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15 Oct 2005

Volume 98, Issue 8, Articles (08xxxx)

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The low-temperature a-SiNx films with high impermeability and high optical gap with application to organic light-emitting diode

Hsu-Yu Chang, Chao-Yu Meng, Ching-Wei Huang, and Si-Chen Lee

J. Appl. Phys. 98, 084501 (2005); http://dx.doi.org/10.1063/1.2089161 (5 pages) | Cited 4 times

Online Publication Date: 18 October 2005

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A low-temperature passivation layer (≦100 °C) with high impermeability and a high optical gap is required in top-emitting organic light-emitting diodes. Hydrogenated amorphous silicon nitride (a-SiNx:H) films fabricated by the plasma-enhanced chemical-vapor deposition of SiH4+NH3+N2 at 100 °C fulfill this requirement. Infrared-absorption spectroscopy was used to monitor the permeability of the a-SiNx:H film. The optimum a-SiNx:H film with an optical gap of 3.1 eV was fabricated at a gas ratio of SiH4:NH3:N2 = 10:3:197 SCCM. The main absorption peaks of the a-SiNx:H films deposited at 70 °C did not change at all, even following a 3000-h-long storage test.
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81.05.Gc Amorphous semiconductors
78.66.Jg Amorphous semiconductors; glasses
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.35.+c Brillouin and Rayleigh scattering; other light scattering
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
52.77.Dq Plasma-based ion implantation and deposition
68.55.A- Nucleation and growth

Rapid-thermal-processing-based intrinsic gettering for nitrogen-doped Czochralski silicon

Xiangyang Ma, Liming Fu, Daxi Tian, and Deren Yang

J. Appl. Phys. 98, 084502 (2005); http://dx.doi.org/10.1063/1.2089167 (4 pages) | Cited 6 times

Online Publication Date: 18 October 2005

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In order to optimize the intrinsic gettering (IG) process based on the rapid thermal processing (RTP) for nitrogen-doped Czochralski (NCZ) silicon wafer, the effects of RTP and the following nucleation anneal on oxygen precipitation in NCZ silicon and conventional CZ silicon wafers have been comparatively investigated. It was found that, for NCZ silicon, the RTP at enough high temperature (e.g., 1250 °C) was necessary for generation of high density of bulk microdefects (BMDs) in the subsequent anneals despite the nitrogen enhancement effect on oxygen precipitation. With enough high concentration of vacancies introduced by the RTP at 1250 °C, for the conventional CZ silicon wafer, the vacancy enhancement effect on the nucleation of oxygen precipitates was most significant at 800 °C; while for NCZ silicon wafer, the vacancies and nitrogen atoms coacted most remarkably to nucleate the oxygen precipitates during the ramping anneal from 800 to 1000 °C. It was further found that the doped nitrogen is superior to the vacancy to enhance the nucleation of oxygen precipitates at temperatures of 900–1000 °C. Moreover, in order to maximize the density of BMDs, the RTP-based IG process for NCZ silicon wafer should be different from that for the conventional CZ silicon wafer.
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81.05.Cy Elemental semiconductors
61.72.Cc Kinetics of defect formation and annealing
61.72.J- Point defects and defect clusters
61.72.uf Ge and Si
81.65.Tx Gettering
81.30.Mh Solid-phase precipitation

Tungstate polyoxometalates as active components of molecular devices

D. Velessiotis, N. Glezos, and V. Ioannou-Sougleridis

J. Appl. Phys. 98, 084503 (2005); http://dx.doi.org/10.1063/1.2103416 (4 pages) | Cited 12 times

Online Publication Date: 18 October 2005

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Polyoxometalates are a class of well-defined metal oxygen clusters mostly known for their catalytic properties. However, their electronic and optical properties have been used in device applications such as electrochromic displays, dopants for conductive polymers, gas and chemical sensors, capacitors, and electrochemical cells. We fabricate nanodevices based on a composite poly(methyl methacrylate) H3SiW12O40 system and we investigate the effects of electrode material, electrode distance, and molecular concentration on the electronic transport characteristics. It is found that in the case of electrode distances smaller than 50 nm, tunneling effects appear, which are discussed using tunneling theory models. These effects are primarily dependent on the electrode distance and molecular concentration, and less on the electrode material.
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85.65.+h Molecular electronic devices

Describing thin-film imaging with a Gaussian beam as potential flow

Asher Klatchko and Peter Pirogovsky

J. Appl. Phys. 98, 084504 (2005); http://dx.doi.org/10.1063/1.2085310 (6 pages) | Cited 2 times

Online Publication Date: 19 October 2005

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It is shown that patterning with a Gaussian beam on thin films is congruent to a diffusion process. In particular, the acuity of the image patterned is tied to a steady state (Laplace equation). For a small spot size, pattern acuity improves locally when the Laplacian approaches zero at the said region. However, when the spot size (blur) of the patterning beams is of the order of the critical dimensions of the geometry, the diffusion equation gives rise to a global steady state. As a corollary, a conformal mapping of the type w = z2 is applied, transforming a 45∘ rotated cross into a contact hole. We discuss the relevance of conformal mapping to corner rounding seen on raster beam pattern generators.
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68.55.-a Thin film structure and morphology
42.30.-d Imaging and optical processing
02.50.Ng Distribution theory and Monte Carlo studies
05.60.-k Transport processes
02.30.Jr Partial differential equations

