On the appearance of induced magnetic moment of β-FeSi2 by doping halogen or oxygen atoms

We have theoretically examined whether magnetic moment of iron atom can be induced or not when highly electronegative elements such as halogen or oxygen atoms are doped. Based on the impurity Anderson model, we evaluated the green function of d electrons. Through the analysis of green function, it is disclosed that a magnetic moment can be induced by impurity potential caused from doping elements, although intrinsic β-FeSi₂ is non-magnetic. In particular, high electronegativity of doping atoms and/or strong Coulomb repulsive interaction between conduction electrons and doped atoms easily induce magnetic moment from non-magnetic state. In the viewpoint of chemical bonding, such an induced magnetic moment appears as a result of decomposition of covalent FeSi bonds by a strong electronegative impurity potential, because the broken bonds create the unpaired 3d electrons.     

Spin filling of valley-orbit states in a silicon quantum dot

We report the demonstration of a low-disorder silicon metal-oxide-semiconductor (Si MOS) quantum dot containing a tunable number of electrons from zero to N = 27. The observed evolution of addition energies with parallel magnetic field reveals the spin filling of electrons into valley-orbit states. We find a splitting of 0.10?meV between the ground and first excited states, consistent with theory and placing a lower bound on the valley splitting. Our results provide optimism for the realisation in the near future of spin qubits based on silicon quantum dots.     

Hole transport due to shallow acceptors along boron doped SiGe quantum wells

Lateral conductivity and magnetotransport measurements were performed with SiGe single quantum well (QW) structures doped with boron in the QW. The conductivity at low temperatures (T) is shown to be due to hopping over B centers while at higher T, it is due to two-stage excitation: thermal activation of holes from the ground to strain-split B states are followed by hole tunneling into the valence band. The tunneling is due to a potential drop across the QW which is due to hole capture at surface states of the Si cap layer making the surface charged. The external potential applied across the QW essentially changes the lateral conductivity as well as the activation energy. The calculations of band profile, free carrier concentration in the QW and acceptor population, as well as an effect on the transverse electric field were carried out taking into account the charging of surface states.     

Novel concepts for GaAs/LiNbO(3) layered systems and their device applications

Thin semiconductor quantum well structures fused onto LiNbO(3 ) substrates using the epitaxial lift-off (ELO) technology offer the possibility of controlling the surface acoustic wave (SAW) velocity via field effect. The tunability of the conductivity in the InGaAs quantum well results in a great change in SAW velocity, in general, accompanied by an attenuation. We show that an additional lateral modulation of the sheet conductivity reduces the SAW attenuation significantly, enhancing device performance. At high SAW intensity the bunching of electrons in the SAW potential also leads to a strong reduction of attenuation. These effects open new possibilities for voltage-controlled SAW devices. We demonstrate a novel, wireless, passive voltage sensor, which can be read out from a remote location.     

Highly efficient charge separation and collection across in situ doped axial VLS-grown Si nanowire p-n junctions

VLS-grown semiconductor nanowires have emerged as a viable prospect for future solar-based energy applications. In this paper, we report highly efficient charge separation and collection across in situ doped Si p-n junction nanowires with a diameter <100 nm grown in a cold wall CVD reactor. Our photoexcitation measurements indicate an internal quantum efficiency of ~50%, whereas scanning photocurrent microscopy measurements reveal effective minority carrier diffusion lengths of ~1.0 μm for electrons and 0.66 μm for holes for as-grown Si nanowires (d(NW) ≈ 65-80 nm), which are an order of magnitude larger than those previously reported for nanowires of similar diameter. Further analysis reveals that the strong suppression of surface recombination is mainly responsible for these relatively long diffusion lengths, with surface recombination velocities (S) calculated to be 2 orders of magnitude lower than found previously for as-grown nanowires, all of which used hot wall reactors. The degree of surface passivation achieved in our as-grown nanowires is comparable to or better than that achieved for nanowires in prior studies at significantly larger diameters. We suggest that the dramatically improved surface recombination velocities may result from the reduced sidewall reactions and deposition in our cold wall CVD reactor.     

