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17 Nov 2008

Volume 93, Issue 20, Articles (20xxxx)

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Appl. Phys. Lett. 93, 201101 (2008); http://dx.doi.org/10.1063/1.3025818 (3 pages)

W. Dai and C. M. Soukoulis
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Raman characterization of Ge distribution in individual Si1−xGex alloy nanowires

Chiharu Nishimura, Go Imamura, Minoru Fujii, Takahiro Kawashima, Tohru Saitoh, and Shinji Hayashi

Appl. Phys. Lett. 93, 203101 (2008); http://dx.doi.org/10.1063/1.3028027 (3 pages) | Cited 2 times

Online Publication Date: 17 November 2008

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The distribution of Ge composition in an individual Si1−xGex alloy nanowire (SiGeNW) grown by the vapor liquid solid (VLS) process was studied by micro-Raman spectroscopy by changing the measurement position from the catalyst side to the substrate side. The Si–Si mode in Raman spectra were found to split into two peaks and the intensity ratio as well as the wavenumbers of the two peaks depended on the measurement position. The Raman data revealed that SiGeNWs have a core-shell structure with a low-Ge composition core grown by the VLS process covered with a high-Ge composition shell grown by the conformal deposition.
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78.67.Lt Quantum wires
78.30.Hv Other nonmetallic inorganics
68.65.La Quantum wires (patterned in quantum wells)
81.16.-c Methods of micro- and nanofabrication and processing
68.35.Fx Diffusion; interface formation

Mechanical and electrical coupling at metal-insulator-metal nanoscale contacts

Doo-In Kim, Namboodiri Pradeep, Frank W. DelRio, and Robert F. Cook

Appl. Phys. Lett. 93, 203102 (2008); http://dx.doi.org/10.1063/1.3009211 (3 pages) | Cited 4 times

Online Publication Date: 18 November 2008

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Mechanical and electrical coupling at nanoscale metallic contacts was investigated using a conducting-probe atomic force microscope (AFM). The current-voltage responses were non-Ohmic, symmetric about zero bias, with conductance values smaller than the quantum conductance limit, which indicate electron tunneling through an insulating layer. Using a self-consistent contact mechanics model and a parabolic tunneling model for thin insulating layers, we determined the contact area, barrier height, and barrier thickness as a function of applied contact load. The results suggest the presence of two insulating layers: an oxide layer on the AFM tip and an organic contaminant layer on the metallic surface.
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73.40.Rw Metal-insulator-metal structures
73.40.Gk Tunneling
73.40.Ns Metal-nonmetal contacts

Improving light output power of InGaN-based light emitting diodes with pattern-nanoporous p-type GaN:Mg surfaces

Chung Chieh Yang, Chia Feng Lin, Chun Min Lin, Cheng Chien Chang, Kuei Ting Chen, Jui Fen Chien, and Chung Ying Chang

Appl. Phys. Lett. 93, 203103 (2008); http://dx.doi.org/10.1063/1.3027068 (3 pages) | Cited 14 times

Online Publication Date: 18 November 2008

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InGaN-based light emitting diodes (LEDs) with a top pattern-nanoporous p-type GaN:Mg surface were fabricated by using a photoelectrochemical (PEC) process. The peak wavelengths of electroluminescence (EL) and operating voltages were measured as 461.2 nm (3.1 V), 459.6 nm (9.2 V), and 460.1 nm (3.3 V) for conventional, nanoporous, and pattern-nanoporous LEDs using 20 mA operation current. The EL spectrum of the nanoporous LED had a larger blueshift phenomenon as a result of a partial compression strain release in the InGaN active layer through the formation of a top nanoporous surface. The light output power had 12.1% and 26.4% enhancements for the nanoporous and the pattern-nanoporous LEDs compared with conventional LEDs. The larger operating voltage of the nanoporous LED was due to the non-ohmic contact on the PEC treated p-type GaN:Mg surface. By using a pattern-nanoporous p-type GaN:Mg structure, the operating voltage of the pattern-nanoporous LED was reduced to 3.3 V. A lower compression strain in the InGaN active layer and a higher light extraction efficiency at the top nanoporous surface were observed in pattern-nanoporous LEDs for higher efficiency nitride-based LED applications.
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85.60.Jb Light-emitting devices
61.43.Gt Powders, porous materials
82.45.Yz Nanostructured materials in electrochemistry
82.50.-m Photochemistry

