Ga Antisite Research Articles

Tailoring-compensated ferrimagnetic state and anomalous Hall effect in quaternary Mn–Ru–V–Ga Heusler compounds

Abstract Cubic Mn2Ru x Ga Heusler compound is a typical example of compensated ferrimagnet with attractive potential for high-density, ultrafast, and low-power spintronic applications. In the form of epitaxial thin films, Mn2Ru x Ga was shown to exhibit high spin polarization and high tunability of compensation temperature by freely varying the Ru content x over a broad range (0.3 < x < 1.0). Here, we systematically studied Mn-Ru-Ga-based polycrystalline bulk buttons prepared by arc melting and found that, in equilibrium bulk form, the cubic structure is unstable when x < 0.75. To overcome this limitation, we alloyed Mn-Ru-Ga with a fourth element V. By adjusting the content of V in the Mn2Ru0.75V y Ga and Mn2.25-y Ru0.75V y Ga quaternary systems, the magnetic compensation temperature was regulated. Compensation was achieved near 300 K which was confirmed by both the magnetic and anomalous Hall effect measurements. Analyses of the anomalous Hall effect scaling in quaternary Mn-Ru-V-Ga alloys revealed the dominant role of skew scattering, notably that contributed by the thermally excited phonons, in contrast to many other 3d ferromagnets and Heusler compounds. We further showed that the Ga antisites and V content can simultaneously control the residual resistivity ratio (RRR) as well as the relative phonon and defect contribution to the anomalous Hall effect a’’/a’ in Mn-Ru-V-Ga, leading to a scaling relation a’’/a’ ∝ RRR1.8.

Crystal structure and site preference, magnetic and elastic properties, and martensitic transformation of Ni- and Fe-doped Co2VGa alloys: A first-principles study

Using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, the crystal structure and site preference, magnetic and elastic properties, and martensitic transformation (MT) are systematically investigated with the three groups of Heusler alloys: (Co2−xMx)VGa (M1x), Co2(V1−xMx)Ga (M2x), and Co2V(Ga1−xMx) (M3x, M = Ni and Fe, 0≤x≤1.0). It is shown that instead of the L21 and XA structures, the fcc one is energetically preferred in the cubic M3x (x≥0.8) alloys. In L21-Ni2x (x≤0.6) and fcc-Ni3x (x=0.8), Ni atoms even prefer the Ga and Co anti-sites, respectively, and the replaced atoms move to the sublattices of the deficient ones. Their total magnetic moment is dominated by the magnetic exchange interactions corresponding to the pairs of two Co atoms on the different sublattices in M = Ni and Fe1x, Co and Fe in Fe2x and Fe3x (x<0.8), and Fe and Fe atoms in Fe3x (x≥0.8) alloys, respectively. These Ni1x, Ni2x, and Fe3x with x≥0.4 as well as Ni3x with x≥0.2 alloys are predicted having the MT behavior and also the better mechanical property relative to Co2VGa. A lower shear modulus (C′=(C11−C12)/2) generally corresponds to a higher MT temperature, and these alloys, which can undergo the MT are further evaluated with C′<36.50 GPa. Both considerable magnetocaloric and magnetovolume effects can be also expected during the MT of these Fe3x alloys (x=0.4 and 0.6). In the remaining Fe1x and Fe2x alloys, the Fe doping disfavors the MT and also improves their brittleness. The structural preference of these cubic alloys and also their stability relative to the tetragonal martensite can be mainly attributed to the number of their minority density of states at the Fermi level: the smaller they are, the more stable their system tends to be.

First-principles study of defects at [formula omitted] grain boundaries in CuGaSe[formula omitted

We present a first-principles computational study of cation–Se Σ3 (112) grain boundaries in CuGaSe2. We discuss the structure of these grain boundaries, as well as the effect of native defects and Na impurities on their electronic properties. The formation energies show that the defects will tend to form preferentially at the grain boundaries, rather than in the grain interiors. We find that in Ga-rich growth conditions Cu vacancies as well as Ga at Cu and Cu at Ga antisites are mainly responsible for having the equilibrium Fermi level pinned toward the middle of the gap, resulting in carrier depletion. The Na at Cu impurity in its +1 charge state contributes to this. In Ga-poor growth conditions, on the other hand, the formation energies of Cu vacancies and Ga at Cu antisites are comparatively too high for any significant influence on carrier density or on the equilibrium Fermi level position. Thus, under these conditions, the Cu at Ga antisites give rise to a p-type grain boundary. Also, their formation energy is lower than the formation energy of Na at Cu impurities. Thus, the latter will fail to act as a hole barrier preventing recombination at the grain boundary, in contrast to what occurs in CuInSe2 grain boundaries. We also discuss the effect of the defects on the electronic properties of bulk CuGaSe2, which we assume reflect the properties of the grain interiors.

