- Email: [email protected]
Characterization of ion-implanted silicon-insulator interfaces by reflected optical second harmonic generation
/ O U R N A L OF
NON-CRYSTALLINE Journal of Non-Crystalline Solids 187 (1995) 227 231
ELSEVIER
Characterization of ion-implanted silicon-insulator interfaces by reflected optical second harmonic generation I.V. K r a v e t s k y * , L . L . K u l y u k , A.V. M i c u , V.I. T s y t s a n u , I.S. V i e r u Institute o f Applied Physics, Academic str. 5, Kishinau 277028, Moldova
Abstract
Optical second harmonic generation has been applied for the study of the ion-implanted SiO2/Si(l 1 1) interface and Si(1 1 1) surfaces. Experimental results of real-time measurements of the second harmonic generation signal during thermal oxide etching are presented and analyzed taking into consideration various contributions to the non-linearoptical response from oxide/silicon interface such as the silicon substrate, the static electric field, the inhomogeneous deformation and the crystalline oxide interlayer. The symmetry properties of the thermal oxide/Si(1 1 I) interface have also been studied by measuring the rotational dependences of the second harmonic generation signal before and after oxide etching. It is shown that second harmonic rotational anisotropy and intensity strongly depend on the disordering of the interface and surface induced by ion implantation.
1. Introduction
The insulator/silicon interface structure is of great interest from both fundamental and technological point of view and has therefore stimulated theoretical and experimental research [1-3]. On the other hand, the increased use of ion implantation in electronic device fabrication has increased interest in diagnostic techniques capable of probing both the implanted ion depth distribution and the structural changes produced by the implantation on silicon surfaces and interfaces. Optical techniques are particularly attractive here, since they
* Corresponding author. Tel: +373-2 739 513. Telefax: +373-2 738 149. E-mail: [email protected].
are non-destructive and contactless. At the same time, these techniques can be used to make realtime measurements with high spatial resolution. Optical second harmonic generation (SHG), in particular, has been identified as a versatile and sensitive probe for surfaces and interfaces [4-6]. This laser-based method has the advantage of being applicable to any object accessible by laser beam and can be used for the control of the technological processes in liquids and at the relatively high gas pressure conditions (conventional surface sensitive techniques are not appropriate for this purpose). This method is especially useful in the case of materials with inversion symmetry (e.g., Si and Ge) because the SHG process is electric dipole forbidden within the bulk of the centrosymmetric media, but allowed in the near-surface region where the inversion symmetry is broken [7]. The SHG
0022-3093/95/$09.50 @ 1995 Elsevier Science B.V. All rights reserved SSD1 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 2 1 5 - 4
228
L 14 Kravetsky et al. / Journal o f Non-Crystalline Solids 187 (1995) 227-231
method is very suitable for the study of the surface and interface symmetry as well as for the study of changes in these symmetries which occur at phase transformations and at any surface modifying process [8-10]. In this paper we report the experimental results obtained by probing ion-implanted thermal oxide/silicon interface structure as well as the silicon surfaces by the S H G method.
b
IOlil
.]
[2iil
10all
4;
12iil
[011.l
lOlil
2. Experimental procedure In our experiments, the Si(1 1 1) and SiO2/ Si(1 1 1) samples implanted with BF~- and As + ions with energy of 130keV and doses of 1013 1015 cm -2 were studied. The thermal oxide thickness was ,~ 100 nm. Thermal annealing of the investigated samples was carried out in an ultrahigh vacuum chamber at a base pressure of ~ 10 lo Torr at the temperature of ~ 600°C. F o r the S H G experiments, a Q-switched YAG: Nd 3+ (neodymium-doped yttrium aluminum garnet) laser operating at )~ = 1064 nm and producing 15 nm pulses at a 5 Hz repetition rate was used. The s-polarized (i.e., polarized perpendicularly to the plane of incidence) laser radiation was incident at an angle of 45 ° onto the sample surface. The power density ( ~ 50 M W c m -2) was well below that of the threshold of any laser-induced damages in the surface. The reflected S H G signal at 2zo, = 532 nm was detected by a photomultiplier and a standard boxcar-integrator technique after being isolated by appropriate spectral filtering. The symmetry properties of the ion-implanted SiO2/Si(1 1 1) interface (before and after oxide etching) and Si(1 1 1) surfaces (before and after thermal annealing) were probed by measuring the S H G intensity while the samples were rotated about their normal at a constant rate. The rotation angle q~ was defined as the angle between the plane of incidence and the projection of the crystallographic axis [ 1 0 0 ] on the (1 1 1) plane. The study of SiO2/Si(1 1 1) interface structure which is based on the in situ control of the slow chemical etching process of oxide layer by the S H G method was carried out. The sample located in an optical cell was treated with the solution
[2iil
~l
[2ill
Fig. 1. p-polarized SH intensity from Si(1 1 1) implanted with BF~ (a, b) and As + (c, d) ions with an energy of 130 keV and dose of 101%m-z before (a, c) and after (b, d) thermal annealing, respectively, as a function of the sample rotation angle ~p.
