E. Alves

Characterisation of Ti1−xSixNy nanocomposite films

Abstract Ti 1− x Si x N y films were synthesised by RF reactive sputtering from Ti and Si elemental targets, in an Ar/N 2 gas mixture. XRD results revealed the development of a two-phase system, composed of a nanocrystalline f.c.c. TiN (phase 1: B1 NaCl type) and a second one (phase 2), where Si atoms replaced some of the Ti ones, inducing a structure that we may call a solid solution. An amorphous phase, supposed to be of silicon nitride, within grain boundaries seems to be also present, especially for high Si contents. TEM experiments confirmed the f.c.c.-type structure for phase 2, which is the only phase that develops without ion bombardment. The higher lattice parameter of phase 1 (∼0.429 nm compared to 0.424 nm for bulk TiN) may be explained by the residual stress effect on peak position. The Ti replacement by Si would explain the low value of the lattice parameter for phase 2 (∼0.418 nm). All samples showed good results for hardness (Hv≥30 GPa), and Ti 0.85 Si 0.15 N 1.03 at a deposition temperature of 300°C showed a value of approximately 47 Gpa, which is approximately double that of pure TiN. For higher deposition temperatures, an increase in hardness is observed, as demonstrated by this same sample, which at 400°C reveals a value of approximately 54 GPa. Similar behaviour was observed in adhesion, where this same sample revealed a critical load for adhesive failure of approximately 90 N. In terms of oxidation resistance, a significant increase has also been observed in comparison with TiN. At 600°C, the oxidation resistance of Ti 0.70 Si 0.30 N 1.10 is already 100 times higher than that of TiN. For higher temperatures this behaviour tends to be even better when compared with other nitrides.

Strain and composition distributions in wurtzite InGaN/GaN layers extracted from x-ray reciprocal space mapping

Strain and composition distributions within wurtzite InGaN/GaN layers are investigated by high-resolution reciprocal space mapping (RSM). We illustrate the potential of RSM to detect composition and strain gradients independently. This information is extracted from the elongation of broadened reciprocal lattice points (RLP) in asymmetric x-ray reflections. Three InxGa1−xN/GaN (nominal x=0.25) samples with layer thickness of 60, 120, and 240 nm, were grown in a commercial metal-organic chemical vapor deposition reactor. The RSMs around the (105) reflection show that the strain profile is nonuniform over depth in InGaN. The directions of “pure” strain relaxation in the reciprocal space, for a given In content (isocomposition lines), are calculated based on elastic theory. Comparison between these directions and measured distributions of the RLP shows that the relaxation process does not follow a specific isocomposition line. The In mole fraction (x) increases as the films relax. At the start of growth all t...

Hard nanocomposite Ti–Si–N coatings prepared by DC reactive magnetron sputtering

Abstract Films resulting from Si additions to TiN matrix were prepared with Si contents in the range 0–19 at.%, using a closed field unbalanced DC magnetron sputtering system. Transmission Electron Microscopy (TEM) analyses revealed the nanocrystalline nature of these coatings, confirming the results of grain size evaluation from X-ray diffraction (XRD) patterns. Nanoindentation tests and scratch tests were carried out for the mechanical characterisation. Regarding the results, the samples show hardness values as high as 45 GPa. Best hardness values were found for Si content in the range 4–10 at.%. Almost all samples showed high critical loads for total adhesion failure, generally higher than 80 N, although the critical load for the first adhesion failure was found to be lower than 20 N for several samples. XRD patterns revealed the presence of only one phase that can be assigned to a cubic B1 NaCl structure, typical for TiN, with a lattice parameter of approximately 0.430 nm. The preferential growth, as a function of Si content, changes from a strong (111) orientation at the lowest Si additions to a weak (200) orientation at the highest Si content. Density values in the range 3.0–3.7 g/cm3 were obtained for most of the samples prepared with deposition rates between 0.5 and 1.1 μm/h, although higher density values were obtained for higher Ti deposition rates, with maximum of approximately 4.7 g/cm3 for the case of samples with low Si content.

Metal-organic vapor phase epitaxy and properties of AlInN in the whole compositional range

The authors present a detailed study of Al1−xInxN layers covering the whole composition range of 0.09<x<1. All layers were grown on GaN on Si(111) templates using metal-organic vapor phase epitaxy. For 0.13<x<0.32 samples grow fully strained and without phase separation. At higher In concentrations, the crystalline quality starts to deteriorate and a transition to three-dimensional growth is observed. A comparison of their experimental data with theoretically predicted phase diagrams reveals that biaxial strain increases the stability of the alloy.

