Aluminum nitride (AlN) is a wide bandgap semiconductor with a direct bandgap of approximately 6.2 eV, making it suitable for applications in deep-ultraviolet optoelectronics, high-power electronics, and high-temperature devices. However, achieving controlled n-type and p-type conductivity in AlN remains a significant challenge due to its intrinsic material properties, including high ionization energies of dopants and strong compensation effects. This article explores the doping mechanisms of AlN for both n-type and p-type conductivity, the associated challenges, and comparisons with other nitride semiconductors like gallium nitride (GaN).

N-type doping in AlN is typically achieved using silicon (Si) or germanium (Ge) as dopants. These elements substitute for aluminum (Al) in the lattice, acting as shallow donors. Silicon is the most commonly used n-type dopant due to its relatively low activation energy compared to other candidates. The ionization energy of Si in AlN is estimated to be around 50-70 meV, which is deeper than in GaN, where Si has an ionization energy of about 12-20 meV. This difference arises from the larger bandgap of AlN, which increases the binding energy of donor electrons. Germanium, another potential n-type dopant, exhibits similar behavior but with slightly higher activation energies due to its larger atomic size and the resulting lattice strain. Despite these challenges, n-type AlN with carrier concentrations in the range of 10^17 to 10^18 cm^-3 has been achieved using Si doping, though achieving higher concentrations is limited by compensation effects and the formation of defects such as nitrogen vacancies (V_N), which act as compensating acceptors.

P-type doping in AlN is considerably more difficult than n-type doping due to the high ionization energies of acceptor impurities and strong self-compensation effects. Magnesium (Mg) and beryllium (Be) are the most studied p-type dopants for AlN. Mg, the standard p-type dopant for GaN, behaves as a deep acceptor in AlN, with an ionization energy of approximately 510-630 meV, significantly higher than its ~200 meV activation energy in GaN. This deep level nature results in very low hole concentrations at room temperature, often below 10^15 cm^-3, even at high Mg doping levels. Beryllium has been explored as an alternative due to its smaller atomic size and theoretically predicted shallower acceptor level. However, experimental results show that Be also forms deep acceptors in AlN, with activation energies around 400-500 meV. Additionally, Be incorporation is challenging due to its high vapor pressure and toxicity, complicating growth processes. Compensation effects further hinder p-type conductivity, as defects such as aluminum vacancies (V_Al) and oxygen impurities (which act as donors) counteract the hole concentration.

The differences in doping behavior between AlN and GaN can be attributed to their distinct electronic structures and bonding characteristics. GaN has a smaller bandgap (3.4 eV) and more covalent bonding compared to AlN, which has a more ionic character. The increased ionicity in AlN leads to stronger localization of dopant states, resulting in deeper impurity levels. Additionally, the higher bond strength in AlN makes it more resistant to defect formation but also reduces dopant solubility and increases compensation. In contrast, GaN exhibits better dopant incorporation and lower activation energies, making it easier to achieve both n-type and p-type conductivity. For example, Mg-doped GaN can achieve hole concentrations above 10^17 cm^-3 after thermal activation, while similar doping in AlN yields orders of magnitude lower carrier densities.

Compensation effects are a major obstacle in AlN doping. Nitrogen vacancies (V_N) are common compensating centers in n-type AlN, while aluminum vacancies (V_Al) and oxygen impurities dominate in p-type material. Oxygen, a ubiquitous contaminant in AlN due to its high affinity for aluminum, acts as a donor and compensates holes in p-type AlN. The high formation energy of V_Al in AlN further limits p-type doping efficiency, as these vacancies are less abundant than their counterparts in GaN. Advanced growth techniques, such as metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), have been employed to minimize unintentional impurities and improve doping control. However, achieving high conductivity remains elusive, particularly for p-type AlN.

Thermal activation is another critical factor in AlN doping. Due to the deep nature of acceptor levels in p-type AlN, high-temperature annealing is often required to activate dopants. However, excessive temperatures can lead to dopant diffusion, defect formation, and degradation of material quality. For Mg-doped AlN, post-growth annealing at temperatures above 1000°C is typically necessary to activate acceptors, but the resulting hole concentrations remain low. In contrast, GaN benefits from lower activation temperatures and more efficient dopant activation, highlighting the inherent challenges of AlN.

Recent research has explored alternative approaches to improve doping efficiency in AlN. Codoping strategies, such as the use of Mg-Si or Mg-O pairs, have been investigated to reduce compensation and enhance p-type conductivity. Additionally, polarization-induced doping in AlN-based heterostructures has shown promise for achieving higher carrier densities without relying solely on impurity doping. However, these methods are still in the experimental stage and require further optimization.

In summary, doping aluminum nitride for n-type and p-type conductivity presents significant challenges due to deep impurity levels, strong compensation effects, and high activation energies. While n-type doping with Si or Ge is relatively more feasible, p-type doping with Mg or Be remains problematic due to the deep acceptor nature and low hole concentrations. Comparisons with GaN underscore the fundamental differences in doping behavior arising from AlN's wider bandgap and more ionic bonding. Overcoming these challenges will require advances in growth techniques, defect engineering, and alternative doping strategies to unlock the full potential of AlN for high-performance electronic and optoelectronic applications.