Studies on Influence of AlN Thin Film Thickness on Its Crystal Structure, Optical Properties, and Thermal Stability +
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Aluminum nitride (AlN), as an ultra wide bandgap semiconductor material, has shown broad application prospects in the fields of electronics and optoelectronic devices due to its excellent properties such as high thermal conductivity, high critical breakdown electric field, and high breakdown voltage. Due to the excellent lattice matching between AlN grown epitaxially and III nitrides such as GaN and AlGaN, it is often used as a high-quality template layer for ultraviolet light-emitting diodes (UV-LEDs) and GaN/AlGaN based electronic and photonic devices. However, a large number of studies have shown that higher dislocation density, strain, and stress in AlN templates can significantly reduce the luminescence efficiency and electrical performance of devices. Therefore, the material quality has a decisive impact on the overall device performance. It is worth noting that film thickness, as a key growth parameter, has a significant regulatory effect on the crystal quality, optical constants, and bandgap energy of AlN.
1. Influence of AlN Thickness on Crystal Quality
Structural characteristic analysis shows that the crystal quality of AlN thin films significantly improves with increasing thickness. The XRD results showed that when the film thickness increased from sub micron to 10μm, the FWHM of the (0002) diffraction peak of the thick film sample significantly decreased, and the dislocation density (about 3.0±0.9×109/cm2) and microstrain (0.023±0.006%) significantly decreased. This indicates that thick films contribute to stress release and defect annihilation. The cross-sectional SEM image further confirms that thicker AlN films (such as 4.29μm) have a denser structure, consistent with the XRD conclusion.
Fig. 1 XRD of sapphire based AlN (0002)
Fig. 2 SEM images of cross-sections of AlNt (1.93µm) and AlNn (4.29µm)
2. Influence of AlN Thickness on Optical Properties
In terms of optical performance, optical transmission (OT) and spectral ellipsometry (SE) measurements jointly reveal the significant influence of thickness on bandgap (Eeg) and Urbach band tail. The difference in the E-g values measured by OT (5.9-6.0 eV) compared to SE (6.1-6.2 eV) is due to the contribution of substrate and defect energy level transitions in OT measurements, while SE reflects the intrinsic properties of the lattice. In addition, the OT results showed that the thick film sample had a wider Urbach band tail, indicating a higher density of defect states. SE analysis also showed that the thickness of the thick film sample had a higher E-g, and the Urbach energy decreased with increasing thickness, which is consistent with the decrease in FWHM in XRD, further confirming the improvement of crystal quality.
Fig. 3 Relationship between Urbach Binding Energy (Eu) and AlN epitaxial film thickness by OT Method
Fig. 4 Comparison of FWHM (0002) of HR-XRD and Urbach binding energy (Eu) of SE with thickness variation of AlN epitaxial film
3. Effect of AlN Film Thickness on High Temperature Stability
High temperature spectroscopic ellipsometry (VT-SE) analysis further confirms that the thermal stability of AlN thin films increases with thickness. When the thickness of the film reaches the micrometer level, the change in bandgap with temperature slows down, and the electron phonon coupling strength (α-B) decreases to about 390 meV, indicating that the thick film has better thermal stability. Variable temperature Raman spectroscopy also observed a decrease in E2 (high) phonon lifetime with increasing temperature, attributed to enhanced phonon phonon interactions and lattice thermal expansion.
The above researches indicate that the crystal quality, optical properties, and high-temperature stability of AlN thin films are significantly dependent on their thickness. Thick film can not only effectively reduce defect density and improve stress state, but also enhance the stability of materials to temperature changes. These studies provide key experimental basis and parameter guidance for the design and preparation of high-performance AlN based optoelectronic devices, such as ultraviolet LEDs and high-temperature sensors.
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References:
- Feng, Z. C., Yang, H., Yin, J., Liu, Y., Wu, F., Dai, J., … & Ferguson, I. T. (2023). Crystalline and optical properties of AlN films with varying thicknesses (0.4-10 µm) grown on sapphire by metalorganic chemical vapor deposition. Thin Solid Films, 780, 139939.
- Yang, Y., Liu, Y., Wang, L., Zhang, S., Lu, H., Peng, Y., … & Sun, W. (2023). Optical and structural properties of aluminum nitride epi-films at room and high temperature. Materials, 16(23), 7442.