Determination of the quasi-fermi-level separation in single-quantum-well p-i-n diodes

The radiative behavior of quantum-well (QW) devices depends upon the quasi-Fermi-level separation Delta E(f) induced in the quantum well. We present a method of obtaining Delta E(f) in absolute units from the emission spectra of optically or electrically biased QWs. Emission spectra are calibrated by comparison with measurements of the limiting photocurrent. A theoretical model is then used to separate out the effects of carrier generation rate and field-dependent QW absorption. We apply the method to the low-temperature photoluminescence spectra of a set of single QW p-i-n photodiodes at different electric fields. We show that modeled emission spectra agree closely with measured spectra in flatband conditions. We also observe a field-dependent loss in emission intensity-leading to a reduction in Delta E(f) of several meV-which we attribute mainly to carrier escape from the QW. The derived values for field-dependent nonradiative efficiency are consistent with independent measurements of low-temperature de photocurrent, and with a simple model for thermally assisted carrier escape, We show how the method can be applied to electroluminescence spectra in order to investigate the dependence of Delta E(f) on applied bias. (C) 1996 American Institute of Physics.