Check out our New Publishers' Select for Free Articles

Applied Surface Science (v.253, #1)

Author Index (pp. i-v).

Subject Index (pp. vi-xviii).

No Title by Bernard Servet Guest Editor (pp. 1-2).

Analysis of mesoporous thin films by X-ray reflectivity, optical reflectivity and grazing incidence small angle X-ray scattering by A. Gibaud; S. Dourdain; G. Vignaud (pp. 3-11).

It is well-established that X-ray reflectivity (XR) is an invaluable tool to investigate the structure of thin films. Indeed, this technique provides under correct analysis, the electron density profile of thin films in the direction perpendicular to the substrate. For thin films that exhibit lateral ordering at the nanometer scale, grazing incidence small angle X-ray scattering (GISAXS) ideally complements the XR technique to measure the scattering in off-specular directions. As typical examples, XR and GISAXS data of mesoporous silica thin films and porous materials are presented. The analysis of the XR curve allows to determine the porosity of the film. We also show that the combination of X-ray and visible optical reflection provides information about the index of refraction of thin films. Finally we report how capillary condensation of water can be monitored by XR and GISAXS.

Keywords: Mesoporous; Reflectivity; Scattering

Thin multilayers characterization by grazing X-ray reflectometry and use of Fourier transform by F. Bridou; J. Gautier; F. Delmotte; M.-F. Ravet; O. Durand; M. Modreanu (pp. 12-16).

Grazing X-ray reflectometry is used in order to characterize thin layer stacks, in particular periodic multilayers. The specular reflectivity depends on the thickness, the complex refractive index of each layer and on the roughness of the interfaces. By a trial and error method, the experimental reflectivity curve can be fitted with a theoretical one, and so, the parameters of the stack can be obtained. This numerical method needs usually initial guess of the kind of results.Fourier transform method allows to obtain directly the values of distances between interfaces, with a good approximation depending on the maximum angular scan of the measure. It can also reveal some particularity of the multilayer, i.e. periodic multilayered structures with more than two layers per period. As an illustration of this characterization method, some examples in XUV optical domain will be shown. This method can also be used for the characterization of many kinds of multilayer stacks, in particular semi-conductor heterostructures ones, under the condition that adjacent layers have sufficient contrast index at the wavelength of the X-ray source.

Keywords: Fourier transform; Grazing; Multilayers

Electron density profile at the interface of SiO₂/Si(0 0 1) by S. Banerjee; S. Ferrari; R. Piagge; S. Spadoni (pp. 17-20).

In this report we present grazing incidence X-ray reflectivity (GIXR) study of SiO₂/Si(0 0 1) system. We have analysed the X-ray reflectivity data using recursive formalism based on matrix method and distorted wave Born approximation (DWBA). From the analysis of the reflectivity data we could obtain the electron density profile (EDP) at the interface of the dielectric SiO₂ film and the Si(0 0 1) substrate. The EDP obtained from the matrix method follows the DWBA scheme only when two transition layers are considered at the interface of SiO₂/Si. The layer which is in proximity with the Si substrate has a higher electron density value than the Si and SiO₂ values and it appears as a maximum in the EDP. The layer which is in proximity with the dielectric SiO₂ layer has an electron density value lower than the SiO₂ value and it appears as a minimum in the EDP. When the thickness of the SiO₂ layer is increased the lower density layer diminishes and the higher density layer persists.

Keywords: SiO; ₂; /Si(0 0 1) system; X-ray reflectivity; Electron density profile

X-ray metrology for advanced silicon processes by C. Wyon; J.P. Gonchond; D. Delille; A. Michallet; J.C. Royer; L. Kwakman; S. Marthon (pp. 21-27).

X-ray reflectivity (XRR), X-ray fluorescence (XRF) and small angle X-ray scattering (SAXS) techniques are used to the monitoring of Cu/porous low κ processes, which are developed for the next generation (≤65nm) integrated circuits. Sensitivity of XRR and XRF is sufficient to detect drifts of the copper barrier layer, copper seed layer and Cu CMP (chemical–mechanical polishing) processes. Their metrology key parameters comply with production requirements. SAXS allows determining the pore structure of low κ films: average pore size and pore size distribution.

Keywords: X-ray reflectivity (XRR); Small angle X-ray scattering (SAXS); X-ray fluorescence (XRF); Cu interconnects; Low κ; Dielectrics

A simple solution to systematic errors in density determination by X-ray reflectivity: The XRR-density evaluation (XRR-DE) method by P. Bergese; E. Bontempi; L.E. Depero (pp. 28-32).

X-ray reflectivity (XRR) is a non-destructive, accurate and fast technique for evaluating film density. Indeed, sample-goniometer alignment is a critical experimental factor and the overriding error source in XRR density determination. With commercial single-wavelength X-ray reflectometers, alignment is difficult to control and strongly depends on the operator. In the present work, the contribution of misalignment on density evaluation error is discussed, and a novel procedure (named XRR-density evaluation or XRR-DE method) to minimize the problem will be presented. The method allows to overcome the alignment step through the extrapolation of the correct density value from appropriate non-specular XRR data sets. This procedure is operator independent and suitable for commercial single-wavelength X-ray reflectometers. To test the XRR-DE method, single crystals of TiO₂ and SrTiO₃ were used. In both cases the determined densities differed from the nominal ones less than 5.5%. Thus, the XRR-DE method can be successfully applied to evaluate the density of thin films for which only optical reflectivity is today used. The advantage is that this method can be considered thickness independent.

Keywords: Thin films; Density, X-ray reflectivity (XRR); Sample-goniometer; XRR-density evaluation (XRR-DE)

XRR and GISAXS study of silicon oxynitride films by S. Bernstorff; P. Dub�?ek; B. Pivac; I. Kova�?ević; A. Sassella; A. Borghesi (pp. 33-37).

Thin films of silicon oxynitride have largely replaced pure silicon oxide films as gate and tunnel oxide films in modern technology due to their superior properties in terms of efficiency as boron barrier, resistance to electrical stress and high dielectric strength. A single chamber system for plasma enhanced chemical vapor deposition was employed to deposit different films of SiO_xN_yH_z with 0.85< x<1.91. All films were previously characterized by Rutherford back-scattering and infrared spectroscopy to determine the stoichiometry and the presence of various bonding configurations of constituent atoms. We used X-ray reflectivity to determine the electron density profile across the depth, and we showed that the top layer is densified. Moreover, grazing incidence small-angle X-ray scattering was used to study inhomogeneities (clustering) in the films, and it is shown that plate-like inhomogeneities exist in the top and sphere-like particles at the bottom part of the film. Their shape and size depend on the stoichiometry of the films.

Keywords: Silicon oxynitride films; X-ray reflectivity; Small-angle X-ray scattering

The ultimate in real-time ellipsometry: Multichannel Mueller matrix spectroscopy by Chi Chen; M.W. Horn; Sean Pursel; C. Ross; R.W. Collins (pp. 38-46).

A review of the techniques and applications of multichannel ellipsometry in the dual-rotating-compensator configuration is given. This ellipsometric approach has been established as the ultimate in real-time, single-spot optical measurement, as it determines the entire 16-element Mueller matrix of a sample over a wide spectral range (up to 1.7–5.3eV) from raw data collected over a single optical period of 0.25s. The sequence of optical elements for this ellipsometer is denoted PC_1r SC_2r A, where P, S, and A represent the polarizer, sample, and analyzer. C_1r and C_2r represent two MgF₂ rotating compensators, either biplates or monoplates that rotate synchronously at frequencies of ω₁=5 ω and ω₂=3 ω, where π/ ω is the fundamental optical period. Previous high-speed Mueller matrix measurements with this instrument have been performed on uniform, weakly anisotropic samples such as (110) Si, in which case one can extract the bulk isotropic and near-surface anisotropic optical responses simultaneously. In such an application, the instrument is operated at its precision/accuracy limits. Here, ex situ results on a strongly anisotropic, locally biaxial film are presented that demonstrate instrument capabilities for real-time analysis of such films during fabrication or modification. In addition, the use of the instrument as a real-time probe to extract surface roughness evolution on three different in-plane scales for an isotropic film surface is demonstrated for the first time.

Keywords: PACS; 07.60.Fs (polarimeters and ellipsometers); 78.20.Ci (optical constants [including refractive index; complex dielectric constant; absorption; reflection and transmission coefficients; emissivity]); 78.20.Ek (optical activity); 68.35.Ct (interface structure and roughness)Multichannel Mueller matrix ellipsometry; Dual-rotating-compensator ellipsometer; Ellipsometry data analysis; Optical anisotropy; Surface roughness evolution

Generalized ellipsometry in unusual configurations by G.E. Jellison Jr.; D.E. Holcomb; J.D. Hunn; C.M. Rouleau; G.W. Wright (pp. 47-51).

Most ellipsometry experiments are performed by shining polarized light onto a sample at a large angle of incidence, and the results are interpreted in terms of thin film thicknesses and isotropic optical functions of the film or substrate. However, it is possible to alter the geometrical arrangement, either by observing the sample in transmission or at normal-incidence reflection. In both cases, the experiment is fundamentally the same, but the interpretation of the results is considerably different. Both configurations can be used in conjunction with microscope optics, allowing for images to be made of the sample. The results of three examples of these different configurations using the two-modulator generalized ellipsometer (2-MGE) are reported: (1) spectroscopic birefringence measurements of ZnO, (2) electric field-induced birefringence (Pockels effect) in GaAs, and (3) normal-incidence reflection anisotropy of highly oriented pyrolytic graphite (HOPG).

Keywords: Ellipsometry; Birefringence; Pockels effect; Optical anisotropy

Investigation of the optical anisotropy of PET and PEN films by VIS-FUV to IR spectroscopic ellipsometry by A. Laskarakis; S. Logothetidis (pp. 52-56).

In the last years, a significant amount of research is being performed in the field of polymer research for novel applications, such as flexible electronic devices, photovoltaic cells, high performance optics, data storage, etc. Toward this direction, in this work, the optical anisotropy of biaxially stretched poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) films has been extensively investigated. The optical properties of the films have been studied in terms of their optical, electronic and vibrational response, by Fourier transform IR spectroscopic ellipsometry (FTIRSE) (900–3500cm⁻¹) and Vis-fUV variable angle SE (1.5–6.5eV) techniques. The films optical anisotropy is the result of the stretching procedure during their fabrication, which results to the structural rearrangement of the macromolecular chains parallel to the stretching direction and to a higher structural symmetry. During the SE spectra analysis, the films have been approximated as uniaxial materials with their optic axis parallel to the sample/ambient interface leading to the accurate determination of the principal components ɛ_||( ω) and ɛ_⊥( ω) of the dielectric function ɛ( ω). The detailed study of the electronic transitions has been performed in the Vis-fUV region, where the characteristic features corresponding to the n→π* electronic transitions of the carbonyl –CO group and the¹A_1g→¹B_1u transition due to the π→π* excitation of the π-electron structures have been identified and analysed. Furthermore, the FTIRSE spectra allowed the accurate identification and assignment of the features of ɛ( ω) to the vibrational modes of the various bonding structures characteristic of the PET and PEN macromolecular chains.

