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21 May 2007

Volume 90, Issue 21, Articles (21xxxx)

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Appl. Phys. Lett. 90, 213501 (2007); http://dx.doi.org/10.1063/1.2742294 (3 pages)

Jong-Hyun Ahn, Hoon-Sik Kim, Etienne Menard, Keon Jae Lee, Zhengtao Zhu, Dae-Hyeong Kim, Ralph G. Nuzzo, John A. Rogers, Islamshah Amlani, Vadim Kushner, Shawn G. Thomas, and Terrisa Duenas
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High-performance bottom-contact devices based on an air-stable n-type organic semiconductor N,N-bis (4-trifluoromethoxybenzyl)-1,4,5,8-naphthalene-tetracarboxylic di-imide

Chia-Chun Kao, Pang Lin, Cheng-Chung Lee, Yi-Kai Wang, Jia-Chong Ho, and Yu-Yuan Shen

Appl. Phys. Lett. 90, 212101 (2007); http://dx.doi.org/10.1063/1.2741414 (3 pages) | Cited 15 times

Online Publication Date: 21 May 2007

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N,N-bis(4-trifluoromethoxybenzyl)-1,4,5,8-naphthalene-tetracarboxylic di-imide was applied to organic semiconductors for bottom-contact thin-film transistors. The carrier mobility was 1.6×10−2 cm2 V−1 s−1, the threshold voltage (VT) was +5.5 V, and the on/off current ratio was 8.6×105. Devices without any further surface treatments were tested in an ambient environment. The threshold voltage shift (ΔVT) was verified by gate bias stress measurements. A prototype compound, N,N-bis(4-trifluoromethylbenzyl)naphthalene-1,4,5,8-tetracarboxylic di-imide, shows direct correlation to the bottom-contact device with the varied molecular structure.
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85.30.Tv Field effect devices

Temperature dependence of the pyroelectric coefficient and the spontaneous polarization of AlN

W. S. Yan, R. Zhang, X. Q. Xiu, Z. L. Xie, P. Han, R. L. Jiang, S. L. Gu, Y. Shi, and Y. D. Zheng

Appl. Phys. Lett. 90, 212102 (2007); http://dx.doi.org/10.1063/1.2741600 (3 pages) | Cited 4 times

Online Publication Date: 21 May 2007

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Using the Debye model and existing experimental data of the pyroelectric coefficient of AlN, the temperature dependence of the pyroelectric coefficient as well as the spontaneous polarization of AlN is calculated over a wide temperature range from 0 to 1000 K. The pyroelectric coefficient is proportional to T3 at low temperature and increases acutely from 0 to around 400 K, and then increases gently from 400 to 1000 K. It makes AlN uniquely suitable for application in high temperature pyroelectric sensors. The spontaneous polarization of AlN changes a little from 0 to 1000 K, which indicates that the features of III-nitrides based devices will hardly be degraded by the change of the spontaneous polarization.
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77.70.+a Pyroelectric and electrocaloric effects
77.22.Ej Polarization and depolarization

Bias-dependent contact resistance in rubrene single-crystal field-effect transistors

Anna Molinari, Ignacio Gutiérrez, Iulian N. Hulea, Saverio Russo, and Alberto F. Morpurgo

Appl. Phys. Lett. 90, 212103 (2007); http://dx.doi.org/10.1063/1.2741411 (3 pages) | Cited 15 times

Online Publication Date: 21 May 2007

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The authors report a systematic study of the bias-dependent contact resistance in rubrene single-crystal field-effect transistors with Ni, Co, Cu, Au, and Pt electrodes. They show that the reproducibility in the values of contact resistance strongly depends on the metal, ranging from a factor of 2 for Ni to more than three orders of magnitude for Au. Surprisingly, field-effect transistors with Ni, Co, and Cu contacts exhibit an unexpected reproducibility of the bias-dependent differential conductance of the contacts once this has been normalized to the value measured at zero bias. This reproducibility may enable the study of microscopic carrier injection processes into organic semiconductors.
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85.30.Tv Field effect devices

