National Institute Of Materials Physics - Romania

The carrier transport mechanisms in as-grown LaAlO3 and La2Hf2O7 high-k insulator layers deposited on n- and p-Si were deduced from temperature dependent C-V and I-V characteristics correlated with photoelectric measurements. Large, parallel shifts in the high frequency C-V curves are explained by the presence of a large density of interface states and an approximate analytical formula relating the density of states to the C-V shift is deduced. The space charge limited current is explained by the existence of impurity channels situated energetically near the conduction or valence band of silicon.

Fatigue is investigated in epitaxial Pb(Zr,Ti)O-3 films grown on SrRuO3/SrTiO3 substrates with Pt or SrRuO3 (SRO) top electrodes. It was experimentally determined that fatigue occurs irrespective of whether the top electrode is Pt or SRO. The fatigue behavior is strongly dependent on the frequency. A polarization recovery was observed for both types of top electrodes, but the recovery is almost complete for a SRO top electrode and only about 40% from the initial polarization value for Pt top electrodes. The results are tentatively explained by the frequency response of the deep traps and by migration of oxygen vacancies.

Nanocrystalline PbS thin films were grown on glass substrates using Chemical Bath Deposition (CBD) method. The presence of nanocrystals was revealed by optical absorption and photocurrent measurements. We have compared the absorption threshold between large size and nanocrystalline PbS. Our study was performed in order to demonstrate that the size of the PbS thin films crystallites affects the photoelectric properties of the material. Using the Scanning Electron Microscopy (SEM) we have determined that the grains inside large size PbS crystallites are 300 nm. From the optical absorption measurements we have determined that the size of PbS nanocrystals is between 5-50 nm. The photoconduction measurements show that large size PbS has a maximum in infrared situated at 2.4 mu m, and in PbS nanocrystalline thin film this maximum is shifted toward smaller wavelengths towards visible and near infrared.

The lead zirconate-titanate (PZT) epitaxial thin films are analyzed as being p-type, wide-gap semiconductors with standard Schottky contacts. The results of the hysteresis, capacitance-voltage (C-V), and current-voltage (I-V) measurements are explained in the frame of the classical model for metal-semiconductor Schottky contacts, in which the effect of the ferroelectric polarization on band-bending was considered. The presence of the deep traps was also considered and their effect on the measured quantities was discussed. The free carrier concentration was estimated to about 3 x 10(18) cm(-3), with a remnant polarization of 40 mu C/cm(2), and with an effective density of the fixed charges in the depletion region of about 1.8 x 10(19) cm(-3).

Detectors processed on epitaxial silicon are a promising solution for the extreme radiation levels in the innermost tracking layers at future particle physics experiments as in the upgraded (S-LHC). In order to systematically investigate their radiation tolerance.. sets of 25 and 50-mu m-thick diodes had been irradiated with 24GeV/c protons up to fluences Of Phi(eq) = 10(16) cm(-2). The full depletion voltage was measured during full isothermal annealing cycles at both 60 and 80 degrees C. A voltage of only 290 V was measured for the 50 mu m diodes after Phi(eq) = 1016cm(-2). In contrast to likewise thin float zone (FZ) diodes, the depletion voltage in epi-devices decreases with annealing during foreseen operational and storage periods. Low-temperature storage during beam-off periods as inevitable for FZ devices could be avoided. The reverse annealing components reveal an overall similar functional time dependence as observed in FZ diodes. However, the most important stable component is drastically different. In contrast to the well-known negative space charge build up observed in FZ diodes, the behavior in epi-diodes can only be explained by an additional formation of positive space charge. The immense improvement of the radiation tolerance of epi-detectors is thus attributed to a compensation effect between acceptor generation and donor creation unique for the epi-material. Charge collection efficiency drops only to about 80% after Phi(eq) = 6 x 10(15)cm(-2) proving that epi-detectors are indeed adequate candidates for the tracking area at extremely large radiation levels. (c) 2005 Elsevier B.V. All rights reserved.

