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18 May 2009

Volume 94, Issue 20, Articles (20xxxx)

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Appl. Phys. Lett. 94, 203301 (2009); http://dx.doi.org/10.1063/1.3133902 (3 pages)

Zihong Liu, Joon Hak Oh, Mark E. Roberts, Peng Wei, Bipul C. Paul, Masaki Okajima, Yoshio Nishi, and Zhenan Bao
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Isotropic transport in an oligothiophene derivative for single-crystal field-effect transistor applications

Colin Reese, Mark E. Roberts, Sean R. Parkin, and Zhenan Bao

Appl. Phys. Lett. 94, 202101 (2009); http://dx.doi.org/10.1063/1.3129162 (3 pages) | Cited 4 times

Online Publication Date: 18 May 2009

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Single-crystal organic semiconductors have proven invaluable tools in the exploration of charge transport in molecular materials. We employ the elastomeric, photolithographically patterned single-crystal field-effect transistor in the study of an alpha-substituted oligothiophene. The terminal units specify a symmetric layered motif, while allowing the oligothiophene cores to pack closely. Angle-resolved measurements of the field-effect mobility reflect the symmetric edge/face interactions and isotropic mobility. These measurements are supported by electronic structure calculations that show nearly equivalent intermolecular interactions along cell diagonals. These results reveal that the transport is diffusive and a minimum of fourfold symmetry is required for in-plane mobility isotropy.
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72.20.Fr Low-field transport and mobility; piezoresistance
66.30.-h Diffusion in solids
85.30.Tv Field effect devices

Pre-edges in oxygen (1s) x-ray absorption spectra: A spectral indicator for electron hole depletion and transport blocking in iron perovskites

Artur Braun, Defne Bayraktar, Selma Erat, Ashley S. Harvey, Daniel Beckel, John A. Purton, Peter Holtappels, Ludwig J. Gauckler, and Thomas Graule

Appl. Phys. Lett. 94, 202102 (2009); http://dx.doi.org/10.1063/1.3122926 (3 pages) | Cited 8 times

Online Publication Date: 19 May 2009

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The eg↑/(t2g↓+eg↓) band ratio in cation-substituted La–Fe oxides is identified in O (1s) x-ray absorption spectra as a linear spectral indicator for conducting electron holes. The t2g↓ and eg↓ bands act as a conductivity inhibitor by ferromagnetic double exchange coupling on the eg↑ electron. Disorder induced by substitution appears to modulate the hole conduction such that an exponential relation is found between the conductivity and the eg↑/(t2g↓+eg↓) ratio and hole concentration. The quantitative correlation of conductivity and x-ray absorption spectra from heterovalent-substituted LaFeO3 lets substitution-driven metal insulator transitions appear in a new light.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
78.70.Dm X-ray absorption spectra
75.30.Et Exchange and superexchange interactions
71.30.+h Metal-insulator transitions and other electronic transitions
72.80.Sk Insulators
75.50.Dd Nonmetallic ferromagnetic materials

A wide-band-gap p-type thermoelectric material based on quaternary chalcogenides of Cu2ZnSnQ4 (Q = S,Se)

Min-Ling Liu, Fu-Qiang Huang, Li-Dong Chen, and I-Wei Chen

Appl. Phys. Lett. 94, 202103 (2009); http://dx.doi.org/10.1063/1.3130718 (3 pages) | Cited 30 times

Online Publication Date: 19 May 2009

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Chalcopyritelike quaternary chalcogenides, Cu2ZnSnQ4 (Q = S,Se), were investigated as an alternative class of wide-band-gap p-type thermoelectric materials. Their distorted diamondlike structure and quaternary compositions are beneficial to lowering lattice thermal conductivities. Meanwhile, partial substitution of Cu for Zn creates more charge carriers and conducting pathways via the CuQ4 network, enhancing electrical conductivity. The power factor and the figure of merit (ZT) increase with the temperature, making these materials suitable for high temperature applications. For Cu2.1Zn0.9SnQ4, ZT reaches about 0.4 at 700 K, rising to 0.9 at 860 K.
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72.20.Pa Thermoelectric and thermomagnetic effects
72.80.Jc Other crystalline inorganic semiconductors
61.72.up Other materials

