[Paper Review] Atomic-Scale Probing of Heterointerface Phonon Bridges in Nitride Semiconductor

Yuehui Li, Ruishi Qi+13 more|arXiv (Cornell University)|Aug 27, 2021

Thermal properties of materials57 references64 citations

TL;DR

This study uses atomically resolved vibrational electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope to probe interfacial phonon modes at AlN/Si and AlN/Al heterointerfaces. It identifies extended and interfacial phonon modes at the AlN/Si interface that act as phonon bridges, significantly enhancing inelastic phonon transport and interface thermal conductance (ITC), while such bridging modes are absent at the AlN/Al interface, where partially extended modes dominate.

ABSTRACT

Interface phonon modes that are generated by several atomic layers at the heterointerface play a major role in the interface thermal conductance for nanoscale high-power devices such as nitride-based high-electron-mobility transistors and light emitting diodes. Here we measure the local phonon spectra across AlN/Si and AlN/Al interfaces using atomically resolved vibrational electron energy-loss spectroscopy in a scanning transmission electron microscope. At the AlN/Si interface, we observe various localized phonon modes, of which the extended and interfacial modes act as bridges to connect the bulk AlN modes and bulk Si modes, and are expected to boost the inelastic phonon transport thus substantially contribute to interface thermal conductance. In comparison, no such phonon bridge is observed at the AlN/Al interface, for which partially extended modes dominate the interface thermal conductivity. This work provides valuable insights into understanding the interfacial thermal transport in nitride semiconductors and useful guidance for thermal management via interface engineering.

Motivation & Objective

• To understand the atomic-scale phonon modes governing interfacial thermal conductance (ITC) in nitride semiconductor heterostructures.
• To identify the dominant phonon mode types—extended, partially extended, interfacial, or isolated—at AlN/Si and AlN/Al interfaces.
• To correlate specific interfacial phonon modes with their contribution to ITC, particularly focusing on bridging mechanisms between bulk phonon modes.
• To provide experimental validation of theoretical predictions about phonon bridge formation and their impact on thermal transport.

Proposed method

• Atomically resolved vibrational electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) with off-axis geometry to enhance spatial and spectral resolution.
• Use of a monochromated electron microscope (Nion U-HERMES200) with 7.5 meV energy resolution and 0.125–0.166 nm spatial resolution to map local phonon spectra.
• Application of advanced data processing: background subtraction using modified Pearson-VII function, Lucy-Richardson deconvolution, and BM3D denoising to improve signal-to-noise ratio.
• Molecular dynamics (MD) simulations using the Stillinger-Weber potential and Dynaphopy software to compute phonon dispersion and mode classification.
• Classification of phonon modes into four types—extended, partially extended, isolated, and interfacial—based on eigenvector localization.
• Interface conductance modal analysis (ICMA) applied to MD data to compute modal contributions to ITC using the formula 𝐺_𝑛 = (1/(𝑘_𝐵𝐴𝑇²)) ∫〈𝐽_𝑛(𝑡)𝐽(0)〉𝑑𝑡.

Experimental results

Research questions

• RQ1 What types of interfacial phonon modes exist at AlN/Si and AlN/Al heterointerfaces at the atomic scale?
• RQ2 Which phonon modes—extended, interfacial, or partially extended—act as bridges to enhance inelastic phonon transport across the interface?
• RQ3 How do the relative contributions of different phonon modes to interface thermal conductance (ITC) differ between AlN/Si and AlN/Al interfaces?
• RQ4 To what extent do extended and interfacial modes contribute to ITC compared to partially extended modes, as predicted by MD simulations?
• RQ5 Can experimental EELS data confirm the existence and role of phonon bridges in enhancing thermal conductance at nitride semiconductor interfaces?

Key findings

• At the AlN/Si interface, extended phonon modes that connect bulk AlN and bulk Si phonon modes act as effective bridges, significantly enhancing inelastic phonon transport.
• Interfacial modes at the AlN/Si interface promote transverse acoustic (TA) modes of Si to penetrate into the AlN layer, further contributing to bridging effects.
• No such phonon bridges are observed at the AlN/Al interface, where partially extended modes dominate the thermal conductance.
• The contribution of extended modes to ITC is estimated to be 2–3 times higher than that of partially extended modes based on MD simulations.
• The AlN/Si interface is predicted to have substantially higher ITC than the AlN/Al interface due to the presence of bridging phonon modes.
• Experimental EELS data confirm the localization and energy distribution of these modes, validating theoretical predictions about interfacial phonon-mediated thermal transport.