bluuebunny/crossref_metadata_2025 huggingface dataset:
Real-time imaging of enzymatic degradation of starch granules by atomic force microscopy
Authors: Thomson N. H. , Miles M. J. , Ring S. G. , Shewry P. R. , Tatham A. S.
Published: July 2002

Atomic force microscopy has been used to image wheat seed starch granules (cultivar Timmo) both in air and under water. In the aqueous environment, the enzyme α-amylase was introduced into the solution cell of the microscope and its progressive attack on a starch granule was followed by collecting successive images of the granule. Each image was recorded over a time period of 105 s. The enzyme created a ‘‘pin-hole’’ in the top of the granule, consistent with previous off-line electron microscopy studies.
New electron excited light emitting materials*
Authors: Vecht A. , Smith D. W. , Chadha S. S. , Gibbons C. S. , Koh James , Morton David
Published: July 2002

A range of bright, multicolor phosphor systems for field emission displays is reported. These include the standard ZnO:Zn and newer, narrow band emitting systems based on green ZnGa2O4:Mn and red CaTiO3:Pr. A modified, blue emitting ZnO:(Zn/Mg) and thin films of ZnGa2O4:Mn have also been prepared.
Space-charge effects in Spindt-type field emission cathodes
Authors: van Veen G. N. A.
Published: July 2002

The field emission characteristic of a Spindt-type cathode, cleaned extensively in a hydrogen atmosphere while emitting, is considered. The maximum emission reached is about 800 μA. At high current levels the I–V curve clearly deviates from a Fowler–Nordheim behavior. This can be explained by the occurrence of space charge. Several models are considered to simulate the experimental results. The fit to the data is good except at the highest emission levels. In this region the difference between the measured currents and the simulation results is due to the neglect of the variation of the field on top of the tip in the presence of space charge.
Force probe characterization using silicon three-dimensional structures formed by focused ion beam lithography
Authors: Edenfeld K. M. , Jarausch K. F. , Stark T. J. , Griffis D. P. , Russell P. E.
Published: July 2002

The creation of a localized etch stop by focused ion beam implantation of Ga+ into Si combined with selective material removal by sputtering has been used to produce submicron size three-dimensional structures. These funnellike structures have been used to characterize probes used in atomic force microscopy.
Electron-beam fabrication and focused ion beam inspection of submicron structured diffractive optical elements
Authors: Dix C. , McKee P. F. , Thurlow A. R. , Towers J. R. , Wood D. C. , Dawes N. J. , Whitney J. T.
Published: July 2002

Diffractive optical elements, designed by computer optimization techniques, can provide high coupling efficiencies between components in optical communications systems, and can be reproduced accurately in large numbers by the same technology employed to make microcircuits. This applications-led development was for optimized f/0.48 Fresnel lenses to couple light from arrays of semiconductor lasers into monomode optical fibers. The lenses were fabricated as four phase level structures in fused quartz using electron-beam lithography and reactive ion etching. The low f number needed to match between the laser and fiber numerical apertures required 0.2 μm feature sizes in the outer region of the lenses etched 2.1 μm deep. Effective use was made of focused ion beam etching and imaging to obtain cross sections of these high aspect ratio structures during process development. Differing lens designs have achieved 34% and 50% coupling efficiencies between 30° full width half peak 1.55 μm lasers and cleaved, monomode system fibers. In the former case, lens to fiber alignment tolerance was ±6 μm, making passive assembly of the lens fiber arrays feasible.
Composition inhomogeneities in the buffer layers of In0.52Al0.48As/InxGa1−xAs/InP multiquantum well structures driven by In segregation
Authors: Peiró F. , Cornet A. , Morante J. R. , Georgakilas A.
Published: July 2002

Transmission electron microscopy was used to study the morphology of InxGa1−xAs/In0.52Al0.48As multiquantum well (MQW) structures epitaxied at 510 °C over a In0.52Al0.48As buffer grown at 550 °C on a (100) InP substrate by molecular beam epitaxy. The plan-view images showed some squared contrast inhomogeneities oriented along the 〈010〉 directions. The projection of such contrast inhomogeneities along the (011) zone axis reveals the presence of contrast fringes along a direction slightly tilted from {111} planes, in particular at ≂35°–45° from (100). These contrast features extend only in the buffer layer, whereas they disappear completely when they reach the MQW structure, i.e., where the growth temperature is reduced. The origin of such a contrast pattern has been related to In composition inhomogeneities alternating on {101} planes, driven by In segregation during the growth of InAlAs at relatively high temperatures.
Lithographic evaluation of the hydrogenated amorphous carbon film
Authors: Martino R. , Ferguson R. , Molless A. , Liebmann L. , Neisser M. , Weed J. , Callegari S.
Published: July 2002

Electrical measurement techniques and aerial image simulation were used to quantify the lithographic benefits of an IBM hydrogenated amorphous carbon attenuated phase shifting mask for 0.25 and 0.2 μm features. Both a chrome and attenuated amorphous carbon blank were patterned with electrically testable structures consisting of varying dimensions and pitch. Detailed characterization was performed on the blanks to confirm phase, transmission, and dimensional accuracy. Process window characterization as a function of image biasing and pitch were evaluated using SPLAT simulations and electrical measurements for SVGL Micrascan II exposures combined with exposure-defocus analysis techniques. The electrical linewidth data showed a 12.5% reduction of depth-of-focus corresponding to a 25 nm offset from optimum bias, demonstrating the importance of accurate mask biasing. Electrical measurements structures were patterned on doped polysilicon films using APEX-E resist. Exposure latitude enhancement was achieved for isolated and nested features producing process window enhancements as large as 50% relative to chrome-based imaging of 0.25 and 0.2 μm features. Finally, depth-of-focus varied systematically with pitch.
InGaAs/GaAs quantum wires and dots defined by low-voltage electron-beam lithography
Authors: Steffen R. , Oshinowo J. , Koch T. , Forchel A.
Published: July 2002

