Effects of Interface States on Ge-On-SOI Photodiodes

The 4.2% mismatch at the Si/Ge interface has a significant impact on Si/Ge photodetectors. However, few researchers have attempted to determine the major noise source or study the effects of the Si/Ge interface on the dark current, the responsivity and the 3-dB bandwidth of these devices. In this letter, we found that the dark current was dominated by generation-recombination processes that were enhanced by trap-assisted-tunneling around the interface below 220 K, with a characteristic tunneling energy of E00 = 14 meV corresponding to an effective mass of m* = 18m0. This behavior can be explained by the rise in the heavy-hole band caused by the compressive strain on the Ge layer. When the temperature increased above 240 K, Shockley-Read-Hall recombination was clearly observed and believed to be dominant. The responsivity, the collection efficiency and the absorption efficiency were all extracted at 850 nm, 1310 nm, and 1550 nm. The absorption coefficient around the interface was found to be lower than that of the bulk material. In addition, comparison of the measured 3-dB frequency (~20.6 GHz @ -0.5 V) with the theoretical value (~29.37 GHz) indicated that defects have little effect on the bandwidth at high frequencies.