Al Adatoms Research Articles

Surface diffusion during metalorganic vapor phase epitaxy of AlN

AbstractThe surface diffusion of Al adatoms during metalorganic vapor phase epitaxy of AlN was investigated, using sapphire substrates with different mis‐orientations. It was found that the diffusion length during modified migration‐enhanced epitaxy (MEE) is about 40 nm and comparable to that of conventional MEE under the current growth conditions. The enhancement of the diffusion length by alternating supply of source precursors was also demonstrated. The screw dislocation density in the modified‐MEE‐grown AlN was extracted to be as low as 2.2 × 106 /cm2 from X‐ray diffraction, while the chemical etching by KOH solution revealed an etch pit density of 1.0 × 106 /cm2, both of which indicated its superior structural properties. The chemical etching also evidenced that the grown AlN has Al polarity. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Accelerated molecular dynamics simulation of thin-film growth with the bond-boostmethod

We review the bond-boost method for accelerated molecular dynamics (MD) simulationand we demonstrate its application to kinetic phenomena relevant to thin-film growth. Toillustrate various aspects of the method, three case studies are presented. We first illustrateaspects of the bond-boost method in studies of the diffusion of Cu atoms onCu(001). In these studies, Cu interactions are described using a semi-empiricalembedded-atom method potential. We recently extended the bond-boost method toperform accelerated ab initio MD (AIMD) simulations and we present resultsfrom preliminary studies in which we applied the bond-boost method in AIMDto uncover diffusion mechanisms of Al adatoms on Al(110). Finally, a probleminherent to many rare-event simulation methods is the ‘small-barrier problem’, inwhich the system resides in a group of states connected by small energy barriersand separated from the rest of phase space by large barriers. We developed thestate-bridging bond-boost method to address this problem and we discuss itsapplication for studying the diffusion of Co clusters on Cu(001). We discuss theoutlook for future applications of the bond-boost method in materials simulation.

Connector model for describing many-body interactions at surfaces

First-principles calculations based on density-functional theory indicate that high-order many-body interactions are significant in Al clusters on Al(110) and Al(100). The large number of many-body interactions renders a full-lattice-gas approach ineffective for such systems. To simplify the description of adsorbate interactions, we utilize two different schemes. First, we find effective parameters for Al adatom interactions using the leave-one-out cross-validation method. Second, we propose the connector model, which is based on additive single-atom connector units. The central idea of the connector model is to combine groups of many-body interactions into important structural units (e.g., step edges) that have a single interaction energy. We find that the connector model is more accurate and efficient in representing high-order many-body interactions than the traditional lattice-gas approach and it may be suitable for describing a variety of surface phenomena such as thin-film and crystal growth, adsorption, phase transitions, and catalysis at surfaces.

Relative stabilities of adsorbed versus substitutional Al atoms in submonolayerAl/SixGe1−x(111)

Using scanning tunneling microscopy, low-energy electron-diffraction observations, and density-functional calculations, the effect of adding Ge to Si(111) substrate on the reconstructions induced by Al adsorption has been studied. It has been found that Ge incorporation alters the relative stability of the reconstructions. In particular, while in the ``pure'' Al/Si(111) system the magic cluster array ($\ensuremath{\alpha}\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ phase) is less stable than the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ reconstruction (to which it irreversibly converts upon heating above $600\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$), in the $\text{Al}/{\text{Si}}_{x}{\text{Ge}}_{1\ensuremath{-}x}(111)$ system magic clusters possess an enhanced thermal stability and persist almost up to the Al desorption temperature of $\ensuremath{\sim}800\ifmmode^\circ\else\textdegree\fi{}$. The results of calculations allow us to track the sequential stages of the substitution of Si atoms by Ge in the $\ensuremath{\alpha}\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ phase. The general trend found is that adding Ge to Si(111) makes the Al substitutional configuration more preferable than the Al adatom configuration, which is opposite to the relative stabilities of the configurations in the pure Al/Si(111) system.

