Electrical and optical measurements of the bandgap energy of a light-emitting diode

International audienceSemiconductors materials are at the basis of electronics. Most electronic devices are made of semiconductors. The operation of these components is well described by quantum physics which is a little mysterious to students. One of the intrinsic parameters of semiconductors is their bandgap energy Eg. In the case of light-emitting diodes (LEDs) Eg fixes the colour of the light emitted by the diodes. We propose in this article a lab work allowing a comparison of Eg of a green LED obtained by both electrical and optical measurements. The two slightly different results can be explained by the theoritical knowledges of the students on solid physics and electronic devices internal architecture

An introduction to photocatalysis through methylene blue photodegradation

International audienceWe described a simple experimental setup for a lab work on the photocatalytic degradation of methylene blue by T iO 2 nanoparticles. The photocatalysis process can be used for many applications. Treatments for diluted wastewaters industries, air purifying in underground car parks, preventing fouling on glass surfaces, these are some of the things this phenomenon has the ability to do. The described experiment is easy to perform and the interpretation can be easily adapted to different level of students: from high school students demonstrating their interests on a sustainable development, to students who are getting Masters in the science department that want to propose a full explanation of all the phenomenon of the photocatalytic process. Starting by a description of the experimental setup , we analysed the photocatalyst nanoparticles and applied the Langmuir-Hinshelwood model to our experimental data. Finally we discussed shortly on the respective energetic levels of the photocatalyst semiconductor and the methylene blue

Mn5Ge3C0.6/Ge(111) Schottky contacts tuned by a n-type ultra-shallow doping layer

International audienceMn 5 Ge 3 Cxcompound is of great interest for spintronics applications. 10 The various parameters of Au/Mn 5 Ge 3 C 0.6 /Ge(111) and Au/Mn 5 Ge 3 C 0.6 /δ-doped Ge(111) Schottky diodes were measured in the temperature range of 30-300 K by using current-voltage and capacitance-voltage techniques. The Schottky barrier heights and ideality factors were found to be temperature dependent. These anomaly behaviours were explained by Schottky barriers inhomogeneities 15 and interpreted by means of Gaussian distributions model of the Schottky barrier heights. Following this approach we show that the Mn 5 Ge 3 C 0.6 /Ge contact is described with a single Gaussian distribution and a conduction mechanism mainly based on the thermoionic emission. On the other hand the Mn 5 Ge 3 C 0.6 /δ-doped Ge contact is depicted with two Gaussian distributions according to the 20 temperature and a thermionic-field emission process. The differences between the two types of contacts are discussed according to the distinctive features of the growth of heavily doped germanium thin films

Magnetic properties of self-organized Co dimer nanolines on Si/Ag(110)

International audienceWe demonstrate the kinetically controlled growth of one-dimensional Co nanomagnets with a high lateral order on a nanopatterned Ag(110) surface. First, self-organized Si nanoribbons are formed upon submonolayer condensation of Si on the anisotropic Ag(110) surface. Depending on the growth temperature, individual or regular arrays (with a pitch of 2 nm) of Si nanoribbons can be grown. Next, the Si/Ag(110) system is used as a novel one-dimensional Si template to guide the growth of Co dimer nanolines on top of the Si nanoribbons, taking advantage of the fact that the thermally activated process of Co diffusion into the Si layer is efficiently hindered at 220 K. Magnetic characterization of the Co nanolines using X-ray magnetic circular dichroism reveals that the first atomic Co layer directly adsorbed onto the Si nanoribbons presents a weak magnetic response. However, the second Co layer exhibits an enhanced magnetization, strongly suggesting a ferromagnetic ordering with an in-plane easy axis of magnetization, which is perpendicular to the Co nanolines

Ferromagnetic resonance and magnetic damping in C-doped Mn5Ge3

International audience2 X-band ferromagnetic resonance (FMR) was used to investigate static and dynamic magnetic properties of Mn5Ge3 and Carbon-doped Mn5Ge3 (C0.1 and C0.2) thin films grown on Ge(111). The temperature dependence of magnetic anisotropy shows an increased perpendicular magneto-crystalline contribution at low temperature with an in-plane easy axis due to the large shape contribution. We find that our samples show as small as 40Oe FMR linewidth (corresponding Gilbert damping α=0.005), for the out-of-plane direction, certifying of their very good structural quality. The perpendicular linewidth shows a minimum around 200K for all samples, which seems not correlated to the C-doping. The magnetic relaxation parameters have been determined and indicate as main extrinsic contribution the two-magnon scattering. A transition from six-fold to twofold plus fourth-fold in-plane anisotropy is observed in the FMR linewidth of Mn5Ge3C0.2 around 200K

