There has been increasing research interest in nonpolar and semipolar GaN for high brightness lightemitting
application. Due to the very limited supply of GaN bulk substrates, the feasible way of obtaining
large-area nonpolar and semipolar GaN material is still through heteroepitaxy on foreign substrates at
present. This paper highlighted the major challenges in the heteroepitaxy of nonpolar and semipolar GaN
on sapphire and presented the progress in reducing the defect density by a two-step growth technique
according to the in situ optical reflectance and the ex situ x-ray analyses and transmission electron
microscopy measurements. A defect reduction model was proposed based on the correlation between the
morphological evolution and the microstructural development during the two-step growth of nonpolar aplane
GaN. The material research status of nonpolar and semipolar GaN was summarized. A promising
approach (orientation controlled epitaxy) was pointed out for a further improvement of nonpolar and
semipolar GaN material quality.
In addition to future applications in electronics, optoelectronics, and biophotonics, synthesis of nanostructures such as
nanowires, nanorods, and quantum dots offer insights regarding the governing principle of crystal growth that can be
applied to a wide range of mesoscopic phenomena. The basis for understanding the morphology of GaN nanosystems
during epitaxy is the (kinetic) Wulff theorem which incorporates the concept of energy minimization into a set of
geometrical rules depicting shape evolution. An appreciation of the Wulff plot for GaN, a three-dimensional diagram (v-plot)
where the radial distance is proportional to the growth velocity along that direction, not only assists the
interpretation but also facilitates a detailed control of nanoepitaxial processes. To map out the kinetic Wulff diagram, we
carried out selective-area growth (SAG) of GaN on polar, nonpolar, and semipolar surfaces under a wide range of
conditions (temperature, pressure, and V/III ratio). Salient features on the kinetic Wulff plot include cusps, saddle points,
and apexes, which all have implications in shaping the nano-objects. Examples will be given to illustrate the utility of
Wulff plots in explaining the topography of nanorods and quantum dots and in aiding a rational design of GaN nonpolar
and semipolar growth for solid state lighting applications.
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