(H20 + N H 4 F + HF) at room temperature and was simultaneously subjected to p u m p radiation at 45 ° incidence. The etching rate varied from 10 to 1 nm m i n - 1 depending on the reagent concentration. The experimental conditions excluded photochemical etching [11].
3. Results Fig. 1 shows p-polarized (i.e., polarized in parallel to the plane of incidence) SH intensity from Si(1 1 1) implanted with BF~ and As + ions with a dose of l015 cm -2 before (Figs. l(a) and (c)) and after (Figs. l(b) and (d)) thermal annealing, respectively, as a function of the sample rotation angle ~p in the polar coordinate system. It should be noted that in the case of the initial Si(1 1 1) surface (assumed C3v symmetry), the IEo~((P)dependence is well approximated by (A + B cos 3tp)2 for p-polarized S H G light, where the isotropic contribution A (it does not depend on the angle ~o) is much less than the anisotropic contribution B to the S H G and, therefore, shows six equally spaced peaks [12].
L V. Kravetsky et al. / Journal of Non-Crystalline Solids 187 (1995) 22~231
229
/
xl
t
0
90
180
9O
' Ao'
270
t
360
I
i
I
i
I
9O
1~
~0
3~
90
I80
270
360
(b)
I i
0
RcrrAa-r~ ANc~.~. ~
(desm)
ROTATION ANCe.e '~
(d~me)
Fig. 2. p-polarized SH intensity from SiO2/Si(1 1 1) samples implanted with BF~- with an energy of 130 keV: with • a dose of 10 14 cm 2 before (a) and after (b) thermal oxide etching, with a dose of 1015 cm - z before (c) and after (d) thermal oxide etching as a function of the sample rotation angle q~. The initial oxide thickness is ~ 100 nm.
Fig. 2 shows p-polarized SH intensity from SiO2/Si(1 1 1) samples implanted with BF~- with a dose of 1014cm -2 before (Fig. 2(a)) and after (Fig. 2(b)) oxide etching as well as with a dose of 1015 cm 2 before (Fig. 2(c)) and after (Fig. 2(d)) oxide etching, respectively, as a function of the sample rotation angle ~p. To probe the SiO2/Si(1 1 1) interface structure the real-time measurements of the second harmonic generation signal during thermal oxide etching were carried out. The intensity of the reflected pump radiation at 2,o = 1064 nm did not change during the oxide etching, which proves the uniformity of the etching process and absence of considerable interference effects in the oxide film. The etching rate was V = (6 + 1) nm m i n - 1. Assuming that the etching process is linear [13], we transformed 12,o(0 to lz,o(dox), where oxide thickness dox = din - V t (where din is initial thermal oxide
thickness, t is the etching time). Fig. 3 shows the p-polarized SH intensity reflected from SIO2/ Si(1 1 1) samples implanted with BF2 with doses of D = 1014cm -2 (curve 1) and D = 1015cm -2 (curve 2) as a function of the oxide thickness.