Carrier dynamics and activation energy of CdxZn(1-x)Te/ZnTe quantum dots on GaAs and Si substrates

We have investigated the carrier dynamics and activation energy of CdxZn(1-x)Te/ZnTe quantum dots (QDs) on GaAs and Si substrates. The carrier dynamics of QDs on GaAs and Si substrates is studied using time-resolved photoluminescence (PL) measurements, revealing shorter exciton lifetimes of QDs on Si substrate. In particular, the activation energy of electrons confined in QDs on the GaAs substrate, as obtained from temperature-dependent PL spectra, is higher than that of electrons confined in QDs on the Si substrate. Both results confirm that defects and dislocations in QDs on the Si substrate provide nonradiative channels.     

Engineering Exchange Coupling in Double Elliptic Quantum Dots

Coupled elliptic quantum dots with different aspect ratios containing up to two electrons are studied using a model confinement potential in the presence of magnetic fields. Single and two-particle Schrodinger equations are solved using numerical exact diagonalization to obtain the exchange energy and chemical potentials. As the ratio between the confinement strengths in directions perpendicular and parallel to the coupling direction of the double dots increases, the exchange energy at zero magnetic field increases, while the magnetic field of the singlet-triplet transition decreases. By investigating the charge stability diagram, we find interdot quantum mechanical coupling increases with the dot aspect ratio, whereas the electrostatic coupling between the two dots remains nearly constant. With increasing interdot detuning, the absolute value of the exchange energy increases superlinearly followed by saturation. This behavior is attributed to the electron density differences between the singlet and triplet states in the asymmetric QD systems     

Activation energy and carrier dynamics of CdTe/ZnTe quantum dots on GaAs and Si substrates

We investigate the activation energy and carrier dynamics of CdTe/ZnTe quantum dots (QDs) grown on GaAs and Si substrates. The activation energy of the electrons confined in QDs on the Si substrate, as obtained from the temperature-dependent photoluminescence (PL) spectra, is lower than that of electrons confined in QDs on the GaAs substrate. Time-resolved PL measurements used to study the carrier dynamics show shorter exciton lifetimes for QDs on the Si substrate. This behavior is attributed to the fact that defects and dislocations in the QDs on the Si substrate provide nonradiative channels.     

Interrogation of a PS1‐Based Photocathode by Means of Scanning Photoelectrochemical Microscopy

In the development of photosystem‐based energy conversion devices, the in‐depth understanding of electron transfer processes involved in photocurrent generation and possible charge recombination is essential as a basis for the development of photo‐bioelectrochemical architectures with increased efficiency. The evaluation of a bio‐photocathode based on photosystem 1 (PS1) integrated within a redox hydrogel by means of scanning photoelectrochemical microscopy (SPECM) is reported. The redox polymer acts as a conducting matrix for the transfer of electrons from the electrode surface to the photo‐oxidized P₇₀₀ centers within PS1, while methyl viologen is used as charge carrier for the collection of electrons at the reduced F_B site of PS1. The analysis of the modified surfaces by SPECM enables the evaluation of electron‐transfer processes by simultaneously monitoring photocurrent generation at the bio‐photoelectrode and the associated generation of reduced charge carriers. The possibility to visualize charge recombination processes is illustrated by using two different electrode materials, namely Au and p‐doped Si, exhibiting substantially different electron transfer kinetics for the reoxidation of the methyl viologen radical cation used as freely diffusing charge carrier. In the case of p‐doped Si, a slower recombination kinetics allows visualization of methyl viologen radical cation concentration profiles from SPECM approach curves.     

Impurity band Mott insulators: a new route to high Tc superconductivity

Abstract

Last century witnessed the birth of semiconductor electronics and nanotechnology. The physics behind these revolutionary developments is certain quantum mechanical behaviour of ‘impurity state electrons’ in crystalline ‘band insulators’, such as Si, Ge, GaAs and GaN, arising from intentionally added (doped) impurities. The present article proposes that certain collective quantum behaviour of these impurity state electrons, arising from Coulomb repulsions, could lead to superconductivity in a parent band insulator, in a way not suspected before. Impurity band resonating valence bond theory of superconductivity in boron doped diamond, recently proposed by us, suggests possibility of superconductivity emerging from impurity band Mott insulators. We use certain key ideas and insights from the field of high-temperature superconductivity in cuprates and organics. Our suggestion also offers new possibilities in the field of semiconductor electronics and nanotechnology. The current level of sophistication in solid state technology and combinatorial materials science is very well capable of realizing our proposal and discover new superconductors.     

