A diamond surface undergoes a transformation in its electronic structure by a vacuum anneal at approximately 900 C. The polished surface has no electronic states in the band gap, whereas the annealed surface has both occupied and unoccupied states in the and gap and exhibits some electrical conductivity. The effect of this transformation on the strength of the diamond metal interface was investigated by measuring the static friction force of an atomically clean meta sphere on a diamond flat in ultrahigh vacuum. It was found that low friction (weak bonding) is associated with the diamond surface devoid of gap states whereas high friction (strong bonding) is associated with the diamond surface with gap states. Exposure of the annealed surface to excited hydrogen also leads to weak bonding. The interfacial bond is discussed in terms of interaction of the metal conduction band electrons with the band gap states on the diamond surface. Effects of surface electrical conductivity on the interfacial bond are also be considered

Pepper, S. V.

English

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effect of Electronic 'A -- -u^re of e
Diamond Surface on the Strength of the
Diamond-Metal Interface
--	 °^► 	 OCT
Stephen V. F
Lewis Research Center	 ^ NASA 8" ^ ^^A	 ^
Cl eveland, Ohio,
Prepared for the
Twenty-eighth National Symposium of the American Vacuum Society
1	 Anaheim, California, November 2-6, 1981
f
Y.1111
YD
EFFECT OF ELECTRONIC STRUCTURE OF THE DIAMOND SURFACE
ON THE STRENGTH ULF THE UTAMUND-MIRY"AG INTERFACE
by
Stephen V. pepper
National Aeronautics and Space Administration
'	 Lewis Research Center
Cleveland, Ohio 44135
Abstract
Recent electron spectroscopic investigations nave shown
teat the diamond surface unaergoes a transformation in its
J
electronic structure by a vacuum anneal at IvIO0 0 C.	 'one
o^
polishea surface has no electronic states in the nand gap,
w
whereas the annealea surface has ootti occupied and unoccupied
states in the band gap.	 in addition, the annealed surface
exhibits some electrical conductivity. 	 The effect of this
' transtormation on the strength of the diamond-metal interface
a
is investigatea by measuring cne static friction force of an
atoidically clean :petal sphere on a diamond flat in ultrahigh
vacuum.	 The friction force is due to interfacial bonding. 	 It
` is found that low friction (weak bonding)	 is associated with
the diamond surface devoid of gap states whereas nigh friction
(strong bonding)	 is associated with the diamond surface with
gap states.	 Exposure of the annealea surface to e3cited
^G,
__ 
 ^..............
hydrogen also leads to weak bonding. rho interfacial bond will
be discussed in terms of interaction of the metal conduction
band electrons with the band gap states on the diamond
surface. Etfects of surface electrical conductivity on the
interfacial bond will also be considered.
W^y.	 Introduction
The electronic structure of the diamond surface has
recently been observed to undergo a transformation by vacuum
annealing at N900 0
 C. Prior to annealing the polished
surtace exhibits no surface states, whereas the annealed
surface exhibits uoth occupied (rei. 1, 2) and unoccupied (ret.
3 0, 4) surtace states. The annealed surtace also exhioits some
electrical conductivity (ref. 5) . Since the electron ► .:
structure of a semiconductor-metal interface is determined by
the alectroniQ structure of the semic:onouctur surface with
wnich the interface is formed (ref. u) , the uiamonu-weLa i
5c11ottky bdrrier snuuld De attected uy tnis transtormaLiul1. All
understanding ut Schottky barrier formation is important in
5ullu state electronics ana has ueen the suojec:t of intense
Investigation in this journai;
A very uitterent area in which the electronic structure of
the diamonu surtace may oe important is the metal worKing
industry. The strength of the diamonu-metal interface is a
factor ueter ,nining the wear of aiamond cutting tools and the
Lice of aiamond-impregnated grinding wheels. Interfacial
strength is affected by interfacial chemical bonds that may be
tormed and these bonds are part of tt:e intertacial electronic
structure. 'Thus the transformation of the electronic structure
of the diamond surface is likely to aftect the strength of the
aiamond-metal interface.
in this paper it is shown that there is a correlation
between the electronic structure or the diamond surtace as
probed by electron energy lo gs spectroscopy (ELS) and the
strength of the diamond-metal interface as probed by the static
coefficient of friction of a metal sphere on a flat diamond in
ultranigh vacuum. We first review the evidence obtained by ELS
for the appearance of unoccupied states in the band gap of the
annealed diamonu surtace. The static friction experiment is
then described and the correlation betwen the appearance of
unoccupied gap states on diamond with high static friction is
demonstrated. The results presented here a.e for the (11U)
surface of diamond, but similar results have been obtained for
the (111) surtace. Finally, the interfacial chemical bond is
discussed in terms of elementary concepts of molecular orbital
tnerory.
