National Institute Of Materials Physics - Romania

We investigated MOCVD growth of c-axis two-layer and three-layer stacked heterostructures on (100) SrTiO3 of SI, IS or SIS type. S denotes high temperature superconductor thin films of YBa2Cu3O7 (Y-123), Bi2Sr2Ca2Cu3O10 (Bi-2223) or Bi2Sr2CaCu2O8 (Bi-2212), while I stands for insulating thin films of (Ba, Ca) CuO2 or (Sr, Ca) CuO2. X-ray diffraction, atomic force microscopy and measurements of resistivity versus temperature were performed after the deposition of each layer. The best results, suitable for further device fabrication, were obtained for the SIS structure (100) SrTiO3/Bi-2223/(Sr, Ca) CuO2/Bi-2212. Similar structures with I = (Ba, Ca) CuO2 have shown higher impurity phases and lower superconducting quality of the top superconducting layer. For two-layer or three-layer structures, Bi-2223 as a top layer could not be obtained and, instead, a phase with Bi-2212 formed. Structures with (100) SrTiO3/Y-123/(Sr, Ca) CuO2/Y-123 or (100) SrTiO3/Y-123/(Ba, Ca) CuO2/Y-123 were also promising, but a current leak often occurred due to greater roughness. Structures with Y-123 are less sensitive to growth temperature than those with BSCCO.

This paper presents some investigations on the electrical transport properties of ITO/single (double) layer organic semiconductor (m-DNB, benzil, PTCDA, Alq3) contacts in SIS-like (ITO/organic/Si) and MIS-like (ITO/organic/metal) heterostructures. The I-V characteristics have emphasised the injection properties of different contacts and the effect of space charge limited currents in correlation with the type and preparation conditions of the contacts. We have studied the influence of the type of contact (In/ITO; In/Al) on the electrical conduction in Alq3/PTCDA/Si/In heterostructure. In a planar grid contact configuration for In/Al/PTCDA/Al/In structure we have observed the effect of the low electric field on the shape of the I-V characteristic. (c) 2007 Elsevier B.V All rights reserved.

We report here on the characteristics of RF-sputtered 300 nm thick films of TiO2-2xNx prepared on glass substrates at 350 degrees C, by adjusting the N-2:Ar partial pressure ratio in the deposition chamber between 0.00 and 0.33. XRD, XPS, AFM and contact angle data were used to derive film structure, elemental composition and oxidation state of Ti, surface morphology and hydrophilicity, respectively. The band gap was derived from spectral data in the 350-450 nm range. Film structure and composition were changed by adjusting the partial pressure of the reactive gases during sputtering and by post-deposition annealing at 400 degrees C in air, for 90 min. The values of the contact angle of films' surface with de-ionized water and of surface free energy per unit area show that films are super-hydrophilic for high-nitrogen content. Correlations are made between film structure, elemental composition, electronic and wettability properties. (c) 2007 Elsevier B.V. All rights reserved.

In-plane zero-magnetic-field current-voltage (I-V) characteristics of oxygen-deficient YBa2Cu3O7-delta films (7-delta similar to 6.5, 6.55, and 6.65) were thoroughly investigated. A good agreement with the quasi two-dimensional (2D) vortex-antivortex unbinding scenario was observed, similar to the behaviour of highly anisotropic Bi2Sr2CaCu2O8+delta films. The temperature variation of the I-V exponent allows the determination of the Berezinskii-Kosterlitz-Thouless transition temperature T-KT at the superconducting Cu-O layer level, and the mean-field critical temperature T-c0. We found that both T-KT and T-c0 increase with increasing doping, and T-c0 remains in the region of the electrical resistivity drop. These results do not support the vortex fluctuation scenario for the superconducting transition in underdoped cuprates. (C) 2007 Elsevier B.V. All rights reserved.

A first sharp diffraction peak (FSDP) is the most sensitive characteristic to medium range order in non-crystalline solids with speculative origin up to now. The compositional trends in the FSDP position and full width at half maximum are tested on the example of pseudo-binary Sb2S3-As2S3 and Sb2S3-GeS2 systems of chalcogenide glasses. The FSDP-related correlations are found for the investigated materials and considered in terms of packing factor of amorphous layers and experimental density of glass.

A versatile method, that of sol-gel deposition was used to produce Si1-xGexO2 films on Si substrates. Samples after annealing at different temperatures are investigated by different techniques including high-resolution transmission electron microscopy and spectral ellipsometry. It was revealed that, by annealing at high temperatures, Ge accumulates in small clusters (5-10nm) inside of the oxide layer. By ellipsometric investigations, valuable data about the temperature evolution of the composition and optical constants were obtained.

The effect of a lateral electric field on the ground-state energy of an electron-hole pair confined in a quantum dot is investigated on different levels of sophistication. Two different regimes occur: a Coulomb dominated one, where the influence of the field is weak, and a regime dominated by field induced effects. Depending on the strength of the Coulomb interaction relative to the single-particle energies, which varies with the quantum dot parameters, the transition between these regimes is either smooth or rather abrupt.

The structure of the chalcogenide glass films of composition As2S3-GeS2 has been investigated bu X-ray diffraction. The relation between the quasi-distance as evidenced from the first sharp diffraction peak, and the composition ration has been estabilished. The Boolchand (intermediate) phase was found ro be characterized by large structuro-compositional fluctuations.

We present a theoretical study of the electronic transport through a many-level quantum dot driven by time-dependent signals applied at the contacts to the leads. If the barriers oscillate out of phase, the system operates like a turnstile pump under a finite constant bias, as observed in the experiments of Kouwenhoven [Phys. Rev. Lett. 67, 1626 (1991)]. The time-dependent currents and their averages over successive pumping periods are computed from the Keldysh formalism for tight-binding models. The calculation considers a sudden application of the pumping potentials at t=0, which leads to transient features of the time-dependent and averaged currents during the first pumping cycles which turn out to be important in the high-frequency regime. We show that in the transient regime, the efficiency of the system as a pump is rather poor because it mainly absorbs charge from both leads in order to fill the levels located below the bias window. Under a finite bias and a low-frequency pumping signal, the charge transferred across the system depends on the number of levels located within the bias window. The internal charge dynamics and the role of energy sidebands are investigated. The so-called satellite peaks of the averaged current are also observed in the transient regime.

In this paper, the trapping phenomena in silicon-based nanocrystalline semiconductors are studied. We propose a general and complete model for optical charging spectroscopy measurements, which takes into account the trapping-detrapping-retrapping processes and all the types of discharge currents that can occur in a nanocrystalline system. The model was applied to the measurements performed on multi-quantum well structures, (nc-Si/CaF2)(50), as well as on nanocrystalline porous silicon. This way, the trap parameters that are not directly measurable were determined. (C) 2007 Elsevier Ltd. All rights reserved.