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We report for the first time on molecular beam epitaxial growth of new hybrid Al(Ga)Sb/InAs/Cd(Mg)Se heterostructures and studies of their structural, luminescence and transport properties. The structures demonstrate intense photoluminescence (PL) in both visible and infra-red spectral regions. The results of luminescence and structural measurements indicate the rather high structural quality of the A3Â5/A2Â6 interface. Theoretical estimations of band line-ups in these structures predict type II band alignment for the InAs/CdSe interface, which transforms to type I with increasing the Mg content in a CdMgSe alloy. A valence band offset as large as 1.6 eV is expected at the InAs/Cd(Mg)Se interface. The results of electron transport measurements along the interface are in good agreement with the theoretical estimation.

The discovery of carrier induced ferromagnetism in p-type doped diluted magnetic semiconductors (DMS) with Curie temperatures actually up to 110 K in Ga 1− x Mn x As, as well as the demonstration of high spin injection efficiency observed from II–VI and III–V DMS [1,2] have recently renewed strongly the interest in DMS. In this paper, we present an overview of the recent experiments we have performed on spin injection light emitting diodes using an n-type doped II–VI DMS layer as spin aligner and on p-type doped II–VI magnetic heterostructures: Cd 1− x Mn x Te quantum wells and Zn 1− x Mn x Te epilayers.

Electrical spin injection (i.e., injection of a spin-polarized current) into semiconductors has been a hot topic in semiconductor physics over the past few years [1] . The reasons are obvious: spin injection would pave the way for an entirely new class of electronic devices, in which the electron's spin, rather than its charge is manipulated for information processing [2] . Such devices could, e.g., combine the advantages of a magnetic hard-disk with semiconductor memory. Also, devices dissipating only minimal amounts of energy could be developed, and—because spin is an intrinsically quantum-mechanical property—spin devices could be used for a solid-state implementation of logical gates in a quantum computer. For several years different research groups have tried to achieve spin injection in top semiconductors from ferromagnetic metal contacts. Although careful experiments were performed, none of them showed convincing results [3,4] . This lack of success can be explained by a simple resistor model which shows that indeed a mismatch in conductivities prevents efficient spin injection in these systems. We have used a dilute magnetic II–VI-semiconductor as a spin aligner to inject highly spin polarized electrons into a GaAs light emitting diode. The circular polarization of the electroluminescence of the diode indicates that we indeed achieve up to 90% of electron spin polarization in our experiment and that DMSs are a class of materials which is highly promising for spin injection into semiconductors.

Single diluted magnetic semiconductor (DMS) quantum dots are studied by means of photoluminescence spectroscopy and magnetoluminescence. The sp-d exchange interaction between a single electron-hole pair and roughly 100 Mn spins within the dot is demonstrated to result in (i) a significant enhancement (more than one order of magnitude) of the emission linewidth and (ii) a strongly modified magnetic field dependence of the polarization degree in a single DMS quantum dot as compared to a non-magnetic reference sample.

Recently, efficient spin injection, being the first step towards semiconductor spin electronics, by using BeMnZnSe as a spin filter was accomplished. Such a spin filter made it possible to align the spin orientation of conduction electrons and subsequently inject them into GaAs. However, controlling spin orientation of conduction electrons by an external voltage would be very desirable for semiconductor-based magnetoelectronics. This can be accomplished by using spin switch structures, based on resonant tunneling through magnetic quantum wells, with two separate spin-up and spin-down resonances. Here we summarize both our recent results on spin injection as well as on spin aligner and magnetic resonant tunneling structures. For accomplishing the latter, we have developed magnetic resonant tunneling diodes based on BeTe–ZnMnSe–BeTe structures. Resonant tunneling diode is meant to serve as a spin switch because of the existence of two separate spin-up and spin-down resonances. The tunneling carriers have subsequently been injected into a nonmagnetic GaAs p–i–n light emitting diode. Circular polarization of the emitted light is an indicator of the spin polarization of injected electrons. At constant magnetic field and current, degree of spin polarization could be changed from 81% to 38% by only varying the voltage across the magnetic resonant tunneling device.

