Carrier Injection Efficiency Research Articles

Ohmic contacts between monolayer WSe2 and two-dimensional titanium carbides

Formation of Ohmic contact is crucial for achieving high device performance in two-dimensional (2D) materials based transistors. By using ab initio electronic structure calculations and quantum transport simulations, we reveal the great potential of 2D titanium carbides as the electrode materials for monolayer (ML) WSe2 device. In the vertical direction, both tunnel barrier and Schottky barrier are absent in all the three hybrid heterostructures of ML WSe2/Ti2C and ML WSe2/Ti2CY2 (Y = F and OH), implying a high vertical carrier injection efficiency. In the lateral direction, although pristine Ti2C and ML WSe2 form a Schottky contact with electron barrier of 0.40 eV, Ti2CF2/ML WSe2 and Ti2C(OH)2/ML WSe2 form p-type quasi-Ohmic and n-type Ohmic contacts, respectively. Therefore, 2D titanium carbides can be promising candidates as electrode materials for the ML WSe2 transistors by proper controlling of the surface functional group.

Electrical metal contacts to atomically thin 2H-phase MoTe2 grown by metal–organic chemical vapor deposition

We grow atomically thin molybdenum ditelluride (MoTe2) films on a SiO2/Si substrate by means of metal–organic chemical vapor deposition (MOCVD). Our Raman spectroscopy measurements reveal the formation of 2H-phase MoTe2 films. Further, transmission electron microscopy and X-ray photoelectron spectroscopy studies indicate a three-atomic-layer structure and the surface element composition of MoTe2 films. In this study, we mainly focus on the influence of metal contacts attached to the films on their electrical performance. We fabricate 2H-phase-MoTe2-based field-effect transistors (FETs) with various metal contacts such as titanium/gold, nickel and palladium, which present p-type semiconductor properties. We also examine the influence of the work functions of the contact metals on the electrical properties of three-atomic-layer-MoTe2-based FET devices. For a p-type MoTe2 semiconductor, higher work functions of the contact metals afford narrower Schottky barrier heights (SBHs) and eventually highly efficient carrier injection through the contacts.

Dissolution and recrystallization of perovskite induced by N-methyl-2-pyrrolidone in a closed steam annealing method

Abstract High quality perovskite films with large columnar grains are greatly desired for efficient perovskite solar cells. Here, low volatility N-methyl-2-pyrrolidone (NMP) was added in MAI/IPA solution in a two-step spin-coating method, which promoted the conversion of lead iodide to perovskite. The perovskite films were annealed by a closed-steam annealing method to prolong the recrystallization process of perovskite films assisted by the residual NMP. It leaded to high quality CH3NH3PbI3 perovskite films with large columnar grains due to its enhancement of the Oswald ripening. The large grain perovskite film leaded to efficient carrier transformation and injection, and low recombination. The photovoltaic performance of the perovskite solar cells was improved significantly.

Effect of Si3N4‐Mediated Inversion Layer on the Electroluminescence Properties of Silicon Nanocrystal Superlattices

AbstractThe achievement of an efficient all‐Si electrically‐pumped light emitter is a major milestone in present optoelectronics still to be fulfilled. Silicon nanocrystals (Si NCs) are an attractive material which, by means of the quantum confinement effect, allow attaining engineered bandgap visible emission from Si by controlling the NC size. In this work, SiO2‐embedded Si NCs are employed as an active layer within a light‐emitting device structure. It is demonstrated that the use of an additional thin Si3N4 interlayer within the metal–insulator–semiconductor device design induces an enhanced minority carrier injection from the substrate, which in turn increases the efficiency of sequential carrier injection under pulsed electrical excitation. This results in a substantial increase in the electroluminescence efficiency of the device. Here, the effect of this Si3N4 interlayer on the structural, optical, electrical, and electro‐optical properties of a Si NC‐based light emitter is reported, and the physics underlying these results is discussed.

