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Diffusion of hydrogen implanted in α-quartz during air annealing
Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 641±645
www.elsevier.nl/locate/nimb
Diusion of hydrogen implanted in a-quartz during air annealing W. Bolse
a,b,*
, M. Gustafsson c, F. Harbsmeier
b,d
, F. Roccaforte
b,d
a
Institut f ur Strahlenphysik, Universit at Stuttgart, D-70569 Stuttgart, Germany b SFB 345, Universit at G ottingen, D-37073 G ottingen, Germany c Accelerator Laboratory, University Helsinki, P.O. Box 43, FIN-00014, Finland d II. Physikalisches Institut, Universit at G ottingen, D-37073 G ottingen, Germany
Abstract 16 2 a-quartz samples were implanted with 20 keV H 2 -ions to ¯uences of 5 10 H/cm at a target temperature of 77 K. Rutherford Backscattering in channeling geometry (RBS-C) revealed that an amorphous surface layer of 1:54 1018 atoms/cm2 forms under these conditions. Resonant nuclear reaction analysis (RNRA) utilizing the 1 H(15 N, ac)-resonance at 6.385 MeV beam energy was used to measure the implanted hydrogen pro®le. The samples were then annealed in air for 1 h at temperatures between 300°C and 950°C. After annealing, RBS-C and RNRA were again employed to study the alterations of the hydrogen pro®le and the amorphous layer induced by the heat treatment. In contrast to the observation with alkali ions no epitaxial regrowth could be detected even after the 950°C annealing. Below about 450°C also no changes of the hydrogen pro®le were observed, while at about 600°C almost all hydrogen has left the sample. This behavior ®ts nicely to the results obtained for other alkali implantations. Hydrogen as the lightest (and smallest) group-I atom becomes mobile at the lowest temperature and also the observed trend that the quality of the regrown layer decreases with decreasing atomic number of the implanted species has been con®rmed, since no epitaxial recrystallization has taken place. Ó 2000 Elsevier Science B.V. All rights reserved.
PACS: 61.80.jh; 81.15.Np; 42.70.Ce; 61.82.Ms Keywords: Hydrogen; Quartz; Ion implantation; Diusion
1. Introduction Recently we have reported on a systematic investigation of the diusion of alkali atoms implanted into a-quartz during annealing in oxygen,
* Corresponding author. Tel.: +49-771-685-3875; fax: +49771-685-3866. E-mail address: [email protected] (W. Bolse).
and its in¯uence on the epitaxial recrystallization behavior of the ion-beam amorphized layer [1±4]. We have found that the incorporation of alkali oxide (as a network modi®er) during annealing of alkali implanted SiO2 allows for epitaxial regrowth of the disordered SiO2 network at temperatures below 900°C, while during annealing in vacuum and also after rare gas or even self-ion (stoichiometric amounts of Si and O) implantation recrystallization could not be achieved. The temperature, at which the alkali atoms become mobile
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 8 1 - 3
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W. Bolse et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 641±645
and the recrystallization process starts, becomes the lower the smaller is the atomic number of the alkali atom. At the same time, however, the crystalline quality of the regrown layer decreases. Hydrogen also belongs to group-I of the periodic system of the elements and we are therefore very much interested whether its diusion behavior is similar to the heavier group-I elements and whether it can also improve the recrystallizability of ion-implanted a-quartz. 2. Experimental H-implantation and also Rutherford Backscattering of 900 keV a-particles in channeling geometry (RBS-C [5]) were carried out at the G ottingen heavy ion implanter IONAS [6], while the Helsinki tandem accelerator EPG-10-II was used for H-depth pro®ling. For the latter, resonant nuclear reaction analysis (RNRA) was employed making use of the proton capture reaction 1 H(15 N,ac)12 C, which exhibits a resonance at ER 6:385 MeV with a width C 1:7 keV [7]. The 4.43 MeV c-ray yield as a function of the energy of the 15 N-beam, was measured by means of the lowlevel c-ray detection facility described in [8]. During the measurement the samples were kept at liquid nitrogen temperature to avoid H-diusion induced by the 