Built-in Electrostatic Potential Research Articles

Effective tailoring of the MoS2 layer number on the surface of CdS nanorods for boosting hydrogen production rate.

Hydrogen fuel plays a ubiquitous role in empowering the sustainable green energy economy. As an eco-friendly production method for hydrogen, photo-assisted water splitting is accepted to be the most reliable. However, the fabrication of stable and efficient photocatalysts is challenging. To overcome this difficulty, here we present a novel and inexpensive oxidant-promoted ultrasonic-assisted liquid phase layer exfoliation technique to fabricate a CdS/H-MoS2 nano hybrid. The newly fabricated CdS/H-MoS2 shows a hydrogen evolution rate of 162.4 mmol g-1h-1, which is 16 times higher compared to that of CdS/Pt and 67 times higher compared to that of bare CdS. Theoretical results clearly demonstrate a built-in electrostatic potential in the heterostructure junction, and that a shift in water reduction potential plays a key role in the enhancement of hydrogen production rate. We believe that the proposed experimental strategies and theoretical studies will open up a new avenue to develop new photocatalysts with high hydrogen evolution efficiency.

Simulating the growth of amorphous organic thin films

Physical vapor deposition is used widely to produce organic electronics devices, in particular for OLEDs in smartphones, TVs and other display applications. Despite its long history, recent years have seen a number of surprising observations, such as deposition-induced emitter orientation and built-in electrostatic potentials in thin organic films. Modeling the details of these effects is complicated by the long time scales involved in the underlying processes. In this paper, we compare two different modeling approaches, which both aim to simulate the physical vapor deposition (PVD) process for small organic molecules. We compare a molecular dynamics approach, based on a classical bead-spring force field and time integration, with a Metropolis Monte Carlo approach, where the intramolecular degrees are limited to the torsional rotations. To analyze the resulting structures, we calculate the density and radial distribution functions (RDF) of all films. We observe a good agreement for the RDFs, but an approximately 10% higher density for the films generated by the molecular dynamics approach. Additionally, we investigate the anisotropic nature of such morphologies by calculating the ordinary and extra-ordinary refractive index for each material. Finally, we calculate electron and hole mobilities with an Kinetic Monte Carlo protocol.

Extracting band edge profiles at semiconductor heterostructures from hard-x-ray core-level photoelectron spectra

Internal electric fields that underpin functioning of multi-component materials systems and devices are coupled to structural and compositional inhomogeneities associated with interfaces in these systems. Hard-x-ray photoelectron spectroscopy is a valuable source of information on band-edge profiles, governed by the distribution of internal fields, deep inside semiconductor thin films and heterojunctions. However, extracting this information requires robust and physically meaningful decomposition of spectra into contributions from individual atomic planes. We present an approach that utilizes the physical requirements of a monotonic dependence of the built-in electrostatic potential on depth and continuity of the potential function and its derivatives. These constraints enable efficient extraction of band-edge profiles and allow one to capture details of the electronic structure, including determination of the signs and magnitudes of the band bending as well as the valence band offsets. The utility of this approach to generate quantitative insight into the electronic structure of complex materials is illustrated for epitaxial hbox {SrTiO}_3 on intrinsic Si(001).

Creating localized plasma waves by ionization of doped semiconductors.

Localized plasma waves can be generated by suddenly ionizing extrinsic semiconductors with spatially periodic dopant densities. The built-in electrostatic potentials at the metallurgical junctions, combined with electron density ripples, offer the exact initial condition for exciting long-lasting plasma waves upon ionization. This method can create plasma waves with a frequency between a few terahertz to subpetahertz without substantial damping. The lingering plasma waves can seed backward Raman amplification in a wide range of resonance frequencies up to the extreme ultraviolet regime. Chirped wave vectors and curved wave fronts allow focusing the amplified beam in both longitudinal and transverse dimensions. The main limitation to this method appears to be obtaining sufficiently low plasma density from solid-state materials to avoid collisional damping.

Influence of strain relaxation in axial nanowire heterostructures on their electronic properties

We present a systematic theoretical study of the influence of elastic strain relaxation on the built-in electrostatic potentials and the electronic properties of axial nanowire (NW) heterostructures. Our simulations reveal that for a sufficiently large ratio between the thickness of the disk and the diameter of the NW, the elastic relaxation leads to a significant reduction of the built-in electrostatic potential in comparison to a planar system of similar layer thickness and In content. In this case, the ground state transition energies approach constant values with increasing thickness of the disk and only depend on the In content, a behavior usually associated to that of a quantum well free of built-in electrostatic potentials. We show that the structures under consideration are by no means field-free, and the built-in potentials continue to play an important role even for ultrathin NWs. In particular, strain and the resulting polarization potentials induce complex confinement features of electrons and holes, which depend on the In content, shape, and dimensions of the heterostructure.

