Carriers In Semiconductors Research Articles

Signature of checkerboard fluctuations in the phonon spectra of a possible polaronic metal La1.2Sr1.8Mn2O7

Charge carriers in low-doped semiconductors may distort the atomic lattice around them and through this interaction form so-called small polarons. High carrier concentrations on the other hand can lead to short-range ordered polarons (large polarons) and even to a long-range charge and orbital order. These ordered systems should be insulating with a large electrical resistivity. However, recently a polaronic pseudogap was found in a metallic phase of La(2-2x)Sr(1+2x)Mn(2)O(7) (ref. 7). This layered manganite is famous for colossal magnetoresistance associated with a phase transition from this low-temperature metallic phase to a high-temperature insulating phase. Broad charge-order peaks due to large polarons in the insulating phase disappear when La(2-2x)Sr(1+2x)Mn(2)O(7) becomes metallic. Investigating how polaronic features survive in the metallic phase, here we report the results of inelastic neutron scattering measurements showing that inside the metallic phase polarons remain as fluctuations that strongly broaden and soften certain phonons near the wavevectors where the charge-order peaks appeared in the insulating phase. Our findings imply that polaronic signatures in metals may generally come from a competing insulating charge-ordered phase. Our findings are highly relevant to cuprate superconductors with both a pseudogap and a similar phonon effect associated with a competing stripe order.

Ambipolar diffusion and spatial and time-resolved spectroscopies in semiconductor heterostructures

An analysis of the hydrodynamic motion of the fluid of photoinjected carriers in polar semiconductors is presented. Experiments of time-resolved photoluminescence, which provide relevant insights into the dynamical behavior of heterostructures, are analyzed. We study the propagation and recombination of carriers in semiconductor devices with a large cap layer, where carriers are photoinjected, and a quantum well where they recombine. The movement of the photoinjected, and away from equilibrium, carriers along such cap layer consists, to a good degree of approximation, in an ambipolar diffusivelike one, which decays in time as a result of recombination and the coupling, via Coulomb interaction, with the optical and acoustic plasma waves. The density of the electrons arriving at the interface with the quantum well can be determined; these electrons are transferred through the interface to recombine in the quantum well, and the resulting intensity of the time-resolved luminescence is obtained. Comparison with experimental data shows a good agreement.

Time-resolved Studies of Carrier and Spin Dynamics in QuantumDots and Wide Band-gap Materials

Time-resolved studies using ultra-short laser pulses unveil interesting aspects of carrier and spin dynamics in compound semiconductors. Here, thermally-activated carrier transfers between CdTe/ZnTe quantum dots (QDs) and ultrafast spin-relaxations in bulk GaN are reported. Carrier transfer among CdTe/ZnTe self-organized QDs was studied using time-resolved photoluminescence measurements. The carriers in the high-energy ground states of small QDs are confirmed to transfer to the lower-energy ground states of larger QDs, even at 10 K. The energy dependence of the PL decay time changes uniquely with increasing temperature. The change in the energy dependence of the PL decay time can be explained by thermally-activated carrier transfer. Excitonic spin-relaxations in hexagonal GaN and cubic GaN are observed. The A-band free exciton in hexagonal GaN shows a sub-picosecond spin-relaxation of 0.47 ps at 150 K. The acceptor-bound exciton in hexagonal GaN shows spin-relaxation times of 1.40 – 1.14 ps at 15 – 50 K. Meanwhile, the spin-relaxation times in cubic GaN at 15 – 75 K are found to be longer than 5 ns. The long nanosecond spin-relaxation time in cubic GaN is consistent with the observation that spin-relaxation time becomes longer for wider-band-gap zincblende semiconductors.

Effects of charge screening and surface properties on signal transduction in field effect nanowire biosensors

A self-consistent numerical model for silicon-based field effect nanowire biosensors is developed to study the impact of various surface-related physical and chemical processes, including transport of semiconductor carriers and electrolyte mobile ions, protonation and deprotonation of surface charge groups, and charges, and orientations and surface binding dynamics of immobilized biomolecules. It is shown that the sensing signal levels are affected by the gate biasing points, nonlinear screening from both electrolytes and surface charge groups, as well as the biomolecule charges and orientations. The critical role of the nanowire surface heterogeneity in determining the sensing input dynamic range is indicated based on correlations with experimental data.

