Abstract
Non-polar a-plane gallium nitride (GaN) films have been grown on r-plane (11¯02) sapphire by metal organic vapour phase epitaxy (MOVPE). A total of five in situ defect reduction techniques for a-plane GaN are compared, including two variants with a low temperature GaN nucleation layer (LTNL) and three variants without LTNL, in which the high-temperature growth of GaN is performed directly on the sapphire using various crystallite sizes. The material quality is investigated by photoluminescence (PL), X-ray diffraction, cathodoluminescence, atomic force and optical microscopy. It is found that all layers are anisotropically strained with threading dislocation densities over 109cm−2. The PL spectrum is typically dominated by emission from basal plane stacking faults. Overall, growth techniques without LTNL do not yield any particular improvement and even result in the creation of new defects, i.e. inversion domains, which are seldom observed if a low temperature GaN nucleation layer is used. The best growth method uses a LTNL combined with a single silicon nitride interlayer.
Highlights
Hexagonal gallium nitride (GaN) presents two stable growth directions inclined at 901 angle to the c-direction
We have studied the quality of a-plane GaN layers grown on r-plane sapphire by different metal organic vapour phase epitaxy (MOVPE) methods
We find that the presence of a GaN low temperature nucleation layer (LTNL) or its absence does not alter significantly the final material quality in terms of 1D or 2D extended defects: threading dislocations densities (TDDs) are in the low 109 cmÀ2 regime and basal plane staking faults densities are greater than 105 cmÀ1
Summary
Hexagonal gallium nitride (GaN) presents two stable growth directions inclined at 901 angle to the c-direction. For (Al)(In)GaN quantum wells or dots grown on these planes, the quantum confined Stark effect is theoretically prevented, leading to shorter radiative lifetimes [4] and a potentially higher efficiency [5]. In-plane anisotropic strain is present in a-plane or m-plane non-polar heterostructures, due to the different lattice mismatch respectively along the in-plane c and m, or c and a directions. This effectively lifts the spatial degeneracy of the valence subbands [6] and results in the emission of linearly polarised light for a-plane [7] and m-plane [8] heterostructures
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