Reflectance Anisotropy Spectroscopy Research Articles

Electronic Properties of Ag Reconstructions on Si(111): Coulomb Blockade Behavior at Room Temperature

The electronic properties and optical response of Si(111)–Ag reconstructions, with distinct interface structures, are studied with scanning tunneling microscopy (STM), reflectance anisotropy spectroscopy (RAS), and infrared spectroscopic ellipsometry (IRSE). The results are compared to previous studies on the Si(111)‐(3 × 1)–Ag surface. Depending on the preparation conditions, STM shows hybrid surfaces comprising (√3 × √3) areas with semiconducting (3 × 1)–Ag regions or metallic Ag islands. RAS of the (√3 × √3)–Ag reconstruction mostly shows a flat response, confirming the isotropic nature of the surface while Ag islands on this surface produce a steep increase in the infrared as well as a strong optical anisotropy at 3.5 eV. Scanning tunneling spectroscopy (STS) of the islands shows a high density of states with almost no structure. STS of Ag nanodots on the (3 × 1)–Ag surface shows distinct Coulomb resonances within the Si bandgap while the Si(111)‐(3 × 1)–Ag surface reveals an expected semiconducting behavior with a band gap and no sign of resonances. Ag islands on the (3 × 1)–Ag surface also show weak Coulombic oscillations indicating that local transport depends on the electronic properties of the interface.

Optical Anisotropy of SrTiO3(110) for Different Surface Terminations

In this work we investigate the SrTiO3(110) surface by reflectance anisotropy spectroscopy (RAS) at varying preparation steps including ex situ wet etching, high temperature oxygen annealing, as well as in situ vacuum annealing. Different surface terminations show distinctly different RAS spectra which are correlated to an altered surface stoichiometry measured by X‐ray photoelectron spectroscopy (XPS) and an altered valence band structure measured by UV‐photoelectron spectroscopy (UPS) in cases of conductive samples. We link the changes in the observed RAS spectra to various surface reconstructions, with particular focus of the signature of (3 × 1) and (1 × 4) reconstructed surfaces, and a metallic surface state observed in vacuum annealed SrTiO3.

Control Over Dimer Orientations on Vicinal Si(100) Surfaces in Hydrogen Ambient: Kinetics Versus Energetics

In chemical vapor ambience, Si(100) surfaces and hydrogen are strongly interacting with each other at relevant process temperatures. Energetic considerations cannot solely describe step and terrace formation sufficiently, as the formation of equilibrium surface reconstructions can be counteracted by surface kinetics. In recent years, the combination of in situ reflection anisotropy spectroscopy (RAS) and complementary UHV‐based surface sensitive techniques helped to understand and control the entire Si(100) surface preparation, in particular step and domain formation, which strongly depend on the applied process conditions and surface misorientation. Here, we discuss vicinal, 6° misoriented Si(100) surfaces in comparison to results on larger terraces in order to derive a general view on how to promote either a kinetically or an energetically driven step formation. Dictated by the dominant driving force, monohydride Si(100) surfaces with “A‐type” (1 × 2) or “B‐type” (2 × 1) majority domain can be prepared, as we confirm with RAS as well as low energy electron diffraction, scanning tunneling microscopy and Fourier‐transform infrared spectroscopy. Well‐ordered DB double layer steps at the vicinal B‐type Si(100) surface cause a step‐related, derivative‐like RAS signal, in contrast to all other surfaces. In general, we confirm that high H2 pressures promote kinetically driven processes, while the impact of energetics increases with offcut magnitude.

Plasmonic reflectance anisotropy spectroscopy of metal nanoclusters in a dielectric multilayer. Theory

A theory of plasmonic reflectance anisotropy spectroscopy (RAS) is developed for nanocluster layer at an interface. The model of identical ellipsoidal metal particles occupying the sites of rectangular lattice is used to calculate the effective plasmonic polarizability of nanoparticles. The anisotropic local field due to optically induced dipole plasmons and their interface-conditioned images is taken into account. Within the theory, resonant reflectance anisotropy spectra recently observed for In nanoclusters on InAs surface are explained, the anisotropy being associated with the difference between frequencies of plasmons with orthogonal in-layer polarizations. The frequency difference is treated in terms of anisotropy of the particles shape or/and the layer structure, its sign being opposite for the two types of anisotropy. The plasmonic RAS is concluded to serve as a method for investigating the anisotropy of nanocluster arrays.

