Understanding the relation between surface morphology
during epitaxy
of GaN:Si and its electrical properties is important from both the
fundamental and application perspectives. This work evidences the
formation of nanostars in highly doped GaN:Si layers with doping level
ranging from 5 × 1019to 1 × 1020cm–3grown by plasma-assisted molecular beam epitaxy (PAMBE).
Nanostars are 50-nm-wide platelets arranged in six-fold symmetry around
the [0001] axis and have different electrical properties from the
surrounding layer. Nanostars are formed in highly doped GaN:Si layers
due to the enhanced growth rate along thea-direction
⟨112̅0⟩. Then, the hexagonal-shaped growth spirals,
typically observed in GaN grown on GaN/sapphire templates, develop
distinct arms that extend in thea-direction ⟨112̅0⟩.
The nanostar surface morphology is reflected in the inhomogeneity
of electrical properties at the nanoscale as evidenced in this work.
Complementary techniques such as electrochemical etching (ECE), atomic
force microscopy (AFM), and scanning spreading resistance microscopy
(SSRM) are used to link the morphology and conductivity variations
across the surface. Additionally, transmission electron microscopy
(TEM) studies with high spatial resolution composition mapping by
energy-dispersive X-ray spectroscopy (EDX) confirmed about 10% lower
incorporation of Si in the hillock arms than in the layer. However,
the lower Si content in the nanostars cannot solely be responsible
for the fact that they are not etched in ECE. The compensation mechanism
in the nanostars observed in GaN:Si is discussed to be an additional
contribution to the local decrease in conductivity at the nanoscale.