Mario Noël

Optical dispersion relationships in amorphous silicon grown by molecular beam epitaxy

We have produced a novel form of amorphous silicon (a-Si) using ultra-high-vacuum molecular beam epitaxy (MBE). From measurements of the specular reflectance spectrum at near normal incidence and the regular transmittance spectrum at normal incidence we have determined the spectral dependence of the refractive index, the extinction coefficient, the optical absorption coefficient, and the real and complex components of the dielectric function. These optical dispersion relationships are contrasted with those corresponding to other forms of a-Si.

Influence of growth temperature on order within silicon films grown by ultrahigh-vacuum evaporation on silica

We study the role that the growth temperature plays in determining the amount of order present within silicon films deposited on fused silica substrates through ultrahigh-vacuum evaporation at growth temperatures ranging from 98 to 572°C. Through measurements of the Raman and optical absorption spectra, we quantitatively determine how the growth temperature influences the order present within 11 such films. We employ three disparate measures of order for the purposes of this study: the breadth of the transverse-optic phonon Raman peak, this being related to the amount of short-range order present; the area under the transverse-acoustic Raman peak divided by the area under the corresponding transverse-optic peak, this being related to the amount of intermediate-range order present; and the breadth of the optical absorption tail, which is a general measure of the overall amount of order present. All three measures of order indicate a dramatic increase in the amount of order present for growth temperatures a...

Optical absorption in an amorphous silicon superlattice grown by molecular beam epitaxy

Abstract We have fabricated amorphous silicon (a-Si) superlattices, comprised of thin layers of a-Si separated by even thinner layers of SiO2 through ultra-high-vacuum molecular beam epitaxy and an ultraviolet ozone process. From measurements of the specular reflection spectrum at near normal incidence, and the regular transmittance spectrum at normal incidence, we have determined the spectral dependence of the optical absorption coefficient corresponding to the a-Si layers within such a superlattice deposited on sapphire. We contrast these results with those corresponding to thin films of a-Si deposited through ultra-high-vacuum molecular beam epitaxy and find that the optical absorption edge of the a-Si layers within the a-Si/SiO2 superlattices is sharper and occurs at higher energies as compared with the thin films of a-Si. We conjecture that both quantum confinement and impurities may be responsible for this effect.

PROCEDURES AND STANDARDS FOR ACCURATE SPECTROPHOTOMETRIC MEASUREMENTS OF SPECULAR REFLECTANCE

Procedures and standards that have been developed at the National Research Council of Canada for the accurate measurement of specular reflectance are discussed for both absolute and relative methods over the spectral range 250 to 2500 nm. There has been an increasing demand for these types of measurements, particularly for coated samples approaching the extremes of 0% reflectance and 100% reflectance. In some applications of these coatings, such as energy conservation and control, conventional methods of measuring specular reflectance give insufficient accuracies for the prediction of optical performance. Details of alignment procedures for both absolute and relative reflectance methods, the preparation and application of several candidate reflectance standards, and the compensation, attenuation, and verification procedures that have been developed to improve the precision and accuracy of specular reflectance measurements are described. Using these various techniques, one can routinely achieve accuracies of 0.3% reflectance at reflectance values as high as 97% and as low as 4%.

Optical characterization of a reference instrument for gloss measurements in both a collimated and a converging beam geometry

A reference instrument has been developed at the National Research Council of Canada for rapid, reproducible specular gloss measurements. The design and validation of this instrument for specular gloss measurements in accordance with standard methods for paints and plastics at 20 degree, 60 degree, and 85 degree geometries [American Society for Testing and Materials (ASTM) D523 and the International Organization for Standards (ISO) 2813] have been recently reported. These standard methods require a collimated beam geometry. Here we present the optical design considerations and characterization of this instrument to extend its gloss measurement capabilities to specular gloss measurements of paper samples at 75 degree geometry in accordance with standard test methods requiring a converging beam geometry (ASTM D1223 and TAPPI T480). This is, to the best of our knowledge, the first reported reference instrument that provides direct traceability for both types of standard gloss method and applications. The design challenge was to convert from a collimated beam to converging beam geometry while meeting the rigorous requirements of beam uniformity at the sample and receptor apertures specified in the 75 degree geometry test methods. We describe the innovative design to achieve this degree of functionality and reference instrument performance. The instruments optical performance has been characterized theoretically and by comparison with measurement results. The light collection and detection systems have been analyzed via Monte Carlo simulation and ray tracing. The instrument validation includes comparison of the measurement results with theoretical gloss values for quartz, black glass, Vitrolite, and mirror gloss working standards, giving agreement of better than 0.32%. Measurement validation also involved participation in the Collaborative Testing Services program interlaboratory comparison measurements of 75 degree gloss for white papers.

