bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron emission from SrTiO3-coated silicon-tip arrays
Authors: Bian H. J. , Chen X. F. , Pan J. S. , Sun Chang Q. , Zhu W.
Published: June 2007

The field emission of electrons from silicon-tip arrays coated with SrTiO3 thin layers has been investigated. It is found that the emission properties are sensitive to the preparation conditions and the extent of nitrogen doping in the magnetron sputtering. Nitrogen addition could reduce the band gap of the SrTiO3 caps to improve the emission properties significantly with a threshold field of 17V∕μm and the stability of emitted current compared with the uncoated silicon tips. Results demonstrate that SrTiO3 thin films with N addition effectively lower the work function of silicon tips. However, oxygen-rich SrTiO3 exhibits a detrimental effect. Findings are explained in terms of valence-density-state modification by the nonbonding lone pairs and the lone-pair-induced dipoles [W. T. Zheng and C. Q. Sun, Prog. Solid State Chem. 34, 1 (2006)].
Etch mechanisms of hybrid low-k material (SiOCH with porogen) in fluorocarbon based plasma
Authors: Eon D. , Darnon M. , Chevolleau T. , David T. , Vallier L. , Joubert O.
Published: June 2007

This study is dedicated to the etching of a low-k material using the late porogen removal approach. In this approach, the porogen is removed by thermal annealing or UV curing after patterning or copper filling. Starting from a CF4 based plasma, the etch rate decreases with an Ar dilution or by adding a polymerizing gas such as CH2F2. On the other hand, the etch rate increases with O2 or N2 addition in the fluorocarbon gas mixture. X-ray photoelectron spectroscopy analyses show that the etching is controlled by the thickness of the fluorocarbon layer formed at the top surface of the hybrid material during the steady state etch regime. An etch stop phenomenon is even observed with highly polymerizing etch chemistries due to the formation of a thick fluorocarbon layer. Infrared spectroscopy analysis shows that the hybrid film is not damaged by the different etching chemistries investigated in contrast to the porous material.
Chemical beam epitaxy of GaAsN∕GaAs multiquantum well solar cell
Authors: Freundlich A. , Fotkatzikis A. , Bhusal L. , Williams L. , Alemu A. , Zhu W. , Coaquira J. A. H. , Feltrin A. , Radhakrishnan G.
Published: June 2007

The authors present preliminary data for a set of GaAsN∕GaAs multiquantum well (MQW) solar cells, grown by radio-frequency (rf) nitrogen plasma-assisted chemical beam epitaxy. The spectral response of this preliminary set of devices extends well below the GaAs band gap, while exhibiting remarkably high photoconversion strength that exceeds that of other MQW-based solar cells with comparable band gaps (1.0–1.2eV). This behavior is consistent with the enhancement of the electron effective mass in III-V dilute nitrides. Although the output current is similar to that of conventional GaInAsN solar cells, the output voltage is significantly higher when compared to that of bulk solar cells of similar wavelengths. The spectral response of as-grown devices is characterized by a deep valley around 1.37–1.4eV, that could be attributed to N contamination of the GaAs barriers. Rapid thermal annealing improves significantly the spectral response in the vicinity of this valley.
Midinfrared InGaAsSb quantum well lasers with digitally grown tensile-strained AlGaAsSb barriers
Authors: Li W. , Shao H. , Moscicka D. , Torfi A. , Wang W. I.
Published: June 2007

The InGaAsSb lasers emitting in the midinfrared with strain-compensated multiple-quantum-well (MQW) active region have been grown by solid-source molecular-beam epitaxy (MBE). Digitally grown tensile-strained AlGaAsSb was employed as barriers to increase the valence-band offset for hole confinement. The laser structures were characterized by reflection high-energy electron diffraction, x-ray diffraction, and photoluminescence. The digital-growth approach for the tensile-strained AlGaAsSb barriers improved the reliability and controllability of MBE growth for the MQW active region. The optical and structural qualities of InGaAsSb MQW were improved significantly as compared to those with random-alloy barriers due to the removal of growth interruption at the barrier/well interfaces in digital growth. As a result, high-performance devices were achieved in the InGaAsSb lasers with digital AlGaAsSb barriers. A low threshold current density of 163A∕cm2 at room temperature was achieved for 1000-μm-long lasers emitting at 2.38μm. An external differential quantum efficiency as high as 61% was achieved for the 880-μm-long lasers, the highest ever reported for any lasers in this wavelength range.
Fabrication and optical characterization of highly ordered InAs∕GaAs quantum dots on nonlithographically patterned substrates
Authors: Guico Rodney S. , Tzolov Marian , Guo Wei , Cloutier Sylvain G. , Beresford Roderic , Xu Jimmy
Published: June 2007

The authors demonstrate efficient light emission from InAs∕GaAs quantum dots (QDs) arranged in highly ordered arrays and fabricated in the presence of a spatial constraint created by nonlithographically patterned substrates. Photoluminescence (PL) bands are observed at 0.50eV from highly ordered QD arrays and they are further explored to monitor the different stages in the dot formation. A filling threshold of 35% is determined for obtaining this PL, since spectral emission from nanopore arrays with a smaller filling fraction is observed near the typical band position for self-assembled QDs. The behavior of emission intensity as a function of the dot growth time is consistent with the pattern-driven growth mechanism. The observed emission energy shift indicates that the dots are composed of InxGa1−xAs, where x≈0.87.
Optical monitoring of molecular beam epitaxy growth of AlN∕GaN using single-wavelength laser interferometry: A simple method of tracking real-time changes in growth rate
Authors: Griffiths C. L. , Weeks K. J.
Published: June 2007

