Search

Electrowetting-based adaptive optics are of great interest for applications ranging from confocal microscopy to LIDAR, but the impact of low-frequency mechanical vibration on these devices remains to be studied. We present a simple theoretical model for predicting the resonance modes induced on the liquid interface in conjunction with a numerical simulation. We experimentally confirm the resonance frequencies by contact angle modulation. They are found to be in excellent agreement with the roots of the zero-order Bessel functions of the first kind. Next, we experimentally verify that external axial vibration of an electrowetting lens filled with density mismatched liquids (Δρ = 250 kg/m3) will exhibit observable Bessel modes on the liquid–liquid interface. An electrowetting lens filled with density matched liquids (Δρ = 4 kg/m3) is robust to external axial vibration and is shown to be useful in mitigating the effect of vibrations in an optical system.

We demonstrate a method that permits wavefront aberration correction using an array of electrowetting prisms. A fixed high fill factor microlens array followed by a lower fill factor adaptive electrowetting prism array is used to correct wavefront aberration. The design and simulation of such aberration correction mechanism is described. Our results show significant improvement to the Strehl ratio by using our aberration correction scheme which results in diffraction limited performance. Compactness and effectiveness of our design can be implemented in many applications that require aberration correction, such as microscopy and consumer electronics.

Magnetic anisotropy modulation is an effective method to simultaneously reduce the switching current and extend the data retention of magnetic tunnel junction (MTJ), which is promising to be used in the next-generation spin transfer torque (STT) magnetic random-access memory. However, to meet the requirements of high storage life and harsh environments, the improved perpendicular magnetic anisotropy of MTJ makes the conventional modulation methods suffer from high breakdown risk owing to the relatively low efficiency. In this paper, a method of phase-change controlled magnetic anisotropy (PCMA) is introduced to a physical model of VO2/CoFeB/MgO/CoFeB perpendicular MTJ with superior modulation capability proved by systematical simulation. The time sequence of phase change pulse and STT pulse is studied, proving that there exists a specific interval to achieve both rapid and low-power switching. With the joint effect of PCMA and STT, low-energy (68.2 fJ), low-error-rate (0.08), and fast (2 ns) write operation can be achieved in the MTJ accompanied by a high thermal stability factor (78). The results demonstrate that the PCMA-STT switching strategy is most suitable for MTJ with large perpendicular magnetic anisotropy, paving a promising way to replace NOR flash memories.

Bipolar plasticity for synaptic emulation of indium gallium zinc oxide thin-film transistors (TFTs) with HfOxNy–HfO2–HfOxNy sandwich-stack films as the gate dielectric was investigated. The postsynaptic current increased when a negative pulse train was applied to the gate of TFTs; when a positive pulse was applied, the postsynaptic current was reduced. This result is discussed based on the charge trapping/releasing process with the assistance of ferroelectric behavior of the enwrapped oxygen-deficient-HfO2 layer. The dual response of this synaptic transistor shows promising prospect in the mimicking of biological neurons.

Photomultiplication-type ultraviolet (UV)/visible photodetectors (PDs) are demonstrated in an electrodeposited Cu2O/C60 hybrid structure. These simple organic/inorganic hybrid PDs exhibit external quantum efficiencies (EQEs) of 1.1 × 104% under illumination of 365 nm UV light at −3 V, indicating a large gain of photocurrent for these devices. Such an EQE is one of the highest values among the reported organic/inorganic hybrid PDs at the same voltage. Cu2O and C60 are found to play different roles in realizing the photomultiplication. Copper vacancies are proposed as the defects in the electrodeposited Cu2O layers, which can trap photogenerated holes. Such trapped holes will trigger the injection of multiple electrons and hence result in the photocurrent gain of the devices while C60 primarily acts as a light absorption media to provide free holes.

