Volume 104, Issue 16, 21 April 2014
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- superconductivity and superconducting electronics
- dielectrics, ferroelectrics, and multiferroics
- nanoscale science and technology
- organic electronics and photonics
- device physics
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
Photovoltaic nano-devices have largely been relying on charge separation in conventional p-n junctions. Junction formation via doping, however, imposes major challenges in process control. Here, we report on a concept for photovoltaic energy conversion at the nano scale without the need for intentional doping. Our approach relies on charge carrier separation in inhomogeneously strained germanium nanowires (Ge NWs). This concept utilizes the strain-induced gradient in bandgap along tapered NWs. Experimental data confirms the feasibility of strain-induced charge separation in individual vapor-liquid-solid grown Ge NW devices with an internal quantum efficiency of ∼5%. The charge separation mechanism, though, is not inherently limited to a distinct material. Our work establishes a class of photovoltaic nano-devices with its opto-electronic properties engineered by size, shape, and applied strain.
- PHOTONICS AND OPTOELECTRONICS
Influences of carrier diffusion and radial mode field pattern on high speed characteristics for microring lasers104(2014); http://dx.doi.org/10.1063/1.4872266View Description Hide Description
High-speed directly modulated microlasers are potential light sources for on-chip optical interconnection and photonic integrated circuits. In this Letter, dynamic characteristics are studied for microring lasers by rate equation analysis considering radial carrier hole burning and diffusion and experimentally. The coupled modes with a wide radial field pattern and the injection current focused in the edge area of microring resonator can greatly improve the high speed response curve due to the less carrier hole burning. The small-signal response curves of a microring laser connected with an output waveguide exhibit a larger 3 dB bandwidth and smaller roll-off at low frequency than that of the microdisk laser with the same radius of 15 μm, which accords with the simulation results.
104(2014); http://dx.doi.org/10.1063/1.4871369View Description Hide Description
Our work presents a slot and a patch array antenna at the front facet of a 4.7 THz quantum cascade laser as extractor, decreasing the facet reflectivity down to 2.6%. The resulting output power increases by a factor 2 and the slope efficiency by a factor 4. The simulated and the measured far-fields are in good agreement.
104(2014); http://dx.doi.org/10.1063/1.4871381View Description Hide Description
Hot carrier dynamics in the Dirac band of n-type epitaxial graphene on a SiC substrate were traced in real time using femtosecond-time-resolved photoemission spectroscopy. The spectral evolution directly reflects the energetically linear density of states superimposed with a Fermi–Dirac distribution. The relaxation time is governed by the internal energy dissipation of electron–electron scattering, and the observed electronic temperature indicates cascade carrier multiplication.
Electrothermally actuated microelectromechanical systems based omega-ring terahertz metamaterial with polarization dependent characteristics104(2014); http://dx.doi.org/10.1063/1.4871999View Description Hide Description
We present the design, simulation, fabrication, and characterization of a continuously tunable Omega-ring terahertz metamaterial. The tunability of metamaterial is obtained by integrating microactuators into the metamaterial unit cell. Electrothermal actuation mechanism is used to provide higher tuning range, larger stroke, and enhanced repeatability. The maximum achieved tuning range for the resonant frequency is around 0.30 THz for the input power of 500 mW. This shows the potential of using electrothermally actuated microactuators based tunable metamaterial design for application such as filters, absorbers, sensors, and spectral imagers.
104(2014); http://dx.doi.org/10.1063/1.4872163View Description Hide Description
We present an experimental study of the magnetic Purcell effect in finite arrays of the wire metamaterial. By directly measuring the spatial-frequency map of the Purcell factor, we explicitly demonstrate how the Purcell factor is enhanced at the Fabry-Pérot resonances of the wire metamaterial block in microwave frequency range. The experimental results are in a good agreement with theoretical and numerical estimations.
104(2014); http://dx.doi.org/10.1063/1.4872170View Description Hide Description
Dispersive Fourier transform imaging is a powerful technique in achieving ultrafast imaging of wide areas. However, system power efficiency is often limited by dispersive components. Here, we demonstrate that a gap-plasmon metasurface (GPM) based blazed grating can be used in dispersive imaging applications to achieve higher power efficiency than conventional gratings. A sub-wavelength GPM-based grating at telecommunication wavelengths has been designed and fabricated. 75.6% power efficiency with ∼0.4°/10 nm spatial dispersion has been measured for TE polarized waves at normal incidence. The fabricated device has been tested in a wide area real-time dispersive imaging system and <300 μm spatial resolution has been demonstrated experimentally.
104(2014); http://dx.doi.org/10.1063/1.4872258View Description Hide Description
Here, we report on the development of an antiresonant graphene-based one-dimensional structure which allows the control of linear and nonlinear device performance through optical confinement. A record average output in excess of 10 W is achieved by integrating this antiresonant graphene saturable absorber mirror into a vertical-external-cavity-surface-emitting-laser at 1030 nm, which leads to strong evidence of mode-locking, generating pulses with energies up to 2.8 nJ and a pulsewidth of 353 fs.