Quasi-zero-dimensional magnetic polarons in single Cd0.93Mn0.07Te/Cd0.6Mg0.4Te quantum dots have been studied by photoluminescence spectroscopy. By comparing the experimental data with model calculations, the energy, the internal magnetic field, and the volume of the magnetic polarons are obtained. Moreover, the magnetic environment of the recombining electron hole pair causes a distinct broadening of the emission line (;4 meV) of one diluted magnetic single quantum dot. The alignment of the Mn-spins in high magnetic fields results in a linewidth narrowing of almost one order of magnitude and the linewidth becomes comparable to that of a nonmagnetic Cd0.93Mg0.07Te/Cd0.6Mg0.4Te reference sample. A variety of electronic or magnetic properties of a solid can be artificially tailored by including magnetic ions in a semiconductor crystal matrix. This has been shown, e.g., in diluted magnetic semiconductors ~DMSs!, where the strong sp -d exchange interaction between the charge carriers and the magnetic Mn 21 ions results in a giant Zeeman splitting of the valence- and conduction-band states, a large Faraday rotation, and the formation of magnetic polarons ~MPs!. 1‐3

The thermopower of the molecular state in a double quantum dot is studied using a mixed model, which combines the classical capacitance model and a valence-electron approach. Our calculations show that close to the triple points where the resonances of the two dots intersect, there are additional steplike structures in the thermopower that result from coupling of the ``topmost'' electronic states. Far away from the triple points, we find that, with increasing detuning, the steplike structures shift toward the center of two neighboring Coulomb blockade oscillations, while very far away from the triple points, the transport resembles that of a single dot of lower electrostatic energy. Our results are in qualitative agreement with experimental findings on the conductance of Blick et al. [Phys. Rev. Lett. 80, 4032 (1998)].

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn1 − x − yBexMnySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.

Experimental studies of electron-beam propagation in a degenerate 2DEG in a GaAs-(Al,Ga)As heterostructure are presented for a wide range of injection energies, 0

The thermopower of a chaotic quantum dot has been studied in the Coulomb and the ballistic transport regime. Theoretical examinations predict distinct features for quantum dots where the electron trajectories are chaotic. Accordingly, we give experimental evidence for a residual effective charging energy in the case of asymmetric ajusted leads (G⩽2e2/h) and for non-Gaussian thermopower fluctuation distributions in the ballistic regime (G=4e2/h).

We have calculated the spin-polarization effects of a current in a two dimensional electron gas which is contacted by two ferromagnetic metals. In the purely diffusive regime, the current may indeed be spin-polarized. However, for a typical device geometry the degree of spin-polarization of the current is limited to less than 0.1%, only. The change in device resistance for parallel and antiparallel magnetization of the contacts is up to quadratically smaller, and will thus be difficult to detect., Revtex, 4 pages, 3 figures (eps), Definition of spin pilarization changed to standard definition in GMR, some straight forward algebra removed. To appear as PRB Rap. Comm. August 15th

Single-electron tunnelling is studied in a narrow submicron metal-oxide-semiconductor field-effect transistor (MOSFET) equipped with a gate on three sides of the channel. Coulomb oscillations are observed below 20 K, while a much broader structure persists up to 50 K. Every conductance peak is found to decrease with temperature, corresponding to a single-level resonance. The observed temperature dependence of the peak height and the presence of an intrinsic line width imply the participation of a constant number of energy levels in the zero-dimensional transport. At the same time, the peak spacing varies strongly from peak to peak, which is further evidence of quantum-mechanical transport through zero-dimensional states.

The field of magnetoelectronics has been growing in practical importance in recent years1. For example, devices that harness electronic spin—such as giant-magnetoresistive sensors and magnetoresistive memory cells—are now appearing on the market2. In contrast, magnetoelectronic devices based on spin-polarized transport in semiconductors are at a much earlier stage of development, largely because of the lack of an efficient means of injecting spin-polarized charge. Much work has focused on the use of ferromagnetic metallic contacts3,4, but it has proved exceedingly difficult to demonstrate polarized spin injection. More recently, two groups5,6 have reported successful spin injection from an NiFe contact, but the observed effects of the spin-polarized transport were quite small (resistance changes of less than 1%). Here we describe a different approach, in which the magnetic semiconductor BexMnyZn1-x-ySe is used as a spin aligner. We achieve injection efficiencies of 90% spin-polarized current into a non-magnetic semiconductor device. The device used in this case is a GaAs/AlGaAs light-emitting diode, and spin polarization is confirmed by the circular polarization state of the emitted light.

