AlN thin films: stress, structural properties and optical phonons

Abstract

Thin aluminum nitride (AlN) films of different thickness are deposited by DC-pulsed magnetron sputtering under identical conditions on sapphire (0001) and silicon (100) substrates, and sputtered polycrystalline chromium layers. An investigation of the residual stress, morphology and structural properties is carried out. The thickness of the films covers the range from 17 nm to ≈3900 nm. A higher compressive residual stress is measured for the thinner films and the presence of a stress gradient is proven. X-ray diffraction (XRD) studies show that all AlN films are achieved with perfect c-axis orientation perpendicular to the film surface and that the films are biaxially strained. XRD rocking curves reveal that AlN films on sapphire are highly oriented for all film thicknesses, whereas AlN film growth on silicon and Cr layers starts highly disoriented while the film quality improves with film thickness. Surface analysis by atomic force microscopy (AFM) shows a continuous film roughening and decrease of grain boundary density with increasing film thickness. The study of the vibrational properties of AlN thin films is performed. The frequencies and lifetimes of the E1(TO) and A1(LO) optical phonons are calculated from Fourier transform infrared spectra using the factorized model of a damped oscillator. Additionally, using the Raman technique the E2-2 phonon mode is investigated. We analyze the structural properties by the XRD technique to correlate the elongation of phonon lifetimes with increasing film thickness. The lifetimes of the phonon modes in AlN thin films are compared to the values in a single crystal. Finally, the route towards the applications of AlN is analyzed. AlN is deposited on various metals and the film quality is investigated by the XRD and Raman techniques. The combination with nanocrystalline diamond (NCD) is elaborated. The surfaces of AlN, and Si as comparison, are exposed to different gas discharge plasmas (Ar, CF4, H2, N2, O2) and the colloidal seeding procedure of the surfaces with nanodiamond (ND) particles is followed. AFM reveals different seeding densities after different plasma pretreatment. It is shown that the Van der Waals (VdW) forces plays a role in ND adhesion to the surface. The surface roughening changes the nucleation density proportionally to the VdW force that varies with roughness. Surface hydrogenation and fluorination enhances the seeding density on Si and AlN surfaces. In the latter case the seeding density improved by almost four orders of magnitude compared to an untreated surface. This allowed to grow 60 nm thick pinhole-free NCD films on AlN.