Electrical properties of thermal oxides on SiC manufactured in an alumina furnace

Licentiatavhandling, 2006

A process to achieve high inversion channel mobility in 4H-SiC MOSFETs is needed to enable manufacturing of devices for high power applications. In this thesis, investigations on the electrical properties of oxides produced in an alumina furnace are presented. MOSFETs with gate oxides produced in this manner exhibit the highest inversion channel mobilities reported so far within the field. However mobile charge in the form of sodium is present in these oxides. Mobile charge reduces the device reliability and gives rise to threshold voltage shift, that is unacceptable in most applications. Comparison of this oxidation process is made to other processes known to increase the inversion channel mobility. Furthermore studies of MOSFETs at elevated temperatures are performed in order to investigate what mechanism is limiting the mobility. The studies show that phonon scattering is limiting the mobility in MOSFETs produced in an alumina furnace. A method to remove the sodium from the samples produced in this manner is presented and by removing the sodium new properties of the SiO_{2}/SiC interface can be investigated. A high density of deep acceptor type interface traps is detected. Their presence does not degrade the saturation mobility but gives rise to large threshold voltage shifts. These traps have slow electron emisson rates and have energy levels deeper in the band-gap than the near interface traps that are normally associated with the mobility reduction in MOSFETs.