Group V adsorbate structures on vicinal Ge(001) surfaces determined from the optical spectrum

Vicinal Ge(001) is the standard substrate for the fabrication of high-performance solar cells by metal-organic vapour phase epitaxy, where growth of the III-V material on single domain Ge surfaces, with a single dimer orientation, minimizes the formation of anti-phase domain defects. Reflectance anisotropy spectroscopy has proved to be a powerful and sensitive optical probe of such anisotropic surface structures, but moving beyond fingerprinting to atomic structure determination from the optical spectra has been held back by the high computational cost. It is shown that an empirical, local-orbital-based hybrid density functional theory approach produces very good agreement between the theory and the experiment for (2 × 1)-As and (2 × 1)-Sb structures grown on vicinal Ge(001). These results, when taken together with previous work on Si interfaces, show that this computationally efficient approach is likely to prove to be an important general technique for determining the structure of anisotropic semiconductor surfaces and interfaces by comparing the experimental and calculated optical spectrum.

Monitoring of stress–strain evolution in thin films by reflection anisotropy spectroscopy and synchrotron X-ray diffraction

With progressing miniaturization of modern electronic devices, interconnects become increasingly smaller. Additionally, as electronic devices move away from rigid substrates toward flexible ones, understanding their mechanical and structural stability is becoming crucial. In this work, a thorough mechanical characterization of copper thin films deposited on flexible substrates was performed with two techniques, namely well-established synchrotron X-ray diffraction (sXRD) and the rather new usage of reflectance anisotropy spectroscopy (RAS) for mechanical characterization of thin films. The comparison of these two techniques shows that RAS can be reliably used for the accurate and prompt yield stress measurements. The acquisition time of RAS is much faster than that of sXRD: 1 second per data point compared to several seconds per data point for sXRD experiments. Moreover, the signal-to-noise ratio of the RAS data is much higher than that of the sXRD. Our results show that yield stress of Cu films increases with the decrease in the film thickness, going from 352 MPa for a 500 nm films to 793 MPa for a 50 nm thick film. Microstructure analyses of the films by electron microscopy allowed correlation of the mechanical behavior of the films to their grain morphologies. We have shown that RAS can supplement sXRD measurements due to a faster acquisition rate which allowed us to analyze the creep behavior of our copper thin film at different strain rates.

Cohesive and adhesive properties of ultrathin amorphous and crystalline Ge2Sb2Te5 films on polyimide substrates

In this work, the onset strains of fragmentation, the fracture strength, and fracture toughness of amorphous and crystalline Ge2Sb2Te5 (GST) films are determined. Using in situ methods, such as resistance measurements, reflectance anisotropy spectroscopy (RAS), and light microscopy during uniaxial tensile loading, we demonstrate that onset strain of fragmentation and delamination depend on both, crystallographic state of the GST film and the film thickness. We observe that amorphous GST fractures at larger strains than crystalline GST. However, due to its small Young's modulus the fracture toughness KIC of amorphous GST is lower than that of crystalline GST (amorphous: KIC=0.4±0.2 MPa m0.5; crystalline: KIC=0.8±0.2 MPa m0.5). The results presented are critically discussed with respect to the potential application of GST films in flexible displays.

Optical anisotropy of quasi-1D rare-earth silicide nanostructures on Si(001)

Rare earth metals are known to interact strongly with Si(001) surfaces to form different types of silicide nanostructures. Using STM to structurally characterize Dy and Tb silicide nanostructures on vicinal Si(001), it will be shown that reflectance anisotropy spectroscopy (RAS) can be used as an optical fingerprint technique to clearly distinguish between the formation of a semiconducting two-dimensional wetting layer and the metallic one-dimensional nanowires. Moreover, the distinctive spectral features can be related to structural units of the nanostructures. RAS spectra of Tb and Dy nanostructures are found to show similar features.

In situ preparation of Si p-n junctions and subsequent surface preparation for III-V heteroepitaxy in MOCVD ambient

III-V integration on active Si-bottom cells promises not only high-efficiency multi-junction solar cells but also lower production costs. In situ preparation of an adequate Si p-n junction in metalorganic chemical vapor deposition ambient is challenging, particularly since the final Si surface should be atomically well-ordered to enable low-defect III-V nucleation. Precisely, a single-domain Si(100) surface with double layer steps needs to be prepared in order to suppress antiphase disorder in subsequently grown III-V layer structures on top of the Si p-n junction. We first investigate the formation of a n+-type collector in Si(100) as a result of annealing in tertiarybutylphosphine (TBP) or tertiarybutylarsine (TBAs) ambient. We illustrate how the n-type doping concentrations and their depth profiles depend on the essential preparation parameters, such as precursor partial pressures, exposure and annealing time, as well as reactor pressure. Subsequently, by applying in situ reflectance anisotropy spectroscopy, we find that exposure of Si(100) to TBP or TBAs leads to atomic disorder on the surface. Further, we apply an additional annealing step without precursor supply leading to predominantly (1×2) reconstructed Si(100) surfaces, which are suitable for subsequent low-defect III-V growth.

