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30 Jan 2012

Volume 100, Issue 5, Articles (05xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 100, 053101 (2012); http://dx.doi.org/10.1063/1.3679127 (3 pages)

Shinya Kano, Yasuyuki Yamada, Kentaro Tanaka, and Yutaka Majima
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Fabrication and characterization of ambipolar devices on an undoped AlGaAs/GaAs heterostructure

J. C. H. Chen, D. Q. Wang, O. Klochan, A. P. Micolich, K. Das Gupta, F. Sfigakis, D. A. Ritchie, D. Reuter, A. D. Wieck, and A. R. Hamilton

Appl. Phys. Lett. 100, 052101 (2012); http://dx.doi.org/10.1063/1.3673837 (3 pages) | Cited 5 times

Online Publication Date: 30 January 2012

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We have fabricated AlGaAs/GaAs heterostructure devices in which the conduction channel can be populated with either electrons or holes simply by changing the polarity of a gate bias. The heterostructures are entirely undoped, and carriers are, instead, induced electrostatically. We use these devices to perform a direct comparison of the scattering mechanisms of two-dimensional electrons (μpeak = 4 × 106 cm2/Vs) and holes (μpeak = 0.8 × 106 cm2/Vs) in the same conduction channel with nominally identical disorder potentials. We find significant discrepancies between electron and hole scattering, with the hole mobility being considerably lower than expected from simple theory.
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85.30.-z Semiconductor devices

Quantum Hall effect in bottom-gated epitaxial graphene grown on the C-face of SiC

B. Jouault, N. Camara, B. Jabakhanji, A. Caboni, C. Consejo, P. Godignon, D. K. Maude, and J. Camassel

Appl. Phys. Lett. 100, 052102 (2012); http://dx.doi.org/10.1063/1.3680564 (4 pages) | Cited 2 times

Online Publication Date: 30 January 2012

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We demonstrate that the carrier concentration of epitaxial graphene devices grown on the C-face of a SiC substrate is efficiently modulated by a buried gate. The gate is fabricated via the implantation of nitrogen atoms in the SiC crystal. The charge neutrality point is observed close to gate voltage zero, and graphene can be populated by either holes or electrons down to low temperature (1.5 K). The hole concentration is hardly tuned by the gate voltage, possibly because of interface states below the Dirac point. A remarkably large quantum Hall plateau is observed for electrons.
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73.43.-f Quantum Hall effects
73.61.Wp Fullerenes and related materials

Influence of charge compensation mechanisms on the sheet electron density at conducting LaAlO3/SrTiO3-interfaces

F. Gunkel, P. Brinks, S. Hoffmann-Eifert, R. Dittmann, M. Huijben, J. E. Kleibeuker, G. Koster, G. Rijnders, and R. Waser

Appl. Phys. Lett. 100, 052103 (2012); http://dx.doi.org/10.1063/1.3679139 (3 pages) | Cited 2 times

Online Publication Date: 30 January 2012

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The equilibrium conductance of LaAlO3/SrTiO3 (LAO/STO)-heterointerfaces was investigated at high temperatures (950 K-1100 K) as a function of ambient oxygen partial pressure (pO2). Metallic LAO/STO-interfaces were obtained for LAO grown on STO single crystals as well as on STO-buffered (La,Sr)(Al,Ta)O3 substrates. For both structures, the high temperature sheet carrier density nS of the LAO/STO-interface saturates at a value of about 1 × 1014 cm−2 for reducing conditions, which indicates the presence of interfacial donor states. A significant decrease of nS is observed at high oxygen partial pressures. According to the defect chemistry model of donor-doped STO, this behavior for oxidizing conditions can be attributed to the formation of Sr-vacancies as charge compensating defects.
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73.40.-c Electronic transport in interface structures
61.72.jd Vacancies
71.55.Ht Other nonmetals
72.80.Sk Insulators

Effect of electric field on the band structure of graphene/boron nitride and boron nitride/boron nitride bilayers

Radhakrishnan Balu, Xiaoliang Zhong, Ravindra Pandey, and Shashi P. Karna

Appl. Phys. Lett. 100, 052104 (2012); http://dx.doi.org/10.1063/1.3679174 (3 pages) | Cited 7 times

