Excitons in ultrathin InAs/InP quantum wells: Interplay between extended and localized states
Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films
We have performed detailed optical measurements of ultrathin InAs/InP quantum wells grown by metalorganic vapor phase epitaxy. Absorption and photoluminescence excitation spectra reveal the excitonic resonances associated with two-and three-monolayer-thick InAs layers. Photoluminescence spectra also show an emission band at intermediate photon energies which is associated with excitons localized in thin InAs quantum dots. Polarization-dependent measurements clearly show the heavy-hole or
... eavy-hole or light-hole nature of the quantum well resonances. Such an identification of both type of transitions provides a test for electronic structure models. We find that the energy positions of the excitonic transitions in ultrathin InAs/InP quantum wells are not consistent with calculations based on the envelope function model. Ultrathin InAs-based quantum wells ͑QW͒ are characterized by high radiative efficiencies and thus have significant potential for optoelectronic devices. 1,2 Moreover, since their optical properties are strongly affected by the presence of monolayer width fluctuations, they can serve as test structures to study exciton localization. 3-12 However, the electronic states in ultrathin QWs are still not well understood. They are usually described with effective mass models within the envelope-function approximation ͑EFA͒. But EFA is highly questionable in the case of QWs which are only a few monolayers ͑ML͒ thick, 13 as indicated by recent tightbinding calculations on the InAs/GaAs system which depart significantly from the EFA approach. 14 Nevertheless, all experimental results on InAs ultrathin QWs have been interpreted to date using EFA models. In the case of the InAs/ GaAs system, the energy position of both heavy-hole ͑hh͒ and light-hole ͑lh͒ excitonic resonances agree with the predictions of EFA models. 4,7,12 However, because of the small value of the critical thickness induced by the high lattice mismatch between InAs and GaAs, these measurements were performed in the limited range of 0-1.6 ML. In this respect, the InAs/InP system is more interesting since InAs layers can in principle be grown pseudomorphically on InP with thicknesses of up to eight MLs. The photoluminescence ͑PL͒ spectra of this system generally show multiple lines which are associated with InAs layers of integer ML thicknesses. 3, 6, 10, 11, 15, 16 The energy position of heavy-hole excitonic transitions deduced from the PL spectra can also be well reproduced by EFA. Emission lines in PL spectra give, however, only a first approximation to the position of excitonic transitions since interface roughness can result in significant Stokes shift in ultrathin QWs. In this article, we present detailed optical measurements of InAs/InP single QWs. The samples were grown by lowpressure metallorganic vapor phase epitaxy on ͑001͒oriented semi-insulating InP:Fe. The growth procedure involved the deposition of InAs layers between a 300-nm-thick InP buffer and a 120-nm-thick InP cap layer. Tertiarybutylarsine ͑TBAs͒, trimethylindium ͑TMI͒, and pure phosphine were used as source materials. All the samples were grown under a reactor pressure of 40 Torr and at a substrate temperature of 600°C. More details of the system and growth procedure have been reported in Refs. 17 and 18. The thickness of the InAs layers can be accurately determined from the relative positions between the main InP Bragg peak and the Pendellösung fringes observed in highresolution x-ray-diffraction ͑HRXRD͒ measurements. 4, 6, 19 HRXRD was performed with a Philips five-crystal diffractometer. The ͑004͒ rocking curves were analyzed assuming a tetragonal deformation 19, 20 and no relaxation of InAs. They gave average thicknesses of 0.7Ϯ0.1 nm for sample A and 0.8Ϯ0.1 nm for sample B, in agreement with the values expected from the growth conditions. Within the continuum elastic theory, the InAs lattice parameter along the growth axis is 0.626 nm, yielding nominal thicknesses of 2.2Ϯ0.3 and 2.6Ϯ0.3 MLs for samples A and B, respectively. For the optical measurements, the samples were mounted strain free in a helium flow cryostat and cooled to 5 K. The PL and PLE spectra were obtained using a Ti:sapphire laser tunable in the range 1.18-1.38 eV. It thus allowed us to study the excitonic resonances of InAs QWs having thick-a͒ Author to whom correspondence should be addressed; electronic mail: firstname.lastname@example.org. 956 956 J.