Sulphurisation of gallium-containing thin-film precursors analysed in-situ

It has been demonstrated that rapid thermal sulphurisation of sputtered Cu/In precursor layers is suitable for industrial production of thin-film photovoltaic modules. The process is relatively straightforward and the underlying fundamental aspects, such as phase formation sequence and reaction rates, have been studied intensively. Using lab-scale preparation technology, incorporation of gallium is known to improve transport properties of the absorber and to enable the fabrication of wide-gap cells. In this work we have used energy dispersive in-situ X-ray diffraction to study the sulphurisation of sputtered Cu:Ga/In precursors. It is the basis for the future development of an industrially feasible production of Cu(In,Ga)S₂ films. Precursor stacking sequence and sulphur partial pressure in relation to precursor temperature have been varied. In many cases, in particular when establishing sulphur partial pressure already at low precursor temperature, we observe a severe reduction of reaction rates after going from pure Cu to Cu:Ga in the precursor. In consequence, single phase films cannot be prepared within the feasible ranges of time and temperature. Adhesion failure and at least intermediate formation of CuIn₅S₈ are other problems frequently encountered. In spite of these problems, promising pathways to single phase Cu(In,Ga)S₂ films prepared from sputtered Cu:Ga/In precursors have now been identified.     

Solid-state gallium NMR characterization of cubic gallium nitride prepared by the reaction of gallium arsenide with ammonia

The reaction of cubic gallium arsenide (GaAs) with ammonia yielded gallium nitride (GaN). Powder X-ray diffraction patterns of the GaN products showed that they are a mixture of c- and w-GaN, while their Ga MAS NMR spectra revealed that they have the other phase of GaN besides c- and w-GaN and the high reaction temperature (≥900 °C) induces nitrogen deficiency in GaN. The peaks at 353 and 347 ppm in the ⁷¹Ga MAS NMR spectra were tentatively assigned to c-GaN and an intermediate of w- and c-GaN in the stacking order, respectively. The observed ⁷¹Ga chemical shifts of GaN, GaP, GaAs and GaSb in cubic phase were well correlated with the reciprocal of their band gaps.     

Effect of copper-deficiency on multi-stage co-evaporated Cu(In,Ga)S2 absorber layers and solar cells

Solar cell absorber films of Cu(In,Ga)S₂ have been fabricated by multi-stage co-evaporation resulting in compositional ratios [Cu]/([In] + [Ga]) = 0.93-0.99 and [Ga]/([In] + [Ga]) = 0.15. Intentional doping is provided by sodium supplied from NaF precursor layers of different thicknesses. Phases, structure and morphology of the resulting films are investigated by X-ray diffraction (XRD) and scanning electron microscopy. The XRD patterns show CuIn₅S₈ thiospinel formation predominantly at the surface in order to accommodate decreasing Cu content. Correlated with the CuIn₅S₈ formation, a Ga-enrichment of the chalcopyrite phase is seen at the surface. Since no CuS layer is present on the as-deposited films, functioning solar cells with CdS buffer and ZnO window layers were fabricated without KCN etch. The open-circuit voltage of solar cells correlates with the copper content and with the amount of sodium supplied. The highest efficiency cell (open-circuit voltage 738 mV, short-circuit current 19.3 mA/cm², fill factor 65%, efficiency 9.3%) is based on the absorber with the least Cu deficiency, [Cu]/([In] + [Ga]) = 0.99. The activation energy of the diode saturation current density of such a cell is extracted from temperature- and illumination-dependent current-voltage measurements. A value of 1.04 eV, less than the band gap, suggests the heterojunction interface as the dominant recombination zone, just as in cells based on Cu-rich grown Cu(In,Ga)S₂.     

Electrodeposition of wide band gap InGaxSyOz thin films for solar cell applications

Indium-Gallium-sulfide-oxide thin films were deposited onto F-doped SnO₂-coated glass by electrochemical deposition from an aqueous bath. The films were deposited at three different ratios of gallium to indium in the precursor bath; namely [Ga/In] = 2/8, 5/5 and 8/2. The impact of the gallium content on the composition, optical transmission, structure, photosensitivity, electrical resistivity and morphology of the deposited films was investigated. The films deposited at [Ga/In] = 5/5 and 8/2 had an energy gap as high as 3.5 eV. The X-ray diffraction spectrum of the film deposited at [Ga/In] = 2/8 contained weak peaks of indium metal, but the In peaks were absent in the spectra of the films deposited at [Ga/In] = 5/5 and 8/2. The photosensitivity of the film was observed by means of photoelectrochemical measurements, which confirmed that all the films showed n-type conduction. Finally, the film has been used as a buffer layer to fabricate a SnS-based thin film solar cell.     

