# Distributed Bragg Reflectors for GaN-Based Vertical-Cavity Surface-Emitting Lasers
## 2 Fabrication Techniques and Design Considerations of Distributed Bragg Reflectors (DBRs)  
### Fabrication Techniques of DBRs  
Prior to 1980s, all of the highly reflective DBR mirrors were produced by the deposition of amorphous dielectric layers with oxides (SiO\({}_{2}\), HfO\({}_{2}\), and Ta\({}_{2}\)O\({}_{5}\)), nitrides (SiN\({}_{x}\)), and fluorides (MgF\({}_{2}\),... ) [21]. This was a branch in the mature field of optical coating [22] for interference filters, anti-reflective coatings, mirrors, and absorbers. In this article we refer this approach of fabricating DBR mirrors as non-epitaxial or dielectric DBRs, interchangeably. After the conceptual proposal of VCSEL by [PERSON] in 1977 [23], dielectric DBRs and/or metal reflectors were the only options toward the realization of this novel device [24; 25; 26; 27]. However, since the semiconductor gain region (a double heterostructure, DH, or multiple quantum wells, MQWs) of a VCSEL needs to be prepared epitaxially on a single crystalline substrate, major fabrication efforts had to be exerted to remove the single crystalline substrates, or to expose the backside of the active region, and to coat both the top and bottom surfaces of the gain region with non-epitaxial DBR mirrors. During this time the overall pursuit of VCSEL devices can be considered more of a laboratory curiosity than for technology development.  
With the advances in modern epitaxial technologies for III-V compound semiconductors, epitaxial growth of AlGaAs/GaAs DBR mirrors was first demonstrated in 1983 using MBE [12] and subsequently with OMVPE [28]. Epitaxial DBRs enable a continuous growth of a full VCSEL structure to greatly simplify the fabrication process. Furthermore, semiconductor epitaxial DBRs could be made electrically and thermally conductive, thus producing significant benefits to the VCSEL performance. With these stated benefits, nevertheless, the pathway to manufacturable DBRs is by no means straightforward. In the epitaxial growth of DBRs, an implicit requirement is the hetero-epitaxy of two \(\lambda\)/4 layers havingsufficiently contrasting dielectric property in order to induce appreciable reflection at each interface. Several technical challenges are often reported in the epitaxial production of DBR mirrors:  
(1) In the case where the DBR layers consists of different cation elements such as Al, Ga, and or In, the difference in diffusion properties of surface adatoms [29] needs to be factored into growth design to ensure two-dimensional growth modes [30; 31].  
(2) In the case where the DBR layers consists of different anion elements including As, P, or Sb, the difference in surface reactivity and sticking coefficients needs to be carefully monitored and controlled [32; 33],  
(3) In the use of ternary or quaternary alloys as DBR layers, it is crucial to maintain uniformity in compositions and thicknesses across the entire wafer [34; 35; 36],  
(4) As a result of (1)-(3), a need to use different, and sometimes compromised growth conditions for the two layers, which often results in interfacial disorder or deterioration in surface morphology [37; 38], and  
(5) Last but not least, a difference in lattice parameters between the constituent layers leading to high-tensile or compressive strains during the growth of DBRs [39; 40].  
Given these challenges, growth of highly-reflective DBRs is often considered both the most challenging task and a symbolic gauge of maturity in epitaxy for a specific heteroepitaxial system. We also want to add that the success in epitaxial DBRs has laid a foundation for modern integrated optics [41] and even quantum optics [42].