Reflectance Difference Spectroscopy Spectra Research Articles

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 characterization of thin nickel films on polymer substrates using reflectance difference spectroscopy

We have used reflectance difference spectroscopy (RDS) and its extension, azimuth-dependent RDS (ADRDS), to study the properties of sputtered and evaporated nickel films on biaxially oriented poly(ethylene terephtalate) (PET) films in a roll to roll web-coating process. From the full set of ADRDS spectra we extract and analyze both the intrinsic RDS spectra and the azimuthal orientation of the effective optical anisotropy of the samples. From the latter, contributions to the RDS spectra arising from the nickel layer and the PET substrate with different orientations of the optical eigenaxes can be inferred. We find an attenuation of the characteristic RDS signal of the PET substrate with increasing nickel film thickness which is in good agreement with the theoretical prediction. For film thicknesses above 20 nm another contribution to the RDS signal attributed to the optical anisotropy of the deposited nickel layers can be observed. Its strength depends on the deposition method, and is considerably larger for evaporated films than for sputtered ones. With increasing nickel film thickness, the azimuthal orientation of the sample anisotropy changes from the initial value of the PET substrate by about 20° toward the machine direction of the foil. We demonstrate that RDS is also a valuable tool for inline monitoring in the roll to roll process, as the attenuation of the RDS signal, under proper consideration of the orientation of the effective anisotropy, is a function of the film thickness and characteristic for the deposited material.

Para-Sexiphenyl thin film growth on Cu(1 1 0) and Cu(1 1 0)–(2 × 1)O surfaces

We present a reflectance difference spectroscopy (RDS) study of para-sexiphenyl ( p-6P) thin film growth on Cu(1 1 0) and Cu(1 1 0)–(2 × 1)O substrates. The RDS spectra show pronounced anisotropies for p-6P films formed on both substrates at room temperature, demonstrating that the molecules are uniaxially aligned within the films. Based on the RD spectra and the evolution of the optical transitions with p-6P coverage the growth mode on both substrates could be identified. From the dominating RDS feature, assigned to the lowest energy HOMO–LUMO transition, the orientation of the molecular chain can be determined. On Cu(1 1 0), the p-6P molecular chains align in the [ 1 ¯ 1 0 ] direction, i.e., along the Cu atomic rows, whereas on the Cu(1 1 0)–(2 × 1)O surface, the molecules are oriented in the orthogonal [0 0 1] direction, i.e., along the “added” Cu–O rows of the Cu(1 1 0)–(2 × 1)O surface. The energetic position and line shape of the main RDS feature differs for the two substrates and varies with p-6P coverage. This fine structure is discussed in terms of different molecular conformations, adlayer structure and vibronic replicas.

Reflectance difference spectroscopy – a powerful tool to study adsorption and growth

Changes in the optical anisotropy of the Cu(110) surface due to adsorption and growth have been studied by reflectance difference spectroscopy (RDS). The optical anisotropy signal at 2.13 eV was found to be extremely sensitive to small quantities of CO, while the signal at 4.3 eV turned out to be particularly sensitive to the transition from a (3×1) to a (2×1) CO superstructure. In the case of the oxygen covered Cu(110), we have successfully monitored and analyzed the transition between two oxygen induced Cu(110) reconstructions: The c(6×2)O and the (2×1)O phases have clearly distinguishable RDS features, and the transition between them leads to a gradual transformation of the related RDS spectra. For the system Co on Cu(110)(2×1)O characteristic features of a CoCuO alloy phase have been identified, which allow to monitor the crucial stages of the Co thin film growth. The alloying and de-alloying are easily recognized with RDS.

Optical and Electronic Anisotropy of a π-Conjugated Molecular Monolayer on the Silicon(001) Surface

The electronic and optical properties of an oriented monolayer of a π-conjugated molecule, 9,10-phenanthrenequinone on a silicon(001) substrate were investigated. The adsorption of 9,10-phenanthrenequinone onto a single-domain, 4°-miscut Si(001) surface leads to an oriented monolayer film with intact π conjugation. Reflectance-difference spectroscopy (RDS) was used to probe the optical properties of the layer of 9,10-phenanthrenequinone, and comparison spectra were obtained on a monolayer 1,2-cyclohexanedione, a nonconjugated molecule that has a nearly identical structure at the Si−O−C interface. RDS spectra of the 9,10-phenanthrenequinone monolayer show a strong absorption feature at 5.2 eV due to π−π * transitions within the molecule, while spectra of the nonconjugated analogue show no feature in this region. Further information about the electronic structure is provided by ultraviolet photoemission spectroscopy of the chemisorbed monolayers and by ultraviolet−visible absorption spectroscopy of the pare...