Very low-temperature epitaxial growth of Mn5Ge3 and Mn5Ge3C0.2 films on Ge(111) using molecular beam epitaxy

International audienceC-doped Mn5Ge3 compound is ferromagnetic at temperature up to 430 K. Hence it is a potential spin injector into group-IV semiconductors. Segregation and diffusion of Mn at the Mn5Ge3 /Ge interface could severely hinder the efficiency of the spin injection. To avoid these two phenomena we investigate the growth of Mn5Ge3 and C-doped Mn5Ge3 films on Ge(111) substrates by molecular beam epitaxy at room-temperature. The reactive deposition epitaxy method is used to deposit these films. Reflection high energy electron diffraction, X-ray diffraction analysis, transmission electron microscopy and atomic force microscopy indicate that the crystalline quality is very high. Magnetic characterizations by superconducting quantum interference device and ferromagnetic resonance reinforce the structural analysis results on the thin films quality

Effect of carbon on structural and magnetic properties of Ge1−xMnx\hbox {Ge}_{1-x}\hbox {Mn}_{x} nanocolumns

International audienceWe have investigated the structural and magnetic properties of Ge 0.94 Mn 0.06−δ C δ films (δ = 0.005, 0.01 and 0.02) using reflexion high-energy electron diffraction (RHEED) technique, transmission electron microscopy (TEM) and superconducting quantum interference device magnetometer. All films have been prepared by co-depositing Ge, Mn and C by molecular beam epitaxy. RHEED pattern shows the increase in sample surface roughness when doping carbon into the GeMn films. TEM analyses indicate that adding carbon greatly reduces the surface diffusion of both Ge and Mn elements. Ferromagnetic ordering in samples containing carbon contents of 0.01 and 0.02, persists at temperatures >400 K. An increase in net magnetization is found for carbon-doped samples with increasing carbon content from 0.01 to 0.02. However, we found a decrease in the net magnetization and the Curie temperature of the samples after annealing at 450 and 650°C. The Curie temperature reduces down to about 300 K, which is comparable to the value of the free-carbon sample showing a harmful effect of the post-annealing on the magnetic properties of carbon-doped GeMn nanocolumns

Gaining insights into the formation of high temperature GeMn nanocolumns: the effect of substrate crystalline orientation on their structural and magnetic properties

International audienceStructural and magnetic characterisations along with first-principles total energy calculations based on the density functional theory have been combined to investigate the effects of (001) and (111)-oriented Ge substrates on the formation of GeMn nanocolumns. The samples were grown by means of molecular beam epitaxy (MBE) at the growth temperature (T S) of 130 °C, the Mn concentration of ∼6% and the film thickness of ∼80 nm. We found that due to the surfactant effect of Mn atoms, Ge 0,94 Mn 0,06 films grown on the Ge(001) substrate exhibit the nanocolumn structure along the growth direction and the Curie temperature (T C) is higher than 400 K. On the other hand, for the Ge 0,94 Mn 0,06 films grown on the Ge(111) substrate, Mn adatoms can easily diffuse into deeper layers through the interstitial sites, resulting in the formation of Ge 3 Mn 5 streaks along the preferred direction [110] and surrounded by diluted matrix. The physical origin of the contrasting behaviour of Mn atoms is therefore strongly related to the different surface reconstruction

Direct evidence for mean-free-path effects in the magnetoresistance of magnetic multilayers with the current perpendicular to the planes

Experimental evidence is presented showing that for magnetic multilayers measured in the CPP mode (current perpendicular to the planes), the magnetic-field dependence of the magnetoresistance MR(H) is dominated by scattering processes in which the electron is scattered by the potential due to pairs of neighboring magnetic layers. It is demonstrated experimentally that curtailing such scattering processes leads to a significant decrease in MR(H). These results are confirmed by a calculation

Quantum transport simulation based on an equation of motion method: An application to current-perpendicular-to-the-plane giant magnetoresistance

International audienceA tight-binding equation of motion ͑EOM͒ method for the simulation of electronic transport in complex, and inhomogeneous systems is presented. Conductance is calculated in the linear response regime where chemical potentials can mimic electrochemical potentials. The technique is first elucidated by application to several simple systems to clarify important issues. A calculation of current-perpendicular-to-the-plane giant magnetoresistance ͑GMR͒ in a Co/ Cu multilayer then follows. A 67% GMR is calculated which originates primarily from spin-dependent interface resistances. The advantages of an EOM method are that complicated geometries can be considered, and interactions such as spin-orbit effects or phonons, for example, may be included easily