Unintentional doping in GaN.

Dept. of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ.
Physical Chemistry Chemical Physics (Impact Factor: 3.83). 06/2012; 14(27):9558-73. DOI: 10.1039/c2cp40998d
Source: PubMed

ABSTRACT The optimisation of GaN-based electronic and optoelectronic devices requires control over the doping of the material. However, device performance, particular for lateral transport electronic devices, is degraded by the presence of unintentional doping, which for heteroepitaxial GaN layers grown in the polar (0001) orientation is mainly confined to a layer adjacent to the GaN/substrate interface. The use of scanning capacitance microscopy (SCM) has demonstrated that this layer forms due to the high rate of incorporation of gas phase impurities, primarily oxygen, during the early stages of growth, when N-rich semi-polar facets are often present. The presence of such facets leads to additional unintentional doping when defect density reduction strategies involving a three-dimensional growth phase (such as epitaxial lateral overgrowth) are employed. Many semi-polar epitaxial layers, on the other hand, exhibit significant unintentional doping throughout their thickness, except when a three-dimensional growth phase is introduced to aid in defect density reduction resulting in the presence of (0001) and non-polar facets which incorporate less dopant. Non-polar epitaxial samples exhibit behaviour more similar to (0001)-oriented material, but oxygen diffusion from the sapphire substrate along prismatic stacking faults also locally affects the extent of the unintentional doping in this case.

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    ABSTRACT: We investigated the properties of a GaN epilayer grown by metalorganic vapour phase epitaxy on a c-plane bulk GaN substrate obtained by ammonothermal growth. X-ray diffraction measurements showed that the epilayer and substrate were fully relaxed, had a miscut angle of 0.3±0.05° towards m and had omega rocking curve width values of 20-30 arcsec, limited by the instrumental broadening. Scanning capacitance microscopy data of the sample in cross-section indicated that the substrate had n-type conductivity with a carrier concentration of at least 1019 cm-3. Combined optical Nomarski microscopy, atomic-force microscopy and scanning electron microscope-cathodoluminescence studies showed the presence of large hexagonal pyramids on the surface, each associated with one or two dislocations with a screw-component threading from the substrate. This observation leads us to calculate a lower limit of the threading dislocation density of 3×102 cm-2. We predict that the formation of such hexagonal hillocks during epitaxy can be avoided with a slightly larger miscut angle of 0.4° or 0.5°. Another type of defect observed were ridge-like surface structures with narrow arrays of edge-type threading defects with a local density of 109 cm-2. However, the absence of threading defects below the regrowth interface at a ridge suggested that this type of structure is linked to (polishing) damage to the substrate surface and is therefore rated as an avoidable problem.
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