Abstract
Wide-bandgap III–nitrides have seen enormous success
in modern electronic, optoelectronic, and even spintronic
devices.[1–5] 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.[6–10] These studies include work on the
inversion domain boundaries (IDBs) of GaN, which separate
adjacent domains of different polarity.[11] 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 (25meVA ° 2).[11]
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.[9,12]
Among these is the remarkable effect, observed by Stutzmann
et al.[9] that the IDB can act as a rectifying junction when
biased by two electrodes placed on adjacent Ga- and 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,[13] 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-A° ngstrom
resolution (ca. 0.8A ° ). We propose also an improved method to
make such an IDB structure intentionally, which may avoid the
problems with conventional approaches.