energy electrons are capable to cause both atomic electron shells ionisation and "atomic shiftu defects formation. As calculation show practically each atom is ...
JOURNAL DE PHYSIQUE page C4-915
CoZZoque C4, suppllment a u nO1O, Tome 42, o c t o b r e 1981
RADIATION-INDUCED DEFECTS FORMATION IN CHALCOGENIDE GLASSES Sh.Sh. Sarsembinov, E.E. Abdulgafarov and K. Tolkanchinov S.M.
Kirov Kaaakh S t a t e U n i v e r s i t y , 480091, Alma-Ata,
U . R.S. S.
Abstract.ilign-energy (2;,ev) electron irradiation may produce excess density of valence-alternation pairs (VAY1s)in chalcogenide glasses. Relativistic electron energy losses and nonequilibrium density of radiation-induced "atomic shiftudefects in vitreous As S and As2Seg are calculated. Zadiation-induced defects formatgo2 scheme is proposed and compared with positron annihilation data. It is shown that radiation-induced defects may cause essential changes in electrical and optical properties of chalcogenide glasses.
Zelativistic electron (C. 5 < E < lO?.Tev)irradiation lead to effective structural changes in chalcogenide glasses, becouse highenergy electrons are capable to cause both atomic electron shells ionisation and "atomic shiftu defects formation. As calculation show electron irra practically each atom is ionised during 2 :Lev diation in vitreous As S and As2Se3. Znergy losses in these materi2 3 als amount to -dE/dx= 0.35.,~ev/nlrn and 0.49A;-ev/m,respectively. Obviously, deep shell electrons ionisation may not cause irradiated glass matrix structural cha~des.tiowever,high-energy electrons ionise only atonric valeme electrons to the conduction band after energy losses in some caskade -1rocesses. In this case chemical bonds are broken, binding peculiarities are affected and new structural defects are formed. Ve suppose that ioiiising effect of electron irradiation results in excess density of the existing charge defects C ; and C; according to the scheme: 2c; 2(c;)* 2c; c; + c;
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Electronic configurations for that reaction represented in the fol10~~ving figure (only for p-electrons ) :
...etastable states of cheloogen atom (C;)* exist only during the electronic irradietion. After the irradiation one of lone-pzir electrons snould better pass into the bonciinr state fonin; a C; defect center
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814199
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JOURNAL DE PHYSIQUE
which transforms into charged defects according to the well known valence-alternation pairs forming reaction( 1 ). Relativistic electron irradiation of chalcogenide glasses produces some "atoinic shiftu defects density too. 2 Mev electrons dissipation differential cross sections ( d ), and "atomic shift" defect densities (iV) for the glass network elements (As2S3 and As2Se3) in table 1 are given. Bere atomic shift energy threshold is supposed to be Ed-25ev, and = 6 DI 0 (according to the(2)), where Y- coefficient with the caskade process taken into account, I=1017 No= 3x 1 0 ~ ~ 8 6EA~ . - the maximu energy, which is transferred from the bombardment electrons to atomic nuclei It should be noted that N Table 1. Element
EA
ev 6 D,sme2 3 ,srn-)
S
402
2.
10-20 4.9
Se
163
1.1
2.5
AS
172
3*7
2*7 lo*'
is the non-equilibrium defects density in the sample under irradiation.After the irradiation turned off the radiation-induced defact concentration decreases to a certain stationary value
1020
lo2'
due to the some relaxation processes and self-recombination.For example, positron annihilation data showed that C; defects equilibrium at 300K after 2 MBV electrons irradiation density is 1.3*10~~srn-~ with dose of 1017* ' m s (before irradiation 0.8 10~~srn-~)(3). Thus it may be supposed that high-energy electrons irradiation of chalcogenide glasses gives rise to the excess valence-alternation pairs defects density, essentially affecting their electrica1,photoelectrical and optical properties. Por instence, we found that conductivity and photoconductivity values of some chalcogenide glasses increase by several orders of magnitude due to a peculiar electron-enhanced electrode matter diffusion to the glass matrix ( 4 ) . We think, that it takes place as a result of specific nature of interaction between additive elements and radiation-induced VAPts defects. So, the following situation may arise from the electron-enhanced diffusion of the Periodic Table I group elements(Cu,Ag,Au - Me) to the glass bulk: (a) additive atoms may form two normal covalent bonds with the glass network elements becouse one d-electron passes into the p-shel1,lieing at a short energy distance from the d-shell (for Cu from the 3d to 4p-shell) (dp-hibridization): c0 dp:vle20 + 2~: B
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-
-
'a.
