Ga-terminated Surfaces Research Articles

Mn induces a 1×3 reconstruction in the ferromagnetic L21 Mn2FeGa (001) surface

The Mn2FeGa compound is an ordered ferrimagnetic Heusler alloy characterized by its unique inverse magnetostriction and exceptional magnetic properties. In this study, we employ density functional theory (DFT) to conduct a theoretical analysis of the atomic structure of the (001) surface of Mn2FeGa. We utilize the surface energy formalism to systematically evaluate and compare the stability of various surface atomic configurations under different chemical potential conditions. Notably, our investigation identifies three energetically stable surfaces: MnGa-terminated, Ga-terminated, and MnGa2-terminated surfaces, featuring a 3×1 surface reconstruction, prevalent under Ga-rich conditions. Understanding the atomic arrangements and surface effects of Mn2FeGa is pivotal for optimizing its properties for specific device applications.

First principles characterization of C-doped magnetically-enhanced D022-Mn[formula omitted]Ga surfaces

The present paper reports the results of a set of first-principles calculations via Density Functional Theory to characterize the structural, electronic, and magnetic properties of the bulk and (001) surface structures of D022-Mn3Ga. It was found that a Mny-Ga-terminated and another Ga-terminated surfaces are most stable surface reconstructions. The role of C diffusion was evaluated. It has a very localized effect on the magnetic enhancement of these surfaces as it diffuses through them. The magnetic enhancement is also discussed in terms of the surface effect, the magnetoelastic effect, the superexchange interaction, and the Density of States of each atom.

Preparation of Ga-terminated negative electron affinity-GaAs (100) surface by HCl-isopropanol treatment for nanoanalysis by scanning tunneling microscopy

Negative electron affinity (NEA) surfaces can emit electrons by low-energy light illumination that is nearly equal to the bandgap energy of a semiconductor because NEA surfaces lower the vacuum level to below the conduction-band minimum. In particular, NEA-GaAs surfaces show distinct characteristics such as high spin polarization, low emittance, short pulsed operation, and high intensity. NEA surfaces are formed by alternating application of Cs and O2 on a clean GaAs surface. Scanning tunneling microscopy (STM) was used to investigate the surface states of NEA-GaAs (100) surfaces prepared using HCl-isopropanol treatment followed by annealing in an ultrahigh vacuum. The results indicated remarkable improvement in the surface quality of the GaAs (100). The authors have been studying the relationship between electron emission properties and the adsorption structures of Cs on Ga-terminated GaAs surfaces. Here, they report the first observation of NEA-Ga-terminated surfaces with Cs adsorption using STM.

Atomic and electronic structures of silicene and germanene on GaAs(111)

By using first-principles method in the density-functional theory, we clarify the atomic and electronic structures of silicene and germanene on 1×1 GaAs(111). We find stable structures for silicene and germanene on both the As-terminated and Ga-terminated GaAs surfaces. The structures of silicene and germanene are similar to those of the free-standing ones, which present a honeycomb-hexagonal geometry. The cohesive energies of silicene and germanene on both As and Ga sides of GaAs surfaces are comparable to those of their bulk structures and/or those on Ag(111) substrates which have been widely observed in experiment, showing the possibility of synthesizing them on both sides of GaAs surfaces in experiment. The corresponding binding energies are in a range of 0.56-1.37 eV per Si (Ge) atom, 10 times larger than the usual van der Waals interaction, showing the covalent interaction between silicene (germanene) and GaAs surfaces. The band structure calculations show that such a covalent interaction induces the absence of Dirac electrons for silicene and germanene on GaAs surfaces. We then explore the method of recovering the Dirac electrons by using hydrogen (H) intercalation. It is found that the intercalated H atoms are chemically bonded to GaAs surface, and the silicene (germanene) shifts upward distance from GaAs surface increasing from 2.50-2.58 Å to 3.49-3.86 Å, where a covalent van-der-Waals interaction transition happens between silicene (germanene) and GaAs surface. Moreover, the distances between silicene (germanene) and H atoms are 30% and 8% larger than the atomic-radius sum of Si (Ge) and H on As-terminated and Ga-terminated GaAs surfaces, respectively. This shows that the interaction between silicene (germanene) and H on the As-terminated GaAs surface is obviously weaker than the typical covalent interaction, while on the Ga-terminated GaAs surface, it is comparable to the typical covalent interaction. This difference is induced by the difference in electronegativity between As and Ga atoms. We further find that the H intercalation recovers the Dirac electrons well on the As-terminated GaAs(111) due to the weaker Si (Ge)-H interaction, while it does not on the Ga-terminated GaAs(111) due to the stronger Si (Ge)-H interaction. The results are confirmed by performing calculations for silicene (germanene) on larger GaAs(111) surfaces, i.e., the 3×3 GaAs surface. Our study provides the theoretical basis for the preparation and application of silicene and germanene on semiconductor surfaces.

