Characterization of field-effect transistors with La2Hf2O7 and HfO2 gate dielectric layers deposited by molecular-beam epitaxy

The electrical properties of La2Hf2O7 (LHO) and HfO2 (HO) high-k dielectric layers deposited by molecular-beam epitaxy are reported. Capacitors and transistors with LHO and HO gate dielectric layers and TaN metal gate electrodes deposited using physical vapor deposition were fabricated. The (SiO2) equivalent oxide thickness (EOT), the electrical oxide thickness in inversion toxinv, and the gate leakage current density (Jg) were determined on large area metal-oxide-semiconductor capacitors and metal-oxide-semiconductor field-effect transistors. HO layers with a physical thickness tph=30Å showed an EOT (toxinv) of 15.1Å (19.3Å) with Jg=8.1×10−6A∕cm2 at Vg=Vfb−1V. LHO layers deposited on SiON showed a minimum EOT (toxinv) of 18.7Å (25Å) with Jg=4×10−8A∕cm2 at Vg=Vfb−1V. The (effective) electron mobility at high E field for LHO layers was observed for a 40-Å-thick LHO layer deposited on Si with μeff=147cm2∕Vs at E=1MV∕cm. For a 30-Å-thick HO layer at identical field, μeff=170cm2∕Vs was found. LHO layers deposited on SiON interface exhibited 5%–10% higher electron mobility at high E field than identical layers deposited on Si. Further, both low E field and high E field mobilities decreased for thicker high-k layers, indicating remote charge scattering from both the interface and the bulk of the high-k dielectric. It was observed that LHO layers showed strongly reduced electron trapping in preexisting bulk defects as compared to HO layers. At E=1MV∕cm (corresponding to Vg=VT+0.6V), the trapped charges per area Ntr were Ntr<5×1010∕cm2 for the LHO layers and Ntr>5×1011∕cm2 for the HO layers. These results show that low-leakage Hf-based gate dielectric layers with low defect density can be obtained by alloying with La.