International audiencePrecession electron diffraction is an efficient technique to measure strain in nanostructures by precessing the electron beam, while maintaining a few nanometre probe size. Here, we show that an advanced diffraction pattern treatment allows reproducible and precise strain measurements to be obtained using a default 512 x 512 DigiSTAR off-axis camera both in advanced or non-corrected transmission electron microscopes. This treatment consists in both projective geometry correction of diffraction pattern distortions and strain Delaunay triangulation based analysis. Precision in the strain measurement is improved and reached 2.7 x 10(-4) with a probe size approaching 4.2 nm in diameter. This method is applied to the study of the strain state in InGaAs quantum-well (QW) devices elaborated on Si substrate. Results show that the GaAs/Si mismatch does not induce in-plane strain fluctuations in the InGaAs QW region. (C) 2014 AIP Publishing LLC
Changes in the lattice parameters, i.e., introduction of strain, can modify material properties greatly. For instance, the band structure is modified with strain and this leads to changes in the transport or optical properties. In microelectronic devices, strain has been used to improve the mobility of charge carriers since 2003, with strain as low as 0.7% improving mobility by 50% [1]. Transmission Electron Microscopy (TEM) is presently the only technique that can measure the strain in individual nano-objects with high spatial resolution (about 1 nm) and high precision (about 10 -4 ). Here we focus on recent developments we have made in two electron diffraction techniques: Off-axis Convergent Beam Electron Diffraction (CBED) and Nanobeam Precession Electron Diffraction (N-PED). Off-axis CBED can give 3D maps of the complete 3D strain tensor but it is computationally and experimentally demanding. In contrast, N-PED is a straightforward and precise technique, but it is limited to the projected 2D strain.In off-axis CBED the originality of our approach is to use both the deficient HOLZ lines of the transmitted beam and the excess HOLZ lines of the diffracted beams to measure the strain [2]. Using Bloch wave calculated CBED patterns as tests, we could retrieve 7 out of the 9 components of the deformation gradient tensor F. In particular, the volume of the cells can be determined (Fig. 1b). By using two different electron beam directions engendering an angle of 22°, we show that it is possible to determine the whole tensor F. In addition, the method can also be extended to the analysis of split HOLZ lines that allow measuring the variations of the strain tensor along the electron beam.Depending on the convergence angle (α), the diffraction spots look like more dots or disks and the diameter of the incident beam varies (Fig. 1). When α is about 2 mrad, the diffracted disks do not have a uniform intensity (Fig. 2f), which can complicate accurate determination of the position of the disks. It is why in Nanobeam Electron diffraction (NBED) α is reduced in order to have spots with more uniform intensities (Fig. 2c) [3]. As these intensity variations are very sensitive to thickness, chemical composition and orientation, it turns out that the positioning of the disk centers is not very robust. This is why we introduced the precession of the beam [4]: the incident beam is tilted by an angle α p and rotated around the original incident direction and a "descan" of the beam is applied after the sample in order to bring back the diffracted spot to their original positions (Fig. 1g-1h). The advantages of introducing precession are manifold: (i) uniformity in the disk intensity, (ii) presence of more diffracted spots, (iii) possibility to work with a relatively large convergence angle in order to reduce the beam diameter ( Fig. 1j-1k); all this leads to an improved precision with a smaller probe [4]. Classically, PED is used in crystallography problems with α p greater than 1°, in order to have diffracted intensities that appro...
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