Improved Carrier Injection Research Articles

Enhanced water splitting on (010) facet-exposed BiVO4 photoanode with improved carrier injection efficiency

Transition metal oxide semiconductors, noted for their stability and suitable bandgap, are promising photoanodes for water splitting. Surface engineering is critical to tackle issues like low carrier mobility and charge recombination, stemming from atomic arrangement and Fermi level differences. While exposing dominant crystal facets boosts photocatalytic capability, it can hinder carrier injection into the electrolyte. In this study, BiVO4 films with various facet exposures were synthesized and characterized using scanning electron microscopy and x-ray diffraction to confirm their morphology and crystalline structure. Mott–Schottky analysis was employed to investigate changes in the band structure near the semiconductor–electrolyte interface, revealing that high (010)-BiVO4 facet exposure enhances carrier separation but reduces injection efficiency. The results from photoconductive atomic force microscopy tests demonstrated that enhanced band bending at the semiconductor interface improves hole transfer. Coating the (010)-BiVO4 photoanode with MoS2 and an amorphous ZrO2 interlayer yielded a photocurrent density of 0.6 mA cm−2 at 1.2 V (vs RHE) under AM 1.5 G illumination, tripling the pristine photoanode's performance and nearly tripling water splitting efficiency. Mechanism revealing the improved photoelectrochemical performance is attributed to a greater band bending on the BiVO4 surface, enhancing hole injection dynamics. This work provides a feasible strategy for a deeper understanding of the intrinsic mechanisms of facet engineering and improving the activity of photoanodes.

High-efficiency electroluminescence devices containing Si nanocrystals/SiC multilayers via improved carrier injection and recombination process

Realizing high efficiency of all Si-based light-emitting devices is currently one of interesting issues in order to develop monolithic opto-electronic integration on chips. Here, we report an electroluminescence device based on phosphorus (P)-doped silicon nanocrystals (Si NCs)/silicon carbide (SiC) multilayers by modulating carrier injection and recombination process. The p+-Si substrate is used instead of p-Si substrate for facilitating the hole injection into Si NCs. Additionally, the influences of annealing temperature on the device performance have been studied, and the optimized annealing temperature is achieved by balancing the crystallinity, defect state density, and recombination process.

Improved Design of Slope-Shaped Hole-Blocking Layer and Electron-Blocking Layer in AlGaN-Based Near-Ultraviolet Laser Diodes.

The injection and leakage of charge carriers have a significant impact on the optoelectronic performance of GaN-based lasers. In order to improve the limitation of the laser on charge carriers, a slope-shape hole-barrier layer (HBL) and electron-barrier layer (EBL) structure are proposed for near-UV (NUV) GaN-based lasers. We used Crosslight LASTIP for the simulation and theoretical analysis of the energy bands of HBL and EBL. Our simulations suggest that the energy bands of slope-shape HBL and EBL structures are modulated, which could effectively suppress carrier leakage, improve carrier injection efficiency, increase stimulated radiation recombination rate in quantum wells, reduce the threshold current, improve optical field distribution, and, ultimately, improve laser output power. Therefore, using slope-shape HBL and EBL structures can achieve the superior electrical and optical performance of lasers.

Phenethylammonium bromide interlayer for high-performance red quantum-dot light emitting diodes.

Interfacial modification is vital to boost the performance of colloidal quantum-dot light-emitting diodes (QLEDs). We introduce phenethylammonium bromide (PEABr) as an interlayer to reduce the trap states and exciton quenching at the interface between the emitting layer (EML) with CdSe/ZnS quantum-dots and the electron transport layer (ETL) with ZnMgO. The presence of PEABr separates the EML and the ETL and thus passivates the surface traps of ZnMgO. Moreover, the interfacial modification also alleviates electron injection, leading to more improved carrier injection balance. Consequently, the external quantum efficiency of the PEABr-based red QLED reached 27.6%, which outperformed those of the previously reported devices. Our results indicate that the halide ion salts are promising to balance charge carrier injection and reduce exciton quenching in the QLEDs.

