5.2. Diamond Detectors 307
CVD technique involves decomposition of hydrocarbon molecules in gaseous state
and then their activation in a high temperature (or energy) environment. Specifically
what is done is that a gas mixture consisting of about 98% hydrogen and 2% methane
is led into the activation volume that is either kept at a very high temperature or
has the ability to cause activation through the microwave-plasma process. The
high energy departed to the molecules decomposes them and activates the chemical
reactions necessary to fuse them into the diamond structure. The reactants are then
transported to the deposition surface, where processes of nucleation and growth
occur. This leads to development of carbon and diamond structures on the substrate.
Since the diamond structures in the substrate are mixed with the graphite structures
therefore they must be purified before use. This is accomplished by the process of
gasification of the graphite by atomic hydrogen. Since diamond is more stable than
graphite for this process therefore most of it survives.
Before we look at how a CVD diamond detector is built, let us study some of the
important properties of this kind of diamond relevant to the detector technology.
5.2.A ChargePairProduction
The process of charge pair production in diamond is essentially the same as in
semiconductors. The only difference is the higherw-value of diamond owing to its
wider band gap. In comparison to silicon, in a diamond detector the same amount
of deposited energy would produce about three times less charge pairs. But if we
compare this to gaseous detectors the number of charge pairs would still be about
three times higher. The good thing about diamond is that the larger band gap also
means lower leakage current because of lower probability of production of thermally
agitated electron hole pairs.
5.2.B Recombination
As with semiconductor materials, CVD diamonds also have crystal defects and im-
purities that produce additional energy levels within the forbidden gap. These levels
facilitate the recombination of excess charge pairs. Furthermore the usual recombi-
nation characterized by fall of an electron from the conduction to the valence band
also occurs in diamond. The lifetimes of charges recombining through these two
processes depend mostly on the quality of the material.
The direct recombination of an electron in the conduction band with a hole in
the valence band is generally known as intrinsic recombination. On the other hand
the extrinsic recombination refers to the process that follows via an intermediate
energy state. The lifetime for the intrinsic process can vary from a fewμsto about
1 swhile that for the extrinsic process can be as low as 0.1 to 10ns. The average
or effective lifetime, as in case of semiconductors, is given by
1
τef f=
1
τint+
1
τext, (5.2.1)
where the subscriptsef f,int,andextrefer respectively to effective, intrinsic, and
extrinsic. It is apparent that for CVD diamond the effective lifetime is dominated
by the extrinsic lifetime, which depends on the crystal defects and impurities. Hence