Real-time in situ optical monitoring of growth rate, refractive index, and layer thickness has been achieved for molecular beam epitaxy growth of AlN∕GaN on GaN templates on Si(111) using laser interferometry at normal incidence. The reflectance data were analyzed using the proprietary (ORS Ltd.) software package R-FIT V2.0, which is capable of real-time fitting of the data to solutions of the equations of thin-film interference, hence yielding optimized values for optical constants (refractive index n, absorption coefficient k), growth rate (and hence film thickness) and surface roughness where appropriate. Changes in optimal fitting during growth are thus able to reveal changes in parameters such as growth rate. The problem of intermittent optical access due to the necessity of using a shutter to minimize view-port deposits was solved in two ways. First data were acquired continuously, only when the shutter was open, (1min in three), leading to widely spaced clusters of several data points. Subsequently the data acquisition was synchronized with the opening of the shutter, resulting in a shutter cycle of 1s in six and yielding one data point per rotation. Both methods were found to result in successful fitting, with stable refractive index values in excellent agreement with the literature. Furthermore, both methods were able to detect changes in growth rate during deposition, whether such changes were due to initial establishment of steady growth or to deliberate changes in the temperature of the Ga Knudsen cell. This demonstrates the value of this technique for process monitoring as it can potentially yield system diagnostic information (e.g., evidence of flux drift) as well as data on materials and growth parameters.
Study of intersubband transitions of ZnxCd1−xSe∕Znx′Cdy′Mg1−x′−y′Se multiple quantum wells grown by molecular beam epitaxy for midinfrared device applications
Authors: Lu H. , Shen A. , Tamargo M. C. , Charles W. , Yokomizo I. , Muñoz M. , Gong Y. , Neumark G. F. , Franz K. J. , Gmachl C. , Song C. Y. , Liu H. C.
Published: June 2007

Two ZnxCd1−xSe∕Znx′Cdy′Mg1−x′−y′Se multiple quantum well structures were grown by molecular beam epitaxy. The quantum well layer thickness of the multiple quantum well region was varied in order to tune the intersubband transition energy. The high crystalline quality of the material was demonstrated by high resolution x-ray diffraction. Contactless electroreflectance (CER) spectroscopy and Fourier transform infrared (FTIR) spectroscopy were used to characterize the intersubband transitions. Excellent agreement between the estimated value obtained by CER and the value measured by FTIR was achieved. Intersubband absorption at 6.89 and 5.37μm was observed demonstrating the ability to tune the properties of these wide band gap II-VI materials for mid-IR intersubband device applications.
Embedded vertically grown carbon nanotubes for field emission applications
Authors: Abdi Yaser , Koohsorkhi Javad , Mohajerzadeh Shams , Darbari Sara , Sanaee Zeinab
Published: June 2007

Vertically aligned carbon nanotubes, embedded in titanium oxide, have been fabricated and their electron emission behavior has been characterized. The growth of carbon nanotubes was achieved using a dc plasma enhanced chemical vapor deposition method with a mixture of acetylene and hydrogen, and nickel was used as the catalyst layer. Titanium oxide, deposited using an atmospheric pressure chemical vapor deposition, was used to encapsulate the grown carbon nanotubes and the physical properties of the as-grown carbon nanotubes as well as the encapsulated structures have been investigated using scanning electron microscopy. By a sequential polishing and plasma ashing, it is possible to open up the top side of the encapsulated carbon nanotubes. Also by means of a reactive ion etching, carbon nanotubes are exposed with an inherent gate surrounding each one. This technique allows the evolution of individually processed nanotubes with no need of nanolithography. The emission of electrons from carbon nanotubes was examined and a preliminary field emission display was prepared.
Fabrication of suspended single crystal diamond devices by electrochemical etch
Authors: Wang C. F. , Hu E. L. , Yang J. , Butler J. E.
Published: June 2007

Suspended single crystal diamond devices, microdisks and beam structures, have been fabricated. Low energy boron ions (180keV) were implanted to create subsurface damage while maintaining an undamaged top surface; homoepitaxial growth was subsequently carried out on the material. The damaged layer was selectively removed by electrochemical etching. Electrodes were deposited adjacent to the devices to reduce the voltage required for etching and to help control the etch profile.
Growth by molecular beam epitaxy of self-assembled InAs quantum dots on InAlAs and InGaAs lattice-matched to InP
Authors: Simmonds P. J. , Beere H. E. , Li H. W. , See P. , Shields A. J. , Ritchie D. A.
Published: June 2007

The authors report the results of a detailed study of the effect of growth conditions, for molecular beam epitaxy, on the structural and optical properties of self-assembled InAs quantum dots (QDs) on In0.524Al0.476As. InAs QDs both buried in, and on top of, In0.524Al0.476As were analyzed using photoluminescence (PL) and atomic force microscopy. InAs QD morphology and peak PL emission wavelength both scale linearly with deposition thickness in monolayers (MLs). InAs deposition thickness can be used to tune QD PL wavelength by 170nm/ML, over a range of almost 700nm. Increasing growth temperature from 440to480°C results in a linear decrease in QD size and a blueshift in peak emission wavelength of 3.5nm∕°C. This is a direct result of the temperature dependence of the In-sticking coefficient. InAs deposited on InP-lattice-matched In0.532Ga0.468As forms larger, lower-density features with longer PL wavelength, as expected from a consideration of the effects of In segregation and intermixing on strain and surface roughness. Choice of buffer material is shown to be critical to QD characteristics.
Kinetic Monte Carlo simulation of InAs quantum dot growth on nonlithographically patterned substrates
Authors: Guo W. , Guico R. S. , Xu J. M. , Beresford R.
Published: June 2007

