Dangling Bond Density Research Articles

Electronic states in a-Si:H/c-Si heterostructures

We have investigated PECVD-deposited ultrathin intrinsic a-Si:H layers on c-Si substrates using UV-excited photoemission spectroscopy (hν=4–8eV) and surface photovoltage measurements. For samples deposited at 230°C, the Urbach energy is minimal, the Fermi level closest to midgap and the interface recombination velocity has a minimum. The a-Si:H/c-Si interface density of states is comparable to that of thermally oxidized silicon interfaces. However, the measured a-Si:H dangling bond densities are generally higher than in thick films and not correlated with the Urbach energy. This is ascribed to additional disorder induced by the proximity of the a-Si:H/c-Si interface and H-rich growth in the film/substrate interface region.

Improvement of Threshold Voltage Degradation Characteristics of a-Si:H TFT by Pre-Electrical Bias-Aging for Amoled Display

AbstractWe propose a pre-electrical bias aging to reduce threshold voltage (Vth) shift of hydrogenated- amorphous silicon thin-film transistor (a-Si:H TFT) for AMOLED display. The quantity of Vth shift in the sample subjected to a bias-aging is reduced due to the reduction of created dangling bond density, compared with a sample without a bias-aging. When an identical stress duration of 50,000 sec is applied to a-Si:H TFT with or without a pre-electrical bias-aging, the created dangling bond density (ΔNDB) after a pre-electrical bias-aging is decreased from 1.38 × 1011/cm2 to 0.685 × 1011/cm2. Our experimental results indicate that after the pre-electrical bias aging, a newly created dangling bond during an electrical stress is decreased because a weak bond density and hydrogen diffusion may be decreased.

EB Induced Stress Relaxation of Tight Network Structure in Silica and Soda Glasses

This paper mainly describes that nano-scale surface structure change in dangling bond density related to free volume caused the reinforcement of inorganic glass irradiated by electron beam. Based on the results of electron spin resonance experiments, EB irradiation enhanced the density of dangling bonds at the glass surface. To confirm the strengthening principle, we have employed EB irradiation of inorganic glass. The EB reinforcement was explained as caused by stress relaxation induced by dangling bonds formation in the tight network structure of silica and soda glasses.

On the sensitivity of open-circuit voltage and fill factor on dangling bond density and Fermi level position in amorphous silicon p–i–n solar cell

Abstract A simple method for calculation of current–voltage characteristics of an amorphous silicon solar cell is described in terms of excitation current, J G , and excitation voltage, V G , the latter being defined in terms of separation of quasi-Fermi levels. Contrary to the usual method of calculating the short-circuit current and dark current separately and assuming a linear superposition, in the present method the calculations are done first in the open circuit where the neutrality of space charge can be assumed and then the current has been calculated in terms of a gradient in the quasi-Fermi levels. We find that depending on other parameters, the open-circuit voltage is a weak function of dangling bond density except in cases of very large degradation. The sensitivity of open-circuit voltage, V oc , to light-induced degradation can further be reduced by moving the thermal equilibrium Fermi level above the upper dangling bond level. Fill factor deterioration is found to be mainly due to conductivity modulation and is higher for the lower values of thermal equilibrium Fermi level.

Rate equation analysis of hydrogen uptake on Si (100) surfaces

We have studied the uptake process of H on Si (100) surfaces by means of rate equation analysis. Flowers’ quasiequilibrium model for adsorption and desorption of H [M. C. Flowers, N. B. H. Jonathan, A. Morris, and S. Wright, Surf. Sci. 396, 227 (1998)] is extended so that in addition to the H abstraction (ABS) and β2-channel thermal desorption (TD) the proposed rate equation further includes the adsorption-induced desorption (AID) and β1-TD. The validity of the model is tested by the experiments of ABS and AID rates in the reaction system H+D∕Si (100). Consequently, we find it can well reproduce the experimental results, validating the proposed model. We find the AID rate curve as a function of surface temperature Ts exhibits a clear anti-correlation with the bulk dangling bond density versus Ts curve reported in the plasma-enhanced chemical vapor deposition (CVD) for amorphous Si films. The significance of the H chemistry in plasma-enhanced CVD is discussed.

