# In-Plane Monolithic Integration of Scaled III-V Photonic Devices
## 3 Hybrid III-V/Si Photonic Crystal Cavity Lasers
### Design and Fabrication  
The 1D PhC structure consists of evenly spaced nanorods made from a high refractive index material such as Si or III-Vs and embedded in SiO\({}_{2}\). The length of the nanorods \(w_{y}\) is quadratically tapered towards the center of the device to form the cavity according to \(w_{y}(i)=w_{y}(0)+i^{2}\) (\(w_{y}(i_{\text{max}})-w_{y}(0)\))/\(i_{\text{max}}\)[2], where \(i\) is the nanorod position. This design is adapted from Ref. [42] and thus highlights how TASE is able to leverage existing and well-optimized PhC cavity designs from the Si photonics community. Additionally, it is well suited for in-plane coupling to SOI waveguides, as required for future integration within PICs. As each rod is individually addressable, it is possible to replace any number of rods with III-V materials, just as it is also possible to replace non-neighboring rods, while leaving the remaining rods in the original silicon. We choose the central rods here in order to achieve a high overlap between the first cavity mode field profile, which is localized in the center of the device, and the gain material. The device resulting from replacing the central Si rods with III-V material is shown in the top-view scanning electron microscopy (SEM) image in Figure 3a, where the III-Vs appear in a brighter color than Si. Two other features can be observed in this SEM as a result of the TASE process: first, the oxide opening from where the Si was etched back; second, the remaining Si seeds at which the III-V growth nucleated. The remaining part of the PhC cavity is untouched by the process, benefitting from the high side-wall quality of the Si processing. A different refractive index of the two materials can be compensated for by adjusting the III-V rod width. Indeed, finite difference time domain (FDTD) simulations show that \(Q\) factors and resonance wavelengths comparable to the Si-only design can be achieved [33]. The \(\mid\)E\({}_{y}\)[2] field intensity profile of the fundamental cavity mode obtained from 3D FDTD simulations (Lumerical) is shown in Figure 3b. Comparing this mode profile with the position of the III-V rods in the SEM above showcases the high overlap between cavity mode and gain material as intended. The cavity center with the oxide windows and non-uniform III-V rods on one side and the Si seed on the other side disturbs the symmetry needed for a high-\(Q\) PhC cavity. The fact that we very consistently observe cavity modes at expected peak positions and featuring similar experimental \(Q\)-factors on the order of 1000 is encouraging. This can also partially be attributed to the lateral heterojunctions implemented in [33], which help to confine the carriers in the nanorod center and thus away from the Si seed. In order to further optimize this, a symmetric device can be achieved by removing the absorbing Si seed nanowires which extend beyond the structures. We realize this by patterning a 500 nm-thick FOX-16 resist to cover a hybrid III-V/Si PhC structure. By electron beam lithography, the resist is cured and it obtains a composition and refractive index similar to silicon oxide. A non-selective Ar-ion milling etch then removes both the Si seed and the slightly irregularly grown III-V rods in a single step, resulting in the symmetric device sketched in Figure 3c. In the corresponding SEM image, the individual rods of the PhC cavity are not visible underneath the oxide bar, but the outline of the cavity is clear. The shapes from the Si seeds and the oxide window are still recognizable but smoothed out, as they are transferred into the BOX layer below the cavity level, in the former case because of the slower etch rate of Si in the ion milling process. The scanning transmission electron microscope (STEM) cross-sectional image in Figure 3d highlights the self-aligned position of the embedded III-V rods within the hybrid PhC cavity.