Aluminum nitride (AlN) is a critical material in the family of III-nitride semiconductors due to its ultra-wide bandgap, high thermal conductivity, and strong piezoelectric properties. When integrated into heterostructures and quantum wells with other nitrides such as gallium nitride (GaN) and boron nitride (BN), AlN enables precise control over electronic and optoelectronic properties through band engineering, polarization effects, and carrier confinement. These structures are foundational for high-performance devices in high-frequency electronics, deep-UV optoelectronics, and quantum technologies.

The formation of AlN-based heterostructures relies on epitaxial growth techniques such as molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). The lattice mismatch between AlN and GaN is approximately 2.4%, which is relatively small compared to other semiconductor pairs, allowing for high-quality interfaces with minimal defect formation. In contrast, the lattice mismatch between AlN and hexagonal boron nitride (hBN) is significantly larger, often leading to challenges in direct epitaxy. However, van der Waals epitaxy and buffer layers can mitigate these issues, enabling the integration of AlN with 2D materials like hBN for novel quantum heterostructures.

Band alignment is a crucial aspect of AlN heterostructures. The conduction band offset between AlN and GaN is approximately 1.8 eV, while the valence band offset is around 0.7 eV, creating a type-I heterojunction favorable for electron and hole confinement. In AlN/BN systems, the band alignment is less well-defined due to the insulating nature of BN, but theoretical studies suggest a type-I alignment with large offsets, enabling deep carrier trapping. The precise measurement of these offsets relies on techniques such as X-ray photoelectron spectroscopy (XPS) and capacitance-voltage profiling.

Polarization effects play a dominant role in AlN-based quantum wells due to the strong spontaneous and piezoelectric polarization inherent in wurtzite nitrides. In AlN/GaN heterostructures, the polarization-induced electric field can exceed several MV/cm, leading to the quantum-confined Stark effect (QCSE), which redshifts emission wavelengths and reduces radiative recombination efficiency. This effect is particularly pronounced in ultrathin quantum wells, where the internal field causes significant band bending. Mitigation strategies include the use of nonpolar or semipolar crystal orientations, which reduce polarization-related field effects.

Carrier confinement in AlN quantum wells is highly efficient due to the large band offsets and polarization fields. Electrons and holes are localized within the well regions, with quantization energies that can be tuned by adjusting the well width. For example, in an AlN/GaN/AlN quantum well with a thickness of 3 nm, the electron ground state energy is typically around 200 meV above the GaN conduction band edge. The heavy-hole and light-hole states exhibit larger quantization energies due to their higher effective masses. The confinement is further enhanced in double heterostructures, where AlN barriers provide nearly impenetrable boundaries for carriers.

The interface quality in AlN heterostructures is critical for minimizing defect-related nonradiative recombination. Atomic-resolution transmission electron microscopy (TEM) reveals that AlN/GaN interfaces grown under optimized conditions exhibit monolayer sharpness with minimal intermixing. In contrast, AlN/BN interfaces often show more disorder due to the lack of strong chemical bonding between AlN and hBN. Advanced growth techniques, such as migration-enhanced epitaxy, can improve interface abruptness by promoting surface adatom mobility.

Thermal properties also influence the performance of AlN heterostructures. The thermal conductivity of AlN is exceptionally high, around 285 W/m·K for bulk crystals, but it can be reduced in thin films due to phonon scattering at interfaces. In AlN/GaN superlattices, the thermal conductivity is further suppressed by interface scattering, which must be accounted for in high-power device designs. Conversely, the integration of AlN with BN, which has anisotropic thermal conductivity, can be leveraged for directional heat dissipation in heterostructures.

Optical characterization of AlN quantum wells reveals sharp excitonic transitions, particularly at low temperatures where phonon broadening is minimized. The exciton binding energy in AlN/GaN quantum wells is enhanced compared to bulk GaN due to confinement, reaching values up to 30 meV. Time-resolved photoluminescence studies show that radiative lifetimes are strongly influenced by the QCSE, with longer lifetimes observed in wider wells where the overlap between electron and hole wavefunctions is reduced.

The strain state of AlN heterostructures is another key consideration. Tensile strain in AlN layers grown on GaN can alter bandgap energies and polarization fields, while compressive strain in GaN layers grown on AlN can modify effective masses and mobility. Strain engineering via substrate selection and buffer layer design allows for precise tuning of these parameters. For instance, the use of AlN templates on sapphire introduces biaxial compressive strain in overlying GaN layers, which can be partially relaxed through careful control of growth conditions.

Advances in AlN heterostructures have enabled breakthroughs in deep-UV optoelectronics, where AlN serves as both a barrier and an active layer. By incorporating thin GaN or AlGaN wells within AlN barriers, emission wavelengths below 250 nm can be achieved. The large band offsets suppress carrier leakage, while the polarization fields enhance electron-hole overlap under forward bias. These structures are pivotal for applications such as water purification and biological sensing, where compact UV sources are required.

In quantum technologies, AlN-based heterostructures are explored for their potential in hosting defect centers and enabling low-dimensional electron systems. The large bandgap of AlN provides deep confinement potentials for spins and charges, while the low intrinsic defect density ensures long coherence times. Coupling AlN quantum wells with superconducting or ferromagnetic materials opens avenues for hybrid quantum devices leveraging both charge and spin degrees of freedom.

The future development of AlN heterostructures will likely focus on improving interface control, reducing defect densities, and exploring new material combinations. The integration of AlN with emerging 2D materials beyond BN, such as transition metal dichalcogenides, could unlock novel functionalities in optoelectronics and quantum computing. Additionally, the use of machine learning for growth optimization may accelerate the discovery of optimal heterostructure designs for specific applications.

In summary, AlN heterostructures and quantum wells represent a versatile platform for engineering electronic and optical properties through precise control of band offsets, polarization, and confinement. Their compatibility with other nitrides and 2D materials enables a wide range of applications, from high-power electronics to quantum technologies, with ongoing research pushing the boundaries of performance and functionality.