  This paper reports on the deposition and photoluminescence of amorphous and
crystalline thin films of zinc gallium oxide with Ga:Zn atomic ratio varied
between 0.3 and 5.7. The films are prepared by reactive direct current
magnetron co-sputtering from liquid/solid gallium/zinc targets onto fused
quartz substrates; the temperature of the substrate is varied from room
temperature (RT) to 800{\deg}C. The sputtering process is effectively
controlled by fixing the sputtering power of one of the targets and controlling
the power of the other target by plasma optical emission spectroscopy. The
method, in conjunction with oxygen flow adjustment, enables the production of
near-stoichiometric films at any temperature used. The composition analysis
suggests a few at.% oxygen deficiency in the films. The resulting deposition
rate is at least an order of magnitude higher compared to the commonly used
radio-frequency sputtering from a ceramic ZnO:Ga2O3 target. Deposited onto
unheated substrates, the films with Ga:Zn {\approx} 2 are X-ray amorphous.
Well-defined X-ray diffraction peaks of spinel ZnGa2O4 start to appear at a
substrate temperature of 300{\deg}C. The surface of the as-deposited films is
dense and exhibits a fine-featured structure observed in electron microscopy
images. Increasing the deposition temperature from RT to 800{\deg}C eliminates
defects and improves crystallinity, which for the films with Ga:Zn ratio close
to 2 results in an increase in the optical band gap from 4.6 eV to 5.1 eV. Room
temperature photoluminescence established the main peak at 3.1 eV (400 nm); a
similar peak in Ga2O3 is ascribed to oxygen-vacancy related transitions. A
prominent feature around 2.9 eV (428 nm) is attributed to self-activation
center of the octahedral Ga-O groups in the spinel lattice of ZnGa2O4. It was
found that photoluminescence from ZnGa2O4 depends significantly on the ratio
Ga:Zn.
