Vacuum Ultraviolet Spectroscopic Ellipsometry Research Articles

Comprehensive characterization of low-damaged GaN surface exposed to NH3 plasma toward plasma-induced metalorganic chemical vapor deposition

To examine the application of plasma-induced metalorganic chemical vapor deposition (MOCVD) toward the epitaxial growth of III-V nitride semiconductors, a GaN surface exposed to NH3 plasma at 0.4 kPa from a novel plasma source was comprehensively characterized using physical and chemical techniques. The exposure time (10–40 min) dependence at 700 °C and substrate temperature (200–700 °C) dependence for 20-min exposure were investigated using various methods, such as photoluminescence (PL), vacuum ultra-violet (VUV) spectroscopic ellipsometry, transmission electron microscopy (TEM), atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and current–voltage (I–V) characteristics. The PL intensity decreased linearly with the increase in exposure time and was maintained at ∼ 60% intensity after an exposure of 10 min. Compared to that of the as-grown GaN, the dielectric function (DF) of the exposed GaN was slightly modulated. For temperature dependence, the PL intensity decreased linearly with temperature and remained at ∼ 80% intensity at 700 °C. The I–V characteristics showed ∼ 50% decrease in conductivity for an exposure time of 20 min at 700 °C. TEM and XPS revealed extremely low modifications. In case of long-term exposure, hexagonal pits and particle-like nanostructures appeared at the surface. Based on the TEM results, the pit is attributed to threading dislocations, and the nanostructure is not really a particle but a protrusion with a GaN crystal structure. From this study, it can be concluded that, compared to the XPS and TEM techniques, optical techniques such as PL measurement and VUV-SE have higher sensitivity for detecting the effect of NH3 plasma exposure on the GaN surface. The comprehensive characterizations revealed that no significant damage is induced by the plasma exposure in the time corresponding to the usual growth time of an InGaN quantum well, that is, several minutes.

Optical spectra and interfacial band offsets of pulse-laser-deposited metal-oxides: SnO2, TiO2, and ZnO

Transparent conducting oxides are electrically conductive materials with high optical transmittance in the visible region of the spectrum and are useful in a wide range of applications. In this study, the optical spectra of a set of single-phase transparent conducting oxides TiO2, ZnO, and SnO2 grown by pulse laser deposition are measured by vacuum ultraviolet spectroscopic ellipsometry and the optical bandgaps are determined to be 3.30 ± 0.05 eV, 3.13 ± 0.05 eV, and 3.95 ± 0.05 eV, respectively. Differences between these values and previous measurements are discussed. SnO2 and ZnO optical responses at the bandgap reveal that they are a direct bandgap, while TiO2 appears to show an indirect type. For the interfacial electronic characteristics, the internal photoemission measurement shows that the electronic barriers of these naturally n-type-doped metal oxides adjacent to an Al2O3 layer originate from the Fermi level in their conduction bands. The band offset determination shows that the barrier heights are similar and have a small internal field dependence. The work functions are then estimated from the measured barrier heights.

Investigation of Cody–Lorentz and Tauc–Lorentz Models in Characterizing Dielectric Function of (HfO2) x (ZrO2)1–x Mixed Thin Film

In the present study, Tauc–Lorentz and Urbach tail assisted Cody–Lorentz models were intuitively derived, explained, and employed to simulate the imaginary dielectric function of the atomic layer deposited (HfO2)x(ZrO2)1–x in the energy range of 0.7–9.3 eV. A fitting procedure is carried out using the Levenberg–Marquardt method for minimizing mean square errors. Both models support the experimental data derived by vacuum ultraviolet spectroscopic ellipsometry. For a zirconium content of 60 and 75%, a weak absorption profile is observed in the visible region and elaborately explained by the Lorentz oscillator model. In order to calculate the band gap of different compositions, Cody plots and Tauc plots are presented. The results show that the Cody–Lorentz model is more accuracy due to its exponential absorption below the band gap energy and modified density of states.

