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Exploiting novel materials with advanced properties is necessary to improve device performance and continue with their scaling for high performance applications at reduced power. Among these materials, topological insulators (TIs) present an exciting opportunity where the highly conductive edge states, which are protected against back scattering, can lead to advances in electronic as well as spin controlled devices. Here, we present first principles results that evaluate topologically protected edge states in MoS 2 nanoribbons. We vary the width of the nanoribbons and show that they transition to trivial insulators below a critical width. Furthermore, the trivial insulator can be a direct or indirect band gap insulator depending on the width and the edge termination. Our results show that including realistic edge terminations can provide valuable insight, especially for narrow width TIs.

The NO anneal has been shown to effectively remove 99% of defects in SiC based devices. However, the details of interactions of NO molecules with amorphous (a)-SiO2 and SiC/SiO2 interface are still poorly understood. We use DFT simulations to investigate the NO incorporation energies in a-SiO2, and how these are affected by the steric environment. The results explain the ease with which NO molecules incorporate into a-SiO2 and give an insight into the diffusion paths they take during annealing. We highlight the importance of exhaustive sampling for exploring NO diffusion pathways.

Using the deterministic version of our hot-carrier degradation (HCD) model, we perform a statistical analysis of the impact of random dopants (RDs) on the HCD in n-FinFETs. For this, we use an ensemble of 200 transistors with different configurations of RDs. Our analysis shows that changes in the linear drain currents have broad distributions, thereby resulting in broad distributions of device lifetimes. While lifetimes are nearly normally distributed at high stress biases, under voltages close to the operating regime, the distribution has a substantially different shape. This observation considerably complicates extrapolation from accelerated stress conditions, thereby suggesting that a comprehensive statistical treatment of the impact of RDs is required.

Silicon-hydrogen bonds play a crucial role in modern microelectronics, especially regarding reliability. At the semiconductor-oxide interface these bonds are broken via interaction with energetic charge carriers, which spoils a MOSFET's performance, for example. This study develops a consistent physical picture of that phenomenon through a bottom-up approach based on quantum mechanical formulations, and also unravels the disparity of that effect in $n\ensuremath{-}$ and $p\ensuremath{-}$MOSFETs. The model is free of empirical parameters and can easily be extended to emerging material combinations.

Root-cause of instability in carbon nanotubes memristors is analyzed employing ultra-short pulse technique in combination with atomic-level material modeling. Separating various factors affecting switching operations allowed to identify structural features and operational conditions leading to improved cell characteristics.

integration requires development of low thermal budget process modules. High-quality SiO 2 interfacial layer (IL), obtained up to now only by high-temperature (≥850°C) oxidation or exposure, is crucial for pMOS NBTI reliability. In unannealed IL’s grown at reduced temperatures, we show that unrelaxed interface strain induces high defect densities, with physics-based NBTI modeling suggesting excessive hydroxyl-E’ defect formation due to Si-O bond stretch. Based on ab-initio theoretical insights, we demonstrate an atomic hydrogen treatment to passivate SiO 2 defects at low temperatures (100-300°C), which is shown to be vastly more effective than high pressure molecular hydrogen exposure, and to yield an SiO 2 quality and reliability surpassing a 900°C oxide.

Charge transfer between oxide defects and device substrate can be described as a non-radiative multiphonon transition. Within this approach oxide defects are usually modelled as 1-dimensional harmonic oscillators. In the classical limit the charge transition dynamics are determined by the crossing point of the corresponding diabatic potential energy curves of the defects. In this work we go beyond the harmonic approximation and present a scheme to locate the minimum energy path between two differently charged defect configurations on multidimensional potential energy surfaces obtained with density functional theory. Using this more accurate method we quantify the accuracy of the harmonic approximation and demonstrate its applicability for common defects in amorphous silica. Furthermore, we compare the resulting transition barriers with experimental data and demonstrate excellent agreement for hydrogen-related defect species.

