Increasing Ga Content Research Articles

Low frequency conductivity study of gallium‐substituted magnesium–copper spinel ferrite

AbstractMixed ferrites with the chemical formula Mg0.5Cu0.5GaxFe2–xO4 (0.0 ≤ x ≤ 0.5) were prepared by a ceramic route. X‐ray diffraction results confirmed the presence of single‐phase spinel structure for these samples. Electrical properties of these ferrites at various compositions were investigated from room temperature up to 550 K in the frequency range 102–105 Hz. The obtained results revealed semiconductor behavior at high frequency and metallic behavior at low frequency in these materials. All studied compositions exhibit a transition with a change in the slope of the conductivity versus temperature curve. The obtained transition temperature TC is found to decrease linearly with increasing Ga concentration. As Ga substitutes Fe in these compounds the activation energy for electrical conduction in the ferrimagnetic region (Ef) has a small variation with frequency. In contrast, the activation energy in the paramagnetic region (Ep) shows undefined behavior reflecting the nature of this disordered state. The relation of the universal exponent s with temperature gives evidence for the presence of the large overlap polaron (LOP) hopping conduction mechanism in these compounds. The obtained results are explained in the light of the cation–anion–cation and cation–cation interactions present over the octahedral B‐sites in the spinel structure. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of Ga content on the wetting reaction and interfacial morphology formed between Sn–8.55Zn–0.5Ag–0.1Al–xGa solders and Cu

Abstract The wetting properties of Sn–8.5Zn–0.5Ag–0.1Al– x Ga lead-free solders were investigated. The wetting time and wetting angle decreased while the wetting force increased with the increase in Ga content. The intermetallic compounds were Al 4.2 Cu 3.2 Zn 0.7 , Cu 5 Zn 8 , Cu 5 Zn 8 and AgZn 3 in the solder. The activation energy and the surface tension of the solder were estimated.

Magnetic properties of Y2Fe17−x Gax and Sm2Fe17−xGax compounds

The magnetic properties of Y2Fe17−x Ga x and Sm2Fe17−xGa x for 3 ≤ x ≤ 7 have been investigated using the 57Fe Mossbauer spectroscopy at room temperature. These compounds have the rhombohedral Th2Zn17 structure. X-ray diffraction analyses of aligned powders show that the easy direction of magnetization is parallel to the c-axis in Y2Fe10Ga7 and Sm2Fe14Ga3 and is perpendicular to the c-axis in Y2Fe14Ga3, Y2Fe12Ga5, Sm2Fe12Ga5 and Sm2Fe10Ga7. Mossbauer studies indicate that all the samples studied are ferromagnetically ordered. The 57Fe hyperfine field decreases with increasing Ga content. This decrease results from the decreased magnetic exchange interactions resulting from Ga substitution. The average isomer shift, δ, for Y2Fe17−xGa x and Sm2Fe17−xGa x at room temperature is positive and the magnitude of δ increases with increasing Ga content.

Synthesis, structure, and magnetic properties of SrMn 1−xGa xO 3−δ ( x=0–0.5) perovskites

We report the synthesis of SrMn 1− x Ga x O 3− δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques derived from thermogravimetric measurements, we have extended the solubility limit of random substitution of Ga 3+ for Mn in the cubic perovskite phase to x = 0.5 . In the cubic perovskite phase the maximum oxygen content is close to 3− x/2, which corresponds to 100% Mn 4+. Maximally oxygenated solid solution compounds are found to order antiferromagnetically for x = 0 – 0.4 , with the transition temperature linearly decreasing as Ga content increases. Increasing the Ga content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn 0.5Ga 0.5O 2.67(3) below 12 K. These properties are markedly different from the long-range antiferromagnetic order below 180 K observed for the layer-ordered compound Sr 2MnGaO 5.50 with nominally identical chemical composition.

PEMOCVD of ZnO thin films, doped by Ga and some of their properties

Zinc oxide (ZnO) is a promising semiconductor material with a great variety of applications, for example for highly conductive films for transparent electronics. Recently, Ga has been proposed as a dopant, exhibiting the advantages of a very similar atomic radius compared to Zn, a smaller reactivity, and a higher resistivity to oxidation compared to its competitor Al. In this study ZnO films, doped by Ga, were produced on Al2O3(0001) substrates by PEMOCVD. The doping was realized with 1, 3, 5 and 10 wt% gallium precursor content in the mixture. The resistivity of the prepared films, as well as the morphology and the transmittance, was investigated. All the deposited films have demonstrated a high optical transmittance above 93% in the range between 400 and 800 nm. A strong correlation between the electrical resistivity and the optical band gap depending on the Ga content was observed. An AFM analysis demonstrated highly uniform and smooth surfaces. The average grain size and route mean square roughness decreased with increasing Ga content.

