A simple and complementary metal-oxide-semiconductor-compatible method for fabricating germanium ͑Ge͒ single-electron transistors (SETs) is proposed, in which the Ge quantum dots (QDs) are naturally formed by selective oxidation of Si 0.95 Ge 0.05 / Si wires on a silicon-on-insulator substrate. Clear Coulomb-blockade oscillations, Coulomb staircase, and negative differential conductances were experimentally observed at room temperature. The tunneling currents through the Ge QDs were simulated by the Anderson model with two energy levels. Analysis of the current-voltage characteristics indicates that the single-electron addition energy of the Ge QD is about 125 meV.
Recessed oxynitride dots deposited on self-assembled Ge dots are demonstrated using liquid-phase deposition ͑LPD͒. By adding ammonia into the solution, the nitrogen atoms can be incorporated into the deposited film. The tensile strain of the Si cap layer directly deposited on Ge dots can enhance the oxynitride nucleation and deposition on Si surface. The tensile strain may also increase the etching rate of the Si cap layer and the recessed dots are formed directly above the Ge dots. The LPD-SiON dots have a higher dot step height as compared to LPD-SiO 2 dots.
In this work, two successive layers of Ge quantum dots separated by a thin Si spacer grown by ultra-highvacuum chemical vapor deposition were demonstrated. With an optimal thickness of the thin Si spacer, the sandwiched Ge (13.1 ML)/Si (28 ML)/Ge (13.1 ML) quantum dots suppress the coarsening of Ge quantum dots and efficiently increase the uniformity of the quantum dots. Cross-sectional transmission electron microscopy shows that the Si/Ge underlayers provide preferred nucleation sites for the overlayer Ge deposition. Such a modification avoids the formation of Ge superdomes and prevents the occurrence of threading dislocations even at such a thin Si spacer thickness. A stronger photoluminescence intensity of these sandwich Ge/Si/Ge quantum dots was observed compared with that of 26.2 eq-ML Ge quantum dots without using any intermediate Si layer. Furthermore, the narrower width of the photoluminescence spectra indicates that the Ge/Si/Ge dots are more uniform compared with the 13.1 and 26.1 eq-ML Ge quantum dots. Five-fold bilayers of Ge/Si/Ge/Si(150 ML) have been achieved to enhance the photoemission efficiency.
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