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24 Jan 2005

Volume 86, Issue 4, Articles (04xxxx)

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Appl. Phys. Lett. 86, 043106 (2005); http://dx.doi.org/10.1063/1.1853514 (3 pages)

William L. Hughes and Zhong L. Wang
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Quasiballistic transport of hot holes in GaAs submicron channels

O. Makarovsky, A. Neumann, D. Walker, A. Patanè, L. Eaves, M. Henini, S. Thoms, and C. D. W. Wilkinson

Appl. Phys. Lett. 86, 042101 (2005); http://dx.doi.org/10.1063/1.1851003 (3 pages) | Cited 1 time

Online Publication Date: 18 January 2005

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We investigate the ballistic motion of hot holes in a submicron channel fabricated from a modulation-doped p-type GaAs/(AlGa)As quantum well heterostructure. We observe peaks in the low temperature differential conductance, which we attribute to ballistic motion of holes limited by the onset of longitudinal-optical (LO) phonon emission when the voltage drop along the channel corresponds to an integer multiple of the LO-phonon energy.
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73.63.Hs Quantum wells
73.61.Ey III-V semiconductors
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.23.Ad Ballistic transport
73.50.Fq High-field and nonlinear effects
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials

Effect of film thickness on the incorporation of Mn interstitials in Ga1−xMnxAs

K. M. Yu, W. Walukiewicz, T. Wojtowicz, J. Denlinger, M. A. Scarpulla, X. Liu, and J. K. Furdyna

Appl. Phys. Lett. 86, 042102 (2005); http://dx.doi.org/10.1063/1.1855430 (3 pages) | Cited 11 times

Online Publication Date: 18 January 2005

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We have investigated the effect of film thickness on the distribution of Mn atoms at various lattice sites in Ga1−xMnxAs thin films. We find that the growth surface acts as a sink facilitating the outdiffusion of Mn interstitials (MnI), and thus reducing its concentration in the film. The outdiffused MnI accumulate on the surface in a surface oxide layer and do not participate in the ferromagnetism of the film. For thin films less than 15 nm thick, no MnI can be detected. Because of the absence of compensating MnI defects, higher TC can be achieved for such extremely thin Ga1−xMnxAs layers. These results agree with our previously suggested Fermi-level-governed upper limit of the TC of III–Mn–V ferromagnetic semiconductors.
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75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
68.55.-a Thin film structure and morphology
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.S- Impurities in crystals
68.35.Fx Diffusion; interface formation
66.30.J- Diffusion of impurities
61.72.J- Point defects and defect clusters

Zinc vacancies in the heteroepitaxy of ZnO on sapphire: Influence of the substrate orientation and layer thickness

A. Zubiaga, F. Tuomisto, F. Plazaola, K. Saarinen, J. A. Garcia, J. F. Rommeluere, J. Zuñiga-Pérez, and V. Muñoz-Sanjosé

Appl. Phys. Lett. 86, 042103 (2005); http://dx.doi.org/10.1063/1.1855412 (3 pages) | Cited 23 times

Online Publication Date: 18 January 2005

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Positron annihilation spectroscopy has been used to study the vacancy-type defects produced in films grown by metalorganic chemical vapor deposition on different sapphire orientations. Zn vacancies are the defects controlling the positron annihilation spectra at room temperature. Close to the interface (<500 nm) their concentration depends on the surface plane of sapphire over which the ZnO film has been grown. The Zn vacancy content in the film decreases with thickness, and above 1 μm it is independent of the substrate orientation.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.-a Thin film structure and morphology
61.72.J- Point defects and defect clusters
61.72.S- Impurities in crystals
78.70.Bj Positron annihilation

Unraveling the conduction mechanism of Al-doped ZnO films by valence band soft x-ray photoemission spectroscopy

Mercedes Gabás, Susana Gota, José Ramón Ramos-Barrado, Miguel Sánchez, Nicholas T. Barrett, José Avila, and Maurizio Sacchi

Appl. Phys. Lett. 86, 042104 (2005); http://dx.doi.org/10.1063/1.1856141 (3 pages) | Cited 22 times

