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23 May 2005

Volume 86, Issue 21, Articles (21xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 86, 213111 (2005); http://dx.doi.org/10.1063/1.1931027 (3 pages)

R. H. Miwa, W. Orellana, and A. Fazzio
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Enhanced stress relaxation in ultrathin SiGe-on-insulator by H+-implantation-assisted oxidation

T. Sadoh, R. Matsuura, M. Miyao, M. Ninomiya, M. Nakamae, and T. Enokida

Appl. Phys. Lett. 86, 211901 (2005); http://dx.doi.org/10.1063/1.1935028 (3 pages) | Cited 4 times

Online Publication Date: 16 May 2005

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Effects of H+ implantation ( ⩽ 5×1016 cm−2) on stress relaxation in an oxidation-induced Ge condensation method have been investigated to form stress-relaxed ultrathin ( ∼ 30 nm) SiGe-on-insulator (SGOI) virtual substrates. High-dose ( ≥ 1015 cm−2) implantation enhanced stress relaxation, which was attributed to bond breaking at the SiGe/buried SiO2 interface. However, oxidation velocity was also enhanced due to irradiation defects. Two-step annealing (500 °C for 30 min and 850 °C for 60 min) before oxidation was proposed to remove irradiation defects. This achieved enhanced stress relaxation in ultrathin SGOI without changing oxidation velocity.
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61.82.Fk Semiconductors
61.80.Jh Ion radiation effects
61.72.uf Ge and Si
61.72.up Other materials
61.72.Cc Kinetics of defect formation and annealing
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.82.Ms Insulators
81.65.Mq Oxidation
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
81.40.Jj Elasticity and anelasticity, stress-strain relations

Low-resistance and highly transparent Ni/indium-tin oxide ohmic contacts to phosphorous-doped p-type ZnO

Soon-Hyung Kang, Dae-Kue Hwang, and Seong-Ju Park

Appl. Phys. Lett. 86, 211902 (2005); http://dx.doi.org/10.1063/1.1935030 (3 pages) | Cited 9 times

Online Publication Date: 16 May 2005

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The use of a Ni (50 Å)/indium tin oxide (ITO) (500 Å) bilayer scheme for low-resistance and transparent ohmic contacts to phosphorous doped p-type ZnO was investigated. Ni/indium tin oxide (ITO) (50/500 Å) layers were prepared by electron-beam evaporation. Although the as-deposited Ni/ITO contact was highly resistant and opaque, contact resistance and transparency were greatly improved by a thermal annealing process. A specific contact resistance as low as 6.2×10−5 Ω cm2 was obtained after thermal annealing at 400 °C for 1 min under an N2 ambient. The measured light transmittance of the Ni/ITO (50/500 Å) bilayer on p-type ZnO was determined to be above 80% at a wavelength of 400–600 nm. These results strongly indicate that the use of a Ni/ITO has considerable promise for ZnO-based optical devices.
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73.40.Cg Contact resistance, contact potential
73.40.Ns Metal-nonmetal contacts
61.72.Cc Kinetics of defect formation and annealing
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

A different way of performing picosecond ultrasonic measurements in thin transparent films based on laser-wavelength effects

A. Devos, R. Côte, G. Caruyer, and A. Lefèvre

Appl. Phys. Lett. 86, 211903 (2005); http://dx.doi.org/10.1063/1.1929869 (3 pages) | Cited 19 times

Online Publication Date: 16 May 2005

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We present a way of using picosecond ultrasonics to simultaneously get the thickness and elastic properties of thin dielectric layers. This is based on the use of a blue probe which is shown to improve the detection of acousto-optic oscillations in the dielectric from which the sound velocity can be measured from the refractive index. At the same wavelength a strong response of the silicon is used to detect the arrival of the acoustic pulse. We apply this scheme to various materials deposited on silicon substrates.
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77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
43.58.Dj
43.35.Yb
62.65.+k Acoustical properties of solids
78.20.hb Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Topological instability as a criterion for design and selection of aluminum-based glass-former alloys

R. D. Sá Lisboa, C. Bolfarini, W. J. Botta F., and C. S. Kiminami

Appl. Phys. Lett. 86, 211904 (2005); http://dx.doi.org/10.1063/1.1931047 (3 pages) | Cited 34 times