We investigated random telegraph signals and 1/f noise in a submicron metal–oxide–semiconductor field-effect transistor at low temperatures in the Coulomb-blockade regime. The rich noise characteristics were studied as a function of gate voltage, drain current, and temperature, both in and beyond the Ohmic regime. The results can be understood within a simple model assuming a uniform potential fluctuation of constant magnitude at the location of the dot. Clear signatures of electron heating are found from the noise at higher currents.

The detailed quantitative theory of electron-electron scattering processes in 2D electron degenerated systems in GaAs(AlGaAs) heterostructures has been developed on the basis of analytical treatment and numerical calculations. We have found the conditions and intervals of values of the characteristic parameters in which specific properties of 2D relaxation predicted previously on the theoretical level manifest themselves. New effects, i.e. a secondary beam of electrons scattered back and a very narrow beam of holes moving in the direction of injection, have been found.

The 39 papers in this special issue focus on the broad area of spintronics, excluding the well-advanced and developed storage media.

We report an inexpensive and simple way of performing nanoimprint-lithography. For this purpose a stamp is fabricated by using electron beam patterning of hydrogen silsesquioxane (HSQ) on a silicon substrate. The developed HSQ-resist can be used directly as a stamp for nanoimprint-lithography. Thus with a simple imprinting setup and an improved stamp fabrication process high resolution imprints can easily be made. The method reported here differs from previous work in the specific method of baking and developing the HSQ-resist and the imprint process. Reproducible imprints can be obtained by pressing the stamp in a layer of heated PMMA spin-coated on a silicon substrate.

The scaling behavior of the charging energy of a quantum dot with asymmetrically adjusted tunnel barriers is measured through the amplitude of the Coulomb oscillations in the thermovoltage. For weak coupling between the dot and the reservoirs, we observe a linear scaling of the effective charging energy when the transmission probability of one tunnel barrier is increased. At higher transmission probabilities, we find a deviation from the linear scaling and a crossover to a constant value. This behavior is caused by the chaotic nature of the electron trajectories within the dot.

We present measurements of the magneto-thermopower fluctuations of a gate-defined, chaotic quantum dot in a (Al,Ga)As-semiconductor heterostructure. Current-heating techniques were employed to build up the necessary temperature difference across the quantum dot for the thermopower measurement. A statistical ensemble is generated by registering the magnetic-field and gate-voltage dependence for a number of slightly different dot sizes. The experimental results are in good agreement with the predicted non-gaussian distribution of the Random Matrix Theory. To our knowledge these are the first transport experiments that directly yield the non-gaussian distribution which are an unambiguous sign for the occurence of chaos.

We report measurements on submicron metal–oxide–semiconductor field effect transistors equipped with a gate on three sides of the channel. At room temperature, a strong suppression of short-channel effects has been achieved for the narrowest channels. At liquid helium temperatures, the same devices exhibit clear conductance oscillations in the subthreshold regime, indicating that a quantum dot has formed in the disordered channel.

We demonstrate experimentally and by Monte Carlo simulation that the negative dips which occur at low magnetic fields on both sides of the main signal in nonlocal electron-beam measurements in semiconductor nanostructures result from electrons following classical rebound trajectories. We propose an alternative measurement geometry that eliminates these effects.

The formation of a Schottky barrier between an eutectic (Ga,In) alloy and a highly doped thiophene oligomer is followed as a function of time using current density–voltage and capacitance–voltage measurements. Within 1 h, the diode characteristics change from almost nonrectifying, leaky behavior into a rectification ratio of 104 with a considerably reduced leakage current. Measurements and energy band diagram calculations show that the depletion width increases with time due to a decrease in the ionizable acceptor density of the semiconductor at the Schottky interface. This is probably caused by a chemical reaction between the in-diffusing metals and the doped oligomer.