15 N-beam, and charging was suppressed by a Ni-grid of 92% transmission, which was put onto the insulating sample surface. The single-crystalline a-quartz samples with caxis orientation had a size of 10 mm ´ 10 mm ´ 1 mm. Each sample was homogeneously implanted 16 ions/ with 20 keV H 2 ions to ¯uences of 2:5 10 cm2 , which is equivalent to an implantation of 5 1016 H/cm2 at an energy of 10 keV. During implantation, the target was kept in contact with a liquid nitrogen reservoir and the beam current was limited below 2.5 lA in order to avoid excessive beam heating. Utilizing RBS-C an amorphized surface layer with a thickness of 1:54 1018 atoms/ cm2 was found after H implantation. In addition, the H-depth pro®le was measured in the as-implanted sample by means of RNRA. The samples were then annealed in air at temperatures between 300°C and 950°C for 1 h and afterwards the H-
distribution was determined in order to investigate the thermally induced migration of the H atoms. For the samples annealed above 500°C, RBS-C was again employed to check for epitaxial regrowth, which however was not observed.
3. Results and discussion In Fig. 1(a) and (b) the measured depth distribution of the implanted hydrogen atoms is shown after implantation and after annealing at various temperatures in a linear and a logarithmic scale. For comparison also the range distribution as simulated by the TRIM Monte Carlo code [9] is given and the thickness of the amorphous layer as
Fig. 1. Hydrogen depth pro®les as determined by RNRA after implantation and annealing in (a) linear and (b) logarithmic presentation. In addition, the ion range distribution as simulated by TRIM and the thickness of the ion beam amorphized layer are given.
W. Bolse et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 641±645
determined by RBS-C is indicated. In contrast to the implantation of similar amounts of alkali ions [1±4], where we observed an almost Gaussianshaped distribution of the implanted species in good agreement with the TRIM calculations, H implantation at 77 K results in a broad, almost box-like pro®le, which does not agree with the simulation. Obviously H-diusion had taken place and spread out the H-pro®le not only over the amorphized layer (indicated in the ®gure) but also into the undamaged bulk to depths of about twice the ion range. This is most probably due to a transient diusion process during ion implantation, since heating of the samples at temperatures T 6 400 C did not result in signi®cant changes as compared to the as-implanted H-pro®le. It therefore looks very much unlikely that the H spreadout occurred during warming up of the sample to room temperature after ion implantation. During annealing at 500°C, however, thermally activated diusion sets in and results in out-gassing of the surface near the amorphous layer, while the pro®le in the crystalline bulk only slightly varies. At higher temperatures signi®cant loss of H also occurs from the non-damaged depths. After annealing at temperatures above 600°C, almost all H has diused out of the sample except for a small amount left in the amorphized layer, indicating a higher H solubility of the amorphous material as compared to its crystalline counterpart.
Fig. 2. Total amount of H-atoms found in the samples after annealing at dierent temperatures. The solid curve is to guide the eye only.
643
The H-content as a function of the annealing temperature is illustrated in Fig. 2. As already seen in Fig. 1 the H-content stays constant up to 400°C but then drops to zero within a temperature interval of 200°C. At 550°C about half of the implanted H atoms have diused out of the sample, and at 700°C almost no H is left in the sample. In Fig. 3 the hydrogen loss is plotted versus 1/T (T is the sample temperature). It turns out that at temperatures below 600°C the H-release can be
Fig. 3. Arrhenius plot of the amount of hydrogen lost from the samples during annealing. The solid curve represents a linear ®t to the data at T 6 600 C.
Fig. 4. Residual amount of group-I elements left in the sample after implantation and annealing [3]. In addition the amount of 18 O found in the sample after annealing is given, normalized to its maximum value [4]. The solid, dashed and dotted curves are to guide the eye only.