Fabrication and characterization of an AlGaN light emitting diode with Al-doped ZnO as a current spreading tunnel junction layer

We fabricated an AlGaN light emitting diode (LED) with a heavily Al-doped n-type ZnO layer on a p-type contact layer as a tunnel junction (TJ) to improve carrier injection into the LED. We characterized its electrical and optical properties and compared them to those of an AlGaN LED without ZnO. From the I–V characteristic of the LED with ZnO, we observed a threshold voltage of circa 2 V, which could be due to Zener breakdown of the type II heterostructure of n-ZnO/p-GaN as a TJ. From the electroluminescence measurement, we observed a similar emission peak in both AlGaN LEDs at ultraviolet (UV) wavelengths, but a broad emission band around 365 nm in the LED with ZnO. This emission could be originating from ZnO photoexcited by the UV LED emission. The dependence of these peak intensities on input currents shows that there is a monotonic increase in the light emission intensity for the UV LED emission, but a saturation behavior after the threshold voltage for the emission from the ZnO. This saturation behavior is attributed to an overflow of photoexcited electron–hole pairs into p-GaN, strongly suggesting that n-ZnO/p-GaN works as a TJ. Electroluminescence data also show that the presence of the ZnO film facilitates current spreading, which enables device operation at large input currents. Therefore, ZnO can work as a current spreading TJ layer and improve the performance of the AlGaN LED.

High-Power, High-Efficiency GaSb-Based Laser with Compositionally Linearly Graded AlGaAsSb Layer

We propose a novel graded AlGaAsSb layer growth method to achieve a super-linear interface by precisely controlling the cell temperature and valve position. Atomically smooth surface and lattice-matched epitaxy was confirmed by AFM and the HRXRD characterization of the graded AlGaAsSb layer sample. With the inserted graded layer between the cladding and waveguide layers, high-power, high-efficiency GaSb-based laser emitters and laser bars were confirmed. The linearly graded interface layer smooths the potential barrier peak between the cladding and waveguide layers, which resulted in a low turn-on voltage of 0.65 V and an ultra-low series resistance of 0.144 Ω. A maximum continuous-wave output power of 1.8 W was obtained with a high power conversion efficiency of 28% at 1.1 A and 12% at 8 A. A facet-coated laser bar was also fabricated with a record-high CW output power of 18 W. A high internal quantum efficiency of 83 was maintained at 40 °C, implying improved carrier injection efficiency, which benefits from the built-in electric field of the composition-graded AlGaAsSb layer.

Reduction of nonradiative recombination for high-power 808 nm laser diode adopting InGaAsP/InGaAsP/GaAsP active region

Reducing nonradiative recombination is a key problem to achieving high-power laser diode. InGaAsP/InGaAsP/GaAsP active region was designed so as to increase injection efficiency and decrease leakage currents which corresponds to a lower nonradiative recombination. The simulation results showed that the output power of 15.59 W and the wall-plug efficiency of 78.95% were obtained for the InGaAsP/InGaAsP/GaAsP active region laser at injection current density 6 kA/cm2 at room temperature of 25 °C by improved carrier injection efficiency. Meanwhile, the temperature drift coefficient of 0.217 nm/∘C was obtained for the peak wavelength of InGaAsP/InGaAsP/GaAsP active region laser at the temperature range from 5 to 65 ∘C. The designed InGaAsP/InGaAsP/GaAsP active region structure is an effective way to reduce the nonradiative recombination loss for achieving high-power laser diode.

Significant improvement of injection efficiency in deep-UV LD structures by light Mg doping in p-core layer

Improvement of carrier injection efficiency is essential to achieve lower threshold and shorter wavelength in deep-ultraviolet laser diodes. We have confirmed that the introduction of electron blocking layer and Mg doping layer into the core layer significantly improves the injection efficiency. In this study, we show that optimizing the Mg doping level in the core layer improves the external quantum efficiency by a factor of about 10 compared to the non-doped sample. The dependence of the external quantum efficiency on Mg flow rate can be interpreted in terms of a reduction of the dip at the p-side core/cladding interface in the conduction band. The dip is expected to be suppressed by ionized Mg activated by the Poole–Frenkel effect, resulting in improved carrier injection efficiency. A remarkable improvement in efficiency is also observed when the Mg doping region is limited to the vicinity of the core/cladding layer interface.

