Carrier Depletion Research Articles

Enhanced electrical characteristics of Zr/diamond Schottky barrier diode with cerium hexaboride interfacial layer

To fabricate a performance-enhanced Schottky barrier diode (SBD) on a diamond, a cerium hexaboride (CeB6) interfacial layer was introduced between the diamond and the Zr/Au electrode. The inserted CeB6 layer was characterized using X-ray photoelectron spectroscopy. Introducing the CeB6 layer significantly improved the performance of the diamond SBD compared with a sample without the layer. The ideality factor (n)/SBH (ΦB) values of the SBD with and without the CeB6 interlayer were 2.24/1.92 eV and 2.82/1.65 eV, representing an improvement of 20.5% and 16.4% for the ideality factor and Schottky barrier height, respectively. The reverse breakdown voltage increased from −86 to −110.5 V, indicating an improvement of 28.5% for breakdown voltage. Additionally, the reverse current of the SBD with CeB6 was suppressed, making it more reliable and stable than that without CeB6 before the breakdown threshold. The carrier concentration and depletion layer width were increased from 1.135 × 1014 cm−3 and 206.7 nm to 2.646 × 1014 cm−3 and 227.6 nm, respectively, with the introduction of CeB6. These results demonstrate the advantages of inserting of CeB6 in vertical diamond SBDs and show the potential of SBDs for future power rectifiers.

Correlation Between Reverse Leakage Current and Electric Field Spreading in GaN Vertical SBD With High-Energy Ion Implanted Guard Rings

This work focuses on the bias-dependent reverse leakage current and carrier depletion process of vertical gallium nitride (GaN)-on-GaN Schottky barrier diodes (SBDs) with fluorine ion-implanted guard rings (GRs). The reverse leakage characteristics in the vertical GaN SBD with GRs sequentially go through ohmic conduction, thermionic field emission (TFE), and space charge limited conduction (SCLC) model as the reverse bias increases gradually. Once the traps in the implanted termination region are fully ionized, the device will undergo large leakage current at high biases. Compared with infinite area ion implanted edge termination (ET), ion-implanted GR can effectively reduce the leakage current at low biases. In addition, there are kinks in the reverse current–voltage curves, which proved to be related to the electric field spreading effect of individual GR. Based on the analysis of reverse leakage current, the electric field modulation mechanism of ion-implanted GRs is reviewed.

Selective Purity Modulation of Semiconducting Single-Walled Carbon Nanotube Networks for High-Performance Thin-Film Transistors

Single-walled carbon nanotube (SWCNT) random networks have become strong candidates for next-generation electronics due to their exceptional mechanical, electrical, and optical properties. However, metallic nanotubes in networks generally incur a trade-off between the charge carrier mobility and on/off ratio, limiting the performance of SWCNT-based devices. Therefore, various methods to increase the purity of semiconducting nanotubes in entire random networks have been reported, but this direction has faced other issues, such as nanotube shortening, higher cost, and higher energy. Here, we introduce SWCNT random network-based thin-film transistors (SWCNT TFTs) with a varying purity profile of semiconducting SWCNTs across the channel, exploiting the superior mobility of metallic SWCNTs by partially tuning the semiconducting SWCNT purity and developing a novel perspective on metallic nanotubes in semiconductor channels. Based on the high-precision drop-on-demand capability of inkjet printing and various concentrations of semiconducting SWCNT ink, we form selectively patterned channel regions with different semiconducting SWCNT purities. The metallic nanotube-dominant region drastically increases the carrier density with a minimized Schottky barrier, while high-purity semiconducting regions at the channel boundaries effectively block off-state leakage through carrier depletion. As a result, the SWCNT TFTs with selectively patterned metallic nanotube regions show superior carrier mobility (75.50 cm2 V–1 s–1) and channel width normalized on-current (34.33 nA μm–1) without compromising the on/off ratio (1.62 × 107). To show the feasibility of our device in high-performance electronics, we demonstrate all-inkjet-printed flexible display driving circuits with two transistors that enable low-power, high-performance operation in display applications.