Investigation of efficiency enhancement in InGaN MQW LED with compositionally step graded GaN/InAlN/GaN multi-layer barrier

The advantage of InGaN multiple Quantum well (MQW) Light emitting diode (LED) on a SiC substrate with compositionally step graded GaN/InAlN/GaN multi-layer barrier (MLB) is studied. The Internal quantum efficiency, Optical power, current-voltage characteristics, spontaneous emission rate and carrier distribution profile in the active region are investigated using Sentaurus TCAD simulation. An analytical model is also developed to describe the QW carrier injection efficiency, by including carrier leakage mechanisms like carrier overflow, thermionic emission and tunnelling. The enhanced electron confinement, reduced carrier asymmetry, and suppressed carrier overflow in the active region of the MLB MQW LED leads to render a superior performance than the conventional GaN barrier MQW LED. The simulation result also elucidates the efficiency droop behaviour in the MLB MQW LED, it suggests that the efficiency droop effect is remarkably improved when the GaN barrier is replaced with GaN/InAlN/GaN MLB barrier. The analysis shows a dominating behaviour of carrier escape mechanism due to tunnelling. Moreover, the lower lattice mismatching of SiC substrate with GaN epitaxial layer is attributed with good crystal quality and reduced polarization effect, ultimately enhances the optical performance of the LEDs.

Copper Phthalocyanine as Contact Layers for Pentacene Films Grown on Coinage Metals

The metal–semiconductor interface determines the efficiency of charge carrier injection into any organic electronics device. Control of this interface, its structure, and its morphology is therefore essential for device improvement. In this study, we analyze the approach of controlling semiconductor morphology at this interface by insertion of a copper phthalocyanine (CuPc) monolayer as a primer between Ag(111), Au(111), and Cu(100) surfaces and the organic semiconductor pentacene (PEN). Controlled monolayer formation is facilitated by thermal desorption of excess multilayers, monitored via thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), and the growth of PEN on the resultant monolayer primers is investigated by near-edge X-ray absorption spectroscopy (NEXAFS), atomic force microscopy (AFM) and STM. While well-ordered CuPc monolayers with flat-lying molecules are formed on Au(111) and Ag(111), no long-range order is observed on Cu(100)...

Effects of Buffer Layer Treatments on the Characteristics and Performances of OLEDs

The authors demonstrated the performance of Organic Light Emitting Diodes (OLEDs) with hexylphosphonic acid (PA) or UV-ozone treatment on their molybdenum trioxide (MoO3) anode buffer layers. The OLEDs with a PA treated (5 mM @1 h) MoO3 layer have lower turn-on voltage and low current efficiency roll-off under high operating current. The hole-only device (ITO/MoO3 with PA or UV-ozone treatment/NPB/Al) was fabricated to calculate the active energy via temperature dependent I-V measurement. When the devices were operated at high temperatures, the activation energy of the UV-ozone treated, and untreated hole-only devices became nonlinear. However, the activation energy of the PA treated devices had a more stable performance at high temperatures. The interfacial resistance of the untreated hole-only devices and the PA and UV-ozone treated devices were calculated by Admittance Spectroscopy (AS) to be 204.5, 363, and 450 Ω. These results indicated that the PA treated devices reduced the activation energy, surface energy mismatch, and interface resistance to enhance the efficiency of carrier injection, which in turn lowers the turn on voltage. Moreover, PA treatment produces a long carbon chain and smooths the morphology of MoO3 to prevent roll-off phenomenon of OLEDs under high operating current.

Quantum Barrier Growth Temperature Affects Deep Traps Spectra of InGaN Blue Light Emitting Diodes

Electroluminescence (EL) efficiency, deep electron and hole traps spectra, microcathodoluminescence (MCL), electron beam induced current (EBIC) imaging, and MCL spectra were studied for blue GaN/InGaN multi-quantum-well (MQW) light emitting diodes differing by the temperature at which the GaN barriers of the MQW active region were grown. It was found that increasing the growth temperature from 850 to 920°C very strongly suppressed the formation of deep electron traps with level at Ec-1 eV in the GaN barriers and of the hole traps with levels at Ev+0.7 eV in InGaN QWs. The suppression of the formation of the Ec-1 eV electron trap, a known prominent nonradiative recombination center in n-GaN, improved the carrier injection efficiency into the InGaN QWs and increased the external quantum efficiency by about 9%. EBIC and MCL imaging showed that the density of threading dislocations and terminating them V-pits was relatively low and similar for both studied growth temperatures, close to 108 cm−2. The cross-sectional dimensions of the V-pits were measurably higher for increased growth temperature. However, the rather low dislocation density and rather high dimensions of the V-pits were believed to result in minor contribution of these defects to the observed EL efficiency changes.

