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15 Dec 2009

Volume 106, Issue 12, Articles (12xxxx)

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In situ monitoring the drying kinetics of knife coated polymer-fullerene films for organic solar cells

B. Schmidt-Hansberg, M. F. G. Klein, K. Peters, F. Buss, J. Pfeifer, S. Walheim, A. Colsmann, U. Lemmer, P. Scharfer, and W. Schabel

J. Appl. Phys. 106, 124501 (2009); http://dx.doi.org/10.1063/1.3270402 (7 pages) | Cited 23 times

Online Publication Date: 17 December 2009

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The efficiency of polymer based bulk heterojunction (BHJ) solar cells mainly depends on the film morphology of the absorption layer and the interface properties between the stacked layers. A comparative study using atomic force microscopy (AFM) and optical in situ thin film drying measurements is performed. The strong impact of distinct drying scenarios on the polymer:fullerene BHJ layer morphology is investigated by AFM. The AFM images show a systematic dependency of structure sizes at the surface on drying kinetics. In addition thin film optical measurements for the determination of thin film drying kinetics and parameters are performed using a dedicated experimental setup. The data are used as the input for a quantitative simulation of the drying process. The film thickness decreases linearly during drying while the solvent mass fraction decreases moderately over a wide range until it drops rapidly. Subsequently the remaining solvent fraction evaporates considerably slower. Our work gives a fundamental understanding of the film formation kinetics and prerequisites for the systematic optimization of the film morphology in solution processed organic photovoltaic devices.
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68.55.am Polymers and organics
68.37.Ps Atomic force microscopy (AFM)
88.40.jr Organic photovoltaics
78.66.Qn Polymers; organic compounds

Thermal excitation of large charge offsets in a single-Cooper-pair transistor

L. R. Simkins, D. G. Rees, P. H. Glasson, V. Antonov, E. Collin, P. G. Frayne, P. J. Meeson, and M. J. Lea

J. Appl. Phys. 106, 124502 (2009); http://dx.doi.org/10.1063/1.3266012 (11 pages) | Cited 1 time

Online Publication Date: 17 December 2009

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Charge offsets and two-level fluctuators are common in single-electron transistors with a typical magnitude of |ΔQ|<0.1e. We present measurements in a 2e-periodic single-Cooper-pair transistor which exhibited hysteretic charge offsets close to 1e. The real-time capture and escape of individual electrons in metastable trapped states was measured at very low temperatures. This enabled the dynamics of the transitions to be investigated in detail, demonstrating thermal excitation to a hysteretic tunneling transition. We show that, allowing for the hysteresis, the metastable states are in thermal equilibrium with each other. The observed temperature dependence and hysteresis can be explained by the coupling of a two-level fluctuator to a quasiparticle trap.
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85.25.-j Superconducting devices
85.35.Gv Single electron devices

Analysis of degradation mechanisms in lattice-matched InAlN/GaN high-electron-mobility transistors

J. Kuzmik, G. Pozzovivo, C. Ostermaier, G. Strasser, D. Pogany, E. Gornik, J.-F. Carlin, M. Gonschorek, E. Feltin, and N. Grandjean

J. Appl. Phys. 106, 124503 (2009); http://dx.doi.org/10.1063/1.3272058 (7 pages) | Cited 18 times

Online Publication Date: 18 December 2009

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We address degradation aspects of lattice-matched unpassivated InAlN/GaN high-electron-mobility transistors (HEMTs). Stress conditions include an off-state stress, a semi-on stress (with a partially opened channel), and a negative gate bias stress (with source and drain contacts grounded). Degradation is analyzed by measuring the drain current, a threshold voltage, a Schottky contact barrier height, a gate leakage and an ideality factor, an access, and an intrinsic channel resistance, respectively. For the drain-gate bias <38 V parameters are only reversibly degraded due to charging of the pre-existing surface states. This is in a clear contrast to reported AlGaN/GaN HEMTs where an irreversible damage and a lattice relaxation have been found for similar conditions. For drain-gate biases over 38 V InAlN/GaN HEMTs show again only temporal changes for the negative gate bias stresses; however, irreversible damage was found for the off-state and for the semi-on stresses. Most severe changes, an increase in the intrinsic channel resistance by one order of magnitude and a decrease in the drain current by ∼ 70%, are found after the off-state ∼ 50 V drain-gate bias stresses. We conclude that in the off-state condition hot electrons may create defects or ionize deep states in the GaN buffer or at the InAlN/GaN interface. If an InAlN/GaN HEMT channel is opened during the stress, lack of the strain in the barrier layer is beneficial for enhancing the device stability.
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85.30.Tv Field effect devices

