Thermal Atomic Layer Etching of Gallium Oxide Using Sequential Exposures of HF and Various Metal Precursors

• Published in: Chemistry of materials Vol. 32; no. 14; pp. 5937 - 5948
• Main Authors: Lee, Younghee, Johnson, Nicholas R, George, Steven M
• Format: Journal Article
• Language: English
• Published: American Chemical Society 28.07.2020
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.0c00131

Gallium oxide (Ga2O3) is a transparent semiconducting oxide with a large band gap that has applications for power electronics and optoelectronics. Ga2O3 device fabrication requires etching for many processing steps. In this work, the thermal atomic layer etching (ALE) of Ga2O3 was performed using hydrofluoric acid (HF) and a wide range of different metal precursors including BCl3, AlCl­(CH3)2, Al­(CH3)3, TiCl4, and Ga­(N­(CH3)2)3. Because Ga2O3 is not a particularly stable oxide, the B-, Al-, or Ti-containing metal precursors can possibly convert the surface of Ga2O3 to B2O3, Al2O3, or TiO2. These metal precursors can also provide Cl, CH3, and N­(CH3)2 ligands for ligand-exchange reactions. Consequently, the thermal ALE of Ga2O3 can occur via “conversion-etch” or fluorination and ligand-exchange reaction pathways. Using sequential HF and BCl3 exposures and in situ spectroscopic ellipsometry techniques, Ga2O3 etch rates were observed to vary from 0.59 to 1.35 Å/cycle at temperatures from 150 to 200 °C, respectively. The Ga2O3 etch rates were also self-limiting versus HF and BCl3 exposure. The lack of BCl3 pressure dependence for the etch rates argued against the conversion-etch mechanism and in favor of a fluorination and ligand-exchange reaction pathway. In situ quartz crystal microbalance techniques also revealed that Ga2O3 could be etched using sequential exposures of HF and various other metal precursors. Ga2O3 etch rates at 250 °C were 1.2, 0.82, 0.85, and 0.23 Å/cycle for AlCl­(CH3)2, Al­(CH3)3, TiCl4, and Ga­(N­(CH3)2)3 as the metal precursors, respectively. The mass changes during the individual exposures of HF and the AlCl­(CH3)2 and Al­(CH3)3 metal precursors argued for a fluorination and ligand-exchange mechanism. The AlCl­(CH3)2 and Al­(CH3)3 exposures may also lead to some conversion of Ga2O3 to Al2O3. In contrast, the mass changes during the HF and TiCl4 exposures were consistent with the conversion of the surface of Ga2O3 to TiO2 and then the spontaneous removal of the TiO2 surface layer by HF. Distinctly different behavior was observed during the HF and Ga­(N­(CH3)2)3 exposures. The large mass gain during the Ga­(N­(CH3)2)3 exposures suggested that Ga­(N­(CH3)2)3 can adsorb on the fluorinated Ga2O3 surface prior to the ligand-exchange reaction. The wide range of metal precursors that can etch Ga2O3 argues that the ability of these precursors to convert Ga2O3 or to undergo ligand-exchange reactions provides multiple pathways for effective thermal Ga2O3 ALE.