bluuebunny/crossref_metadata_2025 huggingface dataset:
A new non‐contact corneal aesthesiometer (NCCA)
Authors: Murphy Paul J. , Patel Sudi , Marshall John
Published: July 2002

A novel method for testing the corneal nerve function, through non‐invasive measurement of corneal sensitivity, is presented. The method of using a controlled pulse of air to stimulate the corneal surface is described. Technical information of the aesthesiometer's construction is given. Examples of the ability of the instrument to accurately locate the corneal sensitivity threshold using either a method of constant stimuli, or a method of limits, is described for 14 normal, non‐lens wearing eyes. Measurements are made in millibars of air pressure required. Mean normal values found were 0.342 ± 0.068 millibars. Discussion is made of the instrument's mode of action, its advantages over other invasive instruments, and potential clinical application.
A new computer assisted objective method for quantifying vascular changes of the bulbar conjunctivae
Authors: Owen Christopher G. , Fitzke Fred W. , Woodward E. Geoffrey
Published: July 2002

A novel computer software method was used to quantify the conjunctival plexus on the scleral background for measurement of the vascular surface area from photographs. A previously described method was used (Palmer, J. R., Owen, C. G., Ford, A. M., Jacobson, R. E. and Woodward, E. G. (1996). Optimal photographic imaging of the bulbar conjunctival vasculature. Ophthal. Physiol. Opt.16, 144–149) to optimise photographic imaging of the bulbar conjunctival vasculature by increasing the information content in the image. Repeatability of this technique was evaluated. Twenty subjects (20 eyes) free from ophthalmological and systemic abnormality were examined on two separate occasions. The maximum 95% confidence limits for repeatability are +8.58/−3.95%. For 10 consecutive estimates of vascularity the maximum 95% confidence interval lie between ±6.54%. To evaluate the technique the lateral‐bulbar conjunctivaein 10 soft (SCL) and 10 rigid gas permeable contact lens (RGPCL) wearers during the first 10 months of contact lens wear, were assessed and compared with subjective grading of hyperaemia. The new method showed sufficient sensitivity in detecting increased hyperaemia in the RGPCL wearing group and demonstrated statistically significant change. Subjective graded assessment of vascularity (using established classifications) detected increased hyperaemia, however, this was not statistically significant. Conjunctival vasculature is a dynamic structure and a source of valuable quantitative information where the ocular environment is varied, or where the ocular surface is affected by disease. Hence it is worthy of further investigation. A simple inexpensive method of computer assisted determination of vascularity is described.
Accommodation during simulation of 3D visual space from stereoscopic pairs of 2D images
Authors: Koh L. H. , Charman W. N
Published: July 2002

Under normal conditions, accommodation and convergence vary synkinetically and are dependent on object distance. In some situations, as when viewing stereoscopic pairs of images, this relationship may break down. The accommodation required to view the 2D images may differ from that normally associated with the convergence demanded by the local disparities in the images. Thus the additional convergence required to view the stereoscopic relief may stimulate inappropriate levels of accommodation. The present experiment looked for such accommodation change during the viewing of 1200″ disparity images of the TNO and Lang Stereotests, using a Canon R1 auto‐refractor to measure the accommodation. Significant differences in mean accommodation were found when a group of 10 subjects viewed either the background or figure of the stereograms. These differences approximate to those appropriate to the apparent depth relief under the viewing conditions in use, for both crossed and uncrossed disparities.
Intracellular chloride activity in corneal endothelial cells
Authors: Turner Helen C. , Wigham Chris G. , Hodson Stuart A.
Published: July 2002

Purpose: Current models of hydration control by the corneal endothelium suggest that HCO−3 movement across the cell is driven by the energy contained in the re‐entry of NA+ into the cell down its electrochemical gradient. The nature of the coupling between HCO−3 and Na+ is not yet fully resolved. One suggestion is that the Na+/K+/2Cl− co‐transporter drives Cl− into the cell which then exits the same membrane via a HCO−3/Cl− exchanger. This would drive HCO−3 into the cell from where it could exit down its own electrochemical gradient into the aqueous. Such a system would also raise Cl− above its electrochemical equilibrium. The purpose of this study was to determine if Cl− is passively distributed across the plasma membrane or elevated above its electrochemical equilibrium.

Method: To determine if this situation was present in rabbit corneal endothelial cells, we measured [Cl−]i directly using potential sensing and Cl− selective microelectrodes. To make the Cl− selective microelectrodes, filamented glass capillaries were silanized and back‐filled with Cl− ionophore (Corning or WPI) immediately before use. Electrode sensitivity was determined using the fixed interference method.

Results: Endothelial membrane potential (PDm) in this series was similar to that in previous studies, 29 ± 0.9 mV (mean ± SEM, n = 22). Chloride potential (PDCl) was used to calculate [Cl−]i, which was 53 ± 2.0 mM (mean ± SEM, n = 22).

Conclusions: The [Cl−]i measurement is significantly higher (P < 0.001) than that predicted from the measured PDm (39 ± 0.9 mM) and suggests that Cl− is not passively distributed in corneal endothelial cells.
Binocular summation in the peripheral visual field
Authors: Whitaker Angela , Pardhan Shahina
Published: July 2002

Purpose: To investigate binocular summation for a light detection task in the central and peripheral retina along 4 meridians.

