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Surface analysis with slow positrons
!facuum/volume 42/number Printed in Great Britain
Surface
13lpages
823 to 825/l
analysis
0042-207x/91 s3.00+.00 c 1991 Pergamon Press plc
991
with
slow
positrons
J Paridaens, D Segers, M Dorikens and L Dorikens-Vanpraet, RUG Proeftuinstraat 86, B-9000 Gent, Belgium
Laboratory
for Nuclear
Physics,
The slow positron annihilation technique is introduced. The LINAC based slow positron beam at the Ghent State University is described. A slow positron intensity of 2 x IO7 e+ s -’ is obtained. An overview of the possible surface applications is illustrated.
1. Introduction The use of positron annihilation as a technique for materials research has been known for quite some time. The annihilation characteristics give information on several properties of the material, such as electronic structure or the presence of certain types of defects. The problem with these classical techniques is that it is impossible to obtain some depth resolved information. The reason for this is that p+-emitting isotopes are being used as a positron source. These sources always have a continuous energy spectrum that ends at a maximum energy of typical!y something between 0.5 and 3.0 MeV. For these high energies positrons are implanted as deep as the bulk of the material. which makes surface investigation impossible. However, if one has a monoenergetic positron source with energies variable from 0 to something like 50 keV it is possible to do more selective material investigation. There have been quite a few experimental and theoretical studies of the implantation profiles of positrons in materials for this energy range, so that these profiles are now reasonably well known’. It is then possible, by just selecting the right energy, to implant positrons to a certain depth in the material for example to investigate an interface between two layers of different material or even to do some research on the surface and the near surface regions of a material. Such a technique now exists. This has been made possible by the developments during the last ten years concerning moderator materials for positrons. High energetic positrons are thermalised in metal moderators. After this thermalisation process, which only takes about 10 ps, they diffuse randomly through the material. Some of the positrons are able to reach the surface of the moderator and can then be re-emitted if the moderator has a negative work function for positrons. This is the case in, for example, copper, molybdenum, nickel and tungsten. The energy with which the positrons are re-emitted then equals the work function. In this way it is possible to obtain low energetic positrons with a very narrow energy spread that can be accelerated towards the sample one wants to investigate. There are several possibilities for primary positron sources as well as for moderator set-ups and transport systems. At the University of Ghent there now exists such a slow positron facility and a description of this facility now follows. 2. Description of the Ghent slow positron beam The Ghent slow positron beam is LINAC based. This means that the primary fast positrons are produced by means of an electron
linear accelerator, With this accelerator electrons are accelerated up to an energy of 90 MeV. The electrons are then stopped in a tantalum Bremsstrahlungstarget (Figure 1) thus producing y-rays and, through pair production, positrons and electrons. The fast positrons are stopped in a set of thin tungsten foils. Those are carefully annealed to increase the positron diffusion length and also to obtain a contaminant free surface. Both factors increase the slow positron yield. The moderator foils are biased to a voltage Vmod which is variable between 0 and 250 V and this accelerates the slow positrons towards the transport system. This transport system is entirely magnetic. It consists of a series of 2 m long straight solenoids combined with some Helmholtz coils to bridge the area between the moderator and the first solenoid and the area where the turbo pumps are connected to the tubes. The whole system is pumped down with turbomolecular pumps to a pressure of 10-j Pa. There are also four quarter toroids present in the transport system. It reaches a total length of 41 m and this was necessary because of architectural reasons. It takes the positrons away from the high radiation background of the accelerator to a high vacuum chamber connected to a turbomolecular and a Ti-sublimation pump. An overview of the whole facility is presented in Figure 2. Transport of slow positrons over such a large distance is not evident. In fact the Ghent slow positron beam is only the fifth of its kind in the whole world, the second in Europe. With its 41 m it certainly is the longest. This is why special precautions had to be taken in the construction of this beam, such as, for example, the compensation for the earth’s magnetic field all along the transport system and the installation of numerous kick coils in the bends’.
I
-----_ P -----
l-l
HT
+HT
water
/LJ
Figure 1. The convertor-moderator positron in Ghent.
