Binary Chalcogenides Research Articles

Role of cation diffusion in the formation mechanism and properties of cobalt-doped n-type pyrite thin films

Binary and ternary chalcogenides are relevant compounds for photovoltaic and thermoelectric energy conversion processes. The prototype material in this wide family is FeS2 (pyrite). In this work, we have investigated the simultaneous formation and doping (n-type) of FeS2 thin films. Sulphuration of Fe over Co metallic bilayers at constant pressure and temperature has allowed to get the necessary experimental information. Through the analysis of the chemical composition (Rutherford backscattering/elastic recoil detection analysis, energy dispersive X-ray) and structural (XRD) experimental data we have been able to propose a detailed mechanism to explain the formation of pyrite doped with Co thin films. In this mechanism the most relevant role is played by the diffusion of cationic (Fe and Co) species. Measured transport properties of the films reflect the different steps of the formation and doping processes.

Topological Insulators – From Materials Design to Reality

AbstractTopological insulators (TIs) are a new quantum state of matter discovered in recent years. They are beyond the spontaneous symmetry‐breaking description by Landau and are instead characterized by topological invariants, and described by topological field theory. Their topological nature is similar to the quantum Hall effect, a major discovery of condensed‐matter physics in 1980s (Klaus von Klitzing, Nobel Prize in Physics, 1985). The manifestation of the topological effect is the existence of robust gapless surface states inside the bulk energy gap. The topological surface states exhibit Dirac‐cone‐like energy dispersion with strong spin‐momentum locking. Potential future applications cover areas such as spintronics, thermoelectrics, quantum computing and beyond.It is remarkable that TIs have been realized in many common materials, without the requirement of extreme conditions such as high magnetic field and low temperature. The first TI was predicted in 2006 and experimentally realized in 2007 in HgTe quantum wells. Soon afterwards, three traditionally well‐known binary chalcogenides, Bi2Se3, Bi2Te3 and Sb2Te3, were predicted and observed to be TIs with a large bulk gap and a metallic surface state consisting of a single Dirac cone. The discovery of these topological materials opened up the exciting field of topological insulators. Extensive experimental and theoretical efforts are devoted to synthesizing and optimizing samples, characterizing the topological states by surface sensitive spectroscopy, transport measurements, device fabrications, and searching for new material candidates.The field of TIs is now expanding at a rapid pace in the communities of physics, chemistry and materials science. In this Focus Issue, we intend to present a high‐quality snapshot of the materials and applications aspect of this field.We present ten Review papers from both experiment and theory aspects. Five experimental papers [1–5] overview recent status and challenges of TI nanostructures [1], magnetotransport and induced superconductivity [2], chemistry of Bi‐based TI materials [3], molecular beam epitaxial growth of TI thin films [4], and angle‐resolved photoemission spectroscopy (ARPES) with circular dichroism [5]. On the other hand, five theoretical papers [6–10] report the progress from different perspectives: materials design by first‐principles calculations [6, 7], the relations between TIs and thermoelectric materials [8], Floquet TIs [9], and the classification of topological states [10].We present ten Letters that cover various aspects, ranging from ARPES, transport measurement and devices, thin film growth to first‐principles simulations and fundamental theory. Letters on ARPES [11–14] report the surface states of HgTe [11], Bi2Se3 [12, 13] and Bi2Te3 [11, 14], in which the surface modification, defect doping and electron–phonon coupling are discussed; a paper on transport experiments [15] demonstrates the coexistence of electron‐ and hole‐type charge carriers in devices of Sb2Te3/Bi2Te3 heterostructures; the growth of YPtSb thin film is reported [16], which is a Heusler compound near the boundary of topological trivial–nontrivial transition. Corresponding to the ARPES experiments, a Letter of band structure calculations [17] also reveals the effect of vacancy defects on Bi2Se3 surface states; another paper [18] shows the dependence of edge state dispersion on edge geometry of graphene. Last but not the least, two papers on phenomenological models [19, 20] report the maximally localized flat‐band Hamiltonians and the spectra flow for Aharonov–Bohm rings, respectively.We hope that this Focus Issue will be helpful for your research and stimulate more activity in the exciting field of topological insulators (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis and characterisation of Cu2ZnSnSe4 thin films prepared via a vacuum evaporation-based route

Different sequentially stacked binary chalcogenide layers (CuSe, ZnSe, and SnSe) deposited by vacuum evaporation onto molybdenum covered soda-lime glass substrates were used as precursors to form Cu2ZnSnSe4 films. The influence of the stacked binary layer sequence, substrate temperature, both the duration and speed of deposition and the post deposition treatment atmosphere on the structural and the morphological parameters of the Cu2ZnSnSe4 thin films was studied. Our results indicate the possibility of replacing the Se2 selenisation with a thermal treatment in an SnSe2 atmosphere to avoid the selenisation of the Mo substrate and MoSe2 formation. This SnSe2 treatment forms p-type Cu2ZnSnSe4 films with an optical band-gap of 1.14eV and a solar cell structure with an efficiency of up to 3%.

