Tunnelling currents through oxygen vacancies in ultrathin SiO2 layer

First principle calculations have been carried out to investigate the effects of oxygen vacancy on the electronic structure of silicon dioxide. Energy levels within the band gap at E _c?0.11 eV and E _v + 1.98 eV has been obtained. Thus, the effects of the oxygen vacancy located at the silicon/silicon dioxide interface on the tunnelling current through the oxide have been calculated. When the oxide thickness is less than 3 nm, the effects of the defect-assisted electron tunnelling on the tunnelling current will increase with the oxide thickness and oxide field, whereas the defect-assisted hole tunnelling on the tunnelling current will decrease with the oxide thickness and oxide field increasing.     

Hot hole stress induced leakage current (SILC) transient in tunneloxides

The mechanisms and transient characteristics of hot hole stress induced leakage current (SILC) in tunnel oxides are investigated. Positive oxide charge assisted tunneling is found to be a dominant SILC mechanism in a hot hole stressed device. The SILC transient is attributed to oxide hole detrapping and thus annihilation of positive charge assisted tunneling centers. Our characterization shows that the leakage current transient in a 100-Å oxide obeys a power law time dependence f^-n with the power factor n significantly less than one. An analytical model accounting for the observed time dependence is proposed     

Atomic Cobalt Vacancy-Cluster Enabling Optimized Electronic Structure for Efficient Water Splitting

Vacancies created on a surface can alter the local electronic structure, thus enabling a higher intrinsic activity for the evolution of hydrogen and oxygen. Conventional strategies for vacancy engineering, however, have a strong focus on non-metal sulfur/oxygen defects, which have often overlooked metallic vacancies. Herein, evidence is provided that cobalt vacancies can be atomically tuned to have different sizes to achieve cobalt vacancy clusters through controlling the migration of iridium single atoms. The coalescence of Co vacancy clusters at the surface of an IrCo alloy results in an increased d-band level and eventually compromises H adsorption, leading to enhanced electrocatalytic activity toward the hydrogen evolution reaction. In addition, the Co vacancy clusters can improve the electronic conductivity with respect to the oxidized Co surface, which substantially aids in strengthening the adsorption of oxygen intermediates toward an effective oxygen evolution reaction at a low overpotential. These collective effects originate from the Co vacancy cluster and specifically enable highly efficient and stable water splitting with a low total overpotential of 384 mV in alkaline media and 365 mV in an acidic environment, achieving a current density of 10 mA cm^–2.     

Investigation of hole-tunneling current through ultrathinoxynitride/oxide stack gate dielectrics in p-MOSFETs

The systematic investigation of hole tunneling current through ultrathin oxide, oxynitride, oxynitride/oxide (N/O) and oxide/oxynitride/oxide (ONO) gate dielectrics in p-MOSFETs using a physical model is reported for the first time. The validity of the model is corroborated by the good agreement between the simulated and experimental results. Under typical inversion biases (|VG|<2 V), hole tunneling current is lower through oxynitride and oxynitride/oxide with about 33 at.% N than through pure oxide and nitride gate dielectrics. This is attributed to the competitive effects of the increase in the dielectric constant, and hence dielectric thickness, and decrease in the hole barrier height at the dielectric/Si interface with increasing with N concentration for a given electrical oxide thickness (EOT). For a N/O stack film with the same N concentration in the oxynitride, the hole tunneling current decreases monotonically with oxynitride thickness under the typical inversion biases. For minimum gate leakage current and maintaining an acceptable dielectric/Si interfacial quality, an N/O stack structure consisting of an oxynitride layer with 33 at.% N and a 3 Å oxide layer is proposed. For a p-MOSFET at an operating voltage of -0.9 V, which is applicable to the 0.7 μm technology node, this structure could be scaled to EOT=12 Å if the maximum allowed gate leakage current is 1 A/cm² and EOT=9 Å if the maximum allowed gate leakage current is 100 A/cm²     