Structural and optical properties of InGaN/GaN layers close to the critical layer thickness

In this work, we investigate structural and optical properties of metalorganic chemical vapor deposition grown wurtzite InxGa1−xN/GaN epitaxial layers with thicknesses that are close to the critical layer thickness (CLT) for strain relaxation. CLT for InxGa1−xN/GaN structures was calculated as a function of the InN content, x, using the energy balance model proposed by People and Bean [Appl. Phys. Lett. 47, 322 (1985)]. Experimentally determined CLT are in good agreement with these calculations. The occurrence of discontinuous strain relaxation (DSR), when the CLT is exceeded, is revealed in the case of a 120 nm thick In0.19Ga0.89N layer by x-ray reciprocal space mapping of an asymmetrical reflection. The effect of DSR on the luminescence of this layer is clear: The luminescence spectrum shows two peaks centered at ∼2.50 and ∼2.67 eV, respectively. These two components of the luminescence of the sample originate in regions of different strain, as discriminated by depth-resolving cathodoluminescence spectr...

Microstructure and mechanical properties of nanocomposite (Ti, Si, Al)N coatings

c ˆ´ ˜ ITN, Departamento de Fisica, E.N. 10, 2686-953 Sacavem, Portugal d ´´ Abstract In this work (Ti,Si,Al)N films were deposited using only rf or a combination of rf and d.c. reactive magnetron sputtering. Chemical composition, thickness, film structure and mechanical properties of the films were investigated by means of Rutherford backscattering (RBS), electron microprobe analysis (EPMA), ball-cratering, X-ray diffraction (XRD) and ultramicroindentation, respectively. All samples showed high hardness values, exceeding, in some cases, 50 GPa. XRD results revealed the formation of a mixture of two phases whose structure is similar to TiN. One phase is noted as being TiN bulk with a lattice parameter of 0.428 nm and develops only in conditions of high surface mobility. This behaviour can be associated with the segregation of the SiN phase, though the formation of an amorphous AlN phase cannot be excluded. Another phase, which is noted as Ti-Si-Al-N x

Compositional dependence of the strain-free optical band gap in InxGa1−xN layers

The effect of strain on the compositional and optical properties of a set of epitaxial single layers of InxGa1−xN was studied. Indium content was measured free from the effects of strain by Rutherford backscattering spectrometry. Accurate knowledge of the In mole fraction, combined with x-ray diffraction measurements, allows perpendicular strain (ezz) to be evaluated. Optical band gaps were determined by absorption spectroscopy and corrected for strain. Following this approach, the strain free dependence of the optical band gap in InxGa1−xN alloys was determined for x⩽0.25. Our results indicate an “anomalous,” linear, dependence of the energy gap on the In content, at room temperature: Eg(x)=3.39–3.57x eV. Extension of this behavior to higher concentrations is discussed on the basis of reported results.

Selectively excited photoluminescence from Eu- implanted GaN

The intensity of Eu-related luminescence from ion-implanted GaN with a 10nm thick AlN cap, both grown epitaxially by metal organic chemical vapor deposition (MOCVD) is increased markedly by high-temperature annealing at 1300°C. Photoluminescence (PL) and PL excitation (PLE) studies reveal a variety of Eu centers with different excitation mechanisms. High-resolution PL spectra at low temperature clearly show that emission lines ascribed to D05-F27 (∼622nm), D05-F37 (∼664nm), and D05-F17 (∼602nm) transitions each consist of several peaks. PL excitation spectra of the spectrally resolved components of the D05-F27 multiplet contain contributions from above-bandedge absorption by the GaN host, a GaN exciton absorption at 356nm, and a broad subedge absorption band centred at ∼385nm. Marked differences in the shape of the D05-F27 PL multiplet are demonstrated by selective excitation via the continuum/exciton states and the below gap absorption band. The four strongest lines of the multiplet are shown to consist ...

Damage formation and annealing at low temperatures in ion implanted ZnO

N, Ar, and Er ions were implanted into ZnO at 15 K within a large fluence range. The Rutherford backscattering technique in the channeling mode was used to study in situ the damage built-up in the Zn sublattice at 15 K. Several stages in the damage formation were observed. From the linear increase of the damage for low implantation fluences, an upper limit of the Zn displacement energy of 65 eV could be estimated for [0001] oriented ZnO. Annealing measurements below room temperature show a significant recovery of the lattice starting at temperatures between 80 and 130 K for a sample implanted with low Er fluence. Samples with higher damage levels do not reveal any damage recovery up to room temperature, pointing to the formation of stable defect complexes.

Photoluminescence and lattice location of Eu and Pr implanted GaN samples

Rare earth (RE) ions implanted GaN films were studied by optical spectroscopy and RBS techniques. Sharp emission lines due to intra-4f n shell transitions can be observed even at room temperature for the Eu 3+ and Pr 3+ .Th e photoluminescence spectra recorded by the above band gap excitation reveal dominant transitions due to the 5 D0- 7 F1,2,3 lines at 6004, 6211 and 6632 ( A for the Eu 3+ and 3 P0,1- 3 F2,3 at 6450 and 6518 ( A, respectively, for the Pr 3+ . We report on the temperature dependence of the intra-ionic emissions as well as on the lattice site location of the RE detailed angular scans through the / 00 0 1S and / 10 % S axial directions; which indicates that for Pr, complete substitutionality on the Ga sites was achieved while for Eu a Ga displaced site was found. r 2001 Elsevier Science B.V. All rights reserved.