Cathodoluminescence Wavelength Imaging Study of Clustering in InAs/GaAs Self-Assembled Quantum Dots
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explore the relationship between the number of layers, interlayer spacing, and the magnitude of the QD peak energy variations in multi-layered vertically self-organized (VSO) samples. EXPERIMENTAL The SAQD samples were grown by MBE in which a total of 1.74 monolayers (ML) of InAs were deposited for each layer at a 500 TC substrate temperature and resulted in a lateral QD density of -350 jim2. In the multi-layered VSO samples, 20- and 36-ML-thick GaAs spacers were grown at 400 TC by migration enhanced epitaxy (MEE) after each InAs deposition. The samples were capped with 170-ML MEE-grown GaAs. Details of the punctuated island growth procedure are discussed in Ref. 3. We have examined three samples grown with 0 T = 2.5 ML and Op = 1.74, 2.0 and 2.5 ML, the last of which is just the case for continuous 2.5 ML growth (i.e., a single stage growth). During the punctuated phase of the growth, the deposition is suspended for 60 s, after which the remaining InAs was deposited unitl the total coverage of 2.5 ML was reached. CL experiments were performed with a modified JEOL-840A scanning electron microscope (SEM). 4 A Ge p-i-n detector was used to detect the luminescence dispersed by a 0.25-m monochromator for CL imaging. CL spectroscopy was performed with an InGaAs linear array detector. CLWI is accomplished by acquiring a series of discrete monochromatic images, constructing a local spectrum at all 640 x 480 scan points within the image, and determining the wavelength, X(X,y), at which there is a peak in the CL spectrum at each scan point (xy). RESULTS AND DISCUSSION We illustrate these results first with a stack plot of CL spectra acquired from a two-layer InAs sample whose spacer thickness is 36 ML. The 21 CL spectra in Fig. 1 are acquired locally along an arbitrary scan line of 57 jim in length by fixing the electron beam at each point, whose relative distance is indicated in the figure. The lineshape of each spectrum is observed to change at each position. The center of gravity of the spectral lineshape is roughly independent of the position. However, small changes in the lineshape give rise to a change in the peak position, XM. The spectra further appear to be composed of multiple components whose relative intensities change from one location to another. In further analysis, a dashed vertical line is marked at the point in each spectrum which has a maximum in intensity. It is therefore evident that the peak wavelength position, X varies along this line, owing to changes in the relative intensity of all components that comprise the spectra. Similar subtle variations in lineshape are observed for the single- and five-layer samples. Conventional monochromatic CL imaging reveals -10% variation in the intensity of luminescence in the SAQD samples, with domain sizes on the order of a few microns, as shown in 174
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Figure 1. Stack plot of CL spectra acquired locally for an electron beam movement along an arbitray line of the two-layer SAQD sample.
Fig. 2(a) for the sing
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