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6 Aug 2007

Volume 91, Issue 6, Articles (06xxxx)

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Appl. Phys. Lett. 91, 063118 (2007); http://dx.doi.org/10.1063/1.2768861 (3 pages)

Douglas C. Meier, Steve Semancik, Bradley Button, Evgheni Strelcov, and Andrei Kolmakov
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Impact of surface chemical treatment on capacitance-voltage characteristics of GaAs metal-oxide-semiconductor capacitors with Al2O3 gate dielectric

Davood Shahrjerdi, Emanuel Tutuc, and Sanjay K. Banerjee

Appl. Phys. Lett. 91, 063501 (2007); http://dx.doi.org/10.1063/1.2764438 (3 pages) | Cited 45 times

Online Publication Date: 6 August 2007

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The authors examine the impact of two different chemical surface treatment methods on capacitance-voltage characteristics of GaAs metal-oxide-semiconductor (MOS) capacitors using NH4OH and (NH4)2S prior to atomic layer deposition (ALD) of Al2O3. In both cases, x-ray photoelectron spectroscopy data confirm the removal of As2O3/As2O6 upon Al2O3 deposition. However, Ga–O bonds appear to incorporate in the final gate stack at the Al2O3/GaAs interface. MOS capacitors exhibit a steep transition from accumulation to depletion as well as very low leakage current density indicating high quality of ALD-Al2O3. The midgap interface trap density was evaluated to be ( ∼ 3–5)×1011/cm2 eV using the Terman method. In addition, quasistatic capacitance-voltage (C-V) measurement confirms the formation of true inversion layer in GaAs using both chemical treatment protocols. However, sulfur-passivated GaAs demonstrates better frequency dispersion behavior and slightly smaller capacitance equivalent thickness than hydroxylated GaAs. A statistical study substantiates the reproducibility of these results.
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84.32.Tt Capacitors
81.65.Rv Passivation

Gain degradation mechanisms in wafer fused AlGaAs/GaAs/GaN heterojunction bipolar transistors

Chuanxin Lian, Huili Grace Xing, Chad S. Wang, David Brown, and Lee McCarthy

Appl. Phys. Lett. 91, 063502 (2007); http://dx.doi.org/10.1063/1.2766961 (3 pages) | Cited 4 times

Online Publication Date: 6 August 2007

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The authors have compared AlGaAs/GaAs/GaN heterojunction bipolar transistors (HBTs) formed by wafer fusion with AlGaAs/GaAs/GaAs as-grown HBTs subject to high temperature annealing conditions similar to those used in the wafer fusion process. The high temperature annealing alone is found to cause gain degradation by a factor of 2–6, a result of reduction in minority carrier lifetime in the base. Detailed analysis indicates that the fused HBTs also suffer from higher recombination in the emitter-base junction, exacerbated base degradation as well as effective potential barriers formed at the GaAs base/GaN collector junction.
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85.30.Pq Bipolar transistors

Superhydrophobic modification of gate dielectrics for densely packed pentacene thin film transistors

Chang Su Kim, Sung Jin Jo, Jong Bok Kim, Seung Yoon Ryu, Joo Hyon Noh, Hong Koo Baik, Se Jong Lee, and Youn Sang Kim

Appl. Phys. Lett. 91, 063503 (2007); http://dx.doi.org/10.1063/1.2767779 (3 pages) | Cited 14 times

Online Publication Date: 7 August 2007

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Pentacene organic thin film transistors (OTFTs) with low-k and high-k hybrid gate dielectrics by CF4 plasma treatment exhibited excellent device performance with field effect mobilities (maximum 1.41 cm2/V s), a low threshold voltage of +1 V, and on/off current ratios of 105 at −5 V gate bias. After CF4 plasma treatment, fluorine atoms diffuse into the interior low-k polymer and eliminate ionic impurities which reduce the leakage current density and overall pentacene initial growth on the superhydrophobic surface is significantly improved. It seems apparent that proper surface treatment is desirable for higher quality pentacene film and improving the performance of OTFTs.
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81.05.Hd Other semiconductors
77.55.-g Dielectric thin films
85.30.Tv Field effect devices

Hysteresis phenomena in nanoscale rectifying diodes: A Monte Carlo interpretation in terms of surface effects

I. Iñiguez-de-la-Torre, T. González, D. Pardo, and J. Mateos

Appl. Phys. Lett. 91, 063504 (2007); http://dx.doi.org/10.1063/1.2768638 (3 pages) | Cited 4 times