Uiamond surtace
i'; ► e understanding of the ciamonu surtace has recently been
turthered by electron spectroscopies. Photoemission
spectroscopy has shown that the surtace annealed at '900° C
exhibits occupied surtace states near the top of the valence
band and possibly in the nand gap (ref. 1, 1). ELS probes
unoccupied surtace states and shows that the annealed surtace
exniuits unoccupied surtace states that appear in the band gap
(ref. 3). The electrical conductivity observed for the
2
r4
annealed surface is evidently related to these surface states
in the vicinity of the band gap. The use of ELS is
particularly appropriate here because the interfacial chemical
bond will be considered in terms of the interaction of metal
electrons with unoccupied surtace states ► on diamond.
Figure 1 presents the energy loss specta for ionization of
C(lS) electrons into empty states (ref. 3, 4). The damaging
effect of the electron beam has been eliminated as reportea
(ref. 3) . The large feature (labeled K l ) is due to a
transition into the lowest maximum in the conduction band
jensity of states. The top of the 5.5 eV band gap has been
positioned at the bottom of the conduction band which was
determined by taking the energy interval of 3.6eV between the
lowest maximum (K l ) and the bottom of the conduction band
Crum a band structure calculation (ref. 7). A polished diamond
surface exhibits no states in the band gap. After annealing at
%9UO° C, the spectrum exhibits an additional feature labeled
K0
 locatea in the band gap. This is obseved for both the
(110) and (111) surfaces. The magnitude of K O
 as a function
of temperature for successive two minute annealing cycles is
indicated for the (11U) surface in Fig. 2. For this annealing
schedule the transformation for the polished surface begins
" 50 0 C and is complete by %900° C.
As indicated in Fig. 1, the K 0 feature could be removed
by exposing the annealed surface to hydrogen excited by the
ionization gauge filament for lU minutes at 5x10 5
 torr (ref.
3
4). When this "regenerates" surface is suojected to the above
annealing schedule, the K0 feature appe p.re first it a lower
temperature than for the polishes surface. The large increase
to the maximum value, however, occurs at the same temperature
for both surfaces. There is thus a difference, not as yet
understood, between the polished and regenerated surface even
though they are both devoid of unoccupied surface statues.
'there are two points in these results that must be
discussed. First, it is widely assumes that the polished
surface is covered witn a cnemisoroed layer of nydrogen that is
responsible for the absence of surface states (ref. 1, 2).
Annealing ( tiyUU° C) or electron bombardment (ref.. 3)
presumauly removes the hydrogen and generates the observed
surface states. The removal of the unoccupied states by
exposure to hydrogen can be taken as evidence nor chemisorbtion
of hydrogen. However, at this time there is still no direct
experimental detection of surtace hydrogen and this must still
be treated as an assumption. Secona, there is the question of
whether the energy of the v-,occupied surface state is in the
band gap of the ground state of the crystal or whether it lies
higher in the conduction band. The difference is due to the
effect of electron-core hole binding energy (exciton) and nas
been encountered in the determination of the energy of surface
states on GaAs, for example (ref. d). The question must be
resolved before a theory of the interfacial chemical bond based
n„ oan g ates on diamond can be developed.
tl
4
To summarize these results, the polished surface exhibits
no unoccupied surface states and is probably covered with
ehemisorbed hydrogen. Annealing the surface to ~900 0 C
introduces unoccupied surface states in the band gap and
removes the hydrogen. Exposure to excited hydrogen removes the
unoccupied surface states, although this regenerated surface
may not be the same as the polished surface.
