Bandgap engineering and structure analysis of ZnO/ Al2O3 superlattices grown by atomic layer deposition

ALD-grown ZnO/Al₂O₃ superlattices (SLS) as analyzed by XRR assisted with GenX fittings exhibit a consistent mass density for the ZnO layers of 5.6 g/cm³, largely that of the bulk crystal. However, for Al₂O₃, the value is 2.95 g/cm³ versus the ideal 3.95 g/cm³. This discrepancy suggests a highly porous Al₂O₃, possibly due to the presence of hydrogen in an AlO(OH) amorphous boehmite phase. TEM imaging portrays the periodic structures consistent with the XRR findings. The ZnO layers are c-textured while Al₂O₃ amorphous. Room-temperature CL measurements showed decreasing ZnO bandgap as the Al₂O₃ cycles increased, hinting at feasible bandgap engineering through SLS structural variations. Amorphous Al₂O₃ is known to have a smaller bandgap of 5.7-7.1 eV as compared to 7.1-8.8 eV for bulk crystals. CL also showed a peak at 5.1 eV, thus consistent with our conjecture of the am-Al₂O₃.