Excess Ga Research Articles

Record High Thermoelectric Figure of Merit of a III-V Semiconductor InGaSb by Defects Engineering via the Addition of Excess Constituent Elements.

Materials with enhanced electron and reduced phonon transport properties are preferred for thermoelectric applications. The defect engineering process can optimize the interrelated electron and phonon transport properties to enhance thermoelectric performance. As the influence of various crystalline defects on the functional properties of materials is diverse, it is crucial to scale, optimize, and understand them experimentally. With this perspective, crystalline defects in InGaSb ternary alloys were engineered and their influence on the thermoelectric properties was studied experimentally. Crystalline defects such as point defects, dislocations, and compositional segregations were induced in In0.95Ga0.05Sb crystals by the addition of excess constituent elements, In, Ga, or Sb. The addition of excess Ga increased point defects, whereas excess Sb reduced dislocation densities. The thermoelectric figure of merit value (ZT) of In0.95Ga0.05Sb+Ga0.02 was recorded to be 0.87 at 573 K, which is the highest among other reported values of III-V semiconductors. The collective interactions of compositional segregations, point defects, and dislocations with electrons and phonons enhanced the ZT in this study.

Effect of Ga on microstructure and properties of Ag50CuSn12 and Al2O3/oxygen-free copper welds

The effects of Ga on the properties of Ag50CuSn12 and brazed joints were studied. The results show that Ga can reduce the melting temperature of the Ag50CuSn12 and improve its wettability. With the increase of Ga content, the spreading area of the solder increases first and then decreases. And the spreading area is the largest when the content of Ga is 3 wt%, which is 48% higher than that without Ga. Ga plays an important role in grain refinement of weld microstructure. The size of the block particles in the weld decreases accompanied by the increase of Ga. The fine strip grains increase is evenly distributed. The refinement effect is also the best when the Ga content is 3 wt%. For this reason, the shear strength of the brazed joint of Al2O3 and oxygen-free copper increases first and then decreases. When the Ga content is 3 wt%, the shear strength is the highest, which is 2.2 times that of the joint without Ga.The analysis suggests that the incorporation of Ga lowers the melting point of the solder, improves its oxidation resistance, and reduces its surface tension and viscosity, thereby enhancing its wettability and diffusivity. However, excessive Ga can hinder the solder's spread.

Modulation of GaAs nanowire growth by pre-treatment of Si substrate using a Ga focused ion beam

We reveal a novel phenomenon observed after self-catalytic growth of GaAs nanowires (NWs) on Si(111) substrates treated with a Ga focused ion beam (FIB). Depending on the ion dose, NW arrays with various geometrical parameters can be obtained. A minor treatment of the substrate enables a slight increase in the surface density of NWs relative to an unmodified substrate area. As the ion dose is increased up to ∼0.1 pC μm−2, the growth of GaAs NWs and nanocrystals is suppressed. However, a further increase in the ion dose stimulates the crystal growth leading to the formation of extremely thin NWs (39 ± 5 nm) with a remarkably high surface density of up to 15 μm−2. Resting upon an analysis of the surface structure before and after stages of ion-beam treatment, ultra-high vacuum annealing and NW growth, we propose a mechanism underlying the phenomenon observed. We assume that the chemical interaction between embedded Ga ions and a native Si oxide layer leads either to the enhancement of the passivation properties of the oxide layer within FIB-modified areas (at low and middle ion doses), or to the etching of the passivating oxide layer by excess Ga atoms, resulting in the formation of pores (at high ion doses). Due to this behavior, local fabrication of GaAs NW arrays with a diverse range of characteristics can be implemented on the same substrate. This approach opens a new way for self-catalytic growth of GaAs NWs.

