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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

First principles investigation of dopants and defect complexes in CdSe$_x$Te$_{1-x}$

Se alloying is a common approach to improve the performance of CdTe solar cells by tuning the bandgap, defect levels, and carrier density. A fundamental understanding of these improvements, specifically the effect of Se alloying on the behavior of defects and dopants in CdTe, remains unclear. Here, in this work, we present a density functional theory (DFT) study of point defect energetics in CdTe and CdSe x Te 1-x with x = 0.25, leading to a comparison of how native defects, dopants (As and Cu), impurities (Cl and O), and related defect complexes behave in CdTe vs CdSe x Te 1-x . Our calculations, performed by combining semi-local and nonlocal hybrid functionals, show a general lowering of the formation energies of native defects as well as substitutional defects formed by As and Cl upon Se addition. For successful p-type doping with As, destabilizing Cl-based defects in the CdSeTe lattice would be essential. We find evidence for some low-energy defect complexes of As, Cl, and O in CdSe 0.25 Te 0.75 . The computed defect formation energies further enable estimates of temperature-dependent defect concentrations and self-consistent Fermi levels. A comparison of defect energetics with the energies of impurity phases reveals that As, Cu, Cl, and O overwhelmingly prefer being segregated to unwanted As 2 O 5 , AsCl 3 , Cd 2 AsCl 2 , and CuO x phases rather than remain at defect sites, but such segregation is less likely to happen in CdSe 0.25 Te 0.75 than in CdTe. Overall, our work presents a list of likely defects and complexes in CdTe and Se-incorporated CdTe, paving the way to explain and mitigate limited dopant activation in experimental observations.

CdTe↗
DOE OSTI2013

Effects of surface oxide layer on nanocavity formation and silver gettering in hydrogen ion implanted silicon

We have made an investigation of the surface oxide effects on nanocavity formation in hydrogen implanted silicon and the influence of resultant nanocavities on diffusion and gettering of implanted silver atoms. A wafer with a 200-nm SiO{sub 2} surface layer was implanted with 22.5 keV H ions to a dose of 1 Multiplication-Sign 10{sup 17} cm{sup -2}, yielding the concentration peak of implanted H ions at {approx}140 nm below the SiO{sub 2}/Si interface. Subsequently, two sets of Si samples were prepared, depending on whether the oxide layer was etched off before (Group-A) or after (Group-B) post-H-implantation annealing. As evidenced by transmission electron microscopy, Group-A samples exhibited an array of large-sized nanocavities in hexagon-like shape, extending from the surface to the depth {approx}140 nm, whereas a narrow band of sphere-shaped nanocavities of small size was present around 140 nm below the surface in Group-B samples. These Si samples with pre-existing nanocavities were further implanted with Ag ions in the surface region ({approx}40 nm projected range) and post-Ag-implantation annealing was conducted in the temperature range between 600 and 900 Degree-Sign C. Measurements based on Rutherford backscattering spectroscopy revealed much different behaviors for Ag redistribution and defect accumulation in these two sets of samples. Compared to the case for Group-B Si, Group-A Si exhibited a lower concentration of residual defects and a slower kinetics in Ag diffusion as well. We discuss the role of thick surface oxide in point defect generation and recombination, and the consequence on nanocavity formation and defect retention in Si. The properties of nanocavities, e.g., their depth distribution, size, and even shape, are believed to be responsible for the observed disparities between these samples, including an interesting contrast of surface vs. bulk diffusion phenomena for implanted Ag atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Low-Temperature epitaxial growth of InGaAs films on InP(100) and InP(411)A substrates

The structural and electrical characteristics of In{sub 0.53}Ga{sub 0.47}As epitaxial films, grown in the low-temperature mode on InP substrates with (100) and (411)A crystallographic orientations at flow ratios of As4 molecules and In and Ga atoms of γ = 29 and 90, have been comprehensively studied. The use of InP(411)A substrates is shown to increase the probability of forming two-dimensional defects (twins, stacking faults, dislocations, and grain boundaries), thus reducing the mobility of free electrons, and As{sub Ga} point defects, which act as donors and increase the free-electron concentration. An increase in γ from 29 to 90 leads to transformation of single-crystal InGaAs films grown on (100) and (411)A substrates into polycrystalline ones.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Electrical degradation of double-Schottky barrier in ZnO varistors

