We report the results of an investigation of the structural properties and relaxation of misfit stress with transmission electron microscopy and x-ray diffraction techniques. Epitaxial films of ZnSe were grown on GaAs by molecular-beam epitaxy of thicknesses ranging from 0.05 to 4.9 μm. The films contain stacking fault defects up to thicknesses of about 150 nm. Above 150 nm perfect misfit dislocations are generated from surface sources and the stacking fault defects to accommodate the lattice mismatch. The majority of dislocations observed are of the 60° type with Lomer edge type dislocations observed in a much lower concentration. The density of misfit dislocations increases with increasing epilayer thickness. Above about 1 μm the films exhibit biaxial tension which we believe is due to thermal expansion differences of ZnSe and GaAs. Good agreement is observed between microscopic and diffraction measurements of the relaxation phenomena.
We present photoluminescence (PL) and PL excitation data for unintentionally doped zinc selenide epilayers grown by molecular-beam epitaxy using ultra-high-purity sources. In particular, we discuss a transition Ir at 22380 cm ' (2.7738 eV) which had been identified as an LO replica of the free excitons in the past. We present data to show that Iy is not related to the formation of free excitons even though it is always observed in samples which show strong freeexciton transitions. Evidence is also presented to show that Ii may be a result of recombinations involving selenium-site-related efects.In this Rapid Communication, we are mainly concerned with the nature of certain low-temperature photoluminescence (PL) features, located below donor-and acceptorbound exciton transition energies, and which are observed especially in those unintentionally doped (n-type) epitaxial ZnSe layers which are grown from ultra-high-purity sources of zinc and selenium. In particular, we are concerned with a transition Jv occurring at 22 380 cm (2.7738 eV) which was previously identified with an LO phonon replica of the free excitons in ZnSe. ' We show that this transition is not related to the recombination processes of either free or bound excitons but, instead, is associated with an independent luminescent transition, possibly involving a defect associated with the selenium sites.
We present low-temperature photoluminescence and transmission electron microscopy data to show that two transitions I0V at ∼2.774 eV and Y0 at ∼2.60 eV, frequently observed in unintentionally doped zinc selenide epitaxial layers, are directly related to structural defects. It is shown that these transitions are strong in those samples which have very low background impurities and high density of structural defects and weak in those cases that have either high background impurities or low density of structural defects.
Stimulated emission at 6 K by optical excitation has been investigated in ZnSe epilayers. A detailed examination of the spectra below threshold indicates that the stimulated emission is due to inelastic exciton-exciton scattering, in contrast with some recent reports that lasing is due to electron-hole plasma luminescence modified by self-absorption. A red shift of the lasing line at high pump intensities has also been modeled in terms of the exciton-exciton and band filling mechanisms, giving excellent agreement with the data.
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