In this letter, we report a pilot study on epitaxy of monolayer graphene nanoribbons (GNRs) on hexagonal boron nitride (h-BN). We found that GNRs grow preferentially from the atomic steps of h-BN, forming in-plane heterostructures. GNRs with well-defined widths ranging from ∼15 nm to ∼150 nm can be obtained reliably. As-grown GNRs on h-BN have high quality with a carrier mobility of ∼20 000 cm2 V−1 s−1 for ∼100-nm-wide GNRs at a temperature of 1.7 K. Besides, a moire pattern induced quasi-one-dimensional superlattice with a periodicity of ∼15 nm for GNR/h-BN was also observed, indicating zero crystallographic twisting angle between GNRs and h-BN substrate. The superlattice induced band structure modification is confirmed by our transport results. These epitaxial GNRs/h-BN with clean surfaces/interfaces and tailored widths provide an ideal platform for high-performance GNR devices.

High-efficiency blue light-emitting diodes were realized based on the blends of poly(dialkylfluorene-co-dibenzothiophene) in poly[2-(2′-ethylhexyloxy)-1,4-phenylene] host. The external quantum efficiency was increased by about four times, relative to devices with the guest polymer alone. Much more balanced carrier injection was assigned as the origin of the performance enhancement. Dramatic narrowing on linewidth, with the full width at half maximum reduced from more than 40 to 14.3 nm, produced more pure blue emission.

Iridium complexes with alkyl substituted 2-phenylpyridine, Ir(Bu-PPy)3, were synthesized. Polymer light emitting diodes with Ir complexes as the guest materials and the substituted polyphenylenes as the host were fabricated. Ir(Bu-PPy)3-doped Poly(2-(6-cyano-6-methyl)- heptyloxy-1,4-phenylene) (CNPPP) device showed generally higher quantum efficiency (QE) than that of Ir(PPy)3-doped device for a given dopant concentration. More importantly, the addition of butyl group into phenylpyridine ligand significantly suppresses the decay of device efficiency at high current density. For instance, for devices made with Ir(Bu-PPy)3-doped CNPPP: the maximum external quantum efficiency, QE, and luminance efficiency reached 5.1% ph/el and 12 cd/A, respectively, at 800 cd/m2 and maintained at 4.2% ph/el and 10 cd/A, respectively, at 2500 cd/m2.

We reported efficient polymer light emitting diodes with alkali and alkaline earth metal fluoride Al cathodes. The quantum efficiencies of these devices are more than two orders of magnitude higher than those in devices without fluoride layer, comparable to the best data reported with similar electroluminescent polymers. The mechanism of the metal fluoride/Al bilayer cathodes was also discussed based on photovoltaic, ac impedance, and photoluminescence efficiency measurement.

Picosecond time-resolved photoluminescence (PL) spectroscopy has been used to investigate the optical properties of an insulating GaN epilayer grown by metalorganic chemical vapor deposition on a sapphire substrate. Two emission lines at 3.503 and 3.512 eV in the continuous wave (cw) PL spectra observed at 10 K under a low excitation intensity (∼23 W/cm2) were identified as the band-to-band transitions involving the A and B valence bands, respectively. A third emission line at 3.491 eV was identified as a band-to-impurity transition involving a shallow donor. The PL decay behavior can be well understood with a model taking into account both the free carriers and impurities. The effective recombination lifetime of the band-to-band transition in GaN was found to be about 3.7 ns. Possible mechanisms for the band-to-band transition being dominant in this high quality insulating GaN epilayer have also been discussed.

Picosecond time-resolved photoluminescence (PL) spectroscopy has been used to investigate the optical properties of GaN pyramids overgrown on hexagonal-patterned GaN(0001) epilayers on sapphire and silicon substrates with AlN buffer layers. We found that: (i) the release of the biaxial compressive strain in GaN pyramids on GaN/AlN/sapphire substrate led to a 7 meV redshift of the spectral peak position with respect to the strained GaN epilayer grown under identical conditions; (ii) in the GaN pyramids on GaN/AlN/sapphire substrate, strong band edge transitions with much narrower linewidths than those in the GaN epilayer have been observed, indicating the improved crystalline quality of the overgrown pyramids; (iii) PL spectra taken from different parts of the pyramids revealed that the top of the pyramid had the highest crystalline quality; and (iv) the presence of strong band-to-impurity transitions in the pyramids were primarily due to the incorporation of the oxygen and silicon impurities from the SiO2 mask.