Crystalline growth and alloying of In[formula omitted]Ga[formula omitted]Sb films by electrodeposition onto liquid metal electrodes

Incorporating indium (In) atoms into the GaSb lattice provides broad tunability of the optical bandgap within the infrared spectral region. This research documents the first instance in the literature of electrochemical liquid–liquid–solid (ec-LLS) growth of a ternary semiconductor alloy. Indium content of deposited InxGa1−xSb samples was regulated by controlling the Ga/In ratio of the liquid metal electrode. All depositions were performed using a single-step growth method under ambient pressure at 90 ± 5 °C. X-ray diffraction analysis showed shifts of the (111) diffraction peaks toward the peak locations of cubic InSb for the x=0.50, 0.60, and 0.70 samples, indicating a consistent increase in lattice parameter. However, the x=0.28 and 0.41 samples displayed unexpected shifts toward larger angles, possibly due to increased presence of Ga vacancies and antisites caused by Ga-rich growth conditions, as well as insufficient mixing of the liquid metal electrode for x< 0.50. Optical bandgaps ranging from 0.594 to 0.707 eV were determined via the Tauc method and Kubelka–Munk theory applied to diffuse reflectance data, and showed general agreement with measured lattice constants. Data obtained in this research supports In incorporation into the GaSb lattice for samples with x≥ 0.50, but with significantly lower In compositions relative to the liquid metal electrode. No general trend was observed for average crystallite size as a function of In composition, likely due to inconsistencies in the film harvesting process. This research provides further support for ec-LLS as a semiconductor growth technique capable of crystalline growth and alloying of III-V semiconductors.

Ab initio analytic calculation of point defects in AlGaN/GaN heterointerfaces

One of the major challenges for the GaN-based high-electron-mobility transistors (HEMTs) used as high power devices is to understand the effect of defects, especially on the band alignment. Using ab initio calculation, herein we investigate the variations of band offsets with interfacial structure, defect position, interface states and Al content in Al x Ga1-x N/GaN heterostructures (x = 0.06, 0.13, 0.19, 0.25). It was found that N vacancy (VN) and Ga anti-site (GaN) introduce nonlocal interface states and the change of valence band offset (VBO) depends on the defect location. While the interface states induced by Ga vacancy (VGa) and N anti-site (NGa) show strong localization behavior, and their impact on VBO is independent on the defect position. The low symmetry of wurtzite nitride and the lattice mismatch between AlGaN and GaN will generate polarization charge (spontaneous polarization and piezoelectric polarization) at the interface. Along the direction of polarization field, VN and GaN lying in the AlGaN side change the VBO most pronouncedly. These theoretical results provide useful guidance for control of point defects in AlGaN/GaN HEMTs, which have profound impact on the performance and reliability of GaN-based devices.

Hole density and acceptor-type defects in MBE-grown GaSb1-x Bix

We study acceptor-type defects in grown by molecular beam epitaxy. The hole density of the layers, from capacitance-voltage measurements of Schottky diodes, is higher than that of the binary alloys and increases linearly up to 1019 with the Bi content. Positron annihilation spectroscopy and ab initio calculations show that both Ga vacancies and Ga antisites contribute to the hole density and that the proportion of the two acceptor-type defects vary in the layers. The modification of the band gap due to Bi incorporation as well as the growth parameters are suggested to affect the concentrations of acceptor-type defects.