A universal theoretical approach for examining the efficiency of doping processes in semiconductors

The efficiency of doping crystalline semiconductors is often associated with the solubility of the dopant. However, the scope of doping is usually a high density of extrinsic carriers, which is not necessarily guaranteed by a high solubility of the dopant but by the high concentrations of a certain type of defects. We present a universal approach to relate the concentrations of defects resulting in extrinsic carriers in crystalline semiconductors as a function of the doping process experimental parameters. Based on quantum multiple scattering, our method features two main theoretical novelties: the transition operators are calculated by iteratively “switching on” the interaction potentials and the temperature effects are introduced as semiclassical vibrations. The bulk and doped configurations are characterized by their ground state electronic structures obtained from first-principles calculations. The dependency on substrate temperature of the carrier concentration calculated with our method is shown to reproduce well the experimental results not only for well-known processes (the n-type doping of ZnO with Al and the p-type doping of GaAs with Be) but also for challenging processes such as the p-type doping of ZnO with P. This proves that our approach is reliable and that it can become a powerful tool in the search for optimal growth condition.

Excitons Surf Along Conjugated Polymer Chains

The energy carriers in an organic polymer semiconductor stay in phase when they move along a single polymer chain at room temperature.

Temperature-Dependent Transfer Characteristics of Amorphous InGaZnO4 Thin-Film Transistors

The transfer characteristics of amorphous InGaZnO4 thin-film transistors (a-IGZO TFTs) were measured at temperatures ranging from 298 to 523 K in order to analyze the behavior of the above-threshold (ON state) and subthreshold regions. For comparison, the transfer characteristics of a hydrogenated amorphous silicon TFT (a-Si:H TFT) were measured in the same temperature range. We developed a simple analytical model that relates the threshold voltage (Vt) decrease due to increasing temperature to the formation of point defects in a-IGZO. It is well known that the formation of point defects results in the generation of free carriers in oxide semiconductors. Incorporating the analytical model with the experimental transfer characteristics data taken at high temperatures over 423 K, we estimated the formation energy to be approximately 1.05 eV. The Vt decrease because of the generation of point defects is peculiar to a-IGZO TFTs, which is not observed in a-Si:H TFTs. The results for the ON-current activation energy suggested that the density of tail states for a-IGZO is much lower than that for a-Si:H.

Specific features of the generation-recombination properties of bistable defects in semiconductors: Manifestation in Hoffmann’s function

Features of the temperature dependence of the concentration of majority free charge carriers in semiconductors on temperature-dependent discharging of bistable centers differing in electrical activity are treated theoretically. Specifically, two-level acceptors, two-level donors, and amphoteric centers are considered. The effects of the level of compensation and the ratio between the binding energies of one and two electrons at a center upon the discharging are studied. The results are represented as features of the behavior of Hoffmann’s function.

Theoretical study on spin-polarized injection of electrical currents into polymer semiconductors

Different from electrons and holes in traditional inorganic semiconductors, the charge carriers in polymer semiconductors are spin polarons and spinless bipolarons. In this paper, a theoretical model is presented to describe the spin-polarized injection of electrical currents from a ferromagnetic contact into a nonmagnetic polymer semiconductor. In this model, a new relation of conductivity to concentration polarization for polymer semiconductors is introduced based on a three-channel model to describe the spin-polarized injection of electrical currents under large electrical current densities. The calculated results of the model reveal the effects of the polaron ratio, the carrier concentration polarization, the interfacial conductance, the bulk conductivity of materials, and the electrical current density, etc. on the spin polarization of electrical currents. As conclusions, the large and matched bulk conductivity of materials, the small spin-dependent interfacial conductance, the thin polymer thickness and the large enough electrical current are critical factors for upgrading the spin polarization of electrical currents in polymer semiconductors. Particularly, when the polaron ratio in polymer semiconductors approaches the concentration polarization of the ferromagnetic contact, a modest concentration polarization is sufficient for achieving a nearly complete spin-polarized injection of electrical currents.