Using the effect of electron focusing we measure the Fermi wave vector of the delocalized electrons in a two-dimensional ~2D! electron system. After obtaining the total electron density from the high-field magnetoresistance we are able to determine the density of localized electrons in the metallic phase at zero magnetic field that form the bandtail of the 2D band. The number of localized electrons does not change appreciably over a wide range of total electron densities ~down to the point of the metal-insulator transition!, in spite of a profound change in the screening properties in the density-range studied. We show that the localized electrons are responsible for the observed shift of the quantum oscillation minima in relatively weak magnetic fields. @S0163-1829~97!50708-9#

By means of spin- and angle-resolved photoelectron spectroscopy we studied the spin structure of thin films of the topological insulator Bi2Se3 grown on InP(111). For thicknesses below six quintuple layers the spin-polarized metallic topological surface states interact with each other via quantum tunneling and a gap opens. Our measurements show that the resulting surface states can be described by massive Dirac cones which are split in a Rashba-like manner due to the substrate induced inversion asymmetry. The inner and the outer Rashba branches have distinct localization in the top and the bottom part of the film, whereas the band apices are delocalized throughout the entire film. Supported by calculations, our observations help in the understanding of the evolution of the surface states at the topological phase transition and provide the groundwork for the realization of two-dimensional spintronic devices based on topological semiconductors.

We use time-resolved two-color Kerr microscopy to study low-temperature electron spin diffusion in bulk GaAs with high spatial and temporal resolution. This method enables direct real-space imaging of the advancing spread of an optically excited electron spin packet by spin diffusion. We observe a high initial expansion rate of the spin packet, which is strongly reduced with increasing time. Comparison with continuous-wave Kerr spectroscopy demonstrates that this decrease is caused by the influence of the transient cooling of hot photocarriers on the electron spin diffusion coefficient.

We have investigated shot noise and conductance of multi-terminal graphene nanoribbon devices at temperatures down to 50 mK. Away from the charge neutrality point, we find a Fano factor $F \approx 0.4$, nearly independent of the charge density. Our shot noise results are consistent with theoretical models for disordered graphene ribbons with a dimensionless scattering strength $K_0 \approx 10$ corresponding to rather strong disorder. Close to charge neutrality, an increase in $F$ up to $\sim 0.7$ is found, which indicates the presence of a dominant Coulomb gap possibly due to a single quantum dot in the transport gap., 5 pages, 5 figures

We report the results of magnetoresistance measurements in vertical organic spin valves with the magnetic field oriented perpendicular to the layer stack. The magnetoresistance measurements were performed after carefully preparing either parallel or antiparallel in-plane magnetization states of the magnetic electrodes in order to observe traces of Hanle precession. Due to the low mobility in organic semiconductors the transit time of spin polarized carriers should allow for precession of the spins in perpendicular fields which in statistical average would quench the magnetoresistance. However, in none of the experiments we do observe any change in resistance while sweeping the perpendicular field, up to the point where the electrode's magnetization starts to reorient. This absence of Hanle type effects indicates that the magnetoresistance is not based on the injection of spin polarized electrons into the organic semiconductor but rather on tunneling through pinholes superimposed with tunneling anisotropic magnetoresistance., Comment: 14 pages, 7 figures

We report on spatially resolved pump-probe MOKE spectroscopy measurements of electron spins in bulk $n$-type GaAs. For low lattice temperatures these measurements are significantly compromised by local changes in the excitonic magneto-optical response which are not related to the electron spin polarization. These local changes in the excitonic Kerr spectrum are due to carrier heating caused by above-band-gap optical excitation. When hot electrons are present, a measurement of the local Kerr rotation at a fixed arbitrary probe wavelength does not necessarily correctly reveal the local electron spin polarization. By analyzing the spatial dependence of the full excitonic Kerr rotation spectrum we determine the local excitonic spin splitting energy from which we obtain the true lateral electron spin polarization profile.

We demonstrate electrical control of the room temperature Faraday effect in a 100-nm-thick film of strained HgTe, which is a topological insulator. The terahertz (THz) response of our device is dominated by the Drude response of carriers with high mobility. The electrical control is achieved by gating the carrier density in a static magnetic field, opening new perspectives for applications like high-speed amplitude and phase modulators in the THz frequency range.

