First-principle Study On The Formation Mechanism Of SiC/SiO₂ Interfaces And Interface Defects

Silicon carbide (SiC) as a wide band gap semiconductor has high thermal conductivity, high electron saturation velocity, and large critical breakdown field, which make it the preferred "successor" of silicon semiconductor material. Because the quality factor of SiC-based high-power electronic devices is 1000 times higher than Si devices, this device used in power system can save 70% energy. For SiC MOS power devices, high-quality insulating film is a basis of SiC MOS devices, in which low interface state density is the performance guarantee. To explore the formation mechanism of SiC/SiO2 interface defects and to clarify the origin and abatement technology of interface states, therefore, have important scientific and practical values.In this thesis, a first-principles thermodynamic method to describe the behavior of SiC surface oxidation under real oxidizing environment was developed to study phase diagram of SiC thermal oxidation during the formation of SiC/SiO2 interface. The composition of SiC/SiO2 interface and possible defect configurations were analyzed within angular resolved X-ray photoelectron spectroscopy (ARXPS) combining with density functional tight-binding method. Using hybrid density functional theory approach, the accurate charge transition levels of typical defects were calculated, and the contribution to near interface states were clarified. Based on these defect levels, the passivation mechanism of P atoms on near interface states was explored. The main research contents and conclusions are as follows.(1) Study on formation mechanism of interface defects during the growth of SiC/SiO2 interfaces. The thermal oxidation of 4H-SiC was investigated under real temperature and pressure conditions using the first-principles thermodynamic method. The calculated results indicated that the first oxygen atom is incorporated into the subsurface region that actuates the formation of a surface oxide on the SiC surface when oxygen coverage increases to 3/4 ML, As the oxidation process contines, the 2-ML configuration with a C=C dimer defect becomes thermodynamically favorable, which provides a possible mechanism for the creation of a C-cluster defect in the interface. For oxidation of stepped SiC(0001) surfaces, a one-dimensional-Si-O-chain structure as a precursor for oxide growth on stepped SiC surfaces is formed along the step edge, promoting further oxidation of the step edges. Following the modified Deal-Grove oxidation model, the oxidation rate at steps is found to be higher than that at terraces by three orders of magnitude. These results further deepen the understanding of the SiC/SiO2 interface defect formation mechanism, providing a theoretical guidance for the control of interface defects.(2) Study on the composition of SiC/SiO2 interface and possible defect configurations. Using ARXPS and density functional theory methods, the composition of SiC/SiO2 interface and possible interface defect configurations were studied. The XPS results showed that the SiC/SiO2 interface formed by thermal oxidation contains an interface transition layer with various SiCxOy species. These SiCxOy species reveal the different depth profile. To more clearly understand the atomic structure of the SiC/SiO2 interface, a non-abrupt interface structure model was constructed by the tight-binding density functional method. In this interface, various possible defect configurations (such as dangling bonds, carbon clusters, silicon clusters, and SiCxOy etc.) are observed. Compared the simulation SiC/SiO2 interface structural information with the ARXPS results, the relative intensity of various SiCxOy components was found to be consistent, indicating the reasonableness of this interface model.(3) Study on the accure energy levels of interface defects. A defect-free structural model of the amorphous SiO2/4H-SiC(0001) interface was presented through first-principle calculations. Following the potential lineup method, the valence- and conduction-band offsets of this interface were calculated. The calculated results are 2.76 eV and 2.84 eV, respectively, in good agreement with the experimental values. Based on this interface model, several typical interface defects were created and the accurate charge transition levels of these defects were estimated within the HSE06 hybrid functional scheme. The results indicated that the silicon interstitial in SiO2 and carbon dimers in both SiC and SiO2 were the possible candidates for the large interface states experimentally observed near the conduction band of 4H-SiC.(4) Study on the P passivation effect and mechanism of SiC/SiO2 interface defects. The passivation effect of P atom on the surface Si dangling bonds and the SiC/SiO2 interface defects were explored using the first principles method. The results showed that the 1/3-ML configuration is most energetically favorable in a reasonable environment, and the Si dangling bonds on the SiC surface could be effectively reduce. For C=C dimer defects, the interface structure with a C atom of C=C dimer defect substituted by a P atom is the most energetically stable. After P passivation, the electronic structure analysis showed that the interface state density near the valence band of SiC increased, while the interface state close to the conduction band of SiC was reduced, and thus improving the mobility of SiC MOS devices. These findings illustrate the atomic mechanism of P atom passivation on interface defects, and provide effective protection for the development of more effective passivation methods.