Cu deficiency in multi-stage co-evaporated Cu(In,Ga)Se2 for solar cells applications: Microstructure and Ga in-depth alloying

The objective of this work is to study the influence of the maximum Cu content during the deposition of Cu(In,Ga)Se₂ (CIGSe) by multi-stage co-evaporation on the phases present in the final film, the film structure and the electrical properties of resulting solar cell devices. The variation of the composition is controlled by the Cu content in stage 2 of the deposition process. The different phases are identified by Raman spectroscopy. The in-depth Ga gradient distribution is investigated by in-depth resolved Raman scattering and secondary neutral mass spectroscopy. The morphology of the devices is studied by scanning electron microscopy. Efficiencies of 9.2% are obtained for ordered-vacancy-compound-based cells with a Cu/(In + Ga) ratio = 0.35, showing the system’s flexibility. This work supports the current growth model: a small amount of Cu excess during the absorber process is required to obtain a quality microstructure and high performance devices.     

Surface Cu depletion of Cu(In,Ga)Se2 films: An investigation by hard X-ray photoelectron spectroscopy

The origin of surface Cu depletion of polycrystalline chalcopyrite thin films and its consequences for the physics of related solar cells have been discussed for the past 15 years. In order to shed light on the composition and thickness of this Cu-depleted surface layer, depth-dependent compositional analysis by hard X-ray photoelectron spectroscopy was performed. The data from Cu-poor grown Cu(In,Ga)Se₂ samples point to a surface layer in the sub-nanometer regime, which is completely depleted of Cu. This result supports the surface reconstruction model proposed by first-principles calculations by other authors. Analysis of the surface morphology of the investigated samples confirms the conjunction of Cu depletion and faceting of the surface. Theoretical considerations show that the apparent surface concentration ratio of [Cu]/([In] + [Ga]) = 1/3 found by conventional photoelectron spectroscopy studies can be explained by the surface reconstruction model.     

Facile route to straight ZnGa2O4 nanowires and their cathodoluminescence properties

Single-crystalline ZnGa₂O₄ nanowires with high purity were grown on the Au and amorphous carbon layers coated Si substrates by a facile carbothermal reduction process. These ZnGa₂O₄ nanowires were straight along their growth direction, and the length and diameter were around 10 μm and 150 nm, respectively. The growth mechanism follows an enhanced vapor–liquid–solid (VLS) process. The carbon layers predeposited on the substrates can strongly enhance the VLS growth process of ZnGa₂O₄ nanowires. The cathodoluminescence (CL) spectrum of an individual nanowire exhibits a broad emission band centered at 537 nm, which can be ascribed to a large quantity of ionized oxygen vacancies.     

An investigation of the effect of structural order on magnetostriction and magnetic behavior of Fe–Ga alloy thin films

This paper reports results from a comprehensive study of Fe–Ga films fabricated over a wide range of growth conditions. Polycrystalline Fe_100?xGa_x films (14 ? x ? 32) were deposited (using three different combinations of growth parameters) on Si(1 0 0) using a co-sputtering and evaporation technique. The growth parameters (sputter power, Ga evaporation rate and chamber pressure) were used primarily to control the Fe:Ga ratio in the films. X-ray diffraction showed that all films had 〈1 1 0〉 crystallographic texture normal to the film plane. The lattice parameter increased with % Ga up to x = 22 and was independent of growth parameters. Conversion electron Mössbauer spectroscopy studies showed a predominance of the disordered A2 phase in all films. It appears that the use of vacuum deposition with appropriate parameters can effectively suppress the D0₃ ordered phase. Systematic studies of the effective magnetostriction constant as a function of composition support this conclusion. It was found that films of high effective saturation magnetostriction constant and low stress could be fabricated using low Ar pressure, irrespective of sputter power or evaporation rate, giving properties useful for application in microelectromechanical systems.     

Energy level diagrams of C60/pentacene/Au and pentacene/C60/Au

The electronic structures of pentacene and C₆₀ interfaces were investigated using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The magnitudes of measured interface dipole were 0.11 eV and 0.07 eV for the C₆₀ deposited on pentacene (C₆₀/pentacene) and the pentacene deposited on C₆₀ (pentacene/C₆₀), respectively. The obtained C 1s spectra on these samples show that no significant chemical bonds at the interface. The offsets of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) at the C₆₀-pentacene interface were 1.29 eV and 0.89 eV for C₆₀/pentacene/Au, while for pentacene/C₆₀/Au they were 1.5 eV and 1.1 eV. In this paper we present the complete energy level diagrams of C₆₀/pentacene/Au and pentacene/C₆₀/Au.     

