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Electron Emission from Clean Solid Surfaces by Fast Ions.

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Annalen der Phvsik. 7. Folge. Band 47. Heft 7. 1990. S. 555-567
J. A. Barth. LeiDzin
Electron Emission from Clean Solid Surfaces
by Fast Ions
By D. HASSELKAMP,
9. HIPPLER,
A. SCHARMB“and T. SCHMEHL
I. Physikalisches Institut der Justus-Liebig Universitat, Giessen
Zurn 200. Jahrestag der Annalen der PhysQ
A b s t r a c t . Total backward electron yields from 27 elemental, non-crystalline, clean solids were
measured during bombardment by H+-, H$-, H$-, He+- and Ax+-ions in the energy range from
100 keV to 800 keV. The yields were found t o exhibit an oscillatory dspendence on the atomic number of the target material correlated with the periods of the periodic system. These 2,-oscillations are
relatively insensitive t o the type of projectile and the impact energy a t the high projectile energies
of this experiment. Present theories of electron emission cannot explain the main experimental
results. The reasons for this failure are discussed.
Ioneninduzierte Elektronenemission von sauberen Metalloberfliichen
I n h a l t s i i b e r s i c h t . Es wurde der Elektronenkoeffizient von 27 elementaren, nichtkristallinen,
sauberen Festkorperobcrflachen beim BeschuB durch H+-,H$-, H$-, He+- und Ar+-Ionen im Energiebereich von 100 keV bis 800 keV gemessen. Die Koeffizienten zeigen ein mit den Perioden des
Periodensystems oszillatorisches Verhalten in Abhangigkeit von der Ordnungszahl des Targetmaterials. Diese 2,-Oszillationen sind bei den benntzten hohen Projektilenergien nur schwvach abhangig von der Art des Projektils und der EinschuBenergie. Die experimentellen Ergebnisse konnen von
gangigen Theorien nicht wiedergegeben werden. Die Griinde werden diskutiert.
1. Introduction
Electron emission from solid surfaces induced by high-energy ions is a result of the
direct energy transfer in projectile-target collisions to the electrons of the solid and of
the projectile. This process, which accompanies all kinds of ion-solid interactions a t
high impact energies, is known as “kinetic electron emission” [l].Within the last ten
years considerable progress was achieved in this field both experimentally [ 2- 61 and
theoretically [7- 111. One open question concerns the problem of t h r material dependence of the total electron yield y which is defined w the mean number of emitted electrons per incoming projectile. I n early works y was assumed to be almost independent
of the target material (e.g. [12]). Later mostly a monotoneous increase of y with the
atomic numbcr 2, of the target was conjectured [ 18-17]. However in the work of
[a, 6, 18, 191 evidence for a nonmonotonic behaviour emerged. Recently we have presented data for thc impact of HT-, €1: - and Ht-ions 011 a variety of different elemental
materials a t an impact energy of 100 keV/proton which gave evidence of an oscillatory
dependence of y with 2, [20]. It is the aim of this contribution to present details of the
experimental met hod and further results of a systematic study for impact energies rang-
Ann. Physik Leipzig 47 (1990) 7
556
ing from 100 keV to 800 keV and for different projectile species. The results allow
t o test the validity of current theories, the data are of relevance for the contrast of
pictures obtained in modern scanning ion microscopes [ 21- 231.
2. Apparatus
The projectiles (singly charged H+-, H$-, H$-, He+- and Ar+-ions) were delivered
by a 1.3 MeV Van de Graaff generator. After passing the analysing magnet the ions were
focussed onto the entrance slits of the UHV-chamber. A cold trap and a combination of
turbomolecular pump, cryo-pump and sublimation pump were mounted in the beamline-system between the accelerator and the first stage of the UHV-system. Here the
ion beam passed a set of apertures before entering the actual target chamber (Fig. 1).
The base pressure in the target chamber was -1-2x
Pa during tlhe experiments
and was provided by a combination of a sublimation and a turbomolecular pump.
The residual gas was analysed by a quadrupol mass spectrometer and was free of
hydrocarbon signals.
Chamber I
1r10-8Pa
Shield
I 1
Collector
1-
n
Chamber 1
5*1O-’Po
1 1
I I
Apertures
Fig. 1. Schematic view of the target-collector assembly for the measurement of the total electron
yields y
Before the ion beam entered the final Faraday-cup-assembly it passed through
two deflectiag plates which were used to eliminate spurious electrons in the beam (electron suppressor). Ion currents varied from 50nA to 500nA a t a beamdiameter of 1.5mm.
The measurements of the (backward) total electron yields y were performed in a Faraday-cup-assembly (Fig. 1). It consisted of the target which was finally struck by the
ion beam at normal incidence and a cylindrical electron collector in front of i t (a =
30 mm with an aperture of 6 mm for the ion beam inlet). The whole assembly was surrounded by a shield on ground potential.
