Influence of Off-Angle Substrate on the Growth and Crystal Quality of AlN Thin Films +

PAM-XIAMEN can grow silicon, sapphire, and silicon carbide based aluminum nitride(AlN) thin films. For details, please consult: [email protected]

As an ultra wide bandgap semiconductor material, AlN is crucial for the development of deep ultraviolet optoelectronic devices. The main challenge facing its epitaxial growth is the severe lattice and thermal mismatch with commonly used substrates such as sapphire, which easily introduces high-density through dislocations in the epitaxial layer, thereby weakening device performance. Recent studies have found that using angled substrates is an effective strategy for regulating the growth of AlN thin films, optimizing their crystal quality and surface morphology. The researchers analyzed the mechanism of different substrate angles in the growth process of AlN.

1. Control of AlN Surface Morphology by Off-Angle Substrate

The deviation angle of the substrate directly affects the growth mode of AlN by changing the surface step structure. When growing c-plane AlN via HVPE, Li et al. found that step flow growth can be achieved on a 0.2° off-angle substrate with a relatively flat surface; The substrate with a 4° off angle leads to the aggregation of steps, forming macroscopic steps and significantly increasing surface roughness, as shown in Fig. 1. In addition, Maosong et al. also pointed out in their study of semi polar (11-22) AlN that introducing a 1° tilted angle at 1530°C can achieve surface fusion and significantly improve surface flatness (Fig. 2D).

Fig. 1 AFM image of HVPE AlN film on: (a), (b) 0.2° mis-cut substrate; (c) (d) 4° miscut substrate

Fig. 2 SEM image of surface fusion at different off-angle substrates

2. Influence of Miscut Angle Substrate on Dislocation Behavior in AlN Thin Films

Deviation angle substrates can not only regulate surface morphology, but also effectively induce dislocation tilt, reaction, and annihilation, thereby reducing the density of penetrating dislocations. Li et al. found through XRD and TEM analysis that a large number of tilted through dislocations appeared in AlN grown on a 4° deviation angle substrate. These dislocations reacted with each other during propagation, significantly reducing dislocation density.

Fig. 3 TEM images of HVPE AlN thin film taken under different beam conditions: (a), (b) 0.2° mis-cut substrate; (c) (d) 4° miscut substrate. The red arrow indicates a tilted threading dislocation.

In epitaxial lateral overgrowth (ELO) AlN, the off-angle substrate can also promote step aggregation and void formation, effectively terminating dislocation propagation. Okada et al. systematically studied the effect of a deviation angle of 0.15° to 2.0° on ELO-AlN and found that as the deviation angle increased, the phenomenon of step aggregation intensified, the efficiency of void formation increased, and the dislocation density significantly decreased. The experiment found that as the deviation angle increased to 0.8°, the dislocation density significantly decreased, especially in the low pit density region, where the etch pit density (EPD) could drop to 3.3 × 10⁷ cm⁻².

Fig. 4 SEM and AFM images of ELO-AlN layers grown on AlN templates with different tilt angles

Fig. 5 Wet etching evaluation of EPD of ELO-AlN layer on different mis-cut angle substrates

3. Deviation Angle and Stress Release Mechanism of AlN

The tilt angle substrate will affect the crystal quality of AlN by regulating the strain release mechanism. Maosong et al. found through X-ray inverse space mapping that the tilted substrate induces the tilt of the epitaxial layer along the [-1-123] direction, thereby promoting the slip of misfit dislocations and reducing strain energy. They further calculated that the misfit dislocation density of the sample grown on a 1° off angle substrate was 6.7 × 10⁵ cm^-2, significantly lower than that of the sample without angle.

The tilted angle substrate significantly improves the surface morphology and crystal quality of AlN thin films through various mechanisms such as regulating the surface step structure, inducing dislocation tilt and reaction, and promoting strain release. In different growth methods (HVPE, MOVPE, ELO), selecting an appropriate angle (such as 0.8°~1°) can effectively reduce dislocation density, improve material quality, and provide an important technical path for the preparation of high-performance AlN based optoelectronic devices.

Whether you need AlN template for research or for industrial applications, please contact us email at [email protected] and [email protected].

 

References:

1. Li, D. D. , Su, X. J. , Chen, J. J. ,  Wang, L. H. ,  Huang, J. , &  Niu, M. T. , et al. (2022). Influence of sapphire substrate miscut on the surface morphology and microstructure of aln films grown by hvpe. Journal of Crystal Growth.
2. Maosong, S., Ting, L., Yong, L., Shuxin, T., Xu, L., Jicai, Z., & Wenhong, S. (2022). Influence of growth temperature and miscut angle of m-plane sapphire substrate on the semi-polar (11–22) AlN film grown by HVPE. Frontiers in Physics, 10, 1076895.
3. Okada, N., Saito, T., Fujikawa, S., Maeda, N., Jo, M., Hirayama, H., & Tadatomo, K. (2022). Investigation of off-cut angle of sapphire for epitaxial lateral overgrowth of AlN and fabrication of high-quality AlN template. Journal of Crystal Growth, 588, 126640.