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DOE OSTI2022

Electronic structure of epitaxially grown and regrown GaN pn junctions characterized by scanning Kelvin probe and capacitance microscopy

Epitaxial regrowth of GaN pn junctions is a key technology for realization of a variety of high-performance GaN power electronic devices. However, the regrowth process can introduce impurities and defects that degrade a device’s performance. Here, we show that scanning Kelvin probe force microscopy and scanning capacitance microscopy can be used in a cross-sectional geometry to probe dopant distributions and an electronic structure in epitaxially grown GaN pn junctions. These measurements enable profiling of potential and dopant distributions across GaN pn junctions produced by uninterrupted epitaxial growth and by regrowth on an etched surface. Clear differences are observed in comparisons to the electronic structure of these two types of junctions that can be correlated with results of complementary characterization of dopant distributions reported for similarly grown structures. These measurements also suggest the presence of defects in etch-and-regrow pn junction structures that extend nearly 1 μm below the regrown interface.

Kim, Tae-Hyeon (ORCID:0000000341530617)↗

DOE OSTI2013

Photoelectrochemistry, Electronic Structure, and Bandgap Sizes of Semiconducting Cu(I)-Niobates and Cu(I)-Tantalates

Semiconducting metal-oxides have remained of intense research interest owing to their potential for achieving efficient solar-driven photocatalytic reactions in aqueous solutions that occur as a result of their bandgap excitation. The photocatalytic reduction of water or carbon dioxide to generate hydrogen or hydrocarbon fuels, respectively, can be driven on p-type (photocathodic) electrodes with suitable band energies. However, metal-oxide semiconductors are typically difficult to dope as p-type with a high mobility of carriers. The supported research led to the discovery of new p-type Cu(I)-niobate and Cu(I)-tantalate film electrodes that can be prepared on FTO glass. New high-purity flux syntheses and the full structural determination of several Cu(I)-containing niobates and tantalates have been completed, as well as new investigations of their optical and photoelectrochemical properties and electronic structures via density-functional theory calculations. For example, CuNbO3, Cu5Ta11O30 and CuNb3O8 were prepared in high purity and their structures were characterized by both single-crystal and powder X-ray diffraction techniques. These two classes of Cu(I)-containing compounds exhibit optical bandgap sizes ranging from ~1.3 eV to ~2.6 eV. Photoelectrochemical measurements of these compounds show strong photon-driven cathodic currents that confirm the p-type semiconductor behavior of CuNbO3, CuNb3O8, and Cu5Ta11O30. Incident-photon-to-current efficiencies are measured that approach greater than ~1%. Electronic-structure calculations based on density functional theory reveal the visible-light absorption stems from a nearly-direct bandgap transition involving a copper-to-niobium or tantalum (d10 to d0) charge-transfer excitations.

14 SOLAR ENERGY↗

DOE OSTI2016

Crystal growth, electronic structure, and properties of Ni-substituted FeGa{sub 3}

Crystals of the Fe{sub 1−x}Ni{sub x}Ga{sub 3} limited solid solution (x<0.045) have been grown from gallium flux. We have explored the electronic structure as well as magnetic and thermoelectric properties of Fe{sub 0.975}Ni{sub 0.025}Ga{sub 3} in comparison with Fe{sub 0.95}Co{sub 0.05}Ga{sub 3}, following the rigid band approach and assuming that one Ni atom donates twice the number of electrons as one Co atom. However, important differences between the Co- and Ni-doped compounds are found below 620 K, which is the temperature of the metal-to-insulator transition for both compounds. We have found that Fe{sub 0.975}Ni{sub 0.025}Ga{sub 3} displays lower degree of spatial inhomogeneity on the local level and exhibits diamagnetic behavior with a broad shallow minimum in the magnetic susceptibility near 35 K, in sharp contrast with the Curie–Weiss paramagnetism of Fe{sub 0.95}Co{sub 0.05}Ga{sub 3}. Transport measurements have shown the maximum of the thermoelectric figure-of-merit ZT of 0.09 and 0.14 at 620 K for Fe{sub 0.975}Ni{sub 0.025}Ga{sub 3} and Fe{sub 0.95}Co{sub 0.05}Ga{sub 3}, respectively. - Graphical abstract: Crystals of Ni-substituted FeGa{sub 3} up to 8 mm long were grown from gallium flux (see Figure for the temperature profile and crystal shape) that allowed studying magnetic and thermoelectric properties of the title solid solution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2011

