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Precise measurement of charged defects in III-V compounds (supplement 2)
Experimental methods and related theory which will permit the measurement of low concentrations of vacancies and other defects in III-V compound semiconductors are discussed. Once the nature of these defects has been determined, this information can be incorporated into a transport theory for devices constructed from these materials, and experiments conducted to test the theory. The vacancies and other defects in the III-V compounds are detected by measurement of the nuclear magnetic resonance (NMR) line width. Most of the III-V compounds have at least one isotope with a nuclear quadrupole moment. In a crystal with a cubic crystal field (characteristic of most III-V compounds) there is no quadrupole splitting of the Zeeman resonance line. However, a defect removes the cubic symmetry locally and causes splitting which result in a change of the NMR width. This change can be used to detect the presence of vacancies.
Modeling charged defects and defects levels in semiconductors and oxides with DFT: An improved inside-out perspective.
Abstract not provided.
Modeling charged defects and defect levels in semiconductors and oxides with DFT: An improved inside-out perspective.
Abstract not provided.
Modeling charged defects and defect levels in semiconductors and oxides with density functional theory ? an improved inside-out perspective.
Abstract not provided.
Fundamental Resolution of Difficulties in the Theory of Charged Point Defects in Semiconductors
A defect’s formation energy is a key theoretical quantity that allows the calculation of equilibrium defect concentrations in solids and aids in the identification of defects that control the properties of materials and device performance, efficiency, and reliability. The theory of formation energies is rigorous only for neutral defects, yet the Coulomb potentials of charged defects require additional ad hoc numerical procedures. In this report, we invoke statistical mechanics to derive a revised theory of charged-defect formation energies, which eliminates the need for ad hoc numerical procedures. Calculations become straightforward and transparent. We introduce calculations demonstrating the significance of the revised theory for defect formation energies and thermodynamic transition levels.
CHARGED POINT DEFECTS AND DEFECT COMPLEXES IN PHOSPHORENE AND STRUCTURAL PREDICTION OF TAU-11, A DENSITY FUNCTIONAL THEORY APPROACH
With the advent of powerful computers, first principle calculations have become a robust and reliable tool to complement experiments in the study of materials. First principle calculations such as density functional theory can be used to guide experiments to save time and effort when it comes to new and advanced materials discovery. Study of charge defects in materials and crystal structure prediction are such areas in which density functional theory calculations facilitate the prediction of defect formation energy, charge transition levels and elemental site occupations. Experimentally, studying charge defects requires inferring results from various techniques. Likewise, determining site occupancies for similar elements using experimental techniques is challenging and sometimes impossible. In this work, we study the charged intrinsic and extrinsic defects and defect complexes in phosphorene and determine the site occupancies of Al, Fe and Si in the τ11 phase using density functional theory. In the defect work, we calculate the defect formation energy, charge transition level, binding energies and Stokes shift for vacancy, dopant substitution and dopant-vacancy defect complexes in phosphorene. We found that vacancy defect in phosphorene becomes negatively charged in n-type doping and may passivate the dopants and reduce carrier concentration and mobility. For non-metal dopants in phosphorene, we predict that O, S and Si prefer to form dopant-vacancy complexes removing the vacancy defect states from the band gap. Mn dopant-vacancy defect complex exhibits possibility of switching between two magnetic spin states. Lastly, using density functional theory, we complement the results from neutron powder diffraction to determine the site occupancies of Al and Si in τ11 phase which was used to determine the alloying element to develop a high temperature and low density alloy.
Electrometry by optical charge conversion of deep defects in 4H-SiC
Optically active point defects in various host materials, such as diamond and silicon carbide (SiC), have shown significant promise as local sensors of magnetic fields, electric fields, strain, and temperature. Modern sensing techniques take advantage of the relaxation and coherence times of the spin state within these defects. Here we show that the defect charge state can also be used to sense the environment, in particular high-frequency (megahertz to gigahertz) electric fields, complementing established spin-based techniques. This is enabled by optical charge conversion of the defects between their photoluminescent and dark charge states, with conversion rate dependent on the electric field (energy density). The technique provides an all-optical high-frequency electrometer which is tested in 4H-SiC for both ensembles of divacancies and silicon vacancies, from cryogenic to room temperature, and with a measured sensitivity of 41±8(V/cm) 2 /√Hz. In conclusion, due to the piezoelectric character of SiC, we obtain spatial 3D maps of surface acoustic wave modes in a mechanical resonator.
