Volume 102, Issue 2, 14 January 2013
- 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
- biophysics and bio-inspired systems
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
Highly mismatched alloys are promising for applications to intermediate-band (IB) solar cells. Here, we report first-principles prediction of intermediate bands in GaP on the basis of hybrid-density-functional theory, which enables to handle large supercells including defects with much better accuracy than semilocal functionals. Calculated optical conductivity reveals that the intermediate states due to co-doped Mg and O have sufficiently high optical transition probability. The multiple gaps are robust against thermalization. Intermediate-band states become more delocalized by hybridization with phosphorus-vacancy states, increasing the optical transition probability.
- PHOTONICS AND OPTOELECTRONICS
102(2013); http://dx.doi.org/10.1063/1.4775578View Description Hide Description
Single-nanowire solar cells (SNSCs) in lying configuration can have external quantum efficiency (EQE) over 100% but always in narrowbands with EQE peaks slightly exceeding unit. We presented a rectangular gallium arsenide (GaAs) SNSC, which provides light absorption efficiency (Qabs) and EQE far beyond 100% for both transverse electric and magnetic illuminations, by optimally engineering the nanowires and introducing an advanced nanoshell design. Electromagnetic and carrier transport calculations show that Qabs and EQE peaks of the designed SNSCs can both be over 200% with averaged EQE ∼ 150% in most of the active spectral band of GaAs.
102(2013); http://dx.doi.org/10.1063/1.4775664View Description Hide Description
Second harmonic (SH) generation from AlGaAs near-wavelength high-contrast gratings was studied in polarization-dependent reflection measurements. The grating design is critical in determining the strength and polarization of the second harmonic signal. The second harmonic response was enhanced by more than 3 orders of magnitude compared to flat AlGaAs surfaces without the gratings. These enhancements are due to the diffractive nature of the gratings, specifically their ability to change the direction of light waves.
102(2013); http://dx.doi.org/10.1063/1.4775666View Description Hide Description
We report the demonstration of a composite right/left-handed (CRLH) metamaterial waveguide for terahertz quantum-cascade (QC) lasers. By incorporating gap capacitors ( ) in the top metallization of a metal-metal waveguide operating in a higher order lateral mode, we have realized a CRLH transmission line that supports traveling modes with negative effective phase indices (i.e., left-handed or backward-wave propagation). The CRLH metamaterial waveguide is employed as an active leaky-wave antenna for a terahertz QC-laser. Directional single-lobed beams launched in the backwards direction at angles of and were experimentally observed at excitation frequencies 2.59 and 2.48 THz, respectively.
102(2013); http://dx.doi.org/10.1063/1.4775668View Description Hide Description
We report an on-chip solid-state Mach-Zehnder interferometer operating on two-dimensional (2D) plasmonic waves at microwave frequencies. Two plasmonic paths are defined with GaAs/AlGaAs 2D electron gas 80 nm below a metallic gate. The gated 2D plasmonic waves achieve a velocity of ∼c/300 (c: free-space light speed). Due to this ultra-subwavelength confinement, the resolution of the 2D plasmonic interferometer is two orders of magnitude higher than that of its electromagnetic counterpart at a given frequency. This gigahertz proof-of-concept at cryogenic temperatures can be scaled to the terahertz–infrared range for room temperature operation, while maintaining the benefits of ultra-subwavelength confinement.
Photoresponse mechanisms of ultraviolet photodetectors based on colloidal ZnO quantum dot-graphene nanocomposites102(2013); http://dx.doi.org/10.1063/1.4776651View Description Hide Description
Ultraviolet (UV) photodetectors were fabricated using the wet spin-coating for ZnO quantum dots (QDs) and the transfer method for the graphene sheet. High-resolution transmission electron microscopy images showed that the ZnO QDs were uniformly distributed between the voids of the surface circumferences on the graphene layers. Current-voltage measurements on the UV photodetector at 300 K showed that the ratio of the photocurrent to the dark current was about 1.1 × 104. The rise and the decay times of the UV photodetector were approximately 2 and 1 s, respectively. The photoresponse mechanisms are described on the basis of the experimental results.
102(2013); http://dx.doi.org/10.1063/1.4776685View Description Hide Description
A flexible silicon barrier diode was fabricated by the transfer printing method. Micro-line patterned p-type single crystalline silicon membranes were created from a silicon on insulator wafer. The dark current of our device was very low, about 1 pA for reverse bias voltages up to 5 V, and showed rectifying behavior with an ideality factor of 1.05. The photo-response and the responsivity was 32 and 0.3 A/W, respectively, for light intensity of 1.2 mW/cm2. Also, the current of the photodetector changed under compressive stress or tensile stress. Our device is functional as the piezotronic sensor as well as the photodetector.
