High Ga Content Research Articles

Origin of Reduced Efficiency in Cu(In,Ga)Se$_2$ Solar Cells With High Ga Concentration: Alloy Solubility Versus Intrinsic Defects

It is well known that adding Ga to CuInSe2 forming CuIn1-xGaxSe2 (CIGS) alloy can significantly improve the solar cell efficiency, but adding too much Ga will lead to a decline of the solar cell efficiency. The exact origin of this puzzling phenomenon is currently still under debate. It is especially unclear whether it is caused by either structural or electronic issues. In this paper, we conclude that the defect issue, especially antisite defects MCu (M = In, Ga), rather than the alloy solubility is the key problem for the reduced efficiency in CIGS. The deep levels that are induced by MCu defects can pin the open-circuit voltage (Voc) of CIGS. Self-compensation in CIGS, which forms 2VCu + M Cu defect complexes, is found to be beneficial to quenching the deep-trap levels induced by MCu in CIGS. Unfortunately, the density of isolated MCu is quite high and cannot be largely converted into 2VCu + MCu complexes under thermal equilibrium condition. Thus, nonequilibrium growth conditions or low growth temperature that can suppress the formation of the deep-trap centers MCu will be necessary to improve the efficiency of CIGS solar cells, especially with high Ga concentrations.

Synthesis of single phase chalcopyrite CuIn1−xGaxSe2 (0 ≤ x ≤ 1) nanoparticles by one-pot method

Single phase chalcopyrite and near stoichiometric CuIn1−xGaxSe2 (0≤x≤1) nanoparticles were successfully synthesized by using a facile and rapid one-pot method. The effects of various Ga contents on crystal phase, morphology, element composition and absorption spectrum of the as-synthesized CuIn1−xGaxSe2 nanoparticles were investigated in detail. The XRD and Raman patterns indicated that the as-synthesized nanoparticles had a single phase chalcopyrite structure, and the diffraction peaks shifted toward larger diffraction angles or higher frequencies with increasing Ga content. The FE-SEM images showed that the as-synthesized nanoparticles were polydispersed in both size and shape, and the nanoparticles with higher Ga content were more prone to aggregate. The Vis–IR absorption spectra showed strong absorption in the entire visible light region. The estimated band gap increased from 1.00eV to 1.68eV as Ga content increasing.

The effects of sodium on the growth of Cu(In,Ga)Se2 thin films using low-temperature three-stage process on polyimide substrate

In this work, Cu(In,Ga)Se2 (CIGS) absorbers were deposited on polyimide substrates using the low-temperature three-stage process, and different sodium incorporation methods were applied to investigate the effects of Na on the CIGS growth. It was found that Na affected the CIGS film growth significantly during the second stage of the process, and resulted in a higher Ga content near the back contact. The CIGS thin films with different Na-incorporation methods exhibited similar electrical parameters. A modelling investigation was also implemented by using the wxAMPS software package to quantitatively analyse the effects of different Ga gradient to the device characteristics. Finally, the device performance showed little difference between the post-deposition treatment of NaF and the co-evaporation of NaF during the third stage.

Modelling of dissolved H in Ga stabilised δ-Pu

The behaviour of hydrogen in Ga stabilised δ-Pu has been investigated using atomistic computer simulation techniques. We have considered only the solid solution of H in Pu–Ga. H diffusivity in the undamaged material was calculated and was shown to depend on the Ga concentration of the Pu–Ga alloy. Furthermore, localised regions of high Ga concentration within the material were shown to block H diffusion pathways. These are important findings and could allow for the possibility to control H diffusion if it were possible to control the Ga configuration within the system. The interaction of H with simple point defects was also investigated and suggests that H will behave differently in cascade damaged systems compared to undamaged systems. Vacancies were observed to trap any H interstitials that enter their vicinity, while the likelihood of dissociation was very low, effectively reducing the H diffusion coefficient to zero. On the other hand, binding energy calculations show that it is energetically unfavourable for a H interstitial to be close to a Pu interstitial. No long range interaction between H and the single point defects was observed.

