High Ga Content Research Articles

Growth and characterization of (Zn,Sn,Ga)As2 thin films grown on GaAs(001) substrate by molecular beam epitaxy

AbstractGa‐incorporated ZnSnAs thin films were grown on semi‐insulating GaAs(001) substrate by molecular beam epitaxy, and the structural, compositional, and electrical properties were characterized with respect to achieving conduction‐type control. Compositional analysis with an electron‐probe micro analyzer indicated that Ga atoms are substituted for cation (Zn and Sn) atoms and that the Ga/Sn beam equivalent pressure ratio has only a small influence on the Ga concentration. In addition, the composition ratios of Zn to all the cation sites were 2.0–5.7% smaller than that of Sn. ‐scan X‐ray diffraction analyses confirmed the epitaxial growth of (Zn,Sn,Ga)As thin films on GaAs(001) substrates. Reciprocal space maps revealed that these thin films are almost fully relaxed, and the lattice constants are in good agreement with Vegard's law. Hall effect measurements of all the (Zn,Sn,Ga)As thin films indicated n‐type conductivity, which was due to the high Ga content of 44.1–47.8% on the cation sites.

Deep-ultraviolet polychromatic emission from three-dimensionally structured AlGaN quantum wells

Three-dimensional (3D) AlGaN/AlN quantum wells (QWs) were fabricated on trench-patterned AlN templates using a regrowth technique based on metalorganic vapor phase epitaxy. The 3D structures are composed of planar (0001) facets, facets, and misoriented (0001) planes with bunched steps. Cathodoluminescence spectroscopy revealed double-peaked deep-ultraviolet (DUV) emissions: the shorter-wavelength emission was attributed to the (0001) facets, whereas the longer-wavelength emission arose from bunched step structures located around the bottom corner of the AlN trench, a region in which the AlGaN QWs possessed a relatively high Ga concentration and a thick well width compared with planar (0001) QWs.

Effect of Ga doping on the thermoelectric performance of Cu3SbSe4

The Cu3SbSe4 compound is an environmentally friendly and low-cost medium-temperature thermoelectric material, which is featured by its low thermal conductivity. The disadvantage of this compound lies in its intrinsic poor electrical transport property. In order to improve the electrical conductivity of Cu3SbSe4, in this work we are to increase its carrier concentration by one to two orders of magnitude though elemental doping. The sample composition of Cu2.95GaxSb1-xSe4 is designed to increase the hole carrier concentration by introducing Cu vacancies and substituting Ga3+ for Sb5+. The Cu2.95GaxSb1-xSe4 (x=0, 0.01, 0.02 and 0.04) samples are prepared by melting-quench method. The X-ray diffraction analysis indicates that the obtained samples are of single-phase with the tetragonal famatinite structure, and the energy-dispersive X-ray spectroscopy results show that the actual compositions of the samples are very close to their nominal compositions. The effect of Ga doping on the thermoelectric performance of Cu3SbSe4 compound is investigated systematically by electrical and thermal transport property measurements. According to our experimental results, the hole concentration of the sample is efficiently increased by substituting Sb with a small amount of Ga (x=0.01), which can not only substantially improve the electrical conductivity but also suppress the intrinsic excitation of the sample. The maximum power factor reaches 10 μW/cm·K2 at 625 K for the Ga doped sample with x=0.01, which is nearly twice as much as that of the sample free of Ga. Although the carrier concentration further increases with increasing Ga content, the hole mobility decreases dramatically with the Ga content increasing due to the increased hole effective mass and point defect scattering. Thus, the electrical transport properties of the samples deteriorate at higher Ga content, and the maximum power factors for the samples with x=0.02 and 0.04 reach 9 and 8 μW/cm·K2 at 625 K, respectively. The lattice thermal conductivities of the samples basically comply with the T-1 relationship, suggesting the phonon U-process is the dominant scattering mechanism in our samples. For the samples with x=0 and 0.01, the lattice thermal conductivities at high temperature deviate slightly from the T-1 curve due to the presence of intrinsic excitation. However, these deviations are eliminated for the samples with x=0.02 and 0.04 because the bipolar effect is effectively suppressed with the increasing of Ga content. Thus, Ga doping can reduce the bipolar thermal conductivity at high temperature by increasing the hole carrier concentration. Furthermore, the point defects introduced by Ga doping can also enhance the scattering of high-frequency phonons, leading to slightly reduced lattice thermal conductivities of Ga-doped samples at higher temperature. Finally, a maximum ZT value of 0.53 at 664 K is achieved in Ga-doped sample, which is 50% higher than that of the sample free of Ga.

