Silicon Dangling Bond Defects Research Articles

Atomic-scale defects generated in the early/intermediate stages of dielectric breakdown in Si/SiO2 transistors

Electrically detected magnetic resonance and near-zero-field magnetoresistance measurements were used to study atomic-scale traps generated during high-field gate stressing in Si/SiO2 MOSFETs. The defects observed are almost certainly important to time-dependent dielectric breakdown. The measurements were made with spin-dependent recombination current involving defects at and near the Si/SiO2 boundary. The interface traps observed are Pb0 and Pb1 centers, which are silicon dangling bond defects. The ratio of Pb0/Pb1 is dependent on the gate stressing polarity. Electrically detected magnetic resonance measurements also reveal generation of E′ oxide defects near the Si/SiO2 interface. Near-zero-field magnetoresistance measurements made throughout stressing reveal that the local hyperfine environment of the interface traps changes with stressing time; these changes are almost certainly due to the redistribution of hydrogen near the interface.

Are dangling bond centers important interface traps in 4H-SiC metal oxide semiconductor field effect transistors?

Silicon carbide (SiC) based metal-oxide-semiconductor field-effect transistors (MOSFETs) have great promise in high power and high temperature applications. Unfortunately, effective channel mobilities remain disappointingly low, typically about 30 cm2/Vs. A major contributor to the disappointing effective channel mobilities is the presence of substantial densities of interface traps at the SiC/SiO2 interface. Many investigators have invoked silicon or carbon dangling bonds to be the dominating source of these interface defects, but very little, if any, direct experimental evidence exists to support this assumption in the SiC/SiO2 system. Cantin et al. [Phys. Rev. Lett. 92, 1 (2004)] have used conventional electron paramagnetic resonance measurements on porous oxidized SiC structures to measure the g tensor for the SiC/SiO2 interface carbon dangling bond. These results provide a particularly straightforward means to search for the presence of carbon dangling bonds in fully processed SiC MOSFETs using electrically detected magnetic resonance. Additionally, simple theory provides guidance to search for silicon dangling bond defects. In this study, we utilize K band electrically detected magnetic resonance via spin dependent charge pumping measurements in which almost all of the SiC band gap at the SiC/SiO2 interface is accessed. Although quite high signal to noise measurements are achieved, we are unable to detect any trace of the carbon dangling bond spectra. However, in very poor quality p-channel devices, we observe a spectrum which could be consistent with silicon dangling bonds. Other defect centers are clearly present and we conclude that these other centers dominate the interface trap density of states.

Band diagram for low-k/Cu interconnects: The starting point for understanding back-end-of-line (BEOL) electrical reliability

The starting point for describing the electrostatic operation of any semiconductor device begins with a band diagram illustrating changes in the semiconductor Fermi level and the alignment of the valence and conduction bands with other interfacing semiconductors, insulating dielectrics and metal contacts. Such diagrams are essential for understanding the behavior and reliability of any semiconductor device. For metal interconnects, the band alignment between the metal conductor and the insulating intermetal and interlayer dielectric (ILD) is equally important. However, relatively few investigations have been made. In this regard, we have investigated the band alignment at the most common interfaces present in traditional single and dual damascene low-k/Cu interconnect structures. We specifically report combined X-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy (REELS) measurements of the Schottky barrier present at the ILD and dielectric Cu capping layer (CCL) interfaces with the Ta(N) via/trench Cu diffusion barrier. We also report similar measurements of the valence and conduction band offsets present at the interface between a-SiN(C):H dielectric CCLs and low-k a-SiOC:H ILDs (porous and non-porous). The combined results point to metal interfaces with the CCL having the lowest interfacial barrier for electron transport. As trap and defect states in low-k dielectrics are also important to understanding low-k/Cu interconnect reliability, we additionally present combined electron paramagnetic resonance (EPR) and electrically detected magnetic resonance (EDMR) measurements to determine the chemical identity and energy level of some electrically active trap/defect states in low-k dielectrics. Combined with the photoemission derived band diagrams, the EPR/EDMR measurements point to mid-gap carbon and silicon dangling bond defects in the low-k ILD and CCL, respectively, playing a role in electronic transport in these materials. We show that in many cases the combined band and defect state diagrams can explain and predict some of the observed reliability issues reported for low-k/Cu interconnects.

