L. F. Lastras-Martínez

Stress-induced optical anisotropies measured by modulated reflectance

In the last few years, the understanding of information delivered by reflectance difference/anisotropy spectroscopy (RAS) has grown considerably. However, a full understanding of this optical technique is not yet achieved because surface, interface and bulk effects are present particularly where heteroepitaxial systems are concerned. This is especially true for the case of resonances at the bulk critical points of the dielectric function, which either resemble the dielectric function or its derivative. Previous RAS experiments on zincblende and diamond structure semiconductors found optical anisotropies in the vicinity of the E0, E1 and E1 + ?1 critical points. In this review, the origin of these structures is discussed and it is shown that anisotropic in-plane strain in the epilayer or bulk induces resonances at these critical points. This in-plane strain is either caused by the boundaries of the epilayer system (i.e. the surface or the interface) or by symmetry breaking via a surface electric field or a preferred orientation of dislocations. These findings are best supported by applying additionally photoreflectance difference spectroscopy (PRD), where the difference between a spectrum taken with linearly polarized light and with unpolarized light is measured. In contrast to RAS, PRD spectroscopy is specific to the symmetry breakdown occurring due to band bending via the surface or interface electric field and stress.

Microreflectance difference spectrometer based on a charge coupled device camera: surface distribution of polishing-related linear defect density in GaAs (001)

We describe a microreflectance difference (microRD) spectrometer based on a charge coupled device (CCD), in contrast to most common RD spectrometers that are based on a photomultiplier or a photodiode as a light detector. The advantage of our instrument over others is the possibility to isolate the RD spectrum of specific areas of the sample; thus topographic maps of the surface can be obtained. In our setup we have a maximum spatial resolution of approximately 2.50 microm x 2.50 microm and a spectral range from 1.2 to 5.5 eV. To illustrate the performance of the spectrometer, we have measured strains in mechanically polished GaAs (001) single crystals.

Linear Electro-Optic Photoreflectance Spectra of GaAs and CdTe around E1 and E1 + Δ1

We report on the measurement of linear electro-optic (LEO) photoreflectance (PR) spectra for both GaAs and CdTe (001) crystals. It is found that the amplitude of the LEO spectra is directly related to the sample doping level, while the sign of these spectra is dependent on the conductivity type of the sample. GaAs LEO spectral lineshapes, in contrast, are found to be independent of the conductivity type in the 1015 to 1017 cm—3 impurity range. These facts, along with a precise theoretical understanding of the LEO spectra, allow for the use of LEO-PR spectroscopy for the characterization of surface and interface electric fields.

A multichannel reflectance anisotropy spectrometer for epitaxial growth monitoring

We report on a reflectance anisotropy (RA) spectrometer capable of measuring reflectance spectra on the 100 ms time-scale and sensitivity in the upper 10−4 range. A multichannel lock-in amplifier was used to acquire 32 wavelengths RA spectra covering the 2.25–3.85 eV photon energy range, where the E 1 and transitions of GaAs and other technologically relevant III–V semiconductor are located. The RA spectra recorded during the first stages of the GaAs homoepitaxial deposition are presented for the first 0.38 monolayers of growth, showing significative changes in the lineshape with low noise. Thanks to the capabilities of this instrument, it is possible to observe in detail, in terms of the evolution of RA spectra, the processes carried out during the migration of surface reconstruction between two stable phases present in the homoepitaxial growth of GaAs.

Linear electro-optic reflectance modulated spectra of GaAs (001) around E1 and E1+Δ1

Abstract We report on the determination of the linear electro-optic (LEO) reflectance modulated spectra of GaAs (101). LEO spectra were obtained from photoreflectance (PR) measurements. Experiments were carried out in an energy range around the E 1 and E 1 +Δ 1 interband transitions. Two samples were employed, an undoped homepitaxial GaAs and a GaAs bulk crystal doped with Te donors in the low 10 17 /cm 3 range. We show that, although LEO spectra amplitudes for the two samples differ for one order of magnitude, their line shape is essentially the same. We further report on reflectance-difference (RD) measurements aimed to obtain LEO spectra. We employed the same GaAs samples for, both, PR and RD measurements in order to contrast the information provided by these two techniques.

