We report reproducible fabrication of InP-InAsP nanowire light emitting diodes in which electron-hole recombination is restricted to a quantum-dot-sized InAsP section. The nanowire geometry naturally self-aligns the quantum dot with the n-InP and p-InP ends of the wire, making these devices promising candidates for electrically-driven quantum optics experiments. We have investigated the operation of these nano-LEDs with a consistent series of experiments at room temperature and at 10 K, demonstrating the potential of this system for single photon applications.Nanowire light emitting diodes (NW LEDs) offer exciting new possibilities for opto-electronic devices. Growth of direct-bandgap NWs on Si 1, 2 will allow optically active elements to be integrated with already highly mature Si technology. For solid-state lighting applications, broad-area LEDs made from NW arrays have higher light-extraction efficiency than traditional planar LEDs 3 , and in the field of quantum optics, NWs offer the possibility to control electron transport at the single-electron level 4 and light emission at the single-photon level 5 .1Since the first demonstration of GaAs NW LEDs in 1992 6 , different geometries and materials have been used to produce NW LEDs operating over a wide range of wavelengths 3, 7-10 . Single-NW LEDs with doping modulation in the axial direction, which is the most interesting geometry for many applications, have been fabricated using GaN-GaInN multi-junctions 3 and a proof-of-principle device has been shown using InP 7 . In this letter we describe the fabrication and characterization of reproducible axial InP NW LED devices, and show that an active InAsP quantum dot region can be incorporated into these devices. The axial geometry allows for controllable injection of electrons and holes into the precisely defined active region, with the additional advantage of high light-extraction efficiency since the optically active region is not embedded in a high refractive index material. Unlike GaInN, InAsP emission can be tuned to infra-red telecommunications wavelengths where there is strong interest in electrically driven single-photon sources 11 .Nanowire p-n junctions were reproducibly grown in the vapor-liquid-solid (VLS) growth mode 12 by use of low-pressure metal-organic vapour-phase epitaxy (MOVPE). 20 nm colloidal Au particles were dispersed on (111)B InP substrates, after which the samples were transferred to a MOVPE system (Aixtron 200), and placed on a RF-heated gas foil rotated graphite disc on a graphite susceptor. The samples were heated to a growth temperature of 420 °C under phosphine (PH 3 ) containing ambient at molar fraction χ PH3 = 8.3×10 -3 , using hydrogen as carrier gas (6 l/min H 2 at 50 mbar). After a 30 s temperature stabilization step, the NW growth was initiated by introducing trimethyl-indium (TMI) into the reactor cell at a molar fraction of χ TMI = 2.2×10 -5 . During the first 20 minutes, hydrogen sulfide (χ H2S = 1.7×10 -6 ) was used for n-type doping, after which the p-type NW part was g...

Gate capacitances of back-gated nanowire field-effect transistors (NW-FETs) are calculated by means of finite element methods and the results are compared with analytical results of the "metallic cylinder on an infinite metal plate model". Completely embedded and non-embedded NW-FETs are considered. It is shown that the use of the analytical expressions also for non-embedded NW-FETs gives carrier mobilities that are nearly two times too small. Furthermore, the electric field amplification of non-embedded NW-FETs and the influence of the cross-section shape of the nanowires are discussed.PACS numbers: 41.20.Cv, 85.30.Tv Semiconducting nanowires (NWs) with diameters from several nanometers up to around 100nm are currently being investigating for their potential towards realization of electronic devices with significantly enhanced performance like enhanced carrier mobility. While much effort is spent on growth and structural characterization, obviously reliable and accurate electrical characterization is equally important. For electronic applications the charge carrier mobility µ is a crucial property. The mobility can be determined from the linear part of the transconductance g = dI sd /dU gs measured in a field-effect transistor device µ = gL 2 /(CU sd ), with L being the channel length, C the gate capacitance, U sd the source-drain, and U gs the gate-source voltage.In the case of NWs usually back-gated nanowire fieldeffect transistors (NW-FETs) are fabricated: [3,4,5,6,7,8,9,10,11,12,13,14, 15] a highly doped Sisubstrate and thermally grown SiO 2 function as the backgate and the gate dielectric, respectively. Then NWs are transferred on the SiO 2 followed by the deposition of the source and drain electrodes. Such a transistor has the advantage that it can be processed relatively easily. However, in practical devices other geometries like the wrap-around NW-FET will be preferred. A cross sectional view along the NW axis with equipotential lines due to the gate voltage is shown in Fig. 1(a). To calculate the gate capacitance, usually the "metallic cylinder on an infinite metal plate model" [1] is used. [6,7,8,9,10,11,12,13,14,15] The cross section geometry and the equipotential lines of this model are depicted in Fig. 1(b). To use this model, the charge density in the nanowire is assumed to be so high that the semiconducting NW can be treated as metallic. It was shown [16] that this approximation yields reasonable results for GaN NWs with doping concentrations above 10 17 cm −3 . Additionally, it is assumed that the NW is much longer than the dielectric layer thickness so that the fringing capacitance at the source and drain contact can be neglected and that there are no movable charges or defects in the dielectric layer or at the NW surface. The model further assumes that the NWs are completely embedded in the dielectric and posses a circular cross section. The latter assumptions are reexamined in detail in this paper.The model yields an analytical equation for the gate capacitance per unit lengthwith ǫ r bein...

