Semiconductor-metal transition in thin VO2 films grown by ozone based atomic layer deposition

Vanadium dioxide (VO2) has the interesting feature that it undergoes a reversible semiconductor-metal transition (SMT) when the temperature is varied near its transition temperature at 68°C.1 The variation in optical constants makes VO2 useful as a coating material for e.g. thermochromic windows,2 while the associated change in resistivity could be interesting for applications in microelectronics, e.g. for resistive switches and memories.3 Due to aggressive scaling and increasing integration complexity, atomic layer deposition (ALD) is gaining importance for depositing oxides in microelectronics. However, attempts to deposit VO2 by ALD result in most cases in the undesirable V2O5. In the present work, we demonstrate the growth of VO2 by using Tetrakis[EthylMethylAmino]Vanadium and ozone in an ALD process at only 150°C. XPS reveals a 4+ oxidation state for the vanadium, related to VO2. Films deposited on SiO2 are amorphous, but during a thermal treatment in inert gas at 450°C VO2(R) is formed as the first and only crystalline phase. The semiconductor-metal transition has been observed both with in-situ X-ray diffraction and resistivity measurements. Near a temperature of 67°C, the crystal structure changes from VO2(M1) below the transition temperature to VO2(R) above with a hysteresis of 12°C. Correlated to this phase change, the resistivity varies over more than 2 orders of magnitude.

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