Applied Physics

Strained Stacking

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Science  28 May 2004:
Vol. 304, Issue 5675, pp. 1213
DOI: 10.1126/science.304.5675.1213d

Both CdSe/ZnSe quantum wells and quantum dots (QDs) have been examined in detail, with one possible use as yellow, blue, or green light-emitting devices. Fabrication using QDs should produce devices that will have a lower lasing threshold and be less susceptible to defect-induced degradation effects than those produced using quantum wells. One challenge, though, is to assemble the QDs so that they have a uniform size distribution and periodic ordering. Stacking layers of dots with a separation layer of matrix material is effective for ordering in III-V compounds, because the strain induced by the lattice mismatch leads to heterogeneous nucleation of the subsequent layers of dots. Schmidt et al. used grazing incidence small-angle x-ray scattering to see if both vertical and lateral coupling of the dots occur in a CdSe system, which is a II-VI material. They grew the first layer of CdSe dots on a ZnSe buffer layer, but then used ZnSSe as the matrix material in order to avoid stacking fault formation. For a fixed spacer layer, they observed ordering when 10 and 5 layers were grown, but not for 3 layers. Similarly, when they kept the number of layers thick, they observed ordering only for the smaller spacer layer thicknesses. These observations confirm the idea that the initial QD layer acts as a seed layer for subsequent growth and thus drives the ordering process. By tuning the induced lattice strain, it should be possible to optimize the growth of the QD array. — MSL

Appl. Phys. Lett. 84, 4367 (2004).

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