Atomic layer deposition of aluminum fluoride using Al(CH3)3 and SF6 plasma

Metal fluorides typically have a low refractive index and a very high transparency and find many applications in optical and optoelectronic devices. Nearly stoichiometric, high-purity AlF3 films were deposited by atomic layer deposition (ALD) using trimethylaluminum [Al(CH3)3] and SF6 plasma. Self-limiting growth was confirmed and the growth per cycle was determined to range from 1.50 Å to 0.55 Å for deposition temperatures between 50 °C and 300 °C. In addition, the film density of ∼2.8 g cm−3 was found to be relatively close to the bulk value of 3.1 g cm−3. Vacuum ultraviolet spectroscopic ellipsometry measurements over the wavelength range of 140–2275 nm showed a refractive index n of 1.35 at 633 nm, and an extinction coefficient k of <10−4 above 300 nm, for all deposition temperatures. Optical emission spectroscopy during the SF6 plasma exposure step of the ALD cycle revealed the formation of C2H2 and CF2 species, resulting from the interaction of the plasma with the surface after Al(CH3)3 exposure. On the basis of these results, a reaction mechanism is proposed in which F radicals from the SF6 plasma participate in the surface reactions. Overall, this work demonstrates that SF6 plasma is a promising co-reactant for ALD of metal fluorides, providing an alternative to co-reactants such as metal fluorides, HF, or HF-pyridine.

Atomic-layer deposited thulium oxide as a passivation layer on germanium

A comprehensive study of atomic-layer deposited thulium oxide (Tm2O3) on germanium has been conducted using x-ray photoelectron spectroscopy (XPS), vacuum ultra-violet variable angle spectroscopic ellipsometry, high-resolution transmission electron microscopy (HRTEM), and electron energy-loss spectroscopy. The valence band offset is found to be 3.05 ± 0.2 eV for Tm2O3/p-Ge from the Tm 4d centroid and Ge 3p3/2 charge-corrected XPS core-level spectra taken at different sputtering times of a single bulk thulium oxide sample. A negligible downward band bending of ∼0.12 eV is observed during progressive differential charging of Tm 4d peaks. The optical band gap is estimated from the absorption edge and found to be 5.77 eV with an apparent Urbach tail signifying band gap tailing at ∼5.3 eV. The latter has been correlated to HRTEM and electron diffraction results corroborating the polycrystalline nature of the Tm2O3 films. The Tm2O3/Ge interface is found to be rather atomically abrupt with sub-nanometer thickness. In addition, the band line-up of reference GeO2/n-Ge stacks obtained by thermal oxidation has been discussed and derived. The observed low reactivity of thulium oxide on germanium as well as the high effective barriers for holes (∼3 eV) and electrons (∼2 eV) identify Tm2O3 as a strong contender for interfacial layer engineering in future generations of scaled high-κ gate stacks on Ge.

Broadband optical properties of large-area monolayer CVD molybdenum disulfide

Recently emerging large-area single-layer ${\mathrm{MoS}}_{2}$ grown by chemical vapor deposition has triggered great interest due to its exciting potential for applications in advanced electronic and optoelectronic devices. Unlike gapless graphene, ${\mathrm{MoS}}_{2}$ has an intrinsic band gap in the visible which crosses over from an indirect to a direct gap when reduced to a single atomic layer. In this paper, we report a comprehensive study of fundamental optical properties of ${\mathrm{MoS}}_{2}$ revealed by optical spectroscopy of Raman, photoluminescence, and vacuum ultraviolet spectroscopic ellipsometry. A band gap of 1.42 eV is determined by the absorption threshold of bulk ${\mathrm{MoS}}_{2}$ that shifts to 1.83 eV in monolayer ${\mathrm{MoS}}_{2}$. We extracted the high precision dielectric function up to 9.0 eV, which leads to the identification of many unique interband transitions at high symmetry points in the ${\mathrm{MoS}}_{2}$ momentum space. The positions of the so-called A and B excitons in single layers are found to shift upwards in energy compared with those of the bulk form and have smaller separation because of the decreased interactions between the layers. A very strong optical critical point predicted to correspond to a quasiparticle gap is observed at 2.86 eV, which is attributed to optical transitions along the parallel bands between the $M$ and $\ensuremath{\Gamma}$ points in the reduced Brillouin zone. The absence of the bulk ${\mathrm{MoS}}_{2}$ spin-orbit interaction peak at \ensuremath{\sim}3.0 eV in monolayer ${\mathrm{MoS}}_{2}$ is, as predicted, the consequence of the coalescence of nearby excitons. A higher energy optical transition at 3.98 eV, commonly occurring in bulk semiconductors, is associated with a combination of several critical points. Additionally, extending into the vacuum ultraviolet energy spectrum are a series of newly observed oscillations representing optical transitions from valence bands to higher conduction bands of the monolayer ${\mathrm{MoS}}_{2}$ complex band structure. These optical transitions herein reported enhance our understanding of monolayer ${\mathrm{MoS}}_{2}$ as well as of two-dimensional systems in general and thus provide informative guidelines for ${\mathrm{MoS}}_{2}$ optical device designs and theoretical considerations.

