D. Araújo

Design of InGaAs linear graded buffer structures

The relaxation of compositionally graded InGaAs buffers, with and without uniform cap layers, has been studied. Simple InGaAs linear‐graded layers on GaAs substrates never reach complete relaxation. The residual strain in these structures produces a dislocation‐free strained top region while the rest of the buffer is nearly completely relaxed through misfit dislocations, as observed by transmission electron microscopy (TEM). This strained top region is analyzed and its thickness compared with theoretical calculations. The effects of different cap layers on the relaxation behavior of the graded buffer has been studied by double crystal x‐ray diffraction, TEM, and low temperature photoluminescence, and results compared with predictions of the models. The optical quality of the cap layer improves when its composition is close to the value that matches the lattice parameter of the strained surface of the grade. The design of linear graded buffers having a strain‐free cap layer with high crystalline quality is...

Critical boron-doping levels for generation of dislocations in synthetic diamond

Defects induced by boron doping in diamond layers were studied by transmission electron microscopy. The existence of a critical boron doping level above which defects are generated is reported. This level is found to be dependent on the CH4/H2 molar ratios and on growth directions. The critical boron concentration lied in the 6.5–17.0 × 1020at/cm3 range in the ⟨111⟩ direction and at 3.2 × 1021 at/cm3 for the ⟨001⟩ one. Strain related effects induced by the doping are shown not to be responsible. From the location of dislocations and their Burger vectors, a model is proposed, together with their generation mechanism.

SiC voids, mosaic microstructure and dislocations distribution in Si carbonized layers

Abstract The defect structure of SiC/Si layers obtained by carbonization of Si is reported by means of transmission electron microscopy in high-resolution (HREM) and conventional (CTEM) modes. 3C–SiC was obtained after a rapid thermal annealing treatment and good interfacial quality is reported in terms of small void dimensions and densities. Moreover, high misfit dislocation densities are observed close to the Si/SiC interface and inside the SiC layer without observable generation of threading dislocations. The mosaic grain structure is also evidenced, with low misorientation with respect to the substrate. These results are encouraging for further growth of III–N alloy heterostructures.

Spatially correlated microstructure and superconductivity in polycrystalline boron-doped diamond

Scanning tunneling spectroscopies are performed below 100 mK on polycrystalline boron-doped diamond films characterized by transmission electron microscopy and transport measurements. We demonstrate a strong correlation between the local superconductivity strength and the granular structure of the films. The study of the spectral shape, amplitude, and temperature dependence of the superconductivity gap enables us to differentiate intrinsically superconducting grains that follow the BCS model, from grains showing a different behavior involving the superconducting proximity effect.

Boron concentration profiling by high angle annular dark field-scanning transmission electron microscopy in homoepitaxial δ-doped diamond layers

To develop further diamond related devices, the concentration and spatial location of dopants should be controlled down to the nanometer scale. Scanning transmission electron microscopy using the high angle annular dark field mode is shown to be sensitive to boron doping in diamond epilayers. An analytical procedure is described, whereby local boron concentrations above 1020 cm−3 were quantitatively derived down to nanometer resolution from the signal dependence on thickness and boron content. Experimental boron local doping profiles measured on diamond p−/p++/p− multilayers are compared to macroscopic profiles obtained by secondary ion mass spectrometry, avoiding reported artefacts.

Electron microscopy study of SiC obtained by the carbonization of Si(111)

A SiC thin film grown by propane carbonization of a Si(111) substrate has been characterized by transmission electron microscopy and scanning electron microscopy techniques. This study reveals the presence of planar defects in the SiC layer and voids in the Si(111) substrate as well as misfit dislocations at the SiC/Si interface. The resulting SiC layer consists of a mosaic structure and is shown to have low stress.

Step-graded buffer layer study of the strain relaxation by transmission electron microscopy

Abstract The lattice relaxation behavior in an In x Ga 1−x As/GaAs linearly step-graded structure is studied by transmission electron microscopy (TEM). From the misfit dislocation densities measured by TEM at each interface the relaxation parameters such as strain and percentage relaxation are deduced for each layer. The obtained results are compared with the predictions of the Dunstan et al. model which describe the dislocation behavior during relaxation in such structures. A different relaxation behavior than that described by Dunstan et al. is observed. This is attributed to the fact that the individual layer thickness is lower than the critical layer thickness of Dunstan et al. Work-hardening processes are found to induce a linear increase in the residual strain with increasing layer thickness.

Stratigraphy of a diamond epitaxial three-dimensional overgrowth using doping superlattices

The selective doped overgrowth of 3D mesa patterns and trenches has become an essential fabrication step of advanced monolithic diamond-based power devices. The methodology here proposed combines the overgrowth of plasma-etched cylindrical mesa structures with the sequential growth of dopingsuperlattices. The latter involve thin heavily borondoped epilayers separating thicker undoped epilayers in a periodic fashion. Besides the classical shape analysis under the scanning electron microscope relying on the appearance of facets corresponding to the main crystallographic directions and their evolution toward slow growing facets, the dopingsuperlattices were used as markers in oriented cross-sectional lamellas prepared by focused ion beam and observed by transmission electron microscopy. This stratigraphic approach is shown here to be applicable to overgrown structures where faceting was not detectable. Intermediate growth directions were detected at different times of the growth process and the periodicity of the superlattice allowed to calculate the growth rates and parameters, providing an original insight into the planarization mechanism. Different configurations of the growth front were obtained for different sample orientations, illustrating the anisotropy of the 3D growth.Dislocations were also observed along the lateral growth fronts with two types of Burger vector:b011¯=12[011¯] and b112=16[112]. Moreover, the clustering of these extended defects in specific regions of the overgrowth prompted a proposal of two different dislocation generation mechanisms.

Multiple quantum well GaAs/AlGaAs solar cells: transport and recombination properties by means of EBIC and cathodoluminescence

Abstract Multiple quantum well (MQW) p-i-n heterostructures are a new alternative to increase the quantum efficiency of solar cell devices. In such structures, the QW carrier capture, carrier escape and radiative recombinations are the phenomena governing the efficiency of the p-i(MQW)-n solar cell. In this contribution, in spite of the photon-induced current mode of work of such device, an electron beam-induced-current (EBIC) study allowing a very localized carrier excitation is reported. The EBIC measurements are shown to be able to estimate with high accuracy the QWs capture–escape and radiative lifetimes. The latter are determined as a function of the carrier injection level fitting experimental EBIC profiles to simulated ones. Values around 10 −10 and 10 −7 s respectively are obtained.

Influence of interface dislocations on surface kinetics during epitaxial growth of InGaAs

The correlation between surface striations and misfit dislocations at the interface has been studied on In x Ga 1-x As single layers (x < 0.25), as a function of the growth parameters (substrate temperature and deposition rate), by means of atomic force and transmission electron microscopies. It is concluded that both features may be initially linked by mechanical causes (elastic displacement fields), but eventually evolve in a different way due to the surface kinetic effects. The range of growth conditions for an optimum surface quality is determined. A simplified treatment of the diffusion equation, in which the effect of the surface on the dislocation stress field is included, has allowed an estimation of the effective mean free path between collisions for the group III adatoms in the range of a few A.