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All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Defect States in Copper Indium Gallium Selenide Solar Cells from Two-Wavelength Excitation Photoluminescence Spectroscopy

We use two-wavelength excitation photoluminescence spectroscopy to probe defect states in CIGS thin films. Above-Eg excitation is combined with a tunable IR bias light that modulates the population of the defect states. We find that IR illumination in the range of 1400-2000 nm (0.62-0.89 eV) causes a reduction of the PL intensity, the magnitude of which scales linearly with IR power. Further, KF post deposition treatment has only a modest influence on the effect of the IR excitation. Initial data suggest that we have developed an optical characterization tool for band-gap defect states.

14 SOLAR ENERGY↗
DOE OSTI2021

Understanding the Origin of Tabula Rasa Process-Induced Defects in CZ n-type c-Si

High-temperature annealing, known as Tabula Rasa (TR) has been the subject of recent studies as a method to make n-type CZ silicon material resistant to temperature-induced and process-induced lifetime degradation during solar cell fabrication. In this work, we study the defects responsible for the lifetime changes after TR treatment. We use temperature and injection dependent lifetime spectroscopy (T-IDLS) and the recently introduce defect parameter contour mapping to identify the Energy Level (E t ) and capture cross-section ratio (k) of the most likely process-induced defect. We also present a closer look at changes in surface recombination velocity before and after TR annealing.

bulk lifetime↗
DOE OSTI2015

Confirmation of the Dominant Defect Mechanism in Amorphous In-Zn-O Through the Application of In Situ Brouwer Analysis

The dominant point defect mechanism of amorphous (a-) indium zinc oxide (IZO) was probed through in situ electrical characterization of sputtered a-IZO thin films in response to changes in oxygen partial pressure (pO2) at 300 degrees C. The results yielded a power law dependence of conductivity (s) versus pOinline image of ~-1/6. This experimental method, known as Brouwer analysis, confirms doubly-charged oxygen vacancies as the dominant defect species in a-IZO. The success of this study suggests that Brouwer analysis is a viable method for studying the defect mechanisms of amorphous oxides.

14 SOLAR ENERGY↗
DOE OSTI2016

X-ray photoelectron spectroscopy for identification of morphological defects and disorders in graphene devices

The progress in the development of graphene devices is promising, and they are now considered as an option for the current Si-based electronics. However, the structural defects in graphene may strongly influence the local electronic and mechanical characteristics. Although there are well-established analytical characterization methods to analyze the chemical and physical parameters of this material, they remain incapable of fully understanding of the morphological disorders. In this study, x-ray photoelectron spectroscopy (XPS) with an external voltage bias across the sample is used for the characterization of morphological defects in large area of a few layers graphene in a chemically specific fashion. For the XPS measurements, an external +6 V bias applied between the two electrodes and areal analysis for three different elements, C1s, O1s, and Au4f, were performed. By monitoring the variations of the binding energy, the authors extract the voltage variations in the graphene layer which reveal information about the structural defects, cracks, impurities, and oxidation levels in graphene layer which are created purposely or not. Raman spectroscopy was also utilized to confirm some of the findings. This methodology the authors offer is simple but provides promising chemically specific electrical and morphological information.

42 ENGINEERING↗
DOE OSTI2016

Defect structure of TiS{sub 3} single crystals of the A-ZrSe{sub 3} type

The defect structure of TiS{sub 3} single crystals of the A-ZrSe{sub 3} type has been determined based on X-ray diffraction data. Shear defects manifest themselves as displacements of ab layers (which can imitate a twin) by ∼0.5a. Regular shears facilitate the formation of a superstructure along the c axis. A model of defect in the layer structure is proposed to explain the atomic displacements at an angle to the layer plane.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Soliton-like defects in nematic liquid crystal thin layers

The nonsingular soliton-like defects in plane nematic liquid crystal (NLC) layers and spherical NLC drops are experimentally detected and studied when the interaction of NLC molecules with a bounding surface is varied. The dynamics and the annihilation of nonsingular defects of opposite signs on a plane surface are investigated. Periodic transformations of the soliton-like defects in NLC drops in an electric field are detected. The theory of elasticity is used to show that the surface energy taken into account in the total free energy of NLC in the case of weak anchoring leads to the possibility of nonsingular solutions of a director equilibrium equation. The calculated pictures of director distribution in a plane NLC layer and in a spherical NLC drop characterized by weak surface anchoring agree well with the results of polarized light optical observations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Atomistic Simulation of the Fission-Fragment-Induced Formation of Defects in a Uranium–Molybdenum Alloy

