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Electrothermal Properties of 2D Materials in Device Applications

To keep downsizing transistors, new materials must be explored since traditional 3D materials begin to experience tunneling and other problematic physical phenomena at small sizes. 2D materials are appealing due to their thinness and bandgap. The relatively weak van der Waals forces between layers in 2D materials allow easy exfoliation and device fabrication but they also result in poor heat transfer to the substrate, which is the main path for heat removal. The impaired thermal coupling is exacerbated in few-layer devices where heat dissipated in the layers further from the substrate encounters additional interlayer thermal resistance before reaching the substrate, which results in self-heating and degradation of mobility. This study explores the electro-thermal properties of five materials (MoS2, MoSe2, WS2, WSe2, and 2D black phosphorous) which have been identified as possible replacements for Si in future sub-5-nm channel-length devices. We have developed a coupled electro-thermal model to calculate device mobility. The carrier wavefunctions and distribution are obtained from solving the coupled Schrodinger and Poisson equations in the cross-plane direction. The screening length is then calculated from the screening wavenumber. We calculate TBC for each layer in the stack into the substrate from a model based on first-principles phonon dispersion. We determine the local temperature in each layer from a ratio of its dissipated energy and its TBC. We simulate various devices with self-heating (Delta T does not equal 0, where Delta T is the temperature rise of the few-layer device) under several parameters and examined the effects on mobility and change in device temperature. The effects are compared to the isothermal case (Delta T = 0). We observe that self-heating has a significant effect on temperature rise, layer-wise drain current, and effective mobility. Black phosphorous performs the best electrothermally and WS2 performs the worst overall. This thesis will inform future thermally aware designs of nanoelectronic devices based on 2D materials.
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