Methods: 10 young subjects aged 22‐30 years (mean 26 years) with healthy eyes and normal visual fields participated in the study. A custom‐designed program on a Humphrey Visual Field Analyser measured thresholds at the fovea and at 5°, 10°, 15°, 25°, 40° and 55° along the horizontal meridian, and at 5°, 10°, 15°, 25°, 40° along the vertical, 45° oblique and 135° oblique meridians. Right and left monocular fields and binocular fields were measured on each subject using white spot stimuli (Goldmann sizes I and III). Binocular summation ratios were calculated for each eccentricity by dividing binocular sensitivity be the better monocular sensitivity.

Results and conclusion: As expected, binocular and monocular sensitivities decreased with increasing eccentricity. The mean binocular summation ratio at the fovea was 1.38 for size I stimulus and 1.42 for size III. Binocular summation was demonstrated in the peripheral retina along all meridians and is of similar magnitude to foveal binocular summation. ANOVA showed that binocular summation was significantly higher in the vertical meridian than horizontal meridian for both size I and III stimuli. There was no significant variation in binocular summation ratios with eccentricity along any of the 4 meridians examined.
Assessment of time‐dependent inter‐relationships between spontaneous eyeblink frequency, average and dominant interblink intervals in man
Authors: Zaman Mohammed L. , Doughty Michael J.
Published: July 2002

Measures of spontaneous eyeblink activity, despite often being measured, have not been standardized and the reliability of the measures has received very little attention. Video recordings were made of 22 healthy volunteers aged 20 to 58 years, while silently fixating on a 2 m distant, 35 mm high target under 350 lux illumination. Using an event marker, significant fluctuations (up to ±35%) in spontaneous eyeblink frequency (SEBF) or inter‐blink intervals (IBI) were observed in most subjects, on a minute‐by‐minute basis, over a 5‐minute period. However, the eyeblink activity did not conform to a minute‐by‐minute periodicity and time‐dependent trends were uncommon. For subjects <40 years of age, correlation's between SEBF and IBI indicated that 3‐5 minute assessments were likely to be adequate, but that at least 5 min periods are generally required for older patients. Modal IBI values correlated well with an adjusted modal calculated SEBF and represent a more accurate estimate of the dominant eyelid activity. Methods have thus been standardized to allow meaningful measures of spontaneous eyeblink activity to be made in health and disease.
Analysis of refractive & visual development from an infant screening programme
Authors: Grounds Annette R.
Published: July 2002

It has been shown by Aurell and Norrsell (1990), that infants who were 4.00D. hypermetropic or more in one meridian at six months showed a 50% tendency to go on to develop squints and/or amblyopia. The other 50% showed emmetropisation characteristic. Similar trends were found by Dobson and Sebris (1989).

A prescribing criteria developed in this clinic for infants with higher refractive errors is presented and shows that the 50% trend towards squint and amblyopia can be dramatically reduced.

Previous studies have shown that the most rapid decline in hypermetropia occurs between 6 months and 2 years in the normally developing infant eye and similar trends have been reported in infant monkey eyes, (Neuringer et al, 1984). Our results generally agree with this trend.

Data will be presented showing the refractive changes of premature and term infants presented separately between 6 months of age and 3 years. Visual acuity and stereo‐acuity development will also be presented from the age of 6 months to 4 years. It is shown that VA development (with normal visual potential), falls in line with the level of prematurity, that is if 3 months early, then at 6 months of age VA level will be at the 3 months level. Visual acuity gradually improves to the expected level, and the time taken will depend on the amount of prematurity. Generally babies who are eight weeks or less premature catch up within their first year. While those greater than eight weeks but less than twelve weeks normalise by eighteen months to two years, with those greater than twelve weeks taking up to three years. This is in agreement with findings of general development delay changes, in prematurity.
A summary of recent developments in computer technology to aid the visually impaired in the workplace
Authors: Joseph Anna , Harper Robert
Published: July 2002

In the UK, an estimated 75% of the visually disabled population of working age are unemployed. The EC is funding research into methods for assisting the visually impaired. One of the main areas of development involves the use of person computers (PCs).

Enlarged characters on a VDU screen can be achieved either by using a large monitor, or by running a Screen Magnification System. Large monitors vary in size from 17″ to 28″; a 20″ screen will give roughly 1.5 times magnification. A screen magnification system is a software package which increases the size of the characters by between 1.2 and 12 times.

A significant technological development is the amalgamation of CCTVs with PCs. Instead of displaying their information on an integral screen, new generation CCTVs can employ a VDU screen, magnifying by 2 to 60 times. A PC screen can be split between the computer display and input from an external CCTV camera. The proportion of the screen dedicated to the CCTV image can be varied, as can its position.

In some situations, audible rather than visual information is preferable. Speech synthesizers enable the text from a PC display to be read to the user. A Stand Alone Optical Character Reader converts printed text into speech via an integral microprocessor and speech synthesizer.

Despite the wide range of aids available, they are used by only a small proportion of employees with visual impairment. A responsibility of the Hospital Optometrist is clearly to promote awareness of the products available and to encourage liaison with Disability Employment Advisors. Full consideration of the individual's needs, together with an awareness of their entitlements through the Access to Work Scheme, will allow those with a visual disability to compete effectively in the workplace.
Training of patients with age‐related macular disease (AMD) using a scanner laser ophthalmoscope (SLO)
Authors: Culham L. E. , Fitzke F.W. , Marshall J.
Published: July 2002

Purpose: AMD is the leading cause of blindness in developed countries. Rehabilitation is predominant in management since medical intervention is appropriate only in limited circumstances. Prescription of low vision devices is standard but additional low visual training using eccentric viewing techniques has been advocated. The SLO allows visualisation on a TV monitor of both the fundus image and presented visual stimuli and hence is ideally suited to undertake training. The aim was to test the concept that SLO training influenced patients' reading ability.