Surface plasmon coupling effects on the behaviors of radiative and non-radiative recombination in an InGaN/GaN quantum well

Although surface plasmon (SP) coupling has been widely used for enhancing the emission efficiency of an InGaN/GaN quantum well (QW) structure, the interplay of the carrier transport behavior in the QW with SP coupling, which is a crucial mechanism controlling the SP-coupling induced QW emission enhancement, is still an issue not well explored yet. To understand the effects of SP coupling on the radiative and non-radiative recombination behaviors of carriers in a QW structure, the temperature-dependent time-resolved photoluminescence spectroscopies of two QW samples of different indium contents with surface Ag nanoparticles are studied. A two-single-exponential model is used for calibrating their radiative and non-radiative decay times. The SP coupling process, which transfers carrier energy from a QW into the SP resonance mode for effective radiation and increases the effective radiative recombination rate, produces energy-dependent carrier depletion and, hence, disturbs the quasi-equilibrium condition of carrier distribution. In this situation, a strong carrier transport process occurs targeting a new quasi-equilibrium condition that enhances non-radiative recombination and, hence, reduces the benefit of using the SP coupling technique. To alleviate this problem of SP-coupling induced energy loss, a weak energy-dependent or broad-spectrum SP coupling process is recommended.

Investigation of the diameter-dependent piezoelectric response of semiconducting ZnO nanowires by Piezoresponse Force Microscopy and FEM simulations

Semiconducting piezoelectric nanowires (NWs) are promising candidates to develop highly efficient mechanical energy transducers made of biocompatible and non-critical materials. The increasing interest in mechanical energy harvesting makes the investigation of the competition between piezoelectricity, free carrier screening and depletion in semiconducting NWs essential. To date, this topic has been scarcely investigated because of the experimental challenges raised by the characterization of the direct piezoelectric effect in these nanostructures. Here we get rid of these limitations using the piezoresponse force microscopy technique in DataCube mode and measuring the effective piezoelectric coefficient through the converse piezoelectric effect. We demonstrate a sharp increase in the effective piezoelectric coefficient of vertically aligned ZnO NWs as their radius decreases. We also present a numerical model which quantitatively explains this behavior by taking into account both the dopants and the surface traps. These results have a strong impact on the characterization and optimization of mechanical energy transducers based on vertically aligned semiconducting NWs.

Edge and Interface Resistances Create Distinct Trade-Offs When Optimizing the Microstructure of Printed van der Waals Thin-Film Transistors

Inks based on two-dimensional (2D) materials could be used to tune the properties of printed electronics while maintaining compatibility with scalable manufacturing processes. However, a very wide range of performances have been reported in printed thin-film transistors in which the 2D channel material exhibits considerable variation in microstructure. The lack of quantitative physics-based relationships between film microstructure and transistor performance limits the codesign of exfoliation, sorting, and printing processes to inefficient empirical approaches. To rationally guide the development of 2D inks and related processing, we report a gate-dependent resistor network model that establishes distinct microstructure-performance relationships created by near-edge and intersheet resistances in printed van der Waals thin-film transistors. The model is calibrated by analyzing electrical output characteristics of model transistors consisting of overlapping 2D nanosheets with varied thicknesses that are mechanically exfoliated and transferred. Kelvin probe force microscopy analysis on the model transistors leads to the discovery that the nanosheet edges, not the intersheet resistance, limit transport due to their impact on charge carrier depletion and scattering. Our model suggests that when transport in a 2D material network is limited by the near-edge resistance, the optimum nanosheet thickness is dictated by a trade-off between charged impurity screening and gate screening, and the film mobilities are more sensitive to variations in printed nanosheet density. Removal of edge states can enable the realization of higher mobilities with thinner nanosheets due to reduced junction resistances and reduced gate screening. Our analysis of the influence of nanosheet edges on the effective film mobility not only examines the prospects of extant exfoliation methods to achieve the optimum microstructure but also provides important perspectives on processes that are essential to maximizing printed film performance.

