Nonlinear terahertz metamaterials with active electrical control Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 G. R. Keiser, N. Karl, P. Q. Liu, C. Tulloss, H.-T. Chen, A. J. Taylor, I. Brener, J. L. Reno, D. M. Mittleman
We present a study of an electrically modulated nonlinear metamaterial consisting of an array of split-ring resonators fabricated on n-type gallium arsenide. The resonant metamaterial nonlinearity appears as an intensity-dependent transmission minimum at terahertz frequencies and arises from the interaction between local electric fields in the split-ring resonator (SRR) capacitive gaps and charge carriers in the n-type substrate. We investigate the active tuning range of the metamaterial device as the incident terahertz field intensity is increased and conversely the effect of an applied DC bias on the terahertz field-induced nonlinear modulation of the metamaterial response. Applying a DC bias to the metamaterial sample alters the nonlinear response and reduces the net nonlinear modulation. Similarly, increasing the incident terahertz field intensity decreases the net modulation induced by an applied DC bias. We interpret these results in terms of DC and terahertz-field-assisted carrier acceleration, scattering, and multiplication processes, highlighting the unique nature of this DC-field modulated terahertz nonlinearity.
Experimental demonstration of Fabry-Perot open resonators in a surface-wave bandgap crystal Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 Zhen Gao, Fei Gao, Hongyi Xu, Youming Zhang, Baile Zhang
We report on the proposal and experimental realization of a type of retardation-based Fabry-Perot (FP) open resonator in a surface-wave bandgap crystal implemented on a single structured metal surface. This surface-wave FP open resonator is formed by introducing a finite line defect in the surface-wave bandgap crystal, whose resonance frequencies lie exactly within the forbidden bandgap of the surrounding crystal. Due to the complete surface-wave forbidden bandgap, a new FP plasmonic resonance mode exhibiting monopolar features which is missing in both traditional FP resonators and plasmonic resonators is emerged. Near-field response spectra and imaged mode profiles are presented to characterize the properties of these FP open resonators in the microwave regime.
Spectral response of steady-state photoluminescence from GaAs1-xPx layers grown on a SiGe/Si system Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 Li Wang, Michael E. Pollard, Mattias Klaus Juhl, Brianna Conrad, Anastasia Soeriyadi, Dun Li, Anthony Lochtefeld, Andrew Gerger, Darren M. Bagnall, Allen Barnett, Ivan Perez-Wurfl
Measuring the spectral response of photoluminescence (SRPL) in solar cells has recently attracted attention as it can be used as a contactless relative measure of external quantum efficiency (EQE) prior to full device fabrication. However, this technique requires that the monitored PL spectrum originates mainly from a region in the solar cell with uniformly distributed majority carriers. For a stack of thin films with a similar material composition, the slightly different emission spectrum from each layer may lead to the superposition of several luminescence peaks. This letter presents the measurement of the SRPL from GaAsP tandem solar cells and outlines a method for separating the individual layer contributions. Good agreement between measured SRPL and EQE at short wavelengths has been achieved, and the deviations at longer wavelengths have been analyzed. This study also reveals unexpected bandgap narrowing resulting from a variable material composition within the active region.
Enhanced optical nonlinearities in CMOS-compatible ultra-silicon-rich nitride photonic crystal waveguides Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 E. Sahin, K. J. A. Ooi, G. F. R. Chen, D. K. T. Ng, C. E. Png, D. T. H. Tan
We present the design, fabrication, and characterization of photonic crystal waveguides (PhCWs) on an ultra-silicon-rich nitride (USRN) platform, with the goal of augmenting the optical nonlinearities. The design goals are to achieve an optimized group index curve on the PhCW band edge with a non-membrane PhCW with symmetric SiO2 undercladding and overcladding, so as to maintain back-end CMOS compatibility and better structural robustness. Linear optical characterization, as well as nonlinear optical characterization of PhCWs on ultra-silicon-rich nitride is performed at the telecommunication wavelengths. USRN's negligible two-photon absorption and free carrier losses at the telecommunication wavelengths ensure that there is no scaling of two-photon related losses with the group index, thus maintaining a high nonlinear efficiency. Self-phase modulation experiments are performed using a 96.6 μm PhCW. A 1.5π phase shift is achieved with an input peak power of 2.5 W implying an effective nonlinear parameter of 1.97 × 104 (W m)−1. This nonlinear parameter represents a 49× enhancement in the nonlinear parameter from the slow light effect, in good agreement with expected scaling from the measured group index.
Homodyne phase sensitive terahertz spectrometer Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-20 S. Rumyantsev, X. Liu, V. Kachorovskii, M. Shur
We present the theory of a field effect transistor (FET) operating as a THz or far infrared detector. We demonstrate that the detected signal is enhanced by orders of magnitudes by using a homodyne detection scheme involving the interference of a weak incoming signal and a strong signal of a local oscillator with the close frequency. We developed a theory valid for the arbitrary relationship between the amplitude of the local oscillator and the gate voltage swing. Remarkably, the response saturates at a high local oscillator intensity at the value which depends on the phase difference between the signals. The observed gain in this regime is over 100, and the predicted maximum gain in this operating regime is on the order of 105. These results show that a FET could be used as a sensitive spectrometer and/or interferometer when exposed to a strong tunable local oscillator signal with the varying frequencies and phases. This regime of the detector operation is very promising for the interferometric and spectroscopic applications in the subterahertz and terahertz ranges.
Resonance-free optical response of a vertical cavity transistor laser Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-22 M. Feng, Cheng-Han Wu, M. K. Wu, Chao-Hsin Wu, N. HolonyakJr.
Optical resonance in a semiconductor laser is a major limitation in high speed data communications, resulting in bit error rate degradation and requiring additional power consuming error-correction circuits to counter these effects. In this work, we report the microwave bandwidth measurement of a vertical cavity transistor laser with an oxide-confined aperture of 4.7 × 5.4 μm2 and demonstrate a 3 dB bandwidth of 11 GHz resonance-free optical response via base-current or collector-voltage modulation. The emission spectra exhibit single-mode operation around 970 nm with a narrow linewidth of Δλ ∼ 0.23 Å (cavity Q of 42 216). The resonance-free optical response is explained by the absence of carrier “accumulating” due to the fast base electron-hole recombination lifetimes and a gradient in the minority carrier charge in the transistor active mode.
Tunable InGaN quantum dot microcavity light emitters with 129 nm tuning range from yellow-green to violet Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-22 Yang Mei, Rong-Bin Xu, Guo-En Weng, Huan Xu, Lei-Ying Ying, Zhi-Wei Zheng, Hao Long, Bao-Ping Zhang, Werner Hofmann, Jian-Ping Liu, Jian ZhangJr., Mo Li, Jian ZhangSr.
An electrically pumped wavelength-tunable InGaN quantum dot (QD) based microcavity (MC) lighter emitter with a large tuning range of 129 nm was demonstrated. The multi-mode emission spectrum was tuned by injected current from 564 nm (yellow-green) to 435 nm (violet). The MC light emitter is featured with a double dielectric distributed Bragg reflector structure and a copper substrate fabricated using substrate transfer and laser lift off techniques. By utilizing an InGaN QD active layer with a tunable broad emission spectrum and a Fabry-Pérot cavity which allows multi-longitudinal mode resonating, the emission spectrum could be tuned among several particular cavity modes, which are decided by the gain enhancement factor. In addition, both the enhancement and suppression of MC emission modes caused by the gain enhancement factor were observed in a single MC device. As the first electrically driven III-V nitride semiconductor based tunable MC light emitter with a tuning range of 129 nm, the device is promising for applications such as in wide-gamut compact displays and projectors.