Methods: Eleven patients were assessed by scotoma and acuity mapping, identification of the preferred retinal location (PRL) and evaluation of fixation stability and letter recognition. A novel SLO training programme totalling 6 hours over a period of 6 weeks emphasised optimal viewing strategies.

Results: Of the ten patients who had a strong PRL an alternative retinal area that provided better performance could not be identified. With training, no significant change was observed in reading speed. However, improvements were recorded in visual acuity for distance (t = 2.765) and near (p < 0.0016), and in fixation stability (p > 0.0316) and letter recognition (p < 0.0009).

Conclusions: SLO training improved patients' performance. This was not due to changing the retinal area used, but possibly was related to psychological benefits associated with training and the acquisition and perfection of new visual strategies.
Isolation of chromatic and achromatic mechanisms: A new approach
Authors: Sharanjeet‐Kaur  , Kulikowski J. J. , Carden D.
Published: July 2002

The purpose of this study was to optimise the testing paradigm for isolating the contributions of chromatic and achromatic mechanisms to the human spectral sensitivity function. Spectral sensitivity was determined for a test spot size of 1.2 deg presented with various spatial and temporal masks on a large, 10 deg background field of moderate intensity (1000 td) and colour temperature, CT = 2700 K. Sinusoidal temporal presentation (1 Hz) and a masking annulus of between 3 and 10 min of arc surrounding the test spot, was found to be most effective in separating chromatic from achromatic mechanisms. Square‐wave (1 Hz) temporal presentation combined with the annulus was slightly less selective. The presence of the annulus did not affect the shape of flicker detection at 25 Hz which is a measure of the luminosity (achromatic) spectral sensitivity function.
Control of the myopic shift in modified Badal optometer
Authors: Kȩdzia Bolesław , Twardowski Piotr
Published: July 2002

Refractive error measurements made with the Badal optometer typically register more myopia than is actually present. In this paper we measured the influence of the image size on the end‐point of a Badal optometer's refractive error measurement. The optometer was modified with a camera's zoom lens in such a way that the retinal image size could be controlled. It was found that retinal image size is a major factor in controlling the subjective endpoint. When retinal image size is diminished rather than increased as the target approaches the subject, as is the case under normal circumstances, the perceived endpoint more closely agrees with the actual refractive error.
Gaze orientation during full‐field and peripheral field passive optokinesis
Authors: Abadi Richard V. , Howard Ian P. , Ohmi Masao
Published: July 2002

During full‐field optokinetic nystagmus the mean position of gaze shifts the eyes in the direction of the fast phase. The driving force for this, pre‐supposes that the preferred locus for the position control system, is shifting gaze into the direction where the motion is coming from. In this study, six subjects were examined to determine whether the absence of the central visual field would influence the mean position of gaze during passive optokinesis. Our findings indicate that a full field and a central field restricted to 20 deg, evoked gaze shifts of up to 7 deg into the direction of the fast phase. However, when the central field was masked by either 12.5 deg or 25 deg the mean gaze position was found to be significantly reduced (p<0.05). This effect was not influenced by the velocity of the stimulus (p>0.05). These results lead us to conclude that gaze orientation during optokinesis is strongly influenced by the area of retina stimulated. The role of the slow eye movement control system and possible cognitive strategies adopted during selective spatial attention are discussed in the light of this finding.
An extension of an accommodation and convergence model of emmetropization to include the effects of illumination intensity
Authors: Blackie C. A. , Howland H. C.
Published: July 2002

The Flitcroft (1998) emmetropization model incorporates a classical model of accommodation and convergence with blur‐driven feedback control of eye growth. We have modified this model by incorporating the effects of illumination (with or without extensive near work) on accommodation, vergence, pupil diameter and emmetropization. In addition to replicating Flitcroft's results, we show that (1) decreased illumination and (2) a low convergence accommodation/convergence (CAC) ratio exacerbate the progression of near‐work‐induced myopia. Our model further indicates that prescription of negative lenses, under these conditions, augments the advancement of myopia.
The Relationship Between Lower Body Strength and Obstructed Gait in Community‐Dwelling Older Adults
Authors: Lamoureux Ecosse L. , Sparrow William A. , Murphy Aron , Newton Robert U.
Published: July 2002

OBJECTIVES:

To determine the relationship between lower body strength of community‐dwelling older adults and the time to negotiate obstructed gait tasks.

DESIGN:

A correlational study.

SETTING:

The Biomechanics Laboratory, Deakin University, Australia.

PARTICIPANTS:

Twenty‐nine women and 16 men aged 62 to 88 were recruited using advertisements placed in local newspapers. The participants were independent community dwellers, healthy and functionally mobile.

MEASUREMENTS:

Maximal isometric strength of the knee extensors and dynamic strength of the hip extensors, hip flexors, hip adductors, hip abductors, knee extensors, knee flexors, and ankle plantar flexors were assessed. The times to negotiate four obstructed gait tasks at three progressively challenging levels on an obstacle course and to complete the course were recorded. The relationship between strength and the crossing times was explored using linear regression models.

