Manfred Beckers

Phase stability of epitaxially grown Ti2AlN thin films

The phase stability of Mn+1AXn phase (M: early transition metal, A: A-group element, and X: C and∕or N) Ti2AlN thin films reactively sputtered onto MgO(111) and Al2O3(0001) substrates has been investigated by in situ x-ray diffraction and Rutherford backscattering. High substrate temperature deposition results in epitaxial Ti2AlN growth with basal planes parallel to the substrate surface. In contrast to reported high thermal stability for bulk Ti–Al–N Mn+1AXn phases in air, Ti2AlN thin films in vacuum decompose already at ∼800°C. The decomposition proceeds by outward Al diffusion and evaporation, followed by detwinning of the as-formed Ti2N atomic layers into cubic TiNx and intermediate phases.

Effects of strain and composition on the lattice parameters and applicability of Vegard's rule in Al-rich Al1-xInxN films grown on sapphire

The lattice parameters and strain evolution in Al1−xInxN films with 0.07⩽x⩽0.22 grown on GaN-buffered sapphire substrates by metal organic vapor phase epitaxy have been studied by reciprocal space mapping. Decoupling of compositional effects on the strain determination was accomplished by measuring the In contents in the films both by Rutherford backscattering spectrometry (RBS) and x-ray diffraction (XRD). Differences between XRD and RBS In contents are discussed in terms of compositions and biaxial strain in the films. It is suggested that strain plays an important role for the observed deviation from Vegard’s rule in the case of pseudomorphic films. On the other hand, a good agreement between the In contents determined by XRD and RBS is found for Al1−xInxN films with low degree of strain or partially relaxed, suggesting applicability of Vegard’s rule in the narrow compositional range around the lattice matching to GaN.

Magnetron sputtering of Ti3SiC2 thin films from a compound target

In this Thesis, I have investigated multifunctional nanostructured Ti-Si-C thin films synthesized by magnetron sputtering in the substrate-temperature range from room temperature to 900 °C. The studies cover high-temperature growth of Ti3SiC2 and Ti4SiC3, low-temperature growth of Ti-Si-C nanocomposites, and Ti-Si-C-based multi¬layers, as well as their electrical, mechanical, and thermal-stability properties. Ti3SiC2 and Ti4SiC3 were synthesized homoepitaxially onto bulk Ti3SiC2 from individual sputtering targets and heteroepitaxially onto Al2O3(0001) substrates from a Ti3SiC2 target at substrate temperatures of 700 – 900 °C. In the latter case, the film composition exhibits excess C compared to the nominal target composition due to differences between species in angular and energy distribution and gas-phase scattering processes. Ti buffering is shown to compensate for this excess C. The electrical-resistivity values of Ti3SiC2 and Ti4SiC3 thin films were measured to 21-32 uOhmcm and ~50 uOhmcm, respectively. The good conductivity is because the presence of Si layers enhances the relative strength of the metallic Ti-Ti bonds. The higher density of Si layers in Ti3SiC2 than in Ti4SiC3 explains why Ti3SiC2 is the better conductor of the two. Ti3SiC2 thin films are shown to be thermally stable up to 1000 – 1100 °C. Annealing at higher temperature results in decomposition of Ti3SiC2 by Si out-diffusion to the surface with subsequent evaporation. Above 1200 °C, TiCx layers recrystallized. Nanocomposites comprising nanocrystalline (nc-)TiC in an amorphous (a-)SiC matrix phase were deposited at substrate temperatures in the range 100 – 300 °C. These nc-TiC/a-SiC films exhibit low contact resistance in electrical contacts and a ductile deformation behavior due to rotation and gliding of nc-TiC grains in the matrix. The ductile mechanical properties of nc-TiC/a-SiC are actually more similar to those of Ti3SiC2, which is very ductile due to kinking and delamination, than to those of the brittle TiC. Epitaxial TiC/SiC multilayers deposited at ~550 °C were shown to contain cubic SiC layers up to a thickness of ~2 nm. Thicker SiC layers gives a-SiC due to the corresponding increase in interfacial strain energy leading to loss of coherent-layer growth. Nanoindentation of epitaxial Ti3SiC2/TiC0.67 nanolaminates showed inhibition of kink-band formation in Ti3SiC2, as the lamination with the less ductile TiC effectively hindered this mechanism.

Microstructure and nonbasal-plane growth of epitaxial Ti2AlN thin films

Thin films of the Mn+1AXn (MAX) phase (M: early transition metal, A:A-group element, X: C and/or N, n=1-3) Ti2AlN were epitaxially grown onto single-crystal MgO(111) and MgO(100) substrates by dc r ...