Instability of the Sb vacancy in GaSb

We demonstrate that the instability of the Sb vacancy in GaSb leads to a further increase in the acceptor-type defect concentration in proton irradiated undoped, $p$-type GaSb. Using positron annihilation spectroscopy in situ with 10 MeV proton irradiation at 35 K, we find that the irradiation produces both native vacancy defects in GaSb. However, the Sb vacancy is unstable above temperatures of 150 K and undergoes a transition resulting in a Ga vacancy and a Ga antisite. The activation energy of this transition is determined to be $0.6\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\ifmmode\pm\else\textpm\fi{}0.1$ eV. Our results are in line with the established amphoteric defect model and prove that the instability of the Sb vacancy in GaSb has a profound role on the native defect concentration in GaSb.

Discriminating a deep gallium antisite defect from shallow acceptors in GaAs using supercell calculations

For the purposes of making reliable first-principles predictions of defect energies in semiconductors, it is crucial to distinguish between effective-mass-like defects, which cannot be treated accurately with existing supercell methods, and deep defects, for which density functional theory calculations can yield reliable predictions of defect energy levels. The gallium antisite defect ${\mathrm{Ga}}_{As}$ is often associated with the 78/203 meV shallow double acceptor in Ga-rich gallium arsenide. Within a conceptual framework of level patterns, analyses of structure and spin stabilization can be used within a supercell approach to distinguish localized deep defect states from shallow acceptors such as ${\mathrm{B}}_{As}$. This systematic approach determines that the gallium antisite supercell results has signatures inconsistent with an effective mass state and cannot be the 78/203 shallow double acceptor. The properties of the Ga antisite in GaAs are described, total energy calculations that explicitly map onto asymptotic discrete localized bulk states predict that the Ga antisite is a deep double acceptor and has at least one deep donor state.

Increased p-type conductivity in GaNxSb1−x, experimental and theoretical aspects

The large increase in the p-type conductivity observed when nitrogen is added to GaSb has been studied using positron annihilation spectroscopy and ab initio calculations. Doppler broadening measurements have been conducted on samples of GaNxSb1−x layers grown by molecular beam epitaxy, and the results have been compared with calculated first-principle results corresponding to different defect structures. From the calculated data, binding energies for nitrogen-related defects have also been estimated. Based on the results, the increase in residual hole concentration is explained by an increase in the fraction of negative acceptor-type defects in the material. As the band gap decreases with increasing N concentration, the ionization levels of the defects move closer to the valence band. Ga vacancy-type defects are found to act as positron trapping defects in the material, and the ratio of Ga vacancy-type defects to Ga antisites is found to be higher than that of the p-type bulk GaSb substrate. Beside Ga vacancies, the calculated results imply that complexes of a Ga vacancy and nitrogen could be present in the material.

Application of the bounds-analysis approach to arsenic and gallium antisite defects in gallium arsenide

A recently developed bounds-analysis approach has been used to interpret density-functional-theory (DFT) results for the As and Ga antisites in GaAs. The bounds analysis and subsequent processing of DFT results for the As antisite yielded levels---defined as the Fermi levels at which the defect charge state changes---in very good agreement with measurements, including the \ensuremath{-}1/0 level which is within 0.1 eV of the conduction-band edge. Good agreement was also obtained for the activation energies to transform the $\mathrm{A}{\mathrm{s}}_{\mathrm{Ga}}$ from its metastable state to its stable state. For the Ga antisite, the bounds analysis revealed that the \ensuremath{-}1 and 0 charge states are hole states weakly bound to a localized \ensuremath{-}2 charge state. The calculated levels are in good agreement with measurements.

Native point defects in GaSb

We have applied positron annihilation spectroscopy to study native point defects in Te-doped n-type and nominally undoped p-type GaSb single crystals. The results show that the dominant vacancy defect trapping positrons in bulk GaSb is the gallium monovacancy. The temperature dependence of the average positron lifetime in both p- and n-type GaSb indicates that negative ion type defects with no associated open volume compete with the Ga vacancies. Based on comparison with theoretical predictions, these negative ions are identified as Ga antisites. The concentrations of these negatively charged defects exceed the Ga vacancy concentrations nearly by an order of magnitude. We conclude that the Ga antisite is the native defect responsible for p-type conductivity in GaSb single crystals.

