A. Rocher

Lateral modulations in zero‐net‐strained GaInAsP multilayers grown by gas source molecular‐beam epitaxy

Compressive GaInAsP multiple quantum wells (MQW) grown by gas source molecular‐beam epitaxy present altered structural and optical characteristics when tensile GaInAsP barriers are used instead of lattice‐matched ones. An alternate tensile/compressive GaInAsP MQW has been examined by transmission electron microscopy. A strong lateral modulation of thickness, strain, and probably chemical composition was shown. This modulation exhibits pronounced anisotropy, with a periodicity of about 50 nm along the [110] direction. Although its origin is not fully accounted for yet, it seems to allow partial elastic relaxation of tensile layers. Based on this analysis, a schematic description of distortion modulation is proposed.

Strain relaxation in the epitaxy of La2/3Sr1/3MnO3 grown by pulsed-laser deposition on SrTiO3(001)

With a Curie point at 370 K, the half-metal (La0.7Sr0.3)MnO3 (LSMO) is one of the most interesting candidates for electronic devices based on tunnel magnetoresistance. SrTiO3 (STO) is up to now the best substrate for the epitaxy of suitable thin films of LSMO. The pseudocubic unit cell of rhombohedral LSMO has a parameter a LSMO such that (a STO − a LSMO)/a LSMO = + 0.83% (where a STO is the parameter of cubic STO) and an angle of 90.26°. As strained growth is tetragonal, relaxation implies recovery of both the pseudocubic parameter and of the original angle. In the LSMO layers that we prepare by pulsed-laser deposition, we show that these two processes are quite independent. The angular distortion is partially recovered by twinning in films 25 nm thick, while recovery of the parameter never occurs in the thickness range that we explored (up to 432 nm). A relaxation, however, takes place above a thickness of 100 nm, associated with a transition from two-dimensional to three-dimensional columnar growth. It is accompanied by chemical fluctuations. Our magnetic measurements exhibit Curie temperatures and magnetic moments very close to the bulk values in those layers where the crystal parameter is strained but the angle partially relaxed.

Solid state interdiffusions in epitaxial Fe/GaAs(001) heterostructures during ultrahigh vacuum annealings up to 450 °C

We have used a set of complementary experimental techniques to characterize an epitaxial structure (25 nm Fe)/GaAs(001) annealed at 450 °C under ultrahigh vacuum conditions. The solid state interdiffusion leads to the formation of an epitaxial reaction layer made of Fe2As patches embedded in a Ga rich Fe3Ga2−XAsX ternary phase. The epitaxial character of this layer explains how the usually reported epitaxial growth of Fe on GaAs performed in the temperature range of 175 to 225 °C is possible in spite of the species intermixing occurring at the interface. Moreover, the observed grains of Fe2As explain the decrease of magnetization at the interface in such contact, since Fe2As is an antiferromagnetic alloy.

SELF-INDUCED LATERALLY MODULATED GAINP/INASP STRUCTURE GROWN BY METAL-ORGANIC VAPOR-PHASE EPITAXY

Zero‐net strained multilayer alternating tensile GaInP and compressive InAsP have been grown on (001)InP by metal‐organic vapor‐phase epitaxy. A structural analysis using transmission electron microscopy (TEM) is reported. A remarkably regular laterally modulated structure has been observed. GaInP‐ and InAsP‐rich vertical zones alternate with a periodicity of 0.28 μm along the lateral [110] direction, thus balancing the mismatch along the [110] rather than the [001] growth direction. TEM experiments suggest that each vertical zone is partially elastically relaxed.

Accommodation of lattice mismatch and threading of dislocations in GaSb films grown at different temperatures on GaAs (001)

Abstract Transmission electron microscopy has been used to investigate the accommodation of lattice mismatch and the threading mechanism of dislocations in GaSb films deposited at 420, 470 and 520°C by molecular beam epitaxy (MBE) on GaAs (001) substrate. The lattice mismatch was relieved by regular 90° dislocations generated during island growth of GaSb films. At high temperature, however, the lattice mismatch was partly accommodated by 60° dislocation arrays which induce a local tilt of GaSb film with respect to the substrate. Even in the best case of misfit accommodation by very regular 90° dislocations, the density of threading dislocations reaches 1 × 10 10 cm -2 . The main source of threading dislocations is attributed to the coalescence of randomly distributed GaSb islands. In particular, at low temperature many 90° dislocations thread directly to the film surface, which is explained by the difference of 90° dislocation spacings in two coalesced islands.