Keywords: Spectroscopic ellipsometry; Polymer; poly(ethylene terephthalate); poly(ethylene naphthalate)

Evaluation strategies for multi-layer, multi-material ellipsometric measurements by O. Polgár; P. Petrik; T. Lohner; M. Fried (pp. 57-64).

In order to extract the physical properties from an ellipsometric measurement, an optical model of the sample has to be assumed first, because the theory of ellipsometry consists on one-directional computation only (there is no reverse function). Then, the ellipsometric evaluation is an iterative optimising procedure with high time consumption feature and the reliability depends strongly on the a-priori information. The faster the computers are today, the more exactly the physical properties of either the sample or the process can be evaluated. However, the increasing number of the parameters and so, the dimensions of the search space leads to a combinatorial explosion. In the case of larger search space is needed (either less a-priori information is available or more parameters are used), the error surface of the parameter space can be quite “hilly�? and may contain even numerous local minima. In the lack of precise a-priori information the Levenberg–Marquardt (LM) gradient search is generally started out of the decreasing area of the global minimum and therefore, it is inappropriate to find the solution. Therefore, there is a hard need of more complex evaluating strategies, which combines the algorithms to make the evaluation more reliable. Different point selection strategies, an extended criteria function and combined algorithms were applied on porous silicon multi-layer and polycrystalline measurements to demonstrate a higher convergence speed (effectiveness) and more reliability.

Keywords: Ellipsometry; Levenberg–Marquardt gradient; Ellipsometric measurements

Optical characterization of ns-SiN:H in the infrared by spectroscopic ellipsometry by Jordi Sancho-Parramon; Salvador Bosch; Adolf Canillas (pp. 65-69).

The optical properties of thin films of amorphous silicon nitride with embedded nanoparticles are determined in the infrared using spectroscopic ellipsometry. In the spectral range of study (950–3500cm⁻¹), the material presents a considerable number of absorption bands, and consequently, a large number of parameters are necessary for the complete description of its optical behaviour. This fact enhances the possibility of reaching good numerical solutions without or with incomplete physical meaning. Particularly, we observe that the common approach consisting of optimising all the parameters in a single step may neglect some of the absorption bands that are evidenced by the experimental data. We propose a fitting strategy based on the progressive fitting of the data, introducing at each step new absorption bands and thus extending the fitted spectral range. This strategy is able to assure a good numerical solution with a correct description for all the absorption bands considered.

Keywords: Spectroscopic ellipsometry; Lorentz oscillator; Plasma enhanced chemical vapour deposition (PECVD)

Analysis of reflectance and modulation spectroscopic lineshapes in optoelectronic device structures by T.J.C. Hosea; S.A. Cripps; T.E. Sale; K. Hild (pp. 70-79).

We discuss the spectral lineshapes of reflectance and modulated reflectance (MR) measurements on optoelectronic device structures such as epi-layers, quantum wells (QWs), vertical-cavity surface emitting-lasers (VCSELs) and resonant-cavity light-emitting diodes (RCLEDs). We consider the various methods for the extraction of built-in electric fields and band-gap energies from Franz-Keldysh oscillations (FKO), using the example of a tensilely strained InGaAs QW system, whose InGaAsP barriers yield strong FKO. We describe how critical point transition energies can be easily obtained by eye from Kramers-Kronig (KK) transforms of low field or QW modulation spectra, using the example of the modulated transmittance spectra of dilute-nitrogen InGaAsN p-i-n structures. We also discuss how the ordinary reflectivity spectrum, usually acquired at the same time as the MR signal, may also be exploited to extract layer thicknesses and compositions, and information about the active QW absorption spectrum in VCSEL and RCLED structures.

Keywords: Optoelectronics; Characterisation; Optical spectroscopy (modulation spectroscopy and reflectivity); Quantum wells; Microcavities

Photoreflectance spectroscopy of semiconductor structures at hydrostatic pressure: A comparison of GaInAs/GaAs and GaInNAs/GaAs single quantum wells by R. Kudrawiec; J. Misiewicz (pp. 80-84).

The pressure dependence of optical transitions in Ga_0.64In_0.36As/GaAs and Ga_0.64In_0.36N_0.01As_0.99/GaAs single quantum well (SQW) structures were studied in photoreflectance (PR) spectroscopy. In order to apply high hydrostatic pressure, up to ∼11kbar, the liquid-filled clamp-pressure cell with a sapphire window for optical access has been adopted in the PR set-up with the so called ‘bright configuration’. It has been found that the linear hydrostatic pressure coefficient for the ground state transition are equal to 8.6 and 7.3meV/kbar for the GaInAs/GaAs and GaInNAs/GaAs SQWs, respectively. This result shows that the incorporation of only 1% of N atoms into GaInAs/GaAs leads to ∼15% decrease in the pressure coefficient. In addition, a non-linearity in the pressure dependence of the ground state transition has been resolved for the GaInNAs/GaAs SQW.

Keywords: Photoreflectance; SQW; Hydrostatic pressure; Dilute nitrides

Ultrathin InAs and modulated InGaAs layers in GaAs grown by MOVPE studied by photomodulated reflectance spectroscopy by P. Hazdra; J. Voves; E. Hulicius; J. Pangrác; Z. Šourek (pp. 85-89).

Photomodulated reflectance spectroscopy (PR) and X-ray diffraction (XRD) were used for the characterization of highly strained ultrathin InAs quantum wells and modulated InGaAs layers in GaAs grown by metal-organic vapor phase epitaxy (MOVPE). Structures were grown in AIXTRON 200 reactor at 500°C on (100) oriented GaAs substrates by sequential growth of InAs and GaAs layers. Various PR spectral features corresponding to optical transitions between ground and excited states in the layers were identified by means of simulation of electronic states in these structures using nextnano³ quantum simulator. Different models of InAs layer growth were used to explain both the XRD and PR data. Results show that the Gaussian distribution of In atoms within few monolayers gives the best fit for our MOVPE grown ultrathin InAs layers.

Keywords: Photomodulated reflectance spectroscopy; X-ray diffraction; Ultrathin InAs layer; MOVPE; Quantum well; Electronic states

Photoreflectance spectroscopy of self-organized InAs/InP(001) quantum sticks emitting at 1.55μm by H. Chouaib; N. Chauvin; C. Bru-Chevallier; C. Monat; P. Regreny; M. Gendry (pp. 90-94).

Photoreflectance (PR) measurements are performed as a function of temperature on self-organized InAs/InP(001) quantum sticks (QSs) grown by solid-source molecular beam epitaxy. With a very weak excitation power, three PR transition energies are arising and associated with the ground state and two excited states, respectively, in good agreement with both photoluminescence (PL) and PL excitation measurements. The temperature dependence of the PR transition energies is in good agreement with the Bose–Einstein behavior.From PL analysis of these InAs/InP QSs, the ground state was assumed to be partially filled because of the residual n-type doping of the InP barrier layers. The PR spectra analysis allows us to further confirm this assumption, considering mainly the relative PR intensity of the different transitions, as well as the Franz Keldysh oscillations (FKO) above the InP bandgap.

Keywords: Photoreflectance spectroscopy; Quantum sticks; Photoluminescence; molecular beam

Recent advances in high-resolution X-ray diffractometry applied to nanostructured oxide thin films: The case of yttria stabilized zirconia epitaxially grown on sapphire by A. Boulle; R. Guinebretière; O. Masson; R. Bachelet; F. Conchon; A. Dauger (pp. 95-105).

The investigation of nanostructured oxide thin films using high-resolution X-ray diffraction (XRD) is considered. Because of the small amount of matter deposited and significant defect densities, such oxide thin film structures can be considered as imperfect materials that require specific data acquisition and data analysis methods. Fast reciprocal space mapping is carried out using a diffractometer based on an 18kW X-ray source, a four-reflection monochromator and a curved position sensitive detector. In order to extract quantitative information concerning the microstructure of the films, an approach is developed that combines a microscopic modelling of dimensional effects (crystallite or island shape, size and size distribution) with a phenomenological description of lattice disorder. Within this approach, simple analytical expressions or expressions implying a simple Fourier transform, can be derived for the XRD intensity distribution in the direction perpendicular to the film surface and parallel to it. Profiles exhibiting damped and/or broadened fringes and profiles exhibiting a two-component line shape can be simulated. Parameters of primary interest, such as the island thickness, thickness distribution function, island in-plane dimensions and the distribution function of the dimensions, the level of disorder, the disorder correlation length and the spatial distribution of disorder, can be extracted. The applicability of the model is illustrated with yttria stabilized zirconia films epitaxially grown on sapphire by sol–gel dip-coating.

Keywords: High resolution X-ray diffraction; Microstructure; Modelling; Size and shape; Lattice disorder

Novel methods and universal software for HRXRD, XRR and GISAXS data interpretation by A. Ulyanenkov (pp. 106-111).

Several novel methods for evaluation and interpretation of X-ray data from modern nanostructures are presented along with their applications. The background of methods and their relations to fundamental problems of X-ray analysis is shortly described. The key features of LEPTOS software, which is designed for the analysis of X-ray data measured with various geometries and setups and implements all discussed techniques, are discussed.

Keywords: X-ray diffraction; X-ray reflectivity; Diffuse scattering; Grazing-incidence small angle scattering

Structural characterisation of Sb-based heterostructures by X-ray scattering methods by C. Renard; O. Durand; X. Marcadet; J. Massies; O. Parillaud (pp. 112-117).

Antimonide-based superlattices dedicated to the elaboration of opto-electronic devices have been studied by X-ray scattering techniques. In particular, specular and non-specular X-ray reflectometry experiments have been performed on two MBE-samples elaborated with different shutter sequences at the interfaces. The results have shown a limitation of the incorporation of Sb species in the subsequent InAs layer for one of the samples, as expected.Then, a study on a InGaAs-cap layer/(InGaAs/AlAsSb)_N superlattice grown on a InGaAs/InP buffer layer by both specular X-ray reflectometry and High resolution X-ray diffraction is reported. In particular, the results have revealed the presence of a highly disturbed thin-layer on top of the MOVPE-made GaInAs, whose presence has been explained by In-concentration modification during the desoxidation procedure at the surface of the MOVPE-made GaInAs.Beside the results on the Sb-based heterostructures, the use of X-ray scattering metrology as a routinely working non-destructive testing method has been emphasized.

Keywords: X-ray reflectometry; High resolution X-ray diffractometry; Antimonide heterostructures; Opto-electronic devices; Thickness determination; Fourier-inversion method; Non-specular X-ray reflection

Simulation of X-ray diffraction profiles in multilayers by direct wave summation: Application to asymmetric reflections by S. Zamir; O. Steinberg; E. Lakin; E. Zolotoyabko (pp. 118-123).

A novel algorithm for the simulation of the X-ray diffraction profiles in multilayers is developed, which can be applied to any multilayered structure, with no limitations. The simulation program in the MATLAB format is based on the direct summation of waves scattered by individual atomic planes. It takes into account the strain and concentration-induced fluctuations of interplanar spacings, interface roughness and buried amorphous layers, and enables adding the diffuse scattering contributions of the Gaussian or Lorentzian types.The summation over individual layers can be done coherently or incoherently, depending on the interface structure. In order to visualize the steps of the fitting procedure, the contribution of each layer can be plotted separately.In this paper the simulation routine is described with a focus on handling asymmetric reflections. We stress that in this case, the effective thickness of the layers, participating in the formation of diffraction signals, can be very different for low or high X-ray incidence angles. We also show that in contrast to symmetric reflections, when treating the asymmetric ones, an additional phase shift depending on the distance between the sample and detector, should be taken into account. The simulation program is applied to fit experimental diffraction profiles, symmetric and asymmetric, taken from the MOVPE-grown heterostructures and superlattices of practical importance, based on the InGaAsP/InP materials system.

