Himwas et al. Nanoscale Research Letters 2011, 6:496 http://www.nanoscalereslett.com/content/6/1/496 Page 3 of 7 (a) QDs (A) ) . u . a ( P e H c {s n C e n c o s e n i D m Q u lto (b) o h P CHP # GaAs 0.9 [1-10] 1.0 (D) (C) data sim. CHP (B) [110] 1.1 1.2 1.3 Energy (eV) # 1.4 1.5 Figure 2 20-K photoluminescence of the controlled QDs, the controlled CHP, and the QDs on CHP samples. (a) PL spectra of the controlled QDs (sample A, in red), the controlled CHP (B, green), and the QDs on CHP samples (C, black and D, blue) at 20 K. (b) The measured (blue) and simulated (black) PL spectra of sample D. Open symbols (circle) are multiple Gaussian function fits. The peak energies as indicated by the arrows from left to right originate from the [1-10]-QDs, the [110]-QDs, the first WL, the CHP, and the second WL. The symbols below the arrows correspond to the structures depicted in the schematic cross-sectional diagram to the left. The vertical axis is logarithmic in (a) and linear in (b). Spectra are offset for clarity. suggests that a significant fraction of excitons are able to combine radiatively. The 1.377-eV peak energy is higher than the bulk In0.13Ga0.87As bandgap of 1.323 eV and agrees well with the electron and hole eigenenergies estimated from self-consistent solutions of coupled Pois- son-Schrodinger equation [29]. Sample C’s PL exhibits a double, lopsided peak feature with the broad, low-energy lobe centered at 1.222 eV overwhelming the narrow, high-energy lobe centered at 1.344 eV. The low-energy lobe results from the 0.96-ML InAs QDs on CHP which emit at energies between those of the 1.7-ML QDs and the CHP. Its broad line- width results from the superposition of two groups of QDs: those nucleated along the [1-10] and [110] MDs. The two peaks are not resolved in macro PL for sample C, but their presence can be deduced from the differ- ence between the rising and falling edges, indicating the different FWHM between the [1-10]- and the [110]- aligned QDs. Different emission energies of QDs nucleated along the two orthogonal MDs have been observed in a similar structure by micro PL [9] and can be explained by direction-dependent, apparent critical thickness for QD formation [30]. The high-energy lobe centered at 1.344 eV to the right of the QDs band is attributed to the wetting layer (WL) between the InAs QDs and the underlying InGaAs CHP surface. For con- ventional QDs grown directly on GaAs, the low-tem- perature WL luminescence is centered at about 1.42 eV [31]. The 1.344 eV observed here is a result of lower confinement potential of InGaAs. Sample C is subject to in situ annealing, and the results are reported in the next subsection. Sample D contains a thinner InAs layer than those in sample C. The resulting smaller QDs would thus emit at greater corresponding energies as clearly observed in Figure 2a. To elucidate the origins of all the PL peaks in sample D, with implications for C, the measured data are fitted to multiple Gaussian functions as shown in Figure 2b. The schematic cross-sectional diagram show- ing the layers responsible for all the PL peaks is given in the inset of the figure. The two lower PL peak energies at 1.250 and 1.296 eV are attributed to the QDs nucleated along the [1-10] and [110] MDs, respectively. The nucleation of QDs along the two orthogonal direc- tions is asymmetrical, and previous studies have shown that QDs along the [1-10] direction are the first to form [30]; hence, their average size is larger and peak energy is smaller than that of the later-formed QDs along the orthogonal [110] direction. These two peaks are resolved in sample D but unresolved in C simply because the InAs layer in D is thinner and the QDs in both direc- tions have not yet saturated. In sample C, the [1-10] QDs are saturated while the [110] QDs are still growing; additional In adatoms will thus get incorporated into the [1-10] QDs at a reduced rate and into the [110] QDs at an enhanced rate. Consequently, the orthogon- ally aligned QDs in sample C are closer in size (PL peaks less well resolved) than in D. The next two higher-energy PL peaks at 1.344 and 1.377 eV originate from the WL and the CHP, respec- tively. The highest-energy PL peak at 1.42 eV is the sec- ond WL formed in the denuded zones between the cross hatches. The second WL is different from the first WL that gives rise to the 1.344-eV PL peak. The first WL is the WL between the over-critical InAs 3D dots and the underlying InGaAs CHP layer which exists only above the MDs. The second WL is the thin InAs 2D film between the InGaAs CHP layer and the overlying GaAs capping layer which exists only in the denuded zones between the cross hatches. This peak is absent in sample C because there are no denuded zones: the criti- cal thickness for QD formation has been reached across the surface, including the areas between the cross