Probing the Dielectric Properties of Ultrathin Al/Al₂O₃/Al Trilayers Fabricated Using in Situ Sputtering and Atomic...

Dielectric properties of ultrathin Al₂O₃ (1.1-4.4 nm) in metal-insulator-metal (M-I-M) Al/Al₂O₃/Al trilayers fabricated in situ using an integrated sputtering and atomic layer deposition (ALD) system were investigated. An M-I interfacial layer (IL) formed during the pre-ALD sample transfer even under high vacuum has a profound effect on the dielectric properties of the Al₂O₃ with a significantly reduced dielectric constant (ε_r) of 0.5-3.3 as compared to the bulk ε_r ∼ 9.2. Moreover, the observed soft-type electric breakdown suggests defects in both the M-I interface and the Al₂O₃ film. By controlling the pre-ALD exposure to reduce the IL to a negligible level, a high ε_r up to 8.9 was obtained on the ALD Al₂O₃ films with thicknesses from 3.3 to 4.4 nm, corresponding to an effective oxide thickness (EOT) of ∼1.4-1.9 nm, respectively, which are comparable to the EOTs found in high-K dielectrics like HfO₂ at 3-4 nm in thickness and further suggest that the ultrathin ALD Al₂O₃ produced in optimal conditions may provide a low-cost alternative gate dielectric for CMOS. While ε_r decreases at a smaller Al₂O₃ thickness, the hard-type dielectric breakdown at 32 MV/cm and in situ scanning tunneling spectroscopy revealed band gap ∼2.63 eV comparable to that of an epitaxial Al₂O₃ film. This suggests that the IL is unlikely a dominant reason for the reduced ε_r at the Al₂O₃ thickness of 1.1-2.2 nm but rather a consequence of the electron tunneling as confirmed in the transport measurement. This result demonstrates the critical importance in controlling the IL to achieving high-performance ultrathin dielectric in MIM structures.