Reflectance Anisotropy Spectroscopy Signal Research Articles

In Situ Studies on the Influence of Surface Symmetry on the Growth of MoSe2 Monolayer on Sapphire Using Reflectance Anisotropy Spectroscopy and Differential Reflectance Spectroscopy

The surface symmetry of the substrate plays an important role in the epitaxial high-quality growth of 2D materials; however, in-depth and in situ studies on these materials during growth are still limited due to the lack of effective in situ monitoring approaches. In this work, taking the growth of MoSe2 as an example, the distinct growth processes on Al2O3 (112¯0) and Al2O3 (0001) are revealed by parallel monitoring using in situ reflectance anisotropy spectroscopy (RAS) and differential reflectance spectroscopy (DRS), respectively, highlighting the dominant role of the surface symmetry. In our previous study, we found that the RAS signal of MoSe2 grown on Al2O3 (112¯0) initially increased and decreased ultimately to the magnitude of bare Al2O3 (112¯0) when the first layer of MoSe2 was fully merged, which is herein verified by the complementary DRS measurement that is directly related to the film coverage. Consequently, the changing rate of reflectance anisotropy (RA) intensity at 2.5 eV is well matched with the dynamic changes in differential reflectance (DR) intensity. Moreover, the surface-dominated uniform orientation of MoSe2 islands at various stages determined by RAS was further investigated by low-energy electron diffraction (LEED) and atomic force microscopy (AFM). By contrast, the RAS signal of MoSe2 grown on Al2O3 (0001) remains at zero during the whole growth, implying that the discontinuous MoSe2 islands have no preferential orientations. This work demonstrates that the combination of in situ RAS and DRS can provide valuable insights into the growth of unidirectional aligned islands and help optimize the fabrication process for single-crystal transition metal dichalcogenide (TMDC) monolayers.

Exploring the range of applicability of anisotropic optical detection in axially coordinated supramolecular structures

Ensuring the highest accuracy in determining the molecular assembling and the preservation of the chemical and physical properties of the molecules during the growth of organic layers requires a real-time monitoring of film formation. In this respect, optical techniques are preferred since they result in minimum organic film damage. Among these techniques, Reflectance Anisotropy Spectroscopy (RAS) proved to be the one with the highest sensitivity due to the development of intrinsic anisotropies in the optical response of molecular films, where molecular packing is driven by Van der Waals interactions. Recently, we proposed an original strategy to enable the growth of organic films via molecular self-assembly through highly directional coordination bonds. Herein, we consider a straightforward supramolecular structure employing axial bonds, based on the 1:1 assembly of a CoII porphyrin (CoTPP) and a linear ligand (DPNDI). This system is characterized by a rather isotropic structure that might, in principle, result in a weak RAS signal. Therefore, in the present work, we critically assess the range of applicability of such a spectroscopy. A number of other surface science techniques, including Low Energy Electron Diffraction (LEED), photoemission spectroscopies (PES and IPES) and Atomic Force Microscopy (AFM) is employed to fully characterize the axially coordinated molecular film.

Fourier Transform Infrared Reflection Anisotropy Spectroscopy of Semiconductor Crystals and Structures: Development and Application in the Mid-Infrared.

A new method of reflection anisotropy spectroscopy (RAS) with increased mid-IR efficiency owing to the use of a Fourier transform infrared (FT-IR) spectrometer has been developed. An optical setup was implemented using a photoelastic modulator (PEM) to modulate the direction of linear polarization of the probe beam originating from the Michelson interferometer. An original measurement algorithm was proposed to eliminate the influence of spectral inhomogeneity of the PEM efficiency on the obtained spectra using appropriate calibration. It was shown that to preserve the sign of the RAS signal, it is necessary to use a specialized procedure for phase correction of the interferogram registered by the FT-IR spectrometer. In the visible range, good agreement was confirmed between the obtained reflection anisotropy (RA) spectra of a semiconductor crystal and the results of independent measurements using a conventional diffraction-grating spectrometer-based setup. The RA spectrum of a III-V semiconductor heterostructure in the mid-infrared range (λ up to 8 µm) is demonstrated. Application of the developed FT-IR RAS method to layered black phosphorus has enabled characterization of anisotropic interband transitions in this graphene-like semiconductor crystal.

