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The structure of oxide films on different faces of a single crystal of copper
THE
STRUCTURE
OF OXIDE
OF A SINGLE KENNETH
R.
LAWLESS
FILMS
CRYSTAL
ON
DIFFERENT
FACES
OF COPPER*
and ALLAN
T. GWATHMEYT
The composition and structure of oxide films formed on electropolished single crystals of copper were studied for eight different crystal faces, using X-ray diffraction techniques. Particular care was taken to assure the best possible surfaces as concerns smoothness and freedom from contaminants. The effects of crystal face, temperature, pressure, oxide thickness, and contaminants were studied. Cu,O was found to be the major oxide formed between 170” and 450°C and at pressures from 0.8 mm of Hg to atmospheric. CuO formed above certain minimum thicknesses, the values of which depended on crystal face and temperature. The degree of orientation varied with all the above variables, being, in general, greater for higher temperatures and for lower pressures. The type of orientation (epitaxy) varied with crystal face, but did not vary with the other variables studied. Four different classes of orientations were found for Cu,O on Cu, and in all cases a [l lo] direction of Cu,O was parallel to a [llO] direction of Cu. CuO when formed was not oriented. LA
STRUCTURE
DES FILMS D’OXYDE D’UN MONOCRYSTAL
SUR DE
LES DIFFfiRENTES CUIVRE
FACES
La composition et la structures des films d’oxydes form& sur des monocristaux de cuivre polis 6lectrolytiquement ont 6th BtudiBes pour huit faces cristallines diffbrentes B l’aide des rayons X. Un soin particulier a BtB pris pour assurer les meilleures surfaces possible en ce qui concerne le poli et l’absence d’agents de contamination. Les effets de face cristalline, temp&ature, pression, d’bpaisseur d’oxyde et d’agents contaminants ont Bt6 BtudiBs. On a trouv6 que Cu,O eat lo principal oxyde form6 entre 170°C et 450°C et entre 0,8 mm de mercure et B la pression atmosphbrqiue. CuO se forme au-dessus de certaines 6paisseurs minima dont les valeurs dependent de la face cristalline et de la tempbrature. Le degr6 d’orientation varie avec tous les parambtres cites ci-deesus. 11 eat en g&&al plus grand pour des temp&atures 61evBes et de basses pressions. Le type d’orientation (6pitaxie) varie avec la face cristalline mais est indbpendant dei autres variables BtudiBes. Quatre classes d’orientations diff&entes ont At6 trouv6es pour Cu,O sur Cu et dans tous les cas, une direction (110) de Cu,O est parall8e 21une direction (110) du cuivre. CuO, lorsqu’il se forme, n’est pas orient& DIE
STRUKTUR
VON
OXYDSCHICHTEN AUFVERSCHIEDENEN EINES KUPFER-EINKRISTALLS
FL&HEN
Mit Hilfe von RGntgen-Beugungsverfahren wurden Zusammensetzung und Strnktur von auf elektrolytisch polierten Kupfereinkristallen gebildeten Oxydschichten fti acht verschiedene Kristallfllichen Besondere Sorgfalt wurde angewandt, urn maglichst glatte und verunreinigungsfreie untersucht. OberflBchen zu erhalten. Der Einfluss der KristallflBche, der Temperatur, des Drucks, der Oxyddicke und der Verunreinigungen wurde untersucht. Es wurde gefunden, dass zwischen 170” und 450°C bei Drncken von 0,8 mm Hg aufw(irts bis zu Atmospharendruck Cu,O das hauptstichlich gebildete Oxyd ist. CuO bildete sich erst oberhalb einer bestimmten Minimaldicke, deren Grijsse van der Kristallfliiche und der Temperatur abhilngt. Der Grad der Ausrichtung (Verwachsung) variiert mit allen obengenannten VeriLnderlichen. Er ist im allgemeinen grBsser bei hBheren Temperaturen und bei niedrigeren Drucken. Der Typ der Verwachsung (Epitaxie) variert mit der Kristallfliiche, aber nicht mit den anderen untersuchten Variablen. Vier verschiedene Arten von Verwachsungen wurden fiir Cu,O auf Cu gefunden. Bei allen war eine (1 lo)Richtung des Cu,O parallel zu einer (1 lO)-Richtung im Cu. CuO, sofern es auftrat, zeigte kein orientiertes Aufwachsen.
INTRODUCTION
The purpose of this study is to determine the nature of the oxide film formed on different faces of a single crystal of copper under various conditions of experiment. A thorough knowledge of the composition and structure (including texture and epitaxy) of the oxide film is required in order generally to understand * Received March 30, 1955; in revised form July 15, t Cobb Chemical Laboratory, University of Virginia. ACTA METALLURGICA,
VOL. 4, MARCH 1956
1955.
the mechanism of oxidation and especially the large differences in rate with face. Although
considerable
to explain
work has been done on the
composition of the oxide formed on polycrystalline (ll 2* 3g 4$ 5, only a few studies have been made “pper, of the composition of the oxide formed on single crystals of copper.(6p ‘1 The structural relationships of the oxides on single crystals of copper have been studied by a number of workers,@, gl lop 11) the most complete study being that of Menzel,(12) which was 153
ACTA
154
made
primarily
general,
most
surfaces
or
on
etched
studies
spherical
have
surfaces
of
been
an
METALLURGICA,
surfaces.
In
made
on etched
unknown
character.
The present studies were made of thin oxide films in the range from 250 A to 5000& The oxidations were technique. carefully
electropolished
single-crystal
effects of temperature, and contaminants
using an X-ray carried out on
pressure,
surfaces.
thickness
on the nature
of oxide,
of the oxide
studied for several faces, and epitaxial were determined
The were
relationships
VOL.
4,
of interference colors
as
1956
colors, using values for the first-order
determined
directly ment
thicknesses
Since both the rate of oxidation of the oxide
film depend
flat,
and
as strain-free
and the structure
on the preparation
as possible.
satisfactory
method
surface for rate or structure adsorbed heating but
foreign
gases
of the
in a vacuum
on the other
best
is no a metal
Dissolved be removed
at an elevated
hand
There
of preparing
studies.
may
the structure
tech-
first
orders
order,
the
cessive
from
This amounted
orders
of reds.
and agreeinterference
For thicknesses
increment
as determined
was used.
from between
the optical to 105OA
Thicknesses accurate
sharp changes
above
successive formulae(14)
between
suc-
determined
in
for these studies,
of composition
and
structure did not occur over narrow thickness ranges.
surface of the metal, great emphasis in these studies was placed on obtaining surfaces which were as clean, completely
light
spectrometer,
estimated
colors was always very good. the
since in general, METHOD
a polarized
with the polarizing
with
this way were sufficiently
for eight different faces.
EXPERIMENTAL
by
nique.o** 15) In some cases, thicknesses were checked
and by
temperature, of a copper
The oxide
structure
and composition
mined by glancing-angle using
Cu
radiation.
stationary-crystal
X-ray Both
techniques
were deter-
diffraction
methods,
oscillating-crystal
and
were used, the radiation
being filtered through Ni foil in the former case.
The
face of the specimen being studied was adjusted normal to the X-ray beam in such a way that the axis of oscillation
was in the plane of that face and
normal to the X-ray beam. The crystal was then rotated about the oscillation axis until the face being
surface so treated is not known at the present time.
studied made a definite angle of inclination
The exact preparation
X-ray beam, this angle being determined by consideration of the Bragg angles for the metal and
and the advantages
of the surface for these studies of using the crystal in the form
of a sphere with flat faces cut on it have been discussed
oxide faces concerned.
by Young and Gwathmey and others.03) It should be emphasized that the use of crystals in
rical film concentric
this form
the oxide
made it possible
to utilize
the oxidation
patterns standard
on the spherical surface as a reference for the conditions used for surface pre-
paration
and carrying
trace
amounts
out the reaction.
of impurities,
gases, or as foreign compounds marked
the surface, the oxidation
Addition
of
as adsorbed
such as grease, caused
patterns were recorded on a cylindabout the axis of oscillation.
was highly
oriented,
the resulting
If films
showed layer-line patterns with X-ray reflections from both the flat face of the substrate and from the oxide
plane parallel to the substrate
zero layer-line, satisfied.
provided
the Bragg
As an additional
falling
on the
conditions
check in determining
oxide orientation,
the homogeneity
of
an angle of about 45” to 90” about an axis perpen-
and this also showed up as a change in
dicular to the flat face, and a second layer-line pattern
the
standard.
of facets destroyed
obtained.
pattern.
was rotated
the
or
from
the specimen
were
Etching
changes
development
whether
The diffraction
with the
through
This gave at once, in many cases, the plane
The crystals were oxidized at temperatures from 170’ to 450°C and at pressures from 0.8 mm of Hg to
of the oxide which was parallel to the given substrate plane. In some cases where a sufficient number of
atmospheric.
X-ray
The oxidation
reactions
were stopped
at a given thickness by evacuating the system with a high-capacity pump and then cooling to room temperature.
