Carrier Injection Efficiency Research Articles

Impact of Metal Contacts on the Performance of Multilayer HfS2 Field-Effect Transistors.

HfS2 is one of the emerging transition metal dichalcogenides and is very promising for low-power nanoelectronics and high-sensitivity optoelectronic device applications. We studied the band structures of 1T-HfS2 with different thicknesses by first principles simulation, and the impact of different metal contacts to the HfS2 device performance has been experimentally studied. Back-gate and top-gate HfS2 field-effect transistors (FETs) were fabricated, and better electrical characteristics have been achieved with the FETs with the Ti/Au contact as compared with the Pt-contacted FETs. Thin layers of Pt and Ti/Au films were deposited on HfS2 flakes to investigate the metal/HfS2 interface by using scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. A smoother Ti/Au film was formed on HfS2, resulting in higher carrier injection and transport efficiency. The phonon behavior being dominated by the interface chemical bonding at the Ti/Au contact region has been confirmed with the more sensitive A1g phonon mode from the bilayer HfS2.

Hybrid indium phosphide-on-silicon nanolaser diode

By exploiting one-dimensional photonic crystal nanocavities, an ultra-compact indium phosphide-on-silicon laser diode with low current threshold, high wall-plug efficiency and high integrability is demonstrated. The most-awaited convergence of microelectronics and photonics promises to bring about a revolution for on-chip data communications and processing1. Among all the optoelectronic devices to be developed, power-efficient nanolaser diodes able to be integrated densely with silicon photonics and electronics are essential to convert electrical data into the optical domain. Here, we report a demonstration of ultracompact laser diodes based on one-dimensional (1D) photonic crystal (PhC) nanocavities2,3,4 made in InP nanoribs heterogeneously integrated on a silicon-waveguide circuitry. The specific nanorib design enables an efficient electrical injection of carriers in the nanocavity without spoiling its optical properties. Room-temperature continuous-wave (CW) single-mode operation is obtained with a low current threshold of 100 µA. Laser emission at 1.56 µm in the silicon waveguides is obtained with wall-plug efficiencies greater than 10%. This result opens up exciting avenues for constructing optical networks at the submillimetre scale for on-chip interconnects and signal processing.

Tuning the Work Function of Printed Polymer Electrodes by Introducing a Fluorinated Polymer To Enhance the Operational Stability in Bottom-Contact Organic Field-Effect Transistors.

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a promising electrode material for organic electronic devices due to its high conductivity, good mechanical flexibility, and feasibility of easy patterning with various printing methods. The work function of PEDOT:PSS needs to be increased for efficient hole injection, and the addition of a fluorine-containing material has been reported to increase the work function of PEDOT:PSS. However, it remains a challenge to print PEDOT:PSS electrodes while simultaneously tuning their work functions. Here, we report work function tunable PEDOT:PSS/Nafion source/drain electrodes formed by electrohydrodynamic printing technique with PEDOT:PSS/Nafion mixture solutions for highly stable bottom-contact organic field-effect transistors (OFETs). The surface properties and work function of the printed electrode can be controlled by varying the Nafion ratio, due to the vertical phase separation of the PEDOT:PSS/Nafion. The PEDOT:PSS/Nafion electrodes exhibit a low hole injection barrier, which leads to efficient charge carrier injection from the electrode to the semiconductor. As a result, pentacene-based OFETs with PEDOT:PSS/Nafion electrodes show increased charge carrier mobilities of 0.39 cm2/(V·s) compared to those of devices with neat PEDOT:PSS electrodes (0.021 cm2/(V·s)). Moreover, the gate-bias stress stability of the OFETs is remarkably improved by employing PEDOT:PSS/Nafion electrodes, as demonstrated by a reduction of the threshold voltage shift from -1.84 V to -0.28 V.

