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Scattering of reactive and inert ion beams from surfaces: energy loss, straggling and skipping
Nuclear Instruments North-Holland
Scattering straggling
and Methods
in Physics
Research
B58 (1991) 365-371
365
of reactive and inert ion beams from surfaces: and skipping
W. Mix, J.-H. Rechtien,
D. Danailov
energy loss,
and K.J. Snowdon
Fachbereich Physik, Unioersitiit Osnabriick, Postfach 4469, W-4500 Osnabriick, FRG
We compare and contrast the scattering behaviour of beams of the ‘chemically active’ species H’ and N+ with that of the ‘chemicalty inert’ species He+ and Ar+. The 3-3.5 keV beams were incident at from l-8O to a Cu(ll1) surface, The scattered neutral particle energy distributions were recorded at scattering angles in the range l-9O with a 0.2“ angular resolution using a time-of-flight (TOF) technique. We analyse the mean energy loss and straggling as a function of incidence, scattering and azimuthal angles, and beam species. Both quantities behave in a complex manner as a function of these variables. No universal behaviour was found. We report regularly spaced peaks in the scattered Ar atom energy distributions and evidence for such structures in the scattered N atom distributions. We tentatively associate the fine structure in at least the Ar distributions with transient adsorption or
skipping motion of a portion of the beam on the surface.
1. Introduction Previous studies of the interaction of low velocity ion beams with well char( Uthermal -Z u +c uFermi) acterized crystal surfaces have primarily utilized inert gas or alkali ion beams [l]. Very often, only those scattered particles which retain their initial charge are detected. This choice tends to filter out those particles which experience strong coupling to the surface during scattering, favouring those whose trajectories are well described by simple potentials. On the other hand, studies of the scattering behaviour of species which at thermal velocities would be chemisorbed, have revealed a wealth of effects whose origins are not yet well understood. Among these studies are those using molecular projectile ions such as Hz, N: and 0; scattered from Ni(lll), Ag(ll1) and Si(OO1) surfaces 121. In such cases, both the atomic and molecular species may be strongly bound to the surface at thermal energies, and dissociation may occur during the scattering event. Only limited studies using ‘chemically active’ atomic ions have to date been performed. Among these studies are those using Si+ ions and a Cu(ll1) surface [3]. In this example, the scattered positive and negative ion energy distributions were characterized by a series of regularly spaced peaks or structures. The possibility that these structures arise from ‘transient adsorption’ or ‘skipping motion’ of the beam on the surface has been extensively discussed in a series of publications [3,4]. The precise mechanisms for such adsorption and desorption of the beam in this specific example remain unclear. Both elastic scattering [5] or inelastic effects [3,4,6] could conceivably be responsible for this type of phenomenon. In addition, 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
theoretical investigations of inelastic energy losses in the limit of strong coupling are in their infancy. For these reasons, we considered it timely to systematically compare and contrast the scattering behaviour .of beams of ‘chemically active’ species with that of ‘chemically inert’ species. This paper represents a summary of those results, obtained using a Cu(ll1) surface. A more detailed publication, including molecular dynamics simulations, will appear elsewhere [7].
2. Apparatus The ion beam was generated in a plasma ion source, accelerated to its final energy of several keV, magnetically mass analysed, and tighty collimated before entry into the UHV chamber cont~ning the crystal and scattering particle diagnostics. The experiments were performed with at worse a beam angular divergence A,Bi (half-cone-angle) of 0.04O. This leads to a maximum relative uncertainty AE,/E, , in the ‘perpendicular energy’ of the beam to the surface E, = E, sir? Bi of AE,/E,
= 2 A Bi cotan 0,.
(I)
Explicit evaluation of eq. (1) for 8, = 1 and 3” gives AE,/E, = 0.08 and 0.03 respectively. The beam was pulsed electrostatically immediately following acceleration by a system similar to that described by Rathmann et al. [8].
B.V. (North-Holland)
II. PARTICLE
SCATTERING
366
W. Mix et al. / Scattering
The UHV target chamber contains a mass spectrometer for residual gas analysis, LEED, a stepper motor controlled crystal manipulator, and the scattered particle detectors. The scattered particle detectors and associated electronics are schematically illustrated in fig. 1. This spectrometer consists of a combination of a TOF spectrometer for measurement of the velocity distribution of scattered neutral particles, and a hemispherical electrostatic energy analyser for measurement of the energy distribution of scattered ionized (positive and negative) particles. The spectrometer can be rotated through scattering angles 0,, from O-9”. The spectrometer angular resolution we used was 0.2”. The incident beam TOF distribution was measured directly by this spectrometer set at zero-degree scattering angle. This allows not only a direct measurement of the incident beam energy, but using Fourier transform techniques, allows deconvolution of the incident beam time profile from the scattered particle distributions. The copper crystal (99.999% purity) was cut and mechanically polished. The crystal was then mounted in the UHV chamber and cleaned and further polished by a combination of glancing incidence sputtering (3 keV Ne+, Bi = 3”) and annealing (TannealI 450” C). An excellent LEED pattern was readily obtained. In contrast, a symmetric ‘surface channeling’ pattern, with pronounced channeling dips in the [liO] direction (3 keV Ne+, Bi = 3“) OS,= 5 ” ), plus a narrow scattered particle angular distribution, both indicators of low adatom,
of reactive and inert ion beams
adsorbate, defect and step densities, was much more time consuming to attain. The base pressure in the scattering chamber was below 2 x 10-l’ mbar. The pressure rose into the mid 10-l’ mbar region during measurements, due alone to the gas load of the pulsed beam.
