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Charge state ratios for helium ions channelled through silicon
Volume 50A, number 4
PHYSICS LETTERS
16 December 1974
CHARGE STATE RATIOS FOR HELIUM IONS CHANNELLED THROUGH SILICON R.J. PETTY and G. DEARNALEY AERE, Harwell, Didcot, UK Received 2 November 1974 4He+f/4He+ is measured for helium ions emerging from channelled and random directions in silicon for The ratio the energy range 0.4—1.2 MeV. The ratio is observed to be higher for the (110) channel direction than for the (111) channel or random directions.
Measurements of the ratio of the stopping powers of fast ions traversing silicon crystals in channelled and random directions have shown that the minimum ratio for 4He ions is actually less than that for protons [2—4].It has been suggested [1] that this may be due to a significantly different charge state distribution for helium ions in channelled, compared with random, directions. In this paper we describe a measurement of the ratio He~/He+for both channelled and random directions: A 4He~beam, collimated to 0.03°divergence, was transmitted through a thin silicon crystal target (mounted on a 3-axis goniometer) and into a 60°cylindrical electrostatic analyser (with entrance slits aligned with the beam direction and subtending an angle of 0.03°at the target) and thence into a channel electron multiplier detector placed at the analyser focus. The analyser electrostatic field was varied continuously from the minimum (about 3 ky/cm) to the maximum (about 45 ky/cm) and the pulses from the channel electron multiplier detector were accumulated in a multichannel analyser addressed by the plate voltage. Thus the channel number (proportional to field strength) gave a direct measure of the energy/unit charge of the detected ions. Beam monitoring was carned out by detecting ions backscattered from the target in a surface barrier detector and accumulating the output in a second multichannel analyser addressed by the same ADC as the first. Division of the energy/ unit charge spectrum by the beam monitor spectrum then allowed for beam fluctuations, electronic lifetime variations and non-linearities in the electrostatic analyser plate voltage scanning system. Several crystals (about 3ji thick) were used in the
experiment, alignment of the crystal axes being obtamed by observing the “star pattern” of the transmitted ions on a luminescent screen and checked by observing their energy spectrum. The geometry of the experiment was such that only very well channelled ions were detected. The random directions were ohtamed by tilting the crystal a few degrees off the axial direction. Both faces of the crystals were used as the ion entry face and all crystals were several months old (i.e., each face must have had 30—40A of amorphous oxide [5]). These conditions preclude the possibility that the charge state differences observed are due to surface effects. The measured ratios He/He~ for beams parallel to the (110) and (111) axes and for random directions are plotted in fig. 1 as a function of the square of the transmitted beam energy. The results for the random direction are in very close agreement with those of Dissanaike [6] and of Armstrong et al. [7] and show the same E2 dependence. The (111> channel results are indistinguishable from the random results while the (110) channel results show a He~/He+ratio which, starting from equality (at about 400keV), becomes increasingly greater than for the (ill) channel and random directions. It should be noted here that in the channelled cases, as only well-channelled ions are detected, the impact parameters for interaction with the silicon atoms are restricted to (approximately) the channel radii (about 1 A and 2A respectively for the (111) and (110) channels). The difference between the resuits for the two channels thus implies a strong im273
Volume 50A. number 4
P1 IYSICS LETTERS
16 December 1974
50 RANDOM DIRECTIONS
+
0
• <111> AXIS 0 <110> AXIS He
0
0
3.O~
2.4
00
+
4 •4-+•
0
to
••
0*00
0
+4.
~ ~
0
0~
-~-
02
03
01,
0.5
06
2 07 E EXIT
08 (MeV)2
09
10
11
1.2
13
1.1.
Fig. 1. Charge state ratios for 4Ue ions after traversing silicon crystals as a function of energy squared.
pact parameter dependence in the electron capture/ loss processes, while the increasing difference with energy clearly implies a velocity dependence. It is to be expected that the probability of electron loss in the reduced electron density in channels will be significantly diminished but it is known that the probability does not vary strongly over the range of impact parameters considered here [11]). Thus the explanation of the results must lie in the electron capture process. Drisko [10] has shown that, for the proton-hydrogen change exchange case, the electron capture
random stopping power ratios in terms of the observed charge state distributions, the results obtained by other laboratories [8,9] for channelled charge state distributions of heavier ions show similar behaviour and consequently increase our confidence in the present results. The authors would like to thank Dr. K. Dettmann for many helpful discussions.