The electronic structure of the CuRh{sub 1-x}Mg{sub x}O{sub 2} thermoelectric materials: An X-ray photoelectron spectroscopy study

A detailed X-ray photoelectron spectroscopy study has been performed for the CuRh{sub 1-x}Mg{sub x}O{sub 2} (x=0, 0.04 and 0.10) series for a better understanding of the role of the Mg{sup 2+} substitution on the electrical properties and the value of the Seebeck coefficient. This study is based on an analysis of different compounds such as Rh{sub 2}O{sub 3}, Sr{sub 2}RhO{sub 4} and CuCrO{sub 2} in order to characterize different oxidation states (Rh{sup 3+} and Rh{sup 4+} in octahedral oxygen environment and Cu{sup +} in a dumbbell O-Cu-O coordination). The Cu2p signal of copper in the non-doped compounds CuCrO{sub 2} and CuRhO{sub 2} reveals different electronic structures. An evolution of the Cu2p core signal with the increase of Mg{sup 2+} content in the CuRh{sub 1-x}Mg{sub x}O{sub 2} is highlighted by XPS. The differences observed, especially for the Cu2p core peaks are discussed for the non-doped compounds CuCrO{sub 2} and CuRhO{sub 2} as for the CuRh{sub 1-x}Mg{sub x}O{sub 2} series upon Mg{sup 2+} substitution. - Graphical Abstract: The role of the Mg substitution in the CuRh{sub 1-x}Mg{sub x}O{sub 2} as a function of x (x=0, 0.04 and 0.1) has been investigated by following the evolution of the Cu2p spectra obtained by X-ray photoelectron spectroscopy. The filled peaks are assigned to main peak A and A{sub sat} satellites, the hatched peaks to main peak B and B{sub sat} satellites. Highlights: > XPS investigations on CuRh{sub 1-x}Mg{sub x}O{sub 2} (x=0-0.04-0.1) delafossites. > Influence of magnesium doping on to the electrical properties and the electronic structures. > Copper cation keeps two oxidation states Cu{sup +}/Cu{sup 2+} in the CuRh{sub 1-x}Mg{sub x}O{sub 2} (x=0-0.4-0.1) with an increase of the Cu{sup 2+}/Cu ratio with the x value. > Cu layer seems to contribute to complex electrical transport in such delafossites.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2012

Comparison of electronic structure of as grown and solar grade silicon samples

A comparison of the electronic structure of two different types of silicon materials viz., (i) as grown silicon and (ii) solar silicon has been carried out utilizing maximum entropy method and pair distribution function using powder X-ray data sets. The precise electron density maps have been elucidated for the two samples. The covalent nature of the bonding between atoms in both the samples is found to be well pronounced and clearly seen from the electron density maps. The electron densities at the middle of the Si-Si bond are found to be 0.47 and 0.45 e/A{sup 3} for as grown silicon and solar silicon respectively. In this work, the local structural information has also been obtained by analyzing the atomic pair distribution functions of these two samples.

36 MATERIALS SCIENCE↗

DOE OSTI2013

Ab initio simulation of the electronic structure of Ta{sub 2}O{sub 5} crystal modifications

Ab initio simulation of the electronic structure crystalline {beta} and {delta} phases of tantalum(V) oxide (Ta{sub 2}O{sub 5}), representing a promising dielectric material for microelectronics, has been carried out. Both ideal crystals and those with neutral oxygen vacancies in various coordination positions have been studied. The simulation has been performed using the density functional theory with hybrid functionals involving the Hartree-Fock exchange energy. This approach gives a correct description of the bandgap width: 4.1 eV for {beta}-Ta{sub 2}O{sub 5} and 3.1 eV for {delta}-Ta{sub 2}O{sub 5}. The energy levels related to oxygen vacancies in various positions have been determined for the spectra of electron states in {beta}- and {delta}-Ta{sub 2}O{sub 5} polymorphs. It is established that the presence of oxygen vacancies in Ta{sub 2}O{sub 5} crystal modifications leads to the formation of characteristic absorption peaks in their electron energy loss spectra.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2012

On the electronic structure and thermoelectric properties of BiTeBr and BiTeI single crystals and of BiTeI with the addition of BiI{sub 3} and CuI

