Research on HVPE Growth of High-Quality Semi-Polar AlN on M-Plane Sapphire Substrate +
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Aluminum nitride (AlN), as the direct bandgap semiconductor material with the widest bandgap (6.2eV) among group III nitrides, has important application prospects in deep ultraviolet optoelectronic devices. Compared with traditional polar c-plane AlN, semi polar AlN (such as (10-13) and (11-22) crystal planes) can effectively suppress the quantum confinement Stark effect (QCSE), thereby improving the luminescence efficiency of the device; However, due to the significant lattice and thermal mismatches between AlN and sapphire substrates, growing high-quality semi polar AlN directly on them still faces challenges. Hydrogenated vapor phase epitaxy (HVPE) technology is highly suitable for the preparation of thick film AlN due to its high growth rate and low cost advantages. Based on this, researchers adopted HVPE technology to optimize the process of growing high-quality semi polar AlN thin films on m-plane sapphire substrates.
1. Effect of High Temperature Nitridion on Orientation and Quality of Semi Polar AlN
High temperature nitriding treatment is a key step in regulating the crystal orientation and quality of semi-polar AlN. Research conducted by Li et al. has shown that samples without nitriding treatment exhibit mixed orientations of (10-13) and (10-11), while samples nitrided at 1400°C exhibit a single (10-13) orientation. The X-ray rocking curve (XRC) showed that the full width at half maximum (FWHM) of the nitrided sample decreased by 47%, indicating that the nitridation treatment significantly improved the crystal quality.
Further research has found that nitriding temperature has a significant impact on the quality of AlN. At a nitriding temperature of 1300°C, the FWHM of (10-13) and (0002) diffraction reached their minimum values of 0.343° and 0.409°, respectively. Through scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations, it was found that during the nitriding process, the surface of sapphire decomposes to form nano pores, whose size and density vary with the nitriding temperature. At 1300°C, the average size of nanopores is about 70nm, with a high density, which helps to promote rapid merging of AlN grains in the early stages of growth.
Fig. 1 XRD comparison of semi polar AlN films without/with nitriding treatment, showing the change in crystal orientation before and after nitriding
2. Synergistic Effect of High-Temperature Buffer Layer and Nitridation on AlN
The introduction of a high-temperature buffer layer further optimized the crystal quality of semi polar AlN. Comparing samples with different combinations of nitriding temperature and buffer layer temperature, it was found that only when both nitriding and buffer layer temperatures were increased to 1300°C (sample D), could a single (10-13) oriented AlN film be obtained. X-ray photoelectron spectroscopy (XPS) analysis showed that high-temperature treatment significantly reduced the bonding ratio of impurities such as Al-O and N-Al-O, increased the Al-N bond content, and thus improved the chemical purity and crystal integrity of the film.
Fig. 2 XPS spectra comparison of AlN on sapphire (samples A, B, C, and D)
3. Influence of Growth Temperature and Substrate Deflection Angle on the Surface Morphology and Dislocation Density of AlN
The growth temperature has a significant impact on the surface morphology and crystal quality of AlN. As the temperature increases from 1380°C to 1530°C, the grain size of AlN increases and the grain density decreases, but the surface is still not completely merged. After introducing an m-plane sapphire substrate with a 1° tilt angle, the surface of sample (D) grown at 1530°C became smoother, and the XRC-FWHM further decreased, indicating an improvement in crystal quality.
Fig. 3 Surface SEM images of AlN on Sapphire (samples A, B, C, and D)
The lattice tilt angle along the [-1-123] direction was analyzed using X-ray reciprocal space mapping (RSM), and the density of misfit dislocations (MD) was calculated. The results indicate that under the conditions of 1530 ° C and 1 ° deviation angle, the MD density is the lowest, at 6.7 × 10⁵cm⁻², which is conducive to stress release and grain coalescence.
Fig. 4 X-ray reciprocal space mapping of AlN film samples
By optimizing the high-temperature nitriding temperature, buffer layer process, growth temperature, and substrate deflection angle, high-quality, single oriented semi-polar AlN thin films can be successfully prepared on m-plane sapphire. The (10-13) AlN obtained at a temperature combination of 1300°C nitriding and buffer layer has the optimal crystal quality; At a growth temperature of 1530°C and substrate conditions with a 1° angle, (11-22) AlN exhibits the lowest dislocation density and optimal surface flatness. These process strategies provide an important technical foundation for the application of semi-polar AlN in deep ultraviolet optoelectronic devices.
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References:
- Li, X., Zhao, J., Liu, T., Lu, Y., & Zhang, J. (2021). Growth of Semi-Polar (10-13) AlN Film on M-Plane Sapphire with High-Temperature Nitridation by HVPE. Materials, 14(7), 1722.
- 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.
- 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.