Four different kinds of silicon materials (standard float zone-FZ, oxygen enriched FZ-DOFZ, Czoehralski-Cz and thin epitaxial layers grown on Cz substrates-Epi/Cz) have been investigated in this work. They have been irradiated with (CO)-C-60-gamma- radiation and 24 GeV/c protons. The differences in the changes observed in the effective doping concentration (N-eff) after proton irradiation of Cz and Epi silicon can be explained by the balance between the formation of two types of defects-a deep acceptor (the I center, referred to as It, in this paper in order to avoid any confusion with interstitials) and a shallow donor (the bistable donor (BD) complex). While the formation of the I-P center depends on the concentration of interstitial oxygen, the BD is generated in materials with high concentration of oxygen dimers. Following the generation of the IO2i defect-detected after low irradiation fluences only in Epi and Cz material-the average dimer concentration in 50 put Epi/Cz was estimated to be of only 2.7 times lower than in Cz diodes. A value of similar to 2.7 x 10(-12) cm(2) for the electron capture cross-section of the Thermal Double Donors (TDDs), present in the Cz material, was also measured for the first time. The positive space charge introduced by ionization of the BD centers in Epi diodes was directly determined from TSC experiments for two irradiation fluences. The determined values show a linear fluence dependence for the formation of BD centers. By taking into account the BD and the I-p center generation as well as the donor removal, the change of the effective doping concentration N-eff at 20 degrees C in Epi/Cz and Cz diodes after an annealing time of 120 min at 60 degrees C can be explained up to a negative space charge introduction rate of similar to 2.2 x 10(-2) cm(-1) thought to be determined by negatively charged clusters. Long-time annealing experiments at 80 degrees C have shown that the generation of BDs represents a stable damage. (c) 2005 Elsevier B.V. All rights reserved.

The influence of uranium doping up to 2 at. % on grain size, electrical resistivity and dielectric constant of barium titanate ceramics has been investigated. The samples were prepared by the conventional ceramic technique using pure raw materials. The mixed oxides were calcined at 1100 degrees C. The pressed pellets were sintered at temperatures between 1300 divided by 1500 degrees C in controlled atmosphere of argon and oxygen. The electrical resistivity drastically decreased, from 10(9) to 10(3) Omega(.)cm, with increasing uranium content up to 1.5 at. % and increased thereafter to about 10(7)Omega center dot cm, for 2 at. % U. The dielectric constant slightly increased with increasing U-content, up to about I at. % U and then shows a sudden increase with about two orders of magnitude, having a sharp maximum at 1.5 at. % U, when sintered in argon atmosphere compared to only half an order of magnitude when sintered in oxygen atmosphere. The results are discussed in terms of the possible A and B sites occupied by uranium as well as the boundary layer enriched with vacancies.

A modified model of metal-semiconductor contacts is applied to analyze the capacitance-voltage and current-voltage characteristics of metal-ferroelectric-metal structures. The ferroelectric polarization is considered as a sheet of surface charge situated at a fixed distance from the interface. The presumable high concentration of structural defects acting as active electric traps is taken into account by introducing a deep acceptorlike level. The model is applied on a set of metal-Pb(Zr,Ti)O-3-metal samples with different Zr/Ti ratios, deposited by different methods, and having different thicknesses, electrode materials, and electrode areas. Values around 10(18) cm(-3) were estimated for the hole concentration from capacitance-voltage measurements. The space-charge density near the electrode, estimated from current-voltage measurements, is in the 10(20)-10(21) cm(-3) range. The total thickness of the interface layer ranges from 3 to 35 nm, depending on the Zr/Ti ratio, on the shape of the hysteresis loop, and on the electrode material. The simulated I-V characteristics is fitted to the experimental one using the potential barrier and Richardson's constant as parameters. The potential barrier is determined to be in the 1.09-1.37 eV range and Richardson's constant is 520 A cm(-2) K-2. (c) 2005 American Institute of Physics.

A model for metal-ferroelectric-metal heterostructures with Schottky contacts is proposed. The model adapts the general theories of metal-semiconductor rectifying contacts for the particular case of metal-ferroelectric contact by introducing the ferroelectric polarization as a sheet of surface charge located at a finite distance from the electrode interface, a deep trapping level of high concentration, and the static and dynamic values of the dielectric constant. Consequences of the proposed model on relevant quantities of the Schottky contact such as builtin voltage, charge density, and depletion width, as well as on the interpretation of the current-voltage and capacitance-voltage characteristics are discussed in detail. (c) 2005 American Institute of Physics.

Ba0.5Sr0.5TiO3 ferroelectric ceramics (BST) were prepared by solid-state reaction. The effect of sintering temperature T, on microwave dielectric properties was studied. Also, the influence of 1 wt % MgO and 1 wt % MnO2 doping on the dielectric parameters was investigated. The morphological aspects of samples were studied by scanning electron microscopy (SEM). The microwave measurements at room temperature revealed dielectric constant around 1.000 and loss smaller than 1 % at 1.1 GHz. Also, the dielectric parameters at low frequency were determined. The results indicate that the BST dielectric ceramics are suitable for manufacturing electric field controlled components such as tunable resonators, phase shifters, steerable antennas etc.