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11 Apr 2005

Volume 86, Issue 15, Articles (15xxxx)

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Appl. Phys. Lett. 86, 152101 (2005); http://dx.doi.org/10.1063/1.1897831 (3 pages)

Walid Hafez and Milton Feng
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Experimental demonstration of pseudomorphic heterojunction bipolar transistors with cutoff frequencies above 600 GHz

Walid Hafez and Milton Feng

Appl. Phys. Lett. 86, 152101 (2005); http://dx.doi.org/10.1063/1.1897831 (3 pages) | Cited 30 times

Online Publication Date: 4 April 2005

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Pseudomorphic InP/InGaAs heterojunction bipolar transistors (PHBTs) using a compositionally graded collector (10% indium grading) and graded base (6% indium grading) to reduce the transit time of the device are reported. A 0.4×6 μm2 HBT achieves excellent ƒT values of 604 GHz (associated ƒMAX = 246 GHz) at a collector current density of 16.8 mA/μm2, with a dc gain of 65 and a breakdown voltage of BVCEO = 1.7 V.
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85.30.Pq Bipolar transistors
85.30.De Semiconductor-device characterization, design, and modeling
73.20.At Surface states, band structure, electron density of states
84.40.-x Radiowave and microwave (including millimeter wave) technology

Postannealing of magnetic tunnel junctions with ion-bombardment-modified exchange bias

V. Höink, M. D. Sacher, J. Schmalhorst, G. Reiss, D. Engel, D. Junk, and A. Ehresmann

Appl. Phys. Lett. 86, 152102 (2005); http://dx.doi.org/10.1063/1.1899771 (3 pages) | Cited 11 times

Online Publication Date: 4 April 2005

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The influence of a postannealing procedure on the transport properties of magnetic tunnel junctions with ion-bombardment-manipulated exchange bias is investigated. The controlled manipulation of the direction of the exchange bias field in magnetic tunnel junctions by He ion bombardment usually is accompanied by a reduction of the tunneling magnetoresistance and an increase in the resistance. Here, we demonstrate that it is possible to reduce these negative effects of the ion bombardment considerably by postannealing without a magnetic field. For optimized combinations of ion dose and postannealing temperature, the tunneling magnetoresistance recovers completely (>50% resistance change) while the exchange bias direction set by the ion bombardement is preserved.
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75.50.Bb Fe and its alloys
75.50.Ee Antiferromagnetics
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.30.Et Exchange and superexchange interactions
75.47.Np Metals and alloys
81.40.Gh Other heat and thermomechanical treatments
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Direct observation of negative electron affinity in hydrogen-terminated diamond surfaces

D. Takeuchi, H. Kato, G. S. Ri, T. Yamada, P. R. Vinod, D. Hwang, C. E. Nebel, H. Okushi, and S. Yamasaki

Appl. Phys. Lett. 86, 152103 (2005); http://dx.doi.org/10.1063/1.1900925 (3 pages) | Cited 45 times

Online Publication Date: 5 April 2005

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Total photoyield experiments are applied to characterize p-, intrinsic, and n-type diamond with hydrogen-terminated surfaces. On all hydrogen-terminated samples a photoelectron threshold energy of 4.4 eV is detected which is discussed in detail in this letter. We attribute this threshold to the energy gap between the valence-band maximum and the vacuum level, which is 1.1 eV below the conduction-band minimum, and generally referred to as ”negative electron affinity” (NEA). Hydrogen terminated p-type and intrinsic diamond show a rise of secondary photoyield in the excitation regime hν>5.47 eV. However, this is not detected on n-type diamond. We ascribe this to the formation of an upward surface band bending in the vicinity of the n-type diamond surface which acts as an energy barrier for electrons.
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73.20.At Surface states, band structure, electron density of states
79.60.Bm Clean metal, semiconductor, and insulator surfaces

