Increase In Barrier Thickness Research Articles

2.5 A/mm/350 GHz aggressively scaled gate engineered Fe-doped AlN/GaN channel HEMT with graded InGaN Backbarrier on SiC-wafer for next generation RF power electronics applications

This study intends to investigate the effects of barrier scaling, a crucial RF GaN-HEMT design element, which is imperative to achieve enhanced performance at nano-scale dimensions. From the performance characterization of the designed 50 nm recessed gated Fe-doped AlN/GaN/SiC HEMT featuring graded InGaN-back barrier (BB), the highest transconductance of 464.7 mS/mm, a maximum fT of 351.5 GHz, peak ID of 1.596 A/mm, & high drain saturation current of 2.527 A/mm at VGS = 3 V is observed for a 6 nm thin AlN barrier due to better gate control, and aided quantum well confinement because of Fe-doped buffer & graded InGaN BB. We observed that as barrier thickness increases, threshold voltage (Vth) becomes more negative and performance is deteriorated due to poor aspect ratio. This work also examines the role of gate-engineering on the electrical properties of the designed HEMT by employing different metals. The TCAD analysis indicates that the Al-gate HEMT had better DC/RF performance because of its lower ϕm (work-function). The performance declines and Vth increases as the Schottky barrier raises due touplifting of the conduction band edge. Since, it is crucial to understand the contribution of Rs & Rd in RF design, so in addition, we have done simulations by scaling source-gate gap (LGS), & drain-gate gap (LGD). It was found that the HEMT delivered better performance at the smallest LGS & LGD due to minimal Rs & Rd. The incredible performance of the novel HEMT reflects enhanced electron transport properties and implies that it possesses the capability of being extremely desirable for future aviation, military, and space applications.

Effect of wells thicknesses disorder on quantum magneto-transport properties in GaAs/AlxGa1-xAs multi-quantum wells near wavelength infrared detectors

We investigate theoretically, the GaAs/Al0.18Ga0.82As multi-quantum wells electronic band-structures and quantum magneto-transport properties at 1.6 ​K, using the envelope function with the effective mass formalism. The effect of well and barrier thicknesses on the band-structures and bandgap were studied. The bandgap decreases when the well thickness increases whereas an increase of barrier thickness leads to narrowing of sub-band width. The carrier’s effective masses were calculated and the electronic transport dimensionality was determinates through the density of states. The inter-plateau widths of transitions in the quantum Hall effect increases with increasing inter-layer tunneling and well thickness disorder. Our results allowed easer interpretation of quantum Hall effect measurements in the literature and are necessary for the design and engineering of multiple quantum wells infrared detectors.

Spin dependent charge transfer in MoSe2/hBN/Ni hybrid structures

We present magneto-photoluminescence measurements in a hybrid two-dimensional semiconductor/ferromagnetic structure consisting of MoSe2/hBN/Ni. When the nickel layer is magnetized, we observe circularly polarized photoluminescence of the trion peak in the MoSe2 monolayer under linearly polarized excitation. This build-up of circular polarization can reach a measured value of about 4% when the magnetization of Ni is saturated perpendicularly to the sample plane and changes its sign when the magnetization is reversed. The circular polarization decreases when the hBN barrier thickness increases. These results are interpreted in terms of a spin-dependent charge transfer between the MoSe2 monolayer and the nickel film. The build-up of circular polarization is observed up to 120 K, mainly limited by the trion emission that vanishes with temperature.

