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3 Sep 2007

Volume 91, Issue 10, Articles (10xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 102101 (2007); http://dx.doi.org/10.1063/1.2778542 (3 pages)

A. Hübel, J. Weis, W. Dietsche, and K. v. Klitzing
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Optical loss and interface morphology in AlGaAs/GaAs distributed Bragg reflectors

Z. Zhang, R. Marcks von Würtemberg, J. Berggren, and M. Hammar

Appl. Phys. Lett. 91, 101101 (2007); http://dx.doi.org/10.1063/1.2779242 (3 pages) | Cited 7 times

Online Publication Date: 4 September 2007

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It is shown that n-type doping of AlGaAs/GaAs distributed Bragg reflectors (DBRs) grown by metal-organic vapor-phase epitaxy has a profound negative impact on the performance of vertical-cavity surface-emitting lasers (VCSELs) based on such mirrors. Using an intracavity contact scheme, 1.3-μm-range InGaAs VCSELs with and without doping in the bottom DBR are directly compared. Doped mirrors lead to lower slope efficiency, lower output power, and higher threshold current. From x-ray diffraction, high-accuracy reflectance measurements, and atomic force microscopy studies, it is suggested that this performance degradation is due to the doping-enhanced Al–Ga interdiffusion, leading to interface roughening and increased scattering loss.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.60.By Design of specific laser systems
42.82.Gw Other integrated-optical elements and systems
42.79.Bh Lenses, prisms and mirrors

Tunable lead-chalcogenide on Si resonant cavity enhanced midinfrared detector

F. Felder, M. Arnold, M. Rahim, C. Ebneter, and H. Zogg

Appl. Phys. Lett. 91, 101102 (2007); http://dx.doi.org/10.1063/1.2779244 (3 pages) | Cited 8 times

Online Publication Date: 4 September 2007

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Midinfrared tunable resonant cavity enhanced detectors have been realized. The linewidths of 0.07 μm are determined by the finesse of the cavities, while the length of the cavity can be changed with a movable mirror. This allows tuning across the 4–5.5 μm midinfrared wavelength range. The thin (0.3 μm) photodiodes inside the cavity are based on lead-chalcogenide narrow gap semiconductor layers grown epitaxially onto a Si substrate. Due to the thin active layer, a higher sensitivity at the higher operation temperatures is achieved as compared to conventional thick photodiodes.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.60.Dw Photodiodes; phototransistors; photoresistors

Tailoring of optical mode profiles of high-power diode lasers evidenced by near-field photocurrent spectroscopy

Anna Kozlowska, Michał Szymański, Emilia Pruszyńska-Karbownik, Maciej Bugajski, Robert Pomraenke, Christoph Lienau, Julien Renard, and Andrzej Maląg

Appl. Phys. Lett. 91, 101103 (2007); http://dx.doi.org/10.1063/1.2779846 (3 pages)

Online Publication Date: 4 September 2007

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Tailoring of optical mode profiles of high-power diode lasers is directly demonstrated by means of near-field photocurrent spectroscopy. Three double barrier separate confinement heterostructures with different confinement geometries are designed and their optical mode profiles are studied both theoretically and experimentally. The near-field spectroscopic results clearly resolve the intended variation in optical mode width. A remaining discrepancy between the designed and experimentally measured mode profiles, manifesting itself in the reduction of their evanescent tails, is attributed to the structure of the antiguiding barrier. The results demonstrate that near-field field photocurrent spectroscopy is a powerful, nondestructive, and quantitative technique for optical waveguide inspection in high-power diode lasers.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

Reduced free carrier absorption loss in midinfrared double heterostructure diode lasers grown by liquid phase epitaxy

M. Yin, A. Krier, R. Jones, and P. J. Carrington

Appl. Phys. Lett. 91, 101104 (2007); http://dx.doi.org/10.1063/1.2779246 (3 pages) | Cited 3 times

Online Publication Date: 4 September 2007

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An improved InAsSb/InAsSbP double heterojunction ridge laser was designed and grown by liquid phase epitaxy. The cladding layer absorption loss was minimized by the introduction of two undoped quaternary layers on either side of the active region to form a five layer epitaxial structure. The inserted layers also helped alleviate interdiffusion of unwanted dopants into the active region and reduced current leakage in the device. The resulting diode lasers operate readily in pulsed mode near 3.5 μm at elevated temperatures and with a threshold current density as low as 118  A cm−2 at 85 K. Compared to the conventional three-layer double heterostructure laser, the modified structure with reduced optical loss increased the maximum lasing temperature by 95–210 K.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)