In this work, the authors use the kinetic Monte Carlo method to simulate the heteroepitaxial submonolayer epitaxial growth process on a prepatterned semiconductor substrate. Experimentally, the GaAs substrates are patterned prior to growth by reactive ion etching through a self-organized nanopore anodized aluminum oxide film. At the early stages of growth, the InAs nuclei appear at the edges of the pores and then continue to grow from these locations, eventually filling the pores entirely. The shape of the pores greatly influences the location of the nuclei. If the pores are smoothed during the high temperature thermal cleaning process, the selectivity of nucleation is reduced or lost. The results of the kinetic Monte Carlo simulation demonstrate that a likely theoretical explanation of the observations is that the nonuniform distribution of the substrate elastic strain energy that is brought in by the lattice mismatch in the InAs wetting layer is the origin of the selectivity in the dot nucleation process.
Low-damage fabrication of high aspect nanocolumns by using neutral beams and ferritin-iron-core mask
Authors: Kubota Tomohiro , Baba Tomohiro , Saito Suguru , Yamasaki Satoshi , Kumagai Shinya , Matsui Takuo , Uraoka Yukiharu , Fuyuki Takashi , Yamashita Ichiro , Samukawa Seiji
Published: June 2007

A silicon-nanocolumn structure was fabricated by using neutral beams and a ferritin-iron-core mask. The etching process was observed by scanning electron microscope and x-ray photoelectron spectroscopy (XPS), which revealed that the iron core can be used as an etching mask for more than 30min of etching. Extrapolation of the XPS results shows that it is possible to fabricate nanocolumns about 180nm high. The diameter at the top of the nanocolumns was about 7nm after etching for 5min or longer, while it was about 12nm after etching for 2.5min. It is thus concluded that the current etching condition is applicable for fabricating nanocolumns about 20–180nm high and 170-nm-high nanocolumn was fabricated. Densely distributed nanocolumns with a narrow gap of 5.5nm between them could be etched successfully. To demonstrate that nanocolumns can be used for devices and circuits, a regularly arranged nanocolumn array was fabricated successfully. Etching damage was investigated by transmission electron microscope (TEM) and electron-spin resonance (ESR). The TEM observation revealed that the interior and subsurface of the nanocolumns kept a single-crystal structure completely, and no crystalline defects were observed. A small amount of Pb centers were found by ESR measurement and attributed to silicon dangling bonds at the nanocolumn sidewalls. The dangling-bond density was not increased by neutral-beam etching and stayed low (i.e., 2.7×1011cm−2) after etching. Damage-free etching was thus achieved by neutral beam. This result may be attributed to the absence of UV photons in the neutral beam. Such a sub-10-nm process with an arranged pattern and low etching damage has great potential in regard to the development of future nanometer-scale devices.
Poly-Si∕TiN∕HfO2 gate stack etching in high-density plasmas
Authors: Le Gouil A. , Joubert O. , Cunge G. , Chevolleau T. , Vallier L. , Chenevier B. , Matko I.
Published: June 2007

The authors have investigated the dry etch mechanisms of complex poly-Si∕TiN∕HfO2 gate stacks and the issues that are correlated with the introduction of a thin metal layer in the gate stack. Based on atomic force microscopy (AFM) and scanning electron microscope measurements, they will first show that a mixture of HBr and Cl2 at low rf bias power is required to successfully pattern the TiN layer without damaging the HfO2 gate oxide. Second, it is demonstrated that the introduction of a metal layer in the gate stack prevents charging effects during the last etching steps of the silicon part of the gate. Transmission electron microscope measurements and x-ray photoelectron spectroscopy analyses of the gate sidewalls show that the thickness of the silicon sidewall passivation layer decreases during the O2 free metal etching step potentially inducing silicon gate profile distortion such as notch. However, the notch can be eliminated by etching the Si∕TiN gate in a single step process instead of stopping at the TiN surface. Finally, AFM measurements show that during the TiN etching step, a low rf bias power is required to prevent damage (punching through) of the HfO2 layer. However, even under these conditions, a significant silicon recess (oxidation of the c-Si beneath the HfO2 layer) is observed even if TiN is etched in an O2-free chemistry.
Novel spin-on hard mask with Si-containing bottom antireflective coating for nanolithography
Authors: Choi Sang-Jun
Published: June 2007

ArF lithography has continued with the increase of numerical aperture (NA) and the decrease of resist thickness, which are causing several problems both in mass production and development stage. NA is going to exceed unity in immersion, which necessitates the use of dual bottom antireflective coating (BARC) with increased process complexity and cost. Resist thickness, on the other hand, is expected to further decrease below 100nm. Therefore, it is inevitable to use additional hard masks, which increases production cost due to chemical vapor deposition (CVD) process. Here the author discloses the novel spin-on hard (SOH) mask system with dual BARC property to overcome both problems aforementioned. This SOH mask composed of two layers of Si-containing BARC and carbon materials shows high etch selectivity between thin resist and several substrates. Composition and etch chemistries of two layers are intensively studied to give CVD-comparable step-by-step etch selectivity to transfer various patterns of thin resist including line/space and contact holes to the various substrates. In addition, optical properties of two layers are finely designed from comprehensive optical simulation to be applied to various generations of ArF lithography from dry to immersion process. In this study, the Si-BARC with high silicon content showed the fastest etching rate related with resist and showed also high etching resistance toward underlying spin-on carbon. The author obtained good etch profile of 60nm L/S patterns using Si-BARC with 36% silicon content as a SOH mask. This novel system is under extensive optimization to be applied to various generations of ArF lithography from mass production to the most pioneering semiconductor devices utilizing immersion lithography.
UV nanoimprint materials: Surface energies, residual layers, and imprint quality
Authors: Schmitt H. , Frey L. , Ryssel H. , Rommel M. , Lehrer C.
Published: June 2007