Online Publication Date: 8 August 2007

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Hysteresis effects taking place in the rectifying current-voltage characteristic of self-switching diodes are studied by means of a semiclassical Monte Carlo simulator. When the applied voltage is higher than a certain threshold, in both negative and positive voltage sweeps, a well-defined hysteresis loop appears which can be exploited for memory applications. An algorithm for the simulation of surface effects has been introduced in the Monte Carlo simulator to explain this behavior in terms of the charging and discharging of the surface states on the etched sidewalls of the structure. Devices with different geometries have been simulated, and a detailed microscopic analysis of the behavior of surface charge and potential profiles has been performed. The reduction of the channel length or the increase of the width improves the discrimination of the memory state (by providing a higher reverse current); however, this induces a decrease of the reverse voltage threshold.
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85.30.Kk Junction diodes

Low temperature a-Si:H photodiodes and flexible image sensor arrays patterned by digital lithography

Tse Nga Ng, Rene A. Lujan, Sanjiv Sambandan, Robert A. Street, Scott Limb, and William S. Wong

Appl. Phys. Lett. 91, 063505 (2007); http://dx.doi.org/10.1063/1.2767981 (3 pages) | Cited 11 times

Online Publication Date: 8 August 2007

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Hydrogenated amorphous silicon-based image sensor arrays were fabricated on polyethylene naphthalate substrates, with photodiodes optimized for process temperatures of 150 °C. An optimal i-layer thickness was determined to minimize carrier recombination and to maintain sufficient light absorption and acceptable leakage current. Patterning of the thin-film transistor backplane was accomplished using ink-jet printed etch masks. A flexible image sensor is demonstrated with 75 dots/in. resolution over 180×180 pixels and with sensitivity of 1.2 pW/cm2.
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85.60.Dw Photodiodes; phototransistors; photoresistors
42.79.Pw Imaging detectors and sensors
85.40.Hp Lithography, masks and pattern transfer

Scaling effect on the operation stability of short-channel organic single-crystal transistors

T. Minari, T. Miyadera, K. Tsukagoshi, T. Hamano, Y. Aoyagi, R. Yasuda, K. Nomoto, T. Nemoto, and S. Isoda

Appl. Phys. Lett. 91, 063506 (2007); http://dx.doi.org/10.1063/1.2767987 (3 pages) | Cited 10 times

Online Publication Date: 8 August 2007

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Organic single-crystal transistors allowed the authors to investigate the essential features of short-channel devices. Rubrene single-crystal transistors with channel lengths of 500 and 100 nm exhibited good field-effect characteristics under extremely low operation voltages, although space charge limited current degrades the subthreshold properties of 100 nm devices. Furthermore, bias-stress measurements revealed the remarkable stability of organic single-crystal transistors regardless of device size. The bias-stress effect was explained by the trapping of gate-induced charges into localized density of states in the single-crystal channel.
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85.30.Tv Field effect devices
72.80.Le Polymers; organic compounds (including organic semiconductors)

Interdigitated back contact silicon heterojunction solar cell and the effect of front surface passivation

Meijun Lu, Stuart Bowden, Ujjwal Das, and Robert Birkmire

Appl. Phys. Lett. 91, 063507 (2007); http://dx.doi.org/10.1063/1.2768635 (3 pages) | Cited 14 times

Online Publication Date: 8 August 2007

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This letter reports interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells which combine the performance benefits of both back contact and heterojunction technologies while reducing their limitations. Low temperature (<200 °C) deposited p- and n-type amorphous silicon used to form interdigitated heteroemitter and contacts in the rear preserves substrate lifetime while minimizes optical losses in the front. The IBC-SHJ structure is ideal for diagnosing surface passivation quality, which is analyzed and measured by internal quantum efficiency and minority carrier lifetime measurements. Initial cells have independently confirmed efficiency of 11.8% under AM1.5 illumination. Simulations indicate efficiencies greater than 20% after optimization.
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84.60.Jt Photoelectric conversion
81.65.Rv Passivation

High-efficiency orange and yellow organic light-emitting devices using platinum(II) complexes containing extended π-conjugated cyclometalated ligands as dopant materials

Bei-Ping Yan, Cecil C. C. Cheung, Steven C. F. Kui, V. A. L. Roy, Chi-Ming Che, and Shi-Jie Xu

Appl. Phys. Lett. 91, 063508 (2007); http://dx.doi.org/10.1063/1.2768868 (3 pages) | Cited 11 times

