# Trends in Performance Limits of the HOT Infrared Photodetectors
## 6 Colloidal Quantum Dot Infrared Photodetectors
### Brief View  
In the last decade, a significant progress in fabrication of CQD photodetectors has been observed. In this approach, an active region is constructed based on 3D quantum confined nanoparticles synthesized by inorganic chemistry. CQDs offer a promising alternative to the single crystal IR materials (InGaAs, InSb, InAsSb, HgCDTe, as well as T2 SLs see Figure 18). These nanoparticles could improve CQD photodetectors performance compared to epitaxial QDs due to many aspects gathered in Table 2[50; 68]. CQD photodetectors are typically fabricated using conducting-polymer/nanocrystal blends, or nanocomposites[13; 50; 69; 70]. Nanocomposites often feature narrow-bandgap, II-VI (HgTe, HgSe)[71; 72], PbSe or PbS[73; 74; 75]. Usually, the reported IR photodetectors use CQDs embedded in conducting polymer matrices, such as poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). It is expected that the extension of application of CQD-based devices will be significant, especially in IR imaging which is currently dominated by epitaxial semiconductor and hybrid technologies[76]. Hybrid technology, due to the complexity of production \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Advantages**} & \multicolumn{2}{c}{**Disadvantages**} \\ \hline \(\bullet\) & control of dot synthesis and absorption & \(\bullet\) & inferior chemical stability and electronic passivation of the nanomaterials in comparison with epitaxial materials \\ \(\bullet\) & much stronger absorption than in & \(\bullet\) & bipolar, interband (or excitonic) transitions across the CQD bandgap (e.g., electrons hopping among QDs and holes \\ \(\bullet\) & considerable elimination of strains & transport through the polymer) contrary to the intraband transitions in the epitaxial QDs \\ \(\bullet\) & by better selection of absorber materials & epitaxial QDs \\ \(\bullet\) & reduction of cost fabrication (using e.g., such solution as spin coating, inject & \(\bullet\) & insulating behaviour due to slow electron transfer through many barrier interfaces in a nanomaterial \\  & printing) compared to epitaxial growth & \(\bullet\) & problems with long term stability due to the large density of interfaces with atoms presenting different or weaker binding \\ \(\bullet\) & deposition methods are compatible with a variety of flexible substrates and sensing & \(\bullet\) & high level of 1/f noise due to disordered granular systems \\ \(\bullet\) & technologies such as CMOS (e.g., direct coating on silicon electronics for imaging) & \(\bullet\) & high level of 1/f noise due to disordered granular systems \\ \hline \hline \end{tabular}
\end{table} 
Table 2: CQD photodetectors advantages and disadvantages in comparison with single crystal QD photodetectors. Figure 18: The wavelengths range that can be detected by materials commonly used in imaging applications. stages, reduces yield and increases overall cost. The IR CQD-based photodetectors are an alternative solution without these limitations. The CQD layers are amorphous what permits fabrication of devices directly onto ROIC substrates, as shown in Figure 19 with no restrictions on pixel or array size and with a day cycle of production. In addition, the monolithic integration of CQD detectors into ROIC does not require any hybridization steps. Individual pixels are defined by the area of the metal pads arranged on the top of ROIC surface. To synthesize colloidal nanocrystals, wet chemistry techniques are used. Reagents are injected into a flask and, the desirable shape and size of nanocrystals are obtained by the control of reagent concentrations, ligand selection, and temperature. This so-called top-surface photodetector offers a 100% fill factor and is compatible with postprocessing at the top of complementary metal-oxide semiconductor (CMOS) electronics.