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1 Oct 2007

Volume 91, Issue 14, Articles (14xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 143101 (2007); http://dx.doi.org/10.1063/1.2789812 (3 pages)

Hyun S. Kim, Hua Qin, and Robert H. Blick
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Dangling-bond defects and hydrogen passivation in germanium

J. R. Weber, A. Janotti, P. Rinke, and C. G. Van de Walle

Appl. Phys. Lett. 91, 142101 (2007); http://dx.doi.org/10.1063/1.2793184 (3 pages) | Cited 46 times

Online Publication Date: 1 October 2007

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The application of germanium in complementary metal-oxide semiconductor technology is hampered by high interface-state densities. Using first-principles calculations, we investigate the effects of dangling bonds (DBs) and their interaction with hydrogen. We find that Ge DBs give rise to electronic levels below the valence-band maximum. They therefore occur exclusively in the negative charge state, explaining why they cannot be observed with electron spin resonance. The associated fixed charge is likely responsible for threshold-voltage shifts and poor performance of n-channel transistors. We also find that passivation of DBs by hydrogen will be ineffective because interstitial hydrogen is also stable exclusively in the negative charge state.
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81.05.Cy Elemental semiconductors
71.55.Cn Elemental semiconductors
71.20.Mq Elemental semiconductors
81.65.Rv Passivation
71.15.-m Methods of electronic structure calculations
76.30.-v Electron paramagnetic resonance and relaxation

The characteristic of strain relaxation on SiGe virtual substrate with thermal annealing

Wu-Ping Huang, Henry H. Cheng, Greg Sun, Re-Fa Lou, J. H. Yeh, and Tzer-Min Shen

Appl. Phys. Lett. 91, 142102 (2007); http://dx.doi.org/10.1063/1.2794016 (3 pages) | Cited 6 times

Online Publication Date: 1 October 2007

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We report the effect of thermal annealing on the strain relaxation of SiGe virtual substrate. The results show that the strain of the SiGe layer relaxes anisotropically with thermal annealing through the formation of the misfit dislocations. The strain field fluctuation persists up to the annealing temperature of 1000 °C, suggesting that the in-plane strain fluctuation could be eliminated by thermal annealing process. In particular, from the analysis of spatially resolved ultraviolet Raman mapping, we found that the normal region relaxes with a faster rate than the crosshatch region as attributed to larger initial strain.
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68.35.Gy Mechanical properties; surface strains
61.72.Cc Kinetics of defect formation and annealing
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)

Characterization of metal-ferroelectric (BiFeO3)-insulator (ZrO2)-silicon capacitors for nonvolatile memory applications

Yu-Wei Chiang and Jenn-Ming Wu

Appl. Phys. Lett. 91, 142103 (2007); http://dx.doi.org/10.1063/1.2794335 (3 pages) | Cited 18 times

Online Publication Date: 1 October 2007

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The electric properties of ferroelectric BiFeO3 (BFO) thin films with ZrO2 insulating buffer on Si substrates fabricated by chemical solution deposition were investigated. ZrO2 demonstrates excellent insulating properties on Si substrates. The metal-ferroelectric-insulator-semiconductor (MFIS) structure exhibits clockwise capacitance-voltage hysteresis loops due to the ferroelectric polarization of BFO. The maximum memory windows are 0.66 and 0.78 V, respectively, for BFO deposited on postannealed ZrO2 and on unannealed ZrO2 insulating layers. The small memory window of the BFO/ZrO2 (postannealed) MFIS is caused by significant interface trap states on the BFO/ZrO2 interface; this is supported by C-V measurements under varying frequency.
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84.32.Tt Capacitors
84.30.Sk Pulse and digital circuits
85.40.Sz Deposition technology

Magnetic field tuning of hot electron resonant capture in a semiconductor device

S. Spasov, G. Allison, A. Patanè, A. Ignatov, L. Eaves, D. K. Maude, M. Hopkinson, and R. Airey

Appl. Phys. Lett. 91, 142104 (2007); http://dx.doi.org/10.1063/1.2794407 (3 pages) | Cited 2 times

