Secondary-electron emission from diamond
When a high energy beam of electrons stikes a surface, electrons are knocked out into vacuum. The ratio of emitted electrons to incoming electrons is called the secondary electron yield, SEY. Natural and CVD diamond have been measured as having SEE values as high as 80, although more typical values are ~10. This means that 10 slow electrons are emitted for every 1 fast incident electron. These emitted electrons can be accelerated using a potential difference to strike a second diamond surface, giving rise to even more electrons. This is the basis for electron multiplication as used in photomultiplier tubes.
Advantages of diamond
- High gain: because of the high SEY or gain of diamond, fewer multiplication stages are required to obtain sufficient electrons to detect as a current, making the devices smaller and cheaper.
- Fast response: The electron emission process occurs on a fs timescale, suggesting that extremely fast-response photomultiplier tubes can be fabricated using diamond.
- Long lifetime: The diamond surface should be inert and so devices should have longer lifetimes than current ones.
Reflection or Transmission mode?
When electrons are emitted from the same surface struck by the incident electron beam, this is called reflection mode. Multiplication in this case occurs by having the surfaces at an angle to the incident beam, with the second surface below this and to one side, in a so-called 'Venetian blind' structure (see figure below, left). Alternatively, if the sample is thin enough, electrons are emitted from the opposite surface to that struck by the incident electron beam - this is transmission mode (see diagram, below right). This mode allows line-of-sight multipliers to be fabricated, which can be simpler than the Venetian blinds. However, it relies on fabricating thin membranes of diamond which are ~100 nm thick.
Measurements of SEY
The original project was a collaboration with Jon Lapington's group at the University of Leicester, Photek and other partners. Now we are measuring the SEY for different types of diamond as a function of doping, surface termination, crystal size, etc, with the aim of fabricating a demonstrator high-gain, fast-response photomultiplier system. We are investigating both reflection and transmission modes using a home-built system.
While most groups use a traditional Faraday cage to collect and measure the secondary electron current, we are using a novel system in which the secondary electrons are accelerated onto a phosphor screen, which then emits light with intensity proportional to the electron beam current. By calibrating this intensity against that obtained from known SEY standard materials (such as Cu), we can obtain absolute SEY values. The system can be run in both modes, with detectors above and below the substrate.
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Schematic diagram of the SEY measurement set-up. |
Photograph of the home-built SEY apparatus. |
Refs:
- J.S. Lapington, P.W. May, N.A. Fox, J. Howorth, JA. Milnes, "Diamond dynodes create new breed of photon detectors", Laser Focus World (Sept 2008).
- J.S. Lapington, D.P. Thompson, P.W. May, N.A. Fox, J. Howorth, J. Milnes, V. Taillandier, "Investigation of the secondary emission characteristics of CVD diamond films for electronamplification", Nucl. Instr. Methods Phys. Res. A, 610 (2009) 253–257.
- R. Vaz, P.W. May, N.A. Fox, C.J. Harwood, V. Chatterjee, J.A. Smith, C.J. Horsfield, J.S. Lapington and S. Osbourne, "Measurement of the secondary electron emission from CVD diamond films using phosphor screen detectors", JINST 10 (2015) P03004