Acceptor removal - Displacement damage effects involving the shallow acceptor doping of p-type silicon devices

Abstract

New sensor technologies are under development to cope with the ever increasing requirements for high energy physics (HEP) detectors. For the High-Luminosity LHC (HL-LHC) and the Future Circular Collider (FCC) higher radiation hardness, improved timing performance and lower cost large area detector technologies are essential. Recent developments of p-type silicon planar sensors, sensors with intrinsic gain for timing applications and monolithic sensors are examples for next generation detector technologies. A commonality of these new sensors is the use of boron doped p-type silicon, while present HEP sensors are essentially based on n-type silicon. Consequently, radiation effects in p-type silicon have been less studied in the detector community, but are now of high relevance. In this paper, we review radiation effects in p-type silicon devices with a focus on acceptor removal effects. In p-type silicon exposure to radiation causes an apparent deactivation of the dopant. This socalled acceptor removal effect is for example evidenced by an initial decrease of depletion voltage in p-type sensors with rising particle fluence, while after exposure to higher fluences the device behavior is dominated by other radiation induced effects. Low Gain Avalanche Detectors (LGADs) are the baseline technology for future solid-state timing layers at the HL-LHC. Their good timing performance is based on the intrinsic amplification of the signal by impact ionization in a highly doped p-type gain layer. Radiation effects reduce the active boron concentration in the gain layer and lead to a reduction in field strength and consequently a loss of intrinsic signal amplification, thus limiting the radiation hardness of this technology. Monolithic detectors, especially High Voltage CMOS, are another technology affected by acceptor removal, as the bulk silicon used for the sensor is typically of p-type. We review the impact of radiation damage on the depleted volume as function of the initial boron concentration and the corresponding change in signal charge and device performance. The origin of the radiation induced deactivation of the boron acceptor is found in the creation of defects that contain boron, that is consequently no longer acting as shallow dopant. Defect spectroscopy techniques like Thermally Stimulated Currents (TSC) and Deep Level Transient Spectroscopy (DLTS) allow detecting these levels, help to understand the defect formation kinetics on the microscopic level and open the door to defect engineering approaches to mitigate the acceptor removal effect. Latest results of the CERN RD50 collaboration in this research field are reviewed.

Bistability of the BiOi complex and its implications on evaluating the “acceptor removal” process in p-type silicon

“…a BiOi concentration of 10 13 cm -3 will generate a 2 pA current at 293 K). 0.0 5.0x10 13 1.0x10 14 1.5x10 14 2.0x10 14 0.0 0.5 The dependence of the BiOi introduction rate on the boron doping concentration, as deduced by C-V measurements, is given in Figure 4. A non-linear dependence on boron content was reported earlier by Vines et al [45].…”

“…In this respect the estimation of defect introduction rate through its A configuration, of gBiOi A = 0.023 cm -1 in this case, is regarded to be the minimum value, as gBiOi should be calculated considering both defect configuration (gBiOi= gBiOi A + gBiOi B ). 1.2x10 13 1.4x10 13 1.6x10 13 1.8x10 13 2.0x10 13 2.2x10 13 2.4x10 Bistable defects have normally more than one stable state which, under certain conditions may reversibly pass over from one state to another and so change their spatial and electronic structure [46][47][48][49]. There are several examples of radiation induced metastable centers in Silicon, some with bistable character (see the [50] review), and not always the different possible electronic states of the defect could be evidenced on the same samples on which the metastability was first detected or with the same investigation techniques.…”

Fabrication of Silicon Sensors Based on Low-Gain Avalanche Diodes

“…The gain almost vanished at irradiation fluences of about 10 15 cm −2 . The effect is attributed to the effective acceptor removal in the gain layer [32]. By means of SIMS measurements [33] it has been shown that the boron ion is in the same position as before irradiation, and most likely inactivated by becoming interstitial and forming defect complexes with oxygen.…”

AC-coupled n-in-p pixel detectors on MCz silicon with atomic layer deposition (ALD) grown thin film

“…[43] However, further work is required to form a full understanding of the microscopic defect formation kinetics causing acceptor removal. [44] Besides the introduction of newer phenomena such as acceptor removal, the validity of traditional radiation damage models is not verified for extreme fluences of > 1×10 16 n eq /cm 2 -for example, a saturation of radiation effects at very high fluences has been observed. [28,45] More extensive research, both experimental and in simulation, is required to determine and quantify the underlying physics behind these observations.…”