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EPR evidence of helium-oxygen-vacancy complexes in crystalline silicon
Volume 125, number 6,7
PHYSICS LETTERSA
23 November 1987
EPR EVIDENCE OF HELIUM-OXYGEN-VACANCY COMPLEXES IN C R Y S T A L L I N E S I L I C O N
Yu.V. GORELKINSKII, N.N. NEVINNY! and S.S. AJAZBAEV Institute olttigh Energ)' Physics. Academ)' (!/ &'ience~ ol lhe Kazakh S'A'R, 4800~2 :t/ma. Ira. ~ ~A'~R
Received 23 March 1987: revised manuscript received 28 July 1987; accepted for publication 22 September 1987 Communicated by J.l. Budnick
Two new (S= 1) EPR spectra, labeled Si-AA7and Si-AAS,arise from a helium-associateddeli:ct in crystalline silicon. Both are produced only in Czochralski-grown silicon by .helium ion implantation at ~ 20 C followed b? annealing at ~ 180:C and are stable to ~ 350°C. The effects of stress applied at high temperature indicate an atomic reorientation process which occurs with an activation energy of 1.05 + 0.05 cV for both centers. The Si-AA7and Si-AA8centers are identified as helium-related vacancytype defects containing oxygen atom(s ).
Recently, we reported [ l ] the discovery of two ( S = 1) EPR spectra, labeled Si-AA5 and Si-AA6, due to association of helium and vacancy-type defects in silicon. Both spectra are produced by helium ion implantation at room temperature and are stable up to 180°C. By i m p l a n t i n g the 3He isotope, the Si-AA6 3He hyperfine spectrum has been observed. These experiments have shown that the individual helium atoms interact actively with vacancy-type defects and their electronic structure is strongly changed by incorporation of helium atoms. It should be noted that at the present time h e l i u m - c o n t a i n i n g EPR centers arising from radioactive 13-decay of tritium incorporated into glycine a m i n o acid (Gly-T in H20 at 77 K) [2] also are known. In this Letter we present two new ( S = 1) EPR spectra, labeled Si-AA7 and Si-AA8, which are dominant in the h e l i u m - i m p l a n t e d layer of Czochralskigrown silicon after annealing of the sample at temperatures above ~ 180 ° C. We implanted ~ 20 MeV helium ( H e : ~ ) ions into high purity silicon ( p ~ 103 f~ cm) as well as into nand p-type ( p ~ 10 ~ cm) Czochralski-grown silicon using a cyclotron. I m p l a n t a t i o n s were performed at room temperature with fluences in the range 10 ~5_ 10 .6 He/cm 2. EPR studies were performed at Q-band and observed at 77 K in absorption. The AA7 and AA8 spectra start to grow at ~ 180°C 354
annealing and reach a m a x i m u m intensity at ~ 2 5 0 ° C . Both spectra disappear after 15 min annealing of samples at 350:C. We note that the annealing behaviour of the AA7 and AA8 spectra is vet'y similar to that of the zero-phonon line (1.012 eV) of photoluminescence in helium-implanted silicon [ 3 - 5 ] . The AA7 and AA8 spectra appear only at sample temperatures below ~ 200 K and satisfacto~ ~ conditions for observation are achieved at 77 K. Their production rate is z 0.05 centers/ion. We note, however, that the production rate of the AA7 is always larger than the AA8 center. These spectra are observed in n- and p-type Czochralski-grown silicon but are absent in high-purity silicon. The intensity of AA7 and AA8 is i n d e p e n d e n t of carbon concentration (in the range 10 '7 to 10 ~s C/cm3), suggesting that carbon is not incorporated in the structure of these defects. However, the fact that AA7 and AA8 centers formed only in Czochralski-grown silicon. which is known to contain oxygen and carbon as impurities, suggests that oxygen is involved in the structure of these defects. The study of the spatial distribution of defects indicates that the AA7 and AA8 centers are located exclusively at the end of the helium ion range (in the h e l i u m - i m p l a n t e d layer of 20 lam thickness). In Czochralski-grown silicon we also observed the previously known oxygen-dependent ( S = I ) Si-AI4 center [6] and Si-P1 (pentava-
Volume 125, number 6,7
PHYSICS LETTERS A