Online Publication Date: 16 May 2005

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This letter proposes a criterion to quantitatively correlate chemical composition with crystallization behaviors of Al-based glass-former alloys. By extending to multicomponent systems the topological instability model of Egami and Waseda [J. Non-Cryst. Solids 64, 113 (1984)] , we introduce a “lambda (λ) criterion” according to which amorphous alloys with λ>0.1 exhibit glassy behavior, whereas alloys with λ<0.1 are nanocrystalline. Nanoglassy alloys occur when λ ≈ 0.1. A large number of experimental findings in the literature supports the λ criterion, rendering it a useful tool for the design and selection of glass-former systems and compositions.
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64.70.K- Solid-solid transitions
64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
61.43.Fs Glasses
61.46.-w Structure of nanoscale materials
82.80.-d Chemical analysis and related physical methods of analysis

Characterization and properties of green-emitting β-SiAlON:Eu2+ powder phosphors for white light-emitting diodes

Naoto Hirosaki, Rong-Jun Xie, Koji Kimoto, Takashi Sekiguchi, Yoshinobu Yamamoto, Takayuki Suehiro, and Mamoru Mitomo

Appl. Phys. Lett. 86, 211905 (2005); http://dx.doi.org/10.1063/1.1935027 (3 pages) | Cited 163 times

Online Publication Date: 17 May 2005

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This letter reports a β-SiAlON:Eu2+ green phosphor with the composition of Eu0.00296Si0.41395Al0.01334O0.0044N0.56528. The phosphor powder exhibits a rod-like morphology with the length of ∼ 4 μm and the diameter of ∼ 0.5 μm. It can be excited efficiently over a broad spectral range between 280 and 480 nm, and has an emission peak at 535 nm with a full width at half maximum of 55 nm. It has a superior color chromaticity of x = 0.32 and y = 0.64. The internal and external quantum efficiencies of this phosphor is 70% and 61% at λex = 303 nm, respectively. This newly developed green phosphor has potential applications in phosphor-converted white LEDs.
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78.60.-b Other luminescence and radiative recombination
85.60.Jb Light-emitting devices
61.43.Gt Powders, porous materials
78.40.Ha Other nonmetallic inorganics

Transfer of the in-plane molecular orientation of polyimide film surface to liquid crystal monolayer

Kiyoaki Usami, Kenji Sakamoto, Yoichi Uehara, and Sukekatsu Ushioda

Appl. Phys. Lett. 86, 211906 (2005); http://dx.doi.org/10.1063/1.1937988 (3 pages) | Cited 11 times

Online Publication Date: 17 May 2005

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We have determined the relationship between the in-plane molecular orientations of a polyimide film and the liquid crystal (LC) monolayer in contact with it. A photoaligned film of polyimide, containing azobenzene in the backbone structure, was used, because its in-plane molecular order can be varied over a wide range without change in the morphology and the chemical nature of the film surface. The in-plane order parameter of the LC molecule was found to be almost equal to that of the polyimide backbone structure. This result shows that the molecular alignment of the LC monolayer is mainly induced by a short-range interaction between the LC and polyimide molecules.
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61.41.+e Polymers, elastomers, and plastics
61.30.-v Liquid crystals

Measurement of microwave electro-optic coefficient in Sr0.61Ba0.39Nb2O6 crystal fiber

Chuanyong Huang, Amar S. Bhalla, and Ruyan Guo

Appl. Phys. Lett. 86, 211907 (2005); http://dx.doi.org/10.1063/1.1937997 (3 pages) | Cited 5 times

Online Publication Date: 17 May 2005

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A method is put forward to measure the linear electro-optic (EO) coefficient at high frequency using a light-coupled microwave cavity perturbation method of standing wave condition. The method is used to determine the EO coefficient of Sr0.61Ba0.39Nb2O6 (SBN) crystal fiber. The electro-optic coefficient r33 of SBN crystal fiber at 10 GHz is measured to be near 200 (pm/V), which is smaller than that measured at low frequency containing secondary contributions. This method is suitable for measuring the electro-optic property in a wide range of high frequencies depending on the availability of microwave cavity and suitable electronics.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
78.20.Jq Electro-optical effects
42.81.Cn Fiber testing and measurement of fiber parameters
84.40.-x Radiowave and microwave (including millimeter wave) technology

Low temperature relaxations and effects on poling efficiencies of dendronized nonlinear optical side-chain polymers

