# Impact of Device Topology on the Performance of High-Speed 1550 nm Wafer-Fused VCSELs
## 2 VCSEL Structure and Fabrication
Although MBE allows one to reach a record pseudomorphic strain in QWs (up to 3.5% [47]), it is supported by tensile-strained barriers [48]. As a result, partial stress compensation in the active region is realized [49]. Such a large compressive strain in QWs is related to an increase in the Al mole fraction [50] and nonradiative Shockley-Read-Hall (SRH) recombination [51; 52]. The latter authors decreased the QWs strain to 1.2-1.7% [28] and aimed to decrease the Al mole fraction and changed to use weakly (\(\varepsilon=-0.02\%\)) tensile-strained (Al\({}_{0.274}\)Ga\({}_{0.203}\)In\({}_{0.523}\)As) barriers. Here, we decreased the compressive strain in relation to InAlGaAs QWs, and at the same time, we eliminated the SRH loss using quaternary InAlGaAs QWs. The serial resistance as well as the selectivity in the wet etching of the mesa were increased by using the bottom n-InGaAs and top n-InP layers instead of the bottom n-InGaAsP and top n-InGaAsP contact layers. Finally, we implemented a composite BT (n\({}^{++}\)/p\({}^{++}\)/p\({}^{++}\) In\({}_{0.53}\)Ga\({}_{0.47}\)As/In\({}_{0.53}\)Ga\({}_{0.47}\)As/In\({}_{0.53}\)Al\({}_{0.16}\)Ga\({}_{0.31}\)As TJ) as compared to the previous n\({}^{++}\)/p\({}^{++}\) In\({}_{0.53}\)Al\({}_{0.20}\)Ga\({}_{0.27}\)As TJ. The doping level of the n-InP overgrown layer was reduced, aiming to increase the modulation performance [48; 53; 54]. To summarize, the design of the WF VCSELs presented here shows some fundamental changes compared to previous MBE-grown 1550 nm WF VCSELs [35]. Scanning transmission electron microscopy (STEM) images of a WF VCSEL structure and a transmission electron microscopy (TEM) image of the active region are presented in Figure 3a. The TEM studies were carried out using a JEM-2100F transmission electron microscope with an accelerating voltage of 200 keV. The samples in the cross-sectional geometry were prepared using a standard method by cleaving the structure along (110) planes, the \"face to face\" gluing of 2 pieces (by epoxy resin), and the subsequent grinding of both (110) sides. At the final stage, the samples were thinned by Ar\({}^{+}\) ions at energies of 3-4 keV at an incidence angle of 7 degrees (by Nanomill 1050). These structural studies were performed using equipment of the Joint Research Center \"Materials science and characterization in advanced technology\" (registration No. : 3317) of the Ioffe Institute. According to the TEM data, after double wafer fusion, no noticeable smearing of the heterointerfaces of the strained quantum wells in the active region was observed. In addition, no formation of dislocations and other extended defects, as well as air voids associated with fused interfaces, were detected. One can find the details about the VCSELs fabrication process in previous publications [35; 42]. Figure 3b shows a schematic view of the device with the double-mesa structure. Figure 2: The 1550 nm InGaAs QW-based WF VCSEL: Dependence of the effective refractive index contrast \(\Delta t_{\text{eff}}\) on the etching depth \(\Delta L_{\text{TJ}}\) of the composite TJ (left panel); an atomic force microscopy image of the overgrown surface and schematic cross-section of the WF VCSEL heterostructure in the microcavity region (right panel). The 1 st mesa was formed by dry etching (inductively coupled plasma, ICP) the top DBR. The second one by two-step wet etching of the InAlGaAsP/InP cavity. Silicon nitride was used for insulation and as a protection/phase-matching layer deposited on the top DBR. The ohmic contacts (Ti/Pt/Au) were formed by a standard lift-off process. A GSG contact configuration was used. Two types of devices with unequal mesa sizes (marked as \(L\)-type and \(S\)-type topologies) were fabricated and studied.