GaN Clusters Research Articles

Single-Exciton Photoluminescence in a GaN Monolayer inside an AlN Nanocolumn.

GaN/AlN heterostructures with thicknesses of one monolayer (ML) are currently considered to be the most promising material for creating UVC light-emitting devices. A unique functional property of these atomically thin quantum wells (QWs) is their ability to maintain stable excitons, resulting in a particularly high radiation yield at room temperature. However, the intrinsic properties of these excitons are substantially masked by the inhomogeneous broadening caused, in particular, by fluctuations in the QWs' thicknesses. In this work, to reduce this effect, we fabricated cylindrical nanocolumns of 50 to 5000 nm in diameter using GaN/AlN single QW heterostructures grown via molecular beam epitaxy while using photolithography with a combination of wet and reactive ion etching. Photoluminescence measurements in an ultrasmall QW region enclosed in a nanocolumn revealed that narrow lines of individual excitons were localized on potential fluctuations attributed to 2-3-monolayer-high GaN clusters, which appear in QWs with an average thickness of 1 ML. The kinetics of luminescence with increasing temperature is determined via the change in the population of localized exciton states. At low temperatures, spin-forbidden dark excitons with lifetimes of ~40 ns predominate, while at temperatures elevated above 120 K, the overlying bright exciton states with much faster recombination dynamics determine the emission.

DFT Study of GaN Clusters Decorated with Rh and Pt Nanoparticles for the Photochemical Reduction of CO2

DFT Study of GaN Clusters Decorated with Rh and Pt Nanoparticles for the Photochemical Reduction of CO₂

Ab-initio calculations on the structural and electronic transport properties of five-atom GaN clusters

The structural and electronic transport properties of [Formula: see text] clusters are studied based on density functional theory (DFT). Their most stable structures are proved to be planar by the particle swarm optimization (PSO) algorithm, and have decreasing binding energies with the increasing number of Ga atom in clusters. The electronic transport properties of these clusters connected with two Al(1 0 0) electrodes are calculated by combining nonequilibrium Green’s-function (NEGF) with DFT. Most of them have an equilibrium conductance of above [Formula: see text], except for [Formula: see text]. Negative differential resistance (NDR) phenomenon of different level is observed in their I–V curves in bias ranges of from [Formula: see text] to [Formula: see text] V and from 0.7 to 1.1 V.

Ab initio calculations on structural and electronic transport properties of six-atom GaN clusters

The structural and electronic transport properties of GaxNy ([Formula: see text]) clusters are investigated in the framework of density functional theory (DFT). To get their most stable structures, a strategy of particle swarm optimization (PSO) algorithm is adopted. It is found that the most stable cluster’s binding energy and HOMO–LUMO gap energy decrease with Ga atom’s number in cluster increasing. The electronic transport properties of the clusters connected with two Al(100) electrodes are obtained by a method of combining nonequilibrium Green’s function (NEGF) with DFT. Equilibrium conductance of all six-atom GaN cluster is low (less than 0.65 G0), and Ga2N4 has the highest one (0.635 G0). Significant negative differential resistance (NDR) phenomenon is observed in configurations with cluster Ga2N4, Ga3N3 and Ga5N1, and these three clusters have almost the same current value in voltage region from 0.8 V to 1.3 V.

A comparative ab intio study on structural evolution, stability and electronic properties of undoped and Al-doped GaxNy ( x + y = 4-25 clusters

We present a comparative study on structure evolution, binding energies, relative stabilities, electronic and vibrational properties of size-selected GaN and Al-doped GaN clusters by employing B3LYP exchange-correlation function with double- $ \xi$ basis set LANL2DZ via the density functional theory (DFT) method. Cube, ring, and rhombus stacks are constructed, and the optimization results suggest that the evolution of basic structural entities of different morphologies from planer rings to nanotubes may be realized by taking into account the stacking mode of stable clusters. We also find that the geometry of each Al-substituted GaN cluster keeps a structure similar to that of the corresponding pure GaN clusters. The stability, IR and Raman activity vary with the growth of units and the stacking mode. The energetic analysis shows that the most stable structure is the 3-dimensional cubic structure. A blue shift in the vibrational frequency of the most intense Raman activity has been observed with variation in size and structure. Moreover, the vibrational properties are significantly affected by the introduction of Al dopants. We also consider parameters, such as HOMO-LUMO gap, ionization potential (IP), electron affinity (EA), dipole moment (DM), chemical potential (CP), chemical hardness (CH), and results were compared and critically discussed for interpretation of the enhanced stability and to extract useful information for their applications.

