Si Dangling Bond Defects Research Articles

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.

On the characteristics of N-polar GaN Schottky barrier contacts with LPCVD SiN interlayers

We study the behavior of N-polar GaN Schottky diodes with low-pressure chemical vapor deposited (LPCVD) SiN interlayers and unveil the important role of an amphoteric miniband formed in this interlayer due to a previously identified and dominating Si dangling bond defect. Through analysis of temperature-dependent current–voltage (I–V–T), capacitance–voltage (C–V), and x-ray photoelectron spectroscopy measurements, we observe that when nickel is deposited on LPCVD SiN pretreated with hydrofluoric acid, the SiN/GaN interface is responsible for determining the overall system's barrier height. By contrast, contact formation on oxidized LPCVD SiN leads to a metal/SiN-dominant barrier. We, consequently, propose band diagrams that account for an amphoteric miniband in LPCVD SiN, leading to a new understanding of LPCVD SiN as a lossy dielectric with surface barrier-dependent behavior.

Electron transport and dielectric breakdown in silicon nitride using a charge transport model

Silicon nitride is an important material used in the electronics industry. As such, the electronic transport and reliability of these materials are important to study and understand. We report on a charge transport model to predict leakage current and failure trends based on previously published data for a stoichiometric silicon nitride dielectric. Failure occurs when the defect density increases to a critical value of approximately 6 × 1025 traps/m3. The model's parameters are determined using voltage ramp data only, and yet, the model is also able to predict constant voltage stress failure over a time scale ranging from minutes to months. The successful fit of the model to the experimental data validates our assumption that the dominant defect in the dielectric is the Si dangling bond, located approximately 2.2 eV below the conduction band. A comparison with previous SiCOH simulations shows SiN and SiCOH have similar defect-related material properties. It is also speculated that, based on the estimated parameter values of 2.75 eV for the defect formation activation energy, the materials' TDDB wear-out are caused by broken Si-H bonds, resulting in Si dangling bond defects.

Generation of Si dangling bond defects at Si/insulator interfaces induced by oxygen scavenging

AbstractIn this work we analyse the influence of the O‐scavenging process on interface traps in (100)Si/SiOx/HfO2(1.8 nm)/TiNx/Si stacks by using electrical measurements and electron spin resonance spectroscopy. The reduction of interfacial SiOx by high‐temperature annealing in O‐free ambient is found to lead to severe interface degradation exposed as generation of additional Si dangling bond defects (paramagnetic Pb0 centres). The density of these centres, well known to be electron traps impairing device operation, increases from ∼1 × 1012 cm−2 – the value typical for thermally oxidized Si – to >3 × 1012 cm−2 upon annealing at 1100 °C. This interface degradation is accompanied by the development of a hysteresis in the MOS capacitance–voltage curves indicating generation of additional oxide traps with areal density in the range of 1012 cm−2. However, the trap generation can be alleviated if the O‐scavenging annealing step is performed in helium ambient, suggesting formation of silicon monoxide molecules at the Si/SiOx interface as the origin of the observed trap generation.

Control of growth process for obtaining high-quality a-SiO:H

Film-growth process of hydrogenated amorphous silicon–oxygen alloys (a-SiO:H) from CO2/(CO2 + SiH4) plasma has been investigated to control the optoelectronic properties in the resulting materials. Optical emission spectroscopy results and simple simulation results for steady-state density of chemical species in the plasma indicate that main film-growth precursors for a-SiO:H are SiH3, OH, and O. Si dangling-bond defect density is drastically increased in a-SiO:H when increasing the CO2 gas ratio in CO2/(CO2 + SiH4) plasma, being caused by the increase in the contribution ratio of Si-related short-lifetime species (SiHx, x < 2) to film growth owing to a severe SiH4-molecule depletion because of high-rate consumption reaction of SiH4 with O produced from CO2 in the plasma. Considering the primary electron impact dissociation reactions of source gas molecules and several secondary chemical reactions in the plasma, the guiding principle for obtaining high quality a-SiO:H has been proposed.

PECVD-AlOx/SiNx Passivation Stacks on Silicon: Effective Charge Dynamics and Interface Defect State Spectroscopy

The charge dynamics and the interface defect state density of AlOx/SiNx passivation stacks deposited by plasma-enhanced chemical vapor deposition (PECVD) on crystalline silicon (c-Si) wafers are investigated. High frequency (1MHz) capacitance voltage (C-V) measurements were performed on stacks in the as deposited state and after an annealing step. C-V sweeps reveal an initially high negative charge density for the as deposited sample, activated by the thermal budget during SiNx deposition. However, this charge state is unstable and reduced owning to electron detrapping and emission into the c-Si upon applying moderate voltages. In the annealed sample, the AlOx/SiNx stack has a stable negative fixed charge. Both for as deposited and for annealed samples, applying a positive or negative constant gate voltage stress (Vstress) enhances or reduces the negative effective charge density (Qox,eff), respectively. Injection of charges from the c-Si into traps in the AlOx/SiNx stack is identified as the mechanism responsible for this behavior. We conclude that in addition to fixed negative charges trapping of negative charges near the interface is a crucial mechanism contributing to the total effective negative charge of the stack. Their contribution depends on the temperature and duration of the thermal treatment. Additionally, a large Vstress leads to generation of additional Si dangling bond defects over the entire c-Si bang gap at the c-Si/AlOx interface.

