Himwas et al. Nanoscale Research Letters 2011, 6:496 http://www.nanoscalereslett.com/content/6/1/496 degradation in the former and the improvement in the latter are discussed. Experiments The structure of InAs QDs on InGaAs CHPs under investigation is shown in the schematic cross section in Figure 1. All growth takes place in a solid-source MBE system (Riber 32P). Epi-ready (100)-GaAs substrates are prepared by standard thermal desorption of native oxi- des at 580°C before the deposition of 300-nm GaAs buf- fer layer at the same temperature followed by 50-nm In0.13Ga0.87As CHP layer at 500°C, a 30-s growth inter- ruption, 0.80 or 0.96 monolayer (ML) of InAs at 500°C at a rate of 0.01 ML/s, another 30-s growth interruption, and the final 50-nm GaAs capping layer at 500°C. Dur- ing the deposition of the InAs layer, the reflection high- energy electron diffraction spots appear, indicating the formation of QDs. The surface of the QDs grown on the CHP layer is shown in the 5 × 5 μm2 atomic force microscopy (AFM) image in Figure 1b. The alignment of QDs along the orthogonal [110] and [1-10] directions occurs as a result of non-uniform surface strain fields originating from the subsurface MDs [23]. To identify the source(s) of changes in optical characteristics upon annealing, two controlled samples are grown: one is a controlled InGaAs CHP sample with identical structure to Figure 1a less the QD layer and the other is a con- trolled InAs QD sample (1.7-ML InAs) with identical structure to Figure 1a less the InGaAs CHP layer. Our in situ annealing follows the same procedures successfully applied to conventional QDs [13,14]: the controlled QDs, the controlled CHP, and the QDs on CHP samples are removed from the growth chamber, cleaved into smaller pieces, re-attached to the molybloc, transferred back into the growth chamber, and annealed at 700°C for 10, 30, and 60 min under As4 partial pres- sure higher than 8 × 10-6 Torr. Such high pressure alle- viates surface As desorption, and after annealing, the surface of all samples remains reflective. For ex situ annealing, the samples are also cleaved into smaller (a) GaAs cap (b) In0.13Ga0.87As GaAs buffer (001)-GaAs [1-10] [110] Figure 1 Structure of InAs QDs on InGaAs CHPs. (a) Schematic cross-sectional diagram of the QDs on CHP structure and (b) a 5 × 5 μm2 AFM image of the uncapped QDs layer (height contrast, 11.3 nm). Page 2 of 7 pieces but later placed in the middle of a quartz tube and heated to 350°C for between 30 min and a few hours under continuous flow of a hydrogen-containing forming gas. The optical properties of as-grown and annealed sam- ples are characterized by macroscopic PL at 20 K. The samples are mounted on the cold finger of a closed- cycle He cryostat and excited by 476.5-nm Ar+ laser at a nominal power density of I0 = 0.45 W/cm2. The PL sig- nal is dispersed in a 1-m monochromator and collected by a cooled InGaAs detector using standard lock-in detection technique. Results and discussion The optical properties of the as-grown and annealed QDs on CHP structure are analyzed against those of the controlled QDs and CHP samples. The results for as- grown samples will first be discussed, followed by those for samples that underwent in situ and ex situ anneal- ing, respectively. As-grown The 20-K photoluminescence of the controlled QDs, the controlled CHP, and the QDs on CHP samples before annealing are shown in Figure 2. The controlled QDs (sample A) show two peaks at 1.075 and 1.117 eV with corresponding full width at half maxima (FWHM) of 31 and 49 meV, respectively. The controlled CHP (sample B) shows a single emission peak at 1.377 eV with an FWHM of 21 meV. The results for QDs on CHPs are obtained from two samples destined for in situ (sample C) or ex situ (sample D) annealing. The InAs QD layer in sample C is 0.96 ML, thicker than 0.80 ML in sample D. The strong luminescence from all unannealed sam- ples indicates that the as-grown materials are of high crystalline quality. Sample A shows two ground states’ (GSs) PL from the 1.7-ML InAs QD layer. This has been confirmed by excitation-dependent measurements. The presence of two GSs indicates that this particular growth condition on flat GaAs substrates results in QDs with a bimodal size distribution [24-26]. At higher excitation power density, two excited-state peaks resulting from the state filling of each of the GS emerge, as expected. Sample B (CHP) is basically an InGaAs quantum well (QW) sandwiched between the GaAs buffer and GaAs capping layers. The lattice-mismatched QW is 50-nm thick, much greater than the critical thickness for strain relaxation by the formation of interfacial MDs which for In0.13Ga0.87As on GaAs is estimated at 15 nm [27]. The dislocations thus formed act as traps and non-radiative recombination centers [28]. PL from such layer is thus expected to be weak or absence. The observed peak at 1.377 eV is indeed weak with respect to sample A, yet it