A DFT Study of TiO<sub>2</sub> Adsorption on GaN(0001)Line Defect Surface

TiO2adsorption on GaN(0001) line defect surface has been explored by employing density functional theory. The total energy, density of states, surface energy (adsorption energy), formation energy and electron density of the various models have been obtained. Our calculations indicate that the model containing [11 line defect and one O atom at the vacancy of line defect for Ga-terminated surfaces prefers to adsorb TiO2, and the smallest adsorption energy is-11.258 eV. This study also shows that the TiO2adsorption on line defect of [11 direction is also associated with the initial adsorption position of TiO2.

First-principles theoretical study of hydrolysis of stepped and kinked Ga-terminated GaN surfaces

We have investigated the initial stage of hydrolysis process of Ga-terminated GaN surfaces by using first-principles theoretical calculations. We found that the activation barrier of H2O dissociation at the kinked site of the Ga-terminated GaN surface is about 0.8 eV, which is significantly lower than that at the stepped site of about 1.2 eV. This is consistent with the experimental observation where a step-terrace structure is observed after the etching process of Ga-terminated GaN surfaces with catalyst-referred etching method. Detailed analysis on the nature of the chemical interaction uring the hydrolysis processes will be discussed.

Structure and chemical reactivity of the polar three-fold surfaces of GaPd: A density-functional study

The polar threefold surfaces of the GaPd compound crystallizing in the B20 (FeSi-type) structure (space group P2(1)3) have been investigated using density-functional methods. Because of the lack of inversion symmetry the B20 structure exists in two enantiomorphic forms denoted as A and B. The threefold {111} surfaces have polar character. In both nonequivalent (111) and (111) directions several surface terminations differing in structure and chemical composition are possible. The formation of the threefold surfaces has been studied by simulated cleavage experiments and by calculations of the surface energies. Because of the polar character of the threefold surfaces calculations for stoichiometric slabs permit only the determination of the average energy of the surfaces exposed on both sides of the slab. Calculations for nonstoichiometric slabs performed in the grand canonical ensemble yield differences of the surface energies for the possible terminations as a function of the chemical potential in the reactive atmosphere above the surface and predict a transition between Ga- and Pd-terminated surfaces as a function of the chemical potential. The {100} surfaces are stoichiometric and uniquely defined. The calculated surface energies are identical to the average energies of the {100} surfaces of the pure metals. The {210} surfaces are also stoichiometric, with an energy very close to that of the {100} surfaces. Assuming that for the {111} surfaces the energies of different possible terminations are in a proportion equal to that of the concentration-weighted energies of the {111} surfaces of the pure metals, surface energies for all possible {111} terminations may be calculated. The preferable termination perpendicular to the A<111> direction consists of a bilayer with three Ga atoms in the upper and three Pd atoms in the lower part. The surface energy of this termination further decreases if the Pd triplet is covered by additional Ga atom. Perpendicular to the A<111> direction the lowest energy has been found for a bilayer with three Ga atoms per surface cell in the upper layer and one Ga and one Pd in the lower part. The calculated surface energies are in agreement with a simulated cleavage experiment. However, cleavage does not result in the formation of the lowest-energy surfaces, because all possible {111} cleavage planes expose a low-energy surface on one, and a high-energy surface on the other side. The prediction of Ga-terminated surfaces has been tested against the available experimental information. The calculated surface electronic density of states is in very good agreement with photo-emission spectroscopy. Calculated STM images of the most stable surfaces agree with all details of the available experimental images. The chemical reactivity of the most stable surfaces has been studied by the adsorption of CO molecules. The adsorption energies and maximum coverages calculated for the Ga-terminated surfaces permit a reasonable interpretation of the observed thermal desorption spectra, whereas for the Pd-terminated surfaces the calculated adsorption energies are far too high.

