bluuebunny/crossref_metadata_2025 huggingface dataset:
InGaP/GaAs/InGaP composite collector double heterojunction bipolar transistor with high breakdown, low offset, and knee voltage
Authors: Lew K. L. , Zhang R. , Yoon S. F.
Published: March 2004

A InGaP/GaAs/InGaP composite collector double heterojunction bipolar transistor (DHBT) has been designed and fabricated to combine the advantages of single heterojunction bipolar transistor (SHBT) and conventional DHBT. The device has low collector–emitter offset voltage (∼60 mV) and knee voltage (<1 V). The breakdown voltage of ∼14 V is as high as that of a conventional DHBT. Comparison was made with a GaAs homojunction bipolar transistor and InGaP/GaAs SHBT. Both homojunction transistor and composite collector DHBT show similar collector–emitter offset voltage dominated by the geometrical factor. The knee voltage of the composite collector DHBT is ∼0.15 V lower than that of the InGaP/GaAs SHBT. The results demonstrate the potential of composite collector DHBTs for practical power amplifier applications.
Effect of the cyclic delivery of (hexafluoroacetylacetonate)Cu(I) ×(3,3-dimethyl-1-butene) pulse and Ar purge gas on the low temperature copper metalorganic chemical vapor deposition
Authors: Kim Kwansoo , Tak Youngjo , Yong Kijung
Published: March 2004

Cu thin films were deposited on the sputter-deposited Ta/Si substrate using cycles of alternate supply of (hexafluoroacetylacetonate)Cu(I)(3,3-dimethyl-1-butene) pulse and argon purge gas. The growth temperature was as low as 70 °C and 200–1000 cycles were performed to deposit films in pulsed-metalorganic chemical vapor deposition (MOCVD) with a growth rate of ∼0.75 Å/cycle. Cross-sectional scanning electron microscopy of the pulsed-MOCVD Cu films showed continuous and uniform films, while the MOCVD Cu films showed void formations. Also, x-ray diffraction patterns of the Cu films showed the preferential crystallographic orientation of the (111) plane. The Cu films grown by MOCVD and pulsed MOCVD, both contained oxygen (O), carbon (C), and fluorine (F) as impurities. The impurities of C and F atoms were hard to detect in the bulk films and only the O atom was detected in both films. The O concentration in pulsed-MOCVD films was much lower than that in MOCVD films with a concentration ratio below 1/3.
Silicon–oxide etching process employing an electron-beam-excited plasma
Authors: Ito Masafumi , Takeda Keigo , Shiina Tatsuo , Okamura Yasuyuki , Nagai Hisao , Hori Masaru , Goto Toshio
Published: March 2004

Silicon–oxide (SiO2) etching employing an electron-beam-excited plasma has been investigated. SiO2 films were etched using self-bias induced by the electron beam for the generating plasma. As a source gas, the CF4 diluted by Ar (CF4/Ar) was employed. The etch rate of 117 nm/min has been obtained at a total pressure of 0.18 Pa and a total flow rate of 43 sccm with a CF4/Ar flow rate ratio of 13%. The etching was carried out at an electron-beam current of 9.5 A and an electron-acceleration voltage of 100 V without any additional bias power supply. The CF4/Ar plasma diagnostics was carried out by using a Langmuir probe and optical-emission spectroscopy. It has been found that the higher electron-beam current for generating plasmas improves the plasma density and sheath potential, resulting in a higher etch rate of SiO2. The results indicate that the electron-beam-excited plasma has a potential for application to micromachining processes.
Application of a segmentation algorithm to quantum dots study
Authors: Belardinelli P. , Capoleoni S. , Tirozzi B. , Coluzza C.
Published: March 2004

A variation of the neural algorithm locally excitatory globally inhibitory network has been developed for automatic recognition, count, and morphological analysis of a large number of quantum dots. In a typical image of 256×256 pixels (500×500 nm2), the program recognizes the dots with an average error of about 3%. It also calculates area and height of each of them. Furthermore, the recognition procedure is very fast (less than 2–3 s on a Pentium III at 600 MHz).
Effects of fluorocarbon gas species on electrical conductivity and chemical structure of deposited polymer in SiO2 etching processes
Authors: Shimmura T. , Soda S. , Samukawa S. , Koyanagi M. , Hane K.
Published: March 2004

High-aspect-ratio SiO2 contact-hole etching is one of the key processes in the fabrication of ultralarge-scale integrated devices. However, there are many serious problems, such as charge-buildup damage, etching-stop, and microloading effects. Charge accumulation in high-aspect-ratio contact holes during etching is one of the main causes of these problems. In SiO2 etching using fluorocarbon gases, it is well known that fluorocarbon film is deposited on the underlayer surface and sidewall of contact holes. It is expected that such deposited fluorocarbon polymer will exert a great influence on the etching characteristics and charge accumulation in SiO2 contact holes. Therefore, it is necessary to measure the conductivity of the sidewall surfaces of contact holes with deposited fluorocarbon polymer. We made a monitoring device on a silicon wafer to evaluate the sidewall current of SiO2 contact holes and determined the relationship between the chemical structure and electrical conductivity of the fluorocarbon films deposited in the contact holes as a function of fluorocarbon gases and incident ion flux. We found that the electrical conductivity of the sidewall surface in SiO2 contact holes depends on the chemical structure of the deposited fluorocarbon polymer. It was also clear that the chemical structure of the deposited fluorocarbon polymer depended on nature of the radical species and ion flux incident on the etching surface. These results indicate that by controlling the chemical structure of the deposited fluorocarbon polymer one may be able to mitigate the influence of charge accumulation.
Activation improvement of ion implanted boron in silicon through fluorine co-implantation
Authors: Shauly Eitan N. , Lachman-Shalem Sivan
Published: March 2004

