Potential Barrier Layer Research Articles

Enhanced temperature stability of threshold current of InAs/GaAs quantum dot lasers by AlGaAs lateral potential barrier layers.

We have investigated the incorporation of an AlGaAs lateral potential barrier layer (LPBL) as a novel approach to improve the temperature stability of the threshold current in InAs/GaAs quantum dot (QD) lasers. This layer serves to increase the energy separation (ΔE) between the ground and excited states of the QD while maintaining efficient vertical carrier injection. Theoretical calculations confirm that the LPBL is effective in increasing ΔE. The LPBLs were successfully formed using the preferential growth properties of AlGaAs induced by the non-uniform distribution of strain effects on the QD surface during molecular beam epitaxy growth. To confirm the usefulness of the LPBLs, we fabricated an InAs/GaAs QD laser incorporating AlGaAs LPBLs, demonstrating that the threshold current at 150°C was significantly reduced by 48% compared to a QD laser without LPBLs. The temperature stabilization achieved by incorporating the LPBLs provides a promising way for establishing high reliability and low power operation of QD lasers in high-temperature environments.

Selective Area Epitaxy of Highly Strained InGaAs Quantum Wells (980-990 nm) in Ultrawide Windows Using Metalorganic Chemical Vapor Deposition.

We employed the selective-area-epitaxy technique using metalorganic chemical vapor deposition to fabricate and study samples of semiconductor heterostructures that incorporate highly strained InGaAs quantum wells (980-990 nm emission wavelength). Selective area epitaxy of InGaAs quantum wells was performed on templates that had a patterned periodic structure consisting of a window (where epitaxial growth occurred) and a passive mask (where epitaxial growth was suppressed), each with a width of 100 µm for every element. Additionally, a selectively grown potential barrier layer was included, which was characterized by an almost parabolic curvature profile of the surface. We conducted a study on the influence of the curvature profile of the growth surface on the optical properties of InGaAs quantum wells and the spatial distribution of composition in an ultrawide window. Our results showed that, under fixed selective-area-epitaxy conditions, the composition of the InxGa1-xAs and the wavelength of the quantum-well emission changed across the width of the window. Our study demonstrates that increasing the curvature profile of the growth surface of highly strained quantum wells leads to a transition in the photoluminescence wavelength distribution profile across the window, from quasi-parabolic to inverted parabolic.

Sputtering Al2O3 enhanced bandgap engineering for integrated photonic devices

We present experimental results of the application of Sputtering enhanced Quantum Well Intermixing (QWI) process, where the bandgap of InGaAsP/InP quantum well laser microstructure is effectively modified by intermixing quantum well and potential barrier layers through sputtering Al2O3-QWI. This technique creates point defects through plasma induced disordering in the process of sputtering Al2O3. The subsequent Rapid thermal Anneal (RTA) process is performed to increase the diffusion rate and quantum well intermixing. This results in an increased transitional energy and hence it modifies the bandgap essential for the development of next generation complex photonic integrated circuits (ICs). We report a large bandgap blue shift of 144 nm is achievable with a very high increase in photoluminescence (PL) intensity. The experimental results of the bandgap shifted waveguides and laser diodes are presented. Photoluminescence (PL) measurements of InGaAsP/InP laser structure were taken before and after Al2O3-QWI process. The proposed technique can provide reliable and fast post-growth wafer level fabrication of photonic devices monolithically.

Unipolar barriers in near-broken-gap heterostructures for high-performance self-powered photodetectors

The two-dimensional heterostructure is a promising research direction in photodetection. However, developing a good photodetector with high responsivity and fast speed is still challenging. Herein, we fabricate a high-performance self-powered broadband (355–1064 nm) photodetector based on a near-broken-gap GeSe/SnS2/InSe heterostructure, where SnS2 is used as a potential hole barrier layer. The device shows an ultrahigh open-circuit voltage (VOC) of 0.57 V, a high power-dependent responsivity of 1.87 A W−1 at 355 nm, and a fast response time of 8 μs in the self-powered mode. Based on the near-broken band alignment, the InSe layer with high electron mobility can efficiently collect the photogenerated electrons from the GeSe layer to improve conversion efficiency. Furthermore, the unipolar hole barrier at the interface can inhibit the Langevin recombination resulting in VOC enhancement. Notably, the anisotropy ratio of photocurrent in our device is also enhanced to ∼3.5, which is higher than GeSe photodetectors and other anisotropic devices counterparts. This work provides an opportunity for the realization of the high-sensitivity polarization-sensitive broadband photodetector.

