Thermoelectric Properties of P-Doped and B-Doped Polycrystalline Silicon Thin Films

Thermoelectric power generation (TEG) is one of the ambient energy sources and a clean power generation method that directly converts heat, an unutilized energy source, into electrical energy. TEG modules are safe, maintenance-free, and can be expected to be used over the long term because they can be operated with a simple structure that has no driving parts.<br/>Silicon (Si) is the 2nd most abundant materials in the Earth’s crust and is relatively cheap and non-toxic. However, its high thermal conductivity <i>κ</i> is a bottleneck for usage as a TE material. Many silicon-based next-generation TE materials have been investigated to reduce <i>κ</i> and improve the Seebeck coefficient <i>S</i>. In TE materials, a structure called PGEC that behaves like a glass for the phonons responsible for thermal conduction and like a crystal for the carriers responsible for electrical conduction is considered suitable. One of the strategies to decrease high <i>κ</i> by phonon-scattering and maintain high electrical conductivity <i>σ</i> is nanostructure by developing polycrystalline Si (poly-Si) from amorphous Si (a-Si).<br/>This study focused on the annealing process of P-doped and B-doped a-Si thin films prepared by plasma-enhanced chemical vapor deposition (PECVD). In PECVD, it is easy to control a doping concentration at the high doping level, since PECVD is a non-equilibrium process. Moreover, the effective <i>κ</i> of thin films can be decreased by reducing the thickness due to the influence of interfacial thermal resistance and surface phonon scattering. After post-annealing, TE properties of P-doped or B-doped poly-Si thin films were measured from room temperature to 873 K.<br/>P-doped or B-doped hydrogenated a-Si (a-Si:H) thin films were prepared on a quartz substrate by PECVD. After deposition, the samples were annealed at 1173 K for 30 minutes under a forming gas atmosphere (N₂: 97%, H₂: 3%). The <i>σ</i>, the <i>S</i>, and thermal diffusivity were evaluated under an Ar atmosphere using a TE property measurement system. The <i>κ</i> was estimated from the data of thermal diffusivity, density, and mass heat capacity of a-Si. From spectroscopic ellipsometry, the thicknesses of P-doped and B-doped poly-Si thin films were estimated to be 480 and 440 nm, respectively. In Raman scattering spectra, the peaks derived from the crystalline silicon phase did not appear in both samples. a sharp peak due to the crystalline silicon phase was observed at 514 cm^-1 for both samples. Crystal volume fractions of P-doped and B-doped thin films were estimated to be 0.32 and 0.51, respectively. The <i>S</i> for both types tended to increase with increasing temperature up to -231 μV/K and 301 μV/K. The <i>σ</i> tended to decrease for P-doped and increase for B-doped with increasing temperature. On the other hand, the <i>κ</i> was not changed with temperature and was lower by about two orders of magnitude compared with that of bulk Si. In the case of P-doped poly-Si, the dimensionless figure of merit <i>ZT</i> exceeded unity around 573 K, and <i>ZT</i> = 1.92 was obtained around 873 K. This <i>ZT</i> value is relatively high compared with Si-based TE materials previously reported. This may be due to the high <i>σ</i> of 629 S/cm with a doping concentration of ~10²¹ cm^-3 and the low <i>κ</i> of 1.51 W･m^-1･K^-1at 873 K. The increase in crystallinity enhances dopant activation, leading to the increase in carrier concentration, and also enhances carrier mobility. That is why high <i>σ</i> was obtained. Moreover, the low <i>κ</i> was obtained, which is as low as that of quartz. This may be due to the thermal interfacial resistance (<i>R</i>_int). When the thickness of the poly-Si thin film and the intrinsic <i>κ</i> of bulk Si at 873 K were assumed to be 480 nm and 38 W･m^-1･K^-1, the <i>R</i>_int was estimated to be 3.05×10^-7 m²･K･W^-1, which is in the range of previously reported <i>R</i>_int at a Si/SiO₂ interface, suggesting that the low <i>κ</i> is due to <i>R</i>_int. In the presentation, we will also provide the dependence of TE properties on crystal volume fraction and doping concentration.