cancy) [ 7 ], which are d o m i n a n t (as well as AA7 and AA8) in the h e l i u m - i m p l a n t e d samples after annealing at ~ 250°C. The AA7 and AA8 spectra can be described as arising from an anisotropic defect which has six equivalent orientations in the cubic silicon lattice with the spin h a m i l t o n i a n
I#.0
15.5
23 November 1987
)f=#BH'g'S+S" D ' S + ~ - A j
"S,
J with effective spin S= 1, and T r D = 0 . The positions o f the fine structure lines are shown versus magnetic field orientation in fig. 1. F r o m this we d e t e r m i n e the principal values o f the g- and D-tensors, which are given in fig. 2 along with the principal axes for one o f the equivalent defect orientations. The s y m m e t r y o f the g- a n d D-tensors indicates that the AA7 and AA8 centers are defects having C2v symmetry. We note that the values o f the g- and D-tensors do not d e p e n d on the helium ion fluence. In these experiments we are able to resolve the 29Si (I= ½, 4.7% a b u n d a n t ) hyperfine (hf) structure o f the AA7 a n d AA8 spectra. The intensity ratio o f the h f line to the corresponding central line is ~ 5% indicating that two equivalent nuclear sites are involved in the interaction for both spectra. F r o m the angular dependence o f the h f satellites we deduce the principal values a n d axes o f A given in fig. 2. There
[ol] gl I DI
]3.0 oI-4 £-.I
g3,D3 ~ ~
0
/ ~ A,, \
111~]i \\
=
/
toll] ga,D-a
12.5 ~-tenso2
[fool I 0
I
I
I
I
i0
20
50
#0
I
I
50 60
i
I
70
80
[olq
AA7
90
~°(DEGREE) Fig. 1. Angular dependence of the Si-AA7 (solid lines) and SiAA8 (dashed lines) spectra with H in the (011 ) plane at 77 K, t,~= 37.21 GHz. The circles correspond to the experimental data, and the lines correspond to the calculated angular dependence using the g- and D-constants. Each fine structure line is labeled by the corresponding defect orientation in the lattice.
AA$
D,A-tensor(10-~cm -I)
(~o.ooo5)
(~Oo5)
g1=2o0096
O1-+576o2
(29S2)
tg2=2o0008
D2:T~06.5
AII=7Z.0
[g~=2.00~p
D3=7269.7
Ai=~6.~
DZ=~+~I0°~ D2=$27~.~
AII=70.O
'gi=2.009~ go=2.001~
I
Fig. 2. Spin hamiltonian constants and axes for one of the six equivalent defect orientations in the silicon lattice ("bc,cb" in
fig. l.). 355
Volume 125, number 6,7
PHYSICS LETTERS A
are two nonparallel (l 12 + 1°) h f axes ( , 4 ) approximately along { 111 ) directions in the { 110} plane. It should be noted that the D (D~) axis is perpendicular to the >Si h f a x e s ( A ) for the AA7 and AA8 (as well as AA5 and AA6 [ 1 ]) centers in contrast to all known spin-triplet centers in irradiated silicon [ 6 , 7 , 8 - 1 0 ] . An analysis, like that done previously on other defects in silicon [6,11], o f the >Si h f interaction indicates that each o f the two equivalent silicon a t o m accounts for ~ 2 8 % o f the total paramagnetic wavefunction in the AA7 and ~ 27% in the AA8 centers. On these a t o m s the wavefunction consists of ~ 19% 3S and ~ 81% 3P for both centers. The large localization ( ~ 55%) o f the p a r a m a g n e t i c wavefunction on the two silicon a t o m s suggests that the AA7 and AA8 centers are vacancy-type defects [6,11]. In these experiments resolved hf interaction was not observed for the 3He ( I = ~) isotope. However. we have found that the AA7 and AA8 spectra have slightly differing values o f D for samples i m p l a n t e d with 3He or 4He and have interpreted this as a slight shift in D due to the differing mass o f the helium isotope which is i n c o r p o r a t e d in the structure o f the centers. The data o f our m e a s u r e m e n t s at 77 K give 6 D / D ~ 4 × 10 3 per unit mass change o f the helium a t o m for the AA7 and AA8 centers. The nature o f these slight shifts in D has been explained by different vibrational frequencies o f isotopes in the solid [ 12,13 ]. In our case the isotope shift in D is a direct argument that helium a t o m s are i n c o r p o r a t e d in the structure o f the AA7 and AA8 centers. An analysis o f the g- and D-tensors indicates that the s p i n - o r b i t interaction cannot be a m a j o r origin for the fine structure o f the AA7 and AA8 spectra, because their D u are not p r o p o r t i o n a l to Ag,, [ 14]. The magnitude and anisotropy o f the observed g- and D-tensors and also the d a t a o f the 2uSi hf interaction suggest that the magnetic d i p l e - d i p o l e interaction between the two unpaired electrons in the AA7 and AA8 centers should dominate. We note that the principal D values o f the AA7 and AA8 centers are about two times as large as that o f the AA5 and AA6 centers [1] or SLI (excited state o f neutral one vac a n c y + o x y g e n center) [8,10]. Lee and Corbett [6] have previously calculated that for a distance corresponding to second nearest neighbours D~ values are ~ 0.05 cm-~ for the o - o r b i t a l and only ~ 0.02 356
