Journal of Non-Crystalline Solids

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1 Journal of Non-Crystalline Solids 377 (2013) Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: locate/ jnoncrysol Gamma ray induced structural effects in bare and Ag doped Ge S thin films for sensor application M. Mitkova a,b,e,, P. Chen a, M. Ailavajhala a, D.P. Butt b,e, D.A. Tenne c, H. Barnaby d, I.S. Esqueda d a Department of Electrical and Computer Engineering, Boise State University, Boise, ID 83725, USA b Department of Materials Science and Engineering, Boise State University, Boise, ID 83725, USA c Department of Physics, Boise State University, Boise, ID 83725, USA d Department of Electrical Engineering, Arizona State University, Tempe, AZ 85287, USA e Center for Advanced Energy Studies, 995 University Blvd., Idaho Falls, ID 83401, USA article info abstract Article history: Received 5 October 2012 Received in revised form 11 December 2012 Available online 31 January 2013 Keywords: Chalcogenide glasses; Radiation sensing; Radiation effects; Radiation induced Ag diffusion We present data on radiation-induced effects in chalcogenide glasses that also trigger radiation induced structural reorganization contributing to silver (Ag) diffusion. To study these effects and silver diffusion, depending on the radiation dose, films were prepared and analyzed using Raman spectroscopy, X-ray diffraction and Energy Dispersion X-ray Spectroscopy. The results show a structural development occurring in films containing 45.4 at.% Ge with increasing radiation dose defined by increase in the edge-sharing/ corner-sharing ratio, higher ethane-like unit values and rise of the amount of Ag diffused within the system. Utilizing these effects, a resistance based radiation sensing device has been created. The I V curves characterizing the sensor operation demonstrate decreased device resistance as a result of the radiation Elsevier B.V. All rights reserved. 1. Introduction One of the widely explored property of chalcogenide glasses are the light induced effects in them, which manifest themselves by bleaching or darkening of the material due to formation of electron hole pairs and even bond switching by the interaction with light [1]. Often these effects are reversible and can be erased by annealing the material [2]. The photoinduced effects extend towards shorter wavelengths, for example by radiation with γ rays [3] by which a variety of radiation-induced phenomena occur, attributed to the freedom and flexibility associated with their atomic structure [4]. Studies show that in the case of Se S thin films, the band gap of the material can decrease by 0.39 ev (from 2.25 ev to 1.86 ev) as a result of γ ray dose of 500 kgy [3]. The traditional explanation of this effect is based on the formation of electron hole pairs created by radiation, which aid in the formation of defects that introduce localized states within the bandgap and contribute to a change in the effective Fermi level due to an increase of carrier concentration. These effects usually recombine shortly after cession of radiation because of the high availability of charged defects in the glass material. The radiation induced effects can be accompanied by bond rearrangement resulting in molecular rearrangements [4], which are time invariant. Another important feature of chalcogenide glasses is related to radiation induced diffusion of mobile metals (for example Ag or Cu) Corresponding author at: Department of Electrical and Computer Engineering, Boise State University, Boise, ID 83725, USA. Tel.: ; fax: address: mariamitkova@boisestate.edu (M. Mitkova). within the host glass [5]. In this case, one can obtain a significant increase in the radiation sensitivity by optical or electrical measurements, for example, Ag additives form states within the band gap of the chalcogenide glasses thus changing the optical properties and conductivity of the glassy medium. Conductivity measurements of Ag doped chalcogenide glasses show, that introduction of a very small amount of Ag drastically changes the conductivity of the hosting glass [6]. The above-mentioned effects make chalcogenide glasses very attractive for short wavelength radiation applications, for example imaging in X-ray radiology [7] or dosimeters for gamma radiation using radiation induced Ag diffusion. With respect to the latter, detailed understanding of the relation between structure, composition and radiation induced effects is important since those are the main factors influencing the performance of the sensor. In this work, we focused our study towards understanding the structural effects of radiation over a range of glass compositions from the Ge S system as bare films as well as Ag diffused films as a direct result of gamma radiation. These effects are studied utilizing Raman spectroscopy and X-ray diffraction (XRD). Energy Dispersion X-ray Spectroscopy (EDS) elucidates the amount of Ag introduced during the gamma radiation. Examples of the electrical performance of structures with non-diffusive metal electrodes and source of Ag are presented in order to illustrate both the potential of the material and the suggested structure for radiation sensing. 2. Experimental Using thermal evaporation (PVD) with Cressington evaporation system and applying a crucible with a semi-knudsen cell structure /$ see front matter 2012 Elsevier B.V. All rights reserved.

