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4 Apr 2005

Volume 86, Issue 14, Articles (14xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 86, 142101 (2005); http://dx.doi.org/10.1063/1.1895476 (3 pages)

M. Hanke, T. Boeck, A.-K. Gerlitzke, F. Syrowatka, F. Heyroth, and R. Köhler
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Efficient charge collection in hybrid polymer/TiO2 solar cells using poly(ethylenedioxythiophene)/polystyrene sulphonate as hole collector

P. Ravirajan, D. D. C. Bradley, J. Nelson, S. A. Haque, J. R. Durrant, H. J. P. Smit, and J. M. Kroon

Appl. Phys. Lett. 86, 143101 (2005); http://dx.doi.org/10.1063/1.1890468 (3 pages) | Cited 41 times

Online Publication Date: 28 March 2005

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We report a study of the optimization of power conversion efficiency in hybrid solar cells based on nanostructured titanium dioxide and a poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) based conjugated polymer. Charge collection efficiency is enhanced by introducing a poly(ethylenedioxythiophene)/polystyrene sulphonate (PEDOT) layer (under the gold electrode) as the hole collector. Device performance is maximized for a device with a net active layer thickness of 100 nm. The optimized device has peak external quantum efficiencies ≈ 40% at the polymer’s maximum absorption wavelength and yield short circuit current density ≥ 2 mA cm−2 for air mass (AM) 1.5 conditions (100 mW cm−2, 1 sun). The AM 1.5 open circuit voltage for this device is 0.64 V and the fill factor is 0.43, resulting in an overall power conversion efficiency of 0.58%.
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84.60.Jt Photoelectric conversion

Fabrication of CdS/Si nanocable heterostructures by one-step thermal evaporation

X. L. Fu, Y. J. Ma, P. G. Li, L. M. Chen, W. H. Tang, X. Wang, and L. H. Li

Appl. Phys. Lett. 86, 143102 (2005); http://dx.doi.org/10.1063/1.1897043 (3 pages) | Cited 7 times

Online Publication Date: 28 March 2005

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Coaxial CdS/Si nanocable heterostructures with a length of hundreds of micrometers and an average diameter of 100 nm were fabricated via one-step thermal evaporation of CdS powder under experimentally controlled conditions. The CdS cores have a hexagonal crystal structure. The Si sheaths are amorphous and can be directly grown on the CdS surfaces from the silicon substrate via a vapor-liquid-solid mechanism without an extra Si source. The photoluminescence of the nanocables presents two emission bands, around 510 and 590 nm. This simple method may be applied to other Si-sheathed heterostructures, which can be used in nanodevices with various functions.
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61.46.-w Structure of nanoscale materials
71.55.Gs II-VI semiconductors
81.16.−c

Fabrication of atomic-scale gold junctions by electrochemical plating using a common medical liquid

A. Umeno and K. Hirakawa

Appl. Phys. Lett. 86, 143103 (2005); http://dx.doi.org/10.1063/1.1897444 (3 pages) | Cited 19 times

Online Publication Date: 28 March 2005

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Fabrication of nanometer-separated gold junctions has been performed using “iodine tincture,” a medical liquid known as a disinfectant, as an etching/deposition electrolyte. In the gold-dissolved iodine tincture, gold electrodes were grown or eroded slowly enough to form quantum point contacts in an atomic scale. The resistance evolution during the electrochemical deposition showed plateaus at integer multiples of the resistance quantum, (2e2/h)−1, at room temperature (e: the elementary charge, h: the Planck constant). Iodine tincture is a commercially available common material, which makes the fabrication process to be simple and cost effective. Moreover, in contrast to the conventional electrochemical approaches, this method is free from highly toxic cyanide compounds or extraordinarily strong acids.
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81.16.-c Methods of micro- and nanofabrication and processing
82.45.Qr Electrodeposition and electrodissolution
85.65.+h Molecular electronic devices

Nanomechanical hydrogen sensing

X. M. H. Huang, M. Manolidis, Seong Chan Jun, and J. Hone

Appl. Phys. Lett. 86, 143104 (2005); http://dx.doi.org/10.1063/1.1897445 (3 pages) | Cited 38 times

