Browsing by Author "Chen, FR"

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Bandgap mapping for III-V quantum well by electron spectroscopy imaging
(2004)
Tsai, JS
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Kai, JJ
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Chang, L
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Chen, FR
;
材料科學與工程學系
The concept of bandgap mapping' was proposed originally to map the inhomogeneity of band energy in III-V semiconductors with a spatial resolution of a few nanometres in a scanning transmission electron microscope with the focus beam mode. In this paper, several techniques were developed to demonstrate the possibility to map the distribution of bandgap energies for GaN/AlN quantum-well structures using electron spectroscopy imaging (ESI). The phase correlation function was used to register different energy-loss images among ESI series with an accuracy of 1 pixel. The energy dispersion of ESI series was improved by a fast Fourier transform interpolation method. An iterative multivariable least square algorithm was derived to refine the fitting of the single scattering distribution to an analytic form of the density of states function alpha(E Eg)(0.5). The inhomogeneity of the band energy of the quantum well can be revealed from the band-energy map. A threshold filter method is applied to estimate the average value and SD of the bandgap energy from barrier and well regions in the energy map. The average bandgap energy of AlN and GaN is determined to be 5.62 +/- 0.35 and 3.87 +/- 0.36 eV, respectively. The effect of delocalization on the accuracy of band-energy determination is discussed. The 2sigma(Eg), accuracy of this analysis is comparable to half of the energy resolution of the ESI experiment.
Convergent electron beam induced growth of copper nanostructures: Evidence of the importance of a soft template
(2004-01-20)
Yen, MY
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Chiu, CW
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Chen, FR
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Kai, JJ
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Lee, CY
In a transmission electron microscope, CuCl particles were irradiated by a convergent electron beam (CEB, 200 kV, 200 pA/cm(2), beam diameter of ca. 1-2 mum) to induce growth of copper nanostructures. Upon CEB irradiation, Cu nanoparticles (5-20 nm in diameter) were formed from commercial CuCl. Using synthesized CuCl encapsulated in permethylpolysilane, growth of polycrystalline copper nanowires (50-200 nm in diameter and up to 30 mum in length) was observed. Evolution of Cu nanoparticles from the synthesized CuCl was observed from the positions with apparent incomplete encapsulation. The effect of polymer encapsulation, acting as a soft template, is shown to be important to influence the nanostructure growth. The observation supports the vapor-solid reaction growth model.
Deposition of heteroepitaxial diamond on 6H-SiC single crystal by bias-enhanced microwave plasma chemical vapor deposition
(2000-04-01)
Chang, L
;
Yan, JE
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Chen, FR
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Kai, JJ
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材料科學與工程學系
Deposition of heteroepitaxial diamond on substrates of 6H-SiC single crystal by a d.c. voltage biased-enhanced microwave plasma chemical vapor deposition method has been attempted. Various concentrations of methane (CH4) up to 10% in hydrogen (H-2) was used to evaluate its effect on epitaxy. Characterization of cross-sectional transmission electron microscopy with electron energy loss spectroscopy shows that an interlayer can form between diamond 6H-SiC, depending on the CH4 concentration. With low CH4 concentration, diamond was directly nucleated on 6H-SiC with an orientation relationship of diamond {111}//6H-SiC {0001} and diamond [110]//6H-SiC [11 (2) over bar 0]. (C) 2000 Elsevier Science S.A. All rights reserved.
Direct observation of structure effect on ferromagnetism in Zn1-xCoxO nanowires
(2006-06-01)
Jian, WB
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Wu, ZY
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Huang, RT
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Chen, FR
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Kai, JJ
Diameter-controllable single-crystalline ZnO nanowires (NWs), with the [0001] growth direction in the plane, have been fabricated by using thermal evaporation method. The as-grown NWs with diameters of similar to 40 nm were implanted with various amounts of Co ions. These Zn1-xCoxO (x <= 0.11) NWs possessed a high density of bombardment-induced orientation variations and stacking faults, and exhibited paramagnetic behavior. After thermal annealing, the structural defects largely disappeared and noticeable hysteresis in the magnetization loops were observed, indicating the presence of apparent ferromagnetic ordering in the NWs. This result provides important insight into the role played by defects in relation to the occurrence of ferromagnetism in diluted magnetic semiconductor NWs.