Charge current polarization and magnetoresistance in ferromagnetic/organic semiconductor/ferromagnetic devices

Spin-polarized injection and transport in ferromagnetic/organic semiconductor/ferromagnetic devices are studied theoretically. Based on the spin diffusion theory and Ohm’s law, we obtain the charge current polarization and the magnetoresistance, which takes into account the special carriers in organic semiconductors. From the calculation, it is found that the charge current polarization decreases exponentially from the ferromagnetic layer into the organic layer and polarons are effective spin carriers in organic semiconductors for polarized charge current. To get an apparent magnetoresistance in an organic device, it is better to adopt a spin-dependent interface, and the thickness of the organic interlayer is much smaller than the spin diffusion length. Spin polarons are effective carriers for gaining remarkable magnetoresistance in ferromagnetic/organic semiconductor/ferromagnetic devices.

Effects of substrate temperature on the optical and electrical properties of Al:ZnO films

Al-doped ZnO films were grown on glass substrates by the pulsed-laser deposition technique with varying substrate temperatures. The optical band gap decreases from 3.64 to 3.46 eV as the substrate temperature increases from 350 to 450 °C, illustrating the increase in Al content in the context of a degenerate semiconductor, and can be explained in the framework of the Burstein–Moss effect. All films show optical transparency greater than 85%. Al:ZnO films show a metal–semiconductor transition to metal-like behavior as the substrate temperature increases from 350 to 450 °C. The observed metal-like and metal–semiconductor transitions are explained by taking into account the Mott phase transition and localization effects due to defects. The resistivity decreases from 896 to 470 µΩ cm as the substrate temperature increases from 350 to 450 °C. In addition, the competition between the thermally activated carriers and scattering effects of free carriers in a degenerate semiconductor can also explain the metal–semiconductor transition.

Kinetics of thermalization of electric charge carriers in organic molecular semiconductors in strong electric fields

The kinetics of photogeneration of electric charge carriers in organic molecular semiconductors in strong electric fields is investigated within a theoretical model that includes the diffusion-drift mechanism of motion of electric charge carriers. In the framework of the proposed model, it is established that the time dependence of the probability of dissociation of a geminal pair exhibits an exponential behavior. The time constant strongly depends on the electric charge carrier hopping distance and the external electric field strength in accordance with the experimental data.

Effect of Carrier Differences on Spin Polarized Injection into Organic and Inorganic Semiconductors

Spin polarized injection into organic and inorganic semiconductors are studied theoretically from the spin diffusion theory and Ohm's law, and the emphases are placed on the effect of the carrier differences on the current spin polarization. The mobility and the spin-flip time of carriers in organic and inorganic semiconductors are different. From the calculation, it is found that current spin polarization at a ferromagnetic/organic interface is higher than that at a ferromagnetic/inorganic interface because of different carriers in them. Effects of the conductivity matching, the spin dependent interfacial resistances, and the bulk spin polarization of the ferromagnetic layer on the spin polarized injection are also discussed.

Electronically Driven Structure Changes of Si Captured by Femtosecond Electron Diffraction

The excitation of a high density of carriers in semiconductors can induce an order-to-disorder phase transition due to changes in the potential-energy landscape of the lattice. We report the first direct resolution of the structural details of this phenomenon in freestanding films of polycrystalline and (001)-oriented crystalline Si, using 200-fs electron pulses. At excitation levels greater than approximately 6% of the valence electron density, the crystalline structure of the lattice is lost in <500 fs, a time scale indicative of an electronically driven phase transition. We find that the relaxation process along the modified potential is not inertial but rather involves multiple scattering towards the disordered state.

