Cross-sectional transmission electron microscope observation has been performed on the microstructure of GaN films grown on a (001) GaAs substrate by metalorgahic vapor phase epitaxy (MOVPE) using 1,1-dimethylhydrazine (DMHy) and trimethylgallium (TMG) as the sources of nitrogen and gallium, respectively. Before the deposition, the surface of the substrate was nitrided with DMHy. High-resolution images and electron diffraction patterns confirmed that the GaN films have a zincblende structure (β-GaN) with the lattice constant of a GaN=0.454 nm, and contain bands of stacking faults parallel to {111} planes. The interface between GaN and GaAs is made of {111} facets with no interlayer. Misfit dislocations are found to be inserted on the interface approximately every five atomic planes of GaAs. The nitridation treatment with only DMHy for 130 min is found to form a thick layer of β-GaN on the (001) GaAs substrate. Nuclei of β-GaN formed by the pretreatment of surface nitridation play an important role in growing GaN in a zincblende structure during the supply of DMHy and TMG. The formation of facets on the top surface of GaN and on the interface of GaN/GaAs is explained in terms of the diffusion of arsenic in β-GaN. The characteristics of the structure of GaN films grown at 600 and 650° C are also presented.

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.Heterostructures between metals and ceramics have been widely used for power electronic devices requiring both high thermal performance and reliability in harsh environments. Since the interfaces play critical roles in many properties such as mechanical strength, thermal conductivity and dielectric strength, a fundamental understanding of the interface structure and the interface formation mechanism is crucially important. So far, 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. However, one of the most important aspects when considering the formation of heterointerfaces is the bonding process 1,7,8 . Thus, in order to understand and control the heterointerface structures and their resultant properties, the actual bonding processes must be considered.Aluminum nitride (AlN) is considered one of the most important materials for power electronic device applications due to its high thermal conductivity, low thermal expansion coefficient and nontoxic nature 9 . Metal aluminum (Al)/AlN heterostructures, fabricated by a direct bonding aluminum (DBA), are now widely used in automobiles as high power modules which can perform under harsh thermal stress conditions 10 . In DBA, the system is heated near the melting point of Al metal to facilitate liquid phase bonding between the molten Al and the AlN substrate. The melting temperature of Al metal can be decre...

2019). A new method for liquid-phase bonding of copper plates to aluminum nitride (AlN) substrates used in high-power modules. AbstractA new method for liquid-phase bonding of copper plates to an aluminum nitride (AlN) substrate was developed in this work. A newly developed proprietary interlayer composed of titanium and silver powders was deposited on the AlN substrate using a screen-printing machine. The eutectic reaction between printed silver and copper at 850°C led to formation of a liquid phase at the joint interface. A total of 42 samples were prepared using 7 and 6 different amounts of silver and titanium, respectively. The microstructures of all samples were analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy, and an ultrasonic flaw detector was used to assess joint integrity. The optimum composition of the Ti-Ag brazing alloy for producing defectfree joints was determined. The formation of a continuous TiN layer was found to be essential for achieving sound joints between the copper plates and AlN substrate.

Scanning electron microscope (SEM) image contrasts have been investigated for dislocations in semiconductor and metal materials. It is revealed that single dislocations can be observed in a high contrast in SEM images formed by backscattered electrons (BSE) under the condition of a normal configuration of SEM. The BSE images of dislocations were compared with those of the transmission electron microscope and scanning transmission electron microscope (STEM) and the dependence of BSE image contrast on the tilting of specimen was examined to discuss the origin of image contrast. From the experimental results, it is concluded that the BSE images of single dislocations are attributed to the diffraction effect and related with high-angle dark-field images of STEM.

A new bonding process has been developed for producing direct-bonded aluminum (DBA) substrates using aluminum nitride (AlN). A transient eutectic liquid phase forms in aluminumX (X = silicon, germanium, silver, or copper) systems at the interface between the aluminum foil and the AlN substrate. The aluminumX liquid phase transiently contacts the AlN substrate prior to isothermal solidification by diffusion of the element X into the aluminum foil. We have prepared DBA substrates using this process and demonstrated that they are highly stable after thermal cycling testing.