Ga Sublattice Research Articles

Influence of excess silicon on polytype selection during metal-mediated epitaxy of GaN nanowires

We have examined the origins of polytype selection during metal-mediated molecular-beam epitaxy of GaN nanowires (NWs). High-angle annular dark-field scanning transmission electron microscopy reveals [111]-oriented zinc blende (ZB) NWs and [0001]-oriented wurtzite (WZ) NWs, with SixNy at the interface between individual NWs and the Si (001) substrate. Quantitative energy dispersive x-ray spectroscopy reveals a notably higher Si concentration of 7.0% ± 2.3% in zinc blende (ZB) NWs than 2.3% ± 1.2% in wurtzite (WZ) NWs. Meanwhile, density functional theory calculations show that incorporation of 8 at. % Si on the Ga sublattice inverts the difference in formation energies between WZ and ZB GaN, such that the ZB polytype of GaN is stabilized. This identification of Si and other ZB polytype stabilizers will enable the development of polytype heterostructures in a wide variety of WZ-preferring compounds.

Atomic-scale investigation of γ-Ga2O3 deposited on MgAl2O4 and its relationship with β-Ga2O3

Nominally phase-pure γ-Ga2O3 was deposited on (100) MgAl2O4 within a narrow temperature window centered at ∼470 °C using metal-organic chemical vapor deposition. The film deposited at 440 °C exhibited either poor crystallization or an amorphous structure; the film grown at 500 °C contained both β-Ga2O3 and γ-Ga2O3. A nominally phase-pure β-Ga2O3 film was obtained at 530 °C. Atomic-resolution scanning transmission electron microscopy (STEM) investigations of the γ-Ga2O3 film grown at 470 °C revealed a high density of antiphase boundaries. A planar defect model developed for γ-Al2O3 was extended to explain the stacking sequences of the Ga sublattice observed in the STEM images of γ-Ga2O3. The presence of the 180° rotational domains and 90° rotational domains of β-Ga2O3 inclusions within the γ-Ga2O3 matrix is discussed within the context of a comprehensive investigation of the epitaxial relationship between those two phases in the as-grown film at 470 °C and the same film annealed at 600 °C. The results led to the hypotheses that (i) incorporation of certain dopants, including Si, Ge, Sn, Mg, Al, and Sc, into β-Ga2O3 locally stabilizes the “γ-phase” and (ii) the site preference(s) for these dopants promotes the formation of “γ-phase” and/or γ-Ga2O3 solid solutions. However, in the absence of such dopants, pure γ-Ga2O3 remains the least stable Ga2O3 polymorph, as indicated by its very narrow growth window, lower growth temperatures relative to other Ga2O3 polymorphs, and the largest calculated difference in Helmholtz free energy per formula unit between γ-Ga2O3 and β-Ga2O3 than all other polymorphs.

High mobility of intrinsic defects in α-Ga2O3

Migration properties of the intrinsic defects were investigated in α-Ga2O3 by controllable introduction of the lattice disorder with ion irradiation and monitoring its evolution as a function of ion dose, flux, and temperature. Already the dose dependence acquired at room temperature suggested prominently high mobility of intrinsic defects in α-Ga2O3, since we observed two distinct disordered regions—near the surface and in the bulk—instead of a Gaussian shape following the ballistic defects production process. Moreover, the disorder accumulation has shown to be highly sensitive to the variation of the ion flux and temperature, known in the literature as the dose-rate effect. Therefore, by monitoring the process as a function of the flux and temperature, we observed such dose-rate effect in α-Ga2O3 with an activation energy of 0.33 ± 0.04 eV, which we attributed to the migration barrier of the intrinsic defects in the Ga sublattice, from where we collected the experimental data. By setting these results in the context of the theoretical data available in the literature, we argued that this energy may be attributed to the migration activation of the Ga self-interstitials in α-Ga2O3.