Low-voltage electron-beam lithography (EBL) of energies of 2.3 keV is a powerful tool for efficient exposure of poly(methylmethacrylate) resist patterned substrate, with negligible proximity effects. InGaAs/GaAs quantum wires and dots have been fabricated with lateral wire widths down to 15 nm, and dot diameters down to 37 nm, respectively. The nanostructures were defined by combining high-resolution low-voltage EBL and deep wet chemical etching. Quantum wires and dots show a dimensionality dependent photoluminescence energy blue shift of up to 12 meV, caused by lateral quantization.
Evaluation of plasma damage using fully processed metal–oxide–semiconductor transistors
Authors: Li Xiao-Yu , Brożek Tomasz , Preuninger Fred , Chan David , Viswanathan Chand R.
Published: July 2002

The device damage during plasma etching is studied in this work using fully processed metal–oxide–semiconductor (MOS) transistors with antennas to accentuate oxide charging effect. The investigation of both charging damage and another damage that occurs during poly gate etching are reported in this article. The susceptibility of plasma damaged n-MOS and p-MOS devices to hot carrier stress is shown to be a function of the etch chemistry and etching tool.
Interferometric lithography of sub-micrometer sparse hole arrays for field-emission display applications
Authors: Chen Xiaolan , Zaidi Saleem H. , Brueck S. R. J. , Devine Daniel J.
Published: July 2002

Interferometric lithography, the use of interactions between coherent laser beams to define sub- wavelength patterns, is well adapted to the periodic nature of field-emitter structures. Techniques to fabricate sparse (hole diameter to pitch ratio of 1:3 or larger) emitter arrays to improve reliability and lifetime are presented. These include: multiple exposures at two different pitches; integration of interferometric and optical imaging lithography; and various multiple beam techniques that both provide a sparse array and result in a two dimensional pattern in a single exposure. Moiré alignment techniques are demonstrated to provide a simple process for aligning multiple levels. Manufacturing related issues such as process latitude and photoresist profiles and their suitability for subsequent processing are also discussed. Exposure-dose process control using latent image monitoring is demonstrated.
Molecular beam epitaxy growth and characterization of InGaAlAs-collector heterojunction bipolar transistors with 140 GHz fmax and 20 V breakdown
Authors: Block T. R. , Wojtowicz M. , Cowles J. , Tran L. , Oki A. K. , Streit D. C.
Published: July 2002

The molecular beam epitaxy (MBE) growth, material characterization, and device performance of InGaAlAs-collector heterojunction bipolar transistors that dramatically increase device breakdown without compromising high-frequency performance is described. This performance is achieved by the use of large band gap InGaAlAs collectors with a pulse-doped layer at the beginning of the graded collector-to-base junction to suppress current-blocking effects. Double-crystal x-ray diffraction measurements indicate these complex structure can be grown using MBE with very close lattice matching to the underlying InP substrate. From photoreflectance spectra the collector composition, field present at the collector-to-base junction, and various transitions associated with the graded emitter can be measured. The use of an In0.53(Ga0.50Al0.50)0.47As collector was found to increase breakdown characteristics (BVcbo=20 V, VA=250 V) substantially compared to an In0.53Ga0.47As collector (BVcbo=11 V, VA=7 V). For the In0.53(Ga0.50Al0.50)0.47As collector current blocking effects were only found at very high current levels (∼100 kA/cm2) allowing this device to achieve very good rf performance (ft=58 GHz and fmax=140 GHz). For higher Al compositions in the collector the breakdown voltage was found to increase further but current-blocking effects became more severe and rf performance began to suffer.
Molecular beam epitaxy growth of InGaP multiple quantum well structures on GaP for optical modulators
Authors: Vogt T. J. , Thiagarajan P. , Robinson G. Y.
Published: July 2002

The epitaxial growth and characterization of multiple quantum well (MQW) optical modulators on GaP substrates is reported. The devices were fabricated using graded composition InGaP strain relieving buffer layers and strain-compensated InGaP MQWs. The devices operate in the 570–620 nm wavelength range with a maximum transmission change of 15% occurring at 594 nm for an applied bias of −10 V. The composition range required for the target wavelength range is near the indirect–direct crossover point for the InGaP material system and the corresponding effects on photoluminescence and absorption data is presented.
Rapid thermal N2O oxynitride on Si(100)
Authors: Lu Z. H. , Hussey R. J. , Graham M. J. , Cao R. , Tay S. P.
Published: July 2002