One electron and discrete excitonic contributions to the optical response of semiconductors around E 1 transition: analysis in the reciprocal space

Spectroscopic ellipsometry (SE) has been utilized during the past decades for the measurement of the dielectric function of semiconductors. By using SE, interband critical point parameters such as energy gaps and broadenings are routinely determined. In the direct-space analysis approach, these parameters are known by taking the numerical energy derivatives of the dielectric function and fitting the spectra by using a Lorenzian line shape. However, in many cases the noise of the spectra does not allow the determination of such parameters as precisely as they are needed. Additionally, the determination of the character of the transitions, which is uncorrelated (one electron) or correlated (discrete excitons), is necessary for the analysis of the dielectric function. For instance, different values for the broadening parameter are obtained by using uncorrelated or correlated line shapes. We use a reciprocal-space analysis instead of the most commonly used direct-space analysis for determining without any uncertainty the character and, consequently, a precise value of the broadening parameter of the E1 transitions of GaP, GaAs, Si, CdTe, GaSb, HgTe, and an alloy semiconductor: Cd0.18Hg0.82Te.

Characterization of Si3N4/Si(111) thin films by reflectance difference spectroscopy

Si3N4 has become an important material with great technological and scientific interests. The lattice symmetry and the crystallinity quality of Si3N4 thin films are fundamental parameters that must be determined for different applications. In order to evaluate the properties of Si3N4 films, we used reflectance difference spectroscopy/reflectance anisotropy spectroscopy (RDS/RAS) to measure the optical anisotropy of Si3N4 thin films (1–2 nm) grown by nitridation of two different Si(111) substrates, one with a 4.2° miscut off towards the direction and another one with a nonintentional miscut. We demonstrate that, by modifying the measurement optical setup, we could increase the RD sensitivity and clearly display the optical response corresponding to the hexagonal symmetry of the Si3N4 thin layer. Our results are in good agreement with reflection high energy electron diffraction (RHEED) measurements for both misoriented and oriented substrates.

Measurement of the shear strain of the Gd2O3/GaAs(001) interface by photoreflectance difference spectroscopy

In this work, we report on photoreflectance (PR) and photoreflectance-difference (PR-D) measurements of GaAs(001) upon deposition of Gd2O3 thin films. The study is focused on two different substrates: a semi-insulating (SI) with Cr impurities and a Si-doped n-type. PR-D results show that Gd2O3 induces a tensile strain on the GaAs surface and a direct piezo-electric dipole is created. Such strain changes the crystal symmetry from cubic to orthorhombic and renders the quadratic electro-optic (QEO) component anisotropic. For the SI substrate, both linear electro-optic (LEO) and QEO components contribute to the PR-D spectrum, whereas the n-type PR-D spectrum is dominated by the LEO component. In both cases, a tensile strain induces a rigid redshift of ∼20 meV to low energies of the E1 and E1 + Δ1 optical transitions.

Structural Characterization of a Capillary Microfluidic Chip Using Microreflectance.

The structural characterization of capillary microfluidic chips is important for reliable applications. In particular, nondestructive diagnostic tools to assess geometrical dimensions and their correlations with control processes are of much importance, preferably if they are implemented in situ. Several techniques to accomplish this task have been reported; namely, optical coherence tomography (OCT) jointly with confocal fluorescence microscopy (CFM) to investigate internal features of lab-on-a-chip technologies. In this paper, we report on the use of a simple optical technique, based on near-normal incidence microreflectance, which allows mapping internal features of a microfluidic chip in a straightforward way. Our setup is based on a charge-coupled device camera that allows a lateral resolution of ∼2.5 µm and allows us to measure in the wavelength range of 640–750 nm. The technique takes advantage of the Fabry–Perot interferences features in the reflectance spectra, which are further analyzed by a discrete Fourier transform. In this way, the amplitude of the Fourier coefficients is modulated by the presence of a microfluidic channel.

Reflectance-difference spectroscopy: a technique for characterization of dislocations in semiconductors

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.