2 196 M. Mitkova et al. / Journal of Non-Crystalline Solids 377 (2013) to establish pressure equilibrium, 60 nm thin films were evaporated from the Ge 20 S 80,Ge 30 S 70,Ge 33 S 67 and Ge 40 S 60 glasses. These films were studied before and after radiation either as-prepared or covered with a 30 nm film of Ag. After radiation, excess topological silver was dissolved with a 0.1 mol/l solution of Fe(NO 3 ) 3 for further compositional and structural studies. To determine the exact elemental composition of the thin films, EDS studies were performed on a LEO 1430VP Scanning Electron Microscope with EDS. Several points (usually 5) were measured across the wafer for each sample to study uniformity. The Raman spectroscopy was accomplished in macro Raman mode using a Horiba-Jobin Yvon T64000, triple spectrometer and macro-backscattering configuration. For excitation a nm line of a He Cd laser was used with 60 mw power, focused into a circular spot of ~1 mm diameter. The samples were measured in an evacuated cryostat (at Torr) at 100 K as described in [8]. To give a proof of the concept for electrically measuring radiation induced effects, a basic lateral device was created, build up by two inert electrodes and a Ag source in close proximity. X-ray diffraction patterns were obtained using a Bruker AXS D8 Discover X-ray Diffractometer equipped with a Hi-Star area detector. The typical setting is 2 frames, 300 s per frame, the X-ray tube and the area detector scan axes are coupled starting from 15 with step size of 20. The final XRD spectra are integrated along Chi. Beam conditions included a Cu anode at 40 kv and 40 ma to produce Cu Kα 1 radiation (λ= Å) through a Göbel mirror producing a collimated beam. Further experimental details are given in [8]. DC current-voltage measurements were performed on fabricated sensors using an Agilent 4146 parameter analyzer with the aid of Labview software to control the sweep parameters. Conditions of the sweep were adjusted such that voltage was varied from 0 to 1 V with 5 mv steps, while simultaneously measuring the current using a Source Monitoring Unit (SMU). 3. Results The results show that the films are quite homogenous since EDS in different areas and different samples yielding a variation of ~1%. Evaporated films tend to be much more Ge-rich than the synthesized bulk glass and the Ge/S ratio does not change with dose or Ag diffusion since gamma rays do not cause structural transmutation. The compositional results for pure and Ag-diffused films are summarized in Table 1. Raman spectra for pure chalcogenide glass films, the mode assignments, cross-section of Ge S and corresponding structural units are summarized, and their positions are represented in Fig. 1. The figure shows a standard development of the glass structure depicting an increase in the intensity of edge-sharing modes () and ethane-like modes () compared to corner-sharing modes () as the samples become more Ge-rich. For samples with greater than 33.3 at.% Ge, the signature of the distorted rock salt double-layer structure () is registered. This specific trend for the GeS 2 glass corresponds to the Raman results for the bulk glass with the same composition [9] and is an evidence for films of good quality. The difference in the intensity of Raman Intensity (arb. units) Ge 34.7 S 65.3 Sulfur Sulfur Wavenumber(cm -1 ) Fig. 1. Raman data for the studied films at different radiation doses. the characteristic vibrations of the structural units in the radiated samples is approximately 2 3%, which is within the error of the fitting method, so it is difficult to say whether these effects are real. However, well expressed difference has been registered for samples with composition where