Electrical Doping Research Articles

Gated Tunability and Hybridization of Localized Plasmons in Nanostructured Graphene

Graphene has emerged as an outstanding material for optoelectronic applications due to its high electronic mobility and unique doping capabilities. Here we demonstrate electrical tunability and hybridization of plasmons in graphene nanodisks and nanorings down to 3.7 μm light wavelength. By electrically doping patterned graphene arrays with an applied gate voltage, we observe radical changes in the plasmon energy and strength, in excellent quantitative agreement with rigorous analytical theory. We further show evidence of an unexpected increase in plasmon lifetime with growing energy. Plasmon hybridization and electrical doping in nanorings of suitably chosen nanoscale dimensions are key elements for bringing the optical response of graphene closer to the near-infrared, where it can provide a robust, integrable platform for light modulation, switching, and sensing.

The role of cesium fluoride as an n-type dopant on electron transport layer in organic light-emitting diodes

We report on the role of cesium fluoride (CsF) doping on the enhanced electron transport properties of tris-(8-hydroxyquinolin) aluminum (Alq3) for organic light-emitting diodes. The electronic structures of CsF-doped Alq3 layers with various doping concentration are characterized by in situ ultraviolet and X-ray photoelectron spectroscopies, showing an n-type electrical doping effect with Fermi level shift towards unoccupied molecular orbital and the formation of chemistry-induced gap-states. The increase in conductivity and reduction in electron injection barrier in CsF-doped Alq3 layer with optimal doping concentration lead to the enhanced electron injection and transport, which are consistent with the improved electrical characteristics of OLEDs.

Doping of organic semiconductors

AbstractThe understanding and applications of organic semiconductors have shown remarkable progress in recent years. This material class has been developed from being a lab curiosity to the basis of first successful products as small organic LED (OLED) displays; other areas of application such as OLED lighting and organic photovoltaics are on the verge of broad commercialization. Organic semiconductors are superior to inorganic ones for low‐cost and large‐area optoelectronics due to their flexibility, easy deposition, and broad variety, making tailor‐made materials possible. However, electrical doping of organic semiconductors, i.e. the controlled adjustment of Fermi level that has been extremely important to the success of inorganic semiconductors, is still in its infancy. This review will discuss recent work on both fundamental principles and applications of doping, focused primarily to doping of evaporated organic layers with molecular dopants. Recently, both p‐ and n‐type molecular dopants have been developed that lead to efficient and stable doping of organic thin films. Due to doping, the conductivity of the doped layers increases several orders of magnitude and allows for quasi‐Ohmic contacts between organic layers and metal electrodes. Besides reducing voltage losses, doping thus also gives design freedom in terms of transport layer thickness and electrode choice. The use of doping in applications like OLEDs and organic solar cells is highlighted in this review. Overall, controlled molecular doping can be considered as key enabling technology for many different organic device types that can lead to significant improvements in efficiencies and lifetimes.

Microscopic density matrix model for optical gain of terahertz quantum cascade lasers: Many-body, nonparabolicity, and resonant tunneling effects

Intersubband semiconductor-Bloch equations are investigated by incorporating many-body Coulomb interaction, nonparabolicity, and coherence of resonant tunneling transport in a quantitative way based on the density matrix theory. The calculations demonstrate the importance of these parameters on optical properties, especially the optical gain spectrum, of terahertz (THz) quantum cascade lasers (QCLs). The results show that the lasing frequency at gain peak calculated by the proposed microscopic density matrix model is closer to the experimentally measured result, compared with that calculated by the existing macroscopic density matrix model. Specifically, both the many-body interaction and nonparabolicity effects red-shift the gain spectrum and reduce the gain peak. In addition, as the injection-coupling strength increases, the gain peak value is enhanced and the spectrum is slightly broadened, while an increase of the extraction-coupling strength reduces the gain peak value and broadens the gain spectrum. The dependence of optical gain of THz QCLs on device parameters such as external electrical bias, dephasing rate, doping density, and temperature is also systematically studied in details. This model provides a more comprehensive picture of the optical properties of THz QCLs from a microscopic point of view and potentially enables a more accurate and faster prediction and calculation of the device performance, e.g., gain spectra, current-voltage characteristics, optical output powers, and nonlinear amplitude-phase coupling.

