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IV-VI colloidal quantum dot photodetectors integrated on silicon waveguide circuitsResearch Area: Heterogeneous integration technology for silicon photonics , Silicon photonics for lab-on-chip spectroscopy Main Researcher: Chen Hu Many molecules that we want to detect or monitor in our environment have bands of absorption lines in the mid-infrared (MIR). These absorption lines form a ¡°fingerprint¡± for a particular molecule, hence MIR spectroscopic sensing systems allow detecting the presence (and concentration) of specific molecules (Figure 1). Traditional high-sensitivity photodetectors used in these spectroscopic systems are based on epitaxial materials, leading to expensive devices. Low-cost MidIR photodetectors based on colloidal quantum dots (QDs) offer an alternative way to realize this functionality, either as discrete components or integrated on photonic integrated circuits. ![]() Figure 1. Illustration of the characteristic absorption lines of molecules in mid-infrared range. Using colloidal QDs as new photonic material gets a lot of attention in the photonic community. The simple hot injection chemical synthesis method allows low-cost production. By tuning the size of the QDs, the electrical and optical properties (such as the absorption cut-off wavelength) can be tuned due to the quantum size effect (Figure 2). Furthermore, the fact that these QDs are available in solution makes it easy to do large-area heterogeneous integration on substrates, using dip coating or printing, which can offer a considerable cost reduction as compared to thermal evaporation or epitaxially grown layer stacks. ![]() Figure 2. Typical CdSe size series, as obtained through hot-injection based synthesis. In this project we propose to use IV-VI or II-VI colloidal quantum dot materials, such as PbS, PbSe and HgTe, for infrared photodetector application. We demonstrate a uniform, ultra-smooth colloidal QD film without any cracks, which is realized by dip coating and subsequent ligand exchange. Metal-free inorganic ligands, such as OH- and S2-, are investigated to facilitate the charge carrier transport. Both PbS and HgTe-based quantum dot photoconductors were fabricated on interdigitated gold electrodes. For PbS-based detectors a responsivity of 200A/W is measured at 1.5¦Ìm, due to the large internal photoconductive gain. A 2.2¦Ìm cut-off wavelength for PbS photodetectors and 2.8¦Ìm for HgTe quantum dot photodetectors are obtained (Figure 3). ![]() Figure 3. Normalized detector responsivity as a function of wavelength for S2- capped and OH- capped PbS QD photodetectors. Other people involved: PhD thesises
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