AVS2011 Session SE-ThM: Nanostructured Thin Films and Coatings

Memristors, nonvolatile bipolar resistive switching devices first intentionally fabricated in 2008 [1], have attracted attention for use in a wide range of applications. To date, most memristors have been fabricated from either TiO_x [1] or TaO_x [2]. However, it is necessary to explore other dielectric materials, because the memristive properties of these dielectrics have not yet been explored.

In this study, memristors were fabricated on both silicon and flexible polyethylene teraphthalate (PET) substrates by using HfO₂, HfSiO₄, and ZrSiO₄ thin films and thermally evaporated aluminum contacts. The dielectric films were formed from sol-gel precursors synthesized based on the recipe of Meena et al. [3]. Following synthesis, the precursors were spun on to substrates, exposed to an oxygen plasma, and annealed in air at 150°C for 24 hours. Memristors with cross sectional areas ranging from 2×2 mm² to 0.5×0.5 mm² were fabricated.

The dielectric films were extensively characterized by using optical profilometry and microscopy, x-ray photoelectron spectroscopy (XPS), and deep ultraviolet spectroscopic ellipsometry (SE). The optical profilometry measurements demonstrate that the films show no evidence of significant inhomogeneities due to the solvent-based sol-gel synthesis methods. According to the XPS measurements, the films are ≈50% carbon. Such high levels of carbon are reasonable given the synthesis approach employed, and similarly high levels of carbon have been observed previously in TiO₂ films synthesized from sol-gels [4]. From the SE measurements, it is evident that the bandgaps of the HfO₂, HfSiO₄, and ZrSiO₄ films are similar to their predicted values, but the densities of the films are ≈0.5 due to the high amounts of excess carbon. Transmission electron microscopy measurements of the films will also be discussed.

Current-voltage (I-V), capacitance-voltage (C-V), capacitance-frequency (C-f), and conductance-frequency (G-f) measurements were performed at room temperature in a commercial probe station. The I‑V measurements indicate that devices fabricated from these films exhibit bipolar resistive switching. Furthermore, optical microscopy images indicate that the switching events are often accompanied by significant changes to the morphology of the active region of the device, particularly when the switching occurs at high voltages. These changes suggest the formation of localized conduction pathways through the dielectric. The C‑f and G-f measurements indicate that the capacitive properties of the devices also change following switching events. Additionally, after 24 days, retention tests show the devices are still in the ON state, thus, they may be considered nonvolatile.

References

[1] D.B. Strukov et al., Nature 438, 80 (2008).

[2] J.J. Yang et al., Appl. Phys. Lett. 97, 232102 (2010).

[3] J.S. Meena et al., 2010 3^rd International Nanoelectronics Conference (INEC), 1-2, 992, 2010.

[4] J.L. Tedesco et al., ECS Trans. 35, 107 (2011).

Nanoporous osmium-ruthenium may be applicable as an improved coating for dispenser cathodes used in travelling wave tubes and other various vacuum tube devices. Osmium-Ruthenium (OsRu) was co-deposited with either magnesium (Mg) or scandium (Sc) as a sacrificial element to form an intermetallic thin film. The sacrificial element was then selectively etched away, which dealloyed the film and left behind a nanoporous structure. Electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy were used to characterize the morphology, structure, and composition of the films in both the as-deposited and dealloyed states. Various film compositions and dealloying conditions were studied to determine the optimal conditions for creating a nanoporous film structure.

The excellent hardness, good abrasive and sliding wear resistance and high thermal stability and oxidation resistance favor Ti_1-xAl_xN to be used as hard protective coating in a variety of industrial applications such as advanced machining and forming tools, automobile and aerospace industry or for semiconductor electronics. Although, thin films based on this material system are investigated in detail, only little is known on the sequential relations between deposition conditions, chemistry, structure and physical and mechanical properties.

Here we show that Ti_1-xAl_xN coatings can be synthesized by magnetron-sputtering in single-phase cubic (NaCl, c) structure for x = Al/(Ti+Al) ratios up to 0.62 and single-phase hexagonal (ZnS-wurtzite, w) structure for x ≥ 0.75. These critical values for the individual favored structures strongly depend on the deposition conditions used such as N₂-partial pressure and resulting growth rate. Generally, a low N₂-partial pressure and a high growth rate favor the cubic structure. Increasing Al content of the single-phase cubic coatings results in increased hardness (to ~35 GPa for c-Ti_0.38Al_0.62N), earlier onset of the decomposition towards the stable phases c-TiN and w-AlN (across the formation of Ti-rich and Al-rich cubic nm-sized domains) and increased oxidation resistance. The single-phase wurtzite-structure coatings exhibit a high thermal stability and better oxidation resistance as compared to the single-phase cubic structure coatings. Even after 40h at 800 °C in ambient atmosphere the w-Ti_0.25Al_0.75N coating exhibits only an ~1 µm thin dense oxide layer on top of the ~2 µm remaining nitride, whereas the other coatings are already fully oxidized.

The coatings with a dual-phase or mixed-phase structure in the as deposited state show a reduced thermal stability and oxidation resistance as compared to the single-phase cubic or wurtzite structure coatings.

Nanocomposite ceramic coatings are being explored based on their good tribological and electrical properties, which can be tailored by changing e.g. the composition, microstructure, and surface morphology. For example, the nc-TiC/a-SiC nanocomposite system has shown promising properties for electrical contact applications [1, 2]. The tendency to form oxides and carbides greatly influences these properties. For instance, NbC has similar electrical properties as TiC, but has a lower tendency to form oxides. Especially for low force electrical contact applications, this makes NbC a suitable choice as replacement for TiC. Ge is in the same group as Si, but does, as a more noble element, not form carbides or oxides as easily. Here, we investigate the Nb-Ge-C material system with respect to nanostructure and electrical properties.

Nb-Ge-C thin films were deposited onto Si(001) and Ni-electroplated bronze substrates using dc magnetron sputtering in an Ar discharge at a low temperature (200 ºC). By use of three elemental targets in current-regulating mode both the Nb/C ratio and the Ge content was varied to study the effect of composition. XRD, TEM, and SEM showed that the NbC films are columnar and exhibit a preferred 111 crystallographic orientation. With increasing Ge content up to 10 at.%, the NbC grain size was reduced to ~5 nm with a random orientation. While the nc-NbC/Ge nanocomposites have no discernable amorphous tissue phase, XPS showed predominantly Ge-Ge with only little Ge-C bonds. In contact resistance measurements the films performed well (1.7-3.7 mΩ) in comparison to Ag reference (1.2 mΩ) at a contact force of 10 N.

[1] Eklund,P., Surface Engineering, 23, (2007), 406

[2] Lauridsen,J.; et al. Surf. Coat. Technol., 205, (2010), 299