In-situ etch-depth control better than 5 nm with reflectance anisotropy spectroscopy (RAS) equipment during reactive ion etching (RIE): A technical RAS application

A measurement technique, i.e. reflectance anisotropy/difference spectroscopy (RAS/RDS), which had originally been developed for in-situ epitaxial growth control, is employed here for in-situ real-time etch-depth control during reactive ion etching (RIE) of cubic crystalline III/V semiconductor samples. Temporal optical Fabry-Perot oscillations of the genuine RAS signal (or of the average reflectivity) during etching due to the ever shrinking layer thicknesses are used to monitor the current etch depth. This way the achievable in-situ etch-depth resolution has been around 15 nm. To improve etch-depth control even further, i.e. down to below 5 nm, we now use the optical equivalent of a mechanical vernier scale– by employing Fabry-Perot oscillations at two different wavelengths or photon energies of the RAS measurement light – 5% apart, which gives a vernier scale resolution of 5%. For the AlGaAs(Sb) material system a 5 nm resolution is an improvement by a factor of 3 and amounts to a precision in in-situ etch-depth control of around 8 lattice constants.

Optical properties of silicene, Si/Ag(111), and Si/Ag(110)

We present a state-of-the-art study of the optical properties of free-standing silicene and of single-layer Si 1D and 2D nanostructures supported on Ag(110) and Ag(111) substrates. Ab initio simulations of reflectance anisotropy spectroscopy (RAS) and surface differential reflectivity spectroscopy (SDRS) applied to the clean Ag surface and Si/Ag interfaces are compared with new measurements. For Si/Ag(110) we confirm a pentagonal nanoribbon geometry, strongly bonded to the substrate, and rule out competing zigzag chain and silicenelike models. For Si/Ag(111) we reproduce the main experimental features and isolate the optical signal of the epitaxial silicene overlayer. The absorption spectrum of a silicene sheet computed including excitonic and local field effects is found to be quite similar to that calculated within an independent particle approximation, and shows strong modifications when adsorbed on a Ag substrate. Important details of the computational approach are examined and the origins of the RAS and SDRS signals are explained in terms of the interface and substrate response functions. Our study does not find any evidence for Si adlayers that retain the properties of freestanding silicene.

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.

Influence of plasma composition on reflectance anisotropy spectra for in situ III–V semiconductor dry-etch monitoring

Reflectance anisotropy spectroscopy (RAS) can be used to monitor (reactive) ion etching (RIE) of semiconductor samples. We present results on the influence of the Cl2 content of the plasma gas on the RAS spectra during reactive ion etching. In a first step GaAs samples have been used and the RAS spectra are compared to results of secondary ion mass spectrometry (SIMS) on sample surfaces and depth profiles. In a second step a III–V semiconductor multilayer system has been investigated using the time-evolution of the average reflected intensity as an indication for the etch rate. In both cases usually even a high amount of Cl2 does not disturb the surface-sensitivity of the RAS signal.

Cu(110) Surface in Hydrochloric Acid Solution: Potential Dependent Chloride Adsorption and Surface Restructuring