Investigation on the role of indium in the removal of metallic gallium from soft and hard sputtered GaN (0001) surfaces

Cleaning of GaN by argon sputtering and subsequent annealing introduces metallic gallium on the GaN surface. Once formed, this metallic gallium can be difficult to remove. It has a strong influence on the Fermi level position in the band gap and poses a problem for subsequent epitaxial growth on the surface. We present a method of removing metallic gallium from moderately damaged GaN surfaces by deposition of indium and formation of an In-Ga alloy that can be desorbed by annealing at ~ 550 °C. After the In-Ga alloy has desorbed, photoemission spectra show that the Ga3d bulk component becomes narrower indicating a smoother and more homogeneous surface. This is also reflected in a sharper low energy electron diffraction pattern. On heavily damaged GaN surfaces, caused by hard sputtering, larger amount of metallic gallium forms after annealing at 600 °C. This gallium readily alloys with deposited indium, but the alloy does not desorb until a temperature of 840 °C is reached and even then, traces of both indium and metallic gallium could be found on the surface.     

Synthesis of GaN nanoparticles from NH4[Ga(OH)2CO3] under a flow of ammonia gas

In this investigation, we report the synthesis of gallium nitride (GaN) nanoparticles from ammonium-carbonato-dihydroxo-gallate (NH₄[Ga(OH)₂CO₃]) in the flow of NH₃ gas in a temperature range of 500-900 °C. The GaN nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR). The FTIR and XPS revealed that the conversion of NH₄[Ga(OH)₂CO₃] to GaN under a flow of ammonia proceeds stepwise via amorphous gallium oxynitrides (GaO_xN_y) intermediates. Nanosized GaN particles with an average diameter of approximately 20-40 nm were obtained. The results obtained demonstrate that the large-quantity nanosized GaN particles can be synthesized from NH₄[Ga(OH)₂CO₃] powders.     

Carbon nanotube composite coated platinum electrode for detection of Ga(III)

This study demonstrates the application of composite multi-walled carbon nanotube (MWCNT) polyvinylchloride (MWNT-PVC) based on 7-(2-hydroxy-5-methoxybenzyl)-5,6,7,8,9,10-hexahydro-2H benzo [b][1,4,7,10,13] dioxa triaza cyclopentadecine-3,11(4H,12H)-dione ionophore for gallium sensor. The sensor shows a good Nernstian slope of 19.68 ± 0.40 mV/decade in a wide linear range concentration of 7.9 × 10(-7) to 3.2 × 10(-2)M of Ga(NO(3))(3). The detection limit of this electrode is 5.2 × 10(-7)M of Ga(NO(3))(3). This proposed sensor is applicable in a pH range of 2.7-5.0. It has a short response time of about 10s and has a good selectivity over nineteen various metal ions. The practical analytical utility of this electrode is demonstrated by measurement of Ga(III) in river water.     

Influence of Cu off-stoichiometry on wide band gap CIGSe solar cells

The electric properties of solar cells based on co-evaporated Cu(In,Ga)Se₂ (CIGSe) thin film show a good tolerance regarding the absorber Cu content (y = [Cu]/([In] + [Ga])) for standard Ga concentration, i.e. x = [Ga] / ([In] + [Ga]) ~ 0.3. In the present contribution, we show that this tolerance is lost when the gallium content is increased. Wide bandgap CIGSe samples (x ~ 0.55) with a variation in y from 0.97 to 0.84 have been grown. The efficiency of the cells decreases from 12.6% to 6.5% for y = 0.97 and 0.84 respectively. For the lowest y, the efficiency is harmed because of a low short-circuit current density (J_sc), an increased voltage dependency in the current collection, which affects the fill factor (FF), and a decrease of the open-circuit voltage (V_oc). For y = 0.97 and 0.84 respectively, the decrease of the activation energy (E_a) from 1.36 to 1.24 eV indicates a shift of the area of the dominant recombination from the space charge region towards the interface. There seems to be evidence that reducing the Cu-content in the CIGSe thin film will cause a decrease in the width of the space charge region. Solar cells based on Cu-rich CIGSe (1.03 < y < 1.09) have also been fabricated and characterized. A strong deterioration of their electrical properties is observed despite the KCN etch of the segregated Cu_2 − xSe binary phases at the surface, suggesting the presence of residual Cu_2 − xSe precipitates within the layer.     

