229 nm UV LEDs on aluminum nitride single crystal substrates using p-type silicon for increased hole injection Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-20 Dong Liu, Sang June Cho, Jeongpil Park, Jung-Hun Seo, Rafael Dalmau, Deyin Zhao, Kwangeun Kim, Jiarui Gong, Munho Kim, In-Kyu Lee, John D. Albrecht, Weidong Zhou, Baxter Moody, Zhenqiang Ma
AlGaN based 229 nm light emitting diodes (LEDs), employing p-type Si to significantly increase hole injection, were fabricated on single crystal bulk aluminum nitride (AlN) substrates. Nitride heterostructures were epitaxially deposited by organometallic vapor phase epitaxy and inherit the low dislocation density of the native substrate. Following epitaxy, a p-Si layer is bonded to the heterostructure. LEDs were characterized both electrically and optically. Owing to the low defect density films, large concentration of holes from p-Si, and efficient hole injection, no efficiency droop was observed up to a current density of 76 A/cm2 under continuous wave operation and without external thermal management. An optical output power of 160 μW was obtained with the corresponding external quantum efficiency of 0.03%. This study demonstrates that by adopting p-type Si nanomembrane contacts as a hole injector, practical levels of hole injection can be realized in UV light-emitting diodes with very high Al composition AlGaN quantum wells, enabling emission wavelengths and power levels that were previously inaccessible using traditional p-i-n structures with poor hole injection efficiency.
Creation and annealing of metastable defect states in CH3NH3PbI3 at low temperatures Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-21 F. Lang, O. Shargaieva, V. V. Brus, J. Rappich, N. H. Nickel
Methylammonium lead iodide (CH3NH3PbI3), an organic-inorganic perovskite widely used for optoelectronic applications, is known to dissociate under illumination with light at photon energies around 2.7 eV and higher. Here, we show that photo-induced dissociation is not limited to ambient temperatures but can be observed even at 5 K. The photo-induced dissociation of N–H bonds results in the formation of metastable states. Photoluminescence (PL) measurements reveal the formation of defect states that are located 100 meV within the bandgap. This is accompanied by a quenching of the band-to-band PL by one order of magnitude. Defect generation is reversible and annealing at 30 K recovers the band-to-band PL, while the light-induced defect states disappear concurrently.
Near-perfect terahertz wave amplitude modulation enabled by impedance matching in VO2 thin films Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-22 Hong-Fu Zhu, Liang-Hui Du, Jiang Li, Qi-Wu Shi, Bo Peng, Ze-Ren Li, Wan-Xia Huang, Li-Guo Zhu
We present a terahertz (THz) amplitude modulation method with near perfect E-field amplitude modulation depths that is based on impedance matching in VO2 thin films during the thermally induced insulator-metal transition (IMT). It was observed that the impedance matching-induced THz amplitude modulation was sensitive to the resistance switching characteristics of the VO2 thin films. By designing the VO2 thin films to have four orders of magnitude of change in resistance during the IMT, we experimentally achieved an E-field amplitude modulation depth of 94.5% (intensity modulation depth of 99.7%) between the insulator phase of VO2 and the impedance matching state, and an E-field amplitude modulation depth of 97.6% (intensity modulation depth of 99.94%) between the impedance matching state and the metallic phase of VO2 at 0.5 THz. The experimental results were consistent with the results of simulations based on the transmission matrix model.
Ultrafast spectral dynamics of dual-color-soliton intracavity collision in a mode-locked fiber laser Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-23 Yuan Wei, Bowen Li, Xiaoming Wei, Ying Yu, Kenneth K. Y. Wong
The single-shot spectral dynamics of dual-color-soliton collisions inside a mode-locked laser is experimentally and numerically investigated. By using the all-optically dispersive Fourier transform, we spectrally unveil the collision-induced soliton self-reshaping process, which features dynamic spectral fringes over the soliton main lobe, and the rebuilding of Kelly sidebands with wavelength drifting. Meanwhile, the numerical simulations validate the experimental observation and provide additional insights into the physical mechanism of the collision-induced spectral dynamics from the temporal domain perspective. It is verified that the dynamic interference between the soliton and the dispersive waves is responsible for the observed collision-induced spectral evolution. These dynamic phenomena not only demonstrate the role of dispersive waves in the sophisticated soliton interaction inside the laser cavity, but also facilitate a deeper understanding of the soliton's inherent stability.
Hybridization regulated metal penetration at transition metal-organic semiconductor contacts Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-20 Tzu-Hung Chuang, Kun-Ta Lu, Chun-I Lu, Yao-Jane Hsu, Der-Hsin Wei
Metal-organic contacts are keys to define the functionalities of hybrid structures, but orbital hybridization at interfaces has made rationalizing their behavior a challenging task. Here, we examined Fe/C60 and Ni/C60 bilayers with X-ray absorption spectra to study the nature of orbital hybridization and the possible correlation with metal penetration. Depositing Fe or Ni on C60 films of sub-nanometer thickness to emulate interfaces, we found that both bilayers show evidence of not only d-π hybridization and metal penetration but also a deeper Ni penetration. The carbon K-edge spectra recorded from C60 films indicate that the deeper Ni penetration is accompanied by a larger donation of electrons from Ni to C60. This finding of hybridization-modulated metal penetration is somewhat counterintuitive but is consistent with a scenario of metal-C60 hybridization competing with metal-metal aggregation. A stronger Ni-C60 hybridization could result in smaller Ni clusters and a greater probability of penetration through the interstitial space between C60 molecules. We conclude that metal penetration can be regulated with orbital hybridization between metal and C60.
Simultaneous formation of multiscale hierarchical surface morphologies through sequential wrinkling and folding Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-22 Yu Wang, Qingyang Sun, Jianliang Xiao
Highly organized hierarchical surface morphologies possess various intriguing properties that could find important potential applications. In this paper, we demonstrate a facile approach to simultaneously form multiscale hierarchical surface morphologies through sequential wrinkling. This method combines surface wrinkling induced by thermal expansion and mechanical strain on a three-layer structure composed of an aluminum film, a hard Polydimethylsiloxane (PDMS) film, and a soft PDMS substrate. Deposition of the aluminum film on hard PDMS induces biaxial wrinkling due to thermal expansion mismatch, and recovering the prestrain in the soft PDMS substrate leads to wrinkling of the hard PDMS film. In total, three orders of wrinkling patterns form in this process, with wavelength and amplitude spanning 3 orders of magnitude in length scale. By increasing the prestrain in the soft PDMS substrate, a hierarchical wrinkling-folding structure was also obtained. This approach can be easily extended to other thin films for fabrication of multiscale hierarchical surface morphologies with potential applications in different areas.
Accurate temperature measurement by temperature field analysis in diamond anvil cell for thermal transport study of matter under high pressures Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-23 Donghui Yue, Tingting Ji, Tianru Qin, Jia Wang, Cailong Liu, Hui Jiao, Lin Zhao, Yonghao Han, Chunxiao Gao
The study on the thermal transport properties of matter under high pressure is important but is hard to fulfill in a diamond anvil cell (DAC) because the accurate measurement of the temperature gradient within the sample of DAC is very difficult. In most cases, the sample temperature can be read accurately from the thermocouples that are directly attached to the lateral edges of diamond anvils because both the sample and diamond anvils can be uniformly heated up to a given temperature. But for the thermal transport property studies in DAC, an artificial temperature distribution along the compression axis is a prerequisite. Obviously, the temperature of the top or bottom surface of the sample cannot be substituted by that of diamond anvils although diamond anvils can be considered as a good medium for heat conduction. With temperature field simulation by finite element analysis, it is found that big measurement errors can occur and are fatal to the correct analysis of thermal transport properties of materials. Thus, a method of combining both the four-thermocouple configuration and temperature field analysis is presented for the accurate temperature distribution measurement in DAC, which is based on the single-function relationship between temperature distribution and sample thermal conductivity.