Air-stable solution processed Cu2ZnSn(Sx,Se(1-x))4 thin film solar cells: influence of ink precursors and preparation process

ABSTRACTQuaternary semiconductors, Cu2ZnSnS4 and Cu2ZnSnSe4 which contain only earth-abundant elements, have been considered as the alternative absorber layers to Cu(In,Ga)Se2 (CIGS) for thin film solar cells although CIGS-based solar cells have achieved efficiencies over 20 %. In this work we report an air-stable route for preparation of Cu2ZnSn(Sx,Se(1-x))4 (CZTSSe) thin film absorbers by a solution process based on the binary and ternary chalcogenide nanoparticle precursors dispersed in organic solvents. The CZTSSe absorber layers were achieved by spin coating of the ink precursors followed by annealing under Ar/Se atmosphere at temperature up to 580°C. We have investigated the influence of the annealing temperature on the reduction or elimination of detrimental secondary phases. X-ray diffraction combined with Raman spectroscopy was utilized to better identify the secondary phases existing in the absorber layers. Solar cells were completed by chemical bath deposited CdS buffer layer followed by sputtered i-ZnO/ZnO: Al bi-layers and evaporated Ni/Al grids.

High-Efficiency Solution-Processed Cu2ZnSn(S,Se)4 Thin-Film Solar Cells Prepared from Binary and Ternary Nanoparticles

A new solution-based method to fabricate Cu(2)ZnSn(S,Se)(4) (CZTSSe) thin films is presented. Binary and ternary chalcogenide nanoparticles were synthesized and used as precursors to form CZTSSe thin films. The composition of the CZTSSe films can be easily controlled by adjusting the ratio of the nanoparticles used. The effect of compositional adjustment on device performance is illustrated. Laboratory-scale photovoltaic cells with 8.5% total-area efficiency (or 9.6% active-area efficiency) were demonstrated without anti-reflective coatings. Material characterization data revealed the formation of a bilayer microstructure during thermal processing and suggested a path forward on device improvement.

STRUCTURAL, ELECTRONIC AND OPTICAL PROPERTIES OF MgSxSe1-x, MgSxTe1-x AND MgSexTe1-x (0 ≤x ≤1) ALLOYS FROM FIRST PRINCIPLES

First principles total energy calculations have been performed using full potential linear augmented plane wave method (FP-LAPW) within density functional theory to study the structural, electronic and optical properties of MgS x Se 1-x, MgS x Te 1-x and MgSe x Te 1-x alloys in the rock salt crystallographic phase. The generalized gradient approximation parameterization scheme has been used for calculating the ground state structural parameters and their deviation from the Vegard's law has been discussed. Full relativistic electronic band structures and density of states have been calculated to study the electronic properties of the end binary compounds and ternary alloys MgS x Se 1-x, MgS x Te 1-x and MgSe x Te 1-x (0.25 < x < 0.75). Optical bowing for these semiconductor alloys has been discussed in term of volume deformation, electronegativity and structural relaxation. Optical properties of the binary and ternary magnesium chalcogenides have been calculated in terms of the complex dielectric function and the results are compared with available theoretical and experimental data.

Trialkylphosphine-Stabilized Copper(I) Gallium(III) Phenylchalcogenolate Complexes: Crystal Structures and Generation of Ternary Semiconductors by Thermolysis

