AIP Advances

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The improvement of the field emission properties from graphene films: Ti transition layer and annealing process

Jun Li (李军), Jiangtao Chen (陈江涛), Baomin Luo (罗保民), Xingbin Yan (阎兴斌), and Qunji Xue (薛群基)

AIP Advances 2, 022101 (2012); http://dx.doi.org/10.1063/1.3702588 (9 pages)

Online Publication Date: 2 April 2012

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Chemical-reduced graphene oxide (rGO) films were deposited on titanium (Ti)-coated silicon substrates by a simple electrophoretic deposition. The rGO films were annealed under argon atmosphere at different temperatures. The morphology and microstructure of the rGO films before and after annealing were characterized using scanning electron microscope, X-ray diffraction and Raman spectroscope. The field emission behaviors from these rGO films were investigated. The results show that, Ti-based transition layer can improve the stability of field emission from the rGO film, and the annealing at appropriate temperature is in favor of the field emission. Particularly, the rGO film displays an unexpected vacuum breakdown phenomenon at a relatively high current density. In addition, it is found that the field emission property of the rGO film is dependent on anode-sample distance and the film exhibits lower turn on field at larger anode-sample distance.

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61.72.Cc	Kinetics of defect formation and annealing
78.30.Am	Elemental semiconductors and insulators
81.15.Pq	Electrodeposition, electroplating
68.55.ag	Semiconductors

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Isolated sub-10 attosecond pulse generation by a 6-fs driving pulse and a 5-fs subharmonic controlling pulse

Yunhui Wang, Haiping Wu, Ying Chen, Zelin Lu, Chao Yu, Qi Shi, Kaiming Deng, and Ruifeng Lu

AIP Advances 2, 022102 (2012); http://dx.doi.org/10.1063/1.3702589 (6 pages)

Online Publication Date: 2 April 2012

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We theoretically study high-order harmonic generation by quantum path control in a special two-color laser field, which is synthesized by a 6 fs/800 nm fundamental pulse and a weaker 5 fs/1600 nm subharmonic controlling pulse. Single quantum path is selected without optimizing any carrier phase, which not only broadens the harmonic bandwidth to 400 eV, but also enhances the harmonic conversion efficiency in comparison with the short-plus-long scheme, which is based on 5 fs/800 nm driving pulse and 6 fs/1600 nm control pulse. An isolated 8-attosecond pulse is produced with currently available ultrafast laser sources.

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42.60.-v	Laser optical systems: design and operation
42.65.Ky	Frequency conversion; harmonic generation, including higher-order harmonic generation
42.65.Re	Ultrafast processes; optical pulse generation and pulse compression

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Electrical characteristics of multiwalled carbon nanotube arrays and influence of pressure

L. T. Singh and K. K. Nanda

AIP Advances 2, 022103 (2012); http://dx.doi.org/10.1063/1.3702777 (7 pages)

Online Publication Date: 3 April 2012

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We have investigated the current-voltage characteristics of carbon nanotube arrays and shown that the current through the arrays increases rapidly with applied voltage before the breakdown occurs. Simultaneous measurements of current and temperature at one end of the arrays suggest that the rapid increase of current is due to Joule heating. The current through the array and the threshold voltage are found to increase with decreasing pressure.

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73.63.Fg

Nanotubes

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Characterizations of photoconductivity of graphene oxide thin films

Shiang-Kuo Chang-Jian, Jeng-Rong Ho, J.-W. John Cheng, and Ya-Ping Hsieh

AIP Advances 2, 022104 (2012); http://dx.doi.org/10.1063/1.3702871 (9 pages)

Online Publication Date: 3 April 2012

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Characterizations of photoresponse of a graphene oxide (GO) thin film to a near infrared laser light were studied. Results showed the photocurrent in the GO thin film was cathodic, always flowing in an opposite direction to the initial current generated by the preset bias voltage that shows a fundamental discrepancy from the photocurrent in the reduced graphene oxide thin film. Light illumination on the GO thin film thus results in more free electrons that offset the initial current. By examining GO thin films reduced at different temperatures, the critical temperature for reversing the photocurrent from cathodic to anodic was found around 187°C. The dynamic photoresponse for the GO thin film was further characterized through the response time constants within the laser on and off durations, denoted as τ_on and τ_off, respectively. τ_on for the GO thin film was comparable to the other carbon-based thin films such as carbon nanotubes and graphenes. τ_off was, however, much larger than that of the other's. This discrepancy was attributable to the retardation of exciton recombination rate thanks to the existing oxygen functional groups and defects in the GO thin films.