Fabrication of compositional graded Si1−xGex layers by using thermal oxidation

J. H. Jang, S. Y. Son, Wantae Lim, M. S. Phen, K. Siebein, S. J. Pearton, and V. Craciun

Appl. Phys. Lett. 94, 202104 (2009); http://dx.doi.org/10.1063/1.3139070 (3 pages) | Cited 4 times

Online Publication Date: 19 May 2009

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Compositional graded and highly relaxed Si1−xGex layers have been fabricated by using thermal oxidation at high temperature. It was found that the behavior of Ge atoms during thermal oxidation was significantly dependent of the oxidation temperature. The Ge accumulation below the oxide occurred at 800 and 900 °C due to a large difference of the heat formation of GeO2 and SiO2. However, Si1−xGex layers oxidized in 1000 °C did not show any Ge accumulation because Ge diffusion efficiently occurred. The compositional graded Si1−xGex layers fabricated by thermal oxidation can be used as virtual substrates for the strained-Si and relaxed-SiGe applications.
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81.65.Mq Oxidation
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.ag Semiconductors
66.30.-h Diffusion in solids

Effect of nanoparticle scattering on thermoelectric power factor

Mona Zebarjadi, Keivan Esfarjani, Ali Shakouri, Je-Hyeong Bahk, Zhixi Bian, Gehong Zeng, John Bowers, Hong Lu, Joshua Zide, and Art Gossard

Appl. Phys. Lett. 94, 202105 (2009); http://dx.doi.org/10.1063/1.3132057 (3 pages) | Cited 36 times

Online Publication Date: 20 May 2009

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The effect of nanoparticles on the thermoelectric power factor is investigated using the relaxation time approximation. The partial-wave technique is used for calculating the nanoparticle scattering cross section exactly. We validate our model by comparing its results to the experimental data obtained for ErAs:InGaAlAs samples. We use the theory to maximize the power factor with respect to nanoparticle and electron concentrations as well as the barrier height. We found that at the optimum of the power factor, the electron concentration is usually higher in the sample with nanoparticles, implying that Seebeck is usually unchanged and conductivity is increased.
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73.50.Lw Thermoelectric effects
72.20.Dp General theory, scattering mechanisms
73.63.-b Electronic transport in nanoscale materials and structures

Understanding the observation of large electrical conductivity in liquid crystal-carbon nanotube composites

V. Jayalakshmi and S. Krishna Prasad

Appl. Phys. Lett. 94, 202106 (2009); http://dx.doi.org/10.1063/1.3133352 (3 pages) | Cited 6 times

Online Publication Date: 20 May 2009

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We report the electrical conductivity and dielectric constant measurements in composites of single-walled carbon nanotubes (CNTs) with nematic liquid crystals, employing electrical and magnetic fields as reorienting fields. Our studies demonstrate that the large magnitude of enhancement in the electrical conductivity reported in voltage-driven reorientation measurements is not due to the inherent property of CNTs, and that dielectric breakdown and local heating effects play an important role. The calculated magnitude of the local heating effect is in agreement with the experimental observation including the nematic-isotropic transition caused by such Joule heating.
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73.63.Fg Nanotubes
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
77.22.Ch Permittivity (dielectric function)
77.22.Jp Dielectric breakdown and space-charge effects

Resistive switching in nanostructured thin films

H. Silva, H. L. Gomes, Yu. G. Pogorelov, P. Stallinga, D. M. de Leeuw, J. P. Araujo, J. B. Sousa, S. C. J. Meskers, G. Kakazei, S. Cardoso, and P. P. Freitas

Appl. Phys. Lett. 94, 202107 (2009); http://dx.doi.org/10.1063/1.3134484 (3 pages) | Cited 8 times