Effect of oxygen vacancy in tungsten oxide on the photocatalytic activity for decomposition of organic materials in the gas phase

The relationship between the oxygen vacancy of tungsten oxide and its ability to decompose organic materials under visible-light irradiation was investigated experimentally. In the field of rechargeable batteries, the highest charge-discharge rate is obtained when tungsten oxide is used as a negative electrode with an O/W ratio of 2.72. This result suggested that the number of oxygen vacancies in tungsten oxide affects the photocatalytic decomposition behavior of organic materials. Therefore, with the aim of increasing the photocatalytic activity of tungsten oxide to decompose organic materials, we attempted to clarify the role of the oxygen vacancy. WO_3 − x nanoparticles, including WO_2.83 and WO_2.72 nanoparticles, were fabricated by changing the annealing temperature in a 10% H₂, 90% N₂ atmosphere to generate different densities of oxygen vacancies. Tungsten oxide with O/W ratios of 2.83 and 2.72 exhibited no photocatalytic activity for the photodecomposition of organic materials. The maximum decomposition rate was obtained for stoichiometric WO₃ (O/W = 3). The reason for the decrease or disappearance of the photodecomposition ability should originate in the increase in the number of electrons generated by the oxygen vacancies. These excess electrons promote the recombination reaction between electrons and holes in WO_3 − x, and hence reduce the lifetime of electron-hole pairs.     

Two-carrier conduction in MOS tunnel oxides—1 Experimental results

A novel device structure incorporating a p-channel MOSFET with a metal/tunnel-oxide/n-silicon device is proposed as a tool for separating electron and hole tunneling currents in ultra-thin silicon dioxide films. With this structure, the electron and hole tunneling currents can be independently measured at the substrate and source terminals, respectively. Furthermore, the injected minority carrier (hole) current which is supplied by the p-MOSFET can be varied independently of the tunnel-oxide bias. As expected, the injected hole current modulates the electron current and “current multiplication” was observed. By correlating experimental results for 22.5 Å SiO₂ films with theoretical calculations, the electron and hole barrier heights were determined to be 3.2 and 3.6 eV, respectively, where a trapezoidal tunneling barrier was assumed and a carrier effective mass of 0.5 m₀ was used.The tunnel-oxide quality and uniformity was evaluated by measuring I–V and C-V curves on two-terminal MOS capacitors of various areas. The results suggest that the oxide films are extremely uniform in thickness, and the measured interface trap density was determined to be less than 10¹¹ cm^?2eV^?1. For the reverse-biased tunnel-oxide, the electron/hole current ratio was found to be less than unity except for the condition when the injected hole current was very small compared to the electron current without any hole injection. In addition, this ratio was found to decrease rapidly with increasing oxide thickness and/or increasing hole injection level.     

Efficient Field Emission from Vertically Aligned Cu2O1‐δ(111) Nanostructure Influenced by Oxygen Vacancy

In the architecture described, cuprous oxide (Cu₂O) is tamed to be highly (111) plane oriented nanostructure through adjusting the deposition postulate by glancing angle deposition technique. In the controlled atmosphere oxygen vacancy is introduced into the Cu₂O crystal subsequently fostering an impurity energy state (E_im) close to the conduction band. Our model of Cu₂O electronic structure using density functional theory suggests that oxygen vacancies enhance the electron donating ability because of unshared d‐electrons of Cu atoms (nearest to the vacancy site), allowing to pin the work function energy level around 0.28 eV compared to the bulk. This result is also complemented by Kelvin probe force microscopy analysis and X‐ray photoelectron spectroscopy method. Oxygen vacancy in Cu₂O (Cu₂O_1‐δ) exhibits promising field emission properties with interesting field electron tunneling behavior at different applied fields. The films show very low turn‐on and threshold voltages of value 0.8 and 2.4 V μm^?1 respectively which is influenced by the oxygen vacancy. Here, a correlation between the work function modulation due to the oxygen vacancy and enhancement of field emission of Cu₂O_1–δ nanostructure is demonstrated. This work reveals a promising new vision for Cu₂O as a low power field emitter device.     