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73.61.Wp	Fullerenes and related materials
81.05.ue	Graphene
73.50.Pz	Photoconduction and photovoltaic effects
72.40.+w	Photoconduction and photovoltaic effects
71.35.-y	Excitons and related phenomena

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Surface plasmon on topological insulator/dielectric interface enhanced ZnO ultraviolet photoluminescence

Zhi-Min Liao, Bing-Hong Han, Han-Chun Wu, L. V. Yashina, Yuan Yan, Yang-Bo Zhou, Ya-Qing Bie, S. I. Bozhko, K. Fleischer, I. V. Shvets, Qing Zhao, and Da-Peng Yu

AIP Advances 2, 022105 (2012); http://dx.doi.org/10.1063/1.3703320 (7 pages)

Online Publication Date: 4 April 2012

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It has recently been predicted that the surface plasmons are allowed to exist on the interface between a topological insulator and vacuum. Surface plasmons can be employed to enhance the optical emission from various illuminants. Here, we study the photoluminescence properties of the ZnO/Bi₂Te₃ hybrid structures. Thin flakes of Bi₂Te₃, a typical three-dimensional topological insulator, were prepared on ZnO crystal surface by mechanical exfoliation method. The ultraviolet emission from ZnO was found to be enhanced by the Bi₂Te₃ thin flakes, which was attributed to the surface plasmon – photon coupling at the Bi₂Te₃/ZnO interface.

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73.20.Mf	Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.25.+i	Surface conductivity and carrier phenomena
78.55.Cr	III-V semiconductors

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Influence of a-Si:H deposition power on surface passivation property and thermal stability of a-Si:H/SiNx:H stacks

Hua Li and Stuart. R. Wenham

AIP Advances 2, 022106 (2012); http://dx.doi.org/10.1063/1.3703659 (10 pages)

Online Publication Date: 5 April 2012

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The effectiveness of hydrogenated amorphous silicon (a-Si:H) layers for passivating crystalline silicon surfaces has been well documented in the literature for well over a decade. One limitation of such layers however has arisen from their inability to withstand temperatures much above their deposition temperature without significant degradation. This limitation is of importance particularly with multicrystalline silicon materials where temperatures of at least 400°C are needed for effective hydrogenation of the crystallographic defects such as grain boundaries. To address this limitation, in this work the surface passivation quality and thermal stability of a stack passivating system, combining a layer of intrinsic a-Si:H and a capping layer of silicon nitride (SiNx:H), on p-type crystalline silicon wafers is studied and optimized. In particular the sensitivity of different microwave (MW) power levels for underlying a-Si:H layer deposition are examined. Both effective minority carrier lifetime (ζ_eff) measurement and Fourier transform infrared (FTIR) spectrometry were employed to study the bonding configurations, passivating quality and thermal stability of the a-Si:H/SiNx:H stacks. It is established that the higher MW power could result in increased as-deposited ζ_eff and implied Voc (iVoc) values, indicating likely improved surface passivation quality, but that this combination degrades more quickly when exposed to prolonged thermal treatments. The more dihydride-rich film composition corresponding to the higher MW power appears to be beneficial for bond restructuring by hydrogen interchanges when exposed to short term annealing, however it also appears more susceptible to providing channels for hydrogen out-effusion which is the likely cause of the poorer thermal stability for prolonged high temperature exposure compared with stacks with underlying a-Si:H deposited with lower MW power.

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81.65.Rv	Passivation
61.72.Mm	Grain and twin boundaries
78.30.Am	Elemental semiconductors and insulators

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Coordination variation of hydrated Cu²⁺/Br¹⁻ ions traversing the interfacial water in mesopores

Q. Wang, X. F. Huang, C. X. Li, L. Q. Pan, Z. H. Wu, T. D. Hu, Z. Jiang, Y. Y. Huang, Z. X. Cao, G. Sun, and K. Q. Lu

AIP Advances 2, 022107 (2012); http://dx.doi.org/10.1063/1.4704545 (10 pages)