-.,
where the subscript denotes the element valency, and the superscript denotes additive toms charge states; (b) the large probability exist that empty orbitals of additive atoms ;nay be occupied by C; centers lone-pair electrons. So the following reaction takes place: ~~isie;+ C; 2Mei + C; This reaction is the source cf excess holes, and all d-orbitals are filled. Similarly, the inte-action of additive elements of other groups with defect centers may be described (in accord'ance with(5)): I?e0 + 2 ~ ; ~e:- + 2 ~ ' and 2,ai; + 3 ~ ; 2~iz-+ 3 ~ ; , d 4 3 where ~ e i -and 3iz- -"inert ionsbb, not forming chemical bonds and not displaying electrical activity in the glass matrix. At large additives densities the C; centers conversion to the C+ centers gives rise to extrinsic p-typo corld-uctivityirrespective 3 of the additive elements origin (the I or 5 group of the Periodic Table or transition elements). The latter is confirmed by thermoemf sign measurements. iie suppose that specific nature of effective interaction between additives elements and charged defect centers result from high electropositive properties of C; centers (becouse of two lone-pair electrons), whose density in the irradiated glass matrix increases considerably. Tie radiation-induced C; centers conversion to C+ centers for 3 the I11 group elements (In,Ga,Al) is hindered, becouse empty orbitals are absent. So electren-enhanced modification of chalcogenide glasoes plectric properties in that case is not realized ( 5 ) . The radiation-induced defects formation caused changes in chalcogenide glasses optical properties. Por vitreous As,S and r 3 AspSe3 the exponential plot ( & =lo-103 sm-I ) of optical absorption edge before and after the electron irradiation in figure 1 are given. 17sm'2) the exponential chaAfter 2 Idev electronic hombardmont (1~10 racter of absorption is unchanged and its edge shift to the long wavelenth range is observed. The absorption edge slope (S) decreases from 22ev-I to 1 gev-I - for As Se and from l8ev-I to 15ev-I - for 2 3 As2S3. The high-energy electrons irradiation influence on the electroabsorption spectral dependences are illustrated by figure 2. Electroabsorption signals decrease as a result of irradiation: 1.4 in As2S3 2 times (bW =2.0ev) times (near h@=I ,5ev) in 2 decreases can be asIt is concluded that thettS"and " b & / & F cribed to growth of intrinsic field value (Y2) in the equation with(6)). which is due to 4&/&r2= ~ (- b ~ ~ ) e ~ (in / 3 accordance ~ ~ the fact that irradiation produces excess C; defect centers density.
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Fi 1 : Spectral dependences of a a l abeorption in vitreous As S, and As ,So, before(1) and after(2) eleCtrbn irradiation.
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: Electroabsorption spectral dependences in vitreous As,S, and As,Se, before(1) and after121 electron irradiation.
Fig.2
References
D. ,Fritzsche ii, Phyrr. Rev.Lett. 37 (1 976) 1504. 2 Kelly B.T. Irradiation damage to Solids, Pergamon Press ,it. Y. ,1965. 3 Sarsembinov Sh. Sh. ,Abdulgafarov E.E. , Vestnik Ad KazSSR,2 (1 980)48. 4 Sareembinov Sh. Sh. ,Abdulgafarov E. E. ,Tmanov I&. A. ,Rogachev N. A. Journal of Non-Crystalline Solids, 35-36 (1980) 877. 5 Sarsembinov Sh. Sh. ,Abdul&afarov E.E. ,Vestnik AN KazSSR, 5 (1 981 )51. 6 Esser B. Phys.Status Solidi, 51 (1972) 735. 1 Kaetner PB. ,Adler
