Browsing by Issue Date, starting with "1997-05-12"

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Mobility enhancements in AlGaN/GaN/SiC with stair-step and graded heterostructures
(1997-05-12)
Lin, CF
;
Cheng, HC
;
Huang, JA
;
Feng, MS
;
Guo, JD
In this letter, we report the observation of the enhanced electron mobility in three different Al0.08Ga0.92N/GaN heterostructures. These structures were deposited on (0001) 6H-SiC substrates by using the low-pressure metalorganic chemical vapor deposition method. The structure was composed of 500 Angstrom AlGaN compositional stair-step layer deposited onto 1.3 mu m GaN epitaxial layer. There is a 100 Angstrom GaN Cap layer on top of the AlGaN layer to prevent the oxidation of the AlGaN layer. By using the van der Pauw method of Hall measurement, the sheet carrier density and mobility for Al0.08Ga0.92N/GaN heterostructure are 6.6x10(12) cm(-2) and 5413 cm(2)/Vs at 77 K, respectively. This high value of mobility is, to the best of our knowledge, the first observed two-dimensional electron gas (2DEG) at the AlGaN/GaN stair-step type of heterostructure. Other interface structures such as graded composition and bulk composition structures were also prepared and 2DEG properties were observed. (C) 1997 American Institute of Physics.
Mobility enhancements in AlGaN/GaN/SiC with stair-step and graded heterostructures
(1997-05-12)
Lin, CF
;
Cheng, HC
;
Huang, JA
;
Feng, MS
;
Guo, JD
In this letter, we report the observation of the enhanced electron mobility in three different Al0.08Ga0.92N/GaN heterostructures. These structures were deposited on (0001) 6H-SiC substrates by using the low-pressure metalorganic chemical vapor deposition method. The structure was composed of 500 Angstrom AlGaN compositional stair-step layer deposited onto 1.3 mu m GaN epitaxial layer. There is a 100 Angstrom GaN Cap layer on top of the AlGaN layer to prevent the oxidation of the AlGaN layer. By using the van der Pauw method of Hall measurement, the sheet carrier density and mobility for Al0.08Ga0.92N/GaN heterostructure are 6.6x10(12) cm(-2) and 5413 cm(2)/Vs at 77 K, respectively. This high value of mobility is, to the best of our knowledge, the first observed two-dimensional electron gas (2DEG) at the AlGaN/GaN stair-step type of heterostructure. Other interface structures such as graded composition and bulk composition structures were also prepared and 2DEG properties were observed. (C) 1997 American Institute of Physics.
Effect of adding Ar on the thermal stability of chemical vapor deposited fluorinated silicon oxide using an indirect fluorinating precursor
(1997-05-12)
Chang, KM
;
Wang, SW
;
Li, CH
;
Yeh, TH
;
Yang, JY
For a low dielectric constant intermetal dielectric application, fluorinated silicon oxide (FxSiOy) films were deposited in an electron cyclotron resonance chemical vapor deposition system, with SiH4, O-2, and CF4 as the reaction gases. Since the CF4 is an indirect fluorinating precursor, the fluorinating mechanism resembles that of the oxide etching by a fluorocarbon plasma. Thermal stability of the incorporated fluorine (and hence, the dielectric constant) relies heavily on the deposition parameters and technologies. According to experimental results, adding Ar gas during deposition can improve the thermal stability of incorporated fluorine. Such an improvement is due to the fact that Ar sputtering enhances the removal of weakly bonded silicon fluoride on the as-deposited film surface, thereby elevating the mean bonding strength of fluoride remaining in the oxide. (C) 1997 American Institute of Physics.
