Optimization of the annealing conditions for thin VO2 ALD films

Vanadium dioxide (VO2) is an intriguing material due to its semiconductor-metal transition (SMT). During this transition, which occurs near 67°C, electrical as well as optical properties change drastically. Possible applications include thermochromic windows, and memories or switches in micro- and optoelectronics. Although atomic layer deposition (ALD) is gaining importance for some of these applications, the growth of VO2 with this technique is not obvious, since in most cases V2O5 is obtained. In our previous work we presented ALD growth of VO2 by using Tetrakis[EthylMethylAmino]Vanadium and ozone at a temperature of 150°C [1]. XPS revealed the 4+ oxidation state of vanadium, indicating growth of VO2. Post-ALD thermal processing proved essential to crystallize the VO2 in the desired tetragonal phase (R). In this work we present the influence of the oxygen partial pressure on phase formation during such thermal processes. Additionally the influence of film thickness and annealing temperature on the post-annealing properties were studied, including morphology and SMT characteristics. During thermal processing a minimum oxygen partial pressure of approximately 1 Pa is indispensable to form crystalline VO2 (R) (figure 1). Oxygen partial pressures above 2 Pa show an intermediate monoclinic phase (B), which transforms to VO2 (R) at higher temperatures. At a value of 35 Pa this VO2 (B) phase finally transforms to V6O13 instead of VO2 (R). For very thin films, the thermal post-processing may result in agglomeration of the VO2 layers on the SiO2 substrate. Samples with a film thickness above 20nm show a typical resistivity ratio during the SMT of more than 2 orders of magnitude when annealed in the range 450°C to 500°C. For thinner films or higher annealing temperatures the resistivity ratio is suppressed and an overall increased resistivity is observed due to agglomeration (figure 2)

Thermal trimming and tuning of hydrogenated amorphous silicon nano-photonic devices

Deposited silicon and, in particular, hydrogenated amorphous silicon forms an attractive alternative platform for realizing compact photonic integrated circuits. In this paper we report on trimming (toward lower wavelengths) and tuning (toward higher wavelengths) of photonic devices through a suitable thermal treatment. The former is achieved by a material density change, the latter through the thermo-optic effect. By using Fourier transform infrared spectroscopy, a change in the hydrogen content is identified as the source of the density change. A total wavelength tuning range of 24.6 nm is achievable, which can be used for compensating fabrication imperfections. c 2010 American Institute of Physics. [doi:10.1063/1.3479918

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