High-resolution x-ray photoelectron spectroscopy (XPS) in conjunction with secondary-ion-mass spectrometry was used to study the chemical nature and distribution of N in oxynitride films formed by rapid thermal N2O processes (RTPs) or conventional furnace methods. The kinetics of furnace oxide growth in N2O are slower than that in O2. During reoxidation the oxidation rate increased to that in pure O2 and the N in the SiO2–Si interface region is displaced into the bulk of the oxide. High-resolution synchrotron Si 2p core-level photoemission spectroscopy (PES) was used to study the oxide–Si(100) interface suboxide structures produced by RTP with and without the presence of N. XPS N 1s studies indicated that there are two types of N in the RTP oxynitride films. The chemical bond configuration of the first type of N is similar to that of N in Si3N4 and is mainly distributed within the first 1 nm from the interface. The second type of N is distributed mainly outside of the first 1 nm region, and the N is likely bonded to two Si and one oxygen atom. PES studies showed that Si formed suboxides with oxygen at the interface for all oxynitride films. It is found that there is no change in the Si+1 structure while there is a dramatic decrease in the Si+2 and Si+3 states with the inclusion of N in the oxide. Both the XPS and PES results are explained in terms of a strain reduction as N is incorporated in the film near the interface region, where Si3N4 functions as a buffer layer which reduces the stress caused by the large Si lattice mismatch between the bulk Si and the oxide overlayer. About 1/5 of the Si+2 and 1/3 of Si+3 atoms at the SiO2–Si interface have been replaced by the Si3N4 buffer layer at the oxynitride–Si interface.
Evolution of deep levels and internal photoemission with annealing temperature at ZnSe/GaAs interfaces
Authors: Yang X. , Brillson L. J. , Raisanen A. D. , Vanzetti L. , Bonanni A. , Franciosi A. , Grundmann M. , Bimberg D.
Published: July 2002

We have measured charge states deep within the ZnSe band gap and localized near the ZnSe/GaAs interface as a function of annealing temperature by means of photoluminescence spectroscopy, as well as cathodoluminescence from the cross section of a ZnSSe/GaAs heterostructure with a scanning electron microscope. Annealing produces new emissions at 1.9 eV and 2.25 eV which increase with annealing temperature and which vary with different growth conditions. Cathodoluminescence spectra show two similar features which originate within ZnSe and whose relative emission intensities vary with depth. Since the 2.25 eV feature appears only under Zn-rich growth conditions, these results indicate a role of both interface diffusion and crystal growth in the formation of this defect. We observe a proportional increase of the 1.9 eV and 0.9 eV peak intensities with annealing, indicating their complementary nature with respect to a common deep level. Finally, a detailed analysis of the relative intensities of ZnSe and GaAs features due to internal photoemission across the heterointerface suggests that the heterojunction band offset changes with formation of these new deep levels. All these results emphasize that multiple deep levels form near the buried interface with annealing which can dominate the heterojunction electronic properties.
Scanning force microscopy for the study of domain structure in ferroelectric thin films
Authors: Gruverman A. , Auciello O. , Tokumoto H.
Published: July 2002

A piezoresponse technique based on scanning force microscopy (SFM) has been used for studying domain structure in ferroelectric thin films. Studies were performed on Pb(Zrx,Ti1−x)O3(PZT) thin films produced by a sol–gel method. The piezoresponse images of the PZT films were taken before and after inducing polarization in the films by applying a direct current voltage between the bottom electrode and the SFM tip. Polarization induced patterns were written with 20 V pulses and subsequently imaged by the SFM piezoresponse technique. The effect of the film structure on the imaging resolution of domains is discussed.
Scanning force microscopy in the dynamic mode using microfabricated capacitive sensors
Authors: Blanc N. , Brugger J. , de Rooij N. F. , Dürig U.
Published: July 2002

We report on the first successful operation of a scanning force microscope using microfabricated capacitive force sensors. The sensors, which are made from single crystal silicon on insulator wafers, consist of a cantilever spring with integrated tip at the free end and an electrically insulated counter electrode. Dynamic force gradient sensing is the preferred operating mode. Here, tip–sample interactions are detected by letting the sensor act as a resonator in a phase controlled oscillator setup and measuring corresponding shifts of the oscillation frequency. Experiments were performed in vacuum using a standard tunneling microscope. A Cr grating on a quartz substrate served as the test sample. Topographic images showing details on a 10 nm scale were obtained operating at a constant force gradient of the order of 0.01 N/m. In addition, critical design parameters are discussed based on an analysis of the electromechanical properties of the sensors.
Thermal anneal activation of near-surface deep level defects in electron cyclotron resonance hydrogen plasma-exposed silicon
Authors: Nam C. W. , Ashok S. , Sekiguchi T.
Published: July 2002

Electron cyclotron resonance hydrogen plasma has been found effective in cleaning Si surfaces in a matter of minutes, with no substrate heating. Deep level transient spectroscopy measurements showed only a broad defect peak with relatively low concentration immediately after the plasma exposure. Subsequent thermal anneals reveal emergence of strong new defects that have apparently been latent until the thermal anneal treatment. After annealing at temperatures above 450 °C, several well-defined defect peaks with concentrations above 1×1013 cm−3 appear near the Si surface. These defect concentrations reach their maximum at an anneal temperature of 500 °C and reduce to negligible levels at 750 °C. Experiments on wafers of different oxygen concentrations show that these defects are unrelated to the presence of oxygen in Si.
Ion projection lithography: Next generation technology?
Authors: Gross G.
Published: July 2002