Point defect balance in epitaxial GaSb

Positron annihilation spectroscopy in both conventional and coincidence Doppler broadening mode is used for studying the effect of growth conditions on the point defect balance in GaSb:Bi epitaxial layers grown by molecular beam epitaxy. Positron annihilation characteristics in GaSb are also calculated using density functional theory and compared to experimental results. We conclude that while the main positron trapping defect in bulk samples is the Ga antisite, the Ga vacancy is the most prominent trap in the samples grown by molecular beam epitaxy. The results suggest that the p–type conductivity is caused by different defects in GaSb grown with different methods.

As vacancies, Ga antisites, and Au impurities in zinc blende and wurtzite GaAs nanowire segments from first principles

In this paper some specific issues related to point defects in GaAs nanowires are addressed with the help of density functional theory calculations. These issues mainly arise from the growth of nanowires under conditions different from those used for thin film or bulk GaAs, such as the coexistence of zinc blende and wurtzite polytypes, the use of gold particles as catalyst, and the arsenic-limited growth regime. Hence, we carry out density functional calculations for As vacancies, Ga${}_{\mathrm{As}}$ antisites, and Au impurities in zinc blende (ZB) and wurtzite (WZ) GaAs crystals. Our results show that As vacancies can diffuse within a ZB GaAs crystal with migration barriers of $\ensuremath{\sim}$1.9 eV. Within WZ GaAs, As vacancy diffusion is found to be anisotropic, with low barriers of 1.60 up to 1.79 eV (depending on doping conditions) in the $ab$ plane, while there are higher barriers of 2.07 to 2.44 eV to diffuse along the $c$ axis. The formation energy of Au impurities is found to be generally much lower than those of arsenic vacancies or Ga${}_{\mathrm{As}}$ antisites. Thus, Au impurities will be the dominant defects formed in Au-catalyzed nanowire growth. Moreover, we find that it is energetically more favorable by 1 to 2 eV for a Au impurity to replace a lattice Ga atom than a lattice As atom in GaAs. A Au substitutional defect for a lattice Ga atom in ZB GaAs is found to create a charge transfer level in the lower half of the band gap. While our calculations locate this level at ${E}_{v}+0.22$ eV, taking into account the inaccuracy of the density functional that ought to be corrected by a downshift of ${E}_{v}$ by about 0.2 eV, results in good agreement with the experimental result of ${E}_{v}+0.4$ eV.

Molecular beam epitaxial growth and characterization of GaSb layers on GaAs (0 0 1) substrates

We report on the growth of GaSb layers on GaAs (001) substrates by molecular beam epitaxy (MBE). We investigate the influence of the GaAs substrate surface treatment, growth temperature, and V/III flux ratios on the crystal quality and the surface morphology of GaSb epilayers. Comparing to Ga-rich GaAs surface preparation, the Sb-rich GaAs surface preparation can promote the growth of higher-quality GaSb material. It is found that the crystal quality, electrical properties, and surface morphology of the GaSb epilayers are highly dependent on the growth temperature, and Sb/Ga flux ratios. Under the optimized growth conditions, we demonstrate the epitaxial growth of high quality GaSb layers on GaAs substrates. The p-type nature of the unintentionally doped GaSb is studied and from the growth conditions dependence of the hole concentrations of the GaSb, we deduce that the main native acceptor in the GaSb is the Ga antisite (GaSb) defect.

Point defects in dilute nitride III-N–As and III-N–P

We provide a brief review of our recent results from optically detected magnetic resonance studies of grown-in non-radiative defects in two most important dilute nitride systems—Ga(In)NAs grown on GaAs substrates and Ga(Al,In)NP grown on Si and GaP substrates. These results have led to the identification of defect complexes in the alloys, involving intrinsic defects such as As Ga antisites and Ga i self-interstitials. They have also shed light on formation mechanisms of the defects and on their role in non-radiative carrier recombination that is harmful to the performance of potential optoelectronic and photonic devices based on these dilute nitrides.

