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

From Jekyll to Hyde and Beyond: Hydrogen's Multifaceted Role in Passivation, H-Induced Breakdown, and Charging of Amorphous Silicon Nitride

In semiconductor devices, hydrogen has traditionally been viewed as a panacea for defects, being adept at neutralizing dangling bonds and consequently purging the related states from the band gap. With amorphous silicon nitride (a-Si3N4)─a material critical for electronic, optical, and mechanical applications─this belief holds true as hydrogen passivates both silicon and nitrogen dangling bonds. However, there is more to the story. Our density functional theory calculations unveil hydrogen's multifaceted role upon incorporation in a-Si3N4. On the "Jekyll" side, hydrogen atoms are indeed restorative, healing coordination defects in a-Si3N4. However, "Hyde" emerges as hydrogen induces Si-N bond breaking, particularly in strained regions of the amorphous network. Beyond these dual roles, our study reveals an intricate balance between hydrogen defect centers and intrinsic charge traps that already exist in pristine a-Si3N4: the excess charges provided by the H atoms result in charging of the a-Si3N4 dielectric layer.

Near-zero-field magnetoresistance measurements: A simple method to track atomic-scale defects involved in metal-oxide-semiconductor device reliability

We demonstrate the ability of a relatively new analytical technique, near-zero-field magnetoresistance (NZFMR), to track atomic-scale phenomena involved in the high-field stressing damage of fully processed Si metal-oxide-semiconductor field-effect transistors. We show that the technique is sensitive to both the P_b0 and P_b1 dangling bond centers and that the presence of both centers can be inferred through NZFMR via hyperfine interactions with the central ²⁹Si atoms of the dangling bonds. The NZFMR results also provide evidence for the redistribution of mobile hydrogen atoms at the Si/SiO₂ interface and also a potential change in the average dipolar coupling constant between electrons in neighboring defects. This work shows that NZFMR offers significant analytical power for studying technologically relevant semiconductor device reliability problems and has advantages in experimental simplicity over comparable techniques.

Defects Induced Charge Trapping/Detrapping and Hysteresis Phenomenon in MoS2 Field-Effect Transistors: Mechanism Revealed by Anharmonic Marcus Charge Transfer Theory

One critical problem inhibiting the application of MoS₂ field-effect transistors (FETs) is the hysteresis in their transfer characteristics, which is typically associated with charge trapping (CT) and charge detrapping (CDT) induced by atomic defects at the MoS₂-dielectric interface. Here, we propose a novel atomistic framework to simulate electronic processes across the MoS₂-SiO₂ interface, demonstrating the distinct CT/CDT behavior of different types of atomic defects and further revealing the defect type(s) that most likely cause hysteresis. An anharmonic approximation of the classical Marcus theory is developed and combined with state-of-the-art density functional theory to calculate the gate bias-dependent CT/CDT rates. All the key electronic quantities are calculated with Heyd-Scuseria-Ernzerhof hybrid functionals. The results show that single Si-dangling bond defects are active electron trapping centers. Single O-dangling bond defects are active hole trapping centers, which are more likely to be responsible for the hysteresis phenomenon due to their significant CT rate and apparent threshold voltage shift. In contrast, double Si-dangling bond defects are not active trap centers. These findings provide fundamental physical insights for understanding the hysteresis behavior of MoS₂ FETs and provide vital support for understanding and solving the reliability of nanoscale devices.

First-Principles Study on the Molecular Mechanism of Solar-Driven CO2 Reduction on H-Terminated Si

Solar-driven conversion of CO₂ with H-terminated silicon has recently attracted increasing interest. However, the molecular mechanism of the reaction is still not well understood. A systematic study of the mechanism has been carried out with first-principles calculations. The formation energies of the intermediates are found to be insensitive to the structure of the surface. On the fully H-terminated Si(111) surface, several pathways for the conversion of CO₂ into CO at a coordinatively saturated Si site are studied, including the conventional COOH* pathway and the direct insertion of CO₂ into Si-H and Si-Si bonds. Although the barrier of the COOH* pathway is lowest among the three pathways, it is higher than that for OH* elimination, which suggests that CO₂ should be converted by other types of active site. The reaction at the isolated and dual coordinatively unsaturated (CUS) Si sites, which can be generated by light illumination, heat, and Pd loading, are then studied. The results suggest that the most efficient pathway to convert CO₂ is to convert it into CO and O* at an isolated CUS Si site before O* reacts with a terminating H* to form adsorbed OH* and generate new isolated CUS Si sites. Therefore, the CUS Si site catalyzes the reaction until all H* is converted into OH*. The results provide new insight into the mechanism of the reaction and should be helpful for the design of more efficient Si-based catalysts for CO₂ conversion.

Modeling of Diffusion and Incorporation of Interstitial Oxygen Ions at the TiN/SiO2 Interface

Silica-based resistive random access memory devices have become an active research area due to complementary metal-oxide-semiconductor compatibility and recent dramatic increases in their performance and endurance. In spite of both experimental and theoretical insights gained into the electroforming process, many atomistic aspects of the set and reset operation of these devices are still poorly understood. Recently a mechanism of electroforming process based on the formation of neutral oxygen vacancies (V_O⁰) and interstitial O ions (O_i^2-) facilitated by electron injection into the oxide has been proposed. In this work, we extend the description of the bulk (O_i^2-) migration to the interface of amorphous SiO₂ with the polycrystaline TiN electrode, using density functional theory simulations. The results demonstrate a strong kinetic and thermodynamic drive for the movement of O_i^2- to the interface, with dramatically reduced incorporation energies and migration barriers close to the interface. The arrival of O_i^2- at the interface is accompanied by preferential oxidation of undercoordinated Ti sites at the interface, forming a Ti-O layer. We investigate how O ions incorporate into a perfect and defective ∑5(012)[100] grain boundary (GB) in TiN oriented perpendicular to the interface. Our simulations demonstrate the preferential incorporation of O_i at defects within the TiN GB and their fast diffusion along a passivated grain boundary. They explain how, as a result of electroforming, the system undergoes very significant structural changes with the oxide being significantly reduced, interface being oxidized, and part of the oxygen leaving the system.

