Abstract

Quantum wells (QWs) exposed to electric fields perpendicular to the layers may significantly change their optical properties.¹⁻³ In this paper we report low temperature picosecond photoluminescence studies for GaAs/Al_0.3Ga_0.7As QWs as a function of the electric field strength for four different nominal thicknesses of 5nm, 10nm, 20nm and 28nm. We provide for the first time a consistent picture of the photoluminescence properties in the quantum-confined Stark regime. Time-resolved experiments were performed with a synchroscan streak camera and a synchronously mode-locked dye laser ( photon energy 1.675eV, intensity 5×10¹² photons per cm² and pulse). Details of the sample preparation and experimental parameters are given in Ref.¹. Time-integrated photoluminescence spectra exhibit a significant low-energy shift of the photoluminescence peak with increasing electric field (Fig.1). Numerical calculations of the Stark-shift in the single particle picture for the finite quantum well (solid lines) give excellent agreement with the experimental points, if slightly smaller values of the QW thicknesses are assumed (4nm, 8nm, 16nm and 20nm thicknesses, respectively). For the calculation the effective electron and hole masses are taken from Ref.² for a conduction band discontinuity of 0.57. Figure 1 (r.s.) displays the corresponding photoluminescence lifetimes as deduced from our measured streak curves. The times are normalized to the values at zero electric field, which are 180ps, 290ps, 660ps and 950ps for the 5nm, 10nm, 20nm and 28nm quantum well. The strong field-induced lifetime enhancement up to a factor of 100 demonstrates efficient charge separation in the wider quantum wells and the corresponding decrease in electron-hole wavefunction overlap. It is excellently described by the solid lines which are calculated with the same theoretical model used in Fig.1 (l.s.) for the energy shift. A sharp decrease of the photoluminescence lifetime is found for the 5nm quantum well at fields above 130kV/cm and is attributed to quantum-mechanical tunneling. Obviously, this tunneling is most important for the thinner QW due to the higher subband energy.

© 1986 Optical Society of America