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1 Oct 2002

Volume 92, Issue 7, pp. 3425-4144

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Morphology and effects of hydrogen etching of porous SiC

Ashutosh Sagar, C. D. Lee, R. M. Feenstra, C. K. Inoki, and T. S. Kuan

J. Appl. Phys. 92, 4070 (2002); http://dx.doi.org/10.1063/1.1501749 (5 pages) | Cited 22 times

Online Publication Date: 18 September 2002

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The morphology of the porous network in porous SiC has been studied. It has been found that pore formation starts with a few pores on the surface and then the porous network grows in a V-shaped branched structure below the surface. The hydrogen etching rates of porous and nonporous SiC have been measured. Etch rates of porous and nonporous wafers of various miscuts are found to be equal within a factor of two, indicating that the rate-limiting step in the etching process arises from the supply of active etching species from the gas phase. The porous SiC etches slightly faster than the nonporous SiC, which is interpreted simply in terms of the reduced average density of the porous material. © 2002 American Institute of Physics.
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81.65.Cf Surface cleaning, etching, patterning
81.05.Rm Porous materials; granular materials

Linewidth determination in local oxidation nanolithography of silicon surfaces

Marta Tello, Fernando García, and Ricardo García

J. Appl. Phys. 92, 4075 (2002); http://dx.doi.org/10.1063/1.1501753 (5 pages) | Cited 8 times

Online Publication Date: 18 September 2002

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We measure the linewidth of structures fabricated by local oxidation lithography on silicon surfaces. Two different structures, isolated and arrays of parallel lines have been generated. The oxide structures have been fabricated in the proximity of sexithiophene islands whose size is comparable to the oxide motives. The comparison between local oxides and sexithiophene islands reveals that atomic force microscopy (AFM) images faithfully reproduce the size and shape of local silicon oxides. The oxide lines have a trapezoidal shape with a flat section at the top. AFM images of the oxide structures show rather small slopes ∼0.05–0.15 which imply angles with the horizontal between 3° and 8°. The shallow angles imply a minimum feature size of 14 nm at the base for an oxide thickness of 1 nm. Linewidths of 7 nm and 20 nm at the top and base, respectively, have been fabricated. We have also demonstrated the ability to pack structures with a periodicity of 13 nm. © 2002 American Institute of Physics.
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81.05.Cy Elemental semiconductors
81.65.Mq Oxidation
81.16.Nd Micro- and nanolithography
85.40.Hp Lithography, masks and pattern transfer
68.37.Ps Atomic force microscopy (AFM)

Low voltage electrowetting-on-dielectric

Hyejin Moon, Sung Kwon Cho, Robin L. Garrell, and Chang-Jin “CJ” Kim

J. Appl. Phys. 92, 4080 (2002); http://dx.doi.org/10.1063/1.1504171 (8 pages) | Cited 173 times

Online Publication Date: 18 September 2002

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This article discusses and experimentally verifies how to lower the operating voltage that drives liquid droplets by the principle of electrowetting on dielectric (EWOD). A significant contact angle change (120°→80°) is desired to reliably pump the droplet in microchannels for applications such as lab-on-a-chip or micrototal analysis systems. Typically, much higher voltages (>100 V) are used to change the wettability of an electrolyte droplet on a dielectric layer compared with a conductive layer. The required voltage can be reduced by increasing the dielectric constant and decreasing the thickness of the dielectric layer, thus increasing the capacitance of the insulating layer. This dependence of applied voltage on dielectric thickness is confirmed through EWOD experiments for three different dielectric materials of varying thickness: Amorphous fluoropolymer (Teflon® AF, Dupont), silicon dioxide (SiO2) and parylene. The dependence on the dielectric constant is confirmed with two different dielectric materials of similar thickness: SiO2 and barium strontium titanate. In all cases, the surface is coated with a very thin (200 Å) layer of amorphous fluoropolymer to provide initial hydrophobicity. Limiting factors such as the dielectric breakdown and electrolysis are also discussed. By using very thin (700 Å) and high dielectric constant (∼180) materials, a significant contact angle change (120°→80°) has been achieved with voltages as low as 15 V. Based on these results, a microfluidic device has been fabricated and tested, demonstrating successful transporting (pumping) of a 460 nL water droplet with only 15 V. © 2002 American Institute of Physics.
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68.08.Bc Wetting
68.03.Cd Surface tension and related phenomena
47.65.-d Magnetohydrodynamics and electrohydrodynamics
77.22.Jp Dielectric breakdown and space-charge effects
82.45.-h Electrochemistry and electrophoresis