Migration enhanced lateral epitaxial overgrowth of AlN and AlGaN for high reliability deep ultraviolet light emitting diodes

We report on the growth of low-defect thick films of AlN and AlGaN on trenched AlGaN/sapphire templates using migration enhanced lateral epitaxial overgrowth. Incoherent coalescence-related defects were alleviated by controlling the tilt angle of growth fronts and by allowing Al adatoms sufficient residence time to incorporate at the most energetically favorable lattice sites. Deep ultraviolet light emitting diode structures (310nm) deposited over fully coalesced thick AlN films exhibited cw output power of 1.6mW at 50mA current with extrapolated lifetime in excess of 5000hours. The results demonstrate substantial improvement in the device lifetime, primarily due to the reduced density of growth defects.

Growth and properties of Al-rich InxAl1−xN ternary alloy grown on GaN template by metalorganic chemical vapour deposition

An Al-rich InxAl1−xN ternary alloy was grown on a GaN template by metal–organic chemical vapour deposition (MOCVD). The GaN template was fabricated on a c-plane sapphire with a low temperature GaN nucleation layer. The growth of the 300 nm thick InxAl1−xN layer was carried out under various growth temperatures and pressures. The surface morphology and the InN molar fraction of the InxAl1−xN layer were assessed by using atomic force microscopy (AFM) and high resolution x-ray diffraction, respectively. The AFM surface images of the InxAl1−xN ternary alloy exhibited quantum dot-like grains caused by the 3D island growth mode. The grains, however, disappeared rapidly by increasing diffusion length and mobility of the Al adatoms with increasing growth temperature and the full width at half maximum value of ternary peaks in HR-XRD decreased with decreasing growth pressure. The MOCVD growth condition with the increased growth temperature and decreased growth pressure would be effective to grow the InxAl1−xN ternary alloy with a smooth surface and improved quality. The optical band edge of InxAl1−xN ternary alloys was estimated by optical absorbance and, based on the results of HR-XRD and optical absorbance measurements, we obtained the bowing parameter of the InxAl1−xN ternary alloy at b = 5.3 eV, which was slightly larger than that of previous reports.

Ab initio modeling of Al adsorption on CaF 2 surfaces

Ab initio simulations of the adsorption of Al atoms on CaF 2 (0 0 1) and (1 1 1) surfaces have been performed for supercells with 7 different atomic configurations, using density functional theory. For (1 1 1) surfaces, a repulsive interaction was observed for most configurations, while a weak attraction was obtained when the Al atom was placed above F atoms. For the Ca-terminated (0 0 1) surface, the adsorption energy was about 5 times larger, whereas for the F-terminated (0 0 1) surface it was about 20 times greater. The comparative analysis indicates that the (0 0 1) surfaces are reactive and have a strong Al adatom bonding (chemisorption), especially for the F-terminated substrate. On the contrary, the (1 1 1) plane may be considered as non-reactive (physisorption), having a weak bonding of the Al adatom above the F site.

Self-Directed Growth of AlGaAs Core−Shell Nanowires for Visible Light Applications

Al0.37Ga0.63As nanowires (NWs) were grown in a molecular beam epitaxy system on GaAs(111)B substrates. Micro-photoluminescence measurements and energy dispersive X-ray spectroscopy indicated a core--shell structure and Al composition gradient along the NW axis, producing a potential minimum for carrier confinement. The core--shell structure formed during growth as a consequence of the different Al and Ga adatom diffusion lengths.

Interactions between Al atoms on Al(110) from first-principles calculations

We quantified pair and trio interactions between Al adatoms on the Al(110) surface using first-principles, density-functional theory, total-energy calculations. We find that the pair interaction is the strongest for the nearest in-channel $[1\overline{1}0]$ neighbor and is attractive due to the formation of direct chemical bonds between the adatoms. Beyond the nearest neighbor, the pair interaction is repulsive and is mediated by elastic distortion of the substrate atoms. The pair interaction is negligible for adatom separations beyond $\ensuremath{\sim}8.00\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Interactions between atoms in a collinear trio chain along the in-channel $[1\overline{1}0]$ direction have both electronic and elastic characters. All other trio interactions are elastic in origin. The long-ranged trio interaction is significant and exhibits damped oscillations between attraction and repulsion. We find several trios that enhance cross-channel [001] attraction. This trio attraction facilitates the formation of two-dimensional islands in Al(110) homoepitaxy, which are not favored by repulsive cross-channel pair interactions. These observations demand a refined approach to study thin-film growth, in which many-body interactions are taken into account.

