EQE larger than 100% has been reported from photodetectors made on many semiconductor materials such as beta-Ga2O3, ZnO, GaN, and black Si, etc. I have never seen this from any types of Ge photodetectors. Is it possible to get over 100% of EQE from Ge photodetectors? Possible mechanism could be deep level defects which makes photoconductive gain. But, I am not sure.
As a suggestion, it could be a multi-electron process in which absorption of a photon (I would think at least twice the bandgap) will excite first one, then a second by secondary interactions to produce two electrons into the conduction band instead of just highly exciting a single electron deep into the conduction band. The resultant charge carrier/absorbed photons ratio would therefore be larger than one-to-one.
EQE = number of charge carriers collected by the solar cell to the number of photons of a given energy
Yes, it can over 100% if you can generate two pairs of carriers in one photon.
But it is hard for Ge to do this. There are many possible solutions. For examples, find a material/structure that can generate two carriers on top of your Ge detectors. Try to generate it within Ge.
It is unclear whether to me whether you are talking about multi-carrier generation by the photon or the delivery to the external circuit of multiple carriers for each phto-generated pair. Either way there a multiple ways for such an effect to occur.
Starting with multiple pair generation per photon: in principle the simplest process is for the photon energy to exceed 2x the bandgap. However, optical and near-UV photons would be absorbed very near the surface, so this is rarely practical other than in a Schottky diode. (At yet shorter wavelengths the absorbtion rate reduces again, so PN diodes can become viable - but photo-generated damage can be a problem here). Reverse-biased devices can have the effective bandgap reduced by incorporation of impurities, but such devices would normally be very leaky in room-temperature germanium. There are more complex schemes, but I'm not certain I would classify any that I know as "germanium detectors".
More commonly, we see multiple carriers released to the external circuit for each generated hole-electron pair. Device types to implement this effect include photoconductors, avalanche photodiodes and phototransistors.
It is unusual for a solar cell to achieve > 100% EQE (under no external bias). However, it is easy for a photo-detector to yield >> 100% EQE, because the applied bias voltage provides the energy. Often light modifies the conductivity of the active region of the detector, while the external field injects electrons into the material. See this paper on Light Effect Transistor.
http://journal.frontiersin.org/article/10.3389/fphy.2016.00008/full
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