Tomoko Gray, René M. Overney, Marnie Haller, Jingdong Luo, and Alex K.-Y. Jen

Appl. Phys. Lett. 86, 211908 (2005); http://dx.doi.org/10.1063/1.1938005 (3 pages) | Cited 7 times

Online Publication Date: 18 May 2005

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Low temperature relaxations in a dendronized nonlinear optical (NLO) side-chain polymer were found to take place at more than 20 °C below the glass transition temperature. Relaxations of localized mobilities, removed from long range relaxations responsible for chromophore aggregation, are shown to offer new gateways for optimized acentric ordering of the chromophores. Supreme electro-optical (EO) activity was achieved by electrical poling close to the critical temperatures of localized mobilities identified as dendronized NLO side-chain relaxations. This study features, in particular, one new instrumental approach to relaxation studies of thin spin coated NLO polymer films; the shear-modulation force microscopy (SM-FM) method. Originating from scanning force microscopy (SFM), the SM-FM method grants access to the detection of low temperature relaxations in constrained thin NLO films not obtainable by conventional means.
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42.70.Jk Polymers and organics
77.22.Ej Polarization and depolarization
78.20.Jq Electro-optical effects
68.55.A- Nucleation and growth
68.37.Ps Atomic force microscopy (AFM)

Controlled generation of neutral, negatively-charged and positively-charged excitons in the same single quantum dot

M. Ediger, P. A. Dalgarno, J. M. Smith, B. D. Gerardot, R. J. Warburton, K. Karrai, and P. M. Petroff

Appl. Phys. Lett. 86, 211909 (2005); http://dx.doi.org/10.1063/1.1937996 (3 pages)

Online Publication Date: 18 May 2005

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We report the controlled generation of neutral, negatively-charged and positively-charged excitons in the same single InAs quantum dot. The control parameters are a vertical electric field applied to a capacitor-like structure, in which the quantum dots are embedded, and optical pump power. The strong Coulomb blockade in quantum dots can be exploited to control the charge of excitons containing one hole, the neutral exciton, X0, and singly negatively charged exciton, X1−. We show here how this concept can be extended to excitons containing two holes, the biexciton, 2X0, and significantly the singly positively charged exciton, X1+. We support all these assignments with a Coulomb blockade model. For all dots, the emission from the X1− is redshifted relative to the neutral exciton, but surprisingly we observe blueshifts as well as small redshifts for X1+.
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81.07.Ta Quantum dots
81.05.Ea III-V semiconductors
71.35.-y Excitons and related phenomena
78.66.Fd III-V semiconductors
73.23.Hk Coulomb blockade; single-electron tunneling

Substitutional diatomic molecules NO, NC, CO, N2, and O2: Their vibrational frequencies and effects on p doping of ZnO

Sukit Limpijumnong, Xiaonan Li, Su-Huai Wei, and S. B. Zhang

Appl. Phys. Lett. 86, 211910 (2005); http://dx.doi.org/10.1063/1.1931823 (3 pages) | Cited 52 times

Online Publication Date: 18 May 2005

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First-principles calculations show that AB defects substituting on an O site in ZnO where A, B = N, O, or C are an important class of defects whose physical properties cannot be described by the usual split interstitials but rather by substitutional diatomic molecules. The molecular natures of the (AB)O defects are reflected in their vibrational frequencies which are redshifted from those of the corresponding free molecules but only by about 10%. These calculated results agree with the frequency range recently observed by IR measurement on N-doped ZnO. Moreover, most (AB)O defects are donors in p-type samples. The (NC)O and (N2)O defects have sufficiently low energies to convert substituional NO acceptors into donors, thereby hindering the efforts of doping ZnO p type.
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81.05.Dz II-VI semiconductors
71.55.Gs II-VI semiconductors
61.72.uj III-V and II-VI semiconductors
63.20.-e Phonons in crystal lattices
78.30.Fs III-V and II-VI semiconductors
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)

Interaction between dislocations and He-implantation-induced voids in GaN epitaxial layers

D. Alquier, C. Bongiorno, F. Roccaforte, and V. Raineri

Appl. Phys. Lett. 86, 211911 (2005); http://dx.doi.org/10.1063/1.1940121 (3 pages) | Cited 5 times

