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23 Jul 2007

Volume 91, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 043103 (2007); http://dx.doi.org/10.1063/1.2760191 (3 pages)

M. Hanke, Yu. I. Mazur, E. Marega, Jr., Z. Y. AbuWaar, G. J. Salamo, P. Schäfer, and M. Schmidbauer
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Energy gaps in zero-dimensional graphene nanoribbons

Philip Shemella, Yiming Zhang, Mitch Mailman, Pulickel M. Ajayan, and Saroj K. Nayak

Appl. Phys. Lett. 91, 042101 (2007); http://dx.doi.org/10.1063/1.2761531 (3 pages) | Cited 36 times

Online Publication Date: 25 July 2007

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The finite size effects on the electronic structure of graphene ribbons are studied using first principles density functional techniques. The energy gap [difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)] dependence for finite width and length is computed for both armchair and zigzag ribbons and compared to their one-dimensional (infinite length) cases. The results suggest, in addition to quantum confinement along the width of the ribbon, an additional finite size effect emerges along the length of ribbons only for metallic armchair ribbons. The origin of additional quantum confinement in these structures is analyzed based on the energy states near the Fermi energy: both HOMO and LUMO energy levels for metallic armchair ribbons are delocalized entirely on the ribbons while for nonmetallic ribbons, these states are localized at the edges only. The results are discussed in light of effect of passivation on the electronic properties of graphenes and their impact on nanoelectronic devices based on graphenes.
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71.20.Ps Other inorganic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
73.22.-f Electronic structure of nanoscale materials and related systems

Effect of interfacial oxynitride layer on the band alignment and thermal stability of LaAlO3 films on SiGe

Y. Y. Mi, S. J. Wang, J. W. Chai, H. L. Seng, J. S. Pan, Y. L. Foo, C. H. A. Huan, and C. K. Ong

Appl. Phys. Lett. 91, 042102 (2007); http://dx.doi.org/10.1063/1.2762277 (3 pages) | Cited 3 times

Online Publication Date: 25 July 2007

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The effect of interfacial oxynitride layer on the band alignment and thermal stability of amorphous LaAlO3/Si0.75Ge0.25 heterojunction has been investigated. The presence of interfacial oxynitride layer shifts the band alignment due to the modification of interfacial dipole. During the thermal annealing, it was found that the interfacial electronic structures were altered, and the valence-band maximum of LaAlO3 films shifted to the lower energy due to the diffusion of nitrogen species from the interfaces into LaAlO3 films at high annealing temperature.
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68.60.Dv Thermal stability; thermal effects
73.20.At Surface states, band structure, electron density of states
79.60.Jv Interfaces; heterostructures; nanostructures

Detection of field-induced single-acceptor ionization in Si by single-hole-tunneling transistor

Zainal A. Burhanudin, Ratno Nuryadi, and Michiharu Tabe

Appl. Phys. Lett. 91, 042103 (2007); http://dx.doi.org/10.1063/1.2762278 (3 pages) | Cited 4 times

Online Publication Date: 25 July 2007

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The authors have detected the ionization of single-acceptors in the underlying p on p+ substrate of a silicon-on-insulator (SOI) wafer using a single-hole-tunneling (SHT) transistor fabricated in the top Si layer of the SOI at low temperatures. It was found that freeze-out boron atoms in the substrate are sequentially ionized from near the buried SiO2/p-Si substrate interface to deeper positions by application of a vertical electric field, creating steplike features in the time-dependent SHT current.
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71.55.Cn Elemental semiconductors
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)

Mobility enhancement in strained p-InGaSb quantum wells

Brian R. Bennett, Mario G. Ancona, J. Brad Boos, and Benjamin V. Shanabrook

Appl. Phys. Lett. 91, 042104 (2007); http://dx.doi.org/10.1063/1.2762279 (3 pages) | Cited 16 times

Online Publication Date: 25 July 2007

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Quantum wells of InGaSb clad by AlGaSb were grown by molecular beam epitaxy. The InGaSb is in compressive strain, resulting in a splitting of the heavy- and light-hole valence bands and an enhancement of the mobility. The mobility was found to increase with increasing InSb mole fraction for values of strain up to 2%. Room-temperature mobilities as high as 1500 cm2/V s were reached for 7.5 nm channels of In0.40Ga0.60Sb. These results are an important step toward the goal of high-performance p-channel field-effect transistors for complementary circuits operating at extremely low power.
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73.63.Hs Quantum wells

W2B and CrB2 diffusion barriers for Ni/Au contacts to p-GaN

L. F. Voss, L. Stafford, J. S. Wright, S. J. Pearton, F. Ren, and I. I. Kravchenko

Appl. Phys. Lett. 91, 042105 (2007); http://dx.doi.org/10.1063/1.2762280 (3 pages) | Cited 2 times

