## 4 All-Silicon Photodetector Structures
### MBA-Based Devices  
MBA has been used to develop integrated optical detectors completely compatible with standard silicon technology and sensitive to C and L optical band wavelengths. [PERSON] _et al._, in a work for the Conference on Optical Fiber Communication in 2003, proposed a monolithic p-i-n waveguide photodiode based on implantation into a silicon-on-insulator wafer [46]. In particular, the implantation of protons into a rib waveguide with a lateral p-i-n diode (Figure 6) was used to create the deep levels. Thus, defects could be controlled in terms of concentration, depth and location via selective masking. Figure 5: Measured value of \(\beta\) (squares) as a function of wavelength for a 125-\(\upmu\)m-thick sample silicon wafer [43]. Data from other sources is also given: circles [44], up triangles [37], down triangles [39], and diamonds [41]. The solid curve represents the best fit based on calculations of [PERSON] and [PERSON] [45]. Reprinted with permission from [PERSON]. [PERSON], _et al_. [43]. (2007). Copyright 2007, American Institute of Physics. On the other hand, shallow implantation and rapid thermal annealing of the p+ and n+ contact regions allowed for the separation of the contacts to be close to the width of the waveguide without introducing excessive free carrier absorption. The proposed 10-mm-long photodiode exhibited a responsivity of about 8 mA/W at 1,550 nm and a leakage current of about 3 nA, in both cases with 1 V of applied reverse bias. [PERSON], [PERSON], and [PERSON] have investigated different factors useful for improving both the performance and repeatability of integrated photodiodes [47, 48]. Due to the high deep trap density, carriers are characterized by a lifetime in the nanosecond range and thus by a diffusion length of the order of a few microns. This implies a dependence of device performance on the separation between the waveguide, where the carriers are generated, and the semiconductor p-n junction [38]. Moreover, in 2006, [PERSON] _et al_. [48] demonstrated the importance of post-implantation annealing on device performance. Figure 7(a,b) reports plots derived by the authors with regards to the dependence of the propagation optical losses and the photocurrent generated on the post-implantation annealing temperature for an integrated photodetector with defects introduced by silicon-ion bombardment. The plots demonstrate that an improvement of device performance can be obtained through low-temperature annealing (around 300 \({}^{\circ}\)C). For these temperatures, carrier recombination centers that do not contribute to the carrier generation process are removed. This should imply a reduction of the competing mechanisms of carrier generation and thus a significant improvement both of the photoresponse and optical loss within the waveguide. Figure 6: Monolithic p-i-n waveguide photodiode proposed by [PERSON] _et al_. [46] in 2003. Different implanted ions were used by [PERSON] _et al_. [49]; in particular, helium ions were chosen for their non-reactivity with silicon and for stability of created defects [50]. [PERSON] _et al_. carried out a detailed analysis of the influences of ion-implantation doses and of annealing temperature, both on the photogenerated current and the optical losses. This analysis led to the fabrication of an optimized integrated photodetector that was 1.7 cm long, and which obtained a maximum value of responsivity of 64 mA/W at 1,440 nm and a leakage current of 0.1 mA, both with a reverse bias of 20 V.  
All the described photodetectors have been characterized by a very large traversal section in the order of tens of \(\upmu\)m\({}^{2}\), and by absorption lengths of 1 to 2 cm. Thus, the bandwidth of these devices was rather limited. Reducing the waveguide dimensions and optimizing the implantation in the silicon waveguide, [PERSON] _et al_. [51] demonstrated the feasibility of robust high-frequency integrated photodiodes. The main concept is related to the decrease of the transversal cross section of the waveguide.