Stress Relaxation of Poly-Si Film Formed by Excimer Laser Annealing

We investigated the low-temperature processed polycrystalline-silicon (poly-Si) film formed by carrying out low-energy-density and multishot excimer laser annealing (ELA) of amorphous Si (a-Si) film on a glass or quartz substrate. The influence of secondary grain growth which grain size becomes larger than the film thickness on the tensile stress relaxation of the poly-Si film was clarified. Relationship between hydrogens which are supplied to the melt-Si from the bottom catalytic-chemical vapor deposition (Cat-CVD) SiN film and the stress relaxation was also examined. The tensile stress in the poly-Si film relaxes upon the appearance of the secondary grains, and the stress decreases with increasing diameter of the secondary grains. However, the hydrogen supply to melt-Si suppresses the secondary grain growth and the stress relaxation becomes marked as the hydrogen concentration decreases. Furthermore, the relationship between the dangling bond density corresponding to the crystal defect density and grain size was investigated, and the defect site was clarified. Lastly, the recrystallization model by ELA at low energy densities was discussed. The model was constructed based on the experimental data that the secondary grain growth occurs suddenly at the critical energy density and the critical shot number, and it was shown that the present model is consistent with the results related to the hydrogen concentration and the secondary grain growth.

Post hydrogenation effect by hot wire method on poly-crystalline silicon based devices

The effect of post hydrogenation by the hot-wire method on both bulk properties and TFT performance is studied and is compared with the effect of thermal annealing. In device applications, the passivation of defects is a key process for attaining high device performance. One of the most effective hydrogenation methods for poly-crystalline silicon and silicon based materials is the hot-wire technique. While the hydrogen concentration in the as-deposited poly-Si film fabricated by reactive thermal CVD is near 0.01%, it is found to increase to 0.6–0.7% by hydrogenation (at a temperature 200 °C) regardless of the crystalline fraction. The dangling bond density is observed to decrease from ∼3.5 × 10 18 to ∼9.0 × 10 17 cm −1. It also decreases by annealing in helium gas at temperatures exceeding 350 °C. The results suggest that hydrogenation by hot-wire method besides the passivation effect also creates dangling bonds, due to breaking of Si–Si bonds by highly reactive hydrogen.

A theoretical study of light induced defect creation, annealing and photoconductivitydegradation in a-Si:H

This work is a theoretical study of the defect creation and annealing, and of thephotoconductivity degradation of an intrinsic a-Si:H film under continuous illumination. Anew model is developed for the Staebler–Wronski effect (SWE). In this model, we considerthat a defect of the SiHD type is locally created by a non-radiative recombinationat a weak bond close to a SiHHSi configuration. Also, it is considered that thelight induced defect (LID) annealing requires a hydrogen diffusion motion. Usingthe fact that the areas of defect Gaussians increase with illumination time whiletheir positions and widths remain unchanged, we present the evolution of thegap state defect density, calculated by the defect pool model, with illuminationtime. This density of states, which varies during illumination, is introduced in thesimulation of the photoconductivity to obtain its degradation. In addition, thevalidity of the proportionality of the photoconductivity to the illumination intensityG and to the inverse dangling bond density is assessed. Our investigations show amonomolecular behaviour of the photoconductivity for moderate intensity. They also show an illumination intensity dependency onGα,with α varying between 0.5 and 1, in agreement with a large number of similar investigations.