The rate of cooling was varied for different
experiments. A fast rate of cooling was used to minimize solid-phase reactions(5) for studies in which the determination of the composition of the oxide was the primary purpose. Experiments indicated that the rate of cooling did not affect the oxide structure appreciably, and in general a moderate to slow rate of cooling was used for the structure studies. Oxide-film thicknesses were determined by means
reflections
was recorded,
the complete
orienta-
tion of the oxide could be determined simply by inspection of one or two oscillation X-ray patterns. In other cases, it was necessary diffraction
patterns,
using
to take a number of
both
oscillating-crystal
and stationary glancing-angle techniques, before the complete orientation could be fixed with certainty. The use of reciprocal lattice relationships simplified the determination of the orientation relationships in cases in which they were not obvious from inspection. The degree of orientation was estimated by comparing a particular diffraction pattern with each
LAWLESS
of a series of X-ray oxide
classified
diffraction
as to
the
ranging from essentially perfect (96100%).
random
to the distribution diffraction become the
arcs
give
a
of
of the classified
perfect
vary
alignment,
the
sharp spots,
but
in length
depending
on
method
can
This
estimate
of
the
degree
the
155
Pressure
25O’C 250°C 250°C 350°C 350°C 350°C 350°C
atm atm atm mm of mm of mm of mm of
Least thickness for which CuO was found
(OOl), (ill),
(Oil),
(113),
20 20 20 20
(112),
the entire ranges of pressure and temperature
(012), (122), and (133), the studies being more complete on the first six faces listed. All faces studied were
experiments,
probable
oxide.
prepared
as follows.
cut on a spherical
%: i 5000 2000 1200 2500
Hg Hg Hg Hg
A A A A
Thermodynamic calculations(5) been very small. reveal that both Cu,O and CuO are stable throughout in these
as flat surfaces
3000 8,
of
Studies were made of the oxide on eight different faces,
Temperature
As the
orientation. crystal
FILMS
TABLE 1
weighted according
disorientation.
reasonable
OXIDE
orientation,
in the arc.
are no longer
which
degree
maxima,
from
maxima
OF
of cuprous
of
(O-1Oo/o) to nearly
of intensity
deviates
STRUCTURE
on the basis of the length of
the arc of the diffraction orientation
patterns
amount
The orientations
series were determined
THE
GWATHMEY:
AND
crystal.
and
the
Cu,O
The formation
covered
is the
more
of CuO is explained
When the rate of formation
of Cu,O has
It should be noted that results were the same on the
become very slow, the number of copper ions available
(100)
for reaction
(OlO), (OiO), etc., planes as for the (001).
Thus
at the oxide-oxygen
interface
is very
it is necessary to indicate results for the faces of only
small.
one unit stereographic
a ready supply of copper ions is not available, the reaction Cu,O + 40, + 2 CuO is more likely to take
triangle.
place.
RESULTS
The
results
of this
study
are divided
sections, the first concerning effects of temperature, contamination degree epitaxy.
the complex
on the composition
Epitaxy
and the
may
mutual orientation
into
two
interrelated
pressure, oxide thickness,
of orientation),
and
and texture second
be generally
relationships
(or
concerning
defined
as the
between two different
crystal substances.
oxide
these experiments patterns closely
AND
formed
d,,,
values
the X-ray which
of
diffraction
agreed
with those given for Cu,O by Swanson
atmosphere
after the rate of oxidation
very
monoclinic
CuO was detected depended
the temperature least thicknesses
on different
amounts
thicknesses expected supply
of
of
CuO
on different at
smaller
copper
very slow at different
crystal
faces, the formation
will
begin
at different
faces.
CuO would
thicknesses
where
a greater
readily
available.
ions
is more
not
be
These results are in general agreement with experiments by both electron diffraction,(4T 5, 6, and electrolytic
reduction(4)
very and
had become
CuO was formed
below
by most
different
Fuyat,(l@ including the very weak 211 reflection. If the crystal was allowed to remain in an oxidizing
which
large
surface
under the conditions
was Cu,O,
giving
slow,
of
used
TEXTURE
of the Oxide Film
The major
Since the rates become
thicknesses
and
methods.
No indication
of
the oxide CuO’ was found. (l) In general, the methods
COMPOSITION
Composition
Since CuO is stable under these conditions
certain
on the particular
oxide
also.
No
thicknesses,
crystal
face and
of oxidation. Table 1 shows the for which CuO ‘was found under the
previous
and forming
for preparing
the
on the metal
were
from those used in these experiments
and
were not comparable. The CuO, whenever polycrystalline, tation.
workers
the oxide
present,
with no evidence
was
completely
of preferred
orien-
The Cu,O was in general oriented, the degree
of orientation
depending on the crystal face of copper,
oxide thickness, oxygen pressure, temperature, and the presence of impurities. The remainder of this paper is devoted variables
have
to the effect which these different on the texture
and epitaxy
of the
cuso. InJluence of Oxygen Pressure on Orientation
specified conditions. Since the X-ray techniques used were not capable of detecting trace amounts of CuO, it was not possible
The effect of oxygen pressure on the degree of orientation of oxide on the (001) and (111) faces is shown in Fig. 1 for oxidation at 200°C. The curves
to say that no CuO was present for smaller film thicknesses, but only that the amount must have
show markedly that for pressures below about 200 mm the degree of orientation increases with decreasing
ACTA
156
METALLURGICA,
For pressures greater than 200 mm, there
pressure.
seems to be no change in the amount with pressure. degree
of
pressure.
In general,
orientation
was
of orientation
for all faces studied, greater
the
lower
the the
This held true over the whole temperature
range studied and for all oxide thicknesses over about
VOL.
4,
1956
tion difficult. oratory
Preliminary
indicate
experiments
in this lab-
that a slower rate of oxidation
at
pressures under 9 mm of Hg may account partially for the improvement in orientation observed for thicker films formed at low pressures. It was found that the type of orientation
(epitaxy)
0 5 w40ii a.
A -(I 1 I) - 400
i
0 - (100) - 700
H
OXIDATION 20-
AT
200°C
FIG. 1. Effect of pressure on the degree of orientation of the oxide on the (100) and (111) faces of Cu.
5OOA.
With decreasing
degree of orientation peratures for
larger
thicknesses
(<25O”C). film
pressure, the increase in the
was greater for the lower temThe increase
thicknesses
less than 500&
was also greater
(>500
A).
For
oxide
the oxide was in general
highly oriented and showed either a very slight increase in the amount of orientation or no appreciable increase.
In no case was the amount
of orientation
found to decrease with decreasing pressure. Since the oxide film formed at atmospheric pressure is highly oriented for low thicknesses due to the strong orienting effect of the substrate, the degree of orientation can increase only slightly with decreasing pressure.
This increase is probably
due to the fact
that, at the lower pressures, few oxide nuclei are formed with unfavorable orientations. The number of possible adsorption sites occupied by oxygen ions will in general be greater at atmospheric pressure than at low pressure. Since more adsorption sites are always available than are needed for oxide of the
did not vary with pressure; crystal given
i.e., when a particular
plane of the oxide substrate
pressure,
the
face same
for
was found oxidation
relationship
parallel
to a
at atmospheric was
present
at
Thickness
on
lower pressures. Influence
of
Temperature
and
P&n
Orientation There was a general increase in the degree of orientation of the oxide with increasing temperature for
all faces
depending crystal face. produce
studied,
the
amount
on oxide thickness,
of the increase
oxygen
pressure,
and
In no case did an increase in temperature
a decrease in the degree of orientation.
The
complex relationships between the degree of orientation, temperature, and oxide thickness may best be described by a type of existence diagram in which points are designated by the amount of orientation for a given temperature and oxide thickness. This is done in Fig. 2 for the (111) face and in Fig. 3 for
observed orientations, the higher pressure will lead to the formation of additional nuclei of different and less stable orientations. The state of knowledge concerning the influence of pressure on the rate of
the (001) face oxidized at atmospheric pressure. For oxidation at atmospheric pressure, the oxide films formed were generally highly oriented for low thicknesses and could be considered as pseudo-
oxidation of single crystals of copper is very limited at the present time, and this makes a more complete interpretation of the effect of pressure on the orienta-
monocrystalline, i.e., consisting of small crystallites aligned in a small angular region about a given orientation.
With increasing
thickness
above
600 A,
LAWLESS
GWATHMEY:
AND
THE
STRUCTURE
amount
the
OF
OXIDE
157
FILMS
of orientation
decreased,
the decrease
being rapid at the lower temperatures at the higher
350
temperatures.
This
but gradual
can be seen in
Oxide on the (012) and (001)
Fig. 4 for the (111) face.
faces showed this same general behavior, i.e., a high initial orientation for low film thicknesses followed ORIENTED
I
I
I
by a fairly rapid decrease in the amount of orientation
43
YODERITELY
with increasing
DRIEWTED
I
I
I
/
I
/
J
;
I’
I
over a certain
range
of thicknesses
in rates
minimum.
all the faces studied because
of reaction
of the
on different
@@I@
@g&g I
a large
Iarge differences
/
I
thickness
It was not possible to compare over
/
I
I
/
/
I
/’
/-
4oool
SLIOIITLI ClRlEWTED
,I
.?ooo2Oc A
/
ATMOSPHERIC .
PRESSURE L
I5c
1
L
1
,
1
I
I
I
a 3000
2000
1000 OXIDE
THICKNESS
(A)
2. Orientation variation with temperature and
Fra.