Room temperature interfacial reactions in electrodeposited Au/Sn couples

Two Au/Sn couples electrodeposited on metallized Si wafers, a Au/Sn couple (Au deposited first) and a Sn/Au couple (Sn deposited first), were employed to study the room temperature interfacial reactions between Au and Sn. The electroplating sequence has a significant effect on the phases and the microstructures of the reaction regions. AuSn₄ and AuSn are formed in the as-deposited Au/Sn couples, while Au₅Sn and AuSn are formed in the as-deposited Sn/Au couples. Upon ageing, AuSn₂ formation depends on the Sn (Au) content (i.e. the Sn (Au) layer thickness) in the Au/Sn (Sn/Au) couples. In aged Au/Sn couples, with 1.5 μm of Au and 3 μm of Sn, AuSn, AuSn₂ and AuSn₄ are sequentially distributed in the reaction region with fine Kirkendall voids at the AuSn/Au interface. In the aged Sn/Au couples, with 15 μm of Sn and 1.5 μm of Au, AuSn is distributed on either side of the original Sn/Au interface, while Au₅Sn only exists on the Au side. The presence of a thin interfacial SnO₂ film significantly affects phase formation and reaction kinetics.     

Effect of the ordering process on pseudoelasticity in Fe3Ga single crystals

The pseudoelastic behaviour of Fe₃Ga single crystals with the D0₃ structure was investigated focusing on the annealing conditions and Ga content. In these D0₃–ordered Fe₃Ga single crystals, uncoupled and paired 1/4[1 1 1] superpartial dislocations moved, dragging the nearest-neighbour and next-nearest-neighbour anti-phase boundaries (APBs), respectively. These APBs pulled back the superpartials during unloading, resulting in pseudoelasticity, though a martensitic transformation never occurred in the crystals. The amount of strain recovery increased with increasing Ga content up to 25.4 at.% when the crystals were annealed at 853–893 K within 10 h. In particular, nearly perfect pseudoelasticity with a recoverable strain of 5% was obtained at 24.2 and 25.4 at.% Ga after an appropriate heat treatment. Isothermal annealing in the D0₃ single phase region was effective in improving pseudoelastic properties. Moreover, the metastable D0₃ phase also exhibited pseudoelasticity, even if the (α + L1₂) or (D0₃ + L1₂) dual phase was thermally stable in the equilibrium state.     

Structure of martensite in Ti-rich Ti–Ni–Cu thin films annealed at different temperatures

After annealing at different temperatures, there are different types of precipitates in Ti-rich Ti–Ni–Cu thin films: plate-like Guinier–Preston (GP) zones, Ti₂Cu precipitates and spherical Ti₂Ni precipitates. The (0 1 1) compound twins and (1 1 1) type I twins are dominant in Ti-rich Ti–Ni–Cu thin films annealed at different temperatures, which suggests that the precipitates do not change the twinning modes of the B19 martensite. However, the amount of the (0 1 1) compound twin increases with increasing annealing temperature due to its small twinning shear. In thin films with GP zones or Ti₂Ni precipitates, the amount of martensite with a single-pair morphology is less than that in thin films without precipitates. And in thin film with Ti₂Cu + Ti₂Ni precipitates, hardly any martensite with a single-pair morphology is observed. For the different types of precipitates at the different annealing temperatures, the obstacle of the precipitates to the growth of the B19 martensite plate also varies. The GP zones slightly hinder the growth in the width of martensite, resulting in wavy twin boundaries at the martensite variant tip. The Ti₂Cu precipitates can change both the width and the direction of the martensite plates. Ti₂Ni precipitates also significantly disturb or impede the growth of the martensite variants. These effects lead to a decrease in the maximum shape recoverable strain with increasing annealing temperature.     

Multilayered InGaN/GaN structure vs. single InGaN layer for solar cell applications: A comparative study

We report a comparison of the morphological, structural and optical properties of both InGaN single-layer and multilayered structures, the latter consisting of periodic thin GaN interlayers inserted during InGaN growth. It is shown that such a structure suppresses the In concentration fluctuations and corresponding different states of strain relaxation with depth, both detrimental to solar cell applications. Measurements performed by X-ray diffraction, cathodoluminescence and photoluminescence demonstrate that this multilayer growth is a promising approach to increase both the InGaN layer total thickness and In content in InGaN epilayers. As an example, single-phase 120 nm thick InGaN with 14.3% In content is obtained and found to possess high structural quality.     