Nucleation and growth of Ti2AlN thin films deposited by reactive magnetron sputtering onto MgO(111)

The nucleation and growth of Ti2AlN thin films on MgO(111) substrates during dual direct current reactive magnetron cosputtering from Ti and Al targets in an Ar∕N2 atmosphere at a substrate temperature of 690 °C have been investigated. Time and thickness dependent in situ specular x-ray reflectivity and x-ray diffraction in combination with cross-sectional transmission electron microscopy and Rutherford backscattering spectroscopy reveal the formation of competing phases for slight N superstoichiometry with respect to Ti2AlN. The stoichiometry deviations initiate the layer-by-layer growth of a ∼380 A thick epitaxial N-substoichiometric cubic (Ti1−xAlx)Ny layer. N-vacancy driven diffusion of Ti and Al leads to decomposition of this metastable solid solution into nanosized cubic TiNy′ and AlNy″ domains as well as to a solid-state reaction with the MgO(111) by formation of a Mg2(Al:Ti)O4 spinel, reducing the transformed (Ti1−xAlx)Ny layer thickness down to ∼60 A. Local AlNy″ domains serve as templates for Ti...

Topotaxial growth of Ti2AlN by solid state reaction in AlN∕Ti(0001) multilayer thin films

The formation of Ti2AlN by solid state reaction between layers of wurtzite-AlN and α-Ti was characterized by in situ x-ray scattering. The sequential deposition of these layers by dual magnetron sputtering onto Al2O3(0001) at 200°C yielded smooth, heteroepitaxial (0001) oriented films, with abrupt AlN∕Ti interfaces as shown by x-ray reflectivity and Rutherford backscattering spectroscopy. Annealing at 400°C led to AlN decomposition and diffusion of released Al and N into the Ti layers, with formation of Ti3AlN. Further annealing at 500°C resulted in a phase transformation into Ti2AlN(0001) after only 5min.

Growth and characterization of TiN/SiN(001) superlattice films

We report the layer structure and composition in recently discovered TiN/SiN(001) superlattices deposited by dual-reactive magnetron sputtering on MgO(001) substrates. High-resolution transmission ...

Deviations from Vegard’s rule in Al1−xInxN (0001) alloy thin films grown by magnetron sputter epitaxy

Al1−xInxN (0001) thin films of the pseudobinary AlN–InN system were grown epitaxially onto (111)-oriented MgO wafers with seed layers of Ti1−yZryN by dual direct current magnetron sputtering under ultrahigh vacuum conditions. The relaxed film c-axis lattice parameters determined by x-ray diffraction were studied as a function of composition in the range of 0.07<x<0.82 measured by Rutherford backscattering spectrometry. We find a relative deviation by as much as 37% from the linear dependency described by Vegard’s rule for the lattice parameter versus film composition. The highest relative deviations were found at low InN mole fractions, while the largest absolute deviation was found at x=0.63. This shows that Vegard’s rule is not directly applicable to determine the compositions in the wurtzite Al1−xInxN system.

The influence of the growth rate on the preferred orientation of magnetron-sputtered Ti–Al–N thin films studied by in situ x-ray diffraction

In situ x-ray diffraction has been used to characterize the growth and microstructure of wear protective Ti1−xAlxN thin films. The films were deposited onto oxidized Si(100) wafers in a sputter chamber mounted onto a six-circle goniometer located at a synchrotron-radiation beam line. Off-plane and in-plane x-ray diffraction data were recorded in situ during growth, in order to follow the development of microstructure and preferred orientation as a function of film thickness. The measurements were supplemented by ex situ cross-sectional transmission electron microscopy analyses. The films were deposited by reactive cosputtering from metallic Ti and Al targets in Ar∕N2 gas mixtures at substrate temperatures of 150 and 300°C, substrate bias voltages of −30 and +10V, and deposition rates between 0.9 and 0.3A∕s. The film composition was changed between pure TiN and Ti0.91Al0.09N. Films deposited at higher deposition rates show columnar structure with competitive growth between (001) and (111) crystalline orien...

In situ x-ray diffraction studies concerning the influence of al concentration on the texture development during sputter deposition of Ti-Al-N thin films

In situ x-ray diffraction was employed during the growth of thin Ti1−xAlxN films, using a deposition chamber installed at a synchrotron radiation beamline. The films were deposited by reactive cosputtering from Ti and Al targets. In a previous experiment, the substrate temperature, bias voltage, and nitrogen partial pressure, and thus growth rate, were varied at constant x≈0.07. High deposition rates of ∼1A∕s lead to the typical crossover behavior between initial (001) and final (111) off-plane preferred orientation. Reducing the deposition rate to <0.5A∕s leads to a reversed behavior with a clear (001) preferred orientation above a film thickness of 600 A, which is essentially independent of the substrate temperature. For the results presented here, the studies were extended to a systemical variation of x from 0 to 0.73 while keeping all the other parameters constant. For a Al concentration up to x∼0.15 the (001) preferred orientation is persistent. On the other hand, at low deposition rates, a (111) pre...