Spectral characterization of yeast cells with an epitaxy-based UV-Vis optical sensor.

The optical spectra of yeast cells in phosphate buffer saline (PBS) were analyzed with an optical UV-vis sensor based on a shallow p(+)n junction realized in a low doped n-type epitaxial silicon layer grown on a strongly doped n(+) substrate. The presence of the n/n(+) interface allows a significantly enhanced sensitivity, due to an increased collection of carriers photogenerated both by short and large wavelengths in the range 250...800 nm. In our experiments the optical absorption of yeast cells was investigated in the wavelength range 250...500 nm as a function of the cells concentration in PBS in the range of 6 x 10(6)-2 x 10(8) cells/ml. The main absorption peaks were found at 310, 350, 400 and 427 nm, respectively. A significant red shift of the wide absorption band at 427 nm has been observed when increasing cell concentration. This red shift behaviour was nonlinear, with saturation observed for yeast concentrations larger than 5 x 10(7) cells/ml. The half-peak bandwidth of this peak also showed a most significant nonlinear variation. These findings suggest that monitoring the parameters of the absorption band at 427 nm versus cells concentration could be used, e.g. using a dedicated integrated spectrometric microsystem, for fast quantitative measurements of yeast cell concentrations in various bio-samples, with possible applications in the food industry.

Thin-film optical sensors with silicon-compatible materials.

Antireflection filters based on multilayer stacks of dielectric and polysilicon films on monocrystalline silicon combined with charge collection in different (poly)Si layers can be used to realize sensors with a programmable spectral response controlled by weighted summing of the photocurrents detected in the polysilicon and the substrate. Thus, employing both interference and selective absorption of light yields increased photoelectric efficiency and improved flexibility of spectral control and enables on-chip integration of the detector(s) with the signal conditioning and processing circuits. The potential of thin-film color sensors has been evaluated for this purpose. However, for practical implementation of such structures the problems associated with the realization of reliable photodetectors in polysilicon must also be considered. Phosphorus passivation of the grain-boundary states has been employed to yield polysilicon photodiodes with improved electrical characteristics and reliable light and color detection. We present the design methods of thin-film color sensors employing silicon-compatible materials only. The measurement results of a fabricated structure fully demonstrate that such sensors can be realized with good spectral selectivity.

Optical properties of thin-film silicon-compatible materials.

The optical performance of photodetectors in silicon strongly depends on the transmission of incident light into Si and the charge collection efficiency therein. Consequently, improving the transmission efficiency of light into Si by means of an interference antireflectant (AR) filter can lead to improved optoelectric conversion efficiency. However, the design of these filters requires the availability of data on the optical characteristics of the materials used. Furthermore, for the realization of such filters for light detectors realized in silicon, the required compatibility with standard microelectronic processing implies that only Si-compatible materials should be used. The range of such Si-compatible materials available for fabricating photoelectric devices in silicon includes thermally grown SiO(2), low-pressure chemical-vapor deposited polysilicon, silicon nitride (low stress and stoichiometric), and oxides (low-temperature oxide, phosphosilicate glass, borosilicate glass, borophosphosilicate glass) as well as plasma-enhanced chemical-vapor deposited oxynitrides. We present the refractive index and the extinction coefficient of these materials in the 380-800-nm range. The data presented enhance the accuracy and applicability of simulation and design tools used to design photodetectors in silicon for the visible range.