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15 Sep 2010

Volume 108, Issue 6, Articles (06xxxx)

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back to top Device Physics

Discussion of some “trap signatures” observed by admittance spectroscopy in CdTe thin-film solar cells

Jian V. Li, Steve W. Johnston, Xiaonan Li, David S. Albin, Timothy A. Gessert, and Dean H. Levi

J. Appl. Phys. 108, 064501 (2010); http://dx.doi.org/10.1063/1.3475373 (5 pages) | Cited 2 times

Online Publication Date: 16 September 2010

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Considerable ambiguity and controversy exist concerning the defect signatures (H1, H2, and H3) frequently observed in admittance spectroscopy of thin-film CdTe solar cells. We prove that the commonly labeled H1 defects, observed in all devices in this study, are actually due to the freeze-out of the majority carriers in the neutral CdTe absorber. This freeze-out is evident in the temperature dependencies of capacitance, carrier concentration, and depletion region width. Contrary to intuitive expectation, the activation energy of freeze-out is less than, not identical to, that of the conductivity. In some other cases, H2 or H3 are observed and attributed to the back-contact potential barrier, rather than to the carrier emission from the traps. We extract the back-contact barrier height from the activation energy of the saturation current determined from the temperature-dependent current-voltage curves using the back-to-back diode model. The back-contact barrier height agrees well with the H2 or H3 energy determined by admittance spectroscopy. We present a more comprehensive and realistic equivalent circuit that includes the admittances from both the back-contact and the neutral absorber.
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88.40.jm Thin film III-V and II-VI based solar cells

The ion capturing effect of 5° SiOx alignment films in liquid crystal devices

Yi Huang, Philip J. Bos, and Achintya Bhowmik

J. Appl. Phys. 108, 064502 (2010); http://dx.doi.org/10.1063/1.3481088 (8 pages) | Cited 5 times

Online Publication Date: 17 September 2010

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We show that SiOx, deposited at 5° to the interior surface of a liquid crystal cell allows for a surprisingly substantial reduction in the ion concentration of liquid crystal devices. We have investigated this effect and found that this type of film, due to its surface morphology, captures ions from the liquid crystal material. Ion adsorption on 5° SiOx film obeys the Langmuir isotherm. Experimental results shown allow estimation of the ion capturing capacity of these films to be more than an order of 10 000/μm2. These types of materials are useful for new types of very low power liquid crystal devices such as e-books.
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42.79.Kr Display devices, liquid-crystal devices

Characterization of dual high transition temperature superconducting quantum interference device first-order planar gradiometers on a chip

Kuen-Lin Chen, Hong-Chang Yang, P. C. Ko, and H. E. Horng

J. Appl. Phys. 108, 064503 (2010); http://dx.doi.org/10.1063/1.3481449 (4 pages)

Online Publication Date: 17 September 2010

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The dual first-order planar gradiometers of high transition temperature (high-Tc) superconducting quantum interference devices (SQUIDs) were designed and fabricated on a 10×10 mm2 SrTiO3 bicrystal substrate. Each gradiometer consists of four bare SQUIDs which are connected to two symmetric pickup loops. Any two of the SQUIDs can be connected in series to obtain better performance. In this study, by selecting the coupling direction of SQUID, a balance resolution of 0.08% was achieved. And, a second-order electronic planar gradiometer was composed of these two first-order gradiometers. Experimental data showed that the noise performance of the second-order electronic planar gradiometer composed of two planar first-order gradiometers can reach 15 μΦ0/Hz1/2 at 1 kHz in an unshielded environment, which is comparable to the noise level of a magnetometer in a shielded environment.
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85.25.Dq Superconducting quantum interference devices (SQUIDs)
07.55.Ge Magnetometers for magnetic field measurements
07.55.Jg Magnetometers for susceptibility, magnetic moment, and magnetization measurements

Quantum transport simulation of nanoscale semiconductor devices based on Wigner Monte Carlo approach