644
W. Bolse et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 641±645
well-described by an Arrhenius curve with an activation energy of EA 0:86 13 eV. A similar behavior was observed earlier for Liand for Cs-implanted a-quartz, as is indicated in Fig. 4. Here the residual content of the implanted species is plotted versus the annealing temperature. The implanted amounts of Cs and Li ions were 2:5 1016 atoms/cm2 . In all cases, curves of similar shape were observed for the out-diusion of these group-I elements. The temperatures at which release of the implanted species occurs, however, dier from each other. Hydrogen as the lightest and smallest atom diuses out of the SiO2 already at temperatures around 550°C, while the heavy Cs become mobile at about 780°C. Li diusion starts at an intermediate temperature of about 610°C. The activation energy found for Cs release was EA 0:98 (1) eV, which nicely compares with the value found for hydrogen in the present work and possibly indicates, that the release process is controlled by the same diusion mechanism. On the other hand, the activation energy reported for Li is smaller by a factor of about two, EA 0:45 9 eV. The large scatter in the Li-data especially at lower temperature, however, might in¯uence the analysis and lead to a somewhat too small value. This behavior of increasing release temperature with increasing atomic number of the group-I element correlates well with the recrystallizability of the amorphized material as a function of the implanted species. Roccaforte et al. [1,2] recently reported that epitaxial regrowth of ion beam amorphized a-quartz can be achieved at moderate temperatures when alkali ions were implanted and the annealing was done in an oxygen-containing atmosphere. As a reason for this improved recrystallizability the authors assumed that during annealing in oxygen the alkali atoms become oxidized and dissolve as network modi®ers in the amorphous SiO2 -network (alkali glass formation). This leads to the formation of non-connected [SiO4 ]-tetrahedral corners, which should improve the recrystallizibility of the network [10]. During recrystallization the alkali atoms diuse out of the sample, while at the same time, as demonstrated by annealing in 18 O atmosphere and subsequent time-of-¯ight ERDA (elastic recoil detection analysis], oxygen from the gas phase diuses into
the sample, leading to a strong isotope exchange [3,4]. The uptake of 18 O is also illustrated in Fig. 4, where the total amount of 18 O normalized to the maximum observed value is plotted versus temperature. Oxygen diusion slightly starts at about 600°C and strongly increases between 700°C and 800°C. This coincides well with the epitaxial regrowth of the amorphous layer and the out-diusion of Cs, and almost perfect recrystallization is achieved. In case of Li-implanted SiO2 out-diusion of the alkali occurs at low temperature and the recrystallization is much less perfect than in the case of Cs. This can probably be ascribed to the lower oxygen diusivity at these temperatures at which a large fraction of the implanted Li has diused out of the sample when oxygen starts to enter. Similar observations were made for Na-implanted a-quartz, where regrowth sets in at about 700°C (according to the above arguments, the onset temperature for Na diusion is expected between those of Li and Cs) and is less perfect than in case of Cs-implantation, but slightly better than in case of Li (probably due to the higher oxygen mobility at 700°C). Our observations after hydrogen implantation nicely ®t to this behavior: hydrogen is the lightest group-I element and starts to diuse at the lowest temperature as compared to the other alkali atoms. Out-diusion of the implanted hydrogen is almost completely ®nished before oxygen enters the sample and, hence, recrystallization does not take place. 4. Conclusions In conclusion, we have shown that hydrogen implanted into a-quartz (which becomes amorphized by the irradiation) becomes mobile at about 550°C and diuses out of the sample surface. The activation energy of this process is EA 0:86 13 eV. By comparison of the behavior of hydrogen during annealing of the implanted samples with that of other group-I elements we found that: 1. The onset temperature for the diusion of the group-I elements is the lower the smaller is the atomic number (i.e., the size of the ion).
