Identification of intrinsic electron trapping sites in bulk amorphous silica from ab initio calculations

Using ab initio calculations we demonstrate that extra electrons in pure amorphous SiO2 can be trapped in deep band gap states. Classical potentials were used to generate amorphous silica models and density functional theory to characterise the geometrical and electronic structures of trapped electrons. Extra electrons can trap spontaneously on pre-existing structural precursors in amorphous SiO2 and produce ≈≈3.2 eV deep states in the band gap. These precursors comprise wide (⩾⩾130°°) O–Si–O angles and elongated Si–O bonds at the tails of corresponding distributions. The electron trapping in amorphous silica structure results in an opening of the O–Si–O angle (up to almost 180°°). We estimate the concentration of these electron trapping sites to be View the MathML source≈5×1019cm-3

Nature of intrinsic and extrinsic electron trapping in SiO 2

Using classical and ab initio calculations we demonstrate that extra electrons can be trapped in pure crystalline and amorphous SiO2 (a-SiO2) in deep band gap states. The structure of trapped electron sites in pure a-SiO2 is similar to that of Ge electron centers and so-called [SiO4/Li]0 centers in α quartz. Classical potentials were used to generate amorphous silica models and density functional theory to characterize the geometrical and electronic structures of trapped electrons in crystalline and amorphous silica. The calculations demonstrate that an extra electron can be trapped at a Ge impurity in α quartz in six different configurations. An electron in the [SiO4/Li]0 center is trapped on a regular Si ion with the Li ion residing nearby. Extra electrons can trap spontaneously on pre-existing structural precursors in amorphous SiO2, while the electron self-trapping in α quartz requires overcoming a barrier of about 0.6 eV. The precursors for electron trapping in amorphous SiO2 comprise wide (≥132∘) O–Si–O angles and elongated Si–O bonds at the tails of corresponding distributions. Using this criterion, we estimate the concentration of these electron trapping sites at ≈4×1019 cm−3

Atomistic Modeling of the Electrical Conductivity of Single‐Walled Carbon Nanotube Junctions

Carbon nanotubes (CNTs) have many interesting properties that make them a focus of research in a wide range of technological applications. In CNT films, the bottleneck in charge transport is typically attributed to higher resistance at CNT junctions, leading to electrical transport characteristics that are quite different from individual CNTs. Previous simulations confirm this; however, a systematic study of transport across junctions is still lacking in the literature. Herein, density functional tight binding (DFTB) theory combined with the nonequilibrium Green's functions (NEGF) method is used to systematically calculate current across a range of CNT junctions. A random sampling approach is used to sample an extensive library of junction structures. The results demonstrate that the conductivity of CNT contacts depends on the overlap area between nanotubes and exponentially on the distances between the carbon atoms of the interacting CNTs. Two models based solely on the atomic positions of carbon atoms within the nanotubes are developed and evaluated: a simple equation using only the smallest C–C separation and a more sophisticated model using the positions of all C atoms. These junction current models can be used to predict transport in larger-scale simulations where the CNT fabric structure is known

Hydrogen-induced rupture of strained Si─O bonds in amorphous silicon dioxide

Using ab initio modeling we demonstrate that H atoms can break strained Si─O bonds in continuous amorphous silicon dioxide (a−SiO2) networks, resulting in a new defect consisting of a threefold-coordinated Si atom with an unpaired electron facing a hydroxyl group, adding to the density of dangling bond defects, such as E′ centers. The energy barriers to form this defect from interstitial H atoms range between 0.5 and 1.3 eV. This discovery of unexpected reactivity of atomic hydrogen may have significant implications for our understanding of processes in silica glass and nanoscaled silica, e.g., in porous low-permittivity insulators, and strained variants of a−SiO2

Effect of electric field on migration of defects in oxides: Vacancies and interstitials in bulk MgO

Dielectric layers composed of metal oxides are routinely subjected to external electric fields during the course of normal operation of electronic devices. Many phenomenological theories suggest that electric fields strongly affect the properties and mobilities of defects in oxide films and can even facilitate the creation of new defects. Although defects in metal oxides have been studied extensively both experimentally and theoretically, the effect of applied electric fields on their structure and migration barriers is not well understood and still remains subject to speculations. Here, we investigate how static, homogeneous electric fields affect migration barriers of canonical defects—oxygen vacancies and interstitial ions—in a prototypical oxide, MgO. Using the modern theory of polarization within density functional theory (DFT), we apply electric fields to defect migration pathways in three different charge states. The effect of the field is characterized by the change of the dipole moment of the system along the migration pathway. The largest changes in the calculated barriers are observed for charged defects, while those for the neutral defects are barely significant. We show that by multiplying the dipole moment difference between the initial and the transition states, which we define as the effective dipole moment, by the field strength, one can obtain an estimate of the barrier change in excellent agreement with the DFT calculated values. These results will help to assess the applicability of phenomenological models and elucidate linear and nonlinear effects of field application in degradation of microelectronic devices, electrocatalysis, batteries, and other applications

A computational study of Si–H bonds as precursors for neutral E' centres in amorphous silica and at the Si/SiO2 interface

Using computational modelling we investigate whether Si–H Bonds can serve as precursors for neutral E′E′ centre formation in amorphous silica and at the Si/SiO2 interface. Classical inter-atomic potentials are used to construct models of a-SiO2 containing Si–H bonds. We then investigate the mechanism of dissociation of a Si–H bond to create a neutral E′E′ defect, that is a 3-coordinated silicon with an unpaired electron localised on it. We show that the Si–H bond is extremely stable, but as a result of hole injection it is significantly weakened and may dissociate, creating a neutral E′E′ centre and a proton attached to one of the nearby oxygen atoms. The proton can diffuse around the E′E′ centre and has a profound effect on the defect levels. We show that at a Si/SiO2 interface, the position of the proton can facilitate electron transfer from the Si substrate onto the defect, making it negatively charged

