Thermoelectric Transport in Semiconducting Nanowires

The objectives of the work presented in this dissertation are to further develop methods for combined structure and thermoelectric (TE) characterizations of semiconducting nanowires (NWs), and to employ these methods to investigate TE transport in NWs and correlate the TE properties with its structural characteristics. Three semiconducting NW systems have been investigated in this dissertation, namely silicon NWs synthesized by electroless etching (EE) method, InAs NWs synthesized by chemical beam epitaxy, and InSb NWs grown at different base pressures using a vapor liquid solid (VLS) method. The characterization results are used to investigate the effects of size confinement on electron and phonon transport in NWs.;TE properties of EE silicon NWs have been measured. A method that combines HF etching, focused electron beam induced platinum deposition, and annealing in hydrogen was developed to make electrical contacts between the NWs and the supporting Pt electrodes on the suspended membrane. A four-probe TE measurement method was used to evaluate and eliminate the error due to contact thermal resistance for three samples. The thermal conductivity of the degenerately doped EE silicon NWs was found to be lower than silicon NWs synthesized by VLS without showing a clear dependence on the NW diameter. It is found in the TEM and tilted SEM analysis that the surface roughness varied among the measured NWs. In particular, one NW was found to be much rougher than the other NWs. The measured thermal conductivity of the NW with very rough surface is much lower than those of other NWs with smoother surfaces. The rough NW with the lowest thermal conductivity was also found to possess the lowest electrical conductivity. Among the other three NWs, a NW with a lower electrical conductivity and a rougher surface was found to have a lower thermal conductivity than another NW with a similar diameter. Hence, it appears that the observed thermal conductivity variation in EE Si NWs is correlated with the variation in the surface roughness.;The thermal conductivity of InAs NWs with a wurtzite structure was characterized. At room temperature and below, the thermal conductivity of the NWs with proper thermal contacts was found to decrease with decreasing diameter, revealing the effect of increased phonon surface scattering with decreasing NW diameter. At temperature above the room temperature, the difference in the thermal conductivity of NWs of different diameters was found to decrease with increasing temperature, suggesting that Umklapp phonon scattering starts to become the dominant scattering process. Compared to bulk InAs of zinc blend phase, a five to ten fold reduction in thermal conductivity was observed in the wurtzite NWs. In addition to a reduced phonon boundary scattering mean free path in the NWs, the thermal conductivity reduction is attributed to a lower average phonon group velocity in the InAs NWs of wurtzite phase, which has more atoms in the primitive unit cell and thus more optical phonon branches of low group velocity than the zinc blende phase. In addition, with the use of one broken InAs NW sample where the broken NW edge makes a point contact to the side surface of the SiNx membrane, the interface thermal resistances was measured and compared with an analytical model.;InSb nanowires (NWs) have been synthesized at three different base pressures varying from ambient pressure to 10-6 Torr. The NWs were found to be zinc-blende structure with growth direction. The Seebeck coefficient was found to increase with decreasing base pressure, indicating that oxygen is an n-type dopant. The thermal conductivity of the NWs was found to be suppressed by diffuse phonon-surface scattering. The ZT of the two NWs is about 10 times lower than the bulk values mainly because of the much higher doping levels in NWs than the bulk as well as mobility suppression in the NWs. In addition, the ZT of one NW grown at a high vacuum base pressure was found to be higher than another NW grown at low vacuum. These results show that it is necessary to better control the impurity doping in order to increase the ZT of the InSb NWs. (Abstract shortened by UMI.).