In this study, boron diffusion and activation characteristics of samples implanted with F co-implantation were studied to meet the challenge of lower sheet resistance. Samples were implanted with F co-implantation in a dose range of 0 (no F) to 5×1015 cm−2, at a fixed energy of 25 keV, followed by 950 °C/10 s rapid thermal annealing. It was found that although the fluorine has a negligible affect on the boron diffusion at the specified conditions, a higher F dose reduced the boron sheet resistance. Using reverse modeling, the boron solid solubility at 950 °C was extracted as function of the F co-implant concentration. For low fluorine doses (0–1×1014 cm−2), the boron solid solubility is similar to that reported in the literature (9×1019 cm−3). At higher doses, boron solid solubility increased by 25% and even 50% for F co-implantation doses of 1×1015 and 5×1015, respectively. We suggest that the F co-implantation terminates some of the defects created by the implantation, inactivating the defects and improving boron activation.
High quality ion-induced secondary electron imaging for MeV nuclear microprobe applications
Authors: Teo E. J. , Breese M. B. H. , Bettiol A. A. , Watt F. , Alves L. C.
Published: March 2004

The image quality of ion-induced secondary electron images generated with a MeV nuclear microprobe has previously been severely limited by the large beam current fluctuations produced by Van de Graaff accelerators. In this article we report the use of a solid state Cockroft–Walton type accelerator (HVEE™ Singletron) to produce high quality secondary electron images in a rapid manner. This has been achieved using a voltage-modulated dc signal imaging process rather than the inefficient pulse counting mode commonly used in nuclear microprobe applications. This is made possible because of the superior voltage stability of the Singletron accelerator resulting in higher ion beam current stability. Excellent topographical contrast has been demonstrated with the newly implemented secondary electron system.
Advantages of energetic cluster evaporation for organic light emitting devices
Authors: Jeong Dong-Seob , Hwang D. K. , Im Seongil
Published: March 2004

Organic light emitting devices (OLEDs) using Alq3 have been fabricated by employing two techniques: energetic cluster evaporation and normal thermal evaporation. Atomic force microscopy taken on the Alq3 film surfaces obtained from both techniques revealed that using energetic clusters was advantageous over normal thermal evaporation in terms of surface roughness particularly when the film grows thicker than 50 nm. OLEDs with the critical thickness of 50 nm, prepared by energetic cluster evaporation, thus showed much better performances in both current–voltage (I–V) and luminance–voltage (L–V) curves than devices prepared by thermal evaporation. The lifetime of the energetic-cluster-prepared OLED was also found to be longer than those of devices prepared by thermal evaporation.
Influence of residual solvent in polymers patterned by nanoimprint lithography
Authors: Gourgon C. , Tortai J. H. , Lazzarino F. , Perret C. , Micouin G. , Joubert O. , Landis S.
Published: March 2004

In nanoimprint lithography, one of the key points to be addressed is the printing uniformity. The limitation of the printing temperature is also of great importance to develop this technology for industrial applications. The thermal behavior of a commercial resist is characterized and related to the printing performance of the polymer. It is shown that the plasticizing effect of the solvent contained in the commercial resist decreases the glass transition temperature of the polymer (Tg) and favors a deeper penetration of the mold into the polymer at a specified printing temperature. We also demonstrate that the control of the residual solvent in the polymer can favor a better pattern uniformity and a decrease of the printing temperature.
Reduction of stacking faults in the ZnSe/GaAs heterostructure with a low-temperature-grown ZnSe buffer layer
Authors: Song J. S. , Oh D. C. , Makino H. , Hanada T. , Cho M. W. , Yao T. , Park Y.-G. , Shindo D. , Chang J. H.
Published: March 2004

We have investigated the effect of a thin low-temperature-grown ZnSe (LT-ZnSe) buffer layer in improving ZnSe crystallinity by inserting it between the high-temperature-grown ZnSe epilayer and the GaAs substrate. In particular, the density of stacking fault defects, which is normally observed in ZnSe-based films grown on GaAs substrate, can be well reduced by inserting a thin LT-ZnSe buffer layer. A ZnSe film with stacking fault densities as low as ∼5.4×104/cm2 was obtained by growing on Zn exposed (2×4) As-stabilized surfaces of GaAs buffer layer with LT-ZnSe buffer, in contrast, ∼7×108/cm2 was obtained by directly growing on GaAs substrate, that is, without Zn exposure, LT-ZnSe, and GaAs buffer layers.
An inlaid electroplated copper coil for on-chip powering of microelectromechanical systems devices
Authors: Wu Jie , Bernstein Gary H.
Published: March 2004