Online Publication Date: 30 January 2012

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Effect of electric field on the band structures of graphene/boron nitride (BN) and BN/BN bilayers is investigated within the framework of density functional theory. The calculated bandgap of the graphene/BN bilayer increases, although by small amount, with applied electric field. In the case of BN/BN bilayer, the bandgap decreases with the applied field in agreement with earlier studies. The modulation of bandgap in graphene/BN bilayers is dominated by the features of graphene and appears to be related to the modification in molecular orbitals as revealed by the calculated projected density of states.
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71.20.Nr Semiconductor compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Pseudobinary Al2Te3-Sb2Te3 material for high speed phase change memory application

Kun Ren, Feng Rao, Zhitang Song, Shilong Lv, Yan Cheng, Liangcai Wu, Cheng Peng, Xilin Zhou, Mengjiao Xia, Bo Liu, and Songlin Feng

Appl. Phys. Lett. 100, 052105 (2012); http://dx.doi.org/10.1063/1.3680580 (3 pages) | Cited 4 times

Online Publication Date: 31 January 2012

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Al2Sb2Te6 is a pseudobinary material constructed by Sb2Te3 (fast crystallization speed but thermally unstable) and Al2Te3 (thermally stable but without memory switching ability). Al2Sb2Te6 material possesses advantages of these two binary compounds showing good memory switching ability with fast switching speed and good thermal stability. These improvements are believed to be closely related to the coordination situations of Al atoms in Al2Sb2Te6 material. Phase change memory device using Al2Sb2Te6 showed high speed (∼5 ns), low power consumption, and high endurance (∼106 cycles). Thus, we consider that Al2Sb2Te6 can be one of the most promising materials for phase change memory use.
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84.30.Sk Pulse and digital circuits
68.60.Dv Thermal stability; thermal effects

Epitaxial Cr on n-SrTiO3(001)—An ideal Ohmic contact

C. Capan, G. Y. Sun, M. E. Bowden, and S. A. Chambers

Appl. Phys. Lett. 100, 052106 (2012); http://dx.doi.org/10.1063/1.3680608 (3 pages) | Cited 1 time

Online Publication Date: 31 January 2012

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Epitaxial Cr metallizations grown on n-SrTiO3(001) by molecular beam epitaxy are shown to result in an ordered interface with Cr bound to O in the terminal TiO2 layer, no reduction of the SrTiO3, and a near-perfect Ohmic contact. Cr/n-SrTiO3(001) thus constitutes an ideal interface between a pure metal and wide gap oxide in which interface redox chemistry does not occur, and the Fermi level remains unpinned.
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73.40.Ns Metal-nonmetal contacts
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

2-bit operation based on modulated Fowler-Nordheim tunneling in charge-trapping flash memory cell

Min-Feng Hung, Yung-Chun Wu, and Jiang-Hung Chen

Appl. Phys. Lett. 100, 052107 (2012); http://dx.doi.org/10.1063/1.3681145 (3 pages)

Online Publication Date: 31 January 2012

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This work proposes a spatially charge tunneling method for program/erase (P/E) based on the drain-modulated Fowler-Nordheim (MFN) tunneling in a polycrystalline silicon channel charge-trap type flash memory device. The program mechanism and the simulation of the MFN are discussed. 2-bit operations based on MFN and channel hot electron injection were demonstrated. MFN 2-bit operation and conventional 2-bit operation present different transfer curves, indicating that charge is injected into the different regions. The MFN P/E operation provides flexibility in flash memory applications.
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85.30.-z Semiconductor devices
84.30.Sk Pulse and digital circuits

Growth and valence band offset measurement of PbTe/InSb heterojunctions

Kyoung-Keun Lee, Weerasinghe Priyantha, and Thomas H. Myers

Appl. Phys. Lett. 100, 052108 (2012); http://dx.doi.org/10.1063/1.3681161 (4 pages) | Cited 2 times

Online Publication Date: 31 January 2012

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InSb (111)B and (211)B substrates were investigated for the growth of PbTe. The PbTe/InSb (111)B showed slip features caused by stain relaxation during cooling down, while growth of PbTe/InSb (211)B resulted in a better surface morphology. The valence band offset of PbTe/InSb (211)B was measured by high resolution x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy with the value of 0.38 ± 0.05 eV. The PbTe/InSb (211)B heterojunction structure forms a type-III—broken gap heterostructure with the conduction band offset of 0.23 ± 0.05 eV.
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71.20.Nr Semiconductor compounds
79.60.Bm Clean metal, semiconductor, and insulator surfaces
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
68.35.bg Semiconductors