Online Publication Date: 25 July 2007

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Ohmic contacts to p-type GaN were fabricated using W2B and CrB2 as diffusion barriers for a traditional Ni/Au contact scheme. The annealing temperature dependence (25–1000 °C) of contact resistance and the thermal aging characteristics at 200 °C were examined. A minimum contact resistance of ∼ 2×10−4 Ω cm2 was achieved after annealing at 700 °C for 60 s. These contacts also showed excellent stability as a function of aging at 200 °C. Auger electron depth profiles reveal a large degree of intermixing at the GaN interface between Ni and Au.
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66.30.Ny Chemical interdiffusion; diffusion barriers
68.35.Fx Diffusion; interface formation

Fully epitaxial (Zn,Co)O/ZnO/(Zn,Co)O junction and its tunnel magnetoresistance

C. Song, X. J. Liu, F. Zeng, and F. Pan

Appl. Phys. Lett. 91, 042106 (2007); http://dx.doi.org/10.1063/1.2762297 (3 pages) | Cited 18 times

Online Publication Date: 25 July 2007

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The authors report tunnel magnetoresistance (TMR) and its bias dependence in fully epitaxial (Zn,Co)O/ZnO/(Zn,Co)O magnetic tunnel junctions. A positive TMR of 20.8% is obtained at 4 K, which can resist up to room temperature with the TMR ratio of 0.35% at 2 T, due to improved crystallinity of barriers and electrode/barrier interfaces. The decay of TMR with bias up to 2 V is significantly small leading to V1/2, for which half of the TMR remains, well over 2 V, shedding promising light on solving readout problems in gigabit-scale magnetoresistive random access memory.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.Pq Other materials
75.50.Pp Magnetic semiconductors

Bimetallic oxide nanoparticles CoxMoyO as charge trapping layer for nonvolatile memory device applications

Chien-Wei Liu, Chin-Lung Cheng, Sung-Wei Huang, Jin-Tsong Jeng, Shiuan-Hua Shiau, and Bau-Tong Dai

Appl. Phys. Lett. 91, 042107 (2007); http://dx.doi.org/10.1063/1.2763962 (3 pages) | Cited 2 times

Online Publication Date: 26 July 2007

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The reduced CoxMoyO bimetallic oxide nanoparticles (BONs) embedded in the hafnium oxynitride high-k dielectric have been developed by means of the chemical vapor deposition method. Capacitance-voltage (C-V) measurements estimate that a charge trap states density of 1.1×1012 cm−2 and a flatband voltage shift of 700 mV were achieved during the C-V hysteresis sweep at ±5 V. Scanning electron microscopy image displays that the CoxMoyO BONs with a diameter of ∼ 4–20 nm and a surface density of ∼ 1×1011 cm−2 were obtained. The writing characteristics measurements illustrate that the memory effect is mainly due to the holes trapping.
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81.16.-c Methods of micro- and nanofabrication and processing
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
73.20.At Surface states, band structure, electron density of states
77.80.Dj Domain structure; hysteresis
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
85.40.Sz Deposition technology
77.55.-g Dielectric thin films

Aligned carbon nanotubes for through-wafer interconnects

Ting Xu, Zhihong Wang, Jianmin Miao, Xiaofeng Chen, and Cher Ming Tan

Appl. Phys. Lett. 91, 042108 (2007); http://dx.doi.org/10.1063/1.2759989 (3 pages) | Cited 18 times

Online Publication Date: 26 July 2007

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Through-wafer interconnects by aligned carbon nanotube for three-dimensional stack integrated chip packaging applications have been reported in this letter. Two silicon wafers are bonded together by tetra-ethyl-ortho-silicate. The top wafer (100 μm thick) with patterned through-holes allows carbon nanotubes to grow vertically from the catalyst layer (Fe) on the bottom wafer. By using thermal chemical vapor deposition technique, the authors have demonstrated the capability of growing aligned carbon nanotube bundles with an average length of 140 μm and a diameter of 30 μm from the through holes. The resistivity of the bundles is measured to be 0.0097 Ω cm by using a nanomanipulator.
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47.55.D- Drops and bubbles
47.11.Df Finite volume methods
47.27.te Turbulent convective heat transfer

Improvement of Pt Schottky contacts to n-type ZnO by KrF excimer laser irradiation

Min-Suk Oh, Dae-Kue Hwang, Jae-Hong Lim, Yong-Seok Choi, and Seong-Ju Park

Appl. Phys. Lett. 91, 042109 (2007); http://dx.doi.org/10.1063/1.2764436 (3 pages) | Cited 15 times