Precise in situ etch depth control of multilayered III-V semiconductor samples with reflectance anisotropy spectroscopy (RAS) equipment.

Reflectance anisotropy spectroscopy (RAS) equipment is applied to monitor dry-etch processes (here specifically reactive ion etching (RIE)) of monocrystalline multilayered III–V semiconductors in situ. The related accuracy of etch depth control is better than 16 nm. Comparison with results of secondary ion mass spectrometry (SIMS) reveals a deviation of only about 4 nm in optimal cases. To illustrate the applicability of the reported method in every day settings for the first time the highly etch depth sensitive lithographic process to form a film lens on the waveguide ridge of a broad area laser (BAL) is presented. This example elucidates the benefits of the method in semiconductor device fabrication and also suggests how to fulfill design requirements for the sample in order to make RAS control possible.

GaP-interlayer formation on epitaxial GaAs(100) surfaces in MOVPE ambient

The challenge to embed a single monolayer of phosphorus during epitaxial gallium arsenide (GaAs) growth triggers numerous questions regarding practical preparation, effective analysis, and fundamental consideration of the resulting interlayers. Beyond better understanding of III-V heterointerface formation processes, precise interlayer incorporation may enable enhanced interface design, effective diffusion barriers, and advanced band structure engineering. We employ metalorganic vapor phase epitaxy (MOVPE) in various growth modes (continuous, with interruptions, pulsed, surface exchange) targeting the most abrupt incorporation of thinnest GaP films in the GaAs(100) matrix. The intensities of higher order interference fringes in high resolution X-ray diffraction (HR-XRD) serve as a measure of the effective GaPxAs1−x film thickness and P concentration, which is compared to compositional analysis based on scanning transmission electron microscopy (STEM). In situ reflection anisotropy spectroscopy (RAS) provided us with insights to the GaAs(100) surface configurations relevant during the P interlayer preparation.

Use of reflectance anisotropy spectroscopy for mapping the anisotropy of lyotropic liquid crystal dispersions in formulations

ABSTRACTLyotropic liquid crystal dispersions have commercial importance in the formulation of cleaning products and pharmaceuticals. The anisotropy of such dispersions is an important aspect of their properties and a simple method for measurement would be valuable to formulators. The relatively new optical technique of reflectance anisotropy spectroscopy (RAS) was considered to have potential in this respect. To test the idea, the anisotropy of a binary mixture of an anionic surfactant Aerosol OT (AOT) and glycerol (used as a model for water) was investigated using RAS. The variation in the measured anisotropy parallels the expected behaviour of a dispersion of lyotropic liquid crystals as a function of concentration and temperature. A response surface of anisotropy generated as a function of temperature and surfactant concentration demonstrates the use of RAS as a tool for mapping liquid anisotropy to facilitate the formulation of structured liquids.

Obtaining solid solutions of InGaAsPInGaAsP solid solutions in the spinoidal decomposition region

Technology for growing layers of InGaAsP solid solutions with Eg ∼⃒ 1.0 - 1.2eV on InP substrates in the spinodal decomposition region by the metalorganic vapour phase epitaxy (MOVPE) technique has been developed. Results of investigation of the obtained solid solution layers by the methods of photoluminescence, reflectance anisotropy spectroscopy and fractal analysis are presented.

The impact of the surface on step-bunching and diffusion of Ga on GaAs (001) in metal-organic vapour phase epitaxy

We studied diffusion by measuring step-bunching, island spacing, and the transition from step-flow growth to two-dimensional island growth of (001) GaAs in metal-organic vapour phase epitaxy and correlated them with the surface reconstruction measured by reflectance anisotropy spectroscopy. The V/III ratio had a small effect, while the square root of the growth rate was anti-proportional to the diffusion length. The thermal activation energy was about 2.3 eV on terraces and 1.6 eV on domains at higher temperatures. Pronounced step-bunching coincided with large domains at the step-edges, causing smoother steps for the [11̅0] misorientation. This Ga-rich reconstruction at the step-edges is needed for the Schwoebel barrier to induce step-bunching. At higher temperatures of domains grow in size, the Schwoebel barrier reduces and nucleation becomes easier on this surface which reduces diffusion length and thus step-bunching.

Extent of hydrogen coverage of Si(001) under chemical vapor deposition conditions from ab initio approaches.

