We report on a rapid, 32-channel reflectance-difference (RD) spectrometer with sub-second spectra acquisition times and ΔR/R sensitivity in the upper 10(-4) range. The spectrometer is based on a 50 kHz photo-elastic modulator for light polarization modulation and on a lock-in amplifier for signal harmonic analysis. Multichannel operation is allowed by multiplexing the 32 outputs of the spectrometer into the input of the lock-in amplifier. The spectrometer spans a wavelength range of 230 nm that can be tuned to cover E(1) and E(1) + Δ(1) transitions for a number of III-V semiconductors at epitaxial growth temperatures, including GaAs, InAs, AlAs, and their alloys. We present two examples of real-time measurements to demonstrate the performance of the RD spectrometer, namely, the evolution of the RD spectrum of GaAs (001) annealed at 500 °C and the time-dependent RD spectrum during the first stages of the epitaxial growth of In(0.3)Ga(0.7)As on GaAs (001) substrates.

We report on real time-resolved Reflectance-difference (RD) spectroscopy of GaAs(001) grown by molecular beam epitaxy, with a time-resolution of 500 ms per spectrum within the 2.3–4.0 eV photon energy range. Through the analysis of transient RD spectra we demonstrated that RD line shapes are comprised of two components with different physical origins and determined their evolution during growth. Such components were ascribed to the subsurface strain induced by surface reconstruction and to surface stoichiometry. Results reported in this paper render RD spectroscopy as a powerful tool for the study of fundamental processes during the epitaxial growth of zincblende semiconductors.

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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.

Reflectance difference spectroscopy (RDS/RAS) constitutes a very sensitive tool for the study of the optical properties of semiconductors. RDS spectra comprise several components of different physical origins. One of these components is the strain induced by the surface. In the present work, we report RD spectra measured on GaAs at room temperature at around E1 and the E1+Δ1 transitions. Two cases were studied, the surface strains induced by the c (4× 4) reconstruction and by the vacancies generated by the annealing of semi insulating GaAs crystal. Fits to the line shape of the c (4× 4) reconstructed and annealed spectra were performed by using a surface strain component for one‐electron and discrete‐exciton lineshapes respectively. The character of the fits were interpreted by estimating the piezoelectric field induced by surface strain. For the c (4× 4) surface the strength of the field partially quenches the exciton while for the annealing surface this field is fifty times lower, and an excitonic lineshape is expected.We also inferred that the RD spectra include a component related to surface roughness. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)