Bilinear Magnetoresistance in HgTe Topological Insulator: Opposite Signs at Opposite Surfaces Demonstrated by Gate Control.

Spin-orbit effects appearing in topological insulators (TI) and at Rashba interfaces are currently revolutionizing how we can manipulate spins and have led to several newly discovered effects, from spin-charge interconversion and spin-orbit torques to novel magnetoresistance phenomena. In particular, a puzzling magnetoresistance has been evidenced as bilinear in electric and magnetic fields. Here, we report the observation of bilinear magnetoresistance (BMR) in strained HgTe, a prototypical TI. We show that both the amplitude and sign of this BMR can be tuned by controlling with an electric gate the relative proportions of the opposite contributions of opposite surfaces. At magnetic fields of 1 T, the magnetoresistance is of the order of 1% and has a larger figure of merit than previously measured TIs. We propose a theoretical model giving a quantitative account of our experimental data. This phenomenon, unique to TI, offers novel opportunities to tune their electrical response for spintronics.

Role of threading dislocations on the growth of HgCdTe epilayers investigated using monochromatic X-ray Bragg diffraction imaging.

High-quality Hg1-xCdxTe (MCT) single crystals are essential for two-dimensional infrared detector arrays. Crystal quality plays an important role on the performance of these devices. Here, the dislocations present at the interface of CdZnTe (CZT) substrates and liquid-phase epitaxy grown MCT epilayers are investigated using X-ray Bragg diffraction imaging (XBDI). The diffraction contributions coming from the threading dislocations (TDs) of the CZT substrate and the MCT epilayers are separated using weak-beam conditions in projection topographs. The results clearly suggest that the lattice parameter of the growing MCT epilayer is, at the growth inception, very close to that of the CZT substrate and gradually departs from the substrate's lattice parameter as the growth advances. Moreover, the relative growth velocity of the MCT epilayer around the TDs is found to be faster by a factor of two to four compared with the matrix. In addition, a fast alternative method to the conventional characterization methods for probing crystals with low dislocation density such as atomic force microscopy and optical interferometry is introduced. A 1.5 mm × 1.5 mm area map of the epilayer defects with sub-micrometre spatial resolution is generated, using section XBDI, by blocking the diffraction contribution of the substrate and scanning the sample spatially.

Using 2D integral breadth to study plastic relaxation in a quasi-lattice-matched HgCdTe/CdZnTe heterostructure.

Micro-Laue diffraction has been used to record cross-section profiles on a quasi-lattice-matched HgCdTe/CdZnTe heterostructure as a function of the stress induced by a flexion machine. The heterostructure may be decomposed into four different regions according to depth. Sufficiently far from the interface, the CdZnTe substrate is undisturbed by the HgCdTe layer, while the region situated 10 µm beneath the interface presents an in-plane lattice parameter adjustment to the +0.02% mismatched layer. The layer has a 2 µm critical thickness and, beyond, misfit dislocations induce a large peak broadening whose main direction changes with depth. The same occurs over the whole heterostructure once flexion-induced plastification has started. Consequently, the usual full width at half-maximum or integral breadth is no longer relevant, and only a newly defined and rotationally invariant 2D integral breadth correctly measures the plastification-induced peak broadening. Taking into account only the critical thickness region, a 15.1 ±â€…0.7 MPa tensile HgCdTe elastic limit was measured, slightly overestimated because of the initial compressive layer stress. It was observed that the plastic onset of the substrate perfectly matches the elastic limit of the layer, despite the fact that the substrate elastic limit is expected to be four times higher: a striking demonstration of the propagation of threading dislocations. The 'plastification easiness' is found to be 2.4 times smaller deep inside the substrate than in the layer critical thickness region, while in the substrate lattice adjustment region, the plastification easiness goes from the substrate to the layer value with a 22-25 MPa transition interval. This novel method using the 2D integral breadth allows for easy critical thickness measurement as well as precise plastic onset determination and plastification easiness assessment. It is a quite general method, since it may be applied to the vast class of epitaxial layers for which the critical thickness is larger than the micro-Laue beam size (currently 250 nm).