3. Experimental results We have measured the TOF distributions of the neutral particles produced by scattering beams of H+, H:, N+, N:, He+ and Ar+ from Cu(ll1). In this paper, we present the results for beam energies from 3-3.5 keV, incidence angles from 1-7 O, scattering angles from l-9 o and azimuthal angles 0, - 20 and - 30 o to the [liO] direction. We summarize the results for the atomic ion projectiles H’, He+ and Ar+ in figs. 2-6. In these figures, we plot both the ‘energy loss’, Eloss, and the ‘straggling’, Ewidth. Eloss was determined from the energy difference of the peak maxima of the incident and scattered particle beams. Ewidth represented the FWHM of the scattered particle energy distribution, after correction for the energy width of the incident beam. The TOF distribution corresponding to that of Ar scattered through 5O for a 3067 eV Ar+ beam incident along the [liO] azimuth at an angle of 3” to the surface plane is shown in fig. 7. It consists of a main peak
TOF/Energy Analyser
Fig. 1. Schematic illustration of the scattered Ramp high voltage ramp, MCS multichannel
particle detectors and associated electronics (CEM channel scaler, TAC time to amplitude converter, PHA pulse height generator).
electron analyser,
multiplier, H.V. PG beam pulse
W. Mix et al. / Scattering
of reactive and inerf ion beams
367
Ei =353keV 0 0
+ :Q=
I-
0 Y
0’ Q=-2b
G
Q = -30’
0
b
d
0
0
0
0
0
0
3-
0
8
~
* 0
l-
I 8,
2
I
=0.97’
, 4
I
I
6
8
0,;3.05’
I
)
0i=4.82’
I
I
6
8
6
8
Fig. 2. Energy loss of the neutral product of the scattering of a 3.53 keV Ar+ beam from a Cu(lll) surface as a function of incidence angle of the beam to the surface, 6,; scattering angle, 6,,; and azimuthal angle, + (+ = 0 o represents the [Ii01 direction).
accompanied by a shoulder toward lower energies. This shoulder exhibits a series of at least six easily visible structures or peaks. All features in the measured scattered particle distribution are broadened by the
A?+Cu IlllJ-Ar
10
time width of the incident beam pulse. This latter dist~bution was directly measured and deconvoluted from the scattered atom distribution using the numerical fast Fourier transform and optimal filtering method
7
+Cu*(ltlI
+:
Q=
o:
Q =-20’
0’
x :
Q =-30’
i
Bi 0.97' q
0
’
2
4
6
8
4
6
8
6
8
Fig. 3. FWHM of the scattered neutral product energy distribution following scattering of a 3.53 keV Ar+ beam from a Cu(lt1) surface as a function of Bi, 8,, and -#B. II. PARTICLE
SCATTERING
368
W. Mix et al. / Scattering
ofreactiveand
inert ion beams
110 -
s s
,sa:
ln90s Lli
O
.b 8
i
70
?
Bi =3.05'
f3fO.97'
I
Oi =L.82’
:p=0’
+
0 : 19=-20’ x :‘o =-30’
l
: t
XII
2
G
6
6
L
6
6
1
L
6
0
Fig. 4. Energy loss of the neutral product of the scattering of a 3.05 keV He+ beam from a Cu(ll1) and 6.
described
in ref. [9]. The
filter is used to remove
significant change in the low-energy components of the spectrum. Convolution of the thus derived ‘true’ TOF distribution NT(t) with the apparatus function using numerical integration yields the measured distribution
high
which arise due to statistical noise in the data and which cause numerical difficulties by the inverse transform. A range of weaker filters causes no frequencies
131 I-
He* +Cu(llll-
surface as a function of 13~.0,
He+Cct(lll)
8, =305’
3,=LB2' ) c
E, =3.05keV c
8,=097 +: 'o= 0'
Ill 3-
0: q =-20’ x: Ip z-30'
+ t +++t
F u
t
5 9(I-
t
t
t
IL?
++
t
1 +*+*
7(I-
‘*c 9+,ttt
5(I-
t 2
L
6
0
L
6
8
6
8
Fig. 5. FWHM of the scattered neutral product energy distribution following scattering of a 3.05 keV He+ beam from a Cu(lll) surface as a function of Bi, S,, and +.
W. Mix et al. / Scattering
of reactiveand inert ion beam
369
H++Cu(111)--H*Cu’f111)
2700
o
Ei =2.99keV
t
Ei =3067eV 6i =3*,e,,=s
8, :3.05 1p =-20’
v =O’
160 0
$ f uo
0 0
Ii
w” Energy [eV] 0
1901
’
L
,
6
12C
ia
a
8
B,,(deg)
Fig. 8. Energy distributions of neutral Ar corresponding to (a) the TOF distribution of fig. 7 and (b) the ‘true’ TOF distribution, after deconvolution of the incident beam time profile (see text). L
6 B,,(deg
B
I
Fig. 6. Energy loss and FWHM of the scattered neutral product energy distribution fotlowing scattering of a 2.99 keV H’ beam
from a Cu(lll) surface as a function of B,, (i?, = 3.05 O, + = - 20 o to [liO] direction).