Online Publication Date: 18 January 2005

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We report on the correlation between the electrical behavior and valence band spectra of undoped and Al-doped ZnO films, obtained by using x-ray photoelectron spectroscopy. Although Al-doping can induce a conductivity increase of two orders of magnitude, we show that the gap persists and there is no semiconductor–metal transition upon doping. For the 3% Al-doped ZnO film, we measure a reduction in the band gap of ∼ 150 meV with respect to the undoped and the 1% doped films. Our results suggest that the band conduction mechanism proposed for undoped ZnO at room temperature still dominates the conduction process in doped films.
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73.61.Ga II-VI semiconductors
73.20.At Surface states, band structure, electron density of states
79.60.Dp Adsorbed layers and thin films
79.60.Bm Clean metal, semiconductor, and insulator surfaces

Humidity effect on electrical performance of organic thin-film transistors

Dawen Li, Evert-Jan Borkent, Robert Nortrup, Hyunsik Moon, Howard Katz, and Zhenan Bao

Appl. Phys. Lett. 86, 042105 (2005); http://dx.doi.org/10.1063/1.1852708 (3 pages) | Cited 95 times

Online Publication Date: 18 January 2005

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Humidity dependence of electrical performance of p-channel organic thin-film transistors (OTFTs) with various semiconductor compounds has been investigated. All devices showed decreased current output and mobility as the relative humidity (RH) was increased. The moisture sensitivity of the OTFT saturation current depends on the device geometry (bottom or top contact device) and channel length. The OTFT configuration with a short channel length and bottom contact was most affected by humidity compared to the top contact and larger channel length OTFT structures. The degradation of electrical performance under high RH is attributed to charge trapping at grain boundaries by polar water molecules reducing the rate of charge transport.
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85.30.Tv Field effect devices
73.61.Ph Polymers; organic compounds
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.50.Dn Low-field transport and mobility; piezoresistance
61.72.Mm Grain and twin boundaries

Nanometer-scale two-terminal semiconductor memory operating at room temperature

A. M. Song, M. Missous, P. Omling, I. Maximov, W. Seifert, and L. Samuelson

Appl. Phys. Lett. 86, 042106 (2005); http://dx.doi.org/10.1063/1.1852711 (3 pages) | Cited 18 times

Online Publication Date: 18 January 2005

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Based on a nanometer-scale semiconductor channel with an intentionally broken geometric symmetry, we have realized a type of memory device that consists of only two terminals, rather than the minimum of three terminals in conventional semiconductor memories. The charge retention time is at least 10 h at cryogenic temperatures and a few minutes at room temperature. Furthermore, the simplicity of the design allows the active part of the devices to be made in a single nanolithography step which, along with the planar structure of the device, provides promising possibilities for a high integration density.
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85.30.Kk Junction diodes
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
81.16.Nd Micro- and nanolithography
85.30.De Semiconductor-device characterization, design, and modeling
85.40.Hp Lithography, masks and pattern transfer
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

On the origin of the two-dimensional electron gas at the AlGaN/GaN heterostructure interface

G. Koley and M. G. Spencer

Appl. Phys. Lett. 86, 042107 (2005); http://dx.doi.org/10.1063/1.1850600 (3 pages) | Cited 26 times

Online Publication Date: 18 January 2005

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Bare surface barrier heights (BSBHs) of AlGaN/GaN heterostructures, with varying AlGaN layer thickness and ∼ 35% Al alloy composition, have been measured using UV laser induced transients. The BSBH has been observed to vary with AlGaN thickness, before it saturates beyond a critical thickness, and the variation found to be very similar to that of the two-dimensional electron gas (2DEG) density at the interface. Such a trend can be explained by considering the presence of surface donor states distributed in the band gap. The density of the surface donor states has been calculated from the variation of the surface barrier with the 2DEG density, and found to be almost constant at ∼ 1.6×1013 cm−2 eV−1 in the energy range of ∼ 1.0–1.8 eV from the conduction band. After SiNx passivation of the surface, the BSBH reduces, and sheet charge density increases, indicating the presence of positive charges in the passivation layer.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
73.20.Hb Impurity and defect levels; energy states of adsorbed species
73.20.At Surface states, band structure, electron density of states
81.65.Rv Passivation
81.05.Ea III-V semiconductors
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Fk Semiconductors

