Formation Of Defect Complexes Research Articles

Tunable Hydrogen-Related Defects in ZnO Nanowires Using Oxygen Plasma Treatment by Ion Energy Adjustment

The chemical bath deposition (CBD) process enables the deposition of ZnO nanowires (NWs) on various substrates with customizable morphology. However, the hydrogen-rich CBD environment introduces numerous hydrogen-related defects, unintentionally doping the ZnO NWs and increasing their electrical conductivity. The oxygen-based plasma treatment can modify the nature and amount of these defects, potentially tailoring the ZnO NW properties for specific applications. This study examines the impact of the average ion energy on the formation of oxygen vacancies (VO) and hydrogen-related defects in ZnO NWs exposed to low-pressure oxygen plasma. Using X-ray photoelectron spectroscopy (XPS), 5 K cathodoluminescence (5K CL), and Raman spectroscopy, a comprehensive understanding of the effect of the oxygen ion energy on the formation of defects and defect complexes was established. A series of associative and dissociative reactions indicated that controlling plasma process parameters, particularly ion energy, is crucial. The XPS data suggested that increasing the ion energy could enhance Fermi level pinning by increasing the amount of VO and favoring the hydroxyl group adsorption, expanding the depletion region of charge carriers. The 5K CL and Raman spectroscopy further demonstrated the potential to adjust the ZnO NW physical properties by varying the oxygen ion energy, affecting various donor- and acceptor-type defect complexes. This study highlights the ability to tune the ZnO NW properties at low temperature by modifying plasma process parameters, offering new possibilities for a wide variety of nanoscale engineering devices fabricated on flexible and/or transparent substrates.

Complex defect formation in Fe doped γ-CuI: Enhancement of thermoelectric properties via band engineering

The doping effect of Fe atoms on CuI, a potential candidate for room temperature thermoelectric materials, was investigated due to the lack of studies for the aspect of band engineering. The addition of Fe atoms was observed to enhance the p-type performance of CuI, although Fe is more suitable as a donor material. Through Mossbauer spectroscopy and Density Functional Theory (DFT) calculations, it was determined that only about 5 % of the added Fe atoms simply substitute Cu sites, while most Fe atoms occupy interstitial sites, acting as defects and promoting Cu vacancies. Despite the trace amount of Fe only contribute to magnetic properties, it was found that multiple defects and deformation in the electronic band structure caused the effective mass to nearly double. As a result, electrical conductivity increased by an order of magnitude, while the decrease in the Seebeck coefficient was less than half, demonstrating a violation of the Pisarenko relation. Additionally, the incorporation of Fe atoms induced nano structuring behavior, increasing boundary scattering in CuI and significantly reducing thermal conductivity. Fe could be suggested as a suitable dopant for the "phonon-glass-electron-crystal" strategy.

Donor-induced electrically charged defect levels: examining the role of indium and n-type defect-complexes in germanium

Defect levels induced by defect-complexes in Ge play important roles in device fabrication, characterization, and processing. However, only a few defect levels induced by defect-complexes have been studied, hence limiting the knowledge of how to control the activities of numerous unknown defect-complexes in Ge. In this study, hybrid density functional theory calculations of defect-complexes involving oversize atom (indium) and n-type impurity atoms in Ge were performed. The formation energies, defect-complex stability, and electrical characteristics of induced defect levels in Ge were predicted. Under equilibrium conditions, the formation energy of the defect-complexes was predicted to be within the range of 5.90–11.38 eV. The defect-complexes formed by P and In atoms are the most stable defects with binding energy in the range of 3.31-3.33 eV. Defect levels acting as donors were induced in the band gap of the host Ge. Additionally, while shallow defect levels close to the conduction band were strongly induced by the interactions of Sb, P, and As interstitials with dopant (In), the double donors resulting from the interactions between P, As, N, and the host atoms including In atom are deep, leading to recombination centers. The results of this study could be applicable in device characterization, where the interaction of In atom and n-type impurities in Ge is essential. This report is important as it provides a theoretical understanding of the formation and control of donor-related defect-complexes in Ge.

