Bob De Schutter

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.

Influence of Carbon Alloying on the Thermal Stability and Resistive Switching Behavior of Copper-Telluride Based CBRAM Cells

We report the improved thermal stability of carbon alloyed Cu0.6Te0.4 for resistive memory applications. Copper-tellurium-based memory cells show enhanced switching behavior, but the complex sequence of phase transformations upon annealing is disadvantageous for integration in a device. We show that addition of about 40 at % carbon to the Cu-telluride layer results in an amorphous material up to 360 °C. This material was then integrated in a TiN/Cu0.6Te0.4-C/Al2O3/Si resistive memory cell, and compared to pure Cu0.6Te0.4. Very attractive endurance (up to 1 × 10(3) cycles) and retention properties (up to 1 × 10(4) s at 85 °C) are observed. The enhanced thermal stability and good switching behavior make this material a promising candidate for integration in memory devices.

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.

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.