Mineralogy and trace element geochemistry of sulfide minerals from the Wocan Hydrothermal Field on the slow-spreading Carlsberg Ridge, Indian Ocean

The basalt-hosted Wocan Hydrothermal Field (WHF), located on the NW slope of an axial volcanic ridge at a depth of ∼3000m at 6°22′N on the slow-spreading Carlsberg Ridge, northwest Indian Ocean, was discovered in 2013 during Chinese DY28th cruise. Preliminary investigations show that the field consists of two hydrothermal sites: Wocan-1, which shows indications for recent high-temperature hydrothermal activity, is located near the peak of the axial volcanic ridge at a water depth of 2970–2990m, and Wocan-2 site, located at a water depth of 3100m, ∼1.7km to the northwest of Wocan-1. The recovered hydrothermal precipitates can be classified into four groups: (i) Cu-rich chimneys; (ii) Cu-rich massive sulfides; (iii) Fe-rich massive sulfides; and (iv) silicified massive sulfides. We conducted mineral texture and assemblage observation and Laser-ablation ICP-MS analyses of the hydrothermal precipitates to study the mineralization processes. Our results show that there are distinct systematic trace element distributions throughout the different minerals in the four sample groups. In general, chalcopyrite from the group (i) is enriched in Pb, As, Mo, Ga, Ge, V, and Sb, metals that are commonly referred to as medium- to low-temperature elements. In contrast these elements are present in low contents in the chalcopyrite grains from other sample groups. Selenium, a typical high-temperature metal, is enriched in chalcopyrite from groups (ii) and (iv), whereas Ag and Sn are enriched only in some silicified massive sulfides. As with chalcopyrite, pyrite also shows distinct trace element associations in grains with different habitus. The low-temperature association of elements (Pb, Mo, Mn, U, Mg, Ag, and Tl) is typically present in colloform/framboidal pyrite, whereas the high-temperature association (Se, Co, and Bi) is enriched in euhedral pyrite. Sphalerite in the groups (i) and (iii) at Wocan-1 is characterized by high concentrations of Ga, Ge, Pb, Cd, As, and Sb, indicating that sphalerite in these sample groups likely precipitated at intermediate temperatures. Early bornite, which mainly occurs in the central part of the Cu-rich chimney, is typically enriched in Sn and In compared to the other minerals. In contrast, late bornite that likely formed during increasing interaction of hydrothermal fluids with cold, oxygenated seawater has low Sn and In, but significantly higher concentrations of Ag, Au, Mo and U. Digenite, also forming in the exterior parts of the samples during the late stages of hydrothermal fluid venting, is poor in most trace elements, except Ag and U. The notable Ag enrichment in the late-stage mineral assemblages at both Wocan-1 and Wocan-2 may therefore be related to lower temperatures and elevated pH. Our results indicate that Wocan-1 has experienced a cycle of heating with Cu-rich chimney growth and subsequent cooling, followed by late seafloor weathering, while Wocan-2 has seen intermediate- to high-temperature mineralization followed by intense silicification of sulfides. Seafloor weathering processes or mixing of hydrothermal fluids with seawater during the waning stages of hydrothermal fluid flow result in significant redistributions of trace elements in sulfide minerals.