Origin of the stress-induced leakage currents in Al–Ta2O5/SiO2–Si structures

N. Novkovski and E. Atanassova

Appl. Phys. Lett. 86, 152104 (2005); http://dx.doi.org/10.1063/1.1900955 (3 pages) | Cited 5 times

Online Publication Date: 5 April 2005

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The leakage currents in the Al–Ta2O5/SiO2–Si structures increase as a result of the current stress. The case of the constant current stress was studied and it was found that the I–V characteristics of both unstressed and stressed samples can be described by the previously developed model involving Poole–Frenkel effect in the Ta2O5 and both hopping conductivity and tunneling in the SiO2. After extracting the parameters for both layers from the fitting of the theoretical curves to the experimental results, it was found that the main reason for the increased leakage is the partial destruction of the SiO2 layer.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.55.-g Dielectric thin films
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
84.32.Tt Capacitors
73.50.Dn Low-field transport and mobility; piezoresistance
73.50.Fq High-field and nonlinear effects
73.40.Gk Tunneling
77.22.Ch Permittivity (dielectric function)
68.35.Ct Interface structure and roughness

Acoustoelectric current in submicron-separated quantum wires

J. Cunningham, M. Pepper, V. I. Talyanskii, and D. A. Ritchie

Appl. Phys. Lett. 86, 152105 (2005); http://dx.doi.org/10.1063/1.1897050 (3 pages) | Cited 4 times

Online Publication Date: 5 April 2005

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We measure acoustoelectric current in two submicron-separated quasi-one-dimensional wires formed in an AlGaAs/GaAs heterostructure. We show that independent control of acoustoelectric current in both wires can be achieved with a suitably chosen geometry of Schottky gates. The implications of the results on two proposed uses of single-electron acoustoelectric current flow are discussed: a single-photon source and an acoustoelectrically driven quantum computer.
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85.30.Tv Field effect devices
73.63.Nm Quantum wires
72.50.+b Acoustoelectric effects
73.21.Hb Quantum wires
73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Ab initio study of the effect of hydrogen and point defects on arsenic segregation at Si(100)/SiO2 interfaces

Karthik Ravichandran and Wolfgang Windl

Appl. Phys. Lett. 86, 152106 (2005); http://dx.doi.org/10.1063/1.1900958 (3 pages) | Cited 5 times

Online Publication Date: 6 April 2005

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The previously suggested segregation model for arsenic at Si/SiO2 interfaces based on a combined trapping∕pairing model [ J. Dabrowski, H.-J. Müssig, V. Zavodinsky, R. Baierle, and M. J. Caldas, Phys. Rev. B 65, 245305 (2002) ] requires high binding energies for interface vacancies, which our results of ∼ 0.2 eV cannot confirm. As an alternative explanation, we present ab initio results that show that As and hydrogen bond with an energy gain of 1.5–3 eV with their minimum-energy position at the interface, which creates additional trapping sites for As segregation. The inclusion of hydrogen into the modeling might thus be able to explain the differences between the previous model and experiments.
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61.72.J- Point defects and defect clusters
68.35.Dv Composition, segregation; defects and impurities
61.72.Bb Theories and models of crystal defects
68.35.Ct Interface structure and roughness
61.72.Qq Microscopic defects (voids, inclusions, etc.)

General theory of acceptor-oxygen-vacancy complex single donor in high-dielectric-constant metallic oxide insulators

W. S. Lau and Taejoon Han

Appl. Phys. Lett. 86, 152107 (2005); http://dx.doi.org/10.1063/1.1897051 (3 pages) | Cited 10 times

