Volume 87, Issue 3, 18 July 2005
- lasers, optics, and optoelectronics
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
We demonstrate single-electron operation of a 1 bit adder circuit using GaAs single-electron tunneling transistors (SETs). GaAs dot and wire coupled structures for the fabrication of SETs were grown by a selective-area metalorganic vapor-phase epitaxy technique. The logic circuit was realized based on a binary decision diagram architecture using Coulomb blockade (CB) in GaAs dots and switching operations were achieved in a single-electron mode because of the CB effects. Through this architecture, a 1 bit adder circuit was realized with three SETs, two of which were for AND logic and one with two input gates for exclusive OR (XOR). Both AND and XOR operations were demonstrated at 1.9 K, which indicated successful fabrication of the 1 bit adder.
- LASERS, OPTICS, AND OPTOELECTRONICS
87(2005); http://dx.doi.org/10.1063/1.1992668View Description Hide Description
We propose a holographic femtosecond laser processing system capable of parallel, arbitrary, and variable patterning. These features are achieved by introducing a spatial light modulator displaying a hologram into the femtosecond laser processing system. We demonstrate the variable parallel processing of a glass sample.
Double interference fluorescence enhancement from reflective slides: Application to bicolor microarrays87(2005); http://dx.doi.org/10.1063/1.1999018View Description Hide Description
A technique for enhancing fluorescence from species close to a substrate is described, based on a multilayer dielectric mirror coating. For proper design, interferences bring improvements to both fluorescence excitation and collection, each about four fold. The overall improvement reaches 10 to 15 fold as compared to a standard glass slide. We apply this to so-called deoxyribonucleic acid (DNA) chips, that is to DNA hybridization assays on microarray platforms, for which two fluorophores most commonly used are cyanine dyes (Cy3™ and Cy5™). Impact of the two-color scheme is discussed. A validation method based on spin-coated fluorescent ultrathin layers is shown to accurately determine the amplification factors of the different tagging dyes.
87(2005); http://dx.doi.org/10.1063/1.1994956View Description Hide Description
We demonstrate the mode splitting of the resonant emission from a symmetric monolithic organic semiconductor microcavity. The device, realized by low-temperature reactive electron-beam evaporation and deposition of a conjugated polymer, exhibits a 100 meV polarization-induced splitting of the transmission and emission resonances for angles larger than 45°. This opens the way for the realization of novel polarized-emitting optoelectronic devices based on plastic materials.
87(2005); http://dx.doi.org/10.1063/1.1994928View Description Hide Description
We report solid-state, optically pumped poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), distributed feedback lasers that operate in the green-yellow spectral region, previously unaddressed with conjugated polymer gain media. The lasers were fabricated by spin coating F8BT (the gain medium) onto one-dimensional gratings patterned in silica substrates. The emission wavelength could be selected to lie within the range from 558 to 591 nm by controlling the F8BT film thickness and grating periodicity. The minimum lasing threshold of 6.5 nJ per pump pulse (10 ns, 10 Hz, 450 nm) was achieved for a 350 nm spatial period grating with a 180 nm thickness F8BT film. The corresponding emission wavelength was 573 nm and the laser slope efficiency was a relatively high 3%.
87(2005); http://dx.doi.org/10.1063/1.1999843View Description Hide Description
An oxide aperture is used to confine optical modes in a micropillar structure. This method overcomes the limitations due to sidewall scattering loss typical in semiconductor etched micropillars. High cavity quality factors up to 48 000 are determined by external Fabry–Perot cavity scanning measurements, a significantly higher value than prior work in III-V etched micropillars. Measured values and estimated mode volumes correspond to a maximum Purcell factor figure of merit value of 72.
87(2005); http://dx.doi.org/10.1063/1.2000328View Description Hide Description
A continuous wave Raman laser, with only of holey fiber as the Raman gain medium, is reported in a linear cavity all-fiber configuration. Slope efficiencies of 29% with a threshold of and 77% with a threshold of were achieved with a maximum output power of at . The estimated holey fiber Raman gain coefficient of is over 40 times larger than that of a standard telecommunication fiber.