New insights into the lowest Xuanwei Formation in eastern Yunnan Province, SW China: Implications for Emeishan large igneous province felsic tuff deposition and the cause of the end-Guadalupian mass extinction

A previous study suggested that the lowest Xuanwei Formation is derived from weathered clastic materials of silicic composition from the Emeishan large igneous province (ELIP) based on chemostratigraphic correlations (Al2O3/TiO2 ratios) between the two. In this study, we have adopted the model that the Emeishan mantle plume commenced and terminated within a short duration and have investigated the detailed mineralogy and geochemistry of carefully sampled rocks from the lower sections of the Xuanwei Formation, eastern Yunnan Province, Southwest China. These samples are intensely argillized and characterized by high proportions of clay minerals and quartz. The samples with Al2O3/TiO2>7 from the lowest Xuanwei Formation have an anomalous natural gamma response and high concentrations of Nb, Ta, Zr, Hf, Th, U, Ga and REY (rare earth elements and yttrium). Our results suggest that the samples with Al2O3/TiO2>7 from the lowest Xuanwei Formation represent felsic volcanic tuff instead of acidic clasts as originally proposed. The lowest Xuanwei Formation and the Wangpo Bed are the felsic tuffaceous layers interbedded with clastic rocks derived from the Emeishan high-Ti basalts. Such volcanic layers most likely represent ELIP felsic tuff originated from the extrusive equivalent of Nb–Zr-enriched alkaline syenitic magmatism at the waning stage of Emeishan mantle plume activity. This study has verified the existence of extensive alkaline felsic volcanism of early Late Permian age. Such alkaline volcanism may have been catastrophic and have contributed to the end-Guadalupian mass extinction.

First discovery of a Palaeoproterozoic A-type granite in southern Wuyishan terrane, Cathaysia Block: evidence from geochronology, geochemistry, and Nd–Hf–O isotopes

ABSTRACTIn this study, a combined study of zircon U–Pb and Hf–O isotopes, as well as whole-rock major and trace elements and Nd isotopes has been conducted for Yangjia gneissic granite from the southern Wuyishan terrane, Southeast China, to constrain its petrogenesis and provide a new window for investigating the tectonic evolution of the Cathaysia basement. U–Pb dating for magmatic zircons yields a 207Pb/206Pb age of ca. 1.80 Ga, interpreted as the emplacement age of the Yangjia granite. The granites have relatively high K2O, Rb, Ga, Zr, Nb, Y, and Ce contents and show low Al2O3, CaO, and Ba concentrations. Their 10,000*Ga/Al ratios range between 2.8 and 3.2. Zircons from the granite have εHf(t) values ranging from −13.2 to −7.2, corresponding to THfDM2 model ages of 2.99 Ga to 2.72 Ga. The zircon δ18O values range between 6.7‰ and 9.1‰ with an average of 7.7‰. In addition, the whole-rock εNd(t) values of the granites range from −6.5 to −5.4 and the TNdDM2 model ages from 2.73 Ga to 2.82 Ga. All these geochemical and Nd–Hf–O isotopic signatures suggest an A-type affinity for the Yangjia granites, and they were likely generated by partial melting of Palaeoproterozoic parametamorphic rocks of the Wuyishan terrane in a post-collisional extensional setting. When our data is combined with existing geochronological data, it provides further evidence for the Palaeoproterozoic basement in the southern Wuyishan terrane, which records a rapid tectonic transition from post-collision to intraplate extension (1.80–1.77 Ga) related to the break-up of the supercontinent Columbia.

Effect of electron-electron correlation and site disorder on the magnetic moment and half-metallicity of Co2FeGa1−xSix alloys

X-ray diffraction studies on Co2Fe(Ga1−xSix) alloys did not show the characteristic L21 super-lattice peaks for alloys with x ≤ 0.5. The saturation magnetization (Ms) of alloy compositions with x = 0.75 and x = 1.00 deviated from the values predicted by Slater-Pauling (S-P) rule. In order to understand this deviation in Ms and its implication on the half-metallic character of these alloys, a systematic ab initio study was carried out using generalized gradient approximation (GGA) and GGA with on-site Coulomb interaction (GGA + U). Total magnetic moment obtained from the GGA calculation agrees well with S-P rule and experimental results obtained for alloys with high Ga concentration (i.e. for x = 0 and 0.25) and gets overestimated by the inclusion of U. However, the value predicted by the S-P rule is obtained for the alloys with x = 0.50, 0.75 and 1.00, only after the inclusion of U. Similar behavior was observed in the appearance of the energy gap in density of states of the spin-down band and hence the half-metallic nature of the alloys. The deviation in measured Ms of the alloy with x = 1.00 is attributed to the presence of small amount of DO3 disorder in the alloy as revealed by the ab initio studies. Comparison of experimental and theoretically estimated properties of Co2Fe(Ga1−xSix) alloys provides insight to the variation in electron-electron interaction and the influence on atomic site disorder in these alloys on their half-metallic character.