UV nanoimprint lithography is attracting more and more interest, because it has the potential of becoming a high-resolution, low-cost patterning technique. The availability of suitable UV curing materials is mandatory for successful imprinting. Within this work, a systematic investigation of commercially available photocuring materials was conducted to provide an overview of the properties of these materials. Their wetting behavior with respect to different substrate surfaces was characterized and their surface tensions were determined from their contact angles against two specifically selected solid surfaces: This method is presented here for the first time. The adhesion properties of the UV curing materials to different substrate surfaces and to the mold were investigated and necessary curing times were estimated. Additionally, the dependence of the residual layer thickness on the viscosity and the initial dispensed volume of UV curing materials was analyzed. It was found that the resist formulation of the UV curing materials strongly influences the surface tension as well as the adhesion to different substrate surfaces. Furthermore, the experiments verified that the thickness of the residual layer for UV curing materials increases with the square root of their viscosity which is predicted by theory. To demonstrate the suitability of the UV curing materials, first imprints with the prototype imprint tool, Nano Patterning Stepper 300 from Süss MicroTec, with pattern sizes down to 50nm are shown.
Formation and morphology of InGaN nanoislands on GaN(0001)
Authors: Gangopadhyay S. , Schmidt Th. , Einfeldt S. , Yamaguchi T. , Hommel D. , Falta J.
Published: June 2007

The morphology and density of InGaN nanoislands can be controlled by the choice of proper growth conditions for metal organic vapor phase epitaxy. Scanning tunneling microscopy has been used to investigate the dependence of InGaN island morphology on the growth parameters. A heterogeneous nucleation of large InGaN islands with a complex structure is observed after growth at 650°C in conjunction with a high In partial pressure. For 600°C and low In partial pressure, however, the homogeneous nucleation of small islands of sizes suitable for three-dimensional quantum confinement is found, with very high densities of 1012cm−2. The influence of the growth temperature and the In partial pressure is discussed in terms of thermally activated diffusion and surface mobility.
Optical properties of molecular-beam-epitaxy-grown InGaMnAs thin films
Authors: Peiris F. C. , Hungerford J. I. , Maksimov O. , Samarth N.
Published: June 2007

The authors have determined the dielectric functions of a series of molecular-beam-epitaxy-grown (In0.5Ga0.5)1−xMnxAs thin films deposited on InP substrates. Two variable angle spectroscopic ellipsometers, covering both the IR and the UV range (0.2–30μm), were used to obtain optical spectra for each of the samples. Using a standard inversion technique, the experimental data were modeled to obtain the dielectric function for each of the quaternary samples. By using a parametric semiconductor model, they deduced the critical point parameters corresponding to the electronic transitions in the Brillouin zone. Their analysis indicates that in this particular quaternary system, while the critical point associated with the fundamental gap, E0, blueshifts as a function of Mn concentration, the E1 critical point shows a redshift with respect to the Mn concentration.
In situ spectroscopic ellipsometry to monitor surface plasmon resonant group-III metals deposited by molecular beam epitaxy
Authors: Wu Pae C , Losurdo Maria , Kim Tong-Ho , Choi Soojeong , Bruno Giovanni , Brown April S.
Published: June 2007

The evolution of the surface plasmon resonance of Al, Ga, and In deposited by molecular beam epitaxy on GaN surfaces was monitored in real-time using spectroscopic ellipsometry. The correlation between the metal plasmon resonance modes, the particle size, and the growth mode is addressed. Ga and In deposited on GaN substrates form nanoparticles while the Al is shown to form a nearly coalesced thin film. The plasmon resonance of the Ga and In nanoparticles redshift with increasing average particle size while the pseudodielectric function of Al approaches that of a Drude metal.
Function of quantum-confinement effect in the AlGaN∕AlN∕GaN heterostructure with an AlN interfacial layer
Authors: Kong Y. C. , Chu R. M. , Zheng Y. D. , Zhou C. H. , Gu S. L. , Zhang R. , Han P. , Shi Y. , Jiang R. L.
Published: June 2007

This is a theoretical study of the distribution of two-dimensional electron gas (2DEG) in an AlGaN∕AlN∕GaN heterostructure. It is shown that the addition of an AlN interfacial layer leads to a slight increase of 2DEG density. The role of quantum-confinement effect in the change of 2DEG distribution with an inserted AlN interfacial layer is revealed by comparing calculated 2DEG distributions in an AlGaN∕(AlN)∕GaN heterostructure with and without considering the quantum-confinement effect. In the classic model, insertion of an AlN interfacial layer has a negligible effect on the 2DEG distribution. While taking the quantum effect into account, we can clearly see that incorporation of an AlN interfacial layer results in a substantial influence of 2DEG distribution, with the electron spillover into barrier layer reduced from 1.35×1012to0.23×1012cm−2, which are 9.8% and 1.5%, respectively, of the total 2DEG sheet density.
Characteristics of atomic-layer-deposited thin HfxZr1−xO2 gate dielectrics
Authors: Triyoso D. H. , Hegde R. I. , Schaeffer J. K. , Gregory R. , Wang X.-D. , Canonico M. , Roan D. , Hebert E. A. , Kim K. , Jiang J. , Rai R. , Kaushik V. , Samavedam S. B. , Rochat N.
Published: June 2007