RESULTS:

Significant associations between the seven strength measures and the times to negotiate each gait task and to walk the entire course at each level were obtained (r= −0.38 to −0.55;P< .05). In addition, the percentage of the variance explained by strength (R2), consistently increased as a function of the progressively challenging level. This increase was particularly marked for the stepping over task (R2= 19.3%, 25.0%, and 27.2%, for levels 1, 2, and 3, respectively) and the raised surface condition (R2= 17.1%, 21.1%, and 30.8%, for levels 1, 2, and 3, respectively).

CONCLUSION:

The findings of the study showed that strength is a critical requirement for obstructed locomotion. That the magnitude of the association increased as a function of the challenging levels suggests that intervention programs aimed at improving strength would potentially be effective in helping community‐dwelling older adults negotiate environmental gait challenges.J Am Geriatr Soc 50:468–473, 2002.
High Genetic Diversity and Differentiation in Relict Lowland Populations ofGentianella austriaca(A. and J. Kern.) Holub (Gentianaceae)
Authors: Greimler J. , Dobeš Ch.
Published: July 2002

Abstract:The biennialGentianella austriaca(A. & J. Kern.) Holub, representing a nutrient‐poor grassland taxon of low competition power, is becoming rare in the lowlands of eastern Austria due to changes in land use. To estimate effects of isolation and decreasing population sizes, as well as evolutionary relationships, we investigated variation in isozymes and morphological characters within and between seven populations from the mountains, foothills, and lowlands. Additionally, data on reproduction, habitat, germination and population sizes were collected to examine possible causes of variation and differentiation. We found highest genetic diversity (va, vgo) in the lowland and foothill populations, and highest genetic differentiation (Dja, Djgo) (i.e., lowest genetic identity: Nei's I) in the lowland populations. The low diversity of the mountain populations might indicate that they are derived from lowland populations. Surprisingly, highest genetic diversity was found in the smallest population. This indicates that in small remnant populations of taxa with a mixed mating system, genetic diversity might be maintained even after many generations after reducing population size dramatically. We found some relationship between genetic diversity and high fitness (germination success) and (inversely) with seed size. Plant size and reproductive success are negatively correlated with altitude, whereas flower size and seed size seem to be subject to other forces of selection. Combining all morphometric, reproductive and genetic traits, the lowland populations are most strongly differentiated and therefore of highest conservation priority.
Trends in Biomass Fractionation in Wheat and Barley from Wild Ancestors to Modern Cultivars
Authors: Wacker L. , Jacomet S. , Körner Ch.
Published: July 2002

Abstract:We tested the hypothesis that cultivar selection during the process of domestication in cereal plants led to a change in dry mass allocation, e.g., less root mass and more leaf mass or more leaf area per unit leaf mass. We divided 24 varieties of diploid, tetraploid and hexaploid winter wheat and two‐rowed winter barley into three categories of domestication levels (wild species, old landraces and modern cultivars) and compared the patterns of dry matter fractionation at the time of anthesis under standardized outdoor growth conditions. In both cereals, total biomass per individual increased significantly with domestication level but, to our surprise, we found no significant change in dry matter investment between domestication levels: neither the dry mass fraction of leaves increased, nor was there a trend of reduced investment in stems and roots, contrary to what we expected. Specific leaf area (SLA) and leaf area ratio (LAR) of modern wheat and barley cultivars were significantly lower compared to wild varieties. Major differences in both cereals were of a purely morphological nature, namely a decrease in the number of stems and ears from wild species to domesticated varieties, along with more synchronous tiller development and therefore similar tiller size. Fertilizer increased total biomass in all domestication levels in both cereals, but influenced the dry matter fractionation only in barley. Tissue nitrogen concentration was unresponsive to both domestication and fertilization. The expected shift in functional traits, conventionally considered to determine plant growth, was not found. Indeed, dry matter fractionation among the major plant biomass components seems to be very conservative.
Electrical and optical studies of dislocation filtering in InGaAs/GaAs strained-layer superlattices
Authors: Fritz I. J. , Gourley P. L. , Dawson L. R. , Schirber J. E.
Published: July 2002

We report electrical transport and optical studies of the efficiency with which an In0.2Ga0.8As/GaAs strained-layer superlattice (SLS) can filter threading dislocations generated in a thick In0.1 Ga0.9 As layer grown on GaAs. The electrical studies, the first of their kind, rely on a novel test structure which allows electrical characterization of just the top portion of the SLS, with the bottom portion acting as the dislocation filter. For optical characterization we detect dislocations directly by photoluminescence microscopy. The electrical results show that ∼3–6 periods of filtering are needed to attain high mobilities. The photoluminescence microimages show a small density of dislocations near the top of an eight-period SLS but no dislocations for 11 or more periods. Filtering with In0.2Ga0.8As/GaAs SLS’s is more effective than with GaAs0.8P0.2/GaAs SLS’s, possibly because of larger interlayer differences in strain and elastic constants for the former.
Growth of InP on Si substrates by molecular beam epitaxy
Authors: Crumbaker T. E. , Lee H. Y. , Hafich M. J. , Robinson G. Y.
Published: July 2002