The quantum spin Hall (QSH) state is a genuinely new state of matter characterized by a non-trivial topology of its band structure. Its key feature is conducting edge channels whose spin polarization has potential for spintronic and quantum information applications. The QSH state was predicted and experimentally demonstrated to exist in HgTe quantum wells. The existence of the edge channels has been inferred from the fact that local and non-local conductance values in sufficiently small devices are close to the quantized values expected for ideal edge channels and from signatures of the spin polarization. The robustness of the edge channels in larger devices and the interplay between the edge channels and a conducting bulk are relatively unexplored experimentally, and are difficult to assess via transport measurements. Here we image the current in large Hallbars made from HgTe quantum wells by probing the magnetic field generated by the current using a scanning superconducting quantum interference device (SQUID). We observe that the current flows along the edge of the device in the QSH regime, and furthermore that an identifiable edge channel exists even in the presence of disorder and considerable bulk conduction as the device is gated or its temperature is raised. Our results represent a versatile method for the characterization of new quantum spin Hall materials systems, and confirm both the existence and the robustness of the predicted edge channels., Comment: 11 pages, 4 figures; v2: link to supplementary information added, see http://www.stanford.edu/group/moler/publications.html

Strained bulk HgTe is a three-dimensional topological insulator, whose surface electrons have a high mobility (30,000 cm^2/Vs), while its bulk is effectively free of mobile charge carriers. These properties enable a study of transport through its unconventional surface states without being hindered by a parallel bulk conductance. Here, we show transport experiments on HgTe-based Josephson junctions to investigate the appearance of the predicted Majorana states at the interface between a topological insulator and a superconductor. Interestingly, we observe a dissipationless supercurrent flow through the topological surface states of HgTe. The current-voltage characteristics are hysteretic at temperatures below 1 K with critical supercurrents of several microamperes. Moreover, we observe a magnetic field induced Fraunhofer pattern of the critical supercurrent, indicating a dominant 2\pi-periodic Josephson effect in the unconventional surface states. Our results show that strained bulk HgTe is a promising material system to get a better understanding of the Josephson effect in topological surface states, and to search for the manifestation of zero-energy Majorana states in transport experiments., Comment: experimental paper; 19 pages, 4 figures

We report on spatially resolved two-color pump-probe Hanle--magneto-optical Kerr effect studies of low-temperature electron spin diffusion in bulk $n$-type GaAs (${n}_{D}=1.4\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$). The influence of the lattice temperature on electron spin diffusion is investigated and it is shown that simple drift-diffusion models do not describe the spin diffusion for above-band-gap optical spin injection correctly. Variation of lattice temperature and excitation energy allow us to trace this discrepancy to the influence of the pump-induced local overheating of the electron system with respect to the lattice which persists over length scales comparable to the spin diffusion length. Consideration of this hot carrier effect is crucial for a reliable extraction of spin propagation parameters from optical experiments.

The discovery of the Quantum Spin Hall state, and topological insulators in general, has sparked strong experimental efforts. Transport studies of the Quantum Spin Hall state confirmed the presence of edge states, showed ballistic edge transport in micron-sized samples and demonstrated the spin polarization of the helical edge states. While these experiments have confirmed the broad theoretical model, the properties of the QSH edge states have not yet been investigated on a local scale. Using Scanning Gate Microscopy to perturb the QSH edge states on a sub-micron scale, we identify well-localized scattering sites which likely limit the expected non-dissipative transport in the helical edge channels. In the micron-sized regions between the scattering sites, the edge states appear to propagate unperturbed as expected for an ideal QSH system and are found to be robust against weak induced potential fluctuations., 12 pages, including Supplemental Material

Using terahertz spectroscopy in external magnetic fields, we investigate the low-temperature charge dynamics of strained HgTe, a three-dimensional topological insulator. In resonator experiments, we observe quantum Hall oscillations at terahertz frequencies, which offer direct access to the unusual electrodynamic properties of the surface states of topological insulators. The 2D density estimated from the period of the quantum Hall oscillations agrees well with dc transport experiments on the topological surface state. The Dirac character of the surface state is further evidenced by the observation of the characteristic Berry phase in the dependence of the Landau levels on magnetic field.

