Aluminum Oxide Photodetector Transfer Standard

Vacuum Ultraviolet Detectors

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Photon Physics Group

Electron & Optical Physics Division

NIST Windowless Aluminum Oxide Photodiodes

Photograph of an aluminum oxide photodetector Aluminum oxide (Al₂O₃) photodetectors are issued by NIST as transfer standard detectors in the spectral region extending from 5 nm to 122 nm. The photocathode 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 photocathode 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:

Si Windowless Solid State (5 nm to 254 nm)
CsTe Windowed Photoemissive (116 nm to 254 nm)

Detector Construction and Circuit Schematic

Schematic drawing of aluminum oxide photodetector measurement circuit 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

Graph of aluminum oxide photodetector typical quantum efficiency 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.