Online Publication Date: 6 April 2005

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Previously, we showed experimentally that Si/O-vacancy complex single donors and C/O-vacancy complex single donors in tantalum oxide films have smaller ionization energies than the first ionization energy of O-vacancy double donors [ W. S. Lau, L. L. Leong, T. Han, and N. P. Sandler, Appl. Phys. Lett. 83, 2835 (2003) ]. In this letter, a theory based on a larger electron orbit for acceptor O-vacancy complex single donors compared to the O-vacancy double donors is proposed to explain the physics behind our previously reported experimental observation and why Si or C may cause an increase in leakage current in tantalum oxide films.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.55.-g Dielectric thin films
61.72.J- Point defects and defect clusters
71.55.Ht Other nonmetals
77.22.Ch Permittivity (dielectric function)
61.72.Bb Theories and models of crystal defects
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Anomalous bias dependence of tunnel magnetoresistance in a magnetic tunnel junction

Soumik Mukhopadhyay, I. Das, S. P. Pai, and P. Raychaudhuri

Appl. Phys. Lett. 86, 152108 (2005); http://dx.doi.org/10.1063/1.1901823 (3 pages) | Cited 9 times

Online Publication Date: 6 April 2005

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We have fabricated a spin-polarized tunneling device based on half-metallic manganites incorporating Ba2LaNbO6 as an insulating barrier. An anomalous bias dependence of tunnel magnetoresistance (TMR) has been observed, the first of its kind in a symmetric electrode tunnel junction with a single insulating barrier. The bias dependence of TMR shows an extremely sharp zero-bias anomaly, which can be considered as a demonstration of the drastic density of states variation around the Fermi level of the half-metal. This serves as strong evidence for the existence of minority-spin tunneling states at the half-metal–insulator interface.
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75.47.Lx Magnetic oxides
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
72.20.My Galvanomagnetic and other magnetotransport effects
72.25.Mk Spin transport through interfaces
73.20.At Surface states, band structure, electron density of states
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Deep level defects in Si-doped AlxGa1−xN films grown by molecular-beam epitaxy

Y. S. Park, C. J. Park, C. M. Park, J. H. Na, J. S. Oh, I. T. Yoon, H. Y. Cho, T. W. Kang, and Jae-Eung Oh

Appl. Phys. Lett. 86, 152109 (2005); http://dx.doi.org/10.1063/1.1887817 (3 pages) | Cited 6 times

Online Publication Date: 6 April 2005

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The deep trap levels of AlxGa1−xN films with x in the range from 0 to 0.15 grown on c-plane sapphire substrates using rf-plasma-assisted molecular-beam epitaxy have been investigated by deep level transient spectroscopy measurements. Two distinct defect levels (denoted as Ei and Di) were observed. The origins of the Ei and the Di are associated with point defects such as the N vacancies and extended defects, such as the threading dislocations, respectively. According to Al content (x), the activation energy and capture cross section for the Di defect ranged from 0.19 to 0.41 eV and 1.1–6.6×10−15 cm2, respectively. The trap energy levels of Di defects in AlxGa1−xN were calculated and the values were nonlinear with Al content. The bowing parameter of AlxGa1−xN films was determined to be b = 1.22.
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71.55.Eq III-V semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.20.At Surface states, band structure, electron density of states
68.55.-a Thin film structure and morphology
61.72.Nn Stacking faults and other planar or extended defects
61.72.J- Point defects and defect clusters
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

Influence of silicon-nanocrystal distribution in SiO2 matrix on charge injection and charge decay

C. Y. Ng, T. P. Chen, M. S. Tse, V. S. W. Lim, S. Fung, and Ampere A. Tseng

Appl. Phys. Lett. 86, 152110 (2005); http://dx.doi.org/10.1063/1.1901831 (3 pages) | Cited 8 times

Online Publication Date: 7 April 2005

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Influence of distribution of silicon nanocrystals (nc-Si) embedded in SiO2 matrix on charge injection and charge decay of the nc-Si has been investigated with electrostatic force microscopy. For nc-Si distributing in the surface region, the size of charge cloud does not change with decay time, and neighboring charges have no influence on the charge decay. In contrast, for nc-Si distributing away from the surface, the size linearly increases with decay time, and the neighboring charges can either accelerate or resist the charge decay depending on their charge signs. In addition, the characteristic decay time for the first distribution is much shorter than that for the second distribution. These results provide an insight into the dissipation mechanism of the charges stored in the nc-Si.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.61.Ng Insulators
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
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
61.46.-w Structure of nanoscale materials
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Ps Atomic force microscopy (AFM)

