Ga Incorporation Research Articles

Effect of Ga incorporation in the As30Se50Te20 glass

The effect of Ga addition on the physical properties and structure of As30−xGaxSe50Te20 alloys is studied using atomic density measurements, differential scanning calorimetry, X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, optical spectroscopy in visible and IR regions, as well as extended X-ray absorption fine structure techniques. The process of As30Se50Te20 glass crystallization owing to Ga additives (1–10at.%) is shown to be related to extraction of droplet-like nanoinclusions in As28Ga2Se50Te20 glassy-like alloy and larger microcrystallites basically with a structure of cubic Ga2Se3 polymorphs as observed for As25Ga5Se50Te20 and As20Ga10Se50Te20. It was possible to introduce only 1at.% of Ga (substituting As) without essential crystallization within the studied system. This glass was successfully drawn into fiber and can be used for rare earth doping. The microcrystalline seeds present in As28Ga2Se50Te20 alloy can serve as a basis for further development of crystallization-controlled ceramization to produce high-reliable far-IR transmitting media.

Solution-Processed Ga-Doped ZnO Nanorod Arrays as Electron Acceptors in Organic Solar Cells

This paper reports the utilization of ZnO nanorod arrays (NRAs) doped with various concentrations of Ga (0, 0.5, 1, 2, and 3 at %) as electron acceptors in organic solar cells. The donor, poly(3-hexylthiophene) (P3HT), was spin coated onto Ga-doped ZnO NRAs that were grown on fluorine-doped tin oxide (FTO) substrates, followed by the deposition of a Ag electrode by a magnetron sputtering method. Adjusting the Ga precursor concentration allowed for the control of the structural and optical properties of ZnO NRAs. The short circuit current density increased with increasing Ga concentration from 0 to 1 at %, mainly because of improved exciton dissociation and increased charge extraction. Meanwhile, the reduced charge recombination and lower hole leakage current led to an increase in the open circuit voltage with Ga concentrations up to 1 at %. The device with the optimum Ga concentration of 1 at % exhibited power conversion efficiency nearly three times higher compared to the device without Ga doping. This finding suggests that the incorporation of Ga can be a simple and effective approach to improve the photovoltaic performance of organic solar cells.

Dielectric Properties and Microstructure of Y-Al-Ga-Si Co-Doped Barium Titanate Ceramics

Multilayer ceramic capacitors (MLCC) are important functional components of electronic information technology. The development of AC MLCC requires low dielectric loss and high AC breakdown voltage. In this paper, Y-Al-Ga-Si co-doped barium titanate ceramics were prepared by conventional solid state method. Microstructures, surface morphology and dielectric properties were investigated by X-ray diffraction, SEM, and LCR analyzer, respectively. Y3+ entered into the lattice of BaTiO3, replaced A-sites and B-sites, suppressed grain growth effectively, and made crystal structure change from tetragonal to pseudo-cubic, which reduced dielectric loss and lowered the Curie peak. The sintering characteristic and permittivity can be improved by the incorporation of Al and Ga. BaTiO3 -0.06Y2O3 - 0.02Ga2O3 -0.01Al2O3 -0.01SiO2 ceramics sintered at 1380°C achieved good dielectric properties: εr= ~2223, tanδ =~1.1% (at 1kHz), ΔC/C25 <~15.26% (from 55°C to 150°C).

Photoelectrochemical Characterizations of CuInS<sub>2</sub> and Cu(In,Ga)S<sub>2</sub> Thin Films Fabricated by a Spray Pyrolysis Method

Polycrystalline CuInS2 chalcopyrite thin films were formed on a Mocoated glass substrate by annealing of a spray deposited precursor film in a sulfur atmosphere at 600 °C. Partial incorporation of Ga in the CuInS2 film with a Ga/In ratio of ca. 0.2 to form a Cu (In,Ga)S2 mixed crystal was also prepared. Photoelectrochemical (PEC) analyses revealed that the Ga incorporation was effective to modulate electric and semiconductive properties of the chalcopyrite film. As a result, relatively large cathodic photocurrent responses in PEC analyses as well as high photovoltaic properties of a solar cell based on the Cu (In,Ga)S2 film were obtained.

