  Gallium oxide films were grown by HVPE on (0001) sapphire substrates with and
without $\alpha$-Cr$_2$O$_3$ buffer produced by RF magnetron sputtering.
Deposition on bare sapphire substrates resulted in a mixture of
$\alpha$-Ga$_2$O$_3$ and $\epsilon$-Ga$_2$O$_3$ phases with a dislocation
density of about $2\cdot10^{10}$ cm$^{-2}$. The insertion of
$\alpha$-Ga$_2$O$_3$ buffer layers resulted in phase-pure $\alpha$-Ga$_2$O$_3$
films and a fourfold reduction of the dislocation density to $5 \cdot 10^9$
cm$^{-2}$.