Online Publication Date: 19 May 2005

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In this letter, we demonstrate that helium high-dose implantation is able to produce voids in GaN and we describe the behavior of material dislocations under these conditions. Two main types of nanovoids are encountered after annealing: cylindrical and pyramidal nanovoids. During their thermal evolution, these vacancy-type defects are interacting with dislocations favoring their local annihilation. The experimental results demonstrate a short-range interaction between nanovoid layer and dislocations, thus having potential applications for the improvement of GaN epitaxial layers quality.
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61.72.Yx Interaction between different crystal defects; gettering effect
61.72.uj III-V and II-VI semiconductors
61.72.Qq Microscopic defects (voids, inclusions, etc.)
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
61.72.Cc Kinetics of defect formation and annealing
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Transverse and longitudinal surface plasmon resonances of a hexagonal array of gold nanorods embedded in an alumina matrix

Brian G. McMillan, Léonard E. A. Berlouis, Francis R. Cruickshank, David Pugh, and Pierre-François Brevet

Appl. Phys. Lett. 86, 211912 (2005); http://dx.doi.org/10.1063/1.1939070 (3 pages) | Cited 7 times

Online Publication Date: 19 May 2005

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The specular reflectance from a hexagonal array of gold nanorods embedded in an alumina matrix supported on an aluminum substrate is reported. The rods were grown by electrodeposition of gold in an alumina template and were oriented with their long axis perpendicular to the film surface. Optical reflectance measurements performed with an incident light beam of S polarization only exhibited the transverse surface plasmon resonance whereas the measurements obtained with P polarization exhibited both transverse and longitudinal resonances. A model for the reflectance from a thin anisotropic film was developed and shown to be in agreement with the experimental data.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.66.Nk Insulators

A comparative study of contact resonance imaging using atomic force microscopy

S. Banerjee, N. Gayathri, S. Dash, A. K. Tyagi, and Baldev Raj

Appl. Phys. Lett. 86, 211913 (2005); http://dx.doi.org/10.1063/1.1927698 (3 pages) | Cited 5 times

Online Publication Date: 19 May 2005

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We present here a comparative study of atomic force microscope (AFM) imaging in contact mode when either the cantilever carrying the probing tip or the sample is excited at ultrasonic frequencies. The cantilever or the sample can be excited by piezoelectric transducers attached to them. When the AFM tip is in contact with the sample surface the contact resonance curve depends on the local tip–sample contact stiffness. By measuring the contact resonance as a function of position one can image the local stiffness of the sample surface. We will report here imaging carried out on piezoelectric material and thin metal film using both the modes. The comparison shows that both give similar results.
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68.37.Ps Atomic force microscopy (AFM)
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
62.20.D- Elasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations
68.55.-a Thin film structure and morphology
77.65.-j Piezoelectricity and electromechanical effects

Effects of weak ordering of InGaPN

K. I. Lin, J. Y. Lee, T. S. Wang, S. H. Hsu, J. S. Hwang, Y. G. Hong, and C. W. Tu

Appl. Phys. Lett. 86, 211914 (2005); http://dx.doi.org/10.1063/1.1940118 (3 pages) | Cited 5 times

Online Publication Date: 20 May 2005

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The effect of weak ordering on InGaPN/GaAs heterostructure grown by gas source molecular-beam epitaxy is quantitatively studied by room-temperature Raman, photoluminescence (PL), and photoreflectance spectroscopy in this work. The PL intensity decreases rapidly as the nitrogen concentration increases, implying that more nonradiative centers are generated by the ordering effect and the degradation of the samples. The band gap of InGaPN decreases dramatically as the nitrogen is incorporated. The Raman modes of InGaPN between 130 and 1000 cm−1 are analyzed. Polarized Raman spectra reveal that the InGaPN layers become more ordered as more nitrogen is incorporated. A broad Raman structure that appeared around 730 cm−1 is attributed to an InGaN-like LO-phonon mode. The transition of the crystal structure from the zinc blende to CuPt structure and the formation of GaN clusters are responsible for the ordering effect in the InGaPN layer.
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64.70.K- Solid-solid transitions
78.55.Cr III-V semiconductors
63.20.-e Phonons in crystal lattices
68.55.-a Thin film structure and morphology
78.20.-e Optical properties of bulk materials and thin films
71.20.Nr Semiconductor compounds
78.30.Fs III-V and II-VI semiconductors

Low-energy ion irradiation during film growth: Kinetic pathways leading to enhanced adatom migration rates