Metal Removal and Defect Passivation Performed on Si Wafers for Solar Cell Use by HCN Treatments

Cleaning of Si wafers for solar cell use with HCN solutions can remove metal contaminants almost completely. Fe and Ni atoms with ∼2 × 1011 and ∼3 × 1010 atoms/cm2 concentrations reside on Si even after formation of pyramidal textured Si surfaces produced by anisotropic alkaline etching. The Fe and Ni concentrations decrease to less than ∼6 × 109 atoms/cm2 (i.e., detection limit of a total-reflection X-ray fluorescence (TXRF) spectrometer for Fe) and ∼1 × 1010 atoms/cm2, respectively, after cleaning with pH 11.5 HCN solutions containing 1.5 M isopropanol (IPA). Minority carrier lifetime for the pyramidal textured p-type single crystalline Si specimens increases from 58 to 220 μs by the HCN treatment with IPA. The micro-roughness on Si(111) surfaces treated with HCN solutions containing IPA is much less (i.e., 0.27 nm) than that without IPA (i.e., 0.52 nm). Adsorption of IPA on Si retards the etching rate from 32.7 to 8.6 nm/min, leading to enhancement of anisotropic etching, thus resulting in the formation of more complete pyramidal structure. Cleaning with pH 11.5 HCN solutions without IPA increases reflectivity due to isotropic etching, while that with IPA slightly decreases it due to the formation of more complete pyramidal textured surfaces. The increase in the minority carrier lifetime caused by HCN cleaning with IPA is attributable to i) removal of metal contaminants, ii) passivation of Si dangling bond defect states by CN− ions, and iii) a decrease in the defect density due to a decrease in the micro-roughness.

The dangling-bond defect in amorphous silicon: Statistical random versus kinetically driven defect geometries

Amorphous and micro-crystalline silicon (a-Si:H, μc-Si) are key materials for resource-saving thin-film solar cells. However, the efficiency of such devices is severely limited by light-induced Si dangling-bond defects, which can be detected by electron paramagnetic resonance (EPR). We report density-functional theory calculations on a set of random dangling bonds created in supercell models of a-Si:H and compare calculated hyperfine and g-tensor distributions to the ones obtained from a recent multi-frequency EPR spectral analysis. Our results show that the g-tensor does not exhibit axial symmetry as has been previously assumed, but is clearly rhombic. The hyperfine coupling to the undercoordinated Si atom, on the other hand, is almost perfectly axial. This apparent discrepancy in the symmetry properties is shown to be a consequence of the underlying coupling mechanisms and how these are influenced by structural disorder.However, the hyperfine distribution calculated from our random models underestimates the experimentally observed 30% red-shift when going from c-Si to a-Si:H. We suggest that only a subset of possible dangling-bond configurations is observed in experiment. We discuss plausible mechanisms that would give rise to such a selection, and new experiments to test these hypotheses.

Native defects in hexagonalβ-Si3N4studied using density functional theory calculations

A comprehensive study of single native point defects in hexagonal silicon nitride ($\ensuremath{\beta}$-Si${}_{3}$${\mathrm{N}}_{4}$) has been carried out based on density functional calculations of formation energies. Both nitrogen- and silicon-rich native defect centers form donor and acceptor states in the band gap of $\ensuremath{\beta}$-Si${}_{3}$${\mathrm{N}}_{4}$, confirming their amphoteric behavior. Silicon dangling bonds resulting from structural nitrogen vacancies (${V}_{\mathrm{N}}$) are the most abundant native defects, in particular, in their acceptor state (${V}_{\mathrm{N}}^{\ensuremath{-}}$ and ${\mathrm{V}}_{\mathrm{N}}^{3\ensuremath{-}}$). Hydrogenation promotes the appearance of Si dangling bond defects in the neutral state consistent with the observation of paramagnetic centers in the gap of amorphous silicon nitrides by electron spin resonance. This explains the utility of silicon nitride as charge trapping layer in nonvolatile memories.

Generation of defects in amorphousSiO2 assisted by two-step absorption on impurity sites

Generation of the Si dangling bond defect in amorphousSiO2 (E′ centre) induced by tunable pulsed UV laser radiation was investigated by in situ opticalabsorption measurements. The defect generation efficiency peaks when the photon energy equals∼5.1 eV, it depends quadratically on laser intensity and is correlated with the native linearabsorption due to Ge impurities. We propose a model in which the generation ofE′ is assisted by a two-step absorption process occurring on Ge impurity sites.