The extent of hydrogen coverage of the Si(001) c(4 × 2) surface in the presence of hydrogen gas has been studied with dispersion corrected density functional theory. Electronic energy contributions are well described using a hybrid functional. The temperature dependence of the coverage in thermodynamic equilibrium was studied computing the phonon spectrum in a supercell approach. As an approximation to these demanding computations, an interpolated phonon approach was found to give comparable accuracy. The simpler ab initio thermodynamic approach is not accurate enough for the system studied, even if corrections by the Einstein model for surface vibrations are considered. The on-set of H2 desorption from the fully hydrogenated surface is predicted to occur at temperatures around 750 K. Strong changes in hydrogen coverage are found between 1000 and 1200 K in good agreement with previous reflectance anisotropy spectroscopy experiments. These findings allow a rational choice for the surface state in the computational treatment of chemical reactions under typical metal organic vapor phase epitaxy conditions on Si(001).

Investigation of spinodal decomposition of InGaAsP solid solutions grown by the MOCVD technique

Technology for growing layers of InGaAsP solid solutions with Eg ∼ 1.0 - 1.2 eV on InP substrates by the MOCVD technique in the spinodal decomposition region has been elaborated. Results of the investigation of solid solution layers by the methods of photoluminescence, reflectance anisotropy spectroscopy and fractal analysis are presented.

Experimental investigation of a flowing superspreader solution using Reflection Anisotropy Spectroscopy

Superspreading is the phenomenon where a drop of trisiloxane surfactant solution rapidly spreads on a moderately hydrophobic substrate over a very large area. However the mechanism of superspreading is unknown to date. In this work Reflection Anisotropy Spectroscopy (RAS) is used for the first time in order to investigate the causes of superspreading. An aqueous solution of the superspreader Evonik S240 (0.1%) is compared with the chemically similar but non superspreading solution of Evonik S233 (0.1%), on a polycarbonate (pcp) substrate. This study is aimed at finding evidence of organised structures, such as micelles, at the contact line of the receding film which could be part of the mechanism of superspreading. The RA signal is monitored as the solutions recede from vertically aligned pcp substrates. The results are then analysed for evidence of anisotropy on the surface arising from any micelle structures located at the contact line of the film.

In situ controlled heteroepitaxy of single-domain GaP on As-modified Si(100)

Metalorganic vapor phase epitaxy of III-V compounds commonly involves arsenic. We study the formation of atomically well-ordered, As-modified Si(100) surfaces and subsequent growth of GaP/Si(100) quasisubstrates in situ with reflection anisotropy spectroscopy. Surface symmetry and chemical composition are measured by low energy electron diffraction and X-ray photoelectron spectroscopy, respectively. A two-step annealing procedure of initially monohydride-terminated, (1 × 2) reconstructed Si(100) in As leads to a predominantly (1 × 2) reconstructed surface. GaP nucleation succeeds analogously to As-free systems and epilayers free of antiphase disorder may be grown subsequently. The GaP sublattice orientation, however, is inverted with respect to GaP growth on monohydride-terminated Si(100).

The reflection anisotropy spectroscopy of the Au(1 1 0) surface structures in liquid environments

The reflection anisotropy (RAS) profiles of the Au(1 1 0)-(1 × 1), (1 × 2) and (1 × 3) surface structures in electrochemical environments are shown to arise mainly from surface dipole transitions directed along the principal axes of the Au(1 1 0) surface. There are weak contributions to the RAS profiles of the Au(1 1 0)-(1 × 1) and (1 × 3) surfaces in the region of 4.0 eV which probably arise from (1 1 1) facets that are either intrinsic to the surface structures or are associated with steps. A transition involving a surface state just above the Fermi level, EF, contributes to the RAS profiles of the (1 × 2) and (1 × 3) surfaces but not to the RAS profile of the (1 × 1) surface. A strong feature at 2.5 eV in the RAS profiles of the Au(1 1 0)-(1 × 1) and (1 × 2) surfaces is attributed to a transition in the vicinity of the L point of the Brillouin zone between the 5d band and the band at EF. It is argued that the applied potential of −0.6 V, which creates the Au(1 1 0)-(1 × 3) surface, lifts EF above the band causing it to become occupied and quenching this contribution to the RAS profile.

Formation of plasmonic nanoparticle arrays –rules and recipes for an ordered growth

AbstractWe review a self‐assembled growth method for plasmonic nanoparticle arrays, based on glancing angle deposition. We produced ordered Ag, Au, and Cu nanoparticle arrays over large areas on different stepped oxide templates. Precise control over the final geometry can be difficult and we provide recipes to obtain macroscopically ordered structures. We discuss the influence of the adsorbate diffusion length and facet termination on the shape and size distributions of metallic nanoparticle arrays and show that an increased ad‐atom mobility leads to more regular arrays of spherical nanoparticles. We also show how in‐situ Reflectance Anisotropy Spectroscopy (RAS) can be used to extract indirect information on the nucleation and ripening the nanoparticles, as well as measure the plasmonic resonance.Schematics of the RAS setup used to in‐situ monitor the growth of plasmonic nanoparticle arrays on stepped oxide templates.