Keywords: X-ray diffraction simulations; Semiconductor multilayers; Non-destructive testing

X-ray triple-axis diffractometry investigation of Si/SiGe/Si on silicon-on-insulator subjected to in situ low-temperature annealing by T.D. Ma; H.L. Tu; G.Y. Hu; B.L. Shao; A.S. Liu (pp. 124-127).

X-ray triple-axis diffractometry (XRTD) was used to characterize heterostructure Si/SiGe/Si on silicon-on-insulator (SOI) subjected to in situ low-temperature annealing. Crystallographic tilt, lattice constant and relaxation percentage were examined, respectively. Two peaks have been observed in (004) reciprocal lattice mappings (RLMs) of Si layers. The (004) RLMs indicate that Si cladding is in tensile strain. We have also found two peaks with different k_∥ and k_⊥ in (113) asymmetric RLMs of Si layers. It is deduced from comprehensive analyses on (004) and (113) RLMs that Ge diffusion and in-plane tensile strain lead to 2 θ shift of the Si layers underneath SiGe layer in (004) RLMs. And the diffusion concentration of Ge accurately determined by XRTD is mole fraction 0.84%.

Keywords: XRTD; Diffraction peaks; Tensile strain; Ge diffusion

Structure of PtFe/Fe double-period multilayers investigated by X-ray diffraction, reflectivity, diffuse scattering and TEM by N. Zotov; J. Feydt; T. Walther; A. Ludwig (pp. 128-132).

Double-period [(Pt 1.7nm/Fe 0.9nm)₅Fe( t_Fe2)]₈ and [(Pt 1.8nm/Fe 0.6nm)₅Fe( t_Fe2)]₈ multilayers with different thickness t_Fe2 (between 0.23 and 4.32nm) of the additional Fe layers, prepared by combinatorial sputter deposition, show differences in the mosaic spread and the vertical interfacial roughness when deposited on native or thermally oxidised Si wafers. Simulations of the wide-angle X-ray scattering intensities revealed the presence of interdiffusion in the (Pt/Fe)₅ bilayers and systematic variations of the grain sizes, perpendicular to the film surface, as well as the rms variations of the two superlattice periods with the total film thickness. A comparison of ω-rocking scans shows an increase of the correlated vertical roughness of the (Pt/Fe)₅ multilayers with the total multilayer thickness.

Keywords: PACS; 61.10.Eq; 61.10.Kw; 74.78.Fk; 68.35.Ct; 68.03.HjX-ray diffraction; Reflectivity; Diffuse scattering; Multilayers; Fe–Pt

Fourier-inversion and wavelet-transform methods applied to X-ray reflectometry and HRXRD profiles from complex thin-layered heterostructures by O. Durand; N. Morizet (pp. 133-137).

We show that X-ray scattering techniques can be used for the assessment of individual layer thicknesses inside complicated semi-conductor heterostructures dedicated to the opto-electronic domain. To this end, we propose methods to overcome two main drawbacks coming from: (1) the complexity of the X-ray profiles and, hence the difficulty to use model-dependent tools such as fitting procedures and (2) large dynamics in intensity due to numerous high diffraction superlattice peaks from superlattices which limit the use of the model-independent Fourier-inversion method.We demonstrate first the reliability of the Fourier-inversion method applied to high-resolution X-ray diffraction profiles curve from quantum well infrared photodetectors heterostructures, complementary to the model-dependent fitting tools. Then, a wavelet-transform-based procedure has been successfully used on X-ray reflectometry profiles containing intense SL Bragg peaks.

Keywords: X-ray reflectometry; High resolution X-ray diffractometry; Semi-conductor heterostructures; Opto-electronic devices; Thickness determination; Fast Fourier transform; Wavelet transform

Nanophotonics and nanometrology with planar X-ray waveguide-resonator by V.K. Egorov; E.V. Egorov (pp. 138-144).

The mechanism of X-ray waveguide-resonance propagation or the radiation superstream model, which can become the ground of X-ray nanophotonics, is discussed briefly. Some attention is devoted to features consideration of the simplest devices characterized by the waveguide-resonance transportation of X-ray beams. The experimental data showing the user possibilities of a simplest waveguide-resonators application for diffractometry are presented. We discuss the main reasons to improve the metrological characteristics for total reflection X-ray fluorescence (TXRF) analytical method in case when the target exciting beam is formed by a waveguide-resonator. Some problems appearing during the waveguide-resonator application are formulated.

Keywords: Interference field of X-ray standing wave; Total reflection X-ray fluorescence (TXRF); Planar X-ray waveguide-resonator (PXWR)

Advances in modulation spectroscopy: State-of-art photoreflectance metrology by M.E. Murtagh; S. Ward; D. Nee; P.V. Kelly (pp. 145-151).

In this paper, technological advances of modulation spectroscopy are presented, exploiting the sensitivity, room-temperature resolution, as well as the rapid and non-contact (non-destructive) nature of laser-induced photoreflectance (PR). A novel method of asynchronous (switching) modulation is presented to overcome laser-induced non-PR background effects, which limit or even obscure the complex (phase) PR response. The solid-state, acousto-optic based method may even be employed for non-uniform samples, and moreover, exhibits evidence for improved signal-to-noise level. Also presented is a novel optical design in order to achieve multiple, independent and simultaneous spectral acquisition, including auto-calibration. Results are demonstrated for heavily doped n-type and p-type GaAs substrates, and also technologically important HBT device structures, with further applications also emphasised for HEMTs, LEDs, etc. The results demonstrate the importance and role of PR as a successful commercial metrology tool for existing state-of-art, as well as next generation semiconductor characterisation and statistical-process-control (SPC) equipment.

Keywords: Photoreflectance; Asynchronous (solid-state) modulation; Multiple (simultaneous) acquisition; Auto-calibration; Metrology

Contactless electroreflectance spectroscopy of Ga(In)NAs/GaAs quantum well structures containing Sb atoms by R. Kudrawiec; M. Gladysiewicz; M. Motyka; J. Misiewicz; H.B. Yuen; S.R. Bank; M.A. Wistey; H.P. Bae; James S. Harris Jr. (pp. 152-157).

Contactless electroreflectance (CER) spectroscopy has been applied to investigate the optical transitions in Ga(In)NAs/GaAs quantum well (QW) structures containing Sb atoms. The identification of the optical transitions has been carried out in accordance with theoretical calculations which have been performed within the framework of the effective mass approximation. Using this method, the bandgap discontinuity for GaN_0.027As_0.863Sb_0.11/GaAs, Ga_0.62In_0.38As_0.954N_0.026Sb_0.02/GaAs, and Ga_0.61In_0.39As_0.963N_0.017Sb_0.02/GaN_0.027As_0.973/GaAs QW structures has been determined. It has been found that the conduction-band offset is ∼50 and ∼80% for GaN_0.027As_0.863Sb_0.11/GaAs and Ga_0.62In_0.38As_0.954N_0.026Sb_0.02/GaAs QWs, respectively. It corresponds to 264 and 296meV depth QW for electrons and heavy-holes in GaN_0.027As_0.863Sb_0.11/GaAs QW; and 520 and 146meV depth QW for electrons and heavy-holes in Ga_0.62In_0.38As_0.954N_0.026Sb_0.02/GaAs QW. In the case of the Ga_0.61In_0.39As_0.963N_0.017Sb_0.02/GaN_0.027As_0.973/GaAs step-like QW structure it has been shown that the depth of electron and heavy-hole Ga_0.61In_0.39As_0.963N_0.017Sb_0.02/GaN_0.027As_0.973 QW is ∼144 and ∼127meV, respectively.

Keywords: Dilute nitrides; Quantum wells; Electroreflectance; Band offset

New calibration method for UV–VIS photothermal deflection spectroscopy set-up by Jordi Sancho-Parramon; Josep Ferré-Borrull; Salvador Bosch; Anna Krasilnikova; Jiri Bulir (pp. 158-162).

Photothermal deflection spectroscopy has emerged as a useful technique for the determination of the absorption of materials with a small absorption coefficient. The technique offers relative values of the material absorptivity and, therefore, requires a calibration procedure in order to determine the absolute values. In this work, we present a new calibration method for a photothermal deflection spectroscopy set-up working in the UV–VIS, spectral range. The method is based on the use of reference samples with different levels of absorption. The samples, consisting of single thin films of amorphous carbon on transparent substrates, are optically characterized by means of spectrophotometric measurements. The accurate characterization of the samples enables the computation of their corresponding optical absorptivity in the PDS set-up. The calibration method is cross-checked by comparison of the measurements for the different reference samples and is finally applied to the study of the absorption of dielectric films in the UV.

Keywords: Photothermal deflection spectroscopy (PDS); Absorption coefficient; Optical spectroscopy

In situ ellipsometry of surface layer of non-metallic transparent materials during its finish processing by Oleksandr Y. Filatov; Leonid V. Poperenko (pp. 163-166).

For modern technology applications it is important to develop non-contact methods of control of the modification of dielectric materials surface layer. The aim of the work is to determine the level of roughness changes in the surface layer of non-metallic material, optical glass BK-7, and to control it by in situ ellipsometry. The probing light spot was formed at a second (lower) reflective surface of the plate being studied during its mechanical processing at direct observation of these changes. The fine mechanical polishing was carried out for 2 hours by using the grinding-polishing machine installed directly on the sample table of ellipsometer LEF-3M. The angle of light incidence was close to 70 degree. The ellipsometric parameters, were determined within the mechanically processed area. For this purpose, the probing light beam passed two times through the sample and then returned to the initial (air) medium, where its polarization state was studied. The polarized beam falls on lower plate surface polished by conventional technology using grinding-polishing CeO₂-based instrument “Aquapol�? (grain size 1 micron). The time dependences of the ellipsometric parameters during the surface layer treatment were studied. In these dependences the tendency of changes of ellipsometric parameters indicates the surface roughness enhancement.

Keywords: Ellipsometry; Surface; Roughness; Polishing

Simultaneous optical measurement of Ge-content and carbon doping in strained epitaxial SiGe films by S. Morris; D. Le Cunff; D. Ristoiu; V. Vachellerie; F. Deleglise; D. Dutartre (pp. 167-172).

We use a novel analytical technique, previously shown to be able to decouple germanium and boron contents, to measure simultaneously the germanium and carbon contents of a set of carbon-doped epitaxial SiGe films. The method is based on the use of “perturbation functions�? to account empirically for the effects that the dopant has upon the fundamental dielectric functions of the material. To demonstrate the technique, a matrix of 10 wafers having variation in both germanium and carbon contents was analyzed. Neglecting the presence of carbon led to large errors in germanium content, whether measured optically or by X-ray diffraction (XRD). However, using the new method the thickness, germanium and carbon contents could be derived together and very good agreement was obtained between measurements on a production-grade optical metrology tool and measurements by secondary ion mass spectrometry (SIMS). To verify the production-worthiness of the approach used, results from two different production metrology tools (i.e. two Therma-Wave Opti-Probe 5220 tools) were compared and some repeatability testing was also performed. The method holds great promise for improving run-to-run process control for advanced epitaxy processes.

