Chemical Behavior of Energetic Deuterium lmplanted into Silicon Carbide

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1 J. Plsm Fusion Res. SERES, Vol.3 (2000) Chemicl Behvior of Energetic Deuterium lmplnted into Silicon Crbide GUCH Kzunri*, MORMOTO Ysutomi, SHMADA Asko, NUDUKA Nobuo, OKUNO Kenji. NAKAMURA Hirohumir nd NSH Mstkr Rdiochemicl Reserch Lbortory, Fculty of Science, Shizuok University, Shizuok , Jpn ltritium Engineering lnbortory, Jpn Atomic Energy Reserch nstitute, brki , Jpn (Received: l8 Jnury 2000 / Accepted: 14 April 2000) Abstrct Studies on chemicl behvior of energetic deuterium (D) ions implnted into SiC were crried out by mens of X-ry Photoelectron Spectroscopy (XPS) nd Therml Desorption Spectroscopy (TDS). From XPS results, it ws suggested tht the implnted D ws ttrcted strongly to Si nd/or C. From TDS results, two peks corresponding to D2 relese ppered ner 470 K (lst pek) nd 1080 K (2nd pek). The ctivtion energies for the two D2 relese processes were lso determined. t ws found from these experimentl results tht the lst pek seemed to be ttributed to deuterium tht existed in interstitil. The 2nd pek ws considered to be cused by deuterium trpped in lttice defects produced through deuterium ion implnttion. Keywords: XPS, TDS, deuterium, desorption, recycling, recovery 1. lntroduction Silicon crbide (SiC) is one of ttrctive cndidte mterils for plsm fcing components of next fusion rectors due to its high temperture strength, good therml conductivity, low induced rdioctivity, nd low plsm impurity contmintion compred with other cndidte mterils, such s high-z mterils l]. The plsm fcing components would be expected to be exposed to energetic deuterium nd tritium becuse those re fced directly to D-T plsm [2]. From the stndpoint of tritium sfety nd tritium recycling on the plsm surfce components, therefore, it is importnt to investigte chemicl behvior, mobility nd existing chemicl sttes of energetic tritium implnted into SiC. Prticulrly, mobility of implnted tritium in SiC is useful knowledge for estimtion of tritium inventory nd recovery s well s tritium recycling. n the present study, we investigted chemicl * C o rre sponding uthor' s e - mil : 15 ip c. s hizuo k. c.i p behvior of deuterium ions implnted into B-SiC crystls by men of X-ry Photoelectron Spectroscopy (XPS) nd Therml Desorption Spectroscopy (TDS). We ccumulted knowledge of chemicl existing sttes of implnted deuterium into SiC using XPS, furthermore, its therml desorption behvior ws investigted by TDS. 2. Experimentl Bet-SiC prepred by CVD method ws provided by TOYO TANSO CO., LTD., the dimeter nd the thickness were x l0-2 m0 nd x 10-3 m, respectively. 2.1 XPS mesurements As pretretment, the SiC surfce ws sputtered with 3 kev rgon (Ar*) ions. After tht, deuterium (D2*) ions were implnted into the SiC, nd XPS by The Jpn Society of Plsm Science nd Nucler Fusion Reserch 337

2 guchi K. et l., Chemicl Behvior of Energetic Deuterium mplnted into Silicon Crbide were performed. n XPS mesurements, chemicl shifts of C-ls, Si-2s nd Si-2p resulting from D2* ions implnttion were mesured. As prmeters, ion energies were kev nd 3 kev, nd ion fluences were vried from 0 to 4 x 102r m-2. From clcultion using the TRM code [3], ion rnge of deuterium is over severl ten nm, while XPS nlysis depth is nm [4]. So XPS spectr cnnot reflect the informtion round the ion rnge region. Therefore, the depth profile of XPS spectr should be crried out through 3 kev Ar* sputtering. 