Characteristics of CuIn1−xGaxS2 thin films synthesized by chemical spray pyrolysis

CuIn1−xGaxS2 multi-component semiconductors thin films were prepared by chemical spray pyrolysis on glass substrates using different concentrations of gallium in the spray solutions (y=([Ga3+]/[In3+]) varying from 0 to 20at% by a step of 5at%). Samples were characterized using X-ray diffraction, Raman spectroscopy, Atomic Force Microscopy, photoluminescence spectroscopy, spectrophotometric and Hall effect measurements. The X-ray spectra reveal that the CuIn1−xGaxS2 thin films are of chalcopyrite crystalline phase with a highly (112) preferential orientation. The best crystallinity is obtained for 10at% Ga incorporation since the maximum (112) peak intensity and grain size are obtained at this Ga incorporation rate. The level of the residual microstrain and dislocation network seems to be reduced respectively to the values 0.09% and 4×108linesmm−2 for an optimum y=10at% for which the crystallinity of CuIn1−xGaxS2 thin layers is the best one. Raman spectra indicate that the sprayed thin films are grown only with CH-ordering. Optical analysis by means of transmission T(λ) and reflection R(λ) measurements allow us to determine the direct band gap energy value which increases by increasing the Ga content and it is in the range 1.39–1.53eV, indicating that CuIn1−xGaxS2 compound has an absorbing property favorable for applications in solar cell devices. Photoluminescence measurements are performed on CuIn1−xGaxS2 crystals and the analysis reveals that the emission is mainly due to donor–acceptor pair transitions. The film resistivity (ρ) and Hall mobility (μ) are strongly affected by Ga incorporation rate. The lowest resistivity (ρ=0.1Ωcm) and maximum value of Hall mobility (μ=0.5cm2V−1s−1) are also obtained for the thin layers prepared with y=10at%. Finally, we reported two new structures for CuInS2/β-In2−xAlxS2/ZnO:Al and CuIn1−xGaxS2 (y=10at%)/β-In2-xAlxS2/ZnO:Al solar cells to investigate the effect of gallium incorporation on the photovoltaic parameters. We found that the Ga-containing cell shows conversion efficiency (η=1.6%) higher than the Ga-free reference cell due to higher open-circuit voltage (Voc=540mV) and short-circuit current density (Jsc=10mAcm−2).

MOVPE growth of AlxGa1‐xN with x ∼ 0.5 on epitaxial laterally overgrown AlN/sapphire templates for UV‐LEDs

AbstractThe crystalline perfection of AlxGa1‐xN layers with Al content of x ∼ 0.5 on epitaxial laterally overgrown (ELO) AlN with low defect density was investigated by spatially and spectrally resolved cathodoluminescence, scanning electron and atomic force microscopy. Cathodoluminescence mapping reveals compositional variations caused by different Ga incorporation during growth over macro‐steps resulting from step bunching on the ELO‐AlN. Introducing AlN interlayers as marker the influence of the macrosteps as well as of the growth rate on the development of Ga‐rich regions was studied. With increasing growth rate from 0.2 to 1 µm/h the spatial extension of the Ga‐rich regions is strongly reduced. At the same time the difference in Ga content is increased. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A conductive ZnO–ZnGaON nanowire-array-on-a-film photoanode for stable and efficient sunlight water splitting

We report highly stable and efficient sunlight water splitting on a ZnO–ZnGaON nanowire-array-on-a-film photoanode without the assistance of any co-catalyst. The single crystalline ZnO–ZnGaON nanowire-array-on-a-film photoanode was synthesized via a high-temperature vapor-phase diffusion reaction of gallium (Ga) and nitrogen (N) on a single crystal domain ZnO nanowire-array-on-a-film structure. The synthesized ZnO–ZnGaON photoanode offers visible light absorption through N incorporation, improved electrical conductivity via Ga incorporation, and structural advantages with the high-aspect-ratio nanowire array. Compared to the chemically unstable ZnO nanowire photoanode, the ZnO–ZnGaON nanowire photoanode significantly improves the anti-photocorrosive ability for water splitting. A highly stable photocurrent density of ∼1.5 mA cm−2 is obtained with the ZnO–ZnGaON nanowire photoanode at an applied bias of 0.8 VRHE under continuous sunlight illumination over five hours without noticeable degradation.