It is generally agreed that optical lithography is the method of choice for production of integrated circuits having minimum dimensions of down to 200 nm. By applying special enhancement techniques and complex resist processes the limit of optical lithography can be shifted down to about 150 nm in production using an exposure wavelength of 193 nm. However, this 193 nm equipment will be a one generation tool, so that new concepts are necessary for exposing structures smaller than 150 nm. The situation for this time (after about 2003) is still unclear. As lithography tools for after the year 2003 are not yet defined, there is an urgent need for preparative work for closing this technology gap. One of the most promising candidates for reaching 100 nm and below in production is ion projection lithography. In this article, the advantages and risks of this technique are compared to other possible candidates like x-ray, e-beam, and extreme ultraviolet. Insoluble production problems are identified. The feasibility of the relevant mask technologies, the availability of resist techniques are throughput and cost competitiveness are discussed.
Visible luminescence from Si/SiO2 superlattices
Authors: Novikov S. V. , Sinkkonen J. , Kilpelä O. , Gastev S. V.
Published: July 2002

Si/SiO 2 superlattices were grown by molecular beam epitaxy using in situ oxidation by the rf-plasma source. Transmission electron microscopy shows high uniformity of the grown layers. Optical band gap obtained from the absorption measurement changes from 1.2 eV for thick samples to 2.3 eV for thin samples, indicating quantum confinement in the Si layers. Room temperature photoluminescence (PL) was observed in the spectral range 1.9–2.3 eV. Two different mechanisms of PL were found. In the Si layer thickness range 1–2 nm the luminescence arises from the quantum confined energy states in the ultrathin silicon layers. For Si layers with thickness more than 3 nm oxygen related defects are the main origin of PL.
Surface nitridation of silicon dioxide with a high density nitrogen plasma
Authors: Kraft R. , Schneider T. P. , Dostalik W. W. , Hattangady S.
Published: July 2002

A high density nitrogen plasma generated with a helicon plasma source has been used to incorporate approximately 15 at. % nitrogen into the top 0.5 nm of a silicon dioxide layer. The surface nitridation was accomplished in 10 s with a high flux of low energy ions which were extracted from the high density nitrogen plasma and accelerated in the plasma sheath towards the surface an electrically floating silicon dioxide surface. A rf compensated Langmuir probe was used to measure the nitrogen ion energy and ion current density as a function of the nitrogen pressure and source power. The nitrogen ion energy, ion current density, and exposure time determine the nitrogen range and dose into the silicon dioxide surface. This process may be advantageous for nitriding the gate oxide in advanced complementary metal–oxide semiconductor process flows.
Growth and fractal scaling nature of copper thin films on TiN surface by metal organic chemical vapor deposition from hexafluoroacethylacetonate Cu(I) vinyltrimethylsilane
Authors: Park Young-Bae , Rhee Shi-Woo , Hong Jae-Hwa
Published: July 2002

The initial stage of growth and fractal scaling nature of copper (Cu) films, deposited by metal organic chemical vapor deposition on TiN substrate using hexafluoroacethylacetonate Cu(I) vinyltrimethylsilane precursor, was investigated by means of Auger electron spectroscopy, atomic force microscopy (AFM), x-ray diffraction, transmission electron microscopy, and scanning electron microscopy (SEM). Cu films were deposited in the surface reaction controlled regime (<200 °C) onto a columnar structure TiN substrate and discontinuous island growth was observed due to oxygen contamination of the TiN surface. Preferred orientation of the deposited Cu film changed as a function of the deposition time and coverage. The roughness exponent (α) and lateral correlation length (ξ) of Cu films grown under different growth stages were studied. From AFM measurement, it was found that the roughness exponent increased with increasing deposition time and the lateral correlation length also increased as the grains coalesced, which was also compared with SEM images. For island growth mode at the early stage of growth, α was below 0.65 and it saturated at 0.75 for the continuous growth mode.
Radial uniformity of an external-coil ionized physical vapor deposition source
Authors: Dickson M. , Zhong G. , Hopwood J.
Published: July 2002

A production-scale ionized physical vapor deposition (I-PVD) source has been developed to investigate the directional metalization of 200 mm diameter wafers by sputtering. The ionization of sputtered metal is accomplished by a 45 cm diameter inductively coupled plasma (ICP). Unlike previously reported I-PVD sources, however, the coil of the ICP is external to the plasma chamber. A plasma-immersed Faraday shield is used to prevent sputtered metal from depositing on the walls of the plasma chamber and electrically shorting the ICP source. Interaction between the Faraday shield and the ICP is found to result in an rf-induced negative self-bias of no more than 15 V dc on the shield. Since the simple internal geometry of this I-PVD system is not complicated by an immersed inductor, factors that control radial uniformity are readily investigated. The spatially resolved flux of aluminum neutrals and ions on 200 mm wafers is measured and compared with two diffusion models. Both the aluminum neutral and ion density are centrally peaked with a profile that is predicted by simple diffusion in a cylindrical chamber. The fraction of ionized aluminum flux is quite uniform, however, since the aluminum neutral density and ion density radially decrease at similar rates.
Comparative study of back surface field contact formation using different lamp configurations in rapid thermal processing
Authors: Singh R. , Vedagarbha V. , Nimmagadda S. V. , Narayanan S.
Published: July 2002

In this article, we have studied the role of photoeffects in the formation of aluminum back surface field (BSF) contacts on p-type silicon wafers by using different lamp configurations in rapid thermal processing (RTP). Use of photons of wavelengths less than 800 nm in conjunction with infrared and visible photons in RTP resulted in the reduction of processing time from 202 to 138 s and the processing temperature from 890 to 850 °C. The reduction in the overall thermal budget used for BSF contact formation is a direct result of photon assisted RTP. The availability of a large number of high energy photons on the metal surface also resulted in improved electrical, structural and mechanical properties of the processed device.
Arcing and voltage breakdown in vacuum microelectronics microwave devices using field emitter arrays: Causes, possible solutions, and recent progress
Authors: Charbonnier Francis
Published: July 2002

