The responsivity of a SiC photodiode was measured with synchrotron radiation in the deep UV and for the first time in the EUV and soft x-ray wavelength regions. A peak responsivity was 200 mA/W at 270 nm wavelength and 60 mA/w at 13 nm in the EUV. Extended measurements at shorter wavelengths demonstrated a responsivity up to 80 mA/W for wavelengths as short as 1.5 nm. The responsivity was calculated by an optical model that accounted for the reflection and absorption of the incident electromagnetic wave, the pair creation energy in the 6H-SiC device, and the variation of the charge collection efficiency (CCE) with depth into the device. The calculated responsivity was in excellent agreement with the measured responsivity and with the structure of the p-n junction photodiode. The measured visible light sensitivity was a factor of 100 lower than that of a silicon photodiode. These new results open up several possible applications for SiC photodiodes, including the selective detection of EUV and soft x-ray radiation without contamination by visible and IR wavelengths. SiC photodiodes have also been proven to withstand prolonged UV exposure and extreme temperatures, thus making them nearly ideal detectors for fiiture solar and space missions where absolutely calibrated EUV and soft x-ray intensities must be accurately measured.
Silicon photodiode detectors with multilayer coatings were characterized using synchrotron radiation. The coatings were composed of thin layers of metals and other materials and were designed to provide wavelength bandpasses in the 17 - 150 angstrom wavelength region. The measured transmittances of the multilayer coatings are in good agreement with the calculated transmittances. The modeling accounts for the transmittance of the multilayer coating and the deposition of the radiation energy in the underlying silicon photodiode. Detectors with the following layer materials (and wavelength bandpasses were characterized: Fe/Al (17 - 30 angstrom), Mn/Al (19 - 30 Angstrom), V/Al (24 - 35 angstrom), Ti/C (27 - 40 angstrom), Pd/Ti (27 - 50 angstrom), Ti/Zr/Al (27 - 50 angstrom), Ag/CaF2/Al (36 - 50 angstrom), and Ti/Mo/C (50 - 150 angstrom).
Silicon photodiodes which operate satisfactorily in the extreme ultraviolet (EUV) have been commercially available for the past few years. These photodiodes also inherently respond to radiation extending from the x-ray region to the near infrared, a property which is undesirable in many EUV applications. The addition of a thin film of a suitable filtering material to the surface of such a photodiode can accomplish the restriction of the sensitivity of the silicon to a much narrower band, or bands, in the EUV. This results in a rugged, yet sensitive photometer for applications in which dominant out-of-band radiation is present. Applications include plasma diagnostics, solar physics, x-ray lithography, x-ray microscopy, and materials science. Previous attempts to produce such devices have resulted in degraded shunt resistance with a corresponding increase in background noise. Prototype detectors have now been fabricated using directly deposited films of aluminum, aluminum/carbon, aluminum/carbon/scandium, silver, tin, and titanium, without degradation of the noise characteristics of the uncoated photodiodes. Measured and theoretical sensitivity data are presented, as well as a discussion of relatively simple methods to reduce the x-ray response of such filtered detectors.
New developments in transmission grating and photodiode technology now make it possible to realize spectrometers in the extreme ultraviolet (EUV) spectral region (λ<1000Å), which are expected to be virtually constant in their diffraction and detector properties. Time-dependent effects associated with reflection gratings are eliminated through the use of free-standing transmission gratings. These gratings together with recently developed and highly stable EUV photodiodes have been utilized to construct a highly stable normal incidence spectrophotometer to monitor the variability and absolute intensity of the solar 304-Å line. Owing to its low weight and compactness, such a spectrometer will be a valuable tool for providing absolute solar irradiance throughout the EUV. This novel instrument will also be useful for cross-calibrating other EUV flight instruments and will be flown on a series of Hitchhiker shuttle flights and on SOHO. A preliminary version of this instrument has been fabricated and characterized, and the results are described.
New developments in transmission grating and photodiode technology now make it possible to realize spectrometers in the extreme ultraviolet (EUV) spectral region (wavelengths less than 1000 A) which are expected to be virtually constant in their diffraction and detector properties. Time dependent effects associated with reflection gratings are eliminated through the use of free standing transmission gratings. These gratings together with recently developed and highly stable EUV photodiodes have been utilized to construct a highly stable normal incidence spectrophotometer to monitor the variability and absolute intensity of the solar 304 A line. Owing to its low weight and compactness, such a spectrometer will be a valuable tool for providing absolute solar irradiance throughout the EUV. This novel instrument will also be useful for cross-calibrating other EUV flight instruments and will be flown on a series of Hitchhiker Shuttle Flights and on SOHO. A preliminary version of this instrument has been fabricated and characterized, and the results are described.
Available photodiodes are reviewed with attention given to the performance parameters, temporal stability, and appropriateness for narrow bandpass applications and certain photon energies. The configuration of XUV photodiodes for use in the EUV and soft X-ray regions is delineated, and the measured parameters are outlined. The photodiodes have stable efficiencies that vary linearly with photon energy and exceed 1 electron/incident photon for photon energies of at least 10 eV. The silicon photodiodes are found to be suitable for EUV and soft X-ray applications and are stable, very highly efficient, and are unaffected by operation under high gas pressures. The silicon dioxide outer surface can be coated with thin films to develop narrow bandpass applications. The present XUV silicon detectors have active areas of 1 or 3 sq cm and can be used with an instrument for measuring photocurrent without external power supplies.
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