V. Kažukauskas

Interaction of deep levels and potential fluctuations in scattering and recombination phenomena in semi‐insulating GaAs

The complex influence of recombination centers and potential fluctuations of the band gap on the scattering and recombination phenomena in n‐type semiinsulating liquid‐ encapsulated‐Czochralski‐grown GaAs were investigated by using the transient photoconductivity and photo‐Hall effects. The inhomogeneities cause a hyperbolic decrease of nonequilibrium carrier concentration and the saturation of Hall mobility, while the exponential parts of the decay appear due to the recharge of deep levels. The mean recombination barrier heights of potential fluctuations were evaluated. We propose a complex ‘‘island’’ model of scattering and recombination centers, consisting of defect clusters and their associations around dislocations, surrounded by potential barriers. At low light intensities and at the temperatures below 330 K they are insulating for majority charge carriers, thus reducing an effective crystal volume and causing percolation transport effects. At the temperature higher than 330–360 K the main barrier o...

Opportunities of deoxyribonucleic acid complexes composites for nonlinear optical applications

In this paper, we illustrate new functionalities for nonlinear optical applications of bio-molecular systems. This study presents DNA complex with new ionic surfactants. These surfactants enabled DNA solubility in solvents other than alcohols, like aromatic and chlorinated ones. Composites with two nonlinear optical (NLO) active dyes are subjects of the second and third harmonic generation experiments. The found effective nonlinear susceptibilities values are much higher than that for standard fused silica. We also demonstrate any influence of the surfactant on NLO properties.

Enhancement of linear and nonlinear optical properties of deoxyribonucleic acid-silica thin films doped with rhodamine

In this work, we present the linear and nonlinear optical properties of DNA as functional material, incorporated into a silica material matrix with rhodamine organic dye. We observed that even low concentration of DNA affects the aggregate behavior of the dyes in silica films. The samples with DNA showed higher transmittance and fluorescence efficiency. Moreover, the presence of DNA has been found to significantly enhance the nonlinear optical response of the systems. In this way, we prove that silica materials can provide suitable matrices for hybridization with functional molecules and can be utilized as active optical waveguide materials with enhanced nonlinear optical properties.

Charge Carrier Trapping in Organic Solar Cell Structures P3HT:PCBM

We have investigated carrier transport and trapping in blends of 6:5 wt. P3HT:PCBM that are important for the development of organic Solar cells. The devices with the inverted layer sequence and solar efficiency of 3.7% were analysed. We demonstrate that, though the fill factor of the IV characteristics is as high as 68%, carrier trapping is effectively involved in the transport phenomena. The evaluated trapping state activation energy is about 0.18 eV and their density is up to 1020 ÷ 7 × 1021 cm−3. At such high density these states may probably act as transport states, limiting carrier mobility. The results were analyzed by taking into account mobility variation according to the Gaussian disorder model as well as carrier thermal generation from traps. The mobility parameters obtained by both methods demonstrate good coincidence.

Electrical and optical properties of the CdS quantum wells of CdS/ZnSe heterostructures

Earlier we reported the investigation of the electrical properties of selectively doped and degenerate CdS/ZnSe quantum heterostructures grown by molecular beam epitaxy [V. Kažukauskas, M. Grun, St. Petillon, A. Storzum, and C. Klingshirn, Appl. Phys. Lett. 74, 395 (1999)]. The maximum Hall mobilities in these heterostructures were found to be less than 400 cm2/Vs. In the present work we analyze in detail the scattering mechanisms in order to increase the carrier mobility and to optimize these quantum structures. We demonstrate that the Hall mobility can reach in the CdS quantum wells at low temperatures 2800 cm2/V s for slightly doped structures, having an effective sheet carrier density 2.6×1011 cm−2. In these structures the mobility is mostly limited by interface alloying scattering. At high doping levels carriers become redistributed between the quantum well and the ZnSe doped layer. This causes the parallel conductivity phenomena, which diminishes the effective mobility. Near room temperature the sca...