Investigation of oxide charge trapping and detrapping in a MOSFETby using a GIDL current technique

We proposed a new measurement technique to investigate oxide charge trapping and detrapping in a hot carrier stressed n-MOSFET by measuring a GIDL current transient. This measurement technique is based on the concept that in a MOSFET the Si surface field and thus GIDL current vary with oxide trapped charge. By monitoring the temporal evolution of GIDL current, the oxide charge trapping/detrapping characteristics can be obtained. An analytical model accounting for the time-dependence of an oxide charge detrapping induced GIDL current transient was derived. A specially designed measurement consisting of oxide trap creation, oxide trap filling with electrons or holes and oxide charge detrapping was performed. Two hot carrier stress methods, channel hot electron injection and band-to-band tunneling induced hot hole injection, were employed in this work. Both electron detrapping and hole detrapping induced GIDL current transients mere observed in the same device. The time-dependence of the transients indicates that oxide charge detrapping is mainly achieved via field enhanced tunneling. In addition, we used this technique to characterize oxide trap growth in the two hot carrier stress conditions. The result reveals that the hot hole stress is about 10⁴ times more efficient in trap generation than the hot electron stress in terms of injected charge     

A comparative analysis of substrate current generation mechanisms in tunneling MOS capacitors

This paper presents a critical analysis of the origin of majority and minority carrier substrate currents in tunneling MOS capacitors. For this purpose, a novel, physically-based model, which is comprehensive in terms of impact ionization and hot carrier photon emission and re-absorption in the substrate, is presented. The model provides a better quantitative understanding of the relative importance of different physical mechanisms on the origin of substrate currents in tunneling MOS capacitors featuring different oxide thickness. The results indicate that for thick oxides, the majority carrier substrate current is dominated by anode, hole injection, while the minority carrier current is consistent with a photon emission-absorption mechanism, at least in the range of oxide voltage and oxide thickness covered by the considered experiments. These two currents appear to be strictly correlated because of the relatively flat ratio between impact ionization and photon emission scattering rates and because of the weak dependence of hole transmission probability on oxide thickness and gate bias. Simulations also suggest that, for thinner oxides and smaller oxide voltage drop, the photon emission mechanism might become dominant in the generation of substrate holes.     

Electronic properties of defects in polycrystalline dielectric materials

Grain boundaries have been implicated in current leakage and dielectric breakdown of CMOS devices. We calculate the electronic properties of oxygen vacancy defects near grain boundaries in the dielectric insulators MgO and HfO₂ using first principles methods. In both materials we find that oxygen vacancies favourably segregate to grain boundaries, in various charge states. Their electronic properties are different from their counterparts in the bulk. At increased concentrations, such defects at grain boundaries may play a key role in processes such as electron tunneling, charge trapping and dielectric breakdown in electronic devices.     

Role of positive trapped charge in stress-induced leakage current for flash EEPROM devices

The transient behavior of hot hole (HH) stress-induced leakage current (SILC) in tunnel oxides is investigated. The dominant SILC mechanism is positive oxide charge-assisted tunneling (PCAT). The transient effect of SILC is attributed to positive oxide charge detrapping and thus the reduction of PCAT current. A correlation between SILC and stress-induced substrate current is observed. Our study shows that both SILC and stress-induced substrate current have power law time-dependence t/sup -n/ with the power factor n about 0.7 and 1, respectively. Numerical analysis for PCAT current incorporating a trapped charge caused Coulombic potential in the tunneling barrier is performed to evaluate the time- and field-dependence of SILC and the substrate current. Based on our model, the evolution of threshold voltage shift with read-disturb time in a flash EEPROM cell is derived. Finally, the dependence of SILC on oxide thickness is explored. As oxide thickness reduces from 100 /spl Aring/ to 53 /spl Aring/, the dominant SILC mechanism is found to change from PCAT to neutral trap-assisted tunneling (TAT).     

