Defect Creation Rate Research Articles

Amorphous-amorphous transformation induced in glasses by intense x-ray beams

The atomic displacements induced by an x-ray beam of relatively low energy, $\ensuremath{\epsilon}\ensuremath{\sim}8$ KeV, are investigated in pure boron oxide and in a set of sodium silicate glasses by means of x-ray photon correlation spectroscopy. We observe the complete x-ray induced transformation of the initial glass into a new amorphous state which remains stable under irradiation. The new phase continues to rearrange under the beam with a stretched exponential relaxation similar to the one observed with macroscopic measurements in the corresponding high-temperature supercooled liquid, suggesting that the new configuration lies in a higher energy minimum of the potential energy landscape. We investigate the temperature dependence of the observed dynamics for a specific sodium concentration and we observe a temperature dependence of the beam induced motion, which suggests that the defect creation rate is thermally activated. The radiation dose needed for the initial structural variation is sample dependent and correlates well with the number of constraints per vertex, within the framework of rigidity theory. This observation provides a quantitative tool to evaluate the efficiency of the radiolytic process in different network topologies.

Geometrical Control of Active Turbulence in Curved Topographies.

We investigate the turbulent dynamics of a two-dimensional active nematic liquid crystal constrained to a curved surface. Using a combination of hydrodynamic and particle-based simulations, we demonstrate that the fundamental structural features of the fluid, such as the topological charge density, the defect number density, the nematic order parameter, and defect creation and annihilation rates, are approximately linear functions of the substrate Gaussian curvature, which then acts as a control parameter for the chaotic flow. Our theoretical predictions are then compared with experiments on microtubule-kinesin suspensions confined on toroidal droplets, finding excellent qualitative agreement.

Importance of Ge surface oxidation with high oxidation rate in obtaining low interface state density at oxide/Ge interfaces

The formation of a gate-insulator/Ge interface with a low interface state density (Dit) starts with oxidation at the Ge surface under appropriate conditions. In this study, a key physical parameter for realizing Dit lower than 1011 eV−1 cm−2 during the oxidation process was clarified by comparison between the experimental results and results calculated based on a proposed model including a time-dependent defect distribution near the Ge-oxide/Ge interface and movement of the interface due to the oxidation at the Ge surface. Dit near the midgap is determined by the competition between the rate of defect creation near the Ge-oxide/Ge interface and the movement rate of the interface, which is the Ge oxidation rate. Oxidation with a Ge oxidation rate of more than 0.1 nm/s can lead to Dit lower than 1011 eV−1 cm−2, taking into account the defect creation rate in a typical Ge device fabrication process, which is 10−2 to 10−4 nm/s.

Anneal treatment to reduce the creation rate of light-induced metastable defects in device-quality hydrogenated amorphous silicon

We observe a dramatic reduction in the Staebler–Wronski metastable defect creation efficiency in device-quality films of hydrogenated amorphous silicon after they undergo a 20 min anneal treatment at 350 to 400 °C. After several hours of rapid degradation with a high intensity pulsed laser, there are about half as many total dangling bond defects in the annealed samples as in unannealed control samples. This reduction is observed in both 1.02-μm- and 0.22-μm-thick films, indicating it is not a surface-related artifact. The improved stability is likely caused by H motion, which restructures the Si–Si network and H-related nanovoids.

Theoretical analysis of defect-mediated turbulence in a catalytic surface reaction

We present a statistical analysis of defect-mediated turbulence in a kinetic model of catalytic CO oxidation on Pt(110). A probabilistic description based on the gain and loss rates of defects is derived. For low values of the CO partial pressure the statistics of topological defects agree with earlier results for the complex Ginzburg-Landau equation. For high values of the CO partial pressure, we observe an additional autocatalytic reproduction of defects that results in a linear dependence of the defect creation rate on the number of defects in the system. The role of correlations between defects of opposite topological charge was found to be weaker than in the experimental system.

Thermal stability of light-induced defects in hydrogenated amorphous silicon: Effect on defect creation kinetics and role of network microstructure

The kinetics of light-induced defect creation in a-Si:H is studied in early-time limit and as function of pre-existing defects of different thermal stability by electron spin resonance and optical spectroscopy techniques. Both for cw and for laser pulse exposures, the early-time kinetics follows sublinear tβ time dependences, similar to the long-time limit. In addition, the overall defect creation rate is not a single function of the total defect number. Instead, it depends on the thermal stability, or annealing energy distribution, of the defects present in the film. Furthermore, creation of the thermally less stable defects is unaffected by the presence of a large number of stable defects introduced by pre-exposure at a higher temperature. These findings question the existing defect creation models. Thermal stability of the light-induced defects depends on the network microstructure, the less stable defects being created in a-Si:H deposited near microcrystalline transition.

