One potential application for semiconductor lasers is in the illumination of color display devices. In any such device wherein it was desired to provide an accurate color display, it would be necessary to provide at least one semiconductor laser emitting red light, at least one other semiconductor laser emitting green light and at least one further semiconductor laser emitting blue light.
Most commercially available blue-light emitting diode-lasers are made from indium gallium nitride (abbreviated InGaN), a II-V semiconductor having a general formula InxGa1−xN, where x is equal to or greater than 0.0 and less than or equal to 1.0. The lasers can be made to emit at a particular wavelength in a spectral range from about 380 nanometers (nm) in the ultraviolet region of the electromagnetic spectrum to about 460 nm in the blue region of that spectrum by selecting an appropriate value for x.
The blue region of the spectrum is defined as having a spectral range extending from about 425 nm (purplish blue) to about 490 nm (greenish blue). Accordingly, InGaN diode-lasers provide can provide light in only in the shortest 50% of the blue region of the spectrum. It would be advantageous to have a diode-laser capable of emitting light in at least the remaining 50% of the blue region of the spectrum.
Diode-lasers grown from II-VI semiconductor materials such as zinc sulfoselenide ZnSxSe1−x and ZnxCd1−xSe (where x is equal to or greater than 0.0 and less than or equal to 1.0) are capable of providing light at wavelengths in a range from about 460 nm in the blue region of the spectrum to about 530 nm in the green region of the spectrum. These lasers, unfortunately, have been found to have relatively short lifetimes, for example less than 1000 hours. It is generally believed that the short lifetime is due to the growth of color centers in the II-VI material originating from doping sites in the material. Doping of the material is necessary to provide the p and n conductive layers which provide the “diode” of the diode-laser. The color centers develop as a result of the passage of current through the diode-laser. A lifetime of less than 1000 hours is at least an order of magnitude shorter than would typically be required for a diode-laser to be commercially viable. There is a need for a blue-light emitting, II-VI semiconductor laser that does not have the limited lifetime problem of prior-art II-VI semiconductor diode-lasers.