Ambipolar Carriers Research Articles

Two-dimensional ambipolar carriers of giant density at the diamond/cubic-BN(111) interfaces: toward complementary logic and quantum applications.

The extremely difficult ambipolar doping activation greatly hinders the outstanding performance of diamond for electronic devices. The main concern has been devoted to surface conduction by two-dimensional (2D) carriers. 2D hole gas (2DHG) in the diamond is induced by surface transfer doping dominated by the adsorbate's status and faces stability issues. Meanwhile, a feasible way to generate the other essential ambipolar carrier-2D electron gas (2DEG) is still lacking. We propose that the well-lattice-matched diamond/cBN(111) interfaces can spontaneously induce 2D ambipolar carriers with a giant density of 4.17 × 1014 cm-2, an order higher than other competitors. 2DEG and 2DHG can be separately achieved near the hetero-interfaces consisting of C-N and C-B bonds, respectively. Interestingly, the robust 2D charges are derived from a novel bulk-induced polarization-discontinuity at the interfaces, which can be attributed to an unexpected non-zero formal polarization of centrosymmetric cBN along the [111] direction. The existence of 2D ambipolar carriers at the diamond/cBN(111) interfaces has resolved the missing n-type conduction in diamond, thus opening up possibilities for complementary logic applications. Additionally, the high density of quantum-confined 2D ambipolar carriers provides an excellent platform for strongly correlated systems, which could lead to novel quantum information processing applications.

Stabilizing p-type conductivity in CuYO2 by co-doping: A first-principles study

This work figures out the importance of p-type conductive materials through first-principle calculations. It is shown that the co-doping mechanism can enhance the p-type conductivity of CuYO2. Substituting Boron at the Yttrium (BY) site, together with Oxygen at interstitial positions (Oi), introduces shallow acceptor levels just above the valence band maximum (VBM). From the thermodynamical calculations, the stability of defects was also confirmed in O-rich synthesis conditions. A detailed analysis of the electronic band structure shows that the VBM and CBM can be tuned by introducing suitable impurities. Likewise, the substitutional impurities Ge and In at Y sites (GaY and InY) are shown to have high electronic conductivity. We have demonstrated the possibility of ambipolar charge carriers in CuYO2 by co-doping.

Metal modulated effects on the optoelectronic and charge transport properties of some graphene nanoribbons

In this study, we have investigated the effects of adding a metal on the nonlinear, mechanical and charge transport properties of some small organic graphene nanoribons (GNRs) molecule. A theoretical investigation is done using the ab initio (rhf) and DFT (B3LYP and BPBE) methods with the 6-31+g(d,p) basis set integrated in Gaussian 09. Results reveals that the metalated GNRs have higher nonlinear optical activity than the virgin counterpart. Interestingly these GNRs metal derivatives also have remarkable nonlinear optical effect in comparison to the classical push–pull organic crystal; para-nitroaniline and urea. Other remarkable results show that as compared to the starting GNRs the metalated counterparts are excellent ambipolar charge transport carriers better than the prototypic electron transport Alq3 (tris(8-hydroxyquinolinato)aluminum(III), and hole tranport TPD (N,N0-diphenyl-N,N0-bis(3-methlphenyl)-(1,10-biphenyl)-4,40-diamine. However, the metal derivative have diminished mechanical stability than the starting molecule. Metalated GNRs could therefore serve as potential candidates for the fabrication of optoelectronic and photonic devices for second and third-order NLO application.

Thickness-dependent carrier mobility of ambipolar MoTe2: Interplay between interface trap and Coulomb scattering

The correlation between the channel thickness and the carrier mobility is investigated by conducting static and low frequency (LF) noise characterization for ambipolar carriers in multilayer MoTe2 transistors. For channel thicknesses in the range of 5–15 nm, both the low-field carrier mobility and the Coulomb-scattering-limited carrier mobility (μC) are maximal at a thickness of ∼10 nm. For LF noise, the interplay of interface trap density (NST), which was minimal at ∼10 nm, and the interfacial Coulomb scattering parameter (αSC), which decreased up to 10 nm and saturated above 10 nm, explained the mobility (μC) peaked near 10 nm by the carrier fluctuation and charge distribution.

Gate‐Controlled BP–WSe2 Heterojunction Diode for Logic Rectifiers and Logic Optoelectronics

p-n junctions play an important role in modern semiconductor electronics and optoelectronics, and field-effect transistors are often used for logic circuits. Here, gate-controlled logic rectifiers and logic optoelectronic devices based on stacked black phosphorus (BP) and tungsten diselenide (WSe2 ) heterojunctions are reported. The gate-tunable ambipolar charge carriers in BP and WSe2 enable a flexible, dynamic, and wide modulation on the heterojunctions as isotype (p-p and n-n) and anisotype (p-n) diodes, which exhibit disparate rectifying and photovoltaic properties. Based on such characteristics, it is demonstrated that BP-WSe2 heterojunction diodes can be developed for high-performance logic rectifiers and logic optoelectronic devices. Logic optoelectronic devices can convert a light signal to an electric one by applied gate voltages. This work should be helpful to expand the applications of 2D crystals.

