Surface/Interface Defect Behaviors And Their Effects On Physical Properties Of Silicon Carbide

As a representative of the third-generation broad-gap semiconductors,silicon carbide?SiC?serves as a key material in the fabrication of new-generation microelectronic devices.SiC-based devices show their irreplaceable advantages in the fields of the high-temperature,high-voltage,high-frequency and large-powder electronic devices as well as the aerospace,military,nuclear energy and other extreme environmental applications.Therefore,SiC as a promising electronic material deserves a deeply theoretical research and a wide exploration of various applications.The electronic device applications are closely related to or even determined by the surface characteristics of SiC and the interfacial bonding and connection behaviors between metal and SiC.This dissertation focuses on the systematically theoretic studies on the formation and evolution mechanisms of vacancy defects from the view of thermodynamics within the different Si-C atomic bilayers of monocrystal 6H-SiC surfaces,and emphasizes the effects of various defects on the electronic properties of SiC and the bonding and jointing performance of Metal/SiC interfaces.Meanwhile,the first-principle calculation methods were used to obtain the interfacial bonding energy values to simulate and evaluate the factors that influence the jointing performance of Metal/SiC.By using ion implantation to generate surface defects,as well,we combine the Monte Carlo theoretic simulation?SRIM method?and the high-temperature wetting experiment to probe the relations of surface defects with the interfacial bonding performance of metal or alloy metal/SiC system.The main contents and results of this thesis can be summarized as the following four aspects:1.We construct various vacancy defects V_n?n=1¹0?on the Hydrogen-passivated?H-P?and Silicon-reconstructed?S-R?6H-SiC?0001?surfaces,calculate their formation energies?E_fs?based on the first-principle methods,compare and disclose their stability and evolution routes of these vacancy defects on the both kinds of selected surfaces.It is found that monovacancy V_Si and V_C within the S-R6H-SiC?0001?surface have relatively lower formation energies than those within the H-P 6H-SiC?0001?surface,while the formation probability of V_Si and V_C in different Si-C atomic bilayers is dependent on the magnitude of their formation energies.The calculation results of E_fs also reveal that isolated V_Si and V_C are more likely to aggregate into a V_SiV_C divacancy on the both kinds of selected surfaces due to its low formation energy,high-energy V_SiV_Si divacancy is difficult to be generated,and that the appearance and thermodynamic evolution of V_CV_C is strongly influenced by the types of SiC surfaces and the layer depth?or layer number from top to down?of Si-C atomic bilayers.The results of optimized spatial configurations and formation energies of multivacancies?V_ns?lead to the clarification of the lowest energy paths?LEPs?of mono-,di-,to multivacancy for two types of selected surfaces.Generally,high symmetric V_ns are easy to appear during the thermodynamic defect evolution because of their lowest located formation energies and high stability.Besides,it is revealed that under energetic excitation the small V_n?n=1⁶?defects tend to expand within the two-dimensional?2D?first Si–C bilayer,but large V_n?n>6?will expend vertically into the underneath Si–C bilayers.The fitted curves of resultant LEPs demonstrates that the formation energies for different lowest-energy V_ns are proportional to the square root of missing atom numbers?n?,i.e.,E_f^n1/2,which means that the 2D evolution of the multivacancy is the main evolution form for the6H–SiC?0001?surfaces.2.The E_fs of several kinds of typical intrinsic and doping point defects are calculated by the first-principle methods for the layer-limited?few-layer?2D 6H-SiC and the effects of point defects on the electronic properties of 2D SiC are demonstrated.The results show that the intrinsic point defects of the as-constructed2D SiC generally possess low formation energies compared to the corresponding defects in the bulk SiC.The types and concentrations of point defects are highly dependent on the chemical environments?e.g.,stoichiometry,Si-rich and C-rich?and the temperatures.The formation of C-Si antisite(C_Si)defects is easy under the both stoichiometry and C-rich conditions,whereas the C-Si antisites?Si _C?are easier to form under the Si-rich environment.It is revealed that these defects in the superficial and inner-layer Si-C bilayers have nearly the same effects on the electronic properties of 2D SiC,and that the transition energy levels of the antisites and interstitial C defects are much closer to the band edges and then make the Fermi Energy level?E_F?near the VBM upper-shift,giving rise to the n-type conductivity.For the surface point defects of dopants,the substitution defects of C sites have low formation energies and tend to form;especially,a negative formation energy value enables the N substitution for C sites?N_C?to form spontaneously within the surface first Si-C bilayer.In the meantime,it is found that the Ag doping at the C sites leads to a high degree of electronic state localization and thus a weak influence on the electronics of SiC,but the N doping generates the transition energy levels closer to the VBM edges and heavily affects the electronic properties of SiC.3.From the aspect of interfacial bonding energies,Metal/SiC system is used as an example to investigate and understand the connection performance of metal/ceramics interfaces as well as to reveal the factors and microscopic mechanisms that determine and improve the interfacial wetting contact.Seven types of Ag_n nanoclusters with different n values and geometric structures are constructed to bind with idea?CL-SiC?,hydrogen-passivated?HP-SiC?and O-reconstructed?O-SiC?6H-SiC?0001?surfaces,respectively,and their interfacial binding energies are calculated and analyzed in detail.The results show that the interaction and binding energy and the large relaxation degree between Ag atoms and SiC substrate are highest for the Ag_n nanoclusters/CL-SiC interface,followed by O-SiC and then HP-SiC surfaces.Moreover,the presence of surface C_Sii antisites is able to increase the binding energy,which is favorable for the connection of Ag/SiC composite system.On the other hand,the coherent interface models are created between Ag?111?and CL-SiC?0001?or HP-SiC?0001?planes,and the Ag/CL-SiC coherent interface is found to have a relatively high binding energy.This means that the epitaxial growth or combination of Ag thin films on the CL-SiC?0001?surface is preferred.As compared,the binding energy of Ag/HP-SiC coherent interface is negative,which makes the Ag thin films grow freely and independently without the atomic binding.4.By using ion implantation technique,an effective method to introduce the high-energy defects and the surface modifications,different ion-beam energies and fluences of Pd,Co and Ni metal ions are implanted into the 6H-SiC?0001?monocrystal substrate,the resulting surface structural features and changes are carefully characterized and analyzed,and the influences of surface modifications induced by the ion implantation on the high-temperature wetting behaviors of Si/SiC and Al-12Si/SiC are especially evaluated and clarified.The results of Rutherford backscattering spectrometry in channeling geometry?RBS/C?,Raman spectroscopy,X-ray photoelectron spectroscopy?XPS?,and Monte Carlo simulation demonstrate that high concentration of surface defects,such as vacancies,lattice disorders and amorphization,and metal ion doping,are generated on the SiC surface,and leads to the increase of solid-vapor interfacial energy(?_SV)on the SiC substrate and the improvement of surface wettability of Si/SiC interface.Wettability improvement to a greater extent of Al-12Si/SiC system after ion implantation can be attributed to the decrease of solid-liquid interfacial energy(?_LV) derived from the existence of metal ions and to the increase of ?_SV of the SiC substrate.