Electron Density In Conduction Band Research Articles

Conduction Band Electron Density Variation Leads to Different Degrees of Ultrafast Laser Ablation of Aluminum-Silver Bimetals in Double Distilled Water

Conduction Band Electron Density Variation Leads to Different Degrees of Ultrafast Laser Ablation of Aluminum-Silver Bimetals in Double Distilled Water

Chiral induction enhances hydrogen generation performance of CdS quantum dots

Chirality is ubiquitous in the nature. Chiral nanomaterials show wide application prospects owing to their special properties. In this study, chirality was introduced into photocatalysis. Chiral L-cysteine was introduced into photocatalytic materials for preparation of chiral CdS quantum dots (QDs) at room temperature. Then the CdS QDs were characterized by ultraviolet-visible spectra, infrared spectra and X-ray photoelectron spectroscopy. The hydrogen production activity was tested within the visible light range. Under visible light irradiation, the hydrogen yields of chiral CdS QDs at 298, 323, 333, 348, 358 K were all higher compared with the nonchiral CdS QDs. The hydrogen yield was maximized to 18.72 mmol/g/h at 333 K, which was significantly higher than that of the nonchiral CdS QDs (10.35 mmol/g/h). The reasons for the higher activity of chiral CdS QDs were that the introduction of the chiral ligand broadened the bandgap of CdS, and the more-negative conduction band strengthened the reducibility of photoelectrons. Moreover, the electron-donating groups that coordinated with the active Cd2+ directionally enhanced the electron density of CdS QDs. Theoretical calculation showed the conduction band electron density of CdS was significantly improved, which is consistent with the above results. For these reasons, the hydrogen production ability of chiral CdS QDs is considerably higher than that of nonchiral CdS QDs.

Graphene loaded TiO2 submicron spheres scattering layer for efficient dye-sensitized solar cell

Dye-Sensitized Solar Cells (DSSCs) have attracted great attention due to environmentally friendly low-cost processing, excellent working ability in diffuse light, and potential to meet the power demands of future buildings due the true class of building integrated photovoltaics (BIPV). Nevertheless, DSSCs have relatively low photoconversion efficiency (PCE) due to multiple issues. Several strategies have been employed to enhance its PCE. For instance, bi-layered structure of photoelectrode i.e., mesoporous TiO2 transparent layer with top scattering layer was introduced which scatter light inside on large angles improves the harvesting ability of photoelectrode thus enhanced PCE. However, scattering layer is composed of aggregated small particles which offer sluggish electron transport due to multiple grain boundaries, consequently, unwanted recombination reaction which leads to poor PCE. This issue has been addressed for transparent layer immensely but ignored for scattering layer. Mostly for scattering layer in previous studies novel structures have been proposed to enhance scattering properties and dye adsorption only. Therefore, in this study for the first time presenting dual functional graphene/TiO2 scattering layer in which solvent exfoliated graphene is incorporated in TiO2 submicron spheres which enhanced electron transport properties, while submicron spheres scatter light effectively. Scattering and electron transport characteristics of DSSCs are thoroughly investigated with the function of graphene loading. Electrochemical impedance spectroscopy (EIS) has revealed that diffusion coefficient length and coefficient and conductivity attained maximum value at 0.01 wt%. while other important parameters such as electron lifetime and electron density in conduction band have been improved till 0.020 wt% graphene loading. However, results indicated that with 0.01 w% graphene 33% higher PCE was achieved than without scattering layer and 13% higher than scattering layer without graphene. The depraving in PCE at >0.01 wt% graphene despite of excellent electron transport improvement is attributed to the loss of diffuse reflectance and higher optical absorption by graphene.