Epitaxial layers of the topological insulator Bi2Se3 have been grown by molecular beam epitaxy on laterally lattice-matched InP(111)B substrates. High resolution X-ray diffraction shows a significant improvement of Bi2Se3 crystal quality compared to layers deposited on other substrates. The measured full width at half maximum of the rocking curve is Delta omega=13 arcsec, and the (omega-2theta) scans exhibit clear layer thickness fringes. Atomic force microscope images show triangular twin domains with sizes increasing with layer thickness. The structural quality of the domains is confirmed on the microscopic level by transmission electron microscopy., 4 pages, 4 figures

We have experimentally studied the diffusion thermopower of a serial double quantum dot, defined electrostatically in a GaAs/AlGaAs heterostructure. We present the thermopower stability diagram for a temperature difference ΔT = (20 ± 10) mK across the device and find a maximum thermovoltage signal of several μV in the vicinity of the triple points. Along a constant energy axis in this regime, the data show a characteristic pattern which is in agreement with Mott's relation and can be well understood within a model of sequential transport.

Topological insulators are a newly discovered phase of matter characterized by a gapped bulk surrounded by novel conducting boundary states. Since their theoretical discovery, these materials have encouraged intense efforts to study their properties and capabilities. Among the most striking results of this activity are proposals to engineer a new variety of superconductor at the surfaces of topological insulators. These topological superconductors would be capable of supporting localized Majorana fermions, particles whose braiding properties have been proposed as the basis of a fault-tolerant quantum computer. Despite the clear theoretical motivation, a conclusive realization of topological superconductivity remains an outstanding experimental goal. Here we present measurements of superconductivity induced in two-dimensional HgTe/HgCdTe quantum wells, a material which becomes a quantum spin Hall insulator when the well width exceeds d_{C}=6.3 nm. In wells that are 7.5 nm wide, we find that supercurrents are confined to the one-dimensional sample edges as the bulk density is depleted. However, when the well width is decreased to 4.5 nm the edge supercurrents cannot be distinguished from those in the bulk. These results provide evidence for superconductivity induced in the helical edges of the quantum spin Hall effect, a promising step toward the demonstration of one-dimensional topological superconductivity. Our results also provide a direct measurement of the widths of these edge channels, which range from 180 nm to 408 nm., Comment: 20 pages, 12 figures

This chapter will focus on the experimental properties of the quantum spin Hall effect in HgTe quantum well structures. HgTe quantum wells above a critical thickness are 2-dimensional topological insulators. The most prominent signature of the non-trivial topology in these systems is the occurrence of the quantum spin Hall effect when the Fermi energy is located inside the bulk band gap. We will present the main experimental results we obtained for transport in the quantum spin Hall regime and discuss how they confirm the prediction of the quantum spin Hall effect as a helical edge state system consisting of two counterpropagating oppositely spin polarized edge states.

It is detected experimentally for the first time that the connection of 2D electron systems with different electrochemical potentials results in long-range (τ 50 μm) electron density disturbances. When a gated region of a Corbino sample is strongly depleted, the amplitude of the magnetoresistance oscillations caused by high density ungated regions is found to increase in direct proportion to the sample resistance, which is dominated by low-density regions with small conductivity. Experiments on samples with an artificial potential profile (antidots and etched rings) below the gate show that the observed effects are not due to contact effects.

The incorporation of carbon in low-temperature (LT) GaAs and GaMnAs layers deposited by molecular-beam epitaxy under various growth conditions has been investigated. The lattice parameter depends on Mn content, C content, and the growth conditions which strongly affect Mn, C, and point defect incorporation. We found optimum growth conditions (Tsub=270°C, the beam equivalent pressure ratio is 5, growth rate is 0.1nm∕s) at which high-quality GaMnAs layers with moderate Mn content as well as LT-GaAs layers were deposited, which were efficiently p-doped by carbon. LT GaAs:C revealed the same electrical activation of carbon of about 50% at a high doping level p=5×1019cm−3 as observed in high-temperature GaAs:C. The Curie temperature as well as the saturation magnetization of GaMnAs decreases with C doping. This suggests a reduced amount of Mn contributing to the ferromagnetic phase in GaMnAs:C.

