AXUV Photodiode Operating Principles: Quantum Efficiency, Responsivity & Silicon Thickness

AXUV photodiodes are windowless silicon detectors built by Opto Diode, a division of ITW, in Camarillo, California. The family measures photons, electrons, or X-rays from 0.0124 nm to 190 nm and detects energy from 100 eV to 50 keV with 100% internal quantum efficiency. They achieve that because the silicon has no surface dead region and the entrance window is a passivating oxide only a few nanometers thick, so essentially every carrier created by absorbed radiation reaches the external circuit. This page explains the physics behind those numbers.

Source note: AXUV Operating Principles and Applications (PDF). That document is the original, undated application note and it predates the current AXUV catalog; it remains online because it holds the quantum-efficiency, responsivity and electron-gain plots. No newer revision of it is published on this site. Where the note and the current product pages differ, the product pages govern, and this page follows them.

How an AXUV photodiode converts radiation into current

Any photon carrying more than 1.12 eV — a wavelength shorter than 1100 nm — creates an electron-hole pair in silicon. The p-n junction field separates the carriers, and a current proportional to the number of pairs flows through the external circuit.

In the XUV band a single photon creates many pairs, because about 3.7 eV is required per pair. Quantum efficiency — electrons seen by an external circuit per incident photon — is therefore much greater than unity and increases linearly with photon energy. The application note gives the approximation Eph/3.7 for most of the XUV region, where Eph is the photon energy in electron-volts.

The same mechanism applies to incident particles. The note characterizes particle response by electron gain Ge, the number of charges generated per incident electron, related to responsivity R and incident energy by Ge = R × ε.

Why the internal quantum efficiency reaches 100%

Two properties separate AXUV silicon from an ordinary photodiode.

The first is the absence of a surface dead region — no recombination of photogenerated carriers in the doped n-region or at the silicon / silicon-dioxide interface. Absorption depths for the majority of XUV photons are less than one micrometer in silicon, so every carrier is generated within reach of the junction field. The result is 100% carrier collection efficiency and near-theoretical quantum efficiency.

The second is the entrance window: an extremely thin, radiation-hard silicon dioxide layer 3 to 7 nm thick that both passivates and protects the junction. It is thin enough that the only quantum-efficiency loss it causes is oxide absorption and reflection, and that loss matters mainly for 7 to 100 eV photons.

Ideal responsivity, and the terms that reduce it

The silicon electron-hole creation energy is 3.71 eV. For photons, that fixes the ideal responsivity of a lossless system at R_A = 1/3.71 electron charge per eV = 0.27 C/J, or 0.27 A/W.

Particles incident on a detector surface carry additional loss mechanisms that photons do not. The note summarizes them in its electron response equation — it describes the response to incident electrons, not the photon response:

R_m = R_A (1 − Δ_DL − Δ_B − Δ_R − Γ)

In AXUV devices the dead-layer and residual losses are minimized. Dead-layer losses in the 30 to 70 Å front oxide window are less than 0.1% above 2 keV, increasing at energies below 2 keV, which leaves backscattering from the front surface as the dominant loss mechanism above 2 keV.

What a datasheet actually gives you. The note publishes responsivity as curves rather than as a single number, and individual AXUV datasheets do the same for the wavelength and energy dependence — while also carrying a production limit. The AXUV100G datasheet, for example, specifies 0.08 A/W typical, 0.07 A/W minimum and 0.09 A/W maximum, with the responsivity graph itself as the stated test condition. Read the value for your own wavelength or beam energy off the curves; treat the single figure as a production limit.

Where the response rolls off: effective silicon thickness

Standard AXUV photodiodes have an effective silicon thickness of 30 to 105 micrometers. At wavelengths shorter than about 0.41 nm or longer than about 700 nm, incident radiation transmits through the active collection region instead of being absorbed in it, and responsivity falls below its ideal value. The exact wavelengths at which this matters depend on the effective silicon thickness. Above 4000 eV a growing fraction of photons transmits straight through the active silicon, reducing quantum efficiency from its designed 100%.

Because that reduction is caused solely by the limited silicon thickness, it is calculable. Once the absorption A(ε_ph) of the silicon layer is known — from published tabulations of photoelectric absorption coefficients — the note gives the responsivity as:

S(ε_ph) = 0.98 · A(ε_ph) / 3.65 (A/W)

Here 3.65 eV is an average electron-hole pair creation energy in silicon, and the 0.98 factor accounts for the roughly 2% X-ray fluorescence yield in silicon above 1838 eV, including an estimate for reabsorption of part of the fluorescent light. The note reports a device with 104 micrometers of effective silicon that was calibrated at PTB and matched this calculation — the basis for its conclusion that an AXUV diode of known silicon thickness can be used as an absolute X-ray device. Effective silicon thickness is not a published parameter on current AXUV datasheets, so contact Opto Diode if your measurement depends on it.

Why AXUV diodes were adopted as XUV transfer standards

The application note records that these diodes were approved as transfer standards in the XUV spectral range on the strength of ease of use, stability, spatial homogeneity of quantum efficiency, dynamic range over eight orders of magnitude, small size, ruggedness and ultrahigh-vacuum compatibility.

Compared with orthodox tube-type XUV detectors, the note credits them with very low noise, no requirement for an external operating voltage, insensitivity to magnetic fields, low mass and a large collection-area-to-size ratio. Being windowless, they are operated in the open-face configuration down to angstrom wavelengths, even in the presence of gases — an advantage over XUV spectrometers built on conventional detectors, which must be used either in vacuum or behind a window.

Applications and the current AXUV models

Opto Diode lists three applications for the AXUV family: electron detection, synchrotron radiation monitoring and electron reticle inspection. The application note adds that quadrant AXUV diodes with central holes and rectangular slit openings were built specifically for synchrotron beam intensity monitoring and position sensing; the current catalog carries a center-hole part (AXUV63HS1-CH) and a quad part (AXUVPS7), so check the datasheets for the geometry your beamline needs.

Where radiation hardness under high-intensity EUV matters more than absolute broadband response, the SXUV family is the one built for that duty. Compare them in the AXUV vs SXUV selection guide, see the SXUV100 EUV photodiode, or read the EUV 13.5 nm power and dose monitoring guide.

Full listing, datasheets and matching sockets: AXUV photodiode range. Working across families? See how to choose a photodetector.

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