Comparing n- and p-type polycrystalline silicon absorbers in thin-film solar cells

We have investigated fine grained polycrystalline silicon thin films grown by direct chemical vapor deposition on oxidized silicon substrates. More specifically, we analyze the influence of the doping type on the properties of this model polycrystalline silicon material. This includes an investigation of defect passivation and benchmarking of minority carrier properties. In our investigation, we use a variety of characterization techniques to probe the properties of the investigated polycrystalline silicon thin films, including Fourier Transform Photoelectron Spectroscopy, Electron Spin Resonance, Conductivity Activation, and Suns-Voc measurements. Amphoteric silicon dangling bond defects are identified as the most prominent defect type present in these layers. They are the primary recombination center in the relatively lowly doped polysilicon thin films at the heart of the current investigation. In contrast with the case of solar cells based on Czochralski silicon or multicrystalline silicon wafers, we conclude that no benefit is found to be associated with the use of n-type dopants over p-type dopants in the active absorber of the investigated polycrystalline silicon thin-film solar cells.

Process-Induced Degradation of SiO<inline-formula><tex-math>$_{\bf 2}$</tex-math></inline-formula> and a-Si:H Passivation Layers for Photovoltaic Applications

The passivation characteristics of thermally grown silicon dioxide (SiO 2 ) and hydrogenated amorphous silicon (a-Si:H) layers are investigated, using a combination of photoluminescence and capacitance-voltage analysis techniques. Key findings are the significant passivation degradation of SiO 2 and a-Si:H layers induced by metallization through electron beam evaporation. The degradation correlates with an increase in silicon dangling bond defect density at the interface with silicon (for both SiO 2 and a-Si:H) or in the passivation layer (a-Si:H). Performing the metallization by thermal evaporation is an effective method to avoid such process-induced damage, as is forming gas annealing at 450°C, which effectively recovers the interface characteristics of SiO 2 layers. Deposition of amorphous silicon on a thermal SiO 2 layer induces bulk and interface defects in the SiO 2 layer-but in this case, a 450°C forming gas anneal is not possible due to the thermal budget limitations of a-Si:H, thereby posing problems for solar cell structures which rely on a combination of PECVD a-Si:H and thermal SiO 2 passivation layers.

Atomic and Electrical Characterisation of Amorphous Silicon Passivation Layers

In this work, an extensive characterisation of intrinsic amorphous silicon (a-Si) passivation layers deposited on n- and p-type silicon is reported. Low temperature capacitance-voltage measurements are utilised to enable parameter extraction from the c-Si/a-Si interface and a-Si bulk. Electron spin resonance enables atomic identification of defects present. Results reveal the presence of electrically active defects at the c-Si/intrinsic a-Si interface (∼1x1012cm-2), and throughout the amorphous silicon layer bulk (∼8x1016cm-3), which are atomically identified as Pb0 centres and D centres silicon dangling bond defects, respectively. The value of this work is the atomic identification of these defects in this stack, coupled with their electrical activity. That they can be detected by these techniques demonstrates the power of the methodology used to assess and quantify these defects. Therein lies the significance of this work: a methodology capable of fundamentally optimising amorphous silicon processing from an atomic perspective.

Effects of vacuum ultraviolet and ultraviolet irradiation on ultrathin hafnium-oxide dielectric layers on (100)Si as measured with electron-spin resonance

The effects of vacuum ultraviolet (VUV) (7.2 eV) and UV (4.9 eV) irradiation on hafnium-oxide dielectric layers were studied with electron-spin resonance to detect defect states. Silicon dangling-bond defects (Pb centers) and positively charged oxygen vacancies (E′ centers) were detected with g-factor fitting. VUV irradiation increases the level of Pb states, while UV decreases the level of Pb states but increases the level of E′ states significantly. Rapid thermal annealing appears to mitigate these effects. Absolute values of the defect-state concentrations are presented.

