  The influence of Ga incorporation into cubic In$_2$O$_3$ on the electronic
and vibrational properties is discussed for (In$_{1-x}$,Ga$_x$)$_2$O$_3$ alloy
films grown by molecular beam epitaxy. Using spectroscopic ellipsometry, a
linear dependence of the absorption onset on the Ga content $x$ is found with a
blueshift of up to 150 meV for $x = 0.1$. Consistently, the fundamental band
gap exhibits a blueshift as determined by hard X-ray photoelectron
spectroscopy. The dependence of the absorption onset and the effective electron
mass on the electron concentration is derived from the infrared dielectric
functions for a Sn doped alloy film. The influence of alloying on phonon modes
is analyzed on the basis of Raman spectroscopic measurements. The frequencies
of several phonon modes are identified as sensitive measures for the
spectroscopic determination of the Ga content.
