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20 Sep 2004

Volume 85, Issue 12, pp. 2157-2437

Issue Cover Spotlight Figure

Appl. Phys. Lett. 85, 2390 (2004); http://dx.doi.org/10.1063/1.1796520 (3 pages)

Stas Polonsky and Alan Weger
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Selective area growth of InAs quantum dots formed on a patterned GaAs substrate

S. Birudavolu, N. Nuntawong, G. Balakrishnan, Y. C. Xin, S. Huang, S. C. Lee, S. R. J. Brueck, C. P. Hains, and D. L. Huffaker

Appl. Phys. Lett. 85, 2337 (2004); http://dx.doi.org/10.1063/1.1792792 (3 pages) | Cited 18 times

Online Publication Date: 24 September 2004

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We describe the growth and characterization of InAs quantum dots (QDs) on a patterned GaAs substrate using metalorganic chemical vapor deposition. The QDs nucleate on the (001) plane atop GaAs truncated pyramids formed by a thin patterned SiO2 mask. The base diameter of the resulting QDs varies from 30 to 40 nm depending on the size of the mask. With specific growth conditions, we are able to form highly crystalline surface QDs that emit at 1.6 μm under room-temperature photopumped conditions. The crystalline uniformity and residual strain is quantified in high-resolution transmission electron microscopy images and high-resolution x-ray reciprocal space mapping. These strained QDs may serve as a template for selective nucleation of a stacked QD active region.
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81.05.Ea III-V semiconductors
81.07.Ta Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
78.67.Hc Quantum dots
68.47.Fg Semiconductor surfaces
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.55.Cr III-V semiconductors
68.37.Lp Transmission electron microscopy (TEM)
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)

Strong luminescence from dislocation-free GaN nanopillars

Y. Inoue, T. Hoshino, S. Takeda, K. Ishino, A. Ishida, H. Fujiyasu, H. Kominami, H. Mimura, Y. Nakanishi, and S. Sakakibara

Appl. Phys. Lett. 85, 2340 (2004); http://dx.doi.org/10.1063/1.1792793 (3 pages) | Cited 15 times

Online Publication Date: 24 September 2004

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GaN nanostructures were prepared on Si(111) by a hot-wall epitaxy technique employing the modified two-step growth method. Isolated hexagonal pillar-like GaN nanostructures (GaN nanopillars) with the typical diameter, height, and density of 200–300 nm, 0.5–1 μm, and 3–4×108 cm−2, respectively, are self-organized without intentional pre-processing to the Si substrate. The photoluminescence and cathodoluminescence (CL) measurements show the strong near-band-edge emissions without the yellow band at room temperature. Stronger CL is obtained from the GaN nanopillars in comparison to single-crystalline GaN. The obtained strong CL is related to high crystal quality of the dislocation-free GaN nanopillars.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Cr III-V semiconductors
78.60.Hk Cathodoluminescence, ionoluminescence
81.07.Bc Nanocrystalline materials
81.05.Ea III-V semiconductors
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.15.Kk Vapor phase epitaxy; growth from vapor phase

Nanocrystal-size selective spectroscopy in SnO2:Eu3+ semiconductor quantum dots

A. C. Yanes, J. Del Castillo, M. Torres, J. Peraza, V. D. Rodríguez, and J. Méndez-Ramos

Appl. Phys. Lett. 85, 2343 (2004); http://dx.doi.org/10.1063/1.1790039 (3 pages) | Cited 50 times

Online Publication Date: 24 September 2004

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Glass ceramics of composition 95SiO2-5SnO2 doped with 0.4 mol% Eu3+ have been prepared by thermal treatment of sol-gel glasses. The segregated SnO2 nanocrystals present a mean size comparable to the bulk exciton Bohr radius (about 2.4 nm), corresponding to a wide band-gap quantum-dot system in an insulator SiO2 glass. A fraction of the Eu3+ ions is incorporated to the SnO2 nanocrystals in the process. In these strong confinement conditions, the energy gap presents a high dependence on the nanocrystal size. Taking advantage of this effect, it has been possible to excite selectively the Eu3+ ions located in the SnO2 nanocrystals, by energy transfer from the host, obtaining emission spectra that depend on the nanocrystal size. The Eu3+ ions environment in small nanocrystals (radius under 2 nm) are very distorted, meanwhile they are like crystalline for nanocrystals with a radius of some nanometers.
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81.05.Pj Glass-based composites, vitroceramics
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.46.-w Structure of nanoscale materials
78.67.Hc Quantum dots
78.55.Qr Amorphous materials; glasses and other disordered solids
61.72.up Other materials
81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
64.75.-g Phase equilibria
81.40.Gh Other heat and thermomechanical treatments

