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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Depth-dependent recovery of thermal conductivity after recrystallization of amorphous silicon

The depth-dependent recovery of silicon thermal conductivity was achieved after the recrystallization of silicon that had been partially amorphized due to ion implantation. Transmission electron microscopy revealed nanoscale amorphous pockets throughout a structurally distorted band of crystalline material. The minimum thermal conductivity of as-implanted composite material was 2.46 W m −1 K −1 and was found to be uniform through the partially amorphized region. X-ray diffraction measurements reveal 60% strain recovery of the crystalline regions after annealing at 450 °C for 30 min and almost full strain recovery and complete recrystallization after annealing at 700 °C for 30 min. In addition to strain recovery, the amorphous band thickness reduced from 240 to 180 nm after the 450 °C step with nanoscale recrystallization within the amorphous band. A novel depth-dependent thermal conductivity measurement technique correlated thermal conductivity with the structural changes, where, upon annealing, the low thermal conductivity region decreases with the distorted layer thickness reduction and the transformed material shows bulk-like thermal conductivity. Full recovery of bulk-like thermal conductivity in silicon was achieved after annealing at 700 °C for 30 min. After the 700 °C anneal, extended defects remain at the implant projected range, but not elsewhere in the layer. Previous results showed that high point-defect density led to reduced thermal conductivity, but here, we show that point defects can either reform into the lattice or evolve into extended defects, such as dislocation loops, and these very localized, low-density defects do not have a significant deleterious impact on thermal conductivity in silicon.

Huynh, Kenny↗
DOE OSTI2014

Microstructure and conductance-slope of InAs/GaSb tunnel diodes

InAs/GaSb and similar materials systems have generated great interest as a heterojunction for tunnel field effect transistors (TFETs) due to favorable band alignment. However, little is currently understood about how such TFETs are affected by materials defects and nonidealities. We present measurements of the conductance slope for various InAs/GaSb heterojunctions via two-terminal electrical measurements, which removes three-terminal parasitics and enables direct study on the effect of microstructure on tunnelling. Using this, we can predict how subthreshold swings in TFETs can depend on microstructure. We also demonstrate growth and electrical characterization for structures grown by metalorganic chemical vapor deposition (MOCVD)—a generally more scalable process compared with molecular beam epitaxy (MBE). We determine that misfit dislocations and point defects near the interface can lead to energy states in the band-gap and local band bending that result in trap-assisted leakage routes and nonuniform band alignment across the junction area that lower the steepness of the conductance slope. Despite the small lattice mismatch, misfit dislocations still form in InAs on GaSb due to relaxation as a result of large strain from intermixed compositions. This can be circumvented by growing GaSb on InAs, straining the GaSb underlayer, or lowering the InAs growth temperature in the region of the interface. The conductance slope can also be improved by annealing the samples at higher temperatures, which we believe acts to annihilate point defects and average out major fluctuations in band alignment across the interface. Using a combination of these techniques, we can greatly improve the steepness of the conductance slope which could result in steeper subthreshold swings in TFETs in the future.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Engineering the formation of spin-defects from first principles

The full realization of spin qubits for quantum technologies relies on the ability to control and design the formation processes of spin defects in semiconductors and insulators. We present a computational protocol to investigate the synthesis of point-defects at the atomistic level, and we apply it to the study of a promising spin-qubit in silicon carbide, the divacancy (VV). Our strategy combines electronic structure calculations based on density functional theory and enhanced sampling techniques coupled with first principles molecular dynamics. We predict the optimal annealing temperatures for the formation of VVs at high temperature and show how to engineer the Fermi level of the material to optimize the defect’s yield for several polytypes of silicon carbide. Our results are in excellent agreement with available experimental data and provide novel atomistic insights into point defect formation and annihilation processes as a function of temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2020

High Performance GaAs Solar Cells Grown at High Growth Rates by Atmospheric-Pressure Dynamic Hydride Vapor Phase Epitaxy

We report GaAs solar cells grown at rates exceeding 300 um/h by atmospheric-pressure dynamic hydride vapor phase epitaxy (D-HVPE). The surface morphology of the GaAs layers grown at those rates is smooth, with root mean square surface roughness of < 1.4 nm. We grew GaAs solar cells at rates from 35 - 309 um/h with negligible degradation in open circuit voltages (VOC), which ranged from 1.04 - 1.07 V. We showed the slight decrease in VOC is due to lack of doping optimization at higher growth rates. Deep level transient spectroscopy measurements showed low point defect densities that increases slightly with growth rate from 35 - 309 um/h. The low point defect concentrations and the high VOCs indicate that high material quality can be maintained at these extremely high growth rates, enabling the deposition of high efficiency cells in mere seconds instead of hours.

