Gallium nitride (GaN) is a critical wide bandgap semiconductor material with applications in high-power electronics, optoelectronics, and high-frequency devices. However, defects and dislocations in GaN significantly influence its performance. The primary defects include threading dislocations, point defects, and stacking faults, which arise during growth and affect electronic and optical properties. Understanding their origins, effects, and mitigation strategies is essential for optimizing GaN-based devices.

Threading dislocations are among the most prevalent defects in GaN, typically originating from lattice mismatch and thermal expansion coefficient differences between GaN and commonly used substrates like sapphire or silicon carbide. These dislocations propagate from the substrate-epilayer interface into the growing GaN layer, forming threading screw dislocations (TSDs), threading edge dislocations (TEDs), and mixed-type dislocations. The density of threading dislocations in GaN grown on sapphire can reach 1e8 to 1e10 cm-2, while growth on native GaN substrates reduces this to below 1e6 cm-2. Threading dislocations act as non-radiative recombination centers, reducing the internal quantum efficiency of light-emitting diodes (LEDs) and increasing leakage currents in high-electron-mobility transistors (HEMTs). They also serve as scattering centers, degrading carrier mobility.

Point defects in GaN include vacancies, interstitials, and antisite defects, with nitrogen vacancies (V_N) and gallium vacancies (V_Ga) being the most studied. Nitrogen vacancies are shallow donors, contributing to unintentional n-type conductivity in undoped GaN. Gallium vacancies, on the other hand, act as deep acceptors and recombination centers. Oxygen and silicon impurities substituting nitrogen sites (O_N, Si_N) further influence conductivity. Point defects also interact with dislocations, forming defect complexes that exacerbate non-radiative recombination. For instance, V_Ga clusters along threading dislocations can create deep-level traps, worsening device reliability.

Stacking faults are planar defects occurring due to errors in the stacking sequence of GaN’s wurtzite crystal structure. Basal plane stacking faults (BSFs) and prismatic stacking faults (PSFs) are common, with densities ranging from 1e3 to 1e5 cm-1 depending on growth conditions. Stacking faults introduce localized states within the bandgap, acting as charge trapping centers and reducing radiative efficiency. In high-power devices, they can initiate premature breakdown under high electric fields.

The origins of these defects are closely tied to growth techniques. Metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are the primary methods for GaN epitaxy, with MOCVD being dominant for commercial applications. The high growth temperatures (1000-1100°C) and ammonia-rich environments in MOCVD influence defect formation. For example, low V/III ratios increase gallium vacancy concentrations, while high growth rates promote threading dislocation propagation. Substrate choice also plays a critical role; sapphire substrates, despite their cost advantage, introduce high dislocation densities due to a 16% lattice mismatch with GaN. Silicon carbide substrates reduce this mismatch to 3.5%, but thermal expansion differences still cause strain-related defects.

Mitigation strategies focus on reducing defect densities and their detrimental effects. Epitaxial lateral overgrowth (ELO) is a widely used technique where a patterned dielectric mask is applied to the substrate before GaN growth. The mask blocks dislocation propagation from the substrate, forcing dislocations to bend and terminate at the mask edges. GaN grows laterally over the mask, producing regions with dislocation densities as low as 1e6 cm-2. Variants of ELO, such as pendeo-epitaxy, further enhance dislocation reduction by exploiting selective growth from sidewalls.

Patterned substrates are another effective approach, where nano- or micro-scale patterns on the substrate surface control strain distribution and dislocation propagation. Nano-patterned sapphire substrates (NPSS) and silicon substrates with engineered pits or grooves have demonstrated dislocation density reductions of up to 80%. The patterns act as dislocation filters, bending and annihilating dislocations before they reach the active device regions.

Buffer layer engineering is critical in defect reduction. Low-temperature GaN or aluminum nitride (AlN) buffer layers on sapphire substrates help accommodate lattice mismatch and reduce threading dislocation densities. Graded AlGaN buffer layers on silicon substrates manage thermal expansion mismatch, preventing cracking and dislocation generation.

Point defect control involves optimizing growth conditions and doping strategies. Increasing the V/III ratio during MOCVD reduces gallium vacancies, while post-growth annealing in nitrogen or ammonia atmospheres passivates nitrogen vacancies. Silicon and magnesium doping can compensate for native point defects, though excessive doping introduces new defects like dopant clusters.

Stacking fault suppression requires precise control of growth kinetics. Lowering growth rates and optimizing substrate off-cut angles minimize stacking sequence errors. In situ monitoring techniques, such as reflectance anisotropy spectroscopy, enable real-time adjustment of growth parameters to prevent fault formation.

The impact of defects on device performance is well-documented. In LEDs, threading dislocations and point defects reduce internal quantum efficiency by up to 50% in high-dislocation-density material. HEMTs suffer from increased leakage currents and reduced breakdown voltages due to dislocation-related trap states. Stacking faults in high-electron-mobility transistors cause current collapse, a reliability issue under high-voltage operation.

Despite these challenges, advances in defect mitigation have enabled GaN devices to achieve remarkable performance. Commercial GaN-based LEDs now exhibit external quantum efficiencies exceeding 80%, while HEMTs operate at frequencies above 100 GHz with power densities surpassing 10 W/mm. Continued research into novel substrate materials, such as bulk GaN and graphene-coated substrates, promises further defect reduction.

In summary, defects and dislocations in GaN materials are intrinsic to heteroepitaxial growth but can be managed through advanced techniques like ELO, patterned substrates, and optimized growth conditions. Their control is essential for unlocking the full potential of GaN in next-generation electronic and optoelectronic devices.