Optical anisotropy of (001)-GaAs surface quantum wells

We report a reflectance difference spectroscopy (RDS) study of the optical anisotropy of GaAs:(001) surface quantum wells consisting of a thin GaAs layer (3--30 nm thick) embedded between an arsenic reconstructed surface and an AlAs barrier. The RDS spectra display anisotropic contributions from the free surface and from the GaAs/AlAs interface. By comparing RDS spectra for the $c(4\ifmmode\times\else\texttimes\fi{}4)$ and $(2\ifmmode\times\else\texttimes\fi{}4)$ surface reconstructions, we separate these two contributions, and demonstrate that the anisotropy around the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ transitions comprises a component originating from modifications of bulk states near the surface. The latter is attributed to anisotropic strains induced by the surface reconstruction. The experimental data are well described by a model for the RDS response of the multilayer structures, which also takes into account the blue energy shifts and the changes in oscillator strength of the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ transitions induced by quantum-well confinement.

In-situ monitoring, structural, and optical properties of ultrathin GaSb/GaAs quantum wells grown by OMVPE

We present a study of the growth of strained ultrathin GaSb quantum well (QW) layers in a GaAs host crystal by organometallic vapor phase epitaxy (OMVPE). We report surface anisotropy features observed by reflectance difference spectroscopy (RDS) during exposure of the GaAs (001) surface to trimethylantimony (TMSb) and during subsequent growth interruption. We demonstrate the formation of a floating layer of Sb during growth of GaAs over GaSb quantum well layers. The periodic nature of the RDS signal during growth of multiple quantum well (MQW) structures allows us to construct time-resolved RDS spectra, detailing the evolution of the surface anisotropy. We show how x-ray diffraction (XRD) data may be used to determine the graded compositional profile resulting from Sb segregation at the GaAs/GaSb interface. Photoluminescence (PL) spectra at 2 K from MQW structures exhibit two peaks below the GaAs bandgap. The lower-energy peak, which we attribute to a type-II transition at the GaSb/GaAs interface, shifts logarithmically with excitation power density. The higher energy peak shows no shift with excitation power, and is attributed to a transition occurring within the graded barrier layers.

Stress in GaAs at the hetero-interface of ZnSe/GaAs/GaAs: a possible effect of pit filling and difference in thermal expansion coefficients

We report on the observation of stress effects on GaAs at the ZnSe–GaAs hetero-interface. Samples with the structure ZnSe/GaAs/GaAs(100) were grown by molecular beam epitaxy (MBE). ZnSe epilayers thickness ranged from 0.08 to 0.6 μm. Hetero-interfacial stress effects were investigated by photoreflectance (PR) and reflectance-difference spectroscopy (RDS). From a comparison between PR spectra and the second energy-derivative of the RDS spectra (SDRD) we conclude that both PR and RDS spectra have two components: (1) a bulk-like signal as for bare GaAs and (2) a signal coming from a strained region near the ZnSe–GaAs hetero-interface. From the theory of PR we estimate that the observed compressive strain giving rise to the second component has a value ε≅−0.0010±0.0004, independent of the thickness of the ZnSe epilayer. Atomic force microscopy (AFM) measurements were carried out on the GaAs epilayer prior to ZnSe growth revealing an almost uniform density of pits for all samples observed. These have irregular cross-section profiles, a situation that tends to preclude coherent growth between the ZnSe and the GaAs. We calculate that there has to be present a strain in the upper atomic layers of the GaAs due to the incoherent growth of ZnSe inside the GaAs pits and to the difference in thermal expansion coefficients between the GaAs and the ZnSe. Both phenomena are expected to produce a total strain of same magnitude as that observed by PR.