Light-induced modification of a-SiOx:H. I: Metastability

Amorphous hydrogenated silicon suboxides (a-SiOx:H) were deposited by plasma enhanced chemical vapor deposition from the source gases SiH4, H2, and CO2. The band gap of the samples can be tuned from 1.9 to 3.0 eV by varying the oxygen content from 0 to 50 at. %. The light-induced degradation of the photoconductivity and the generation of metastable dangling bond defects, known as the Staebler–Wronski effect in hydrogenated amorphous silicon (a-Si:H), were studied in a-SiOx:H for a wide range of oxygen contents up to 44 at. % as a function of illumination time and intensity. Upon intensive illumination above their respective band gaps for several hours at comparable light intensities, all silicon suboxides displayed a similar increase of the subgap defect density and a corresponding decrease of the photocurrent. Also the temporal evolution of the dangling bond density and photoconductivity were found to resemble those of pure amorphous silicon. Apart from an irreversible redshift of the absorption edge for higher oxygen contents (comparable to the effect of photodarkening in chalcogenide glasses), the effects of light degradation could be reversed by an appropriate annealing at temperatures below 200 °C for all oxygen concentrations studied. However, for higher oxygen concentrations ([O]≳30 at. %) partial irreversibilities were observed after repeated degradation cycles.

Microcrystalline silicon.: Growth and device application

Processes used to grow hydrogenated microcrystalline silicon (μc-Si:H) from a H 2/SiH 4-glow discharge plasma are explained in comparison to those for hydrogenated amorphous silicon (a-Si:H). Differences and similarities between μc-Si:H and a-Si:H-growth reactions in the plasma and on the film-growing surface are discussed, and the nucleus-formation process followed by epitaxial-like crystal growth process is illustrated as unique processes for the formulation of μc-Si:H. Determination of the effect of dangling-bond defect density on the propagation of the resulting μc-Si:H films is also discussed in parallel with the effect on a-Si:H in order to obtain a clue to improve opto-electronic properties of those materials for device applications especially for thin-film-silicon solar cells. Material issues to produce low cost and high efficiency solar cells are described, and finally recent progress in those issues is demonstrated.

Evolution of D0 and non-D0 Light Induced Defect States in a-Si:H Materials and Their Respective Contribution to Carrier Recombination

A study has been carried out on the evolution of light induced defects in protocrystalline (diluted) a-Si:H films under 1 sun illumination. A room temperature reversal is observed in the photocurrents at 25°C, which is consistent with the relaxation in the recombination currents on corresponding p-i-n solar cells. It is also consistent with the pressure of “fast” states such as have been observed after high intensity illumination. Even with the limitations imposed by the relaxation in the light induced changes on the subgap absorption measurements, the evolution of distinctly different gap states centered around 0.9 and 1.15eV from the conduction band was identified. The kinetics of the electron occupied states, kN(E), at these two energies is compared with that of the neutral dangling bond (D0) densities as measured with electron spin resonance. Because of the similarity between the preliminary results of these kinetics it has not been possible to identify which states correspond to the D0 nor to draw any reliable conclusions about the nature of the different states.

The growth mechanism of GaN grown by hydride vapor phase epitaxy in N 2 and H 2 carrier gas

This paper investigates the growth mechanism of GaN for the hydride vapor phase epitaxy (HVPE) process in the N 2 and H 2 carrier gas ambients. Differences are observed between the surface morphologies of GaN films grown in N 2 carrier gas and those grown in H 2 carrier gas. It is determined that the GaN growth direction and growth rate are both affected by H 2 flow rate. In N 2 ambient, the {1 1 ̄ 0 1} facet is stable, because of the N-polarity and high dangling bond density (DB) of {1 1 ̄ 0 1} facet. On the other hand, in H 2 ambient, and the {1 1 ̄ 0 0} facet becomes stable. In this paper, this phenomenon will be explained by the atomic configuration model of the GaN wurtzite structure.