N++Cu(1111-N +Cu+lll) _ E,=2610eV
within the statistical error in the measurement. The ‘true’ TOF distribution A$( t) of scattered Ar was converted to an energy distribution Nr( E) via the transformation.
=ct3iv,(t),
N,(E)
2200
where c is an arbitrary constant. Both this distribution, and the energy distribution corresponding to the ‘raw’ distribution (fig. 7) are shown for comparison in fig. 8. The true scattered particle energy dist~bution consists of a series of regularly spaced peaks, up to eleven of which are readily visible. The scattered neutral-particle energy distributions from N+, NT, Hl and Ar+ are all characterized by a
1000
2300
2400
Energy
2600
2500
[eV]
Fig. 9. Energy distribution of the neutral product scattering of a 2.61 keV N+ beam from a Cu(ll1) (8,=1O, l9,=3o, r$=O”).
of the surface
series of regularly spaced structures or peaks. The data for the molecular projectiles are very complex, and is discussed in detail elsewhere [lo]. The spectrum of N atoms produced by scattering of a 2.61 keV N+ beam from Cu(ll1) is shown in fig. 9.
-
4. Discussion 600
-
3 90
Time
of Flight
3.80
[ps]
Fig. 7. TOF distribution of neutral kr scattered through 5”. The 3067 eV At-+ beam was incident at 3O to the surface along the [liO] azimuth.
A glance at the data of figs. 2-6 reveals a very complex behaviour. The data for Ar exhibits only a very weak dependence on crystal azimuth, while that for He exhibits a strong dependence, particularly the straggling at the larger incidence angles. We note that, dependent on projectile, azimuth and incidence angle, the energy loss can increase, decrease, or go through a maximum as a function of the scattering angle. In addition to this range of behaviour, the straggling can also go through a minimum. The absolute energy loss and straggling appears to increase from Ar through He to H. In fig. 10 II. PARTICLE
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we plot the ratio Eloss/Ew,dth for a selection of the data of figs. 2-6. The absolute value of this ratio is strongly dependent on projectile, azimuth and scattering angle. The ratio is largest for Ar. The total energy loss contains contributions from both elastic and inelastic processes. Our molecular dynamics simulations [ll] show that elastic losses can be ignored in the case of H and He. This is also true for Ar at the smallest incidence angle. For incidence angles of 3.05 and 4.82”, however, Eloss is of order 0.9 and 4.4 eV, respectively. Simple subtraction of these losses from the respective curves in fig. 2 leads almost to a universal curve for all three incidence and azimuthal angles. In other words, the angle of incidence (in the case of Ar) appears only to be of secondary importance in determining the inelastic energy loss for a given scattering angle. At present we only possess accurate values for the contribution of elastic processes to the straggling for a T = 0 K surface. These are much too small (0.14 eV at Bi = 3.05’ and 0.66 eV at 19;= 4.82”) to explain the observed differences in the straggling as a function of incidence angle. Our molecular dynamics simulations also show that at 0, = 4.82”, a portion of the He+ beam penetrates below the position of the nuclei of the outermost atomic layer. However, the particles which do this have trajectories which, after scattering, lie outside the plane in which the detector moves. Adatoms, steps and defects should also lead to large angle scattering. Therefore, we believe that the data of figs. 2-6 represent the energy loss and straggling of particles scattered from the surface of Cu(ll1).
!
0-
1.6
c + Ar
x He
OH ‘9= -20’
0-
L
6
8
1.2
o.
of reactiue and inert ion beams The shape of the energy distribution of slow ions scattered by metal surfaces has been discussed in some detail by Kato [12]. He notes that the uncertainty in the scattered particles energy is determined by two effects viz., the uncertainty in the path taken by the ions (trajectory effect), and the stochastic nature of the excitation process itself (stochastic effect). In addition to these effects, the experimental distributions can be further influenced by the intrinsic resolution of the measurement apparatus. Our deconvolution procedure removes most of the latter contribution. We expect the ‘trajectory effect’ to be particularly serious for the ‘smaller’ projectiles (He+ and H+) at large angles of incidence along low index crystallographic directions. This contention is supported by a comparison of the He+ and Ar+ data. The near absence of azimuthal dependences in the Art data suggests that the corresponding distributions are primarily defined by the stochastic nature of the excitation process itself, i.e. by the statistical nature of the phonon, plasmon, and electron-hole pair excitation process. With this in mind, the comparison of the energy loss to straggling ratio for the three projectiles Art, He+ and H+ (fig. 10) may suggest that the ‘stochastic effect’ contributes little to the uncertainty in the scattered particle energy. The main uncertainty may come from the ‘trajectory effect’. A quantitative discussion of the ‘trajectory effect’ is beyond the scope of this paper. We wish merely to discuss the possible inelastic processes which may be responsible for the observed mean losses, ‘stochastic width’ (in the case of Art), and multipeak structure of the Ar and N atom distributions.
ooo
f
3 0
+
0 0
+
L
6
8
6
L
6
8
0Jdegl Fig. 10. Dependence of the ratio of energy loss to straggling for Ar, He and H on scattering angle and azimuth. The incidence angle 0. was 3.05 o in all cases.
