High performance platinum contacts on high‐fux CdZnTe detectors.pdf
1 Vol.:(0123456789) Scientific Reports |        (2023) 13:17963   | https://doi.org/10.1038/s41598-023-45331-9 www.nature.com/scientificreports High performance platinum  contacts on high‑flux CdZnTe  detectors Manuele Bettelli 1*, Silvia Zanettini 2, Leonardo Abbene 3, Francesca Casoli 1, Lucia Nasi 1,  Giovanna Trevisi 1, Fabio Principato 3, Antonino Buttacavoli 3 & Andrea Zappettini 1 The need for direct X-ray detection under high photon flux with moderate or high energies (30– 100 keV range) has strongly increased with the rise of the 4th Generation Synchrotron Light Sources,  characterised by extremely brilliant beamlines, and of other applications such as spectral computed  tomography in medicine and non-destructive tests for industry. The novel Cadmium Zinc Telluride  (CZT) developed by Redlen Technologies can be considered the reference material for high-flux  applications (HF-CZT). The enhanced charge transport properties of the holes allow the mitigation of  the effects of radiation induced polarization phenomena, typically observed in standard CZT materials  (LF-CZT) under high photon flux. However, standard LF-CZT electrical contacts led to inacceptable  high dark leakage currents on HF-CZT devices. In this work, a detailed study on the characteristics of  new optimized sputtered platinum electrical contacts on HF-CZT detectors is reported. The results  from electrical and spectroscopic investigations, showed the best performances on HF-CZT detectors  with platinum anode, coupled with both platinum or gold cathode. The morphology, structure, and  composition of Pt/CZT contact have been analysed by means of Transmission Electron Microscopy  (TEM) on microscopic lamellas obtained by Focused Ion Beam (FIB), highlighting the presence of  ­CdTeO3 oxide at the metal semiconductor interface. Despite several decades of progress in developing CdZnTe-based radiation detectors, both crystal growth and  contact deposition can still be considered hot topics in this field of research. Recently, Redlen Technologies  (Canada) proposed a new grade of CdZnTe material able to withstand high radiation fluxes to satisfy the ever- growing needs of application fields like medical and security ­imaging1. The high-flux CdZnTe (HF-CZT) can  operate at continuous X-ray fluxes > ­108 photons/mm2/s2 and at far higher levels of instantaneous flux tested with  extreme intensities delivered by a free electron laser (FEL)3. HF-CZT is also characterized by excellent spatial  ­uniformity4, outstanding linear response and time stability at fluxes up to ­1010 photons/mm2/s5 as was recently  tested. It has been well demonstrated by now that such astonishing performances at intense radiation fluxes are  related to a hole’s lifetime increase of an order of magnitude ( τh ∼2.5 µs)6–8 with respect to standard grade CZT  (LF-CZT) and a hole’s mobility-lifetime product µhτ h exceeding ­10–4 ­cm2/V9. The development and improvement of new contact deposition techniques is stimulated by the availability of  new promising materials. One of the main challenges posed by CdZnTe is the fact that the same contact deposi- tion procedure may lead to strongly different results on CZT crystals with slightly different physico-chemical  properties, trap characteristics and Fermi-level ­position10. Selecting the appropriate electrode material is essential  for minimizing the leakage currents. Despite several studies have been carried out reporting electrical charac- terization of contacts on standard LF-CZT ­detectors11–16, only few studies were performed on HF-CZT17. The first attempts in realising blocking contacts on this material by means of the technique developed at  IMEM-CNR, i.e., gold electroless deposition technique from alcoholic ­solutions13, were unsuccessful. This deposi- tion technique allowed to obtain performing and robust contacts on both boron-encapsulated vertical-Bridgman  grown CZT and Redlen LF-CZT, as shown in previous ­publications18–20, but producing sensors with extremely  high leakage currents even at low bias voltage when used on HF-CZT. The platinum electroless contacts devel- oped by IMEM-CNR and reported in Bettelli et al.16 were also tested. However, similarly to the gold electroless  contacts, these contacts induced excessively high leakage currents, making them unsuitable for radiation detec- tor applications. OPEN 1IMEM-CNR, 43124  Parma, Italy. 2due2lab S.R.L., 42019  Scandiano, RE, Italy. 3Department of Physics and  Chemistry (DiFC) ‑ Emilio Segrè, University of Palermo, 90128  Palermo, Italy. *email: manuele.bettelli@ imem.cnr.it 2 Vol:.