# Integrated Photodetectors Based on Group IV and Colloidal Semiconductors: Current State of Affairs
## 3 Group IV Semiconductors
### Si  
Although in the visible spectrum the PDs in Si have reached the phase of commercial development, in NIR band the use of Si for the realization of PD is difficult due to its transparency at wavelengths greater than 1.1 \(\upmu\)m. Generally, the integration of germanium (Ge) in a SiGe heterostructure is used to overcome this drawback [36; 37], however a reticular superlattice mismatch of 4.3% leads to a very high leakage current, affecting the performance of the device. A two-step epitaxial growth technique can mitigate these effects [36; 37] but, unfortunately, does not completely remove the defect center responsible for the high leakage current. Furthermore, the grown buffer layer gives rise to thermal and flatness problems [38], which prevent the possibility of a monolithic integration of Ge on Si. Additionally, in p-i-n structures the Ge needs a very thick intrinsic layer (especially with respect to the gallium arsenide (InGaAs)) because at 1550 nm the Ge shows a lower absorption. In this paragraph, we present a brief overview on all-Si photodetectors at NIR wavelengths. In these PDs, the most used physical effects, that allow the absorption at sub-band wavelength, are the internal photoemission absorption (IPA) in presence of positive charged particles or molecules, two photon absorption (TPA), mid-bandgap absorption (MBA), and surface state absorption (SSA). A quantitative comparison of PDs in the literature, divided by absorption effects are shown in Table 2. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Responsivity (A/N)** & **Vbias (V)** & **Operating Wavelength (nm)** & **Dark Current/Leakage** & **Effect** & **Ref.** \\ \hline \(4.6\times 10^{-3}\) & \(-1\) & 1550 & 3 nA & IPA NiSi/p-Si Schottky barrier & [39] \\ \(8\times 10^{-3}\) & \(-1\) & 1550 & \(\sim\)3 nA & MBA Proton implantation & [40] \\ \(0.8\times 10^{-3}\) & \(-0.1\) & 1550 & 6 \(\mu\)A & IPA Surface plasmon polariton & [41] \\ \(64\times 10^{-3}\) & \(-20\) & 1440 & 0.1 nA & MBA +2\({}^{th}\) implantation & [42] \\ \(0.1\) & \(-2\) & 1549 & 0.1 nA & MBA Si + implantation & [43] \\ \(8\times 10^{-6}\) & \(-0.1\) & 1550 & - & IPA Cu/p-Si Schottky barrier & [44] \\ \(0.08\times 10^{-3}\) & \(-1\) & 1550 & 10 nA & IPA Cu/p-Si Schottky barrier & [45] \\ \(0.5\)-\(0.8\) & \(-5\) & 1550 & 2.5 nA/mm & MBA Si + implantation & [46, 67] \\ \(50\times 10^{-3}\) & \(-0.5\) & 1330 & 120 u/cm\({}^{2}\) & MBA Laser irradiation in presence of SF\({}_{6}\) & [48] \\ \(36\times 10^{-3}\) & \(-11\) & 1575 & 0.12 uA & SSA & [49] \\ \(0.25\times 10^{-3}\) & \(-15\) & 1541.5 & 2.5 nA & SSA-ring resonator & [50] \\ \(6\times 10^{-3}\) & \(-3\) & 1550 & 15 pA & TPA Photonic crystal resonators & [51] \\ \(2\times 10^{-6}\) & 1 & 1300 & - & TPA Hemispherical structure & [52] \\ \(0.18\) & \(-6\) & 1550 & 5 \(\mu\)A & IPA Al-porous Si Schottky barrier & [53] \\ \(35\times 10^{-3}\) & \(-0.5\) & 1550 & 120 μA/cm\({}^{2}\) & MBA in presence of SF\({}_{6}\) & [54] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Some physical and optical properties at room temperature of narrow gap semiconductors. To improve the Si based photodetectors performance, many approaches have been considered and proposed. Among them, encouraging results have been obtained by exploiting the internal photo absorption effect (IPA), that is, the use of photon-assisted transmission of hot carriers across a potential barrier at metal-semiconductor interfaces. Several new structures have been reported in literature combining, for example, IPA with nanoscale metallic structures, comprising Si nanoparticles (NPs) [39], metal stripes allowing surface plasmon polaritons (SPPs) [40; 41], metallic gratings [42] and new structures based on two-dimensional materials (like graphene) capable to substitute metal in the Schottky junction [43]. Moreover, as reported in some complete review on this topic [44; 45], due to the unipolar nature of the Schottky junction, IPE based PDs are very fast and can be monolithically integrated with Si-based CCD for IR applications [46].