Geert Rampelberg

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.

Composition influence on the physical and electrical properties of SrxTi1−xOy-based metal-insulator-metal capacitors prepared by atomic layer deposition using TiN bottom electrodes

In this work, the physical and electrical properties of SrxTi1−xOy (STO)-based metal-insulator-metal capacitors (MIMcaps) with various compositions are studied in detail. While most recent studies on STO were done on noblelike metal electrodes (Ru, Pt), this work focuses on a low temperature (250 °C) atomic layer deposition (ALD) process, using an alternative precursor set and carefully optimized processing conditions, enabling the use of low-cost, manufacturable-friendly TiN electrodes. Physical analyses show that the film crystallization temperature, its texture and morphology strongly depends on the Sr/Ti ratio. Such physical variations have a direct impact on the electric properties of SrxTi1−xOy based capacitors. It is found that Sr-enrichment result in a monotonous decrease in the dielectric constant and leakage current as predicted by ab initio calculations. The intercept of the EOT vs physical thickness plot further indicates that increasing the Sr-content at the film interface with the bottom TiN...

Partially fluorinated MIL-47 and Al-MIL-53 frameworks: influence of functionalization on sorption and breathing properties

Two perfluorinated metal hydroxo terephthalates [M(III)(OH)(BDC-F)]·n(guests) (M(III) = V, MIL-47-F-AS or 1-AS; Al, Al-MIL-53-F-AS or 2-AS) (BDC-F = 2-fluoro-1,4-benzenedicarboxylate; AS = as-synthesized) have been synthesized by a hydrothermal method using microwave irradiation (1-AS) or conventional electric heating (2-AS), respectively. The unreacted or occluded H(2)BDC-F molecules can be removed under vacuum by direct thermal activation or exchange of guest molecules followed by thermal treatment leading to the empty-pore forms of the title compounds [V(IV)(O)(BDC-F)] (MIL-47-F, 1) and [Al(III)(OH)(BDC-F)] (Al-MIL-53-F, 2). Thermogravimetric analysis (TGA) and temperature-dependent XRPD (TDXRPD) experiments indicate that the compounds are stable up to 385 and 480 °C, respectively. Both of the thermally activated compounds exhibit significant microporosity, as verified by N(2), CO(2), n-hexane, o- and p-xylene sorption analyses. The structural changes of 2 upon adsorption of CO(2), n-hexane, o- and p-xylene were highly influenced due to functionalization by -F groups, as compared to parent Al-MIL-53. The -F groups also introduce a certain degree of hydrophobicity into the frameworks, as demonstrated by the H(2)O sorption analyses.

In situ X-ray diffraction study of the controlled oxidation and reduction in the V–O system for the synthesis of VO2 and V2O3 thin films

VO2 and V2O3 thin films have been prepared by controlled oxidation and reduction reactions in the vanadium–oxygen system. During these reactions, crystalline phase formation and stability were characterized by means of in situ X-ray diffraction. Oxidation of vanadium thin films was carried out over a wide range of oxygen partial pressures between 0.2 and 200 mbar and temperatures between 430 °C and 615 °C. Depending on the oxygen partial pressures and temperatures, VO2, V6O13 and V2O5 could be obtained as pure or mixed phases. Reduction of V2O5 in 50 mbar H2 resulted in a continuous reduction to V2O3. Stabilization of the VO2 phase was obtained by adding low O2 concentrations in the range from 0.2 to 2 mbar to the H2 gas, a method which proved to be successful also for the controlled oxidation of vanadium to VO2. The semiconductor−metal transition was observed by means of temperature dependent sheet resistance measurements. VO2 films prepared by the oxidation of vanadium at low oxygen partial pressures were characterized by a 3 orders of magnitude decrease in resistance during transition. Annealing in air only yielded comparable switching ratios when the annealing time was carefully optimized. Both the VO2 films prepared by oxidation of vanadium or reduction of V2O5 in the mixture of H2 and O2 showed 4 to 5 orders of magnitude switching, which is close to the best reported values for bulk, single-crystal VO2. This excellent switching performance is believed to originate from a decreased level of defects at grain boundaries and in the bulk. In addition, the V2O3 films prepared by reduction of V2O5 showed a 3 orders of magnitude increase in resistance near −100 °C. Our results provide methods for transforming vanadium oxide phases into VO2 and V2O3 with high resistance switching ratios.

Low temperature plasma-enhanced atomic layer deposition of thin vanadium nitride layers for copper diffusion barriers

Thin vanadium nitride (VN) layers were grown by atomic layer deposition using tetrakis(ethylmethylamino)vanadium and NH3 plasma at deposition temperatures between 70 °C and 150 °C on silicon substrates and polymer foil. X-ray photoelectron spectroscopy revealed a composition close to stoichiometric VN, while x-ray diffraction showed the δ-VN crystal structure. The resistivity was as low as 200 μΩ cm for the as deposited films and further reduced to 143 μΩ cm and 93 μΩ cm by annealing in N2 and H2/He/N2, respectively. A 5 nm VN layer proved to be effective as a diffusion barrier for copper up to a temperature of 720 °C.