2.2 TDS mesurements The drwing of the pprtus is shown in Fig. l. Deuterium ions were implnted into the smple, nd then, the qurtz tube (QT) ws evcuted for 2 hours to remove residul gses. After tht, the smple ws heted using n infrred heter (RH). The temperture of the smple ws incresed by iso-rte mnner. The deuterium gs (D2) relesed from the SiC ws detected by qudrupole mss spectrometer (QMS). The mss clibrtion of the QMS ws performed using D, gs before mesurements. The dependence of ion fluence on TDS ws mesured in the rnge from x lo2t to 1.2 x 1022 m-2 of deuterium toms by 3 kev Dr* implnttion. The therml desorption ws crried out by iso-rte heting of 30 K min-'. The dependence of ion energy on TDS ws mesured in the rnge from 1 to 5 kev of ion energy by ion fluence of 1.2 x 1022 m-2. The therml desorption ws crried out by iso-rte heting of 30 K min-r. n the experiments for determining the ctivtion energies, heting rte of smples, where deuterium ws implnted with ion energy of 3 kev nd ion fluence of 8 x l02r m-2, ws vried from l0 to 50 K min-r. Shifts of the pek top temperture were obtined, nd ctivtion energies were determined using Redhed's Eqution [5]. 3. Results nd Discussion Figure 2 shows chemicl pek shifts of C-ls, Si-2s, nd Si-2p of SiC with D2* ion fluence incresing nd D2* ion energy of 1 kev. t cn be seen from the figure tht these peks shift to higher energy region. Binding energy of Si-2s, 2p shifted until ion fluence reched x l02r m-2, nd then they become constnt pproximtely. Besides, binding energy of C-ls lso shifted until ion fluence becme two times lrger (2 x l02l m-2) thn for Si-2s, 2p, nd then it becme lso chngeless E & c/ \.o oo :l o. Y o_o-x.a-^"")6 *'- ^"'th-o. - ', ' 'b--o QMS : Qudrupole mse spectroscopy TMP Turbo,moleculr pump RT i Rotry pump G oniztionvcuum guge D, Deuterium tnk TC i R-type thermocouple QT Qurtz tube RH nfrred heter Fig. 1 Drwing of experirnentl pprtus for TDS Deuterium ion fluence (tdt m'') Fig. 2 Chemicl pek shift of C-1s, Si-2s, nd Si-2p by implnttion of 1 kev D2* ions. 338

3 guchi K. et l., Chemicl Behvior of Energetic Deuterium mplnted into Silicon Crbide pproximtely. Figure 3 shows depth profile for chemicl pek shifts of C-ls, Si-2s, nd Si-2p by 3 kev Ar+ sputtering fter D2+ implnttion with ion fluence of 4 x l02r m-2 nd ion energy of kev. On the figure, shifts of C-1s, Si-2s, nd Si-2p becme the mximum ner the surfce, while they becme smller s the depth being deeper. By the wy, it hs been reported for SiC tht C would be sputtered more esily thn Si below ion energy of 100 ev [6]. The fct suggested for the SiC tht the chemicl composition of the surfce could vry by deuterium implnttion within the ion energy used in this study. To confirm the chemicl composition of the surfce, Si : C rtio with the ion implnttion ws obtined using ech photoelectron pek re before nd fter implnttion. t ws understood from experimentl results tht no selective sputtering for both elements ws observed in the present experimentl conditions, becuse the Si : C rtio hd no chnge. The fct suggested tht chemicl pek shift ttributed to vriety of the surfce chemicl composition could be ignorble. On the other hnd, implnted deuterium ions interct chemiclly with the other elements in SiC, or dsorb chemiclly on the surfce of SiC. These could cuse the chemicl chnge of constitutive elements. The chnge of chemicl sttes of constitutive elements in Fig. 2 would be considered to be cused through chemicl interction mong Si nd/or C, D. According to the literture vlue [7], Si-2p photoelectron peks of SiC nd SiH pper round 100-l0l ev nd ev, respectively. t is suggested tht the Si-2p pek shifts to higher energy region when Si of SiC bonds to H. Hence, s shown in Fig. 2, since the Si-2p pek shifted to higher energy region with