Improvement of contact resistance at carbon electrode/organic semiconductor interfaces through chemical doping

Organic thin-film transistors (OTFTs) are promising building blocks for low cost, low-environmental load, and lightweight electronic devices. Carbon-based conductors can be potentially used as TFT electrodes. However, a concern is that the carbon electrode is unsuitable for carrier injection into organic semiconductors due to the difficulty in precise work function control. Herein, we have demonstrated that molecular dopants in carbon networks can improve carrier injection with a reasonably low contact resistance of 510 Ω·cm, which constitutes a key step in the realization of noble-metal-free electronic devices.

Contact resistance in sub-micron and nanoscale organic thin-film transistors

Two improved methods for evaluating contact resistance in organic thin-film transistors (TFTs) are presented. These methods can be applied to TFTs with any arbitrary geometry and are also suitable for retroactive analysis of current-voltage characteristics. The first method is based on the analysis of the differential conductance of TFTs and in separating the conductivity contributions from the contacts and the channel. This method provides accurate results when the contact resistance is approximately 0.1–10 times the channel resistance. The second method is especially applicable to nanoscale and short channel length TFTs. The minimum value of differential on-resistance as a function of drain bias is used to separate the contributions of the channel and contact resistances at various drain voltages. This method is accurate when the contact resistance is approximately 0.3–10 times the channel resistance, as verified by simulations. It works even when there is no distinct saturation in current-voltage characteristics with increasing drain voltage. Both methods have been applied to experimental data from organic TFTs with channel lengths in the range 10 nm to 5 μm. • Two novel methods of extracting contact resistance in OTFTs are introduced. • Contact resistance of various channel lengths OTFTs are extracted and analyzed. • Effects of contact resistance in short channel pentacene TFTs are discussed. • New design of ultra-short channel TFTs is proposed to improve carrier injection.

High-performance thin-film transistor device architecture for flexible and printed electronics

A device design paradigm for thin-film transistors (TFTs) suitable for fabrication using methods available for flexible and printed electronics devices and circuits is described. The TFT architecture utilizes an array of nanospike-shaped electrodes as the source and drain electrodes. This results in improved carrier injection, greater gate control of the drain current, and lower threshold and operating voltage. The on-currents are also higher in comparison with standard flat edge electrode TFTs with equivalent channel dimensions. Importantly, the design is very tolerant of thick gate insulators. The proposed architecture requires one level of relatively high resolution patterning of the source and drain contacts, which can be potentially realized with methods that have been previously employed in flexible electronics such as nanoimprint lithography or roll-to-roll photolithography. The experimental data presented in this paper were obtained from TFTs fabricated using conventional fabrication methods, as the emphasis in this paper is on the device design and in demonstrating the advantageous features of the new architecture in future flexible systems.

Anisotropic interface characteristics of bilayer GeSe based field effect transistors

Moderate direct band gap and strong stability render bilayer gemanium selenide (GeSe) more suitable for the channel material in the electronic and optoelectronic devices. To improve carrier injection efficiency in an actual two dimensional device, the choice of the metal electrode is a key issue. Here using both electronic band calculations and quantum transport simulation, we comprehensively investigate the interface characteristics of bilayer GeSe contacted with six representative metals. The projected band structures of bilayer GeSe on these metals illustrate that all of these bilayer GeSe have undergone metallization, resulting in the contact natures are ohmic in the vertical interface. While，anisotropic lateral Schottky contacts are formed between the Ag, Au, Pd, Pt, Cu and bilayer GeSe along the armchair and the zigzag direction. The Ni electrode forms p-type quasi-Ohmic contact with bilayer GeSe with a small hole Schottky barrier height of 0.04 (0.08) eV along the armchair (zigzag) direction. Thus, the high performance of bilayer GeSe based FET would be realized by choosing the Ni as source/drain electrodes.

