Distribution Of Point Defects Research Articles

Non-metricity in the continuum limit of randomly-distributed point defects

We present a homogenization theorem for isotropically-distributed point defects, by considering a sequence of manifolds with increasingly dense point defects. The loci of the defects are chosen randomly according to a weighted Poisson point process, making it a continuous version of the first passage percolation model. We show that the sequence of manifolds converges to a smooth Riemannian manifold, while the Levi-Civita connections converge to a non-metric connection on the limit manifold. Thus, we obtain rigorously the emergence of a non-metricity tensor, which was postulated in the literature to represent continuous distribution of point defects.

Single Metal Ohmic and Rectifying Contacts to ZnO Nanowires: A Defect Based Approach

AbstractOhmic and rectifying metal contacts to semiconductor nanowires are integral to electronic device structures and typically require different metals and different process techniques to form. Here we show how a noble metal ion beam of Pt commonly used to pattern conducting contacts in electron microscopes can form both ohmic and Schottky/blocking contacts on ZnO nanowires by controlling native point defects at the intimate metal‐semiconductor interface. Spatially‐resolved cathodoluminescence spectroscopy on a nanoscale both laterally and in depth gauges the nature, density, and spatial distribution of specific native point defects inside the nanowires and at their metal interfaces. Combinations of electron and ion beam deposition, annealing, and sculpting of the same nanowire provide either low contact resistivity ohmic contacts or a high Schottky/blocking barrier with a single metal source. These results highlight the importance of native point defects distributed inside nanowires and their variation near interfaces with sculpting and annealing.

Direct Observation of Oxygen Vacancy Distribution across Yttria-Stabilized Zirconia Grain Boundaries.

Crystalline interfaces in materials often govern the macroscopic functional properties owing to their complex structure and chemical inhomogeneity. For ionic crystals, however, such understanding has been precluded by the debatable local anion distribution across crystal interfaces. In this study, using yttria-stabilized zirconia as a model material, the oxygen vacancy distribution across individual grain boundaries was directly quantified by atomic-resolution scanning transmission electron microscopy with ultrahigh-sensitive energy-dispersive X-ray spectroscopy. Combined with dynamical scattering calculations, we unambiguously show that the relative oxygen concentrations increase in four high-angle grain boundaries, indicating that the oxygen vacancies are actually depleted near the grain boundary cores. These results experimentally evidence that the long-range electric interaction is the dominant factor to determine the local point defect distribution at ionic crystal interfaces.

Nonuniform electromechanical efficiency and point defects in lithium niobate

The electromechanical efficiency (EME) and distribution of point defects (DPD) are measured from LiNbO3 samples at room temperature. The EME is estimated by reading an electric potential (V) generated while applying a mechanical force to a local point. The DPD is measured by Photoluminescence (PL) taken from samples. PL is excited by 310-nm light and optical spectra are registered from 350 to 900 nm. The defects are identified by their characteristic peaks. The applied mechanical force and intensity of 310-nm light are kept constant while scanning V and PL along Z-axis or normal to Z. The F-center and Fe+ have been shown to be sensitive to local electrical polarization. Thus, they may influence EME in a local point of crystal. The experimental data from bulk and wafer samples show a nonuniform distribution of the F-center and Fe+ defects. For example, a 1.9-mm-thick Y-cut wafer demonstrates periodicity in defect concentration of 1 to 2 mm along the Z-axis. Similar periodicity is detected in the potential V along the same crystallographic axis. The revealed correlation between defects and EME along with their nonuniform distribution may be a physical basis of nonlinear phenomena involving piezoelectricity including nonlinear elasticity, acoustical memory, etc.

Impact of defect distribution on IrOx/ZnO interface doping and Schottky barriers

We used depth-resolved cathodoluminescence spectroscopy (DRCLS) to measure the nature and spatial distribution of native point defects at Zn- and O-polar ZnO interfaces with iridium oxide (IrOx) and their impact on Schottky barrier formation. IrOx and other metal oxides exhibit higher Schottky barriers than their pure metal counterparts, consistent with wider depletion regions and potentially useful for ohmic contacts to p-type semiconductors. DRCLS with I-V and 1/C2-V barrier height and carrier profile measurements showed high zinc vacancy VZn and CuZn defect densities that compensate free carrier densities, increase depletion widths, and form higher effective barriers than Ir/ZnO contacts. Zn-polar versus O-polar ZnO interfaces with IrOx exhibit 40% higher VZn + CuZn interface segregation and lower carrier densities within a wider depletion region, accounting for the significantly higher (0.89 vs. 0.67 eV) barrier heights. Both the depth of VZn density segregation and the Zn-deficient layer thickness measured microscopically match the depletion width and applied electric fields comparable to spontaneous polarization fields across similar layers displaying analogous defect segregation. These results account for the difference in polarity-dependent segregation due to the electric field-driven diffusion of native defects near ZnO interfaces.

