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24 May 2004

Volume 84, Issue 21, pp. 4141-4340

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Appl. Phys. Lett. 84, 4316 (2004); http://dx.doi.org/10.1063/1.1756684 (3 pages)

Seok Pil Jang and Stephen U. S. Choi
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Investigation of forward and reverse current conduction in GaN films by conductive atomic force microscopy

J. Spradlin, S. Dogan, J. Xie, R. Molnar, A. A. Baski, and H. Morkoç

Appl. Phys. Lett. 84, 4150 (2004); http://dx.doi.org/10.1063/1.1751609 (3 pages) | Cited 16 times

Online Publication Date: 6 May 2004

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We have used conductive atomic force microscopy (C–AFM) to investigate the forward and reverse bias current conduction of homo- and heteroepitaxial GaN-based films grown by molecular beam epitaxy. In the case of homoepitaxy, C–AFM shows enhanced current conduction at the centers of ∼30% of spiral hillocks, which are associated with screw dislocations. Local current–voltage spectra taken by C–AFM on and off such hillocks indicate Frenkel–Poole and field emission mechanisms, respectively, for low current levels in forward conduction. In the case of heteroepitaxial GaN films grown on sapphire, the correlation between conduction pathways and topography is more complex. We do observe, however, that films with more rectifying nominal Schottky behavior (less reverse leakage current) produce forward and reverse bias C–AFM images with strong asymmetry. © 2004 American Institute of Physics.
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73.61.Ey III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.37.Ps Atomic force microscopy (AFM)
79.70.+q Field emission, ionization, evaporation, and desorption

Unpinning of Fermi level in nanocrystalline semiconductors

Cesare Malagù, Vincenzo Guidi, Maria Cristina Carotta, and Giuliano Martinelli

Appl. Phys. Lett. 84, 4158 (2004); http://dx.doi.org/10.1063/1.1755419 (3 pages) | Cited 14 times

Online Publication Date: 6 May 2004

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A theoretical model has been developed to interpret the size dependent behavior of nanostructured metal-oxide semiconductors. It is based on the determination of the surface-state density, which pins the Fermi level of the semiconductor, thus removing the linear relationship between the work function and the Schottky barrier. To provide the model with numerical input, measurements of the Schottky barrier height were performed at different temperatures on nanocrystalline SnO2 and TiO2 films. The obtained solution predicts the unpinning of the Fermi level when the material can be considered as nanostructured, and the evidence confirms it. © 2004 American Institute of Physics.
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71.20.Nr Semiconductor compounds
73.20.At Surface states, band structure, electron density of states
73.30.+y Surface double layers, Schottky barriers, and work functions
73.22.Dj Single particle states

Effects of finite deformed length in carbon nanotubes

Jun-Qiang Lu, Jian Wu, Wenhui Duan, and Bing-Lin Gu

Appl. Phys. Lett. 84, 4203 (2004); http://dx.doi.org/10.1063/1.1751608 (3 pages) | Cited 13 times

Online Publication Date: 7 May 2004

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The effect of finite deformed length is demonstrated by squashing an armchair (10,10) single-walled carbon nanotube with two finite tips. Only when the deformed length is long enough, an effectual metal–semiconductor–metal heterojunction can be formed in the metallic tube. The effect of finite deformed length is explained by the quantum tunneling effect. Furthermore, some conceptual designs of nanoscale devices are proposed from the metal–semiconductor–metal heterojunction. © 2004 American Institute of Physics.
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73.20.At Surface states, band structure, electron density of states
73.22.-f Electronic structure of nanoscale materials and related systems
61.46.-w Structure of nanoscale materials

Temperature stability of the refractive index and the direct bandedge in TlInGaAs quaternary alloys

A. Imada, H.-J. Lee, A. Fujiwara, T. Mukai, S. Hasegawa, and H. Asahi

Appl. Phys. Lett. 84, 4212 (2004); http://dx.doi.org/10.1063/1.1755415 (3 pages) | Cited 3 times

