Thickness dependent optical properties of amorphous/polycrystalline Ga 2 O 3 thin films grown by plasma-enhanced atomic layer deposition

Amorphous/polycrystalline Ga 2 O 3 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 Ga 2 O 3. Spectroscopic ellipsometry (SE) is the preferred technique for determining optical constants that are vital for device design. The bandgap energy of Ga 2 O 3 is close to the upper energy limit of commercial ellipsometers, making it challenging to determine the optical constants of Ga 2 O 3 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 Ga 2 O 3 films deposited by plasma-enhanced atomic layer deposition on Si, sapphire, and Si/SiO 2 substrates. The refractive index of the Ga 2 O 3 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 Ga 2 O 3 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.