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

from states deep within the band gap, which appears at 0.99 eV, is associatedwith phosphorus vacancies.Recent optically detected magnetic resonance (ODMR) results [12]show that unetched InP QDs have phosphorous vacancies both at the surfaceand in the QD core and that these defects act as radiative traps for photogeneratedelectrons. Treatment of the QDs with HF eliminates the ODMRsignal that results from phosphorous vacancies at the surface, but it leaves asmall ODMR signal due to the small population of phosphorous vacancies inthe QD core. The peaks of the ODMR and PL spectra coincide, both for thedeep-trap emission and band-edge emission in untreated and HF-treated InPQDs, respectively Thus, the ODMR experiments confirm that the strongdeep-trap emission of untreated InP QDs arises from phosphorous vacanciesand that when the QDs are treated with HF, the phosphorous vacancies at thesurface are passivated, leaving very weak or nonexistent PL emission from thelow residual phosphorous vacancy population in the core. Additional electronparamagnetic resonance (EPR) experiments [48] support this conclusionand provide additional information about the nature of nonradiative holetraps near the valence band that are involved in the anti-Stokes PL (PLupconversion) that is observed with QDs (see Sect. III.C).B. Size-Selected PhotoluminescenceIf the PL excitation energy is restricted to the onset region of the absorptionspectrum of the QD ensemble, then a much narrower range of QD sizes isexcited that have the larger particle sizes in the ensemble. Consequently, thePL spectra from this type of excitation show narrower linewidths and smallerred shifts with respect to the excitation energy. This technique is termedfluorescence line narrowing (FLN)—the resulting PL spectra being considerablynarrowed.Fluorescence line narrowing spectra at 10 K are shown in Figs. 8a–8e forInP QDs with a mean diameter of 32 A˚ . FLN/PL spectra are shown for aseries of excitation energies (1.895–2.07 eV) spanning the absorption tail nearthe onset of absorption for this sample [16]. Also shown is the global PLspectrum produced when the excitation energy (2.41 eV) is deep into the highenergyregion of the absorption spectrum (Fig. 8f).Fluorescence line narrowing spectra can be combined with photoluminescenceexcitation (PLE) spectra to determine the resonant red shiftassociated with true band-edge emission [10]. The experiment is done asfollows: (1) A photon energy is selected in the onset region of the absorptionspectrum of the QD ensemble spectrum, and this energy is set as the detectedphoton energy in the PLE; (2) the PLE spectrum is then obtained, and the firstpeak of the PLE spectrum is taken to be the lowest-energy excitonic transitionfor the QDs capable of emitting photons at the selected energy; (3) an FLNCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.