Online Publication Date: 1 October 2007

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We study hot-electron capture on resonant N-impurities in the dilute nitride Ga(AsN) alloy under high magnetic fields up to 23 T. We show that when the ratio of electric and magnetic fields reaches a critical value, the trajectory of conduction electrons becomes fully localized in real space; this leads to a negative differential conductance and current instabilities tuneable by magnetic field.
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85.30.Kk Junction diodes

Stability of polymeric thin film transistors for x-ray imaging applications

Christopher R. Newman, Henning Sirringhaus, James C. Blakesley, and Robert Speller

Appl. Phys. Lett. 91, 142105 (2007); http://dx.doi.org/10.1063/1.2785946 (3 pages) | Cited 12 times

Online Publication Date: 3 October 2007

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The effects of x-ray irradiation on the electrical performance of organic thin film transistors (OTFTs) have been examined up to equivalent lifetime doses for medical digital x-ray imaging backplanes. Transistor performance metrics have been assessed before and after exposures up to 500 Gy for bottom-gate, bottom-contact OTFTs of p-channel polytriarylamine and polyfluorene semiconductors. There is no discernible degradation in device performance for either material and no indication of the creation of additional trap states. In situ measurements indicate that x-ray exposure tends to reverse undesirable shifts in the threshold voltage due to prolonged atmospheric exposure and bias stress.
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85.30.Tv Field effect devices

Charge carrier mobility of mixed-layer organic light-emitting diodes

Shun-Wei Liu, Jiun-Haw Lee, Chih-Chien Lee, Chin-Ti Chen, and Juen-Kai Wang

Appl. Phys. Lett. 91, 142106 (2007); http://dx.doi.org/10.1063/1.2787890 (3 pages) | Cited 25 times

Online Publication Date: 3 October 2007

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The authors report the investigation of the charge transport behaviors in mixed thin films of N,N′-diphenyl-N,N′-bis(1-napthyl)-1,1′-biphenyl-4,4′-diamine and tris(8-hydroxyquinoline) aluminum. The extracted electron and hole drift mobility were found to be sensitive to the compositional fraction and interpreted by energy levels, charge mobilities of neat compounds, and microscopic networks within the mixed systems. The carrier conduction characteristics, therefore, were used to illustrate the electrical and optical properties of the organic light emitting devices with a mixed layer and present direct evidences on the role of the mixed layer in these devices.
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85.60.Jb Light-emitting devices

Identification of bulk and surface sulfur impurities in TiO2 by synchrotron x-ray absorption near edge structure

M. F. Smith, Kongthip Setwong, Rungnapa Tongpool, Darin Onkaw, Sutassana Na-phattalung, Sukit Limpijumnong, and Saroj Rujirawat

Appl. Phys. Lett. 91, 142107 (2007); http://dx.doi.org/10.1063/1.2793627 (3 pages) | Cited 4 times

Online Publication Date: 3 October 2007

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Synchrotron x-ray absorption near edge structure (XANES) measurements of Ti and S K edges, combined with first principles simulations, are used to characterize S-doped TiO2 prepared by oxidative annealing of TiS2 at various temperatures. Ti-edge XANES and x-ray powder diffraction data indicate that samples annealed above 300 °C have an anatase TiO2 crystal structure with no trace of TiS2 domains. S-edge XANES data reveal that the local structure seen by S atoms evolves gradually, from TiS2 to a qualitatively different structure, as the annealing temperature is increased from 200 to 500 °C. For samples annealed at 500 °C, the spectrum appears to have features that can be assigned to S on the surface in the form of SO4 and S defects in the bulk (most likely S interstitials) of TiO2.
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78.70.Dm X-ray absorption spectra
61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling
61.66.Fn Inorganic compounds
61.72.J- Point defects and defect clusters

Bridged ZnO nanowires across trenched electrodes

Pu-Xian Gao, Jin Liu, Brent A. Buchine, Benjamin Weintraub, Z. L. Wang, and J. L. Lee

Appl. Phys. Lett. 91, 142108 (2007); http://dx.doi.org/10.1063/1.2794417 (3 pages) | Cited 14 times