Two currents were measured simultaneously by a computer-controlled interface :
the target current I,,which was the sum of the true ion current and the current of emitted electrons, and the (negativ) collector current I , , which corresponded to the current
of emitted electrons. For efficient collection of electrons the collector was biased a t
+lo0 V while thr targct \+as kept on ground potential. The electron yield is then given
by y = - - I C / ( I t- I,.). Data reduction was directly accomplished by the computer.
The absolute uncertainty in the determination of y is estimated to be below lo%,
the reproducibility within the whole data-set was f 3%. A comparison of the present
data with former measurements of our group with a different set-up [24] showed good
agreement .
D. HASSELKAMP
et al., Electron Emission by Fast Ions
667
27 different elemental solids were investigated: Be, c', Mg, Al, Si, Ti, Cr, Mn, Fe,
Co, Ni, Cu, Zn, Ge, Zr, Nb, Mo, Ag, Cd, In, Sn, Sb, Ta, W, Pt, Au and Pb. The targets
were massive samples of 1mm thickness and 0 = 10 mm. 14 targets were mounted on
a target holder a t a time and were positioned by a manipulator in front of the sputter
gun, the CMA or in the ion beam position, respectively. Before the experiment started
the targets were sputter-cleaned by a scannable sputter gun with Ar+-ions a t 5 keV.
The surfaces were then analysed in-situ by Auger-Electron-Spectroscopy with a commercial Varian-spectrometer. I n general the residual surface contamination (mainly
by carbon and oxygen) was below 2%. Exceptions were the carbon target (pyro-graphite)
for which the oxygen content could not be reduced below 1670,the Ti-target for which
the contamination by carbon and oxygen was 56y0 and <5y0, respectively, and the
Zr-target for which the carbon content a t the surface was 1 4 . 5 % .
3. Results
Typical results of this investigation are summarized in Figs. 2-4 in which the electron yields y for impact by H+, He+ and Ar+ are plotted as a function of the atomic
number 2, of the target material for four different projectile energies [25]. It is found
that the dependence of y = j(2,) is not a monotonic function; rather an oscillatory behaviour is observed that is correlated with the periods of the periodic system and that
is relatively independent of the impact energy. The overall-dependence is roughly the
same for all ions investigated. I n some cases a fine structure is observed (e.g. around
2, = 28(Ni)). Data for H2f- and H$-impact, which are not presented graphically, show
the same behaviour as the proton-data [20]; molecular effects are observed similar t o
the ones reported in [24, 261 but will not be discussed here. All data are for sputtercleaned target surfaces. Limited results are available for contaminated or oxydized targets in which cases the electron yields are in general larger than those reported here.
Only metals and semi-conductors have been studied in this work. It is well-known
that the yields from insulators are larger than for metal targets. The inclusion of insulating materials may therefore lead to larger variations of the yields y as j(2,).
The results confirm conjectures in [a, 6, 18, 201 of an oscillatory dependence of
y vs. 2, and disprove a monotonic behaviour as suggested in [13-171. The data are also
in approximate agreement with measurements a t low impact energies [18, 271 and agree
I
100 keV
200 keV
tiSOOkeV
b 800 keV
x-
0-
~
0
20
LO
60
80
ATOMIC NUMBER OF TARGET Z2
Pig. 2. Total yields y as a function of the atomic number 2, of tho target material for proton impact at different energies. Here and in the following figures thc lines are only meant to guide the eye
558
Ann. Physik Leipzig 47 (1990) 7
partially with recent high-energy results obtained in beam-foil experiments [191.
Excellent agreement is observed between the present data for proton impact and those
for electron impact at 2 keV [28] in the case of the transition elements. This is not SUTprising since at high projectile velocities the primary excitation process induced by
protons and electrons, respectively, will be similar.
Only small differences are observable when the results for different ions are compared. The oscillations are more pronounced for light ion impact than for heavy ion impact.
A closer inspection also reveals differences for different projectile ions in the period from
Ti to Ge. Furthermore the observed maximum for elements of the last period studied
occurs a t Au for proton and helium impact but a t Pt for argon impact. Thus there is
evidence that the 2,-dependence of the total yields y is slightly influenced by the projectile type. This result is in qualitative agreement with detailed work on the projectiledependence of the total yields of one single target a t low impact energies which led t o
the observation of pronounced 2,-oscillations [29, 301. These oscillations were tentatively
100 keV
0200 keV
0500 keV
A-800
keV
-x
r
0
0
20
I
Projectile He+
40
60
ATOMIC NUMBER OF TARGET 22
1
80
Fig. 3. Total yields y as a function of the atomic number 2,of the target material for helium ion
impact at different energies
1 *0D-
100 keV
Projectile . A r +
ZOO keV
So0 kev
i
+
OO
20
40
60
ATOMIC NUMBER OF TARGET
80
Z2
Fig. 4. Total yields y as a function of the atomic number 2, of the target material for argon ion impact a t different energiefj
559
D. HASSELKAMP
et al., Electron Emission by Fast Ions
explained as due to shell-effects in the primary collision event which depend on the electronic structures of target and projectile. The present oscillations depend mainly on the
target material and are different from those observed in [29, 301.