Researches on electrical degradation of double-Schottky barrier in ZnO varistors are reviewed, aimed at the constitution of a full picture of universal degradation mechanism within the perspective of defect. Recent advances in study of ZnO materials by atomic-scale first-principles calculations are partly included and discussed, which brings to our attention distinct cognition on the native point defects and their profound impact on degradation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Structural and optical properties of dense vertically aligned ZnO nanorods grown onto silver and gold thin films by galvanic effect with iron contamination

Highlights: • ZnO nanorods were grown on Au and Ag films in aqueous solution by galvanic effect. • The method is prone to metal contamination which can influence the ZnO properties. • Iron doping improves the lattice matching between ZnO and the substrate. • Energy levels of point defects are lowered and the light emission is red-shifted. • Galvanic-induced nucleation starts and proceeds continuously during the growth. - Abstract: Dense arrays of vertically aligned ZnO nanorods have been grown onto either silver or gold seedless substrates trough a simple hydrothermal method by exploiting the galvanic effect between the substrate and metallic parts. The nanorods exhibit larger bases and more defined hexagonal shapes, in comparison with standard non-galvanic wet-chemistry synthesis. X-ray diffraction (XRD) shows that the iron contamination, associated with the galvanic contact, significantly improves the in-plane compatibility of ZnO with the Au and Ag cubic lattice. Photoluminescence (PL) measurements indicate that the contamination does not affect the number density of localized defects, but lowers their energy levels uniformly; differently, the band-edge emission is not altered appreciably. Finally, we have found that the ZnO hetero-nucleation by galvanic effect initiates at different times in different sites of the substrate area. Our results can be useful for the fabrication of high performance piezonanodevices comprising high-density metal-to-ZnO nanoscaled junctions without intermediate polycrystalline layers.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1993

Electronic characterization of defects in narrow gap semiconductors

The study of point defects in semiconductors has a long and honorable history. In particular, the detailed understanding of shallow defects in common semiconductors traces back to the classic work of Kohn and Luttinger. However, the study of defects in narrow gap semiconductors represents a much less clear story. Here, both shallow defects (caused by long range potentials) and deep defects (from short range potentials) are far from being completely understood. In this study, all results are calculational and our focus is on the chemical trend of deep levels in narrow gap semiconductors. We study substitutional (including antisite), interstitial and ideal vacancy defects. For substitutional and interstitial impurities, the efects of relaxation are included. For materials like Hg(1-x)Cd(x)Te, we study how the deep levels vary with x, of particular interest is what substitutional and interstitial atoms yield energy levels in the gap i.e. actually produce deep ionized levels. Also, since the main technique utilized is Green's functions, we include some summary of that method.

Patterson, James D.↗
DOE OSTI2014

Suppression of tin precipitation in SiSn alloy layers by implanted carbon

By combining transmission electron microscopy and Rutherford backscattering spectrometry, we have identified carbon related suppression of dislocations and tin precipitation in supersaturated molecular-beam epitaxial grown SiSn alloy layers. Secondary ion mass spectrometry has exposed the accumulation of carbon in the SiSn layers after high temperature carbon implantation and high temperature thermal treatment. Strain-enhanced separation of point defects and formation of dopant-defect complexes are suggested to be responsible for the effects. The possibility for carbon assisted segregation-free high temperature growth of heteroepitaxial SiSn/Si and GeSn/Si structures is argued.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Tuning thermal conductivity in homoepitaxial SrTiO{sub 3} films via defects

We demonstrate the ability to tune the thermal conductivity of homoepitaxial SrTiO{sub 3} films deposited by reactive molecular-beam epitaxy by varying growth temperature, oxidation environment, and cation stoichiometry. Both point defects and planar defects decrease the longitudinal thermal conductivity (k{sub 33}), with the greatest decrease in films of the same composition observed for films containing planar defects oriented perpendicular to the direction of heat flow. The longitudinal thermal conductivity can be modified by as much as 80%—from 11.5 W m{sup −1}K{sup −1} for stoichiometric homoepitaxial SrTiO{sub 3} to 2 W m{sup −1}K{sup −1} for strontium-rich homoepitaxial Sr{sub 1+δ}TiO{sub x} films—by incorporating (SrO){sub 2} Ruddlesden-Popper planar defects.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Prediction of Thermal Conductivity for Irradiated SiC/SiC Composites by Informing Continuum Models with Molecular Dynamics Data