Nanoscale active tuning of the second harmonic generation efficiency in semiconductors from super-low to gigantic values

Second harmonic generation efficiency (SHGE) strongly depends on the length of the interaction material along the beam propagation axis. Since a nanoscale interaction length is considered too short even in the optical wavelength scale, the attained SHGE through nanomaterials is usually too low to be of practical use. In this study, it will be shown that by properly adjusting the conduction-band electron density in a semiconductor nanomaterial under a certain optical pumping rate (active tuning), the SHGE can be effectively tuned from being super-low to being ultra-high. Such sharp tunability is only valid for small-scale materials as their density of conduction-band electrons can be rapidly switched between high and low under moderate optical pumping. Using an experimentally verified computational model, we have observed that at a given frequency, for a certain range of conduction-band electron densities, the SHGE can reach up to 1080% for Ga-As and 230% for silicon nanomaterials under active tuning, with respect to the intensity of the first harmonic of the input signal. Such SHGEs are unprecedented, which is very promising for generating higher harmonics via cascaded second harmonic generation performed via adaptive tuning of the conduction band electron density at each stage.

Top Electrode Engineering for Freedom in Design and Implementation of Ferroelectric Tunnel Junctions Based on Hf1–xZrxO2

Ferroelectric tunnel junctions (FTJs) based on ultrathin HfO2 have great potential as a fast and energy-efficient memory technology compatible with complementary metal oxide semiconductors. FTJs consist of a ferroelectric film sandwiched between two distinct electrodes, the properties of which are intricately linked to the electrical properties of the FTJs. Here we utilize a W crystallization electrode (CE) to achieve a high and reproducible remanent polarization, combined with a metal replacement process in which the W is carefully removed and replaced by another top electrode (TE). In this way we separate the ferroelectric film properties from the device design and can thereby evaluate the effect of the TE work function (WF) and conduction band electron density (ne) on the tunneling electroresistance (TER) and device reliability. We compare FTJs designed with a TiN bottom electrode and W, Cr, or Ni TE and find that the use of high electron density metals such as Ni or Cr as TE allows for an improved TER, albeit at the cost of reliability due to a large built-in electric field. To bypass this effect, a bilayer Cr/Ni TE is implemented, which allows for a high TER and minimal built-in field, leading to excellent retention and endurance beyond 108 cycles. The results presented here thus highlight a process flow for reliable design and implementation of FTJs.

Plasmonic Surface Lattice Resonances: Theory and Computation.

Plasmonic surface lattice resonances (SLRs) are mixed light-matter states emergent in a system of periodically arranged metallic nanoparticles (NPs) under the constraint that the array spacing is able to support a standing wave of optical-frequency light. The properties of SLRs derive from two separate physical effects; the electromagnetic (plasmonic) response of metal NPs and the electromagnetic states (photonic cavity modes) associated with the array of NPs. Metal NPs, especially free-electron metals such as silver, gold, aluminum, and alkali metals, support optical-frequency electron density oscillations known as localized surface plasmons (LSPs). The high density of conduction-band electrons in these metals gives rise to plasmon excitations that strongly couple to light even for particles that are several orders of magnitude smaller than the wavelength of the excitation source. In this sense, LSPs have the remarkable ability to squeeze far-field light into intensely localized electric near-fields that can enhance the intensity of light by factors of ∼103 or more. Moreover, as a result of advances in the synthesis and fabrication of NPs, the intrinsic dependence of LSPs on the NP geometry, composition, and size can readily be exploited to design NPs with a wide range of optical properties. One drawback in using LSPs to enhance optical, electronic, or chemical processes is the losses introduced into the system by dephasing and Ohmic damping-an effect that must either be tolerated or mitigated. Plasmonic SLRs enable the mitigation of loss effects through the coupling of LSPs to diffractive states that arise from arrays satisfying Bragg scattering conditions, also known as Rayleigh anomalies. Bragg modes are well-known for arrays of dielectric NPs, where they funnel and trap incoming light into the plane of the lattice, defining a photonic cavity. The low losses and narrow linewidths associated with dielectric NPs produce Bragg modes that oscillate for ∼103-104 cycles before decaying. These modes are of great interest to the metamaterials community but have relatively weak electric fields associated with dielectric NPs and therefore are not used for applications where local field enhancements are needed. Plasmonic lattices, i.e., photonic crystals composed of metallic NPs, combine the characteristics of both LSPs and diffractive states, enabling both enhanced local fields and narrow-linewidth excitations, in many respects providing the best advantages of both materials. Thus, by control of the periodicity and global symmetry of the lattice in addition to the material composition and shape of the constituent NPs, SLRs can be designed to simultaneously survive for up to 103 cycles while maintaining the electric field enhancements near the NP surface that have made the use of LSPs ubiquitous in nanoscience. Modern fabrication methods allow for square-centimeter-scale patches of two-dimensional arrays that are composed of approximately one trillion NPs, making them effectively infinite at the nanoscale. Because of these advances, it is now possible to experimentally realize SLRs with properties that approach those predicted by idealized theoretical models. In this Account, we introduce the fundamental theory of both SLRs and SLR-mediated lasing, where the latter is one of the most important applications of plasmonic SLRs that has emerged to date. The focus of this Account is on theoretical concepts for describing plasmonic SLRs and computational methods used for their study, but throughout we emphasize physical insights provided by the theory that aid in making applications.

