investigation of weakly damaged 〈110〉, 〈111〉 and 〈100〉 silicon by means of temperature...

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Nuclear Instruments and Methods in Physics Research B 118 (I 996) 113- 118

RIOMB Beam Interactions

with Materials 8 Atoms

Investigation of weakly damaged ( 110)) ( 111) and ( 100) silicon by means of temperature dependent dechanneling measurements

B. Weber *, E. Wendler, K. Girtner, D.M. Stock, W. Wesch

Institut fir Festkiirperphysik, Friedrich-Schiller-Uniuersitiit Jena. Mar-Wien-Pluto 1. D-07743 Jmu. Grrmuny

Abstract

Weakly damaged ( 100). ( 110) and (111) Si layers produced by multiple implantation of B + ions at room temperature

with energies between 50 and 300 keV at various fluences are investigated by means of temperature dependent RBS channeling measurements. Using the computer code DICADA the relative concentration of displaced atoms and their mean displacement distance perpendicular to the low index atomic rows are determined. The results are different for the three low

index directions considered. By MD simulations the lattice distortion of the Si crystal due to a divacancy or a (110) split

interstitial is calculated. The spectrum of the displacement distances is used to get information about the kind of the point defects in the weakly damaged Si.

1. Introduction

Since about 25 years Rutherford backscattering (RBS) of light energetic ions in combination with channeling has

been used for defect studies in crystals, and different models have been developed to determine the kind and the concentration of the defects [l-6]. Crystal defects increase the relative Rutherford backscattering minimum yield xmin which is caused by direct backscattering and dechanneling of the ions interacting with the displaced atoms. The dechanneling mechanisms are different for different kinds of defects which are characterized by different correlations between the displacements of the lattice atoms.

The weakly damaged crystals investigated previously

[7,8] and to be considered in this paper are characterized by a small increase of the minimum yield without a

pronounced direct backscattering contribution. Investiga- tions of the energy dependence of the minimum yield [ 111 and TEM micrographs showed that point defects and small point defect clusters are the dominating defects in these crystals [7]. Therefore, with respect to dechanneling the defects in the weakly damaged crystals can be described as uncorrelatedly displaced lattice atoms [9] characterized by

their displacement distance ra perpendicular to the channeling direction and their relative concentration ndat. For the determination of the two quantities the knowledge of

* Corresponding author. Tel. + 49 3641 6358 14, fax + 49 3641 In order to get more information about the defects

635854, e-mail [email protected]. produced by B+ implantation in Si, RBS investigations of

xmi, as a function of depth is not sufficient. Theoretical considerations of Matsunami et al. [5] and Gartner et al. [6] showed that the temperature dependence of dechanneling is strongly affected by the value of the displacement

distance ra. Thus RBS channeling measurements performed at different temperatures allow to determine displacement distances and the depth distribution of the dis-

placed atoms [8-l 11. Temperature dependent measurements on He+ implanted silicon were first done by Howe et al. [12] showing a stronger temperature dependence of xmin for ( 110) Si than for ( 111) Si. Subsequent analysis of these data by Glrtner et al. [6] yielded a mean displacement distance of 0.5 A perpendicular to the (111) atomic row. For the (110) direction larger displacement distances dominate. These values of ra could be understood by assuming (110) split self interstitials experimentally found

by GGtz and Sommer [ 131 for Si implanted with 150 keV B+ at T, = 77 K. Glaser et al. [14,15] found in (111) Si implanted with 150 keV B+ ions at room temperature that also for the highest dose used (1 ,X lOI cm- *) small displacement distances of about 0.3 A are dominant, which could be attributed to divacancies and higher vacancy

complexes by EPR measurements [15]. Recent investigations of weakly damaged B+ implanted (111) Si [8]

showed that the temperature dependence of the minimum yield does not change with the ion dose yielding the same displacement distance of 0.4-0.5 A for all implantation doses. By optical transmission measurements the existence of divacancies in these layers was proved.

0168-583X/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved SsO/Ol68-583X(95)OlJ98-6 11. RBS, ETC.

114 B. Weber et al./Nucl. Instr. and Merh. in Phys. Res. B 118 (1996) 113-118

(loo), (110) and (111) Si crystals implanted under iden- tical conditions, were performed at room temperature and 100 K. The data were analyzed using the computer code DICADA [6], providing the concentration of the displaced atoms and the displacement distances. Molecular dynamics simulations using the computer code developed by Stock [ 16,171 were performed to calculate the lattice distortion caused by divacancies and by ( 110) split interstitials in Si, the most probable isolated point defects in Si.