D. Adamovic, E. P. Münger, V. Chirita, L. Hultman, and J. E. Greene

Appl. Phys. Lett. 86, 211915 (2005); http://dx.doi.org/10.1063/1.1940122 (3 pages) | Cited 7 times

Online Publication Date: 20 May 2005

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Embedded-atom molecular dynamics simulations are used to investigate the effects of low-energy self-ion irradiation of Pt adatoms on Pt(111). Here, we concentrate on self-bombardment dynamics, i.e., isolating and monitoring the atomic processes, induced by normally incident Pt atoms with energies E ranging from 5 to 50 eV, that can affect intra- and interlayer mass transport.. We find that adatom scattering, surface channeling, and dimer formation occur at all energies. Atomic intermixing events involving incident and terrace atoms are observed at energies ≥ 15 eV, while the collateral formation of residual surface vacancies is observed only with E>40 eV. The overall effect of low-energy self-ion irradiation is to enhance lateral adatom and terrace atom migration.
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68.43.Jk Diffusion of adsorbates, kinetics of coarsening and aggregation
61.72.J- Point defects and defect clusters
61.80.Jh Ion radiation effects
61.82.Bg Metals and alloys
66.30.Qa Electromigration
61.85.+p Channeling phenomena (blocking, energy loss, etc.)
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Epitaxial tilting of GaN grown on vicinal surfaces of sapphire

X. R. Huang, J. Bai, M. Dudley, R. D. Dupuis, and U. Chowdhury

Appl. Phys. Lett. 86, 211916 (2005); http://dx.doi.org/10.1063/1.1940123 (3 pages) | Cited 14 times

Online Publication Date: 20 May 2005

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The epitaxial tilting effect of GaN films grown on vicinal (0001) surfaces of sapphire and its relationship with the offcut angles and the substrate surface steps have been revealed using synchrotron Laue method and high-resolution x-ray diffraction. This effect is a general consequence of the large out-of-plane lattice mismatch between GaN and sapphire and can be explained by the extended Nagai theory based on the step configurations. The large lattice tilts and their formation mechanism indicate that the substrate surface morphology may be a very important factor that influences the epitaxy process and the crystalline quality of GaN films in vicinal surface epitaxy.
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81.05.Ea III-V semiconductors
68.55.-a Thin film structure and morphology
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.47.Fg Semiconductor surfaces

High-spatial-resolution strain measurements by Auger electron spectroscopy in epitaxial-lateral-overgrowth GaN

Duanjun Cai, Fuchun Xu, Junyong Kang, Pierre Gibart, and Bernard Beaumont

Appl. Phys. Lett. 86, 211917 (2005); http://dx.doi.org/10.1063/1.1929866 (3 pages) | Cited 6 times

Online Publication Date: 20 May 2005

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A high-spatial-resolution strain measurement by Auger electron spectroscopy was established and applied to analyze the strain distributions in epitaxial-lateral-overgrowth (ELO) GaN. The theoretical N KVV Auger line was set by fitting the experimental data and then the relation between Auger shift and strain was obtained. By this relation the local strain distributions in regions of special interest were measured, which well interpret the mechanism of ELO process. A crucial stage for strain release was found within a distance range above the mask. This evidence confirms the existence of the Auger physical shift.
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81.05.Ea III-V semiconductors
79.20.Fv Electron impact: Auger emission
68.60.Bs Mechanical and acoustical properties
62.20.-x Mechanical properties of solids

Enhanced photoconductivity of ZnO films Co-doped with nitrogen and tellurium

H. L. Porter, A. L. Cai, J. F. Muth, and J. Narayan

Appl. Phys. Lett. 86, 211918 (2005); http://dx.doi.org/10.1063/1.1923194 (3 pages) | Cited 23 times

Online Publication Date: 20 May 2005

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Zinc oxide films are typically found to be n type, and conductive under most growth conditions and growth methods. Co-doping with multiple elements is one strategy for improving the electrical and optical properties of zinc oxide materials for optoelectronic device applications. Using pulsed-laser deposition, thin ZnO films were grown on c-axis oriented sapphire. The films were co-doped with nitrogen and tellurium. Depending on the relative concentrations of the dopants, the resistivity of the films was observed to increase by several orders of magnitude, significantly improving the photoconductive response. The enhancement of the photosensitivity reached a maximum at a tellurium concentration of around 1020 cm−3.
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81.05.Dz II-VI semiconductors
61.72.uj III-V and II-VI semiconductors
73.61.Ga II-VI semiconductors
61.72.S- Impurities in crystals
72.40.+w Photoconduction and photovoltaic effects
73.50.Pz Photoconduction and photovoltaic effects