Origin of magnetocrystalline anisotropy in Ni-Mn-Ga-Co-Cu tetragonal martensite

We investigate the origin of magnetocrystalline anisotropy (MCA) in non-modulated martensite of Ni-Mn-Ga-Co-Cu exhibiting magnetic-field-induced strain up to 12%. Experiments as well as theoretical calculations using density functional theory show that Co and Cu doping or deviation from Ni2MnGa stoichiometry decreases the MCA. As follows from the calculations, the decrease of MCA is much stronger for Cu in Ga sublattice in comparison to Cu in Mn sublattice. The decreasing effect of Co on the MCA is only indirect caused by deficiency in Ni, which is the main element governing the MCA. For further insight, we calculated MCA and magnetic moment as a function of lattice tetragonality c/a. The MCA reaches a maximum at the same c/a where Ni magnetic moment is maximum. However, the tetragonality of equilibrium does not coincide with these maxima. Consequently, in contrast to common expectation, decreasing tetragonality from equilibrium can increase the MCA.

Effects of Ni and Cu Antisite Substitution on the Phase Stability of CuGa2 from Liquid Ga/Cu–Ni Interfacial Reaction

Ga and Ga-based alloys have received significant attention for applications in the liquid state and also for their potential as a bonding material in microelectronic assemblies. This study investigates the phase stability of the CuGa2 phase as a product of the interfacial reaction between liquid Ga and Cu-10Ni substrates at room temperature. In the binary Ga-Cu system, CuGa2 is decomposed into liquid Ga and Cu9Ga4 as the temperature increases to around 260 °C, which prevents the widespread application of this alloy. In contrast to CuGa2 grown from a pure Cu substrate, CuGa2 from the Cu-10Ni substrate shows an increase in the decomposition temperature during heating from 25 to 300 °C. According to our first-principle calculations, there is only a minor difference in the total free energy between Ni solute at the Cu sublattice and the Ga sublattice in the tetragonal CuGa2 crystal structure. This result indicates that both of the sublattices can accommodate the dilute Ni solute with comparable probability. Regardless of the sublattice where the Ni impurities are located, the presence of diluted Ni in the matrix stabilizes the CuGa2 system by inducing some localized Ni 3d states at energy levels near the Fermi level. It is also shown that the formation of Cu antisite defects, which also stabilizes CuGa2, is preferable if the CuGa2 matrix is grown on a Ni-containing substrate.

Pressure-induced disruption of the local environment of Fe-Fe dimers inFeGa3accompanied by metallization

The semiconducting gap in the $\mathrm{FeG}{\mathrm{a}}_{3}$ intermetallic originates from $\mathrm{Fe}(3d)/\mathrm{Ga}(4p)$ hybridization. Pressures of 15--20 GPa initiate a disruption of this semiconducting tetragonal $P{4}_{2}/mnm$ structure and an emergence of a high-pressure metallic phase, estimated to be fully stabilized just beyond \ensuremath{\sim}35 GPa. An accompanying pronounced \ensuremath{\sim}17% volume collapse occurs at the structural transition. The high-pressure metallic phase has a ${T}^{1/2}$ temperature dependence of the resistivity below its minimum at 8--12 K, symptomatic of disorder. There is a corresponding weak high-temperature dependence of the resistivity and resultant broad maximum at \ensuremath{\sim}250 K to yield ``bad-metal'' values of \ensuremath{\sim}0.5 m\ensuremath{\Omega} cm at room temperature. This is shown to signify that the high-pressure phase is a low carrier density metal on the verge of an Anderson transition. Ga $K$-edge absorption spectroscopy and Fe M\ossbauer spectroscopy local probes indicate that the atomic disorder stems from a pressure-instigated rearrangement of the Ga sublattice at the structural transition.