Hybrid density functional theory simulation of sodium impurity and impurity–vacancy defect complexes in germanium: perspectives of defect engineering for activation of shallow donors

At present, germanium (Ge) as a high-mobility material is considered as a possible replacement for Si in microelectronics. The main obstacle in the method is the large vacancy concentration obtained after the implantation of shallow donors. This prevents the formation of n+ regions and has a negative impact on device performance. One way to eliminate the detrimental effects of such defects is through the codoping approach. In this study, the behavior of Na impurities in germanium together with the formation of defect complexes with vacancies, divacancies and shallow donors (P and As) were investigated using hybrid density functional theory calculations. The site preference, diffusion barriers, charge states and binding energies of various complexes, as well as the activation of donor–vacancy complexes with both F and Na codoping were investigated. For this purpose, two extreme cases were considered. Firstly, we calculate the concentrations of substitutional and interstitial Na together with monovacancies in the case of quasi-equilibrium conditions realized for doping from melt. Another extreme case is Na codoping during ion implantation. Our calculations show that Na passivates vacancies, divacancies and donor–vacancy complexes (E-centers) with a large energy gain, which prevents rapid donor diffusion similar to the case of doping with fluorine. We have shown that the passivation of E-centers with both F and Na impurities leads to the back transformation of such complexes into shallow donors. This means that codoping with Na may neutralize the harmful effects of vacancies and is a perspective for defect engineering.

Oxygen Diffusion in Li(Nb,Ta)O3 Single Crystals

Oxygen tracer self‐diffusion in LiNbO3, LiTaO3, and LiNb0.15Ta0.85O3 single crystals between 880 and 1050 °C is investigated. 18O2 isotope‐exchanged samples are analyzed by secondary ion mass spectrometry. The diffusivities of each of the three different materials can be described by the Arrhenius law with an activation enthalpy of diffusion of about 3.2–3.5 eV. The diffusivities are highest for LiNbO3 and are lower by about one order of magnitude for LiNb0.15Ta0.85O3 and LiTaO3. The change of the pre‐exponential factor is identified as the reason for the difference in diffusivities. The experimental results are compared to defect formation energy calculations as given in literature and to energy barrier calculations for the diffusion of a single O vacancy as determined by nudged elastic band calculations based on density‐functional theory. An oxygen vacancy mechanism is suggested to govern diffusion. The difference in diffusivities is tentatively attributed to a different number of freely migrating vacancies, probably due to defect complex formation.

Tuning enhanced dielectric properties of (Sc3+–Ta5+) substituted TiO2 via insulating surface layers

In this study, we achieved significantly enhanced giant dielectric properties (EG-DPs) in Sc3+–Ta5+ co-doped rutile-TiO2 (STTO) ceramics with a low loss tangent (tanδ ≈ 0.05) and high dielectric permittivity (ε′ ≈ 2.4 × 104 at 1 kHz). We focused on investigating the influence of insulating surface layers on the nonlinear electrical properties and the giant dielectric response. Our experimental observations revealed that these properties are not directly correlated with the grain size of the ceramics. Furthermore, first-principles calculations indicated the preferred formation of complex defects, specifically 2Ta diamond and 2ScVo triangular-shaped complexes, within the rutile structure of STTO; however, these too showed no correlation. Consequently, the non-Ohmic properties and EG-DPs of STTO ceramics cannot be predominantly attributed to the grain boundary barrier layer capacitor model or to electron-pinned defect-dipole effects. We also found that the semiconducting grains in STTO ceramics primarily arise from Ta5+, while Sc3+ plays a crucial role in forming a highly resistive outer surface layer. Notably, a significant impact of grain boundary resistance on the nonlinear electrical properties was observed only at lower co-dopant concentrations in STTO ceramics (1 at%). The combination of low tanδ values and high ε′ in these ceramics is primarily associated with a highly resistive, thin outer-surface layer, which substantially influences their non-Ohmic characteristics.

