Defect characterization in silicon particle detectors irradiated with Li ions

Transcription

1 Defect characterization in silicon particle detectors irradiated with Li ions M. Scaringella, M. Bruzzi, D. Menichelli, A. Candelori, R. Rando Abstract--High Energy Physics experiments at future very high luminosity colliders will require ultra radiation-hard silicon detectors, able to withstand fast hadron fluences up to cm -2. In order to test the detectors radiation hardness in this fluence range, long irradiation times are required at the currently available proton irradiation facilities. Energetic (58 MeV) lithium ions, with experimental hardness factor about two orders of magnitude higher than 24 GeV protons for µm thick detectors, could represent a promising alternative radiation source. In this study, the degradation mechanisms in single pad p + -n Standard Float Zone (STFZ) and Diffusion Oxygenated Float Zone (DOFZ) Si detectors irradiated with Li ions up to the fluence of Li/cm 2 have been investigated by means of Photo Induced Current Transient Spectroscopy and Thermally Stimulated Currents. Results are compared with the radiation damage induced by 24 GeV proton, 1 MeV neutron and 60 Co γ- ray irradiation in order to compare the radiation hardness of STFZ and DOFZ Si under different beams with different energies. Keywords: Silicon; Radiation Detectors; Defect Characterization; Ion irradiation effects. I. INTRODUCTION Silicon-based devices are widely used in a variety of fields of application characterized by hostile radiation levels. This applies in particular to tracker detectors for High Energy Physics (HEP) experiments at the Large Hadron Collider (LHC) [1]. LHC is planned to provide proton-proton collisions with a centre of mass energy of 14 TeV at the unprecedented luminosity of cm -2 s -1, possibly increased to cm -2 s -1 in a future upgrade (SuperLHC) [2]. Experiments projected to run at LHC are designed to have a highly advanced central tracking detector around the collision region, where high spatial precision and time resolution will be achieved using pixel and microstrip silicon detectors. At SuperLHC the high luminosity will produce fast hadron fluences up to cm -2 at This work was performed in the framework of the CERN RD50 Collaboration and was financially supported by the SMART INFN group V project. Monica Scaringella, Mara Bruzzi and David Menichelli are with Istituto Nazionale di Fisica Nucleare and Dipartimento di Energetica, Università di Firenze, via S. Marta 3, 50139, Firenze, Italy (telephone: , m.scaringella@ing.unifi.it, bruzzi@fi.infn.it, and menichelli@ingfi1.ing.unifi.it). Andrea Candelori and Riccardo Rando are with Istituto Nazionale di Fisica Nucleare and Dipartimento di Fisica, Università di Padova, via Marzolo 8, 35131, Padova, Italy (telephone: , surname@pd.infn.it). the innermost radius after five years of operation [2]. Detailed radiation damage tests must be carried out to study the performance and the degradation of Si detectors in such a high fluence range, and specific radiation hardening technologies are necessary to ensure an efficient and reliable tracking detector system [3]. The high fluence level presents as a strong constraint the long irradiation times required at the currently available proton irradiation facilities for detector prototype testing. Recent results [4],[5] have shown that 58 MeV Lithium (Li) ions are a suitable radiation source for radiation hardness studies. The ratio of the damage constants for leakage current and effective doping concentration, α and β parameters, as measured for devices irradiated by 58 MeV Li ions and 27 MeV protons suggests that the damaged induced by Li ions and protons can be opportunely scaled for HEP radiation hardness tests [5]. In particular, 58 MeV Li ions present an experimental hardness factor 45 with respect to 1 MeV neutrons in µm thick devices [5]. This suggests that cost-effective radiation damage tests can be easily performed with Li ions in the fluence range of interest for the SuperLHC applications. However, in order to assess the equivalence between irradiation with hadrons and Li ions it is necessary to check if the damage induced in the Si bulk is similar. In this study we present the microscopic characterization of Si detectors, produced with Standard Float Zone (STFZ) and Diffusion Oxygenated FZ (DOFZ) n-type material, after irradiation with Li ions up to the fluence of Li/cm 2. The investigation of the energy level distribution