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Dechanneling due to light ion microbeam induced swelling in single crystals
NuclearInstirmnents &Methods in Physics Research
Nuclear Instruments and Methods in Physics Research B66 (1992) 369-373 North-Holland
Section B
Dechannelling due to light ion microbeam induced swelling in single crystals Sean P . Dooley and David N. Jamieson
Micro Analytical Research Ceture, School of Physics, University of Melbourne, Parkrille 3052, Australia
Received 17 September 1991 and in revised form 8 November 1991
Beam induced swelling can be the most significant cause of dechannelling for He' and H' microprobe measurements of the crystallinity of single crystals for irradiated regions of sizes typical of microprobe scans ( 100x 100 Rm=). Swelling causes dechannelling by tilting the crystal axis at the edge of the irradiated region. The present work shows that this effect only becomes significant, relative to dechannelling from beam induced point defects, above a threshold dose, D,. For 2 MeV He' incident along the (100) axis of Si, D, is - 2x 10 17 /cm` and D, is - 4x 10 17 /cm2 for GaAs. Since swelling induced dechannelling is confined to the edge of the irradiated area, increasing the beam scan size is a useful means of minimizing its effect on Xmm measurements of small regions of crystal . 1 . Introduction In nuclear microprobe analysis, it is vitally important to ensure that changes to the target induced by the analysis beam are kept to a minimum. In the channelling analysis of crystals, there is an increase in the Xmin with ion beam dose, which must be minimized for accurate analysis. This has been attributed to the creation of point defects by the analysis beam [1] . The present work shows that swelling is another major mechanism for increasing the Xmm of an irradiated region under similar dosage conditions to those used in some of the previous studies [1-5]. The large amount of damage produced by an ion beam in the end of range region results in a volume change which causes swelling of the target surface [6,7]. This is due to the greater specific volume of damaged or amorphous material in comparison to undamaged crystal and, to a lesser extent, the incorporation of gas atoms from the ion beam into the lattice [6] . The effect of irradiating a small region of crystal is to raise the surface layers of the crystal in the irradiated area relative to the rest of the crystal as shown in fig . l . At the edge of the irradiated region of the crystal (regions A and C), the height difference must be accommodated by the tilting of the surface layers, and therefore the tilting of the crystal axis at the edge . If the target is being analysed by a beam channelled along the crystal axis of the undamaged material, then the edges of the scanned region will be misoriented with respect to the beam . As the dose of ions increases, the amount of swelling, and hence the degree of tilt
END-OF-RANGE DAMAGE
Fig. I . The swelling produced by damage at the end of range of the beam. The crystal planes are tilted at the edge of the irradiated region (A and C), resulting in dechannelling of the normal beam. Region B remains in alignment with the analysis beam. An "off-normal" beam can channel into region A to measure the misalignment of the crystal axis. The damage curve was calculated from the TRIM 110] program, and is the depth profile of the number of matrix atoms displaced from lattice sites by the beam . Although the large amount of damage at the end of range cannot affect channelling measurements directly, it does cause swelling.
0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
370
S.P. Dooley, D.N. Jamieson / Dechannelling due to beans induced swelling
and/or the width of the tilted region must increase . When the tilt of the edge region reaches only a few tenths of a degree, the beam at the edge will be significantly dechaanclled . The dose at which significant dechannelling at the edge of the irradiated region first occurs, which we define as D depends on the critical angle für channelling 41, and the susceptibility of the crystal to swelling and tilting. These factors depend on the crystal properties, crystal temperature, beam type and beam energy. The effect of swelling on a plot of Xmm against dose for the irradiation of a small region of crystal is thus to cause a rapid increase in measured X.,,, starting at D, This rise is more dramatic for smailler irradiated regions, where the edge comprises a greater fraction of the irradiated area . Channelling contrast microscopy (CCM) [5] has not been used previously to determine the effect of swelling on microbeam channelling measurements although it has been used to image microprobe induced damage [4] at doses above D, The CCM images presented in this report clearly show that swelling is the most significant dechannelling mechanism in microbeam channelling experiments carried out at doses above D, for small scans . Spectra were extracted from subregions of these maps, allowing quantitative Xmm measurements of features on the maps.
