A thin-film storage transistor in a NOR memory string has a gate dielectric layer that includes a silicon oxide nitride (SiON) tunnel dielectric layer. In one embodiment, the SiON tunnel dielectric layer has a thickness between 0.5 to 5.0 nm thick and an index of refraction between 1.5 and 1.9. The SiON tunnel dielectric layer may be deposited at between 720° C. and 900° C. and between 100 and 800 mTorr vapor pressure, using an LPCVD technique under DCS, N2O, and NH3 gas flows. The SiON tunnel dielectric layer may have a nitrogen content of 1-30 atomic percent (at %).