Synthesis of QDs self-modified Bi2MoO6/Bi4Ti3O12 photocatalysts via controlling charge unidirectional flow for effective degradation of organic pollutants

Reinforcing separation of charge carriers and increasing electron density played a leading role in improving photocatalytic performance. In this work, incorporating homojunction and heterojunction in quantum dots (QDs) self-modified Bi2MoO6/Bi4Ti3O12 (BMO(Q)/BTO) composites had been rationally designed, and a newfangled photocatalyst had been acquired which could realize the fine manipulation of charge carriers flow direction. Among multitudinous samples, the 15-BMO(Q)/BTO composite exhibited the optimal photocatalytic properties and pervasive applicability for removing the various pollutants (dye, mercaptan and heavy metal) under visible light. We utilized the three-dimensional excitation-emission matrix fluorescence spectroscopy (3D EEMs) to fully grasp the degradation behaviors of RhB. The encouraging increase in photocatalytic activity was mainly attributed to the unified transfer of electrons generated by BMO QDs and BTO to the conduction band of BMO, which effectively increased charge carrier lifetime and improved the conduction band electron density. Furthermore, the photocatalytic degradation mechanism over 15-BMO(Q)/BTO composite was systematically investigated by the calculation, trapping experimental and ESR. The ·OH undertook the active constituent in the process of degradation pollutants. The result suggested that the construction of homojunction/heterojunction photocatalysts to enhance electrons density was a reliable design idea.

Analysis of electron transport in the nano-scaled Si, SOI and III-V MOSFETs: Si/SiO2 interface charges and quantum mechanical effects

The ITRS predicts that the scaling of planar CMOS (Complementary Metal Oxide Semiconductor) technology will continue till the 22 nm technology node [1] and a possible extension beyond is appealing [2]. In this work, we investigate the effect of electron confinement [3] in nanoscaled transistor channels of 25 nm surface channel Si and 32 nm SOI (Silicon on Insulator) and 15 nm IF (Implant Free) III-V MOSFETs using a self-consistent solution of 1 D Poisson - Schrödinger equation [4,5]. For simulat ion and development with accuracy of nano-scaled of 25 nm gate length Si MOSFET (Metal Oxide Semiconductor Field Effect Transistor), 32 nm SOI Implant Free (IF) MOSFET, and 15nm Implant Free III-V MOSFET transistors, we investigated the bandstructure and quantum confinement effects occurring near the oxide-semiconductor interface inmetal-Oxide-Semiconductor (MOS) structure of Si MOSFET device. These investigation have been carried out using a selfconsistent solution of 1D Poisson-Schrödinger equation across the channel of conventional Si / SOI / III-V MOSFET Transistors. To solve self-consistently 1D Poisson-Schrödinger equations across the channel of a conventional Si, SOI, and an Implant Free III-V MOSFETs to determine the conduction and valence band profiles, electron density, electron sheet density, eigenstate and eigenfunctions in these structures. We present the simulat ion results of conduction band profile, electron density (classical and quantum mechanical), eigenstate and eigenfunctions for Si, SOI and III-V MOSFET structures at two different bias voltages of 0.5 V and 1.0 V. For comparison, we calculate the electron sheet density (quantum mechanically) as a function of the applied gate voltages.