Energy-tunable transmission x-ray microscope for differential contrast imaging with near 60 nm resolution tomography
(2006-06-12)
Yin, GC
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Tang, MT
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Song, YF
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Chen, FR
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Liang, KS
An energy-tunable transmission hard x-ray microscope with close to 60 nm spatial resolution in three dimensions (3D) has been developed. With a cone beam illumination, a zone plate of 50 nm outmost zone width, a stable mechanical design, and software feedback, we obtained tomographic data sets that are close to 60 nm spatial resolution. Meanwhile, the element specific imaging was also obtained by a differential absorption contrast technique used below and above the absorption of the element. Examples of advanced intergraded circuit devices are used to demonstrate the element selectivity and spatial resolution in 3D of the microscope.
Extension of HRTEM resolution by semi-blind deconvolution method and Gerchberg-Saxton algorithm: application to grain boundary and interface
(2001)
Chen, FR
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Ichnose, H
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Kai, JJ
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Chang, L
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材料科學與工程學系
A generalized maximum entropy method coupled with Gerchberg-Saxton algorithm has been developed to extend the resolution from high-resolution TEM image(s) for weak objects. The Gerchberg-Saxton algorithm restores spatial resolution by operating real space and reciprocal space projections cyclically. in our methodology, a generalized maximum entropy method (Kullback-Leibler cross entropy) dealing with weak objects is used as a real space (P1) projection. After P1 projection, not only are the phases within the input spatial frequencies improved, but also the phases in the next higher frequencies are extrapolated. An example of semi-blind deconvolution (P1 project only) to improve the resolution in SiC twin boundary is shown. The nature of the bonding in this twin boundary is Si-C but it was rotated 180degrees along the boundary normal. The optimum solution from P1 projection can be further improved by a P2 projection. The square roots of diffraction intensities from a diffraction pattern are then substituted to complete a cycle operation of the Gerchberg-Saxton algorithm. Application examples of Gerchberg-Saxton algorithm to solve the atomic structure of defects (2 x 1 interfacial reconstruction and dislocation) in NiSi2 / Si interfaces will be shown also.
Four-dimensional dielectric property image obtained from electron spectroscopic imaging series
(2001)
Lo, SC
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Kai, JJ
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Chen, FR
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Chang, L
;
Chen, LC
We have demonstrated a new quantitative method to characterize two- dimensional distributions of energy-dependent dielectric function of materials from low loss electron spectroscopic image (ESI) series. Two problems associated with extracted image-spectrum from the low-loss image series, under-sampling and loss of energy resolution, were overcome by using fast Fourier transformation (FFT) interpolation and maximum entropy deconvolution method. in this study, Black Diamond(TM)/Si3N4/SiO2/Si-substrate dielectric layer designed for copper metallization was used as the sample. We show that the reconstructed (FFT interpolated and maximum entropy deconvoluted) image-spectrum obtained from ESI series images can be quantified with the same accuracy as conventional electron energy-loss spectroscopy spectra. Since the analysis of the dielectric function is sensitive to the local thickness of the specimen using Kramers-Kronig analysis, we also developed a new method to quantitatively determine the dielectric constant for low-k materials. We have determined the thickness of the Black Diamond using the extrapolated thickness method from the materials of known dielectric constants. Using Kramers-Kronig formula, the dielectric function map can be deduced front two-dimensional reconstructed single scattering spectra with providing the information of thickness. We proposed a four-dimensional data presentation for revealing the uniformity of the energy dependent property. The accuracy of our methods depends on the thickness determination and on the quality of the reconstructed spectra from the image series.
Growth and characterization of Cd1-x-yZnxMnyTe crystals
(1996-12-01)
Chou, WC
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Chen, FR
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Chiang, TY
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Shin, HY
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Sun, CY
The Cd1-x-yZnxMnyTe crystals, having potential in the development of magnetic field sensitive devices, were grown by the temperature gradient solution method at 940 degrees C. All crystals were p-type with resistivity of 10(7) Omega . cm. They have novel magneto-optical properties, and could be substitutes for the Cd1-xZnxTe ternary compounds in certain optoelectronic/integrated-optical device applications.