Tip-enhanced Raman spectroscopy for investigating adsorbed species on a single-crystal surface using electrochemically prepared Au tips

Xi Wang, Zheng Liu, Mu-De Zhuang, Hai-Ming Zhang, Xiang Wang, Zhao-Xiong Xie, De-Yin Wu, Bin Ren, and Zhong-Qun Tian

Appl. Phys. Lett. 91, 101105 (2007); http://dx.doi.org/10.1063/1.2776860 (3 pages) | Cited 25 times

Online Publication Date: 4 September 2007

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A tip-enhanced Raman instrument was set up based on a homemade optical fiber Raman head, a dispersive spectrograph, and a scanning tunneling microscope (STM) system. Electrochemical preparation of tip-enhanced Raman spectroscopy (TERS) Au tips was refined by using the etching current as ending point control, resulting in a success rate as high as 90%. The high quality Au tips allow the recording of STM images with molecular resolution and TERS spectra of nonresonant surface species on a single-crystal surface.
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78.30.-j Infrared and Raman spectra
68.43.Mn Adsorption kinetics
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
82.45.-h Electrochemistry and electrophoresis

Carrier capture in InGaAsSb/InAs/InGaSb type-II laser heterostructures

L. Shterengas, R. Kaspi, A. P. Ongstad, S. Suchalkin, and G. Belenky

Appl. Phys. Lett. 91, 101106 (2007); http://dx.doi.org/10.1063/1.2771037 (3 pages) | Cited 4 times

Online Publication Date: 5 September 2007

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Experimental studies of the electron and hole concentration dynamics in the barrier of GaSb-based type-II quantum-well (QW) heterostructures were performed. Capture of electrons and holes was studied separately in specially designed and grown laser heterostructures with QWs only for electrons or only for holes. The difference between electron and hole relaxation rates is explained by corresponding QW carrier confinement energies.
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73.63.Hs Quantum wells
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
42.65.Pc Optical bistability, multistability, and switching, including local field effects

Detection of THz radiation with semiconductor diode lasers

C. Brenner, S. Hoffmann, M. R. Hofmann, M. Salhi, M. Koch, A. Klehr, G. Erbert, G. Tränkle, J. T. Steiner, M. Kira, and S. W. Koch

Appl. Phys. Lett. 91, 101107 (2007); http://dx.doi.org/10.1063/1.2783172 (3 pages) | Cited 5 times

Online Publication Date: 6 September 2007

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As a consequence of the strong many-body interactions in the electron-hole plasma, a semiconductor laser efficiently interacts with terahertz radiation. The injection of terahertz laser radiation into the active region of a diode laser induces a measurable variation of the voltage over the p-n junction, indicating the potential of a semiconductor laser to act as a terahertz detector.
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07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.25.Pb Superconducting infrared, submillimeter and millimeter wave detectors
42.55.Px Semiconductor lasers; laser diodes

Self-assembled biomimetic antireflection coatings

Nicholas C. Linn, Chih-Hung Sun, Peng Jiang, and Bin Jiang

Appl. Phys. Lett. 91, 101108 (2007); http://dx.doi.org/10.1063/1.2783475 (3 pages) | Cited 28 times

Online Publication Date: 6 September 2007

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The authors report a simple self-assembly technique for fabricating antireflection coatings that mimic antireflective moth eyes. Wafer-scale, nonclose-packed colloidal crystals with remarkable large hexagonal domains are created by a spin-coating technology. The resulting polymer-embedded colloidal crystals exhibit highly ordered surface modulation and can be used directly as templates to cast poly(dimethylsiloxane) (PDMS) molds. Moth-eye antireflection coatings with adjustable reflectivity can then be molded against the PDMS master. The specular reflection of replicated nipple arrays matches the theoretical prediction using a thin-film multilayer model. These biomimetic films may find important technological application in optical coatings and solar cells.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
81.16.Dn Self-assembly
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.55.A- Nucleation and growth
78.66.Qn Polymers; organic compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Temperature and angle tuning of second harmonic generation in media with a short-range order