There is growing interest in high current field emitter arrays (FEAs) capable of delivering high current density and high conductance electron beams, particularly for microwave applications. Large, high packing density molybdenum and silicon FEAs have been placed in ultrahigh vacuum chambers, carefully conditioned, and tested for maximum performance and have yielded total FEA currents of 20–200 mA and beam current densities of 5–2000 A/cm2 at gate voltages of 80–150 V. However similar Mo and Si FEAs, and GaAs edge arrays, when placed in a prototype 10 GHz klystrode amplifier, have failed at 1–4 mA, even for pulsed operation at a low duty factor. Hence, the current must be increased by 25–50 in order to meet klystrode design objectives. We compare the intrinsic FE stable current limits of various materials: Mo, Si, GaAs, ZrC, and ZrC films on Mo emitters. We conclude that Mo FEAs have a high FE current limit but demand an extremely clean environment, Si or GaAs FEAs are more tolerant of a poorer environment but have a relatively low FE current limit, while ZrC/Mo FEAs have at least as equally high a FE current limit as Mo FEAs but are much more robust and tolerate a much poorer environment. We hypothesize that failures of high density Mo FEAs at relatively low current levels (below 1 μA per tip) in a microwave tube are due to ion bombardment of the FEA creating sharp nanoprotrusions on the sides of the emitters, which emit intense, focused electron beams directed at the gate, leading to a vacuum arc. Binh’s recent study presents strong evidence for the ongoing creation and destruction of nanoprotrusions on Mo FEAs. A ZrC thin film coating protects and passivates the Mo FEA surface, thus minimizing nanoprotrusion formation and allowing more stable operation at higher current levels and/or in more degraded environments. Studies of carbide (ZrC, HfC) film coatings on W, Mo, and Si single tips and on Mo and Si FEAs show that indeed the carbide film reduces the work function and operating voltage (by about 25% and 40%, respectively) and increases stability. Other studies still in progress show that blunt ZrC or ZrC/Mo single tips can operate for reasonable periods (∼1 h) at currents averaging 400 μA in ultrahigh vacuum or 200 μA in 10−5 Torr air. More extensive studies are planned to verify and to quantify the advantages of carbide film coatings for producing lower voltage, higher current, environment tolerant, and long life FEAs for various applications.
Prebreakdown and breakdown investigation of broad area electrodes in the micrometric regime
Authors: Ma Xianyun , Sudarshan T. S.
Published: July 2002

The dc prebreakdown and breakdown characteristics of micrometric gaps varying from 25 to 1000 μm, between highly polished 2 cm diam spherical electrodes, were extensively investigated at ∼10−6 Torr. The current–voltage characteristics of the above gaps obey Fowler–Nordheim behavior, confirming that the prebreakdown conduction was dominated by high field electron tunneling. The breakdown voltage derived from the prebreakdown data took the form of a power relation of gap distance as Vb=Kdα . Experimental results showed that highly polished metal electrodes can withstand very high fields in relatively poor vacuum: 220 V/μm for a 50 μm gap, 130 V/μm for a 300 μm gap. The breakdown of a narrow (⩽ 200 μm) gap resulted in rapid degradation, causing damage to both anode and cathode, while wider gaps (400–1000 μm) were found to exhibit spark conditioning, with the dc breakdown voltage increasing after each successive breakdown, causing damage only to the anode. A dc-glow-discharge treatment improved the gap insulation capability significantly and could recover the degraded gap insulation capability caused by spark discharge breakdown in small gaps. The experimental results also showed that the electrode surface roughness per se does not play a key role in influencing the breakdown characteristics after the dc glow-discharge pretreatment.
Cross-sectional scanning tunneling microscopy characterization of molecular beam epitaxy grown InAs/GaSb/AlSb heterostructures for mid-infrared interband cascade lasers
Authors: Harper J. , Weimer M. , Zhang D. , Lin C.-H. , Pei S. S.
Published: July 2002

We have used cross-sectional scanning tunneling microscopy (STM) to examine two important aspects of material and interface quality in the mixed-anion InAs/GaSb/AlSb system under growth conditions similar to those presently employed for type-II quantum well and interband cascade lasers: (1) anion cross incorporation in the bulk (specifically, unintended substitution of As for Sb within the GaSb layers); and (2) the roughness spectrum at the common-anion AlSb-on-GaSb heterojunction. Two apparently different anion defects are noted in the GaSb layers, but the demonstration of a linear correlation between the bulk defect density observed with STM and the arsenic valve setting during antimonide layer growth clearly establishes background arsenic incorporation as the common origin for both of these defects. The roughness spectrum at the AlSb-on-GaSb interface displays a surprisingly short correlation length that is dominated by heterogeneity in the cation rather than anion sublattice. The relationship of these observations to fundamental materials-based performance limitations in type-II interband cascade lasers is briefly discussed.
Resistivity effect of ZnGa2O4:Mn phosphor screen on cathodoluminescence characteristics of field emission display
Authors: Kim Shin-Sung , Cho Sung Hee , Yoo Jae Soo , Jo Sung Ho , Lee Jong Duk
Published: July 2002

