Diamond sensors for particle detector applications

Diamond offers unique possibilities for particle detector applications. 
This is due to its tightly bound lattice which results in extraordinary electrical, mechanical and thermal properties. Its atomic structure results in few free charge carriers and hence very low leakage currents in the presence of an external electric field.

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The electrical properties are determined largely by its bandgap of 5.45 eV, which is so much larger than typical thermal energies at room temperature. The pure or “intrinsic” material has a resistivity in the region 10E+13 – 10E+16 ohm cm, making it in electrical terms an insulator, although it is often referred to as a semiconductor because of the close similarity to conventional semiconductors like silicon and germanium. As with these materials, doping is possible, the introduction of boron yielding p-type diamond.

However attempts to generate n-type diamond, usually by phosphorus doping, have encountered difficulties. Its high carrier mobility also suggests that as a simple ionization detector intrinsic diamond should work well. Table 1 lists some of the physical parameters of diamond along with silicon and gallium arsenide. One property which is not immediately apparent from the table is the radiation tolerance of the material. We will return to this later. One of the few drawbacks of diamond is that the number of electron-hole pairs created by a minimum ionizing particle in diamond is smaller than in silicon (see Table 1).

This smaller signal stems from the larger band gap in diamond and hence the larger energy necessary to create a pair. However, when the signal collection distance, the average distance of separation before electron and hole recombine, approaches the thickness of the material, the signal in diamond will be half as big as that in silicon. Then the many other advantages of diamond become real assets.

The other potential drawback is the cost of acquiring sufficient diamond to instrument a significant surface area. Advances in the Chemical Vapour Deposition (CVD) of diamond over the last decade make the production of large sheets of synthetic diamond relatively affordable. As a result of development work undertaken to produce high-energy physics detector prototypes the charge collection distance in CVD diamond has increased from a fraction of a micron six years ago, to over several hundred microns today.

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The improvement comes from learning to grow CVD diamond with low defect densities and large crystallites. One way to accommodate a lower signal in a sensor is to reduce the noise in the readout electronics. Here, several of the physical properties of diamond are true assets. The low leakage currents which result from the material of high resistivity imply reduced shot noise. The dielectric constant of diamond is half that of silicon (eC = 5.7, eSi = 11.9) thus all load capacities, in a comparably laid-out diamond sensor, will be half those found in silicon. These facts lead to lower noise figures than for comparable silicon detectors.

Electron and hole mobilities in CVD-diamond are large (1800 cm2/Vs and 1600 cm2/Vs, respectively) leading to fast collection times. In addition, diamond is an excellent thermal conductor, a property which can possibly be exploited for intelligent integrated cooling concepts of pixel vertex detectors.

Tab.1 Comparison of materials properties between diamond and other semiconducting materials

Of course, the main interest in diamond, as a replacement for silicon, lies in its significantly greater radiation tolerance. Work continues to understand the ultimate dose that diamond will survive in a particle detector.

There are three primary manifestations of radiation damage in solid state sensors. One is the trapping of moving charges which reduces the signal, the second is an increase in the effective doping concentration Neff which changes the space charge in the detector substrate material

The tightly bound lattice structure of diamond suggests that it will be insensitive to large doses of radiation. Previous studies of neutrons and 1.5 MeV electrons have shown that natural diamond is very resistant to both types of damage. Exposing CVD diamond samples to photons (1.2 and 1.3 MeV) from a 60Co source and to 5 MeV alphas, no damage was seen from the photons, while the charge collection properties were unchanged beyond fluences of 10E+13 alphas per cm2.

This leads us to conclude that diamond sensors survive fluences beyond 10E+15 cm2, which should allow their use at radii of less than 10 cm in a general purpose LHC tracker.  At this stage it only remains to test material of the highest possible quality to ensure that it is no less radiation tolerant.

In conclusion we can assert that silicon sensors are the heart of the next generation of high-energy physics trackers. Plans to construct strip and pixel detectors and configure them in the LHC experiments are well advanced. Still radiation resistance in the inner-most regions of these experiments remains a concern. CVD diamond sensors offer a possible cure for the finite lifetime of silicon in these regions. In the coming year it should be possible, by actively working with manufacturers, to obtain large-area high-quality CVD diamonds.

References

1. N. Wermes Characterization of a Single Crystal Diamond Pixel Detector in a High Energy Particle Beam , http://iopscience.iop.org/0034-4885/63/8/203/pdf/0034-4885_63_8_203.pdf. July 2006.
2. M. Krammer,, et al CVD diamond sensors for charged particle detection, Diamond and Related Materials, Volume 10, Issues 9-10, September-October 2001, Pages 1778-1782