Vojtech Nádaždy

New experimental facts on the Staebler–Wronski effect

Abstract The affect of light soaking on the gap-state distribution of undoped hydrogenated amorphous silicon (a-Si:H) has been investigated using the charge deep level transient spectroscopy (DLTS) technique. Measured spectra directly show the change in all the gap states (Dh, Dz, De) due to the prolonged illumination with low intensity white light. Results are tentatively interpreted evoking the pecularities of hydrogen diffusion in silicon.

Grain Boundary Effect on Charge Transport in Pentacene Thin Films

We report on charge transport properties of polycrystalline pentacene films with variable average grain size in the range from 0.1 to 0.3 µm controlled by the preparation technology. We illustrate with the organic field-effect transistors decrease of the effective mobility and presence of traps with decrease of the grain size. Analysis of the charge transfer excitons reveals decrease of the mobile charge density and the steady-state voltammetry showed significant increase of oxygen- and hydrogen-related defects. We also briefly discuss accumulation of the defects on the grain boundary and show relation between the defect density and grain boundary length.

Orientation Ordering of Nanoparticle Ag/Co Cores Controlled by Electric and Magnetic Fields

The effect of electric and magnetic fields on the sandwich structure Pt/hydrogenated amorphous silicon (a-Si:H)/stearic acid monolayer/Langmuir-Blodgett film of Ag/Co nanoparticles encapsulated in an organic envelope is studied. This structure is used as a working electrode in an electrochemical cell filled with NaCl solution (1 mM) and equipped with an Ag/AgCl reference electrode. Reversible changes in voltammograms are observed due to treatments (negative or positive bias voltage and simultaneous laser irradiation) applied to the designed structure before measurements. As an explanation of the observed phenomena we suggest that both the Co-up and Ag-up (on the a-Si:H surface) orientation orderings of nanoparticle Ag/Co cores are repeatedly reached. The role of the photovoltaic material (a-Si:H) in the observed behavior is explained. Voltammetric measurements with an applied magnetic field support our idea about the orientation ordering of nanoparticle cores.

Small-signal deep level transient spectroscopy on hydrogenated amorphous silicon based metal/insulator/semiconductor structures

The high-temperature tail of small-signal deep level transient spectroscopy (DLTS) spectra of hydrogenated amorphous silicon (a-Si:H) based metal/insulator/semiconductor structures has been analyzed in terms of oxidation of some negatively charged mobile hydrogen species at the Al/a-Si:H interface, the excitation pulses of an amplitude of 0.1 V being applied repetitively between the top Al and the bottom (gate) electrodes. The concentration of the mobile hydrogen in the a-Si:H layer has been estimated to an amount up to ≈2×1020 cm−3, taking the earlier published data on the diffusion coefficient. Having chosen an appropriate polarity of the excitation pulses, one could observe a DLTS peak moving with applied bias on the temperature axis. The latter is pointing to an energy distribution of states at the Al/a-Si:H interface. The interface state density increases with increasing energy depth of the states. At the ultimate experimentally observable energy of E≈0.95 eV below the electron mobility edge Ec a cor...

Effect of crystallinity on UV degradability of poly[methyl(phenyl)silane] by energy-resolved electrochemical impedance spectroscopy

Low stability and degradability of polymers by ambient air, UV irradiation or charge transport are major problems of molecular electronics devices. Recent research tentatively suggests that the presence of a crystalline phase may increase polymer stability due to an intensive energy trapping in the ordered phase. Using the UV degradability, we demonstrate this effect on an archetypal model σ bonded polymer - poly[methyl(phenyl)silane] (PMPSi) - with partially crystalline and amorphous-like layers. UV degradation with 345 nm, derived from the branching state generation rate, was inversely proportional to the crystalline phase content, changing from 4.8x1011 s-1 (partially crystalline phase) to 1.8x1013 s-1 (amorphous-like phase). A model is proposed where crystallites formed by molecular packing act as effective excitation energy traps with a suppressed nonradiative recombination improving thus PMPSi film stability. The molecular packing and higher crystalline phase proportion may be a general approach for...

Determination of the flatband voltage and the electron threshold voltage of a-Si:H based MIS structures by the feedback charge C–V method

Abstract The spectroscopic feature of the feedback charge capacitance as a function of voltage (C–V) method is demonstrated using a simplified energy band diagram of a high resistivity semiconductor with a negligible space charge interfaced with an insulating layer. To transform the gate bias scale to an energetic one, one has to find the flatband voltage of the device. The position of the equilibrium Fermi level in the a-Si:H with respect to the mobility edge is deduced from the difference between the electron threshold and flatband voltages of the thin film transistor.

Electrochemical impedance spectroscopy for study of electronic structure in disordered organic semiconductors—Possibilities and limitations

There is potential in applying conjugated polymers in novel organic optoelectronic devices, where a comprehensive understanding of the fundamental processes and energetics involved during transport and recombination is still lacking, limiting further device optimization. The electronic transport modeling and its optimization need the energy distribution of transport and defect states, expressed by the energy distribution of the Density of States (DOS) function, as input/comparative parameters. We present the Energy Resolved-Electrochemical Impedance Spectroscopy (ER-EIS) method for the study of transport and defect electronic states in organic materials. The method allows mapping over unprecedentedly wide energy and DOS ranges. The ER-EIS spectroscopic method is based on the small signal interaction between the surface of the organic film and the liquid electrolyte containing reduction-oxidation (redox) species, which is similar to the extraction of an electron by an acceptor and capture of an electron by...

On the influence of an ultrathin Al overlayer on GaAs plasma oxide growth kinetics

The growth rate of GaAs anodic oxide in radio-frequency plasma at low temperatures can be strongly affected by an ultrathin aluminum layer (<3 nm) evaporated on the semiconductor before oxidation. The effect of this layer on the properties of the resulting oxide-GaAs interface has been investigated. Comparing the structures grown by means of the Al overlayer to the sample with a pure native oxide, the corresponding deep level transient spectra differ markedly and the GaAs lattice contraction below the double oxide is reduced. Application of a fitting procedure to a simple layer growth model leads to interpretation of the investigated process by increasing both the migration coefficient of the negative oxygen ions in the oxide and concentration of O- ions on the dielectric surface during the oxidation procedure.

Diffusion effects at PtGaAs interfaces

Abstract The effect of γ irradiation and electric field on the properties of PtGaAs contacts is studied. It is found that such treatments cause reversible diffusion of platinum at the interface. This accounts for the relaxation of mechanical stresses in the contact.

Electrochemically induced charge injection in disordered organic conductive polymers

Slovak Research and Development Agency [APVV-14-0891, APVV-0096-11]; Scientific Grant Agency VEGA [1/0501/15, 2/0163/17, 2/0081/18]; project Efficient controlling of the production and consumption of energy from renewable sources, ITMS [26240220028]; Research and Development Operational Programme - ERDF