Peculiar behavior of magnetoresistance in HgSe single crystal with low electron concentration Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-20 A. T. Lonchakov, S. B. Bobin, V. V. Deryushkin, V. I. Okulov, T. E. Govorkova, V. N. Neverov
Magnetoresistive properties of the single crystal of HgSe with a low electron concentration were studied in a wide range of temperatures and magnetic fields. Some fundamental parameters of the spectrum and scattering of electrons were experimentally determined. Two important features of magnetic transport were found—strong transverse magnetoresistance (MR) and negative longitudinal MR, which can indicate the existence of the topological phase of the Weyl semimetal (WSM) in HgSe. Taking this hypothesis into account, we suggest a modified band diagram of mercury selenide at low electron energies. The obtained results are essential for the deeper understanding of both physics of gapless semiconductors and WSMs—promising materials for various applications in electronics, spintronics, computer, and laser technologies.
Lateral-electric-field-induced spin polarization in a suspended GaAs quantum point contact Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-20 D. A. Pokhabov, A. G. Pogosov, E. Yu. Zhdanov, A. A. Shevyrin, A. K. Bakarov, A. A. Shklyaev
The conductance of a GaAs-based suspended quantum point contact (QPC) equipped with lateral side gates has been experimentally studied in the absence of the external magnetic field. The half-integer conductance plateau ( 0.5 × 2 e 2 / h ) has been observed when an asymmetric voltage between the side gates is applied. The appearance of this plateau has been attributed to the spin degeneracy lifting caused by the spin-orbit coupling associated with the lateral electric field in the asymmetrically biased QPC. We have experimentally demonstrated that, despite the relatively small g-factor in GaAs, the observation of the spin polarization in the GaAs-based QPC became possible after the suspension due to the enhancement of the electron-electron interaction and the effect of the electric field guiding. These features are caused by a partial confinement of the electric field lines within a suspended semiconductor layer with a high dielectric constant.
Electron effective mass in In0.33Ga0.67N determined by mid-infrared optical Hall effect Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-20 Nerijus Armakavicius, Vallery Stanishev, Sean Knight, Philipp Kühne, Mathias Schubert, Vanya Darakchieva
Mid-infrared optical Hall effect measurements are used to determine the free charge carrier parameters of an unintentionally doped wurtzite-structure c-plane oriented In0.33Ga0.67N epitaxial layer. Room temperature electron effective mass parameters of m ⊥ * = ( 0.205 ± 0.013 ) m 0 and m ∥ * = ( 0.204 ± 0.016 ) m 0 for polarization perpendicular and parallel to the c-axis, respectively, were determined. The free electron concentration was obtained as (1.7 ± 0.2) × 1019 cm−3. Within our uncertainty limits, we detect no anisotropy for the electron effective mass parameter and we estimate the upper limit of the possible effective mass anisotropy as 7%. We discuss the influence of conduction band nonparabolicity on the electron effective mass parameter as a function of In content. The effective mass parameter is consistent with a linear interpolation scheme between the conduction band mass parameters in GaN and InN when the strong nonparabolicity in InN is included. The In0.33Ga0.67N electron mobility parameter was found to be anisotropic, supporting previous experimental findings for wurtzite-structure GaN, InN, and AlxGa1−xN epitaxial layers with c-plane growth orientation.
Effect of 1.5 MeV electron irradiation on β-Ga2O3 carrier lifetime and diffusion length Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-21 Jonathan Lee, Elena Flitsiyan, Leonid Chernyak, Jiancheng Yang, Fan Ren, Stephen J. Pearton, Boris Meyler, Y. Joseph Salzman
The influence of 1.5 MeV electron irradiation on minority transport properties of Si doped β-Ga2O3 vertical Schottky rectifiers was observed for fluences up to 1.43 × 1016 cm−2. The Electron Beam-Induced Current technique was used to determine the minority hole diffusion length as a function of temperature for each irradiation dose. This revealed activation energies related to shallow donors at 40.9 meV and radiation-induced defects with energies at 18.1 and 13.6 meV. Time-resolved cathodoluminescence measurements showed an ultrafast 210 ps decay lifetime and reduction in carrier lifetime with increased irradiation.
Enhanced magnetoelectric effect in M-type hexaferrites by Co substitution into trigonal bi-pyramidal sites Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-20 J. E. Beevers, C. J. Love, V. K. Lazarov, S. A. Cavill, H. Izadkhah, C. Vittoria, R. Fan, G. van der Laan, S. S. Dhesi
The magnetoelectric effect in M-type Ti-Co doped strontium hexaferrite has been studied using a combination of magnetometry and element specific soft X-ray spectroscopies. A large increase (>×30) in the magnetoelectric coefficient is found when Co2+ enters the trigonal bi-pyramidal site. The 5-fold trigonal bi-pyramidal site has been shown to provide an unusual mechanism for electric polarization based on the displacement of magnetic transition metal (TM) ions. For Co entering this site, an off-centre displacement of the cation may induce a large local electric dipole as well as providing an increased magnetostriction enhancing the magnetoelectric effect.
Giant magnetostriction in nanoheterogeneous Fe-Al alloys Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-22 Yongjun Han, Hui Wang, Tianli Zhang, Yangkun He, Chengbao Jiang
As a potential magnetostrictive material, Fe-Al alloys exhibit excellent mechanical properties, low cost, and moderate magnetostriction, but the magnetostriction mechanism is still a mystery. Here, we elucidate the structural origin of magnetostriction in Fe-Al alloys and further improve the magnetostriction five-fold via Tb doping. Nanoinclusions with a size of 3–5 nm were found dispersed in the A2 matrix in Fe82Al18 ribbons. The structure of the nanoinclusions is identified to be tetragonally modified-D03 (L60), which are considered to create the tetragonal distortion of the matrix, leading to the enhanced magnetostriction. Furthermore, a drastic enhancement of the magnetostriction up to 5 times was achieved by trace Tb doping (0.2 at. %). Synchrotron X-ray diffraction directly revealed the increased tetragonal distortion of the matrix caused by these Tb dopants. The results further enrich the heterogeneous magnetostriction and guide the development of magnetostrictive materials.
Impact of ultrafast demagnetization process on magnetization reversal in L10 FePt revealed using double laser pulse excitation Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-23 J. Y. Shi, M. Tang, Z. Zhang, L. Ma, L. Sun, C. Zhou, X. F. Hu, Z. Zheng, L. Q. Shen, S. M. Zhou, Y. Z. Wu, L. Y. Chen, H. B. Zhao
Ultrafast laser induced magnetization reversal in L10 FePt films with high perpendicular magnetic anisotropy was investigated using single- and double-pulse excitations. Single-pulse excitation beyond 10 mJ cm−2 caused magnetization (M) reversal at the applied fields much smaller than the static coercivity of the films. For double-pulse excitation, both coercivity reduction and reversal percentage showed a rapid and large decrease with the increasing time interval (Δt) of the two pulses in the range of 0–2 ps. In this Δt range, the maximum demagnetization (ΔMp) was also strongly attenuated, whereas the integrated demagnetization signals over more than 10 ps, corresponding to the average lattice heat effect, showed little change. These results indicate that laser induced M reversal in FePt films critically relies on ΔMp. Because ΔMp is determined by spin temperature, which is higher than lattice temperature, utilizing an ultrafast laser instead of a continuous-wave laser in laser-assisted M reversal may reduce the overall deposited energy and increase the speed of recording. The effective control of M reversal by slightly tuning the time delay of two laser pulses may also be useful for ultrafast spin manipulation.
Gradient chemical order in the relaxor Pb(Mg1∕3Nb2∕3)O3 Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-20 Matthew J. Cabral, Shujun Zhang, Elizabeth C. Dickey, James M. LeBeau
Here, we apply aberration-corrected scanning transmission electron microscopy to quantify chemical ordering in the relaxor Pb(Mg1∕3Nb2∕3)O3 (PMN). We find that contrary to the prevailing model of a binary distribution of chemically ordered regions within a disordered matrix, the degree of ordering smoothly varies within an ordered domain and approaches a minimum at anti-phase boundaries. These results provide direct insight into the nature of cation ordering in this important prototypical relaxor material.
