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15 Jul 2009

Volume 106, Issue 2, Articles (02xxxx)

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Characterization of midwave infrared InAs/GaSb superlattice photodiode

C. Cervera, J. B. Rodriguez, R. Chaghi, H. Aït-Kaci, and P. Christol

J. Appl. Phys. 106, 024501 (2009); http://dx.doi.org/10.1063/1.3174439 (5 pages) | Cited 12 times

Online Publication Date: 16 July 2009

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We report on structural, electrical, and optical characterizations of midwave infrared InAs/GaSb superlattice (SL) p-i-n photodiodes. High-quality SL samples, with 1 μm thick active region (220 SL periods), exhibited a cut-off wavelength of 4.9 μm at 80 K. Using a capacitance-voltage measurement technique performed on mesa diode, the residual background concentration in the nonintentionally doped region was determined to be 3×1015 cm−3 at 80 K. Extracted from current-voltage characteristics, R0A products above 4×105 Ω cm2 at 80 K were measured, and the quantitative analysis of the J-V curves showed that the dark current density of SL photodiode is dominated by generation-recombination processes. Front-side illuminated photodiodes produced responsivity at 80 K equal to 360 mA/W at 4.5 μm.
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85.60.Dw Photodiodes; phototransistors; photoresistors
81.05.Ea III-V semiconductors

Correlation between defect density and current leakage in InAs/GaAs quantum dot-in-well structures

A. M. Sanchez, R. Beanland, N. F. Hasbullah, M. Hopkinson, and J. P. R. David

J. Appl. Phys. 106, 024502 (2009); http://dx.doi.org/10.1063/1.3168492 (4 pages) | Cited 5 times

Online Publication Date: 20 July 2009

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We present a study of InAs/GaAs quantum dot-in-well (DWELL) material using transmission electron microscopy and leakage current-voltage measurements. The spacer layers between the DWELL layers have a variety of annealing and growth temperatures. We show that there is a strong correlation between spacer layer, annealing temperature, defect density, and these leakage currents, with the most defective sample having 30 times more defects and a leakage current several orders of magnitude above that of the least defective. Cross section transmission electron microscope (TEM) shows that surface roughness above defective dots is responsible for the high defect densities. However, even in the best sample the reverse bias leakage current is several orders of magnitude above that typically seen in quantum well materials and a measurable density of defective dots are observed in planar view TEM.
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73.63.Hs Quantum wells
68.35.bg Semiconductors
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
68.37.Lp Transmission electron microscopy (TEM)
71.55.Eq III-V semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)
73.63.Kv Quantum dots

Emitting-layer design of white organic light-emitting devices with single-host material

Chih-Hung Hsiao and Jiun-Haw Lee

J. Appl. Phys. 106, 024503 (2009); http://dx.doi.org/10.1063/1.3176486 (7 pages) | Cited 12 times

Online Publication Date: 20 July 2009

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Efficient phosphorescence sensitization (PS) consisting of tris(phenylpyridine)iridium (Ir(ppy)3) sensitizers and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) (DCJTB) exciton acceptors in a host material, N,N′-dicarbazolyl-3,5-benzene, was implemented by a simplified emitting-layer structure with selectively doped DCJTB in the Ir(ppy)3-doped region. This codoped region away from the recombination zone peak in the Ir(ppy)3-doped region significantly improved the PS efficiency without affecting the carrier transport characteristics. Coupled with an efficiently phosphorescent blue emitter, iridium(III)bis[(4,6-di-fluoropheny)- pyridinato-N,C2′]picolinate, a white organic light-emitting device utilizing this PS approach was demonstrated to have 10.94% external quantum efficiency, 17.4 cd/A and 12.2 lm/W at an applied voltage of 4.5 V, and a brightness of 107 cd/m2 with Commission Internationale de l'Eclairage coordinates of (0.293, 0.427). With a 3 nm spacer for manipulating the exciton diffusion in the emitting layer, a slight CIE coordinates variation of (−0.008, −0.006) was obtained at practical luminance levels of 1000–4000 cd/m2, evidence that PS was successfully achieved.
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85.60.Jb Light-emitting devices

