IDENTIFICATION OF BAND-EDGE OPTICAL-TRANSITION TYPES IN TENSILE-STRAINED QUANTUM-WELLS

Either electron-to-light-hole e-lh or electron-to-heavy-hole e-hh transitions can define the optical gap in tensile strained quantum wells. Transition types near the band-edge must thus be determined experimentally for these structures, as the lowest energy transition can not be assumed to be of the e-hh type as in unstrained and compressively strained structures. We have developed a new characterization technique based on photoluminescence excitation spectroscopy (PLE) which allows simple and direct identification of band-edge transition types in quantum wells. A ''ratio curve'' is generated by pointwise division of one PLE spectrum by a second PLE spectrum. The two PLE spectra are obtained using orthogonal polarizations of the excitation beam, which is incident on the sample surface at an oblique angle, and the transition types near the band edge are identified by simple visual inspection of the ratio curve. In this paper, we describe and assess the theoretical foundation for our ratio method, detail our experimental procedure, present PLE spectra and ratio curves for several quantum wells, and determine optimum experimental conditions and the physical origin of features in the ratio curve through investigation of the influence of several experimental parameters.