Online Publication Date: 3 October 2007

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Using a hydrothermal synthesis approach, large scale and laterally bridged nanowires have been grown across trenched Au/Si, Au/SiO2/Si, and ZnO/Si electrodes. This technique shows a low temperature (80 °C) approach for growing ZnO nanowires on a prepatterned substrate, showing its potential for integrating with silicon based technology. The I-V characteristics of the nanowires have been measured and their nonlinear behavior has been analyzed. It is suggested that the nonlinear behavior might be due to the dominant phonon scattering and the impurity involved grain boundaries in the multinanowire bridges. The bridged nanowire arrays could be useful for fabricating gas, chemical, or biochemical nanosensor arrays.
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68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
81.16.-c Methods of micro- and nanofabrication and processing
73.63.-b Electronic transport in nanoscale materials and structures
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
61.72.Mm Grain and twin boundaries

Multiphoton absorption pumped ultraviolet stimulated emission from ZnO microtubes

C. F. Zhang, Z. W. Dong, K. J. Liu, Y. L. Yan, S. X. Qian, and H. Deng

Appl. Phys. Lett. 91, 142109 (2007); http://dx.doi.org/10.1063/1.2794776 (3 pages) | Cited 9 times

Online Publication Date: 3 October 2007

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We report observation of stimulated emission from ZnO microtubes pumped by multiphoton absorption using intense femtosecond pulses at wavelength around 800 nm. Band edge emission, together with second harmonic generation, was observed and several stimulated emission peaks with linewidths of 0.5–0.7 nm appeared at high excitation fluence. The stimulated emission from the microtubes were attributed to the excitonic effect at intermediate fluence (50–100 mJ/cm2) and electron-hole plasma at high fluence (>100 mJ/cm2), respectively. Possible Fabry-Perot cavity effects were also discussed.
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78.67.Ch Nanotubes
78.45.+h Stimulated emission
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
71.35.-y Excitons and related phenomena

Nanopillar ferromagnetic nanostructure as highly efficient spin injector into semiconductor

S. Bala Kumar, S. G. Tan, M. B. A. Jalil, and J. Guo

Appl. Phys. Lett. 91, 142110 (2007); http://dx.doi.org/10.1063/1.2795341 (3 pages) | Cited 4 times

Online Publication Date: 3 October 2007

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We report the prediction of a high spin injection ratio γ into a semiconductor (SC) contact, by using a ferromagnetic (FM), small-sized, and cylindrical nanopillar as the spin injector (SI). The increase in spin scattering within the FM nanopillar injects spin current into the SC, effectually mitigates the blockage of SI due to conductance mismatch. To minimize the spreading resistance (SR) which arises due to areal discontinuity at FM-SC interface, a thin low resistance metal is inserted at the interface such that SR is contained within it. With the insertion, we obtained γ of as high as 40%, compared to just 3% without insertion, and an even lower value of 0.5% without the nanopillar patterning.
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72.25.Dc Spin polarized transport in semiconductors
75.50.Tt Fine-particle systems; nanocrystalline materials

Electrostatic force microscopy mapping of electrical conductivity of hydrogen-terminated diamond films

A. Volodin, C. Toma, G. Bogdan, W. Deferme, K. Haenen, M. Nesládek, and C. Van Haesendonck

Appl. Phys. Lett. 91, 142111 (2007); http://dx.doi.org/10.1063/1.2795342 (3 pages)

Online Publication Date: 3 October 2007

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A method for mapping the surface conductivity of hydrogen-terminated (H-terminated) diamond on a sub-100 nm scale is presented. The measuring technique relies on electrostatic force microscopy imaging of the voltage distribution of a current-carrying H-terminated diamond film. The uniform linear voltage drop in highly conductive H-terminated diamond surface layers indicates that the layers behave as homogeneous, diffusive conductors with a well-defined value of the sheet resistance. On the other hand, we observe conductive as well as insulating regions that coexist for not perfectly H-terminated diamond surfaces with poor electric conductivity.
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73.61.Cw Elemental semiconductors
73.25.+i Surface conductivity and carrier phenomena

