Density Of Point Defects Research Articles

Quasi-chemical analysis of point defect structure іn Mn-doped CdTe single crystals

Electrical properties of CdTe single crystals, doped by Mn (1 × 1017–1 × 1019 at/cm3), were studied in the wide temperature range using high-temperature Hall effect measurements.To explain the obtained electrical properties the numerical simulation of the structure of both native and foreign point defect densities in the crystals was applied. Between the possibilities of whether predominantly Mn-single donors only or combined with Mn-containing associates occurrence in the crystals the second possibility was confirmed.High-temperature electrical measurements were explained in the framework of the Kröger's theory of quasi-chemical reactions between point defects in solids. It was done by investigating the series of differently Mn-doped CdTe single crystals. In the 1 × 1017–1 × 1019 at/cm3 Mn concentration range the measured free electron densities at high-temperature defect equilibrium were almost identical indicating clear donor activity of the dopant.

Impact of face-to-face annealed sputtered AlN on the optical properties of AlGaN multiple quantum wells

The impact of a face-to-face annealed sputtered AlN/sapphire (FFA Sp-AlN) template with threading-dislocation densities (TDDs) of 2 × 108 cm−2 and an n-type AlGaN (n-AlGaN) underlayer on optical properties of AlGaN multiple quantum wells (MQWs) with an ultraviolet C (UVC) emission is investigated comprehensively. For comparison of the FFA Sp-AlN template with low TDDs, a conventional MOVPE (metalorganic vapor phase epitaxially)-grown AlN/sapphire (MOVPE-AlN) template with TDDs of 1 × 109 cm−2 was prepared. Consequently, cathodoluminescence (CL), temperature-dependent photoluminescence (PL), and time-resolved PL (TR-PL) measurements verified that both the FFA Sp-AlN template and n-AlGaN underlayer are indispensable for obtaining MQWs with high internal quantum efficiencies, which decrease the TDDs and point defect (PD) densities. Our results revealed that 10-period quantum wells (10QWs)/n-AlGaN/AlN grown on the FFA Sp-AlN template exhibit a lower dark spot density in CL panchromatic intensity maps, a higher integrated emission intensity ratio from the temperature-dependent PL (from 15 to 300 K), and a longer nonradiative lifetime from the TR-PL measurements at 300 K compared with those grown on the MOVPE-AlN template. Moreover, we found that the optical properties of 10QWs/AlN in FFA Sp-AlN and MOVPE-AlN templates do not exhibit a significant difference because of the existence of numerous PDs. Our experimental results demonstrate the favorable impact of the FFA Sp-AlN template for low-TDDs and the n-AlGaN underlayer for low-PDs, which holds promise for highly efficient AlGaN deep-ultraviolet light-emitting devices.

Setting Carriers Free: Healing Faulty Interfaces Promotes Delocalization and Transport in Nanocrystal Solids.

Superlattices of epitaxially connected nanocrystals (NCs) are model systems to study electronic and optical properties of NC arrays. Using elemental analysis and structural analysis by in situ X-ray fluorescence and grazing-incidence small-angle scattering, respectively, we show that epitaxial superlattices of PbSe NCs keep their structural integrity up to temperatures of 300 °C; an ideal starting point to assess the effect of gentle thermal annealing on the superlattice properties. We find that annealing such superlattices between 75 and 150 °C induces a marked red shift of the NC band-edge transition. In fact, the post-annealing band-edge reflects theoretical predictions on the impact of charge carrier delocalization in these epitaxial superlattices. In addition, we observe a pronounced enhancement of the charge carrier mobility and a reduction of the hopping activation energy after mild annealing. While the superstructure remains intact at these temperatures, structural defect studies through X-ray diffraction indicate that annealing markedly decreases the density of point defects and edge dislocations. This indicates that the connections between NCs in as-synthesized superlattices still form a major source of grain boundaries and defects, which prevent carrier delocalization over multiple NCs and hamper NC-to-NC transport. Overcoming the limitations imposed by interfacial defects is therefore an essential next step in the development of high-quality optoelectronic devices based on NC solids.