14 SOLAR ENERGY↗
NASA NTRS1994

CoSb(sub 3)-IrSb(sub 3)Solid Solutions: Preparations and Characterization

The binary antimonide compounds of the skutterudite family have a good potential for thermoelectric applications as recent results at JPL have shown. However, the room temperature thermal conductivity of these materials is about 10 Wm(sup -1)K(sup -1) with an 80 % contribution from the lattice. To achieve maximum ZT values, the lattice thermal conductivity needs to be lowered. Because the lattice thermal conductivity of solid solutions can substantially decrease due to the addition of point defect phonon scattering (all state-of-the-art thermoelectric materials are solid solutions), a study of the formation of solid solutions between the antimonide skutterudite compounds is of interest. The preparation and characterization of both p-type and n-type CoSb(sub 3)-IrSb(sub 3) solid solutions by several techniques is reviewed in this paper. Experimental results showed that solid solutions were successfully formed in a wide range of compositions. The changes in the thermoelectric properties indicated the influence of a strong point defect scattering, resulting in large decreases in Hall mobility and thermal conductivity. The potential for high ZT values in CoSb(sub 3)-IrSb(sub 3) solid solutions is discussed.

mobility↗
DOE OSTI2015

Slow relaxation of cascade-induced defects in Fe

On-the-fly kinetic Monte Carlo (KMC) simulations are performed to investigate slow relaxation of non-equilibrium systems. Point defects induced by 25 keV cascades in α -Fe are shown to lead to a characteristic time-evolution, described by the replenish and relax mechanism. Then, we produce an atomistically-based assessment of models proposed to explain the slow structural relaxation by focusing on the aggregation of 50 vacancies and 25 self-interstital atoms (SIA) in 10-lattice-parameter α-Fe boxes, two processes that are closely related to cascade annealing and exhibit similar time signature. Four atomistic effects explain the timescales involved in the evolution: defect concentration heterogeneities, concentration-enhanced mobility, cluster-size dependent bond energies and defect-induced pressure. In conclusion, these findings suggest that the two main classes of models to explain slow structural relaxation, the Eyring model and the Gibbs model, both play a role to limit the rate of relaxation of these simple point-defect systems.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Combined Impact of Heterogeneous Lifetime and Gettering on Solar Cell Performance

We couple numerical process and device simulations to provide a framework for understanding the combined effects of as-grown wafer impurity distribution, processing parameters, and solar cell architecture. For this study, we added the Impurity-to-Efficiency simulator to Synopsys’ Sentaurus Process software using the Alagator Scripting Language. Our results quantify how advanced processing can eliminate differences in efficiency due to different as-grown impurity concentrations and due to different area fractions of defective wafer regions. We identify combinations of as-grown impurity distributions and process parameters that produce solar cells limited by point defects and those that are limited by precipitated impurities. Gettering targeted at either point defect or precipitate reduction can then be designed and applied to increase cell efficiency. We also visualize the post-processing iron and total recombination distributions in 2D maps of the wafer cross-section. PV researchers and companies can input their initial iron distributions and processing parameters into our software and couple the resulting process simulation results with a solar cell device design of interest to conduct their own analyses. The Alagator scripts we developed are freely available online at http://pv.mit.edu/impurity-to-efficiency-i2e-simulator-for-sentaurus-tcad/.