Controlling the screening process of a nanoscaled space charge region by minority carriers.

The miniaturization of future electronic devices is intimately connected to the ability to control electric fields on the atomic scale. In a nanoscopic system defined by a limited number of charges, the combined dynamics of bound and free charges become important. Here we present a model system based on the electrostatic interaction between a metallic tip of a scanning tunnelling microscope and a GaAs(110) semiconductor surface. The system is driven out of equilibrium by optical excitation, which provides ambipolar free charge carriers, and by an optically induced unipolar tunnel current. This combination enables the active control of the density and spatial distribution of free and bound charge in the space-charge region, that is, modifying the screening processes. Temporal fluctuations of single dopants are modified, meaning we are able to control the noise of the system. It is found that free charge carriers suppress the noise level in field-controlled, nanoscopic systems.

Imaging ambipolar two-dimensional carriers induced by the spontaneous electric polarization of a polar semiconductor BiTeI

Two-dimensional (2D) mobile carriers are a wellspring of quantum phenomena. Among various 2D-carrier systems, such as field effect transistors and heterostructures, polar materials hold a unique potential; the spontaneous electric polarization in the bulk could generate positive and negative 2D carriers at the surface. Although several experiments have shown ambipolar carriers at the surface of a polar semiconductor BiTeI, their origin is yet to be specified. Here we provide compelling experimental evidences that the ambipolar 2D carriers at the surface of BiTeI are induced by the spontaneous electric polarization. By imaging electron standing waves with spectroscopic imaging scanning tunneling microscopy, we find that positive or negative carriers with Rashba-type spin splitting emerge at the surface corresponding to the polar directions in the bulk. The electron densities at the surface are constant independently of those in the bulk, corroborating that the 2D carriers are induced by the spontaneous electric polarization. We also successfully image that lateral $p\ensuremath{-}n$ junctions are formed along the boundaries of submicron-scale domains with opposite polar directions. Our study presents a means to endow nonvolatile, spin-polarized, and ambipolar 2D carriers as well as, without elaborate fabrication, lateral $p\ensuremath{-}n$ junctions of those carriers at atomically sharp interfaces.

Improved color stability of white organic light-emitting diodes without interlayer between red, orange and blue emission layers

We demonstrated color stability improved white phosphorescent organic light-emitting diodes (WOLEDs) based on red, orange and blue emission layers. Iridium(III) Bis(3,5-diflouro)-2-(2-pyridyl)phenyl-(2-carboxypyridyl) was doped into red emission layer (R-EML) and orange emission layer (O-EML) to lower the electrons injection barrier and facilitate the ambipolar charge carriers balance. Consequently, the recombination region was extended to the R-EML and O-EML, leading to the excellently stable spectra and the reduction of triplet–triplet annihilation. Then the resulting device with a negligible Commission International de L'Eclairage coordinates shift of (0.003, 0.007) within a wide luminance range as well as a high color rendering index of 90 was gained, which was comparable to the profit caused by the conventional method of introducing the interlayer. And the emission mechanism of the WOLEDs was also discussed.

High Mobility Pentacene/C60-Based Ambipolar OTFTs by Thickness Optimization of Bottom Pentacene Layer

In this paper, we investigated the dependence of ambipolar carriers transport on the thickness of bottom pentacene layers in a series of pentacene/C 60 heterojection based ambipolar organic thin-film transistors (OTFTs). It was found that hole transport increased monotonically, while the electron transport increased at first and then decreased, with the increasing bottom pentacene layer thickness from 1.5 to 9 nm. The morphologies evolution of both pentacene and C 60 layers, investigated by atomic force microscopy measurement, demonstrated the effect mechanism of the bottom pentacene layer thickness on the device performance of ambipolar OTFTs. The almost saturated hole mobility of 0.39 cm 2 /Vs and the highest electron mobility of 6.3 cm 2 /Vs were achieved in an optimized ambipolar OTFT with a 4.5-nm-thick pentacene layer.

