**2. Ge/TiO2 composite thin films**

TiO2 mainly has crystal structures of rutile, anatase and brookite. It is believed that the anatase structure is favorable for the matrix, since carrier mobility and photoconductivity in the anatase structure exceed those in the rutile structure (Tang et al., 1994). It is difficult to forecast how the crystal structure of the TiO2 matrix will be formed in such composite films. In fact, Ge/TiO2 films prepared by rf sputtering employing a mixture target of TiO2 and Ge powder hitherto contained anatase- and rutile -structure almost equally (Chatterjee, 2008). Hence, it is investigated here that the composition of Ge/TiO2 films is thoroughly varied for preparing the anatase structure of the TiO2 matrix while retaining vis-NIR absorption of Ge quantum dots.
