Recombination processes associated with “Deep states” in gallium phosphide

Abstract

“Deep” bound states introduced by impurities in direct gap semiconductors like GaP greatly enhance the oscillator strength for the radiative recombination of electrons and holes. The quantum efficiency for luminescence associated with recombinations at such deep states may be high, even at 300° K, since these transitions may compete effectively with nonradiative processes at other inadvertently present impurities or native lattice defects. Other characteristics of deep states are (a) strong phonon cooperation for the electronic transitions they induce and (b) large binding energies due to the large positive central cell interaction responsible for the other properties. Isoelectronic traps may possess small binding energies and yet induce a large oscillator strength for near-interband transitions in an indirect semiconductor, with moderate phonon cooperation compared to conventional deep states. These differences are due to the relatively large proportion of the wave function confined near the central cell for a particle trapped by short range forces at a neutral center. These properties are of great importance for the design of efficient semiconductor diode lamps. Shallow isoelectronic traps introduce large scattering cross-sections for free excitons or free electron-hole pairs. This effect may be detected through studies of electrical transport or, more sensitively, through the associated features in optical absorption spectra. Cross-sections induced by N, As, Sb, Bi and S vary qualitatively as expected from these considerations, provided that an enhancement factor for electron scattering due to the form of the band structure of GaP is considered. Some properties of conventional deep impurity states due to the O donor and Si acceptor are contrasted with those of a shallow isoelectronic trap such as N and Bi.