Atomic-level investigation of Al and Ni thin film growth on Ni(1 1 1) surface: Molecular dynamics simulation

Using molecular dynamics (MD) simulation, the structural characteristics of Al and Ni thin film growth on Ni(1 1 1) substrate according to the incident energy of adatoms were investigated. In case of Al on Ni(1 1 1), Al adatoms were grown basically through the layer-by-layer growth mode. On the other hand, Ni thin films on Ni(1 1 1) surface at low incident energy were shown to favor island growth. The steering effect due to atomic attraction, which results in rougher surface, was significantly observed at low incident energy. The growth mode of Ni film was, however, changed to follow layer-by-layer growth mode for the incident energy of 6 eV. The different aspects of surface morphology between Al and Ni deposition on Ni(1 1 1) surface could be successfully explained by the surface diffusion and impact cascade diffusion.

MOCVD Growth and Characterization of EpitaxialAlxGa1−xN Films

We study the growth of AlxGa1−x Nepilayers on (0001) sapphire by low-pressure MOCVD, using alow-temperature AlN buffer. By varying the input flow rates oftrimethylgallium (TMGa), we obtain crack-free AlGaN films in the wholerange of composition. A linear relationship between gas and solid Alcontent is observed. The structural properties of the layers (x = 0−1)are investigated by x-ray diffraction, atomic force microscopy (AFM)and scanning electron microscopy (SEM). It is found that atwo-direction growth appears along the c-axis and the (101̄1) directionsfor x⩾0.45. From the results of Raman spectroscopy, we suggest thatthe compressive stain and the lack of mobility of Al adatoms can inducethe formation of (101̄1) grains.

Dynamical properties of the Ni 3Al low index surfaces with and without Ni or Al adatoms from molecular dynamics simulations

We present molecular dynamics simulation results on the vibrational properties of the Ni 3Al low index surfaces with and without Ni/Al adatoms. We found that the mean square displacements of both surface atoms and adatoms depend linearly upon temperature, the adatoms on the (1 1 0) face exhibiting vibrational amplitudes twice as large as on the other two surfaces. In addition, we found that all surfaces studied are expanded and rumpled, the rippling effect being more pronounced in the case of the (0 0 1) face with the Al atoms occupying the topmost position. In the cases of the (1 1 0) and the (1 1 1) surfaces the rippling is visible at several atomic layers beneath the surface. The adatoms are found to be contracted at ambient conditions, an effect that is significant when deposited on the (1 1 0) face, attaining the value of −10%. Moreover, from the calculated phonon density of states (DOS) and phonon spectral densities at room temperature, we found that the presence of surfaces modifies the bulk phonon DOS, exhibiting new modes some of which are located in the bulk phonon gap. The present results are in line with available experimental and theoretical results.

The effects of LT AlN buffer thickness on the optical properties of AlGaN grown by MOCVD and Al composition inhomogeneity analysis

We have investigated the growth of AlGaN epilayers on a sapphire substrate by metalorganic chemical vapour deposition using various low-temperature (LT) AlN buffer thicknesses. Combined scanning electron microscopy and cathodoluminescence investigations reveal the correlation between the surface morphology and optical properties of AlGaN films in a microscopic scale. It is found that the suitable thickness of the LT AlN buffer for high quality AlGaN growth is around 20 nm. The Al compositional inhomogeneity of the AlGaN epilayer is attributed to the low lateral mobility of Al adatoms on the growing surface.

