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

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.

Before selecting the SiB growth temperature, a preliminary study on deposition temperature, from 400 to 850°C, was carried out.XRD data show that temperatures above 800 o C are required for good crystallization.This is demonstrated by the increased intensity and sharpness of the Si (111) reflection observed for higher temperatures, as well as the presence of the Si (311) and ( 220) peaks only at temperatures above 700°C.
The presence of Au in the film volume, which is not removable with the KI solution, has been studied in a deposition temperature series previous to the one presented in the main text.For this, electron backscattering imaging with a scanning electron microscope (EBSSEM) is used.The EBS-SEM images on cross-section of the samples is shown in Fig. S1 indicating the presence of embedded Au clusters in the film volume.These Au clusters increase the electrical conductivity while decrease the Seebeck coefficient and the power factor.For temperatures between 400-600 o C the Au is not migrating to the surface and shortcircuits the film.At 700 o C the situation is improved but still a large Au content is observed.
For 800 o C and above only single unconnected particles are seen.For temperatures above S2 800 o C an adequate migration of the Au to the film surface is observed.The gold layer at the surface can then be removed well with KI solution.

Figure S 1 :
Figure S 1: Cross-section EBS-SEM of samples for different growth temperatures.The bright spots correspond to Au (due to the high atomic number) trapped inside the Si film.In addition to the SEM imaging, the surface has been characterized with atomic force microscope (AFM).Two 5µm x 5µm images corresponding to two samples with different boron RF power are shown in Fig. S2.The observed granular structure is similar to the one seen in the SEM images.The RMS roughness of the samples is of 60nm.Additional Raman data shown in Fig. S3 also support the use of deposition temperature above 800 o C. The spectrum of a sample deposited at 600 o C in Fig. S3a shows a very broad peak at 470cm −1 corresponding to amorphous Si compared to the sharp and high intense peaks at 530cm −1 for the 800 o C and 700 o C grown layers corresponding to crystallized Si.Moreover, Fig. S3b demonstrates the higher symmetry of the Si peak for the 800 o C sample and the presence of a shoulder reminiscent of the amorphous Si for the 700 o C sample.In conclusion, we can conclude from the XRD and Raman spectra measurements that the lower temperature samples (500 o C and 600 o C) present a low partial crystallization for the Si film and the fully crystallization of the Si film is obtained for the high temperature samples with an optimum crystallization at 800 o C and above.

Figure S 2 :
Figure S 2: AFM images (5µm x 5µm) of SiB films deposited with a RF power of 40 and 60W.

Fig
Fig.S4shows, in double logarithmic scale, the dependence of the resistivity on the carrier concentration.The qualitative behavior and absolute values are similar to the ones reported for boron-doped Si.1,2The mobility values for the charge carrier in our samples are of 3-6 cm 2 /Vs.These values do not change significantly with doping content.The values are lower than the ones reported for boron-doped Si, 2 owing probably to the granular structure of our films.

Figure S 3
Figure S 3: a) Raman spectra for the 600 o C, 700 o C and 800 o C samples.b) Linear scale for the highest temperature samples highlighting the asymmetry of the 700 o C Si peak compared to the 800 o C one.