The origin of negative charging in amorphous Al2O3 films: the role of native defects

Amorphous aluminum oxide Al₂O₃ (a-Al₂O₃) layers grown by various deposition techniques contain a significant density of negative charges. In spite of several experimental and theoretical studies, the origin of these charges still remains unclear. We report the results of extensive density functional theory calculations of native defects-O and Al vacancies and interstitials, as well as H interstitial centers-in different charge states in both crystalline α-Al₂O₃ and in a-Al₂O₃. The results demonstrate that both the charging process and the energy distribution of traps responsible for negative charging of a-Al₂O₃ films (Zahid et al 2010 IEEE Trans. Electron Devices 57 2907) can be understood assuming that the negatively charged O_i and V_Al defects are nearly compensated by the positively charged H_i, V_O and Al_i defects in as prepared samples. Following electron injection, the states of Al_i, V_O or H_i in the band gap become occupied by electrons and sample becomes negatively charged. The optical excitation energies from these states into the oxide conduction band agree with the results of exhaustive photo-depopulation spectroscopy measurements (Zahid et al 2010 IEEE Trans. Electron Devices 57 2907). This new understanding of the origin of negative charging of a-Al₂O₃ films is important for further development of nanoelectronic devices and solar cells.

Quantum-chemical simulation of the adsorption-induced reduction of strength of siloxane bonds

Mechanical strength of silicate glasses is known to decrease markedly due to the adsorption of molecules from the environment, especially in aqueous alkali solutions. This effect, known as the adsorption-induced reduction of strength (AIRS), has not yet been fully understood. Here, the dependence on the chemical nature and electronic properties of adsorbates of the AIRS of siloxane bonds in silica was studied by means of quantum-chemical calculations at the wB97X-D3/def2-TZVP level of theory. A siloxane bond was modelled by H₃Si-O-SiH₃ and (HO)₃Si-O-Si(OH)₃ clusters, and the AIRS was simulated by a linear tensile deformation of the siloxane bond in the presence of the following adsorbates: OH^-, Cl^-, H₂O, H⁺ and H₃O⁺. Potential energy profiles and derivative force curves of the siloxane bond rupture were obtained. The varying effect of the adsorbates on the energy-force characteristics of the AIRS can be explained by changes in the bond lengths and electron occupancy. It is shown that the AIRS of the siloxane bonds increases with an increase in the nucleophilicity of the adsorbates, and correlates with an adsorbate-induced redistribution of electron density.

Photoinduced Hysteresis of Graphene Field-Effect Transistors Due to Hydrogen-Complexed Defects in Silicon Dioxide

Photoinduced hysteresis (PIH) of graphene field-effect transistors (G-FETs) has attracted attention because of its potential in developing photoelectronic or nonvolatile memory devices. In this work, we focused on the role of SiO₂ dielectric layer on PIH, where G-FETs have only a SiO₂ dielectric layer. Adsorbates are effectively removed before the PIH test. The effects of laser wavelength, laser power density, and temperature on the PIH are systematically investigated. The PIH is significantly enhanced by increasing the hydrogen flow in a hydrogen-atmosphere device thermal annealing. This strongly suggests proton-related defects that play a key role. The pure electronic process for PIH is further ruled out by the significant dependence of the doping rate on the temperature. A mechanism of PIH based on proton generation after hole trapping at [O₃≡Si-H] is proposed. The proposed mechanism is well-supported by our experimental data: (1) the observed threshold photon energy for PIH is between 2.76 and 2.34 eV, which is close to the energy barrier for [O₃≡Si-H], releasing a proton. (2) No obvious carrier mobility degradation after the PIH process suggests that the bulk defects in SiO₂ are the major contributors rather than graphene/SiO₂ interface defects. (3) The dependence of the doping rate on the temperature and the laser power density matches a theoretical model based on the random hopping of H⁺. The results in this work are also valuable for the study of degradation of other oxide dielectric materials in various field-effect transistors.

Dielectric Breakdown and Post-Breakdown Dissolution of Si/SiO2 Cathodes in Acidic Aqueous Electrochemical Environment

Understanding the conducting mechanisms of dielectric materials under various conditions is of increasing importance. Here, we report the dielectric breakdown (DB) and post-breakdown mechanism of Si/SiO₂, a widely used semiconductor and dielectric, in an acidic aqueous electrochemical environment. Cathodic breakdown was found to generate conduction spots on the Si/SiO₂ surface. Using scanning electrochemical microscopy (SECM), the size and number of conduction spots are confirmed to increase from nanometer to micrometer scale during the application of negative voltage. The morphologies of these conduction spots reveal locally recessed inverted-pyramidal structures with exposed Si{111} sidewalls. The pits generation preceded by DB is considered to occur via cathodic dissolution of Si and exfoliation of SiO₂ that are induced by local pH increases due to the hydrogen evolution reaction (HER) at the conduction spots. The HER at the conduction spots is more sluggish due to strongly hydrogen-terminated Si{111} surfaces.

Role of hydrogen in volatile behaviour of defects in SiO2-based electronic devices

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.