Doping nitride semiconductors such as gallium nitride (GaN) and aluminum nitride (AlN) is a critical process for enabling their use in electronic and optoelectronic applications. However, achieving controlled and efficient n-type and p-type doping in these materials presents significant challenges due to their wide bandgaps, high bond strengths, and intrinsic defects. The primary dopants for n-type conductivity are silicon (Si) and germanium (Ge), while magnesium (Mg) is the most common p-type dopant. Iron (Fe) is sometimes used as a compensating dopant to control unintentional conductivity. Each dopant introduces unique activation mechanisms, compensation effects, and deep-level traps that influence the electrical properties of the material.

N-type doping in GaN is relatively well-established compared to p-type doping. Silicon is the most widely used n-type dopant due to its low activation energy and high solubility in GaN. When Si substitutes for gallium (Ga) in the lattice, it donates an extra electron to the conduction band, increasing electron concentration. The activation energy of Si in GaN is approximately 12-17 meV, making it highly efficient. Germanium has also been explored as an alternative, with similar activation behavior but slightly higher ionization energy. However, n-type doping in AlN is more challenging due to its even wider bandgap (6.2 eV compared to 3.4 eV for GaN). The activation energy for Si in AlN is significantly higher, around 50-70 meV, leading to lower ionization efficiency at room temperature. This necessitates higher doping concentrations or elevated temperatures to achieve sufficient carrier densities.

P-type doping in GaN and AlN is considerably more difficult than n-type doping. Magnesium is the primary p-type dopant for GaN, but its activation is hindered by several factors. When Mg substitutes for Ga, it creates an acceptor level approximately 170-200 meV above the valence band, meaning only a small fraction of Mg atoms are ionized at room temperature. To activate Mg acceptors, post-growth annealing in nitrogen or oxygen ambient is required to passivate hydrogen complexes that form during growth. Even after activation, the hole concentration is typically limited to the mid-10^17 cm^-3 range due to compensation effects. In AlN, p-type doping with Mg is even less efficient because the acceptor level is deeper, estimated at 500-630 meV. This results in extremely low hole concentrations, often below 10^15 cm^-3, making p-type AlN highly resistive.

Compensation effects are a major challenge in both n-type and p-type doping of nitride semiconductors. Unintentional impurities such as oxygen, carbon, and hydrogen can act as donors or acceptors, counteracting the intended doping. For example, oxygen is a common unintentional donor in GaN and AlN, compensating Mg acceptors in p-type material. Carbon, on the other hand, can act as a deep acceptor in GaN, reducing electron concentration in n-type layers. Hydrogen is particularly problematic for p-type doping because it forms neutral complexes with Mg, passivating the acceptors until annealed out. The presence of native defects like nitrogen vacancies (V_N) and gallium vacancies (V_Ga) further complicates the doping process by introducing additional energy levels that trap carriers.

Deep-level traps associated with dopants and defects significantly impact the electrical properties of nitride semiconductors. In n-type GaN, Si doping can introduce shallow donor levels, but residual defects such as V_Ga or carbon-related centers create deep traps that reduce electron mobility. In p-type GaN, Mg-related defects can form deep acceptor levels that trap holes, lowering conductivity. Iron is sometimes intentionally introduced to create deep-level traps that compensate background n-type conductivity in semi-insulating GaN. Fe introduces a deep donor level around 0.5-0.6 eV below the conduction band, effectively trapping electrons and increasing resistivity. However, Fe doping must be carefully controlled to avoid excessive carrier scattering or unintended recombination centers.

The growth method plays a crucial role in determining doping efficiency and uniformity. Metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are the most common techniques for growing doped GaN and AlN layers. MOCVD offers higher growth rates and better scalability but may introduce more carbon and hydrogen impurities. MBE provides cleaner growth conditions and more precise doping control but is less suitable for mass production. The choice of precursor gases and growth parameters such as temperature, pressure, and V/III ratio also affects dopant incorporation and activation. For example, higher growth temperatures can enhance Mg incorporation in GaN but may also increase defect formation.

Doping challenges are further exacerbated in ternary and quaternary nitride alloys such as AlGaN and InAlGaN. The varying compositions introduce additional lattice strain and bandgap fluctuations that influence dopant behavior. In AlGaN, for instance, the activation energy of Mg increases with aluminum content, making p-type doping progressively harder as Al concentration rises. Similarly, n-type doping efficiency decreases due to the higher ionization energy of Si in Al-rich alloys. Strain-induced polarization fields in these materials can also spatially separate electrons and holes, reducing recombination efficiency in optoelectronic devices.

Despite these challenges, advances in doping techniques continue to improve the performance of nitride semiconductors. Delta doping, where dopants are confined to ultrathin layers, has been explored to enhance carrier concentrations while minimizing defect formation. Co-doping strategies, such as using Mg and Si together in specific ratios, have shown promise in reducing compensation effects. Additionally, alternative p-type dopants like beryllium (Be) and zinc (Zn) have been investigated, though they come with their own limitations in terms of solubility and activation energy.

In summary, doping GaN and AlN remains a complex but essential aspect of their development for practical applications. N-type doping with Si is relatively efficient in GaN but becomes less effective in AlN due to higher activation energies. P-type doping with Mg faces significant hurdles in both materials, particularly in AlN, where deep acceptor levels severely limit hole concentrations. Compensation effects from unintentional impurities and native defects further complicate the doping process, while deep-level traps degrade carrier mobility and lifetime. Continued research into growth techniques, dopant activation methods, and defect engineering is necessary to overcome these challenges and unlock the full potential of nitride semiconductors.