Carrier Depletion High-Speed Tunable Dual-Mode Converter for Mode-Division Multiplexing Networks

Mode-division multiplexing (MDM) is a promising technique to further enhance the channel capacity in modern optical communications. Tunability of the mode converter in MDM systems is very important for flexible network deployment. In this letter, we demonstrate a high-speed tunable dual-mode converter on silicon-on-insulator (SOI) platform for MDM networks. The proposed device is based on carrier depletion mechanism, which enables ultrafast conversion between TE0 and TE1 mode on the order of sub-nanoseconds. The experimental results show that the insertion losses are less than 4.6 dB and the crosstalk values are lower than −19 dB in the wavelength range from 1525 nm to 1565 nm. To verify the function of high-speed mode transition, 10 Gb/s on-off key optical payload signal is performed to control the dual-mode converter. The performance indicates this device has great potential to be a pivotal component in high-speed MDM networks.

Targeted Suppression of Peptide Degradation in Ag-Based Surface-Enhanced Raman Spectra by Depletion of Hot Carriers.

Sample degradation, in particular of biomolecules, frequently occurs in surface-enhanced Raman spectroscopy (SERS) utilizing supported silver SERS substrates. Currently, thermal and/or photocatalytic effects are considered to cause sample degradation. This paper establishes the efficient inhibition of sample degradation using iodide which is demonstrated by a systematic SERS study of a small peptide in aqueous solution. Remarkably, a distinct charge separation-induced surface potential difference is observed for SERS substrates under laser irradiation using Kelvin probe force microscopy. This directly unveils the photocatalytic effect of Ag-SERS substrates. Based on the presented results, it is proposed that plasmonic photocatalysis dominates sample degradation in SERS experiments and the suppression of typical SERS sample degradation by iodide is discussed by means of the energy levels of the substrate under mild irradiation conditions. This approach paves the way toward more reliable and reproducible SERS studies of biomolecules under physiological conditions.

Inverse Design of Silicon Photonic Modulators

A novel shape optimization procedure is formulated for obtaining a higher phase change while maintaining high transmission in phase modulators. The formulation is implemented as a shape optimization tool. Subwavelength grating-based Si carrier depletion and carrier injection modulators are considered for testing the optimizer. Carrier transport simulations were performed to extract the carrier concentration profiles and were used in the optimizer. The optimized structures are then validated using simulation for transmission and phase change. The results show that the optimization tool developed can optimize both carrier injection and depletion type modulators, in terms of both transmission and phase modulation. The optimized structures can be placed in a ring or Mach-Zehnder modulator to make compact modulators with low drive voltages.

Ion-gating analysis on conduction mechanisms in oxide semiconductors

• Self-gated field-effect by ion adsorption at the solid–liquid interface (ion-gating). • Resistive factors of indium tin oxide controlled depending on sputtering conditions. • A fundamental correlation between the ion-gating effect and the resistive factors. • Ion-gating analysis on the resistance-dependent behavior of electrons in the electrode. • Identification of conduction mechanisms in amorphous oxide semiconductors. The electric field-effect is one of the common methods to tune the conductivity of materials in semiconductor devices because it can control the accumulation or depletion of charge carriers. However, to efficaciously dominate the electrical properties of the semiconductor in such a way, the application of an external voltage through the gate is an inevitable condition. Here, we propose an ion-dynamics-driven (ionovoltaic) transducer focusing on a self-gated field-effect by ion adsorption at the solid–liquid interface (an ion-gating effect without applied electric field). Considering an effective resistance derived from the generated signal, we establish an equivalent circuit of the ion-gating field-effect transducer to specifically analyze the conduction behavior of traveling electrons in an amorphous indium tin oxide film. As a result, using water droplet flow, we provide a convenient method to identify inherent conduction mechanisms of amorphous oxide semiconductors. This analysis has great potential as a tool to leap forward into a new interdisciplinary approach covering interface science and solid-state physics from a peculiar concept based on the ion-gating effect.