4. Discussion
Inspection of the results in Figs. 2-4 leads to two qualitative conclusions: First,
since the overall-variations of y = f(2,)are essentially independent of the type of projectile and its energy and further since proton arid electron impact [28] yield the same
results for the transition elements, we conclude that the observed oscillations are mainly
dependent on properties of the target material; a comparison between the data for proton
and argon impact indicates however that also the type of projectile plays a certain role.
Second, at first sight the variations of y vs. 2, within each period of the periodic system
resemble roughly those known for the number density of target atoms (as noted before
(z),
for electron impact in [28, 311) or for the electronic stopping power - of the projectile
in the solid.
For a quantitative understanding a comparison with theoretical predictions is
desirable. Unfortunately a complete theory is still lacking. Detailled work so far is
only available for some special cases, mainly for proton impact on aluminium [9,10].
For the present purpose we must therefore use those formulations which result in general
expressions for the total electron yield, i.e. the semi-empirical theory [Z,5, 181, the
results obtained by Sternglass [12], by Parilis and Kishinevskii [32] and by Schou [7, 111.
In these theories electron emission is described by a three-step-process: 1)the excitation
process, which at high impact energies is usually connected to the mean (electronic)
stopping power of the projectile in the target, 2) the diffusion of excited electrons in
the target, either by introducing a characteristic length for absorption or more realistically, the stopping power of low-energy electrons including cascade multiplication, 3) penetration of electrons through the solid-vacuum-interface in the plane-barrier approximation.
Since the basic predictions of the semi-empirical-, the Sternglass- and the Parilis
and Kishinevskii-theories are equivalent we will use only the first theory and the transport theory of Schou for the discussion. In the semi-empirical theory the total electron
yield y is given by the following formula (which assumes an isotropic distribution of excited electrons inside the target)
The underlying three-step-mechanism is clearly evident : i) the excitation process is
described by the inelastic stopping power
(2)
-
which, divided by the energy J needed
e
to produce one excited electron, gives the mean number of liberated electrons per unit
depth, ii) the diffusion process which is introduced by a characteristic mean length L
for absorption of excited electron in the solid and iii) the mean escape probability P
for an electron to surmount the surface barrier. Eq. (1) predicts a proportionality
(3,
bctween the total yield y and the electronic stopping power - in qualitative agrcemeiit with the conclusion reached a t the beginning of this section. The yield is further
determined by the quantities P , L and J which must be assumed to depend on the material and which will add t o the variation of y vs. 2,. Though there is little doubt that
eq. (1) describes electron rmissioii in a simplified yet correct way (strictly speaking
eq. (I) is valid for a source of monoenergetic electrons) any attempt t o use eq. (1)for
Ann. Physik Leipzig 47 (1990)7
560
a prediction of the material dependence of the yields y must fail because the input quantities are ill-defined : indeed, because the energy distribution of electrons inside the target
and its dependence on the target material (and the type of projectile) are neglected,
P,L and J are 'mean' quantities related to some mean energy of excited electrons which
(3
is not known. We have to conclude that with the exception of - the input quantities
in eq. (1) are presently not accessible to the necessary accuracy.
In the transport theory developed by Schou "7,111 the yield y is expressed as
+
= As x (0, 0,)
(2)
0,and 0,denote the amount of inelastically deposited energy near the target surface
7
by direct projectile-target interactions and by recoil ionization, respectively. The factor
takes account of the diffusion of electrons on the basis of the low-energy electron
stopping power and of the escape process. I n case that recoil ionization can be neglected,
e.g. for energetic proton impact, the yield is given by
AS
Thus y is again predicted to be proportional to the electronic stopping power
me
and to a material parameter As.The only principal difference to the expression from the
semi-empirical theory (eq. (1)) is the factor @ whichiaccounts for energy transport by
excited electrons or by reflected primaries and which is predicted to be a slowly varying
function of the projectile velocity (for proton impact j9 M 0.3 from 100 keV t o 1 MeV
[ll]).A prediction of the material dependence of the yields y on the basis of eqs. (2 3)
is difficult since the input quantities are not easily evaluted: 0,and 0,as well as 9, have
to be calculated by a transport theory, the main input into&! is the low-energy stopping
power of excited electrons which is not well known [ll].So far a direct comparison of
3) with the experimental data in Figs. 2-4 is not possible.
eqs. (2
Besides the fact that input quantities in current theories are not well-defined or
are difficult to evaluate it should be kept in mind that the theoretical expressions in
eqs. (1- 3) neglect certain aspects of the complicated phenomenon of electron emission.
