Positron spectroscopy of high-k dielectric films on SiC

A correlation has been observed between the structural integrity and growth temperature of ∼80 nm-thick TiO2 films grown on SiO2/4H-SiC substrates as high-k dielectric stacks for power device applications, using variable-energy positron annihilation spectroscopy. The structural change, tentatively attributed to a decrease in oxygen vacancy concentration, was observed as the TiO2 growth temperature was increased in the range 700 to 1000 °C, whereas grain boundaries form in the polycrystalline material grown at the highest temperatures. It is proposed that the optimal electrical performance for films grown at 800 °C reflects a balance between decreasing vacancy concentration and increasing grain boundary formation. Because positrons are swept to the SiO2/SiC interface by a field in the substrate, VEPAS is strongly sensitive to interface states, which can have important consequences for channel mobility and thus the electrical performance of the devices. The use of a number of lineshape parameters (beyond the standard S and W) has been applied to these measurements, enabling clearer insights to be gained. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)