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Carbon layers are deposited on 100 nm thick atomically clean (001) lead zirconate titanate (PZT) in ultrahigh vacuum, ruling out the presence of any contaminants. X-ray photoelectron spectroscopy is used to assess the substrate surface or interface composition, substrate polarization and the thickness of carbon layers, which ranges from less than one monolayer (1 ML) of graphene to several monolayers. Atomically clean PZT(001) exhibit inwards polarization, and this polarization reverses the sign upon carbon deposition. Cationic vacancies are detected near the PZT surface, consistent with heavy p doping of these films near the surface. The carbon layers exhibited a consistent proportion of atoms forming in-plane sp2 bonds, as detected by near-edge absorption fine structure (NEXAFS) analysis and confirmed partially by scanning tunneling microscopy (STM). In situ poling with simultaneous in-plane transport measurements revealed the presence of resistance anti-hysteresis versus the polarization orientation for films with less than 1 ML carbon amount, evolving towards ‘normal’ hysteresis for thicker carbon films. The anti-hysteresis is explained in terms of a mixed screening mechanism, involving charge carriers from the sp2 carbon layers together with holes or ionized acceptors in PZT(001) near the interface. For thicker films, the compensation mechanism becomes extrinsic, involving mostly electrons and holes from carbon, yielding the expected hysteresis.

Multi-layered structures, composed of thin films from materials with different compositions or physical properties, represents a way to obtain enhanced properties or even new functionalities. In this work, lead zirconate titanate PbZrxTi1-xO3 (PZT; x = 0.20, 0.52, 0.80) multilayers were grown by pulsed laser deposition (PLD) on a single crystal strontium titanate (SrTiO3, STO) substrate, using a strontium ruthenate (SrRuO3, SRO) film as buffer layer for epitaxial growth, and also as back electrode. Up and down multi-layers were grown and their physical and structural properties were compared, up being the structure in which Zr concentration was varied from 20% near the STO substrate to 80% at the surface, while down is for the structure in which the Zr concentration starts with 80% near the substrate and ends with 20% at the surface. It was found that the electric and pyroelectric properties of the two graded structures are significantly different. The up structure presents electric properties that are comparable with those of single composition PZT films while the properties of the down structure are deteriorated, especially in terms of the leakage current magnitude. Pyroelectric signal could be measured only for the up structure. These differences were attributed to larger density of structural defects in the down structure compared to the up one. This is due to the different growth sequence: up structure starts with tetragonal PZT on cubic substrate (lower lattice mismatch, 1.1%) while down structure starts with rhombohedral PZT on cubic substrate (larger lattice mismatch, almost 5%).

Polarization switching in ferroelectric films is exploited in many applications, such as non-volatile memories and negative capacitance field affect transistors. This can be inhomogeneous or homogeneous, depending on if ferroelectric domains are forming or not during the switching process. The relation between the polarization switching, the structural quality of the films and the negative capacitance was not studied in depth. Here, Pb(Zr0.2Ti0.8)O3 (PZT) layers were deposited by pulse laser deposition (PLD) and sol-gel (SG) on single crystal SrTiO3 (STO) and Si substrates, respectively. The structural quality was analyzed by X-ray diffraction and transmission electron microscopy, while the electric properties were investigated by performing hysteresis, dynamic dielectric measurements, and piezo-electric force microscopy analysis. It was found that the PZT layers grown by PLD on SRO/STO substrates are epitaxial while the layers deposited by SG on Pt/Si are polycrystalline. The polarization value decreases as the structure changes from epitaxial to polycrystalline, as well as the magnitude of the leakage current and of the differential negative capacitance, while the switching changes from homogeneous to inhomogeneous. The results are explained by the compensation rate of the depolarization field during the switching process, which is much faster in epitaxial films than in polycrystalline ones.

Composites of magnetite (Fe3O4) nanoparticles dispersed in a polydimethylsiloxane (PDMS) matrix were prepared by a molding process. Two types of samples were obtained by free polymerization with randomly dispersed particles and by polymerization in an applied magnetic field. The magnetite nanoparticles were obtained from magnetic micrograins of acicular goethite (α-FeOOH) and spherical hematite (α-Fe2O3), as demonstrated by XRD measurements. The evaluation of morphological and compositional properties of the PDMS:Fe3O4 composites, performed by SEM and EDX, showed that the magnetic particles were uniformly distributed in the polymer matrix. Addition of magnetic dispersions promotes an increase of thermal conductivity compared with pristine PDMS, while further orienting the powders in a magnetic field during the polymerization process induces a decrease of the thermal conductivity compared with the un-oriented samples. The shape of the magnetic dispersions is an important factor, acicular dispersions providing a higher value for thermal conductivity compared with classic commercial powders with almost spherical shapes.

