Influence of Substrate Crystal Plane and High-Temperature Annealing on AlN Structure and Optical Properties +

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AlN thin films are showing great potential for applications in fields such as ultraviolet optoelectronics, high-frequency acoustic devices, and high-power electronics. However, its device performance heavily relies on the high crystalline quality and low defect density of the thin film. Due to the lack of large-sized and high-quality AlN single crystal substrates, heteroepitaxial growth has become the mainstream solution, among which sapphire has become a commonly used substrate due to its lattice matching and stability. Based on current research, this article explores the effects of different sapphire substrate crystal planes (c-plane, a-side, r-plane) and high-temperature thermal annealing (HTTA) on the structure and optical properties of AlN thin films prepared by magnetron sputtering.

1. Substrate Crystal Face Affects on Properties of AlN Thin Films

Yin et al. prepared AlN thin films on c-plane, a-side, and r-plane sapphire (referred to as ANc, ANa, ANr) by radio frequency magnetron sputtering. XRD analysis shows that the ANa sample exhibits the strongest (0002) diffraction peak, indicating the best crystal quality, while the diffraction peaks of ANc and ANr are weaker, suggesting the presence of more defects in its crystal. The grain size of ANa was calculated to be 22.41nm using the Scherrer formula, with a dislocation density of 2.0nm⁻ ² and a microstrain of 1.57%.

Fig. 1 XRD of AlN thin films on sapphire substrates with different faces

The ellipsometer (SE) test showed that the band gaps of ANc, ANr, and ANa were 6.03 eV, 6.08 eV, and 6.17 eV, respectively, with ANc having the smallest roughness (4.90 nm) and ANr having the largest roughness (19.39 nm). XPS analysis further showed that the oxide layer on the surface of ANa was the thickest (33.64 Å) and ANr was the thinnest (19.00 Å), indicating that the Al atoms on the AlN surface on the a-side substrate were more easily oxidized, which may be related to surface energy and dangling bond density, indicating that different substrates have a significant impact on the surface oxygen adsorption ability of AlN.

Fig. 2 XPS measurement spectra of AlN thin films (ANa, ANr, and ANc)

2. Improvement of High-Temperature Annealing on AlN Thin Films

Peng et al. investigated the effect of high-temperature annealing (up to 1700°C) on sputtered AlN thin films. After annealing, the FWHM of AlN diffraction peak significantly narrowed, especially on c-plane sapphire, from 252 arcsec to 68 arcsec, approaching the level of MOCVD growth. AFM images show that columnar grains merge after annealing, with varying surface roughness changes, and even atomic level flat areas appear in the a-plane sapphire based AlN sample (as shown in Fig. (d)).

Fig. 3 Surface morphology of AlN sputtered on c, a, and r-face sapphire: (a, c, e) before annealing; (b, d, f) After annealing

The optical transmittance test showed that the band gap of all samples increased after annealing, such as the c-plane sample increasing from 6.107eV to 6.114eV, which is closer to the theoretical band gap of AlN of 6.2 eV, indicating a reduction in point defects (such as nitrogen vacancies VN) and an improvement in UV transmittance. The appearance and enhancement of the E2 (high) peak in Raman spectroscopy also confirm the improvement of crystal quality. XPS analysis revealed that after annealing, the number of Al-Al erroneous bonds decreased, and the binding energy of Al-N bonds shifted towards higher energy, indicating an increase in the oxidation state of Al and a reduction in defects.

Fig. 4 Raman spectra of AlN sputtered on c, a, and r-afce sapphire substrates: (a) before annealing; (b) After annealing

Fig. 5 XPS analysis of AlN sputtered on c, a, and r-face sapphire substrates: (a, c, e) before annealing; (b, d, f) After annealing

From above, it can be seen that the crystal surface of sapphire substrate significantly affects the crystal quality, surface morphology, and optical properties of sputtered AlN thin films, among which the a-face sapphire substrate is more conducive to the epitaxial growth of high-quality AlN. High temperature annealing post-treatment can effectively promote grain merging, reduce dislocations and point defects, improve the crystallization quality and UV transmittance of thin films, and is a universal and effective post optimization method.

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References:

1. Yin, J., Zhou, B., Li, L., Liu, Y., Guo, W., Talwar, D. N., … & Feng, Z. C. (2021). Optical and structural properties of AlN thin films deposited on different faces of sapphire substrates. Semiconductor Science and Technology, 36(4), 045012.
2. Peng, X., Sun, J., Liu, H., Li, L., Wang, Q., Wu, L., … & Ye, J. (2022). Structural and optical properties of AlN sputtering deposited on sapphire substrates with various orientations. Journal of Semiconductors, 43(2), 022801.