Neutral dangling bond depletion in amorphous SiN films induced by magnetic rare-earth elements

Amorphous silicon–nitrogen (a-SiN) thin films doped with rare-earth elements (RE=Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Yb, and Lu) were prepared by cosputtering and studied by means of electron spin resonance. It was found that the neutral dangling bond density, [ D 0], of the a-SiN films decreases with the presence of magnetic REs and the drop in [ D 0] approximately scales with the de Gennes and/or the spin factor of each RE element. Similar to the decrease in T c in RE-doped superconductors, our experimental results strongly suggest that an exchange-like interaction, H∼J RE-D 0 S RE · S D 0 , between the spin of the magnetic REs and that of the D 0 is taking place.

EPR studies of some thick film ferrates, aluminates and their correlation with humidity sensitivity

Ceramic types of humidity sensors using aluminates and ferrates have attracted attention of workers as stable sensors. It is expected that there are some active centers on the surface of the sensors which are responsible to break the water vapor molecule in to H 3O + and OH − ions. These active centers can be unpaired spins or dangling bonds which can be measured using electron paramagnetic resonance (EPR). In the present paper, the dangling bond density of various samples of magnesium aluminates, magnesium ferrates and their mixtures prepared at different temperatures is monitored and correlated with the sensitivity of the humidity sensor prepared. It is found that the sensitivity is increasing as the dangling bond density increases.

Structural properties and recombination processes in hydrogenated polymorphous silicon

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to powder formation, a new type of material, consisting of an amorphous matrix in which silicon nanocrystallites are embedded is obtained. This material, named hydrogenated polymorphous silicon (pm-Si:H), exhibits enhanced transport properties with respect to state-of-the-art hydrogenated amorphous silicon (a-Si:H). In order to understand the origin of such improved properties, we investigated structural characterization of pm-Si:H films. High resolution transmission electron microscopy (HRTEM) micrograph, micro-Raman and infrared spectra of the films are presented. The crystallite sizes deduced from the Raman spectra are supported by the HRTEM measurements. The infrared stretching modes of pm-Si:H films present a band at ~2030 cm −1 attributed to hydrogen platelets. Two approaches are then given in order to explain the enhanced photoconductivity properties. The first one, qualitative, appeals to low density of states at the Fermi level and low capture cross-section of electrons due to the improved amorphous matrix. The second, more quantitative, suggests that recombination mainly occur at dangling bonds at the surface of the crystallites. Considering the dangling bond density at silicon surface passivated by hydrogen and the description of multiphonon carrier capture, both given in the literature, we derive and values in good agreement with experiments.

SIMS characterization of hydrogen transport through SiO2 by low-temperature hydrogen annealing

The variation of hydrogen distribution at the SiO2/Si interface by low-temperature hydrogen annealing was investigated using secondary ion mass spectrometry (SIMS). As the amount of hydrogen atoms incorporated at SiO2/Si is considered to be comparable to the silicon dangling bond density (1×1010 to 1×1012 atoms/cm2), an analytical method with a high sensitivity is necessary for the detection of hydrogen at SiO2/Si. In this study, the experimental conditions of SIMS were optimized in order to obtain a sufficient reproducibility of interfacial hydrogen ion intensity. There are two main causes that influence the measurement reproducibility: (1) misalignment of the relative irradiation areas of the electron beam and ion beam and (2) the contribution of background hydrogen from surface contaminants and residual gas. We obtained a high measurement reproducibility within a 5.5% relative standard deviation (2σ). This enabled us to observe an increase of hydrogen at SiO2/Si by hydrogen annealing at 400°C, which resulted from the incorporation of hydrogen from the ambient. From the results of nuclear reaction analysis (NRA) and thermal desorption spectroscopy (TDS), it was also found that the incorporated hydrogen had two chemical states.

