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29 Aug 2005

Volume 87, Issue 9, Articles (09xxxx)

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Appl. Phys. Lett. 87, 093109 (2005); http://dx.doi.org/10.1063/1.2035332 (3 pages)

J. Noborisaka, J. Motohisa, S. Hara, and T. Fukui
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Investigation of the humidity effect on the electrical properties of single-walled carbon nanotube transistors

Pil Sun Na, Hyojin Kim, Hye-Mi So, Ki-Jeong Kong, Hyunju Chang, Beyong Hwan Ryu, Youngmin Choi, Jeong-O Lee, Byoung-Kye Kim, Ju-Jin Kim, and Jinhee Kim

Appl. Phys. Lett. 87, 093101 (2005); http://dx.doi.org/10.1063/1.2032594 (3 pages) | Cited 39 times

Online Publication Date: 22 August 2005

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We investigated the effect of humidity on the electrical transport properties of single-walled carbon nanotube field effect transistors (SWNT-FETs). Water molecules are found to behave as electron donors to the nanotube: Current through the p-type carbon nanotube device is found to decrease under a modest humidity, and starts to increase as the humidity increases over 65%, which is believed to be due to the opening of electron channels. Through first principles calculations based on the density functional theory, we found that water molecules can donate electrons to the carbon nanotube. Moreover, a hydrogen-bonded water monolayer will be formed around the nanotube at fully covered conditions. We suggest that this result could provide a systematic understanding of the humidity effect on SWNT-FETs, which has been believed to be essential in the development of nanotube-based sensors.
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85.35.Kt Nanotube devices
85.65.+h Molecular electronic devices

Intense terahertz radiation from longitudinal optical phonons in GaAs/AlAs multiple quantum wells

K. Mizoguchi, T. Furuichi, O. Kojima, M. Nakayama, S. Saito, A. Syouji, and K. Sakai

Appl. Phys. Lett. 87, 093102 (2005); http://dx.doi.org/10.1063/1.2033138 (3 pages) | Cited 12 times

Online Publication Date: 22 August 2005

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We report the generation of intense terahertz (THz) electromagnetic waves from GaAs/AlAs multiple quantum wells (MQWs) using ultrashort laser pulses. Pronounced THz waves from the coherent GaAs-like longitudinal optical (LO) phonons with a frequency of 8.8 THz were observed in the time region over 6 ps, which indicates the long dephasing time. The power of the THz waves from the coherent GaAs-like LO phonons in the GaAs/AlAs MQW was enhanced to be about 100 times larger than that of those from coherent LO phonons in an epitaxial-growth GaAs film. We discuss the origin of the enhancement of the THz wave from coherent GaAs-like LO phonon in the MQW.
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78.66.Fd III-V semiconductors
78.67.De Quantum wells
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
78.47.-p Spectroscopy of solid state dynamics

Nanoscale mapping of the space charge layer of Au/GaAs(110) contacts

T. C. G. Reusch, M. Wenderoth, L. Winking, N. Quaas, and R. G. Ulbrich

Appl. Phys. Lett. 87, 093103 (2005); http://dx.doi.org/10.1063/1.2034097 (3 pages) | Cited 4 times

Online Publication Date: 22 August 2005

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We have investigated Au/GaAs(110) Schottky contacts with scanning tunneling spectroscopy in cross-sectional configuration. The tunneling spectra show that both band edges are shifted and modified by the electrostatic potential of the space charge layer. The shifts were extracted and compared with simulations of the tunneling current including a local potential in the semiconductor. The resulting potential landscape is mapped on a near-atomic scale in the plane perpendicular to the metal-semiconductor interface.
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73.30.+y Surface double layers, Schottky barriers, and work functions
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)

Observation of intershell and hybridized energy states in InAs/GaAs quantum dots

F. Z. Wang, Z. H. Chen, L. H. Bai, S. H. Huang, H. Xiong, S. C. Shen, J. Sun, P. Jin, and Z. G. Wang

Appl. Phys. Lett. 87, 093104 (2005); http://dx.doi.org/10.1063/1.2034104 (3 pages) | Cited 3 times