(1234567890) Scientific Reports |        (2023) 13:17963  |  https://doi.org/10.1038/s41598-023-45331-9 www.nature.com/scientificreports/ This limitation stimulated the development of new optimized contacts for HF-CZT detectors. In this context,  we developed new platinum sputtered contacts, obtaining HF-CZT detectors with low leakage currents and  interesting spectroscopic performance. Methods Sample preparation Several detectors were fabricated on HF-CZT, with different electrode configurations. HF-CZT crystals, provided  by Redlen Technologies, are oriented along the <111> crystallographic direction and have two polar faces, A-face  or Cd-face (cadmium-rich face) and B-face or Te-face (tellurium-rich face). Usually Cd-face is more p-type and  is conventionally chosen as anode, while Te-face is more n-type and typically chosen as ­cathode21. As discussed  in the introduction, the transport properties of HF-CZT differ from those of standard spectroscopic CZT (LF- CZT), with mobility-lifetime products of ­10–3 and ­10–4 ­cm2/V for electrons and holes, ­respectively6. Detectors equipped with platinum sputtered contacts and gold electroless contacts were realised. Platinum  contacts were deposited at IMEM-CNR using the sputtering technique (base vacuum: 3 × ­10–8 mbar; Ar pres- sure: 8 × ­10–3 mbar, target voltage: 1000 V). The resulting Pt layer appears homogeneous, shiny, and off-white.  The layer thickness is about 100 nm, the surface roughness is 5.0 nm and 4.0 nm for ­Rq and ­Ra respectively as  measured with AFM (see Fig. SI_1 in Supplementary Info). Gold contacts were instead deposited following the  electroless procedure developed by Benassi et al.13 that ensures high mechanical stability. Four detectors were fabricated starting from a single chip of HF-CZT cut into four 5 × 5 × 1.5 ­mm3 crystals  oriented along the <111> direction. Each sample was lapped and polished before depositing the contacts. The  detectors had a full-area cathode and a customised pixelated anode as shown in Fig. 1. Pixel size is 500 µm, gaps  between pixels and guardpad are 200 µm large. Pixels area is 0.25 ­mm2. The pattern is composed of two single  isolated pixels (surrounded by guardpad) and a 2 × 2 pixel matrix. The four detectors had the same geometry  (Fig. 1) but different combinations of metal contacts at the anode (Cd-face) and cathode (Te-face). We carried  out a comparative study to evaluate their characteristics. All possible electrode configurations were tested and, subsequently, the best detectors were investigated  extensively to characterise their behaviour. Samples are named reporting their cathode/CZT/anode structure  in the following way: 1.	 Pt/CZT/Pt detector, where both contacts were made of platinum, is called PP. 2.	 Au/CZT/Au detector, where both contacts were gold, is called AA. 3.	 Au/CZT/Pt detector, where cathode was gold, while anode was platinum, is called AP. 4.	 Pt/CZT/Au detector, where cathode was platinum while anode was gold, is called PA. Current–voltage measurements and modelling Preliminary electrical characterization was performed at IMEM-CNR Parma (Italy), in order to first check  leakage currents of the detectors. Current–voltage (I–V) measurements were carried out in a probe station  equipped with a metallic plate and two probe tips mounted on micromanipulators and connected to a Keithley  2410 sourcemeter and a Keithley 6548 picoammeter. The system is located inside a Faraday cage that allows to  ensure dark measurement conditions and to reach very low noise levels. To investigate the electrical properties of the detectors at different temperatures, a second set of electrical  measurements was performed at the Department of Physics and Chemistry (DiFC) of University of Palermo by  using a temperature-controlled system. The I–V curves were measured with the CAEN NDT1471 power supply  connected to the cathode and the Keithley 2635B, configured as an electrometer, connected to the pixel anode.  The guard electrode was forced to the ground potential. I–V measurements were performed in both reverse  (i.e., by applying a negative voltage to the full-area electrode) and forward biasing (i.e., by applying a positive  voltage to the full-area electrode). All measurements were performed with the detectors enclosed in a shielded  box under a nitrogen atmosphere with a temperature control system. Figure 1.   Ramm3D model of Pt anode (left); photograph of the Pt full-area cathode and the pixellated Pt anode  (right). 9 Vol.:(0123456789) Scientific Reports |        (2023) 13:17963  |  https://doi.org/10.1038/s41598-023-45331-9 www.nature.com/scientificreports/ Data availability All data necessary to replicate the experiments are included in the paper. Datasets generated and acquired during  the current study are available from the corresponding author on a reasonable request. Received: 30 August 2023; Accepted: 18 October 2023 Figure 10.   (a) 109Cd, (b) 241Am, (c) 57Co energy spectra of a tested pixel of the AP detector. Figure 11.   Time stability of 241Am energy spectra. 10 Vol:.