implnted ions incresing, it ws suggested tht Si ttrcted D strongly in the SiC. Besides, for some crbide whose composition rtio is :, such szrc, TiC, VC, nd WC, binding energies of C-1s re28l.l ev, ev, nd ey,282.8 ev [7], respectively. n ddition, binding energy for CH is round 286 ev [7]. Similr to Si, when C bonds to D, C-1s pek is considered to shift to higher energy region. As shown in Fig. 2, since the C-ls pek shifted to higher energy region with implnted ions incresing, it ws suggested tht C ttrcted D strongly in the SiC. Here, in order to cler desorption behvior of deuterium from SiC, TDS mesurements were performed. As shown in Fig. 4, there re two peks tht ppered ner 47O K (lst pek) nd 1080 K (2nd pek). The fct suggested tht there would be two desorption process for implnted deuterium. The process for the 1st pek would be relted to recycling, nd could be recovered in low temperture. On the other hnd, the process for the 2nd pek would not be relted to the recycling, nd could not be recovered in low temperture. TDS spectr were nlyzed by Gussin distribution function, nd pek top temperture nd pek intensity were mesured from such spectr. The dependence of ion fluence on TDS spectr is shown in Fig. 5. The pek intensity ws growing with ion fluence incresing, while, there ws no shift of pek top temperture for both peks. qr q & o) : 3.ox 3 z.w = l.(h onfluerrce (rdn- -) Fig. 3 Depth profile for chemicl pek shifts of C-1s, Si- 2s. nd Si-2p by Ar- sputtering fter Dr* ions implnttions. Fig. 4 TDS spectrum fter deuterium implnttion with ion energy of 3 kev nd ion fluence of 8 x lgt m'2, nd heting rte of 30 K min-l. 339

4 guchi K. et l., Chemicl Behvior of Energetic Deuterium mplnted into Silicon Crbide o 2.0x on fluence : X l02r m-2....' 6X 102rm'2 h 2X l02r m' X 10'zr m'' X lozr m2 i il. i.z't i,/ \ i i! i j.'. i!!!.' '.i!!!'.ii it i.'--r ',1 t:.o '-t l. 7 \r t Temperture (K) Fig.5 The dependence of the ion fluence on TDS spectrum with ion energy of 3 kev nd heting rte of 30 K min-l x10 ' 3.0x10'' 1.8 t \ 1.4 g t z Pek 1 O Pek 2 ft /J Activtion energy : o.29 ey; Activtion energy : 0.61 t0.19 ev) t t E log To Fig.7 Pek top tempreture (4) for both peks ws plotted logrithmiclly ginst heting rcte Wl. 1.0x10 Temperture (K) Fig.6 The depndence ofthe ion energy on TDS spectr with ion fluence of 8 x 1021 m-2 nd heting rte of 30 K min-l. Figure 6 shows the dependence of ion energy on TDS spectr. With incresing ion energy, the intensity nd the pek top temperture for the lst pek vried little, while the intensity for the 2nd pek tended to increse, nd the pek top temperture for the 2nd pek shifted to higher temperture region s ion energy incresing The dependence ofheting rte on the TDS spectr ws investigted to determine ctivtion energies for the two peks. The experimentl results showed shift of ech pek top temperture by chnging heting rte from 10 to 50 K min-r. Here, in order to determine ctivtion energies of therml desorption process corresponded to two peks, Eq. (l) ws used, E / RTr+2 = d (log B) d (loet)151, (1) where, E is ctivtion energy, B heting rte, R gs constnt, nd Tp pek top temperture. As shown in Fig. 7, logb ws plotted s function of logzp, nd ctivtion energy ws determined from the grdient. The ctivtion energies of the lst pek nd the 2nd pek were l9 ev nd, O.29 ey, respectively. The ctivtion energy of the 2nd pek is comprble to the vlue of 2.87 ey reported for the tritium desorption from SiC exposed to tritium by R.A. Cusey, et l [8]. From TDS results, these two peks were interpreted s followings. For the lst pek, both relese mount nd relese temperture depended scrcely on ion