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 q uasi-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 e mission spectra of optically or electrically biased QWs. Emission spec tra are calibrated by comparison with measurements of the limiting pho tocurrent. A theoretical model is then used to separate out the effect s of carrier generation rate and field-dependent QW absorption. We app ly the method to the low-temperature photoluminescence spectra of a se t 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 emissio n 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 indepe ndent measurements of low-temperature de photocurrent, and with a simp le 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 Insti tute of Physics.