Measurement of the vacuum-ultraviolet absorption spectrum of low-k dielectrics using X-ray reflectivity Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-22 F. A. Choudhury, H. M. Nguyen, S. W. King, C. H. Lee, Y. H. Lin, H. S. Fung, C. C. Chen, W. Li, D. Benjamin, J. M. Blatz, Y. Nishi, J. L. Shohet
During plasma processing, low-k dielectrics are exposed to high levels of vacuum ultraviolet (VUV) radiation that can cause severe damage to dielectric materials. The degree and nature of VUV-induced damage depend on the VUV photon energies and fluence. In this work, we examine the VUV-absorption spectrum of low-k organosilicate glass using specular X-ray reflectivity (XRR). Low-k SiCOH films were exposed to synchrotron VUV radiation with energies ranging from 7 to 21 eV, and the density vs. depth profile of the VUV-irradiated films was extracted from fitting the XRR experimental data. The results show that the depth of the VUV-induced damage layer is a function of the photon energy. Between 7 and 11 eV, the depth of the damaged layer decreases sharply from 110 nm to 60 nm and then gradually increases to 85 nm at 21 eV. The maximum VUV absorption in low-k films occurs between 11 and 15 eV. The depth of the damaged layer was found to increase with film porosity.
Impact of semiconducting electrodes on the electroresistance of ferroelectric tunnel junctions Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-23 M. Asa, R. Bertacco
Ferroelectric tunnel junctions are promising candidates for the realization of energy-efficient digital memories and analog memcomputing devices. In this work, we investigate the impact of a semiconducting layer in series to the junction on the sign of electroresistance. To this scope, we compare tunnel junctions fabricated out of Pt/BaTiO3/La1/3Sr2/3MnO3 (LSMO) and Pt/BaTiO3/Nb:SrTiO3 (Nb:STO) heterostructures, displaying an opposite sign of the electroresistance. By capacitance-voltage profiling, we observe a behavior typical of Metal-Oxide-Semiconductor tunnel devices in both cases but compatible with the opposite sign of charge carriers in the semiconducting layer. While Nb:STO displays the expected n-type semiconducting character, metallic LSMO develops an interfacial p-type semiconducting layer. The different types of carriers at the semiconducting interfaces and the modulation of the depleted region by the ferroelectric charge have a deep impact on electroresistance, possibly accounting for the different sign observed in the two systems.
Determining the spring constant of arbitrarily shaped cantilevers in viscous environments Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-20 A. F. Payam, W. Trewby, K. Voïtchovsky
Accurate calibration of the flexural spring constant of microcantilevers is crucial for sensing devices, microactuators, and atomic force microscopy (AFM). Existing methods rely on precise knowledge of cantilever geometry, make significant simplifications, or require potentially damaging contact with the sample. Here, we develop a simple equation to calculate the flexural spring constants of arbitrarily shaped cantilevers in fluid. Our approach, verified here with AFM, only requires the measurement of two resonance frequencies of the cantilever in air and in a liquid, with no need for additional input or knowledge about the system. We validate the method with cantilevers of different shapes and compare its predictions with existing models. We also show how the method's accuracy can be considerably improved, especially in more viscous liquids, if the effective width of the cantilever is known. Significantly, the developed equations can be extended to calculate the spring constants of the cantilever's higher eigenmodes.
Quantum beats of a multiexciton state in rubrene single crystals Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-20 Eric A. Wolf, Drew M. Finton, Vincent Zoutenbier, Ivan Biaggio
We observe quantum beats in the nanosecond-scale photoluminescence decay of rubrene single crystals after photoexcitation with short laser pulses in a magnetic field in the range of 0.1 to 0.3 T. The relative amplitude of the quantum beats is of the order of 5%. Their frequency is 1.3 GHz when the magnetic field is oriented parallel to the twofold rotation axis of the rubrene molecules and decreases to 0.6 GHz when the magnetic field is rotated to the crystal's molecular stacking direction. The amplitude of the quantum beats decays alongside the non-oscillatory photoluminescence background, which at low excitation densities has an exponential decay time of 4.0 ± 0.2 ns. We interpret this as the effective lifetime of a multiexciton state that originates from singlet-fission and can undergo geminate recombination back to the singlet state.
Tunable hole injection of solution-processed polymeric carbon nitride towards efficient organic light-emitting diode Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-21 Xiaowen Zhang, Qinghong Zheng, Zhenyu Tang, Wanshu Li, Yan Zhang, Kai Xu, Xiaogang Xue, Jiwen Xu, Hua Wang, Bin Wei
Polymeric carbon nitride (CNxHy) has been facilely synthesized from dicyandiamide and functions as a solution-processed hole injection layer in organic light-emitting diodes (OLEDs). The measurements using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and impedance spectroscopy elucidate that CNxHy exhibits superior film morphology and extra electric properties such as tailored work function and tunable hole injection. The luminous efficiency of CNxHy-based OLED is found to improve by 76.6% in comparison to the counterpart using favorite solution-processed poly(ethylene dioxythiophene):poly(styrene sulfonate) as the hole injection layer. Our results also pave a way for broadening carbon nitride applications in organic electronics using the solution process.
High thermoelectric power factor from multilayer solution-processed organic films Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-21 Guangzheng Zuo, Olof Andersson, Hassan Abdalla, Martijn Kemerink
We investigate the suitability of the “sequential doping” method of organic semiconductors for thermoelectric applications. The method consists of depositing a dopant (F4TCNQ) containing solution on a previously cast semiconductor (P3HT) thin film to achieve high conductivity, while preserving the morphology. For very thin films (∼25 nm), we achieve a high power factor around 8 μW/mK−2 with a conductivity over 500 S/m. For the increasing film thickness, conductivity and power factor show a decreasing trend, which we attribute to the inability to dope the deeper parts of the film. Since thick films are required to extract significant power from thermoelectric generators, we developed a simple additive technique that allows the deposition of an arbitrary number of layers without significant loss in conductivity or power factor that, for 5 subsequent layers, remain at ∼300 S/m and ∼5 μW/mK−2, respectively, whereas the power output increases almost one order of magnitude as compared to a single layer. The efficient doping in multilayers is further confirmed by an increased intensity of (bi)polaronic features in the UV-Vis spectra.
Characterization of microchannel anechoic corners formed by surface acoustic waves Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-21 Ghulam Destgeer, Ashar Alam, Husnain Ahmed, Jinsoo Park, Jin Ho Jung, Kwangseok Park, Hyung Jin Sung
Surface acoustic waves (SAWs) generated in a piezoelectric substrate couple with a liquid according to Snell's law such that a compressional acoustic wave propagates obliquely at a Rayleigh angle ( θ t ) inside the microchannel to form a region devoid of a direct acoustic field, which is termed a microchannel anechoic corner (MAC). In the present study, we used microchannels with various heights and widths to characterize the width of the MAC region formed by a single travelling SAW. The attenuation of high-frequency SAWs produced a strong acoustic streaming flow that moved the particles in and out of the MAC region, whereas reflections of the acoustic waves within the microchannel resulted in standing acoustic waves that trapped particles at acoustic pressure nodes located within or outside of the MAC region. A range of actuation frequencies and particle diameters were used to investigate the effects of the acoustic streaming flow and the direct acoustic radiation forces by the travelling as well as standing waves on the particle motion with respect to the MAC region. The width of the MAC ( w c ), measured experimentally by tracing the particles, increased with the height of the microchannel ( h m ) according to a simple trigonometric equation w c = h m × tan ( θ t ) .