600-
E g
600-
:-::-\,i A
Q I
I
_
z
oxide thickness for Cu,O on (111)Cu.
A
A A
1 u ; 400-
A-200°c
A - 25O’C
ki ;i 0
A 0 - 325’C 200 -
/ /
I
I:::
/’ @
0
/
/
HICHLI
/ I
MODERATELY
60 ORIENTATION
40 PERCENT
IO 0
60
,
of Hg were highly
,
/
ORlENTED
I
Oxide films formed at pressures of 0.8 mm to 25 mm
/
I
I
/
/
6LlOHTLY
/
/
/ ORlENTED
oriented
2OOA up to 3000-5000
,
I
20
A
ON (I I I)
FIG. 4. Variation of the degree of orientation with thickness for Cu,O on (111) face of Cu.
/ ORlENTED
OXIDE
,
/
.
amount [oxide
of
orientation
on (OOl), (ill),
from The
either (012),
increased
slightly
and (011) planes]
or
remained about the same [oxide on (112) and (113) planes] with increasing temperature. The disorientation amounted
/’
for all thicknesses
A on all faces studied.
to less than 10“ of arc on the X-ray
diffraction film in all cases, with the oxide on the (001) showing the greatest amount of disorientation.
/
/
/
‘@
/’
,/
OXIDE
(001)
It should
cu
ATMOSPHERIC
I
I
L
1
I
2000 THICKNESS
,
s I 3oOf
(A)
FIG. 3. Orientation variation with temperature and
oxide thickness for Cu,O on (0Ol)Cu.
4
be pointed
out
that
it is difficult
to
separate the effects of temperature from those due to initial low-pressure oxidation, when oxidizing at atmospheric pressure. In general, 0.7 to 0.8 set was
PRESSURE I
1000
/
/
required for the pressure in the reacting system to reach 200 mm of Hg; and at temperatures of 300°C and over, an oxide film greater than 5OOA formed in this short time. The evidence indicates that a
158
ACTA
METALLURGICA,
large part of the increase in degree of orientation observed with increasing temperature over 300% for oxidation at so-called atmospheric pressure is due to the initial low-pressure oxidation. Two factors in general could lead to disorientation of the oxide film with increasing thickness. The first of these is the rapid falling off of the substrate influence with increasing oxide thickness. The second factor is the rearrangement of the oxide due to slip or rotation in such a way as to relieve compressive stresses in the oxide film. The first of these would be the same whether the oxidation was carried out at low or atmospheric pressure. The second, however, could depend on pressure, since oxide films formed at atmospheric pressure contain more imperfections than those formed at low pressures. This would enable slip or rotation of the oxide to take place at lower compressive forces than would be the case for oxide films formed at low pressures. This means that the high initial orientation can extend to greater thicknesses for oxidation at low pressures, as is observed. The greater amount of energy available to the crystal at higher temperatures gives greater mobility to the oxide. Since the less favorably oriented units of oxide are more weakly bound, it is more likely that these will move about under the in~uen~e of temperature than the stronger bound, favored orientations. This then leads to an overall increase in the degree of orientation with increasing temperature.
Trace amounts of such contaminants as SO,, H,S, silicone grease, phosphate from polishing solution, residual gases from gas used in glass blowing, silicates or chloride from wash-water, and many others, caused marked changes in the oxidation patterns on the crystal sphere. Crystals showing an oxidation pattern ditl?ering from a “normal” or “standard” pattern could then be considered as contaminated, and in many cases the contaminant could be identified from the oxide pattern. In all cases, at temperatures of 200” to 25O”C, it was found that the oxide, when formed in the presence of impurities, showed a marked decrease in the amount of preferred orientation for all thicknesses. In general, the amount of disorientation was almost complete, i.e., the oxide was polycrystalline, with random orientation. For oxidation at 3OO*C to 4OO”C, the oxide showed a marked decrease in the amount of orientation, but not the randomness shown for the lower temperatures.
VOL.
4,
I950
In general, it would be expected that a contaminant which is adsorbed on the surface of the metal would reduce the orientation of the oxide by a simple blocking and reduction of the substrate influence. In general, less than a monolayer of contaminant could cause effective disorientation by blocking the adsorption sites on the metal surface. Gaseous contaminants could also produce disorientation by creating imperfections in the oxide as it is formed. EPITAXY
The epitaxial relationships reported in this section are those of C!u,O formed on electropolish~ flat faces of a single crystal. The type of orientation observed on a particular face did not change with temperature, pressure, or oxide thickness within the ranges covered in these experiments [170°C4500C; 0.8 mm of Hg to atmospheric pressure; 2OOA to 5000~]. The degree of orientation did change with these variables, as described previously, The epitaxial relationships observed are summarized in Table 2, the second column showing the plane of the oxide which is parallel to the Cu plane listed in column 1, and the third column giving the directional relationships necessary to define the complete orientation, i.e., the alignment of given rows of atoms of the oxide with row-s of atoms of the substrate. All the orientations observed can be placed in one of four different classes. These are designated as I (A, B, C, and C’), II (D), III (E, E’, and Z’), and IV (P). These orientations are shown in Figs. 5 (a, b, c, and d). The diagrams represent a section through the oxide and the copper normal to the particular face of copper indicated. The [Ii01 direction is normal to the page in each case. The rectangular figures in each drawing serve as an aid to visualizing the relative orientations of corresponding units of the metal and oxide. Fig. 6 is a schematic representation of these orientations for six different faces of copper. The large areas of Fig. 6 represent the plane faces of copper, and the small figures represent the oxide with all faces shown being cube faces. Orientations A, B, C, and C’ oocurred only on the (001) face of copper. The (001) plane of the oxide is parallel to the (111) plane of Cu in A and to the (111) plane of copper in B. In C the (010) plane of Cu,O is parallel to the (ill) plane of Cu. Orientation I), in which the oxide has the same orientation as the Cu substrate, occurred in only minor amounts in a few cases on the (001) plane of Cu, but was a major orientation of the oxide on the
LAWLESS
AND
+
+
t
t
GWATHMEY:
THE
STRUCTURE
OF
OXIDE
t
t +
+
+
159
FILMS
+
+
+
is?J&~~~oo,,~~ll ~~~&:_:ll,: ... .
“_-___ . ‘-._--.___
.
.
-.
.
.
.
.
.
IYI
.
.
.
C?
9
. .
.
. ‘--.‘---l22i~ -.
.
.
. .
8
‘13311
. .
++
(a) +
+
+
+
P 0
+ -t-
+
+
+
+
cq.ZSlk’ b-?-d+ ‘l.blA+
-t-tolIh[iool
. . .H .. .
.
-
l
’
.
(010
Pm. 5. Positions of Cu ions
.
,ooO
.
.
.
8
(c) (+)
.
. .
(4
of Cu,O with respect to Cu atoms (0) of single-crystal substrate viewed the faces indicated and with the [IlO] direction normal to the page. (a) orientations A and B on (001)Cu; tions D and E on (lll)Cu; (0) orientation D on (011)Cu; (d) orientation F on (113)Cu.
ccid
FIG. 6. Schematic
representation
of oxide orientations
on Cu.
parallel to (b) orienta-
ACTA
160
__ _
._
(001)
-. (111)
.a.
(111)
A
[ liO]Cu,O//[ liO]Cu
(001)
B c c’ D
[oil]cu,o/~[lro]cu ~loi]c~~o~~~llo]~ [oli]cu~o~/[l lo]cu [lio]cu~o/~[lio]~u
(111)
D
[iropu,o/piop
E
[ilo]cu,o//[lio]cu
22
[~io]~u~o~/[~io~c~
(011)
_:.
(113)
Major directional relationships
_i_
fllot (44i)
(112)
E’
[llo]cu,0//[0il]cu
E”
[llO]Cu,O//[O11]Cu
I
/
p
[lio~~*o~/~lio]~
_j
p
[lio]cu~o~/[lio]cu
_.
(474)
(132)
:=
1956
_____ ChASS
~-.--.-.
~-
Remarks
/
/_. I
_-
A,
B, C, and G’ are exactly equivalent orientations.
II
Found only in a few casas
II
Orientation E expressed as:
II
may
also
be
(iii)cu,o//(iii)cu piopu,o//pTop~
III
/ / ’ E’ and E” are equivalent orientations, but are not equivalent to D. E’ may also be expressed as: (120)cu,0//(012)cu
piop,o//[lTo]cu
I -_ __-
I
IV IV
I I
III
_7z [lio]cu,o//[lio]cu
_.
(133)
4,
D ~~io]~~o/~~iioi~
tow
(012)
VOL.
TAULE 2.