Online Publication Date: 22 August 2005

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We have investigated the evolution of exciton state filling as a function of excitation power density in InAs/GaAs quantum dots (QDs). In addition to the emission bands of exciton recombination corresponding to the atom-like S, P, and D, etc. shells of quantum dots, it was observed that some extra states, P′ (between the S and P shells) and D′ (between the P and D shells), appear in the spectra with increasing number of excitons occupying the QDs. The emergence of these intershell excitonic levels is an experimental demonstration of strong exciton–exciton exchange interaction and coupling as well as state mixing and hybridization of a multiexciton system in quantum dots.
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73.21.La Quantum dots
71.70.Gm Exchange interactions
71.35.-y Excitons and related phenomena
78.55.Cr III-V semiconductors
78.67.Hc Quantum dots

Controlled formation and size-selected deposition of indium nanoparticles from a microwave flow reactor on semiconductor surfaces

Klemens Hitzbleck, Hartmut Wiggers, and Paul Roth

Appl. Phys. Lett. 87, 093105 (2005); http://dx.doi.org/10.1063/1.2012516 (3 pages) | Cited 5 times

Online Publication Date: 22 August 2005

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Indium nanoparticles were synthesized in a microwave flow reactor by thermal decomposition of trimethylindium. The particles were extracted from the gas phase by molecular beam sampling, deflected in an electric field, and deposited on a semiconductor surface. The size of the deposited particles was selected by adjusting the deflection voltage. The geometric standard deviation of the size-selected particles was found to be smaller than 10%. The deposition method is compatible with epitaxial growth methods and enhances their potentials with nanoparticle technology.
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81.07.Bc Nanocrystalline materials
81.16.Be Chemical synthesis methods
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Spatial ordering of quantum dots in microdisks

Z. G. Xie and G. S. Solomon

Appl. Phys. Lett. 87, 093106 (2005); http://dx.doi.org/10.1063/1.2034111 (3 pages) | Cited 12 times

Online Publication Date: 22 August 2005

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We demonstrate the spatial localization of optically active InAs quantum dots in the narrow whispering gallery mode region of a GaAs microdisk cavity. This is achieved through preferential In adatom surface diffusion on a partial cavity structure in a way that can be generalized to other optical cavities. In agreement with our model, and contrary to regrowth on mesa structures, we observe decreasing perimeter quantum dot densities with decreasing microdisk diameters. Photoluminescence measurements indicate optically-active single-quantum-dot emission.
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68.65.Hb Quantum dots (patterned in quantum wells)
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
78.67.Hc Quantum dots
78.55.Cr III-V semiconductors
68.43.Jk Diffusion of adsorbates, kinetics of coarsening and aggregation

Control of injected carriers in tetracyano-p-quinodimethane encapsulated carbon nanotube transistors

Masashi Shiraishi, Shuichi Nakamura, Tomohiro Fukao, Taishi Takenobu, Hiromichi Kataura, and Yoshihiro Iwasa

Appl. Phys. Lett. 87, 093107 (2005); http://dx.doi.org/10.1063/1.2035331 (3 pages) | Cited 10 times

Online Publication Date: 23 August 2005

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We examined transistor characteristics of tetracyano-p-quinodimethane encapsulated single-walled carbon nanotubes (TCNQ@SWNTs). In device operations, a clear conversion to a p-type character was observed and the stability of carriers, previously doped into SWNTs, were simultaneously clarified. Because of an energy band shift, between the electrodes and the doped SWNTs induced by the doping, electron injection was achieved only by application of a high source-drain voltage, while holes were easily injected because of decrease in hole barrier height.
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85.35.Kt Nanotube devices
85.40.Ry Impurity doping, diffusion and ion implantation technology

Time-resolved photoluminescence study of the stimulated emission in ZnO nanoneedles

W. M. Kwok, A. B. Djurišić, Y. H. Leung, W. K. Chan, and D. L. Phillips

Appl. Phys. Lett. 87, 093108 (2005); http://dx.doi.org/10.1063/1.2035871 (3 pages) | Cited 26 times

