We have fabricated Ge on Si PDs but they do not show any response under a white light while doing IV characterization. Unfortunately we do not have a dedicated system for optical characterization yet and we are working on it but we just want to have a quick look into the optical response of these PDs. We understand that it might not comprise of that high intensity wavelength component corresponding to Ge bandgap but even a small fraction must produce at least some response. Can someone shed some light on it?
Note: No anti-reflecting coating has been used but the devices have been unpassivated and passivated (SiO2, Al2O3 passivating layers).
I'm not sure what you're trying to do but we have a high pressure doping process where we can implant element atoms into Si and then power up the implanted area to measure the wavelength. Our dopant is actually Al2O3 because that gives us light.
You may have to try a longer wavelength. Depending on the device structure, if the carriers are generated too close to the surface or interface, then they might non-radiatively recombine and not contribute to the photocurrent. I faced a similar issue when we fabricated InP phototransistors, and going from 660 nm to 780 nm made a large difference. For Germanium, you want to be operating around 1.3-1.55 um. White light has longest wavelength around 700 nm, which is far too energetic to penetrate deeply into the germanium. You may also want to consider the dark current of your device: if the dark current is really high, then you may have to use a lock in amplifier or integrate for a long time to get rid of shot noise from the large dark current and actually be able to distinguish your photocurrent.
MSM photodetectors usually have very small sensitive area, therefore they may need rather high light intensity to observe significant photocurrent
Thank you all for your valuable suggestions.
@ Indrasen - Maybe what you described about the high energetic beam is an issue with us and yes, our dark current is too high, in mAs although the device size is also large due to limitations imposed by the lithography techniques we use.
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