A model of electrical conduction across the grain boundaries in polycrystalline-silicon thin film transistors and metal oxide semiconductor field effect transistors

Kiran Sharma and D. P. Joshi

J. Appl. Phys. 106, 024504 (2009); http://dx.doi.org/10.1063/1.3173179 (9 pages) | Cited 1 time

Online Publication Date: 21 July 2009

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An electrical conduction model of carrier transport across the grain boundaries (GBs) in polycrystalline silicon (PX-Si) films is developed by considering four conduction mechanisms, a Gaussian energy distribution for GB interface states and the GB scattering effects. The model is applicable over a wide range of temperature and grain size. It is found that the GB scattering potential and the GB distribution parameter are function of temperature but are independent of doping density and grain size. The conduction model is able to explain the dependence of transfer and output characteristics of thin film transistors (TFTs) on the temperature and grain size in the strong inversion regime. The variation of effective mobility and drain current for n-channel TFTs and metal oxide semiconductor field effect transistors with gate bias voltage and grain sizes is also studied. A satisfactory agreement is obtained between the theoretical investigations and the available experimental data.
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72.80.Cw Elemental semiconductors
73.61.Cw Elemental semiconductors
61.72.Mm Grain and twin boundaries
68.55.ag Semiconductors
73.20.At Surface states, band structure, electron density of states
85.30.Tv Field effect devices
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
72.20.Fr Low-field transport and mobility; piezoresistance
73.50.Dn Low-field transport and mobility; piezoresistance

Displacement current analysis of carrier behavior in pentacene field effect transistor with poly(vinylidene fluoride and tetrafluoroethylene) gate insulator

Shuhei Yoshita, Ryousuke Tamura, Dai Taguchi, Martin Weis, Eunju Lim, Takaaki Manaka, and Mitsumasa Iwamoto

J. Appl. Phys. 106, 024505 (2009); http://dx.doi.org/10.1063/1.3176487 (4 pages) | Cited 6 times

Online Publication Date: 23 July 2009

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The carrier injection mechanism was investigated using a field effect transistor (FET) system with a ferroelectric gate insulator: poly(vinylidene fluoride and tetrafluoroethylene). The carrier injection and the turnover voltage of the spontaneous polarization were measured by application of ramp gate voltage with various sweep rates. Three peaks appeared in the gate current-gate voltage characteristics (Ig-Vg) because of the turnover of the spontaneous polarization and the long-range carrier motion of injected carriers. We accounted for the peaks generated and analyzed the Ig-Vg characteristics. For FETs with Al electrodes, the increase in the sweep rate caused a larger turnover voltage. For FETs with Au electrodes, the turnover voltage was constant. Results show that the difference of the injection property well accounted for these results.
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85.30.Tv Field effect devices
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Improving the lifetime of white polymeric organic light-emitting diodes

