Field-effect-assisted photoconductivity in PbS films deposited on silicon dioxide

Cataloged from PDF version of article.Lead sulfide (PbS) thin films were deposited from a chemical bath onto SiO2/Si (n-type) substrates. Pseudo-metal-oxide-semiconductor devices were obtained by evaporating source and drain gold electrodes on a PbS surface and aluminum gate electrode on a Si substrate. Field-effect-assisted photoconductivity in the PbS layer was investigated at room temperature, in the 800-2700-nm-wavelength domain for different values and polarities of the drain and gate voltages. The best results were obtained for a positive gate, when both semiconductors are in depletion. An enhancement of about 25% of the photoconductive signal is obtained compared with the case when the gate electrode is absent or is not used. A simple model is proposed that explains the behavior of the dark current and photoconductive signal in PbS film with changing the gate voltage. (C) 2002 American Institute of Physics

Is Forensic Paint Analysis Affected by Environmentally Friendly Products?

The forensic comparison of paint often provides key evidence in crime scene investigations. Paint analysis involves using attenuated total reflectance infrared spectroscopy (ATR-FTIR) to compare the chemical makeup of different chips of paint. Environmentally friendly paints are becoming more common in homes, and this study was conducted to determine if the brand ECOS Paints, which absorbs potentially harmful compounds from the air, changes its chemical makeup over time. ECOS Paints claims that its products are uniquely formulated without the harsh chemicals that can be found in other paint brands. The company claims that its paints have a molecular sieve designed to stop harmful compounds from being released into the air (Air Pure Learn). Four paints were chosen from ECOS Paints, the air-purifying primer, air-purifying drywall primer, anti-formaldehyde paint, and the air-purifying paint. The paint was applied to wood sticks in two coats and every two weeks the paint was analyzed using ATR-FTIR to determine if there was a change in the chemical makeup. These results will be presented at the conference

Plastic Water Bottles, Source of Phthalates?

Phthalates are plasticizers which are added to many materials to increase their flexibility, durability, longevity, and transparency. These compounds are commonly found in personal-care products, plastic bottles, medical tubing, and in foods (Centers for Disease Control, n.d). Due to their prevalence in our everyday lives, and their ability to act as endocrine disruptors in humans, knowing the exact source of phthalate exposure is of interest to public health. One common source is the contact between food we consume and the plastic containers they are stored in. This study was conducted to determine thepresence of phthalates in disposable water bottles after incubation at three different temperatures to determine the optimal storage temperature that would expel the least amount of phthalates. Over the course of eight weeks, water bottles were incubated at 4°C, 29°C, and 40°C. The water samples from each condition were extracted every two weeks to detect the level of dimethyl phthalate (DMP), using high performance liquid chromatography (HPLC) with UV detection. The relative concentration was then plotted over time. It was hypothesized that water stored at higher temperatures would have higher levels of DMP compared to water stored in colder temperatures (Al-Saleh, 2011). However, we found that bottled water stored at higher temperatures over the course of eight weeks contained lower levels of DMP over time compared to bottled water stored at cooler temperatures. The phthalate levels obtained were low, however these chemicals should still not be dismissed due to the health issues which could arise from exposure

Highly (111)-oriented Ge thin films on insulators formed by Al-induced crystallization

Remarkable current-enhanced photoconductivity in oxygen-deficient La7/8Sr1/8MnO3−δ thin film Appl. Phys. Lett. 101, 042413 (2012) Ideal transparent conductors for full spectrum photovoltaics J. Appl. Phys. 111, 123505 (2012) Insights on the influence of surface roughness on photovoltaic properties of state of the art copper indium gallium diselenide thin films solar cells J. Appl. Phys. 111, 114509 (2012) Additional information on J. Appl. Phys

Comparison of near-interface traps in Al2_2O3_3/4H-SiC and Al2_2O3_3/SiO2_2/4H-SiC structures

Aluminum oxide (Al2O3) has been grown by atomic layer deposition on n-type 4H-SiC with and without a thin silicon dioxide (SiO2) intermediate layer. By means of Capacitance Voltage and Thermal Dielectric Relaxation Current measurements, the interface properties have been investigated. Whereas for the samples with an interfacial SiO2 layer the highest near-interface trap density is found at 0.3 eV below the conduction band edge, Ec, the samples with only the Al2O3 dielectric exhibit a nearly trap free region close to Ec. For the Al2O3/SiC interface, the highest trap density appears between 0.4 to 0.6 eV below Ec. The results indicate the possibility for SiC-based MOSFETs with Al2O3 as the gate dielectric layer in future high performance devices.Comment: 3 figures. Applied Physics Letters, accepted for publicatio

First-Principles Calculation of Born Effective Charges and Spontaneous Polarization of Ferroelectric Bismuth Titanate

