Bellini, M.

Berdermann, E.

Cindro, V.

Corsi, C.

Fischer, M.

Gsell, S.

Kanxheri, K.

Kis, M.

Lagomarsino, S.

Morozzi, A.

Passeri, D.

Schmidt, C. J.

Schreck, M.

Sciortino, S.

Servoli, L.

Träger, M.

Visinka, R.

GSI Repository

 Fabrication and characterization of efficiency and radiation tolerance of 3D di-amond detectors* S. Sciortino1,2,#, S.Lagomarsino1,2, M. Bellini3, C. Corsi1,4, V. Cindro5, K. Kanxheri6,7, A. Morozzi6,7, D. Passeri6,7, L. Servoli7, E. Berdermann8, C. J. Schmidt8, M. Kis8, M. Träger8, R. Visinka8, M. Schreck9, S. Gsell9, M. Fischer9 1Department of Physics, Florence, Italy; 2 INFN, Florence, Italy; 3INO-CNR Florence, Italy; 4LENS, Florence, Italy; 5Jožef Stefan Institute, Ljubljana, Slovenia; 6University of Perugia, Italy; 7INFN, Perugia, Italy; 8GSI, Darmstadt, Ger-many; 9University of Augsburg, Germany; 3D diamond detectors combine the intrinsic proper-ties of radiation tolerance of diamond with the ad-vantages of the 3D architecture: a shorter inter-electrode distance and, consequently, lower trapping probability and operating voltage bias. 3D diamond electrodes can be fabricated by laser irradiation [1]. Femtosecond laser irradiation is required, because with longer laser widths, at least in the nanosecond range, a more defective modified material is obtained [2]. We have previously reported [1] on 3D monocrys-talline Chemical Vapour Deposited (scCVD) diamond detectors, which exhibited a full collection to beta Minimum Ionizing Particles (MIP) at a voltage bias of a few volts, i.e., one order of magnitude lower than the bias level required for a two 2D sensor prepared with the same material. Since scCVD diamond are present-ly only obtained in very small areas, usually less than 1 cm2, we decided to investigate the performances of 3D detectors fabricated on polycrystalline (pCVD) dia-mond, before and after neutron irradiation up to the highest fluence ever reported for diamond. From our results 3D diamond detectors seem the most radiation-hard detectors available at present. Heteroepitaxial growth of Diamond On Iridium (DOI) seems a very promising technique to obtain a high quality material, more homogeneous in its electronic properties than the pCVD one and not limited in surface area. A 3D sensor implemented on DOI material has also been tested and reported in this work.   Sample preparation ant tests 3D radiation sensors have been implemented on four pCVD samples (5×5×0.5 mm3) by laser irradiation. Bulk electrodes were fabricated with an 800 nm Ti:Sa 30 fs laser with an energy density of 12 J/cm2. The columns were started from one of the 5×5 mm2 faces and termi-nated about 80 m away from the opposite one. They were connected by graphitic combs written on the surfac-es with a Nd:YAG ns laser (schematics in Fig. 1). Two 3D sensors and a 2D reference sensor have been fabricated in each of the four sample as shown in Figure 1. The 3D electrodes are patterned by repeating two elementary cells of sizes 100×160 m2 (3D100×160 sensors) or 70×114 m (3D70×114 sensors). The CCE has been measured by means of a 90Sr beta source, producing 18000 electron-hole pairs per MIP in the sample. The saturation collected charge was evaluated as 8700±300, 7200±300, and 7100±200 for the 3D70x114, the 3D100x160 and the 2D sensor, respectively.  Figure 1: One pCVD sample with the 2D reference sen-sors and the two 3D sensors. On the right: the unit cell. The 90% of these values were reached at 45 V, 60 V and 450 V, respectively. Hence, we assessed also for pCVD diamond, that the required bias voltage is an order of magnitude lower for the 3D than for the 2D sensors. The maximum signal for the 3D100x160 and the 2D sensor have about the same value, but the 3D70x114 sensors yield a collection efficiency about 20% higher. The CCE vs. volt-age bias has been fitted by use of the following expres-sion for the mean free path of the carriers:  ଵఒ ൌଵఒ೒ ൅ଵ௩ఛ   (1)  where  is the lifetime of the carriers, limited by the intra-grain defects, v is the drift velocity, which depends on the applied electric field (the bias voltage), g is a mean path limited by the grain boundaries. The parameter values resulting from the fit of the 2D sensors data are ns and g =160 m. We