Malte C. Gather, Sebastian Köber, Susanne Heun, and Klaus Meerholz

J. Appl. Phys. 106, 024506 (2009); http://dx.doi.org/10.1063/1.3176502 (10 pages) | Cited 2 times

Online Publication Date: 23 July 2009

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We report on efficient polymeric white organic light-emitting diodes with unprecedented stability. The investigated devices are based on an electroluminescent copolymer of electron and hole-transporting units and red-, green-, and blue-emitting chromophores. We find that the glass transition of the polymer (Tg = 182.5 °C) is the process determining the relation between thermal annealing during fabrication and device lifetime. For devices annealed below Tg, the device lifetime significantly increases with increasing annealing temperature. For annealing temperatures above Tg, however, the current density in the devices rapidly increases while their lifetime slightly decreases. Insight into the underlying processes is provided by atomic force microscopy phase imaging and by UV/visible and fluorescence spectroscopy. We also investigated the influence of the operating temperature of the device: besides the commonly known fact that elevated operating temperatures reduce the lifetime, we discovered that the acceleration coefficient, which determines the scaling of the device lifetime with applied current density, was reduced. At the glass transition, the device lifetime no longer depended on the current density. The device lifetime was improved even further by introducing an additional cross-linkable hole-transport layer. Optimized devices achieve a half-luminance lifetime of 1860 h when operated at room temperature and at an initial luminance of 500 cd m−2. As a result of the relatively balanced stability of the three chromophores, the emission spectrum remains virtually unchanged over the entire device lifetime. Finally, to reduce the time required for the lifetime measurements, we propose to analyze the voltage increase over the first 10–50 h of the lifetime test and find that this allows precisely estimating the lifetime of our devices.
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85.60.Jb Light-emitting devices

Photocurrent transients in all-polymer solar cells: Trapping and detrapping effects

Christopher R. McNeill, Inchan Hwang, and Neil C. Greenham

J. Appl. Phys. 106, 024507 (2009); http://dx.doi.org/10.1063/1.3177337 (8 pages) | Cited 17 times

Online Publication Date: 23 July 2009

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We have studied photocurrent transients in all-polymer bulk-heterojunction solar cells based on poly(3-hexylthiophene) and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl). By illuminating devices with square pulses of light of varying intensity, we reveal nonlinear photocurrent transients on the timescale of tens of microseconds. These microsecond photocurrent transients are attributed to the effects of trapping and detrapping of charges on this timescale, in particular, electrons. The buildup of trapped electrons results in the appearance of a peak in the photocurrent at high intensities at ∼ 10 μs after turn on. This trapped charge produces a local reduction in the strength of the internal electric field near the anode resulting in a net decrease in charge separation efficiency and an increase in the likelihood of bimolecular recombination due to increased and overlapping electron and hole densities. After turn off, a long photocurrent tail is observed with charge still being extracted after 0.5 ms consistent with the detrapping of deeply trapped charges. We are able to reproduce the observed transient photocurrent features using a time-dependent drift-diffusion model incorporating the trapping and detrapping of electrons.
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84.60.Jt Photoelectric conversion
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.40.+w Photoconduction and photovoltaic effects

A simplified approach to estimating total trap contributions in negative bias temperature instability

R. A. B. Devine, J. K. Mee, H. P. Hjalmarson, M. A. Quevedo-Lopez, and H. N. Alshareef

J. Appl. Phys. 106, 024508 (2009); http://dx.doi.org/10.1063/1.3160330 (5 pages) | Cited 1 time

Online Publication Date: 23 July 2009

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A simplified approach is used to determine the relative importance of slow (>3 s) and fast (<3 s) charged trap contributions to threshold voltage shifts (ΔVth) induced by negative bias temperature instability in HfSiON gate dielectric p-channel field effect transistors. For the devices under study the relative importance of the two components is approximately the same. Total trap density induced threshold voltage shifts from measurements at 368, 398, and 428 K can be fitted to a simplified law of the form ΔVth = Ae−EA/kTtα with EA = 0.085±0.012 eV and α = 0.186±0.003. The importance of ignoring fast trap effects in overestimating reliability lifetimes is discussed.
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85.30.Tv Field effect devices
85.75.Hh Spin polarized field effect transistors

Modeling charge transport in graphene nanoribbons and carbon nanotubes using a Schrödinger-Poisson solver