Controlling color emission of InGaN/AlGaN nanowire light-emitting diodes grown by molecular beam epitaxy

The authors report on the achievement of full-color nanowire light-emitting diodes (LEDs), with the incorporation of InGaN/AlGaN nanowire heterostructures grown directly on the Si (111) substrates by molecular beam epitaxy. Multiple color emission across nearly the entire visible wavelength range can be realized by varying the In composition in the InGaN quantum dot active region. Moreover, multiple AlGaN shell layers are spontaneously formed during the growth of InGaN/AlGaN quantum dots, leading to the drastically reduced nonradiative surface recombination, and enhanced carrier injection efficiency. Such core–shell nanowire structures exhibit significantly increased carrier lifetime and massively enhanced photoluminescence intensity compared to conventional InGaN/GaN nanowire LEDs. A high color rendering index of ∼98 was recorded for white-light emitted from such phosphor-free core–shell nanowire LEDs.

Surface modification of printed silver electrodes for efficient carrier injection in organic thin-film transistors

The modification of printed silver electrode surfaces for use as the bottom-contact electrodes of organic thin-film transistors (OTFTs) is reported. Printed silver electrodes fabricated using the surface photoreactive nanometal printing (SuPR-NaP) technique are inevitably covered with an inert surface layer of alkylamines that is originally used for encapsulation of the silver nanoparticles (AgNPs). However, it may act as a built-in protective layer against carrier injections. We demonstrate that a simple vapor exposure method is sufficient for converting the protective layer into a layer that assists carrier injection. As modifiers, we used various types of fluorinated benzenethiols that exhibit a stronger coordination with the silver surfaces than the alkylamimes. We detected the chemical conversion from alkylamine encapsulation to thiol coordination by surface enhanced Raman spectroscopy (SERS) and evaluated the improvement in the carrier injection using a transfer length method (TLM) for the OTFTs. Among the modifiers, the pentafluorobenzenethiol (PFBT) treatment significantly improves the device performance and stability of the OTFTs.

50-Fold EQE Improvement up to 6.27% of Solution-Processed All-Inorganic Perovskite CsPbBr3 QLEDs via Surface Ligand Density Control.

Solution-processed CsPbBr3 quantum-dot light-emitting diodes with a 50-fold external quantum efficiency improvement (up to 6.27%) are achieved through balancing surface passivation and carrier injection via ligand density control (treating with hexane/ethyl acetate mixed solvent), which induces the coexistence of high levels of ink stability, photoluminescence quantum yields, thin-film uniformity, and carrier-injection efficiency.

Organic Spin-Valves and Beyond: Spin Injection and Transport in Organic Semiconductors and the Effect of Interfacial Engineering.

Since the first observation of the spin-valve effect through organic semiconductors, efforts to realize novel spintronic technologies based on organic semiconductors have been rapidly growing. However, a complete understanding of spin-polarized carrier injection and transport in organic semiconductors is still lacking and under debate. For example, there is still no clear understanding of major spin-flip mechanisms in organic semiconductors and the role of hybrid metal-organic interfaces in spin injection. Recent findings suggest that organic single crystals can provide spin-transport media with much less structural disorder relative to organic thin films, thus reducing momentum scattering. Additionally, modification of the band energetics, morphology, and even spin magnetic moment at the metal-organic interface by interface engineering can greatly impact the efficiency of spin-polarized carrier injection. Here, progress on efficient spin-polarized carrier injection into organic semiconductors from ferromagnetic metals by using various interface engineering techniques is presented, such as inserting a metallic interlayer, a molecular self-assembled monolayer (SAM), and a ballistic carrier emitter. In addition, efforts to realize long spin transport in single-crystalline organic semiconductors are discussed. The focus here is on understanding and maximizing spin-polarized carrier injection and transport in organic semiconductors and insight is provided for the realization of emerging organic spintronics technologies.