Recovery of gallium and vanadium from gasification fly ash

The Puertollano Integrated Coal Gasification Combined Cycle (IGCC) Power Plant (Spain) fly ash is characterized by a relatively high content of Ga and V, which occurs mainly as Ga2O3 and as Ga3+ and V3+ substituting for Al3+ in the Al-Si fly ash glass matrix. Investigations focused on evaluating the potential recovery of Ga and V from these fly ashes. Several NaOH based extraction tests were performed on the IGCC fly ash, at different temperatures, NaOH/fly ash (NaOH/FA) ratios, NaOH concentrations and extraction times. The optimal Ga extraction conditions was determined as 25 degrees C, NaOH 0.7-1 M, NaOH/FA ratio of 5 L/kg and 6 h, attaining Ga extraction yields of 60-86%, equivalent to 197-275 mg of Ga/kg of fly ash. Re-circulation of leachates increased initial Ga concentrations (25-38 mg/L) to 188-215 mg/L, while reducing both content of impurities and NaOH consumption. Carbonation of concentrated Ga leachate demonstrated that 99% of the bulk Ga content in the leachate precipitates at pH 7.4. At pH 10.5 significant proportions of impurities, mainly Al (91%), co-precipitate while >98% of the bulk Ga remains in solution. A second carbonation of the remaining solution (at pH 7.5) recovers the 98.8% of the bulk Ga. Re-dissolution (at pH 0) of the precipitate increases Ga purity from 7 to 30%, this being a suitable Ga end product for further purification by electrolysis. This method produces higher recovery efficiency than currently applied for Ga on an industrial scale. In contrast, low V extraction yields (<64%) were obtained even when using extreme alkaline extraction conditions, which given the current marked price of this element, limits considerably the feasibility of V recovery from IGCC fly ash.     

Effects of laser scans on the diffusion depth and diffusivity of gallium in n-type 4H-SiC during laser doping

An n-type 4H-SiC substrate has been doped with gallium using a continuous wave Nd:YAG laser to heat the sample to high temperatures but below the peritectic temperature of SiC. Mathematical models have been presented for the temperature and Ga concentration distributions in the sample. The Ga atoms, which are produced due to the thermal decomposition of a metallorganic precursor, diffuse into the sample by the solid-phase diffusion process at high temperatures. This process is modeled by considering the temperature-dependent diffusion coefficient and the Ga concentration profile was measured by the secondary ion mass spectrometry (SIMS). The concentration of Ga (6.25 × 10²⁰ cm⁻³) at the substrate surface was found to exceed the solid solubility limit (1.8 × 10¹⁹ cm⁻³) of Ga in SiC. Comparing the SIMS data to the results of the diffusion model, the activation energy, pre-exponential factor and diffusion coefficient of Ga were determined for different doping conditions. Four doped samples were produced by scanning the samples with a laser beam for different number of passes. The sample prepared with four passes showed the highest diffusion coefficient of 5.53 × 10⁻⁷ cm²/s with activation energy 1.84 eV and pre-exponential factor 1.05 × 10⁻² cm²/s. The diffusion coefficient is five orders of magnitude higher than the typical diffusion coefficient of Ga in SiC. This indicates that the laser doping process enhances the diffusion coefficient of dopant significantly.     

Preparation of wide gap Cu(In,Ga)S2 films on ZnO coated substrates

We have prepared Cu(In,Ga)S₂ films at growth temperatures from 300 °C to 580 °C with a homogeneous gallium depth distribution (estimated band gap 1.67 eV) onto soda lime glass (SLG) substrates with one of three different kinds of back contact: Mo(1000 nm), ZnO(500 nm), and Mo(30 nm)/ZnO(500 nm), respectively. We have also investigated the depth profiles of Zn and Na (diffused from SLG) in Cu(In,Ga)S₂ films by secondary ion mass spectroscopy (SIMS). The efficiency of solar cells on Mo increases with increasing growth temperature. It is higher on Mo/ZnO than on ZnO, and increases from 350 °C to 450 °C, then decreases above 450 °C. It was observed by SIMS that the amount of Zn in Cu(In,Ga)S₂ on Mo/ZnO is lower than it is on ZnO up to 450 °C, and a large amount of Zn diffuses into absorbers over 450 °C, which contributes to decreasing efficiency. The amount of Na in the back contact increases with growth temperature. The depth distribution of Na in Cu(In,Ga)S₂ films on Mo is almost constant in the order of 10¹⁷-10¹⁸ cm^− 3, on ZnO and Mo/ZnO the Na concentration increases towards the surface and is in the range of 10¹⁵-10¹⁷ cm^− 3.     