W. MIX et al. / Scattering In the case of Ar+ scattering under the conditions of fig. 7, we observe up to eleven peaks in the scattered Ar distribution with a mean separation of 21 + 3 eV. The FWHM of the dominant peak is 13 eV. The width of the peaks seems to increase slightly with increasing mean loss. It would be tempting to suggest multiple discrete excitations of quantum - 20 eV as an explanation for this behaviour, were it not for the continuous dependence of the energy loss of the dominant peak on scattering angle (fig. 2). Similar considerations led to the rejection of such mechanisms as an explanation of the Si+/Cu(lll) scattering data reported earlier [3]. We therefore suggest that a fraction of the Ar beam may also undergo skipping motion [13]. The apparently ‘continuous’ dependence of the energy loss of the projectiles Ar’, He+ and I-I+ on scattering angle suggests that the excitation quantum has a typical energy less than a few eV. This is consistent with phonon, plasmon, and electron-hole pair excitation processes. However, we do not wish to exclude the possibility that the ‘excitation quantum’ for Ni/Cu(lll) scattering is larger and that we have partially resolved this ‘quantum’ in fig. 9. The form of the N atoms distribution (fig. 9) is fundamentally different from that for both scattered Si i: and Ar from the same surface, and which we can reconcile with transient adsorption or skipping motion.
5. Conclusion We summarize the results of an ongoing systematic investigation of the small angle scattering behaviour of ‘chemically active’ and ‘chemically inert’ species on metal crystal surfaces. We find little evidence for universal behaviour, even among the inert gas ion projectiles. In contrast, we find complex dependences on incidence angle, scattering angle, and crystal azimuth. The mean energy loss to straggling ratios also show such complex behaviour, and differ significantly in magnitude for the various ions. We argue that the form of our energy distributions for Ar are principally determined by the stochastic nature of the excitation process itself. We report a series of regularly spaced peaks in the scattered atom energy distributions for Ar+ scattering, and evidence for such structures in Nf scattering. We suggest that a portion of the Ar beam may also undergo the skipping motion reported earlier for Si+ scattering from Cu(ll1). It is not yet clear why one projectile apparently undergoes skipping motion while others apparently do not. Our main indicator for such behaviour is a series of regularly spaced peaks in the scattered particle energy distribution. However, if the mean energy loss to straggling ratio is small, such peaks will not be resolved in an
of reactiveand
371
inert ion beams
experiment. We must extend our data set further, however, we speculate already that transient adsorption or skipping motion may in fact be a quite general phenomena in glancing-incidence beam-surface scattering, whose identification is simply hampered by inadequate separation of the individual energy loss peaks. In any case, these results, combined with a recent theoretical analysis 161, reveal the complexity of small angle scattering at surfaces, independent of the ‘chemical activity’ of the projectile at that surface.
Acknowledgements This work was generously supported by the Deutsche Forschungsgemeinschaft. We thank P. Hertel for invaluable advice in the use of FFT techniques.
References ill See for example: Proc. of Seventh International Workshop on Inelastic Ion-Surface Collisions, Radiat. Eff. Defects Sol. 109 (1989). 121 B. Willerding, W. Heiland and K.J. Snowdon, Phys. Rev. Lett. 53 (1984) 2031; P.H.F. Reijnen, P.J. van den Hoek, A.W. Kleyn, U. Imke and K.J. Snowdon. Surf. Sci. 221 (1989) 427; J.-H. Rechtien, U. Imke, K.J. Snowdon, P.H.F. Reijnen, P.J. van den Hoek, A.W. Kleyn and A. Nan&i, Surf. Sci. 227 (1990) 35. Phys. [31 K.J. Snowdon, D.J. O’Connor and R.J. MacDonald, Rev. Lett. 61 (1988) 1760; Surf. Sci. 221 (1989) 465. [41 K.J. Snowdon, D.J. O’Connor, M. Kato and R.J. MacDonald, Nucl. Instr. and Meth. B48 (1990) 327. ISI Y.H. Ohtsuki, K. Koyama and Y. Yamamura, Phys. Rev. B20 (1979) 5044. [61 M. Kato, K.J. Snowdon, D.J. O’Connor and R.J. MacDonald, Nucl. Instr. and Meth. B48 (1990) 39. [71 W. Mix, J.-H. Rechtien, D. Danailov, K.J. Snowdon, in preparation. 181 D. Rathmann, N. Exeler and B. Willerding, J. Phys. El8 (1985) 17. [91 W.H. Press, B.P. Flannery, S.A. Teukolsky and T. Vetterling, Numerical Recipes (Cambridge University Press, Cambridge, 1988). DOI J.-H. Rechtien, W. Mix, K.J. Snowdon, Surf. Sci. submitted. 1111 We sofve Newtons equations of motion using a potential constructed from pair potentials of the ‘universal’ type described by Biersack with the Ziegler screening length; J.P. Biersack and J.F. Ziegler, in: Ion Implantation Techniques, eds., H. Ryssel and H. Glawischnig, Springer Series in Electrophysics, Vol. 10 (Springer, Berlin, 1982) pp. 122-156. WI M. Kato, 3. Phys. Sac. Jpn. 55 (1986) 1011. [I31 J.-H. Rechtien, W. Mix and K.J. Snowdon, Surf. Sci., in press.