Inductive powering is promising for freeing microsystems incorporating microelectromechanical systems devices from wire tethering. Planar integrated coils used to intercept electromagnetic energy typically have large internal resistance, and hence low power transfer efficiency. We report a fabrication procedure developed to electroplate metal microstructures of large dimensions inlaid into silicon substrates. With a modified Bosch process, silicon substrates can be used to form electroplating molds of high lateral aspect ratio, so metal microstructures with thicknesses comparable to that from the LIGA process can be realized. In this work, copper coils of centimeter side length and several tens of micrometers thick are fabricated, and inlaid coils can be potentially integrated with circuits. The silicon molds can also be selectively removed to realize stand-alone metal structures. Three types of integrated coils are fabricated to compare their performance as a part of inductive link for power delivery. Power transfer efficiency is greatly improved by increased copper thickness, and is not noticeably affected by coil parasitic capacitance from silicon sidewalls. Also, higher voltage amplification at resonance was obtained with an electroplated coil.
Two-step surface treatment technique: Realization of nonalloyed low-resistance Ti/Al/Ti/Au ohmic contact to n-GaN
Authors: Motayed Abhishek , Jah Muzar , Sharma Ashok , Anderson Wallace T. , Litton Cole W. , Mohammad S. Noor
Published: March 2004

A novel two-step surface treatment method has been developed to realize low resistance nonalloyed ohmic contact to n-type GaN doped with Si to 6×1017 cm−3. The removal of native oxide (oxides and hydroxides) formed on GaN surface is crucial for successful creation of nonalloyed low resistance ohmic contact. In the case of GaN, plasma etching of the material surface prior to metal deposition holds promise for developing such nonalloyed ohmic contacts. In this article, the effects of the postetch chemical treatment of the n-type GaN surface on the Ti based nonalloyed contact performance have been investigated. Contacts on samples without reactive ion etching (RIE) showed Schottky behavior. However, contacts on samples with 15 s of RIE using Cl2 showed ohmic behavior. The contact resistivity of this contact reached to ρs=1.2×10−3 Ω cm2. Treating the RIE etched sample in boiling aqua regia for 5 min yielded a contact resistivity on the order of 3.6×10−4 Ω cm2. Dramatic improvement in current–voltage characteristics was observed after boiling RIE etched samples in KOH for 5 min. It yielded contact resistivity on the order of ρs∼7×10−5 Ω cm2, which is comparable to the contact resistivity of alloyed contacts. It was demonstrated, for the first time, that chemical treatment of postetched GaN surfaces employing KOH is very promising for new generation of nonalloyed ohmic contacts. This study suggested that the reduction of contact resistivity results from the effective removal of surface oxides and hydroxides, which act as an impeding barrier for electron transport.
Materials growth for InAs high electron mobility transistors and circuits
Authors: Bennett Brian R. , Tinkham Brad P. , Boos J. Brad , Lange Michael D. , Tsai Roger
Published: March 2004

High electron mobility transistors (HEMTs) with InAs channels and antimonide barriers were grown by molecular beam epitaxy. Both Si and Te were successfully employed as n-type dopants. Sheet resistances of 90–150 Ω/□ were routinely achieved on a variety of heterostructures with nonuniformities as low as 1.5% across a 75 mm wafer. X-ray diffraction measurements show that the InAs channels are in tension, coherently strained to the Al(Ga)Sb buffer layers. Atomic force microscopy measurements demonstrate that the surfaces are relatively smooth, with rms roughness of 8–26 Å over a 5×5 μm2 area. These results demonstrate that the growth of InAs HEMTs has progressed to the point that the fabrication of circuits should be feasible.
Comparison of Ir and Ni-based Ohmic contacts for AlGaN/GaN high electron mobility transistors
Authors: Fitch R. C. , Gillespie J. K. , Moser N. , Jessen G. , Jenkins T. , Dettmer R. , Via D. , Crespo A. , Dabiran A. M. , Chow P. P. , Osinsky A. , La Roche J. R. , Ren F. , Pearton S. J.
Published: March 2004

A direct comparison of Ti/Al/Ir/Au Ohmic contacts on AlGaN/GaN high electron mobility transistors with the more conventional Ti/Al/Ni/Au metallization is reported. The Ir-based contacts produce lower specific contact resistance (4.6×10−5 Ω cm2 compared to 2×10−4Ω cm2) after annealing at 850 °C for 30 s, lower interdevice isolation currents, higher peak transconductance (133 mS/mm compared to 121 mS/mm), and higher device breakdown voltage (31 compared to 23 V) than the Ni-based contacts fabricated on the same wafer. The results are based on mapping of a large number of high electron mobility transistors (∼250) on a 2-in.-diam wafer and show strong evidence that a relatively simple change in Ohmic metallurgy provides benefits in device dc and rf performance.
Effect of dislocations on luminescence properties of silicon-doped GaN grown by metalorganic chemical vapor deposition method
Authors: Alam Jahangir , Bathe Ravi , Vispute R. D. , Zavada John M. , Litton Cole W. , Iliadis Agis A. , Mohammad S. Noor
Published: March 2004

The effect of dislocations on the structural and electro-optic characteristics of n-GaN has been studied. X-ray diffraction, Hall measurement, photoluminescence spectroscopy, Raman spectroscopy, and transmission electron microscopy (TEM) have been performed to understand the interdependence of strain, dislocation, and doping concentration. The most remarkable observation of the study is the blueshift observed at a doping level ND∼1018 cm−3 as a result of the relaxation of strain. The TEM results reveal a higher dislocation defect density at lower doping levels (∼1017 cm−3) than at moderate doping levels. Blueshift is found to result from the redistribution of the dislocation density due to the reduction in strain at a certain optimum doping level(s). This distribution of the dislocation is such that there are virtually no dislocations at some locations, as evident from the sample MD27 with doping ⩾1018 cm−3.
High conversion efficiency mass-limited Sn-based laser plasma source for extreme ultraviolet lithography
Authors: Richardson M. , Koay C.-S. , Takenoshita K. , Keyser C. , Al-Rabban M.
Published: March 2004