-400
c 300
v
200 " ~ 100 = 0
-~i -50
0 Position
(Um)
50
2 . Experimental An optical micrograph of the swelling produced by a 2 MeV He' microbeam scanned over a 60 wm square to a total dose of 10 1 "/cm= (- 5D) is shown in fig . 2a. The target was (100) Si with a 100 nm epitaxial CoSi, layer on it. The irradiation was performed along the <100> axis at room temperature . The micrograph was taken with a light source positioned so that there was almost a specular reflection off the target surface, producing high contrast for small variations in surface orientation that could not be obtained with scanning electron microscopy . A Sloane Dektak trace of the irradiated region (fig. 2b) shows that it was swollen to a height of 320 nm, with the centre of the irradiated region remaining relatively flat . A CCM image of the damaged region is shown in fig. 2c . The edges of the scan show far more backscattering counts due to swelling induced misalignment . The extent of the dechannelling was not the same for all edges as the beam was not perfectly aligned with the axis. To determine the relative importance of swelling and point defect creation as mechanisms for dechannelling in Si, a (100) oriented silicon single crystal was irradiated at room temperature by a 2 McV He' microbeam incident along the (100) axis, and scanned in a 60 Wm square. The Xmin of the edge (50% of the scan area), centre (25%) and the total Xm,n for the scan
Fig. 2. (a) An optical micrograph, (b) a Sloane Dektak profile and (c) a CCM image, of the swelling produced in a 60x60 Wm square by a dose of 10 18 2 MeV He + /cml on a (100) oriented 100 nm epitaxial CoSi, layer on a Si substrate. The CCM image was taken with a 180x 180 wm scan in the normal direction, and shows severe loss of channelling at the edge of the swollen square (light region) but good channelling in the centre of the swollen square and the background (dark region). Measurements by "off-normal" CCM imaging found that the crystal axis at the edge of the swelling was tilted by about 0.9° away from the axis of undamaged crystal, in agreement with estimates of the slope of the edge from (b). The Dektak profile and optical micrograph cover the same region as the CCM image .
S.P. Dooley, D.N. Jamieson / Dechannelling due to beam induced swelling
Fig. 3. The increase in Xmm with dose of a 60X60 Wm square of silicon irradiated by a 2 MeV He' beam. The Xmm was measured at the centre (triangles) and the edge (stars) of the irradiated region. The drastic increase in Xmm at the edge observed above D, is due to swelling induced misalignment. The area weighted sum (squares) corresponds to the Xmeasured with the entire scan. This rises drastically above D, due to the swelling induced misalignment at the edge . The lines are to guide the eye . is plotted as a function of dose in fig . 3. The Xmin was computed from spectra extracted from the regions of highest and lowest counts of CCM images taken from
37 1
the scans that were actually used to inflict the damage. At the edge, swelling induced misalignment was the dominant dechannelling mechanism, while point defect accumulation was the dominant mechanism in the centre . At a dose of 4 .3 X 10 17 /em=, which was just greater than 2Ds, the Xmm at the edge was 23% compared to 5 .0% at the centre and 3 .5% for virgin crystal. The area weighted average Xmm of the scan was 16 .6%, so the increase was clearly dominated by swelling induced dechannelling. To show that the dechannelling at the edge of the irradiated region was due to misalignment of the crystal rather than severe crystal damage, the sample was tilted so that the beam channelled into an edge of the swollen region, as shown by the "off-normal" beam in fig . 1 . The Si sample described above was tilted by 0.5° around the (010) axis and an "off-normal" CCM image was taken (fig. 4a). Spectra extracted from the regions in this image are shown in fig. 4b. These data show substantially planar channelling in the background, centre and right edge of the original 60 pm square scan. The left vertical edge shows far fewer counts because the beam was axially channelling. The upper and lower edges show a greater backscattering yield because these edges were tilted away from the planar channel by swelling. Dektak profiles and optical microscopy showed that the surface of this crystal was swollen to a height of 200 ± 10 nm, and that the
Fig. 4. An "off-normal" CCM image of the swelling produced in a 60x60 Wm square (indicated by black outline) irradiated by a dose of 5X 10 17 He '/cm z in (100) oriented silicon. The sample was oriented so that an off normal analysis beam channeled into the tilted crystal on the left edge of the swollen region (dark region in the CCM image) . This edge corresponds to region A in fig. 1 . The required tilt was 0.5° from the (100) normal axis. With this tilt the analysis beam undergoes near planar channelling in most of the crystal (mottled grey regions), represented by regions B, C and the undamaged crystal in fig . l . The tilted crystal on the upper and lower edges of the swollen region has the axis even more misaligned with the beam (light regions) . (b) shows spectra extracted from the sub-regions in the CCM image . The spectrum from the left edge (dark region) shows a spectrum characteristic of axial channelling (squares). The upper and lower edges (light regions) are tilted out of the planar alignment, and produce a yield (triangles) which is near that of randomly oriented crystal . The background and right edge (mottled grey regions) show a spectrum similar to that of planar channelling (circles). Spectra from (100) aligned virgin crystal and randomly oriented crystal (histograms) are shown for comparison.