High quality factor multichannel filter of electrons based on defective CdMnTe/CdTe multi-quantum wells

Using the transfer matrix (TM) method, the propagation of electrons is studied, in a one-dimensional multi-quantum well composed of alternating layers of barriers and wells. In the first time, we proposed system with a single barrier CdMnTe is embedded between two semi-infinite substrates. Hence, the corresponding Eigen modes energies were found to be shifted towards lower energies as the barrier thickness increases. The resulting Eigen modes are characterized by a high transmission rates but a very low quality factor values, which highlight the importance of the system periodicity. On the other hand, in order to create localized states in the band gaps, a defective system is exanimated. Indeed, a defect barrier is inserted in a perfect multi-quantum wells (MQWs) system, basically made of repeated CdMnTe barriers and CdTe wells. This defective system can create multi defect modes in the electronic band gaps. The analysis of the transmittance spectra of the defect states evidences a shift to lower energies by increasing the defect thickness with fixed parameters of the system. These defect states move toward higher energy regions, when the concentration of defect increases. In addition, the energy of the defect states varying from 600 meV to 785 meV. We found one defect state for a defect concentration equal to 0.7, when the defect thickness is equal to 5 A°. Otherwise, for a defect concentration equal to 0.1, with a defect thickness of 150 A°, we found two defect states appeared inside the gaps. While, three defect states were found for a concentration interval between 0.3 and 0.5 and a defect thickness equal respectively to 127 A° and 145 A°. These defect states have higher quality factors reaching 7630000. Moreover, it shown that the number of gaps modes depends on the defect concentration’s and thickness’ parameters. High quality factors are reached, and an electron multi-channel filtering system is then proposed.

Serratiopeptidase, A Serine Protease Anti-Inflammatory, Fibrinolytic, and Mucolytic Drug, Can Be a Useful Adjuvant for Management in COVID-19.

Serratiopeptidase, A Serine Protease Anti-Inflammatory, Fibrinolytic, and Mucolytic Drug, Can Be a Useful Adjuvant for Management in COVID-19.

Waste Foundry Sand as Permeable and Low Permeable Barrier for Restriction of the Propagation of Lead and Nickel Ions in Groundwater

This work aims to investigate the ability of using waste foundry sand (WFS) resulting as inexpensive by-product from steel industry in the low permeability barrier (LPB) and permeable reactive barrier (PRB) technologies for restriction of the movement of lead and nickel ions in the groundwater. Outputs of flask and tank tests certified that this material could capture these ions with sorption efficiency greater than 95% at time, pH, sorbent dosage, and speed equal to 60 min, 4 for lead and 6 for nickel, 2.5 g/100 mL, and 250 rpm, respectively. Sorption isotherm measurements were represented in a good manner by Langmuir model in comparison with Freundlich model with coefficient of determination (R2) greater than 0.99. So, the chemisorption was the predominant mechanism which could be supported by O-H, H-O-H, C-O, O-Si-O, and Si-O functional groups based on the Fourier transform infrared analysis. The maximum sorption capacity of WFS was 13.966 and 4.227 mg/g for lead and nickel ions, respectively, with corresponding affinities equal to 0.647 and 0.099 L/mg. Measurements signified that the hydraulic conductivity of WFS was 3.8 × 10−7 cm/s which satisfies the requirements of LPB. To obtain the acceptable values of permeability and reactivity, PRB was prepared from mixing 18% WFS with 82% filter sand. COMSOL software was able to simulate the measurements of two-dimensional tank packed with Iraqi soil aquifer in combination with WFS-LPB and WFS-filter sand PRB. Thicker barriers have a high ability in the protection of locations in the down-gradient side because their longevity increased dramatically with increase of barrier thickness.

Numerical Investigation on the Effects of Dielectric Barrier on a Nanosecond Pulsed Surface Dielectric Barrier Discharge.

In order to understand the impacts of dielectric barrier on the discharge characteristics of a nanosecond pulsed surface dielectric barrier discharge (NS-DBD), the effects of dielectric constant and dielectric barrier thickness are numerically investigated by using a three-equation drift–diffusion model with a 4-species 4-reaction air chemistry. When the dielectric constant increases, while the dielectric barrier thickness is fixed, the streamer propagation speed (V), the maximum streamer length (L), the discharge energy (), and the gas heating () of a pulse increase, but the plasma sheath thickness (h), the fast gas heating efficiency , and the charge densities on the wall surface decrease. When the dielectric barrier thickness increases, while the dielectric constant is fixed, V, L, , and of a pulse decrease, but h, , and the charge densities on the wall surface increase. It can be concluded that the increase of the dielectric constant or the decrease of the dielectric barrier thickness results in the increase of the capacitance of the dielectric barrier, which enhances the discharge intensity. Increasing the dielectric constant and thinning the dielectric barrier layer improve the performance of the NS-DBD actuators.