In this study, the authors investigated the addition of zirconium (Zr) into HfO2 to improve its dielectric properties. HfxZr1−xO2 films were deposited by atomic-layer deposition at 200–350°C and annealed in a nitrogen ambient environment at 1000°C. Extensive physical characterization of the impact of alloying Zr into HfO2 is studied using vacuum ultraviolet spectroscopy ellipsometry, attenuated total reflectance Fourier transform infrared spectroscopy, secondary-ion mass spectrometry, transmission electron microscopy, atomic force microscopy, x-ray diffraction, Rutherford backscattering spectrometry, and x-ray reflectometry. HfxZr1−xO2 transistors are fabricated to characterize the impact of Zr addition on electrical thickness, mobility, and reliability. Zr addition into HfO2 leads to changes in film microstructure and grain-size distribution. HfxZr1−xO2 films have smaller and more uniform grain size compared to HfO2 for all deposition temperatures explored here. As Zr content and deposition temperature are increased, stabilization of the tetragonal phase is observed. A monotonic decrease in band gap is observed as ZrO2 content is increased. The chlorine impurity in the films is strongly dependent on deposition temperature and independent of film composition. TEM images of transistors showed excellent thermal stability as revealed by a sharp HfxZr1−xO2∕Si interface and no Zr silicide formation. Significant improvement in device properties such as lower electrical thickness (higher permittivities), lower threshold voltage (Vt) shift after stress (improved reliability), and higher mobilities are observed with Zr addition into HfO2. All of these results show HfxZr1−xO2 to be a promising candidate for SiO2 replacement.
Thin, crystalline MgO on hexagonal 6H-SiC(0001) by molecular beam epitaxy for functional oxide integration
Authors: Goodrich T. L. , Cai Z. , Losego M. D. , Maria J.-P. , Ziemer K. S.
Published: June 2007

MgO thin films are proposed as a template for the effective integration of three and four element oxides on wide band gap SiC for next generation multifunctional devices. Oriented, crystalline MgO(111) of 20–380Å is grown on 6H-SiC(0001) by molecular beam epitaxy at a substrate temperature of 140°C using a magnesium effusion cell and a remote oxygen plasma source with ion deflection plates located at the end of the plasma discharge tube and approximately 7in. from the sample surface. Films are conformal to the steps of the cleaned SiC surface with a rms roughness of 0.45±0.05nm. Magnesium adsorption controls the growth rate in an excess oxygen environment with Mg:O flux ratios of 1:99–1:20, where the oxygen flux is the equivalent molecular oxygen. The oxygen plasma, which was determined to be free of ions when the ion deflection plates are energized, does impact nucleation and initial stages of the MgO film formation, and there may be evidence of etching mechanisms involved in the thicker film growth. Chemical and structural thermal stability of 20Å MgO(111)‖6H-SiC(0001) was demonstrated up to 740°C in vacuum for 90min through reflection high-energy electron diffraction and x-ray photoelectron spectroscopy analyses. X-ray diffraction was used to further test the thermal stability of 380Å films in vacuum and in an oxygen environment up to 790°C. As a proof of concept for MgO(111) as an interface for aligned functional oxide growth, barium titanate (111) was deposited on 100Å MgO(111)‖6H-SiC(0001) by rf magnetron sputtering.
Molecular-beam epitaxy of phosphor-free 1.3μm InAlGaAs multiple-quantum-well lasers on InP (100)
Authors: Li W. , Moscicka D. , Torfi A. , Wang W. I.
Published: June 2007

1.3 μ m InAlGaAs strain-compensated multiple-quantum-well lasers without any phosphor-containing layers in the epitaxial structure have been grown by solid-source molecular-beam epitaxy (MBE). Substrate temperature was found to be a critical growth parameter for lattice-matched InAl(Ga)As layers in the laser structures. In the temperature range of 485–520°C, spontaneously ordered superlattices (SLs) with periods around 7–10nm were formed in the bulk InAl(Ga)As layers, as evidenced by x-ray diffraction measurements. Based on photoluminescence (PL) measurements, a large band gap reduction of 300meV and a broadened PL peak were observed for the In0.52Al0.48As layers with SL as compared to those without SL. The undesirable spontaneously ordered SL can be avoided in MBE growth with temperatures higher than 530°C, resulting in a high laser performance. Threshold-current density as low as 690A∕cm2 and T0 as high as 80K were achieved for InAlGaAs laser bars emitting at 1310nm.
Reproducible reflection high energy electron diffraction signatures for improvement of AlN using in situ growth regime characterization
Authors: Burnham Shawn D. , Namkoong Gon , Lee Kyoung-Keun , Doolittle W. Alan
Published: June 2007

