V. Kirchner

X-ray diffraction analysis of the defect structure in epitaxial GaN

High-resolution x-ray diffraction has been used to analyze the type and density of threading dislocations in (001)-oriented GaN epitaxial layers. For this, (00l) and (hkl) Bragg reflections with h or k nonzero were studied, the latter one measured in skew symmetric diffraction geometry. The defect analysis was applied to a variety of GaN layers grown by molecular-beam epitaxy under very different conditions. The outcome is a fundamental correlation between the densities of edge- and screw-type dislocations.

Strain relaxation in AlGaN under tensile plane stress

Relaxation of tensile strain in AlxGa1−xN layers of different compositions epitaxially grown on GaN/sapphire is investigated. Extended crack channels along 〈211¯0〉 directions are formed if the aluminum content exceeds a critical value, which decreases with increasing layer thickness. This process is found to limit the average strain energy density to a maximum value of 4 J/m2. By calculating the stress distribution between cracks and the strain energy release rate for crack propagation, the relaxed strain as measured by x-ray diffraction is correlated to the crack density, and the onsets of crack channeling and layer decohesion are fitted to a fracture toughness of 9 J/m2. Moreover, the crack opening at the surface is found to linearly increase with the stress. Annealing of samples above the growth temperature introduces additional tensile stress due to the mismatch in thermal expansion coefficients between the layer and substrate. This stress is shown to relieve not only by the formation of additional cr...

INCORPORATION OF INDIUM DURING MOLECULAR BEAM EPITAXY OF INGAN

We report on the incorporation of In during growth of InxGa1−xN by molecular beam epitaxy under varying In/Ga flux ratios and with different film thicknesses. The incorporation efficiency studied by energy dispersive x-ray microanalysis, high-resolution x-ray diffraction and photoluminescence spectroscopy is strongly affected by the chosen fluxes of Ga and N and is limited by the excess of nitrogen compared to gallium. Furthermore, thick films exhibit a decrease of the In content in growth direction. The behavior can be explained by considering the different stabilities of the two binary compounds InN and GaN.

Strain relaxation in AlGaN/GaN superlattices grown on GaN

Lattice relaxation of strained AlxGa1−xN/GaN superlattices grown on thick GaN buffer layers is investigated using optical microscopy, x-ray diffraction, and photoluminescence spectroscopy. The results are compared to strained bulk AlxGa1−xN layers particularly with regard to the impact of the superlattice period and the Al content. A relaxation process which keeps the coherency between AlxGa1−xN barriers and GaN wells in the superlattice is found and it is attributed to misfit dislocations at the buffer/superlattice interface. Additionally, the AlxGa1−xN barriers relax via crack channels which form beyond a critical Al content and limit the additional strain energy compared to a free-standing superlattice to a maximum value. Cracks relieve tensile plane stress to an extent similar as in bulk layers, i.e., they do not put the GaN wells of the superlattice under additional plane compression. This is explained by misfit dislocations which nucleate at crack faces and glide into the superlattice at the well/ba...

Analysis of the Defect Structure of Epitaxial GaN

The type and density of threading dislocations in GaN epitaxial layers grown on c-plane sapphire have been analyzed by using nondestructive high resolution X-ray diffraction. The highly distorted GaN layers were described as mosaic crystals characterized by a mean tilt and twist angle between the mosaic blocks which are correlated with the densities of screw and edge type threading dislocations, respectively. Triple axis rocking curves of (00l) reflections for varying l-indices were used to determine the tilt angle, while the twist was extrapolated from ω-scans for (hkl) Bragg reflections with h or k nonzero, measured in skew symmetric diffraction geometry. This defect analysis was applied to selected GaN layers grown by molecular beam epitaxy (MBE) and metalorganic chemical vapour deposition (MOCVD) revealing clear differences between both sample types.

Thermal expansion of bulk and homoepitaxial GaN

The thermal behavior of Mg-doped and intentionally undoped bulk crystals and homoepitaxial GaN was investigated in a wide temperature range from 12 to 600 K. With high-resolution x-ray diffraction, both lattice parameters a and c were determined and the thermal expansion coefficients were calculated. Within the experimental accuracy, mean values were extracted for the temperature ranges 12–100, 100–250, and 250–600 K. These values are essential, especially, for the interpretation of measurements of other GaN properties performed at low temperatures.

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

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.

Influence of buffer layers on the structural properties of molecular beam epitaxy grown GaN layers

Abstract The influence of low temperature buffer layers on the structural characteristics of GaN grown by molecular beam epitaxy on sapphire (0001) substrates was investigated. Layers grown on GaN and AlN buffers were studied by high-resolution X-ray diffraction and transmission electron microscopy (TEM). For both buffer materials, the variation of the buffer parameters, like their thickness and growth temperature, is reflected in a clear change of the GaN (0002) rocking curve width. For strongly decreased as well as for increased Bragg reflection width a deterioration of optical and electrical properties of GaN layers grown on buffers with respect to reference samples without buffer layers was observed. Moreover, layers grown on thin GaN buffer layers show extremely narrow ω scans and layer thickness interferences in 2θ/ω direction, while TEM reveals a high defect density throughout the entire layer. Therefore, not only the width of the rocking curves but also their shape has to be considered for the estimation of the defect densities by X-ray diffraction.

Thermally induced strain in MBE grown GaN layers

A study of thermally induced strain in GaN layers grown on c-plane sapphire by molecular beam epitaxy is presented. For this, high-resolution X-ray diffraction at variable temperature (10-630 K) has been used, which is the most direct method to investigate this strain. A linear change of the lattice constant was observed for a 3 μm thick GaN layer for temperatures above 300 K which is characterized by a thermal expansion coefficient of 3.8 x 10 -6 K -1 . In contrast to this, the thermal expansion was found to be negligible for temperatures below 100 K.