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2 Apr 2012

Volume 100, Issue 14, Articles (14xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 100, 144101 (2012); http://dx.doi.org/10.1063/1.3697983 (4 pages)

H. Xu (徐涵), Wei Yu (余玮), M. Y. Yu (郁明阳), A. Y. Wong (黄燿煇), Z. M. Sheng (盛政明), M. Murakami (村上匡且), and J. Zhang (张杰)
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Correlation between interface traps and paramagnetic defects in c-Si/a-Si:H heterojunctions

N. H. Thoan, M. Jivanescu, B. J. O’Sullivan, L. Pantisano, I. Gordon, V. V. Afanas’ev, and A. Stesmans

Appl. Phys. Lett. 100, 142101 (2012); http://dx.doi.org/10.1063/1.3698386 (4 pages) | Cited 1 time

Online Publication Date: 2 April 2012

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Low-temperature (77 K) capacitance-voltage measurements are proposed as a technique to quantify the densities of traps in c-Si/a-Si:H heterojunction solar cell structures. By comparing the inferred trap densities to the results of electron spin resonance spectroscopy, we found that the dangling bonds of silicon atoms at the surface of the (100)Si substrate (Pb0 centers) and in a-Si:H layer (D-centers) provide the most significant contributions to the density of traps.
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88.40.H- Solar cells (photovoltaics)

Semiconducting-like filament formation in TiN/HfO2/TiN resistive switching random access memories

F. De Stefano, M. Houssa, J. A. Kittl, M. Jurczak, V. V. Afanas’ev, and A. Stesmans

Appl. Phys. Lett. 100, 142102 (2012); http://dx.doi.org/10.1063/1.3696672 (3 pages) | Cited 3 times

Online Publication Date: 2 April 2012

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The conduction mechanisms and nature of the filament formed in the low resistive state of TiN/HfO2/TiN resistive random access memories are studied from the temperature-dependence of their current-voltage characteristics. These characteristics are analyzed using the percolation theory of conductor networks, allowing us to extract the temperature-dependence of their effective resistivity. The results suggest a semiconducting-like nature of the filament in these devices. By considering the formation of a large density (about 10 at. %) of oxygen vacancies in HfO2, the electronic properties of such O-deficient HfOx filaments are computed using first-principles calculations. These calculations confirm the likely semiconducting nature of the filament formed in these devices.
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84.30.Sk Pulse and digital circuits

Semiconducting behavior of niobium-doped titanium oxide in the amorphous state

Kyung-Chul Ok, Joseph Park, Ju Ho Lee, Byung Du Ahn, Je Hun Lee, Kwun-Bum Chung, and Jin-Seong Park

Appl. Phys. Lett. 100, 142103 (2012); http://dx.doi.org/10.1063/1.3698389 (3 pages) | Cited 2 times

Online Publication Date: 2 April 2012

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Electrical properties of Nb-doped titanium oxide films were evaluated with respect to annealing temperatures. Although an amorphous phase is preserved up to 450 °C, x-ray absorption spectroscopy analyses indicate that crystal field splitting in the conduction band begins to take place at this temperature. Such molecular orbital ordering effects induce a semiconducting behavior, which is manifested by working thin film transistor devices with field effect mobility values as high as 0.64 cm2/Vs. X-ray photoelectron spectroscopy studies disclose a drastic increase in Nb+5 states upon heat treatment, and these may be attributed to oxygen deficient states that generate free electrons.
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73.61.Le Other inorganic semiconductors
78.70.Dm X-ray absorption spectra
85.30.Tv Field effect devices
61.72.Cc Kinetics of defect formation and annealing
71.70.Ch Crystal and ligand fields
73.61.Jc Amorphous semiconductors; glasses

Fluorinated copper-phthalocyanine/cobalt-phthalocyaine organic heterojunctions: Charge transport and Kelvin probe studies

A. K. Debnath, Arvind Kumar, S. Samanta, R. Prasad, A. Singh, A. K. Chauhan, P. Veerender, S. Singh, S. Basu, D. K. Aswal, and S. K. Gupta

Appl. Phys. Lett. 100, 142104 (2012); http://dx.doi.org/10.1063/1.3699272 (4 pages)

Online Publication Date: 2 April 2012

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Organic heterojunctions comprising of n-type fluorinated copper-phthalocyanine (F16CuPc) and p-type cobalt-phthalocyanine (CoPc) layers were prepared on (001) LaAlO3 substrates. In the entire temperature range of 300–30 K, F16CuPc/CoPc heterojunctions showed an ohmic conduction with three order of magnitude lower resistivity than the individual layers. This indicates formation of a charge accumulation layer at the interface. Kelvin probe studies showed that charge accumulation layer is ∼10 nm thick on both the sides of the interface.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.ag Semiconductors

Monte Carlo calculation of electron diffusion coefficient in wurtzite indium nitride