Temperature-dependent efficiency droop analysis of InGaN MQW light-emitting diode with modified ABC model

In this work, the origin of the efficiency droop at high injection current in an InGaN multiple-quantum-well light-emitting diode is suggested to be saturation of the radiative efficiency and insufficient carrier injection efficiency. A simple internal quantum efficiency (IQE) estimation method is developed by modifying the conventional ABC model to include carrier leakage mechanisms such as thermionic emission and carrier overflow, to account for the carrier injection efficiency at high current densities. The results show that thermionic emission and carrier overflow play a dominant role in the carrier leakage mechanism at high injection current. The data obtained from this analysis enable inference of the temperature dependence of the radiative/nonradiative coefficients and carrier leakage mechanisms. In addition, the model predicts the temperature dependence and rationale behind the degradation of the IQE at higher injection currents. Furthermore, the modeled output results show good fit with experimental data.

Hole transport in c-plane InGaN-based green laser diodes

Hole transport in c-plane InGaN-based green laser diodes (LDs) has been investigated by both simulations and experiments. It is found that holes can overflow from the green double quantum wells (DQWs) at high current density, which reduces carrier injection efficiency of c-plane InGaN-based green LDs. A heavily silicon-doped layer right below the green DQWs can effectively suppress hole overflow from the green DQWs.

Charge Injection at the Heterointerface in Perovskite CH3NH3PbI3 Solar Cells Studied by Simultaneous Microscopic Photoluminescence and Photocurrent Imaging Spectroscopy.

Charge carrier dynamics in perovskite CH3NH3PbI3 solar cells were studied by means of microscopic photoluminescence (PL) and photocurrent (PC) imaging spectroscopy. The PL intensity, PL lifetime, and PC intensity varied spatially on the order of several tens of micrometers. Simultaneous PL and PC image measurements revealed a positive correlation between the PL intensity and PL lifetime, and a negative correlation between PL and PC intensities. These correlations were due to the competition between photocarrier injection from the CH3NH3PbI3 layer into the charge transport layer and photocarrier recombination within the CH3NH3PbI3 layer. Furthermore, we found that the decrease in the carrier injection efficiency under prolonged light illumination leads to a reduction in PC, resulting in light-induced degradation of solar cell devices. Our findings provide important insights for understanding carrier injection at the interface and light-induced degradation in perovskite solar cells.

C70/Pentacene Organic Heterojunction as Charge Generator to Realize Highly Efficient Charge Carrier Injection in Organic Light‐Emitting Diodes: Performance and Mechanism Analysis

Highly efficient charge carrier injection from electrodes is basically required for high‐performance organic light‐emitting diodes (OLEDs). However, the mismatch of metal work function and energy level of charge carrier transport layers inevitably introduces the injection barrier and also greatly restricts the choice of the electrode metals. Here, a planar organic heterojunction (OHJ) C70/pentacene is used as charge generator to realize the highly efficient injection of charge carriers in OLEDs, not only achieving the high efficiency but also significantly improving the stability. More importantly, the resulting devices show an electrode work function‐independent efficiency property, greatly broadening the selectivity of electrode metals. Detailed studies are presented on the transport properties of electrons and holes in single‐carrier devices based on C70/pentacene OHJ by current density–voltage (J–V) measurements at various temperatures. It can be seen that Fowler–Nordheim (F–N) tunnel model can be used to well demonstrate the J–V properties, indicating that the charge injection based on OHJ is a tunnel process. The impedance measurements found that the lower lowest unoccupied molecular orbit energy level and higher electron mobility properties are very necessary for the used electron‐transport materials to reduce the space charge region width, thus realizing highly efficient electron injection.