Droplet heteroepitaxy of GaN quantum dots by metal-organic chemical vapor deposition

M. Gherasimova, G. Cui, S.-R. Jeon, Z. Ren, D. Martos, J. Han, Y. He, and A. V. Nurmikko

Appl. Phys. Lett. 85, 2346 (2004); http://dx.doi.org/10.1063/1.1793343 (3 pages) | Cited 3 times

Online Publication Date: 24 September 2004

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Optically active GaN quantum dots on conductive AlGaN templates are synthesized by droplet heteroepitaxy, whereby the Ga droplets are converted to GaN islands in the presence of ammonia at 600 °C. We have investigated the evolution of metallic Ga layers on AlGaN, obtaining the optimal surface densities and size distribution of the Ga droplets. The stability of GaN islands is influenced by the surface kinetics and the initial droplet size; the condition of Ga deposition and subsequent nitrogen exposure is identified, which preserves the initial density of the Ga droplets. A nitrogen-rich environment is identified as a necessary condition for maintaining the optimal GaN morphology by suppressing the Ga surface diffusion and preventing two-dimensional layer growth.
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81.07.Ta Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
68.35.Fx Diffusion; interface formation
78.67.Hc Quantum dots
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.55.Cr III-V semiconductors

Clarifying the origin of near-infrared electroluminescence peaks for nanocrystalline germanium in metal-insulator-silicon structures

E. W. H. Kan, W. K. Chim, C. H. Lee, W. K. Choi, and T. H. Ng

Appl. Phys. Lett. 85, 2349 (2004); http://dx.doi.org/10.1063/1.1793348 (3 pages) | Cited 12 times

Online Publication Date: 24 September 2004

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The origin of room-temperature near-infrared electroluminescence (EL) of nanocrystalline germanium (Ge) embedded in oxide was investigated. The nanocrystals were synthesized by partial oxidation of silicon-germanium, Si0.54Ge0.46, films. Under constant current density bias in accumulation and inversion, Ge nanocrystals with diameters of 5 and 10 nm exhibit strong luminescence at 1350 nm. The 1350 nm EL peak is only observed in the presence of elemental Ge and is enhanced with the formation of Ge nanocrystals. The introduction of hydrogen during annealing passivates the dangling bonds at the interfaces of nanocrystals, thus minimizing the energy spread in the 1350 nm peak. The 1350 nm peak intensity is a function of the injected carrier density, while the peak location remains constant in energy and independent of the applied bias. The results are a clear indication that the luminescence peak originates from radiative recombination of excitons confined in the nanocrystals.
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81.05.Cy Elemental semiconductors
81.07.Bc Nanocrystalline materials
61.46.-w Structure of nanoscale materials
78.66.Db Elemental semiconductors and insulators
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
71.35.-y Excitons and related phenomena
71.55.Cn Elemental semiconductors
81.65.Rv Passivation
81.65.Mq Oxidation
78.60.Fi Electroluminescence
81.40.Gh Other heat and thermomechanical treatments

Formation and thermal stability of sub-10-nm carbon templates on Si(100)

Olivier Guise, Joachim Ahner, John Yates, and Jeremy Levy

Appl. Phys. Lett. 85, 2352 (2004); http://dx.doi.org/10.1063/1.1794369 (3 pages) | Cited 18 times