Recently published methods that answer the previously unresolved critical issue of in situ growth regime determination during molecular beam epitaxy of AlN are used to address issues of material quality and intergrowth nonuniformity for improved repeatability using a modulated flux technique. A shutter modulation growth technique, defined as metal modulation epitaxy (MME), using the previously published reflection high-energy electron diffraction (RHEED) signatures was developed with the goal of obtaining materials with the properties of droplet regime materials, without the adverse effect of droplets. The films grown using MME were compared to films grown with no shutter modulation, and the surface roughness determined by atomic force microscopy was improved. For an unmodulated sample without droplets, the rms surface roughness was 6.9nm, while a sample with droplets had a rms surface roughness of 1.2nm. For the same Al flux that resulted in droplets with the unmodulated sample, the MME sample had no droplets and had a surface roughness of 3.3nm. Furthermore, while a nearly 20% increase in the Al flux still did not result in droplets for MME, a nearly 50% increase in the al flux did finally result in droplets. Therefore, by using MME, a wider range of Al flux is allowed for Al-rich growths without droplets. The results from the recently published RHEED transient characterization provide a powerful new tool that can be used as repeatable growth indicators that can possibly be used to standardize growths with techniques such as MME presented herein.
Low bias reactive ion etching of GaAs with a SiCl4∕N2∕O2 time-multiplexed process
Authors: Golka S. , Schartner S. , Schrenk W. , Strasser G.
Published: June 2007

An inductively coupled plasma reactive ion etching process was developed for transferring patterns from a thin intermediate mask consisting of Ni or SiNx into GaAs. Smoothed out etch floors and sidewalls can be achieved under an approximately 200V bias by switching between an anisotropic etch phase and a deposition phase by gas chopping. The deposition of Si–O based film that protects the sidewall from chlorine attack is promoted by the addition of O2 to an SiCl4∕N2 gas mixture. The total achieved etch depth was approximately 20μm in this work, but the process can in principle be adopted for a larger depth and other chlorine-etchable III-V semiconductors. SiCl4 is shown to act as a suitable deposition gas as well as an etch gas.
High-adhesive back side metallization of ultrathin wafers
Authors: Felmetsger Valery V.
Published: June 2007

In order to realize enhanced performance, advanced power devices such as metal-oxide-semiconductor field effect transistors and insulated gate bipolar transistors require using ultrathin wafers down to 50μm. Because the wafer’s back side is an active part of the majority of modern power devices, innovations in the wafer back side processing technology enable further device improvement. In this work, the author investigated how adhesion of Ag∕Ni∕Ti film stack deposited by S-Gun dc magnetrons depends on sputtering conditions of the Ti underlayer. The author found that high-adhesive back side metallization can be realized when Ti is deposited with relatively low rf substrate bias power without predeposition rf plasma etch of the wafer. rf induced bias voltage in the range of −50to−250V ensured the best adhesion property of the film stack. A high-resolution transmission electron microscopy investigation with a quantitative x-ray energy dispersive spectroscopy compositional analysis of the interface between the Ti layer and Si substrate revealed an interfacial 1.5–2nm thick Si–Ti mixed layer having ten times higher concentration of Ti (about 50at.%) compared to the no-bias case. Hence, Ti deposition with rf substrate bias enhances intermixing between Ti and Si atoms by low-energy ion bombardment without accumulation of Ar atoms in the interface area as is inherent to metallization with predeposition rf plasma etch. As a result, an essentially Ti-enriched crystalline Si layer is formed on the substrate, ensuring better bonding with the Ti film, thus improving the adhesion strength of the film stack.
Restoration and pore sealing of plasma damaged porous organosilicate low k dielectrics with phenyl containing agents
Authors: Liu J. , Kim W. , Bao J. , Shi H. , Baek W. , Ho P. S.
Published: June 2007

Silylation-based techniques are being actively investigated for dielectric recovery after plasma processing of porous ultralow dielectric constant (low k) dielectrics. A two-step pore sealing and dielectric recovery process was attempted on plasma damaged porous organosilicates (OSG). In the sealing step, an OSG low k monomer, phenyltrimethoxysilane, was used to form a thin low k sealant layer. Scanning transmission electron microscopy and transmission electron microscopy images indicated a much improved low k/barrier interface as a result of pore sealing. In the surface silylation step, two phenyl-containing agents, diphenyltetramethyldisilasane and phenyldimethylchlorosilane, were employed to restore surface hydrophobicity. The process was found to be capable of restoring surface hydrophobicity and partially blocking the pathway for moisture adsorption without much adverse effect on the subsequent atomic layer deposition of Cu barrier.
Nanosoldering of carbon nanotubes on metal electrodes with an atomic force microscope
Authors: Lee Sungjun , Park Ji-Yong , Cho Yong Sung , Park Yong Dae , Kuk Young , Chung Jin Wan
Published: June 2007

Contacts of carbon nanotubes to the prefabricated metal electrodes are improved by field evaporation of metals from a tip of an atomic force microscope such as soldering and/or the thermal treatment. The resultant contact resistance is as good as ∼100kΩ with nanoscale Au dots on Au electrodes, similar to the result of thermal treatment of the device. The contact resistance is found to be lower with Au dots than with Ni or Al dots. This method can be applied to local soldering of nanowires and nanotubes to the metal electrodes.
Fabrication and material properties of NiTi nanofluid
Authors: Chang Ho , Hsu Yi-Chun
Published: June 2007

This article describes the fabrication of the NiTi nanofluid using pure titanium and nickel rods as the anode and cathode electrodes, respectively, to further investigate the submerged arc nanofluid synthesis system the authors have developed. Using this kind of nanofluid, this article tests and analyzes the magnetic and photocatalytic properties of the nanofluid. The fabricated NiTi nanofluid has a pH value of 7, but the Brunauer, Emmett, and Teller surface area of the NiTi nanoparticles reaches 92.3m2∕g. Furthermore, this nanofluid can maintain a long-term and stable suspension status for more than six months. Testing of the NiTi nanofluid magnetism by superconducting quantum interferences device magnetometer indicates that paramagnetism appears in these nanoparticles. The saturated magnetization strength of NiTi nanoparticles is 2355Oe, and their saturated magnetization is 0.72emu∕g in average, which is over 11 times the bulk. After oxidization of the NiTi nanofluid, NiTiO3 is formed, and its energy gap is 4.9eV. In the photocatalysis experiment the NiTiO3 nanofluid has better photodecomposition ability for methylene blue than the general TiO2 nanoparticles for commercial use.
In
0.68
Ga
0.32
As
∕
Al
0.64
In
0.36
As
∕
In
P
4.5μm quantum cascade lasers grown by solid phosphorus molecular beam epitaxy
Authors: Chen J. , Malis O. , Sergent A. M. , Sivco D. L. , Weimann N. , Cho A. Y.
Published: June 2007

