P. Horváth

Thin plasma-polymerized films of dichloro(methyl)phenylsilane

Thin plasma polymer films were deposited from a mixture of dichloro(methyl)phenylsilane (DCMPS) vapour and gaseous hydrogen in a r.f. (13.56 MHz) capacitive coupling deposition system on pieces of silicon wafers. Some samples were annealed in vacuum at the temperature ranging from 450 to 700°C. Chemical composition, structure and surface morphology of annealed samples and those stored in air at room temperature were studied by FTIR, XPS, SEM, and optical microscopy. Thermal stability and a decomposition of the plasma polymer with increasing temperature were characterized by thermogravimetry together with mass spectrometry. The plasma polymer was stable up to a temperature of 300°C. Above that temperature the material started to decompose together with additional cross-linking due to incorporation of extra oxygen atoms forming new siloxane bonds. The plasma polymer was tough at room temperature but much more brittle at elevated temperatures.

Visible photoluminescence and electroluminescence in wide-bandgap hydrogenated amorphous silicon

Abstract The work describes basic photoluminescence (PL) and electroluminescence (EL) properties of wide-bandgap (2.0 eV or greater) hydrogenated amorphous silicon (a-Si: H). Thin films of wide-bandgap (high-hydrogen-content) a-Si: H were prepared by microwave electron cyclotron resonance plasma-enhanced chemical vapour deposition from SiH4 under high dilution with He. The films exhibit spectrally broad (full width at half-maximum, 0.4 eV or greater) visible PL at room temperature, peaked at about 1.5eV. On the basis of measurements of the PL temperature dependence, PL dynamics, infrared absorption spectra, picosecond pump-and-probe experiments and hydrogen thermal desorption spectroscopy the dominant microscopic mechanism of visible PL has been revealed to be the radiative de-excitation of oligosilane (-(SiH2)2-) units or of a specific defect in their close vicinity. EL has been investigated in sandwich p+‒i‒n+ and p+‒p‒n‒n+ structures with (Cr‒Ni)/indium tin oxide contacts. The as-grown structures exhibit good rectifying properties, low conductivity and no EL. After being subjected to a ‘forming’ procedure (application of a sufficiently high voltage), the current flowing through the structures increases abruptly by several orders of magnitude, up to about 10 mA, and the structures begin to emit weak EL at room temperature. The EL occurs in reverse bias only and its external quantum efficiency is about 10−5%. It is concluded that the forming procedure leads to partial crystallization of a-Si: H between the contacts and impact lattice ionization can participate in the light emission.

Luminescence as a tool for crosslinking determination in plasma polysilylenes prepared from organosilanes

Abstract The photoluminescence (PL) in polysilanes during the change from linear one-dimensional (1D) Si chain to amorphous three-dimensional (3D) Si network was studied. The excitonic absorption observed in 1D Si at 320 nm undergoes a blue-shift and broadening of main-chain σ–σ* transitions on introducing branching and networking defects. With the gradual transition to the 3D structure the extensive redistribution of oscillator intensity along the absorption edge with the loss of the resolved σ–σ* band edge. Further, the gradual disappearance of main-chain exciton σ–σ* and appearance of the defect luminescence centred approximately at 450 nm and exhibiting red-shift with increasing 3D networking are observed.

Nanostructural effects in plasmatically prepared polysilylenes

The photoluminescence of the plasma prepared polysilylenes from corresponding monomers, exempli.ed by poly[(methyl)phenylsilylene], is examined. For the preparation two quantitatively di erent plasmas, were used: radio frequency discharge (13:5 MHz) and microwave discharge (2:45 GHz) in the regime of electron cyclotron resonance. We concentrated on the conditions where the 1D features of the polymers were preserved. Results for both optimised groups of materials exhibit similar results: the excitonic absorption band with maximum at 320 –360 nm, depending on the conditions of preparation. We explain the observed phenomena by the occurrence of the nanosized 1D Si polymers embedded in 3D covalently bonded Si network. ? 2002 Elsevier Science B.V. All rights reserved.

Wide Gap Hydrogenated Amorphous Silicon for Visible Light Emission

A series of samples of hydrogenated amorphous silicon (a-Si:H) was prepared from silane diluted highly with He by the microwave electron-cyclotron-resonance PE CVD. Such a wide gap (E≥2.0 eV) a-Si:H emits room temperature photoluminescence (PL) in the visible region. We attempt to reveal the microscopic origin of this PL by monitoring variations of PL intensity vs frequency of infrared vibrations in the vicinity of 2100 cm−1. We find that oligosilanes -(SiH2)n- act as one type of possible luminescence centres. We report also on room temperature electroluminescence (EL) from p-i-n junctions. Surprisingly, and unlike p-i-n structures from standard a-Si:H, weak EL radiation with external quantum efficiency of the order of 10−5% is emitted under reverse bias only. EL and PL emission spectra resemble strongly each other, except high energy wing of the EL spectrum. This high energy widening indicates the participation of hot electrons in the EL excitation mechanism.

Luminescence in plasma polysilylenes prepared from organosilanes

Abstract The photoluminescence in polysilanes (exemplified on poly(methylphenylsilylene) – PMPSi) during the change from linear 1D Si chain to amorphous 3D Si networks was studied. The excitonic absorption observed in 1D Si at 320 nm undergoes a blue shift on introducing branching and networking defects. With the transition to the 3D structure the redistribution of oscillator intensity along the absorption edge with the loss of the resolved σ–σ * band edge occurs. Further, the disappearance of main-chain exciton σ–σ * and appearance of the defect luminescence at 450 nm exhibiting red shift with increasing 3D networking are observed.

Silicon-based materials for optoelectronics: visible photoluminescence and electroluminescence from amorphous silicon

Room temperature visible photoluminescence (PL) and electroluminescence (EL) of wide band gap hydrogenated amorphous silicon thin films prepared in SiH4 microwave plasma strongly diluted with He is reported. The emission spectra are peaked at approximately 1.5 eV. Films were characterized by means of optical and IR absorption and hydrogen thermal desorption. The band gap of a-Si:H films varies within the interval 2.0-2.2 eV. The strong evidence for two distinct types of PL processes is presented: one being linked with oligosilanes and the second one attributed to electron-hole recombination in tail states. EL has been investigated in p-i-n and p+-p-n-n+ structures with CrNi/ITO contacts. The EL occurs after initial forming in reverse bias only and its external quantum efficiency is approximately 10-5 percent. The shape and spectral position of EL spectra suggest that the light emission in this case is probably due to simultaneous excitation of the oligosilane units and impact ionization by hot electrons.