Overcoming the compensation of acceptors in GaN:Mg by defect complex formation

In GaN:Mg, the MgGa acceptor is compensated extensively by the formation of nitrogen vacancies (VN) and Mg interstitials (Mgi). However, we show that such compensation can be overcome by forming two kinds of Mg-rich complexes: one that contains VN and the other that contains only MgGa and Mgi. Such complexing not only neutralizes VN and Mgi but also forms better complex acceptors that have lower formation energies and smaller hole localization energies than isolated MgGa. Our results help explain the different doping behaviors in samples grown by different methods.

A density functional theory study of the CiN and the CiNOi complexes in silicon

Nitrogen (N) is an important impurity in silicon (Si), which associates with impurities as well as with other defects to form defect complexes. The knowledge of the properties and behavior of defect structures containing carbon (C), N and oxygen (O) is important for the Si–based electronic technology. Here, we employ density functional theory (DFT) calculations to investigate the association of nitrogen with carbon and oxygen defects to form the CiN and CiNOi defects. We provide evidence of the formation of these defects and additional details of their structure, their density of states (DOS) and Bader charges. Therefore, CiN and CiNOi defects are now well characterized.

γ‐Irradiation Damage Mechanism of InGaAs/InP p–i–n Focal Plane Array Investigated by Spatially Resolved and Temperature‐Dependent Photoluminescence

InGaAs infrared photodetectors subjected to irradiation environments undergo microstructural modifications and concomitant degradation, yet the underlying microscopic mechanism has not been fully studied. Herein, the influence of γ irradiation (total dose of 20 krad(Si)) on an In0.53Ga0.47 As/InP p–i–n focal plane array is studied by spatially resolved and temperature‐dependent (3–290 K) photoluminescence (PL) measurements. By comparative PL studies of pre‐irradiation and post‐irradiation, the spatially resolved PL results of irradiation indicate that the in‐plane uniformity of all PL features presents bigger fluctuations, meanwhile, the results of temperature‐dependence PL demonstrate that the PL integral intensity related to impurities and interface‐bound states is significantly weakened after irradiation. This can be attributed to the enhanced migration and reaction of defects caused by γ irradiation. Some mobile defects tend to migrate to lower energy regions, such as interfaces, and form defect complexes. In addition, some impurities combine with mobile defects and form inactive impurity–defect complexes. The findings reveal the effects of low‐dose γ irradiation on InGaAs devices and may provide useful information for enhancing radiation resistance.

Interplay Effects in the Co-Doping of ZnO Nanowires with Al and Ga Using Chemical Bath Deposition

The simultaneous co-doping of ZnO nanowires grown by chemical bath deposition is of high interest for a large number of engineering devices, but the process conditions required and the resulting physicochemical processes are still largely unknown. Herein, we show that the simultaneous co-doping of ZnO nanowires with Al and Ga following the addition of Al(NO3)3 and Ga(NO3)3 in the chemical bath operates in a narrow range of conditions in the high-pH region, where the adsorption processes of respective Al(OH)4- and Ga(OH4)- complexes on the positively charged m-plane sidewalls are driven by attractive electrostatic forces. The structural morphology and properties of ZnO nanowires are significantly affected by the co-doping and mainly governed by the effect of Al(III) species. The incorporation processes of Al and Ga dopants are characterized by significant interplay effects, and the amount of incorporated Ga dopants into ZnO nanowires is found to be larger than the amount of incorporated Al dopants owing to energetic considerations. The Al and Ga dopants are located in the bulk of ZnO nanowires, but a part of Al and Ga lies on their surfaces, their incorporation processes in the bulk being enhanced by thermal annealing under oxygen atmosphere. Eventually, the Al and Ga dopants directly affect the incorporation of hydrogen-related defects, notably by annihilating the formation of VZn-nH defect complexes. These findings present an efficient strategy to proceed with the co-doping of ZnO nanowires grown by chemical bath deposition, opening perspectives to control their electronic structure properties with a higher precision.