Study of cubic GaN clusters in hexagonal GaN layers and their dependence with the growth temperature

The inclusion of cubic phase in MOVPE-grown hexagonal GaN on GaAs substrate and its dependence with the growth temperature are investigated by high-resolution X-ray diffraction (HR-XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and cathodoluminescence (CL). It is observed that the GaN layers surface exhibits 3D-grains structure. The density and shape of these grains are largely dependent on the growth temperature. HR-XRD study reveals the presence of cubic GaN clusters in the hexagonal GaN layer. Using CL we show that the cubic inclusions are not localized at the substrate/epilayer interface but propagate throughout the film.

Insertion of benzonitrile into Al-N and ga-N bonds: formation of fused carbatriaza-gallanes/alanes and their subsequent synthesis from amidines and trimethyl-gallium/aluminum.

Insertion of aromatic nitriles into Al-N and Ga-N bonds are reported. Sterically less hindered aluminum amide [PhNHAlMe2]2 (1) undergoes C≡N insertion with benzonitrile to give an isomeric mixture of tetracyclic triazaalanes {[PhNC(Ph)N]3[PhNC(Ph)NH]Al[AlMe][AlMe2]2} (2 and 3). A similar reaction with analogous gallium amide affords a tetracyclic triazagallane {[PhNC(Ph)N]3[PhNC(Ph)NH]Ga[GaMe][GaMe2]2} (6) along with a novel bowl shaped carbon containing Ga-N cluster {[PhNC(Ph)N][PhN][GaMe]2}3 (5). On the other hand, when sterically bulky gallium amide (Dipp on N, Dipp = 2,6-diisopropylphenyl) is employed, a tetrameric gallium amidinate {[(Dipp)NC(Ph)N]GaMe}4 (8) is obtained. Tetracyclic triazagallazane 6 is also synthesized from the condensation reaction of N-phenylbenzamidine with GaMe3·OEt2. Unlike AlMe3, this reaction produces only one isomer. In case of amidines with bulkier substituents on N such as Dipp, formation of a bicyclic triazagallane {[(Dipp)NC(Ph)NH]2[(Dipp)NC(Ph)N][GaMe]2} (14) is also observed along with tetrameric gallium amidinate 8, whereas N-tert-butylbenzamidine affords exclusively a tetrameric gallium amidinate {[(tert-Bu)NC(Ph)N]GaMe}4 (15) similar to its Al analogue. However, treating N-(Dipp)acetamidine with GaMe3·OEt2 gives only a bicyclic triazagallane {[(Dipp)NC(Me)NH]2[(Dipp)NC(Me)N][GaMe]2} (16). An intermediate [(tert-Bu)N(H) C(Ph)NGaMe2]2 (17), which is involved in the formation of tetrameric gallium amidinate 15, is also characterized. A comparison of the structural parameters of Ga-N-C and Al-N-C frameworks synthesized in this study is reported.

From Stable ZnO and GaN Clusters to Novel Double Bubbles and Frameworks

A bottom up approach is employed in the design of novel materials: first, gas-phase “double bubble” clusters are constructed from high symmetry, Th, 24 and 96 atom, single bubbles of ZnO and GaN. These are used to construct bulk frameworks. Upon geometry optimization—minimisation of energies and forces computed using density functional theory—the symmetry of the double bubble clusters is reduced to either C1 or C2, and the average bond lengths for the outer bubbles are 1.9 Å, whereas the average bonds for the inner bubble are larger for ZnO than for GaN; 2.0 Å and 1.9 Å, respectively. A careful analysis of the bond distributions reveals that the inter-bubble bonds are bi-modal, and that there is a greater distortion for ZnO. Similar bond distributions are found for the corresponding frameworks. The distortion of the ZnO double bubble is found to be related to the increased flexibility of the outer bubble when composed of ZnO rather than GaN, which is reflected in their bulk moduli. The energetics suggest that (ZnO)12@(GaN)48 is more stable both in gas phase and bulk frameworks than (ZnO)12@(ZnO)48 and (GaN)12@(GaN)48. Formation enthalpies are similar to those found for carbon fullerenes.

Effects of trimethylaluminium and tetrakis(ethylmethylamino) hafnium in the early stages of the atomic-layer-deposition of aluminum oxide and hafnium oxide on hydroxylated GaN nanoclusters

We calculate the interactions of two atomic layer deposition (ALD) reactants, trimethylaluminium (TMA) and tetrakis(ethylmethylamino) hafnium (TEMAH) with the hydroxylated Ga-face of GaN clusters when aluminum oxide and hafnium oxide, respectively, are being deposited. The GaN clusters are suitable as testbeds for the actual Ga-face on practical GaN nanocrystals of importance not only in electronics but for several other applications in nanotechnology. We find that TMA spontaneously interacts with hydroxylated GaN; however it does not follow the atomic layer deposition reaction path unless there is an excess in potential energy introduced in the clusters at the beginning of the optimization, for instance, using larger bond lengths of various bonds in the initial structures. TEMAH also does not interact with hydroxylated GaN, unless there is an excess in potential energy. The formation of a Ga-N(CH3)(CH2CH3) bond during the ALD of HfO2 using TEMAH as the reactant without breaking the Hf-N bond could be the key part of the mechanism behind the formation of an interface layer at the HfO2/GaN interface.