The cathodoluminescent characteristics of phosphor screens excited by cold electrons from field emitter arrays can be influenced by the resistivity of the phosphor screen. The resistivity of ZnGa2O4:Mn phosphor screens was determined experimentally and the correlation between the screen resistivity and the luminance was obtained. Addition of WO3 to ZnGa2O4:Mn phosphor was one way of controlling the resistivity, and subsequently the luminance. In field emission displays (FEDs), the resistivity of the phosphor screen gives rise to a voltage drop on the anode side and modifies the emission characteristics of emitters, which leads to a negative effect on the luminance. The luminance has been generally expressed as L=ηIanodeVanode. But with the resistivity of the phosphor screen taken into consideration, it should be expressed as L=ηIanodeVphosphor. For either reason, the resistivity of the phosphor needs to be held as low as possible to get the maximum luminance out of FED. Experiments show that the resistivity of ZnGa2O4:Mn phosphor is typically 1010 Ω cm.
Simulation study on performance of field emitter array
Authors: Wei Lei , Baoping Wang , Hanchun Yin
Published: July 2002

An improved finite difference method is used to study the field emitter arrays in both diode and triode modes. In the simulation, the boundary of the emitter can be fitted with irregular meshes. The emission performance of the emitter array is analyzed in this article. Simulation results show that the electric field at tips in the emitter array is not uniform. The emission currents from tips are also different. Due to the influence of the interaction and fringe field, the emission currents from tips near the edge of the array are larger than that from tips on the center part of the array. However, the presence of the gate can reduce the interaction between neighboring tips in the triode structure. Thus, it is possible to achieve a higher current density in the triode mode.
Application of novel O- and H-atom sources in molecular beam epitaxy
Authors: Hoflund Gar B. , Wolan John T.
Published: July 2002

A novel source based on electron stimulated desorption (ESD) has been developed for the production of O-atom and H-atom fluxes. The fluxes produced by these sources are greater than 1015 atoms/cm2 s with an ion-to-atom ratio of about 10−8, and no other contaminants are present. During operation in a typical molecular beam epitaxial (MBE) system, the pressure remains below 10−9 Torr. The energies of the atoms range from about 1 to 4 eV, and no high energy species, which would damage a surface, are present in the flux. Therefore, these ESD atom sources are superior to plasma sources in all respects. The application of these sources for the in situ, room-temperature cleaning of GaAs and InP surfaces, the room-temperature growth of an insulating oxide layer on GaAs(001), and the room-temperature MBE growth of ZrO2 are described.
Nanolithography using a 100 kV electron beam lithography system with a Schottky emitter
Authors: Kamp M. , Emmerling M. , Kuhn S. , Forchel A.
Published: July 2002

We present lithography results with a new electron beam lithography system using a W/ZrO Schottky emitter operating at 100 kV acceleration voltage. The wide range of available beam currents between 100 pA and 50 nA allows both high resolution nanolithography and large area patterning. Resolution tests with line and dot arrays patterned by lift off using a poly(methyl/methacrylate) resist showed a minimum linewidth of 12 nm for 36 nm line pitch written with a beam current of 1.1 nA, 15 nm for 50 nm pitch written with 3 nA and 30 nm for 100 nm pitch written at 12 nA. The smallest spacing between features at 1.1 nA current was 36 nm for lines and 32 nm for dot arrays. The high current density of the beam at very small spot sizes makes the system a promising tool for nanofabrication using both conventional resist as well as high resolution but low sensitivity inorganic resist.
Nitrogen influence on dangling-bond configuration in silicon-rich SiOx:N,H thin films
Authors: Pivac B. , Rakvin B. , Borghesi A. , Sassella A. , Bacchetta M. , Zanotti L.
Published: July 2002

Paramagnetic centers were studied in different SiOx:N,H films deposited by plasma-enhanced chemical vapor deposition in a wide composition range. The total dangling-bond concentration is detected to be proportional to the oxygen content. Moreover, sample irradiation by ultraviolet light revealed also that nitrogen impurities play some role. In particular, the presence of N–H bonds induces a release of the film stress, which seems to be related to the concentration of dangling bonds. It is also shown that both weak Si–Si and Si–H bonds may serve as precursors for the dangling-bond formation.
Effect of surface treatment by (NH4)2Sx solution on the reduction of ohmic contact resistivity of p-type GaN
Authors: Kim Jong Kyu , Lee Jong-Lam , Lee Jae Won , Park Yong Jo , Kim Taeil
Published: July 2002

Surface treatment using aqua regia and (NH4)2Sx solution in sequence prior to Pd metal deposition was effective in reducing the contact resistivity of Pd to p-type GaN. The contact resistivity was drastically decreased from 3.6×10−1 to 2.9×10−4 Ω cm2 by the treatment. The surface oxides formed on p-type GaN during epitaxial growth were effectively removed using aqua regia, and the following (NH4)2Sx treatment protected the surface from the formation of oxides during air exposure. The reduction of the contact resistivity is due to the direct contact of Pd to the clean surface of p-type GaN, via shift of the Fermi level to an energy level near the valence band, resulting in the reduction of the barrier height for holes at the interface of Pd/p-type GaN.
Engineering phosphors for field emission displays
Authors: Vecht Aron , Gibbons Carol , Davies Dominic , Jing Xiping , Marsh Paul , Ireland Terry , Silver Jack , Newport Anne , Barber David
Published: July 2002

