Reflectance Difference Spectroscopy Research Articles

Probing ultrafast carrier dynamics, nonlinear absorption and refraction in core–shell silicon nanowires

We investigate the relaxation dynamics of photogenerated carriers in silicon nanowires consisting of a crystalline core and a surrounding amorphous shell, using femtosecond time-resolved differential reflectivity and transmission spectroscopy at photon energies of 3.15 eV and 1.57 eV. The complex behavior of the differential transmission and reflectivity transients is the mixed contributions from the crystalline core and the amorphous silicon on the nanowire surface and the substrate where competing effects of state filling and photoinduced absorption govern the carrier dynamics. Faster relaxation rates are observed on increasing the photo-generated carrier density. Independent experimental results on crystalline silicon-on-sapphire help us in separating the contributions from the carrier dynamics in crystalline core and the amorphous regions in the nanowire samples. Further, single beam z-scan nonlinear transmission experiments at 1.57 eV in both open and close aperture configurations yield two-photon absorption coefficient \$beta$ (~3 cm/GW) and nonlinear refraction coefficient \$gamma$ (-2.5x10^-4 cm2/GW).

Monitoring preparation and phase transitions of carburized W(1 1 0) by reflectance difference spectroscopy

Reflectance difference spectroscopy (RDS) is applied to follow in situ the preparation of clean and carburized W(110) surfaces and to study the temperature-induced transition between the R(15×3) and R(15×12) carbon/tungsten surface phases. RDS data for this transition are compared to data obtained from Auger-electron spectroscopy and low-energy electron diffraction. All techniques reveal that this transition, occurring around 1870K, is reversible with a small hysteresis, indicating a first-order-like behaviour. The present results also prove a high surface sensitivity of RDS, which is attributed to the excitation of electronic p-like surface resonances of W(110).

Errata: Monitoring the reactivity of Ag nanoparticles in oxygen atmosphere by using <italic>in situ</italic> and real-time optical spectroscopy

Surface differential reflectance spectroscopy (SDRS) is sensitive enough to observe the minute changes in the surface plasmon resonance (SPR) of noble metal nanoparticles (NPs), which is extremely dependent on the morphology and organization of the NPs as well as on the chemical atmosphere surrounding them. Taking this SPR as a signature, we have studied the reactivity of Ag NPs using a dedicated in situ SDRS setup adapted on a magnetron sputtering deposition machine. This configuration allowed us to analyze the SPR modifications in real-time, not only during the growth of Ag NPs, but also during their exposure to molecular O2 and during their capping by a dielectric (Si3N4) matrix. Real-time SDRS analysis reveals that, upon exposure of the Ag NPs to O2, their SPR characteristics (position, amplitude, and width of the absorption band) alter immediately, indicating the instantaneous reactive interactions between Ag NPs and adsorbed O2 molecules. In addition, during the deposition of the Si3N4 matrix, real-time SDRS reveals possible breaking of Ag-O2 interactions. Moreover, with increasing Ag NP size, SPR modifications are seen to be reduced in O2 atmosphere, suggesting the diminution of Ag-O2 reactive interactions in the bigger NPs compared to the smaller ones.

Structural and Optical Properties of Mixed Diindenoperylene–Perfluoropentacene Thin Films

Mixed films of perfluorinated pentacene and diindenoperylene are studied for their structural and optical properties on SiO2 and glass substrates. Grazing incidence X-ray diffraction shows that the compounds mix on the molecular level. Two structures are identified. Their out-of-plane lattice spacing is determined by X-ray reflectivity, and their growth morphology is characterized using atomic force microscopy. It is found that one structure consists of molecules in a standing-up and the other in a lying-down orientation. The uniaxial dielectric function of mixed films is determined with spectroscopic ellipsometry, and the in-plane optical extinction coefficient is measured by real-time differential reflectance spectroscopy during growth. The question of intermolecular coupling between perfluorinated pentacene and diindenoperylene is discussed.

Investigation of Wetting Layers in InAs/GaAs Self-Assembled Nanostructures with Reflectance Difference Spectroscopy

As a system with both profound physics and promising application potentials, the strain-induced self-assembled semiconductor nanostructures have been investigated for decades of years. The optical and electrical properties of this system are mainly determined by the nanostructures, but can be greatly affected by the generally existent two dimensional structures from which the nanostructures evolve, the so called wetting layer (WL). The WL configurations can be varied with different growth conditions, and further influence the nanostructure morphology and properties. This is a reviewing article introducing some recent progresses in investigating the evolution of WLs in InAs/GaAs system with reflectance difference spectroscopy. Different kinds of WL evolution processes in Stranski-Krastanov growth mode are introduced. The segregation and desorption of indium atoms and the effect of growth interruption are mentioned. The existence and the evolution process of wetting layers in droplet epitaxy method are also discussed.