Impact of opioid-free analgesia on pain severity and patient satisfaction after discharge from surgery: multispecialty, prospective cohort study in 25 countries

Background: Balancing opioid stewardship and the need for adequate analgesia following discharge after surgery is challenging. This study aimed to compare the outcomes for patients discharged with opioid versus opioid-free analgesia after common surgical procedures.Methods: This international, multicentre, prospective cohort study collected data from patients undergoing common acute and elective general surgical, urological, gynaecological, and orthopaedic procedures. The primary outcomes were patient-reported time in severe pain measured on a numerical analogue scale from 0 to 100% and patient-reported satisfaction with pain relief during the first week following discharge. Data were collected by in-hospital chart review and patient telephone interview 1 week after discharge.Results: The study recruited 4273 patients from 144 centres in 25 countries; 1311 patients (30.7%) were prescribed opioid analgesia at discharge. Patients reported being in severe pain for 10 (i.q.r. 1-30)% of the first week after discharge and rated satisfaction with analgesia as 90 (i.q.r. 80-100) of 100. After adjustment for confounders, opioid analgesia on discharge was independently associated with increased pain severity (risk ratio 1.52, 95% c.i. 1.31 to 1.76; P < 0.001) and re-presentation to healthcare providers owing to side-effects of medication (OR 2.38, 95% c.i. 1.36 to 4.17; P = 0.004), but not with satisfaction with analgesia (beta coefficient 0.92, 95% c.i. -1.52 to 3.36; P = 0.468) compared with opioid-free analgesia. Although opioid prescribing varied greatly between high-income and low- and middle-income countries, patient-reported outcomes did not.Conclusion: Opioid analgesia prescription on surgical discharge is associated with a higher risk of re-presentation owing to side-effects of medication and increased patient-reported pain, but not with changes in patient-reported satisfaction. Opioid-free discharge analgesia should be adopted routinely

Theoretical models of hydrogen-induced defects in amorphous silicon dioxide

We used density functional theory (DFT) calculations to model the interaction of hydrogen atoms and molecules with strained bonds and neutral oxygen vacancies in amorphous silica (a-SiO2). The results demonstrate that the interaction of atomic hydrogen with strained Si–O bonds in defect-free a-SiO2 networks results in the formation of two distinct defect structures, which are referred to as the [SiO4/H]0 and the hydroxyl E center. To study the distribution of each defect’s properties, up to 116 configurations of each center were calculated. We show that the hydroxyl E center can be thermodynamically stable in the neutral charge state. In order to understand the origins and reactions of this defect, different mechanisms of formation, passivation, and depassivation have been investigated. The interaction of H with a single-oxygen vacancy in a-SiO2 was studied in 144 configurations, all resulting in the hydrogen bridge defect. The reaction of the hydrogen bridge defect with the second H atom is barrierless and fully passivates the O vacancy. The latter defect reacts with atomic H with a small barrier, restoring the hydrogen bridge defect. These results provide a better understanding of how atomic and molecular hydrogen can both passivate existing defects and create new electrically active defects in amorphous-silica matrices.status: publishe

Hole trapping at hydrogenic defects in amorphous silicon dioxide

We used ab initio calculations to investigate the hole trapping reactions at a neutral defect generated in amorphous silicon dioxide networks by the interaction of strained Si-O bonds with atomic hydrogen, a so-called hydroxyl E' center. It was found that the hole trapping at this defect leads to two distinct charged configurations. The first one consists of an H atom bound to a bridging O in a hydronium-like configuration. The second configuration involves relaxation of a Si atom through the plane of its oxygen neighbors facilitated by a weak interaction with a 2-coordinated O atom. The distribution of total energy differences between these two configurations calculated for a number of amorphous network models has a width of about 1.0 eV. These hole trapping reactions are discussed in the context of Si complementary metal-oxide-semiconductor device reliability issues. © 2015 Published by Elsevier B.V

Intrinsic charge trapping in amorphous oxide films: status and challenges

We review the current understanding of intrinsic electron and hole trapping in insulating amorphous oxide films on semiconductor and metal substrates. The experimental and theoretical evidences are provided for the existence of intrinsic deep electron and hole trap states stemming from the disorder of amorphous metal oxide networks. We start from presenting the results for amorphous (a) HfO2, chosen due to the availability of highest purity amorphous films, which is vital for studying their intrinsic electronic properties. Exhaustive photo-depopulation spectroscopy measurements and theoretical calculations using density functional theory shed light on the atomic nature of electronic gap states responsible for deep electron trapping observed in a-HfO2. We review theoretical methods used for creating models of amorphous structures and electronic structure calculations of amorphous oxides and outline some of the challenges in modeling defects in amorphous materials. We then discuss theoretical models of electron polarons and bi-polarons in a-HfO2 and demonstrate that these intrinsic states originate from low-coordinated ions and elongated metal-oxygen bonds in the amorphous oxide network. Similarly, holes can be captured at under-coordinated O sites. We then discuss electron and hole trapping in other amorphous oxides, such as a-SiO2, a-Al2O3, a-TiO2. We propose that the presence of low-coordinated ions in amorphous oxides with electron states of significant p and d character near the conduction band minimum can lead to electron trapping and that deep hole trapping should be common to all amorphous oxides. Finally, we demonstrate that bi-electron trapping in a-HfO2 and a-SiO2 weakens Hf(Si)-O bonds and significantly reduces barriers for forming Frenkel defects, neutral O vacancies and O2- ions in these materials. These results should be useful for better understanding of electronic properties and structural evolution of thin amorphous films under carrier injection conditions.status: publishe