AlN Islands Research Articles

Nonpolar 4H-AlN grown on 4H-SiC (11¯00) with reduced stacking fault density realized by persistent layer-by-layer growth

Nonpolar AlN layers were grown on 4H-SiC (11¯00) substrates by plasma-assisted molecular-beam epitaxy. By using SiC substrates with well-formed step-and-terrace structures, stable layer-by-layer growth of 4H-AlN (11¯00) can be realized. The layer-by-layer growth is confirmed by observations of anisotropic two-dimensional AlN islands on the grown surface as well as persistent reflection high-energy electron diffraction intensity oscillations. Cross-sectional transmission electron microscopy observations reveal that stacking fault generation during growth is suppressed and the stacking fault density is reduced to 1×106 cm−1.

Competitive growth mechanisms of aluminum nitride thin films deposited by off-normal reactive magnetron sputtering

We have recently shown that AlN (0002) (c axis) is tilted abruptly toward the deposition direction as N2 concentration is increased in N2/Ar sputtering gas mixtures. Here, we present a Monte Carlo simulation model to describe the phenomenon of sudden c-axis AlN tilt. The model is based on the assumption that AlN islands with their c axis parallel to substrate normal and AlN islands with tilted c axis coexist at the initial stages of the growth and they can provide the adatoms with different surface mobilities. It is believed that the adatom mobilities are quenched when N2 concentration reaches a certain amount in the reactive sputtering of AlN. Our model further assumes that adatom mobility differences on different islands result in a growth rate difference of the islands. At the initial stages of the growth, AlN islands with tilted c axis grow taller due to the lower adatom mobility on these islands. As they grow taller, they win the competition and stop the further growth of AlN islands with their c axis parallel to substrate normal due to shadowing effect. Monte Carlo simulations revealed that the shadowing effect combined with different adatom mobilities promotes the sudden c-axis tilt in AlN thin films.

Formation of Aluminum Nitrides by Precipitate-Accommodated Plasma Nitriding

AbstractPrecipitate accommodated nitriding is proposed as a new, non-traditional surface treatment of aluminum alloys. In the case of nitriding the aluminum-copper alloys, Al2Cu plays an important role as a nucleation site to form AlN by the two step reactions of Al2Cu + 2N → 2AlN + Cu and Cu + 2Al → Al2Cu and as a helper to preserve the fast nitrogen diffusion path. In this precipitate accommodation, AlN islands form on Al2Cu in coherent, and, gradually cover the matrix surface by the AlN layer. This Al2Cu accommodation in nucleation reduces the pre-sputtering time and shortens the incubation time prior to formation of AlN layer. Furthermore, the interface between Al2Cu and AlN plays a nitrogen diffusion path across the nitrided layer. This Al2Cu accommodation in growth of AlN accelerates the growth rate of AlN layer in practice. The formation rate of AlN approaches to 7 μm/ks in the present nitriding. This reduction of processing time is much favored for industrial manufacturing of automotive parts and electrical components in practice. Low friction and wear is also attained in dry sliding condition by this AlN-layered specimen: μ = 0.13 and Ws = 1.5 × 10−5 mm3N−1m−1. Since Al2Cu is common precipitate to most of aluminum alloys, this AlN-layer formation is also favored to protective coating for automotive parts and electric components.

Impact of silicon incorporation on the formation of structural defects in AlN

The impact of Si impurities on the structural properties of AlN, grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire is studied. Under nitrogen-rich growth conditions silicon can be homogeneously incorporated up to Si concentrations of [Si]=5.2×1021 cm−3. The presence of silicon on the surface during the growth process is demonstrated to be beneficial for the surface morphology and the structural properties of the AlN films. For [Si] up to (5±3)×1020 cm−3, this surfactant behavior results in a decrease of the surface roughness from 8 nm for undoped layers grown in a nitrogen-rich regime to less than 1 nm. In addition, high resolution x-ray diffraction studies reveal an increase of the average lateral crystal size from 300 nm to more than 1 μm and a simultaneous decrease of the screw dislocation density from 3.8×108 cm−2 for (comparably) weakly doped samples to 2×107 cm−2. At the same [Si] the heterogeneous stress shows a minimum of less than 50 MPa and drastically increases for higher [Si]. The analysis of edge dislocations as a function of [Si] reveals that their density is directly related to Si-induced compressive biaxial stress which increases up to (2.0±0.15) GPa, independently determined by x-ray diffraction and Raman spectroscopy. While edge dislocations are found to be strongly correlated with the release of stress, screw dislocations are formed due to the coalescence of AlN islands with different stacking order, as their density is decreasing with increasing lateral crystal size. For AlN films with [Si]<1.2×1021 cm−3, a Poisson ratio ν=0.525±0.022 is determined.

