Studies On The Preparation Of Al₂O₃/4H-SiC MOS Structure And Its Related Properties

SiC can be applied in high temperature, high frequency and high power devices due to its excellent semiconductor properties, such as wide band gap, high critical breakdown field, high thermal conductivity, and high carrier saturation drift velocity. On the other hand, SiC is the only wide band gap compound semiconductor which can be thermally oxidized to form SiO₂ film. This feature makes SiC MOSFET can be realized on the mature silicon technology. SiC MOSFET is a kind of important power device and is also an important part of SiC IGBT. However, the low channel mobility in SiC MOSFET, due to the high interface state density of SiO₂/SiC, is hard to resolve. Atom layer deposition（ALD） preparation of Al₂O₃ is a possible solution to replace SiO₂. However, the Al₂O₃/4H-SiC MOS structure also faces the problem of high trap density. In this master thesis, the effect of different annealing methods on the electronic transport properties of Al₂O₃/4H-SiC MOS structure is investigated based on the ALD preparation method, and the two step annealing process is proposed.Al₂O₃/4H-SiC MOS structure is prepared by using the as-deposited Al₂O₃. It is found that the oxygen vacancy is the main cause for the F-P emission in the MOS structure. The effects of different annealing methods and temperatures on the morphology and electrical properties of Al₂O₃ film are thoroughly investigated. Experimental results suggest that slow thermal annealing be superior to the rapid thermal annealing in maintaining the surface morphology. Less than 500°C annealing in an oxygen atmosphere has no effect on the F-P emission, while more than 500°C annealing can reduce the density of the oxygen vacancy and suppress the F-P emission. Two-step-annealing process is proposed, in which the perfect morphology of Al₂O₃ is maintained at the lower annealing temperature and the concentration of oxygen vacancies is reduced at higher temperature. The influence of O₂ and NO atmospheres on the electrical and interfacial properties of MOS structure is investigated. NO atmosphere lowers the passivation temperature to ⁷00 °C, the interface state density at the flant band is reduced from 1013（before annealing） to 1012cm-2eV-1（after annealing at 700 °C）, and the border interface trap density is reduced from 2.32×1012 to 2.1×1011/cm2eV-1. Compared to O₂ anealing, a lower leakage is achieved for MOS structure after NO annealing, which demonstrates that the NO annealing has an advantage in Al₂O₃/4H-SiC interface passivation.MOS structures are prepared by using Al₂O₃ and SiO₂ as gate materials. The distribution of the interface states are calculated by Terman method. The results show that the distribution is similar each other, in which the interface state density is gradually decreased from the conduction band to the mid-gap. This phenomenon is related to the defect, such as the near interface trap. The oxygen vacancy level, located in the 1.6 eV below the Al₂O₃ conduction band, just falls into the Ec-Et⁰.16 eV of the 4H-SiC band gap. The exchange rate between the substrate and the near interface state is slow enough, which could be passived by NO and O₂ annealing. Hill-Coleman method is also applied to calculate the interface density. It is found that Al₂O₃ has a low interface density compared to SiO₂. We believe that Al₂O₃ will become an ideal gate material to fabricate 4H-SiC MOS device in the respect of lower interface state and the future integration.