A Study of Complex Defect Formation in Silicon Doped With Nickel

A Study of Complex Defect Formation in Silicon Doped With Nickel

Study of the influence of gamma irradiation on internal friction in dislocation silicon

The effect of radiation defects formed during gamma irradiation of single-crystal silicon on the internal friction in dislocations was studied. It was found that in dislocation silicon, after irradiation with gamma rays of a source of ionizing radiation 60Co, at first the internal friction (Q-1) increases to a maximum value and then Q-1 gradually decreases to the initial values. It is shown that, as a result of gamma irradiation of silicon, the resulting change in Q-1 is associated with the process of relaxation of vacancies with the formation of vacancy defect complexes near dislocation lines. The data of the acoustic emission method (obtained AE spectra) confirm that AE signals in gamma-irradiated silicon arise due to moving dislocations, and the intensity of AE signals initially increases during the first 1.5–2 h, which is also observed with an increase in internal friction depending on Q-1(t), and then the AE signals decrease in the form of pulses of discrete AE signals.

Multifrequency EPR spectroscopy study of Mn, Fe, and Cu doped nanocrystalline ZnO

Mn, Fe, and Cu ions, when doped into ZnO nanocrystals, impart magnetic phenomena to their semiconducting property. Although notable in the dilute magnetic semiconductor community, transition metal (TM) ion-doped ZnO lacks investigations that inform researchers on the local lattice structure around the dopant ion, its spin-exchange phenomena, and the interaction between its intrinsic defects and the doped metal ion. The current study presents a detailed multi-frequency (X- and Q-band) EPR investigation that clarifies the localization of the dopant ion, its site symmetry, and the formation of intrinsic-extrinsic defect complexes in ZnO:TM. The incorporation of TM ion is observed to modify the intrinsic defect structure of ZnO nanocrystals. Particularly, a deviation from the core-shell model is observed for ZnO:TM, and the appearance of intrinsic-extrinsic defect complexes that may contribute to a peculiar spin-exchange phenomenon are noticed. Additionally, the localization as observed from the resonance lines of defect complexes comprising Cu2+ is different from those of Mn2+ and Fe3+, showing charge selective substitutions in the matrix.

Dynamic doping and Cottrell atmosphere optimize the thermoelectric performance of n-type PbTe over a broad temperature interval

High thermoelectric energy conversion efficiency requires a large figure-of-merit, zT, over a broad temperature range. To achieve this, we optimize the carrier concentrations of n-type PbTe from room up to hot-end temperatures by co-doping Bi and Ag. Bi is an efficient n-type dopant in PbTe, often leading to excessive carrier concentration at room temperature. As revealed by density functional theory calculations, the formation of Bi and Ag defect complexes is exploited to optimize the room temperature carrier concentration. At elevated temperatures, we demonstrate the dynamic dissolution of Ag2Te precipitates in PbTe in situ by heating in a scanning transmission electron microscope. The release of n-type Ag interstitials with increasing temperature fulfills the requirement of higher carrier concentrations at the hot end. Moreover, as characterized by atom probe tomography, Ag atoms aggregate along parallel dislocation arrays to form Cottrell atmospheres. This results in enhanced phonon scattering and leads to a low lattice thermal conductivity. As a result of the synergy of dynamic doping and phonon scattering at decorated dislocations, an average zT of 1.0 is achieved in n-type Bi/Ag-codoped PbTe between 400 and 825 K. Introducing dopants with temperature-dependent solubility and strong interaction with dislocation cores enables simultaneous optimization of the average power factor and thermal conductivity, providing a new concept to exploit in the field of thermoelectrics.

Factors influencing the structure of the complex-defects in AF2: RE3+ (A= Ca, Sr and Ba): A first-principles study

As aliovalent doping materials, RE3+ doped AF2 (A = Ca, Sr and Ba), which show not only high thermal conductivity and low nonlinear refractive index ascribed to fluorides, but also unique spectroscopic properties related to the RE3+ ions, are promising ultra-strong and ultra-fast laser crystal candidates. It is noted that the stability of the C3v complex-defect, in which Fi− compensates at the next-nearest interstitial site of RE3+, increases with the ratio of the lattice parameter to the dopant ionic radius. In this work, systematic first-principles calculations are performed to resolve factors influencing the structure of the RE-Fi monomers. The formation energy, the reorientation energy and the local structure of the monomers are obtained, and the Coulomb interaction and the sub-lattice distortion are quantified. It is concluded that the sub-lattice distortion determines the defect structure even though the Coulomb interaction is the fundamental basis for the complex-defect formation in aliovalent doping materials. On this basis, the cluster aggregation process in AF2: RE3+ are elucidated and the octahedral arrangement of the hexamer is specially explained. Our calculation results and analytical methods not only provide new insights for the defect formation mechanism in AF2: RE3+, but also guide the improvement of aliovalent doping materials.