Plasma nitriding design for aluminium and aluminium alloys

Plasma nitriding for aluminium and aluminium alloys is a promising processing to improve the wear resistance for automotive parts. Normal plasma nitriding is characterised by three processes: presputtering, aluminium nitride nucleation and nitrided layer growth processes. N2+ presputtering is used to effectively eliminate the preexisting oxide films of Al2O3, covering the surface of aluminium matrix. Relatively long incubation time is required for nucleation process to form AlN islands or nodules on its surface. In addition, formation rate becomes very slow owing to low nitrogen diffusion coefficient in the nitrided layer. Physical and chemical modification methods to this normal nitriding processing are proposed to accelerate the formation rate of nitrided layer. Refinement of grain size in the aluminium matrix increases the formation rate by enlarging grain boundary area as a diffusion path. Crystallographic coherency between TiN and AlN reflects on enhancement of nucleation process by coformation of TiN with AlN. Standing on the nitriding design by physical and chemical modification of inner nitriding mechanism, an alternative plasma nitriding is proposed as the third processing for copper bearing aluminium alloys. In this processing, reduction of duration for nucleation and acceleration of growth rate are attained with the aid of the precipitate, Al2Cu. Crystallographic coherency between AlN and Al2Cu is effective to enhance the formation of AlN nodules and islands. Solid state reaction between Al2Cu and penetrating nitrogen is also significant to form the fine interfacial boundaries as a nitrogen diffusion path and to accelerate the formation rate of nitrided layer.

AlGaN films grown on (0001) sapphire by a two-step method

A two-step growth method, commonly used for GaN on sapphire, was applied to grow high-quality Al0.2Ga0.8N on sapphire. Comparing to the one grown on a low-temperature grown AlN buffer layer, the decomposition, recrystallization, and islands coalescence processes of the two-step growth increased the surface flatness, the crystal quality, the electrical property, suppressed the phase separation, and released the biaxial tensile strain. A 2.0μm thick high-quality crack-free nearly GaN-free Al0.2Ga0.8N epilayer was obtained.

Evolution of stress in GaN heteroepitaxy on AlN∕Si(111): From hydrostatic compressive to biaxial tensile

The initial steps of GaN growth on an AlN buffer layer on Si(111) substrates by metalorganic vapor phase epitaxy were investigated using field emission scanning electron microscopy, micro-photoluminescence, as well as by conventional and grazing incidence x-ray diffraction. A series of GaN layers was grown for various times ranging from 7.5 s to several minutes, doubling the growth time for each step. The AlN buffer layer is noncontinuous and consists of (0001)-oriented AlN islands with a mean diameter of about 50 nm. On top of these nucleation centers three-dimensional growth of GaN was observed. With increasing growth times up to 30 s these islands further expanded and their distribution became more homogeneous. At 60 s coalescence started with homogeneously distributed islands, and after 120 s the layer was fully coalesced. The layers grown for 7.5 and 15 s are under a high compressive hydrostatic pressure, which might be enhanced by the lattice mismatch between AlN and GaN. For longer growth times a biaxial tensile stress is observed. The occurrence of the biaxial tensile stress correlates with the onset of island coalescence. The x-ray results are in agreement with low-temperature optical measurements showing a consistent energy shift of the near band gap luminescence and longitudinal optical Raman modes with respect to relaxed GaN.

Transmission electron microscopy study of an AlN nucleation layer for the growth of GaN on a 7-degree off-oriented (0 0 1) Si substrate by metalorganic vapor phase epitaxy

We have investigated the morphology of the high-temperature-grown AlN nucleation layer and its role in the early stage of GaN growth, by means of transmission electron microscopy. The nitride was selectively grown on a 7-degree off-oriented (0 0 1) patterned Si substrate by metalorganic vapor phase epitaxy. AlN was deposited on the inclined unmasked (1 1 1) facet in the form of islands. The size of the islands varied along the slope, which is attributable to the diffusion of the growth species in the vapor phase. The GaN nucleation occurred at the region where rounded AlN islands formed densely. The threading dislocations were observed to generate in the GaN nucleated region.

Nucleation and growth kinetics of AlN films on atomically smooth 6H–SiC (0001) surfaces

Nucleation and growth kinetics of AlN films on atomically smooth 6H–SiC (0001) surfaces, which were obtained by HCl etching at elevated temperatures prior to growth, were investigated. The surface morphology and the defect density of AlN films on such surfaces were significantly improved compared to those on as-received SiC surfaces. This is due to enhanced diffusion length and reduced incoherent boundaries at the coalescence regions of the AlN islands. AlN nuclei on the as-received SiC surface were crystallographically misaligned and thus induced incoherent boundaries at the coalescence stage, resulting in the delay of the two-dimensional growth mode transition and defect formation in AlN films.