Development of Surface Hall Potentiometry to Reveal the Variation of Drift Velocity of Carriers in Semiconductor Materials

Surface Hall potentiometry is a method of detecting the Hall effect occurring in a semiconductor material as the surface potentials change. An experimental setup to detect surface Hall potentials by the Kelvin method is introduced. After we reveal the basic characteristics of surface Hall potentials, a silicon sample is prepared to induce the variation of the drift velocity of carriers. Line-profile measurements of surface Hall potentials with this sample demonstrate that our method reveals the distribution of drift velocities of carriers in semiconductors, which reflects electronic or structural properties such as the carrier type (hole or electron), current density, and thickness.

Using hydrostatic pressure for evaluating the effect of a fluctuating potential on the energy spectrum of charge carriers in crystalline semiconductors

As known [1, 2], doped compensated semiconductors (in which the concentration of free charge carriers is small compared to the concentration of ionized impurities) exhibit smooth, large-scale fluctuations of random potential with certain characteristic amplitudes ( γ ). The role of this chaotic potential increases with decreasing temperature, while at a fixed temperature it grows under the action of pressure as the free carrier concentration decreases [3]. It is important to develop a method for evaluating the effect of the chaotic potential on the energy spectrum of charge carriers and to assess the correctness of relations obtained for defect-free crystals so as to provide a quantitative analysis of experimental data in each particular case. It should be noted that the pressure coefficients of the energy gaps in semiconductors are virtually pressure-independent at not very high pressures. Moreover, the pressure-induced changes of the deepest minima e Γ , e L , and e X in the conduction bands of various semiconductors (IV, II‐VI, III‐V, IV‐VI, and II–IV–V 2 ) are also approximately the same [4‐8]. Previously, the pressure coefficients of the e Γ , e L , and e X extrema relative to the values in absolute vacuum were determined [5] based on a concept according to which the energy of deep strongly localized states in some semiconductors is independent of the uniform (hydrostatic) pressure.

Energy spectrum of carriers in Kane-type semiconductors in a certain magnetic field

We present compact analytical solutions for the energy spectrum of carriers in Kane-type semiconductors in a certain varying magnetic field. We find an analytical expression for the effective g-factor of electrons in the three-dimensional semiconductors in a certain magnetic field.

Quasiequilibrium optical nonlinearities from spin-polarized carriers in GaAs

Semiconductor Bloch equations, which microscopically describe the dynamics of a Coulomb interacting, spin-unpolarized electron-hole plasma, can be solved in two limits: the coherent and the quasiequilibrium regimes. These equations have been recently extended to include the spin degree of freedom and used to explain spin dynamics in the coherent regime. In the quasiequilibrium limit, one solves the Bethe-Salpeter equation in a two-band model to describe how optical absorption is affected by Coulomb interactions within a spin unpolarized plasma of arbitrary density. In this work, we modified the solution of the Bethe-Salpeter equation to include spin polarization and light holes in a three-band model, which allowed us to account for spin-polarized versions of many-body effects in absorption. The calculated absorption reproduced the spin-dependent, density-dependent, and spectral trends observed in bulk GaAs at room temperature, in a recent pump-probe experiment with circularly polarized light. Hence, our results may be useful in the microscopic modeling of density-dependent optical nonlinearities due to spin-polarized carriers in semiconductors.

Predicting the ionization threshold for carriers in excited semiconductors

A simple set of formulas are presented which allow prediction of the fraction of ionized carriers in an electron–hole–exciton gas in a photoexcited semiconductor. These results are related to recent experiments with excitons in single and double quantum wells.

NUMERICAL SIMULATION OF THE CONDUCTIVITY RELAXATION IN THE HIGH RESISTIVITY SEMICONDUCTOR

A theoretical model describing the relaxation of charge carriers in semiconductors of high resistance under the influence of the laser pulses is presented. It is demonstrated that parameters of the trapping states relevant to the processes of the conductivity relaxation can be defined by fitting the experimental data. Time evolution of the conductivity of the GaAs bulk semiconductor under the influence of nanosecond and picosecond laser pulses is considered. Effect of two laser pulses, when the first one results in population of the trapping state and the second one induces its depopulation, is also considered.