Morphological changes of Au nanoparticles in Au–polystyrene nanocomposites: A combined spectroscopy,atomic force microscopy and X-ray scattering study

Au nanoparticles sputter deposited on polystyrene-coated Si and fused quartz substrates have been studied using optical spectroscopy, atomic force microscopy and X-ray reflectivity. Under the same deposition conditions, both spectroscopy and atomic force microscopy indicate clearly that the nanoparticles undergo a shape transition from near-spherical to progressively ellipsoidal as the polystyrene film becomes thinner than 4R_g, R_g being the radius of gyration of the polymer. There is a gradual increase in the in-plane ellipticity a/b, a(b) = semi-major (semi-minor) axis, with decrease in polystyrene film thickness from 230 to 20 nm, where b remains almost invariant for a particular deposition time while the semi-major axis increases in dimension. Electron density profiles along the depth of the films, extracted from X-ray reflectivity data, show that the Au nanoparticles sit on the top of the polystyrene film with c, the third ellipsoid axis having a dimension of about 3.0 nm irrespective of film thickness or deposition times used (7, 10, 12 and 15 s).     

Formation of oxide nanotubes via oxidation of Fe,Cu and Ni nanowires and their structural stability: Difference in formation and shrinkage behavior of interior pores

Changes in the morphology of Fe, Cu and Ni nanowires with a diameter of 55 nm during oxidation at 423–923 K were studied by transmission electron microscopy. Oxide nanotubes with a cylindrical interior pore of uniform diameter were formed after the oxidation of Fe and Cu nanowires in air at 573 and 423 K, respectively, while the Ni nanowires became bamboo-like nanowires of NiO with separate interior voids after oxidation at 673–773 K. Oxide nanotubes of Fe and Cu and the bamboo structures of NiO showed a tendency to shrink into solid oxide nanowires after annealing at higher temperatures in air. In the shrinking process of Fe₃O₄ nanotubes, however, an array of additional nanovoids was observed along the inner wall of the nanotubes, suggesting the formation of a duplex porous nanostructure. This can be explained by the recombination of vacancies diffusing outward from the inner cylindrical pore.     

Synthesis of (V,Cr)(C,N) nanopowders via carbothermal reduction nitridation method: Influence factors and morphology evolution

(V,Cr)(C,N) nanopowders, in the form of globular-like with an average grain diameter of about 60 nm, were successfully synthesized via carbothermal reduction nitridation (CRN) of the precursor of ammonium vanadate (NH₄VO₃), ammonium dichromate ((NH₄)₂CrO₄) and nano carbon black. The influence factors, including synthesis temperature, isothermal holding time and nitrogen pressure, were studied through X-ray diffraction (XRD), and the morphology evolution was investigated by a scanning electron microscope (SEM). The results show that the phase evolution follows: amorphous → VO₂ → V₃O₅ + V₂O₃ + Cr₂O₃ → V₂O₃ + Cr₂O₃ + (V,Cr)(C,N) → (V,Cr)(C,N) with temperature increases. Long isothermal holding time is beneficial to the dissolution of Cr atom into (V,Cr)(C,N), but it leads to grain growth and nitrogen content reduction of the product. On the contrary, high nitrogen pressure contributes to grain refinement and nitrogen increment of (V,Cr)(C,N). In addition, synthesis temperature has a significant effect on the morphology evolution of (V,Cr)(C,N) nanopowders. The microstructure and chemical composition of the product were also studied.     

Epitaxial Ni–Mn–Ga/MgO(1 0 0) thin films ranging in thickness from 10 to 100 nm

Thin films of Ni–Mn–Ga alloy ranging in thickness from 10 to 100 nm have been epitaxially grown on MgO(1 0 0) substrate. Temperature-dependent X-ray diffraction measurements combined with room-temperature atomic force microscopy and transmission electron microscopy highlight the structural features of the martensitic structure from the atomic level to the microscopic scale, in particular the relationship between crystallographic orientations and twin formation. Depending on the film thickness, different crystallographic and microstructural behaviours have been observed: for thinner Ni–Mn–Ga films (10 and 20 nm), the L2₁ austenitic cubic phase is present throughout the temperature range being constrained to the substrate. When the thickness of the film exceeds the critical value of 40 nm, the austenite-to-martensite phase transition is allowed. The martensitic phase is present with the unique axis of the pseudo-orthorhombic 7M modulated martensitic structure perpendicular to the film plane. A second critical thickness has been identified at 100 nm where the unique axis has been found both perpendicular and parallel to the film plane. Magnetic force microscopy reveals the out-of-plane magnetic domain structure for thick films. For the film thickness below 40 nm, no magnetic contrast is observed, indicating an in-plane orientation of the magnetization.     