High Electrical Resistivity $1.55~\mu$ m Photoconductive Switch Using a Barrier Structure

We propose a low temperature grown Be-doped InGaAs vertically aligned barrier structure to increase the dark electrical resistivity of the 1.55 μm photoconductive switch. The insertion of a 100 nm-thick AlAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.56</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.44</sub> barrier layer with a large conduction band offset to In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As blocks the flow of the majority carrier. In this study, samples are grown using a molecular beam epitaxy system. I-V measurement shows that the effective dark electrical resistivity of the samples increases from 0.5Ωcm to 8.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> Ωcm. The increase of barrier thickness from 100 nm to 400 nm does not increase the dark electrical resistivity further. The photo-response of the samples is also measured and analyzed. This study demonstrates a new approach to achieve high electrical resistivity in a photoconductive switch.

Thermal conductance by Dirac fermions in a normal-insulator-superconductor junction of silicene

We theoretically study the properties of thermal conductance in a normal-insulator-superconductor junction of silicene for both thin and thick barrier limit. We show that while thermal conductance displays the conventional exponential dependence on temperature, it manifests a nontrivial oscillatory dependence on the strength of the barrier region. The tunability of the thermal conductance by an external electric field is also investigated. Moreover, we explore the effect of doping concentration on thermal conductance. In the thin barrier limit, the period of oscillations of the thermal conductance as a function of the barrier strength comes out be $\pi/2$ when doping concentration in the normal silicene region is small. On the other hand, the period gradually converts to $\pi$ with the enhancement of the doping concentration. Such change of periodicity of the thermal response with doping can be a possible probe to identify the crossover from specular to retro Andreev reflection in Dirac materials. In the thick barrier limit, thermal conductance exhibits oscillatory behavior as a function of barrier thickness $d$ and barrier height $V_0$ while the period of oscillation becomes $V_0$ dependent. However, amplitude of the oscillations, unlike in tunneling conductance, gradually decays with the increase of barrier thickness for arbitrary height $V_0$ in the highly doped regime. We discuss experimental relevance of our results.

Effect of Barrier Layer Thickness on Gate Leakage Current in AlGaN/GaN HEMTs

Gate leakage current mechanisms in Al0.28Ga0.72N/GaN high-electron mobility transistors with different barrier thicknesses are studied using temperature-dependent current–voltage characteristics. Poole–Frenkel emission and Fowler–Nordheim tunneling are observed in the reverse bias. Beyond the threshold voltage, increase in gate leakage current with the increase in barrier thickness is observed. Furthermore, the gate leakage current has negative temperature dependence over a wide range of temperatures especially for the devices with a thicker barrier. This has been qualitatively explained with the help of secondary electron–hole pair generation in the channel.

Impact of barrier thickness on gate capacitance—modeling and comparative analysis of GaN based MOSHEMTs

A mathematical model is developed predicting the behavior of gate capacitance with the nanoscale variation of barrier thickness in AlN/GaN MOSHEMT and its effect on gate capacitances of AlInN/GaN and AlGaN/GaN MOSHEMTs through TCAD simulations is compared analytically. AlN/GaN and AlInN/GaN MOSHEMTs have an advantage of a significant decrease in gate capacitance up to 108 fF/μm2 with an increase in barrier thickness up to 10 nm as compared to conventional AlGaN/GaN MOSHEMT. This decrease in gate capacitance leads to improved RF performance and hence reduced propagation delay.

