Vertical Switching Research Articles

High‐precision target echo generation technology based on one‐dimensional linear array radar

The high-precision target echo generation in the field of radar systems is particularly important for large-scale, comprehensive testing of radars. Conventional echo generation techniques often use planar array air feed to simulate echoes. However, it is complicated and expensive to achieve the echo generation with traditional planar array. Therefore, this study proposes a high-precision target generation technique based on one-dimensional linear arrays. The simulation system achieves the target radial high-precision simulation through DRFM technology; the target azimuth high-precision simulation is achieved through differential GPS technology; and the target elevation high-precision simulation is realised through vertical one-dimensional linear array antenna switching. The semi-physical simulation system presented here has the advantages of simple control, low cost, low link loss, easy maintenance, and high simulation target accuracy. Finally, the system was tested in the field to verify the theoretical concept of the article.

(Invited) X-Ray Metrology of AlN Single Crystal Substrates

As development of GaN and SiC material systems for applications in power switching progresses, the ultrawide-bandgap (UWBG) materials, with bandgaps greater than GaN, are gaining increased attention, in pursuit of improved power device performance. Substrate technology is critical to improve this field. High-quality native substrates, possessing low lattice and thermal expansion mismatches to epitaxial active layers, enable epitaxial growth with low densities of extended defects, leading to improved device performance and lifetime. Lateral and vertical power switching devices have been demonstrated in the emerging AlGaN alloy system [1]. Aluminum nitride (AlN) possesses a close lattice match to high Al mole fraction AlGaN alloys and a high thermal conductivity, which make it an excellent substrate for UWBG AlGaN-based devices. Physical vapor transport (PVT) growth has emerged as the most promising technique for the production of large, high-quality AlN single crystal substrates. Seeded growth technology developed by HexaTech enables iterative expansion of single crystal size, while suppressing low angle grain boundary (LAGB) formation, a primary source of grown-in dislocations. X-ray diffraction (XRD) is well suited to accurately and non-destructively probe the extended defect structure, composition, and strain of single crystals and epitaxial films. A variety of x-ray techniques were used throughout AlN substrate processing, from evaluation of as-grown crystals, through substrate polishing, to analysis of epitaxial films grown on AlN. High-resolution x-ray rocking curves (HRXRRC) and x-ray topography (XRT) were used to determine the primary dislocation types in AlN bulk crystals and their likely formation mechanisms [2, 3]. Synchrotron-based XRT images demonstrated that the expanded diameter of freestanding AlN boules contained low densities of threading dislocations (<104 cm-3), and no basal plane dislocations or LAGBs [4]. Furthermore, XRT images acquired on a commercial diffractometer were used for quality control of AlN seed crystals. The advent of a new generation of x-ray area detectors, capable of a wide dynamic range and rapid data acquisition, opened the possibility for using XRT in production. HRXRRC and surface-sensitive reciprocal space maps were used to monitor sub-surface damage removal during substrate polishing [5]. These results, together with atomic force microscopy and defect selective etching images, demonstrated that chemo-mechanically polished AlN surfaces were free of plastic damage [4]. Polished AlN surfaces were also studied by x-ray reflectivity, yielding long-range data about surface roughness and density. Finally, epitaxial AlGaN films and heterostructures deposited on AlN substrates were studied by a combination of these x-ray techniques, providing essential information on composition, thickness, relaxation, and heterostructure design. XRD has become a valuable metrology tool in the development of AlN technology. This talk will review the implementation of complementary x-ray based measurements in the processing of AlN, from boule growth to AlGaN-based devices.

(Invited) Selective Area Growth of p-Type GaN for Gallium Nitride Power Switching Transistors

