Recombination emission in inorganic solids

Recombination emission studies in semiconductors have added another and powerful parameter to the study of electronic processes in such solids. In addition they have linked semiconductor problems and their solution to those of high-band-gap photoconducting phosphors, the subjects of much older investigations. This article is concerned with the important features of recombination emission such as spectra for band-to-band transitions of a direct and indirect (phonon-assisted) kind, exciton formation and annihilation either in the free or bound states, radiative transitions via donors, acceptors and other impurities (including those in donor-acceptor pairs), and also intra-band transitions and those involving structural defects such as dislocations. Studies of recombination lifetimes provide a valuable corollary to other techniques for carrier lifetime measurements.

Mechanisms of carrier excitation such as those involving photon absorption, high-energy particles, carrier injection in diode emitters and the reverse-bias impact ionization processes are discussed. The production of coherent radiation by electron, photon or carrier-injection excitation has assumed a very great importance in recent years, for example the gallium arsenide diode laser, and is briefly touched on though the main emphasis is on spontaneous emission and the variety of recombination mechanisms often suppressed in laser emission.

After a discussion of the several theoretical mechanisms for recombination radiation a selection of solids is made and the experimental data on them are reviewed. This selection includes monatomic solids such as diamond, silicon, germanium and tellurium and compounds such as silicon carbide, arsenides, phosphides and antimonides, some of the IIb-VIb compounds like zinc and cadmium sulphides, selenides and tellurides, and finally lead sulphide, selenide and telluride and lead-tin tellurides. In a final section some comments are made on recombination radiation in semiconducting lasers and on the problems of utilizing the high-band-gap visible emitters as carrier-injection sources of radiation.