Online Publication Date: 24 September 2004

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We report a lithographic process for creating high-resolution (<10 nm) carbon templates on Si(100). A scanning electron microscope, operating under low vacuum (10−6 mbar), produces a carbon-containing deposit (“contamination resist”) on the silicon surface via electron-stimulated dissociation of ambient hydrocarbons, water, and other adsorbed molecules. Subsequent annealing at temperatures up to 1320 K in ultrahigh vacuum removes SiO2 and other contaminants, with no observable change in dot shape. The annealed structures are compatible with subsequent growth of semiconductors and complex oxides. Carbon dots with diameter as low as 3.5 nm are obtained with a 200 μs electron-beam exposure time.
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68.47.Fg Semiconductor surfaces
85.40.Hp Lithography, masks and pattern transfer
61.72.Cc Kinetics of defect formation and annealing
81.40.Gh Other heat and thermomechanical treatments
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Ps Atomic force microscopy (AFM)

Scanning tunneling microscope study of capped quantum dots

H. Z. Song, M. Kawabe, Y. Okada, R. Yoshizaki, T. Usuki, Y. Nakata, T. Ohshima, and N. Yokoyama

Appl. Phys. Lett. 85, 2355 (2004); http://dx.doi.org/10.1063/1.1791340 (3 pages) | Cited 1 time

Online Publication Date: 24 September 2004

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On thinly capped InGaAs∕GaAs quantum dots (QDs), a simultaneous study of both the microscopic and electronic structures was carried out using scanning tunneling microscopy (STM). Although the surface is morphologically flat, the STM image of the embedded QDs can be clearly observed at cryogenic temperatures and is distinguishable up to room temperature. Such images are available in a particular bias range, which corresponds to the occurrence of QD-associated current, as demonstrated in scanning tunneling spectroscopy.
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81.05.Ea III-V semiconductors
81.07.Ta Quantum dots
71.20.Nr Semiconductor compounds
68.47.Fg Semiconductor surfaces
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)

Growth and characterization of tungsten carbide nanowires by thermal annealing of sputter-deposited WCx films

Shui-Jinn Wang, Chao-Hsuing Chen, Shu-Cheng Chang, Kai-Ming Uang, Chuan-Ping Juan, and Huang-Chung Cheng

Appl. Phys. Lett. 85, 2358 (2004); http://dx.doi.org/10.1063/1.1791322 (3 pages) | Cited 7 times

Online Publication Date: 24 September 2004

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In this letter, the growth of dense W2C nanowires by a simple thermal annealing of sputter-deposited WCx films in nitrogen ambient is reported. Straight nanowires with a density of 250–260 μm−2 and length∕diameter in the range of 0.2–0.3 μm∕13–15 nm were obtained from the 700°C-annealed samples, which exhibit good electron field emission characteristics with a typical turn-on field of about 1.7 V∕μm. The self-catalytic growth of W2C nanowires is attributed to the formation of α-W2C phase caused by carbon depletion in the WCx films during thermal annealing.
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68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
79.70.+q Field emission, ionization, evaporation, and desorption
81.16.Hc Catalytic methods
81.40.Gh Other heat and thermomechanical treatments
81.15.Cd Deposition by sputtering

Lasing in ZnS nanowires grown on anodic aluminum oxide templates

J. X. Ding, J. A. Zapien, W. W. Chen, Y. Lifshitz, S. T. Lee, and X. M. Meng

Appl. Phys. Lett. 85, 2361 (2004); http://dx.doi.org/10.1063/1.1791326 (3 pages) | Cited 52 times

Online Publication Date: 24 September 2004

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High-density and uniform-sized gold particle arrays have been prepared electrochemically on anodic aluminum oxide (AAO) templates. The gold particles were used as catalysts to synthesize ZnS nanowires. The as-grown nanowires had a wurtzite single-crystal structure and were aligned perpendicularly to the AAO template. Under high-power density optical excitation (266 nm), the nanowire array showed an intense, narrow [full width at half maximum (FWHM) of 2.2 nm] photoluminescent peak at 338 nm composed of a superposition of optical resonant modes (FWHM ∼0.3 nm) resulting from the collective emission of a large number of nanowires. These results indicate that the ZnS nanowires act as optical waveguide resonators.
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81.05.Dz II-VI semiconductors
61.46.-w Structure of nanoscale materials
42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
68.35.B- Structure of clean surfaces (and surface reconstruction)
78.55.Et II-VI semiconductors
81.15.Pq Electrodeposition, electroplating
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures

