# Integrated Photodetectors Based on Group IV and Colloidal Semiconductors: Current State of Affairs
## 1 Introduction
PD can be used in different configurations for different purposes:  
* Single light sensors: A single sensor is used to detect overall light levels, it is useful to quantify the total optical power or light intensity;
* 1-D array light sensor: A line of PD is used to quantify the distribution of optical power or light intensity along a line. Combined with a wavelength splitter it can be used in a spectrophotometer; and
* 2-D array light sensor: A NxM matrix of photodetectors can be used to form images with NxM resolution. Photodetectors are built by semiconductor materials which are able to absorb emitted photons and generate electron-hole pairs under the light excitation. A typical configuration is made of inorganic semiconductors [14], for example, GaN-based photodetectors are used for the ultraviolet sub-band (0.25-0.4 \(\mu\)m), Si-based photodetectors are implemented for the visible sub-band (0.45-0.8 \(\mu\)m), and InGaAs-based photodetectors are developed for near-infrared (NIR) sub-band (0.9-1.7 \(\mu\)m). An interesting and explicative graph that summarizes composition, energy gap and wavelength for most used material systems for infrared (IR) photodetectors is reported in Figure 1[15], while some physical and optical properties at room temperature of narrow gap semiconductors are reported in Table 1 (Energy gap, \(\mathrm{E_{g}}\), quantum efficiency, \(\mathrm{n_{i}}\), dielectric permittivity, \(\varepsilon\), and magnetic permeability, \(\mu_{\mathrm{e}}\) and \(\mu_{\mathrm{h}}\)). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & \(\mathbf{E_{g}}\) **(eV)** & \(\mathbf{n_{i}}\) **(cm\({}^{-3}\))** & \(\varepsilon\) & \(\mathbf{\mu_{e}}\) **(10\({}^{4}\) cm\({}^{2}\)/Vs)** & \(\mathbf{\mu_{h}}\) **(10\({}^{4}\) cm\({}^{2}\)/Vs)** \\ \hline InAs & 0.359 & \(9.3\times 10^{14}\) & 14.5 & 3 & 0.02 \\ InSb & 0.18 & \(1.9\times 10^{16}\) & 17.9 & 8 & 0.08 \\ PbS & 0.42 & \(1.0\times 10^{15}\) & 172 & 0.05 & 0.06 \\ PbSe & 0.28 & \(2.0\times 10^{16}\) & 227 & 0.10 & 0.10 \\ PbTe & 0.31 & \(1.5\times 10^{16}\) & 428 & 0.17 & 0.08 \\ In\({}_{0.53}\)Ga\({}_{0.47}\)As & 0.75 & \(5.4\times 10^{11}\) & 14.6 & 1.38 & 0.05 \\ Pb\({}_{0.44}\)Sn\({}_{0.56}\)Te & 0.1 & \(2.0\times 10^{16}\) & 400 & 0.12 & 0.08 \\ Hg\({}_{1-x}\)Cd\({}_{x}\)Te & 0.07–0.25 & (0.23–2.3) \(\times 10^{16}\) & 16.7–18.0 & 0.6–1.0 & 0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Some physical and optical properties at room temperature of narrow gap semiconductors [15]. While the InSb has narrow band gaps in the mid-wave IR (MWIR), where it dominates as a semiconductor for PDs, PDs based on Hg\({}_{1\text{-}}\)X\({}_{\text{Cd}}\)X\({}_{\text{Te}}\) (MCT) have the great advantage of being able to tune the wavelength of operation from MWIR to long-wave IR (LWIR), adjusting the composition. As it is known, semiconductor materials have a crystalline solid structure, but to have a device with optimal performance, this structure must not have defects and it should be single crystal. To achieve this, crystals must be grown by coupling the crystal lattice to that of the substrate. This is achieved either with molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) and involves high costs for industrial production. Using heterostructured semiconductors, whose molecular structure has a broadband super-lattice, it was possible to obtain infrared absorption with intersubband transitions within one of the wells of the superlattice (GaAs/AlGaAs). PD based on this effect are called Quantum Well Photodetectors (QWP) [16] or Strained Layer Super-lattice Photodetectors (SLSP). Another approach has been the development of infrared quantum dot detectors (QDIP) based on self-assembled epitaxial quantum dots [17; 18; 19; 20; 21]. However, initially the use of QD in photodetectors presented two main difficulties: Limited control of QD size and low density of QD grown epitaxially. To date, InSb and MCT are the most used materials for the production of commercial quantum detectors [22].