Matrix-theoretical analysis in the Laplace domain for the time lags and mean first passage times for reaction-diffusion transport
(1997-05-15)
Chen, JS
;
Chang, WY
;
應用化學系
;
Department of Applied Chemistry
Siegel's matrix analysis of membrane transport in the Laplace domain [J. Phys. Chem. 95, 2556 (1991)], which is restricted to zero initial distribution, has been extended to including the case of nonzero initial distribution. This extension leads to a more general transport equation with Siegel's results as a special case. The new transport equation allows us to formulate the mean-first-passage time (t) over bar for various boundary conditions, if the initial distribution is stipulated to be of the Dirac delta-function type; and the steady-state permeability P and time lag t(L), if zero initial distribution is employed. Based on this matrix analysis we also propose an algorithm for quick and effective numerical computations of P, t(L), and ST Examples are given to demonstrate the application of this algorithm, and the numerical results are compared with the theoretical ones. The validity of the transport equation is also checked by a Green's function. (C) 1997 American Institute of Physics.
Translationally invariant polarons in conjugated conducting polymers
(1997-05-15)
Meng, HF
;
Lai, CM
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
A simple effective Hamiltonian for the electron-phonon coupling in the conjugated polymers is derived from the Su-Schrieffer-Heeger model with quantized lattice motions. Phonons correspond to lattice quantum fluctuations around the dimerized positions. Both acoustic and optical modes are considered. The energy-momentum relation for the polaron is calculated variationally, with the aid of Lee-Low-Pines transformation. The discrete translational invariance of an ideal conjugated polymer is. explicitly enforced. Analytical expression for the polaron effective mass m* is obtained. m* turns out to be, in general, much larger than the conduction-band effective mass.
陶藝夫妻野菜香
(1997-05-15)
李展平
Crystalline structure changes in GaN films grown at different temperatures
(1997-05-15)
Lin, HC
;
Ou, J
;
Chen, WK
;
Chen, WH
;
Lee, MC
Thin GaN films were grown on (0001) sapphire at various temperatures between 520 and 1050 degrees C using metalorganic chemical vapor deposition (MOCVD). Optical properties and crystalline structures of the films were investigated by means of photoluminescence, Raman scattering, and X-ray diffraction. A noticeable structure transition occurred at around 750 degrees C with higher growth temperatures favoring the hexagonal structure and lower ones the cubic. Defect formation was also seen to be temperature dependent. The yellow luminescence which was clearly observed in our 700-850 degrees C films was attributable to the cubic and hexagonal structure mixing. The drastic reduction of yellow luminescence and the substantial enhancement of near band edge emission in the 950-1050 degrees C films indicated that this temperature range is optimum for growing high quality wurtzite films.
Raman spectroscopy study of ZnSe and Zn0.84Fe0.16Se at high pressures
(1997-05-15)
Lin, CM
;
Chuu, DS
;
Yang, TJ
;
Chou, WC
;
Xu, JA
The ZnSe powder and Zn0.84Fe0.16Se crystal were studied by Raman scattering spectroscopy at pressures up to 36.0 and 32.0 GPa, respectively. For ZnSe powder at 4.7 and 9.1 GPa, two phase transitions were observed. However, the resulted phases have not been identified yet. As the pressure was increased to 14.4 GPa, the LO phonon peak disappeared while the TO phonon peak was still visible until the metallization pressure, 17.0 GPa was reached. In addition, three unidentified Raman peaks were still observable above the metallization pressure. For Zn0.84Fe0.16Se crystal, the structure transition from possible zinc blende to sodium chloride phase (B-1) was identified by the disappearance of Fe local mode and longitudinal optical (LO) phonon mode at 10.9 GPa. In addition, an unidentified phase transition at 4.7 GPa was observed. The TO phonon and the split TO phonons were still observable at a pressure above the phase transition pressure up to 32.0 GPa. The existence of Fe impurity in the ZnSe up to a concentration of 0.16 reduced the semiconductor-metal phase transition pressure to 10.9 GPa. According to the calculation of Gruneisen parameters, Zn0.84Fe0.16Se was found to have a higher ionicity than ZnSe. Reasons for the observation of Raman peaks at a pressure above the metallization pressure are still unknown.
