## 4 All-Silicon Photodetector Structures
### IPA-Based Devices  
For all the previous described methods, a photon at a wavelength of 1.55 um that travels in the silicon waveguide is efficiently absorbed if its energy allows an inter- or intra-band transition of an electron (or hole). Another way to absorb the photon is to lower the energy gap that the electrons (holes) must overcome. This approach is based on the use of a semiconductor/metal interface. The potentialities of such a method for the realization of an all-silicon photodetector have been demonstrated by the theoretical work of [PERSON] _et al._[29], and recently the effect has been employed for an integrated structure as well [63]. In Figure 14, the integrated photodetector proposed by [PERSON] _et al._[63] is schematically illustrated. A rib waveguide was terminated on a deep trench that  
Figure 13: **(a)** Top view of the ring resonator detector. **(b)** Sketch of the device and contour of optical mode [62]. Reprinted with permission from [PERSON]. [PERSON], _et al._ Copyright 2009, American Institute of Physics. reached down to the buried oxide layer of the SOI wafer. A Cu/p-Si Schottky contact was fabricated on the vertical surface of the deep trench. **Figure 14.** Schematic view of the Cu/p-Si Schottky-barrier-based integrated photodetector proposed by [PERSON] _et al._[63]. Reprinted with permission from [PERSON], _et al._Copyright 2010, American Institute of Physics. By means of this technological solution, a very narrow semiconductor/metal barrier transverse to the optical field coming out from the waveguide has been achieved. The integrated photodetector was characterized by a responsivity of 0.08 mA/W at a wavelength of 1,550 nm with a reverse bias of \(-1\) V. Measured dark current at \(-1\) V was about 10 nA. Moreover, the authors assert that the thinness of the Cu/p-Si Schottky barrier could enable a speed operation in the gigahertz range. An indirect evaluation of the bandwidth of the detector was reported to confirm the operation speed potentialities. A bandwidth of about 3 GHz was measured by [PERSON] _et al._[64] on a Schottky-barrier-based integrated photodetector, where the junction was achieved by a nickel silicide layer (NiSi\({}_{2}\)) on silicon [65,66] (Figure 15). **Figure 15. (a)** Schematic structure of the waveguide-based silicide Schottky-barrier photodetector proposed by [PERSON] _et al._; (**b)** XTEM of the thin absorbing NiSi\({}_{2}\) layer on the silicon waveguide [64]. Reprinted with permission from [PERSON], _et al._Copyright 2008, American Institute of Physics. In order to achieve both a suitable optical absorption and an efficient photoexcitation of metal electrons or holes across the silicide/silicon interface, the authors proposed the lengthening of a thin silicide layer on the surface of a SOI waveguide. The authors reported a detailed analysis on the influence of the silicide layer dimension on the performance of both NiSi\({}_{2}\)/p-Si and NiSi\({}_{2}\)/n-Si diodes. The overall performance of the NiSi\({}_{2}\)/p-Si structure has resulted better than that relative to the NiSi\({}_{2}\)/n-Si interface, due to the lower Schottky barrier height of the NiSi\({}_{2}\)/p-Si structure. In particular, a responsivity of about 4.6 mA/W at a wavelength of 1,550 nm and a reverse bias of \(-\)1 V was estimated for the NiSi\({}_{2}\)/p-Si diode against the value of 2.3 mA/W of the NiSi\({}_{2}\)/n-Si junction. Moreover, a measured dark current of 3 nA was reported. Recently, [PERSON] _et al._[67] have proposed enhancing the interaction between the optical field and the metal/semiconductor barrier by the use of surface plasmon polariton (SPP) [68,69]; _i.e._, an optical surface wave propagating along the interface between a metal and a dielectric [70]. The performances achieved are interesting and comparable with other Schottky-barrier-based integrated photodetectors.