Morphological, Structural, and Optical Properties of Single-Phase Cu(In,Ga)Se2Thin Films from the Selenization of Thermally Evaporated InSe/Cu/GaSe Precursors

The relatively small band gap values (~1 eV) of CuInSe2thin films limit the conversion efficiencies of completed CuInSe2/CdS/ZnO solar cell devices. In the case of traditional two-stage growth techniques, limited success has been achieved to homogeneously increase the band gap by substituting indium with gallium. In this study, thermal evaporation of InSe/Cu/Gase precursors was exposed to an elemental Se vapour under defined conditions. This technique produced large-grained, single-phase Cu(In,Ga)Se2thin films with a high degree of in-depth compositional uniformity. The selenization temperature, ramp time, reaction period, and the effusion cell temperature with respect to the Cu(In,Ga)Se2films were optimized in this study. The homogeneous incorporation of Ga into CuInSe2led to a systematic shift in the lattice spacing parameters and band gap of the absorber films. Under optimized conditions, gallium in cooperation resulted only in a marginal decrease in the grain size, X-ray diffraction studies confirmed single-phase Cu(In,Ga)Se2material, and X-ray photoluminescence spectroscopy in-depth profiling revealed a uniform distribution of the elements through the entire depth of the alloy. From these studies optimum selenization conditions were determined for the deposition of homogeneous Cu(In,Ga)Se2thin films with optimum band gap values between 1.01 and 1.21 eV.

Effect of Ga Incorporation and Film Thickness on the Optical Properties of as-Deposited Amorphous GaxSe1-x Thin Films

Flash evaporated amorphous GaxSe1-x (x = 0, 1, 2, 3, and 4 atomic %) on glass substrates have been investigated within a 500nm-1500nm spectral range. Film thicknesses explored were ; 200±10nm, 265±10nm, 330±10nm, and 400±10nm. The effect of film thickness and gallium content on the as-deposited thin films has been established. As the gallium content increases, both the optical transmittance and band gap energy decrease. Increase in film thickness led to a decrease in optical transmittance and an increase in the band gap energy. The other optical parameters like the absorption coefficient, extinction coefficient, refractive index, real part and imaginary part of dielectric constant increase with increase in gallium content and film thickness.

Determination of Ga auto-incorporation in nominal InAlN epilayers grown by MOCVD

We report on the consistent measurement of gallium incorporation in nominal InAlN layers using various complimentary techniques, underpinned by X-ray diffraction. Nominal InAlN layers with similar growth conditions were prepared, and the change in unintended Ga content in the group III sublattice ranged from ∼24% to ∼12% when the total reactor flow rate was increased from 8000 to 24 000 standard cubic centimetres per minute. Ultra-thin InAlN/GaN HEMT layers were grown in a clean reactor to minimize Ga auto-incorporation, and measured using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The implications of Ga incorporation in InAlN layers within optoelectronic and power devices is discussed.

V-defects formation and optical properties of InGaN/GaN multiple quantum well LED grown on patterned sapphire substrate

To enhance light extraction efficiency, high-quality InGaN/GaN multiple quantum well (MQW) LED was grown on cone-shaped patterned sapphire substrate (CPSS) by using metal organic chemical vapor deposition (MOCVD). In the growth of InGaN multiple quantum well structure, all V-defects are not always connected with threading dislocations (TDs) at their bottom, also many V-defects are generated from the stacking mismatch boundaries induced by stacking faults which are formed within the MQW due to the strain relaxation. The formation of the V-defect is kinetically controlled by reduced Ga incorporation on the pyramid walls ({101¯1} planes). The improvement of luminous intensity and reverse I–V characteristic (the leakage currents of the LEDs grown on CPSS and USS were −0.23μA and −0.76μA, respectively, at the reverse voltage of −10V) was explained by considering not only an increase of the extraction efficiency via the suppressed total internal reflection at the CPSS interface but also a decrease of V-defect density.

Tuning the gallium content of metal precursors for Cu(In,Ga)Se2 thin film solar cells by electrodeposition from a deep eutectic solvent

