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We report accurate calculations, performed over the last three years, of the optical gap of small silicon nanocrystals (Si dots). In these calculations the Si dangling bonds on the surface of the nanocrystals are passivated by hydrogen and (more recently) by hydrogen and oxygen. The actual diameters of the nanocrystals range from about 2 Å (SiH) to 25 Å (SiH). The results for the optical gap can be safely (judging from the quality of the fit) extrapolated up to much larger diameters.

We have used a large variety of real-space theoretical techniques, including configuration-interaction singlets (CIS) and multi-reference second-order perturbation theory (MR-MP2). The bulk of the calculations has been performed in the framework of density functional theory (DFT) using advanced methods ranging from screened Coulomb interactions and ΔSCF calculations to DFT/CIS and time-dependent DFT (TDDFT).

In all DFT methods we have used the hybrid nonlocal exchange-correlation functional of Becke and Lee, Yang and Parr, which includes partially exact Hartree—Fock exchange (B3LYP). Our results are in excellent agreement with accurate recent and earlier experimental data. We have found that the diameter of the smallest oxygen-free nanocrystal that could emit photoluminescence in the visible region of the spectrum is around 22 Å, whereas the largest diameter falls in the range of 84–85 Å. The high level and the resulting high accuracy (estimated within 0.3 eV for the optical gap) of our calculations have led to the resolution of existing experimental and/or theoretical discrepancies. Our (most recent) results also clarify unambiguously and confirm earlier predictions about the role of oxygen on the gap size.

The fabrication of Ge(Si) self-assembled islands on Si substrates has attracted much attention in the last several years because of their great potential for practical application. In this paper we present the results of investigations into the growth and optical properties of single and multilayer structures with Ge(Si)/Si(001) self-assembled islands, grown at different Ge deposition temperatures (). It is shown that the sizes and surface density of the islands can be varied in a wide range by altering : from nanometer scale islands for high to small quantum dots with a height of ∼ 1 nm and a surface density of >10 cm. The GeSi alloy formation in islands grown at >580° C was observed by X-ray diffraction, Raman scattering and selective etching. The alloy formation is caused by a strain-driven Si diffusion in islands. The dependence of island composition on was experimentally measured and found to drastically affect the sizes and shapes of the islands.

A stable field electron emission was obtained from Ge nanocluster structures grown on Si(100) by molecular-beam epitaxy. The size of the clusters was ≤10 nm and cluster density was ∼10 cm. The emission current-voltage characteristics showed current peaks, presumably due to resonance electron tunneling the energy levels of the nanocluster potential well. For Ge cluster multilayers embedded in Si, the field emission current showed a considerable temperature sensitivity, as well as photosensitivity in the wavelength range from 0.4 to 10 µm.

Silicon plays a dominant role in microelectronics. Si nanostructures are intensively investigated because of their compatibility with Si technology. In the present work, we report on the optical emission properties of Si quantum dots. The energy states are calculated within the effective mass approximation. Both first and second order transitions are included in the calculation. The sizes of the quantum dots that we have considered are in the range 2–5 nm. The magnitude of the lifetime is found to be very sensitive to the structural parameters of the dots such as the shape, the cross section and the crystallographic direction. Lifetimes varying from the order of μsec to the order of msec have been obtained. The results of our calculations provide further insight in the photoluminescence behavior of Si nanostructures and porous Si.

Strain-driven decomposition of an alloy layer is investigated as a means to control the structural and electronic properties of self-organized quantum dots. Coherent InAs/GaAs islands overgrown with an InGaAs alloy layer serve as a model system. Cross-sectional and plan-view transmission electron microscopy, as well as photoluminescence studies, consistently indicate an increase in height and width of the quantum dots with increasing indium content and/or thickness of the alloy layer. The increase in dot size is attributed to the phase separation of the alloy layer driven by the surface strain introduced by the initial InAs islands. The density of large dislocated clusters in the quantum dot samples can be dramatically reduced by special annealing (defect reduction) techniques. A laser based on multiple (up to 10) layers of InAs/GaAs quantum dots grown under optimized conditions with the use of defect reduction techniques show considerably enhanced optical gain and improved performance. Differential efficiency as high as 88% is achieved in the lasers. An emission wavelength of 1280 nm, a threshold current density of 147 A/cm, a differential efficiency of 80%, and a characteristic temperature of 150 K are realized simultaneously in one device. Wavelength extension up to 1500 nm for InAs quantum dot lasers on GaAs substrates is possible by using metamorphic (plastically relaxed) buffer InGaAs layers. In cases when the threading dislocations are avoided in the active region, high-performance operation with quantum efficiency exceeding 60–70% and pulsed output powers up to 7 W are realized.