Manganese is deposited at high temperature on (001) oriented ferroelectric lead zirco-titanate prepared in two different ways: sputter-annealed or just simply annealed in ultrahigh vacuum. Room temperature ferromagnetism (FM) is obtained for Mn deposited on sputter-annealed substrates, while for the other sample preparation a paramagnetic behaviour is obtained. Also, for the first case a clear inwards component of the polarization P(−) is observed by X-ray photoelectron spectroscopy and piezoresponse force microscopy. Composition analysis evidenced formation of Pb vacancies in the case of FM – P(−) sample, consistent with hole formation near the surface, needed both to stabilize the inwards polarization state and to intermediate ferromagnetism between Mn2+ ions. The indirect exchange ferromagnetism mediated by holes is stronger, most probably because the interaction energy is proportional with the carrier effective mass. Also, whereas in the case of unsputtered substrate a stable surface Mn oxide is formed, defect formation by sputtering seems to favor Mn migration inside the sample. This also induces the formation of a thin film where ferromagnetism and the orientation of ferroelectric polarization might have the same origin, i. e. holes accumulated near the outer surface.

Ferroelectric field effect transistors (FeFETs) based on lead zirconate titanate (PZT) ferroelectric material and amorphous-indium-gallium-zinc oxide (a-IGZO) were developed and characterized. The PZT material was processed by a sol-gel method and then used as ferroelectric gate. The a-IGZO thin films, having the role of channel semiconductor, were deposited by radio-frequency magnetron sputtering, at a temperature of ~50°C. Characteristics of a typical field effect transistor with SiO2 gate insulator, grown on highly doped silicon, and of the PZT-based FeFET were compared. It was proven that the FeFETs had promising performances in terms of Ion/Ioff ratio (i.e., 106) and IDS retention behavior.

This work involves the synthesis and characterization of Cu2ZnSnS4 (CZTS) layers. The films were prepared on ITO/glass substrate by ecofriendly and simple single-step electrodeposition method followed by sulfurization and annealing at 500 °C under Argon flow. By using two different complexing agents, the electrodeposition process can give better results. Therefore, the effect of combining the trisodium citrate - TC to multiple complexing agents (cetyl trimethyl ammonium bromide - CTAB, ethylene diamine tetra acetic acid - EDTA, Boric Acid - BA, Glutamic Acid - GA and Tartaric Acid - TA) is investigated. The characterization of the absorber films was done by X-ray diffraction (XRD) analysis, Raman spectroscopy, Scanning Electron Microscopy and Diffuse Reflectivity. The combination of TC and CTAB is suggested to be the best pair of complexing agents within the combinations used in this work.

By platinum deposition on a 150 nm thick film of lead zirco-titanate oriented PZT(001), grown on strontium titanate (001) single crystals with a strontium ruthenate buffer layer, which did not show initial preferential out-of-plane orientation of its ferroelectric polarization, a band bending near the interface towards lower energies is observed using photoelectron spectroscopy, by following all core levels from the substrate (Pb 4f, Zr 3d, Ti 2p, O 1s). This is unexpected given the fact that platinum has a larger work function than PZT and a rectifying contact for electrons is expected to be built at the interface. This observation may have two explanations: (i) platinum forms an alloy with elements from PZT yielding a metal with considerable lower work function; (ii) platinum provides electrons to the substrate which are able to compensate the depolarization field generated by the outwards polarization state. Several arguments are brought in favor of the second hypothesis, especially the attenuation of core levels from the substrate which is well described by exponential functions with reasonable values of the photoelectron inelastic mean free path, suggesting the formation of a sharp interface. High resolution transmission electron microscopy confirmed the sharpness of the interface.

Un-doped ZnO (UDZO) and Li-doped ZnO (LZO) polycrystalline thin films were grown on platinized silicon by pulsed laser deposition (PLD). The electrical properties were investigated on as-grown and annealed UDZO and LZO films with capacitor configuration, using top and bottom platinum electrodes. In the case of the as-grown films it was found that the introduction of Li increases the resistivity of ZnO and induces butterfly shape in the C-V characteristic, suggesting ferroelectric-like behavior in LZO films. The properties of LZO samples does not significantly changes after thermal annealing while the properties of UDZO samples show significant changes upon annealing, manifested in a butterfly shape of the C-V characteristic and resistive-like switching. However, the butterfly shape disappears if long delay time is used in the C-V measurement, the characteristic remaining non-linear. Pyroelectric signal could be measured only on annealed films. Comparing the UDZO results with those obtained in the case of Li:ZnO, it was found that the pyroelectric properties are considerably enhanced by Li doping, leading to pyroelectric signal with about one order of magnitude larger at low modulation frequencies than for un-doped samples. Although the results of this study hint towards a ferroelectric-like behavior of Li doped ZnO, the presence of real ferroelectricity in this material remains controversial.