Superior electronic material properties of GaN semiconductors compared to Si will enable the foundational diodes and transistors for the next revolution in power management technology. To date, high-performance GaN diodes have been demonstrated with breakdown voltages near 5 kV and specific on-resistances of less than 5 mΩ-cm2 [1]. However, a viable GaN vertical power switching transistor is the missing element necessary to realize advanced power management systems utilizing GaN. While GaN transistors are now ubiquitous in RF systems, their lateral device architecture inherently limits these devices to niche applications in power systems. Rather, high-voltage systems rely on vertical device architectures where current flow is normal to the semiconductor surface. Vertical power electronics devices such as junction field effect transistors (JFETs) require selective areas of p-type semiconductor surrounded by lightly-doped n-type semiconductor. For Si power devices, p-type regions are implanted in n-type drift regions to create p/n junctions that act as a gate and pinch-off current. Ion implantation presents challenges in GaN and requires specialized equipment and processes, including high-pressure and high-temperature annealing, for dopant activation [2]. Our previous work using metal-organic chemical vapor deposition (MOCVD) where p-type GaN was regrown on a lightly-doped, non-etched n-type drift layer demonstrated planar p/n diodes with the same electrical performance as continuously-grown devices. However, when dry-etching processes are used before the regrowth of the p-type GaN layer, as would be needed to define a regrown area, planar diodes demonstrate higher forward and reverse leakage currents, and lower breakdown voltage. Deep Level Optical Spectroscopy (DLOS) was used to identify the source of the higher forward and reverse leakage currents observed in diodes with p-type anodes re-grown on inductively coupled plasma (ICP) dry-etched n-type drift layers. We will report DLOS studies of various diode structures that show the formation of a mid-gap trap located ~ 1.7 eV below the conduction band in dry-etched, lightly doped (Nd-Na = 2x1016 cm-3) drift layers. We also will show that various wet-chemical treatments of the dry-etched n-type drift layer prior to p-anode regrowth results in diodes with 1 to 2 orders of magnitude lower reverse leakage current. DLOS measurements of regrown diodes showed a 3 times reduction in the density of the mid-gap deep level trap following wet-chemical treatment. These studies establish the connection between high reverse and forward leakage currents with residual damage caused by dry-etching and that this damage can be mitigated by with wet-chemical treatments prior to regrowth of the p-anode. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES programs directed by Dr. Isik Kizilyalli. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. H. Ohta et al., J. J. Appl. Phys., 57, 04FG09 (2018).T.J. Anderson et al., Elec. Lett., 50(3), 197 (2014).

Wavelength dependent light tunable resistive switching graphene oxide nonvolatile memory devices

This paper reports on the first optically tunable graphene oxide memristor device. Modulation of resistive switching memory by light opens the route to new optoelectronic devices that can be switched optically and read electronically. Applications include integrated circuits with memory elements switchable by light and optically reconfigurable and tunable synaptic circuits for neuromorphic computing and brain-inspired, artificial intelligence systems. In this report, planar and vertical structured optical resistive switching memristors based on graphene oxide are reported. The device is switchable by either optical or electronic means, or by a combination of both. In addition the devices exhibit a unique wavelength dependence that produces reversible and irreversible properties depending on whether the irradiation is long or short wavelength light, respectively. For long wavelength light, the reversible photoconductance effect permits short-term dynamic modulation of the resistive switching properties of the light, which has application as short-term memory in neuromorphic computing. In contrast, short wavelength light induces both the reversible photoconductance effect and an irreversible change in the memristance due to reduction of the graphene oxide. This has important application in the fabrication of cloned neural networks with factory defined weights, enabling the fast replication of artificial intelligent chips with pre-trained information.

Structural and Electrical Characterization of 2" Ammonothermal Free-Standing GaN Wafers. Progress toward Pilot Production.

Free-standing gallium nitride (GaN) substrates are in high demand for power devices, laser diodes, and high-power light emitting diodes (LEDs). SixPoint Materials Inc. has begun producing 2” GaN substrates through our proprietary Near Equilibrium AmmonoThermal (NEAT) growth technology. In a single 90 day growth, eleven c-plane GaN boules were grown from free-standing hydride vapor phase epitaxy (HVPE) GaN substrates. The boules had an average X-ray rocking curve full width at half maximum (FWHM) of 33 ± 4 in the 002 reflection and 44 ± 6 in the 201 reflection using 0.3 mm divergence slits. The boules had an average radius of curvature of 10.16 ± 3.63 m. The quality of the boules was highly correlated to the quality of the seeds. A PIN diode grown at Georgia Tech on a NEAT GaN substrate had an ideality factor of 2.08, a high breakdown voltage of 1430 V, and Baliga’s Figure of Merit of >9.2 GW/cm2. These initial results demonstrate the suitability of using NEAT GaN substrates for high-quality MOCVD growth and fabrication of high-power vertical GaN switching devices.