14 SOLAR ENERGY↗
DOE OSTI2016

Evolution of small defect clusters in ion-irradiated 3C-SiC: Combined cluster dynamics modeling and experimental study

Distribution of black spot defects and small clusters in 1 MeV krypton irradiated 3C-SiC has been investigated using advanced scanning transmission electron microscopy (STEM) and TEM. We find that two thirds of clusters smaller than 1 nm identified in STEM are invisible in TEM images. For clusters that are larger than 1 nm, STEM and TEM results match very well. A cluster dynamics model has been developed for SiC to reveal processes that contribute to evolution of defect clusters and validated against the (S)TEM results. Simulations showed that a model based on established properties of point defects (PDs) generation, reaction, clustering, and cluster dissociation, is unable to predict black spot defects distribution consistent with STEM observations. This failure suggests that additional phenomena not included in a simple point-defect picture may contribute to radiation-induced evolution of defect clusters in SiC and using our model we have determined the effects of a number of these additional phenomena on cluster evolution. Using these additional phenomena it is possible to fit parameters within physically justifiable ranges that yield agreement between cluster distributions predicted by simulations and those measured experimentally.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Intrinsic Defects in MoS 2 Grown by Pulsed Laser Deposition: From Monolayers to Bilayers

Pulsed laser deposition (PLD) can be considered a powerful method for the growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) into van der Waals heterostructures. However, despite significant progress, the defects in 2D TMDs grown by PLD remain largely unknown and yet to be explored. In this work, we combine atomic resolution images and first-principles calculations to reveal the atomic structure of defects, grains, and grain boundaries in mono- and bilayer MoS 2 grown by PLD. We find that sulfur vacancies and MoS antisites are the predominant point defects in 2D MoS 2 . We predict that the aforementioned point defects are thermodynamically favorable under a Mo-rich/S-poor environment. The MoS 2 monolayers are polycrystalline and feature nanometer size grains connected by a high density of grain boundaries. In particular, the coalescence of nanometer grains results in the formation of 180° mirror twin boundaries consisting of distinct 4- and 8-membered rings. We show that PLD synthesis of bilayer MoS 2 results in various structural symmetries, including AA' and AB, but also turbostratic with characteristic moiré patterns. Moreover, we report on the experimental demonstration of an electron beam-driven transition between the AB and AA' stacking orientations in bilayer MoS 2 . These results provide a detailed insight into the atomic structure of monolayer MoS 2 and the role of the grain boundaries on the growth of bilayer MoS 2 , which has importance for future applications in optoelectronics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Computationally predicted energies and properties of defects in GaN

Recent developments in theoretical techniques have significantly improved the predictive power of density-functional-based calculations. In this article, we discuss how such advancements have enabled improved understanding of native point defects in GaN. We review the methodologies for the calculation of point defects, and discuss how techniques for overcoming the band-gap problem of density functional theory affect native defect calculations. In particular, we examine to what extent calculations performed with semilocal functionals (such as the generalized gradient approximation), combined with correction schemes, can produce accurate results. The properties of vacancy, interstitial, and antisite defects in GaN are described, as well as their interaction with common impurities. We also connect the first-principles results to experimental observations, and discuss how native defects and their complexes impact the performance of nitride devices. Overall, we find that lower-cost functionals, such as the generalized gradient approximation, combined with band-edge correction schemes can produce results that are qualitatively correct. However, important physics may be missed in some important cases, particularly for optical transitions and when carrier localization occurs.

36 MATERIALS SCIENCE↗
DOE OSTI2016

A low-temperature study of manganese-induced ferromagnetism and valence band convergence in tin telluride

SnTe is renowned for its promise in advancing energy-related technologies based on thermoelectricity and for its topological crystalline insulator character. Here, we demonstrate that each Mn atom introduces ∼4 μ{sub B} (Bohr magneton) of magnetic moment to Sn{sub 1−x}Mn{sub x}Te. The Curie temperature T{sub C} reaches ∼14 K for x = 0.12, as observed in the field dependent hysteresis of magnetization and the anomalous Hall effect. In accordance with a modified two-band electronic Kane model, the light L-valence-band and the heavy Σ-valence-band gradually converge in energy with increasing Mn concentration, leading to a decreasing ordinary Hall coefficient R{sub H} and a favorably enhanced Seebeck coefficient S at the same time. With the thermal conductivity κ lowered chiefly via point defects associated with the incorporation of Mn, the strategy of Mn doping also bodes well for efficient thermoelectric applications at elevated temperatures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Fracture modes in tubular LSFCO ceramic membranes under graded reducing conditions

