Non-Equivalent Atomic Vibrations at Interfaces in a Nitride Superlattice

The structural and chemical discontinuities at interfaces in III-V material heterostructures are what lead to their emergent electronic properties. However, the spectrum of vibrations in the materials often have large mismatch and large interface thermal resistances can limit device performances. The unique structure and chemistry at interfaces in these materials do not only lead to unique electronic properties, but also unique atomic vibrations. In the case of heterostructures made from AlN-(Al_0.65Ga_0.35)N -AlN, the nonequivalence of the two interfaces has long been recognized in that they host different two-dimensional carrier gasses.¹ The nonequivalence of the corresponding atomic vibrations, however, has not been investigated so far due to a lack of experimental techniques with both high spatial and high spectral resolution. Herein we experimentally demonstrate the nonequivalence of AlN-(Al_0.65Ga_0.35)N and (Al_0.65Ga_0.35)N-AlN interface vibrations using monochromated electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS)^2-4 and density-functional-theory (DFT). We demonstrate that angle-resolved STEM-EELS possesses mixed real- and reciprocal-space selectivity of the vibrational response, which enables direct mapping of the nonequivalent interface phonons between materials with different stacking order. The physical origin of the localized and nonequivalent interface behavior is then unraveled through the perspective of DFT. The results have implications on the vibrational properties of heterostructures where interfaces states increase thermal conductivity.^3,5<br/><b><u>References</u></b><br/>1. Ambacher, O. <i>et al.</i> Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures. <i>Journal of Applied Physics</i> <b>85</b>, 3222–3233 (1999).<br/>2. Hoglund, E. R. <i>et al.</i> Emergent interface vibrational structure of oxide superlattices. <i>Nature</i> <b>601</b>, 556–561 (2022).<br/>3. Cheng, Z. <i>et al.</i> Experimental observation of localized interfacial phonon modes. <i>Nat. Commun.</i> <b>12</b>, 6901–10 (2021).<br/>4. Wu, M. <i>et al.</i> Effects of Localized Interface Phonons on Heat Conductivity in Ingredient Heterogeneous Solids. <i>Chinese Phys. Lett.</i> <b>40</b>, 036801 (2023).<br/>5. Ravichandran, J. <i>et al.</i> Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices. <i>Nat. Mater.</i> <b>13</b>, 168–172 (2014).