Yan Sheng, Junhong Dou, Jingjuan Li, Dongli Ma, Bingying Cheng, and Daozhong Zhang

Appl. Phys. Lett. 91, 101109 (2007); http://dx.doi.org/10.1063/1.2783225 (3 pages) | Cited 10 times

Online Publication Date: 7 September 2007

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The authors theoretically study the nonlinear optical characterization of the short-range ordered structure and deduce the analytical expression of the effective nonlinear optical coefficient. The dependences of the second harmonic conversion efficiency on crystal temperature, incident angle, and reversed domain cycle are measured. They find that for a substantial range of parameters, the short-range ordered structure has a broader tuning response than the periodic or the quasiperiodic structures. The experimental results accord well with the theory.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.70.Mp Nonlinear optical crystals

Organic-based distributed feedback lasers by direct electron-beam lithography on conjugated polymers

Ripalta Stabile, Andrea Camposeo, Luana Persano, Silvia Tavazzi, Roberto Cingolani, and Dario Pisignano

Appl. Phys. Lett. 91, 101110 (2007); http://dx.doi.org/10.1063/1.2779104 (3 pages) | Cited 5 times

Online Publication Date: 7 September 2007

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The authors demonstrate direct electron-beam writing on conjugated polymers as patterning route to realize plastic optoelectronic devices. Lithography was carried out by a 20 kV electron beam dose in the range of 0–360 μC/cm2, with no need for masking or development/etching processes. The features could be employed for the fabrication of polymer distributed feedback lasers, exhibiting optically pumped lasing in the range of 607–620 nm, with a spectral linewidth around 1 nm and a threshold excitation fluence of 34 μJ/cm2.
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42.55.Rz Doped-insulator lasers and other solid state lasers
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Simulation for generation of 15 fs laser pulses by Raman backscatter in plasmas

Min Sup Hur, Jaehoon Kim, Devki N. Gupta, Hyo Jae Jang, and Hyyong Suk

Appl. Phys. Lett. 91, 101501 (2007); http://dx.doi.org/10.1063/1.2779926 (3 pages) | Cited 2 times

Online Publication Date: 5 September 2007

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Pulse compression using the Raman backscatter (RBS) in plasmas was numerically investigated for the strong kinetic regime. It was found that shortening of a seed pulse is more effective when the interaction length is smaller, which is contradictory to the general expectation. In a representative case, compression of up to 14 fs could be obtained from the RBS interaction length less than 0.1 mm. Behavior of the Raman amplification system for such a short interaction distance was not addressed before. Estimation with realistic parameters indicates that the output power can reach tens of terawatts.
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52.38.Bv Rayleigh scattering; stimulated Brillouin and Raman scattering
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.25.Dg Plasma kinetic equations
52.65.Rr Particle-in-cell method
42.65.Dr Stimulated Raman scattering; CARS
42.65.Es Stimulated Brillouin and Rayleigh scattering
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
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First-principles study of pressure-induced metal-insulator transition in BiNiO3

M. Q. Cai, G. W. Yang, X. Tan, Y. L. Cao, L. L. Wang, W. Y. Hu, and Y. G. Wang

Appl. Phys. Lett. 91, 101901 (2007); http://dx.doi.org/10.1063/1.2779925 (3 pages) | Cited 11 times

Online Publication Date: 4 September 2007

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First-principles calculation is used to study the pressure-induced metal-insulator transition in BiNiO3. It is found that the G-antiferromagnetic insulator triclinic phase in BiNiO3 transforms to the ferromagnetic half-metallic orthorhombic phase with a volume collapse of 6.2% when the applied pressure is 3.46 GPa. The interaction between neighbor Ni atoms creates the energy band gap of 1.96 eV in the majority density of states. The pressure suppresses the charge disproportion of Bi. The strong hybridizations of Ni–O and Bi–O lead to the decrease of the spin magnetic moment with 1.74μB compared with the Ni3+ with d7 configuration 3μB.
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71.30.+h Metal-insulator transitions and other electronic transitions
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Cr Saturation moments and magnetic susceptibilities
64.70.K- Solid-solid transitions

Polymer constraint effect for electrothermal bimorph microactuators

T. Chu Duc, G. K. Lau, and P. M. Sarro

Appl. Phys. Lett. 91, 101902 (2007); http://dx.doi.org/10.1063/1.2779929 (3 pages) | Cited 4 times