W. Bolse et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 641±645
2. The epitaxial recrystallizability of the ion beam amorphized layer improves with increasing atomic number of the implanted group-I species (almost perfect recrystallization for Cs, intermediate quality of the regrown layer for Na and Li, no recrystallization for H). The latter seems to be governed by the oxygen mobility at the onset temperature of the diusion of the group-I element (high for Cs, vanishing for H).
Acknowledgements The authors would like to acknowledge J. Keinonen and K.P. Lieb for help and stimulating discussions. This work has partly been supported by the Deutsche Akademischer Austauschdienst (DAAD) and the Finnish Academy of Sciences.
645
References [1] F. Roccaforte, W. Bolse, K.P. Lieb, Appl. Phys. Lett. 73 (1998) 1349. [2] F. Roccaforte, W. Bolse, K.P. Lieb, Nucl. Instr. and Meth. B 148 (1999) 692. [3] F. Roccaforte, M. Gustafsson, W. Bolse, J. Keinonen, K.P. Lieb, Nucl. Instr. and Meth. B 166±167 (2000), in press. [4] M. Gustafsson, F. Roccaforte, J. Keinonen, W. Bolse, L. Ziegeler, K.P. Lieb, Phys. Rev. B, in press. [5] W.K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, Academic Press, Orlando, USA, 1978. [6] M. Uhrmacher et al., Nucl. Instr. and Meth. B 9 (1985) 234. [7] T. Osipowicz, K.P. Lieb, S. Br ussermann, Nucl. Instr. and Meth. B 18 (1987) 232. [8] P. Torri, J. Keinonen, K. Nordlund, Nucl. Instr. and Meth. B 84 (1994) 105. [9] J.F. Ziegler, J.P. Biersack, W. Littmark, The Stopping and Ranges of Ions in Solids, Vol. 1, Pergamon Press, New York, USA, 1985. [10] L.W. Hobbs, Nucl. Instr. and Meth. B 91 (1994) 30.
www.elsevier.nl/locate/nimb
Diusion of hydrogen implanted in a-quartz during air annealing W. Bolse
a,b,*
, M. Gustafsson c, F. Harbsmeier
b,d
, F. Roccaforte
b,d
a
Institut f ur Strahlenphysik, Universit at Stuttgart, D-70569 Stuttgart, Germany b SFB 345, Universit at G ottingen, D-37073 G ottingen, Germany c Accelerator Laboratory, University Helsinki, P.O. Box 43, FIN-00014, Finland d II. Physikalisches Institut, Universit at G ottingen, D-37073 G ottingen, Germany
Abstract 16 2 a-quartz samples were implanted with 20 keV H 2 -ions to ¯uences of 5 10 H/cm at a target temperature of 77 K. Rutherford Backscattering in channeling geometry (RBS-C) revealed that an amorphous surface layer of 1:54 1018 atoms/cm2 forms under these conditions. Resonant nuclear reaction analysis (RNRA) utilizing the 1 H(15 N, ac)-resonance at 6.385 MeV beam energy was used to measure the implanted hydrogen pro®le. The samples were then annealed in air for 1 h at temperatures between 300°C and 950°C. After annealing, RBS-C and RNRA were again employed to study the alterations of the hydrogen pro®le and the amorphous layer induced by the heat treatment. In contrast to the observation with alkali ions no epitaxial regrowth could be detected even after the 950°C annealing. Below about 450°C also no changes of the hydrogen pro®le were observed, while at about 600°C almost all hydrogen has left the sample. This behavior ®ts nicely to the results obtained for other alkali implantations. Hydrogen as the lightest (and smallest) group-I atom becomes mobile at the lowest temperature and also the observed trend that the quality of the regrown layer decreases with decreasing atomic number of the implanted species has been con®rmed, since no epitaxial recrystallization has taken place. Ó 2000 Elsevier Science B.V. All rights reserved.