Online Publication Date: 11 April 2012

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Resolution of the atomistic and electronic details about the coordination structure variation of hydrated ions in the interfacial water is still a tough challenge, which is, however, essentially important for the understanding of ion adsorption, permeation and other similar processes in aqueous solutions. Here we report the tracing of coordination structure variation for hydrated Cu²⁺/Br^1- ions traversing the interfacial water in Vycor mesopores (ϕ = 7.6 nm) by employing both X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies. By controlled desorption/adsorption of water, the filling fraction of the mesopores, thus the water layer thickness, can be adjusted, which in turn effects the variation of coordination structure of the ions therein. It is found that both Cu²⁺ and Br^1- ions prefer staying exclusively in the core water, and in this circumstance no ion pairs have been detected in the solution of concentrations up to 1.0 M. Following capillary decondensation occurring at a filling fraction of ∼35% which corresponds to a water layer of about three monolayers, Br^1- ions begin immediately to reconstruct their first coordination shell, characterized by ionic dehydration, shrinkage of ion-water bond length, and formation of ion pairs. In contrast, Cu²⁺ ions can retain a bulk-like coordination structure till being driven to bond directly to the pore surface when the filling fraction is below 20%. At the final stage of dehydration via thermal vacuum treatment at 110°C, Cu²⁺ ions can be completely reduced to the Cu¹⁺ state, and recover at room temperature only when the filling fraction is above 14%. These results may be inspirable for the investigation of similar problems concerning hydrated ions in water solution under different confining conditions.

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82.30.Nr	Association, addition, insertion, cluster formation
68.43.Nr	Desorption kinetics
68.03.Fg	Evaporation and condensation of liquids
68.43.Mn	Adsorption kinetics
78.70.Dm	X-ray absorption spectra
82.30.Lp	Decomposition reactions (pyrolysis, dissociation, and fragmentation)

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Soft matrix and fixed point of Lennard-Jones potentials for different hard-clusters in size at glass transition

Jia Lin Wu

AIP Advances 2, 022108 (2012); http://dx.doi.org/10.1063/1.4704662 (14 pages)

Online Publication Date: 11 April 2012

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The existence of fixed point in self-similar Lennard-Jones (L-J) potentials has been proved based on the mosaic geometric structure theory of glass transition (GT) [ J. L. Wu, Soft Nanoscience letters, 1, 3, 86 (2011) ]. A geometric local-global mode-coupling recursive equation, different from the current Mode-Coupling Theories, has been introduced to find out the non-integrable induced potential structure of boson peak at GT. The recursively defined variable in reduced recursive equation is the potential fluctuation of reduced L-J potentials associated with reduced geometric phase potentials. A series of results have been deduced directly at GT. (i) There are only 8 orders of molecule-clusters. (ii) Two orthogonally fast-slow reduced phase potentials, 3/8 and 5/8, are accompanied with density fluctuation and clusters hop-delocalization along 8 geodesics. (iii) The stability condition of potential fluctuation is the Lindemann ratio. (iv) A new reduced attractive potential of –17/16, lower than reduced potential well energy –1, occurs.

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64.70.Q-	Theory and modeling of the glass transition
05.30.Jp	Boson systems

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Membrane metamaterial resonators with a sharp resonance: A comprehensive study towards practical terahertz filters and sensors

Yongyao Chen, Ibraheem A. I. Al-Naib, Jianqiang Gu, Mingwei Wang, Tsuneyuki Ozaki, Roberto Morandotti, and Weili Zhang

AIP Advances 2, 022109 (2012); http://dx.doi.org/10.1063/1.4704549 (9 pages)

Online Publication Date: 13 April 2012

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We investigate the resonant properties of high quality-factor membrane-based metamaterial resonators functioning in the terahertz regime. A number of factors, including the resonator geometry, dielectric loss, and most importantly the membrane thickness are found to extensively influence the resonance strength and quality factor of the sharp resonance. Further studies on the membrane thickness-dependent-sensitivity for sensing applications reveal that high quality-factor membrane metamaterials with a moderate thickness ranging from 10 to 50 μm are the most promising option towards developing realistic integrated terahertz filters and sensors.