Synthesis of monodispersed La2Ce2O7 nanocrystals via hydrothermal method: A study of crystal growth and sintering behavior

Monodispersed and uniformly distributed La₂Ce₂O₇ nanocrystals were synthesized via the hydrothermal method using polyethyleneglycol (PEG) as surfactant. X-ray diffraction (XRD), Thermogravimetric analysis/Differential scanning calorimeter (TG/DSC), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and High resolution transmission electron microscopy (HRTEM) were utilized to characterize the thermal decomposition, phase structure and morphology of the products. Qualitative analysis indicates that the products are comprised of well-dispersed square particles with cubic fluorite structure. The specific surface area and the average crystallite size of the as-prepared products are 195.59 m² g^− 1 and 10–15 nm, respectively. The low effective activation energy (15.27 ± 0.03 kJ mol^− 1) for crystal growth was obtained in the calcination temperature range of 700–1300 °C. The sintering behavior of the compacted body was also investigated, revealing that La₂Ce₂O₇ has a low relative density and open channel morphology.     

Structural dependence of the piezoelectric properties of KNbO3 nanowires synthesized by the hydrothermal method

Because of the presence of OH^? and H₂O in the KN unit cell, tetragonal KNbO₃ (KN) nanowires were formed when the synthesis was carried out at 120 °C for 48 h. However, when the fabrication was conducted at high temperatures (?150 °C) or at 120 °C for a long period of time (?72 h), orthorhombic KN nanowires were formed. Moreover, the KN nanowires synthesized at 120 °C for 60 h showed a morphotropic phase boundary (MPB) structure in which both tetragonal and orthorhombic structures coexisted. Tetragonal, orthorhombic and MPB KN nanowires were also grown on the Nb⁵⁺-doped SrTiO₃ substrate, and their d₃₃ values were measured for the first time. A tetragonal KN nanowire exhibited a d₃₃ value of 23.5 pm V^?1, which is larger than that of the orthorhombic KN nanowire (11.6 pm V^?1), probably because of the softening effect of the metal vacancies. The MPB KN nanowires exhibited a larger d₃₃ value of 40.0 pm V^?1. The d₃₃ values of KN nanowires increased to 104.5, 137.1 and 146.0 pm V^?1 for the orthorhombic, tetragonal and MPB KN nanowires, respectively, after the KN nanowires were poled along the [1 0 0] direction by application of a DC voltage of 10 V.     

Variation of atomic spacing and thermomechanical properties in Ni–Mn–Ga/alumina film composites

Ni_51.4Mn_28.3Ga_20.3 thin films deposited on alumina ceramics have been studied by X-ray powder diffraction and dynamic mechanical analysis. Substantial temperature vs. film thickness dependencies of interatomic spacing measured in the direction of the film normal are observed in the range of 25–200 °C and 0.1–5 μm, respectively. The coefficient of thermal expansion (CTE) of the film in the paramagnetic cubic phase has been determined to be equal to (15 ± 1) × 10⁻⁶ K⁻¹ for all the films, in agreement with the CTE of bulk material. The thickness dependent shrinkage of the pseudo-cubic lattice along the film normal direction is attributed to the thermally induced tensile stress in the film plane. The thickness dependence of the elastic modulus of submicron films is obtained. It is shown that the internal stresses result in both the thickness dependence of martensitic transformation temperature and the reversible, thermally induced change in shape of the Ni–Mn–Ga/alumina cantilever actuator.     