Ion-beam damage and non-radiative exciton decay in LiNbO 3

A model to account for the defects generated by ion irradiation in the electronic loss regime, and based on non-radiative decay of self-trapped excitons, is discussed and compared to experiments. It takes into account the competing role of the light-emission (radiative) and defect-creation (non-radiative) decay channels. Calculations are applied to LiNbO 3, a useful electro-optic crystal, where a large number of relevant experimental data are available. The model explains the strong nonlinear dependence of the defect creation rates as a function of electronic stopping power ( thresholding behavior). It also satisfactorily accounts for the formation and growth of amorphous layers by ion-beam irradiation at moderate fluences, φ>10 13 cm −2. Moreover, it also provides the right trend and reasonable quantitative accordance to data showing the dependence of the track radius on stopping power in single-impact experiments. Finally, the model determines the light emission yield during irradiation. In particular, it predicts that the number of photons emitted by ion impact first increases and then decreases monotonically with increasing electronic stopping power.

Light-induced defect creation in hydrogenated polymorphous silicon during repeated cycles of illumination and annealing

We have examined how the intensity of the ESR signal associated with dangling bonds in hydrogenated polymorphous silicon is affected by repeating light-soaking (LS) and 100°C annealing cycles. It was found that the light-induced degradation efficiency decreases with repeated LS–100°C annealing cycles. This result is accounted for in terms of the termination of dangling bonds by mobile hydrogen, i.e. the termination is enhanced by more mobile hydrogen as a result of modification of the amorphous network by the LS–annealing cycles, and consequently the net light-induced defect creation rate is reduced. This is in contrast with previous models in which the decrease in the light-induced degradation efficiency on repeated LS–annealing cycles is attributed to an increase in the amount of hydrogen being ineffective for light-induced defect creation.

Threshold voltage instability of amorphous silicon thin-film transistors under constant current stress

We investigate the time-dependent shift in the threshold voltage of amorphous silicon thin-film transistor stressed with constant drain current. We observe a nonsaturating power-law time dependence, which is in contrast to the conventional stretched exponential that saturates at prolonged stress time. The result is consistent with the carrier-induced defect creation model and corroborates the nonlinear dependence of the rate of defect creation on the band-tail carrier density.

Kinetics of defect creation in amorphous silicon thin film transistors

We have developed a theoretical model to account for the kinetics of defect state creation in amorphous silicon thin film transistors, subjected to gate bias stress. The defect forming reaction is a transition with an exponential distribution of energy barriers. We show that a single-hop limit for these transitions can describe the defect creation kinetics well, provided the backward reaction and the charge states of the formed defects are properly taken into account. The model predicts a rate of defect creation given by (NBT)α(t/t0)(β−1), with the key result that α=3β. The time constant t0 is also found to depend on band-tail carrier density. Both results are in excellent agreement with experimental data. The t0 dependence means that comparing defect creation kinetics for different thin film transistors can only be done for the same value of band-tail carrier density. Normalization of bias stress data on different thin film transistors made at different band-tail densities is not possible.

Physical origin of ion mixing induced amorphization and associated asymmetric growth in the binary metal systems

It has been clarified that the physical origin for a crystalline metal to transform into amorphous is the collapsing of the crystalline lattice while the solute atoms exceeding its critical solid solubility. In ion beam mixing, there are two major factors influencing the amorphization behavior, i.e. the critical solid solubilities and defect (or damage) creation rates of the two constituent metals and it is accordingly predicted that if there exists any difference between the two metals in achieving their respective critical solid solubilities, asymmetric growth would take place. For the Ni–Sc system, the solid solubilities are almost identical, yet the damage creation rate in Ni is about 3 times of that in Sc. Whereas for the Ni–W system, the damage creation rates are basically the same, yet the solid solubility of Ni in W lattice was much smaller than W in Ni. Carefully performed ion beam mixing experiments indeed revealed an asymmetric amorphization in the two systems with predicted preferential directions. Moreover, similar asymmetric amorphization observed in some other systems could also be well explained by the above principle.

Statistics of defect motion in spatiotemporal chaos in inclined layer convection.

We report experiments on defect-tracking in the state of undulation chaos observed in thermal convection of an inclined fluid layer. We characterize the ensemble of defect trajectories according to their velocities, relative positions, diffusion, and gain and loss rates. In particular, the defects exhibit incidents of rapid transverse motion which result in power law distributions for a number of quantitative measures. We examine connections between this behavior and Levy flights and anomalous diffusion. In addition, we describe time-reversal and system size invariance for defect creation and annihilation rates.

Evolution of the Free Volume during Homogeneous Flow of a Metallic Glass

ABSTRACTGlassy Pd41Ni10Cu29P20 was creep-tested in compression at constant true stress. The defect concentrations were shown to be reversible with reproducible kinetics. The results could be fit better with a defect creation rate that is proportional to the applied power density than with one that depends on the strain rate only. The dilatation mechanism for creation of free volume is inefficient in energy and generation of free volume.

