The M2 Phase of Free-Standing Ga-Doped VO2 Single Crystals Over Wide Temperature Ranges

Abstract

For more than six decades, VO₂ has been one of the most impressive electronic materials due to its spectacular insulator metal transition (IMT) close to room temperature, accompanied by a structural transition and significant changes in optical properties which provided a list of potential applications such as electrical and optical switches, electrical and optical memories, smart windows, neural devices and thermal actuators. Despite a comprehensive investigation effort invested by scientists and engineers, as of today, there is no useful device on sale made of VO₂. The main cause is cracks induced by the large strain-stress developed upon cycling through IMT of clamped samples. Another weakness is the high sensitivity of VO₂ to foreign cations and deviations from oxygen stoichiometry, which may induce multiple coexisting phases. To become useful in device technology, the phase diagram of doped VO₂ should be adjusted to provide a wide range of temperatures below IMT in which the material is single-phase and not under strain or stress induced by the substrate and contacts. Low- current R(T), DC, and pulsed I–V measurements conducted on free-standing Ga-doped VO₂ single crystals provided encouraging results and proved the advantage of Ga over, e.g., Al doping.