# In-Plane Monolithic Integration of Scaled III-V Photonic Devices
## 4 Monolithic Detectors  
TASE is equally well suited to realize efficient III-V-based detectors which lie in-plane with Si waveguides. Together with emitters, these are the main components needed to obtain a full optical link. Recently, electrical functionality for monolithic In\({}_{50}\)Ga\({}_{50}\)As detectors [34] was demonstrated using the same TASE technique, which is shown in Figure 6. In this work the focus was on small form-factor devices for low-capacitance high-speed performance. We grew the III-V active region using the same method as for the emitters, but this time the height was only 60 nm. This means that, although the devices are coupled directly to a silicon nanowire, it is too thin to support a propagating mode and the metal contacts on top will induce absorption losses. In future designs, interfacing the \(i\)-region of the photodetector directly with a Si waveguide perpendicular to it might prove beneficial. Here, the responsivity and bandwidth measurements were performed through free space coupling from an optical fiber.  
The 1 \(\upmu\)m-long devices contain an in situ \(p\)-\(i\)-\(n\)-doping profile with an intrinsic absorption region of roughly 300 nm length, as pointed out in the SEM and STEM in Figure 6b. If we consider that only the \(i\)-region contributes to the absorption, we calculate a responsivity up to 0.65 A/W (2 V reverse bias) depending on the operating conditions. This is the ideal case and should be considered as an upper limit. The devices show relatively low dark  
Figure 6: (**a**) The last step in addition to the process flow in Figure 1 consists of depositing Ni/Au contacts. The grown III-V material for this device is a \(p\)-\(i\)-\(n\) heterostructure, as indicated in (**b**) by the false-colored top-view SEM and cross-sectional TEM. The Si seed for growth nucleation is clearly visible underneath the deposited contact. (**c**) Spectral response of the monolithic InGaAs photodetector in reverse bias (detection) and forward bias (emission), on different scales. (**b,c**) are adapted with permission from [34].  
currents in the range of a few A/cm\({}^{2}\) and a strong RF response, whereby the measured \(f\)3 dB > 25 GHz is limited by our setup.  
We observed a rather non-linear spectral response with peaks in the O- and C-band, as shown in Figure 6c. In particular, the strong peak in absorption at 1350 nm is somewhat surprising based on the 50% In content. Extensive simulations carried out in Synopsys Sentaurus by collaborators at the Swiss Federal Institute of Technology (ETH) in Zurich showed this to be a result of the scaled geometry and device architecture. This results in the reflections at the various film interfaces as well as at the contact edges and illustrates an important point about nanoscale photonic devices--namely, that such effects might dominate device performance in minute geometries, as compared to conventional bulk-like devices. Thus, nanoscale effects need to be considered in the device design so that it might be turned to an advantage. This is the case here, where these reflections cause an amplification of the response in the important region for datacom around 1350 nm.  
We also evaluated the operation of those devices under forward bias for light emission. The electroluminescence is further in the near-IR spectral region, so we are only able to observe the tails around 1600 nm in our setup.