Influence of ions irradiation on the microstructural evolution, mechanical and tribological properties of Zr-4 alloy

The wear of fuel cladding due to flow induced vibration in nuclear reactor presents a safety concern, and the impact of neutron irradiation on wear behavior is not fully understood. In this study, Kr+ and Zr+ ions bombardment were used to simulate the neutron irradiation on Zr-4 alloys, and the impacts of ion irradiation on the microstructure evolution, mechanical and tribological behaviors of Zr-4 alloy were investigated. The HRTEM observation reveals that high density of point defects, dislocations as well as small dislocation loops formed below the surface within 260 nm depth after Zr+ irradiated with the damage level up to 30 dpa. The nano-indentation tests suggests that the hardness of Zr-4 alloy increases with increasing of irradiation dose, resulting from the inhibition of slip of dislocation under the large amount of defects induced by irradiation. The fretting wear tests show that the friction coefficient of Zr-4 alloy increases significantly from 0.4 to above 1, due to the irradiation hardening and embrittlement, the repeated sliding during fretting wear leads to intergranular fracture at surface and the growth of cracks under load contact area, leaving deeper furrows and higher hardness debris, which results in higher friction coefficient and severe wear of Zr-4 alloy after irradiation.

Role of passive film in dominating the electrochemical corrosion behavior of FeCrMoCBY amorphous coating

FeCrMoCBY amorphous coating with a high volume fraction of amorphous phase (90%) was prepared on Q235 steel by atmospheric plasma spraying. Influence of passivation potential on the corrosion resistance of passive films on the coating surface was estimated by electrochemical measurements, capacitance analysis, AFM and XPS technique. The results revealed that increasing potential promoted the growth of a more compact and thicker film due to the formation of more bounded water and oxides. Reduction in hydroxides further decreased point defects density significantly in the passive layer when passivated at a higher potential. A low diffusivity (1.67 × 10−15 cm2 s−1) corresponding to point defects could suppress both the growth and degradation processes of passive film in chloride containing electrolyte, and thus enhance the film resistance to local thinning.

Long term semiconducting and passive film properties of a novel dense FeCrMoCBY amorphous coating by atmospheric plasma spraying

The long term electrochemical corrosion behavior and passive film properties of a novel dense Fe48Cr15Mo14C15B6Y2 (at.%) amorphous coating were investigated using electrochemical measurement, Mott-Schottky test, effective capacitance model and XPS technique. The coating exhibited a superior corrosion resistance with a relative low current density of 5.305 × 10−6 A cm−2 even after 1000 h immersion in 3.5 wt% NaCl solution. The film composition changed slightly and made no difference to the corrosion behavior. Noting that point defect density in the passive film and film thickness played a dominant role in the corrosion resistance of the amorphous coating in chloride electrolyte. The enhanced long term corrosion resistance of the coating may be attributed to an inactive mass transport behavior in the passive film in the early immersion age.

High Resolution Investigation of Stacking Fault Density by HRXRD and STEM

The effect of varying growth rate on the formation of defects in homo-epitaxially grown cubic silicon carbide (3C-SiC) is studied. Three growth rates are considered (30, 60 and 90 μm/hr) demonstrating that as the growth rate increases the density of point defects, as demonstrated by photo- luminescence, and stacking faults (SFs), as measured by a KOH etching procedure, increase. Scanning transmission electron microscopy images demonstrate generation, annihilation and closure of SFs as a function film thickness. High resolution X-ray diffraction is used to uncover the higher quality of homo-epitaxial with respect hetero-epitaxial films through the examination of the sample mosaicity and SF density.

Thermal Annealing of High Dose P Implantation in 4H-SiC

In this work, we have studied the crystal defectiveness and doping activation subsequent to ion implantation and post-annealing by using various techniques including photoluminescence (PL), Raman spectroscopy and transmission electron microscopy (TEM). The aim of this work was to test the effectiveness of double step annealing to reduce the density of point defects generated during the annealing of a P implanted 4H-SiC epitaxial layer. The outcome of this work evidences that neither the first 1 hour isochronal annealing at 1650 - 1700 - 1750 °C, nor the second one at 1500 °C for times between 4 hour and 14 hour were able to recover a satisfactory crystallinity of the sample and achieve dopant activations exceeding 1%.