Electronic structure of interfaces between copper-hexadecafluoro-phthalocyanine and 2,5-bis(4-biphenylyl) bithiophene

Yongli Gao, Huanjun Ding, Haibo Wang, and Donghang Yan

Appl. Phys. Lett. 91, 142112 (2007); http://dx.doi.org/10.1063/1.2784944 (3 pages) | Cited 9 times

Online Publication Date: 4 October 2007

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The interfaces formed between copper-hexadecafluoro-phthalocyanine (F16CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T) were examined using photoemission and inverse photoemission spectroscopy. It is observed that in F16CuPc/BP2T the heterojunction is characterized by band bending in both materials, while in BP2T/F16CuPc the band bending is confined in BP2T only. The combination of the band bending and finite Debye lengths provides an explanation to the observed ambipolar behavior of the organic thin film transistors based on such heterojunctions.
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85.30.Pq Bipolar transistors
85.30.Tv Field effect devices

Schottky contact on a ZnO (0001) single crystal with conducting polymer

M. Nakano, A. Tsukazaki, R. Y. Gunji, K. Ueno, A. Ohtomo, T. Fukumura, and M. Kawasaki

Appl. Phys. Lett. 91, 142113 (2007); http://dx.doi.org/10.1063/1.2789697 (3 pages) | Cited 32 times

Online Publication Date: 4 October 2007

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High quality Schottky junctions were fabricated on a ZnO (0001) bulk single crystal by spin coating a commercial conducting polymer, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), as the metal electrodes. The junctions exhibited excellent rectifying behavior with a typical ideality factor of 1.2. Such parameters as Schottky barrier height (ϕb) and built-in potential (Vbi) show negligible variation among junctions. The electron affinity of ZnO derived from ϕb and qVbi values show a slight deviation ( ∼ 0.2 eV), suggesting the existence of spontaneously formed interfacial dipole layer between ZnO (0001) polar surface and anionic PSS molecules.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ei Rectification
73.40.Ns Metal-nonmetal contacts

Photoresponse properties of Al/n-β-FeSi2 Schottky diodes using β-FeSi2 single crystals

Teruhisa Ootsuka, Yasunori Fudamoto, Masato Osamura, Takashi Suemasu, Yunosuke Makita, Yasuhiro Fukuzawa, and Yasuhiko Nakayama

Appl. Phys. Lett. 91, 142114 (2007); http://dx.doi.org/10.1063/1.2789706 (3 pages) | Cited 14 times

Online Publication Date: 4 October 2007

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We have clearly observed photoresponse properties in an Al/n-β-FeSi2 structure using β-FeSi2 single crystals grown by chemical vapor transport. A photocurrent is observed for photons with energies greater than 0.68 eV. It increases sharply with increasing photon energy and attains a maximum at approximately 0.95 eV (1.31 μm). The photocurrent originated from the photoexcited electrons in the Al and the band-to-band photoexcited carriers in the β-FeSi2 located under the Al contact. The photoresponsivity increased upon high-temperature annealing, reaching 58 mA/W at 0.95 eV after annealing at 800 °C for 8 h.
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85.30.Kk Junction diodes

Bias-induced orbital hybridization in diblock co-oligomer diodes

G. C. Hu, J. H. Wei, and S. J. Xie

Appl. Phys. Lett. 91, 142115 (2007); http://dx.doi.org/10.1063/1.2790076 (3 pages) | Cited 8 times

Online Publication Date: 4 October 2007

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We investigate current rectification in diblock co-oligomer diode molecules on the basis of the Su-Schrieffer-Heeger model [ Phys. Rev. B 22, 2099 (1980) ] combined with the nonequilibrium Green’s function formalism. The current rectification observed in experiment [ M. K. Ng et al., J. Am. Chem. Soc 124, 11862 (2002) ] is well explained by the mechanism of bias-induced asymmetric hybridization of molecular orbitals. The positive bias tends to delocalize molecular orbitals through the hybridization, which produces a sharp increase in current at the threshold voltage; while the negative bias enhances the mismatch of energy levels and has no effect on the hybridization and the current.
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85.65.+h Molecular electronic devices
84.47.+w Vacuum tubes

