Reduction Of Barrier Height Research Articles

Analytical study of Metal-Insulator-Semiconductor contacts for both p- and n-InGaN

This paper has studied the metal–insulator-semiconductor (MIS) contact for both p- and n-InGaN. We found that the insulator layer thickness has a remarkable effect on the Fermi level pinning and barrier height reduction in MIS contacts. Schottky's formation with doped InGaN is explained by extending the MIS contacts' using the metal-induced gap states (MIGS) model. The J-V characteristics clarify the current transport mechanism through the MIS contact with InGaN semiconductor for different temperatures. We observed less control on the barrier height reduction in the case of n-InGaN through changing the thickness of the interfacial layer. The calculated contact resistivities of MIS contact with p- and n-InGaN show better results than the metal–semiconductor contact counterparts. These findings are highly significant to design and fabricate the InGaN based switching devices for converter applications.

New nano-composite based on carbon dots (CDots) decorated magnesium oxide (MgO) nano-particles (CDots@MgO) sensor for high H2S gas sensitivity performance

Schottky device based on carbon dots (CDots) decorated magnesium oxide (MgO) nano-particles (CDots@MgO) has been engineered for H2S gas sensing applications. TEM microscope proves the decoration of CDots on the surface of MgO nano-particles. The sensor device has been tested for various reducing gases. However, CDots@MgO device displays high response against H2S gas, as a resultof reducing the barrier height between CDots@MgO crystals. I–V behavior of the engineered sensor device is examined under both open air and H2S gas environments. The parameters (series resistance Rs, effective barrier height ϕB, and ideality factor n) of the schottky diode are determined for the CDots@MgO and MgO based sensors. Interestingly, under the exposure of 120 ppm of H2S, CDots@MgO sensor has shown response current value 11 times higher than MgO sensor at external biasing voltage = −0.7 V. The reduction of barrier height can be observed with increasing the (Tapplied) up to 200 °C and then increases. The obtained results prove that both device response and sensitivity are strongly influenced by the change in the barrier height. Finally, this effort introduces an invaluable methodology aiming at sensing the harmful gases by engineering a new gas sensor based on Schottky device of metal oxides decorated with CDots for high response, low power-consumption, high repeatability and high stability.

How Oxygen Exposure Improves the Back Contact and Performance of Antimony Selenide Solar Cells.

The improvement of antimony selenide solar cells by short-term air exposure is explained using complementary cell and material studies. We demonstrate that exposure to air yields a relative efficiency improvement of n-type Sb2Se3 solar cells of ca. 10% by oxidation of the back surface and a reduction in the back contact barrier height (measured by J-V-T) from 320 to 280 meV. X-ray photoelectron spectroscopy (XPS) measurements of the back surface reveal that during 5 days in air, Sb2O3 content at the sample surface increased by 27%, leaving a more Se-rich Sb2Se3 film along with a 4% increase in elemental Se. Conversely, exposure to 5 days of vacuum resulted in a loss of Se from the Sb2Se3 film, which increased the back contact barrier height to 370 meV. Inclusion of a thermally evaporated thin film of Sb2O3 and Se at the back of the Sb2Se3 absorber achieved a peak solar cell efficiency of 5.87%. These results demonstrate the importance of a Se-rich back surface for high-efficiency devices and the positive effects of an ultrathin antimony oxide layer. This study reveals a possible role of back contact etching in exposing a beneficial back surface and provides a route to increasing device efficiency.

Fabrication and Characterization of Oxygenated AlN/4H-SiC Heterojunction Diodes.

The effects of rapid thermal annealing (RTA) on Schottky barrier diodes (SBDs) made from oxygenated aluminum nitride (AlN) thin films deposited on a silicon carbide (SiC) substrate using radio frequency sputtering were investigated. The annealed SBD devices exhibited a 10x increase in the on/off current ratio vs. non-annealed devices for measurement temperatures ranging from 300 K to 450 K. The ideality factor, derived from the current density–voltage (J-V) characterization, increased by a factor of ~2.2 after annealing, whereas the barrier height decreased from ~0.91 to ~0.68 eV. Additionally, Auger electron spectroscopy indicated decreased concentrations of atomic oxygen in the AlN thin film, from ~36% before, to ~24% after annealing. This may have contributed to the reduced barrier height and improved on/off ratio in the annealed AlN/SiC diodes.