Extreme ultraviolet lithography requires a high-efficiency light source at 13 nm that is free from debris. Our mass-limited Sn-based laser plasma source shows 1.2% conversion efficiency. Emission spectra from the source were obtained to observe the effects of Sn concentration and effects of laser intensity. Debris measurements were analyzed, and an enhanced repeller field configuration shows marked improvement in mitigating debris.
Use of the focused ion beam technique to produce a sharp spherical diamond indenter for sub-10 nm nanoindentation measurements
Authors: Yu Ning , Polycarpou Andreas A.
Published: March 2004

Very sharp and well-defined rigid diamond indenters are critical for accurate sub-10 nm nanoindentation mechanical property measurements. The focused ion beam milling technique is adopted to create such a very sharp diamond indenter. With ion milling patterning, a spherical well-defined super tip is obtained at the apex of the indenter, with a radius of 53.4 nm. This sharpened indenter is applied to sub-10 nm nanoindentation measurements on fused quartz and silicon (100) samples. The elastic moduli measured at sub-10 nm depths are consistent with literature bulk values. In addition, the measured hardness values are qualitatively consistent with indentation experiments using conventional larger radii indenters. Finally, due to the small size of the indenter tip, the transition from elastic to plastic contact occurs at very shallow contact depths.
Chemical-mechanical photoresist drying in supercritical carbon dioxide with hydrocarbon surfactants
Authors: Zhang Xiaogang , Pham Joseph Q. , Ryza Nick , Green Peter F. , Johnston Keith P.
Published: March 2004

Supercritical CO2 containing hydrocarbon surfactants was utilized to remove water from photoresists without pattern collapse due to capillary forces. The addition of highly branched hydrocarbon surfactants to CO2 reduces the amount of solvent required for drying, and lowers the interfacial tension from 20 mN/m to approximately 1 mN/m, thus reducing pattern collapse. The drying time and amount of solvent were further reduced in a chemical/mechanical method, in which solvent, drained by gravity forces, is removed by hydrodynamic forces resulting from flowing CO2. In situ ellipsometry measurements show that the resist films swollen with CO2 return to their original thickness upon depressurization, indicating CO2 is compatible with these resists.
Facet evolution in selective epitaxial growth of Si by cold-wall ultrahigh vacuum chemical vapor deposition
Authors: Lim Seung-Hyun , Song Sukchan , Lee Gun-Do , Yoon Euijoon , Lee Jong-Ho
Published: March 2004

Si epitaxial layers were selectively grown on local-oxidation-of-silicon-patterned Si (100) substrates by cold-wall ultrahigh vacuum chemical vapor deposition. The Si windows were aligned along the [110] direction on Si (100) surface. As growth temperature increased from 550 to 650 °C, the development of (111) facets was dramatically suppressed, and the Si growth on sidewall facet planes was decreased. It is believed that surface diffusion of Si adatoms plays an important role in the morphological evolution of selective epitaxial growth (SEG). We propose a model to explain our experimental observations, and to clarify the effect of growth temperature on the facet morphology in terms of the surface mass transport and mass accumulation processes on facet surfaces. (211) facet formation between (311) and (111) facets in Si SEG is reported, and the stability of the (211) plane is also discussed. Finally, we investigated the changes in facet morphology with Si layer thickness, which supports our model for the facet evolution observed in Si SEG.
Texture and electromigration performance in damascene interconnects formed by reflow sputtered Cu film
Authors: Abe Kazuhide , Harada Yusuke , Yoshimaru Masaki , Onoda Hiroshi
Published: March 2004

We have investigated the Cu texture in damascene interconnect structures formed by the reflow method of sputtered Cu film and electromigration (EM) performance in Cu damascene interconnects. The texture of reflowed Cu film depends on Cu deposition temperature and line width. Improved 〈111〉 texture in blanket films is obtained when Cu is deposited at temperatures over 200 °C. However, Cu films form voids in the trench patterns at deposition temperatures of 350 °C or more, and the voids cannot be filled even after the reflow process. Thus, it is necessary to choose the optimum deposition temperature to fabricate void-free Cu damascene interconnects. It was found that the Cu (111) peak intensity in reflowed Cu lines measured by using x-ray diffraction decreases as trench width decreases. This can be explained by aspect ratio changes of Cu lines in trenches, which affect the dominating Cu texture components originating from sidewall or bottom nucleations. Underlayer metals at sidewalls affect the Cu texture. In damascene interconnects, Cu 〈111〉 orientation normal to the substrate surface degrades in the edge regions near the sidewalls more than it does in the center region of the trenches. Even in these damascene structures, highly reliable Cu interconnects can be obtained by using an rapid-thermal-nitrided TiN (RTN-TiN) underlayer with a strong 〈111〉 orientation in both wide polycrystalline Cu lines and narrow bamboo-like Cu lines. An RTN-TiN underlayer decreases the void growth rate in Cu interconnects during EM testing. The improved EM lifetime can be explained mainly by the reduced Cu migration along the Cu interface and the underlayer metal. The Cu migration along the RTN-TiN underlayer progresses more slowly than it does along a reactively sputtered TiN underlayer, because the strong crystallographic bonding between Cu and the RTN-TiN underlayer decreases Cu migration. In wide polycrystalline lines, the difference in Cu migration at grain boundaries also affects the EM lifetime. In this case, the use of an RTN-TiN underlayer also leads to a strong Cu 〈111〉 texture with low-angle grain boundaries and reduces Cu migration along grain boundaries.
Two-dimensional electron gas formation in undoped In0.75Ga0.25As/In0.75Al0.25As quantum wells
Authors: Capotondi F. , Biasiol G. , Vobornik I. , Sorba L. , Giazotto F. , Cavallini A. , Fraboni B.
Published: March 2004