Shulong Wang, Hongxia Liu, Bo Gao, and Huimin Cai

Appl. Phys. Lett. 100, 142105 (2012); http://dx.doi.org/10.1063/1.3700720 (3 pages) | Cited 1 time

Online Publication Date: 3 April 2012

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This paper presents the theoretical analysis of the diffusivity of electrons in wurtzite (WZ) indium nitride (InN) using an ensemble Monte Carlo (EMC) method. The electron diffusion coefficient D(E,d,ω) as the functions of the electric field (E), doping concentration (d), and frequency (ω) is investigated in detail. The research results show that the maximum zero-field diffusion coefficient is about 250 cm2/s. The diffusion coefficient parallel to the electric field is smaller than that of perpendicular to the electric field. Further investigation shows that the diffusion coefficient decreases greatly at high frequency. Using Einstein equation, we obtain the zero-field mobility and make comparisons with the references.
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66.30.hp Molecular crystals
72.80.Ey III-V and II-VI semiconductors
72.20.Fr Low-field transport and mobility; piezoresistance
61.72.uj III-V and II-VI semiconductors

Improved switching uniformity in resistive random access memory containing metal-doped electrolyte due to thermally agglomerated metallic filaments

Wootae Lee, Jubong Park, Seonghyun Kim, Jiyong Woo, Jungho Shin, Daeseok Lee, Euijun Cha, and Hyunsang Hwang

Appl. Phys. Lett. 100, 142106 (2012); http://dx.doi.org/10.1063/1.3700730 (4 pages) | Cited 1 time

Online Publication Date: 3 April 2012

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We demonstrate improved switching uniformity in resistive random-access memory (RRAM) containing metal-doped electrolyte due to thermally agglomerated metallic filaments. Rapid thermal annealing (RTA) produced copper-doped carbon (CuC) devices that exhibited better switching parameters, such as on/off resistance and set/reset voltage, than a control device. High-resolution transmission electron microscopy, electron dispersive spectroscopy, and conductive atomic force microscopy revealed that Cu atoms were agglomerated during the RTA process and formed a Cu filament in the CuC film. Consequently, the forming process can be eliminated, which is desirable for practical RRAM applications.
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84.30.Sk Pulse and digital circuits

Nanocrystal-based Ohmic contacts on n and p-type germanium

V. Pavan Kishore, Prashanth Paramahans, Sunny Sadana, Udayan Ganguly, and Saurabh Lodha

Appl. Phys. Lett. 100, 142107 (2012); http://dx.doi.org/10.1063/1.3700965 (5 pages)

Online Publication Date: 3 April 2012

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We report a simple method of forming Ohmic contacts to n and p-type germanium (Ge) simultaneously using Au nanocrystals embedded in Ti contact metal. The electric field due to the work-function difference of Ti and Au reduces the tunneling resistance at the Ti-Au/Ge interface for n and p-type contacts. The nanocrystal-based contacts on n-Ge exhibit quasi-Ohmic current-voltage characteristics with >1000× increase in current density and an apparent reduction of 0.18 eV in the barrier height over Ti/n-Ge and Au/n-Ge contacts that show rectifying characteristics due to Fermi level pinning. On n+-Ge, the nanocrystal-based contacts exhibit Ohmic characteristics with >35× reduction in zero-bias resistance over Ti/n+-Ge contacts. Similar to Ti and Au contacts on p-Ge, Au nanocrystals embedded in Ti continue to exhibit Ohmic characteristics. This is in contrast to methods that convert p-type contacts from Ohmic to rectifying in the process of forming an Ohmic contact on n-Ge by inserting a thin dielectric layer between the metal and Ge. Device simulations reinforce the observed decrease in tunneling resistance due to the enhanced electric field at the Ti-Au/Ge interface.
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73.40.Ns Metal-nonmetal contacts
81.07.Lk Nanocontacts
73.40.Ei Rectification
73.40.Cg Contact resistance, contact potential
73.40.Gk Tunneling

Free carrier accumulation at cubic AlGaN/GaN heterojunctions

Q. Y. Wei, T. Li, J. Y. Huang, F. A. Ponce, E. Tschumak, A. Zado, and D. J. As

Appl. Phys. Lett. 100, 142108 (2012); http://dx.doi.org/10.1063/1.3700968 (4 pages) | Cited 1 time

