# In-Plane Monolithic Integration of Scaled III-V Photonic Devices
## 3 Hybrid III-V/Si Photonic Crystal Cavity Lasers
### Characterization of Hybrid III-V/Si PhC Cavities  
The PhC cavities are characterized by micro-photoluminescence (\(\mu\)-PL) spectroscopy at 100, 130, and 150 K for indium phosphide (InP)-based and room temperature for indium gallium arsenide (InGaAs)-based devices. A picosecond-pulsed supercontinuum laser at a pump wavelength of 750 nm (InP) or 850 nm (InGaAs) is focused onto the device by a 100\(\times\) objective (NA = 0.6). The emission is collected with the same objective and analyzed by a spectrometer with a cooled InGaAs detector, using a filter which removes the pump signal. First, we characterize hybrid III-V/Si PhC cavities with InGaAs as a gain material. Figure 4a shows the measured PL spectra for cavities with different PhC lattice constants. By tailoring this PhC parameter (lattice constant \(a\) = 325-390 nm, height \(h\) = 220 nm, rod width \(w_{x}\) = 0.3\(a\), outer rod length \(w_{y}\) = 5\(a\)), we control the resonant wavelength which in  
Figure 3: Fabrication of the hybrid III-V/Si photonic crystal cavities. **(a)** Schematic of the device center where some Si rods were replaced with active material. This is visible in the SEM top view (top), where the III-V material appears in a brighter color. **(b)** 3D FDTD simulated \(\,{}^{1}\,\mathrm{E}_{y}\,{}^{1}\,{}^{2}\) field profile of the fundamental mode. A comparison with **(a)** emphasizes the high gain overlap of the selected five rods with the cavity mode. **(c)** In an additional step, a symmetric cavity is achieved by defining the device outline with FOX-16 resist and removing the surrounding oxide. The resulting structure is visible in the SEM image with the cavity hidden underneath. **(d)** A STEM cross-section along the dashed line shows that the III-V rods directly inherit the Si shape. **(a,b)** are reprinted (adapted) with permission from [33] © 2020 American Chemical Society. total covers the entire telecom bands. The measured wavelength of the modes fits well with simulated values, also depicted by the shaded area. An uncertainty when comparing simulated and experimental results arises from the refractive index of our grown III-V materials, which we estimate to lie in the range of \(n=3.2\)-\(3.5\), whereas the Si refractive index ranges from \(n=3.46\) to \(n=3.61\) for wavelengths of 1550 nm and 900 nm, respectively [43]. It is remarkable that all the measured devices behave similarly despite the non-uniform length of the grown InGaAs rods, as seen in Figure 3a. We believe this further proves the benefit from introducing III-V material locally in the center of the device. This region where the symmetry of the PhC is disturbed only takes up a small part of the entire device, whereas a large part remains Si. Here, is it appropriate to mention that the non-uniform length of the III-V rods in the PhC structures is not intrinsic to TASE technology, past work on electronic devices showed uniform lengths of III-V features also when grown in large arrays. Recently, oxygen contamination of our reactor has led to some variation in the nucleation efficiency, which translates into a variation of total length. This affected those works, but has since been remedied. A similar study is performed on devices with InP as the gain material [33], as shown in Figure 4b,c. The PhC parameters for this material was adjusted for resonant emission within the InP gain bandwidth. Additionally, these PhC cavities possess a slightly different geometry since the adiabatic tapering is realized by quadratically modulating the lattice constant outwards from the cavity center according to \(a(i)=a(0)+i^{2}\left(a(i_{\text{max}})-a(0)\right)/i_{\text{max}}{}^{2}\). Figure 4b shows the PL intensity under increasing pump power for a PhC cavity (\(a=215\) nm, \(h=120\) nm, \(w_{x}=0.5a\), \(w_{y}=2.5a\)) with seven InP rods in the center measured at 130 K. At low pump intensity, the spontaneous emission from InP, centered around 900 nm, is visible. Upon increasing pump power, a peak appears at 835 nm and continues to increase in intensity.