Luminescence in crystals of zinc sulpho -selenide

Abstract

Zinc selenide and zinc sulphide are wide band-gap II-VI compound semiconductors which are capable of emitting visible luminescence. These two compounds form a series of solid solutions throughout the whole range of composition, with band-gaps ranging from 2.7 eV in ZnSe to 3.6 eV in ZnS at 300 K. The main purpose of the research reported in this thesis was to study the luminescence of Zn(S,Se) crystals grown in the department, with the eventual aim of producing light-emitting diodes capable of covering a colour range from the red to the blue. The undoped mixed crystals show two emission bands, whereas ZnSe and ZnS emit single self-activated bands. These emission bands are interpreted as being due to the presence of two distinct self-activated centres in the mixed crystals. It is concluded that the self-activated centre in ZnSe is not directly comparable to the centre in ZnS. Several samples were doped with copper during the growth. These crystals showed two separate emission bands throughout the range of composition which shift from the red (6450 Ǻ) and the green (5350 Ǻ) in ZnSe through to the green (5320 Ǻ) and the blue (4400 Ǻ) in ZnS. There is a continuous shift in the position of the peaks with varying composition; this contrasts with other reports for the copper emission. Samples were doped with manganese since this produces an efficient luminescence centre in ZnSe and ZnS. The Mn(^2+) emission occurs near 5860 Ǻ with a half-width of 0.15 eV at 85 K throughout the range of composition of the mixed alloys. This has been established by using 5300 Ǻ radiation in the lowest energy excitation band to stimulate the luminescence. The Mn(^2+) emission is quenched in ZnSe following treatment in zinc/copper and the red copper emission predominates. The copper emission in such a sample is excited by characteristic manganese excitation radiation, and it is suggested that the Mn(^2+) emission may be quenched by a de-excitation to a copper centre.