Nm-thin Film Research Articles

Resolving buried interfaces with low energy ion scattering

We investigate the use of low energy ion scattering (LEIS) to characterize buried interfaces of ultrathin films. LEIS spectra contain depth-resolved information in the so-called subsurface signal. However, the exact correlation between the subsurface signal and the sample’s depth composition is still unknown. For this reason, LEIS spectra so far only provided qualitative information about buried interfaces. In this study, we investigate nm-thin films of Si-on-W and Si-on-Mo, where we compare simulated data to LEIS spectra. We present a method to extract depth-sensitive compositional changes—resolving buried interfaces—from LEIS spectra for the first few nanometers of a thin-film sample. In the case of Si-on-Mo, the simulation of the LEIS subsurface signal allows obtaining a quantitative measurement of the interface profile that matches the value determined using the LEIS layer growth profile method with an accuracy of 0.1 nm. These results pave the way to further extend the use of LEIS for the characterization of features buried inside the first few nanometers of a sample.

Influence of molecular weight of polycation polydimethyldiallylammonium and carbon nanotube content on electric conductivity of layer-by-layer films

For biological and engineering applications, nm-thin films with high electrical conductivity and tunable sheet resistance are desirable. Multilayers of polydimethyldiallylammonium chloride (PDADMA) with two different molecular weights (322 and 44.3 kDa) and oxidized carbon nanotubes (CNTs) were constructed using the layer-by-layer technique. The surface coverage of the CNTs was monitored with a selected visible near infrared absorption peak. Both the film thickness and the surface coverage of the CNTs increased linearly with the number of CNT/PDADMA bilayers deposited (film thickness up to 80 nm). Atomic force microscopy images showed a predominantly surface-parallel orientation of CNTs. Ohmic behavior with constant electrical conductivity of each CNT/PDADMA film and conductivity up to 4 · 103 S/m was found. A change in PDADMA molecular weight by almost a factor of ten has no effect on the film thickness and electrical conductivity, only the film/air roughness is reduced. However, increasing CNT concentration in the deposition dispersion from 0.15 up to 0.25 mg/ml results in an increased thickness of a CNT/PDADMA bilayer (by a factor of three). The increased bilayer thickness is accompanied by a decreased electrical conductivity (by a factor of four). The decreased conductivity is attributed to the increased monomer/CNT ratio.

Chemically vapor deposited Eu3+:Y2O3 thin films as a material platform for quantum technologies

Rare earth ions hosted in solids are good candidates for quantum technologies due to their chemical stability and optical and spin transitions exhibiting long coherence lifetimes. While bulk oxide crystals are usually the preferred host material, the development of a scalable silicon-compatible thin film platform would be desirable. In this paper, we report on the growth of Y2(1−x)Eu2xO3 thin films on silicon in the full range of Eu3+ concentration by direct liquid injection chemical vapor deposition (CVD). Our sub-micrometer polycrystalline films with a strong-(111) texture were grown for all compositions into the bixbyite cubic phase. The variation of growth rates with temperature and flow indicated that deposition occurred through a mass-transport controlled regime. Optical assessment of the Eu-doped thin films showed inhomogeneous linewidths as narrow as 50 GHz and fluorescence lifetimes of 1 ms for the lowest concentrations. Finally, a spectral hole was successfully burned in a 200 nm-thin film with a 2% Eu doping leading to a homogeneous linewidth of 11 MHz. These values are still below those reported for bulk single crystals indicating that additional decoherence mechanisms exist in such nanometric films, which might be alleviated by further improvement of the crystalline quality. Nevertheless, these results pave the way to the use of CVD-grown Eu:Y2O3 thin films as a platform for integrated quantum devices.

Two-Step Approach for Conformal Chemical Vapor-Phase Deposition of Ultra-Thin Conductive Silver Films.

Conductive ultra-thin silver films are commonly fabricated by physical vapor deposition methods such as evaporation or sputtering. The line-of-sight geometry of these techniques impedes the conformal growth on substrates with complex morphology. In order to overcome this issue, volume deposition technologies such as chemical vapor deposition or atomic layer deposition are usually preferred. However, the silver films fabricated using these methods are generally non-electrically conductive for thicknesses below 20-50 nm due to island formation. Here, we demonstrate a novel approach for producing ultra-thin conductive silver layers on complex substrates. Relying on chemical vapor-phase deposition and plasma post-treatment, this two-step technique allows the synthesis of highly conductive and uniform silver films with a critical thickness lower than 15 nm and a sheet resistance of 1.6 Ω/□ for a 40 nm-thin film, corresponding to a resistivity of 6.4 μΩ·cm. The high infrared reflectance further demonstrates the optical quality of the films, despite a still large root-mean-square roughness of 8.9 nm. We successfully demonstrate the highly conformal deposition in lateral structures with an aspect ratio of up to 100. This two-step deposition method could be extended to other metals and open new opportunities for depositing electrically conductive films in complex 3D structures.

X-ray diffraction and stress relaxations to study thermal and stress-assisted annealings in nanocrystalline gold thin films

The dependence on thermal history of the plasticity mechanisms occurring in nanocrystalline gold thin films is evidenced thanks to relaxation tests combined with in-situ synchrotron X-ray diffraction. The two techniques complement one another. The activation parameters show that the films deform mainly by dislocations and give indications about their mean free paths, whereas the Bragg peak positions, widths and areas bring invaluable information on residual stress as well as on some plasticity properties, like dislocation storage inside the grains or grain rotations. For the demonstration, two sputter-deposited nanocrystalline 50 nm-thin films deposited onto stretchable substrates are studied. It is shown that an as-grown sample (at a homologous temperature of 0.22) presents a stress-assisted annealing, thus decreasing its initial defect density, whereas if a thermal annealing (3 h at 200∘C, corresponding to a homologous temperature of 0.35) has been applied to the sample before the tensile test, it deforms by conventional plasticity, and the dislocations are not stored inside the grains. These mechanisms lead to different work-hardening properties. This work shows how a moderate annealing can have a profound influence on the mechanical behaviour of these thin films.