W-vanes
set-up of the LINAC based slow 823
J Paridaens
ef a/: Surface
analysis
Figure 2. General view of the slow positron beamline
The slow positron intensity in the experimental chamber 1s about 2 x IO’ SK’, for an accelerator current of 85 PA and an electron energy of 45 MeV. The energy distribution of the slow positrons is shown in Figure 3 for a moderator voltage of 200 V. The FWHM is 5 CV which gives a value of AE/E of 2.6%. The slow positron beam is pulsed with a frequency of 300 Hz and a maximum pulse length of 3 ,us because of the accelerator characteristics. This means that in each pulse there arc about 6.7 x 10” slow positrons or an instant intensity of 2.2 x 10’” Bq. This causes tremendous pile-up in all detection systems and this is why an attempt was made to stretch the pulse length of the beam. A Penning trap was installed and it is shown in Figure 4. The intention is to store and confine all the slow positrons produced during one LINAC pulse and then to release them slowly inbetween the pulses. Radical confinement is achieved by an axial magnetic field of 10 mT, axial confinement electrostatically by means of two cylinders which can be biased to voltages exceeding the slow positron energy. In this way confinement times up to I ms have been achieved’. Further improvements on this system are in progress. However, the first results obtained with this Penning trap are sufficient to start the first experiments. 3. Applications of the slow positron technique The field of slow positrons and their applications is comparatively new. During the last decade a number of techniques has been proposed and partly developed. This has also been the case for those methods that concern investigation of surfaces and near
160
180 Energy
190 200 (eV)
210
Figure 3. Energy distribution of the slow positrons after transport. The points represent the measured slow positron intensity as function of their energy. The solid line is the negative derivative of the dotted curve and represents the energy spectrum of the slow positrons.
surfaces. Some of these techniques are more or less similar to more familiar electron techniques. Slow positron techniques arc usually somewhat less easy to do, primarily because of slow positron beam availability and intensity. However. there arc some advantages to them, for example, due to the absence 01‘ exchange effects. REPELS”.’ is an acronym which stands for re-emitted positron energy-loss spectroscopy, which is similar to the more familiar electron energy-loss spectroscopy (EELS). Positrons of a few keV are implanted into a material so that up to 50% of them can diffuse back to its surface. The implantation depth has typically an order of magnitude of 10 nm. depending of course on the specific implantation energy of the positrons and the density of the material. Re-emission of these positrons is then possible if the work function of the material for positrons is negative. This re-emission is clearly a surface effect and the energy of the reemitted positrons is, for example. very much dependent on the
Figure 4. Schematic view of the Penning trap. VI and V3 represent the voltages on the outer cylinders confinement time. V2 is a constant voltage used to influence the positron energy in the trap. 824
170
as function
of time in the ideal case of a 3.3 ms
J Paridaens
et al: Surface
analysis
presence of overlayers on the surface, the presence of surface defects, etc. Another technique is positronium time of flight spectroscopy’.‘. Positronium is a bound state between a positron and an electron. It can be formed, for example, at the surface of a material with a surface electron. The kinetic energy of the Ps is representative of the density of states of the electron from which the Ps is formed. This makes the technique a probe for surface electron structure investigation. PRM, or positron re-emission microscopy”.‘, is another possibility. Low energy positrons bunched in a microspot are implanted in a material and may diffuse back to the surface. There they can be re-emitted and through a series of lenses an enlarged image is formed on a phosphor screen. The re-emission is sensitive to defects, adsorbates, overlayer islands and thin film overlayers. Resolution can be as good as 0.1 nm or even better. LEPD is the acronym for low energy positron diffraction and the technique is very similar to LEED. The main problem here again is the relatively low quality of the positron beam. This can be overcome, however, by remoderating the slow positron beam twice’“. The full power of LEPD is yet to be demonstrated but it looks a very promising new technique which in combination with LEED might lead to interesting results. PAES, or positron induced Auger electron spectroscopy is also an interesting development”. A beam of low energy positrons of the order of about IO eV can be used to create core holes at the surface of a sample. The positron simply annihilates with a core electron thus creating a hole after which an Auger emission can take place. Probabilities for this process can be as high as a few per cent. The technique has some advantages to electron induced Auger electron spectroscopy. The incident particles have very low energy. Therefore, there is no high electron background as is the case in the classical Auger spectroscopy. Moreover, this technique is very surface specific, all Auger electrons are created at the surface of the sample because of the low implantation depth of the slow positrons. A disadvantage might be the problem of energy resolution when looking at the emitted Auger electrons
in the presence of magnetic fields. Since many slow positron beams are magnetically guided on to the sample this is a problem. However, using an i! x B type energy filter this problem has partly been overcome. Other possibilities are measurements of the Ps-fraction” or lifetime measurements on o-Ps’~. Some of these techniques can and will be used in the Ghent slow positron facility during the time to come, thus making from this slow positron beam a quite unique new tool for surface analysis besides lots of other applications such as defect profiling’, studies of interfaces and thin overlayers’ or investigations in the field of atomic and molecular physics“‘.“. Acknowledgement This work is part of the research programme Brussels. Financial support is acknowledged.
