Thermal and electrical properties of Si/Si0.8Ge0.2 and B4C/B9C films

Two-dimensional quantum wells have been synthesized by alternating layers of B₄C and B₉C in one system and alternating layers of Si and Si_0.8Ge_0.2 in another system. Such nanostructures are being investigated as candidate thermoelectric materials for high figures of merit (Z). The predicted enhancement is attributed to the confined motion of charge carriers and phonons in the two dimensions and separating them from the ion scattering centers. MBE and sputtering techniques have been used to prepare these multilayer films. Films have been deposited on single-crystal silicon substrates. The α and ρ properties of these films have been determined over a broad range of temperatures from 4.2 K to 1200 K and were previously reported. The α²/ρ values for these P type B-C and N type SiGe films were more than a factor of 10 to 30 times higher than bulk P type B-C and N type SiGe. Recently, thermal conductivities have also been measured with a modified 3-ω method. The room temperature thermal conductivity of Si and Si_0.8Ge_0.2 were also encouraging smaller, and about one third (1/3) of the bulk values and in line with theoretical predictions. The performance of the MBE films have been systematically compared with bulk materials. Preliminary thermoelectric measurements of the multilayer structures, lead us to believe that significant gains in the thermoelectric figure of merit (Z) may be possible with this approach. The first quantum well thermoelectric couple with N-type Si/Si_0.8Ge_0.2 and P-type B₄C/B₉C was fabricated from these films. The test results generated continue to indicate that much higher thermoelectric efficiencies can be achieved in the quantum wells compared to the bulk materials. Also, the potential cost of fabricating quantum well modules will be discussed and are expected to be much lower than present bulk thermoelectric module on a cost/watt basis