Number 74 (Story #3), April 2, 1992 by Phillip F. Schewe and Ben Stein TYPE II-VI SEMICONDUCTOR device performance is improving. Silicon (residing in column IV of the Periodic Table) does not readily emit light; compounds using column III and V elements (such as GaAs) produce light, but chiefly in the infrared. To get shorter wavelengths in the visible range, scientists have been studying compounds (such as ZnSe) which combine elements from columns II and VI; bandgap energies in these compounds usually exceed 2 eV. Problems with doping and with establishing a suitable crystalline matchup between the light-emitting material (II-VI) and substrate material (III-V) have hindered the development of devices. New epitaxial techniques have partially changed this. For example, type II-VI lasers have operated at power levels up to 700 mW, at temperatures up to room temperature, and at duty cycles (the fraction of time the laser is on) as high as 40%. Light emitting diodes made from type II-VI semiconductors have radiated at wavelengths as short as 490 nm. (Physics World, Mar. 1992.)