Effect of low constant current stress treatment on the performance of the Cu/ZrO2/Pt resistive switching device

In this paper, the effect of a low constant current stress (CCS) treatment on the performance of a Cu/ZrO2/Pt resistive switching device is investigated. The conductance of the device increases about two orders of magnitude after CCS treatment, indicating that some defects are introduced into the ZrO2 matrix and the CCS treatment can be regarded as an electrical doping process. Benefiting from these introduced defects, better resistive switching performance is obtained after CCS treatment, including low forming voltage, low reset current, uniform resistive switching and good endurance characteristics.

Interfacial doping for efficient charge injection in organic semiconductors

AbstractInterfacial doping in organic semiconductors (OSs) is an important technique to achieve efficient organic electronic devices. In this paper, we discuss how the charge injection into an OS can be enhanced by the insertion of a thin interfacial layer or an electrically doped OS layer between an electrode and an undoped OS. We present that the vacuum level shift and Fermi level modification by the electrical doping is the origin of the efficient charge injection through a metal–organic junction and an organic–organic junction. Application to organic electronics such as organic light‐emitting diodes (OLEDs) and organic photovoltaics (OPVs) is briefly summarized.

A study on the performance of metal-oxide-semiconductor-field-effect-transistors with asymmetric junction doping structure

Diode currents of MOSFET were studied and characterized in detail for the ion implanted pn junction of short channel MOSFETs with shallow drain junction doping structure. The diode current in MOSFET junctions was analyzed on the point of view of the gate-induced-drain leakage (GIDL) current. We could found the GIDL current is generated by the band-to-band tunneling (BTBT) of electrons through the reverse biased channel-to-drain junction and had good agreement with BTBT equation. The effect of the lateral electric field on the GIDL current according to the body bias voltage is characterized and discussed. We measured the electrical doping profiling of MOSFETs with a short gate length, ultra thin oxide thickness and asymmetric doped drain structure and checked the profile had good agreement with simulation result. An accurate effective mobility of an asymmetric source–drain junction transistor was successfully extracted by using the split C–V technique.

Publisher's Note: Persistent electrical doping of Bi2Sr2CaCu2O8+xmesa structures [Phys. Rev. B85, 144519 (2012)

Publisher's Note: Persistent electrical doping of Bi<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>2</mml:mn></mml:msub></mml:math>Sr<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>2</mml:mn></mml:msub></mml:math>CaCu<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>2</mml:mn></mml:msub></mml:math>O<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mrow><mml:mn>8</mml:mn><mml:mo>+</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>mesa structures [Phys. Rev. B<b>85</b>, 144519 (2012)

Understanding metal doping for organic electron transport layers

This work concerns the physical mechanisms of metal n-doping in charge transport layers for optoelectronic devices, for which the doping level is constrained by transparency requirements so as to avoid parasitic absorption. Comparing various metal dopants, we claim that enhanced conductivity at low doping is initiated by the electrical doping effect, namely, metal-semiconductor charge donation. Electrical measurements show that doping effects at low concentration strongly depend on the work function of the introduced metal, and not every metal works as an efficient dopant. Practical applicability is demonstrated by introducing doped transport layers in prototypical bilayer solar cells in conventional and inverted architectures.

Intermolecular Hybridization Governs Molecular Electrical Doping

Current models for molecular electrical doping of organic semiconductors are found to be at odds with other well-established concepts in that field, like polaron formation. Addressing these inconsistencies for prototypical systems, we present experimental and theoretical evidence for intermolecular hybridization of organic semiconductor and dopant frontier molecular orbitals. Common doping-related observations are attributed to this phenomenon, and controlling the degree of hybridization emerges as a strategy for overcoming the present limitations in the yield of doping-induced charge carriers.

Multilayer Graphene-Based Carbon Interconnect

ABSTRACT3D stacked (or uncorrelated) multilayer graphene (s-MLG) is investigated as a potential material platform for carbon-based on-chip interconnects. S-MLG samples are prepared by repeatedly transferring and stacking the large-area CVD-grown graphene monolayers, followed by wire patterning and oxygen plasma etching of graphene. We observed superior wire conduction of s-MLG over that of monolayer graphene or ABAB-stacked multilayer graphene. Further reduction of s-MLG resistivity is anticipated with increasing number of stacked layers. Electrical stress-induced doping technique is used to engineer the Dirac point, as well as to reduce graphene-to-metal contact resistance, improving the overall performance metrics of the s-MLG system. Breakdown experiments show that the current-carrying capacity of s-MLG is significantly enhanced as compared with that of monolayer graphene.