Microstructure and properties of a novel Cu20Ni 20Mn-xGa alloy with high strength, high elasticity and high plasticity

The Cu20Ni20Mn alloy possesses both high strength and excellent elastic modulus, which has the potential to work in some extreme environments. In our study, by adding Ga into the Cu-Ni-Mn alloy, a new kind of Cu20Ni20Mn-xGa (x = 0, 1, 5 wt%) alloys were produced. After undergoing different thermal-mechanical treatments, these alloys were comprehensively investigated particularly for their mechanical properties and microstructures. While at the peak aging condition, the ultimate tensile strength and total elongation of the Cu20Ni20Mn-1Ga alloy are 1476 MPa and 7.9% respectively, both are higher than that of the Ga-free case (1391 MPa and 3.9%). The Ga added in small amount facilitates the nucleation of NiMn nanoprecipitates inside the grain instead of on the grain boundary, which provides enormous precipitation strengthening and greatly enhances the alloy. However, with addition of excessive Ga (e. g. 5 wt%), discontinuous Ni2MnGa precipitates are observed at grain boundaries, which greatly reduce the ductility of the alloy. Based on our research, we believe that adding appropriate amount of Ga is beneficial for the mechanical properties of the Cu-Ni-Mn alloys, which will become a potential strengthening method for Cu alloys in the future.

Crystalline phase purity and twinning of Mg-doped zincblende GaN thin films

Zincblende (zb) GaN thin films doped with Mg were grown by molecular beam epitaxy on GaAs under Ga- and N-rich conditions and different Mg cell temperatures. X-ray diffraction and electron-backscatter diffraction determined the ideal orientation of the zb-GaN, its textural relationship to the substrate, wurtzite and twin content, and their interfaces with the GaN matrix. A comparison to pole figure simulations allowed a thorough description of orientation of wurtzite inclusions and twins, forming interfaces along the 〈111〉 directions of the matrix. The results revealed zincblende volumetric purities as high as 99.9 % and twinning as low as 1.3 %. Ga-rich conditions and lower doping cell temperatures led to the highest phase purities. Moreover, crystalline and surface quality are also deeply linked to the wurtzite inclusions and twinning obtained. Films with the least amounts of these displayed the highest quality. This is likely due to excess Ga atoms on the surface helping the Mg doping atoms incorporate more orderly into the growing film. The results from this investigation provide a deeper understanding of the effects of Mg doping on the crystalline structure of zb-GaN films, paving the way for practical applications of zincblende nitrides in modern optoelectronic devices.

Improving thermoelectric performance of half-Heusler Ti0.2Hf0.8CoSb0.8Sn0.2 compounds via the introduction of excessive Ga and Co-deficiencies

(Ti/Zr/Hf)CoSb0.8Sn0.2 compounds are promising p-type thermoelectric materials. In this work, excessive Ga and Co deficiencies were introduced into Ti0.2Hf0.8CoSb0.8Sn0.2 compounds. The microstructure, electrical, and thermal transport properties were investigated in the temperature range of 300 K < T < 1000 K. The excessive Ga formed nanoinclusions with Ti and Sn at the grain boundaries. The Co deficiencies lowered the thermal conductivity and simultaneously improved the electrical conductivity. As a result, the power factor was enhanced to 3.2 mW/m K2, and the maximum Z.T. of ∼0.93 at 988 K was obtained in the Ti0.2Hf0.8Co0.99Sb0.8Sn0.2–0.01Ga sample, which is an increase of ∼48% compared to that of the pristine Ti0.2Hf0.8CoSb0.8Sn0.2 sample.

Transcriptional responses to gibberellin in the maize tassel and control by DELLA domain proteins.

