Laser properties and carrier collection in ultrathin quantum-well heterostructures

Stimulated emission from a series of AlGaAs-GaAs single-quantum-well heterostructures is demonstrated for well-widths as thin as 8.5 Å at 22 K. These undoped samples, grown by molecular beam epitaxy, are the thinnest single quantum wells reported to support stimulated emission. Laser thresholds are quite low despite the fact that the single well is undoped and of dimensions that were previously thought to be too small to collect excess carriers effectively (L _z≪scattering path length). It is shown that current models for carrier collection would agree with these unexpected experimental results if the spatial extent of the wave function replaced the well width; that is, carrier collection becomes ineffective when the spatial extend of the wave function≪scattering path length. The simple square well model if sound to predict correctly the experimentally measured energy levels of ultrathin quantum wells (both lattice-matched AlGaAs-GaAs and strained-layer InAs-GaAs). It is shown that under certain conditions accurate solutions for ultrathin wells can be obtained with minimal information about the band structure