Shunsuke Koba, Ryō Aoyagi, and Hideaki Tsuchiya

J. Appl. Phys. 108, 064504 (2010); http://dx.doi.org/10.1063/1.3487482 (7 pages) | Cited 1 time

Online Publication Date: 17 September 2010

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In this paper, we present quantum transport simulation of nanoscale semiconductor devices based on Wigner Monte Carlo (WMC) approach. We have found that the WMC approach can accurately handle higher-order quantized subbands, tunneling, quantum reflection, and decoherence processes occurring in nanoscale semiconductor devices. Furthermore, we have demonstrated that carrier quantum transport in source electrode plays an important role in devices extremely downscaled into the nanometer regime.
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73.63.-b Electronic transport in nanoscale materials and structures
73.40.Gk Tunneling
05.60.-k Transport processes

Insight on the SU-8 resist as passivation layer for transparent Ga2O3–In2O3–ZnO thin-film transistors

Antonis Olziersky, Pedro Barquinha, Anna Vilà, Luís Pereira, Gonçalo Gonçalves, Elvira Fortunato, Rodrigo Martins, and Juan R. Morante

J. Appl. Phys. 108, 064505 (2010); http://dx.doi.org/10.1063/1.3477192 (7 pages) | Cited 5 times

Online Publication Date: 17 September 2010

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A nonvacuum and low temperature process for passivating transparent metal oxides based thin-film transistors is presented. This process uses the epoxy-based SU-8 resist which prevents device degradation against environmental conditions, vacuum or sputtering surface damage. The incorporation of SU-8 as a passivation layer is based on the ability of this polymer to provide features with high mechanical and chemical stability. With this approach, lithography is performed to pattern the resist over the active area of the device in order to form the passivation layer. The resulting transistors demonstrate very good electrical characteristics, such as μFE = 61 cm2/V s, VON = −3 V, ON/OFF = 4.4×109, and S = 0.28 V/dec. Electrical behavior due to the SU-8/metal oxide interface characteristics is also reported on the basis of Fourier transform infrared analysis. In contrast, we demonstrate how sputtering of SiO2 as a passivation layer results in severely degraded devices that cannot be switched-off. In order to obtain proper working devices, it is shown that SU-8 should be hard baked at 200 °C for 1 h in order to obtain a highly cross-linked polymer network. The stability of SU-8 passivated devices over the time of storage, under current bias stress and vacuum conditions is also demonstrated.
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85.40.Hp Lithography, masks and pattern transfer
85.30.Tv Field effect devices
81.65.Rv Passivation

The explanation of barrier height inhomogeneities in Au/n-Si Schottky barrier diodes with organic thin interfacial layer

İlke Taşçıoğlu, Umut Aydemir, and Şemsettin Altındal

J. Appl. Phys. 108, 064506 (2010); http://dx.doi.org/10.1063/1.3468376 (7 pages) | Cited 12 times

Online Publication Date: 17 September 2010

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The forward bias current-voltage (I-V) characteristics of Au/n-Si Schottky barrier diodes (SBDs) with Zn doped poly(vinyl alcohol) (PVA:Zn) interfacial layer have been investigated in the wide temperature range of 80–400 K. The conventional Richardson plot of the ln(Io/T2) versus q/kT has two linear regions: the first region (200–400 K) and the second region (80–170 K). The values of activation energy (Ea) and Richardson constant (A∗) were obtained from this plot and especially the values of A∗ are much lower than the known theoretical value for n-type Si. Also the value of Ea is almost equal to the half of the band gap energy of Si. Therefore, the Φap versus q/2kT plot was drawn to obtain the evidence of a Gaussian distribution (GD) of barrier heights (BHs) and it shows two linear region similar to ln(Io)/T2 versus q/kT plot. The analysis of I-V data based on thermionic emission of the Au/PVA:Zn/n-Si SBDs has revealed the existence of double GD with mean BH values (mathB0) of 1.06 eV and 0.86 eV with standard deviation (σ) of 0.110 eV and 0.087 V, respectively. Thus, we modified ln(Io/T2)−(qσ)2/2(kT)2 versus q/kT plot for two temperature regions (200–400 K and 80–170 K) and it gives renewed mean BHs mathB0 values as 1.06 eV and 0.85 eV with Richardson constant (A∗) values 121 A/cm2 K2 and 80.4 A/cm2 K2, respectively. This obtained value of A∗ = 121 A/cm2 K2 is very close to the known theoretical value of 120 A/cm2 K2 for n-type Si.
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85.30.Kk Junction diodes