Improving the effective photovoltaic performance in dye-sensitized solar cells using an azobenzenecarboxylic acid-based system

In this research, an azobenzenecarboxylic acid was used as a sufficient co-adsorbent in combination with N719 dye. As it is found from the results, an optimized concentration of the co-absorbent leads to the highest efficiency. The dye-sensitized solar cells (DSSCs) parameters such as short-circuit current (Jsc), open-circuit voltage (Voc) and conversion efficiency (η) were obtained -14.87 mA/cm2, 0.765 V and 5.20% respectively. Based on the results, the N719/Azobenzenecarboxylic-based system shows a significant increase in Voc and Jsc, resulting in an ∼21% improvement in the efficiency. A higher conversion efficiency for the co-adsorbent-based systems was assigned to their enhanced η, which is attributed to reduced dye aggregation, higher electron injection and increased Voc. This corresponded to the improved electron density in the TiO2 conduction band of the photoanode and reduced charge recombination revealed through electrochemical impedance spectroscopy measurements. Also, evidence was provided for a long charge life time and a high resistance of charge recombination for the co-absorbed solar cells.

From random inhomogeneities to periodic nanostructures induced in bulk silica by ultrashort laser

Femtosecond laser-induced volume nanograting formation is numerically investigated. The developed model solves nonlinear Maxwell's equations coupled with multiple rate free carrier density equations in the presence of randomly distributed inhomogeneities in fused silica. As a result of the performed calculations, conduction band electron density is shown to form nanoplanes elongated perpendicular to the laser polarization. Two types of nanoplanes are identified. The structures of the first type have a characteristic period of the laser wavelength in glass and are attributed to the interference of the incident and the inhomogeneity-scattered light waves. Field components induced by coherent multiple scattering in directions perpendicular to the laser polarization are shown to be responsible for the formation of the second type of structures with a subwavelength periodicity. In this case, the influence of the inhomogeneity concentration on the period of nanoplanes is shown. The calculation results not only help to identify the physical origin of the self-organized nanogratings, but also explain their period and orientation.

Ion induced electron emission from chemically cleaned Si and Ge

A compelling yet basic question regarding ion induced electron emission (IIEE) from semiconductors has never been experimentally investigated: Can a larger number of electrons in the conduction band (closer in energy to the vacuum level) cause a larger IIEE yield (γ)? The lack of fundamental measurements of γ under varying surface and sub-surface circumstances has left this question open to speculation. Recent measurements related to IIEE from semiconductors have suggested that deep sub-surface conditions may in fact lead to significant variations in γ. In this work, the IIEE yields of chemically cleaned silicon and germanium bombarded with a variety of noble gas ions have been measured at energies between 25 and 300 eV. Our measurements indicate that while the IIEE yields depend strongly on the ion being tested (due to the ion’s potential energy) they do not increase with increasing conduction band electron density. These measurements are consistent with the assumption made in the most widely accepted theory for IIEE, and with our extension of this theory to incorporate conduction band electrons.

Efficient co-sensitization of dye-sensitized solar cells by novel porphyrin/triazine dye and tertiary aryl-amine organic dye

A novel porphyrin dye ZnP-triazine-(gly)2, consisting of a zinc-metallated porphyrin unit covalently linked through its peripheral aryl-amino group with a 1,3,5-triazine group which is functionalized by two carboxylic acid groups of glycine moieties, has been synthesized. Photophysical and electrochemical measurements, as well as theoretical DFT calculations, suggest that the compound exhibits appropriate light absorption characteristics and frontier molecular orbital levels for use as sensitizer in dye-sensitized solar cells (DSSCs). The ZnP-triazine-(gly)2 based solar cell was found to exhibit a power conversion efficiency (PCE) value of 4.72%. A significant improvement of the overall photovoltaic efficiency of the solar cell was achieved up to 7.34% upon co-sensitization with a tertiary aryl-amine D with two ethynyl-pyridine substituents and cyano-acetic acid anchoring group, which exhibits complementary light absorption characteristics with the porphyrin dye. The higher PCE value of the co-sensitized solar cell is attributed to its enhanced short circuit current (Jsc), which is due to improved light harvesting efficiency, reduced porphyrin aggregation, and faster electron injection and charge collection, as well as its enhanced open circuit voltage (Voc), which is due to increased electron density in the TiO2 conduction band of the photoanode. These results are in accordance with electrochemical impedance spectra (EIS) of the solar cells, which revealed higher charge recombination resistance (Rrec), longer electron lifetime (τe), and shorter electron transport time (τd) for the co-sensitized solar cell.