UV optical absorption spectra analysis of beryl crystals from Brazil

The spectral decomposition analysis was applied to the optical absorption spectra of green and colorless beryl crystals from the Brazilian Eastern Pegmatitic province in the natural state, submitted to heat treatment and irradiated with UV light. The attributions of the lines were made taking into account highly accurate quantum mechanical calculations. The deconvolution of the green beryl spectra revealed four lines, two of them around 12,000 cm −1 (1.5 eV) and two of them around 34,000 cm −1 (4.2 eV) attributed to Fe 2+ and Fe 3+, respectively. The deconvolution of the colorless beryl spectra without any treatment, after heating and for the same heat treatment followed by UV light irradiation revealed five lines. The analysis of ratio relations showed that the lines at 36,400 cm −1 (4.5 eV) and 41,400 cm −1 (5.1 eV) belongs to a single defect attributed to a silicon dangling bond defect (Si:). Discussions and comparison with reported defects in quartz have supported the allocation of the lines at 61,000 cm −1 (7.6 eV) and 43,800 cm −1 (5.4 eV) to diamagnetic oxygen vacancy defect (Si–Si) and unrelaxed (Si⋯Si) defect, respectively. Finally, the line at 39,100 cm −1 (4.8 eV), quite polarized along the c-axis, was attributed to a (Fe 2+OH −) defect in the structural channels.

Direct observation of electrically active interfacial layer defects which may cause threshold voltage instabilities in HfO2 based metal-oxide-silicon field-effect transistors

We show that a Si∕HfO2 interfacial layer defect with an electron spin resonance spectrum similar to that of some E′ center variants responds to oxide bias consistent with an amphoteric defect. The spectrum is weakly orientation dependent indicating that the defect does not reside in a completely amorphous matrix. The defect’s spin lattice relaxation time is much shorter than that of conventional E′ centers suggesting that the defect involves some coupling of a Hf atom to a nearby oxygen deficient silicon dangling bond defect. This defect very likely plays an important role in widely reported instabilities in HfO2 based transistors.

Identification of the atomic-scale defects involved in the negative bias temperature instability in plasma-nitrided p-channel metal-oxide-silicon field-effect transistors

We utilize a combination of DC gate-controlled diode recombination current measurements as well as two very sensitive electrically detected magnetic resonance techniques, spin-dependent recombination and spin-dependent tunneling, to identify atomic-scale defects involved in the negative bias temperature instability (NBTI) in 2.3nm plasma-nitrided SiO2-based p-channel metal-oxide-silicon field-effect transistors. We demonstrate that the dominating NBTI-induced defect in the plasma-nitrided devices is fundamentally different than those observed in pure SiO2-based devices. (In pure SiO2 devices, we observe NBTI-induced Pb0 and Pb1 defects.) Our measurements indicate that the NBTI-induced defect in the plasma-nitrided devices extends into the gate dielectric. The defect participates in both spin-dependent recombination and spin-dependent tunneling. The defect also has a density of states which is more narrowly peaked than that of Pb centers near the middle of the band gap. The high sensitivity of our spin-dependent tunneling measurements allow for an identification of the physical and chemical nature of this defect through observations of Si29 hyperfine interactions. We identify these defects as silicon dangling bond defects in which the central silicon is back bonded to nitrogen atoms. We assign these NBTI-induced defects as KN centers because of their similarity to K centers observed in silicon nitride. (The silicon nitride K centers are also silicon dangling bond defects in which the silicon atom is back-bonded to nitrogen atoms.) The defect identification in plasma-nitrided devices helps to explain (1) why NBTI is exacerbated in nitrided devices, (2) conflicting reports of NBTI-induced interface states and/or bulk traps, and (3) fluorine’s ineffectiveness in reducing NBTI in nitrided devices.

Microscopic aspects of the variations in the retention times of dynamic random access memory

The authors have examined the retention time of memory bits. Silicon dangling bond defects are shown to be consistent with retention time observations. Interactions between hydrogen and silicon dangling bond defect complexes are calculated for several model cases using first-principles density functional theory. Variable retention time is explained in terms of hydrogen interacting with a silicon dangling bond defect at the Si–SiO2 interface.

