Direct Observation of Inversion Domain Boundaries of GaN on c-Sapphire at Sub-ångstrom Resolution

Wide-bandgap III–nitrides have seen enormous success in modern electronic, optoelectronic, and even spintronic devices. Recently, interest has grown in manipulating the crystal polarity of GaN having a wurtzite structure, which provides a new degree of freedom for investigating III–nitrides and their novel devices. These studies include work on the inversion domain boundaries (IDBs) of GaN, which separate adjacent domains of different polarity. Ten years ago, Northrup et al. performed first-principles calculations of domain-boundary energies and proposed the structure shown in Figure 1, based on its very low energy (25 meV Å ). Since then, no direct, indisputable test has been carried out to determine the exact boundary structure. At the same time, many unique properties have been observed at the IDBs. Among these is the remarkable effect, observed by Stutzmann et al. that the IDB can act as a rectifying junction when biased by two electrodes placed on adjacent Gaand N-face regions. The boundary between two adjacent domains with different polarity has been shown to be a very efficient radiative recombination center, which may have potential application for novel light-emitting devices. Meanwhile, it was found that the rectifying behavior of the IDBs can be explained by ab initio density functional calculations, assuming the IDBs have a structure as shown in Figure 1 (where a thin AlN layer is used to invert the polarity of GaN). However, the IDB structure has never been determined directly and their real structure remains unconfirmed. Here we show that we have determined directly the IDB structure, including the determination of GaN polarity, by aberration-corrected scanning transmission electron microscopy (STEM) at sub-Ångstrom