Boundary-enhanced momentum relaxation of longitudinal optical phonons in GaN

N. Ma, B. Shen, L. W. Lu, F. J. Xu, L. Guo, X. Q. Wang, F. Lin, Z. H. Feng, S. B. Dun, and B. Liu

Appl. Phys. Lett. 100, 052109 (2012); http://dx.doi.org/10.1063/1.3681373 (3 pages)

Online Publication Date: 1 February 2012

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The influence of the channel geometry on the high-field electron transport properties has been investigated in n-type GaN at room temperature. It is found that electrons in narrow channels drift much faster than those in wide channels. This discrepancy is attributed to the boundary-enhanced momentum relaxation of longitudinal optical (LO) phonons, which increases the electron drift velocity through enhancing the electron energy dissipation while weakening the momentum relaxation. In narrow channels, the effective LO-phonon momentum relaxation time is about ∼0.67 ps. The suggested theoretical analysis coincides well with the important features of the experimental results.
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63.20.D- Phonon states and bands, normal modes, and phonon dispersion
68.55.ag Semiconductors
73.61.Ey III-V semiconductors
81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Modeling the transport properties of epitaxially grown thermoelectric oxide thin films using spectroscopic ellipsometry

S. R. Sarath Kumar, Anas I. Abutaha, M. N. Hedhili, and H. N. Alshareef

Appl. Phys. Lett. 100, 052110 (2012); http://dx.doi.org/10.1063/1.3678186 (4 pages)

Online Publication Date: 1 February 2012

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The influence of oxygen vacancies on the transport properties of epitaxial thermoelectric (Sr,La)TiO3 thin films is determined using electrical and spectroscopic ellipsometry (SE) measurements. Oxygen vacancy concentration was varied by ex-situ annealing in Ar and Ar/H2. All films exhibited degenerate semiconducting behavior, and electrical conductivity decreased (258–133 S cm−1) with increasing oxygen content. Similar decrease in the Seebeck coefficient is observed and attributed to a decrease in effective mass (7.8–3.2 me), as determined by SE. Excellent agreement between transport properties deduced from SE and direct electrical measurements suggests that SE is an effective tool for studying oxide thin film thermoelectrics.
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73.50.Lw Thermoelectric effects
73.61.Le Other inorganic semiconductors
61.72.jd Vacancies
68.55.ag Semiconductors
61.72.Cc Kinetics of defect formation and annealing
72.20.Pa Thermoelectric and thermomagnetic effects

Electronic transport anisotropy of buckling graphene under uniaxial compressive strain: Ab initio study

Yang Xu, Haiyuan Gao, Huabin Chen, You Yuan, Kaicheng Zhu, Hongsheng Chen, Zhonghe Jin, and Bin Yu

Appl. Phys. Lett. 100, 052111 (2012); http://dx.doi.org/10.1063/1.3680092 (3 pages) | Cited 1 time

Online Publication Date: 1 February 2012

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Electronic transport properties of graphene under uniaxial compressive strain are studied using ab initio calculations. With approximate thermal perturbation, buckling occurs when strain exceeds a threshold, comparing to flat unperturbed structures. Transmissions of flat graphene compressed along zigzag direction (ZCG) and buckled graphene compressed along armchair direction (ACG) are insensitive to strain, whereas those of buckled ZCG and flat ACG are negatively correlated to strain. Flat graphene has anisotropic resistance along the strain direction, while buckling suppresses the anisotropy by releasing the strain. The insensitivity of buckled graphene on strain direction and out-of-plane deformation makes feasible to implement flexible electronics.
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81.40.Lm Deformation, plasticity, and creep
62.20.mq Buckling

Negative differential spin conductance in doped zigzag graphene nanoribbons

Ting-Ting Wu, Xue-Feng Wang, Ming-Xing Zhai, Hua Liu, Liping Zhou, and Yong-Jin Jiang

Appl. Phys. Lett. 100, 052112 (2012); http://dx.doi.org/10.1063/1.3681775 (4 pages) | Cited 2 times

Online Publication Date: 2 February 2012

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The spin dependent charge transport in zigzag graphene nanoribbons (ZGNRs) has been investigated by the nonequilibrium Green’s function method combined with the density functional theory at the local spin density approximation. The current versus voltage curve shows distinguished behaviors for symmetric and asymmetric ZGNRs, and the doping on the ZGNR edges can manipulate the spin transport. In special cases that a Be atom is substitutionally doped on one edge of the symmetric ZGNRs, one spin current shows negative differential resistance, whereas the other increases monotonically with the bias. This property might be used to design spin oscillators or other devices for spintronics.
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72.25.-b Spin polarized transport
72.80.Vp Electronic transport in graphene
72.20.Ht High-field and nonlinear effects

