Paper EH-ThM10
Gate-Modulated Thermoelectric Characteristics of Silicon Nanowires on Plastic Substrates

Thursday, December 11, 2014, 11:00 am, Room Lehua

Recently, several novel concepts of thermoelectric technologies have been reported as a promising source of next-generation regeneratable energy. Moreover, silicon nanowire (Si NW) based thermoelectric devices are an attractive research area due to their advantages such as non-toxicity, abundance, and low-cost manufacturing, which can lead to new opportunities in broad areas of commercial electronics. The efficiency of energy conversion has been intensively studied by modulating the dimensionless thermoelectric figure of merit, ZT = σS²T/κ, where σ is the electrical conductivity, S is the thermoelectric power (Seebeck coefficient), κ is the thermal conductivity, and T is the absolute temperature. Numerous studies indicate that the use of Si NW enables the improvement of ZT due to their low-dimensional properties. Nevertheless, it is still difficult to achieve high ZT value because σ, S and κ are interdependent. Therefore, the modification of these parameters in Si NW can enhance the thermoelectric properties. Among various methods, the field-effect modulation, applying electric fields on thermoelectric materials, may be one of the prominent ways to modulate the thermoelectric parameters. In this study, we fabricate a gated Si NW thermoelectric device constructed on a thermal-insulating plastic substrate, and examine the field-effect modulation of thermoelectric characteristics.

Figure 1 shows the optical image of our gated thermoelectric device on a plastic substrate. The structure of the device with a NW channel length of 50 μm is similar to a NW field-effect transistor. N-type Si NWs with a doped concentration of 10¹⁷ cm^-3 are present between the electrodes and a gate electrode is located at the middle of the Si NWs. And a heater electrode is used as a heating source. We investigate the seebeck voltage variation of the gated thermoelectric device with the gate bias is applied. Figure 2(a) exhibits the seebeck voltage versus temperature variation characteristics of the device with V_GS from 0 to 5 V. Our device shows the good gate controllability of the channel potential, which implies the capability of the electrical conductivity modulation for the Si NWs. Thus, as V_GS decreases, the conductivity of the Si NWs also decreases accordingly, leading to the increase of the seebeck voltage with the same temperature variation. Therefore, as shown in Figure 2(b), the seebeck coefficient, defined as –ΔV/ΔT, is able to modulate with the gate voltage variation. Our study suggests that the gated thermoelectric device with n-type Si NWs on a plastic substrate reveals its potential use as a next-generation energy harvesting device.