Adjustable optical anisotropy in porous GaAs

Vladimir Kochergin, Marc Christophersen, and Helmut Föll

Appl. Phys. Lett. 86, 042108 (2005); http://dx.doi.org/10.1063/1.1849846 (3 pages) | Cited 4 times

Online Publication Date: 19 January 2005

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We report a theoretical investigation of the optical properties of porous (100) GaAs having crystallographic pores. An effective medium approach is used for the calculations. A biaxial anisotropy of the material is predicted for most of the material parameters. The parameter windows for different kinds of uniaxial anisotropy are predicted as well. It is shown that the type and value of the optical anisotropy, and even the direction of the optical axes, can be controlled by GaAs etching parameters, and this letter gives the theoretical blueprint for the overall pore morphology required for that.
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42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.43.Gt Powders, porous materials
68.35.B- Structure of clean surfaces (and surface reconstruction)

Halogen n-type doping of chalcopyrite semiconductors

Stephan Lany, Yu-Jun Zhao, Clas Persson, and Alex Zunger

Appl. Phys. Lett. 86, 042109 (2005); http://dx.doi.org/10.1063/1.1854218 (3 pages) | Cited 11 times

Online Publication Date: 19 January 2005

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We theoretically identify the chemical thermodynamic boundary conditions that will produce n-type CuInSe2 via halogen doping. Remarkably, we find that due to the low formation energies of the intrinsic defects, VCu and InCu in CuInSe2, the growth conditions that maximize the halogen donor incorporation do not yield n-type conductivity, whereas the conditions that maximize the concentration of the intrinsic donor InCu do yield n-type conductivity. Under the latter conditions, however, the contribution of the halogen donors to the net donor concentration stays significantly below that of InCu.
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61.72.up Other materials
61.72.S- Impurities in crystals
71.55.Ht Other nonmetals
72.80.Jc Other crystalline inorganic semiconductors
65.40.G- Other thermodynamical quantities

Characteristics of Schottky contacts to ZnO:N layers grown by molecular-beam epitaxy

D. C. Oh, J. J. Kim, H. Makino, T. Hanada, M. W. Cho, T. Yao, and H. J. Ko

Appl. Phys. Lett. 86, 042110 (2005); http://dx.doi.org/10.1063/1.1854191 (3 pages) | Cited 19 times

Online Publication Date: 20 January 2005

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We have investigated the characteristics of Au Schottky contacts to ZnO:N layers grown on (0001) GaN/Al2O3 substrates by plasma-assisted molecular-beam epitaxy. It is found that the Schottky characteristics are dependent on the growth temperature and polar direction of ZnO:N layers. The Schottky barrier height for the Au contact to a ZnO:N layer (300 °C, Zn-polar) is estimated to be 0.66 and 0.69 eV by current–voltage measurements and capacitance–voltage measurements, respectively. It is found that the Schottky barrier height is proportional to the resistivity and incorporated N concentration of ZnO:N layers. Consequently, we believe that the low growth temperature and Zn-polar direction are favored for N incorporation in the growth of ZnO:N layers, which contributes to the increased resistivity in ZnO:N layers and results in good Schottky characteristics.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts
73.61.Ga II-VI semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.S- Impurities in crystals

Synthesis and transport properties of HfFe4Sb12

G. S. Nolas, G. Yoon, H. Sellinschegg, A. Smalley, and D. C. Johnson

Appl. Phys. Lett. 86, 042111 (2005); http://dx.doi.org/10.1063/1.1854747 (2 pages) | Cited 7 times