Major shortcomings are : 1) neglection of distinct excitation mechanisms which have
been experimentally identified as there are : one-electron plasmon decay [33- 351,
electron loss from the projectile [26, 361 or excitation by quasi-molecular effects [29, 301;
these processes will change the probability of electron excitation and the corresponding
energy spectrum; 2. in the semi-empirical theory the value for the stopping power is uncertain: due to non-equilibrium effects the stopping power near the surface can be different from that in the bulk (e.g. [37]); this deficiency is partly removed in the theory
of Schou (factor ,3 in eq. (3)) ; 3. recoil ionization and transport phenomena are included
only in the theory of Schou [7].
For a further qualitative discussion we come back to one prediction of eqs. (1-3):
it is the proposed factorization on the right-hand sides of the expressions for the yields
into a factor describing the excitation process and another factor which is predicted
to depend mainly on material properties. Indeed, as long as the energy distribution of
electrons in the target is neglected (or is independent of the impact energy) the factor
PL/J in eq. (1) involves quantities related t o target properties in the semi-empirical
theory. Also the factor Asp is predicted to be a material parameter in the transport
theory of Schou as long as ,3 is constant. A t this point it is convenient to introduce
the quantity
+
+
1).HASSELKAIVIP
et a!.,
Electron Emission by Fast Ions
(z)),
561
based on experimental values for y and data from the literature for - [38, 391 in
order to test if this quantity is a material parameter as predicted by theory. We note
that AexPis a measure of the efficiency with which inelastically deposited energy is converted into detectable electrons for a given projectile-target combination and impact
energy (given in number of emitted electrons per unit of deposited inelastic energy per
unit length). I n the following we will investigate the energy- and material-dependence
of AExP.
We firstly constrain the discussion to proton impact. The energy dependence of the
parameter AexP was studied for all target materials in the energy range from about
100 keV to 800 keV
((2).
- was taken from the tabulation in [39]
1.
Without giving the
corresponding plots we summarize that AexPis nearly independent of the primary proton
energy for all materials studied [40];this finding is in agreement with former investigations covering a large energy interval1 (e.g. [S]). We conclude that the results for proton
impact support the interpretation of theory that the ratio ye)!(/
has the physical mean-
ing of a material parameter. This result can further be interpreted within the framework
of the semi-empirical theory. I n recent measurements of the energy distribution curves of
emitted electrons i t was demonstrated that the shapes of the low-energy spectra for a
given target material were independent of the projectile energy for proton impact [41].
It follows from this observation that also the mean energies of emitted electrons, and
consequently, the mean escape depth L and the mean escape probability P in eq. (1)
will indeed be independent of the ion energy for proton impact.
We now turn to the question of the material dependence of the ratio y
/g)e
*
The
corresponding parameters AexPfor proton impact are depicted as a function of the atomic
number 2, of the target material in Fig. 5 and are summarized in Table 1. It is found
that AexPvaries with Z,, maximal and minimal values for the targets investigated vary
by as much as a factor of two. From the viewpoint of the efficiency of the transformation
of deposited inelastic energy into detectable secondary electrons we have elements with
a low efficiency (e.g. Be, Si, Cr, Zr, Nb, Mo, Ta) and those with a high efficiency (e.g. C,
Mg, Zn, Cd, In, Pb). A mean value of the material parameter can be taken from Fig. 5
t o be
-
A"P m O.l&eV
(5)
in agreement with [2] in which the same mean value was derived for low-energy proton
impact. Eq. (5) allows an estimation of y from known stopping powers (and vice versa)
t o a precision of 50% for clean metals and semiconductors.
I n Tab. 2 we compare published values of the parameter A e x p which were obtained
in different intervals of the proton energy. I n general good agreement is observed between
the data-sets for massive targets (rows 2-5 and 7). Some of the differences may be due
to different sources of the stopping powers used in the evaluation of AexPby the original
authors. Also included in Tab. 2 are recent results of backward electron yields obtained
from proton hombardrncnt of thin foils [44]. I n [44] the parameterAexp is given in units
of mg
MeV-'. For the comparison with the other data in Tab. 2 this iinit was converted to the usual A/eV by using mass densities given in [45]. Only for Cu there is good
agreement with the results for massive targets. I n the other cases the values obtained
from the foil-experiment are larger by a factor two. The high value for A1 is difficult t o
understand. Though a detailed comparison of yield-data from massive targets and from
thin foils has not yct been performed it secms to be a general fact that reported back-
Ann. Physik Leipzig 47 (1990) i
562
ward yields from foils are larger than those obtained for massive targets. This may be
due to different states of surface and bulk in the two types of experiments. The problem
is however not yet settled.