Diamond - Copper Friction
The strength of the diamond-metal interface is assessed by
the static friction of an atomically clean copper sphere on the
flat diamond as a function of diamond surface condition. The
basis for this approach is the adhesion theory of friction
(ret. 9) according to which the tangential force is due to the
rupture of interfacial adhesive bonds formed at asperity
contacts. This approach to understanding interfacial strength
has been used to investigate the Al 20 3
 - metal system (ref.
1U). The trends observed in the static friction experiments
have also been observed in thin film adhesion so that under
these Experimental conditions static friction is considered at
least as a reliable probe of interfacial strength as is thin
film adhesion.
The experimental apparatus has been described before (ref.
10) and consists of the copper sphere contacting the flat
diamond at a pressure ~5x10 -9 torr in the same chamber used
5
for ,surface analysis. The diamond is subjected to the thermal
treatment discussed above and the sphere is cleaned Loy argon
ion bombardment after each anneal. The normal load of the
sphere on the flat is 57 gmf. and the tangential force
developed is sensed by a piezo-electric torce transducer on the
diamond mount. This force is developed within a few
micrometers of relative displacement of the sphere with respect
to the flat after which the force no longer, inereasPT and the
system starts to slide. In our experiments the static and
kinetic friction forces were the same. This maximum friction
force is expressed as a fraction of the normal force (coeffi-
cient of friction) and is used here simply to indicate the
relative strength of the interface as a function of diamond
surface condition, an increase in fiction implying an increase
in interfacial strength.
In Fig. 3 the coefficient of friction u as a tunetion of
annealing temperature is presented. The value of u for the
polishes surface annealed at ^050° C is about 0.1i. Uther
experimental trials have yielded u ti 0.05, outstandingly
low values for an atomically clean solid interface. The
friction is then observed to increase to %U.5 after annealing
the diamond at %850° C, the same annealing temperature for
which unoccupied surface states appeared in the energy loss
spectrum (Fig. 2). Thus there is a correlation between
increased interfacial strength and the appearance of unoccupied
surface states in the band gap. This correlation was observed
L . ,
6
7
	 it
with both the (111) and (110) surfaces. it was observed before
the transformation of the stirtace electronic structure was
known and in fact prompted the surface analysis that led to the
observation of surface states (ref. 3). This lends confidence
to the interpretation of the friction in terms of surface
effects.
The increase in friction was also observed after bombarding
the polished diamond surface with 500eV electrons. Since it
has been shown that unoccupied gap states are generated by
electron oombaromerit, this supports the above correlation.
At1er the friction attained the high value, the diamond was
exposed to excited hydrogen, after which the copper sphere was
recleaned. As ina.cated in Fig. 3, the friction was reduced,
although not to the initial value of %0.11. The reduction in
friction is in accord with the above correlation since the
unoccupied surface states are removed by hydrogen exposure.
The fact that it does not return to its initial value may be
related to the appearance of a small amplitude of K 0 at
temperatures .tower than that of the initial transformation. In
any case the friction does again increase to its high value
after annealing at the same temperature ( 1450 0 C) for which
the increase was first observed.
Discussion
E-
The correlation of high 	 strength with the
presence of unoccupied surface stbtes in the band gap leads to
consideration of a possible interfacial chemical bond based on
the energy level diagram of Fig. 4. First, recall from
elementary molecular-orbital theory of chemical bond formation
that a bond is formed by partially occupied orbitals of similar
energy (ref. 11). The orbitals that constitute the valence
band of diamond are fully occupied, while the orbitals that
constitute the conduction band are empty. Thus for the diamond
surface, without gap states, the fermi level electrons - those
electrons with the highest energy in tr metal - must interact
with the (empty) Londuction band orbital: in the diamond to
torm a chemical bona. It may be that the energy ditterence
between the bottom of the conduction band and the Fermi level
is too large to allow a bond to be formed and this may account
for the low friction. Un the other hand, the unoccupied states
in the band gap of the annealed surface lie much closer in
energy to the Fermi level and this smaller energy difference
may allow a bond to be formed by the Fermi level metal
electrons and the empty gap states. Such a bond would increase
interfacial strength and lead to nigher triction. Since
electron bombardment of diamond generates unoccupied gap
--testes, interfacial bonds should be possible after this
atment as well, leading to the observed higher friction.