Controlling the Ga incorporation of Cu-In-Ga metal precursors for Cu(In,Ga)Se2 (CIGS) solar cells is one of the main challenges for low cost electrodeposition processes, mainly due to the difficulty in electrodepositing metallic Ga from aqueous electrolytes. In this work we use the deep eutectic solvent (DES) Choline Chloride : Urea (ChCl : U - 1 : 2) to efficiently codeposit In-Ga on Cu and Mo electrodes. We control the Ga/(Ga+In) (Ga/III) ratio of the films via the mass fluxes. The electrochemical behavior of ChCl : U containing GaCl3 and InCl3 is studied by rotating disk electrode cyclic voltammetry (CV) on Mo and Cu electrodes. CV revealed on both Mo and Cu electrodes that the electrochemical behavior of the ChCl : U-GaCl3-InCl3 system is the superposition of the individual In and Ga electrochemistry. On a Cu electrode the morphology, crystal structure and element distribution of the deposits were a function of the Ga/III ratio. We demonstrate the precise control of Ga incorporation over a large composition range from 0.1 ≤ Ga/III ≤ 0.9 and proved that ED from DES is a straightforward, robust and efficient process. First solar cells based on Mo/Cu/In-Ga metal stacks achieved efficiencies as high as 7.9% with a Voc of 520 mV.

Doping Effect of Electron Transport Layer on Nanoscale Phase Separation and Charge Transport in Bulk Heterojunction Solar Cells

We investigated the influence of the Ga doping in the ZnO interlayer as an electron transport layer (ZnO ETL) on the nanoscale phase separation in the bulk heterjunction (BHJ) layer coated on the ETL as well as the morphological and electrical properties of a low temperature sol–gel-derived pristine ZnO ETL (P-ETL) and Ga-doped ZnO ETL (G-ETL), which affect the performance of inverted organic solar cells (IOSCs). X-ray photoelectron spectroscopy (XPS) confirms the successful incorporation of the element Ga in the ZnO ETL. The short circuit current densities (JSC) of IOSCs fabricated using a G-ETL were significantly improved from those of IOSCs fabricated with a P-ETL. The maximum JSC was obtained at 2 at. % Ga doping. The IOSCs fabricated with a 2 at. % G-ETL demonstrated power conversion efficiencies of 3.51% (P3HT:PC60BM) and 5.43% (PCDTBT:PC70BM), which were higher than the power conversion efficiencies of 2.88% (P3HT:PC60BM) and 4.90% (PCDTBT:PC70BM) of the IOSCs fabricated with a P-ETL under simulated air mass 1.5 global full-sun illumination. The better performance was attributed to the improved electrical properties of the G-ETL and the enhanced nanoscale phase separation in the BHJ active layer.

Origins of unintentional incorporation of gallium in AlInN layers during epitaxial growth, part I: Growth of AlInN on AlN and effects of prior coating

We propose a new origin of unintentional gallium (Ga) incorporation during the epitaxial growth of AlInN layers. We observed substantial amount of Ga in AlInN layers on GaN-free underlying layers and wafer carrier grown by metalorganic chemical vapor deposition, even though the Ga precursor was not introduced during the growth. The Al(Ga)InN layers were characterized by high-resolution X-ray diffraction and Rutherford backscattering spectrometry. The mole fraction of Ga in the Al(Ga)InN layers showed strong dependence on conditions of the growth chamber, making it difficult to maintain controlled alloy compositions only by growth process parameters. Also, Ga incorporation was not observed under the same growth parameters and conditions, when the indium (In) precursor was absent. The formation of In–Ga eutectic system between metallic Ga from the deposition on the chamber surrounding surfaces and adsorbed In is suggested as a possible pathway for the Ga incorporation. Finally, we performed an intentional chamber coating process prior to growth of AlInN to achieve repeatable Al(Ga)InN growth with stable alloy compositions.

Marked suppression of In incorporation in heavily Si-doped InxGa1−xN (x ∼ 0.3) grown on GaN/α-Al2O3(0001) template

InxGa1−xN (x ∼ 0.3) films doped with Si up to 1022 cm−3 are grown on GaN/α-Al2O3(0001) templates by the metal-organic vapor phase epitaxy using monomethylsilane (MMSi) as a Si source. The In composition in InGaN is markedly decreased with increasing MMSi flow rate; In compositions are 0.32 and 0.12 for 0 and 4.3 μmol/min flow rates, respectively. The sum of concentrations of Si and In in InGaN is independent on MMSi flow rate and the Ga incorporation is not affected by the Si doping, suggesting that Si atoms are preferably incorporated into sites where In atoms should be incorporated.