Galvanic effects in Si-based microelectromechanical systems: Thick oxide formation and its implications for fatigue reliability

O. N. Pierron, D. D. Macdonald, and C. L. Muhlstein

Appl. Phys. Lett. 86, 211919 (2005); http://dx.doi.org/10.1063/1.1939072 (3 pages) | Cited 15 times

Online Publication Date: 20 May 2005

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Nanometer-scale reaction layers have a profound impact on the fracture and fatigue resistance of the Si films used in microelectromechanical systems (MEMS). This letter presents experimental evidence that thick (i.e., greater than 10 nm) oxides can form at room temperature during manufacturing due to a galvanic effect between n+-type Si and Au. The growth of such oxides in concentrated HF solutions that are usually associated with oxide dissolution can be predicted from the measured current density–voltage (i–V) behavior and geometry of the galvanic couple. These results can account for unexplained findings in the literature and can be used to improve the performance of MEMS.
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81.05.Cy Elemental semiconductors
62.20.M- Structural failure of materials
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
68.60.-p Physical properties of thin films, nonelectronic
73.61.Cw Elemental semiconductors
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Temperature dependence of electrical conductance in single-crystalline boron nanobelts

K. Kirihara, Z. Wang, K. Kawaguchi, Y. Shimizu, T. Sasaki, N. Koshizaki, K. Soga, and K. Kimura

Appl. Phys. Lett. 86, 212101 (2005); http://dx.doi.org/10.1063/1.1935036 (3 pages) | Cited 16 times

Online Publication Date: 16 May 2005

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We studied electrical transport in single-crystalline boron nanobelts with α-tetragonal crystalline structure. We obtained ohmic contacts to the boron nanobelts by metal electrodes of Ni/Au bilayer. From the temperature dependence of electrical conductance, we found that the boron nanobelt is a semiconductor. The electrical conductivity was of the order of 10−3 (Ω cm)−1 at 295 K. Fitting the results to variable-range-hopping conduction revealed a high density of localized states at the Fermi level compared with bulk β-rhombohedral boron.
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72.20.Ee Mobility edges; hopping transport
73.63.Bd Nanocrystalline materials
73.40.Ns Metal-nonmetal contacts
61.46.-w Structure of nanoscale materials
61.66.Bi Elemental solids

Where does photocurrent flow in polycrystalline CdS?

D. Azulay, O. Millo, S. Silbert, I. Balberg, and N. Naghavi

Appl. Phys. Lett. 86, 212102 (2005); http://dx.doi.org/10.1063/1.1923157 (3 pages) | Cited 10 times

Online Publication Date: 16 May 2005

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We have studied the local photoconductance in polycrystalline CdS films using conductance atomic force microscopy under illumination, and found that photoconductivity along the grain boundaries is excited at photon energies significantly smaller than the CdS band gap, Eg, whereas phototransport through the grains is detected only above Eg. In addition, we observed a rather strong persistent photoconductivity effect at both conduction channels. The implications of these findings regarding the band tails in CdS films and photovoltaic applications are discussed.
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72.40.+w Photoconduction and photovoltaic effects
73.61.Ga II-VI semiconductors
71.20.Nr Semiconductor compounds
61.72.Mm Grain and twin boundaries
68.37.Ps Atomic force microscopy (AFM)

Operation and high-frequency performance of nanoscale unipolar rectifying diodes

J. Mateos, B. G. Vasallo, D. Pardo, and T. González

Appl. Phys. Lett. 86, 212103 (2005); http://dx.doi.org/10.1063/1.1931051 (3 pages) | Cited 20 times

Online Publication Date: 16 May 2005

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By means of the microscopic transport description supplied by a semiclassical two-dimensional Monte Carlo simulator, we provide an in depth explanation of the operation (based on electrostatic effects) of the nanoscale unipolar rectifying diode, so called self-switching diode, recently proposed in A. M. Song, M. Missous, P. Omling, A. R. Peaker, L. Samuelson, and W. Seifert, Appl. Phys. Lett. 83, 1881 (2003) . The simple downscaling of this device and the intrinsically high electron velocity of InGaAs channels allows one to envisaging the fabrication of structures working in the THz range. We analyze the high-frequency performance of the diodes and provide design considerations for the optimization of the downscaling process.
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85.30.Kk Junction diodes
73.40.Ei Rectification