Ionization, ion distribution, and ion focusing in laser plasmas from atomic and diatomic targets

Charge-resolved measurements of the total number of particles from plasmas produced from planar, monoatomic targets of copper and tungsten as well as the binary targets of copper and tungsten are reported, using a 125mJ, 5ns, Nd:YAG laser, at a laser intensity of about 1010W∕cm2. The measurements show a severe quenching of the ionization states in the case of the diatomic targets. These measurements and their variations with ionization state support the theoretical investigations of plasma motion under the influence of the viscous force in case of plasmas consisting of light and heavy particles. Gaussian width measurements of the angular particle distribution showed a focusing effect towards the target normal, the width decreasing as the ion mass and ionization state increased. From the analysis of the theoretical results on self-similarity expansion it is concluded that the ion acceleration due to the built-in electrostatic potential is not significant.

How the photoluminescence of submicron epitaxial n^+-n-i structures of III-V compounds depends on their electrophysical characteristics

This paper presents the results of photoluminescence (PL) studies of submicron epitaxial n^+-n-i structures of GaAs. A mathematical model of the PL of submicron epitaxial III-V structures is proposed that makes it possible to determine the theoretical dependences of the PL intensity on the surface recombination rate, the mobility of the minority nonequilibrium charge carriers, the absorption and self-absorption coefficients, the magnitude and sign of the built-in electrostatic potential between the n^+ and n layers, and also the thicknesses of the epitaxial layers. It is pointed out that there is good agreement of the experimental and theoretical results. © 2005 Optical Society of America

The effects of adjacent dislocations on the electronic and optical properties of GaN/AlN quantum dots

We present a theory of the electronic energy levels and wave functions within GaN quantum dots (QDs), including the effect of the strain field of a nearby dislocation and a full treatment of the built-in electrostatic potential. The QD carrier spectra and wave functions are calculated using a plane-wave expansion method within an 8-band k · p model. The GaN QDs are truncated hexagonal pyramids on a wetting layer with an edge dislocation adjacent to each dot. The built-in piezoelectric potential strongly influences the localization of the carrier wave functions. The potential pushes the electrons to the top of the dot, the holes to the bottom and, additionally, causes strong lateral confinement of the carriers near the dot center. The strong lateral confinement of the carriers indicates that the effect of the strain field due to the dislocation is relatively small. The size of the dot influences the energies and overlaps, but the presence of the dislocation has minimal effect. The dependence of the ground state optical transition energy on the size of the dot is in good agreement with experimental data.

The influence of Coulomb effects on the electron emission and capture in InGaAs/GaAs self-assembled quantum dots

The influence of built-in electrostatic potential of interface dipole on barrier energies for the emission and capture of electrons by InGaAs/GaAs quantum dots (QDs) has been found. It was shown that the formation of the interface dipole from carriers localized in QDs and ionized deep level defects depended on isochronous annealing under bias-on-bias-off cooling conditions. The dependence of the height of the capture barrier on the filling pulse duration, related to a manifestation of the Coulomb blockade effects on the capture of electrons into the ground and excited states of the dots, has also been revealed.

Theory of the electronic structure of GaN/AlN hexagonal quantum dots

We present a theory of the electronic structure of GaN/AlN quantum dots (QD's), including built-in strain and electric-field effects. A Green's function technique is developed to calculate the three-dimensional (3D) strain distribution in semiconductor QD structures of arbitrary shape and of wurtzite (hexagonal) crystal symmetry. We derive an analytical expression for the Fourier transform of the QD strain tensor, valid for the case when the elastic constants of the QD and matrix materials are equal. A simple iteration procedure is described, which can treat differences in the elastic constants. An analytical formula is also derived for the Fourier transform of the built-in electrostatic potential, including the strain-induced piezoelectric contribution and a term associated with spontaneous polarization. The QD carrier spectra and wave functions are calculated using a plane-wave expansion method we have developed, and a multiband $\mathbf{k}\ensuremath{\cdot}\mathbf{P}$ model. The method used is very efficient, because the strain and built-in electric fields can be included analytically through their Fourier transforms. We consider in detail the case of GaN/AlN QD's in the shape of truncated hexagonal pyramids. We present the calculated 3D strain and electrostatic potential distributions, the carrier spectra, and wave functions in the QD's. Due to the strong built-in electric field, the holes are localized in the wetting layer just below the QD bottom, while electrons are pushed up to the pyramid top. Both also experience an additional lateral confinement due to the built-in field. We examine the influence of several key factors on the calculated confined state energies. Use of a one-band, effective-mass Hamiltonian overestimates the electron confinement energies by \ensuremath{\sim}100 meV, because of conduction-band nonparabolicity effects. By contrast, a one-band valence Hamiltonian provides good agreement with the calculated multiband ground-state energy. Varying the QD shape has comparatively little effect on the calculated levels, because of the strong lateral built-in electric field. Overall, the transition energies depend most strongly on the assumed built-in electric field. The calculated variation of transition energy with quantum dot size is in good agreement with the available experimental data.