Online Publication Date: 23 August 2005

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ZnO nanoneedles were fabricated by thermal evaporation of Zn nanoparticles at 800 °C and atmospheric pressure. The samples showed strong ultraviolet photoluminescence and weak orange defect luminescence. Time-resolved photoluminescence (TRPL) was measured using the Kerr-gated fluorescence technique in order to probe the ultrafast carrier dynamics in exciton-exciton scattering and electron hole plasma (EHP) regimes. In both regimes, the decay time of the photoluminescence is very fast ( ∼ 1 ps). Even though no structure is detected in the time-integrated spectra of the EHP emission, the TRPL reveals the coexistence of the excitons and free carriers. Possible reasons for the observed phenomena are discussed.
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78.55.Et II-VI semiconductors
78.47.-p Spectroscopy of solid state dynamics
78.45.+h Stimulated emission
72.30.+q High-frequency effects; plasma effects
61.46.-w Structure of nanoscale materials
71.35.Ee Electron-hole drops and electron-hole plasma

Fabrication and characterization of freestanding GaAs/AlGaAs core-shell nanowires and AlGaAs nanotubes by using selective-area metalorganic vapor phase epitaxy

J. Noborisaka, J. Motohisa, S. Hara, and T. Fukui

Appl. Phys. Lett. 87, 093109 (2005); http://dx.doi.org/10.1063/1.2035332 (3 pages) | Cited 63 times

Online Publication Date: 24 August 2005

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We fabricated GaAs/AlGaAs core-shell nanowires by using selective-area metalorganic vapor phase epitaxy. First, GaAs nanowires were selectively grown on partially masked GaAs (111)B substrates; then AlGaAs was grown to form freestanding heterostructured nanowires. Investigation of nanowire diameter as a function of AlGaAs growth time suggested that the AlGaAs was grown on the sidewalls of the GaAs nanowires, forming GaAs/AlGaAs core-shell structures. Microphotoluminescence measurements of GaAs and GaAs/AlGaAs core-shell nanowires reveal an enhancement of photoluminescence intensity in GaAs/AlGaAs core-shell structures. Based on these core-shell nanowires, AlGaAs nanotubes were formed by using anisotropic dry etching and wet chemical preferential etching to confirm the formation of a core-shell structure and to explore a new class of materials.
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81.07.Vb Quantum wires
81.07.De Nanotubes
68.65.La Quantum wires (patterned in quantum wells)
81.05.Ea III-V semiconductors
81.16.-c Methods of micro- and nanofabrication and processing
81.65.Cf Surface cleaning, etching, patterning
61.46.-w Structure of nanoscale materials
78.67.Ch Nanotubes
78.55.Cr III-V semiconductors

Quasi-aligned ZnO nanotubes grown on Si substrates

W. Z. Xu, Z. Z. Ye, D. W. Ma, H. M. Lu, L. P. Zhu, B. H. Zhao, X. D. Yang, and Z. Y. Xu

Appl. Phys. Lett. 87, 093110 (2005); http://dx.doi.org/10.1063/1.2035868 (3 pages) | Cited 27 times

Online Publication Date: 24 August 2005

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Quasi-aligned ZnO nanotubes have been grown on silicon substrates by metalorganic chemical vapor deposition without using any catalyst. Two kinds of ZnO nanotubular structures were found: Nanotubes with single walls and nanotubes with double walls. The nanotubes were grown along the [001] direction. Room-temperature photoluminescence measurements of the ZnO nanotubes indicate strong ultraviolet emission and weak green emission. A new growth mode for these ZnO nanotubes is proposed, which can be used to prepare other nanotubular structures.
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78.55.Et II-VI semiconductors
61.46.-w Structure of nanoscale materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Pulsed-ion-beam nitriding and smoothing of titanium surface in a vacuum

X. P. Zhu, Hisayuki Suematsu, Weihua Jiang, Kiyoshi Yatsui, and M. K. Lei

Appl. Phys. Lett. 87, 093111 (2005); http://dx.doi.org/10.1063/1.2035876 (3 pages) | Cited 5 times

