Lixin Yi

Si rings, Si clusters, and Si nanocrystals—different states of ultrathin SiOx layers

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.

Phase separation of thin SiO layers in amorphous SiO/SiO2 superlattices during annealing

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 .

Size-Controlled Si Nanocrystals for Photonic and Electronic Applications

A new approach for the fabrication of ordered Si quantum dots fully compat ible with normal Si technology is presented. The preparation of SiO/SiO 2 superlattices represents a simple and efficient method for fabricating highly luminescent Si nanocryst als and allows independent control of size, size distribution, and density. The Si nanocrystals can be arrange d 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 /cm. 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 w idth at half maximum of 0.6 nm. We report on TEM images showing early states of phase separati on in SiO/SiO2 superlattices and combine these results with IR and PL investigations. Three differe nt states of phase separation are distinguished and correlated to specific luminescence and infrared fe atures. Photoluminescence experiments after crystallization show a size-dependent blue shif t of the luminescence from 950 to 750 nm and a luminescence intensity comparable to porous Si. The nearly s ize-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/ SiO2 superlattices is reported which shows a similar size dependent luminescence after crystalliz ation.

Silicon Technology Used For Size-Controlled Silicon Nanocrystals

Nanoscience represents a rapidly expanding area of research that includes aspects of many areas such as physical science, materials science and engineering. The growth and investigation of low dimensional semiconductor structures offer new possibilities for light emitting devices. In a bulk semiconductor electrons and holes can move freely through the crystal. Reducing the semiconductors dimensions may drastically change their recombination properties. Size dependent optical properties, greater electron/hole overlap for enhanced photoluminescence efficiency, and discrete single electron-hole charging are some of the consequences of carrier confinement.

Highly luminescent Si quantum dots: new ways for size, position, and density control

Phase separation and thermal crystallization of SiO/SiO2 superlattices result in ordered arranged silicon nanocrystals. The preparation method enables independent control of particle size as well as of particle density and spatial position by using a constant stoichiometry of the layers. Infrared absorption and photoluminescence spectra are measured as a function of annealing temperature to study the phase separation process. Three photoluminescence emission bands are observed. A band centered at 560 nm is found in as-prepared samples and vanishes for annealing above 700oC. A second band around 760 nm to 890 nm is detected for annealing temperatures above 500oC. The superlattices show a strong luminescence and a size dependent blue shift in the visible and near-infrared region after crystallization for temperatures above 900oC. The origin of the different photoluminescence bands at different phase separation stages of ultra thin SiOx layers are discussed based on transmission electron microscopy investigations and on correlations seen in photoluminescence spectra and infrared absorption. In addition, we report the PECVD preparation of amorphous SiO/SiO2 superlattices which show a similar size dependent luminescence after crystallization.

Structure Study of Amorphous SiOx Films

Two FTIR spectra bands of amorphous SiOx films prepared by sputtering technology were detected. Different structures corresponding to them were studied according to the CFM mode and RBM mode.

Organic solar cells with improved spectral coverage based on copper phthalocyanine : MEH-PPV : C60 bulk heterojunctions

The photo-current of the polymer solar cell based on MEH-PPV is enhanced by adding CuPc dyes in the active layer due to improved spectra coverage as indicated in the absorption spectra.

Synthesis and size control of Si nanocrystals by SiO/SiO2 superlattices and Er doping

The synthesis of nc-Si by reactive evaporation of SiO and subsequent thermal induced phase separation is reported. The size control of nc-Si is realized by evaporation of SiO/SiO2 superlattices. By this method an independent control of crystal size and density is possible. The phase separation of SiO into SiO2 and nc-Si in the limit of ultrathin layers is investigated. Different steps of this phase separation are characterized by photoluminescence, infrared absorption and transmission electron microscopy measurements. The strong room temperature luminescence of nc-Si shows a strong blueshift of the photoluminescence signal from 850 to 750 nm with decreasing crystal size. Several size dependent properties of this luminescence signal, like decreasing radiative lifetime and increasing no-phonon transition properties with decreasing crystal size are in good agreement with the quantum confinement model. Er doping of the nc-Si shows an enhancement of the Er luminescence at 1.54 µm by a factor of 5000 compared to doped SiO2 layers. The decreasing transfer time for the nc-Si to Er transition with decreasing crystal size can be understood as additional proof of increasing recombination probability within the nc-Si for decreasing crystal size.