Unintentional Ga incorporation in metalorganic vapor phase epitaxy of In-containing III-nitride semiconductors

We prepared InAlN barrier layer films on GaN buffer layers using the metalorganic vapor phase epitaxy (MOVPE) method and investigated the InAlN/GaN heterointerfaces. Secondary ion spectroscopy experiments revealed that a quaternary alloy of InAlGaN is grown on GaN even when trimethylindium (TMIn) and trimethylaluminum (TMAl) are exclusively supplied as group-III precursors, indicating that Ga is unintentionally incorporated into the InAlN layers. This Ga incorporation is also observed in InGaN/GaN heterostructures. Our systematic investigations of the growth condition dependence, such as the TMIn flow rate, indicate that the Ga is supplied by a transmetalation reaction between TMIn and residual Ga on the flow distributor in the reactor. Here, we show that the Ga incorporation can be eliminated by adopting an elaborate growth sequence, including reactor cleaning and regrowth processes. This study provides guides for designing the MOVPE reactor configuration, as well as the growth sequences, for the growth of device structures with In-containing nitride layers.

Competitive In and Ga incorporations for InxGa1-xN (0.29<x<0.36) nanorods grown at a moderate temperature

The kinetics of In and Ga incorporation into wurtzite InxGa1−xN nanorods, grown by plasma-assisted MBE under N-rich conditions at a moderate temperature, has been systematically investigated with Ga-flux set as a growth parameter at three distinct values while varying In-flux. The interplay of Ga and In fluxes in their contributions to the incorporation was found to disagree with the empirical Böttcher's formula, of which the reliability is based on the assumption of preeminent Ga incorporation. The competition between Ga and In for incorporations involves, we believe, the displacement of In from the weaker In-N bonds by Ga to form the Ga-N bonds at high In and Ga fluxes.

Low temperature growth of GaAs1−yBiy epitaxial layers

A comparative study is presented of the effect of reactant sequencing during the metalorganic vapor phase epitaxy of GaAs1−yBiy on epilayer composition and structural properties. The simultaneous introduction of precursors was compared with a pulsed or alternating reactant flow. Pulsed growth resulted in a more controlled incorporation of Bi into the GaAs epitaxial layers and led to a more well-defined superlattice structure as determined by X-ray diffraction. The effect of growth temperature (370–420°C) and precursors flow rate on the film properties and Bi incorporation was determined. While growth rate of GaAs decreased with decreasing growth temperature, the GaAs1−yBiy growth rate was almost temperature insensitive over this investigated temperature range. The catalytic effect of Bi metal or the trimethyl bismuth reactant on the decomposition and incorporation of Ga is considered to rationalize this observed behavior. The specific choice of reactant flow and temperature can strongly influence the material properties of the GaAs1−yBiy films. These observations suggest the possibility of the growth of GaAs1−yBiy hetero-structures at the temperatures below what was assumed to be the lowest possible temperature for the growth of GaAs.

Transient and self-limited nanostructures on patterned surfaces

Site-controlled quantum dots formed during the deposition of (Al)GaAs layers by metalorganic vapor-phase epitaxy on GaAs(111)B substrates patterned with inverted pyramids result in geometric and compositional self-ordering along the vertical axis of the template. We describe a theoretical scheme that reproduces the experimentally-observed time-dependent behavior of this process, including the evolution of the recess and the increase of Ga incorporation along the base of the template to stationary values determined by alloy composition and other growth parameters. Our work clarifies the interplay between kinetics and geometry for the development of self-ordered nanostructures on patterned surfaces, which is essential for the reliable on-demand design of confined systems for applications to quantum optics.

Dramatic Effects of Gallium Promotion on Methanol Steam Reforming Cu–ZnO Catalyst for Hydrogen Production: Formation of 5 Å Copper Clusters from Cu–ZnGaOx

A new class of copper, zinc, and gallium mixed oxides (CuZnGaOx) with different chemical compositions obtained by a coprecipitation technique is identified as a highly active catalyst for the low-temperature, direct steam reforming of methanol to supply hydrogen gas to portable fuel cell devices. Their catalytic activity and selectivity are found to be critically dependent on the copper surface area, catalyst structure, and metal–support interaction, etc. As a result, temperature-programmed reduction has been used to investigate the copper ion reducibility and resulting copper speciation; N2O chemisorption and advanced microscopies to determine specific copper surface area, dispersion, and particle size; XRD to investigate the catalyst structure; EPR spectroscopy to probe the environment of Cu2+ species; and AC impedance spectroscopy to probe the mobility of trapped ions in solids. It is proposed that Ga incorporation into Cu–Zn oxide leads to the formation of a nonstoichiometric cubic spinel phase contai...