Using cyclic voltammetry (CV), reflectance anisotropy spectroscopy (RAS), and in situ electrochemical scanning tunneling microscopy (EC-STM), we have studied the structure and structural transitions at a Cu(110) electrode surface in 10 mM HCl solution as a function of the applied electrode potential. While at potentials lower than −550 mV vs Ag/AgCl the in situ STM images reveal the adsorbate-free, unreconstructed structure of the Cu(110) surface, at increasing potential ≥ −550 mV chloride adsorption, as indicated by CV, leads first to the formation of grooves followed by the growth of added stripes. Both are aligned in the [001] direction as shown by EC-STM—and supported by ex situ low energy electron diffraction (LEED)—and are the result of a severe but fully reversible restructuring of the surface. This faceting is accompanied by an optical anisotropy peak in RAS centered at about 500 nm (2.48 eV), having a maximum when the linearly polarized light is aligned along the [11̅0] direction, i.e., perpendicular to the stripes detected with in situ STM under the same conditions. By cycling the electrode potential through a full cyclic voltammogram and monitoring simultaneously the RAS signal we obtain a hysteresis-like curve which supports a two-step kinetics of the restructuring process in agreement with the CV and STM data. The investigations demonstrate the power of combined RAS and in situ STM measurements to shine light on potential-driven processes at metal–electrolyte interfaces.

Monitoring of (reactive) ion etching (RIE) with reflectance anisotropy spectroscopy (RAS) equipment

Experimental results on the application of reflectance anisotropy spectroscopy (RAS) to the monitoring of (reactive) ion etching of monocrystalline semiconductor samples are described. To show the potential of this technique RAS signals collected during etching of GaAs/AlxGa1−xAs multilayer samples are compared to RAS data obtained before during molecular-beam epitaxial (MBE) growth of these very samples. A change of the RIE-RAS spectrum can be attributed to a change of material composition. And the current etch depth can be monitored with an accuracy at least down to several tens of nanometers – f. e. by recording the average reflected intensity.

Reflectance anisotropy spectroscopy of magnetite (110) surfaces

Reflectance anisotropy spectroscopy (RAS) has been used to measure the optical anisotropies of bulk and thin-film Fe3O4(110) surfaces. The spectra indicate that small shifts in energy of the optical transitions, associated with anisotropic strain or electric field gradients caused by the (110) surface termination or a native oxide layer, are responsible for the strong signal observed. The RAS response was then measured as a function of temperature. A distinct change in the RAS line-shape amplitude was observed in the spectral range from 0.8 to 1.6 eV for temperatures below the Verwey transition of the crystal. Finally, thin-film magnetite was grown by molecular beam epitaxy on MgO(110) substrates. Changes in the RAS spectra were found for different film thickness, suggesting that RAS can be used to monitor the growth of magnetite (110) films in situ. The thickness dependence of the RAS is discussed in terms of various models for the origin of the RAS signal.

Early oxidation stages of the strained Ge/Si(105) surface: A reflectance anisotropy spectroscopy study

We have applied reflectance anisotropy spectroscopy (RAS) to investigate the rebonded-step (RS) reconstructed strained Ge/Si(105) surface. RAS spectra display a marked peak at 2.1 eV that is completely cancelled after exposure to about 1200 L of molecular oxygen. Ab initio calculations of the RAS signal explain the experimental spectra in terms of optical transitions between filled bulk-like states and empty surface bands. The early stages of exposure of the clean surface to molecular oxygen have been monitored by RAS and scanning tunneling microscopy (STM), correlating the evolution of the optical anisotropy to the bonding configuration of the adsorbed gas. Oxygen atoms are proposed to first adsorb onto the arms and later on the head dimers of the “horseshoe” motifs that comprise the RS reconstruction, thus gradually destroying the local order.

Electronic structures of GaP(100) surface reconstructions probed with two-photon photoemission spectroscopy

The electronic structures of two significant atomically well-defined (100)-surface reconstructions of gallium phosphide were investigated with two-photon-photoemission spectroscopy (2PPE). We allocated a series of occupied and unoccupied surface states and deduced the influence of each particular reconstruction on the electronic structure of the surface. Photoemission signals arising from bulk optical transitions were distinguished from surface-state related signals by studying the influence of oxygen exposure to the surfaces and by comparing our results to reflectance anisotropy spectroscopy (RAS) measurements. This revealed that the features in the RAS signal correlate with the transition energies between unoccupied and occupied surface-related states that were observed in the 2PPE measurements. An anisotropy around the ${E}_{0}^{\ensuremath{'}}$ critical point, previously known from RA spectra, was also investigated with 2PPE and is ascribed to modifications of the bulk electronic structure in the vicinity of the surface.