Growth and characterization of Cd1-x-yZnxMnyTe crystals
(1996-12-01)
Chou, WC
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Chen, FR
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Chiang, TY
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Shin, HY
;
Sun, CY
The Cd1-x-yZnxMnyTe crystals, having potential in the development of magnetic field sensitive devices, were grown by the temperature gradient solution method at 940 degrees C. All crystals were p-type with resistivity of 10(7) Omega . cm. They have novel magneto-optical properties, and could be substitutes for the Cd1-xZnxTe ternary compounds in certain optoelectronic/integrated-optical device applications.
High resolution and energy filtering TEM study of interfacial structure and reaction in advanced materials processing
(1998-04-10)
Chang, HL
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Chang, L
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Chen, FR
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Kai, JJ
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Tzou, E
High resolution TEM becomes more powerful when coupled with a Gatan imaging filler (GIF). It gives both structural and compositional information simultaneously with subnanometer and nanometer resolution. respectively. By means of these combined techniques, the failure mechanism of diffusion barrier layers in IC devices, having the structure of Al-1%Si-0.5%CuiTiNITi/Si substrate, was investigated. In non-oxygen stuffed samples thermally stressed at 550 degrees C for 1 h serious Al spikes below contacts were observed where metal layer Ti also reacted with Si substrate to form C49-TiSi2 and was consumed completely. TiN and Al become unstable due to high temperature thermal stress and reacted into TiAl3 and AlN. Decomposition of TiN degraded the function of the barrier layer, and so a diffusion path at the weakest point of the contact corner was opened for metal Al alloy to diffuse through the barrier layer and to form a spike below the TiN layer. We also showed that the oxygen-stuffing processes after each TiN/Ti deposition enhances the barrier capability against Al/Si diffusion in thermal stress. It was found that Ti and TiN were oxidized to form titanium oxide which reacted with the Al alloy. As a result, Al2O3 and TiAl3 were found above the TiN layer. Therefore, it is concluded that oxidation during oxygen-stuffing processes provides a means to inhibit diffusion in thermal stress processes, and maintains integrity of the stack structure. (C) 1998 Acta Metallurgica Inc.
HRTEM and EFTEM studies of the evolution of Cu/Ta/SiO2/Si interfaces in ULSI devices
(1999)
Yin, KM
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Chang, L
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Chen, FR
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Kai, JJ
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Chuang, CCCG
The Cu/Ta/SiO2/Si films have been annealed at temperatures from 500 degreesC to 600 degreesC in various vacuum conditions. Transmission electron microscopy has been used to characterize the microstructure of the films after annealing. It shows that different thicknesses of amorphous interlayer were formed between Cu and Ta under various vacuum conditions. Energy dispersive spectroscopy confirmed this interlayer to be tantalum oxide. It has also found that tantalum oxidation is caused by oxygen from the outside atmosphere diffusing along grain boundaries in copper films.
Improvement of resolution by maximum entropy linear image restoration for NiSi2/Si interface
(1999)
Chen, FR
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Kai, JJ
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Chang, L
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Wang, JY
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Chen, WJ
A linear imaging constrained-maximum entropy method has been developed to extend the resolution from a series of defocused high-resolution images and one diffraction pattern. Our method is basically the Gerchberg-Saxton algorithm, which restores spatial information by imposing real space and Fourier space constraints cyclically. A constrained-maximum entropy method (constrained-MEM) was developed for real space constraints. This constrained-MEM finds an optimum solution of phases such that simulated images resemble the experimental image under some constraints in real space. These constraints include conservation of charge as well as a minimum of the chi(2) function, which is a measure of the level of satisfaction between simulated images and experimental images. For the Fourier space constraint, the square root intensities of diffraction spots were substituted. In this paper, we demonstrate that this method is able to extend the resolution from a series of high-resolution images and one diffraction pattern from a periodic NiSi2/Si interface. The atomic structure of the NiSi2/Si interface can be directly read out from the MEM image without a priori knowledge of the interface.