Factors affecting the synthesis and properties of a new generation of fine particle low voltage phosphors in field emission displays are reviewed. The morphology and particle size, the composition and stoichiometry, the stability, together with the nature and shape of the particle surface, all play important roles in the performance of the final phosphor. Initial new results from novel synthetic methodology are presented and discussed. Their implications in the light of the known literature point the way to the successful conclusion of the current thrust of phosphor research for good red, green, and blue low voltage, high definition phosphors.
High pressure, high temperature scanning tunneling microscopy
Authors: Jensen John A. , Rider Keith B. , Chen Yong , Salmeron Miquel , Somorjai Gabor A.
Published: July 2002

The design and performance of a high pressure, high temperature scanning tunneling microscope (HPHT-STM) system is described. The system combines an ultrahigh vacuum surface analysis/preparation chamber with a variable pressure (5×10−10 Torr–1 atm) and temperature (300–675 K) STM. The STM chamber can be isolated by three gate valves and filled with 1 atm of any gas mixture. The composition of the gas can be monitored by gas chromatography. A load-lock mechanism allows the transfer of samples and tips into the chamber without exposing it to air. Heating at high pressure is achieved using a halogen lamp beneath the sample. By switching between tungsten and gold tips, the microscope can be used in both oxidizing and reducing environments at room temperature, and reducing environments at elevated temperatures. This instrument allows the exploration of surfaces in a pressure regime that few other surface sensitive techniques can work in.
Kinetics of MnAs growth on GaAs(001) and interface structure
Authors: Schippan F. , Trampert A. , Däweritz L. , Ploog K. H.
Published: July 2002

On different As-rich GaAs(001) templates, well characterized by reflectance difference spectroscopy, nucleation and growth of NiAs-type MnAs is investigated in real time by reflection high-energy electron diffraction. Using very high As4/Mn flux ratios and low growth rates, one of the two occurring azimuthal alignments of the (1̄100) orientation can be nearly suppressed even in the nucleation stage, and it vanishes completely with further growth. Annealing is found to be very effective in surface smoothing. In dependence on the As/Mn ratio the MnAs(1̄100) surface develops different reconstructions. This finding is important for further investigations in the growth of double heterostructures. High-resolution transmission electron microscopy of as-grown MnAs/GaAs samples reveals an abrupt interface. The lattice mismatch accommodation is anisotropic with regularly arranged misfit dislocations along the [1̄10] direction and less localized coherency strain in the [110] direction, consistent with a near-coincidence-site lattice model.
Effect of defect density on the electrical characteristics of n-type GaN Schottky contacts
Authors: Shiojima Kenji , Woodall Jerry M. , Eiting Christopher J. , Grudowski Paul A. , Dupuis Russell D.
Published: July 2002

The effect of defects in n-type GaN epitaxial layers on the electrical characteristics of Ni contacts was studied. The defect density of GaN layers was characterized, and Ni dots deposited on four n-GaN wafers with different mobilities were examined by current–voltage (I–V) and capacitance–voltage (C–V) measurements. Ni contacts deposited on undoped GaN with a mobility of 6.7 cm2/V s showed ohmic behavior with a specific contact resistance on the order of 0.1 Ω cm2. In contrast, Ni contacts deposited on Si-doped GaN with a mobility of over 100 cm2/V s exhibited Schottky behavior with a Schottky barrier height of 0.75 eV from I–V and 1.10 eV from C–V. These results suggest the formation of small areas with low barrier height at the interface due to defects and dislocations.
Fabrication of a new broadband waveguide polarizer with a double-layer 190 nm period metal-gratings using nanoimprint lithography
Authors: Wang Jian , Schablitsky Steven , Yu Zhaoning , Wu Wei , Chou Stephen Y.
Published: July 2002

We fabricated a new waveguide polarizer, that has a double-layer 190 nm period metal grating at the interface between the waveguide core and waveguide cladding layer. Both silicon nitride and polymethylmethancrylate were used as waveguide cores. 190 nm period gratings were patterned by nanoimprint lithography. The new type waveguide polarizer works as a broadband and highly efficient transverse magnetic-pass polarizer with transverse electric-polarized light highly attenuated. The new polarizer structure can be applied to almost any waveguide structures. The simple fabrication process, which is compatible with conventional device processing, makes it very attractive for integrated optoelectronic system.
Large scale nanolithography using nanoimprint lithography
Authors: Heidari Babak , Maximov Ivan , Sarwe Eva-Lena , Montelius Lars
Published: July 2002

Nanoimprint lithography is a promising technique for fabrication of nanometer-sized structures with an eventual throughput capacity similar to UV-lithography based production of integrated circuits. In this article we address the possibility for wafer scale nanoimprint lithography, and the results presented here are all obtained from 2 in. sized substrate and stamp wafers. Our nanoimprint lithography equipment is described and some of its characteristics are discussed. These include ultrafast imprint cycle times of less than 2 min, good temperature monitoring, and control possibility of the substrate temperature. We show complete results after imprint, removal of remaining resist, and liftoff for 2 in. wafers. Furthermore, in this article the relation between polymer sticking to the stamp applied pressure, stamp depth, etc. is studied in detail.
Scanning tunneling microscopy of passivated Au nanocrystals immobilized on Au(111) surfaces
Authors: Harrell L. E. , Bigioni T. P. , Cullen W. G. , Whetten R. L. , First P.N.
Published: July 2002