372
S. P. D.oley, D.N. Jamies., l Dccluunnclli,g due to beam induced swelling 60 50 40
u Area weighted sum Centre Edge
liminary measurements [8], of dechannelling in (110) oriented diamond irradiated by a 1 .4 MeV H' beam scanned over a 32 Wm square, showed that swelling was the only significant mechanism for dechannelling . In general, dug to the smaller critical angle for H' channelling and much lower rate of point defect production, the relative importance of dechannelling due to swelling is much greater for H' beams than for He' beams.
i d
c_30 20 10
00
3 Discussion 20
40
60
80
100
Dose (10îe/Cm2) Fig . 5 . The increase in Xn n with dose of a 611X611 gm square of GaAs irradiated by a scanned 2 MeV He' microbeam. The Xmm was measured at the centre (triangles) and the edge (stars) of the irradiated region at a dose of 9 X 10 16 He =/cm = . The increase in Xm,n at the centre is larger than in Si due to the greater rate of point defect creation in GaAs. Swelling is still a significant dechannelling mechanism at high doses, as the increase in Xn,in at the edge is much greater than in the centre . The lines are to guide the eye. surface at the edges was tilted by 0 .57°, which was sufficient to cause significant dechannelling. The swelling produced in the crystal shown in fig . 2 was also imaged with an "off-normal" beam . Optimum channeling into the edge of this swollen region occurred at a tilt of 0.9° from the (100) axis, consistent with the greater dose this sample received . The same dechanncling mechanisms were found to apply to GaAs, but the rate of point defect creation by the beam was about 20 times greater than in Si . A GaAs crystal at room temperature was irradiated along the <100) axis by a 2 McV He' microbeam scanned in a 60 wm square at a beam current of 1 nA . The increase in Xm,n is plotted against dose in fig. 5 . This shows similar behaviour to that for Si, except that there is a much greater rise in Xm,n at low doses and in the centre of the scan, due to a much higher point defect production rate . From a CCM image taken at a dose of 9 .3 X 10' 7 /cm= , the Xm,n was measured to be increased by 26% at the centre and 49% at the edge . The dechannelling mechanisms of point defect creation anc: swelling induced misalignment make comparable contributions to the overall dechannelling at high dose, for this scan size . The value of D, for 2 MeV He' on GaAs is 4 X 10 17 /cm= . As far as dose limitations for analysis are concerned, dechannelling due to point defects is the major problem, but that can be minimized with lower beam fluxes [1]. Swelling effects due to H' beams have been observed in many crystals [6] and can thus be expected to produce dechannelling at the edges of scans . Our pre-
The present results suggest that the dechannelling produced by the surface swelling had a significant effect on the results of previous MeV energy He + ion beam damage studies on silicon carried out with high doses [1-5] . All of these previous studies were carried out with small regions of irradiation (size < 100 wm) where the present study shows that swelling induced misalignment is the dominant dechannelling mechanism. The previous studies showed a slow increase in - 2 X 10 17 He 2/cm = Xm,n of Si until a critical dose (D, at 2 McV) where the Xm,n rapidly increased . Previous workers suggested that this rapid increase was due to the trapping of defects at stable damage centres [1], but the present measurements show that this effect can be explained by the onset of swelling induced misalignment . Other studies [9] did not reach D and so were not significantly affected by swelling. Recent work on 400 keV He' irradiation of Si revealed that the onset of a rapid rise in Xmin occurs at a dose of 1 X 10 1"/cm =, reaching the yield expected for random orientation by a dose of 2 .5 X 10 1"/cm'- for a 40 wm X 30 Wm scan [4] . These doses were correlated with the dose at which surface swelling was observed, but dechannelling mechanisms were not discussed . The higher critical dose reported for a 400 keV beam is consistent with the present 2 MeV result due to the larger critical angle for channelling at lower energy, which means that more swelling is required to induce dechannelling. 4. Conclusions The effect of dechannelling due to swelling induced misalignment at the edge of the irradiated region is not a significant problem for large scans or low doses used in routine microbeam channelling analysis. It is significant, and hitherto largely ignored, in damage studies [1 -3,5] where high doses or small scans arc employed, leading to excessively conservative dose limits being proposed. f n CCM analysis at doses above D the edges of the scan should be about 30 wm clear of any structure of interest to avoid the dechannelling caused