Specular Gloss Scales Comparison Between the National Institute of Standards and Technology and the National Research Council of Canada

The gloss value of a test sample is determined relative to a standard, generally a polished piece of black glass. Therefore, gloss is a dimensionless quantity whose accurate determination requires standardized experimental conditions such as spectral distribution of the incident beam of light, incident and viewing angles, and a gloss standard. To help manufacturers monitor and assess specular gloss, the National Institute of Standards and Technology (NIST) Physics Laboratory provides a special test service to calibrate gloss reference standards. This facility is built around a newly rebuilt reference goniophotometer—an instrument that measures flux as a function of angles of illumination or observation—and a newly developed primary gloss standard—three wedges of highly polished, high-quality optical glass. The system has an overall (k=2) uncertainty of 0.4%. The new service offers calibration measurements of industry working gloss standards at the specular geometries of 20°, 60°, and 85°, in compliance with the International Standards Organization (ISO) 2813 and the American Society for Testing and Materials (ASTM) D 523 documentary standards. This article describes a bilateral comparison of specular gloss scales between NIST and the National Research Council of Canada (NRCC) that has been performed. The results of this comparison show agreement within the combined uncertainties for the measurement of specular gloss of highly polished black glass.

An amorphous-to-crystalline phase transition within thin silicon films grown by ultra-high-vacuum evaporation and its impact on the optical response

A number of thin silicon films are deposited on crystalline silicon, native oxidized crystalline silicon, and optical quality fused quartz substrates through the use of ultra-high-vacuum evaporation at growth temperatures ranging from 98 to 572 °C. An analysis of their grazing incidence X-ray diffraction and Raman spectra indicates that a phase transition, from amorphous-to-crystalline, occurs as the growth temperature is increased. Through a peak decomposition process, applied to the Raman spectroscopy results, the crystalline volume fractions associated with these samples are plotted as a function of the growth temperature for the different substrates considered. It is noted that the samples grown on the crystalline silicon substrates have the lowest crystallanity onset temperature, whereas those grown on the optical quality fused quartz substrates have the highest crystallanity onset temperature; the samples grown on the native oxidized crystalline silicon substrates have a crystallanity onset temperat...

Near-infrared photoluminescence of orange color standards – then and now

The presence of near-infrared (NIR) photoluminescence has been recently reported in some of the second series of Ceramic Color Standards (CCSII) that are widely used in the calibration and performance evaluation of color measuring instruments. The impact of this photoluminescence effect can cause significant colorimetric errors particularly for broadband measurements using a detector with high spectral responsivity in the NIR region. The magnitude of this effect has been demonstrated for specific color standards and specific instrument systems but has not been unambiguously quantified to allow general predictions or absolute comparisons of different instrument designs or different ceramic tiles. Here we present absolute NIR photoluminescence measurements on three different formulations of the CCSII orange ceramic color standard using the National Research Council of Canada (NRC) Reference Spectrofluorimeter whose spectral range has been recently extended to 1000 nm. The validation for this extended spectral range is shown by comparison of an independent method of instrument calibration using a different combination of physical standards. It is convincingly shown that the two different leaded formulations of this ceramic orange standard issued in 2000 and 2011 have no significant photoluminescence and thus can be used for calibration with any type of spectrophotometer design whereas the unleaded formulation issued in 2011 has significant NIR photoluminescence and should not be used for instrument calibration and validation over an extended range into the NIR for certain spectrophotometers with relatively high throughput in the NIR region, such as a spectrophotometer with polychromatic illumination mode using a xenon source or with monochromatic illumination mode using a Si detector. It is shown that for colorimetric applications, the impact of this NIR fluorescence is only significant for the latter spectrophotometer design with broadband detection with a Si or spectrally flat detector and is negligible with a narrowband PMT detector. These calculated colorimetric results are also consistent with previously estimated colorimetric errors for this type of orange CCSII ceramic tile used to transfer calibration between these two types of detector systems.

Characterization of a versatile reference instrument for traceable fluorescence measurements using different illumination and viewing geometries specified in practical colorimetry—part 1: bidirectional geometry (45:0)

For highest accuracy fluorescence colorimetry, standardizing organizations recommend the use of a two-monochromator method with a bidirectional illumination and viewing geometry (45:0 or 0:45). For this reason, reference fluorescence instruments developed by National Measurement Institutes (NMIs) have largely conformed to this bidirectional geometry. However, for many practical applications in colorimetry where the samples exhibit texture, surface roughness or other spatial non-uniformities, the relevant standard test methods specify a sphere geometry with diffuse illumination or viewing (e.g. d:8 or 8:d) which gives improved measurement precision. This difference in the measurement geometry between the primary instrument used to realize the fluorescence scale and the secondary testing instruments used for practical measurements, compromises the traceability of these fluorescence calibrations. To address this metrology issue, a two-monochromator goniospectrofluorimeter instrument has been developed at the National Research Council of Canada (NRC). This instrument can be configured for different illumination and viewing geometries to conform with international standards for different colorimetric applications. To improve the traceability chain for measurements using different geometries, the instrument has been thoroughly characterized and validated by means of comparison measurements with NRCs other spectrophotometric and fluorescence reference instruments. This uncertainty analysis has been carried out in a step-wise manner; first, for a bidirectional geometry (45:0) and then for a sphere geometry (8:d) to provide an uninterrupted traceability to primary radiometric scales. The first paper in this two paper series reviews the background to this work and provides details of the basic design of the new instrument and its characterization for measurements using a bidirectional geometry (45:0), including a representative uncertainty budget. In part 2, the major sources of sphere error are described and minimized in a modified sphere design. The instrument characterization and validation are then extended to a sphere geometry (8:d) to provide direct traceability for practical fluorescence colorimetry.