Boron-doped a-Si:H∕c-Si interface passivation: Degradation mechanism

The authors report that for a-Si:H∕c-Si heterostructure solar cell fabrication the presence of a boron-doped a-Si:H(p+) overlayer may cause H2 effusion from a (few nanometers) thin underlying intrinsic a-Si:H(i) film at moderate temperatures. This phenomenon is in agreement with losses in the electronic passivation quality of c-Si∕a-Si:H(i)∕a-Si:H(p+) structures occurring during low temperature (⩽260°C) postdeposition annealing. Consequently, it is argued that such passivation degradation is due to Si–H rupture in the a-Si:H(i) film, likely resulting in Si dangling bond defects, mediated by the presence of the doped layer.

Modelling of paramagnetic trivalent silicon defect centres in amorphous silica and atSi–SiO2 interfaces

We review the structural and hyperfine properties of Si dangling bond defects occurring in amorphousSiO2 and atvarious Si–SiO2 interfaces. These defects have in common a singly occupied orbital on a trivalent Si centre.We first briefly summarize the most important methodologies for calculating hyperfineparameters, pointing out their advantages and drawbacks. The properties of the defectcentres composed of kernels, with n varying from 0 to 3, are then discussed in a systematic manner. We present three importantfactors affecting the hybrid state of the unpaired dangling bond: the local geometry aroundthe defect centre, the electronegativity of the first-neighbour atoms, and the polarizationeffect due to the oxide environment around the dangling bond. We demonstrate that thecage polarization effect significantly increases the Fermi contact term of the defect Si atom,and discuss the relevant implications. We also quantify the interaction between thedangling orbital and the oxygen atoms belonging to the oxide cage by focusing on the17O hyperfine couplings. For various defects in amorphousSiO2 andat Si–SiO2 interfaces, we discuss assignments to structural units in relation to available experimental data. In particular, we address the charge stateof the Eγ′ centre, the peculiar hyperfine properties of theS centre, and the atomic structures of the threePb-type centres.

Stability of fully deuterated amorphous silicon thin-film transistors

The threshold voltage stability of fully deuterated (a-Si:D) and hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) is compared. The difference in the kinetic energy of D+ and H+ ions upon impact with the growing surface during radio-frequency plasma-enhanced chemical vapor deposition leads to material having different physical properties for the same nominal deposition conditions. However, a-Si:D and a-Si:H grown at the same growth rate by adjusting the gas pressure have almost identical properties. By using the growth rate as a normalizing parameter for comparing a-Si:H and a-Si:D TFTs, it is shown that there is no difference in the stability of a-Si:D compared with a-Si:H TFTs. This study rules out the possibility of a giant isotopic effect in amorphous silicon TFTs, and supports the model for Si dangling bond defect creation in a-Si:H where the breaking of weak Si–Si bonds is the rate-limiting step.

Admittance spectroscopy of traps at the interfaces of (1 0 0)Si with Al 2O 3, ZrO 2, and HfO 2

Admittance (ac) measurements were carried out to determine the interface trap density ( D it) as a function of energy E in the Si bandgap at interfaces of Si with different insulating oxides (Al 2O 3, ZrO 2, HfO 2). The results are compared to those of the conventional thermal SiO 2/Si interface. The results show that a significant portion of the interface trap density in the as-deposited and de-hydrogenated samples is related to the amphoteric Si dangling bond defects ( P b0 -centers). The D it is much enhanced for the Al-containing insulators as compared to Si/SiO 2 but can be reduced by annealing in O 2. As to annealing in H 2, efficient passivation of P b0 centers by hydrogen is achieved for Si/ZrO 2 and Si/HfO 2 interfaces, yet it fails for Si/Al-containing oxide entities. Among the insulators studied, the results suggest HfO 2 to be the best choice of an alternative insulator.

The O – (Al 2 ) centre in topaz and its relation to the blue colour

Colour-enhanced blue topaz is one of the most traded gemstones. Naturally very rare, mostly topaz is irradiated by neutrons, electrons, gamma radiation and combinations of them. The colour centre is still not identified. It was speculated that it is related to a Si dangling bond defect occupied by two electrons with spin S = 0. We investigated natural blue as well as colourless topaz from different regions in Brazil by electron paramagnetic resonance (EPR), optical absorption and Raman spectroscopy. The results are compared with neutron and gamma-irradiated blue topaz. By EPR two paramagnetic defects are identified in all samples: (i) the peroxy radical (O2–) measured at room temperature and (ii) an (O–) hole centre interacting with two equivalent Al neighbours measured at low temperature. Blue samples show an absorption band centred at 620 nm which is responsible for the blue colour. From our investigation we find that the O–(Al2) hole centre has nearly the same thermal stability as the optical absorption band. However, we cannot say whether it is responsible for the absorption band and the blue colour. We suggest that at least it plays a dominant role in the stabilization of the blue colour. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Energy distribution of the (100)Si/HfO2 interface states