Single step electrosynthesis of NiMnGa alloys

An electrochemical synthesis route for NiMnGa alloys is presented. Thin films of NiMnGa were fabricated by single step electrodeposition from aqueous electrolytes using direct current over a range of current densities. By electrolyte tuning, homogeneous films with high Ga and Mn content could be achieved at current densities as high as −400mAcm−2. Detailed compositional analysis of the alloys showed that growth was homogeneous and oxygen content was minimized. Films plated at very low current densities were found to be nanocrystalline/amorphous. In order to obtain fully crystalline samples, thermal annealing was carried out. Mechanical characterization was assessed by nanoindentation, and the effect of Ga content on mechanical properties was investigated.

Single crystal studies of binary compounds Ta/Ga – A system with experimental and crystallographic peculiarities

We report on single crystal growth and characterisation of binary intermetallics TaxGay. Single crystals were obtained from mixtures of the elements which were handled under various conditions. Several new compounds were identified. The highest Ga-content is found for the new binary phases Ta6Ga31 and Ta8Ga41. Both compounds evolve as intergrown crystals. Ta8Ga41 belongs to the V8Ga41 type (R3¯, Z=3, a=14.311Å, c=15.344Å), Ta6Ga31 to the triclinic Mo6Ga31-type (P1¯, Z=2, a=9.697Å, b=9.698Å, c=14.879Å, α=87.18°, β=80.83°, γ=85.18°). TaGa3 bases on the tetragonal TiAl3-type (I4/mmm, Z=2, a=3.769Å, c=8.718Å) but shows stacking faults leading to an increased Ta content Ta1+xGa3–x. This interpretation is supported by the structure model of an orthorhombic superstructure with an idealized composition Ta2Ga5 (Cmcm, Z=4, a=3.769, b=31.37, c=3.770Å). Ta2–xGa5+x (x≈0.38) is a representative of the Mn2Hg5-type (P4/mbm, Z=2, a=9.3213(13), c=2.7572(6)Å). Ta4Ga5 represents a new compound with a novel crystal structure (P4/mbm, Z=16, a=11.793(2), c=16.967(3)Å). The complex structure contains polyhedra with coordination numbers between 11 and 14. Ta3Ga2 (P42/mnm, Z=2, a=6.8382(4), c=3.4963(2) Å) belongs to the U3Si2 type. For the composition Ta5Ga3 three different structure types were confirmed but with some differences. The tetragonal W5Si3 type (I4/mcm, Z=4, a=10.2199(7), c=5.1121(4)Å) is a stoichiometric binary compound, the hexagonal Mn5Si3-type (P63/mcm, Z=2, a=7.7023(4), c=5.3062(3) Å) contains a small amount of oxygen (Ta5Ga3O0.4) and in the tetragonal Cr5B3-type (I4/mcm, Z=4, a=6.5986(9), c=11.931(2)Å) one of the Ga-sites shows a significant underoccupation of 40% (“Ta2.2Ga”). Compositions were confirmed by EDX measurements.

Nature of tonsteins in the Azeisk deposit of the Irkutsk Coal Basin (Siberia, Russia)

