Y. J. He

THERMOELECTRIC EFFECT OF SILICON FILMS WITH SHALLOW- AND DEEP-LEVEL ACCEPTORS

Crystalline Si films with both shallow- and deep-level acceptors, Al and Cu, have been prepared on glass and quartz substrates by the methods of magnetron sputtering and Al-induced crystallization. Al and Cu are co-added in the Si films intermittently by regular pulse sputtering of Al and Cu targets during deposition of the Si films. By regulating the sputtering times of Al and Cu targets, the amounts of Al and Cu in the Si films can be controlled, and thus the Seebeck coefficient and electrical resistivity of the silicon films can be adjusted. It is found that the Al and Cu co-doped Si film has a larger Seebeck coefficient and a lower electrical resistivity at higher temperatures, as compared with that of only Al-doped Si film. As a result, the thermoelectric power factor of the Al and Cu co-doped Si film is greatly enhanced. The present experimental results will not only help us to understand the basic thermoelectric properties of semiconductors doubly doped with shallow- and deep-level impurities, but also open the possibility of enhancement of thermoelectric power factor by using this concept.

INFLUENCE OF SILICON ADDITION ON SEEBECK COEFFICIENT OF MnSi1.7 FILMS

MnSi1.7 films with different thicknesses (16–242 nm) are prepared by magnetron sputtering and electron beam evaporation. When the MnSi1.7 film thickness is about 40 nm or above, MnSi1.7 films are p-type in the whole temperature range (300–700 K) in agreement with reports in literature. By co-sputtering of MnSi1.85 and silicon targets or deposition of Si/Mn multi-layers with a larger thickness ratio, silicon is added to the films and the Seebeck coefficients transform from positive to negative with increasing temperature. The Seebeck coefficients at room temperature and 633 K are +0.098 mV/K and -0.358 mV/K, respectively. By reducing the MnSi1.7 film thickness to 27 nm, the transition of Seebeck coefficient from positive to negative is also observed although silicon is not added intentionally. When an ultra-thin aluminum layer is deposited between MnSix(x < 1.7) and Si layers to enhance silicon diffusion, the p- to n-type transition temperature decreases about 100 K. The silicon-added MnSi1.7 films usually have higher electrical resistivity.

Thermoelectric properties of MnSi 1.7

Higher manganese silicide (MnSi1.7) bulk and thin-film materials have been prepared by the hot-pressed and electron-beam evaporation methods, respectively. The Seebeck coefficient, electrical resistivity, and thermal conductivity (for bulk material only) are measured from room temperature to 783 K. The un-doped bulk and thin-film MnSi1.7 are p-type. The figure of merit, ZT, of the bulk material can be reached to 0.29 at 723 K. N-type MnSi1.7 film can be obtained by addition of iron into the film. It is found that the thermoelectric properties of n-type MnSi1.7 are better than those of p-type MnSi1.7, which is consistent with the recent thoeretical prediction.