All-optical control of charge-trapping defects in rare-earth doped oxides
Abstract Charge-trapping defects in crystalline solids play important roles in applications ranging from microelectronics, optical storage, sensing and quantum technologies. On one hand, depleting trapped charges in the host matrix reduces charge noise and enhances coherence of solid-state quantum emitters. On the other hand, stable charge traps can enable high-density optical storage systems. Here we report all-optical control of charge-trapping defects via optical charge trapping (OCT) spectroscopy of a rare-earth ion doped oxide (Y 2 O 3 ). Charge trapping is realized by low intensity optical excitation in the 200–375 nm range. Charge detrapping or depletion is carried out by optically stimulated luminescence (OSL) under 532 nm stimulation. Using a Pr-doped Y 2 O 3 polycrystalline ceramic host matrix, we observe charging pathways via the inter-band optical absorption of Y 2 O 3 and via the 4f-5d transitions of Pr 3+ . We demonstrate effective control of the density of trapped charges within the Y 2 O 3 matrix at ambient environment. These results point to a viable method for controlling the local charge environment in rare-earth doped crystals via all-optical means, and pave the way for further development of efficient optical storage technologies with ultrahigh storage capacity, as well as for the localized control of quantum coherence in rare-earth doped solids.
Chemical Interpretation of Charged Point Defects in Semiconductors: A Case Study of Mg 2 Si
The electronic structures of charged point defects influence electrical and optical properties of semiconductors. Understanding the orbital interactions responsible for the electronic structures of defects therefore promotes a chemical intuition for defect-driven mechanisms in semiconductors. In this tutorial, we discuss a molecular orbital theory-based framework for understanding defect-induced electronic states based on local chemical interactions between the defect and the atoms surrounding the defect site. By using Mg 2 Si as a case study, we show how both the chemical interactions and molecular orbitals (i. e., wave functions) responsible for the charge state(s) of a defect can be understood from the bonding symmetry of the defect site. We anticipate that a chemistry-based perspective of charged defects will enrich defect engineering efforts for electronic and optical materials.
Self-regulation mechanism for charged point defects in hybrid halide perovskites
Hybrid halide perovskites such as methylammonium lead iodide (CH 3 NH 3 PbI 3 ) exhibit unusually low free-carrier concentrations despite being processed at low-temperatures from solution. We demonstrate, through quantum mechanical calculations, that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to self-regulate the concentration of charge carriers through ionic compensation of charged point defects. The equilibrium charged vacancy concentration is predicted to exceed 0.4 % at room temperature. Furthermore, this behavior, which goes against established defect conventions for inorganic semiconductors, has implications for photovoltaic performance.
Motional characteristics of positively charged muonium defects in In{sub 2}O{sub 3}
We report on a study of the motional characteristics of positively charged muonium defect centers in In{sub 2}O{sub 3} powder. Zero field muon spin relaxation (ZF-MuSR) measurements were taken from 2 K to 950 K. Results show the positively charged muonium defects occupying two states (Mu{sub 1} and Mu{sub 2}) at low temperatures while a third state (Mu{sub 3}) is introduced as the temperature is increased. Mu{sub 1} is occupied more heavily than Mu{sub 2} at low temperatures by a ratio of ∼8:1. The Mu1 state changes to the Mu{sub 2} state starting at 300 K with site change energy of 0.46±0.11 eV and is essentially not occupied above 500 K. The Mu2 state becomes diffusively mobile with a barrier of 0.78±0.07 eV at 350 K and begins trapping at Mu{sub 3} at 400 K with a capture energy of 0.56±0.019 eV. A metastable region is observed between 500 K and 650 K in which the ratio between Mu{sub 2} and Mu{sub 3} amplitudes and the hop rate of Mu{sub 2} are both roughly constant, implying a steady state trap and release balance between mobile Mu{sub 2} centers and the Mu{sub 3} trap state. Above 650 K, the muonium defects release from the Mu{sub 3} trap to the Mu{sub 2} diffusive state with a dissociation energy of 0.901±0.003 eV. The upper limit on the high temperature diffusion barrier is determined, from the Mu{sub 2} hop rate, to be 0.43±0.03 eV.
Quantum defects in silicon
The dataset used in the publication titled “High-throughput identification of spin-photon interfaces in silicon. Using an automated first-principles computational workflow, we construct a dataset that details simple charged defects in silicon that are relevant for spin-photon interface applications. It encompasses properties vital for quantum defect design, including charged defect stability, excitation energy (determined using the single-shot hybrid functional approach), emission brightness, electronic structures, and more. The dataset encompasses all charged defects stable within the energy gap. We emphasized the importance of defect-bound excitons as telecom band emitters and proposed promising candidates for spin-photon interfaces.