Electrochemical and XPS analytical investigation of the accelerative effect of bicarbonate/carbonate ions on AISI 304 in alkaline environment

Abstract Passivation behavior and surface chemistry of 304 stainless steel in alkaline solution with different HCO 3 − /CO 3 2– concentrations are studied with a combined electrochemical and XPS analytical investigation. Cathodic and anodic reactions are inhibited and accelerated, respectively, attributed to the depletion of acceptable oxygen and variation of passive film properties. Increasing HCO 3 − /CO 3 2– concentration does not alter the bilayer structure but leads to an intensification in electric field strength and composition changes including enrichment of oxidized Cr, increase of Fe(II)/Fe(III) ratio, and decrease of OH − /O 2– ratio. Meanwhile, the film thickness is reduced and the point defect density is increased due to the enhanced film dissolution. High HCO 3 − /CO 3 2– concentration increases the critical chloride concentration of 304 SS in alkaline solution, attributed to the inhibition of the localized acidification within the pits and suppression of the stable pit development.

In-line chemiresistors based on multilayer graphene for cadmium dication sensing in water

We report that multilayer graphene exfoliated from graphite at sufficiently low point-defect density, and intercalated by poly(ethylene glycol)-block-poly(propylene glycol)-block-poly (ethylene glycol) (PEG-PPG-PEG) block copolymers of appropriate chain length, functions as a sorbent and detector for cadmium dications in water, which remain trapped within the graphene interlayers, thus changing the device’s electrical resistivity. Graphitic samples at different defectiveness – synthetic graphite and high-quality natural graphite – are exfoliated by PEG-PPG-PEG at different chain lengths (Mn ≈ 1100–14,600 amu). The resulting multilayer graphene “cakes” are incorporated into gap-cell chemiresistors assembled onto microporous supports. The resulting devices are nanoporous and can be inserted in-line in water filtration systems. We have monitored their resistivity as a function of the concentration of specific metal dications (Cd2+, Hg2+ and Mn2+) in the water passing through the devices. Although dication concentrations (of the order of a few ppb) are too low to directly affect the solution ionic conductivity, the solid-state resistivity of our graphene-based chemiresistors decreased at increasing Cd2+ concentration, while the effect was less significant for the other types of ions. We attribute this to Cd2+ intercalation within graphene stacks. It appears that Cd2+ possesses the optimal hydrodynamic radius for being trapped within graphene layers intercalated by PEG-PPG-PEG of appropriate chain length, leading to enhanced Cd2+ sensing. Our chemiresistors are of low cost and can be easily computer controlled.

Neutron studies of gauge field and charge in Ih heavy-water ice

The distinctive character of water ice results from the partially disordered combination of covalent and hydrogen bonds in the network of hydrogen and oxygen atoms. The nontrivial hydrogen correlations we report in diffuse neutron scattering are analytically fit via a description of this state as a topological system exhibiting an emergent gauge field. This allows for the density of correlation-terminating point defects to be determined as one defect per 500 oxygen sites at 30 K. Application of an analytical model of ice paves the way towards a detailed understanding of this ubiquitous solid.

Quantum Efficiency of III-Nitride Emitters: Evidence for Defect-Assisted Nonradiative Recombination and its Effect on the Green Gap

Carrier lifetime measurements reveal that, contrary to common expectations, the high-current non-radiative recombination (droop) in III-Nitride light emitters is comprised of two contributions which scale with the cube of the carrier density: an intrinsic recombination --most likely standard Auger scattering-- and an extrinsic recombination which is proportional to the density of point defects. This second droop mechanism, which hasn't previously been observed, is likely caused by an impurity-assisted Auger process. Further, it is shown that longer-wavelength emitters suffer from higher point defect recombinations, in turn causing an increase in the extrinsic droop process. It is proposed that this effect leads to the green gap, and that point defect reduction is a strategy to both vanquish the green gap and more generally improve quantum efficiency at high current.