Hysteretic effects are investigated in perovskite solar cells in the standard FTO/TiO2/CH3NH3PbI3–xClx/spiro-OMeTAD/Au configuration. We report normal (NH) and inverted hysteresis (IH) in the J–V characteristics occurring for the same device structure, and the behavior strictly depends on the prepoling bias. NH typically appears at prepoling biases larger than the open circuit bias, while pronounced IH occurs for negative bias prepoling. The transition from NH to IH is marked by an intermediate mixed hysteresis behavior characterized by a crossing point in the J–V characteristics. The measured J–V characteristics are explained quantitatively by the dynamic electrical model. Furthermore, the influence of the bias scan rate on the NH/IH hysteresis is discussed based on the time evolution of the accumulated ionic and electronic polarization charge at the interfaces. Introducing a three-step measurement protocol, which includes stabilization, prepoling, and measurement, we put forward the difficulties and pos...

The substantial increase in the power conversion efficiency of hybrid perovskite solar cells, to date reaching more than 20% in the laboratory, has strongly motivated research on this class of organic-inorganic materials and related devices, particularly based on CH3NH3PbI3-xXx/TiO2 heterostructures (X = Cl,Br). Taking under consideration that a ferroelectric substrate may act as an efficient electron transporter, positively influencing charge collection across the interface and allowing the tuning of the halide perovskite (HP) - ferroelectric junction, we performed extensive density functional theory calculations on CH3NH3PbI3-xClx layers deposited on tetragonal PbTiO3 (PTO) (001) surfaces, to study their structural and electronic properties. The main findings of this study are as follows. (i) A ferroelectric polarization pointing from the PTO/HP interface to the PTO is favorable for the photogenerated electrons transfer across the interface and their transport to the collecting electrode. (ii) The PTO internal electric field leads to a position dependent energy levels diagram. (iii) The HP gap may be tuned by chlorine concentration at the interface, as well as the by the surface terminations of PbTiO3 and hybrid perovskite layers. (iv) The presence of the PTO ferroelectric surface is likely to have just a slight orientational effect on the (CH3NH3)+ dipoles. (Graph Presented).

Germanium Metal-Ferroelectric-Semiconductor (MFS) capacitors based on ferroelectric Hf1−xZrxO2 (HZO) with clean, oxide free Ge/HZO interfaces emerge as an interesting layer structure for the fabrication of ferroelectric field effect transistor (FeFET) non-volatile memory devices. It is shown that, at low temperature (

In this paper, potassium based triple cation mixed-halide perovskite films were explored in order to enhance the stability and photovoltaic performance of perovskite based solar cells. It was found that adding potassium (K+) to a double cation mixed halide perovskite (FA0.80MA0.20PbI2.8Cl0.2), structural, morphological and optoelectronic properties of perovskites are improved. The perovskite films were prepared by one-step spin coating method with and without K+ and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy (PL). The results indicate that potassium incorporation reduces significantly the yellow non-perovskite δ-phase formation and improves the perovskite film quality, thus contributing to the reduction of hysteresis, improves the stability and increases the PCE up to 12.51%. Furthermore, the doped devices exhibit reduced hysteresis and provide remarkable shelf stability by retaining more than 70% of the initial efficiency with low humidity over 850 h.

We monitored the evolution in time of pinhole-free structures based on FTO/TiO2/CH3NH3PbI2.6Cl0.4 layers, with and without spiro-OMeTAD and counter electrodes (Ag, Mo/Ag, and Au), aged at 24 °C in a dark nitrogen atmosphere. In the absence of electrodes, no degradation occurs. While devices with Au show only a 10% drop in power conversion efficiency, remaining stable after a further overheating at 70 °C, >90% is lost when using Ag, with the process being slower for Mo/Ag. We demonstrate that iodine is dislocated by the electric field between the electrodes, and this is an intrinsic cause for electromigration of I– from the perovskite until it reaches the anode. The iodine exhaustion in the perovskite layer is produced when using Ag electrodes, and AgI is formed. We hypothesize that in the presence of Au the iodine migration is limited due to the buildup of I– negative space charge accumulated at the perovskite–OMeTAD interface.

