We report on the presence of optically active stacking faults on basal and prismatic planes in epitaxially laterally overgrown GaN (ELOG) on {112¯2} facets. The structure of the faults has been analyzed using diffraction contrast electron microscopy. We show that stacking faults on {112¯0} prismatic planes involve a lattice displacement of 12⟨11¯01⟩, parallel to the fault plane. They appear as jogs connecting basal-plane stacking faults, the latter with a lattice displacement of 16⟨202¯3⟩. These faults are observed only in the laterally overgrown regions that grow on {112¯2} planes, which indicates that the stacking fault formation is closely related to the orientation of the growth surface. Possible formation mechanisms of these faults are discussed. Direct correlation between TEM and cathodoluminescence shows that these prismatic-plane and basal-plane stacking faults are optically active with light emission at 3.30 and 3.41eV, respectively.

The authors report on pulsed lateral epitaxial overgrowth of aluminum nitride films on basal plane sapphire substrates. This approach, at temperatures in excess of 1150°C, enhanced the adatom migration, thereby significantly increasing the lateral growth rates. This enabled a full coalescence in wing regions as wide as 4–10μm. Atomic force microscopy and cross-section transmission electron microscopy were used to establish the reduction of threading dislocations in the lateral growth. Cross-sectional monochromatic cathodoluminescence and photoluminescence measurements confirmed the improved optical properties of the laterally overgrown aluminum nitride films.

We have found that, in the absence of threading dislocations in In x Ga 1Àx N/GaN heterostructures, coherent generation of misfit dislocations occurs for x > 0:11 in $100-nm-thick epilayers. We focus this report on In 0:17 Ga 0:83 N grown on a lowdefect-density GaN free-standing substrate (with 1.9% lattice mismatch). A diffraction contrast analysis carried out in the transmission electron microscope showed straight line defects with Burgers vectors 2/3 h11 " 2 20i (i.e., 2a, where a is the hexagonal plane lattice parameter), which extended many micrometers approximately along h1 " 1 100i directions and with an average lateral spacing of 90 nm. Although these defects were complex and mostly sessile, evidence was found that they can dissociate into glissile misfit dislocations with Burgers vectors of 1/3 h11 " 2 20i. It is proposed that the defects are generated by a punch-out mechanism involving slip on inclined prismatic planes. The properties of these defects and their role in relieving misfit strains are discussed.

Articles you may be interested inTrace analysis of non-basal plane misfit stress relaxation in ( 20 2 ¯ 1 ) and ( 30 3 ¯ 1 ¯ ) semipolar InGaN/GaN heterostructures Appl.Evidence of lattice tilt and slip in m-plane InGaN/GaN heterostructure Appl.The authors have observed that for In x Ga 1−x N epitaxial layers grown on bulk GaN substrates exhibit slip on the basal plane, when in the presence of free surfaces that intercept the heterointerface and for indium compositions x ജ 0.07. This leads to almost complete relaxation of the local misfit strain by generation of radial-shape dislocation half loops. For x ജ 0.17, generation of straight misfit dislocations by glide on the secondary ͗1123͘ ͕1122͖ slip system is observed, in addition to the radial-shape half loops at surface pits. These two mechanisms act independently with no observed interaction between them, leading to the conclusion that slip on the basal plane occurs first during the growth process. The secondary slip system is activated later and involves a significantly higher critical stress energy.

Articles you may be interested inIn situ X-ray investigation of changing barrier growth temperatures on InGaN single quantum wells in metalorganic vapor phase epitaxy

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A direct correlation has been established between the spatial variation of spectral luminescence and the distribution of threading dislocations in a green-light-emitting InGaN quantum well structure grown on a sapphire substrate. Transmission electron microscopy and monochromatic cathodoluminescence images, taken from the same region, indicate that the nature of the quantum well emission is influenced by the microstructure of the underlying GaN. The microstructure is defined by threading dislocations that reflect a columnar structure with low-angle grain boundaries. A strong correlation is observed between this microstructure and the peak and low-energy portion of the quantum well luminescence, with threading dislocations as boundaries between bright and darker regions. The high-energy portion of the luminescence is localized and is generally complementary to the rest of the spectrum.

The joining of defect-free AlN stripes is observed to trigger the generation of a large density of threading dislocations in the vicinity of the coalescence point. The AlN structure was grown by pulsed lateral epitaxy on shallow-grooved sapphire substrates. In the precoalescence stage, the dislocation density in the lateral epitaxial region (<108cm−2) is over two orders of magnitude less than in standard c-plane epitaxy. Basal-plane dislocations (b=a=13<112¯0>) are generated at the coalescence point as a result of relaxation of compressive stress that develops due to temperature gradients during growth. They bend toward the surface during the postcoalescence growth stage, leading to a high density of pure-edge threading dislocations in the lateral growth regions. Some threading dislocations form loops on prismatic planes in the crystal and the basal segments are observed to glide in the [0001] direction under the electron beam in the microscope.