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Characterization of ZnO structures by optical and X-ray methods
Abstract
ZnO thin films doped by Ga and In as well as multilayer structures of ZnO/Al2O3 have been investigated by X-ray fluorescence, Raman spectrometry, spectroscopic ellipsometry and vacuum ultra violet reflectometry. Systematic changes in the optical properties have been revealed even for Ga concentrations below 1%. The Raman active phonon mode of Ga doping at 580 cm−1 shows a correlation with the Ga concentration. Optical models with surface nanoroughness correction and different parameterizations of the dielectric function have been investigated. There was a good agreement between the dielectric functions determined by the Herzinger–Johs polynomial parameterization and by direct inversion. It has been shown that the correction of the nanoroughness significantly influences the accuracy of the determination of the layer properties. The band gap and peak amplitude of the imaginary part of the dielectric function corresponding to the excitonic transition changes systematically with the Ga-content and with annealing even for low concentrations.
... This study is part of a European project, intended to be a step towards the establishment of validated reference methodologies for a reliable characterization of key optoelectronic materials (IND07, "Metrology for the manufacturing of thin films") in the European Metrology Research Program of EURAMET. Furthermore, our investigations aim for the development of reference samples with controlled defect concentration and morphology or methods for elemental depth profiling [9]. ...
... The ellipsometric measurements were used to calculate the film thicknesses of the different layers, because using the reflectometric principle it is possible to calculate only one sample parameter (refractive index, extinction coefficient or thickness). The transparent and opaque photon energy ranges can clearly be distinguished by the interference oscillations characteristic to the transparent range [9] in the ellipsometric measurement. Hence, a simple Cauchy dispersion (n=A+B/λ 2 +C/λ 4 , where n denotes the refractive index, and A, B and C are the Cauchy parameters) using an optical model of c-Si/SiO 2 /ZnO, whereas the refractive index of ZnO can be described by the Cauchy model, was used to calculate the layer thickness and surface nanoroughness from the ellipsometric data. ...
ZnO has a huge potential and is already a crucial material in a range of key technologies from photovoltaics to opto and printed electronics. ZnO is being characterized by versatile metrologies to reveal electrical, optical, structural and other parameters with the aim of process optimization for best device performance. The aim of the present work is to reveal the capabilities of vacuum ultra-violet (VUV) reflectometry for the characterization of ZnO films of nominally 50 nm, doped by Ga and In. Optical metrologies have already shown to be able to sensitively measure the gap energy, the exciton strength, the density, the surface nanoroughness and a range of technologically important structural and material parameters. It has also been shown that these optical properties closely correlate with the most important electrical properties like the carrier density and hence the specific resistance of the film. We show that VUV reflectometry is a highly sensitive optical method that is capable of the characterization of crucial film properties. Our results have been cross-checked by reference methods such as ellipsometry and X-ray fluorescence.
We report the manufacturing of thin zinc oxide films by reactive magnetron sputtering at room temperature, and examine their structural and optical properties. We show that the partial oxygen pressure in DC mode can have dramatic effect on absorption and refractive index (RI) of the films in a broad spectral range. In particular, the change of the oxygen pressure from 7% to 5% can lead to either conventional crystalline ZnO films having low absorption and characteristic descending dependence of RI from 2.4-2.7 RIU in the visible to 1.8-2 RIU in the near-infrared (1600 nm) range, or to untypical films, composed of ZnO nano-crystals embedded into amorphous matrix, exhibiting unexpectedly high absorption in the visible-infrared region and ascending dependence of RI with values varying from 1.5 RIU in the visible to 4 RIU in the IR (1600 nm), respectively. Untypical optical characteristics in the second case are explained by defects in ZnO structure arising due to under-oxidation of ZnO crystals. We also show that the observed defect-related film structure remains stable even after annealing of films under relatively high temperatures (30 min under 450 C). We assume that both types of films can be of importance for photovoltaic (as contact or active layers, respectively), as well as for chemical or biological sensing, optoelectronics etc.
