Ultrahigh purity (UHP) silicon nitride (Si3N4) was applied as a barrier layer to reduce the reaction of high-k materials with the underlying silicon channel. UHP Si3N4 was grown by rapid thermal nitridation (RTN) in chemically scrubbed ammonia in an ultrahigh vacuum (UHV) chamber. The grown thickness nearly saturates for nitridation times greater than 10 s. This self-limiting thickness increases by about 0.2 nm per 100 °C nitridation temperature from 500 to 900 °C. In situ Auger show that UHP nitride has less than 1% oxygen contamination. The effective charge density of UHP Si3N4 was found to increase with the thickness of nitride layer. The thinnest nitride layers (0.5 nm) have a charge density of approximately 5×1011 cm-2. The peak mobility of field effect transistors made from such a layer are 180 cm2/V s (n channel) and 50 cm2/V s (p channel). Both are about 70% of the value predicted by the universal curve. © 2004 American Vacuum Society.