Nitrogen vacancies as major point defects in gallium nitride

We present results of ab initio calculations for vacancies and divacancies in GaN. Particular attention is paid to nitrogen vacancies and mixed Ga-N divacancies in negatively charged states, which in n-type GaN are found to be energetically comparable with gallium vacancies. We also demonstrate that the activation energy for self-diffusion over the nitrogen sublattice is lower than over the gallium one for all Fermi-level positions, which implies the nitrogen vacancies are major defects in samples annealed at high temperatures. Possibilities for direct observations of nitrogen vacancies through positron annihilation experiments are discussed.     

Vacancies in GaN bulk and nanowires: effect of self-interaction corrections

We investigate gallium and nitrogen vacancies in gallium nitride (GaN) bulk and nanowires using self-interaction corrected pseudopotentials (SIC). In particular, we examine the band structures to compare and contrast differences between the SIC results and standard density functional theory (DFT) results using a generalized gradient approximation (GGA) (Perdew et al 1996 Phys. Rev. Lett. 77 3865) functional. For pure nanowires, we observed similar trends in the bandgap behaviour, with the gap decreasing for increasing nanowire diameters (with larger bandgaps using SIC pseudopotentials). For gallium vacancies in bulk GaN and GaN nanowires, SIC results are similar to DFT-GGA results, albeit with larger bandgaps. Nitrogen vacancies in bulk GaN show similar defect-induced states near the conduction band, whilst a lower lying defect state is observed below the valence band for the DFT-GGA calculations and above the valence band for the SIC results. For nitrogen vacancies in GaN nanowires, similar defect states are observed near the conduction band, however, while the SIC calculations also show a defect state/s above the valence band, we were unable to locate this state for the DFT-GGA calculations (possibly because it is hybridized with edge states and buried below the valence band).     

First-principle study on electronic structure and magnetic properties of organic transition metal compound: Trans-tetrachloro-bis-(pyridine)-rhenium

The electronic structure and magnetic properties of the trans-tetrachloro-bis-(pyridine)-rhenium compound with the Re atom as the metallic magnetic center, were studied using the full potential linearized augmented plane wave method (FP-LAPW) within the density functional theory. The calculated total energies revealed that the compound has a stable antiferromagnetic (AFM) ground state, which is in agreement with the experiment. The band structure of the compound has a semiconductor character. The calculated magnetic moment per molecule is 3.00 μ_B, the magnetic moments are mainly from the Re atoms with a 5d³ electronic configuration. The AFM interaction between ferromagnetically coupled Re atom layers passes through the p orbitals of the Cl ligands near Re atoms.     

Simulation of kinetics of water temperature-programmed desorption from n-GaAs(100) and n-GaP(100) semiconductor surfaces

The energy spectra of adsorption centers on the n-GaAs(100) and n-GaP(100) surfaces are studied using temperature-programmed desorption of water. The desorption spectra are analyzed in terms of the model of discrete adsorption centers, which assumes the presence of a loosely bound precursor state. The values of frequency factors and activation energies of desorption are in good agreement with the frequency of electronic transitions and energies of surface electronic states in gallium arsenide and gallium phosphide. It is concluded that the water desorption kinetics is limited by slow electronic processes on the surface of the semiconductors.     

Reconstruction control of magnetic properties during epitaxial growth of ferromagnetic Mn3-deltaGa on Wurtzite GaN(0001)

Binary ferromagnetic Mn(3-delta)Ga (1.2<3-delta< or =1.5) crystalline thin films have been epitaxially grown on wurtzite GaN(0001) surfaces using rf N-plasma molecular beam epitaxy. The film structure is face-centered tetragonal with CuAu type-I (L1(0)) ordering with (111) orientation. The in-plane epitaxial relationship to GaN is nearly ideal with [110](MnGa) parallel[1100](GaN) and [112](MnGa) parallel[1120](GaN). We observe magnetic anisotropy along both the in-plane and out-of-plane directions. The magnetic moments are found to depend on the Mn/(Mn+Ga) flux ratio and can be controlled by observation of the surface reconstruction during growth, which varies from 1x1 to 2x2 with increasing Mn stoichiometry.     

Nucleation of III nitride semiconductors in heteroepitaxy

The nucleation of III nitride semiconductors in heteroepitaxy is theoretically investigated using GaN nucleation on the AlN surface as an example. It is inferred that the mechanism of this process is determined by the temperature at the initial stage of the layer formation (T). At low temperatures (T<500°C), liquid gallium droplets appear and the chemical reaction between the Ga and N atoms results in the formation of GaN nuclei. At substrate temperatures T>650°C, there arise only GaN nuclei. It is revealed that the GaN nucleation is governed by the generalized diffusion coefficient of GaN, which is a combination of the diffusion coefficients for gallium and nitrogen atoms. It is shown that the generalized diffusion coefficient of GaN on the crystal surface increases by seven orders of magnitude as the growth temperature increases from 600 to 800°C. This is accompanied by a change in the growth mechanism of the III nitride semiconductor epitaxial layers.     