23 November 1987
cm ~ for the n-orbital. In our case the D~ values are 0.0576cm ~and0.0510cm -~fortheAA7andAA8 centers, respectively, and correspond to the G-orbital at a distance between nearest neighbors better than other ones. Application of compressional uniaxial stress (2150 kg/cm -~) along the { l l 0 ) direction at T = 7 0 - 8 0 : C produces a preferential alignment o f the AA7 and AA8 centers which can be studied by cooling to room t e m p e r a t u r e with stress on. removing the stress, and m o n i t o r i n g the relative EPR intensities. We have observed that a quenched-in alignment for the AA7 and AA8 (as well as AA5 and AA6) centers has taken place favoring the defect whose D, ( g , ) axis (fig. 2) is parallel to the stress axis, i.e. the intensity o f the "bc,cb" line (fig. 1 ) is increased while the intensity o f other lines are decreased, contrary to the previous observation for the { l l 0 } - s y m m e t r i c spin-triplet centers in irradiated silicon [6], where the intensity of the "bc,cb" line is decreased. This fact has confirmed the unique direction o f the D axis relative to t h e A (-~Si) axis (D LA ) for the AA7 and AA8 as well as for the AA5 and AA6 centers. The energy o f a defect in an applied strain field can be written as E = ~ ,iB,,ej,, where ei, are strain tensor c o m p o n e n t s and Bi, are the c o m p o n e n t s o f a symmetric second-rank " p i e z o s p e c t r o s c o p i c " [ 15 ] tensor B. The analysis of the observed alignment was performed for the defect with C,, symmetry, as has been done previously by Lee and Corbett [ 6,16] for other defects in irradiated silicon. We have estimated the traceless c o m p o n e n t s of the elastic coupling tensor, B~,, B,_> B~ which represent the change in the defect energy due to the stress along the [011 ], [011] and [100] axes, respectively (fig. 2). Their value are given in table 1 along with other p a r a m eters. Watkins and Corbett [ I 1,17,18 ] have previously argued that the energy change in the pairbonding orbitals plays a m a j o r role in preferential alignment o f the defects. If we apply their results to the AA7 and AA8 centers (assuming B ~ = 0 ) . we should expect at least three parallel pair-bonding orbitals which must be parallel to the D~ (g~) axis. Recovery from the alignment was studied by a series of isothermal anneals. The results for AA5 - AA8 centers are shown in fig. 3 and given in table 1. The thermally activated reorientation for all o f the helium-containing centers ( A A 5 - A A 8 ) occurs at 8 0 -
Volume 125, number 6,7
PHYSICS LETTERS A
23 November 1987
Table 1 Parameters of the piezospectroscopic tensor under the < 110 > uniaxial compression in eV/(unit strain) and the values of activation energy for the reorientation process of the helium-containing centers. Spectrum
T,~
Si-AA5 Si-AA6 Si-AA7 Si-AA8
80 80 85 70
('C)
n ~I n
B,,
B,2
B33
E (eV)
% (s)
3.3 3.3 3.4 3.0
35.0 34.0 36.1 30.0
-16.5 -16.3 -17.4 -14.5
-18.5 -17.7 --18.7 -15.5
1.12 1.12 1.05 1.05
4.4X10 -'3 7.0X 10-~4 2.4X 10-~2 1.8X10 ~3
90°C. This t e m p e r a t u r e is lower than for all the previously known multivacancy defects in silicon which reorient in the t e m p e r a t u r e range 1 4 0 - 2 2 0 ° C
T°C i00
90
80
70
60
50
I
I
I
I
I
I
lo 5
/
T= %.
~t5
e (aE/kT)
kg7
/
/
/
/
/
/
#
,// /
i0~
lo3
lo2
/ AA6 /
," ,,/ ,,/
,,,)" / ' / / / l
2.7
,
i
2.8
i
J
2.9
i
t
5.0
,
k
3.1
I/T (IO-3K -I)
Fig. 3. Characteristic reorientation time z for the Si-AA5-Si-AA8 centers versus temperature.
