The GaN platelets were produced by epitaxial growth and reformation of GaN nanowires, effectively blocking the propagation of threading dislocations (TDs) from the underlying substrate. These structures have an excellent homogeneity over an entire array. We demonstrate the ability to completely avoid structural defects including dislocations in AlxGa1-xN layers with x up to 0.90 and thicknesses over 100 nm, grown directly on submicron-sized, flat-topped GaN platelets. The main sources of the dislocations are growth on non-native substrates and heteroepitaxial growth of lattice-mismatched layers. To improve the performance and efficiency of Al containing III-Nitride-based devices, a number of issues must be addressed, especially the presence and generation of dislocations and other structural defects. To further reduce dislocation density and improve crystal quality, proposed ways of controlling the introduction of dislocations (substrate patterning, buffer layer and compliant substrate methods) are highlighted.

The dislocation density has been reduced to 6×10⁵ cm⁻², based on the use of a micrometric laser-pierced hole array, a method similar to patterned nucleation growth. Next, methods to reduce dislocation density are summarized, including increasing dislocation reactions (increasing the diamond film thickness and off-axis substrate growth), removing dislocations (conventional epitaxial lateral growth, pendeoepitaxial lateral growth and patterned nucleation growth), and other methods (three-dimensional growth, metal-assisted termination and using a pyramidal substrate). We first discuss several aspects of dislocations in heteroepitaxial diamond nucleation and growth, including their generation, types and characterization. This especially holds for diamond growth on iridium substrates with a large lattice constant difference of 7.1%. Minimizing the dislocation density is a significant challenge for using diamond in electronics. In the sputtered AlN thickness range of 10–25 nm, the LOP of UV LED with 15-nm-thick sputtered AlN NL was the highest, revealing that optimum thickness of the sputtered AlN NL is around 15 nm.ĭislocations are considered crucial linear defects in the synthesis of heteroepitaxial single crystal diamond. Using a sputtered AlN NL instead of the AlGaN NL, the LOP of UV LED was further enhanced by 11.3%, which is attributed to reduced TD density in InGaN/AlInGaN active region. The light output power (LOP) of UV LED with AlGaN NL was 18.2% higher than that of UV LED with GaN NL owing to a decrease in the absorption of 375 nm UV light in the AlGaN NL with a larger bandgap. The threading dislocation (TD) densities in GaN-based UV LEDs with GaN/AlGaN/sputtered AlN NLs were determined by high-resolution X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM), which revealed that the TD density in UV LED with AlGaN NL was the highest, whereas that in UV LED with sputtered AlN NL was the lowest. We report on the demonstration of GaN-based ultraviolet light-emitting diodes (UV LEDs) emitting at 375 nm grown on patterned sapphire substrate (PSS) with in-situ low temperature GaN/AlGaN nucleation layers (NLs) and ex-situ sputtered AlN NL. The synchrotron diffraction and the micro-Raman spectroscopy revealed the depth profiles of dislocation density and lattice parameters. The transmission electron microscopy provided information about the kind of microstructure defects and their mutual interaction.

In order to be able to visualize and to quantify the microstructure changes in individual parts of the sandwich-like structure, the samples were investigated using nanofocused synchrotron diffraction, confocal micro-Raman spectroscopy, and transmission electron microscopy. The uppermost GaN layer mainly contains bunched threading dislocations terminating large areas of almost defect-free GaN. The next GaN layer, which is internally structured and defect-rich, bends and bunches these dislocations and facilitates their annihilation. Thus, it contains a high number of randomly distributed threading dislocations. The first GaN layer accommodates the lattice misfit between sapphire and gallium nitride.

The GaN crystals under study were deposited in three steps with different process temperatures, growth rates, and ammonia flows. In this study, this general approach is applied to reduce the density of threading dislocations in GaN crystals grown using high-temperature vapor phase epitaxy directly on (0001)-oriented sapphire substrates. The interaction of microstructure defects is regarded as a possible tool for the reduction of the defect density and improvement of the crystal quality.

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