Substrate nanopatterning by e-beam lithography to growth ordered arrays of III-nitride nanodetectors, white light emitters, and solar cells

III-nitride nanorods have attracted much scientific interest during the last decade because of their unique optical and electrical properties [1,2]. The high crystal quality and the absence of extended defects make them ideal candidates for the fabrication of high efficiency opto-electronic devices such as nano-photodetectors, light-emitting diodes, and solar cells [1-3]. Nitride nanorods are commonly grown in the self-assembled mode by plasma-assisted molecular beam epitaxy (MBE) [4]. However, self-assembled nanorods are characterized by inhomogeneous heights and diameters, which render the device processing very difficult and negatively affect the electronic transport properties of the final device. For this reason, the selective area growth (SAG) mode has been proposed, where the nanorods preferentially grow with high order on pre-defined sites on a pre-patterned substrate

E-beam nanopatterning for the selective area growth of III-V nitride nanorods

GaN/InGaN nanorods have attracted much scientific interest during the last decade because of their unique optical and electrical properties [1,2]. The high crystal quality and the absence of extended defects make them ideal candidates for the fabrication of high efficiency opto-electronic devices such as nano-photodetectors, light-emitting diodes, and solar cells [1-3]. Nitrides nanorods are commonly grown in the self-assembled mode by plasma-assisted molecular beam epitaxy (MBE) [4]. However, self-assembled nanorods are characterized by inhomogeneous heights and diameters, which render the device processing very difficult and negatively affect the electronic transport properties of the final device. For this reason, the selective area growth (SAG) mode has been proposed, where the nanorods preferentially grow on pre-defined sites on a pre-patterned substrate [5].

Cu(In,Ga)Se2 absorber thinning and the homo-interface model: Influence of Mo back contact and 3-stage process on device characteristics

Thinning the absorber layer is one of the possibilities envisaged to further decrease the production costs of Cu(In,Ga)Se2 (CIGSe) thin films solar cell technology. In the present study, the electronic transport in submicron CIGSe-based devices has been investigated and compared to that of standard devices. It is observed that when the absorber is around 0.5 μm-thick, tunnelling enhanced interface recombination dominates, which harms cells energy conversion efficiency. It is also shown that by varying either the properties of the Mo back contact or the characteristics of 3-stage growth processing, one can shift the dominating recombination mechanism from interface to space charge region and thereby improve the cells efficiency. Discussions on these experimental facts led to the conclusions that 3-stage process implies the formation of a CIGSe/CIGSe homo-interface, whose location as well as properties rule the device operation; its influence is enhanced in submicron CIGSe based solar cells.

Fano-Rashba effect in quantum dots

We consider the electronic transport through a Rashba quantum dot coupled to ferromagnetic leads. We show that the interference of localized electron states with resonant electron states leads to the appearance of the Fano-Rashba effect. This effect occurs due to the interference of bound levels of spin-polarized electrons with the continuum of electronic states with an opposite spin polarization. We investigate this Fano-Rashba effect as a function of the applied magnetic field and Rashba spin-orbit coupling.

Analysis of the Kondo effect in ferromagnetic atomic-sized contacts

Atomic contacts made of ferromagnetic metals present zero-bias anomalies in the differential conductance due to the Kondo effect. These systems provide a unique opportunity to perform a statistical analysis of the Kondo parameters in nanostructures since a large number of contacts can be easily fabricated using break-junction techniques. The details of the atomic structure differ from one contact to another so a large number of different configurations can be statistically analyzed. Here we present such a statistical analysis of the Kondo effect in atomic contacts made from the ferromagnetic transition metals Ni, Co, and Fe. Our analysis shows clear differences between materials that can be understood by fundamental theoretical considerations. This combination of experiments and theory allows us to extract information about the origin and nature of the Kondo effect in these systems and to explore the influence of geometry and valence in the Kondo screening of atomic-sized nanostructures.