Online Publication Date: 26 July 2007

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High quality Pt Schottky contact to n-type ZnO was formed using KrF excimer laser. A pulsed laser irradiation of n-type ZnO in O2 pressure of 0.1 Mtorr, prior to Pt metal deposition, considerably improved the rectifying characteristics. The Schottky barrier heights of 0.73 and 0.85 eV were obtained from the current-voltage (I-V) and capacitance-voltage (C-V) measurements, respectively. The cathodoluminescence and Auger electron spectroscopy results indicated that the improvement in rectifying characteristics can be attributed to a removal of surface carbon and hydrogen contaminants and a reduction of subsurface donorlike point defects by the KrF excimer laser irradiation.
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73.40.Ns Metal-nonmetal contacts
73.30.+y Surface double layers, Schottky barriers, and work functions
78.60.Hk Cathodoluminescence, ionoluminescence
42.62.-b Laser applications

Study of InAs quantum dots in AlGaAs/GaAs heterostructure by ballistic electron emission microscopy/spectroscopy

J. Walachová, J. Zelinka, V. Malina, J. Vaniš, F. Šroubek, J. Pangrác, K. Melichar, and E. Hulicius

Appl. Phys. Lett. 91, 042110 (2007); http://dx.doi.org/10.1063/1.2760134 (3 pages) | Cited 2 times

Online Publication Date: 26 July 2007

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Self-assembled InAs quantum dots in GaAs/GaAlAs structures were examined by ballistic electron emission microscopy/spectroscopy. The studied structures were grown by metal-organic chemical vapor deposition. Quantum dots with an image of elliptical shape were studied. Ballistic current-voltage characteristics through the quantum dot and outside the quantum dot are compared in the voltage range of 0.55–2 V. In the voltage range from 0.55 to 0.8 V examples of ballistic characteristics and their derivatives are given. In a detailed study, measurements with 1 mV step in the energy range from 0.55 to 0.63 V are presented.
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81.07.Ta Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
73.63.Kv Quantum dots
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.23.Ad Ballistic transport

Ni-imprint induced solid-phase crystallization in Si1−xGex (x: 0–1) on insulator

Kaoru Toko, Hiroshi Kanno, Atsushi Kenjo, Taizoh Sadoh, Tanemasa Asano, and Masanobu Miyao

Appl. Phys. Lett. 91, 042111 (2007); http://dx.doi.org/10.1063/1.2764447 (2 pages) | Cited 18 times

Online Publication Date: 27 July 2007

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Position control of solid-phase crystallization in the amorphous Si1−xGex (x: 0–1) films on insulating substrates was investigated by using Ni-imprint technique. Crystal nucleation at the imprinted positions proceeded approximately 2–20 times, depending on Ge fraction, faster than the conventional solid-phase crystallization, which was due to the catalytic effect of Ni. As a result, large SiGe crystal regions ( ∼ 2 μm) were obtained at controlled positions. On the other hand, the growth velocity did not changed, which suggested that grown regions contained few residual Ni atoms.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.A- Nucleation and growth

Polarization-induced valence-band alignments at cation- and anion-polar InN/GaN heterojunctions

Chung-Lin Wu, Hong-Mao Lee, Cheng-Tai Kuo, Shangjr Gwo, and Chia-Hung Hsu

Appl. Phys. Lett. 91, 042112 (2007); http://dx.doi.org/10.1063/1.2764448 (3 pages) | Cited 23 times

Online Publication Date: 27 July 2007

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The authors show that the existence of polarization discontinuities at polar III-nitride heterointerfaces can lead to large core-level shifts of photoelectrons and modification of apparent valence-band offsets. In this letter, large Ga core-level shifts, resulting from the interface dipole fields, have been directly measured by photoelectron spectroscopy on In/Ga-polar (0001)- and N-polar (000 math)-oriented InN/GaN heterojunctions with monolayer abrupt, nearly fully relaxed lattices. Combined with the photoelectron spectroscopic measurements of InN and GaN bulk epilayers, the determined valence-band offsets are 1.04 and 0.54 eV for In/Ga- and N-polar heterojunctions, respectively.
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73.20.At Surface states, band structure, electron density of states
79.60.Jv Interfaces; heterostructures; nanostructures

Organic thin-film transistors using suspended source/drain electrode structure

Yong-Hoon Kim, Sang-Myeon Han, Woocheul Lee, Min-Koo Han, Yong Uk Lee, and Jeong-In Han

Appl. Phys. Lett. 91, 042113 (2007); http://dx.doi.org/10.1063/1.2760142 (3 pages) | Cited 4 times

Online Publication Date: 27 July 2007

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The electrical properties of triisopropylsilyl pentacene organic thin-film transistors such as field-effect mobility, on/off ratio, threshold voltage, and subthreshold slope were markedly improved by employing suspended source/drain (SSD) electrode structure. The SSD structure was fabricated by using Cr/Au double layer where Cr was used as a sacrificial layer. Using the SSD structure, the field-effect mobility in the linear region increases from 0.007 to 0.29 cm2/V s, on/off ratio from 104 to 107, threshold voltage decreases from +9 to −3 V, and subthreshold slope decreases from 4.5 to 0.6 V/decade.
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85.30.Tv Field effect devices