[6,16,18,19]. We suggest, therefore, that the previously known multivacancy defects were not involved in the structure of the A A 5 - A A 8 centers. The fact that for all helium-containing centers ( A A 5 - A A 8 ) the activation energy required for the reorientation process is almost the same within error ( _+0.05 eV) suggests that the reorientation process is controlled by the binding energy between helium and an intrinsic vacancy-type defect. We presume that differences in the preexponential factors o f the AA7 and AA8 as well as AA5 and AA6 (see table 1 ) are due to different numbers o f helium atoms incorporated in the centers. F o r example, the AA5 and AA7 presumably contain one helium atom while the AA6 and AA8 are associations of vacancy-type defects and two heliums atoms. The large principal values o f the D-tensors suggest that the defects responsible for the AA7 and AA8 spectra m a y be one-vacancy-oxygen centers incorporating helium a t o m s ( s ) . However, in this case it is necessary to suppose that helium changes the electronic structure o f the one-vacancy-oxygen centers so that the A (29Si) hf axes (dangling b o n d axes [11,17]) are perpendicular to the D~ axis (dip o l e - d i p o l e interaction axis). Moreover, it is necessary to suppose that the energy o f the defect is changed by helium so that the c o m p o n e n t s o f the elastic tensor (for example B~,) are about three times as large as that for the one pair-bonding orbitals ( S i - O - S i ) in the vacancy-oxygen center [ 17 ]. There is another posible variant including a multivacancy, if the d i p o l e - d i p o l e interaction occurs in the < 1 10 > direction between unpaired electrons on two silicon atoms, each o f which resides in a different parallel {110} plane at a distance corresponding to second nearest neighbors. The partially dissociated divacancy o f H o r n s t r a [20,21] is a possible condidate for this variant o f helium-containing AA7 and AA8 357
Volume 125, number 6,7
PHYSICS LETTERS A
centers (for example with two oxygen atoms).
It is
clear, h o w e v e r , t h a t t h e o r e t i c a l a n d e x p e r i m e n t a l investigations of the nature of the interaction of hel i u m w i t h i n t r i n s i c d e f e c t s a r e r e q u i r e d in o r d e r t o establish exact models of these centers.
References [ l ] Yu.V. Gorelkinskii, N.N. Nevinnyi and S.S. Ajazbaev, Phys. Lett. A 110 (1985) 157. [2] V.A. Legasov. A.F. Usatyi, R.A. lbragimov and N.F. Myasoedov, J. Chem. Phys. 76 ( 1982 ) 91. [3] V,D. Tkachev, A.V. Mud~,i, and N.S. Minaev, Phys. Slat. Sol. (a) 81 (1984) 313. [4] L.N. Saphronov, in: Radiation effects in semiconductors, ed. L.S. Smirnov (Nauka, Novosibirsk, 1979) p. 101 [in Russian]. [5] N. Biirger, K. Thonke, R. Sauer and G. Pensl, Phys. Roy. Lelt. 52 (1984) 1645. [61Y.H. Lee and J.W. Corbett, Phys. Rev. B 13 (1976) 2653.
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23 November 1987
[ 7 ] W. Jung and G.S. Nevell, Ph:ys. Rev. 132 ( 1963 ) 648. [8] K.L. Brower, Phys. Rev. B4 (1971) 1968. [9] Y.H. Lee, Y.M. Kim and J.W. Corbett, Radial. Efl~ 15 (1972) 77. [10] K.L. Brower, Radial. Eff. 8 (1971) 213. [ I I ] G . D , Watkins and J.W. Corbctt, Phys. Rex,. 134 (1964) A1360. [I 2] S.A. Marshall, ,I.A. Hodges and R.A. Serway, Phys. Rex. 133 11964) A1427. [13] G.D. Watkins, Phys. Rcv. B 12 (1975) 4383. [14] M.H.L. Priyce. Proc. Phys. Soc. A 63 (1950) 25. [ 15 ] A.A. Kaplyanskii, Opl. Spektrosk. 16 ( 1964 ) 602. [ 16] Y.H. Lee and J.W. Corbetl, Phys. Rex'. B 9 (1974) 4351. [17]G.D. Watkins and J.W. (orbetl, Phys. Rcv. 121 (19611 1001. [18]G.D. Watkins and J.W. (orbett, Phys. Rcv. 138 (1965'~ A543. [19] Y.H. Lee, J.W. Colbell and K.L. Brower, Phys. Slat. Sol. (a)41 (1977)637. [20] J.W. Corbctt and ,I.C. Bourgoin, in: Point defects in solids. Vol. 2, eds. J.H. Crawford .It-. and L.M. Slifkin (Plenum. New York, 19751 p./. [21] J,W. Corbett. J.P. Karins and T.Y. Tan, Nucl. lnstrum. Methods 182/183 (1981)457.