We present the first physics-based approach to modeling the effect of random dopants on hot-carrier degradation (HCD) in FinFETs, which is based on a statistical analysis of HCD performed over an ensemble of 200 transistors with different random dopant configurations. As a reference, the results obtained with the deterministic version of our HCD model are used. The statistical analysis shows that degradation traces and device lifetimes have quite broad distributions and that the deterministic model tends to overestimate HCD and makes pessimistic predictions on device lifetime. Moreover, lifetime distributions evaluated for high stress voltages and for biases close to the operating regimes have different shapes which makes backward lifetime extrapolation challenging, thereby demonstrating that full physics-based HCD treatment is of crucial importance.

We present a stochastic description of hot-carrier degradation (HCD) which captures the impact of random traps (RTs) and random dopants (RDs) using our deterministic physical model for HCD. For each combination of stress voltages and stress time we generate 10,000 different samples with each of them having a unique configuration of RTs and RDs. Our analysis shows that both RTs and RDs broaden the set of degradation traces and device lifetimes, herewith resulting in average (over the sample ensemble) changes in the linear drain current lower than the nominal values from the deterministic model. Although at higher stress voltages device lifetimes follow bimodal normal distributions, at stress biases close to the operating regime the distributions are substantially different. Therefore, a proper modeling of HCD should be based on a full statistical description.

We experimentally and theoretically study the impact of interface and border traps on the transfer characteristics of 4H-SiC transistors measured over a wide temperature range of 200-350 K. Quite apparently, the experimental current-voltage characteristics have drain currents which are much lower than those obtained from simulations performed without traps. Moreover, currents increase with temperature over the entire gate voltage range, while the threshold voltage shifts towards lower values as temperature increases. We show that although interface traps can explain ${I}_{{\mathrm {d}}}-{V}_{{\mathrm {gs}}}$ curves measured at room temperature with good accuracy it fails for lower temperatures. Inclusion of border traps, on the other hand, results in good agreement between experimental and simulated current-voltage characteristics over the entire temperature range. For the first time we were able to successfully represent transfer characteristics of a 4H-SiC transistor at temperatures substantially below 300 K. Therefore, we conclude that border traps are responsible for the complicated behavior of $\mathrm{I}_{{\mathrm {d}}}-\mathrm{V}_{{\mathrm {gs}}}$ characteristics.

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) HfO

We investigated possible mechanisms for correlated defect production in amorphous (a) SiO 2 and HfO 2 films under applied stress bias using ab initio simulations. During bias application, electron injection into these films may lead to the localization of up to two electrons at intrinsic trapping sites which are present due to the natural structural disorder in amorphous structures. Trapping two electrons weakens Si-O and Hf-O bonds to such an extent that the thermally activated creation of Frenkel defects, O vacancies and O2- interstitial ions, becomes efficient even at room temperature. Bias application affects defect creation barriers and O2- interstitial diffusion. The density of trapping sites is different in a-SiO 2 and a-HfO 2 . This leads to qualitatively different degradation kinetics, which results from different correlation in defect creation in the two materials. These effects affect TDDB statistics and its dependence on the film thickness.

Display Omitted HighlightsWide O-Si-O angles in bulk amorphous silica are shown to trap electrons.Trapped electron levels appear 3.2eV below the bottom of the silica conduction band.Estimated concentration of electron trapping sites is 5i?1019. 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 ? 3.2eV 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 ? 5 i? 10 19 cm - 3 .

Display Omitted HighlightsHole-trapping on Si-H bond facilitates creation of neutral E ' centres.Proton released from Si-H bond has a dramatic effect on the defect's levels.A proton nearby the neutral E ' defect can facilitate electron transfer from Si substrate onto defect. Using computational modelling we investigate whether Si-H Bonds can serve as precursors for neutral 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 ' 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 ' centre and a proton attached to one of the nearby oxygen atoms. The proton can diffuse around the 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.

