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11 Jul 2005

Volume 87, Issue 2, Articles (02xxxx)

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Appl. Phys. Lett. 87, 024103 (2005); http://dx.doi.org/10.1063/1.1984098 (3 pages)

A. Dupuis, J. Léopoldès, D. G. Bucknall, and J. M. Yeomans
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GaAs on Si(111)—crystal shape and strain relaxation in nanoscale patterned growth

S. C. Lee, L. R. Dawson, S. R. J. Brueck, and Y.-B. Jiang

Appl. Phys. Lett. 87, 023101 (2005); http://dx.doi.org/10.1063/1.1984100 (3 pages) | Cited 2 times

Online Publication Date: 6 July 2005

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Nanoscale patterned growth of GaAs on Si(111) by molecular beam epitaxy is examined. A 355 nm period two-dimensional array of circular holes (diameter ∼ 200–250 nm) is fabricated into a 45-nm-thick SiO2 film on a Si(111) substrate by large-area interferometric lithography and dry etching. For 300 nm deposition, the GaAs epilayer selectively deposited within each hole on the patterned substrate is surrounded by {1math0}-type sidewalls perpendicular to Si(111), resulting in a hexagon-based prismatic pillar, without significant lateral overgrowth. At the initial stage of growth, twins parallel to Si(111) and an aperiodic mixture of cubic and hexagonal phases are observed but most of the GaAs pillars are terminated with a cubic phase region. Raman scattering reveals that the individual nanoscale GaAs pillars are completely strain relaxed.
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81.05.Ea III-V semiconductors
81.16.Rf Micro- and nanoscale pattern formation
81.16.Nd Micro- and nanolithography
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.65.Cf Surface cleaning, etching, patterning
78.30.Fs III-V and II-VI semiconductors
78.66.Fd III-V semiconductors
68.60.Bs Mechanical and acoustical properties
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
61.72.Mm Grain and twin boundaries

Approach to nonphotoperturbed differential capacitance measurements: A front-wing cantilever

M. N. Chang, C. Y. Chen, W. J. Huang, and T. C. Cheng

Appl. Phys. Lett. 87, 023102 (2005); http://dx.doi.org/10.1063/1.1994949 (3 pages) | Cited 4 times

Online Publication Date: 6 July 2005

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We have developed a front-wing (FW) cantilever structure that can significantly suppress photoperturbation effects during scanning capacitance microscopy (SCM) and scanning capacitance spectroscopy (SCS) measurements. The FW cantilever provides an effective shadow area that fully covers the scan region, allowing us to synchronously obtain SCM images and the corresponding topographic images without photoperturbation problems. Nonphotoperturbed differential capacitance characteristics versus tip biases were also obtained for SCS by the use of these FW cantilevers. This means that nonphotoperturbed SCM and SCS measurements can be carried out during the typical SCM operations.
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07.79.-v Scanning probe microscopes and components
07.68.+m Photography, photographic instruments; xerography

Efficient field emission from α-Fe2O3 nanoflakes on an atomic force microscope tip

Y. W. Zhu, T. Yu, C. H. Sow, Y. J. Liu, A. T. S. Wee, X. J. Xu, C. T. Lim, and J. T. L. Thong

Appl. Phys. Lett. 87, 023103 (2005); http://dx.doi.org/10.1063/1.1991978 (3 pages) | Cited 46 times

Online Publication Date: 7 July 2005

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Aligned arrays of flake-shaped hematite (α-Fe2O3) nanostructure have been fabricated on an atomic force microscope (AFM) tip. They are created by simply heating an iron-coated AFM tip in ambience on a hot plate. These nanoflakes are characterized as α-Fe2O3 single crystalline structures with tip radii as small as several nanometers and are highly effective as electron field emitters. With a vacuum gap of about 150 μm, field emission measurements of α-Fe2O3 nanoflakes on AFM tips show a low turn-on voltage of about 400–600 V and a high current density of 1.6 A cm−2 under 900 V. Such high emission current density is attributed to the nanoscale sharp tips of the as-grown nanoflakes. Based on the Fowler–Nordheim theory, it is demonstrated the enhancement factor of α-Fe2O3 nanoflakes on AFM tips is comparable to that of carbon nanotubes. Our findings suggest that α-Fe2O3 nanoflakes are potentially useful as candidates for future electron field emission devices.
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79.70.+q Field emission, ionization, evaporation, and desorption
61.46.-w Structure of nanoscale materials
81.07.Bc Nanocrystalline materials

Hydrogen as origin of compressive intrinsic stress in hydrogenated amorphous silicon: The contribution of clustered forms