One-component semiconductor photocatalysts have low photocatalytic life because of the recombination of photo generated electrons and holes. Through compositing of two components can significantly improve the photocatalytic efficiency. In this paper, the water bath heating method has been used to synthesise alumina doped zinc oxide nanoparticles supported on hydroxyapatite (ZAO/HAP) nanocomposite materials. The ZAO/HAP nanocomposite materials have better photocatalytic efficiency than ZAO nanoparticles. The morphologies of HAP have significant impact on photocatalytic efficiency of ZAO/HAP composite catalysts.
Being one of the most sensitive methods for optical thin film metrology ellipsometry is widely used for the characterization of zinc oxide (ZnO), a key material for optoelectronics, photovoltaics, and printable electronics and in a range of critical applications. The dielectric function of ZnO has a special feature around the band gap dominated by a relatively sharp absorption feature and an excitonic peak. In this work we summarize and compare direct (point-by-point) and parametric approaches for the description of the dielectric function. We also investigate how the choice of the wavelength range influences the result, the fit quality and the sensitivity. Results on ZnO layers prepared by sputtering are presented.
Optical techniques have been intensively developed for many decades in terms of both experimental and modeling capabilities. In spectroscopy and scatterometry material structures can be measured and modeled from the atomic (binding configurations, electronic band structure) through nanometer (nanocrystals, long range order) to micron scales (photonic structures, gratings, critical dimension measurements). Using optical techniques, atomic scale structures, morphology, crystallinity, doping and a range of other properties that can be related to the changes of the electronic band structure can most sensitively be measured for materials having interband transition energies in the optical photon energy range. This will be demonstrated by different models for the dielectric function of ZnO, a key material in optoelectronics and in numerous other fields. Using polarimetry such as spectroscopic ellipsometry, sub-nanometer precision has long been revealed for the thickness of optical quality layers. The lateral resolution of spectroscopic ellipsometry is limited (> 50 μm) by the use of incoherent light sources, but using single-wavelength imaging ellipsometry, a sub-micron lateral resolution can be reached. In case of sub-wavelength structures, the morphology (of e.g. porous or nanocrystalline materials) can be characterized using the effective medium theory. For structure sizes comparable to the wavelength, scatterometry is applied in a broad versatility of configurations from specular to angle resolved, from coherent to incoherent, from monochromatic to spectroscopic, from reectometric to polarimetric. In this work, we also present an application of coherent Fourier scatterometry for the characterization of periodic lateral structures.
Layers of ZnO nanoparticles with thicknesses of about 40 nm were prepared on Si substrates. It was shown that UV laser irradiation is suitable for consolidation and significant densification of the ZnO particle layers under ambient conditions. Both experiments and simulations show that an underlying SiO2 particle layer has a beneficial effect in inhibiting heat transfer towards the substrate and thus enables the application of temperature-sensitive carrier substrates like polymer foils despite the extremely high melting temperature of ZnO.
The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by
Bunn [Proc. Phys. Soc. London 47, 836 (1935)
], studies of its vibrational properties with Raman scattering in 1966 by
Damen et al. [Phys. Rev. 142, 570 (1966)
], detailed optical studies in 1954 by
Mollwo [Z. Angew. Phys. 6, 257 (1954)
], and its growth by chemical-vapor transport in 1970 by
Galli and Coker [Appl. Phys. Lett. 16, 439 (1970)
]. In terms of devices, Au Schottky barriers in 1965 by
Mead [Phys. Lett. 18, 218 (1965)
], demonstration of light-emitting diodes (1967) by
Drapak [Semiconductors 2, 624 (1968)
], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by
Minami et al. [Jpn. J. Appl. Phys. 13, 1475 (1974)
], ZnO/ZnSe n-p junctions (1975) by
Tsurkan et al. [Semiconductors 6, 1183 (1975)
], and Al/Au Ohmic contacts by
Brillson [J. Vac. Sci. Technol. 15, 1378 (1978)
] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by
Look and Claflin [Phys. Status Solidi B 241, 624 (2004)
]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures.