Coherent nanocavity structures for enhancement in internal quantum efficiency of III-nitride multiple quantum wells104(2014); http://dx.doi.org/10.1063/1.4873161View Description Hide Description
A “coherent” nanocavity structure has been designed on two-dimensional well-ordered InGaN/GaN nanodisk arrays with an emission wavelength in the green spectral region, leading to a massive enhancement in resonance mode in the green spectra region. By means of a cost-effective nanosphere lithography technique, we have fabricated such a structure on an InGaN/GaN multiple quantum well epiwafer and have observed the “coherent” nanocavity effect, which leads to an enhanced spontaneous emission (SE) rate. The enhanced SE rate has been confirmed by time resolved photoluminescence measurements. Due to the coherent nanocavity effect, we have achieved a massive improvement in internal quantum efficiency with a factor of 88, compared with the as-grown sample, which could be significant to bridge the “green gap” in solid-state lighting.
Characterization of coplanar poled electro optic polymer films for Si-photonic devices with multiphoton microscopy104(2014); http://dx.doi.org/10.1063/1.4872165View Description Hide Description
We imaged coplanar poled electro optic (EO) polymer films on transparent substrates with a multiple-photon microscope in reflection and correlated the second-harmonic light intensity with the results of Pockels coefficient (r 33) measurements. This allowed us to make quantitative measurements of poled polymer films on non-transparent substrates like silicon, which are not accessible with traditional Pockels coefficient measurement techniques. Phase modulators consisting of silicon waveguide devices with EO polymer claddings with a known Pockels coefficient (from measurements) were used to validate the correlation between the second-harmonic signal and r 33. This also allowed us to locally map the r 33 coefficient in the poled area.
104(2014); http://dx.doi.org/10.1063/1.4872319View Description Hide Description
This paper demonstrates the enhanced photoacoustic sensing of surface-bound light absorbing molecules and metal nanoparticles using a one-dimensional photonic crystal (PC) substrate. The PC structure functions as an optical resonator at the wavelength where the analyte absorption is strong. The optical resonance of the PC sensor provides an intensified evanescent field with respect to the excitation light source and results in enhanced optical absorption by surface-immobilized samples. For the analysis of a light absorbing dye deposited on the PC surface, the intensity of photoacoustic signal was enhanced by more than 10-fold in comparison to an un-patterned acrylic substrate. The technique was also applied to detect gold nanorods and exhibited more than 40 times stronger photoacoustic signals. The demonstrated approach represents a potential path towards single molecule absorption spectroscopy with greater performance and inexpensive instrumentation.
104(2014); http://dx.doi.org/10.1063/1.4873356View Description Hide Description
We report on the growth, fabrication, and experimental study of distributed feed-back antimonide diode lasers with buried grating. A second order index-coupled grating was defined by interferometric lithography on the top of the laser waveguide and dry etched by reactive ion etching. The grating was then buried thanks to an overgrowth of the top cladding layer using molecular beam epitaxy. The wafer was then processed using standard photolithography and wet etching into 15 μm-wide laser ridges. Single frequency laser emission at a wavelength of 2.2 μm was measured with a side mode suppression ratio of 34 dB, a maximum output power of 30 mW, and a total continuous tuning range of 6.5 nm.
Exciplex emission and decay of co-deposited 4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine:tris-[3-(3-pyridyl)mesityl]borane organic light-emitting devices with different electron transporting layer thicknesses104(2014); http://dx.doi.org/10.1063/1.4870492View Description Hide Description
Highly efficient fluorescence organic light-emitting diodes (OLEDs) based on the mixed 4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine:tris-[3-(3-pyridyl)mesityl]borane (1:1) system are reported. The electroluminescence due to the exciplex emission is red shifted when the thickness of the electron-transporting layer increases. The prepared OLEDs achieve a low turn-on voltage of 2.1 V, a high current efficiency of 36.79 cd/A, and a very high luminescence of 17 100 cd/m2, as well as a low efficiency roll-off. The current efficiency of the optimized OLED is maintained at more than 28.33 cd/A up to 10 000 cd m−2. The detailed recombination mechanism of the prepared OLEDs is investigated by the transient electroluminescence method. It is concluded that there are no contributions from trapped charges and annihilations of triplet-triplet excitons to the detected electroluminescence.
Simultaneous enhancement of electron overflow reduction and hole injection promotion by tailoring the last quantum barrier in InGaN/GaN light-emitting diodes104(2014); http://dx.doi.org/10.1063/1.4873395View Description Hide Description
A three-step graded undoped-InGaN layers embedded between the GaN last quantum barrier layer and the p-AlGaN electron blocking layer was proposed and its effect on the performance of InGaN/GaN light-emitting diodes was investigated both experimentally and theoretically. In the proposed structure, the electron leakage is found to be effectively reduced, while the hole injection efficiency is simultaneously increased significantly, hence enabling a greatly enhanced radiative recombination rate within the active region. As a result, improvements of 12.25% in the optical output power and 11.98% in the external quantum efficiency are obtained from the proposed device with the respect to the reference device.