4. Discussion
It is seen that the I2~(~0) dependences change considerably: the threefold (1 1 1) symmetry is clearly broken (see Figs. l(a) and (c)), but after thermal annealing the symmetry is reconstructed (Figs. l(b) and (d)). It is also seen (comparing Fig. l(a) with Fig. l(c)) that implantation of BF~changes the Si(1 1 1) symmetry more than the implantation of As + ions. For s-s polarization geometry (s-polarized SHG signal is registered) only a week isotropic signal was detected from both
230
L V. Kravetsky et al. / Journal o f Non-Crystalline Solids 187 (1995) 22 ~ 231
1.0
.~
0.5
0.0
2
I 100
x2
50
0
Oxide thickness, nm
Fig. 3. Variation of the SH intensity reflected from SiO2/ Si(1 1 1) samples implanted with BF~ with an energy of 130 keV and doses of D = 1014cm 2 (curve 1) and D = 1015cm -2 (curve 2) during thermal oxide etching. The initial oxide thickness is ~ 100nm.
BF2~ and As + implanted Si(1 1 1) samples, but after thermal annealing the symmetry properties of Si(1 1 1) were reconstructed and corresponded to initial not implanted Si(1 1 1) symmetry. The s-polarized SHG intensity was proportional to sin2(3~0) and thus showed six equivalent maxima that were equally spaced [9, 12]. The results obtained (see Fig. 2) show a strong effect of the ion implantation on the redistribution of the peak amplitudes (compare Fig. 2(a) with Fig. 2(c)). It was observed that the increasing of the implantation dose leads to the increasing of the SHG isotropic contribution which is connected with the increase of the interface disordering induced by ion implantation. Examining the 12~o(dox) dependences in Fig. 3, we suppose that the slight drop at the beginning of both curves 1 and 2 is connected with the decrease in the thermoelastic strain contribution to the S H G [2] as well as in the contribution of the static electric field which takes place in the oxide/silicon interface [14, 15]. It is known that maximum thermoelastic strain concentration (,-~ 10 kbar) is on the SiO2/Si interface [2]. Therefore, when the oxide layer is becoming very thin, the thermoelastic strain breaks off it, which induces the creation of the microcrystal oxide islands. Such island-shaped crystalline regions, in their turn, can cause an increase in the reflected SH signal [16] (see the peak in
curve 1). The abrupt decrease of the SH intensity (curve 1) at the end of the etching process shows that the non-linear response is mainly determined by the thin transition oxide layer of ~ 5 nm thickness as well as that the SiO2/Si(1 1 1) interface is sharp within an atomic dimension [1, 17, 18]. The silicon substrate contribution to the nonlinear response of the BF2~ implanted SiOz/Si(1 1 1) interface is ~ 15% of the total value. The different behavior of the SH intensity in the case of SiO2/Si(1 1 1) sample implanted with D = 10 ~5 cm 2 (curve 2) suggests that the dose of 1015 c m - 2 is enough to destroy the interface sharpness and to relax the thermoelastic strains on the interface. So the monotonic decrease of SH intensity at the end of the etching process is observed (curve 2). In this case, non-linear optical response is mainly determined by the oxide layer of ,~ 25 nm thickness and the silicon substrate contribution to the non-linear response of the ion-implanted SiO2/Si(l 1 1) interface does not exceed 10% of the total value.
5. Conclusions The in situ control of the selective etching of the thermal oxide made it possible to discover the qualitative difference between the non-linear-optical responses from SiO2/Si(1 1 1) interfaces implanted with BF~ in doses of 1014cm -2 and 1015 cm 2. In both cases the non-linear optical response is mainly determined by the transition oxide layer ( ~ 5 nm and ~ 25 nm) which is adjacent to silicon substrate. The high contribution of this layer to the S H G can be explained by the existence of a space-charge region [14, 153 and a crystalline oxide interlayer with non-centrosymmetric structures [1, 17] situated between a thick amorphous oxide layer and the silicon substrate. It is shown that SHG method is sensitive and suitable for the characterization of ion-implanted silicon surfaces and thermal oxide/silicon interface structures. The correlation between isotropic and anisotropic contributions to the SHG could be used as quantitative criteria for the surface and interface disordering induced by ion implantation or other factors.