Dielectric functions of solution-processed GdAlO(x)/Si films measured with vacuum ultra-violet spectroscopic ellipsometry.

The dielectric functions of amorphous GdAlO(x) (GAO) films grown by the sol-gel process were investigated from 1.12 to 8.5 eV as a function of annealing temperature using spectroscopic ellipsometry (SE). A GAO precursor sol with a molar ratio of Gd:Al = 1:1 was prepared. Thin layers were formed by spin-coating on p-type Si substrates. The layers were sintered at 400 degrees C for 2 h in an ambient atmosphere, then rapid-thermal-annealed (RTA) at 700 or 800 degrees C for 1 min in an N2 ambient. The optical properties were measured via variable angle SE, at room temperature. The angle of incidence was varied from 50 to 70 degrees in 10 degrees steps. The dielectric functions of the resulting GAO films were obtained from the measured pseudodielectric functions by multilayer-structure calculations using the Tauc-Lorentz (TL) dispersion relation. The real and imaginary parts of the dielectric functions were found to increase with increasing RTA temperature. The film thicknesses and TL parameters (threshold energy E(g) and broadening C) decrease with increasing RTA temperature.

Ge interface engineering using ultra-thin La2O3 and Y2O3 films: A study into the effect of deposition temperature

A study into the optimal deposition temperature for ultra-thin La2O3/Ge and Y2O3/Ge gate stacks has been conducted in this paper with the aim to tailor the interfacial layer for effective passivation of the Ge interface. A detailed comparison between the two lanthanide oxides (La2O3 and Y2O3) in terms of band line-up, interfacial features, and reactivity to Ge using medium energy ion scattering, vacuum ultra-violet variable angle spectroscopic ellipsometry (VUV-VASE), X-ray photoelectron spectroscopy, and X-ray diffraction is shown. La2O3 has been found to be more reactive to Ge than Y2O3, forming LaGeOx and a Ge sub-oxide at the interface for all deposition temperature studied, in the range from 44 °C to 400 °C. In contrast, Y2O3/Ge deposited at 400 °C allows for an ultra-thin GeO2 layer at the interface, which can be eliminated during annealing at temperatures higher than 525 °C leaving a pristine YGeOx/Ge interface. The Y2O3/Ge gate stack deposited at lower temperature shows a sub-band gap absorption feature fitted to an Urbach tail of energy 1.1 eV. The latter correlates to a sub-stoichiometric germanium oxide layer at the interface. The optical band gap for the Y2O3/Ge stacks has been estimated to be 5.7 ± 0.1 eV from Tauc-Lorentz modelling of VUV-VASE experimental data. For the optimal deposition temperature (400 °C), the Y2O3/Ge stack exhibits a higher conduction band offset (>2.3 eV) than the La2O3/Ge (∼2 eV), has a larger band gap (by about 0.3 eV), a germanium sub-oxide free interface, and leakage current (∼10−7 A/cm2 at 1 V) five orders of magnitude lower than the respective La2O3/Ge stack. Our study strongly points to the superiority of the Y2O3/Ge system for germanium interface engineering to achieve high performance Ge Complementary Metal Oxide Semiconductor technology.

