Growth and characterization of hexagonal (Zn,Mg)(S,Se) bulk substrates

Abstract

Hexagonal bulk II–VI crystals may have advantages over cubic crystals as substrates for producing long lifetime blue laser diodes. Quaternary (Zn,Mg)(S,Se) bulk crystals with Mg composition of 8%, 12% and 16% have been grown by zone melting techniques. Optical tests of birefringence along the [1 1 2 0] direction and uniaxiality show that the material with 8% Mg has a cubic structure while the 12% and 16% Mg crystals have hexagonal symmetry. These tests indicate that the 16% Mg crystal is more perfectly hexagonal while the 12% Mg crystal contains a high density of stacking faults. The hexagonal structure was confirmed by TEM. From powder X-ray diffraction we estimate that the 12% Mg crystal has a lattice constant a 0 of 4.013 Å and c 0/ a 0 ratio of 1.634 while the 16% Mg crystal has values of 4.008 Å and 1.640, respectively, consistent with those of ZnSe. Good crystalline quality is also indicated by X-ray θ–2 θ scans and double crystal X-ray rocking curves. The 77K PL spectra show near band-edge emissions of 2.93 eV for the 12% Mg and 3.06 eV for the 16% Mg crystals and a strong emission ∼90 meV below the band gap, proposed to be a DAP transition. Single layers of ZnSe and ZnCdSe/ZnSe quantum wells have been grown on cleaved or cut and polished [1 1 2 0] surfaces of the 12% Mg crystals by MBE. The epitaxial layers exhibit hexagonal symmetry.