Effects of steps and ordered defects on Cu(110) surface states

The effects of steps and ordered defects on the surface states supported by Cu(110) terraces are investigated by a combination of reflection anisotropy spectroscopy (RAS) and scanning tunneling microscopy (STM). For several vicinal (110)-type surfaces, we measure the 2.1-eV RAS peak arising from transitions between surface states. A Poelsema-Comsa scattering model is used to relate the intensity of this RAS signal to the surface morphology (the distributions of terrace widths and step-edge roughness) observed by STM, providing a measure of the ability of the surface defects to scatter the Shockley-type surface state. A scattering cross section of area equivalent to 20 unit cells is obtained---a value consistent with previous results obtained from other types of defect for the Cu(110) surface. We find that the Poelsema-Comsa scattering model, originally developed for random distributions of defects, is also applicable to the modeling of RAS intensities of surfaces with ordered and partially ordered defects: specifically steps. Our results highlight the growing importance of the Poelsema-Comsa methodology in combination with RAS data for extracting topographic information associated with surface defects, particularly from surfaces in hostile environments, where RAS can access as a real-time in situ probe.

Controlling the formation of a monolayer of cytochrome P450 reductase onto Au surfaces

The conditions necessary for the formation of a monolayer and a bilayer of a mutated form (P499C) of human cytochrome P450 reductase on a Au(110)/electrolyte interface have been determined using a quartz crystal microbalance with dissipation, atomic force microscopy, and reflection anisotropy spectroscopy (RAS). The molecules adsorb through a Au-S linkage and, for the monolayer, adopt an ordered structure on the Au(110) substrate in which the optical axes of the dipoles contributing to the RAS signal are aligned roughly along the optical axes of the Au(110) substrate. Differences between the absorption spectrum of the molecules in a solution and the RAS profile of the adsorbed monolayer are attributed to surface order in the orientation of dipoles that contribute in the low energy region of the spectrum, a roughly vertical orientation on the surface of the long axes of the isoalloxazine rings and the lack of any preferred orientation in the molecular structure of the dipoles in the aromatic amino acids. Our studies establish an important proof of principle for immobilizing large biological macromolecules to gold surfaces. This opens up detailed studies of the dynamics of biological macromolecules by RAS, which have general applications in studies of biological redox chemistry that are coupled to protein dynamics.

The use of reflection anisotropy spectroscopy to assess the alignment of collagen

The alignment of collagen fibres in tissue has a major influence on their mechanical properties. This study investigated the ability of reflection anisotropy spectroscopy (RAS) to determine the degree of alignment of collagen fibres deposited onto surfaces and secreted by mouse fibroblast cells in vitro. Aligned nanofibres of polytetrafluoroethylene were deposited on glass coverslips using a simple friction transfer method. These linear parallel nanofibres were used as topographical cues to orientate and align L929 fibroblasts and their deposited collagen. The strength of the RAS signal was demonstrated to correlate with the degree of collagen alignment. Immunochemical staining and atomic force microscopy were used to visualize the topography of the fibres and confirm that the RAS signal was as a result of collagen fibres. Collagen deposited onto glass coverslips from a solution that had been subjected to dialysis that caused ‘nanofibrillar’ collagen to form also resulted in a strong RAS signal whereas collagen adsorbed from a simple solution of collagen in which collagen fibres are not formed resulted in no RAS signal. It was concluded that the RAS signal could be used to determine the degree of alignment of collagen and that this could have a potential application in the assessment of collagen orientation in tissue repair.