Temperature dependence of DC transport characteristics for a two-dimensional electron gas in an undoped Si/SiGe heterostructure Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-21 Kuan-Yu Chou, Nai-Wen Hsu, Yi-Hsin Su, Chung-Tao Chou, Po-Yuan Chiu, Yen Chuang, Jiun-Yun Li
We investigate DC characteristics of a two-dimensional electron gas (2DEG) in an undoped Si/SiGe heterostructure and its temperature dependence. An insulated-gate field-effect transistor was fabricated, and transfer characteristics were measured at 4 K–300 K. At low temperatures (T < 45 K), source electrons are injected into the buried 2DEG channel first and drain current increases with the gate voltage. By increasing the gate voltage further, the current saturates followed by a negative transconductance observed, which can be attributed to electron tunneling from the buried channel to the surface channel. Finally, the drain current is saturated again at large gate biases due to parallel conduction of buried and surface channels. By increasing the temperature, an abrupt increase in threshold voltage is observed at T ∼ 45 K and it is speculated that negatively charged impurities at the Al2O3/Si interface are responsible for the threshold voltage shift. At T > 45 K, the current saturation and negative transconductance disappear and the device acts as a normal transistor.
Improving the performance of galloping micro-power generators by passively manipulating the trailing edge Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-22 J. Noel, R. Yadav, G. Li, M. F. Daqaq
Recent trends in distributed sensing networks have generated significant interest in the design of scalable micro-power generators. One such device exploits the galloping oscillations of a prism to harness energy from a moving fluid. Performance of galloping harvester's depends on the flow patterns around the prism, which, in turn, depend on its geometry and the Reynolds number of the flow. In this letter, we demonstrate that the useful range of the galloping instability can be extended by attaching a rigid splitter plate to the rear face of the prism. The plate provides a secondary flow reattachment point, which serves to improve the oscillation amplitude and power output of the generator. Experimental results demonstrate as much as 67% power enhancement for some prism geometries and a significant reduction in the cut-in wind speed of the generator.
Subnanopore filling during water vapor adsorption on microporous silica thin films as seen by low-energy positron annihilation Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-20 Kenji Ito, Shigeru Yoshimoto, Brian E. O'Rourke, Nagayasu Oshima, Kazuhiro Kumagai
Positron annihilation lifetime spectroscopy (PALS) using a low-energy positron microbeam extracted into air was applied to elucidating molecular-level pore structures formed in silicon-oxide-backboned microporous thin films under controlled humidity conditions; as a result, a direct observation of the interstitial spaces in the micropores filled with water molecules was achieved. It was demonstrated that PALS using a microbeam extracted into air in combination with water vapor adsorption is a powerful tool for the in-situ elucidation of both open and closed subnanoscaled pores of functional thin materials under practical conditions.
Electrostatically actuated thermal switch device for caloric film Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-21 Morgan Almanza, Lucas Depreux, Fabien Parrain, Martino LoBue
An innovative thermal switch device using a thin metallic film electrostatically actuated by an electrode mainly conceived for caloric cooling is studied. Our study focuses on the characterization of the thermal conductance at the interface for the “on” and “off” states. Our setup uses the current passing through the metallization of the film as a heater, while the temperature is deduced from the measurement of its electrical resistivity. Using a thermal diffusion model and our measurements, we deduce the on and off state thermal conductances, and we achieve an on/off conductance ratio of 103. Lastly, we use a simple finite-time thermodynamic model to estimate the efficiency at maximum power, and we would obtain by integrating a standard electrocaloric film in our thermal switch. The result is a micro-refrigerator working at 85% of Carnot efficiency with a power density of 228 W g−1 which is far more than what it has been currently demonstrated.
Excitonic nature of optical transitions in electroabsorption spectra of perovskite solar cells Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-22 Fabian Ruf, Alice Magin, Moritz Schultes, Erik Ahlswede, Heinz Kalt, Michael Hetterich
We investigate the electronic structure of solution-processed perovskite solar cells using temperature-dependent electroabsorption (EA) spectroscopy. Simultaneous measurements of absorption and electromodulated spectra of semitransparent methylammonium lead iodide solar cells facilitate a direct comparison of the specific features. The EA spectra can be transformed to peak-like line shapes utilizing an approach based on the Kramers–Kronig relations. The resulting peak positions correspond well to the discrete excitonic—rather than the continuum—contribution of the absorption spectra derived from generalized Elliott fits. This indicates the excitonic nature of the observed EA resonance and is found to be consistent over the whole temperature range investigated (from T = 10 K up to room temperature). To further confirm these findings, a line shape analysis of the measured EA spectra was performed. The best agreement was achieved using a first-derivative-like functional form which is expected for excitonic systems and supports the conclusion of an excitonic optical transition. Exciton binding energies EB are estimated for the orthorhombic and tetragonal phases as 26 meV and 19 meV, respectively. Nevertheless, power-conversion efficiencies η up to 13% (11.5% stabilized) demonstrate good charge-carrier separation in the devices due to sufficient thermal dissociation and Sommerfeld-enhanced absorption.
Generation of multiple vortex beam by means of active diffraction gratings Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-20 Ruben D. Muelas-Hurtado, Joao L. Ealo, Jhon F. Pazos-Ospina, Karen Volke-Sepúlveda
Acoustic vortices have attracted a great deal of attention in recent years due to their numerous applications. We introduce a highly efficient method for the generation of acoustic Bessel vortices in air, using spiral-shaped active diffraction gratings, which can be operated within a broad spectral range of ultrasonic frequencies. Using a single-arm active spiral source, we achieve the simultaneous generation of vortices of different topological charges, well separated among each other along the propagation axis. With a theoretical analysis, numerical simulations, and experiments, we demonstrate some features about spiral diffraction gratings, such as the equivalence between specific diffraction orders of an m-armed spiral and a single-arm spiral, and the annihilation of prescribed diffraction orders by tuning the width to pitch ratio.
Coexistence of multiple multimode nonlinear mixing regimes in a microelectromechanical device Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-23 Adarsh Ganesan, Ashwin Seshia
This paper builds on the recent demonstrations of two-mode three-wave mixing and three-mode four-wave mixing pathways. In these individual mixing mechanisms, the drive and resonant frequencies intrinsically couple to generate frequency combs with spacing defined by the separation between drive and resonant frequencies. Such frequency combs resulting from N -mode N + 1 -wave mixing processes possess spectral characteristics which are strikingly different from those of nominal N -mode parametric resonances. Now, in this paper, we experimentally show the possibility to simultaneously trigger one N = N 1 -mode N = N 1 + 1 -wave mixing and one nominal N = N 2 -mode parametric resonance through the significant drive of a single phonon mode. This specific demonstration, when set alongside previous studies, represents an important step towards understanding phononic frequency comb processes.
Micropillars with a controlled number of site-controlled quantum dots Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-12 Arsenty Kaganskiy, Fabian Gericke, Tobias Heuser, Tobias Heindel, Xavier Porte, Stephan Reitzenstein
We report on the realization of micropillars with site-controlled quantum dots (SCQDs) in the active layer. The SCQDs are grown via the buried stressor approach which allows for the positioned growth and device integration of a controllable number of QDs with high optical quality. This concept is very powerful as the number and the position of SCQDs in the cavity can be simultaneously controlled by the design of the buried-stressor. The fabricated micropillars exhibit a high degree of position control for the QDs above the buried stressor and Q-factors of up to 12 000 at an emission wavelength of around 930 nm. We experimentally analyze and numerically model the cavity Q-factor, the mode volume, the Purcell factor, and the photon-extraction efficiency as a function of the aperture diameter of the buried stressor. Exploiting these SCQD micropillars, we experimentally observe a Purcell enhancement in the single-QD regime with FP = 4.3 ± 0.3.
Electro-optic guided-mode resonance tuning suppressible by optically induced screening in a vertically coupled hybrid GaN/Si microring resonator Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-13 B. Thubthimthong, T. Sasaki, K. Hane
GaN as a nanophotonic material has gained much attention in recent years. Using the hybrid GaN/Si platform, we report the electro-optic tuning of guided-mode resonance in a vertically coupled hybrid GaN/Si microring resonator operating in the 1.5 μm window with up to a 6 dB extinction ratio and a 1.5 MHz modulation frequency (test equipment limit). The electro-optic tuning could be optically suppressed by electron-hole-originated screening induced by an ultraviolet excitation at 325 nm. Our work may benefit in externally intervenable optical interconnects for uninterrupted secure photonic networks.