Modification of Schottky barrier height on Si (111) by Ga-termination

The method of “partisan interlayer” (PI) was recently applied to the As-, S- and Cl-terminated surfaces of silicon and was demonstrated effective in reducing the n-type Schottky barrier height (SBH) and increasing the p-type SBH. These SBH shifts were likely driven by the expected transfer of charge from silicon to the adsorbates with larger electronegativity. In the present work, SBHs for Ag, Au and In on Si(111)3×3-Ga R30° and Si(111)6.3×6.3-Ga surfaces are studied and found to be shifted by as much as 0.2eV from those on clean Si(111)7×7, but in directions opposite to those observed previously. The present results are thus in agreement with the expected Ga to Si charge transfer and illustrate the general validity of the PI technique. However, large degrees of SBH inhomogeneity are observed for diodes on Ga-terminated surfaces, likely due to insufficient stability of these surfaces to withstand metal interaction, i.e., breakage of silicon–adsorbate bonds on the interface and the simultaneous/subsequent bond formation between metal and silicon and/or adsorbate atoms, as a result of metal deposition.

Preserving stable 100% spin polarization at (111) heterostructures of half-metallic Heusler alloy Co2VGa with semiconductor PbS

Based on the half-metallicity confirmed experimentally in bulk Heusler alloy Co2VGa, we use the first-principles calculations to extend our previous studies on the Co2VGa (111) surfacial properties to the interfaces of Co2VGa/PbS (111) heterostructures in which four possible interfacial structures are considered between the V- and Ga-terminated (111) surfaces of Co2VGa and S- and Pb-terminated (111) surfaces of PbS. From the atomic density of states, it is shown that the half-metallicity of the bulk system is destroyed at Ga-S and Ga-Pb configurations while V-S and V-Pb configurations exhibit 100% spin polarization and nearly 100% spin polarization, respectively. Furthermore, the structure relaxation and the calculated interfacial adhesion energies indicate that V-S configuration is the most stable structure among them. In addition, we also discuss the changes of the atom magnetic moments at interface and subinterface layers with respect to the corresponding bulk values.

First-principles study on the half-metallicity of full-Heusler alloy Co2VGa (111) surface

A recent experimental study indicates that full-Heusler alloy Co2VGa exhibits half-metallic ferromagnetism and the magnetic moment measured at 5 K is independent of high pressure up to 12.1 kbar [T. Kanomata et al. Phys. Rev. B 82, 144415 (2010)]. Using the full potential linearized augmented plane-wave method, we here further investigate the structural, electronic, and magnetic properties of all possible (111) surfaces of Co2VGa. Both structural relaxation and calculated surface energy reveal that Ga-terminated surface is more stable than other terminations. From the analysis of the spin-polarized density of states, we find that the half-metallicity confirmed in bulk Co2VGa is lost at the Co-terminated surface but still maintained at the V- and Ga-terminated surfaces. Moreover, we also find that the atomic magnetic moments at the (111) surfaces are greatly different from the bulk values.

