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)

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

Growth and characterization of Y-BA-CU-O high-Tc superconductor thin films

Two types of growth processes of Y-Ba-Cu-O thin films were investigated: three step processes involving post deposition high temperature anneals, and in situ growth processes. Films were deposited by sequential ion beam sputtering from elemental Y, Ba and Cu targets, and characterized by x-ray diffraction, transmission and scanning electron microscopy, energy dispersive x-ray analysis, Rutherford backscattering spectrometry, and low temperature resistivity measurements. In the three step process, multilayers of ~60 Å periodicity were deposited on (001) SrTiO3, annealed in oxygen at 850-900°C, and subsequently at 400-500°C, to obtain the superconducting YBa2Cu3O7-δ phase. The films were epitaxial, predominantly single phase YBa2Cu3O7-δ, with different orientations. The nucleation and growth of Y-Ba-Cu-O films deposited on (001) SrTiO3 by magnetron sputtering from separate Y, BaF2 and Cu sources and grown by a three step process, was investigated by transmission electron microscopy. The in situ growth of YBa2Cu3O7-δ films by sequential ion beam sputtering was investigated. The films were deposited following the stacking sequence of YBa2Cu3O7-δ, with the individual layer thicknesses nominally equal to one monolayer, at temperatures between 550 and 750°C. O2 was supplied during growth. Epitaxial, c-axis oriented YBa2Cu3O7-δ films were obtained on MgO and SrTiO3. The correlations between deposition parameters, and structural and electrical properties were investigated. The films had expanded c-axis lattice parameters. The superconducting transition temperatures decreased with the enlargement of the c-lattice parameter. The deposition temperature was the main parameter controlling the lattice expansion. This was later interpreted in terms of the thermally activated dissociation of O2 at the film surface. We proposed that the expansion of the c-lattice parameter was a consequence of kinetic limitations to the incorporation of oxygen into the films during growth. This led to a consistent description of the results obtained in this work, and results reported in the literature for other in situ growth techniques. The films also presented inhomogeneous lattice distortions along the c-direction, that were larger for films with large lattice parameters. The superconducting transitions were broader for films with large inhomogeneous strains. The microstructure of films grown on several substrates (SrTiO3, MgO, SiO2/Si) under different growth conditions was investigated

Nonequilibrium Partitioning During Rapid Solidification of Si-As Alloys

The velocity dependence of the partition coefficient was measured for rapid solidification of polycrystalline Si-4.5 at% As and Si-9 at% As alloys induced by pulsed laser melting. The results constitute the first test of partitioning models both for the high velocity regime and for non-dilute alloys. The continuous growth model (CGM) of Aziz and Kaplan fits the data well, but with an unusually low diffusive speed of 0.46 m/s. The data show negligible dependence of partitioning on concentration, also consistent with the CGM. The predictions of the Hillert-Sundman model are inconsistent with partitioning results. Using the aperiodic stepwise growth model (ASGM) of Goldman and Aziz, an average over crystallographic orientations with parameters from independent single-crystal experiments is shown to be reasonably consistent with these polycrystalline partitioning results. The results, combined with others, indicate that the CGM without solute drag and its extension to lateral ledge motion, the ASGM, are the only models that fit the data for both solute partioning and kinetic undercooling interface response functions. No current solute drag models can match both partitioning and undercooling measurements.Engineering and Applied Science