A. 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.

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.

Study of the crystal quality and Ga-segregation in ZnSe films grown by molecular beam epitaxy on AlxGa1−xAs and InxGa1−xAs buffer layers on GaAs substrates

Abstract In the present work, we report a study of the molecular beam epitaxial growth of ZnSe on GaAs substrates using Al x Ga 1− x As and In x Ga 1− x As ternary alloys as buffer layers. When growing ZnSe directly on a thermally desorbed GaAs substrate, surface segregation of Ga across the film towards the ZnSe surface was observed by secondary ion mass spectroscopy. We demonstrate that the use of AlGaAs buffer layers is very effective to suppress the Ga surface segregation. The characterization of the films by reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, photoluminescence and photoreflectance spectroscopy revealed that the best crystal quality ZnSe films were obtained for buffer layers with In or Al concentrations of 1%.

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.

Growth of self-assembled InAs quantum dots on Si exposed GaAs substrates by molecular beam epitaxy

Summary form only given. Semiconductor quantum dot (QDs) structures have received increasing attention over the last few years because they are expected to lead to significant improvements in optical and electronic device applications. Nowadays, all efforts in this area are directed toward controlling sizes, densities and the spatial arrangement of QDs, since all of these are crucial factors on the effectiveness of QD-based optoelectronic devices. In this work, we investigated the effects induced by Si during the formation of self-assembled InAs QDs. The samples were prepared in a Riber 32D MBE system. AFM and photoluminescence were used to characterize the samples.

Self-Assembled GaAs Quantum Dots on Pseudomorphic Si Layers Grown on AlGaAs by Molecular Beam Epitaxy.

A new method to obtain self-assembled GaAs quantum dots is presented. In this method, one monolayer (ML) of Si is pseudomorphically grown on an AlGaAs layer by molecular beam epitaxy. The subsequent GaAs overgrowth proceeds in a three dimensional (3D) mode on the pseudomorphic Si layer. For a GaAs growth equivalent to 2 ML, the self-assembled 3D islands presented an average height of 1.5 nm, with an average lateral size of 20 nm, and a density in the order of 1×1011/cm2. The transition to 3D GaAs growth strongly depends on the Si interlayer thickness.