Atomic layer deposition of insulating nitride interfacial layers for germanium metal oxide semiconductor field effect transistors with high-κ oxide/tungsten nitride gate stacks

Kyoung H. Kim, Roy G. Gordon, Andrew Ritenour, and Dimitri A. Antoniadis

Appl. Phys. Lett. 90, 212104 (2007); http://dx.doi.org/10.1063/1.2741609 (3 pages) | Cited 12 times

Online Publication Date: 21 May 2007

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Atomic layer deposition (ALD) was used to deposit passivating interfacial nitride layers between Ge and high-κ oxides. High-κ oxides on Ge surfaces passivated by ultrathin (1–2 nm) ALD Hf3N4 or AlN layers exhibited well-behaved C-V characteristics with an equivalent oxide thickness as low as 0.8 nm, no significant flatband voltage shifts, and midgap density of interface states values of 2×1012 cm−1 eV−1. Functional n-channel and p-channel Ge field effect transistors with nitride interlayer/high-κ oxide/metal gate stacks are demonstrated.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.-a Thin film structure and morphology
73.20.At Surface states, band structure, electron density of states
85.30.Tv Field effect devices
81.65.Rv Passivation
77.55.-g Dielectric thin films

Spatially indirect excitons in type-II quantum dots

Justino R. Madureira, Márcio P. F. de Godoy, Maria J. S. P. Brasil, and Fernando Iikawa

Appl. Phys. Lett. 90, 212105 (2007); http://dx.doi.org/10.1063/1.2741601 (3 pages) | Cited 11 times

Online Publication Date: 22 May 2007

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The authors have calculated the electronic structure for type-II InP/GaAs quantum dot systems considering a three-dimensional geometry including the wetting layer and the electron-hole interaction, which is the only responsible for the hole localization. Their results for the InP/GaAs structure show the electron confined inside the dot and the hole in the GaAs layer, partially above and below the dot. The authors propose structures with InGaAs or InGaP layers, where the hole wave function forms a ring around the dot walls. The electron-hole overlap, and therefore, the carrier lifetimes are very sensitive to the structural geometry, which is an important tool for device engineering.
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73.21.La Quantum dots
71.35.-y Excitons and related phenomena
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.63.Kv Quantum dots

Fabrication of highly conductive Ti1−xNbxO2 polycrystalline films on glass substrates via crystallization of amorphous phase grown by pulsed laser deposition

T. Hitosugi, A. Ueda, S. Nakao, N. Yamada, Y. Furubayashi, Y. Hirose, T. Shimada, and T. Hasegawa

Appl. Phys. Lett. 90, 212106 (2007); http://dx.doi.org/10.1063/1.2742310 (3 pages) | Cited 44 times

Online Publication Date: 22 May 2007

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Nb-doped anatase TiO2 [Ti0.94Nb0.06O2 (TNO)] films with high electrical conductivity and transparency were fabricated on nonalkali glass using pulsed laser deposition and subsequent annealing in a H2 atmosphere. The amorphous films as deposited on unheated substrates were found to crystallize, forming polycrystalline films at around 350 °C. The films annealed at 500 °C showed resistivity down to 4.6×10−4 Ω cm at room temperature and optical transmittance of 60%–80% in the visible region, which are comparable to those of epitaxial films. These results indicate that TNO films have the potential to be practical transparent conducting oxides that could replace indium tin oxide.
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81.15.Fg Pulsed laser ablation deposition
64.70.K- Solid-solid transitions
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.40.Gh Other heat and thermomechanical treatments
68.55.-a Thin film structure and morphology