Metal-insulator-metal antennas in the far-infrared range based on highly doped InAsSb Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-22 F. Omeis, R. Smaali, F. Gonzalez-Posada, L. Cerutti, T. Taliercio, E. Centeno
Plasmonic behavior in the far-infrared (IR) and terahertz (THz) ranges can facilitate a lot of applications in communication, imaging or sensing, security, and biomedical domains. However, simple scaling laws cannot be applied to design noble metal-based plasmonic systems operating at far-IR or THz frequencies. To overcome this issue, we numerically and experimentally explore the plasmonic properties in the spectral range between 25 and 40 μm (12 and 7.5 THz) of metal-insulator-metal (MIM) antennas made of InAsSb a highly Si-doped semiconductor. We demonstrate that these MIM antennas sustain a gap plasmon mode that is responsible for high light absorption. By tracking this peculiar plasmonic signature for various antennas' widths, we prove that Si-doped InAsSb microstructures realized on large areas by laser lithography and the wet etching process are a low cost, reproducible, and readily CMOS compatible approach.
Spatially resolved chemical analysis of photodecomposition and doping effect of fluoropolymer-covered graphene Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-18 Mihyun Yang, Kyuwook Ihm, Soo Young Kim, Joung Real Ahn
We have studied the photo-decomposition of fluoropolymer-covered graphene and its effects on the electrical properties of embedded graphene using spatially resolved X-ray photoemission spectroscopy. From the comparative approach to the photo-decomposition and chemical analysis, we clearly prove that the fluorine atoms are desorbed from the sample surface by photon irradiation, resulting in a change of difluoride into a monofluoride form. As this photo-induced chemical modification proceeds, the dipole field changes strongly, which is responsible for the field-driven Dirac point realignment of the graphene layer. The desorption temperature of the photo-modified fluoropolymer was similar to that without photon irradiation (286 °C; ∼0.047 eV); this similarity means that photo-modification did not cause chemical interactions between the fluoropolymer and graphene.
Tailoring Al-SiO2 interfacial work function using an organophosphonate nanolayer Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 Matthew Kwan, Roy Winter, P. Hubert Mutin, Moshe Eizenberg, Ganpati Ramanath
We show that introducing a mercaptan-terminated organophosphonate nanomolecular layer (NML) at the Al-SiO2 interface decreases the effective metal work function Φeff by 0.67 eV. In contrast, introducing a methyl-terminated organophosphonate NML has a negligible impact on Φeff. Photoelectron spectroscopy of NML-tailored surfaces and Al-NML-SiO2 interfaces indicate that Al bonds with oxidized mercaptan moieties form Al-O-S bridges, which determine the Φeff shift. Our findings should be useful for molecularly tailoring the electronic properties of metal-ceramic interfaces for electronics and energy device applications.
Mimicking glide symmetry dispersion with coupled slot metasurfaces Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-20 Miguel Camacho, Rhiannon C. Mitchell-Thomas, Alastair P. Hibbins, J. Roy Sambles, Oscar Quevedo-Teruel
In this letter, we demonstrate that the dispersion properties associated with glide symmetry can be achieved in systems that only possess reflection symmetry by balancing the influence of two sublattices. We apply this approach to a pair of coupled slots cut into an infinite perfectly conducting plane. Each slot is notched on either edge, with the complete two-slot system having only mirror symmetry. By modifying the relative size of the notches on either side of the slots, we show that a linear dispersion relation with a degeneracy with non-zero group velocity at the Brillouin zone boundary can be achieved. These properties, until now, only found in systems with glide symmetry are numerically and experimentally validated. We also show that these results can be used for the design of ultra-wideband one-dimensional leaky wave antennas in coplanar waveguide technology.
A large-strain, fast-response, and easy-to-manufacture electrothermal actuator based on laser-reduced graphene oxide Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-18 Tian-Yu Zhang, Qian Wang, Ning-Qin Deng, Hai-Ming Zhao, Dan-Yang Wang, Zhen Yang, Ying Liu, Yi Yang, Tian-Ling Ren
In this paper, we have developed a high-performance graphene electrothermal actuator (ETA). The fabrication method is easy, fast, environmentally friendly, and suitable for preparing both large-size and miniature graphene ETAs. When applied with the driving voltage of 65 V, the graphene ETA achieves a large bending angle of 270° with a fast response of 8 s and the recovery process costs 19 s. The large bending deformation is reversible and can be precisely controlled by the driving voltage. A simple robotic hand prepared by using a single graphene ETA can hold the object, which is more than ten times the weight of itself. By virtue of its large-strain, fast response, and easy-to-manufacture, we believe that the graphene ETA has tremendous potential in extensive applications involving biomimetic robotics, artificial muscles, switches, and microsensors in both macroscopic and microscopic fields.
The dielectric signature of glass density Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 M. Rams-Baron, Z. Wojnarowska, J. Knapik-Kowalczuk, K. Jurkiewicz, A. Burian, M. Wojtyniak, J. Pionteck, M. Jaworska, C. Rodríguez-Tinoco, M. Paluch
At present, we are witnessing a renewed interest in the properties of densified glasses prepared by isobaric cooling of a liquid at elevated pressure. As high-pressure densification emerges as a promising approach in the development of glasses with customized features, understanding and controlling their unique properties represent a contemporary scientific and technological goal. The results presented herein indicate that the applied high-pressure preparation route leads to a glassy state with higher density (∼1%) and a reduced free volume of about 7%. We show that these subtle structural changes remarkably influence the dielectric response and spectral features of β-relaxation in etoricoxib glass. Our study, combining dynamical and structural techniques, reveal that β-relaxation in etoricoxib is extremely sensitive to the variations in molecular packing and can be used to probe the changes in glass density. Such connection is technologically relevant and may advance further progress in the field.
Hybrid 2D patterning using UV laser direct writing and aerosol jet printing of UV curable polydimethylsiloxane Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-20 Kotaro Obata, Adam Schonewille, Shayna Slobin, Arndt Hohnholz, Claudia Unger, Jürgen Koch, Oliver Suttmann, Ludger Overmeyer
The hybrid technique of aerosol jet printing and ultraviolet (UV) laser direct writing was developed for 2D patterning of thin film UV curable polydimethylsiloxane (PDMS). A dual atomizer module in an aerosol jet printing system generated aerosol jet streams from material components of the UV curable PDMS individually and enables the mixing in a controlled ratio. Precise control of the aerosol jet printing achieved the layer thickness of UV curable PDMS as thin as 1.6 μm. This aerosol jet printing system is advantageous because of its ability to print uniform thin-film coatings of UV curable PDMS on planar surfaces as well as free-form surfaces without the use of solvents. In addition, the hybrid 2D patterning using the combination of UV laser direct writing and aerosol jet printing achieved selective photo-initiated polymerization of the UV curable PDMS layer with an X-Y resolution of 17.5 μm.
Buckling analysis of stiff thin films suspended on a substrate with tripod surface relief structure Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-20 Qingmin Yu, Furong Chen, Ming Li, Huanyu Cheng
A wavy configuration is a simple yet powerful structural design strategy, which has been widely used in flexible and stretchable electronics. A buckled structure created from a prestretch-contact-release process represents an early effort. Substrates with engineered surface relief structures (e.g., rectangular islands or tripod structure) have enabled stretchability to the devices without sacrificing their electric performance (e.g., high areal coverage for LEDs/photovoltaics/batteries/supercapacitors). In particular, the substrate with a tripod surface relief structure allows wrinkled devices to be suspended on a soft tripod substrate. This minimizes the contact area between devices and the deformed substrate, which contributes to a significantly reduced interfacial stress/strain. To uncover the underlying mechanism of such a design, we exploit the energy method to analytically investigate the buckling and postbuckling behaviors of stiff films suspended on a stretchable polymeric substrate with a tripod surface relief structure. Validated by finite element analysis, the predications from such an analytical study elucidate the deformed profile and maximum strain in the buckled and postbuckled stiff thin device films, providing a useful toolkit for future experimental designs.
Carrier diffusion in thin-film CH3NH3PbI3 perovskite measured using four-wave mixing Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-21 D. Webber, C. Clegg, A. W. Mason, S. A. March, I. G. Hill, K. C. Hall
We report the application of femtosecond four-wave mixing (FWM) to the study of carrier transport in solution-processed CH3NH3PbI3. The diffusion coefficient was extracted through direct detection of the lateral diffusion of carriers utilizing the transient grating technique, coupled with the simultaneous measurement of decay kinetics exploiting the versatility of the boxcar excitation beam geometry. The observation of the exponential decay of the transient grating versus interpulse delay indicates diffusive transport with negligible trapping within the first nanosecond following excitation. The in-plane transport geometry in our experiments enabled the diffusion length to be compared directly with the grain size, indicating that carriers move across multiple grain boundaries prior to recombination. Our experiments illustrate the broad utility of FWM spectroscopy for rapid characterization of macroscopic film transport properties.