The plant hormone gibberellin (GA) impacts plant growth and development differently depending on the developmental context. In the maize (Zea mays) tassel, application of GA alters floral development, resulting in the persistence of pistils. GA signaling is achieved by the GA-dependent turnover of DELLA domain transcription factors, encoded by dwarf8 (d8) and dwarf9 (d9) in maize. The D8-Mpl and D9-1 alleles disrupt GA signaling, resulting in short plants and normal tassel floret development in the presence of excess GA. However, D9-1 mutants are unable to block GA-induced pistil development. Gene expression in developing tassels of D8-Mpl and D9-1 mutants and their wild-type siblings was determined upon excess GA3 and mock treatments. Using GA-sensitive transcripts as reporters of GA signaling, we identified a weak loss of repression under mock conditions in both mutants, with the effect in D9-1 being greater. D9-1 was also less able to repress GA signaling in the presence of excess GA3 . We treated a diverse set of maize inbred lines with excess GA3 and measured the phenotypic consequences on multiple aspects of development (e.g., height and pistil persistence in tassel florets). Genotype affected all GA-regulated phenotypes but there was no correlation between any of the GA-affected phenotypes, indicating that the complexity of the relationship between GA and development extends beyond the two-gene epistasis previously demonstrated for GA and brassinosteroid biosynthetic mutants.

Mechanism of Early Flowering in a Landrace Naked Barley eam8.l Mutant

The EARLY MATURITY 8 (EAM8) gene of barley is homologous to the EARLY FLOWERING 3 (ELF3) gene in Arabidopsis, as loss-of-function mutations in this circadian clock gene promote rapid flowering. A previous study demonstrated that the early flowering phenotype of a hulless barley, Lalu, was due to allele eam8.l carrying an alternative splicing mutation in intron 3 that led to intron retention. In the present study, we verified that eam8.l encoded a truncated protein. Although EAM8 was expressed at a higher level in Lalu than in other barley lines with a longer growth period, it did not negatively regulate flowering time. This result further proved that the eam8.l protein was nonfunctional in regulating flowering in barley. The early flowering phenotype of Lalu plants was strongly dependent on the biosynthesis of gibberellin (GA). The eam8.l mutation stopped the suppression of GA biosynthesis, and Lalu accumulated excessive GA, especially in leaves. This was achieved through the upregulated expression of genes in the GA pathway, including GA20ox2, LFY1, SOC1, PAP2, and FPF3. The mutation of the EAM8 gene also abolished the inhibition of FLOWERING LOCUS T-like (FT1) gene expression at night. During the night, expression levels of the FT1 gene were higher than those during the day in Lalu. However, the GA-dependent pathway and FT1 gene mechanism are two independent pathways that promote flowering in Lalu. Alleles of EAM8, therefore, demonstrated an important breeding value in barley, which is probably effective in many other day-length-sensitive crop plants as well; thus, they could be used to tune adaptation in different geographic regions and climatic conditions, a critical issue in times of global warming.

Structure of Ge&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;92–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; glasses studied by high-resolution X-ray photoelectron spectroscopy and Raman scattering