II. PARTICLE
SCATTERING
Scattering straggling
and Methods
in Physics
Research
B58 (1991) 365-371
365
of reactive and inert ion beams from surfaces: and skipping
W. Mix, J.-H. Rechtien,
D. Danailov
energy loss,
and K.J. Snowdon
Fachbereich Physik, Unioersitiit Osnabriick, Postfach 4469, W-4500 Osnabriick, FRG
We compare and contrast the scattering behaviour of beams of the ‘chemically active’ species H’ and N+ with that of the ‘chemicalty inert’ species He+ and Ar+. The 3-3.5 keV beams were incident at from l-8O to a Cu(ll1) surface, The scattered neutral particle energy distributions were recorded at scattering angles in the range l-9O with a 0.2“ angular resolution using a time-of-flight (TOF) technique. We analyse the mean energy loss and straggling as a function of incidence, scattering and azimuthal angles, and beam species. Both quantities behave in a complex manner as a function of these variables. No universal behaviour was found. We report regularly spaced peaks in the scattered Ar atom energy distributions and evidence for such structures in the scattered N atom distributions. We tentatively associate the fine structure in at least the Ar distributions with transient adsorption or
skipping motion of a portion of the beam on the surface.
1. Introduction Previous studies of the interaction of low velocity ion beams with well char( Uthermal -Z u +c uFermi) acterized crystal surfaces have primarily utilized inert gas or alkali ion beams [l]. Very often, only those scattered particles which retain their initial charge are detected. This choice tends to filter out those particles which experience strong coupling to the surface during scattering, favouring those whose trajectories are well described by simple potentials. On the other hand, studies of the scattering behaviour of species which at thermal velocities would be chemisorbed, have revealed a wealth of effects whose origins are not yet well understood. Among these studies are those using molecular projectile ions such as Hz, N: and 0; scattered from Ni(lll), Ag(ll1) and Si(OO1) surfaces 121. In such cases, both the atomic and molecular species may be strongly bound to the surface at thermal energies, and dissociation may occur during the scattering event. Only limited studies using ‘chemically active’ atomic ions have to date been performed. Among these studies are those using Si+ ions and a Cu(ll1) surface [3]. In this example, the scattered positive and negative ion energy distributions were characterized by a series of regularly spaced peaks or structures. The possibility that these structures arise from ‘transient adsorption’ or ‘skipping motion’ of the beam on the surface has been extensively discussed in a series of publications [3,4]. The precise mechanisms for such adsorption and desorption of the beam in this specific example remain unclear. Both elastic scattering [5] or inelastic effects [3,4,6] could conceivably be responsible for this type of phenomenon. In addition, 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
theoretical investigations of inelastic energy losses in the limit of strong coupling are in their infancy. For these reasons, we considered it timely to systematically compare and contrast the scattering behaviour .of beams of ‘chemically active’ species with that of ‘chemically inert’ species. This paper represents a summary of those results, obtained using a Cu(ll1) surface. A more detailed publication, including molecular dynamics simulations, will appear elsewhere [7].
2. Apparatus The ion beam was generated in a plasma ion source, accelerated to its final energy of several keV, magnetically mass analysed, and tighty collimated before entry into the UHV chamber cont~ning the crystal and scattering particle diagnostics. The experiments were performed with at worse a beam angular divergence A,Bi (half-cone-angle) of 0.04O. This leads to a maximum relative uncertainty AE,/E, , in the ‘perpendicular energy’ of the beam to the surface E, = E, sir? Bi of AE,/E,
= 2 A Bi cotan 0,.
(I)
Explicit evaluation of eq. (1) for 8, = 1 and 3” gives AE,/E, = 0.08 and 0.03 respectively. The beam was pulsed electrostatically immediately following acceleration by a system similar to that described by Rathmann et al. [8].
B.V. (North-Holland)
II. PARTICLE
SCATTERING
366
W. Mix et al. / Scattering
The UHV target chamber contains a mass spectrometer for residual gas analysis, LEED, a stepper motor controlled crystal manipulator, and the scattered particle detectors. The scattered particle detectors and associated electronics are schematically illustrated in fig. 1. This spectrometer consists of a combination of a TOF spectrometer for measurement of the velocity distribution of scattered neutral particles, and a hemispherical electrostatic energy analyser for measurement of the energy distribution of scattered ionized (positive and negative) particles. The spectrometer can be rotated through scattering angles 0,, from O-9”. The spectrometer angular resolution we used was 0.2”. The incident beam TOF distribution was measured directly by this spectrometer set at zero-degree scattering angle. This allows not only a direct measurement of the incident beam energy, but using Fourier transform techniques, allows deconvolution of the incident beam time profile from the scattered particle distributions. The copper crystal (99.999% purity) was cut and mechanically polished. The crystal was then mounted in the UHV chamber and cleaned and further polished by a combination of glancing incidence sputtering (3 keV Ne+, Bi = 3”) and annealing (TannealI 450” C). An excellent LEED pattern was readily obtained. In contrast, a symmetric ‘surface channeling’ pattern, with pronounced channeling dips in the [liO] direction (3 keV Ne+, Bi = 3“) OS,= 5 ” ), plus a narrow scattered particle angular distribution, both indicators of low adatom,
of reactive and inert ion beams
adsorbate, defect and step densities, was much more time consuming to attain. The base pressure in the scattering chamber was below 2 x 10-l’ mbar. The pressure rose into the mid 10-l’ mbar region during measurements, due alone to the gas load of the pulsed beam.
