Polyoxide as an insulator for SiC MOS devices

Date of Completion

January 2005

Keywords

Engineering, Electronics and Electrical

Degree

Ph.D.

Abstract

Oxide grown from polysilicon, referred to as polyoxide, has been used as an insulating dielectric in silicon-based floating gate MOS devices. However, the properties of polyoxide in SiC MOS devices have rarely been studied. Polyoxide may be used in SiC UMOSFETs to avoid the problems caused by non-uniform native oxide growth in the trench. The other potential benefits of polyoxide include faster oxidation and less carbon involvement. ^ We have produced and characterized MOS capacitors on n-type 4H-SiC with native oxide and polyoxide as insulators. We have observed a lower interface state density for wet oxidation than that for dry oxidation, which also applies to polyoxides. The polyoxide samples grown by dry oxidation without an anneal had a high interface state density (Dit) of 1.8x1012 cm-2 eV-1 and the polyoxide samples grown by wet oxidation had a lower Dit of 1.2x1012 cm -2 eV-1 (both at 0.5 eV below the conduction band). After 1 hour Ar annealing, the Dit of wet polyoxide was reduced significantly to 2.6x1011 cm-2 eV-1 (at 0.5 eV below the conduction band). Dry polyoxide exhibits higher breakdown electric fields than wet polyoxide. The maximum breakdown field for dry polyoxide is 6.1 MV/cm. The maximum breakdown field for wet polyoxide is 4.4 MV/cm. The dielectric strength of the polyoxide is lower than that of the native oxide on SiC. The interface quality and breakdown characteristics of polyoxides are comparable to published results of low-temperature CVD deposited oxides. ^ The better interface quality of wet polyoxides could be related to passivation by H and OH in wet oxidation. The reduction of Dit by high temperature Ar annealing may be associated with stress relaxation and partial removal of interfacial oxide defects. The low breakdown field of polyoxide could be due to the locally enhanced electric fields caused by the rough surface. Further improvement for polyoxide quality is possible with post-oxidation annealing. ^

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