Density functional calculations of the binding energies and adatom diffusion on strained AlN (0001) and GaN (0001) surfaces

AbstractDensity functional theory calculations were carried out to study the binding energies and diffusion barriers for various adatoms on AlN and GaN (0001) surfaces. The binding energies and potential energy surfaces were investigated for Al, Ga, and N adatoms on both Al (Ga) terminated and N terminated (0001) surfaces of AlN (GaN). Calculations for the diffusion paths and diffusion energy barriers for Al, Ga, and N adatoms on AlN and GaN were performed. It was found that the N adatom on N terminated AlN and GaN surfaces faces a high diffusion barrier due to strong N-N bond. The Al and Ga adatom on Al (Ga) terminated AlN (GaN) surfaces showed lower diffusion barriers due to the weak metallic bonds. However, the diffusion barrier for an Al adatom was always larger than that of a Ga adatom on any surface. The surfaces were also subjected to a hydrostatic compressive and tensile strain in the range of 0 to 5% to investigate the effect of strain on diffusion barriers. The diffusion energy barrier for N adatom on N terminated AlN and GaN surfaces decreased when the strain state was changed from tensile to compressive. In contrast, Al and Ga adatoms show continuous increase in diffusion barriers from tensile to compressively strained Al (Ga) terminated AlN (GaN) surfaces.

Low-energy sputtering events at free surfaces near anti-phase and grain boundaries in Ni3Al

Atomic recoil events on free surfaces orthogonal to two different anti-phase boundaries (APBs) and two grain boundaries (GBs) in Ni3Al are simulated using molecular dynamics methods. The threshold energy for sputtering, E sp, and adatom creation, E ad, are determined as a function of recoil direction. The study is relevant to FEG STEM (a scanning transmission electron microscope fitted with a field emission gun) experiments on preferential Al sputtering and/or enhancement of the Ni–Al ratio near boundaries. Surfaces intersected by {110} and {111} APBs have minimum E sp of 6.5 eV for an Al atom on the Ni–Al mixed (M) surface, which is close to the value of 6.0 eV for a perfect M surface. High values of E sp of an Al atom generally occur at a large angle to the surface normal and depend strongly on the detailed atomic configuration of the surface. The mean E sp, averaged over all recoil directions, reveals that APBs have a small effect on the threshold sputtering. However, the results for E ad imply that an electron beam could create more Al adatoms on surfaces intersected by APBs than on those without. The equilibrium, minimum energy structures for a (001) surface intersected by either Σ5[001](210) or Σ25[001](340) symmetric tilt grain boundaries are computed. E sp for surface Al atoms near these GBs increases monotonically with increasing recoil angle to the surface normal, with a minimum value, which is only about 1 eV different from that obtained for a perfect surface. Temperature up to 300 K has no effect on this result. It is concluded that the experimental observations of preferential sputtering are due to effects beyond those for E sp studied here. Possible reasons for this are discussed.

Surface Diffusion and Incorporation Process of Adatom in Fe-Al Multilayer System

Using the ab initio pseudopotential calculations, the surface diffusion and incorporation process at the interface of Fe-Al multilayer system were quantitatively investigated. The hollow site was most stable adsorption site on both Al (001) and Fe (001) surface. The adsorption energies were 8.62 eV for Fe/Al (001) and 5.30 eV for Al/Fe (001) system. The calculated energy barriers for the surface diffusion of adatom were 0.89 eV and 0.61 eV for each system. The energy barrier for the incorporation of Fe adatom into the Al substrate was calculated to be 0.38 eV and the energy gain of the system was 0.49 eV. However, the Al adatom required relatively large energy barrier, 0.99 eV for the incorporation into the Fe substrate resulting in 0.13 eV increase in total energy of the system.