Effects of inter-nanocrystal distance on luminescence quantum yield in ensembles of Si nanocrystals

The absolute photoluminescence (PL) quantum yield (QY) of multilayers of Silicon nanocrystals (SiNCs) separated by SiO2 barriers were thoroughly studied as function of the barrier thickness, excitation wavelength, and temperature. By mastering the plasma-enhanced chemical vapor deposition growth, we produce a series of samples with the same size-distribution of SiNCs but variable interlayer barrier distance. These samples enable us to clearly demonstrate that the increase of barrier thickness from ∼1 to larger than 2 nm induces doubling of the PL QY value, which corresponds to the change of number of close neighbors in the hcp structure. The temperature dependence of PL QY suggests that the PL QY changes are due to a thermally activated transport of excitation into non-radiative centers in dark NCs or in the matrix. We estimate that dark NCs represent about 68% of the ensemble of NCs. The PL QY excitation spectra show no significant changes upon changing the barrier thickness and no clear carrier multiplication effects. The dominant effect is the gradual decrease of the PL QY with increasing excitation photon energy.

Thickness dependent barrier performance of permeation barriers made from atomic layer deposited alumina for organic devices

Organic devices like organic light emitting diodes (OLEDs) or organic solar cells degrade fast when exposed to ambient air. Hence, thin-films acting as permeation barriers are needed for their protection. Atomic layer deposition (ALD) is known to be one of the best technologies to reach barriers with a low defect density at gentle process conditions. As well, ALD is reported to be one of the thinnest barrier layers, with a critical thickness – defining a continuous barrier film – as low as 5–10nm for ALD processed Al2O3. In this work, we investigate the barrier performance of Al2O3 films processed by ALD at 80°C with trimethylaluminum and ozone as precursors. The coverage of defects in such films is investigated on a 5nm thick Al2O3 film, i.e. below the critical thickness, on calcium using atomic force microscopy (AFM). We find for this sub-critical thickness regime that all spots giving raise to water ingress on the 20×20μm2 scan range are positioned on nearly flat surface sites without the presence of particles or large substrate features. Hence below the critical thickness, ALD leaves open or at least weakly covered spots even on feature-free surface sites. The thickness dependent performance of these barrier films is investigated for thicknesses ranging from 15 to 100nm, i.e. above the assumed critical film thickness of this system. To measure the barrier performance, electrical calcium corrosion tests are used in order to measure the water vapor transmission rate (WVTR), electrodeposition is used in order to decorate and count defects, and dark spot growth on OLEDs is used in order to confirm the results for real devices. For 15–25nm barrier thickness, we observe an exponential decrease in defect density with barrier thickness which explains the likewise observed exponential decrease in WVTR and OLED degradation rate. Above 25nm, a further increase in barrier thickness leads to a further exponential decrease in defect density, but an only sub-exponential decrease in WVTR and OLED degradation rate. In conclusion, the performance of the thin Al2O3 permeation barrier is dominated by its defect density. This defect density is reduced exponentially with increasing barrier thickness for alumina thicknesses of up to at least 25nm.

Lateral shifts and photon tunneling in a frustrated total internal reflection structure with a negative-zero-positive index metamaterial

Motivated by the realization of the Dirac point (DP) with a double-cone structure for optical field in the negative-zero-positive index metamaterial (NZPIM), the lateral shift and tunneling time of photon tunneling through a frustrated total internal reflection structure containing a NZPIM barrier are investigated by employing Artman's stationary phase method. Near the DP, the lateral shift can vary from positive to negative and the photon tunneling displays a superluminal dynamic. Because of the Hartman effect, both lateral shift and tunneling time tend to a saturation value when the barrier thickness increases. These results may lead to potential applications in integral optics and optical-based devices and also suggest analogous phenomena of valance electron in single-layered carbon graphene.