PHYSICS LETTERSA
23 November 1987
EPR EVIDENCE OF HELIUM-OXYGEN-VACANCY COMPLEXES IN C R Y S T A L L I N E S I L I C O N
Yu.V. GORELKINSKII, N.N. NEVINNY! and S.S. AJAZBAEV Institute olttigh Energ)' Physics. Academ)' (!/ &'ience~ ol lhe Kazakh S'A'R, 4800~2 :t/ma. Ira. ~ ~A'~R
Received 23 March 1987: revised manuscript received 28 July 1987; accepted for publication 22 September 1987 Communicated by J.l. Budnick
Two new (S= 1) EPR spectra, labeled Si-AA7and Si-AAS,arise from a helium-associateddeli:ct in crystalline silicon. Both are produced only in Czochralski-grown silicon by .helium ion implantation at ~ 20 C followed b? annealing at ~ 180:C and are stable to ~ 350°C. The effects of stress applied at high temperature indicate an atomic reorientation process which occurs with an activation energy of 1.05 + 0.05 cV for both centers. The Si-AA7and Si-AA8centers are identified as helium-related vacancytype defects containing oxygen atom(s ).
Recently, we reported [ l ] the discovery of two ( S = 1) EPR spectra, labeled Si-AA5 and Si-AA6, due to association of helium and vacancy-type defects in silicon. Both spectra are produced by helium ion implantation at room temperature and are stable up to 180°C. By i m p l a n t i n g the 3He isotope, the Si-AA6 3He hyperfine spectrum has been observed. These experiments have shown that the individual helium atoms interact actively with vacancy-type defects and their electronic structure is strongly changed by incorporation of helium atoms. It should be noted that at the present time h e l i u m - c o n t a i n i n g EPR centers arising from radioactive 13-decay of tritium incorporated into glycine a m i n o acid (Gly-T in H20 at 77 K) [2] also are known. In this Letter we present two new ( S = 1) EPR spectra, labeled Si-AA7 and Si-AA8, which are dominant in the h e l i u m - i m p l a n t e d layer of Czochralskigrown silicon after annealing of the sample at temperatures above ~ 180 ° C. We implanted ~ 20 MeV helium ( H e : ~ ) ions into high purity silicon ( p ~ 103 f~ cm) as well as into nand p-type ( p ~ 10 ~ cm) Czochralski-grown silicon using a cyclotron. I m p l a n t a t i o n s were performed at room temperature with fluences in the range 10 ~5_ 10 .6 He/cm 2. EPR studies were performed at Q-band and observed at 77 K in absorption. The AA7 and AA8 spectra start to grow at ~ 180°C 354
annealing and reach a m a x i m u m intensity at ~ 2 5 0 ° C . Both spectra disappear after 15 min annealing of samples at 350:C. We note that the annealing behaviour of the AA7 and AA8 spectra is vet'y similar to that of the zero-phonon line (1.012 eV) of photoluminescence in helium-implanted silicon [ 3 - 5 ] . The AA7 and AA8 spectra appear only at sample temperatures below ~ 200 K and satisfacto~ ~ conditions for observation are achieved at 77 K. Their production rate is z 0.05 centers/ion. We note, however, that the production rate of the AA7 is always larger than the AA8 center. These spectra are observed in n- and p-type Czochralski-grown silicon but are absent in high-purity silicon. The intensity of AA7 and AA8 is i n d e p e n d e n t of carbon concentration (in the range 10 '7 to 10 ~s C/cm3), suggesting that carbon is not incorporated in the structure of these defects. However, the fact that AA7 and AA8 centers formed only in Czochralski-grown silicon. which is known to contain oxygen and carbon as impurities, suggests that oxygen is involved in the structure of these defects. The study of the spatial distribution of defects indicates that the AA7 and AA8 centers are located exclusively at the end of the helium ion range (in the h e l i u m - i m p l a n t e d layer of 20 lam thickness). In Czochralski-grown silicon we also observed the previously known oxygen-dependent ( S = I ) Si-AI4 center [6] and Si-P1 (pentava-
Volume 125, number 6,7
PHYSICS LETTERS A
cancy) [ 7 ], which are d o m i n a n t (as well as AA7 and AA8) in the h e l i u m - i m p l a n t e d samples after annealing at ~ 250°C. The AA7 and AA8 spectra can be described as arising from an anisotropic defect which has six equivalent orientations in the cubic silicon lattice with the spin h a m i l t o n i a n
