Studies of Gamma Radiation Induced Effects in Ge-rich
Studies of Gamma Radiation Induced Effects in Ge-rich Chalcogenide Thin Films D. Nesheva1, M. Ailavajhala2, P. Chen2, D. A. Tenne3, H. Barnaby4, M. Mitkova2* Institute of Solid State Physics, Bulgarian Academy of Science, 1784 Sofia, Bulgaria 1 Dept. of Electrical and Computer Engineering, Boise State University, Boise, ID 83725, USA 3 Dept. of Physics, Boise State University, Boise, ID 83725-1570, USA 4 School of Electrical, Computer, and Energy Engineering, Arizona State University, Tempe, AZ 85287-5706, USA 1 Outlines Motivation Experimental details Raman scattering data
Dark and photoconductivity data Conclusions Chalcogenide glasses These glasses are based on the chalcogen elements S, Se, and Te and are formed by the addition of other elements such as Ge, As, Sb, Ga, etc. They are: generally transparent from the visible up to infrared; show phase change memory effects; can be doped by rare-earth elements and numerous applications of active optical devices have been proposed; are optically highly non-linear and could therefore be useful for all-optical switching; are sensitive to the absorption of electromagnetic radiation and particle irradiation and show a variety of photoinduced effects as a result of illumination: photodarkening, photobleaching, photocrystallization, photoplastic effects, etc. Photodiffusion in Chalcogenide Glasses
EF Ag film Ag+ EC Ag film EV Glass film Radiation sensor, BSU February 24, 2011 Scheme of electron/ion generation at the metal/metal-doped chalcogenide interface Experimental details
Synthesis of bulk Ge40Se60 glasses and thermal evaporation in vacuum, film thickness 100 nm. EDS composition check by a LEO 1430VP Scanning Electron Microscope with EDS accessory. Gamma irradiation in a closed cylindrical cavity by concentrically established 60Co (average energy E = 1.25 MeV) radioisotope capsule at a dose rate of 12 rad/sec. The applied -ray doses were ranging from 20 ray doses were ranging from 20 krad(Si) to 3 Mrad(Si). Raman scattering measured by a Horiba Jobin Yvon T64000 triple monochromator, = 441.6 nm blue line of a He-ray doses were ranging from 20 Cd lase, power 60 mW on ~ 0.2 mm diameter circle area, Tm=100K. Electrical conductivity measured in the 293-ray doses were ranging from 20 423 K range; co-ray doses were ranging from 20 planar aquadag (carbon) contacts on the top surface of the layers; the contacts produce a gap cell with an active area of ~ (110) mm2. There are two types of effects occurring in chalcogenide glasses upon irradiation*:irradiation*: reversible which decay after ceasing the radiation irreversible which are stable with the time. The reported measurements have been carried out after the radiation has been stopped, so that they can be regarded as capturing only the irreversible changes caused by radiation.
* O. I. Shpotyuk, in Semiconducting Chalcogenide Glass I: Glass formation, Structure and simulated transformations in chalcogenide glasses, edited by R. Fairman and B. Ushkov (Elsevier Academic Press, New York, 2004), p. 215. Raman scattering Ge40Se60 0.9 100 krad 20 krad virgin (a) 1.2 Normalized intensity Normalized intensity 1.2 1.5 0.6 0.3
250 300 350 -1 Raman shift (cm ) Raman scattering spectra (symbols) of virgin and -irradiated Ge40Se60 films: (a) low-dose range, (b) high-dose range. The dashed lines represent the deconvolution results for the samples irradiated with 100 krad (a) and 1.3 Mrad (b). GeSe2 irradiation*: main Raman modes Symmetric, 150-230 cm-1 ETH units Se3-Ge-Ge-Se3 178 cm-1 216 cm-2 Asymmetric 230-330 cm-1 1.0 Ge40Se60
CS ETH 0.8 ES 0.6 0.4 100 krad 20krad virgin 175 200 -1 Raman shift (cm ) 225 Irradiation induced decrease of the ethane-like units i.e. of the weakest Ge-Ge bonds. Area ratio of ES/CS structure units
Normalized intensity Raman scattering symmetric vibrations 0.48 Ge40Se60 0.45 0.42 0.39 0.36 20 100 200 1300 0 Gamma irradiation dose (krad) Dependence of the area ratio ES/CS structural units for Ge40Se60 films at different irradiation doses. Results from different evaporation batches are presented. Raman scattering 1.5 Normalized intensity