PACS: 61.80.jh; 81.15.Np; 42.70.Ce; 61.82.Ms Keywords: Hydrogen; Quartz; Ion implantation; Diusion
1. Introduction Recently we have reported on a systematic investigation of the diusion of alkali atoms implanted into a-quartz during annealing in oxygen,
* Corresponding author. Tel.: +49-771-685-3875; fax: +49771-685-3866. E-mail address: [email protected] (W. Bolse).
and its in¯uence on the epitaxial recrystallization behavior of the ion-beam amorphized layer [1±4]. We have found that the incorporation of alkali oxide (as a network modi®er) during annealing of alkali implanted SiO2 allows for epitaxial regrowth of the disordered SiO2 network at temperatures below 900°C, while during annealing in vacuum and also after rare gas or even self-ion (stoichiometric amounts of Si and O) implantation recrystallization could not be achieved. The temperature, at which the alkali atoms become mobile
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 8 1 - 3
642
W. Bolse et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 641±645
and the recrystallization process starts, becomes the lower the smaller is the atomic number of the alkali atom. At the same time, however, the crystalline quality of the regrown layer decreases. Hydrogen also belongs to group-I of the periodic system of the elements and we are therefore very much interested whether its diusion behavior is similar to the heavier group-I elements and whether it can also improve the recrystallizability of ion-implanted a-quartz. 2. Experimental H-implantation and also Rutherford Backscattering of 900 keV a-particles in channeling geometry (RBS-C [5]) were carried out at the G ottingen heavy ion implanter IONAS [6], while the Helsinki tandem accelerator EPG-10-II was used for H-depth pro®ling. For the latter, resonant nuclear reaction analysis (RNRA) was employed making use of the proton capture reaction 1 H(15 N,ac)12 C, which exhibits a resonance at ER 6:385 MeV with a width C 1:7 keV [7]. The 4.43 MeV c-ray yield as a function of the energy of the 15 N-beam, was measured by means of the lowlevel c-ray detection facility described in [8]. During the measurement the samples were kept at liquid nitrogen temperature to avoid H-diusion induced by the 15 N-beam, and charging was suppressed by a Ni-grid of 92% transmission, which was put onto the insulating sample surface. The single-crystalline a-quartz samples with caxis orientation had a size of 10 mm ´ 10 mm ´ 1 mm. Each sample was homogeneously implanted 16 ions/ with 20 keV H 2 ions to ¯uences of 2:5 10 cm2 , which is equivalent to an implantation of 5 1016 H/cm2 at an energy of 10 keV. During implantation, the target was kept in contact with a liquid nitrogen reservoir and the beam current was limited below 2.5 lA in order to avoid excessive beam heating. Utilizing RBS-C an amorphized surface layer with a thickness of 1:54 1018 atoms/ cm2 was found after H implantation. In addition, the H-depth pro®le was measured in the as-implanted sample by means of RNRA. The samples were then annealed in air at temperatures between 300°C and 950°C for 1 h and afterwards the H-
distribution was determined in order to investigate the thermally induced migration of the H atoms. For the samples annealed above 500°C, RBS-C was again employed to check for epitaxial regrowth, which however was not observed.
3. Results and discussion In Fig. 1(a) and (b) the measured depth distribution of the implanted hydrogen atoms is shown after implantation and after annealing at various temperatures in a linear and a logarithmic scale. For comparison also the range distribution as simulated by the TRIM Monte Carlo code [9] is given and the thickness of the amorphous layer as
Fig. 1. Hydrogen depth pro®les as determined by RNRA after implantation and annealing in (a) linear and (b) logarithmic presentation. In addition, the ion range distribution as simulated by TRIM and the thickness of the ion beam amorphized layer are given.