Onset of energy dissipation in ballistic atomic wires

Electronic transport at finite voltages in free-standing gold atomic chains of up to seven atoms in length is studied at low temperatures using a scanning tunneling microscope. The conductance vs voltage curves show that transport in these single-mode ballistic atomic wires is nondissipative up to a finite voltage threshold of the order of several mV. The onset of dissipation and resistance within the wire corresponds to the excitation of the atomic vibrations by the electrons traversing the wire and is very sensitive to strain.

Cotunneling theory of atomic spin inelastic electron tunneling spectroscopy

We propose cotunneling as the microscopic mechanism that makes possible inelastic electron tunneling spectroscopy of magnetic atoms in surfaces for a wide range of systems, including single magnetic adatoms, molecules, and molecular stacks. We describe electronic transport between the scanning tip and the conducting surface through the magnetic system (MS) with a generalized Anderson model, without making use of effective spin models. Transport and spin dynamics are described with an effective cotunneling Hamiltonian in which the correlations in the magnetic system are calculated exactly and the coupling to the electrodes is included up to second order in the tip MS and MS substrate. In the adequate limit our approach is equivalent to the phenomenological Kondo exchange model that successfully describes the experiments. We apply our method to study in detail inelastic transport in two systems, stacks of cobalt phthalocyanines and a single Mn atom on Cu2N. Our method accounts for both the large contribution of the inelastic spin exchange events to the conductance and the observed conductance asymmetry.

Giant magnetoresistance in ultrasmall graphene based devices

By computing spin-polarized electronic transport across a finite zigzag graphene ribbon bridging two metallic graphene electrodes, we demonstrate, as a proof of principle, that devices featuring 100% magnetoresistance can be built entirely out of carbon. In the ground state a short zigzag ribbon is an antiferromagnetic insulator which, when connecting two metallic electrodes, acts as a tunnel barrier that suppresses the conductance. The application of a magnetic field makes the ribbon ferromagnetic and conductive, increasing dramatically the current between electrodes. We predict large magnetoresistance in this system at liquid nitrogen temperature and 10 T or at liquid helium temperature and 300 G.

Spin-filtered edge states in graphene

Spin–orbit coupling changes graphene, in principle, into a two-dimensional topological insulator, also known as quantum spin Hall insulator. One of the expected consequences is the existence of spin-filtered edge states that carry dissipationless spin currents and undergo no backscattering in the presence of non-magnetic disorder, leading to quantization of conductance. Whereas, due to the small size of spin–orbit coupling in graphene, the experimental observation of these remarkable predictions is unlikely, the theoretical understanding of these spin-filtered states is shedding light on the electronic properties of edge states in other two-dimensional quantum spin Hall insulators. Here we review the effect of a variety of perturbations, like curvature, disorder, edge reconstruction, edge crystallographic orientation, and Coulomb interactions on the electronic properties of these spin filtered states.

Understanding the structure of the first atomic contact in gold

We have studied experimentally jump-to-contact (JC) and jump-out-of-contact (JOC) phenomena in gold electrodes. JC can be observed at first contact when two metals approach each other, while JOC occurs in the last contact before breaking. When the indentation depth between the electrodes is limited to a certain value of conductance, a highly reproducible behaviour in the evolution of the conductance can be obtained for hundreds of cycles of formation and rupture. Molecular dynamics simulations of this process show how the two metallic electrodes are shaped into tips of a well-defined crystallographic structure formed through a mechanical annealing mechanism. We report a detailed analysis of the atomic configurations obtained before contact and rupture of these stable structures and obtained their conductance using first-principles quantum transport calculations. These results help us understand the values of conductance obtained experimentally in the JC and JOC phenomena and improve our understanding of atomic-sized contacts and the evolution of their structural characteristics.

