Abstract
We have fabricated an edge-coupled InGaAs p-i-n photodiode (EC-PD) with its pseudowindow defined by conventional photolithographic processes and its facet formed by etching. Through fine tuning the window thickness, the transit-time-limited bandwidth was largely increased and device bandwidth was improved from ∼8 GHz toward ∼20 GHz. Such a tuning process is in fact a controlled selective chemical etching process, and optimizes the window thickness through reducing the thickness of undepleted absorption region. Although after tuning, the device preserves low leakage, the anisotropic chemical etching results in a sloped and reentrant facet that degrades the optical coupling efficiency and thus the device responsivity, which drops from 0.5 to 0.4 A/W at 1.3-μm wavelength for a device without an anti-reflection coating. For the EC-PD with the optical input facet formed by etching instead of the cleavage process, the device yield can be improved and direct die separation is feasible, which amounts to a huge cost reduction. Furthermore, an edge-coupled photodiode array, which requires several reliable diodes in series, can be realized.