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 (n_s) 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 n_s=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 n_s 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 n_s. 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).