Fast electrical detection of Hg(II) ions with AlGaN/GaN high electron mobility transistors

Hung-Ta Wang, B. S. Kang, T. F. Chancellor, Jr., T. P. Lele, Y. Tseng, F. Ren, S. J. Pearton, W. J. Johnson, P. Rajagopal, J. C. Roberts, E. L. Piner, and K. J. Linthicum

Appl. Phys. Lett. 91, 042114 (2007); http://dx.doi.org/10.1063/1.2764554 (3 pages) | Cited 9 times

Online Publication Date: 27 July 2007

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Bare Au gated and thioglycolic acid functionalized Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect mercury (II) ions. Fast detection of less than 5 s was achieved for thioglycolic acid functionalized sensors. This is the shortest response time ever reported for mercury detection. Thioglycolic acid functionalized Au-gated AlGaN/GaN HEMT based sensors showed 2.5 times larger response than bare Au-gated based sensors. The sensors were able to detect mercury (II) ion concentration as low as 10−7M. The sensors showed an excellent sensing selectivity of more than 100 for detecting mercury ions over sodium or magnesium ions. The dimensions of the active area of the sensor and the entire sensor chip are 50×50 μm2 and 1×5 mm2, respectively. Therefore, portable, fast response, and wireless based heavy metal ion detectors can be realized with AlGaN/GaN HEMT based sensors.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
85.30.Tv Field effect devices

Influence of electron injection on the temporal response of ZnO homojunction photodiodes

O. Lopatiuk-Tirpak, G. Nootz, E. Flitsiyan, L. Chernyak, L. J. Mandalapu, Z. Yang, J. L. Liu, K. Gartsman, and A. Osinsky

Appl. Phys. Lett. 91, 042115 (2007); http://dx.doi.org/10.1063/1.2764559 (3 pages)

Online Publication Date: 27 July 2007

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The effects of solid-state electron injection on the peak amplitude and decay time of photosignal in a ZnO-based homojunction UV photodiode were studied using temporal photoresponse measurements under femtosecond pulses of 355 nm radiation. The injection of about 50 C of charge, carried out by applying forward bias to the junction, resulted in a nearly twofold increase of the peak photoresponse and a corresponding increase of the decay constant. Both observations are shown to be a consequence of electron trapping. The long-term stability of the induced changes is also discussed.
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85.60.Dw Photodiodes; phototransistors; photoresistors
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Degradation of hole injection at the contact between a conducting polymer and a fluorene copolymer

Alexios Papadimitratos, Hon Hang Fong, George G. Malliaras, Aharon Yakimov, and Anil Duggal

Appl. Phys. Lett. 91, 042116 (2007); http://dx.doi.org/10.1063/1.2760170 (3 pages) | Cited 4 times

Online Publication Date: 27 July 2007

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Hole injection was studied in devices that utilize the contact between poly[(ethylenedioxy)thiophene]/poly(styrenesulfonate) (PEDOT:PSS) and poly(9,9-dioctylfluorene-co-N,N’-bis(4-butylphenyl)-N,N’-diphenyl-1,4-phenylenediamine) (PFB). The transient space charge limited current injection technique was used to measure the hole mobility in the PFB layer and to estimate the hole injection efficiency at the contact. Prolonged electrical stressing of the devices did not affect hole mobility in PFB, while the injection efficiency decreased by an order of magnitude. These observations show that degradation occurs at the PEDOT:PSS/PFB contact, rather than the bulk of the PFB.
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73.61.Ph Polymers; organic compounds
72.80.Le Polymers; organic compounds (including organic semiconductors)
73.50.Dn Low-field transport and mobility; piezoresistance

On the electric-dipole contribution to the valence-band offsets in semiconductor-oxide heterostructures

Winfried Mönch

Appl. Phys. Lett. 91, 042117 (2007); http://dx.doi.org/10.1063/1.2760176 (3 pages) | Cited 15 times

Online Publication Date: 27 July 2007

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The continua of interface-induced gap states (IFIGS) determine the band-structure lineup at semiconductor interfaces. Hence, both barrier heights of Schottky contacts and valence-band offsets in semiconductor heterostructures are composed of a zero-charge-transfer and an electric-dipole term. The analysis of experimental barrier heights and valence-band offsets yields IFIGS branch-point energies of 3.51±0.07 eV for SiO2 and 2.98±0.26 eV, 2.27±0.14, and 2.87±0.14 eV for the high-κ oxides Al2O3, HfO2, and ZrO2, respectively. In the oxide heterostructures, the electric-dipole terms come up to 30% of the valence-band offsets and can thus not be neglected.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
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