Relative variation of the integrated intensity of the scattered light in the 230-330 cm-1 range with respect to the sum of the integrated intensities of the cornershared (ICS) and edge-shared (ICS) modes. Each group of symbols corresponds to different evaporation batch. Dark and photoconductivity Ip, virgin Ge40Se60 Ip, 3Mrad -9 10 Id, virgin -1 -1
Conductivity ( cm ) Id, 3Mrad 10 -10 dark current, Id Ead(virgin)=0.75 eV 10 -11 Ead(3Mrad)=0.74 eV photocurrent, Ip 10 Eap(virgin)=0.26 eV -12 Eap(3Mrad)=0.28 eV 2.1 2.4
2.7 3 3.0 -1 10 /T (K ) 3.3 Temperature dependences of dc dark conductivity and steady-state photoconductivity of virgin and -irradiated (dose of 3 Mrad) Ge40Se60 thin films. The solid lines represent a linear fit of the experimental points (symbols). Conclusions In Ge40Se60 thin films the gamma irradiation induces : o decrease of the weakest Ge-Ge bonds; o increase in the ES/CS ratio and at high irradiation doses an overall disorder decrease. Both effects could be important for Ag diffusion in the films; The disorder decrease is similar to the effect of the
temperature treatment but it is related to strong electronphonon interaction rather than to sample heating. Thank you for your attention Interaction of pureChalcogenide glasses with electromagnetic radiation Creation of electron-hole pairs Separation of electron-hole pair, thus contributing to electrical response of the material Photoconductivity Radiative recombination of the electron-hole pair Photolumines cence Non-radiative recombination of the electron-hole pair - this brings material in a different state than the
non-illuminated one Radiation sensor, BSU February 24, 2011 15 Gamma-induced optical changes in GeSe thin films Absorption spectra of (1,2) gammairradiated and (3) unirradiated a-GeSe films (1.25 m) (a) before and (b) after storage for a year; D= (1)1 MGy and (2) 10 kGy. (1, 2) Eopt and (3, 4) n as functions of gamma dose for a-GeSe films (2, 3) before and (1, 4) after storage for a year. R. R. Romanyuk et al., Inorganic Materials, 2007, Vol. 43, No. 6, pp. 584587. Chalcogenide glasses These glasses are based on the chalcogen elements S, Se, and Te and are formed by the addition of other elements such as Ge, As, Sb, Ga, etc. They are generally transparent from the visible up to infrared
can be doped by rare-earth elements and numerous applications of active optical devices have been proposed. are optically highly non-linear and could therefore be useful for all-optical switching. are sensitive to the absorption of electromagnetic radiation and particle irradiation and show a variety of photoinduced effects as a result of illumination: photodarkening, photobleaching, photocrystallization, photoplastic effects, etc. 3D AFM picture (a) and a cross-section (b) of the pattern obtained on 0.5 lm thick aSe layer irradiated with 60 keV Xe24+ ions. 3D AFM picture (a) and a cross-section (b) of the pattern recorded on Se/As2S3 nanolayered film with total thickness d = 900 nm, illuminated through a microgrid with a 17 mW, 532 nm diode laser for 30 min. S. Kokenyesi et al. / Journal of Non-Crystalline Solids 353 (2007) 14701473. Cross Section of a Pixel of a X-ray Imaging Array S. A. Kasap, J.A. Rolands, IEE Proc. Circuits Devices Syst., 149 (2002) 85. Radiation sensor, BSU February 24, 2011 Solid electrolyte The solid state electrolyte material is formed by Ag diffusion in Ge Chalcogenide glasses (ChG). This electrolyte allows the movement of silver ions under the influence of an electric field. The electrically stimulated injection
and removal of metal in the solid electrolyte at very low voltage is the basis of Programmable Metallization Cell memory PMCm devices technology. Schematic of (a) PMCm device modeled as an electrochemical cell. a. Unwritten device. (b) Device during WRITE: A very small voltage rapidly injects into the electrolyte excess silver ions, which are reduced by the electron current to form highly stable silver dendrites within the electrolyte. (c) Written Device: The electrondeposition process stops when a conducting link is formed. Metallic link reduced the resistance of the structure by many orders of magnitude.(d) Device during ERASE: A very small reverse voltage (a few hundred mV) removes excess silver from the electrolyte. Excess silver is backed on the silver electrode by an easily reversible reaction.
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