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The benefit of La-Al-O higher-k gate dielectrics is maximized by optimizing the Al/(La+Al) atomic ratio. The Al/(La+Al) atomic ratio modulates the band gap, dielectric constant, and interface dipoles, resulting in change of energy band alignment. The La-Al-O film with an Al/(La+Al) atomic ratio of 0.25 yields a maximum leakage current reduction exceeding HfO2 and La2O3, owing to the optimized band alignment structure. Moreover, a SiON underlayer instead of SiO2 enables additional equivalent oxide thickness scaling down to 7.4 Å while maintaining the fully reduced leakage current with a gate-first process.