Online Publication Date: 3 April 2012

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Cubic Al0.3Ga0.7N/GaN heterostructures were grown by plasma-assisted molecular beam epitaxy on 3C-SiC (001) substrates. A profile of the electrostatic potential across the cubic-AlGaN/GaN heterojunction was obtained using electron holography in the transmission electron microscope. The experimental potential profile indicates that the unintentionally doped layers show n-type behavior and accumulation of free electrons at the interface with a density of 5.1 × 1011/cm2, about one order of magnitude less than in wurtzite AlGaN/GaN junctions. A combination of electron holography and cathodoluminescence measurements yields a conduction-to-valence band offset ratio of 5:1 for the cubic AlGaN/GaN interface, which also promotes the electron accumulation. Band diagram simulations show that the donor states in the AlGaN layer provide the positive charges that to a great extent balance the two-dimensional electron gas.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
78.60.Hk Cathodoluminescence, ionoluminescence
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Observation of the photoinduced anomalous Hall effect spectra in insulating InGaAs/AlGaAs quantum wells at room temperature

J. L. Yu, Y. H. Chen, C. Y. Jiang, Y. Liu, H. Ma, and L. P. Zhu

Appl. Phys. Lett. 100, 142109 (2012); http://dx.doi.org/10.1063/1.3701281 (4 pages)

Online Publication Date: 3 April 2012

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The photocurrent spectra of inter-band photoinduced anomalous Hall effect (AHE) in insulating InGaAs/AlGaAs quantum wells have been observed at room temperature. The AHE current changes linearly with longitudinal electric fields. The anomalous Hall conductivity corresponding to excitonic state 1HH-1E (the first valence subband of heavy hole to the first conduction) has the same sign with that of excitonic state 1LH-1E, while the sign inverses when the photon energy is larger than the excitonic state 1LH-1E. The reason for this interesting phenomenon has been discussed. Due to the strong Rashba spin-orbit coupling in this perfect crystal, the photoinduced AHE is supposed to be intrinsic.
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73.21.Fg Quantum wells
79.60.Bm Clean metal, semiconductor, and insulator surfaces
81.05.Ea III-V semiconductors
71.35.-y Excitons and related phenomena
72.20.My Galvanomagnetic and other magnetotransport effects
72.40.+w Photoconduction and photovoltaic effects

Role of nitrogen vacancies in the luminescence of Mg-doped GaN

Qimin Yan, Anderson Janotti, Matthias Scheffler, and Chris G. Van de Walle

Appl. Phys. Lett. 100, 142110 (2012); http://dx.doi.org/10.1063/1.3699009 (4 pages) | Cited 4 times

Online Publication Date: 3 April 2012

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Defects may cause compensation or act as recombination centers in Mg-doped GaN. Using hybrid functional calculations, we investigate the effects of nitrogen vacancies (VN) and Mg-vacancy complexes (MgGa-VN) on the electrical and optical properties of GaN. We find that MgGa-VN are compensating centers in p-type but electrically inactive in n-type GaN. They give rise to a broad emission at 1.8 eV, explaining the red luminescence that is frequently observed in Mg-doped GaN, regardless of the Fermi level. Nitrogen vacancies are also compensating centers in p-type GaN and likely contribute to the yellow luminescence that has been observed in Mg-doped GaN.
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78.55.Cr III-V semiconductors
61.72.jd Vacancies
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.Nr Semiconductor compounds
71.55.Eq III-V semiconductors
72.20.Fr Low-field transport and mobility; piezoresistance

Influence of the SET current on the resistive switching properties of tantalum oxide created by oxygen implantation

S. M. Bishop, H. Bakhru, J. O. Capulong, and N. C. Cady

Appl. Phys. Lett. 100, 142111 (2012); http://dx.doi.org/10.1063/1.3701154 (4 pages) | Cited 4 times

Online Publication Date: 4 April 2012

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Resistive memory devices fabricated from oxygen implanted tantalum exhibited bipolar switching without a forming voltage. The influence of the current limit during SET on the switching properties has been studied using endurance measurements. The SET/RESET voltages did not change with the current limit. The RESET current increased proportionally to the SET current, while the on-state resistance varied inversely to the SET current. These results are consistent with a RESET process that is directly linked to the peak power during SET. The trade-off between the switching endurance and memory window that results from the SET process is also shown.
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84.30.Sk Pulse and digital circuits

A model for the bandgap energy of the N-rich GaNAs(0 ≤ x ≤ 0.07)

Chuan-Zhen Zhao, Na-Na Li, Tong Wei, Chun-Xiao Tang, and Ke-Qing Lu

Appl. Phys. Lett. 100, 142112 (2012); http://dx.doi.org/10.1063/1.3701284 (3 pages) | Cited 2 times

Online Publication Date: 4 April 2012

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A model for the bandgap energy of the N-rich GaNAs is developed. We find that the evolution of the conduction band minimum and the valence band maximum in the N-rich GaNAs is due to two factors. One is intraband coupling within the conduction band and separately within the valence band. The other one is the As level. It can pin the valence band maximum near the As level when the As content is large enough. It is also found that the character of the As impurity band is similar to the P impurity band in the N-rich GaNP.
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71.20.Nr Semiconductor compounds
71.55.Eq III-V semiconductors