Adsorption configuration of magnesium on wurtzite gallium nitride surface using first-principles calculations

Abstract First-principles calculations of magnesium adsorption at the Ga-terminated and N-terminated {0 0 0 1} basal plane wurtzite gallium nitride surfaces have been carried out to explain the atomic-scale insight into the initial adsorption processes of magnesium doping in gallium nitride. The results reveal that magnesium adsorption on N-terminated surfaces is preferred than that on Ga-terminated surfaces. Furthermore, the surface diffusivity of magnesium atom on the N-terminated surface is much lower than that on the Ga-terminated surface, which is due to both the larger average adsorption energies and the lower adsorption distance on N-terminated surface than that on Ga-terminated surface. The results indicate that the p-type doping on the Ga-terminated surface will be better distributed than that on the N-terminated surface.

Band bending and adsorption/desorption kinetics on N-polar GaN surfaces

Highly reactive N-polar [000−1] GaN surfaces were analyzed using spectroscopic ellipsometry. Following exposure to air, observed changes in the pseudodielectric function near the GaN band edge indicate that surface contamination reduces the band bending. A subsequent Ga adsorption/desorption experiment on pristine N-polar GaN indicates that it contains a mixture of Ga-terminated and N-terminated surfaces. During deposition, Ga adatoms preferentially bond to the dangling bonds on the N-terminated surface: the measured 3.19 eV desorption activation energy equals the Ga–N decomposition energy. Further deposition forms a 1 ML Ga wetting layer whose 2.78 eV desorption activation energy is comparable to the Ga sublimation energy.

Studies of Effects of Adsorption of Silicon or Germanium on the Electronic States of (100) GaAs Surfaces

The electronic states of both bare Ga-terminated and As-terminated (100)-(1×1) GaAs surfaces and the same surfaces with adsorbed Si or Ge were studied by molecular orbital calculations using small cluster models and the discrete variational Xα method. The most stable adsorption sites for Si and Ge are bridge sites on the (100)-GaAs surface, regardless of whether they are Ga- or As-terminated. When Si or Ge atoms are adsorbed, the density of the surface states in the forbidden band of the Ga or As surface layers markedly decreases. This suggests that both the Ga-terminated and As-terminated surfaces have a clear band gap when either Si or Ge are adsorbed on the surfaces. These adsorbates have states in the lower half of the band gap or in the valence band.

Structure of clean and arsenic-covered GaN(0 0 0 1) surfaces

The effect of trace arsenic on the growth and surface structure of GaN(0 0 0 1) has been studied. We find that a partial pressure of only 10 −9 Torr of arsenic during molecular beam epitaxial growth significantly modifies the growth kinetics. Such a small background pressure of arsenic leads to an arsenic-terminated surface displaying a 2×2 reconstruction during growth which is absent for the clean surface. First-principles theoretical calculations show that As-terminated surfaces are energetically more favorable than Ga-terminated surfaces for arsenic pressures of 10 −9 Torr, and structural models for the As-adatom 2×2 reconstruction are presented.

Surface composition and structure of GaN epilayers on sapphire

The surface composition and structure of GaN films grown on sapphire substrates by organometallic vapor-phase epitaxy (OMVPE) have been determined through the use of time-of-flight scattering and recoiling spectrometry (TOF-SARS), classical ion trajectory simulations, and low-energy electron diffraction (LEED). TOF-SARS spectra of scattered and recoiled ions plus fast neutrals were collected using 4 keV Ar+ primary ions. The scattering results were simulated by means of the three-dimensional scattering and recoiling imaging code (SARIC). This data leads to the conclusions that both N-terminated {001̄}-(1×1) and Ga-terminated {0001}-(1×1) surfaces occur, however no evidence was obtained for mixed terminations. No relaxation or reconstruction was detected on either surface, although both surfaces exhibited two structural domains. The {0001̄} surfaces are well-ordered and contained hydrogen atoms bound to the N atoms of the outermost layer. The {0001} surfaces are highly reactive towards adsorption of carbon and oxygen from residual gases, however unlike the {0001̄} surfaces, they adsorb very little hydrogen. These Ga-terminated surfaces are stabilized and obtain more ordered structures as a result of the contamination.