Online Publication Date: 20 May 2009

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Planar capacitor structures based on granular films composed of nanometric ferromagnetic grains embedded in an insulating Al2O3 matrix can switch between a high-conductance and a low-conductance state. The switching properties are induced by a forming process. The ON/OFF resistance ratio is as high as 104 under an electrical field of only 15 kV/m. This resistive switching is accompanied by a capacitive switching between two well-defined voltage-independent states, a behavior that is not readily explained by the filamentary type of conduction.
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73.63.Bd Nanocrystalline materials

Ga2O3(Gd2O3) on Ge without interfacial layers: Energy-band parameters and metal oxide semiconductor devices

L. K. Chu, T. D. Lin, M. L. Huang, R. L. Chu, C. C. Chang, J. Kwo, and M. Hong

Appl. Phys. Lett. 94, 202108 (2009); http://dx.doi.org/10.1063/1.3139772 (3 pages) | Cited 8 times

Online Publication Date: 20 May 2009

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Ga2O3(Gd2O3) (GGO) directly deposited on Ge substrate in ultrahigh vacuum, without a passivation layer such as GeOxNy or Si, has demonstrated excellent electrical performances and thermodynamic stability. Energy-band parameters of GGO/Ge have been determined by in situ x-ray photoelectron spectroscopy in conjunction with reflection electron energy loss spectroscopy and current transport of Fowler–Nordheim tunneling. A conduction-band offset and a valence-band offset of ∼ 2.3 and ∼ 2.42 eV, respectively, have been obtained. Moreover, self-aligned Ge pMOSFETs of 1-μm-gate length using Al2O3/GGO as the gate dielectrics have shown a high drain current and a peak transconductance of 252 mA/mm, and 143 mS/mm, respectively.
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85.30.Tv Field effect devices
73.20.At Surface states, band structure, electron density of states
73.40.Gk Tunneling
65.40.G- Other thermodynamical quantities
79.60.Jv Interfaces; heterostructures; nanostructures
79.20.Uv Electron energy loss spectroscopy

Measuring spin diffusion of electrons in bulk n-GaAs using circularly dichromatic absorption difference spectroscopy of spin gratings

Hua-Liang Yu, Xiu-Min Zhang, Peng-Fei Wang, Hai-Qiao Ni, Zhi-Chuan Niu, and Tianshu Lai

Appl. Phys. Lett. 94, 202109 (2009); http://dx.doi.org/10.1063/1.3141483 (3 pages) | Cited 2 times

Online Publication Date: 21 May 2009

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Circular dichromatic absorption difference spectroscopy is developed to measure the spin diffusion dynamics of electrons in bulk n-GaAs. This spectroscopy has higher detection sensitivity over homodyne detection of spin-grating-diffracted signal. A model to describe circular dichromatic absorption difference signal is derived and used to fit experimental signal to retrieve decaying rate of spin gratings. A spin diffusion constant of Ds = 201±25 cm2/s for bulk n-GaAs has been measured at room temperature using this technique and is close to electron diffusion constant (Dc), which is much different from the case in GaAs quantum wells where Ds is markedly less than Dc.
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72.25.Dc Spin polarized transport in semiconductors
72.80.Ey III-V and II-VI semiconductors
66.30.H- Self-diffusion and ionic conduction in nonmetals
78.20.Fm Birefringence

Energy barriers at interfaces between (100) InxGa1−xAs (0 ≤ x ≤ 0.53) and atomic-layer deposited Al2O3 and HfO2

V. V. Afanas’ev, A. Stesmans, G. Brammertz, A. Delabie, S. Sionke, A. O’Mahony, I. M. Povey, M. E. Pemble, E. O’Connor, P. K. Hurley, and S. B. Newcomb

Appl. Phys. Lett. 94, 202110 (2009); http://dx.doi.org/10.1063/1.3137187 (3 pages) | Cited 11 times