In this paper data on the mineral and chemical composition of tonsteins from the Jurassic Azeisk deposit located in the Irkutsk Basin (South Siberia, Russia) are presented. The mineral composition includes kaolinite (87–89%); quartz (3.0–4.0%), cristobalite (4.9–5.3%), illite (1.9–2.1%) and feldspar (1.0–2.0%). Accessory minerals comprise zircon, monazite, uranium oxides, native minerals (gold, silver, nickel, zinc, tungsten, and silicon), sulfides and intermetallic compounds. The tonsteins and coals of the Azeisk deposit have high concentrations of REE, Y, Zr, Hf, U, Th, Ta, Sn, Ga, Cu, Pb, Se, Hg, Sb, and Te. The highest concentrations of these elements were found at the contact with coals. The enrichment of the coal with lithophile rare metals is possibly caused by their leaching from the tonsteins. The accumulation of these trace elements is a characteristic feature in coals both above and below the tonsteins. The width of the enrichment in the coal zone depends on the properties of chemical elements and features of the coal properties and character of interaction between the coals and tonsteins. The authors verify the pyroclastic nature of the initial matter of the tonsteins of the Azeisk deposit. Mineralogical and geochemical data from the tonsteins suggest a felsic (rhyolite) composition for the original volcanic matter that was altered in the aggressive acidic environments of the peat bogs. As a result of the burial of this material in a paleo-peatland some of the elements typical of felsic pyroclastics accumulated.

The annual meeting of Coal Ash Asia (2015) in Shuozhou, Shanxi Province, China

The annual meeting of Coal Ash Asia was held in Shuozhou, Shanxi Province, China, from September 21st to 24th, 2015 (CAA 2015). More than 700 attendees of 300 organizations from 15 different countries attended the 4 days’ conference, exhibition and seminars. The CAA 2015 focused on the comprehensive utilization technology of powder coal ash and desulfurized gypsum, with the aim to bring researchers, managers, and industry leaders together to share knowledge and develop highest value solutions for managing and utilizing coal combustion products. High-aluminum coal, which is abundant in Shanxi and Inner Mongolia, not only contains unusually high level of Al but also has high concentrations of Li, Ga, and REY (Sun et al., 2016). These elements with high concentrations in high-Al coal are very valuable source to various industries, and therefor make perfect economic sense and add extra benefits into coal mines and power plants if they can be recovered by beneficiation processes (Sun et al., 2012). The research team from Hebei University of Engineering has taken part in the CAA 2015. They gave series of oral presentations related to the integrated extraction of lithium (carbonate) with aluminum, silicon, and possibly gallium, which represents a feasible way for the high value-added The annual meeting of Coal Ash Asia (2015) in Shuozhou, Shanxi Province, China

The determining factor of a preferred orientation of GaN domains grown on m-plane sapphire substrates.

Epitaxial lateral overgrowth in tandem with the first-principles calculation was employed to investigate the determining factor of a preferred orientation of GaN on SiO2-patterned m-plane sapphire substrates. We found that the (100)-orientation is favored over the (10)-orientation in the region with a small filling factor of SiO2, while the latter orientation becomes preferred in the region with a large filling factor. This result suggests that the effective concentration determines the preferred orientation of GaN: the (100)- and (10)-orientations preferred at their low and high concentrations, respectively. Our computational study revealed that at a low coverage of Ga and N atoms, the local atomic arrangement resembles that on the (10) surface, although the (100) surface is more stable at their full coverage. Such a (10)-like atomic configuration crosses over to the local structure resembling that on the (100) surface as the coverage increases. Based on results, we determined that high effective concentration of Ga and N sources expedites the growth of the (10)-orientation while keeping from transition to the (100)-orientation. At low effective concentration, on the other hand, there is a sufficient time for the added Ga and N sources to rearrange the initial (10)-like orientation to form the (100)-orientation.

Occurrence of some valuable elements in the unique ‘high-aluminium coals’ from the Jungar coalfield, China

In China, a unique type of coal with extremely high aluminium (Al) content commonly occurs in the southern region of the Yinshan Oldland, Jungar Basin. The typical Al2O3 content in the coal ash is ~45%, and exceeds 60% in some cases. This type of coal forms a league of its own and is named as ‘high-Al coal’. This high-Al coal, not only contains unusually high level of Al but also has high concentrations of Li, Ga, and REY (REE+Y). All these elements (i.e. Al, Li, Ga, REY) with high concentrations in this type of coal are very valuable source to various industries and therefor make perfect economic sense and add extra benefits into coal mines if they can be recovered by beneficiation processes.The mineral compositions in these coals mainly consist of kaolinite, boehmite, chlorite-group minerals, along with minor amounts of quartz, calcite, pyrite, siderite and amorphous clay materials. The Al and rare elements show different patterns of distribution and affinity relationships with organic and inorganic matter. The strong peraluminous granites and moyite from the Yinshan Oldland may be the main source of the Al and rare elements in the coals. Bauxite from the Benxi Formation may also be another source for the Al and rare elements in the coal of the Jungar coalfield.