Nonlinear Spectroscopic Markers of Structural Change during Charge Accumulation in Organic Field-Effect Transistors

Vibrational sum frequency generation (VSFG) is used to characterize the buried polymer–dielectric interface in poly(triarylamine) (PTAA) organic field-effect transistors (oFETs) over a spectral range of more than 1100 cm–1. The FTIR and Raman spectra are presented for the neutral and chemically oxidized thin films of this polymer as a starting point for identifying the potential vibrational changes that are induced by doping. In the VSFG spectra collected as a function of applied gate bias, we find evidence for interfacial electric fields, polaronic absorbances, and a strong VSFG-active band that is a spectral signature of the interfacial molecules that are perturbed by electrical doping. In most cases, the unipolar electrical behavior of PTAA oFETs is directly correlated with intensity changes in this structural perturbation band regardless of the dielectric (SiO2) surface functionalization. However, in a few selected examples, the VSFG measurements demonstrate that accumulation of either holes or electrons can be achieved with this organic semiconductor, even in the absence of measurable source-drain currents. These results highlight the potential for PTAA to achieve ambipolar device operation and the power of nonlinear spectroscopy to provide the feedback needed to optimize this performance when electrical measurements cannot.

Local electrical stress-induced doping and formation of monolayer graphene P-N junction

We demonstrated doping in monolayer graphene via local electrical stressing. The doping, confirmed by the resistance-voltage transfer characteristics of the graphene system, is observed to continuously tunable from N-type to P-type as the electrical stressing level (voltage) increases. Two major physical mechanisms are proposed to interpret the observed phenomena: modifications of surface chemistry for N-type doping (at low-level stressing) and thermally-activated charge transfer from graphene to SiO2 substrate for P-type doping (at high-level stressing). The formation of P-N junction on two-dimensional graphene monolayer is demonstrated with complementary doping based on locally applied electrical stressing.

Mechanism of Cs2CO3 as an n-type dopant in organic electron-transport film

The electronic structures of cesium carbonate (Cs2CO3) doped 4,7-diphenyl-1,10-phenanthroline (BPhen) films with various doping concentration are characterized by in situ ultraviolet and x-ray photoelectron spectroscopies, in an attempt to understand the mechanism of electron-transport enhancement in Cs2CO3-doped organic electron-transport layer for organic optoelectronic devices. The n-type electrical doping effect is evidenced by the Fermi level shift in the Cs2CO3-doped BPhen films toward unoccupied molecular states with increasing doping concentration, leading to increase in electron concentration in the electron-transport layer and reduction in electron injection barrier height. These findings originate from energetically favorable electron transfer from Cs2CO3 to BPhen.

High‐Performance, Phosphorescent, Top‐Emitting Organic Light‐Emitting Diodes with p–i–n Homojunctions

AbstractHigh‐performance, green, orange, and red top‐emitting organic light‐emitting diodes (TOLEDs) with p–i–n homojunction are demonstrated. An excellent ambipolar host, 2,5‐bis(2‐(9H‐carbazol‐9‐yl)phenyl)‐1,3,4‐oxadiazole (o‐CzOXD), which has good thermal and morphological stabilities, a high triplet energy level, and equally high electron and hole mobilities, is chosen as the organic host material for the homojunction devices. By electrical doping, the carrier injection and transporting characteristics are greatly improved. The optical structure is optimized in view of light emission of different colors to enhance the color purity and improve the view characte­ristics. As a result, high efficiency p–i–n homojunction TOLEDs with saturated intrinsic emission of the emitting materials and angular independence of the emission are realized. The performances of these p–i–n homojunction TOLEDs are even higher than the multi‐layer heterojunction bottom‐emitting devices using the same emitting layers.

