The electronic structure of La1−x_{1-x}Srx_{x}MnO3_{3} thin films and its TcT_c dependence as studied by angle-resolved photoemission

We present angle-resolved photoemission spectroscopy results for thin films of the three-dimensional manganese perovskite La$_{1-x}$Sr$_{x}$MnO$_{3}$. We show that the transition temperature ($T_c$) from the paramagnetic insulating to ferromagnetic metallic state is closely related to details of the electronic structure, particularly to the spectral weight at the ${\bf k}$-point, where the sharpest step at the Fermi level was observed. We found that this ${\bf k}$-point is the same for all the samples, despite their different $T_c$. The change of $T_c$ is discussed in terms of kinetic energy optimization. Our ARPES results suggest that the change of the electronic structure for the samples having different transition temperatures is different from the rigid band shift.Comment: Accepted by Journal of Physics: Condensed Matte

Total Angular Momentum Conservation During Tunnelling through Semiconductor Barriers

We have investigated the electrical transport through strained p-Si/Si_{1-x}Ge_x double-barrier resonant tunnelling diodes. The confinement shift for diodes with different well width, the shift due to a central potential spike in a well, and magnetotunnelling spectroscopy demonstrate that the first two resonances are due to tunnelling through heavy hole levels, whereas there is no sign of tunnelling through the first light hole state. This demonstrates for the first time the conservation of the total angular momentum in valence band resonant tunnelling. It is also shown that conduction through light hole states is possible in many structures due to tunnelling of carriers from bulk emitter states.Comment: 4 pages, 4 figure

Atomic scale strain relaxation in axial semiconductor III-V nanowire heterostructures

Combination of mismatched materials in semiconductor nanowire heterostructures offers a freedom of bandstructure engineering that is impossible in standard planar epitaxy. Nevertheless, the presence of strain and structural defects directly control the optoelectronic properties of these nanomaterials. Understanding with atomic accuracy how mismatched heterostructures release or accommodate strain, therefore, is highly desirable. By using atomic resolution high angle annular dark field scanning transmission electron microscopy combined with geometrical phase analyses and computer simulations, we are able to establish the relaxation mechanisms (including both elastic and plastic deformations) to release the mismatch strain in axial nanowire heterostructures. Formation of misfit dislocations, diffusion of atomic species, polarity transfer, and induced structural transformations are studied with atomic resolution at the intermediate ternary interfaces. Two nanowire heterostructure systems with promising applications (InAs/InSb and GaAs/GaSb) have been selected as key examples

Pulsed laser deposition of atomically flat La1-xSrxMnO3 thin films using a novel target geometry

A new ablation target geometry is presented that was used to produce thin films of La1-xSrxMnO3 grown heteroepitaxially on SrTiO3 by pulsed reactive crossed-beam laser ablation. The films were grown in order to perform angle-resolved photoelectron spectroscopy, which demands that the surface be atomically flat. In situ and ex situ analysis shows that this condition was met, even after depositing to a thickness of over 100n

A Structural Characterization of GaAs MBE Grown on Si Pillars

Growth on deeply patterned substrates, i.e. on pillars instead of a continuous substrate, is expected to be very promising to get crack free epilayers on wafers without any bowing. We report here on a structural investigation of GaAs MBE deposited on patterned (001) offcut Si, consisting of pillars 8 μm high and 5 to 9 μm wide, to check mostly the behaviour of the threading dislocations. It is found that only very rarely they propagate up to the GaAs top that will serve as active region in devices. Twins were also detected which sometimes reached the topmost part of GaAs. However, as twins have no associated dangling bonds, they should not be electrically active. Rare antiphase boundaries exist at the interface, hence not harmful for device operation