Determination of the charge trap energy distribution in conjugated polymer MDMO-PPV

The trap distribution depending on the exciting light spectral range and applied electrical field in the MDMO-PPV ([poly-(2-methoxyl),5-(3,77dimethyloctyloxy)] paraphenylenevinylene) was investigated by the thermally stimulated current (TSC) technique. To ensure selective excitation of the defect states we have used the long-pass colour filters with cut-on energies from 1.77 up to 3.10 eV. In order to evaluate the trap parameters of MDMO-PPV, curve fitting was done. The analysis revealed the Gaussian distribution of the defect states; the estimated effective activation energies were about 0.22 and 0.40 eV.

GaAs peculiarities related with inhomogeneities and the methods for reveal of their properties

Abstract The inhomogeneities in as-grown and irradiated GaAs crystals and structures, their properties and role have been analysed. The influence of inhomogeneities on the properties of crystal and Schottky diode structure are presented. The possibilities of different methods for material and structure characterisation (Hall effect and magnetoresistance, noise spectra, thermally stimulated current and polarisation, photoconductivity kinetics and spectrum, light-induced diffraction) are discussed. Existence of the inhomogeneity of the crystal structure has been revealed by diffusion of copper through the GaAs wafer, and a percolation of it through doped crystal has been determined by means of the Hall effect. Modification of the recombination-induced defect system has been analysed to get deeper insight into the crystal and device degradation under illumination by ionising radiation. Role of free carriers in local electric field generation has been analysed via modification of the EL2 centre in GaAs. Complicated behaviour of non-equilibrium conductivity during quenching and activation of EL2 centres has been determined by precise measurement of the photoconductivity spectra in different crystals. Influence of the EL2 centre on the non-linear polarisation of sample and on the additional non-active light absorption has been analysed.

Charge trapping in [poly-(2-methoxyl),5-(3,77dimethyloctyloxy)] paraphenylenevinylene synthesized in different routes affected by the energetical distribution of trapping states

In this study, the thermally stimulated current measurements are used as a direct photoelectrical method to prove distribution of the trapping states in MDMO-PPV ([poly-(2-methoxyl),5-(3,77dimethyloctyloxy)] paraphenylenevinylene) polymers synthesized in the “Gilch” and “Sulfinyl” routes. It was demonstrated that two traps with the Gaussian distribution of the states and effective mean activation energies of about 0.22 and 0.40 eV are prevailing in MDMO-PPV synthetized in the “gilch” route, meanwhile in the material synthesized in the “sulfinyl” route the best expressed traps appear at 0.28 eV.

Correlation between temperature activation of charge-carrier generation efficiency and hole mobility in small-molecule donor materials.

In organic solar cells, free charge carriers are generated at the interface between an electron-donating and an electron-accepting material. The detailed mechanisms of the generation of free charge carriers are still under discussion. In this work, we investigate the influence of temperature on the generation efficiency of free charge carriers in blends of dicyanovinyl substituted oligothiophene (DCVnT) molecules and C60 by quasi-steady-state photoinduced absorption (PIA) measurements. The observed positive temperature dependence of charge-carrier generation can be directly correlated to the charge-transport behavior. The determined activation energy scales inversely with the hole mobility for all investigated DCVnT derivatives, suggesting higher dissociation probability of bound interfacial charge pairs at high mobility. Furthermore, the energetic disorder parameter, σ, determined by CELIV (charge extraction by linearly increasing voltage) measurements for a DCV6T derivative, matches the activation energy from the PIA measurements. In conclusion, these results underline the need for high-mobility donor materials for optimal charge-pair dissociation in organic solar cells.

Experimental observation of two-dimensional electron gas in the CdS quantum wells of CdS/ZnSe heterostructures

We report the observation of a two-dimensional electron gas (2DEG) in the CdS quantum wells of CdS/ZnSe quantum heterostructures grown by molecular beam epitaxy. The CdS layers are about 4 nm thick; the thicknesses of the ZnSe-doped layers and the growth temperatures of the heterostructures were varied in order to investigate their influence on the 2DEG parameters. The effective sheet concentration of the 2DEGs reached up to 1.5×1013 cm−2. The maximal Hall mobilities were nevertheless still inferior or equal to 380 cm2/V s. Possible scattering mechanisms that cause such low values are discussed. Most probably, it is scattering by stacking faults or by interface roughness.