Photoluminescent, undoped ZnO films have been fabricated using spray pyrolysis of zinc nitrate solution. The luminescent films had a polycrystalline hexagonal wurtzite type structure with no preferred orientation. Photoluminescence intensity was critically dependent on substrate temperature during spray pyrolysis and on post-annealing temperature. Green, photoluminescent films possessed a porous structure while orange films possessed a close packed granular morphology. Green luminescence appears to be due to oxygen vacancies in a layer just below the crystallite surface. © 1998 American Institute of Physics.
A modified Deal Grove model for the oxidation of 4H-SiC is presented, which includes the removal of the carbon species. The model is applied to data on the oxidation rates for the (0001) Si, (000) C, and (110) a faces, which are performed in 1 atm dry oxygen and in the temperature range 950–1150 °C. Analysis within the model provides a physical explanation for the large crystal-face dependent oxidation rates observed. © 2004 American Institute of Physics.
Thin films of ZnO have been deposited on glass and silicon substrates by the pulsed laser deposition technique employing a KrF laser (λ=248 nm). The influence of the deposition parameters, such as substrate temperature, oxygen pressure, and laser fluence on the properties of the grown films, has been studied. All the films grown over a rather wide range of deposition conditions were found to be optically transparent, electrically conductive, and c‐axis oriented, with the full width at half‐maximum (FWHM) of the (002) x‐ray reflection line being very often less than 0.25°. Under optimized laser fluence and oxygen pressure conditions, highly c‐axis oriented films having a FWHM value less than 0.15° and optical transmittance around 85% in the visible region of the spectrum have been grown at a substrate temperature of only 350 °C. These are among the best properties yet reported for ZnO films grown by any technique at such a low temperature. © 1994 American Institute of Physics.
We fabricated ZnO thin-film transistors by rf magnetron sputtering on Si substrates held near room temperature. The best devices had field-effect mobility of more than 2 cm2/V s and an on/off ratio ≫106. These ZnO films had resistivity ∼105 ohm cm , with high optical transparency (≫80% for wavelength ≫400 nm), and compressive stress ≪0.5 GPa. The combination of transparency in the visible, excellent transistor characteristics, and low-temperature processing makes ZnO thin-film transistors attractive for flexible electronics on temperature sensitive substrates. © 2003 American Institute of Physics.
Spectral ellipsometry at 300 K, in the range 0.8-5.5 eV, has been used
to study the bulk and surface oxide properties of a
molecular-beam-epitaxy grown Zn0.53Cd0.47Se/InP
film (~1 μm thick). We have observed the direct gap E0,
which exhibits a well-defined excitonic structure, its spin-orbit split
component E0+Δ0, as well as the spin-orbit
split E1, E1+Δ1 doublet. The
experimental data over the entire measured spectral range (after oxide
removal) have been fit using a model dielectric function based on the
electronic energy-band structure near critical points plus excitonic and
Coulomb enhancement effects. The influence of a native oxide on the
optical properties also was investigated.
ZnO thin films were grown on silicon (100) by pulsed laser deposition. Highly textured crystalline ZnO thin films can be grown at 600 °C. The films were then annealed at 600 °C in oxygen. The effects of annealing on chemical composition of the ZnO films were investigated by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XPS spectra indicate that water has been adsorbed and then dissociated into H and OH groups. The surface properties of ZnO were studied both by scanning tunneling microscopy and scanning tunneling spectroscopy (STS). A narrow potential well has been formed on the surface of the ZnO thin films due to high density of surface states and negatively biasing the ZnO thin films during STS measurement. The discrete energy levels can be measured by STS.