In this study, we present the results of our first-principles calculations of the band structure, density of states and the Born effective charge tensors for the ferroelectric (ground state B1a1) and paraelectric (I4/mmm) phases of bismuth titanate. The calculations are done using the generalized gradient approximation (GGA) as well as the local density approximation (LDA) of the density functional theory. In contrast to the literature, our calculations on B1a1 structure using GGA and LDA yield smaller indirect band gaps as compared to the direct band gaps, in agreement with the experimental data. The density of states shows considerable hybridization among Ti 3d, Bi 6p and O 2p states indicating covalent nature of the bonds leading to the ferroelectric instability. The Born effective charge tensors of the constituent ions for the ground state (B1a1) and paraelectric (I4/mmm) structures were calculated using the Berry phase method. This is followed by the calculation of the spontaneous polarization for the ferroelectric B1a1 phase using the Born effective charge tensors of the individual ions. The calculated value for the spontaneous polarization of ferroelectric bismuth titanate using different Born effective charges was found to be in the range of 55+/-13 $\mu$C/cm2 in comparison to the reported experimental value of (50+/-10 $\mu$C/cm2) for single crystals. The origin of ferroelectricity is attributed to the relatively large displacements of those oxygen ions in the TiO6 octahedra that lie along the a-axis of the bismuth titanate crystal.Comment: 36 pages, 5 figure

Progress and perspective on different strategies to achieve wake-up-free ferroelectric hafnia and zirconia-based thin films

In the last decade orthorhombic hafnia and zirconia films have attracted tremendous attention arising from the discovery of ferroelectricity at the nanoscale. However, an initial wake-up pre-cycling is usually needed to achieve a ferroelectric behaviour in these films. Recently, different strategies, such as microstructure tailoring, defect, bulk and interface engineering, doping, NH3 plasma treatment and epitaxial growth, have been employed to obtain wake-up free orthorhombic ferroelectric hafnia and zirconia films. In this work we review recent developments in obtaining polar hafnia and zirconia-based thin films without the need of any wake-up cycling. In particular, we discuss the rhombohedral phase of hafnia/ zirconia, which under a constrained environment exhibits wake-up-free ferroelectric behaviour. This phase could have a strong impact on the current investigations of ferroelectric binary oxide materials and pave the way toward exploiting ferroelectric behaviour for next-generation memory and logic gate applications.This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding Contract UIDB/04650/2020 and by DST-SERB, Govt. of India through Grant Nr. ECR/2017/00006. R. F. Negrea and L. Pintilie acknowledge funding through project CEPROFER/ PN-III-P4-ID-PCCF-2016-0047 (contract 16/2018, funded by UEFISCDI). J.L.M-D. thanks the Royal Academy of Engineering Chair in Emerging Technologies Grant, CIET1819_24, the EPSRC grant EP/T012218/1- ECCS – EPSRC, and the grant EU-H2020-ERC-ADG # 882929, EROS

Metal-Ferroelectric-Metal heterostructures with Schottky contacts I. Influence of the ferroelectric properties

A model for Metal-Ferroelectric-Metal structures 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; the static and dynamic values of the dielectric constant. Consequences of the proposed model on relevant quantities of the Schottky contact such as built-in voltage, charge density and depletion width, as well as on the interpretation of the current-voltage and capacitance-voltage characteristics are discussed in detail.Comment: 14 pages with 4 figures, manuscript under revision at Journal of Applied Physics for more than 1 year (submitted May 2004, first revision September 2004, second revision May 2005

Influence of long-range dipolar interactions on the phase stability and hysteresis shapes of ferroelectric and antiferroelectric multilayers

Phase transition and field driven hysteresis evolution of a two-dimensional Ising grid consisting of ferroelectric-antiferroelectric multilayers that take into account the long range dipolar interactions were simulated by a Monte-Carlo method. Simulations were carried out for a 1+1 bilayer and a 5+5 superlattice. Phase stabilities of components comprising the structures with an electrostatic-like coupling term were also studied. An electrostatic-like coupling, in the absence of an applied field, can drive the ferroelectric layers towards 180º domains with very flat domain interfaces mainly due to the competition between this term and the dipole-dipole interaction. The antiferroelectric layers do not undergo an antiferroelectric-to-ferroelectric transition under the influence of an electrostatic-like coupling between layers as the ferroelectric layer splits into periodic domains at the expense of the domain wall energy. The long-range interactions become significant near the interfaces. For high periodicity structures with several interfaces, the interlayer long-range interactions substantially impact the configuration of the ferroelectric layers while the antiferroelectric layers remain quite stable unless these layers are near the Neel temperature. In systems investigated with several interfaces, the hysteresis loops do not exhibit a clear presence of antiferroelectricity that could be expected in the presence of anti-parallel dipoles, i. e., the switching takes place abruptly. Some recent experimental observations in ferroelectric-antiferroelectric multilayers are discussed where we conclude that the different electrical properties of bilayers and superlattices are not only due to strain effects alone but also long-range interactions. The latter manifests itself particularly in superlattices where layers are periodically exposed to each other at the interfaces