Microstructure of V3Ga Superconducting Wire Using High Ga Content Ti-Ga/V Precursor Composite Material

Our co-workers, Hishinuma et. al. have fabricated the V3Ga compound wire by a new process using a high Ga content Ti-Ga compound in order to improve the superconducting property of the wire. In this study, we investigated microstructures of this wire to clarify the existence of V3Ga phase. The different contrasts of the matrix, two V-Ga phases and Ti-Ga core were observed by SEM observation. Two V-Ga phases were also confirmed. The ratio of V to Ga for two V-Ga phases was respectively 3:1 and 6:5 according to the EDS result. And these two phases were confirmed as V3Ga and V6Ga5.

Effect of Cu Addition Into $\hbox{TiGa}_{3}$ Compound on Superconducting Properties and Microstructure of $ \hbox{V}_{3}\hbox{Ga}$ Multifilamentary Wires Synthesized Through High Ga Content Diffusion Process

The decay time of induced radioactivity in V3Ga is remarkably shorter than in Nb-based compounds, making the V3Ga compound as one of the candidate and suitable superconducting magnet materials for the future nuclear fusion system. In order to improve the superconducting properties of V3Ga multifilamentary wires, we investigated high Ga content TiGa3 compound with various Cu addition as the Ga source material. The Cu additions into TiGa3 compounds were 10, 30, and 50 wt.%, respectively. The volume fraction of V3Ga phase was increased with increasing of Cu addition. We confirmed that Cu addition into TiGa3 compound promoted the formation of the V3Ga phase. The optimum Cu addition into TiGa3 compound on superconducting properties was obtained to be 30 wt. %, and Hc2 value of the 30 wt.%Cu sample was estimated to be above 24 T. This value was 3 T higher than conventional “Bronzed route” processed V3Ga wires.

Effect of Ga content and growth temperature on Cu(In,Ga)Se2 thin film deposited on heat‐resistant glass substrates

AbstractCu(In,Ga)Se2 (CIGS) thin films were deposited on the Mo/heat‐resistant glasses (SS‐8, Nippon Electric Glass) by the three‐stage evaporation process. The coefficient of thermal expansion (CTE) of SS‐8 is 8.4×10–6/K, strain point of SS‐8 is 582 °C. SS‐8 has the CTE similar to that of SLG but higher thermal tolerance. Substrate temperature (Tsub) during deposition and Ga/(In+Ga) atomic ratio of CIGS thin films were controlled to 550∼600 °C and 0∼0.7, respectively. The effect of Tsub and Ga/(In+Ga) atomic ratios of CIGS films have been characterized by electron backscatter diffraction (EBSD) measurements. Grain size and ∑3 grain boundary ratio increased at higher Tsub. ∑3 grain boundary ratio was calculated from the fraction of the length of ∑3 grain boundary by the length of the total grain boundaries on EBSD image. There was the correlation among Ga contents, Tsub, grain size and efficiency. It is difficult to distinguish between effect of grain size and effect of grain boundary to efficiency. However, it is found that high Ga content up to 0.7 results in both high efficiency and ∑3 grain boundary ratio. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Regional Tectonic Transformation in East Kunlun Orogenic Belt in Early Paleozoic: Constraints from the Geochronology and Geochemistry of Helegangnaren Alkali‐feldspar Granite

Abstract:The Helegangnaren feldspar granite exposed in the eastern part of East Kunlun, is characterized by high concentrations of SiO2 and alkaline, low abundances of Fe, Mg and Ca, metaluminous‐weak peraluminous. Trace elements analysis shows that the granite is depleted extremely in Ba, Sr and Eu, and rich in some large‐ion lithophile elements and high field strength elements. Besides, the granite has high Ga contents, the values of 104(Ga/Al) vary from 2.50 to 2.77, which is mainly greater than the lower limit of A‐type granites (2.6), and is higher than the I‐ and S‐type granites’ average (2.1 and 2.28, respectively). Rare earth element (REE) is characterized by relatively high fractionations of light REE (LREE) and heavy REE (HREE) (LREE/HREE=9.3–13.60, (La/Yb)N=10.92–18.02), pronounced negative Eu anomalies (δEu=0.08–0.13), and exhibits right‐dipping gull pattern. Major elements, rare elements and trace elements features show the granite is ascribed to A‐type granite and A2 subtype in tectonic genetic type. They are plotted into post‐collision or within‐plate area in a variety of tectonic discriminations. Geological and geochemical data comprehensively suggest that the granite is formed in a post‐collision extensive tectonic setting. Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) zircon U‐Pb dating yields a weighted mean age of 425 Ma, belonging to Middle Silurian, which is similar to the age of the post‐collision geological events in the region. The differences of magmatic rocks in formation age, rocks assemblage and rocks series systematically indicate that the regional tectonic stress regime in the East Kunlun orogenic belt experienced a major transformation from compress to extension in Middle Silurianin, and the Helegangnaren feldspar granite intruded in the early stage of tectonic transformation.

