Wouter Devulder

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.

Comparison of two single-image phase-retrieval algorithms for in-line x-ray phase-contrast imaging

The attenuation of x-rays in a material forms the basis of x-ray radiography and tomography. By measuring the transmission of the x-rays over a large amount of raypaths, the three-dimensional (3D) distribution of the x-ray linear attenuation coefficient can be reconstructed in a 3D volume. In x-ray microtomography (μCT), however, the x-ray refraction yields a significant signal in the transmission image and the 3D distribution of the refractive index can be reconstructed in a 3D volume. To do so, several methods exist, on both a hardware and software level. In this paper, we compare two similar software methods, the modified Bronnikov algorithm and the simultaneous phase-and-amplitude retrieval. The first method assumes a pure phase object, whereas the latter assumes a homogeneous object. Although these assumptions seem very restrictive, both methods have proven to yield good results on experimental data.

Combinatorial Study of Ag–Te Thin Films and Their Application as Cation Supply Layer in CBRAM Cells

In this work, we investigate binary Ag-Te thin films and their functionality as a cation supply layer in conductive bridge random access memory devices. A combinatorial sputter deposition technique is used to deposit a graded Ag(x)Te(1-x) (0 < x < 1) layer with varying composition as a function of the position on the substrate. The crystallinity, surface morphology, and material stability under thermal treatment as a function of the composition of the material are investigated. From this screening, a narrow composition range between 33 and 38 at% Te is selected which shows a good morphology and a high melting temperature. Functionality of a single Ag(2-δ)Te composition as cation supply layer in CBRAM with dedicated Al2O3 switching layer is then investigated by implementing it in 580 μm diameter dot Pt/Ag(2-δ)Te/Al2O3/Si cells. Switching properties are investigated and compared to cells with a pure Ag cation supply layer. An improved cycling behavior is observed when Te is added compared to pure Ag, which we relate to the ionic conducting properties of Ag2Te and the preferred formation of Ag-Te phases.

Controllable nitrogen doping in as deposited TiO2 film and its effect on post deposition annealing

In order to narrow the band gap of TiO2, nitrogen doping by combining thermal atomic layer deposition (TALD) of TiO2 and plasma enhanced atomic layer deposition (PEALD) of TiN has been implemented. By altering the ratio between TALD TiO2 and PEALD TiN, the as synthesized TiOxNy films showed different band gaps (from 1.91 eV to 3.14 eV). In situ x-ray diffraction characterization showed that the crystallization behavior of these films changed after nitrogen doping. After annealing in helium, nitrogen doped TiO2 films crystallized into rutile phase while for the samples annealed in air a preferential growth of the anatase TiO2 along (001) orientation was observed. Photocatalytic tests of the degradation of stearic acid were done to evaluate the effect of N doping on the photocatalytic activity.

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.

Electronic defect study on low temperature processed Cu(In,Ga)Se2 thin-film solar cells and the influence of an Sb layer

A way to lower the manufacturing cost of Cu(In,Ga)Se2 (CIGS) thin-film solar cells is to use flexible polymer substrates instead of rigid glass. Because such substrates require lower temperature during absorber deposition, the grain growth of the absorber layer can be hindered which leads to a lower cell performance. Partial compensation of this efficiency loss might be accomplished by growing the absorber in the presence of Sb, which is reported to promote grain growth. In this work CIGS solar cells, deposited on glass substrates, at a reduced substrate temperature with a thin Sb layer (7, 12 nm) on top of the Mo contact are investigated. The diffusion profile of Sb is measured with plasma profiling time of flight mass spectrometry. The beneficial effect of Sb on efficiency and grain size is shown in quantum efficiency measurements and with scanning electron microscopy, respectively. Electric spectroscopy is used to explore the possible effects on the defect structure, more in particular on the dominant shallow acceptor. Admittance spectra exhibit a capacitance step to the geometric capacitance plateau at low temperature (5–60 K). Analyzing this capacitance step, we obtained a good estimate of the activation energy of the intrinsic defects that provide the p-type conductivity of the CIGS absorber. The measurements did not show a change in the nature of the dominant acceptor upon Sb treatment.

Te-based chalcogenide materials for selector applications

The implementation of dense, one-selector one-resistor (1S1R), resistive switching memory arrays, can be achieved with an appropriate selector for correct information storage and retrieval. Ovonic threshold switches (OTS) based on chalcogenide materials are a strong candidate, but their low thermal stability is one of the key factors that prevents rapid adoption by emerging resistive switching memory technologies. A previously developed map for phase change materials is expanded and improved for OTS materials. Selected materials from different areas of the map, belonging to binary Ge-Te and Si-Te systems, are explored. Several routes, including Si doping and reduction of Te amount, are used to increase the crystallization temperature. Selector devices, with areas as small as 55 × 55 nm2, were electrically assessed. Sub-threshold conduction models, based on Poole-Frenkel conduction mechanism, are applied to fresh samples in order to extract as-processed material parameters, such as trap height and density of defects, tailoring of which could be an important element for designing a suitable OTS material. Finally, a glass transition temperature estimation model is applied to Te-based materials in order to predict materials that might have the required thermal stability. A lower average number of p-electrons is correlated with a good thermal stability.

Study of amorphous Cu-Te-Si thin films showing high thermal stability for application as a cation supply layer in conductive bridge random access memory devices

In this work we demonstrate a thermally stable copper supply layer by Si alloying of Cu0.6Te0.4 for application in Conductive Bridge Random Access Memory (CBRAM) cells. A good thermal stability of the copper supply layer is necessary to allow its implementation in future memory devices. In situ X-ray diffraction is used to investigate the crystallization behaviour of Cu0.6Te0.4 layers with Si contents up to 20 at%. Low Si concentrations result in crystallization, phase separation and transformations at temperatures below 400 °C, whereas addition of 20 at% Si results in a layer that remains amorphous up to temperatures exceeding 500 °C, making it compatible with back end of line temperatures. Moreover, atomic force microscopy measurements show a very smooth surface morphology up to temperatures exceeding 400 °C. The absence of grain boundaries in the amorphous layer is expected to contribute to the uniformity of the supply layer, and hence it should be beneficial for integration in scaled devices. We attribute the good ability of Si to keep the material amorphous to the high coordination number of Si and the formation of strong bonds which are difficult to break, making rearrangement in a lattice more difficult to proceed. This is further evidenced by XPS measurements, which suggest the occurrence of both Si–Si and Si–Te bonds. CBRAM functionality of this composition is demonstrated by integrating the material in 580 μm diameter dot Pt/Cu–Te–Si/Al2O3/n+ Si CBRAM cells.

Influence of alloying the copper supply layer on the retention of CBRAM

Conductive Bridge Random Access Memory (CBRAM) is one of the emerging technologies for future memory devices. However, one of the main challenges is to ensure high temperature data retention. In this work, the use of different copper alloys as cation supply layer and their influence on the retention properties of CBRAM cells are presented. Also the thermal stability of the Cu alloys, which is important to sustain the temperatures applied during device fabrication, is investigated.