Himwas et al. Nanoscale Research Letters 2011, 6:496 http://www.nanoscalereslett.com/content/6/1/496 hatches. The presence of two WLs is thus unique to sample D, but we believe that it is a general phenom- enon for all Stranski-Krastanow QDs grown on cross- hatch patterns. Their existence which has not been identified until now possibly explains the more complex carrier dynamics than those exhibited in conventional QDs [32,33]. Sample D is subject to ex situ annealing, and the results are reported in the last subsection. In situ annealed After 700°C in situ annealing, the luminescence from the QDs on CHP (sample C) is severely degraded: its PL can no longer be observed even with the shortest experimental annealing time of 10 min, in contrast to the slowly degraded PL of the controlled QDs (sample A), but similar to the controlled CHP (sample B) subject to the same annealing conditions. The degradation of the optical quality of annealed QDs on CHP thus unequivocally originates from the degradation of the CHP itself. Figure 3a shows the PL of the controlled QDs after 700°C in situ annealing for 0, 10 and 30 min. The PL spectra of the 0- and 10-min annealed samples can be well fitted to a double Gaussian function whereas that of the 30-min annealed sample can be fitted to a single (a) QDs ) . u . a ( e c n e c s e n i m u to l (b) CHP o h 30 min P 10 min in-situ anneal 30 min 10 min 0 min x32 x3.2 x1 0 min 1.2 1.3 Energy (eV) 1.0 1.1 1.4 1.5 Figure 3 PL spectra of samples (a) A [the controlled QDs] and (b) B [the controlled CHP]. The samples are subject to 700°C in situ annealing for 0, 10, and 30 min. Spectra are offset for clarity. Symbols in (a) are multiple Gaussian function fits. Page 4 of 7 Gaussian function. The bimodal size distribution is thus maintained during the initial stages of annealing but is transformed into a monomodal one after extended annealing. The solid arrows in Figure 3a indicate that upon 10- min annealing, the lower-energy GS peak blueshifts by 4 meV from 1.075 to 1.079 eV and the higher-energy GS peak by 13 meV from 1.117 to 1.130 eV. The observed blueshifts are much smaller than the 140 to 250 meV reported for conventional, monomodal QDs [13-19] or other nanostructures [20], yet the underlying mechan- isms for the blueshifts are the same: interdiffusion and intermixing of group III cations at elevated temperatures lead to QD volume expansion, reduced confinement energy, and subsequent increased in confined electron and hole energies which have recently been modeled [34]. Prolonged annealing, however, adversely affects the optical quality of monomodal QDs [13,19] and is also the case in our bimodal QDs: the 30-min annealed sam- ple has six times lower integrated intensity than the 10- min annealed one. The dashed arrows in Figure 3a indicates that upon 30-min annealing, the bimodal distribution changes into a monomodal one. This is evident from two observa- tions. First, the change in form of Gaussian fitting - from a double to a single distribution - signifies that an intermixing threshold has been reached where expanded bimodal QDs cannot be statistically distinguished. Sec- ond, the values of FWHM of the two annealed condi- tions are closely related. The 10-min annealed QDs with maintained bimodality exhibit two GS peaks with FWHM of 30.8 and 56.5 meV, whereas the 30-min annealed QDs exhibit one GS peak with FWHM of 87.1 meV, almost an exact linear combination of the two GS FWHM. Given finite experimental and fitting errors, the above data lead us to establish that the threshold for annealing induced transformation from bi- to monomo- dal QD size distribution occurs when the FWHM of the monomodal distribution equates the combined FWHM of the bimodal distribution. Figure 3b shows the PL of the controlled CHP after 0- , 10-, and 30-min annealing. The narrow QW peak at 1.377 eV of the unannealed sample significantly broad- ens and is slightly red-shifted with reduced peak inten- sity upon 10-min annealing. Additional peak at around 1.5 eV emerges as a result of annealing. This value cor- responds to exciton combination in bulk GaAs. Upon 30-min annealing, this bulk GaAs emission strengthens, whereas the QW peak weakens so much that it is below detection limits. The GaAs peak however hovers above the noise level by only a small margin, indicating a poor structural integrity. The rapid degradation of the CHP layer and the insig- nificant improvement of the GaAs layers are related and