Electronic defect and trap-related current of (Ba0.4Sr0.6)TiO3 thin films
(1997-05-15)
Wang, YP
;
Tseng, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
By employing a deep level transient spectroscopy (DLTS) technique, this work investigates the deep trap levels of rf-sputtered (Ba0.4Sr0.6)TiO3 (BST) thin films deposited at various temperatures. Arrhenius plots of DLTS spectra detect a single trap located at 0.45 eV in 450 degrees C deposited films, whereas two traps located at 0.2 and 0.40 eV appear in 550 degrees C deposited films. On the other hand, examining the I-V characteristics of the films at the temperature range of 298-403 K reveals the presence of two conduction regions in the BST him capacitors, having ohmic behavior at low voltage (<1V) and Schottky-emission or Poole-Frenkel mechanism at high voltage (>6V). The barrier height and trapped level are, respectively, estimated to be 0.46 and 0.51 eV, corresponding to the trap activation energy 0.4-0.45 eV obtained from our DLTS measurements. Compared with previous published reports, the trap distributed at 0.4-0.5 eV should be an intrinsic defect of BST and possibly ascribed to be oxygen vacancies. Meanwhile, the trap plays a prominent role in the leakage current of BST films. (C) 1997 American Institute of Physics.
Plasmons in graphite and stage-1 graphite intercalation compounds
(1997-05-15)
Lin, MF
;
Huang, CS
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
The pi-electronic excitations df graphite layers are studied within the random-phase approximation. They principally reflect the pi-band characteristics, the strong wave-vector dependence, the anisotropic behavior, and the special symmetry. The pi plasmons in graphite have strong dispersion relations with the transferred momentum (q). They behave as an optical plasmon in a three-dimensional electron gas at small q. Moreover, the anisotropic behavior at the plane is apparent at large q. For a single graphite layer, the pi plasmons would disappear at very small q, and their frequencies are obviously reduced. The absence of interlayer Coulomb interactions is the main reason for this. The stage-1 graphite intercalation compounds (GIC's), as compared with graphite, exhibit the richer excitation spectra and the lower pi-plasmon frequencies. They have the intraband plasmon as well as the interband pi plasmon. These two kinds of plasmons are quite different from each other in certain respects, e.g., the cause of the plasmon. The enhanced interlayer distances could effectively reduce the pi-plasmon frequency, but not the transferred charges. The calculated plasmon frequencies are consistent with the experimental measurements on graphite and stage-1 GIC's.
Cordial labeling of mK(n)
(1997-05-15)
Kuo, D
;
Chang, GJ
;
Kwong, YHH
;
應用數學系
;
Department of Applied Mathematics
Suppose G = (V,E) is a graph with vertex set V and edge set E. A vertex labeling f: V --> (0, 1) induces an edge labeling f*: E --> (0, 1) defined by f*(ny) = If(x)- f(y)l. For i is an element of (0, 1), let v(f)(i) and e(f)(i) be the number of vertices v and edges e with f(v) = i and f*(e) = i, respectively. A graph G is cordial if there exists a vertex labeling f such that v(f)(0) - v(f)(1)less than or equal to 1 and e(f)(0) - e(f)(1) less than or equal to 1. This paper determines all m and n for which mK(n) is cordial.
Matrix-theoretical analysis in the Laplace domain for the time lags and mean first passage times for reaction-diffusion transport
(1997-05-15)
Chen, JS
;
Chang, WY
;
交大名義發表
;
應用化學系
;
National Chiao Tung University
Siegel's matrix analysis of membrane transport in the Laplace domain [J. Phys. Chem. 95, 2556 (1991)], which is restricted to zero initial distribution, has been extended to including the case of nonzero initial distribution. This extension leads to a more general transport equation with Siegel's results as a special case. The new transport equation allows us to formulate the mean-first-passage time (t) over bar for various boundary conditions, if the initial distribution is stipulated to be of the Dirac delta-function type; and the steady-state permeability P and time lag t(L), if zero initial distribution is employed. Based on this matrix analysis we also propose an algorithm for quick and effective numerical computations of P, t(L), and ST Examples are given to demonstrate the application of this algorithm, and the numerical results are compared with the theoretical ones. The validity of the transport equation is also checked by a Green's function. (C) 1997 American Institute of Physics.