The formation of defects in a uranium–molybdenum alloy induced by xenon and zirconium ion irradiation with energies typical of fission fragments has been simulated within the two-temperature atomistic model and Monte Carlo method. This has allowed estimating the total number of primary radiation defects formed at the “thermal spike” stage and in collision cascades for different temperatures. The calculated threshold stopping powers of ions responsible for the formation of defects at the thermal spike stage are in good agreement with the available theoretical and experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Temperature dependence of the atomic structure and electrical activity of defects in ZnSb thermoelectric lightly doped with copper

A model for describing the temperature dependence of the defect microstructure in high-efficiency ZnSb thermoelectric with a copper content of 0.1 at % is chosen. The temperature dependences of the chargecarrier density and mobility for thermal cycle I (300–700–300 K) are analyzed taking into account the features of the crystal structure and covalent chemical bond in ZnSb. The basic defect structure (at temperatures of T = 560–605 K) is the state when all Cu atoms are equally distributed between sites of both sublattices and behave as acceptors, and the number of intrinsic donor and acceptor defects is much smaller. The effect of the latter becomes noticeable when the temperature goes beyond the above-mentioned range. At T > 605 K, extra acceptors (antisite Zn{sub Sb}) occur; upon cooling below 560 K, Cu{sub 2} dimers arise and the electrical activity of the impurity lowers. Dimer decay upon heating leads to growth in the concentration with temperature up to saturation in the above-mentioned range. Additional thermal cycles II–VIII are performed; the observed changes in the temperature dependences of the hole concentration and mobility are discussed in the context of the investigated model.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of the Electric Mode and γ Irradiation on Surface-Defect Formation at the Si–SiO{sub 2} Interface in a MOS Transistor

The results of experimental investigation into surface-defect formation under the effect of gamma-radiation with a dose rate P = 0.1 rad(Si)/s on MOS (metal-oxide-semiconductor) transistors with the n-type channel in the passive and active modes are presented. Two stages of surface-defect formation are observed. A qualitative model is proposed to explain the effect of the the drain transistor voltage on the defect formation process.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Variation of a Defect Structure of Lithium Tetraborate (Li{sub 2}B{sub 4}O{sub 7}) in an External Electric Field

The variation of a defect structure of a lithium tetraborate single crystal under the influence of a high-strength external electric field applied along polar direction [001] has been studied by the X-ray diffraction (XDR) method. The conductivity kinetics has been measured; it is found to agree with changes in the diffraction peak parameters. Application of the electric field with the strength of 300–500 V/mm leads to a sharp broadening of the rocking curve and the increase in the integral intensity by several times, but the curve position and shape are only slightly changed. At higher electric fields from 500 to 1500 V/mm, the process of broadening the curve slows down; however, the shape asymmetry appears and the peak shifts to smaller angles, which is due to an increase in the lattice parameter along axis c. In this case, the changes become irreversible, since the distorted structure is partially recovered with a very low rate (for several months). Two types of the dependences of the rocking curves parameters variation under an external field are interpreted as the manifestation of two mechanisms of the ionic conduction due to mobile lithium (Li{sup +}) ions at low fields and oxygen vacancies (V{sub O}{sup 2+}) at higher fields. The charge carrier migration leads to an increase in the defect concentration and structural changes in a near-surface crystal region. The obtained results have practical importance from the point of view of the controlled change in the defect structure in the crystals with ionic conductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Luminescence of Pr{sup 3+} Impurity Centers and Defects in Sr{sub 9}Sc (PO{sub 4}){sub 7}:Pr{sup 3+}

The spectroscopic characteristics and luminescence decay kinetics of Sr{sub 9}Sc(PO{sub 4}){sub 7}:Pr{sup 3+}(1%) were investigated using synchrotron radiation of ultraviolet (UV), vacuum ultraviolet (VUV) and X-ray ranges, as well as pulsed cathodoluminescence (PCL) methods at temperatures of 10, 90, and 295 K. The luminescence spectra contain three groups of bands and lines. In the UV range, the observed bands correspond to interconfigurational 5d → 4f transitions in Pr{sup 3+} ions. In the visible spectrum, the wide luminescence band of defects dominates, as well as narrow lines associated with intraconfigurational 4f → 4f transitions in Pr{sup 3+} ions. When excited by an electron beam (5 Hz), the main component with a decay time τ ∼ 17 ns dominates in the decay kinetics of the 5d → 4f luminescence. The decay kinetics of 5d → 4f luminescence upon excitation with high-frequency (τ ∼ 8 MHz) synchrotron radiation of the X-ray range contains a fast component (τ ∼ 15 ns) against the background of the slow component of the μs-range. An effective nonradiative energy transfer is observed from impurity centers to defects, as well as from the host to Pr{sup 3+} ions and defects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Defect Mode in Microwave Waveguide Bragg Structures with Metal Pins

