A. V. Vasin

Color control of white photoluminescence from carbon-incorporated silicon oxide

Color control of the white photoluminescence (PL) from carbon-incorporated silicon oxide is demonstrated. The carbon-incorporated silicon oxide was fabricated by carbonization of porous silicon in acetylene flow (at 650 and 850 °C) followed by wet oxidation (at 650 and 800 °C). It was shown that PL color can be controlled in the range of blue-white and yellow-white by selecting the porosity of starting porous silicon as well as the carbonization and oxidation temperatures. Low-temperature oxidation resulted in bluish light emission in lower porosity series, while high-temperature oxidation promoted yellow-white light emission. The maximal integral intensity of PL was observed after oxidation at 800 °C. It was shown that white PL from carbon-incorporated silicon oxide has blue and yellow-white PL bands originating from different light-emitting centers. The origin of blue PL is attributed to defects in silicon dioxide. Some trap levels at the interface of the carbon clusters and silicon oxide are suggested ...

Strong white photoluminescence from carbon-incorporated silicon oxide fabricated by preferential oxidation of silicon in nano-structured Si:C layer

A new approach to development of light-emitting SiO2:C layers on Si wafer is demonstrated. Carbon-incorporated silicon oxide was fabricated by three-step procedure: (1) formation of the porous silicon (por-Si) layer by ordinary anodization in HF:ethanol solution; (2) carbonization at 1000 °C in acetylene flow (formation of por-Si:C layer); (3) oxidation in the flow of moisturized argon at 800 °C (formation of SiO2:C layer). Resulting SiO2:C layer exhibited very strong and stable white photoluminescence at room temperature. It is shown that high reactivity of water vapor with nano-crystalline silicon and inertness with amorphous carbon play a key role in the formation of light-emitting SiO2:C layer.

Structure, paramagnetic defects and light-emission of carbon-rich a-SiC:H films

The effect of vacuum annealing on local structure reconstruction, evolution of photoluminescence (PL) and paramagnetic defects in carbon-rich a-Si1−xCx:H films (x=0.7) was studied. Strong enhancement of visible (white-green) PL was observed after annealing in the temperature range of 400–500 °C. Such enhancement was correlated with increasing of the concentration of carbon-hydrogen bonds in Si:CHn accompanied with increase in the fluctuation of the interatomic potential. Complete disappearance of PL, “graphitization” of the carbon precipitates, and a strong increase in the concentration of the paramagnetic states were observed after annealing at 650 °C and above. The enhancement and the degradation of PL after different-temperature treatments are explained by the following competing effects: (1) enhancement of the radiative recombination due to passivation of paramagnetic defects with hydrogen and increase of localization of photoexcited electron-hole pairs due to formation of new Si:CH and (2) enhancem...

Study of the processes of carbonization and oxidation of porous silicon by Raman and IR spectroscopy

Porous silicon layers were produced by electrochemical etching of single-crystal silicon wafers with the resistivity 10 Ω cm in the aqueous-alcohol solution of hydrofluoric acid. Raman spectroscopy and infrared absorption spectroscopy are used to study the processes of interaction of porous silicon with undiluted acetylene at low temperatures and the processes of oxidation of carbonized porous silicon by water vapors. It is established that, even at the temperature 550°C, the silicon-carbon bonds are formed at the pore surface and the graphite-like carbon condensate emerges. It is shown that the carbon condensate inhibits oxidation of porous silicon by water vapors and contributes to quenching of white photoluminescence in the oxidized carbonized porous silicon nanocomposite layer.

Effects of hydrogen bond redistribution on photoluminescence of a-SiC:H films under thermal treatment

Hydrogenated amorphous silicon carbide (a-SiC:H) films have been deposited using magnetron sputtering technique. An integrated investigation of the effect of vacuum annealing temperature on photoluminescence properties and paramagnetic defects and its correlation with structural transformation of a-SiC:H has been performed. Significantly enhanced light emission efficiency after low-temperature vacuum treatment (450°C) is found due to enhanced passivation of paramagnetic defects associated with carbon-rich chemically disordered structure. Subsequent high-temperature vacuum annealing results in a decrease of luminescent intensity that is associated with an increase of carbon-related paramagnetic defect states, shown to be the primary nonradiative recombination centers.