Flexible strain sensors with high performance based on metallic glass thin film Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-21 H. J. Xian, C. R. Cao, J. A. Shi, X. S. Zhu, Y. C. Hu, Y. F. Huang, S. Meng, L. Gu, Y. H. Liu, H. Y. Bai, W. H. Wang
Searching strain sensitive materials for electronic skin is of crucial significance because of the restrictions of current materials such as poor electrical conductivity, large energy consumption, complex manufacturing process, and high cost. Here, we report a flexible strain sensor based on the Zr55Cu30Ni5Al10 metallic glass thin film which we name metallic glass skin. The metallic glass skin, synthesized by ion beam deposition, exhibits piezoresistance effects with a gauge factor of around 2.86, a large detectable strain range (∼1% or 180° bending angle), and good conductivity. Compared to other e-skin materials, the temperature coefficient of resistance of the metallic glass skin is extremely low (9.04 × 10−6 K−1), which is essential for the reduction in thermal drift. In addition, the metallic glass skin exhibits distinct antibacterial behavior desired for medical applications, also excellent reproducibility and repeatability (over 1000 times), nearly perfect linearity, low manufacturing cost, and negligible energy consumption, all of which are required for electronic skin for practical applications.
Impeded thermal transport in composition graded SiGe nanowires Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-21 Honggang Zhang, Haoxue Han, Shiyun Xiong, Hongyan Wang, Sebastian Volz, Yuxiang Ni
Composition graded nanowires (NWs) have attracted increasing research interest in the application of optoelectronic devices, due to their graded bandgaps caused by the changing composition. However, the thermal transport property of composition graded NWs is not clear, which is critical for their potential applications in electronics and thermoelectrics. In this Letter taking SiGe NW as an example, we explore the thermal transport property of composition graded NWs. Molecular dynamics simulations reveal that the thermal conductivities (κ) of the composition graded SiGe NWs can be reduced up to 57% compared with that of the corresponding SiGe NW with abrupt interfaces. The κ reduction stems from the shortening of phonon mean free paths due to the inhomogeneous composition distributions. The phonon wave packet propagation analysis reveals that the composition gradient can reflect more than 70% of the wave packet energy, and phonon localization is observed in the composition graded region. Our findings suggest a promising prospect of composition graded NWs in the use of thermoelectrics and high temperature coatings, where low thermal conductivity is expected.
Full control of heat transfer in single-particle structural materials Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-22 S. Yang, L. J. Xu, R. Z. Wang, J. P. Huang
Thermal metamaterials have been applied to implement thermal phenomena, such as invisibility, illusion, and refraction. However, during the fabrication, they probably have complicated issues which are on account of the complicated structures. To get around this, here we put forward a single-particle structure. The theory helps to simplify the existing methods, which will undoubtedly contribute to the efficiency of fabrication. For clarity, we show the simulation and experimental results of thermal invisibility and illusion based on our proposed single-particle structural materials. Moreover, by tailoring the shape factor of the single particle appropriately, we can simultaneously realize thermal invisibility and illusion with only one device. The adjustable area fraction also indicates that these types of structural materials are highly adaptable. Such a single-particle device may have broad applications in misleading infrared detection. This work not only opens an avenue to design thermal materials based on single-particle structures but also holds for other physical fields like electrostatics, magnetostatics, and particle dynamics.
Synthesis and optical properties of (GaAs)yGe5-2y alloys assembled from molecular building blocks Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 P. E. Sims, P. M. Wallace, Chi Xu, C. D. Poweleit, B. Claflin, J. Kouvetakis, J. Menéndez
Monocrystalline alloys of GaAs and Ge with compositions (GaAs)yGe5–2y have been synthesized following a chemical vapor deposition approach that promotes the incorporation of Ga and As atoms as isolated donor-acceptor pairs. The structural and optical properties show distinct behavior relative to (GaAs)1-xGe2x counterparts produced by conventional routes. Strong band gap photoluminescence is observed in the 0.5–0.6 eV range for samples whose compositions approach the GaAsGe3 limit for isolated Ga-As pairs. In such samples, the Ge-like Raman modes appear at higher frequencies and are considerably narrower than those observed in samples with higher Ge concentrations. These results suggest that the growth mechanism may favor the formation of ordered phases comprising Ga-As-Ge3 tetrahedra. In contrast with the diamond-to-zincblende ordering transition previously reported for III-V-IV alloys, ordered structures built from Ga-As-Ge3 tetrahedra feature III-III and V-V pairs as third-nearest neighbors, and therefore both the III- and V-components are equally present in each of two fcc sublattices of the average diamond-like structure. These bonding arrangements likely lead to the observed optical response, indicating potential applications of these materials in mid-IR technologies integrated on Si.
Initial leakage current paths in the vertical-type GaN-on-GaN Schottky barrier diodes Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 Liwen Sang, Bing Ren, Masatomo Sumiya, Meiyong Liao, Yasuo Koide, Atsushi Tanaka, Yujin Cho, Yoshitomo Harada, Toshihide Nabatame, Takashi Sekiguchi, Shigeyoshi Usami, Yoshio Honda, Hiroshi Amano
Electrical characteristics of leakage current paths in vertical-type n-GaN Schottky barrier diodes (SBDs) on free-standing GaN substrates are investigated by using photon emission microscopy (PEM). The PEM mapping shows that the initial failure of the SBD devices at low voltages is due to the leakage current paths from polygonal pits in the GaN epilayers. It is observed that these polygonal pits originate from carbon impurity accumulation to the dislocations with a screw-type component by microstructure analysis. For the SBD without polygonal pits, no initial failure is observed and the first leakage appeals at the edge of electrodes as a result of electric field concentration. The mechanism of leakage at pits is explained in terms of trap assisted tunneling through fitting current-voltage characteristics.
Study on the band alignment of GaN/CH3NH3PbBr3 heterojunction by x-ray photoelectron spectroscopy Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 Jinhui Gong, Shitao Liu, Yuandan He, Xingcan Feng, Xuefeng Xia, Zhijue Quan, Li Wang
A GaN/CH3NH3PbBr3 heterojunction was fabricated by depositing a GaN thin layer on a CH3NH3PbBr3 single crystal by plasma enhanced atomic layer deposition. The band alignment of the GaN/CH3NH3PbBr3 heterojunction was studied by x-ray photoelectron spectroscopy. The valance band offset (VBO) is directly determined to be 0.13 ± 0.08 eV. The conduction band offset is deduced from the VBO and the band gaps, which turned out to be 1.39 ± 0.12 eV. Thus, the band alignment of the GaN/CH3NH3PbBr3 heterojunction is determined to be type-I. These results show that GaN is a promising material for carrier confinement in halide perovskite based light emitting devices.
A nonmagnetic topological Weyl semimetal in quaternary Heusler compound CrAlTiV Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-20 Xiaoxiong Liu, Lei Li, Yvgui Cui, Jianbo Deng, Xiaoma Tao
We predict that the paramagnetic state of inversion-breaking quaternary Heusler alloy CrVTiAl is a Weyl semimetal candidate. There are 24 Weyl nodes (WN) appearing at the same energy in the first Brillouin zone without spin-orbit coupling (SOC). One WN splits into a pair of Weyl nodes (WN1 and WN2) of the same chirality when considering SOC because SOC removes the spin degeneracy. Clear surface state Fermi arc structures on the (111) surface are also obtained. The formation mechanism of the Weyl nodes is independent of point symmetries, i.e., the number and chirality of Weyl nodes are not affected when the point symmetries are broken slightly.