This article proposes a new method to estimate the thermal conductivity of SiC/SiC composites subjected to neutron irradiation. The modeling method bridges different scales from the atomic scale to the scale of a 2D SiC/SiC composite. First, it studies the irradiation-induced point defects in perfect crystalline SiC using molecular dynamics (MD) simulations to compute the defect thermal resistance as a function of vacancy concentration and irradiation dose. The concept of defect thermal resistance is explored explicitly in the MD data using vacancy concentrations and thermal conductivity decrements due to phonon scattering. Point defect-induced swelling for chemical vapor deposited (CVD) SiC as a function of irradiation dose is approximated by scaling the corresponding MD results for perfect crystal β-SiC to experimental data for CVD-SiC at various temperatures. The computed thermal defect resistance, thermal conductivity as a function of grain size, and definition of defect thermal resistance are used to compute the thermal conductivities of CVD-SiC, isothermal chemical vapor infiltrated (ICVI) SiC and nearly-stoichiometric SiC fibers. The computed fiber and ICVI-SiC matrix thermal conductivities are then used as input for an Eshelby-Mori-Tanaka approach to compute the thermal conductivities of 2D SiC/SiC composites subjected to neutron irradiation within the same irradiation doses. Predicted thermal conductivities for an irradiated Tyranno-SA/ICVI-SiC composite are found to be comparable to available experimental data for a similar composite ICVI-processed with these fibers.

SiC/SiC composites, thermal conductivity, fiber/ma↗
DOE OSTI2021

Minimizing hydrogen vacancies to enable highly efficient hybrid perovskites

Defect-induced non-radiative losses are currently limiting the performance of hybrid perovskite devices. Experimental reports have indicated the existence of point defects that act as detrimental non-radiative recombination centres under iodine-poor synthesis conditions. However, the microscopic nature of these defects is still unknown. Here we demonstrate that hydrogen vacancies can be present in high densities under iodine-poor conditions in the prototypical hybrid perovskite MAPbI 3 (MA = CH 3 NH 3 ). They act as very efficient non-radiative recombination centres with an exceptionally high carrier capture coefficient of 10 –4 cm 3 s –1 . By contrast, the hydrogen vacancies in FAPbI 3 [FA = CH(NH 2 ) 2 ] are much more difficult to form and have a capture coefficient that is three orders of magnitude lower. Our study unveils the critical but overlooked role of hydrogenvacancies in hybrid perovskites and rationalizes why FA is essential for realizing high efficiency in hybrid perovskite solar cells. Furthermore, minimizing the incorporation of hydrogen vacancies is key to enabling the best performance of hybrid perovskites.

14 SOLAR ENERGY↗
DOE OSTI2015

Rondorfite-type structure — XPS and UV–vis study

Highlights: • Structural and spectroscopic characterization of chlorosilicate mineral, rondorfite. • Characterization of main photoemission lines and valence band spectra. • The study of color origin’s using UV–vis spectroscopy. • Analysis of structural changes in context of origin of natural fluorescence. • Discussion of a new application possibilities of analyzed mineral - Abstract: This paper focuses on X-ray diffraction, X-ray photoelectron and UV–vis spectroscopy of two different (green, orange) rondorfite samples. The differences in the sample color originate from various O/Cl ratios. The orange color was found to be related either to the isomorphic substitution of Fe{sup 3+}/Al{sup 3+} for Mg{sup 2+}, the presence of atypical [MgO{sub 4}] tetrahedrons in crystal structure or electronegativity of the sample. The tetrahedron is known to be very prone to accumulation of impurities and substitute atoms. Moreover, the XPS data showed tetrahedrally coordinated Mg{sup 2+} and isomorphic substitution of Al{sup 3+}/Fe{sup 3+} for Mg{sup 2+}, which influences local disordering and the point defects density and distribution. Non-equilibrium chlorine positions inside the crystal cages as well as Ca-Cl bonds have also been found. The XPS measurements as a function of temperature indicate occurrence of a structural transformation at about 770 K which is accompanied by a rotation of silicate tetrahedra within magnesiosilicate pentamer and luminescence disappearance.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Exploring the relationship between deposition method, microstructure, and performance of Nb/Si-based superconducting coplanar waveguide resonators