2. Methods

(lOO), (110) and (111) Si samples were simultane- ously implanted with B+ ions at room temperature 7” off axis. In order to produce thick weakly damaged layers ions were subsequently implanted with energies of 300, 150, and 50 keV at a constant current density of 0.5 pA cmm2. For the highest energy ion fluences of 3 X 1014, 1 X 10i5, 5 X 10” and 1 X lOI6 cme2 were used. At the lower energies the dose was reduced to 50% (150 keV) and 46% (50 keV), respectively.

The samples were analyzed by RBS-channeling measurements at 300 and 100 K using 1.4 MeV He+ ions and a backscattering angle of 177”. To exclude the influence of the damage production by the analyzing ion beam itself, the charge dependence of the backscattering yield was investigated and a linear extrapolation to zero-charge was performed.

The evaluation of the measured backscattering spectra is done in two steps. First, the difference in minimum yield

A Xti& z, Ti,,) = Xmi”( z* Ti,,) - Xmin,peti ( z, Ti,,) ( Xmin = Ya*/Yra, ~ti”,~ti = Y,r,&Yra with Ytil CL,, - backscattering yield in aligned direction for the implanted and the perfect crystal; Y, - backscattering yield in ran- dom direction) for T,,, = 100 K was used to calculate the relative concentrations of displaced atoms ndat( z) for a set of different values of ra with the computer code DICADA developed by Grtner et al. [6]. In the second step, A Xti,,( ~,Lr,,~s~) was calculated for This,, = 300 K using the set of (r,, ndat(z)). A comparison with the measured A X,J z,Thigh) provides ra and ndat( z>. The resulting value of ra has to be understood as a mean value of the actually existing distribution of displacement distances. This method can be used for displacement distances up to about 0.7 A because the temperature dependence of the dechanneling is sensitive to the value of ra in this region [6]. For larger displacement distances ndat(z) can be calculated using DICADA if the value of r, is known. In this case the displaced atoms are near the middle of the channel and therefore the azimuthal direction of the displacements may be important. However, large displacement distances are considered here only in connection with clusters of differently displaced atoms. In this case the influence of the azimuthal direction of the different displacements is con- siderably reduced (averaging). For simplicity it is not taken

into account in these investigations. Additional thermal vibrations of the displaced atoms are not considered here because the amplitude of the thermal vibration of the displaced atoms is not known. Their influence is largest for small values of r, [6]. Assuming the same vibrational amplitude as for the lattice atoms Xmbl is increased by about 30% in the worst case (ra = 0.2 A, T= 300 K) and less in the other cases.

For the MD simulations we considered a Si crystal of the size of 8 X 8 X 8 unit cells (4096 lattice sites) contain- ing either 4096 - 2 atoms in the case of the divacancy or 4096 + 1 atoms in the case of the (110) split interstitial. The atomic interaction is described by the Stillinger-Weber potential [18]. After equilibration for 2 ps at 1000 K dynamical quenching is performed to obtain the relaxed ground state configuration [17].

3. Results and discussion

Fig. 1 shows the backscattering spectra of virgin and implanted (110) Si samples measured at room temperature. The aligned spectra show a slight increase of dechanneling with depth and dose. Besides the case of the highest dose, no damage peak appears which is characteristic for weakly damaged layers [ 1 l]. The analysis of the backscattering spectra with DICADA is demonstrated for the exam- ple of the B+ implantation with a fluence of 9.8 X 10” cmm2. From the data in Fig. 1, AX,,,& 2,300 K) is determined. The corresponding RBS spectra for 100 K (not shown) provide A X,,,i,( 2,100 K). The results for the exam- ple chosen are given in Fig. 2a. For 5 different ra values the corresponding relative concentrations of displaced atoms ndat(z) are calculated from the measured

1.5x1 0’ I I

0.0

channel number

Fig. 1. Backscattering spectra measured at room temperature for B+ implanted (110) Si with different ion fluences.

3. Weberetal./Nucl. Instr. andMeth. in Phys. Rex B 118(1996) 113-118 115

---- r I I

(110) si 0

O-S.&l O’hm-’

t-

,. 1

i-

IL 0

I I I 1

.O 0.2 0.4 0.6 0.8 1

0.20

0.15

f O.lC Q

0.05

o.oc

1.0 B

C

g 0.8

ij

lo 0.6 .E D

6

5 0.4

f

P fj 0.2

I! 0.0

b \

.._

I-

O.