I#.0
15.5
23 November 1987
)f=#BH'g'S+S" D ' S + ~ - A j
"S,
J with effective spin S= 1, and T r D = 0 . The positions o f the fine structure lines are shown versus magnetic field orientation in fig. 1. F r o m this we d e t e r m i n e the principal values o f the g- and D-tensors, which are given in fig. 2 along with the principal axes for one o f the equivalent defect orientations. The s y m m e t r y o f the g- a n d D-tensors indicates that the AA7 and AA8 centers are defects having C2v symmetry. We note that the values o f the g- and D-tensors do not d e p e n d on the helium ion fluence. In these experiments we are able to resolve the 29Si (I= ½, 4.7% a b u n d a n t ) hyperfine (hf) structure o f the AA7 a n d AA8 spectra. The intensity ratio o f the h f line to the corresponding central line is ~ 5% indicating that two equivalent nuclear sites are involved in the interaction for both spectra. F r o m the angular dependence o f the h f satellites we deduce the principal values a n d axes o f A given in fig. 2. There
[ol] gl I DI
]3.0 oI-4 £-.I
g3,D3 ~ ~
0
/ ~ A,, \
111~]i \\
=
/
toll] ga,D-a
12.5 ~-tenso2
[fool I 0
I
I
I
I
i0
20
50
#0
I
I
50 60
i
I
70
80
[olq
AA7
90
~°(DEGREE) Fig. 1. Angular dependence of the Si-AA7 (solid lines) and SiAA8 (dashed lines) spectra with H in the (011 ) plane at 77 K, t,~= 37.21 GHz. The circles correspond to the experimental data, and the lines correspond to the calculated angular dependence using the g- and D-constants. Each fine structure line is labeled by the corresponding defect orientation in the lattice.
AA$
D,A-tensor(10-~cm -I)
(~o.ooo5)
(~Oo5)
g1=2o0096
O1-+576o2
(29S2)
tg2=2o0008
D2:T~06.5
AII=7Z.0
[g~=2.00~p
D3=7269.7
Ai=~6.~
DZ=~+~I0°~ D2=$27~.~
AII=70.O
'gi=2.009~ go=2.001~
I
Fig. 2. Spin hamiltonian constants and axes for one of the six equivalent defect orientations in the silicon lattice ("bc,cb" in
fig. l.). 355
Volume 125, number 6,7
PHYSICS LETTERS A
are two nonparallel (l 12 + 1°) h f axes ( , 4 ) approximately along { 111 ) directions in the { 110} plane. It should be noted that the D (D~) axis is perpendicular to the >Si h f a x e s ( A ) for the AA7 and AA8 (as well as AA5 and AA6 [ 1 ]) centers in contrast to all known spin-triplet centers in irradiated silicon [ 6 , 7 , 8 - 1 0 ] . An analysis, like that done previously on other defects in silicon [6,11], o f the >Si h f interaction indicates that each o f the two equivalent silicon a t o m accounts for ~ 2 8 % o f the total paramagnetic wavefunction in the AA7 and ~ 27% in the AA8 centers. On these a t o m s the wavefunction consists of ~ 19% 3S and ~ 81% 3P for both centers. The large localization ( ~ 55%) o f the p a r a m a g n e t i c wavefunction on the two silicon a t o m s suggests that the AA7 and AA8 centers are vacancy-type defects [6,11]. In these experiments resolved hf interaction was not observed for the 3He ( I = ~) isotope. However. we have found that the AA7 and AA8 spectra have slightly differing values o f D for samples i m p l a n t e d with 3He or 4He and have interpreted this as a slight shift in D due to the differing mass o f the helium isotope which is i n c o r p o r a t e d in the structure o f the centers. The data o f our m e a s u r e m e n t s at 77 K give 6 D / D ~ 4 × 10 3 per unit mass change o f the helium a t o m for the AA7 and AA8 centers. The nature o f these slight shifts in D has been explained by different vibrational frequencies o f isotopes in the solid [ 12,13 ]. In our case the isotope shift in D is a direct argument that helium a t o m s are i n c o r p o r a t e d in the structure o f the AA7 and AA8 centers. An analysis o f the g- and D-tensors indicates that the s p i n - o r b i t interaction cannot be a m a j o r origin for the fine structure o f the AA7 and AA8 spectra, because their D u are not p r o p o r t i o n a l to Ag,, [ 14]. The magnitude and anisotropy o f the observed g- and D-tensors and also the d a t a o f the 2uSi hf interaction suggest that the magnetic d i p l e - d i p o l e interaction between the two unpaired electrons in the AA7 and AA8 centers should dominate. We note that the principal D values o f the AA7 and AA8 centers are about two times as large as that o f the AA5 and AA6 centers [1] or SLI (excited state o f neutral one vac a n c y + o x y g e n center) [8,10]. Lee and Corbett [6] have previously calculated that for a distance corresponding to second nearest neighbours D~ values are ~ 0.05 cm-~ for the o - o r b i t a l and only ~ 0.02 356