Monitoring the reactivity of Ag nanoparticles in oxygen atmosphere by using <italic>in situ</italic> and real-time optical spectroscopy

Surface differential reflectance spectroscopy (SDRS) is sensitive enough to observe the minute changes in the surface plasmon resonance (SPR) of noble metal nanoparticles (NPs), which is extremely dependent on the morphology and organization of the NPs as well as on the chemical atmosphere surrounding them. Taking this SPR as a signature, we have studied the reactivity of Ag NPs using a dedicated in situ SDRS setup adapted on a magnetron sputtering deposition machine. This configuration allowed us to analyze the SPR modifications in real-time, not only during the growth of Ag NPs, but also during their exposure to molecular O2 and during their capping by a dielectric (Si3N4) matrix. Real-time SDRS analysis reveals that, upon exposure of the Ag NPs to O2, their SPR characteristics (position, amplitude, and width of the absorption band) alter immediately, indicating the instantaneous reactive interactions between Ag NPs and adsorbed O2 molecules. In addition, during the deposition of the Si3N4 matrix, real-time SDRS reveals possible breaking of Ag-O2 interactions. Moreover, with increasing Ag NP size, SPR modifications are seen to be reduced in O2 atmosphere, suggesting the diminution of Ag-O2 reactive interactions in the bigger NPs compared to the smaller ones.

Growth kinetics and size-dependent wetting of Ag/α-Al2O3(0001) nanoparticles studied via the plasmonic response

The growth of vapour-deposited silver nanoparticles on α-Al2O3 was studied in situ from 190 to 675 K by surface differential reflectivity spectroscopy in the UV–visible range. Changes in size, shape and density were derived from the plasmonic response modelled in the framework of interface susceptibilities by assuming that supported clusters were in the form of truncated spheres. The sticking coefficient of silver on alumina is close to one up to T ≃ 575 K before entering a regime of incomplete condensation. The Arrhenius dependence of the saturation density indicates a nucleation on defects at low temperature (T ≤ 300 K) and detrapping above. The particle size D evolution follows temporal power laws, independent of temperature and flux, which characterize the growth (D ∼ t0.31) and coalescence (D ∼ t0.55) of the film. These are indicative of the growth of isolated particles at constant density and dynamic coalescence, respectively. The wetting angle of the silver clusters is shown to increase during the growth regime, which is assigned to a combination of surface stress and mismatch-induced strain, and to decrease upon coalescence, which is attributed to plastic relaxation. For particles larger than 10 nm in size, the values of contact angle and adhesion energy level off with asymptotic limits (θc = 127.5° ± 1° and 0.48 ± 0.02 J m−2) that nicely agree with tabulated data. This work highlights the ability of nanoplasmics to monitor in situ the growth kinetics of thin supported films.

Excitonic diffusion dynamics in ZnO

We investigate excitonic carrier diffusion in both bulk ZnO and nanorods (NRs). Using time-resolved differential reflectivity spectroscopy, we observe a fast decaying component together with a longer exponential relaxation. In bulk ZnO, we find that the fast decay term (∼1 ps) originates from excitonic diffusion along the growth direction. By probing at both the A and B excitons, we find different diffusion coefficients for each. In ZnO nanorods, the diffusion contribution is missing. We attribute this to two effects: (1) defects in the nanorods substantially slow the diffusion process and (2) excitons in nanorods are generated more uniformly than in bulk.

Micro reflectance difference techniques: Optical probes for surface exploration

AbstractMicro reflectance difference spectroscopy (µ‐RDS) is a promising tool for the in‐situ and ex‐situ characterization of semiconductors surfaces and interfaces. We discuss and compare two different approaches used to measure µ‐RD spectra. One is based on a charge‐coupled device (CCD) camera, while the other uses a laser and a XY translation stage. To show the performance of these systems, we have measured surface optical anisotropies of GaSb(001) sample on which anisotropic strains have been generated by preferential mechanical polishing along [110] and $[1{\bar {1}}0]$ directions. The spectrometers are complementary and the selection of one of them depends on the sample to be investigated and on experimental conditions.