5 mol% Ce3+-doped barium titanate nanoshell tubes were prepared via sol–gel method using as template a polycarbonate membrane with channels of 200 nm diameter. FE-SEM analyses showed that continuous and uniform green tubes with length up to 15 μm, an average outer diameter of 188.6 nm and wall thickness of 15.1 nm, were obtained. After calcination at 700 °C for 1 h, these amorphous 1D nanostructures were converted into polycrystalline tubes with an average outer diameter of 157.4 nm and a grain size of 43.4 nm, as high resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) indicated. Thermo-Raman investigations pointed out diffuse phase transitions, inducing the preservation of a stable polar state at higher temperatures (up to 200 °C). Piezoresponse force microscopy (PFM) investigations on 5 mol% Ce3+-doped BaTiO3 nanoshell tubes revealed ferroelectric and piezoelectric behaviour which recommends these tubes for microelectronic devices.

Plasma assisted atomic oxygen deposition was used to grow polycrystalline ferroelectric Hf1-xZrxO2 (x = 0.5–0.7) on technologically important (100) Germanium substrates showing sharp crystalline interfaces free of interfacial amorphous layers and strong evidence for the presence of a predominately orthorhombic phase. The electrical properties, evaluated using metal-ferroelectric-semiconductor (MFS) capacitors, show symmetric and robust ferroelectric hysteresis with weak or no wake-up effects. The MFS capacitors with x = 0.58 show very large remanent polarization up to 34.4 μC/cm2 or 30.6 μC/cm2 after correction for leakage and parasitics, combined with good endurance reaching 105 cycles at a cycling field of 2.3 MV/cm. The results show good prospects for the fabrication of Ge ferroelectric field effect transistors (FeFETs) for use in 1 T FeFET embedded nonvolatile memory cells with improved endurance.

The orientation of polarization in ferroelectric thin films is important for applications of these materials in nonvolatile memory and field-effect transistors with ferroelectric gates, and in catalysis and photocatalysis. One means of control is to choose a well defined substrate, but the influence of a substrate's electronic properties on polarization orientation is not well understood. In this investigation of model systems, the experimental findings are integrated with electrostatic models to determine the magnitudes and orientations of the interfacial electric fields that are useful for setting the polarization of the thin ferroelectric epilayer.

Electrochemical deposition allows the preparation of ZnO nanostructures with precisely controlled morphology and properties, by finely tuning the process parameters. ZnO nanowires were deposited onto gold substrates by electrodeposition from a low concentration zinc nitrate bath. Photolithography was employed for patterning interdigitated electrode systems onto silicon/silicon dioxide substrates and ZnO electrodeposition lead to wires connected to each other by bridging neighboring interdigits allowing electronic transport characterization. Optical measurements, i.e. reflection and photoluminescence spectroscopy, were performed and the results were correlated to electronic transport data. We found that we deal with a system for which one can apply a model of space charge limited currents with different traps energy distribution as a consequence of electrodeposition rate. Current versus temperature measurements show different behavior for lower and higher range of temperatures. Such nanowires, fabricated and contacted in a straightforward way, allow a wide area of applications ranging from conductometric bio- or chemo-sensors to optoelectronic devices.

Aluminum doped zinc oxide (AZO) thin films were obtained by pulsed laser deposition on (001) SrTiO3 (STO) on a range of substrate temperatures during ablation between 300 °C and 600 °C. A hexagonal system lying on a cubic one should be difficult to be obtained in epitaxial form. The geometrical selection of the AZO growth on (001) STO is not giving a unique preferential orientation. Two orientations, c-axis (along [001]) and 110, have been observed experimentally with different ratios at different substrate temperature. Discussions are made with respect to the temperature dependence of lattice mismatch between the two cases and the cubic surface of the substrate, and to the substrate surface morphology and terminating atomic layer composition. The 110 AZO is the main phase at deposition temperature of 550 °C, while for other substrate temperatures the 001 is the preferential orientation. The conductive character of 110 AZO thin film have been inferred from both ellipsometry spectra and current–voltage measurements. Excepting the samples deposited at 300 °C, the lowest resistivity is recorded for the samples with 110 AZO as the main phase.

The emerging field of electronics based on ferro-functional materials relies on driving effectively and predictably a ferroelectric system between different polarization states through bias applied to metallic contacts. This requires detailed understanding of the growth mechanisms and electronic properties of the interface, including ferroelectric and material – dependent band alignment and Schottky barrier heights. Whether the major contribution at the interface band alignment comes from the work function difference or from the ferroelectric state is still under debate. Here, using X-ray photoemsion and ab-initio calculations, we derive the complex microscopic picture of metal/ferroelectric interface formation, including growth mechanism, valence alteration, ferroelectric-dependent electrostatic potential and thickness – dependent compensation mechanisms of ferroelectricity, starting from the ultrathin growth of Cu up to 100 A on BaTiO3. One establishes the evolution of the band bending and of the build-in potential from the initial probed thickness of the ferroelectric in the range of 3 λ ( λ – the inelastic mean free path) while gradually approaching the contact region with the metal at higher thickness of the top layer. We find that the well-defined orientation of the ferroelectric polarization lead to a band bending at the interface, which add at the bending expected from the work function difference of the two joining materials.