Determination of refractive index and layer thickness of nm-thin films via ellipsometry.

Ellipsometric measurements give information on two film properties with high precision, thickness and refractive index. In the simplest case, the substrate is covered with a single homogenous, transparent film. Yet, with ellipsometry, it is only possible to determine the two film properties thickness and refractive simultaneously if the layer thickness exceeds 15 nm - a restriction well known for a century. Here we present a technique to cross this limitation: A series expansion of the ellipsometric ratio ρ to the second order of the layer thickness relative to the wavelength reveals the first and second ellipsometric moment. These moments are properties of the thin film and independent of incident angle. Using both moments and one additional reference measurement enables to determine simultaneously both thickness and refractive index of ultra-thin films down to 5 nm thickness.

Temperature dependent behavior of thermal conductivity of sub-5 nm Ir film: Defect-electron scattering quantified by residual thermal resistivity

By studying the temperature-dependent behavior (300 K down to 43 K) of electron thermal conductivity (κ) in a 3.2 nm-thin Ir film, we quantify the extremely confined defect-electron scatterings and isolate the intrinsic phonon-electron scattering that is shared by the bulk Ir. At low temperatures below 50 K, κ of the film has almost two orders of magnitude reduction from that of bulk Ir. The film has ∂κ/∂T > 0, while the bulk Ir has ∂κ/∂T < 0. We introduce a unified thermal resistivity (Θ = T/κ) to interpret these completely different κ ∼ T relations. It is found that the film and the bulk Ir share a very similar Θ ∼ T trend, while they have a different residual part (Θ0) at 0 K limit: Θ0 ∼ 0 for the bulk Ir, and Θ0 = 5.5 m·K2/W for the film. The Ir film and the bulk Ir have very close ∂Θ/∂T (75–290 K): 6.33 × 10−3 m K/W for the film and 7.62 × 10−3 m K/W for the bulk Ir. This strongly confirms the similar phonon-electron scattering in them. Therefore, the residual thermal resistivity provides an unprecedented way to quantitatively evaluating defect-electron scattering (Θ0) in heat conduction. Moreover, the interfacial thermal conductance across the grain boundaries is found larger than that of Al/Cu interface, and its value is proportional to temperature, largely due to the electron's specific heat. A unified interfacial thermal conductance is also defined and firmly proves this relation. Additionally, the electron reflection coefficient is found to be large (88%) and almost temperature independent.

Characterization in a Wide Frequency Range (40 MHz–67 GHz) of a KTa0.65Nb0.35O3 Thin Film for Tunable Applications

A (100) oriented KTa0.65Nb0.35O3 400 nm-thin film has been deposited by Pulsed Laser Deposition on MgO substrate. Microwave measurements, performed on InterDigitated Capacitors, show a paraelectric phase at room temperature with a tunability for the devices of 64% under an electric field of 400 kV/cm. Then, using a specific de-embedding method, the complex permittivity of the KTN thin film has been extracted from 40 MHz up to 67 GHz on coplanar waveguides. As promising applications are pointed out at 60 GHz, such as indoor communications, material characterizations are expected in this spectrum.

Pulsed laser deposition of functional oxides - towards a transparent electronics

Metal oxides, in particular with transition metals, show strong electronic correlations which determine a huge variety of electronic properties, together with other functionalities. For example, ZnO and Ga2O3 as wide-bandgap semiconductors have a high application potential as transparent functional layers in future oxide electronics [1-2]. Other oxides of current interest are ferrimagnetic spinels of the type MFe2O4 (M=Zn,Co,Ni), see K. Brachwitz et al. Appl. Phys. Lett. 102, 172104 (2013), or highly correlated iridate films, see M. Jenderka et al. Phys. Rev. B 88, 045111 (2013). Furthermore, combinations of ferroelectric and magnetic oxides in multiferroic composites and multilayers show promising magnetoelectric coupling. For the exploratory growth of the above mentioned novel oxides into nm-thin films, pulsed laser deposition (PLD) appears as the method of choice because of its extremely high flexibility in terms of material and growth conditions, high growth rate and excellent structural properties [3]. This talk highlights recent developments of new functional oxides using unique large-area PLD processes running for more than two decades in the lab of the author [3].

Surface molecularly imprinted polydopamine films for recognition of immunoglobulin G

We have prepared a surface imprinted polymer (SIP) film for label-free recognition of immunoglobulin G (IgG). The IgG-SIPs were obtained by covalent immobilization of IgG via a cleavable covalent bond and a suitable spacer unit to a gold electrode, followed by electrodepostion of a nm-thin film of polydopamine (PDA). The IgG was then removed by destruction of the cleavable bond so that complementary binding sites were created on the surface of the film. IgG-SIPs with various thicknesses of the PDA films were compared with respect to their affinity to IgG using a quartz crystal microbalance combined with flow injection analysis. The films were also characterized by cyclic voltammetry and scanning electron microscopy. The IgG-SIPs with a film thickness of around 17 nm showed the most pronounced imprinting effect (IF 1.66) and a binding constant of 296 nM.