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18 Jun 2007

Volume 90, Issue 25, Articles (25xxxx)

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Appl. Phys. Lett. 90, 253101 (2007); http://dx.doi.org/10.1063/1.2749418 (3 pages)

G. D. Dice, M. J. Brett, D. Wang, and J. M. Buriak
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Investigation of the electronic structure at interfaces of crystalline and amorphous Gd2O3 thin layers with silicon substrates of different orientations

M. Badylevich, S. Shamuilia, V. V. Afanas’ev, A. Stesmans, A. Laha, H. J. Osten, and A. Fissel

Appl. Phys. Lett. 90, 252101 (2007); http://dx.doi.org/10.1063/1.2746419 (3 pages) | Cited 22 times

Online Publication Date: 18 June 2007

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Internal photoemission, photoconductivity, and spectroscopic ellipsometry experiments were carried out to characterize the electronic structure of interfaces of (001) and (111)-oriented Si with crystalline (epitaxially grown) and amorphous Gd2O3 insulators. The energy barriers for electrons and holes (3.2 and 3.9 eV, respectively) appear to be sensitive neither to the orientation of the Si crystal surface nor to the oxide phase (crystalline or amorphous). This result indicates that despite the difference in Si–O bond density in going from (001) to (111)Si, the interface dipoles do not ensue any measurable effect on the electronic structure of the interface and the associated band offsets.
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73.20.At Surface states, band structure, electron density of states
79.60.Jv Interfaces; heterostructures; nanostructures
72.40.+w Photoconduction and photovoltaic effects
68.35.Ct Interface structure and roughness

Fermi level shift in La1−xSrxMO3 (M = Mn, Fe, Co, and Ni) probed by Schottky-like heteroepitaxial junctions with SrTi0.99Nb0.01O3

A. Sawa, A. Yamamoto, H. Yamada, T. Fujii, M. Kawasaki, J. Matsuno, and Y. Tokura

Appl. Phys. Lett. 90, 252102 (2007); http://dx.doi.org/10.1063/1.2749431 (3 pages) | Cited 18 times

Online Publication Date: 18 June 2007

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The authors have studied electrical properties of perovskite heteroepitaxial junctions consisting of transition metal oxides La1−xSrxMO3 (LSMO: M = Mn, Fe, Co, and Ni) and an n-type semiconductor SrTi0.99Nb0.01O3 (Nb:STO). The junctions showed rectifying current-voltage characteristics that could be analyzed by taking into account a Schottky-like barrier formed in the Nb:STO at the interfaces. As the doping level x is increased, the Schottky barrier height and built-in potential increase as ∼ x (eV), indicating the downward shift of the Fermi level position in the LSMO. The Fermi level position in the LSMO with the same doping level x tends to be deepened with increasing the atomic number of M, in the order of Mn, Fe, Co, and Ni.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.30.+y Surface double layers, Schottky barriers, and work functions
73.20.At Surface states, band structure, electron density of states
61.72.up Other materials

Low-resistivity Au/Ni Ohmic contacts to Sb-doped p-type ZnO

L. J. Mandalapu, Z. Yang, and J. L. Liu

Appl. Phys. Lett. 90, 252103 (2007); http://dx.doi.org/10.1063/1.2750400 (3 pages) | Cited 14 times

Online Publication Date: 18 June 2007

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Au/Ni contacts were fabricated on Sb-doped p-type ZnO film, which was grown on n-type Si (100) substrate with a thin undoped ZnO buffer layer by molecular beam epitaxy. As-deposited contacts were rectifying while Ohmic behavior was achieved after thermally annealing the contacts in nitrogen environment. Contact resistance was determined by linear transmission line method and it decreased with the increase of annealing temperature. Low specific contact resistivity of 3.0×10−4 Ω cm2 was obtained for sample annealed at 800 °C for 60 s. Secondary ion mass spectroscopy was used to analyze elemental profiles of the contacts before and after annealing. Zn vacancies created by outdiffusion of Zn are believed to couple with activated Sb atoms to increase the surface hole concentration enabling Ohmic contact formation.
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73.40.Ns Metal-nonmetal contacts
73.40.Cg Contact resistance, contact potential
61.72.Cc Kinetics of defect formation and annealing
61.72.J- Point defects and defect clusters
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Effects of high carrier densities on phonon and carrier lifetimes in Si by time-resolved anti-Stokes Raman scattering

Jeffrey J. Letcher, Kwangu Kang, David G. Cahill, and Dana D. Dlott

Appl. Phys. Lett. 90, 252104 (2007); http://dx.doi.org/10.1063/1.2749728 (3 pages) | Cited 9 times