Four-step heating process for solid-phase crystallization of Ge leading to high carrier mobility

Solid-phase crystallization is rapidly developing as a method for obtaining high-quality polycrystalline Ge layers. We propose a four-step heating process, focusing on post-deposition annealing (PDA), which is performed between the film deposition and crystallization. PDA at an appropriate temperature (500 °C) reduces the grain size while improving the crystallinity of Ge (150 nm thickness), which leads to the reduction of the grain boundary barrier height (5 meV). The resulting hole mobility (530 cm2 V−1 s−1) is higher than that of any other semiconductor thin films (<300 nm) and even single-crystal Si wafers. These findings will further improve Ge thin-film transistors for advanced devices.

Modeling of Short-Channel Effects in GaN HEMTs

In this article, we propose an explicit and analytic charge-based model for estimating short-channel effects (SCEs) in GaN high-electron-mobility transistor (HEMT) devices. The proposed model is derived from the physical charge-based core of the Ecole Polytechnique Federale de Lausanne (EPFL) HEMT model, which treats HEMT as a generalized MOSFET. The main emphasis of this article is to estimate SCEs by effectively capturing 2-D channel potential distribution to calculate the reduced barrier height, drain-induced barrier lowering (DIBL), velocity saturation, and channel length modulation (CLM). The model is validated with TCAD simulation results and agreed with measurement data in all regions of operation. This represents the main step toward the design of high-frequency and ultralow-noise HEMT devices using AlGaN/GaN heterostructures.

Terahertz Transmission Limiters Using Randomly Oriented Carbon Nanotubes Network Near Percolation Threshold

We demonstrated a dramatic reduction in the terahertz (THz) transmittance and enhancement in the conductance of the carbon nanotube (CNT) random network system at high radiation intensities near the critical density limit. Amplification of the mesoscopic tunneling and rectification mechanism at the junctions between the adjacent nanotubes into the network induces the macroscopic phenomenon–optical limiting effect. In the percolative networks, the film conductance is dominantly determined by the electron tunneling at the CNT junctions, and the symmetry of the junctions breaks down by the randomness and defects in the network structure. This junction asymmetry enables the optical rectification and reduction of the tunneling barrier height when THz light impinges on the CNT network, and in turn enhancement in the CNT’s and their junctions’ conductance. Consequently, as the radiation intensity increases, the network becomes more conductive and less transparent.

Specific Contact Resistivity Improvement by As Preamorphization Implantation for Ti-Based Ohmic Contacts on n+-Si

As preamorphization implant (PAI) for Ti-based ohmic contacts on n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -Si is explored in this article. With As implantation at 3 keV/5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , the specific contact resistivity (ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of TiSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> on 1.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> P-doped (Nd) n+-Si is significantly reduced by ~33.8%, down to ~2.33 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> -cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . This ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> improvement is a result of manipulation between the reduction of effective Schottky barrier height to electrons (φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bn,eff</sub> ) and the retardation of Ti silicidation, which are both strongly correlated with As segregation. In comparison with the widely adopted Ge PAI, As PAI shows superior performance for nMOS contact engineering, featuring with lower ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bn,eff</sub> as well as better thermal stability.

Vertical P-TFET With a P-Type SiGe Pocket

Among all material systems currently being exploited for tunnel field-effect transistor (TFET) applications, silicon germanium (SiGe) is most likely to be adopted for the future low power very-large-scale integration (VLSI) technologies due to its VLSI-compatibility, mature synthesis techniques, and tunable bandgap. This article demonstrates experimentally the enhancement of the drive current ( ${I}_{ \mathrm{ON}}$ ) and the reduction in the subthreshold swing (SS) of vertical Si p-TFET by inserting a thin layer of boron-doped Si0.8Ge0.2 ( ${E}_{\text {G}}\sim {1}$ eV) between the source and the channel. Owing to the reduction in the tunneling barrier height and the increased lateral electric field, carrier transport across the tunneling junction is significantly enhanced. By improving the material quality, further enhancement in the device performance is observed. Simulation suggests that the proposed structure serves as an effective method to boost the device performance of TFETs.

Increased radiative recombination of AlGaN-based deep ultraviolet laser diodes with convex quantum wells

An AlGaN-based deep ultraviolet laser diode with convex quantum wells structure is proposed. The advantage of using a convex quantum wells structure is that the radiation recombination is significantly improved. The improvement is attributed to the increase of the effective barrier height for electrons and the reduction of the effective barrier height for holes, which results in an increased hole injection efficiency and a decreased electron leakage into the p-type region. Particularly, comparisons with the convex quantum barriers structure and the reference structure show that the convex quantum wells structure has the best performance in all respects.