Optical Properties and van der Waals-London Dispersion Interactions in Inorganic and Biomolecular Assemblies

ABSTRACTThe optical properties and electronic structure of AlPO4, SiO2, Type I collagen, and DNA were examined to gain insight into the van der Waals-London dispersion behavior of these materials. Interband optical properties of AlPO4 and SiO2 were derived from vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and showed a strong dependence on the crystals’ constituent tetrahedral units, with strong implications for the role of phosphate groups in biological materials. The UV-Vis decadic molar absorption of four DNA oligonucleotides was measured, and showed a strong dependence on composition and stacking sequence. A film of Type I collagen was studied using spectroscopic ellipsometry, and showed a characteristic shoulder in the fundamental absorption edge at 6.05 eV. Ab initio calculations based on density functional theory corroborated the experimental results and provided further insights into the electronic structures, interband transitions and vdW-Ld interaction potentials for these materials.

Infrared to vacuum-ultraviolet ellipsometry and optical Hall-effect study of free-charge carrier parameters in Mg-doped InN

Infrared to vacuum-ultraviolet spectroscopic ellipsometry and far-infrared optical Hall-effect measurements are applied to conclude on successful p-type doping of InN films. A representative set of In-polar Mg-doped InN films with Mg concentrations ranging from 1.2×1016 cm−3 to 3.9×1021 cm−3 is investigated. The data are compared and discussed in dependence of the Mg concentration. Differences between n-type and p-type conducting samples are identified and explained. p-type conductivity in the Mg concentration range between 1.1×1018 cm−3 and 2.9×1019 cm−3 is indicated by the appearance of a dip structure in the infrared spectral region related to a loss in reflectivity of p-polarized light as a consequence of reduced LO phonon plasmon coupling, by vanishing free-charge carrier induced birefringence in the optical Hall-effect measurements, and by a sudden change in phonon-plasmon broadening behavior despite continuous change in the Mg concentration. By modeling the near-infrared-to-vacuum-ultraviolet ellipsometry data, information about layer thickness, electronic interband transitions, as well as surface roughness is extracted in dependence of the Mg concentration. A parameterized model that accounts for the phonon-plasmon coupling is applied for the infrared spectral range to determine the free-charge carrier concentration and mobility parameters in the doped bulk InN layer as well as the GaN template and undoped InN buffer layer. The optical Hall-effect best-match model parameters are consistent with those obtained from infrared ellipsometry analysis.

Parametric modeling of the dielectric function and identification of the critical point of a CdMgTe alloy in the vacuum ultraviolet spectral range

We report the parameters necessary to construct the dielectric functions of Cd1−xMgxTe ternary alloys at room temperature for arbitrary compositions from x = 0 to x = 0.5. The experimental spectra were measured by using vacuum ultraviolet spectroscopic ellipsometry in the spectral range from 0.7 to 9.0 eV. By performing a band structure calculation with the linear augmented Slater-type orbital method, we newly identify the four higher band gaps as E2 + Δ2, E2(Δ), E2(Σ), and E′1 transitions.

Identification of electrically active defects in thin dielectric films by spectroscopic ellipsometry

A study of resonant photo-absorption features in high-k dielectric film stacks using vacuum ultraviolet spectroscopic ellipsometry demonstrates that all optically observable dielectric-related defects are located in the interfacial SiO2 layer rather than in the bulk high-k film. The defects, located at 2.9 eV, 3.6 eV, 3.9 eV, and 4.75 eV within the bandgap of this bottom interface, are found to be strongly affected by processing conditions. These results are supported by both electrical and physical characterization measurements that identify a consistent trend in the evolution of charge trapping defects for samples subjected to identical processing conditions. In addition, evidence is provided correlating the optically active 2.9 eV defect to positively charged oxygen vacancies in the bottom interfacial layer that have recently been proposed as contributing to the flatband voltage roll-off phenomenon. The close connection between these results and both ab initio calculations and experimental findings substantiate the use of spectroscopic ellipsometry as a unique characterization method for identifying process-induced defects during development and fabrication of dielectric film stacks.