A waveguide Bragg structure containing equidistant cylindrical pins that are galvanically coupled to a wide wall of the waveguide is used to implement frequency response functions characterized by the presence of a band gap. Characteristics of a defect mode of the microwave photonic crystal with a pin element as a defect with an n–i–p–i–n structure with controlled conductivity placed in the capacitive gap are experimentally studied and calculated. Controlled reflectance of a microwave signal with a dynamic range of greater than 50 dB is obtained at the frequency of the defect mode.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Defect Structure of GaAs Layers Implanted with Nitrogen Ions

Structural defects formed in epitaxial GaAs layers as a result of 250-keV N{sup +} ion implantation to doses within 5 × 10{sup 14}–5 × 10{sup 16} cm{sup –2} have been studied by the X-ray diffraction (XRD) and transmission electron microscopy techniques. No amorphization of the ion-implanted layer took place in the entire dose range studied. The implantation to doses of 5 × 10{sup 14} and 5 × 10{sup 15} cm{sup –2} led to the appearance of an additional peak on XRD curves, which was related to the formation of a stressed GaAs layer with positive deformation arising due to the formation of point-defect clusters. The implantation to a dose of 5 × 10{sup 16} cm{sup –2} led to the formation of a dense structure of extended defects in the implanted layer, which was accompanied by the relaxation of macrostresses to the initial state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

A Study of the Influence Exerted by Structural Defects on Photoluminescence Spectra in n-3C-SiC

Photoluminescence (PL) spectra have been studied in 3C-SiC/4H-SiC heterostructures and 3C‑SiC single crystals. It was shown that epitaxial 3C-SiC layers grown on 4H-SiC substrates have a markedly poorer crystal perfection than do 3C-SiC single crystals. It was found that doping with aluminum gives rise to a characteristic PL both in epitaxial layers and in 3C-SiC single crystals. At the same time, the electron irradiation of epitaxial layers does not give rise to defect-related PL, in contrast to what is observed for single crystals. An assumption is made that the twin boundaries existing in epitaxial 3C-SiC layers can serve as getters of radiation defects that are components of donor–acceptor pairs responsible for the “defect-related” PL.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Migration of defect clusters and xenon-vacancy clusters in uranium dioxide

The possible transition states, minimum energy paths and migration mechanisms of defect clusters and xenon-vacancy defect clusters in uranium dioxide have been investigated using the dimer and the nudged elastic-band methods. The nearby O atom can easily hop into the oxygen vacancy position by overcoming a small energy barrier, which is much lower than that for the migration of a uranium vacancy. A simulation for a vacancy cluster consisting of two oxygen vacancies reveals that the energy barrier of the divacancy migration tends to decrease with increasing the separation distance of divacancy. For an oxygen interstitial, the migration barrier for the hopping mechanism is almost three times larger than that for the exchange mechanism. Xe moving between two interstitial sites is unlikely a dominant migration mechanism considering the higher energy barrier. A net migration process of a Xe-vacancy pair containing an oxygen vacancy and a xenon interstitial is identified by the NEB method. We expect the oxygen vacancy-assisted migration mechanism to possibly lead to a long distance migration of the Xe interstitials in UO2. The migration of defect clusters involving Xe substitution indicates that Xe atom migrating away from the uranium vacancy site is difficult.

Defect clusters↗
DOE OSTI2014

Defects in Epitaxial Lift-Off Thin Si Films/Wafers and Their Influence on the Solar Cell Performance

In this paper, we will describe the nature of defects and impurities in thick epitaxial-Si layers and their influence on the cell efficiency. These wafers have very low average dislocation density. Stacking faults (SFs) are the main defect in epi layers. They can occur in many configurations - be isolated, intersecting, and nested. When nested, they can be accompanied by formation of coherent twins resulting in dendritic growth, with pyramids protruding out of the wafer surface. Such pyramids create large local stresses and punch out dislocations. The main mechanism of dislocation formation is through pyramids. Stacking faults degrade solar cell performance. Analyses of the solar cells have revealed that the nested SFs have a controlling effect on the solar cell performance. A well-controlled growth can minimize defect generation and produce wafers that can yield cell efficiencies close to 20%.

14 SOLAR ENERGY↗
DOE OSTI2014

Universality of phase transition dynamics: topological defects from symmetry breaking

In the course of a non-equilibrium continuous phase transition, the dynamics ceases to be adiabatic in the vicinity of the critical point as a result of the critical slowing down (the divergence of the relaxation time in the neighborhood of the critical point). This enforces a local choice of the broken symmetry and can lead to the formation of topological defects. The Kibble-Zurek mechanism (KZM) was developed to describe the associated nonequilibrium dynamics and to estimate the density of defects as a function of the quench rate through the transition. During recent years, several new experiments investigating formation of defects in phase transitions induced by a quench both in classical and quantum mechanical systems were carried out. At the same time, some established results were called into question. We review and analyze the Kibble-Zurek mechanism focusing in particular on this surge of activity, and suggest possible directions for further progress.

74 ATOMIC AND MOLECULAR PHYSICS↗