energy, moreover, its relese mount ws incresing with ion fluence growth. Here, diffusion ctivtion energy for H in Si nd C re 0.48 ev [9] nd ev [10], respectively, nd ctivtion energy of ev for the 1st pek ws ner to these vlue. Consequently, it ws suggested tht the lst pek ws cused by interstitil deuterium. For the 2nd pek, the pek top shifted to higher temperture region with ion energy incresing s shown in Fig. 6. Thus, it ws considered tht deuterium ws trpped deeper plce with ion energy incresing, nd then it ws required longer time for the deuterium to diffuse from trp site to the surfce. The time lg cn be relted to desorption temperture shift towrd higher region, therefore the trp site of D for the 2nd pek ws considered to exist in bulk. n ddition, the ctivtion energy of ev ws reltive lrger thn tht of the lst pek. So the 2nd pek should not be due to 340

5 guchi K. et l., Chemicl Behvior of Energetic Deuterium mplnted into Silicon Crbide interstitil deuterium but due to tht trpped in lttice defects produced by deuterium implnttion. Therefore, the phenomen tht desorption mount incresed with ion energy increse would be ttributed to the fct tht ion energy incresing cused the increse of defects number, nd then, trpped deuterium mounts incresed. About the dependence on ion fluence, the increse of desorption mount for the 2nd pek seemed to be due to the fct tht ion fluence qrowth cused the increse of defects number. From literture, binding energy of C-ls nd Si-2p for compound constituted of only C or Si, nd H re 2.94 llll nd 2.50 ev [2], respectively. On the other hnd, binding energy of C-ls nd Si-2p for compound constituted of C or Si, H, nd other elements, re ev [3] nd ev [4], respectively. The ctivtion energy of ey for the 2nd pek is similr to the vlue of C-ls for compound constituted of C nd H. But, in view of tht SiC consists Si nd C, it seems tht the comprison with compounds constituted of Si or C, H, nd other elements is suitble, so existing sttes of deuterium in SiC my be Si-D. However, it is difficult to conclude the existing sttes t the moment. The investigtion of Si nd C s well s SiC re lso need for mking cler the existing sttes of deuterium in SiC. 4. Summry XPS experiments were performed fter energetic deuterium ws implnted into SiC. From XPS results, it ws suggested tht the implnted D ws ttrcted strongly to Si nd/or C. TDS experiments were lso performed fter energetic deuterium ws implnted into SiC, nd two peks ppered. The lst pek seemed to be ttributed to deuterium tht existed in interstitil. The 2nd pek ws considered to be cused by deuterium trpped in lttice defects produced through deuterium ion implnttion. References tll K. Hojou, S. Furuno er l., Nucl. nstr. nd Meth. in Phys. Res. B 91, 534 (1994). [2] S.W. Tm, J.P. Kopsz et l., J. Nucl. Mter. 219, 87 (l995). [3] J.F. Ziegler, J.P. Biersck et l., The stopping nd Rnge of ons in Solids (Pergmon Press. New York 1985). [4] The Surfce Science of Jpn, X-ry Photo Electron Spectroscopy, MARUZEN. t5l P.A. Redhed, Vcuum 12,203-2ll (1962). t6l H. Plnk et cl., Nucl. nstr. And Meth.8124,23-30 (1997). [7] Perkin-elemer Hndbook of X-ry photoelectron Spectroscopy Corportion Physicl electronics division. [8] R.A. Cusey et l.,j. Nucl. Mter. 203, (lee3). t9l R.Bisws, B.C. Pn et l.,phys. Rev. B57, 2253 (l998). tl0l G. Hnsli, J.P. Biberin et l.,j. Nucl. Mter. 171, 395 fl990). llll A.J. Robell et l., J. Phys. Chem. 6E, (1964\. tl2l M. Myers er /., Phys. Rev. B45, (1992). [13] R. Wlsh et l., Acc. Chem. Res. 14,246 (1981). [4] R.C. West, CRC-hndbook of Chemistry nd Physics. 34r

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