Online Publication Date: 4 September 2007

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The authors report on the analysis of the polymer constraint effect and its use for a micromachined electrothermal bimorph actuator. The actuated displacement is enhanced due to the polymer constraint effect. Both the thermal expansion and apparent Young’s modulus of the constrained polymer blocks are significantly improved, compared with the no constraint case. The calculation that agrees well with experimental results provides the means to optimize the design of the constrained polymer stack electrothermal microactuator.
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07.07.Tw Servo and control equipment; robots
07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

In situ Raman characterization of reversible phase transition in stress-induced amorphous silicon

Kehui Wu, X. Q. Yan, and M. W. Chen

Appl. Phys. Lett. 91, 101903 (2007); http://dx.doi.org/10.1063/1.2779933 (3 pages) | Cited 2 times

Online Publication Date: 4 September 2007

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The authors report a reversible phase transition of stress-induced amorphous silicon subjected to laser heating. In situ Raman characterization suggested that a metastable crystalline phase precipitates from the amorphous silicon upon annealing at ∼ 400 °C and vanishes after subsequent cooling at room temperature. The unusual reversible phase transition is most likely associated with the unique atomic structure of stress-induced amorphous silicon and high residual stresses within amorphous imprints.
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64.70.K- Solid-solid transitions
78.30.Am Elemental semiconductors and insulators

Elastic and electronic properties of TcB2 and superhard ReB2: First-principles calculations

Yuan Xu Wang

Appl. Phys. Lett. 91, 101904 (2007); http://dx.doi.org/10.1063/1.2780077 (3 pages) | Cited 42 times

Online Publication Date: 4 September 2007

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The structural, elastic, and electronic properties of TcB2 and recent synthesized superhard ReB2 were studied by the first-principles calculations. Both ReB2 and TcB2 are found to be elastically stable with the hexagonal and orthorhombic structures. For the two materials, the hexagonal structure is more stable than the orthorhombic one. Moreover, hexagonal ReB2 and TcB2 have stronger directional bonding between ions than OsB2, IrN2, OsN2, and PtN2. The author’s calculations show that both ReB2 and TcB2 are potentially superhard materials on the basis of their large bulk moduli and also their large shear to bulk modulus ratios. The band structure shows that experimentally synthesized hexagonal ReB2 is metallic. A pseudogap appears around the Fermi level of the total density of states of hexagonal ReB2, which may contribute to its stability.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
71.20.Ps Other inorganic compounds
71.15.-m Methods of electronic structure calculations
61.66.Fn Inorganic compounds

Influence of random roughness on the adhesion between metal surfaces due to capillary condensation

P. J. van Zwol, G. Palasantzas, and J. Th. M. De Hosson

Appl. Phys. Lett. 91, 101905 (2007); http://dx.doi.org/10.1063/1.2768919 (3 pages) | Cited 12 times

Online Publication Date: 5 September 2007

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The capillary force was measured by atomic force microscopy between a gold coated sphere mounted on a cantilever and gold surfaces with different roughnesses. For smooth surfaces the capillary adhesive force surpasses in magnitude any dispersion, e.g., van der Waals/Casimir and/or electrostatic forces. A substantial decrease in the capillary force was observed by increasing the roughness ampltitude a few nanometers in the range of 1–10 nm. From these measurements two limits can be defined: a smooth limit where a closely macroscopic size contact surface interacts through the capillary force and a rough limit where only a few asperities give a capillary contribution.
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68.35.Np Adhesion
68.35.B- Structure of clean surfaces (and surface reconstruction)

Direct hcp→bcc structural phase transition observed in titanium alloy at high pressure

Nenad Velisavljevic and Gary N. Chesnut

Appl. Phys. Lett. 91, 101906 (2007); http://dx.doi.org/10.1063/1.2780078 (3 pages) | Cited 3 times

Online Publication Date: 5 September 2007

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Structural stability of Ti alloy, stabilized as a mixture of bcc (β) and hcp (α) phases, was investigated up to 71 GPa. Over this pressure, β phase remains stable, while for α phase, a decrease in c/a ratio from 1.601 at ambient pressure to 1.568 at 36 GPa is observed, followed by an increase to 1.613 at 44 GPa. A steady value is then observed up to 67 GPa. Above 58 GPa, the authors also observe that α phase peak intensity is decreasing. At 67 GPa, the authors only observe β phase peaks, which indicates that α→β structural phase transition is complete.
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62.50.-p High-pressure effects in solids and liquids
64.70.K- Solid-solid transitions