W. Bolse et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 641±645
determined by RBS-C is indicated. In contrast to the implantation of similar amounts of alkali ions [1±4], where we observed an almost Gaussianshaped distribution of the implanted species in good agreement with the TRIM calculations, H implantation at 77 K results in a broad, almost box-like pro®le, which does not agree with the simulation. Obviously H-diusion had taken place and spread out the H-pro®le not only over the amorphized layer (indicated in the ®gure) but also into the undamaged bulk to depths of about twice the ion range. This is most probably due to a transient diusion process during ion implantation, since heating of the samples at temperatures T 6 400 C did not result in signi®cant changes as compared to the as-implanted H-pro®le. It therefore looks very much unlikely that the H spreadout occurred during warming up of the sample to room temperature after ion implantation. During annealing at 500°C, however, thermally activated diusion sets in and results in out-gassing of the surface near the amorphous layer, while the pro®le in the crystalline bulk only slightly varies. At higher temperatures signi®cant loss of H also occurs from the non-damaged depths. After annealing at temperatures above 600°C, almost all H has diused out of the sample except for a small amount left in the amorphized layer, indicating a higher H solubility of the amorphous material as compared to its crystalline counterpart.
Fig. 2. Total amount of H-atoms found in the samples after annealing at dierent temperatures. The solid curve is to guide the eye only.
643
The H-content as a function of the annealing temperature is illustrated in Fig. 2. As already seen in Fig. 1 the H-content stays constant up to 400°C but then drops to zero within a temperature interval of 200°C. At 550°C about half of the implanted H atoms have diused out of the sample, and at 700°C almost no H is left in the sample. In Fig. 3 the hydrogen loss is plotted versus 1/T (T is the sample temperature). It turns out that at temperatures below 600°C the H-release can be
Fig. 3. Arrhenius plot of the amount of hydrogen lost from the samples during annealing. The solid curve represents a linear ®t to the data at T 6 600 C.
Fig. 4. Residual amount of group-I elements left in the sample after implantation and annealing [3]. In addition the amount of 18 O found in the sample after annealing is given, normalized to its maximum value [4]. The solid, dashed and dotted curves are to guide the eye only.
644
W. Bolse et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 641±645
well-described by an Arrhenius curve with an activation energy of EA 0:86 13 eV. A similar behavior was observed earlier for Liand for Cs-implanted a-quartz, as is indicated in Fig. 4. Here the residual content of the implanted species is plotted versus the annealing temperature. The implanted amounts of Cs and Li ions were 2:5 1016 atoms/cm2 . In all cases, curves of similar shape were observed for the out-diusion of these group-I elements. The temperatures at which release of the implanted species occurs, however, dier from each other. Hydrogen as the lightest and smallest atom diuses out of the SiO2 already at temperatures around 550°C, while the heavy Cs become mobile at about 780°C. Li diusion starts at an intermediate temperature of about 610°C. The activation energy found for Cs release was EA 0:98 (1) eV, which nicely compares with the value found for hydrogen in the present work and possibly indicates, that the release process is controlled by the same diusion mechanism. On the other hand, the activation energy reported for Li is smaller by a factor of about two, EA 0:45 9 eV. The large scatter in the Li-data especially at lower temperature, however, might in¯uence the analysis and lead to a somewhat too small value. This behavior of increasing release temperature with increasing atomic number of the group-I element correlates well with the recrystallizability of the amorphized material as a function of the implanted species. Roccaforte et al. [1,2] recently reported that epitaxial regrowth of ion beam amorphized a-quartz can be achieved at moderate temperatures when alkali ions were implanted and the annealing was done in an oxygen-containing atmosphere. As a reason for this improved recrystallizability the authors assumed that during annealing in oxygen the alkali atoms become oxidized and dissolve as network modi®ers in the amorphous SiO2 -network (alkali glass formation). This leads to the formation of non-connected [SiO4 ]-tetrahedral corners, which should improve the recrystallizibility of the network [10]. During recrystallization the alkali atoms diuse out of the sample, while at the same time, as demonstrated by annealing in 18 O atmosphere and subsequent time-of-¯ight ERDA (elastic recoil detection analysis], oxygen from the gas phase diuses into
