Please use this identifier to cite or link to this item: http://scholarbank.nus.edu.sg/handle/10635/33308
Title: Electrical-thermal energy transfer and energy conversion in semiconductor nanowires
Authors: SHI LIHONG
Keywords: thermoelectric, semiconductor nanowires, Seebeck coefficient, figure of merit, power factor, DFT
Issue Date: 5-Aug-2011
Source: SHI LIHONG (2011-08-05). Electrical-thermal energy transfer and energy conversion in semiconductor nanowires. ScholarBank@NUS Repository.
Abstract: In this thesis, we firstly combine the Boltzmann Transport Theory and the first principle method to investigate the size dependence of thermoelectric properties of silicon nanowires (SiNWs). With cross section area increasing, the electrical conductivity increases slowly, while the Seebeck coefficient reduces remarkably. This leads to a quick reduction of cooling power factor with diameter. Moreover, the figure of merit also decreases with transverse size. Our results demonstrate that in thermoelectric application, NW with small diameter is preferred.We also predict that isotopic doping can increase the value of ZT significantly. With 50% 29Si doping (28Si0.5 29Si0.5 NW), the ZT can be increased by 31%. Besides the Si NWs, we also use first-principles electronic structure calculation and Boltzmann transport equation to investigate composition effects on the thermoelectric properties of silicon-germanium Si1-xGex NWs. The power factor and figure of merit in n-type Si1-xGex wires are much larger than those in their p-type counterparts with the same Ge content and doping concentration. Moreover, the maximal obtainable figure of merit can be increased by a factor of 4.3 in n-type Si0.5Ge0.5 NWs, compared with the corresponding values in pure silicon nanowires (SiNWs). Given the fact that the measured ZT of n-type SiNW is 0.6 . -1.0, we expect ZT value of n-type Si1.xGex NWs to be 2.5 . 4.0. Recently, Znic Oxide (ZnO) nanowires (NWs) have shown promise for nanodevice applications. However, rare researches are concerning about the thermoelectric properties of ZnO wires. In this thesis, we use the first-principle electronic structure calculation and Boltzmann transport equation to investigate the impacts of phase transition and Gallium (Ga) doping on the thermoelectric properties of [0001] ZnO NWs. The phase transition has played an important role in electronic conduction and thermal conduction in ZnO NWs, but this effect on thermoelectric is still unclear. Our results show that the electronic band gap of ZnO NWs for Wurtzite (W) phase is larger than that of Hexagonal (H) phase. For a certain carrier concentration, the Seebeck coefficient S for W-phase is larger than that for H-phase, while electrical conductivity with H-Phase is much higher than that of W-Phase because of the higher electron mobility in H-Phase. There is an optimal carrier concentration to achieve the maximum value of power factor P for both W and H phases. The maximum value of P (Pmax) for H phase (Pmax = 1638?W/m - K2) is larger than that of W phase (Pmax = 1213?W/m-K2) due to its high electrical conductivity. Provided that the thermal conductivity for H phase is about 20% larger than that for W phase, the maximum achievable value of figure of merit ZT for H phase is larger than that for W phase (1.1 times).We also study the impact of the Ga doping effect on the thermoelectric properties of [0001] ZnO NWs. Our results show that the thermoelectric performance of the Ga-doped ZnO (Zn1-xGaxO ) NWs is strongly dependent on the Ga contents. The maximum achieved room-temperature thermoelectric figure of merit in Zn1-xGaxO can be increased by a factor 2.5 at Ga content is 0.04, compared with the corresponding pure ZnO wires. Finally, we investigate the thermoelectric figure of merit in [001] Si0:5Ge0:5 superlattice (SL) nanowires (NWs). In this work, we combine the charge transport and the phonon transport to study the interface effect on the thermoelectric properties of this SL NWs.
URI: http://scholarbank.nus.edu.sg/handle/10635/33308
Appears in Collections:Ph.D Theses (Open)

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