We show the epitaxial integration of III-V semiconductor nanowires with silicon technology. The wires are grown by the VLS mechanism with laser ablation as well as metal-organic vapour phase epitaxy. The hetero-epitaxial growth of the III-V nanowires on silicon was confirmed with x-ray diffraction pole figures and cross-sectional transmission electron microscopy. We show preliminary results of two-terminal electrical measurements of III-V nanowires grown on silicon. E-beam lithography was used to predefine the position of the nanowires.

Based on model and ab initio calculations we discuss the effect of resonant interface states on the conductance of epitaxial tunnel junctions. In particular we show that the ''hot spots'' found by several groups in ab initio calculations of symmetrical barriers of the k ʈ -resolved conductance can be explained by the formation of bonding and antibonding hybrids between the interface states on both sides of the barrier. If the resonance condition for these hybrid states is met, the electron tunnels through the barrier without attenuation. Even when both hybrid states move together and form a single resonance, strongly enhanced transmission is still observed. The effect explains why, for intermediate barrier thicknesses, the tunneling conductance can be dominated by interface states, although hot spots only occur in a tiny fraction of the surface Brillouin zone. DOI: 10.1103/PhysRevB.65.064425 PACS number͑s͒: 72.25.Ϫb, 73.20.Ϫr, 73.40.Rw, 73.40.Gk The tunneling magnetoresistance ͑TMR͒ of magnetic tunnel junctions consisting of ferromagnet͉insulator͉ferro-magnet layers has attracted a strong scientific interest, partly due to their potential application as magnetic random access memories. Miyazaki and Tezuka 1 and Moodera et al. 2 were able to obtain TMR ratios up to 20% in room-temperature experiments and recently room-temperature values of more than 50% were reported by various groups. The understanding of the TMR and of the electronic structure has not progressed equally quickly. Model calculations 3,4 have shed light on various aspects of the effect, but only recently have ab initio calculations of the electronic structure and the spindependent transport been reported. [5][6][7][8] In this paper, we will consider the tunneling through epitaxial junctions, which are characterized by two-dimensional periodicity. Here recent ab initio calculations of the k ʈ -resolved conductance show a very interesting phenomenon: for certain discrete k ʈ values ''hot spots'' or ''spikes'' appear in the transmitted intensity, showing that electrons with such k ʈ values can apparently tunnel through the junction with no or very little attenuation while all other states are very strongly damped. [9][10][11] This effect occurs only in the minority band of the ferromagnet and only for ferromagnetic coupling. If present, it can dominate the tunnel characteristics for intermediate thicknesses. For large thicknesses, in the asymptotic limit, the behavior is determined by the complex band structure of the insulator, 12 i.e., by those metal-inducedgap states, which have the smallest imaginary part of the perpendicular component k z of the Bloch vector. An example for such hot spots is given in Fig. 1, showing the results of ab initio Korringa-Kohn-Rostoker calculations for a junction consisting of two fcc Co͑001͒ half-crystals separated by 4 monolayers ͑ML͒ of vacuum. The results are based on density-functional theory in the local-density approximation and the Landauer formula for the conductance. We have chosen a vacuum layer as the ...