Online Publication Date: 24 August 2005

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Both nitriding and smoothing of titanium have been achieved under irradiation of intense pulsed ion beam in a vacuum of 2×10−2 Pa. Applying a screening method, we find that medium ion-beam intensity and multi-shot irradiation are effective for the processing, where repetitive surface melting with limited ablation favored Ti nitride formation as well as surface smoothing. The present results demonstrate that ambient gas atoms∕molecules can be efficiently incorporated in metal matrices to form compounds under the ion-beam irradiation. The finding is of great significance for extending application scope of the ion-beam technique in materials research and processing, combined with the recent success in introducing ambient gas into the processing chamber.
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81.65.Lp Surface hardening: nitridation, carburization, carbonitridation
61.80.Jh Ion radiation effects

Transport properties of InN nanowires

Chih-Yang Chang, Gou-Chung Chi, Wei-Ming Wang, Li-Chyong Chen, Kuei-Hsien Chen, F. Ren, and S. J. Pearton

Appl. Phys. Lett. 87, 093112 (2005); http://dx.doi.org/10.1063/1.2037850 (3 pages) | Cited 28 times

Online Publication Date: 24 August 2005

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The transport properties of single InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of both length/square of radius ratio and temperature. The resistivity of the n-type InN nanowires with diameter >100 nm was measured by the transmission line method and the value was on the order of 4×10−4 Ω cm. The specific contact resistivity for unalloyed Pd/Ti/Pt/Au ohmic contacts was near 1.09×10−7 Ω cm2. The temperature dependence of resistance showed a positive temperature coefficient and a functional form characteristic of metallic conduction in the InN.
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73.63.Nm Quantum wires
73.61.Ey III-V semiconductors
73.40.Cg Contact resistance, contact potential
73.40.Ns Metal-nonmetal contacts

Zinc oxide nanoarrays in nanoporous nickel phosphate with a huge blueshift ultraviolet-visible exciton absorption peak

Zhi Chen, Qiuming Gao, Meiling Ruan, and Jianlin Shi

Appl. Phys. Lett. 87, 093113 (2005); http://dx.doi.org/10.1063/1.2035318 (3 pages) | Cited 5 times

Online Publication Date: 25 August 2005

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Zinc oxide (ZnO) nanoarray prepared in the template of nanoporous nickel(II) phosphate Versailles Santa Barbara-1 (VSB-1) crystal exhibits an unusual ultraviolet-visible exciton absorption peak and an absorption shoulder. The exciton absorption peak at 266 nm with the largest blueshift of 122 nm up to now and narrow absorption peak with a full width at half maximum (FWHM) of about 34 nm and the narrow photoluminescence emission peak 380 nm with a FWHM of about 30 nm, compared with that of bulk ZnO, are due to the quantum-confinement effects and the narrow size distribution of ZnO in the pores of VSB-1. This material presents potential applications in the theoretical research and fabrication of future quantum devices.
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78.66.Hf II-VI semiconductors
78.55.Et II-VI semiconductors
78.55.Mb Porous materials
78.40.Ha Other nonmetallic inorganics

Generation of frequency-tunable nanoacoustic waves by optical coherent control

Cheng-Ta Yu, Kung-Hsuan Lin, Chia-Lung Hsieh, Chang-Chi Pan, Jen-Inn Chyi, and Chi-Kuang Sun

Appl. Phys. Lett. 87, 093114 (2005); http://dx.doi.org/10.1063/1.2035882 (3 pages) | Cited 6 times

Online Publication Date: 25 August 2005

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We have developed a system to generate arbitrary wave-form nanoacoustic waves (NAWs) with a piezoelectric InGaN/GaN single-quantum well. Based on an optical coherent control technique, acoustic frequency tunability in the subterahertz range is realized within only one fixed sample. The acoustic generation mechanisms, especially the in-well piezoelectric field Coulomb screening which tends to be saturated at high carrier concentrations, are discussed with optical power dependency. With the generated NAWs propagating in the c axis of a GaN thin film, the lifetime of the 500 GHz longitudinal-acoustic phonon pulses in GaN is measured to be longer than 420 ps, corresponding to a GaN depth more than 3.3 μm.
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78.66.Fd III-V semiconductors
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
78.47.-p Spectroscopy of solid state dynamics