Effect of tunneling layers on the performances of floating-gate based organic thin-film transistor nonvolatile memories

Two types of floating-gate based organic thin-film transistor nonvolatile memories (FG-OTFT-NVMs) were demonstrated, with poly(methyl methacrylate co glycidyl methacrylate) (P(MMA-GMA)) and tetratetracontane (TTC) as the tunneling layer, respectively. Their device performances were measured and compared. In the memory with a P(MMA-GMA) tunneling layer, typical unipolar hole transport was obtained with a relatively small mobility of 0.16 cm2/V s. The unidirectional shift of turn-on voltage (Von) due to only holes trapped/detrapped in/from the floating gate resulted in a small memory window of 12.5 V at programming/erasing voltages (VP/VE) of ±100 V and a nonzero reading voltage. Benefited from the well-ordered molecule orientation and the trap-free surface of TTC layer, a considerably high hole mobility of 1.7 cm2/V s and a visible feature of electrons accumulated in channel and trapped in floating-gate were achieved in the memory with a TTC tunneling layer. High hole mobility resulted in a high on current and a large memory on/off ratio of 600 at the VP/VE of ±100 V. Both holes and electrons were injected into floating-gate and overwritten each other, which resulted in a bidirectional Von shift. As a result, an enlarged memory window of 28.6 V at the VP/VE of ±100 V and a zero reading voltage were achieved. Based on our results, a strategy is proposed to optimize FG-OTFT-NVMs by choosing a right tunneling layer to improve the majority carrier mobility and realize ambipolar carriers injecting and trapping in the floating-gate.

Modeling an antenna-coupled graphene field-effect terahertz detector

The effect of ambipolar carriers on terahertz self-mixing is theoretically investigated in an antenna-coupled graphene field-effect terahertz (GFET) detector by taking into account the spatial distributions of the charge carriers and the terahertz field. The model predicts that the charge and field distributions can be tuned by the gate voltage so that they match up with each other and enhance the photocurrent. Such a cooperative self-mixing does not occur in unipolar FET detectors. A GFET detector with a moderate carrier mobility could offer current responsivity of a few A/W and noise-equivalent power below 50 pW/Hz at room temperature.

Electroluminescence at pure blue region from a new anthracene-contained polymer

We report an electroluminescence at pure blue region from a new anthracene-contained polymer. Ter(9,9-diarylfluorene)s of this polymeric unit is capable of ambipolar carriers transportation. The polymer also contains both electron-withdrawing oxadiazole unit and electron-rich carbazole unit. Most of all, the anthracene part of polymer is designed for keeping emission at pure blue region. The light-emitting diode fabricated with the emitter results to a luminous efficiency of 6.91cdA−1. The maximum luminescence is 13,159.8cdm−2. Moreover, the main peak of electroluminescence spectrum is at 446.2nm, which is in the pure blue region.

ESR studies of ambipolar charge carriers in metal–insulator–semiconductor diodes of regioregular poly(3-hexylthiophene)/PCBM composites

We have applied field-induced electron spin resonance (FI-ESR) method to organic metal–insulator–semiconductor (MIS) diodes using regioregular poly(3-hexylthiophene) (RR-P3HT) and [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) composites as the active layer, Al 2O 3 as the gate insulator and gold (Au) or aluminum (Al) as the top electrode. In the case of Al electrode, clear FI-ESR signals are observed with g ∼ 2.002 and 1.999 for negative and positive gate voltage ( V G), respectively, demonstrating that these signals with former and the latter g-values correspond to field-induced positive polarons of RR-P3HT and PCBM radical anions, respectively. Both RR-P3HT and PCBM signals monotonically increase as | V G| increases. The threshold voltages ( V th) for field-induced ambipolar charge carriers coincide well with those of capacitance measurements with hysteresis behaviors. On the other hand, in the case of Au electrode, only positive polarons signals are clearly observed and PCBM signals are nearly undetected. These different behaviors of FI-ESR signals between Al and Au top electrodes are discussed in relation to the difference of work functions.

Ambipolar Carrier Injection Characteristics of Erbium-Silicided n-Type Schottky Barrier Metal–Oxide–Semiconductor Field-Effect Transistors

Erbium-silicided 50-nm-gate-length n-type Schottky barrier metal–oxide–semiconductor field-effect transistors (SB-MOSFETs) with a 5 nm gate oxide thickness are manufactured. Their on/off-current ratio is higher than 105 with low leakage current less than 10 nA/um. The abnormal increase of drain current with a negative gate voltage is explained by the hole carrier injection from drain into the channel. In SB-MOSFETs, ambipolar carriers, i.e., electrons and holes, can be injected into the channel depending on gate voltage polarity.

Experimental and Theoretical Investigations of Free Exciton Transport in Si

An all optical Time-of-Flight method is used to study transport properties of laser pulse generated ambipolar carrier systems. Exciting carrier pulses at the front of Si wafers and probing the carrier density by a thin layer at the back yields a high temporal and spatial resolution.The evaluation with a simple hydrodynamical and a kinetic model yields the mobility and the distribution function as a function of space and time at different temperatures.The most important feature of surface generated ambipolar carriers is the motion in a nonequilibrium state over relatively large distances, though no electric field is applied. The moving carriers are identified as free excitons showing the classical mobility μ(T) ∼ T–1.5 by scattering with acoustic phonons via the deformation potential.