In this paper, the structures of chalcogenide glasses Ge&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;92–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; (&lt;i&gt;x&lt;/i&gt; = 24%, 26.67%, 29.6%, 32% and 36%) at a fixed Ga atomic content of 8% are studied by high-resolution X-ray photoelectron spectroscopy and Raman scattering spectra. In order to quantify the evolutions of the different structural units in Ge&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;92–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; glasses, the number of double peaks in the Ge 3d, Ga 3d and S 2p spectra are determined by iterative fitting method, the binding energy and the full width at half maximum of each peak, and the relative ratio of the integral area of each decomposed peak to that of the whole area of the X-ray photoelectron spectroscopy are thus achieved. On the other hand, the Raman scattering spectra of Ge&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;92–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; glass are decomposed into multiple Gaussians based on the structural units. We use the iterative method to simulate the position of peak center, full width at half maximum, and height of each Raman peak. By analyzing the evolution of each unit structure in the glasses, it is found that the network structure of glass network is mainly formed by S atom bridging the tetrahedral structure of GeS&lt;sub&gt;4&lt;/sub&gt; and GaS&lt;sub&gt;4&lt;/sub&gt;. The S chains or rings structural units are formed in Ge&lt;sub&gt;24&lt;/sub&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;68&lt;/sub&gt; glass, indicating that S atoms are in excess in the chemical composition of the glass, so there are enough S atoms around Ge and Ga atoms, forming heteropolar Ge—S and Ga—S bonds. With the gradual increase of Ge content, S chains or rings structure units rapidly disappear in Ge&lt;sub&gt;26.67&lt;/sub&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;65.33&lt;/sub&gt; glass. The Ge—Ge homopolar bonds in the ethane-like structure S&lt;sub&gt;3&lt;/sub&gt;Ge—GeS&lt;sub&gt;3&lt;/sub&gt; and the &lt;i&gt;M&lt;/i&gt;—&lt;i&gt;M&lt;/i&gt; (Ge—Ge, Ga—Ga or Ge—Ga) homopolar bonds in the S&lt;sub&gt;3&lt;/sub&gt;Ge/Ga—Ga/GeS&lt;sub&gt;3&lt;/sub&gt; structure simultaneous appear in the Ge&lt;sub&gt;29.6&lt;/sub&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;62.4&lt;/sub&gt; glass, and the number of structures increases gradually with the increase of Ge content. This is mainly due to the insufficient number of S atoms in the Ge-Ga-S glass. Once S atoms are lacking, the excess Ge and Ga atoms can only combine with themselves to form the homopolar bond &lt;i&gt;M&lt;/i&gt;—&lt;i&gt;M&lt;/i&gt;. It can be concluded below. Firstly, Ge and Ga atoms appear mainly in the form of 4-coordination, while S atoms occur mainly in the form of 2-coordination in the chalcogenide glasses of Ge&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;92–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;. Secondly, the existence of &lt;i&gt;M&lt;/i&gt;—&lt;i&gt;M&lt;/i&gt; bond leads the nanophase to separate, and the ordering degree of glass network structure to decrease .

Alloying and magnetic disordering effects on phase stability of Co2 Y Ga (Y = Cr, V, and Ni) alloys: A first-principles study

The alloying and magnetic disordering effects on site occupation, elastic property, and phase stability of Co2 YGa (Y = Cr, V, and Ni) shape memory alloys are systematically investigated using the first-principles exact muffin-tin orbitals method. It is shown that with the increasing magnetic disordering degree y, their tetragonal shear elastic constant C′ (i.e., (C 11 – C 12)/2) of the L21 phase decreases whereas the elastic anisotropy A increases, and upon tetragonal distortions the cubic phase gets more and more unstable. Co2CrGa and Co2VGa alloys with y ≥ 0.2 thus can show the martensitic transformation (MT) from L21 to D022 as well as Co2NiGa. In off-stoichiometric alloys, the site preference is controlled by both the alloying and magnetic effects. At the ferromagnetism state, the excessive Ga atoms always tend to take the Y sublattices, whereas the excessive Co atom favor the Y sites when Y = Cr, and the excessive Y atoms prefer the Co sites when Y = Ni. The Ga-deficient Y = V alloys can also occur the MT at the ferromagnetism state by means of Co or V doping, and the MT temperature T M should increase with their addition. In the corresponding ferromagnetism Y = Cr alloys, nevertheless, with Co or Cr substituting for Ga, the reentrant MT (RMT) from D022 to L21 is promoted and then T M for the RMT should decrease. The alloying effect on the MT of these alloys is finally well explained by means of the Jahn–Teller effect at the paramagnetic state. At the ferromagnetism state, it may originate from the competition between the austenite and martensite about their strength of the covalent banding between Co and Ga as well as Y and Ga.

Effect of Ga excess concentration on the structural, electronic, magnetic, elastic and thermoelectric properties of Fe2−xNiGa1+x Heusler alloys: results of FP-LAPW calculations

Effect of Ga excess concentration on the structural, electronic, magnetic, elastic and thermoelectric properties of Fe2−xNiGa1+x Heusler alloys: results of FP-LAPW calculations

In situ reforming of lignite pyrolysis volatiles for enriching light aromatics over Ga substituted HZSM-5

