Non-thermal to thermal electric field effects induced by DC and current pulses in pure and doped VO2 single crystals

Abstract

Pure VO2, GexV1−xO2 (x < 0.2), and AlxV1−xO2 (x ≤ 0.02) exhibit above room temperature the well-known insulator-metal-transition (IMT) (at TIMT ≅ 340 K). In addition, AlxV1−xO2 exhibits an insulator-insulator-transition (IIT) at TIIT < TIMT. Both transitions can be induced either by external or by Joule heating. Both leave prominent marks on the I–V characteristics of the investigated samples, IIT at low currents and IMT at high currents. Reported herein is a comparative study of the I–V characteristics of needle-like pure and doped VO2 single crystals obtained with pulsed currents of various durations and with DC, in an attempt to separate thermal from non-thermal electric field effects. Identification of the Poole–Frenkel effect (a non-thermal electric field effect) in the I–V characteristics of the doped crystals may play an important role in a long-standing debate on the mechanism of IMT. Such an identification was obtained in this investigation for several Ge- and Al-doped VO2 crystals, so far only in narrow ranges of electric fields (<2000 V/cm) and pulse durations (≤1 ms) above which the non-linear conductivity of the crystals is dominated by Joule heating.