Electrical conduction behavior of proton conductor BaCe 1-xSm xO 3-δ in the intermediate temperature range

Determining the relationship between electrical conductivity and doping level in high temperature proton conductors is of great significance to accelerate their process of practicality and develop new applications. In this work, BaCe 1-xSm xO 3-δ ( x = 0.01–0.50) was synthesized via citric-nitrate method and their electrical conduction behavior in 5% H 2/Ar in the intermediate temperature range was investigated. The solubility of Sm in BaCeO 3 was between 0.30 and 0.40. Both the bulk and the grain boundary conductivity increased with Sm content up to x = 0.20 and then decreased. The infrared spectra results indicated that the degree of hydrogen bonding decreased with Sm content ( x = 0.20–0.30), which should be responsible for the descending bulk conductivity of samples with x = 0.25 and 0.30. BaCe 0.80Sm 0.20O 3-δ, with electrical conductivity of 0.017 S cm −1 at 600 °C, was a promising electrolyte for intermediate temperature solid oxide fuel cells.

Electrical transport properties of the Si-doped cubic boron nitride thin films prepared by in situ cosputtering

Si-doped cubic boron nitride (c-BN) films with various Si concentrations were achieved by in situ cosputtering during ion beam assisted deposition. Effects of the Si concentration and rapid thermal annealing (RTA) conditions on the electrical transport properties of Si-doped c-BN thin films were investigated systematically. The results suggest that the optimum RTA condition is at the temperature of 1000 °C for 3 min. The resistance of Si-doped c-BN films gradually decreases as the Si concentration increases, indicating an electrical doping effect of the Si impurity. The temperature dependent electrical conductivity of the Si-doped c-BN films suggests that different conduction mechanisms are dominant over the different temperature ranges. Based on the Davis–Mott model, we propose that the extended-state conduction, band tail-state conduction and short-range hopping conduction are responsible for the respective temperature ranges. In addition, the reduction in activation energy of Si impurities is observed as the Si concentration increases.

Electrical Extractions of One Dimensional Doping Profile and Effective Mobility for Metal–Oxide–Semiconductor Field-Effect Transistors

In this study, an attempt is made to provide a framework to assess and improve metal–oxide–semiconductor field-effect transistor (MOSFET) reliability from the early stage of the design to the completion of the product. A small gate area has very small capacitances that are difficult to measure, making capacitance–voltage (C–V) based techniques difficult or impossible. In view of these experimental difficulties, we tried electrical doping profiling measurement for MOSFET with short gate length, ultra thin oxide thickness and asymmetric source/drain structure and checked the agreement with simulation result. We could get the effective mobility by simple drain current versus drain bias voltage measurement. The calculated effective mobility was smaller than expected value and we explained some reasons. An accurate effective mobility for asymmetric source–drain junction transistor was successfully extracted by using the split C–V technique, with the capacitance measured between the gate and source–drain and between the gate and the substrate.

Atmospheric Oxygen Binding and Hole Doping in Deformed Graphene on a SiO2 Substrate

Using micro-Raman spectroscopy and scanning tunneling microscopy, we study the relationship between structural distortion and electrical hole doping of graphene on a silicon dioxide substrate. The observed upshift of the Raman G band represents charge doping and not compressive strain. Two independent factors control the doping: (1) the degree of graphene coupling to the substrate, and (2) exposure to oxygen and moisture. Thermal annealing induces a pronounced structural distortion due to close coupling to SiO2 and activates the ability of diatomic oxygen to accept charge from graphene. Gas flow experiments show that dry oxygen reversibly dopes graphene; doping becomes stronger and more irreversible in the presence of moisture and over long periods of time. We propose that oxygen molecular anions are stabilized by water solvation and electrostatic binding to the silicon dioxide surface.

Tailoring the properties of semiconductor nanowires using ion beams

AbstractThis review demonstrates that ion irradiation is a very useful tool in order to tailor the properties of semiconductor nanowires. Besides optical and electrical doping provided by adequate ion species and ion energies, one can use ion beams also for the controlled shaping of the morphology of nanostructures. Here, one utilizes the commonly as ‘negative’ described characteristics of ion implantation: defect formation and sputtering. We show that ion beams can be even used for an alignment of the nanowires. Furthermore, we report here on several successful experiments in order to modify the electrical and optical properties in a controlled manner of ZnO semiconductor nanowires by the use of transition metals, rare earth elements and hydrogen ions. magnified image Schematic illustration of ion beam doping of a single contacted nanowire.