These results can partly be compared with theoretical predictions. We first note that
a direct calculation of A = y
/rg)e
within the framework of the semi-empirical theory
is not possible because as noted above most input quantities are not accessible t o any
accuracy. The same holds for the Sternglass-theory [12] which however contains an
additional term which takes account of the energy transport by recoiling &electrons.
02r--O0 0
"
20
'
Projectile
'
HI +
'
40 O
60
b
'
1
b
80
ATOMIC NUMBER OF TARGET 22
-
Fig. 5. The parameter A e x p - y/(9,as
for proton impact
a function of the atomic number Z2of the target material
Table 1. The mean value of the parameter A e z p = y / K ) @ and its standard deviation in the energy
rangc from 100 keV t o 800 keV for proton impact,
Element
Z,
Aexp in &eV
Be
4
6
12
13
14
22
24
26
26
27
28
29
30
32
0.078
0.113
0.152
0.107
0.082
0.069
0.057
0.070
0.072
0.078
0.077
0.094
0.127
0.094
c
Mg
A1
Si
Ti
Cr
MI1
Ft;
co
Xi
Cll
%I1
Ge
& 0.003
& 0.006
& 0.005
k 0.00:!
k 0.004
& 0.005
& 0.003
f 0.004
0.004
k 0.004
f 0.003
f. 0.003
2 0.004
k 0.004
*
Element
z
2
A e x p in
Zr
Nb
Mo
40
41
42
47
48
49
50
51
73
i4
78
79
S2
0.070
0.065
0.068
0.129
0.145
0.141
0.115
0.113
0.070
0.074
0.092
0.109
0.118
4
Cd
In
Sn
Sb
T 2.
w
Pt
AU
Pb
&eV
f 0.004
k
k
ztr
&
f
k
k
&
f
f
&
&
0.003
0.003
0.005
0.003
0.006
0.004
0.006
0.004
0.0012
0.004
0.003
0.006
563
D. HASSELKAMP
e t al., Electron Emission by Fast Ions
Table 2. Comparison of published values of the parameter AexP --= y/fg),for
Target
AexP in &eV
Ref. [lS]") Refs. [42, 431 Ref. [24Ib) Ref. [47Ic) Ref. [44Id) this work
-
Be
C
-
Mg
A1
Ti
Cr
Ni
cu
Ag
W
Au
protol; impact
0.11
0.11
-
0.077 1 R )
-
-
-
0.133
-
0.40
0.17
0.09
0.13
0.07
0.10
0.082
0.125
0.13
0.105
0.075
0.08
0.125
0.12
-
0.27
-
0.098
Theory
Ref. [Ille)
0.077
0.078
0.113
0.152
0.107
0.069
0.057
0.077
0.094
0.129
0.074
0.109
-
0.105
0.099
-
-
-
") 6-50 keV; b, SO--1000 keV; c , 4-12 MeV; d, 300-1200 keV; e, 100-1000 keV;
[42];
R)
f,
10-350keV
10-400 keV 1431
From this term it follows that the ratio y /($)eshould
decrease with increasing projec-
tile energy. Such an behaviour is not observed-in the experimental data of this work
for proton impact and in fact is also not found for data covering a very large impact
energy interval [S]. I n the transport theory of Schou [7,11] the relevant quantity is the
product ASP (eq. (3)) with which the experimental data A E x p have t o be compared. Unfortunately both As and P are not easily available. Only for the target materials Be,
Mg and A1 a comparison is possible since for these metals the results of a recent calculation are available [ll]. These data are included in Tab. 2 in the last row. Excellent agreement between experimental and theoretical values is found for Be and Al, the agreement is not very good for Mg. Nevertheless this result is encouraging from the viewpoint
of theory.
We now turn to the case of heavy ion impact and use our results for argon ion impact as an example. Contrary t o the proton case many authors have noted that the total
electron yield y is not proportional t o the electronic stopping power with respect to
its energy dependence for primary energies > 100 keV [24, 42, 43, 46, 481. Furthermore
many authors report thatAexPis also dependent on the type of projectile at high energies
[24, 42, 43, 461.
The energy dependence of t h e parameter A e z P will be discussed a t first. In Fig. 6a+b
we present typical examples from our work. Fig. 6 b depicts the parameter Aez?)as a
function of the Ar+-energy for the targets Cr, Zn and Au, Fig. 6 a for Si, Ti and Sn.