S.P. Dooley, D.N. Jarnieson / Dechannelling due to beam induced swelling by the swelling induced tilting of the crystal axis at the edge. Such a strategy allows higher doses to be used
than those suggested from a simple extrapolation of the results of damage studies [1,21 carried out on regions of a few tens of microns .
Acknowledgements S.P. Dooley wishes to acknowledge financial support from an Australian Postgraduate Research Award. This work was supported by a University of Melbourne Special Initiative Grant.
References [11 R.A. Brown, J.C. McCallum and J.S. Williams, Nucl . Instr. and Meth. B54 (1991) 197.
373
[21 J .C. McCallum, PhD Thesis, University of Melbourne. (1988) unpublished . [3] J. Thornton, R.E . Harper and D.M. Albury, Nucl . Instr . and Meth. B29 (1987) 515. [41 K. Inoue, M. Takai, K. Ishibashi, K. Hirai, Y. Kawata and S. Namba, Nucl . Instr. and Meth . B54 (1991) 231. [5) J.C . McCallum, R.A. Brown, E. Nygren, J .S . Williams and G.L. Olson, Mater. Res. Soc. Symp. Proc. 69 (1986) 305. [61 C. Ascheron, A. Schindler, R. Flagmeyer and G. Otto, Nucl. Instr. and Meth . B36 (1979) 163. [71 M. Takai, K. Hirai, A. Kinomura, S. Manba, K. Ishibashi, K. Inoue and Y. Kawata, Nucl. Insu. and Meth . B54 (1991) 209. [81 S.P . Dooley (unpublished) [91 M. Piette and F. Bodart, Nucl. lnstr. and Meth. B54 (1991) 204. [101 J.B. Biersack and L.G . Haggmark, Nucl. Instr. and Meth . 174 (1980) 257.
Nuclear Instruments and Methods in Physics Research B66 (1992) 369-373 North-Holland
Section B
Dechannelling due to light ion microbeam induced swelling in single crystals Sean P . Dooley and David N. Jamieson
Micro Analytical Research Ceture, School of Physics, University of Melbourne, Parkrille 3052, Australia
Received 17 September 1991 and in revised form 8 November 1991
Beam induced swelling can be the most significant cause of dechannelling for He' and H' microprobe measurements of the crystallinity of single crystals for irradiated regions of sizes typical of microprobe scans ( 100x 100 Rm=). Swelling causes dechannelling by tilting the crystal axis at the edge of the irradiated region. The present work shows that this effect only becomes significant, relative to dechannelling from beam induced point defects, above a threshold dose, D,. For 2 MeV He' incident along the (100) axis of Si, D, is - 2x 10 17 /cm` and D, is - 4x 10 17 /cm2 for GaAs. Since swelling induced dechannelling is confined to the edge of the irradiated area, increasing the beam scan size is a useful means of minimizing its effect on Xmm measurements of small regions of crystal . 1 . Introduction In nuclear microprobe analysis, it is vitally important to ensure that changes to the target induced by the analysis beam are kept to a minimum. In the channelling analysis of crystals, there is an increase in the Xmin with ion beam dose, which must be minimized for accurate analysis. This has been attributed to the creation of point defects by the analysis beam [1] . The present work shows that swelling is another major mechanism for increasing the Xmm of an irradiated region under similar dosage conditions to those used in some of the previous studies [1-5]. The large amount of damage produced by an ion beam in the end of range region results in a volume change which causes swelling of the target surface [6,7]. This is due to the greater specific volume of damaged or amorphous material in comparison to undamaged crystal and, to a lesser extent, the incorporation of gas atoms from the ion beam into the lattice [6] . The effect of irradiating a small region of crystal is to raise the surface layers of the crystal in the irradiated area relative to the rest of the crystal as shown in fig . l . At the edge of the irradiated region of the crystal (regions A and C), the height difference must be accommodated by the tilting of the surface layers, and therefore the tilting of the crystal axis at the edge . If the target is being analysed by a beam channelled along the crystal axis of the undamaged material, then the edges of the scanned region will be misoriented with respect to the beam . As the dose of ions increases, the amount of swelling, and hence the degree of tilt