It is well known that polycrystals exhibit abnormal Hall-Petch dependence when their grain sizes fall into the submicron range. A plausible explanation is that the scale dependence of the yield stress of submicron polycrystals is mediated not only by the internal energy of the dislocation patterns, but also by their configurational entropy. Two-level dislocation arrangements are explored which naturally embed a grain size. Detailed calculations of the internal energy and the configurational entropy are facilitated by the dislocation pileup theory and the Ashby model of geometric necessary dislocations. It is shown that the present model is capable of interpreting the abnormal Hall-Petch dependence for grain sizes in the submicron range.

The built‐in electric field and surface Fermi level in the InAlAs surface‐intrinsic‐n+ structures were studied by room‐temperature photoreflectance. The samples were grown by molecular beam epitaxy with an undoped layer thickness of 1000 A. The undoped layer was subsequently etched to 800, 600, 400, and 200 A. Different chemical solutions were used in the etching process and the built‐in electric field is found independent of the etching process. While the surface Fermi level, in general, varies with the undoped layer thickness, there exists, for each Al concentration, a certain range of thicknesses within which the surface Fermi level is weakly pinned. From the dependence of electric field and surface Fermi level on the undoped layer thickness, we conclude that the surface states distribute over two separate regions within the energy band gap and the densities of surface states are as low as 1.02±0.05×1011 cm−2 for the distribution near the conduction band and 2.91±0.05×1011 cm−2 for the distribution nea...

Large signal dynamics of passive mode locking of semiconductor lasers is studied in the time domain. A two‐section structure is modeled by numerical evaluation of the traveling wave rate equations. The results show simultaneous occurrence of mode locking and self‐pulsation over a large parameter range. Typical transient time for mode locking is about 0.2–1.0 ns, and pulse widths of 1.5–8 ps are observed.

An AlGaAs/GaAs ridge directional coupler electro‐optic modulator with a voltage‐length (VL) product of only 7.0 V mm (L=3.5 mm, V=2 V) is reported. The low VL product has resulted from the efficient electrode configuration as well as the large overlap between the optical and electrical fields in the waveguides. Propagation loss of 3.4 dB/cm and the measured I‐V characteristics confirm that the contributions from the Franz–Keldysh, charge‐carrier, and thermal effects are small. To our knowledge, this is the lowest VL product ever reported for an electro‐optic directional coupler modulator utilizing the Pockels effect. An extinction ratio of 13 dB was measured for the modulator at 0.83 μm wavelength.

Identifying the role of defects that limits graphene's quality is important for various graphene devices on SiO2. In this paper, monolayer graphene samples with defect densities varying from ∼0.04 μm−2 to ∼10 μm−2 on SiO2 are characterized by both microscopic imaging and electrical transport measurements. We found that the height of graphene on SiO2 is directly related to its defect densities with a reverse correlation, which in turn degrade graphene's quality through a complicated mechanism rather than defects scattering only. We suggest that, at relative high defect density regime, graphene-SiO2 coupling is greatly enhanced causing an increasing charged impurity scattering significantly.

In this Letter, we report on the design criteria of plasmonic nano-lasers based on hybrid waveguides using semiconductor gain to compensate for the metal loss. A quantitative measure of the plasmonic-like character in nano-lasers was discussed with the distance of operation wavelength to plasmon resonance, herein defined as “plasmonicity” ( Θ=ω/ωsp). We found that the effective index (confinement) increased with plasmonicity rapidly when approaching resonance and the associated giant modal loss prohibited lasing. We further studied the design criteria of a practicable green nano-laser, based on a vertical p-n junction combined with a lateral semiconductor/dielectric/metal core-shell structure. The influence of thickness and permittivity of dielectric spacer and also the critical thickness of the active region were investigated numerically. The approach proposed here will shed light on the future fabrication of electrical injection nano-lasers with high plasmonicity.

Electrical characteristics of pentacene-based ambipolar organic field-effect transistors (OFETs) are examined and are shown to significantly differ from those of unipolar OFETs. The electrical and hysteresis characteristics of the ambipolar OFETs depend on the applied source-drain bias and sweeping gate voltage range. The ambipolar OFET characteristics, such as charge mobility, subthreshold swing, threshold voltage, and off-current level, are controlled by the dual-carrier recombination and release process, through which opposite-sign charges can capture and release majority charges. This study contributes to advancing the development of more applications based on ambipolar OFETs.