Online Publication Date: 18 June 2007

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The relaxation times T1 of nonequilibrium populations of elementary excitations in Si are studied by time-resolved Raman scattering using a subpicosecond pump-probe method. Incoherent anti-Stokes Raman scattering is used to monitor the decay of the nonequilibrium populations of holes and the generation and decay of zone-center longitudinal optical (LO) phonons. At lower levels of laser excitation, hole and LO phonon T1 lifetimes are less than 0.2 ps and greater than 1.6 ps, respectively. At higher laser intensities, the lifetime of holes increases and the lifetime of LO phonons decrease toward a common value of T1 ∼ 0.4 ps.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
78.30.Am Elemental semiconductors and insulators
78.47.-p Spectroscopy of solid state dynamics

Probing electronic excitations in organic light-emitting diodes via Raman scattering

M. Arif, S. Guha, A. Tsami, and U. Scherf

Appl. Phys. Lett. 90, 252105 (2007); http://dx.doi.org/10.1063/1.2749872 (3 pages) | Cited 2 times

Online Publication Date: 19 June 2007

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The authors present Raman scattering studies from ethyl-hexyl substituted polyfluorene (PF)-based light-emitting diodes in the presence of injected and photogenerated charge carriers. The Raman background systematically increases with increased charge density and the Raman peaks exhibit asymmetric line shapes characteristic of a Breit-Wigner-Fano (BWF) resonance, indicating interference effects between the electronic continuum and phonons. A complete BWF line shape analysis of the intraring stretch mode at 1605 cm−1 is provided. These results are compared with p-doped PF; by increasing the doping concentration, it is seen that the center of the electronic continuum lies at ∼ 0.2 eV.
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85.60.Jb Light-emitting devices

Huge positive magnetoresistance of GaAs/AlGaAs high electron mobility transistor structures at high temperatures

Chien-Chung Wang, C.-T. Liang, Yu-Ting Jiang, Y. F. Chen, N. R. Cooper, M. Y. Simmons, and D. A. Ritchie

Appl. Phys. Lett. 90, 252106 (2007); http://dx.doi.org/10.1063/1.2750388 (3 pages) | Cited 3 times

Online Publication Date: 19 June 2007

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The authors have performed magnetoresistivity measurements ρxx(B) on GaAs/AlGaAs high electron mobility transistor (HEMT) structures at high temperatures T. These HEMT structures show huge positive magnetoresistance (MR). For B = ±6 T, the MR values are >1300% and >200% at T = 20 and 80 K, respectively. Since a GaAs-based HEMT structure is not susceptible to ferromagnetic noise which appears to represent a fundamental challenge to the scalability of magnetic MR devices to ultrahigh area densities, the experimental results pave the way for the integration of scalable nonmagnetic MR devices with the mature HEMT technology using the same material system.
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85.30.Tv Field effect devices

Growth behavior of self-formed barrier at Cu–Mn/SiO2 interface at 250–450 °C

M. Haneda, J. Iijima, and J. Koike

Appl. Phys. Lett. 90, 252107 (2007); http://dx.doi.org/10.1063/1.2750402 (3 pages) | Cited 30 times

Online Publication Date: 19 June 2007

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A diffusion barrier layer was self-formed at the interface between Cu–Mn alloy and tetraethylorthosilicate oxide layers at 250–450 °C. No interdiffusion occurred across the self-formed barrier layer during annealing at these temperatures up to 100 h. The growth of the barrier layer obeyed a logarithmic law and depended on manganese concentration. The barrier thickness could be controlled in the range of 2–8 nm.
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66.30.Ny Chemical interdiffusion; diffusion barriers
68.35.Fx Diffusion; interface formation
85.40.Ls Metallization, contacts, interconnects; device isolation

Metal-like conductivity in transparent Al:ZnO films

O. Bamiduro, H. Mustafa, R. Mundle, R. B. Konda, and A. K. Pradhan

Appl. Phys. Lett. 90, 252108 (2007); http://dx.doi.org/10.1063/1.2749836 (3 pages) | Cited 50 times

Online Publication Date: 20 June 2007

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This letter reports on the metal-like conductivity down to 125 K in highly crystalline transparent ( ≥ 85% in the visible region) Al:ZnO films grown on sapphire and glass substrates by pulsed-laser deposition technique. Significantly different surface morphologies were found on both types of films. Temperature dependent resistivity measurements of the films grown at 450 °C on sapphire and glass show metal-like conductivity with electrical resistivity, ∼ 1.77×10−4 and ∼ 3.92×10−4 Ω cm, respectively, at room temperature followed by either a residual conductivity or a metal-semiconductor transition at low temperature due to the localization effect caused by the defects.
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73.61.Ga II-VI semiconductors
72.60.+g Mixed conductivity and conductivity transitions
71.30.+h Metal-insulator transitions and other electronic transitions
81.15.Fg Pulsed laser ablation deposition
68.55.A- Nucleation and growth