(Invited) Halide Vapor Phase Epitaxy for b- and a-Ga2O3 Power Electronics Devices

Monoclinic b-Ga2O3 and rhombohedral a-Ga2O3 have recently emerged as promising materials for “ultrawide” bandgap electronics for next-generation high voltage lateral and vertical power switching devices, thanks to their large bandgaps (4.9eV and 5.3eV, respectively), high expected critical breakdown fields (8MV/cm and 10MV/cm, respectively), and their correspondingly large high voltage and high frequency Baliga figures of merit (FOMs). These large FOMs ultimately translate to higher efficiencies for high power (>1kW) switching devices, which will enable more compact, higher performing power electronics systems. In order to realize the high voltages required for the most demanding applications, further development of epitaxial growth techniques which can produce thick Ga2O3 films is required. Halide vapor phase epitaxy (HVPE) is a scalable and inexpensive growth technique which currently appears to be well positioned to address such a need. Kyma Technologies has developed a low-cost HVPE reactor for the growth of thick Ga2O3 films which boasts high growth rates and smoothness while simultaneously being able to be lightly and controllably doped with Si and free of carbon. The reactor has been configured to grow both b-Ga2O3 and a-Ga2O3, and in this talk we will outline our recent growth results from both. Effects of substrate preparation, epi-readiness, and growth conditions on epilayer quality as well as on the corresponding Schottky barrier diode device performance will be presented.

Vertical ferroelectric switching by in-plane sliding of two-dimensional bilayer WTe2.

Based on first-principles calculations, we studied the ferroelectric properties of bilayer 1T'-WTe2. In this work, we discovered that the polarization stems from uncompensated out-of-plane interlayer charge transfer, which can be switched upon interlayer sliding of an in-plane translation. Our differential charge density results also confirmed that such ferroelectricity in the bilayer WTe2 is derived from interlayer charge transfer. The ferroelectric polarization directions further control the spin texture of the bilayer WTe2, which may have important applications in spintronics. Therefore, we propose a spin field effect transistor (spin-FET) design that may effectively improve the spin-polarized injection rate. In addition, the lattice strain has been found to have an important influence on the ferroelectric properties of the bilayer WTe2. One can effectively increase the polarization with a maximum at 3% tensile strain, whereas a 3% compressive strain can transform the bilayer WTe2 from the ferroelectric to paraelectric phase.

(Invited) Polarization-Induced Doping in Graded AlGaN Epilayers Grown on AlN Single Crystal Substrates

Wide-bandgap nitride semiconductor-based lateral and vertical power switching devices are reaching a mature state and gaining increasing commercial acceptance. This is due to the nitrides’ outstanding properties, including high critical electric fields, mobilities, and saturation velocities, and to progress made in the understanding of key issues, such as controllable doping and polarization engineering. Since many of the figures of merit for power device performance scale with the critical electric field, itself a strong function of the bandgap, the ultrawide-bandgap (UWBG) materials, with bandgaps beyond the 3.4 eV of GaN, have the potential for improved power device performance [1]. However, research and development into these emerging materials is still at a relatively immature stage. Significant challenges remain in the growth of high Al composition AlGaN, including efficient n-type doping at very high Al compositions, and, even more so, p-type doping in general. In spite of these challenges, high-quality, controllably-doped AlGaN epilayers grown on AlN single crystal substrates have been demonstrated in devices, including UV emitters [2] and Schottky diodes [3]. Improved approaches to doping the high Al composition AlGaN layers required for operation of these devices will lead to increased device performance. AlGaN epilayers are commonly doped with substitutional impurities, typically Si for n-type and Mg for p-type, which require thermal activation to generate free carriers. As the Al composition, x, in AlxGa1-xN increases, the Mg impurity activation energy increases, while the Si impurity undergoes a DX transition beyond x~ 0.84, leading to rapidly decreasing free carrier concentrations in high Al content layers. In contrast to this approach, doping in compositionally graded AlGaN epilayers by taking advantage of the distributed polarization charge has been demonstrated for both n-type [4] and p-type [5] doping. The polarization-doped layers thus formed exhibit high sheet carrier densities and are resistant to carrier freeze-out at low temperatures, since they do not depend on thermal activation. We have investigated graded AlGaN epilayers grown on Al-polar AlN single crystal substrates by metal-organic chemical vapor deposition (MOCVD), and measured n-type or p-type doping in increasing or decreasing Al composition layers, respectively. The structural quality and compositional grading were assessed by high-resolution x-ray diffraction (HRXRD) and secondary ion mass spectrometry (SIMS). The space charge profile in an n-type layer graded from x= 0.66 to x= 1.0 was calculated from capacitance-voltage (CV) measurements of a Schottky diode structure. Furthermore, the temperature-dependent resistivity, carrier concentration, and mobility in p-type layers was determined by Hall effect measurements using the Van der Pauw method, and a room-temperature resistivity of 1 Ωcm was obtained for a p-type layer graded from x= 1.0 to x= 0.36. This talk will review MOCVD growth and characterization of both n-type and p-type doped, compositionally graded AlGaN epilayers by HRXRD, SIMS, CV, and Hall effect measurements. Results of integrating these layers into UWBG device structures will also be discussed. [1] R. J. Kaplar, A. A. Allerman, A. M. Armstrong, M. H. Crawford, J. R. Dickerson, A. J. Fischer, A. G. Baca, and E. A. Douglas, ECS J. Solid State Sci. and Technol., 6(2), Q3061 (2017). [2] T. Kinoshita, K. Hironaka, T. Obata, T. Nagashima, R. Dalmau, R. Schlesser, B. Moody, J. Xie, S. Inoue, Y. Kumagai, A. Koukitu, and Z. Sitar, Appl. Phys. Express, 5, 122101 (2012). [3] R. Dalmau, H. S. Craft, R. Schlesser, S. Mita, J. Smart, C. Hitchcock, G. Pandaey, T. P. Chow, and B. Moody, ECS Trans., 80(7), 217 (2017). [4] D. Jena, S. Heikman, D. Green, D. Buttari, R. Coffie, H. Xing, S. Keller, S. DenBaars, J. S. Speck, U. K. Mishra, Appl. Phys. Lett., 81, 4395 (2002). [5] J. Simon, V. Prosatenko, C. Lian, H. Xing, and D. Jena, Science, 327, 60 (2010).