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84.40.Az

Waveguides, transmission lines, striplines

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DNA-decorated carbon nanotube-based FETs as ultrasensitive chemical sensors: Discrimination of homologues, structural isomers, and optical isomers

S. M. Khamis, R. A. Jones, A. T. C. Johnson, G. Preti, J. Kwak, and A. Gelperin

AIP Advances 2, 022110 (2012); http://dx.doi.org/10.1063/1.4705394 (11 pages)

Online Publication Date: 17 April 2012

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We have explored the abilities of all-electronic DNA-carbon nanotube (DNA-NT) vapor sensors to discriminate very similar classes of molecules. We screened hundreds of DNA-NT devices against a panel of compounds chosen because of their similarities. We demonstrated that DNA-NT vapor sensors readily discriminate between series of chemical homologues that differ by single methyl groups. DNA-NT devices also discriminate among structural isomers and optical isomers, a trait common in biological olfactory systems, but only recently demonstrated for electronic FET based chemical sensors.

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85.30.Tv	Field effect devices
85.35.Kt	Nanotube devices
07.07.Df	Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
87.14.gk	DNA
87.80.-y	Biophysical techniques (research methods)

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Lateral energy band profile modulation in tunnel field effect transistors based on gate structure engineering

Ning Cui, Renrong Liang, Jing Wang, and Jun Xu

AIP Advances 2, 022111 (2012); http://dx.doi.org/10.1063/1.4705398 (16 pages)

Online Publication Date: 17 April 2012

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Choosing novel materials and structures is important for enhancing the on-state current in tunnel field-effect transistors (TFETs). In this paper, we reveal that the on-state performance of TFETs is mainly determined by the energy band profile of the channel. According to this interpretation, we present a new concept of energy band profile modulation (BPM) achieved with gate structure engineering. It is believed that this approach can be used to suppress the ambipolar effect. Based on this method, a Si TFET device with a symmetrical tri-material-gate (TMG) structure is proposed. Two-dimensional numerical simulations demonstrated that the special band profile in this device can boost on-state performance, and it also suppresses the off-state current induced by the ambipolar effect. These unique advantages are maintained over a wide range of gate lengths and supply voltages. The BPM concept can serve as a guideline for improving the performance of nanoscale TFET devices.

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85.30.Tv

Field effect devices

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Development of a versatile capacitive tactile sensor based on transparent flexible materials integrating an excellent sensitivity and a high resolution

H. Z. Zhang, Q. Y. Tang, and Y. C. Chan

AIP Advances 2, 022112 (2012); http://dx.doi.org/10.1063/1.4706011 (6 pages)

Online Publication Date: 17 April 2012

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A versatile capacitive tactile sensor based on transparent flexible materials is developed in a simple and low-cost fabrication process. The sensor shows an excellent sensitivity (S=2.05 N^-1), and is highly sensitive to the load as low as about 3 mN. Moreover, it exhibits a prominent resolution. The excellent device performance is attributed to the creative design of polydimethylsiloxane (PDMS) polymer layer, used as the structural material of the sensor, in which each sensing section acting as a sensor unit is a concave square with hemispheric micro-structured PDMS arrays. Meanwhile, other sections without any PDMS arrays serving as perfect natural wall-barriers can make each sensor unit separated effectively.

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07.07.Df	Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
78.20.Ci	Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

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Source/drain electrodes contact effect on the stability of bottom-contact pentacene field-effect transistors

Xinge Yu, Junsheng Yu, Wei Huang, Lin Zhang, and Hongjuan Zeng

AIP Advances 2, 022113 (2012); http://dx.doi.org/10.1063/1.4707164 (6 pages)

Online Publication Date: 18 April 2012

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Bottom-contact pentacene field-effect transistors were fabricated with a PMMA dielectric layer, and the air stability of the transistors was investigated. To characterize the device stability, the field-effect transistors were exposed to ambient conditions for 30 days and subsequently characterized. The degradation of electrical performance was traced to study the variation of field-effect mobility, saturation current and off-state current. By investigating the morphology variance of the pentacene film at the channel and source/drain (S/D) contact regions by atomic force microscopy, it was clear that the morphology of the pentacene film adhered to the S/D degenerated dramatically. Moreover, by studying the variation of contact resistance in detail, it was found that the S/D contact effect was the main reason for the degradation in performance.