We report on the achievement of a two-dimensional electron gas in completely undoped In0.75Al0.25As/In0.75Ga0.25As metamorphic quantum wells. Using these structures we were able to reduce the carrier density, with respect to reported values in similar modulation-doped structures, to about 2–3×1011 cm−2 with mobilities of up to 2.15×105 cm2 (V s)−1. We found experimentally that the electronic charge in the quantum well is likely due to a deep-level donor state in the In0.75Al0.25As barrier band gap, whose energy lies within the In0.75Ga0.25As/In0.75Al0.25As conduction band discontinuity. This result is further confirmed through a Poisson–Schrödinger simulation of the two-dimensional electron gas structure.
Investigation of nanocrystal-(Ti,Al)N1−x/amorphous-SiNy composite films
Authors: Yau Bao-Shun , Huang Jow-Lay , Kan Ming-Chi , Lii Ding-Fwu
Published: March 2004

Nanocrystal-(Ti,Al)N1−x/amorphous-SiNy composite films were prepared by the magnetron sputtering codeposition process. The effects of deposition parameters on the composition, microstructure, and mechanical properties of nanocomposite films were investigated. Results indicated that the composition of nanocomposite films could be controlled through the codeposition process. Nanocrystal (Ti,Al)N1−x embedded in amorphous SiNy matrix with (200) preferred orientation was observed. The content of amorphous SiNy has substantial influence on the nanocrystallite size, columnar structure, surface morphology, and hardness.
Study of InGaP/InGaAs double delta-doped channel heterostructure field-effect transistors
Authors: Chuang Hung-Ming , Cheng Shiou-Ying , Chen Chun-Yuan , Liao Xin-Da , Lai Po-Hsien , Kao Chung-I , Liu Wen-Chau
Published: March 2004

The device properties of InGaP/InGaAs double delta-doped channel heterostructure field-effect transistors (DDDCHFETs) are comprehensively analyzed and demonstrated. Based on the variations of delta-doped densities of InGaAs double channels and GaAs spacer thickness, the dc and rf characteristics are compared and studied. Due to the employed InGaAs DDDC structure and Schottky behaviors of InGaP “insulator,” good pinch-off and saturation characteristics, higher and linear transconductance, and good rf performances are obtained. Experimentally, for comparison, a practical DDDCHFET with good device properties is fabricated successfully. It is known that the experimental results are very consistent with theoretical simulation data.
Stability and composition of Ni–germanosilicided Si1−xGex films
Authors: Pey K. L. , Chattopadhyay S. , Choi W. K. , Miron Y. , Fitzgerald E. A. , Antoniadis D. A. , Osipowicz T.
Published: March 2004

The stability and composition of the Ni–germanosilicided films formed on relaxed Si1−xGex alloy has been studied in the temperature range of 400–900 °C. During the solid phase thermal reaction between Ni and Si1−xGex, a nickel–germanosilicide Niy(Si1−wGew)1−y ternary phase (w⩽x and y≈0.5) and a Ge-rich Si1−zGez phase (z>x) have been found. In the lower annealing temperature range of 500 °C, the Ge composition in the nickel–germanosilicide phase is similar to that of the Si0.75Ge0.25 substrate. At the same time, germination of Si1−zGez (z>x) takes place within the germanosilicide film. At higher annealing temperatures, Ni thermodynamically prefers to react with Si compared to Ge, and as a result, Ge segregates out from the germanosilicide grains to enrich Ge in the formed Si1−zGez (z>x) grains in between the germanosilicide grains. On the other hand, the size of the germanosilicide grains increases almost linearly with annealing temperature while that for the Si1−zGez grains remains almost constant up to an annealing temperature of 700 °C, and above which it increases sharply. As a result, the Ge-rich Si1−zGez grains make the germanosilicide film discontinuous, leading to an increase in the sheet resistance of the germanosilicide film.
Fabrication of two-dimensional InP-based photonic crystals by chlorine based chemically assisted ion beam etching
Authors: Mulot M. , Anand S. , Ferrini R. , Wild B. , Houdré R. , Moosburger J. , Forchel A.
Published: March 2004

Two-dimensional photonic crystals (PhCs) were etched into InP/(Ga,In)(As,P) planar waveguides using chlorine-based chemical assisted ion beam etching (CAIBE). The processed PhCs were optically characterized by measuring transmission through simple slabs and one-dimensional cavities. The optical performances inside the photonic band gap are much better compared to both previously reported CAIBE results and results obtained with other etching methods. In particular, we measured a record quality factor of 310 for one-dimensional cavities fabricated in this material system.
Study of direct current characteristics of carbon-doped GaInP/GaAs heterojunction bipolar transistor grown by solid source molecular beam epitaxy
Authors: Zhang R. , Lew K. L. , Yoon S. F. , Tan K. H. , Sun Z. Z.
Published: March 2004