Putting ScTGa5(T = Fe, Co, Ni) on the Map: How Electron Counts and Chemical Pressure Shape the Stability Range of the HoCoGa5Type

We explore the factors stabilizing one member of the diverse structures encountered in Ln–T–E systems (Ln = lanthanide or similar early d-block element, T = transition metal, E = p-block element): the HoCoGa5 type, an arrangement of atoms associated with unconventional superconductivity. We first probe the boundaries of its stability range through the growth and characterization of ScTGa5 crystals (T = Fe, Co, Ni). After confirming that these compounds adopt the HoCoGa5 type, we analyze their electronic structure using density functional theory (DFT) and DFT-calibrated Hückel calculations. The observed valence electron count range of the HoCoGa5 type is explained in terms of the 18-n rule, with n = 6 for the Ln atoms and n = 2 for the T sites. The role of atomic sizes is investigated with DFT-chemical pressure (DFT-CP) analysis of ScNiGa5, which reveals negative pressures within the Ga sublattice as it stretches to accommodate the Sc and T atoms. This CP scheme is consistent with HoCoGa5-type gallides only being observed for relatively small Ln and T atoms. These conclusions account for the relative positions of the HoCoGa5, BaMg4Si3, and Ce2NiGa10 types in a structure map, demonstrating how combining the 18-n and CP schemes can guide our understanding of Ln–T–E systems.

Ab Initio Study of Properties of Co- and Cu- Doped Ni-Mn-Ga Alloys

The influence of Co and Cu doping on Ni-Mn-Ga alloy is investigated using the first-principles exact muffin-tin orbital method in combination with the coherent-potential approximation. The energy difference between the austenite (A) and the nonmodulated (NM) martensite ΔEA-NM depends linearly on the Cu concentration and distribution, with a minimum for all Cu at Mn sites and a maximum for all Cu at Ga sites. For alloys simultaneously doped by Co in Ni sublattice and Cu in Mn or Ga sublattice, the effects of the individual dopants on ΔEA-NM and (c/a)NM and are almost independent. The alloy with composition Ni46Co4Mn24Ga22Cu4 exhibits decreased equilibrium (c/a)NM and increased ΔEA-NM in comparison with Ni50Mn25Ga25, which is in agreement with the previous experimental results.

Identification of As-vacancy complexes in Zn-diffused GaAs

We have used positron annihilation spectroscopy to study the introduction of point defects in Zn-diffused semi-insulating GaAs. The diffusion was performed by annealing the samples for 2 h at 950 °C. The samples were etched in steps of 7 μm. Both Doppler broadening using slow positron beam and lifetime spectroscopy studies were performed after each etching step. Both techniques showed the existence of vacancy-type defects in a layer of about 45 μm. Secondary ion mass spectroscopy measurements illustrated the presence of Zn at high level in the sample almost up to the same depth. Vacancy-like defects as well as shallow positron traps were observed by lifetime measurements. We distinguish two kinds of defects: As vacancy belongs to defect complex, bound to most likely one Zn atom incorporated on Ga sublattice, and negative-ion-type positron traps. Zn acceptors explained the observation of shallow traps. The effect of Zn was evidenced by probing GaAs samples annealed under similar conditions but without Zn treatment. A defect-free bulk lifetime value is detected in this sample. Moreover, our positron annihilation spectroscopy measurements demonstrate that Zn diffusion in GaAs system is governed by kick-out mechanism.

Electronic and magnetic properties of CeCoGa

We report the magnetic and electronic transport properties of CeCoGa, using magnetic susceptibility, magnetization, specific heat and electrical resistivity investigations. We also present X-ray photoemission valence band spectra (VB-XPS) as well as ab-initio band structure calculations for the ferromagnetic and antiferromagnetic ordering. The dc and ac magnetic susceptibility data reveal a magnetic phase transition at TN = 4.3 K. For T < TN magnetization M vs. magnetic field B isoterms have two metamagnetic transitions. Above TN susceptibility shows evidence of spin-glass like behavior. The magnetic entropy is strongly reduced at TN due to the on-site Kondo coupling. For CeCoGa the Kondo temperature is ∼20 K, much larger than the temperature of the magnetic ordering. Our calculations show that within bare local spin density approximation (LSDA) the ground state of CeCoGa is weakly magnetic, however, the 4f correlations when they are included within LSDA + U approach lead to stable antiferromagnetic ground state. Calculations for moderate U parameter indicate that magnetic ordering is present not only at the Ce sublattice, but also for the Co sites, moreover small induced magnetic moment is also present at the Ga sites. All sublattices exhibit antiferromagnetic ordering with different values of local magnetic moments what explains the ferrimagnetic behavior of CeCoGa. The magnetization M vs. magnetic field B shows at T = 2 K two metamagnetic features at B = 0.1 T and B = 5 T, which well correlates with the calculated magnetic moments of Co and Ga. The first transition can be attributed to spin–flop within the magnetic moments induced in Ga sublattice, whereas the anomaly at B = 5 T is related to the spin–flop within the Co sublattice.

