A Si-based light-emitting diode (LED) containing strained GaSb quantum dots (QDs) embedded in Si within the active region was fabricated by means of Si molecular-beam epitaxy. An external quantum efficiency of 0.3% was obtained for near bandedge luminescence at 11 K. The high luminosity of Si-based QD-LED was also evidenced by the fact that direct imaging on an infrared camera of standard sensitivity was successful. Characteristics of the QD-LED operating at room temperature are described.
Upon varying the thickness of liquid crystal (LC) cells and alternating their surface chemical and physical environments, phase transition behaviors of the rod-disc molecule (RD12, where 12 is the number of carbon atoms in each alkyl chain linkage between the rod and the disc mesogens) were dramatically changed. From the cross-polarized optical microscopic observations and analyses, it was realized that the macroscopically oriented nematic (N) phase of RD12 was obtained by the surface anchoring confinement and the crystallization of RD12 was completely suppressed. On the basis of the systematic experimental investigations, it was concluded that the glassy N phase was formed because the interaction between surface alignment layer and RD12 (a surface anchoring force) is bigger than that of RD12 themselves (a driving force of the crystallization). The finely tuned molecular orientations and anisotropic physical properties of the programmed RD12 building compound can allow us to fabricate smart optical and electrical thin films for practical applications in electro-optical applications.
The morphological evolution of InAs and InSb quantum dots grown on Si (001) by molecular beam epitaxy was studied by reflection high-energy electron diffraction and scanning probe microscopy. For the InSb/Si system, the maximum density of 1 × 10 10 cm -2 was achieved at a growth temperature of 200 °C with the average base length and dot height of 80 nm and 1 nm, respectively. A broad luminescence band extending over 1.1 -1.4 µm was observed at low temperature from the In-based quantum dots embedded in Si. The decay lifetimes of luminescence were of the order of submicroseconds, which indicates an indirect transition both in real-and in k-space. Excessive thermal budget after the growth only resulted in the development of luminescence features characteristic of In-doped Si.1 Introduction There has been sustained research interest in the formation of self-assembled quantum dots (QDs) of lattice-mismatched semiconductor heterostructures, motivated by the desire to create a new class of devices utilizing quantum effects [1 -12].Such self-assembling QD systems as those which spontaneously develop due to elastic strain relief have been so far limited to the combinations of allied elements or the same columns, e.g., InAs/GaAs [1 -5] and Ge/Si [6][7][8][9][10][11]. The introduction of compound semiconductor QDs into the Si matrix, as opposed to the well-known GaAs-on-Si technology where Si stands as a mere substrate [13 -15], offers a unique possibility of utilizing the direct band-gap properties of III-V QDs in the otherwise optically less active Si, paving an easier way toward realistic Si-based light emitters [12].In terms of the efficiency of radiative recombination, the best match would be such a configuration that direct band-gap QDs with an energy gap smaller than that of Si are encompassed by Si in a type-I potential line-up. The QDs will then provide the potential traps for the electrons and holes, thereby reducing the carrier dissipation.In this study, an attempt is made to grow such heterogeneous QD systems, with primary focus on the morphological evolution of In-based III-V QDs that spontaneously develop on Si (001) substrate.
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