Fine modulation of the energy band strategy to control the carrier confinement capability of digital alloys

In this paper, a strategy to finely modulate the energy band structure to control the carrier confinement capability of digital alloys (DA) is proposed. Strain analysis shows that As and Sb atoms are exchanged within the AlAsSb DA. The bottom of the corrected potential well is low on the left and high on the right in the growth direction, resulting in a higher band offset of the AlSb potential barrier layer on the left side of the potential well than on the right side. The modulation of the band leads to a higher probability of electron tunneling in DA under the action of an electric field opposite to the growth direction. Conversely, it is difficult for the electrons to tunnel into the lower energy level potential wells. The I–V curve of DA shows that the current value under positive bias is significantly smaller than the value under negative bias when the voltage is higher. The measured results correspond perfectly with the modified energy band model, which verifies the feasibility of energy band modulation. This is important for the structural design of DA and the reduction of dark current in optoelectronic devices.

Enhanced Photoluminescence of 1.3 μm InAs Quantum Dots Grown on Ultrathin GaAs Buffer/Si Templates by Suppressing Interfacial Defect Emission.

We report on the photoluminescence enhancement of 1.3 μm InAs quantum dots (QDs) epitaxially grown on an ultrathin 250 nm GaAs buffer on a Si substrate. Decreasing the GaAs buffer thickness from 1000 to 250 nm was found to not only increase the coalesced QD density from 6.5 × 108 to 1.9 × 109 cm-2 but also decrease the QD photoluminescence emission intensity dramatically. Inserting an Al0.4Ga0.6As potential barrier layer maintained strong photoluminescence from the QDs by effectively suppressing carrier leakage to the GaAs/Si interfacial region even when the GaAs buffer was thinned to 250 nm. We then fabricated a light-emitting diode using the ultrathin 250 nm GaAs buffer on Si and confirmed strong electroluminescence peaking at 1.28 μm without interfacial defect emission at room temperature. We believe that this work is promising for monolithically integrated evanescent Si lasers using InAs/GaAs QDs.

Wigner time for electromagnetic radiation in plasma

Wave tunneling is an intriguing phenomenon spanning different branches of physics, from quantum mechanics to classical electrodynamics and optics. The Wigner (or phase) time is proved to be an adequate measure to describe wave transit through a potential barrier or material layer in the tunneling regime. Here, we analytically and numerically calculate the Wigner time for electromagnetic-radiation propagation through the layer of both lossless and lossy plasmas. It is shown that the plasma frequency is the key parameter governing the value of Wigner time allowing us to interpret tunneling as due to the reaction of plasma as a whole. We analyze the Wigner time for obliquely incident waves of TE and TM polarizations and discuss the meaning of negative Wigner times appearing in the lossy case in the low-frequency range and close to the plasma frequency. The results show that plasma deserves attention as a perspective object for tunneling studies.

(Invited) Developing Barrier Layer Free Oxygen Electrode for Solid Oxide Cells

High temperature solid oxide cell (SOC) technology is attracting considerable interest in recent decades for clean power generation and hydrogen production (green hydrogen) when combined with low-cost renewable energy. The key driver for such a strong interest is its superior thermodynamic efficiency and high-rate power and hydrogen production. However, the current scale-up and commercialization efforts are hindered by the insufficient durability and high cost, both of which are resulted from the high operating temperature (700-800oC). A further reduction in operation temperature to, e.g. below 700oC, is the best solution to address the durability and cost issues. As the operating temperature is reduced, the resistances of cell components increase correspondingly. To decrease the resistances, one can thin out or use high-conductivity electrolyte membranes, open up microstructure of fuel (or hydrogen) electrodes substrate and employ oxygen electrocatalytically active oxygen electrodes. Unfortunately, a direct use of active oxygen electrodes in SOCs has been proven formidable due to their reaction with electrolytes (ZrO2-based) and very high thermal expansion coefficient. To address the first problem, applying a CeO2-based barrier layer between electrolytes and oxygen electrodes is currently the solution. However, introduction of such a barrier layer considerably increases the ohmic resistance (sometimes as high as 2-3x), thus defeating the purpose of boosting cell performance. To address the second problem, mixing the active oxygen electrode with CeO2-based material or infiltrating it into a thermally compatible skeleton is currently the solution, but CeO2-based barrier layer is still needed.Therefore, a key development for reduced-temperature ZrO2-based SOCs is to find an active oxygen electrode without the need of a barrier layer. Here we show that a composite consisting of La0.8Sr0.2MnO3 (LSM), an excellent electronic conductor, and (Bi0.75Y0.25)0.93Ce0.07O1.5±δ (BYC), an excellent ionic conductor, is a potential barrier layer free oxygen electrode for reduced-temperature SOCs. We also show fabrication of open structured hydrogen electrode substrate to facilitate H2/H2O diffusion and increase the performance of HE.