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85.30.Tv	Field effect devices
73.40.Cg	Contact resistance, contact potential

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Localization effects in the tunnel barriers of phosphorus-doped silicon quantum dots

T. Ferrus, A. Rossi, W. Lin, D. A. Williams, T. Kodera, and S. Oda

AIP Advances 2, 022114 (2012); http://dx.doi.org/10.1063/1.4707165 (9 pages)

Online Publication Date: 18 April 2012

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We have observed a negative differential conductance with singular gate and source-drain bias dependences in a phosphorus-doped silicon quantum dot. Its origin is discussed within the framework of weak localization. By measuring the current-voltage characteristics at different temperatures as well as simulating the tunneling rates dependences on energy, we demonstrate that the presence of shallow energy defects together with an enhancement of localization satisfactory explain our observations. Effects observed in magnetic fields are also discussed.

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72.80.Cw	Elemental semiconductors
73.63.Kv	Quantum dots
61.72.uf	Ge and Si
71.55.Cn	Elemental semiconductors

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Density functional investigations on electronic structures, magnetic ordering and ferroelectric phase transition in multiferroic Bi₂NiMnO₆

Hong Jian Zhao, Xiao Qiang Liu, and Xiang Ming Chen

AIP Advances 2, 022115 (2012); http://dx.doi.org/10.1063/1.4709401 (9 pages)

Online Publication Date: 23 April 2012

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Using the full-potential linearised augmented-plane wave (FP-LAPW) method based on density functional theory (DFT), we have investigated the electronic structures, the magnetic behavior, and the ferroelectric origin of multiferroic Bi₂NiMnO₆. The calculated ferromagnetic Curie temperature of Bi₂NiMnO₆ is very sensitive to the Mn⁴⁺—O^2-—Ni²⁺ length. When average Mn⁴⁺—O^2-—Ni²⁺ length increases from 3.82 to 4.05 Å, the Curie temperature increases from 179 to 295 K. The Mn⁴⁺—O^2-—Ni²⁺ superexchange interaction due to the virtual hopping of electrons from O-2p filled states to Mn-/Ni-3d empty states is enhanced when the band gap formed by crystal-field splitting decreases, thus the effective exchange parameters and Curie temperature increase as Mn⁴⁺—O^2-—Ni²⁺ length increases. The ferroelectric distortion in Bi₂NiMnO₆ is directly from the hybridization of Bi-6p and O-2p states. The role of Bi-6s² lone pairs electrons may be that hybridized O-2p with Bi-6s orbitals may be more appropriate in compatible symmetry with Bi-6p orbital than O-2p orbital only. Furthermore, the route of ferroelectric distortion in Bi₂NiMnO₆ from paraelectric P2₁/n phase to ferroelectric C2 phase is discussed.

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77.80.B-	Phase transitions and Curie point
75.30.Kz	Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.40.-s	Critical-point effects, specific heats, short-range order
77.55.Nv	Multiferroic/magnetoelectric films
71.15.Ap	Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
71.15.Mb	Density functional theory, local density approximation, gradient and other corrections

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A new type of low temperature conductivity in InSb doped with Mn

Sergey A. Obukhov

AIP Advances 2, 022116 (2012); http://dx.doi.org/10.1063/1.4711041 (5 pages)

Online Publication Date: 25 April 2012

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We investigated unusual low temperature transport properties of InSb single crystals doped with manganese to concentrations in the range of 10¹⁷<N_Mn<2*10¹⁷cm^-3 which includes the critical concentration for metal-insulator transition N_cr= 2*10¹⁷cm^-3. It was found that in the temperature range T=10÷1.5K the resisitivity had an unusual behavior described by an exponential quadratic function ρ ∼ exp(Δ₁/kT)² where Δ₁ varied from Δ₁=0.3meV to Δ₁=0. Coexistence of electrons and holes at temperatures around the metal-insulator transition in p-InSb(Mn) crystals gives the ground to explain the unusual temperature - resistivity dependence in the framework of excitonic insulator phase model (L. V. Keldysh and Yu. V. Kopaev, Sov. Phys. Solid State 6, 2219 (1965)).