The energy distributions of interface states were determined in (100)Si/HfO2 using capacitance–voltage measurements on structures with the periphery of the metal electrode switched to inversion by controlled application of corona discharge. Together with independently applied ac conductance spectroscopy they reveal a strong impact of the HfO2 deposition technique on the interface trap density. This includes the enhancement of the Si-dangling bond defect (Pb0 centers) density and a contribution of insulator-related traps in samples deposited using a nitrogen-containing precursor.

Absence of enhanced stability in deuterated amorphous silicon thin film transistors

ABSTRACTA comparison of the threshold voltage shift after gate-bias stress in hydrogenated and fully deuterated amorphous silicon thin film transistors (TFTs) is presented. A series of fully deuterated bottom gate TFTs consisting of a deuterated n+ contact layer, deuterated intrinsic amorphous silicon (deposited at a range of pressures) and deuterated silicon nitride gate insulator have been produced. A similar series of fully hydrogenated bottom gate TFTs have also been produced, and the stability of the two sets of devices compared. Deuterated and hydrogenated amorphous silicon deposited under the same process conditions do not have the same material properties due to the difference in the ion energy of H and D in the plasma. However, deuterated and hydrogenated material deposited at the same growth rate have almost identical structural properties. Hydrogenated and deuterated TFTs are found to exhibit the same variation in stability as a function of growth rate. In particular, there is no evidence for increased stability in deuterated TFTs. Previous reports of more stable deuterated TFTs, by other groups, can be explained by a change in the Si network properties due to the higher ion energy of deuterium in comparison with hydrogen, when using similar deposition conditions. The implication of our experimental results is that, for the same amorphous network and hydrogen/deuterium concentration, the stability is identical for hydrogenated and deuterated TFTs. Therefore, there is no giant isotopic effect in amorphous silicon TFTs. The study also further supports the idea that Si-Si bond breaking is the rate-limiting step for Si dangling bond defect creation, rather than Si-H bond breaking.

Interface state energy distribution in (1 0 0)Si/HfO 2

Energy distributions of interface states in (1 0 0)Si/HfO 2 entities were determined using capacitance–voltage measurements on structures with the periphery of the metal electrode switched to inversion by controlled application of corona discharge. This method was applied in combination with ac conductance spectroscopy to assess (1 0 0)Si/HfO 2 interface traps. The significant impact of the HfO 2 deposition process on the interface trap density is revealed. The latter includes both the enhancement of the Si-dangling bond defect (P b0 centers) density and an additional contribution of insulator-related traps in samples deposited from a nitrogen-containing precursor.

Kinetics ofH2passivation of Si nanocrystals inSiO2

Time-resolved photoluminescence measurements were used to study the passivation kinetics of luminescence-quenching defects, associated with Si nanocrystals in SiO 2 , during isothermal and isochronal annealing in molecularhydrogen. The passivation of these defects was modeled using the generalized simple thermal model of simultaneous passivation and dissociation, proposed by Stesmans. Values for the reaction-rate parameters were determined and found to be in excellent agreement with values previously determined for paramagnetic Si dangling-bond defects (P b -type centers) found at planar Si/SiO 2 interfaces; supporting the view that nonradiative recombination in Si nanocrystals is dominated by such defects.

First-principles modeling of paramagnetic Si dangling-bond defects in amorphousSiO2

We modeled paramagnetic Si dangling-bond defects in amorphous SiO2 using a generalized-gradient density-functional approach. By creating single oxygen vacancies in a periodic model of amorphous SiO2, we first generated several model structures in which the core of the defect consists of a threefold coordinated Si atom carrying a dangling bond. These model structures were then fully relaxed and the hyperfine parameters calculated. We found that the hyperfine parameters of such model defects, in both the neutral and positive charge states, reproduced those characteristic of the E', in accord with experimental observations for amorphous SiO2. By eliminating a second O atom in the nearest-neighbor shell of these defect centers, we then generated model defects in which the Si atom carrying the dangling bond forms bonds with two O atoms and one Si atom. In this defect, the spin density is found to delocalize over the Si-Si dimer bond, giving rise to two important hyperfine interactions. These properties match the characteristics of the hyperfine spectrum measured for the S center. Our results are complemented by the calculation of hyperfine interactions for small cluster models which serve the threefold purpose of comparing different electronic-structure schemes for the calculation of hyperfine interactions, estimating the size of core-polarization effects, and determining the reliability of cluster approximations used in the literature.