Efficient CH3NH3PbI3 perovskite/fullerene planar heterojunction hybrid solar cells with oxidized Ni/Au/Cu transparent electrode Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-13 Wei-Chih Lai, Kun-Wei Lin, Tzung-Fang Guo, Peter Chen, Yuan-Yu Liao
We demonstrated the performance of inverted CH3NH3PbI3 perovskite-based solar cells (SCs) with a thermally oxidized nickel/gold/copper (Ni/Au/Cu) trilayer transparent electrode. Oxidized Ni/Au/Cu is a high transparent layer and has less resistance than the oxidized Ni/Au layer. Like the oxidized Ni/Au layer, oxidized Ni and Cu in oxidized Ni/Au/Cu could perform as a hole transport layer of the perovskite-based SCs. It leads to improved perovskite SC performance on an open circuit voltage of 1.01 V, a short circuit current density of 14.36 mA/cm2, a fill factor of 76.7%, and a power conversion efficiency (η%) of 11.1%. The η% of perovskite SCs with oxidized Ni (10 nm)/Au (6 nm)/Cu (1 nm) improved by approximately 10% compared with that of perovskite SCs with oxidized Ni/Au.
On-chip optical true time delay lines featuring one-dimensional fishbone photonic crystal waveguide Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-14 Chi-Jui Chung, Xiaochuan Xu, Gencheng Wang, Zeyu Pan, Ray T. Chen
In this paper, we present on-chip optical true time delay lines based on slow light one-dimensional (1D) fishbone photonic crystal waveguides (FPCWs). The structural slow light is generated by modulating the index guided optical mode with periodically arranged sidewalls along the propagation direction. Due to the reduced mode overlap with the rough etched surface, the propagation loss of the 1D FPCW is significantly reduced compared to the two-dimensional photonic crystal waveguide. A delay time of 65 ps/mm is observed experimentally.
On-chip III-V monolithic integration of heralded single photon sources and beamsplitters Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-14 J. Belhassen, F. Baboux, Q. Yao, M. Amanti, I. Favero, A. Lemaître, W. S. Kolthammer, I. A. Walmsley, S. Ducci
We demonstrate a monolithic III-V photonic circuit combining a heralded single photon source with a beamsplitter, at room temperature and telecom wavelength. Pulsed parametric down-conversion in an AlGaAs waveguide generates counterpropagating photons, one of which is used to herald the injection of its twin into the beamsplitter. We use this configuration to implement an integrated Hanbury-Brown and Twiss experiment, yielding a heralded second-order correlation g her ( 2 ) ( 0 ) = 0.10 ± 0.02 that confirms single-photon operation. The demonstrated generation and manipulation of quantum states on a single III-V semiconductor chip opens promising avenues towards real-world applications in quantum information.
Imaging of acoustic pressure modes in opto-mechano-fluidic resonators with a single particle probe Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-15 Jeewon Suh, Kewen Han, Gaurav Bahl
Opto-mechano-fluidic resonators (OMFRs) are a platform for high-throughput sensing of the mechanical properties of freely flowing microparticles in arbitrary media. Experimental extraction of OMFR mode shapes, especially the acoustic pressure field within the fluidic core, is essential for determining sensitivity and for extracting the particle parameters. Here, we demonstrate an imaging technique for simultaneously capturing the spatially distributed acoustic pressure fields of multiple vibrational modes in the OMFR system. The mechanism operates using microparticles as perturbative imaging probes and potentially reveals the inverse path towards multimode inertial detection of the particles themselves.
Tunnel-injected sub 290 nm ultra-violet light emitting diodes with 2.8% external quantum efficiency Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-15 Yuewei Zhang, Zane Jamal-Eddine, Fatih Akyol, Sanyam Bajaj, Jared M. Johnson, Gabriel Calderon, Andrew A. Allerman, Michael W. Moseley, Andrew M. Armstrong, Jinwoo Hwang, Siddharth Rajan
We report on the high efficiency tunnel-injected ultraviolet light emitting diodes (UV LEDs) emitting at 287 nm. Deep UV LED performance has been limited by the severe internal light absorption in the p-type contact layers and low electrical injection efficiency due to poor p-type conduction. In this work, a polarization engineered Al0.65Ga0.35N/In0.2Ga0.8N tunnel junction layer is adopted for non-equilibrium hole injection to replace the conventionally used direct p-type contact. A reverse-graded AlGaN contact layer is further introduced to realize a low resistance contact to the top n-AlGaN layer. This led to the demonstration of a low tunnel junction resistance of 1.9 × 10−3 Ω cm2 obtained at 1 kA/cm2. Light emission at 287 nm with an on-wafer peak external quantum efficiency of 2.8% and a wall-plug efficiency of 1.1% was achieved. The measured power density at 1 kA/cm2 was 54.4 W/cm2, confirming the efficient hole injection through interband tunneling. With the benefits of the minimized internal absorption and efficient hole injection, a tunnel-injected UV LED structure could enable future high efficiency UV emitters.
Plasmonic-enhanced targeted nanohealing of metallic nanostructures Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-16 Hangbo Yang, Jinsheng Lu, Pintu Ghosh, Ziyao Chen, Wei Wang, Hui Ye, Qian Yu, Min Qiu, Qiang Li
Healing defects of metallic structures is an essential procedure for manufacturing and maintaining integrated devices. Current nanocomposite-assisted microhealing methodologies are inadequate for nanoscopic applications because of their concomitant contamination and limited operation accuracy. In this paper, we propose an optically controllable targeted nanohealing technique by utilizing the plasmonic-enhanced photothermal effect. The healing of nanogaps between two silver nanowires (NWs) is achieved by increasing the incident laser power in steps. Partial connection of NWs can be readily obtained using this technique, while near-perfect connection of NWs with the same crystal orientations is obtained only when the lattices on the two opposing facets are matched after recrystallization. This non-contaminating nanohealing technique not only provides deeper insight into the heat/mass transfer assisted by plasmonic photothermal conversion in the nanoscale but also suggests avenues for recovering mechanical, electronic, and photonic properties of defected metallic nanodevices.
Photopolymerization of complex emulsions with irregular shapes fabricated by multiplex coaxial flow focusing Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-12 Qiang Wu, Chaoyu Yang, Jianxin Yang, Fangsheng Huang, Guangli Liu, Zhiqiang Zhu, Ting Si, Ronald X. Xu
We fabricate complex emulsions with irregular shapes in the microscale by a simple but effective multiplex coaxial flow focusing process. A multiphase cone-jet structure is steadily formed, and the compound liquid jet eventually breaks up into Janus microdroplets due to the perturbations propagating along the jet interfaces. The microdroplet shapes can be exclusively controlled by interfacial tensions of adjacent phases. Crescent-moon-shaped microparticles and microcapsules with designated structural characteristics are further produced under ultraviolet light of photopolymerization after removing one hemisphere of the Janus microdroplets. These complex emulsions have potential applications in bioscience, food, functional materials, and controlled drug delivery.
Enhanced water collection through a periodic array of tiny holes in dropwise condensation Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-13 Kyungjun Song, Gyeonghee Kim, Sunjong Oh, Hyuneui Lim
This paper introduces a simple method of water collection by increasing the coalescence effects in dropwise condensation with the use of microscale holes. The tiny holes modified the surface free energy states of the droplets on the plate, yielding a surface free energy barrier between the flat solid surface and the holes. The spatial difference in the surface free energy of the droplets enabled the droplets to move toward the adjacent droplets, thus increasing the possibility of coalescence. The water collection experiments were performed using a Peltier-based cooling system at 2 °C inside a chamber at 30 °C and 70% humidity. The results demonstrated that the perforated plates without any additional treatment provided the water collection rate of up to 22.64 L/m2 day, which shows an increase of 30% compared to that demonstrated by the bare plate. By comparing the experimental results for the surface of filmwise condensation, it was proved that the dominant water collecting improvement results from the increased coalescence effects. This simple technique can enhance the performance of systems exposed to water condensation, including water collection, heat-transfer, and dehumidifying systems.