Using density functional theory (DFT) calculations we demonstrate how electron injection can facilitate the creation of Frenkel defects in amorphous (a)-SiO

Charge capture and emission by point defects in gate oxides of metal–oxide–semiconductor field-effect transistors (MOSFETs) strongly affect reliability and performance of electronic devices. Recent advances in experimental techniques used for probing defect properties have led to new insights into their characteristics. In particular, these experimental data show a repeated dis- and reappearance (the so-called volatility) of the defect-related signals. We use multiscale modelling to explain the charge capture and emission as well as defect volatility in amorphous SiO2 gate dielectrics. We first briefly discuss the recent experimental results and use a multiphonon charge capture model to describe the charge-trapping behaviour of defects in silicon-based MOSFETs. We then link this model to ab initio calculations that investigate the three most promising defect candidates. Statistical distributions of defect characteristics obtained from ab initio calculations in amorphous SiO2 are compared with the experimentally measured statistical properties of charge traps. This allows us to suggest an atomistic mechanism to explain the experimentally observed volatile behaviour of defects. We conclude that the hydroxyl-E′ centre is a promising candidate to explain all the observed features, including defect volatility.

While the defects constituting the recoverable component R of NBTI have been very well analyzed recently, the slower defects forming the more “permanent” component P are much less understood. Using a pragmatic definition for P, we study the evolution of P at elevated temperatures in the range 200°C to 350°C to accelerate these very slow processes. We demonstrate for the first time that P not only clearly saturates, with the saturation value depending on the gate bias, but also that the degradation at constant gate bias can also slowly reverse. Furthermore, at temperatures higher than about 300° C, a significant amount of additional defects is created, which are primarily uncharged around V th but contribute strongly to P at higher V G . Our new data are consistent with our recently suggested hydrogen release model which will be studied in detail using newly acquired long-term data.

The negative bias temperature instability (NBTI) in pMOS transistors is typically assumed to consist of a recoverable (R) and a so-called permanent (P) component. While R has been studied in great detail, the investigation of P is much more difficult due to the large time constants involved and the fact that P is almost always obscured by R. As such, it is not really clear how to measure P and whether it will in the end dominate device lifetime. We address these questions by introducing a pragmatic definition of P, which allows us to collect long-term data on both large and nanoscale devices. Our results suggest that (i) P is considerably smaller than R, (ii) that P is dominated by oxide rather than interface traps and therefore (iii) shows a very similar bias dependence as R, and finally (iv) that P is unlikely to dominate device lifetime. We argue that a hydrogen-release mechanism from the gate-side of the oxide, which has been suspected to cause reliability problems for a long time [1-6], is consistent with our data. Based on these results as well as our density-functional-theory (DFT) calculations we suggest a microscopic model to project the results to operating conditions.

Hole trapping in the gate insulator of pMOS transistors has been linked to a wide range of detrimental phenomena, including random telegraph noise (RTN), 1/ f noise, negative bias temperature instability (NBTI), stress-induced leakage currents (SILC) and hot-carrier degradation. Recently we were able to show that the hydrogen bridge (HB) and hydroxyl E′ centers (H-E′centers) are likely candidates for BTI defects in amorphous silicon dioxide (a-SiO 2 ). In time-dependent defect spectroscopy (TDDS) measurements, it was observed that defects tend to dis- and reappear in the measurements. This so called volatility is not a rare event, but occurs for a majority of the defects. In this work we investigate whether this particular behavior could be explained by an extension of our four-state model. As both of the investigated defect candidates contain hydrogen, we propose that the behavior could be explained by the hydrogen atom moving away from the defect site onto a neighboring oxygen atom and back again. Our results show that the suggested mechanism is likely to occur for hydroxyl E′ centers, but not for the hydrogen bridges.