Online Publication Date: 28 March 2005

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A nanomechanical beam resonator is used as a sensitive, specific hydrogen sensor. The beam is fabricated from AuPd alloy and tested by magnetomotive transduction at room temperature. The fundamental resonance frequency decreases significantly and reversibly at hydrogen pressures above 10−5 Torr, whereas the frequency shifts observed for other gases are significantly smaller. The large frequency shift is likely due to the formation of interstitial hydrogen in the metal alloy lattice, which relieves the built-in tensile stress in the resonator beam. The uptake of hydrogen as measured by frequency shift is consistent with previous studies.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Surface-charge-induced asymmetric electrokinetic transport in confined silicon nanochannels

R. Qiao and N. R. Aluru

Appl. Phys. Lett. 86, 143105 (2005); http://dx.doi.org/10.1063/1.1897430 (3 pages) | Cited 14 times

Online Publication Date: 28 March 2005

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Molecular dynamics simulations of a NaF solution transport in a confined silicon nanochannel indicated that the water flux and the ionic conductivity through two oppositely charged silicon channels, that are otherwise similar, differ by a factor of more than three, and the co-ion fluxes are in the opposite direction. Such a behavior cannot be predicted by the classical electrokinetic transport theory, and is found to originate from the asymmetric dependence of the transport properties of water near the charged silicon surface.
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47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.60.-i Flow phenomena in quasi-one-dimensional systems
66.10.-x Diffusion and ionic conduction in liquids

Selective etching of InGaAs∕GaAs(100) multilayers of quantum-dot chains

Zh. M. Wang, L. Zhang, K. Holmes, and G. J. Salamo

Appl. Phys. Lett. 86, 143106 (2005); http://dx.doi.org/10.1063/1.1898425 (3 pages) | Cited 8 times

Online Publication Date: 29 March 2005

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We report selective chemical etching as a promising procedure to study the buried quantum dots in multiple InGaAs∕GaAs layers. The dot layer-by-dot layer etching is demonstrated using a mixed solution of NH4OH:H2O2:H2O. Regular plan-view atomic force microscopy reveals that all of the exposed InGaAs layers have a chain-like lateral ordering despite the potential of significant In–Ga intermixing during capping. The vertical self-correlation of quantum dots in the chains is observed.
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81.05.Ea III-V semiconductors
81.65.Cf Surface cleaning, etching, patterning
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.65.Hb Quantum dots (patterned in quantum wells)
68.65.Ac Multilayers
68.37.Ps Atomic force microscopy (AFM)

Effect of hydrogen passivation on charge storage in silicon quantum dots embedded in silicon nitride film

Chang-Hee Cho, Baek-Hyun Kim, Tae-Wook Kim, Seong-Ju Park, Nae-Man Park, and Gun-Yong Sung

Appl. Phys. Lett. 86, 143107 (2005); http://dx.doi.org/10.1063/1.1894595 (3 pages) | Cited 24 times

Online Publication Date: 29 March 2005

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The effect of hydrogen passivation on the charge storage characteristics of two types of silicon nitride films containing silicon quantum dots (Si QDs) grown by SiH4+N2 and SiH4+NH3 plasma was investigated. The transmission electron microscope analysis and the capacitance-voltage measurement showed that the silicon nitride film grown by SiH4+NH3 plasma has a lower interface trap density and a higher density of Si QDs compared to that grown by SiH4+N2 plasma. It was also found that the charge retention characteristics in the Si QDs were greatly enhanced in the samples grown by means of SiH4+NH3 plasma, due to the hydrogen passivation of the defects in the silicon nitride films by NH3 during the growth of the Si QDs.
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81.07.Ta Quantum dots
81.05.Cy Elemental semiconductors
73.21.La Quantum dots
81.15.Jj Ion and electron beam-assisted deposition; ion plating
52.77.Dq Plasma-based ion implantation and deposition
68.37.Lp Transmission electron microscopy (TEM)

Long-wavelength light emission and lasing from InAs/GaAs quantum dots covered by a GaAsSb strain-reducing layer