Ab initio investigation of local magnetic structures around substitutional 3d transition metal impurities at cation sites in III-V and II-VI semiconductors

The local magnetic structures around substitutional 3d transition metal impurities at cation sites in zinc blende structures of III-V (GaN, GaAs) and II-VI (ZnTe) semiconductors are investigated by using a spin-polarized density functional theory. We find that Cr-, Co-, Cu-doped GaN, Cr-, Mn-doped GaAs and Cr-, Fe-, Ni-doped ZnTe are half metallic with 100% spin polarization. The magnetic moments due to these 3d transition metal (TM) ions are delocalized quite significantly on the surrounding ions of host semiconductors. These doped TM ions have long range interactions mediated through the induced magnetic moments in anions and cations of host semiconductors. For low impurity concentrations Mn in GaAs also has zero magnetic moment state due to Jahn-Teller structural distortions. Based upon half metallic character and delocalization of magnetic moments in the anions and cations of host semiconductors these above mentioned 3d TM-doped GaN, GaAs and ZnTe seem to be good candidates for spintronic applications.     

Magnetic properties of Mn-doped GaN with defects: ab-initio calculations

According to first-principles density functional calculations,we have investigated the magnetic properties of Mn-doped GaN with defects,Ga 1-x-y V Gx Mn y N 1-z-t V Nz O t with Mn substituted at Ga sites,nitrogen vacancies V N,gallium vacancies V G and oxygen substituted at nitrogen sites.The magnetic interaction in Mn-doped GaN favours the ferromagnetic coupling via the double exchange mechanism.The ground state is found to be well described by a model based on a Mn 3+-d 5 in a high spin state coupled via a double exchange to a partially delocalized hole accommodated in the 2p states of neighbouring nitrogen ions.The effect of defects on ferromagnetic coupling is investigated.It is found that in the presence of donor defects,such as oxygen substituted at nitrogen sites,nitrogen vacancy antiferromagnetic interactions appear,while in the case of Ga vacancies,the interactions remain ferromagnetic;in the case of acceptor defects like Mg and Zn codoping,ferromagnetism is stabilized.The formation energies of these defects are computed.Furthermore,the half-metallic behaviours appear in some studied compounds.     

Surface order dependent magnetic thin film growth: Fe on GaN(0 0 0 1)

X-ray photoemission spectroscopy and X-ray magnetic circular dichroism have been used to study the growth process, chemical composition and magnetic character of iron deposited on ordered and disordered GaN(0 0 0 1) surfaces. On the (1 × 1) ordered surface the Fe grows uniformly but with disruption to the substrate surface, subsequently nitrogen desorbs from the surface, some of which diffuses into the Fe overlayer. The film is magnetically fractured, with high magnetic coercivity and broad switching fields. Conversely, the gallium rich disordered surface protects the underlying substrate from any disruption and initially induces non-uniform growth, the Fe clusters coalesce at ∼12 Å, to produce a uniform film with desirable magnetic characteristics. Films beyond this point (>12 Å) indicate sharp hysteresis loops with low coercivities. For the resultant film (36 Å) we measure a magnetic moment of 2.02 μ_B, in agreement with bulk bcc iron (2.068 μ_B.)     

Magnetic moments at the surface of antiferromagnetic NiO(100)

The orientation of magnetic moments at the (100) surface of antiferromagnetic NiO single crystals is studied by x-ray linear magnetic dichroism in photoemission microscopy. T domains are observed terminating at the surface, with domain boundaries running mostly along in-plane [10] directions. From the detailed polarization dependence we find that the magnetic surface structure of a cleaved crystal is bulk terminated. This is in contrast to sputtered surfaces, where magnetic moments lie within the surface plane, forming a magnetically relaxed structure. These findings are of importance for understanding the exchange bias phenomenon.     

Nitrogen vacancy effects on the ferromagnetism of Mn doped GaN films

The amorphous gallium nitride thin films doped with Mn were deposited by Laser assisted Molecular Beam Epitaxy (LMBE). After annealing at different NH₃ flow rates, the high-quality GaMnN crystalline films with different concentration of nitrogen vacancies (V _N) were obtained, which were confirmed by the X-ray diffraction spectroscopy and Raman measurements. The magnetic behaviors of these films were also obtained to investigate the effects of nitrogen vacancies. It indicates that V _N play a significant role in the origin of ferromagnetism. The stronger room-temperature ferromagnetism is given with the higher V _N concentration when it is not beyond a critical concentration. Moreover, from our M(T) curves and Raman analysis, the films present high resistivity. The magnetism of films with high resistivity varies with concentration of nitrogen vacancies, which can be explained by the bound magnetic polarons (BMP) theory.     

