International Radiation Detectors, Inc.

Electron Detectors

Silicon electron detectors (AXUV-series, p-n junction photodiodes) have been developed by IRD for detection of electrons and low energy ions. Unlike common photodiodes, these diodes do not have a doped dead-region and have zero surface recombination resulting in near theoretical quantum efficiencies for low energy electrons and ions.

The AXUV photodiode behavior is characterized by electron gain G_e, which is the number of charges generated per electron incident upon the detector, or responsivity R, the number of charges generated per incident energy ε. The following relation exists between G_e and R for monoenergetic electrons incident upon the detector:

G_e = R x ε

Responsivity is proportional to the number of electron hole pairs generated in the AXUV photodiode. As the photodiode is exposed to electrons and ions, electron-hole pairs (carriers) are created; the electric field in the p-n junction separates the charges and drives the current in the external circuit. The silicon electron-hole creation energy is found to be 3.71 eV in silicon [1]. For photons, this value can be used to calculate the ideal responsivity in a lossless system, R_A = 1/3.71 electron charge/eV = 0.27 C/J = 0.27 A/W.

Particles incident upon a detector surface have additional loss mechanisms not present for photons. For electrons, these losses are summarized in Equation 1, taken from Reference [1].

R_m = R_A (1 - Δ_DL - Δ_B - Δ_R - Γ)

Equation 1:Electron response equation, including terms for responsivity loss.

In the AXUV series photodiodess, dead layer and residual losses are minimized. Residual losses are dominated by recombination of the electron-hole pairs generation in the silicon-silicon oxide interface, which is non-existant in the 100% internal efficency AXUV photodiodes. Dead layer losses in the the 30 - 70 Â front oxide window are less than 0.1% at ε > 2 keV, increasing at energies lower than 2 keV [1]. This leaves backcattering from the front surface as the dominant loss mechanism at ε > 2 keV. As absorption depths for the low energy ions and electrons are less than 1 micrometer in silicon, when losses from the front oxide window and backscattered electrons are subtracted measured data indicates 100% internal carrier collection efficiency and near theoretical gain/responsivity. Measured responsivity data is shown in Figure 1, while Figure 2 shows the same results in terms of electron gain.

Figure 1: Photon and electron responsivity for the AXUV photodiode.

Figure 2: Electron gain for the AXUV photodiode.

References:

1] H. O. Funsten, D. M. Suszcynsky, S. M. Ritzau, and R. Korde,
"Response of 100% Internal Quantum Efficiency Silicon Photodiodes to 200 eV to 40 keV Electrons."
IEEE Transactions on Nuclear Science, Vol. 44, No. 6, December 1997, 2561-2565

2] Herbert O. Funsten, Stephen M. Ritzau, Ronnie W. Harper, and Raj Korde,
"Response of 100% Internal Carrier Collection Efficiency Silicon Photodiodes to Low-Energy Ions"
IEEE Transactions on Nuclear Science, Vol. 48, No. 6, p. 1785-1789, 2001

3] H. O. Funsten, S. M. Ritzau, R. W. Harper, and R. Korde
"Fundamental limits to detection of low-energy ions using silicon solid-state detectors."
Applied Physics Letters, Vol. 84, No. 18, p. 3552-3554, 2004