Amorphous In-Ga-Zn-O (a-IGZO) is expected as a backplane transistor material to drive next-generation flat-panel and flexible displays. It has been elucidated that thermal annealing even at low temperatures <200 °C reduces deep subgap defects and those at ≥300 °C further improve device characteristics, stability, and uniformity. These temperatures are much lower than the reported crystallization temperature (TX ∼ 600 °C). In this work, we investigate effects of thermal annealing on the structural and optical properties of a-IGZO thin films. We performed classical molecular dynamics simulation (CMD) and optical interference analyses including spectroscopic ellipsometry (SE). CMD reproduced the x-ray diffraction pattern of a-IGZO and exhibited a glass transition. Experimentally, it was found that TX depends largely on deposition methods and conditions, probably due to different chemical compositions. Sputter-deposited a-IGZO films exhibited onset TX ∼ 600 °C and crystalline volume fraction XC increased linearly from 600 °C. 1.2% of film densification occurred even at <TX, and crystallization caused larger densification, which is consistent with the film density measured by x-ray reflectivity spectroscopy. Bandgap increased in two temperature regions; i.e., (i) at <400 °C due to structural relaxation and (ii) at >600 °C due to crystallization. High-temperature in situ SE measurements did not detect a symptom of a glass transition temperature (Tg) presumably because the TX is close to Tg similar to the case of amorphous metals.
We have investigated the effects of nitrogen annealing on the structural, electrical, and optical properties of Al-doped ZnO (ZnO:Al) thin films deposited by RF magnetron sputtering at room temperature. From the X-ray diffraction observations, films have highly crystalline structure with a c-axis preferred orientation. The most improvements in the electrical and optical properties of the ZnO:Al films were obtained by nitrogen annealing. The ZnO:Al films exhibited an average optical transmittance of 90–95% in the visible range and a sharp fundamental absorption edge. Spectroscopic ellipsometry (SE) was used to extract the optical constants of thin films. The optical characteristics of ZnO:Al films were modeled using a Tauc–Lorentz based dielectric function. The bandgap energy increased with the increases in nitrogen annealing temperature, which change in accordance with the Burstein–Moss effect, and was consistent with the observed changes in the transport properties.
Transparent conductive Al-doped ZnO (AZO) thin films with various thicknesses between 520 and 1420 nm were deposited on quartz substrates by radio frequency (RF) magnetron sputtering at room temperature for thin film solar cells as transparent conductive oxide (TCO) electrode layers. After deposition, the samples were annealed in a vacuum ambient at temperatures between 250 and 550 °C for a period of 30 min. The structural, electrical, and optical properties of these films have been analyzed as a function of the thickness and the annealing temperature by a series of characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Hall effect measurements and spectrophotometry. All of these samples exhibited strong (002) diffraction peaks and the visible range transmittance was over 80%. In addition, with the increase of thickness, the Hall mobility increased from 4.88 to 7.86 cm2/V, the resistivity decreased from 1.2 × 10−2 Ω cm to 4.2 × 10−3 Ω cm. Annealing in vacuum improved the crystallinity together with some changes of the electrical resistance that depended on the annealing temperature. The best characteristics have been obtained at 450 °C, where the lowest resistivity was 2.7 × 10−3 Ω cm for the thickest films.
We fabricated high-performance ZnO thin-film transistors on gate dielectrics of HfO2, HfSiOx, and Al2O3, grown by atomic layer deposition (ALD). Devices on HfO2 had a mobility of 12.2 cm2∕V s with a threshold voltage of 2.6 V and subthreshold slope of 0.5 V∕decade. Device performance on Al2O3 depended on synthesis temperature. For 100 nm thick Al2O3, synthesized at 200 °C, ZnO devices had a mobility of 17.6 cm2∕V s with a threshold voltage of 6 V and less than ∼0.1 nA gate leakage at 20 V. The overall trends were that devices on Hf oxides had a lower threshold voltage, while the gate leakage current density was lower on Al2O3. Device characteristics for all ALD dielectrics exhibited negligibly small hysteresis, suggesting a low defect density at the interface of ZnO with the gate dielectric.
