The Bifeo <sub> 3 </ Sub> Multiferroic Thin Films And Gan-based Semiconductor Integrated Growth And Performance

The rapid development of the information technology requires electronic systems to have smaller volume, faster speed, more enhanced functionality and lower power consumption, which leads to the miniaturization, monolithic and multi-functional integration of electronic components. BiFeO₃(BFO) is a multiferroic coexistence of ferroelectric and ferromagnetic at the room temperature, with large ferroelectric polarization and high Curie temperature, having great potential in integrated electronics. As a third-generation semiconductor, GaN having excellent characteristics such as wide band gap, high electron saturation drift velocity, small dielectric constant and high breakdown field, is very suitable for making high-frequency, high-speed, high power, anti-radiation, high integration electronic devices and circuits. If BFO and GaN are integrated to form the BFO/GaN integrated film, not only can the multi-fuctional integration of dielectric passive components and semiconductor active components be realized, but also we would exploit the coupling effect at the interface to develop some new devices. However, the two materials have different lattice structure and non-compatible growth processes, the realization of BFO epitaxially grown on GaN faces a series of technical problems. In this thesis, we systematically study the growth of BFO on GaN by pulsed laser deposition technique, providing a basis of practical utilization of the BFO/GaN integrated films.1. The effect of inserting the SrTiO₃(STO)/TiO₂ double buffer layers on the growth of BFO on sapphire substrate was studied. It was found that the BFO directly grown on sapphire substrate was polycarystalline thin film. It could be realized that the BFO (111)-oriented epitaxial grown on sapphire substrate and improved crystalline quality of films by inserting the buffer layers. The epitaxial relationships of the BFO/STO/TiO₂/Al₂O₃ heterostructures could be confiemed as follows: BFO (111) / / SrTiO₃ (111) / / TiO₂ (100) / / Al₂O₃ (0001); BFO [1-10] / SrTiO₃ [1 - 10] / / TiO₂ [001] / / Al₂O₃ [10-10], plus a twin variant related by a 60°in-plane rotation. Electrical performance tests showed that the double buffer layers can improve the ferroelectric properties of BFO thin films and reduce the film's leakage current density. These conclusions validated the effect of the STO/TiO₂ double buffer layers preliminarily, and laid the foundation of BFO&GaN integration.2. On the basis of the realization of BFO epitaxially grown on sapphire substrate by inserting the STO/TiO₂ double buffer layers, we explored the effect of the main process parameters (oxygen partial pressure and growth temperature) on the structure and properties of BFO films, and optimized the growth process of epitaxial BFO films.3. The growth of BFO on GaN(0002) substrate was studied. It was found that BFO was not grown in accordance with the growth orientation of the smaller in-plane mismatch, but inclined to rotated 30°in-plane according to the BFO (111) [1-10] / / GaN (0002) [11-20 ] epitaxial growth relationship. However, due to the large lattice mismatch of BFO and GaN in the relationship, BFO films grown in island mode. So, the BFO films were polycrystalline and had a large number of defects, showing poor electrical properties. It could be realized that the BFO(111) thin films epitaxially grown on GaN substrate by inserting the STO/TiO₂ double buffer layers. The epitaxial relationship of the heterostructure could be established as follows: BFO (111) // SrTiO₃ (111) // TiO₂ (100) // GaN (0001);BFO [1-10] / SrTiO₃ [1-10] // TiO₂ [001] // GaN[11-20], plus a twin variant related by a 60°in-plane rotation. Performance tests showed that the ferroelectric and insulation properties of BFO epitaxial films prepared on GaN with the double buffer layers is comparable to that of BFO grown on SrTiO₃ single crustal substrates, providing the possibility of the realization of GaN-based ferroelectric memory and ferroelectric field effect transistor.4. The BFO/STO/TiO₂/AlGaN/GaN heterostructure was prepared and the effect of ferroelectric performance of the epitaxial BFO thin films on the intrinsic electrical properties of AlGaN/GaN was explored preliminarily.