Polarization effects in thallium bromide x-ray detectors

A. Kozorezov, V. Gostilo, A. Owens, F. Quarati, M. Shorohov, M. A. Webb, and J. K. Wigmore

J. Appl. Phys. 108, 064507 (2010); http://dx.doi.org/10.1063/1.3481433 (10 pages) | Cited 2 times

Online Publication Date: 20 September 2010

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We present the results of a detailed experimental study of polarization effects in thallium bromide planar x-ray detectors. Measurements were carried out in the range 10–100 keV by scanning a highly focused x-ray beam, 50 μm in diameter, from a synchrotron source across the detector. Above a certain radiation threshold the detector response showed a systematic degradation of its spectroscopic characteristics, peak channel position, peak height, and energy resolution. Using a pump-and-probe technique, we studied the dynamics of spectral degradation, the spatial extent and relaxation of the polarized region, and the dependence of the detector response on bias voltage and temperature. For comparison, we modeled polarization effects induced by the charging of traps by both electrons and holes using a model based on recent theoretical work of Bale and Szeles. We calculated the charge collection efficiency and spectral line shapes as functions of exposure time, beam position, count rate, and photon energy, and obtained credible agreement with experimental results.
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85.25.Oj Superconducting optical, X-ray, and γ-ray detectors (SIS, NIS, transition edge)
85.60.Gz Photodetectors (including infrared and CCD detectors)
07.85.Qe Synchrotron radiation instrumentation

Determination of interface structure and atomic arrangements for strained InAs/Ga1−xInxSb superlattices by high-resolution transmission electron microscopy

Maohua Quan, Fengyun Guo, Meicheng Li, and Liancheng Zhao

J. Appl. Phys. 108, 064508 (2010); http://dx.doi.org/10.1063/1.3481098 (3 pages) | Cited 1 time

Online Publication Date: 21 September 2010

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The lattice structure of the InAs/Ga1−xInxSb interface has been studied in cross-section by high-resolution transmission electron microscopy. The atomic arrangement at the plane of the interface is analyzed based on the image characteristics. Possible bonding configurations are discussed. The results suggest that interface formation is first driven by charge balance. The shift in the interplanar separations associated with this modulation may lead to distortions of the interfacial structure of Ga1−xInxAs-like. The morphological evolution at GaAs-like interface is accompanied by interface misfit dislocations and compositional fluctuations near the interface associated with segregation.
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68.65.Cd Superlattices
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.35.Ct Interface structure and roughness

Red-emitting fluorescent organic light emitting diodes with low sensitivity to self-quenching

S. Forget, S. Chenais, D. Tondelier, B. Geffroy, I. Gozhyk, M. Lebental, and E. Ishow

J. Appl. Phys. 108, 064509 (2010); http://dx.doi.org/10.1063/1.3481460 (6 pages) | Cited 6 times

Online Publication Date: 21 September 2010

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Concentration quenching is a major impediment to efficient organic light-emitting devices (OLEDs). We herein report on OLEDs based on a fluorescent amorphous red-emitting starbust triarylamine molecule [4-di(4′-tert-butylbiphenyl-4-yl)amino-4′-dicyanovinylbenzene, named FVIN], exhibiting a very small sensitivity to concentration quenching. OLEDs are fabricated with various doping levels of FVIN into Alq3, and show a remarkably stable external quantum efficiency of 1.5% for doping rates ranging from 5% up to 40%, which strongly relaxes the technological constraints on the doping accuracy. An efficiency of 1% is obtained for a pure undoped active region, along with deep red emission (x = 0.6; y = 0.35 in the Commission Internationale de l'Energie (CIE) coordinates). A comparison of FVIN with the archetypal 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye is presented in an identical multilayer OLED structure.
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85.60.Jb Light-emitting devices