Ideality factor and barrier height for a GaN nanomembrane electrically contacted with a tungsten nano-tip in a TEM

In the present article, the electrical characteristics of a freestanding gallium nitride nanomembrane in contact with a tungsten nanoprobe are evaluated using scanning tunneling microscopy in an aberration-corrected transmission electron microscope without any lithographic patterning. We report here barrier height (ΦB=0.33±0.05eV and ideality factor (ηW/GaN−NM=1.620±0.07) parameters as extracted from I–V characteristic curve. Our experimental findings, combined with analytical calculations, show that the use of nanosized edge contacts results in a reduced barrier height, which is very promising for achieving a high ‘on’ current, large photoresponse, and high-frequency operation in FET devices.

Performance deterioration of GaN-based laser diode by V-pits in the upper waveguide layer

Abstract The effect of V-pits in the upper waveguide (UWG) on device performance of GaN-based laser diodes (LDs) has been studied. Experimental results demonstrate that in comparison with the LDs with u-In0.017Ga0.983N/u-GaN multiple UWG or u-In0.017Ga0.983N one, the LDs with a single u-GaN UWG has the best device performance. They have a smaller threshold current density, and a larger and more stable output optical power. The lowest threshold current density is as low as 1.3 kA/cm2, and the optical power reaches to 2.77 W. Furthermore, atomic force microscopy suggests that the deterioration of device performance of former kinds of devices may be attributed to the increase of V-pits’ size and quantity in the undoped-In0.017Ga0.983N UWG layer, and these V-pits could introduce more nonradiative recombination centers and exacerbate the inhomogeneity of injection current. Moreover, theoretical calculation results indicate that the increase of leakage current and optical loss are additional reasons for the device performance deterioration, which may be caused by a reduction of the potential barrier height for electrons in the quantum wells and by an increased background electron concentration in UWG.

The improvement of Mo/4H-SiC Schottky diodes via a P2O5 surface passivation treatment

Molybdenum (Mo)/4H-silicon carbide (SiC) Schottky barrier diodes have been fabricated with a phosphorus pentoxide (P2O5) surface passivation treatment performed on the SiC surface prior to metallization. Compared to the untreated diodes, the P2O5-treated diodes were found to have a lower Schottky barrier height by 0.11 eV and a lower leakage current by two to three orders of magnitude. Physical characterization of the P2O5-treated Mo/SiC interfaces revealed that there are two primary causes for the improvement in electrical performance. First, transmission electron microscopy imaging showed that nanopits filled with silicon dioxide had formed at the surface after the P2O5 treatment that terminates potential leakage paths. Second, secondary ion mass spectroscopy revealed a high concentration of phosphorus atoms near the interface. While only a fraction of these are active, a small increase in doping at the interface is responsible for the reduction in barrier height. Comparisons were made between the P2O5 pretreatment and oxygen (O2) and nitrous oxide (N2O) pretreatments that do not form the same nanopits and do not reduce leakage current. X-ray photoelectron spectroscopy shows that SiC beneath the deposited P2O5 oxide retains a Si-rich interface unlike the N2O and O2 treatments that consume SiC and trap carbon at the interface. Finally, after annealing, the Mo/SiC interface forms almost no silicide, leaving the enhancement to the subsurface in place, explaining why the P2O5 treatment has had no effect on nickel- or titanium-SiC contacts.

Enhancement of UV Photodetection Properties of Hierarchical Core-Shell Heterostructures of a Natural Sericin Biopolymer with the Addition of ZnO Fabricated on Ultra-Nanocrystalline Diamond Layers.