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72.80.Ey	III-V and II-VI semiconductors
61.72.uj	III-V and II-VI semiconductors
71.30.+h	Metal-insulator transitions and other electronic transitions
72.60.+g	Mixed conductivity and conductivity transitions

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In-line Si_1-xGe_x epitaxial process monitoring and diagnostics using multiwavelength high resolution micro-Raman spectroscopy

Chun-Wei Chang, Min-Hao Hong, Wei-Fan Lee, Kuan-Ching Lee, Shen-Min Yang, Ming-Shan Tsai, Yen Chuang, Yu-Ta Fan, Noriyuki Hasuike, Hiroshi Harima, Takeshi Ueda, Toshikazu Ishigaki, Kitaek Kang, and Woo Sik Yoo

AIP Advances 2, 022117 (2012); http://dx.doi.org/10.1063/1.4711043 (11 pages)

Online Publication Date: 25 April 2012

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Multiwavelength, high resolution micro-Raman spectroscopy was applied to in-line process monitoring and diagnostics of undoped and B-doped Si_1-xGe_x epitaxy on Si(100) device wafers. This noncontact technique was used to monitor the Ge content, B concentration and thickness of single and double Si_1-xGe_x epitaxial layers. Epitaxial process problems were diagnosed nondestructively. Raman peak positions and full-width-at-half-maximum of the Si-Si peak(s) from the Si_1-xGe_x epitaxial layer(s) and Si substrates, in the wavenumber range of 475 ∼ 535 cm^-1, were monitored under ultraviolet and visible excitation wavelengths. The Ge content, B concentration and Si_1-xGe_x epitaxial film structures were verified by secondary ion mass spectroscopy (SIMS) depth profiling results. In-line monitoring of Si-Si and Si Raman peaks is very effective in noncontact material property characterization, epitaxial process optimization, and quality control applications.

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61.72.uf	Ge and Si
68.55.ag	Semiconductors
78.30.Am	Elemental semiconductors and insulators
78.55.Ap	Elemental semiconductors
79.20.Rf	Atomic, molecular, and ion beam impact and interactions with surfaces

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Evolution of electrical performance of ZnO-based thin-film transistors by low temperature annealing

J. Zhang, X. F. Li, J. G. Lu, P. Wu, J. Huang, Q. Wang, B. Lu, Y. Z. Zhang, B. H. Zhao, and Z. Z. Ye

AIP Advances 2, 022118 (2012); http://dx.doi.org/10.1063/1.4711046 (6 pages)

Online Publication Date: 25 April 2012

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The effects of post-annealing on performance of ZnO-based thin-film transistors (TFTs) fabricated at room temperature were investigated. It was observed that high-temperature annealing resulted in a large decrease in resistivity of the ZnO channel layer and caused a large off-state current for ZnO TFTs, while low-temperature annealing had little effect on the off-state current. The evolution of electrical performance of ZnO TFTs annealed at a lower temperature showed that the threshold voltage decreased greatly and the sub-threshold slope improved evidently without great change of the resistivity of the ZnO channel as the annealing time prolonged. The possible mechanism is that the traps have been removed without activating the donor defects in the ZnO channel layer.

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85.30.Tv	Field effect devices
61.72.Cc	Kinetics of defect formation and annealing

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High temperature operation of short wavelength InAs-based quantum cascade lasers

P. Laffaille, J. C. Moreno, R. Teissier, M. Bahriz, and A. N. Baranov

AIP Advances 2, 022119 (2012); http://dx.doi.org/10.1063/1.4714363 (6 pages)

Online Publication Date: 2 May 2012

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InAs/AlSb quantum cascade lasers emitting at 3.06 and 3.22 μm at room temperature has been studied. The lasers with high reflection coating on back facets operated in pulse mode up to 400 and 423 K, respectively. The obtained results showed no dramatic performance degradation of the InAs-based QCLs with decreasing emission wavelength down to 3 μm.

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42.55.Px	Semiconductor lasers; laser diodes
42.60.By	Design of specific laser systems

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Puzzling phonon dispersion curves and vibrational mode instability in superconducting MgCNi₃

Prafulla K. Jha, Sanjay D. Gupta, and Sanjeev K. Gupta

AIP Advances 2, 022120 (2012); http://dx.doi.org/10.1063/1.4714366 (9 pages)

Online Publication Date: 2 May 2012

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A first principles calculation of the lattice dynamical properties of superconducting MgCNi₃ has been performed using density functional perturbation theory (DFPT). The calculated phonon dispersion curves and phonon density of states have been compared with inelastic x-ray scattering (IXS) and inelastic neutron scattering (INS) measurements. We show for the first time that phonon dispersion curves for MgCNi₃ in whole Brillouin zone are positive (stable phonon modes) and in good agreement with the experimental data. The phonon DOS shows absence of phonon density of states at zero energy unlike earlier calculations. There is a good agreement between calculated and experimental electron-phonon parameter and superconducting transition temperature. The Eliasberg function is quantitatively as well as qualitatively different from the phonon density of states. The lattice specific heat and Debye temperature do not show any anomalous behaviour.