The electronic structures were calculated for BiTeBr and BiTeI using the density-functional theory approach and accounting for the strong spin-orbital interaction. Qualitatively, the band structures for two compounds are similar, showing strong mixing of the p states of all elements in vicinity of the Fermi level, with the band gaps of 0.595 and 0.478 eV for BiTeBr and BiTeI, respectively. The optimized crystal structures show a tendency for the Bi-X (X=Br, I) bond elongation compared to the Bi-Te one. Both compounds are intrinsic n-type semiconductors but display a metallic-like conductivity coupled to rather large thermopower, which is rationalized within the frames of the acoustic phonons scattering model. Because of larger thermopower BiTeBr exhibits a twice higher thermoelectric figure-of-merit near room temperature, ZT=0.17, compared to BiTeI. The addition of 1 mass% of BiI{sub 3} or CuI to BiTeI decreases the mobility of electrons by two orders of magnitude, leading to significantly lower electrical conductivity, but at the same time effectively reduces the thermal conductivity. The prospects of further enhancing the thermoelectric efficiency are briefly discussed. - Graphical abstract: View of the crystal structure of BiTeBr is shown in the figure The optimized crystal structures show a tendency for the Bi-X (X=Br, I) bond elongation compared to the Bi-Te one. The electronic structures were calculated for BiTeBr and BiTeI using the density-functional theory approach and accounting for the strong spin-orbital interaction. Qualitatively, the band structures for two compounds are similar, showing strong mixing of the p states of all elements in vicinity of the Fermi level, with the band gaps of 0.595 and 0.478 eV for BiTeBr and BiTeI, respectively. Both compounds are intrinsic n-type semiconductors but display a metallic-like conductivity coupled to rather large thermopower, which is rationalized within the frames of the acoustic phonons scattering model. The addition of 1 mass% of BiI{sub 3} or CuI to BiTeI effectively reduces the thermal conductivity. The prospects of further enhancing the thermoelectric efficiency are briefly discussed. Highlights: Black-Right-Pointing-Pointer BiTeBr and BiTeI feature mixing of p states of Bi, Te, and halogen near Fermi level. Black-Right-Pointing-Pointer BiTeBr has thermoelectric figure-of-merit ZT=0.17, two times that of BiTeI. Black-Right-Pointing-Pointer 1% CuI or BiI{sub 3} decrease dramatically electron mobility in BiTeI. Black-Right-Pointing-Pointer 1% CuI decreases thermal conductivity of BiTeI by a factor of 4, reaching 0.5 W m{sup -1} K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

Electronic structure basis for the extraordinary magnetoresistance in WTe 2

The electronic structure basis of the extremely large magnetoresistance in layered non-magnetic tungsten ditelluride has been investigated by angle-resolved photoelectron spectroscopy. Hole and electron pockets of approximately the same size were found at the Fermi level, suggesting that carrier compensation should be considered the primary source of the effect. The material exhibits a highly anisotropic, quasi one-dimensional Fermi surface from which the pronounced anisotropy of the magnetoresistance follows. As a result, a change in the Fermi surface with temperature was found and a high-density-of-states band that may take over conduction at higher temperatures and cause the observed turn-on behavior of the magnetoresistance in WTe₂ was identified.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2013

DFT investigation on the electronic structure of Faujasite

We report here first-principle pseudopotential DFT calculations to investigate relevant aspects of the electronic structure of zeolites based FAU. Fundamental molecular issues of the band-gap and electronic population analysis were reviewed under GGA/RPBE level of theory, corroborated with a DZP basis set and Troullier-Martins norm conserving pseudo-potentials. The atom-projected density of states and the analysis of HOMO-LUMO frontier orbitals at Gamma point were performed. Their electronic transfers are discussed through the alignment and relative positions of orbitals in order to determine the way that the molecule interacts with adsorbed molecules and other practical applications. Mulliken population analysis was employed for describing atomic charge distribution in the chosen systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Electronic Structure of LiCoO 2 Surfaces and Effect of Al Substitution

