Impurity-induced layer disordering of quantum-well heterostructures: discovery and prospects

The circumstances leading to the discovery in 1980 of impurity-induced layer disordering (IILD) of AlAs-GaAs (Al/sub x/Ga/sub 1-x/As) quantum-well heterostructures (QWHs) and superlattices (SLs) are described. In view of the great stability of a QWH or SL (AlAs-GaAs) against ordinary thermal annealing, IILD came as a surprise, i.e., the lower temperature (selective) change from red-gap QW crystal to yellow-gap bulk crystal. Layer disordering can be carried out most effectively, via diffusion or implantation, with two-site dopants such as Zn (acceptor) or Si (donor), but is not restricted to active impurities alone. This maskable planar technology, which (with crystal conservation) transforms a coarser layered III-V "alloy" to a smoother stochastic alloy, and higher bandgap, is capable of forming, as desired, regions that confine carriers and photons. Accordingly, IILD has broad and growing use in optoelectronics (lasers, waveguides, etc.), particularly for III-V systems employing Al and Ga which easily substitute for one another and are sensitive to IILD. The atomic rearrangement of diffusion, a small scale (microscopic) lattice change, is in essence "amplified" by IILD into a large scale (macroscopic) layer change (patterned) that provides a method to study III-V diffusion mechanisms. IILD, a planar technology and growing area of work, is useful in optoelectronics applications as well as for basic diffusion studies in III-Vs and potentially other crystal systems.