The growth of single-crystal InP films on (100) Si substrates by molecular beam epitaxy (MBE) is described. Three different buffer layers were grown by gas-source MBE in order to reduce the density of dislocations created by the 8% InP-Si lattice mismatch. Double-crystal x-ray diffraction revealed that all buffer layers produced large-area single-crystal (100) InP films with the InP lattice tilted towards the 〈100〉 Si directions. A buffer layer of four Inx Ga1−x P/Iny Ga1−y P strained superlattices produced a specular InP film with an estimated dislocation density of 108–109 cm−2 and a residual stress of less than 5×10−4.
ac losses in R
EBa2Cu3O7−y superconductors
Authors: Xu Yunhui , Guan Weiyan , Zeibig K.
Published: July 2002

We present the results of hysteretic ac losses of REBa2Cu3O7−y (RE=Gd, Dy, Er, Eu, Y) calculated from our magnetization data graphically at various temperatures (4.2 K≤T<Tc ) and amplitudes of sweeping field (10−4–4.7 T). We find that the relation of ac loss w versus amplitude of sweeping field Hm follows a power law: w∞Hnm, where n varies from 7 to 1 in various regions of sweeping field Hm. We find that at a fixed field amplitude Hm≫Hp (Hp is the field of full penetration), the dependence of ac losses w on temperature T follows an exponential law: w=αe−βT, which cannot be treated within a mechanism controlled by flux creep.
Preparation and growth of (Bi,Pb)-Sr-Ca-Cu-O superconductor fibers
Authors: Carrillo-Cabrera Wilder , Göpel Wolfgang , de la Fuente Germán F. , Verdún Horacio R.
Published: July 2002

The laser floating zone (LFZ) method has been used to grow (Bi,Pb)-Sr-Ca-Cu-O superconductor fibers from solution-derived ceramic precursors (nominal Bi1.8Pb0.35Sr1.87Ca2Cu3Ox), prepared under different sintering conditions. The as-grown LFZ fibers were semiconducting. After annealing at 855 and 800 °C in air, the fibers were metallic. A second anneal at these temperatures optimized the superconducting properties of the fibers [Tc (midpoint)=111 K and Tc (zero)=106 K]. The LFZ-grown fibers contain highly oriented superconductor grains their electrical properties depend strongly on precursor thermal processing.
Tunneling and thermal emission of electrons from a distribution of shallow traps in SiO2
Authors: Thompson Scott E. , Nishida Toshikazu
Published: July 2002

Charging of existing oxide electron traps is found to be strongly oxide electric field dependent. The transition rate of trapped electrons tunneling into the oxide conduction band increases exponentially with increasing oxide field due to barrier thinning. Electron detrapping via tunnel emission is measured at 77 K using a new isochronal electric-field-stimulated emission technique which gives the energy spectrum of the charged oxide electron traps. The experimental spectrum peaks at 0.9 eV while the measured thermal activation for electron detrapping is about 0.4 eV which are consistent with the Mott ratio of the Jahn–Teller effect for the polar SiO2, 1.9.
Double-junction amorphous silicon-based solar cells with 11% stable efficiency
Authors: Yang J. , Guha S.
Published: July 2002

The performance of dual-band gap, double-junction amorphous silicon alloy-based solar cells in which the component cells have different degrees of current mismatch has been studied under annealed and light-soaked conditions. Using a profiled band gap amorphous silicon-germanium alloy in the bottom cell, a stabilized active-area efficiency of 11.16% has been achieved.
Atomic layer etching of GaAs(110) with Br2 studied by scanning tunneling microscopy
Authors: Patrin J. C. , Li Y. Z. , Chander M. , Weaver J. H.
Published: July 2002

Scanning tunneling microscopy studies of GaAs(110) exposed to Br2 at 720 K show preferential etching at single-height [1̄12] and [001] steps with little etching at double-height steps. Etching in the [11̄0] direction is at least ∼4.5 times faster than in the [001] direction, producing rectangular etch pits. For higher Br2 exposures, etching is dominated by single-height step flow but triangular double-layer etch pits also form on extended terraces.
Microwave gap solitons and bistability in magnetostatic periodic structures
Authors: Dragoman M. , Jäger D.
Published: July 2002

The existence of bistability, gap solitons, and self-localized solitons in a magnetostatic periodic structure are predicted. The very low threshold power necessary for the observation of such phenomenas at microwave frequencies makes these structures very attractive for microwave signal processing.
Novel 1×N and N×N integrated optical switches using self-imaging multimode GaAs/AlGaAs waveguides
Authors: Jenkins R. M. , Heaton J. M. , Wight D. R. , Parker J. T. , Birbeck J. C. H. , Smith G. W. , Hilton K. P.
Published: July 2002

We report the demonstration of novel multiway GaAs/AlGaAs electro-optic waveguide switches which incorporate self-imaging planar multimode waveguide splitters and recombiners interconnected by single-mode guides. Each device consists of one or more input guides, a multiway splitter, an array of individually addressed electro-optic waveguide phase shifters, a multiway recombiner, and an array of output guides. By controlling the voltage applied to the electro-optic guides, light from any one input guide can be switched to any one output guide. We present experimental results for 1×10 and 10×10 devices. Typical values for switching uniformity, maximum crosstalk, and insertion loss were ±9%, −10 dB, and −12 dB, respectively.
Thickness dependence of ferroelectric domains in thin crystalline films
Authors: Tsai Feng , Cowley J. M.
Published: July 2002