Optically-induced charge storage in self-assembled InAs quantum dots

We present results on spectrally resolved photo-resistance studies of optically-induced charge storage effects in self-organized InAs quantum dots (QDs). The stored charge can be detected and erased electrically. The investigated structure designed for electron or hole storage in the QDs consists of a modulation doped two-dimensional channel which was grown on top of a layer of InAs QDs, separated by an asymmetric tunnel barrier. Our results show that optical QD charging with spectral resolution provides information on the charging dynamics and on the quantity and spectral dependence of stored charges in the QDs. This is a novel technique by which QD excitation spectra can be studied. Spectrally resolved storage effect measurements on electrons as well as on holes allowed to investigate thermal redistribution of carriers in the quantum dot layer. It was found that only at low temperatures carriers can be stored selectively over long time scales in the InAs QDs. The charge storage effect is observable for several hours at temperatures up to 170 K, for several seconds up to 250 K due to an increase in thermal emission of stored charges.     

Tunable SiO<Subscript>2</Subscript>/Si-based nanostructures

We model the formation of a nanoscale potential well with quantum wires on the semiconductor surface near the SiO₂/Si interface owing to a special charge distribution in the oxide. We consider an SiO₂/Si structure in the form of a cylindrical substrate covered with a coaxial oxide layer. The charge distribution in the oxide is taken to have the form of charged circular rings of finite thickness, coaxial with the cylindrical substrate. The parameters of the quantum wires are analyzed in relation to the charge distribution and density. Reducing the separation between two charged rings decreases the width of the quantum wires produced on the semiconductor surface and increases their depth.     

Anisotropic Superconductivity of Quasi-Two-Dimensional Tight-Binding Electrons in a Strong Magnetic Field

Based on the mean field theory, we have investigated the transition temperature T _c (H) of anisotropic superconductivity in quasi-two-dimensional (Q2D) tight-binding electrons in a strong magnetic field, where we assume the nearest-site attractive interaction. By taking account of the quantum effect of electronic motion in a strong magnetic field parallel to the 2D conducting plane, T _c (H) of the Q2D superconductor has been shown to increase in an oscillatory manner as the magnetic field becomes large and to reach T _c (0) in a strong magnetic field limit for the spin-triplet superconductor. We get the different magnetic field dependencies from that of on-site case.     

Two carriers in vertically coupled quantum dots: magnetic field effect

We study a two-charge-carrier (two holes or two electrons) quantum dot molecule in a magnetic field. In comparison with the electron states in the double quantum dot, the switching between the hole states is achieved by changing both the inter-dot distance and magnetic field. We use harmonic potentials to model the confining of two charge carriers and calculate the energy difference delta E between the two lowest energy states with the Hund-Mulliken technique, including the Coulomb interaction. Introducing the Zeeman effect, we note a ground-state crossing, which can be observed as a pronounced jump in the magnetization at a perpendicular magnetic field of a few Tesla. The ground states of the molecule provide a possible realization for a quantum gate.     

Spin Polarization Measurements of InAs-Based LEDs

We have investigated electroluminescence (EL) characteristics of hybrid II–VI/III–V light emitting diodes (LEDs) at low temperatures and in magnetic fields up to 10 T. Spin-polarized or unpolarized electrons are injected from n-type Cd(Mn)Se layers into a wide quantum well of InAs where they undergo radiative recombination with unpolarized holes injected via p-type InAs/AlAsSb layers. Measurements of the circular polarization properties of the emitted mid-infrared EL have been made to investigate spin-injection from the Brillouin paramagnet CdMnSe into InAs; a “non-magnetic” CdSe injector is used for comparison. To infer spin polarization from the circular polarization degree, details of the InAs band structure in a magnetic field have to be taken into account due to the large electron g-factors and, more importantly, because radiative recombination and spin relaxation of injected carriers occur on similar timescales. As a result optical and spin polarization are not simply related to each other. Experimentally, the circular polarization degrees of magnetic and non-magnetic structures are observed to be very similar. In addition, broad, multi-component EL features, as well as significant carrier heating complicate the quantitative analysis.