Ohmic contacts to p-type GaN based on TaN, TiN, and ZrN

L. F. Voss, L. Stafford, R. Khanna, B. P. Gila, C. R. Abernathy, S. J. Pearton, F. Ren, and I. I. Kravchenko

Appl. Phys. Lett. 90, 212107 (2007); http://dx.doi.org/10.1063/1.2742572 (3 pages) | Cited 6 times

Online Publication Date: 22 May 2007

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Ohmic contacts to p-GaN using a Ni/Au/X/Ti/Au metallization scheme, where X is TaN, TiN, or ZrN, are reported. The dependence of the contact properties on annealing temperature (25–1000 °C) in N2 is examined. For annealing temperatures greater than 500 °C, the contacts display Ohmic characteristics and reach a minimum of about 2×10−4 Ω cm2 after annealing at 700 °C for 60 s in a N2 ambient. The specific contact resistance is stable on annealing up to at least 1000 °C. However, at high temperatures the morphology of the contacts are very rough and there is a large degree of intermixing between the metallic layers. The thermal stability of these contacts are superior as compared to conventional Ni/Au, which display poor characteristics at annealing temperatures greater than 500 °C.
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73.40.Ns Metal-nonmetal contacts
73.40.Cg Contact resistance, contact potential
61.72.Cc Kinetics of defect formation and annealing
85.40.Ls Metallization, contacts, interconnects; device isolation

Using double layer CoSi2 nanocrystals to improve the memory effects of nonvolatile memory devices

F. M. Yang, T. C. Chang, P. T. Liu, P. H. Yeh, U. S. Chen, Y. C. Yu, J. Y. Lin, S. M. Sze, and J. C. Lou

Appl. Phys. Lett. 90, 212108 (2007); http://dx.doi.org/10.1063/1.2742573 (3 pages) | Cited 15 times

Online Publication Date: 22 May 2007

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The nonvolatile memory device with multilayer nanocrystals has advantages such as the memory effects can be increased by the increasing density of the nanocrystals and the whole retention characteristic can be improved. There are much more electrons that can be stored in the double layer than single layer nanocrystal memory device. The double layer CoSi2 nanocrystals have better retention characteristic than the single layer. The good retention characteristic of the double layer device is due to the Coulomb-blockage effects on the top layer nanocrystals from the bottom layer nanocrystals. So, the memory effects of the nonvolatile memory device can be improved by using the double layer nanocrystals.
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84.30.Sk Pulse and digital circuits
85.35.Ds Quantum interference devices

Dislocation reduction in GaN with multiple MgxNy/GaN buffer layers by metal organic chemical vapor deposition

C. J. Tun, C. H. Kuo, Y. K. Fu, C. W. Kuo, C. J. Pan, and G. C. Chi

Appl. Phys. Lett. 90, 212109 (2007); http://dx.doi.org/10.1063/1.2742590 (3 pages) | Cited 6 times

Online Publication Date: 22 May 2007

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Unintentionally doped GaN epitaxial layers with a conventional single low temperature (LT) GaN buffer layer and with multiple MgxNy/GaN buffer layers were grown on sapphire substrates by metal organic chemical vapor deposition. The multiple MgxNy/GaN buffer layers exhibit a low nuclei density, increasing the volume of defect-free regions and reducing the dislocations associated with the grain boundaries. Therefore, the GaN with multiple MgxNy/GaN buffer layers reveals an asymmetrical reflection (102) with a small full width at half maximum, and a higher mobility, lower background concentration, and lower etching pit density than the GaN with the LT GaN buffer layer.
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81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
61.72.Mm Grain and twin boundaries
81.65.Cf Surface cleaning, etching, patterning

Scanning near-field electron beam induced current microscopy: Application to III-V heterostructures and quantum dots

M. Troyon and K. Smaali

Appl. Phys. Lett. 90, 212110 (2007); http://dx.doi.org/10.1063/1.2742638 (3 pages) | Cited 8 times