Online Publication Date: 20 January 2005

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We have synthesized, characterized, and measured the low-temperature transport properties of the metastable skutterudite HfFe4Sb12. HfFe4Sb12 was synthesized by depositing successive very thin elemental layers of Hf, Fe, and Sb, in the appropriate stoichiometric quantities, over a relatively large polymer coated substrate. Low-temperature annealing of this film resulted in the direct crystallization of the desired skutterudite. The skutterudite film was removed from the substrate by dissolving the polymer film and the resulting powder was densified utilizing standard powder metallurgical techniques. The HfFe4Sb12 skutterudite possesses a very low thermal conductivity; lower than that of other lanthanide-filled skutterudites while exhibiting p-type conduction. Although the room-temperature dimensionless thermoelectric figure of merit of this compound is low compared to thermoelectric materials in use today, it is in the range of the best p-type skutterudite compounds thus far reported. More importantly, this synthesis technique opens the possibility of producing bulk specimens of metastable compounds that are of interest for thermoelectric and other technological applications.
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81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
81.05.Hd Other semiconductors
72.20.Pa Thermoelectric and thermomagnetic effects
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
81.40.Gh Other heat and thermomechanical treatments
61.72.Cc Kinetics of defect formation and annealing
61.66.Bi Elemental solids
61.66.Dk Alloys
64.75.-g Phase equilibria

Evolution of the defect structure in helium implanted SiGe/Si heterostructures investigated by in situ annealing in a transmission electron microscope

N. Hueging, M. Luysberg, K. Urban, D. Buca, and S. Mantl

Appl. Phys. Lett. 86, 042112 (2005); http://dx.doi.org/10.1063/1.1852705 (3 pages) | Cited 2 times

Online Publication Date: 20 January 2005

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The evolution of the He-implantation induced defect structure in SiGe/Si heterostructures is observed during in situ annealing at 650 and 800 °C within a transmission electron microscope. The He implantation and annealing results in the formation of He precipitates below the SiGe/Si interface, which at first show a platelike shape and subsequently decay into spherical bubbles. The coarsening mechanism of the He bubbles is revealed as coalescence via movement of entire bubbles. The nucleation of dislocation loops at overpressurized He platelets and their propagation into the heterostructure could be observed as well. We found distinctly different velocities of the dislocations which we attribute to glide and climb processes. The in situ experiments clearly show that the He platelets act as internal dislocation sources and play a key role in the relaxation of SiGe layers.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.A- Nucleation and growth
61.72.Cc Kinetics of defect formation and annealing
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
61.72.Lk Linear defects: dislocations, disclinations
61.72.Qq Microscopic defects (voids, inclusions, etc.)
68.37.Lp Transmission electron microscopy (TEM)

Pd versus Au as evaporated metal contacts to molecules

Hossam Haick, Jamal Ghabboun, and David Cahen

Appl. Phys. Lett. 86, 042113 (2005); http://dx.doi.org/10.1063/1.1854742 (3 pages) | Cited 19 times

Online Publication Date: 21 January 2005

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Indirect e-beam evaporation of metal on a cooled substrate that allows making reproducible and gentle electrical contact to molecular films of organic molecules yields strikingly different results with Pd and Au. This is attributed to different growth modes of the metals, which lead to different molecule/metal interactions and to Au penetration in between the molecules. These differences can radically change the effect of the molecules on the resulting junctions.
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68.55.A- Nucleation and growth
81.05.Bx Metals, semimetals, and alloys
73.40.Ns Metal-nonmetal contacts
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy

Analysis of compacted and sintered metal powders by temperature-dependent resistivity measurements

Martin Gabl, Norbert Memmel, and Erminald Bertel

Appl. Phys. Lett. 86, 042114 (2005); http://dx.doi.org/10.1063/1.1857094 (3 pages) | Cited 3 times

Online Publication Date: 21 January 2005

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It is shown that temperature-dependent resistivity measurements of compacted and sintered powders allow for extracting independent information on sample density on the one hand and sintering-induced changes of grain-to-grain contacts on the other hand. The temperature gradient of the resistivity as a function of sample density follows (semi-) universal laws derived from percolation theory.
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72.15.Eb Electrical and thermal conduction in crystalline metals and alloys
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