Keywords: Germanium; Carbon; Epitaxial SiGe films

Optical models for the ellipsometric characterization of carbon nitride layers prepared by inverse pulsed laser deposition by P. Petrik; T. Lohner; L. Égerházi; Zs. Geretovszky (pp. 173-176).

Amorphous carbon nitride (CN_x) films were prepared by KrF excimer laser ablation of a graphite target in a nitrogen atmosphere in the inverse PLD geometry. From the ellipsometric point of view, the challenging properties of these films were their exponentially decaying thickness as a function of distance from the ablation source, accompanied by a laterally varying chemical composition and structure. Optical models were developed to accurately describe the dependence of film properties on distance from the ablation, layer thickness, and nitrogen pressure. Multi-layer models were used to characterize the surface roughness as well as lateral inhomogeneities. Multiple angles of incidence and multiple wavelengths were applied in the ranges of 66–72° and 250–1000nm, respectively. A microspot capability of the spectroscopic ellipsometer (with a spot size of about 100μm) was exploited to decrease the error caused by the lateral inhomogeneity within the measurement spot. Material properties were derived using the empirical Cauchy dispersion model as well as the Tauc–Lorentz parametric dielectric function model. These models allowed the quantitative determination of the band gap and the oscillator parameters in addition to the layer thicknesses and dielectric functions.

Keywords: Carbon nitride; Inverse pulsed laser deposition; Spectroscopic ellipsometry; Tauc–Lorentz model

Changes in the shapes of self-organized PbSe quantum dots during PbEuTe overgrowth investigated by anomalous X-ray diffraction by V. Holý; T.U. Schülli; R.T. Lechner; G. Springholz; G. Bauer (pp. 177-181).

Anomalous X-ray diffraction was used for the investigation of shape and chemical composition of self-organized PbSe quantum dots covered by PbEuTe capping layers. From reciprocal-space maps of diffracted intensities measured at two energies of the primary radiation, we discriminated the contributions of the dot volumes and the surrounding crystal lattice to the diffracted intensity. We have found that the presence of Eu atoms suppresses the flattening of the dots during their overgrowth.

Keywords: PACS; 61.10.; −; i; 68.65.; −; k; 81.07.TaNanostructures; Surfaces and interfaces; Semiconductors; C. X-ray diffraction

X-ray scattering: A powerful probe of lattice strain in materials with small dimensions by Olivier Thomas; Audrey Loubens; Patrice Gergaud; Stéphane Labat (pp. 182-187).

X-ray diffraction was recognized from the early days as highly sensitive to atomic displacements. Indeed structural crystallography has been very successful in locating with great precision the position of atoms within an individual unit cell. In disordered systems, it is the average structure and fluctuations about it that may be determined. In the field of mechanics, diffraction may thus be used to evaluate elastic displacement fields. In this short overview, we give examples from recent work where X-ray diffraction has been used to investigate average strains in lines, films or multilayers. In small objects, the proximity of surfaces or interfaces may create very inhomogeneous displacement fields. X-ray scattering is again one of the best methods to determine such distributions. The need to characterize displacement fields in nano-structures together with the advent of third generation synchrotron radiation sources has generated new and powerful methods (anomalous diffraction, coherent diffraction, micro-diffraction, etc.). We review some of the recent and promising results in the field of strain measurements in small dimensions via X-ray diffraction.

Keywords: X-ray diffraction; Nano-structures; strain/stress

Crystallite misorientation analysis in semiconductor wafers and ELO samples by rocking curve imaging by P. Mikulík; D. Lübbert; P. Pernot; L. Helfen; T. Baumbach (pp. 188-193).

Rocking curve imaging is based on measuring a series of Bragg-reflection digital topographs by monochromatic parallel-beam synchrotron radiation in order to quantify local crystal lattice rotations within a large surface area with high angular and high spatial resolution. In this paper we apply the method to map local lattice tilts in two distinct semiconductor sample types with lattice misorientations up to0.5° and with spatial resolution from 30μm down to 1μm. We analyse the measured surface-tilt data volumes for samples with almost smoothly varying specific misoriented defect formation in GaAs wafers and for an inherent subsurface grain structure of epitaxial lateral overgrowth wings in GaN. Back-projected tilt maps and histograms provide both local and global characteristics of the microcrystallinity.

Keywords: X-ray diffraction; X-ray topography; Microdiffraction; Crystal growth; Microstructure; GaAs; GaN

Photoreflectance study at the micrometer scale by C. Bru-Chevallier; H. Chouaib; A. Bakouboula; T. Benyattou (pp. 194-199).

Photoreflectance (PR) spectroscopy has proven to be a very efficient non-destructive tool to get information on various semiconducting epitaxial structures as it is very sensitive to every direct optical transitions in semiconducting quantum structures and allows as well to optically measure internal electric fields in space charge layers, through Franz–Keldysh oscillation (FKO) analysis. We have developed an experimental setup to get micro-PR spectra on epitaxial structures or devices on a few micrometer size spots. Due to very low signal intensity, experimental conditions have to be very carefully controlled: the signal/noise ratio strongly depends on the pump–probe power ratio.We give experimental micro-PR results recorded on antimonide-based heterojunction bipolar transistors (HBTs), which give the local electric field at the emitter–base junction under different biasing conditions. A second part of the paper is devoted to micro-PR analysis performed on tuneable vertical cavity surface emitting layers (VCSELs) with InP/air Bragg mirrors. In such VCSELs, both the cavity Fabry–Perot peak and the active region quantum well ground state are giving transitions in the micro-PR spectrum. This is very useful in the case of a tuneable structure. Feasibility of micro-PR analysis at the device scale is demonstrated.

Keywords: Photoreflectance spectroscopy; Micro-PR analysis; VCSELs

Ellipsometric characterization of nanocrystals in porous silicon by P. Petrik; M. Fried; É. Vázsonyi; T. Lohner; E. Horváth; O. Polgár; P. Basa; I. Bársony; J. Gyulai (pp. 200-203).

Porous silicon layers (PSLs) were prepared by electrochemical etching of p-type single-crystalline silicon (c-Si) wafers having different dopant concentrations to obtain systematically changing sizes of nanocrystals (walls). The microstructure of the porous material was characterized using spectroscopic ellipsometry with multi-layer effective medium approximation (EMA) models. The dielectric function of PSL is conventionally calculated using EMA mixtures of c-Si and voids. The porosity is described by the concentration of voids. Some PSL structures can be described only by adding fine-grained polycrystalline silicon (nc-Si) reference material to the EMA model. Modified model dielectric functions (MDF) of c-Si have been shown to fit composite materials containing nanocrystalline regions, either by fitting only the broadening parameter or also other parameters of the parametric oscillator in MDF. The broadening parameter correlates with the long-range order in the crystalline material, and, as a consequence, with the size of nanocrystals. EMA and MDF models were used to describe systematically changing nanostructure of PSLs. Volume fraction of nc-Si in EMA and broadening parameter in MDF provide information on the nanocrystal size. The longer-term goal of this work is to provide a method for the quantitative characterization of nanocrystal size using quick, sensitive and non-destructive optical techniques.

Keywords: Porous silicon layers; Effective medium approximation; Model dielectric functions

Quantitative methods for nanopowders characterization by T. Wejrzanowski; R. Pielaszek; A. Opalińska; H. Matysiak; W. �?ojkowski; K.J. Kurzydłowski (pp. 204-208).

The size, shape and surface topology have a strong influence on powders properties, such as: mechanical, optical, catalytic, etc. In addition, when particles have a nanometer size, the dispersion of these features plays an important role.There are a number of techniques, which could be used in order to characterize powders in terms of particle size and shape. However, due to the scale of analysis, well beyond the wavelength of visible light, most of them cannot be applied for investigations of nanopowders.In this paper, transmission electron microscopy (TEM) image analysis and X-ray methods are presented as promising and complementary techniques. An example of their application to ZrO₂ nanopowder is shown. The advantages and limitations of each method are described.

Keywords: Powder characterization; Image analysis; X-ray diffraction

X-ray topographic imaging of (Al, Ga)N/GaN based electronic device structures on SiC by L. Kirste; S. Müller; R. Kiefer; R. Quay; K. Köhler; N. Herres (pp. 209-213).

Structural defects and their impact on the performance, lifetime and reliability of electronic devices are of permanent interest for crystal growers and device manufacturers. This is especially true for epitaxial (Al, Ga)N/GaN based high electron mobility transistor (HEMT) structures on 4H-SiC (0001) substrates. This work points out how micropipes, dislocations and grain boundaries present in a 4H-SiC (0001) wafer and subsequently overgrown with an (Al, Ga)N–GaN-HEMT layer sequence show up in X-ray topographic images and two-dimensional XRD maps. Using X-ray topography in transmission geometry, micropipes and other structural defects are localized non-destructively below structured metallization layers with a spatial resolution of a few tens of micrometers.

Keywords: X-ray topography; X-ray diffraction; Structural defect; SiC; GaN; HEMT

GaN epilayers on nanopatterned GaN/Si(111) templates: Structural and optical characterization by L.S. Wang; S. Tripathy; B.Z. Wang; S.J. Chua (pp. 214-218).

Template-based nanoscale epitaxy has been explored to realize high-quality GaN on Si(111) substrates. We have employed polystyrene-based nanosphere lithography to form the nano-hole array patterns on GaN/Si(111) template and then, subsequent regrowth of GaN is carried out by metalorganic chemical vapor deposition (MOCVD). During the initial growth stage of GaN on such nanopatterned substrates, we have observed formation of nanoislands with hexagonal pyramid shape due to selective area epitaxy. With further epitaxial regrowth, these nanoislands coalesce and form continuous GaN film. The overgrown GaN on patterned and non-patterned regions is characterized by high-resolution X-ray diffraction (HRXRD) and high-spatial resolution optical spectroscopic methods. Micro-photoluminescence (PL), micro-Raman scattering and scanning electron microscopy (SEM) have been used to assess the microstructural and optical properties of GaN. Combined PL and Raman data analyses show improved optical quality when compared to GaN simultaneously grown on non-patterned bulk Si(111). Such thicker GaN templates would be useful to achieve III-nitride-based opto- and electronic devices integrated on Si substrates.

Keywords: PACS; 81.15.Gh; 81.16.Nd; 78.55.Cr; 68.55.JkGaN; Nanosphere lithography; X-ray diffraction; Optical spectroscopy

Real time ellipsometry for monitoring plasma-assisted epitaxial growth of GaN by Giovanni Bruno; Maria Losurdo; Maria M. Giangregorio; Pio Capezzuto; April S. Brown; Tong-Ho Kim; Soojeong Choi (pp. 219-223).

GaN is grown on Si-face 4H-SiC(0001) substrates using remote plasma-assisted methods including metalorganic chemical vapour deposition (RP-MOCVD) and molecular beam epitaxy (MBE). Real time spectroscopic ellipsometry is used for monitoring all the steps of substrate pre-treatments and the heteroepitaxial growth of GaN on SiC. Our characterization emphasis is on understanding the nucleation mechanism and the GaN growth mode, which depend on the SiC surface preparation.