induced by radiation in the two materials is carried out by Thermally Stimulated Currents (TSC) and Photo Induced Current Transient Spectroscopy (PICTS). II. EXPERIMENTAL PROCEDURE Tested devices are p + -n silicon diodes processed by ST Microelectronics [6] on STFZ ([O] cm -3 ) and DOFZ ([O] cm -3 ) n-type high resistivity (2kΩ cm), <100>, 300 µm thick silicon substrates from Wacker [7]. The diodes have been irradiated by 58 MeV Li ions, at the SIRAD irradiation facility of the INFN National Laboratory of Legnaro (Padova, Italy), with an ion flux of Li/(cm 2 s). Our experiments are carried out on samples irradiated to the fluences of Li/cm 2, Li/cm 2 (annealed for 4 minutes at 80

2 C), and Li/cm 2 (annealed for 2048 minutes at 80 C), see [5] for details. The scheme of the PICTS experimental setup is described in Fig.1. The sample excitation is provided by a Light Emitting Diode (LED), with wavelength λ=940 nm, driven by a pulse generator (Systron Donner 110D), which allows to vary the excitation pulse duration t p starting from 500 ns. The current transients generated by the pulsed light across the sample are measured using a custom readout circuit connected to a digital oscilloscope (Tektronix TD520D, 500MHz). The oscilloscope samples the transients to produce the PICTS spectrum. The sample is located inside a vacuum chamber, where a pressure lower than 10-3 mbar is maintained, and cooled by a cryogenerator (Officine Galileo K1). The heating is provided by a resistor wounded around the sample support. spectra of the STFZ and DOFZ samples exhibit the same spectral features, which correspond to the deep levels commonly observed in irradiated silicon [8]. However, a peak close to 120K, not reported before, is found only in the STFZ sample. The peaks observed by TSC below 80 K are due to the wellknown Vacancy-Oxygen (VO) [8] complex at 70 K and to a group of deep levels (with activation energy around 0.1eV) formerly detected in γ-ray irradiated diodes [9]. In the high temperature range the contribution of C i O i [10] and V 2 -/0 [11] can be distinguished. Fig.1. PICTS experimental setup. Thick line: fast analog signals; thin line: slow analog signals; dashed line: digital signals. TSC measurements have been performed using an electrometer (Keithley 6517A) which provides sample bias V rev, low temperature forward injection and current reading. TSC measurements are carried out in the temperature range K, with a maximum reverse voltage of 200 V. In this case the cooling apparatus makes use of a sample holder immersed in vapors inside a liquid helium dewar. In both TSC and PICTS experiments the temperature sensor is a silicon diode (Lake Shore DT-470-CU11) driven by a temperature controller (Lake Shore DRC91C). III. EXPERIMENTAL RESULTS AND DISCUSSION A. TSC Measurements The TSC spectra of the samples irradiated to Li/cm 2 are shown in Fig.2. After irradiation the two samples are almost intrinsic, with effective doping concentration N eff cm -3 at room temperature [5]. All the measurements have been carried out with different reverse biases, also to study the evolution of N eff with temperature. The Fig. 2. TSC spectra of the STFZ (upper plot) and DOFZ sample (lower plot), irradiated to Li/cm 2. The fits (dashed lines) corresponding to the dominant deep levels are also shown. The signal below 80 K can be saturated with a reverse bias lower than 200 V. On the contrary, between 140 K and 180 K, the space charge density inside the depleted region is so high that full depletion cannot be achieved in our voltage range. The high and positive N eff value deduced at 160 K from these measurements cannot be compared to the results of capacitance-voltage measurement at room temperature [5], which predict a quasi-intrinsic bulk. This indicates that further deep levels close to midgap become ionized between the highest temperature investigated by our TSC and room temperature. In the DOFZ sample the total concentration of the group below VO is cm -3, and the VO concentration is cm -3. The cumulative concentration [V 2 ]+[C i O i ] is at least cm -3, as deduced from the measurement carried out with V rev =200 V. In the STFZ sample the total concentration of the group below VO ( cm -3 ), and the