B. Pantchev, P. Danesh, and B. Schmidt

Appl. Phys. Lett. 87, 023104 (2005); http://dx.doi.org/10.1063/1.1999007 (3 pages) | Cited 2 times

Online Publication Date: 8 July 2005

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The role of hydrogen in hydrogenated amorphous silicon (a‐Si:H) has been studied from the viewpoint of its specific contribution to mechanical stress in the material. Hydrogen ion implantation has been used to increase the hydrogen concentration. In order to distinguish the effect of the changed hydrogen concentration∕bonding configuration from the accompanying implantation-induced defects, a‐Si:H samples with corresponding number of displacements have been studied, created using proper doses of silicon ion implantation. The experimental results have shown that it is the silicon-bonded hydrogen that essentially affects the stress, as the major contribution has its clustered bonding configuration.
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61.72.uf Ge and Si
68.60.Bs Mechanical and acoustical properties
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.50.Lt Crystal binding; cohesive energy

Enhanced thermoelectric performance in PbTe-based superlattice structures from reduction of lattice thermal conductivity

J. C. Caylor, K. Coonley, J. Stuart, T. Colpitts, and R. Venkatasubramanian

Appl. Phys. Lett. 87, 023105 (2005); http://dx.doi.org/10.1063/1.1992662 (3 pages) | Cited 45 times

Online Publication Date: 8 July 2005

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We have fabricated two-dimensional n-type PbTe/PbTe0.75Se0.25 structures using an evaporation process. In optimized films exhibiting a high-quality superlattice structure, a significant reduction in lattice thermal conductivity has been experimentally measured. The reduction would indicate enhanced thermoelectric device performance compared to standard PbTeSe alloys given that the electrical components, specifically, the Seebeck coefficient and electrical resistivity, were not observed to deteriorate from bulk values. The analysis of these films shows continuous layers with a true two-dimensional superlattice structure, as opposed to the PbTe/PbSe system that exhibits zero-dimensional structures from self-assembly. The room-temperature measurement of cross-plane figure-of-merit in a n-type PbTe/PbTe0.75Se0.25 device structure by the transient method has been combined with temperature-dependent measurements of in-plane resistivity and Seebeck coefficient to yield evidence of enhanced thermoelectric performance. The similarities and differences between the superlattice in the PbTe/PbTe0.75Se0.25 system and the Bi2Te3/Sb2Te3 material system are presented.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.05.Hd Other semiconductors
73.50.Lw Thermoelectric effects
72.20.Pa Thermoelectric and thermomagnetic effects
73.61.Le Other inorganic semiconductors
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
68.55.A- Nucleation and growth
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy

Ultrafast carrier dynamics in ZnO nanorods

Chi-Kuang Sun, Shih-Ze Sun, Kung-Hsuan Lin, Kenneth Yi-Jie Zhang, Hsiang-Lin Liu, Sai-Chang Liu, and Jih-Jen Wu

Appl. Phys. Lett. 87, 023106 (2005); http://dx.doi.org/10.1063/1.1989444 (3 pages) | Cited 19 times

Online Publication Date: 8 July 2005

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Free exciton and above-band-gap free carrier dynamics in ZnO nanorods have been investigated at room temperature with a femtosecond transient transmission measurement. Following the photoexcitation of above-band-gap free carriers, an extremely fast external thermalization time on the order of 200 fs can be observed. Under high excitation, hot phonon effects were found to delay the carrier cooling process. While the photoexcitation energy was tuned to match the free exciton transition, stable exciton formation can be uncovered while no evident exciton ionization process can be found unless the photoexcited exciton density exceeded the Mott density.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
78.47.-p Spectroscopy of solid state dynamics
78.40.Fy Semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
71.35.-y Excitons and related phenomena
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials

Ballistic thermal and electrical conductance measurements on individual multiwall carbon nanotubes

Elisabetta Brown, Ling Hao, John C. Gallop, and John C. Macfarlane

Appl. Phys. Lett. 87, 023107 (2005); http://dx.doi.org/10.1063/1.1993768 (3 pages) | Cited 26 times

Online Publication Date: 8 July 2005

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We report thermal measurements on individual carbon nanotubes using a temperature sensing scanned microscope probe. An arc-grown bundle of multiwalled nanotubes (MWNTs) is mechanically attached to a thermal probe. The heat flow down individual MWNTs is recorded as a function of the temperature difference across them. Simultaneous measurements of thermal and electrical conductance are recorded. The size of the conductance steps observed at room temperature and the correlation between electrical and thermal conductance steps are discussed and we present evidence for ballistic transport of both phonons and electrons in these tubes.
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73.23.Ad Ballistic transport
73.63.Fg Nanotubes
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
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