assumed the same value of  for the 3D sensors and evaluated numerical the CCE, by means of a three-dimensional finite element simulation of the electric field. The resulting values for g were 29 and 25 m for 3D100x160 and 3D70x114, respectively. The above values correlate well with the structure of the grains of the samples,  which exhibit a truncated cone shape along  the  ___________________________________________  * Work supported by EU (HadronPhysics3 project No. 283286) andGSI (Detector Technology and Systems Platform) and INFN Italy(experiment 3D_SOD) #silvio.sciortino@unifi.it GSI SCIENTIFIC REPORT 2014 FG-GENERAL-38DOI:10.15120/GR-2015-1-FG-GENERAL-38 455  Figure 2: CCE vs, neutron irradiation fluence, 1 MeV equivalent. diamond thickness, as evidenced by a morphological analysis carried out by optical profilometry. In the case of the 2D sensors the carriers drift almost parallel to the grain boundaries while in the case of 3D sensors they are strongly limited by the polycrystalline material in their path between the columnar electrodes. Radiation Tolerance of the 3D sensors We irradiated the four diamond samples at the experi-mental nuclear reactor of the Jožef Stefan Institute of Ljubljana, with fast neutron (neutrons of energy greater than 100 keV) at fluences ranging from 2.2×1014 cm-2 to 1016 cm-2. Figure 2 shows the variation of the CCE with the equivalent fluence of 1 MeV neutrons, for the 2D and the 3D sensors. In order to prove that the all-carbon elec-trodes are not degraded by the neutron irradiation, we have also fabricated a new 2D sensor and new 3D 3D70x114 sensor, on the sample irradiated at 3×1015 cm-2 1MeV-eq. neutrons. These new sensors exhibited the same response as the older ones after irradiation (see Fig. 2). The results from the planar sensors allow the immedi-ate evaluation of the hardness factor K of the material, defined by the following relation: 	 ଵ஼஼ா ൌଵ஼஼ாబ ൅ ܭܮ߶   (2)  where  is the fluence and L = 500 m the thickness of the material. The result, K= (4.7±0.2) 10-18 m-1 cm2, is in good agreement with that previously reported in [3]. The data represented in Fig. 2 have been fitted by use of a modification of expression (1): ଵఒ ൌଵఒ೒ ൅ଵ௩	ቀଵఛ ൅ ݇߶ቁ   (3)   Figure 3: Pulse height spectrum of DOI 2D (at 600 V) and 3D (at 140 V) sensors. A k value of about 1.5 10-6 cm2 Hz can be used to fit consistently the three curves. The factor 3 in efficiency gained by the irradiated 3D detectors with the smaller pitch can be explained by the assumption  that neutron damage results in the introduction of new intra-bandgap levels reducing the bulk mean lifetime of the charge carri-ers in the conduction and in the valence band, leaving unaffected the grain boundaries-limited mean free path g.  Diamond On Iridium  3D sensors A 3D sensor of the type 3D70×114 has been implemented on a DOI plate together with a reference planar 2D sen-sor. The sample is about 500 m thick, hence the overall generated charge from a MIP beta source is 18000 elec-trons. The collected charge was 7600 e corresponding to a CCE=42% for the 2D sensor at a bias voltage of 600 V. The 3D sensor yielded a mean value of 11000 e, corre-sponding to a CCE of 61%, at a bias voltage of 140 V.  The relative gain from 2D to 3D is quite high. Figure 3 shows the pulse high spectrum for the two sensors at maximum collection. The DOI sample exhibits relevant pumping effect and polarization. The above results corre-spond to the pumped state (>100 Gy beta-irradiation). The 2D signal before pumping was 4000 e (21 % CCE). The study of the radiation tolerance of these sensors is under way.  References [1] S. Lagomarsino et al., Diamond Relat. Mater. 43 (2014) 23. [2] S. Lagomarsino et al., Appl. Phys. Lett. 103 (2013) 233507.  [3] G. Kramberger, Radiation damage in diamond detec-tors, September 2012, Vertex 2012, Jeju, Korea FG-GENERAL-38 GSI SCIENTIFIC REPORT 2014456 DOI:10.15120/GR-2015-1-FG-GENERAL-38

Fabrication and characterization of efficiency and radiation tolerance of 3D diamond detectors

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Fabrication and characterization of efficiency and radiation tolerance of 3D diamond detectors

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