Soft X-ray photoemission study of nitrogen diffusion in TiN/HfO:N gate stacks

The impact of HfO:N post nitridation anneal (PNA) and gate fabrication on the physico-chemical properties of the TiN/HfO:N/SiO2/Si stack are investigated using Soft X-ray Photoelectron Spectroscopy (S-XPS) and Vacuum UltraViolet Spectroscopic Ellipsometry (VUV-SE). Defects created in the high-k during plasma nitridation are passivated by PNA under O2. Both oxygen and nitrogen diffusion is observed towards the bottom SiO2/Si interface together with a regrowth of the SiO2. These defects play a major role regarding nitrogen diffusion during gate fabrication. Without PNA, no diffusion is observed because O and N atoms are trapped inside the high-k. With PNA and simultaneous defects passivation, nitrogen from both metal gate and high-k diffuses towards the bottom SiO2/Si interface.

Vacuum ultra-violet spectroscopic ellipsometry study of sputtered BeZnO thin films

This paper reports on a systematic investigation of the optical properties of BeZnO thin films fabricated by radio frequency reactive magnetron sputtering technique using vacuum ultraviolet spectroscopic ellipsometry (VUV-SE). The thicknesses and optical constants of the thin films were determined in the wavelength range 138–1650 nm, using VUV-SE through the Tauc–Lorentz and Gaussian models. Refractive indices and extinction coefficients of the thin films were determined to be in the range n = 1.58–1.99 and κ = 1.0 × 10 −27–0.37, respectively. The absorption coefficient and the optical bandgap energy were then calculated. Measurement of the polarized optical properties reveals a high transmissivity (>90%) and very low absorptivity (<4%) for BeZnO films in the visible and near infrared regions at different angles of incidence. From the angle dependence of the p-polarized reflectivity we deduced a Brewster angle of about 58.5°.

Ellipsometric study of single-crystal γ-InSe from 1.5 to 9.2 eV

We report the component E⃑⊥ĉ of the pseudodielectric-function tensor ⟨ε(E)⟩=⟨ε1(E)⟩+i⟨ε2(E)⟩ of γ-phase single-crystal InSe, obtained from 1.5 to 9.2 eV by vacuum-ultraviolet spectroscopic ellipsometry with the sample at room temperature. Overlayer artifacts were reduced as far as possible by measuring fresh surfaces prepared by cleavage. Accurate critical-point energies of observed structures were obtained by a combined method of spectral analysis.

Vacuum Ultra Violet Spectroscopic Ellipsometry를 이용한 BaSm2Ti4O12의 광 특성 연구

본 연구에서는 분광타원분석법을 이용하여 최근 주목 받고 있는 microwave dielectric materials 인 <TEX>$BaSm_2Ti_4O_{12}$</TEX> 박막의 광 특성을 <TEX>$0.92{\sim}8.6\;eV$</TEX> 에너지 영역에서 분석하였다. 광역 에너지영역에서 측정이 가능한 Vacuum Ultra Violet spectroscopic ellipsometer를 사용하여 시료의 광 스펙트럼을 측정 하였으며, 측정된 스펙트럼으로부터 <TEX>$BaSm_2Ti_4O_{12}$</TEX> 박막의 광 특성을 얻기 위하여 Tauc-Lorentz 분산 함수를 이용하였고, 고 에너지 영역대의 새로운 피크구조 (structure) 를 최초로 발견하였다. We performed a study on optical properties of <TEX>$BaSm_2Ti_4O_{12}$</TEX> thin films by vacuum ultra violet spectroscopic ellipsometry in the <TEX>$0.92{\sim}8.6\;eV$</TEX> energy range. For the analysis of the measured ellipsometric spectra, a 5-layer model was applied where optical property of the <TEX>$BaSm_2Ti_4O_{12}$</TEX> layer was well represented by a Tauc-Lorentz dispersion function. Our analysis clearly showed new structure in high energy region at about 7.5 eV Consistent changes of refractive index & extinction coefficient of the <TEX>$BaSm_2Ti_4O_{12}$</TEX> thin film by the growth and annealing temperatures were also confirmed.

