## 4 All-Silicon Photodetector Structures
### SSA-Based Devices  
In 2007, [PERSON] _et al._[51] observed a low photoresponse for an unimplanted diode and attributed it to a surface effect of the waveguide even if its performances were a strong function of the operating environment (such as humidity and surface chemistry). [PERSON] _et al._[61] enhanced the effect of the surface states optimizing the overlap between the optical mode and the surface of the waveguide. The SEM image and a top view of one of the devices characterized by the authors are shown in Figures 12(a,b), respectively.  
Small conductions arms intersects the waveguide defining the active regions of the photodetector. The waveguide was developed with a cross section of 500 nm \(\times\) 100 nm in order to increase overlap  
Figure 11: Bulk photodiode proposed by [PERSON] _et al._[60] in 2005. Copyright © 2005 Optical Society of America.  
Figure 12: Top view SEM image **(a)** and optical image **(b)** of the detector proposed by [PERSON] _et al._[61]. Copyright © 2008 Optical Society of America.  
between the optical mode and the waveguide surface. In this way, part of the optical mode is absorbed by the surface states and a change in conductivity of the device is achieved. For the device illustrated in Figure 12, a responsivity of 36 mA/W for a bias of 11 V at a wavelength of 1,575 nm was measured. A device leakage current of 0.12 \(\upmu\)A at \(-\)11 V is reported as well. Moreover, the authors reported a detailed analysis to demonstrate that the measured photocurrent is due to the energy state of the surface defects and not to other effects such as heating, two-photon absorption or internal photoemission.  
In order to enhance the photocurrent generated by SSA, [PERSON] _et al._[62] used a p-i-n diode embedded into a silicon micro-ring resonator. Figure 13(a) shows a top view of [PERSON]'s proposed device; whereas in Figure 13(b), the waveguide dimension obtained on a SOI wafer with a 1-\(\upmu\)m-thick buried oxide layer is illustrated. A low-temperature deposited oxide layer with a thickness of 1 \(\upmu\)m has been used to cover the waveguide. In Figure 13(b), the intensity contour of the simulated TE optical mode field is reported and the spatial overlapping with the Si/SiO\({}_{2}\) interfaces can be noted. This overlapping makes possible achievement of photogenerated current by SSA.  
The ring resonator was characterized by a Q factor of about 8,000. With this value, the responsivity detected at the resonance wavelength (1,541.5 nm) for 0 V bias was about 0.12 mA/W and resulted twenty times that of the responsivity measured for off-resonance wavelengths. Increasing the bias voltage to \(-\)15 V, the responsivity increased to 0.25 mA/W but the dark current reached 2.5 nA. Due to the presence of a resonator, the device became more sensitive to temperature changes. In particular, the authors measured a redshift of the resonance wavelength that was explained by the thermo-optical effect and the heating due to the carrier recombination. Finally, the authors noted a slight linear absorption-induced photovoltaic effect, with a power generation efficiency of about 0.05 mW/W.