National Institute Of Materials Physics - Romania

This paper investigates the conduction mechanism in Cr2O3 gas sensing thick films. A citrate combustion method was used for the preparation of the metal oxide and ethanol vapour as a test gas. The manner in which surface reactions induced electrical changes are affecting the sensor signals inputs was explored by simultaneous dc and work function changes (Kelvin probe method). The identification of the contributions to conduction of the different sensing layer elements was made possible by ac impedance spectroscopy measurements. A conduction model, which qualitatively explains the experimental findings, was elaborated on the basis of the acquired experimental data and the information provided in literature. The model validity should apply, besides Cr2O3, to all p-type metal oxides used as gas sensitive materials. (C) 2008 Elsevier B.V. All rights reserved.

This paper demonstrates an analytical expression for the quasistatic capacitance of a quantum dot layer embedded in a junction, where the reverse bias is used to discharge the initially occupied energy levels. This analysis can be used to determine the position and the Gaussian homogeneous broadening of the energy levels in the conduction band, and is applied for an InGaAs/GaAs quantum dot structure grown by metal organic chemical vapor deposition. It is shown that the Gaussian broadening of the conduction band levels is significantly larger than the broadening of the interband photoluminescence (PL) transitions involving both conduction and hole states. The analysis also reveals a contribution from the wetting layer both in PL and modeled C-V profiles which is much stronger than in typical molecular beam epitaxy grown dots. The presence of a built-in local field oriented from the apex of the dot toward its base, contrary to the direction expected for a strained dot with uniform composition (negative dipole), is also derived from fitting of the C- V experimental data. (c) 2008 American Institute of Physics.

Several hole trapped centres, namely Tl2+, self-trapped hole and A-type centres, were observed by ESR in PbBr2:Tl single crystals after X-ray irradiation at 77 K. The corresponding spectra are visible up to 200 K for all three defect centres. The STH centres are observed at higher temperatures than in the undoped PbBr2 single crystals, probably due to the stabilising effect of the Tl+ impurities. The ESR parameters of the Tl2+ centre point to stronger covalency effects in PbBr2 than in the isostructural PbCl2 crystals. All hole trapped centres started decaying around 160 K, probably due to recombination with nonparamagnetic electron trapped centres. Besides a small concentration of self-trapped electron centres, no other electron trapped centres were observed. (c) 2008 Elsevier B.V. All rights reserved.

Single crystallites of superhard, semiconducting, n-type, amber colored and p-type, blue colored, Be-doped cubic boron nitride have been irradiated either with UV (350 nm) light or with an intense beam of accelerated (1 MeV) electrons. The examination of the irradiated samples at low temperatures by high frequency W (95 GHz)-band electron spin resonance reveals several new, radiation-induced, isotropic paramagnetic centers. The UV irradiation of both types of crystals yields centers involving very likely protons. In the amber c-BN crystals the irradiation with I MeV electrons results in the formation of vacancy associated paramagnetic defects and quasi-free electrons in colloidal particles. (C) 2008 Elsevier B.V. All rights reserved.

Mono and multilayer TiO2(Fe, PEG(600)) films were deposited by the dip-coating on SiO2/glass substrate using sol-gel method. In an attempt to improve the antibacterial properties of doped TiO2 films, the influence of the iron oxides and polyethilenglycol (PEG(600)) on the morphological, optical, surface chemical composition and biological properties of nanostructured layers was studied. Complementary measurements were performed including Spectroscopic Ellipsometry (SE), Scanning Electron Microscopy (SEM) coupled with the fractal analysis, X-Ray Photoelectron Spectroscopy (XPS) and antibacterial tests. It was found that different concentrations of Fe and PEG(600) added to coating solution strongly influence the porosity and morphology at nanometric scale related to fractal behaviour and the elemental and chemical states of the surfaces as well. The thermal treatment under oxidative atmosphere leads to films densification and oxides phase stabilization. The antibacterial activity of coatings against Escherichia Coli bacteria was examined by specific antibacterial tests.