Mechanism of Si outdiffusion in plasma‐assisted molecular beam epitaxy of GaN on Si

AbstractThe mechanism of Si outdiffusion related to the formation of Si‐rich GaN clusters during the growth of GaN‐on‐Si by plasma‐assisted molecular beam epitaxy (PAMBE) has been identified. GaN thin films were grown, on high resistivity Si (111) substrates at Ga‐rich conditions to favour the 2D step‐flow growth mode. Hall‐effect measurements revealed very high electron background concentrations, ranging from 2 x 1018 cm‐3 to 3 x 1019 cm‐3, with degraded carrier mobility in the range of 5‐30 cm2/Vs. SEM examination of surfaces and cross‐sections of the samples revealed the presence of hollow GaN clusters on the film surface and voids at the Si/III‐nitride interface. EDX spot analysis revealed dissolution of Si material in the GaN clusters. Growth of GaN on Si under stoichiometric conditions resulted in films without GaN clusters, which exhibited a serious reduction in carrier concentration, down to 1.4 x 1016 cm‐3, with a simultaneous increase of the carrier mobility to 137 cm2/Vs. C‐V carrier profile measurements were consistent with Hall‐effect measurements and revealed a uniform electron concentration in the GaN film, down to the AlN nucleation layer (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ab Initio Analysis of the Interactions of GaN Clusters with Oxygen and Water

We calculate the interactions of oxygen and water with the Ga-face of GaN clusters, which could be used as testbeds for the actual Ga-face on GaN crystals of importance in electronics; however, our additional goal is the analysis of the nanoclusters for several other applications in nanotechnology. Our results show that the local spin plays an important role in these interactions. It is found that the most stable interaction of O2 and the GaN clusters results in the complete dissociation of the O2 molecule to form two Ga–O–Ga bonds, while the most stable interaction between a H2O molecule and the GaN clusters is the complete dissociation of one of the O–H bonds to form a Ga–O–H bond and a Ga–H bond.

Local Transport Property of GaN Cluster as a Model of Nanowire

The validity of our local conductivity density analysis method is shown by using a GaN cluster model, which attempts to mimic a GaN nanowire. Conductive properties of this model are investigated by our novel quantity, a local conductive density tensor. For the analysis of nano-materials, such as nanowires, this local quantity is very useful. To use this, electronic current is required to be represented as quantum states. We have developed our new code for this purpose. Conductive states are included as plain waves in the code. We confirm that the local conductivity realizes some reasonable conductive properties, e.g., the conductive path between the nearest atoms. In addition, the rotational behavior of conductivity is found, which appears as complex eigenvalues of a conductivity tensor. This behavior can only be studied by the analysis using the tensor.

Local Transport Property of GaN Cluster as a Model of Nanowire

Local Transport Property of GaN Cluster as a Model of Nanowire

On the bonding of Ga2, structures of Gan clusters and the relation to the bulk structure of gallium

Gallium is an unusual metal, sometimes described as a molecular metal, composed of covalently bonded-dimers, with metallic bonding primarily in the plane perpendicular to the average alignment of the dimers. We analyse the interactions between Ga2 molecules, and compare the calculated structures of small clusters to the complicated structure of the bulk. Both density functional and wavefunction-based methods are employed. The structures of small neutral clusters are shown to be remarkably similar to certain features of the bulk (α-Ga) structure. DFT calculations perform well in comparison to coupled cluster calculations in the size range considered.

Structural and energy characteristics of native vacancy-type defects in the biaxially stressed GaN lattice

For biaxially stressed GaN clusters, the structure, charges, and energies of the formation of intrinsic Ga and N vacancies are calculated by the quantum-chemical method in the SCF MO LCAO approximation taking into account relaxation of the crystalline surroundings. It is established that the use of substrates introducing compressive or tensile stresses into the epitaxial layer affects the concentrations of intrinsic vacancy-type point defects. This effect most clearly manifests itself in the nitrogen sublattice in the GaN crystal lattice, especially in the case of its tension, i.e., during the epitaxial grown of GaN on the Si substrate. Redistribution of the electron density in the defect region in the case of lattice compression or tension can be the cause of variation in the location of electronic levels of defects in the band gap of the crystal.

