Transverse p x n type thermoelectrics: Type ii superlattices and their thermal conductivity characterization.

摘要

This dissertation proposes a paradigm for p x n type transverse thermoelectrics, in which the induced heat flow is orthogonal to the applied electric current or conversely, the generated electric field is orthogonal to the applied temperature gradient. p x n type materials have a net p-type Seebeck coefficient in one direction and a net n-type Seebeck coefficient in an orthogonal direction, breaking the symmetry of the Seebeck tensor and inducing a transverse Seebeck coefficient. p x n type transverse hermoelectrics have advantages in microscale devices, cryogenic temperature operations and large temperature difference refrigeration, where the more standard longitudinal thermoelectrics have limited use. The InAs/GaSb type II superlattice is shown to be one candidate pxn type material. In a p x n type material, the anisotropic electron and hole conduction gives rise to a transverse Seebeck coefficient without the need for an external magnetic field. p x n type transverse thermoelectrics can enhance the thermoelectric performance via geometric shaping, so the figure of merit ZT no longer limits the performance of a thermoelectric material. pxn type devices can realize cooling to arbitrarily low temperature with an exponentially tapered cooler, or generating large voltage from a small temperature difference with a meander-shaped generator. The transport tensors and differential heat flow equations are derived, which are different from those of the related Nernst-Ettingshausen effect and stacked synthetic transverse thermoelectrics. The geometric enhancement of p x n type transverse thermoelectric cooling is studied, and the concept of a crossover electric field in an exponentially tapered cooler is introduced to distinguish optimal performance in thin and thick samples. The InAs/GaSb type II superlattice (T2SL) is proposed to be a promising candidate as a pxn type material based on its tunable bandgap and anisotropic electrical conductivity tensor. To estimate its p x n type transverse thermoelectric performance, the Seebeck coefficient and electrical conductivity tensors are calculated. The band structure of T2SL is simulated using 8x8 k˙p envelope function for various InAs and GaSb layer thicknesses. General forms of thermoelectric tensors, including the electrical conductivity tensor and the Seebeck tensor, for 3D, 2D and 1D cases are deduced. The electrical conductivity tensor and Seebeck tensor maps are then calculated via the equations of corresponding cases using the simulated band structure parameters. The transverse thermoelectrics power factor is then optimized for various InAs and GaSb layer thicknesses. Another important thermoelectric property, the thermal conductivity of T2SLs, is experimentally characterized using the 2-wire 3o method. So the calculated Seebeck and electrical conductivity tensors and the measured thermal conductivity tensor of T2SL allow its transverse thermoelectric figure of merit ZT to be estimated. GaAs and AlGaAs thin film and superlattice are first measured to calibrate experimental setup of the 2-wire 3o method. An error analysis method is introduced to calculate the best fit parameters and estimate the error bars of the measurement. The thermal conductivity tensors of related T2SLs are then characterized with the same experimental setup and analysis method. Since these superlattices also have wide application in high-power infrared lasers and photodiodes, the temperature dependent thermal conductivity is used to estimate the temperature distribution in the active region.