Two-dimensional reconstruction theory of thermal conductivity profiles based on the thermal wave technique

Q. B. Zhou, Y. K. Lu, S. Y. Zhang, J. C. Cheng, and X. J. Shui

J. Appl. Phys. 92, 4088 (2002); http://dx.doi.org/10.1063/1.1503172 (7 pages)

Online Publication Date: 18 September 2002

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A two-dimensional (2D) theory of thermal wave reconstruction is presented. By using a pulse spectrum technique and a Newton-like iteration method, the 2D inverse problem is expressed by a 2D Fredholm integral equation of the first kind. Further, the integral equation is approximated by a set of ill-posed linear algebraic equations, and a regularization method is introduced to overcome the singularity of the ill-posed linear algebraic equations. Finally, an error function is defined to choose the regularization parameter automatically to converge the iteration rapidly. The important developments in 2D inverse problem, compared with the one-dimensional problem, are that a line source is used instead of the source with a large cross section, and a finite difference method is adopted for numerical solution of the 2D heat equation. Numerical simulations demonstrate that this approach is effective and stable even with 5% random noise disturbance in the signal. © 2002 American Institute of Physics.
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07.20.-n Thermal instruments and apparatus
02.60.Jh Numerical differentiation and integration
02.60.Nm Integral and integrodifferential equations

Influence of nitrogen on diamond growth in oxyacetylene combustion chemical vapor deposition

M. Okkerse, M. H. J. M. de Croon, C. R. Kleijn, G. B. Marin, and H. E. A. van den Akker

J. Appl. Phys. 92, 4095 (2002); http://dx.doi.org/10.1063/1.1502925 (8 pages) | Cited 4 times

Online Publication Date: 18 September 2002

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Addition of di-nitrogen to the feed gas has been shown to greatly influence growth rates and morphology of the deposited layer in various diamond chemical vapor deposition (CVD) techniques. In this article, several hypotheses for these phenomena, as presented in literature, are tested for the case of diamond combustion CVD with the aid of an atmospheric pressure oxyacetylene flame. For this purpose, one-dimensional and two-dimensional simulations are performed of the hydrodynamics, the combustion and deposition chemistry, and the nitrogen chemistry. Based on the simulation results, several proposed hypotheses can be ruled out as possible explanations for the observed phenomena. It is concluded, that the most likely hypotheses are: (i) the presence of nitrogen atoms in the diamond lattice, enhancing diamond growth by acting on the electron structure of surface dimer bonds, and (ii) selective adsorption of nitrogen-containing species on the surface, selectively increasing growth in the (100) direction. It is found that possible gas phase candidates for affecting diamond growth are NH, NH2, NH3, CN, HCN, H2CN, and NCO.© 2002 American Institute of Physics.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
82.33.Ya Chemistry of MOCVD and other vapor deposition methods
82.33.Vx Reactions in flames, combustion, and explosions
81.05.U- Carbon/carbon-based materials
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
81.05.Cy Elemental semiconductors
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
82.20.Wt Computational modeling; simulation