Intrinsic defect properties in GaN calculated byab initioand empirical potential methods

Density functional theory (DFT) has been used to investigate the formation, properties, and atomic configurations of vacancies, antisite defects and interstitials in GaN, and the DFT results are compared with those calculated by molecular dynamics simulations using two representative potentials. The DFT calculations reveal that the relaxation of vacancies is generally small, but the relaxation around antisite defects is large, especially for the Ga antisite that is not stable and converts to a ${\mathrm{N}}^{+}\ensuremath{-}\mathrm{N}⟨0001⟩$ split interstitial plus a Ga vacancy at the original site. The N interstitials, starting from all possible sites, eventually relax into a ${\mathrm{N}}^{+}\ensuremath{-}\mathrm{N}⟨11\overline{2}0⟩$ split interstitial. In the case of Ga interstitials, the most stable configuration is a Ga octahedral interstitial, but the energy difference among all the interstitials is small. The ${\mathrm{Ga}}^{+}\ensuremath{-}\mathrm{Ga}⟨11\overline{2}0⟩$ split interstitial can bridge the gap between nonbonded Ga atoms, thereby leading to a chain of four Ga atoms along the $⟨11\overline{2}0⟩$ direction in GaN. The formation energies of vacancies and antisite defects obtained using the Stillinger-Weber (SW) potential are in reasonable agreement with those obtained by DFT calculations, whereas the Tersoff-Brenner (TB) potential better describes the behavior of N interstitials. In the case of Ga interstitials, the most stable configuration predicted by the TB model is a ${\mathrm{Ga}}^{+}\ensuremath{-}\mathrm{N}⟨11\overline{2}0⟩$ split interstitial; while for the SW model the Ga tetrahedral configuration is more stable, which is in contrast to DFT results.

Effects of Photowashing Treatment on Gate Leakage Current of GaAs Metal-Semiconductor Field-Effect Transistors

Effects of photowashing treatment on gate leakage current (IGD) of a GaAs metal-semiconductor field-effect transistor were studied by observing changes in atomic composition and band bending at the surface of GaAs through X-ray photoemission spectroscopy. The photowashing treatment produces Ga2O3 on the surface of GaAs, leaving acceptor-type Ga antisites behind under the oxide. The Ga antisites played a role in reducing the maximum electric field at the drain edge of the gate, leading to the decrease of IGD. The longer photowashing time produced thicker oxide on the surface of GaAs, acting as a conducting pass for electrons, leading to the increase of IGD.

Native point defects in non-stoichiometric GaAs doped with beryllium

Native point defects in non-stoichiometric GaAs layers doped with Be were observed. Ga vacancies were identified by positron annihilation. As Ga antisite defects were investigated by near infrared absorption and magnetic circular dichroism of absorption, respectively. We found the properties of the As Ga antisites very similar to that of As Ga in bulk GaAs, i.e. they exhibit the same MCDA spectra and quenching behavior. The concentration of native point defects was not influenced by the Be doping. Therefore, the thermal stability of non-stoichiometric GaAs : Be can not be explained with a different incorporation of native defects.

Evolution from point defects to arsenic clusters in low-temperature grown GaAs/AlGaAs multiple quantum wells

Optical transient current spectroscopy (OTCS), photoluminescence (PL) spectroscopy and excitonic electroabsorption spectroscopy have been used to investigate the evolution of defects in the low-temperature grown GaAs/AlGaAs multiple quantum well structures during the postgrowth rapid thermal annealing. The sample was grown at 350°C by molecular beam epitaxy on miscut (3.4° off (0 0 1) towards (1 1 1) A) (0 0 1) GaAs substrate. After growth, the sample was subjected to 30 s rapid thermal annealing in the range of 500–800°C. It is found that the integrated PL intensity first decreases with the annealing temperature, then gets a minimum at 600°C and finally recovers at higher temperatures. OTCS measurement shows that besides As Ga antisites and arsenic clusters, there are several relatively shallower deep levels with excitation energies less than 0.3 eV in the as-grown and 500°C-annealed samples. Above 600°C, OTCS signals from As Ga antisites and shallower deep levels become weaker, indicating the decrease of these defects. It is argued that the excess arsenic atoms group together to form arsenic clusters during annealing.

Observation of Ga vacancies in silicon δ-doping superlattices in (001) GaAs

Positron annihilation experiments have been performed to investigate the compensating defects in silicon δ-doping superlattices in (001) GaAs. The results reveal vacancies and ion-type defects, which are located between the delta planes in undoped GaAs. The vacancy defect is identified as the Ga vacancy and the negative ion is attributed to the Ga antisite. The concentrations of these defects increase strongly, when the areal concentrations of free carriers are reduced at the delta planes.