InGaAs/Si heterostructure photodetectors using direct wafer bonding

Previous works have shown that InGaAs/Si heterostructures fabricated using wafer fusion technology have resulted in PIN and APD detectors of high quantum efficiency and bandwidth. At least two bonding processes have been reported by different laboratories so far. One method directly fuses InGaAs/Si material first and then removes the InP substrate by wet etch; and the other method removes the InP substrate first from temporarily bonded InGaAs/Si wafers and then uses thermal annealing to achieve covalent bonding. The first part of the talk includes discussions on the pros and cons of both methods based on our own experience. For practical concerns, we finally chose the former method to make the heterostructure for interface characterization and photodetectors. The variable temperature I-V and PL characteristics show that the properties of the heterojunction are sensitive to the interface oxygen concentration as well as the properties of misfit dislocations. Making use of the bonding interface and tailoring the electric field profiles in InGaAs and Si, we have proposed and demonstrated new PIN and APD structures. The preliminary results showed that the internal quantum efficiency was greater than 60% at low bias voltage for the PIN structure. The estimated frequency response was about 12 GHz, limited by the RC delay. For nearly the same device structure, avalanche gain was observed at very low bias voltage (about 10 to 15 V). APD of low operating voltage is attractive because it simplifies the bias circuit and as a result, reduces the cost of the receiver. The low operating voltage is obtained by using heavily n-doped Si substrate as the anode and the InGaAs/Si heterojunction itself as an equivalent of a delta-doped p-region. It can be shown from the simulation results that both the injection efficiency and the multiplication factor can be enhanced using such a structure. One key remaining issue is the noise characteristics of the APD. Different designs will be discussed for the purpose of dark current reduction to improve the noise figure of the device