Heterojunction photodiode fabricated from multiwalled carbon nanotube/ZnO nanowire/p-silicon composite structure102(2013); http://dx.doi.org/10.1063/1.4776691View Description Hide Description
A heterojunction photodiode was fabricated from multiwalled carbon nanotubes (MWCNTs)/ZnO nanowires/p-Si (100) substrate composite structure. The heterojunction photodiode demonstrated a faster transient response and higher responsivity than the reference sample without deposition of MWCNTs, which is attributed to improved carrier collection and transport efficiency through the MWCNTs network. The high photoresponsivities of the devices are explained in terms of operation as a hybrid of photodiode and photoconductor modes. The spectral response of the devices showed dependence on voltage polarity and is attributed to the high valance band offset in the interfacial region of ZnO and p-Si substrate.
Light-emitting devices based on erbium-doped TiO2/p +-Si heterostructures: Engineering of electroluminescence via aluminum co-doping102(2013); http://dx.doi.org/10.1063/1.4788679View Description Hide Description
We have recently reported erbium (Er)-related visible and infrared (∼1540 nm) electroluminescence (EL) from the light-emitting device (LED) based on Er-doped TiO2 (TiO2:Er)/p +-Si heterostructure, triggered by the energy transferred from oxygen-vacancy-related self-trapped excitons (STEs) to Er3+ ions in anatase TiO2. Herein, we further co-dope aluminum (Al) into the TiO2:Er film, which is also used to form heterostructure with p +-Si. The LED based on such heterostructure features the Er-related EL with the substantially suppressed visible emissions and the remarkably enhanced ∼1540 nm emission. The Al co-doping is proved not to substantially affect the amounts of oxygen-vacancy-related STEs and Er3+ ions in anatase TiO2. In this context, the above-mentioned engineering of Er-related EL is tentatively ascribed to the modification of crystal field around the Er3+ ions in anatase TiO2 by the Al co-doping.
102(2013); http://dx.doi.org/10.1063/1.4788682View Description Hide Description
Random lasing action and weak localization of light were observed in multiple visible wavelengths with highly transparent, Nd+3 doped phosphate glass gain media, with strong energy redistribution between different frequencies under different pumping powers. A tentative model based on photoinduced scattering was proposed and verified with a He-Ne laser beam in probing the lasing zone. The remarkable coherency and intensity changes of the probing laser beam confirmed the local, random scattering behind the photoinduced random lasing and the localization of light.
Cavity piezooptomechanics: Piezoelectrically excited, optically transduced optomechanical resonators102(2013); http://dx.doi.org/10.1063/1.4788724View Description Hide Description
We present a monolithic integrated aluminum nitride (AlN) optomechanical resonator in which the mechanical motion is actuated by piezoelectric force and the displacement is transduced by a high-Q optical cavity. The AlN optomechanical resonator is excited from a radio-frequency electrode via a small air gap to eliminate resonator-to-electrode loss. We observe the electrically excited mechanical motion at 47.3 MHz, 1.04 GHz, and 3.12 GHz, corresponding to the 1st, 2nd, and 4th radial-contour mode of the wheel resonator, respectively. An equivalent circuit model is developed to describe the observed Fano-like resonance spectrum.
102(2013); http://dx.doi.org/10.1063/1.4788727View Description Hide Description
For conventional imaging, shaking of the imaging system or the target leads to the degradation of imaging resolution. The influence of the target's shaking to Fourier-transform ghost diffraction (FGD) is investigated and phase-retrieval method is used to recover the target's imaging. The analytical results, which are backed up by numerical simulation and experiments, demonstrate that the quiver of target has no effect on the resolution of FGD and high-resolution imaging can be always achieved by phase-retrieval method from FGD patterns. This approach can be applied in high-precision imaging systems, to overcome the influence of the system's shaking to imaging resolution.
- SURFACES AND INTERFACES
102(2013); http://dx.doi.org/10.1063/1.4775667View Description Hide Description
We found inverse-hexagonal packing pattern from self-assembled anodic aluminum oxide and exploited the pattern to obtain triangular pore array. By replicating the curved interface between aluminum and porous alumina, we fabricated a pattern with the opposite packing structure as well as the inversed pattern curvature. Anodization from the replicated structure formed triangular pores in inverse-hexagonal packing, whereas that from the original pattern produces circular pores in hexagonal packing. Our finding highlights the importance of the curvature as well as packing structure of pre-patterns in pore formation and achievement in the control via a simple replication process.
Controlling interfacial states in amorphous/crystalline LaAlO3/SrTiO3 heterostructures by electric fields102(2013); http://dx.doi.org/10.1063/1.4775669View Description Hide Description
The tunable metal-insulator transition in crystalline LaAlO3/SrTiO3 heterostructures constitutes a central element in the range of remarkable interface properties that has made this oxide system subject to extensive research. Recently, metallic interfaces have also been realized when depositing amorphous LaAlO3 films on SrTiO3. Here, we present a non-volatile and reversible tuning of the interface conductivity by more than 3 orders of magnitude at room temperature by applying an electric field to such amorphous/crystalline heterostructures with amorphous LaAlO3 film thicknesses of ∼2 nm. We show that the tunability is strongly temperature dependent, and demonstrate a simple protocol for enhancing the tunability.
