# Long-Wavelength VCSELs: Status and Prospects
## 2 Distributed Bragg Reflectors for the 1300-1550 nm Spectral Range  
Monolithic DBRs lattice-matched to InP based on InGaAsP/InP [17; 18], AlInGaAs/AlInAs [19], or AlGaInAs/InP [20] heterostructures suffer from very low refractive index contrast (\(\Delta\)n). At 1550 nm, the refractive index contrast is \(\sim\)0.3. At 1300 nm, \(\Delta\)n is only \(\sim\)0.2 [21], since the band gap of the layers must be increased to avoid absorption. Larger maximum \(\Delta\)n values are provided by AlAs/GaAs pairs (\(\Delta\)n \(\sim\) 0.5) and AlGaAsSb/AlAsSb pairs (\(\Delta\)n \(\sim\) 0.49). Growth of, e.g., a bottom mirror with 99.9% reflectivity requires epitaxy of 63 pairs of InAlGaAs/InP (\(\Delta\)n \(\sim\) 0.19) [21]. DBRs on GaAs substrates provide the same reflectivity with only 23 AlAs/GaAs pairs. AlAs/GaAs can also be used for the 1550 nm spectral range. AlGaAsSb-based DBRs combined with aperture formation based on an air gap fabricated by selective etching of an imbedded tunnel junction (TJ) allowed the development of LW VCSELs [22]. A disadvantage of this approach is the low thermal conductivity of such mirrors, which leads to early turn-over of the output power with increasing current and their predisposition to oxidation. An alternative approach for mirrors with high reflectivity for LW VCSELs is DBRs based on an InP/Air gap combination [21]. The refractive index contrast in such DBRs is 2.2, and only three pairs of quarter-wavelength layers are required to reach 99.9% reflectivity. However, the thermal and current conductivities of InP/Air pairs of such mirrors are extremely low. Only configurations with double intra-cavity contacts and a thick heat dissipation layer based on InP were reported until now. In addition, a design with air gaps is quite fragile. Since the thermal resistance of DBRs is directly proportional to the number of pairs in the DBR and also depends on the composition of the layers [23], dielectric DBRs are also advantageous as compared to DBRs based on AlAs/GaAs while achieving the same mirror reflectivity. Hybrid DBRs combining several pairs of dielectric DBRs and a layer of gold enable a compromise from the point of view of minimizing the thermal resistance of LW VCSELs DBRs. The thermal conductivities of various materials have been studied, such as amorphous silica (a-Si), MgF\({}_{2}\), CaF\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and ZnS [23] ranging from 6.83 WK\({}^{-1}\)m\({}^{-1}\) (for ZnS [23]) to 2.4 WK\({}^{-1}\)m\({}^{-1}\) (for a-Si [23]). Al\({}_{2}\)O\({}_{3}\)/a-Si pairs show the best thermal behavior, but \(\Delta\)n is only 1.6 and high loss is typical. 2.5 pairs of CaF\({}_{2}\)/a-Si (\(\Delta\)n = 1.9) provide a thermal resistance of less than a third of the value of the corresponding epitaxial mirror (7.4 \(\times\) 10\({}^{3}\) K/W) [23; 24]. 2.5 MgF/a-Si pairs (\(\Delta\)n = 1.9) enable reducing the thermal resistance down to 5.6 \(\times\) 10\({}^{3}\) K/W [25; 26]. However, due to significant losses of a-Si at 1550 nm (400 cm\({}^{-1}\)[23; 25]), 2.5 pairs of CaF\({}_{2}\)/ZnS are used [2] to develop hybrid DBRs for 1550 nm VCSELs with 99.85% reflectivity. The thermal resistance here is only 6.8 \(\times\) 10\({}^{3}\) K/W. In general, before p-type contact fabrication, natural oxides are removed from the contact layer surface by wet etching in diluted hydrochloric acid (HCl) or ammonia (NH\({}_{3}\)). Moreover, the InP substrate can be removed by wet etching in HCl. Due to the solvent possibility of CaF\({}_{2}\) in HCl and NH\({}_{3}\) in further works, authors began using DBRs based on AlF\({}_{3}\)/ZnS for the development of 1550 nm VCSELs. AlF\({}_{3}\)/ZnS DBRs are compatible with the HCl cleaning process but have a lower \(\Delta\)n (about 1.0) in relation to CaF\({}_{2}\)/ZnS [27]. The penetration depth in AlF\({}_{3}\)/ZnS DBR is about 300 nm [28]. The use of five AlF\({}_{3}\)/ZnS pairs provides a reflectivity of 99%. Here, the mirror thickness is about 2.4 \(\upmu\)m, as compared to 7 \(\upmu\)m for a semiconductor DBR based on 30 AlGaInAs/AlInAs pairs [29].