Amorphous SiO/SiO2 superlattices were prepared by reactive evaporation of SiO powder in an oxygen atmosphere. Infrared absorption and photoluminescence spectra were measured as a function of annealing temperature. Three photoluminescence emission bands were observed. A band centered at 560 nm is present in as-prepared samples and vanishes for annealing above 700 °C. The second band around 760 nm to 890 nm is detected for annealing temperatures above 500 °C. A strong red luminescence is observed for annealing temperatures above 900 °C. The origin of the different photoluminescence bands and different states of the phase separation of ultrathin SiOx layers is discussed.
Stimulated emission from silicon-nanocrystal planar waveguides grown via phase separation and thermal crystallization of SiO/ SiO 2 superlattices is presented. Under high power pulsed excitation, positive optical gain can be observed once a good optical confinement in the waveguide is achieved and the silicon nanocrystals have proper size. A critical tradeoff between Auger nonradiative recombination processes and stimulated emission is observed. The measured large gain values are explained by the small size dispersion in these silicon nanocrystals.
The preparation of ordered and arranged Si quantum dots using a SiO/SiO 2 superlattice approach is presented. The different processes of phase separation and crystallization are studied in detail by infrared (IR) absorption and photoluminescence (PL) spectroscopy for different annealing temperatures from 300 to 1100 • C. IR spectra show a continuous shift of the Si-O-Si asymmetric stretching mode to higher energies with increasing annealing temperature, which is a sign of phase separation to Si and SiO 2. Three PL bands are distinguished and correspond to the three processes of phase separation. A band centred at 2.2 eV is present in as-prepared samples and vanishes for annealing above 800 • C which is clearly correlated with defects. The second band shifting from 1.7 to 1.4 eV is detected for annealing temperatures between 300 and 900 • C. A strong red luminescence due to quantum confinement is observed for annealing above 900 • C. Our results indicate that the different and seemingly contradictory PL observations in the literature could originate from different states of network reorganization during the phase separation and crystallization processes. The origins of the different IR and PL bands are discussed in comparison with those of bulk crystalline SiO and SiO 2 .
Abstract.A new approach for the fabrication of ordered Si quantum dots fully compatible with normal Si technology is presented. The preparation of SiO/SiO 2 superlattices represents a simple and efficient method for fabricating highly luminescent Si nanocrystals and allows independent control of size, size distribution, and density. The Si nanocrystals can be arranged to a specific depth and for a specific number of layers with a nanometer adjustment. The density of the Si nanocrystals is in the range of 10 19 /cm 3 . TEM and XRD investigations confirm control of the upper limit of the nanocrystal size to an average size of below 2.5 nm with a full width at half maximum of 0.6 nm. We report on TEM images showing early states of phase separation in SiO/SiO 2 superlattices and combine these results with IR and PL investigations. Three different states of phase separation are distinguished and correlated to specific luminescence and infrared features. Photoluminescence experiments after crystallization show a size-dependent blue shift of the luminescence from 950 to 750 nm and a luminescence intensity comparable to porous Si. The nearly size-independent PL intensity observed in our SiO/SiO 2 superlattices indicates the achievement of independent control of crystal size and number. In addition, PECVD preparation of amorphous SiO/SiO 2 superlattices is reported which shows a similar size dependent luminescence after crystallization.
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