A series of organometallic trialkylphosphine-stabilized copper gallium phenylchalcogenolate complexes [(R(3)P)(m)Cu(n)Me(2-x)Ga(EPh)(n+x+1)] (R = Me, Et, (i)Pr, (t)Bu; E = S, Se, Te; x = 0, 1) has been prepared and structurally characterized by X-ray diffraction. From their molecular structures three groups of compounds can be distinguished: ionic compounds, ring systems, and cage structures. All these complexes contain one gallium atom bound to one or two methyl groups, whereas the number of copper atoms, and therefore the nuclearity of the complexes, is variable and depends mainly on size and amount of phosphine ligand used in synthesis. The Ga-E bonds are relatively rigid, in contrast to flexible Cu-E bonds. The lengths of the latter are controlled by the coordination number and steric influences. The Ga-E bond lengths depend systematically on the number of methyl groups bound to the gallium atom, with somewhat shorter bonds in monomethyl compounds compared to dimethyl compounds. Quantum chemical computations reproduce this trend and show furthermore that the rotation of one phenyl group around the Ga-E bond is a low energy process with two distinct minima, corresponding to two different conformations found experimentally. Mixtures of different types of chalcogen atoms on molecular scale are possible, and then ligand exchange reactions in solution lead to mixed site occupation. In thermogravimetric studies the complexes were converted into the ternary semiconductors CuGaE(2). The thermolysis reaction is completed at temperatures between 250 and 400 °C, typically with lower temperatures for the heavier chalcogens. Because of significant release of Me(3)Ga during the thermolysis process, and especially in case of copper excess in the precursor complexes, binary copper chalcogenides are obtained as additional thermolysis products. Quaternary semiconductors can be obtained from mixed chalcogen precursors.

Transition from optically inactive to active Mg-chalcogenides: A first principle study

Density functional calculations of the structural, electronic and optical properties of MgSxSe1−x, MgSxTe1−x and MgSexTe1−x, in the composition range from x=0 to x=1, are carried out using the highly accurate mBJ potential with the full potential linearized augmented plane waves (FP-LAPW) method. The calculated lattice constants follow Vigard’s law while other properties like bulk moduli, band gaps and optical properties vary nonlinearly with the concentration (x). The binary magnesium chalcogenides, MgS, MgSe and MgTe, are wide and indirect band gap materials, whereas it is found that their ternaries are of direct band gap nature. Indirect band gaps materials are inactive material and could not be used as luminescent materials. The wide and direct band gaps of these alloys predict that they can be efficiently used in optoelectronic devices. Therefore, the optical properties of direct band gap ternary alloys are also calculated.

Reaction pathways for the formation of Cu2ZnSn(Se,S)4 absorber materials from liquid-phase hydrazine-based precursor inks

Kesterite Cu2ZnSn(Se,S)4 (CZTSSe) is rapidly becoming an important photovoltaic material due to the abundance and industrial compatibility of its constituent elements. Hydrazine-based slurry deposition has taken a leading role in producing high efficiency devices from this material system, outperforming even high vacuum deposition methods. In this paper, we study the reaction mechanisms involved in the overall transformation from the precursor ink to the solid-state framework and finally to the CZTSSe phase during deposition and subsequent thermal treatment. X-ray diffraction and Raman spectroscopy have been employed to track the various stages of the reaction pathway, and to mark the formation and consumption of precursor phases as they interact to form the final material. It was found that drying the precursor ink at room temperature results in the integration of copper and tin chalcogenide complexes to form a bimetallic framework, with hydrazine and hydrazinium molecules as spacers. After mild thermal annealing, the spacers are removed and the Cu2Sn(Se,S)3 + Zn(Se,S) → Cu2ZnSn(Se,S)4 reaction is triggered. This reaction pathway contains far fewer steps than most deposition processes, which typically start with elemental or binary chalcogenides. As the formation of secondary phases such as Cu2−xS, SnSe, and SnSe2 is no longer necessary to produce the final Cu2ZnSn(Se,S)4 phase, the relative simplicity of this formation mechanism is likely beneficial for the performance of the resulting solar cells.

Atom-specific spin mapping and buried topological states in a homologous series of topological insulators

A topological insulator is a state of quantum matter that, while being an insulator in the bulk, hosts topologically protected electronic states at the surface. These states open the opportunity to realize a number of new applications in spintronics and quantum computing. To take advantage of their peculiar properties, topological insulators should be tuned in such a way that ideal and isolated Dirac cones are located within the topological transport regime without any scattering channels. Here we report ab-initio calculations, spin-resolved photoemission and scanning tunnelling microscopy experiments that demonstrate that the conducting states can effectively tuned within the concept of a homologous series that is formed by the binary chalcogenides (Bi(2)Te(3), Bi(2)Se(3) and Sb(2)Te(3)), with the addition of a third element of the group IV.

Mössbauer studies of two-electron centers with negative correlation energy in crystalline and amorphous semiconductors

The results of the study of donor U −-centers of tin and germanium in lead chalcogenides by Mossbauer emission spectroscopy are discussed. The published data regarding the identification of amphoteric U −-centers of tin in glassy binary arsenic and germanium chalcogenides using Mossbauer emission spectroscopy, and in multicomponent chalcogenide glasses using Mossbauer absorption spectroscopy are considered. Published data concerning the identification of two-atom U −-centers of copper in lattices of semimetal copper oxides by Mossbauer emission spectroscopy are analyzed. The published data on the detection of spatial inhomogeneity of the Bose-Einstein condensate in superconducting semiconductors and semimetal compounds, and on the existence of the correlation between the electron density in lattice sites and the superconducting transition temperature are presented. The principal possibility of using Mossbauer U −-centers as a tool for studying the Bose-Einstein condensation of electron pairs during the superconducting phase transition in semiconductors and semimetals is considered.