END-OF-RANGE DAMAGE
Fig. I . The swelling produced by damage at the end of range of the beam. The crystal planes are tilted at the edge of the irradiated region (A and C), resulting in dechannelling of the normal beam. Region B remains in alignment with the analysis beam. An "off-normal" beam can channel into region A to measure the misalignment of the crystal axis. The damage curve was calculated from the TRIM 110] program, and is the depth profile of the number of matrix atoms displaced from lattice sites by the beam . Although the large amount of damage at the end of range cannot affect channelling measurements directly, it does cause swelling.
0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
370
S.P. Dooley, D.N. Jamieson / Dechannelling due to beans induced swelling
and/or the width of the tilted region must increase . When the tilt of the edge region reaches only a few tenths of a degree, the beam at the edge will be significantly dechaanclled . The dose at which significant dechannelling at the edge of the irradiated region first occurs, which we define as D depends on the critical angle für channelling 41, and the susceptibility of the crystal to swelling and tilting. These factors depend on the crystal properties, crystal temperature, beam type and beam energy. The effect of swelling on a plot of Xmm against dose for the irradiation of a small region of crystal is thus to cause a rapid increase in measured X.,,, starting at D, This rise is more dramatic for smailler irradiated regions, where the edge comprises a greater fraction of the irradiated area . Channelling contrast microscopy (CCM) [5] has not been used previously to determine the effect of swelling on microbeam channelling measurements although it has been used to image microprobe induced damage [4] at doses above D, The CCM images presented in this report clearly show that swelling is the most significant dechannelling mechanism in microbeam channelling experiments carried out at doses above D, for small scans . Spectra were extracted from subregions of these maps, allowing quantitative Xmm measurements of features on the maps.
-400
c 300
v
200 " ~ 100 = 0
-~i -50
0 Position
(Um)
50
2 . Experimental An optical micrograph of the swelling produced by a 2 MeV He' microbeam scanned over a 60 wm square to a total dose of 10 1 "/cm= (- 5D) is shown in fig . 2a. The target was (100) Si with a 100 nm epitaxial CoSi, layer on it. The irradiation was performed along the <100> axis at room temperature . The micrograph was taken with a light source positioned so that there was almost a specular reflection off the target surface, producing high contrast for small variations in surface orientation that could not be obtained with scanning electron microscopy . A Sloane Dektak trace of the irradiated region (fig. 2b) shows that it was swollen to a height of 320 nm, with the centre of the irradiated region remaining relatively flat . A CCM image of the damaged region is shown in fig. 2c . The edges of the scan show far more backscattering counts due to swelling induced misalignment . The extent of the dechannelling was not the same for all edges as the beam was not perfectly aligned with the axis. To determine the relative importance of swelling and point defect creation as mechanisms for dechannelling in Si, a (100) oriented silicon single crystal was irradiated at room temperature by a 2 McV He' microbeam incident along the (100) axis, and scanned in a 60 Wm square. The Xmin of the edge (50% of the scan area), centre (25%) and the total Xm,n for the scan
Fig. 2. (a) An optical micrograph, (b) a Sloane Dektak profile and (c) a CCM image, of the swelling produced in a 60x60 Wm square by a dose of 10 18 2 MeV He + /cml on a (100) oriented 100 nm epitaxial CoSi, layer on a Si substrate. The CCM image was taken with a 180x 180 wm scan in the normal direction, and shows severe loss of channelling at the edge of the swollen square (light region) but good channelling in the centre of the swollen square and the background (dark region). Measurements by "off-normal" CCM imaging found that the crystal axis at the edge of the swelling was tilted by about 0.9° away from the axis of undamaged crystal, in agreement with estimates of the slope of the edge from (b). The Dektak profile and optical micrograph cover the same region as the CCM image .
