Spatial Distribution Of Charge Carriers Research Articles

Mapping the spatial distribution of charge carriers in LaAlO3/SrTiO3 heterostructures

At the interface between complex insulating oxides, novel phases with interesting properties may occur, such as the metallic state reported in the LaAlO(3)/SrTiO(3) system . Although this state has been predicted and reported to be confined at the interface, some studies indicate a much broader spatial extension, thereby questioning its origin. Here, we provide for the first time a direct determination of the carrier density profile of this system through resistance profile mappings collected in cross-section LaAlO(3)/SrTiO(3) samples with a conducting-tip atomic force microscope (CT-AFM). We find that, depending on specific growth protocols, the spatial extension of the high-mobility electron gas can be varied from hundreds of micrometres into SrTiO(3) to a few nanometres next to the LaAlO(3)/SrTiO(3) interface. Our results emphasize the potential of CT-AFM as a novel tool to characterize complex oxide interfaces and provide us with a definitive and conclusive way to reconcile the body of experimental data in this system.

Numerical modeling and simulations of electrical characteristics of multi‐layer organic light emitting diodes

Theoretical simulations of spatial distribution of charge carriers and recombination rate, and J‐V characteristics of the multi‐layer organic light emitting diodes are carried out. Drift‐diffusion current transport, field‐dependent carrier mobility, exponential and Gaussian trap distribution, and Langevin recombination models are included in this computer model. The simulated results show good agreement with the experimental data confirming the validity of the physical models for organic light emitting diodes.

Imaging theory for X-ray pixel detectors

In this paper we discuss the physical processes from the first interaction of an X-ray photon to the spatial distribution of charge carriers at the electrodes of a pixel detector. The implications of these processes on the achievable position and energy resolution have been investigated with simulation calculations and with measurements employing the Medipix detectors. As the next generation detectors will be capable of energy resolution we present two methods of energy exploitation in X-ray imaging, i.e. energy weighting and material reconstruction.

Binding of charge carriers in the extendedt−Jmodel

We have examined the effects of extended hopping parameters in the $t\text{\ensuremath{-}}{t}^{\ensuremath{'}}\text{\ensuremath{-}}{t}^{\ensuremath{''}}\text{\ensuremath{-}}J$ model on the binding and spatial distribution of charge carriers. Previous studies have shown that a negative next-nearest-neighbor hopping in the physical parameter range serves to weaken striped behavior in the model. We show using exact diagonalization that a physically relevant set of parameters for the hole doped cuprates that includes next-next-nearest-neighbor hopping strengthens the binding of the holes and enhances stripe behavior in the underdoped regime.

P‐86: Numerical Device Simulation of Bilayer Organic Light Emitting Diodes

AbstractTheoretical simulations of spatial distribution of charge carriers and recombination rate, and I‐V characteristics of the bilayer organic light emitting diodes are carried out. Drift‐diffusion current transport, field‐dependent carrier mobility, exponential trap distribution, and Langevin recombination models are included in this computer model. The simulated results show good agreement with the experimental data indicating the validity of the physical models for organic light emitting diodes.

Raman-spectroscopy investigations of photoinduced changes of the spatial charge-carrier distribution in p-type modulation-doped quantum-well structures.

Raman-spectroscopy investigations of photoinduced changes of the spatial charge-carrier distribution in p-type modulation-doped quantum-well structures.

Influence of spatial charge carrier distribution near interface between organic crystal and water on electrokinetic potential

Influence of spatial charge carrier distribution near interface between organic crystal and water on electrokinetic potential

Spectroscopic investigations of photoinduced changes of the spatial distribution of charge carriers in modulation-doped quantum-well structures.

We investigated the photoinduced changes of the spatial distribution of the free holes in p-type modulation-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructures by measuring the energy of the quantum-well luminescence, which is conditioned by the many-particle effect of band-gap renormalization and the excitonic interactions between the electrons and holes. A conventional chopper technique with millisecond time resolution permitted us to study the dynamics of the hole density variations in the quantum wells. The strength of excitonic effects on the luminescence signals could be estimated by application of a magnetic field oriented parallel to the growth direction of the samples. \textcopyright{} 1996 The American Physical Society.