Superconducting quantum circuits (SQC) are one of the most promising hardware platforms for quantum computing, yet their performance is currently limited by the presence of various structural defects inside the circuit's structure. Despite impressive progress in the past decade, a precise understanding of the origin of these defects from various fabrication processes and their impact on coherence is still lacking. Here, in this study, we performed a comprehensive investigation on the microstructure, superconductivity, and resonator quality factor of Nb films deposited by high-power impulse magnetron sputtering (HiPIMS) and direct current (DC) magnetron sputtering. A suite of characterization techniques, including electron microscopy with spectroscopy, secondary ion mass spectrometry, magneto-optical microscopy, and pump-probe reflectivity spectroscopy is used. We reveal that niobium (Nb) resonators fabricated using HiPIMS exhibit a smaller average grain size, thicker surface oxide with larger thickness variations (rougher surface), and a thicker amorphous Nb/Si interface layer compared to samples deposited by DC sputtering. We identified that the amorphous Nb oxides (mainly located at the Nb surface and along the grain boundaries) and Nb-Si amorphous layers (at the Nb/Si interface) are major and potential sources of two-level system (TLS), while off-stochiometric oxides and suboxides of Nb close to the surface, crystalline defects (i.e., dislocations at grain boundary, point defects introduced during deposition) are main contributors of non-TLS sources. Our findings clarify the relationship between different defects and coherence loss mechanisms, highlighting the importance of material microstructure control on performance optimization in SQC.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Efficient carrier transport in halide perovskites: theoretical perspectives

Recently, halide perovskites have been shown to exhibit excellent carrier transport properties. Density functional calculations are performed to study the electronic structure, dielectric properties, and defect properties of β-CH 3 NH 3 PbI 3 . Our results show that Pb chemistry plays an important role in a wide range of material properties, i.e., small effective masses, enhanced Born effective charges and lattice polarization, and the suppression of the formation of deep defect levels, all of which contribute to the exceptionally good carrier transport properties observed in CH 3 NH 3 PbI 3 . Defect calculations show that, among native point defects (including vacancies, interstitials, and antisites), only iodine vacancy is a low-energy deep trap and non-radiative recombination centre. Finaly, alloying iodide with chloride reduces the lattice constant of the iodide and significantly increases the formation energy of interstitial defects, which explains the observed substantial increase in carrier diffusion length in mixed halide CH 3 NH 3 PbI 3 Cl compared to that in CH 3 NH 3 PbI 3 .

36 MATERIALS SCIENCE↗
DOE OSTI2014

Shockley-Read-Hall lifetimes in CdTe

A combination of first principles electronic structure calculations, Green's function method, and empirical tight-binding Hamiltonian method is used to evaluate the minority carrier lifetimes of CdTe due to recombination via native point defects in CdTe. For defect energy levels near mid-gap, our calculated value of the Shockley-Read-Hall capture cross section for both electrons and holes is ~10⁻¹³ cm², which is considerably different from the most commonly employed values. We further find that minority carrier lifetimes in doped CdTe are affected more by defect levels closer to the Fermi level than those in the mid-gap.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Atomistic simulation of Er irradiation induced defects in GaN nanowires

Classical molecular dynamics simulation was used to irradiate a GaN nanowire with rear-earth erbium (Er). Ten cumulative irradiations were done using an ion energy of 37.5 keV on a 10 × 10 nm²surface area which corresponds to a fluence of 1 × 10¹³ cm⁻². We studied the location and types of defects produced in the irradiation. Er implantation leads to a net positive (expansion) strain in the nanowire and especially at the top region a clear expansion has been observed in the lateral and axial directions. The lattice expansion is due to the hydrostatic strain imposed by a large number of radiation induced defects at the top of the NW. Due to the large surface-to-volume ratio, most of the defects were concentrated at the surface region, which suggests that the experimentally observed yellow luminescence (YL) in ion implanted GaN NWs arises from surface defects. We observed big clusters of point defects and vacancy clusters which are correlated with stable lattice strain and the YL band, respectively.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Low-noise, radiation tolerant SiCpms