Temperature dependence of the band gap of zinc nitride observed in photoluminescence measurements Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-20 A. Trapalis, I. Farrer, K. Kennedy, A. Kean, J. Sharman, J. Heffernan
We report the photoluminescence properties of DC sputtered zinc nitride thin films in the temperature range of 3.7–300 K. Zinc nitride samples grown at 150 °C exhibited a narrow photoluminescence band at 1.38 eV and a broad band at 0.90 eV, which were attributed to the recombination of free carriers with a bound state and deep-level defect states, respectively. The high-energy band followed the Varshni equation with temperature and became saturated at high excitation powers. These results indicate that the high-energy band originates from shallow defect states in a narrow bandgap. Furthermore, a red-shift of the observed features with increasing excitation power suggested the presence of inhomogeneities within the samples.
Band alignment of B0.14Al0.86N/Al0.7Ga0.3N heterojunction Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-21 Haiding Sun, Young Jae Park, Kuang-Hui Li, C. G. Torres Castanedo, Abdulmohsen Alowayed, Theeradetch Detchprohm, Russell D. Dupuis, Xiaohang Li
Owing to large bandgaps of BAlN and AlGaN alloys, their heterojunctions have the potential to be used in deep ultraviolet and power electronic device applications. However, the band alignment of such junctions has not been identified. In this work, we investigated the band-offset parameters of a B0.14Al0.86 N/Al0.7Ga0.3N heterojunction grown by metalorganic vapor phase epitaxy. These specific compositions were chosen to ensure a sufficiently large band offset for deep ultraviolet and power electronic applications. High resolution transmission electron microscopy confirmed the high structural quality of the heterojunction with an abrupt interface and uniform element distribution. We employed high resolution X-ray photoemission spectroscopy to measure the core level binding energies of B 1s and Ga 2p3/2 with respect to the valence band maximum of B0.14Al0.86N and Al0.7Ga0.3N layers, respectively. Then, we measured the energy separation between the B 1s and Ga 2p3/2 core levels at the interface of the heterojunction. The valence band offset was determined to be 0.40 ± 0.05 eV. As a consequence, we identified a staggered-gap (type-II) heterojunction with the conduction band offset of 1.10 ± 0.05 eV. The determination of the band alignment of the B0.14Al0.86N/Al0.7Ga0.3N heterojunction facilitates the design of optical and electronic devices based on such junctions.
High efficiency low threshold current 1.3 μm InAs quantum dot lasers on on-axis (001) GaP/Si Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-21 Daehwan Jung, Justin Norman, M. J. Kennedy, Chen Shang, Bongki Shin, Yating Wan, Arthur C. Gossard, John E. Bowers
We demonstrate highly efficient, low threshold InAs quantum dot lasers epitaxially grown on on-axis (001) GaP/Si substrates using molecular beam epitaxy. Electron channeling contrast imaging measurements show a threading dislocation density of 7.3 × 106 cm−2 from an optimized GaAs template grown on GaP/Si. The high-quality GaAs templates enable as-cleaved quantum dot lasers to achieve a room-temperature continuous-wave (CW) threshold current of 9.5 mA, a threshold current density as low as 132 A/cm2, a single-side output power of 175 mW, and a wall-plug-efficiency of 38.4% at room temperature. As-cleaved QD lasers show ground-state CW lasing up to 80 °C. The application of a 95% high-reflectivity coating on one laser facet results in a CW threshold current of 6.7 mA, which is a record-low value for any kind of Fabry-Perot laser grown on Si.
Temperature dependent quasi-static capacitance-voltage characterization of SiO2/β-Ga2O3 interface on different crystal orientations Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-21 Ke Zeng, Uttam Singisetti
The interface trap density (Dit) of the SiO2/β-Ga2O3 interface in (2¯01), (010), and (001) orientations is obtained by the Hi-Lo method with the low frequency capacitance measured using the Quasi-Static Capacitance-Voltage (QSCV) technique. QSCV measurements are carried out at higher temperatures to increase the measured energy range of Dit in the bandgap. At room temperature, higher Dit is observed near the band edge for all three orientations. The measurement at higher temperatures led to an annealing effect that reduced the Dit value for all samples. Comparison with the conductance method and frequency dispersion of the capacitance suggests that the traps at the band edge are slow traps which respond to low frequency signals.
Highly stable amorphous zinc tin oxynitride thin film transistors under positive bias stress Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-22 K. M. Niang, B. C. Bayer, J. C. Meyer, A. J. Flewitt
The stability of amorphous zinc tin oxynitride thin film transistors (a-ZTON TFTs) under positive bias stress (PBS) is investigated. Thin films are deposited by remote plasma reactive sputtering and are annealed at 300 °C in air for 1 h, after which films are confirmed to be highly amorphous by transmission electron microscopy. Typical a-ZTON TFTs exhibit a threshold voltage of 2.5 V, a field effect mobility of 3.3 cm2 V−1 s−1, a sub-threshold slope of 0.55 V dec−1, and a switching ratio over 106. Using a thermalization energy analysis, the threshold voltage shift under PBS is analysed. A maximum energy barrier to defect conversion up to 0.91 eV is found, which is significantly greater than that of the ∼0.75 eV energy barrier for amorphous indium gallium zinc oxide and amorphous zinc tin oxide TFTs previously reported. The improved stability of these oxynitride TFTs over amorphous oxide TFTs is explained by the elimination of less stable oxygen vacancies due to the passivation of oxygen vacancies with nitrogen. The higher attempt-to-escape frequency of 108 to 109 s−1 in a-ZTON TFTs compared with 107 s−1 in amorphous oxide semiconductor TFTs, on the other hand, is attributed to the high homogeneity of the amorphous film leading to strong carrier localization in the band tails.
A switchable spin-wave signal splitter for magnonic networks Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-18 F. Heussner, A. A. Serga, T. Brächer, B. Hillebrands, P. Pirro
The influence of an inhomogeneous magnetization distribution on the propagation of caustic-like spin-wave beams in unpatterned magnetic films has been investigated by utilizing micromagnetic simulations. Our study reveals a locally controllable and reconfigurable tractability of the beam directions. This feature is used to design a device combining split and switch functionalities for spin-wave signals on the micrometer scale. A coherent transmission of spin-wave signals through the device is verified. This attests the applicability in magnonic networks where the information is encoded in the phase of the spin waves.
Plasmonic diabolo cavity enhanced spin pumping Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-18 Jie Qian, Peng Gou, Y. S. Gui, C. M. Hu, Zhenghua An
Low spin-current generation efficiency has impeded further progress in practical spin devices, especially in the form of wireless excitation. To tackle this problem, a unique Plasmonic Diabolo Cavity (PDC) is proposed to enhance the spin pumping (SP) signal. The SP microwave photovoltage is enhanced ∼22-fold by PDC at ferromagnetic resonance (FMR). This improvement owes to the localization of the microwave magnetic field, which drives the spin precession process to more effectively generate photovoltage at the FMR condition. The in-plane anisotropy of spin pumping is found to be suppressed by PDC. Our work suggests that metamaterial resonant structures exhibit rich interactions with spin dynamics and could potentially be applied in future high-frequency spintronics.
Magnetization, ESR, and giant magnetocaloric effects in nanocrystals of Haldane-chain compound Gd2BaNiO5 Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-20 M. Y. Ruan, Z. W. Ouyang, Z. X. Wang, Z. C. Xia, G. H. Rao
The magnetization and electron spin resonance (ESR) in nanocrystals of Haldane-chain antiferromagnet Gd2BaNiO5 have been investigated. It is revealed that a reduction in crystal size results in an enhancement of magnetization due to a large number of paramagnetic Gd3+ and Ni2+ ions forming on the surfaces of nanocrystals. The smallest nanoparticles with an average size of 45 nm behave like a paramagnet, as evidenced by our ESR data. Upon application of an external magnetic field, the weakly coupled spins can be well aligned along the direction of the magnetic field, giving rise to a giant entropy change of −ΔSm = 36 J⋅kg−1⋅K−1 at 2 K in a field range of 0–7 T. This value is larger than those of most rare-earth-based compounds reported. The large value of −ΔSm, together with the absence of thermal and field hysteresis, makes Gd2BaNiO5 nanocrystals very promising candidates for low-temperature magnetic refrigeration.