Radiation Hardness of 4H-SiC P-N Junction UV Photo-Detector.

4H-SiC based p-n junction UV photo-detectors were irradiated with 600 keV He+ in the fluence range of 5 × 1011 ÷ 5 × 1014 ion/cm2 in order to investigate their radiation hardness. The effects of irradiation on the electro-optical performance were monitored in dark condition and in the UV (200 ÷ 400 nm) range, as well as in the visible region confirming the typical visible blindness of unirradiated and irradiated SiC photo-sensors. A decrease of UV optical responsivity occurred after irradiation and two fluence regimes were identified. At low fluence (<1013 ions/cm2), a considerable reduction of optical responsivity (of about 50%) was measured despite the absence of relevant dark current changes. The presence of irradiation induced point defects and then the reduction of photo-generated charge lifetime are responsible for a reduction of the charge collection efficiency and then of the relevant optical response reduction: point defects act as recombination centers for the photo-generated charges, which recombine during the drift/diffusion toward the electrodes. At higher irradiation fluence, the optical responsivity is strongly reduced due to the formation of complex defects. The threshold between low and high fluence is about 100 kGy, confirming the radiation hardness of SiC photo-sensors.

Enhancement of the electrocaloric effect in PbZr0.7Ti0.3O3 ceramics via La doping: Driven by phase co-existence or defect effects?

Lattice defects and their effects have been pivotal in studies of phase transitions in a wide range of materials. Introduction of such defects into a ferroelectric material through doping of secondary elements can be tailored towards specific applications but the mechanism through which the bulk properties change is seldom scrutinized. Here we study the effect of systematic La substitution into PbZr0.7Ti0.3O3 (PZT 70/30) ceramics whereby we analyzed the temperature dependent properties and estimated the temperature changes that could be induced upon application of an external electric field, namely the electrocaloric effect (ECE). Expecting the entropic changes to be maximal under an applied field, the suitability of the La doped PZT 70/30 system for EC applications had been a motivation to undertake the current task as this composition reportedly can host a rich variety of phases depending on La content including relaxor and antiferroelectric (AFE) states. An electrocaloric (EC) temperature change of 1.15 °C in a wide range of temperatures for 8% La doping at 45 kV/cm applied field was estimated from experimental data, the possible origins of which is discussed. We were able to explain the experimental results by adopting a Landau-Ginzburg based computational approach coupled with elasticity and electrostatics whereby La sites are treated as point defects in a PZT 70/30 lattice. The gradual slanting of the hystereses and reduction of the transition temperature in the samples with increasing La content is claimed to be a direct consequence of the electrical fields due to formation of dipolar defect complexes as backed by our simulations. The ECE is discussed in the light of the simulations and recent results for AFE ceramics.

Interactions of oxygen with intrinsic defects in L1[formula omitted][formula omitted]-TiAl in presence of substitutional solutes: Influence on diffusion kinetics

This work reexamines the insertion of O atoms in the L10γ-TiAl system using first-principles calculations and thermodynamic modeling in the independent point defect approximation. It includes a study of intrinsic point defects, the insertion of many alloying elements (more than twenty were considered), as well as a study of their interaction with oxygen. The formation of complex defects composed of either vacancies, anti-sites or solute elements is then studied. Results at the atomic scale show a high segregation of oxygen in titanium-rich environments: oxygen easily segregates onto Ti anti-sites (TiAl) and alloying elements are located in the vicinity of Al sub-lattices. DFT point-defect energetics shows that there is a clear correlation between the nature and site preference of an alloying element, and the oxygen segregation energy in the vicinity of this solute. The thermodynamic model shows that at equilibrium, oxygen does not occupy isolated interstitial sites but prefers to be located in the vicinity of Ti anti-sites or alloying elements. The effect of this strong segregation on oxygen diffusivity is discussed hereinafter. Results show a strong slowdown in oxygen diffusivity due to intrinsic defects. For Ti/Al >0.5 ratios, the traps for O diffusion are mainly constituted by Ti anti-sites, and the addition of solutes does not contribute much to the trapping of diffusing O atoms. For Ti/Al < 0.5 ratios however, the contribution of solutes to trapping phenomena can be very important, and a decrease by 1–2 orders of magnitude of effective O diffusion coefficients can be observed for temperatures around 800–1100 K.