L V. Kravetsky et al. / Journal of Non-Crystalline Solids 187 (1995) 227 231
References [1] A. Ourmazd, P.H. Fuoss, J. Bevk and J.F. Morar, Appl. Surf. Sci. 41&42 (1989) 365. [2] S.V. Govorcov, V.I. Emel'yanov, N.I. Koroteev, G.I. Petrov, I.L. Shumay and V.V. Yakovlev, J. Opt. Soc. Am. B6 (1989) 1117. [3] B. Schubert, P. Avouris and R. Hoffmann, J. Chem. Phys. 98 (1993) 7593. I-4] J.F. McGilp, J. Phys.: Condens. Matter 2 (1990) 7985. 1-5] G. Lupke, D.J. Bottomley and H.M. van Driel, Phys. Rev. B47 (1993) 10389. [6] T.F. Heinz, M.M,T. Loy and W.A. Thompson, J. Vac. Sci. Technol. B3 (1985) 1467. 1-7] Y.R. Shen, The Principles of Nonlinear Optics (Wiley, New York, 1984). [8] T.F. Heinz, M.M,T. Loy and W.A. Thompson, Phys. Rev. Lett. 54 (1985) 63. [9] C.W. van Hasselt, M.A. Verheijen and Th. Rasing, Phys. Rev. B42 (1990) 9263.
231
[10] G.G. Malliaras, H.A. Wierenga and Th. Rasing, Surf. Sci. 287&288 (1993) 703. [11] S. Affrossman, R.T. Bailey, F.R. Cramer, F.R. Cruickchank, J.M.R. Macallister and J. Alderman, Appl. Phys. A49 (1989) 533. [12] J.A. Litwin, J.E. Sipe and H.M. van Driel, Phys. Rev. B31 (1985) 5543. [13] Y. Hayafuji and K. Kajiwara, J. Electrochem. Soc. 129 (1982) 2102. [14] C.H. Lee, R.K. Chang and N. Bloembergen, Phys. Rev. Lett. 18 (1967) 167. [15] N.F. Mott, Adv. Phys. 26 (1977) 363. [16] C.K. Chen, A.B.R. Castro and Y.R. Shen, Phys. Rev. Lett. 46 (1981) 145. [17] A. Ourmazd, D.W. Taylor, J.A. Rentschler and J. Bevk, Phys. Rev. Lett. 59 (1987) 213. [18] O.L. Krivanek and J.H. Mazur, Appl. Phys. Lett. 37 (1980) 392.
NON-CRYSTALLINE Journal of Non-Crystalline Solids 187 (1995) 227 231
ELSEVIER
Characterization of ion-implanted silicon-insulator interfaces by reflected optical second harmonic generation I.V. K r a v e t s k y * , L . L . K u l y u k , A.V. M i c u , V.I. T s y t s a n u , I.S. V i e r u Institute o f Applied Physics, Academic str. 5, Kishinau 277028, Moldova
Abstract
Optical second harmonic generation has been applied for the study of the ion-implanted SiO2/Si(l 1 1) interface and Si(1 1 1) surfaces. Experimental results of real-time measurements of the second harmonic generation signal during thermal oxide etching are presented and analyzed taking into consideration various contributions to the non-linearoptical response from oxide/silicon interface such as the silicon substrate, the static electric field, the inhomogeneous deformation and the crystalline oxide interlayer. The symmetry properties of the thermal oxide/Si(1 1 I) interface have also been studied by measuring the rotational dependences of the second harmonic generation signal before and after oxide etching. It is shown that second harmonic rotational anisotropy and intensity strongly depend on the disordering of the interface and surface induced by ion implantation.
1. Introduction
The insulator/silicon interface structure is of great interest from both fundamental and technological point of view and has therefore stimulated theoretical and experimental research [1-3]. On the other hand, the increased use of ion implantation in electronic device fabrication has increased interest in diagnostic techniques capable of probing both the implanted ion depth distribution and the structural changes produced by the implantation on silicon surfaces and interfaces. Optical techniques are particularly attractive here, since they
* Corresponding author. Tel: +373-2 739 513. Telefax: +373-2 738 149. E-mail: [email protected].