Bias sputtered NbN and superconducting nanowire devices Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-18 Andrew E. Dane, Adam N. McCaughan, Di Zhu, Qingyuan Zhao, Chung-Soo Kim, Niccolo Calandri, Akshay Agarwal, Francesco Bellei, Karl K. Berggren
Superconducting nanowire single photon detectors (SNSPDs) promise to combine near-unity quantum efficiency with >100 megacounts per second rates, picosecond timing jitter, and sensitivity ranging from x-ray to mid-infrared wavelengths. However, this promise is not yet fulfilled, as superior performance in all metrics is yet to be combined into one device. The highest single-pixel detection efficiency and the widest bias windows for saturated quantum efficiency have been achieved in SNSPDs based on amorphous materials, while the lowest timing jitter and highest counting rates were demonstrated in devices made from polycrystalline materials. Broadly speaking, the amorphous superconductors that have been used to make SNSPDs have higher resistivities and lower critical temperature (Tc) values than typical polycrystalline materials. Here, we demonstrate a method of preparing niobium nitride (NbN) that has lower-than-typical superconducting transition temperature and higher-than-typical resistivity. As we will show, NbN deposited onto unheated SiO2 has a low Tc and high resistivity but is too rough for fabricating unconstricted nanowires, and Tc is too low to yield SNSPDs that can operate well at liquid helium temperatures. By adding a 50 W RF bias to the substrate holder during sputtering, the Tc of the unheated NbN films was increased by up to 73%, and the roughness was substantially reduced. After optimizing the deposition for nitrogen flow rates, we obtained 5 nm thick NbN films with a Tc of 7.8 K and a resistivity of 253 μΩ cm. We used this bias sputtered room temperature NbN to fabricate SNSPDs. Measurements were performed at 2.5 K using 1550 nm light. Photon count rates appeared to saturate at bias currents approaching the critical current, indicating that the device's quantum efficiency was approaching unity. We measured a single-ended timing jitter of 38 ps. The optical coupling to these devices was not optimized; however, integration with front-side optical structures to improve absorption should be straightforward. This material preparation was further used to fabricate nanocryotrons and a large-area imager device, reported elsewhere. The simplicity of the preparation and promising device performance should enable future high-performance devices.
Tunable frequency response of tunnel-type magneto-dielectric effect in Co−MgF2 granular films with different content of Co Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-18 Y. Cao, A. Umetsu, N. Kobayashi, S. Ohnuma, H. Masumoto
We have demonstrated the frequency dependence of the tunnel-type magneto-dielectric (TMD) effect in superparamagnetic Cox−(MgF2)1–x granular nanostructures by precise variations of x from 0.06 to 0.2. The structures consist of the nanometer-sized Co granules embedded in a crystallized MgF2 dielectric matrix. We observed an increased peak dielectric change Δε′/ε′0 from 0.8% to 3% at a specific frequency fTMD, and tunable fTMD was achieved from 8 kHz to 6.6 MHz by increasing x. Theoretical fittings predict that the narrow distribution of relaxation time gave rise to an enhanced Δε′/ε′0 and the narrowing of fTMD; the position variation of fTMD was attributed to the change in the intergranular distance between a pair of two neighboring granules. This study may help understand the fundamental physics between the TMD effect and nanometric structure and indicate that the films may work at higher frequency for devices with tunable dielectrics.
Single orthorhombic b axis orientation and antiferromagnetic ordering type in multiferroic CaMnO3 thin film with La0.67Ca0.33MnO3 buffer layer Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 F. Wang, B. J. Dong, Y. Q. Zhang, W. Liu, H. R. Zhang, Y. Bai, S. K. Li, T. Yang, J. R. Sun, Z. J. Wang, Z. D. Zhang
The detailed crystal structure and antiferromagnetic properties of a 42 nm thick CaMnO3 film grown on a LaAlO3 substrate with a 9 nm La0.67Ca0.33MnO3 buffer layer have been investigated. Compared with a CaMnO3 film directly grown on a LaAlO3 substrate, only one kind of orthorhombic b axis orientation along the  axis of the substrate is observed in the CaMnO3 film with a La0.67Ca0.33MnO3 buffer layer. To determine the antiferromagnetic ordering type of our CaMnO3 film with a buffer layer, the first-principles calculations were carried out with the results, indicating that the CaMnO3 film, even under a tensile strain of 1.9%, is still a compensated G-type antiferromagnetic order, the same as the bulk. Moreover, the exchange bias effect is observed at the interface of the CaMnO3/La0.67Ca0.33MnO3 film, further confirming the antiferromagnetic ordering of the CaMnO3 film with a buffer layer. In addition, it is concluded that the exchange bias effect originates from the spin glass state at the La0.67Ca0.33MnO3/CaMnO3 interface, which arises from a competition between the double-exchange ferromagnetic La0.67Ca0.33MnO3 and super-exchange antiferromagnetic CaMnO3 below the spin glass freezing temperature.
Physically based DC lifetime model for lead zirconate titanate films Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-21 Lauren M. Garten, Manabu Hagiwara, Song Won Ko, Susan Trolier-McKinstry
Accurate lifetime predictions for Pb(Zr0.52Ti0.48)O3 thin films are critical for a number of applications, but current reliability models are not consistent with the resistance degradation mechanisms in lead zirconate titanate. In this work, the reliability and lifetime of chemical solution deposited (CSD) and sputtered Pb(Zr0.52Ti0.48)O3 thin films are characterized using highly accelerated lifetime testing (HALT) and leakage current-voltage (I-V) measurements. Temperature dependent HALT results and impedance spectroscopy show activation energies of approximately 1.2 eV for the CSD films and 0.6 eV for the sputtered films. The voltage dependent HALT results are consistent with previous reports, but do not clearly indicate what causes device failure. To understand more about the underlying physical mechanisms leading to degradation, the I-V data are fit to known conduction mechanisms, with Schottky emission having the best-fit and realistic extracted material parameters. Using the Schottky emission equation as a base, a unique model is developed to predict the lifetime under highly accelerated testing conditions based on the physical mechanisms of degradation.
Highly efficient solid state magnetoelectric gyrators Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-21 Chung Ming Leung, Xin Zhuang, Daniel Friedrichs, Jiefang Li, Robert W. Erickson, V. Laletin, M. Popov, G. Srinivasan, D. Viehland
An enhancement in the power-conversion-efficiency (η) of a magneto-electric (ME) gyrator has been found by the use of Mn-substituted nickel zinc ferrite. A trilayer gyrator of Mn-doped Ni0.8Zn0.2Fe2O3 and Pb(Zr,Ti)O3 has η = 85% at low power conditions (∼20 mW/in3) and η ≥ 80% at high power conditions (∼5 W/in3). It works close to fundamental electromechanical resonance in both direct and converse modes. The value of η is by far the highest reported so far, which is due to the high mechanical quality factor (Qm) of the magnetostrictive ferrite. Such highly efficient ME gyrators with a significant power density could become important elements in power electronics, potentially replacing electromagnetic and piezoelectric transformers.
Enhanced concentrations of nitrogen-vacancy centers in diamond through TEM irradiation Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 D. Farfurnik, N. Alfasi, S. Masis, Y. Kauffmann, E. Farchi, Y. Romach, Y. Hovav, E. Buks, N. Bar-Gill
The studies of many-body dynamics of interacting spin ensembles, as well as quantum sensing in solid state systems, are often limited by the need for high spin concentrations, along with efficient decoupling of the spin ensemble from its environment. In particular, for an ensemble of nitrogen-vacancy (NV) centers in diamond, high conversion efficiencies between nitrogen (P1) defects and NV centers are essential while maintaining long coherence times of an NV ensemble. In this work, we study the effect of electron irradiation on the conversion efficiency and the coherence time of various types of diamond samples with different initial nitrogen concentrations. The samples were irradiated using a 200 keV transmission electron microscope. Our study reveals that the efficiency of NV creation strongly depends on the initial conversion efficiency and on the initial nitrogen concentration. The irradiation of the examined samples exhibits an order of magnitude improvement in the NV concentration (up to ∼1011 NV/cm2), without degradation in their coherence time of ∼180 μs. We address the potential of this technique toward the study of many-body physics of NV ensembles and the creation of non-classical spin states for quantum sensing.
