Strain-related structural and vibrational properties of thin epitaxialAlNlayers

Abstract

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.

Infrared dielectric functions and optical phonons of wurtzite YxAl1−xN (0 ⩽ x ⩽ 0.22)

“…Figure 6 shows the high-frequency limit dielectric constant ε ∞ obtained from the IRSE analysis as well as the static dielectric constant ε 0 obtained from the Lyddane-Sachs-Teller relation: (Table I), as function of yttrium content x. The ε ∞ value of 3.78, for the film with x=0, is in good agreement with the reported data for high-quality wurtzite AlN [25]. It is seen from Fig.…”

“…6) which is somehow lower than the previously reported values of 2.47 -2.53 from experiments and calculations [24,29]. A ZB value lower than 3 for AlN is consistent with the partly covalent character of the bonding in AlN [25].…”

“…According to the MREI model, for an alloy to exhibit two-mode behaviour, the following relationship must be fulfilled: AlN films with rocking curve widths as low as 12 arcsec [25] and bulk AlN [26]. It is seen from Fig.…”

Anisotropic strain and phonon deformation potentials in GaN

“…For our films it is necessary to apply a generalized ellipsometry approach since both GaN and sapphire are anisotropic materials and because of the non-c-axis orientation of the films and substrates. 30,[34][35][36] In the generalized ellipsometry situation Eq. ͑7͒ depends on the polarization state of the incident plane wave and the respective parameters ⌿ ij and ⌬ ij ͑i , j = p , s͒ are defined by the following ratios of the polarized light reflection coefficients:…”

Effect of Mg doping on the structural and free-charge carrier properties of InN films

“…It is most probably due to strain relaxation along the InN films' thicknesses and the presence of InN buffer layer with a larger c-lattice parameter. 59 As a result of lattice and thermal expansion coefficient mismatches, a compressive in-plan strain and tensile out-of-plane strain is generated in InN films on GaN/sapphire. Thus, larger c-lattice parameters would be expected in the biaxially strained InN films as suggested above.…”