References
cross-section has the form °~
1] K. Dettmann, private communication.
=f 2xb ~2L ~i6r b~ vJ La~i .K
2
b
Ui db
(1)
2[~_.~~1
where b = impact parameter, aB and UB = and velocity respectively, v = ion velocity
Bohr radius and K2 is a
modified Bessel function. From this it can be shown that, at low velocities the capture cross-section is significant even for quite large impact parameters while as v is increased capture becomes limited to smaller and smaller impact parameters —- in agreement with the results obtained.
If a dependence of this nature is the explanation of the results then, at higher energies, the He~/He~ ratio in the (11 1) channel should become greater than
in random directions. An experiment is in progress to test this prediction. While it is difficult to understand the channelled/
274
[2] F. Fisen, G.J. Garke, J. B~ttigerand J.M. Poate, AERE report R-6949. G. Della Mes, A.V. Drigo, S. Lo Russo, P. Mazzoldi and ~ Bentini, Phys. Rev. Lett. 27 (1971) 1194. 14] G. Della Mes, A.V. Drigo, S. Lo Russo, P. Mazzoldi and
131
G.G. Bentini, Rad. Eff. 13(1972)115. R.J. Archer, J. Electrochem. Soc. 104 (1947) 619. [61 GA. Dissanaike, Phil. Mag. 7 (1953) 1051. [7] J.C. Armstrong, J.V. Mullendore, W.R. Flarris and ill. Marion, Proc. Phys. Soc. 86 (1965) 1283. [8] S. Datz, LW. Martin, C.D. Moak, B.R. Appleton and L.B. Bridwell, Rad. Eff. 12 (1972) 163. 19] C.D. Moak, 5.Datz, B.R. Appleton, J.A. Biggcrstaff,
151
M.D. Brown, H.F. Krause and T.S. Noggle, Phys. Rev., to be published. [10] R.M. Drisko, Ph.D. Thesis, Carnegie Inst. of Technology (1955), unpublished. [11] Electronic and ionic impact phenomena, Vol. IV. Massey and Gilbody (Clarendon Press, 1974).
PHYSICS LETTERS
16 December 1974
CHARGE STATE RATIOS FOR HELIUM IONS CHANNELLED THROUGH SILICON R.J. PETTY and G. DEARNALEY AERE, Harwell, Didcot, UK Received 2 November 1974 4He+f/4He+ is measured for helium ions emerging from channelled and random directions in silicon for The ratio the energy range 0.4—1.2 MeV. The ratio is observed to be higher for the (110) channel direction than for the (111) channel or random directions.
Measurements of the ratio of the stopping powers of fast ions traversing silicon crystals in channelled and random directions have shown that the minimum ratio for 4He ions is actually less than that for protons [2—4].It has been suggested [1] that this may be due to a significantly different charge state distribution for helium ions in channelled, compared with random, directions. In this paper we describe a measurement of the ratio He~/He+for both channelled and random directions: A 4He~beam, collimated to 0.03°divergence, was transmitted through a thin silicon crystal target (mounted on a 3-axis goniometer) and into a 60°cylindrical electrostatic analyser (with entrance slits aligned with the beam direction and subtending an angle of 0.03°at the target) and thence into a channel electron multiplier detector placed at the analyser focus. The analyser electrostatic field was varied continuously from the minimum (about 3 ky/cm) to the maximum (about 45 ky/cm) and the pulses from the channel electron multiplier detector were accumulated in a multichannel analyser addressed by the plate voltage. Thus the channel number (proportional to field strength) gave a direct measure of the energy/unit charge of the detected ions. Beam monitoring was carned out by detecting ions backscattered from the target in a surface barrier detector and accumulating the output in a second multichannel analyser addressed by the same ADC as the first. Division of the energy/ unit charge spectrum by the beam monitor spectrum then allowed for beam fluctuations, electronic lifetime variations and non-linearities in the electrostatic analyser plate voltage scanning system. Several crystals (about 3ji thick) were used in the