This article reports the dc characteristics of a GaInP/GaAs heterojunction bipolar transistor (HBT) with carbon-doped GaAs base layer grown by solid source molecular beam epitaxy using carbon tetrabromide (CBr4) as p-type dopant precursor. Hydrofluoric acid (HF) was used to passivate the GaInP/GaAs HBTs. At base bias voltages below 0.9 V in the Gummel plot, the base current of large area devices after HF passivation, was greatly reduced indicating that the extrinsic base surface recombination current was significantly reduced. After HF passivation, detailed dc characterization of the device performance within the temperature range of 300–380 K was carried out, and the carrier transport properties were investigated. The base current and collector current ideality factors at 300 K were 1.12 and 1.01, respectively. This indicates that a space charge region recombination current is insignificant in the base. From the temperature dependent Gummel plot, the activation energies of the collector current and base current were obtained. For the collector current, the activation energy is 1.4 eV, which is close to the band gap of the GaAs base. This indicates that the collector current is determined by the drift-diffusion process. For the base current, the activation energy is also 1.4 eV, indicating that band-to-band recombination plays a dominant role in the base current. No trap-related recombination was observed in the base and collector currents, which further indicates the good base material quality in the HBT structure. The current gain versus collector current characteristics at different temperatures were also investigated and analyzed.
Three-dimensional micro- and nanostructuring by combination of nanoimprint and x-ray lithography
Authors: Tormen Massimo , Romanato Filippo , Altissimo Matteo , Businaro Luca , Candeloro Patrizio , Di Fabrizio Enzo M.
Published: March 2004

We present results on a lithographic approach that combines nanoimprint (NIL) and x-ray lithography (XRL) for fabricating unconventional three-dimensional (3D) polymer structures. The use of XRL for structuring a prepatterned resist by NIL gives rise to high-resolution high-aspect-ratio structures whose overall profile is enveloped by the original 3D imprinted profile. The technological potential of this method has been demonstrated by patterning several different types of structures with XRL on an hexagonal array of hemispheres previously obtained by nanoimprinting.
Temperature dependent characteristics of bulk GaN Schottky rectifiers on free-standing GaN substrates
Authors: Kang B. S. , Ren F. , Irokawa Y. , Baik K. W. , Pearton S. J. , Pan C.-C. , Chen G.-T. , Chyi J.-I. , Ko H.-J. , Lee H.-Y.
Published: March 2004

The performance of Schottky rectifiers fabricated with dielectric overlap edge termination on epitaxial layers grown on a free-standing GaN template is reported. The power figure-of-merit (VB)2/RON where VB is the reverse breakdown voltage and RON is the on-state resistance was 11.5 MW cm−2. The forward turn-on voltage was ∼3.5 V at 25 °C, with an on-state resistance of ∼5×10−3 Ω cm2. The reverse recovery time was ⩽50 ns in switching from forward bias to reverse bias. The reverse breakdown showed a temperature coefficient of −0.45 V/C.
Degradation of superhard nanocomposites by built-in impurities
Authors: Veprek S. , Männling H.-D. , Niederhofer A. , Ma D. , Mukherjee S.
Published: March 2004

Impurities such as oxygen and chlorine can strongly decrease the hardness of superhard nc-TiN/a-Si3N4 and similar nanocomposites when incorporated into the coatings during their deposition. It is shown that 1–1.5 at. % of oxygen causes a hardness decrease to about 30 GPa, as compared to 45–55 GPa for the pure material. This may explain some of the contradictory results found by other authors, particularly for coatings deposited by physical vapor deposition at relatively low nitrogen pressure, deposition temperature, and deposition rates.
Extreme ultraviolet lithography based nanofabrication using a bilevel photoresist
Authors: Talin A. A. , Cardinale G. F. , Wallow T. I. , Dentinger P. , Pathak S. , Chinn D. , Folk D. R.
Published: March 2004

We describe the use and characterization of a bilevel photoresist for extreme ultraviolet lithography (EUVL). The bilevel photoresist consists of a combination of a commercially available polydimethylglutarimide (PMGI) bottom layer and an experimental EUVL photoresist top (imaging) layer. We measure the sensitivity of PMGI to EUV exposure dose as a function of photoresist prebake temperature, and using this data, optimize a metal liftoff process. Reliable fabrication of 700 Å thick Au structures with sub-1000 Å critical dimensions is achieved, even without the use of a Au adhesion layer, such as Ti. Using the bilevel photoresist process, we fabricate an electrode array test structure, designed for electrical characterization of molecules and nanocrystals.
Gettering by Ba films in color cathode ray tubes
Authors: den Engelsen Daniel , Ferrario Bruno
Published: March 2004

Ba getters are applied to maintain high vacuum during the life of a cathode ray tube (CRT). In a color CRT the shadow mask is electron bombarded and electrons stimulated desorption of gas from the shadow mask (and other tube components) is the result. When the Ba film is covering the shadow mask, this desorption is largely prevented because of the effective gettering of the Ba film. We found that the consumption of Ba in the getter film during life in this case cannot be attributed only to the reaction between Ba and reactive residual gases, but also, to some extent, to a reaction between Ba and the oxide layer that covers the shadow mask. This conclusion could be drawn after analyzing the getter capacity of Ba films in 26 in. CPTs with H2 as a test gas.
Bandwidth of spectral resolution for two-formant synthetic vowels and two-tone complex signals
Authors: Xu Qiang , Jacewicz Ewa , Feth Lawrence L. , Krishnamurthy Ashok K.
Published: March 2004