Plasmons in graphite and stage-1 graphite intercalation compounds
(1997-05-15)
Lin, MF
;
Huang, CS
;
Chuu, DS
;
交大名義發表
;
電子物理學系
The pi-electronic excitations df graphite layers are studied within the random-phase approximation. They principally reflect the pi-band characteristics, the strong wave-vector dependence, the anisotropic behavior, and the special symmetry. The pi plasmons in graphite have strong dispersion relations with the transferred momentum (q). They behave as an optical plasmon in a three-dimensional electron gas at small q. Moreover, the anisotropic behavior at the plane is apparent at large q. For a single graphite layer, the pi plasmons would disappear at very small q, and their frequencies are obviously reduced. The absence of interlayer Coulomb interactions is the main reason for this. The stage-1 graphite intercalation compounds (GIC's), as compared with graphite, exhibit the richer excitation spectra and the lower pi-plasmon frequencies. They have the intraband plasmon as well as the interband pi plasmon. These two kinds of plasmons are quite different from each other in certain respects, e.g., the cause of the plasmon. The enhanced interlayer distances could effectively reduce the pi-plasmon frequency, but not the transferred charges. The calculated plasmon frequencies are consistent with the experimental measurements on graphite and stage-1 GIC's.
Translationally invariant polarons in conjugated conducting polymers
(1997-05-15)
Meng, HF
;
Lai, CM
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
A simple effective Hamiltonian for the electron-phonon coupling in the conjugated polymers is derived from the Su-Schrieffer-Heeger model with quantized lattice motions. Phonons correspond to lattice quantum fluctuations around the dimerized positions. Both acoustic and optical modes are considered. The energy-momentum relation for the polaron is calculated variationally, with the aid of Lee-Low-Pines transformation. The discrete translational invariance of an ideal conjugated polymer is. explicitly enforced. Analytical expression for the polaron effective mass m* is obtained. m* turns out to be, in general, much larger than the conduction-band effective mass.
Structural amendment and stereoselective synthesis of mutisianthol
(1997-05-16)
Ho, TL
;
Lee, KY
;
Chen, CK
;
應用化學系
;
Department of Applied Chemistry
cis-1-(5-Acetoxy-3,6-dimethyl-1-indanyl)-2-methyl-1-propene synthesized from 3,6-dimethyl-1-indanone was found to be different from mutisianthol by spectral comparison. The presence of a high-field signal in the NMR spectrum of the final product and various intermediates, characteristic of the cis-1,3-dialkylindanes but absent in the spectrum of the natural terpene, suggests a revision of the structure of mutisianthol to the trans isomer. The trans-indane which was subsequently obtained indeed exhibits data fully agreeable with mutisianthol. A similar stereochemical revision for jungianol is also indicated.
Structural amendment and stereoselective synthesis of mutisianthol
(1997-05-16)
Ho, TL
;
Lee, KY
;
Chen, CK
;
交大名義發表
;
應用化學系
cis-1-(5-Acetoxy-3,6-dimethyl-1-indanyl)-2-methyl-1-propene synthesized from 3,6-dimethyl-1-indanone was found to be different from mutisianthol by spectral comparison. The presence of a high-field signal in the NMR spectrum of the final product and various intermediates, characteristic of the cis-1,3-dialkylindanes but absent in the spectrum of the natural terpene, suggests a revision of the structure of mutisianthol to the trans isomer. The trans-indane which was subsequently obtained indeed exhibits data fully agreeable with mutisianthol. A similar stereochemical revision for jungianol is also indicated.