The thickness dependence of ferroelectric domains in thin free-standing BaTiO3 crystalline films has been studied by transmission electron microscopy. It is found that the widths of ferroelectric domains decrease as the film thickness decreases. This phenomenon may indicate that the ferroelectric properties of thin films are weakened due to surface relaxation effects, including lattice relaxation and a change of spontaneous polarization and charge compensation. The weakening of ferroelectric domains is suggested as a transition state from ferroelectric to paraelectric phase of the BaTiO3 thin film. The thickness of the surface relaxation layer of totally nonferroelectric film is on the order of 10 nm.
Effect of poling on piezoelectric properties of lead zirconate titanate thin films formed by sputtering
Authors: Watanabe Shunji , Fujiu Takamitsu , Fujii Toru
Published: July 2002

Lead zirconate titanate (PZT) thin film was formed on Pt/Ta/Si3N4/Si(100) substrate at 400 °C by sputtering and then annealed at 650 °C in air. The PZT film was 1 μm thick and had dielectric permittivity of 980, loss tangent of 0.05, remanent polarization of 31 μC/cm2, and coercive field of 110 kV/cm. Piezoelectric property of the film formed on silicon cantilever was estimated from the converse effect. Poling at 5 kV/mm increased the property by a factor of 1.2 to 3.4, resulting in converse piezoelectric constant (d31) varying from −84 to −102 pC/N.
Compact efficient modular cesium atomic beam oven
Authors: Tompa G. S. , Lopes J. L. , Wohlrab G.
Published: July 2002

A simple atomic beam oven is described. It uses a modified commercially available all metal sealed valve to break a glass cesium ampoule. Standard fittings are used to provide modularity and cost efficiency. Although designed for use with cesium ampoules, application to any volatile material is straightforward.
Two-level liquid-nitrogen-cooled sample mounting assembly for surface analysis
Authors: Chusuei C. C. , Gwynn L. K. , Schreifels J. A.
Published: July 2002

A design for a sample mounting assembly is discussed that permits standard ultrahigh vacuum surface analysis experiments to be performed both on well defined surfaces, such as single crystals, and on externally prepared surfaces.
Ion sources for ion beam assisted deposition of thin films and coatings (invited) (abstract)
Authors: Ensinger W.
Published: July 2002

Ion beam assisted deposition (IBAD) is a coating technique which combines a thin-film deposition method such as evaporation with irradiation by highly energetic ions from an ion source. Application of an ion source and a vapor source which are operated independently of each other render the IBAD technique highly controllable, reproducible, and flexible. Ion flux, atom flux, ion energy, ion impact angle, and other parameters can be controlled independently over a wide range. In order to take advantage of the beneficial features of this technique and obtain optimum process conditions, ion sources with special properties are required. In this paper different ion source types and equipment which has been used for IBAD to date are presented and their special features discussed. Ion sources which should be applicable for IBAD are described. Finally, general requirements of IBAD ion sources are discussed.
Series operation of power MOSFETs for high-speed, high-voltage switching applications
Authors: Baker R. J. , Johnson B. P.
Published: July 2002

Series operation of power metal-oxide semiconductor field-effect transistors (MOSFETs) to increase their effective hold off voltage is described. The design procedure presented is a modification of a recently reported [Baker and Johnson, Rev. Sci. Instrum. 63, 5799 (1992)] method. Comments are made on implementing MOSFET stacks in various types of instrumentation.
Cooling of silicon crystal monochromator for the undulator x rays at the beamline NE3
Authors: Mochizuki Tetsuro , Zhang Xiaowei , Sugiyama Hirosi , Zhao Jiyong , Ando Masami , Yoda Yositake
Published: July 2002

A double crystal monochromator accommodating water or liquid nitrogen cooling device for a first monochromator crystal was designed and installed in the x-ray undulator beamline NE3 of the 6.5 GeV TRISTAN accumulation ring. In a series of cooling experiments at the BL-NE3 the silicon crystals were set at an incident angle of 7.9° to reflect 14.4 keV photons and its higher harmonics, and its maximum power density on the crystal surface was 4.3 W/mm2 (815 W total power). The water cooled crystal behaved properly only when the peak power density was below 1.0 W/mm2 (185 W total power), while the LN2 cooled one showed no apparent changes in width of rocking curve and its reflectivity against the maximum power and the maximum peak power density 815 W and 4.3 W/mm2, respectively, and no deformation in the pattern of a diffraction spot.
Dissipation of mechanical energy in fused silica fibers
Authors: Gretarsson Andri M. , Harry Gregory M.
Published: July 2002

To determine the dissipation induced by the surface of fused silica fibers, we measured the quality factor of fibers having various diameters. We measured a maximum quality factor of 21 million and extrapolated to obtain an intrinsic quality factor for fused silica of 30 million. Dissipation in the surface dominated at diameters less than about 1 mm. We developed a method for characterizing surface-induced dissipation that is independent of sample geometry or mode shape.
Evidence of carrier number fluctuation as origin of 1/f noise in polycrystalline silicon thin film transistors
Authors: Corradetti A. , Leoni R. , Carluccio R. , Fortunato G. , Reita C. , Plais F. , Pribat D.
Published: July 2002

