Formation of 4H and 8H polytypes in bulk Zn1-xMgxSe crystals

X-ray diffraction, photoluminescence and high-resolution transmission elect ron microscopy (HRTEM) were used to show that several as-grown Zn1-xMgxSe c rystals prepared by the high-pressure Bridgman method exhibit polytypism fo r x in the range from 0.17 to 0.21. The largest contribution of polytypes i s found in two unannealed crystal plates characterised by x=0.17 (polytype 8H) and 0.18 (polytype 4H). Crystal domains with disordered stacking were a lso found. A small quantity of 4H or 8H is detected in other sphalerite typ e crystals of various x values lower than 0.17 and for wurtzite type crysta ls for x from 0.18 up to about 0.25. Exciton transition energy for the give n Mg content depends on the polytype. With Mg content increasing along the crystal, the polytype sequence observed at diffractograms tin agreement wit h HRTEM observations and with luminescence spectral is 3C-->8H-->4H-->2H, T he presence of higher polytypes (4H and 8H) depends on thermal history of t he crystal growth, Annealing of the as-grown crystals containing these two polytypes results in full or almost full transformation to 3C, 2H or their mixture, depending on the Mg concentration.. (C) 1999 Elsevier Science S.A. All rights reserved.