Comparison of Fe and Si doping of GaN: An EXAFS and Raman study

Abstract The effect of Fe and Si doping in GaN grown epitaxially on Al 2 O 3 is studied using Extended X-ray Absorption Fine Structure (EXAFS) and Raman spectroscopies. The EXAFS analysis shows that in GaN samples grown by Molecular Beam Epitaxy (MBE) at 750 °C the Ga–Ga distance is about 0.19% longer than the corresponding distance in samples grown by Metalorganic Vapour Phase Epitaxy (MOVPE) at 1050 °C. As this distance is affected strongly by the value of the a lattice constant, its smaller value complies with the higher compressive biaxial thermal strain, present in samples grown at a higher temperature. This finding is in line with the smaller blue shift observed for the E 2 Raman peak in the MBE samples. Si doping, contrary to Fe doping, reduces the mean square relative displacement of the Ga atoms in the second nearest neighboring shell of Ga. Most probably a stress relaxation mechanism, e.g. formation of dislocations, affects the c / a ratio which in turn modifies the difference of the Ga–Ga distances in- and out- of the c -plane. Contrary to Fe doping, the incorporation of Si atoms in the Ga sublattice induces compressive hydrostatic strain that blue-shifts the E 2 Raman peak. Finally, the carrier concentration determined by proper fitting of the A 1 (LO) Raman peak, that takes into account the phonon–plasmon coupling, shows that continuous Fe doping compensates slightly the n-type conductivity of GaN.

Magnetoelastic effects inNi2Mn1+xGa1−xalloys from first-principles calculations

The magnetic coupling between Mn atoms on Ga sublattice (Mn-Ga) and Mn atoms on Mn sublattice (Mn-Mn) in Ni2Mn1+xGa1-x alloy and its effect on the elastic modulus of the alloy are investigated by t ...

The effect of a material growth technique on ion-implanted Mn diffusion in GaAs

Diffusion of ion-implanted Mn in semi-insulating (SI) and liquid encapsulated Czochralski (LEC)-grown GaAs has been determined employing the modified radiotracer technique. The effect of the growth technique and conditions on Mn diffusion in low temperature molecular beam epitaxy (LT-MBE)-grown GaAs has also been studied. Two distinct diffusion components appear in ion-implanted Mn diffusion in GaAs: slow and fast. As the diffusivity for the SI material is slightly higher than that for the LT-grown material, it is observed that the diffusivity of the fast component retards with increasing initial concentrations of Ga sublattice defects. At the same time the Mn concentration in the tail part of the diffusion profile is higher in the LT-grown material. Ga vacancy-assisted clustering of Mn is proposed as a likely reason for the observed effects.

Effect of Growth Temperature and Mn Incorporation on GaN:Mn Thin Films Grown by Plasma-Assisted MOCVD