Chemical potential pinning due to equilibrium electron transfer at metal/C60-doped polymer interfaces

We report electroabsorption measurements of the built-in electrostatic potential in metal/C60-doped polymer/metal structures to investigate chemical potential pinning due to equilibrium electron transfer from a metal contact to the electron acceptor energy level of C60 molecules in the polymer film. The built-in potentials of a series of structures employing thin films of both undoped and C60-doped poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) were measured. For undoped MEH-PPV, which has an energy gap of about 2.4 eV, the maximum built-in potential is about 2.1 eV, whereas for C60-doped MEH-PPV the maximum built-in potential decreases to 1.5 eV. Electron transfer to the C60 molecules close to the metal interface pins the chemical potential of the metal contact near the electron acceptor energy level of C60 and decreases the built-in potential of the structure. From the systematic dependence of the built-in potential on the metal work function we find that the electron acceptor energy level of C60 in MEH-PPV is about 1.7 eV above the hole polaron energy level of MEH-PPV.

Probing electronic state charging in organic electronic devices using electroabsorption spectroscopy

In metal/organic-film/metal device structures with different metal contacts there is a built-in electrostatic potential at equilibrium due to the asymmetric contacts. At thermal equilibrium the electrochemical potential is constant across the device structure. The electrochemical potential can be divided into the sum of two parts, the electrostatic potential and the chemical potential. By measuring the built-in electrostatic potential change across a structure at equilibrium, one can determine the change in chemical potential across the structure. Measuring this built-in electrostatic potential for devices with different contact metals provides a way of changing the chemical potential ( μ) in the organic material and identifying the values of μ at which charged excitations are populated. Such measurements can be used to gain information on the energy spectrum of intrinsic charged excitations, charged trap states and charged interface states. We apply an electroabsorption technique to measure the built-in potentials of metal/organic-film/metal structures fabricated from poly[2-methoxy,5-(2′-ethyl-hexyloxy)-1,4 phenylene vinylene] (MEH-PPV), C 60-doped MEH-PPV, and 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) and discuss what is learned about the charged excitations in these materials from the results.

Control of carrier lifetimes in PbTe doping superlattices

Following subnanosecond pulsed laser excitation, time-resolved photoconductivity has been measured in PbTe doping superlattices in the temperature range 50–200 K. The carrier lifetime changes drastically by about three orders of magnitude during the recombination processes indicating an instantaneous lifetime which varies from a ns time scale up to about 1 μs. The temporal evolution is explained by the initially complete flattening of the built-in superlattice electrostatic potential and its recovery with the decrease in the nonequilibrium carrier concentration. The temperature dependence of the recombination is determined by both the bulk band structure of PbTe and the superlattice potential. The effective lifetime initially increases with temperature due to a decrease in the electron-hole recombination by tunneling, but decreases at higher temperatures where a thermal activation process dominates.

Kinetics of oxide film growth on metal crystals: Space-charge-modified thermal electron emission and ionic diffusion. Part 2. Numerical results

Presented are results obtained by a coupled-currents numerical evaluation of an oxidation model based on ionic diffusion and thermal electron emission in the presence of space charge of the diffusing ionic species. The pertinent equations for this model were given in Part 1. Part 2 focuses on the predictions of the model as deduced by numerical evaluation. The various figures illustrate well the range of physical quantities, such as diffusing defect concentration profiles, electric fields within the growing oxide layer, and built-in electrostatic potentials, which can be obtained by a coupled-currents calculation for any model once the ionic and electronic transport currents have been specified. Of special interest in the present work are the electron energy barriers obtained for oxides of various thicknesses and various growth rates. The space-charge-modified kinetics deduced for the present model, when compared with the significantly different kinetics exhibited in published homogeneous field calculations, illustrate the following: (a) In the early growth stage, the negative surface-charge field is an order of magnitude or so larger than its homogeneous field counterpart; this significantly aids the injection of rate-limiting electronic carriers into the conduction band of the oxide, and the oxide growth rate is thereby enhanced. (b) In the later stages of growth where the ionic species becomes rate-limiting, the space charge of the diffusing ions causes a marked retardation of the ionic current and the accompanying oxide film growth. (c) The transition from electron rate-limited growth to ionic rate-limited growth occurs shortly after the classical electron energy barrier maximum switches from inside the film (Schottky-type emission) to a space-charge-produced barrier maximum at the outer interface of the film.

The Built-In Electrostatic Potential, Field, and Field Gradient in Diffused p-n Junctions

First Page