Nanoscale interfacial defect shedding in a growing nematic droplet.

Interfacial defect shedding is the most recent known mechanism for defect formation in a thermally driven isotropic-to-nematic phase transition. It manifests in nematic-isotropic interfaces going through an anchoring switch. Numerical computations in planar geometry established that a growing nematic droplet can undergo interfacial defect shedding, nucleating interfacial defect structures that shed into the bulk as +1/2 point defects. By extending the study of interfacial defect shedding in a growing nematic droplet to larger length and time scales, and to three dimensions, we unveil an oscillatory growth mode involving shape and anchoring transitions that results in a controllable regular distributions of point defects in planar geometry, and complex structures of disclination lines in three dimensions.

Quasi-freestanding, striped WS2 monolayer with an invariable band gap on Au(001)

Revealing the structural/electronic features and interfacial interactions of monolayer MoS2 and WS2 on metals is essential to evaluating the performance of related devices. In this study, we focused on the atomic-scale features of monolayer WS2 on Au(001) synthesized via chemical vapor deposition. Scanning tunneling microscopy and spectroscopy reveal that the WS2/Au(001) system exhibits a striped superstructure similar to that of MoS2/Au(001) but weaker interfacial interactions, as evidenced by experimental and theoretical investigations. Specifically, the WS2/Au(001) band gap exhibits a relatively intrinsic value of ∼2.0 eV. However, the band gap can gradually decrease to ∼1.5 eV when the sample annealing temperature increases from ∼370 to 720 °C. In addition, the doping level (or Fermi energy) of monolayer WS2/Au(001) varies little over the valley and ridge regions of the striped patterns because of the homogenous distributions of point defects introduced by annealing. Briefly, this work provides an in-depth investigation into the interfacial interactions and electronic properties of monolayer MX2 on metal substrates.

Nonuniform distribution of point defects in ferroelectric phononic crystal

The photoluminescence (PL) from point defects in ferroelectric phononic crystal (FPC) is investigated at room temperature. The FPC consists of periodically poled domains 0.45-mm-long each along the x-axis in the 0.5-mm-thick z-cut LiNbO3 wafer. The spectra of PL are excited by 310 nm ultraviolet light and registered in the range of 350 to 900 nm. The PL spectra reveal different point defects including F-center, Ba, Ar, Ne, Cr, K, Fe+, Xe, and others. The electrically active defects such as F-center and Fe+ are expected to be sensitive on a local electric polarization. In a FPC, the ferroelectric neighboring domains are inversely poled and have an opposite electric polarization. The change from polarization “up” to polarization “down” happens across so called interdomain wall. The point defect concentrations along the neighboring domains are researched by PL-scanning, consisting of taking PL-spectra from narrow zones across the domain structure along the x-axis. This scanning reveal a nonuniform distribution of defects along the FPC. The striking result is that some of the defects such as F-center and Fe+ have respectively narrow extrema in PL-intensity right in the interdomain wall location. Engineering application of these findings may be new non-destructive characterization method for ferroelectric phononic crystals.

Tunable Oxygen Diffusion and Electronic Conduction in SrTiO3 by Dislocation‐Induced Space Charge Fields

Plastic strain engineering was applied to induce controllable changes in electronic and oxygen ion conductivity in oxides by orders of magnitude, without changing their nominal composition. By using SrTiO3 as a model system of technological importance, and by combining electrical and chemical tracer diffusion experiments with computational modeling, it is revealed that dislocations alter the equilibrium concentration and distribution of electronic and ionic defects. The easier reducibility of the dislocation cores increases the n‐type conductivity by 50 times at oxygen pressures below 10−5 atm at 650 °C. At higher oxygen pressures the p‐type conductivity decreases by 50 times and the oxygen diffusion coefficient reduces by three orders of magnitude. The strongly altered electrical and oxygen diffusion properties in SrTiO3 arise because of the existence of overlapping electrostatic fields around the positively charged dislocation cores. The findings and the approach are broadly important and have the potential for significantly impacting the functionalities of electrochemical and/or electronic applications such as thin film oxide electronics, memristive systems, sensors, micro‐solid oxide fuel cells, and catalysts, whose functionalities rely on the concentration and distribution of charged point defects.