Estimating the range of influence of point defects on Cu (110) surface states

By utilizing reflection anisotropy spectroscopy (RAS) and scanning tunneling microscopy (STM) measurements of the ion-bombarded Cu (110) surface at low temperatures, we have developed a simple methodology for estimating the effective surface area over which irradiation-induced defects perturb surface states, leading to a reduction in the intensity of the 2.1 eV RAS peak of this surface. Each composite defect decorating an ion-impact site quenches the RAS signal in proportion to an area equivalent to approximately 170 unit cells. We estimate that an atomic defect has an effective RAS cross section with area approximately equal to that of a circle with a radius of 0.75 nm, an area equivalent to that of around 19 unit cells. Accurate determination of the coverage and spatial distribution of surface defects is a prerequisite for a coherent analytical approach to modeling the RAS data of this system.

Effect of surface defects and adsorbates on the optical anisotropy of Cu(110)

We investigate the effects of defects on the optical response of the Cu(110) surface in the spectral region dominated by transitions between surface states. By modeling the variation of reflection anisotropy spectroscopy (RAS) signal with defect coverage we investigate the length scale over which a defect quenches the 2.1 eV RAS peak. We consider a number of different defect types; adatoms, vacancies, and molecular adsorbates, and find that all quench the RAS signal in this region by similar amounts.

Molecular orientation studied by reflection anisotropy spectroscopy

AbstractA description of the reflection anisotropy spectroscopy (RAS) results from oriented thin films is presented. The RAS signals of a 1 nm thick film are simulated to investigate the effect of molecular tilt on the RAS response of the system. Whilst the spectroscopic measurements themselves appear to be relatively insensitive to the effects of the tilt, the variation of the RAS signal with rotation of the sample azimuthal angle sees sensitivity to out of surface plane anisotropy, characterized by a sin θs effect.

In situ antiphase domain quantification applied on heteroepitaxial GaP growth on Si(100)

Via a particular postgrowth annealing procedure applied to heteroepitaxial GaP films grown on Si(100) substrates by metal-organic vapor phase epitaxy, ex situ atomic force microscopy (AFM) provides insight into the spatial distribution of antiphase domains (APDs). On a specific sample, the AFM characterization reveals a variation of the APD concentration over the sample’s surface. In situ reflectance anisotropy spectroscopy (RAS) was used as a complementary technique for the quantification of APDs on the P-rich prepared GaP surface. Besides the expected linear reduction of the RAS signal according to the presence of antiphase disorder, the comparison with the reflection anisotropy (RA) spectrum of an identically prepared, (2×2)/c(4×2) reconstructed surface of a homoepitaxial GaP(100) reference revealed further characteristic deviations. In principle, they originated from the additional reflection at the GaP/Si(100) heterointerface. Mainly, its interference with the surface reflection affected the normalization of the RAS signal. Corresponding interference corrections improved the agreement of the GaP/Si(100) RA spectra with the homoepitaxially grown GaP(100) reference in general and, in particular, the accuracy and reliability of the in situ APD quantification via RAS. Finally, the statistical evaluation of comprehensive ex situ AFM characterization agreed well with RAS results after interference corrections, which represent an in situ technique for the APD quantification over a macroscopic spot size of a few millimeters.

In situ investigation of CuPc thin films grown on vicinal Si(111)

In order to characterize the growth process of copper phthalocyanine (CuPc) thin films on vicinal Si(111) substrates, in situ spectroscopic ellipsometry (SE) and reflection anisotropy spectroscopy (RAS) were combined. The analysis of the in situ SE data implies a structural change occurring with increasing film thickness during growth. The ex situ SE data are fitted using a uniaxial model, and different out-of-plane molecular orientations are found: lying molecules on Si(111)-6° and standing molecules on Si(111)-0.35°. The average tilt angles of molecules relative to the substrate surface are calculated to be 41.5°±1.0° and 81.1°±3.5°, respectively. The in situ RAS spectra show that the optical anisotropy of CuPc/Si(111)-6° is induced by the substrate anisotropy, and the strength of the RAS signal of CuPc films is proportional to the film thickness.