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| Main Authors: | , , , , |
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| Format: | Preprint |
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2025
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| Online Access: | https://arxiv.org/abs/2505.04443 |
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| _version_ | 1866908720597827584 |
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| author | Chen, Xin Luo, Xin Wang, Duo Cheng, Xu Cui, Peng |
| author_facet | Chen, Xin Luo, Xin Wang, Duo Cheng, Xu Cui, Peng |
| contents | In our recent experimental work (Appl. Phys. Lett. 125, 122109 (2024)), we observed that crystalline Si$_3$N$_4$ cap layers a few nanometers thick can form in situ on GaN surfaces. Compared with amorphous SiO$_2$ and Al$_2$O$_3$ caps, these crystalline caps yield cleaner GaN/Si$_3$N$_4$ interfaces with fewer defects and improved device performance. These observations raise two questions: why does Si$_3$N$_4$ away from the interface become amorphous as the cap thickens, and what is the actual crystal structure of the interfacial Si$_3$N$_4$? Previous work proposed a defect-wurtzite (DW) model constructed heuristically from $β$-Si$_3$N$_4$ and the AlGaN lattice constants, but this model is significantly higher in energy than $β$-Si$_3$N$_4$ and disagrees with experiment in both interlayer spacings and electronic gap. Using a systematic structure-search approach under in-plane lattice constraints commensurate with AlGaN, we identify a lower-energy configuration, denoted Lam-Si$_3$N$_4$, with quasi-two-dimensional (laminar) stacking normal to the interface. Under AlGaN-matched metrics, Lam-Si$_3$N$_4$ is about 60 meV/atom more stable than DW-Si$_3$N$_4$ and reproduces the experimentally observed interlayer spacings more closely. The substantial lattice mismatch explains amorphization when the crystalline cap grows far from the interface. Upon full relaxation, both DW- and Lam-Si$_3$N$_4$ exhibit wide $\sim$4 eV band gaps. Under AlGaN constraints, the DW gap collapses to $\sim$1.88 eV whereas Lam-Si$_3$N$_4$ maintains a larger $\sim$2.70 eV gap (for reference, PBE gaps: GaN 1.73 eV, AlN 4.05 eV). The wider gap and improved structural match of Lam-Si$_3$N$_4$ rationalize the superior capping performance and provide guidance for optimizing AlGaN/GaN device encapsulation. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2505_04443 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Revisiting the epitaxial Si$_3$N$_4$ crystalline cap on AlGaN/GaN via evolutionary structure search Chen, Xin Luo, Xin Wang, Duo Cheng, Xu Cui, Peng Materials Science In our recent experimental work (Appl. Phys. Lett. 125, 122109 (2024)), we observed that crystalline Si$_3$N$_4$ cap layers a few nanometers thick can form in situ on GaN surfaces. Compared with amorphous SiO$_2$ and Al$_2$O$_3$ caps, these crystalline caps yield cleaner GaN/Si$_3$N$_4$ interfaces with fewer defects and improved device performance. These observations raise two questions: why does Si$_3$N$_4$ away from the interface become amorphous as the cap thickens, and what is the actual crystal structure of the interfacial Si$_3$N$_4$? Previous work proposed a defect-wurtzite (DW) model constructed heuristically from $β$-Si$_3$N$_4$ and the AlGaN lattice constants, but this model is significantly higher in energy than $β$-Si$_3$N$_4$ and disagrees with experiment in both interlayer spacings and electronic gap. Using a systematic structure-search approach under in-plane lattice constraints commensurate with AlGaN, we identify a lower-energy configuration, denoted Lam-Si$_3$N$_4$, with quasi-two-dimensional (laminar) stacking normal to the interface. Under AlGaN-matched metrics, Lam-Si$_3$N$_4$ is about 60 meV/atom more stable than DW-Si$_3$N$_4$ and reproduces the experimentally observed interlayer spacings more closely. The substantial lattice mismatch explains amorphization when the crystalline cap grows far from the interface. Upon full relaxation, both DW- and Lam-Si$_3$N$_4$ exhibit wide $\sim$4 eV band gaps. Under AlGaN constraints, the DW gap collapses to $\sim$1.88 eV whereas Lam-Si$_3$N$_4$ maintains a larger $\sim$2.70 eV gap (for reference, PBE gaps: GaN 1.73 eV, AlN 4.05 eV). The wider gap and improved structural match of Lam-Si$_3$N$_4$ rationalize the superior capping performance and provide guidance for optimizing AlGaN/GaN device encapsulation. |
| title | Revisiting the epitaxial Si$_3$N$_4$ crystalline cap on AlGaN/GaN via evolutionary structure search |
| topic | Materials Science |
| url | https://arxiv.org/abs/2505.04443 |