Behavior and Role of Oxygen Diffusion in Aluminum Nitride Thin Films +

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AlN possesses a wide bandgap (6.1eV), high critical electric field, and good thermal conductivity, making it important for deep-ultraviolet light-emitting devices and high-power electronic devices. However, oxygen is the most common and unavoidable impurity in AlN thin films, and its form and concentration directly determine the electrical, optical, and structural properties of the material. The source of oxygen varies with the fabrication method: in low-temperature atomic layer deposition, oxygen mainly comes from precursor residues or chamber wall adsorption; in metalorganic vapor phase epitaxy (MOVPE) and subsequent high-temperature annealing, oxygen may diffuse from the sapphire substrate decomposition into the film. Understanding the diffusion behavior of oxygen under different conditions and its interaction with point defects and dislocations is central to achieving controlled performance of AlN.

1. Oxygen Incorporation during Low-Temperature Growth: PEALD AlN

1.1 Oxygen Sources and Plasma Control

In plasma-enhanced atomic layer deposition (PEALD), AlN thin films are typically grown at temperatures below 300 °C. Oxygen impurities originate mainly from oxygen species adsorbed on the reaction chamber walls, residual water vapor, and oxygen-containing groups in the precursors. Gungor and Alevli found that when pure N₂ plasma is used as the nitrogen source, the oxygen concentration in AlN films reaches as high as 55 at.%. Such high oxygen content results in the film no longer being pure AlN but forming a mixed AlON phase.

To suppress oxygen incorporation, H₂ was introduced into the N₂ plasma. Experiments show that at an N₂/H₂ flow ratio of 50/25 SCCM, the oxygen concentration in the film drops to about 18 at.%; further increasing H₂ to 50 SCCM raises the oxygen concentration only slightly to about 20 at.%, with no further reduction. This indicates that the ability of H₂ plasma to remove oxygen species via reduction has a saturation threshold. Fig. 1 shows Al 2p XPS spectra of AlN films under different plasma conditions. It can be clearly seen that the sample grown with pure N₂ plasma exhibits distinct Al–O bonds on the surface, while after introducing H₂, the Al–O peak disappears, leaving only Al–N bonds.

Fig. 1 Al 2p XPS spectra of AlN film surfaces under different plasma conditions

1.2 Effect of Oxygen on Chemical Composition and Bonding State

XPS depth profiling further reveals differences in oxygen distribution between the surface and bulk of the films. The sample grown with pure N₂ plasma has a bulk oxygen concentration as high as 55 at.% and a nitrogen content of only about 4 at.%, indicating severe oxidation of the film. In contrast, the sample grown with N₂/H₂ plasma has a bulk oxygen concentration stable at 18–20 at.%, a nitrogen content of about 30 at.%, and an aluminum content of about 50 at.%, closer to the stoichiometric ratio of AlN.

Fig. 2 Surface and bulk elemental composition (at.%) of AlN films under different N₂/H₂ flow ratios

High-resolution N 1s and O 1s spectra show that in the film grown with pure N₂ plasma, N is mainly present as N–Al–O bonds, whereas in the film grown with N₂/H₂ plasma, N–Al bonds dominate, indicating that oxygen is incorporated mainly by substituting nitrogen sites, while also forming a small amount of Al–O–N transition structures. The figure below shows that the N₂/H₂ sample is dominated by the N–Al peak (~396.2eV), while the pure N₂ sample shows only the N–Al–O peak (~399.8eV), indicating that H₂ promotes effective nitrogen incorporation.

Fig. 3 Bulk N 1s XPS spectra of AlN films under different plasma conditions

1.3 Effect of Oxygen on Morphology and Phonon Behavior

Atomic force microscopy (AFM) morphology analysis shows that oxygen concentration significantly affects surface roughness and grain size. The sample grown with pure N₂ plasma (55 at.% O) exhibits a surface roughness as high as 14.87nm and a mixed columnar‑cluster structure; in contrast, the sample grown with N₂/H₂ plasma (~20 at.% O) shows a surface roughness of about 1nm and well‑aligned c‑axis oriented regular columnar grains. Grain size distribution analysis indicates that high oxygen concentration increases the average grain radius from about 3nm to about 10nm, suggesting that oxygen promotes grain coarsening.

Fig. 4 AFM three‑dimensional morphology images of AlN films with different oxygen concentrations: (a–c) 55 at.% O: rough mixed structure; (d–f) 18 at.% O: smooth c‑axis columnar grains; (g–i) 20 at.% O: similar regular columnar grains

Infrared spectroscopic ellipsometry (IRSE) measurements further reveal the effect of oxygen on lattice vibrations. The sample with an oxygen concentration of ~20 at.% clearly shows E₁(TO) and E₁(LO) phonon modes at frequencies of about 660cm⁻¹ and 910 cm⁻¹, respectively. When the oxygen concentration increases to 55 at.%, these phonon peaks completely disappear, indicating loss of long‑range order and formation of an amorphous AlON phase. In addition, the valence band maximum (VBM) shifts from 2.14eV to 1.47eV as the oxygen concentration decreases, reflecting bandgap narrowing.

