# Impact of Device Topology on the Performance of High-Speed 1550 nm Wafer-Fused VCSELs
## 2 VCSEL Structure and Fabrication
In contrast to MOVPE growth [42], there is no planarization of the surface relief of the TJ during the MBE overgrowth process; thus, the height difference of the overgrown surface persists \(\Delta L_{\text{SR}}\) and coincides with the initial etching depths of the TJ layers \(\Delta L_{\text{TJ}}\). Calculations within the effective refractive index model [43], presented in Figure 2, indicate that even at relatively small etching depths \(\Delta L_{\text{TJ}}\) of the composite TJ, a stronger lateral waveguide effect (effective refractive index contrast \(\Delta n_{\text{eff}}\)) is realized than for the case of near-IR oxide-confined (In)AlGaAs/GaAs VCSELs [44]. This fact significantly limits the size of the BTJ mesa, at which stable SM lasing can be realized for LW VCSELs. However, an atomic force microscopy (AFM) analysis (cf. Figure 2) shows an increase in the lateral sizes of the surface relief (about 1.5-2.0 times), accompanied by a lateral smoothing of the interface with the height difference after the TJ regrowth. As a result, an efficient waveguide with a gradient refractive index profile is formed, which makes it possible to significantly reduce the optical confinement factor (\(\Gamma\)-factor) for high-order modes. This reduction contributes to SM lasing at larger BTJ sizes due to an increase in the difference in the modal gain between the fundamental mode and high-order modes [45]. However, for small diameters of the BTJ, it is also necessary to consider the drop in the \(\Gamma\)-factor for the fundamental mode, which is accompanied by an increase in the level of optical losses in the non-pumped peripheral part of the active region. As a compromise, we formed circular mesas with a diameter of 6 \(\upmu\)m and an etching depth of \(\sim\)20 nm. The choice of the doping level of the n-InP overgrowth layer is associated with the search for a compromise between low electrical resistance to reduce the effect of self-heating, an increase in the VCSEL energy efficiency, and a low parasitic capacitance \(C_{\text{P}}\) of the reverse-biased p\({}^{+}\)n-junction region outside the BTJ mesa to improve the parasitic cutoff frequency and thus laser dynamics. The series resistance \(R_{\text{s}}\) describing the longitudinal current flow through the BTJ region is predominantly determined by the highly resistive p-InAlAs emitter. At the same time, the spreading resistance \(R_{\text{sp}}\), which considers the lateral current flow through IC-layers, depends on the electrical resistance of the n-InP layers. The capacitance \(C_{\text{P}}\) is the space-charge capacitance arising between the p\({}^{++}\)-InGaAs and the heavily doped n-InP contact layer and depends on the donor concentration of the overgrown InP and the bias voltage [46]. For an average doping level of 1 \(\times\) 10\({}^{18}\) cm\({}^{-3}\) of the n-InP overgrown layer (excluding the area for a highly doped contact layer), the spreading resistance \(R_{\text{sp}}\) is about 30 Ohm, and the operating voltage does not exceed 2.5 V. However, the huge capacitance \(C_{\text{P}}\) (up to 1.5 pF) limits the parasitic cutoff frequency to several GHz. Reducing the n-InP layer doping down to 1 \(\times\) 10\({}^{17}\) cm\({}^{-3}\) of the overgrowth contributes to an expansion of the space-charge region and consequently reduces the capacitance \(C_{\text{P}}\)[17]. This allows a significant boost of the modulation bandwidth for the short-cavity VCSEL design [24]. However, WF VCSELs are characterized by a more extended device topology, which, together with the need for precision etching, leads to a dramatic increase in spreading resistance \(R_{\text{sp}}\). As a compromise, we reduced the doping level of the n-InP overgrown layer to 5 \(\times\) 10\({}^{17}\) cm\({}^{-3}\) and exploited a different device topology. The number of InGaAs QWs was increased from 7 to 10 to increase the modal gain. The total mechanical strain of the active region was increased by minimizing the barrier thickness (from 12 nm to 7 nm) and no strain compensating barriers were used.