Operation and properties of ambipolar organic field-effect transistors

G. Paasch, Th. Lindner, C. Rost-Bietsch, S. Karg, W. Riess, and S. Scheinert

J. Appl. Phys. 98, 084505 (2005); http://dx.doi.org/10.1063/1.2085314 (13 pages) | Cited 16 times

Online Publication Date: 19 October 2005

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Two-dimensional simulations of ambipolar or double-injection field-effect transistors (FETs) based on organic materials as active layer are presented in this article. These organic FETs are of interest because of the direct recombination accompanied by light emission. When modeling such FETs, one problem is that even advanced existing models based on the Pao-Sah description neglect both the actual contact properties and the recombination process. We present numerical simulations taking account these peculiarities. Current-voltage characteristics are used as reference data, recently obtained on an organic heterostructure ambipolar FET with a bottom Au contact and a Mg top contact [ Rost et al., J. Appl. Phys. 95 5872 (2004) ]. To investigate the basic ambipolar effects without the additional influences of the special structure, a single-layer model structure is considered in this simulation study. Simulated current-voltage characteristics indeed feature all specific characteristics arising from the ambipolar operation. Their origin becomes clear by inspecting the internal field and concentration profiles. The simulations reveal a complex interplay between the properties of the source and gate contacts, interface charges, the ratio of the electron and hole mobilities, doping, and recombination. Thereby, pronounced ambipolar operation depends sensitively on the combination of the barrier of the electron-injecting contact and the ratio of hole to electron mobilities. Possibilities and limitations of direct parameter extraction based on simple analytical expressions are demonstrated.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling

High mobility top-gated pentacene thin-film transistors

Christopher R. Newman, Reid J. Chesterfield, Matthew J. Panzer, and C. Daniel Frisbie

J. Appl. Phys. 98, 084506 (2005); http://dx.doi.org/10.1063/1.2076429 (6 pages) | Cited 25 times

Online Publication Date: 20 October 2005

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A common device geometry for measuring the electrical characteristics of organic semiconductors is the thin-film organic field-effect transistor (OTFT). Mostly for reasons of cost, convenience, and availability, this usually involves depositing the organic material on a prefabricated gate structure such as Si/SiO2, the surface chemistry of which is often modified with self-assembled monolayers. The interactions between these surfaces and the deposited organic can have a profound effect on thin-film growth and the resulting electrical characteristics since most of the charge transport in these structures occurs near the organic-insulator interface. An alternative to this traditional technique is to assemble the transistor on top of the organic semiconductor. We have used chemical-vapor deposition of the polymeric dielectric material parylene to create pentacene OTFTs with gate electrodes both on top of and below the semiconductor film, with field-effect mobilities as high as 0.1 cm2/V s and Ion/Ioff ratios greater than 106 in the top-gated OTFTs. Comparing the electronic properties of top- and bottom-gate structures yields insight into the charge transport characteristics as well as the effects of device geometry, contacts, and surface roughness of the organic thin film.
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85.30.Tv Field effect devices

Spin-polarized conduction in localized ferromagnetic materials: The case of Fe3O4 on MgO(100)

E. Vescovo, H.-J. Kim, J. M. Ablett, and S. A. Chambers

J. Appl. Phys. 98, 084507 (2005); http://dx.doi.org/10.1063/1.2085316 (5 pages) | Cited 5 times

Online Publication Date: 21 October 2005

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The surface electronic structure of 1500-Å-thick Fe3O4(100) films has been investigated by spin-resolved photoemission spectroscopy with vacuum ultraviolet synchrotron radiation. The films, epitaxially grown on MgO(100) substrates using plasma-assisted molecular-beam deposition, are briefly exposed to air during transfer to the photoemission chamber. It is shown that clean surfaces of Fe3O4 films after exposure to air can be recovered through mild annealing in an oxygen atmosphere. The presence of the characteristic Verwey transition, as detected in the valence-band photoemission spectra, provides evidence for the excellent electronic quality at the surface of these films. The top of the valence band is found to be negatively spin polarized, with a value of ≈ −50%; this result strongly points towards the localized nature of the 3d states in this system.
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75.50.Dd Nonmetallic ferromagnetic materials
73.20.At Surface states, band structure, electron density of states
75.47.-m Magnetotransport phenomena; materials for magnetotransport
79.60.Bm Clean metal, semiconductor, and insulator surfaces
75.70.Ak Magnetic properties of monolayers and thin films
71.30.+h Metal-insulator transitions and other electronic transitions

Characterization and analysis of single-mode high-power continuous-wave quantum-cascade laser

W. W. Bewley, I. Vurgaftman, C. S. Kim, J. R. Meyer, J. Nguyen, A. Evans, J. S. Yu, S. R. Darvish, S. Slivken, and M. Razeghi