Synthesis,morphology, microstructure and magnetic properties of hematite submicron particles

We report on hydrothermal synthesis, plate-like morphology, microstructure and magnetic properties of hematite (α-Fe₂O₃) plate-like particles. The sample is obtained immediately after the hydrothermal process without using any template and without further heat treatment. The so-obtained sample is characterized by X-ray powder diffraction (XRPD), energy-dispersive X-ray spectroscopy (EDX), field-emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and superconducting quantum interference device (SQUID) magnetometer. XRPD confirms the formation of a single-phase hematite sample whereas EDX reveals that iron and oxygen are the only components of the sample. SEM, FE-SEM, TEM and HRTEM show that the sample is composed of plate-like particles. The width of the particles is ～500 nm whereas thickness is ～100 nm (aspect ratio 5:1). The HRTEM images exhibit well defined lattice fringes of α-Fe₂O₃ particles that confirm their high crystallinity. Moreover, the HRTEM analysis indicates the plate-like particles preferring crystal growth along [0 1 2] direction. Magnetic measurements display significant hysteretic behavior at room temperature with coercivity H_C = 1140 Oe, remanent magnetization M_r = 0.125 emu/g and saturation magnetization M_S = 2.15 emu/g as well as the Morin transition at T_M ～ 250 K. The magnetic properties are discussed with respect to morphology and microstructure of the particles. The results and comparison with urchin-like, rods, spherical, hexagonal, star-like, dendrites, platelets, irregular, nanoplatelets, nanocolumns and nanospheres hematites reveal that the plate-like particles possess good magnetic properties. One may conjecture that the shape anisotropy plays an important role in the magnetic properties of the sample.     

On the role of Fe in the growth of single crystalline heteroepitaxial Au thin films on sapphire

Thin Au–Fe bilayers were deposited on c-plane sapphire (α-Al₂O₃) substrates at room temperature employing the electron beam deposition method. The layers were found to be single crystalline (i.e. the grain size was much larger than the film thickness), with a [1 1 1] and [1 1 0] texture for Au and Fe, respectively, and strong heteroepitaxy to the substrate. Au films deposited on sapphire and Au–Fe bilayers deposited on amorphous SiO₂ were polycrystalline and exhibited random in-plane orientation of the grains. The effects of Fe and the Fe–sapphire interface on the microstructure of the Au film were investigated and discussed in terms of the orientation relationships, in-plane strain, interface energy and adhesion. The microstructures of annealed and as-deposited films were very similar, indicating that as-deposited films are close to thermodynamic equilibrium in terms of the orientation relationship with the substrate. This is uncommon for non-equilibrium thin film deposition processes, which usually result in a high density of defects in the as-deposited films.     

Magnetic and structural properties of BaFe12−xGaxO19 nanoparticles

The structural and magnetic properties of barium hexaferrite nanoparticles (BaFe_12?xGa_xO₁₉) with x = 0.0–1.0, prepared by ball milling were investigated using XRD, TEM, and VSM. It was found that the particles and crystallites have similar mean size of ～41 nm for all investigated samples. The saturation magnetization decreased slightly and nonlinearly with increasing x, and this was attributed to different preferential site occupation of Ga at low and high concentration ranges. The coercivity decreased slightly with increasing x for low concentrations of Ga (x ≤ 0.2), and then increased with increasing Ga concentration up to x = 1.0. This behavior of the coercivity was attributed to the change in the exchange coupling, which was confirmed by the variation of SFD, remanence ratio and Curie temperature with Ga concentration in the samples.     

Effect of additives on the properties of plasma electrolytic oxidation coatings formed on AM50 magnesium alloy in electrolytes containing K2ZrF6

A plasma electrolytic oxidation (PEO) coating on AM50 magnesium alloy was obtained in a K₂ZrF₆–NH₄H₂PO₄–KF–C₆H₅O₇Na₃ electrolyte solution. The influence of the electrolytes on the properties of the PEO coating had been investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), roughness determination and electrochemical measurements. The experimental results show that 10 g L^? 1 K₂ZrF₆ and the presence of 8 g L^? 1 NH₄H₂PO₄ are beneficial for the passivation effect of the solution on AM50 Mg alloy as well as the compactness of the PEO coating. Many pores with large dimension on the surface of the coating are filled with coating compounds (mainly MgF₂), and the characteristics of these pores are influenced by the concentration of KF. The addition of KF and C₆H₅O₇Na₃ enhances the growth rate of the coating. The coating shows high corrosion resistance in the presence of 10 g L^? 1 K₂ZrF₆ combined with 8 g L^? 1 NH₄H₂PO₄, 3 g L^? 1 KF and 5 g L^? 1 C₆H₅O₇Na₃.