(1234567890) Scientific Reports |        (2023) 13:17963  |  https://doi.org/10.1038/s41598-023-45331-9 www.nature.com/scientificreports/ References 	 1.	 Iniewski, K. CZT sensors for computed tomography: From crystal growth to image quality. J. Instrum. 11, C12034 (2016). 	 2.	 Prokesch, M., Soldner, S. A. & Sundaram, A. G. CdZnTe detectors for gamma spectroscopy and X-ray photon counting at 250 ×  106 photons/(mm2 s). J. Appl. Phys. 124, 044503 (2018). 	 3.	 Veale, M. C. et al. Cadmium zinc telluride pixel detectors for high-intensity X-ray imaging at free electron lasers. J. Phys. Appl.  Phys. 52, 085106 (2019). 	 4.	 Veale, M. C. et al. Characterization of the uniformity of high-flux CdZnTe material. Sensors 20, 2747 (2020). 	 5.	 Baussens, O. et al. Characterization of high-flux CdZnTe with optimized electrodes for 4th generation synchrotrons. J. Instrum.  17, C11008 (2022). 	 6.	 Thomas, B. et al. Characterisation of Redlen high-flux CdZnTe. J. Instrum. 12, C12045 (2017). 	 7.	 Li, Y. et al. Effects of deep-level traps on the transport properties of high-flux X-ray CdZnTe detectors. Mater. Sci. Semicond. Process.  133, 105974 (2021). 	 8.	 Abbene, L. et al. 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This work was also supported by the Italian Ministry for  University and Research (MUR), under PRIN2022 project CUP B53D23004720006, by the European Union  (EU) under the project—FESR o FSE, PON Ricerca e Innovazione 2014–2020—DM 1062/2021 and FFR2023. Author contributions A.Z., M.B. and S.Z. planned the experiments. M.B., S.Z., F.C. and G.T. contributed to samples preparation. G.T.  and L.N. performed the structural characterization. M.B., S.Z., L.A., F.P. and A.B. performed the electrical char- acterization. F.P., L.A. and A.B. tested the spectroscopic performance of samples. A.Z. supervised the project.  M.B. and S.Z. wrote the manuscript with input from all authors. All authors discussed the results and reviewed  the manuscript. Competing interests  The authors declare no competing interests. Additional information Supplementary Information The online version contains supplementary material available at https://​doi.​org/​ 10.​1038/​s41598-​023-​45331-9. Correspondence and requests for materials should be addressed to M.B. Reprints and permissions information is available at www.nature.com/reprints. Publisher’s note  Springer Nature remains neutral with regard to jurisdictional claims in published maps and  institutional affiliations. 11 Vol.:(0123456789) Scientific Reports |        (2023) 13:17963  |  https://doi.org/10.1038/s41598-023-45331-9 www.nature.com/scientificreports/ Open Access   This article is licensed under a Creative Commons Attribution 4.0 International  License, which permits use, sharing, adaptation, distribution and reproduction in any medium or  format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the  Creative Commons licence, and indicate if changes were made. The images or other third party material in this  article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the  material. 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Title: High performance platinum contacts on high-flux CdZnTe detectors ​
Authors: Manuele Bettelli, Silvia Zanettini, Leonardo Abbene, Francesca Casoli, Lucia Nasi, Giovanna Trevisi, Fabio Principato, Antonino Buttacavoli, Andrea Zappettini ​
Publisher: Nature Scientific Reports
Date: 2023-10-18 00:00:00
Abstract: The need for direct X-ray detection under high photon flux with moderate or high energies (30–100 keV range) has strongly increased with the rise of the 4th Generation Synchrotron Light Sources, characterized by extremely brilliant beamlines, and of other applications such as spectral computed tomography in medicine and non-destructive tests for industry. ​ The novel Cadmium Zinc Telluride (CZT) developed by Redlen Technologies can be considered the reference material for high-flux applications (HF-CZT). ​ The enhanced charge transport properties of the holes allow the mitigation of the effects of radiation-induced polarization phenomena, typically observed in standard CZT materials (LF-CZT) under high photon flux. ​ However, standard LF-CZT electrical contacts led to unacceptable high dark leakage currents on HF-CZT devices. ​ In this work, a detailed study on the characteristics of new optimized sputtered platinum electrical contacts on HF-CZT detectors is reported. ​ The results from electrical and spectroscopic investigations showed the best performances on HF-CZT detectors with platinum anode, coupled with both platinum or gold cathode. ​ The morphology, structure, and composition of Pt/CZT contact have been analyzed by means of Transmission Electron Microscopy (TEM) on microscopic lamellas obtained by Focused Ion Beam (FIB), highlighting the presence of CdTeO3 oxide at the metal-semiconductor interface.