Vertically aligned carbon nanotube heterojunctions

Alan M. Cassell, Jun Li, Ramsey M. D. Stevens, Jessica E. Koehne, Lance Delzeit, Hou Tee Ng, Qi Ye, Jie Han, and M. Meyyappan

Appl. Phys. Lett. 85, 2364 (2004); http://dx.doi.org/10.1063/1.1794356 (3 pages) | Cited 22 times

Online Publication Date: 24 September 2004

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The bottom-up fabrication and electrical properties of end-to-end contacted multiwalled carbon nanotube (MWCNT) heterojunctions are reported. The vertically aligned MWCNT heterojunction arrays are formed via successive plasma-enhanced chemical vapor deposition processing to achieve the layered junction architecture. Electron microscopy and current-sensing atomic force microscopy are used to reveal the physical nature of the junctions. Symmetric, nonlinear I–V curves of the as-fabricated junctions indicate that a tunnel barrier is formed between the end-to-end contacted MWCNTs. Repeated high bias I–V scans of many devices connected in parallel fuses the heterojunctions, as manifested by a shift to linear I–V characteristics.
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81.07.De Nanotubes
61.48.-c Structure of fullerenes and related hollow and planar molecular structures
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
73.63.Fg Nanotubes
81.05.ub Fullerenes and related materials
68.37.Ps Atomic force microscopy (AFM)
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)

Magnetic soliton-based logic with fan-out and crossover functions

Pooja Wadhwa and M. B. A. Jalil

Appl. Phys. Lett. 85, 2367 (2004); http://dx.doi.org/10.1063/1.1794850 (3 pages)

Online Publication Date: 24 September 2004

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A micromagnetic simulation is performed of a device consisting of arrays of square magnetic nanostructures, which is capable not only of basic logic operations but also controlled information transfer in linear, fan-out, and crossover manner by means of magnetic “solitons.” The square elements exhibit shape anisotropy, with easy axes along the diagonals. This enables the solitons to propagate around a bend in an array, or to fan-out into two distinct directions in Y-shaped arrays, in the presence of particular external field sequence and direction. Finally, independent soliton crossover at the intersection of two arrays is also demonstrated, which completes the device’s logical functionality.
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85.70.Ay Magnetic device characterization, design, and modeling
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
75.30.Gw Magnetic anisotropy
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.30.Ds Spin waves
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
75.40.Mg Numerical simulation studies

Bonding silicon-on-insulator to glass wafers for integrated bio-electronic circuits

Hyun S. Kim, Robert H. Blick, D. M. Kim, and C. B. Eom

Appl. Phys. Lett. 85, 2370 (2004); http://dx.doi.org/10.1063/1.1794855 (3 pages) | Cited 4 times

Online Publication Date: 24 September 2004

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We report a method for bonding silicon-on-insulator wafers onto glass wafers. After pre-cleaning the wafers by an ozone and ultraviolet exposure, followed by mega-sonic water rinse, the SOI wafers are bonded to glass wafers in a vacuum chamber. This is performed at a temperature of 400 °C under an applied voltage of 700 V. The interface between the glass and SOI wafer is tested mechanically and inspected by electron beam microscopy. Furthermore, we demonstrate removal of the silicon bulk layer after wafer bonding. The quality of the single crystalline Si thin film on the glass wafers has been verified by four-circle x-ray diffraction and scanning electron microscopy. This process will allow us the integration of thin-film electronics in biological sensor applications.
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81.65.Cf Surface cleaning, etching, patterning
85.40.Hp Lithography, masks and pattern transfer
85.65.+h Molecular electronic devices
87.80.-y Biophysical techniques (research methods)

Multi-dot floating-gates for nonvolatile semiconductor memories: Their ion beam synthesis and morphology

T. Müller, K.-H. Heinig, W. Möller, C. Bonafos, H. Coffin, N. Cherkashin, G. Ben Assayag, S. Schamm, G. Zanchi, A. Claverie, M. Tencé, and C. Colliex

Appl. Phys. Lett. 85, 2373 (2004); http://dx.doi.org/10.1063/1.1794856 (3 pages) | Cited 18 times

