International Radiation Detectors, Inc.

AXUV Photodiodes Operating Principles

When these diodes are exposed to photons of energy greater than 1.12 eV (wavelength less than 1100 nm) electron-hole pairs (carriers) are created. These photogenerated carriers are separated by the p-n junction electric field and a current proportional to the number of electron-hole pairs created flows through an external circuit. For the majority of XUV photons, about 3.7 eV energy is required to generate one electron-hole pair. Thus more than one electron-hole pair is generally created by these photons. This results in device quantum efficiencies (electrons seen by an external circuit per incident photon) much greater than unity, which increase linearly with photon energy.

Two unique properties of the AXUV photodiodes provide previously unattainable stable, high quantum efficiencies for XUV photons. The first property is the absence of a surface dead region i.e. no recombination of photogenerated carriers in the doped n-region or at the silicon-silicon dioxide interface. As absorption depths for the majority of XUV photons are less than 1 micrometer in silicon, the absence of a dead region yields complete collection of the photogenerated carriers by an external circuit resulting in 100% carrier collection efficiency and near theoretical quantum efficiency.

The second unique property of the AXUV diodes is their extremely thin (3 to 7 nm), radiation-hard silicon dioxide junction passivating, protective entrance window. Owing to these two outstanding properties, the quantum efficiency of AXUV diodes can be approximately predicted in most of the XUV region by the theoretical expression Eph/3.7, where Eph is the photon energy in electron-volts. The only quantum efficiency loss is due to the front (3 to 7 nm) silicon dioxide window at wavelengths for which (mainly for 7 to 100 eV photons) oxide absorption and reflection are not negligible.

At wavelengths shorter then about 0.41 nm or longer then about 700 nm, the response of the photodiode will be limited by the "effective silicon thickness" diode. The exact wavelengths where the effect becomes important depends on the effective silicon thickness. Incident light at these wavelengths will transmit through the active collection region of the silicon and responsivity will be reduced from its ideal value. The effective silicon thickness can be measured at IRD's radiometric characterization facility; due to the absolute nature of the detectors, this thickness can be used to calculate diode responsivity up to energies of 50 keV (see the Absolute X-ray Detectors page).

Figure 1 shows typical quantum efficiency plots for AXUV photodiodes with various oxide and silicon thicknesses.

Typical quantum efficiency of the AXUV photodiodes.

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Owing to their extremely thin (3 to 7 nm) entrance window, AXUV diodes exhibit near theoretical response to low energy electrons and hydrogen ions. The following figure shows the responsivity of AXUV photodiodes to photons with 10 to 4000 eV energy and to electrons and hydrogen ions with 100 to 40,000 eV energy.

Typical responsivity of the AXUV photodiodes to photons, electrons and hydrogen ions

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Typical UV/Visible responsivity of the AXUV photodiode,
50 µm effective Si Thickness

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