Reflectance Anisotropy Spectroscopy Research Articles

Quantification of the As/P content in GaAsP during MOVPE growth

Epitaxial integration of direct-bandgap III–V compound semiconductors with silicon requires overcoming a significant lattice mismatch. To this end, GaAsP step-graded buffer layers are commonly applied. The thickness and composition of the individual layers are decisive for the envisaged strain relaxation. We study GaAsP growth by metalorganic vapor phase epitaxy in situ with reflection anisotropy spectroscopy. We find that the growth surface exhibits optical fingerprints of atomically well-ordered surfaces. These allow for tuning the interface preparation between adjacent layers. The spectral position of the characteristic peaks in the RA spectra, which are related to surface-modified bulk transitions, behaves similarly upon an increased As content as does the E1 interband transition of GaAsP at the growth temperature. The impact of strain on this shift is negligible. We thus monitor a bulk property via the surface reconstruction. An empiric model enables quantification of the As content of individual layers directly in situ without growth interruptions and for various surface reconstructions. Our findings are suitable for a simplified optimization of the GaAsP buffer growth for high-efficiency devices.

In situ analysis of Bi terminated GaAs (0 0 1) and Ga(As,Bi) surfaces during growth by MOVPE

The influence of trimethylbismuth (TMBi) on the GaAs (001) surface reconstruction is studied in situ by reflection anisotropy spectroscopy (RAS) in a metal organic vapor phase epitaxy (MOVPE) system. During supply of TMBi the RAS spectra indicate a change of the arsenic rich c(4 × 4)β surface reconstruction to a bismuth terminated surface reconstruction. This Bi terminated surface is correlated to the current understanding of growth mechanisms for III/V semiconductors containing small amounts of bismuth. The formation, thermal stability and influence of the ambient conditions on this surface is analyzed. The RAS results are correlated to mass spectrometric studies with a real time fast Fourier transform quadrupole ion trap mass spectrometer (iTrap), which is used inline in the same MOVPE system. Both results indicate an about 40 °C lower decomposition temperature of TMBi compared to TBAs. The decomposition temperatures are determined by mass spectrometry with 290 °C for TMBi and 330 °C for TBAs. Furthermore, the surface reconstruction is studied during growth of Ga(As,Bi) bulk layers grown on GaAs. Under ideal conditions, the RAS measurement of the Ga(As,Bi) growth surface shows the signal of a c(4 × 4)β surface reconstruction which is shifted to lower energies as compared to the GaAs surface reconstruction.

Real-time reflectance anisotropy spectroscopy of GaAs homoepitaxy.

Reflectance anisotropy spectroscopy (RAS) is a highly sensitive optical probe for the real-time study of the epitaxial growth of zincblende semiconductors. Here we report on (1) non-equilibrium RAS spectra acquired in real time during the homoepitaxial growth of GaAs, and (2) RAS spectra for GaAs surfaces under equilibrium with several arsenic overpressures. We show that in both cases RAS spectra can be decomposed into two basic components, each with a characteristic line shape. We further show that both dynamic and equilibrium RAS spectra are described by the same pair of basic components. We conclude that the time evolution of non-equilibrium RAS spectra acquired during the epitaxial growth can be described in terms of RAS spectra for equilibrium surfaces. The results reported here should be useful for the interpretation of the physics underlying the rapid time evolution of dynamic RAS spectra during the first monolayer growth. Thus, we show that RAS constitutes a valuable tool for the study of epitaxial growth mechanisms.

