Epitaxy Engineering Of Metastable Phased Gallium Oxide By Mist-CVD Technique

As an ultra-wide band gap semiconductor material,gallium oxide?Ga₂O₃?has attracted extensive attention for the applications of power electronic devices and solar blind photodetectors owing to its unique characteristics of high breakdown electric field and wide band.Despite of extensive efforts on the material growth and device fabrication based on the thermodynamic stable?-phase Ga₂O₃,the synthesis and fundamental investigation of the metastable?-Ga₂O₃ is still insufficient.In this paper,a low-cost mist chemical vapor deposition?Mist-CVD?epitaxial system has been constructed to realize high-quality heteroepitaxy of?-Ga₂O₃ on sapphire.The evolution of dislocations in?-Ga₂O₃ epilayers have been revealed and the crack-free?-Ga₂O₃thick epilayer have been achieved,which provides an alternative strategy for the development of high power electronic devices.The innovative achievements are summarized as follows.1.A low-cost Mist-CVD epitaxial system has been established,and the optimization of the hetero-epitaxy of metastable?-Ga₂O₃ on sapphire substrate has been systematically performed.Considering the incomplete decomposition of gallium acetylacetonate at low temperature,the chosen of oxygen as the carrier gas and the hydrogen peroxide mixed into the precursor solution is an effective way to suppress the residual carbon impurities in epitaxial films.The epitaxy of?-Ga₂O₃ exhibits a narrow growth window:Low growth temperature leads to the incomplete crystallization or amorphous film while high growth temperature facilitate the phase transition to?and?phases.The optimized growth temperature in this work is determined to be 570°C,at which the epilayer in phase-pure?-Ga₂O₃ is achieved with a typical epitaxial relationship of[0001]?-Ga₂O₃//[0001]?-Al₂O₃ and[11-20]?-Ga₂O₃//[11-20]?-Al₂O₃.The full-wide at half-maximum?FWHM?of the?-scanning of?0006?plane is as small as 36 arc seconds with its corresponding screw dislocation density of about 10⁶cm^-2.The result of rapid thermal annealing shows that?-Ga₂O₃ has excellent thermal stability below 700°C.2.The dependence of dislocation type and dislocation density on the thickness of epitaxial layer has been investigated.Owing to the strain induced by the combination of lattice mismatch and thermal expansion mismatch,cracks are easily observed in thick?-Ga₂O₃ epilayers.It was found that the post-growth cooling down process from growth temperature to room temperature will induce tensile stress and then convert to compressive stress,and the accumulated tensile strain is maximized when temperature drops to 225?.Therefore,by reducing the cooling rate,the cracking in epitaxial films can be effectively suppressed.It is also shown that with increasing film thickness,screw dislocations exhibit an independent characteristic with film thickness and have a low density of about 1.8×10⁶ cm^-2,while edge dislocation densities are inversely proportional to the thickness of the epitaxial film,which decreases from 1.1×10¹¹ cm^-2for a 80nm-thick layer down to 2.1×10⁹ cm^-2 in density for a 8?m-thick?-Ga₂O₃ layer.In the framework of glide analytic model,parallel edge dislocations are generated at the interface due to the misfit-induced strain relaxation,while the dislocation glide and coalescence result in the annihilation and fusion behaviors.3.The influence of miscut angle of c-plane sapphire substrate on the growth mode and dislocation formation of?-Ga₂O₃ epilayers has been investigated.The?-Ga₂O₃epilayer shows a larger off-axis angle than the substrate,that is,the epitaxial film has a lattice tilt relative to the substrate.The tilt angles between epilayer and substrate have an approximately linear relationship with the off-axis angle of the substrate,which is coincided with the prediction by the Nagai model.In comparison,the XRD rocking curves exhibits asymmetric broadening feature in the thin epilayers of about 100nm,which is absent in the thick epitaxial films?about 700nm?.This phenomenon is resulted from the inhomogeneous strain along the terrace induced by the lattice mismatch at the interface between the epitaxial film and the substrate.It is also shown that when the off-axis angle is 6°,there is the lowest edge dislocation density in the epitaxial film.