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74.25.Kc	Phonons
63.20.D-	Phonon states and bands, normal modes, and phonon dispersion
71.15.Mb	Density functional theory, local density approximation, gradient and other corrections
74.62.-c	Transition temperature variations, phase diagrams

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Hybrid carbon nanostructure assemblage for high performance pseudo-capacitors

A. K. Mishra and S. Ramaprabhu

AIP Advances 2, 022121 (2012); http://dx.doi.org/10.1063/1.4717490 (7 pages)

Online Publication Date: 4 May 2012

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Investigation of novel nanocomposites for pseudo-capacitors with high capacitance and energy density is the spotlight of current energy research. In the present work, hybrid carbon nanostructure assemblage of graphene and multiwalled carbon nanotubes has been used as carbon support to nanostructured RuO₂ and polyaniline for high energy supercapacitors. Maximum specific capacitances of 110, 235 and 440 F g⁻¹ at the voltage sweep rate of 10 mV s⁻¹ and maximum energy densities of 7, 12.5 and 20.5 Wh kg⁻¹ were observed for carbon assemblage and its RuO₂ and polyanilne decorated nanocomposites, respectively, with 1M H₂SO₄ as electrolyte.

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84.32.Tt	Capacitors
88.80.fh	Supercapacitors
81.07.Pr	Organic-inorganic hybrid nanostructures
82.45.Gj	Electrolytes
82.47.Uv	Electrochemical capacitors; supercapacitors

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Influence of laser lift-off on optical and structural properties of InGaN/GaN vertical blue light emitting diodes

M. H. Doan, S. Kim, J. J. Lee, H. Lim, F. Rotermund, and Kihong Kim

AIP Advances 2, 022122 (2012); http://dx.doi.org/10.1063/1.4717493 (8 pages)

Online Publication Date: 4 May 2012

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The influences of the laser lift-off (LLO) process on the InGaN/GaN blue light emitting diode (LED) structures, grown on sapphire substrates by low-pressure metalorganic chemical vapor deposition, have been comprehensively investigated. The vertical LED structures on Cu carriers are fabricated using electroplating, LLO, and inductively coupled plasma etching processes sequentially. A detailed study is performed on the variation of defect concentration and optical properties, before and after the LLO process, employing high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) observations, cathodoluminescence (CL), photoluminescence (PL), and high-resolution X-ray diffraction (HRXRD) measurements. The SEM observations on the distribution of dislocations after the LLO show well that even the GaN layer near to the multiple quantum wells (MQWs) is damaged. The CL measurements reveal that the peak energy of the InGaN/GaN MQW emission exhibits a blue-shift after the LLO process in addition to a reduced intensity. These behaviors are attributed to a diffusion of indium through the defects created by the LLO and creation of non-radiative recombination centers. The observed phenomena thus suggest that the MQWs, the active region of the InGaN/GaN light emitting diodes, may be damaged by the LLO process when thickness of the GaN layer below the MQW is made to be 5 μm, a conventional thickness. The CL images on the boundary between the KrF irradiated and non-irradiated regions suggest that the propagation of the KrF laser beam and an accompanied recombination enhanced defect reaction, rather than the propagation of a thermal shock wave, are the main origin of the damage effects of the LLO process on the InGaN/GaN MQWs and the n-GaN layer as well.

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85.60.Jb	Light-emitting devices
68.65.Fg	Quantum wells
78.55.Cr	III-V semiconductors
78.67.De	Quantum wells
81.15.Gh	Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

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Surface sensing behavior and band edge properties of AgAlS₂: Experimental observations in optical, chemical, and thermoreflectance spectroscopy

Ching-Hwa Ho and Chia-Chi Pan

AIP Advances 2, 022123 (2012); http://dx.doi.org/10.1063/1.4718342 (14 pages)