Microstructural investigation of oxidized Ni/Au ohmic contact to p-type GaN
(1999-01-01)
Chen, LC
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Chen, FR
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Kai, JJ
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Chang, L
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Ho, JK
The phase evolution of oxidized (10nm) Ni/(5nm) Au films on p-GaN was examined with a field emission gun transmission electron microscope in conjunction with composition analyses to explore the mechanism of formation of low resistance ohmic contact to p-GaN. The p-GaN/Ni/Au sample heat treated above 500 degreesC in air was mainly composed of a mixture of crystalline NiO, Au and amorphous Ni-Ga-O phases. Small voids adjacent to the p-GaN film were also found. Moreover, NiO partially contacts to p-GaN as well as Au islands and the amorphous Ni-Ga-O phases. The results suggest that the crystalline NiO and/or amorphous Ni-Ga-O phases may play a significant role in the formation of a low resistance ohmic contact to p-GaN.
Microstructural investigation of oxidized Ni/Au ohmic contact to p-type GaN
(1999)
Chen, LC
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Chen, FR
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Kai, JJ
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Chang, L
;
Ho, JK
The microstructure of oxidized Ni/Au films on p-GaN was examined to elucidate the formation of a low resistance ohmic contact to p-GaN with a field-emission gun transmission electron microscope in conjunction with composition analyses. The p-GaN/Ni/Au samples were heat treated at 500 degrees C in air mainly composed of a mixture of crystalline NiO, Au, and amorphous Ni-Ga-O phases. Small voids adjacent to the p-GaN film were also observed. The as-deposited Au film converted into discontinuous islands containing small amounts of Ni that connect with p-GaN. NiO formed a continuous film at the surface that covers the Au islands and the amorphous Ni-Ga-O phases. Moreover, NiO partially contacts p-GaN as well as Au islands and the amorphous Ni-Ga-O phase. The orientation relationship of the crystalline NiO, Au-rich islands, and p-GaN film was identified as NiO(111)//Au(11 (1) over bar)//GaN(0002) and NiO[1 (1) over bar 0]//Au[1 (1) over bar 0]//GaN[11 (2) over bar 0]. The results suggested that Ni atoms diffuse through the Au layer onto the surface and react with oxygen to form NiO, whereas Au atoms diffuse towards the inside to form a Au-Ni alloy. The microstructural examination indicated that the crystalline NiO and/or the amorphous Ni-Ga-O phases may significantly affect the low resistance ohmic contact to p-GaN. (C) 1999 American Institute of Physics. [S0021-8979(99)05219-6].
Observations of Al segregation around dislocations in AlGaN
(2001-08-13)
Chang, L
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Lai, SK
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Chen, FR
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Kai, JJ
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材料科學與工程學系
Transmission electron microscopy has been used to observe Al segregation around the threading dislocations in Al0.1Ga0.9N and Al0.3Ga0.7N grown by metalorganic chemical vapor deposition on 6H-SiC. Dislocation lines were found to have up to 70% more Al concentration than those regions free of dislocations in the matrix. The Al-depleted regions around the dislocations are shown to be within a few nanometers from the dislocation lines. The results also show that more Al segregate to edge dislocations than to screw ones. (C) 2001 American Institute of Physics.
Observations of segregation of Al in AlGaN alloys
(2001-11-23)
Chang, L
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Lai, SK
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Chen, FR
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Kai, JJ
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材料科學與工程學系
Transmission electron microscopy has been used to characterize Al segregation in Al0.1Ga0.9N and Al0.3Ga0.7N alloys grown by metal organic chemical vapor deposition on 6H-SiC. It has been found that an interlayer of AlGaN alloy with much higher Al content was formed at first, followed by normal growth of nominal composition of AlGaN alloy. In Al0.1Ga0.9N and Al0.3Ga0.7N films. dislocation lines were also found to have more Al segregated than those regions free of dislocations in the matrix. Furthermore, it shows that more Al atoms segregate to an edge dislocation than to a screw one.
Orbital susceptibilities of PbSe quantum dots
(2006-02-14)
Jian, WB
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Lu, WG
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Fang, JY
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Chiang, SJ
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Lan, MD
Different sizes of three-dimensional PbSe quantum dots have been synthesized for the study of orbital magnetic susceptibilities. Two types of orbital susceptibilities have been found, including the Curie susceptibility and finite-size corrections to the Landau susceptibility. The Curie term of a quantum dot manifests itself in the temperature dependence of magnetic susceptibility at low temperatures, while the field dependence of differential susceptibility at high temperatures shows finite-size corrections to the Landau susceptibility. Both of the two kinds of orbital susceptibility, estimated per quantum dot, show linear dependence on the size. (c) 2006 American Institute of Physics.