Online Publication Date: 8 August 2007

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Two luminescent platinum(II) complexes 1 and 2 containing extended π-conjugated cyclometalated ligands have been used as dopant materials for the construction of two high-efficiency organic light-emitting devices I and II. Device I (containing dopant 1) emits orange emission and exhibits a maximum external quantum efficiency of 12.4%, a maximum luminous efficiency of 32.3 cd/A, and a maximum power efficiency of 11.2 lm/W. Device II (containing dopant 2) emits yellow light and exhibits a maximum external quantum efficiency of 16.1%, a maximum luminous efficiency of 51.8 cd/A, and a maximum power efficiency of 23.2 lm/W.
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85.60.Jb Light-emitting devices

Improving polymer light-emitting diodes efficiency using interlayers based on cross-linkable polymers

G. Bernardo, A. Charas, L. Alcácer, and J. Morgado

Appl. Phys. Lett. 91, 063509 (2007); http://dx.doi.org/10.1063/1.2768636 (3 pages) | Cited 6 times

Online Publication Date: 8 August 2007

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The efficiency of light-emitting diodes based on poly[(2-methoxy)-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] increases upon formation of interlayers, on top of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS), made of acid-initiated cross-linkable polyfluorenes. The use of this type of polymers allows for the formation of thicker interlayers leading to higher efficiencies when comparing with similar devices with interlayers formed by the parent non-cross-linkable polymers. This efficiency increase is attributed to a combination of electron and exciton confinement away from PEDOT:PSS.
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85.60.Jb Light-emitting devices

Highly efficient p-i-n-type organic light emitting diodes on ZnO:Al substrates

Yuto Tomita, Christian May, Michael Toerker, Joerg Amelung, Michael Eritt, Frank Loeffler, Claus Luber, Karl Leo, Karsten Walzer, Karsten Fehse, and Qiang Huang

Appl. Phys. Lett. 91, 063510 (2007); http://dx.doi.org/10.1063/1.2768865 (3 pages) | Cited 7 times

Online Publication Date: 8 August 2007

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Aluminum doped zinc oxide (ZAO) is presented in this letter as an alternative transparent electrode: optimized ZAO films offer excellent parameters for organic light emitting diodes (OLEDs). The ZAO films are applied to various p-i-n-type OLEDs. By using green phosphorescent molecules in a double emitter structure, very high efficiencies were obtained, namely, 54.6 cd/A and 61.5 lm/W for 100 cd/m2 at 2.78 V. Additionally, white OLEDs on ZAO demonstrated pure white emission independent of the luminance and high efficiencies of 12.6 cd/A and 14.5 lm/W for 100 cd/m2 at 2.6 V, which is comparable to indium-tin-oxide based white OLEDs.
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85.60.Jb Light-emitting devices

Switch-on transient behavior of vanadium phthalocyanine based organic transistors

Lijuan Wang, Guojun Liu, Haibo Wang, De Song, Bo Yu, and Donghang Yan

Appl. Phys. Lett. 91, 063511 (2007); http://dx.doi.org/10.1063/1.2768886 (3 pages) | Cited 3 times

Online Publication Date: 8 August 2007

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The authors investigated the switch-on transient properties of p-type vanadium phthalocyanine (VOPc) transistors, which were fabricated by weak epitaxy growth on ordered para-sexiphenyl (p-6P) layer. The overshoot phenomenon of drain current had been observed in the VOPc/p-6P transistors, which was explained by the filling of carriers in traps of organic films. The small overshoot value of about 35% and transient duration time of 2 ms demonstrated the low trap concentration in organic films, which were comparable to the reported hydrogenated amorphous-silicon thin-film transistors. Therefore, the VOPc/p-6P transistors can be applied in active matrix liquid crystal display as switch elements.
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85.30.Tv Field effect devices

Parity effect in Al and Nb single electron transistors in a tunable environment

A. M. Savin, M. Meschke, J. P. Pekola, Yu. A. Pashkin, T. F. Li, H. Im, and J. S. Tsai

Appl. Phys. Lett. 91, 063512 (2007); http://dx.doi.org/10.1063/1.2768897 (3 pages) | Cited 4 times

Online Publication Date: 9 August 2007

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Two different types of Cooper pair transistors, with Al and Nb islands, have been investigated in a tunable electromagnetic environment. The device with an Al island demonstrates gate charge modulation with 2e periodicity in a wide range of environmental impedances at bath temperatures below 340 mK. Contrary to the results of the Al sample, the authors were not able to detect 2e periodicity under any conditions on similar samples with Nb island. The authors attribute this to the material properties of Nb.
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85.25.-j Superconducting devices
85.35.Gv Single electron devices