D. I. Odili, Y. Wu, P. A. Childs, and D. C. Herbert

J. Appl. Phys. 106, 024509 (2009); http://dx.doi.org/10.1063/1.3174430 (5 pages) | Cited 2 times

Online Publication Date: 27 July 2009

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Interest in carbon-based electronics has been stimulated in recent years, initially through the discovery of carbon nanotubes, but recently with the formation of graphene layers. In this paper metal-oxide-semiconductor (MOS) systems based on these carbon structures are used to model and compare charge transport within them. Schrödinger’s equation is solved self-consistently with Poisson’s equation, using the scattering matrix method. A tight-binding model is used to determine the energy band structure in graphene. The current-voltage characteristics of MOS devices based on graphene and those based on carbon nanotubes demonstrate significant differences associated with their respective transmission probabilities.
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61.46.Fg Nanotubes
85.30.Tv Field effect devices
71.20.Tx Fullerenes and related materials; intercalation compounds
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
72.80.Cw Elemental semiconductors
72.20.Fr Low-field transport and mobility; piezoresistance

A combined reaction-separation lab-on-a-chip device for low Péclet number applications

A. Weddemann, B. Eickenberg, F. Wittbracht, A. Auge, and A. Hütten

J. Appl. Phys. 106, 024510 (2009); http://dx.doi.org/10.1063/1.3176942 (5 pages) | Cited 1 time

Online Publication Date: 27 July 2009

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A microfluidic continuous flow lab-on-a-chip structure is presented, for combined reaction and separation implementation. Thereby, the geometry is optimized with respect to a long duration time in the reaction area and a narrow preconditioning along the separation site to enhance the separation yield. We theoretically predict high stability of separation and transport properties of the device against particle diffusion whereas the particle dynamics in the reaction site maintain a diffusive character. We derive a formula setting the limitations of the structure, relating the desired duration of the particles in the reaction site to the particle size under the condition of a narrow spatial particle distribution along the separation site. We experimentally realize the low diffusion limit to prove the theoretically predicted properties of the device.
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47.85.Np Fluidics
07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
87.80.Ek Mechanical and micromechanical techniques

Hole mobility enhancement by double-gate mode in ultrathin-body silicon-on-insulator p-type metal-oxide-semiconductor field-effect transistors

Shigeki Kobayashi, Masumi Saitoh, and Ken Uchida

J. Appl. Phys. 106, 024511 (2009); http://dx.doi.org/10.1063/1.3182792 (6 pages) | Cited 4 times

Online Publication Date: 28 July 2009

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Hole mobility enhancement in double-gate (DG) ultrathin-body (UTB) silicon-on-insulator (SOI) p-type metal-oxide-semiconductor field-effect transistors (MOSFETs) is investigated systematically in comparison with single-gate (SG) UTB MOSFETs for various SOI thicknesses (TSOI) ranging from 2 to 30 nm. It is found that mobility in DG mode (μDG) is higher than that in SG mode (μSG) in all the measured ranges of TSOI and the surface carrier concentrations (Ns). In particular, enhancement of mobility of the sub-10-nm-thick TSOI devices is greater at higher Ns. As a result, it is demonstrated that μDG of the 8.1-nm-thick TSOI device is 12.9% greater than the universal mobility when Ns is 6×1012 cm−2. Higher μDG is attributed to the average effective mass reduction in DG mode because of the increased population in the light hole (LH) bands. Careful investigation also clarifies that mobility in the sub-10-nm-thick TSOI devices is enhanced greatly at higher Ns, since the DG mode-induced increase in the hole population in LH bands is enhanced at higher Ns; the reduction in the hole population in LH bands due to the high-Ns-enhanced carrier confinement in the normal direction is larger in SG mode than in DG mode.
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85.30.Tv Field effect devices
72.20.Fr Low-field transport and mobility; piezoresistance

Investigation of external noise and its rejection in magnetoelectric sensor design

Zengping Xing, Junyi Zhai, Jiefang Li, and Dwight Viehland

J. Appl. Phys. 106, 024512 (2009); http://dx.doi.org/10.1063/1.3176500 (7 pages) | Cited 2 times

Online Publication Date: 28 July 2009

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Laminated magnetoelectric (ME) composites consisting of magnetostrictive and piezoelectric layers are an important class of magnetic sensors. Here, we will present a means to design ME sensors with in-built capabilities to cancel environmental noise, based on designing the sensor signal and the noise in different fundamental modes. We then show that some composite designs offer unique capabilities to reject acoustic or thermal noise sources.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
77.55.-g Dielectric thin films
75.80.+q Magnetomechanical effects, magnetostriction
85.70.Ec Magnetostrictive, magnetoacoustic, and magnetostatic devices