The effect of growth temperature of GaAs nucleation layer on InAs/GaAs quantum dots monolithically grown on Ge substrates

T. Wang, A. Lee, F. Tutu, A. Seeds, H. Liu, Q. Jiang, K. Groom, and R. Hogg

Appl. Phys. Lett. 100, 052113 (2012); http://dx.doi.org/10.1063/1.3682314 (4 pages) | Cited 2 times

Online Publication Date: 2 February 2012

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The effect of the growth temperature of the GaAs nucleation layer on the properties of 1.3-μm InAs/GaAs quantum dots (QDs) monolithically grown on a Ge substrate is investigated by using transmission electron microscopy, etch pit density, and photoluminescence (PL) measurements. The photoluminescence intensity for Ge-based InAs/GaAs quantum dots is very sensitive to the initial GaAs nucleation temperature with the strongest room-temperature emission at 380 °C, due to the lower density of defects generated at the GaAs/Ge interface and prorogating into the III-V layers at this temperature. Furthermore, lasing operation up to 100 °C was achieved for Ge-based 1.3-μm InAs/GaAs quantum-dot diodes with the initial GaAs layer nucleated at 380 °C.
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81.07.Ta Quantum dots
78.67.Hc Quantum dots
42.55.Px Semiconductor lasers; laser diodes
71.55.Eq III-V semiconductors
78.55.Cr III-V semiconductors
81.05.Ea III-V semiconductors

Deep traps in nonpolar m-plane GaN grown by ammonia-based molecular beam epitaxy

Z. Zhang, C. A. Hurni, A. R. Arehart, J. Yang, R. C. Myers, J. S. Speck, and S. A. Ringel

Appl. Phys. Lett. 100, 052114 (2012); http://dx.doi.org/10.1063/1.3682528 (4 pages) | Cited 1 time

Online Publication Date: 3 February 2012

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Deep level defects in nonpolar m-plane GaN grown by ammonia-based molecular beam epitaxy were characterized using deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) and compared with polar c-plane GaN that was grown simultaneously in the same growth run. Significant differences in both the levels present and their concentrations were observed upon comparison of both growth orientations. DLTS revealed electron traps with activation energies of 0.14 eV, 0.20 eV, and 0.66 eV in the m-plane material, with concentrations that were ∼10−50 × higher than traps of similar activation energies in the c-plane material. Likewise, DLOS measurements showed ∼20 × higher concentrations of both a CN acceptor-like state at EC − 3.26 eV, which correlates with a high background carbon concentration observed by secondary ion mass spectroscopy for the m-plane material [A. Armstrong, A. R. Arehart, B. Moran, S. P. DenBaars, U. K. Mishra, J. S. Speck, and S. A. Ringel, Appl. Phys. Lett. 84, 374 (2004)], and the VGa-related state level at EC − 2.49 eV, which is consistent with an enhanced yellow luminescence observed by photoluminescence. The findings suggest a strong impact of growth dynamics on the incorporation of impurities and electrically active native point defects as a function of GaN growth plane polarity.
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71.55.Eq III-V semiconductors
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Role of deep and shallow donor levels on n-type conductivity of hydrothermal ZnO

S. Brochen, C. Granier, G. Feuillet, and J. Pernot

Appl. Phys. Lett. 100, 052115 (2012); http://dx.doi.org/10.1063/1.3681168 (4 pages) | Cited 2 times

Online Publication Date: 3 February 2012

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The residual n-type conductivity of O-polar hydrothermally grown ZnO single crystals and the role of annealing on the transport properties are assessed by temperature dependent Hall measurements on a wide 20–800 K temperature range. A deep level lying 250 meV below the conduction band is responsible for the residual n-type conductivity of unannealed samples. After annealing, a shallow donor level with 25 meV ionization energy becomes responsible for the sample conductivity in the room temperature range. Thanks to high temperature Hall measurement, the coexistence of the deep and the shallow level has been demonstrated in the case of annealed sample.
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72.20.Fr Low-field transport and mobility; piezoresistance
72.20.My Galvanomagnetic and other magnetotransport effects
72.80.Ey III-V and II-VI semiconductors
73.20.At Surface states, band structure, electron density of states
61.72.Cc Kinetics of defect formation and annealing
71.55.Gs II-VI semiconductors
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