A Changing-Reference Parasitic-Matching Sensing Circuit for 3-D Vertical RRAM

The 3-D vertical array architecture is considered to be a promising technology for emerging nonvolatile memories. But in 3-D vertical emerging nonvolatile memories, planar parasitic elements, vertical parasitic elements, and the sneak currents of the half-selected memory cells result in the delay of the read operation and read errors. This paper refers to the case of the 3-D vertical resistive switching random access memory (RRAM) technology. A read scheme, the memory core design, and the read path are proposed or analyzed. The factors that affect the read operation are concluded. A changing-reference parasitic-matching sensing circuit is, therefore, proposed. In the proposed circuit, the reference side and the read side share similar sneak currents and read paths. Simulated in a 40-nm CMOS process, the sensing time of 128-Mb 3-D vertical RRAM is 8.54 ns compared to the conventional 34.26 ns. Monte Carlo simulations show a 130.21-ns worst sensing time compared to the conventional 401.95 ns. The read errors are reduced by 100% and 95.31% under the regular and the worst RRAM resistance, respectively.

Aligned P(VDF-TrFE) Nanofibers for Enhanced Piezoelectric Directional Strain Sensing.

Piezoelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) nanofibers fabricated by electrospinning have drawn increasing levels of attention in the fields of flexible sensors and nanogenerators. However, the directional dependence of piezoelectricity of electrospun nanofibers remains elusive. In this study, the piezoelectric performances of individual nanofibers are characterized by piezoresponse force microscopy (PFM), while the effects of annealing on β-phase crystallinities are investigated by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The experimental results reveal that the as-spun P(VDF-TrFE) nanofibers form higher content of β-phase compared with spin-coated films, and the content of β-phase increases by annealing. The annealed P(VDF-TrFE) nanofiber exhibits distinct vertical polarization switching characteristics. The high piezoelectric output in the thickness direction and low piezoelectric output in the longitudinal direction of the nanofiber mats further confirm that the preferential dipole orientation of electrospun P(VDF-TrFE) nanofibers is normal to the surface of the substrate. Highly aligned P(VDF-TrFE) nanofibers show directional strain sensing ability due to the piezoelectric and mechanical anisotropy.