AlGaN Light‐Emitting Diodes on AlN Substrates Emitting at 230 nm

Light‐emitting diodes with emission wavelengths between 230 and 240 nm are grown on AlN substrates and characterized optically and electrically. Optimized doping of the n‐AlGaN with Si has resulted in a mean forward voltage at 200 mA of 9 V. The electroluminescence spectra is demonstrated to be free of long wavelength parasitic emission. An envelope function in the output power versus peak wavelength is shown, demonstrating the increasing dominance of in‐plane emission at shorter wavelengths as light extraction becomes more difficult. However, variations in device structure show that further optimization of the structure for carrier confinement and injection efficiency are also able to push performance beyond the current state of the art.

Graphene/PbS-Quantum Dots/Graphene Sandwich Structures Enabled by Laser Shock Imprinting for High Performance Photodetectors.

Quantum dots (QDs) integrated 2-dimensional (2D) materials have great potential for photodetector applications due to the excellent light absorption of QDs and ultrafast carrier transportation of 2D materials. However, there is a main issue that prevents efficient carrier transportation and ideal performance of photodetectors: the high interfacial resistance between 2D materials and QDs due to the bad contacts between 2D/0D interface, which makes sluggish carrier transfer from QDs to 2D materials. Here, a sandwich structure (graphene/PbS-QDs/graphene) with seamless 2D/0D contact was fabricated by laser shock imprinting, which opto-mechanically tunes the morphology of 2D materials to perfectly wrap on 0D materials and efficiently collect carriers from the PbS-QDs. It is found that this seamless integrated 2D/0D/2D structure significantly enhanced the carrier transmission, photoresponse gain (by 2×), response time (by 20×), and photoresponse speed (by 13×). The response time (∼30 ms) and Ip/ Id ratio (13.2) are both over 10× better than the reported hybrid graphene photodetectors. This is due to the tight contact and efficient gate-modulated carrier injection from PbS-QDs to graphene. The gate voltage dictates whether electrons or holes dominate the carrier injection from PbS-QDs to graphene.

Effect of Electron Blocking Layer Doping and Composition on the Performance of 310 nm Light Emitting Diodes.

The effects of composition and p-doping profile of the AlGaN:Mg electron blocking layer (EBL) in 310 nm ultraviolet B (UV-B) light emitting diodes (LEDs) have been investigated. The carrier injection and internal quantum efficiency of the LEDs were simulated and compared to electroluminescence measurements. The light output power depends strongly on the temporal biscyclopentadienylmagnesium (CpMg) carrier gas flow profile during growth as well as on the aluminum profile of the AlGaN:Mg EBL. The highest emission power has been found for an EBL with the highest CpMg carrier gas flow and a gradually decreasing aluminum content in direction to the p-side of the LED. This effect is attributed to an improved carrier injection and confinement that prevents electron leakage into the p-doped region of the LED with a simultaneously enhanced carrier injection into the active region.

In situ assembly of well-defined Au nanoparticles in TiO2 films for plasmon-enhanced quantum dot sensitized solar cells

A photoanode composed of homogeneously dispersed Au nanoparticles (NPs) within TiO2 mesoporous film (TiO2/Au) was fabricated via an in situ assembly process for plasmon-enhanced CdS/CdSe quantum dot sensitized solar cells (QDSCs). The incorporation of Au NPs in the photoanode for QDSCs shows improvement not only in the photocurrent density with increased photon capturing but also in photovoltage arising from reduced charge recombination. TiO2/Au based photocanode exhibits excellent injection and transport efficiency of photoinduced charge carriers, which likely arose from the core-shell structure of CdS and CdSe QDs assembled on the surface of Au NPs, preventing Au NPs from contacting directly with electrolyte. As a result, the power conversion efficiency of the QDSCs with TiO2/Au photoelectrodes reached 6.00%, which is about 50% enhancement of the efficiency for the solar cells without Au NPs (4.04%).