a clear tendency towards increase of the / area ratio has been well expressed as shown in Fig. 2. The Raman spectra of films that were doped with Ag, due to radiation, show that the,, and modes remain. We suggest that these are Ge S br modes since the sulfur atoms are two-fold coordinated, connecting two Ge atoms. Adding Ag atoms to the backbone structure tends to break sulfur bridges and form Ag cations terminated by S anion pairs. The tentative mode assignments of the Ge S t modes are shown in Fig. 3. For these samples, the spectra decreases in counts and shows a sloped background by increasing the Ag content. There are a number of terminal Ge S modes, which progressively grow in scattering strength with higher radiation dose. This further proves that when silver enters the network, it preferably breaks sulfur bridges instead of Ge\Ge bonds, leading to a predominant increase of modes compared to other modes. The reaction products forming after Ag diffusion at room temperature are studied by XRD and the results are presented in Fig. 4. In general, the films are amorphous and there are no strongly expressed crystalline molecular peaks. Using the JCPDS card , we could identify the binary composition Ag 2 S which is only present in the spectra for films. There are some peaks which could be associated with the presence of Ag 2 GeS 3 (JCPDS card ) but they are wide and with a small intensity indicating that the crystalline Table 1 EDS results on GeS and Ag/GeS samples. Bulk glass composition Evaporated non-ag samples, Ge x S 1-x Ag-containing samples, Ag y (Ge x S 1-x ) 1-y Ge (x) Ge (x) Ag (y) Ge 20 S ±0.1% 33.4±0.1% 0.7% 3.2% 3.4% Ge 30 S ±0.3% 35.1±0.2% 0.8% 3.8% 3.5% 3.2% 4.0% Ge 33 S ±1.0% 43.0±0.4% 1.1% 4.5% 6.0% Ge 40 S ±0.8% 46.1±1.7% 0.7% 4.0% 4.6% 5.7% 7.2%

3 M. Mitkova et al. / Journal of Non-Crystalline Solids 377 (2013) Area ratio of / structure units clusters related to them are very small and the structure is predominantly amorphous. There are also some small peaks that have been identified as pure Ag (JCPDS card ) and we assume that those are traces of non-dissolved Ag clusters from the surface of the samples. Fig. 5 shows the DC current voltage characteristic of lateral device, pre and post irradiation. There is a decrease of the resistivity of the material after radiation credited to Ag diffusion. This effect is best expressed for the based devices, which corresponds with the introduction of a highest amount of Ag as shown in the compositional analysis. 4. Discussion Mrad Fig. 2. Dependence of the / Raman modes ratio for the studied films at different doses. XRD intensity (Arb. units) (a) Ag/Ge S (b) Ag/Ge S (c) Ag/ (d) α-ag 2 S, PDF Ag (111) Ag (200) Ag PDF (e) Ag 8 GeS 6, PDF Ag (220) θ (deg.) Fig. 4. XRD data for the studied films at different radiation doses. The major Raman peak for the S rich glasses is located at around 340 cm 1 which represents the breathing mode of the Ge(S 1/2 ) 4 corner sharing tetrahedra in which Ge is fourfold coordinated and S Ag/ is twofold coordinated. Its dominance fades with increasing the relative amount of Ge over 41.0 at.% when the edge sharing tetrahedra, Raman Intensity (arb. units) Difference spectrum Ag/ Difference spectrum Ag/ meta- dipyro- GeS 3- GeS pyro- meta- di- GeS I D (A) (a) (b) pre rad post rad (100k) Resistance decrease after γ-ray irradiation ΔR/R = 24% pre rad post rad (100k) Difference spectrum pyro- meta- di Resistance decrease after γ-ray irradiation ΔR/R = 86% Wavenumber (cm -1 ) Fig. 3. Raman spectra of Ag containing films after irradiation compared to non-ag containing films and their difference spectra V D (V) Fig. 5. I V curves of radiation sensing structures before and after γ irradiation.