Characterization of Cu(In,Ga)Se2 (CIGS) films with varying gallium ratios

Cu(In1−x,Gax)Se2 (CIGS) absorber layers were deposited on molybdenum (Mo) coated soda-lime glass substrates with varying Ga content (described as Ga/(In + Ga) ratios) with respect to depth. As the responsible mechanisms for the limitation of the performance of the CIGS solar cells with high Ga contents are not well understood, the goal of this work was to investigate different properties of CIGS absorber films with Ga/(In + Ga) ratios varied between 0.29 and 0.41 (as determined by X-ray florescence spectroscopy (XRF)) in order to better understand the role that the Ga content has on film quality. The Ga grading in the CIGS layer has the effect causing a higher band gap toward the surface and Mo contact while the band gap in the middle of the CIGS layer is lower. Also, a wider and larger Ga/(In + Ga) grading dip located deeper in the CIGS absorber layers tend to produce larger grains in the regions of the films that have lower Ga/(In + Ga) ratios. Moreover, it was found that surface roughness decreases from 51.2 nm to 41.0 nm with increasing Ga/(In + Ga) ratios. However, the surface roughness generally decreases if the Ga grading occurs deeper in the absorber layer.

Martensite Transformation and Magnetic Properties of Ni-Fe-Ga Heusler Alloys

Compositional instability and phase formation in Ni-Fe-Ga Heusler alloys are investigated. The alloys are synthesized into two-phase microstructure. Their structures are identified as fcc and L21, respectively. The γ-phase formation could be suppressed with higher Ga-content in the alloy as Ga stabilizes austenite phase, but Ga lowers the martensite transformation temperature. The increase of Fe content improves the magnetization value and the increase of Ni from 52 to 55 at. pct raises the martensite transformation temperature from 216 K to 357 K (−57 °C to 84 °C). Magnetic properties and martensitic transformation behavior in Ni-Fe-Ga Heusler alloys follow opposite trends, while Ni replaces either Fe or Ga, whereas they follow similar trends, while Fe replaces Ga. Modulated martensite structure has low twinning stress and high magneto crystalline anisotropic properties. Thus, the observation of 10- and 14 M-modulated martensite structures in the studied Ni-Fe-Ga Heusler alloys is beneficial for shape memory applications. The interdependency of alloy composition, phase formation, magnetic properties, and martensite transformation are discussed.

Strong effects of minor Ga addition on liquid-state Sn–Ga/Co interfacial reactions

Liquid-state interfacial reactions between Co and Sn–Ga solders at 250 °C were examined with addition of various Ga contents (0.1wt.%–1wt.%). Minor Ga addition in Sn-based solders strongly affected thegrowth of intermetallic compounds (IMCs) at the interface and even the IMC species. With addition of less than 0.3wt.%Ga, one continuous CoSn3 layer was formed. Most importantly, the CoSn3 growth was greatly decreased by 95% with only 0.2wt.%Ga addition. Multi-stage interfacial reactions with different solders demonstrated that the strong inhibition of IMC growth was likely due to minor Ga doping in the CoSn3 phase and a possible mechanism was proposed. When the Ga content was increased to 0.4wt.%–0.6wt.%, only a thin uniform CoGa layer, being ∼1 μm thick, was stably present at the interface. Even after aging for 12 h, it had no significant growth. At higher Ga contents (0.7wt.%–1wt.%), the dominant phase changed to the CoGa3 phase in the initial reaction. Subsequently, the CoGa3 gradually transformed to the CoGa. In addition, the partial isothermal section (less than 40at.%Co) of the Sn–Ga–Co phase diagram at 250 °C was experimentally determined. Two three-phase regions, L–CoSn3–CoGa and L–CoGa–CoGa3 tie-triangles, were identified. The phase equilibria relationship can reasonably interpret the transitions of interfacial products with varying Ga contents.