3. Experimental results We have measured the TOF distributions of the neutral particles produced by scattering beams of H+, H:, N+, N:, He+ and Ar+ from Cu(ll1). In this paper, we present the results for beam energies from 3-3.5 keV, incidence angles from 1-7 O, scattering angles from l-9 o and azimuthal angles 0, - 20 and - 30 o to the [liO] direction. We summarize the results for the atomic ion projectiles H’, He+ and Ar+ in figs. 2-6. In these figures, we plot both the ‘energy loss’, Eloss, and the ‘straggling’, Ewidth. Eloss was determined from the energy difference of the peak maxima of the incident and scattered particle beams. Ewidth represented the FWHM of the scattered particle energy distribution, after correction for the energy width of the incident beam. The TOF distribution corresponding to that of Ar scattered through 5O for a 3067 eV Ar+ beam incident along the [liO] azimuth at an angle of 3” to the surface plane is shown in fig. 7. It consists of a main peak
TOF/Energy Analyser
Fig. 1. Schematic illustration of the scattered Ramp high voltage ramp, MCS multichannel
particle detectors and associated electronics (CEM channel scaler, TAC time to amplitude converter, PHA pulse height generator).
electron analyser,
multiplier, H.V. PG beam pulse
W. Mix et al. / Scattering
of reactive and inerf ion beams
367
Ei =353keV 0 0
+ :Q=
I-
0 Y
0’ Q=-2b
G
Q = -30’
0
b
d
0
0
0
0
0
0
3-
0
8
~
* 0
l-
I 8,
2
I
=0.97’
, 4
I
I
6
8
0,;3.05’
I
)
0i=4.82’
I
I
6
8
6
8
Fig. 2. Energy loss of the neutral product of the scattering of a 3.53 keV Ar+ beam from a Cu(lll) surface as a function of incidence angle of the beam to the surface, 6,; scattering angle, 6,,; and azimuthal angle, + (+ = 0 o represents the [Ii01 direction).
accompanied by a shoulder toward lower energies. This shoulder exhibits a series of at least six easily visible structures or peaks. All features in the measured scattered particle distribution are broadened by the
A?+Cu IlllJ-Ar
10
time width of the incident beam pulse. This latter dist~bution was directly measured and deconvoluted from the scattered atom distribution using the numerical fast Fourier transform and optimal filtering method
7
+Cu*(ltlI
+:
Q=
o:
Q =-20’
0’
x :
Q =-30’
i
Bi 0.97' q
0
’
2
4
6
8
4
6
8
6
8
Fig. 3. FWHM of the scattered neutral product energy distribution following scattering of a 3.53 keV Ar+ beam from a Cu(lt1) surface as a function of Bi, 8,, and -#B. II. PARTICLE
SCATTERING
368
W. Mix et al. / Scattering
ofreactiveand
inert ion beams
110 -
s s
,sa:
ln90s Lli
O
.b 8
i
70
?
Bi =3.05'
f3fO.97'
I
Oi =L.82’
:p=0’
+
0 : 19=-20’ x :‘o =-30’
l
: t
XII
2
G
6
6
L
6
6
1
L
6
0
Fig. 4. Energy loss of the neutral product of the scattering of a 3.05 keV He+ beam from a Cu(ll1) and 6.
described
in ref. [9]. The
filter is used to remove
significant change in the low-energy components of the spectrum. Convolution of the thus derived ‘true’ TOF distribution NT(t) with the apparatus function using numerical integration yields the measured distribution
high
which arise due to statistical noise in the data and which cause numerical difficulties by the inverse transform. A range of weaker filters causes no frequencies
131 I-
He* +Cu(llll-
surface as a function of 13~.0,
He+Cct(lll)
8, =305’
3,=LB2' ) c
E, =3.05keV c
8,=097 +: 'o= 0'
Ill 3-
0: q =-20’ x: Ip z-30'
+ t +++t
F u
t
5 9(I-
t
t
t
IL?
++
t
1 +*+*
7(I-
‘*c 9+,ttt
5(I-
t 2
L
6
0
L
6
8
6
8
Fig. 5. FWHM of the scattered neutral product energy distribution following scattering of a 3.05 keV He+ beam from a Cu(lll) surface as a function of Bi, S,, and +.
W. Mix et al. / Scattering
of reactiveand inert ion beam
369
H++Cu(111)--H*Cu’f111)
2700
o
Ei =2.99keV
t
Ei =3067eV 6i =3*,e,,=s
8, :3.05 1p =-20’
v =O’
160 0
$ f uo
0 0
Ii
w” Energy [eV] 0
1901
’
L
,
6
12C
ia
a
8
B,,(deg)
Fig. 8. Energy distributions of neutral Ar corresponding to (a) the TOF distribution of fig. 7 and (b) the ‘true’ TOF distribution, after deconvolution of the incident beam time profile (see text). L
6 B,,(deg
B
I
Fig. 6. Energy loss and FWHM of the scattered neutral product energy distribution fotlowing scattering of a 2.99 keV H’ beam
from a Cu(lll) surface as a function of B,, (i?, = 3.05 O, + = - 20 o to [liO] direction).
