Diamond Detectors for Ionizing Radiation - HEPHY

CHAPTER 5. DETECTOR MATERIAL COMPARISON 21
lifetime, summed for both carriers,
d c = d c;e + d c;h = v e e + v h h =( e e + h h )E : (5.1)
Taking the border limits into account, the collection distance obtained from measurements
is smaller than the average mean free path because at the electrodes, electrons and
holes are drained and do no longer contribute to the drift path, thus reducing the total
drift length or the signal induced at the electrodes, respectively.
The number of charges (electron-hole pairs) generated by a MIP is [8]
Q p = q p D with q p =36em ,1 : (5.2)
The value of q p includes not only the primary excitation, but also the contribution of
secondary interactions by eventually generated electrons. The charge collected at the
electrodes is approximately represented by the ratio of the carrier drift length, or charge
collection distance, to the lm thickness,
Substituting Q p with the expression in eq. 5.2 results in
Q c Q p
d c
D : (5.3)
Q c q p d c : (5.4)
The charge collection eciency, which is dened as the ratio of measured charge to the
total generated charge, is given by
cce d c
D : (5.5)
Eq. 5.4 tells that the charge collected at the electrodes is a function of the mean collection
distance only. However, with thicker lms more charge is generated, thus more
charge is collected and the charge collection increases. Thus, the charge collection distance,
together with the sample thickness, state the material quality.
In order to increase the signal size, the diamond lm can be grown thicker. On
the other hand, tracking detectors must be kept as thin as possible. The solution that
complies with both requirements is to grow a rather thick diamond lm and then remove,
by lapping, material from the substrate side, where the collection distance is very low.
Due to surface limits, the mean charge collection passes its maximum and decreases, if too
much material is removed. It has been shown by theory and experiment [23] that there
is an optimal remaining thickness for given detector parameters. The collection distance
increase using this technique ranges up to 40% with present diamond samples. Fig. 5.1
shows the charge collection distances of two dierent diamond samples after several steps
of lapping. The measured values agree with the theory well. For the application as a
tracking detector is the target to achieve a thin detector with sucient signal output.
Apart from the local collection distance dependending on the depth as discussed above,
the diamond lm is considered to be laterally homogeneous. Measurements have shown