  In$_2$Se$_3$ in the three-dimensional (3D) hexagonal crystal structure with
space group $P6_1$ ($\gamma$-In$_2$Se$_3$) has a direct band gap of $\sim$1.8
eV and high absorption coefficient, making it a promising semiconductor
material for optoelectronics. Incorporating Te allows for tuning the band gap,
adding flexibility to device design and extending the application range. Here
we report the growth and characterization of $\gamma$-In$_2$Se$_3$ thin films,
and results of hybrid density functional theory calculations to assess the
electronic and optical properties of $\gamma$-In$_2$Se$_3$ and
$\gamma$-In$_2$(Se$_{1-x}$Te$_x$)$_3$ alloys. The calculated band gap of 1.84
eV for $\gamma$-In$_2$Se$_3$ is in good agreement with data from the absorption
spectrum, and the absorption coefficient is found to be as high as that of
direct band gap conventional III-V and II-VI semiconductors. Incorporation of
Te in the form of $\gamma$-In$_2$(Se$_{1-x}$Te$_x$)$_3$ alloys is an effective
way to tune the band gap from 1.84 eV down to 1.23 eV, thus covering the
optimal band gap range for solar cells. We also discuss band gap bowing and
mixing enthalpies, aiming at adding $\gamma$-In$_2$Se$_3$ and
$\gamma$-In$_2$(Se$_{1-x}$Te$_x$)$_3$ alloys to the available toolbox of
materials for solar cells and other optoelectronic devices.