Online Publication Date: 24 September 2004

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Scalability and performance of current flash memories can be improved substantially by replacing the floating polycrystalline-silicon gate by a layer of Si dots. Here, we present both experimental and theoretical studies on ion beam synthesis of multi-dot layers consisting of Si nanocrystals (NCs) embedded in the gate oxide. Former studies have suffered from the weak Z contrast between Si and SiO2 in transmission electron microscopy (TEM). This letter maps Si plasmon losses with a scanning TEM equipped with a parallel electron energy loss spectroscopy system. Kinetic Monte Carlo simulations of Si phase separation have been performed and compared with Si plasmon maps. Predicted and measured Si morphologies agree remarkably well, both change with increasing ion fluence from isolated NCs to spinodal pattern. However, the predicted fluences are lower than the experimental ones. We identify as the main reason of this discrepancy the partial oxidation of implanted Si by atmospheric humidity, which penetrates into the as-implanted SiO2.
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84.30.Sk Pulse and digital circuits
85.30.-z Semiconductor devices
61.46.-w Structure of nanoscale materials
61.72.uf Ge and Si
68.35.B- Structure of clean surfaces (and surface reconstruction)
61.72.Cc Kinetics of defect formation and annealing
61.80.Jh Ion radiation effects
81.65.Mq Oxidation
81.40.Gh Other heat and thermomechanical treatments
79.20.Uv Electron energy loss spectroscopy
68.37.Lp Transmission electron microscopy (TEM)

Growth of epitaxial nanowires by controlled coarsening of strained islands

V. B. Shenoy

Appl. Phys. Lett. 85, 2376 (2004); http://dx.doi.org/10.1063/1.1792374 (3 pages) | Cited 5 times

Online Publication Date: 24 September 2004

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We show that elongated nanowires can be grown on crystal surfaces by allowing large strained two-dimensional islands to desorb by varying the adatom supersaturation or chemical potential. The width of the wires formed in this process is determined by a competition between the repulsive elastic interactions of the long edges of the wires and the thermodynamic driving force which tends to decrease the distance between these edges. The proposed mechanism allows for control of the wire sizes by changing the growth conditions, in particular, the vapor pressure of the material that is being deposited.
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81.07.Bc Nanocrystalline materials
61.46.-w Structure of nanoscale materials
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
68.35.Gy Mechanical properties; surface strains
81.40.Lm Deformation, plasticity, and creep
81.40.Jj Elasticity and anelasticity, stress-strain relations
65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
62.20.F- Deformation and plasticity
62.20.D- Elasticity
68.43.Mn Adsorption kinetics

Alcohol detection using carbon nanotubes acoustic and optical sensors

M. Penza, G. Cassano, P. Aversa, F. Antolini, A. Cusano, A. Cutolo, M. Giordano, and L. Nicolais

Appl. Phys. Lett. 85, 2379 (2004); http://dx.doi.org/10.1063/1.1784872 (3 pages) | Cited 44 times

Online Publication Date: 24 September 2004

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We demonstrate the integration of single-walled carbon nanotubes (SWCNTs) onto quartz crystal microbalance (QCM) and standard silica optical fiber (SOF) sensor for alcohol detection at room temperature. Different transducing mechanisms have been used in order to outline the sensing properties of this class of nanomaterials, in particular the attention has been focused on two key parameters in sensing applications: mass and refractive index changes due to gas absorption. Here, Langmuir–Blodgett (LB) films consisting of tangled bundles of SWCNTs without surfactant molecules have been successfully transferred onto QCM and SOF. Mass-sensitive 10 MHz QCM SWCNTs sensor exhibited a resonant frequency decreasing upon tested alcohols exposure; also the normalized optoelectronic signal (λ=1310 nm) of the refractive index-sensitive SOF SWCNTs sensor was found to decrease upon alcohols ambient. Highly sensitive, repeatable and reversible responses of the QCM and SOF SWCNTs sensors indicate that the detection, at room temperature, in a wide mmHg vapor pressures range of alcohols and potentially other volatile organic compounds is feasible.
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85.35.Kt Nanotube devices
42.81.Pa Sensors, gyros
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
43.60.Vx Acoustic sensing and acquisition
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