Reflectance difference spectroscopy and magneto-optical analysis of digital magnetic heterostructures

We report on the investigation of CdTe/Cd0.75Mg0.25Te quantum wells grown by molecular-beam epitaxy and containing a digital distribution of magnetic MnTe fractional monolayers in the well. The influence of electronic confinement and of exciton primary localization on the formation of ferromagnetic ordering on the scale of small spin clusters (polarons) has been investigated by performing selective excitation of the exciton luminescence. The calculated temperature dependence of the free magnetic polarons binding energy F is found to qualitatively agree with the photoluminescence excitation data and the ferromagnetic clusters result to be more stable in these magnetic structures of reduced dimensionality, with respect to the three-dimensional case. The magneto-optical Kerr effect is discussed in the case of reflectance difference spectroscopy (RDS) and from the angular dependence of the RDS spectra in magnetic field, signals due to interband Faraday rotation and signals originating from crystal anisotropy could be distinguished. The Faraday rotation in these structures is found to follow the Curie–Weiss law with a transition temperature of −6 K corresponding to the nominal ion spin concentration in the magnetic barriers.

Time-resolved reflectance difference spectroscopy of InAs growth under alternating flow conditions

We report a time-resolved reflectance difference spectroscopy (RDS) study of the growth of InAs during alternating flow conditions typical of atomic layer epitaxy (ALE). The precursors used were trimethylindium (TMIn) and tertiarybutylarsine (TBAs). For ALE growth we observe that the InAs surface remains As-rich for an appreciable fraction of the 1 monolayer (ML) TMIn pulse. This is similar to results obtained for the growth of GaAs by ALE using trimethylgallium. Three distinct RDS spectra are observed, two corresponding to As-rich phases, and one which is In-rich. In both InAs and GaAs the extra As layer appears to play a key role in maintaining self-limiting behaviour.

Numerical simulation of ZnSe/GaAs interface reflectance difference spectroscopy

A numerical method based on the matrices established by Yeh [Optical Waves in Layered Media (Wiley, New York, 1988)] is developed to simulate the non-normal incidence reflectance difference spectroscopy (RDS) spectra of biaxial anisotropic (εx≠εy≠εz) multilayer systems. The main features of the RDS spectra obtained from the biaxial anisotropic ZnSe/GaAs interface are reproduced by the numerical method. It has demonstrated that in the cases of near-normal incidence and when the anisotropy within the layer plane (in-plane anisotropy) is small (εx−εy≪εx) a RDS spectrum can be separated into two spectra, namely, the in-plane anisotropic spectrum and the off-plane anisotropic spectrum. The reflectance of the s wave and the p wave can be calculated separately when the in-plane principal axes are at certain orientations, making it possible to obtain the anisotropic dielectric tensor directly from the measured spectra.

Influence of hydrogen adsorption on the optical properties of the GaAs(100)-c(4 x 4) surface.

We have investigated the influence of hydrogen adsorption on the optical anisotropy and the surface reconstruction of the GaAs(100)-c(4\ifmmode\times\else\texttimes\fi{}4) surface using reflectance difference spectroscopy (RDS) and low-energy electron diffraction (LEED), respectively, At low exposures the optical anisotropy of the c(4\ifmmode\times\else\texttimes\fi{}4) surface is removed almost completely by hydrogen adsorption due to removal of the As dimers in the outermost As layer, while the LEED pattern changes from c(4\ifmmode\times\else\texttimes\fi{}4) to (1\ifmmode\times\else\texttimes\fi{}1). At intermediate hydrogen exposures, the surface again becomes optically anisotropic showing the signature of Ga dimers in the RDS spectra, while LEED indicates a mixture of (1\ifmmode\times\else\texttimes\fi{}2) and (\ensuremath{\surd}2 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 ) reconstructions. Finally, at even higher dosages the Ga dimers are broken by hydrogenation. The LEED pattern shows a (1\ifmmode\times\else\texttimes\fi{}1) symmetry and the RDS features are correlated with the critical points of the bulk electronic band structure.

Comparison of reflectance difference spectroscopy and surface photoabsorption used for the investigation of anisotropic surfaces

Reflectance difference spectroscopy (RDS) and surface photoabsorption (SPA) are two optical characterization techniques that are now in common use for monitoring and controlling epitaxial growth. Here we establish the connection of SPA data to the surface dielectric response, using an anisotropic three phase model (substrate, anisotropic overlayer, ambient). In contrast to the generally accepted picture, our calculations show, that the contribution to the SPA signal from the dielectric component normal to the surface is negligible, and that the major contribution arises from the projection of the dielectric tensor on the line formed by the intersection of the surface and the plane of incidence. It follows, that the difference between SPA data taken with the plane of incidence along the different principal axis is related to, and in fact can be calculated from, RDS spectra. By comparing published data to our database of RD spectra, we are able to find the surface reconstruction present during flow modulated organo metallic chemical vapour epitaxial growth of GaAs epilayers.