experiment, alignment of the crystal axes being obtamed by observing the “star pattern” of the transmitted ions on a luminescent screen and checked by observing their energy spectrum. The geometry of the experiment was such that only very well channelled ions were detected. The random directions were ohtamed by tilting the crystal a few degrees off the axial direction. Both faces of the crystals were used as the ion entry face and all crystals were several months old (i.e., each face must have had 30—40A of amorphous oxide [5]). These conditions preclude the possibility that the charge state differences observed are due to surface effects. The measured ratios He/He~ for beams parallel to the (110) and (111) axes and for random directions are plotted in fig. 1 as a function of the square of the transmitted beam energy. The results for the random direction are in very close agreement with those of Dissanaike [6] and of Armstrong et al. [7] and show the same E2 dependence. The (111> channel results are indistinguishable from the random results while the (110) channel results show a He~/He+ratio which, starting from equality (at about 400keV), becomes increasingly greater than for the (ill) channel and random directions. It should be noted here that in the channelled cases, as only well-channelled ions are detected, the impact parameters for interaction with the silicon atoms are restricted to (approximately) the channel radii (about 1 A and 2A respectively for the (111) and (110) channels). The difference between the resuits for the two channels thus implies a strong im273
Volume 50A. number 4
P1 IYSICS LETTERS
16 December 1974
50 RANDOM DIRECTIONS
+
0
• <111> AXIS 0 <110> AXIS He
0
0
3.O~
2.4
00
+
4 •4-+•
0
to
••
0*00
0
+4.
~ ~
0
0~
-~-
02
03
01,
0.5
06
2 07 E EXIT
08 (MeV)2
09
10
11
1.2
13
1.1.
Fig. 1. Charge state ratios for 4Ue ions after traversing silicon crystals as a function of energy squared.
pact parameter dependence in the electron capture/ loss processes, while the increasing difference with energy clearly implies a velocity dependence. It is to be expected that the probability of electron loss in the reduced electron density in channels will be significantly diminished but it is known that the probability does not vary strongly over the range of impact parameters considered here [11]). Thus the explanation of the results must lie in the electron capture process. Drisko [10] has shown that, for the proton-hydrogen change exchange case, the electron capture
random stopping power ratios in terms of the observed charge state distributions, the results obtained by other laboratories [8,9] for channelled charge state distributions of heavier ions show similar behaviour and consequently increase our confidence in the present results. The authors would like to thank Dr. K. Dettmann for many helpful discussions.
References
cross-section has the form °~
1] K. Dettmann, private communication.
=f 2xb ~2L ~i6r b~ vJ La~i .K
2
b
Ui db
(1)
2[~_.~~1
where b = impact parameter, aB and UB = and velocity respectively, v = ion velocity
Bohr radius and K2 is a
modified Bessel function. From this it can be shown that, at low velocities the capture cross-section is significant even for quite large impact parameters while as v is increased capture becomes limited to smaller and smaller impact parameters —- in agreement with the results obtained.
If a dependence of this nature is the explanation of the results then, at higher energies, the He~/He~ ratio in the (11 1) channel should become greater than
in random directions. An experiment is in progress to test this prediction. While it is difficult to understand the channelled/
274
[2] F. Fisen, G.J. Garke, J. B~ttigerand J.M. Poate, AERE report R-6949. G. Della Mes, A.V. Drigo, S. Lo Russo, P. Mazzoldi and ~ Bentini, Phys. Rev. Lett. 27 (1971) 1194. 14] G. Della Mes, A.V. Drigo, S. Lo Russo, P. Mazzoldi and
131
G.G. Bentini, Rad. Eff. 13(1972)115. R.J. Archer, J. Electrochem. Soc. 104 (1947) 619. [61 GA. Dissanaike, Phil. Mag. 7 (1953) 1051. [7] J.C. Armstrong, J.V. Mullendore, W.R. Flarris and ill. Marion, Proc. Phys. Soc. 86 (1965) 1283. [8] S. Datz, LW. Martin, C.D. Moak, B.R. Appleton and L.B. Bridwell, Rad. Eff. 12 (1972) 163. 19] C.D. Moak, 5.Datz, B.R. Appleton, J.A. Biggcrstaff,
151
M.D. Brown, H.F. Krause and T.S. Noggle, Phys. Rev., to be published. [10] R.M. Drisko, Ph.D. Thesis, Carnegie Inst. of Technology (1955), unpublished. [11] Electronic and ionic impact phenomena, Vol. IV. Massey and Gilbody (Clarendon Press, 1974).