Investigations on the structural and magnetic properties of doubly substituted Nd 2Fe 17− x− yTi xGa y compounds (0 ≤ x ≤ 1.0, 0 ≤ y ≤ 3)

The structural and magnetic properties of doubly substituted Nd 2Fe 17− x− y Ti x Ga y (0 ≤ x ≤ 1.0, 0 ≤ y ≤ 3) compounds have been investigated by a combined technique of X-ray diffraction, neutron diffraction and magnetic measurements. Rietveld refinements of the diffraction data indicate that all the samples crystallize in the rhombohedral Th 2Zn 17-type structure. For a given Ti content ( x), the lattice parameters a and c, and unit cell volume V of Nd 2Fe 17− x− y Ti x Ga y all increase linearly with increasing Ga content. The addition of Ti initially has a considerably positive effect on the increasing rates of a, c, and unit cell volume V, but later the effect becomes very slight and even negative with the further increase of Ti content. The site occupancies of Ti and Ga in the crystallographic sites change a little compared to what is observed in the corresponding singly substituted compounds. The effects of Ti and Ga on the bond lengths of the doubly substituted compounds are quite different from that of the singly substituted compounds. Magnetic measurements show the T C of Nd 2Fe 17− x− y Ti x Ga y increases with increasing Ti content for a low Ga content while it behaves conversely for a higher Ga content. The T C of Nd 2Fe 17− x− y Ti x Ga y increases with increasing Ga content for a particular Ti content, while the addition of Ti results in a slower increase of T C on the Ga content ( y ≤ 3). For a given Ti content, the M s of Nd 2Fe 17− x− y Ti x Ga y first increases a little and then decreases with the increase of Ga content, while for a given Ga content the M s of Nd 2Fe 17− x− y Ti x Ga y decreases with the increase of Ti content.

Structural and electro‐optical characteristics of Ga‐doped ZnO films prepared by thermal oxidation of metallic Zn100–xGax films

AbstractGa‐doped ZnO thin films have been obtained by thermal oxidation of electron beam evaporated metallic Zn100–xGax thin films. The structural, optical, and electrical resistivity of the oxidized films depend on Ga content. The value of oxidation temperature shifts to higher temperatures with increasing Ga content. After oxidation, polycrystalline ZnO phase is formed and no Ga2O3 peaks are found. The Ga content is found to affect transmittance strongly and the optical bandgap slightly. The optical bandgap values of the oxidized Ga‐doped films are comparable at higher oxidation temperatures. Depending on the Ga content, the electrical resistivity increases with increasing oxidation temperature. At 550 °C all the Ga‐doped films have electrical resistivity values around 105 Ω cm. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Flowering of the Grass Lolium perenne. Effects of Vernalization and Long Days on Gibberellin Biosynthesis and Signaling

Almost 50 years ago, it was shown that gibberellin (GA) applications caused flowering in species normally responding to cold (vernalization) and long day (LD). The implication that GAs are involved with vernalization and LD responses is examined here with the grass Lolium perenne. This species has an obligatory requirement for exposure to both vernalization and LD for its flowering (inflorescence initiation). Specific effects of vernalization or LD on GA synthesis, content, and action have been documented using four treatment pairs: nonvernalized or vernalized plants exposed to short days (SDs) or LDs. Irrespective of vernalization status, exposure to two LDs increased expression of L. perenne GA 20-oxidase-1 (LpGA20ox1), a critical GA biosynthetic gene, with endogenous GAs increasing by up to 5-fold in leaf and shoot. In parallel, LD led to degradation of a DELLA protein, SLENDER (within 48 h of LD or within 2 h of GA application). There was no effect on GA catabolism or abscisic acid content. Loss of SLENDER, which is a repressor of GA signaling, confirms the physiological relevance of increased GA content in LD. For flowering, applied GA replaced the need for LD but not that for vernalization. Thus, GAs may be an LD, leaf-sourced hormonal signal for flowering of L. perenne. By contrast, vernalization had little impact on GA or SLENDER levels or on SLENDER degradation following GA application. Thus, although vernalization and GA are both required for flowering of L. perenne, GA signaling is independent of vernalization that apparently impacts on unrelated processes.

