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Abstract
A low-temperature (4–2 K) photoluminescence (PL) analysis combined with transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) microanalysis study was conducted on samples of ZnSe which were grown at very low growth rates on GaAs (100) substrates by molecular beam epitaxy and exhibited unusual surface colouring effects. PL measurements from bluish-coloured areas showed that the material was optically ‘dead’. However, reddish-coloured areas, which were found to exist in close proximity to the blue areas, exhibited strong dominant free-exciton-related emission centred at 2-8007 eV.
This study revealed that the differently coloured regions of ZnSe epilayer surfaces and the drastically different PL which results from these areas were associated with particles formed at the free surface of the epilayer. A correlation between the PL, TEM and EDX results was established. The crystallographic structure of the particles was different from that of the host ZnSe film, and they were enriched in Ga and depleted in Zn, producing a Se-rich stoichiometry. Thus, anomalously small values of Zn-to-Se ratios (0·40 ± 0·20) were recorded from areas on the particles, this ratio being 1·3 ± 0·08 for the reddish-coloured areas. The Ga-to-As ratios were 1·90 ± 0·19 from the areas on the particles, and 1·30 ± 0·10 in the reddish-coloured areas.
Enhanced segregation along preferential paths, occurring at the very slow growth rates employed in this study, is suggested as a possible cause for the observed phenomenon.