H. Y. Liu, M. J. Steer, T. J. Badcock, D. J. Mowbray, M. S. Skolnick, P. Navaretti, K. M. Groom, M. Hopkinson, and R. A. Hogg

Appl. Phys. Lett. 86, 143108 (2005); http://dx.doi.org/10.1063/1.1897850 (3 pages) | Cited 39 times

Online Publication Date: 30 March 2005

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The effects of a thin GaAsSb strain-reducing layer on the optical properties of InAs/GaAs quantum dots (QDs) are investigated. With increasing Sb composition, the room-temperature emission wavelength of the InAs QDs increases to ∼ 1.43 μm. For Sb compositions above 14%, the system becomes Type II, with a decrease of the photoluminescence (PL) efficiency. At a composition of 14%, the room-temperature PL efficiency is maximized, and is also significantly enhanced when compared to that of conventional InGaAs-capped InAs QDs grown under the same conditions. Room-temperature ground-state lasing at 1.292 μm is demonstrated for an InAs/GaAsSb/GaAs structure.
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42.55.Px Semiconductor lasers; laser diodes
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
78.67.Hc Quantum dots
78.55.Cr III-V semiconductors
78.60.Fi Electroluminescence

Lithographically defined metal-semiconductor-hybrid nanoscrolls

O. Schumacher, S. Mendach, H. Welsch, A. Schramm, Ch. Heyn, and W. Hansen

Appl. Phys. Lett. 86, 143109 (2005); http://dx.doi.org/10.1063/1.1897056 (3 pages) | Cited 31 times

Online Publication Date: 30 March 2005

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We report on two-layer metal-semiconductor-hybrid scrolls fabricated from rolled-up strained metal∕InGaAs-layers. As the central approach, the metallic layer itself acts as a stressor in contact with the semiconductor. Position and length of the scrolls can be precisely tuned by patterning the e-beam-evaporated metallic stressor with conventional lithographic techniques. The thickness of the metallization determines the radius of the resulting scrolls. This fabrication technique significantly improves the reliability and simplifies the fabrication of metal∕semiconductor three-dimensional objects which employ bending up layers. Even more important, using this technique the bending radius of such three-dimensional objects can easily be downsized to very small radii in the nanometer scale, e.g. in order to build nano-electro-mechanical systems.
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82.45.Vp Semiconductor materials in electrochemistry
81.05.Bx Metals, semimetals, and alloys

Epitaxial DySi2 nanowire formation on stepped Si(111)

Zhian He, David J. Smith, and P. A. Bennett

Appl. Phys. Lett. 86, 143110 (2005); http://dx.doi.org/10.1063/1.1897424 (3 pages) | Cited 19 times

Online Publication Date: 30 March 2005

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We report the growth of epitaxial DySi2 nanowires (NW) with a single orientation on miscut Si(111). Using high-resolution electron microscopy, we determine that the islands are hexagonal DySi2 with orientation DySi2(0001)‖Si(111), corresponding to a near-perfect lattice match. The NW islands develop extended defects that correlate perfectly with individual step bunches at the buried interface, produced during growth. By contrast, islands grown on step-free substrates develop a broad, two-dimensional shape with no defects. We suggest that the NW shape results from the energy cost of extended defects, which inhibits growth across step edges.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.15.Np Solid phase epitaxy; growth from solid phases

Air flow technique for large scale dispersion and alignment of carbon nanotubes on various substrates

James Hedberg, Lifeng Dong, and Jun Jiao

Appl. Phys. Lett. 86, 143111 (2005); http://dx.doi.org/10.1063/1.1897435 (3 pages) | Cited 17 times

Online Publication Date: 30 March 2005

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Herein we present a method to disperse and align carbon nanotubes on various substrate surfaces. Using the shear forces associated with a rapidly moving fluid, nanoscale objects were positioned in a direction corresponding to the flow vector of the fluid. Dispersion of carbon nanotubes on microaddressable electrodes via the gas flow method creates opportunities for scaling up the production of nanoscale devices. In this letter, we demonstrate the feasibility of the method and the electrical characterization results obtained after the fabrication of carbon nanotube testing structures.
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81.07.De Nanotubes
81.05.U- Carbon/carbon-based materials