Influence of the Ga Content on the Optical and Electrical Properties of CuIn $_{{\bm 1}\hbox{--}{\bm x}}$Ga $_{\bm x}$Se$_{\bm 2}$ Thin-Film Solar Cells

Thin-film solar cells that are based on Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS) absorbers with Ga/(Ga+In)-ratios from 0 to 1 are fabricated, and their optical and electrical properties are investigated by macroscopic (current density-voltage, external quantum efficiency) and microscopic (Kelvin probe force microscopy on untreated cross sections of solar cells) measurements. Combining all results, the diffusion voltages of individual solar cells are deduced and compared with the directly measured open-circuit voltages. An increasing splitoff between the diffusion voltage and the open-circuit voltage is observed for Ga addition, which indicates a higher recombination rate of photogenerated charge carriers in solar cells with higher Ga content.

Synchrotron nanoimaging of single In-rich InGaN nanowires

This work reports on the elemental distribution and local structure of single InxGa1–xN nanowires (NWs) grown by molecular beam epitaxy on Si (111) substrates using X-ray fluorescence nanoprobe. Ga and In maps reveal an inhomogeneous elemental distribution along the NWs, with a higher Ga concentration at the bottom of the NW. Scanning electron microscopy images show that the inhomogeneous axial distribution is not correlated with a X-ray beam induced damage, and therefore, should be an intrinsic characteristic of the NWs arising from the growth process. Spatially resolved X-ray absorption near edge structure spectroscopy data acquired around the In K-edge show that the tetrahedral structure is preserved around the absorbing In-atoms all along the NW, and suggests that the compositional modulation could be affecting its long-range order.

Large-scale first-principles determination of anisotropic mechanical properties of magnetostrictive Fe–Ga alloys

Magnetostrictive Fe1−xGax alloys, as a new class of smart materials, have great potential in sensing and actuator applications. However, the fundamental understanding of the anisotropic elastic responses at high Ga concentration remains one of the most challenging problems for the binary alloys. Here, we apply the density functional theory and large-scale ab initio molecular dynamics simulation to investigate the effect of high Ga concentration on the elastic anisotropy of the Fe–Ga alloys with supercell models obtained by non-linear and non-uniform annealing processes. It is demonstrated that the formation of D03-like structures has an important effect on the softness of the tetragonal shear modulus and a negligible influence on the rhombohedral shear modulus. Meanwhile, the Fe dangling bond to its nearest Ga atoms results in a decrease in the Young’s modulus and the negative Poisson’s ratio in the [110] direction. The improved Young’s modulus in the [110] direction compared to that in the [100] direction is attributed to the different arrangement of the pure Fe layer and the Fe–Ga mixed layer along the [110] and [100] axes. Furthermore, the ductility of Fe1−xGax alloys is enhanced at high Ga content, playing a key role in the enhanced magnetostriction.

Luminescence and Luminescence Quenching in Gd3(Ga,Al)5O12 Scintillators Doped with Ce3+

The optical properties of gadolinium gallium aluminum garnet, Gd3(Ga,Al)5O12, doped with Ce(3+) are investigated as a function of the Ga/Al ratio, aimed at an improved understanding of the energy flow and luminescence quenching in these materials. A decrease of both the crystal field strength and band gap with increasing content of Ga(3+) is observed and explained by the geometrical influence of Ga(3+) on the crystal field splitting of the 5d level in line with theoretical work of Muñoz-García et al. ( uñoz-García, A. B.; Seijo, L. Phys. Rev. B 2010, 82, 184118 ). Thermal quenching results in shorter decay times as well as reduced emission intensities for all samples in the temperature range from 100 to 500 K. An activation energy for emission quenching is calculated from the data. The band gap of the host is measured upon Ga substitution and the decrease in band gap is related to Ga(3+) substitution into tetrahedral sites after all octahedral sites are occupied in the garnet material. Based on the change in band gap and crystal field splitting, band diagrams can be constructed explaining the low thermal quenching temperatures in the samples with high Ga content. The highest luminescence intensity is found for Gd3(Ga,Al)5O12 with 40% of Al(3+) replaced by Ga(3+).