Efficient Nondoped Pure Red/Near-Infrared TADF OLEDs by Designing and Adjusting Double Quantum Wells Structure

The selection of host materials has a great impact on the performance of doping devices, especially near-infrared (NIR) emitters. In this investigation, four diverse host materials serve as a potential barrier layer (PBL) to confine and balance holes and electrons within the potential well layer (PWL), TPA-DCPP, for fabricating nondoped NIR thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) (NNT-OLEDs) with double quantum wells’ (DQWs) structure. The hole-type host (mCP) forms an optimum interface energy barrier (IEB) and disperses carriers and excitons in each well, which helps to widen the recombination interval of carriers and restrain the quenching of excitons. Finally, OLEDs with pure red emission and a maximum external quantum efficiency (EQEmax) of nearly 15% (with 0.5 nm well width), deep-red emission with an EQEmax of 12% (with 1.0 nm well width), and NIR emission with an EQEmax of 3.3% (with 5.5 nm well width) are achieved with tunable emission peaks within the range of 641–700 nm by adjusting well width, which are significantly superior to those of doped devices based on the same emitter. This investigation demonstrates a simple, feasible, and effective design strategy for achieving efficient fluorescent OLEDs with controllable wavelength, which solves the blue shift problem in traditional NIR devices of host–guest structure.

Hierarchical WO3–NiO microflower for high sensitivity detection of SF6 decomposition byproduct H2S

In this work, hierarchical WO3–NiO microflowers have been designed and prepared through a controllable hydrothermal route for high sensitivity detection of H2S produced by SF6 decomposition. The hierarchical flower-like nanostructures assembled with numerous nanosheets were influenced by the introduction of WO3, which is regarded as a significant strategy for improving the gas sensing properties. The synthesized nanostructures were tested by various characterization to investigate the different microstructures of the nanocomposites. The H2S sensing performances of the sensors fabricated with these flower-like nanostructures were measured, which indicated that 3.0 at% WO3–NiO microflower based sensor possessed excellent properties such as higher gas responses and more prominent repeatability compared with those of other fabricated sensors. The enhanced performances might be mainly ascribed to the creation of the heterojunction at n-type WO3 and p-type NiO interface, which caused the improvement of the potential barrier and depletion layer. In addition, the larger specific surface area of flower-like nanostructures also possessed abundant sites for surface reaction.

Sealing capacity of siderite-bearing strata: The effect of pore dimension on abundance and micromorphology type of siderite in the Lopingian (Late Permian) coal-bearing strata, western Guizhou province

We conducted a series of core observation, thin section evaluation, and integrated with low-temperature nitrogen adsorption/desorption, porosity, permeability, breakthrough pressure analysis of the siderite-bearing strata. This paper aims to discuss the sealing capacity of siderite-bearing strata as a function of siderite abundance andmicromorphology type. The siderite-bearing strata can be potential gas barrier layers in multiply-superposed gas-bearing systems. In the transitional Lopingian coal-bearing succession of western Guizhou, the siderite content of siderite-bearing mudstone is 18.6–90.7% (wt.), with an average 53.4% (wt.). There are three morphologies of siderite, including S1 (gelatinous aggregates), S2 (microcrystal-silt size crystal aggregates), and S3 (sphaerosiderites). It shows that different abundance of siderite and type of siderite micromorphology affected the pore structure. The sealing capacity of siderite-bearing mudstone at the study area is somewhat or significantly larger, and the sealing capacity of the siderite-free mudstone is moderate. The development of siderite-bearing strata is controlled by the stratigraphy of the sequence.