The corresponding electronic stopping powers were taken from [38]. For most of the
materials studied we find a decrease of Ae.zI)with increasing projectile energy (Fig. 6b) in
agreement with most of the previous work. However it is clearly demonstrated in Fig. 6a
that in special cases also an energy independent value of Aexpis observed in the energy
interval studied. From these results it follows that in general the ratio y
/(g)*
cannot
(z)e
be interpreted as a material parameter which is contrary to the proton case discussed
above. This result was to be foreseen : the electronic stopping power
-
does not take
Ann. Physik Leipzig 47 (1990) 7
664
account of secondary effects like recoil ionization or the reflection of primaries which
may be important sources of electron excitation for argon bombardment. Since these
effects are most efficient a t the lower energies used in this experiment their neglection
may explain the observed energy dependence of AexP(recoil ionization may be neglected
at 800 keV but not at 100 keV [42, 431). A more valid treatment would be to use the
(2Ie
total deposited energy a t the surface instead of - as proposed in eq. (2). Attempts
to verify y / D (where D denotes the total deposited energy a t the surface) as a material
parameter have been made with a somewhat uncertain result [42, 43, 461 since no agreement with values obtained for proton bombardment could be reached (see below).
The puzzling observation that AexPis nearly constant for the target materials Si, Ti, Ge
and Sn (Figs. 6a and 7) remains unexplained a t the moment. This fact may be either
due to the uncertainty in the used stopping powers [38] or to shell effects which are
specific to the projectile-target combination.
Ar - Si
Ar- Ti
Ar-Sn
-x
O-
A
-
0.1 0
0.05
1, 1, I
xxx-rx--X-7-x
I
0
200
,
I
,
LOO 600 800
ENERGY/ keV
",:!',
,
0
200
,
,
,I
400 600 800
ENERGY/keV
as a function of the projectile energy for argon ion impact
Fig. 6. The parameter AexP = y / ( 9
e
on different target materials
I n Fig. 7 we have plotted AexP for argon ion impact as a function of the atomic
number Z , of the target material a t 100 keV and 800 keV. Thus besides the material
dependence also the rough energy dependence of AexPis recognized. I n comparison with
the proton data we find the following features for argon impact : The overall dependence
of AexPas a function of 2, is similar for proton and argon bombardment, the oscillations
are however less pronounced for argon impact. Larger deviations are observed in the
sequence Mg-Al-Si
and for Z, > 74 which indicate a dependence on the specific
projectile-target combination. Mean values of AexPare wO.OBA/eV for the lower 2,region and w0.075afeV a t higher Z , for 800 keV argon impact (where contributions
by recoil ionization and reflection of primaries are of minor importance). These values
are lower than the corresponding mean estimate for proton impact (eq. (5)). Recalling
that the parameter AexP may be interpreted as the efficiency of the transformation of
inelastically deposited energy into detectable electrons we note that this efficiency is in
D. HASSELKAMP
e t al., Electron Emission by Fast Ions
565
general smaller for argon ions (at 800 keV) than for protons. A decrease of Aexp with
increasing 2, of the impacting ion (including effects of recoil ionization) was already noted
in [42, 43, 461 for bombardment of aluminium arid copper by noble gas ions.
021
I
I
0
20
I
I
I
r
I
I
I
I
40
60
80
ATOMIC NUMBER OF TARGET Z2
Fig. 7. The parameter AexP = y / E ) e a s a function of the atomic number Z2of the target material
for argon ion impact at 100 keV and 800 keV
The foregoing discussion leads t o the conclusion that the interpretation of AexP as a
material parameter arid the factorization of the expressions for the total electron yield
(eq. (1-3), see above) are questionable for heavy ion impact even if recoil ionization
is taken into account. Especially the observation of Ae“P(Ar+)< Aex”(H+)for high impact
energies is not predicted. There are however several reasons why current theories may
fail in the case of heavy ion impact: (a) It was shown in [41] that the energy spectrum
of electrons from several materials under argon bombardment differed from that observed for proton impact and further that the shape of the spectra variied with impact
energy. Thus there is evidence that the ‘inner’ energy distribution of electrons is dependent on the type and energy of the projectiles for heavy ion impact. This fact is not included in the derivation of eqs. (1-3). (b) Non-equilibration of the projectile charge in
the first layers of the solid may lead t o effective stopping powers different of those for
the bulk [37, 42, 431. (c) Quasi-molecular processes in projectile-target collisions are
not included in current theories. (d) Inner-shell ionization of heavy projectiles will contribute t o the stopping power but may lead to an energy transport into the solid. I n
this manner energy may be lost from the near-surface region. As an example we consider
the creation of a 2p-vacancy in the Ar-projectile near the surface at an impact energy of
500 keV. The lifetime is crudely estimated as zzp m 5 x
s [49, 501. I n this case part
of the energy lost near the surface will be deposited some 75 nm below the surface which
is larger than the mean escape depth. A decrease of the amount of inelastically deposited
energy at the surface and a decreasing electron yield with increasing projectile velocity
will be a consequence.