Low Resistance Tunnel Junctions for Efficient Electrically Pumped Nanolasers

Extremely compact nanoscale devices such as electrically pumped nanolasers are difficult to operate at room temperature due to the high electrical resistance inherent to small cavities. As a consequence, large voltages are necessary to reach the lasing threshold, which generates heat and reduces device efficiency. The poor heat sinking of small devices makes matters worse, dramatically reducing the laser efficiency. Instead of looking for solutions to dissipate heat from small structures more efficiently, designing nanolasers to produce less heat in the first place is an important goal. Here we propose and theoretically analyze the effect of adding an InGaAsP tunnel junction for efficient carrier injection in metallo-dielectric nanolasers. With our theoretical model we show that the device resistance is reduced by a factor of ∼6.5. The applied voltage at the room temperature lasing threshold is reduced from 3.05 to 1.35 V, a reduction of 69% in heat generation, whereas the Q-factor and gain threshold of the cavity are not degraded.

Sub-bandgap response of graphene/SiC Schottky emitter bipolar phototransistor examined by scanning photocurrent microscopy

Graphene layers grown epitaxially on SiC substrates are attractive for a variety of sensing and optoelectronic applications because the graphene acts as a transparent, conductive, and chemically responsive layer that is mated to a wide-bandgap semiconductor with large breakdown voltage. Recent advances in control of epitaxial growth and doping of SiC epilayers have increased the range of electronic device architectures that are accessible with this system. In particular, a recently-introduced Schottky-emitter bipolar phototransistor (SEPT) based on an epitaxial graphene (EG) emitter grown on a p-SiC base epilayer has been found to exhibit a maximum common emitter current gain of 113 and a UV responsivity of 7.1 A W−1. The behavior of this device, formed on an n+-SiC substrate that serves as the collector, was attributed to a very large minority carrier injection efficiency at the EG/p-SiC Schottky contact. This large minority carrier injection efficiency is in turn related to the large built-in potential found at a EG/p-SiC Schottky junction. The high performance of this device makes it critically important to analyze the sub bandgap visible response of the device, which provides information on impurity states and polytype inclusions in the crystal. Here, we employ scanning photocurrent microscopy (SPCM) with sub-bandgap light as well as a variety of other techniques to clearly demonstrate a localized response based on the graphene transparent electrode and an approximately 1000-fold difference in responsivity between 365 nm and 444 nm excitation. A stacking fault propagating from the substrate/epilayer interface, assigned as a single layer of the 8H-SiC polytype within the 4H-SiC matrix, is found to locally increase the photocurrent substantially. The discovery of this polytype heterojunction opens the potential for further development of heteropolytype devices based on the SEPT architecture.

Threading dislocation density effect on the electrical and optical properties of InGaN light-emitting diodes

The electrical and optical properties of InGaN LEDs with different threading dislocation densities (TDDs) were investigated. LEDs with low TDDs exhibited low forward voltage and high injection efficiency compared with LEDs with high TDDs. The effect of TDDs on the electrical properties of InGaN LEDs was attributed to efficient carrier injection into the QWs brought about by increased transverse carrier mobility in the InGaN layer resulting from reductions in carrier scattering around dislocation cores. In terms of optical properties, LEDs with low TDDs exhibited high peak efficiency and substantial efficiency droops under increased current densities, whereas LEDs with high TDDs showed low peak efficiency and minimal droops under the same condition. These trends can be explained by the correlations of high TDDs with increased dominance of nonradiative recombination and pronounced suppression of peak efficiency under low current densities.

Analysis of threshold current of uniaxially tensile stressed bulk Ge and Ge/SiGe quantum well lasers.

