Oxygen Effect and Mechanism of Insulator-to-Metal Transition using Indirect Band in Vanadium Dioxide

The vanadium dioxide VO$_{2}$ is known as the Mott insulator and it undergoes the insulator-to-metal transition (IMT) near $T_{c}=$340 K. Under the external influence (doping, strain etc.) the $T_{c}$ is reduced. To explain this behavior, we have used the Mott criterion proposing the critical carrier density $n_{c}$ at MIT (metal to insulator). In this case, Mott derived $n_{c}$ for the transition from metal to insulator. Therefore, $n_{c}$ is regarded as the minimum carrier density in metal phase. However, in the reverse transition (insulator to metal), $n_{c}$ cannot clarify the above behavior. Thus, a new model has been required. Here, we have grown VO$_{2}$ thin films under the different oxygen partial pressure conditions and studied the influence of oxygen deficiency on $T_{c}$ of the IMT. For the analysis, we have measured the Hall voltage near $T_{c}$ and a change of work function. Based on the experimental data we have proposed the model explaining $T_{c}$ changes in VO$_{2}$. This model describes the excitation process of bounded charges in direct and indirect energy bands as a criterion of the IMT. Moreover, it is shown that it can serve as universal mechanism that describes the physics of the IMT in many MIT materials from the new point of view, different from that, suggested by Mott. It is the fundament for operation of new devices.