Study of Ga incorporation in glassy arsenic selenides by high-resolution XPS and EXAFS

Effect of Ga addition on the structure of vitreous As2Se3 is studied using high-resolution X-ray photoelectron spectroscopy and extended X-ray absorption fine structure techniques. The "8-N" rule is shown to be violated for Ga atoms and, possibly, for certain number of As atoms. On the contrary, Se keeps its 2-fold coordination according to "8-N" rule in the amorphous phase throughout all the compositions. Crystalline inclusions appear in the amorphous structure of the investigated glasses at Ga concentrations greater than 3 at. %. These inclusions are presumably associated with Ga2Se3 crystallites and transition phases/defects formed at the boundaries of these crystallites and host amorphous matrix. The existence of Ga-As and Se-Se bonds in the samples with higher Ga content is supported by present studies.

Fabrication of Cu(In,Ga)Se2 solar cells with a single graded band profile

It was found that Cu(In,Ga)Se2 (CIGS) solar cells with single graded (SG) band profile and average bandgap Eg(avg) of 1.15 eV has higher efficiency than double graded (DG) band profile with the same Eg(avg), despite no intentional bandgap‐widening at a CdS/CIGS interface in SG‐CIGS. To explain this contradiction, we focused on the band structure at the CdS/CIGS interface and proposed that the contradiction can be explained by the formation of an ordered vacancy chalcopyrite compound (OVC) layer on the surface of CIGS. Device simulation using wxAMPS revealed that if an OVC layer exists at the interface, conversion efficiency of SG‐CIGS solar cells overcomes that of DG‐CIGS solar cells due to suppression of surface recombination at the interface, leading to open‐circuit voltage (VOC) improvement. We applied the SG structure to CIGS solar cells with high Ga contents. As a result, the conversion efficiency of 15.1% for the SG‐CIGS solar cell with Eg(avg) of 1.4 eV was achieved, although in contrast, that of the DG‐CIGS solar cells was 12.9%. This efficiency is comparable to the highest efficiency in CIGS solar cells with high Ga contents. Therefore, SG band profile is promising structure and it is expected to achieve high efficiency predicted essentially in CIGS solar cells with high Ga contents.

Investigation of the properties of deep-level defect in Cu(In,Ga)Se2 thin films by steady-state photocapacitance and time-resolved photoluminescence measurements

The quantitative characterization of the deep-level defect located at around 0.8 eV above the valence band in Cu(In1−x,Gax)Se2 (CIGS) thin films was performed by steady-state photocapacitance (SSPC) and time-resolved photoluminescence (TRPL) measurements. The defect concentration, photoionization cross section and electron capture cross section were evaluated to be on the order of 1013–1014 cm−3, 10−17–10−16 cm2, and 10−14 cm2, respectively, in the Ga content x range from 0.30 to 0.80; the first two values increased with increasing Ga content while the last one remained almost constant. By comparing the obtained values with the actual device performance, the cause of the degradation of the conversion efficiency of CIGS solar cells with higher Ga content was discussed in relation to the 0.8 eV defect.

Synthesis and characterization of single-crystal Cu(In,Ga)Se2nanowires: high Ga contents and growth behaviour

Precise control over the defect density, a high Ga content, and uniform stoichiometry are critical for controlling the physical and optical properties of Cu(In,Ga)Se2 (CIGS) nanowires (NWs). In this study, we investigated the synthesis of epitaxially grown, single-crystal CIGS NWs by a vapour-phase transport method using multiple sources of Ga2Se3, In2Se3, and Cu2Se as the precursors. No catalysts were employed, and r-cut Al2O3 substrates were used for the fabrication of the NWs. The synthesized CIGS NWs had a uniform composition along their length, and the NWs with the highest Ga/(In + Ga) content ratio (0.8) had a chalcopyrite structure. The bandgap energy of the CIGS NWs was higher than that of typical CIGS thin films grown by co-evaporation methods because of the high Ga content ratio. These single-crystal CIGS NWs offer an attractive platform for exploring various concepts related to hierarchical nanostructures and devices based on fully epitaxial semiconductor structures.