8
Note that this chemical bonding explanation requires the
energy of the unoccupied surface states to be in the band gap
of the cryRtalline ground state. If in fact the energy lies in
the conduction band (as discussed above), then the chemical
bond is unlikely to be formed and a different explanation for
the change in interfacial strength must be sought. For
example, one might consiaer the occupied (ref. i t 2) instead or
the unoccupied surface states. It is these states that
presumably lead to the observed electrical conductivity. It
has been shown by Ferrante and Smith (reef. 12) that the
interaction of the free electrons at a bimetallic interface
leads to a strong a&isive bond. It the annealed surface is
indeed metallic, then a strong bona with the metal might
result. However such an explanation requires a determination
of whether the surface is metallic or semiconducting, a matter
that is still in question even for well studied silicon
surfaces.
An understanding of the diamond-metal interfacial strength
may ue considered in the same light as the attempts to
understand the Schottky barrier of the semiconductor-metal
interface. botn interfacial strength and potential barrier are
determined by interfacial electronic structure. Theories of
the Schottky barrier have relied heavily on the understanding
of the silicon surface, a field which is well developed.
However, the investigation of the diamond surface is just
beginning and much more definite information is required before
a theory of interfacial strength can be realistically developed.
9
r
lReferences
1. B. J. Himpsel, D. E. Eastman, and J. F. van der Veen, J.
Vac. Sci. Technol. 17, 1065 (1980) and F. J. Himpsel, D. E.
Eastman, P. Heimann, and J. F. van der Veen, Phys. Rev. 8
(to be published).
1. B. B. Pate, P. M. Stefan, C. Sinns, P. J. Jupiter, M. L.
bhek, I. Lindau, and W. E. Spicer, 4. Vac. Sci. 'Technol.
(to be published).
3. S. V. Pepper, Appl. Phys. Lett. 38, 344 (1961).
49 s. V. Pepper, J. Vac. Sci. Technol. (submitted for
publication).
5. D. K. wheeler and S. V. Pepper, bull. Am. Phys. Soc. 26P
353 (19dl).
6. J. Bardeen, Phys. Rev. 71, 717 (1947).
7. U. S. painter, U. E. Ellis, and h. K. Lubinskyo Phys. Rev.
d 4, 3610 (1971).
d. J. C. McMenamin and K. S. Bauer, J. Vac. Sci. Technol. 15,
1261 (1974) and references contained therein.
9. F. P. Bowden and D. Tabor, The Friction and Lubrication of
Solids (Clarendon Press, Oxford, 1950).
1U. S. V. Pepper, J. Appl. Phys. 50, 8062 (1979).
q	 11. C. A. Coulson, Valence, (Oxford University Press, London,
r
1961) 0 2nd ed., Chap. 4.
12. J. Ferrante and J. R. Smith, Phys. Rev. B 19, 3911 (1979).
10
eIGUKE LEGENDS
Figure 1. - Ionization loss spectra of diamond (110). In (a)
the surface is either freshly polished or exposed to excited
hydrogen. In (o) the diamond has been annealed %900 0 C
to develop K0 to its maximum size of 0.11 K 1 . The
values of the energy loss depicted are taken at the
arithmetic mean of the maximum and minimum in these
derivative spectra.
relative to
annealing
ling cycles. O:
exposed to
to L,yurOgen and
Figure 2. - Magnitude of band gap feature K0
the maximum value attained as a function of
temperature for successive two minute annaa
polishea surface. O : transformea surface
nyarogen. 0 : transformed surface exposed
the y: annealed.
Figure 3. - Copper-diamond sta ,:ic friction coefficient as a
function of diamona annealing temperature. O: polished
surface.	 O: transformed surface exposed to excited
hydrogen. 0 : transformed surface exposed to nyarogen and
then annealed.
Figure 4. - Schematic representation of the density of states
for the copper and diamond surface near the energy gap of
diamond. The diamond has been depicted with both occupied
ana unoccupied surface states in the gap, which are absent on
the as-polished surface.
11
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Effect of electronic structure of the diamond surface on the strength of the diamond-metal interface

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