Heterojunction injection lasers (review)

Considerable success has been achieved in semiconductor lasers by the use of hetero junctions. This use was first suggested back in 1963, soon after the development of the injection laser. However, the advantages of the heterojunction lasers were realized only after the mastering of the epitaxial growth of multilayer structures based on (Al,Ga)As solid solutions. The superiority of the heterojunctions formed in this system is due to the fact that these solid solutions, including pure gallium arsenide, have practically identical crystal lattices irrespective of their composition. This is a consequence of the similarity of the covalent radii of gallium and aluminum. In this way a series of solid solutions is formed in which the interchange of gallium and aluminum results only in small changes in the lattice constant, whereas the forbidden-band width varies quite strongly. Two solid-solution systems – (Al,Ga)As and (Al,Ga)(As,P) – have been used successfully in injection lasers. Heterostructures suitable for injection laser applications can be built from one or several heterojunction and p–n junctions. Double-sided heterojunction structures of the n(Al,Ga)As–pGaAs–p(Al,Ga)As type are characterized by the lowest ever room-temperature values of the threshold current density (less than 1 kA/cm²). This has made it possible to achieve continuous emission from injection lasers at room temperature and up to 355°K. The differential efficiency of uncooled heterojunction lasers has recently been raised to 0.7.