The surface mobility of alkanethiol-passivated Au nanocrystals on inert surfaces makes them challenging to image by scanning tunneling microscopy (STM). However, the potential for obtaining information on the density of states of these nanocrystals from tunneling spectroscopy measurements justifies the effort to find sample preparations that effectively immobilize them. We have explored the use of dithiol molecules as a means of strengthening the interaction between the substrate and the nanocrystal, with the goal of imaging isolated nanocrystals on a conducting substrate. Modifying the nanocrystals by substituting dithiol molecules into the passivation layer allowed the nanocrystals to bind strongly to the Au(111) surface. The formation of a self-assembled monolayer of xylenedithiol on the Au substrate allowed unmodified nanocrystals to more strongly adsorb to this modified surface. In both cases, isolated 1.7 nm nanocrystals were easily imaged by STM on Au(111), even at room temperature. These are significant steps toward the goal of obtaining high quality tunneling spectra from this class of materials.
Process development of gated field emitter arrays with dry etched amorphous silicon microtips on glass substrates
Authors: Choi J. H. , Park Y. J. , Lee H. W. , Oh H. W. , Kim J. W. , Lee N. S. , Cha S. N. , Jung J. E. , Choi Y. S. , Kim J. M.
Published: July 2002

A low temperature fabrication process for gated field emitter arrays is developed with etched amorphous silicon microtips on glass substrates. Reactive ion etching using a SF6/O2 plasma is employed to sharpen microtips without any thermal oxidation or wet etching technique. Field emitter arrays with a triode structure are successively fabricated by utilizing plasma enhanced chemical vapor deposition and electron beam evaporation for a SiO2 gate oxide and a niobium gate, respectively.
Growth of CdS self-organized quantum dots by molecular beam epitaxy and application to light emitting diode structures
Authors: Kobayashi M. , Kitamura K. , Umeya H. , Jia A. W. , Yoshikawa A. , Shimotomai M. , Kato Y. , Takahashi K.
Published: July 2002

CdS quantum dots (QDs) were formed on ZnSe and ZnSxSe1−x. The nucleation process was monitored by reflection high energy electron diffraction, and it was suggested that QDs were formed without the wetting layer. The low temperature (13 K) photoluminescence (PL) measurement showed that the peak energy could be controlled by the amount of the CdS deposit. The PL peak energy covered most of visible range, indicating that the band lineup between CdS QD and ZnSe (or ZnSxSe1−x) host would be type II. Bright green and blue light emitting diode (LED) structures were fabricated in which CdS QDs were embedded in pn junctions of ZnSe and ZnSxSe1−x. The control of QD size as well as the band gap of the host material enabled the luminescence peak wavelength of the LED to be varied.
Detection of Widespread Fluids in the Tibetan Crust by Magnetotelluric Studies
Authors: Wei Wenbo , Unsworth Martyn , Jones Alan , Booker John , Tan Handong , Nelson Doug , Chen Leshou , Li Shenghui , Solon Kurt , Bedrosian Paul , Jin Sheng , Deng Ming , Ledo Juanjo , Kay David , Roberts Brian
Published: July 2002

Magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north-to-south width of the Tibetan plateau. The integrated conductivity (conductance) of the Tibetan crust ranges from 3000 to greater than 20,000 siemens. In contrast, stable continental regions typically exhibit conductances from 20 to 1000 siemens, averaging 100 siemens. Such pervasively high conductance suggests that partial melt and/or aqueous fluids are widespread within the Tibetan crust. In southern Tibet, the high-conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust. In northern Tibet, the conductive layer is at 30 to 40 kilometers and is due to partial melting. Zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow.
Space-Geodetic Constraints on Glacial Isostatic Adjustment in Fennoscandia
Authors: Milne G. A. , Davis J. L. , Mitrovica Jerry X. , Scherneck H.-G. , Johansson J. M. , Vermeer M. , Koivula H.
Published: July 2002

Analysis of Global Positioning System (GPS) data demonstrates that ongoing three-dimensional crustal deformation in Fennoscandia is dominated by glacial isostatic adjustment. Our comparison of these GPS observations with numerical predictions yields an Earth model that satisfies independent geologic constraints and bounds both the average viscosity in the upper mantle (5 × 10
20
to 1 × 10
21
pascal seconds) and the elastic thickness of the lithosphere (90 to 170 kilometers). We combined GPS-derived radial motions with Fennoscandian tide gauge records to estimate a regional sea surface rise of 2.1 ± 0.3 mm/year. Furthermore, ongoing horizontal tectonic motions greater than ∼1 mm/year are ruled out on the basis of the GPS-derived three-dimensional crustal velocity field.
Ecological Degradation in Protected Areas: The Case of Wolong Nature Reserve for Giant Pandas
Authors: Liu Jianguo , Linderman Marc , Ouyang Zhiyun , An Li , Yang Jian , Zhang Hemin
Published: July 2002

It is generally perceived that biodiversity is better protected from human activities after an area is designated as a protected area. However, we found that this common perception was not true in Wolong Nature Reserve (southwestern China), which was established in 1975 as a “flagship” protected area for the world-renowned endangered giant pandas. Analyses of remote sensing data from pre- and post-establishment periods indicate that the reserve has become more fragmented and less suitable for giant panda habitation. The rate of loss of high-quality habitat after the reserve's establishment was much higher than before the reserve was created, and the fragmentation of high-quality habitat became far more severe. After the creation of the reserve, rates of habitat loss and fragmentation inside the reserve unexpectedly increased to levels that were similar to or higher than those outside the reserve, in contrast to the situation before the reserve was created.