B1-to-B2 Structural Transitions in Rock Salt Intergrowth Structures

The rock salt (B1) structure of binary oxides or chalcogenides transforms to the CsCl (B2) structure under high pressure, with critical pressures P(s) depending on the cation to anion size ratio (R(c)/R(a)). We investigated structural changes of A(2)MO(3) (A = Sr, Ca; M = Cu, Pd) comprising alternate 7-fold B1 AO blocks and corner-shared MO(2) square-planar chains under pressure. All of the examined compounds exhibit a structural transition at P(s) = 29-41 GPa involving a change in the A-site geometry to an 8-fold B2 coordination. This observation demonstrates, together with the high pressure study on the structurally related Sr(3)Fe(2)O(5), that the B1-to-B2 transition generally occurs in these intergrowth structures. An empirical relation of P(s) and the R(c)/R(a) ratio for the binary system holds well for the intergrowth structure also, which means that P(s) is predominantly determined by the rock salt blocks. However, a large deviation from the relation is found in LaSrNiO(3.4), where oxygen atoms partially occupy the apical site of the MO(4) square plane. We predict furthermore the occurrence of the same structural transition for Ruddlesden-Popper-type layered perovskite oxides (AO)(AMO(3))(n), under higher pressures. For investigating the effect on the physical properties, an electrical resistivity of Sr(2)CuO(3) is studied.

Nano-scaled top-down of bismuth chalcogenides based on electrochemical lithium intercalation

A two-step method has been used to fabricate nano-particles of layer-structured bismuth chalcogenide compounds, including Bi2Te3, Bi2Se3, and Bi2Se0.3Te2.7, through a nano-scaled top-down route. In the first step, lithium (Li) atoms are intercalated between the van der Waals bonded quintuple layers of bismuth chalcogenide compounds by controllable electrochemical process inside self-designed lithium ion batteries. And in the second step, the Li intercalated bismuth chalcogenides are subsequently exposed to ethanol, in which process the intercalated Li atoms would explode like atom-scaled bombs to exfoliate original microscaled powder into nano-scaled particles with size around 10 nm. The influence of lithium intercalation speed and amount to three types of bismuth chalcogenide compounds are compared and the optimized intercalation conditions are explored. As to maintain the phase purity of the final nano-particle product, the intercalation lithium amount should be well controlled in Se contained bismuth chalcogenide compounds. Besides, compared with binary bismuth chalcogenide compound, lower lithium intercalation speed should be applied in ternary bismuth chalcogenide compound.

Systematics in the optical and electronic properties of the binary lanthanide halide, chalcogenide and pnictide compounds: an overview

The basic optical, electrical and chemical properties of the binary divalent, trivalent and tetravalent lanthanide (Ln = La,… , Lu) halides (F, Cl, Br and I), chalcogenides (O, S, Se and Te) and pnictides (N, P, As and Sb) were studied using an empirical model that utilizes the systematic behaviour in the energy difference between the localized 4fn-states, the 5d-derived conduction band and the mp-derived valence band (m = 2,…,5) over the Lanthanide series. As the wide variety in the electrical and optical properties of Ln materials is to a large extent controlled by the energy of these states relative to each other, it is possible to simultaneously predict insulating, semiconducting or metallic behaviour, the nature and magnitude of bandgap energies and the chemical stability of Ln materials as well as valence and valence changes of Ln ions. It can thus be used to predict the basic electronic structure of materials for which no experimental data can be found, and may be a guideline for theoretical modelling.

Peculiar Linear Dispersive Bands Observed in Angle-Resolved Photoemission Spectra of Tl-Based Ternary Chalcogenide TlGaTe2

Electronic energy bands of the Tl-based ternary chalcogenide TlGaTe2 with a quasi one-dimensional crystalline structure have been studied by means of high resolution angle-resolved photoemission spectroscopy (ARPES) in order to check for dispersive structures similar to the Dirac cone observed in the surface bands of Bi-based binary chalcogenides. Two linear dispersive structures which are not reproduced in band calculations for bulk material have been observed in the energy band along the Γ–N direction perpendicular to the chains. These dispersions form a cross-type structure that is centered at the Γ point and extends along the Γ–H–T direction parallel to the chains, reflecting, in our opinion, one-dimensional features of surface morphology of TlGaTe2. The cross-type structure, the energy position of which linearly varies with excitation photon energy, is observed only for high-grade quality surfaces of TlGaTe2. It is therefore assumed that the observed peculiar dispersive structure is caused by the Dirac-type dispersion of high-lying surface conduction bands and that ARPES detects the joint density of states.