4 198 M. Mitkova et al. / Journal of Non-Crystalline Solids 377 (2013) ethane-like structures and outrigger raft structure concurrently develop. It is worth mentioning that the frequency of the corner sharing mode that appears at 340 cm 1 for the composition undergoes a small shift to 345 cm 1 with increase of the Ge concentration, which is derived from intertetrahedral couplings [10]. This factor is an important feature in the interaction of the glasses with radiation. The coupling with the nearby tetrahedra creates a high connectivity of the system with high concentration of S keeping it intact and the radiation at the conditions of this study does not result in significant bond breaking that could be detected. Based on our radiation data we suggest that for the S rich glasses, formation of electron hole pairs as a result of radiation is the major mechanism of their reaction to radiation. It is empowered by the high concentration of chalcogen atoms that contain lone pair electrons. This creates internal electric fields produced by non-equilibrium, radiation induced effects such as C 1 + and C 3 + centers [11]. They are the reason for the reduction of the optical band gap reported by Xia et.al. [12] and hence increased conductivity. At the conditions of our experiments there is no significant bond breaking and structural reorganization for chalcogen-rich glasses. However structural reorganization has been obtained for sulfur-rich Ge Sb S glasses that have been radiated with a 7.7 mgy dose which is much higher than the dose, used in our experiments [13]. For the glasses with 45.4 at.% Ge, due to the reduced amount of nearby tetrahedra, restructuring of the system is possible. In this case, Ge 2+ can be regarded as a modifier in the system which contributes to breaking up the bridging sulfur. It is for this reason that radiation induces formation of a higher number of edge sharing structural units in the Ge-rich films by breaking some of the existing bonds. This has the important consequence of opening the entire structure of the films. For the Ge-rich glasses, the disconnection of the network and decrease of the S bridging atoms makes the rigid structure more susceptible to bond reorganization. Zhao et al. [14] also have reported an increase of the sensitivity with increase of the Ge concentration. When Ag film is in contact with the chalcogenide film in the presence of radiation, radiation induced diffusion takes place. Once introduced into a non-crystalline or glassy phase, Ag could form stoichiometric solids and could be included as an additive in the base network [15]. These additives can either segregate [15,16] as separate phases or uniformly mix [15] with the base glass to form homogeneous solid electrolyte glasses. The fate of Ag strongly depends upon the matrix in which it is introduced. As revealed by the XRD studies for the case when Ag diffuses in film, part of its phase separates and forms Ag 2 S reacting with S from the S chains and rings. This Ag 2 S forms big clusters which can be sensed with the XRD resolution, but are not visible on the Raman spectra since Ag 2 s Raman silent. However, we suggest that there is only a small fraction of Ag which forms Ag 2 S since there is a large change in the Raman spectrum of the hosting glass Fig. 3a, suggesting reorganization of the chalcogenide network to accommodate Ag that did not form Ag 2 S. There are several specifics that we want to point out: (i) there is intensity growth of the mode at 250 cm 1 indicating the formation of higher number structural units and significant sulfur depletion of the initial composition of the hosting backbone. The number of units is limited in the case of host with 32.8 at.% Ge because of the partial consumption of Ag atoms in formation of Ag 2 S. In the case of glasses with higher concentration of Ge, the growth of this mode is better expressed because of the higher consuming of the network building blocks for Ag incorporation and the higher concentration of Ag introduced in the glass-phases (Table 1). (ii) The relative intensity of the mode at 343 cm 1 is reduced while the vibrations at 370 cm 1 and 400 cm 1 are strengthened. This tendency develops with the increased Ag concentration which was traced by the EDS results. We attribute the latter modes to the development of thiogermanate bonds (GeS ) forming pyro- (GeS 3 3.5), meta- (GeS 2 3)anddi-(GeS 2.5) thiogermanate tetrahedra as suggested by Kamitsos et al. [17]. Note the dominance of the metathiogermanate tetrahedra, which