Highlights: ► Microstructural evolution in LSFCO membranes under graded environment is reported. ► The role of chemically induced stresses and oxygen deficiency is evaluated. ► The stress distribution is modeled by a point defect model. - Abstract: Chromium (III) oxide (Cr{sub 2}O{sub 3})-doped LaSrFeO{sub 3} perovskite, La{sub 0.2}Sr{sub 0.8}Fe{sub 0.8}Cr{sub 0.2}O{sub 3−δ} (LSFCO), is being considered as a potential material for applications in solid oxide fuel cells, gas separation membranes, and electrochemical reactors because of its high electro-catalytic activity. Similar to other perovskites, the performance and mechanical strength of LSFCO materials are significantly affected by environment and temperature. Here, we report a fracture gradient phenomenon in tubular C-ring-shaped LSFCO ceramic membranes under graded reducing conditions. The graded reducing condition was produced by flushing N{sub 2} on the outer side of the C-ring membranes at 1000 °C while keeping the inner side untreated. The rings were then diametrically compressed to fracture, and the resultant fracture morphology was analyzed with a scanning electron microscope (SEM). A fracture gradient with three distinct regions across the thickness of the membranes was identified on the split surfaces. In the outer region of the C-ring specimen exposed to N{sub 2}, a mixed inter/transgranular fracture with a predominant intergranular pattern was observed. In the middle section of the fracture surface, a characteristic transgranular fracture of the perovskite grains was found. At the inner region of the ring, a mixed inter/transgranular fracture with a predominant transgranular pattern occurred. The mechanism of gradient fractures was attributed both to chemically induced stresses caused by oxygen diffusion and to the formation of a separate phase of oxygen-deficient perovskite in the parent perovskite. The stresses generated were modeled by a point defect model. This work provides significant information on microstructure evolutions of tubular LSFCO membranes under graded reducing atmospheres.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Impact of pre-existing disorder on radiation defect dynamics in Si

The effect of pre-existing lattice defects on radiation defect dynamics in solids remains unexplored. Here, we use a pulsed beam method to measure the time constant of defect relaxation for 500 keV Ar ion bombardment of Si at 100 °C with the following two representative types of pre- existing lattice disorder: (i) point defect clusters and (ii) so-called “clamshell” defects consisting of a high density of dislocations. Results show that point defect clusters slow down defect relaxation processes, while regions with dislocations exhibit faster defect interaction dynamics. These experimental observations demonstrate that the dynamic aspects of damage buildup, attributed to defect trapping-detrapping processes, can be controlled by defect engineering.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Incoherent twin boundary migration induced by ion irradiation in Cu

Grain boundaries can act as sinks for radiation-induced point defects. The sink capability is dependent on the atomic structures and varies with the type of point defects. Using high-resolution transmission electron microscopy, we observed that {Sigma}3{l_brace}112{r_brace} incoherent twin boundary (ITB) in Cu films migrates under Cu{sup 3+} ion irradiation. Using atomistic modeling, we found that {Sigma}3{l_brace}112{r_brace} ITB has the preferred sites for adsorbing interstitials and the preferential diffusion channels along the Shockley partial dislocations. Coupling with the high mobility of grain boundary Shockley dislocations within {Sigma}3{l_brace}112{r_brace} ITB, we infer that {Sigma}3{l_brace}112{r_brace} ITB migrates through the collective glide of grain boundary Shockley dislocations, driven by a concurrent reduction in the density of radiation-induced defects, which is demonstrated by the distribution of nearby radiation-induced defects.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Successes and failures of Hubbard-corrected density functional theory: The case of Mg doped LiCoO{sub 2}

We have evaluated the successes and failures of the Hubbard-corrected density functional theory approach to study Mg doping of LiCoO{sub 2}. We computed the effect of the U parameter on the energetic, geometric, and electronic properties of two possible doping mechanisms: (1) substitution of Mg onto a Co (or Li) site with an associated impurity state and (2) formation of impurity-state-free complexes of substitutional Mg and point defects in LiCoO{sub 2}. We find that formation of impurity states results in changes on the valency of Co in LiCoO{sub 2}. Variation of the Co U shifts the energy of the impurity state, resulting in energetic, geometric, and electronic properties that depend significantly on the specific value of U. In contrast, the properties of the impurity-state-free complexes are insensitive to U. These results identify reasons for the strong dependence on the doping properties on the chosen value of U and for the overall difficulty of achieving agreement with the experimentally known energetic and electronic properties of doped transition metal oxides such as LiCoO{sub 2}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Impact of implanted phosphorus on the diffusivity of boron and its applicability to silicon solar cells