Observation of delocalized transport and low-dimensionality effects in disordered carbon thin films

Somnath Bhattacharyya

Appl. Phys. Lett. 91, 142116 (2007); http://dx.doi.org/10.1063/1.2790844 (3 pages) | Cited 4 times

Online Publication Date: 4 October 2007

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Delocalized transport in a batch of amorphous carbon thin films is shown via a positive slope of the reduced activation energy below a certain temperature and a large value of conductivity minima. A crossover of dimensionality from three-dimensional (3D) to two-dimensional (2D) weak localization below the transition temperature and a signature of electron-electron interactions in 3D can be found in these films, which is significantly different from many other forms of carbon. The dimensionality crossover suggests 2D resonant tunneling between carbon clusters due to the increase of the phase coherence length and a low tunnel effective mass of electrons.
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73.61.Ng Insulators
73.40.Gk Tunneling
73.20.Fz Weak or Anderson localization

Coulomb blockade in a Si channel gated by an Al single-electron transistor

L. Sun, K. R. Brown, and B. E. Kane

Appl. Phys. Lett. 91, 142117 (2007); http://dx.doi.org/10.1063/1.2793712 (3 pages) | Cited 3 times

Online Publication Date: 4 October 2007

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We incorporate an Al–AlOx–Al single-electron transistor as the gate of a narrow ( ∼ 100 nm) metal-oxide-semiconductor field-effect transistor (MOSFET). Near the MOSFET channel conductance threshold, we observe oscillations in the conductance associated with Coulomb blockade in the channel, revealing the formation of a Si single-electron transistor. Abrupt steps present in sweeps of the Al transistor conductance versus gate voltage are correlated with single-electron charging events in the Si transistor, and vice versa. Analysis of these correlations using a simple electrostatic model demonstrates that the two single-electron transistor islands are closely aligned, with an interisland capacitance approximately equal to 1/3 of the total capacitance of the Si transistor island, indicating that the Si transistor is strongly coupled to the Al transistor.
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85.35.Gv Single electron devices
85.30.Tv Field effect devices

The effects of nickel germanosilicide contacts on the biaxial compressive stress in thin epitaxial silicon-germanium alloys on silicon

Saurabh Chopra, Mehmet C. Ozturk, Veena Misra, Zhongqiao Ren, and L. E. McNeil

Appl. Phys. Lett. 91, 142118 (2007); http://dx.doi.org/10.1063/1.2795346 (3 pages) | Cited 1 time

Online Publication Date: 4 October 2007

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When a thin Si1−xGex epitaxial layer is grown on Si, it is under biaxial compression. In this letter, it is shown that a nickel germanosilicide (NiSi1−xGex) layer formed on Si1−xGex can significantly reduce the in-plane compressive strain in Si1−xGex. It is proposed that the observed reduction is due to the biaxial tensile stress applied by the NiSi1−xGex layer. Because the Si1−xGex bandgap is a strong function of the strain, this is expected to have a strong impact on the metal-semiconductor barrier height and the contact resistivity of the interface if the metal Fermi level is pinned near the Si1−xGex midgap.
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81.05.Hd Other semiconductors
68.60.Bs Mechanical and acoustical properties
68.55.-a Thin film structure and morphology
81.40.Lm Deformation, plasticity, and creep
73.40.Cg Contact resistance, contact potential
73.40.Ns Metal-nonmetal contacts

Analysis of a wafer bonded Ge/Si heterojunction by transmission electron microscopy

Hiroshi Kanbe, Masayuki Miyaji, Mami Hirose, Noriko Nitta, and Masafumi Taniwaki

Appl. Phys. Lett. 91, 142119 (2007); http://dx.doi.org/10.1063/1.2795797 (3 pages) | Cited 3 times