Deep Ultraviolet AlGaN-Based Light-Emitting Diodes with p-AlGaN/AlGaN Superlattice Hole Injection Structures

The p-AlGaN/AlGaN superlattice (SL) hole injection structure was introduced into deep ultraviolet (DUV) light-emitting diodes (LEDs) to enhance their performances. The period thicknesses of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs affected the performances of the DUV LEDs. The appropriate period thickness of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL may enhance the hole injection of DUV LEDs. Therefore, compared with the reference LEDs, the DUV LEDs with the 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL presented forward voltage reduction of 0.23 V and light output power improvement of 15% at a current of 350 mA. Furthermore, the 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL could slightly suppress the Auger recombination and current overflow of the DUV LEDs in a high-current operation region. In addition to improved carrier injection, the DUV LEDs with the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL hole injection structure showed reduced light absorption at their emission wavelength compared with the reference LEDs. Therefore, the DUV LEDs with p-Al0.8Ga0.2N/Al0.48Ga0.52N SL may exhibit better light extraction efficiency than the reference LEDs. The enhancement of p-Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL may contribute to improvements in light extraction and hole injection.

Stacked GaN/AlN last quantum barrier for high-efficiency InGaN-based green light-emitting diodes.

Realization of efficient InGaN-based green light-emitting diodes (LEDs) is highly desirable in solid-state lighting industry. Here, we propose a stacked last quantum barrier (SLQB) GaN/AlN layer for green (∼550nm) LEDs grown on patterned sapphire substrate to improve the device performance. A green LED with a SLQB achieves a light output power (LOP) of 13 mW at 15 mA, which is 35% higher than that of LEDs with a conventional last quantum barrier (CLQB) GaN layer. In addition, the forward voltage of the green LED with the SLQB is reduced by 0.16 V compared with green LEDs with the CLQB. Simulation results demonstrate that the AlN layer in the SLQB increases the effective barrier height for electrons and alleviates the electron leakage effect. It also enables an intraband tunneling process for holes to inject into the active region, promoting the hole injection efficiency. As a result, we successfully obtain InGaN-based green LEDs with remarkably improved carrier injection efficiency by adopting the SLQB structure.

Reducing the polarization mismatch between the last quantum barrier and p-EBL to enhance the carrier injection for AlGaN-based DUV LEDs

In this work, we report an AlGaN-based ∼275 nm deep ultraviolet light-emitting diode (DUV LED) that has AlGaN based quantum barriers with a properly large Al composition. It is known that the increased conduction band barrier height helps to enhance the electron concentration in the active region. However, we find that the promoted hole injection efficiency is also enabled for the proposed DUV LED when the Al composition increases. This is attributed to the reduced positive polarization charge density at the last quantum barrier (LQB) and p-type electron blocking layer (p-EBL) interface, which can suppress the hole depletion effect in the p-EBL. Thus, the hole concentration in the p-EBL gets promoted, which is very helpful to reduce the hole blocking effect caused by the p-EBL. Therefore, thanks to the improved carrier injection, the proposed DUV LED increases the optical power and reduces the forward voltage when compared with the conventional DUV LED.

Vertically-Stacked Si0.2Ge0.8 Nanosheet Tunnel FET With 70 mV/Dec Average Subthreshold Swing

A novel CMOS-compatible SiGe Tunnel Field-Effect Transistor (TFET) with a high current drivability is demonstrated, which features a vertically-stacked SiGe nanosheet (NS) with high Ge content and gate-all-around(GAA) structure to improve carrier injection efficiency and gate controllability. A multi-NS Si 0.2 Ge0.8 channel is formed by the Si selective etching and Ge condensation technique. The measured data shows both the 50 mV/dec minimum subthreshold swing (SS) and 70 mV/dec average SS over 3 decades of drain current change. Compared to previously reported devices, it is clear that the proposed SiGe nanosheet TFET can achieve steeper switching and low-level leakage current with a low drive voltage as an alternative to conventional MOSFET.