The authors report the growth and fabrication of 4.5μm quantum cascade lasers based on strain balanced In0.68Ga0.32As∕Al0.64In0.36As heterostructures by molecular beam epitaxy. A device with 12.5μm waveguide width and 2.25mm cavity length has a threshold current density of 5kA∕cm2 and can deliver power from one facet of about 90mW at room temperature under pulse mode. The characteristic temperature is 124K. The measured waveguide losses at room temperature are 11.5 and 13.2cm−1 with ridge width of 14 and 11μm, respectively. The temperature dependence of the waveguide loss is correlated with the change of the carrier mobility.
A laboratory experiment on internal solitary waves
Authors: Bourgault Daniel , Richards Clark
Published: June 2007

A simple laboratory experiment is designed to show the properties of internal solitary waves. The procedure and analysis are suited for a senior undergraduate laboratory course, though the techniques described may also be used for demonstration purposes in a fluid mechanics course. The measurements collected can be compared to the weakly nonlinear Korteweg–deVries (KdV) theory for the wave shape, lengthscale-amplitude relationship, and phase speed. The experiment provides a good introduction to internal solitary waves in the ocean, along with an exploration of error analysis and the limits of applicability of a theory.
The lock and key paradox and the limits of rigidity in special relativity
Authors: Pierce Evan
Published: June 2007

According to special relativity, a (longitudinally) moving stick is Lorentz contracted. When it is brought to rest, it must expand to its proper length. But, exactly how this expansion unfolds depends on how the stick is stopped. If the front end hits a brick wall, the rear end must (briefly) continue moving, and the stick contracts even further before expanding; if instead the rear end is suddenly stopped, the front end (briefly) continues, and (surprisingly) the stick overexpands before settling into its proper length. These effects of overexpanding and overcontracting are independent of any classical or molecular elasticity, but are derived entirely from the limits of information travel time imposed by special relativity. I explore these phenomena, inspired by the little known “lock and key” paradox.
Experimental study of the gear effect in ball collisions
Authors: Cross Rod , Nathan Alan M.
Published: June 2007

The gear effect arises when a ball collides with an object and the object recoils with a component of its acceleration in a direction parallel to the two contacting surfaces. Two experiments are described that show how the surfaces of the ball and the object can engage without slip similar to two gears. The effect is used in golf to correct for off-center impacts and it plays a minor role in baseball.
What is a reversible process?
Authors: Samiullah Mohammad
Published: June 2007

The definitions of reversible processes given in introductory physics books are found to be vague and misleading. An operational definition suitable for introductory texts is presented that avoids these problems. It stresses that to properly describe reversible processes, the second law of thermodynamics must be used. The constancy of entropy, which defines a reversible process, also distinguishes reversible processes from quasi-static processes.
Spontaneous symmetry breaking in quantum mechanics
Authors: van Wezel Jasper , van den Brink Jeroen
Published: June 2007

We present a mathematically simple procedure to explain spontaneous symmetry breaking in quantum systems. The procedure is applicable to a wide range of models and can be easily used to explain the existence of a symmetry broken state in crystals, antiferromagnets, and even superconductors. It has the advantage that it automatically brings to the fore the main players in spontaneous symmetry breaking: the symmetry-breaking field, the thermodynamic limit, and the global excitations of a “thin” spectrum.
Modeling corneal transparency
Authors: Smith T. B.
Published: June 2007

The mammalian cornea is very nearly transparent to light, a fact that is not obvious from its structure. The explanation lies with the degree of coherence in the scattering of light as it passes through the cornea. This article analyzes this explanation with an approach based on undergraduate electromagnetism concepts.
A storm in a wineglass
Authors: Joubert Stephan V. , Fay Temple H. , Voges Esme L.
Published: June 2007

The qualitative effect of a major disturbance such as an earthquake or a hurricane or, on a lesser scale, a powerboat moving along the edge of a bay, harbor, or lake, can be observed in a partially filled wineglass. We simulate this small-scale disturbance and do a quantitative analysis to explain how a resonance can occur on the liquid surface in the wineglass. An explanation is also given why such circumstances can occur in scaled-up situations such as bays, rivers, and harbors.
Modulation of Neuronal Interactions Through Neuronal Synchronization
Authors: Womelsdorf Thilo , Schoffelen Jan-Mathijs , Oostenveld Robert , Singer Wolf , Desimone Robert , Engel Andreas K. , Fries Pascal
Published: June 2007

Brain processing depends on the interactions between neuronal groups. Those interactions are governed by the pattern of anatomical connections and by yet unknown mechanisms that modulate the effective strength of a given connection. We found that the mutual influence among neuronal groups depends on the phase relation between rhythmic activities within the groups. Phase relations supporting interactions between the groups preceded those interactions by a few milliseconds, consistent with a mechanistic role. These effects were specific in time, frequency, and space, and we therefore propose that the pattern of synchronization flexibly determines the pattern of neuronal interactions.
Resistance-Mediating
Plasmodium falciparum pfcrt
and
pfmdr1
Alleles after Treatment with Artesunate-Amodiaquine in Uganda
Authors: Nsobya Samuel L. , Dokomajilar Christian , Joloba Moses , Dorsey Grant , Rosenthal Philip J.
Published: June 2007