Computational study of wavelength conversion based on XGM by photonic crystal semiconductor optical amplifier

In this paper, a numerical study of wavelength conversion based on cross-gain modulation (XGM) has been conducted for the first time using the photonic crystal semiconductor optical amplifier (PC-SOA). A PC-SOA and an optical filter have been used in this design. Nonlinear effects, such as carrier depletion (CD), carrier heating (CH), and spectral hole burning (SHB), have been included in the PC-SOA. The rate and propagation equations have been solved using the finite difference method (FDM). The optimal wavelength conversion based on XGM has been obtained in the PC-SOA through the application of 3.2fJ and 1.0pJ energies for the input and the continuous wave (CW) signals, respectively, and a 15mA injection current for the biasing. The effects of the injection current and input signal energies on wavelength conversion have also been investigated for further examination of the impact of the XGM mechanism on wavelength conversion. It has finally been demonstrated that the PC-SOA exhibits better conditions and performance than the conventional bulk SOA in wavelength conversion based on XGM. Furthermore, the PC-SOA provides a far more appropriate alternative than the bulk SOA for all-optical integrated circuits due to the smaller length of the device and the lower value of injected current.

Photoluminescence Enhancement by Band Alignment Engineering in MoS2/FePS3 van der Waals Heterostructures.

Single-layer semiconducting transition metal dichalcogenides (2H-TMDs) display robust excitonic photoluminescence emission, which can be improved by controlled changes to the environment and the chemical potential of the material. However, a drastic emission quench has been generally observed when TMDs are stacked in van der Waals heterostructures, which often favor the nonradiative recombination of photocarriers. Herein, we achieve an enhancement of the photoluminescence of single-layer MoS2 on top of van der Waals FePS3. The optimal energy band alignment of this heterostructure preserves light emission of MoS2 against nonradiative interlayer recombination processes and favors the charge transfer from MoS2, an n-type semiconductor, to FePS3, a p-type narrow-gap semiconductor. The strong depletion of carriers in the MoS2 layer is evidenced by a dramatic increase in the spectral weight of neutral excitons, which is strongly modulated by the thickness of the FePS3 underneath, leading to the increase of photoluminescence intensity. The present results demonstrate the potential for the rational design of van der Waals heterostructures with advanced optoelectronic properties.

New n-p Junction Floating Gate to Enhance the Operation Performance of a Semiconductor Memory Device

To lower the charge leakage of a floating gate device and improve the operation performance of memory devices toward a smaller structure size and a higher component capability, two new types of floating gates composed of pn-type polysilicon or np-type polysilicon were developed in this study. Their microstructure and elemental compositions were investigated, and the sheet resistance, threshold voltages and erasing voltages were measured. The experimental results and charge simulation indicated that, by forming an n-p junction in the floating gate, the sheet resistance was increased, and the charge leakage was reduced because of the formation of a carrier depletion zone at the junction interface serving as an intrinsic potential barrier. Additionally, the threshold voltage and erasing voltage of the np-type floating gate were elevated, suggesting that the performance of the floating gate in the operation of memory devices can be effectively improved without the application of new materials or changes to the physical structure.

Thermal synthesis of Ga2O3/SnO2 core–shell nanowires and their structural characterization

Ga2O3 core–SnO2 shell nanowires were synthesized by thermal evaporation of the metallic sources (Ga, Sn) in a horizontal quartz tube furnace at atmospheric pressure and in an oxidizing atmosphere. Au thin layers and 200 ppm Au colloid were applied as growth catalyst of Ga2O3 nanowires in vapor–liquid–solid method. Then the SnO2 shells were grown on the Ga2O3 nanowires cores. The configuration with highly sensitive shell and a relatively low conductivity core should enhance the efficiency of sensors based on these materials. Such narrowing of conductive area where a depletion of the charge carriers may occur, should increase the sensitivity, response and recovery time. The structural and material composition characterization of nanowires was performed by X-ray diffraction, energy-dispersive X-ray and Raman spectroscopy. The lattice parameters and strain of SnO2 shell were calculated. Using the catalyst in the form of a 200 ppm Au colloid compared to Au thin film, Ga2O3/SnO2 nanowires more similar to the core–shell structures were obtained.