In this paper, the growth of GaN:Mn thin films by plasma-assisted metalorganic chemical vapor deposition (PAMOCVD) method is reported. The method used in this study, utilizes a microwave cavity as a cracking cell to produce nitrogen radicals, which in turn reduce the growth temperature. Trimethylgallium (TMGa), nitrogen (N2) and cyclopentadienyl manganese tricarbonyl (CpMnT) were used as a source of Ga, N and Mn, respectively, while hydrogen gas was used as a carrier gas for both TMGa and CpMnT. The effect of growth temperature and Mn incorporation on structural properties and surface morphology of GaN:Mn films are presented. The growth of GaN:Mn thin films were conducted at varied growth temperature in range of 625 o C to 700 o C and the Mn/Ga molar fraction in the range of 0.2 to 0.5. Energy dispersive of X-ray (EDX) and X-ray diffraction (XRD) methods were used to analyze atomic composition and crystal structure of the grown films, respectively. The surface morphology was then characterized using both atomic force microscopy (AFM) and scanning electron microscopy (SEM) images. A systematic XRD analysis reveal that maximum Mn incorporation that still produces single phase GaN:Mn (0002) is 6.4 % and 3.2 % for the film grown at 650 o C and 700 o C, respectively. The lattice constant and full width at half maximum (FWHM) of the single phase films depend on the Mn concentration. The decrease in lattice constant accompanied by the increase in FWHM is due to incorporation of substitutional Mn on the Ga sub-lattice. The maximum values of doped Mn atoms incorporated in the wurtzite structure of GaN:Mn as substitutional atoms on Ga sub-lattice are 2.0 % and 2.5 % at 650 o C and 700 o C, respectively. AFM and SEM images show that the film grown at lower growth temperature and Mn concentration has a better surface than that of film grown at higher growth temperature and Mn concentration.

Introduction and recovery of Ga and N sublattice defects in electron-irradiated GaN

We have used positron annihilation spectroscopy to study the introduction and recovery of point defects introduced by 0.45 and $2\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ electron irradiation at room temperature in $n$-type GaN. Isochronal annealings were performed up to $1220\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We observe vacancy defects with specific lifetime of ${\ensuremath{\tau}}_{V}=190\ifmmode\pm\else\textpm\fi{}15\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ that we tentatively identify as N vacancies or related complexes in the neutral charge state in the samples irradiated with $0.45\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ electrons. The N vacancies are produced at a rate ${\ensuremath{\Sigma}}_{\mathrm{N}}^{0.45}\ensuremath{\simeq}0.25\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The irradiation with $2\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ electrons produces negatively charged Ga vacancies and negative nonopen volume defects (negative ions) originating from the Ga sublattice, at a rate ${\ensuremath{\Sigma}}_{\mathrm{Ga}}^{2.0}\ensuremath{\simeq}5\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The irradiation-induced N vacancies anneal out of the samples at around $600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, possibly due to the motion of the irradiation-induced N interstitials. Half of the irradiation-induced Ga vacancies anneal out of the samples also around $600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and this is interpreted as the isolated Ga vacancies becoming mobile with a migration barrier of ${E}_{M}^{V,\mathrm{Ga}}=1.8\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Interestingly, we observe a change of charge state of the irradiation-induced negative ions from $2\ensuremath{-}$ to $1\ensuremath{-}$ likely due to a reconstruction of the defects in two stages at annealing temperatures of about 600 and $700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The negative ions anneal out of the samples together with the other half of the Ga vacancies (stabilized by, e.g., N vacancies and/or hydrogen) in thermal annealings at $800--1100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Ab initio calculations of transport properties of epitaxial (Ga,Mn)As systems

We calculate transport properties of thin ( Ga , Mn ) As layers sandwiched between two Cr ( 0 0 1 ) leads as a function of Mn concentration and layer thickness, special attention is paid to the spin polarization of the current-perpendicular-to-plane (CPP) conductance. We compare our results to bulk properties. Doping GaAs with Mn increases occurrence of As antisites on the Ga sublattice. Our method allows us to take into account even very small defect concentration changes, and we show that it may have a dramatic impact on the CPP spin polarization, apart from its already known influence on Curie temperatures.