Adsorbate Isotherm Analysis by Reflection Anisotropy Spectroscopy on Copper (110) in Hydrochloric Acid

Reflectance anisotropy spectroscopy (RAS) is a powerful optical probe that works on a polarization contrast basis. It can be operated in any environment, ranging from ultrahigh vacuum to vapor phases and liquids. The measured optical anisotropies are caused by several symmetry breaking effects and are exclusively assigned to the surface for otherwise bulk isotropic materials. In this work, we present a systematic study comprising in situ RAS-transient to assess the surface thermodynamics of the chloride adsorption on Cu(110) upon systematic variations of the applied electrode potentials in comparison to cyclic voltammetry (CV). Numerical time-derivatives of the measured RAS-transients are shown to be exclusively associated with electrical currents of those electrochemical reactions, which change the properties of the electrode surface. The recorded transient line-shapes track the Frumkin type isotherm properties related to chloride coverage. Both connections are theoretically discussed. Owing to the surface and interface specificity, RAS is shown to exhibit a high surface sensitivity. In particular, processes taking place in parallel, namely, the hydrogen evolution reaction (HER) as well as the copper dissolution as Cu+ and Cu2+, do not contribute to the RAS response.

Effect of growth technological conditions on the heterointerface thickness in the InAs/GaSb strained-layer superlattices grown by MOCVD

In this article, we investigated the effect of technological growth parameters by metal-organic chemical vapor deposition (MOCVD) method on the thickness of the transition layers, which affect the value of tensile strain in the structure, in the InAs/GaSb superlattice. We consider that the thickness of transition layers depends on the roughness of the growth surface and the technological conditions of growing single layers in the superlattice. At the first stage, we have determined the optimal annealing parameters for the minimization of the substrate roughness as T = 650 °C and t = 8 min, with a minimum value of 1.1 nm. At the second stage, studies were carried out on the effect of the sequence of elements and the delay time of reagent supply during the growth of InAs and GaSb layers on the quality of heterointerface. The stress analysis of the obtained structures was performed by reflectance anisotropy spectroscopy. It was shown that a superlattice becomes less tense with the increase in the number of pairs of alternating layers.

Gold nanoclusters on GaAs(001) surface: atomic force microscopy and optical spectroscopy of plasmons

Gold nanoclusters of two different kinds are found to occur on annealing of thin Au films deposited on either oxidized or nitridized GaAs(001) surfaces. The morphology of Au/GaAs interfaces is characterized, and the gold nanoclusters are established to cause two resonant peaks in optical reflectance spectra at the energies of 1.6 eV and 2.15 eV. Using the data of reflection spectroscopy and theoretical analysis, we assign the latter peak to localized plasmons of Au nanoislands located on Au/GaAs surface. The former peak is attributed to plasmons of prolate Au nanoclusters buried in GaAs crystal just near its surface. As well, plasmonic anisotropy of Au nanoclusters formed on nitridized GaAs surfaces is detected using reflectance anisotropy spectroscopy.

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.

The impact of non-ideal surfaces on the solid-water interaction: a time-resolved adsorption study

The initial interaction of water with semiconductors determines the electronic structure of the solid–liquid interface. The exact nature of this interaction is, however, often unknown. Here, we study gallium phosphide-based surfaces exposed to H_22O by means of in situ reflection anisotropy spectroscopy. We show that the introduction of typical imperfections in the form of surface steps or trace contaminants not only changes the dynamics of the interaction, but also its qualitative nature. This emphasises the challenges for the comparability of experiments with (idealised) electronic structure models for electrochemistry.

An algorithm for the in situ analysis of optical reflectance anisotropy spectra

We report on a computer algorithm for the in situ analysis of reflectance anisotropy (RA) spectra in a time frame compatible with the epitaxial growth of cubic semiconductors. This algorithm allows for the in situ acquisition of RA spectra and their decomposition into two components whose amplitude depends on the As coverage of the semiconductor surface. One of such components is associated with the surface orthorhombic strain due to the surface reconstruction and has an amplitude that strongly depends with surface reconstruction and thus As coverage. This fact opens the possibility of using reflectance anisotropy spectroscopy (RAS) as an optical probe to characterize the As surface coverage in real time. To demonstrate the performance of the algorithm we report on RA measurements carried out during the homoepitaxial growth of GaAs (001). We show that the algorithm is capable of analyzing a set of 500 RA spectra in a time span of about 10 s. This allows for a range of applications for the developed algorithm, including the surface characterization and fine tuning of the substrate stoichiometry just before epitaxial growth, during the growth of the buffer layer.