Design of a High-Bandwidth Traveling-Wave III–V/Si Hybrid MOS Optical Modulator

The III–V/Si hybrid Metal-Oxide-Semiconductor (MOS) optical modulator is promising for high-efficiency, low-energy and high-speed optical modulation. As of now, all the III–V/Si hybrid MOS optical modulators demonstrated were driven by the RC-limited lumped electrodes. In order to break the RC limitation posed by the driving scheme, we designed a traveling-wave III–V/Si hybrid MOS optical modulator. Based on the equivalent circuit model developed, the traveling-wave electrode geometry, doping concentration and equivalent oxide thickness were optimized numerically to achieve the velocity and impedance matching for high modulation bandwidth with a low <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V_{pp}$</tex-math></inline-formula> of 2 V. High modulation bandwidths of 75 GHz and 102 GHz were predicted for carrier-accumulation mode and carrier depletion mode, respectively. The expected high modulation bandwidth makes the III–V/Si hybrid MOS optical modulator more attractive in the high-data-rate optical transmission.

Effect of defect-rich Co-CeOx OER cocatalyst on the photocarrier dynamics and electronic structure of Sb-doped TiO2 nanorods photoanode

This work demonstrates the in-depth mechanism of enhanced photoelectrochemical (PEC) water oxidation of Sb-doped rutile TiO2 nanorods (NRs) photoanode coupled with oxygen vacancy defect-rich Co-doped CeOx (Co-CeOx) oxygen evolution reaction (OER) cocatalyst. The defect-rich Co-CeOx cocatalyst modification improves the conductivity, light absorption, charge transfer efficiency, and surface photovoltage generation of the Co-CeOx/Sb-TiO2 hybrid NRs photoanode. The Co-CeOx cocatalyst also serves as the surface passivating overlayer for the Sb-TiO2 photoanode, which suppresses the surface states mediated recombination of electron-hole pairs in the NRs. The PEC studies further indicate that Co-CeOx cocatalyst induces remarkably large band bending at the semiconductor/electrolyte interface and shortens the carrier diffusion length and depletion layer width, facilitating the rapid separation and transportation of the photocarriers for the surface PEC reactions. The experimental and theoretical studies confirm that the Co-doping in CeOx cocatalyst enhances the surface oxygen vacancy defects, which provides active catalytic sites for OH− adsorption and charge transportation for enhanced OER kinetics. The density functional theory (DFT) calculations demonstrate a higher conductivity of the Co-CeOx cocatalyst, advantageous for rapid charge transfer capability during PEC reactions. The synergy between all these merits helps the optimized Co-CeOx/Sb-TiO2 photoanode to deliver a maximum photocurrent density of 1.41 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (VRHE) and an ultra-low turn on potential (Von) of 0.1 VRHE under AM 1.5G solar illumination compared to the Sb-TiO2 NRs (0.96 mA cm−2 at 1.23 VRHE and Von = 0.42 VRHE). This work demonstrates the design of an efficient defect-rich cocatalyst modified photoanode for solar energy harvesting.

Intrinsic Polarization Super Junctions: Design of Single and Multichannel GaN Structures

Super junctions (SJs) have enabled unprecedented performance in Silicon power devices, which could be further improved by applying this concept to wide bandgap semiconductors like gallium nitride (GaN). Currently, polarization super junctions (PSJs) are the most promising candidates for GaN SJs. Yet, until now, p-type doping of the GaN cap layer was required to ensure the presence of a 2-D hole gas (2DHG), and the proper charge matching between the 2DHG and 2-D electron gas (2DEG), which is fundamental to the operation of SJs. This approach, however, requires precise control of the p-GaN doping level, which is very challenging and has hindered the demonstration of high-performance PSJs. Besides, while PSJs are particularly promising for multichannel structures aimed at reducing the sheet resistance, achieving proper charge matching combined with large electron density for all the buried channels is challenging. Here, we propose a simple and robust platform for intrinsic PSJs (i-PSJs) that enables excellent charge matching for a wide range of structures, without relying on doping. We show that surface donor states are the origin of charge mismatch and provide a strategy to minimize their impact. Simulated devices based on this structure show optimal carrier depletion with a flat electric field profile in the whole drift region. Finally, we extend this concept to multichannel i-PSJ structures. We demonstrate a much-reduced sheet resistance down to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$58 \Omega $ </tex-math></inline-formula> /sq and present a robust strategy to achieve charge balance, which enables reducing the ON-resistance without degrading the OFF-state performance, thus greatly improving the device figure-of-merit.

