||RF magnetron sputtering has been used to deposit thin films of TiO2/HfO2 superlattices|
in alternating sequence on c-oriented sapphire substrates using titanium, or titanium
dioxide, and Hafnium dioxide targets. The thin films thus obtained have higher
dielectric constants and better insulation properties. The dielectric characteristics were
studied on samples of different numbers of cycles and thickness ratios of the two oxides.
Two series of samples, classified as series-A for [(TiO21nm/HfO21nm)10cycles and
(TiO23nm/HfO21nm)10cycles], and as series-B for [(TiO21nm/HfO21nm)5cycles and
(TiO23nm/HfO21nm)5cycles]. X-ray diffractometry (XRD) and transmission electron
microscopy (TEM) have been conducted to examine the structural properties of the
samples in relation to the dielectric properties, and lock-in amplifier used to measure
the internal impedance of the samples from 0.1 Hz to 100kHz. Change of voltage as a
function of frequency was used to calculate the values the frequency dependence of
involved capacitance and inductance.
TEM electron diffraction patterns showed that the HfO2 layers are of a few
nanometer thick and grow epitaxially on the TiO2 thin film by assuming the same
crystal structure of the underlying TiO2 layer. Traditional wisdom suggests that TiO2
thin films grown on c-sapphire should at best be twinned, if epitaxial at all. However,
in this work, the structure of the obtained TiO2 was found to be epitaxial and twin-free.
This is interpreted as associated with the overall chemical bonding forces among the Ti,
O, and Al atoms of the TiO2 (011)-plane on sapphire (0001)-plane. This led to an ideal
epitaxy with orientation relations of (TiO2)(011)//Al2O3(0001), TiO2// Al2O3
 and TiO2// Al2O3 .