Improvement of MAPbI3 perovskite blend with TiO2 nanoparticles as ReRAM device

Reducing pinholes formation and enhancing surface coverage of hybrid organic-inorganic halide perovskite (MAPbI3) film is critical in most application such as photovoltaics and memory device. These applications usually incorporate Titanium Dioxide (TiO2) which is widely used to improve carrier injection as well as film morphology. Taking into account the fabrication and preparation cost, rather than forming a bilayer device, this work demonstrates the design of a facile two-step coating method in producing higher surface quality of single layer MAPbI3 modified with the addition of TiO2 nanoparticles. The study of this novel MAPbI3-TiO2 compound reveals an enhanced movement of iodide vacancies towards the ITO interfaces while considering the effect of grain size and film uniformity. Hence, the Resistive Random Access Memory (ReRAM) devices employing the MAPbI3-TiO2 compound shows to aid in improving VSET and VRESET voltage values in which molar ratio of Pb to Ti of 5% in MAPbI3-TiO2 layer shows improved VSET and VRESET values of 3.6 V and −1.1 V. This study is believed to aid as a forefront in understanding the applicability of a single layer MAPbI3-TiO2 in multiple applications.

Double Beneficial Role of Fluorinated Fullerene Dopants on Organic Thin-Film Transistors: Structural Stability and Improved Performance.

The present work assesses improved carrier injection in organic field-effect transistors by contact doping and provides fundamental insight into the multiple impacts that the dopant/semiconductor interface details have on the long-term and thermal stability of devices. We investigate donor [1]benzothieno[3,2-b]-[1]benzothiophene (BTBT) derivatives with one and two octyl side chains attached to the core, therefore constituting asymmetric (BTBT-C8) and symmetric (C8-BTBT-C8) molecules, respectively. Our results reveal that films formed out of the asymmetric BTBT-C8 expose the same alkyl-terminated surface as the C8-BTBT-C8 films do. In both cases, the consequence of depositing fluorinated fullerene (C60F48) as a molecular p-dopant is the formation of C60F48 crystalline islands decorating the step edges of the underlying semiconductor film surface. We demonstrate that local work function changes along with a peculiar nanomorphology lead to the double beneficial effect of lowering the contact resistance and providing long-term and enhanced thermal stability of the devices.

Reduction in efficiency droop/decline of green GaN-based light-emitting diodes by employing heterostructure cap layer

In this study, we theoretically studied the enhancement of carrier injection by employing heterostructure layers before p-GaN in green light-emitting diodes. It is evident from the simulation results that improved carrier injection results in high internal quantum efficiency and light output power in the structures using heterostructures on the p-side. In comparison to conventional structure, reduced efficiency droop is observed suggesting promising high-performance green light-emitting diodes.

Improved carrier injection and balance in solution-processed blue phosphorescent organic light emitting diodes based on mixed host system and their transient electroluminescence

Solution processed blue phosphorescent organic devices based on mixed host system were fabricated in this paper. A set of blue phosphorescent organic light emitting diodes (PhOLEDs) utilizing two small molecular materials of 3,3-Di(9H-carbazol-9-yl) biphenyl (mCBP) and 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) as mixed host doped with phosphor bis[(4,6-difluorophenyl)-pyridina-to-N,C2](picolinate) iridium(III) (FIrpic) are fabricated. The results show that the performance of mixed host system devices is improved compared with two single host devices. The max luminance of 23,340 cd/m2 is achieved in the mCBP:TCTA (2:1) mixed host device while the 14,400 cd/m2 in mCBP single host devices and the 12,968 cd/m2 in TCTA, and the current efficiency is improved from 8.17 cd/A(mCBP), 9.61 cd/A (TCTA) to 13.62 cd/A at the luminance of 1000 cd/m2. The transient electroluminescence measurement was utilized to detect the carrier injection between two kinds of PhOLEDs and penetrate how the trap charges works in the mixed host devices. This presents that the mixed host system contributes to the hole injection and the equilibrium of the carrier transport, reduces trapped charges, thus brings about high efficiency devices.