Mechanical response of freestanding Au nanopillars under compression

Luis A. Zepeda-Ruiz, Babak Sadigh, Juergen Biener, Andrea M. Hodge, and Alex V. Hamza

Appl. Phys. Lett. 91, 101907 (2007); http://dx.doi.org/10.1063/1.2778761 (3 pages) | Cited 16 times

Online Publication Date: 6 September 2007

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We employ molecular dynamics simulations of defect-free nanopillars with realistic cylindrical geometries to obtain an atomic-level picture of their deformation behavior under compression. We find that dislocations are nucleated in the two outermost surface layers. Furthermore, plastic yield depends crucially on the particular arrangement of steps and facets at the surface of the nanopillars. We show that different facet orientations can differ dramatically in their response to external stresses. Freestanding nanopillars exhibit a highly nonuniform distribution of stresses along their height. This causes an elastic deformation that leads to a barrel-like shape attained by the nanopillars under compression. The stress concentration at the center of the pillars due to barreling causes dislocations to preferentially nucleate in this region.
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62.20.D- Elasticity
62.20.F- Deformation and plasticity
62.25.-g Mechanical properties of nanoscale systems
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
61.46.-w Structure of nanoscale materials

High resolution transmission electron microscopy of InN

T. P. Bartel, C. Kisielowski, P. Specht, T. V. Shubina, V. N. Jmerik, and S. V. Ivanov

Appl. Phys. Lett. 91, 101908 (2007); http://dx.doi.org/10.1063/1.2779843 (3 pages) | Cited 11 times

Online Publication Date: 6 September 2007

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Hexagonal InN layers were grown by molecular beam epitaxy and studied by high resolution electron microscopy and by photoluminescence spectroscopy. Inclusions of a few nanometers in diameter were found, which are among the smallest reported. Image simulation, beam sensitivity, and photoluminescence of the samples corroborate that these inclusions are indeed metallic indium. This letter provides evidence that nanoscopic metallic indium inclusions can be present in InN crystals and have a strong influence on its optical properties.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Investigation of the structural transformation behavior of Ge2Sb2Te5 thin films using high resolution electron microscopy

Eun Tae Kim, Jeong Yong Lee, and Yong Tae Kim

Appl. Phys. Lett. 91, 101909 (2007); http://dx.doi.org/10.1063/1.2783478 (3 pages) | Cited 6 times

Online Publication Date: 6 September 2007

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Structural transformation of the Ge2Sb2Te5 was investigated by a high resolution transmission electron microscopy (HRTEM). It was found that Ge atoms undergo umbrella-flip motion from a tetrahedral site into an octahedral site in transforming from the amorphous to the metastable phase of Ge2Sb2Te5. The presence of a twin boundary between fcc and hexagonal structured Ge2Sb2Te5 was also confirmed through the HRTEM observations. These results support the umbrella-flip model proposed by Kolobov et al. [Nat. Mater. 3, 703 (2004)] and the epitaxial growth model proposed by Park et al. [Appl. Surf. Sci. 256, 8102 (2006)] .
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68.55.-a Thin film structure and morphology
64.70.M- Transitions in liquid crystals
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
61.72.Mm Grain and twin boundaries
81.05.Bx Metals, semimetals, and alloys

Ti- and Sr-rich surfaces of SrTiO3 studied by grazing incidence x-ray diffraction

A. Fragneto, G. M. De Luca, R. Di Capua, U. Scotti di Uccio, M. Salluzzo, X. Torrelles, Tien-Lin Lee, and J. Zegenhagen

Appl. Phys. Lett. 91, 101910 (2007); http://dx.doi.org/10.1063/1.2779972 (3 pages) | Cited 10 times

Online Publication Date: 7 September 2007

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The structure of SrTiO3 (100) (STO) single crystal surfaces has been investigated by grazing incidence x-ray diffraction. The authors found that chemically etched STO is composed ∼ 75% of an ideal TiO2 surface layer and ∼ 25% of SrO and it is very stable when annealed in ultrahigh vacuum or in O2 at high temperatures. A monolayer of SrO, epitaxially grown by pulsed laser deposition, is found to cover only a fraction of the TiO2 layers and exhibits a 2×2 reconstruction with strong structural changes after annealing in conditions typically used for the deposition of oxides.
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68.35.B- Structure of clean surfaces (and surface reconstruction)
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Two laterally arranged quantum dot systems with strong capacitive interdot coupling