Electrical transport and thermometry of electroformed titanium dioxide memristive switches

Julien Borghetti, Dmitri B. Strukov, Matthew D. Pickett, J. Joshua Yang, Duncan R. Stewart, and R. Stanley Williams

J. Appl. Phys. 106, 124504 (2009); http://dx.doi.org/10.1063/1.3264621 (5 pages) | Cited 17 times

Online Publication Date: 22 December 2009

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We investigated the electrical transport of electroformed titanium dioxide memristive switches from liquid helium to room temperatures in order to better understand their internal states. After electroforming, we observed a continuous transition between two distinct limiting behaviors: a nearly Ohmic “ON”-state and an “OFF”-state characterized by conduction through a barrier. We interpret our data in terms of a model in which the electroforming step creates a conducting channel that does not completely bridge the metal contacts on the titanium dioxide film. The switching then occurs as a result of voltage-induced changes in the oxygen vacancy concentration in the gap between the tip of the channel and the adjacent metal contact. We used the metallic resistivity of the conduction channel as an in situ thermometer to measure the local device temperature, thus revealing an important implicit state variable.
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84.32.Dd Connectors, relays, and switches
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.20.Dt Thermometers

Modeling of metal-ferroelectric-insulator-semiconductor structures based on Langmuir–Blodgett copolymer films

Timothy J. Reece and Stephen Ducharme

J. Appl. Phys. 106, 124505 (2009); http://dx.doi.org/10.1063/1.3271581 (4 pages) | Cited 2 times

Online Publication Date: 22 December 2009

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Among the ferroelectric thin films used in field-effect transistor devices; the ferroelectric copolymer of polyvinylidene fluoride (PVDF) (–CH2–CF2–), with trifluoroethylene (TrFE) (–CHF–CF2–), has distinct advantages, including low dielectric constant, low processing temperature, low cost, and compatibility with organic semiconductors. The operation of a metal-ferroelectric-insulator-semiconductor structure with P(VDF-TrFE) as the ferroelectric layer was analyzed and optimized by numerical solution of the Miller and McWhorter model. A model device consisting of 20 nm PVDF/TrFE on a 10-nm-thick high-k dielectric buffer exhibits a memory window of 5 V with an operating voltage of ±15 V. The operating voltage can be reduced to ±12 V by reducing the ferroelectric and dielectric thicknesses to 10 and 5 nm, respectively.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
77.55.F- High-permittivity capacitive films
77.80.-e Ferroelectricity and antiferroelectricity
77.84.Jd Polymers; organic compounds
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
85.50.Gk Non-volatile ferroelectric memories

Demonstration on GaN-based light-emitting diodes grown on 3C-SiC/Si(111)