Exploring ferroelectric and magnetic properties of Tb-substituted m = 5 layered Aurivillius phase thin films

Here, we report the effect of A-site substitution of Tb at the expense of Bi on the ferroelectric and magnetic properties in m = 5 layered 2-D Aurivillius Bi6Ti3Fe2O18 thin films. The nominal stoichiometry of the prepared compound is Tb0.40Bi5.6Fe2Ti3O18, Tb0.90Bi5.1Fe2Ti3O18, and Bi6Ti3Fe2O18. Phase examination reveals that only 0.40 mol. % is successfully substituted forming Tb0.40Bi5.6Fe2Ti3O18 thin films. Lateral and vertical piezoresponse switching loops up to 200 °C reveal responses for Bi6Ti3Fe2O18, Tb substituted Tb0.40Bi5.6Fe2Ti3O18, and Tb0.90Bi5.1Fe2Ti3O18 thin films along the in-plane (±42.31 pm/V, 88 pm/V and ±134 pm/V, respectively) compared with the out-of-plane (±6.15 pm/V, 19.83 pm/V and ±37.52 pm/V, respectively). The macroscopic in-plane polarization loops reveal in-plane saturation (Ps) and remanence polarization (Pr) for Bi6Ti3Fe2O18 of ±26.16 μC/cm2 and ±22 μC/cm2, whereas, ±32.75 μC/cm2 and ±22.11 μC/cm2, ±40.30 μC/cm2 and ±28.5 μC/cm2 for Tb0.40Bi5.6Fe2Ti3O18 and Tb0.90Bi5.1Fe2Ti3O18 thin films, respectively. No ferromagnetic signatures were observed for Bi6Ti3Fe2O18 and Tb0.40Bi5.6Fe2Ti3O18. However, a weak response was observed for the Tb0.90Bi5.1Fe2Ti3O18 at 2 K. Microstructural analysis of Tb0.90Bi5.1Fe2Ti3O18 revealed that it contains 4 vol. % Fe:Tb rich areas forming FexTbyOz, which accounts for the observed magnetic moment. This study demonstrates the importance of thorough microstructural analysis when determining whether magnetic signatures can be reliably assigned to the single-phase system. We conclude that Tb0.40Bi5.6Fe2Ti3O18 and Tb0.90Bi5.1Fe2Ti3O18 samples are not multiferroic but demonstrate the potential for Fe-RAM applications.

Differentiator application in altitude control for an indoor blimp robot

ABSTRACTThis paper presents design of altitude controller with disturbance compensation for an indoor blimp robot and its realisation. Due to hardware restrictions, the altitude control behaviour of blimp is modelled as a switched system with a constant time-delay complemented with uncertain bounded disturbances. In order to achieve state estimation, four differentiators are applied and compared, then HOMD (homogeneous finite-time) differentiator is chosen as an observer for vertical velocity and switching signal estimation. Next, a predictor-based controller is conceived, and in order to compensate the perturbation, the method for disturbance evaluation is designed still with the help of HOMD differentiator. Control scheme is implemented by Matlab Simulink, and finally, the performance of blimp altitude controller is verified in experiments.

Molecular characteristics of Streptococcus agalactiae in a mother-baby prospective cohort study: Implication for vaccine development and insights into vertical transmission

BackgroundStreptococcus agalactiae (GBS) is a leading cause of neonatal sepsis and meningitis in many countries. This study aimed to determine the molecular characteristics of GBS colonized in mothers and their infants so as to provide implication for vaccine strategies and confirm vertical transmission. MethodsA prospective cohort study was conducted to recruit 1815 mother-neonate pairs. All GBS isolates from pregnant women and her infants were tested for serotypes, multilocus sequence types and virulence genes. The relationship between multiple molecular characteristics of GBS isolates was tested by the correspondence analysis, and the agreement between mother-neonate paired data in molecular characteristics was analyzed using Kappa tests. ResultsThe predominant serotypes were III, Ia and V, and the most prevalent sequence types (STs) were ST19, ST17, ST10, and ST12. All isolates carried at least one pilus island (PI). The most common combination of PIs was PI-2b alone, followed by PI-1+PI-2a and PI-2a alone, and the most prevalent alpha-like protein (alp) genes were rib, epsilon and alphaC. Moreover, a strong relationship was noted between STs, serotypes, alp genes and PIs, including ST17 associated with serotype-III/rib/PI-2b, ST19 with serotype-III/rib/PI-1+PI-2a, and ST485 with serotype-Ia/epsilon/PI-2b. The rate of GBS vertical transmission was 14.1%, and the kappa test revealed good agreement in multiple molecular characteristics among GBS-positive mother-neonate pairs. Notably, the switching of molecular characteristics was found during vertical transmission. ConclusionsOur findings underscore the value of monitoring multiple molecular characteristics so as to provide implication for multivalent strategies and gain insights into GBS vertical transmission and vertical characteristic switching.

