Direct evidence for selective impurity incorporation at the crystal domain boundaries in epitaxial ZnO layers

Abstract

A direct correlation of structural properties with the spatial distribution of bound exciton luminescence in ZnO epitaxial layers has been achieved on a microscopic scale using highly spatially and spectrally resolved cathodoluminescence. The morphology of the high quality ZnO layer is characterized by a distinct domain structure. While the laterally integrated cathodoluminescence spectrum shows narrow (full width at half maximum <3meV) I8 luminescence, a pronounced emission line at I0∕I1 emerges in the local spectra taken at domain boundaries. In complete contrast, no I0∕I1 emission is found inside the domains. Monochromatic images further evidence the selective incorporation of impurities at the grain boundaries of domains. Micro mappings of the I8 peak wavelength directly visualize the strain relaxation across the domains toward their very center, where a drop in quantum efficiency indicates enhanced defect concentration.

Zinc Oxide Nanoparticles with Defects

“…The same observation has been reported for ZnO polycrystalline films studied by cathodoluminescence. [49] In accordance with these literature data, in our ZnO samples some UV and ªgreenº luminescence is observed only after annealing of the majority of defects at temperatures as high as 800 C. This result is an additional illustration that the sum of defects introduced by the kinetically controlled, low-temperature preparation of ZnO from organometallic precursors changes the optical properties of the resulting nanoparticles significantly. In this context, the intentional introduction of additional impurities has already been discussed previously.…”

Electrical and optical properties of ZnO films grown by molecular beam epitaxy

“…4. The variation of the NBE/DL with the growth temperature can be understood as follows: Just as we have noted, the density of grain boundary in the ZnO films decreases with increasing the growth temperature, and impurities and/or defects have a preferential accumulation at the domain boundaries [10,14]. These impurities or defects will capture carriers, thus quench the NBE emission and enhance the DL emission.…”

“…The crystallites are roughly hexagonal-shaped, note that the hexagonal patterns on the surface are an aid to see the shape of the grains. Such hexagonal crystallites are thought to be individual ZnO columns [14]. The crystallites are almost undetectable when the growth temperature reaches 800 and 900 8C.…”

Compared optical properties of ZnO heteroepitaxial, homoepitaxial 2D layers and nanowires

“…two orders of magnitude lower than the dislocation density [4]. A possible explanation relies on the columnar structure of the ZnO epilayer when grown on heterosubstrate [7]. The sub-grain boundaries between columns are composed of threading dislocations.…”