are non-destructive and contactless. At the same time, these techniques can be used to make realtime measurements with high spatial resolution. Optical second harmonic generation (SHG), in particular, has been identified as a versatile and sensitive probe for surfaces and interfaces [4-6]. This laser-based method has the advantage of being applicable to any object accessible by laser beam and can be used for the control of the technological processes in liquids and at the relatively high gas pressure conditions (conventional surface sensitive techniques are not appropriate for this purpose). This method is especially useful in the case of materials with inversion symmetry (e.g., Si and Ge) because the SHG process is electric dipole forbidden within the bulk of the centrosymmetric media, but allowed in the near-surface region where the inversion symmetry is broken [7]. The SHG
0022-3093/95/$09.50 @ 1995 Elsevier Science B.V. All rights reserved SSD1 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 2 1 5 - 4
228
L 14 Kravetsky et al. / Journal o f Non-Crystalline Solids 187 (1995) 227-231
method is very suitable for the study of the surface and interface symmetry as well as for the study of changes in these symmetries which occur at phase transformations and at any surface modifying process [8-10]. In this paper we report the experimental results obtained by probing ion-implanted thermal oxide/silicon interface structure as well as the silicon surfaces by the S H G method.
b
IOlil
.]
[2iil
10all
4;
12iil
[011.l
lOlil
2. Experimental procedure In our experiments, the Si(1 1 1) and SiO2/ Si(1 1 1) samples implanted with BF~- and As + ions with energy of 130keV and doses of 1013 1015 cm -2 were studied. The thermal oxide thickness was ,~ 100 nm. Thermal annealing of the investigated samples was carried out in an ultrahigh vacuum chamber at a base pressure of ~ 10 lo Torr at the temperature of ~ 600°C. F o r the S H G experiments, a Q-switched YAG: Nd 3+ (neodymium-doped yttrium aluminum garnet) laser operating at )~ = 1064 nm and producing 15 nm pulses at a 5 Hz repetition rate was used. The s-polarized (i.e., polarized perpendicularly to the plane of incidence) laser radiation was incident at an angle of 45 ° onto the sample surface. The power density ( ~ 50 M W c m -2) was well below that of the threshold of any laser-induced damages in the surface. The reflected S H G signal at 2zo, = 532 nm was detected by a photomultiplier and a standard boxcar-integrator technique after being isolated by appropriate spectral filtering. The symmetry properties of the ion-implanted SiO2/Si(1 1 1) interface (before and after oxide etching) and Si(1 1 1) surfaces (before and after thermal annealing) were probed by measuring the S H G intensity while the samples were rotated about their normal at a constant rate. The rotation angle q~ was defined as the angle between the plane of incidence and the projection of the crystallographic axis [ 1 0 0 ] on the (1 1 1) plane. The study of SiO2/Si(1 1 1) interface structure which is based on the in situ control of the slow chemical etching process of oxide layer by the S H G method was carried out. The sample located in an optical cell was treated with the solution
[2iil
~l
[2ill
Fig. 1. p-polarized SH intensity from Si(1 1 1) implanted with BF~ (a, b) and As + (c, d) ions with an energy of 130 keV and dose of 101%m-z before (a, c) and after (b, d) thermal annealing, respectively, as a function of the sample rotation angle ~p.
(H20 + N H 4 F + HF) at room temperature and was simultaneously subjected to p u m p radiation at 45 ° incidence. The etching rate varied from 10 to 1 nm m i n - 1 depending on the reagent concentration. The experimental conditions excluded photochemical etching [11].
3. Results Fig. 1 shows p-polarized (i.e., polarized in parallel to the plane of incidence) SH intensity from Si(1 1 1) implanted with BF~ and As + ions with a dose of l015 cm -2 before (Figs. l(a) and (c)) and after (Figs. l(b) and (d)) thermal annealing, respectively, as a function of the sample rotation angle ~p in the polar coordinate system. It should be noted that in the case of the initial Si(1 1 1) surface (assumed C3v symmetry), the IEo~((P)dependence is well approximated by (A + B cos 3tp)2 for p-polarized S H G light, where the isotropic contribution A (it does not depend on the angle ~o) is much less than the anisotropic contribution B to the S H G and, therefore, shows six equally spaced peaks [12].