23 November 1987
cm ~ for the n-orbital. In our case the D~ values are 0.0576cm ~and0.0510cm -~fortheAA7andAA8 centers, respectively, and correspond to the G-orbital at a distance between nearest neighbors better than other ones. Application of compressional uniaxial stress (2150 kg/cm -~) along the { l l 0 ) direction at T = 7 0 - 8 0 : C produces a preferential alignment o f the AA7 and AA8 centers which can be studied by cooling to room t e m p e r a t u r e with stress on. removing the stress, and m o n i t o r i n g the relative EPR intensities. We have observed that a quenched-in alignment for the AA7 and AA8 (as well as AA5 and AA6) centers has taken place favoring the defect whose D, ( g , ) axis (fig. 2) is parallel to the stress axis, i.e. the intensity o f the "bc,cb" line (fig. 1 ) is increased while the intensity o f other lines are decreased, contrary to the previous observation for the { l l 0 } - s y m m e t r i c spin-triplet centers in irradiated silicon [6], where the intensity of the "bc,cb" line is decreased. This fact has confirmed the unique direction o f the D axis relative to t h e A (-~Si) axis (D LA ) for the AA7 and AA8 as well as for the AA5 and AA6 centers. The energy o f a defect in an applied strain field can be written as E = ~ ,iB,,ej,, where ei, are strain tensor c o m p o n e n t s and Bi, are the c o m p o n e n t s o f a symmetric second-rank " p i e z o s p e c t r o s c o p i c " [ 15 ] tensor B. The analysis of the observed alignment was performed for the defect with C,, symmetry, as has been done previously by Lee and Corbett [ 6,16] for other defects in irradiated silicon. We have estimated the traceless c o m p o n e n t s of the elastic coupling tensor, B~,, B,_> B~ which represent the change in the defect energy due to the stress along the [011 ], [011] and [100] axes, respectively (fig. 2). Their value are given in table 1 along with other p a r a m eters. Watkins and Corbett [ I 1,17,18 ] have previously argued that the energy change in the pairbonding orbitals plays a m a j o r role in preferential alignment o f the defects. If we apply their results to the AA7 and AA8 centers (assuming B ~ = 0 ) . we should expect at least three parallel pair-bonding orbitals which must be parallel to the D~ (g~) axis. Recovery from the alignment was studied by a series of isothermal anneals. The results for AA5 - AA8 centers are shown in fig. 3 and given in table 1. The thermally activated reorientation for all o f the helium-containing centers ( A A 5 - A A 8 ) occurs at 8 0 -
Volume 125, number 6,7
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23 November 1987
Table 1 Parameters of the piezospectroscopic tensor under the < 110 > uniaxial compression in eV/(unit strain) and the values of activation energy for the reorientation process of the helium-containing centers. Spectrum
T,~
Si-AA5 Si-AA6 Si-AA7 Si-AA8
80 80 85 70
('C)
n ~I n
B,,
B,2
B33
E (eV)
% (s)
3.3 3.3 3.4 3.0
35.0 34.0 36.1 30.0
-16.5 -16.3 -17.4 -14.5
-18.5 -17.7 --18.7 -15.5
1.12 1.12 1.05 1.05
4.4X10 -'3 7.0X 10-~4 2.4X 10-~2 1.8X10 ~3
90°C. This t e m p e r a t u r e is lower than for all the previously known multivacancy defects in silicon which reorient in the t e m p e r a t u r e range 1 4 0 - 2 2 0 ° C
T°C i00
90
80
70
60
50
I
I
I
I
I
I
lo 5
/
T= %.
~t5
e (aE/kT)
kg7
/
/
/
/
/
/
#
,// /
i0~
lo3
lo2
/ AA6 /
," ,,/ ,,/
,,,)" / ' / / / l
2.7
,
i
2.8
i
J
2.9
i
t
5.0
,
k
3.1
I/T (IO-3K -I)
Fig. 3. Characteristic reorientation time z for the Si-AA5-Si-AA8 centers versus temperature.
