Reduction of threading dislocation densities in AlN∕sapphire epilayers driven by growth mode modification

Abstract

A strategy to reduce the density of threading dislocations (TDs) in AlN epilayers grown on sapphire substrates is reported. The TDs experience a redirection of their line orientation which is found to coincide with imposed increases in both of V/III ratio and overall flux rate leading to the formation of an internal subinterface delineated by the changes in dislocation orientation. Threading dislocations either experience large kinks and then redirect into threading orientation or form dipole half loops via annihilation of redirected threading segments of opposite sign with the latter leading to a significant dislocation density reduction. These phenomena can be accounted for by a transition of growth mode from atomic step flow to two-dimensional layer-by-layer growth which accompanies the imposed changes in V/III ratio and flux. As this occurs, macrosteps (several atomic layers thick) laterally overgrow pre-existing dislocation outcrops. Image forces initiate the redirection processes and create trailing segments parallel to the interface between the advancing macrostep and the surface outcrop. This horizontal segment can be forced to redirect into threading orientation should another macrostep traveling in the opposite direction be encountered. Image forces again nucleate the redirected segment which is then replicated as the crystal grows. A dipole half loop will form if two dislocations with opposite sign are redirected so as to meet each other.

Structural and spectroscopic properties of AlN layers grown by MOVPE

“…In recent years, various growth techniques have been reported by several groups [6][7][8][9][10]. Nonetheless, still lot of ambiguities are prevailing in the relation between basic epitaxial parameters, the respective growth kinetics and resulting crystal quality of AlN layers.…”

Evolution of threading screw dislocation conversion during solution growth of 4H-SiC

“…The macrosteps formed on the growth surface have been reported to be attributed to the threading dislocation conversion. 7,17,18 Lateral growth of macrosteps would lead to the TSD conversion during the solution growth of SiC. 7 Figure 3 shows the X-ray topography image of the seed crystal and the grown crystal taken at the same position.…”

GaN:-Mg grown by MOVPE: Structural properties and their effect on the electronic and optical behavior

“…2D or 3D nucleation can crop the TDs, whereas if the delta layers add in an atomic step-flow mode, their density will not be reduced. 32 At our growth conditions, the Mg dopants tend to occupy the Ga sites without any nucleation, consequently the TDs are not reduced and for this reason the improved morphology of the GaN:δ-Mg samples must be not TDrelated. TDs in GaN, acting as charging centers and gathering Ga vacancies, are likely to attract the Mg ions.…”