Y. H. Zhu, J. C. Zhang, Z. T. Chen, and T. Egawa

J. Appl. Phys. 106, 124506 (2009); http://dx.doi.org/10.1063/1.3273311 (4 pages) | Cited 2 times

Online Publication Date: 23 December 2009

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GaN-based light-emitting diodes (LEDs) grown on template of 3C-SiC/Si(111) were demonstrated. The structural properties have been investigated systematically by means of atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It is found that the intermediate layer (IL) of 3C-SiC leads to not only a significant improvement in the crystalline quality of GaN, but also better interfaces between the buffer layer and the initial layers of strained-layer superlattice. The device properties were also evaluated using the measurements of current-voltage, electroluminescence, and light output power-current. Compared to conventional LEDs that do not contain 3C-SiC IL, the device with IL exhibits enhanced output power by more than 200% at an injection current of 20 mA, and the operating voltage is slightly increased from 3.7 to 3.9 V. These results indicate that using 3C-SiC as IL is one of the promising approaches to improve the performance of LEDs on silicon.
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85.60.Jb Light-emitting devices
68.37.Ps Atomic force microscopy (AFM)
68.37.Lp Transmission electron microscopy (TEM)
68.55.-a Thin film structure and morphology
78.60.Fi Electroluminescence
73.21.Cd Superlattices

Optimization and calibration of atomic force microscopy sensitivity in terms of tip-sample interactions in high-order dynamic atomic force microscopy

Yu Liu, Qiuquan Guo, Heng-Yong Nie, W. M. Lau, and Jun Yang

J. Appl. Phys. 106, 124507 (2009); http://dx.doi.org/10.1063/1.3269703 (9 pages) | Cited 2 times

Online Publication Date: 28 December 2009

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The mechanism of dynamic force modes has been successfully applied to many atomic force microscopy (AFM) applications, such as tapping mode and phase imaging. The high-order flexural vibration modes are recent advancement of AFM dynamic force modes. AFM optical lever detection sensitivity plays a major role in dynamic force modes because it determines the accuracy in mapping surface morphology, distinguishing various tip-surface interactions, and measuring the strength of the tip-surface interactions. In this work, we have analyzed optimization and calibration of the optical lever detection sensitivity for an AFM cantilever-tip ensemble vibrating in high-order flexural modes and simultaneously experiencing a wide range and variety of tip-sample interactions. It is found that the optimal detection sensitivity depends on the vibration mode, the ratio of the force constant of tip-sample interactions to the cantilever stiffness, as well as the incident laser spot size and its location on the cantilever. It is also found that the optimal detection sensitivity is less dependent on the strength of tip-sample interactions for high-order flexural modes relative to the fundamental mode, i.e., tapping mode. When the force constant of tip-sample interactions significantly exceeds the cantilever stiffness, the optimal detection sensitivity occurs only when the laser spot locates at a certain distance from the cantilever-tip end. Thus, in addition to the “globally optimized detection sensitivity,” the “tip optimized detection sensitivity” is also determined. Finally, we have proposed a calibration method to determine the actual AFM detection sensitivity in high-order flexural vibration modes against the static end-load sensitivity that is obtained traditionally by measuring a force-distance curve on a hard substrate in the contact mode.
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68.37.Ps Atomic force microscopy (AFM)
68.35.B- Structure of clean surfaces (and surface reconstruction)
62.25.Jk Mechanical modes of vibration
06.20.fb Standards and calibration

Atomically abrupt and unpinned Al2O3/In0.53Ga0.47As interfaces: Experiment and simulation

Eun Ji Kim, Evgueni Chagarov, Joël Cagnon, Yu Yuan, Andrew C. Kummel, Peter M. Asbeck, Susanne Stemmer, Krishna C. Saraswat, and Paul C. McIntyre