The impact of defect density, grain size, and Cu orientation on thermal oxidation of graphene-coated Cu

Graphene has been shown to be a promising barrier for thermal corrosion due to its low gas and liquid permeability; however, there have been contradictory reports in the literature regarding the mechanisms of oxidation of graphene-coated Cu. This work systematically investigates the effect of chemical vapor deposited graphene grain size, point defect density, and underlying Cu orientation on the thermal oxidation of the underlying Cu in air. For graphene with either small grain size (~0.04–0.4 μm2) or large point defect density (~800 μm−2), oxidizers have relatively unhindered access through these defects to corrode the underlying Cu, and the corrosion is relatively independent of Cu orientation. For graphene with low defect density, the rate of Cu oxidation depends on the Cu crystal orientation on which the graphene is synthesized. Graphene-coated Cu (110) is completely corroded in just 2 min of thermal oxidation at 250 °C, while no surface oxidation for graphene-coated Cu (111) is observed for the same conditions. Thus, corrosion of graphene that is synthesized to achieve large grain size (>~170 μm2) is limited by the weakest graphene/Cu orientation interaction, not the size of the graphene grains, as was previously assumed.

Au and Ag ion irradiation effects on the carbide precipitation and Ar bubble formation in solubilized AISI 316L alloys

This contribution reports on transmission electron microscopy observations of Au and Ag ion irradiated complex metal alloys obtained by solution annealing treatments in AISI 316L foils. Two sets of samples are used: i) pristine solution annealed ones, and ii) solution annealed samples implanted with Ar to form a dense array of small bubbles located in a 250 nm thick surface layer. The irradiations were done in samples kept at 550 °C using 5 MeV Au and 3.5 MeV Ag accelerated ions and fluences calculated to produce damage levels equivalent to 20 and 40 dpa. The results obtained demonstrate that the irradiations cause the formation of 7 times larger cavities in the pristine samples and bubble enlargement 30% superior in Ar contained samples for Au ions as compared to the Ag case. This means that Au irradiations produce a larger number of excess vacancies than Ag at the same dpa values. This phenomenon is discussed in terms of the point defect densities produced by individual ion induced displacement cascades. In addition, for Au irradiations, we observe the formation of MC phase carbide precipitation only in samples containing Ar bubbles. This effect is discussed considering that the nucleation kinetic of the MC precipitates can be controlled by vacancy supersaturation.

Quantitative Principles for Precise Engineering of Sensitivity in Graphene Electrochemical Sensors.

A major difficulty in implementing carbon-based electrode arrays with high device-packing density is to ensure homogeneous and high sensitivities across the array. Overcoming this obstacle requires quantitative microscopic models that can accurately predict electrode sensitivity from its material structure. Such models are currently lacking. Here, it is shown that the sensitivity of graphene electrodes to dopamine and serotonin neurochemicals in fast-scan cyclic voltammetry measurements is strongly linked to point defects, whereas it is unaffected by line defects. Using the physics of point defects in graphene, a microscopic model is introduced that explains how point defects determine sensitivity. The predictions of this model match the empirical observation that sensitivity linearly increases with the density of point defects. This model is used to guide the nanoengineering of graphene structures for optimum sensitivity. This approach achieves reproducible fabrication of miniaturized sensors with extraordinarily higher sensitivity than conventional materials. These results lay the foundation for new integrated electrochemical sensor arrays based on nanoengineered graphene.

Defect Manipulation To Control ZnO Micro-/Nanowire-Metal Contacts.

Surface states that induce depletion regions are commonly believed to control the transport of charged carriers through semiconductor nanowires. However, direct, localized optical, and electrical measurements of ZnO nanowires show that native point defects inside the nanowire bulk and created at metal-semiconductor interfaces are electrically active and play a dominant role electronically, altering the semiconductor doping, the carrier density along the wire length, and the injection of charge into the wire. We used depth-resolved cathodoluminescence spectroscopy to measure the densities of multiple point defects inside ZnO nanowires, substitutional Cu on Zn sites, zinc vacancy, and oxygen vacancy defects, showing that their densities varied strongly both radially and lengthwise for tapered wires. These defect profiles and their variation with wire diameter produce trap-assisted tunneling and acceptor trapping of free carriers, the balance of which determines the low contact resistivity (2.6 × 10-3 Ω·cm-2) ohmic, Schottky (Φ ≥ 0.35 eV) or blocking nature of Pt contacts to a single nano/microwire. We show how these defects can now be manipulated by ion beam methods and nanowire design, opening new avenues to control nanowire charge injection and transport.

