Julian López-Vidrier, Yonder Berencén, Bernat Mundet, Sergi Hernández, Sebastian Gutsch, Daniel Hiller, Philipp Löper, Manuel Schnabel, Stefan Janz, Margit Zacharias, Blas Garrido
The electrical and electroluminescence (EL) properties of Si-rich oxynitride (SRON)/SiO2 superlattices are studied for different silicon excess and layer thicknesses. The precipitation and crystallization of the Si excess present within the SRON layers is induced by a post-deposition annealing treatment, in order to form Si nanocrystals (Si-NCs). The electrical characterization performed in dark conditions allowed for deducing the charge transport mechanism through the superlattice structure, found to follow the Poole-Frenkel law. In addition, the EL investigation revealed the correlation between EL excitation and transport mechanisms, suggesting that impact ionization of high-energy conduction electrons dominates the whole frame. The reduction of the SiO2 barrier thickness and the increase in the Si excess were found to enhance the carrier transport through the superlattices due to the reduction of the electrons mean free path, which, in turn, modifies the EL properties.
An experimental study of the temperature dependence of photoluminescence time decay in size-controlled silicon
nanocrystals in silicon nanocrystal/SiO2 superlattices is reported. The samples were prepared using thermal
evaporation and subsequent thermally induced phase separation. The slow (microseconds) decay line shape is
described well by a stretched exponential. The temperature dependence of the photoluminescence dynamics
can be understood in terms of thermal activation of recombination processes, including hopping of carriers
between localized states. Additional hydrogen treatment causes an increase in both parameters of the stretched
exponential function. This behavior is interpreted as a consequence of H2-passivation of dangling bonds defects.
Silicon nanocrystal quantum dots in a dielectric matrix form a material with higher band gap than silicon, but still
compatible with silicon technology. So far, devices using silicon nanocrystals have been realized either on silicon
wafers, or using in-situ doping in the superlattice deposition which may hinder the nanocrystal formation. In this paper, a
vertical PIN device is presented which allows to investigated the electrical and photovoltaic properties of nanocrystal
quantum dot layers. The device structure circumvents any influence of a substrate wafer or dopants and provides full
flexibility in the material choice of both, i.e. electron and hole, contacts. Furthermore, not-high-temperature stable
contact materials can be applied. Devices have been realized using SiC/Si nanocrystal multilayers as the i-region and
doped a-SixC1-x:H layers as electron and hole contacts. First devices show open-circuit voltage of up to 400mV.
Pavel Kashkarov, Olga Shalygina, Denis Zhigunov, Dmitri Sapun, Sergei Teterukov, Victor Timoshenko, Johannes Heitmann, Michael Schmidt, Margit Zacharias, Kenji Imakita, Minoru Fujii, Shinji Hayashi
Photoluminescence properties of Er-doped nanocrystalline Si/ Si02 structures have been investigated under strong optical excitation. The energy of optical excitation of Si nanocrystals was shown to be almost completely transferred to Er3+ ions in surrounding Si02. It was found that at high pump intensity the energy transfer process competes successfully with nonradiative Auger-recombination in Si nanocrystals. At high excitation level the population inversion of Er3+ ions was achieved and a decrease of the decay time of the photoluminescence at 1.5 μm was observed. Possible mechanisms of the shortening of the Er3+ ion lifetime are discussed.
We present and characterize hexagonal point defects in a two dimensional photonic crystal based on macroporous silicon. These point defects are prepatterned periodically, forming a superstructure within the photonic crystal after electrochemical etching. Spatially resolved, optical investigations related to morphological properties, like defect concentration and pore radius, are compared to bandstructure calculations. The confined defect states are identified and their interaction is evaluated quantitatively.
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.
Ordered Si nanocrystals showing a strong room temperature luminescence are prepared using a novel SiO/SiO2 superlattice approach fully compatible to Si technology. This enables independent control of particle size as well as particle density and particle position. Size control is demonstrated for nanocrystal sizes of 3.8 nm to 2 nm. A size-dependent blue shift of the luminescence from 900 to 750 nm and a luminescence intensity comparable to porous Si are observed. Experiments using high power excitation show a saturation behavior, a blue shift of the PL peak position by 61-87 meV and an increase in full width at half maximum depending on size. A blue shift by around 60 meV is found for luminescence at 5 K. Erbium doping of the superlattice structure for photonic applications is discussed.
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