Character of the Reaction between Molecular Hydrogen and a Silicon Dangling Bond in Amorphous SiO2

The passivation by diffusing H2 of silicon dangling bond defects (E' centers, induced by laser irradiation in amorphous SiO_2 (silica), is investigated in situ at several temperatures. It is found that the reaction between E' center and H_2 requires an activation energy of 0.38eV, and that its kinetics is not diffusion-limited. The results are compared with previous findings on the other fundamental paramagnetic point defect in silica, the non bridging oxygen hole center, which features completely different reaction properties with H_2. Besides, a comparison is proposed with literature data on the reaction properties of surface E' centers, of E' centers embedded in silica films, and with theoretical calculations. In particular, the close agreement with the reaction properties of surface E' centers with H_2 leads to conclude that the bulk and surface E' varieties are indistinguishable from their reaction properties with molecular hydrogen.

The impact of negative-bias-temperature-instability on the carrier generation lifetime of metal-oxynitride-silicon capacitors

N -type metal-oxynitride-silicon capacitors were subjected to a negative bias voltage at an elevated temperature. Under this condition, Negative-bias-temperature-instability (NBTI) was evidenced in capacitance-voltage (CV) characteristics with an accompanying decrease, as determined from inversion layer-related (i.e., generation lifetime) deep-level-transient-spectroscopy (DLTS) measurements, in the carrier generation lifetime. An analysis of the dominant defect supplying minority carriers to the inversion layer at the silicon-oxynitride interface revealed a significant change brought about by NBTI. A full recovery of the carrier generation lifetime was observed after several days at zero bias voltage, and room temperature conditions had elapsed following NBTI. Concurrently, a passivation, as inferred from conventional DLTS measurements, of the NBTI-induced trap density occurred at the silicon-oxynitride interface. These relaxation and deactivation phenomena were discussed in the context of a plausible model involving a reaction at the silicon-oxynitride interface between trivalent silicon dangling bond defects and hydrogenous species.

The Concentration of (SiH2)n Sites in Low and High Defect Density a-Si:H

AbstractThe concentration of polysilane chains (SiH2)n, where n≥1, is estimated for higher quality hydrogenated amorphous silicon (a-Si:H) by pulsed proton nuclear magnetic resonance techniques (1H NMR). Our measurements indicate the minimum hydrogen content of approximately 10% of the total hydrogen is in the (SiH2)n configuration. Similar measurements in a high defect density sample (1017 silicon dangling bond defects cm-3) show that (SiH2)n sites account for ~ 15% of the total hydrogen. While the (SiH2)n infrared absorption (IR) modes are observed in the highly defective sample, no such modes are seen in the higher quality material. The results indicate that a significant amount of the total hydrogen content exists as (SiH2)n regardless of film quality.

Atomic scale defects involved in MOS reliability problems

For quite some time, it has been known that two types of silicon dangling bond centers play important roles in metal-oxide-silicon reliability problems. Silicon dangling bonds at the Si/SiO 2 boundary defects (called P b centers) dominate Si/SiO 2 interface trapping. Silicon dangling bond defects (called E′ centers) dominate deep levels in the oxide. Recent observations provide a more detailed understanding of the relationship between the structure and electronic properties of the P b interface centers. Recent work also helps explain the frequently noted correlation between oxide deep level defects and Si/SiO 2 interface trap defects.

Magnetic resonance measurements in hydrogenated amorphous silicon

Abstract Nuclear magnetic resonance (NMR) and electron spin resonance (ESR) are useful techniques for obtaining information about local structural order and defects in hydrogenated amorphous silicon (a-Si:H). In particular, NMR is a useful probe of the bonding of hydrogen to silicon and of the trapping of molecular hydrogen (H 2 ) in a-Si:H. ESR is a useful probe of localized electronic states, such as silicon dangling bond defects and optically excited electrons and holes trapped in the conduction and valence band tails, respectively. Recent NMR measurements have shown that H 2 exists in two environments, one clustered and one dilute. The fraction of H 2 , which is greater than previously thought, is approximately 10% of the total hydrogen concentration. ESR measurements of the growth and decay of silicon dangling bonds after optical excitation at low temperatures (one probe of the Staebler–Wronski effect) show that these defects are created at a slower rate and anneal at lower temperatures than those created at 300 K. Recent NMR measurements suggest the presence of a paired-hydrogen site, which may be the stabilizing center for the defects that control the Staebler–Wronski effect. ESR results show that the decay of optically excited, band-tail electrons and holes is a universal feature of amorphous semiconductors for which the electron–lattice interaction can be neglected. Although the research is less developed in microcrystalline silicon, NMR and ESR are proving to be useful probes of local order and defects in this material as well.