HZSM-5 was hydrothermally synthesized by Ga substitution to regulate the texture properties for light aromatics such as benzene, toluene, ethylbenzene, xylenes and naphthalene (BTEXN) during the lignite volatiles catalytic upgrading process. High BTEXN yield was obtained over xGa/H5(A) (added NaOH in crystallization) and the value increased with raising the amount of Ga incorporation when the Ga content was<1.26 g. 1.26 Ga/H5(A) (1.26 g Ga(NO3)3) displayed the highest yield of BTEXN (29.3 mg/g-daf coal) since Ga species and acidic protons promoted the aromatization process. However, excessive Ga brought a negative effect on the production of aromatics due to the weakened synergistic effect between Ga species and acid sites of H5. Additionally, 1.26 Ga/H5(A) with the interconnected meso-micropores and moderate acidity distribution exhibited excellent reaction stability during the 6 cycle experiments. xGa/H5(A) provided a potential prospect for enhancing the BTEXN formation during catalytic upgrading of lignite pyrolysis volatiles.

Planar and three-dimensional damage-free etching of β-Ga2O3 using atomic gallium flux

In situ etching using Ga flux in an ultra-high vacuum environment like molecular beam epitaxy is introduced as a method to make high aspect ratio three-dimensional (3D) structures in β-Ga2O3. Etching of β-Ga2O3 due to excess Ga adatoms on the epilayer surface had been viewed as non-ideal for epitaxial growth especially since it results in plateauing and lowering of the growth rate. In this study, we use this well-known reaction from epitaxial growth to intentionally etch β-Ga2O3. We demonstrate etch rate ranging from 2.9 to 30 nm/min with the highest reported etch rate being only limited by the highest Ga flux used. Patterned in situ etching is also demonstrated and used to study the effect of fin orientation on the sidewall profiles and dopant (Si) segregation on the etched surface. Using in situ Ga etching, we also demonstrate 150 nm wide fins and 200 nm wide nanopillars with high aspect ratio. This etching method could enable future development of highly scaled vertical and lateral 3D devices in β-Ga2O3.

Defect species in Ga-doped ZnO films characterized by photoluminescence

Photoluminescence (PL) spectra of Ga-doped ZnO (GZO) films were investigated with reference to those of undoped ZnO films to elucidate the effect of high-level Ga3+ doping. A transition from ZnO-like to GZO-like spectra occurred at a Ga content around 2 at. %. The room temperature PL spectra of sufficiently oxidized GZO films exhibited band edge and violet components, while emissions at wavelengths longer than 480 nm were sharply cut off. The close resemblance of the spectral shapes of the GZO and Zn-rich ZnO films indicated disordering of the ZnO lattice by excess Ga dopants. Deposition under a reducing atmosphere at 100 and 200 °C produced oxygen-deficient GZO films with additional emission signals corresponding to oxygen vacancy (VO) defects between 480 and 600 nm. For GZO films with Ga content larger than 4 at. %, increasing the deposition temperature above 400 °C or postannealing at 500 °C smeared out deep-level emission signals, suppressed the near-band edge emission, and deactivated the donor role of Ga3+. These changes can be ascribed to outdiffusion of Ga3+ from the cation sites and rearrangement of the ZnO crystal network. Argon plasma treatment of GZO films generated a VO-related emission band through preferential sputtering of oxygen atoms. Hydrogen donors trapped at the resulting VOs would be the origin of a slight increase in carrier concentration, by 1 × 1020 cm−3.

Valorization of rice husk to aromatics via thermocatalytic conversion in the presence of decomposed methane

