Correlation between thermal hysteresis width and broadening of metal-insulator transition in Cr- and Nb-doped VO2 films

We investigated the effects of Cr and Nb doping on the metal-insulator transition (MIT) of single-oriented VO2 films deposited on Al2O3 substrates. The MIT temperature (TMI) increased with trivalent Cr doping, whereas it decreased with pentavalent Nb doping. The temperature coefficient of resistance (TCR) and the thermal hysteresis width of the MIT (ΔTMI) were suppressed by Cr and Nb doping, and Nb doping was about twice as effective as Cr doping for decreasing TCR and ΔTMI. We found that the maximum TCR and ΔTMI for the doped VO2 films have a correlation with the lattice constant irrespective of doping elements, suggesting that the lattice deformation caused by metal-ion doping is involved in the decreases in TCR and ΔTMI. There is also an apparent correlation between the maximum TCR and ΔTMI for the doped VO2 films. The present findings suggest that the combination of metal-ion doping with other techniques such as strain control is required to achieve nonhysteretic MIT with a large TCR in VO2 films.