Self-assembled vertical alignment layer of bifunctional amphiphiles for fast electro-optical switching of nematic liquid crystals

ABSTRACTWe present a novel homeotropic alignment method that simultaneously improves the ability of vertical alignment and fast switching of liquid crystals (LCs) in the electro-optical performance. The proposed method utilizes dual self-assembly of 4-(4-heptylphenyl)benzoic acid and 2-carboxyethyl acrylate via hydrogen bonding with ITO substrate. The 4-(4-heptylphenyl)benzoic acid composed of aromatic moieties and alkyl chains induce the homeotropic alignment of the LC molecules. Photopolymerization of 2-carboxylethyl acrylate with a reactive acrylate group under UV irradiation gives the LC molecules a fast response. Our proposed alignment method allows for fabrication of LC devices exhibiting fast LC switching without a conventional polyimide layer.

ON-state evolution in lateral and vertical VO2 threshold switching devices

We report the results of finite element simulations of the ON state characteristic of VO2-based threshold switching devices and compare the results with experimental data. The model is based on thermally induced threshold switching (thermal runaway) and successfully reproduces the I–V characteristics showing the formation and growth of the conductive filament in the ON state. Furthermore, we compare the I–V characteristics for two VO2 films with different electrical conductivities in the insulating and metallic phases as well as those based on TaOx and NbOx functional layers.

(Invited) Progress and Challenges of AlGaN Schottky Diodes Grown on AlN Substrates

Lateral and vertical power switching devices being intensively developed in GaN and SiC materials are beginning to reach a state of maturity and to displace traditional Si devices in certain applications. The relative advantage of these wide bandgap materials, which stems from intrinsic materials properties, can be compared in terms of an applicable figure of merit (FOM), indicating theoretical limits [1]. The most commonly discussed FOM for power applications is the unipolar FOM [2], based on reduction of conduction losses in the on-state and high voltage blocking in the off-state of vertical devices, VB 2/Ron,sp = εμnEc 3/4, where VB is the off-state avalanche breakdown voltage, Ron,sp is the specific on-state resistance, ε is the semiconductor permittivity, μn is the electron mobility, and Ec is the critical electric field. The unipolar FOM scales as the cube of Ec , which is itself a strong function of the bandgap [3], leading to the key advantage for wide bandgap materials. Possessing bandgaps wider than GaN (3.4 eV), AlGaN alloys are expected to provide improved performance. We have investigated quasi-vertical Schottky barrier diodes, consisting of nominal Al0.8Ga0.2N drift layers grown on high quality AlN single crystal substrates. HexaTech has developed proprietary seeded AlN crystal growth technology based on physical vapor transport (PVT), allowing iterative expansion of single crystal size, while maintaining low dislocation density (<103 cm-2), as determined by x-ray diffraction and defect-selective etching. AlN boules were oriented, sliced, lapped, and chemo-mechanically polished (CMP) to produce epi-ready c-plane substrates. Due to the highly insulating nature of the Al-face AlN substrates (resistivity >1013 Ωcm at room temperature), quasi-vertical Schottky diode structures were grown by metal-organic chemical vapor deposition (MOCVD). The device structure consisted of an AlN homoepitaxial layer, followed by a 0.6 µm thick pseudomorphic Si-doped Al0.8Ga0.2N contact layer ([Si] ~2x1019 cm-3), a 1 µm thick low-doped Al0.8Ga0.2N drift layer, and a 0.3 µm Mg-doped AlN cap layer ([Mg] ~2x1018 cm-3), serving as a junction termination extension (JTE). Despite the high activation energy of the Mg dopant in AlN, the Mg acceptors are expected to be ionized under high reverse bias, due to the strong band bending. Following epitaxial growth and dopant activation, circular device mesas were defined by dry etching down to the n-AlGaN contact layer, the JTE region atop the mesa was defined into a series of lateral steps and the Schottky contact area was etched down to the drift layer, Ohmic contacts were deposited and annealed, circular Schottky contacts were deposited, and a blanket polyimide passivation layer was deposited and patterned. Schottky diodes with diameters ranging from 0.3 to 1.0 mm were fabricated. Capacitance-voltage measurements were used to determine the free carrier concentration of the drift region, 2.0x1017 cm-3. The forward and reverse current-voltage characteristics were measured as a function of temperature. The specific on-state resistance was 125 mΩcm2 at room temperature and 190 mΩcm2 at 150 °C, which is partly attributed to high contact resistance. Diodes exhibited a current rectification ratio of ~106. The off-state breakdown voltage was up to 500 V. Breakdown tended to be destructive, likely limited by the polyimide passivation, suggesting improved blocking performance could be achieved with a better dielectric and more developed edge termination strategy. Field calculations based on the measured carrier concentration and breakdown voltage indicated a trapezoidal electric field in the drift region with a 7 MV/cm field at the Schottky junction. This talk will review growth and expansion of high quality AlN single crystals by PVT, and fabrication of epi-ready AlN substrates. Results for high Al composition Schottky diodes grown on these substrates will be presented. Challenges for this emerging technology, including doping limitations and compensation in AlGaN alloys, Ohmic contacts, and device termination will be discussed. [1] T. P. Chow, I. Omura, M. Higashiwaki, H. Kawarada, and V. Pala, IEEE Trans. Electron Devices, 64, 856 (2017). [2] K. Shenai, R. S. Scott, and B. J. Baliga, IEEE Trans. Electron Devices, 36, 1811 (1989). [3] J. L. Hudgins, G. S. Simin, E. Santi, and M. A. Khan, IEEE Trans. Power Electron., 18, 907 (2003).

