2-D Precursors and Interdiffusion in CdSe/ZnSe Self-Assembled Quantum Dots
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Mat. Res. Soc. Symp. Proc. Vol. 618 ©2000 Materials Research Society
CdSe deposition [7] showed the existence of two distinctly different types of islands formed in succession, suggesting that the dot formation process may follow a slightly different path than in the case of InAs dots formed on GaAs. This paper describes the evolution of CdSe quantum dots studied using the Z-contrast imaging technique in a scanning transmission electron microscope. These images, together with PL and micro-PL data carried out on the same samples, clearly indicate the existence of platelet-like 2D islands which form at a very early stage and act as precursors for the 3-D islands which form at higher CdSe coverages. This result is in full accord with the model proposed by Priester and Lannoo [8]. Their approach was based on total energy calculations, using a valence force field for the elastic part of the energy, with the surface contribution being added separately. They show that the existence of a narrow distribution of sizes and shapes in the dot population can be explained by the preliminary formation of stable 2D platelets, which act as precursors for the formation of coherent 3D islands. These platelets grow with increasing coverage until they reach some limiting distribution, and at that critical coverage they spontaneously transform into 3D islands with the same distribution. EXPERIMENT The samples which we investigated were grown by molecular beam epitaxy (MBE) in a Riber 32 R&D MBE machine equipped with elemental sources. A ZnSe buffer was first grown at 300 'C on (100) GaAs substrates to a thickness of approximately 2 gtm. During the growth of ZnSe the RHEED pattern showed a consistently streaky 2xl reconstruction throughout ZnSe deposition, together with well resolved RHEED oscillations of the specular spot. We take this to be a strong indication of layer-by-layer growth, and thus of a smooth ZnSe surface, with little or no surface roughness at the time when the ZnSe growth was interrupted for subsequent CdSe deposition. For depositing CdSe, the substrate temperature was raised to 350 °C. Results described in this paper were obtained on nine such MBE-grown specimens, with nominal thicknesses of the CdSe layers ranging from 0.5 to 2.6 monolayers. CdSe was deposited at a very slow rate of 14 sec/ML. After deposition of the CdSe layers the growth was interrupted for 2 sec., and CdSe was then capped (at the same 350 °C) by 50 nm of ZnSe. Since we are concerned with the effect of varying the CdSe layer thickness (on the scale of a few monolayers) on the development of QDs, it is crucial to deposit well-controlled amounts of material. This demands precise determination of the CdSe deposition rate. This was accomplished by growing a calibration CdSe epilayer and monitoring RHEED intensity oscillations of the specular spot during the growth, each intensity oscillation corresponding to deposition of 1 ML of CdSe. With the CdSe growth rate established, the deposition of CdSe during the QD growth was timed to correspond to the
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