Charged point defect benchmark of Hybrid and GGA-PBE
Data used by the publication "High-throughput calculations of charged point defect properties with semi-local density functional theory - performance benchmarks for materials screening applications." This work presented an in-depth benchmark analysis of automated, semi-local point defect calculations with a-posteriori corrections, compared to 245 “gold standard” hybrid calculations previously published. We considered three different a-posteriori correction sets for semi-local calculations, implemented in a fully automated workflow, and consider the qualitative and quantitative differences for four different categories of defect information: thermodynamic transition levels, formation energies, fermi levels, and dopability limits. We highlighted the type of qualitative information about point defect properties that can be extracted from high-throughput calculations based on semi-local DFT methods, while also demonstrating the limits of quantitative accuracy that can be achieved by these approaches.
Chloride Reduction of Mn 3+ in Mild Hydrothermal Synthesis of a Charge Ordered Defect Pyrochlore, CsMn 2+ Mn 3+ F 6 , a Canted Antiferromagnet with a Hard Ferromagnetic Component
Geometrically frustrated systems play an important role in studying new physical phenomena and unconventional thermodynamics. Charge ordered defect pyrochlores AM 2+ M 3+ F 6 offer a convenient platform for probing the interplay between electron distribution over M 2+ and M 3+ sites and structural distortions; however, they are limited to compounds with M 2+/3+ = V, Fe, Ni, and Cu due to difficulties in the simultaneous stabilization of other 3d elements in the +2 and +3 oxidation states. Herein, we employ Cl– anions under hydrothermal conditions for the mild reduction of Mn 2 O 3 in concentrated HF to obtain the CsMn 2+ Mn 3+ F 6 composition as a phase pure sample and study its properties. The magnetism of CsMn 2 F 6 was characterized by measuring the magnetic susceptibility and isothermal magnetization data, and a magnetic transition to a canted antiferromagnet state was found at 24.1 K. We determined the magnetic structure of CsMn 2 F 6 using powder neutron diffraction, which revealed successive long-range ordering of the Mn 2+ and Mn 3+ sites that is accompanied by a second transition. As a result, the role and strength of magnetic exchange interactions were characterized using DFT calculations.
Influence of structural defects on charge density waves in 1T-TaS2
Abstract The influence of intrinsic defects of 1T-TaS 2 on charge density waves (CDWs) is studied using scanning tunneling microscopy and spectroscopy (STM, STS), angle-resolved photoelectron spectroscopy (ARPES), and density functional theory (DFT). We identify several types of structural defects and find that most have a local character limited to a single CDW site, with a single exception which effectively behaves as a dopant, leading to band-bending and affecting multiple neighboring sites. While only one type of defect can be observed by STM topographic imaging, all defects are easily resolved in STS mapping. Our results indicate modulation of the Mott band gap commensurate with the CDW and breaking of the three-fold symmetry of electronic states. DFT calculations (with included Coulomb interactions) are used to investigate the electronic structure, focusing on both sulfur vacancy and oxygen-sulfur substitution. The sulfur vacancy system, characterized with a metallic behavior, is identified as the origin of one of the experimentally observed defects. Additionally, the effect of oxidation of 1T-TaS 2 depends on the substitution site, leading to the heterogeneity of electronic properties.
Milestone Report: Grain growth and fission gas behaviour in doped UO 2 M2MS-18LA0201034
Here, the microstructure of UO 2 can be modified during nuclear fuel fabrication by using additives. Several dopants (e.g. Cr, Ti, V, Mg, Nb) are used to enhance grain growth and densification during sintering. In previous work, we used atomic scale simulation techniques to identify a common interstitial solution mechanism for a range of dopants that form positively charged defects at sintering temperatures. As a result, negatively/positively charged defect concentrations were enhanced/suppressed relative to undoped fuel under the same conditions (temperature and oxygen partial pressure). Importantly U and fission gas migration is vacancy mediated, such that the increase in the concentration uranium vacancies (which have negative charge), due to doping, causes an increase in U and fission gas diffusivity. This is important because U diffusivity is the rate limiting step in mass transport during sintering and bulk fission gas diffusivity is the underlying property that governs fission gas release. In this work, we implement analytical expressions for uranium vacancy concentrations in doped fuel (based on atomic scale results) into mesoscale and engineering scale simulations of sintering and fission gas release. It is shown that densification is dramatically enhanced at 1850 K and 1950 K due to dopant solution and that Mn-doping has a greater effect than the more widely used Cr-doping. The effect of both dopants is strongly temperature dependent and is negligible below approximately 1800 K. The effect of enlarged grain size and enhanced fission gas diffusivity has been included in BISON. Fuel temperatures under normal operating conditions are not sufficient to activate the enhanced fission gas diffusivity mechanism predicted by atomic scale calculations. Therefore, the benefits of enlarged grains for reduced fission gas release should not undermined by enhanced fission gas diffusivity for doped UO 2 .