SCIPP 05/09 Operation of Short-Strip Silicon Detectors based on p-type Wafers in the ATLAS Upgrade ID

Based on recent data of radiation effects in p-type detectors, the expected performance of planned short silicon strip detectors (SSSD) in the ATLAS upgrade tracking detector are evaluated. The signal-to-noise ratio and power generated will be presented as a function of a set of realistic values for the operating parameters: fluence (3·10, 1·10, 3·10 neq/cm), operating temperature (-30C, -20C, -10C) and bias voltage (400, 600, 800V), detector thickness (200, 300 μm) for both Float Zone (FZ) and Magnetic Czochralski silicon p-type detectors. 1. Reasons for using p-type SSD Based on the charge collection results from RD50 [1,2], we anticipate p-type (n-onp) detectors will be the main contender for the Silicon Strip Detectors (SSD) technology in the intermediate region at the LHC upgrade. In that region with radius from ~20 to ~55 cm, occupancy considerations dictate the use of short strip detectors of about 3 cm length. If proven to be reliable, affordable (single-sided processing!) and supported by highvolume manufacturers, p-type might get used in the long strips in the outer region of the upgrade detector beyond a 55 cm radius as well. The main radiation damage phenomena to SSDs are [3]: an increase of the leakage current, a change in the effective doping concentration at heavy fluences leading to increased depletion voltage, a shortening of the carrier lifetimes due to increased trapping at radiation-induced defects, responsible for a loss of charge, and the appearance of surface effects causing changes in the interstrip capacitance and resistance. An important effect in radiation damage is the annealing, which can significantly change the detector properties after the end of radiation. Since the times characterizing annealing effects tend to depend exponentially on the temperature, the temperature of operating and maintaining the detectors is constrained by these annealing times [4,5]. As shown below, the character and size of these effects are very much the same in ptype and n-type at very high fluences [6]. A significant difference resides in the fact that defects induced by radiation in Si are dominantly deep acceptor like traps [7]. Radiation damage in p-on-n-detectors will therefore eventually lead to an inversion of the space charge sign (SCSI), so that the region of high field will shift from the strip implants (which are connected to the readout) to the rear electrode of the detector. On the contrary, in p-type Si radiation will increase the effective doping concentration without causing type inversion. For this reason, at high fluences, when the SCSI occurrs, n-type (p-on-n) detectors need to be fully depleted or over-depleted to maximize the collected charge [8]. Thus, in terms of the operation, p-type SSD (like the more expensive n-on-n detectors used in the ATLAS Pixel System [9]) have a critical advantage over the customary n-type SSD: because the signals are collected on the junction side throughout the life time of the detector, the detector can be operated under-depleted, i.e. the bias voltage does not have to exceed the depletion voltage for the detector to be efficient, as in the p-on-n detectors after type inversion. If the bias voltage is less than the depletion voltage, only part of the detector can be depleted, and only charge created in this “active” thickness can be collected. This applies to n-on-p detectors, but not to p-on-n detectors, which require full depletion. Trapping of the charge during drift to the electrodes adds to the loss of charge, and here the p-type detectors have the advantage of collecting electrons, which have an almost three times higher mobility than holes. 2. Data on Radiation Damage Much of the radiation damage data has been collected using charged hadron beams, e.g. the 24 GeV proton beam at CERN. On the other hand, expected fluence levels are often expressed in terms of 1 MeV neutron equivalent. There are important differences between the radiation damage by neutrons and protons, e.g. in the formation of a doublejunction in n-type detectors, which will not be discussed here [10]. For most effects, the neutron fluence required to do the same damage as high energy protons is well described by the NIEL hypothesis and scales as 0.62 [5]. Here, the effects are evaluated for protons and attributed to a neutron equivalent fluence of 0.62 magnitude. To investigate a possible beneficial effect of the increased oxygen concentration in terms of radiation induced defects, within the RD50 CERN collaboration the radiation hardness of both magnetic Czochralski (MCz), Diffused Oxygenated Float Zone (DOFZ) and standard Float Zone (FZ) materials are now investigated as p-on-n and n-on-p detectors [2,7]. It is well known in fact that MCz has a concentration of interstitial oxygen of about 10-10cm, while in standard FZ Si it is 10-10cm. Indeed there is experimental evidence that under irradiation with charged hadrons, n-type DOFZ and MCz Si are less prone to type inversion than pure FZ [3,7,8]. This has been related both to a depressed accumulation of the deep radiation induced defect with energy level close to 0.5eV [11] and, especially in MCz, to the creation with radiation of shallow donors [12].