PAM-XIAMEN can supply AlN thin films, additional specifications please see https://www.powerwaywafer.com/2-inch-aluminum-nitride-aln-template-on-sapphire.html.

1. Research Background of Incoherent Interfaces

Functional material interfaces have attracted much attention due to their often exhibiting novel physical and chemical phenomena and properties that differ from bulk materials. For example, two-dimensional electron gas, interface superconductivity, interface luminescence, and interface magnetism have been discovered at material interfaces. These interesting interface phenomena and properties are usually attributed to strong physical and chemical interactions at the interface, so they mostly occur at coherent and semi coherent interfaces.

From the coherent interface to the semi coherent interface, and then to the incoherent interface, the lattice mismatch at the interface continues to increase, resulting in different lattice mismatch adjustment mechanisms and interface structures at the material interface. The lattice mismatch of the coherent interface is small, and the interface mismatch is adjusted by the elastic deformation of two adjacent lattices, forming a perfectly matched interface structure between atoms on the interface; The lattice mismatch at the semi coherent interface is moderate, compensated for by the formation of periodic arrangement of interface mismatch dislocations. The lattice mismatch at incoherent interfaces is very large, and adjacent crystals on both sides of the interface will maintain their original lattice and be rigidly stacked together, making it difficult to form interface mismatch dislocations. Although incoherent interfaces are more common than the other two types of interfaces, due to their poor lattice matching and weak interface bonding strength, the interaction on the interface is very weak. Therefore, incoherent interfaces rarely exhibit unique interface phenomena and properties, which greatly limits the research and application of incoherent interfaces.

2. Research on Interface Phenomena and Properties of AlN/Al2O3 Incoherent Interface

In order to explore novel interface phenomena and properties on incoherent interfaces, a research team has conducted systematic research on the atomic and electronic structures and interface interactions at incoherent interfaces. It has been found that there are unusual strong interface interactions on the non coherent interface of AlN/Al2O3 (0001) with large lattice mismatch (~12%). The strong interface interaction significantly regulates the atomic and electronic structure and luminescent properties of the AlN/Al2O3 interface. The research results of transmission electron microscopy microstructure characterization indicate that interface mismatch dislocation networks and stacking faults are formed on the incoherent interface of AlN/Al2O3, which is rare on other incoherent interfaces.

Fig. 1 Microscopic structure of AlN/Al2O3 (0001) incoherent interface. (a, b) Transmission electron microscopy bright field images and selected area electron diffraction patterns of cross-sectional samples. The epitaxial growth of AlN thin film on Al2O3 substrate resulted in an uneven contrast between light and dark at the interface, indicating the presence of stress concentration at the interface. (c, d) Transmission electron microscopy bright field images and selected area electron diffraction patterns of planar samples. An interface mismatch dislocation network is formed on the interface.

The atomic layer resolved valence electron energy loss spectrum shows that the bandgap at the incoherent interface of AlN/Al2O3 decreases to ~3.9 eV, significantly smaller than the bandgap of AlN and Al2O3 bulk materials (5.4 eV and 8.0 eV, respectively). First principles calculations indicate that the reduction in bandgap at the interface is mainly due to the formation of distorted AlN3O tetrahedra and AlN3O3 octahedra at the interface, resulting in competition between Al-N and Al-O bonds and an increase in bond length.

Fig. 2 Atomic and electronic structures of AlN/Al2O3 interface without stacking faults. (a, b) Scanning transmission electron microscopy HAADF and ABF images. The Al atomic surface of AlN is directly bonded to the O atomic surface of Al2O3 at the interface. The lattice of AlN and Al2O3 is rigidly stacked, with 8 AlN atomic surfaces matching 9 Al2O3 atomic surfaces. Atomic reconstruction and splitting of Al atomic columns occur at the interface (indicated by the red arrow). (c) Atomic layer resolved valence electron energy loss spectrum. The bandgap at the interface decreased to~3.9 eV, significantly smaller than that of AlN and Al2O3 bulk materials.

Fig. 3 Atomic and electronic structures of AlN/Al2O3 interface fault zones. (a, b) Scanning transmission electron microscopy HAADF and ABF images. The interface stacking fault is formed on the side of Al2O3, but it does not change the lattice matching of the materials on both sides of the interface. The interface still has 8 AlN atomic faces matching 9 Al2O3 atomic faces. (c) Atomic layer resolved valence electron energy loss spectrum. The bandgap at the interface decreased to ~3.9 eV, significantly smaller than that of AlN and Al2O3 bulk materials.

Fig. 4 First principles calculations of the atomic and electronic structures at the AlN/Al2O3 interface. (a-c) Atomic model without stacking faults, electronic density of states, and differential charge density of Al atoms. Atomic model of (a-c) stacking fault zone, electronic density of states, and differential charge density of Al atoms. The band gaps in the non stacking fault zone and the stacking fault zone are 3.3 eV and 3.4 eV, respectively. The bonding strength at the interface is high, forming distorted AlN3O tetrahedra and AlN3O3 octahedra, with competition between Al-N and Al-O bonds.

Cathode fluorescence spectroscopy analysis shows that the non coherent interface has interface luminescence characteristics, which can emit ultraviolet light with a wavelength of 320 nm, and the luminescence intensity is much higher than the intrinsic luminescence of AlN thin films. This study indicates that non coherent interfaces with large lattice mismatches can exhibit strong interface interactions and unique interface properties, deepening and expanding people’s understanding of non coherent interfaces. It can provide reference and guidance for the development of advanced heterojunction materials and devices based on incoherent interfaces.

Fig. 5 Cathodic fluorescence measurement at the AlN/Al2O3 interface. (a) Scanning electron microscopy secondary electron images, (b) cathodic fluorescence spectra, (c, d) cathodic fluorescence distribution maps measured by 210 nm and 320 nm lasers. 210nm light excitation comes from AlN thin films, and 320nm light excitation comes from interfaces. The interface luminescence intensity is significantly higher than the intrinsic luminescence of AlN thin films.

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