The Origin of Low Contact Resistance in Monolayer Organic Field‐Effect Transistors with van der Waals Electrodes

The successful commercialization of organic field‐effect transistors (OFETs) for advanced integrated organic electronics requires reducing device sizes, which inevitably clashes with the constraints imposed by the contact effects. Herein, it is demonstrated that the contact resistance in OFETs based on monolayer organic semiconductors is extremely low, especially at mild biasing conditions. The contributions of the access resistance and the metal–organic interface resistance are successfully disentangled for the first time. It is shown that, contrary to the conventional view, the contact resistance of monolayer OFETs in the saturation regime exhibits a very weak dependence on the source electrode length. In the monolayer OFETs based on 2,9‐didecyldinaphtho[2,3‐b:2’,3’‐f]thieno[3,2‐b]thiophene (C10‐DNTT), a gate‐voltage‐independent access resistivity (2.2 × 10−2 Ω cm2) at VDS = −1 mV is obtained, while the interfacial metal–organic Schottky contact resistance is found to be negligible. The depletion of a diode associated with the metal–organic interface expands with increasing VDS and eventually bottlenecks the device performance. Finally, how to overcome such a carrier depletion contact resistance bottleneck and achieve OFETs with outstanding performance are shown. These findings pave the way toward sophisticated organic electronic applications based on the use of monolayer OFETs.

Hot carrier dynamics in MoS2/WS2 heterostructure

TMDs based heterostructure have drawn much attention for its potential application in photoelectric devices benefiting from the rapid and effective carrier separation and ultra-long interlayer exciton lifetime. Recent studies on carrier dynamics of TMDs based heterostructures are mainly focused on the transfer process of photo-generated carriers across the interface and lifetime of interlayer exciton but little attention is paid on the dynamics of hot carriers. Here, the carrier dynamics of hot carriers in MoS2/WS2 heterostructure is investigated by transient absorption spectra. Rapid separation of electron and hole is observed. More importantly, hot carriers of C exciton, which contribute to the absorption of most of the visible light, could compensate for the carrier loss in the band edge exciton energy band through the intervalley transfer process. This re-injection process of hot carriers of C exciton could compensate for carrier depletion in photoelectric devices, thus may greatly improve the light utilization in optoelectronic devices.

High RF Performance GaN-on-Si HEMTs With Passivation Implanted Termination

This work reports recent progress in the sub-6 GHz power performance of GaN-based HEMTs grown on high resistivity silicon substrates with passivation implanted termination (PIT) process. Thanks to the mitigated electric field crowding at the gate edge and the suppressed negative fixed charge-induced carrier depletion, the fabricated HEMTs demonstrate a low leakage current, a high ON/OFF current ratio of 10⁸, and improved breakdown voltage associated with a current collapse at 40 V drain quiescent condition of as low as 5.6&#x0025;. S-band continuous-wave large signal measurements yield a high power-added efficiency (PAE) of 69&#x0025;, a drain efficiency (DE) of 72&#x0025;, and an output power density (<inline-formula> <tex-math notation="LaTeX">${P}_{\text{out}}$ </tex-math></inline-formula>) of 7.2 W/mm at <inline-formula> <tex-math notation="LaTeX">${V}_{\text{DS}}=30$ </tex-math></inline-formula> V. Moreover, the transistor delivers a maximum <inline-formula> <tex-math notation="LaTeX">${P}_{\text{out}}$ </tex-math></inline-formula> up to 10.2 W/mm with a peak PAE of 63.8&#x0025; at <inline-formula> <tex-math notation="LaTeX">${V}_{\text{DS}}=40$ </tex-math></inline-formula> V. The PAE and <inline-formula> <tex-math notation="LaTeX">${P}_{\text{out}}$ </tex-math></inline-formula> as a function of drain bias indicate that the transistors remain constant high PAE and linearly increased <inline-formula> <tex-math notation="LaTeX">${P}_{\text{out}}$ </tex-math></inline-formula> over a wide range of drain voltage variation. These excellent results have demonstrated the PIT process could be an attractive technique to facilitate the application of high-performance and cost-competitive GaN-on-Si HEMTs for 5G wireless base stations.