Influence of the atom source operating parameters on the structural and optical properties of InxGa1−xN nanowires grown by plasma-assisted molecular beam epitaxy

The influence of the atom source operating parameters on the structural and optical properties of InxGa1−xN/GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy is investigated. Electron microscopy and photoluminescence spectroscopy reveal a change of the NW tip morphology and an enhancement of the local indium incorporation with increasing nitrogen flux. Tuning the density ratio of atomic-to-excited molecular nitrogen to lower values minimizes the point defect density, which results in a decrease of the non-radiative recombination rate as demonstrated by a combination of continuous wave and time-resolved photoluminescence spectroscopy.

PH dependent passivation behavior of niobium in acid fluoride-containing solutions

In this work, the passivation behavior of Nb in acid fluoride-containing solutions with pH of 4, 5 and 6 was investigated. Both electrochemical measurements and surface characterization were systematically used to explore the pH effect on the electrochemical stability of Nb in aggressive oral cavity environment. The point defect model (PDM) and surface charge approach (SCA) were used as theoretical basis to analyze the passive film growth and dissolution kinetics. As the pH value decreases, the passive film becomes more reactive and suffers more severe dissolution, leading to higher point defect density across the passive film and more pronounced surface charge behavior at the film/solution interface. Furthermore, the surface becomes rougher and the passive film is thinner with less amount of suboxide as the solution is more acid. Since high electrochemical stability of the passive films is desirable for Nb implants, the results from this work shed insight into the use of fluoride-containing prophylactic substances.

Degradation behaviors and mechanisms of MoS2 crystals relevant to bioabsorbable electronics

Monolayer molybdenum disulfide (MoS2) exhibits unique semiconducting and bioresorption properties, giving this material enormous potential for electronic/biomedical applications, such as bioabsorbable electronics. In this regard, understanding the degradation performance of monolayer MoS2 in biofluids allows modulation of the properties and lifetime of related bioabsorbable devices and systems. Herein, the degradation behaviors and mechanisms of monolayer MoS2 crystals with different misorientation angles are explored. High-angle grain boundaries (HAGBs) biodegrade faster than low-angle grain boundaries (LAGBs), exhibiting degraded edges with wedge and zigzag shapes, respectively. Triangular pits that formed in the degraded grains have orientations opposite to those of the parent crystals, and these pits grow into larger pits laterally. These behaviors indicate that the degradation is induced and propagated based on intrinsic defects, such as grain boundaries and point defects, because of their high chemical reactivity due to lattice breakage and the formation of dangling bonds. High densities of dislocations and point defects lead to high chemical reactivity and faster degradation. The structural cause of MoS2 degradation is studied, and a feasible approach to study changes in the properties and lifetime of MoS2 by controlling the defect type and density is presented. The results can thus be used to promote the widespread use of two-dimensional materials in bioabsorption applications.

Detection of local vibrational modes induced by intrinsic defects in undoped BaSi2 light absorber layers using Raman spectroscopy

We fabricate BaSi2 epitaxial films on Si(111) substrates by molecular beam epitaxy and investigate point defects inside the films using Raman spectroscopy with the help of first-principles calculation. Point defects such as Ba substituted for Si antisites, Si vacancies, and Si interstitials are considered as candidates for native point defects in BaSi2. Vibration analysis based on first-principles calculation suggests that local vibrational modes caused by these point defects appear at around 430, 480, and 560 cm−1, respectively, and are in good agreement with Raman peak positions. Comparing calculations with Raman spectra of the films formed with different Ba to Si deposition rate ratios RBa/RSi from 1.0 to 5.1, we conclude that the density of point defects reaches a minimum at RBa/RSi = 2.2. Furthermore, the position of Raman peaks at approximately 490 cm−1 shifts to a lower wavenumber, depending on RBa/RSi and thereby the density of point defects.