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Windowless Aluminum Oxide (Al2O3) Photodiodes


Photograph of an Al Oxide photodiode calibrated by the EUV Transfer Standard Detector Program. Aluminum oxide (Al2O3) photodetectors are issued by NIST as transfer standard detectors in the spectral region extending from 5 nm to 122 nm. The photocathodes is a thin film of aluminum (150 nm to 200 nm thick) deposited on a nominally 25.4 mm diameter by 1.6 mm thick quartz disc. The approximately 5 nm natural oxide layer is then thickened in an anodizing bath to about 15 nm. After anodizing, the photocathodes is assembled, along with a stainless steel anode, into a Teflon body piece to become a NIST windowless photodetector.

The sensitivity of an Al2O3 photocathode is typically 0.05 to 0.17 electrons / photon when radiation of 5 nm to 100 nm is incident, but falls to an efficiency of about 0.01 electrons / photon when 122 nm radiation is incident. 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. These detectors are essentially solar blind (not responsive to visible radiation), and offer extremely low dark current and noise characteristics. However, the device can be operated only in vacuum and is susceptible to surface contamination.

A fee schedule and ordering information are available.

In addition to these Al2O3 detectors, the Vacuum UV Transfer Standard Program issues other types of transfer standard detectors:

Detector Construction and Circuit Schematic


Schematic diagram and measurement circuit appropriate for an Al oxide photodiode
Radiation incident on the Al2O3 photocathode generates photoelectrons in the film. An anode biased 60 V to 100 V above ground generates an electric field that enhances the emission of electrons from the surface. The photocurrent should be measured in the cathode circuit to ensure that all emitted electrons are detected.


Typical Efficiency of a NIST Al2O3 Transfer Standard


Plot of a typical quantum efficiency curve for an Al oxide photodiode.

Detector efficiency is expressed in terms of electrons per incident photon, and is a function of the incident radiation wavelength. The oscillations at wavelengths between 30 nm and 70 nm are caused by thin film interference effects that change the reflectivity of the cathode film on a quartz substrate.