Effect of Quantum Confinement on the Thermoelectric Properties of Semiconductor 2D Thin Films and 1D Wires

With device dimensions shrinking to nanoscales, quantum effects such as confinement and tunneling become significant in electron transport. In addition, scattering effects such as electron-phonon scattering, electron-impurity scattering also affect carrier transport in small-scale devices. Commonly used quantum transport models involve quantum corrections to the drift-diffusion equations while models based on the solution to the Schrodinger wave equation can be computationally intensive. While most of these transport models are not robust enough to incorporate rigorous scattering effects, the NEGF formalism has been found to be very efficient in coupling quantum and scattering effects. In this paper the NEGF model is used to assess the device characteristics silicon and SiGe superlattice thin films and wires whose applications include thermoelectric cooling of electronic and optoelectronic systems. The effect of quantum confinement on the electrical transport and its impact on the thermoelectric figure of merit is studied in the two cases. Increased confinement causes a drastic reduction in the overall available density of states leading to a decrease in the electrical conductivity while increased boundary scattering of phonons causes the thermal conductivity also to decrease. Results show a competing effect between the decrease in the electrical and thermal conductivity on the overall figure of merit leading to a two to four orders of magnitude decrease in the value of ZT for the case of the two dimensionally confined wires