AVS2010 Session TF+EM-MoM: Nanostructuring Thin Films

Ultrathin metallic heterostructures show richness in magnetic behavior driven, in part, by exchange, interface, proximity, size and dimensionality effects. One of the most widely studied and technologically important, but still not well-understood, magnetic behavior is the phenomenon of exchange bias (EB) observed in ferromagnetic (FM)/antiferromagnetic(AFM) structures. In this talk, I will first discuss epitaxial growth of such MnPd/Fe bilayers with tailored AFM spin-lattices (compensated and uncompensated) at the interface exhibiting in-plane EB and (Co_xÅPt_yÅ)_n/IrMn multilayers with perpendicular EB. Assymetric magnetization reversals in exchange-biased Fe/MnPd bilayers were confirmed by photoemission electron microscopy (PEEM) with x-ray magnetic circular dichroism (XMCD) for magnetic contrast. Further, the element-specific magnetic structure of the Mn₅₂Pd₄₈/Fe bilayer was investigated with atomic-layer depth sensitivity at the antiferromagnet/ferromagnet interface by soft-XMCD and magnetic reflectivity (XMR). A complex magnetic interfacial configuration, consisting of a 2-monolayer-thick induced ferromagnetic region, and pinned uncompensated Mn moments that reach far deeper (~13 Å), both in the antiferromagnet, were found. These epitaxial EB samples also show in-plane reorientation transitions, determined by the competition between the interface exchange coupling and the intrinsic uniaxial energies, and is driven by the temperature, as well as the thickness of MnPd and Fe layers. Complementing these results, work on multilayers show that perpendicular EB arise from a complex interplay between unidirectional anisotropy at the terminating FM/AFM interface, the perpendicular anisotropy of the FM/nonmagnet(NM) multilayer stack and the overall magnetostatic energy of the structure. Finally, we have developed and implemented nanoimprint lithography (NIL) to fabricate wire and particle arrays of exchange biased elements at the 300nm length scale. A mask transfer NIL process has also been developed to grow epitaxial structures. The role of size and dimensionality as well as competing anisotropies, introduced by particle shape, on EB heterostructured elements has been studied. Details will be presented.

The synthesis and assembly of functional metallic nanomaterials is critical for realizing many important applications of nanoscience and nanotechnology. In this study, we investigated thin film dewetting of metal films via pulsed nanosecond laser melting. We studied film instabilities induced by capillary forces and the subsequent mass transport which can lead to thin film break-up and subsequent nanoparticle formation. We have also explored the dewetting and nanopattern formation of nano-lithographically patterned thin films of various shapes to understand how various initial and boundary conditions can guide the assembly. In this presentation we will discuss the various spinodal and Rayleigh-Plateau instability mechanisms and the inertial, visco-inertial and viscous flow regimes involved in the thin metal film assembly. Finally we will discuss how nano-lithographically defined features can be used to direct the assembly of multi-functional nanoparticle ensembles.

Transition metal nitrides (TMN) are well known to have a remarkable range of unique physical properties. One method used to further enhance the physical properties of many transition metal nitrides is to alloy them with a second, thermodynamically immiscible nitride. The most famous example is Ti_1-xAl_xN; many have reported drastically enhanced physical properties including super hardness, increased oxidation resistance, age-hardening behavior, and the formation nanoscale compositional modulations during film growth and post annealing experiments. However, very little has been reported on the ability to control this nanostructure, nor has the effect of these nanostructures on the physical properties of the films. We have chosen Hf_1-xAl_xN as a model system to study the nanostructures of interest. We begin by reporting on the effects of nanostructure on the opto-electronic, thermal transport and elastic constant properties of Hf_1-xAl_xN single crystal layers grown on MgO(001) using ellipsometry, temperature dependent hall effect, and picosecond probe acoustic transport measurements, respectively. We continue by summarizing a systematic study into the effects of growth parameters (ion energy, E_i, ion-to-metal flux ratio, J_ion/J_me, and substrate temperature, T_s) on single crystal reactively sputtered Hf_0.7Al_0.3N/MgO(001) layers in order to controllably manipulate the nanostructure and study its effects on the physical properties. Films are deposited from a Hf/Al 70/30 (at. %) target in 5% N₂/Ar mixtures while J_ion/J_me is varied from 0.7 to 12.6, T_s from 400 to 700 ° C, and E_i from 10 to 80 eV. J_ion/J_me has a strong effect on the formation of 3D nanoscale (2-3nm) compositional modulations as indicated by HR-XRD and HR-TEM. Nanoindentation experiments reveal an increase in film hardness from 31.7 ± 0.6 GPa to 38.9±0.6 GPa. E_i has a strong effect on the AlN incorporation probability, which can be adjusted between ~ 0 and 100% by varying the ion energy (E_i) incident at the growing film over a narrow range, 10-40 eV. Epitaxial film compositions vary from x = 0.30 with E_i = 10 eV, to 0.27 with E_i = 20 eV, 0.17 with E_i = 30 eV, and ≤ 0.002 with E_i ≥ 40 eV. This extraordinary range in real-time manipulation of film chemistry during deposition is due to the efficient resputtering of Al atoms (27 amu) by Ar⁺ ions (40 amu) backscattered from Hf atoms (178.5 amu). We demonstrate that this effect can be used to grow superlattices with abrupt interfaces at high deposition rates from a single target by switching E_i. We grew superlattices with bilayer thicknesess from 1-6nm and films exhibited an increase in hardness from 32.5±0.9 GPa to 37.8±1.2 GPa.

