Sputtering Codeposition and Metal-Induced Crystallization to Enhance the Power Factor of Nanocrystalline Silicon

Abstract

The power factor of highly boron-doped nanocrystalline Si thin films with controlled doping concentration is investigated. We achieve a high degree of tuning of boron content with a charge carrier concentration from 10¹⁸ to 10²¹/cm³ and with the electrical conductivity by varying the boron magnetron power from 10 to 60 W while maintaining the power of a SiB source constant during codeposition from two independent sputtering sources. Along with the increase in the electrical conductivity with increased boron doping, we observe a steady decrease in the Seebeck coefficient from 500 to 100 μV/K. These values result in power factors that exhibit a marked maximum of 5 mW/K²m for a carrier concentration of around 10²¹/cm³ at room temperature. Temperature-dependent measurements up to 650 °C show, with increasing doping concentration, a change of the resistivity from a semiconducting to a metallic behavior and an increase of both Seebeck coefficient and power factor, with this last one peaking at 9.8 mW/K²m in the 350-550 °C temperature range. For higher concentrations, scanning electron microscopy and energy-dispersive X-ray spectroscopy show a partial segregation of boron on particles on the surface. These results exemplify the great advantage of sputtering codeposition methods to easily tune and optimize the thermoelectric performance in thin films, obtaining in our specific case highly competitive power factors in a simple and reliable manner.

Figure 1

X-ray diffraction patterns of a 115 nm thick boron-doped Si film deposited with a boron RF power of 30 W using metal-induced crystallization with a Au layer. The black lines correspond to the as-deposited state, and the red line shows the situation after the chemical etching of the Au layer.

Figure 2

Dependence of the film conductivity (a) and the charge carrier concentration (b) on the RF power of the boron source. Data are obtained at room temperature. (c) Dependence of the normalized resistivity on the temperature for Si films with different boron content.

Figure 3

(a) SEM image showing the nanocrystalline nature of the Si thin films. The image corresponds to a sample with a boron RF power of 40 W. (b) Large-scale SEM image of a film deposited at 60 W, showing the formation of particles on the surface. (c) Detailed SEM images of the particles on the same sample. (d) EDX spectra for a sample deposited at 60 W boron source power with a high doping concentration. The image shows the spectra that were taken at four different areas of the sample. Spectra 1 and 2 (black) were measured in particle-free areas; 3 and 4 (red) were taken on the particles.

Figure 4

(a) Dependence of the Si thin film conductivity at room temperature on the charge carrier concentration. (b) Relationship of the measured Seebeck coefficient at room temperature with the charge carrier concentration. (c) Temperature dependence of the Seebeck coefficient for samples with varying boron content.

Figure 5

(a) Dependence of the PF on the charge carrier concentration. The lines are only a guide for the eye. (b) Dependence of the power factor PF on the temperature for samples with different boron content.