Effect of vacancies on nucleation of oxide precipitates in silicon

The use of installed, tailored vacancy concentration profiles (MDZ^®) to control oxygen precipitation behavior in silicon wafers has become an important technology for the 300 mm era. This paper presents a new model of one of the central physical processes important to the MDZ^® system: of the role played by vacancies to control the nucleation of oxygen precipitates. Steady-state nucleation of oxide precipitates is described as a random walk in a 2D space of the two basic size variables of an oxygen cluster: the number n of agglomerated oxygen atoms, and the number m of consumed vacancies. The model can account well for the basic experimental results: a high sensitivity of the nucleation rate to the vacancy concentration and a low sensitivity to the oxygen content.     

Effects of neglecting carrier tunneling on electrostatic potentialin calculating direct tunneling gate current in deep submicron MOSFETs

We investigate the validity of the assumption of neglecting carrier tunneling effects on the self-consistent electrostatic potential in calculating the direct tunneling gate current in deep submicron MOSFETs. A comparison between simulated and experimental results shows that for accurate modeling of direct tunneling current, tunneling effects on potential profile need to be considered. The relative error in gate current due to neglecting carrier tunneling is higher at higher gate voltages and increases with decreasing oxide thickness. We also study the direct tunneling gate current in MOSFETs with high-K gate dielectrics     

Tunneling leakage current in oxynitride: dependence onoxygen/nitrogen content

It is widely known that the addition of nitrogen in silicon oxide, or the addition of oxygen in silicon nitride, affects its reliability as a gate dielectric. The authors examine the gate leakage current as a function of the oxygen and nitrogen contents in ultrathin silicon oxynitride films on Si substrates. It is shown that, provided that electron tunneling is the dominant current conduction mechanism, the gate leakage current in the direct tunneling regime increases monotonically with the oxygen content for a given equivalent oxide thickness (EOT), such that pure silicon nitride passes the least amount of current while pure silicon oxide is the leakiest     

锐钛矿型TiO2(101)面吸附CO2分子的光学气敏传感机理

对金属氧化物光学气敏传感材料TiO2的探索与应用是当前研究的热点问题。采用基于密度泛函理论(DFT)中的平面波超软赝势方法, 模拟计算CO2分子在锐钛矿型TiO2(101)表面的吸附行为, 对吸附能, 吸附距离, 电子态密度以及光学性质进行分析。结果表明: CO2分子在含O空位表面的吸附效果优于无氧空位表面, 且表面O空位的浓度越高, 吸附效果越明显;分子平行于表面放置模型的吸附能为正值, 吸附后的结构稳定, 且以O端吸附为主, 为此, 分子平行于表面放置O端吸附于含两个O空位表面为最可能吸附模型;对电子态密度分析发现, 当最佳模型吸附稳定后, 含O空位表面为P型杂质, 又有CO2分子中的2p电子掺入, 在费米能级附近出现新峰值, 改善了TiO2材料的光学性质, 体现出较好的光学气敏传感特性。     

Enhanced Hole Gate Direct Tunneling Current in Process-Induced Uniaxial Compressive Stress p-MOSFETs

On a nominally 1.27-nm-thick gate oxide p-MOSFET with shallow trench isolation (STI) longitudinal compressive mechanical stress, hole gate direct tunneling current in inversion is measured across the wafer. The resulting average gate current exhibits an increasing trend with STI compressive stress. However, this is exactly contrary to the currently recognized trend: hole gate direct tunneling current decreases with externally applied compressive stress, which is due to the strain-altered valence-band splitting. To determine the mechanisms responsible, a quantum strain simulator is established, and its validity is confirmed. The simulator then systematically leads us to the finding of the origin: a reduction in the physical gate oxide thickness, with the accuracy identified down to 0.001 nm, occurs under the influence of the STI compressive stress. The strain-retarded oxide growth rate can significantly enhance hole direct tunneling and thereby reverse the conventional trend due to the strain-altered valence-band splitting.      