Fig. 5 Infrared spectroscopic ellipsometry parameters of AlN films with different oxygen concentrations

2. Oxygen Diffusion during High‑Temperature Annealing: Driving Force for Dislocation Density Reduction

2.1 Oxygen Sources and Diffusion Pathways

For AlN epitaxial films grown on sapphire substrates, high‑temperature annealing (1500–1740°C) is an effective method to reduce dislocation density. Cancellara et al. found that after annealing, the oxygen concentration in AlN films increases significantly and forms a uniform plateau region, with the concentration increasing from ~10¹⁸cm⁻³ to ~10²⁰cm⁻³ as the annealing temperature rises. SIMS depth profiles show that all annealed samples exhibit three characteristic regions: a near‑surface plateau, a shoulder region, and a substrate region. The existence of the plateau cannot be explained by simple bulk diffusion – calculations show that within a 1‑hour anneal, the bulk diffusion length is only 50–500nm, much smaller than the film thickness, so faster diffusion pathways must exist.

Fig. 6 Oxygen SIMS depth profiles of MOVPE‑grown AlN films annealed at different temperatures

The authors propose a hybrid model combining dislocation‑core pipe diffusion and radial bulk diffusion. Dislocation cores act as fast diffusion pathways; oxygen atoms are rapidly transported along dislocation lines and then undergo radial bulk diffusion from the dislocation cores into the surrounding lattice, as shown in Fig. 7. This mechanism allows oxygen to fill the entire film within a relatively short time.

Fig. 7 Schematic diagram of the dislocation‑core pipe diffusion and radial bulk diffusion model for oxygen diffusion

2.2 Formation of Oxygen‑Aluminum Vacancy Complexes

Comparing the experimentally measured plateau oxygen concentrations with theoretical solubility curves, the data agree best with a compensation model involving aluminum‑vacancy‑dioxygen complexes (V_Al-2O_N), with a formation enthalpy of only 0.75eV. This means that at high temperatures, these complexes have a very high equilibrium concentration. In contrast, simple oxygen substitution on nitrogen sites (O_N) requires a formation energy above 3eV and cannot explain the experimentally observed high oxygen concentrations. Therefore, oxygen must combine with aluminum vacancies to form complexes, while keeping the Fermi level near mid‑gap.

Fig. 8 Comparison of experimental average plateau oxygen concentrations (black squares) with theoretical solubility curves

2.3 Oxygen‑Driven Dislocation Climb and Density Reduction

Transmission electron microscopy (TEM) analysis shows that after annealing, the dislocation density decreases from an initial 2×10¹⁰ cm⁻² to 2×10⁹cm⁻² after annealing at 1700°C. The reduction rate of a‑type dislocations (Burgers vector 1/3〈1120〉) dominates the overall evolution of dislocation density. The dislocation reduction follows a climb‑controlled recovery kinetics model, with a fitted activation energy of 5.4eV, which is much higher than the formation enthalpy of the V_Al-2O_N complex. This indicates that the rate‑limiting step for dislocation climb is not the formation of the complex but either the migration of the complex or the climb process itself.

Fig. 9 TEM images and dislocation density evolution of AlN films annealed at different temperatures

Oxygen plays a dual role in this process: first, the rapid diffusion of oxygen along dislocation cores supplies the point defects required for climb; second, the formation of V_Al-2O_N complexes modifies the local stress field, promoting annihilation and coalescence of dislocation segments. It is worth noting that even at high oxygen concentrations (5.5×10²⁰cm⁻³), the concentration of vacancy complexes formed is still lower than the theoretically required value, suggesting that in addition to climb, other mechanisms such as glide or cross‑slip may also contribute.

3. Conclusion

Comparing low‑temperature PEALD growth and high‑temperature annealing, it is found that the behavior of oxygen in AlN exhibits a strong temperature dependence, yet also intrinsic commonalities. In PEALD at around 200°C, oxygen is mainly incorporated as an impurity from chamber residuals, and its concentration can be controlled by H₂ plasma to a saturation value of about 20 at.%; in high‑temperature annealing above 1500°C, decomposition of the sapphire substrate becomes the main oxygen source, and oxygen diffuses rapidly along dislocation cores and forms V_Al-2O_N complexes, driving the dislocation density down from 2×10¹⁰cm⁻² to 2×10⁹cm⁻².

Although the temperature ranges are vastly different, the strong interaction between oxygen and aluminum vacancies is a common feature in both processes: during low‑temperature growth, oxygen occupies nitrogen sites and attracts aluminum vacancies to form complex defects; during high‑temperature annealing, oxygen directly combines with aluminum vacancies to form V_Al-2O_N This commonality indicates that controlling the concentration and distribution of oxygen is key to achieving structural perfection and controllable performance of AlN thin films.

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References:

1. Yan, Q., Lyons, J. L., Gordon, L., Janotti, A., & Van de Walle, C. G. (2025). Oxygen impurities in AlN and their impact on optical absorption. Applied Physics Letters, 126(6).
2. Cancellara, L., Markurt, T., Schulz, T., Albrecht, M., Hagedorn, S., Walde, S., … & Sitar, Z. (2021). Role of oxygen diffusion in the dislocation reduction of epitaxial AlN on sapphire during high-temperature annealing. Journal of Applied Physics, 130(20).