Temperature-dependent Raman spectra and thermal conductivity of multi-walled MoS2 nanotubes Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 Dongqing Yu, Siqi Li, Weihong Qi, Mingpu Wang
In this letter, we investigated the temperature dependence of the in-plane (E12g) and out-of-plane (A1g) Raman modes of multi-walled MoS2 nanotubes fabricated from anodic aluminum oxide (AAO) templates. The frequencies of these two phonon modes were found to vary linearly with temperature. In this linear region, the first-order temperature coefficients for the A1g and E12g modes equaled –(1.15 ± 0.06)×10−2 cm−1/K and –(1.11 ± 0.1)×10−2 cm−1/K, respectively. The thermal conductivity of the multi-walled MoS2 nanotubes at room temperature was estimated to be in the range of 4.8 ± 0.1 to 11.1 ± 0.2 W m−1 K−1.
Solution-processed polymer-sorted semiconducting carbon nanotube network transistors with low-k /high-k bilayer polymer dielectrics Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-20 Seung-Hoon Lee, Dong-Yu Kim, Yong-Young Noh
Solution-processed semiconducting carbon nanotube transistors with a high mobility and an ON/OFF ratio are the most promising for use in flexible electronics. In this paper, we report low-k/high-k bilayer polymer dielectrics for solution-processed semiconducting single-walled carbon nanotube (s-SWNT) field-effect transistors (s-SWNT-FETs) with efficient charge transport and operation at low voltage. Thin low-k polystyrene (10 nm) is used for the first contact insulator with a channel in order to passivate the dipolar disorder induced by high-k insulators. The second gate insulator for low voltage operation is cyanoethyl pullulan (CEP), which is an environmentally friendly high-k insulator based on cellulose. Moreover, poly[(vinylidenefluoride-co-trifluoroethylene) is chosen as a single layer dielectric for comparison. A reasonably low operational voltage (<10 V) and high operational stability are achieved by the s-SWNT-FETs with polystyrene/CEP bilayer gate dielectrics. In addition, this indicates that the interface between the s-SWNTs and the low-k insulator is of critical importance for efficient charge transport.
Color atomic force microscopy: A method to acquire three independent potential parameters to generate a color image Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-20 P. E. Allain, D. Damiron, Y. Miyazaki, K. Kaminishi, F. V. Pop, D. Kobayashi, N. Sasaki, H. Kawakatsu
Atomic force microscopy has enabled imaging at the sub-molecular level, and 3D mapping of the tip-surface potential field. However, fast identification of the surface still remains a challenging topic for the microscope to enjoy widespread use as a tool with chemical contrast. In this paper, as a step towards implementation of such function, we introduce a control scheme and mathematical treatment of the acquired data that enable retrieval of essential information characterizing this potential field, leading to fast acquisition of images with chemical contrast. The control scheme is based on the tip sample distance modulation at an angular frequency ω, and null-control of the ω component of the measured self-excitation frequency of the oscillator. It is demonstrated that this control is robust, and that effective Morse Parameters that give satisfactory curve fit to the measured frequency shift can be calculated at rates comparable to the scan. Atomic features with similar topography were distinguished by differences in these parameters. The decay length parameter was resolved with a resolution of 10 pm. The method was demonstrated on quenched silicon at a scan rate comparable to conventional imaging.
Nonlinear dynamics for estimating the tip radius in atomic force microscopy Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-21 E. Rull Trinidad, T. W. Gribnau, P. Belardinelli, U. Staufer, F. Alijani
The accuracy of measurements in Amplitude Modulation Atomic Force Microscopy (AFM) is directly related to the geometry of the tip. The AFM tip is characterized by its radius of curvature, which could suffer from alterations due to repetitive mechanical contact with the surface. An estimation of the tip change would allow the user to assess the quality during imaging. In this work, we introduce a method for tip radius evaluation based on the nonlinear dynamic response of the AFM cantilever. A nonlinear fitting procedure is used to match several curves with softening nonlinearity in the noncontact regime. By performing measurements in this regime, we are able to maximize the influence of the tip radius on the AFM probe response, and this can be exploited to estimate with good accuracy the AFM tip radius.
Reflective small angle electron scattering to characterize nanostructures on opaque substrates Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-22 Lawrence H. Friedman, Wen-Li Wu, Wei-En Fu, Yunsan Chien
Feature sizes in integrated circuits (ICs) are often at the scale of 10 nm and are ever shrinking. ICs appearing in today's computers and hand held devices are perhaps the most prominent examples. These smaller feature sizes demand equivalent advances in fast and accurate dimensional metrology for both development and manufacturing. Techniques in use and continuing to be developed include X-ray based techniques, optical scattering, and of course the electron and scanning probe microscopy techniques. Each of these techniques has their advantages and limitations. Here, the use of small angle electron beam scattering measurements in a reflection mode (RSAES) to characterize the dimensions and the shape of nanostructures on flat and opaque substrates is demonstrated using both experimental and theoretical evidence. In RSAES, focused electrons are scattered at angles smaller than 1° with the assistance of electron optics typically used in transmission electron microscopy. A proof-of-concept experiment is combined with rigorous electron reflection simulations to demonstrate the efficiency and accuracy of RSAES as a method of non-destructive measurement of shapes of features less than 10 nm in size on flat and opaque substrates.
Refractory titanium nitride two-dimensional structures with extremely narrow surface lattice resonances at telecommunication wavelengths Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-22 Vadim I. Zakomirnyi, Ilia L. Rasskazov, Valeriy S. Gerasimov, Alexander E. Ershov, Sergey P. Polyutov, Sergei V. Karpov
Regular arrays of plasmonic nanoparticles have brought significant attention over the last decade due to their ability to support localized surface plasmons (LSPs) and exhibit diffractive grating behavior simultaneously. For a specific set of parameters (i.e., period, particle shape, size, and material), it is possible to generate super-narrow surface lattice resonances (SLRs) that are caused by interference of the LSP and the grating Rayleigh anomaly. In this letter, we propose plasmonic structures based on regular 2D arrays of TiN nanodisks to generate high-Q SLRs in an important telecommunication range, which is quite difficult to achieve with conventional plasmonic materials. The position of the SLR peak can be tailored within the whole telecommunication bandwidth (from ≈ 1.26 μm to ≈ 1.62 μm) by varying the lattice period, while the Q-factor is controlled by changing nanodisk sizes. We show that the Q-factor of SLRs can reach a value of 2 × 103, which is the highest reported Q-factor for SLRs at telecommunication wavelengths so far. Tunability of optical properties, refractory behavior, and low-cost fabrication of TiN nanoparticles paves the way for manufacturing cheap nanostructures with extremely stable and adjustable electromagnetic response at telecommunication wavelengths for a large number of applications.
Thermocompression bonding technology for multilayer superconducting quantum circuits Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-18 C. R. H. McRae, J. H. Béjanin, Z. Pagel, A. O. Abdallah, T. G. McConkey, C. T. Earnest, J. R. Rinehart, M. Mariantoni
Extensible quantum computing architectures require a large array of quantum bits operating with low error rates. A quantum processor based on superconducting devices can be scaled up by stacking microchips that perform wiring, shielding, and computational functionalities. In this article, we demonstrate a vacuum thermocompression bonding technology that utilizes thin indium films as a welding agent to attach pairs of lithographically patterned chips. At 10 mK, we find a specific dc bond resistance of 49.2 μΩ cm2. We show good transmission up to 6.8 GHz in a tunnel-capped, bonded device as compared to a similar uncapped device. Finally, we fabricate and measure a set of tunnel-capped superconducting resonators, demonstrating that our bonding technology can be used in quantum computing applications.