The energy band alignment at interfaces between different materials is a key factor, which determines the function of electronic devices. While the energy band alignment of conventional semiconductors is quite well understood, systematic experimental studies on oxides are still missing. This work presents an extensive study on the intrinsic energy band alignment of a wide range of functional oxides using photoelectron spectroscopy with in-situ sample preparation. The studied materials have particular technological importance in diverse fields as solar cells, piezotronics, multiferroics, photoelectrochemistry and oxide electronics. Particular efforts have been made to verify the validity of transitivity, in order to confirm the intrinsic nature of the obtained band alignment and to understand the underlying principles. Valence band offsets up to 1.6 eV are observed. The large variation of valence band maximum energy can be explained by the different orbital contributions to the density of states in the valence band. The framework provided by this work enables the general understanding and prediction of energy band alignment at oxide interfaces, and furthermore the tailoring of energy level matching for charge transfer in functional oxides. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The fundamental phenomena at ferroelectric interfaces have been the subject of thorough theoretical and computational studies due to their usefulness in a large variety of emergent electronic devices, solar cells and catalysts. Ferroelectricity determines interface band-bending and shifts in electron energies, which can be beneficial or detrimental to device performance. However, the underlying mechanisms are still the subject of debate and investigation, as a deeper understanding of the electrochemistry is required to develop bona fide design principles for functional ferroelectric surfaces and interfaces. Here, using first principles calculations within the GGA + U formalism, we investigate the problem of band alignment in non-defective, asymmetric SrRuO3/PbTiO3/SrRuO3 capacitors with ultra-thin ferroelectric layers. The effects of the dielectric size on the polar distortion stability and interface-specific properties are analyzed. It is shown that the critical size of the dielectric for polarization switching is ≈ 2 nm (5 PbTiO3 u.c.). Below this limit there is no bulk-like region in the dielectric, the space charge accumulated at interfaces leads to the presence of gap states in the whole PbTiO3 layer and ferroelectricity vanishes. We draw the band alignment diagrams as given by the band line-up and band structure terms, as well as by taking Ti 3s semi-core states as reference. In the ferroelectric structures, both approaches predict a strong effect of band-bending on the type of contact, Schottky or Ohmic, at the asymmetric interfaces. The effect of interface states on the interface dipole amplitude and band alignment is discussed.

A carbon-based layer was deposited by spraying on top of a ferroelectric layer grown by sol-gel on Si (001) substrate and its properties as electrode and absorber for pyroelectric detection were tested. It was found that the electric properties of the ferroelectric capacitor with top carbon-based sprayed electrode (CBSE) are comparable with those of the capacitors with standard top SrRuO3 (SRO)/Au electrode. Pyroelectric measurements show that the pyroelectric signal recorded on ferroelectric capacitors with top CBSE electrode is 2.5 times greater than for top SRO/Au electrode for low frequency range. The value of the pyroelectric coefficient was estimated to 9.73·10-4 C/m2K for CBSE electrodes and 3.36·10-4 C/m2K for SRO/Au respectively. The fabrication process of CBSE is of low cost, easy to implement and with high throughput making it attractive for manufacturing various devices like pyroelectric detector, thermal imaging, solar cells, etc.

The band bending at Cu/PZT(001) interfaces is investigated by X-ray photoelectron spectroscopy (XPS) for a PZT(001) layer which exhibits initial outwards ferroelectric polarization. Two competitive processes are identified: (a) formation of the Schottky barrier between the ferroelectric and unconnected Cu islands, and (b) coalescence of the Cu islands, realisation of an electrical contact to the ground of the system, inducing the apparent loss of the component of the ferroelectric polarization perpendicular to the sample surface, at least as it manifests in band bending. Three mechanisms are proposed to explain this loss of band bending when a full metal layer connected to ground is formed on the surface: (i) over-compensation of depolarization field in the sub-surface region, (ii) formation of domains with in-plane orientation of the polarization vector and (iii) loss of polarization in the near-surface layers of the ferroelectric due to electrons provided by the metal. These result in a non-monotonous variation of binding energies with the amount of Cu deposited. High resolution transmission electron microscopy and piezoresponse force microscopy confirmed these hypotheses. The XPS data allowed also to derive the surface PZT composition, its evolution with the deposition of copper and the formation of surface compounds.