The authors undertook to compile a database of recent values of the atomic parameters required for fundamental parameters (FP) calculation of X-ray fluorescence (XRF) spectra, calculation of X-ray absorption in crystals and other samples, and correction of X-ray absorption spectra for self-absorption effects. All values were obtained from published sources and include the elements hydrogen (atomic number 1) through californium (atomic number 98). The data were collected into a single unstructured ASCII text file.
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Al doped ZnO (ZAO) thin films (with Al-doping levels 2 at.%) were deposited at different deposition parameters on silicon substrate by reactive magnetron sputtering for solar cell contacts, and samples were investigated by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS) and spectroscopic ellipsometry (SE). Specific resistances were measured by the well known 4-pin method. Well visible columnar structure and in most cases voided other regions were observed at the grain boundaries by TEM. EELS measurements were carried out to characterize the grain boundaries, and the results show spacing voids between columnar grains at samples with high specific resistance, while no spacing voids were observed at highly conductive samples. SE measurements were evaluated by using the analytical expression suggested by Yoshikawa and Adachi [H. Yoshikawa, S. Adachi, Japanese Journal of Applied Physics 36 (1997) 6237], and the results show correlation between specific resistance and band gap energy and direct exciton strength parameter.
The development of an optical constant library for Hg1−xCdxTe as a function of composition (x=0–0.5) and temperature (T=0–250°C) which is suitable for precise composition control by spectroscopic ellipsometry (SE) during MBE growth is described. An efficient methodology for acquiring in situ optical constants as a function of composition and temperature is first presented. Optical constants extracted from these in situ measurements, as well as literature data from room temperature values, were used to obtain internally Kramers-Kronig consistent parametric optical constant models at discrete compositions and temperatures. Then a global data analysis over temperature `T' and composition `x' was performed in which the internal parameters of the optical constant model were fitted as polynomials in T and x. This parametric model was developed to replace, without compromising the quality of ellipsometric data fits, the usual tabulated optical constant lists while using a reasonably small set of adjustable parameters. The model is flexible enough to describe the complicated critical point structures of semiconductors, yet stable enough to generate optical constants as a function of composition and temperature and permit limited extrapolation outside the measured range.
Aluminum doped Zinc Oxide (AZO) and Lithium doped Zinc Oxide (LZO) thin films are obtained by Pulsed Laser Deposition (PLD) method. These films are characterized by using Spectroscopic Ellipsometry (SE), X-ray Diffraction (XRD) and Photoluminescence (PL). By modeling the ellipsometry spectra we get the dielectric functions, the optical band gap E(g), and the electrical properties. Our results show the influence of the processing parameters on the optical and structural properties of doped ZnO thin films. The post-annealing treatment applied to AZO thin films, changes strongly the optical properties, by lowering the resistivity and red-shifting the band gap. (C) 2009 Elsevier B.V. All rights reserved.