Embrittlement Analysis of sum {{{{5}left[ {{21}0} right]} mathord{left/ {vphantom {{{5}left[ {{21}0} right]} {left( {overline{1}overline{2}0} right)}}} right. kern-nulldelimiterspace} {left( {-{1}-{2}0} right)}}} FeAl Grain Boundary in Presence of Defects: An Ab Initio Study

Iron aluminide (FeAl) inter-metallic compounds are potential candidates for structural applications at high temperatures owing to their superior corrosion resistance, high temperature oxidation, low density and inexpensive material cost. However, the presence of defects can lead to reduction in the strength and ductility of FeAl-based materials. Here we present a density functional theory (DFT) study of the effect of the presence of defects including Fe and Al vacancies as well as H dopants at the substitutional and interstitial sites at a sum {{{{5}left[ {{21}0} right]} mathord{left/ {vphantom {{{5}left[ {{21}0} right]} {left( {overline{1}overline{2}0} right)}}} right. kern-nulldelimiterspace} {left( {overline{1}overline{2}0} right)}}} FeAl grain boundary focusing on the energetics. The plane wave pseudopotential code Vienna Ab initio Simulation Package (VASP) in the generalized gradient approximation (GGA) is used to carry out the computations. The formation energy calculations showed that intrinsic defects such as Fe and Al vacancies probably form at the GB, indicated by their negative formation energies. These vacancies can further form defect complexes with H impurities, indicated by lowered formation energies, compact bonds and charge gain of H atoms. Electronic structure analysis showed stronger hybridization of 1s orbitals of H with Fe and Al atoms, which leads to the stabilization of these defects resulting in degradation of material strength.

Self-diffusion mechanisms based defect complexes in MoSi2

MoSi2 is widely concerned due to excellent electrical conductivity, oxidation resistance as a typical transition metal silicide. The high-temperature diffusion behavior is one of the important factors for the degradation of MoSi2 coatings. However, the diffusion mechanism in MoSi2 is still unclear. Prior theoretical work mostly focused on defect formation energy, but these are not consistent with the self-diffusion experiments because the migration behaviors were not considered. Therefore, the purpose of this work was to investigate the microscopic diffusion mechanisms of Mo and Si atoms in MoSi2 using density functional theory and the CI-NEB method. We confirmed that the temperature-dependent vibrational contribution has a significant impact on the defect formation free energy. The isolated point defects in MoSi2 will tend to aggregate to form defect complexes, which participate in the atomic diffusion as mediators. The defect migration behaviors of atoms for vacancy mediated, vacancy complex mediated, and antisite assisted jumps were obtained based on electronic structures analysis. The results show that Si diffusion is mediated by intrasublattice jumps of the nearest neighbor Si vacancies. Moreover, the destroyed covalent Mo–Si bonds by Si vacancies and the non-directional weak metal bonds formed by the Mo antisites and Mo atoms could improve the mobility of the Mo atom which results in the low migration barrier. The agreement between our calculations and the reported experimental results indicates that the dominant diffusion mechanism for Mo atoms is mediated by vacancy complex mediated jumps and antisite assisted jumps. It is concluded that the Si vacancy-based defect complexes are likely the diffusion mediators for Mo atom self-diffusion in MoSi2. This work provides a deeper insight into the connection between the atomic mechanism and the macroscopic behavior for the diffusion in the MoSi2, and establishes the basis for further optimizing high-temperature coating materials.