Oxidation of Ta diffusion barrier layer for Cu metallization in thermal annealing
(2001-06-01)
Yin, KM
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Chang, L
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Chen, FR
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Kai, JJ
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Chiang, CC
This work examines the thermal stability of Ta barrier layer for Cu metallization with the effect of oxygen. The Cu/Ta/SiO2/Si films were annealed at temperatures ranging from 400 to 600 degreesC under various vacuum conditions. Transmission electron microscopy has been performed to characterize the microstructure of the films after annealing. The results show that an amorphous interlayer of oxide between Cu and Ta can be formed at 400 degreesC in a vacuum of 10(-2) mbar. X-Ray energy dispersive spectroscopy and electron energy loss spectroscopy confirm that this interlayer is tantalum oxide. This interlayer transformed into a crystalline phase of Ta-Cu oxide at 600 degreesC. In addition, formation of tantalum oxide interlayer is more thermodynamically favorable than that of copper oxide layer at the Cu/Ta interface. Growth of the amorphous interlayer is atmosphere-dependent, as evidenced by the thickness of tantalum oxide being decreased with better vacuum or argon gas. This observation suggests that the oxidation source may arise from the annealing atmosphere rather than from interior SiO2. Furthermore, it has been observed that oxygen diffuses along grain boundaries in copper films to cause tantalum oxidation. (C) 2001 Elsevier Science B.V. All rights reserved.
The effect of heat treatment on Ni/Au ohmic contacts to p-type GaN
(1999-11-16)
Chen, LC
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Ho, JK
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Chen, FR
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Kai, JJ
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Chang, L
The effect of heat treatment temperature on the microstructure and specific contact resistance of oxidized Ni(5 nm)/Au(5 nm) contacts to p-type GaN was investigated.,The: minimum specific contact resistance (Q(c)) obtained was 4 x 10(-6) Omega cm(2) after heat treating at 500 degrees C in air for 10 min. The cross-sectional microstructure of heat treated Ni/Au films on p-type GaN was examined with transmission electron microscope (TEM) in conjunction with compositional analyses. The high value of Q(c) for samples heat treated at lower temperatures (< 400 degrees C) was attributed to the fact that Au islands and crystalline NiO detached from the p-type GaN. When the temperature increased to 500 degrees C, NiO films and Au islands epitaxially constructed on p-type GaN matrix. The crystalline NiO may play a crucial role in the formation of low-resistance ohmic contact to p-GaN. Increasing the temperature further to 600 degrees C, large voids adjacent to p-GaN were observed which resulted in the reduction of contacting area deteriorating the contact resistance.
The effect of oxygen in the annealing ambient on interfacial reactions of Cu/Ta/Si multilayers
(2001-06-01)
Yin, KM
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Chang, L
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Chen, FR
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Kai, JJ
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Chiang, CC
Interfacial reactions of Cu/Ta/Si multilayers after thermal treatment were investigated using transmission electron microscopy. The Cu and Ta films were deposited onto Si wafer by ionized metal plasma technique. The samples were then annealed at 400, 500, 550 and 600 degreesC in purified Ar atmosphere for 30 min. The effect of oxygen in the atmosphere on the thermal stability is studied. An interlayer of Ta oxide was observed between Cu and Ta after annealing at 400, 500 and 550 degreesC. It is evident that oxygen as residual gas from furnace ambient can diffuse through Cu grain boundaries to form the Ta oxide layer. After annealing at 600 degreesC, Si reacted with Ta to form TaSi2 at the interface of Ta and Si, in the meantime Cu3Si with surrounding SiO2 formed in the Si substrate. The thermal stability of the Cu/Ta/Si samples was also examined in a two-step annealing treatment of 400 degreesC for 30 min, followed by 600 degreesC for 30 min. Even though interlayers of crystalline Ta-Cu oxide and Ta silicide were formed, Cu silicides were not observed. Formation of TaOx interlayer at the first stage of 400 degreesC annealing may inhibit Cu diffusion into the Si substrate in the second stage of the 600 degreesC annealing process. (C) 2001 Elsevier Science B.V. All rights reserved.