Visible and Infrared Light Detection Using 2D Materials

Abstract

Two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) open up the potential for pushing electronic devices, from transistors to photovoltaics, to the atomic limit. In particular, many transition metal dichalcogenides such as tungsten diselenide (WSe₂) have properties such as a large absorption coefficient, ambipolar nature, and a bandgap near ~1.34 eV that make them suitable for solar cell applications. With respect to longer wavelength detection, application of 2D crystals and other ultrathin materials such as ALD grown Hafnium Zirconium Oxide (Hf₀.₅Zr₀.₅O₂) open up the possibility of designing fast (<1 ms thermal time constant) thermal detectors while still maintaining high specific detectivity. This thesis focuses on developing novel light detectors and harvesters based on 2D materials, including multilayer Tungsten Diselenide (WSe₂) solar cells, ultrathin bolometers, and Bernal stacked bilayer graphene photoconductor devices for hyperspectral imaging within the 10 µm - 20 µm band.

Specifically, schottky junction thin-film Platinum (Pt)/WSe₂/Gold (Au) were shown to exhibit large improvements in the short circuit current and open circuit voltage due to antireflection coating effects, surface doping, and surface trap passivation. A single absorber solar cell with an open circuit voltage (Voc) of 380 mV and short circuit current density (Jsc) of 10.7 mA/cm² was demonstrated, thanks to the absorber coating. Shifting focus to infrared detectors, this work demonstrates suspended 50 nm Al₂O₃/10 nm TiN/10 nm HZO/10 nm TiN/100 nm SiO₂ films that act as pyroelectric detectors with thermal time constant down to 0.625 ms. In addition, a Hafnium Zirconium Oxide (Hf₀.₅Zr₀.₅O₂) gated MoS₂ transistor was shown to have temperature coefficient of resistance (TCR) magnitudes > 0.03 K⁻¹ when biased in subthreshold, making this technology competitive with the state-of-the-art vanadium oxide bolometers. In parallel, this work characterizes the temperature dependent IV characteristics and noise performance of novel metal-nanogap-metal thermomechanical bolometers which have been predicted to obtain TCR magnitudes > 2.0 K⁻¹ and have an experimentally demonstrated TCR of down to -0.39 K⁻¹. Finally, this thesis demonstrates through simulation and early stage experiments that, using just a few programmed states, data obtained from a Bernal stacked bilayer graphene device can be used to reconstruct chemical absorption lines that allow accurate fingerprinting of chemical species.