N++Cu(1111-N +Cu+lll) _ E,=2610eV
within the statistical error in the measurement. The ‘true’ TOF distribution A$( t) of scattered Ar was converted to an energy distribution Nr( E) via the transformation.
=ct3iv,(t),
N,(E)
2200
where c is an arbitrary constant. Both this distribution, and the energy distribution corresponding to the ‘raw’ distribution (fig. 7) are shown for comparison in fig. 8. The true scattered particle energy dist~bution consists of a series of regularly spaced peaks, up to eleven of which are readily visible. The scattered neutral-particle energy distributions from N+, NT, Hl and Ar+ are all characterized by a
1000
2300
2400
Energy
2600
2500
[eV]
Fig. 9. Energy distribution of the neutral product scattering of a 2.61 keV N+ beam from a Cu(ll1) (8,=1O, l9,=3o, r$=O”).
of the surface
series of regularly spaced structures or peaks. The data for the molecular projectiles are very complex, and is discussed in detail elsewhere [lo]. The spectrum of N atoms produced by scattering of a 2.61 keV N+ beam from Cu(ll1) is shown in fig. 9.
-
4. Discussion 600
-
3 90
Time
of Flight
3.80
[ps]
Fig. 7. TOF distribution of neutral kr scattered through 5”. The 3067 eV At-+ beam was incident at 3O to the surface along the [liO] azimuth.
A glance at the data of figs. 2-6 reveals a very complex behaviour. The data for Ar exhibits only a very weak dependence on crystal azimuth, while that for He exhibits a strong dependence, particularly the straggling at the larger incidence angles. We note that, dependent on projectile, azimuth and incidence angle, the energy loss can increase, decrease, or go through a maximum as a function of the scattering angle. In addition to this range of behaviour, the straggling can also go through a minimum. The absolute energy loss and straggling appears to increase from Ar through He to H. In fig. 10 II. PARTICLE
SCATTERING
370
W. Mix et al. / Scattering
we plot the ratio Eloss/Ew,dth for a selection of the data of figs. 2-6. The absolute value of this ratio is strongly dependent on projectile, azimuth and scattering angle. The ratio is largest for Ar. The total energy loss contains contributions from both elastic and inelastic processes. Our molecular dynamics simulations [ll] show that elastic losses can be ignored in the case of H and He. This is also true for Ar at the smallest incidence angle. For incidence angles of 3.05 and 4.82”, however, Eloss is of order 0.9 and 4.4 eV, respectively. Simple subtraction of these losses from the respective curves in fig. 2 leads almost to a universal curve for all three incidence and azimuthal angles. In other words, the angle of incidence (in the case of Ar) appears only to be of secondary importance in determining the inelastic energy loss for a given scattering angle. At present we only possess accurate values for the contribution of elastic processes to the straggling for a T = 0 K surface. These are much too small (0.14 eV at Bi = 3.05’ and 0.66 eV at 19;= 4.82”) to explain the observed differences in the straggling as a function of incidence angle. Our molecular dynamics simulations also show that at 0, = 4.82”, a portion of the He+ beam penetrates below the position of the nuclei of the outermost atomic layer. However, the particles which do this have trajectories which, after scattering, lie outside the plane in which the detector moves. Adatoms, steps and defects should also lead to large angle scattering. Therefore, we believe that the data of figs. 2-6 represent the energy loss and straggling of particles scattered from the surface of Cu(ll1).
!
0-
1.6
c + Ar
x He
OH ‘9= -20’
0-
L
6
8
1.2
o.
of reactiue and inert ion beams The shape of the energy distribution of slow ions scattered by metal surfaces has been discussed in some detail by Kato [12]. He notes that the uncertainty in the scattered particles energy is determined by two effects viz., the uncertainty in the path taken by the ions (trajectory effect), and the stochastic nature of the excitation process itself (stochastic effect). In addition to these effects, the experimental distributions can be further influenced by the intrinsic resolution of the measurement apparatus. Our deconvolution procedure removes most of the latter contribution. We expect the ‘trajectory effect’ to be particularly serious for the ‘smaller’ projectiles (He+ and H+) at large angles of incidence along low index crystallographic directions. This contention is supported by a comparison of the He+ and Ar+ data. The near absence of azimuthal dependences in the Art data suggests that the corresponding distributions are primarily defined by the stochastic nature of the excitation process itself, i.e. by the statistical nature of the phonon, plasmon, and electron-hole pair excitation process. With this in mind, the comparison of the energy loss to straggling ratio for the three projectiles Art, He+ and H+ (fig. 10) may suggest that the ‘stochastic effect’ contributes little to the uncertainty in the scattered particle energy. The main uncertainty may come from the ‘trajectory effect’. A quantitative discussion of the ‘trajectory effect’ is beyond the scope of this paper. We wish merely to discuss the possible inelastic processes which may be responsible for the observed mean losses, ‘stochastic width’ (in the case of Art), and multipeak structure of the Ar and N atom distributions.
ooo
f
3 0
+
0 0
+
L
6
8
6
L
6
8
0Jdegl Fig. 10. Dependence of the ratio of energy loss to straggling for Ar, He and H on scattering angle and azimuth. The incidence angle 0. was 3.05 o in all cases.