Plasmonic Resonance Sensitivity in Phase Transition of Ga Nanoparticle Arrays Grown by Glancing Angle Deposition Technique

Gallium is a liquid metal at near room temperature and in recent decades has become a key element in both electronic and optoelectronic applications. We demonstrate for the first time highly ordered spherical Ga nanoparticle (NP) arrays were fabricated by glancing angle deposition (GLAD) technique. GLAD is a simple method based on self-assembly that can produce highly ordered one-dimensional plasmonic NP chains. The real time (in-situ) monitoring of optical properties of Ga NPs plays an important role in measurement of optical behaviour in phase change. Also this provides information on the growth mechanism and allows production of structures with the desired optical characteristics. Reflectance anisotropy spectroscopy (RAS) has been used for monitoring of in-situ optical properties of the liquid-solid transition in Ga NP arrays. The results show stability and phase transition sensitivity of plasmonic resonance of Ga NP arrays. In order to confirm experimental results an analytical model was adapted by using Transfer Matrix Formalism and experimental parameters of Ga NP arrays were used for simulation.

Double-layer stepped Si(1 0 0) surfaces prepared in As-rich CVD ambience

Low-defect III-V integration on Si(1 0 0) requires atomically well-ordered heterointerfaces. To this end, we study the interaction of As with Si(1 0 0) surfaces and the formation of dimerized Si(1 0 0):As surfaces in situ with reflection anisotropy spectroscopy during metalorganic chemical vapor deposition (MOCVD). Control of surface processes allows us to intentionally ‘rotate’ the dimer orientation and choose between predominantly (1 × 2) and (2 × 1) reconstructed Si(1 0 0):As surfaces both for low and higher surface misorientation. We also verify that the (2 × 1) surface reconstruction becomes energetically more favorable for vicinal Si(1 0 0):As surfaces. We explain the preparation of Si(1 0 0):As surfaces displaying an almost single-domain, (1 × 2) reconstructed surface with an RMS roughness ≤1 Å and exhibiting broad, atomically flat terraces as well as regular double-layer steps. This surface is highly suitable for subsequent low-defect III-V integration.

Reflectance anisotropy spectroscopy of Fe3O4 (110): Anisotropic strain

We present a study of (110)-terminated magnetite using a range of surface-sensitive techniques, the foremost of those being reflectance anisotropy spectroscopy (RAS). The anisotropic optical response of the as-polished and row-reconstructed terminations are investigated. The responses are interpreted to result from termination-induced anisotropic shifts in energy of the bulklike optical transitions. Density functional theory (DFT) calculations demonstrate that the row reconstruction of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(110) exhibits anisotropic strain in its terminating layers, and this strain is concluded to be the origin of the RAS response. X-ray photoelectron spectroscopy measurements demonstrate that the row reconstruction is reduced and the stoichiometry in the surface region is relatively easily altered by in situ preparation procedures. This is correlated to altered surface electronic properties identified by scanning tunneling spectroscopy measurements. RAS is sensitive to this change in surface structure, with the magnitude of the RAS response being altered.

Reflectance anisotropy spectroscopy applied to organic thin films: The role of the substrate

