Trapping of carriers in single quantum wells with different configurations of the confinement layers

This paper reports detailed experimental studies of low-temperature carrier trapping in GaAs/${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$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-μm-thick ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$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.