# Integrated Photodetectors Based on Group IV and Colloidal Semiconductors: Current State of Affairs
## 3 Group IV Semiconductors  
The main semiconductors used to realize a photodetector, are summarized in the table of elements reported in Figure 2; in particular, the group IV semiconductors are currently the main materials for electronic and optoelectronic devices fabrication. Historically, efficient detection is provided by exploiting the direct bandgap of all III-V semiconductors; however, the integration of materials outer to group IV semiconductor with Si integrated circuit (IC) is complicated [30]. As example, in an integrated system the PD has to be closely connected to the biasing system and the electronic amplifying circuits [31], in order to exhibit the best possible performance.  
The compatibility with Si technology of the IV Group semiconductors together with the efficient near-infrared light detection, enable fabrication of PDs and Si CMOS circuits in a simultaneously and monolithically integrated way [32]. Among PD systems, metal-semiconductor-metal (MSM) structures, with group IV semiconductor, are promising candidates for sensor optoelectronic integrated circuits (OEICs) due to the low detector capacitance, large device bandwidth, internal gain and,  
Figure 2: Table of elements involved in semiconductor based photodetectors (PDs) production.  
obviously, easiness of integration with electronic circuits. In spite of that, a lower bandgap increases the dark current associated with a low Schottky barrier in MSMs where the semiconductor is Ge or Si. As reported before, in order to limits the dark current effect, a reverse bias has to be applied leading to an extra power consumption. The connected heat generation increases the already high temperature of the Si IC substrate that absorbs the heat of the whole device. In particular, in Ref. [33] authors show both theoretically and experimentally a significant suppression of MSM-PD dark current by applying asymmetric metal electrodes and by modifying the Schottky barrier heights.  
A similar technique has also been applied for MSM III-V PDs [34]. On the other hand, the only use of a wider bandgap semiconductor layer within the metal-semiconductor contacts can increase the Schottky barrier in MSMs and, thus, enhance the dark current effect [35].  
Bulk silicon is naturally important for its main role in integrated circuits, but substantial challenges also derive from other elements by the group IV bulk materials and their alloys, nanocomposites, nanostructures, films of different thickness, and heterostructures. Advances in device performance are obtained by means of new defect engineering production, novel growth techniques, and improvements in diagnostic tools.  
The advantage to consider group IV semiconductors is that they are widely studied thus modeling of defect generation, modeling of crystal growth, growth of group IV alloy crystals, low quality polycrystalline silicon refinement, including control of dopants and wafering technologies, and defect evolution in wafering processes are all well-known issues. Moreover, nanostructures of/on group IV semiconductors are largely developed, as well as modeling and simulation of epitaxial structures, heterogeneous integration of Si or Ge with III-V epitaxial device quality layers, growth of 2D materials (e.g., graphene, silicene, and germanene) on silicon, deposition of amorphous and crystalline thin layers and silicon membranes. Fundamental research on point defects and extended defects in group IV semiconductors is continuously in progress, as well as the study of dislocation engineering by substrate and process optimization. Additionally, considering the several technological applications for group IV semiconductors, such as silicon on insulator (SOI) devices, high speed and high frequency electronic devices, photonics and light emitting devices, and power devices, it is natural to invest resources to enhance the performance of devices based on these semiconductors.