Theory of amorphous silicon solar cell (a): numerical analysis

Abstract A theoretical analysis of recombination kinetics and space charge distribution in amorphous silicon is carried out with a view to bring out the underlying physics. A uniform excitation with a flat quasi-Fermi level and a constant np product has been used as a probe to estimate the relative importance of various parameters. Recombination rates have been calculated for various ratios of capture rates for Coulomb attractive and neutral traps. In practice a large ratio of capture rates exists and for this case two peaks of recombination maxima are found to lie in the space charge regions corresponding to transitions at the energy level E1 (for D+–D0 transition) at the p–i edge and for E2 energy level (corresponding to D0–D− transition) at the i–n interface. A two independent level model therefore holds to a good approximation. The dangling bond density is found to determine both the space charge distribution and the recombination rate. Based on space charge density distribution i-layer can be divided in the five parts. The two recombination rate peaks are found to exist at the p–i and i–n space charge transitions respectively. This enables us to develop a simple model for the i-layer of the p–i–n diode.

A comprehensive electron paramagnetic resonance study of influence of annealing on defect center in phosphorus ion-implanted C 60 films

Electron paramagnetic resonance (EPR) measurements have been performed to investigate the effect of annealing on paramagnetic defect center in P +-implanted C 60 films, in order to control and improve the electronic properties of the implanted films towards photovoltaic applications. We have found a reduction in the dangling bond density upon annealing by approximately a one order of magnitude, in the temperature range between 100°C and 700°C, regardless of the annealing media whether vacuum or forming gas (N 2/H 2). The reduction in spin defect density was ascribed to the decrease in disordered dangling bond as a consequence of the reconstruction of the less stable defect sites. Indeed the modification in the spin density is accompanied with an improvement in the electrical conductivity and band structure of the films. Also, in the annealed carbon films, a correlation was observed among linewidth, relaxation times, and optical gap. In addition, we report about the temperature dependence of the linewidth, signal intensity and the susceptibility of annealed films. The susceptibility follows the Curie-law at sufficiently low temperature, while above 180 K a deviation was observed. The prime novelty of this study is that it is the first EPR study of effects of annealing on defect center in P +- implanted C 60 films.

Functionalized ion-beam carbon for magnetic disks

Summary form only given. Carbon overcoat thickness is continuously being decreased to achieve higher areal density in magnetic media. For present and future products, the thickness of the carbon overcoat has been reduced to 30 /spl Aring/ or less. These thin carbon films have to be robust enough to effectively protect the media against wear and corrosion. The carbon overcoat consists of two interfaces: an interface with the magnetic layer and the carbon-lubricant interface. Both mechanical/chemical properties of the carbon overcoat and its surface reactivity affect the carbon-lubricant bonding which subsequently affects the lubricant-head interaction. Functionalized carbon overcoat consists of a base layer of ion beam carbon (IBC) and a top thinner layer of nitrogenated sputtered carbon. In the IBC process, hydrocarbon (C/sub x/H/sub y/) gas is used as a precursor. High hydrogen content in the film increases the electrical resistivity which could potentially lead to charge buildup on the surface, resulting in head damage. High hydrogen content also can decrease the dangling bond density in the carbon layer resulting in lower carbon lubricant bonding, leading to lube pick-up by the head. With the use of thin hard carbon overcoats, glide head-disk interactions (glide noise) and head degradation issues are becoming severe. These issues could be addressed by doping the top layer of the functional carbon layer (sputtered) with nitrogen.

Electronic structure of carbon nanotubes studied by photoelectron spectromicroscopy

The electronic structure of multiwalled carbon nanotubes aligned perpendicularly on a Si substrate was studied by means of photoelectron spectromicroscopy. The valence band and the C $1s$ spectra, measured systematically from spatially selected regions along the tube axes, were the fingerprint for lateral variations in the electron density of states and in the band bending, respectively. It was found that the tips have a larger density of states near the Fermi level than the sidewalls, whereas band bending, which would explain such a spectral difference, was not observed. It is suggested that the different density of states near the Fermi level is due to a larger dangling bond density at the tips.