Online Publication Date: 22 May 2007

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High resolution electron beam induced current (EBIC) analyses were carried out on InAs quantum dots (QDs) grown on GaAs substrate, in a scanning electron microscope (SEM) and a conducting atomic force microscope (C-AFM) hybrid system. This scanning near-field EBIC microscope allows one to image a sample conventionally by SEM, to investigate by AFM the local topography and to simultaneously perform EBIC imaging. The EBIC capabilities of this combined instrument are evidenced by imaging plastic slip lines created by nanoindentation, and its performance in resolution is demonstrated to be of the order of 20 nm by imaging the induced current flowing through the InAs/GaAs QDs and the ringlike structures that surround them. The influence of the presence of QDs on the minority carrier diffusion length is also investigated. It is shown that the presence of QDs close to the surface sample increases the diffusion length.
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68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
73.63.Kv Quantum dots
68.37.Ps Atomic force microscopy (AFM)
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.47.Fg Semiconductor surfaces
68.35.Gy Mechanical properties; surface strains

Epitaxial behavior and transport properties of PrBaCo2O5 thin films on (001) SrTiO3

Z. Yuan, J. Liu, C. L. Chen, C. H. Wang, X. G. Luo, X. H. Chen, G. T. Kim, D. X. Huang, S. S. Wang, A. J. Jacobson, and W. Donner

Appl. Phys. Lett. 90, 212111 (2007); http://dx.doi.org/10.1063/1.2741407 (3 pages) | Cited 7 times

Online Publication Date: 24 May 2007

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Highly conductive PrBaCo2O5.5+δ thin films were grown on (001) SrTiO3 by pulsed laser deposition. Microstructural studies from synchrotron x-ray diffraction and transmission electron microscopy reveal that the as-grown PrBaCo2O5.5+δ films are a-axis oriented with excellent single crystallinity. Two domain structures with c axes in the plane directions parallel to the substrate surface were found with an atomic sharp interface. Transport property and isothermal magnetoresistance measurements for both as-grown and post-annealing were done to understand the physical properties of the films. An abnormal positive magnetoresistance effect was observed in the postannealed films, which suggests that the spins in the system are polarized easier.
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68.55.-a Thin film structure and morphology
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
81.15.Fg Pulsed laser ablation deposition
68.37.Lp Transmission electron microscopy (TEM)
61.72.Cc Kinetics of defect formation and annealing

Electronic properties of organic semiconductor blends: Ambipolar mixtures of phthalocyanine and fullerene

Andreas Opitz, Markus Bronner, Wolfgang Brütting, Marcel Himmerlich, Juergen A. Schaefer, and Stefan Krischok

Appl. Phys. Lett. 90, 212112 (2007); http://dx.doi.org/10.1063/1.2742640 (3 pages) | Cited 15 times

Online Publication Date: 24 May 2007

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Mixtures of electron and hole transporting organic materials are widely used for ambipolar organic field-effect transistors and photovoltaic cells. One particular material system used in both device types is a blend of n-conducting fullerene and p-conducting copper-phthalocyanine. The electronic properties of these blends were analyzed by x-ray and ultraviolet photoelectron spectroscopy in dependence on the mixing ratio. The energies of the highest occupied molecular orbitals, the core levels, and the vacuum level are found to vary linearly with the mixing ratio. This energy shift is related to a common work function in the molecular blends.
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73.21.Ac Multilayers
73.20.At Surface states, band structure, electron density of states
73.30.+y Surface double layers, Schottky barriers, and work functions
79.60.Bm Clean metal, semiconductor, and insulator surfaces
85.30.Tv Field effect devices
85.60.-q Optoelectronic devices

Metamorphic InAsyP1−y (y = 0.30–0.75) and AlδIn1−δAsyP1−y buffer layers on InP substrates

Steven S. Bui, Henry P. Lee, and Kin Man Yu

Appl. Phys. Lett. 90, 212113 (2007); http://dx.doi.org/10.1063/1.2742649 (3 pages) | Cited 1 time

