# Impact of Device Topology on the Performance of High-Speed 1550 nm Wafer-Fused VCSELs
## 2 VCSEL Structure and Fabrication  
Undoped DBRs consisting of 22.5 and 35.5 pairs of Al\({}_{0.9}\)Ga\({}_{0.1}\)As/GaAs layers are used to confine an InAlGaAs/InP optical cavity with the length of 2.5\(\lambda\). The optical cavity and DBR structures were grown by MBE on a Riber 49 mass production 4-inch system at Connector Optics LLC. We excluded using an n-InGaAsP layer due to possible problems with the reliable formation of ohmic contacts and the resulting additional potential barriers for carriers, which might lead to higher operating voltages and an increased temperature dependence of the current-voltage characteristics [38]. The longitudinal distribution of electromagnetic field intensity as well as the schematic cross-section of the microcavity region are presented in Figure 1. We used 10 of the 2.4 nm thick compressively strained In\({}_{0.74}\)Ga\({}_{0.26}\)As QWs grown with 1.44% lattice mismatch, \(\varepsilon\), separated by 7 nm thick In\({}_{0.53}\)Al\({}_{0.16}\)Ga\({}_{0.31}\)As barrier layers lattice-matched (\(\varepsilon=0.02\%\)) to InP as the active region. It should be noted that for the MOVPE-grown 1550 nm WF VCSEL, tensile-strained barriers (\(\varepsilon=-0.8\%\)) were previously used together with compressively strained thick InAlGaAs QWs (\(\varepsilon=1.0\%\)) [34] to compensate the total stress of the active region. The optical cavity and DBR thicknesses were chosen such that the cavity resonance was shifted to a longer wavelength by \(\sim\)20-30 nm relative to the active region gain peak (so-called gain-to-cavity detuning, GCD). Previously, it was shown that small GCD allows one to increase the D-factor by better matching the differential gain peak in respect to the gain peak [28]. Figure 1: The 1550 nm InGaAs QW-based WF VCSEL: longitudinal distribution of electromagnetic field intensity of the fundamental mode and refractive index (left panel) and zoom of the refractive index and doping profile of the optical cavity (right panel). The schematic cross-section of the microcavity region shows the distributed Bragg reflector, intracavity contact layer, active region, tunnel junction. The n-InGaAs layer close to the QWs is used to obtain an ohmic contact to the lower n-doped IC layer, to reduce free-carrier absorption and to control the etching process to stop at high doping level (selective wet etching). Dependence of the effective refractive index contrast \(\Delta n_{\text{eff}}\) on the etching depth \(\Delta L_{\text{TJ}}\) of the composite TJ. Figure 1 is adapted, with permission, from Ref. [39], 2023, IEEE. Buried tunnel junctions (BTJs) are used for transverse current/optical confinement. Usually, both for MOVPE- and MBE-grown 1550 nm VCSELs, n\({}^{+}\)/p\({}^{+}\) AlGaInAs TJs are used to minimize internal optical losses [34; 35]. For the MBE regrowth of TJs, a problem related to the oxidation of InAlGaAs surfaces during TJ mesa etching occurs. In general, hydrogen cleaning procedures [40; 41] can resolve this problem if an RF plasma source is installed into an MBE system providing atomic hydrogen. Here, we used our original composite (multi-component) In(Al)GaAs TJ (n\({}^{+}\)/p\({}^{++}\)/p\({}^{++}\) 37 nm thick In\({}_{0.53}\)Ga\({}_{0.47}\)As/In\({}_{0.53}\)Ga\({}_{0.47}\)As/In\({}_{0.53}\)Al\({}_{0.16}\)Ga\({}_{0.31}\)As) aimed to eliminate the hydrogen cleaning process during the MBE regrowth of TJs. The success of n\({}^{+}\)GaInAs/p\({}^{+}\)AlGaInAs TJs for 1550 nm VCSELs was previously attributed to the Burstein-Moss effect in n\({}^{+}\)-InGaAs [31]. Using photolithography and wet etching of a surface relief the composite TJ is formed, which is a set of circular mesas with a well-controlled diameter and etching depth \(\Delta L_{\text{TJ}}\). The surface relief of the TJ layer was overgrown by MBE with a top n-type IC layer with a modulated doping profile. The surface relief parameters provide a compromise between the small mode volume required to increase the VCSEL high-speed performance and a large single-mode output power.