Herein, the thermocatalytic conversion of rice husk is studied as a valorization method for the production of benzene, toluene, ethylbenzene, and xylene (BTEX). The reaction environment, the type of zeolite, the SiO2/Al2O3 ratio of the zeolite, and the Ga loading on the zeolite are considered in order to maximize the BTEX yield. Five distinct reaction media are tested, including N2, CH4, and gas streams evolved from the ex-situ decomposition of CH4 at 650, 725, and 800 °C (designated as CH4-D(650), CH4-D(725), and CH4-D(800), respectively). The thermocatalytic conversion of rice husk in CH4-D(650) with a H2/CH4 molar ratio of 0.88 is shown to provide the highest BTEX yield. In addition, the use of the HZSM-5 zeolite, with its relatively low SiO2/Al2O3 ratio, is shown to provide a further increase in the BTEX yield due to the higher density of Brønsted acid sites. The highest yield of BTEX (18.3 wt%) is achieved in the presence of Ga/HZSM-5 (Ga loading: 1 wt%) under the CH4-D(650) atmosphere. The further addition of Ga to the HZSM-5 (2 and 5 wt% Ga) is shown to decrease the BTEX yield because excess Ga loading on the zeolite decreases the number of available Brønsted acid sites. The present study provides useful information on the effects of various factors upon the thermocatalytic conversion of waste biomass such as rice husk to achieve high yields of value-added chemicals such as BTEX.

Phase relations and extrinsic magnetic properties of Sm–(Fe,Co)–Ti–(Ga)-based alloys for ThMn12-type permanent magnets

Realizing a non-ferromagnetic intergranular phase for the ThMn12-type Sm(Fe,Co,Ti)12 compound is a prerequisite for developing high coercive magnets. In this work, we demonstrate that the addition of Ga into the Sm-(Fe,Co)-Ti system results in the formation of non-ferromagnetic Sm–Ga-rich intergranular phases with good wettability on Sm(Fe,Co,Ti)12 grains. In the Sm-(Fe,Co)-Ti system, when the alloy composition was varied to the Sm-rich region as Sm1+w(Fe0.8Co0.2)11Ti (w = 0–0.7), soft ferromagnetic C15-type Sm(Fe,Co)2 intergranular phase and TbCu7-type Sm(Fe,Co,Ti)~7 phase formed along with the ThMn12-type main phase in the as-cast alloy. We demonstrated that by introducing Ga, the formation of these secondary soft ferromagnetic phases can be hindered as the excess Sm and Ga formed non-ferromagnetic Ba5Si3-type Sm5Ga3 and TII-type SmGa intergranular phases with an excellent wettability on Sm(Fe,Co,Ti)12 grains. Fine Sm(Fe,Co,Ti)12 grains of 1–2 µm, well isolated by the Sm–Ga-rich intergranular phase, were realized by melt-spinning the Ga-doped alloy and the maximum coercivity obtained was 0.5 T. According to the detailed microstructure characterizations, the weak-links to the coercivity of this novel microstructure of well-isolated and fine Sm(Fe,Co,Ti)12-based grains is the presence of defects at the interfaces which could lead to a locally reduced magneto-crystalline anisotropy.

Depth profile study on Ti/Al bilayer Ohmic contacts to AlGaN/GaN

The electrical performance of double layered Ohmic contacts not only depends on layer thicknesses, annealing temperatures and annealing time etc. but also Ohmic contact metals and reactions with substrates and each other. Furthermore, oxidation tendency of Ohmic contact metals is highly important for contact quality and stability. In this study, we have investigated Ti/Al Ohmic contact formation on undoped AlGaN substrates by X-ray photoelectron spectroscopy (XPS) depth profile analysis. We aimed to reveal the oxide compounds and reaction products of metals and dependencies to the contact depth. In survey mode and depth profile mode five atoms core level (Ga 3d, Al 2p, N 1 s, Ti 2p and O 1 s) was observed and the XPS core level peaks were convoluted with Gaussian profiles to identify the origin of the peaks. Our results show that Ohmic contact metals strongly incorporated with oxygen, leading to high specific contact resistivity (ρs) confirmed by the TLM measurement. Also it is found that thermal treatments cause a TiAl3 metallic alloy in the Ti/Al interface and the presence of excess Ga and TiN indicates that out diffusion of N atoms at the metal semiconductor (MS) interface.