Spectral integration refers to the summation of activity beyond the bandwidth of the peripheral auditory filter. Several experimental lines have sought to determine the bandwidth of this “supracritical” band phenomenon. This paper reports on two experiments which tested the limit on spectral integration in the same listeners. Experiment 1 verified the critical separation of 3.5 bark in two-formant synthetic vowels as advocated by the center-of-gravity (COG) hypothesis. According to the COG effect, two formants are integrated into a single perceived peak if their separation does not exceed approximately 3.5 bark. With several modifications to the methods of a classic COG matching task, the present listeners responded to changes in pitch in two-formant synthetic vowels, not estimating their phonetic quality. By changing the amplitude ratio of the formants, the frequency of the perceived peak was closer to that of the stronger formant. This COG effect disappeared with larger formant separation. In a second experiment, auditory spectral resolution bandwidths were measured for the same listeners using common-envelope, two-tone complex signals. Results showed that the limits of spectral averaging in two-formant vowels and two-tone spectral resolution bandwidth were related for two of the three listeners. The third failed to perform the discrimination task. For the two subjects who completed both tasks, the results suggest that the critical region in vowel task and the complex-tone discriminability estimates are linked to a common mechanism, i.e., to an auditory spectral resolving power. A signal-processing model is proposed to predict the COG effect in two-formant synthetic vowels. The model introduces two modifications to Hermansky’s [J. Acoust. Soc. Am. 87, 1738–1752 (1990)] perceptual linear predictive (PLP) model. The model predictions are generally compatible with the present experimental results and with the predictions of several earlier models accounting for the COG effect.
Effects of stimulus and noise rate variability on speech perception by younger and older adults
Authors: Gordon-Salant Sandra , Fitzgibbons Peter J.
Published: March 2004

The present experiments examine the effects of listener age and hearing sensitivity on the ability to understand temporally altered speech in quiet when the proportion of a sentence processed by time compression is varied. Additional conditions in noise investigate whether or not listeners are affected by alterations in the presentation rate of background speech babble, relative to the presentation rate of the target speech signal. Younger and older adults with normal hearing and with mild-to-moderate sensorineural hearing losses served as listeners. Speech stimuli included sentences, syntactic sets, and random-order words. Presentation rate was altered via time compression applied to the entire stimulus or to selected phrases within the stimulus. Older listeners performed more poorly than younger listeners in most conditions involving time compression, and their performance decreased progressively with the proportion of the stimulus that was processed with time compression. Older listeners also performed more poorly than younger listeners in all noise conditions, but both age groups demonstrated better performance in conditions incorporating a mismatch in the presentation rate between target signal and background babble compared to conditions with matched rates. The age effects in quiet are consistent with the generalized slowing hypothesis of aging. Performance patterns in noise tentatively support the notion that altered rates of speech signal and background babble may provide a cue to enhance auditory figure–ground perception by both younger and older listeners.
Extension of measurement surface in near-field acoustic holography
Authors: Sarkissian Angie
Published: March 2004

In near-field acoustic holography, when measurements are made over a limited surface, there has been recent interest in numerically enlarging the measurement surface tangentially. Current algorithms use iterative methods to extrapolate the field tangentially outward. An algorithm based on the method of superposition is applied here which may be used to either extrapolate the field to enlarge the measurement surface or to interpolate in case there is a “hole” in the measurement surface where data are not available.
Sub-bottom profiling using ocean ambient noise
Authors: Harrison Chris H.
Published: March 2004

Spectral factorization is shown to restore the phase of an incoherent layered sediment reflection coefficient so that its Fourier transform is the minimum phase impulse response at each angle. The method requires the reflection coefficient to be known over a range of frequencies and the grazing angles in question to be above critical. It is developed here in the context of another recently established technique for extracting the seabed’s plane wave reflection coefficient from ambient noise data measured on a moored or drifting vertical array (VLA). Thus it offers the possibility of sub-bottom profiling from a single platform with no sound source. Limitations of the phase restoration method are discussed and, using modeled data, comparisons are made between the “true” impulse response derived from the known complex reflection coefficient and the result of applying spectral factorization to the absolute value of the reflection coefficient. The method is also demonstrated on experimental reflection loss inferred from ambient noise measurements at three moored VLA sites and one VLA drift track in the Mediterranean Sea. Sub-bottom profiles (impulse response versus position) are shown for the drift track demonstrating that one can indeed survey with only a single directional receiver. The technique appears to perform well when compared with other profiling techniques and published results.
Modal expansions for sound propagation in the nocturnal boundary layer
Authors: Waxler Roger
Published: March 2004

A modal model is developed for the propagation of sound over an impedance ground plane in a stratified atmosphere which is downward refracting near the ground but upward refracting at high altitudes. The sound’s interaction with the ground is modeled by an impedance with both real and imaginary parts so that the ground is lossy as well as compliant. Such sound speed profiles are typical of the atmospheric boundary layer at night and, together with the ground impedance, have been used extensively to model ground to ground sound propagation in the nocturnal environment. Applications range from community noise and bioacoustics to meteorology. The downward refraction near the ground causes the propagation to be ducted, suggesting that the long range propagation is modal in nature. This duct is, however, leaky due to the upward refraction at high altitudes. The modal model presented here accounts for both the attenuation of sound by the ground as well as the leaky nature of the duct.
A theoretical model for a finite-size acoustic receiver
Authors: Neild Adrian , Hutchins David A.
Published: March 2004