Extremely uniform, microscopically smooth, large-grained polycrystalline Si films saturation doped with phosphorous to a carrier concentration n=3.3×1020 cm-3 were prepared by a combination of low-pressure chemical-vapor-deposition and heat-treating processes to obtain samples suitable for measuring the small effects of large impurity concentrations on the above-band-gap dielectric properties of Si. From 1.5 to 2.5 eV, the effects of heavy doping are described by the Drude free-carrier model with lifetimes determined by scattering from lattice vibrations. Above 3 eV, the dielectric function spectrum of heavily doped Si is remarkably similar to that of undoped material except that the E1 and E2 structures are broadened and shifted to lower energies. The impurity-induced broadening is much larger for E1 than for E2, indicating that a metastable excitonic resonance is well developed for the former but not for the latter. Present and previous data of the authors, together with previous data of Viña and Cardona, are consistent with an approximately linear shift of the threshold energy with impurity concentration. This indicates that the dominant mechanism responsible for the shifts is the effect of the random impurities on the crystal potential. While the possibility of electron-electron and electron-ion interactions on the electron and hole self-energies cannot be ruled out, comparison with recent calculations of Berggren and Sernelius show that these effects must be much smaller at higher interband transitions than at the fundamental indirect band gap. Using our data as a reference, we show that the anomalous absorption observed by Jellison and co-workers for As-implanted and laser-annealed c-Si is due to microscopically rough surface layers on their samples. Consequently, the differences that they observe between P and As dopants must involve differences in recrystallization and regrowth. Dielectric-function spectra for amorphous Si films doped in situ with phosphorus are essentially identical within microstructural effects to those of undoped material, indicating no significant influence from heavy doping. At energies below 3.2 eV, the data expressed as (αnE)1/3 or as (ε2)1/2 both vary nearly linearly with energy, suggesting nondirect transitions between simple parabolic bands with momentum matrix elements linearly proportional to E or with dipole elements independent of E.
The optical functions of uniaxial ZnO have been determined using two-modulator generalized ellipsometry, where a single measurement is sufficient to determine the optical functions from appropriately aligned uniaxial crystals. Above the direct band edge (∼3.3eV), this technique produces the most accurate values of the optical functions of ZnO presently available, while the refractive indices determined below the direct band edge agree with minimum-deviation methods. Near the direct band edge, the optical functions are modified by the excitonic interaction with a three-dimensional critical point. The optical dielectric response functions are fit to a recent formulation by Holden et al. [Phys. Rev. B 56, 4037 (1997)]. One isotropic point in the spectrum was observed at 3.114 eV, and a near-isotropic point near 3.31–3.34 eV.
ZnO layers doped simultaneously with Ga and N (codoping), and magnetic elements (V, Co) were characterized by Raman scattering to study their structural stability. Five impurity modes were observed in range 200–900 cm−1 in the doped samples, and showed characteristic variation with the doping level. It is shown that these modes can be used as a good measure of lattice defects induced by doping. The Raman spectra showed that the magnetic elements were incorporated up to 5 mol% without serious deterioration in crystallinity.
Efficiency and response functions were determined for a modern commercial energy-dispersive x-ray spectrometer in the photon energy range 0.1–5 keV using dispersed and undispersed synchrotron radiation. The spectrometer was equipped with an Si(Li) detector crystal, a thin-film window and a digital pulse processor. Monochromatized synchrotron radiation from the PTB VUV radiometry beamline at the electron storage ring BESSY was used for the absolute determination of the detection efficiency with a typical relative uncertainty of 1–2% in the photon energy range 0.1–1.5 keV by direct comparison with calibrated photodiodes. At higher photon energies, the efficiency was found by comparison of the measured and calculated undispersed synchrotron radiation spectrum. The absolute intensity of the synchrotron radiation was known with a relative uncertainty of less than 1%. In the overlapping region, the results from the two independent experiments are in full agreement. The energy dependence of the measured efficiency can be explained only with a detector model assuming that there is no dead layer. A simple model for the effect of incomplete charge collection (ICC) was applied to describe the measured response functions. Consequences of the ICC such as broadening and shift of low-energy peaks and redistribution of counts around 1.84 keV are explained with the model, in accordance with the experimental results. Copyright © 2001 John Wiley & Sons, Ltd.
After a systematic analysis of different databases and a comparison with empirical data, we decided to describe photoelectric absorption coefficients, coherent and incoherent scattering coefficients and mass attenuation coefficients in the energy range from 1 to 300 keV for elements from Z = 1 to 94 by a modified version of Scofield's tables in combination with scattering coefficients from Elam et al. Modifications include check and correction of edge positions, description of M4- and M5-ranges of elements from promethium upwards by sawtooth responses and reduction of data by introduction of least-squares fits of fifth order to Scofield's and Elam et al.'s numerical data. Our database was developed especially for application in fundamental parameter programs for quantitative x-ray analysis (XRFA, EPMA, XPS and TEY). Copyright © 2003 John Wiley & Sons, Ltd.