. exe. 300K = ex;. 1OOK

depth z &m>

I

I I 1 I

0 0.2 0.6 0.8 ’ depth z km>

Axmin(z); (a) measured at 100 and 300 K (symbols) and calculated at 300 K for different values ra (lines) and the correspondiug relative concentration of displaced atoms (b) for (1 IO) Si implanted with 9.8X 1015 BC cm-‘.

_.. .!_ ._

x

.O

a-

\ =*

I .o

dependence of Axmi, at a depth z - 1 pm expressed by A 2 Xmin = Ax&l pm,300 K) - Axmin(l pm.100 K). The different tow index directions show a quite different behaviour as also discussed by Howe et al. [ 121. For the (111) direction a nearly constant positive A2 ,ymin value of about 0.011 over the dose range investigated is obtained which is in agreement with recent measurements for similar implantation conditions [8]. This leads to a displacement distance r, - 0.4 ,& equal for all doses, connected

with an increase of the corresponding mean relative con-

centratton ndat with the ion dose (Figs. 3b and 3~). In the case of channeling measurements along the (100) direction a negative temperature dependence of Axmin was observed for the two lowest doses, which was also found for weakly damaged GaAs [7]. The displacement distances calculated are 0.2 and 0.28 A, respectively. With increasing dose A, xmin increases up to about 0.018 connected with an increase of ra up to 0.45 ,& (Fig. 3b). The

Ax,,,J z.100 K). They are depicted in Fig. 2b. For each ra

and the corresponding ndal, A x,,,~,( z,300 K) is calculated. ‘Ihey are given by the curves in Fig. 2a. Comparing these curves with the measured Ax,,,J 2,300 K) it ca”. be seen that the best agreement is obtained for ra - 0.45 A (uncer- tainty of r, is less than f0.05 A). Fig. 2a also shows that the method is rather sensitive with respect to the determination of the value of ra in the region of ra considered. lhe final result is that the implantation with 9.8 X lOI5 B + cm - 2 into ( 110) Si produces a nearly homogeneously damaged layer with about 32% of the l@ce atoms being displaced by a mean distance of 0.45 A perpendicular to the ( 110) Si atomic row. The results obtained after analyzing all measured spectra by this way are summarized in Figs. 3a-3c. Fig. 3a shows the measured temperature

A

x

A i

o.oI loo 10’

D/l 0’5cm-2

Fig. 3. Difference of Axmin at 300 and 100 K for (lOO), (I IO>, and (1 I I> Si at a depth of 1 pm versus ion dose (a); mean displacement distances r, calculated by the computer program DICADA [6] for all samples implanted (b); corresponding mean relative concentration of displaced atoms perpendicular to the low index directions (c>_

II. RBS. ETC.

116 B. Weber et al./Nucl. Instr. andbfeth. in Phys. Res. B 118 (1996) 113-118

correspondmg values of ndSI are nearly constant up to the dose of 9.8 X 10” cme2 and decrease for the highest dose (Fig. 3~). A similar behaviour was found in Ref. [7] for weakly damaged ( 111) GaAs generated by N+ implantation of different fluences which was explained as follows. Defect analysis was done with two distinct displacement distances ra (0.18 and 0.65 A). The concentration of displaced atoms with the low ra decreases with dose and ndat of atoms with the higher ra increases with dose. This could be understood by the formation of different defects and different ratios between formation, transformation and annealing of defects for different ion doses up to the formation of amorphous clusters in case of the highest dose implanted. Finally, in the case of the (110) Si samples A, xmin is always positive and increases with dose. The r,-values adequately increase with dose up to 0.6 A, the largest value determined here. The relative concentration of the displaced atoms increases with the ion dose, only in the case of the highest dose ndat decreases which only can be understood by assuming the formation of other defects or defect complexes. By different additional measurements (determination of the sub-gap absorp- tion coefficient by conventional transmission measurements and with photothermal transmission spectroscopy [8b positron-annilihation [19]) a high concentration of divacancies (about lo*’ cmv3) is found, showing a slight decrease for the highest dose. There was no difference between the differently oriented samples. In the case of (300 + 150) keV Si+ implanted (100) silicon in a dose range from 8.4 X lo’* to 2 8 X 1015 cmd2 Zammit et al. [20] found by thermal deflection spectroscopy a pronounced maximum of the divacancy concentration at a dose of 2.8 X lOr4 cm-*. The decrease at higher doses was explained by the formation of higher order vacancy complexes. It is remarkable that the concentration of displaced lattice atoms is about two orders of magnitude higher than the concentration of divacancies. Sealy et al. [2 I] proved the existence of vacancy complexes and interstitial-like defects in B+ implanted Si by EPR measurements and strain measurements, respectively.