Highly efficient and stable luminescence of nanocrystalline porous silicon treated by high-pressure water vapor annealing87(2005); http://dx.doi.org/10.1063/1.2001136View Description Hide Description
The effects of a treatment based on high-pressure water vapor annealing (HWA) on nanocrystalline porous silicon have been investigated in terms of the photoluminescence(PL) efficiency and stability. For originally nonluminescent samples with a relatively low porosity, the treatment produces highly efficient and stable luminescent nanocrystalline-Si (nc-Si) layers without affecting the emission wavelength. Under appropriate conditions of pressure (2.6 MPa) and temperature (260 °C), the PL external quantum efficiency reaches 23% at room temperature. Electron-spin-resonance and infrared absorption analyses show that the HWA treatment promotes surface oxidation of nc-Si under a minimized mechanical stress and consequently generates sufficiently passivated nc- interfaces with an extremely low nonradiative defect density. This causes a drastic enhancement in the PL efficiency associated with a strong localization of excitons in nc-Si. As a practical approach, the HWA technique is very useful for fabrication of efficient and stable optoelectronic nc-Si devices.
Investigation of strontium silicate yellow phosphors for white light emitting diodes from a combinatorial chemistry87(2005); http://dx.doi.org/10.1063/1.1984103View Description Hide Description
In order to develop yellow phosphors that emit efficiently under the 450–470 nm excitation range, combinatorial chemistry was used based on silicate materials and investigated in an attempt to develop white light-emitting diodes(LEDs) by combining it with an InGaN blue LED chip (460 nm). Quaternary and ternary combinatorial libraries were developed to synthesize, process, and screen for silicate materials. Our combinatorial chemistry system consists of solution-based combinatorial synthesis and characterization, enabling the swift scanning of luminance. As a consequence of the combinatorial approach, several candidates were found to show high luminance under the 450–470 nm excitation range.
87(2005); http://dx.doi.org/10.1063/1.1997282View Description Hide Description
In this letter, we demonstrate a single-mode vertical-cavity surface-emitting laser(VCSEL) with a ring-shaped light-emitting aperture, which is realized by the Zndiffusion technique, at a wavelength of . Relative to the control VCSEL with an ordinary circular aperture and the same geometry and size, the demonstrated device can suppress the higher-order transverse mode more effectively without affecting the threshold current and output power. Compared with typical reported single-mode VCSELs, a larger light-emitting aperture and current-confined area with a smaller divergence angle of the output beam, and lower differential resistance are achieved with the present structure.
87(2005); http://dx.doi.org/10.1063/1.1999837View Description Hide Description
A high-definition vertically aligned liquid crystal (LC) microdisplay exhibits a excellent contrast ratio, but its fringing field effect splits the bright state unevenly and leads to a very slow response time. By utilizing a circularly polarized light instead of conventional linearly polarized light, we have overcome the long-standing problems of poor sharpness, low brightness, and slow response time. Confirming computer simulations agree with the experimental results well. This approach can be applied to both reflective and transmissive LC microdisplays.
87(2005); http://dx.doi.org/10.1063/1.1997270View Description Hide Description
An internal high-reflectivity omni-directional reflector (ODR) for the visible spectrum is realized by the combination of total internal reflection using a low-refractive-index (low- ) material and reflection from a one-dimensional photonic crystal (1D PC). The low- layer limits the range of angles in the 1D PC to values below the Brewster angle, thereby enabling high reflectivity and omni-directionality. This ODR is demonstrated using GaP as ambient, nanoporous with a very low refractive index , and a four-pair / multilayer stack. The results indicate a two orders of magnitude lower angle-integrated transverse-electric-transverse-magnetic polarization averaged mirror loss of the ODR compared with conventional distributed Bragg reflectors and metal reflectors. This indicates the high potential of the internal ODRs for optoelectronic semiconductor devices, e.g., light-emitting diodes.
87(2005); http://dx.doi.org/10.1063/1.1999850View Description Hide Description
Stable and unstable oscillations of broad-area semiconductor laser with optical feedback are experimentally examined. We observe stable oscillations and also coexistent states between stable oscillations and low-frequency fluctuations depending on the feedback positions of the laser beam in the active layer. The laser shows single mode operations both for the oscillation frequency and the spatial mode when it is stabilized.
- STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER
87(2005); http://dx.doi.org/10.1063/1.1990269View Description Hide Description
The specific heat of multiwalled titanium dioxide (anatase phase) nanotubes has been measured between 1.5 and . Bulk anatase and rutile were also measured. The nanotubespecific heat approaches that of bulk anatase at high temperatures. Below about the nanotubespecific heat begins to show large enhancements compared to bulk. Using an isotropic elastic continuum model, this can be understood qualitatively as a transition to low-dimensional behavior. Below about there is a second transition and the nanotubespecific heat becomes nearly constant, exceeding bulk anatase by an order of magnitude or more at .
