Process-dependent electronic states at Mo/hafnium oxide/Si interfaces

Abstract

The authors have used low energy electron-excited nanoscale depth-resolved cathodoluminescence spectroscopy (DRCLS) to probe the bulk and interface defect states of ultrathin Mo∕HfO2∕Si with eight different process sequences. After atomic layer deposition of 4nm HfO2 on Si and an O2 post-treatment, they deposited 10nm Mo using either plasma vapor or electron beam deposition, with or without a subsequent 1000°C N2 anneal and with or without a forming gas anneal. DRCLS revealed pronounced gap state emissions within the ultrathin films and their interfaces with Mo and Si. There are multiple deep level emissions below the ∼5.9eV near band edge, including peak emissions at 3.4, 3.5, and 3.9–4.3eV that can be associated with HfO2 oxygen vacancies in different charge states predicted theoretically. In addition, states at 2–2.6eV that resemble known SiO2-related nonbonding oxygen hole centers and E′ (positively charged O vacancy) native defects increase with depth within the 4nm HfO2 film, suggesting the formation of a Hf silicate at the HfO2∕Si interface. No metal-specific interface states at the HfO2∕Mo interface are evident. Furthermore, different process steps produce large changes in these states and for at least one sequence, a dramatic decrease in both types of defects. The differences between process sequences can be understood in terms of known reactions at HfO2–Si interfaces.