Hemispherical silicon quantum dots (QDs) have been self-assembled with an areal density as high as ~2−1011 cm−2 on SiO2/Si(100) and quartz substrates by controlling the early states of low pressure chemical vapor deposition (LPCVD) of pure silane. It is found that, for the thermally-oxidized Si QDs, when the mean Si dot height is decreased from 6.3 nm to 1~2 nm, the photoluminescence (PL) peak energy is increased from 1.2 to 1.4 eV at room temperature while the optical absorption edge determined by photothermal deflection spectroscopy is shifted from 1.9 to 2.5 eV. In addition to the observed Stokes shift as large as 0.7−1.1 eV, a weak temperature dependence of the broad luminescence band and non-exponential luminescence decay with a mean life time of sub-msec even at room temperature suggest that localized, radiative recombination centers existing presumably in the SiO2/Si dot interface are responsible for the efficient PL from the Si QDs. From the change in room temperature PL by SiO2 thinning and removal in a dilute HF solution, it is demonstrated that the surface passivation of Si QDs plays an important role for the efficient light emission at room temperature.