Stable hydrogen sensors from AlGaN/GaN heterostructure diodes with TiB2-based Ohmic contacts

Hung-Ta Wang, T. J. Anderson, B. S. Kang, F. Ren, Changzhi Li, Zhen-Ning Low, Jenshan Lin, B. P. Gila, S. J. Pearton, A. Osinsky, and Amir Dabiran

Appl. Phys. Lett. 90, 252109 (2007); http://dx.doi.org/10.1063/1.2751107 (3 pages) | Cited 14 times

Online Publication Date: 20 June 2007

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The use of TiB2-based Ohmic contacts on Pt-gate AlGaN/GaN heterostructure diode hydrogen sensors is shown to provide very stable operation for detection of 1% H2 in air under field conditions where temperature is allowed to vary. In contrast, the use of more conventional Ti/Al/Pt/Au Ohmic contacts led to higher background variations in current that affect the ultimate detection threshold of the sensors. Combined with a differential pair geometry that compares current from an active diode with Pt-gate contact and a passive diode with Ti/Au gate, the more stable TiB2-based Ohmic contacts reduce false alarms due to ambient temperature changes. The diodes exhibit a change in forward current of more than 1 mA at 1.5 V when 1% H2 is introduced into an air ambient.
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85.30.Kk Junction diodes

Improved photostability of organic thin film transistors with tantalum oxide/poly(4-vinylphenol) double gate insulators

Yihua Zhao, Guifang Dong, Liduo Wang, and Yong Qiu

Appl. Phys. Lett. 90, 252110 (2007); http://dx.doi.org/10.1063/1.2750545 (3 pages) | Cited 7 times

Online Publication Date: 20 June 2007

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In this letter, the authors investigated the photostability of pentacene organic thin film transistors (OTFTs) with Ta2O5 gate insulators. Under illumination, the threshold voltage shift of the Ta2O5-based OTFTs reached 5.51 V. The obvious photoinstability was attributed to the electron trapping ability of the Ta2O5 film. To solve this problem, poly(4-vinylphenol) (PVP), a type of polymer with low trap density and high photostability was used to modify Ta2O5. It was found that OTFTs with Ta2O5/PVP double gate insulators showed improved performance, and the voltage shift after illumination was greatly reduced to 0.04 V.
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85.30.Tv Field effect devices

Ultrafast hot electron relaxation time anomaly in InN epitaxial films

Tsong-Ru Tsai, Chih-Fu Chang, and S. Gwo

Appl. Phys. Lett. 90, 252111 (2007); http://dx.doi.org/10.1063/1.2751110 (3 pages) | Cited 16 times

Online Publication Date: 20 June 2007

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Ultrafast carrier dynamics in InN epitaxial films was investigated by using femtosecond time-resolved pump-probe reflectivity measurements with a photon energy of 1.58 eV. The hot electron relaxation time decreased with increasing electron density (n), measuring at n−0.5. The result was contradictory to what was expected from the hot phonon effect and the screening effect. The authors attributed this result to the important role played by electron-electron scattering in hot electron relaxation.
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73.61.Ey III-V semiconductors
73.50.Fq High-field and nonlinear effects
78.66.Fd III-V semiconductors
78.47.-p Spectroscopy of solid state dynamics
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
71.70.Gm Exchange interactions

Polymeric nanocomposite infrared photovoltaics enhanced by pentacene

Ram Thapa, Kaushik Roy Choudhury, Won Jin Kim, Yudhisthira Sahoo, A. N. Cartwright, and Paras N. Prasad

Appl. Phys. Lett. 90, 252112 (2007); http://dx.doi.org/10.1063/1.2746082 (3 pages) | Cited 8 times

Online Publication Date: 21 June 2007

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An infrared active thin film polymeric photovoltaic device is fabricated from regioregular poly(3-hexylthiophene), PbSe quantum dots, and the organic semiconductor pentacene. The PbSe quantum dots are infrared photosensitizers. Pentacene is incorporated into the formulation in a soluble precursor form. The current-voltage measurements of the device show that the photovoltaic performance is significantly increased by the introduction of pentacene, with both short-circuit current density and open-circuit voltage increased by a factor of 2. The improved performance of the device is attributed to the high mobility of charge carriers in pentacene probably due to conducting domains provided by it.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
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