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

Hexagonal bulk II-VI crystals may have advantages over cubic crystals as su bstrates 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) under bar 0] direction and uniaxiality show that the material with 8% Mg has a cubic structure while the 12% and 16% Mg crystals have hexagonal sym metry. These tests indicate that the 16% Mg crystal is more perfectly hexag onal while the 12% Mg crystal contains a high density of stacking faults. T he 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 An gstrom and c(0)/a(0) ratio of 1.634 while the 16% Mg crystal has values of 4.008 Angstrom and 1.640, respectively, consistent with those of ZnSe, Good crystalline quality is also indicated by X-ray theta-2 theta scans and dou ble crystal X-ray rocking curves. The 77K PL spectra show near band-edge em issions of 2.93 eV for the 12% Mg and 3.06 eV for the 16% Mg crystals and a strong emission similar to 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) under bar 0] surfaces of the 12% Mg crystals by MBE. The epitaxial layers exhibit hexagonal symmetry. (C ) 2000 Elsevier Science B.V. All rights reserved.