An all‐reflector spectrometer is used to measure the refractive indices of the cubic crystals ZnS, GaP, SrTiO 3 , and Y 3 Al 5 O 12 ; the tetragonal crystals rutile, CaWO 4 , SrMoO 4 , CaMoO 4 ; the hexagonal crystals LiTaO 3 , CdSe, ZnO, and AlPO 4 ; and the monoclinic crystal ZnWO 4 . Most of these are measured in the wavelength range 0.4 to 4 μ.
A parameterization of the optical functions of amorphous semiconductors and insulators is presented in which the imaginary part of the dielectric function ϵ 2 is determined by multiplying the Tauc joint density of states by the ϵ 2 obtained from the Lorentz oscillator model. The real part of the dielectric function ϵ 1 is calculated from ϵ 2 using Kramers–Kronig integration. The parameters of this model are fit to n and k data for amorphous Si (2 data sets), SiO, As 2 S 3 , and Si 3 N 4 . Comparative fits are made with a similar parameterization presented earlier by Forouhi and Bloomer [Phys. Rev. B 34, 7018 (1986)]. In all cases, the new parameterization fits the data better. © 1996 American Institute of Physics.
Quantitative measurements of X-rays require absolutely calibrated detectors. In the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage ring BESSY II in Berlin, X-ray detectors can be calibrated with low uncertainties. Two complementary approaches are realized for energy-dispersive detectors. One approach uses the storage ring as primary source standard and the detector response to the calculable spectral photon flux is measured in the undispersed synchrotron radiation. This requires the operation of the storage ring with only a few stored electrons. The other approach is the comparison to a primary detector standard, mainly a cryogenic electrical substitution radiometer. Here, monochromatized radiation of very high spectral purity has to be used which is available at different beamlines in the laboratory in the photon energy range from the UV up to about 120 keV. This method can also be applied for the calibration of non-energy-dispersive detectors. With monochromatized radiation, the response function of energy-dispersive detectors is recorded and compared to Monte Carlo simulations. A photon-energy-dependent parameterization of the response function allows deconvoluting the experimental spectra. The responsivity of Si photodiodes and the detection efficiency of Si(Li), HPGe and CdTe detectors are determined with low uncertainties in a broad spectral range.
Monochromatized synchrotron radiation of the electron storage ring BESSY II has been used for the non-destructive thickness determination of nanolayered materials by two different methods. The aim of these investigations was the comparison of completely reference-free fundamental parameter-based X-ray fluorescence analysis with X-ray reflectometry to validate the quantification of X-ray fluorescence analysis as an absolute method. For this purpose, Cu and Ni layers with a thickness varying between 5 nm and 50 nm as well as double layers of both metals deposited on Si have been studied. In X-ray reflectometry characterization experiments, the tunability of the photon energy allows the determination of not only the total layer thickness but also the individual layer thicknesses of the Cu/Ni double-layer systems. Reference-free X-ray fluorescence analysis involves both the fundamental parameter approach and the knowledge of all relevant experimental parameters obtained by instrumentation calibrated absolutely.The layer thickness determined by both methods agreed within their combined uncertainties. In view of the limits of X-ray reflectometry for very thin layers, laterally inhomogeneous samples, and multi-elemental layer compositions, reference-free X-ray fluorescence analysis offers the potential for the thickness determination of such samples.
A four-crystal monochromator beamline has been installed by the Physikalisch-Technische Bundesanstalt at a bending magnet of the electron storage ring BESSY II. The monochromatic radiation with very high spectral purity and high spectral resolution in the photon energy range from 1.75 to 10 keV is used to calibrate detectors by comparison to a cryogenic electrical substitution radiometer as primary detector standard with relative uncertainties well below 1%. This is one order of magnitude better than all calibrations of non-energy-dispersive detectors in this spectral range previously performed. The beamline is also used for the characterization of optical components.