Thickness inhomogenities in the organometallic chemical vapor deposition of GaP

X. Liu and D. E. Aspnes

Appl. Phys. Lett. 93, 203104 (2008); http://dx.doi.org/10.1063/1.3029742 (3 pages) | Cited 3 times

Online Publication Date: 18 November 2008

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We analyze exponential lateral-thickness variations observed in the growth of GaP on (001) GaAs, thermally generated SiO2, (001) Si, and nanoscopically roughened Si surfaces by organometallic chemical vapor deposition, using as a reference the polycrystalline GaP deposited on the Mo susceptor surrounding the 2 in. wafers. We find these variations to be due to differences in the chemical reactivities of the various surfaces toward the generation of a precursor, probably a H–P = Ga–CH3 dimer adduct, by heterogeneous catalysis followed by desorption and diffusion through the gas phase.
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68.55.ag Semiconductors
68.55.jd Thickness
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.43.Nr Desorption kinetics
66.30.-h Diffusion in solids

Dynamic-gate operation in nanoelectronic amplifiers due to reduced screening

L. Worschech, D. Hartmann, and A. Forchel

Appl. Phys. Lett. 93, 203105 (2008); http://dx.doi.org/10.1063/1.3035852 (3 pages) | Cited 1 time

Online Publication Date: 19 November 2008

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Semiconductor nanostructures in shape of a Y were patterned lithographically in a two-dimensional electron gas. One of the branches was pinched off, so it can act as a gate for the other branch. Such nanoelectronic amplifiers show switching voltages smaller than the thermal limit and tunable bistability. The authors related their observations to a reduced-screening induced dynamic capacitance, which can exceed the contribution from the geometric capacitance.
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85.35.-p Nanoelectronic devices

Codoping of boron and phosphorus in silicon nanowires synthesized by laser ablation

N. Fukata, M. Mitome, Y. Bando, M. Seoka, S. Matsushita, K. Murakami, J. Chen, and T. Sekiguchi

Appl. Phys. Lett. 93, 203106 (2008); http://dx.doi.org/10.1063/1.3033226 (3 pages) | Cited 11 times

Online Publication Date: 19 November 2008

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Codoping of boron (B) and phosphorus (P) atoms was performed during the synthesis of silicon nanowires (SiNWs) by laser ablation. The observation of a local vibrational mode of B clearly showed B doping in codoped SiNWs, while Fano broadening due to heavy B doping disappeared, indicating compensation by P donors. The electrospin resonance signal of conduction electrons also disappeared due to compensation by B acceptors. These results indicate that codoping of B and P atoms was achieved in SiNWs during laser ablation.
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81.16.-c Methods of micro- and nanofabrication and processing
81.05.Cy Elemental semiconductors
61.72.uf Ge and Si
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)

Magnetoresistance studies on Co/AlOX/Au and Co/AlOX/Ni/Au tunnel structures

R. S. Liu, C. M. Canali, L. Samuelson, and H. Pettersson

Appl. Phys. Lett. 93, 203107 (2008); http://dx.doi.org/10.1063/1.3000614 (3 pages) | Cited 3 times