Odd filling factor quantum Hall sequence in magnetic type-III quantum wells

H. Buhmann, E. G. Novik, V. Daumer, J. Liu, Y. S. Gui, C. R. Becker, and L. W. Molenkamp

Appl. Phys. Lett. 86, 212104 (2005); http://dx.doi.org/10.1063/1.1935753 (3 pages)

Online Publication Date: 17 May 2005

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The magnetotransport properties of a series of Hg1−yMnyTe quantum wells (y = 2%) reveal anomalous sequences of quantum Hall plateaus and corresponding minima in the longitudinal magnetoresistance which correspond solely to odd filling factors. This anomaly originates from the very large spin splitting in this material, which exceeds the cyclotron energy even at moderate magnetic fields (B>2T). An 8×8 band k∙p approach has been used to calculate the magnetic field dependence of the Landau levels. Finite temperature and Landau level broadening considerations confirm that subband crossings in the vicinity of the Fermi energy are responsible for the suppression of even filling factor quantum Hall states.
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81.07.St Quantum wells
81.05.Dz II-VI semiconductors
75.50.Pp Magnetic semiconductors
73.63.Hs Quantum wells
73.43.Qt Magnetoresistance
73.21.Fg Quantum wells
72.20.My Galvanomagnetic and other magnetotransport effects
75.75.-c Magnetic properties of nanostructures
71.70.Di Landau levels
71.15.-m Methods of electronic structure calculations
75.47.Pq Other materials

Study on anomalous n-type conduction of P-doped ZnO using P2O5 dopant source

Zhi Gen Yu, Hao Gong, and Ping Wu

Appl. Phys. Lett. 86, 212105 (2005); http://dx.doi.org/10.1063/1.1938249 (3 pages) | Cited 12 times

Online Publication Date: 17 May 2005

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The unexpected n-type conduction observed in P-doped ZnO thin films fabricated from rf magnetron sputtering, was studied systematically through a combined approach of experiment and computer modeling. The carrier stability was predicted from first-principles density functional theory and chemical thermodynamic calculations. It demonstrated that, under oxygen-poor growth condition and low temperature, the stable doping defect PO−1 may have negative effect on n-type conduction and, under oxygen-poor growth condition and high temperature, the stable doping defect may contribute significantly to the n-type conduction. Furthermore, under oxygen-rich growth condition, the stable doping defect PZn1 may help to maintain the n-type conduction at high oxygen partial pressures. Our model predictions are in good agreement with experimental observations in anomalous conduction of P2O5-doped ZnO thin films and provide scientific explanation. This research not only revealed increased fundamental understanding on electronic behaviors but also provided a fabrication strategy for P-doped n-type ZnO.
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81.05.Dz II-VI semiconductors
73.61.Ga II-VI semiconductors
61.72.uj III-V and II-VI semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.15.Cd Deposition by sputtering
65.40.G- Other thermodynamical quantities

Arsenic activation in molecular beam epitaxy grown, in situ doped HgCdTe(211)

P. Boieriu, C. H. Grein, H. S. Jung, J. Garland, and V. Nathan

Appl. Phys. Lett. 86, 212106 (2005); http://dx.doi.org/10.1063/1.1940119 (3 pages) | Cited 13 times

Online Publication Date: 19 May 2005

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Photovoltaic p-n junctions are the most significant active components of both current infrared photodetectors and advanced ones being developed. It is of the utmost importance to control both p- and n-type extrinsic doping. This letter addresses the issue of activating arsenic as a p-type dopant of Hg1−xCdxTe at temperatures sufficiently low that the integrity of p-n junctions and the intrinsic advantages of molecular beam epitaxy as a growth technique will not be compromised. The p-type activation of arsenic in (211)B Hg1−xCdxTe is reported after a two-stage anneal at temperatures below 300 °C for Cd compositions suitable for the sensing of long wavelength infrared radiation.
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61.72.uj III-V and II-VI semiconductors
61.72.Cc Kinetics of defect formation and annealing
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
85.60.Gz Photodetectors (including infrared and CCD detectors)
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