Effect of interlayer coupling in CoFeB/Ta/NiFe free layers on the critical switching current of MgO-based magnetic tunnel junctions

Kangho Lee, Wei-Chuan Chen, Xiaochun Zhu, Xia Li, and Seung H. Kang

J. Appl. Phys. 106, 024513 (2009); http://dx.doi.org/10.1063/1.3184423 (4 pages) | Cited 4 times

Online Publication Date: 29 July 2009

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This paper reports the current-induced magnetization reversal characteristics of MgO-based magnetic tunnel junctions (MTJs) with CoFeB/Ta/NiFe composite free layers designed for spin-transfer-torque magnetoresistive random access memory. As the Ta spacer thickness ( ≤ 8 Å) was increased, the MTJs embedded into nanoscale integrated circuits demonstrated not only higher tunneling magnetoresistance ratios but also lower intrinsic critical switching currents. This suggests that promoting weak interlayer exchange coupling between CoFeB and NiFe is desirable for reducing the intrinsic critical switching current of CoFeB/Ta/NiFe. While the energy barrier was also reduced with a thicker Ta spacer, it was maintained at an adequate level ( ∼ 57kBT) even for the thickest Ta (8 Å) of this work.
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75.47.Np Metals and alloys
75.60.Jk Magnetization reversal mechanisms
75.30.Et Exchange and superexchange interactions
85.70.Ec Magnetostrictive, magnetoacoustic, and magnetostatic devices
85.75.Dd Magnetic memory using magnetic tunnel junctions
75.50.Bb Fe and its alloys

On the Hooge relation in semiconductors and metals

A. P. Dmitriev, M. E. Levinshtein, and S. L. Rumyantsev

J. Appl. Phys. 106, 024514 (2009); http://dx.doi.org/10.1063/1.3186620 (5 pages) | Cited 4 times

Online Publication Date: 30 July 2009

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The expressions describing the low frequency noise caused by defects in semiconductors and metals have been obtained in the framework of a general unified approach for both fluctuations in the number of carriers and their mobility. When fluctuations in the number of carriers are responsible for noise, the spectral noise density is inversely proportional to the carrier concentration squared and to the volume of the sample. The spectral density of the noise caused by mobility fluctuations is inversely proportional to the sample volume, and does not depend either on carrier concentration or on the total number of carriers. In the case when both mechanisms contribute to noise and they are correlated, the dependence of the noise on the number of carriers depends on the relative contribution of these two noise mechanisms. The expressions obtained can be associated with corresponding cases of the 1/f noise. The physical basis and limitations of the Hooge formula are discussed.
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72.20.Fr Low-field transport and mobility; piezoresistance
72.70.+m Noise processes and phenomena

Enhancement of ballistic efficiency due to source to channel heterojunction barrier in Si metal oxide semiconductor field effect transistors

W. Wang, H. Tsuchiya, and M. Ogawa

J. Appl. Phys. 106, 024515 (2009); http://dx.doi.org/10.1063/1.3186028 (6 pages)

Online Publication Date: 31 July 2009

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In this paper, we study the influences of a channel source-end potential profile on the ballistic transport of carriers in Si metal oxide semiconductor field effect transistors (MOSFETs) based on a quantum-corrected Monte Carlo device simulation. As a result, we found that higher ballistic efficiency is expected in MOSFETs with a heterojunction bottleneck barrier, such as Schottky source/drain MOSFETs, compared to that with the conventional p-n junction source and drain. Such a superior ballistic behavior is demonstrated due to the narrower bottleneck potential profile formed at the source-channel interface.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
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