S. Pereira

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...

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...

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.

Structural analysis of InGaN epilayers

The structural properties of InGaN have attracted interest on account of the recent widespread use of the material in visible light-emitting devices. A key topic has been the indirect determination of the composition by x-ray diffraction (XRD). We examine critically the several levels of approximation involved in this procedure. It is shown by extended x-ray absorption fine structure (EXAFS) measurements that the local structure of InGaN is independent of the composition, in the range of InN fraction, from about 15 to 40%, that corresponds to blue to infrared light emission from this material. EXAFS-determined ratios of the numbers of indium and gallium atoms in the first metal co-ordination shell, M1, show very good agreement with the composition measured by established techniques, both structural and chemical, on similar samples. On the other hand, the atomic separations deviate markedly from values calculated using Vegards law. In particular, the average radial separations, In-N1=2.11(2) A and In-M1=3.28(3) A, do not vary significantly with In/Ga ratio in the examined composition range. We conclude with some brief comments on the uncertain but challenging topic of InGaN nanostructure.

Thermal conductivity of silicon bulk and nanowires: Effects of isotopic composition, phonon confinement, and surface roughness

We present a rigorous analysis of the thermal conductivity of bulk silicon (Si) and Si nanowires (Si NWs) which takes into account the exact physical nature of the various acoustic and optical phonon mechanisms. Following the Callaway solution for the Boltzmann equation, where resistive and nonresistive phonon mechanisms are discriminated, we derived formalism for the lattice thermal conductivity that takes into account the phonon incidence angles. The phonon scattering processes are represented by frequency-dependent relaxation time. In addition to the commonly considered acoustic three-phonon processes, a detailed analysis of the role of the optical phonon decay into acoustic phonons is performed. This optical phonon decay mechanism is considered to act as acoustic phonon generation rate partially counteracting the acoustic phonon scattering rates. We have derived the analytical expression describing this physical mechanism which should be included in the general formalism as a correction to the resisti...

Theoretical investigation of size and shape effects on the melting temperature of ZnO nanostructures

We report a theoretical investigation concerning the melting temperature, Tm, of ZnO and Zn nanoparticles (NPs), nanowires (NWs) and nanotubes (NTs). The shapes considered here for the zinc oxide low dimensional structures include spherical NPs, NWs with circular, rectangular (nanobelts) and hexagonal sections and NTs with circular and hexagonal sections. A comparison between ZnO and Zn nanostructures demonstrates a higher stability of ZnO for most size and shape ranges considered. Moreover, the size effect on the melting temperature for ZnO is found to be quite strong: for a spherical ZnO NP with a radius of 5 nm, the size effect on Tm corresponds to a decrease of ∼36% relative to the bulk melting temperature, whereas the reduction for the case of a metallic Zn NP with the same dimension is ∼13%. Based on Tm estimations as a function of size and shape, we predict that certain ZnO nanostructures, such as small (<10 nm) NTs, may not be viable for nanoelectronics or nanophotonic devices, since Tm is too close to, or in some cases even below, room temperature. The influence of the surface tension uncertainties on the calculated melting temperatures is also discussed. Finally, based on the determination of Tm at the nanoscale, the maximal intrinsic residual stress in a hexagonal ZnO NW and in a cylindrical Zn NW is estimated to be ∼45 MPa and ∼1.9 GPa, respectively.

Interpretation of double x-ray diffraction peaks from InGaN layers

The presence of two, or more, x-ray diffraction (XRD) peaks from an InGaN epilayer is sometimes regarded as an indicator of phase segregation. Nevertheless, detailed characterization of an InGaN/GaN bilayer by a combination of XRD and Rutherford backscattering spectrometry (RBS) shows that splitting of the XRD peak may be completely unrelated to phase decomposition. Wurtzite InGaN/GaN layers were grown in a commercial reactor. An XRD reciprocal space map performed on the (105) plane shows that one component of the partially resolved InGaN double peak is practically aligned with that of the GaN buffer, indicating that part of the layer is pseudomorphic to the GaN template. The other XRD component is shown to have the same indium content as the pseudomorphic component, from a consideration of the effect of strain on the c- and a-lattice constants. The composition deduced from XRD measurements is confirmed by RBS. Depth-resolving RBS channeling angular scans also show that the region closer to the GaN/InGaN ...

New light from hybrid inorganic-organic emitters

We present the highlights of a research programme on hybrid inorganic?organic light emitters. These devices combine recent developments in III?V nitride technology (including UV emitting micro-arrays and specifically tailored quantum wells) with conjugated polymers to access the entire visible spectrum. Two types of devices are studied, those based on down conversion of the quantum well emission by radiative transfer and those based on non-radiative resonant energy transfer. The spectral and operating characteristics of the devices are described in detail. Selectable colour micro-arrays and bar emitters are demonstrated. The nature of the non-radiative energy transfer process has also been studied and we find transfer efficiencies of up to 43% at 15?K, with a 1/R2 dependence on the distance between quantum well and polymer layer, suggesting a plane?plane interaction. The relative importance of the non-radiative resonant energy transfer process increases with temperature to be up to 20 times more efficient, at 300?K, than the radiative transfer process.

In situ optical reflectometry applied to growth of indium gallium nitride epilayers and multi-quantum well structures

Reflectometry using a white light source has been applied to in situ monitoring of metal organic vapour phase epitaxy of InGaN alloy structures on GaN buffer layers. Both InGaN epilayers 60-350 nm in thickness and InGaN/GaN multi-quantum-well (MQW) structures with periods of order 10 nm were studied. The InGaN epilayers have indium mole fractions between 0.105 and 0.240, determined principally by the growth temperature. The standard method of deriving film growth rates from in situ reflectance data is a useful predictor of InGaN epilayer thicknesses, and monitoring at wavelengths of 600 or 800 nm minimizes complications caused by absorption and scattering. For a set of seven InGaN epilayers, the average agreement between reflectance-derived thicknesses and estimates based on Rutherford backscattering is within 5%. Uncertainties in these measurements arise from the significant surface roughness of the films, an imprecise knowledge of optical constants and apparent short-term fluctuations in growth rates. Growth rates obtained from in situ monitoring of InGaN epilayers and GaN grown under the same conditions as MQW barriers can be used to successfully predict layer thicknesses in actual QW structures. We illustrate this methodology by comparing predicted layer thicknesses in 10- and 18-period MQW structures with results from conventional ex situ characterization, using transmission electron microscopy and x-ray diffraction.

A green method for the preparation of fluorescent hybrid structures of gold and corrole

Gold/soap nanostructures were prepared by a green methodology using saponified household sunflower oil, as reducing and organic dispersing agent of auric acid. The incorporation of hydrophobic molecules on the novel water-soluble gold nanoparticles was followed by fluorescence lifetime imaging microscopy, using as model hydrophobic compound 5,10,15-tris-(pentafluorophenyl)corrolatogallium(III)(pyridine) (GaPFC), a highly fluorescent corrole. The results showed the hydrophobic GaPFC located between the organic bilayer surrounding several Au nanoparticles, which in turn were coated with fatty acids salts anchored by the double bond at the gold’s surface.Graphical Abstract