L V. Kravetsky et al. / Journal of Non-Crystalline Solids 187 (1995) 22~231
229
/
xl
t
0
90
180
9O
' Ao'
270
t
360
I
i
I
i
I
9O
1~
~0
3~
90
I80
270
360
(b)
I i
0
RcrrAa-r~ ANc~.~. ~
(desm)
ROTATION ANCe.e '~
(d~me)
Fig. 2. p-polarized SH intensity from SiO2/Si(1 1 1) samples implanted with BF~- with an energy of 130 keV: with • a dose of 10 14 cm 2 before (a) and after (b) thermal oxide etching, with a dose of 1015 cm - z before (c) and after (d) thermal oxide etching as a function of the sample rotation angle q~. The initial oxide thickness is ~ 100 nm.
Fig. 2 shows p-polarized SH intensity from SiO2/Si(1 1 1) samples implanted with BF~- with a dose of 1014cm -2 before (Fig. 2(a)) and after (Fig. 2(b)) oxide etching as well as with a dose of 1015 cm 2 before (Fig. 2(c)) and after (Fig. 2(d)) oxide etching, respectively, as a function of the sample rotation angle ~p. To probe the SiO2/Si(1 1 1) interface structure the real-time measurements of the second harmonic generation signal during thermal oxide etching were carried out. The intensity of the reflected pump radiation at 2,o = 1064 nm did not change during the oxide etching, which proves the uniformity of the etching process and absence of considerable interference effects in the oxide film. The etching rate was V = (6 + 1) nm m i n - 1. Assuming that the etching process is linear [13], we transformed 12,o(0 to lz,o(dox), where oxide thickness dox = din - V t (where din is initial thermal oxide
thickness, t is the etching time). Fig. 3 shows the p-polarized SH intensity reflected from SIO2/ Si(1 1 1) samples implanted with BF2 with doses of D = 1014cm -2 (curve 1) and D = 1015cm -2 (curve 2) as a function of the oxide thickness.
4. Discussion
It is seen that the I2~(~0) dependences change considerably: the threefold (1 1 1) symmetry is clearly broken (see Figs. l(a) and (c)), but after thermal annealing the symmetry is reconstructed (Figs. l(b) and (d)). It is also seen (comparing Fig. l(a) with Fig. l(c)) that implantation of BF~changes the Si(1 1 1) symmetry more than the implantation of As + ions. For s-s polarization geometry (s-polarized SHG signal is registered) only a week isotropic signal was detected from both
230
L V. Kravetsky et al. / Journal o f Non-Crystalline Solids 187 (1995) 22 ~ 231
1.0
.~
0.5
0.0
2
I 100
x2
50
0
Oxide thickness, nm
Fig. 3. Variation of the SH intensity reflected from SiO2/ Si(1 1 1) samples implanted with BF~ with an energy of 130 keV and doses of D = 1014cm 2 (curve 1) and D = 1015cm -2 (curve 2) during thermal oxide etching. The initial oxide thickness is ~ 100nm.
BF2~ and As + implanted Si(1 1 1) samples, but after thermal annealing the symmetry properties of Si(1 1 1) were reconstructed and corresponded to initial not implanted Si(1 1 1) symmetry. The s-polarized SHG intensity was proportional to sin2(3~0) and thus showed six equivalent maxima that were equally spaced [9, 12]. The results obtained (see Fig. 2) show a strong effect of the ion implantation on the redistribution of the peak amplitudes (compare Fig. 2(a) with Fig. 2(c)). It was observed that the increasing of the implantation dose leads to the increasing of the SHG isotropic contribution which is connected with the increase of the interface disordering induced by ion implantation. Examining the 12~o(dox) dependences in Fig. 3, we suppose that the slight drop at the beginning of both curves 1 and 2 is connected with the decrease in the thermoelastic strain contribution to the S H G [2] as well as in the contribution of the static electric field which takes place in the oxide/silicon interface [14, 15]. It is known that maximum thermoelastic strain concentration (,-~ 10 kbar) is on the SiO2/Si interface [2]. Therefore, when the oxide layer is becoming very thin, the thermoelastic strain breaks off it, which induces the creation of the microcrystal oxide islands. Such island-shaped crystalline regions, in their turn, can cause an increase in the reflected SH signal [16] (see the peak in
curve 1). The abrupt decrease of the SH intensity (curve 1) at the end of the etching process shows that the non-linear response is mainly determined by the thin transition oxide layer of ~ 5 nm thickness as well as that the SiO2/Si(1 1 1) interface is sharp within an atomic dimension [1, 17, 18]. The silicon substrate contribution to the nonlinear response of the BF2~ implanted SiOz/Si(1 1 1) interface is ~ 15% of the total value. The different behavior of the SH intensity in the case of SiO2/Si(1 1 1) sample implanted with D = 10 ~5 cm 2 (curve 2) suggests that the dose of 1015 c m - 2 is enough to destroy the interface sharpness and to relax the thermoelastic strains on the interface. So the monotonic decrease of SH intensity at the end of the etching process is observed (curve 2). In this case, non-linear optical response is mainly determined by the oxide layer of ,~ 25 nm thickness and the silicon substrate contribution to the non-linear response of the ion-implanted SiO2/Si(l 1 1) interface does not exceed 10% of the total value.