Oxygen Vacancy Creation,Drift, and Aggregation in TiO2‐Based Resistive Switches at Low Temperature and Voltage

Transmission electron microscopy with in situ biasing has been performed on TiN/single‐crystal rutile TiO₂/Pt resistive switching structures. Three elementary processes essential for switching: i) creation of oxygen vacancies by electrochemical reactions at low temperatures (<150 °C), ii) their drift in the electric field, and iii) their coalescence into planar faults (and dissociation from them) have been documented. The faults have a form of vacancy discs in {110} and {121} planes, are bound by partial dislocation loops, and are identical to Wadsley defects observed in nonstoichiometric TiO₂ annealed at high temperatures. The faults can be regarded as a precursor to the formation of oxygen‐deficient Magnéli phases, but 3D secondary phase inclusions have not been detected. Together, the observations shed light on the behavior of oxygen vacancies in relatively low electric fields and temperatures, suggesting that, in addition to the rather accepted notion of oxygen vacancy motion during the writing processes in resistive switching devices, such motion may occur even during reading, and may be accompanied by significant oxygen vacancy creation at modest device excitation levels.     

Oxide thickness dependence of nitridation effects on TDDB characteristics

Wet oxide thicknesses dependence of nitridation effects on electrical characteristics, charge trapping properties and TDDB (Time Dependent Dielectric Breakdown) characteristics have been investigated. It is found that the difference of conduction current between the wet and nitrided wet oxide increases with increasing oxide thickness both for negative and positive bias to the gate until constant current stress is applied. After the stress, with decreasing oxide thickness both in wet and nitrided wet oxide leakage current increases. Up to 60 Å no difference was observed between the wet and nitrided wet oxide but at 50 Å nitrided wet oxide has less increase of current comparing to the wet oxide for the same stress. In wet oxide with increasing stress current density initial hole trap decreases but electron trap increases whereas in nitrided wet oxide has less initial hole trap and also electron trap is less comparing to the wet oxide. Both in wet and nitrided wet oxide for negative bias stress, time to 50 % breakdown decreases with decreasing thickness but at 50 Å a turn-around effect was observed due to nitridation i.e., the 50 % breakdown time is greater for nitrided wet oxide comparing to the wet oxide. On the contrary, for positive bias stress 50 % breakdown time increases with decreasing oxide thickness both in wet and nitrided wet oxide. For positive bias also a turn-around effect is observed at 50 Å i.e., 50% breakdown time is less in nitrided wet oxide comparing to the wet oxide. The improved reliability of nitrided wet oxide at the thin region of 50 Å seems to be due to the increase of more Si---N bond to the interface of oxide and Si comparing to the thick oxide of above 60 Å for the same nitridation conditions.     

Model for the oxide thickness dependence of SILC generation based on anode hole injection process

A new model for the oxide thickness dependence of the SILC generation has been proposed on the basis of defects created by anode hole injection. This model which has been validated on oxides with thickness down to ≈3nm, allows the explanation of the bell-shaped behavior of the SILC variation with oxide thickness and predicts a strong reduction of the SILC intensity for ultra thin oxides operated in the direct tunneling regime.     

Evidence of hole flow in silicon nitride for positive gate voltage

This paper discusses the conduction mechanism of silicon nitride. n-channel transistors and MOS capacitors with the top-oxide/ nitride/bottom-oxide dielectric structure were used to characterize the dielectric conduction. Top and bottom oxides were found to have different effects on the dielectric leakage current and electron and hole tunneling. This implies that the dominant charge carriers across the top and bottom oxides are different. We claim the conduction through a bottom oxide is dominated by electron flow and conduction through a top oxide and the nitride is dominated by hole flow for positive gate voltage. Energy band diagrams are presented to discuss the effective trap level for hole conduction in the nitride and holes and electrons tunneling through the oxide/nitride/oxide dielectrics.