Structural characterization of zincblende epilayers grown by molecular beam epitaxy on (001) GaAs substrates87(2005); http://dx.doi.org/10.1063/1.1995945View Description Hide Description
Zincblende -type epilayers, grown with and without buffer layers using plasma-assisted molecular beam epitaxy on (001) oriented GaAs substrates, have been investigated using a variety of complementary transmission electron microscopy techniques. The epilayers were found to contain a high anisotropic density of stacking faults and microtwins. MnAs inclusions were identified at the interface extending into the substrate. The use of buffer layers was found to inhibit the formation of these inclusions.
Atomically straight steps on vicinal Si(111) surfaces prepared by step-parallel current in the kink-up direction87(2005); http://dx.doi.org/10.1063/1.1995946View Description Hide Description
We demonstrate that annealing of a vicinal Si(111) surface at about with a direct current in the direction that ascends the kinks enhances the formation of atomically straight step edges over micrometer lengths, while annealing with a current in the opposite direction does not. Every straight step edge has the same atomic configuration U(2, 0), which is useful as a template for the formation of a variety of nanostructures. A phenomenological model based on electromigration of charged mobile atoms explains the observed current-polarity dependent behavior.
87(2005); http://dx.doi.org/10.1063/1.1995950View Description Hide Description
The quality factors of quantum bit (qubit) rotations in single self-assembled quantum dots were investigated using photoluminescence spectroscopy. Rotations with azimuthal angle along the equator of the qubit sphere were implemented by varying the time delay between two pulses, resulting in a “free rotation” quality factor . Qubit rotations with polar angle as large as along the meridian of the qubit sphere (Rabi oscillations with zero detuning) were realized with a quality factor , in experiments probing an excited excitonic state.
87(2005); http://dx.doi.org/10.1063/1.1996849View Description Hide Description
Annealing of aluminum/amorphous silicon bilayers below the eutectic temperature of the aluminum/silicon system leads to an exchange of the layer positions and a concurrent crystallization of silicon (aluminum-induced layer exchange). This letter discusses a model for the self-limited suppression of nucleation during the process. This characteristic feature is the reason why large grain sizes can be obtained. In our experiments, we combine nucleation caused by supersaturation and undercooling. Si depletion regions around existing grains are made visible. These experiments give direct proof of the idea that the suppression of nucleation occurs by Si depletion in the aluminum-induced layer exchange process.
87(2005); http://dx.doi.org/10.1063/1.1999011View Description Hide Description
GaN epilayers grown on sapphire substrate were irradiated with various dosages of neutrons and were characterized using Micro-Raman and photoluminescence. It was found that the peak in the Raman spectra clearly shifted with neutron irradiation dosage. Careful curve fitting of the Raman data was carried out to obtain the carrier concentration which was found to vary with the neutron irradiation dosage. The variation of the full width at half maximum height of the photoluminescence was consistent with the Raman results. The neutronirradiation-inducedstructural defects (likely to be ) give rise to carrier trap centers which are responsible for the observed reduction in carrier concentration of the irradiatedGaN.
87(2005); http://dx.doi.org/10.1063/1.1957121View Description Hide Description
The formation of biaxial texture in uniaxially texturedNithin films via Ar-ion irradiation is reported. The ion-beamirradiation was not simultaneous with deposition. Instead, the ion beamirradiates the uniaxially texturedfilm surface with no impinging deposition flux, which differs from conventional ion-beam-assisted deposition. The uniaxial texture is established via a nonion beam process, with the in-plane texture imposed on the uniaxial film via ion beam bombardment. Within this sequential ion beamtexturing method, grain alignment is driven by selective etching and grain overgrowth.
Aligned silver nanorod arrays produce high sensitivity surface-enhanced Raman spectroscopy substrates87(2005); http://dx.doi.org/10.1063/1.1988980View Description Hide Description
Substrates consisting of silvernanorod arrays with an irregular surface lattice (i.e., random nucleation sites) and with varying rod lengths were fabricated by an oblique angle vapor deposition method. These arrays were evaluated as potential surface-enhanced Raman spectroscopy (SERS) substrates using trans-1,2-bis(4-pyridyl)ethene as a reported molecule. SERS activity was shown to depend upon the length of the nanorods. The Agnanorods with average lengths of , and having aspect ratios of exhibited the maximum SERS enhancement factors of greater than . Theoretical calculations indicate that this large SERS enhancement may be partially explained by the shape, density, and lateral arrangement of the Agnanorod arrays.