Research on Mechanism and Suppression Strategy of Crack Formation in AlN on Silicon +

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AlN, as a wide bandgap semiconductor material, has great potential for applications in power electronics, optoelectronics, sensing, and energy harvesting due to its excellent physical and chemical properties. However, when AlN is epitaxially grown on cost-effective and large-area Si substrates, cracks are prone to occur due to lattice mismatch and differences in thermal expansion coefficients, which limits the performance and reliability of AlN/Si heterostructures. Researchers have delved into the reasons for crack formation during the growth of AlN thin films on Si substrates and proposed relevant measures.

1. Mechanism of Crack Formation in AlN on Si

1.1 Lattice Mismatch and Thermal Expansion Coefficient Mismatch

The main reasons for the growth and cracking of AlN on Si substrates are lattice mismatch and thermal expansion coefficient mismatch. AlN (0001) is compatible with the Si (111) crystal plane structure, but there is a significant difference in lattice constants (a_aln=3.110 Å, a_si=3.840 Å), which causes AlN to experience in-plane tensile strain of approximately +0.235 Å during the early stages of growth, resulting in strain energy accumulation. In addition, the thermal expansion coefficients of the two are not matched (AlN: 4.2×10^-6K^-1, Si: 3.6×10^-6K^-1), and AlN shrinks more violently during the cooling process, further increasing tensile strain and exacerbating strain energy accumulation.

1.2 Effects of Dislocations, Thickness and Temperature

The formation of misfit dislocations can alleviate lattice strain, but it has a complex relationship with crack formation. According to the Matthews Blakeslee model, the critical thickness of AlN on Si (111) is extremely low (about 1.72 Å), and a large number of dislocations are generated in the early stages of growth, causing the film to transition from a fully strain state to partial relaxation, significantly reducing the strain energy caused by lattice mismatch and suppressing early cracks.

However, the relaxation ability of dislocations is limited, and they may themselves become the starting point of cracks. When the thickness of the film exceeds about 400nm, the tensile strain caused by thermal mismatch significantly increases, and the total strain energy may exceed the cleavage energy, leading to crack formation, which is consistent with the phenomenon observed in experiments where cracks often occur in the range of 400-500nm. The temperature drop further increases the strain energy, while the cleavage energy remains basically unchanged. During the cooling process, the accumulation of strain energy ultimately leads to cracking.

2. Measures to Avoid or Reduce Cracks in AlN Films on Si Substrates

To avoid or reduce the formation of cracks in AlN thin films on Si substrates, a multi pronged strain management strategy should be adopted:

1) Control film thickness

Strictly controlling the thickness of the film is crucial, and it should be maintained below the critical thickness of 400nm to ensure that the total strain energy caused by mismatch is always lower than the material’s cleavage energy, thereby fundamentally avoiding the occurrence of cracks.

2) Optimize growth temperature and cooling process

Optimizing growth and cooling processes is crucial, and relatively low deposition temperatures (such as<250 °C) can be used when growing thicker films. In addition, a slow and controlled cooling process is combined to minimize the accumulation of thermal strain during the cooling phase due to differences in thermal expansion coefficients.

3) Utilizing dislocations for strain relaxation

Actively guiding and utilizing dislocations for strain relaxation is another core approach. By introducing well-designed seed or buffer layers, an effective dislocation network can be formed in the early stages of growth, thereby relaxing lattice mismatch strain and reducing the total strain of the material. Explore the use of strain engineering methods to guide dislocations to form at specific locations or densities, thereby dispersing strain without causing macroscopic cracks.

4) Explore New Intermediate Layers or Substrates

If cost and technology permit, consider using substrates with smaller lattice mismatch with AlN (such as SiC or potential AlN self substrates, despite high costs and limited supply). In addition, it is also possible to consider introducing one or more intermediate layers (such as the tough intermediate layer) between AlN and Si, whose lattice parameters and thermal expansion coefficients are between AlN and Si, or which can effectively absorb and disperse strain, thereby reducing the strain of AlN thin films.

3. Experimental of Introducing A Ductile Metal Intermediate Layer

To overcome the crack problem of AlN on Si substrates, Aqib et al. adopted a strategy of introducing a ductile metal intermediate layer and actively regulating the strain state to achieve crack free growth. This method utilizes the mechanical and structural properties of the intermediate layer to effectively alleviate or even convert the original tensile strain (ɛ>0) in AlN into compressive strain (ɛ<0), thereby suppressing crack formation.

The study selected gold (Au) as the intermediate layer material, and its (111) crystal plane has good epitaxial compatibility with Si (111) and AlN (0001), which is conducive to high-quality epitaxy. The high ductility of Au enables it to relax strain through plastic deformation and transform the strain of AlN from +0.235 to -0.0746 compressive strain.

The experiment adopts a multi-layer structure: first deposit 250 nm AlN as a transition layer on Si (111), then evaporate a 10 nm Au intermediate layer, cover another 250 nm AlN, and finally grow a 1.25 μm thick AlN main layer via PA-MBE technology. The ultra-thin Au layer plays a stress buffering role by relaxing substrate strain through the formation of misfit dislocations.

The experimental results show that the crack free surface morphology, XRD peak shift, and TEM epitaxial relationship verification all indicate that the ductile intermediate layer strategy is an effective way to achieve crack free growth of thick AlN.

Fig. 1 Surface morphology of AlN (0001) thin film grown on Si (111) substrate: (a) AlN multilayer structure with Au intermediate layer; (b) AlN thin film without Au interlayer

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References:

1. Aqib, M., Pouladi, S., Moradnia, M., Kumar, R. P. R., Kim, N. I., & Ryou, J. H. (2024). Strain accumulation and relaxation on crack formation in epitaxial AlN film on Si (111) substrate. Applied Physics Letters, 124(4).

2. Aqib, M., Moradnia, M., Ji, M., Parameshwaran, V. S., Sarney, W. L., Pouladi, S., … & Ryou, J. H. (2024). Crack-free> 1-μm AlN layer on Si substrate using ductile interlayer for strain modification in epitaxial film. Applied Physics Letters, 125(11).