Atomically clean lead zirco-titanate PbZr0.2Ti0.8O3 (001) layers exhibit a polarization oriented inwards P(−), visible by a band bending of all core levels towards lower binding energies, whereas as introduced layers exhibit P(+) polarization under air or in ultrahigh vacuum. The magnitude of the inwards polarization decreases when the temperature is increased at 700 K. CO adsorption on P(−) polarized surfaces saturates at about one quarter of a monolayer of carbon, and occurs in both molecular (oxidized) and dissociated (reduced) states of carbon, with a large majority of reduced state. The sticking of CO on the surface in ultrahigh vacuum is found to be directly related to the P(−) polarization state of the surface. A simple electrostatic mechanism is proposed to explain these dissociation processes and the sticking of carbon on P(−) polarized areas. Carbon desorbs also when the surface is irradiated with soft X-rays. Carbon desorption when the polarization is lost proceeds most probably in form of CO2. Upon carbon desorption cycles, the ferroelectric surface is depleted in oxygen and at some point reverses its polarization, owing to electrons provided by oxygen vacancies which are able to screen the depolarization field produced by positive fixed charges at the surface.

The results of the electric and magnetic measurements performed on PbZr0.2Ti0.8O3–BiFeO3 symmetric structures, deposited on Pt/Si wafers, were compared for different number of layers in order to analyse the effect of interfaces over the macroscopic properties. It was found that the shape and magnitude of the capacitance–voltage characteristic, as well as the shape and parameters of the ferroelectric and magnetic hysteresis, depend on the number of interfaces in the intended multilayer structure. A temperature induced gradual transition from a magnetically disordered spin glass like phase of low temperature to an uncompensated antiferromagnetic phase at room temperature takes place in the BiFeO3 films, under low applied magnetic fields. A partial ferromagnetic like order can be obtained at low temperatures by increasing the field. The observed changes in the electric and magnetic behaviour of the systems were related to an increased degree of disorder for electric dipoles and magnetic moments, due to the increased number of layers and crystallization treatments.

It is foreseen to significantly increase the luminosity of the Large Hadron Collider (LHC) at CERN around 2018 by upgrading the LHC towards the SLHC (Super-LHC). Due to the radiation damage to the silicon detectors used, the physics experiments will require new tracking detectors for SLHC operation. All-silicon central trackers are being studied in ATLAS, CMS and LHCb, with extremely radiation hard silicon sensors for the innermost layers. The radiation hardness of these new sensors must surpass the one of LHC detectors by roughly one order of magnitude. Within the CERN RD50 collaboration, a massive R&D program is underway to develop silicon sensors with sufficient radiation tolerance. We will report on recent results obtained by RD50 from tests of several detector technologies and silicon materials at radiation levels corresponding to SLHC fluences. Based on these results, we will give recommendations for the silicon detector technologies to be used at the different radii of SLHC tracking systems.

Carbon layers grown on lead zirco-titanate (PZT) weakly interact with the substrate and exhibit a nearly two dimensional character, up to a carbon surface density approaching that of graphene. The first feature is evidenced by X-ray photoelectron spectroscopy, and the second by angle resolved near-edge-absorption spectroscopy (NEXAFS). The binding energies and lineshape parameters of C 1s are similar to that of graphene. The dichroism of C K-edge NEXAFS shows the prevalence of in-plane sp2 bonds for layers whose effective coverage is below the graphene surface density. The polarization state of the substrate, oriented outwards, is preserved upon carbon deposition. The surface Pb content is strongly affected by the carbon ad-layers.

A dynamic electrical model is introduced to investigate the hysteretic effects in the I-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of measured I-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. In particular, the presence of the current over-shoot observed in the reverse characteristics is correlated with the solar cell pre-conditioning. Furthermore, the dynamic hysteresis is analyzed with respect to changing the bias scan rate, the obtained results being consistent with experimentally reported data: the hysteresis amplitude is maximum at intermediate scan rates, while at very slow and very fast ones it becomes negligible. The effects induced by different relaxation time scales are assessed. The proposed dynamic electrical model offers a comprehensive view of the solar cell operation, being a practical tool for future calibration of tentative microscopic descriptions., Comment: 8 pages, 6 figures, Supplemental material, to appear in Solar Energy Materials & Solar Cells