A novel self-assembled hierarchical heterostructure is derived from cocoon-derived sericin biopolymer (CSP) biowaste with ZnO deposited on ultra-nanocrystalline diamond (UNCD) substrates using a scalable chemical deposition technique. Then, high-performance long-life UV photodetectors are fabricated using this hybrid sericin, diamond, and ZnO (SDZ) nanostructure. The microstructural analysis reveals a several nanometer-thick CSP shell coated with a highly uniform ZnO nanorod (ZNR) array grown on the UNCD substrate. The CSP shell also contains columnar nanograins on top of the ZNR as well as vertical sidewalls with unique alignments. The hierarchical core-shell SDZ heterostructures reveal superior UV diode performance, with an ultrahigh UV switching ratio of 1.1 × 105 at 5 V, an increase of up to 49 900% greater than that of as-grown ZNRs (220). High UV responsivity is observed around 3.6 A W-1 under 365 nm UV light illumination. The perfect distribution of the sericin in the ZNRs on the UNCD substrates resulted in the ultrafast electron-hole recombination. The sericin dopants and the UNCD interlayer enabled the device to reach new energy levels in the conduction band, with the reduced barrier height allowing for improved charge carrier transportation during UV light illumination. It is believed that the sericin dopants and the UNCD layer increased the UV adsorptivity and the amount of conducting carbon dopants within the ZNRs was sufficient for s0tability. These noteworthy features make the SDZ heterostructures promising candidates for the fabrication of cost-efficient biopolymers and UNCD hybrid-based UV photodetectors.

Microstructure and Electrical Properties of Low-Voltage Barium Titanate Doped Zinc Oxide Varistor Ceramics

In the present, varistor ceramics through the combination of zinc oxide (ZnO) with a perovskite material have become widespread because of their unique properties for a wide range of applications in electronic protection devices. Low-voltage zinc oxide (ZnO) varistors with fast response and highly nonlinear electrical properties for overvoltage protection in an integrated circuit are increasingly significant in the application of low-voltage electronics. The present study highlights the interaction between barium titanate (BaTiO3) and ZnO varistors through the employment of solid-state reaction method in the production of low-voltage varistors. The effects of BaTiO3 on the microstructure of ZnO varistors were analyzed through scanning electron microscopy (SEM), energy dispersive X-ray analysis spectroscopy (EDS) and X-ray diffraction (XRD). The EDS analysis and XRD measurements suggest the presence of ZnO and BaTiO3 phases. The electrical properties of BaTiO3-doped ZnO varistors were examined based on the current density-electric field (J-E) characteristics measurement. The varistor properties showed the nonlinear coefficient (α) from 1.8 to 4.8 with the barrier height (φB) ranged from 0.70 to 0.88 eV. The used of BaTiO3 additive in ZnO varistors produced varistor voltages of 4.7 to 14.1 V/mm with the voltage per grain boundary (Vgb) was measured in the ranges 0.03 to 0.05 V. The lowest leakage current density was 348 μA/cm2, obtained at the samples containing 12 wt.% BaTiO3 with high barrier height. The reduction in barrier height with increasing BaTiO3 content was associated with the excessive amount of BaTiO3 phase, hence cause the deterioration of active grain boundary due to the variation of oxygen (O) vacancies in the grain boundary.

The effect of chemical disorder on defect formation and migration in disordered max phases

MAX phases have attracted increased attention due to their unique combination of ceramic and metallic properties. Point-defects are known to play a vital role in the structural, electronic and transport properties of alloys in general and this system in particular. As some MAX phases have been shown to be stable in non-stoichiometric compositions, it is likely that such alloying effects will affect the behavior of lattice point defects. This problem, however, remains relatively unexplored. In this work, we investigate the alloying effects on the structural-stability, energy-stability, electronic-structure, and diffusion barrier for point defects in MAX phases within a first-principles density functional theory framework. The vacancy (VM, VA, VX) and antisite (M-A; M-X) defects are considered with M and A site disorder in (Zr-M)2(AA’)C, where M=Cr,Nb,Ti and AA’=Al, Al-Sn, Pb-Bi. Our calculations suggest that the chemical disorder helps lower the VA formation energies compared to VM and VX. The VA diffusion barrier is also significantly reduced for M-site disorder compared to their ordered counterpart. This is a very important finding because the reduced barrier height will ease the Al diffusion at high-operating temperatures, which will help the formation of passivating oxide layer (i.e., Al2O3 in aluminum-based MAX phases) and will slow down or stop the material degradation. We believe that our study provides a fundamental understanding and an approach to tailor the key properties that can lead to the discovery of new MAX phases.