Key parasite polymorphisms were assessed in subjects treated for malaria with artesunate-amodiaquine in Tororo, Uganda. For
pfcrt
, all of the isolates tested had the CVIET haplotype. For
pfmdr1
, 86Y and 1246Y were common at baseline and their prevalences were significantly higher in new isolates after therapy, indicating that treatment selected for mutations associated with a decreased response to amodiaquine.
Pharmacokinetics of the Antiviral Agent β-
d
-2′-Deoxy-2′-Fluoro-2′-
C
-Methylcytidine in Rhesus Monkeys
Authors: Asif Ghazia , Hurwitz Selwyn J. , Shi Junxing , Hernandez-Santiago Brenda I. , Schinazi Raymond F.
Published: June 2007

β-
d
-2′-Deoxy-2′-fluoro-2′-
C
-methylcytidine (PSI-6130) is an effective inhibitor of hepatitis C virus (HCV) replication in vitro. The purpose of this study was to evaluate the single-dose pharmacokinetics of PSΙ-6130 in rhesus monkeys following intravenous (i.v.) and oral administration. Noncompartmental analysis of the serum data obtained following oral and i.v. administration was performed. Pharmacokinetic studies with rhesus monkeys indicated slow and incomplete absorption with a mean absorption time (MAT) of 4.6 h and an oral bioavailability of 24.0% ± 14.3% (mean ± standard deviation), with comparable mean apparent half-lives following i.v. (4.54 ± 3.98 h) and oral (5.64 ± 1.13 h) administrations. The average percentages of the total dose recovered unchanged and in deaminated form in the urine were 32.9% ± 12.6% and 18.9% ± 6.6% (i.v.) and 6.0% ± 3.9% and 3.9% ± 1.0% (oral), respectively. The total bioavailability, taking into account the parent drug and its deaminated metabolite 2′-deoxy-2′-fluoro-2′-
C
-methyluridine (PSI-6206), was 64% ± 26%. PSI-6130 was present in the cerebrospinal fluid after oral and i.v. dosing. However, no deamination of radiolabeled PSI-6130 was detected after 8 h of incubation in monkey and human whole blood. An
N
4
-modified prodrug of PSI-6130 (PSI-6419) was orally administered to monkeys, but it failed to improve the oral bioavailability of PSI-6130. Further studies are warranted to improve the oral bioavailability and reduce the deamination of PSI-6130 in order to explore the potential of this drug for the treatment of HCV-infected individuals.
Increased Survival after Gemfibrozil Treatment of Severe Mouse Influenza
Authors: Budd Alison , Alleva Lisa , Alsharifi Mohammed , Koskinen Aulikki , Smythe Victoria , Müllbacher Arno , Wood Jeff , Clark Ian
Published: June 2007

Gemfibrozil, an agent that inhibits production of proinflammatory cytokines in addition to its clinically useful lipid-lowering activity, increased survival in BALB/c mice that were already ill from infection by influenza virus A/Japan/305/57 (H2N2). Gemfibrozil was administered intraperitoneally once daily from days 4 to 10 after intranasal exposure to the virus. Survival increased from 26% in vehicle-treated mice (
n
= 50) to 52% in mice given gemfibrozil at 60 mg/kg/day (
n
= 46) (
P
= 0.0026). If this principle translates to patients, a drug already approved for human use, albeit by a different route for another purpose, might be adapted relatively fast for use against influenza, conceivably including human infection with a derivative of the avian H5N1 strain.
2-
tert
-Butyl-8-Quinolinamines Exhibit Potent Blood Schizontocidal Antimalarial Activity via Inhibition of Heme Crystallization
Authors: Huy Nguyen Tien , Mizunuma Keisuke , Kaur Kirandeep , Nhien Nguyen Thanh Thuy , Jain Meenakshi , Uyen Dinh Thanh , Harada Shigeharu , Jain Rahul , Kamei Kaeko
Published: June 2007

We have recently reported that the attachment of a bulky metabolically stable
tert
-butyl group at the C-2 position of a quinoline ring in primaquine results in a tremendous improvement in the blood schizontocidal antimalarial activity of 8-quinolinamine. Because free heme released from hemoglobin catabolism in a malarial parasite is highly toxic, the parasite protects itself mainly by crystallization of heme into insoluble nontoxic hemozoin. We now demonstrate the ability of 2-
tert
-butylprimaquine to inhibit in vitro beta-hematin formation, to form a complex with heme with a stoichiometry of 1:1, and to enhance heme-induced hemolysis. The results described herein indicate that a major improvement in the blood-schizontocidal antimalarial activity of 2-
tert
-butylprimaquine might be due to a disturbance of heme catabolism pathway in the malarial parasite.
Retapamulin Inhibition of Translation and 50S Ribosomal Subunit Formation in
Staphylococcus aureus
Cells
Authors: Champney W. Scott , Rodgers Ward K.
Published: June 2007

Retapamulin inhibited protein biosynthesis and cell viability in methicillin-sensitive and methicillin-resistant
Staphylococcus aureus
organisms. A specific inhibitory effect on 50S ribosomal subunit formation was also found. Pulse-chase labeling experiments confirmed the specific inhibition of 50S subunit biogenesis. Turnover of 23S rRNA was found, with no effect on 16S rRNA amounts.