Electrical characterization of Er- and Pr-implanted GaN films

S. F. Song, W. D. Chen, Chunguang Zhang, Liufang Bian, C. C. Hsu, L. W. Lu, Y. H. Zhang, and Jianjun Zhu

Appl. Phys. Lett. 86, 152111 (2005); http://dx.doi.org/10.1063/1.1901828 (3 pages) | Cited 5 times

Online Publication Date: 7 April 2005

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Hall, current–voltage, and deep-level transient spectroscopy measurements were used to characterize the electrical properties of metalorganic chemical vapor deposition grown undoped, Er- and Pr-implanted GaN films. Only one deep level located at 0.270 eV below the conduction band was found in the as-grown GaN films. However, four defect levels located at 0.300, 0.188, 0.600, and 0.410 eV below the conduction band were found in the Er-implanted GaN films after annealing at 900 °C for 30 min, and four defect levels located at 0.280, 0.190, 0.610, and 0.390 eV below the conduction band were found in the Pr-implanted GaN films after annealing at 1050 °C for 30 min. The origins of the deep defect levels are discussed.
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73.61.Ey III-V semiconductors
72.20.My Galvanomagnetic and other magnetotransport effects
73.50.Dn Low-field transport and mobility; piezoresistance
71.55.Eq III-V semiconductors
61.72.Cc Kinetics of defect formation and annealing

Dopant-segregation-controlled ZnO single-grain-boundary varistors

Yukio Sato, Masatada Yodogawa, Takahisa Yamamoto, Naoya Shibata, and Yuichi Ikuhara

Appl. Phys. Lett. 86, 152112 (2005); http://dx.doi.org/10.1063/1.1899762 (3 pages) | Cited 5 times

Online Publication Date: 7 April 2005

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A ZnO single-grain-boundary varistor was designed using a bicrystal. Pr and Co dopant cosegregation at the boundary, key to obtain high varistic property, is optimized by controlling grain-boundary misorientation and, hence, grain-boundary atomic structure. Thus obtained single grain boundary exhibited sufficiently high varistic property. The present result opens up the possibility of single-grain-boundary varistors required for future nanoscale electronic devices.
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84.32.Ff Conductors, resistors (including thermistors, varistors, and photoresistors)
85.30.De Semiconductor-device characterization, design, and modeling
61.72.S- Impurities in crystals
61.72.Mm Grain and twin boundaries
64.75.-g Phase equilibria

Evidence of formation of Si–C bonds during growth of Si-doped III–V semiconductor compounds

J. Bettini and M. M. G de Carvalho

Appl. Phys. Lett. 86, 152113 (2005); http://dx.doi.org/10.1063/1.1905783 (3 pages)

Online Publication Date: 8 April 2005

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In this work, we demonstrate that Si–C bonds are formed in III–V semiconductor compounds grown by chemical beam epitaxy. Our results suggest that the formation of Si–C bonds occurs in III–V epitaxial layers with acceptor Carbon residual concentration and high Si concentrations (>1017 cm−3). The main consequence of Si–C bonds is the generation of defects along [111] direction. These defects produce carrier concentration saturation, reduction of electrical mobility, crystal quality degradation, and surface defects.
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81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.A- Nucleation and growth
73.61.Ey III-V semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.-a Thin film structure and morphology
61.72.S- Impurities in crystals
68.35.B- Structure of clean surfaces (and surface reconstruction)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
73.50.Dn Low-field transport and mobility; piezoresistance
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)

p-type conductivity in cubic (Ga,Mn)N thin films

K. W. Edmonds, S. V. Novikov, M. Sawicki, R. P. Campion, C. R. Staddon, A. D. Giddings, L. X. Zhao, K. Y. Wang, T. Dietl, C. T. Foxon, and B. L. Gallagher