Boron diffusivity reduction in extrinsically doped silicon was investigated in the context of a process combination consisting of BBr{sub 3} furnace diffusion and preceding Phosphorus ion implantation. The implantation of Phosphorus leads to a substantial blocking of Boron during the subsequent Boron diffusion. First, the influences of ion implantation induced point defects as well as the initial P doping on B diffusivity were studied independently. Here, it was found that not the defects created during ion implantation but the P doping itself results in the observed B diffusion retardation. The influence of the initial P concentration was investigated in more detail by varying the P implantation dose. A secondary ion mass spectrometry (SIMS) analysis of the BSG layer after the B diffusion revealed that the B diffusion retardation is not due to potential P content in the BSG layer but rather caused by the n-type doping of the crystalline silicon itself. Based on the observations the B diffusion retardation was classified into three groups: (i) no reduction of B diffusivity, (ii) reduced B diffusivity, and (iii) blocking of the B diffusion. The retardation of B diffusion can well be explained by the phosphorus doping level resulting in a Fermi level shift and pairing of B and P ions, both reducing the B diffusivity. Besides these main influences, there are probably additional transient phenomena responsible for the blocking of boron. Those might be an interstitial transport mechanism caused by P diffusion that reduces interstitial concentration at the surface or the silicon/BSG interface shift due to oxidation during the BBr{sub 3} diffusion process. Lifetime measurements revealed that the residual (non-blocked) B leads to an increased dark saturation current density in the P doped region. Nevertheless, electrical quality is on a high level and was further increased by reducing the B dose as well as by removing the first few nanometers of the silicon surface after the BBr{sub 3} diffusion.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Ultrafast laser and swift heavy ion irradiation: Response of Gd 2 O 3 and ZrO 2 to intense electronic excitation

In order to investigate the response of materials to extreme conditions, there are several approaches to depositing extremely high concentrations of energy into very small volumes of material, including ultrafast laser and swift heavy ion (SHI) irradiation. In this study, crystalline-to-crystalline phase transformations in cubic Gd 2 O 3 and monoclinic ZrO 2 have been investigated using ultrafast laser irradiation. The phases produced by the extreme conditions of irradiation were characterized by grazing incidence x-ray diffraction (GIXRD) and Raman spectroscopy. Gd 2 O 3 exhibited a cubic-to-monoclinic phase transformation, as evidenced by the appearance of the monoclinic (40$\bar{2}$), (003), (310), and (112$\bar{2}$) peaks in the GIXRD pattern and of four A g and three B g Raman modes. ZrO 2 underwent a monoclinic-to-tetragonal phase transformation, as evidenced by the emergence of the tetragonal (101) peak in the GIXRD pattern and of E g and A 1g Raman modes. The new phases formed by ultrafast laser irradiation are high temperature polymorphs of the two materials. No evidence of amorphization was seen in the GIXRD data, though Raman spectroscopy indicated point defect accumulation. These results are identical to those produced by irradiation with SHIs, which also deposit energy in materials primarily through electronic excitation. Finally, the similarity in damage process and material response between ultrafast laser and SHI irradiation suggests a fundamental relationship between these two techniques.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Radiation-induced vacancy injection in heterogeneous multiphase materials

Understanding the synergy between corrosion and defect dynamics is a key consideration in the development of advanced materials for extreme environments. Here, we reveal a surprising phenomenon for the transport of point defects induced by irradiation in a heterogeneous multiphase structure of a metal and an oxide, similar to that formed under metal corrosion that takes place in most environments. Despite the confinement of the produced damage within the oxide, vacancies were injected into the unirradiated metal and coarsened with dose. Furthermore, the results show that the nature of the oxide layer dictates the defect evolution in the metal layer. This work reveals an interesting mechanism for point defect interactions in multiphase materials, with broad implications in many fields, while also emphasizing the complex coupling between corrosion and irradiation. Corrosion leads to the formation of multiphase materials, while irradiation enhances diffusion within the heterogeneous phases, which can impact corrosion rates.

36 MATERIALS SCIENCE↗