A remarkable effect of C-O-C bond angle strain on the regioselective double nucleophilic substitution of the acetal group of tetraacetal tetraoxa-cages and a novel hydride rearrangement of tetraoxa-cages
(1997-05-16)
Wu, HJ
;
Chern, JH
;
交大名義發表
;
應用化學系
;
National Chiao Tung University
A remarkable effect of C-O-C bond angle strain on the regioselective double nucleophilic substitution of the acetal group of tetraacetal tetraoxa-cages and a novel regioselective and stereoselective hydride rearrangement of tetraoxa-cages are reported. Reaction of the tetraacetal tetraoxa-cages 1 with 3 equiv of triethylsilane (at -78 degrees C), cyanotrimethylsilane (at 25 degrees C), and allyltrimethylsilane (at -78 degrees C) in dichloromethane in the presence of TiCl4 gave the double nucleophilic substitution products 2, 6, and 7 in 85-90% yields, respectively. No detectable amount of other regioisomers was obtained. Reaction of 1a with (methylthio)trimethylsilane and (phenylthio)trimethylsilane in dichloromethane in the presence of TiCl4 at -78 degrees C gave the symmetric products 10a,b and the unsymmetric products 11a,b in ratios of 8-10:1. The stereochemistry of the symmetric substitution products was proven by X-ray analysis of the crystalline compound 10a. The mechanism of the double nucleophilic substitution of the tetraoxa-cages 1 are discussed. Treatment of the tetraoxa-cages 1a,c and 22a-c with 2 equiv of TiCl4 or MeSO3H in dichloromethane at 25 degrees C for 3 h regioselectively and stereoselectively gave the novel hydride rearrangement products 16a,b and 23a-c respectively. No detectable amount of other regioisomers was observed. The stereochemistry of the hydride rearrangement was proven by DIBAL-H reduction of 16 and 23 and X-ray analysis of the reduction product 24a. We attribute the high regioselectivity of the double nucleophilic substitution and the hydride rearrangement of the tetraoxa-cages 1 to the bond angle strain of the unusually large bond angle of C(3)-O(4)-C(5) of the tetraoxa-cages.
Effects of aspect ratio on vortex flow patterns in mixed convection of air through a bottom-heated horizontal rectangular duct
(1997-05-23)
Yu, CH
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Buoyancy-induced vortex flow structures and the associated heat transfer were numerically investigated in a mixed convective airflow in a bottom-heated horizontal rectangular duct of different aspect ratios. The unsteady three-dimensional Navier-Stokes and energy equations were directly solved by a higher order upwind finite difference scheme. Results were presented in particular for Reynolds numbers ranging from 5 to 15, Rayleigh numbers up to 9000, and aspect ratios from 4 to 12. The predicted results clearly show significant differences in vortex structures induced in ducts with small and large aspect ratios. For an aspect ratio less than 6 the transverse vortex rolls are periodically generated in the duct entry and gradually transform into longitudinal rolls when moving downstream. The resulting vortex flow eventually evolves to a time periodic state with the upstream and downstream portions of the duct dominated by the transverse rolls and longitudinal rolls, respectively. For a large aspect ratio (A > 9) the transverse rolls prevail in the duct core, with two to three longitudinal rolls existing near each sidewall. The flow oscillation in the region dominated by the transverse rolls is much higher than that dominated by the longitudinal rolls. At high Ra the flow becomes chaotic in time, and the duct is filled with unstable irregular vortex rolls.
Resetting vector clocks in distributed systems
(1997-05-25)
Yen, LH
;
Huang, TL
;
交大名義發表
;
資訊工程學系
;
National Chiao Tung University
This paper establishes the necessary and sufficient condition for a correct clock resetting such that the functionality of vector clocks can be preserved. A clock reset protocol is presented with its applicability and limitation discussed. Our result indicates that for some applications, the potential of clock overflow can be completely prevented by carefully choosing the condition for initiating the clock reset protocol. (C) 1997 Academic Press.
Resetting vector clocks in distributed systems
(1997-05-25)
Yen, LH
;
Huang, TL
;
資訊工程學系
;
Department of Computer Science
This paper establishes the necessary and sufficient condition for a correct clock resetting such that the functionality of vector clocks can be preserved. A clock reset protocol is presented with its applicability and limitation discussed. Our result indicates that for some applications, the potential of clock overflow can be completely prevented by carefully choosing the condition for initiating the clock reset protocol. (C) 1997 Academic Press.