J. Appl. Phys. 98, 084508 (2005); http://dx.doi.org/10.1063/1.2112170 (4 pages) | Cited 2 times

Online Publication Date: 24 October 2005

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We measured and modeled the performance characteristics of a distributed-feedback quantum-cascade laser exhibiting high-power continuous-wave (CW) operation in a single spectral mode at λ ≈ 4.8 μm and temperatures up to 333 K. The sidemode suppression ratio exceeds 25 dB, and the emission remains robustly single mode at all currents and temperatures tested. CW output powers of 99 mW at 298 K and 357 mW at 200 K are obtained at currents well below the thermal rollover point. The slope efficiency and subthreshold amplified spontaneous emission spectra are shown to be consistent with a coupling coefficient of no more than κL ≈ 4–5, which is substantially lower than the estimate of 9 based on the nominal grating fabrication parameters.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Magnetic-field tunable terahertz quantum well infrared photodetector

Ivana Savić, Vitomir Milanović, Nenad Vukmirović, Vladimir D. Jovanović, Zoran Ikonić, Dragan Indjin, and Paul Harrison

J. Appl. Phys. 98, 084509 (2005); http://dx.doi.org/10.1063/1.2085309 (8 pages) | Cited 10 times

Online Publication Date: 26 October 2005

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A theoretical model and a design of a magnetic-field tunable CdMnTe/CdMgTe terahertz quantum well infrared photodetector are presented. The energy levels and the corresponding wave functions were computed from the envelope function Schrödinger equation using the effective-mass approximation and accounting for Landau quantization and the giant Zeeman effect induced by magnetic confinement. The electron dynamics were modeled within the self-consistent coupled rate equations approach, with all relevant electron-longitudinal-optical phonon and electron-longitudinal-acoustic phonon scatterings included. A perpendicular magnetic field varying between 0 and 5 T, at a temperature of 1.5 K, was found to enable a large shift of the detection energy, yielding a tuning range between 24.1 and 34.3 meV, equivalent to 51.4–36.1 μm wavelengths. For magnetic fields between 1 and 5 T, when the electron population of the quantum well infrared photodetector is spin polarized, a reasonably low dark current of ≤ 1.4×10−2 A/cm2 and a large responsivity of 0.36−0.64 A/W are predicted.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Rapid silicon outdiffusion from SiC substrates during molecular-beam epitaxial growth of AlGaN/GaN/AlN transistor structures

W. E. Hoke, A. Torabi, J. J. Mosca, R. B. Hallock, and T. D. Kennedy

J. Appl. Phys. 98, 084510 (2005); http://dx.doi.org/10.1063/1.2099512 (5 pages) | Cited 11 times

Online Publication Date: 26 October 2005

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AlGaN/GaN/AlN transistor structures were grown onto SiC substrates by molecular-beam epitaxy. Under aluminum-rich growth conditions for the AlN nucleation layer, undesirable n-type conduction is observed near the GaN/AlN interface for even thick (>1000 Å) AlN layers. Silicon is identified as the unwanted dopant from secondary-ion mass spectroscopy measurements. Atomic force microscopy surface maps reveal free aluminum metal on AlN surfaces grown under modest aluminum-rich conditions. It is proposed that rapid silicon migration is caused by molten aluminum reacting with the SiC substrate resulting in dissolved silicon that rapidly migrates through the growing AlN layer. This behavior is significantly reduced using a growth flux ratio of aluminum to reactive nitrogen close to unity. The resulting buffer leakage current of the GaN high electron mobility transistor structure is reduced by more than four orders of magnitude.
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85.30.Tv Field effect devices
81.05.Ea III-V semiconductors
68.55.A- Nucleation and growth
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
66.30.H- Self-diffusion and ionic conduction in nonmetals
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.47.Fg Semiconductor surfaces
68.35.Fx Diffusion; interface formation
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
64.75.-g Phase equilibria

A pentacene ambipolar transistor: Experiment and theory

Roland Schmechel, Marcus Ahles, and Heinz von Seggern

J. Appl. Phys. 98, 084511 (2005); http://dx.doi.org/10.1063/1.2106009 (6 pages) | Cited 53 times

Online Publication Date: 31 October 2005

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An ambipolar pentacene transistor with top-gold and top-calcium contacts has been realized by utilizing a parallactic shadow mask effect during vapor deposition. The pentacene deposited on top of a silicon dioxide gate insulator is doped by Ca at the pentacene/SiO2 interface in order to compensate electron traps. An equivalent circuit model based on a resistor-capacitor network has been developed to describe the basic electrical properties of the transistor. Shockley-like analytical expressions for the output and transfer characteristic, as well as an analytical expression for the potential and charge-carrier distribution in the channel, are derived under the assumption of a high electron-hole recombination probability. The model has been fitted to our experimental results and yields comparable mobilities for both holes and electrons in the order of 0.1 cm2/V s. The increasing threshold voltages, with an increase in gate voltage, are discussed as an indication for trapped charge carriers within the insulator (SiO2).
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85.30.Tv Field effect devices
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