Three-dimensional double deck meshlike dye-sensitized solar cells

Yuanhao Wang, Hongxing Yang, and Lin Lu

J. Appl. Phys. 108, 064510 (2010); http://dx.doi.org/10.1063/1.3486222 (6 pages) | Cited 2 times

Online Publication Date: 22 September 2010

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In this paper, we develop a new type of three-dimensional dye-sensitized solar cells (3D DSSCs) with double deck cylindrical Ti meshes as the substrates. One of the Ti meshes is anodized to in situ synthesize the self-organized TiO2 nanotube layer as the photoanode materials. Another Ti mesh is platinized through electrodeposition as the counter electrode. The morphologies of the electrodes are characterized by scanning electron microscopy. We investigate the effect of the mesh number on the 3D DSSCs with the dye adsorption, cyclic voltammetry, and electrochemical impedance spectroscopy. The results show that with the increase in the mesh number, the dye-loadings on the photoanode and the active surface area of Pt on the counter electrode are increased, while the diffusion of the electrolyte becomes more difficult due to the reduced diameter of the openings in the mesh. It has also been demonstrated that the performance of this 3D DSSC is independent of the incident solar beam angle due to its axial symmetrical structure. In the I-V measurement, the 3D DSSC based on the 90-mesh photoanode and the 120-mesh counter electrode shows the highest conversion efficiency of 5.5% under standard AM 1.5 sunlight. The problems of electrical insulator layer are discussed and further investigation is expected.
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88.40.H- Solar cells (photovoltaics)
81.16.Dn Self-assembly
81.15.Pq Electrodeposition, electroplating
81.65.-b Surface treatments
82.45.Rr Electroanalytical chemistry
82.45.Cc Anodic films

Spin dependent tunneling spectroscopy in 1.2 nm dielectrics

J. T. Ryan, P. M. Lenahan, A. T. Krishnan, and S. Krishnan

J. Appl. Phys. 108, 064511 (2010); http://dx.doi.org/10.1063/1.3482071 (6 pages) | Cited 2 times

Online Publication Date: 22 September 2010

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We demonstrate voltage controlled spin dependent tunneling in 1.2 nm effective oxide thickness silicon oxynitride films. Our observations introduce a simple method to link point defect structure and energy levels in a very direct way in materials of great technological importance. We obtain defect energy level resolution by exploiting the enormous difference between the capacitance of the very thin dielectric and the capacitance of the depletion layer of moderately doped silicon. The simplicity of the technique and the robust character of the response make it, at least potentially, of widespread utility in the understanding of defects important in solid state electronics. Since the specific defect observed is generated by high electric field stressing, an important device instability in present day integrated circuitry, the observations are of considerable importance for present day technology. Since the observations involve inherent high sensitivity and tunneling, and since the process can be turned on and off with the application of a narrow range of voltage, our results may also be relevant to the development of spin based quantum computing.
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73.40.Gk Tunneling
77.55.-g Dielectric thin films
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.61.Ng Insulators

Analytical low-frequency noise model in the linear region of lightly doped nanoscale double-gate metal-oxide-semiconductor field-effect transistors

E. G. Ioannidis, C. G. Theodorou, A. Tsormpatzoglou, D. H. Tassis, K. Papathanasiou, C. A. Dimitriadis, J. Jomaah, and G. Ghibaudo

J. Appl. Phys. 108, 064512 (2010); http://dx.doi.org/10.1063/1.3483279 (7 pages) | Cited 2 times

Online Publication Date: 22 September 2010

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An analytical model for the transconductance to drain current ratio (gm/Id) of lightly doped nanoscale double-gate metal-oxide-semiconductor field-effect transistors (DG MOSFETs) has been developed in the weak inversion and from linear to saturation region, using the conductive path potential approach. The obtained analytical model for gm/Id in the weak inversion has been extended in the strong inversion and in the linear region including the short-channel effects, as well as the surface roughness scattering, series resistance, and saturation velocity effects. The obtained gm/Id model from weak to strong inversion has been verified by comparing simulation and experimental results of DG MOSFET with gate length 50 nm and it has been implemented in modeling the 1/f low-frequency noise. The introduced noise model has been validated by developing a Verilog-A transistor noise model, which is in good agreement with the experimental noise results of DG MOSFET with gate length 50 nm in the linear region from weak to strong inversion.
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85.30.Tv Field effect devices

Reducing carrier escape in the InAs/GaAs quantum dot intermediate band solar cell