Detecting and tuning anisotropic mode splitting induced by birefringence in an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser

The mode splitting induced by birefringence in an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser (VCSEL) has been studied by reflectance difference spectroscopy (RDS) at room temperature. The mode splitting, anisotropic broadening width, and the anisotropic integrated area of the cavity mode are determined. Uniaxial strain on the order of 10−4 is introduced to tune the mode splitting. The mode splitting can be linearly tuned by the uniaxial strain, which agrees very well with theoretical calculations using a Jones matrix approach. We demonstrate that the RDS is a powerful, nondestructive tool to study the cavity anisotropy of VCSELs.

Effects of substrate orientation on the optical anisotropy spectra of GaN/AlN/Si heterostructures in the energy range from 2.0 to 3.5 eV

We report reflectance (R) and reflectance difference spectroscopy (RDS) spectra of wurtzite heterostructures grown on Si(111) and Si(110) substrates in the energy range from 2.0 to 3.5 eV. Due to the threefold symmetry of the Si(111) surface, the heterostructures grown on this surface will relax isotropically through the formation of misfit dislocations, preferably at the AlN/Si interface, and no in-plane anisotropies are expected. In fact, only a small in-plane anisotropy of reflectance is observed, due to the initial residual off-cut of the silicon substrate that leads to wurtzite layers with a c-axis slightly tilted with respect to the surface. In contrast, for the Si(110) substrate, strong differences in lattice parameters appear between silicon and GaN/AlN depending on the considered in-plane direction, leading to a large in-plane anisotropy. By using a multiple reflection model for R and the in-plane anisotropies described, we developed a model to describe the RDS spectra in the vicinity and below the fundamental gap of GaN.

Optical differential reflectance spectroscopy on thin molecular films

Optical spectroscopy is a powerful tool to study in depth manifold physical processes occurring in molecular solids, at interfaces between molecules and substrates, and at interfaces between different molecular species. Apart from probing the optical interactions themselves, also structural information can be deduced due to the sensitive dependence of the optical properties of molecular assemblies on the respective structure. In our contribution we review the method Differential Reflectance Spectroscopy (DRS), with special emphasis on the interpretation of the spectral data and the extraction of the material's properties, i.e., the complex dielectric function. We further elucidate which physical effects can be investigated. For this purpose we assess recent progress of DRS experiments and explain several selected results achieved with this technique.

Layer resolved evolution of the optical properties of α-sexithiophene thin films

We report a combined reflectance difference spectroscopy and scanning tunneling microscopy study of ultrathin α-sexithiophene (6T) films deposited on the Cu(110)-(2×1)O surface. The correlation between the layer resolved crystalline structure and the corresponding optical spectra data reveals a highly sensitive dependence of the excitonic optical properties on the layer thickness and crystalline structure of the 6T film.

Optical evidence for intermolecular coupling in mixed films of pentacene and perfluoropentacene

We present optical absorption spectra of mixed films of pentacene (PEN) and perfluoropentacene (PFP) grown on SiO2. We investigated the influence of intermolecular coupling between PEN and PFP on the optical spectra by analyzing samples with five different mixing ratios of PFP:PEN with variable angle spectroscopic ellipsometry (VASE) and differential reflectance spectroscopy (DRS). The data show how the spectral shape is influenced by changes in the volume ratio of the two components. By comparison with the pure film spectra an attempt is made to distinguish transitions due to intermolecular coupling between PEN and PFP from transitions caused by interactions of PEN (PFP) with other molecules of the same type. We observe a new transition at 1.6 eV which is not found in the pure film spectra and which we assign to coupling of PFP and PEN.

Growth and optical properties of Ag clusters deposited on poly(ethylene terephthalate)

The growth and concomitant evolution of the optical properties of Ag nano-clustersdeposited on biaxially extruded poly(ethylene terephthalate) films is studied by reflectancedifference spectroscopy. It is demonstrated by low energy ion scattering and simulatedoptical spectra that the clusters form a two-dimensional layer buried beneath the surface ofthe substrate. The experimental spectra are described by simulations in which differentconfigurations of the host such as anisotropy, amorphization, and dilution areconsidered in an effective medium approach. The contribution of the anisotropicsubstrate is used to explain the resulting line shapes. We also discuss the roleof the rate of change of the filling fraction with Ag coverage in the evolutionof the spectra and the detection of the onset of coalescence by optical means.