---

     

Colloidal synthesis of ultrathin two-dimensional semiconductor nanocrystals

2D semiconductor quantum wells have been recognized as potential candidates for various quantum devices. In quantum wells, electrons and holes are spatially confined within a finite thickness and freely move in 2D space. Much effort has focused on shape control of colloidal semiconductor nanocrystals(NCs), and synthesis of 2D colloidal NCs has been achieved very recently. Here, recent advances in colloidal synthesis of uniform and ultrathin 2D CdSeNCs are highlighted. Structural and optical property characterization of these quantum-sized 2D CdSe NCs is discussed. Additionally, 2D CdSe NCs doped with Mn 2+ ions for dilute magnetic semiconductors (DMS) are presented.These 2D CdSe-based NCs can be used as model systems for studying quantum-well structures.     

Analysis of electron energy States in a thin quantum well in a parallel magnetic field

We carry out a preliminary investigation of electron dynamics in a quantum well subject to a uniform magnetic field applied parallel to its walls. To achieve exact expressions for the Green's function and associated dispersion relation, we study the narrow limit of a delta-function well profile. The Dyson equation is solved and the dispersion relation is examined for weak and strong (quantizing) magnetic fields.     

Real-Time Current of a Charge Carrier in a Strongly Correlated System Coupled to Phonons, Driven by a Uniform Electric Field

We perform a quantum time evolution of a single charge carrier doped into strongly correlated system and coupled to phonons, driven by a uniform electric field. We study a two-dimensional t?CJ-Holstein model and calculate the real-time current. At large values of electric field, the response of the system exhibits damped Bloch oscillations, and the values of the steady current decrease with increasing electric field. On the other hand, the maximal amplitude of the real-time current increase with increasing field. We discuss the appearance of the negative differential resistivity regime, observed recently in various nonequilibrium studies of interacting many-body systems.     

Terahertz emission of SiGe/Si quantum wells

THz emission of stimulated character was observed in Si/SiGe/Si quantum well (QW) structures doped with boron. The resonance cavity formed by extremely parallel-structure planes due to total internal reflection, is necessary for the emission. The mechanism for the possible population inversion of strain-split acceptor levels is proposed.     

Ferromagnetism of ZnO and GaN: A Review

The observation of ferromagnetism in magnetic ion doped II–VI diluted magnetic semiconductors (DMSs) and oxides, and later in (Ga,Mn)As materials has inspired a great deal of research interest in a field dubbed “spintronics” of late, which could pave the way to exploit spin in addition to charge in semiconductor devices. The main challenge for practical application of the DMS materials is the attainment of a Curie temperature at or preferably above room temperature to be compatible with junction temperatures. Among the studies of transition-metal doped conventional III–V and II–VI semiconductors, transition-metal-doped ZnO and GaN became the most extensively studied topical materials since the prediction by Dietl et al., based on mean field theory, as promising candidates to realize a diluted magnetic material with Curie temperature above room temperature. The underlying assumptions, however, such as transition metal concentrations in excess of 5% and hole concentrations of about 10²⁰ cm⁻³, have not gotten as much attention. The particular predictions are predicated on the assumption that hole mediated exchange interaction is responsible for magnetic ordering. Among the additional advantages of ZnO-and GaN-based DMSs are that they can be readily incorporated in the existing semiconductor heterostructure systems, where a number of optical and electronic devices have been realized, thus allowing the exploration of the underlying physics and applications based on previously unavailable combinations of quantum structures and magnetism in semiconductors. This review focuses primarily on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs. One of the desirable outcomes is to obtain carrier mediated magnetism, so that the magnetic properties can be manipulated by charge control, for example through external electrical voltage. We shall first describe the basic theories forwarded for the mechanisms producing ferromagnetic behavior in DMS materials, and then review the theoretical results dealing with ZnO and GaN. The rest of the review is devoted to the structural, optical, and magnetic properties of ZnO- and GaN-based DMS materials reported in the literature. A critical review of the question concerning the origin of ferromagnetism in diluted magnetic semiconductors is given. In a similar vein, limitations and problems for identifying novel ferromagnetic DMS are briefly discussed, followed by challenges and a few examples of potential devices.