of the IIKW-
References
‘P J Schultz and K G Lynn, Rev Mod Phys, 60 (3), 701 (1988). ‘J Paridaens, D Segers, M Dorikens and L Dorikens-Vanprdet, Nucl Instrum Meth, A287, 359 (1990). ‘J Paridaens, D Segers, M Dorikens and L Dorikens-Vanpraet, Nucl Instrum Meth, A295, 39 (1990). ‘D A Fischer, K G Lynn and W E Frieze, Phys Rat? Lett, 50, I 149 (I 983). 5D A Fischer, K G Lynn and D W Gidley, Phys Rev, B33, 4479 (1986). ‘A P Mills Jr and L Pfeiffer. Phys Rev, B32, 53 (I 985). ‘A P Mills Jr, L Pfeiffer and P M Platzman, Ph.vs Rez: Lrtt, 51, 1085 (1983). ‘J Van House and A Rich, Phys Rep Lett, 61,488 (1988). ‘G R Brandes. K F Canter and A P Mills Jr, PhJ.7 Rrr Lett, 61, 492 (1988). I0 W E Frieze, D W Gidley and K G Lynn, Phys Rev, 831, 5628 (1985). ’ ’ A Weiss, R Mayer, M Jibaly, C Lei, D Mehl and K G Lynn, Phy.7 Rev Lett, 61, 2245 (1988). “R Mayer and K G Lynn, Phv.7 Rec. 833, 3507 (1986). ’ ’ C Dauwe, Vacuum (1990). “‘F Ebel, W Faust, C Hahn, M Riickert, H Schneider, A Singe and I Tobehn. Phys Lett, A140, 114 (1989). j5F Ebel, W Faust, C Hahn, M Riickert, H Schneider, A Singe and I Tobehn, Nucl Ins/rum Meth, B50, 328 (1990).
825
Surface
13lpages
823 to 825/l
analysis
0042-207x/91 s3.00+.00 c 1991 Pergamon Press plc
991
with
slow
positrons
J Paridaens, D Segers, M Dorikens and L Dorikens-Vanpraet, RUG Proeftuinstraat 86, B-9000 Gent, Belgium
Laboratory
for Nuclear
Physics,
The slow positron annihilation technique is introduced. The LINAC based slow positron beam at the Ghent State University is described. A slow positron intensity of 2 x IO7 e+ s -’ is obtained. An overview of the possible surface applications is illustrated.
1. Introduction The use of positron annihilation as a technique for materials research has been known for quite some time. The annihilation characteristics give information on several properties of the material, such as electronic structure or the presence of certain types of defects. The problem with these classical techniques is that it is impossible to obtain some depth resolved information. The reason for this is that p+-emitting isotopes are being used as a positron source. These sources always have a continuous energy spectrum that ends at a maximum energy of typical!y something between 0.5 and 3.0 MeV. For these high energies positrons are implanted as deep as the bulk of the material. which makes surface investigation impossible. However, if one has a monoenergetic positron source with energies variable from 0 to something like 50 keV it is possible to do more selective material investigation. There have been quite a few experimental and theoretical studies of the implantation profiles of positrons in materials for this energy range, so that these profiles are now reasonably well known’. It is then possible, by just selecting the right energy, to implant positrons to a certain depth in the material for example to investigate an interface between two layers of different material or even to do some research on the surface and the near surface regions of a material. Such a technique now exists. This has been made possible by the developments during the last ten years concerning moderator materials for positrons. High energetic positrons are thermalised in metal moderators. After this thermalisation process, which only takes about 10 ps, they diffuse randomly through the material. Some of the positrons are able to reach the surface of the moderator and can then be re-emitted if the moderator has a negative work function for positrons. This is the case in, for example, copper, molybdenum, nickel and tungsten. The energy with which the positrons are re-emitted then equals the work function. In this way it is possible to obtain low energetic positrons with a very narrow energy spread that can be accelerated towards the sample one wants to investigate. There are several possibilities for primary positron sources as well as for moderator set-ups and transport systems. At the University of Ghent there now exists such a slow positron facility and a description of this facility now follows. 2. Description of the Ghent slow positron beam The Ghent slow positron beam is LINAC based. This means that the primary fast positrons are produced by means of an electron