Dynamics of Electron Recombination and Transport in Water-Splitting Dye-Sensitized Photoanodes

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) use visible light to split water using molecular sensitizers and water oxidation catalysts codeposited onto mesoporous TiO2 electrodes. Despite a high quantum yield of charge injection and low requirement for the catalytic turnover rate, the quantum yield of water splitting in WS-DSPECs is typically low (<1%). Here we examine the charge separation and recombination processes in WS-DSPECs photoanodes functionalized with varying amounts of IrO2 nanoparticle catalyst. Charge extraction and transient open-circuit voltage decay measurements provide insight into the relationship between light intensity, conduction band electron density, open-circuit photovoltage, and recombination time scale. We correlate these results with electrochemical impedance spectroscopy and present the first complete equivalent circuit model for a WS-DSPEC. The data show quantitatively that recombination of photoinjected electrons with oxidized sensitizer molecules a...

Electrical activation and electron spin resonance measurements of arsenic implanted in silicon

The electrical activation of arsenic (As) implanted in Si is investigated with electron spin resonance (ESR), spreading resistance (SR), and secondary ion mass spectroscopy (SIMS). The As ions were implanted with a dose of 1 × 1012 cm−2 and subsequently annealed at various temperatures in the range of 500–1100 °C. The ESR measurements at 10 K show that the density of the As donor electrons for all the annealing temperatures is less than 10% of the As atom concentration measured by SIMS. The SR data indicate that the density of conduction band electrons is several times larger than that of the As donor electrons. These results strongly suggest that most of the As donor electrons are ESR inactive at low temperatures.

150 fs~10 ps脉宽下熔石英激光损伤的仿真与分析

Based on the reduced growth rate equation of electron density, a theoretical model is established to describe the change of the conduction band electron density of fused silica with the laser pulse duration. Using the concept of the critical electron density, a scope of the laser-induced damage of fused silica is calculated with the pulse width from 150 fs to 10 ps. The analysis shows that the avalanche ionization still takes a leading role from 5 ps to 10 ps. Instead of electrons provided by the fused silica itself, the initial electrons produced by the photoionization make a contribution to the impact ionization. It reaches a balance between avalanche ionization and photoionization when the pulse width shrinks to 4 ps. After that, the photoionization plays a main role in the growth of the conduction band electron density. Through the fitting curve, a new pulse width law is obtained to satisfy the laser-induced damage threshold changing from 150 fs to 10 ps. The results demonstrate that the threshold is proportional to tp0.38(tp is the pulse width), and changes to tp0.34 when the temperature dependence is concerned.

Hybrid polymer-based solar cells with metal oxides as the main electron acceptor and transporter

Hybrid polymer-based solar cells (HPSCs) that use conjugate polymers as electron donor (D) and inorganic semiconductor nanocrystals as electron acceptor (A) are novel photovoltaic devices. HPSCs integrate the properties of organic polymer (flexibility, ease of film formation, high absorption coefficient) and inorganic nanostructures (high electron mobility, high electron affinity, and good stability), and have the extra advantages, such as the rich sources of synthesized nanostructures by wet chemistry, tunable and complementary properties of assembled components, solution-processibility on a large scale at low cost and light-weight, etc. Amongst various inorganic semiconductor materials, the nanostructured metal oxides are the promising electron acceptors for HPSCs, because they are environment-friendly, transparent in visible spectrum and easy to be synthesized. After a brief introduction to the current research status, working principles, device architecture, steady-state and dynamic characterizations of HPSCs, this paper mainly reviews our recent research advances in the HPSCs using ZnO and TiO2 nanostructures as main electron acceptor and transporter, with emphasis on the theoretical models for charge carrier transport dynamics, design and preparation of efficient materials and devices, and the device performance related with nanostructural characteristics. Finally, the main challenges in the development of efficient HPSCs in basic researches and practical applications are also discussed. The main conclusions from our studies are summarized as follows: (i) IMPS and IMVS are powerful dynamic photoelectrochemical methods for studying the charge transport dynamics in HPSCs, and our theoretical models enable the IMPS to serve as an effective tool for the mechanistic characterization and optimization of HPSC devices. (ii) Using a multicomponent photoactive layer with complementary properties is an effective strategy to achieve efficient HPSCs. (iii) Using the complementary property of components, enhancing the dissociation efficiency of excitons, and improving the transport properties of the acceptor channels with reduced energy loss to increase collection efficiency all are the effective measures to access a high photocurrent generation in HPSCs. (iv) The band levels of components in the photoactive layer of HPSCs are aligned into type II heterojunctions, in which the nanostructured component with the lowest conduction band edge acts as the main acceptor/transporter; the maximum open-circuit voltage (Voc) in HPSCs is determined by the energy difference between the highest occupied molecular orbital (HOMO) level of conjugated polymer and the conduction band edge of the main acceptor, but the Voc in practical devices correlates strongly with the quasi-Fermi levels of the electrons in the main acceptor and the holes in the polymer. While passivating the surface defects on the main acceptor, increasing spatial e-h separation, and enhancing the electron density in conduction band of the main acceptor will facilitate the increase in Voc. (v) There is no direct correlation among Voc, photogenerated voltage (Vph) and electron lifetime (τe), and they may change in the same or the opposite trend when the same or different factors affect them, therefore one should get insight into the intrinsic factors that influence them when discussing the changes in Voc, V_{ph} and τe that are subject to nanostructural characteristics.