Kinetics of the plasmon optical response of Au nanoparticles/TiO2 catalyst under O2 and H2 followed by differential diffuse reflectance spectroscopy

The changes in the optical properties of Au/TiO2 powder catalyst, prepared by the deposition-precipitation method, are measured in the visible range by use of a home-made diffuse differential reflectance spectrometer, when the ambient atmosphere is switched between H2 and O2. Two main features are observed: (i) a short wavelength one, located between 400 and 600 nm, is shown to be related to the modifications of the plasmon resonance of the 3-dimensional metallic gold particles with mean size around 4 nm, induced by the exposure to oxygen or by its removal; (ii) a second feature, observed at long wavelengths between 600 and 1100 nm, is extremely sensitive to the exposure to oxygen. This optical feature could be due to the presence in the catalyst, either of Au0 clusters of several tens atoms, which are expected to display molecular-like transitions in this optical range, or to specific sites of the Au particles (edge atoms or peripheral atoms at the interface with the TiO2 support), which could be highly reactive to oxygen.

Observation of strong anisotropic forbidden transitions in (001) InGaAs/GaAs single-quantum well by reflectance-difference spectroscopy and its behavior under uniaxial strain

The strong anisotropic forbidden transition has been observed in a series of InGaAs/GaAs single-quantum well with well width ranging between 3 nm and 7 nm at 80 K. Numerical calculations within the envelope function framework have been performed to analyze the origin of the optical anisotropic forbidden transition. It is found that the optical anisotropy of this transition can be mainly attributed to indium segregation effect. The effect of uniaxial strain on in-plane optical anisotropy (IPOA) is also investigated. The IPOA of the forbidden transition changes little with strain, while that of the allowed transition shows a linear dependence on strain.PACS 78.66.Fd, 78.20.Bh, 78.20.Fm

Room-temperature spin photocurrent spectra at interband excitation and comparison with reflectance-difference spectroscopy in InGaAs/AlGaAs quantum wells

Spectra of the interband spin photocurrent due to Rashba and Dresselhaus spin splittings have been experimentally investigated in InGaAs/AlGaAs quantum wells at room temperature. The Rashba- and Dresselhaus-induced circular photogalvanic effect (CPGE) spectra are found to be quite similar in the spectral regions corresponding to the transitions 1e1hh (the first conduction to the first valence sub-band of heavy hole) and 1e2hh. The ratio of Rashba- and Dresselhaus-induced CPGE currents for the transition 1e1hh is estimated to be 4.95. The magnitude of the Rashba-induced CPGE current is up to several tens of nA/W for the transition 1e1hh, which is 1 order of magnitude larger than that obtained in GaN/AlGaN superlattices. Comparing the CPGE spectrum with reflectance-difference and photoreflectance spectra, we find that the large Rashba spin splitting is mainly induced by a large indium atom segregation effect and by the internal field in the quantum wells.

SiO 2 / Si interfaces on high-index surfaces: Re-evaluation of trap densities and characterization of bonding structures

Interface trap densities, Dit, at the thermally oxidized Si surfaces were investigated for the (001), (111), (110), (120), (331), and (113) orientations. The oxides were formed by dry or wet oxidation in the temperature range of 700–950 °C. Dit took a maximum not only on the (111) surfaces but also on (110). Low Dit values were obtained on wet-oxidized high-index surfaces. Correlation between Dit and the interface anisotropy observed by reflectance difference spectroscopy suggests preferential oxidation of the Si atoms with the (111)-like bonding geometry on the (113) surfaces.

The transition from two-stage to three-stage evolution of wetting layer of InAs/GaAs quantum dots caused by postgrowth annealing

For the InAs/GaAs quantum dot (QD) system, the evolution of wetting layer (WL) with InAs deposition thickness has been studied under different postgrowth annealing (PGA) durations using reflectance difference spectroscopy. For the sample without PGA, WL thickness remains constant after the formation of QDs, exhibiting a typical two-stage evolution, whereas for the samples with PGA, WL thickness continuously increases linearly with a reduced slope after the formation of QDs and is fixed in ripening growth, indicating a three-stage evolution. By adopting a theoretical model, we have well simulated the two kinds of evolution and found that the variations of QD’s morphology and the interaction of QDs occurring during PGA lead to the different evolution behaviors of WL.