Modeling contact formation between atomic-sized gold tips via molecular dynamics

The formation and rupture of atomic-sized contacts is modelled by means of molecular dynamics simulations. Such nano-contacts are realized in scanning tunnelling microscope and mechanically controlled break junction experiments. These instruments routinely measure the conductance across the nano-sized electrodes as they are brought into contact and separated, permitting conductance traces to be recorded that are plots of conductance versus the distance between the electrodes. One interesting feature of the conductance traces is that for some metals and geometric configurations a jump in the value of the conductance is observed right before contact between the electrodes, a phenomenon known as jump-to-contact. This paper considers, from a computational point of view, the dynamics of contact between two gold nano-electrodes. Repeated indentation of the two surfaces on each other is performed in two crystallographic orientations of face-centred cubic gold, namely (001) and (111). Ultimately, the intention is to identify the structures at the atomic level at the moment of first contact between the surfaces, since the value of the conductance is related to the minimum cross-section in the contact region. Conductance values obtained in this way are determined using first principles electronic transport calculations, with atomic configurations taken from the molecular dynamics simulations serving as input structures.

Rashba spin precession in a magnetic field

Spin precession due to Rashba spin-orbit coupling in a two-dimension electron gas is the basis for the spin field effect transistor, in which the overall perfect spin-polarized current modulation could be acquired. There is a prerequisite, however, that a strong transverse confinement potential should be imposed on the electron gas or the width of the confined quantum well must be narrow. We propose relieving this rather strict limitation by applying an external magnetic field perpendicular to the plane of the electron gas because the effect of the magnetic field on the conductance of the system is equivalent to the enhancement of the lateral confining potential. Our results show that the applied magnetic field has little effect on the spin precession length or period although in this case Rashba spin-orbit coupling could lead to a Zeeman-type spin splitting of the energy band.

Duality of weak and strong scatterer in a Luttinger liquid coupled to massless bosons

We study electronic transport in a Luttinger liquid with an embedded impurity, which is either a weak scatterer (WS) or a weak link (WL), when interacting electrons are coupled to one-dimensional massless bosons (e.g., acoustic phonons). We find that the duality relation, ?WS?WL=1, between scaling dimensions of the electron backscattering in the WS and WL limits, established for the standard Luttinger liquid, holds in the presence of the additional coupling for an arbitrary fixed strength of boson scattering from the impurity. This means that at low temperatures such a system remains either an ideal insulator or an ideal metal, regardless of the scattering strength. On the other hand, when fermion and boson scattering from the impurity are correlated, the system has a rich phase diagram that includes a metal-insulator transition at some intermediate values of the scattering.

First-principles calculation of carrier-phonon scattering in n-type Si1−xGex alloys

First-principles electronic structure methods are used to find the rates of inelastic intravalley and intervalley n-type carrier scattering in Si1-xGex alloys. Scattering parameters for all relevant Delta and L intra- and intervalley scattering are calculated. The short-wavelength acoustic and the optical phonon modes in the alloy are computed using the random mass approximation, with interatomic forces calculated in the virtual crystal approximation using density functional perturbation theory. Optical phonon and intervalley scattering matrix elements are calculated from these modes of the disordered alloy. It is found that alloy disorder has only a small effect on the overall inelastic intervalley scattering rate at room temperature. Intravalley acoustic scattering rates are calculated within the deformation potential approximation. The acoustic deformation potentials are found directly and the range of validity of the deformation potential approximation verified in long-wavelength frozen phonon calculations. Details of the calculation of elastic alloy scattering rates presented in an earlier paper are also given. Elastic alloy disorder scattering is found to dominate over inelastic scattering, except for almost pure silicon (x approximate to 0) or almost pure germanium (x approximate to 1), where acoustic phonon scattering is predominant. The n-type carrier mobility, calculated from the total (elastic plus inelastic) scattering rate, using the Boltzmann transport equation in the relaxation time approximation, is in excellent agreement with experiments on bulk, unstrained alloys..

First-principles calculation of P-type alloy scattering in Si1-xGex

The p-type carrier scattering rate due to alloy disorder in Si1-xGex alloys is obtained from first principles. The required alloy scattering matrix elements are calculated from the energy splitting of the valence bands, which arise when one average host atom is replaced by a Ge or Si atom in supercells containing up to 128 atoms. Alloy scattering within the valence bands is found to be characterized by a single scattering parameter. The hole mobility is calculated from the scattering rate using the Boltzmann transport equation in the relaxation time approximation. The results are in good agreement with experiments on bulk, unstrained alloys..