Effects of nanograin and micro-engineering on charge and thermal transport properties in thermoelectric SiGe thin films

With the rapid development of microelectronic and optical devices, thermal management for those devices is becoming increasingly critical. However, most of the current technologies used for cooling are often overdesigned, and not efficient in eliminating the localized hot spots. Thin film thermoelectric coolers can easily adapt to the hot spot area and actively cool it with very high cooling capacity. SiGe thin films are chosen as the target thermoelectric materials as opposed to the conventional Bi2Te3 because the former is fully compatible with microelectronics fabrication process and has more potential to improve the performance with micro- and nano-engineering. The focus of this work is to investigate the influence of nanograin and micro-engineering on the thermoelectric transport properties of SiGe thin films and develop the fundamental understanding of charge and heat transports in these engineered films. First of all, two solid ZT test structures have been developed to enable the simultaneous measurement of all the thermoelectric properties. Thereafter, boron-doped and phosphorus-doped SiGe thin films have been grown using low-pressure chemical deposition technique and their thermoelectric properties were characterized and optimized. The influences of growth condition, thermal treatment, doping level and temperature on thermoelectric transport properties have been studied. It is found that the figures of merit of optimized samples at room temperature are improved by almost 100% and 60% over the reported values of bulk counterparts for p- and n-type materials, respectively, mainly due to the significantly suppressed thermal conductivity. The improved thermoelectric performance makes the SiGe thin film quite promising in the cooling and power generation applications. Moreover, transport properties of charges and phonons are characterized and modeled to understand at the fundamental level the mobility change with different doping levels as well as the phonon suppression by different scattering mechanisms. It is found that the naturally formed columnar nanograins significantly influence both charge and phonon transport. The energy barrier on the grain boundaries can lead to unusual dependence of charge mobility on doping level and temperature. Meanwhile, the unique columnar grain structures result in remarkable thermal conductivity anisotropy with the in-plane thermal conductivities of SiGe films about 50% lower than the cross-plane values. The improvement of figure of merit is mainly due to the suppression of phonon transport by nanograin boundaries. Since the dominant phonon mean free path in SiGe is at micron level while that of charge is at nanometer level, feature size less than a few microns might benefit the thermoelectric performance of thin film. Therefore, porous thin films were fabricated using micro fabrication techniques and they were characterized with a suspended test structure. It is found that both electrical and thermal conductivities are decreased, but the ratio of the two is increased compared with the solid reference. The new finding indicates that by proper arrangement of the pores, it is possible to improve the thermoelectric performance by micro pore structures. For future studies, the composite material system comprised of SiGe matrix and silicide inclusions is proposed, which can provide a much higher electrical to thermal conductivity ratio to benefit ZT. Overall, the work has provided systematic studies to optimize the thermoelectric performance of SiGe thin films, which can be useful in guiding future work on silicide thin films for thermoelectric applications. The test structures are helpful platforms for charge and thermal transport measurement, and can be extended to the measurement of other thin film properties easily and efficiently. The modeling work has provided some insights on the physics of transport properties, which is contribution to enable further discussion on the performance enhancement strategies. Last but not least, the micro-engineering work has provided first-hand information of phonon engineering strategy towards a higher thermoelectric performance, and the proposed approach of blending a second phase into the SiGe matrix has shed light on further improvement of the thin film thermoelectric materials.