AlN templates with reduced defect densities for optoelectronic devices operating in the ultra-violet spectral range
Fig. 1: ELO-AlN(m) template: cross-sectional dark-field micrograph showing threading dislocations (left); AFM image of the surface with periodic macrosteps.
Fig. 2: Cross-sectional ADF STEM image showing dislocation lines in ELO-AlN(a) template grown on sapphire with an offcut of 0.25° to a-plane. Vertical coalescence boundaries lead to nearly homogeneous dislocation distribution.
Thick AlN template layers with low dislocation density and smooth surfaces are required for fabrication of high-efficiency light emitting diodes (LEDs) operating in the ultra-violet spectral range. One of the methods leading to the necessary reduction of the threading dislocation density is epitaxial lateral overgrowth (ELO). Subsequent growth of thick layers after the ELO process leads to a further reduction of defect density by mutual dislocation annihilation. Besides the favorable decrease in TDD the ELO method allows for growth of crack-free layers with a thickness of a few micrometers compared to the growth on planar sapphire substrates, which is limited to 700 nm. Using the ELO process the FBH achieved an effective defect reduction in AlN templates by three orders of magnitude from 1011 cm-2 down to 108 cm-2. The final dislocation distribution at the AlN surface strongly depends on offcut direction of the sapphire substrate.
In particular, a sapphire offcut of 0.25° to m-plane leads to formation of inclined coalescence boundaries in ELO-AlN(m) templates, which results in an inhomogeneous defect distribution on the layer surface (Fig. 1 left). This defect distribution produces anisotropy in electrical and optical properties of the grown layers, which has to be taken into account during device fabrication. The successful use of these templates for LEDs operating in the wavelength region between 320 nm and 280 nm as well as for solar-blind AlGaN based UV photodetectors has been demonstrated.
The achieved defect reduction is accompanied by a number of challenges arising from the offcut of sapphire substrates. For example, the sapphire offcut and the applied pattern geometry result in formation of periodically arranged macrosteps on the surface of the coalesced ELO-AlN(m) layers (Fig. 1 right). The presence of surface macrosteps can have a deteriorating influence on the quality of subsequently grown AlGaN layers with a high Ga content in terms of increasing defect density and compositional inhomogeneity.
In contrast, the sapphire offcut to the perpendicular direction (to a-plane) leads to formation of vertical coalescence boundaries in AlN, which propagate through the whole layer and lead to a more homogeneous defect distribution on the surface (Fig. 2). These templates exhibit smooth layer morphology and are promising for fabrication of LEDs operating in the wavelength region below 280 nm.
FBH research: 16.06.2014