linear accelerator, With this accelerator electrons are accelerated up to an energy of 90 MeV. The electrons are then stopped in a tantalum Bremsstrahlungstarget (Figure 1) thus producing y-rays and, through pair production, positrons and electrons. The fast positrons are stopped in a set of thin tungsten foils. Those are carefully annealed to increase the positron diffusion length and also to obtain a contaminant free surface. Both factors increase the slow positron yield. The moderator foils are biased to a voltage Vmod which is variable between 0 and 250 V and this accelerates the slow positrons towards the transport system. This transport system is entirely magnetic. It consists of a series of 2 m long straight solenoids combined with some Helmholtz coils to bridge the area between the moderator and the first solenoid and the area where the turbo pumps are connected to the tubes. The whole system is pumped down with turbomolecular pumps to a pressure of 10-j Pa. There are also four quarter toroids present in the transport system. It reaches a total length of 41 m and this was necessary because of architectural reasons. It takes the positrons away from the high radiation background of the accelerator to a high vacuum chamber connected to a turbomolecular and a Ti-sublimation pump. An overview of the whole facility is presented in Figure 2. Transport of slow positrons over such a large distance is not evident. In fact the Ghent slow positron beam is only the fifth of its kind in the whole world, the second in Europe. With its 41 m it certainly is the longest. This is why special precautions had to be taken in the construction of this beam, such as, for example, the compensation for the earth’s magnetic field all along the transport system and the installation of numerous kick coils in the bends’.
I
-----_ P -----
l-l
HT
+HT
water
/LJ
Figure 1. The convertor-moderator positron in Ghent.
W-vanes
set-up of the LINAC based slow 823
J Paridaens
ef a/: Surface
analysis
Figure 2. General view of the slow positron beamline
The slow positron intensity in the experimental chamber 1s about 2 x IO’ SK’, for an accelerator current of 85 PA and an electron energy of 45 MeV. The energy distribution of the slow positrons is shown in Figure 3 for a moderator voltage of 200 V. The FWHM is 5 CV which gives a value of AE/E of 2.6%. The slow positron beam is pulsed with a frequency of 300 Hz and a maximum pulse length of 3 ,us because of the accelerator characteristics. This means that in each pulse there arc about 6.7 x 10” slow positrons or an instant intensity of 2.2 x 10’” Bq. This causes tremendous pile-up in all detection systems and this is why an attempt was made to stretch the pulse length of the beam. A Penning trap was installed and it is shown in Figure 4. The intention is to store and confine all the slow positrons produced during one LINAC pulse and then to release them slowly inbetween the pulses. Radical confinement is achieved by an axial magnetic field of 10 mT, axial confinement electrostatically by means of two cylinders which can be biased to voltages exceeding the slow positron energy. In this way confinement times up to I ms have been achieved’. Further improvements on this system are in progress. However, the first results obtained with this Penning trap are sufficient to start the first experiments. 3. Applications of the slow positron technique The field of slow positrons and their applications is comparatively new. During the last decade a number of techniques has been proposed and partly developed. This has also been the case for those methods that concern investigation of surfaces and near
160
180 Energy
190 200 (eV)
210
Figure 3. Energy distribution of the slow positrons after transport. The points represent the measured slow positron intensity as function of their energy. The solid line is the negative derivative of the dotted curve and represents the energy spectrum of the slow positrons.