the sample, leading to a strong isotope exchange [3,4]. The uptake of 18 O is also illustrated in Fig. 4, where the total amount of 18 O normalized to the maximum observed value is plotted versus temperature. Oxygen diusion slightly starts at about 600°C and strongly increases between 700°C and 800°C. This coincides well with the epitaxial regrowth of the amorphous layer and the out-diusion of Cs, and almost perfect recrystallization is achieved. In case of Li-implanted SiO2 out-diusion of the alkali occurs at low temperature and the recrystallization is much less perfect than in the case of Cs. This can probably be ascribed to the lower oxygen diusivity at these temperatures at which a large fraction of the implanted Li has diused out of the sample when oxygen starts to enter. Similar observations were made for Na-implanted a-quartz, where regrowth sets in at about 700°C (according to the above arguments, the onset temperature for Na diusion is expected between those of Li and Cs) and is less perfect than in case of Cs-implantation, but slightly better than in case of Li (probably due to the higher oxygen mobility at 700°C). Our observations after hydrogen implantation nicely ®t to this behavior: hydrogen is the lightest group-I element and starts to diuse at the lowest temperature as compared to the other alkali atoms. Out-diusion of the implanted hydrogen is almost completely ®nished before oxygen enters the sample and, hence, recrystallization does not take place. 4. Conclusions In conclusion, we have shown that hydrogen implanted into a-quartz (which becomes amorphized by the irradiation) becomes mobile at about 550°C and diuses out of the sample surface. The activation energy of this process is EA 0:86 13 eV. By comparison of the behavior of hydrogen during annealing of the implanted samples with that of other group-I elements we found that: 1. The onset temperature for the diusion of the group-I elements is the lower the smaller is the atomic number (i.e., the size of the ion).
W. Bolse et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 641±645
2. The epitaxial recrystallizability of the ion beam amorphized layer improves with increasing atomic number of the implanted group-I species (almost perfect recrystallization for Cs, intermediate quality of the regrown layer for Na and Li, no recrystallization for H). The latter seems to be governed by the oxygen mobility at the onset temperature of the diusion of the group-I element (high for Cs, vanishing for H).
Acknowledgements The authors would like to acknowledge J. Keinonen and K.P. Lieb for help and stimulating discussions. This work has partly been supported by the Deutsche Akademischer Austauschdienst (DAAD) and the Finnish Academy of Sciences.
645
References [1] F. Roccaforte, W. Bolse, K.P. Lieb, Appl. Phys. Lett. 73 (1998) 1349. [2] F. Roccaforte, W. Bolse, K.P. Lieb, Nucl. Instr. and Meth. B 148 (1999) 692. [3] F. Roccaforte, M. Gustafsson, W. Bolse, J. Keinonen, K.P. Lieb, Nucl. Instr. and Meth. B 166±167 (2000), in press. [4] M. Gustafsson, F. Roccaforte, J. Keinonen, W. Bolse, L. Ziegeler, K.P. Lieb, Phys. Rev. B, in press. [5] W.K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, Academic Press, Orlando, USA, 1978. [6] M. Uhrmacher et al., Nucl. Instr. and Meth. B 9 (1985) 234. [7] T. Osipowicz, K.P. Lieb, S. Br ussermann, Nucl. Instr. and Meth. B 18 (1987) 232. [8] P. Torri, J. Keinonen, K. Nordlund, Nucl. Instr. and Meth. B 84 (1994) 105. [9] J.F. Ziegler, J.P. Biersack, W. Littmark, The Stopping and Ranges of Ions in Solids, Vol. 1, Pergamon Press, New York, USA, 1985. [10] L.W. Hobbs, Nucl. Instr. and Meth. B 91 (1994) 30.