Unveiling Phonon Dispersion Behavior of AlN/GaN Heterostructures Using EELS

III-V semiconductor heterostructures are commonly used in modern electronic devices due to their bandgap tunability, high electron mobility and thermal stability [1].These heterostructures are made of different semiconductor layers forming one (i.e., single junction) or several interfaces (i.e., quantum well structure). Lattice vibrations (phonons) at those interfaces play a key role in heat dissipation and electron mobility processes [1,2]. The study of interface phonon properties requires the use of nanoscale-sized probes to characterize its local response. Thanks to the development of novel technologies for monochromators and spectrometers, atom-wide monochromatic probes have become available in electron microscopes, thus offering vibrational scattering information over a wide spectral section of the infrared range with an unmatched spatial sensitivity [3]. For instance, inelastic phonon scattering from nanomaterials can be obtained in the real [4] and reciprocal spaces[5]. We present a momentum-resolved EELS study of the phonon dispersion across AlN/GaN interfaces using a nanometer-sized electron probe. The spectroscopy and imaging work was performed using a scanning transmission electron microscope (STEM) equipped with a monochromator and an aberration corrector operated at an accelerating voltage of 60 kV. An energy resolution of ⁓10 meV was obtained using a momentum-resolved setting. The structural characterization was conducted using High-Angle Annular Dark Field (HAADF) STEM. STEM images were acquired considering 31 mrad convergence half-angle probe and 72 mrad of collection half-angle. For the spectroscopy work, the conditions include a convergence semi-angle of 2 mrad which results in a ⁓2 nm probe size. The EELS collection conditions were configured to create adequate spacing between Bragg disks in the image projected at the EELS detector. The scattering signal was selected along specific crystallographic directions using a slot entrance aperture. Bulk AlN/GaN heterostructures were grown along the [0001] direction on a sapphire substrate using Molecular Beam Epitaxy. Two types of heterostructures were investigated: an interface between an AlN and a GaN crystals and a 2D structure of a few atomic layers of GaN sandwiched between two AlN crystals. The samples were prepared using conventional Focused Ion Beam microscopy. Figure 1a shows a HAADF-STEM image of an AlN/GaN interface oriented along the [112¯0] direction. We found an intermixing region of ⁓1 nm in width at the interface, which led to a reduction of its sharpness character. Stacking faults near the interface were identified on both sides of the interface. Figure 1b shows a HAADF-STEM image of an atom wide GaN monolayer sandwiched between two AlN crystals. Notice the high contrast associated with the presence of Ga atoms. An analysis of the width of the 2D GaN layered system showed an average of 7 atomic layers along 1 μm. We also found stacking faults adjacent to the GaN. Our momentum-resolved EELS work was focused on the AlN/GaN interface. We conducted phonon dispersion measurements across the interface, selecting phonon scattering along the ΓM direction of the wurtzite Brillouin zone. We resolved a phonon gap between 70 to 80 meV in AlN (Figure 2 a) and 40 to 70 meV in GaN (Figure 2 b). The optical phonon band extends between 80 and 100 meV and 70 and 95 meV for AlN and GaN, respectively. Our phonon dispersion results are in agreement with bulk measurements obtained using Inelastic X-ray Scattering [6]. The phonon dispersion at the interface displays the apparent absence of a phonon gap, which contrast with the behavior obtained from regions adjacent to the interface. This suggests the possible formation of hybrid AlN and GaN bulk phonon modes. Further analysis in the interface is in progress. In summary, we analyzed the sharpness character of an AlN/GaN interface and identified a ⁓1nm mixed region at the interface. We imaged the formation of single and multiple GaN layers between AlN. Stacking faults near the interface are present in both structures. We probed the bulk phonon dispersions of AlN and GaN and resolved the phonon gap that exists in both materials. Our phonon dispersion measurement at the interface suggests the appearance of new modes. Our work represents progress toward the exploration of phonon modes in the deep region of the mid-IR range and brings physical insights into the available channels for energy transfer in the contexts of heat transport and charge carrier mobility [7]. (a) HAADF STEM image of the AlN/GaN single interface heterostructure. The intermixing region is indicated by the dashed lines. (b) HAADF STEM image of a quantum well heterostructure of a single GaN layer embedded in AlN. (a) AlN bulk phonon dispersion with a gap between 70 and 80 meV, (b) GaN bulk phonon dispersion with the larger gap from 45 meV to 75 meV.

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