Pure and l -alanine doped Triglycine sulphate (TGS) crystals were grown in paraelectric phase (∼52 °C). Doped crystals show unequal growth rates along the ferroelectric axis. Pure TGS crystals show peculiar dielectric behavior in the ferroelectric phase, after crossing up and down the Curie point in two successive runs between room temperature and 80 °C. Much higher and unstable permittivity was found returning in the ferroelectric phase. At constant temperature (35 °C), permittivity follows a relaxation process, characterized by two relaxation times. l -Alanine doped TGS crystal shows more than one order of magnitude smaller permittivity and dielectric losses. Internal bias field of ∼1 kV/cm, induced by the dopant, made the crystal almost monodomain and pined polarization in one direction. Pyroelectric coefficient measurements were performed at constant heating rate of the samples, using a computer controlled He cryostat and Keithley 6517 electrometer. The temperature dependence of P + polarization component, obtained by computer integration of the pyroelectric coefficient, was measured on a large temperature interval (−20/+80 °C). Pyroelectric coefficient of the doped samples was also measured by the same procedure, using a dc bias electric field, pointing in the opposite direction to the pined polarization. The polarization could be reversed, on the whole temperature range, by dc fields higher than bias or coercive field. Surprisingly, for the first time, the pyroelectric coefficient ( p ) was found constant on quite large temperature intervals. Doped TGS crystals show much smaller values of permittivity ɛ r versus the pure one and consequently, get higher figure of merit M = p / ɛ r . The pyroelectric coefficient of this material can be tailored to become constant on a defined temperature range, under a dc field control. This characteristic makes this material valuable to be used as pyroelectric material for IR devices.

PbZrxTi1-xO3 (PZT) is one of the technologically most important ferroelectric materials. Bulk single-domain single crystals of PZT have never been synthesized for a significant compositional range across the solid-solution phase diagram. This leaves the fundamental properties of PZT under debate. Synthesis of defect-free single crystalline films enables us to unambiguously determine the intrinsic quantities that describe ferroelectric materials, such as spontaneous polarization and dielectric constant. Defect-free single crystalline, strained PbZr0.2Ti0.8O3 thin films were grown by pulsed-laser deposition (PLD) onto vicinal SrTiO3 (001) single crystal substrates. Single crystalline SrRuO3 films fabricated by PLD were employed as bottom electrode. The PZT films have square shape hysteresis loops and remnant polarization values of up to Pr= 110 micro C/cm2, which is considerably higher than the theoretical value predicted for (unstrained) bulk single crystalline PZT of the investigated composition. The films have a dielectric constant eps33= 90 and a piezoelectric coefficient d33= 50 pm/V., 15 pages, 3 figures

Recent results from the CERN RD50 Collaboration for the development of radiation-hard detectors for the LHC upgrade (Super-LHC) and in general for very high luminosity colliders are reviewed, summarized and discussed. Particularly, the attention is focused on emerging technologies (Czochralski and Magnetic Czochralski silicon, thinned detectors, highly doped thin epitaxial layer on Czochralski substrate, pre-irradiated silicon, p-type substrate devices) and new detector structures (3D, 3D-STC, Semi-3D, and Stripixel).

Pyroelectric current spectroscopy (PCS) was used to investigate the homogeneity of the Nb doped lead zirconate–titanate Pb(Zr 0.92 Ti 0.08 )O 3 , (PZT) ceramics intended to be used in the infrared (IR) pyroelectric detectors manufacturing. The non-homogeneous composition in the ceramic volume is reflected by the occurrence of several peaks in the PCS spectra, suggesting the presence of PZT phases with different Zr/Ti ratio, thus having different transition temperatures. The non-homogeneity was confirmed by compositional analysis performed using an Energy Dispersive X-ray Spectrometer (EDAX) attached to a Scanning Electron Microscope (SEM). PCS can be a useful tool in optimizing the calcination and sintering parameters (temperature and time) during ceramic preparation.

Triglycine sulfate crystals (TGS) doped with l - and d -alanine, respectively, where grown in paraelectric phase at 52 °C. They show stable parameters versus pure TGS crystals, lower permittivities and losses, i.e. higher pyroelectric figure of merit. l - and d -alanine-doped crystals show a mirror asymmetry of the growth habit along the ferroelectric axis. Hysteresis loop of pure and doped samples were automatically recorded. Internal bias field of about 1 kV/cm, induced by the two dopants, stabilizes the polarization components P+ and P− in opposite directions. Spontaneous polarization of doped crystals shows two unequal P+/P− components, having a peculiar dependence on the ac applied electric field. Dopage simulation of pure TGS samples, using several bias fields, shows even more complicated pattern. Pyroelectric coefficients were evaluated from pyroelectric current measurements. A remarkable asymmetry of the pyroelectric coefficient peak values for the enantiomorphous-doped crystals was noticed, crossing up and down the Curie point. This fact suggests a non-equivalent l / d -alanine dopant substitution of the glycine group GI in the host lattice. It has been shown that the pyroelectric coefficient can be increased at room temperature, under an optimized dc electric field applied on the pyroelectric wafer. Doped crystals are suitable for IR detector and for more stable gas sensors applications in pollution monitoring.