Appl. Phys. Lett. 86, 152114 (2005); http://dx.doi.org/10.1063/1.1900924 (3 pages) | Cited 16 times

Online Publication Date: 8 April 2005

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The electrical properties of p-type cubic (Ga,Mn)N films are reported. Hole concentrations above 1018 cm−3 at room temperature are observed. Activated behavior is observed down to 150 K, with an acceptor ionization energy of around 45–60 meV. The dependence of hole concentration and ionization energy on Mn concentration indicates that the shallow acceptor level is not simply due to unintentional codoping. Thermopower measurements on freestanding films, capacitance–voltage profilometry, and the dependence of conductivity on thickness and growth temperature, all show that this is not due to diffusion into the substrate. We therefore associate the p-type conductivity with the presence of the Mn in the cubic GaN films.
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75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
73.61.Ey III-V semiconductors
73.50.Dn Low-field transport and mobility; piezoresistance
73.50.Lw Thermoelectric effects
71.55.Eq III-V semiconductors
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.-a Thin film structure and morphology

Shallow trap states in pentacene thin films from molecular sliding

Joo H. Kang, Demetrio da Silva Filho, Jean-Luc Bredas, and X.-Y. Zhu

Appl. Phys. Lett. 86, 152115 (2005); http://dx.doi.org/10.1063/1.1900944 (3 pages) | Cited 52 times

Online Publication Date: 8 April 2005

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Petacene is one of the most promising organic semiconductors for thin-film transistors. Transport measurements in the past have established the presence of shallow traps but their origins have remained a mystery. Here we show that shallow traps in vapor-deposited crystalline pentacene thin films are due to local defects resulting from the sliding of pentacene molecules along their long molecular axis, while two-dimensional crystalline packing is maintained. Electronic structural calculation confirms that these sliding defects are shallow-charge traps with energies ⩽ 100 meV above (below) the valence band maximum (conduction band minimum).
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85.30.Tv Field effect devices
71.55.-i Impurity and defect levels
73.61.Ph Polymers; organic compounds
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
85.65.+h Molecular electronic devices
73.20.At Surface states, band structure, electron density of states

Effects of AlOx-cap layer on the luminescence and photoconductivity of ZnO thin films

K. C. Hui, H. C. Ong, P. F. Lee, and J. Y. Dai

Appl. Phys. Lett. 86, 152116 (2005); http://dx.doi.org/10.1063/1.1900945 (3 pages) | Cited 14 times

Online Publication Date: 8 April 2005

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The effects of AlOx-cap layer on the optical and photoelectrical properties of ZnO films have been studied by cathodoluminescence (CL), photoluminescence (PL), and photoconductivity (PC). Both the PL and CL show that the cap layer improves the emission characteristics of ZnO by enhancing the band-edge emission while at the same time reducing the deep-level emissions. To study the origin of improvement, depth-resolved CL has been carried out to map out the emissions at different depths. It shows that the improvement occurs primarily at the film surface, which indicates the cap layer acts as a passivation layer that suppresses the detrimental surface states. The PC measurement on the capped ZnO at room temperature shows a distinctive excitonic feature at 3.29 eV and an overall increment of photoresponse above the band gap. Therefore, our results suggest a higher sensitivity of UV detection can be achieved in ZnO simply be employing a thin AlOx-cap layer.
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78.66.Hf II-VI semiconductors
78.55.Et II-VI semiconductors
78.60.Hk Cathodoluminescence, ionoluminescence
73.50.Pz Photoconduction and photovoltaic effects
73.20.At Surface states, band structure, electron density of states
68.55.-a Thin film structure and morphology
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