Nanoseed Assisted PVT Growth of Ultrathin 2D Pentacene Molecular Crystal Directly onto SiO2 Substrate

High order of molecular packing and perfect semiconductor/dielectric interface are two key factors to achieve high performance for organic field-effect transistors (OFET). Moreover, the thin crystal offers an improved efficiency of carrier injection for OFETs. To this aim, formation of thin and large single crystal directly on dielectrics is the basis to obtain the ideal crystal OFETs. Herein, we report the controlled growth of ultrathin 2D Pentacene (Pn) crystal via nanoseed assisted physical vapor transport (PVT) method grown directly on SiO2. The size, thickness, and density of Pn crystals are systematically studied. Potentially effective parameters such as initially lowered Pn coverage and decreased supersaturation with the aid of carrier gas flow were optimized to grow large, ultrathin 2D Pn crystalline flakes efficient for the fabrication of crystal OFETs. The typical size and thickness of as-grown Pn crystalline flakes can be controlled to be large and thin enough. Device of ultrathin crystal with ...

Soft-Etching Copper and Silver Electrodes for Significant Device Performance Improvement toward Facile, Cost-Effective, Bottom-Contacted, Organic Field-Effect Transistors.

Poor charge injection and transport at the electrode/semiconductor contacts has been so far a severe performance hurdle for bottom-contact bottom-gate (BCBG) organic field-effect transistors (OFETs). Here, we have developed a simple, economic, and effective method to improve the carrier injection efficiency and obtained high-performance devices with low cost and widely used source/drain (S/D) electrodes (Ag/Cu). Through the simple electrode etching process, the work function of the electrodes is more aligned with the semiconductors, which reduces the energy barrier and facilitates the charge injection. Besides, the formation of the thinned electrode edge with desirable micro/nanostructures not only leads to the enlarged contact side area beneficial for the carrier injection but also is in favor of the molecular self-organization for continuous crystal growth at the contact/active channel interface, which is better for the charge injection and transport. These effects give rise to the great reduction of contact resistance and the amazing improvement of the low-cost bottom-contact configuration OFETs performance.

Investigation of yellow/green II‐VI compound semiconductor laser diode structures on InP substrates

AbstractII‐VI‐compound‐semiconductor laser diode (LD) structures on InP substrates were investigated using device simulations and waveguide analysis. Our simulations showed that electron injection from the n‐cladding into the active layer is hindered by the n‐side barrier layer between the n‐cladding and active layer. Consequently, holes are not injected into the active layer but instead leak to the n‐side layers. It was shown that carrier injection efficiency can be improved by removing the n‐barrier. On the contrary, no large differences were observed between the optical confinement factors of the LD structures with and without the n‐barrier layer. In experiments, we have fabricated the LD structures with and without the n‐barrier layer on InP substrates using molecular beam epitaxy. The turn‐on voltage of the device without the n‐barrier was smaller than that for the device with the n‐barrier by about 5 V. Spontaneous orange emissions around 603 nm were observed for the devices without the n‐barrier. In contrast, no emission was observed for the devices with the n‐barrier. These results prove that the carrier injection into the active layer is enhanced by the removal of the n‐barrier, leading to improved the device performances. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photon correlation study of background suppressed single InGaN nanocolumns

We report on a linearly polarized non-classical light emission from a single InGaN/GaN nanocolumn, which is a site-controlled nanostructure allowing for pixel-like large-scale integration. We have developed a shadow mask technique to reduce background emissions arising from nitride deposits around single nanocolumns and defect states of GaN. The signal to background ratio is improved from to , which allows for detailed polarization-dependent measurement and photon-correlation measurements. Polarization-dependent measurements show that linearly polarized emissions arise from excitonic recombination involving a heavy-hole-like electronic state, corresponding to the bulk exciton of an in-plane polarized A exciton. The second-order coherence function at time zero g(2)(0) is 0.52 at 20 K without background correction. This value is explained in terms of a statistical mixture of a single-photon emission with residual weak background emissions, as well as efficient carrier injection from other localized states.