.-. I._ Parallel face Cu,O
Crystal face Cu substrate
(011)
METALLURGICA,
-I-
High index fa& near (112)
)
23 rlio]cu,o~~rlio]cu
(111) and (012) faces in all cases, and was the only orientation found on the (011) face of Cu. It is probable that the occurrence of D on the (001) face was due to sub-microscopic etching with the development of (110) or (111) facets prior to oxidation. Urien~tions E, E’, and E” were found in addition to D on both the (111) and (012) planes of Cu. It was also the only orientation found on the (122) and (133) faces of Cu. The orientations D and E on the (111) face are exactly equivalent as far as the surface layer of atoms is concerned. E may be derived from D on the (111) face by a 180’ rotation of the oxide about the [ill] axis. On the (012) face of Cu, E is not equivalent to D, and may be thought of as being derived from D by a rotation of the oxide of 131’ 49’ about the normal to the (012) face (counter-clockwise rotation giving E’ and clockwise giving E”). The counter-clockwise rotation brings the densely-packed row [121] of Cu,O into coincidence with the [12i] row of Cu, the (111) plane of Cu,O with the (111) plane of Cu, and makes three pair of [IlO] directions parallel. On the (113) and (112) faces of Cu, the orientation -F” was found. For both of these faces, the (110) plane of the oxide is accurately parallel to the (113) face of the copper, and the [liO] directions of oxide and metal are parallel. No other densely-packed
III
rows of atoms or planes of atoms coincided. It should be emphasized that the results reported in this paper were obtained on flat electropolished and annealed surfaces, and both the conditions of oxidation and the nature of the metal surfaoe were different from those used by previous workers.(sp g*lop I1712) The results are thus not strictly comparable. Neither the preparation of the surface nor the conditions of the oxidation are reported by Mehl, M~Candle~, and Rhines.(*) Their observation, that the Gus0 grows with its cube axes parallel to the cube axes of the substrate, was borne out for the parallel orientations on the (Oil), (012), and (111) faces, but not for any other faces studied. Thiessen and Schiitza used surfaces which had been etched in HNO, and then annealed at 1000°C before oxidation was carried out at 325°C and 150-mm pressure. Their results on the (OOl), (Oil), and (111) faces are in agreement with those reported here. Yamaguti(i*) etched his specimens in HNO, and HCI, and then oxidized them in air at 1000°G. The orientations observed camrot be compared directly, since the faces used were not specified except for one specimen. On a (001) face the orientation observed did not agree with that found here. Elarn(ll~ used surfaces which were characterized
LAWLESS
as “polished,”
AND
but no other specifications
In some cases specimens
THE
GWATHMEY:
STRUCTURE
OF
seem to be in good agreement
those found here. Menzelu2) carried 50-mm
pressure,
out
oxidations
followed
by
including
preparations,
etchants, electropolishing, vacuum
treatment.
general
at 450°C
by
application results
with those
is made
c
.
density
facets
strate
face,
are
in cu20
lattice
on the
parallel
in
and
oxide. define
with respect other
to a given
conditions
the
must
complete
+a
cu k
i-0
l
+.
l+
4-
be
oxide
to consider the fitting of the oxide metal
substrate,
and
to
determine or for two
directions
There
is controlling
the orientation.
two ways of considering
this.
adsorption
and the
second
positions
in the oxide
+, t o+ .+ ta -k at lto
are
The first considers the
misfit between oxygen positions
Consideration
*
sub-
whether small misfit for a given direction
most probable
[33iJ
t
l +o
l+
4-
[ooi]
are the ones
the metal
determining
orientation. It is necessary
CU cu,o
(001' Cu
(A’
if con-
does not completely
one or more for
[Ii01
and
developed
being
directions
in both
orientation
important
0 +
various
of this paper
directions
The [llO]
Since this one condition the oxide
+O
present in all cases is that of at
least one pair of [llO] of greatest
0
heat
the different methods of surface preparation. Consideration of the data of Table 2 reveals that
metal and oxide.
l+
l +o+e
l+
a
+
a variety
of
obtained
of the various
the only relationship
161
and high-temperature-high-
The
agreement
sideration
with
a high-vacuum
treatment,, for surfaces which had received of
FILMS
were given.
were used from which the
oxide had been removed by heating in a high vacuum. The orientations
OXIDE
in the oxide and the
sites on the copper surface,
considers
the matching
of copper
of the first of these indicates
good match or fit for oxygen
O-t
to
with those in the substrate. that a
f
ions on the adsorption
It
.
.
l+
't .
.
0
l
sites on the metal surface is not necessary for oriented growth of the oxide film. If the copper-ion
positions
(Cl
(III) cu
of the oxide planes are
superimposed on the copper positions of the substrate faces, as in Fig. 7, according to the orientations of Table 2, it is seen that a misfit of 18% is present in at least one [llO]
direction
in each case.
The misfit
here is defined as b/a - 1, where b and a are the distances between lattice points for the oxide and metal
respectively
the misfit seen that oxide on misfit is oxide on
is being
in a given
direction
determined.
From
for Fig.
which 7 it is
the matching of the two lattices is good for the (111) and (011) planes of copper (the 18% in any direction on the surface). For the (001) and (113) planes of copper, the
matching of lattices is not so good, but there is present a striking near-coincidence of the rows of closestpacked Cu ions of the oxide with the closest-packed
'
l+
P
(D)
(110)
0
Cu
FIG. 7. Positions of Cu ions (+) of Cu,O superimposed on Cu atoms (0) of the substrate viewed normal to faces indicated.
ACTA
162
rows
of the metal
between
face.
Table
the closest-packed
in the interface
METALLURGICA,
3 lists the misfits
rows of atoms
and ions
plane for seven substrate faces.
The
VOL.
oxide
the value
The type of orientation
packed rows in the oxide-metal
oxide
Table
thickness,
of which
depended
on
(i.e., the major face of the
oxide tending to be parallel to a particular face)
From
1956
crystal face and temperature.
misfit here is defined as above, except that B and A are the perpendicular distances between closestinterface.
4,
did not
vary
thickness,
with temperature,
but
did
vary
The degree of orientation
3 it is seen that this misfit between rows for oxide on the (001) is only 2%,
variables.
It was essentially
and for oxide
on the (113) it has the exceptionally
face
given
oxide
with
crystal
or
face.
varied with all the above
closest-packed
and
substrate
pressure,
constant
thickness)
change
in
small value of 0.50/O. These small misfits are apparently the determining factor in fixing the oxide orientation
pressure above
on these two planes.
creasing pressure for all faces and at all temperatures
It misfit
is also
apparent
between
from
Table
3 that
rows in the interface
a small
plane
cannot
be the deciding factor for oxide on the (112) and (122) planes. The (112) face can be considered as composed
The orientation
of the oxide on these faces seems to be controlled these steps, specifically
by
the (113) for the (112) face
different
crystal
techniques. pressure, studied,
of
and structure
of oxide
copper
studied
faces,
were using
X-ray
films on for
eight
diffraction
The effects of crystal face, temperature, oxide
thickness,
using
carefully
and
increased
studied.
The
with increasing
degree
of
markedly
orientation
of
temperature
with the
de-
oxide
in all cases,
the amount of the increase being greater for oxidation at atmospheric
pressure and large oxide thicknesses.
Oxide films formed
at atmospheric
pressure were
highly oriented for the first 500 to 600 A, but became disoriented
with
Oxide films formed
increasing
thickness
above
this.
at pressures from 0.8 to 25 mm up to
3000 to 5000 A. Contaminants in general caused a marked decrease
SUMMARY
crystals
but below this, the
of Hg were highly oriented for all thicknesses
and the (111) for the (122) and (133) faces.
The composition
of orientation
increased
of steps of (111) and (113) faces, the (122)
and (133) of (111) and (011) faces.
single
degree
200 mm of Hg;
(for a given
with
contaminants
prepared
were
in the amount of orientation Four
different
classes
of the oxide.
of
epitaxial
relationships
were found for Cu,O on the eight crystal faces studied. The controlling
factors in determining
tions can be summarized
electropolished
these orienta-
as follows:
1. At least one pair of [llO] directions packed
Studies between 170” and 450°C showed that C&O was the major oxide which was formed on all
parallel. This gives 18% misfit in one direction. 2. For low-index planes, a good fit (small misfit)
faces initially.
between
CuO formed above a certain minimum
directions
for both
(the closest-
specimens.
closely-packed
Cu and Cu,O)
rows
of
must be
atoms
is
more
TABLE 3.
Crystal plane cu
Parallel plane C%O
(001)
(111)
(111)
(111)
(011)
(011)
(012)
(012)
(012)
(012)
(113)
(110)
! I I I I I
(47%)
____ Cu,O (B)
1 (A, B, C, c’)
2.55
2.61
II (I)), III (E)
2.21
2.61
3.61
4.25
4.03
4.75
11 CD) 11 CD)
._ ._
III (E’, E”)
3.29
IV (F)
4.23
III
I
parallel plane
Cu (A)
IV (F)
(112)
(122)
Distance between closest-packed rows of Cu atoms in boundary (in A)
Orientation class
I
Misfit
B-A A
(%)
IL 18 -__
18
3.88
I
4.25
6.25 12.60 (2A) 18.75 (3A)
17.27
15.31 7.65 (2A)
18.78
18
18 0.5
38 -8
(E) ~.__
1
23 _. --
LAWLESS
important
than
densely-packed
the
requirement
planes.