Structural and dynamic properties of short-period GaN/AlN superlattices grown by submonolayer digital epitaxy

Structural and dynamic properties of short-period GaN/AlN superlattices with the thicknesses of the constituent layers varying from two to several monolayers, grown using the method of submonolayer digital molecular beam epitaxy, are experimentally and theoretically studied. It is established that in the grown samples there are two types of periodicity. One type is formed by periodic sequence of GaN and AlN layers in the superlattice, and the second one is related with periodic interruptions in the growth of superlattice for the evaporation of excess Ga metal. The dependences of the positions and intensities of the lines in the Raman spectra on the period of the superlattice are determined, and microscopic nature of optical phonon modes is established. The doublet structure of the E(TO) lines localized in the GaN and AlN layers of superlattice, genetically related to the E 2(high) and E1 phonon branches of the bulk crystal, is first discovered and explained. A strong dependence of the polar modes localized in the AlN layer on the thickness of layer forming the superlattice is revealed. The results of complex studies will improve the accuracy of quantitative estimation of important parameters of superlattice structures and can be used to optimize growth parameters for the fabrication of structurally perfect short-period GaN/AlN superlattices.

Structural, magnetic, and magnetocaloric properties of the Ni54(FeMn)19Ga27 Heusler alloy

The structural, magnetic, and magnetocaloric properties of the Ni54(FeMn)19Ga27 Heusler alloy have been presented, which has both Fe and Mn atoms at the 4a site of its austenite structure. The alloy is compared with the ternary alloys of the Ni54Fe19Ga27 and the Ni54Mn19Ga27. The alloy has a mixed-phase, which is L21 cubic and 14 M tetragonal at room temperature. The temperature-dependent magnetization curve exhibits ferromagnetic to paramagnetic phase transition in the range of 380–450 K. The obtained Curie temperature is 384 K. Fe and Mn are located at 4a site with the excess Ni and Ga in the non-stoichiometric alloy and therefore, ferromagnetic and antiferromagnetic interactions cause the decrease in the magnetization of the alloy and the Fe–Mn and Ni–Mn interactions led to high Curie temperature compared with that of both the Ni54Fe19Ga27 and Ni54Mn19Ga27. The saturation magnetization of the alloy is 53.97 Am2kg−1 under 2 T magnetic field at room temperature. The hysteresis curve is narrow and the coercivity is 0.19 mT under 2 T, and the alloy is a soft magnetic material. The Arrott plot curves indicate that the alloy shows the second-order phase transition and the magnetic entropy change is about 1.5 J kg−1K−1 under 2 T magnetic field.

Comprehensive Analysis of Metal Modulated Epitaxial GaN.

While metal modulated epitaxy (MME) has been shown useful for hyperdoping, where hole concentrations 40 times higher than other techniques have been demonstrated, and the ability to control phase separation in immiscible III-nitrides, the complexity of the dynamically changing surface conditions during the cyclic growth is poorly understood. While MME is capable of superb crystal quality, performing MME in an improper growth regime can result in defective material. These complications have made the transfer of MME knowledge challenging. This work provides a comprehensive study of the conditions necessary for achieving the benefits of MME while avoiding undesirable defects. The effects of growth temperature, Ga/N ratio, and excess Ga dose per MME growth cycle on the morphological, structural, electronic, and optical properties of unintentionally doped (UID) MME grown gallium nitride (GaN) have been investigated. Optimal structural and electrical quality were achieved for GaN films grown at ∼650 °C, at pre-bilayer Ga coverage and at the moderate droplet regime. However, high defect concentrations were observed at the lowest growth temperatures, and counter to traditional MBE, as the excess Ga dose transitioned from bilayer coverage to the low droplet regime. Optoelectronic properties were optimal for films grown at intermediate growth temperatures, an excess Ga dose condition just before the droplet formation, and, at a III/V ratio of 1.3. Optimization of growth temperatures, Ga/N ratios, and excess Ga dose results in a range of growth conditions achieving smooth surfaces, step-flow surface morphology, and high crystalline quality films with low threading dislocation densities, allowing researchers to utilize the extensive advantages of MME.