Structural modification of covalent-bonded networks: on some methodological resolutions for binary chalcogenide glasses

New methodology to estimate efficiency of externally-induced structural modification in chalcogenide glasses is developed. This approach is grounded on the assumption that externally-induced structural modification is fully associated with destruction-polymerization transformations, which reveal themselves as local misbalances in covalent bond distribution, normal atomic coordination and intrinsic electrical fields. The input of each of these components into the total value of structural modification efficiency was probed for quasibinary (As2S3)100−x(Sb2S3)x ChG.

Influence of Triethanolamine on the Chemical Bath Deposited NiS Thin Films

Problem statement: Recently, many scientists looking for new chalcogenide materials for the solar cell applications. Nowadays, silicon-based solar cell became dominant products in the market. Because of expensive silicon-based solar cells, scientists hope replaces it with cheaper chalcogenide materials. Approach: The binary chalcogenide materials were deposited onto microscope glass slide using simple chemical bath deposition method. Here, we study the influence of complexing agent in the preparation of thin films. The structural and morphological of the deposited films have been studied using X-ray diffraction and scanning electron microscopy, respectively. Results: The X-ray diffraction data showed that the films had polycrystalline in nature with hexagonal structure. The films deposited using 0.1 M of triethanolamine showed more NiS peaks and larger grain sizes as compared with 0.05M and 0.2 M triethanolamine based on the X-ray diffraction and scanning electron microscopy analysis, respectively. Conclusion: The complexing agent played important role during the deposition process.

Valence band structure of binary chalcogenide vitreous semiconductors by high-resolution XPS

High-resolution X-ray photoelectron spectroscopy (XPS) is used to study regularities in the formation of valence band electronic structure in binary As x Se100 − x , As x S100 − x , Ge x Se100 − x and Ge x S100 − x chalcogenide vitreous semiconductors. It is shown that the highest occupied energetic states in the valence band of these materials are formed by lone pair electrons of chalcogen atoms, which play dominant role in the formation of valence band electronic structure of chalcogen-rich glasses. A well-expressed contribution from chalcogen bonding p electrons and more deep s orbitals are also recorded in the experimental valence band XPS spectra. Compositional dependences of the observed bands are qualitatively analyzed from structural and compositional points of view.

RF sputtering of epitaxial lead chalcogenide films in argon and krypton plasma

The measurements of sputtering etch rates for monocrystalline (1 1 1)-oriented epitaxial films of semiconductor binary compounds PbTe, PbSe, PbS in RF high-density low-pressure inductively coupled argon and krypton plasma were performed. Films with 1–5 µm thickness were grown on Si(1 1 1) and BaF2(1 1 1) substrates using molecular beam epitaxy. Sputtering was carried out with the energy of Ar+ and Kr+ ions of 20–400 eV. The sputtering etch rates of the binary lead chalcogenides are demonstrated to have abnormally high values in comparison with the basic semiconductor materials of microelectronics. The sputtering yield values for PbTe, PbSe, PbS for the average energy of the argon ions of 200 eV are practically equal (0.46 ± 0.05 molecule/ion) and vary linearly with the variation of the ion energy. Substitution of the plasma discharge gas from the argon to krypton does not result in a significant change in the sputtering yield of lead chalcogenides. The physical principles of the observed phenomena are discussed.

Study of Cu 2ZnSnSe 4 monograin formation in molten KI starting from binary chalcogenides

The present study deals with the possible chemical interactions of CuSe, SnSe, and ZnSe in molten KI as a flux material in vacuum ampoules. The aim is to find suitable preparation conditions for the synthesis of Cu 2ZnSnSe 4 monograin powders as an absorber material for solar cells. Impurities in the precursor materials and in the product powders were determined by inductively coupled plasma mass spectrometry. Differential thermal analysis was used to determine thermal effects. Phase composition in the mixtures of binary precursors and KI was determined by X-ray diffraction and Raman spectroscopy. It was found that no new compound was formed in the CuSe/KI, ZnSe/KI and SnSe/KI mixtures. The solubility of the binary chalcogenides in KI at 740 °C was determined.