Keywords: GaN; Epitaxy; Spectroscopic ellipsometry

“Anomalous�? pseudodielectric function of GaN: Experiment, modelling and application to the study of surface properties by S. Shokhovets; G. Gobsch; V. Lebedev; O. Ambacher (pp. 224-227).

Studying GaN films exposed to Ar plasma by spectroscopic ellipsometry and reflectance, we found an “anomalous�? pseudodielectric function (PDF) for which the imaginary part is significantly higher as compared to GaN, while the real part of the PDF remains close to the value for GaN. In addition, a higher reflectance at low angles of incidence was observed. The data are explained in terms of a thin highly absorbing surface layer arising due to non-stoichiometry in the near-surface region. Comparison to samples grown by molecular beam epitaxy shows that similar mechanisms are responsible for optical properties of the surfaces of films obtained under Ga-rich conditions.

Keywords: Optical properties; Non-stoichiometry; Pseudodielectric function

Structural characterisation of GaAlN/GaN HEMT heterostructures by N. Sarazin; O. Durand; M. Magis; M.-A. di Forte Poisson; J. Di Persio (pp. 228-231).

(GaN/GaAlN/GaN)//Al₂O₃(00.1) HEMT heterostructures have been studied by X-ray scattering techniques, transmission electron microscopy and atomic force microscopy. X-ray reflectometry has been used to determine with a high accuracy both the individual layer thicknesses and the interfacial roughness, in spite of the weak electronic density contrast between layers. From the Fourier inversion method and using a simulation software, the roughness of the interface corresponding to the two-dimensional electron gas location has been determined equal to 0.5nm. Both high resolution X-ray diffraction and transmission electron microscopy experiments have shown the excellent crystallinity of the heterostructures. Finally, the surface morphology has been inferred using atomic force microscopy experiments.

Keywords: X-ray reflectometry; GaAlN/GaN heterostructures; High electron mobility transistors; Thickness determination; Fourier inversion method

Structural and optical characterization of GaN heteroepitaxial films on SiC substrates by M. Morse; P. Wu; S. Choi; T.H. Kim; A.S. Brown; M. Losurdo; G. Bruno (pp. 232-235).

We have estimated the threading dislocation density and type via X-ray diffraction and Williamson–Hall analysis to elicit qualitative information directly related to the electrical and optical quality of GaN epitaxial layers grown by PAMBE on 4H- and 6H-SiC substrates. The substrate surface preparation and buffer choice, specifically: Ga flashing for SiC oxide removal, controlled nitridation of SiC, and use of AlN buffer layers all impact the resultant screw dislocation density, but do not significantly influence the edge dislocation density. We show that modification of the substrate surface strongly affects the screw dislocation density, presumably due to impact on nucleation during the initial stages of heteroepitaxy.

Keywords: GaN epitaxial layer; SiC substrate; Nitridation

Characterization of GaN layers grown on silicon-on-insulator substrates by S. Tripathy; L.S. Wang; S.J. Chua (pp. 236-240).

In this study, we report growth and characterization of GaN layers on (100)- and (111)-oriented silicon-on-insulator (SOI) substrates. Using metalorganic chemical vapor deposition (MOCVD) technique, GaN layers are grown on KOH treated Si (100) overlayers of thin SIMOX SOI substrates. Growth of GaN on such surface with an AlN buffer leads to c-axis orientated textured GaN. This is evident from high-resolution X-ray diffraction (HRXRD) measurement, which shows a much broader rocking curve linewidth. Significantly enhanced photoluminescence (PL) intensity and partial stress relaxation is observed in GaN layers grown on these SOI substrates. Furthermore, GaN grown on (111)-oriented bonded SOI substrates shows good surface morphology and improved optical quality. Micro-Raman, micro-PL, and HRXRD measurements reveal single crystalline hexagonal GaN oriented along (0001) direction. We also report growth and characterization of InGaN/GaN multi-quantum well structures on (111)-oriented bonded SOI. Such an approach to realize nitride epilayers would be useful to fabricate GaN-based micro-opto-electromechanical systems (MOEMS) and sensors.

Keywords: GaN; Silicon-on-insulator (SOI); X-ray diffraction; Optical spectroscopy

High resolution X-ray diffraction of GaN grown on Si (111) by MOVPE by N. Chaaben; T. Boufaden; A. Fouzri; M.S. Bergaoui; B. El Jani (pp. 241-245).

High temperature GaN layers have been grown on Si (111) substrate by metalorganic vapor phase epitaxy (MOVPE). AlN was used as a buffer layer and studied as a function of thickness and growth temperature. The growth was monitored by in situ laser reflectometry. High resolution X-ray diffraction (HRXRD) revealed that optimized monocrystalline GaN was obtained for a 40nm AlN grown at 1080°C. This is in good agreement with the results of morphological study by scanning electron microscopy (SEM) and also confirmed by atomic force microscopy (AFM) observations. The best morphology of AlN with columnar structure and lower rms surface roughness is greatly advantageous to the coalescence of the GaN epilayer. Symmetric and asymmetric GaN reflections were combined for twist and stress measurements in monocrystalline GaN. It was found that mosaicity and biaxial tensile stress are still high in 1.7μm GaN. Curvature radius measurement was also done and correlated to the cracks observations over the GaN surface.

Keywords: GaN; AlN buffer; HRXRD; Stress

Band structure investigations of GaN films using modulation spectroscopy by V.P. Makhniy; M.M. Slyotov; V.V. Gorley; P.P. Horley; Yu.V. Vorobiev; J. González-Hernández (pp. 246-248).

The paper presents investigation results concerning band structure of gallium nitride and position of intrinsic and associate defect levels. Main optical characteristics (transmission, reflection and luminescence) were measured in both ordinary and λ-modulation mode for epitaxy-grown GaN films, allowing to determine valence band splitting caused by spin–orbital interaction (48meV) and crystalline field (10meV). Analysis of photoluminescence spectra made it possible to identify main recombination mechanisms involving donor and acceptor levels formed by intrinsic point defectsVN,V′Ga, and their associates.

Keywords: GaN films; Modulation spectroscopy; Photoluminescence spectra

On the mixed nature of the 740cm⁻¹ band in wurtzite GaN films: A polarized Raman scattering investigation by P.C. Ricci; C.M. Carbonaro; R. Corpino; A. Anedda (pp. 249-253).

A detailed study of the polarized Raman scattering of wurtzite GaN films is presented, focusing on the nature of the band centered at 740cm⁻¹ observed in the X( Z, Z) X configuration. The origin of this band is ascribed to the mixed contribution of the A₁ and E₁ longitudinal phonon modes coupled with the free carrier excitation. The spectral profile of the 740cm⁻¹ Raman band has been successfully reconstructed through a linear combination of the A₁–E₁ longitudinal phonon plasmon-coupled modes, leading to a free carrier concentration in good agreement with Hall effect measurements.

Keywords: GaN films; Raman scattering; Polarized

Optical characterization of In_xGa1−_xN alloys by M. Gartner; C. Kruse; M. Modreanu; A. Tausendfreund; C. Roder; D. Hommel (pp. 254-257).

InGaN layers were grown by molecular beam epitaxy (MBE) either directly on (0001) sapphire substrates or on GaN-template layers deposited by metal-organic vapor-phase epitaxy (MOVPE). We combined spectroscopic ellipsometry (SE), Raman spectroscopy (RS), photoluminescence (PL) and atomic force microscopy (AFM) measurements to investigate optical properties, microstructure, vibrational and mechanical properties of the InGaN/GaN/sapphire layers.The analysis of SE data was done using a parametric dielectric function model, established by in situ and ex situ measurements. A dielectric function database, optical band gap, the microstructure and the alloy composition of the layers were derived. The variation of the InGaN band gap with the In content ( x) in the 0< x≤0.14 range was found to follow the linear law E_g=3.44–4.5 x.The purity and the stability of the GaN and InGaN crystalline phase were investigated by RS.

Keywords: InGaN; MBE; MOVPE; Spectroscopic ellipsometry; Raman spectroscopy

Modeling of laser reflectance evolution during metalorganic vapor phase epitaxy growth of GaN using SiN treatment by H. Fitouri; Z. Benzarti; I. Halidou; T. Boufaden; B. El Jani (pp. 258-260).

We present the simulation of laser reflectance measurements performed during GaN growth by metalorganic vapor phase epitaxy (MOVPE). We used the scattering theory approximation to determine the root mean square (rms) surface roughness versus growth time. In the region of large roughness, the determined rms roughness exceeds the maximum value authorized by the Rayleigh criterion limiting the validity of the macroscopic roughness model. Another approach based on the effective medium approximation is used to simulate the entire reflectance signal evolution. An effective refractive index and growth rate profiles are determined.

Keywords: Metalorganic vapor phase epitaxy; Rayleigh criterion; Ellipsometry

Structural characterization of In_xGa1−_xAs/Inp layers under different stresses by J. Bak-Misiuk; K. Orlińska; J. Kaniewski; A. Shalimov; E. Lusakowska; A. Misiuk; J. Muszalski; W. Wierzchowski; K. Wieteska; W. Graeff (pp. 261-265).

In_xGa1−_xAs layers on InP substrate can be subjected to compressive or tensile strain due to lattice parameter differences depending on the alloy composition. In order to examine in details the strain of InGaAs/InP epiatxial layers and its evolution after subjecting the layers to annealing at high pressure, X-ray synchrotron topography, high resolution X-ray diffraction and atomic force microscopy have been employed. The data show that the changes of structural properties of the InGaAs layers subjected to high temperature–high pressure treatment at 670K–1.2Gpa, strongly depend on initial strain state and defect structure. The annealing of samples under high pressure results in change of strain in tensile layers only. The behaviour of observed defects is discussed.

Keywords: X-ray diffraction; Tensile strain; High pressure

Photoreflectance and contactless electroreflectance spectroscopy of GaAs-based structures: The below band gap oscillation features by R. Kudrawiec; M. Motyka; M. Gladysiewicz; P. Sitarek; J. Misiewicz (pp. 266-270).

GaAs-based structures characterized below band gap oscillation features (OFs) in photoreflectance (PR) are studied in both PR and contactless electro-reflectance (CER) spectroscopies. It has been shown that the OFs are usually very strong for structures grown on n-type GaAs substrate. The origin of the OFs is the modulation of the refractive index in the sample due to a generation of additional carriers by the modulated pump beam. The presence of OFs in PR spectra complicates the analysis of PR signal related to quantum well transitions. Therefore, PR spectroscopy is often limited to samples grown on semi-insolating (SI) type substrates. However, sometimes the OFs could be observed for structures grown on SI-type GaAs substrates. In this paper we show that the OFs could be successfully eliminated by applying the CER technique instead of PR one because during CER measurements any additional carriers are not generated and hence CER spectra are free of OFs. This advantage of CER spectroscopy is very important in investigations of all structures for which OFs are present in PR spectra.

Keywords: Contactless electroreflectance; Photoreflectance; Quantum wells

Absorbance spectra of polycrystalline samples and twinned crystals of oligothiophenes by L. Raimondo; M. Campione; M. Laicini; M. Moret; A. Sassella; P. Spearman; S. Tavazzi (pp. 271-274).

