Thickness dependent optical properties of amorphous/polycrystalline Ga2O3 thin films grown by plasma-enhanced atomic layer deposition

•Ga2O3 films with variable thickness were deposited by atomic layer deposition.•The optical constants of the Ga2O3 were determined using spectroscopic ellipsometry.•Comparison of structure and permittivity of the amorphous and polycrystalline films obtained.•The bandgap of the films increases as thickness decreases. Amorphous/polycrystalline Ga2O3 is attracting considerable attention because its low cost and convenience of use make it a promising material for applications of solar blind ultraviolet detectors and power electronics, except for the widely investigated α/β phase Ga2O3. Spectroscopic ellipsometry (SE) is the preferred technique for determining optical constants that are vital for device design. The bandgap energy of Ga2O3 is close to the upper energy limit of commercial ellipsometers, making it challenging to determine the optical constants of Ga2O3 films. Considering the results obtained using the point-by-point method, we developed a strategy using 2 Tauc-Lorentz (2TL) oscillators that reasonably describes the optical behavior of the amorphous/polycrystalline Ga2O3 films deposited by plasma-enhanced atomic layer deposition on Si, sapphire, and Si/SiO2 substrates. The refractive index of the Ga2O3 films increases after annealing due to the reduced thickness and polycrystalline structure of the films. The polycrystalline samples have larger bandgaps than amorphous samples. The optical constants of Ga2O3 films with thicknesses ranging from ∼ 3 to ∼ 28 nm were extracted through SE fitting by the developed 2TL method. For both the as-deposited and annealed films, the bandgap increases as the film thickness decreases. The obtained results are useful for the design of photodetectors and power devices.