The influence of the gate voltage on photoconductivity in P-Si/SiO2/(nano)crystalline PbS, Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET)-like structures is investigated. The effect on the photo-signal in the wavelength range 500-1180 nm is opposite compared to the effect in the wavelength range 1600-2500 nm: a negative gate voltage increases the signal in the short wavelength range and decreases it for the long wavelength range. The opposite is valid for a positive gate bias. An on-off ratio better than 85 times is obtained for the spectral range 500-1180 nm, corresponding to the absorption in p-Si and better than 3.5 times for the spectral range 1600-2500 nm corresponding to the absorption in the (nano)crystal line PbS region, respectively. (c) 2007 Elsevier B.V. All rights reserved.

Leakage currents determine bumps in the capacitance-voltage characteristics of metal-oxide-semiconductor-type heterostructures with capacitance values larger than the insulator capacitance. A nearly parallel shift with frequency is also observed. These aspects are connected with the presence of a high density of interface states. We illustrate these effects for the case of nano-PbS/SiO2/Si heterostructures. The phenomena were simulated and we propose a method to quickly estimate the interface state density level of a heterostructure when leakage channels are present. (C) 2008 American Institute of Physics.

The nano-crystalline PbS/n-Si heterostructure was studied using photocurrent spectra, I-V and C-V characteristics and admittance spectroscopy. The frequency dependent junction capacitance and conductance measurements show the presence of two contributions: first, a defect related mechanism which we attribute to a deep trap level with a large cross-section in nano-crystals with smaller sizes around 5 nm at the interface with Si and a second mechanism with an activation energy of about 250 meV, attributed to carrier transport in relatively large PbS grains of about 15 nm in size. (c) 2007 Elsevier B.V. All fights reserved.

Dark noise plays an important role in electronic devices, especially in detectors. Performance of infrared detectors is reflected by parameters such as detectivity and specific detectivity, strongly related to the noise spectral density. This work summarizes the results on characterization of high-performance PbS photo detectors, using a certification pending method developed by the laboratory, in agreement to the standard ISO/CEI 17025:2005. The method is validated for a resistor of known resistance, by comparing the Johnson thermal noise using two different methods: first, the fast Fourier transform measured with a Stanford Research Systems SR785 Spectrum Analyzer and second using a Stanford SR830 lock-in amplifier which measures the rms noise for a certain frequency. We studied the influence of physical dimensions on the performance of thin film polycrystalline PbS photoresistors in the planar geometry. Measurements were performed to compare the dark noise of PbS photoresistors with that in high quality metal thin film resistors. The laboratory setup used for the measurements is appropriate to accurately determine typical parameters of infrared detectors such as spectral responsivity, blackbody responsivity, specific detectivity, and noise spectral density.

Self assembled InGaAs/GaAs quantum dots (QD) have a great potential for high performance optoelectronic devices such as low threshold laser diodes, infrared detectors, modulators, memories. In order to characterize the behavior of the QD system, we use two p(+)-n structures grown epitaxially on GaAs under similar conditions. The first structure acts as reference while in the second structure a single QD self-assembled layer is introduced in the middle of the n-GaAs matrix layer. The structure is designed such that for OV applied bias the QD layer lies outside the depleted region. When the reverse bias is increased, the charge from the QD system is removed and the depletion layer moves further into the GaAs matrix material. The electronic structure of the QD is investigated using two methods: photoluminescence, in order to characterize the transition energies between electron and hole levels in the QD system and capacitance spectroscopy in order to study the electron levels in the conduction band only. In addition, admittance spectroscopy spectra are measured in order to characterize the carrier transport mechanism. There is no evident step in the capacitance versus frequency behavior at room temperature in the range 1 Hz-1 MHz, indicating a large carrier cross section caption and/or a low activation energy for the carrier transport between the dot system and the wetting layer and GaAs barriers. The plateau in the C-V behavior, due to charging or discharging of the QD system is modeled using the solution of a Poisson equation and the resulting energy of the electron states within the conduction band and QD size distribution are correlated with results from photoluminescence studies, which involve transitions between energy levels both from the conduction and valence bands.