Magnetic properties of R 2Fe 17− xGa x (R=Y and Gd) compounds

The magnetic properties of Y 2Fe 17− x Ga x for 3≤ x≤7 and Gd 2Fe 17− x Ga x for 5≤ x≤7 have been investigated using 57Fe Mössbauer spectroscopy. These compounds have the rhombohedral Th 2Zn 17 structure. X-ray diffraction analysis of aligned powders shows that the easy direction of magnetization is parallel to the c-axis in Y 2Fe 10Ga 7 and Gd 2Fe 10Ga 7 and is perpendicular to the c-axis in Y 2Fe 14Ga 3, Y 2Fe 12Ga 5, Gd 2Fe 12Ga 5 and Gd 2Fe 11Ga 6. Mössbauer studies indicate that those samples are ordered ferromagnetically. The 57Fe hyperfine field decreases with increasing Ga content. This decrease results from the decreased magnetic exchange interactions resulting from Ga substitution. The average isomer shift, δ, for R 2Fe 17− x Ga x (R=Y and Gd) at room temperature is positive and the magnitude of δ increases with increasing Ga content.

Field emission properties of self-assembled InN nano-structures: Effect of Ga incorporation

This work reports the field emission properties of InN and In-rich InGaN nano-structures. Pyramid InN with a tip-like feature exhibits field emission characteristics in a vacuum. Adding Ga into InN reduces the surface roughness and degrades the turn-on electric field. However, when the Ga content is as low as 2%, the same roughness of the film is almost maintained and the barrier height is lower than that of InN. Ultraviolet photoemission spectroscopic analyses show a gradual reduction on the surface work function of InGaN films when the Ga content increases. From the perspectives of surface roughness and surface work function, the In 0.98Ga 0.02N sample has the best field emission properties of all of the samples studied herein. A turn-on field of 11 V/μm and an emission current density of 1×10 −2 A/cm 2 (at 30 V/μm) in the In 0.98Ga 0.02N epilayer were achieved.

Cation distribution, hyperfine parameters and conduction mechanism in the ferrimagnetic system Cu0.5Co0.5GaxFe2–xO4

AbstractCompositions having the chemical formula Cu0.5Co0.5GaxFe2–xO4with (0.0 ≤x≤ 0.5) have been prepared by the usual ceramic processing from high‐purity oxides. X‐ray diffraction measurements are used for analyzing the composition, lattice parameter, density, porosity and grain size of these compounds. The area ratio of Fe3+at the tetrahedral A‐ and octahedral B‐sites deduced from the spectral analysis of Mössbauer measurements on the present spinel system gives evidence that Ga replaces Fe at A‐site in the range 0.0 <x≤ 0.3. As the Ga content increases (x> 0.3), it replaces Fe at B‐site. This change in the site preference reflects an abrupt change in the hyperfine parameters with Ga content atx> 0.3. The electrical measurements as a function of temperature and frequency on these materials revealed semiconducting behavior. These results show a significant increase in the frequency dependence of conductivity as Ga content increases. Variation of the universal exponentswith temperature indicates the presence of small‐polaron (SP) hopping mechanism at low Ga content. As Ga substitution progressed, the overlap large polaron (OLP) becomes the predominant hopping mechanism in these compounds. The determined transition temperatureTCof these samples is found to decrease linearly with increasing Ga contentx. The results obtained in this study are interpreted in light of the cation–cation and cation–anion–cation interactions in the spinel structure. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A magnetization study of ErMn6Sn6−xGax single crystals (0.11 ≤ x ≤ 1.20)

Dc and ac magnetic measurements have been performed on single crystals ofHfFe6Ge6 type ErMn6Sn6−xGax compounds prepared by the flux method. At low temperature all the compoundsdisplay easy-axis magnetization and undergo a spin reorientation transition toeasy-plane magnetization below room temperature. The complete ferrimagnetic order(μsat = 4 μB fu−1) is reached at 10 K only in the easy direction. From low field magnetization measurementsat 10 K, a first order magnetic process (FOMP) has been detected for all the compoundsand ascribed to the competition between the higher order magnetocrystalline terms. Anadditional transition detected from the imaginary part of the ac susceptibility for thex = 0.85 and 1.20 compositions was explained in terms of displacement of Bloch walls. Theanisotropy constants, evaluated, following the Sucksmith and Thompson method,for some compositions at room temperature, decrease at increasing Ga content.

