Bandgap Engineering and Structure Analysis of ZnO/ Al2O3 Superlattices Grown by Atomic Layer Deposition
Wan-Chen Hsieh1*, Quark Y. Chen1,4, Paritosh V. Wadekar1, Chun-Fu Chang1, Hui-Chun Huang2, Cheng-Min Shiau1, Yen-Ping Cheng1, Yu-Syuan Hong1, Chun-Yuan Dang1, Po-Cheng Kung1, Chiao-Han Lee1, Shih-Hao Huang1, Zong-Yu Wu1, Yi-Ying Liang1, Che-Min Lin1, Shou-Ting You1, Li-Wei Tu1, New-Jin Ho2, Hye-Won Seo3, Wei-Kan Chu4
1Department of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan
2Department of Material Science and Optoelectronics, National Sun Yat-Sen University, Kaohsiung, Taiwan
3Department of Physics, Jeju National University, Jeju-si, Korea
4Texas Center of Superconductivity and Department of Physics, University of Houston, Houston, TX, USA
* presenting author:Wan-Chen Hsieh,
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 amorphous Al2O3.

Keywords: Superlattices, ZnO Al2O3, Cathodoluminescence, X-ray reflectivity, Bandgap engineering