A. Hübel, J. Weis, W. Dietsche, and K. v. Klitzing

Appl. Phys. Lett. 91, 102101 (2007); http://dx.doi.org/10.1063/1.2778542 (3 pages) | Cited 10 times

Online Publication Date: 4 September 2007

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A method has been developed to form two quantum dot systems in lateral arrangement in a two-dimensional electron system of a GaAs–AlGaAs heterostructure with strong capacitive interdot coupling. In the authors’ design, the interdot capacitance can reach more than one-third of the single-dot capacitance while tunneling between the dots is excluded. This has been achieved by a floating metallic electrode covering both quantum dots, a method already used in split-gate designs before. Here, however, they have reduced the capacitive coupling of this floating gate to other electrodes in the surroundings by an etching technique to obtain a large interdot coupling.
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73.63.Kv Quantum dots
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.40.Gk Tunneling
81.07.Ta Quantum dots
81.05.Ea III-V semiconductors
81.65.Cf Surface cleaning, etching, patterning

Numbers of donors and acceptors from transport measurements in graphene

D. S. Novikov

Appl. Phys. Lett. 91, 102102 (2007); http://dx.doi.org/10.1063/1.2779107 (3 pages) | Cited 38 times

Online Publication Date: 4 September 2007

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A method is suggested to separately determine the surface density of positively and negatively charged impurities that limit the mobility in a graphene monolayer. The method is based on the exact result for the transport cross section, according to which the massless carriers are scattered more strongly when they are attracted to a charged impurity than when they are repelled from it.
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72.20.Fr Low-field transport and mobility; piezoresistance

Electron mobility enhancement in strained-germanium n-channel metal-oxide-semiconductor field-effect transistors

Y.-J. Yang, W. S. Ho, C.-F. Huang, S. T. Chang, and C. W. Liu

Appl. Phys. Lett. 91, 102103 (2007); http://dx.doi.org/10.1063/1.2779845 (3 pages) | Cited 27 times

Online Publication Date: 4 September 2007

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The dependence of electron mobility on strain, channel direction, and substrate orientation is theoretically studied for the germanium n-channel metal-oxide-semiconductor field-effect transistors. For the unstrained channel, (111) substrate can provide the highest mobility among the three orientations, mainly due to its largest quantization mass and smallest conductivity mass in L valley. The tensile strain parallel to the [math10] channel direction on (111) substrate gives 4.1 times mobility of Si at 1 MV/cm, and the mobility enhancement starts to saturate for the strain larger than 0.5%. The compressive strain of ∼ 1.5% transverse to [math10] on (111) substrate yields 2.9 times mobility enhancement at 1 MV/cm.
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85.30.Tv Field effect devices

Variation in the properties of the interface in a CoFeB/MgO/CoFeB tunnel junction during thermal annealing

Youngman Jang, Chunghee Nam, Ki-Su Lee, B. K. Cho, Y. J. Cho, Kwang-Seok Kim, and K. W. Kim

Appl. Phys. Lett. 91, 102104 (2007); http://dx.doi.org/10.1063/1.2779915 (3 pages) | Cited 12 times

Online Publication Date: 4 September 2007

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Variation in the quality of the interface in a CoFeB/MgO/CoFeB tunnel junction during thermal annealing was investigated using x-ray photoemission spectroscopy. The formation of B oxide and the reduction of Fe oxide at the bottom interface after thermal annealing near Ta = 300 °C were found to enhance the tunneling magnetoresistance ratio significantly. At the same time, an asymmetry of the conductance (dV/dI) in the bias polarity and a local minimum of conductance in a positive bias state were measured which were attributed to the presence of a minority state at the bottom interface. The authors believe that the existence of the Bloch state was also responsible for the failure of the application of the Brinkman-Dynes-Rowell or Simmons models to the CoFeB/MgO/CoFeB junction.
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73.40.Rw Metal-insulator-metal structures
79.60.Jv Interfaces; heterostructures; nanostructures
81.40.Gh Other heat and thermomechanical treatments
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