Highly-Ordered 3D Vertical Resistive Switching Memory Arrays with Ultralow Power Consumption and Ultrahigh Density.

Resistive switching random access memories (RRAM) have attracted great scientific and industrial attention for next generation data storage because of their advantages of nonvolatile properties, high density, low power consumption, fast writing/erasing speed, good endurance, and simple and small operation system. Here, by using a template-assisted technique, we demonstrate a three-dimensional highly ordered vertical RRAM device array with density as high as that of the nanopores of the template (10(8)-10(9) cm(-2)), which can also be fabricated in large area. The high crystallinity of the materials, the large contact area and the intimate semiconductor/electrode interface (3 nm interfacial layer) make the ultralow voltage operation (millivolt magnitude) and ultralow power consumption (picowatt) possible. Our procedure for fabrication of the nanodevice arrays in large area can be used for producing many other different materials and such three-dimensional electronic device arrays with the capability to adjust the device densities can be extended to other applications of the next generation nanodevice technology.

Tunable polarised fluorescence of quantum dot doped nematic liquid crystals

ABSTRACTThe concentration, excitation photon wavelength, and polarisation dependent fluorescence of quantum dot (QD)–liquid crystal (LC) mixtures has been studied at room temperature using high-resolution, steady-state fluorescence spectroscopy. The fluorescence of QD–LC mixture increases with increasing QD’s concentration but the spectral red shift of ~10 nm relative to the stock QD–Toluene solution remains independent of concentration. In vertical switching (VS) cells, an external electric field changes the LC alignment direction and enhances the apparent fluorescence intensity. The apparent fluorescence anisotropy compared to that at zero applied electric field monotonically increases up to ~27% at an applied electric field of 0.6 V/µm. These results are consistent with the formation of disc-like assemblages of QDs oriented on planes perpendicular to the director of the nematic liquid crystal (NLC). These findings have important utility in polarisation sensitive photonic devices.

Flexible Supercapacitor Stackable with Solar Cells Based on PEDOT-Carbon Nanotube Nanocomposite Electrodes Using Ionic Liquid Gel Electrolytes for Solar Electricity Storage