Magnetic properties of 3d impurities in GaAs

Electronic structure, thermodynamic, and magnetic properties of 3d-transition metal (TM) impurities in GaAs have been studied from first principles using Green's function approach. The studied TM impurities (V, Cr, Mn, and Fe) are found to form substitutional alloys on the Ga sublattice. The possibility of raising the Curie temperature T C in (GaMn)As by co-doping it with Cr impurities was examined on the basis of total energy difference between the disordered local moment (DLM) and the ferromagnetically ordered (FM) spin configurations. The calculated Curie temperature and magnetic moment have maxima for GaAs doped with Cr and Mn. The magnetic properties of Mn-doped GaAs are shown to be more sensitive to antisite As defects than those of Cr-doped GaAs. However, the Cr impurities are sensitive to the presence of acceptor defects, such as vacancies on the Ga sublattice. The investigation of the electronic structure of pseudo-ternary alloys (Ga (1− x − y )Mn x Cr y )As has shown a mutual compensation of Mn and Cr impurities. Therefore, in order to reach the highest critical temperature, GaAs has to be separately doped with Cr or Mn impurities. The GaAs doped with Fe is found to be non-ferromagnetic.

Onset of itinerant electron magnetism inCo(Ga1−xNix)andCo(Ga1−xCux)alloys

Onset of the magnetic order in $\mathrm{Co}({\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})$ and $\mathrm{Co}({\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x})$ alloys caused by Ni and Cu substitutions is studied in ab initio framework using density functional theory and the coherent potential approximation. It is found that Ni and Cu atoms energetically prefer Ga sites and, contrary to the earlier interpretation, the magnetism develops on the Co sublattice rather then being induced by magnetic Co antisites atoms moved to the Ga sublattice. The changes in the local electronic density of states of Co caused by Ni and Cu substitutions on Ga sublattice lead to the Stoner instability at some critical alloy composition and weak itinerant ferromagnetism, similar to that observed in ${\mathrm{ZnZn}}_{2}$ and ${\mathrm{Ni}}_{3}\mathrm{Al}$ compounds, develops.

Nitrogen related vacancy formation in annealed GaInNAs quantum well superlattices

We have used positrons annihilation spectroscopy to study vacancy type defects in GaAs based superlattices used for 1.3 μ m quantum well lasers. The samples were molecular beam epitaxy grown on p-type GaAs substrates. Three sets of samples were used for the experiment. For each set the superlattice structure consisted of 10 periods of the quantum well material separated by a GaAs buffer. As quantum well material we used GaInAs, GaNAs and GaInNAs. The growth temperature was 420 ∘ C for each sample set and some of the samples were annealed. For annealed samples, it could be shown that vacancies were formed in the superlattice structure or at interface between the GaAs cap and the superlattice structure in the annealing process and that the formation was related to the nitrogen content. Annealed GaNAs and GaInNAs superlattice samples were found to contain vacancies, most likely in the Ga sublattice.

Diffusion of Mn in gallium arsenide

The aim of this work is to compare diffusion coefficient and shape of the manganese atomic profile in GaAs treated under different annealing conditions. Diffusion was performed from implanted Mn as well as from external source of Mn. Manganese implanted GaAs, GaAs:Zn and GaAs:Te bulk samples were investigated. Implantation was performed at room temperature to a dose of 10 16/cm 2 at an energy of 150 keV. The samples, were protected with AlN layers, and annealed at 800 °C with RTA (rapid thermal annealing) method as well as at 700 and 900 °C in a sealed quartz ampoule. After removing the AlN films the extent of diffusion of the species was characterized using the secondary ion mass spectrometry (SIMS) technique. The depth profiles of in-diffused manganese in RTA experiment strongly indicate that the diffusion coefficient D is concentration-dependent. For both: quartz ampoule and RTA annealings of implanted samples, the Mn diffusivity was found larger when GaAs was annealed with the AlN cap than that annealed without a cap. Over 10 times shallower diffusion range in uncovered samples than those encapsulated with AlN is interpreted in terms of generation of additional vacancies in the Ga sub-lattice. Mn atoms incorporate in Ga sites lowering thus the diffusion coefficient. In case of diffusion from external source into differently doped GaAs, the largest diffusion coefficient was found for GaAs:Zn. This result indicates highest Mn diffusivity in the sample with a low Fermi level which provides lowest concentration of ionized Ga vacancies. Both results confirm an interstitial diffusion mechanism and decreasing diffusion coefficient with increasing gallium vacancy concentration. The Boltzmann–Matano analysis was employed to evaluate the concentration-dependent diffusion coefficient of Mn in GaAs.