Quantitative analysis of strain field in thin films from HRTEM micrographs

Abstract A method for measuring and mapping displacement fields and strain fields has been developed. It is based on the Fourier analysis of a HRTEM lattice image selecting a strong Bragg reflection and performing an inverse Fourier transform. The phase component of the resulting complex image gives information on local displacements of atomic planes. The two-dimensional (2D) displacement field can then be derived by applying the method to two non-collinear Fourier components. Local strain tensor components e xx ( r → ), e yy ( r → ), e xy ( r → ) and e yx ( r → ) can be obtained by analysing the derivative of the displacement field. Applied to HRTEM images of thin films, this technique gives quantitative information on the variations of the strain relaxations in the different layers. The method is illustrated studying a fully relaxed GaSb/GaAs interface.

Gas source molecular beam epitaxy of alternated tensile / compressive strained GaInAsP multiple quantum wells emitting at 1.5 μm

Abstract Compressive and tensile strained GaInAsP layers as well as zero-net strain multiple quantum wells, grown by gas source molecular beam epitaxy, have been investigated. Reflection high energy electron diffraction patterns show two-dimensional growth for compressive strained layers ( Δa a . Three-dimensional growth is observed after a few nanometers for tensile strained layers even for low tensile value ( Δa a . Transmission electron microscopy shows that three-dimensional growth of a tensile strained layer is related to a quasi-periodic composition modulation along the [110] in the epitaxial plane.

Structural and electrical investigations of silicon wafer bonding interfaces

Abstract The structure and the electrical properties of Si/Si interfaces obtained by wafer bonding are studied by transmission electron microscopy (TEM), electron-beam-induced current (EBIC) and spreading resistance (SR). Boron-doped Czochralsky-silicon (Cz-Si) (001) wafers and float zone-silicon (FZ-Si) (001) wafers are bonded after hydrophobic cleaning of the surfaces and annealed at 1200 °C for 2 h. The TEM experiments show that these interfaces are made of two dislocation networks: a grid of screw dislocations accommodating the misorientation between the two wafers and a set of 60 ° dislocations compensating a tilt component related to the vicinal surfaces. These dislocations are all located at the interface and no extended defects propagate in the Si wafers. The EBIC experiments show a large electrical activity of the Cz interfaces; SR profiles show a variation of carriers concentration near the interface. Large density of oxide precipitates located at the interface are seen for Cz-Si wafers. EBIC observations performed on FZ wafer show a weak electrical activity which seems to be related to a low content of oxygen in the interface.

Atomic Structure of the Interfaces Between Silicon Directly Bonded Wafers

The so-called Direct Wafer Bonding (DWB) technique opens new possibilities for the electronic industry but still suffers from the poor knowledge we have of the microstructure of these interfaces and hence of their electrical activity. In this work, we have extensively used Transmission Electron Microscopy techniques in plan-view and cross-section to identify the structure of the interfaces found between two bonded silicon wafers. The general structure of these interfaces is that of a perfect grain boundary and evidently depends on the misorientation between the two bonded wafers. A twist component in the range 0>θ>13° creates a square network of pure screw dislocation whereas an unavoidable tilt component (<0.5°) is compensated by a periodic array of 60° dislocation lines perpendicular to the tilt direction. Therefore, the regularity of these networks can be disrupted by the presence of steps (of up to several nanometers) in the interface plane. Silicon oxide precipitates are seen heterogeneously distributed on the interface with preferential nucleation sites on the dislocations.

Direct measurement of lateral elastic modulations in a zero‐net strained GaInAsP/InP multilayer

A zero‐net strained GaInAsP/InP(001) multilayer grown by gas source molecular beam epitaxy exhibits large interfacial undulations and an orthorhombic modulated distortion, which are attributed to elastic relaxation of tensile layers. It is examined by high resolution transmission electron microscopy in order to directly determine the strain distribution in the (110) plane. The interplanar spacings are found to be laterally modulated along the [110] direction within tensile and compressive layers. The interplanar spacing modulation is perfectly correlated to the interfacial morphology. Therefore, the strain is not homogeneous but concentrated along [110] oriented lines. Large variations of (110) interplanar spacings up to 3.5% have been measured, while the lattice mismatch between tensile and compressive layers is only 2%. The observed strain distribution is qualitatively consistent with an elastic relaxation mechanism of the tensile layer. Different other effects are reviewed. Surface relaxation effects...