Heavily B-doped SiGe thin films was reported to have large thermoelectric power and power factor at room temperature after annealing. In this paper, we investigated the structures that give rise to the large thermoelectric power and power factor. The thin films were prepared by ionbeam sputtering method. The annealing temperature dependence of structural properties was investigated. The thin films exhibited large thermoelectric power (1.4 mVK À1) and power factor (6:8 Â 10 À3 Wm À1 K À2) at room temperature after annealing at around 900 K. At that region, crystallite diameter was below 10 nm. The structure changed from amorphous to microcrystalline over 900 K. It is considered that quantum size effect enhances the thermoelectric power and power factor.
structured thin films were prepared by using the ion-beam sputtering (IBS) method with changing the growth temperatures from room temperature to 873 K. Then their thermoelectric properties were estimated and the effects of boron on Si/Ge superlattice structure were investigated. It was shown from XRD spectra that samples prepared below 673 K kept the periodic structure, however at exceeding 773 K the structure was broken. Si/Ge multilayer thin films showed larger thermoelectric power than that of bulk-SiGe materials and also Si/GeB multilayer showed decreases of thermoelectric power with increasing growth temperature, which is related to the increase of carrier concentration by boron activation. As for the resistivity, since the Si/GeB multilayers had lower values of the resistivity than that of Si/Ge multilayers, a boron doping effect was identified. In both Si/Ge and Si/GeB, the resistivity had a minimum at 673 K. Since the multilayer structure holds up to this temperature, the decrease in the resistivity can be attributed to the increase in the carrier mobility of the layer with a lower band gap. As a result, Si/GeB showed the larger power factor than that of Si/Ge multilayer at 673 K.
Si/GeB multilayers were prepared by ion-beam sputtering technique and the effect of the crystallinity on thermoelectric properties was investigated. The crystallinity of the samples was controlled using Si buffer and SiO2 layers. Samples prepared on the Si buffer layer show similar properties to samples prepared on a Si wafer without the buffer layer. Moreover, it is found that the resistivity of the samples prepared on the Si buffer layer is lower than that of the samples prepared on the Si wafer without the buffer layer because of the surface cleaning effect. On the other hand, samples prepared on SiO2 show poor thermoelectric properties. These results are caused by the change in the crystallinity and periodicity, which are related to the carrier confinement effect. After thermal annealing, it is found that the multilayer structure is degraded rapidly and that the sign of the thermoelectric power is changed. This is considered to be caused by crystalline defects in the interfaces due to the collapse of the multilayer structure.
Thermoelectric properties of epitaxially grown SiGe based thin films prepared by ion beam sputtering technique were investigated. Although carrier concentration was considered to be the highest in B doped SiGe sample, it showed the highest thermoelectric power of $1 mVK À1 corresponding to the three times larger value than bulk SiGe. On the other hand, the electrical resistivity decreased by increasing the growth temperature due to the impurity activation and crystallization. As a result, SiGeB epitaxial films prepared at growth temperatures of more than 773 K showed twice larger power factor than bulk SiGe at 300 K. Moreover, the thermoelectric power was found to decrease and resistivity to increase in the polycrystalline phase, concluding that the improvement of thermoelectric performances was achieved by introducing the epitaxially grown phase.
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