Electrically conducting polymers (ECP) showing pseudo-capacitive property are actively researched as supercapacitor electrodes for energy storage. Nanocomposites with structured carbons contribute additional charge storage through electric double layer component enhancing the charge storage ability. The nanocomposites with CNTs hold additional promise to compensate for weak mechanical property of ECPs useful for flexible supercapacitor devices and enhance its electronic conductivity. Conventionally, the supercapacitor performance is evaluated using anion dopants in aqueous medium having voltage limitation and in organic electrolytes which are toxic. Ionic liquid gel electrolytes are promising but not widely studied for supercapacitor applications. Besides enabling stable high voltage operation, ionic liquid gel electrolyte makes supercapacitor assembly simple which can be configured as flat stackable layered device. These attributes are exploited in integration with the solar cells for the storage of solar electricity. We have investigated electrochemical and energy storage performance of poly(3,4-ethylenedioxy-thiophene) (PEDOT) and CNT nanocomposite electrodes based supercapacitors in the flexible solid-state format with ionic liquid gel electrolyte. In this work nanocomposite electrode was formed by co-deposition of PEDOT and CNT over the flexible graphite substrates using the galvanic pulsed electro-polymerization. The method involves ultrasonic dispersion of 1 wt% anionic CNT for 2 h in acetonitrile to which 0.1 M EDOT monomer and 0.2 M LiClO4 dopant was added. The PEDOT-CNT composite deposition is initiated as unipolar anodic current pulses of 4 mA.cm-2 amplitude with 10 ms ON time interspaced with 100 ms OFF time were sequentially applied for 20k cycles providing a unique way to control the electrode morphology. The symmetrical supercapacitor device was fabricated using ionic liquid 1-butyl-3-methylimidazolium tetrafluoro-borate ([BMIm]BF4) immobilized in poly(vinylidenefluoride -co-hexafluoropropylene (PVdF-HFP) forming a gel which was applied uniformly as ~50-70 μ layer sandwiched between the two PEDOT-CNT composite electrodes. With such stackable flat plate- supercapacitor device design, enhanced energy density is obtained and direct charge storage performance was realized when connected to solar cell module while mounted at the rear without affecting the form factor. The electrode morphology show randomly dispersed CNT of average diameter of 20-30 nm embedded within and encased by copolymerized PEDOT forming a 3-D network structure. The CNT is observed to enhance the PEDOT polymerization by facilitating charge transport at the substrate and also increase the electrode conductivity. Raman shifts in the 1425 cm−1 band assigned to symmetric Cα=Cβ (−O) stretching show in the formation of nanocomposite, the CNTs interact with the ethylenedioxy group of PEDOT resulting in high degree of conjugation and enhancrd charge transfer between CNTs and PEDOT molecular chains. Conjugated interaction is further confirmed by shift in disordered A1g mode of CNT. The cyclic voltammetry curves recorded over -1 to +1 V range showed fast and near vertical current switching at end potentials over wide 10-100 mV s-1 scan rates signifying highly capacitive property. Supercapacitor showed areal capacitance density of 357 mF.cm-2 yielding high single electrode specific capacitance of 297 F.g-1. The high areal specific capacitance in the PEDOT-CNT composite is due to dopant anion aggregation along the CNTs and enhanced conduction of dopant ions in ionic liquid through embedded CNT nanopores providing rapid pathways to delocalize the electrons from along the PEDOT molecular chain. Impedance data as Nyquist plots feature near vertical trend in the low frequency region and lower real impedance cutoff suggestive of highly capacitive property and low electrode-electrolyte resistance. Occurrence of high knee frequency of > 10 Hz suggests compared to PEDOT, the CNT nanocomposite has minimal diffusion limitation. Charge-discharge (CD) data at various currents cycled between 0.05-1.0V are triangular with low ESR of 20 Ohm.cm2 and Coulomb efficiency > 90%. Long-term CD cycling for 2000 cycles showed overall degradation of 21% with a nominal aggravation of merely 7% occurring in initial 400 cycles. These data show comparatively higher stability of CNT nanocomposite compared to PEDOT. The supercapacitor device functional properties were evaluated in terms of energy and power density parameters and presented as Ragone plots. Energy density of 18.3 Wh.kg-1 and corresponding power density of 5.1kW.kg-1 was recorded from CD data at 0.5 mA.cm-2 for 1V operation which is reasonably high for a gel electrolyte supercapacitor device. This paper would present detailed analysis of the pulse polymerization synthesis mechanism, microstructure and electrochemical properties of PEDOT-CNT nanocomposite. The supercapacitor device data on CV, CD, impedance and Ragone plots will be presented along with its efficacy in storing the solar electricity in combination with the solar cells.

Titanium oxide vertical resistive random‐access memory device

Pt/TiO2/Pt vertical resistive random-access memory switching devices were fabricated in a vertical three-dimensional structure by combining conventional photolithography, electron-beam evaporation for electrodes and atomic layer deposition for dielectric layers. The active switching cross-sectional area was ∼0.02 µm2, which is comparable to nanosized devices that require more elaborative fabrication processes. Structural integrity and electrical characteristics of the vertical memory device were analysed by cross-sectional scanning, transmission electron microscopy and current–voltage characteristics.