An investigation into the stability of metal-insulator-semiconductor (MIS) transistors based on alpha-sexithiophene is reported. In particular, the kinetics of the threshold voltage shift upon application of a gate bias has been determined. The kinetics follow stretched-hyperbola-type behavior, in agreement with the formalism developed to explain metastability in amorphous-silicon thin-film transistors. Using this model, quantification of device stability is possible. Temperature-dependent measurements show that there are two processes involved in the threshold voltage shift, one occurring at Tapproximate to220 K and the other at Tapproximate to300 K. The latter process is found to be sample dependent. This suggests a relation between device stability and processing parameters. (C) 2004 American Institute of Physics.

We have fabricated organic thin-film transistors (OTFTs) using a microcontact printing technique (μCP) that employs thin polydimethylsiloxane stamps on a rigid silicon substrate in order to reduce macroscopic distortions. Systematic variation of the printing pressure, printing time, and concentration of eicosanethiol, the “molecular ink” in the μCP process, permits the fabrication of devices with smaller channel lengths (Leff) than nominally defined by the stamp. Interdigitated Ti/Au electrode structures with Leff down to 100 nm could be fabricated which, after additional surface treatment and vacuum deposition of αα′-dihexylquaterthiophene, yield OTFTs with excellent characteristics.

Real-time control of self-organized growth of InAs nanostructures has been achieved by employing epitaxial stationary shadow masks in a molecular-beam-epitaxy process. The method is based on the surface diffusion of group-III adatoms governed by the group-V surface concentration. Lateral control is achieved by the geometry of the mask and the incidence angles of the molecular beams. We apply the method to self-organized growth of nanoscale InAs quantum structures at the edge of the incidence region of the arsenic beam. The high quality of the in situ fabricated nanostructures is confirmed by bright cathodoluminescence of InAs quantum wire embedded in GaAs barriers.

We report on the fabrication of nano-contact spin-torque oscillators based on single layers of the epitaxially grown half-metal NiMnSb with ultralow spin wave damping. We demonstrate magnetization auto-oscillations at microwave frequencies in the 1–3 GHz range in out-of-plane magnetic fields. Threshold current densities as low as 3 × 1011 A m−2 are observed as well as minimum oscillation linewidths of 200 kHz, both of which are much lower than the values achieved in conventional metallic spin-valve-based devices of comparable dimensions. These results enable the fabrication of spin transfer torque driven magnonic devices with low current density requirements, improved signal linewidths, and in a simplified single-layer geometry.

Extrinsic disorder strongly affects the performance of graphene-based quantum dots. The standard SiO2 substrate is generally considered to be one major factor besides edge-induced disorder. In this report we present the fabrication of lithographically defined quantum dots on SiO2 with short and narrow constrictions and different central island sizes. Low temperature transport measurements display distinct Coulomb-blockade peaks with amplitudes exceeding what is commonly observed experimentally. The analysis of the normalized Coulomb-blockade peak spacing shows a size dependence, which has not previously been observed for devices on SiO2. Furthermore, a quantitative comparison of the peak spacing distribution to the literature shows that one of the two devices compares favorably to a similar sized dot placed on hexagonal boron nitride, which is known to reduce the substrate disorder. Our findings suggest that the other sources of extrinsic disorder, such as lithography residues, may play an important role ...

Schottky-diode action in thiophene oligomer is investigated by current-density--voltage (J-V) and capacitance-voltage (C-V) measurements. An energy-band diagram is deduced that explains the diode characteristics for both unintentionally and highly doped thiophene oligomers. We conclude that the diode consists of a thin layer of low ionizable acceptor density (${\mathit{p}}^{\mathrm{\ensuremath{-}}}$) at the metal-oligomer interface and a semiconductor bulk layer that has a higher dopant concentration (${\mathit{p}}^{+}$). The presence of the lower doped ${\mathit{p}}^{\mathrm{\ensuremath{-}}}$ layer leads to a built-in voltage of 0.5 V, which is both experimentally observed and predicted using standard Schottky theory. Differences in J-V characteristics upon doping the thiophene semiconductor are explained by the Schottky-barrier lowering effect for the reverse current density, and by a higher conductivity of the bulk for the forward current density.