Researches on Preparation of High Performance AlN Template +

PAM-XIAMEN can provide sapphire, silicon, and SiC based AlN (aluminum nitride) templates. For details, please consult: [email protected]

Due to the significant lattice mismatch and thermal expansion coefficient difference between AlN and commonly used substrates such as sapphire, the dislocation density and stress in the epitaxial layer are high, seriously affecting device performance. In recent years, various techniques have been proposed to improve the quality of AlN templates, including optimization of nucleation layers, introduction of micro pit structures, substrate selection, and polarity control.

1. Influence of Nucleation Layer Type on Dislocation and Strain Evolution of AlN Template

Chen et al. compared the differences in epitaxial growth of AlN on AlN nucleation layers (NL) prepared by MOCVD and sputtering methods. The AlN with sputtered nucleation layer (Sample B) has smaller and more uniform grains, resulting in smaller interface changes and lower dislocation density. XRD analysis showed that the FWHM of the (0002) and (1012) rocking curves of AlN grown on the sputtered nucleation layer were 122 and 430 arcsec, respectively, which were much better than those of the MOCVD nucleation layer sample (Sample A). However, the sputtered nucleation layer also introduces higher tensile stress, leading to cracks in the sample during the later stages of growth. Through geometric phase analysis (GPA), it was found that the bending, merging, and local strain concentration of dislocations are the main mechanisms of strain release.

Fig. 1 Rocking Curve Diffraction: (a) AlN (0002), (b) AlN (1012); Optical microscope images: (c) Sample B; (d) Sample A

2. Micro Pit Layer Technology in Reducing Dislocation Density

On the basis of optimizing the nucleation layer, Okada et al. proposed a method for preparing high-quality AlN templates through metal organic vapor phase epitaxy (MOVPE). They prepared AlN epitaxial layers with self formed micro pits on sapphire substrates by adjusting the V/III ratio and growth time. This structure can promote epitaxial lateral overgrowth (ELO) and significantly reduce the threading dislocation density (TDD). Research has shown that AlN thin films grown on single-layer micro pits have a penetration dislocation density of 7.3×10⁹cm^-2, which can be further reduced to 2.6×10⁸cm^-2 after adopting a multi-layer structure. In addition, the structure can release stress by forming small gaps, increase the curvature radius of the sample, and suppress crack formation.

Fig. 2 (a) ABF-STEM image of AlN layer cross-section with tiny pit layer, (b) schematic model for reducing dislocations, (c) SEM image of the top surface

Fig. 3 AlN with a double-layer micro pit layer: (a) cross-section SEM; (b) TEM image

3. Influence of Substrate Type and Temperature on AlN Properties

Wei et al. systematically studied the optical and structural properties of AlN thin films grown on flat substrates (CFSS) and nano patterned substrates (NPSS). The results indicate that AlN grown on nano patterned substrates has lower surface oxidation rates and better crystal quality. Raman spectroscopy shows that the full width at half maximum (FWHM) of the E₂ (high) mode in the nano patterned substrate based sample is smaller, indicating smaller lattice changes and higher quality. In addition, as the temperature increases, the curvature radius of AlN thin films increases and the warpage decreases, especially on nano patterned substrates where the changes are more significant.

Fig. 4 Raman spectra of AlN sample thin films on S1 flat substrate and S2 nano patterned substrate

Fig. 5 AlN on S1 flat substrate and S2 nano patterned substrate (a) variation of curvature radius with temperature; (b) Warping changes with temperature

4. AlN Polarity Control and Impurity Behavior

On the basis of structural optimization, polarity control has become the key to further improving the performance of AlN. Chen et al. reported the successful preparation of an N-polarized AlN layer on a sapphire substrate with a tilt angle of 2 ° using polarity reversal technique. The Al polar micro pit layer can be used as a template to grow N-polar AlN, significantly improving its crystal quality. SIMS analysis shows that there are peaks of impurities such as C, H, and Si at the polarity reversal interface, which may inhibit the formation of AlON layer and stabilize N polarity. Although the surface roughness has slightly increased, the overall crystal quality is better than that of traditional N-polarized AlN templates.

Fig. 6 SIMS of N-polar/Al polar AlN

All the experimental results show that by optimizing the nucleation layer, introducing micro pit structures, substrate and temperature control, and polarity control, the quality of AlN epitaxial layers can be significantly improved.

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

 

References:

1. Chen, Q., Gao, J., Chen, C., Ye, F., Gao, G., Xu, C., … & Guo, W. (2024). Evolution of dislocations and strains in AlN grown by high-temperature metal–organic chemical vapor deposition. Crystal Growth & Design, 24(4), 1784-1791.
2. Okada, N., Hidaka, R., Kowaki, T., Saito, T., Sugawara, Y., Yokoe, D., … & Tadatomo, K. (2024). Fabrication of high-quality Al-polar and N-polar AlN templates through self-forming tiny-pit layers and polarity inversion. Journal of Applied Physics, 136(2).
3. Wei, W., Peng, Y., Hu, Y., Xu, X., & Xie, Q. (2024). Impact of temperature and substrate type on the optical and structural properties of AlN Epilayers: a cross-sectional analysis using advanced characterization techniques. Molecules, 29(22), 5249.