Though it is not possible at the moment to estimate the importance of the individual
processes mentioned above on the electron emission induced by heavy ions it is clear
that the case of heavy ion impact is more complicated than the proton case. While
for proton impact several predictions of theories are at least in reasonable qualitative
566
Ann. Phyaik Leipzig 47 (1990) 7
(and sometimes even quantitative) agreement with the experimental results this is not
the case for argon impact. It must therefore be questionned if present theories can handle
the heavy ion case properly.
6. Conclusions
Total (backward) secondary electron yields were measured from 27 clean, elemental
solids under bombardment by high energy ions. The yields were found to exhibit an
oscillatory dependence on the atomic number of the target materials correlated with
the periods of the periodic system. The overall-behaviour is relative insensitive to variations of the type of ion and the ion energy in the range from 100 keV to 800 keV. The
periodicity of the yields is only partly due t o the well-known variations of the electronic
stopping power a t high impact energies. A comparison with the predictions of current
theories is difficult since the corresponding input quantities are either ill-defined or not
readily available. A quantitative comparison is only possible for proton bombardment
of Be, Mg and A1 where a t least for Be and A1 good agreement is found with recent results
of a transport theory. It is further concluded that present theories are basically to crude
to account for the complicated features of heavy-ion induced electron emission.
A c k n o w l e d g e m e n t s . The authors gratefully acknowledge the use of a cyrosublimation pumping unit by Dr. R. Hippler and Prof. Dr. H. Lutz, University of Bielefeld. They also express their thanks to Prof. Dr. P. Mokler and H. Folger, GSI, Darmstadt, for providing some of the metal targets, and to G. Trylat for the skillful operation
of the accelerator.
References
[l]KREBS,K. H.: Fortschr. Phys. 16 (1968) 419.
[a]
BARAGIOLA,
R. A.; ALONSO,
E.v.; FERR6N, J.; OLIVA-FLORIO,
A.: Surface. Sci. 90 (1979) 240.
[3] BENAZETH,
N. : Nucl. Instrum. Meth. 194 (1982) 405.
[4] KREBS,K. H.: Vacuum 33 (1983) 555.
[5] HASSELKAMP,
D. : Habilitationsschrift, GieBen, FRG, 1985.
[6] HASSBLKAMP,
D.: Comments At. Mol. Phys. 21 (1988) 241.
[7] SCHOU,
J.: Phys. Rev. B 22 (1980) 2141.
[8] SIGMUND,
P.; TOUOAARD,
S.: In: “Inelastic Particle-Surface Collisions” (E. Taglauer and W.
HEILAND
eds.) Berlin: Springer-Verlag 1981, pp. 2.
191 R~SLER,
M.; BRAUER,W.: Phys. stat. sol. (b) 104 (1981) 161, 576; ibid. 126 (1984) 629; ibid.
148 (1988) 213.
[lo] DUBUS,A.; DEVOOOHT,
J.; DEHAES,
J. C.: Nucl. Instrum. Meth. B 13 (1986) 623.
[ll] SCHOU,
J.: Scanning Microscopy 2 (1988) 607.
[12] STERNGLASS,
E. J.: Phys. Rev. 108 (1957) 1.
[13] LARGE,
L. N.; WHITLOCK,
W. S.: Proc. Phys. SOC.79 (1962) 148.
J. L.; TRAPP,C. M.: J. Appl. Phys. 43 (1972) 3318.
[14] WURTZ,
[15] DOROZHKIN,
A. A.; MISHIN, A. N.; PETROV,
N. N.: Bull. Acad. Sci. USSR, Phys. Ser. 38 (1974)
60.
[16] KOYAMA,
A.; SHIKATA,T.; SAKAIRI,H.; YAGI,E.: Jap. J. Appl. Phys. 21 (1982) 586.
[17] THOMAS,
E. W.: Nucl. Fus. Spec. Issue 94 (1984) 94
[IS] BSRAOIOLA,
R. A.; ALONSO,
E. V.; OLIVA-FLORIO,
A.: Phys. Rev. B 19 (1979) 121.
[19] CLOUVAS,
A.; ROTKARD,
H.; BURKHARD,
M.; KRONEBERGER,
K.; BIEDERMANN,
C.; KEMMLER,
J.; GROENEVELD,
K. 0.; KIRSCH,R.; MISAELIDES, P.; KATSANOS,
A.: Phys. Rev. B 39 (1989)
6316.
D.; SCHARMANN,
A.: Nucl. Instrum. Meth. B 34 (1988) 518.
[20] HIPPLER,S.; HASSELKAMP,
D. HASSELKAMP
et al., Electron Emission by Fast Ions
Xi
[21] KNEIS,H.; MARTIN,B.; NOBILING,
R.; POVH,B.; TRAYEL,
I<.: Nucl. Iiistriim. Meth. 197 (198;')
79.
