Self-Assembling Formation of Silicon Quantum Dots

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Nanometer-size silicon dots were spontaneously formed on thermally-grown SiO_2/c-Si by low-pressure chemical vapor deposition (LPCVD) of pure silane (SiH_4) in the temperature range from 525 to 650ﾟC.The size distribution and the area density of Si dots were evaluated by using atomic fore microscopy (AFM) and transmission electron microscopy (TEM). The Si-dot diameter and dot height on as-grown SiO_2 are rate-limited by the thermal decomposition of SiH_4 and the cohesive action of adsorbed precursors on Si nucleation sites, respectively. It has been also found that in the Si dot formation on OH-terminated SiO_2 surface the nucleation density is dramatically enhanced and consequently the dot size and its distribution become small. The tunneling current through SiO_2/Si-dot/SiO_2 double barrier structures was measured by using a conducting AFM probe. As result, negative conductance characteristics are clearly seen at room temperature, indicating that the resonant tunneling occurs through a single Si quantum dot. It was also demonstrated that Si quantum dots covered with SiO_2 exhibit high efficiency luminescence at room temperature. No differences in luminescence peak energy and spectral shape are observable between excitations with a photon energy of 2.54and 3.81eV.It is suggested that the luminescence mainly originates from Si dots whose optical bandgap are under 2.54eV.