5. Conclusions The in situ control of the selective etching of the thermal oxide made it possible to discover the qualitative difference between the non-linear-optical responses from SiO2/Si(1 1 1) interfaces implanted with BF~ in doses of 1014cm -2 and 1015 cm 2. In both cases the non-linear optical response is mainly determined by the transition oxide layer ( ~ 5 nm and ~ 25 nm) which is adjacent to silicon substrate. The high contribution of this layer to the S H G can be explained by the existence of a space-charge region [14, 153 and a crystalline oxide interlayer with non-centrosymmetric structures [1, 17] situated between a thick amorphous oxide layer and the silicon substrate. It is shown that SHG method is sensitive and suitable for the characterization of ion-implanted silicon surfaces and thermal oxide/silicon interface structures. The correlation between isotropic and anisotropic contributions to the SHG could be used as quantitative criteria for the surface and interface disordering induced by ion implantation or other factors.
L V. Kravetsky et al. / Journal of Non-Crystalline Solids 187 (1995) 227 231
References [1] A. Ourmazd, P.H. Fuoss, J. Bevk and J.F. Morar, Appl. Surf. Sci. 41&42 (1989) 365. [2] S.V. Govorcov, V.I. Emel'yanov, N.I. Koroteev, G.I. Petrov, I.L. Shumay and V.V. Yakovlev, J. Opt. Soc. Am. B6 (1989) 1117. [3] B. Schubert, P. Avouris and R. Hoffmann, J. Chem. Phys. 98 (1993) 7593. I-4] J.F. McGilp, J. Phys.: Condens. Matter 2 (1990) 7985. 1-5] G. Lupke, D.J. Bottomley and H.M. van Driel, Phys. Rev. B47 (1993) 10389. [6] T.F. Heinz, M.M,T. Loy and W.A. Thompson, J. Vac. Sci. Technol. B3 (1985) 1467. 1-7] Y.R. Shen, The Principles of Nonlinear Optics (Wiley, New York, 1984). [8] T.F. Heinz, M.M,T. Loy and W.A. Thompson, Phys. Rev. Lett. 54 (1985) 63. [9] C.W. van Hasselt, M.A. Verheijen and Th. Rasing, Phys. Rev. B42 (1990) 9263.
231
[10] G.G. Malliaras, H.A. Wierenga and Th. Rasing, Surf. Sci. 287&288 (1993) 703. [11] S. Affrossman, R.T. Bailey, F.R. Cramer, F.R. Cruickchank, J.M.R. Macallister and J. Alderman, Appl. Phys. A49 (1989) 533. [12] J.A. Litwin, J.E. Sipe and H.M. van Driel, Phys. Rev. B31 (1985) 5543. [13] Y. Hayafuji and K. Kajiwara, J. Electrochem. Soc. 129 (1982) 2102. [14] C.H. Lee, R.K. Chang and N. Bloembergen, Phys. Rev. Lett. 18 (1967) 167. [15] N.F. Mott, Adv. Phys. 26 (1977) 363. [16] C.K. Chen, A.B.R. Castro and Y.R. Shen, Phys. Rev. Lett. 46 (1981) 145. [17] A. Ourmazd, D.W. Taylor, J.A. Rentschler and J. Bevk, Phys. Rev. Lett. 59 (1987) 213. [18] O.L. Krivanek and J.H. Mazur, Appl. Phys. Lett. 37 (1980) 392.