Excitonic luminescence of Cu(In,Ga)Se 2

In the present work, we report the results of photoluminescence (PL) studies on Cu(In,Ga)Se 2 thin films with varying Ga contents. The samples are epitaxially grown on a GaAs substrate by means of metal-organic vapour phase epitaxy (MOVPE) at slight Cu excess ([Cu]/[III]≈1,1). The polycrystalline samples are prepared by coevaporation on glass. To determine the nature of the observed emissions, PL experiments are performed with varying excitation intensities and temperatures. For the first time, excitonic luminescence is observed in Cu(In,Ga)Se 2. It is seen for [Ga]/([Ga]+[In]) ratios (GGI) ≤0.25 and ≥0.75 that exciton binding energies and band gaps are found to rise with increasing Ga content and to be in good accordance with the hydrogen model. Emissions at lower energies are shown to be due to a donor–acceptor pair (DAP) transition equivalent to the DA1 transition in CuInSe 2 and CuGaSe 2, as well as their phonon replicas. The defect depth was found to increase with increasing Ga content, as expected from the hydrogen model. The study is supplemented by cathodoluminescence (CL) investigations of epitaxial layers and by PL measurements of polycrystalline thin films.

Optical properties of Er-doped Gax(Ge0.3Se0.7)100−x glasses

Optical properties of Ga x (Ge 0.3 Se 0.7 ) 100− x glasses doped with Er 2 S 3 have been investigated as a function of Ga and Er concentrations. The increase of Ga concentration from 0 to 13 at.% has decreased the optical gap E g 03 (corresponding to an absorption coefficient 10 3 cm −1 ) from 2.35 to 2.13 eV. The Urbach tail energy width has been observed to increase only slightly with Ga addition, and it is in the range 70–80 meV. Three absorption lines of Er 3+ ions corresponding to 4 I 15/2 → 4 I 13/2 , 4 I 15/2 → 4 I 11/2 and 4 I 15/2 → 4 I 9/2 transitions have been observed at 1538, 984 and 806 nm. In some samples with low content of Ga the line at 660 nm corresponding to transition 4 I 15/2 → 4 F 9/2 has been observed. The efficiency of absorption at 1538 nm (calculated as the ratio of integral 4 I 15/2 → 4 I 13/2 absorption per atomic concentration of embedded Er) showed a strong dependence on the Ga concentration ( C Ga ). At C Ga > 9 at.% this value was the highest and did not depend on Er concentration. The estimation of Judd–Ofelt parameters gave the values of Ω 2 = (13 ± 2) × 10 −20 cm 2 , Ω 4 = (3.4 ± 0.2) × 10 −20 cm 2 and Ω 6 = (1.3 ± 0.1) × 10 −20 cm 2 , which are in good agreement with data published in literature for other chalcogenide glasses.

In -rich In1−xGaxN films by metalorganic vapor phase epitaxy

Single crystalline In1−xGaxN films containing high In content (70%–100%) were grown by metalorganic vapor phase epitaxy. A linear relation was observed between the lattice constants and gas phase Ga∕In ratios. The surface morphology changed from pyramid for InN to more planar ones for the InGaN alloys with increasing Ga content. The electron mobility decreased rapidly from 1200cm2∕Vs for InN to less than 100cm2∕Vs for In0.7Ga0.3N, with a carrier concentration of low- 1019cm−3 for all the as-grown films. Using photoluminescence a single emission peak was observed at 1.4–1.6μm for the In-rich InGaN with decreasing wavelengths up to below 20% of Ga. Two peaks were observed for the In0.80Ga0.20N, however, indicating possible phase separation. The x-ray photoelectron spectroscopic measurement showed shifts to higher binding energies for both In and Ga with increasing Ga content. The estimated alloy composition, however, depended sensitively on the sputtering conditions of the samples.