Surface Effects and Quantum Confinement in Nanosized GaN Clusters: Theoretical Predictions

The structure and the electronic properties of stoichiometric (GaN)n clusters (with 6 ≤ n ≤ 48) were investigated by means of quantum-chemical hybrid density functional theory (DFT) using the B3LYP functional. Particular emphasis was put on the investigation of the evolution of the physical properties of the clusters as a function of their size. Two types of model clusters were studied. Cage-type structures were found to be the most stable for smaller cluster sizes, whereas for larger sizes conformations cut out from the GaN wurtzite crystal were favorable. The study of the electronic structure shows that the energy gap of the clusters tends to become larger as the dimensions of the clusters increase. The vertical electronic absorption energies were calculated by means of time-dependent (TD) DFT. For such small clusters, probably due to the predominant amount of surface atoms, well-defined quantum confinement effects, as commonly observed in crystalline quantum dots, are not apparent.

Interface and Wetting Layer Effect on the Catalyst‐Free Nucleation and Growth of GaN Nanowires

To avoid catalyst-induced contaminations that might alter the electronic properties of the material, catalyst-free growth is preferable. However, the nucleation and growth mechanisms of GaN wires in the catalyst-free procedure are still under debate. Two mechanisms are usually invoked for the nucleation and the growth of NWs. One is based on the Ga-droplet formation followed by the well-established vapor–liquid–solid mechanism, [13] and the other is based on small GaN clusters as nucleation seeds and a vapor–solid growth process. [12] In the present work the formation of crystalline GaN nanoclusters as possible NW precursors in catalyst-free plasma-assisted MBE (PAMBE) growth is studied by high-resolution transmission electron microscopy (HRTEM) imaging. Details of the interface between the GaN layer and the substrate are investigated and discussed in connection with the mechanism of catalyst-free NW growth. GaN NWs were grown at 7808C by PAMBE without the use of catalysts, on clean Si(111) and oxidized Si(100) substrates, according to the procedure described elsewhere. [6] The GaN NWs were investigated by HRTEM using a JEOL 3000F

Density functional analysis of the structural evolution of Gan (n=30–55) clusters and its influence on the melting characteristics

Recent experimental results have reported surprising variations in the shapes of the heat capacity curves and melting temperatures of gallium clusters in the size range of 30-55 atoms [G. A. Breaux et al., J. Am. Chem. Soc. 126, 8628 (2004)]. In the present work, we have carried out an extensive density functional investigation on ten selected clusters in the above mentioned size range. In particular, we have analyzed the ground state geometry and the nature of bonding in these clusters using electron localization function. We demonstrate that the existence or otherwise of a large island of atoms bonded with similar strength (i.e., the local order) in the ground state geometry is responsible for the variation in the shape of the heat capacity curve. We attribute the observed higher melting temperatures of some of the clusters (viz., Ga45-Ga48) to the presence of a distinct core and strong covalent bonds between the core and surface atoms. The present work clearly demonstrates that it is possible to understand the general trends observed in the heat capacity curves across the entire series on the basis of the analysis of their ground state.

Magnetic properties of M-doped (M=Ti, V, or Cr) GaN clusters

We study the magnetic properties of M-doped (M=Ti,V,Cr) (GaN) x ( x⩽3) clusters using the first principles of density functional theory (DFT). Our calculations reveal that although the magnetic coupling between M atoms is ferromagnetic (with total magnetic moments ranging from 0 to 10 μ B), the antiferromagnetic arrangements are favored for these clusters. Spin density analysis reveals that the stabilization of the antiferromagnetic phase is caused by the antiferromagnetic coupling between the d-states of M atoms and the p-states of N atoms. This also makes the M atoms tend to bind more strongly to N atoms than to Ga atoms. This study may explain the role of the anions in diluted magnetic semiconductors (DMS).

Optical properties of amorphous GaAs1−xNx film sputtering with different N2 partial pressures

We study the optical properties of amorphous GaAs1−xNx films grown by radio frequency magnetron sputtering method with different N2 partial pressures. The surface morphology, the optical absorption, the Raman spectra, and optical constants of the films with different N2 partial pressures are reported. The appearance of the Raman peak at 245cm−1 of “GaAslike” and a shoulder at about 750cm−1 related to GaN indicates the formation of GaN clusters in GaAs matrix. The roughness decreases and the optical band gap of amorphous GaAs1−xNx films moves to short wavelength with increasing N2 partial pressure. The refractive index and the extinction coefficient of the films decrease with increasing N2 partial pressure, and it has been found that the amorphous GaAs1−xNx films with nonzero N2 partial pressure are transparent in red and near infrared wavelength regions.