surfaces. Some of these techniques are more or less similar to more familiar electron techniques. Slow positron techniques arc usually somewhat less easy to do, primarily because of slow positron beam availability and intensity. However. there arc some advantages to them, for example, due to the absence 01‘ exchange effects. REPELS”.’ is an acronym which stands for re-emitted positron energy-loss spectroscopy, which is similar to the more familiar electron energy-loss spectroscopy (EELS). Positrons of a few keV are implanted into a material so that up to 50% of them can diffuse back to its surface. The implantation depth has typically an order of magnitude of 10 nm. depending of course on the specific implantation energy of the positrons and the density of the material. Re-emission of these positrons is then possible if the work function of the material for positrons is negative. This re-emission is clearly a surface effect and the energy of the reemitted positrons is, for example. very much dependent on the
Figure 4. Schematic view of the Penning trap. VI and V3 represent the voltages on the outer cylinders confinement time. V2 is a constant voltage used to influence the positron energy in the trap. 824
170
as function
of time in the ideal case of a 3.3 ms
J Paridaens
et al: Surface
analysis
presence of overlayers on the surface, the presence of surface defects, etc. Another technique is positronium time of flight spectroscopy’.‘. Positronium is a bound state between a positron and an electron. It can be formed, for example, at the surface of a material with a surface electron. The kinetic energy of the Ps is representative of the density of states of the electron from which the Ps is formed. This makes the technique a probe for surface electron structure investigation. PRM, or positron re-emission microscopy”.‘, is another possibility. Low energy positrons bunched in a microspot are implanted in a material and may diffuse back to the surface. There they can be re-emitted and through a series of lenses an enlarged image is formed on a phosphor screen. The re-emission is sensitive to defects, adsorbates, overlayer islands and thin film overlayers. Resolution can be as good as 0.1 nm or even better. LEPD is the acronym for low energy positron diffraction and the technique is very similar to LEED. The main problem here again is the relatively low quality of the positron beam. This can be overcome, however, by remoderating the slow positron beam twice’“. The full power of LEPD is yet to be demonstrated but it looks a very promising new technique which in combination with LEED might lead to interesting results. PAES, or positron induced Auger electron spectroscopy is also an interesting development”. A beam of low energy positrons of the order of about IO eV can be used to create core holes at the surface of a sample. The positron simply annihilates with a core electron thus creating a hole after which an Auger emission can take place. Probabilities for this process can be as high as a few per cent. The technique has some advantages to electron induced Auger electron spectroscopy. The incident particles have very low energy. Therefore, there is no high electron background as is the case in the classical Auger spectroscopy. Moreover, this technique is very surface specific, all Auger electrons are created at the surface of the sample because of the low implantation depth of the slow positrons. A disadvantage might be the problem of energy resolution when looking at the emitted Auger electrons
in the presence of magnetic fields. Since many slow positron beams are magnetically guided on to the sample this is a problem. However, using an i! x B type energy filter this problem has partly been overcome. Other possibilities are measurements of the Ps-fraction” or lifetime measurements on o-Ps’~. Some of these techniques can and will be used in the Ghent slow positron facility during the time to come, thus making from this slow positron beam a quite unique new tool for surface analysis besides lots of other applications such as defect profiling’, studies of interfaces and thin overlayers’ or investigations in the field of atomic and molecular physics“‘.“. Acknowledgement This work is part of the research programme Brussels. Financial support is acknowledged.
of the IIKW-
References
‘P J Schultz and K G Lynn, Rev Mod Phys, 60 (3), 701 (1988). ‘J Paridaens, D Segers, M Dorikens and L Dorikens-Vanprdet, Nucl Instrum Meth, A287, 359 (1990). ‘J Paridaens, D Segers, M Dorikens and L Dorikens-Vanpraet, Nucl Instrum Meth, A295, 39 (1990). ‘D A Fischer, K G Lynn and W E Frieze, Phys Rat? Lett, 50, I 149 (I 983). 5D A Fischer, K G Lynn and D W Gidley, Phys Rev, B33, 4479 (1986). ‘A P Mills Jr and L Pfeiffer. Phys Rev, B32, 53 (I 985). ‘A P Mills Jr, L Pfeiffer and P M Platzman, Ph.vs Rez: Lrtt, 51, 1085 (1983). ‘J Van House and A Rich, Phys Rep Lett, 61,488 (1988). ‘G R Brandes. K F Canter and A P Mills Jr, PhJ.7 Rrr Lett, 61, 492 (1988). I0 W E Frieze, D W Gidley and K G Lynn, Phys Rev, 831, 5628 (1985). ’ ’ A Weiss, R Mayer, M Jibaly, C Lei, D Mehl and K G Lynn, Phy.7 Rev Lett, 61, 2245 (1988). “R Mayer and K G Lynn, Phv.7 Rec. 833, 3507 (1986). ’ ’ C Dauwe, Vacuum (1990). “‘F Ebel, W Faust, C Hahn, M Riickert, H Schneider, A Singe and I Tobehn. Phys Lett, A140, 114 (1989). j5F Ebel, W Faust, C Hahn, M Riickert, H Schneider, A Singe and I Tobehn, Nucl Ins/rum Meth, B50, 328 (1990).
825