High efficiency and brightness of blue light emission from dislocation-free InGaN/GaN quantum well nanorod arrays

Yuanping Sun, Yong-Hoon Cho, Hwa-Mok Kim, and Tae Won Kang

Appl. Phys. Lett. 87, 093115 (2005); http://dx.doi.org/10.1063/1.2037203 (3 pages) | Cited 24 times

Online Publication Date: 25 August 2005

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We report on the optical properties of blue-light-emitting, dislocation-free InGaN/GaN multiple quantum well (MQW) nanorod arrays (NRAs) with high brightness and high efficiency. The InGaN MQW NRAs were grown by metal-organic hydride vapor phase epitaxy and the optical properties were investigated in detail by photoluminescence (PL), PL excitation (PLE) and time resolved PL. We observed a large Stokes-like shift between InGaN PL emission and PLE absorption edge due to the influence of built-in internal electrical field, reflecting the coherent growth of MQW along the NRA growth direction. From the temperature-dependent PL, we extracted a PL intensity ratio at 300 to 15 K of ∼ 55.4% and large thermal activation energy of ∼ 171 meV from the InGaN/GaN MQW NRAs. Time-resolved PL results showed almost the same decay time of the InGaN emission at 20 and 300 K. From the results, the optical properties are dominated by the radiative recombination process and are insensitive to temperature due to large thermal activation energy, indicating that carriers in InGaN wells are well confined by the GaN barriers without an influence of other non-radiative processes. Therefore, we conclude that internal quantum efficiency and extraction efficiency of MQW NRAs are significantly enhanced by a drastic suppression of non-radiative centers inside NRAs and a large surface area to active volume area ratio of NRAs, respectively.
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78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
78.67.De Quantum wells
78.47.-p Spectroscopy of solid state dynamics
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Formation of GaAs/AlGaAs and InGaAs/GaAs nanorings by droplet molecular-beam epitaxy

Z. Gong, Z. C. Niu, S. S. Huang, Z. D. Fang, B. Q. Sun, and J. B. Xia

Appl. Phys. Lett. 87, 093116 (2005); http://dx.doi.org/10.1063/1.2037193 (3 pages) | Cited 34 times

Online Publication Date: 26 August 2005

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GaAs/AlGaAs lattice-matched nanorings are formed on GaAs (100) substrates by droplet epitaxy. The crucial step in the formation of nanorings is annealing Ga droplets under As flux for proper time. The observed morphologic evolution of Ga droplets during annealing does not support the hypothesis that As atoms preferentially react with Ga around the periphery of the droplets, but somehow relates to a dewetting process similar to that of unstable films. Photoluminescene (PL) test results confirm the quantum-confinement effect of these GaAs nanorings. Using similar methods, we have fabricated InGaAs/GaAs lattice-mismatched rings.
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81.05.Ea III-V semiconductors
81.07.Bc Nanocrystalline materials
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
61.72.Cc Kinetics of defect formation and annealing
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Cr III-V semiconductors
68.08.Bc Wetting

Self-organization of polyaniline nanorods: Towards achieving a higher conductivity

Debangshu Chaudhuri, Suwarna Datar, Ranjani Viswanatha, D. D. Sarma, and Heinz Amenitsch

Appl. Phys. Lett. 87, 093117 (2005); http://dx.doi.org/10.1063/1.2037195 (3 pages) | Cited 7 times

Online Publication Date: 26 August 2005

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We present a scanning tunnelling microscopy and spectroscopy study of polyaniline nanostructure. Our results show the possibility of achieving a high degree of self-organization in polyaniline nanorods in the undoped as well as the doped states. The spectroscopy results clearly establish a direct correlation between the extent of ordering and the density of states at the Fermi energy of the doped samples. Small angle x-ray scattering experiment helps to understand the formation of such self-organized structures.
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81.16.Dn Self-assembly
73.61.Ph Polymers; organic compounds
61.41.+e Polymers, elastomers, and plastics
71.20.Rv Polymers and organic compounds
78.70.Ck X-ray scattering
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