104(2014); http://dx.doi.org/10.1063/1.4873541View Description Hide Description
Hybrid graphene-metal gratings with tunable Fano resonance are proposed and theoretically investigated in THz band. The grating contains alternately aligned metal and graphene stripes, which could be viewed as the superposition of two kinds of gratings with the same period. Due to different material properties, the resonance coupling between the metal and graphene parts forms typical Fano-type transmitting spectra. The related physical mechanism is studied by inspecting the induced dipole moment and local surface charge distributions at different wavelengths. Both of the resonance amplitude and frequency of the structure thus are adjustable by tuning graphene's Fermi energy and the grating's geometrical parameters. Furthermore, the Fano-type spectra are also quite sensitive to environmental indices, which supply another kind of tunability. All these features should have promising applications in tunable THz filters, switches, and modulators.
104(2014); http://dx.doi.org/10.1063/1.4873935View Description Hide Description
The optical properties and the Franz-Keldysh effect at the direct band gap of GeSn alloys with Sn concentrations up to 4.2% at room temperature were investigated. The GeSn material was embedded in the intrinsic region of a Ge heterojunction photodetector on Si substrates. The layer structure was grown by means of ultra-low temperature molecular beam epitaxy. The absorption coefficient as function of photon energy and the direct bandgap energies were determined. In all investigated samples, the Franz-Keldysh effect can be observed. A maximum absorption ratio of 1.5 was determined for 2% Sn for a voltage swing of 3 V.
- SURFACES AND INTERFACES
104(2014); http://dx.doi.org/10.1063/1.4872168View Description Hide Description
Narrow-gap lead telluride crystal is an important thermoelectric and mid-infrared material in which phonon functionality is a critical issue to be explored. In this Letter, efficient phonon blockage by forming a polar CdTe/PbTe heterojunction is explicitly observed by Raman scattering. The unique phonon screening effect can be interpreted by recent discovery of high-density two dimensional electrons at the polar CdTe/PbTe(111) interface which paves a way for design and fabrication of thermoelectric devices.
Band alignment and interfacial structure of ZnO/Si heterojunction with Al2O3 and HfO2 as interlayers104(2014); http://dx.doi.org/10.1063/1.4872175View Description Hide Description
Energy band alignment of ZnO/Si heterojunction with thin interlayers Al2O3 and HfO2 grown by atomic layer deposition has been studied using x-ray photoelectron spectroscopy. The valence band offsets of ZnO/Al2O3 and ZnO/HfO2 heterojunctions have been determined to be 0.43 and 0.22 eV, respectively. Accordingly, the band alignment ZnO/Si heterojunction is then modified to be 0.34 and 0.50 eV through inserting a thin Al2O3 and HfO2 layer, respectively. The feasibility to tune the band structure of ZnO/Si heterojunction by selecting a proper interlayer shows great advantage in improving the performance of the ZnO-based optoelectronic devices.
104(2014); http://dx.doi.org/10.1063/1.4872320View Description Hide Description
We demonstrate pressureless wafer bonding using silver abnormal grain growth caused by stress migration at 250 °C, which is very low for a direct solid-state bonding temperature. The bonding achieved a die-shear strength of more than 50 MPa, which exceeds the fracture toughness of Si wafer. Various deposition temperatures for the silver films, i.e., initial residual stress, reveal that the bonding process is driven by thermomechanical stress. Abnormal grain growth is induced at the contact interface instead of hillocks growing on the film surface. Pressureless wafer bonding can be applied to advanced devices such as thin-wafer multi-chip integrations.
104(2014); http://dx.doi.org/10.1063/1.4873113View Description Hide Description
The effects of thermal annealing process on the interface in p+-Si/n-SiC heterojunctions fabricated by using surface-activated bonding are investigated. It is found by measuring their current-voltage (I-V) characteristics that the reverse-bias current and the ideality factor decreased to 2.98 × 10−6 mA/cm2 and 1.03, respectively, by annealing the junctions at 1000 °C. Observation by using transmission electron microscopy indicates that an amorphous layer with a thickness of ∼6 nm is formed at the unannealed interface, which vanishes after annealing at 1000 °C. No structural defects at the interface are observed even after annealing at such a high temperature.
104(2014); http://dx.doi.org/10.1063/1.4873116View Description Hide Description
We describe a process for the growth of a single, electronically decoupled graphene layer on SiC(0001). The method involves annealing in disilane to (1) prepare flat, clean substrates, (2) grow a single graphene layer, and (3) electronically decouple the graphene from the substrate. This approach uses a single process gas, at μTorr pressures, with modest substrate temperatures, thus affecting a drastic simplification over other processes described in the literature.