Low-Temperature As-Grown Crystalline β-Ga₂O₃ Films via Plasma-Enhanced Atomic Layer Deposition

We report on the low-temperature growth of crystalline Ga₂O₃ films on Si, sapphire, and glass substrates using plasma-enhanced atomic layer deposition (PEALD) featuring a hollow-cathode plasma source. Films were deposited by using triethylgallium (TEG) and Ar/O₂ plasma as metal precursor and oxygen co-reactant, respectively. Growth experiments have been performed within 150-240 °C substrate temperature and 30-300 W radio-frequency (rf) plasma power ranges. Additionally, each unit AB-type ALD cycle was followed by an in situ Ar plasma annealing treatment, which consisted of an extra (50-300 W) Ar plasma exposure for 20 s ending just before the next TEG pulse. The growth per cycle (GPC) of the films without Ar plasma annealing step ranged between 0.69 and 1.31 Å/cycle, and as-grown refractive indices were between 1.67 and 1.75 within the scanned plasma power range. X-ray diffraction (XRD) measurements showed that Ga₂O₃ films grown without in situ Ar plasma annealing exhibited amorphous character irrespective of substrate temperature and rf power values. With the incorporation of the in situ Ar plasma annealing process, the GPC of Ga₂O₃ films ranged between 0.76 and 1.03 Å/cycle along with higher refractive index values of 1.75-1.79. The increased refractive index (1.79) and slightly reduced GPC (1.03 Å/cycle) at 250 W plasma annealing indicated possible densification and crystallization of the films. Indeed, X-ray measurements confirmed that in situ plasma annealed films grow in a monoclinic β-Ga₂O₃ crystal phase. The film crystallinity and density further enhance (from 5.11 to 5.60 g/cm³) by increasing the rf power value used during in situ Ar plasma annealing process. X-ray photoelectron spectroscopy (XPS) measurement of the β-Ga₂O₃ sample grown under optimal in situ plasma annealing power (250 W) revealed near-ideal film stoichiometry (O/Ga of ∼1.44) with relatively low carbon content (∼5 at. %), whereas 50 W rf power treated film was highly non-stoichiometric (O/Ga of ∼2.31) with considerably elevated carbon content. Our results demonstrate the effectiveness of in situ Ar plasma annealing process to transform amorphous wide bandgap oxide semiconductors into crystalline films without needing high-temperature post-deposition annealing treatment.