On the kinetics of growth of highly defective GaN epilayers and the origin of the deep trap responsible for yellow-band luminescence

The kinetics of growth of GaN/(0001) sapphire heteroepitaxial films have been examined in the relatively low substrate temperature range, 560–640 °C, using the reflection high energy electron diffraction (RHEED) specular reflection intensity monitoring technique. In particular, an alternate element exposure method of growth was employed in which Ga and N atoms were supplied separately (rather than simultaneously, as in conventional molecular beam epitaxy) to the substrate with the inclusion of a time delay between successive Ga flux and N flux exposures. We interpret the observed time dependent recovery of the RHEED specular reflection intensity during the time delay phases to be associated with Ga–N surface molecule migration on Ga-terminated surfaces and the activation energy for this migration process was determined to be 1.45±0.25 eV.

LCAO calculations of surface electronic structure of zinc-blende-type GaN(100)

Layer resolved density of states has been calculated for N- and Ga-terminated gallium nitride (100) surfaces, using the semi-empirical LCAO method with self-consistently corrected parameters for the surfaces. Surface states on both N- and Ga-terminated surfaces have been found. Localization properties of these states have also been examined.

In-situ observation of Ga adsorption during TMGa exposure on GaAs(001) surfaces with various As coverages

We measured transients of surface photoabsorption signals during TMGa exposure on c(4 × 4)-like, 2 × 4-like and Ga-terminated GaAs(001) surfaces in the MOCVD reactor. Ga-containing species were continuously deposited during TMGa exposure on Ga-terminated surfaces. On the other hand, self-limiting deposition was observed on c(4 × 4)-like and 2 × 4-like surfaces. The growth thickness in one cycle of ALE process on the 2 × 4-like surface was 0.7–0.8 compared to that on the c(4 × 4)-like surface. The stable duration of the SPA signal under TMGa exposure was longer than the residency time of methyl groups on the surface which was obtained from the transients during purge after TMGa exposure. Hence the methyl groups are continuously supplied to the surface by decomposition of TMGa during the exposure period under self-limiting condition.

Study of self-limiting growth mechanism in chloride ALE

The growth process of chloride atomic layer epitaxy (ALE) using GaCl is studied using the temperature programmed desorption method (TPD), the surface photo-absorption (SPA) method and ab initio molecular orbital calculations. From the TPD measurements, the GaCl adsorption energy is calculated to be 32 kcal mol -1 and 38 kcal mol -1 for Ga-terminated and As-terminated surfaces respectively. In the absence of H 2, the Ga-Cl bond appears to be very stable in the adsorption state. Process simulation using a simple cluster model of (AsH 2) 2-GaCl indicates that adsorption is completed by forming σ and π covalent bonds, and that there is hardly any self-dissociation of the Ga-Cl bond. From SPA measurements in the ALE system with ambient H 2, a strong similarity is found between the spectra of the GaCl-supplying surface and the Ga-covered surface. Based on these results, a model of the chemical process in chloride ALE is proposed where adsorbed GaCl molecules can react immediately with hydrogen and Cl is released from the surface as HCl. A self-limiting mechanism results from the very short residence time of GaCl (∼ 10 -3s at 450 °C) on the Ga-terminated surface.

Barrierless dimer breaking at semiconductor surfaces by chlorine atoms.

The interaction of chlorine with semiconductor surfaces is investigated using the first-principles pseudopotential method. We determine the adsorption geometries of Cl atoms on both GaAs(001) and Si(001) dimerized surfaces in the high-coverage limit. It is found that Cl atoms can break Ga dimers at Ga-terminated GaAs surfaces without any potential barrier. The Cl adsorption simultaneously weakens the Ga-As back bonds at the surface, which contributes to the etching process. The effect of Cl adsorption on the bonding character of semiconductor surfaces is elucidated.