The Epitaxial Growth And Properties Of Dielectric Oxide Films On GaN Semiconductors

Recently, electronic information systems are quickly developed to further miniaturization and monolithic integration in order to realize smaller volume and enhanced multifunction. In order to satisfy the demand of system develop trend, the electronic films and devices must be miniaturized and integrated. The integration of multifunctional oxide dielectrics possessing spontaneous polarization with GaN semiconductors put forward a new direction of developing electronic devices with higher performances. However, the dielectric oxide and GaN semiconductors are quite different from each other. It will cause many problems when the two kind materials are integrated together. However, little is known about the physical phenomena and mechanism in this heterostructure. Especially, the lack of related research about epitaxial growth and interface control of dielectric oxide film on GaN has hampered the development of the integrated films and devices.In this dissertation, SrTiO₃ (STO) dielectric oxide films were fabricated by laser molecular beam epitaxy (LMBE) to investigate the epitaxial mechanism and interface control method. Bufferlayers at nanometer scale were designed and fabricated to optimize the crystalline quality of STO epitaxial film. Based on these results, the electric properties of STO and other dielectric films were studied.1. The growth behaviors and interface microstructures were systematically studied. It was found that STO can be epitaxially grown on GaN at 700℃and the epitaxial relationship was STO(111)[110]//GaN(0002)[1120]. The lattice mismatch under this alignment was calculated to be -13.3%, which leads to 3D island growth mode and poor crystalline quality. The analyses of interface energy show that the bonding energy can compensates the strain energy and make this alignment more stable. From the RHEED images, it was found that STO films show a twin variant related by a 180°in-plane rotation. This in-plane structure was caused by the different symmetry index of STO (111) and GaN (0002). On the other hand, an interface layer was observed at STO/GaN interface. The formation of interface layer was due to the instability of STO with Ga-terminated GaN. As indicated by these results, Lattice mismatch and interface diffusions are the two major obstacles hindered the integration growth of dielectric oxide films on GaN.2. The effect of TiO₂ template layer on STO epitaxial growth and microstructure was studied. TiO₂ was epitaxially grown on GaN(0002) surface in layer by layer mode. The TiO₂ surface was smooth and uniform with a root-mean-square roughness (RMS) less than 0.5nm. A sharp interface was observed between TiO₂/GaN. It was found that the epitaxial growth temperature was decreased and the orientation uniformity of STO was improved on TiO₂ coated GaN. These results indicated that the epitaxial growth of STO film was enhanced by TiO₂ template layer because of the reduced lattice mismatch and similar Ti-O6 octahedron structure between TiO₂ and STO. By inserting a strained layer of TiO₂ below its critical thickness, the crystalline quality was further improved. The optimal thickness of TiO₂ was about 2nm. The interfaces of STO/TiO₂/GaN integrated films were sharp as confirmed by HRTEM and XPS. These results demonstrated that the interface of oxide/GaN was optimized and the crystalline quality was improved by the design of TiO₂ template layer.3. The impacts of STO/TiO₂ bufferlayer on the epitaxial growth and properties of GaN-based ferroelectric films were carried out. It was found that these ferroelectric films deposited on GaN directly show polycrystalline structure. In contrast, three kind ferroelectric films with different crystal structure, BaTiO₃,Hf-doped Bi4Ti3O12 and BiFeO₃, were epitaxially grown on STO/TiO₂ buffered GaN. Simultaneously, these epitaxial films show much better electric properties than that of polycrystalline ones, such as enlarged polarization, reduced leakage and enhanced fatigue endurance.4. The low temperature fabrication of MgO and its effect on STO/GaN integrated films were studied. It was found that the strong ionic characteristics of MgO makes its epitaxial temperature can be as low as room temperature. This special feature reduced the interface diffusion between oxide and GaN remarkably. On the other hand, STO/TiO₂/MgO gate stack show reduced leakage than that of STO/TiO₂. It is concluded that the presence of MgO barrier layer increases band offsets and reduces the leakage current density effectively.