P-doping and efficient carrier injection induced by graphene oxide for high performing WSe2 rectification devices

In this work, we fabricated multi-layer WSe2 rectifying diodes using graphene oxide (GO) as p-doping material on one side of the contacting electrodes. This GO layer can reduce the contact resistance by forming a tunneling barrier for efficient hole injection, while it increases the contact resistance for the injection of electrons. Results of Raman shift spectra and the opto-electric response of the device confirmed the p-doping effect caused by the GO layer and the formation of a barrier, respectively. We observed a gate tunable rectification effect with a forward/reverse current ratio of 104 and low reverse bias current of 10−10 A. Applying a GO layer in the fabrication of two-dimensional transition metal dichalcogenides based devices is a very useful method in the applications in future nanotechnologies.

Minimization of reverse recovery charge and forward voltage of silicon p–i–n diodes

The forward voltage and reverse recovery charge of silicon p–i–n diodes are controlled on the basis of carrier lifetime or carrier injection efficiency to reduce power loss. In this study, the ideal carrier lifetime for minimizing power loss is calculated by theoretical analysis. The ideal carrier lifetime depends on the current density, forward voltage, and trap energy level in the intrinsic layer. A decrease in the current density leads to an increase in the ideal lifetime. In addition, a shallow trap level has similar current dependence on ideal lifetime. Using the appropriate lifetime is important for designing a p–i–n diode, and a shallow trap level can achieve greater ideal current density dependency.

Modulation of carrier dynamics and threshold characteristics in 1.3-μm quantum dot photonic crystal nanocavity lasers

A self-consistent all-pathway quantum dot (QD) rate equation model, in which all possible relaxation pathways are considered, is used to investigate the influence of quality (Q) factor on the carrier dynamics of 1.3-μm InAs/GaAs QD photonic crystal (PhC) nanolasers. It is found that Q factor not only affects the photon lifetime, but also modulates the carrier occupation in QDs. About three times increases of carrier injection efficiency in QD ground state can be realized in nanocavity with high Q factor. However, it also reveals that over 90% improvement of threshold current happens when Q factor increases from 2000 to 7000, which means it might be not necessary to pursuit for ultrahigh Q factor for the purpose of low threshold current.

Improved electroluminescence with reversed bilayers of thiophene/phenylene co-oligomer derivatives

Organic electroluminescence (EL) devices were fabricated with bilayered thiophene/phenylene co-oligomer (TPCO) derivatives: p-type 5,5′-bis(4-biphenylyl)-2,2′-bithiophene (BP2T) and n-type 5,5′-bis(4′-cyanobiphenyl-4-yl)-2,2′-bithiophene (BP2T-CN). Two types of EL device, Al:Li/BP2T-CN/BP2T/ITO (device A) and Au/BP2T/BP2T-CN/ITO (device B), were investigated using vapor-deposited films of BP2T and BP2T-CN by changing the order of deposition onto the indium–tin-oxide (ITO)/glass substrate. The EL performance was significantly improved for device B in which the two molecules had a lying orientation suitable for efficient carrier injection and transport as well as surface emission. In device A, on the other hand, the standing BP2T orientation resulted in much lower current density and EL intensity.

Improved Electrical Transport and Electroluminescence Properties of p-ZnO/n-Si Heterojunction via Introduction of Patterned SiO2 Intermediate Layer

The authors report on the fabrication and temperature-dependent current–voltage and electroluminescence properties of p-ZnO:As/n-Si heterojunction diodes. The As-doped p-ZnO material was prepared by out-diffusion of arsenic atoms from a sandwiched GaAs interlayer on patterned SiO2/Si substrates. The introduction of hollow-shaped SiO2 patterned layer promotes the efficiency of carrier injection into the active layer and considerably lowers the emission onset of the studied diode. The current–voltage characteristics of the heterojunction were detailedly studied in the temperature range of 21–120 °C to determine the dominant carrier transport mechanisms in different bias regions. The reverse saturation current, barrier height, and ideality factor were estimated from the thermionic emission model and found to be highly temperature dependent. An improved electroluminescence performance of the studied diode featuring an ultralow emission onset and an acceptable operation stability shows the potential of our app...