Online Publication Date: 16 May 2012

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Optical examination of a chaocogenide compound AgAlS₂ which can spontaneously transfer to a AgAlO₂ oxide has been investigated by thermoreflectance (TR) spectroscopy herein. The single crystals of AgAlS₂ were grown by chemical vapor transport (CVT) method using ICl₃ as a transport agent sealed in evacuated quartz tubes. The as-grown AgAlS₂ crystals essentially possess a transparent and white color in vacuum. The crystal surface of AgAlS₂ becomes darkened and brownish when putting AgAlS₂ into atmosphere for reacting with water vapor or hydrogen gas. Undergoing the chemical reaction process, oxygen deficient AgAlO_2-2x with brownish and reddish-like color on surface of AgAlS₂ forms. The transition energy of deficient AgAlO_2-2x was evaluated by TR experiment. The value was determined to be ∼2.452 eV at 300 K. If the sample is kept dry and moved away from moisture, AgAlS₂ crystal can stop forming more deficient AgAlO_2-2x surface oxides. The experimental TR spectra for the surface-reacted sample show clearly two transition features at E_W=2.452 eV for deficient AgAlO_2-2x and E_U=3.186 eV for AgAlS₂, respectively. The E_U transition belongs to direct band-edge exciton of AgAlS₂. Alternatively, for surface-oxidation process of AgAlS₂ lasting for a long time, a AgAlO₂ crystal with yellowish color will eventually form. The TR measurements show mainly a ground-state band edge exciton of E OX 1 detected for AgAlO₂. The energy was determined to be E OX 1=2.792 eV at 300 K. The valence-band electronic structure of AgAlS₂ has been detailed characterized using polarized-thermoreflectance (PTR) measurements in the temperature range between 30 and 340 K. Physical chemistry behaviors of AgAlS₂ and AgAlO₂ have been comprehensively studied via detailed analyses of PTR and TR spectra. Based on the experimental analyses, optical and chemical behaviors of the AgAlS₂ crystals under atmosphere are realized. A possible optical-detecting scheme for using AgAlS₂ as a humidity sensor has also been proposed.

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78.20.N-	Thermo-optic effects
71.35.-y	Excitons and related phenomena
73.20.Mf	Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
81.10.Bk	Growth from vapor

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Enhanced three-photon near-infrared quantum splitting in β-NaYF₄:Ho³⁺ by codoping Yb³⁺

D. C. Yu, S. Ye, X. Y. Huang, and Q. Y. Zhang

AIP Advances 2, 022124 (2012); http://dx.doi.org/10.1063/1.4718412 (7 pages)

Online Publication Date: 8 May 2012

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An efficient three-step sequential three-photon near-infrared (NIR) quantum splitting (QS), involving the emission of three NIR photons from an absorbed ultraviolet (UV) photon in β-NaYF₄:Ho³⁺, has been demonstrated. Codoping Yb³⁺ can increase the NIR emissions significantly through the first- and second-step cross-relaxation. The involved mechanisms of three-step NIR QS in Ho³⁺-Yb³⁺ have been well demonstrated by the static and dynamic photoluminescence, monitored excitation and time-resolved fluorescence spectra. An internal quantum yield (QY) of 246% for the Yb³⁺ codoped β-NaYF₄:Ho³⁺ is estimated on the basis of experimental data and theoretical considerations, which is much more than the total QY of 124% for the β-NaYF₄:Ho³⁺ three-photon NIR QS. This research may open up promising new perspectives for designing efficient photonic devices.

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78.55.Hx	Other solid inorganic materials
78.47.jd	Time resolved luminescence

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Modulation effect of hydrogen and fluorine decoration on the surface work function of BN sheets

N Jiao, Chaoyu He, C. X. Zhang, Xiangyang Peng, K. W. Zhang, and L. Z. Sun

AIP Advances 2, 022125 (2012); http://dx.doi.org/10.1063/1.4719097 (8 pages)

Online Publication Date: 10 May 2012

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Using first-principles calculations within the framework of density-functional theory, we studied the modulation effect of hydrogen/fluorine chemical decoration on the surface work function of BN sheets. We found that the difference in the work function (ΔW_BN) between two surfaces of the chair structure varies with the different decoration. Geometric distortion and chemical effects cause opposite modulation effects, and the chemical effect plays a leading role by inducing charge redistribution in the system.

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73.30.+y	Surface double layers, Schottky barriers, and work functions
72.80.Ey	III-V and II-VI semiconductors
71.15.Mb	Density functional theory, local density approximation, gradient and other corrections

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