Texture development in Galfenol wire

Galfenol (Fe-Ga alloy) wire fabrication provides a low cost alternative to directional solidification methods. This work evaluates the compositional dependence of the wire drawing suitability of Fe-Ga and characterizes the microstructural and magnetic properties of these wires. Wire has been produced with Ga contents between 10 at. % and 17 at. % to allow determination of the ductile to brittle transition (DTBT) in wire manufacture. Published results on chill cast bend specimens indicated that a DTBT occurs at roughly 15 at. % Ga. This DTBT was observed under tensile loading with a corresponding change in fracture behavior from transverse fracture to intergranular fracture. For improved magnetostrictive performance, higher Ga contents are desired, closer to the 17 at. % Ga evaluated in this work. Electron backscattered diffraction B-H loop and resonance measurements as a function of magnetic field (to determine modulus and coupling factor) are presented for as-drawn, furnace, and direct current (DC) annealed wire. Galfenol wire produced via traditional drawing methods is found to have a strong 〈110〉 (α) texture parallel to the drawing direction. As-drawn wire was observed to have a lower magnetic permeability and larger hysteresis than DC annealed wire. This is attributed to the presence of a large volume of crystalline defects; such as vacancies and dislocations.

Structure, mechanical properties, corrosion behavior and cytotoxicity of biodegradable Mg–X (X = Sn, Ga, In) alloys

As-cast Mg–Sn, Mg–Ga and Mg–In alloys containing 1–7wt.% of alloying elements were studied in this work. Structural and chemical analysis of the alloys was performed by using light and scanning electron microscopy, energy dispersive spectrometry, x-ray diffraction, x-ray photoelectron spectroscopy and glow discharge spectrometry. Mechanical properties were determined by Vickers hardness measurements and tensile testing. Corrosion behavior in a simulated physiological solution (9g/l NaCl) was studied by immersion tests and potentiodynamic measurements. The cytotoxicity effect of the alloys on human osteosarcoma cells (U-2 OS) was determined by an indirect contact assay. Structural investigation revealed the dendritic morphology of the as-cast alloys with the presence of secondary eutectic phases in the Mg–Sn and Mg–Ga alloys. All the alloying elements showed hardening and strengthening effects on magnesium. This effect was the most pronounced in the case of Ga. All the alloying elements at low concentrations of approximately 1wt.% were also shown to positively affect the corrosion resistance of Mg. But at higher concentrations of Ga and Sn the corrosion resistance worsened due to galvanic effects of secondary phases. Cytotoxicity tests indicated that Ga had the lowest toxicity, followed by Sn. The most severe toxicity was observed in the case of In.

Direct observation of substitutional Ga after ion implantation in Ge by means of extended x-ray absorption fine structure

We present an experimental lattice location study of Ga atoms in Ge after ion implantation at elevated temperature (250°C). Using extended x-ray absorption fine structure (EXAFS) experiments and a dedicated sample preparation method, we have studied the lattice location of Ga atoms in Ge with a concentration ranging from 0.5 at. % down to 0.005 at. %. At Ga concentrations ≤0.05 at.%, all Ga dopants are substitutional directly after ion implantation, without the need for post-implantation thermal annealing. At higher Ga concentrations, a reduction in the EXAFS amplitude is observed, indicating that a fraction of the Ga atoms is located in a defective environment. The local strain induced by the Ga atoms in the Ge matrix is independent of the Ga concentration and extends only to the first nearest neighbor Ge shell, where a 1% contraction in bond length has been measured, in agreement with density functional theory calculations.

Effects of the oxygen vacancy concentration in InGaZnO-based resistance random access memory

Resistance random access memory (RRAM) composed of stacked aluminum (Al)/InGaZnO(IGZO)/Al is investigated with different gallium concentrations. The stoichiometric ratio (x) of gallium in the InGaxZnO is varied from 0 to 4 for intentional control of the concentration of the oxygen vacancies (VO), which influences the electrical characteristics of the RRAM. No Ga in the IGZO (x = 0) significantly increases the value of VO and leads to a breakdown of the IGZO. In contrast, a high Ga concentration (x = 4) suppresses the generation of VO; hence, resistive switching is disabled. The optimal value of x is 2. Accordingly, enduring RRAM characteristics are achieved.