Abstract When light is reflected by a solid sample, the collected signal includes the optical response of all the matter within the light penetration depth. Studying films thinner than this length, the substrate thus contributes to the reflected beam, and affects optical spectroscopy measurements. This is particularly true when polarized light is used, as in reflectance anisotropy spectroscopy (RAS), where substrate symmetry properties play a crucial part. A detailed comprehension of the substrate role in the optical response of a sample has important consequences for correctly extracting the optical contribution of the layer grown on it. This problem becomes delicate in organics, where the choice of the substrate is often fundamental for obtaining an overlayer with a desired structure. In particular, the use of organic single crystal substrates, often structurally anisotropic, is a powerful tool for obtaining highly ordered crystalline films by organic epitaxy. To achieve the film optical properties from an experimental RAS spectrum, an interpretative key is necessary: but the most used model describing the interaction of light with the layered system (the so called three-layer model) fails when the substrate is anisotropic. This care in the application of RAS to organics has not been discussed in details yet, in particular to understand the contribution of the substrate to the total anisotropy signal. Within a phenomenological approach to RAS, we have here weighed the substrate contribution, by considering five different and carefully chosen samples, i.e. a crystalline overlayer of the same material and thickness but on five different substrates. The data show that the substrate clearly influences the readability of the spectra by influencing their lineshape, in some complex cases limiting the possibility of extracting the film properties. Consequently, the correct choice of the substrate - that is a fundamental task for obtaining specific properties of the growing layer - is also a delicate element for a correct interpretation of RAS results.

Influence of UDMHy on GaAs (0 0 1) surface reconstruction before and during growth of Ga(NAs) by MOVPE

III–V semiconductors containing small amounts of Nitrogen (“dilute nitrides”) are very promising material systems for optoelectronic applications. There are many studies about growth characterization of Ga(NAs) in metalorganic vapor phase epitaxy (MOVPE), but very little is known about the nitridation process on the GaAs (0 0 1) surface and the influence of the surface reconstruction on the Nitrogen (N) incorporation. Therefore, we investigated GaAs (0 0 1) surfaces under different tertiarybutylarsine (TBAs) and 1,1-dimethylhydrazine (UDMHy) ambient conditions in MOVPE. For in situ surface characterization reflectance anisotropy spectroscopy (RAS) was used. Under sufficient TBAs stabilization the surface forms an As rich c(4 × 4)β surface reconstruction. This changes towards a more Ga rich (2 × 6)/(6 × 6)-like reconstruction if additional N is supplied. Therefore, UDMHy seems to enhance the As desorption from the surface or the surface reconstruction changes due to incorporated N. This conversion occurs rapidly within 5 s when UDMHy is supplied and was observed in a temperature range from 450 °C to 600 °C. The measured RAS spectra are then compared to the RAS spectra obtained during Ga(NAs) growth. These also exhibit a (2 × 6)/(6 × 6)-like surface reconstruction which seems to be necessary to incorporate a sufficient amount of N into the GaAs crystal.

Influence of anisotropic Si(111)-(4 × 1)-In surface on growth of nanoscale In islands

Reflectance anisotropy spectroscopy (RAS) and scanning tunneling microscopy (STM) are used to study the growth of indium (In) on the anisotropic Si(111)−(4 × 1)−In surface at room temperature. RAS shows that epitaxial growth of In is accompanied by the disappearance of the surface optical anisotropy at 1.9 eV which is the fingerprint for the (4 × 1)-In surface reconstruction and the appearance of a large minimum at 1.4 eV which is at the same energy as interband transitions observed on bulk crystalline In. Subsequent spectra taken on the surface, over 3 h, show that this minimum decreases and eventually disappears along with the reappearance of the original RAS signature of the (4 × 1)-In surface. STM of this surface shows elongated, anisotropic In crystal islands on top of a (4 × 1)-In reconstructed surface. Upon annealing the surface to 720 K, the surface reconstruction changes with STM showing regions covered with a phase that resembles the (√7 × √3)-In reconstruction and RAS showing a large positive anisotropy at about 1.5 eV. The epitaxial In islands now show a hexagonal shape, unlike on the (4 × 1) surface. Thus, the growth morphology of the islands is shown to be dependent on the initial surface reconstruction. The authors attribute these findings to Ostwald ripening of the In islands mediated by diffusion, which is dependent on the structurally different In wetting layers on the Si substrate.

