can be produced with no need for complicated lift-off or flip-chip processes. These properties combined present a unique opportunity to build GaN VCSELs. High-reflectance NP GaN DBR mirrors [117; 118; 119; 120; 121; 122], optically pumped VCSELs [117; 118], and resonant-cavity LEDs (RC-LEDs) [123; 124] have been demonstrated by many groups in recently years. The ultimate viability of NP GaN DBRs will be determined by their long-term stability against thermal and electrical stresses. Another common concern with the use of porous medium in semiconductor lasers is the potentially limited thermal conductivity due to a reduced phonon mean free path [125]. Further studies regarding the design tradeoffs between porosity (for index engineering) and the thickness of pore walls [126] for enhanced thermal and electrical transport will be critical. Lastly, during the preparation of this review, an electrically injected VCSEL including NP GaN DBR has also been reported to provide initial validation of this novel concept [127]. Figure 12: Cross-sectional SEM images of (**a**) a NP GaN/GaN DBR structure, (**b**) NP GaN/GaN DBR at a higher magnification, (**c**) Reflectance from three NP-GaN DBRs in the blue, green, and red wavelength regions. Black dashed curve is the simulated reflectance spectrum of the blue DBR.