Interplay between developing flow length and bubble departure diameter during macroconvection enhanced pool boiling Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-15 A. Jaikumar, T. S. Emery, S. G. Kandlikar
Enhanced boiling structures based on the concept of separate liquid-vapor (L-V) pathways rely on the motion of the bubbles departing from the nucleating regions (NRs) to induce a macroconvective liquid jet impingement flow over adjacent non-boiling regions. Heat transfer in the non-boiling regions can be improved by incorporating microchannels which act as feeder channels (FCs) that also improve liquid directionality towards the NR. We hypothesize that the single-phase flow characteristics in the developing region of the FC contribute to the boiling enhancement and explore the interplay between the FC length, developing flow length, and departure bubble diameter. FC lengths shorter than the developing flow length benefit from the enhancement due to developing boundary layers over their entire length. However, FC lengths shorter than the departure bubble diameter suffer from bubble interference while FC lengths that are considerably longer than the developing flow length exhibit lower heat transfer rates in the fully developed region. This hypothesis was verified by conducting pool boiling experiments with four feeder channel lengths between 1 mm and 3 mm using HFE-7000, PP1, PP1C, and water. Three distinct regions: (i) interfering bubble, (ii) efficient L-V pathways, and (iii) diminished jet were identified to explain the boiling performance enhancement. This analysis will be beneficial in the pursuit to enhance critical heat flux (CHF) and heat transfer coefficient (HTC) on surfaces utilizing macroconvection mechanisms during boiling with different liquids.
Determination of the structural phase and octahedral rotation angle in halide perovskites Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-12 Roberto dos Reis, Hao Yang, Colin Ophus, Peter Ercius, Gregory Bizarri, Didier Perrodin, Tetiana Shalapska, Edith Bourret, Jim Ciston, Ulrich Dahmen
A key to the unique combination of electronic and optical properties in halide perovskite materials lies in their rich structural complexity. However, their radiation sensitive nature limits nanoscale structural characterization requiring dose efficient microscopic techniques in order to determine their structures precisely. In this work, we determine the space-group and directly image the Br halide sites of CsPbBr3, a promising material for optoelectronic applications. Based on the symmetry of high-order Laue zone reflections of convergent-beam electron diffraction, we identify the tetragonal (I4/mcm) structural phase of CsPbBr3 at cryogenic temperature. Electron ptychography provides a highly sensitive phase contrast measurement of the halide positions under low electron-dose conditions, enabling imaging of the elongated Br sites originating from the out-of-phase octahedral rotation viewed along the  direction of I4/mcm persisting at room temperature. The measurement of these features and comparison with simulations yield an octahedral rotation angle of 6.5°(±1.5°). The approach demonstrated here opens up opportunities for understanding the atomic scale structural phenomena applying advanced characterization tools on a wide range of radiation sensitive halide-based all-inorganic and hybrid organic-inorganic perovskites.
Gold fillings unravel the vacancy role in the phase transition of GeTe Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-12 Jinlong Feng, Meng Xu, Xiaojie Wang, Qi Lin, Xiaomin Cheng, Ming Xu, Hao Tong, Xiangshui Miao
Phase change memory (PCM) is an important candidate for future memory devices. The crystalline phase of PCM materials contains abundant intrinsic vacancies, which plays an important role in the rapid phase transition upon memory switching. However, few experimental efforts have been invested to study these invisible entities. In this work, Au dopants are alloyed into the crystalline GeTe to fill the intrinsic Ge vacancies so that the role of these vacancies in the amorphization of GeTe can be indirectly studied. As a result, the reduction of Ge vacancies induced by Au dopants hampers the amorphization of GeTe as the activation energy of this process becomes higher. This is because the vacancy-interrupted lattice can be “repaired” by Au dopants with the recovery of bond connectivity. Our results demonstrate the importance of vacancies in the phase transition of chalcogenides, and we employ the percolation theory to explain the impact of these intrinsic defects on this vacancy-ridden crystal quantitatively. Specifically, the threshold of amorphization increases with the decrease in vacancies. The understanding of the vacancy effect sheds light on the long-standing puzzle of the mechanism of ultra-fast phase transition in PCMs. It also paves the way for designing low-power-consumption electronic devices by reducing the threshold of amorphization in chalcogenides.
Determination of the spin orbit coupling and crystal field splitting in wurtzite InP by polarization resolved photoluminescence Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-13 Nicolas Chauvin, Amaury Mavel, Ali Jaffal, Gilles Patriarche, Michel Gendry
Excitation photoluminescence spectroscopy is usually used to extract the crystal field splitting (ΔCR) and spin orbit coupling (ΔSO) parameters of wurtzite (Wz) InP nanowires (NWs). However, the equations expressing the valence band splitting are symmetric with respect to these two parameters, and a choice ΔCR > ΔSO or ΔCR < ΔSO has to be taken into account in order to assign the numerical values. To solve this issue, polarization resolved micro-photoluminescence was performed on vertically aligned and untapered Wz InP NWs grown on silicon. The experimental results combined with a theoretical model and finite difference time domain calculations allow us to conclude that ΔCR > ΔSO in Wz InP.
Defect localization of metal interconnection lines in 3-dimensional through-silicon-via structures by differential scanning photocapacitance microscopy Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-14 K. J. P. Jacobs, M. Stucchi, V. V. Afanas'ev, M. Gonzalez, K. Croes, I. De Wolf, E. Beyne
We report a differential scanning photocapacitance microscopy technique based on the detection of light-induced capacitance changes allowing mapping of metal interconnection line defects in through-silicon-via (TSV) structures used in three-dimensional (3-D) integration technology. Due to the photosensitive silicon depletion capacitance, observation of the photocapacitance response enables non-destructive two-dimensional (2-D) visualization of metallization line ruptures in TSV structures. We demonstrate the application of the proposed method on a TSV chain structure and reveal the location of the open metallization rupture.
Optical AND operation in n-AlGaAs/GaAs heterojunction field effect transistor Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-12 T. Kawazu, T. Noda, Y. Sakuma
The near-infrared photoresponses of an n-AlGaAs/GaAs heterojunction field-effect transistor (FET) were investigated for the irradiation of two lights: (A) a laser beam with the energy above the Schottky-barrier which uniformly illuminates the gate region and (B) a laser beam with the energy above the GaAs bandgap which locally illuminates the ungate region. We measured a lateral photocurrent in the two dimensional electron gas (2DEG) channel at the n-AlGaAs/GaAs heterojunction and found that the FET acts as an optical AND element; the lateral photocurrent is generated only when both the light A and B simultaneously illuminate the FET. The lateral current flows from left to right when the left side of the FET is illuminated with the light B, while the right side irradiation leads to the current from right to left. The experimental findings are well explained by a theory based on the current-continuity equation, where the lateral current in the 2DEG channel is driven by an asymmetric electron transfer resulting from the simultaneous irradiation of the light A and B.
Precision measurement of the quantized anomalous Hall resistance at zero magnetic field Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-13 Martin Götz, Kajetan M. Fijalkowski, Eckart Pesel, Matthias Hartl, Steffen Schreyeck, Martin Winnerlein, Stefan Grauer, Hansjörg Scherer, Karl Brunner, Charles Gould, Franz J. Ahlers, Laurens W. Molenkamp
In the quantum anomalous Hall effect, the edge states of a ferromagnetically doped topological insulator exhibit quantized Hall resistance and dissipationless transport at zero magnetic field. Up to now, however, the resistance was experimentally assessed using standard transport measurement techniques which are difficult to trace to the von-Klitzing constant RK with high precision. Here, we present a metrologically comprehensive measurement, including a full uncertainty budget, of the resistance quantization of V-doped (Bi,Sb)2Te3 devices without the external magnetic field. For the deviation of the quantized anomalous Hall resistance from RK, we determined a value of 0.17 ± 0.25 ppm, the smallest and most precise value reported to date. This is a step towards realization of a practical zero-field quantum resistance standard which in combination with the Josephson effect could provide the universal quantum units standard in the future.