102(2013); http://dx.doi.org/10.1063/1.4775671View Description Hide Description
The formation and development processes of dislocation in graphene are investigated by performing tight-binding molecular dynamics (TBMD) simulation and ab initio total energy calculation. It is found that the coalescence of pentagon-heptagon (5-7) pairs with vacancy defects induces the formation of dislocation due to the separation of two 5-7 pairs. In TBMD simulations, adatoms are ejected and evaporated from graphene surface so that the dislocation is developed. It is observed that diffusing carbon atoms nearby dangling bonds help non-hexagonal rings change into stable hexagonal rings. These results might give some ideas for the control of structural properties by inducing defect structures.
Silicon surface texturing with a combination of potassium hydroxide and tetra-methyl ammonium hydroxide etching102(2013); http://dx.doi.org/10.1063/1.4776733View Description Hide Description
A two step silicon surface texturing, consisting of potassium hydroxide (KOH) etching followed by tetra-methyl ammonium hydroxide etching is presented. This combined texturing results in 13.8% reflectivity at 600 nm compared to 16.1% reflectivity for KOH etching due to the modification of microstructure of etched pyramids. This combined etching also results in significantly lower flat-band voltage (VFB ) (−0.19 V compared to −1.3 V) and interface trap density (Dit ) (2.13 × 1012 cm−2 eV−1 compared to 3.2 × 1012 cm−2 eV−1).
102(2013); http://dx.doi.org/10.1063/1.4775762View Description Hide Description
Post-deposition molecular rearrangement in thin organic films is revealed by in situ real-time photoelectron spectroscopy during organic molecular beam deposition. Agreement between real time spectroscopy and Monte Carlo modeling confirms the role of nearest-neighbor molecular attraction in driving a time-dependent morphology for oriented films of tin phthalocyanine (SnPc) on a range of substrates. The time-dependent molecular self-organization occurs over timescales comparable to the growth rates and is therefore an important factor in the degradation of thin films of organic semiconductors typically considered for the fabrication of multilayer semiconductor devices.
An in situ x-ray photoelectron spectroscopy study of the initial stages of rf magnetron sputter deposition of indium tin oxide on p-type Si substrate102(2013); http://dx.doi.org/10.1063/1.4774404View Description Hide Description
The interface between indium tin oxide and p-type silicon is studied by in situ X-ray photoelectron spectroscopy (XPS). This is done by performing XPS without breaking vacuum after deposition of ultrathin layers in sequences. Elemental tin and indium are shown to be present at the interface, both after 2 and 10 s of deposition. In addition, the silicon oxide layer at the interface is shown to be composed of mainly silicon suboxides rather than silicon dioxide.
102(2013); http://dx.doi.org/10.1063/1.4781768View Description Hide Description
Ni films on (0001) and ( ) InN exhibited different reaction kinetics upon annealing at 673K. Structural and chemical analysis using grazing incidence X-ray diffraction, transmission electron microscopy, and X-ray energy dispersive spectrometry indicated that an interfacial reaction did not occur between the Ni film and the In-polar (0001) InN layer. However, the N-polar face reacted with Ni to form the Ni 3InNx ternary phase with an anti-perovskite structure. The difference in reactivity for Ni on In-face and N-face InN indicates that polarity alters the reaction and may also affect interactions between other metals and group III-nitride semiconductors.
102(2013); http://dx.doi.org/10.1063/1.4776674View Description Hide Description
The existence of intrinsic surface states, the position of the Fermi level, and the size of the surface bandgap of the non-polar ZnO cleavage surfaces were investigated by scanning tunneling microscopy and spectroscopy. The comparison of spectroscopic measurements performed on atomically flat and stepped surfaces reveals the absence of intrinsic surface states within the fundamental bulk bandgap, but shows the occurrence of step-induced gap states. These states lead to a pinning of the Fermi level at the surface within the bandgap and generate a significant defect-related tunnel current, narrowing the measured apparent bandgap.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
102(2013); http://dx.doi.org/10.1063/1.4775596View Description Hide Description
Rare earth element doping of chromium is much desired for various applications, but is technically difficult because of dopant segregation. Using a room temperature mechanical alloying method, dilute yttrium doping into nanosized chromium was achieved. Synchrotron-based high-pressure X-ray diffraction indicated that the Cr-Y alloy (Cr0.97Y0.03) was stable at up to 39 GPa, and the bulk modulus was 203 ± 2.6 GPa. The experimental results were consistent with first-principles density functional theory simulation. The diffraction line broadening profiles indicated the deformation anisotropy of the nanoalloy. This study suggests that Cr0.97Y0.03 alloy is promising for ultrahigh stress applications such as airplane engines and land-based turbines.