S.P. Dooley, D.N. Jamieson / Dechannelling due to beam induced swelling
Fig. 3. The increase in Xmm with dose of a 60X60 Wm square of silicon irradiated by a 2 MeV He' beam. The Xmm was measured at the centre (triangles) and the edge (stars) of the irradiated region. The drastic increase in Xmm at the edge observed above D, is due to swelling induced misalignment. The area weighted sum (squares) corresponds to the Xmeasured with the entire scan. This rises drastically above D, due to the swelling induced misalignment at the edge . The lines are to guide the eye . is plotted as a function of dose in fig . 3. The Xmin was computed from spectra extracted from the regions of highest and lowest counts of CCM images taken from
37 1
the scans that were actually used to inflict the damage. At the edge, swelling induced misalignment was the dominant dechannelling mechanism, while point defect accumulation was the dominant mechanism in the centre . At a dose of 4 .3 X 10 17 /em=, which was just greater than 2Ds, the Xmm at the edge was 23% compared to 5 .0% at the centre and 3 .5% for virgin crystal. The area weighted average Xmm of the scan was 16 .6%, so the increase was clearly dominated by swelling induced dechannelling. To show that the dechannelling at the edge of the irradiated region was due to misalignment of the crystal rather than severe crystal damage, the sample was tilted so that the beam channelled into an edge of the swollen region, as shown by the "off-normal" beam in fig . 1 . The Si sample described above was tilted by 0.5° around the (010) axis and an "off-normal" CCM image was taken (fig. 4a). Spectra extracted from the regions in this image are shown in fig. 4b. These data show substantially planar channelling in the background, centre and right edge of the original 60 pm square scan. The left vertical edge shows far fewer counts because the beam was axially channelling. The upper and lower edges show a greater backscattering yield because these edges were tilted away from the planar channel by swelling. Dektak profiles and optical microscopy showed that the surface of this crystal was swollen to a height of 200 ± 10 nm, and that the
Fig. 4. An "off-normal" CCM image of the swelling produced in a 60x60 Wm square (indicated by black outline) irradiated by a dose of 5X 10 17 He '/cm z in (100) oriented silicon. The sample was oriented so that an off normal analysis beam channeled into the tilted crystal on the left edge of the swollen region (dark region in the CCM image) . This edge corresponds to region A in fig. 1 . The required tilt was 0.5° from the (100) normal axis. With this tilt the analysis beam undergoes near planar channelling in most of the crystal (mottled grey regions), represented by regions B, C and the undamaged crystal in fig . l . The tilted crystal on the upper and lower edges of the swollen region has the axis even more misaligned with the beam (light regions) . (b) shows spectra extracted from the sub-regions in the CCM image . The spectrum from the left edge (dark region) shows a spectrum characteristic of axial channelling (squares). The upper and lower edges (light regions) are tilted out of the planar alignment, and produce a yield (triangles) which is near that of randomly oriented crystal . The background and right edge (mottled grey regions) show a spectrum similar to that of planar channelling (circles). Spectra from (100) aligned virgin crystal and randomly oriented crystal (histograms) are shown for comparison.
372
S. P. D.oley, D.N. Jamies., l Dccluunnclli,g due to beam induced swelling 60 50 40
u Area weighted sum Centre Edge
liminary measurements [8], of dechannelling in (110) oriented diamond irradiated by a 1 .4 MeV H' beam scanned over a 32 Wm square, showed that swelling was the only significant mechanism for dechannelling . In general, dug to the smaller critical angle for H' channelling and much lower rate of point defect production, the relative importance of dechannelling due to swelling is much greater for H' beams than for He' beams.
