# Integrated Photodetectors Based on Group IV and Colloidal Semiconductors: Current State of Affairs
## 3 Group IV Semiconductors
### Carbon
#### 3.3.2 Graphene  
Since its discovery in 2004, graphene, a single layer of carbon atoms arranged in a closely packed two-dimensional honeycomb lattice, has attracted a lot of research interests due to its intriguing physical properties such as ultrahigh mobility (200,000 cm\({}^{-2}\)/V-s at room temperature), high electrical conductivity (10\({}^{8}\) S/m) and an exceptionally large thermal conductivity up to 5300 W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\)[94]. Moreover, graphene has would be involved in fabricating thinner and faster response speed optoelectronic devices [95; 96]. Conversely, even if graphene can act as an absorber in ultrafast and ultra-broadband detectors, graphene-based photodetectors show relatively low responsivity, since a single sheet of carbon atoms has a relatively low absorbance (only 2.3%) from the ultraviolet (UV) to near infrared (NIR) region and short light-matter interaction length. Moreover, in pure graphene, due to its gapless nature, lifetime of excitons is extremely short giving rise to fast carrier recombination, which limits the efficient generation of photocurrent or photovoltage [97; 98]. To overcome this problem, Schottky junction based photodiodes have been proposed in several combination of nanostructures such as II-VI (ZnO [99; 100], ZnTe [101]), IV (Si [102; 103], Ge [104], GeTe [105]), and III-VI (GaN [106], InAs [107], Al\({}_{2}\)O\({}_{3}\)[108]), leading to improvements of both sensitivity and response speed. A very fascinating feature of the Schottky junction is that, due to its photovoltaic characteristics, most of the semiconductor-graphene devices are able to detect light irradiation without power supply [80]. An interesting and complete review of PDs based on graphene/semiconductor hybrid heterostructures, comprising device physics, design, performance, and process technologies for the optimization of PDs was recently published by [PERSON] and [PERSON] [109]. Considering that generally an all-Si approach is desired to take advantage by the existing CMOS technologies, a detailed review on the emerging field of the NIR internal photoemission effect-based graphene/Si PDs is presented by [PERSON] [110]. Figure 4: Photorespone of the PD. (**a**) Single normalized cycle measured at 5 kHz for evaluating both rise (\(\tau_{\rm{r}}\)) and fall time (\(\tau_{\rm{f}}\)). (**b**) Normalized photocurrent versus switching frequency. (**c**) Responsivity and detectivity of the PD as a function of bias voltage. (**d**) Responsivity and detectivity of the PD as a function of light intensity. (Adapted from [80]). Regarding silicon-graphene heterostructures, in 2018, [PERSON] et al. [111] realized and characterized a graphene/silicon (Gr/Si) (2D/3D) van der Waals heterostructure for high-performance PDs. Here, graphene works as an efficient carrier collector and Si as a photon absorption layer, allowing to obtain a barrier height of 0.76 eV and good performance as a self-powered detector under the mechanism of photovoltaic effect, working at 532 nm with zero bias. The authors measured a responsivity up to 510 mA W\({}^{-1}\), a photo switching ratio of 10\({}^{5}\) and a response time of 130 us. These good results have been ascribed to the Schottky barrier which extends the lifetime of photo-excited carriers leading to a fast separation and transport of photoexcited carriers [111]. More recently, a compact PD operating as a metal-graphene-metal photoconductive detector has been co-integrated with a Si photonic waveguide, demonstrating extremely efficient and high-speed plasmonically enhanced waveguide-integrated photodetector [112]. A Si waveguide allows the light enters and evanescently coupled into the graphene-based PD, made with a 6 um long layer of graphene. Then, the in-plane electric field is enhanced by the integrated nanosized bowtie-shaped metallic structures, as reported in Figure 5a,b.