Advancements in Mid-Infrared Colloidal Quantum Dot Photodetectors: Optimization, Metamaterials, and Fabrication

Abstract

Photodetectors operating in the mid-infrared spectral wavelength regime are crucial for numerous technologies and scientific advancements. They are integral to a wide array of applications, including biomedical imaging, environmental monitoring, thermal imaging, spectroscopy, and defense systems. However, developing photodetectors that are both highly sensitive and cost-effective remains a significant challenge. Therefore, this doctoral thesis explores the use of low-cost absorber materials in combination with performance enhancement schemes to overcome these limitations. The cost-efficient material used in this thesis are colloidal quantum dots. These zero-dimensional nanometer-sized crystalline materials can be synthesized in solution and then deposited on various substrates using simple methods. Furthermore, their tunable absorption spectrum, achieved by altering their size, makes them ideal for a wide range of applications. This makes them an ideal candidate to reduce cost. However, photodetectors employing these materials often exhibit poor responsivities and detectivities. Thus, narrow-band resonant metamaterials were used to overcome these performance shortcomings. They were designed to increase the light absorption while simultaneously improving charge extraction properties and reducing noise. The first photodetectors presented in this work were fabricated using lead selenide and lead sulfide colloidal quantum dots. A method was developed to sinter these quantum dots into a solid layer, resulting in improved charge transport properties and an extended absorption spectra to longer wavelengths. This sintered lead selenide layer was then covered by an additional, also sintered, lead sulfide layer to form a heterostructure. By stacking these layers, it was possible to enhance the photo-gain in the lead selenide layer and double the photoresponse. In the final step, this layer stack was combined with a metallic metamaterial perfect absorber. This enabled narrow-band tunable absorption enhancement with peak absorptivity’s reaching 98%. As a result, the responsivity was increased up to twenty-fold, reaching 375 A/W and 4 A/W at wavelengths of 2710 nm and 4250 nm, respectively. The second type of photodetector presented in this thesis utilizes mercury telluride colloidal quantum dots as an absorber layer. These colloidal quantum dots were also combined with metamaterials, which were systematically improved using electro-optical simulations. This enabled the optimization of the metamaterial to increase the responsivity while simultaneously decreasing the noise spectral current density. Specifically, this was achieved by enhancing the photogenerated charge carrier collection efficiency and reducing the active material volume without compromising near-unity absorption. The metamaterial optimization process started with a common disc resonator design. In a first step straight contacts were placed in between the disc resonators and then wrapped around them. Finally, the disc resonators and contacts were merged forming a narrow slot metamaterial. This design optimization process resulted in an approximate 13-fold increase in responsivity and a 345-fold increase in detectivity. The final metamaterial design achieves a responsivity of 16.2 A/W and a detectivity of 6×108 Jones at a wavelength of 2710 nm. This analysis provides a pathway to significantly improve the responsivity and noise characteristics of photodetectors based on cost-efficient cQDs.