In Situ MOCVD Growth of Dielectric Al₂O₃ on β-(Al_xGa_1-x)₂O₃: Interfaces and Band Offsets

β-AlGaO has recently been emerged as a promising semiconductor material for next generation high power and radio frequency electronic applications due to its tunable energy bandgap, high breakdown field strength and the ability to achieve large electron mobility by modulation doping at β-AlGaO/GaO interfaces. Metalorganic chemical vapor deposition (MOCVD) of β-AlGaO films have been demonstrated with a great control of uniformity, composition, and high purity [1-4]. While high quality and high-Al composition β-AlGaO films are advantageous for the development of high-power devices, the quality of the dielectric-semiconductor interfaces is also crucial for reliable high-performance transistors. Due to its good compatibility with β-Ga₂O₃, Al₂O₃ has been widely used as a gate dielectric for β-Ga₂O₃ and β-AlGaO based devices. The deposition of Al₂O₃is mostly performed by using ex-situ method, such as atomic layer deposition (ALD) technique, which can lead to interface contamination due to the exposure to the ambient. Recently, in-situ MOCVD growth of dielectric Al₂O₃/(010) β-Ga₂O₃ has been demonstrated with improved interface quality as compared to other conventional dielectric deposition techniques [5]. However, the investigation of the interface quality and the band offsets at in-situ MOCVD deposited Al₂O₃/β-AlGaO interfaces are still lacking. In this work, for the first time, we have investigated the interface quality and experimentally determined the band offsets of in-situ MOCVD dielectric Al₂O₃/β-AlGaO interfaces with various Al compositions along different orientations. Moreover, the band offsets determined for in-situ Al₂O₃are also compared with ALD (ex-situ) Al₂O₃/β-AlGaO interfaces.<br/><br/>Phase pure β-AlGaO epitaxial films [(010), Al = 0, 0.17, 0.35], [(100), Al = 0, 0.17, 0.52] and [(-201), Al = 0, 0.17, 0.48] were grown on (010), (100) and (-201) β-Ga₂O₃substrates by MOCVD. Al₂O₃layer was deposited on top of β-AlGaO films by using both in-situ (MOCVD) and ex-situ (ALD) deposition techniques. The valence (VBO) and conduction (CBO) band offsets at Al₂O₃/β-AlGaO interfaces were determined by XPS. The XRD and high-resolution HR-STEM imaging confirm the growth of amorphous Al₂O₃ films with abrupt Al₂O₃/β-AlGaO interfaces. The bandgaps estimated by examining the onset of the inelastic energy loss spectra are measured to be 6.91 eV and 6.88 eV for in-situ and ex-situ deposited Al₂O₃, respectively, which are consistent with previous reports on ALD Al₂O₃. The CBOs varying from 2.0 eV (Al=0) to 1.53 eV (Al=0.35) were determined from in-situ Al₂O₃/(010) β-AlGaO interfaces. Similarly, the CBO of 2.18 eV (Al=0) and 1.07 eV (Al=0.52) were measured for (100) orientation. The CBOs at Al₂O₃/(-201) β-AlGaO interfaces vary from 2.14 eV (Al=0) to 1.06 eV (Al=0.48). For ex-situ ALD deposited Al₂O₃, the CBOs were revealed to be varied between 2.09-1.73 eV [(010), Al=0-0.35], 2.32-1.23 eV [(100), Al=0-0.52] and 3.01-1.96 eV [(-201), Al=0-0.48], indicating a distinct dependence of the band offsets on different orientations and dielectric deposition techniques.<br/><br/>In summary, we investigated the interface quality and band offsets of in-situ MOCVD and ex-situ ALD deposited Al₂O₃/β-AlGaO interfaces for different orientations with the variation of Al compositions. Findings from this study will guide future designs of β-Ga₂O₃and β-AlGaO based structures and devices.<br/><b>Acknowledgment</b>: The authors acknowledge the funding support from the Air Force Office of Scientific Research No. FA9550-18-1-0479 (AFOSR, Dr. Ali Sayir), and NSF (1810041, 2019753).<br/><b>References</b>:<br/>1. Bhuiyan et al., APL Mater. 8, 031104 (2020).<br/>2. Bhuiyan et al., Cryst. Growth Des. 20, 6722 (2020).<br/>3. Bhuiyan et al., Appl. Phys. Lett. 117, 142107 (2020).<br/>4. Bhuiyan et al., J. Vac. Sci. Technol. A 39, 063207 (2021).<br/>5. Roy et al., arXiv:2103.15280 (2021).