Steady distribution structure of point defects near crystal-melt interface under pulling stop of CZ Si crystal

In order to reveal a steady distribution structure of point defects of no growing Si on the solid-liquid interface, the crystals were grown at a high pulling rate, which Vs becomes predominant, and the pulling was suddenly stopped. After restoring the variations of the crystal by the pulling-stop, the crystals were then left in prolonged contact with the melt. Finally, the crystals were detached and rapidly cooled to freeze point defects and then a distribution of the point defects of the as-grown crystals was observed. As a result, a dislocation loop (DL) region, which is formed by the aggregation of interstitials (Is), was formed over the solid-liquid interface and was surrounded with a Vs-and-Is-free recombination region (Rc-region), although the entire crystals had been Vs rich in the beginning. It was also revealed that the crystal on the solid-liquid interface after the prolonged contact with the melt can partially have a Rc-region to be directly in contact with the melt, unlike a defect distribution of a solid-liquid interface that has been growing. This experimental result contradicts a hypothesis of Voronkov's diffusion model, which always assumes the equilibrium concentrations of Vs and Is as the boundary condition for distribution of point defects on the growth interface. The results were disscussed from a qualitative point of view of temperature distribution and thermal stress by the pulling-stop.

On the determination of defect dipoles from atomistic simulations using periodic boundary conditions

By solving the problem of a periodic distribution of point defects in general anisotropic media, we give an alternative, more direct proof, of the relatively recent procedure that extracts dipole tensors from the stress acting on the cell of atomistic simulations performed under periodic boundary conditions. Moreover, we show that naive superposition of individual defect fields is not a solution of the problem, though correction terms can be identified; as a byproduct, analysis of the latter allows us to reveal a spurious contribution to the elastic interaction energy as calculated in current literature procedures, that therefore must be subtracted in order to obtain correct results.

Combined Effect of Rapid Thermal Annealing and Crystal Nature on the Gate Oxide Reliability of Czochralski Silicon

Internal gettering (IG) of metallic impurities (MI) in Czochralski silicon (CZ-Si) is one of the key techniques in modern microelectronic engineering to manufacture high-performance devices with high process yield. Various silicon substrates such as EPI wafers with heavily boron-doped substrates, ion-implanted wafers, and rapid-thermal annealing (RTA) treated wafers have been used to enhance IG ability [1-2] and retain denuded zone (DZ) for the active area of devices. Especially, RTA-treatment is an easy way to form IG and DZ simultaneously with a rapid heat process that injects the intrinsic point defects, mainly vacancies, into the bulk. However, it was reported that vacancy clusters, or ‘nano voids’ whose size was less than 50nm [2-3] could be formed in the near-surface region after RTA process via vacancy self-trapping [4], which can possibly degrade the performance and yield of device. In this study, we investigated the effect of RTA treatment on the gate oxide reliability and their relation to the crystal nature, i.e., as-grown point defect distribution in silicon wafers.As shown in Fig. 1 a), we performed the Cu-contamination method [5] on the prepared samples to verify the crystal nature of each sample. After that, we separated the samples into two groups whether they treated by RTA or not, and carried out time-zero dielectric breakdown (TZDB) reliability tests on the patterned poly-Si/oxide/Semiconductor capacitors with the gate oxide thickness of 10 nm. As shown in Fig. 1 b), distribution of TZDB failed cell on the wafer was closely related to the crystal nature. Moreover, it was revealed that a vacancy-dominant region tends to have lower breakdown field than an interstitial-dominant region. These phenomena were only found in RTA-treated wafers and entire fail rate was drastically increases when RTA effect was combined with vacancy-dominated region, which was shown in Fig. 1 b). We assumed that as-grown vacancies contained in the vacancy-dominated region tend to enhance the supersaturation of vacancy beneath the oxide-semiconductor interface, so the formation of nano voids was accelerated and the breakdown field was lowered due to the presence of such defects. A detailed mechanism of the experimental result will be discussed. Figure 1. a) Crystal nature measured by Cu contamination method and b) TZDB frequency with or without RTP

