Charge neutrality in quantum well structures

Abstract

In calculations of recombination rates and optical gain in semiconductors it is necessary to relate the quasi-Fermi level positions for electrons and holes at a given injection level. Usually this is done by requiring the volume densities of electrons and holes to be equal but when this charge neutrality condition is applied to a quantum well a charge imbalance is implied in the barrier regions. A more satisfactory approach is to require the whole active region, comprising wells and barriers, to be neutral overall; nevertheless, the band bending implied by both of these models is inconsistent with the use of spatially constant band edge energies. The authors have explored the range of validity of these two neutrality conditions and their influence upon the calculated gain-current characteristic for a 25 AA wide GaAs quantum well with Al0.35Ga0.65As barriers. While the choice of neutrality condition influences the individual electron and hole densities at a given quasi-Fermi level separation the radiative recombination rate, which depends upon the (np) product, is not significantly changed. Similarly, the gain as a function of spontaneous current is not strongly affected, though the calculation of other recombination currents which depend primarily upon the density of only one type of carrier, such as recombination via traps and Auger recombination, are influenced by the choice of charge neutrality condition.