PbS films were chemically deposited on glass substrates from chemical reducing bath doped with Bi. The growth and propertiesof PbS layers are investigated. The effect of the reducer and Bi ions on the deposition process was studied with a quartz- 3 q crystal microbalance technique. The reducer introduces a new deposition mechanism of electrochemical nature, which allows forthicker films. On the other hand, Bi ions introduce nucleation centers in the solution 3 q (Bi (OH )), which accelerate the 3 homogeneous precipitation and diminish the film thickness. The PbS grain size within the layer increases with increasing the Bicontent in the deposition bath. A considerable enhancement of the photoconductive signal was found in case of PbS filmsdeposited from Bi doped bath. The signal has nonlinear behavior with the Bi content in the chemical bath. There is an optimumquantity of Bi for which the photoconductive characteristic reaches maximum. 2003 Elsevier Science B.V. All rights reserved.

We propose an improved method for the analysis of Thermally Stimulated Currents (TSC) measured on highly irradiated silicon diodes. The proposed TSC formula for the evaluation of a set of TSC spectra obtained with different reverse biases leads not only to the concentration of electron and hole traps visible in the spectra but also gives an estimation for the concentration of defects which not give rise to a peak in the 30–220 K TSC temperature range (very shallow or very deep levels). The method is applied to a diode irradiated with a neutron fluence of φn=1.82×1013 n/cm2.

The luminosity upgrade of the Large Hadron Collider (LHC) at CERN to the Super LHC (sLHC) will increase the radiation dose at the experiments by roughly an order of magnitude. The elevated radiation levels require the LHC experiments to upgrade their tracking systems with extremely radiation hard detectors. Recent results on defect characterization of silicon materials and radiation hard technologies developed by the RD50 Collaboration for sLHC use are reported. Studies on n - and p -type silicon with Float Zone (FZ) and Czochralski technologies as well as 3D silicon detector designs are presented. Properties such as charge collection efficiency, electron signal, effective doping concentration (N eff ) and full depletion voltage (V fd ) after radiation exposure are compared to assess the performance of the different technologies.

6H-SiC bulk single crystals grown by physical vapor transport (PVT) were investigated by deep-level transient spectroscopy (DLTS) and electron paramagnetic resonance (EPR). One of the observed deep level defects was identified as isolated tungsten on Si sites by EPR. The electron spin of 1 2 could be explained by W5+ (5d1). This is equivalent to the single positive charge state of a double donor when taking into account the Fermi level position in the n-type samples. The interpretation is also consistent with the DLTS detection of a W related deep level which showed a behavior of the capture of electrons and holes that hints at a double donor. In addition a tantalum related deep level is tentatively discussed. W and Ta were incorporated on electrically active sites in 6H-SiC only in low concentrations (2–4×1014 cm−3) during crystal growth by PVT.

Photoconductive properties of some sol-gel deposited, ferroelectric thin films are investigated in the ultraviolet (UV) domain. The investigated materials are: Bi 4 Ti 3 O 12 (BiT) ; SrBi 2 Ta 2 O 9 (SBT) and Pb(Zr 0.45 Ti 0.55 )O 3 (PZT). It was found that all these materials exhibit pronounced photoconductivity in the 250–500 nm domain. The best current responsivity was obtained for Bi 4 Ti 3 O 12 (BiT) films.

PbS films were obtained on glass substrates using the chemical bath deposition (CBD) method. The infra red (IR) photosensitivity increases of about 1000 times when a reducing agent (hydroxylamine hydrochloride NH 2 OHHCl) is added in the deposition bath. A further enhancement was tried by adding small amounts of antimony chloride (SbCl 3 ) solution in the deposition bath. The result was just opposite. It was found that the photosensitivity decreases with increasing the Sb salt content. The changes in PbS photoconductive properties are correlated with the changes in film morphology due to the two compounds added in the bath — the reducing agent and the Sb salt.

For the luminosity upgrade of the LHC, the SLHC, the tracking systems of the LHC experiments need to be replaced and upgraded. A main concern is the extreme radiation hardness requirements, up to 1×1016 cm−2. This paper describes recent results on the radiation hardening technologies developed within the CERN RD50 Collaboration for the LHC upgrade. Silicon detectors have been designed and produced on n- and p-type 4/6 in. wafers made by Float Zone, epitaxy and Czochralski technology. Their charge collection efficiency after irradiation with protons, neutron and mixed irradiation has been studied. Novel detector concepts, as 3D detectors, have been designed, produced and studied as well. Radiation induced microscopic disorder has been also investigated in Si detectors with fluences up to 1016 cm−2 and correlated with the performance degradation of irradiated detectors.