Temperature dependent electrical properties of pulse laser deposited Au/Ni/β-(AlGa)2O3 Schottky diode Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-13 Qian Feng, Zhaoqing Feng, Zhuangzhuang Hu, Xiangyu Xing, Guangshuo Yan, Jincheng Zhang, Yongkuan Xu, Xiaozheng Lian, Yue Hao
We have demonstrated the epitaxial growth of a β-(Al0.08Ga0.92)2O3 film on a β-Ga2O3 (010) substrate through pulsed laser deposition. The temperature-dependent electrical characteristics of Au/Ni/β-(Al0.08Ga0.92)2O3 Schottky diodes were investigated in the temperature range of 300–573 K, using thermionic emission theory to calculate the Schottky diode parameters. The barrier height ϕb was found to increase, while the ideality factor n and the series resistance Rs were found to decrease with increasing temperatures. The calculated values of ϕb and n varied from 0.81 eV and 2.29 at 300 K to 1.02 eV and 1.65 at 573 K. The temperature-dependent I-V characteristics of the Schottky diode have shown the Gaussian distribution, yielding a mean barrier height of 1.23 eV and a standard deviation of 0.147 V, respectively. A modified Richardson plot of ln ( I s / T 2 ) − ( q 2 σ s 2 / 2 k 2 T 2 ) versus q/2kT gives ϕ b 0 ¯ and A* as 1.24 eV and 44.3 A cm−2 K−2, showing the promise of Ni/β-(AlGa)2O3 as a Schottky diode rectifier.
Exciton-phonon coupling in a CsPbBr3 single nanocrystal Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-14 Julien Ramade, Léon Marcel Andriambariarijaona, Violette Steinmetz, Nicolas Goubet, Laurent Legrand, Thierry Barisien, Frédérick Bernardot, Christophe Testelin, Emmanuel Lhuillier, Alberto Bramati, Maria Chamarro
We have performed micro-photoluminescence measurements on a single CsPbBr3 nanocrystal (NC) with a size comparable to the Bohr diameter (7 nm). When the NC has an orthorhombic crystal symmetry, we observe an exciton fine structure composed of three peaks linearly polarized. We took advantage of the polarization properties of micro-photoluminescence to monitor in situ both the energy and linewidth of individual peaks when increasing temperature. We reveal that two regimes exist, at low and high temperature, which are dominated by acoustic or longitudinal optical phonon (Fröhlich term) couplings, respectively. The acoustic contribution does not change when the energy of the excitonic transition varies in the range of 2.46–2.62 eV, i.e., with NC sizes corresponding to this range. We find that line broadening is mainly ruled by the Fröhlich term, which is consistent with the polar nature of CsPbBr3.
Enhancement of hot-carrier photoluminescence with intense terahertz pulses Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-15 D. N. Purschke, M. Na, A. Longman, L. V. Titova, F. A. Hegmann
Intense terahertz (THz) pulses have been shown to induce photoluminescence (PL) quenching in bulk semiconductors. We show that in addition to PL quenching near the bandgap, intense THz pulses enhance the high-energy tail of the PL in GaAs. Furthermore, we propose a simple model that accounts for both PL quenching and enhancement where THz-induced hot carriers directly enhance high-energy PL but reduce overall radiative efficiency due to ultrafast diffusion. Exploring the interplay between THz-induced PL enhancement and quenching over a range of excitation parameters reveals a reduction of integrated PL at low photoexcitation fluence, while at higher fluences, the amplitude of the PL quenching is balanced by that of the PL enhancement.
Semiconducting SWNTs sorted by polymer wrapping: How pure are they? Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-16 Vladimir Derenskyi, Widianta Gomulya, Jia Gao, Satria Zulkarnaen Bisri, Mariacecilia Pasini, Yueh-Lin Loo, Maria Antonietta Loi
Short-channel field-effect transistors (FETs) prepared from semiconducting single-walled carbon nanotube (s-SWNT) dispersions sorted with poly(2,5-dimethylidynenitrilo-3,4-didodecylthienylene) are demonstrated. Electrical analysis of the FETs shows no evidence of metallic tubes out of a total number of 646 SWNTs tested, implying an estimated purity of our semiconducting SWNT solution higher than 99.85%. These findings confirm the effectiveness of the polymer-wrapping technique in selecting semiconducting SWNTs, as well as the potential of sorted nanotubes for the fabrication of short channel FETs comprising from 1 to up to 15 nanotubes without inter-nanotube junctions.
Mobility spectrum analytical approach for the type-II Weyl semimetal Td-MoTe2 Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-12 Q. L. Pei, X. Luo, F. C. Chen, H. Y. Lv, Y. Sun, W. J. Lu, P. Tong, Z. G. Sheng, Y. Y. Han, W. H. Song, X. B. Zhu, Y. P. Sun
The extreme magnetoresistance (XMR) in orthorhombic W/MoTe2 arises from the combination of the perfect electron-hole (e-h) compensation effect and the unique orbital texture topology, which have comprised an intriguing research field in materials physics. Herein, we apply a special analytical approach as a function of mobility (μ-spectrum) without any hypothesis. Based on the interpretations of longitudinal and transverse electric transport of Td-MoTe2, the types and the numbers of carriers can be obtained. There are three observations: the large residual resistivity ratio can be observed in the MoTe2 single crystal sample, which indicates that the studied crystal is of high quality; we observed three electron-pockets and three hole-ones from the μ-spectrum and that the ratio of h/e is much less than 1, which shows that MoTe2 is more e-like; different from the separated peaks obtained from the hole-like μ-spectrum, those of the electron-like one are continuous, which may indicate the topological feature of electron-pockets in Td-MoTe2. The present results may provide an important clue to understanding the mechanism of the XMR effect in Td-MoTe2.
Epitaxial contact Andreev reflection spectroscopy of NbN/Co2FeSi layered devices Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-12 Iduru Shigeta, Takahide Kubota, Yuya Sakuraba, Cor G. Molenaar, Joost N. Beukers, Shojiro Kimura, Alexander A. Golubov, Alexander Brinkman, Satoshi Awaji, Koki Takanashi, Masahiko Hiroi
We investigated the spin polarization P of Co-based Heusler alloy Co2FeSi by epitaxial contact Andreev reflection (ECAR) spectroscopy using epitaxially grown superconductor NbN and Heusler alloy Co2FeSi layered devices. Ferromagnetic Co2FeSi possesses the highest Curie temperature (TC ≈ 1100 K) and the largest spontaneous magnetic moment (ps ≈ 6 μB) in the class of Heusler alloys. The ECAR measurements revealed that the P value of Co2FeSi was 54 ± 2% with a finite barrier parameter Z, indicating that an intrinsic P value in ECAR spectroscopy would exceed reported values in point-contact Andreev reflection spectroscopy. We therefore established not only the epitaxial integration of ferromagnetic Co2FeSi with superconductor NbN on an MgO substrate but also the fabrication and evaluation techniques of their ECAR devices. This highly versatile superconducting spintronic system enables fundamental superconducting spintronic studies, and it is also a candidate for practical superconducting spintronic devices.