Mechanism and kinetics of early growth stages of a GaN film

The growth of GaN islands on the substrate surface covered with an AlN buffer layer is theoretically investigated at the stages of nucleation and Ostwald ripening in the temperature range 480–1000°C. The following inferences are made from analyzing the results obtained. (1) At temperatures T>650°C, the growth of islands is controlled by the chemical reaction of formation of GaN molecules around the periphery of the island surface. Islands nucleated at these temperatures are characterized by a large spread in their sizes. (2) At temperatures T<600°C, the island growth is governed by the surface diffusion of nitrogen atoms. Islands nucleated at these temperatures are virtually identical in size. (3) In the temperature range 600–650°C, the mechanism of island growth gradually changes over from the mechanism associated with the surface diffusion of nitrogen atoms with a large mean free path to the mechanism determined by the diffusion of gallium atoms with a smaller mean free path. The supersaturation, flux, and size distribution functions of GaN nuclei are calculated at different substrate temperatures. The phase diagrams describing the evolution in the phase composition of GaN islands with variations in temperature are constructed.     

Stability and electronic properties of the O-terminated Cu2O(111) surfaces: First-principles investigation

The effect of different vacancies on the morphology of the O-terminated Cu₂O(111) surface has been studied through the first-principles calculations. Our results show that Cu and O vacancies trigger large relaxations and formation of two different facets. Our emulated STM images are in consistent with the experimental STM patterns. A sizeable magnetic moment (∼1.0 μ_B) was found for surfaces with either Cu or O vacancies. The calculated formation energies of surface vacancies indicate that the deficient surfaces are more stable than the stoichiometric O-terminated (1×1) surface, which is also in line with our results obtained from ab initio atomistic thermodynamics studies.     

Ferromagnetism induced by intrinsic defects and nitrogen substitution in SnO2 nanotube

The possibilities of magnetism induced by intrinsic defects and nitrogen substitution in (5,5) single-wall SnO₂ nanotube are investigated by ab initio calculations. The calculated results indicate that a stoichiometric SnO₂ nanotube is nonmagnetic. The tin (Sn) vacancy can induce the magnetic moments rather than oxygen vacancy, which is originated from the polarization of O 2p electrons. A couple of tin vacancies can lead to the ferromagnetic coupling. A nitrogen substitution for oxygen also produces magnetic moments. When substituting two nitrogen atoms, the characteristics of exchange coupling depend upon the distance of two nitrogen atoms. The longer distance of two nitrogen atoms prefers the ferromagnetic coupling, whereas the short distance leads to the antiferromagnetic coupling.     

Structure and electronic properties of native and defected gallium nitride nanotubes

Structure and electronic properties of GaN nanotubes (GaNNTs) are investigated by using ab initio density functional theory. By full optimization, the optimized structures (bond-lengths and angles between them) of zigzag GaNNTs (n,0) and armchair GaNNTs (n,n) (4<n<11) are calculated. The difference between nitrogen ring diameter and gallium ring diameter (buckling distance) and semiconducting energy gap in term of diameter for zigzag and armchair GaNNTs have also been calculated. We found that buckling distance decreases by increasing nanotube diameter. Furthermore, we have investigated the effects of nitrogen and gallium vacancies on structure and electronic properties of zigzag GaNNT (5,0) using spin dependent density functional theory. By calculating the formation energy, we found that N vacancy in GaNNT (5,0) is more favorable than Ga vacancy. The nitrogen vacancy in zigzag GaNNT induces a 1.0μB magnetization and makes a polarized structure. We have shown that in polarized GaNNT a flat band near the Fermi energy splits to occupied spin up and unoccupied spin down levels.     

Clusters and magnetic anchoring points in (Ga,Fe)N condensed magnetic semiconductors

The stability and magnetic properties of Fe clusters in the (Ga,Fe)N magnetic semiconductor is investigated by using first-principles density functional theory and local spin density+Hubbard U theoretical methods. The present results reveal the existence of ferrimagnetic clusters formed by three or four peripheral Fe atoms neighboring a central Fe atom acting as a robust magnetic anchoring point. These clusters have magnetic moments 2 or 3 times that of a single Fe atom and, when connected by sharing peripheral Fe atoms, can form stable, ordered magnetic regions where all of the central atoms are ferromagnetically coupled. The formation of these ferrimagnetic clusters is proposed here to be at the origin of the ferromagnetic behavior observed in (Ga,Fe)N samples showing chemical phase separation.     

