Ge-on-Si devices offer a promising potential alternative to the currently market leading III-V-semiconductors for the detection of SWIR-light up to 1700nm. They combine the SWIR-absorbing qualities of germanium with the largely established infrastructure of the silicon industry. A thin Ge layer enables the fabrication of detectors with a high f3dB-bandwidth as well as the integration into CMOS manufacturing processes. However, for thin Ge layers of a few hundred nanometers the SWIR-absorption is rather low. The application of micro- and nanostructures on such Ge-on-Si photodiodes gives a possible solution to overcome this problem.

Fig. 1: Example angular scattering distribution S(θ) of light coupled through a b-Si structure calculated by FDTD simulations.

A first approach lays in the application of stochastic needle-like structures, called Black Silicon (b-Si), on the backside of back-illuminated Ge-on-Si photodiodes. Besides the antireflection effect of b-Si, coupling the light through the nanostructured backside leads to a broad transmissive scattering into the Si substrate (see fig. 1). This results in a light trapping effect due to the high refractive index of Si which extends the propagation length through the Ge layer and hence increases the absorption within the photodiode (see fig. 2).

Fig. 2: Light trapping within Ge-on-Si with b-Si structured backside.

While the advantage of b-Si structures lays in the absence of a lithographic mask, other microstructures, like diffraction gratings, CGHs or plasmonic structures, might be more elaborate in manufacturing but also better adaptable to the concrete specifications. By applying such structures on photodetectors, we can also draw inferences about their optical performance in the SWIR range.
For these investigations we established a complete process chain for the fabrication of single photodiodes as well as photodiode arrays starting from the epitaxial layer stack. A combination of rigorous calculations with ray optic simulations enables pre-estimations on the optical performance and the design of the specific geometrical parameters.

Fig. 3: Trenching and back-thinning reduces the optical crosstalk and guides the light to the designated pixels (not drawn to scale!).

Fig. 4: Investigating the the optical performance of a Ge-on-Si photodiode array in the ray tracing software Zemax.

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