Physics and Engineering of Radiation Detection

306 Chapter 5. Solid State Detectors

5.2 DiamondDetectors............................

The most problematic thing with semiconductor detectors is their vulnerability to
radiation damage. Radiation hard semiconductors, though available, are expensive
to fabricate and have their own engineering difficulties. Another solid state material
that has been shown to perform better than semiconductors in hostile radiation
environments is diamond.
Table.5.2.1 lists some of the properties of diamond relevant for its use as an active
medium for radiation detection. For comparison, the properties of silicon are also
given. It can be seen that the energy needed to create an electron-hole pair in
diamond is significantly larger than in silicon. This is due to its higher band gap
energy, which is almost five times higher than silicon. This implies that the output
signal from a diamond detector will be significantly less than from a silicon detector.
However the higher band gap energy also has a positive side, that is, the leakage or
dark current is also lower than in semiconductors. This implies less noise at the same
bias voltage and therefore it can be expected that the signal to noise ratio will still
remain acceptable. Another positive point is diamond’s lower dielectric constant,
which means lower capacitance as compared to silicon having similar dimensions.

Table 5.2.1: Comparison of some important properties of diamond with those of
silicon (4).

Property Diamond Silicon

Mass Density (gm−cm−^3 ) 3.52 2.33

Dielectric Constant 5.7 11.9

Displacement Energy (eV /atom) 43 13-20

W-value (eV) 13 3.6

Band Gap (eV) 5.5 1.12

Breakdown Field (V/cm) 107 3 × 105

Resistivity (Ωcm) > 1011 2. 3 × 105

Electron Mobility (cm^2 V−^1 s−^1 ) 1800 1350

Hole Mobility (cm^2 V−^1 s−^1 ) 1200 480

The diamonds used as detector material are synthetically produced in labora-
tory through the process of Chemical Vapor Deposition (CVD). The diamond thus
produced is referred to as a CVD diamond. Though going into the details of this
process is out of the scope of this book but for the interest of reader we will give
here a brief introduction.