STEM-Cathodoluminescence of SnO2 nanowires and powders

For the first time, ultra-high spatial resolution STEM-cathodoluminescence has been applied to SnO2 nanowires and nanoparticles in order to study the spatial distribution of point defects that form deep levels in the band gap and often dictate their gas-sensor performance. SnO2 nanowires consistently had emission signals centered at 1.76, 1.99, and 2.45eV. Emission at 1.99eV was found to be more intense with the STEM probe at the edges of nanowires and in higher surface-area nanoparticles compared to emission most intense at 2.45eV with the probe in the center of a nanowire or particle. This result is contrary to recent studies proposing that both emission energies are associated with two different surface oxygen vacancy types. The ability to spatially map the luminescence of a single nanostructure also facilitated much less-ambiguous spectral deconvolution consistent across many particles which gives confidence in properly assigning the energies of each defect state. Individually-probed SnO2 nanowires were found to have much more consistent and reproducible defect states compared to commercial nanoparticles which has implications in stability as gas sensors and efforts to model interactions between analyte gases and the oxide surface.

Non-metric Connection and Metric Anomalies in Materially Uniform Elastic Solids

Metric anomalies arising from a distribution of point defects (intrinsic interstitials, vacancies, point stacking faults), thermal deformation, biological growth, etc. are well known sources of material inhomogeneity and internal stress. By emphasizing the geometric nature of such anomalies we seek their representations for materially uniform crystalline elastic solids. In particular, we introduce a quasi-plastic deformation framework where the multiplicative decomposition of the total deformation gradient into an elastic and a plastic deformation is established such that the plastic deformation is further decomposed multiplicatively in terms of a deformation due to dislocations and another due to metric anomalies. We discuss our work in the context of quasi-plastic strain formulation and Weyl geometry. We also derive a general form of metric anomalies which yield a zero stress field in the absence of other inhomogeneities and any external sources of stress.

Grain boundary segregation and the formation mechanism of secondary-phase in (Ce,Nb)-codoped TiO2 ceramics

Secondary phases are common phenomena in doped titanium dioxide ceramics. Effects of secondary phases on the structure and performance of the doped TiO2 ceramic are crucial. Both boundaries segregation and formation mechanism of secondary phases should be comprehensively explored to enhance properties and application. (Ce,Nb)-codoped TiO2 ceramic samples were prepared from TiO2, Nb2O5, and CeO2 oxide powders by a traditional solid-state sintering method. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and wavelength-dispersive X-ray spectroscopy (WDS) were conducted to determine the sample microstructures, crystal structures, ionic valences, and chemical composition. The point defect distributions at the grain boundaries were calculated via point defect thermodynamic analysis, and the effects of Ce4+ and Ce3+ on segregation at the grain boundaries were compared. The XPS analysis showed that both ions are present in the TiO2 ceramics, and that the Ce3+ content increases with increasing sintering temperature. Further, secondary phases originate from segregation of the NbTi· and Ce´Ti point defects at the grain boundaries. The driving force for segregation is primarily the elastic strain energy, and CeNbO4 and CeNbTiO6 secondary phases occur in the (Ce,Nb)-codoped TiO2 ceramics. With increased sintering temperature, the secondary phase transforms from CeNbO4 to CeNbTiO6.

Analysis of Point Defect Distributions in AlGaN/GaN Heterostructures via Spectroscopic Photo Current-Voltage Measurements

Raman spectroscopy, photoluminescence (PL), and spectroscopic photo current-voltage measurements were performed to characterize point defects at surface/interface and bulk in AlGaN/GaN high electron mobility transistors (HEMTs) wafer. Raman spectroscopy displayed that two samples have similar crystalline quality and stress. PL measurements revealed that samples possessed strong near band edge peak at around 3.42 eV and broad blue luminescence. No yellow luminescence band was observed. The spectroscopic photo current-voltage (I-V) measurements with sub-bandgap illumination exhibited the existence of sub-bandgap defects with different activation energies for each sample. The depth-resolved ultra-violet (UV) spectroscopic photo current-voltage (I-V) (DR-UV-SPIV) measurements revealed that the depth dependent distribution of the defects for these samples are different. It was demonstrated that even though electrically active defects for two devices on the same pieces of AlGaN/GaN HEMTs heterostructures cannot be distinguished in Raman spectroscopy and PL measurements, they can be differentiated by using the spectroscopic photo I-V with sub-bandgap illumination and DR-UV-SPIV measurements. It was concluded that DR-UV-SPIV and spectroscopic photo I-V measurements with sub-bandgap illumination can be used to assess the uniformity of in-depth and spectral distribution of sub-bandgap point defects across the AlGaN/GaN wafers, respectively.