Amorphous and Crystalline Vanadium Oxides as High-Energy and High-Power Cathodes for Three-Dimensional Thin-Film Lithium Ion Batteries

Flexible wearable electronics and on-chip energy storage for wireless sensors drive rechargeable batteries toward thin-film lithium ion batteries. To enable more charge storage on a given surface, higher energy density materials are required, while faster energy storage and release can be obtained by going to thinner films. Vanadium oxides have been examined as cathodes in classical and thin-film lithium ion batteries for decades, but amorphous vanadium oxide thin films have been mostly discarded. Here, we investigate the use of atomic layer deposition, which enables electrode deposition on complex three-dimensional (3D) battery architectures, to obtain both amorphous and crystalline VO2 and V2O5, and we evaluate their thin-film cathode performance. Very high volumetric capacities are found, alongside excellent kinetics and good cycling stability. Better kinetics and higher volumetric capacities were observed for the amorphous vanadium oxides compared to their crystalline counterparts. The conformal deposition of these vanadium oxides on silicon micropillar structures is demonstrated. This study shows the promising potential of these atomic layer deposited vanadium oxides as cathodes for 3D all-solid-state thin-film lithium ion batteries.

Improved thermal stability and retention properties of Cu–Te based CBRAM by Ge alloying

In this work we investigate the influence of Ge as an alloying element in Cu–Te based thin films for application as a cation supply layer in Conductive Bridge Random Access Memory (CBRAM). The thermal stability of the alloys and their functionality as a copper supply layer in CBRAM are investigated. The thermal stability is studied by means of in situ X-ray diffraction, which reveals information on phase separation, phase transformations and melting of the material. We demonstrate that addition of Ge to Cu0.6Te0.4 inhibits crystallization up to 300 °C. However, phase separation occurs upon crystallization, which might result in device to device variability when this occurs in memory devices. This is solved by using Cu2GeTe3 that forms a single phase upon crystallization. The most promising alloys are implemented in 580 μm diameter dot Pt/CuxTeyGe1−x−y/Al2O3/Si CBRAM cells. Their functionality is verified by DC cycling and the influence of Ge is studied by comparing the switching to binary Cu0.6Te0.4 based memory cells. The retention of the programmed memory states is measured at 85 °C. Functional CBRAM is demonstrated, and improved filament stability and retention properties are observed for the Ge containing cells compared to Cu0.6Te0.4. We mainly attribute this to the Ge–Te bonds that are formed in the supply layer. This lowers the tendency for Cu–Te formation which results in a lower driving force for the Cu to go back to the supply layer, and hence contributing to a more stable filament. The formation of Ge–Te bonds was confirmed by XPS measurements.

VO2, a Metal-Insulator Transition Material for Nanoelectronic Applications

Nanoelectronic Applications K. Martens, I. P. Radu, G. Rampelberg, J. Verbruggen, S. Cosemans, S. Mertens, S. Xiaoping, M. Schaekers, C. Huyghebaert, C. Detavernier, S. De Gendt, M. Heyns, J. Kittl a IMEC, Kapeldreef 75, Leuven, Belgium b ESAT Dept., KULeuven, Leuven, Belgium c Physics Dept., K. U. Leuven, Leuven, Belgium d Dept. of Solid State Sciences, UGent, Gent, Belgium e Chemistry Dept. KULeuven f Materials Engineering Dept., KULeuven

Perpendicular magnetic anisotropy of Co\Pt bilayers on ALD HfO2

Perpendicular Magnetic Anisotropy (PMA) is a key requirement for state of the art Magnetic Random Access Memories (MRAM). Currently, PMA has been widely reported in standard Magnetic Tunnel Junction material stacks using MgO as a dielectric. In this contribution, we present the first report of PMA at the interface with a high-κ dielectric grown by Atomic Layer Deposition, HfO2. The PMA appears after annealing a HfO2\Co\Pt\Ru stack in N2 with the Keff of 0.25 mJ/m2 as determined by Vibrating Sample Magnetometry. X-Ray Diffraction and Transmission Electron Microscopy show that the appearance of PMA coincides with interdiffusion and the epitaxial ordering of the Co\Pt bilayer. High-κ dielectrics are especially interesting for Voltage Control of Magnetic Anisotropy applications and are of potential interest for low-power MRAM and spintronics technologies.

Atomic layer deposition of vanadium oxides for thin-film lithium-ion battery applications

Amorphous VO2 thin films are deposited by atomic layer deposition (ALD) using tetrakis[ethylmethylamino]vanadium (TEMAV) as vanadium precursor and water or ozone as the oxygen source. The crystallisation and oxidation behaviour is investigated for different oxygen partial pressures between ambient air and 3.7 Pa, resulting in phase formation diagrams on SiO2, TiN and Pt substrates, demonstrating a series of stable vanadium oxide phases in the VO2–V2O5 series. Most of the obtained phases exhibit lithium intercalation behaviour in the 1.5–4.5 V vs. Li+/Li potential range, and demonstrate high volumetric capacities in the order of V2O5 < VO2 (B) < V6O13 < V3O7 < V4O9, with the latter at more than twice the capacity of the best commercial cathode materials.