J. Appl. Phys. 106, 124508 (2009); http://dx.doi.org/10.1063/1.3266006 (8 pages) | Cited 28 times

Online Publication Date: 28 December 2009

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III-V semiconductor field effect transistors require an insulator/channel interface with a low density of electrically active defects and a minimal interface dipole to avoid Fermi level pinning. We demonstrate that an atomically abrupt and unpinned interface can be formed between an In0.53Ga0.47As (100) channel and an Al2O3 dielectric layer grown by atomic layer deposition (ALD) when oxidation of the substrate surface is prevented before and during oxide deposition. X-ray photoelectron spectra and electron microscopy indicate that in situ desorption of a protective As2 layer on the In0.53Ga0.47As (100)−4×2 surface followed by ALD of Al2O3 produced an atomically abrupt interface without Fermi level pinning. Temperature-dependent and frequency-dependent capacitance-voltage and conductance-voltage analysis of the resulting Pt/Al2O3/InGaAs capacitors are consistent with movement of the Fermi level through the InGaAs band gap. Moreover, the nearly ideal flat band voltages observed for gate metals of widely varying work function indicate a small oxide/semiconductor interface dipole. Density functional theory calculations of the electronic structure of an ideal amorphous Al2O3/InGaAs (100) interface predict a weak perturbation of the InGaAs electronic structure if its oxidation is avoided, consistent with experiment.
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85.30.Tv Field effect devices
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.65.Mq Oxidation
84.32.Tt Capacitors
73.30.+y Surface double layers, Schottky barriers, and work functions

SnO2 thick films for room temperature gas sensing applications

Kamalpreet Khun Khun, Aman Mahajan, and R. K. Bedi

J. Appl. Phys. 106, 124509 (2009); http://dx.doi.org/10.1063/1.3273323 (5 pages) | Cited 8 times

Online Publication Date: 29 December 2009

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Porous nanosized SnO2 powder has been synthesized by a simple nonaqueous sol gel method using SnCl2⋅2H2O and C2H5OH as precursors. Thermal stabilization of the gel is investigated by thermogravimetric/differential thermal analysis. SnO2 powder has been obtained by calcining the gel at 500 °C for 3 h and studied for its structural properties using x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). XRD observations confirm the formation of rutile structured SnO2. On an average, 35 nm size particles have been found in TEM micrographs of SnO2 powder. FESEM of the powder reveals the formation of a porous network formed by weak aggregation of nanoparticles. An attempt has been made to fabricate gas sensor by depositing thick SnO2 films on glass substrate. Gas sensing studies show that the sensing response of SnO2 sensor toward ammonia is comparatively higher at room temperature as compared to that toward acetone and ethanol.
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61.43.Gt Powders, porous materials
68.55.aj Insulators
81.16.-c Methods of micro- and nanofabrication and processing
61.46.-w Structure of nanoscale materials
61.66.Fn Inorganic compounds

Triplet annihilation exceeding spin statistical limit in highly efficient fluorescent organic light-emitting diodes

D. Y. Kondakov, T. D. Pawlik, T. K. Hatwar, and J. P. Spindler

J. Appl. Phys. 106, 124510 (2009); http://dx.doi.org/10.1063/1.3273407 (7 pages) | Cited 26 times

Online Publication Date: 29 December 2009

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We have demonstrated that the exemplary red fluorescent organic light-emitting diodes (OLEDs) gain as much as half of their electroluminescence from annihilation of triplet states generated by recombining charge carriers. The magnitude of triplet-triplet annihilation (TTA) contribution in combination with the remarkably high total efficiencies [>11% external quantum efficiency (EQE)] indicates that the absolute amount of electroluminescence attributable to TTA substantially exceeds the limit imposed by spin statistics, which was independently confirmed by studying magnetic field effects on delayed luminescence. We determined the value of 1.3 for the ratio of the rate constants of singlet and triplet channels of annihilation, which is indeed substantially higher than the value of 0.33 expected for a purely statistical annihilation process. It is, however, in an excellent quantitative agreement with the extent of the experimental contribution of delayed luminescence to steady-state electroluminescence. The nonstatistical branching ratio of the two annihilation channels is attributed to the favorable relationship between the energies of the excited singlet and triplet states of rubrene—emissive layer host. We surmise that, with the appropriate emissive layer materials, the fluorescent OLED devices are capable of using a considerably larger fraction of triplet states than was previously believed. In principle, the upper limit for the singlet excited state yield in the TTA process is 0.5, which makes the maximum internal quantum efficiency of fluorescent OLEDs to be 25%+0.5×75% = 62.5%. The estimates of maximum EQE of the fluorescent OLEDs should be revised to at least 0.2×62.5% = 12.5% and, likely, even higher to account for optical outcoupling exceeding 0.2.
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85.60.Jb Light-emitting devices
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
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