On the initial stages of AlN thin-film growth onto (0001) oriented Al2O3 substrates by molecular beam epitaxy

Conventional and high-resolution transmission electron microscopy are used to characterize the initial stages of AlN thin-film growth. AlN films are deposited by molecular beam epitaxy onto annealed (0001) oriented α-Al2O3 (sapphire) substrates. During the initial stages of film growth (film thickness ∼25 nm) AlN forms islands of varying alignment with the Al2O3 substrate. Some of the AlN islands are well aligned with the [112̄0]AlN∥[101̄0] Al2O3 and (0001)AlN∥(0001)Al2O3, which matches closed-packed planes and directions. Other islands exhibit either an alignment of one set of planes, i.e., grains are aligned with the (11̄01)AlN∥(112̄0) Al2O3, or are misaligned with respect to the Al2O3 substrate. As the AlN film grows in thickness (film thickness ∼100 nm), the film becomes continuous, and the closed-packed planes and directions of the film and substrate are aligned for the majority of the film. Islands of AlN with an alignment other than this predominant orientation disturb the growth near the AlN/Al2O3 interface and create displacements along the [0001] AlN direction in overlying AlN grains. These misaligned AlN grains provide one source for the formation of planar defects in the epitaxial AlN films. The evolution of the AlN film microstructure and the reasons for the observed orientation relationships are discussed.

Structural investigation of sapphire surface after nitridation

Nitridation process of a sapphire surface was investigated with atomic force microscopy (AFM), reflection high-energy electron diffraction (RHEED), and X-ray photoelectron spectroscopy (XPS) in order to reveal a general nitridation mechanism. It was found that the nitridation process consists of two steps. First, inter-mixing between nitrogen-related species and sapphire surface occurs forming hydrogenated Al oxynitride. This step does not change the surface morphology significantly. Second, crystalline AlN islands are gradually formed by further nitridation of hydrogenated Al oxynitride, resulting in a very rough surface.

Growth of AlBN solid solutions by organometallic vapor-phase epitaxy

Layers of AlBN were grown on sapphire by organometallic vapor-phase epitaxy at 1050 °C using triethylboron, trimethylaluminum, and ammonia as precursors. It is shown that boron is readily incorporated into the layers and its concentration in the solid phase can be as high as 40%. However, single phase Al1−xBxN films can only be grown for compositions not exceeding x=0.01. For higher boron concentrations in the solid the second B-rich phase is formed. This phase was identified as wurtzite BN based on the results of transmission electron microscopy and x-ray diffraction. The growth of this thermodynamically unfavorable phase becomes possible, most probably, because it occurs within the framework provided by wurtzite AlN islands first formed on the surface and setting up the sites for lateral growth of wurtzite BN. That leads to formation of columnar structure of AlN and BN crystallites oriented in the basal plane and existing side by side.

Adsorption and Thermal Decomposition of Hydrazoic Acid on Al(111)

The surface chemistry of hydrazoic acid (HN 3 ) on Al(111) was investigated from 100 to 800 K with temperature-programmed desorption, infrared reflection-absorption spectroscopy (IRRAS), Auger electron spectroscopy, and low-energy electron diffraction (LEED). IRRAS suggests that the first monolayer dissociatively chemisorbs as N 2 (ads) and NH(ads) at 120 K. Subsequently, HN 3 adsorbs molecularly in two phases, a physisorbed second layer and a condensed multilayer. In the former, the N 3 chain of the HN 3 is aligned normal to the substrate surface, while in the latter HN 3 exhibits a random orientation. Molecular HN 3 desorbs at 125 K, and two decomposition products, N 2 and H 2 , desorb at 295 and 615 K, respectively. Mixed-isotope desorption experiments show that N2 is derived from an intact NN species, rather than from recombinative desorption of N(ads), and is consistent with the IRRAS observation of chemisorbed N 2 . IRRAS indicates that the NH species dissociates into N(ads) and H(ads) above 320 K, with H 2 thermal desorption resulting from dissociation of the AlH x (1 ≤ x ≤ 3) moiety. After the sample was annealed at 800 K, N was observed with Auger spectroscopy. The partially nitrided Al(111) surface exhibits a concentric, double hexagonal LEED pattern indicating AlN(0001) islands. IRRAS indicates that the islands are Al-terminated and capped with H at temperatures as high as 700 K.