Investigation of Laminated High-k Oxides by Using Vacuum Ultraviolet Spectroscopic Ellipsometry

Zirconium oxide (ZrO2) is suggested as a good candidate for a high-k dielectric. For the storage capacitor in a dynamic random access memory, a multi-stack of ZrO2 and Al2O3 is reported to reduce the leakage current and to secure a high capacitance. In this case, however, the thickness and the properties of each layer need to be precisely controlled in order to deposit a well-de ned laminated structure. Although conventional spectroscopic ellipsometry is one of the best techniques to characterize multilayer lms, it shows poor sensitivity to high-k materials due to its limited spectral range. Thus, in this work, vacuum ultraviolet spectroscopic ellipsometry (VUV SE) and high-resolution transmission electron microscopy were employed for the laminated structure of `ZrO2/Al2O3/ZrO2' on a silicon substrate, where the oxide layers were prepared by using an atomic layer deposition technique. As the optical properties of the ZrO2 lms were so sensitive to the preparation process, many considerations were required for the analysis of the VUV SE data. From the analysis, we found that the optical properties of the bottom ZrO2 lm depended on its own thickness, as well as on the deposition temperature of the subsequent Al2O3 layer. Meanwhile, those of the top ZrO2 layer showed a dependence on the crystalline structure of the bottom ZrO2 and on the thickness of the interfacial Al2O3 layer.

Vacuum ultraviolet ellipsometry investigation of ultrathin organic films and their heterostructures

Thin layers and multilayer structures of the DNA bases guanine and cytosine on ZnO substrates were prepared by organic molecular beam deposition under ultra-high vacuum conditions and measured in situ by means of vacuum ultraviolet spectroscopic ellipsometry at the synchrotron source BESSY. Using the dielectric function of the individual layers the optical response of the G/C and C/G heterostructures was modeled. Deviations between simulated and experimental data were mainly attributed to the ordering of cytosine over guanine in the G/C and C/G structures and to the influence of the substrate.

Bandgap engineering in amorphous BexZnyO thin films

Reactive sputtering was used to grow thin films of BexZnyO on Si (100) substrates. X-ray diffraction patterns of the films revealed no structure, suggesting that the films have an amorphous nature. The optical “bandgap” energy of the amorphous BexZnyO (a-BexZnyO) films was derived from vacuum ultraviolet variable angle spectroscopic ellipsometry measurements. The value of the energy bandgap of the films can be efficiently engineered to vary from the amorphous ZnO bandgap of 3.35to7.91eV by changing the Be doping level in the a-BeZnO. The a-BeZnO films could be used for fabricating excellent a-ZnO based electronic devices.

Detection of ultrathin biological films using vacuum ultraviolet spectroscopic ellipsometry

Spectroscopic ellipsometry (SE) is a non-contact and a non-destructive optical technique used in characterization of thin films. It is widely used to determine optical constants, thickness in multilayer stacks and microstructure (voids, alloy fraction, or mixed phase composition). This paper reports on a systematic investigation of the optical properties of two different kinds of silane compounds: 3-aminopropyltriethoxysilane (APTES) and 3-glycidoxypropyltriethoxy-silane (GPS) as well as for immunoglobulin G (IgG) attached to these modified samples using vacuum ultraviolet spectroscopic ellipsometry (VUV-SE). VUV-SE is a newly developed technique and used to evaluate the strength and energy of the interband electronic excitations/transitions in these biofilms. The shorter wavelengths of VUV-SE increase sensitivity for detection of extremely thin adsorbed films at an interface (<10 nm) and accurately determine optical properties of biological interactions/moieties on the surface. A Cauchy dispersion model was used to determine layer thicknesses in multilayer stacks and adsorption was accounted by including Gaussian shaped oscillators in the optical model. No measurable optical anisotropy is found for these films. The dielectric properties of the adsorbed films are reported in the 0.73–9.43 eV optical range.