Online Publication Date: 20 November 2008

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We report on magnetoresistance (MR) studies on Co/AlOX/Au and Co/AlOX/Ni/Au magnetic tunnel junctions. In spite of the fact that the difference between the two samples is merely a 3 nm thick Ni layer, there is a sharp contrast in MR behavior indicating that the electronic structure at the interface between the ferromagnetic electrodes and the insulating barrier dominates the MR signal. The former sample exhibits a clear tunneling anisotropic MR (TAMR), with the characteristic correlation between resistance and current direction, in contrast to the latter sample which displays a conventional tunneling MR (TMR) dominated by the relative orientation between the magnetization directions of the two electrodes. In addition, the TAMR has a much stronger temperature dependence than the TMR, indicating a much faster drop-off of the tunneling density of states anisotropy than the tunneling electron spin polarization with increasing temperature. Finally, we propose a possible simple way to distinguish TAMR from normal TMR by measuring the resistance of the device at different angles of the external magnetic field.
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75.47.-m Magnetotransport phenomena; materials for magnetotransport
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.50.Cc Other ferromagnetic metals and alloys
72.25.-b Spin polarized transport
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Doping of Ge–SixGe1−x core-shell nanowires using low energy ion implantation

Junghyo Nah, K. Varahramyan, E.-S. Liu, S. K. Banerjee, and E. Tutuc

Appl. Phys. Lett. 93, 203108 (2008); http://dx.doi.org/10.1063/1.3013335 (3 pages) | Cited 8 times

Online Publication Date: 20 November 2008

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We investigate the doping of germanium (Ge)–silicon germanium (SixGe1−x) core-shell nanowires (NWs) by low energy (3 keV) boron (B) ion implantation. Ge–SixGe1−x core-shell NWs were implanted with B atoms at different doses from 1×1014 to 1×1015 cm−2, and subsequently annealed for dopant activation. Using four-point, gate-dependent resistance measurements, we determine the resistivity, doping levels, and contacts resistance of the B-doped Ge–SixGe1−x NWs. Our findings show that depending on the implantation dose, the doping level of B-doped NWs ranges from 1×1018 to 2×1020 cm−3.
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61.72.up Other materials
61.72.sd Impurity concentration
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
73.63.Nm Quantum wires
73.40.Cg Contact resistance, contact potential

Integration of vertical InAs nanowire arrays on insulator-on-silicon for electrical isolation

Shadi A. Dayeh, Peng Chen, Yi Jing, Edward T. Yu, S. S. Lau, and Deli Wang

Appl. Phys. Lett. 93, 203109 (2008); http://dx.doi.org/10.1063/1.3013566 (3 pages) | Cited 6 times

Online Publication Date: 20 November 2008

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Vertical and electrically isolated InAs nanowires (NWs) are integrated with Si in a technique that bypasses structural defects and transport barriers at the Si–III–V NW interface. Smart-cut® technique is used to transfer a thin InAs layer onto SiO2/Si and is subsequently used for ordered organometallic vapor phase epitaxy of InAs NWs. The InAs layer in the regions between the InAs NWs is etched resulting in ordered, vertical, and electrically isolated InAs NW arrays. This transfer and fabrication technique enables heteroepitaxy of three dimensional III–V structures on Si and allows the realization of vertical devices with unprecedented control over their architectures.
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81.05.Ea III-V semiconductors
81.07.-b Nanoscale materials and structures: fabrication and characterization
73.63.Nm Quantum wires
81.16.-c Methods of micro- and nanofabrication and processing
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)

GaAs/AlGaAs quantum dot laser fabricated on GaAs (311)A substrate by droplet epitaxy

T. Mano, T. Kuroda, K. Mitsuishi, Y. Nakayama, T. Noda, and K. Sakoda

Appl. Phys. Lett. 93, 203110 (2008); http://dx.doi.org/10.1063/1.3026174 (3 pages) | Cited 10 times

Online Publication Date: 20 November 2008

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We have demonstrated photopumped laser action of self-assembled GaAs/AlGaAs quantum dots (QDs) grown on GaAs (311)A substrate by droplet epitaxy. Due to the short migration distance of Ga adatoms across the (311)A surface, high-density QDs were created with high uniformity. The QDs exhibited a narrow spectral band of intense photoluminescence from the QD ensemble, reflecting their small size distribution and high quality. Using the QDs on the (311)A surface as an active laser medium, we observed multimodal stimulated emissions at temperatures of up to 300 K.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Electrical characterization of gold island films: A route to control of nanoparticle deposition