Gallium nitride nanoparticle synthesis using nonthermal plasma with gallium vapor

Gallium nitride (GaN) nanoparticles are synthesized by the gallium particle trapping effect in a N₂ nonthermal plasma with metallic Ga vapor. A proposed method has an advantage of synthesized GaN nanoparticle purity because the gallium vapor from the inductively heated tungsten boat does not contain any impurity source. The synthesized particle size can be controlled by the amount of Ga vapor, which is adjusted using the plasma emission ratio of nitrogen to gallium, owing to the particle trapping effect. The synthesized nanoparticles are investigated by electron microscopy studies. High-resolution transmission electron microscopy (HRTEM) studies confirm that the synthesized GaN nanoparticles (10–40 nm) crystallize in a single-phase wurtzite structure. Room-temperature photoluminescence (PL) measurements indicate the band-edge emission of GaN at around 378 nm without yellow emission, which implies that the synthesized GaN nanoparticles have high crystallinity.     

Ga vacancy induced ferromagnetism enhancement and electronic structures of RE-doped GaN

Because of their possible applications in spintronic and optoelectronic devices, GaN dilute magnetic semiconductors (DMSs) doped by rare-earth (RE) elements have attracted much attention since the high Curie temperature was obtained in RE-doped GaN DMSs and a colossal magnetic moment was observed in the Gd-doped GaN thin film. We have systemically studied the GaN DMSs doped by RE elements (La, Ce–Yb) using the full-potential linearized augmented plane wave method within the framework of density functional theory and adding the considerations of the electronic correlation and the spin-orbital coupling effects. We have studied the electronic structures of DMSs, especially for the contribution from f electrons. The origin of magnetism, magnetic interaction and the possible mechanism of the colossal magnetic moment were explored. We found that, for materials containing f electrons, electronic correlation was usually strong and the spin–orbital coupling was sometimes crucial in determining the magnetic ground state. It was found that GaN doped by La was non-magnetic. GaN doped by Ce, Nd, Pm, Eu, Gd, Tb and Tm are stabilized at antiferromagnetic phase, while GaN doped by other RE elements show strong ferromagnetism which is suitable materials for spintronic devices. Moreover, we have identified that the observed large enhancement of magnetic moment in GaN is mainly caused by Ga vacancies (3.0μB per Ga vacancy), instead of the spin polarization by magnetic ions or originating from N vacancies. Various defects, such as substitutional Mg for Ga, O for N under the RE doping were found to bring a reduction of ferromagnetism. In addition, intermediate bands were observed in some systems of GaN:RE and GaN with intrinsic defects, which possibly opens the potential application of RE-doped semiconductors in the third generation high efficiency photovoltaic devices.     

(139La, 55Mn) NMR and magnetic susceptibility data on microscopic phase separation in La0.9MnO3 single crystal

Magnetic susceptibility of the La_0.9MnO₃ single crystal was measured and its (¹³⁹La, ⁵⁵Mn) NMR spectra were recorded. The data obtained indicate that the areas with an A-type antiferromagnetic order (T _N=140 K) and magnetic moments aligned with the b axis occupy a major part of the sample volume in manganite with a considerable concentration of cationic vacancies; simultaneously, the clusters with a canted magnetic sublattice and ferromagnetic interaction between magnetic moments are formed near the vacancies. Charge distribution in these clusters is materially different from that in the antiferromagnetic areas. Magnetic state and relative concentration of the clusters are discussed.     

Ferromagnetism driven by cation vacancy in GaN thin films and nanowires

The first-principles calculations have been performed to understand the origin of magnetism in undoped GaN thin films. The results show that Ga vacancy, rather than that of N contributes the observed magnetism, and the magnetic moments mainly come from the unpaired 2p electrons at nearest-neighbor N atoms of the Ga vacancy. Calculations and discussions are also extended to bare and passivated GaN nanowires, We find that per Ga vacancy on the surface sites products the total magnetic moment of 1.0  while that inside of the nanowires can lead to the formation of a net moment of 3.0 . The coupling between two Ga vacancies is also studied and we found that the coupling is ferromagnetic coupling. The surface passivation with hydrogen is shown to strongly enhance the ferromagnetism. Our theoretical study not only demonstrates that GaN nanowire can be magnetic even without transition-metal doping, but also suggests that introducing Ga vacancy is a natural and an effective way to fabricate low-dimensional magnetic GaN nanostructures.