A systematic study of the noise performances of polycrystalline silicon thin film transistors is presented. The drain current spectral density of these devices shows an evident 1/ f behavior and scales, when operating in the linear regime, with the square of the mean value of the drain current. The origin of the noise can be ascribed to carrier number fluctuations related to the dynamic trapping and detrapping of the oxide traps.
Imaging ferromagnetic tracers with an ac biosusceptometer
Authors: Moreira M. , Murta L. O. , Baffa O.
Published: July 2002

The aim of this work was to study the feasibility of obtaining images of the distribution of ferromagnetic tracers using an ac biosusceptometer (ACB). The images were generated by mapping the response of the ACB at short distances due to the presence of a ferrite powder dispersed in a planar gel matrix with different shapes. The ACB was scanned over the sample and the voltages generated by the variation in ferrite concentration were sampled. Methods to render and image the distribution of ferromagnetic particles were studied. The generalized Wiener parametric method gave the best results. The system has a potential to be used in functional images of the gastrointestinal tract where a moderate resolution is required. We conclude that this biomagnetic method can be successfully used to generate planar functional images of magnetic particles in the near field.
Broadening of the excitonic linewidth due to scattering of two-dimensional free carriers
Authors: Liu Wei , Jiang Desheng , Luo Kejian , Zhang Yaohui , Yang Xiaoping
Published: July 2002

By photoluminescence measurements we find that at low temperature the linewidth of the excitonic luminescence broadens with increasing electron density in the wider well from a photoexcited type-I-type-II mixed GaAs/AlAs asymmetric double quantum well structure, which even makes the excitonic linewidth at 77 K larger than at 300 K above a certain excitation intensity. We verify that the broadening is due to the scattering of two-dimensional carriers to excitonic states. Based on the theory of the scattering of carriers to excitonic states, we calculate the broadening of the excitonic linewidth. Our experimental results are convincing for verifying the theoretical prediction.
Observation of ferromagnetic resonance in a microscopic sample using magnetic resonance force microscopy
Authors: Zhang Z. , Hammel P. C. , Wigen P. E.
Published: July 2002

We report the observation of a ferromagnetic resonance signal arising from a microscopic (∼20 μm×40 μm) particle of thin (3μm) yttrium iron garnet film using magnetic resonance force microscopy (MRFM). The large signal intensity in the resonance spectra suggests that MRFM could become a powerful microscopic ferromagnetic resonance technique with a micron or sub-micron resolution. We also observe a very strong nonresonance signal which occurs in the field regime where the sample magnetization readily reorients in response to the modulation of the magnetic field. This signal will be the main noise source in applications where a magnet is mounted on the cantilever.
Ultrasound transducer with low synthetic quality factor
Authors: Fiorillo Antonino S.
Published: July 2002

An ultrasound transducer with low synthetic quality factor, for in-air applications, is presented. It is realized by curving a thin sheet of polyvinylidenefluoride and by assembling the so shaped piezo-polymer film onto a mobile support. The resonance frequency, which is inversely proportional to the bending radius, can be easily varied on a range of some tens of kHz, 30–65 kHz, by mechanically controlling the opening arc angle of the curved film. An ultrasound transducer with both controlled resonance and controlled axial resolution is obtained. The combination of the different resonance frequencies, related to different opening angles, leads to a synthetic broad-band transducer with unity quality factor Q.
Deposition of diamond-like carbon films by electrolysis of methanol solution
Authors: Wang Hao , Shen Ming-Rong , Ning Zhao-Yuan , Ye Chao , Cao Chuan-Bao , Dang Hai-Yan , Zhu He-Sun
Published: July 2002

By electrolysis of the methanol solution, an attempt was made to deposit diamondlike carbon (DLC) films on silicon substrate at temperature of less than 60 °C. Substrates were negatively biased with a dc potential of 0 to −3000 V. IR spectra showed that the O–H, C–H, and C–O vibration bands of electrolyte decreased remarkably after electrolysis and a new peak characterized as the C=C bond appeared. The deposited films were characterized as DLC films by Raman spectroscopy.
Effects of B doping on hydrogen desorption from Si(001) during gas-source molecular-beam epitaxy from Si2H6 and B2H6
Authors: Kim H. , Glass G. , Park S. Y. , Spila T. , Taylor N. , Abelson J. R. , Greene J. E.
Published: July 2002

Boron doping concentrations ≳6×1019 cm−3 were found to increase Si(001) growth rates RSi at low temperatures while decreasing RSi at higher temperatures during gas-source molecular beam epitaxy (GS-MBE) from Si2H6 and B2H6. In order to probe the mechanisms governing these effects, Si(001) samples with B coverages θB ranging from <0.05 to ≂0.5 ML were prepared by exposing clean Si(001)2×1 wafers to B2H6 doses between 2×1017 and 4×1020 cm−2 at 200–400 °C. The samples were then heated to 700 °C to desorb the hydrogen, cooled to 200 °C, and exposed to atomic deuterium until saturation coverage. D2 temperature programmed desorption spectra exhibit β2 and β1 peaks due to dideuteride and monodeuteride desorption at 405 and 515 °C as well as new B-induced peaks, β2* and β1*, at 330 and 470 °C. Increasing θB increases the area under β2* and β1* at the expense of β2 and β1. Moreover, the total D coverage continuously decreases from ≂1.23 ML in the absence of B to ≂0.74 ML at θB=0.5 ML. We propose that the observed B-induced decrease in the Si*-D bond strength, where Si* represents surface Si atoms bonded to second-layer B atoms, is due to charge transfer and increased Si* dimer strain. The Si* to B charge transfer also deactivates Si surface dangling bonds causing the decrease in θD. These results are used to explain the GS-MBE growth kinetics.
Angle etch control for silicon carbide power devices
Authors: Lanois F. , Lassagne P. , Planson D. , Locatelli M. L.
Published: July 2002

