Quantum Efficiency (QE) in High Energy CCD detectors
Understand QE in a high-energy CCD
The QE of the CCD is governed by the ability of the incident photons to be absorbed in the depletion region of the sensor. To understand how
the photon is absorbed in this key region, or not, is a key factor in understanding the QE range of the variety of sensors on offer. Although
there are other factors which effect the final shape of the QE curve such as the different materials and their different absorption coefficient
and the reflectance of the silicon the most dominant and straightforward impact on the sensor QE is the structure of the actual CCD. If we
look at the CCD in cross-section it helps to illustrate this point as shown in Fig. 1.
In a front illuminated device, photons falling on the CCD must travel
through the region of the gate electrodes structures. These structures
are very strong absorbers of lower energy X-ray photons, which can be
absorbed within a few nm of material.
As a general rule, the absorption length of the incident X-ray increases,
that is lengthens, with increasing energy. If we apply this simple rule
to photons incident on the CCD structure, we can see that in a front
illuminated device photons must travel through the regions of the gate
electrodes structures, AR coating (if applicable) and the oxide layer,
effectively making up a ‘dead zone’ before entering the depletion zone.
The effective depth of this zone will also delineate the probability of
absorption of the higher energy photons, with the incident photons
energies increasing to a point where their absorption length is longer
than the depth, hence the probability of capture approaches zero. This
also explains the QE strengthening to a peak then reducing as the
energy increases. Another point to note is that the discontinuities or
edges seen in the QE are the result of the intrinsic properties of the silicon, the absorption edges L-edge at 100.6 eV and K-edge at 1.8 keV.
Deep Depletion sensors, as the name suggests, extend the depth of the depletion zone and thereby increase the higher energy range that they
can directly detect.
PBack-illuminated sensors are sensors that have been effectively inverted so the electrode structure is no longer on the top surface but the
bottom. The bulk of silicon is also removed leaving only a very thin ‘dead zone’ layer, thus allowing the softer X-ray detection.