We report on in situ Reflectance Difference Spectroscopy measurements carried out on GaAs (001). Measurements were
performed at temperatures of 580 °C and 430 °C, in both n and p-type doped films and for both (2x4) and c(4x4)
reconstructions. Samples employed were grown by Molecular Beam Epitaxy with doping levels in the range from
1016 - 1019 cm-3. We demonstrate the potential of Reflectance Difference Spectroscopy for impurity level determinations under growth conditions.
Reflectance Difference Spectroscopy (RDS) is a powerful tool for the optical characterization of cubic semiconductors.
Several physical mechanisms have been identified to contribute to the RDS signal. Among these we can count on
surface electric fields, lineal defects, and surface strains. The RDS setups reported so far, use photodiodes and
photomultipliers as light detectors and lock-in techniques to process the signal. In the present work we describe a new
instrument based on a charged-coupled device (CCD) as light detector. By focusing the light on the CCD, it is possible
to obtain the RD spectra coming from different regions of the semiconductor surface, by analyzing the spectra for a
group of pixels of the CCD. The instrument can be used to obtain a topographic map of the surface of the semiconductor.
The present paper discusses an improvement for the method by division of cells which is used in multiplexed computer generated holograms (CGH's). Such improvement allows increasing the final number of codified images into a single hologram. Some important properties of images are saved because they will be the key to perform the adequate operations involved in the reconstruction process. The experimental results demonstrate the effectiveness of the suggested procedure.
We report on the growth by MBE and characterization of optically-pumped mid-infrared Vertical Cavity Surface Emitting Lasers (VCSELs), where the optical cavity is formed by a semiconductor Sb-based Bragg mirror, an air gap and a high reflectivity dielectric concave mirror. These lasers operate between 2 μm and 2.5 μm in continuous wave regime at room temperature with a circular TEM00 beam. Two different fabrication processes are tested and the properties of the corresponding devices are compared.
We report on the application of reflectance-difference (RD) spectroscopy to the characterization of 60 degree dislocations in zincblend semiconductors. We discuss a physical model based on dislocation induced anisotropic strains which predict a RD lineshape proportional to the first energy derivative of the semiconductor reflectance spectrum. We present RD spectra for semi-insulating GaAs:Cr (100) crystals in the 1.2 - 3.5 eV energy range, which show a first derivative component in accordance to our model. From a fitting of the experimental RD spectra to the theoretical lineshape we obtain average values for the strains associated to 60 degree dislocations. We also show that for the samples reported in this paper the dislocation-induced anisotropic strain results in a normalized effective change in lattice constant in the range from 10-5 to 10-4.
We describe both reflectance anisotropy and electroreflectance measurements carried out to
determine the physical origin of the anisotropies observed in the reflectance spectrum of (001) and
(110) GaAs. We find an anisotropy component which depends on impurity concentration for both
(001) and (110) surfaces [and on conductivity type for (001) GaAs]. This component is actually a
bulk-related electro-optic effect produced by the electric field present at the semiconductor surface.
This electric field is due to the pinning of the Fermi level at surface states. We find that a linear
electro-optic effect is responsible for the impurity-dependent aniso tropies observed in GaAs (001),
while a quadratic electro- optic effect is responsible for those observed in GaAs (110). We give an
estimate for the linear electro-optic coefficients of GaAs at energies around the E1 and E1 + z