Over the last 10 years, our R&D effort has resulted in a steady progress on improving Geiger detector array technology performance, verifying their reliability and developing three cell layout libraries containing more than 2200 primitive cells for Si and SiC Geiger based single-photon detectors and silicon ROIC array fabrication. Awards from DOE, NASA and DOD, as wells as commercial R&D orders supported much progress, relevant to this program. The expertise gained and the technologies developed or qualified during these programs provided the design database and expertise to carry out the development of this new SiC photomultiplier (SiCpm) technology, successful fabrication of SiCpm detectors, and evaluation of their radiation hardness. After irradiation with protons and neutrons at 1015cm-2 and 1014cm-2 respectively, SiCpms proved radiation hardness, exceeding the expected radiation tolerance The defects induced by irradiation were mainly point defects. A new improved DLTS method (fDLTS – full wave DLTS) hardware and data processing software specific to investigating radiation damage in SiC was developed. Further work is needed to better characterize and control defects induced by charged particles. We recommend further funding of radiation hardened technology development and validation; qualification of device-specific characterization methods like f-DLTS, development of more methods to evaluate defects in SiC, more active involvement of the national labs in evaluating the SiCpm devices, and DOE support of technology transfer to a qualified foundry.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Effect of Mg doping on the structural and free-charge carrier properties of InN films

We present a comprehensive study of free-charge carrier and structural properties of two sets of InN films grown by molecular beam epitaxy and systematically doped with Mg from 1.0 × 10{sup 18} cm{sup −3} to 3.9 × 10{sup 21} cm{sup −3}. The free electron and hole concentration, mobility, and plasmon broadening parameters are determined by infrared spectroscopic ellipsometry. The lattice parameters, microstructure, and surface morphology are determined by high-resolution X-ray diffraction and atomic force microscopy. Consistent results on the free-charge carrier type are found in the two sets of InN films and it is inferred that p-type conductivity could be achieved for 1.0 × 10{sup 18} cm{sup −3} ≲ [Mg] ≲ 9.0 × 10{sup 19} cm{sup −3}. The systematic change of free-charge carrier properties with Mg concentration is discussed in relation to the evolution of extended defect density and growth mode. A comparison between the structural characteristics and free electron concentrations in the films provides insights in the role of extended and point defects for the n-type conductivity in InN. It further allows to suggest pathways for achieving compensated InN material with relatively high electron mobility and low defect densities. The critical values of Mg concentration for which polarity inversion and formation of zinc-blende InN occurred are determined. Finally, the effect of Mg doping on the lattice parameters is established and different contributions to the strain in the films are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Photoluminescent properties of electrochemically synthetized ZnO nanotubes

ZnO nanotubes were prepared by a sequential combination of electrochemical deposition, chemical attack and regeneration. ZnO nanocolumns were initially electrodeposited on conductive substrates and then converted into nanotubes by a process involving chemical etching and subsequent regrowth. The morphology of these ZnO nanocolumns and derived nanotubes was monitored by Scanning Electron Microscopy and their optical properties was studied by photoluminescence spectroscopy. Photoluminescence were measured as a function of temperature, from 6 to 300 K, for both nanocolumns and nanotubes. In order to study the behaviour of induced intrinsic defect all ZnO films were annealed in air at 400 °C and their photoluminescent properties were also registered before and after annealing. The behaviour of photoluminescence is explained taking into account the contribution of different point defects. A band energy diagram related to intrinsic defects was proposed to describe the behaviour of photoluminescence spectra. - Highlights: •ZnO nanotubes were obtained after etching and regrowth of electrodeposited ZnO films. •Photoluminescence spectra contain two parts involving excitonic and defects transitions. •Annealing produces a blue shift in the PL peaks in both ZnO nanocolumns and nanotubes. •Etching causes a blue shift in PL peaks due to confinement effect in nanotubes walls.

36 MATERIALS SCIENCE↗