This
not
important.
of parallelism
is illustrated
planes
such as the
The
orientation
mining
plane,
face.
by
i.e., by the If there is a
(110) and (111) planes as the deterthe (111) seems most
the two orientations non-parallel
directed
orientation
likely;
and of
by the (111) plane, the
seems
to be the preferred
one. In general, CuO, when present, was polycrystalline and completely Consideration
disoriented. of these results in terms of the rates
of oxidation
on different crystal faces reveals one interesting correlation. Although the rates of oxidation of the (001) and (111) faces are considerably
different,
both faces have (111) planes of the oxide parallel to them. lent
There are, however, orientations
on the
only two on the (111).
four energetically (OOl), whereas
diffusion
oxidizing
(011)
through
and
equiva-
there
are
Oxide on the (001) therefore
shows more grain boundaries enhanced
orientation
OF
OXIDE
of the oxide.
FILMS
163
This may partially
account
for the large rate differences between different crystal
and
plane is
is determined
faces near the high-index
choice between
of the
(122)
of misfit in the interface
the steps of which the face is composed, major
by
STRUCTURE
faces.
(11o)cu,o//(ll3)cu. 3. For high-index (133), the amount
THE
AND GWATHMEY:
(113)
and the possibility the faces
oxide. show
The only
of
slowone
ACKNOWLEDGMENT
This work
was supported
in part
by the Office
of Naval Research. REFERENCES 1. C. A. MURISON, Phil. Msg., 17,96 (1934). 2. G. D. PRESTON and L. L. BIRCUMSHAW. Phil. Mao.. ” I 20. ---, 706 (1935). 3. H. DUNHOLTER and H. KERSTEN, J. Applied Phye., 10, 523 (1939). 4. C. G. CRUZAN and H. A. MILEY, J. Applied Phys., 11, 631 (1940). 5. E. A, GULBRANSEN and W. R. MCMILLAN, J. Electrochm. sot., 10,393 (1952). 6. H. FRISBY, Compt. rend., 228, 1291 (1949). 7. G. P. THOMSON, Proc. Roy.Soc. (London), A133.1 (1931). 8. R. F. MEHL, E. L. M&AND&S, and. F. N: R&NE& Nature, 184,1009 (1934). 9. P. A. THIESSEN and H. SCH~~TZA, 2. anoTg. U. allgem. Chem., 233, 35 (1937). 10. T. YAMACWTI, Proc. Phys. Math. Sot. Japan, 20, 230 (1938). 11. C. F. ELAM, Trans. Furuday Sot., 32, 1604 (1936). 12. E. MENZEL, Ann. Phys. [Leipzig] 5, 163 (1949). 13. F. W. YOUNG, Jr., J. V. CATHCART, and A. T. GWATRMEY; Submitted for publication this journal. 14. A. B. WINTERBOTTOM, Trans. FaradaySoc., 42,487 (1946). 15. A. T. GWATHMEY and F. W. You~a, Jr., Rev. Mdtallurgie, 48, 434 (1951). 16. H. E. SWANSON and R. K. FUYAT, National Bureau of Standards Circular 539, Vol. II, 23 (1953).
STRUCTURE
OF OXIDE
OF A SINGLE KENNETH
R.
LAWLESS
FILMS
CRYSTAL
ON
DIFFERENT
FACES
OF COPPER*
and ALLAN
T. GWATHMEYT
The composition and structure of oxide films formed on electropolished single crystals of copper were studied for eight different crystal faces, using X-ray diffraction techniques. Particular care was taken to assure the best possible surfaces as concerns smoothness and freedom from contaminants. The effects of crystal face, temperature, pressure, oxide thickness, and contaminants were studied. Cu,O was found to be the major oxide formed between 170” and 450°C and at pressures from 0.8 mm of Hg to atmospheric. CuO formed above certain minimum thicknesses, the values of which depended on crystal face and temperature. The degree of orientation varied with all the above variables, being, in general, greater for higher temperatures and for lower pressures. The type of orientation (epitaxy) varied with crystal face, but did not vary with the other variables studied. Four different classes of orientations were found for Cu,O on Cu, and in all cases a [l lo] direction of Cu,O was parallel to a [llO] direction of Cu. CuO when formed was not oriented. LA
STRUCTURE
DES FILMS D’OXYDE D’UN MONOCRYSTAL
SUR DE
LES DIFFfiRENTES CUIVRE
FACES
La composition et la structures des films d’oxydes form& sur des monocristaux de cuivre polis 6lectrolytiquement ont 6th BtudiBes pour huit faces cristallines diffbrentes B l’aide des rayons X. Un soin particulier a BtB pris pour assurer les meilleures surfaces possible en ce qui concerne le poli et l’absence d’agents de contamination. Les effets de face cristalline, temp&ature, pression, d’bpaisseur d’oxyde et d’agents contaminants ont Bt6 BtudiBs. On a trouv6 que Cu,O eat lo principal oxyde form6 entre 170°C et 450°C et entre 0,8 mm de mercure et B la pression atmosphbrqiue. CuO se forme au-dessus de certaines 6paisseurs minima dont les valeurs dependent de la face cristalline et de la tempbrature. Le degr6 d’orientation varie avec tous les parambtres cites ci-deesus. 11 eat en g&&al plus grand pour des temp&atures 61evBes et de basses pressions. Le type d’orientation (6pitaxie) varie avec la face cristalline mais est indbpendant dei autres variables BtudiBes. Quatre classes d’orientations diff&entes ont At6 trouv6es pour Cu,O sur Cu et dans tous les cas, une direction (110) de Cu,O est parall8e 21une direction (110) du cuivre. CuO, lorsqu’il se forme, n’est pas orient& DIE
STRUKTUR
VON
OXYDSCHICHTEN AUFVERSCHIEDENEN EINES KUPFER-EINKRISTALLS
FL&HEN
Mit Hilfe von RGntgen-Beugungsverfahren wurden Zusammensetzung und Strnktur von auf elektrolytisch polierten Kupfereinkristallen gebildeten Oxydschichten fti acht verschiedene Kristallfllichen Besondere Sorgfalt wurde angewandt, urn maglichst glatte und verunreinigungsfreie untersucht. OberflBchen zu erhalten. Der Einfluss der KristallflBche, der Temperatur, des Drucks, der Oxyddicke und der Verunreinigungen wurde untersucht. Es wurde gefunden, dass zwischen 170” und 450°C bei Drncken von 0,8 mm Hg aufw(irts bis zu Atmospharendruck Cu,O das hauptstichlich gebildete Oxyd ist. CuO bildete sich erst oberhalb einer bestimmten Minimaldicke, deren Grijsse van der Kristallfliiche und der Temperatur abhilngt. Der Grad der Ausrichtung (Verwachsung) variiert mit allen obengenannten VeriLnderlichen. Er ist im allgemeinen grBsser bei hBheren Temperaturen und bei niedrigeren Drucken. Der Typ der Verwachsung (Epitaxie) variert mit der Kristallfliiche, aber nicht mit den anderen untersuchten Variablen. Vier verschiedene Arten von Verwachsungen wurden fiir Cu,O auf Cu gefunden. Bei allen war eine (1 lo)Richtung des Cu,O parallel zu einer (1 lO)-Richtung im Cu. CuO, sofern es auftrat, zeigte kein orientiertes Aufwachsen.
INTRODUCTION
The purpose of this study is to determine the nature of the oxide film formed on different faces of a single crystal of copper under various conditions of experiment. A thorough knowledge of the composition and structure (including texture and epitaxy) of the oxide film is required in order generally to understand * Received March 30, 1955; in revised form July 15, t Cobb Chemical Laboratory, University of Virginia. ACTA METALLURGICA,
VOL. 4, MARCH 1956
1955.
the mechanism of oxidation and especially the large differences in rate with face. Although
considerable
to explain
work has been done on the
composition of the oxide formed on polycrystalline (ll 2* 3g 4$ 5, only a few studies have been made “pper, of the composition of the oxide formed on single crystals of copper.(6p ‘1 The structural relationships of the oxides on single crystals of copper have been studied by a number of workers,@, gl lop 11) the most complete study being that of Menzel,(12) which was 153
ACTA
154
made
primarily
general,
most
surfaces
or
on
etched
studies
spherical
have
surfaces
of
been
an
METALLURGICA,
surfaces.
In
made
on etched
unknown
character.
The present studies were made of thin oxide films in the range from 250 A to 5000& The oxidations were technique. carefully
electropolished
single-crystal
effects of temperature, and contaminants
using an X-ray carried out on
pressure,
surfaces.
thickness
on the nature
of oxide,
of the oxide
studied for several faces, and epitaxial were determined
The were
relationships
VOL.
4,
of interference colors
as
1956
colors, using values for the first-order
determined
directly ment
thicknesses
Since both the rate of oxidation of the oxide
film depend
flat,
and
as strain-free
and the structure
on the preparation
as possible.
satisfactory
method
surface for rate or structure adsorbed heating but
foreign
gases
of the
in a vacuum
on the other
best
is no a metal
Dissolved be removed
at an elevated
hand
There
of preparing
studies.
may
the structure
tech-
first
orders
order,
the
cessive
from
This amounted
orders
of reds.
and agreeinterference
For thicknesses
increment
as determined
was used.
from between
the optical to 105OA
Thicknesses accurate
sharp changes
above
successive formulae(14)
between
suc-
determined
in
for these studies,
of composition
and
structure did not occur over narrow thickness ranges.
surface of the metal, great emphasis in these studies was placed on obtaining surfaces which were as clean, completely
light
spectrometer,
estimated
colors was always very good. the
since in general, METHOD
a polarized
with the polarizing
with
this way were sufficiently
for eight different faces.