Online Publication Date: 21 May 2009

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The electron energy band alignment at interfaces of InxGa1−xAs (0 ≤ x ≤ 0.53) with atomic-layer deposited insulators Al2O3 and HfO2 is characterized using internal photoemission and photoconductivity experiments. The energy of the InxGa1−xAs valence band top is found to be only marginally influenced by the semiconductor composition. This result suggests that the known bandgap narrowing from 1.42 to 0.75 eV when the In content increases from 0 to 0.53 occurs mostly through downshift of the semiconductor conduction band bottom. It finds support from both electron and hole photoemission data. Similarly to the GaAs case, electron states originating from the interfacial oxidation of InxGa1−xAs lead to reduction in the electron barrier at the semiconductor/oxide interface.
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71.20.Nr Semiconductor compounds
73.61.Ey III-V semiconductors
72.40.+w Photoconduction and photovoltaic effects
79.60.Ht Disordered structures
79.60.Jv Interfaces; heterostructures; nanostructures
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Transition into a gapless state and concentration anomalies in the properties of Bi1−xSbx solid solutions

E. I. Rogacheva, A. A. Drozdova, O. N. Nashchekina, M. S. Dresselhaus, and G. Dresselhaus

Appl. Phys. Lett. 94, 202111 (2009); http://dx.doi.org/10.1063/1.3139076 (3 pages) | Cited 2 times

Online Publication Date: 21 May 2009

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For Bi1−xSbx solid solutions (x = 0.01–0.05), the dependences of the electrical conductivity, Hall coefficient, charge carrier mobility, magnetoresistance, Seebeck coefficient, and microhardness on the Sb concentration have been obtained in the temperature range of 77–300 K. In the isotherms of these properties, distinct extrema or inflection points have been observed in the range of x = 0.025–0.035. The presence of these concentration anomalies is attributed to the critical phenomena, which accompany the transition into a gapless state under increasing Sb concentration.
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72.15.Eb Electrical and thermal conduction in crystalline metals and alloys
72.15.Gd Galvanomagnetic and other magnetotransport effects
71.30.+h Metal-insulator transitions and other electronic transitions
72.15.Jf Thermoelectric and thermomagnetic effects
62.20.Qp Friction, tribology, and hardness
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure

Origin of flatband voltage shift and unusual minority carrier generation in thermally grown GeO2/Ge metal-oxide-semiconductor devices

Takuji Hosoi, Katsuhiro Kutsuki, Gaku Okamoto, Marina Saito, Takayoshi Shimura, and Heiji Watanabe

Appl. Phys. Lett. 94, 202112 (2009); http://dx.doi.org/10.1063/1.3143627 (3 pages) | Cited 27 times

Online Publication Date: 22 May 2009

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Improvement in electrical properties of thermally grown GeO2/Ge metal-oxide-semiconductor (MOS) capacitors, such as significantly reduced flatband voltage (VFB) shift, small hysteresis, and minimized minority carrier response in capacitance-voltage (C-V) characteristics, has been demonstrated by in situ low temperature vacuum annealing prior to gate electrode deposition. Thermal desorption analysis has revealed that not only water but also hydrocarbons are easily infiltrated into GeO2 layers during air exposure and desorbed at around 300 °C, indicating that organic molecules within GeO2/Ge MOS structures are possible origins of electrical defects. The inversion capacitance, indicative of minority carrier generation, increases with air exposure time for Au/GeO2/Ge MOS capacitors, while maintaining an interface state density (Dit) of about a few 1011 cm−2 eV−1. Unusual increase in inversion capacitance was found to be suppressed by Al2O3 capping (Au/Al2O3/GeO2/Ge structures). This suggests that electrical defects induced outside the Au electrode by infiltrated molecules may enhance the minority carrier generation, and thus acting as a minority carrier source just like MOS field-effect transistors.
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84.32.Tt Capacitors
61.72.Cc Kinetics of defect formation and annealing
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
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