after accommodation of Ag forms the stoichiometry that is specific for this system the Ag 2 GeS 3 ternary. It is not visible on the XRD spectra because it becomes part of the amorphous network. A question can arise inquiring the reasoning behind the high concentration of introduced Ag in films with the highest concentration of Ge since there is not enough S to attract Ag. The EDS data show that indeed the real composition of this film is. For this composition, we [5] have demonstrated the formation of a layered structure in which both Ge and S are threefold coordinated through formation of dative bonds. The possibility of a threefold coordination of both Ge and the chalcogen (Se) has been recently confirmed as well for liquid GeSe 2 by first-principle molecular dynamic calculation [18]. The dative bonds are very weak and easily destroyable by radiation. This results in a high negative charge concentrated on the chalcogen atoms, which together with the channeled structure characteristic for this composition [5], is a big driving factor for Ag + diffusion into this glass. In addition, the demonstrated opening of the structure through reorganization of to structural units helps to further the introduction of Ag into the glassy matrix. It is for this reason that the conductivity increases much more for this particular glass compared to the cases with higher concentration of S. Regarded in the context of radiation sensing elements, this particular structure will result in a better expressed difference in their electrical performance during a radiation event. The large reduction in resistance in the Ge-rich devices, shown in Fig. 5, is supported by the structural reorganization and the increase in the Ag + diffusion in these films. 5. Conclusions We have studied the effect of radiation on various compositions of Ge x S 100 x in bare and radiation induced silver doped films, as well as devices that couple the radiation induced effects and silver diffusion. Bare Ge x S 100 x and Ag doped films have been characterized using EDS, Raman and XRD, which show two characteristic behaviors depending on the composition: in systems that are chalcogen-rich, we did not observe a significant structural change. While in the Ge-rich films, an increase in / ratio has been registered including a tendency to incorporate more Ag with an increase in radiation dose. The electrical performance of the devices fabricated from the studied glass films show a decrease in resistance after radiation. This is primarily attributed to Ag diffusion which is assisted by radiation induced changes presented in this paper. Acknowledgment This work is supported by a grant from Battelle Energy Alliance under Blanket Master Contract No The Raman and XRD instruments have been supported by National Science Foundation (NSF) DMR and MRI , respectively. We would also like to acknowledge Brian Jaques for the help with XRD measurements. References [1] A.V. Kolobov, S.R. Elliott, Adv. Phys. 40 (1991) [2] A.V. Kolobov, K. Tanaka, Semiconductors 32 (1998) [3] O.I. Shpotyuk, in: R. Fairman, B. Ushkov (Eds.), Properties of Chalcogenide Glasses, Elsevier Acad. Press, 2004, pp [4] M. El-Hagary, M. Emam-Ismail, E.R. Shaaban, A. El-Taher, J. Radiat. Phys. Chem. 81 (2012) [5] T. Kawaguchi, S. Maruno, S.R. Elliott, J. Appl. Phys. 79 (1996) [6] M. Ribes, E. Bychkov, A. Pradel, J. Optoelectron. Adv. Mater. 3 (2001) [7] S.O. Kasap, J.A. Rowlands, IEEE Proceedings Circuits, Devices and Systems, 149, 2002, pp [8] M. Mitkova, Y. Sakaguchi, D. Tenne, S.K. Bhagat, T.L. Alford, Phys. Status Solidi A 207 (2010) [9] L. Cai, P. Boolchand, Philos. Mag. Part B 82 (2002) [10] X. Feng, W.J. Bresser, P. Boolchand, Phys. Rev. Lett. 78 (1997) [11] H. Fritzsche, in: P. Boolchand (Ed.), Insulating and Semiconducting Glasses, World Scientific, 2000, pp

5 M. Mitkova et al. / Journal of Non-Crystalline Solids 377 (2013) [12] F. Xia, S. Baccaro, D. Zhao, M. Falconieri, G. Chen, Nucl. Inst. Methods Phys. Res. B 234 (2005) [13] A. Kovalskiy, H. Jain, A.C. Miller, R.Y. Golovchak, O.I. Shpotyuk, J. Phys. Chem. B 110 (2006) [14] D. Zhao, H. Wang, G. Chen, S. Baccaro, A. Cecilia, M. Falconieri, L. Pilloni, J. Am. Ceram. Soc. 89 (2006) [15] C.A. Angell, K.L. Ngai, G.B. McKenna, P.F. McMillan, S.W. Martin, J. Appl. Phys. 88 (2000) [16] K.L. Ngai, S.W. Martin, Phys. Rev. B 40 (1989) [17] E.I. Kamitsos, J.A. Kapoutsis, G.D. Chryssikos, G. Taillades, A. Pradel, M. Ribes, J. Solid State Chem. 112 (1994) [18] M. Micoulaut, S. Le Roux, C. Massobrio, J. Chem. Phys. 136 (2012)