Structure and thermoelectric properties of boron doped nanocrystalline Si_0.8Ge_0.2 thin film

The structure and thermoelectric properties of boron doped nanocrystalline Si_0.8Ge_0.2 thin films are investigated for potential application in microthermoelectric devices. Nanocrystalline Si_0.8Ge_0.2 thin films are grown by low-pressure chemical vapor deposition on a sandwich of Si₃N₄/SiO₂/Si₃N₄ films deposited on a Si (100) substrate. The Si_0.8Ge_0.2 film is doped with boron by ion implantation. The structure of the thin film is studied by means of atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It is found that the film has column-shaped crystal grains ∼100 nm in diameter oriented along the thickness of the film. The electrical conductivity and Seebeck coefficient are measured in the temperature range between 80–300 and 130–300 K, respectively. The thermal conductivity is measured at room temperature by a 3ω method. As compared with bulk silicon-germanium and microcrystalline film alloys of nearly the same Si/Ge ratio and doping concentrations, the Si_0.8Ge_0.2 nanocrystalline film exhibits a twofold reduction in the thermal conductivitity, an enhancement in the Seebeck coefficient, and a reduction in the electrical conductivity. Enhanced heat carrier scattering due to the nanocrystalline structure of the films and a combined effect of boron segregation and carrier trapping at grain boundaries are believed to be res- ponsible for the measured reductions in the thermal and electrical conductivities, respectively.