To get more detailed information about possible defect configurations being responsible for the measured displacement distances, MD simulations have been performed to find out the full displacement field around the two most probable isolated point defects, the divacancy and the (110) split interstitial. In the following only Si atoms shifted more than 0.2 A from their lattice position in the perfect crystal are taken into account because less displaced atoms hardly contribute to the dechanneling. Their displacement vectors are projected on the (lOO), (110) and (111) planes, and ail equivalent orientations of the two defects are taken into account. The data necessary for the defect analysis with DICADA are given in Table 1. A divatancy causes 18 lattice atoms to be shifted more than 0.2 A from their lattice place whereas in the case of a (110) split interstitial only 10 atoms are shifted more than

Table 1 Results of MD simulations: number of atoms displace! by a divacancy and a (1 IO) split interstitial by more than 0.2 A from their ideal lattice position; distances of the displaced lattice atoms projected to the low index planes and their relative number

Divacancy (1 IO) split interstitial 18 displaced atoms 10 displaced atoms

ra I.9 rel. number ra [Al tel. number

Projection 0.22 0.78 0.24 0.80 onto ( 100) 0.54 0.22 0.78 0.20

Projection 0.22 0.70 0.27 0.81 onto(ll1) 0.53 0.30 0.86 0.19

Projection 0.23 0.75 0.27 0.76 onto(ll0) 0.54 0.25 0.83 0.12

1.44 0.12

0.2 A,. The full spectrum of the ra values calculated arrange in two or three groups, the values within a group differ only slightly. Therefore, a weighted average value rB is calculated for each group which is given in Table 1 together with the fraction of atoms displaced by this ra relatively to the whole number of displaced atoms considered. It can be seen that the lattice distortion and conse- quently the dechanneling efficiency by a divacancy is very similar in the three directions. There is no hint to any asymmetry. It is interesting to note that a divacancy causes not only very low displacement distances of 0.2-0.3 A as previously discussed 110,141 but also larger distances of 0.54 A to a relative fraction of about 0.25. The lattice distortion caused by the (110) split interstitial could be responsible for a different dechanneling behaviour in the different directions, especially in the ( 110) direction where a large value of ra = 1.44 A was calculated.

In the following, the ra values (see Table 1) of the divacancy alone, of the interstitial alone and of one divacancy plus one interstitial, respectively, were used to eval- uate the measured Axmin spectra from an improved point of view. Comparing these results with the results obtained for one mean displacement distance it follows that in cases where ra - 0.35 A or ra - 0.4 i was found, the displacement distances of the divacancy also reproduce the temperature dependence of Axmin (Fig. 4a for instance). For increasing single ra (2 0.45 W> the best agreement between measured and simulated Ax,.,,~,, spectra was found using the displacement distances of a divacancy plus an interstitial (Fig. 4bl. In the case of the largest mean vahte of ra found the calculation using the ra values provided by the interstitial alone agrees well with the experiment (Fig. 4c). That means large displacement distances will be dominant. For the lowest displacement distances obtained (0.2 A, 0.28 A> the negative temperature dependence of A,ymin could not be verified by one of the defects regarded.

B. Weber et al./Nucl. Instr. und Meth. in Phys. Rrs. B I18 (19961 113-l 18 117

0.25

o.oc

0.20

f 0.10 a

0.05

0.00

0.30

0.25

0.20

2 0.15 a

0.10

0.05

0.00

- r =0.48, --- d:vaconcv

. erp. 306K 1 exp. 1OOK

I

0 0.2 0.4 0.6 0.8 1.0

depth z (pm)

(110) li

1 I- -r-- b

D~9..3+10’%n-a

*

--- divac.+ (110) split . sxp. 300K = erp. 1OOK

I I I I .o 0.2 0.4 0.6 0.8 1 .o

depth z (pm)

1 I I I

(110) si c

1 - D=2r10”cm-a

I exp. 1OOK

I I I I I .O 0.2 0.4 0.6 0.8 1.0

depth z (wm)

Fig. 4. A x,,,~,( Z) measured at 300 K (0) and 100 K ( X ) together

with the best fits of Ax&z) at 300 K calculated using a mean displacement distance r, and using displacement distances calculated for special defects: (a): 2X lOI B+ cm-* in (1 I I) Si, (b):

9.8XlO’5 B+ cm-’ in (110) Si, (c): 2X lOI B+ cm-* in

(1 IO) Si.