R. : Scanning Electron Microsc. 1 (1983) 1.
[22] LEVI-SETTI,
[23] LAI, S. Y.; BRIGGS,
D.; BROWN,
A.; VICKERMAN,
J. C.: Surf. Interf. Anal. 8 (1986) 93.
U.; LANG,
K. G.; SCHARMANN,
A.; STILLER,
N.: SucI. Instrum. Xeth. 180 (1981)
[24] HASSELKAMP,
349.
[25] Data tables are available on request.
[26] HASSELICAMP,
D.; HIPPLER,S.; SCHARMANN,
A; SCHARTNER,
I L K : Z.Pliysik D 6 (1987) 269.
1271 ZALM,P. C.; BECKERS,
L. J.: Philips J. Res. 39 (1984) 61.
[28] MAKAROV,V. V.; PETROV,
N. N.: Soviet Phys.-Solid State 58 (1981) 1028.
[29] THUM,
F.; HOFER,
W. 0.:Nucl. Instrum. Meth. B 2 (1984) 531.
[30] FERGUSON,
M. M.; HOFER,
W. 0.: Rad. Effects Def. Solids 109 (1989) 273.
M. V.; LETUNOV,
N. L.: Soviet Phys.-Solid State 7 (1965) 316.
[31] GOMOYUNOVA,
1321 PARILIS,E. S.; KISHINEVSILII,
L. M.: Soviet Phys.-Solid State 3 (1960) 2046.
[33] HASSELKAMP,
D. ; SCHARMANN,
A.: Surface Sci. 119 (1982) L388.
[34] HASSELKAMP,
D.; SCHARMANN,
A.: Vak.-Tech. 35 (1983) 9.
[35] BURKHARD,
M. F.; ROTHARD,
H.; GROENEVELD,
K.-0.: phys. stat. sol. (b) 147 (1988) 689.
[36] HASSELKAMP,
D.; HIPPLER,
S.; SCHARMANN,
A.: Nucl. Instrum. Meth. B 5 (1984) 475.
[37] KOSCRAR,
P.; KRONENBERGER,
K.; CLOUVAS,
A.; BURKILLRD,
M.;MECILBACH,
W.; HEIL,0.;
KENMLER,
J.; ROTHARD,
H.; GROENEVELD,
K. 0.: Phys. Rev. A 10 (1989) 3632.
J. F.; BIERSACK,
J. P.; LITTMARK,
U.: The Stopping and Range of Ions in Solids.
[38] ZIEQLER,
New York: Pergamon Press, 1985.
J. F.: At. Data Nucl. Data Tables 27 (1982) 314.
[39] JANNI,
[40] HIPPLEE,
S.: Thesis, Giessen, FRG, 1988.
[41] HASSELKANP,
D.; HIPPLER,
S.; SCHARIVIANX,
A.: Nucl. Iiistrum. Meth. B 1s (1987) 561.
B.; HOLMI~N,
G.: J. Appl. Phys. 52 (1981) 6928.
1421 SVENSSON,
[43] HOLMAN,
G.; SVENSSON,
B.; BURBN,A.: Nucl. lnstrum. Meth. Id5 (1981) 623.
[44] ROTHARD,
H. ; KRONEBEROER,
K. ; BURKHARD,
N.; HEMMLER,
J. ; KOSCHAR,
P. ; HEIL,0. ;
BIEDERMANN,
C. ; LENCINAS,
S. ; KELLER,
N. ; LORENZEN,
P. ; HOTMANN,
D. ; CLOUVA~,
A. ;
GROENEVELD,K. 0.; VEJE, E.: Rad. Effects Uef. Solids 109 (1989) 281.
[45] BURPHARD,
M,: Thesis, Frankfurt, FRG, 1986.
G.; SVENSSON,
B.; SCHOU,
J.; SIGMUND,
P.: Phys. Rev. B 20 (1979) 2227.
[46] HOLM~N,
[47] KOYAMA,
A.; SKUUTA, T.; SAEAIRI, H.: Jap. J. Appl. Phys. 20 (1981) 65.
[48j FRISCHKORN,
H. J.; GROENEVELD,K. 0. : Physica Scripta T 6 (1983) 89.
D. L.; BHALLA,
C. B.: Phys. Rev. A 4 (1971) 2164.
[49] WALTER,
[SO] SCHARTNER,
K. H.; HOOGKAMER,
TH. P.; WOERLE,
P.; SARIS,F. W.: Nucl. Instrum. Alcth. IH:!
(1976) 36.
Bei der Redaktion eingegangen am 18. Januar 1990.
Anschr. d. Verf. : Dr. D. HASSELKAMP,
Dr. S. HIPPLER,
Prof. Dr. A. SCHARMANN,
DipLPhys. T. SCHMEHL
I. Physikalisches Institiit der
Justus-Liebig-Universita t
Heinrich-Buff-Ring 16
D-6300 Oiessen, BRU
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