Thin film vanadium oxide is used as the active layer in un-cooled, infrared imaging devices based on microbolometer structures. In this type of imaging device, infrared radiation is detected via a temperature driven resistivity change in the active layer. Underlying readout circuitry and the need for increased detection sensitivity require low electrical resistivity material with high thermal coefficient of resistivity (TCR). Vanadium oxide thin films deposited via reactive pulsed direct current (DC) sputtering have properties in the range of interest with room temperature resistivity varying from 0.01 to 6×10⁴ Ohm cm and TCR’s varying from -0.1 to -4.2 %/°C. Films with resistivity in the range of interest (0.1 to 1.0 Ohm cm) contain the rocksalt structured FCC VO_x phase (0.8 < x < 1.3) accompanied by significant structural disorder, while those with greater resistivity are amorphous. The relationship between TCR and room temperature resistivity is exponential, and throughout the present series of films is fixed, i.e. a film with a given resistivity has a set TCR. Energy filtered electron diffraction patterns collected in the transmission electron microscope (TEM) reveal a diffuse scattering feature at low scattering angle for both amorphous and nanocrystalline films, suggesting that films containing nanocrystals also contain an amorphous phase. Raman spectroscopy results from amorphous films show a broad feature around ~890 cm^-1, while spectra from nanocrystalline films exhibit the “amorphous” feature and a second broad feature at ~300 cm^-1. The feature at ~300 cm^-1 was the only feature present in the most crystalline of the films, suggesting it represents the disordered FCC VO_x phase. Film stoichiometry, as measured with Rutherford backscattering spectroscopy, puts the overall chemistry of the nanocrystalline films outside the FCC VO_x phase field, with many of the samples having a V:O ratio greater than 1.3. A pre-peak feature in the V-K edge was observed with X-ray absorption spectroscopy (XAS), and the intensity of the pre-peak, which is known to result from local octahedral disorder, increased with increasing oxygen content and correlates with diffraction observations of increasing disorder. The combined TEM, RBS, Raman, and XAS analyses suggest vanadium oxide films with properties in the range of interest for microbolometer-based devices consist of a two-phase material containing FCC VO_x nanocrystals embedded in an oxygen-rich amorphous matrix. Films lacking sufficient disorder have resistivity in the range of interest but insufficient TCR magnitude, suggesting the need for both the nano-crystalline phase and the amorphous matrix.

Stress shielding due to uneven load distributions at the bone–prosthesis interface affect joint prostheses and can lead to wear and loosening. Commonly used cobalt–chromium–molybdenum alloys can degrade during wear at an average rate of 0.02–0.06 mm/year. Other alloys such as titanium–aluminium–vanadium although biocompatible and highly corrosion resistant, exhibit relatively low mechanical properties and poor wear resistance. Nanocomposite nanocrystalline (nc-) Ti(N,C)/ amorphous diamond-like-carbon (a-C:H) coatings exhibit high hardness (H), low friction coefficients, high wear resistance and resilience to substrate deformation thus making them promising candidates for prosthetic implant applications. In this work we investigate the influence of the microstructure of nc-Ti(N,C)/a-C:H coatings on the mechanical, tribological and biological properties with the aim of using such materials not only as wear resistant films in biomedical implants, but also as a bioactive surface that can promote bone ingrowth at areas of medical implants, such as the femoral stem or the acetabular cell in hip replacement joints, that are in direct contact with bone. Approximately 2 μm thick, nc-Ti(N,C)/ a-C:H coatings were deposited on Si wafer and implant alloy coupons using low temperature (~ 200 ^oC) DC reactive magnetron sputtering. The carbon content was varied from 41 to 57 at % and the obtained a-C:H phase ranged from 31 – 47 at. % in order to form the desired nanocomposite structure of 2-4 nm wide Ti(N,C) with 1 to 2 monolayer coverage of a-C:H. An increase in the amorphous phase results in a decrease in mechanical and a decline in tribological performance, however the change in structure and surface morphology at increased carbon content favours the bioactivity of the films. Coating chemical composition and microstructure was investigated by means of x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The mechanical and tribological properties of the films where determined using nanoindentation and nanotriboscope methods. In order to assess the biocompatibility of nc-Ti(N,C)/a-C:H coatings and investigate their osteoblast - attachment properties and thus determine their efficacy as implant coatings, the osteoblastogenic osteosarcoma immortalised cell lines Saos-2 and Hos were seeded and allowed to grow on the coating surface. Cell attachment properties were assessed in terms of viability of seeded cells. Viable attached cells were quantified by a mitochondrial enzymatic activity-based colorimetric assay against cultures seeded on conventional tissue-culture treated plastic surface.