The surface properties of LiCoO 2 nanoplatelets, and their chemical modifications with Al 3+ , were studied using combined experimental and theoretical approaches. Our model shows that the electronic structures of several LiCoO 2 surface facets are significantly different from those of bulk LiCoO 2 , due to altered spin states of surface Co 3+ atoms. O K -edge X-ray absorption revealed a prominent splitting of the Co 3d -O 2p states, which is attributed to the presence of intermediate-state and/or high-spin-state Co 3+ . at the surface. In particular, the nonpolar (104) surface with intermediate-spin-state Co 3+ exhibits a strong two-dimensional character of splitting in these states, whereas the polar (001) surface with low-spin-state Co 3+ has a similar electronic structure to bulk LiCoO 2 . Here, partial substitution of Co 3+ by Al 3+ through the formation of core-shell architectures increases the ratio of low-spin-state Co3+, resulting in a distinct change in the intensity ratio of the split Co 3d -O 2p states, as revealed by spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Layer-Dependent Pressure Effect on the Electronic Structure of 2D Black Phosphorus

Through infrared spectroscopy, we systematically study the pressure effect on electronic structures of few-layer black phosphorus (BP) with layer number ranging from 2 to 13. We reveal that the pressure-induced shift of optical transitions exhibits strong layer dependence. In sharp contrast to the bulk counterpart which undergoes a semiconductor to semimetal transition under ~1.8 GPa, the band gap of 2 L increases with increasing pressure until beyond 2 GPa. Meanwhile, for a sample with a given layer number, the pressure-induced shift also differs for transitions with different indices. Through the tight-binding model in conjunction with a Morse potential for the interlayer coupling, this layer- and transition-index-dependent pressure effect can be fully accounted. Our study paves a way for versatile van der Waals engineering of two-dimensional BP.

2-dimensional systems↗

DOE OSTI2019

Structural analysis of Gd 6 FeBi 2 from single-crystal X-ray diffraction methods and electronic structure calculations

The crystal structure of the gadolinium iron bismuthide Gd 6 FeBi 2 has been characterized by single-crystal X-ray diffraction data and analyzed in detail using first-principles calculations. The structure is isotypic with the Zr 6 CoAl 2 structure, which is a variant of the ZrNiAl structure and its binary prototype Fe 2 P (Pearson code hP 9, Wyckoff sequence g f d a ). As such, the structure is best viewed as an array of tricapped trigonal prisms of Gd atoms centered alternately by Fe and Bi. The magnetic-ordering temperature of this compound ( ca 350 K) is much higher than that of other rare-earth metal-rich phases with the same or related structures. It is also higher than the ordering temperature of many other Gd-rich ternary phases, where the magnetic exchange is typically governed by Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions. First-principles calculations reveal a larger than expected Gd magnetic moment, with the additional contribution arising from the Gd 5 d electrons. The electronic structure analysis suggests strong Gd 5 d –Fe 3 d hybridization to be the cause of this effect, rather than weak interactions between Gd and Bi. These details are of importance for understanding the magnetic response and explaining the high ordering temperature in this material.

Chemistry↗

DOE OSTI2015

Crystal and Electronic Structures of Complex Bismuth Iodides A 3 Bi 2 I 9 ( A = K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics

Ternary bismuth halides form an interesting functional materials class in the context of the closely related Pb halide perovskite photovoltaics, especially given the significantly reduced toxicity of Bi when compared with Pb. The compounds A 3 Bi 2 I 9 (A = K, Rb, Cs) examined here crystallize in two different structure types: the layered defect-perovskite K 3 Bi 2 I 9 type, and the Cs 3 Cr 2 Cl 9 type. The latter structure type features isolated Bi 2 I 9 3– anions. Here, the crystal structures of the ternary iodides are redetermined and a corrected structural model for Rb 3 Bi 2 I 9 , as established by single crystal X-ray diffraction and solid state 87Rb NMR spectroscopy and supported by density functional theory (DFT) calculations is presented. A variety of facile preparation techniques for single crystals, bulk materials, as well as solution-processed thin films are described. The optical properties and electronic structures are investigated experimentally by optical absorption and ultraviolet photoemission spectroscopy and computationally by DFT calculations. Absolute band positions of the valence and conduction bands of these semiconductors, with excellent agreement of experimental and calculated values, are reported, constituting a useful input for the rational interface design of efficient electronic and optoelectronic devices. The different structural connectivity in the two different structure types, somewhat surprisingly, appears to not impact band positions or band gaps in a significant manner. Computed dielectric properties, including the finding of anomalously large Born effective charge tensors on Bi 3+ , suggest proximal structural instabilities arising from the Bi 3+ 6s 2 lone pair. These anomalous Born effective charges are promising for defect screening and effective charge carrier transport. The structural, electronic, and optical properties of the complex bismuth iodides are to some extent similar to the related lead iodide perovskites. In conclusion, the deeper valence band positions in the complex bismuth iodides point to the need for different choices of hole transport materials for Bi-iodide based solar cell architectures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2015