Broad spectral responsivity in highly photoconductive InZnO/MoS2 heterojunction phototransistor with ultrathin transparent metal electrode

An amorphous InZnO/MoS2 heterojunction-based phototransistor with excellent photoconductive gain and responsivity over the entire visible range has been demonstrated. The photogenerated current of the InZnO phototransistor at long light wavelength (>600 nm) was significantly improved by utilizing narrow bandgap MoS2 as the capping layer (1.3 eV). At lower wavelength, photocarriers are generated due to the optical absorption of both InZnO and MoS2 layers, whereas the latter ensures significant photocarrier generation even at the higher wavelength region of the visible spectrum. The photogenerated carriers subsequently transfer to the underlying InZnO layer of superior carrier mobility that has a high channel conduction of additional electrons from the optically-induced doubly positively charged oxygen vacancies (Vo++) where the gate field is screening, thereby leading to the higher photoconductive gain of the InZnO/MoS2 phototransistors. The dynamic photosensitivity behaviour of the aforesaid phototransistor reveals the presence of persistent photoconductivity (PPC) due to the oxygen vacancy associated with InZnO which can be removed by applying a reset gate pulse from −15 to +5 V. The optical properties of these phototransistors were further enhanced by replacing the opaque Ti/Au electrode by an ultrathin transparent Ti/Au electrode. Utilization of the transparent electrode results in enhanced electron injection from source to channel due to a reduced barrier height under illumination giving rise to a ten-fold improvement in the photocurrent and responsivity of the phototransistors. A position-dependent study of the photocurrent w.r.t beam position also reveals that the enhancement in photocurrent is strongly dependent on the position and is at its maximum when the beam is placed near the source region.

Improved optical performance of multi-layer MoS2 phototransistor with see-through metal electrode

In recent years, MoS2 has emerged as a prime material for photodetector as well as phototransistor applications. Usually, the higher density of state and relatively narrow bandgap of multi-layer MoS2 give it an edge over monolayer MoS2 for phototransistor applications. However, MoS2 demonstrates thickness-dependent energy bandgap properties, with multi-layer MoS2 having indirect bandgap characteristics and therefore possess inferior optical properties. Herein, we investigate the electrical as well as optical properties of single-layer and multi-layer MoS2-based phototransistors and demonstrate improved optical properties of multi-layer MoS2 phototransistor through the use of see-through metal electrode instead of the traditional global bottom gate or patterned local bottom gate structures. The see-through metal electrode utilized in this study shows transmittance of more than 70% under 532 nm visible light, thereby allowing the incident light to reach the entire active area below the source and drain electrodes. The effect of contact electrodes on the MoS2 phototransistors was investigated further by comparing the proposed electrode with conventional opaque electrodes and transparent IZO electrodes. A position-dependent photocurrent measurement was also carried out by locally illuminating the MoS2 channel at different positions in order to gain better insight into the behavior of the photocurrent mechanism of the multi-layer MoS2 phototransistor with the transparent metal. It was observed that more electrons are injected from the source when the beam is placed on the source side due to the reduced barrier height, giving rise to a significant enhancement of the photocurrent.

Humidity effect on electrical properties of graphene oxide back-to-back Schottky diode

A Schottky diode-based sensor is a promising structure for high sensitive and low power sensor. This paper investigates a device called back-to-back Schottky diode (BBSD) for humidity sensing operation. The BBSD provides simpler device configuration that can be fabricated using less complicated process. The current-voltage characteristic of the fabricated BBSD was measured at different relative humidity. From the obtained characteristics, series resistance, barrier height and ideality factor was analyzed. The device current increased at higher humidity level. The current increase could be associated to the decrease in series resistance, barrier height and ideality factor. When humidity decreased from 11 % to 97%, the barrier height showed reduction of 0.1 eV. The barrier height reduction was explained by considering electric field-induced reduction of graphene oxide. The observed result confirmed the device feasibility as promising simple and low cost humidity sensor.

A Horizontal-Gate Monolayer MoS2 Transistor Based on Image Force Barrier Reduction.

Transition metal dichalcogenides (TMDCs) have received wide attention as a new generation of semiconductor materials. However, there are still many problems to be solved, such as low carrier mobility, contact characteristics between metal and two-dimensional materials, and complicated fabrication processes. In order to overcome these problems, a large amount of research has been carried out so that the performance of the device has been greatly improved. However, most of these studies are based on complicated fabrication processes which are not conducive to the improvement of integration. In view of this problem, a horizontal-gate monolayer MoS2 transistor based on image force barrier reduction is proposed, in which the gate is in the same plane as the source and drain and comparable to back-gated transistors on-off ratios up to 1 × 104 have been obtained. Subsequently, by combining the Y-Function method (YFM) and the proposed diode equivalent model, it is verified that Schottky barrier height reduction is the main reason giving rise to the observed source-drain current variations. The proposed structure of the device not only provides a new idea for the high integration of two-dimensional devices, but also provides some help for the study of contact characteristics between two-dimensional materials and metals.