EXPERIMENTAL
by
nique.o** 15) In some cases, thicknesses were checked
and by
temperature, of a copper
The oxide
structure
and composition
mined by glancing-angle using
Cu
radiation.
stationary-crystal
X-ray Both
techniques
were deter-
diffraction
methods,
oscillating-crystal
and
were used, the radiation
being filtered through Ni foil in the former case.
The
face of the specimen being studied was adjusted normal to the X-ray beam in such a way that the axis of oscillation
was in the plane of that face and
normal to the X-ray beam. The crystal was then rotated about the oscillation axis until the face being
surface so treated is not known at the present time.
studied made a definite angle of inclination
The exact preparation
X-ray beam, this angle being determined by consideration of the Bragg angles for the metal and
and the advantages
of the surface for these studies of using the crystal in the form
of a sphere with flat faces cut on it have been discussed
oxide faces concerned.
by Young and Gwathmey and others.03) It should be emphasized that the use of crystals in
rical film concentric
this form
the oxide
made it possible
to utilize
the oxidation
patterns standard
on the spherical surface as a reference for the conditions used for surface pre-
paration
and carrying
trace
amounts
out the reaction.
of impurities,
gases, or as foreign compounds marked
the surface, the oxidation
Addition
of
as adsorbed
such as grease, caused
patterns were recorded on a cylindabout the axis of oscillation.
was highly
oriented,
the resulting
If films
showed layer-line patterns with X-ray reflections from both the flat face of the substrate and from the oxide
plane parallel to the substrate
zero layer-line, satisfied.
provided
the Bragg
As an additional
falling
on the
conditions
check in determining
oxide orientation,
the homogeneity
of
an angle of about 45” to 90” about an axis perpen-
and this also showed up as a change in
dicular to the flat face, and a second layer-line pattern
the
standard.
of facets destroyed
obtained.
pattern.
was rotated
the
or
from
the specimen
were
Etching
changes
development
whether
The diffraction
with the
through
This gave at once, in many cases, the plane
The crystals were oxidized at temperatures from 170’ to 450°C and at pressures from 0.8 mm of Hg to
of the oxide which was parallel to the given substrate plane. In some cases where a sufficient number of
atmospheric.
X-ray
The oxidation
reactions
were stopped
at a given thickness by evacuating the system with a high-capacity pump and then cooling to room temperature.
The rate of cooling was varied for different
experiments. A fast rate of cooling was used to minimize solid-phase reactions(5) for studies in which the determination of the composition of the oxide was the primary purpose. Experiments indicated that the rate of cooling did not affect the oxide structure appreciably, and in general a moderate to slow rate of cooling was used for the structure studies. Oxide-film thicknesses were determined by means
reflections
was recorded,
the complete
orienta-
tion of the oxide could be determined simply by inspection of one or two oscillation X-ray patterns. In other cases, it was necessary diffraction
patterns,
using
to take a number of
both
oscillating-crystal
and stationary glancing-angle techniques, before the complete orientation could be fixed with certainty. The use of reciprocal lattice relationships simplified the determination of the orientation relationships in cases in which they were not obvious from inspection. The degree of orientation was estimated by comparing a particular diffraction pattern with each
LAWLESS
of a series of X-ray oxide
classified
diffraction
as to
the
ranging from essentially perfect (96100%).
random
to the distribution diffraction become the
arcs
give
a
of
of the classified
perfect
vary
alignment,
the
sharp spots,
but
in length
depending
on
method
can
This
estimate
of
the
degree
the
155
Pressure
25O’C 250°C 250°C 350°C 350°C 350°C 350°C
atm atm atm mm of mm of mm of mm of
Least thickness for which CuO was found
(OOl), (ill),
(Oil),
(113),
20 20 20 20
(112),
the entire ranges of pressure and temperature
(012), (122), and (133), the studies being more complete on the first six faces listed. All faces studied were
experiments,
probable
oxide.
prepared
as follows.
cut on a spherical
%: i 5000 2000 1200 2500
Hg Hg Hg Hg
A A A A
Thermodynamic calculations(5) been very small. reveal that both Cu,O and CuO are stable throughout in these
as flat surfaces
3000 8,
of
Studies were made of the oxide on eight different faces,
Temperature
As the
orientation. crystal
FILMS
TABLE 1
weighted according
disorientation.
reasonable
OXIDE
orientation,
in the arc.
are no longer
which
degree
maxima,
from
maxima
OF
of cuprous
of
(O-1Oo/o) to nearly
of intensity
deviates
STRUCTURE
on the basis of the length of
the arc of the diffraction orientation
patterns
amount
The orientations
series were determined
THE
GWATHMEY:
AND
crystal.
and
the
Cu,O
The formation
covered
is the
more
of CuO is explained
When the rate of formation
of Cu,O has
It should be noted that results were the same on the
become very slow, the number of copper ions available
(100)
for reaction
(OlO), (OiO), etc., planes as for the (001).
Thus
at the oxide-oxygen
interface
is very
it is necessary to indicate results for the faces of only
small.
one unit stereographic
a ready supply of copper ions is not available, the reaction Cu,O + 40, + 2 CuO is more likely to take
triangle.
place.
RESULTS
The
results
of this
study
are divided
sections, the first concerning effects of temperature, contamination degree epitaxy.
the complex
on the composition
Epitaxy
and the
may
mutual orientation
into
two
interrelated
pressure, oxide thickness,
of orientation),
and
and texture second
be generally
relationships
(or
concerning
defined
as the
between two different
crystal substances.
oxide
these experiments patterns closely
AND
formed
d,,,
values
the X-ray which
of
diffraction
agreed
with those given for Cu,O by Swanson
atmosphere
after the rate of oxidation
very
monoclinic
CuO was detected depended
the temperature least thicknesses
on different
amounts
thicknesses expected supply
of
of
CuO
on different at
smaller
copper
very slow at different
crystal
faces, the formation
will
begin
at different
faces.
CuO would
thicknesses
where
a greater
readily
available.
ions
is more
not
be
These results are in general agreement with experiments by both electron diffraction,(4T 5, 6, and electrolytic
reduction(4)
very and
had become
CuO was formed
below
by most
different
Fuyat,(l@ including the very weak 211 reflection. If the crystal was allowed to remain in an oxidizing
which
large
surface
under the conditions
was Cu,O,
giving
slow,
of
used
TEXTURE
of the Oxide Film
The major
Since the rates become
thicknesses
and
methods.
No indication
of
the oxide CuO’ was found. (l) In general, the methods
COMPOSITION
Composition
Since CuO is stable under these conditions
certain
on the particular
oxide
also.
No
thicknesses,
crystal
face and
of oxidation. Table 1 shows the for which CuO ‘was found under the
previous
and forming
for preparing
the
on the metal
were
from those used in these experiments
and
were not comparable. The CuO, whenever polycrystalline, tation.
workers
the oxide
present,
with no evidence
was
completely
of preferred
orien-
The Cu,O was in general oriented, the degree
of orientation
depending on the crystal face of copper,
oxide thickness, oxygen pressure, temperature, and the presence of impurities. The remainder of this paper is devoted variables
have
to the effect which these different on the texture
and epitaxy
of the
cuso. InJluence of Oxygen Pressure on Orientation
specified conditions. Since the X-ray techniques used were not capable of detecting trace amounts of CuO, it was not possible
The effect of oxygen pressure on the degree of orientation of oxide on the (001) and (111) faces is shown in Fig. 1 for oxidation at 200°C. The curves
to say that no CuO was present for smaller film thicknesses, but only that the amount must have
show markedly that for pressures below about 200 mm the degree of orientation increases with decreasing
ACTA
156
METALLURGICA,
For pressures greater than 200 mm, there
pressure.
seems to be no change in the amount with pressure. degree
of
pressure.
In general,
orientation
was
of orientation
for all faces studied, greater
the
lower
the the
This held true over the whole temperature
range studied and for all oxide thicknesses over about
VOL.
4,
1956
tion difficult. oratory
Preliminary
indicate
experiments
in this lab-
that a slower rate of oxidation
at
pressures under 9 mm of Hg may account partially for the improvement in orientation observed for thicker films formed at low pressures. It was found that the type of orientation
(epitaxy)
0 5 w40ii a.
A -(I 1 I) - 400
i
0 - (100) - 700
H
OXIDATION 20-
AT
200°C
FIG. 1. Effect of pressure on the degree of orientation of the oxide on the (100) and (111) faces of Cu.
5OOA.
With decreasing
degree of orientation peratures for
larger
thicknesses
(<25O”C). film
pressure, the increase in the
was greater for the lower temThe increase
thicknesses
less than 500&
was also greater
(>500
A).
For
oxide
the oxide was in general
highly oriented and showed either a very slight increase in the amount of orientation or no appreciable increase.
In no case was the amount
of orientation
found to decrease with decreasing pressure. Since the oxide film formed at atmospheric pressure is highly oriented for low thicknesses due to the strong orienting effect of the substrate, the degree of orientation can increase only slightly with decreasing pressure.
