Study on Detecting Phonon Bridges at Heterojunction Interface of Nitride Semiconductor

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1. Why Detect Phonon Bridges at Heterojunction Interface of Nitride Semiconductor?

Currently, the rapid development of information technology has put forward higher requirements for the thermal management of semiconductor devices: on the one hand, better heat dissipation materials (such as graphene, diamond, etc.) need to be used, and on the other hand, it is necessary to reduce the contact interface thermal resistance. For small-sized high-power devices, the thermal conductivity of the interface has actually become a bottleneck that restricts the performance improvement of the device. Therefore, studying the interface thermal conductivity mechanism is particularly important. In semiconductor devices, interface thermal conductivity is mainly determined by the interface phonons generated by several atomic layers near the heterojunction interface. But so far, little is known about how interface phonons affect interface thermal conductivity, mainly due to the lack of effective experimental means to measure interface heat or phonons.

A research team has developed a four-dimensional electron energy loss spectroscopy technique that combines spatial and momentum resolution capabilities, and demonstrated its application in measuring phonon dispersion at heterojunction interfaces (Nature 2021, 559, 399). Researchers used this spectroscopic method to measure the lattice dynamics behavior of interfaces between third-generation semiconductor aluminum nitride (AlN) and silicon (Si) substrates, as well as metal aluminum (Al) electrodes, and explored the phonon transport behavior at different interfaces and their contribution to interface thermal conductivity.

2. Research and Discovery on Phonon Bridges at Heterojunction Interface of Nitride Semiconductor

The research team found that the interface phonon modes of AlN/Si and AlN/Al are significantly different, leading to several fold differences in interface thermal conductivity. Usually, interface phonons can be divided into four categories: extended mode, localized mode, partially extended mode, and isolated mode. Among them, the extended mode and local mode have a strong correlation with the volume phonons on both sides of the interface, allowing the phonons on one side to cross the interface through elastic/inelastic scattering and reach the other side, serving as a bridge connecting the volume phonons on both sides, thereby helping to improve the interface thermal conductivity; However, some extended modes and isolated modes contribute very little to the interface thermal conductivity.

Fig. 1 Atomic structure and phonon spectrum of AlN/Si interface: (A) HAADF images and structures of the AlN/Si interface of atoms; (B) Mapping of phonon EEL spectra across AlN/Si interfaces; (C) Phonon dispersion, Brillouin zone projection, and in-plane components of PPDOS for Si and AlN

Fig. 2 Phonons on the AlN/Si interface: (A) The measured EEL spectrum; (B) The in-plane components of the AlN, AlN/Si interfaces, and the Si PPDOS were calculated. The arrow indicates the blue shift of Si-TA1 mode and AlN-TA2 mode on the interface; (C) The position of phonon peaks at the AlN/Si interface; (D) The peak intensity of phonons across the AlN/Si interface. Using Gaussian peak fitting method to extract peak position and intensity; (E) Intensity maps of different phonon modes and corresponding intensity line profiles; (F) Eigenvector displacement of interface phonon modes

The research team first observed a significant bridging effect of interface modes at the AlN/Si heterojunction interface: there are localized phonon modes at the atomic scale at the interface, which can undergo inelastic scattering and exchange energy with bulk phonons of different energies of AlN and Si on both sides of the interface. In addition, clear interface extension patterns were also observed. Both of these modes can effectively promote the transfer of interface heat. At the AlN/Al interface, no obvious phonon bridges composed of localized or extended modes were observed, and the interface phonon modes were mainly partially extended modes, resulting in lower heat transfer efficiency.

These results explain why the interfacial thermal conductivity of AlN/Al is much lower than that of AlN/Si. This work deepens the understanding of interface phonon and thermal transport, especially providing valuable information for the thermal management of high-power semiconductor devices such as nitride based high electron mobility transistors and high-power light-emitting diodes.

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