Influence of AlGaN barrier thickness on electrical and device properties in Al0.14Ga0.86N/GaN high electron mobility transistor structures

We investigate the influence of the AlGaN barrier thickness in Al0.14Ga0.86N/GaN heterostructures on both the electrical properties of the heterostructure itself as well as on high electron mobility transistors fabricated on these structures. With increasing barrier thickness, we observe decreasing sheet resistances, transconductances, and threshold voltages. The observed changes are well-described by modelling. We demonstrate that an increase in the barrier thickness in AlGaN/GaN heterostructures results in an increase in available high-frequency input power swing before turn-on of the Schottky gate. The device long-term stability under direct current stress is not affected by the increase in barrier thickness as shown by on-wafer reliability tests at 150 °C base plate temperature. These results pave the way towards AlGaN/GaN transistors offering high robustness under extreme mismatch conditions as well as excellent high-frequency power performance.

Characterization of blue-green m-plane InGaN light emitting diodes

High indium content blue-green (460–520 nm) m-plane InGaN light emitting diodes (LEDs) were grown on low defect-density m-plane GaN substrates. Systematic studies were performed on packaged blue-green LED lamps by using a range of well and barrier thicknesses. Photoluminance and electroluminance peak wavelengths increased while the well width was increased from 2 to 4 nm. The highest output power was achieved for well width of 2.5 nm. The output power improved significantly with the increase in barrier thickness. Nearly blueshift-free emission was observed in all LEDs from 1–400 A/cm2 current density under pulsed operation.

The properties of photon tunnelling in a frustrated total internal reflection structure with an indefinite metamaterial slab

The tunnelling time and lateral shift of photon tunnelling through the frustrated total internal reflection structure containing indefinite metamaterials are obtained by employing the stationary-phase approximation. It is found that the photon tunnelling only occurs when the barrier is thin enough. For the indefinite metamaterial with one-sheet hyperbolic dispersion, the anomalous total reflection occurs, and the negative lateral shift and tunnelling time can be obtained. The negative tunnelling time indicates that superluminal propagation could be observed when a photon tunnels through the indefinite metamaterial. Both lateral shift and tunnelling time tend to a saturation value, respectively, as the barrier thickness increases. Moreover, the dependence of properties of four kinds of indefinite metamaterial on the thickness, the permittivity and permeability tensors is discussed.

나노 양자우물구조를 이용한 광통신용 방향성 결합기의 설계

An optical directional coupler, which consists of quantum wells with nanothickness, is designed by using Modal Transmission Line Theory (MTLT). To demonstrate the validity and usefulness, the propagation characteristics and the coupling efficiencies are rigorously evaluated at nanoscale couplers, which consist of double quantum wells with different effective masses. The numerical result reveals that the coupling efficiency of nanoscale couplers is maximized at a coupling length 2052.3 nm, if the total electron energy is 83.9 meV. Furthermore, the coupler operates as a filter with narrower band as the barrier thickness increases.

Optical property of self-assembled InAs quantum dots in Al0.5Ga0.5As barriers grown by molecular beam epitaxy

Photoluminescence (PL) of quantum dots embedded in high potential barriers is studied as functions of barrier thickness and temperature. With the increase of barrier thickness, both the strengthened two-dimensional electron gas (2DEG) structure and strongly localized electron wave functions can increase the carrier recombination. The optical properties of two different-barrier-thickness samples exhibit different characteristics with the decreased measurement temperatures. The PL recombination characteristic of the samples with the barriers adjacent to a Si-doping GaAs layer is different from that of samples with the barrier adjacent to an i-GaAs layer.

Raman scattering in multilayer structures with CdTe quantum dots in ZnTe

Raman spectra in superlattices composed of layers of self-assembled CdTe quantum dots separated by ZnTe barriers are investigated. As the barrier thickness increases, a high-frequency shift of all peaks is observed, which is explained by a decrease in the lattice constant averaged over the volume of the entire structure. Peaks are found at a CdTe TO mode frequency of 140 cm−1 and also at 120 cm−1. The first peak is assigned to the symmetric Coulomb (interface) mode of the quantum dot material, and the low-frequency peak is assigned to the symmetric mode of the phonons captured in the quantum dot. This combination of modes in structures with quantum dots has not been observed previously.