We propose optical absorption technique at oblique incidence as one of the spectroscopic tools that allow experimentally recognizing the macroscopic order and structural features of molecular solids of conjugated molecules, from single crystals to polycrystalline or twinned samples. We apply this spectroscopy to quaterthiophene as representative of a wide class of materials that usually possess optical transitions of Frenkel exciton origin with strong directional dispersion. The comparison between experimental and simulated data gives evidence of the high sensitivity of this technique for determining quantitatively the polycrystallinity of the measured samples, whose domains may show mirror-like orientation of the unit cell with respect to one of its faces.Frenkel exciton; Oligothiophene, Optical properties

Keywords: Frenkel exciton; Oligothiophene; Optical properties

Laser reflectometry in situ monitoring of InGaAs grown by atmospheric pressure metalorganic vapour phase epitaxy by M.M. Habchi; A. Rebey; A. Fouzri; B. El Jani (pp. 275-278).

InGaAs layers on undoped GaAs (001) substrates were grown by atmospheric pressure metalorganic vapour phase epitaxy (AP-MOVPE). In order to obtain films with different indium composition ( x_In), the growth temperature as a growth parameter, was varied from 420 to 680°C. Furthermore, high-resolution X-ray diffraction (HRXRD) measurements were used to quantify the change of x_In. Crystal quality has been also studied as a function of growth conditions. On the other hand, laser reflectometry (LR) at 632.8nm wavelength, was employed to in situ monitor epitaxy. Reflectivity–time signal was enabled to evaluate structural and optical properties of samples. We have fitted experimental data to determine optical constants and growth rate of InGaAs at 632.8nm. In addition, the fitting provided InGaAs thickness as a function of growth time. Based on ex situ characterization by scanning electronic microscopy (SEM) and HRXRD, we propose a practical method, relating the contrast of first reflectivity maximum with the X-ray diffraction peak angular difference between the substrate and epitaxial layer, to determine in situ the In solid composition in InGaAs alloys.

Keywords: Laser reflectometry; InGaAs; HRXRD; AP-MOVPE

Structural and optical characterization of the propolis films by S.I. Drapak; A.P. Bakhtinov; S.V. Gavrylyuk; I.T. Drapak; Z.D. Kovalyuk (pp. 279-282).

We have performed structural and optical characterizations of the propolis (an organic entity of biological nature) films grown on various non-organic substrates. The films were grown from a propolis melt or a propolis alcohol solution. The crystal structure has been observed in the films precipitated from the solution onto substrates such as an amorphous glass and sapphire or semiconductor indium monoselenide. For any growth method, the propolis film is a semiconductor with the bandgap of 3.07eV at 300K that is confirmed by a maximum in photoluminescence spectra at 2.86eV. We argue that propolis films might be used in various optoelectronic device applications.

Keywords: Heterostructure; Morphology; Semiconductor

X-ray reflectivity study of hydrogen implanted silicon by P. Dub�?ek; B. Pivac; S. Bernstorff; F. Corni; R. Tonini; G. Ottaviani (pp. 283-286).

The X-ray reflectivity (XRR) technique was used to study monocrystalline silicon samples implanted with H₂ ions at an energy of 31keV and to the dose of 2×10¹⁶hydrogen atoms/cm². All samples were subsequently isochronally annealed in vacuum at different temperatures in the range from 100 to 900°C. Although the hydrogen depth distribution was expected to be smooth initially, fringes in the XRR spectra were observed already in the implanted but not annealed sample, revealing the presence of a well-defined film-like structure. Annealing enhances the film top to bottom interface correlation due to structural relaxation, resulting in the appearance of fringes in the larger angular range, already at low annealing temperatures. The thickness of the film decreases slowly up to 350°C where substantial changes in the roughness are observed, probably due to the onset of larger clusters formation. Further annealing at higher temperatures restores the high correlation of the film interfaces, while the thickness decreases with the temperature more rapidly.

Keywords: Silicon; Hydrogen; Ion implantation; X-ray reflectivity

An optical study of the correlation between growth kinetics and microstructure of μc-Si grown by SiH₄-H₂ PECVD by M.M. Giangregorio; M. Losurdo; A. Sacchetti; P. Capezzuto; F. Giorgis; G. Bruno (pp. 287-291).

Fully microcrystalline silicon, μc-Si, thin films have been deposited on corning glass by plasma enhanced chemical vapor deposition (PECVD) using SiH₄-H₂. The effects of the surface treatment and of the deposition temperature on microstructure of μc-Si films are investigated by “in situ�? laser reflectance interferometry (LRI), “ex situ�? spectroscopic ellipsometry (SE) and Raman spectroscopy. LRI indicated the existence of a “crystalline seeding time�?, which is indicative of the crystallite nucleation, and depends on substrate treatments. Longer “crystalline seeding time�? results in a lower density of crystalline nuclei, which grow laterally, yielding to complete suppression of the amorphous incubation layer and to growth of very dense, fully crystalline layer at a growth temperature as low as 120°C.

Keywords: μc-Si; PECVD; Ellipsometry; Raman spectroscopy

Photoluminescence study in step-graded composition In_xAl1−_xAs/GaAs by N. Yahyaoui; S. Aloulou; R. Chtourou; A. Sfaxi; M. Oueslati (pp. 292-295).

We report on the lattice-mismatched growth of step-graded In_xAl1−_xAs buffer layers on GaAs (001) substrates by molecular beam epitay (MBE). The approach to growing highly lattice-mismatched epilayers is to interpose a buffer layer between the substrate and the active layer. Two samples G30 and G40 with active layer compositions, respectively, x=0.46 and x=0.41, are studied by photoluminescence (PL). At low temperature, the PL spectra show a large broadened band whose energy and intensity depend on the active layer composition. The step-graded layer compositions improved the crystalline quality of these structures and increase the active layer PL band intensity.

Keywords: Photoluminescence; Dislocation; Lattice-mismatch; Graded composition

Evidence of polarized charge-transfer transitions by probing the weak dielectric tensor components of oligothiophene crystals by S. Tavazzi; M. Laicini; L. Raimondo; P. Spearman; A. Borghesi; A. Papagni; S. Trabattoni (pp. 296-299).

The absorption spectra of different oligothiophene single crystals without saturation effects in a wide spectral range are reported and compared. The origin of the main ac polarized broad band is discussed in terms of Frenkel excitons. Exploiting experimental configurations which correspond to the excitation of the weak dielectric tensor components, bands of different nature are also detected at different positions for orthogonal polarizations and attributed to transitions of charge-transfer character.

Keywords: Charge-transfer; Oligothiophene; Polarized

Donor–acceptor pairs and excitons recombinations in AgGaS₂ by M. Marceddu; A. Anedda; C.M. Carbonaro; D. Chiriu; R. Corpino; P.C. Ricci (pp. 300-305).

Silver thiogallate (AgGaS₂) is a ternary semiconductor which crystallizes in the chalcopyrite structure. Silver thiogallate has been widely used in different applications for its interesting physical properties: wide transparency range (from 0.5 to 12μm), high non-linear optical coefficient combined with good mechanical properties.The direct band gap in this compound is of about 2.7eV and emissions due to free and bound excitons had been observed. Photoluminescence spectrum is also characterized by a wide emission band centred at 496nm (2.50eV) due to donor–acceptor pairs recombination (DAP).We performed photoluminescence (PL) measurements exciting with the third harmonic (3.5eV) of a Nd:YAG laser from room temperature down to 10K at different excitation power.In this work, we report the dependence of the photoluminescence features of AgGaS₂ on the excitation power at various temperatures: ionization energy of defects are estimated on the basis DAP theoretical model and of thermal quenching of the photoluminescence; evidences of non-radiative processes competitive to DAP is also presented.

Keywords: Silver thiogallate; Photoluminescence properties; Donor–acceptor pairs

Optimization of annealing conditions of In₂S₃ thin films deposited by vacuum thermal evaporation by A. Timoumi; H. Bouzouita; R. Brini; M. Kanzari; B. Rezig (pp. 306-310).

In₂S₃ thin films were grown on glass substrates by means of the vacuum thermal evaporation technique and subsequently thermally annealed in nitrogen and free air atmosphere from 250 to 350°C for different durations. Experimental parameters have been adjusted in order to optimize the annealing conditions, and to obtain high band gap energy at low deposition temperature, as required for photovoltaic applications. In order to improve our understanding of the influence of the deposition and annealing parameters on device performance, we have investigated our indium sulfide material by X-ray diffraction, energy dispersive X-ray analysis (EDAX), atomic force microscopy (AFM) and spectrophotometry. The optical and structural properties of the films were studied as a function of the annealing temperature and durations. X-ray diffraction analysis shows the initial amorphous nature of deposited In–S thin films and the phase transition into crystalline In₂S₃ upon thermal annealing. Films show a good homogeneity and optical direct band gap energy about 2.2eV. An annealing temperature of 350°C during 60min in air atmosphere were the optimal conditions.

Keywords: In; ₂; S; ₃; Thermal annealing; Thin films; Structural properties; Optical study

Band edge electronic structure of transition metal/rare earth oxide dielectrics by Gerald Lucovsky (pp. 311-321).

This article addresses band edge electronic structure of transition metal/rare earth (TM/RE) non-crystalline and nano-crystalline elemental and complex oxide high- k dielectrics for advanced semiconductor devices. Experimental approaches include X-ray absorption spectroscopy (XAS) from TM, RE and oxygen core states, photoconductivity (PC), and visible/vacuum ultra-violet (UV) spectroscopic ellipsometry (SE) combined with ab initio theory is applied to small clusters. These measurements are complemented by Fourier transform infra-red absorption (FTIR), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). Two issues are highlighted: Jahn–Teller term splittings that remove d-state degeneracies of states at the bottom of the conduction band, and chemical phase separation and crystallinity in Zr and Hf silicates and ternary (Zr(Hf)O₂)_x(Si₃N₄)_y(SiO₂)1− x−_y alloys. Engineering solutions for optimization of both classes of high- k dielectric films, including limits imposed on the continued and ultimate scaling of the equivalent oxide thickness (EOT) are addressed.

Keywords: X-ray absorption spectroscopy; X-ray photoelectron spectroscopy; X-ray diffraction; Transition metal and rare earth elemental and complex oxides, Non-crystalline and nano-crystalline dielectrics

Structural–optical study of high-dielectric-constant oxide films by M. Losurdo; M.M. Giangregorio; M. Luchena; P. Capezzuto; G. Bruno; R.G. Toro; G. Malandrino; I.L. Fragalà; R. Lo Nigro (pp. 322-327).

High- k polycrystalline Pr₂O₃ and amorphous LaAlO₃ oxide thin films deposited on Si(001) are studied. The microstructure is investigated using X-ray diffraction and scanning electron microscopy. Optical properties are determined in the 0.75–6.5eV photon energy range using spectroscopic ellipsometry. The polycrystalline Pr₂O₃ films have an optical gap of 3.86eV and a dielectric constant of 16–26, which increases with film thickness. Similarly, very thin amorphous LaAlO₃ films have the optical gap of 5.8eV, and a dielectric constant below 14 which also increases with film thickness. The lower dielectric constant compared to crystalline material is an intrinsic characteristic of amorphous films.

