# Distributed Bragg Reflectors for GaN-Based Vertical-Cavity Surface-Emitting Lasers
## 3 Epitaxial DBRs for GaN Vertical Cavity Surface-Emitting Lasers (VCSELs)
### AlInN/GaN DBRs  
Ternary AlInN is an interesting alloy in the III-nitride family that is much less explored compared with ternary AlGaN. Early work of MOCVD growth of AlInN [70; 71] identified the challenges in the growth of this ternary compound due to both the large mismatch between InN and AlN covalent bonds, as well as a vast difference in optimal growth conditions between InN (\(\sim\)600 C) and AlN (\(>\) 1100 \({}^{\circ}\)C) using MOCVD. In spite of these difficulties, AlInN with an Al fraction of around 80% has drawn a lot of attention [72] due to its ability to be made completely lattice-matched to GaN, raising the hope of a duplicate of the success in AlGaAs/GaAs material system.  
In 2003, the Ecole Polytechnique Federale de Lausanne (EPFL) demonstrated lattice-matched AlInN(Al \(\sim\)0.82)/GaN distributed Bragg reflectors (20 pairs) with a peak reflectance of \(\sim\)90% [44]. Two years later they improved the reflectance to 99% and demonstrated a planar microcavity with top and bottom epitaxial AlInN/GaN DBRs [73].  
The typical growth rate of AlInN by OMVPE was very low (less than 0.2 \(\upmu\)m/h) due to vastly different supersaturation (6 orders of magnitude! [44]) of Al and In species at growth surface. A narrow growth window is typically identified to simultaneously allow a very limited diffusion of Al adatoms while accommodating the incorporation of In atoms. Due to such a low growth rate, it can take up to one full day to synthesize a 40-pair AlInN/GaN DBR with over 99% of peak reflectance. After almost 10 years from their initial report of AlInN, EPFL reported an electrically injected GaN VCSEL in 2012 with a bottom Al\({}_{0.82}\)In\({}_{0.18}\)N/GaN DBR and a top TiO\({}_{2}\)/SiO\({}_{2}\) DBR [74], attesting the inherent challenges in growing this ternary compound.  
Figure 6: Transmission electron microscope (TEM) cross-sectional images of AlN/GaN DBR grown on sapphire substrate (**a**) with AlN/GaN SLIL, (**b**) without AlN/GaN SLIL, and (**c**) the corresponding reflectance spectrum [69].  
Subsequently, AlInN DBRs were pursued by two groups. Otto-von-Guericke-Universitat Magdeburg studied the critical conditions of lattice matching between AlInN and GaN [75]. They also proposed a two-temperature growth procedure [76] and reported the need to use GaN capping layer to improve the interfacial transitions. Meijo University performed very detailed study of OMVPE growth conditions for AlInN and identified a new window for high growth rate (\(\sim\)0.5 \(\upmu\)m/h) [77]. The growth condition for a high growth rate requires short gas mixing time, low In/Al ratio and relatively high growth temperature. With such a growth condition, Meijo University demonstrated AlInN/GaN DBRs with more than 99% reflectance in a much-reduced growth time (\(\sim\)12 h) (Figure 7).  
In 2016, Meijo University achieved high conductivity in AlInN/GaN DBRs by using modulation doping and composition grading at the GaN/AlInN interfaces [80]. Immediately after the demonstration, they developed VCSELs with conductive AlInN/GaN DBRs with back-side contact, showing the feasibility of GaN-based VCSELs with vertical current injections through conducting DBRs [78]. Since 2017, Stanley Corporation, in collaboration with Meijo University, has further advanced the performance of AlInN-based GaN VCSELs with record high optical power and slope efficiencies [81]. The current trend in AlInN/GaN-based VCSEL structure appears to shift toward longer cavity and non-conductive DBRs [79] toward manufacturing of high-power VCSEL devices for lighting applications. Large-scale commercial manufacturability of VCSEL devices involving AlInN DBRs should become clear in the next few years.