Beryllium-containing materials for II-VI laser diodes

ABSTRACT Beryllium containing ZnSe-based compound semiconductors introduce substantial additional degrees offreedom for the design of wide gap il-VI heterostructures. Interesting aspects are the lattice matching ofBeTe with its high lying valence band and high p-type dopability for the growth of graded gap contactscapable of carrying high current densities, as well as the expected strengthening of quatemary berylliumcompounds like BeMgZnSe as compared to the il-VI materials used on the basis of ZnMgSSe. They havea large covalency and therefore large bond energy. The covalency of BeSe e.g. is expected to be as high asthe one of GaN. The fabrication of light emitting devices like LEDs and laser diodes is reported. 1.Introduction:Since the first demonstration of ZnSe based blue-green emitters by M.A. Haase et al. in 1991 [1], asubstantial progress has been made concerning the fabrication of such light emitting devices. The developement of ZnSe based laser diodes has essentially been carried on by three main developements: theincorporation of plasma activated nitrogen doping [2,3], the introduction of magnesium for the use ofZnMgSSe as cladding layers in laser diodes[4], as well as the introduction of ZnSe-ZnTe pseudogradingsfor ohmic contact layers to p-type ZnSe [5].In order to increase the lifetime of the laser diodes, first of all the device design had to be optimized. Asthe threshold current densities decreased it became clear that degradation due to grown in defects mainlyoriginating at the GaAs-ZnSe interface and their propagation were responsible for the limited devicelifetime. Subsequently, the epitaxy of the Il-VI growth on GaAs was substantially improved resulting indefect densities as low as i03 cm2 [6]. Such low defect densities clearly demonstrate that the heteroepitaxyof Il-VI on 111-V substrates can be under control to an extent necessary for growing laser diodes, and is nolimiting issue for the quality of the epilayer. Nowadays DC operation at room temperature is possible, andthe lifetime has steadily improved up to values of above 100 h at present [7].An additional approach for improving the lifetime is to strenghen the Il-VI lattice. Recently, C.Verie haschecked the elastic constants of beryllium chalcogenides, concluding that beryllium chalcogenides are