X-ray scattering from GaN epitaxial layers - an example of highly anisotropic coherence

Abstract

GaN(0001) epitaxial layers were grown by molecular beam epitaxy on a few-nanometres thick low-temperature GaN nucleation layers on c-plane sapphire. Despite extremely high densities of extended defects, the layers show a narrow (002) x-ray diffraction peak, superimposed by broad diffuse scattering. Triple-axis transverse and radial scans were measured for (00l) reflections of different orders and for various GaN layer thicknesses. The results can be described by an interfacial displacement-difference correlation function. Its microscopic origin is assigned to either inversion domain boundaries or edge-type threading dislocations in the GaN layers, in agreement with findings of transmission electron microscopy. These defects are associated with an only weak rotational disorder perpendicular to the growth plane as proven by the x-ray scattering characteristics.

Growth of high crystal quality InN by ENABLE‐MBE

“…4. Both curves show a specific shape with a strong and narrow coherent central peak and a broad diffuse peak [14][15][16]. The full width at half maximum (FWHM) of the central peak (7.2-8.4 arcsec) reveals extremely low ω tilt between adjacent crystal domains.…”

“…1b). The [11][12][13][14][15][16][17][18][19][20] micrograph ( Fig. 1b) also demonstrates the columnar nature of AlN and InN films.…”

“…These structural features decidedly reveal almost perfectly textured epitaxial layers with negligibly tilted small (in lateral directions) crystal domains. X-ray diffraction on such layers has to lead to extended strong interference pattern on ω-2θ RCs and a very narrow central coherent peak on ω RCs, typical for single-crystal epitaxial layers with high spatial coherence [7,[11][12][13][14][15][16]. Such coherent scattering is possible, because in contrast to 60° dislocation loops and their dislocation trenches in cubic compounds, vertically propagated edge and screw dislocations in hexagon layers do not deteriorate spatial coherence of epitaxial layers, while diffused radiation, scattered by closed dislocation loops at the bottom of the layers (Fig.…”

Growth of high quality ZnO thin films with a homonucleation on sapphire

“…24,25 Based on our understanding, the second number correlates specifically with the density of closed dislocation loops at the bottom of the layer, while the FWHM of the narrow central coherent peak directly correlates with the lateral coherence length ($4 lm in this case), and not the density of dislocation loops. In case of GaN layer, it may reach 900-1000 nm, 30,31 in case of AlN $200 to 250 nm. 26,27 Typical ZnO layer grown on sapphire substrate has open primary and secondary threading dislocation loops (most probably, screw type), created at the initial stages of epitaxial growth.…”

Crystalline perfection of GaN and AlN epitaxial layers and the main features of structural transformation of crystalline defects

“…In spite of a number of studies that address crystalline perfection and defect generation at initial stages of epitaxial growth [9] and in thicker layers [10,11], these phenomena are still not completely understood. In this paper, we report high-resolution X-ray diffraction (XRD) studies of the defect morphology in GaN substrates.…”