Thermal expansion of rock-salt cubic AlN

Acta Materialia

Metastable rock-salt (face centered cubic, c-) AlN can be grown in CrN/AlN multilayers when the AlN layer is thin enough. Exceeding a certain critical thickness, the thermodynamically stable wurtzite (w) structure grows. In this work, a bilayer-period-gradient (21 repeated blocks, each consisting of 10 bilayers with AlN layer-thicknesses ranging from 1.0 nm to 10.0 nm), ~2.0 mm-thick, reactively magnetron sputtered multilayer was characterized in detail with a spherical aberration-corrected transmission electron microscope (TEM). The studies are complemented by DFT (density functional theory) calculations. The high resolution TEM (HRTEM) studies reveal that the <111> growth-orientation is not as effective as the <110> and <100> growth-orientations in stabilizing the metastable c-AlN. The critical thickness for the c-AlN layers (before the thermodynamically stable w-AlN forms) is around ~2.0 nm for the <111> growth-orientation but reaches as high as 4.1 nm for both <110> and <100> growth-orientations. Contrary to the <111> orientation, in both <110> and <100> orientations several unusually highly mismatched c-CrN/w-AlN interface structures form as soon as w-AlN is present. DFT studies suggest that the larger critical thickness of the AlN layers in <100> and <110> orientation is allowed by the lower surface energy and higher cubic/wurtzite interfacial energy. The combination of HRTEM and DFT studies allows answering open questions on the impact of crystallographic orientations and interface structures, and also provides a better understanding on the growth mechanisms of c-AlN, necessary for the outstanding mechanical properties of AlN-containing multilayers.

Journal of Applied Physics

AlN thin films are enabling significant progress in modern optoelectronics, power electronics, and microelectromechanical systems. The various AlN growth methods and conditions lead to different film microstructures. In this report, phonon scattering mechanisms that impact the cross-plane (κz; along the c-axis) and in-plane (κr; parallel to the c-plane) thermal conductivities of AlN thin films prepared by various synthesis techniques are investigated. In contrast to bulk single crystal AlN with an isotropic thermal conductivity of ∼330 W/m K, a strong anisotropy in the thermal conductivity is observed in the thin films. The κz shows a strong film thickness dependence due to phonon-boundary scattering. Electron microscopy reveals the presence of grain boundaries and dislocations that limit the κr. For instance, oriented films prepared by reactive sputtering possess lateral crystalline grain sizes ranging from 20 to 40 nm that significantly lower the κr to ∼30 W/m K. Simulation result...

Journal of Physics D: Applied Physics, 2013

We combine the finite element (ABAQUS) and ab initio methods to study and predict the equilibrium critical thickness up to which the metastable cubic (c) AlN phase is energetically favoured to the stable wurtzite (w) variant in TiN/AlN and CrN/AlN bi-layer systems. The results show that the w-AlN phase is preferred for all thicknesses in the free-standing configuration (without a substrate) when grown on TiN, while 4 nm thick c-AlN is predicted for CrN bi-layer material systems. The substrate helps to stabilize c-AlN up to 15.8 nm, and for an incoherent interface between the substrate and the TiN or CrN interlayer, the stabilization mechanically supports the interlayer against relaxation. For a coherent interface to the substrate, a small lattice constant (as, e.g., in the case of Al substrate) helps to stabilize c-AlN, whereas a large lattice constant (as, e.g., in the case of MgO) promotes w-AlN. Finally, we predict that 1 1 1 orientated specimens allow for thicker c-AlN layers than those grown along the 1 0 0 direction.

Physical Review B, 2004

The effect of film thickness on the strain and structural properties of thin epitaxial AlN films has been investigated by high resolution x-ray diffraction techniques and transmission electron microscopy. As a result a sublayer model of the degree of strain and related defects for all films is proposed. A sublayer with low defect density and a strain gradient is found to be present in all films and it reaches a maximum thickness of 65 nm. The films are compressively strained and the strain relaxation after a thickness of 65 nm is shown to be accompanied by misfit dislocation generation and increase of the mosaic tilt. The vibrational properties of the films have been studied by generalized infrared spectroscopic ellipsometry. The proposed sublayer model has been successfully applied to the analysis of the ellipsometry data through model calculations of the infrared dielectric function which confirm the sublayer model. It is found that the strain gradient results in a gradient of the phonon mode frequencies and broadening parameter. The initial strain relaxation in the films leads to narrowing of the observable infrared modes, while further strain relaxation broadens the modes when substantial defect generation occurs.

We report the reduction in residual stress of AlN thin films and also the crystal structure, surface morphology and nanomechanical properties of magnetron sputtered as a function of substrate temperature (T s , 35-600 • C). The residual stress of these films was calculated by sin 2 ψ technique and found that they are varying from tensile to compression with temperature (T s). Evolution of crystalline growth of AlN films was studied by GIXRD and transmission electron microscopy (TEM) and a preferred a-axis orientation was observed at 400 • C. The cross-sectional TEM micrograph and selected area electron diffraction (SAED) of this film exhibited a high degree of orientation as well as a columnar structure. Hardness (H) measured by Nanoindentation technique on these films ranged between 12.8-19 GPa.

Materials Chemistry and Physics

We report the reduction in residual stress of AlN thin films and also the crystal structure, surface morphology and nanomechanical properties of magnetron sputtered as a function of substrate temperature (T s , 35-600 • C). The residual stress of these films was calculated by sin 2 ψ technique and found that they are varying from tensile to compression with temperature (T s). Evolution of crystalline growth of AlN films was studied by GIXRD and transmission electron microscopy (TEM) and a preferred a-axis orientation was observed at 400 • C. The cross-sectional TEM micrograph and selected area electron diffraction (SAED) of this film exhibited a high degree of orientation as well as a columnar structure. Hardness (H) measured by Nanoindentation technique on these films ranged between 12.8-19 GPa.

Applied Physics Letters, 2006

Journal of Crystal Growth, 2010

Atomically smooth cubic AlN (c-AlN) layers were grown by plasma assisted molecular beam epitaxy (PAMBE) using freestanding 3C-SiC substrates. A model based on reflection high electron energy diffraction (RHEED) transients has been developed to lead the way to optimal growth conditions. Confirmation of the cubic structure of the AlN layers was gained by high resolution X-ray diffraction (HRXRD) measurements yielding a lattice parameter of 4.373Å. Finally atomic force microscopy (AFM) scans revealed an atomically smooth surface with a roughness of 0.2 nm RMS.

AlN thin film was prepared over different metal substrates using DC sputtering at various sputtering parameters. The XRD spectra revealed the presence of mixed (cubic and hexagonal) phases for all samples other than samples prepared at 300W with Ar:N 2 gas ratio of 14:6. The intensities of cubic phases observed at copper (Cu) substrates increased drastically with high sputtering power and N 2 gas flow. Low intensive peak was observed at gas mixer ratio of 14:6. N 2 flow and sputtering power influenced the crystallinity of the AlN thin film with respect to the substrates. Mixed residual stress (compressive and tensile) was observed for all samples and high values were observed at 300W sputtering power at low N 2 gas flow. Crystallite size of AlN thin film varied with respect to sputtering power, gas flow ratio and also substrates. AlN thin film prepared at 250 W showed high dislocation density at high N 2 gas flow ratio. Atomic force microscope results showed rough surface for AlN thin film coated over Al substrates and increased high value was observed at high N 2 gas flow ratio. The particle size of the AlN thin films increased with N 2 gas flow increased with respect to sputtering power and high value was observed with Al substrates.

APL Materials, 2018