Defect creation kinetics in amorphous silicon thin film transistors

The rate of defect creation in amorphous silicon thin film transistors, under gate bias stress, is proportional to N BT α t β−1 , where N BT is the bandtail carrier density. Experimentally α is 1.5–1.9, while β is 0.5–0.6. We have developed a model to account for this dependence, based on an exponential distribution of barriers to defect creation. The key new feature of our model is that we include the backward reaction, as well as the forward reaction, and also the effect of the charge-state of the formed defects. Considering the forward reaction only, leads to α= β, while a full analysis leads to the simple new result that α≈3 β, which is in excellent agreement with experiments.

Photocreated Defects in Very Thin Hydrogenated Amorphous Silicon Films

ESR measurements were carried out for a 10-nm-thick a-Si:H film. The results show that the high-density defects in the near-surface layer can be treated in the same manner as the bulk defects in the case of light soaking, if both the penetration depth of the light and the dependence of the defect creation rate on the defect density are taken into account.

Determination of the defect creation mechanism in the mono-silicated fluoroapatite. Disorder modeling under repository conditions

Amorphization resulting from α-decay events can strongly reduce the chemical durability of nuclear waste matrices. The creation rate of defects produced by α recoils in mono-silicated fluoroapatite was determined using a transmission electron microscope (TEM) on line with an ion implanter and compared to previous results obtained in fully phosphated fluoroapatite. In both materials, the defect creation is controlled at room temperature, by the amorphization process directly in the cascade. Furthermore, it has been shown previously that in mono-silicated fluoroapatite, the disorder recovery proceeds mainly via α-annealing. Taking into account our already published data and new results on the defect creation in the mono-silicated fluoroapatite, we have modeled the amorphization level evolution versus time under repository conditions. The main conclusion is the following: thanks to α-annealing, mono-silicated fluoroapatite loaded with 244Cm, will maintain a disorder at a level low enough to prevent the total amorphization of the host lattice during the long-term disposal.

Stress-induced leakage current generation kinetics based on anode hole injection and hole dispersive transport

An approach giving a physical understanding of the stress-induced leakage current (SILC) generation kinetics based on anode hole injection and hole dispersive transport in the oxide is presented. It is shown that the SILC is not directly correlated to the defects responsible for oxide breakdown. However, it is suggested that trapped holes can impact on the defect creation rate and, in turn, on the breakdown generation.

Relative importance of the Si–Si bond and Si–H bond for the stability of amorphous silicon thin film transistors

We investigate the mechanism for Si dangling bond defect creation in amorphous silicon thin film transistors as a result of bias stress. We show that the rate of defect creation does not depend on the total hydrogen content or the type of hydrogen bonding in the amorphous silicon. However, the rate of defect creation does show a clear correlation with the Urbach energy and the intrinsic stress in the film. These important results support a localized model for defect creation, i.e., where a Si–Si bond breaks and a nearby H atom switches to stabilize the broken bond, as opposed to models involving the long-range diffusion of hydrogen. Our experimental results demonstrate the importance of optimizing the intrinsic stress in the films to obtain maximum stability and mobility. An important implication is that a deposition process where intrinsic stress can be independently controlled, such as an ion-energy controlled deposition should be beneficial, particularly for deposition temperatures below 300 °C.

Early stages of IBAD-film growth: Differences between (1 0 0) and polycrystalline Mo substrates

The effects of 250 eV Ar + ion assistance on the growth of ultrathin molybdenum films on Mo (1 0 0) and on well-annealed polycrystalline molybdenum substrates (largely (110)-oriented) are investigated by concurrent thermal desorption spectrometry of incorporated Ar and He, the latter species brought into the film after deposition as probe particles to decorate the point defects. Ar entrapment, defect concentration, and the transition from planar to columnar growth are quantitatively determined. The overall defect creation rate of 250 eV Ar + assistance is 0.1 defects per ion, including self-trapping. The difference between the substrate orientations shows up mainly in the growth morphology, the defect concentration, and the immediate subsurface trapping of Ar and He.

Monte Carlo simulations of ion-enhanced island coarsening

Monte Carlo simulations of growth and ion bombardment have been performed to explore the atomistic processes that occur during ion-assisted growth. The primary elements of the simulation are (1) creation of surface defects (vacancies and adatoms) by ion bombardment and deposition, (2) thermally activated motion of surface defects and (3) recombination of surface vacancies and adatoms. We find that a balance of ion bombardment and deposition (where the creation rate of adatom defects is equal to that of surface vacancies) leads to larger islands and more rapid island coarsening than thermal coarsening with no flux of defects. The presence of the defect flux enhances the breakup of clusters, leading to a broad distribution of island sizes. In comparison, thermal coarsening leads to a more uniform distribution of island sizes that increases in size much more slowly. Histograms of the evolution of the island size distribution provide a quantitative measure of the ion-induced increase in the rate of coarsening.