Characterization of Zn-(1-y)MgySxSe(1-x) epilayers grown by low-pressure metalorganic vapour phase epitaxy

Optical properties of Zn1-yMgySxSe1-x epilayers grown on GaAs substrates by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE) were investigated with photoluminescence spectroscopy, reflection spectroscopy and X-ray diffraction. The sulphur (x) and magnesium (y) content in the samples measured by EPMA (electron probe micro analysis) was varied in the range 0 less than or equal to x, y less than or equal to 0.25, respectively. The photoluminescence study of these quaternary epilayers was performed in the temperature range between 14 and 300 K. Low and negligible concentration of deep levels was achieved. Comparison of the measured halfwidths of the band edge emissions with calculated minimum alloy broadening values allowed the estimation of the composition fluctuation of the majority of the epilayers to be less than 1%. However, at large x or large y (x, y greater than or equal to 0.15) broadening in the band edge emission indicates an increased composition deviation from randomness. Bandgap reduction up to 75 meV with respect to the theoretically expected bandgap in the case of total randomness was observed. Enhanced phase separation and ordering effects especially at large x or large y seem to be the reason for the emission broadening as well as for the bandgap reduction.