A. Lassesson, S. A. Brown, J. van Lith, and M. Schulze

Appl. Phys. Lett. 93, 203111 (2008); http://dx.doi.org/10.1063/1.3027463 (3 pages) | Cited 1 time

Online Publication Date: 20 November 2008

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The evaporation of gold onto SiN or SiOx leads to the formation of nanoscale islands separated by tunneling barriers. As the deposited thickness of Au increases, there is a linear increase in island size and an exponential decrease in film resistance, resulting from coalescence of neighboring islands. This understanding of film morphology allows the construction of a simple model of film resistivity. The model also accounts for the exponential decrease of resistance observed during deposition of Pd nanoparticles onto Au island films. The results suggest the use of discontinuous Au island films as a sensitive way of controlling nanoparticle deposition.
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73.61.At Metal and metallic alloys
68.55.at Other materials
68.55.J- Morphology of films
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.07.Wx Nanopowders

Influence of strong electric fields on the growth, modification, and destruction of thin oxide films at the titanium surface

S. A. Kovalevskii, F. I. Dalidchik, M. V. Grishin, and A. K. Gatin

Appl. Phys. Lett. 93, 203112 (2008); http://dx.doi.org/10.1063/1.3028026 (3 pages) | Cited 4 times

Online Publication Date: 20 November 2008

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The influence of strong electric fields ( ∼ 107 V/cm) on the growth, modification, and destruction of thin oxide films at the metal surfaces are studied for a titanium tip of a scanning tunneling microscope (STM) oxidized in oxygen at different tip potentials. The spectroscopic measurements show that, in sufficiently strong fields (V ≤ −7.5 V) preventing migration of oxygen ions from the surface to bulk, the dielectric oxide films can form even for exposures of ∼ 10 L. At voltages V>−7.5 V, i.e., at lower fields, as well as for the opposite polarity, the conducting oxide films containing predominantly Ti+2 and Ti+3 ions are formed at the tip for the same exposures. The possibility of structural modification in the electric field of STM is demonstrated for titanium nanooxides.
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68.55.aj Insulators
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
77.55.-g Dielectric thin films
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
68.43.Mn Adsorption kinetics
61.46.-w Structure of nanoscale materials

Origin of unusual rapid oxidation process for ultrathin oxidation (<2 nm) of silicon

H. Cui, C. X. Wang, G. W. Yang, and D. Jiang

Appl. Phys. Lett. 93, 203113 (2008); http://dx.doi.org/10.1063/1.3030985 (3 pages) | Cited 2 times

Online Publication Date: 20 November 2008

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We propose a kinetic model to reveal the origin of unusual rapid oxidation behavior for the ultrathin (<2 nm) oxidation of silicon demonstrated by [ Enta et al., Appl. Phys. Lett. 92, 012110 (2008) ] by considering the space-charge drift of O ions and the intrinsic concentration gradient diffusion of residual O ions that dominate the initial rapid oxidation process following the slow oxidation state, respectively. Importantly, the present model well describes the whole experiment data with different temperatures and pressures and diffusion activation energies for two regimes attained by our model, which coincides with the reported experiment data and theory calculation.
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81.65.Mq Oxidation
77.22.Jp Dielectric breakdown and space-charge effects
68.35.Fx Diffusion; interface formation

Reconfiguration of microring resonators by liquid adhesion

C. A. Barrios, M. Holgado, O. Guarneros, K. B. Gylfason, B. Sánchez, R. Casquel, and H. Sohlström

Appl. Phys. Lett. 93, 203114 (2008); http://dx.doi.org/10.1063/1.3033528 (3 pages) | Cited 2 times

