Electrical properties of MOS devices fabricated on 4H carbon-face silicon carbide.

摘要

It has been over 20 years since research groups started to investigate SiC as the candidate to replace silicon. Many advantages of SiC over silicon have been well recognized. Its wide band-gap (3.3eV vs. 1.1eV for silicon) allows SiC to operate at higher temperature. The high thermal conductivity (3.7W/cm-K vs. 1.5W/cm-K for silicon) can significantly reduce the amount of cooling power required in a system. In addition, SiC has a high electric breakdown field (2.1MV/cm vs. 0.3MV/cm for silicon), which enables SiC to block the same voltage as Si with a 7 times thinner layer, thereby providing a much lower drift resistance for drift layer of similar doping concentration. Moreover, among the wide band gap semiconductors, a unique property of SiC is its native oxide is SiO2, which is the same as the native oxide of Si. This implies that the current silicon MOS device technology can be adopted for SiC MOS device fabrication without much effort in the development of new processing methods.;With its advantages and disadvantages, SiC has been widely investigated. Among many polytypes of SiC, 4H-SiC attracts much interest because this polytype has the largest band-gap energy and a high bulk, almost isotropic bulk mobility. The (0001¯) or carbon-terminated face 4H-SiC has been much less studied than the (0001) Si-terminated face. However, the carbon face 4H-SiC has a higher oxidation rate (×9 higher), which can significantly reduce the fabrication time for SiC MOS devices. Such characteristics would make the carbon face 4H-SiC an ideal candidate of power MOSFETs.;In this dissertation, the basic properties of SiC will be discussed in Chapter 1. The physics of MOS devices will be presented in Chapter 2, and the characteristics of SiC-based MOS devices will be discussed in Chapter 3. The processes and techniques used to fabricate SiC MOS devices will be described in Chapter 4. The results of measurements for MOS capacitors and MOSFETs fabricated on the 4H carbon face will be presented in Chapter 5 to provide an overview of (0001¯) characteristics compared to (0001). Both implanted and epitaxial layers are used to build MOSFETs. The oxide layer is grown thermally in furnace at 1150°C, followed by post-oxidation annealing to passivate the O-S interface. High-purity Mo is sputtered as the gate metal, and source and drain ohmic contacts for the lateral test MOSFETs are produced by sputtering Ni on heavily implanted regions (nitrogen at 6×19cm -3), followed by an anneal at 950°°C for 4min in Ar. Hi-lo capacitance-voltage measurements at both 23°C and 300°C are used to obtain the interface trap density (Dit). Current-voltage measurements at room temperature are used to collect information about oxide leakage and breakdown field (Ebd). A three-probe I-V system is employed to determine Id-V g characteristics of the MOSFETs at room temperature, and the inversion channel mobility (μ) is extracted from these characteristics.;Results are compared for different post-oxidation interface passivation anneals, with the combination of nitric oxide (NO) and H2 giving the lowest trap density Dit in the upper half of the band gap. Wet-reoxidation plus NO passivation produces the most reliable oxide, but the measured breakdown field of 6MV/cm is still approximately 2MV/cm lower than the average field measured for the silicon face. Compared to the values reported by Fukuda, et al., our low field mobility value is not remarkable. However, the high field mobilities are similar. It was observed that the presence of mobile ions can increase our low field channel mobility significantly. For example, after negative bias stress at 250°C to remove possible mobile ions from the O-S interface, the mobility peak value drops from 65cm2/V-s to 35cm2/V-s. These results suggest that the effective channel mobility for the carbon face may not be significantly higher compared to the silicon face. (Abstract shortened by UMI.).