Perfecting the Al2O3/In0.53Ga0.47As interfacial electronic structure in pushing metal-oxide-semiconductor field-effect-transistor device limits using in-situ atomic-layer-deposition Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 M. Hong, H. W. Wan, K. Y. Lin, Y. C. Chang, M. H. Chen, Y. H. Lin, T. D. Lin, T. W. Pi, J. Kwo
We performed interfacial electric and electronic studies of both in-situ and ex-situ atomic-layer deposited (ALD) Al2O3 films on InGaAs. Self-aligned inversion-channel metal-oxide-semiconductor field-effect-transistors (MOSFETs) with a 1 μm gate length (Lg) from the in-situ sample have extrinsic drain currents (Id) of 1.8 mA/μm, transconductances (Gm) of 0.98 mS/μm, and an effective mobility (μeff) of 1250 cm2/V s. MOSFETs that employ ex-situ ALD-Al2O3 have an Id of 0.56 mA/μm, Gm of 0.28 mS/μm, and μeff of 410 cm2/V s. Synchrotron radiation photoemission reveals no AsOx residue at the Al2O3/InGaAs interface using the in-situ approach, whereas some AsOx residue is detected using the ex-situ method.
Experimental demonstration of improving resonant-multipactor threshold by three-dimensional wavy surface Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-21
A proof-of-principle experiment is presented demonstrating the suppression of multipactor breakdown in a coaxial multipactor device with three-dimensional periodic wavy surfaces. By changing the power and pulse width of the microwave source, threshold behavior near breakdown was obtained for this wavy-surface structure and a smooth-surface structure used for comparison. With a wide pulse width at a suitable power, the coefficient of reflection for the smooth-surface structure was found to increase, whereas the coefficient of transmission decreased. For the wavy-surface structure, a similar behavior appeared, only when the microwave pulse had a width of order of a few seconds. Accompanied by changes in transmission power characteristics, distinct increases in the second and third harmonic components were evident for the smooth-surface structure. These experimental results demonstrate that the wavy-surface structure effectively suppresses multipactor breakdown with the suppression increasing with the pulse width.
Electrospray characterization based on an emitter of cone-shaped porous medium for the high-throughput microliter aerosol generation Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-21 Chan Byon, Sookhee Ku, Woojong Lee, Jae Hee Jung, Woojin Kim
A compact electrospray system has been developed using a cone-shaped porous medium. The cone-jet conditions with a stable spraying mode were measured using ethanol and lavender oil, and the average diameter and size distribution of the sprayed droplets were investigated according to the applied voltage. In addition, the spray stability over time was analyzed because the fluid was supplied with a capillary action without the use of a pump. The average diameter of the droplets was 3.6–6.7 μm (ethanol) and 6.4–8.6 μm (lavender oil). We observed that the average particle size clearly increased with the applied voltage; the cause was determined based on the electric stress due to the potential difference (“electric pressure”) between the tip of the porous medium and ground ring. Moreover, the mean droplet size according to the electrical potential was proportional to E0.5 (ethanol) and E0.85 (lavender oil). The penetration time in the porous medium of the proposed system was controlled by the physical properties of the fluid, such as surface tension and viscosity, and affected the spray stability when spraying for a long time. Thus, the spray stability could be improved by controlling the applied voltage in the cone-jet mode region.
Reduced clot debris size using standing waves formed via high intensity focused ultrasound Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-22 Shifang Guo, Xuan Du, Xin Wang, Shukuan Lu, Aiwei Shi, Shanshan Xu, Ayache Bouakaz, Mingxi Wan
The feasibility of utilizing high intensity focused ultrasound (HIFU) to induce thrombolysis has been demonstrated previously. However, clinical concerns still remain related to the clot debris produced via fragmentation of the original clot potentially being too large and hence occluding downstream vessels, causing hazardous emboli. This study investigates the use of standing wave fields formed via HIFU to disintegrate the thrombus while achieving a reduced clot debris size in vitro. The results showed that the average diameter of the clot debris calculated by volume percentage was smaller in the standing wave mode than in the travelling wave mode at identical ultrasound thrombolysis settings. Furthermore, the inertial cavitation dose was shown to be lower in the standing wave mode, while the estimated cavitation bubble size distribution was similar in both modes. These results show that a reduction of the clot debris size with standing waves may be attributed to the particle trapping of the acoustic potential well which contributed to particle fragmentation.
Dual role of TiO2 buffer layer in Pt catalyzed BiFeO3 photocathodes: Efficiency enhancement and surface protection Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-22 Huanyu Shen, Xiaoxue Zhou, Wen Dong, Xiaodong Su, Liang Fang, Xi Wu, Mingrong Shen
Polycrystalline ferroelectric BiFeO3 (BFO) films deposited on transparent indium tin oxide (ITO) electrodes have shown to be an interesting photocathode for photoelectrochemical (PEC) water splitting; however, its PEC performance and stability are far from perfection. Herein, we reported an amorphous TiO2 buffer layer, inserted between BFO and Pt catalyst, improves significantly both its PEC activity and stability. A photocathodic current density of −460 μA/cm2 at 0 V vs. reversible hydrogen electrode (RHE) and an onset potential of 1.25 V vs. RHE were obtained in ITO/BFO/TiO2/Pt photocathode under 100 mW/cm2 Xe-lamp illumination. TiO2 functions as a buffer layer to remove the upward barrier between BFO and Pt, and makes the photogenerated carriers separate efficiently. The photocathode also shows high stability in acid solution after a 10-h PEC continuous testing.
Bi-stability of micro-plates: A sensitive mechanism for differential pressure measurements Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-19 Banafsheh Sajadi, Johannes (Hans) Goosen, Fred van Keulen
The electrostatic instability (pull-in) of a flat electrode in a parallel plate capacitor has been shown to be highly sensitive to external mechanical loads such as pressure. In this paper, we substantiate the possibility of prompting additional unstable configurations in such a system, with a remarkable sensitivity to the applied pressure. This additional instability has significant advantageous properties for sensing purposes. In addition to the high sensitivity and robustness of the pull-in voltage measurements, it can be adjusted so that after the unstable configuration is met, a snap-through to a new stable configuration occurs. As a result of this bi-stable behavior, the contact between the electrodes, which is the main drawback of pull-in phenomena, will be easily avoided. The results of this paper particularly suggest the suitability of this mechanism for two different methods of pressure measurements.
Structural and optical characterization of AlGaN multiple quantum wells grown on semipolar (20-21) bulk AlN substrate Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-11 Thomas Wunderer, Zhihong Yang, Martin Feneberg, Max Batres, Mark Teepe, Noble Johnson
Heterostructures of AlGaN with multiple quantum wells were grown by metal-organic vapor phase epitaxy on semipolar (20-21) bulk AlN substrates. Smooth epitaxial surfaces with excellent heterostructure interfaces were demonstrated. Luminescence from the AlGaN multiple quantum wells emitting at λ = 237 nm show a substantial degree of polarization of about 35% as determined by low-temperature photoluminescence measurements.
Torsional frequency mixing and sensing in optomechanical resonators Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-12 J. G. Huang, H. Cai, Y. D. Gu, L. K. Chin, J. H. Wu, T. N. Chen, Z. C. Yang, Y. L. Hao, A. Q. Liu
In this letter, a torsional optomechanical resonator for torque sensing and torsional mechanical frequency mixing is experimentally demonstrated. The torsional mechanical resonator is embedded into a split optical racetrack resonator, which provides high sensitivity in measuring torsional mechanical motion. Using this high sensitivity, torsional mechanical frequency mixing is observed without regenerative mechanical motion. The displacement noise floor of the torsional mechanical resonator is 50 fm/Hz0.5, which demonstrates a resonant torque sensitivity of 3.58 × 10−21 N m/Hz0.5. This demonstration will benefit potential applications for on-chip RF signal modulation using optical mechanical resonators.
Enhanced green fluorescent protein in optofluidic Fabry-Perot microcavity to detect laser induced temperature changes in a bacterial culture Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-12 F. Lahoz, I. R. Martín, D. Walo, R. Freire, J. Gil-Rostra, F. Yubero, A. R. Gonzalez-Elipe
Thermal therapy using laser sources can be used in combination with other cancer therapies to eliminate tumors. However, high precision temperature control is required to avoid damage in healthy surrounding tissues. Therefore, in order to detect laser induced temperature changes, we have used the fluorescence signal of the enhanced Green Fluorescent Protein (eGFP) over-expressed in an E. coli bacterial culture. For that purpose, the bacteria expressing eGFP are injected in a Fabry-Perot (FP) optofluidic planar microcavity. In order to locally heat the bacterial culture, external infrared or ultraviolet lasers were used. Shifts in the wavelengths of the resonant FP modes are used to determine the temperature increase as a function of the heating laser pump power. Laser induced local temperature increments up to 6–7 °C were measured. These results show a relatively easy way to measure laser induced local temperature changes using a FP microcavity and using eGFP as a molecular probe instead of external nanoparticles, which could damage/alter the cell. Therefore, we believe that this approach can be of interest for the study of thermal effects in laser induced thermal therapies.
