Optical Micro-Characterization of Complex GaN Structures

Abstract

Various techniques of pattern-controlled epitaxy like epitaxial lateral overgrowth (ELO) and lateral seeding epitaxy have successfully been used to overcome the lattice mismatch problem, however, resulting in a complex system of self-organized growth domains. For a detailed understanding a correlation of the structural, electronic, and optical properties on a micro-scale is mandatory. Scanning cathodoluminescence (CL) microscopy provides a powerful tool compiling low temperatures, spatial resolution Dx < 45 nm and high spectral resolution. ELO GaN on stripe patterned SiO 2 and W masks were characterized by CL microscopy, directly visualizing the formation of different growth domains: 1. the coherently grown regions between the stripes; 2. the ELO areas coalescing on the center of the masks and showing a blue shifted and strongly broadened CL where m-Raman evidences the reduction of local strain and probes the free carrier concentration which is dramatically changed over the masks; 3. the coalescence regions with poor epitaxial perfection including voids and strong incorporation of impurities; 4. the transition regions between 1. and 2. at the very edges of the masks characterized by high defect density and strongly blue shifted CL. Plan view CL directly images the local areas of improved crystal perfection only involving part of the total ELO regions. The formation of specific micro-domains in 5 mm thick crack-free Al 0.19 Ga 0.81 N, exhibiting different Al concentrations, during the initial stage of MOVPE grown on patterned GaN substrates and the transition to finally homogeneous growth is directly visualized by cross-sectional CL.Introduction For a detailed understanding of complex semiconductor heterostructures and the physics of devices based on them a systematic determination and correlation of the structural, chemical, electronic, and optical properties on a micro-or nanoscale is essential. Luminescence techniques belong to the most sensitive, non-destructive methods of analyzing both, intrinsic and extrinsic semiconductor properties. Combining the luminescence spectroscopy with the high spatial resolution of a scanning electron microscope (SEM), realized by the technique of cathodoluminescence (CL) microscopy, provides a powerful tool for the nano-characterization of semiconductors, their heterostructures as well as their interfaces [1].Employing the technique of Epitaxial Lateral Overgrowth (ELO) to the group-III nitrides has been proven successful in significantly reducing the concentration of threading dislocations emanating from the underlying buffer layer. The ELO approach consists of masking parts of the defective crystalline substrate GaN "seed" layer with an amorphous layer preventing the dislocations from propagating into the overlayer during subsequent re-growth. While impurities are unintentional incorporated in the initial stages of ELOG, the biaxial strain and defect concentration are reduced on the top of the mask [2][3][4][5].The maskless heteroepitaxy on pre-patterned substrates has been pr...

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A -plane GaN epitaxial lateral overgrowth structures: Growth domains, morphological defects, and impurity incorporation directly imaged by cathodoluminescence microscopy

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