Finite element modelling to evaluate the cross-plane thermal conductivity and seebeck coefficient of Ge/SiGe heterostructure

Finite element modelling (FEM) is used to evaluate a new experimental procedure for extracting the thermal conductivity and Seebeck coefficients from micro-fabricated structures with integrated heaters, thermometers and Ohmic contacts. This experimental technique was designed to simultaneously produce and measure the differential temperature and voltage output for the determination of both thermoelectric properties of Ge/SiGeheterostructures. This study demonstrates the reliability of the FEM modelling by comparing the simulation results with experiment. It demonstrates how FEM can be implemented to complement the experiments: it can be used to observe the heat distribution in the heterostructure. Therefore all the mechanisms of parasitic heat loss in the devices can be evaluated in order to improve the accuracy of the extraction of thermal coefficients and especially the thermal conductivity. This will allow for further analysis to be taken to optimize the ZT and power factor for the Ge/SiGeheterostructures.The micro-fabricated structures consisted of an etched mesa of the Ge/SiGeheterostructure with Ohmic contacts, for measuring the voltages, at the top and bottom. On the top of the mesa, a 30 nm thick Si3N4 insulator was applied followed by a Ti/Pt thermometer, another 70 nm of Si3N4 and then a NiCr heater. A second thermometer was micro-fabricated at the bottom of the mesa to allow the ▵T across the material to be measured as a function of the heater power [1]. The results obtained from the FEM studies closely matches the experimental results in [1]