Keywords: Pr; ₂; O; ₃; LaAlO; ₃; Spectroscopic ellipsometry; Optical properties; Thin films

Investigation of thermal annealing effects on microstructural and optical properties of HfO₂ thin films by M. Modreanu; J. Sancho-Parramon; O. Durand; B. Servet; M. Stchakovsky; C. Eypert; C. Naudin; A. Knowles; F. Bridou; M.-F. Ravet (pp. 328-334).

In the present paper, we investigate the effect of thermal annealing on optical and microstructural properties of HfO₂ thin films (from 20 to 190nm) obtained by plasma ion assisted deposition (PIAD). After deposition, the HfO₂ films were annealed in N₂ ambient for 3h at 300, 350, 450, 500 and 750°C. Several characterisation techniques including X-ray reflectometry (XRR), X-ray diffraction (XRD), spectroscopic ellipsometry (SE), UV Raman and FTIR were used for the physical characterisation of the as-deposited and annealed HfO₂ thin films. The results indicate that as-deposited PIAD HfO₂ films are mainly amorphous and a transition to a crystalline phase occurs at a temperature higher than 450°C depending on the layer thickness. The crystalline grains consist of cubic and monoclinic phases already classified in literature but this work provides the first evidence of amorphous-cubic phase transition at a temperature as low as 500°C. According to SE, XRR and FTIR results, an increase in the interfacial layer thickness can be observed only for high temperature annealing. The SE results show that the amorphous phase of HfO₂ (in 20nm thick samples) has an optical bandgap of 5.51eV. Following its transition to a crystalline phase upon annealing at 750°C, the optical bandgap increases to 5.85eV.

Keywords: PACS; 77.55.+f; 78.20.-e; 78.20.Ci; 78.30.-j; 61.10.Kw; 61.10.NzPlasma ion assisted deposition; HfO; ₂; Solid phase crystallization; Optical properties

High- k Mg-doped ZST for microwave applications by A. Ioachim; M.G. Banciu; M.I. Toacsen; L. Nedelcu; D. Ghetu; H.V. Alexandru; C. Berbecaru; A. Dutu; G. Stoica (pp. 335-338).

The (Zr_0.8Sn_0.2)TiO₄ material (ZST), has been prepared by solid state reaction and characterized. The samples were sintered in the temperature range of 1260–1320°C for 2h. The effects of sintering parameters like sintering temperature ( T_s) and MgO addition (0.2wt.%) on structural and dielectric properties were investigated. Bulk density increases from 4900 to 5050kg/m³ with the increase of sintering temperature. The effect of MgO addition is to lower the sintering temperature in order to obtain well sintered samples with high value of bulk density. The material exhibits a dielectric constant ɛ_r∼37 and high values of the Q× f product, greater than 45,000, at microwave frequencies. The dielectric properties make the ZST material very attractive for microwave applications such as dielectric resonators, filters, dielectric antennas, substrates for hybrid microwave integrated circuits, etc.

Keywords: High-; k; dielectrics; Mg-doped ZST; Dielectric resonators; Microwaves

Nanostructure characterization of high k materials by spectroscopic ellipsometry by L. Pereira; H. �?guas; E. Fortunato; R. Martins (pp. 339-343).

In this work, the optical and structural properties of high k materials such as tantalum oxide and titanium oxide were studied by spectroscopic ellipsometry, where a Tauc–Lorentz dispersion model based in one (amorphous films) or two oscillators (microcrystalline films) was used. The samples were deposited at room temperature by radio frequency magnetron sputtering and then annealed at temperatures from 100 to 500°C. Concerning the tantalum oxide films, the increase of the annealing temperature, up to 500°C does not change the amorphous nature of the films, increasing, however, their density. The same does not happen with the titanium oxide films that are microcrystalline, even when deposited at room temperature. Data concerning the use of a four-layer model based on one and two Tauc–Lorentz dispersions is also discussed, emphasizing its use for the detection of an amorphous incubation layer, normally present on microcrystalline films grown by sputtering.

Keywords: Tantalum oxide; Titanium oxide; Spectroscopic ellipsometry

Optical characterization and microstructure of BaTiO₃ thin films obtained by RF-magnetron sputtering by A. Ianculescu; M. Gartner; B. Despax; V. Bley; Th. Lebey; R. Gavrilă; M. Modreanu (pp. 344-348).

BaTiO₃ thin films were deposited on Pt/Ti/SiO₂/Si by rf planar-magnetron sputtering. The films thickness increases with the decrease of both deposition pressure and sample-discharge centre distance. The films annealed at 900°C, for 8h, present direct band gap energy ranged between 3.57 and 3.59eV. The dependence of the structure and microstructure (texture, degree of crystallinity), as well as of the optical characteristics on the deposition parameters, was analysed. Using spectroscopic ellipsometry (SE) measurements coupled with the Bruggeman Effective Medium Approximation (B-EMA), the layer structure and the surface roughness, were determined. The root mean square roughness values of the surface layer, estimated by atomic force microscopy (AFM) analyses, are ranged between 10 and 20nm and were in good agreement with SE data.The obtained films have tetragonal unit cell and show densely packed, non-columnar morphology and hexagon-like crystallite shape.

Keywords: Rf-sputtering; BaTiO; ₃; film; Ellipsometry; Optical properties; Microstructure

Optical and X-ray characterization of ferroelectric strontium–bismuth–tantalate (SBT) thin films by M. Fried; P. Petrik; Z.E. Horváth; T. Lohner; C. Schmidt; C. Schneider; H. Ryssel (pp. 349-353).

Metal-organic chemical vapor deposition (MOCVD) made layers of strontium–bismuth–tantalate (SBT) were characterized by spectroscopic ellipsometry (SE) using the Adachi model [S. Adachi, Phys. Rev. B 35 (1987) 7454–7463]. The evaluated optical parameters were correlated with the physical and chemical behavior examined by X-ray diffraction (XRD).As a result, it was possible to fit the measured spectra with the Adachi model in a wide range covering the region of the band gap. The Adachi model provides electronic layer parameters like the transition energy E₀ and broadening Γ. Our investigations established a correlation between XRD-determined average grain size and the electronic layer parameters.

Keywords: Spectroscopic ellipsometry; X-ray diffraction; Grain size; Ferroelectric materials

BST solid solutions, temperature evolution of the ferroelectric transitions by H.V. Alexandru; C. Berbecaru; A. Ioachim; L. Nedelcu; A. Dutu (pp. 354-357).

Solid solutions Ba1−_xSr_xTiO₃ (BST) are of high technological importance, particularly in microwave domain. Barium titanate has “naturally�? three transitions, between four stable ferroelectric phases: (C) cubic, (T) tetragonal, (O) orthorhombic, (R) rhombohedral. Jaffe et al . [B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, 1971] has given the dependence of the transition temperatures up to 30% of Sr content. We have extrapolated these temperatures and we have found that some phases might disappear at higher Sr concentrations. A family of solid solutions with x=25, 50, 75, 90% was prepared by standard solid-state reaction and sintered at 1230 and 1260°C, respectively. The permitivities and the dielectric losses were measured with a self-acting bridge (1kHz), on a large temperature range (±200°C). The composition x=25% shows three peak values of permittivity as expected, while the composition x=50%, only two peak values, corresponding to phase transitions cubic–tetragonal–rhombohedral, phase orthorhombic being excluded. Compositions with x≥63%, Sr shows only one peak value corresponding to a genuine transition cubic–rhombohedral. The cubic transition to several lower phases shows almost a linear decrease of the Curie point with the increase of Sr fraction. For Sr concentration x≥80%, the Curie point appears to fall more rapidly than linear. To our best knowledge, there is for the first time, this effect is reported.

Keywords: Ferroelectric transitions; Barium strontium titanate solid solutions

Pyroelectric coefficient manipulation in doped TGS crystals by H.V. Alexandru; C. Berbecaru; L. Ion; A. Dutu; F. Ion; L. Pintilie; R.C. Radulescu (pp. 358-362).

Pure andl-alanine doped Triglycine sulphate (TGS) crystals were grown in paraelectric phase (∼52°C). Doped crystals show unequal growth rates along the ferroelectric axis. Pure TGS crystals show peculiar dielectric behavior in the ferroelectric phase, after crossing up and down the Curie point in two successive runs between room temperature and 80°C. Much higher and unstable permittivity was found returning in the ferroelectric phase. At constant temperature (35°C), permittivity follows a relaxation process, characterized by two relaxation times.l-Alanine doped TGS crystal shows more than one order of magnitude smaller permittivity and dielectric losses. Internal bias field of ∼1kV/cm, induced by the dopant, made the crystal almost monodomain and pined polarization in one direction. Pyroelectric coefficient measurements were performed at constant heating rate of the samples, using a computer controlled He cryostat and Keithley 6517 electrometer. The temperature dependence of P⁺ polarization component, obtained by computer integration of the pyroelectric coefficient, was measured on a large temperature interval (−20/+80°C). Pyroelectric coefficient of the doped samples was also measured by the same procedure, using a dc bias electric field, pointing in the opposite direction to the pined polarization. The polarization could be reversed, on the whole temperature range, by dc fields higher than bias or coercive field. Surprisingly, for the first time, the pyroelectric coefficient ( p) was found constant on quite large temperature intervals. Doped TGS crystals show much smaller values of permittivity ɛ_r versus the pure one and consequently, get higher figure of merit M= p/ ɛ_r. The pyroelectric coefficient of this material can be tailored to become constant on a defined temperature range, under a dc field control. This characteristic makes this material valuable to be used as pyroelectric material for IR devices.

Keywords: Ferroelectrics; Triglycine sulphate crystals; Alanine dopage; Pyroelectric coefficient control

Density, thickness and composition measurements of TiO₂SiO₂ thin films by coupling X-ray reflectometry, ellipsometry and electron probe microanalysis-X by A. Hodroj; H. Roussel; A. Crisci; F. Robaut; U. Gottlieb; J.L. Deschanvres (pp. 363-366).

Mixed TiO₂SiO₂ thin films were deposited by aerosol atmospheric CVD method by using di-acetoxi di-butoxi silane (DADBS) and Ti tetra-butoxide as precursors. By varying the deposition temperatures between 470 and 600°C and the ratios between the Si and Ti precursors (Si/Ti) from 2 up to 16, films with different compositions and thicknesses were deposited. The coupled analysis of the results of different characterisation methods was used in order to determine the variation of the composition, the thickness and the density of the films. First EPMA measurements were performed at different acceleration voltages with a Cameca SX50 system. By analysing, with specific software, the evolution of the intensity ratio I_x/ I_std versus the voltage, the composition and the mass thickness (product of density by the thickness) were determined. In order to measure independently the density, X-ray reflectometry experiments were performed. By analysing the value of the critical angle and the Kiessig fringes, the density and the thickness of the layers were determined. The refractive index and the thickness of the films were also measured by ellipsometry. By assuming a linear interpolation between the index value of the pure SiO₂ and TiO₂ films, the film composition was deduced from the refractive index value. XPS measurements were also performed in order to obtain an independent value of the composition. A good agreement between the ways to measure the density is obtained.

Keywords: TiO; ₂; –SiO; ₂; Ellipsometry; XPS measurements; X-ray reflectometry; EPMA

Stabilization of the anatase phase in TiO₂(Fe³⁺, PEG) nanostructured coatings by C. Trapalis; M. Gartner; M. Modreanu; G. Kordas; M. Anastasescu; R. Scurtu; M. Zaharescu (pp. 367-371).