Correlation between the lattice parameter and the dielectric tunability in nonepitaxial Ba0.5Sr0.5TiO3 thin films

Sébastien Luc Delprat, Christophe Durand, JaeHo Oh, Mohamed Chaker, and Ke Wu

Appl. Phys. Lett. 91, 063513 (2007); http://dx.doi.org/10.1063/1.2768898 (3 pages) | Cited 6 times

Online Publication Date: 9 August 2007

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Nonepitaxial Ba0.5Sr0.5TiO3 (BST) films intended for tunable-microwave applications are deposited on alumina substrate by reactive pulsed laser deposition. A direct correlation is established between the lattice parameter and the dielectric tunability (measured at 3 GHz and 3 V μm−1) independently of the parameters used to synthesize the films (oxygen deposition pressure, deposition/crystallization temperature, and W–Al doping level). This correlation is explained in terms of elastic strain effects inside the Ba0.5Sr0.5TiO3 grains. According to this study, a broad tunability can be achieved on low-cost microwave devices based on nonepitaxial BST films provided the internal elastic stress of the film is minimal.
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77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)
61.66.Fn Inorganic compounds

High mobility solution processed 6,13-bis(triisopropyl-silylethynyl) pentacene organic thin film transistors

Sung Kyu Park, Thomas N. Jackson, John E. Anthony, and Devin A. Mourey

Appl. Phys. Lett. 91, 063514 (2007); http://dx.doi.org/10.1063/1.2768934 (3 pages) | Cited 166 times

Online Publication Date: 9 August 2007

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Using the small molecule organic semiconductor 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene), the authors have fabricated the solution-processed organic thin film transistors (OTFTs) with carrier mobility >1 cm2/V s, current on/off ratio greater than 107, and subthreshold slope <0.3 V/decade. The high mobility TIPS-pentacene solution-processed films are deposited from high boiling point solvents and show strong molecular ordering including molecular terracing. Film ordering varies substantially for different solvents and film deposition techniques and OTFT mobility correlates well with film ordering.
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85.30.Tv Field effect devices

High-performance thin film transistors from semiconducting liquid crystalline phases by solution processes

Fapei Zhang, Masahiro Funahashi, and Nobuyuki Tamaoki

Appl. Phys. Lett. 91, 063515 (2007); http://dx.doi.org/10.1063/1.2768307 (3 pages) | Cited 11 times

Online Publication Date: 9 August 2007

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The authors have prepared liquid crystalline (LC) semiconducting thin films with smectic phases at room temperature from 5-propyl-5″-(4-pentylphenyl)-2,2′:5′,2″-terthiophene (3-TTPPh-5) by spin-coating process. The films consist of large-size (approximately several tens of micrometers) LC domains with a lamellar structure. The ordering of molecular packing within the smectic layer is enhanced after annealing. The performance of thin film transistors (TFTs) based on 3-TTPPh-5 is influenced largely by silane agent modification of the dielectric. The octyltrichlorosilane (OTS) treatment enhances performance and leads to a high hole mobility of more than 4×10−2 cm2/V s in ambient air. It is attributed to the formation of highly ordered domains with the size comparable to channel length of the TFT as well as the favorable interaction between OTS and LC molecules.
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61.30.-v Liquid crystals
85.30.Tv Field effect devices

Alignment and switching behaviors of liquid crystal on a-SiOx thin films deposited by a filtered cathodic arc process

P. J. Martin, A. Bendavid, C. Comte, H. Miyata, Y. Asao, Y. Ishida, and A. Sakai

Appl. Phys. Lett. 91, 063516 (2007); http://dx.doi.org/10.1063/1.2768308 (3 pages) | Cited 11 times

Online Publication Date: 9 August 2007

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A new method is described for the preparation of silicon oxide layers, which produces vertical alignment of liquid crystal with controlled pretilt angles, by a reactive filtered cathodic arc deposition under oblique incidence geometry. The pretilt angle is dependent on the angle of deposition, but is not simply caused by the surface roughness. The achievable pretilt angle by this method is ∼ 5.5°, which allows uniform switching behavior under an applied electric field.
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42.79.Kr Display devices, liquid-crystal devices
61.30.-v Liquid crystals
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