Molecular beam epitaxy (MBE) is a highly controlled thin-film deposition technique widely used for growing high-quality nitride semiconductors such as gallium nitride (GaN) and aluminum nitride (AlN). Unlike metalorganic chemical vapor deposition (MOCVD), MBE operates under ultra-high vacuum (UHV) conditions, enabling precise control over layer composition, doping, and interface abruptness. The growth of GaN and AlN via MBE presents unique challenges, particularly in managing nitrogen incorporation, reducing dislocations, and achieving controlled doping. These materials are critical for power electronics and ultraviolet (UV) optoelectronics due to their wide bandgaps, high breakdown voltages, and thermal stability.

Plasma-assisted MBE (PAMBE) is the dominant approach for growing nitride semiconductors due to the difficulty of thermally decomposing molecular nitrogen (N₂) at conventional MBE temperatures. In PAMBE, a radio frequency (RF) or electron cyclotron resonance (ECR) plasma source activates nitrogen, generating reactive nitrogen species (atomic N or N₂*) that readily incorporate into the growing film. The plasma source parameters, such as RF power and nitrogen flow rate, significantly influence the growth kinetics. For instance, higher RF power increases the dissociation efficiency of N₂ but may also introduce ion-induced damage if not optimized. AlN growth typically requires higher plasma activation energy than GaN due to aluminum's stronger bonding with nitrogen.

Nitrogen source selection is critical in PAMBE. While N₂ is the most common source, alternatives like ammonia (NH₃) have been explored to reduce plasma-induced defects. However, NH₃ requires higher cracking temperatures and can introduce hydrogen-related impurities. The choice between RF and ECR plasma sources also affects film quality. ECR sources generally provide higher dissociation efficiency at lower pressures, reducing ion damage, but require careful tuning to avoid excessive nitrogen vacancies. The V/III flux ratio must be precisely controlled to prevent either nitrogen-deficient or metal-rich growth conditions, both of which degrade crystal quality.

Buffer layer strategies are essential for mitigating lattice and thermal mismatch between nitride films and common substrates like sapphire, silicon, or silicon carbide. Low-temperature AlN or GaN nucleation layers are commonly employed to promote two-dimensional growth and reduce threading dislocation densities (TDD). For example, a thin AlN buffer grown at 500–600°C on silicon helps prevent melt-back etching and forms a template for subsequent high-temperature GaN growth. Step-graded AlGaN buffers can further reduce strain-induced cracks in thick GaN layers on silicon. On sapphire, a high-temperature AlN buffer followed by a GaN layer has been shown to reduce TDD to the mid-10⁸ cm⁻² range, which is critical for device performance.

Dislocation reduction remains a major challenge in MBE-grown nitrides. Techniques such as epitaxial lateral overgrowth (ELO) and patterned substrate growth are less common in MBE compared to MOCVD due to the line-of-sight nature of MBE deposition. Instead, MBE relies on optimizing growth conditions, such as substrate temperature and III/V flux ratio, to promote step-flow growth and reduce defect formation. For instance, slightly nitrogen-rich conditions during GaN growth can enhance adatom mobility, leading to smoother films with fewer pits and dislocations. In-situ monitoring tools like reflection high-energy electron diffraction (RHEED) are invaluable for real-time assessment of surface morphology and growth mode transitions.

Doping challenges in MBE-grown nitrides stem from the high activation energies of impurities and compensation effects. Silicon is the most common n-type dopant, introduced via a solid-source effusion cell or gas-phase precursor. However, silicon incorporation efficiency can vary with growth temperature and V/III ratio. For p-type doping, magnesium (Mg) is used, but its activation requires post-growth annealing due to hydrogen passivation and deep acceptor levels. The low hole mobility in p-GaN further complicates device design. Co-doping strategies, such as using Mg with indium, have been explored to improve acceptor activation, though these approaches require precise flux control to avoid phase separation.

The applications of MBE-grown GaN and AlN are particularly prominent in power electronics and UV optoelectronics. In power devices, the high electron mobility and critical electric field of GaN enable efficient high-voltage switching. AlN’s ultra-wide bandgap (6.2 eV) makes it suitable for deep-UV photonics, where its transparency below 200 nm is critical for applications like UV-C disinfection and environmental monitoring. The precise control over heterostructures in MBE allows for the engineering of high-electron-mobility transistors (HEMTs) with AlGaN barriers and two-dimensional electron gases (2DEGs) exhibiting sheet carrier densities above 10¹³ cm⁻². For UV emitters, AlGaN-based multiple quantum wells (MQWs) grown by MBE show promise due to their abrupt interfaces and minimized impurity incorporation.

Despite its advantages, MBE faces scalability challenges compared to MOCVD, particularly for large-area commercial production. However, its strengths in research and development, such as the ability to grow metastable phases and finely tune doping profiles, make it indispensable for advancing nitride semiconductor technology. Continued improvements in plasma sources, in-situ diagnostics, and defect engineering will further solidify MBE’s role in enabling next-generation power and optoelectronic devices.