3 VO concentration ( cm -3 ) is lower than in the DOFZ devices. The cumulative concentration [V 2 ]+[C i O i ] in the STFZ sample is at least cm -3. Above 140 K, the shape of the signal changes with reverse bias, indicating that the distribution of defects is not uniform in the bulk. This observation is consistent with a picture in which the samples are not inverted and the concentration of V 2 related defects increases toward the backside of the sample (n + -side). In fact, as discussed in [12], the range of 58 MeV Li ions in Si is 400 µm and the specific energy loss of Li ions is increasing by a factor 1.7 at the end of a 300 µm thick transversing path. As a consequence the damage produced by Li ions in these samples increases with the depth in the Si bulk. As shown in Fig.2 by the fits plotted with dashed lines, the main features of the TSC spectra at low Li ion fluence can be accounted for in terms of contributions from point defects. This observation is reinforced by the comparison with the TSC spectrum of a DOFZ silicon sample irradiated by 300 Mrad of 60 Co γ-rays which does not produce defect clusters but only point defects [13]. The comparison is shown in Fig.3. It is evident that, apart from a different relative height of the various peaks, the shape of the spectra is very similar. The distortion of the C i O i -V -/0 2 group at 150 K after Li ion irradiation is due to a modulation of the space charge width [14]. Fig. 3. A comparison between the TSC spectra of two DOFZ silicon samples irradiated with Li ions at the fluence of Li/cm 2 and with 300 Mrad of 60 Co γ-rays. Devices were biased at V rev=100 V during the measurements. After γ-ray irradiation, a signal from the I deep level at 0.58 ev is usually observed in STFZ silicon, whereas it is strongly suppressed in DOFZ [15]. It is responsible for the different behavior of N eff at room temperature in STFZ and DOFZ silicon after γ-ray irradiation. In our measurements the I peak should occur at about 220 K in TSC spectra, but it cannot be detected due to the increase of the leakage current occurring in the high temperature range. B. PICTS Measurements It is well known that irradiation with hadrons and ions is able to produce displacement cascades in such a way that the primary state of damage involves extended regions of the semiconductor [16],[17]. The thermally activated rearrangement of the lattice defects should result in both isolated point defects as well as defect clusters [18],[19]. In TSC and DLTS spectra, defect clusters produce a broadening of V -/0 2 peak [20] as well as the formation of new features at high temperature. Both for proton, neutron and Si ion irradiation, a fluence threshold is observed for the formation of these deep levels, while only point defects can be observed below the critical fluence. In the case of 1 MeV neutrons [20] the critical fluence is between n/cm 2 and n/cm 2, the broadening of the V 2 peak can be accounted for by the formation of a subband of deep levels around the V 2 level at 0.42 ev, in the energy range ev with cross section σ=(1-10) cm 2. In [20] the growth of new features at higher temperature in the TSC spectra has been related to the formation of a continuous distribution of deep levels in the range E= ev with cross section σ=(3-10) cm 2. After 24 GeV proton irradiation [11], a wide broadening of the DLTS spectrum has been observed at the fluence of p/cm 2 but not at p/cm 2. Finally a threshold for cluster formation in silicon samples irradiated by 4 MeV Si ions has been observed by DLTS measurements at Si/cm 2 [21]. After such a fluence a broad structure with a peak related to a deep level at 0.44 ev with σ= cm 2 appears in the DLTS spectra. Similar results have been also obtained after Ge and Sn implant [22]. In all the previous cases [21],[22] only point defects are observed below the critical fluence of ions/cm 2. In another study [23], evidence of interaction between defects generated in subsequent collision cascades, producing a different residual damage, has been measured above a Si ion fluence threshold in the range Si/cm 2, for Si ion irradiation energies of 145 kev and 1.2 MeV, respectively. We will show that a similar behavior can be observed also with Li ions. The PICTS spectra of the STFZ and DOFZ samples irradiated to the fluences Li/cm 2 and Li/cm 2 are shown in Fig.4. An enhanced formation of clusters for both STFZ and DOFZ devices irradiated at the highest fluence can be inferred by the appearing of a broad peak in the range K, which is not present for the samples irradiated at the lowest fluence. It is evident that the critical fluence for the formation of new extended defects by 58 MeV Li ions irradiation is comprised within this fluence range ( Li/cm 2 ).