Large moments in bcc FexCoyMnz ternary alloy thin films Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-13 R. J. Snow, H. Bhatkar, A. T. N'Diaye, E. Arenholz, Y. U. Idzerda
The elemental magnetic moments and the average atomic moment of 10–20 nm thick single crystal bcc (bct) FexCoyMnz films deposited on MgO(001) have been determined as a function of a broad range of compositions. Thin film epitaxy stabilized the bcc structure for 80% of the available ternary compositional space compared to only a 23% stability region for the bulk. The films that display ferromagnetism represent 60% of the available compositional possibilities compared to 25% for the bulk. A maximum average atomic moment of 3.25 ± 0.3 μB/atom was observed for a bcc Fe9Co62Mn29 film (well above the limit of the Slater-Pauling binary alloy curve of 2.45 μB/atom). The FexCoyMnz ternary alloys that exhibit high moments can only be synthesized as ultrathin films since the bcc structure is not stable in the bulk for those compositions.
Temperature dependence of interlayer coupling in perpendicular magnetic tunnel junctions with GdOX barriers Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-13 T. Newhouse-Illige, Y. H. Xu, Y. H. Liu, S. Huang, H. Kato, C. Bi, M. Xu, B. J. LeRoy, W. G. Wang
Perpendicular magnetic tunnel junctions with GdOX tunneling barriers have shown a unique voltage controllable interlayer magnetic coupling effect. Here, we investigate the quality of the GdOX barrier and the coupling mechanism in these junctions by examining the temperature dependence of the tunneling magnetoresistance and the interlayer coupling from room temperature down to 11 K. The barrier is shown to be of good quality with the spin independent conductance only contributing a small portion, 14%, to the total room temperature conductance, similar to AlOX and MgO barriers. The interlayer coupling, however, shows an anomalously strong temperature dependence including sign changes below 80 K. This non-trivial temperature dependence is not described by previous models of interlayer coupling and may be due to the large induced magnetic moment of the Gd ions in the barrier.
Critical behavior of two-dimensional intrinsically ferromagnetic semiconductor CrI3 Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-14 G. T. Lin, X. Luo, F. C. Chen, J. Yan, J. J. Gao, Y. Sun, W. Tong, P. Tong, W. J. Lu, Z. G. Sheng, W. H. Song, X. B. Zhu, Y. P. Sun
CrI3, which belongs to a rare category of two-dimensional (2D) ferromagnetic semiconductors, is of great interest for spintronic device applications. Unlike CrCl3 whose magnetism presents a 2D-Heisenberg behavior, CrI3 exhibits a larger van der Waals gap, smaller cleavage energy, and stronger magnetic anisotropy which could lead to a 3D magnetic characteristic. Hence, we investigate the critical behavior of CrI3 in the vicinity of magnetic transition. We use the modified Arrott plot and Kouvel-Fisher method and conduct critical isotherm analysis to estimate the critical exponents near the ferromagnetic phase transition. This shows that the magnetism of CrI3 follows the crossover behavior of a 3D-Ising behavior with mean field type interactions where the critical exponents β, γ, and δ are 0.323 ± 0.006, 0.835 ± 0.005, and 3.585 ± 0.006, respectively, at the Curie temperature of 64 K. We propose that the crossover behavior can be attributed to the strong uniaxial anisotropy and inevitable interlayer coupling. Our experiment demonstrates the applicability of crossover behavior to a 2D ferromagnetic semiconductor.
Plasmon-induced demagnetization and magnetic switching in nickel nanoparticle arrays Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-14 Mikko Kataja, Francisco Freire-Fernández, Jorn P. Witteveen, Tommi K. Hakala, Päivi Törmä, Sebastiaan van Dijken
We report on the manipulation of magnetization by femtosecond laser pulses in a periodic array of cylindrical nickel nanoparticles. By performing experiments at different wavelengths, we show that the excitation of collective surface plasmon resonances triggers demagnetization in zero field or magnetic switching in a small perpendicular field. Both magnetic effects are explained by plasmon-induced heating of the nickel nanoparticles to their Curie temperature. Model calculations confirm the strong correlation between the excitation of surface plasmon modes and laser-induced changes in magnetization.
Control of reversible magnetization switching by pulsed circular magnetic field in glass-coated amorphous microwires Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-15 Alexander Chizhik, Arkady Zhukov, Julian Gonzalez, Andrzej Stupakiewicz
Magnetization reversal in magnetic microwires was studied in the presence of external mechanical stress and helical magnetic fields using the magneto-optical Kerr effect. It was found that a combination of tuned magnetic anisotropy and a direct current or pulsed circular magnetic field activated different types of magnetization reversal scenarios. The application of the pulsed magnetic field of 10 ns time duration induced a transient controlling action to switch the magnetic states without activating a domain wall motion. This created a promising method for tuning the giant magneto-impedance effect.
Nanoscale control of stripe-ordered magnetic domain walls by vertical spin transfer torque in La0.67Sr0.33MnO3 film Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-15 Jing Wang, Shizhe Wu, Ji Ma, Lishan Xie, Chuanshou Wang, Iftikhar Ahmed Malik, Yuelin Zhang, Ke Xia, Ce-Wen Nan, Jinxing Zhang
Stripe-ordered domains with perpendicular magnetic anisotropy have been intensively investigated due to their potential applications in high-density magnetic data-storage devices. However, the conventional control methods (e.g., epitaxial strain, local heating, magnetic field, and magnetoelectric effect) of the stripe-ordered domain walls either cannot meet the demands for miniaturization and low power consumption of spintronic devices or require high strength of the electric field due to the small value of the magnetoelectric effect at room temperature. Here, a domain-wall resistive effect of 0.1% was clarified in La0.67Sr0.33MnO3 thin films between the configurations of current in the plane and perpendicular to the plane of walls. Furthermore, a reversible nanoscale control of the domain-wall re-orientation by vertical spin transfer torque across the probe/film interface was achieved, where a probe voltage of 0.1 V was applied on a manganite-based capacitor. We also demonstrated that the stripe-ordered magnetic domain-wall re-orientation strongly depends on the AC frequency of the scanning probe voltage which was applied on the capacitor.
Effects of hydrostatic pressure on spin-lattice coupling in two-dimensional ferromagnetic Cr2Ge2Te6 Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-16 Y. Sun, R. C. Xiao, G. T. Lin, R. R. Zhang, L. S. Ling, Z. W. Ma, X. Luo, W. J. Lu, Y. P. Sun, Z. G. Sheng
Spin-lattice coupling plays an important role in both formation and understanding of the magnetism in two-dimensional magnetic semiconductors (2DMS). In this paper, the steady pressure effects on the lattice structure, Raman resonances, and magnetization of a 2DMS Cr2Ge2Te6 have been studied by both experiments and first principles calculations. It is found that the bond length of Cr-Cr decreases, the angle of Cr-Te-Cr diverges from 90°, and the Raman modes E g 3 and A g 1 show an increase with the application of external pressure. Consequently, the magnetic phase transition temperature TC decreases from 66.6 K to 60.6 K (∼9%) as the pressure increases from 0 to 1 GPa. These pressure effects not only confirm the existence of strong spin-lattice coupling but also reveal the detailed information about the lattice deformation effect on the magnetic properties in such 2DMS, which would be a benefit for the further understanding and manipulation of the magnetism in 2D materials.
Phase-resolved spin-wave tomography Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-02-16 Yusuke Hashimoto, Tom H. Johansen, Eiji Saitoh
The propagation dynamics of spin waves are represented by their dispersion relations. Recently, we have developed a method, called spin-wave tomography (SWaT), to obtain a dispersion relation of spin waves in the long wavelength regime, the so-called pure magnetostatic waves. In our previous studies on SWaT, phase information of spin waves is disregarded. In this report, we demonstrate an advanced SWaT analysis, called phase-resolved spin-wave tomography (PSWaT), to realize the direct observation of the amplitude and the phase of spin waves. The PSWaT spectra are obtained by separating the real and the imaginary components of the complex Fourier transform in the SWaT analysis. We demonstrate the PSWaT spectra of spin waves excited by photo-induced demagnetization in a Bi-doped garnet film, reflecting the characteristic features of the complex dynamical susceptibility affected by magnetostatic coupling in the film.
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