Influence of absorber copper concentration on the Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS based solar cells performance

The present contribution deals with the influence of the copper concentration in Cu(In,Ga)Se₂ (CIGSe) on the solar cells based on CIGSe/(PVD)In₂S₃ and CIGSe/(CBD)CdS. We find that, depending on the buffer layer, the optimum open circuit voltage (Voc) is not reached for the same copper concentration. The values of Voc for the CIGSe/(CBD)CdS solar cells are higher when the copper content is very close to stoichiometry (25%), whereas, the Voc values for CIGSe/(PVD)In₂S₃ solar cells attain their maximum for lower copper contents. On the other hand, contrary to the case of the (CBD)CdS buffer, the Jsc is strongly hindered for the (PVD)In₂S₃ buffered cells when the copper content is lowered. The study has been made for different absorber gallium contents and the evolution is coherent with the presence of a cliff at the CIGSe/(PVD)In₂S₃ interface.     

Simple synthesis of ZrO2/SiOx core/shell nanofibers using ZrSi2 with gallium

ZrO₂/SiO_x core/shell nanofibers with diameter ~ 50 nm were synthesized by the thermal oxidation of ZrSi₂ substrates with gallium. The crystalline ZrO₂ cores were grown with amorphous SiO_x shells. It is proposed that the growth of crystalline ZrO₂ core was guided by the prior supersaturation of Zr species in the molten gallium film, whereas the amorphous SiO_x shell could be attributed to the deposition of SiO vapor on the surface of ZrO₂ core. In addition, the ZrO₂/SiO_x core/shell nanofibers show a wide visible photoluminescence (PL) emission at 480 nm, which should originate from the SiO_x shells.     

Determination of the scattering lengths of gallium isotopes by neutron interferometry with PNO-apparatus in JRR-3M

For carrying out experiments in the field of the so-called precise neutron optics (PNO), we have implemented special multi-purpose apparatus called the “PNO-apparatus” at JRR-3M. Making use of an Si triple-Laue (LLL) neutron interferometer with the PNO-apparatus, we successfully determined the coherent neutron scattering lengths of gallium isotopes, ⁶⁹Ga and ⁷¹Ga. The results are 8.053±0.013 fm for ⁶⁹Ga and 6.170±0.011 fm for ⁷¹Ga, respectively.     

Transport properties of CuGaSe2-based thin-film solar cells as a function of absorber composition

The transport properties of thin-film solar cells based on wide-gap CuGaSe₂ absorbers have been investigated as a function of the bulk [Ga]/[Cu] ratio ranging from 1.01 to 1.33. We find that (i) the recombination processes in devices prepared from absorbers with a composition close to stoichiometry ([Ga]/[Cu] = 1.01) are strongly tunnelling assisted resulting in low recombination activation energies (E_a) of approx. 0.95 eV in the dark and 1.36 eV under illumination. (ii) With an increasing [Ga]/[Cu] ratio, the transport mechanism changes to be dominated by thermally activated Shockley-Read-Hall recombination with similar E_a values of approx. 1.52-1.57 eV for bulk [Ga]/[Cu] ratios of 1.12-1.33. The dominant recombination processes take place at the interface between CdS buffer and CuGaSe₂ absorber independently from the absorber composition. The increase of E_a with the [Ga]/[Cu] ratio correlates with the open circuit voltage and explains the better performance of corresponding solar cells.     

Method of obtaining gallium from aluminate solution by electrolysis

Abstract

Gallium is an increasingly important material in the fields of semiconductors and energy transfer. A prime source of gallium is the aluminate solution that remains after the purification of bauxite. The authors have sought a way of reclaiming gallium economically by electrolysis of a laboratory aluminate solution without having to use a cathode of mercury – an environmental pollutant. Cathodes of copper, indium, 70In–30Ga, Wood's alloy, and mercury (for comparison) were used with a wide range of anodes. The study accounted for the effects of electrode material, temperature, current density, and initial gallium concentration on the yield, energy consumption, and utilization of both current and electrode. The best results were obtained with indium or In–Ga cathodes and with platinum, Pt–Ti, or stainless steel anodes, at 75°C and a current density of ~100 Am^?2. Electrolysis was more efficient the higher the gallium concentration, demonstrating that commercial–scale electrolysis of aluminate solution is unlikely to be economically viable without prior concentration of gallium.

