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Solid State Silicon Standard Photodiodes
Silicon (Si) photodiodes are issued by NIST
as transfer standard detectors throughout the vacuum ultraviolet spectral region
(5 nm to 254 nm). Each device is an n on p junction with a thin (less
than 15 nm) oxide layer grown on the front surface and specially processed
to improve the radiation hardness. Once the chip is manufactured, it is mounted
in a ceramic case. Redundant leads are wire bonded to the chip to make an
electrical connection to two pins where user electronics may be easily attached.
Incident radiation generates electron-hole pairs in the Si which cause an
electric current to flow across the junction.
In recent years, the availability of Si standards from NIST has been
restricted to the spectral region below 50 nm in wavelength. This was due
to significant losses in efficiency at longer wavelengths when exposed to
radiation which the oxide strongly absorbs. Recent progress in the production
of radiation hardened Si devices now enables NIST to issue standards with
acceptable stability characteristics at all vacuum ultraviolet wavelengths.
These specially processed, hardened transfer standards are only available
from NIST.
The sensitivity of a Si photodiode is typically greater than
50 electrons / photon when radiation with 5 nm wavelength
is incident, but falls to an efficiency of about 5 electrons/photon when
50 nm radiation is incident. The efficiency at longer wavelengths falls
to below 1 electron/photon, but is strongly dependent on interference effects
in the reflectivity of the oxide/substrate system. Calibration uncertainties
(2-sigma) range from 7% to 15% over most of the 5 nm to 122 nm
region, with slightly larger uncertainties below 6.5 nm and between
45 nm and 50 nm. At wavelengths longer than 125 nm,
uncertainties are 2%. These detectors have much higher sensitivity than
photoemissive detectors, require no bias voltage, and are small and
lightweight. However, they are noisier and exhibit higher dark currents
than photoemissive standards and have broadband response from the
infrared to the x-ray region.
A fee schedule and ordering information are available.
In addition to these Si detectors, the
Vacuum UV
Transfer Standard Program issues other types of transfer standard detectors:
Detector Construction and Circuit Schematic
Radiation incident on the detector front surface generates
electron-hole pairs in the Si. These carrier pairs are separated by the
internal electric field and a photocurrent flows across the p-n junction.
Typical Efficiency of a NIST Silicon Transfer Standard
Detector efficiency is expressed in terms of electrons per
incident photon, and is a function of the incident radiation wavelength and
the device oxide layer thickness. The extreme ultraviolet
(5 nm to 50 nm) quantum efficiency of a Si photodiode is essentially
the photon energy divided by the electron-hole pair creation energy
(about 3.63 eV), with an additional wavelength dependent loss factor
due to absorption in the oxide.
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