Interpretation of recent transient on-state data in thin chalcogenide glass and NbO₂ threshold switching material

Abstract

Recent measurements studying the on-regime of the transient on-state charac-teristics (TONC) of a thin amorphous semiconductor chalcogenide threshold switch, employing precisely balanced circuitry and isolated device voltage determination, have shown that the low-current subregime known as the blocked on-state develops when the decreasing ramp voltage rate dV/dt is equal to – 7×10³Vns⁻¹. For a more rapid ramp rate of approximately -21 × 10⁻³ Vns⁻¹ the blocked on-state does not develop and the I-V curve will display metal-like behaviour. We have also observed TONCs for polycrystalline and single crystal NbO₂. In this paper we present a band model based on amphoteric charged defects which gives a good description of these transient I-V characteristics and of the general switch-on and switch-off behaviour in thin films of these materials. The band model assumes eaual densities of amphoteric charged defects in the off-state which allow localized conduction in the donor and acceptor bands of the defects simultaneous with extended-state band conduction. In the on-state, the charged trapping states are converted to their neutral counterparts. The defects may arise in a number of ways including Mott-Street dangling bonds, Kastner valence-alternation pairs, and Wauliewkicz intrinsic native vacancies. The switching to the on-state is essentially a Mott-Anderson transition.