Characterization of Photocurrent Generation Dynamics in Polymer Solar Cells Based on ZnO/CdS-Core/Shell Nanoarrays by Intensity Modulated Photocurrent Spectroscopy: Theoretical Modeling

A theoretical model is developed for the dynamic characterization of hybrid polymer-based solar cells (HPSCs) based on vertically aligned ZnO/CdS-core/shell nanorod arrays (ZC-NAs) by intensity modulated photocurrent spectroscopy (IMPS). The model describes the effects of CdS shell formation on charge generation and transport dynamics. Particularly, an analytical expression for the ineffective polymer phase model in nanoarray solar cells is developed and introduced into IMPS model for the first time. The main expectations of the IMPS model are confirmed by the experimental data of the polymer/ZC-NA cells with the CdS shell thickness (L) of 3–8 nm. It is shown that the contributions from CdS absorption (f1) and polymer absorption (f2) to charge generation are determined by the core/shell nanoarray structure and the intrinsic polymer property, while the optimal CdS shell thickness (Lopt) depends on the interspacing between ZnO core nanorods and the exciton diffusion length of the polymer. The photocurrent generation is dominantly the competitive results of f1 and f2 contributions subjected to the change in L, with the polymer as a dominant absorption material. Fittings of the measured IMPS responses to the IMPS model reveal that the L-dependence of photocurrent generation dominantly originates from f1, f2, and the polymer exciton dissociation rate S at the polymer/CdS interface. Moreover, the first-order rate constants for the surface defects to trap and detrap the injected electrons in ZnO core nanorods are found to decrease with CdS shell growth and become saturated at Lopt. Furthermore, it is demonstrated that the effective electron diffusion coefficient De in the ZnO nanorods reaches a peak value at Lopt as the result of the largest photogenerated electron density in conduction band. Those results provide a guide to the design of efficient HPSCs based on the core/shell nanoarrays with complementary properties.

Fundamental mechanisms of laser damage of dielectric crystals by ultrashort pulse: ionization dynamics for the Keldysh model

Laser-induced ionization is a major process that initiates and drives the initial stages of laser-induced damage (LID) of high-quality transparent solids. The ionization and its contribution to LID are characterized in terms of the time-dependent ionization rate and conduction-band electron density. Considering femtosecond pulses of various durations (from 35 to 706 fs) and variable peak irradiances (from 0.01 to 60 TW/cm2), we use a single-rate equation to simulate time variations of conduction-band electron density and rates of the photoionization and impact ionization. The photoionization rate is evaluated with the Keldysh equation. At low irradiance, the electron density and total ionization rate demonstrate power scaling characteristic of multiphoton ionization. With the increase of irradiance, there is observed a saturation of the photoionization rate due to photoionization suppression by the Keldysh-type singularity during the increase in the number of simultaneously absorbed photons by 1. A striking result is that the saturation is followed by a stepwise transition from the ionization regime which is completely dominated by the photoionization to a regime totally dominated by the impact ionization. The transition results in the increase of the electron density by a few orders of magnitude induced by a variation of peak laser irradiance by about 15% to 20%. The physical effects that are involved are discussed.