Solar cells based on CCSVT-grown CuGaSe 2—absorber and device properties

For solar cells based on CuGaSe 2 (CGSe) polycrystalline thin films, a novel efficient chemical close-spaced vapor transport (CCSVT) technique is used to deposit the CGSe absorber. Clean and Mo-coated soda lime glass (SLG) substrates are used for the CGSe deposition. The CGSe thickness ranges from 1.6 to 1.9 μm and the corresponding [Ga]/[Cu] ratio of the thin films is adjusted within the range of 0.9–1.3. The high bulk homogeneity of the as-grown CGSe films is shown. Transmittance and reflectance measurements were performed to monitor the changes in the CGSe band gap as a function of composition. The optical spectra reveal a shift of the absorption edge towards longer wavelengths with increasing Ga content, as well as a broadening of the distinct structure corresponding to three band-to-band transitions characteristic for CGSe. These results are used for a discussion of the behaviour of the ZnO/CdS/CuGaSe 2 solar cell photovoltaic parameters as well of the current transport. The transport mechanism is analysed on an our best solar cell device with an active area efficiency of 8.7%. For the first time, a thermally activated Shockley–Read–Hall recombination mechanism is observed for the CGSe-based solar cells in a large temperature region.

The effect of Ga-grading in CIGS thin film solar cells

In this paper the effect of an in depth variation of the Ga/(In+Ga) ratio in CIGS based thin film solar cells is presented. The conclusions made are based on a review of earlier publications, theoretical considerations and results from a large set of new devices. For standard devices with normally thick CIGS films (1.5–2 μm) deposited at a relatively long deposition time (60 min) an improved efficiency of around 0.4% units for the devices with an increased Ga/(In+Ga) ratio towards the back contact is observed. This improvement is due to a field assisted carrier collection resulting in an improved QE response at long wavelengths. When the CIGS thickness is reduced the importance of the increased Ga/(In+Ga) ratio towards the back contact is enhanced and at a CIGS thickness of 0.5 μm a gain of 2.5% units is obtained. The gain is due to an improved V oc and FF. The main reason for the improvement is passivation of the back contact, which becomes increasingly detrimental for the device performance as the CIGS thickness is reduced. Also for pure CIS a significant improvement of the device performance is obtained by introducing an increased Ga concentration towards the back contact. This improvement is, however, more related to the introduction of Ga itself than the gradient of the Ga-concentration. From many simulations the largest gain is predicted for an increased Ga/(In+Ga) ratio towards the CIGS surface. However, neither in the literature nor from our own experiments we can find evidence for an improved device performance due to an increased Ga-concentration towards the CIGS surface.

Physical, Magnetic and Electrical Properties of Ga Substituted Mn-Ferrites

Samples with the chemical formula MnGaxFe2-xO4 (x = 0.0, 0.1, 0.3, 0.5 and 0.7) were prepared by the usual ceramic method. X-ray analysis revealed the presence of single spinel phase. The lattice parameters as well as the porosity were found to decrease while the density and grain size were increased with increasing Ga concentration (x). The magnetization, the initial permeability and Curie temperature are decreased with increasing x. On the other hand, the dc electrical resistivity is increased with increasing Ga concentration. The results are explained according to the assumed cation distribution.

Microstructure and mechanical properties of low Ga content Sn–8.55Zn–0.5Ag–0.1Al–xGa solders

Abstract The Sn–8.5Zn–0.5Ag–0.1Al– x Ga solders were investigated. The results showed that the microstructure of the solders were eutectic. The addition of Ga dramatically decreased the melting point of solders. Notably, the increase in Ga content of the solder improved the solderability, the tensile strength and reduced the ductility.

Composition, temperature and frequency dependence of dielectric parameters in Ga-substituted Co-Cu mixed ferrites

Gallium-substituted Co–Cu mixed ferrites, with different Ga contents, were prepared by a conventional ceramic technique. Single-phase spinel structure was confirmed using X-ray diffraction. The effect of Ga substitution on the structure and dielectric properties of the compounds was studied, where dielectric permittivities (e′, e″) and dielectric loss tangent (tan δ) were measured as a function of frequency and temperature. These parameters were found to be strongly dependent on Ga concentration. The frequency dependence of the dielectric loss tangent showed abnormal behaviour where more than one relaxation peak was observed at different frequencies. This behaviour was attributed to the hopping processes between cation–anion–cation interactions over the octahedral sites in the spinel structure. The Curie temperature Tc determined from measurements of dielectric loss versus temperature was found to decrease with increasing Ga content. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)