i d
c_30 20 10
00
3 Discussion 20
40
60
80
100
Dose (10îe/Cm2) Fig . 5 . The increase in Xn n with dose of a 611X611 gm square of GaAs irradiated by a scanned 2 MeV He' microbeam. The Xmm was measured at the centre (triangles) and the edge (stars) of the irradiated region at a dose of 9 X 10 16 He =/cm = . The increase in Xm,n at the centre is larger than in Si due to the greater rate of point defect creation in GaAs. Swelling is still a significant dechannelling mechanism at high doses, as the increase in Xn,in at the edge is much greater than in the centre . The lines are to guide the eye. surface at the edges was tilted by 0 .57°, which was sufficient to cause significant dechannelling. The swelling produced in the crystal shown in fig . 2 was also imaged with an "off-normal" beam . Optimum channeling into the edge of this swollen region occurred at a tilt of 0.9° from the (100) axis, consistent with the greater dose this sample received . The same dechanncling mechanisms were found to apply to GaAs, but the rate of point defect creation by the beam was about 20 times greater than in Si . A GaAs crystal at room temperature was irradiated along the <100) axis by a 2 McV He' microbeam scanned in a 60 wm square at a beam current of 1 nA . The increase in Xm,n is plotted against dose in fig. 5 . This shows similar behaviour to that for Si, except that there is a much greater rise in Xm,n at low doses and in the centre of the scan, due to a much higher point defect production rate . From a CCM image taken at a dose of 9 .3 X 10' 7 /cm= , the Xm,n was measured to be increased by 26% at the centre and 49% at the edge . The dechannelling mechanisms of point defect creation anc: swelling induced misalignment make comparable contributions to the overall dechannelling at high dose, for this scan size . The value of D, for 2 MeV He' on GaAs is 4 X 10 17 /cm= . As far as dose limitations for analysis are concerned, dechannelling due to point defects is the major problem, but that can be minimized with lower beam fluxes [1]. Swelling effects due to H' beams have been observed in many crystals [6] and can thus be expected to produce dechannelling at the edges of scans . Our pre-
The present results suggest that the dechannelling produced by the surface swelling had a significant effect on the results of previous MeV energy He + ion beam damage studies on silicon carried out with high doses [1-5] . All of these previous studies were carried out with small regions of irradiation (size < 100 wm) where the present study shows that swelling induced misalignment is the dominant dechannelling mechanism. The previous studies showed a slow increase in - 2 X 10 17 He 2/cm = Xm,n of Si until a critical dose (D, at 2 McV) where the Xm,n rapidly increased . Previous workers suggested that this rapid increase was due to the trapping of defects at stable damage centres [1], but the present measurements show that this effect can be explained by the onset of swelling induced misalignment . Other studies [9] did not reach D and so were not significantly affected by swelling. Recent work on 400 keV He' irradiation of Si revealed that the onset of a rapid rise in Xmin occurs at a dose of 1 X 10 1"/cm =, reaching the yield expected for random orientation by a dose of 2 .5 X 10 1"/cm'- for a 40 wm X 30 Wm scan [4] . These doses were correlated with the dose at which surface swelling was observed, but dechannelling mechanisms were not discussed . The higher critical dose reported for a 400 keV beam is consistent with the present 2 MeV result due to the larger critical angle for channelling at lower energy, which means that more swelling is required to induce dechannelling. 4. Conclusions The effect of dechannelling due to swelling induced misalignment at the edge of the irradiated region is not a significant problem for large scans or low doses used in routine microbeam channelling analysis. It is significant, and hitherto largely ignored, in damage studies [1 -3,5] where high doses or small scans arc employed, leading to excessively conservative dose limits being proposed. f n CCM analysis at doses above D the edges of the scan should be about 30 wm clear of any structure of interest to avoid the dechannelling caused
S.P. Dooley, D.N. Jarnieson / Dechannelling due to beam induced swelling by the swelling induced tilting of the crystal axis at the edge. Such a strategy allows higher doses to be used
than those suggested from a simple extrapolation of the results of damage studies [1,21 carried out on regions of a few tens of microns .
Acknowledgements S.P. Dooley wishes to acknowledge financial support from an Australian Postgraduate Research Award. This work was supported by a University of Melbourne Special Initiative Grant.
References [11 R.A. Brown, J.C. McCallum and J.S. Williams, Nucl . Instr. and Meth. B54 (1991) 197.
373
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