E. Antolín, A. Martí, C. D. Farmer, P. G. Linares, E. Hernández, A. M. Sánchez, T. Ben, S. I. Molina, C. R. Stanley, and A. Luque

J. Appl. Phys. 108, 064513 (2010); http://dx.doi.org/10.1063/1.3468520 (7 pages) | Cited 21 times

Online Publication Date: 23 September 2010

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Intermediate band solar cells (IBSCs) fabricated to date from In(Ga)As/GaAs quantum dot arrays (QD-IBSC) exhibit a quantum efficiency (QE) that extends to below bandgap energies. However, the production of sub-bandgap photocurrent relies often on the thermal and/or tunneling escape of carriers from the QDs, which is incompatible with preservation of the output voltage. In this work, we test the effectiveness of introducing a thick GaAs spacer in addition to an InAlGaAs strain relief layer (SRL) over the QDs to reduce carrier escape. From an analysis of the QE at different temperatures, it is concluded that escape via tunneling can be completely blocked under short-circuit conditions, and that carriers confined in QDs with an InAlGaAs SRL exhibit a thermal escape activation energy over 100 meV larger than in the case of InAs QDs capped only with GaAs.
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88.40.H- Solar cells (photovoltaics)
72.40.+w Photoconduction and photovoltaic effects

Conductive path formation in glasses of phase change memory

M. Simon, M. Nardone, V. G. Karpov, and I. V. Karpov

J. Appl. Phys. 108, 064514 (2010); http://dx.doi.org/10.1063/1.3478713 (9 pages) | Cited 5 times

Online Publication Date: 23 September 2010

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We present a model of data retention for phase change memory devices in which the active medium is a thin layer of chalcogenide glass. Data retention capability is compromised when a crystalline path is spontaneously formed in the glassy host, essentially shunting the device. We determine the probability and statistics of device failure for systems in which the crystalline volume fraction is below the critical volume fraction of percolation theory. In that regime, we show that rectilinear crystalline path formation is favored and we determine the criteria for when such paths dominate over the typical percolation cluster scenario. Our analytical approach, based on modeling the formation of such paths in terms of a half-space random walk, leads to closed form expressions that relate data retention characteristics to device parameters. The model is used to examine the effects of device geometry, temperature, and external fields. The temporal statistics of device reliability are also considered for several failure mechanisms. A computer simulation is employed that supports our derived relationships between failure probability and device parameters.
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85.30.De Semiconductor-device characterization, design, and modeling
85.40.Qx Microcircuit quality, noise, performance, and failure analysis

Crystallization behavior of amorphous Alx(Ge2Sb2Te5)1−x thin films

Jae-Hee Seo (재희서), Ki-Ho Song (기호송), and Hyun-Yong Lee (현용이)

J. Appl. Phys. 108, 064515 (2010); http://dx.doi.org/10.1063/1.3471799 (6 pages) | Cited 5 times

Online Publication Date: 24 September 2010

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Crystallization properties of thermally deposited amorphous Alx(Ge2Sb2Te5)1−x (x = 0.06 and 0.10) films were investigated. The crystallization was performed by both macroscopic thermal annealing and nanopulse laser illumination (λ = 658 nm and beam diameter <2 μm). The Al0.10(Ge2Sb2Te5)0.90 film exhibited a very stable one-step phase transition from amorphous→face-centered cubic (fcc) in the annealing temperature range of 100–300 °C. The Al0.10(Ge2Sb2Te5)0.90 film had a higher sheet resistances (RS) in both the amorphous and crystalline phases compared to the Ge2Sb2Te5 film, resulting in lower set and reset programming currents in the phase-change random-access memory. The crystallization speed (v) of the amorphous films was quantitatively and qualitatively evaluated through the analysis of the surface images and the nanopulse reflection-response curves. Conclusively, the Al atom added into Ge2Sb2Te5 serves as a center for suppression of the fcc-to-hexagonal phase transition and the v-value was largely improved by the proper addition of Al, e.g., v[Al0.10(Ge2Sb2Te5)0.90]>v[Ge2Sb2Te5]. Additionally, the improved v was believed to result from improvements in both the nucleation and growth processes.
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64.70.kj Glasses
64.60.qj Studies of nucleation in specific substances
68.55.ag Semiconductors
73.61.Jc Amorphous semiconductors; glasses
73.61.Le Other inorganic semiconductors
81.40.Gh Other heat and thermomechanical treatments