W. MIX et al. / Scattering In the case of Ar+ scattering under the conditions of fig. 7, we observe up to eleven peaks in the scattered Ar distribution with a mean separation of 21 + 3 eV. The FWHM of the dominant peak is 13 eV. The width of the peaks seems to increase slightly with increasing mean loss. It would be tempting to suggest multiple discrete excitations of quantum - 20 eV as an explanation for this behaviour, were it not for the continuous dependence of the energy loss of the dominant peak on scattering angle (fig. 2). Similar considerations led to the rejection of such mechanisms as an explanation of the Si+/Cu(lll) scattering data reported earlier [3]. We therefore suggest that a fraction of the Ar beam may also undergo skipping motion [13]. The apparently ‘continuous’ dependence of the energy loss of the projectiles Ar’, He+ and I-I+ on scattering angle suggests that the excitation quantum has a typical energy less than a few eV. This is consistent with phonon, plasmon, and electron-hole pair excitation processes. However, we do not wish to exclude the possibility that the ‘excitation quantum’ for Ni/Cu(lll) scattering is larger and that we have partially resolved this ‘quantum’ in fig. 9. The form of the N atoms distribution (fig. 9) is fundamentally different from that for both scattered Si i: and Ar from the same surface, and which we can reconcile with transient adsorption or skipping motion.
5. Conclusion We summarize the results of an ongoing systematic investigation of the small angle scattering behaviour of ‘chemically active’ and ‘chemically inert’ species on metal crystal surfaces. We find little evidence for universal behaviour, even among the inert gas ion projectiles. In contrast, we find complex dependences on incidence angle, scattering angle, and crystal azimuth. The mean energy loss to straggling ratios also show such complex behaviour, and differ significantly in magnitude for the various ions. We argue that the form of our energy distributions for Ar are principally determined by the stochastic nature of the excitation process itself. We report a series of regularly spaced peaks in the scattered atom energy distributions for Ar+ scattering, and evidence for such structures in Nf scattering. We suggest that a portion of the Ar beam may also undergo the skipping motion reported earlier for Si+ scattering from Cu(ll1). It is not yet clear why one projectile apparently undergoes skipping motion while others apparently do not. Our main indicator for such behaviour is a series of regularly spaced peaks in the scattered particle energy distribution. However, if the mean energy loss to straggling ratio is small, such peaks will not be resolved in an
of reactiveand
371
inert ion beams
experiment. We must extend our data set further, however, we speculate already that transient adsorption or skipping motion may in fact be a quite general phenomena in glancing-incidence beam-surface scattering, whose identification is simply hampered by inadequate separation of the individual energy loss peaks. In any case, these results, combined with a recent theoretical analysis 161, reveal the complexity of small angle scattering at surfaces, independent of the ‘chemical activity’ of the projectile at that surface.
Acknowledgements This work was generously supported by the Deutsche Forschungsgemeinschaft. We thank P. Hertel for invaluable advice in the use of FFT techniques.
References ill See for example: Proc. of Seventh International Workshop on Inelastic Ion-Surface Collisions, Radiat. Eff. Defects Sol. 109 (1989). 121 B. Willerding, W. Heiland and K.J. Snowdon, Phys. Rev. Lett. 53 (1984) 2031; P.H.F. Reijnen, P.J. van den Hoek, A.W. Kleyn, U. Imke and K.J. Snowdon. Surf. Sci. 221 (1989) 427; J.-H. Rechtien, U. Imke, K.J. Snowdon, P.H.F. Reijnen, P.J. van den Hoek, A.W. Kleyn and A. Nan&i, Surf. Sci. 227 (1990) 35. Phys. [31 K.J. Snowdon, D.J. O’Connor and R.J. MacDonald, Rev. Lett. 61 (1988) 1760; Surf. Sci. 221 (1989) 465. [41 K.J. Snowdon, D.J. O’Connor, M. Kato and R.J. MacDonald, Nucl. Instr. and Meth. B48 (1990) 327. ISI Y.H. Ohtsuki, K. Koyama and Y. Yamamura, Phys. Rev. B20 (1979) 5044. [61 M. Kato, K.J. Snowdon, D.J. O’Connor and R.J. MacDonald, Nucl. Instr. and Meth. B48 (1990) 39. [71 W. Mix, J.-H. Rechtien, D. Danailov, K.J. Snowdon, in preparation. 181 D. Rathmann, N. Exeler and B. Willerding, J. Phys. El8 (1985) 17. [91 W.H. Press, B.P. Flannery, S.A. Teukolsky and T. Vetterling, Numerical Recipes (Cambridge University Press, Cambridge, 1988). DOI J.-H. Rechtien, W. Mix, K.J. Snowdon, Surf. Sci. submitted. 1111 We sofve Newtons equations of motion using a potential constructed from pair potentials of the ‘universal’ type described by Biersack with the Ziegler screening length; J.P. Biersack and J.F. Ziegler, in: Ion Implantation Techniques, eds., H. Ryssel and H. Glawischnig, Springer Series in Electrophysics, Vol. 10 (Springer, Berlin, 1982) pp. 122-156. WI M. Kato, 3. Phys. Sac. Jpn. 55 (1986) 1011. [I31 J.-H. Rechtien, W. Mix and K.J. Snowdon, Surf. Sci., in press.
II. PARTICLE
SCATTERING