Ion-to-CH3 flux ratio in diamond chemical-vapor deposition

Kungen Teii, Masaru Hori, and Toshio Goto

J. Appl. Phys. 92, 4103 (2002); http://dx.doi.org/10.1063/1.1506384 (6 pages) | Cited 6 times

Online Publication Date: 18 September 2002

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Methyl radicals (CH3) and positive ionic species in a low-pressure inductively coupled plasma under diamond-depositing conditions have been detected by using a quadrupole mass spectrometer. Absolute calibration of the fluxes of CH3 and ionic species was made by the threshold ionization technique and Langmuir probe measurement, respectively. The CH3 density increased by two to three times with a small addition of carbon monoxide to a methane–hydrogen plasma and was on the order of 1011–1012 cm−3. As the pressure decreased from 60 to 10 mTorr, the ion-to-CH3 flux ratio increased from 0.2 to 4.3, accompanied by an increase in the fraction of light ions such as Hx+ (x=1–3). The average ion energy in the ion energy distribution at a grounded electrode was compared with the sheath potential and the discrepancy was found to be 0.5–2 eV depending on pressure and ion mass. The results were used to describe the specific surface process dominated by energetic (∼ several eV) ions rather than thermal neutrals. © 2002 American Institute of Physics.
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81.05.U- Carbon/carbon-based materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
52.77.Dq Plasma-based ion implantation and deposition
52.70.Nc Particle measurements
52.40.Kh Plasma sheaths
52.70.Ds Electric and magnetic measurements

Spectral shape of in situ mass spectra of sympathetically cooled molecular ions

Takashi Baba and Izumi Waki

J. Appl. Phys. 92, 4109 (2002); http://dx.doi.org/10.1063/1.1506005 (8 pages) | Cited 9 times

Online Publication Date: 18 September 2002

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We formulated a model to reproduce the spectral shape of in situ mass spectra of sympathetically cooled molecular ions trapped in a linear radio-frequency-quadrupole. The molecular ions are sympathetically cooled by laser cooled ions. The mass spectrum is obtained by observing fluorescence emitted from the laser-cooled ions as we excite the resonant motion of the sympathetically cooled molecular ions in the trap. The model, which uses a pseudopotential and space-charge approach, reproduces the mass spectra of molecules. The model suggests that the variation of the space-charge density of the laser-cooled ions can produce in-trap kinetic excitation without the loss of the excited ions from the trap. © 2002 American Institute of Physics.
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37.10.Mn Slowing and cooling of molecules
37.10.Pq Trapping of molecules
33.15.Ta Mass spectra
33.70.Jg Line and band widths, shapes, and shifts
33.50.Dq Fluorescence and phosphorescence spectra

Improving solar cell efficiencies by up-conversion of sub-band-gap light

T. Trupke, M. A. Green, and P. Würfel

J. Appl. Phys. 92, 4117 (2002); http://dx.doi.org/10.1063/1.1505677 (6 pages) | Cited 157 times

Online Publication Date: 18 September 2002

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A system for solar energy conversion using the up-conversion of sub-band-gap photons to increase the maximum efficiency of a single-junction conventional, bifacial solar cell is discussed. An up-converter is located behind a solar cell and absorbs transmitted sub-band-gap photons via sequential ground state absorption/excited state absorption processes in a three-level system. This generates an excited state in the up-converter from which photons are emitted which are subsequently absorbed in the solar cell and generate electron-hole pairs. The solar energy conversion efficiency of this system in the radiative limit is calculated for different cell geometries and different illumination conditions using a detailed balance model. It is shown that in contrast to an impurity photovoltaic solar cell the conditions of photon selectivity and of complete absorption of high-energy photons can be met simultaneously in this system by restricting the widths of the bands in the up-converter. The upper limit of the energy conversion efficiency of the system is found to be 63.2% for concentrated sunlight and 47.6% for nonconcentrated sunlight. © 2002 American Institute of Physics.
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84.60.Jt Photoelectric conversion
42.79.Ek Solar collectors and concentrators
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