G. Peter

The fundamental relationship between radiative lifetime and spectral linewidth of free excitons is demonstrated theoretically and experimentally for quasi 2D excitons in GaAs/AlGaAs quantum wells.Received 24 August 1987DOI:https://doi.org/10.1103/PhysRevLett.59.2337©1987 American Physical Society

A detailed experimental study of the real-space \ensuremath{\Gamma}-X transfer in type-II GaAs/AlAs short-period superlattices and in type-II ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/AlAs multiple-quantum-well structures is presented. Transfer times on a subpicosecond and picosecond time scale are observed. The time constants critically depend on the thickness of the (Al,Ga)As layers, but not on the AlAs-layer thickness in the samples studied. The \ensuremath{\Gamma}-X transfer rate is determined by the spatial overlap of the \ensuremath{\Gamma} and X wave functions confined in the different layers. Intensity- and temperature-dependent measurements provide insight into the scattering mechanism. We conclude that electron--LO-phonon scattering is the dominant scattering process for samples with thick (Al,Ga)As layers (>100 \AA{}). In contrast, interface scattering due to the interface mixing potential (\ensuremath{\Gamma}-${\mathit{X}}_{\mathit{z}}$ mixing) and/or due to potential fluctuations caused by interface roughness (\ensuremath{\Gamma}-${\mathit{X}}_{\mathit{x},}$y mixing) probably dominates for samples with thin (Al,Ga)As layers (35 \AA{}).

The trapping efficiency and trapping dynamics of photoexcited carriers in GaAs/AlGaAs single quantum wells with different confinement structures are examined at low temperature by means of picosecond luminescence as well as photoluminescence excitation spectroscopy. The trapping efficiency is 100% only in graded-index separate confinement heterostructures with a linear band-gap profile. The lower trapping efficiency of other confinement structures is due to radiative and nonradiative recombination in the confinement layers.

This paper reports detailed experimental studies of low-temperature carrier trapping in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As single quantum wells with 5 nm and 1.2 nm thickness, respectively, with different confinement structures. Trapping efficiency and trapping dynamics are studied by means of photoluminescence, photoluminescence excitation spectroscopy, and picosecond luminescence spectroscopy. We obtain trapping efficiencies of about 40% for both the single quantum wells without additional confinement and separate-confinement-heterostructure quantum wells. The percentage of trapped carriers increases to about 60% to 80% for quantum wells cladded by graded-index separate-confinement heterostructures with parabolic band-gap profile. The maximum trapping efficiency of about 100% has been observed for a separate-confinement heterostructure with a linear band-gap profile. Interlayer well width fluctuations are found to be unimportant for the trapping behavior in our samples. Trapping times are appreciably shorter than the rise times of the quantum well photoluminescence, which are between 60 and 100 ps for the different structures. Surface recombination in the 0.2-\ensuremath{\mu}m-thick ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As cladding layer does not reduce the trapping efficiency of the single quantum well without additional confinement compared with the separate-confinement heterostructure. An effective trapping area with about 80 nm width can be deduced on the basis of these results for the quantum-well structures with ungraded cladding layers.