Atomic structures of a liquid-phase bonded metal/nitride heterointerface.

Akihito Kumamoto,Naoya Shibata,Tetsuya Tohei,Yuichi Ikuhara,Kazuhiro Akiyama,Toshiyuki Nagase,Yoshiyuki Nagatomo,Yoshirou Kuromitsu,Nobuyuki Terasaki,Kei-Ichiro Nayuki

doi:10.1038/srep22936

Abstract

Liquid-phase bonding is a technologically important method to fabricate high-performance metal/ceramic heterostructures used for power electronic devices. However, the atomic-scale mechanisms of how these two dissimilar crystals specifically bond at the interfaces are still not well understood. Here we analyse the atomically-resolved structure of a liquid-phase bonded heterointerface between Al alloy and AlN single crystal using aberration corrected scanning transmission electron microscopy (STEM). In addition, energy-dispersive X-ray microanalysis, using dual silicon drift X-ray detectors in STEM, was performed to analyze the local chemistry of the interface. We find that a monolayer of MgO is spontaneously formed on the AlN substrate surface and that a polarity-inverted monolayer of AlN is grown on top of it. Thus, the Al alloy is bonded with the polarity-inverted AlN monolayer, creating a complex atomic-scale layered structure, facilitating the bonding between the two dissimilar crystals during liquid-phase bonding processes. Density-functional-theory calculations confirm that the bonding stability is strongly dependent on the polarity and stacking of AlN and MgO monolayers. Understanding the spontaneous formation of layered transition structures at the heterointerface will be key in fabricating very stable Al alloy/AlN heterointerface required for high reliability power electronic devices.

Highlights

Heterostructures between metals and ceramics have been widely used for power electronic devices requiring both high thermal performance and reliability in harsh environments
A selected area electron diffraction (SAED) pattern taken from the Aluminum nitride (AlN) single crystal region indicates the[1120] zone axis of AlN
Since the interface appears to be flat with no gaps and/or pores, and it is assumed that significant morphological changes between Al alloy and AlN crystal did not occur during the bonding processes, while a large step formation was found in an Al/Al2O3 interface with the same bonding method[11]

Summary

Introduction

Heterostructures between metals and ceramics have been widely used for power electronic devices requiring both high thermal performance and reliability in harsh environments. Several experimental and theoretical studies on metal/ceramic interfaces have been performed, down to atomistic dimensions[1,2,3,4,5,6] These studies suggested that there are several factors affecting the structures of heterointerfaces, such as lattice mismatches, chemical bonding states and dopant/impurity segregation. Given the polar nature of the crystal structure, surface/interfaces of AlN crystal have received considerable attention as fundamental physics[3,12] It is still an open question how the dopants/impurities in this system affect the bonding mechanisms of the interface at atomic dimensions and how these heterointerfaces are formed during the liquid bonding processes. In order to achieve the model liquid phase bonded Al alloy/AlN interface suitable for STEM observations, we used single crystalline AlN substrate as a base material. For atomic-resolution STEM observations, we used an aberration-corrected STEM equipped with dual silicon drift detectors for energy-dispersive X-ray spectroscopy (EDS), operated at 200 kV

Methods

Results

Conclusion