Online Publication Date: 7 May 2004

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TlInGaAs quaternary alloy layers were grown on InP substrates by gas-source molecular-beam epitaxy. Refractive index dispersions were determined at the temperature range of 300–340 K in the photon-energy region below and a little above the direct bandedge E0 by the optical reflectance measurements. The temperature dependence of the refractive index was analyzed with the first-order Sellmeier equation. The temperature dependence of the E0 edge was also determined by the absorption measurements. It was found that the temperature coefficients of both refractive index and E0 edge of TlInGaAs are much smaller than those for InGaAs. These results facilitate the fabrication of the temperature-stable-wavelength optoelectronic devices using this alloy system. © 2004 American Institute of Physics.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.66.Fd III-V semiconductors
85.60.-q Optoelectronic devices
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Reduction of spin transfer by synthetic antiferromagnets

N. C. Emley, F. J. Albert, E. M. Ryan, I. N. Krivorotov, D. C. Ralph, R. A. Buhrman, J. M. Daughton, and A. Jander

Appl. Phys. Lett. 84, 4257 (2004); http://dx.doi.org/10.1063/1.1757638 (3 pages) | Cited 23 times

Online Publication Date: 7 May 2004

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Synthetic antiferromagnetic layers (SAF) are incorporated into spin transfer nanopillars giving a layer composition [Cobottom/Ru/Cofixed]/Cu/Cofree, where square brackets indicate the SAF. The Cobottom and Cofixed layers are aligned antiparallel (AP) by strong indirect exchange coupling through the Ru spacer. All three magnetic layers are patterned, so this AP alignment reduces undesirable dipole fields on the Cofree layer. Adding the Cobottom/Ru layers reduces the spin polarization of the electron current passing through the nanopillar, leading to a decreased spin-torque per unit current incident on the Cofree layer. This may be advantageous for device applications requiring a reduction of the effects of a spin-torque, such as nanoscale current-perpendicular-to-plane magnetoresistive read heads. © 2004 American Institute of Physics.
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75.50.Ee Antiferromagnetics
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.47.De Giant magnetoresistance

Quantized charge pumping through a quantum dot by surface acoustic waves

J. Ebbecke, N. E. Fletcher, T. J. B. M. Janssen, F. J. Ahlers, M. Pepper, H. E. Beere, and D. A. Ritchie

Appl. Phys. Lett. 84, 4319 (2004); http://dx.doi.org/10.1063/1.1757016 (3 pages) | Cited 13 times

Online Publication Date: 7 May 2004

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We present a realization of quantized charge pumping. A lateral quantum dot is defined by metallic split gates in a GaAs/AlGaAs heterostructure. A surface acoustic wave whose wavelength is twice the dot length is used to pump single electrons through the dot at a frequency f = 3 GHz. The pumped current shows a regular pattern of quantization at values I = nef over a range of gate voltage and wave amplitude settings. The observed values of n, the number of electrons transported per wave cycle, are determined by the number of electronic states in the quantum dot brought into resonance with the Fermi level of the electron reservoirs during the pumping cycle.
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73.21.La Quantum dots
73.20.At Surface states, band structure, electron density of states

Direct evidence of the Fermi-energy-dependent formation of Mn interstitials in modulation-doped Ga1−yAlyAs/Ga1−xMnxAs/Ga1−yAlyAs heterostructures

K. M. Yu, W. Walukiewicz, T. Wojtowicz, W. L. Lim, X. Liu, M. Dobrowolska, and J. K. Furdyna

Appl. Phys. Lett. 84, 4325 (2004); http://dx.doi.org/10.1063/1.1758291 (3 pages) | Cited 8 times

Online Publication Date: 7 May 2004

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Using ion channeling techniques, we investigate the lattice locations of Mn in Ga1−xMnxAs quantum wells between Be-doped Ga1−yAlyAs barriers. Our earlier results showed that the Curie temperature TC depends on the growth sequence of the epitaxial layers. A lower TC was found in heterostructures in which the Ga1−xMnxAs layer is grown after the modulation-doped barrier. Here, we provide direct evidence that this reduction in TC is directly correlated with an increased formation of magnetically inactive Mn interstitials. The formation of interstitials is induced by a shift of the Fermi energy as a result of the transfer of holes from the barrier to the quantum well during the growth. © 2004 American Institute of Physics.
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75.50.Pp Magnetic semiconductors
68.65.Fg Quantum wells
75.50.Dd Nonmetallic ferromagnetic materials
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
73.21.Fg Quantum wells
61.72.J- Point defects and defect clusters
72.20.Fr Low-field transport and mobility; piezoresistance
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