This increase is probably
due to the fact
that, at the lower pressures, few oxide nuclei are formed with unfavorable orientations. The number of possible adsorption sites occupied by oxygen ions will in general be greater at atmospheric pressure than at low pressure. Since more adsorption sites are always available than are needed for oxide of the
did not vary with pressure; crystal given
i.e., when a particular
plane of the oxide substrate
pressure,
the
face same
for
was found oxidation
relationship
parallel
to a
at atmospheric was
present
at
Thickness
on
lower pressures. Influence
of
Temperature
and
P&n
Orientation There was a general increase in the degree of orientation of the oxide with increasing temperature for
all faces
depending crystal face. produce
studied,
the
amount
on oxide thickness,
of the increase
oxygen
pressure,
and
In no case did an increase in temperature
a decrease in the degree of orientation.
The
complex relationships between the degree of orientation, temperature, and oxide thickness may best be described by a type of existence diagram in which points are designated by the amount of orientation for a given temperature and oxide thickness. This is done in Fig. 2 for the (111) face and in Fig. 3 for
observed orientations, the higher pressure will lead to the formation of additional nuclei of different and less stable orientations. The state of knowledge concerning the influence of pressure on the rate of
the (001) face oxidized at atmospheric pressure. For oxidation at atmospheric pressure, the oxide films formed were generally highly oriented for low thicknesses and could be considered as pseudo-
oxidation of single crystals of copper is very limited at the present time, and this makes a more complete interpretation of the effect of pressure on the orienta-
monocrystalline, i.e., consisting of small crystallites aligned in a small angular region about a given orientation.
With increasing
thickness
above
600 A,
LAWLESS
GWATHMEY:
AND
THE
STRUCTURE
amount
the
OF
OXIDE
157
FILMS
of orientation
decreased,
the decrease
being rapid at the lower temperatures at the higher
350
temperatures.
This
but gradual
can be seen in
Oxide on the (012) and (001)
Fig. 4 for the (111) face.
faces showed this same general behavior, i.e., a high initial orientation for low film thicknesses followed ORIENTED
I
I
I
by a fairly rapid decrease in the amount of orientation
43
YODERITELY
with increasing
DRIEWTED
I
I
I
/
I
/
J
;
I’
I
over a certain
range
of thicknesses
in rates
minimum.
all the faces studied because
of reaction
of the
on different
@@I@
@g&g I
a large
Iarge differences
/
I
thickness
It was not possible to compare over
/
I
I
/
/
I
/’
/-
4oool
SLIOIITLI ClRlEWTED
,I
.?ooo2Oc A
/
ATMOSPHERIC .
PRESSURE L
I5c
1
L
1
,
1
I
I
I
a 3000
2000
1000 OXIDE
THICKNESS
(A)
2. Orientation variation with temperature and
Fra.
600-
E g
600-
:-::-\,i A
Q I
I
_
z
oxide thickness for Cu,O on (111)Cu.
A
A A
1 u ; 400-
A-200°c
A - 25O’C
ki ;i 0
A 0 - 325’C 200 -
/ /
I
I:::
/’ @
0
/
/
HICHLI
/ I
MODERATELY
60 ORIENTATION
40 PERCENT
IO 0
60
,
of Hg were highly
,
/
ORlENTED
I
Oxide films formed at pressures of 0.8 mm to 25 mm
/
I
I
/
/
6LlOHTLY
/
/
/ ORlENTED
oriented
2OOA up to 3000-5000
,
I
20
A
ON (I I I)
FIG. 4. Variation of the degree of orientation with thickness for Cu,O on (111) face of Cu.
/ ORlENTED
OXIDE
,
/
.
amount [oxide
of
orientation
on (OOl), (ill),
from The
either (012),
increased
slightly
and (011) planes]
or
remained about the same [oxide on (112) and (113) planes] with increasing temperature. The disorientation amounted
/’
for all thicknesses
A on all faces studied.
to less than 10“ of arc on the X-ray
diffraction film in all cases, with the oxide on the (001) showing the greatest amount of disorientation.
/
/
/
‘@
/’
,/
OXIDE
(001)
It should
cu
ATMOSPHERIC
I
I
L
1
I
2000 THICKNESS
,
s I 3oOf
(A)
FIG. 3. Orientation variation with temperature and
oxide thickness for Cu,O on (0Ol)Cu.
4
be pointed
out
that
it is difficult
to
separate the effects of temperature from those due to initial low-pressure oxidation, when oxidizing at atmospheric pressure. In general, 0.7 to 0.8 set was
PRESSURE I
1000
/
/
required for the pressure in the reacting system to reach 200 mm of Hg; and at temperatures of 300°C and over, an oxide film greater than 5OOA formed in this short time. The evidence indicates that a
158
ACTA
METALLURGICA,
large part of the increase in degree of orientation observed with increasing temperature over 300% for oxidation at so-called atmospheric pressure is due to the initial low-pressure oxidation. Two factors in general could lead to disorientation of the oxide film with increasing thickness. The first of these is the rapid falling off of the substrate influence with increasing oxide thickness. The second factor is the rearrangement of the oxide due to slip or rotation in such a way as to relieve compressive stresses in the oxide film. The first of these would be the same whether the oxidation was carried out at low or atmospheric pressure. The second, however, could depend on pressure, since oxide films formed at atmospheric pressure contain more imperfections than those formed at low pressures. This would enable slip or rotation of the oxide to take place at lower compressive forces than would be the case for oxide films formed at low pressures. This means that the high initial orientation can extend to greater thicknesses for oxidation at low pressures, as is observed. The greater amount of energy available to the crystal at higher temperatures gives greater mobility to the oxide. Since the less favorably oriented units of oxide are more weakly bound, it is more likely that these will move about under the in~uen~e of temperature than the stronger bound, favored orientations. This then leads to an overall increase in the degree of orientation with increasing temperature.
Trace amounts of such contaminants as SO,, H,S, silicone grease, phosphate from polishing solution, residual gases from gas used in glass blowing, silicates or chloride from wash-water, and many others, caused marked changes in the oxidation patterns on the crystal sphere. Crystals showing an oxidation pattern ditl?ering from a “normal” or “standard” pattern could then be considered as contaminated, and in many cases the contaminant could be identified from the oxide pattern. In all cases, at temperatures of 200” to 25O”C, it was found that the oxide, when formed in the presence of impurities, showed a marked decrease in the amount of preferred orientation for all thicknesses. In general, the amount of disorientation was almost complete, i.e., the oxide was polycrystalline, with random orientation. For oxidation at 3OO*C to 4OO”C, the oxide showed a marked decrease in the amount of orientation, but not the randomness shown for the lower temperatures.
VOL.
4,
I950
In general, it would be expected that a contaminant which is adsorbed on the surface of the metal would reduce the orientation of the oxide by a simple blocking and reduction of the substrate influence. In general, less than a monolayer of contaminant could cause effective disorientation by blocking the adsorption sites on the metal surface. Gaseous contaminants could also produce disorientation by creating imperfections in the oxide as it is formed. EPITAXY
The epitaxial relationships reported in this section are those of C!u,O formed on electropolish~ flat faces of a single crystal. The type of orientation observed on a particular face did not change with temperature, pressure, or oxide thickness within the ranges covered in these experiments [170°C4500C; 0.8 mm of Hg to atmospheric pressure; 2OOA to 5000~]. The degree of orientation did change with these variables, as described previously, The epitaxial relationships observed are summarized in Table 2, the second column showing the plane of the oxide which is parallel to the Cu plane listed in column 1, and the third column giving the directional relationships necessary to define the complete orientation, i.e., the alignment of given rows of atoms of the oxide with row-s of atoms of the substrate. All the orientations observed can be placed in one of four different classes. These are designated as I (A, B, C, and C’), II (D), III (E, E’, and Z’), and IV (P). These orientations are shown in Figs. 5 (a, b, c, and d). The diagrams represent a section through the oxide and the copper normal to the particular face of copper indicated. The [Ii01 direction is normal to the page in each case. The rectangular figures in each drawing serve as an aid to visualizing the relative orientations of corresponding units of the metal and oxide. Fig. 6 is a schematic representation of these orientations for six different faces of copper. The large areas of Fig. 6 represent the plane faces of copper, and the small figures represent the oxide with all faces shown being cube faces. Orientations A, B, C, and C’ oocurred only on the (001) face of copper. The (001) plane of the oxide is parallel to the (111) plane of Cu in A and to the (111) plane of copper in B. In C the (010) plane of Cu,O is parallel to the (ill) plane of Cu. Orientation I), in which the oxide has the same orientation as the Cu substrate, occurred in only minor amounts in a few cases on the (001) plane of Cu, but was a major orientation of the oxide on the
LAWLESS
AND
+
+
t
t
GWATHMEY:
THE
STRUCTURE
OF
OXIDE
t
t +
+
+
159
FILMS
+
+
+
is?J&~~~oo,,~~ll ~~~&:_:ll,: ... .
“_-___ . ‘-._--.___
.
.
-.
.
.
.
.
.
IYI
.
.
.
C?
9
. .
.
. ‘--.‘---l22i~ -.
.
.
. .
8
‘13311
. .