Online Publication Date: 4 October 2007

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A wafer-bonded Ge/Si heterojunction was observed using transmission electron microscopy to analyze its crystallographic properties and to reveal atomic profiles at the interface by energy dispersive x-ray spectroscopy. There was a 2 nm thick transition layer at the heterojunction, where an aligned lattice image from Si to Ge together with a disordered lattice image could be observed. In the Si layer close to the interface, islandlike modified regions were observed to exist, where a large amount of Ge was detected. Oxygen was also detected accumulated at the interface.
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85.60.Dw Photodiodes; phototransistors; photoresistors
85.60.Gz Photodetectors (including infrared and CCD detectors)
85.30.Kk Junction diodes

Donor binding energy and thermally activated persistent photoconductivity in high mobility (001) AlAs quantum wells

S. Dasgupta, C. Knaak, J. Moser, M. Bichler, S. F. Roth, A. Fontcuberta i Morral, G. Abstreiter, and M. Grayson

Appl. Phys. Lett. 91, 142120 (2007); http://dx.doi.org/10.1063/1.2794012 (3 pages) | Cited 8 times

Online Publication Date: 5 October 2007

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A doping series of AlAs (001) quantum wells with Si δ-modulation doping on both sides reveals different dark and postillumination saturation densities, as well as temperature dependent photoconductivity. The lower dark two-dimensional electron density saturation is explained assuming deep binding energy of ΔDK = 65.2 meV for Si donors in the dark. Persistent photoconductivity (PPC) is observed upon illumination, with higher saturation density indicating shallow postillumination donor binding energy. The photoconductivity is thermally activated, with 4 K illumination requiring postillumination annealing to T = 30 K to saturate the PPC. Dark and postillumination doping efficiencies are reported.
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73.63.Hs Quantum wells
72.40.+w Photoconduction and photovoltaic effects
61.72.uj III-V and II-VI semiconductors

Determination of the electronic band structure for a graded modulation-doped AlGaN/AlN/GaN superlattice

Z. H. Wu, F. A. Ponce, Joachim Hertkorn, and Ferdinand Scholz

Appl. Phys. Lett. 91, 142121 (2007); http://dx.doi.org/10.1063/1.2795794 (3 pages) | Cited 7 times

Online Publication Date: 5 October 2007

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The electronic band structure of a modulation-doped AlGaN/AlN/GaN superlattice structure where the AlGaN layer is compositionally graded has been experimentally determined by electron holography. It is shown that all periods in the superlattice have a similar two-dimensional-electron-gas distribution, indicating no degradation in the quality of the heterostructures during growth. High-resolution potential energy profiles show that the nominally linear grading of the AlGaN barrier layers results in a parabolic profile in Al composition. Knowledge of the nature of energy barriers for electron transfer between channels is important in the optimization of the perpendicular conductivity of AlGaN/GaN superlattice structures.
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68.65.Cd Superlattices
73.21.Cd Superlattices

Interface studies of GaAs metal-oxide-semiconductor structures using atomic-layer-deposited HfO2/Al2O3 nanolaminate gate dielectric

T. Yang, Y. Xuan, D. Zemlyanov, T. Shen, Y. Q. Wu, J. M. Woodall, P. D. Ye, F. S. Aguirre-Tostado, M. Milojevic, S. McDonnell, and R. M. Wallace

Appl. Phys. Lett. 91, 142122 (2007); http://dx.doi.org/10.1063/1.2798499 (3 pages) | Cited 25 times

Online Publication Date: 5 October 2007

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A systematic capacitance-voltage study has been performed on GaAs metal-oxide-semiconductor (MOS) structures with atomic-layer-deposited HfO2/Al2O3 nanolaminates as gate dielectrics. A HfO2/Al2O3 nanolaminate gate dielectric improves the GaAs MOS characteristics such as dielectric constant, breakdown voltage, and frequency dispersion. A possible origin for the widely observed larger frequency dispersion on n-type GaAs than p-type GaAs is discussed. Further experiments show that the observed hysteresis is mainly from the mobile changes and traps induced by HfO2 in bulk oxide instead of those at oxide/GaAs interface.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
77.55.-g Dielectric thin films
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.22.Ch Permittivity (dielectric function)
77.22.Jp Dielectric breakdown and space-charge effects
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