Excitation-wavelength scaling of terahertz radiation in alkali vapor plasmas Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-12 Shijing Zhang, Liangliang Zhang, Hang Zhao, Tong Wu, Cunlin Zhang, Yuejin Zhao
By using a wavelength-tunable near-infrared femtosecond laser, we demonstrate a wavelength scaling mechanism in which the efficiency of terahertz (THz) radiation production by Rb and Cs vapor plasmas increases with increasing excitation laser wavelength. Due to the low ionization energies of alkali metals, the THz radiation energy generated by Cs is an order of magnitude higher than that produced N2 at a wavelength of 1500 nm. The experimental results are well explained by the model of photocurrent as being induced by electron tunneling ionization.
Photo-physical properties of He-related color centers in diamond Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-13 G. Prestopino, M. Marinelli, E. Milani, C. Verona, G. Verona-Rinati, P. Traina, E. Moreva, I. P. Degiovanni, M. Genovese, S. Ditalia Tchernij, F. Picollo, P. Olivero, J. Forneris
Diamond is a promising platform for the development of technological applications in quantum optics and photonics. The quest for color centers with optimal photo-physical properties has led in recent years to the search for novel impurity-related defects in this material. Here, we report on a systematic investigation of the photo-physical properties of two He-related (HR) emission lines at 535.2 nm and 559.7 nm (as measured at a temperature of 25 K) created in three different diamond substrates upon implantation with 1.3 MeV He+ ions and subsequent annealing. The spectral features of the HR centers were studied in an “optical grade” diamond substrate as a function of several physical parameters, namely, the measurement temperature, the excitation wavelength, and the intensity of external electric fields. The emission lifetimes of the 535.2 nm and 559.7 nm lines were also measured by means of time-gated photoluminescence measurements. The Stark shifting of the HR centers under the application of an external electrical field was observed in a CVD diamond film equipped with buried graphitic electrodes, suggesting the lack of inversion symmetry in the defects' structure. Furthermore, the photoluminescence mapping under 405 nm excitation of a “detector grade” diamond sample implanted at a He+ ion fluence of 1 × 1010 cm−2 enabled us to identify the spectral features of both the HR emission lines from the same localized optical spots. The reported results provide an insight into the structure of He-related defects in diamond and their possible utilization in practical applications.
Optical gain from vertical Ge-on-Si resonant-cavity light emitting diodes with dual active regions Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-14 Guangyang Lin, Jiaqi Wang, Zhiwei Huang, Yichen Mao, Cheng Li, Wei Huang, Songyan Chen, Hongkai Lai, Shihao Huang
Vertical resonant-cavity light emitting diodes with dual active regions consisting of highly n-doped Ge/GeSi multiple quantum wells (MQWs) and a Ge epilayer are proposed to improve the light emitting efficiency. The MQWs are designed to optically pump the underlying Ge epilayer under electric injection. Abundant excess carriers can be optically pumped into the Γ valley of the Ge epilayer apart from electric pumping. With the combination of a vertical cavity, the efficiency of the optical-pumping process was effectively improved due to the elongation of the optical length in the cavity. With the unique feature, optical gain from the Ge epilayer is observed between 1625 and 1700 nm at injection current densities of >1.528 kA/cm2. The demonstration of optical gain from the Ge epilayer indicates that this strategy can be generally useful for Si-based light sources with indirect band materials.
Two-well terahertz quantum cascade lasers with suppressed carrier leakage Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-14 Asaf Albo, Yuri V. Flores, Qing Hu, John L. Reno
The mechanisms that limit the temperature performance of diagonal GaAs/Al0.15GaAs0.85-based terahertz quantum cascade lasers (THz-QCLs) have been identified as thermally activated leakage of charge carriers through excited states into the continuum. THz-QCLs with energetically higher-laying excited states supported by sufficiently high barriers aim to eliminate these leakage mechanisms and lead to improved temperature performance. Although suppression of thermally activated carrier leakage was realized in a three-well THz-QCL based on a resonant-phonon scheme, no improvement in the temperature performance was reported thus far. Here, we report a major improvement in the temperature performance of a two-quantum-well direct-phonon THz-QCL structure. We show that the improved laser performance is due to the suppression of the thermally activated carrier leakage into the continuum with the increase in the injection barrier height. Moreover, we demonstrate that high-barrier two-well structures can support a clean three-level laser system at elevated temperatures, which opens the opportunity to achieve temperature performance beyond the state-of-the-art.
Highly transparent twist polarizer metasurface Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-15 Ihar Faniayeu, Sergei Khakhomov, Igor Semchenko, Vygantas Mizeikis
A twist polarizer metasurface for polarization rotation by an angle of 90 ° is proposed and realized at microwave frequencies. The metasurface consists of sub-wavelength metallic helices arranged periodically in a single layer and operates in transmission geometry with a nearly unity cross-polarization conversion coefficient at resonance. The structure exhibits low reflectivity R < 0.06 within a decade-spanning frequency range of 0.1–5.5 GHz and is insensitive to the polarization orientation of the incident wave. Moreover, it can operate with high efficiency at oblique incidence angles of up to 35 ° . Such twist polarizer metasurfaces are potentially applicable as electromagnetic/optical isolators and frequency-selective polarization antennas.
Ultrathin polarization-insensitive wide-angle broadband near-perfect absorber in the visible regime based on few-layer MoS2 films Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-15 Yuebian Zhang, Wenwei Liu, Zhancheng Li, Hua Cheng, Yanbang Zhang, Guozhi Jia, Shuqi Chen, Jianguo Tian
We report the design, characterization, and experimental demonstration of a broadband near-perfect absorber in the visible regime based on strong interference in ultrathin molybdenum disulfide films obtained by the spin-coating method. The absorber is polarization-insensitive, and the absorption peak maintains a high value for large angles of incidence, which provides more efficient absorption for nonpolarized or oblique incident beams. The experimental results show that the absorption can reach more than 87% between 400 nm and 640 nm, which is in reasonable agreement with the simulated results. This work may offer a further step in the development of solar absorption-based nano-optoelectronic devices.
Boosting surface charge-transfer doping efficiency and robustness of diamond with WO3 and ReO3 Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-09-12 Moshe Tordjman, Kamira Weinfeld, Rafi Kalish
An advanced charge-transfer yield is demonstrated by employing single monolayers of transition-metal oxides—tungsten trioxide (WO3) and rhenium trioxide (ReO3)—deposited on the hydrogenated diamond surface, resulting in improved p-type sheet conductivity and thermal stability. Surface conductivities, as determined by Hall effect measurements as a function of temperature for WO3, yield a record sheet hole carrier concentration value of up to 2.52 × 1014 cm−2 at room temperature for only a few monolayers of coverage. Transfer doping with ReO3 exhibits a consistent narrow sheet carrier concentration value of around 3 × 1013 cm−2, exhibiting a thermal stability of up to 450 °C. This enhanced conductivity and temperature robustness exceed those reported for previously exposed surface electron acceptor materials used so far on a diamond surface. X-ray photoelectron spectroscopy measurements of the C1s core level shift as a function of WO3 and ReO3 layer thicknesses are used to determine the respective increase in surface band bending of the accumulation layers, leading to a different sub-surface two-dimensional hole gas formation efficiency in both cases. This substantial difference in charge-exchange efficiency is unexpected since both surface acceptors have very close work functions. Consequently, these results lead us to consider additional factors influencing the transfer doping mechanism. Transfer doping with WO3 reveals the highest yet reported transfer doping efficiency per minimal surface acceptor coverage. This improved surface conductivity performance and thermal stability will promote the realization of 2D diamond-based electronic devices facing process fabrication challenges.
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