Aluminum oxide as passivation and gate insulator in GaAs-based field-effect transistors prepared in situ by metal-organic vapor deposition

P. Kordoš, R. Kúdela, R. Stoklas, K. Čičo, M. Mikulics, D. Gregušová, and J. Novák

Appl. Phys. Lett. 100, 142113 (2012); http://dx.doi.org/10.1063/1.3701584 (3 pages) | Cited 1 time

Online Publication Date: 5 April 2012

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Application of GaAs-based metal-oxide-semiconductor (MOS) structures, as a “high carrier mobility” alternative to conventional Si MOS transistors, is still hindered due to difficulties in their preparation with low surface/interface defect states. Here, aluminum oxide as a passivation and gate insulator was formed by room temperature oxidation of a thin Al layer prepared in situ by metal-organic chemical vapor deposition. The GaAs-based MOS structures yielded two-times higher sheet charge density and saturation drain current, i.e., up to 4 × 1012 cm−2 and 480 mA/mm, respectively, than the counterparts without an oxide surface layer. The highest electron mobility in transistor channel was found to be 6050 cm2/V s. Capacitance measurements, performed in the range from 1 kHz to 1 MHz, showed their negligible frequency dispersion. All these results indicate an efficient suppression of the defect states by in situ preparation of the semiconductor structure and aluminum oxide used as a passivation and gate insulator.
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85.30.Tv Field effect devices

Amorphization and amorphous stability of Bi2Te3 chalcogenide films

C. Ju, X. M. Cheng, and X. S. Miao

Appl. Phys. Lett. 100, 142114 (2012); http://dx.doi.org/10.1063/1.3701275 (4 pages) | Cited 1 time

Online Publication Date: 6 April 2012

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Si doping is proposed to be an effective way to improve the amorphous stability of Bi2Te3 thin film. Structural changes in doped crystalline phases are revealed by the bonding nature given by x-ray photoelectron spectroscopy results. Based on ab initio simulations, the energy, electronic and structural changes induced by Si doping are studied. The results show that both nucleation and growth of crystallite are suppressed by Si doping, so that the stability of amorphous Bi2Te3 is improved.
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61.43.Er Other amorphous solids
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
71.15.-m Methods of electronic structure calculations
71.23.-k Electronic structure of disordered solids
79.60.Bm Clean metal, semiconductor, and insulator surfaces
68.55.A- Nucleation and growth

Coaxial nanowire resonant tunneling diodes from non-polar AlN/GaN on silicon

S. D. Carnevale, C. Marginean, P. J. Phillips, T. F. Kent, A. T. M. G. Sarwar, M. J. Mills, and R. C. Myers

Appl. Phys. Lett. 100, 142115 (2012); http://dx.doi.org/10.1063/1.3701586 (4 pages)

Online Publication Date: 6 April 2012

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Resonant tunneling diodes are formed using AlN/GaN core-shell nanowire heterostructures grown by plasma assisted molecular beam epitaxy on n-Si(111) substrates. By using a coaxial geometry, these devices take advantage of non-polar (m-plane) nanowire sidewalls. Device modeling predicts non-polar orientation should enhance resonant tunneling compared to a polar structure, and that AlN double barriers will lead to higher peak-to-valley current ratios compared to AlGaN barriers. Electrical measurements of ensembles of nanowires show negative differential resistance appearing only at cryogenic temperature. Individual nanowire measurements show negative differential resistance at room temperature with peak current density of 5 × 105 A/cm2.
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85.30.Kk Junction diodes
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
85.30.De Semiconductor-device characterization, design, and modeling

GaSb/GaAs quantum dot formation and demolition studied with cross-sectional scanning tunneling microscopy

E. P. Smakman, J. K. Garleff, R. J. Young, M. Hayne, P. Rambabu, and P. M. Koenraad

Appl. Phys. Lett. 100, 142116 (2012); http://dx.doi.org/10.1063/1.3701614 (3 pages) | Cited 6 times

Online Publication Date: 6 April 2012

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We present a cross-sectional scanning tunneling microscopy study of GaSb/GaAs quantum dots grown by molecular beam epitaxy. Various nanostructures are observed as a function of the growth parameters. During growth, relaxation of the high local strain fields of the nanostructures plays an important role in their formation. Pyramidal dots with a high Sb content are often accompanied by threading dislocations above them. GaSb ring formation is favored by the use of a thin GaAs first cap layer and a high growth temperature of the second cap layer. At these capping conditions, strain-driven Sb diffusion combined with As/Sb exchange and Sb segregation remove the center of a nanostructure, creating a ring. Clusters of GaSb without a well defined morphology also appear regularly, often with a highly inhomogeneous structure which is sometimes divided up in fragments.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
66.30.-h Diffusion in solids
68.65.Hb Quantum dots (patterned in quantum wells)
81.05.Ea III-V semiconductors
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