Radiation-induced leakage currents: atomic scale mechanisms

We link atomic-scale MOS gate oxide silicon dangling bond defects called E' centers to radiation-induced leakage current (RILC). We present evidence that strongly suggests that RILC tolerance is processing dependent and that this tolerance appears to be correlated with lower E' center generation. We furthermore note that in oxides subjected to quite high irradiation levels, the density of (generally electrically neutral) E' centers is far greater than would be expected for the hole trap E' centers involved in the radiation-induced positive charge build-up observed in thicker oxides.

Defect-related photoinduced absorption in amorphous silicon

Photoinduced optical absorption in thin films of hydrogenated amorphous silicon (a-Si : H) is studied over the energy range related to electronic transitions involving the silicon dangling-bond defects, by using the dual-beam Constant Photocurrent Method (CPM). The subgap absorption in the range 0.8–1.4 eV is observed to increase with bias light intensity. Since CPM measures only transitions contributing to the photocurrent, the subgap absorption spectrum in undoped a-Si:H is dominated by transitions between singly occupied D 0 or doubly occupied D − defect states and conduction band extended states. The photoinduced changes in the subgap absorption are consistent with numerical calculations with a recombination model for a-Si : H which yield dangling bond electronic occupations as a function of photogeneration rate. The results suggest that the observed changes in the optical absorption spectra are mainly determined by changes in the occupation of defects induced by the bias-light.

Fingerprints of two distinct defects causing light-induced photoconductivity degradation in hydrogenated amorphous silicon

We find distinct experimental fingerprints of two metastable defects created during illumination of hydrogenated amorphous silicon. The well-studied threefold-coordinated silicon dangling bond defect has an anneal activation energy near 1.1 eV and dominates annealing experiments above about 110 °C. The second defect created by illumination is the “primary recombination” (pr) center, which causes most of the light-induced photoconductivity decrease and dominates annealing experiments below about 110 °C. Because the pr centers are created in linear proportion to the dangling bond defects, they are difficult to distinguish during degradation. However, we observe clear fingerprints of the pr center during their low T annealing: (1) an anneal activation energy of 0.85 eV; (2) a sharp increase in photoconductivity; and (3) a surprising increase in hν⩽1.1 eV optical absorption.

Si (100)– SiO 2 interface properties following rapid thermal processing

An experimental examination of the properties of the Si(100)–SiO2 interface measured following rapid thermal processing (RTP) is presented. The interface properties have been examined using high frequency and quasi-static capacitance-voltage (CV) analysis of metal-oxide-silicon (MOS) capacitor structures immediately following either rapid thermal oxidation (RTO) or rapid thermal annealing (RTA). The experimental results reveal a characteristic peak in the CV response measured following dry RTO and RTA (T>800 °C), as the Fermi level at the Si(100)–SiO2 interface approaches the conduction band edge. Analysis of the QSCV responses reveals a high interface state density across the energy gap following dry RTO and RTA processing, with a characteristic peak density in the range 5.5×1012 to 1.7×1013 cm−2 eV−1 located at approximately 0.85–0.88 eV above the valence band edge. When the background density of states for a hydrogen-passivated interface is subtracted, another peak of lower density (3×1012 to 7×1012 cm−2 eV−1) is observed at approximately 0.25–0.33 eV above the valence band edge. The experimental results point to a common interface state defect present after processes involving rapid cooling (101–102 °C/s) from a temperature of 800 °C or above, in a hydrogen free ambient. This work demonstrates that the interface states measured following RTP (T>800 °C) are the net contribution of the Pb0/Pb1 silicon dangling bond defects for the oxidized Si(100) orientation. An important conclusion arising from this work is that the primary effect of an RTA in nitrogen (600–1050 °C) is to cause hydrogen desorption from pre-existing Pb0/Pb1 silicon dangling bond defects. The implications of this work to the study of the Si–SiO2 interface, and the technological implications for silicon based MOS processes, are briefly discussed. The significance of these new results to thin oxide growth and optimization by RTO are also considered.