We propose and design uniaxially tensile stressed bulk Ge and Ge/SiGe quantum well lasers with the stress along <100> direction. The micro-bridge structure is adapted for introducing uniaxial stress in Ge/SiGe quantum well. To enhance the fabrication tolerance, full-etched circular gratings with high reflectivity bandwidths of ~500 nm are deployed in laser cavities. We compare and analyze the density of state, the number of states between Γ- and L-points, the carrier injection efficiency, and the threshold current density for the uniaxially tensile stressed bulk Ge and Ge/SiGe quantum well lasers. Simulation results show that the threshold current density of the Ge/SiGe quantum well laser is much higher than that of the bulk Ge laser, even combined with high uniaxial tensile stress owing to the larger number of states between Γ- and L- points and extremely low carrier injection efficiency. Electrical transport simulation reveals that the reduced effective mass of the hole and the small conduction band offset cause the low carrier injection efficiency of the Ge/SiGe quantum well laser. Our theoretical results imply that unlike III-V material, uniaxially tensile stressed bulk Ge outperforms a Ge/SiGe quantum well with the same strain level and is a promising approach for Si-compatible light sources.

In situ characterization of radiation sensors based on GaN LED structure by pulsed capacitance technique and luminescence spectroscopy

We report on variations of the electrical and optical characteristics of commercial GaN blue LEDs, acting as radiation sensors or dosimeters, during 1.6MeV proton irradiation. Transients of the barrier and storage capacitance have been examined simultaneously with proton-excited luminescence. The pulsed capacitance technique was shown to be a useful tool for the control of changes of the effective density of dopants in the LED structure during proton irradiation, due to introduced radiation defects. The minority carrier injection efficiency can be estimated by measuring the charging current transients of the barrier and storage capacitance. The transient current measurements imply rather fast response of the GaN LED hadron radiation sensors. Also, our results show that the intensity and spectral variations of the proton-excited blue luminescence of the LEDs can be used for the estimation of hadron flux and fluence, respectively. This implies that GaN LED-based sensors can be used for the synchronous detection of hadron irradiation by recording the optical and electrical signals.

Direct Vapor Growth of Perovskite CsPbBr3 Nanoplate Electroluminescence Devices.

Metal halide perovskite nanostructures hold great promises as nanoscale light sources for integrated photonics due to their excellent optoelectronic properties. However, it remains a great challenge to fabricate halide perovskite nanodevices using traditional lithographic methods because the halide perovskites can be dissolved in polar solvents that are required in the traditional device fabrication process. Herein, we report single CsPbBr3 nanoplate electroluminescence (EL) devices fabricated by directly growing CsPbBr3 nanoplates on prepatterned indium tin oxide (ITO) electrodes via a vapor-phase deposition. Bright EL occurs in the region near the negatively biased contact, with a turn-on voltage of ∼3 V, a narrow full width at half-maximum of 22 nm, and an external quantum efficiency of ∼0.2%. Moreover, through scanning photocurrent microscopy and surface electrostatic potential measurements, we found that the formation of ITO/p-type CsPbBr3 Schottky barriers with highly efficient carrier injection is essential in realizing the EL. The formation of the ITO/p-type CsPbBr3 Schottky diode is also confirmed by the corresponding transistor characteristics. The achievement of EL nanodevices enabled by directly grown perovskite nanostructures could find applications in on-chip integrated photonics circuits and systems.

Influence of annealing temperature for ZnO layer on photoconversion efficiency of organic devices

ABSTRACTSince an interface barrier between a ZnO layer and an adjacent organic active layer affects the carrier injection efficiency, we investigated the relationship between the annealing temperature for the ZnO layer and the photovoltaic performance of inverted cell. An optimized annealing temperature was ranged from 125 to 200°C due to both the sufficient chemical reaction of ZnO precursor solution and the remained organic component, which were evaluated from the atomic force microscope, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. As a result, a highest photoconversion efficiency of 9.34% was achieved for a simple bulk heterojunction structure.

Suppression of recombination in waveguide in c-plane InGaN-based green laser diodes

Recombination in InGaN upper waveguide in c-plane InGaN-based green laser diodes (LDs) has been investigated by both simulation and experiments. It is found that a potential barrier located at the interface between GaN last barrier and InGaN upper waveguide layer causes strong recombination in InGaN upper waveguide layer and lowers the carrier injection efficiency of InGaN-based green LDs when In content is higher than 4%. A suitable reduction of indium content in InGaN upper waveguide layer can effectively suppress recombination in waveguide and thus reduce the threshold current of green LDs.