Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

eV stems from recombining carriers confined in the unmodulated region ofthe near-surface QW, whereas the peak at 1.65 is due to recombination ofcarriers in the SIQDs. Due to the strain imposed on the QW region under theInP S-K islands, the QW is expanded, leading to a decrease of the bandgapenergy and the formation of SIQDs in the QW region. Luminescence from theGaAs substrate and/or the buffer layer appears at and below 1.5 eV. The sizeand shape distribution of the stressors leads to a corresponding distribution ofquantized states within the SIQDs; this results in a rather broad luminescenceline. The full width at half-maximum (FWHM) of the SIQD peak is 43.5 meV.The QW peak has a FWHM of 32.8 meV, which we believe is due to thicknessfluctuations within the QW.The spectrum of a single SIQD at low excitation intensity is compared tothe multidot spectrum in Fig. 14b. The single-dot spectrum consists of severallines, two of which are most prominent and very narrow. The FWHM of thelowest-energy line at 1.645 eV is 1.6 meV, and the single most intense higherenergypeak, at 1.66 eV, has a FWHM of 0.6 meV. The emission energies ofthe QD fall well within the range shown by the averaged spectrum for emissionfrom multiple QDs. It is unlikely that the single SIQD under investigationwas actually a closely spaced double dot, because the area from which thePL for this spectrum is collected is much smaller than the imaged size of asingle SIQD. Thus, the higher-energy peaks are due to recombination fromcarriers occupying excited states of the same SIQD.The PL of single SIQDs shows blinking of an unusual type. Not onlydoes the ground-state PL peak blink but also the excited state shows a temporalintensity modulation, which is phase shifted to the ground state by 180j.This effect is called two-color blinking (TCB).To illustrate the TCB, a series of spectra from a single SIQD underconstant experimental conditions were taken consecutively about every secondwithout changing any parameter. The result is shown in Fig. 15, where theright-hand panel shows spectra at four different times as indicated and the leftside shows a gray-scale plot of the intensity as a function of energy and time.In Fig. 15b, all spectra have been vertically offset to enhance clarity. Thefirst spectrum shows the initial luminescence detected from the SIQD with themain peak at 1.656 eV (FWHM = 1.27 meV) and a small peak at lowerenergy, around 1.649 eV (FWHM = 1.1 meV). The main peak at this timeclearly dominates the spectrum. At a later time (10 s), the intensity ratio of thetwo peaks is reversed and the low-energy peak is now much stronger than thehigh-energy line. At around 15 s, the higher-energy peak is again muchstronger than the low-energy luminescence, and then the intensities arereversed again at around 20 s.Figure 15a gives a gray-scale representation of the whole set of data asmeasured for this SIQD; a spectral shift in the luminescence is not observed.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.