Heat Transfer Mechanisms in Silicon Nanostructured Materials

Abstract

As energy supply and demand problems increases, energy harvesting technologies have been receiving a lot of attention around the world. Among them, thermoelectric devices (TEDs), which directly convert thermal energy into electrical energy, are a promising technology for newly renewable energy. Using a thermoelectric device, we can produce electricity by converting it into electrical energy in places where there is a temperature difference. Conversely, cooling can also be achieved by creating a temperature difference using electrical energy. However, this technology has low energy conversion efficiency and are not widely used. Currently, it is used in aerospace fields, including artificial satellites, small refrigerators, and water purifiers. To overcome these limitations in thermoelectric efficiency, various thermoelectric devices have been developed. Among them, nanotechnology is in the spotlight as it has succeeded in overcoming the material limitations of thermoelectric devices and proving a higher figure of merit. The SnSe, BiTe, and PbTe based materials which have been mainly used in TEDs are compound semiconductors and they have been produced in limited quantities because they are difficult to utilize the semiconductor process. However, Silicon (Si) nanostructures can be manufactured in various ways using the complementary metal-oxide-semiconductor (CMOS) processing. Additionally, because it has advantages in manufacturing TED devices, many studies have been conducted in parallel with processes such as lithography. We conducted research on the development of thermoelectric devices (TEDs) using silicon nanowires (SiNWs). In previously reported SiNWs, thermal conductivity (к) was reduced mainly by inducing scattering through diameter reduction and doping process. In particular, a surface modulation effect was additionally used to reduce thermal conductivity. Through surface modulation using corrugated surfaces, we induced phonons to be scattered backwards and found that these collisions reduced the thermal conductivity by almost two orders of magnitude compared to cylindrical SiNWs. As a result, undoped CSiNW (Silicon nanowire with corrugated surfaces) with DNW of 201 nm shows к of 16.8 W·m–1·K–1 and P-doped CSiNWs with DNW of 223 and 173 nm have к of 9.64 and 6.1 W·m– 1·K–1. Lastly, B-doped CSiNWs with DNW of 230 nm and 191 nm show к of 7.6 and 4.9 W·m–1·K–1 at 300 K. Electrical conductivity (σ) and Seebeck coefficient (S) were also measured at 20K intervals from 200 to 400K. Electrical conductivity was measured using the 4-point probe method, and it was confirmed that there was no particular effect on surface shape or roughness. In the same context, the Seebeck coefficient is similar to electrical conductivity. Consequently, two P-doped CSiNWs with DNW of 223 and 173 nm demonstrate σ values of 1660 and 1515 S·cm–1. Similarly, two B-doped CSiNWs with DNW of 230 and 191 nm exhibit σ values of 885 and 780 S·cm–1, respectively, at 300 K. The S values are found to be 135 and 143 μV·K–1 for two P-doped CSiNWs with 223 and 173nm, respectively. Also, in B-doped CSiNWs with 230 and 191nm, S values demonstrate 180, 188 μV·K–1 at 300 K. The values of the figure of merit (ZT) were calculated from the measured σ, к, and S. ZT values have 0.314 (DNW:173 nm), 0.237 (DNW: 223 nm) for P-doped, and 0.268 (DNW:191 nm) and 0.215 (DNW: 230 nm) for B-doped CSiNWs at 400 K, respectively.