Electronic and Structural Characteristics of Diffusion Process in Fe/Al(001) and Al/Fe(001) Systems

Diffusion behaviors that occurred in the initial stage of Fe–Al multilayer fabrication were investigated using ab initio pseudopotential calculations. The hollow site was the most stable adsorption site on both Al(001) and Fe(001) surfaces and the calculated energy barriers for the surface diffusion were 0.89 eV for Fe/Al(001) and 0.64 eV for Al/Fe(001) systems. The energy barrier for the incorporation of the Fe adatom into the Al substrate was calculated to be 0.38 eV and the energy gain of the system was 0.49 eV. A maximum charge depletion of 0.35 bohr-3 and a maximum charge accumulation of 0.17 bohr-3 were found in the Fe–Al bonds. The Al adatom required relatively a large energy barrier, i.e., 0.99 eV, for its incorporation into the Fe substrate and the total energy of the system was increased by 0.13 eV.

Island formation during Al deposition on 5-fold Al–Cu–Fe quasicrystalline surfaces: Kinetic Monte Carlo simulation of a disordered-bond-network lattice-gas model

Scanning tunneling microscopy experiments have previously revealed the formation of pseudomorphic starfish-shaped islands during the initial stages of Al deposition on 5-fold icosohedral Al–Cu–Fe quasicrystal surfaces. To simulate this process, we first identify appropriate 5-fold surface terminations of Al–Cu–Fe from a model for bulk structure, and construct associated potential energy surfaces for the binding of Al adatoms on these terminations. We then identify a ‘disordered-bond-network’ (DBN) connecting neighboring local adsorption sites for Al on Al–Cu–Fe, and determine site binding energies as well as activation barriers for Al adatom hopping between neighboring sites. Al–Al adsorbate interactions, which stabilize islands, are also prescribed. Then, within the framework of a DBN lattice-gas model, we simulate the deposition and diffusion of Al on Al–Cu–Fe. We explore the competition between starfish and incomplete starfish ensembles of sites (which provide traps in the form of deep potential energy wells for diffusing Al) and isolated trap sites, with regard to the heterogeneous nucleation and aggregation of Al into islands.

Modelling of the energetics and kinetics of Al deposition on 5-fold Al-rich quasicrystal surfaces

We determine potential energy surfaces for the binding of Al adatoms on 5-fold AlPdMn and AlCuFe quasicrystal surfaces. Appropriate geometric models are used to select physical surface terminations. The interaction between the Al adsorbate and the substrate is described using Lennard–Jones pair-wise interactions, with parameters chosen to fit ab initio energetics for Al on relevant single crystal substrates. We identify a ‘disordered-bond-network’ (DBN) of local adsorption sites, which includes deep ‘starfish’ and ‘incomplete starfish’ ensembles. Our primary interest is in the kinetics of deposition and aggregation of Al atoms, and the possible formation of pseudomorphic starfish islands at starfish ensembles (observed for Al deposition on AlCuFe). The deposition process is modeled within the framework of a DBN lattice-gas model, wherein we specify rates for deposition and for hopping between neighboring sites of the DBN, as well as Al–Al adsorbate interactions, which stabilize islands. We, thus, present a picture for the formation of starfish islands subject to the competition between various deep sites for Al adatoms.

Growth and Polarity Control of GaN and AlN on Carbon-face SiC by Metalorganic Vapor Phase Epitaxy

ABSTRACTGrowth and polarity control of GaN and AlN on carbon-face SiC (C-SiC) by metalorganic vapor phase epitaxy (MOVPE) are reported. The polarities of GaN and AlN layers were found to be strongly dependent on the pre-growth treatment of C-SiC substrates. A pre-flow of trimethyaluminum (TMAl) or a very low NH3/TMAl ratio results in Al(Ga)-polarity layers on C-SiC. Otherwise, N-polarity layers resulted. The polarities of AlN and GaN layers were conveniently determined by their etching rate in KOH or H3PO4, a method reported earlier. We suggest that the Al adatoms, which have a high sticking coefficient on SiC, form several Al adlayers on C-SiC and change the incorporation sequence of Ga(Al) and N leading to metal polarity surface. In addition, the hexagonal pyramids, typical on N-polarity GaN surface, are absent on N-polarity GaN on off-axis C-SiC owing to high density of terraces on off-axis C-SiC. The properties of GaN layers grown on C-SiC are studied by X-ray diffraction.