The complex dielectric functions, epsilon(E) = epsilon(1)(E) + i epsilon(2)(E), of ZnO have been measured by spectroscopic ellipsometry (SE) in the photon-energy range between 1.5 and 5.0 eV at room temperature. The SE measurements are carried out on the surface parallel to the optic axis c, which allows the determination of the optical constants for light polarized perpendicular (E perpendicular to c) and parallel to the c-axis (E parallel to c). The measured SE spectra show the exciton peaks at similar to 3.4 eV (E-0 edge). These E(E) spectra are analyzed on the basis of a simplified model of the interband transitions. Excellent agreement is achieved between the calculated and experimental results over the entire range of photon energies. Dielectric-function-related optical constants, such as the complex refractive index n*(E) = n(E) + ik(E), absorption coefficient alpha(E) and normal-incidence reflectivity R(E), of ZnO have also been reported.
Atomic layer deposition (ALD) which has emerged as an important technique for depositing thin films for a variety of applications has been reported. The necessity for continuous and pinhole-free films in semiconductor devices has driven the advancement of ALD. ALD is able to meet the needs for atomic layer control and conformal deposition using sequential, self-limiting surface reactions. The ALD of Al2O3 has developed as a model ALD system. ALD processing is also extendible to very large substrates and to parallel processing of multiple substrates. ALD is a gas phase method based on sequential, selflimiting surface reactions. ALD can deposit very conformal and ultrathin films on substrates with very high aspect ratios. ALD on high aspect ratio structures was then considered including an examination of the times required for conformal growth on high aspect ratio structures. The number of applications for ALD also continues to grow outside of the semiconductor arena.
Total reflection X-ray fluorescence (TXRF) analysis is a well-established method to monitor lowest level contamination on semiconductor surfaces. Even light elements on a wafer surface can be excited effectively when using high-flux synchrotron radiation in the soft X-ray range. To meet current industrial requirements in nondestructive semiconductor analysis, the Physikalisch-Technische Bundesanstalt (PTB) operates dedicated instrumentation for analyzing light element contamination on wafer pieces as well as on 200- and 300-mm silicon wafer surfaces. This instrumentation is also suited for grazing incidence X-ray fluorescence analysis and conventional energy-dispersive X-ray fluorescence analysis of buried and surface nanolayered structures, respectively. The most prominent features are a high-vacuum load-lock combined with an equipment front end module and a UHV irradiation chamber with an electrostatic chuck mounted on an eight-axis manipulator. Here, the entire surface of a 200- or a 300-mm wafer can be scanned by monochromatized radiation provided by the plane grating monochromator beamline for undulator radiation in the PTB laboratory at the electron storage ring BESSY II. This beamline provides high spectral purity and high photon flux in the range of 0.078-1.86 keV. In addition, absolutely calibrated photodiodes and Si(Li) detectors are used to monitor the exciting radiant power respectively the fluorescence radiation. Furthermore, the footprint of the excitation radiation at the wafer surface is well-known due to beam profile recordings by a CCD during special operation conditions at BESSY II that allow for drastically reduced electron beam currents. Thus, all the requirements of completely reference-free quantitation of TXRF analysis are fulfilled and are to be presented in the present work. The perspectives to arrange for reference-free quantitation using X-ray tube-based, table-top TXRF analysis are also addressed.
- G Jellison
- L A Boatner
G. Jellison, Jr., L. A. Boatner, Physical Review B 58 (1998) 3586.
College of Engineering, A. Science, Handbook of Thin Film Materials, College of Engineering and Applied Science
- T Lichtenstein
- U Rochester
T. Lichtenstein, U. of Rochester. College of Engineering, A. Science,
Handbook of Thin Film Materials, College of Engineering and Applied
Science, University of Rochester, 1979.
- V Craciun
- J Elders
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