Precipitation of Ge nanoparticles from GeO2 glasses in transmission electron microscope

Nan Jiang, Jianrong Qiu, and John C. H. Spence

Appl. Phys. Lett. 86, 143112 (2005); http://dx.doi.org/10.1063/1.1898423 (3 pages) | Cited 4 times

Online Publication Date: 31 March 2005

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We show, using spatially resolved energy loss spectroscopy in a transmission electron microscopy (TEM), that GeO2 and GeO2–SiO2 glasses are extremely sensitive to high energy electrons. Ge nanoparticles can be precipitated in GeO2 glasses efficiently by the high-energy electron beam of a TEM. This is relevant to TEM characterization of luminescent Ge nanoparticles in silicate glasses, which may produce artificial results.
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64.75.-g Phase equilibria
61.43.Fs Glasses
79.20.Uv Electron energy loss spectroscopy
61.80.Fe Electron and positron radiation effects
61.82.-d Radiation effects on specific materials
61.46.-w Structure of nanoscale materials
68.37.Lp Transmission electron microscopy (TEM)

MgxZn1−xO(0 ⩽ x<0.2) nanowire arrays on sapphire grown by high-pressure pulsed-laser deposition

M. Lorenz, E. M. Kaidashev, A. Rahm, Th. Nobis, J. Lenzner, G. Wagner, D. Spemann, H. Hochmuth, and M. Grundmann

Appl. Phys. Lett. 86, 143113 (2005); http://dx.doi.org/10.1063/1.1898433 (3 pages) | Cited 105 times

Online Publication Date: 31 March 2005

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MgxZn1−xO nanowires with Mg-content x from 0 to 0.2 have been grown by high-pressure pulsed-laser deposition (PLD) on gold-covered sapphire single crystals. The PLD process allows for a unique wide-range control of morphology, diameter, and composition of the MgxZn1−xO nanowires. The diameter of single ZnO wires could be varied between about 50 and 3000 nm, and the Mg content x of MgxZn1−xO wire arrays was controlled via the PLD gas pressure. The microscopic homogeneity of Mg content is displayed by cathodoluminescence (CL) imaging of the excitonic peak energy. The fluctuation of CL peak energy between individual wires is about an order of magnitude smaller than the alloy broadening.
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81.07.Bc Nanocrystalline materials
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
61.46.-w Structure of nanoscale materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.15.Fg Pulsed laser ablation deposition
81.16.Mk Laser-assisted deposition
78.60.Hk Cathodoluminescence, ionoluminescence

Germanium diffusion and nanocrystal formation in silicon oxide on silicon substrate under rapid thermal annealing

W. K. Choi, V. Ho, V. Ng, Y. W. Ho, S. P. Ng, and W. K. Chim

Appl. Phys. Lett. 86, 143114 (2005); http://dx.doi.org/10.1063/1.1891290 (3 pages) | Cited 24 times

Online Publication Date: 31 March 2005

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The effect of rapid thermal annealing temperature on the diffusion of silicon (Si) and germanium (Ge) and the formation of Ge nanocrystals in a silicon oxide matrix was investigated. The formation of Ge nanocrystals was attributed mainly to the reduction of Ge suboxides by Si diffused from the Si substrate. For samples annealed at 800 °C, the nanocrystals were uniform in size and distributed evenly in the bulk of the oxide but became denser nearer to the silicon–silicon oxide (Si–SiO2) interface. When the sample was annealed at 900 °C, two regions with different nanocrystal densities and size distributions separated by a region void of nanocrystals were observed. The region of denser nanocrystals was located near the Si–SiO2 interface. For annealing at 1000 °C, nanocrystals were only observed at the Si–SiO2 interface and these have significant size variation, with the rest of the oxide being void of nanocrystals. The nanocrystals formed at 900 and 1000 °C were generally found to be defective.
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81.07.Bc Nanocrystalline materials
81.16.-c Methods of micro- and nanofabrication and processing
66.30.Pa Diffusion in nanoscale solids
66.30.J- Diffusion of impurities
66.30.Lw Diffusion of other defects
61.72.S- Impurities in crystals
81.40.Gh Other heat and thermomechanical treatments
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Photon emission in CuInSe2 thin films observed by scanning tunneling microscopy