Online Publication Date: 24 May 2007

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The authors report the growth and characterization of InAsyP1−y and AlδIn1−δAsyP1−y buffer layers on InP by metal-organic vapor phase epitaxy. Under optimized growth conditions, they achieved sheet resistances of 2.8×105 and 4.8×104 Ω/sq for single layer InAs0.44P0.56 (0.5 μm) and step-graded InAs0.75P0.25/InAs0.42P0.58 (0.075/0.5 μm) layers, respectively. A bowing parameter for InAsyP1−y of −0.22 eV is found based on photoreflectance measurement. When 0.5 μm thick Al0.11In0.89As.62P.38 is grown, they obtain sheet resistance and sheet carrier concentration of 7.76×105 Ω/sq and 7.92×109 cm−2, respectively. This opens interesting possibilities for realizing high-performance metamorphic field-effect transistors based on strained InAs or InAsyP1−y (0.5<y<0.75) channel and AlInAsP buffer.
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81.05.Ea III-V semiconductors
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
73.61.Ey III-V semiconductors
78.66.Fd III-V semiconductors
68.35.B- Structure of clean surfaces (and surface reconstruction)

High mobility bottom gate InGaZnO thin film transistors with SiOx etch stopper

Minkyu Kim, Jong Han Jeong, Hun Jung Lee, Tae Kyung Ahn, Hyun Soo Shin, Jin-Seong Park, Jae Kyeong Jeong, Yeon-Gon Mo, and Hye Dong Kim

Appl. Phys. Lett. 90, 212114 (2007); http://dx.doi.org/10.1063/1.2742790 (3 pages) | Cited 128 times

Online Publication Date: 24 May 2007

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The authors report on the fabrication of thin film transistors (TFTs), which use an amorphous indium gallium zinc oxide (a-IGZO) channel, by rf sputtering at room temperature and for which the channel length and width are patterned by photolithography and dry etching. To prevent plasma damage to the active channel, a 100-nm-thick SiOx layer deposited by plasma enhanced chemical vapor deposition was adopted as an etch stopper structure. The a-IGZO TFT (W/L = 10 μm/50 μm) fabricated on glass exhibited a high field-effect mobility of 35.8 cm2/V s, a subthreshold gate swing value of 0.59 V/decade, a thrseshold voltage of 5.9 V, and an Ion/off ratio of 4.9×106, which is acceptable for use as the switching transistor of an active-matrix TFT backplane.
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85.30.Tv Field effect devices
81.16.Nd Micro- and nanolithography
81.15.Cd Deposition by sputtering

Efficient terahertz emission from ballistic transport enhanced n-i-p-n-i-p superlattice photomixers

S. Preu, F. H. Renner, S. Malzer, G. H. Döhler, L. J. Wang, M. Hanson, A. C. Gossard, T. L. J. Wilkinson, and E. R. Brown

Appl. Phys. Lett. 90, 212115 (2007); http://dx.doi.org/10.1063/1.2743400 (3 pages) | Cited 11 times

Online Publication Date: 25 May 2007

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The authors report on photomixing terahertz sources that overcome the transit time versus RC-time trade-off and allow for independent optimization of both of them, using a n-i-p-n-i-p superlattice. Furthermore, they take advantage of ballistic transport for reduced transit times. Apart from more favorable material parameters, In(Al)GaAs photomixers benefit from the advanced telecommunication laser technology around 1.55 μm as compared to GaAs. In such devices, a terahertz-power output of 1 μW has been achieved at 0.4 THz at a photocurrent of 3.8 mA. A comparison between corresponding GaAs- and InGaAs-based n-i-p-n-i-p photomixers reveals an improvement of performance by at least an order of magnitude for the latter one.
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84.30.Qi Modulators and demodulators; discriminators, comparators, mixers, limiters, and compressors
84.40.-x Radiowave and microwave (including millimeter wave) technology
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
42.62.-b Laser applications
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