Ultrasensitive and fast single wavelength plasmonic hydrogen sensing with anisotropic nanostructured Pd films

Anisotropic nanostructured porous Pd films are fabricated using oblique angle deposition in vacuum on a glass substrate. They display a dichroic response, due to localised surface plasmon resonances (LSPR) within the nanoparticles forming the film, dependent on the incident light polarisation. Ultrasensitive hydrogen sensing is reached by using these films in conjunction with a differential optical technique derived from the reflectance anisotropy spectroscopy. The evolution of the samples’ optical responses is monitored during the formation of Pd hydride in both the dilute α-phase and the dense β-phase, whilst the samples are exposed to different concentration of H2 in Ar (from 100% H2 to a few ppm). The measurements are performed at a single wavelength in the visible range and at 22 °C. The results show that a quantitative measurement of the hydrogen concentration in a carrier gas can be measured throughout the concentration range. The limit of detection is 10 ppm and the time for detecting the presence of H2 in the carrying gas is below one second at concentration down to 0.25% of H2 in Ar. Furthermore, the optical anisotropy of the samples and its evolution with exposure to H2 are correctly reproduced with an effective medium theory.

Molecular Reorientation during the Initial Growth of Perfluoropentacene on Ag(110).

Perfluoropentacene (PFP) is an organic material that has been widely studied over the last years and has already found applications in organic electronics. However, fundamental physical questions, such as the structural formation and the preferential orientation of the molecules during deposition on metal surfaces, are still not fully understood. In this work, we report on a unique in-plane molecular reorientation during the completion of the first monolayer of PFP on the Ag(110) surface. To characterize the molecular alignment, we have monitored the deposition process in real time using polarization-dependent differential reflectance spectroscopy and reflectance anisotropy spectroscopy. Abrupt changes in the optical signals reveal an intricate sequence of reorientation transitions of the PFP molecules upon monolayer completion and during the formation of the second monolayer, eventually leading to a full alignment of the long molecular axis along the [001] direction of the substrate and an enhanced structural ordering. Scanning tunneling microscopy and low-energy electron diffraction confirm the observed molecular reorientation upon monolayer compression and provide further details on the structural and orientational ordering of the PFP monolayer before and after compression.

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.

Controlling the Local Electronic Properties of Si(553)-Au through Hydrogen Doping.

We propose a quantitative and reversible method for tuning the charge localization of Au-stabilized stepped Si surfaces by site-specific hydrogenation. This is demonstrated for Si(553)-Au as a model system by combining density functional theory simulations and reflectance anisotropy spectroscopy experiments. We find that controlled H passivation is a two-step process: step-edge adsorption drives excess charge into the conducting metal chain "reservoir" and renders it insulating, while surplus H recovers metallic behavior. Our approach illustrates a route towards microscopic manipulation of the local surface charge distribution and establishes a reversible switch of site-specific chemical reactivity and magnetic properties on vicinal surfaces.

Water adsorption on the P-rich GaP(100) surface: optical spectroscopy from first principles

The contact of water with semiconductors typically changes its surface electronic structure by oxidation or corrosion processes. A detailed knowledge—or even control of—the surface structure is highly desirable, as it impacts the performance of opto-electronic devices from gas-sensing to energy conversion applications. It is also a prerequisite for density functional theory-based modelling of the electronic structure in contact with an electrolyte. The P-rich GaP(100) surface is extraordinary with respect to its contact with gas-phase water, as it undergoes a surface reordering, but does not oxidise. We investigate the underlying changes of the surface in contact with water by means of theoretically derived reflection anisotropy spectroscopy (RAS). A comparison of our results with experiment reveals that a water-induced hydrogen-rich phase on the surface is compatible with the boundary conditions from experiment, reproducing the optical spectra. We discuss potential reaction paths that comprise a water-enhanced hydrogen mobility on the surface. Our results also show that computational RAS—required for the interpretation of experimental signatures—is feasible for GaP in contact with water double layers. Here, RAS is sensitive to surface electric fields, which are an important ingredient of the Helmholtz-layer. This paves the way for future investigations of RAS at the semiconductor–electrolyte interface.