Thermoelectric transport in semiconducting nanowires
The objective of this work is to develop methods to investigate the thermoelectric (TE) transport in semiconducting nanowires (NWs). The thermal conductivity of degenerately doped electrochemically-etched (EE) silicon NWs was measured to be lower than silicon NWs synthesized by a vapor-liquid-solid (VLS) method without showing a clear dependence on the NW diameter. The thermoelectric figure of merit (ZT) at near room temperature obtained from the three measured TE properties on the same EE Si NW was found to be between 0.01 of a very rough NW and 0.08 of a relatively smooth NW, the latter of which is about four times higher than that reported for bulk p-type Si at the optimum doping concentration. In addition, the NW samples could be contaminated or oxidized during the device processing. Based on the TEM characterization, they have relatively thick oxide layer and small surface roughness, and are apparently different from the EE Si NWs that a Berkeley team reported. Typical rough NWs reported by the Berkeley team have thin oxide layer and are free of major structural defects. Hence, given the significant structural differences in the samples, it would be scientifically inappropriate to compare the transport properties obtained from the two studies. In addition, a five to ten fold reduction in thermal conductivity was observed in wurtzite InAs NWs compared to bulk InAs of zinc blend phase, and is mainly attributed to diffuse surface scattering of phonons. Moreover, InSb NWs have been synthesized at three different base pressures. The NWs were found to be zinc-blende structure with <110> growth direction. 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. The ZT of one NW grown at a high vacuum base pressure is 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.