Online Publication Date: 20 November 2008

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We present a procedure to reconfigure microring resonators based on liquid surface adhesion. Droplets of organic solvents were deposited over Si3N4/SiO2 strip- and slot-waveguide ring resonators, and the transmission spectra were measured several hours after the evaporation of the droplets at room temperature. Our measurements show that the optical properties of the microrings are significantly modified by liquid adhered to the waveguides, persisting even 96 h after bulk evaporation. Liquid-solid interfacial forces slow down liquid evaporation at the nanoscale, enabling permanent photonic configurations. Rewriteability is achieved by removing the adhered liquid with heat.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
84.40.Az Waveguides, transmission lines, striplines

Quantitative scanning thermal microscopy using double scan technique

Kyeongtae Kim, Jaehoon Chung, Jongbo Won, Ohmyoung Kwon, Joon Sik Lee, Seung Ho Park, and Young Ki Choi

Appl. Phys. Lett. 93, 203115 (2008); http://dx.doi.org/10.1063/1.3033545 (3 pages) | Cited 9 times

Online Publication Date: 20 November 2008

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Although scanning thermal microscope has shown the highest spatial resolution in local temperature and thermophysical property measurement, its usefulness has been severely limited due to difficulties in quantitative measurement. We propose a double scan technique that measures temperature only from the heat transfer through the tip-sample contact by the subtraction of the signal due to the heat transfer through the air. A rigorous theoretical model for this technique is derived. The effectiveness of the double scan technique in quantitative temperature measurement is demonstrated experimentally.
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07.20.Dt Thermometers
44.90.+c Other topics in heat transfer (restricted to new topics in section 44)

Controlling multidomain states to enable sub-10-nm magnetic force microscopy

Nissim Amos, Rabee Ikkawi, Robert Haddon, Dmitri Litvinov, and Sakhrat Khizroev

Appl. Phys. Lett. 93, 203116 (2008); http://dx.doi.org/10.1063/1.3036533 (3 pages) | Cited 2 times

Online Publication Date: 21 November 2008

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The letter reports experimental data to demonstrate magnetic force microscopy (MFM) with sub-10-nm resolution under ambient conditions. To achieve this record high resolution, multidomain states in a nanomagnetic probe were controlled. Two demagnetized (multidomain) FePt (45/55) films sputtered on a silicon probe and separated by an 8 nm thick MgO layer were further annealed at temperature of 650 °C to trigger the high-anisotropy L10 phase. A field of above 2 T was applied to drive the probes into a saturated “single-domain” state. The multidomain probes were equivalently compared with state-of-the-art conventional MFM probes via comparative imaging of benchmark magnetic recording disks.
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75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.50.Ss Magnetic recording materials
75.30.Gw Magnetic anisotropy
75.60.Ch Domain walls and domain structure

Temperature dependent photoluminescence from ZnO nanowires and nanosheets on brass substrate

Xiuquan Gu, Kaifu Huo, Guixiang Qian, Jijiang Fu, and Paul K. Chu

Appl. Phys. Lett. 93, 203117 (2008); http://dx.doi.org/10.1063/1.3033823 (3 pages) | Cited 17 times

Online Publication Date: 21 November 2008

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The temperature-dependent optical properties of ZnO nanosheets and nanowires fabricated on conductive brass substrates with different surface-to-volume ratios and morphologies are investigated. The near band edge and deep-level emission mechanisms are studied. The blueshifted donor bound exciton D 0X peak and enhanced deep-level emission in the low-temperature photoluminescence spectrum of the nanosheets are due to the large surface-to-volume ratios. Although D 0X is the dominant emission from both the nanowires and nanosheets at low temperature, the room-temperature spectra are dominated by D 0X (nanowires) and first order longitudinal optical phonon replica of free exciton (nanosheets). The decay in the D 0X peak intensity stems from the thermal dissociation of D 0X to free exciton.
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78.55.Et II-VI semiconductors
78.66.Hf II-VI semiconductors
71.55.Gs II-VI semiconductors
71.35.-y Excitons and related phenomena
81.16.-c Methods of micro- and nanofabrication and processing
73.21.Hb Quantum wires
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