The semiconductor laser: a thirty-five-year perspective

Starting in 1962 with GaAs and the alloy GaAsP, the prototype III-V alloy the semiconductor laser developed slowly from a pulse-operated simple p-n junction to a continuously operated (300-K) double heterojunction in 1970. Then, in 1977, in another metamorphosis, it became a quantum-well (QW) device of reduced dimensionality (two dimensional) and improved density of states and performance. The fact that the semiconductor laser is based on an ideal form of “lamp”, a p-n junction of potentially 100% quantum efficiency in conversion of electron-hole pairs to photons, gives it a built-in advantage over all other forms of lasers-all the indirectly excited lasers. Also, being a condensed system, it can be small-smaller than all other forms of lasers-and yet cover a great wavelength and power range, which continue to expand. Because of its third-layer form, a QW heterostructure (the product of more or less easily controlled epitaxial crystal growth) is amenable to modification by relatively simple processing operations such as impurity-induced layer disordering and now “wet” oxidation of Al-based layers, which makes possible a new generation of high-performance vertical-cavity surface emitting lasers (VCSELs). The VCSEL, with its oxide-defined high-Q flat cavity, reduced mode density, and approach now to instant “turn-on” (μA currents), indicates that semiconductor lasers may not become much better than VCSELs, even if there is a change (not certain) in form from two to one to zero dimensional (and, if so, to problems in uniformity and reproducibility). If the technologies of the high-brightness transparent-substrate light emitting diode (LED) and the VCSEL converge much more, it is likely that the VCSEL-LED will become an addressable array (a display), a projection television, or a low- or high-power lamp, perhaps eventually on a large “diameter” Si platform. The study of the semiconductor laser, the first practical QW device, is not complete, nor is its development, which is certain to continue