[6,16,18,19]. We suggest, therefore, that the previously known multivacancy defects were not involved in the structure of the A A 5 - A A 8 centers. The fact that for all helium-containing centers ( A A 5 - A A 8 ) the activation energy required for the reorientation process is almost the same within error ( _+0.05 eV) suggests that the reorientation process is controlled by the binding energy between helium and an intrinsic vacancy-type defect. We presume that differences in the preexponential factors o f the AA7 and AA8 as well as AA5 and AA6 (see table 1 ) are due to different numbers o f helium atoms incorporated in the centers. F o r example, the AA5 and AA7 presumably contain one helium atom while the AA6 and AA8 are associations of vacancy-type defects and two heliums atoms. The large principal values o f the D-tensors suggest that the defects responsible for the AA7 and AA8 spectra m a y be one-vacancy-oxygen centers incorporating helium a t o m s ( s ) . However, in this case it is necessary to suppose that helium changes the electronic structure o f the one-vacancy-oxygen centers so that the A (29Si) hf axes (dangling b o n d axes [11,17]) are perpendicular to the D~ axis (dip o l e - d i p o l e interaction axis). Moreover, it is necessary to suppose that the energy o f the defect is changed by helium so that the c o m p o n e n t s o f the elastic tensor (for example B~,) are about three times as large as that for the one pair-bonding orbitals ( S i - O - S i ) in the vacancy-oxygen center [ 17 ]. There is another posible variant including a multivacancy, if the d i p o l e - d i p o l e interaction occurs in the < 1 10 > direction between unpaired electrons on two silicon atoms, each o f which resides in a different parallel {110} plane at a distance corresponding to second nearest neighbors. The partially dissociated divacancy o f H o r n s t r a [20,21] is a possible condidate for this variant o f helium-containing AA7 and AA8 357
Volume 125, number 6,7
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centers (for example with two oxygen atoms).
It is
clear, h o w e v e r , t h a t t h e o r e t i c a l a n d e x p e r i m e n t a l investigations of the nature of the interaction of hel i u m w i t h i n t r i n s i c d e f e c t s a r e r e q u i r e d in o r d e r t o establish exact models of these centers.
References [ l ] Yu.V. Gorelkinskii, N.N. Nevinnyi and S.S. Ajazbaev, Phys. Lett. A 110 (1985) 157. [2] V.A. Legasov. A.F. Usatyi, R.A. lbragimov and N.F. Myasoedov, J. Chem. Phys. 76 ( 1982 ) 91. [3] V,D. Tkachev, A.V. Mud~,i, and N.S. Minaev, Phys. Slat. Sol. (a) 81 (1984) 313. [4] L.N. Saphronov, in: Radiation effects in semiconductors, ed. L.S. Smirnov (Nauka, Novosibirsk, 1979) p. 101 [in Russian]. [5] N. Biirger, K. Thonke, R. Sauer and G. Pensl, Phys. Roy. Lelt. 52 (1984) 1645. [61Y.H. Lee and J.W. Corbett, Phys. Rev. B 13 (1976) 2653.
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[ 7 ] W. Jung and G.S. Nevell, Ph:ys. Rev. 132 ( 1963 ) 648. [8] K.L. Brower, Phys. Rev. B4 (1971) 1968. [9] Y.H. Lee, Y.M. Kim and J.W. Corbett, Radial. Efl~ 15 (1972) 77. [10] K.L. Brower, Radial. Eff. 8 (1971) 213. [ I I ] G . D , Watkins and J.W. Corbctt, Phys. Rex,. 134 (1964) A1360. [I 2] S.A. Marshall, ,I.A. Hodges and R.A. Serway, Phys. Rex. 133 11964) A1427. [13] G.D. Watkins, Phys. Rcv. B 12 (1975) 4383. [14] M.H.L. Priyce. Proc. Phys. Soc. A 63 (1950) 25. [ 15 ] A.A. Kaplyanskii, Opl. Spektrosk. 16 ( 1964 ) 602. [ 16] Y.H. Lee and J.W. Corbetl, Phys. Rex'. B 9 (1974) 4351. [17]G.D. Watkins and J.W. (orbetl, Phys. Rcv. 121 (19611 1001. [18]G.D. Watkins and J.W. (orbett, Phys. Rcv. 138 (1965'~ A543. [19] Y.H. Lee, J.W. Colbell and K.L. Brower, Phys. Slat. Sol. (a)41 (1977)637. [20] J.W. Corbctt and ,I.C. Bourgoin, in: Point defects in solids. Vol. 2, eds. J.H. Crawford .It-. and L.M. Slifkin (Plenum. New York, 19751 p./. [21] J,W. Corbett. J.P. Karins and T.Y. Tan, Nucl. lnstrum. Methods 182/183 (1981)457.