Cathodoluminescence Characterization of Point Defects in Optical Fibers

We evaluate the potential of the cathodoluminescence (CL) spectroscopy to characterize the nature and the spatial distribution of point defects in the main classes of optical fibers (OFs): Telecom-grade, radiation-hardened, and radiation sensitive. Canonical samples, that are differently doped in their cores (Ge, N, P, Ce) or their claddings (F), have been investigated through CL technique using a 10-keV electron beam. Obtained results are compared with those obtained by photoluminescence spectroscopy. CL benefits and limits are discussed on the basis of the obtained experimental data. CL is shown to be efficient to investigate the kinetics of defect generation and bleaching under the electron exposure, being the unique technique allowing us to determine in situ the spatial distribution of emitting defects in the fiber transverse cross sections. New insights are given for some of the defects related to the Ge, P, Ce and N dopants.

Visualizing Point Defects in Transition-Metal Dichalcogenides Using Optical Microscopy.

While transmission electron microscopy and scanning tunneling microscopy reveal atomic structures of point defect and grain boundary in monolayer transition-metal dichalcogenides (TMDs), information on point defect distribution in macroscale is still not available. Herein, we visualize the point defect distribution of monolayer TMDs using dark-field optical microscopy. This was realized by anchoring silver nanoparticles on defect sites of MoS2 under light illumination. The optical images clearly revealed that the point defect distribution varies with light power and exposure time. The number of silver nanoparticles increased initially and reached a plateau in response to light power or exposure time. The size of silver nanoparticles was a few hundred nanometers in the plateau region as observed using optical microscopy. The measured defect density in macroscale was ∼2 × 10(10) cm(-2), slightly lower than the observed value (4 × 10(11) cm(-2)) from scanning tunneling microscopy.

Investigations of interstitial generations near growth interface depending on crystal pulling rates during CZ silicon growth by detaching from the melt

In conventional CZ crystal growth, since formation of a cone tail takes a long time, from such a crystal to have been subject the long heat treatment it is not possible to observe actual distribution of vacancies (Vs) and interstitial atoms (Is) in a straight body of a crystal during growth. This experiment attempted to observe point defect distribution frozen by rapidly cooling a crystal that had been detached from a melt during growth to eliminate the effect of the time delay. Comparison between the experimental results of these specimens and the defect distributions of a conventionally pulled crystals revealed that Vs are introduced at a growth interface and the concentration of the Vs does not depend on the pulling rate. In addition, when the pulling rate is low, Is are generated by thermal stress which increases with increasing thermal gradient G because the amount of heat transfer by mass transfer is decreased and the crystal surface near the growth interface is cooled for longer period. As a result, the generation of Is due to the increase of the thermal stress is observed in an area referred to as an interstitial generation area (IGA) located above the vacancy region on the growth interface, where the crystal temperature is 1300°C or more. This paper describes the recombination (Rc) mechanism by which these Is created in the IGA are recombined with Vs transformed through the growth interface, thereby creating an observable Rc area at a location where no defect can be detected.

Nanoscale origin and evolution of kinetically induced defects in carbon spheres

Synthesis of carbon nanostructures with controlled defect distribution holds tremendous promise in engineering novel materials for specific device applications and catalysis. Carbon spheres (CS) can be synthesized by controlled thermal pyrolysis of polyethylene (PE) via a series of self-generated pressure reactions. Here, we show that point defect distribution in these CS can be tailored by controlling the kinetics during this synthesis procedure. Using accelerated molecular dynamics, we demonstrate that cooling kinetics during synthesis exercises significant control over the reaction pathways, and consequently, over nucleation and growth of carbon nanostructures. The long-range order of these nanostructures is strongly influenced by the cooling rate. At slower rates <1.4 K/ps, well-ordered carbon onions form with very few defects, while at higher rates >60 K/ps, amorphous carbon with 8–10 membered rings reminiscent of as-quenched carbon melts are seen. In the intermediate regime, the distribution of the defects can be tuned with excellent control. These findings are corroborated by electron microscopy observations of defect formations in synthesized CS. SEM and TEM on CS when cooled from ∼973 K (700 °C) to room temperature indicate smooth surface morphologies with a highly defective structure. In contrast, when cooled from ∼3073 K (2800 °C), rough surfaces with pronounced 6-membered rings graphitic ordering are observed.