Minimizing the size of force-controlled point contacts on silicon for carrier profiling

The spatial distribution of charge carriers in semiconducting structures can be derived from conductive properties. In this report silicon cantilevers with integrated tips and coated with boron‐doped chemical vapor deposited diamond have been used for the first time to establish electrically reliable point contacts on silicon. The relation between the applied force and the nanoindentation characteristics has been investigated in a range from 10 to 150 μN. The threshold value necessary to pierce the native oxide layer has been obtained and is found to depend on the peculiar coating of each tip. The data are correlated with the current–voltage characteristics of the contacts. The impedance of point contacts on homogeneously doped p‐ and n‐type silicon with known resistivity has been measured and calibration curves have been plotted for nanospreading resistance probe measurements. It is found that only above the threshold for plastic deformation can useful electrical characterizations be performed.

Lateral and vertical dopant profiling in semiconductors by atomic force microscopy using conducting tips

The determination of the spatial distribution of charge carriers in semiconducting structures with an atomic force microscope (AFM) is presented. This new technique is based on the measurement of the spreading resistance of a conducting AFM tip on a sample as the tip is scanned over the surface. The high spatial resolution of this method allows for its application on the cross section of a device providing one-dimensional (vertical) and two-dimensional (lateral) profile information. First, a detailed study is presented regarding the properties of the tip (tip material and preparation, lifetime) and the evolution of I–V characteristics with pressure on uniformly doped silicon. From this work a model for the electric properties of the microcontact is deduced. It includes an ohmic contribution to the overall resistance that is related to the plastically deformed area, and contributions from a barrier as well as from tunneling. Through the determination of a calibration curve, whereby the resistance was measured on homogeneously doped substrates with varying resistivities, the quantification properties of the method are determined. Finally, this method has been applied for junction delineation on the cross section of a device. The results show a 50 nm resolution and a sufficient sensitivity to encompass a dynamic range of concentrations between 1014 and 1019 cm−3. These measurements were found to be in good agreement with those performed by the classical spreading resistance profiling technique and by secondary ion mass spectroscopy.

Characterization of a point-contact on silicon using force microscopy-supported resistance measurements

A conductive atomic force microscope (AFM) tip based on B-implanted diamond has been developed for the determination of the spatial distribution of charge carriers in semiconducting structures. The characteristics of this tip have been determined by studying the current–voltage behavior as a function of substrate resistivity and tip load. From this work a model of the electrical properties of the microcontact is emerging. It includes an Ohmic contribution to the overall resistance, which is related to the plastically deformed area, and contributions from a barrier. The tip imprints have been imaged with AFM and their physical dimensions are seen to match the requirements of the model. From resistance measurements on uniformly doped silicon a calibration curve has been established which can be used as a standard to convert measured resistance into resistivity.

Ion mobility measurements in dielectric liquids

Ion mobility measurements were made on n-hexane and a 0.22M solution of biphenyl in n-hexane using a time-of-flight technique. A collimated beam of x rays from a flash x-ray generator was used to produce a layer of ions near one electrode of a parallel-plate conductivity cell. Part of the charge layer was swept under the influence of an electric field to the opposite electrode, and the motion of the charge was observed by means of a current-versus-time measurement. Mobilities have been determined by fitting theoretical current–time curves to experimental ones. The theoretical current–time curves were obtained by solving numerically the differential equations governing carrier motion during a mobility measurement. Diffusion, bimolecular recombination, motion under an applied electric field, and induced bulk liquid motion were included in the theoretical model. The measured spatial dose distribution, radiation yield data, applied voltage, and electrode separation were input constants to the theoretical calculation, while values for the anion mobility, cation mobility, and induced liquid acceleration were variable parameters. The theoretical model yields the spatial distribution of charge carriers and its time dependence during the charge transport. It also describes the electric field between the electrodes as a function of space and time and demonstrates the effects of space charge. The current–time curves generated from the model are in good agreement with those obtained experimentally. The theoretical model that includes the effects of induced bulk liquid motion yields values of mobility that are significantly lower than those determined experimentally by use of inflection or ’’half-steady-state’’ points. Measurements made on purified n-hexane showed that a radiation pulse produces more than one slow carrier of each sign. When biphenyl was added to the sample, one negative and two positive species were observed. Our theoretical fits to experimental curves obtained from the irradiation of a 0.22M solution of biphenyl in n-hexane yields a mobility value for the negative species of 3.6×10−4 cm2 V−1⋅sec−1. This negative species is believed to be the biphenylide anion.