MST/305     

Comparative study of Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS heterojunction based solar cells by admittance spectroscopy, current-voltage and spectral response measurements

Co-evaporated Cu(In,Ga)Se₂ (CIGSe) based solar cells with Physical Vapour Deposited (PVD) Indium Sulphide (In₂S₃) as buffer layer have been studied by admittance spectroscopy and current-voltage characteristics measurements. The results have been compared to those obtained with a reference CBD-CdS/CIGSe device. In darkness, the PVD-In₂S₃ buffer layer devices exhibit higher densities of trapping defects and low values of shunt resistance. However, under illumination we have observed an important improvement of the In₂S₃/CIGSe electronic transport properties. This behavior seems to be linked to the presence of a metastable defect with activation energy of 0.3 eV.     

Epitaxial growth of gallium nitride by ion-beam-assisted evaporation

The epitaxial growth of gallium nitride thin film was obtained on the inclined Si(111) substrates by the process of ion-beam-assisted evaporation (IBAE) at the low temperature of 500 °C. The film composition determined by Rutherford backscattering spectrometry shows that the synthesized film is a stoichiometric nitride. The epitaxial quality of GaN film is enhanced by minimizing the bombardment-induced film damage by decreasing the ion flux. However, the crystallinity of the GaN film becomes very poor when the ion flux is not sufficient to densify the film. The optimum flux ratio of N⁺₂ to Ga and the energy of incident N⁺₂ ions for the epitaxial growth were found to be 3.4 and 50 eV, respectively. The GaN film deposited on the 4 °-inclined Si (111) with respect to substrate surface shows much better crystalline quality compared with that on the 0 ° inclined Si(111) due to many stable nucleation sites. A thin amorphous layer exists at the interface between GaN and Si(111) substrate and acts as a buffer zone enabling the subsequent epitaxial growth of GaN by relaxing the large misfit strain (23%) in the early stage of film growth. The epitaxial GaN film shows an orientational relation with the Si(111) substrate.     

Plasma-plasmonics synergy in the Ga-catalyzed growth of Si-nanowires

This paper reports on the growth of Si nanowires (NWs) by SiH₄/H₂ plasmas using the non-noble Ga-nanoparticles (NPs) catalysts. A comparative investigation of conventional Si-NWs vapour–liquid–solid (VLS) growth catalyzed by Au NPs is also reported. We investigate the use of a hydrogen plasma and of a SiH₄/H₂ plasma for removing Ga oxide shell and for enhancing the Si dissolution into the catalyst, respectively. By exploiting the Ga NPs surface plasmon resonance (SPR) sensitivity to their surface chemistry, the SPR characteristic of Ga NPs has been monitored by real time spectroscopic ellipsometry in order to control the hydrogen plasma/Ga NPs interaction and the involved processes (oxide removal and NPs dissolution by volatile gallium hydride). Using in situ laser reflectance interferometry the metal catalyzed Si NWs growth process has been investigated to find the effect of the plasma activation on the growth kinetics. The role of atomic hydrogen in the NWs growth mechanism and, in particular, in the SiH₄ dissolution into the catalysts, is discussed. We show that while Au catalysts because of the re-aggregation of NPs yields NWs that do not correspond to the original size of the Au NPs catalyst, the NWs grown by the Ga catalyst retains the diameter dictated by the size of the Ga NPs. Therefore, the advantage of Ga NPs as catalysts for controlling NWs diameter is demonstrated.     

Characterizations of gallium-doped ZnO films on glass substrate prepared by atmospheric pressure metal-organic chemical vapor deposition

Ga-doped zinc oxide (ZnO:Ga) films were grown on glass substrate by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) using diethylzinc and water as reactant gases and triethyl gallium (TEG) as an n-type dopant gas. The structural, electrical and optical properties of ZnO:Ga films obtained at various flow rates of TEG ranging from 1.5 to 10 sccm were investigated. X-ray diffraction patterns and scanning electron microscopy images indicated that Ga-doping plays an important role in forming microstructures in ZnO films. A smooth surface with a predominant orientation of (101) was obtained for the ZnO:Ga film grown at a flow rate of TEG = 7.5 sccm. Moreover, a lowest resistivity of 3.6 × 10^− 4 Ω cm and a highest mobility of 30.4 cm² V^− 1 s^− 1 were presented by the same sample, as evaluated by Hall measurement. Otherwise, as the flow rate of TEG was increased, the average transmittance of ZnO:Ga films increased from 75% to more than 85% in the wavelength range of 400-800 nm, simultaneously with a blue-shift in the absorption edge. The results obtained suggest that low-resistivity and high-transparency ZnO films can be obtained by AP-MOCVD using Ga-doping sufficiently to make the films grow degenerate and effect the Burstein-Moss shift to raise the band-gap energy from 3.26 to 3.71 eV.