Himwas et al. Nanoscale Research Letters 2011, 6:496 http://www.nanoscalereslett.com/content/6/1/496 the non-Fickian diffusion description of Ryu et al. or that cation interdiffusion readily occurs even at 350°C. The total QD intensity in Figure 4b is obtained simply by adding the two constituent QD emissions along the [1-10] and [110] directions. Measurements by macro PL do not allow meaningful interpretation of both constitu- ents separately since the excited beam diameter covers large areas of cross hatches and MD line densities vary across the surface. Thus, only explanation regarding the total QD emission is attempted. The total QD emission remains unchanged during the initial stages of annealing but increases slowly with annealing time after 1 h. The mechanism responsible for increased QD intensity is the same as those for increased WL intensity, only to a much smaller scale due to the comparatively lower sur- face coverage. The peak energies of both constituents thus remain unchanged as shown by the vertical dotted lines in Figure 4a. Figure 4c shows the FWHM of the 1.42-eV WL peak, and the 1.250-eV [1-10]-aligned and the 1.296-eV [110]- aligned QD peaks as a function of annealing time. The changes in QDs’ FWHM are non-monotonous, small, and most likely due to surface inhomogeneity, not to annealing. In contrast, the change in WL’s FWHM is monotonous and large, dropping by over one third from 61 to 38 meV. The scale of change is only possible because of the relative large areal coverage of the upper (GaAs) barrier which becomes more homogeneous as more hydrogen atoms are driven to bond with random defects. The same mechanism also gives rise to the more homogeneous lower (InGaAs) layer which, being lower in energy than and adjacent to the second WL, can effectively compete for carriers and possibly explains the decrease in WL intensity and the increase in QD intensity after 120-min annealing seen in Figure 4b. The decrease in WL intensity is due to carrier transfer to the more energetically favorable InGaAs CHP. The increase in QD intensity is due to the InGaAs CHP channeling some of these new carriers through the 1.344-eV WL where they are subsequently captured by the QDs. Low- temperature ex situ annealing thus proves to be a viable approach for enhancing optical emissions from InAs QDs on InGaAs CHPs while maintaining the rich opti- cal feature. Conclusions InAs QDs on InGaAs CHPs are grown by MBE, charac- terized by low-temperature PL, and found to be optically active in the 1.1 to 1.4 eV range with distinct emission peaks from the orthogonally aligned [1-10] and [110] InAs QDs, two different wetting layers, and the InGaAs CHP. The PL spectra of the QDs on CHPs are quenched when the structure is subject to 700°C in situ annealing. In separate controlled experiments, QDs are Page 6 of 7 found to survive the same treatments whereas the CHP deteriorated. The quenching thus results from CHP deterioration, most likely driven by strain relaxation via the formation of additional misfit and threading disloca- tions which are effective carrier traps. When subject to 350°C ex situ hydrogen annealing, however, the struc- ture shows an increase in overall PL intensity, a small blueshift accompanied by spectral narrowing for the 1.42-eV WL. Hydrogen bonding is believed to cause such improvement as it is effective at neutralizing defects and relieving associated strains which frees up carriers and smoothens band discontinuities along heterointerfaces. Abbreviations AFM: atomic force microscopy; CHP: cross-hatch pattern; FWHM: full width at half maximum; GS: ground state; MBE: molecular beam epitaxy; MD: misfit dislocation; ML: monolayer; PL: photoluminescence; QD: quantum dot; QW: quantum well; TD: threading dislocation; WL: wetting layer. Acknowledgements S. Thainoi and P. Changmoang are acknowledged for maintaining the MBE and PL systems. This work is funded by Industry/University Cooperative Research Center (I/UCRC) in HDD Component, the Faculty of Engineering, Khon Kaen University (CPN R&D 01-18-53); NSTDA via Nectec and Nanotec; the 90th anniversary of Chulalongkorn University fund (Ratchadaphiseksomphot endowment fund); Office of the Higher Education Commission and Thailand Research Fund (DPG5380002). Authors’ contributions CH grew and measured the MBE samples, and interpreted the PL spectra. SP provided helps, obtained funding, and supervised the group. SK conceived, designed, and supervised the experiments; obtained funding; analyzed the data; and wrote the manuscript. Competing interests The authors declare that they have no competing interests. Received: 20 June 2011 Accepted: 17 August 2011 Published: 17 August 2011 References 1. Bimberg D, Grundmann M, Ledentsov NN: A. Quantum Dot Heterostructures New York: Wiley; 1999. 2. Grundmann M, Ledentsov NN, Stier O, Bimberg D, Ustinov VM, Kop’ev PS, Alferov ZI: Excited states in self-organized InAs/GaAs quantum dots: theory and experiment. Appl Phys Lett 1996, 68:979-981. 3. Wang ZM, Mazur YI, Seydmohamadi S, Salamo GJ, Kissel H: Photoluminescence linewidths from multiple layers of laterally self- ordered InGaAs quantum dots. Appl Phys Lett 2005, 87:213105. 4. 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