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 $({\text{a-SiO}}_{2})$. The results demonstrate that the interaction of atomic hydrogen with strained Si--O bonds in defect-free ${\text{a-SiO}}_{2}$ networks results in the formation of two distinct defect structures, which are referred to as the ${[{\text{SiO}}_{4}/\text{H}]}^{0}$ and the hydroxyl ${\text{E}}^{\ensuremath{'}}$ center. To study the distribution of each defect's properties, up to 116 configurations of each center were calculated. We show that the hydroxyl ${E}^{\ensuremath{'}}$ 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 ${\text{a-SiO}}_{2}$ 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.

We measure and analyse the optical absorption spectra of three silica glass samples irradiated with 1 MeV electrons at 80 K, where self-trapped holes are stable, and use ab initio calculations to demonstrate that these spectra contain a signature of intrinsic electron traps created as counterparts to the holes. In particular, we argue that optical absorption bands peaking at 3.7, 4.7, and 6.4 eV belong to strongly localised electrons trapped at precursor sites in amorphous structure characterized by strained Si–O bonds and O–Si–O angles greater than 132°. These results are important for our understanding of the properties of silica glass and other silicates as well as the reliability of electronic and optical devices and for luminescence dating.

Recent studies have clearly shown that oxide defects are more complicated than typically assumed in simple two-state models, which only consider a neutral and a charged state. In particular, oxide defects can be volatile, meaning that they can be deactivated and re-activated at the same site with the same properties. In addition, these defects can transform and change their properties. The details of all these processes are presently unknown and poorly characterized. Here we employ time-dependent defect spectroscopy (TDDS) to more closely study the changes occurring at the defect sites. Our findings suggest that these changes are ubiquitous and must be an essential aspect of our understanding of oxide defects. Using density-functional-theory (DFT) calculations, we propose hydrogen-defect interactions consistent with our observations. Our results suggest that standard defect characterization methods, such as the analysis of random telegraph noise (RTN), will typically only provide a snapshot of the defect landscape which is subject to change anytime during device operation.

Using ab initio modeling we demonstrate that H atoms can break strained Si{O bonds in continuous amorphous (a)-SiO2 networks, resulting in a defect consisting of a 3-coordinated Si atom with an unpaired electron facing a hydroxyl group, adding to the density of dangling bond defects, such as E0 centres. The 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 signifcant implications for our understanding of processes in nano-scaled silica, e.g., in porous low-permittivity insulators, and strained variants of a-SiO2. ispartof: Physical Review Letters vol:114 issue:11 pages:1-5 ispartof: location:United States status: published

Display Omitted We model hole trapping at hydroxyl E' centers in amorphous silicon dioxide.Two distinct hole trapping configurations: (1) Protonic configuration; (2) Back-projected configuration.Calculated barriers for transformation between both configurations.Electron trapping at these hole traps results in two distinct neutral defect configurations. 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 1.0eV. These hole trapping reactions are discussed in the context of Si complementary metal-oxide-semiconductor device reliability issues.

Hole trapping in the gate insulator of pMOS transistors has been linked to a wide range of detrimental phenomena, including random telegraph noise (RTN), 1/ f noise, negative bias temperature instability (NBTI), stress-induced leakage currents (SILC) and hot carrier degradation. Since the dynamics of hole trapping appear similar in various oxides such as pure SiO 2 , SiON, and high-k, the responsible defects should have a related microscopic structure. While a number of defects have been suspected to be responsible for these phenomena, such as oxygen vacancies/E′ centers, K centers, hydrogen bridges or hydrogen-related defects in general, the chemical nature of the dominant charge trap remains controversial. Based on extended time-dependent defect spectroscopy (TDDS) data, we investigate the statistical properties of a number of defect candidates using density functional theory (DFT) calculations. Our results suggest hydrogen bridges and hydroxyl E′ centers to be very likely candidates.

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.

Capture and emission of carriers by point defects in gate dielectrics, such as SiO2 and HfO2, and at their interfaces with the substrate are thought to be responsible for performance and reliability issues in MOS devices, particularly dielectric degradation and the bias temperature instability (BTI). Ultra-thin silicon dioxide films are present at the interface between Si and high-κ oxides; thus it is hoped that understanding the defects in silica which contribute to BTI will also aid the reliability of devices containing high-κ oxides. This chapter reviews the state of the art of modeling oxygen deficiency defects implicated in both electron and hole trapping in amorphous silica (a-SiO2).

