Atomic layer annealing enables low-temperature PEALD growth of highly c-axis-oriented GaN thin films on sapphire

Low-temperature growth of crystalline GaN remains challenging for plasma-enhanced atomic layer deposition (PEALD) because limited adatom mobility often leads to poorly ordered films. Here, we investigate a cycle-integrated atomic layer annealing (ALA) strategy to enhance the crystallinity of GaN thin films grown on Al 2 O 3 (0001) by PEALD at 300 °C. Trimethylgallium and NH 3 plasma were used as the Ga and N sources, and an Ar plasma ALA step was inserted after each PEALD cycle to promote plasma-assisted atomic rearrangement. The ALA duration was varied from 0 to 60 s for non-pretreated samples, and NH 3 -plasma-pretreated samples were examined at 30 and 90 s ALA. X-ray diffraction and rocking-curve analysis show that the GaN(002) rocking-curve FWHM decreases from approximately 5.9° without ALA to 0.050° at 60 s ALA, indicating strongly enhanced out-of-plane c-axis orientation. At the same 30 s ALA duration, NH 3 plasma pre-treatment suppresses the secondary GaN-related diffraction component observed in the non-pretreated film, while the fitted FWHM values of ALA30 (0.083°) and P-ALA30 (0.078°) differ only slightly and are not interpreted as statistically different. Within the pretreated series, P-ALA90 exhibits a broader rocking-curve FWHM (0.091°) than P-ALA30; because a non-pretreated ALA90 control was not included, no specific degradation mechanism is assigned. Cross-sectional TEM and selected AFM and XPS measurements provide complementary microstructural, morphological, and chemical-state characterization. These results establish an ALA-duration-dependent process-window perspective for the low-temperature PEALD growth of highly c-axis-oriented GaN thin films on sapphire.