4 Fig. 4. PICTS spectra for STFZ and DOFZ samples biased at V rev=50 V and irradiated at two different fluences. The sampling times are t=6 ms and t=18 ms, respectively. In Fig.5 the PICTS spectra of STFZ samples obtained after 58 MeV Li ion irradiation, are compared to the spectra obtained after 1 MeV neutron and 24 GeV proton irradiation beyond the critical fluence. After the Li ion critical fluence, the corresponding PICTS spectra resemble those obtained by neutron and proton irradiated samples. The dominant spectral feature is a structure between 170 K and 270 K, which can be accounted for by the contributions from C i O i, V -/0 2 and by the deep level distribution in the range ev extrapolated from the analysis of the neutron irradiated sample. The position of the spectral components from these deep levels is shown in the lower part of the figure. The critical fluence measured after Li ion irradiation is consistent with the values reported for hadrons if the fluences are rescaled by the experimental hardness factors for 1 MeV neutrons, 24 GeV protons and 58 MeV Li ions in µm thick samples [5]. IV. CONCLUSIONS TSC and PICTS spectra have been measured with STFZ and DOFZ silicon diodes irradiated with 58 MeV Li ions at the fluences Li/cm 2, Li/cm 2 and Li/cm 2. The deep level population grows with the fluence in a fashion similar to the case of irradiation with neutrons and protons. At the lowest fluence mainly point defects are generated, which are the same observed after 60 Co γ-ray irradiation. Above the critical fluence in the range (2-29) Li/cm 2 a conspicuous formation of extended defects is observed. The critical fluence for Li ions scales with the value measured after 1 MeV neutron irradiation, which is in the range (4.8-47) n/cm 2, if the experimental hardness factors of Li ions in µm thick diodes is considered for the scaling. Fig.5. PICTS spectra measured in STFZ silicon diodes after various fluences of 58 MeV Li ions (dashed lines), MeV neutrons/cm 2 and GeV protons/cm 2. The presented results show that the similarities observed after proton and Li ion irradiation by macroscopic characterization [5] are due to the equivalence of microscopic damage. A difference is observed in the distribution of radiation induced defects along the detector thickness, as microscopic damage of 58 MeV Li ions is higher towards the backside of the device. For this reason, Li ion irradiation is best suited for thinner devices, with thickness in the range µm [24]. For thicker diodes, the change in energy loss along the thickness could be useful to investigate effects as space charge sign inversion at low temperature. V. ACKNOWLEDGMENT We wish to warmly thank Eckhart Fretwurst from University of Hamburg, Ioana Pintilie from University of Bucharest and Z. Li, from BNL Upton, NY for providing us the proton, neutron, and γ-ray irradiated samples used for comparison in this study. VI. REFERENCES [1] The Large Hadron Collider, conceptual design, The LHC study group, CERN/AC/95-05 (LHC), 20 October [2] F. Gianotti et al., hep-ph/ , April [3] The RD50 Proposal, LHCC /P6, CERN, 15 February [4] A. Candelori, D. Bisello, P. Giubilato, A. Kaminski, A. Litovchenko, M. Lozano, M. Ullán, R. Rando, and J. Wyss, "Lithium ion-induced damage in silicon detectors," Nucl. Instr. and Meth. in Phys. Res. A, vol. 518, no. 1-2, pp , [5] A. Candelori, D. Bisello, G.-F. Dalla Betta, P. Giubilato, A. Kaminski, A. Litovchenko, M. Lozano, J. R. Petrie, R. Rando, M. Ullàn, and J. Wyss, "Lithium ion irradiation of standard and oxygenated silicon diodes," IEEE Trans. Nucl. Sci., vol. 51, no. 5, pp. 1-7, [6] ST Microelectronics, Stradale Primosole 50, I

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