4. Conclusion

From the fact that the dechanneling behaviour in the

BC implanted Si as a function of the ion dose is different for the three low index directions it can be concluded that the defects formed cause different displacement distribu-

tions perpendicular to these directions. In the case of the (I 11) and (I 10) direction the defects created at the low

doses can be attributed to displacement distances calcu-

lated for the divacancy by MD simulations. Looking along the (100) direction the lattice distortion is smaller than

that calculated for the divacancy. That means that even at

the lowest implantation dose the interaction of the defects as well as additional defects cause a lattice distortion

different from that of single divacancies. This is in agree-

ment with an estimation of the concentration of divacan- ties from the concentration of displaced atoms yielding a value an order of magnitude greater than the value found

from optical transmission measurements [8]. Increasing the implantation dose larger displacement distances contribute to dechanneling in the case of (100) and (I IO) channeling measurement, which can be attributed to interstitial like defects. These defects are less visible in the (I I I) direction. The formation of defect clusters such as higher

order vacancy clusters or boron-interstitial complexes which were proved by other experimental methods may be responsible for the observed lattice distortion.

References

[II [21

I31 [41 [51

I61

[71

[Sl

191

II01

[I 11

[I21

[I31

[141

E. Btigh, Can. J. Phys. 46 (1968) 653.

I.E. Westmoreland, J.W. Mayer, F.H. Eisen and B. Welch,

Radiat. Eff. 6 ( 1970) 16 1.

S.T. Picraux, J. Appl. Phys. 44 (1973) 587.

N. Matsunami and N. Itoh, Phys. Lett. A 43 (1973) 435.

N. Matsunami, T. Gotoh and N. Itoh, Nucl. Instr. and Meth.

I49 ( 1978) 430.

K. Gamer, K. Hehl and G. Schlotzhauer. Nucl. Instr. and

Meth. B 4 (1984) 55.

W. Wesch, A. Jordanov, K. CZrmer and G. Gljtz, Nucl. Instr.

and Meth. B 39 (1989) 445.

E. Wendler, K. Gktner, W. Wesch. U. Zammit and K.N.

Madhusoodanan, Nucl. Instr. and Meth. B 85 (1994) 528.

K. Gtimer, W. Wesch and G. G&z, Nucl. Instr. and Meth. B

48 (1990) 192.

W. Wesch, K. G%lner, E. Wendler and G. Giitz. Nucl. Instr.

and Meth. B 15 (1986) 431.

T. Bachmann. W. Wesch, K. GIrtner and E. Wendler. Nucl.

Instr. and Meth. B 63 (1992) 64.

L.M. Howe, M.L. Swanson and A.F. Quenneville, Nucl.

Instr. and Meth. 132 (1976) 297.

G. Gijtz and G. Sommer, Phys. Status Solidi B 32 (1975) K

151.

E. Glaser, G. G&z and K. Hehl, Proc. ICACS77, Vol. 2,

Moscow, 1977.

II. RBS, ETC.

118 B. Weber et al, / Nucl. Instr. and Meth. in Phys. Res. B I1 8 (1996) 113-l I8

[I.51 E. Glaser, Ci. G&z, N. Sobolev and W. Wesch, Phys. Status [I 9] R. Krause-Rehberg, Martin-Luther-University Halle, private Solidi A 69 (1982) 603. communication.

[16] G.H. Gilmer, T. Diaz de la Rubia, D.M. Stock and M. Jaraiz. Nucl. Instr. and Meth. B 102 (1995) 247.

[17] D.M. Stock, G.H. Gilmer, M. Jaraiz and T. Diaz de la Rubia. Nucl. Instr. and Meth. B 102 (1995) 207.

[ 181 F.H. Stillinger and T.A. Weber, Phys. Rev. B 31 (1985) 5262.

[20] U. Zammit, K.N. Madhusoodanan, M. Marinelli, F. Scudieri, R. Pizzoferrato, F. Mercuri, E. Wendler and W. Wesch, Phys. Rev. B 49 (1994) 14322.

[21] L. Sealy, R.C. Barkhe, G. Lulli, R. Nipoti, R. Balboni, S. Milita and M. Servidori, Nucl. Instr. and Meth. B 96 (199.5) 215.

investigation of weakly damaged 〈110〉, 〈111〉 and 〈100〉 silicon by means of temperature...

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