  We investigated the influence of vacuum annealing on interface properties of
silicon carbide (SiC) metal-oxide-semiconductor (MOS) structures. For
as-oxidized and nitric oxide (NO)-annealed samples, the interface state density
($D_{\rm it}$) near the conduction band edge ($E_{\rm C}$) of SiC did not
increase by subsequent vacuum annealing. For phosphoryl chloride
(POCl$_3$)-annealed samples, in contrast, $D_{\rm it}$ at $E_{\rm C}-0.2$ eV
increased from $1.3\times10^{10}$ to $2.2\times10^{12}$ cm$^{-2}$eV$^{-1}$ by
the vacuum annealing, and the channel mobility of MOS field effect transistors
(MOSFETs) decreased from 109 to 44 cm$^2$V$^{-1}$s$^{-1}$. Mechanism of the
observed increase in $D_{\rm it}$ was discussed based on the results of
secondary ion mass spectrometry measurement.
