The effects of free carriers on the photoluminescence and photoluminescence excitation spectra of InGaAs-InP quantum wells

Abstract

Photoluminescence (PL) and photoluminescence excitation (PLE) spectra have been studied as a function of sheet electron density (ns) from 0 to 1.4*1011 cm-2 in a Schottky-gated 110 AA wide InGaAs-lnP quantum well grown by solid-source molecular beam epitaxy. It is shown that there are major free-carrier effects on both the PL and PLE spectra, even at these relatively low electron densities. In the PL spectra, a high energy cut-off at the electron Fermi energy is observed at ns=1.4*1011 cm-2. With decreasing carrier density the line narrows from the high-energy side as expected for free-carrier broadening. For temperatures above 10 K, a defect-related peak is observed below the exciton PL line (X), at an energy separation below X increasing from 11 meV at 15 K to 18 meV at 35 K. It is tentatively ascribed to electron recombination with neutral acceptors, with a range of binding energies corresponding to locations from the well edge to well centre. The PLE spectra in the region of the n=1 exciton transition show a marked decrease in oscillator strength and broadening for ns approximately=1011 cm-2. These observations are discussed in terms of phase space filling and exchange screening effects, and broadening of the transitions by the occurrence of many electron Auger processes at finite ns. In the presence of free carriers, the many body nature of the PLE transitions at the Fermi energy is demonstrated by the anomalous temperature dependence of the n=1 heavy-hole 'exciton' transition, as reported recently for GaAs-GaAlAs modulation-doped QWS by Lee and co-workers (1987) and Livescu and co-workers (1988).