Magnetic Resonance and Optical Study of Carbonized Silica Obtained by Pyrolysis of Surface Compounds

The carbonized silica (SiO2:C) nanopowders were prepared by chemical modification of fumed silica (aerosil) by phenyltrimethoxysilane followed by thermal annealing at temperature in range of 500-800 °C in nitrogen flow. Their magnetic properties were investigated by electron paramagnetic resonance (EPR) in the temperature range from 4.2 K to 292 K. The initial and annealed SiO2:C samples revealed carbon (C) related defects. The carbon related radicals (CRR) in annealed SiO2:C nanopowders with g-factors 2.0042, 2.0039 were attributed to the oxygen (O)-centered CRR and C-centered CRR with a nearby O heteroatom, respectively. The EPR data were compared with infrared (IR) and photoluminescence (PL) data. It was found that the position of the PL band depends on the type of CRR formed after sample annealing. The PL with maximum intensity at 440 nm was found for the sample annealed at 500°C in which O-centered CRR was observed while in the sample annealed at 600°C in which C-centered CRR with a nearby O heteroatom was observed and graphite-like amorphous C clusters were appeared the peak of the PL band was shifted to the 510-520 nm.

The Effect of Incorporation of Hydrocarbon Groups on Visible Photoluminescence of Thermally Treated Fumed Silica

Methyl, methoxy and alcoxy groups with different number of carbon atoms were chemically grafted onto the surface of fumed silica nanoparticles. After chemical modification the nanopowders were annealed in vacuum at 700 °C. The effect of the amount of carbon atoms in grafted hydrocarbon groups and type of bonding to silica surface (Si-C v.s. Si-O-C) were studied. It was demonstrated that carbon incorporation results in the development of broad band photoluminescence that covers the wholevisible spectral range. Increasing of carbon incorporation resulted in increasing of photoluminescence intensity and red shift of the photoluminescence band maximum.

The nature of white luminescence in SiO2:C layers

Silicon oxide layers containing incorporated carbon (SiO2:C) have been obtained by sequential thermal carbonization/oxidation of porous silicon. The as-synthesized SiO2:C layers exhibit intense white photoluminescence (PL). The characteristics of excitation, emission, and relaxation of the white luminescence in the SiO2:C layers have been studied. It is established that the observed broad PL band in fact consists of at least two subbands with the maxima of intensity in the green and blue spectral regions. Based on the experimental data, a model of the PL excitation and radiative recombination in SiO2:C layers is proposed and justified.

Nano-Order Structural Analysis of White Light-Emitting Silicon Oxide Prepared by Successive Thermal Carbonization/Oxidation of the Porous Silicon

Recently the present authors’ group found that porous silicon showed strong and stable white/white-blue light emission after successive thermal carbonization and oxidation by water vapor. This material can be considered as a price-competitive solid-state white-light source. We examined these layers by electron energy-loss spectroscopy (EELS), energy-filtering transmission electron microscopy (EFTEM). The EEL spectra indicated that the silicon skeleton in the porous layer was completely oxidized by the thermal treatment in wet argon ambient and multi-types of carbon phases were present in the 1073 K oxidized sample of stronger emission, while carbon complexes including Si and/or O were formed in the 1223 K oxidized sample of weaker light emission. EF-TEM images showed that carbon/oxygen were more uniformly distributed in the 1223 K oxidized sample. It is assumed that the strong light-emitting properties are controlled by the size and internal chemical bonding states of carbon clusters incorporated.

Carbonization of porous silicon for 3C-SiC growth

In the present work, the carbonization of porous silicon for the subsequent 3C-SiC growth has been systematically studied. The effect of temperature and acetylene flow-rate on the chemical state of the surface and structure relaxation was studied. It was found that the porous nano-crystalline morphology is unstable and tends to recrystallize in temperature range typical of 3C-SiC growth on Si (10000C-13000C). The carbonization impedes recrystallization at 10000C, but at 13000C the full recrystallization takes place. Pyrolytic amorphous graphite-like carbon was found on porous silicon carbonized at temperature and with acetylene flow-rate above critical values.