Interface and stability analysis of Tantalum- and Titanium nitride thin films onto Lithiumniobate

Modern surface-acoustic-wave (SAW) devices are characterized by their trend to higher frequencies, power densities and new applications. For this, a shrinking of the dimensions is necessary resulting in an increased power density and temperature within the metallization.To reduce the emerging damaging effects (acustomigration, diffusion etc.) additional barrier layers between substrates and electrodes are necessary, especially for high temperature applications.In this context, we present results of detailed chemical interface analysis of sputtered TaN and TiN thin films as potential barrier layers onto SAW-substrate material (LiNbO3) with respect to their temporal (up to 8h) and thermal stability up to 600°C in vacuum. We report good stability of both systems. The main technique for analysis was non-destructive and surface sensitive angle-resolved X-ray photoelectron spectroscopy (AR-XPS).

Interfacial reactions at the joints of CoSb3-based thermoelectric devices

CoSb3-based alloys are among the most promising thermoelectric materials. The Ag-Cu eutectic alloy, Ag-39.9 at%Cu, has a melting point at 779.1 °C and is a good braze candidate for thermoelectric modules used in mid-temperatures. Since the Ag-39.9 at%Cu alloy reacts significantly with the CoSb3 substrate, the introduction of a barrier layer is necessary, and Co, Ni and Ti are potential barrier layers. This study systematically examines the reactions at the joints, Ag-Cu/Co/CoSb3, Ag-Cu/Ni/CoSb3 and Ag-Cu/Ti/CoSb3. Interfacial reactions are observed between the CoSb3 substrate and barrier layers at 450 °C and 600 °C, although they are significantly reduced with the introduction of barrier layers. CoSb2 and CoSb phases are formed at the Co/CoSb3 contact, Ni5Sb2 and (Co,Ni)Sb phases at Ni/CoSb3, and TiSb, TiSb2 and TiCoSb phases at Ti/CoSb3. After reaction at 450 °C for one day, the reaction layer thick is 2 μm, 24 μm, and 0.4 μm in the Co/CoSb3, Ni/CoSb3 and Ti/CoSb3 couples, respectively. Co has both good adhesion and low interfacial reaction rates with CoSb3 and Ag-39.9 at%Cu alloy, and it is concluded that Co is a good barrier layer at 450 °C for the CoSb3-based modules when Ag-Cu eutectic braze is used.

High efficiency yellow organic light-emitting diodes with optimized barrier layers

High efficiency Iridium (III) bis (4-phenylthieno [3,2-c] pyridinato-N,C2′) acetylacetonate (PO-01) based yellow organic light-emitting devices are fabricated by employing multiple emission layers. The efficiency of the device using 4,4′,4″-tris(N-carbazolyl) triphenylamine (TCTA) as potential barrier layer (PBL) outperforms those devices based on other PBLs and detailed analysis is carried out to reveal the mechanisms. A forward-viewing current efficiency (CE) of 65.21cd/A, which corresponds to a maximum total CE of 110.85cd/A is achieved at 335.8cd/m2 in the optimized device without any outcoupling enhancement structures.

Enhanced preferential orientation and electrical property of fluorine-doped SnO2 thin films via barrier layer

Polycrystalline fluorine-doped SnO2 thin films with SiCxOy or SixSnyO2 barrier layer are deposited on glass substrates by atmospheric pressure chemical vapor deposition (APCVD) method. The effect of barrier layer on structure and electrical property of FTO films was investigated. Results show that the inserting of barrier layer, especially the SiCxOy layer, has led to the improved crystallinity and the enhanced preferential orientation along the (200) crystallographic plane. SnO2:F/SiCxOy/Glass films with larger grain size and a columnar growth structure exhibited lower resistivity (~4.9×10−4), higher reflectance in the mid-far-infrared region (~80%) and lower emissivity (0.16), while maintaining high transmittance in the visible range. The SiCxOy film has therefore been considered as a more ideal potential barrier layer for FTO thin film production.

基于量子阱结构的混合型白光有机发光器件

Hybrid white organic light-emitting device based on fluorescent layer combining with phosphorescent doping layer was fabricated,in which fluorescent material BePP2 acted as blue emitter layer,phosphorescent material GIrl and R-4B doped into CBP∶ Bphen bipolar type host acted as green and red emitter layer,respectively. The ultrathin spacer layer was constructed by inserting a2. 0 nm thin layer of TPBI between red and green emitting layer. In the quantum well structure,BePP2 and TCTA acted as the potential well layer and the potential barrier layer,respectively. The maximum luminance and current efficiency are 21 682. 5 cd /m2 and 23. 73 cd /A,respectively. The Commission Internationale de I'Eclairage( CIE) coordinates of the device vary from( 0. 345,0. 350) at 7 V to(0. 340,0. 342) at 14 V when the number of potential barrier layer is two. In comparison with the reference device without quantum well structure,the device with two barrier layers achieves a maximum power efficiency of 8. 07 lm/W,a low CIE coordinates changing of ±(0. 005,0. 008),and a high color rendering index of 83.