Technical report on "BES Early Career. Control Graphene Electronic Structure for Energy Technology"

Graphene, a one-atom thick sheet of carbon, exhibits incredible structural flexibility, electrical transport, and optical responses. And remarkably, the graphene electronic structure can be varied through interlayer coupling, nanoscale patterning, and electrical gating. In this project we made significant contribution to better understand and control physical properties of graphene and other novel two-dimensional layered materials.

36 MATERIALS SCIENCE↗

DOE OSTI2018

The stability, electronic structure, and optical absorption of boron-nitride diamondoids predicted with first-principles calculations

Although diamondoids are broadly studied for their fundamental properties and applications, boron-nitride-based diamondoids are scarcely explored.In this report we predict the stability, electronic structure, and optical absorption spectra of six boron-nitride (BN) diamondoids with first-principles methods based on pseudopotential density functional theory and many-body perturbation methods implemented with a real-space formalism. We find that four of them are thermodynamically stable at room temperature, while B10N8H24 and B6N4H16 show thermodynamic instability in molecular dynamics simulations. With the GW approximation, we predicted the ionization energies and electron affinities of BN-diamondoids and find that the evolution of the electronic structure with size does not follow the same trend as diamondoids, owing to the unbalanced numbers of boron and nitrogen atoms. We show strong photoabsorption of BN-triamantane and BN-adamantane in the infrared and visible ranges and analyze the features of low-energy absorption by examining the characteristics of related orbitals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Simulation of the Electronic Structure of Graphene–Polyvinylidene Fluoride Composite Material

Theoretical models of shockproof composite materials based on two-layer graphenes and multilayer polyvinylidene fluoride C{sub 124}H{sub 40} + n(H–(C{sub 2}H{sub 2}F{sub 2}){sub 5}–H) (n = (1, 8)-bar ) are constructed. The electronic structure is studied using the semiempirical PM3 method that is one of version of the molecular orbital method. The orbital energies, ionization potentials, total electron energies, strength, and other properties of the considered material are calculated based on the theoretical models. The outlooks for application of these materials in the military field for manufacturing superstrong and lightweight flak jackets are considered.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Atomic-layer-resolved composition and electronic structure of the cuprate B i 2 S r 2 CaC u 2 O 8 + δ from soft x-ray standing-wave photoemission

A major remaining challenge in the superconducting cuprates is the unambiguous differentiation of the composition and electronic structure of the CuO 2 layers and those of the intermediate layers. The large c axis for these materials permits employing soft x-ray (930.3 eV) standing wave (SW) excitation in photoemission that yields atomic layer-by-layer depth resolution of these properties. Applying SW photoemission to Bi 2 Sr 2 CaCu 2 O 8+δ yields the depth distribution of atomic composition and the layer-resolved densities of states. We detect significant Ca presence in the SrO layers and oxygen bonding to three different cations. The layer-resolved valence electronic structure is found to be strongly influenced by the atomic supermodulation structure, as determined by comparison to density functional theory calculations, by Ca-Sr intermixing, and by correlation effects associated with the Cu 3 d -3 d Coulomb interaction, further clarifying the complex interactions in this prototypical cuprate. Here, measurements of this type for other quasi-two-dimensional materials with large c represent a promising future direction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Multiband semimetallic electronic structure of superconducting Ta 2 PdSe 5

We report the electronic structure and related properties of the superconductor Ta 2 PdSe 5 as determined from density functional calculations. The Fermi surface has two disconnected sheets, both derived from bands of primarily chalcogenide p states. These are a corrugated hole cylinder and a heavier complex shaped electron sheet. The sheets contain 0.048 holes and a compensating number of electrons per formula unit, making the material a semimetallic superconductor. The results support the presence of two band superconductivity, although a discrepancy in the specific heat is noted. This discrepancy is discussed as a possible consequence of Pd deficiency in samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