Aluminum nitride (AlN) is a critical wide bandgap semiconductor with a hexagonal wurtzite crystal structure, widely used in high-power electronics, optoelectronics, and high-frequency devices due to its excellent thermal conductivity, electrical insulation, and piezoelectric properties. Understanding its crystal structure and defect behavior is essential for optimizing performance in applications such as UV LEDs, high-electron-mobility transistors (HEMTs), and acoustic wave devices.

The wurtzite structure of AlN belongs to the P6₃mc space group, characterized by a hexagonal close-packed (hcp) arrangement of alternating aluminum and nitrogen sublattices. Each aluminum atom is tetrahedrally coordinated with four nitrogen atoms, and vice versa, forming layers stacked along the c-axis in an ABAB sequence. The lattice parameters are well-documented, with a ≈ 3.11 Å and c ≈ 4.98 Å at room temperature. The c/a ratio of ~1.6 is close to the ideal wurtzite value of 1.633, indicating minimal distortion. The stacking sequence involves a bilayer of Al and N atoms, where the nitrogen atoms in one layer sit above the tetrahedral voids of the aluminum layer below, creating a polar crystal with spontaneous polarization along the [0001] direction.

Intrinsic defects in AlN arise from deviations in stoichiometry or atomic arrangement during synthesis. Vacancies are among the most common, with nitrogen vacancies (Vₙ) and aluminum vacancies (V_{Al}) exhibiting different formation energies. Nitrogen vacancies are more prevalent due to the lower energy required for their formation, and they act as shallow donors, contributing to n-type conductivity. Aluminum vacancies, on the other hand, behave as deep acceptors but are less frequent due to higher formation energies. Interstitials, where atoms occupy non-lattice sites, also occur, with aluminum interstitials (Alᵢ) and nitrogen interstitials (Nᵢ) influencing carrier trapping and recombination. Antisite defects, where Al occupies N sites (Al_N) or vice versa (N_{Al}), are less common but can introduce deep levels in the bandgap, affecting carrier mobility and optical emission.

Extrinsic defects are introduced intentionally via doping or unintentionally through impurities. Silicon (Si) and oxygen (O) are common unintentional impurities in AlN. Silicon substitutes for aluminum (Si_{Al}), acting as a shallow donor and increasing n-type conductivity. Oxygen often replaces nitrogen (O_N), forming a deep donor state that can compensate p-type doping. Intentional doping with elements like magnesium (Mg) or carbon (C) is used to modify electrical properties. Magnesium substitutes for aluminum (Mg_{Al}), creating a deep acceptor level essential for p-type conductivity, though its activation efficiency is limited by high ionization energy. Carbon can occupy either aluminum or nitrogen sites (C_{Al} or C_N), influencing carrier lifetime and defect-assisted recombination.

Defects significantly alter AlN's electrical properties. Nitrogen vacancies and silicon impurities enhance n-type conductivity, while aluminum vacancies and magnesium doping introduce acceptor states, though achieving high hole concentrations remains challenging due to deep levels. Defects also act as scattering centers, reducing carrier mobility. Thermally, point defects and dislocations impede phonon transport, lowering thermal conductivity. Oxygen impurities, for instance, can reduce thermal conductivity by up to 20% at high concentrations. Optically, defects create mid-gap states that influence luminescence. Nitrogen vacancies are associated with yellow luminescence (~2.2 eV), while aluminum vacancies contribute to blue emission.

Characterization techniques are crucial for identifying and quantifying defects in AlN. X-ray diffraction (XRD) reveals lattice distortions and strain from defects through peak broadening and shifts in Bragg angles. High-resolution XRD can detect screw and edge dislocations via rocking curve analysis. Transmission electron microscopy (TEM) provides direct imaging of dislocations, stacking faults, and grain boundaries. High-angle annular dark-field (HAADF) TEM distinguishes between Al and N columns, aiding antisite defect identification. Secondary ion mass spectrometry (SIMS) profiles impurity concentrations, while photoluminescence (PL) spectroscopy maps defect-related emission bands.

In summary, AlN's wurtzite structure and defect landscape govern its functional properties. Intrinsic defects like vacancies and antisites, along with extrinsic dopants and impurities, dictate electrical conductivity, thermal transport, and optical behavior. Advanced characterization methods such as XRD and TEM are indispensable for defect analysis, enabling precise material optimization for next-generation semiconductor devices.