The influence of type-I and type-II triplet multiple quantum well structure on white organic light-emitting diodes.

We demonstrate high-efficient white organic light-emitting diodes (WOLEDs) based on triplet multiple quantum well (MQW) structure and focus on the influence on WOLEDs through employing different potential barrier materials to form type-I and type-II MQWs, respectively. It is found that type-I MQW structure WOLEDs based on 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene as potential barrier layer (PBL) offers high electroluminescent (EL) performance. That is to say, maximum current efficiency and power efficiency are achieved at about 1,000 cd/m2 with 16.4 cd/A and 8.3 lm/W, which increase by 53.3% and 50.9% over traditional three-layer structure WOLEDs, respectively, and a maximum luminance of 17,700 cd/m2 is earned simultaneously. The achievement of high EL performance would be attributed to uniform distribution and better confinement of carriers within the emitting layer (EML). However, when 4,7-diphenyl-1,10-phenanthroline or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline is used as PBL to form type-II MQW structure, poor EL performance is obtained. We attribute that to improper energy level alignment between the interface of EML/PBL, which leads to incomplete confinement and low recombination efficiency of carriers, a more detailed mechanism was argued.

High efficient white organic light-emitting diodes based on triplet multiple quantum well structure

We demonstrate the high efficient triplet multiple quantum well (MQW) structure white organic light-emitting diodes (WOLEDs) (MQW-WOLEDs) by introducing phosphorescent Ir-complex material. In the triplet MQW-WOLEDs, 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene functions as the potential barrier layer; fluorescent dopant layer and Ir-complex phosphorescent dopant layer act as the blue-emitting and orange-emitting potential well layer, respectively. The maximum luminance, current efficiency, and power efficiency are 19 000 cd/m2, 14.5 cd/A, and 5.4 lm/W, respectively; and the efficiency roll-off is especially improved by ∼15% over reference device. The emission spectra almost do not change with the changing drive voltage; that is, Commission Internationale de I’Eclairage coordinates x and y vary only ±0.002 and ±0.008 from 8 to 14 V. The improvement of electroluminescence performances is attributed to the balanced electron-hole injection and transport, confinement of carriers and excitions within potential well; as well more detail of improvement mechanism is discussed.

Non-doped phosphorescent white organic light-emitting devices with a quadruple-quantum-well structure

Non-doped white organic light-emitting devices (WOLEDs) with a quadruple-quantum-well structure were fabricated. An alternate layer of ultrathin blue and yellow iridium complexes was employed as the potential well layer, while potential barrier layers (PBLs) were chosen to be 2,2',2''-(1,3,5-benzenetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) or N,N'-dicarbazolyl-3,5-benzene (mCP) combined TPBi. On adjusting the PBLs for device performance comparison, the results showed that the device with all-TPBi PBLs exhibited a yellow emission with the color coordinates of (0.50,0.47) at a luminance of 1000cd/m2, while stable white emission with the color coordinates of (0.36,0.44) was observed in the device using mCP combined TPBi as the PBLs. Meanwhile, for the WOLED, with a reduced efficiency roll-off, a maximum luminance, luminous efficiency, and external quantum efficiency of 12,610cd/m2, 10.2cd/A, and 4.4%, respectively, were achieved. The performance improvement by the introduction of mCP PBL was ascribed to the well confined exciton and the reduced exciton quenching effect in the multiple emission regions.

Fabrication and characterization of single Nb/NbxSi1-x/Nb Josephson junction for voltage standard

The core device of Modern programmable Josephson voltage standard is Josephson junction array. The most advantageous Josephson junction array is Nb/NbxSi1-x/Nb material array. The advantages of Nb/NbxSi1-x/Nb material Josephson junction are that three-layer film production process is simple, Nb and NbxSi1-x etching processes are the same and NbxSi1-x potential barrier layer components can be easily adjusted. In this paper, we investigate the NbxSi1-x/Nb single Josephson junction in National Institute of Metrology. Through measuring the dc current-voltage characteristics under low temperature (4.2 K), superconducting tunneling current and a zero voltage state jumping to voltage state are observed clearly, finally the measurement results are discussed. The work is the first study on Nb/NbxSi1-x/Nb single Josephson junction in China.