Suppression of roll-off characteristics of electroluminescence at high current densities in organic light emitting diodes by introducing reduced carrier injection barriers

Yousuke Setoguchi and Chihaya Adachi

J. Appl. Phys. 108, 064516 (2010); http://dx.doi.org/10.1063/1.3488883 (7 pages) | Cited 3 times

Online Publication Date: 24 September 2010

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We experimentally investigated suppression of the roll-off characteristics of the electroluminescence efficiency at high current densities in organic light emitting diodes (OLEDs). To increase exciton density, we propose a nonheterostructure OLED that consists of a single emitting layer of 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) and layers locally with doped donors/acceptors on the cathode and anode sides. The OLED exhibited suppression of the roll-off characteristics at high current densities over 100 A/cm2 with balanced bipolar injection and transport, resulting in the high exciton density of 1024 cm−3 s−1. Furthermore, amplified spontaneous emission with a relatively low threshold of Eth = 24 μJ/cm2 was obtained by optically pumping the single-layer device. However, to realize electrical excitation it was necessary to reduce the lasing threshold by two orders of magnitude or to inject a 100 times higher current density.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence

A simple design of an artificial electromagnetic black hole

Wanli Lu, JunFeng Jin, Zhifang Lin, and Huanyang Chen

J. Appl. Phys. 108, 064517 (2010); http://dx.doi.org/10.1063/1.3485819 (4 pages) | Cited 12 times

Online Publication Date: 27 September 2010

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We conduct a rigorous study on the properties of an artificial electromagnetic black hole for transverse magnetic modes. A multilayered structure of such a black hole is then proposed as a reduced variety for easy experimental implementations. An actual design of composite materials based on the effective medium theory is given with only five kinds of real isotropic materials. The finite element method confirms the functionality of such a simple design.
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04.70.-s Physics of black holes
02.70.Dh Finite-element and Galerkin methods

The properties of tris (8-hydroxyquinoline) aluminum organic light emitting diode with undoped zinc oxide anode layer

G. Luka, P. Stakhira, V. Cherpak, D. Volynyuk, Z. Hotra, M. Godlewski, E. Guziewicz, B. Witkowski, W. Paszkowicz, and A. Kostruba

J. Appl. Phys. 108, 064518 (2010); http://dx.doi.org/10.1063/1.3486058 (4 pages) | Cited 7 times

Online Publication Date: 28 September 2010

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Transparent and conductive undoped zinc oxide films were prepared by atomic layer deposition method for use in tris (8-hydroxyquinoline) aluminum (Alq3)-based organic light emitting diodes. The properties of the ZnO layers were investigated. The ZnO/CuI/Alq3/poly(ethylene glycol) dimethyl ether/Al device turned on at 7.9 V and demonstrated external quantum efficiency of 1.5% which is better comparing to the same structure but with indium tin oxide as anode layer.
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85.60.Jb Light-emitting devices

Flexural wave propagation in double-layered nanoplates with small scale effects

Yi-Ze Wang, Feng-Ming Li, and Kikuo Kishimoto

J. Appl. Phys. 108, 064519 (2010); http://dx.doi.org/10.1063/1.3481438 (6 pages) | Cited 13 times

Online Publication Date: 28 September 2010

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In this work, the flexural wave propagation in doubled-layered nanoplates is studied. Based on the nonlocal continuum theory, the equation of wave motion is derived. The frequency, phase velocity, group velocity, and their ratio with different scale coefficients and wave numbers are presented. From the results, it can be observed that the small scale effects should be considered for higher frequencies. The dispersion properties for mode I and mode II are different. The van der Walls (vdW) interaction has significant influence on the wave characteristics for the higher mode, which is similar to the vibration properties of nanoplates. However, not all of the characteristics for mode II can be dominated by the vdW interaction, they can be affected by the wave number and the scale coefficients.
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62.25.Jk Mechanical modes of vibration
62.30.+d Mechanical and elastic waves; vibrations
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