Multilayered TiO₂(Fe³⁺, PEG) films were deposited on glass and SiO₂/glass substrates by sol–gel dipping method. The influence of Fe³⁺ and PEG(polyethylene glycol) concentrations, the number of layers, the thermal treatment time and the temperature on the optical and microstructural properties of the TiO₂ films were studied.As-deposited TiO₂(Fe³⁺, PEG) films were very porous, but after the thermal treatment at 500°C, the PEG decomposed and burned out to porosity decreasing. Homogeneous nanostructured films were obtained, where the amorphous and the anatase phases coexist. XRD analysis showed that no rutile phase is observed in the films deposited on SiO₂/glass as compared with those deposited directly on glass and that the presence of the anatase phase in the films without PEG is more evident in the three-layers film. The XRD intensity of the main peak of anatase from 25° decreases with the increase of PEG concentration.The optical gap of the TiO₂(Fe³⁺, PEG) films is found in 2.52–2.56eV range and does not essentially depend on the PEG content.

Keywords: TiO; ₂; (Fe; ³⁺; , PEG) films; Sol–gel method; SEM; XRD; Spectroscopic ellipsometry

Characterization of oxide thin films using optical techniques by J.H. Hao; J. Gao (pp. 372-375).

Thin films of oxide materials are playing a growing role as critical elements in optoelectronic devices and nanoscale devices. In this work, thin films of some typical oxides such as WO₃, Ga₂O₃ and SrTiO₃ were investigated. We present measurements of those films, using various optical techniques like photoconductivity transients over a wide time range and photo-Hall measurements. Analysis of the photo-Hall and photoconductivity data permits the determination of the contribution to the photoconductivity made by the carrier mobility and concentration. A model for dispersive carrier transport was proposed to explain the relaxation of the photoconductivity in oxide thin films. In addition, photoluminescence characterization was used to study microstructures and energy band in oxide thin films. The broad emission from oxide host, consisting of several band peaks, was likely due to a recombination process with several possible paths. The dependence of the luminescent intensity on the annealing atmosphere was associated with the presence of oxygen vacancies. It is suggested that our optical analysis efforts have improved the understanding of oxide thin films, and this should lead to the necessary advancements in a variety of devices.

Keywords: PACS; 78.66−W; 07.60−jThin films; Oxides; Optical analysis; Microstructure

Effects of UV photon irradiation on SiO_x (0< x<2) structural properties by Nicolae Tomozeiu (pp. 376-380).

Thin films of a-SiO_x (0< x<2) were prepared by reactive r.f. magnetron sputtering from a polycrystalline-silicon target in an Ar/O₂ gas mixture. The oxygen partial pressure in the deposition chamber was varied so as to obtain films with different values of x. The plasma was monitored, during depositions, by optical emission spectroscopy (OES) system. Energy dispersive X-ray (EDX) measurements and infra-red (IR) spectroscopy were used to study the compositional and structural properties of the deposited layers.Structural modifications of SiO_x thin films have been induced by UV photons’ bombardment (wavelength of 248nm) using a pulsed laser. IR spectroscopy and X-ray photoemission spectroscopy (XPS) were used to investigate the structural changes as a function of x value and incident energy. SiO_x phase separation by spinodal decomposition was revealed. The IR peak position shifted towards high wavenumber values when the laser energy is increased. Values corresponding to the SiO₂ material (only Si⁴⁺) have been found for laser irradiated samples, independently on the original x value. The phase separation process has a threshold energy that is in agreement with theoretical values calculated for the dissociation energy of the investigated material.For high values of the laser energy, crystalline silicon embedded in oxygen-rich silicon oxide was revealed by Raman spectroscopy.

Keywords: Optical emission spectroscopy (OES); X-ray photoemission spectroscopy (XPS); R.f. magnetron sputtering

An XPS study on ion beam induced oxidation of titanium silicide by P. Osiceanu (pp. 381-384).

Titanium silicides (TiSi₂) films grown on Si(100) substrate were investigated by ex situ XPS depth profiling after athermal ion beam induced oxidation (IBO) at 12keV O₂⁺ incident energy and normal incidence. The composition and stoichiometry of these films were quantitatively determined as chemical state relative concentrations versus sputter time. “In depth�? silicon and titanium oxidation states have been obtained after spectra deconvolution, showing a mixture of silicon dioxide, titanium dioxide, titanium suboxides, elemental titanium and residual traces of titanium nitride. Thermochemical data based on the corresponding enthalpies of formation of the oxides cannot explain our experimental results as in the case of low energy IBO, an oxygen defective altered layer is formed, presenting features of a reduced TiO_x phase.

Keywords: Titanium silicide; Ion beam oxidation; XPS depth profiling; Oxidized altered layer; “In depth�? chemical states; Titanium reactivity

Characterization of Si nanocrystals into SiO₂ matrix by C. Gravalidis; S. Logothetidis; N. Hatziaras; A. Laskarakis; I. Tsiaoussis; N. Frangis (pp. 385-388).

Silicon nanocrystals (nc-Si) have gained great interest due to their excellent optical and electronic properties and their applications in optoelectronics. The aim of this work is the study of growth mechanism of nc-Si into a-SiO₂ matrix from SiO/SiO₂ multilayer annealing, using non-destructive and destructive techniques. The multilayer were grown by e-beam evaporation from SiO and SiO₂ materials and annealing at temperatures up to 1100°C in N₂ atmosphere. X-rays reflectivity (XRR) and high resolution transmission electron microscopy (HRTEM) were used for the structural characterization and spectroscopic ellipsometry in IR (FTIRSE) energy region for the study of the bonding structure. The ellipsometric results gave a clear evidence of the formation of an a-SiO₂ matrix after the annealing process. The XRR data showed that the density is being increased in the range from 25 to 1100°C. Finally, the HRTEM characterization proved the formation of nc-Si. Using the above results, we describe the growth mechanism of nc-Si into SiO₂ matrix under N₂ atmosphere.

Keywords: Si nanocrystals; X-rays reflectivity; High resolution transmission electron microscopy; Optical density

Application of spectroscopic ellipsometry to the investigation of the optical properties of cobalt-nanostructured silica thin layers by M. Gilliot; A. En Naciri; L. Johann; C. d’Orleans; D. Muller; J.P. Stoquert; J.J. Grob (pp. 389-394).

Spectroscopic ellipsometry is used to investigate optical properties of cobalt-implanted silica thin films. The films under investigation are 250nm thick thermal SiO₂ layers on Si substrates implanted with Co⁺ ions at energy of 160keV and at fluences of 10¹⁷ions/cm² for different temperatures of substrate during implantation (77 and 295K). Changes due to Co⁺ implantation are clearly observed in the optical response of the films. Optical behaviours are furthermore different for the three implantation temperatures. To understand the optical responses of these layers, the ellipsometric experimental data are compared to different models including interference effects and metal inclusions effects into the dielectric layer. The simulated ellipsometric data are obtained by calculating the interferences of an inhomogeneous layer on a Si substrate. The material within this layer is considered as an effective medium which dielectric function is calculated using the Maxwell-Garnett effective medium approximation. We show that although the structures of these layers are very complicated because of ion-implantation mechanisms, quite simple models can provide relatively good agreement. The possibilities of ellipsometry for the study of the optical properties of such clusters-embedded films are discussed. We especially provide the evidence that ellipsometry can give interesting information about the optical properties of nanostructured layers. This is of special interest in the field of nanostructured layered systems where ellipsometry appears to be a suitable optical characterization technique.

Keywords: Spectroscopic ellipsometry; Cobalt; SiO; ₂

Transformation of hydrogen silsesquioxane properties with RIE plasma treatment for advanced multiple-gate MOSFETs by J. Penaud; F. Fruleux; E. Dubois (pp. 395-399).

Keeping in line with Moore's law requires increasing efforts in the development of alternative electronic devices. Multiple-gate transistors are very promising in order to suppress short-channel effects and to increase the current drive. Nevertheless, the fabrication of such devices represents a strong challenge for silicon process technology. One of the key steps of the process consists in shrouding the silicon fins in an isolating matrix, using a flowable oxide, namely hydrogen silsesquioxane (HSQ). The general objective of this work is to show that the properties of HSQ can be modified by appropriate thermal or plasma treatments to modulate its characteristics in terms of etching selectivity and surface topography. SEM characterization has shown that HSQ exhibits excellent planarization and gap fill capabilities, while AFM analysis, on 100nm thick HSQ films deposited by spin on, reveals a roughness as low as 3nm. Various oxygen plasma treatments have been applied to densify the HSQ films. Fourier transform infra-red spectroscopy (FT-IR) has shown very interesting qualitative and quantitative informations. Chemical and physical transformations from a Si–O–Si cage-like structure into an Si–O–Si network one have been observed. It is shown that exposure to oxygen plasma at high power (290W) for a long time (20min) or thermal curing at high temperature improves the resistance to wet etching using 1% hydrofluoric acid (HF). This densification technique holds the remarkable property to transform HSQ into an SiO₂-like structure.

Keywords: Hydrogen silsesquioxane; Multiple-gate transistor

Spectroscopic and X-ray diffraction study of high T_c epitaxial YBCO thin films obtained by pulsed laser deposition by M. Branescu; A. Vailionis; M. Gartner; M. Anastasescu (pp. 400-404).

We report spectroscopic characterization of epitaxial YBCO thin films grown on LaAlO₃ by pulsed laser deposition. Raman spectroscopy and spectroscopic ellipsometry were used for film characterization and the results were correlated with X-ray diffraction measurements. The mentioned techniques allowed us to analyze crystallographic, micro-structural, and morphological properties of YBCO thin films. We also demonstrated that relatively low resolution Raman spectroscopy and spectroscopic ellipsometry are reliable techniques for a rapid and non-destructive characterization of epitaxial YBCO thin films.

Keywords: YBCO films; Low resolution Raman spectroscopy; Ellipsometry; X-ray diffraction

Optical emission spectroscopy during fabrication of indium-tin-oxynitride films by RF-sputtering by M. Koufaki; M. Sifakis; E. Iliopoulos; N. Pelekanos; M. Modreanu; V. Cimalla; G. Ecke; E. Aperathitis (pp. 405-408).

Indium-tin-oxide (ITO) and indium-tin-oxynitride (ITON) films have been deposited on glass by rf-sputtering from an ITO target, using Ar plasma and N₂ plasma, respectively, and different rf-power. Optical emission spectroscopy (OES) was employed to identify the species present in the plasma and to correlate them with the properties of the ITO and ITON thin films. Emission lines of ionic In could only be detected in N₂ plasma, whereas in the Ar plasma additional lines corresponding to atomic In and InO, were detected. The deposition rate of thin films was correlated with the In species, rather than the nitrogen species, emission intensity in the plasma. The higher resistivity and lower carrier concentration of the ITON films, as compared to the respective properties of the ITO films, were attributed to the incorporation of nitrogen, instead of oxygen, in the ITON structure.

Keywords: Optical emission spectroscopy (OES); Indium-tin-oxide (ITO); Oxynitrides; Rf-sputtering

Sections

Personal tools

Applied Surface Science (v.253, #1)

Sections

Personal tools

Local resources

Applied Surface Science (v.253, #1)