Influence of Surface States on Electronic Band Structure and Electron Density in InAs Nanowires and InAs Shell Nanowires

Semiconductor nanowires are recently of special interest due to the strong electron confinement in their one-dimensional structure, which, in turn, implies easily tunable transport properties of charge carriers. This is especially important for use of nanowires in electrical sensing devices as well as field-effect transistors. High surface-to-volume ratio typical for nanowires leads to strong dependence of their electrical properties on surface states. The quantum confinement of the charge carriers in nanowires might be even more strengthened due to manufacturing shell nanowires via the chemical etching of a core in core-shell structures [1]. The higher confinement in the shell nanowires in comparison to typical nanowires implies also higher surface-to-volume ratio, thus even stronger impact of surface states on electronic band structure and electron population of the shell structures. Here, we report on a theoretical study of impact of surface states on the electron band structure and electron density in intrinsic cylindrical InAs nanowires and InAs shell nanowires (Fig., inset). For the calculation of the electron conductivity, we describe the density of the surface states as a function of energy by the U-type dependence with the minimum corresponding to the neutrality level at the surface. We assume that in accordance with the experimental data for surface of bulk InAs presented in [2] the neutrality level is located about 160 meV above the conduction band edge. We calculate the band structure of the conduction band by the self-consistent solution of Poisson and Schroedinger equations in cylindrical coordinate system where the Schroedinger equation is solved for the envelope functions within the effective mass approach.We show that for both nanowires and shell nanowires the conduction band bending and electron density in the nanowires are strongly affected by the charged surface states at both low and room temperature. Due to the location of the neutrality level above the conduction band edge the conduction band in both nanowires and shell nanowires is bent downwards at the surface giving rise to the formation of the electron accumulation channel at the surface (Fig., inset). The conduction band bending increases with increase of total density of surface states and for the density exceeding 1012 cm-2 the band profile remains almost without changing. Then, Fermi level at the surface in both types of structures is pinned in the vicinity of the neutrality level. The electron density in both nanowires and shell nanowires increases with increase in the total surface states density due to the donor-type states delivering electrons in the structures. The electron density reaches its maximum value for the surface states density above 1012 cm-2. The figure displays that for the total density of surface states above 1012 cm-2 intrinsic nanowires and shell nanowires show significant two-dimensional electron density which increases with the external nanowire radius. Donor-type states of the internal surface in shell nanowires introduce additional electrons into the structure volume, thus increasing their two-dimensional electron density in comparison with conventional nanowires, This does not hold only for shell structures with the shell thickness below 15 nm which are less populated then the nanowires due to the strong electron confinement. The study delivers criteria for development of a building block for tube-like electrical devices.Fig.: Two-dimensional electron density vs external radius InAs nanowires (symbols) shell nanowires (solid line) with the shell thickness of 10, 20 and 30 nm at 300 K and the total surface states density above 1012 cm-2; inset: sketch of InAs nanowire and shell nanowire and their band structure.[1] T. Rieger et al. Nano Lett. 12, 5559 (2012). [2] H. Hasegawa and T. Sawada, J. Vac Sci. Technol. 21, 457 (1982).

Plasmon Resonance Effects in GaAs/AlGaAs Heterojunction Devices: An Analysis Based on Spectral Element Simulation

The effect of surface plasmons is investigated in III-V devices that incorporate a subsurface heterojunction to guide electron transport, in a structure analogous to that of the high-electron mobility transistor. The use of the spectral element method results in a highly efficient computational approach; perturbations in the electric potential resulting from surface plasmonic effects are included in a self-consistent solution of the Schrödinger-Poisson equations. The results of calculations of electron conduction band edge and electron density distribution are presented, and the effect of the plasmonic penetration depth on electron density distribution in the 2-D electron gas at the heterojunction is studied. This approach has broad applicability in the design and simulation of III-V optoelectronic sensors and transducers used for physiochemical and biological sensing and imaging.