Silicon carbide plasma etching results are reported. Etching experiments are performed in a distributed electron cyclotron resonance reactor, using a SF6/O2 gas mixture, on both 3C- and 6H-SiC. A special interest has been given to the slope of the etched sidewalls. Slopes between 30° and 80° have been achieved by varying selectivities between SiC and the SiO2 masking layer. Two parameters have been investigated to modulate selectivity: bias voltage and O2 additive flow. A wide range of selectivities (from 1 to 6.5) has been obtained for suitable etch rate (100 to 270 nm/min) with very smooth surfaces.
Low-temperature nitriding of iron through a thin nickel layer
Authors: Inia D. K. , Pröpper M. H. , Arnoldbik W. M. , Vredenberg A. M. , Boerma D. O.
Published: July 2002

We present a new method for nitriding iron at low temperatures. First, iron is coated with a thin layer of nickel (∼36 nm), after which it is exposed to an NH3 atmosphere at temperatures below 300 °C. Underneath the nickel layer ε-Fe3−xN is formed at temperatures as low as 225 °C, while uncovered iron samples show a large uptake of oxygen after the same treatment. The nickel layer prevents the oxidation of iron by impurities in the NH3 gas, and acts as a catalyst for the decomposition of NH3. After decomposition the atomic nitrogen diffuses through the nickel layer towards the iron. With the process described, pore-free iron nitrides can be formed at low temperatures.
Semiconductor nanostructures formed by the Turing instability
Authors: Temmyo Jiro , Nötzel Richard , Tamamura Toshiaki
Published: July 2002

We describe the surface topography domain in a strained InGaAs/AlGaAs system on the GaAs (311)B substrate during metalorganic-vapor-phase-epitaxial growth. The surface rearrangement resulting in the formation of nanostructures seems to belong to a Turing-type self-organization phenomenon that results from a spontaneous symmetry-breaking instability in nonlinear dynamical systems. The unique surface morphologies on the high Miller index faces suggest a novel fourth growth mode due to Turing-type self-organization.
Bi
4
Ti
3
O
12
ferroelectric thin film ultraviolet detectors
Authors: Pintilie L. , Alexe M. , Pignolet A. , Hesse D.
Published: July 2002

Bi 4 Ti 3 O 12 (BiT) ferroelectric thin films were used as photoconductive or photovoltaic ultraviolet detectors. The maximum of the spectral distribution lies around 370 or 390 nm, depending on whether the incident light is continuous or modulated. The maximum current responsivity obtained for the films annealed at 700 °C is about 0.01 A/W. The specific detectivity is 5.5×108 cm W−1 Hz1/2 at a modulation frequency of 20 Hz. The frequency characteristics of the photovoltaic signal measured in modulated light suggests a 1/f frequency dependence of this signal.
Green photoluminescence from cubic In0.4Ga0.6N grown by radio frequency plasma-assisted molecular beam epitaxy
Authors: Müllhäuser J. R. , Brandt O. , Trampert A. , Jenichen B. , Ploog K. H.
Published: July 2002

We investigate the structural and optical properties of a cubic (In, Ga)N/GaN/(Al, Ga)N heterostructure containing a 185 nm thick In0.4Ga0.6N layer which dominates the optical properties of the sample. The phase purity of the structure is verified by means of transmission electron microscopy while the In content is measured by x-ray diffraction and secondary ion mass spectrometry. The room-temperature band gap of the In0.4Ga0.6N layer is determined by transmission and reflectance measurements to be 2.46±0.03 eV. This value agrees with the spectral position of the dominating green photoluminescence at 300 K.
Charge accumulation at a threading edge dislocation in gallium nitride
Authors: Leung K. , Wright A. F. , Stechel E. B.
Published: July 2002

We have performed Monte Carlo calculations to determine the charge accumulation on threading edge dislocations in GaN as a function of the dislocation density and background dopant density. Four possible core structures have been examined, each of which produces defect levels in the gap and may therefore act as electron or hole traps. Our results indicate that charge accumulation, and the resulting electrostatic interactions, can change the relative stabilities of the different core structures. Structures having Ga and N vacancies at the dislocation core are predicted to be stable under nitrogen-rich and gallium-rich growth conditions, respectively. Due to dopant depletion at high dislocation density and the multitude of charge states, the line charge exhibits complex crossover behavior as the dopant and dislocation densities vary.
Electromigration path in Cu thin-film lines
Authors: Hu C.-K. , Rosenberg R. , Lee K. Y.
Published: July 2002

Electromigration in 0.15–10 μm wide and 0.3 μm thick Cu lines deposited by physical vapor deposition has been investigated using both resistance and edge displacement techniques in the sample temperature range 255–405 °C. For wide polycrystalline lines (>1 μm), the dominant diffusion mechanism is a mixture of grain boundary and surface diffusion, while in narrow lines (<1 μm) the dominant mechanism is surface transport. The activation energy for grain-boundary transport is approximately 0.2 eV higher than that of surface transport.