Large excited state Stokes shift in crescent-shaped AlGaAs/GaAs quantum wires

Xue-Lun Wang, Mutsuo Ogura, Hirofumi Matsuhata, Ali Hamoudi
1997 Applied Physics Letters  
The Stokes shifts of the ground and the excited states in a crescent-shaped AlGaAs/GaAs quantum wire ͑QWR͒ are investigated using photoluminescence ͑PL͒ and PL excitation spectroscopy. The first excited electron to heavy-hole transition showed a Stokes shift ͑ϳ17 meV͒ considerably larger than that of ground state-related transitions ͑ϳ4 meV͒. This is a quite different phenomenon than that observed in two dimensional quantum well structures, and can be explained by the spatial separation of wave
more » ... functions with different confinement energies in crescent-shaped QWRs. It is well known that the heterointerface quality has a severe influence on the performance of optical and electronic devices involving semiconductor heterostructures. The heterointerface inhomogeneities of semiconductor quantum structures can usually be detected as a peak energy difference between photoluminescence ͑PL͒ and PL excitation ͑PLE͒ or optical absorption spectra, i.e., the Stokes shift. 1 On the other hand, the information about the excited states of semiconductor quantum structures is very important for devices utilizing excited state transitions, for example the recently developed quantum cascade laser. 2 In two dimensional quantum wells ͑QWLs͒, the Stokes shifts for quantum states with different confinement energies mainly depend on the variation of confinement energy with well thickness since the wave functions of the ground and the excited states extend over the same heterointerfaces. However, in the cases of quantum wire ͑QWR͒ and quantum dot ͑QD͒ structures, 3 which are objects currently under intensive investigation, the situation could be much different. Most of the QWRs and QDs fabricated by the conventional growth techniques have an irregular cross-sectional shape 4,5 which will cause spatial separation of wave functions for the ground and the excited states. This fact means that the wave functions of the ground and the excited states in QWRs and QDs could be exposed to heterointerfaces with different structural quality. In this letter, we show that the spatially separated wave function distribution can result in a large difference in the Stokes shift between the ground and the excited states in crescent-shaped AlGaAs/GaAs QWRs. The sample used here is a 4.5-nm-thick single QWR grown on a 4.8 m pitch V-grooved GaAs substrate by metalorganic vapor phase epitaxy ͑MOVPE͒ at 630°C. The GaAs wire layer was grown by flow rate modulation epitaxy ͑FME͒, a modified MOVPE growth technique, which was found to have superior low temperature growth selectivity and layer thickness controllability compared with the conventional MOVPE selective growth. 5,6 The sample structure has been described in a previous publication. 7 Figure 1͑a͒ shows the high resolution transmission electron microscopy ͑HRTEM͒ image of a vertically stacked multiple QWR sample which was grown under the same conditions as the
doi:10.1063/1.119358 fatcat:6e55pob365dpvmslqyvcdlfp34