We demonstrate the distorted K1 soft mode for the phonon instability of single layer hexagonal-BN sheet under tension, which is different from the well known K1 mode failure mechanism of graphene. We also investigate the influences of charge doping on the soft mode phonon instability. Meanwhile, the ideal strength is found to be larger for both p- and n-type doped h-BN sheet, reflecting the electronic strengthening effect as similar as graphene. Our results reveal a different soft mode for h-BN sheet from that of graphene and can serve as references for future applications of h-BN sheet.

We report the stimulated emission related anomalous behavior of electrical parameters in GaN-based laser diodes based on both experiments and self-consistent calculations. In particular, the separation of quasi-Fermi levels of electrons and holes across active region shows a drop (or “pinning”) in threshold region, followed by an increase with current. The start and end points correspond to the two kink points of the first derivative of the optical output power-current (P-I) curve. This abnormal change of electrical parameters always satisfies the need of sufficient carriers for stimulated emission in the process of transition to lasing as well as lasing enhancement.

The exact relationship between voltage and frequency when negative capacitance (NC) appears in light-emitting diodes containing InGaN was determined both experimentally and by solving the physical equations for the complete equivalent circuit of a symmetrical p-n junction combined with its boundary condition. Results from the calculation revealed that NC depends on the recombination lifetime of carriers; therefore, we can accurately determine recombination lifetime from the appearance of NC.

Graphene shows promise on strain sensor applications, but the piezoresistive sensitivity of perfect graphene is low due to its weak electrical conductivity response upon structural deformation. In this paper, we used nanographene films for ultra-sensitive strain sensors. The piezoresistive sensitivity of nanographene films with different thicknesses and conductivities was systematically investigated and a nearly inverse proportional correlation was found. A gauge factor over 300, the highest so far for graphene-based strain sensors, was achieved. A charge tunneling model was used to explain the piezoresistive characteristics of nanographene films, which indicates our results provide a different rout toward ultra-sensitive strain sensors.

Related Articles Emissive properties of SiO2 thin films through photonic windows Appl. Phys. Lett. 100, 091901 (2012) Size-dependent hydrogen uptake behavior of Pd nanoparticles revealed by photonic crystal surface waves Appl. Phys. Lett. 100, 083108 (2012) Achieving all-dielectric left-handed metamaterials via single-sized dielectric resonators J. Appl. Phys. 111, 044903 (2012) SiGe superlattice nanocrystal infrared and Raman spectra: A density functional theory study J. Appl. Phys. 111, 044306 (2012) Spectral reflectance switching of colloidal photonic crystal structure composed of positively charged TiO2 nanoparticles Appl. Phys. Lett. 100, 063113 (2012)

We studied the influence of a tapered AlGaN electron blocking layer (EBL) with step-graded aluminum composition on hole injection and electron overflow effects in InGaN-based laser diodes (LDs) theoretically. Schrodinger-Poisson self-consistent method together with transfer matrix method was applied to calculate carrier distribution and transport properties for both electrons and holes in tapered EBL and conventional EBL. The results indicate that the new structure favors the tunneling of low energy holes from the p-side to the active region. Meanwhile, more uniform carrier distribution and better balance between electrons and holes are obtained for the tapered structure by proper modification of band diagrams. An advanced device simulation shows the elimination of electron overflow even at a current of 180 mA in the LD with tapered EBL. Decrease of threshold current density from 2.0 kA/cm2 to 1.6 kA/cm2 is benefited from the more uniform local gain profile.

The performance of bilayer heterojunction, pentacene and perylene tetracarboxylic diimide organic solar cells (OSC) is dramatically enhanced by performing nanoimprinting on the hole transport layer in which poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) nanogratings are built. A standard OSC embeded with PEDOT:PSS gratings enables photocarriers to move toward electrodes. In particular, the visualization of morphologies for these heterojunction layers reveals pillar-like grains that are induced by the geometric effect of PEDOT:PSS gratings. Toghther with analyzing absorption and time-resolved photoluminescence spectra, a consistent correlation between OSC performance, photophysical data, and pillar-like morphology has been obtained for the device with imprinted PEDOT:PSS nanogratings.

