Diamond Detectors for Ionizing Radiation - HEPHY

Chapter 5
Detector Material Comparison
5.1 Diamond Detectors
With a band gap of E g = 5:5eV, diamond is regarded as an insulator. This implies
negligible intrinsic carrier densities even at room temperature, allowing to operate intrinsic
diamond lm as a detector. Electrodes are applied to the diamond lm on opposite sides
to form Ohmic contacts. As there is no pn-junction, the polarity of the electric eld is
irrelevant. The dark current of the diamond samples, including both bulk and surface
currents, is less than 1 nA cm ,2 at an electric eld of 1 V m ,1 [22].
According to the high carrier mobilities in diamond, the charge collection is very fast,
taking about 1 ns in detectors of approximately 500 m thickness. It has been shown that
CVD diamond detectors are able to count heavy ion rates above 10 8 cm ,2 s ,1 with a single
readout channel.
5.1.1 Charge Collection Distance
Due to the polycrystalline nature of CVD diamond, the charge collection is not straightforward
as in homogeneous detector materials. The grain structure (g. 3.3) results in a
quality gradient along the y coordinate (depth axis). The grain boundaries are suspected
to provide charge trapping and recombination centers.
On the substrate side (y = 0), the lateral grain size is at its minimum, resulting in
a large amount of traps. Thus, the mean free path for the carriers is very short. With
ascending y the single-crystal volumes expand, causing the trap density to shrink and the
mean free path to increase. A linear model has been proposed [23] for the local mean free
path as a function of y, starting from (almost) zero at y = 0 up to a certain value for
y = D. This model satises experimental data [24].
Neglecting border limits, the sum of the mean free paths for electrons and holes gives
the overall average distance that electrons and holes drift apart in an electric eld. This
value has been established as the charge collection distance d c , describing the quality of
the diamond sample. The border limits are irrelevant as long as d c D. The collection
distance, or sum mean free path, can be stated as the product of carrier velocity and
20