Manuel J. Romero, Chun-Sheng Jiang, Rommel Noufi, and Mowafak Al-Jassim

Appl. Phys. Lett. 86, 143115 (2005); http://dx.doi.org/10.1063/1.1897048 (3 pages) | Cited 11 times

Online Publication Date: 1 April 2005

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We report on the observation of photon emission from CuInSe2 (CIS) thin films by scanning tunneling microscopy (STM), which results from the radiative recombination induced by tunneling electrons. Scanning tunneling luminescence (STL) spectroscopy suggests that photons are emitted near the surface of CIS. STL is excited by recombination of tunneling electrons with available holes in CIS or electron-hole recombination by impact ionization—unipolar and bipolar excitation, respectively. Which process becomes predominant depends on the voltage applied to the STM tip. Under unipolar excitation, the photon intensity decreases on grain boundaries when compared to grain interiors. Under bipolar excitation, on the other hand, no differences are observed in photon intensity. A reduction of the density of holes in grain boundaries, relative to grain interiors, can explain the observed behavior.
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78.66.Li Other semiconductors
73.61.Le Other inorganic semiconductors
61.72.Mm Grain and twin boundaries
73.40.Gk Tunneling
73.50.Fq High-field and nonlinear effects
78.68.+m Optical properties of surfaces
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
78.60.Hk Cathodoluminescence, ionoluminescence
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.-a Thin film structure and morphology
72.40.+w Photoconduction and photovoltaic effects
84.60.Jt Photoelectric conversion

Effective removal of hydrogen resists used to pattern devices in silicon using scanning tunneling microscopy

T. Hallam, F. J. Rueß, N. J. Curson, K. E. J. Goh, L. Oberbeck, M. Y. Simmons, and R. G. Clark

Appl. Phys. Lett. 86, 143116 (2005); http://dx.doi.org/10.1063/1.1897064 (3 pages) | Cited 4 times

Online Publication Date: 1 April 2005

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We present a high resolution scanning tunneling microscope (STM) study of the thermal desorption of hydrogen resist layers used for STM-based lithography on the Si(001)2×1 surface. From this study we determine the optimum annealing conditions for removing the hydrogen resist in one step. We demonstrate that this thermal process can completely remove the hydrogen resist from a phosphorus doped surface structure created using STM-lithography, without disturbing the lithographically defined structure. We investigate the effectiveness of the removal process by performing electrical measurements of a buried STM-patterned device created using the optimized thermal desorption process and demonstrate that we can achieve phase coherence lengths of ∼ 40 nm, comparable to that in P in Si delta-doped layers where no hydrogen resist or STM patterning has been used.
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81.05.Cy Elemental semiconductors
81.65.Cf Surface cleaning, etching, patterning
85.40.Hp Lithography, masks and pattern transfer
61.72.Cc Kinetics of defect formation and annealing
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.43.Vx Thermal desorption
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Single-shot readout with the radio-frequency single-electron transistor in the presence of charge noise

T. M. Buehler, D. J. Reilly, R. P. Starrett, Andrew D. Greentree, A. R. Hamilton, A. S. Dzurak, and R. G. Clark

Appl. Phys. Lett. 86, 143117 (2005); http://dx.doi.org/10.1063/1.1897423 (3 pages) | Cited 16 times

Online Publication Date: 1 April 2005

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The radio-frequency single-electron transistor (rf-SET) possesses key requirements necessary for reading out a solid state quantum computer. This work explores the use of the rf-SET as a single-shot readout device in the presence of 1/f and telegraph charge noise. For a typical spectrum of 1/f noise we find that high fidelity, single-shot measurements are possible for signals Δq>0.01e. For the case of telegraph noise, we present a cross-correlation measurement technique that uses two rf-SETs to suppress the effect of random switching events on readout. We demonstrate this technique by monitoring the charge state of a metal double dot system on microsecond time scales. Such a scheme will be advantageous in achieving high readout fidelity in a solid-state quantum computer.
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85.35.Gv Single electron devices
85.30.De Semiconductor-device characterization, design, and modeling
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