Effects of Ga concentration on electronic and optical properties of Ga-doped ZnO from first principles calculations

This study evaluated the electronic and optical properties of Ga-doped ZnO with various concentrations of gallium, employing first principles calculations based on density functional theory and the Hubbard U (DFT+Ud+Up). The lattice constants and band gap of ZnO calculated in this study are in agreement with experimental values. Results show that donor concentration increases with an increase in Ga concentration; however, electrical conductivity is reduced when localized states close to the Fermi level and higher scattering probability of free electrons occur with high Ga concentration. Following the incorporation of Ga into ZnO (1.4–6.3at.%), the average transmittance of light in both the visible and UV ranges exceeds that of ZnO. However, the stronger and wider donor states obtained from high doping levels (12.5–25at.%) significantly decreases the average transmittance. Thus, selecting a suitable doping level is crucial to optimizing the photoelectric performance of Ga-doped ZnO. This study also provides a theoretical explanation for the factors influencing these properties.

Obtaining mixed ionic/electronic conductivity in perovskite oxides in a reducing environment: A computational prediction for doped SrTiO3

The electronic conductivity and thermodynamic stability of mixed p- and n-doped SrTiO3 perovskites have been investigated under anodic solid oxide fuel cell conditions using density functional theory (DFT). In particular, constrained ab initio thermodynamic calculations have been performed to evaluate the phase stability of various Ga- and La-doped SrTiO3 at synthesized and anodic SOFC conditions. The density of states (DOS) of these materials was analyzed to determine the number of charge carriers and the degree of electronic conductivity. We find that a mixed ionic/electronic conductor can be obtained when doping SrTiO3 perovskite oxide with both p-type and n-type dopants. Calculations show that 10% Ga- and 20% La-doped SrTiO3 exhibit mixed ionic/electronic conductivity at high temperature and low oxygen partial pressure whereas doping with higher concentrations of Ga, e.g., 20%, diminishes the electronic conductivity of the material. Furthermore, changing the n-dopant from La (A-site) to Nb (B-site) does not significantly affect the reducibility and number of charge carriers in p- and n-doped SrTiO3. However, a higher degree of oxygen vacancy clustering is observed for the La-doped material which reduces the oxygen ion diffusion rate and traps electrons. Nevertheless, our findings suggest that independent of doping site, mixed ionic/electronic conductivity can be obtained in SrTiO3 perovskite oxides under reducing conditions and high temperatures when using a mixed p- and n-type doping strategy that uses a p-dopant concentration smaller than the n-dopant concentration.

Improvement of the Band Profile of Cu(In,Ga)Se2 Solar Cells with High-Ga Content Prepared Using a Five-Stage Method

The efficiency of Cu(InGa)Se2 (CIGS) solar cells with high Ga content fabricated by the three-stage method is lower than that with low Ga content in spite of a better matching solar spectrum. Secondary ion mass spectrometry (SIMS) measurement revealed that the band profile of CIGS films with high Ga content had a deep notch around 0.5 µm from the CdS/CIGS interface. In order to decrease the notch depth of the CIGS with high Ga content, the five-stage method was employed instead of the conventional three-stage method. As a result, we successfully obtained the efficiency of 14.9% using the CIGS absorber with an average band gap of 1.40 eV prepared by the five-stage method. Theoretical simulation revealed the effects of the notch location and depth on solar cell performance characteristics.

Control of Ga Distribution in Cu(In,Ga)Se2 Photovoltaic Absorber by Solid-State Selenizatoin of CuGa/In/Se/In/CuGa Stack

Among many key parameters required to obtain a record-efficiency Cu(In,Ga)Se2 (CIGS) cell, the band-gap of CIGS should have a double-graded profile in which the band-gap increases toward both of the back and front of the absorber. In an effort to obtain an increased Ga content near the junction area which will raise the band-gap energy of CIGS, a novel metal precursor layered with predetermined amount of Se was annealed in N2 ambient. By inserting the Se layer in between metallic precursor layers, it was found that the front band-gap was increased due to the high Ga content by changing the direction of selenization reaction from inside to outside of metallic precursor. The proposed method is expected to provide a simple process for high quality CIGS photovoltaic absorber layer. The conversion efficiency of 6.80% with Jsc = 37.65 mA/cm2, Voc = 0.51 V, and FF= 35.4% in an active area of 0.48 cm2 was achieved.