A new formulation is presented for the prediction of the signals transmitted by a rectangular acoustic source, and detected by a finite-sized rectangular receiver. This problem is encountered in measurements where the receiver is of a finite size compared to the shortest wavelength emitted by the source. The paper examines this problem theoretically, and a new formulation is presented in which the geometrical aspects are considered in some detail. The result is a simplified approach, in which the overall impulse response can be obtained from a combination of certain contributions defined geometrically. Theoretical field predictions are compared to experimental measurements for ultrasonic signals radiated into air, to demonstrate the usefulness of the approach.
Acoustic backscattering form function of absorbing cylinder targets (L)
Authors: Mitri Farid G. , Fellah Z. E. A. , Chapelon J. Y.
Published: March 2004

A new expression of the backscattering form function |f∞(k0a,θ)| for cylindrical targets, suspended in an inviscid fluid in a plane incident sound field, is presented. The theory is modified to include the effects of absorption of shear and compressional waves in viscoelastic materials. The numerical results presented show how damping effect due to ultrasound absorption influences the cylinder’s material properties.
Discussions on “Radial vibrations of orthotropic laminated hollow spheres” [J. Acoust. Soc. Am. 113, 847–851 (2003)] (L)
Authors: Ding H. J. , Wang H. M.
Published: March 2004

With the assumption of radial vibration, there exists only a trivial solution for a general orthotropic hollow sphere and the nonzero solution can be obtained only for a spherically isotropic hollow sphere (a special case of the orthotropic hollow sphere) and the corresponding proofs are presented. So, the numerical results given by Stavsky and Greenberg [J. Acoust. Soc. Am. 113, 847–851 (2003)] for a general orthotropic hollow sphere are unreasonable.
Do tests for cochlear dead regions provide important information for fitting hearing aids? (L)
Authors: Summers Van
Published: March 2004

For listeners with cochlear hearing loss, cochlear damage may include “dead regions” with no functioning inner hair cells and/or associated neurons. Recent studies indicate that amplifying frequencies more than 1.7 times the edge frequency (1.7Fe) of a high-frequency dead region is unlikely to improve (and may reduce) speech scores [Vickers et al., J. Acoust. Soc. Am. 110, 1164–1175 (2001); Baer et al., J. Acoust. Soc. Am. 112, 1133–1144 (2002)]. These results were taken as evidence that tests to identify dead regions could improve hearing aid fitting. In the current study, practicing audiologists examined audiograms of listeners diagnosed as having high-frequency dead regions. The audiologists were given no specific information regarding dead regions for any individual, and were asked to base amplification decisions entirely on the audiograms. Most audiologists did not recommend amplification of frequencies with hearing losses exceeding 90 dB HL. Reexamination of speech results reported by Vickers et al. and Baer et al. indicated that limiting amplification based on audiograms alone (90-dB rule) or on specific testing for dead regions (1.7Fe rule) produced similar performance. Thus, testing for dead regions may not provide important information for hearing aid fitting that is not already available in the audiogram.
Three-dimensional ray tracing of laser ultrasound for weld penetration sensing
Authors: Mi Bao , Ume I. Charles
Published: March 2004

The time-of-flight (TOF) method is an ultrasonic nondestructive testing (NDT) technique. The TOF of an ultrasonic wave can be correlated to weld penetration depth, and hence weld quality. Changes in material properties due to temperature gradients will cause ultrasonic speed to vary during welding, which causes a curved propagation path. A ray tracing algorithm is required in order to study how ultrasound propagates within a weld sample. In this paper, a three-dimensional (3-D) ray tracing algorithm based on Fermat’s principle is presented. First, ray equations are derived using the calculus of variation. Then, a numerical algorithm is developed to solve the derived ray equations and obtain the curved propagation path. This algorithm includes finite element analysis (FEA) to obtain the transient temperature distribution during the welding and shooting method to solve the boundary value problem. After the curved ray path is obtained, the TOF can be found by integrating the time variable along the ray path. An analytical relationship between the TOF and penetration depth can be established by repeating the ray tracing algorithm for different penetration depths. Experimental measurements of TOF have been performed, and this data is to be used to validate the numerical results.
Acoustic remote sensing of swimbladder orientation and species mix in the oreo population on the Chatham Rise
Authors: Coombs Roger F. , Barr Richard
Published: March 2004

A method for combining in situ measurements and theoretical swimbladder-derived estimates of target strength of the deep-water fish, black and smooth oreos, is described. The technique uses Monte Carlo simulation and yields fish length–target strength relationships suitable for use in estimating biomass from echo integration acoustic surveys. The relationships are derived from estimates of the mean and standard deviation of the tilt angle distributions of the wild fish generated by the method. The relationships may also be used to estimate proportions of the two oreo species in the wild. The mean tilt angle of black oreos in the wild was about 10° with a standard deviation of 8°. For smooth oreos it was close to zero with a standard deviation of about 4°. The target strength relationships derived for biomass estimation purposes were TSB=−78.05+25.3 log10(L)+1.62 sin(0.0815L+0.238) and TSS=−82.16+24.6 log10(L)+1.03 sin(0.1165L−1.765), where L is the fish length and TSB and TSS are the target strengths of black and smooth oreos respectively.