X-ray characterization of composition and relaxation of AlxGa1−xN(≤x≤1) layers grown on GaN/sapphire templates by low pressure organometallic vapor phase epitaxy

Abstract

A characterization procedure was developed to determine the alloy composition and strain state of AlxGa1−xN/GaN(0<x<1) heterostructures deposited on c-plane sapphire substrates by low pressure organometalic vapor phase epitaxy. Motivated by a method suggested by Bowen and Tanner for separating the contributions of strain and composition in cubic crystals, we extended the technique to the case of hexagonal crystals by first principles derivation from elastic strain theory. The technique was evaluated using double-axis and triple-axis reciprocal space maps of 200 and 30 nm AlxGa1−xN layers. The procedure did not require absolute lattice parameter measurements and relied instead on relative measurements of the layer and substrate peak positions. Symmetric and asymmetric reflections of the film and substrate were measured in the double-axis configuration with ω–2θ scans. From the peak separation, the strained lattice parameters were determined. Assuming biaxial strain and linear variation of the relaxed lattice parameter with alloy composition, an equation was derived for composition in terms of the strained lattice parameters. The relaxation distribution in the AlGaN was found to be bimodal. Films with mole fraction of Al greater than 0.4 were completely relaxed and thoroughly cracked, while films with mole fraction of Al less than 0.4 exhibited compositionally dependent relaxation about the fully strained state.

The role of surface kinetics on composition and quality of AlGaN

“…Similar trends were observed in AlN homoepitaxial films. 11 Figure 4 includes films misoriented toward [1-100], [11][12][13][14][15][16][17][18][19][20], and variations in between, which shows that the general trends for step height and terrace width were not affected by the misorientation azimuth.…”

“…The Al content of the AlGaN films was determined using reciprocal space mapping by high resolution x-ray diffraction (HRXRD) and a technique developed by Tweedie et al that allows for separation of strain and composition effects in XRD. 17 The HRXRD measurements were conducted using a Philips X'Pert Materials Research Diffractometer with a Cu K α x-ray source. Conventional transmission electron microscopy (TEM) was implemented to study extended defects in these films using a JEOL 2000FX operating at 200 kV.…”

Progress on n‐type doping of AlGaN alloys on AlN single crystal substrates for UV optoelectronic applications

“…No cracking or buckling was observed on the samples after being removed from the reactor. The Al content in the AlGaN films was determined by the x-ray diffraction (XRD) method described by Tweedie et al [11]. The (0002) XRD ω-rocking curves were indistinguishable from those of the substrate for the films grown on AlN wafers.…”

Measuring the depth profiles of strain/composition in AlGaN-graded layer by high-resolution x-ray diffraction

“…The chemical composition and strain are extracted from accurate measurements of the in-plane and out-of-plane lattice parameters. Several techniques [14][15][16] have been proposed to measure the composition and strain in non-graded Al x Ga 1Àx N layers. In this study, we extend the XRD method to the case of graded Al x Ga 1Àx N layers.…”