Using density functional theory (DFT) calculations we demonstrate how electron injection can facilitate the creation of Frenkel defects in amorphous (a)-SiO2. The precursor sites composed of wide O-Si-O bond angles in amorphous SiO2 act as deep electron traps and can accommodate up to two extra electrons. Trapping of two electrons at these intrinsic sites results in weakening of a Si-O bond and creates an efficient bond breaking pathway for producing neutral O vacancies and [Formula: see text] interstitial ions characterized by low transition barriers. The low barriers for the migration of [Formula: see text] ions of about 0.2 eV facilitate the separation of created defects. This mechanism may have important implications for our understanding of dielectric breakdown and resistance switching in a-SiO2 based electronic and memory devices.

In a project to develop hydroxyapatites for bone replacement, biological and synthetic types were prepared at 600 degrees C, ground to 300-600 microns and immersed in a pooled human serum for periods up to 1 month at 4 degrees C to assess material interaction. It was found that the levels of calcium in the serum were reduced at 6 h immersion, followed by an increase to reach maximum at 48 h and then stability up to 1 month. Phosphorus levels showed the opposite behaviour. Both apatites showed similar trends, although higher values were recorded for the synthetic type, suggesting higher activity. Infrared spectral analysis complemented the biochemical values, where the optical densities (O.D.) of phosphate groups were reduced, reflecting the increased phosphorus in serum and denoting leaching. Also, O.D. values of both CO3(2-) and OH- groups were reduced at 10 h, then returned to original levels. Scanning electron microscopy revealed a spongy appearance parallel with reduced O.D. and higher levels of serum Ca2+. At longer periods (48 h) the concentric needles of hydroxyapatite are clearly shown to be deposited on biological apatite. Differences in responses were attributed to their original crystalline structure assessed by X-ray diffraction analysis, as well as pore analysis using a mercury porosimeter.

We present a statistical analysis of the cumulative impact of random traps (RTs) and dopants (RDs) on hot-carrier degradation (HCD) in n-channel FinFETs. Calculations are performed at three combinations of high stress voltages and for conditions close to the operating regime. We generate 200 different configurations of devices with RDs and subsequently solve the Boltzmann transport equation to obtain the continuous interface trap concentration ${N} _{\text {it}}$ . These deterministic densities ${N} _{\text {it}}$ for each individual configuration are randomized and converted to 200 different configurations of RTs, yielding a total amount of 40,000 samples in our study. The analysis shows that at high stress voltages (with both RTs and RDs taken into account) probability densities of linear drain currents and device lifetimes are close to a bi-modal normal distribution, while in the operating regime such a trend is not visible.

Hot-carrier degradation (HCD) of Ge pNWFETs has been shown to be significantly lower and different than that of Si pNWFETs. Here we accurately model degradation and the time-to-failure (TTF) measured during HC stress in Ge pNWFETs. For this, we use our HCD framework validated against hot-carrier degradation in Si devices, and first we show here that it thoroughly represents HCD Si pNWFETs. This framework is naturally extendable to incorporate new defects (O-vacancies) and their precursors (Ge-O bonds) which are involved in HCD in Ge transistors. These Ge-O bonds are present due to segregation of Ge through the Si cap into the SiO 2 film of the high-k gate stack with subsequent formation of the SiGe/Si x Ge y O 2(x + y) interface. The Ge-H bonds are argued to be unstable while a low concentration of Si-H bonds with a broad distribution of the bonding energy should still be considered. Our unified model has been demonstrated to accurately capture degradation and TTFs in both Ge and Si pNWFETs, with the superior slope of the TTF vs. drain current dependency in the Ge devices determined by the defect creation energetics, hereby supported by ab initio calculations.