INSTITUTE OF APPLIED PHYSICS - Black Silicon enhanced Ge-on-Si photodiodes

Black Silicon enhanced Ge-on-Si photodiodes

Bild1_Martin Steglich
Fig. 1. Ge-on-Si detector concept with Black Silicon light-trapping for enhancement of intrinsic Ge absorptance. An epitaxial Ge PIN photodiode is arranged on the silicon substrate. Illumination is performed over the Black Silicon structured rear side of the device.

Fast internet access is a crucial economic factor in the modern knowledge society. An even more rapid development of the persistent telecommunication infrastructure fails due to the prohibitively high investments costs. Cheaper photodiodes for the conversion of fiber-delivered light signals into electrical signals would significantly contribute to their reduction.
A promising detector concept is represented by Germanium photodiodes being epitaxially integrated on a silicon chip. However, Germanium is only weakly absorbing in the relevant spectral range from 1300 nm to 1700 nm, thus necessitating high layer thicknesses of a few µm for a sufficient photocurrent yield. On the other hand, application in fast telecommunication systems requires very high detector bandwidths (> 10 GHz). This, in turn, can only be achieved by application of thin Germanium layers of a few 100 nm thickness because of the truncated photocarrier transit time.
Application of Black Silicon, a needle-like, self-organized and thus cost-efficiently manufacturable silicon nanostructure, can solve this problem. Placed on the rear of the silicon chip - which is transparent in this wavelength range - a light-trap is implemented that strongly increases light absorptance in thin Germanium (Fig. 1). By that, the responsivity of ultrafast Ge-on-Si detectors with bandwidths of up to 100 GHz can be increased by a factor of 3 to 10.
In particular, common problems that are related to the application of silicon nanostructures are circumvented with this approach. Since photocurrent generation takes place in the Germanium, the raised surface recombination velocity of the silicon nanostructures imposes no drawback. Thus, theoretically predicted absorptance enhancements through the applied Black Silicon light-trapping can be directly transferred into equivalent responsivity enhancements (Fig. 2). Furthermore, the fabrication of Black Silicon can be carried out uncomplicated on the chip's rear during back-end processing.

Bild2_Martin Steglich
Fig. 2. Theoretically predicted absorptance increase due to rear Black
Silicon light-trapping for an absorber thickness of 500 nm (dashed
lines) and corresponding responsivity increase proven at the device
(dots). Deviations in the vicinity of the Ge bandgap around 1550 nm
can be ascribed to strain in the epitaxial Ge layer.