In this study, an enhancement-mode (E-mode) AlGaN/GaN high electron mobility transistor (HEMT) using N2O plasma oxidation process has been performed. Before the gate metal deposition, the AlGaN barrier layer was treated by 135 W N2O plasma for 240 s. The surface native oxide of AlGaN was removed and a 4 nm high quality Ga2O3/Al2O3 compound insulator was simultaneously formed. The threshold voltage of conventional depletion-mode (D-mode) GaN HEMT was −3.6 V and this value can be shifted to +0.17 after N2O plasma oxidation treatment beneath the gate metal. This 4 nm Ga2O3/Al2O3 compound insulator oxidized by N2O can suppress the polarization charge density for E-mode operation of gate terminal and the gate leakage was also reduced simultaneously.

When multiferroic terbium manganite (TbMnO3) crystal cylinders are periodically arranged in a square lattice, the resulting two-dimensional (2D) system exhibits photonic band gaps (PBGs). The absolute PBG originating from the Mie resonance is modulated from closed to open by applying an external static magnetic field, which is attributed to the electromagnon depression of the dielectric constant by the rearrangement of antiferromagnetic order. Tunable electromagnetic band structure may be realized by controlling the magnetic transition of manganese spins in TbMnO3.

Polymorphism control of organic semiconductor films greatly affects optoelectronic device performance. The thin-film phase is always the dominant phase in the early growth of pentacene films; however, the performance of the bulk phase is superior to that of the thin-film phase. In the present study, a nanoimprinted polyimide layer is used to induce the growth of the bulk phase in the early stage. The bulk phase appears in the initial few layers and its dominance of the polymorphisms in the pentacene film increases with increasing thickness. The mechanism of the phase transformation is explained.

We demonstrated GaN-based light emitting diodes (LEDs) with different embedded heights of SiO2 pillars and air gap array structures. The air gap on top of the SiO2 pillars were also realized using the enhanced epitaxial lateral overgrowth mode. With the embedded SiO2 pillars and air gap array structures, we achieved a smaller reverse leakage current due to the lateral growth-induced crystal quality improvement. Moreover, under 20 mA current injections, the output powers were 3.04, 4.23, 4.66, and 4.44 mW for conventional LED, LEDs with embedded 200 and 500 nm height of SiO2 pillars and air gaps, 500 nm height of SiO2 pillars and air gaps, and 700 and 400 nm height of SiO2 pillars and air gaps, respectively. We found that the embedded 500 nm height SiO2 pillars and 500 nm height air gap array structures could enhance LED output power by more than 50% due to the enhanced guided-light scattering efficiency in our study.

We integrated pentacene- and NN′-diheptyl-3,4,9,10-perylenebiscarboximide-based transistors into an organic complementary metal oxide semiconductor (O-CMOS) whose gate-dielectric surface was modified by polyimide (PI). The hysteresis behaviors in metal-oxide-semiconductors, field-effect transistors, and O-CMOS were reported clearly. Measurements of hysteresis showed that the PI exhibited high trapping and detrapping speeds for charge carriers, including holes and electrons, to result in high performance transistors and O-CMOSs; moreover, the trapping and detrapping speeds were matched. Finally, a PI-modified organic inverter with little hysteresis, low static power dissipation, high noise margins, and switching voltage near VDD/2 was achieved simultaneously.

The charge carrier transport efficiency and issues of patterning in organic semiconductors limit the potential range of microelectronic and optoelectronic applications of organic devices in nanoscale. We demonstrate high-performance organic field-effect transistors (OFETs) with a mobility of approximately 2.5 cm2/V s using nanogroove gate-dielectrics formed by nanoimprinting. The preferred flow of charge carriers in OFETs parallel to the nanogrooves yields a high mobility anisotropic ratio (above 220), providing a built-in autopattern organic semiconductor function with nanoscale resolution. This nanostructure embedded device has great potential for use in the manufacture and lithography-free patterning of organic semiconductor films in integrated circuits.