Effect of Nitridation Treatment on AlN Epitaxial Growth +

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Ultra-wide bandgap AlN semiconductor material plays a core role in deep ultraviolet optoelectronic devices and high-power electronic devices. However, when AlN is epitaxially grown on heterogeneous substrates such as sapphire, significant lattice and thermal mismatches often lead to high-density threading dislocations (TDs) in the thin film, severely limiting device performance. Nitridation, which exposes the substrate to a nitrogen-containing atmosphere for surface modification before growth, has been proven to be an effective means of improving the crystal quality of AlN epitaxial layers.

1. Mechanism of Nitriding Treatment: Formation of AlON Transition Layer

The core function of nitriding treatment is to form a transition layer of aluminum oxynitride (AlON) on the surface of sapphire (Al₂O₃). This thin layer can effectively improve the atomic arrangement of subsequent AlN epitaxial growth.

As described by Yusop et al., during the nitridation process, NH3 reacts with Al₂O₃ surface to form an AlON layer, which can serve as a template to “correct” the arrangement of AlN atoms on it, laying the foundation for low dislocation density epitaxial growth. The XPS analysis results clearly demonstrate the presence of N-Al-O bonds, providing direct evidence for the formation of AlON layers. This study further suggests that the stability of AlON layers is closely related to the nitriding time, and their optimal state corresponds to the lowest threading dislocation density (TDD).

Fig. 1 N1s XPS spectrum of nitrided sapphire surface

2. Control of Nitridion Parameters on Quality of AlN Crystals

The effect of nitridion treatment is mainly controlled by nitriding time and temperature, which together determine the crystalline quality of AlN epitaxial layer. In terms of time, there is a clear “optimization window”: Yusop et al. found that 20 minutes of nitriding can achieve the lowest penetration dislocation density (~10¹⁰cm⁻², Fig. 2c) on sapphire at a 0.5° angle, while exceeding 40 minutes will damage the AlON layer, trigger N polarity growth, and instead increase dislocations.

Fig. 2 Effect of nitriding time and sapphire deviation angle on AlN through dislocation density

In terms of temperature, Zhang et al. found that low-temperature nitridation (1050°C) combined with low-temperature buffer layer leads to the growth of AlN polycrystals (Fig. 3a), while high-temperature nitridation (1300°C) combined with a high-temperature buffer layer can prepare high-quality AlN single crystals with a single orientation. The XRD full width at half maximum is significantly reduced (Fig. 3d), indicating a significant improvement in crystal integrity.

Fig. 3 XRD patterns of semi polar AlN at different nitriding temperatures

3. Improvement of Surface Morphology and Lattice Stress by Nitriding Treatment

The improvement of AlN crystal quality by nitriding treatment is reflected in the synergistic improvement of surface morphology and lattice stress. For off-angle sapphire substrates, surface steps are prone to form rough macroscopic steps in epitaxy. However, the AFM study by Yusop et al. showed that nitriding treatment can significantly reduce the surface roughness of AlN, suppress the formation of macroscopic steps, promote two-dimensional layered growth, and provide a flat surface for subsequent device fabrication.

Fig. 4 AFM morphology of AlN layer surface grown under different conditions

Meanwhile, the Raman spectroscopy analysis by Zhang et al. showed that the (10-13) AlN thin film prepared by high-temperature nitriding and buffer layer process had the E₂(high) peak position closest to the stress free state, proving that optimizing the nitriding process can effectively release compressive stress, reduce crack and defect generation, and thus improve device reliability.

Fig. 5 Raman spectroscopy analysis of AlN thin films at different nitriding and buffer layer temperatures

4. Universality and Specificity of Nitriding Treatment in Different Application Scenarios

Nitriding treatment also exhibits good effects on complex substrate structures, and its effectiveness is influenced by specific conditions. On patterned sapphire substrates (PSS) covered with SiO₂, Zhang et al. found that appropriate nitridation (such as 50 W/60 s) before sputtering AlN can significantly improve the quality of the AlN layer. SEM images show that after nitridation, AlN covers the graphic structure more uniformly and completely, creating favorable conditions for the subsequent epitaxial laterally overgrown (ELOG) of GaN. ELOG is the key to blocking through dislocations and improving device performance. In addition, there is a synergistic effect between the nitriding effect and the substrate deflection angle. Yusop et al. found that nitriding on a 0.5° angle substrate has the best effect on reducing the density of through dislocations, indicating that the process needs to be adjusted according to the characteristics of the substrate to achieve the optimal formation conditions for the AlON transition layer.

Fig. 6 SEM images of GaN/AlN grown under different nitriding conditions on composite PSS

Nitriding treatment, as a key interface engineering technique, can simultaneously optimize the crystal quality, surface flatness, and stress state of AlN epitaxial layers by forming AlON layers in situ. Research has clearly shown that nitriding time, temperature, and substrate deflection angle together constitute the key process window that determines the final quality. In the future, a deep understanding of its internal mechanism and precise control of the process will be the core to promote the development of high-performance AlN optoelectronic devices.

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References:

1. Yusop, N., Yusuf, Y., Samsudin, M. E. A., Azmi, N. S., Ahmad, M. A., Chanlek, N., & Zainal, N. (2025). Influence of nitridation time on growth of AlN layers on different sapphire substrate off-cut angles. Materials Science in Semiconductor Processing, 187, 109130.
2. Zhang, Q., Li, X., Zhao, J., Sun, Z., Lu, Y., Liu, T., & Zhang, J. (2021). Effect of High-Temperature Nitridation and Buffer Layer on Semi-Polar (10–13) AlN Grown on Sapphire by HVPE. Micromachines, 12(10), 1153.
3. Zhang, Y., Zhu, G., Wang, J., & Le, Z. (2023). The effect of nitridation on sputtering AlN on composited patterned sapphire substrate. Materials, 16(3), 1104.