++
(a) +
+
+
+
P 0
+ -t-
+
+
+
+
cq.ZSlk’ b-?-d+ ‘l.blA+
-t-tolIh[iool
. . .H .. .
.
-
l
’
.
(010
Pm. 5. Positions of Cu ions
.
,ooO
.
.
.
8
(c) (+)
.
. .
(4
of Cu,O with respect to Cu atoms (0) of single-crystal substrate viewed the faces indicated and with the [IlO] direction normal to the page. (a) orientations A and B on (001)Cu; tions D and E on (lll)Cu; (0) orientation D on (011)Cu; (d) orientation F on (113)Cu.
ccid
FIG. 6. Schematic
representation
of oxide orientations
on Cu.
parallel to (b) orienta-
ACTA
160
__ _
._
(001)
-. (111)
.a.
(111)
A
[ liO]Cu,O//[ liO]Cu
(001)
B c c’ D
[oil]cu,o/~[lro]cu ~loi]c~~o~~~llo]~ [oli]cu~o~/[l lo]cu [lio]cu~o/~[lio]~u
(111)
D
[iropu,o/piop
E
[ilo]cu,o//[lio]cu
22
[~io]~u~o~/[~io~c~
(011)
_:.
(113)
Major directional relationships
_i_
fllot (44i)
(112)
E’
[llo]cu,0//[0il]cu
E”
[llO]Cu,O//[O11]Cu
I
/
p
[lio~~*o~/~lio]~
_j
p
[lio]cu~o~/[lio]cu
_.
(474)
(132)
:=
1956
_____ ChASS
~-.--.-.
~-
Remarks
/
/_. I
_-
A,
B, C, and G’ are exactly equivalent orientations.
II
Found only in a few casas
II
Orientation E expressed as:
II
may
also
be
(iii)cu,o//(iii)cu piopu,o//pTop~
III
/ / ’ E’ and E” are equivalent orientations, but are not equivalent to D. E’ may also be expressed as: (120)cu,0//(012)cu
piop,o//[lTo]cu
I -_ __-
I
IV IV
I I
III
_7z [lio]cu,o//[lio]cu
_.
(133)
4,
D ~~io]~~o/~~iioi~
tow
(012)
VOL.
TAULE 2.
.-. I._ Parallel face Cu,O
Crystal face Cu substrate
(011)
METALLURGICA,
-I-
High index fa& near (112)
)
23 rlio]cu,o~~rlio]cu
(111) and (012) faces in all cases, and was the only orientation found on the (011) face of Cu. It is probable that the occurrence of D on the (001) face was due to sub-microscopic etching with the development of (110) or (111) facets prior to oxidation. Urien~tions E, E’, and E” were found in addition to D on both the (111) and (012) planes of Cu. It was also the only orientation found on the (122) and (133) faces of Cu. The orientations D and E on the (111) face are exactly equivalent as far as the surface layer of atoms is concerned. E may be derived from D on the (111) face by a 180’ rotation of the oxide about the [ill] axis. On the (012) face of Cu, E is not equivalent to D, and may be thought of as being derived from D by a rotation of the oxide of 131’ 49’ about the normal to the (012) face (counter-clockwise rotation giving E’ and clockwise giving E”). The counter-clockwise rotation brings the densely-packed row [121] of Cu,O into coincidence with the [12i] row of Cu, the (111) plane of Cu,O with the (111) plane of Cu, and makes three pair of [IlO] directions parallel. On the (113) and (112) faces of Cu, the orientation -F” was found. For both of these faces, the (110) plane of the oxide is accurately parallel to the (113) face of the copper, and the [liO] directions of oxide and metal are parallel. No other densely-packed
III
rows of atoms or planes of atoms coincided. It should be emphasized that the results reported in this paper were obtained on flat electropolished and annealed surfaces, and both the conditions of oxidation and the nature of the metal surfaoe were different from those used by previous workers.(sp g*lop I1712) The results are thus not strictly comparable. Neither the preparation of the surface nor the conditions of the oxidation are reported by Mehl, M~Candle~, and Rhines.(*) Their observation, that the Gus0 grows with its cube axes parallel to the cube axes of the substrate, was borne out for the parallel orientations on the (Oil), (012), and (111) faces, but not for any other faces studied. Thiessen and Schiitza used surfaces which had been etched in HNO, and then annealed at 1000°C before oxidation was carried out at 325°C and 150-mm pressure. Their results on the (OOl), (Oil), and (111) faces are in agreement with those reported here. Yamaguti(i*) etched his specimens in HNO, and HCI, and then oxidized them in air at 1000°G. The orientations observed camrot be compared directly, since the faces used were not specified except for one specimen. On a (001) face the orientation observed did not agree with that found here. Elarn(ll~ used surfaces which were characterized
LAWLESS
as “polished,”
AND
but no other specifications
In some cases specimens
THE
GWATHMEY:
STRUCTURE
OF
seem to be in good agreement
those found here. Menzelu2) carried 50-mm
pressure,
out
oxidations
followed
by
including
preparations,
etchants, electropolishing, vacuum
treatment.
general
at 450°C
by
application results
with those
is made
c
.
density
facets
strate
face,
are
in cu20
lattice
on the
parallel
in
and
oxide. define
with respect other
to a given
conditions
the
must
complete
+a
cu k
i-0
l
+.
l+
4-
be
oxide
to consider the fitting of the oxide metal
substrate,
and
to
determine or for two
directions
There
is controlling
the orientation.
two ways of considering
this.
adsorption
and the
second
positions
in the oxide
+, t o+ .+ ta -k at lto
are
The first considers the
misfit between oxygen positions
Consideration
*
sub-
whether small misfit for a given direction
most probable
[33iJ
t
l +o
l+
4-
[ooi]
are the ones
the metal
determining
orientation. It is necessary
CU cu,o
(001' Cu
(A’
if con-
does not completely
one or more for
[Ii01
and
developed
being
directions
in both
orientation
important
0 +
various
of this paper
directions
The [llO]
Since this one condition the oxide
+O
present in all cases is that of at
least one pair of [llO] of greatest
0
heat
the different methods of surface preparation. Consideration of the data of Table 2 reveals that
metal and oxide.
l+
l +o+e
l+
a
+
a variety
of
obtained
of the various
the only relationship
161
and high-temperature-high-
The
agreement
sideration
with
a high-vacuum
treatment,, for surfaces which had received of
FILMS
were given.
were used from which the
oxide had been removed by heating in a high vacuum. The orientations
OXIDE
in the oxide and the
sites on the copper surface,
considers
the matching
of copper
of the first of these indicates
good match or fit for oxygen
O-t
to
with those in the substrate. that a
f
ions on the adsorption
It
.
.
l+
't .
.
0
l
sites on the metal surface is not necessary for oriented growth of the oxide film. If the copper-ion
positions
(Cl
(III) cu
of the oxide planes are
superimposed on the copper positions of the substrate faces, as in Fig. 7, according to the orientations of Table 2, it is seen that a misfit of 18% is present in at least one [llO]
direction
in each case.
The misfit
here is defined as b/a - 1, where b and a are the distances between lattice points for the oxide and metal
respectively
the misfit seen that oxide on misfit is oxide on
is being
in a given
direction
determined.
From
for Fig.
which 7 it is
the matching of the two lattices is good for the (111) and (011) planes of copper (the 18% in any direction on the surface). For the (001) and (113) planes of copper, the
matching of lattices is not so good, but there is present a striking near-coincidence of the rows of closestpacked Cu ions of the oxide with the closest-packed
'
l+
P
(D)
(110)
0
Cu
FIG. 7. Positions of Cu ions (+) of Cu,O superimposed on Cu atoms (0) of the substrate viewed normal to faces indicated.
ACTA
162
rows
of the metal
between
face.
Table
the closest-packed
in the interface
METALLURGICA,
3 lists the misfits
rows of atoms
and ions
plane for seven substrate faces.
The
VOL.
oxide
the value
The type of orientation
packed rows in the oxide-metal
oxide
Table
thickness,
of which
depended
on
(i.e., the major face of the
oxide tending to be parallel to a particular face)
From
1956
crystal face and temperature.
misfit here is defined as above, except that B and A are the perpendicular distances between closestinterface.
4,
did not
vary
thickness,
with temperature,
but
did
vary
The degree of orientation
3 it is seen that this misfit between rows for oxide on the (001) is only 2%,
variables.
It was essentially
and for oxide
on the (113) it has the exceptionally
face
given
oxide
with
crystal
or
face.
varied with all the above
closest-packed
and
substrate
pressure,
constant
thickness)
change
in
small value of 0.50/O. These small misfits are apparently the determining factor in fixing the oxide orientation
pressure above
on these two planes.
creasing pressure for all faces and at all temperatures
It misfit
is also
apparent
between
from
Table
3 that
rows in the interface
a small
plane
cannot
be the deciding factor for oxide on the (112) and (122) planes. The (112) face can be considered as composed
The orientation
of the oxide on these faces seems to be controlled these steps, specifically
by
the (113) for the (112) face
different
crystal
techniques. pressure, studied,
of
and structure
of oxide
copper
studied
faces,
were using
X-ray
films on for
eight
diffraction
The effects of crystal face, temperature, oxide
thickness,
using
carefully
and
increased
studied.
The
with increasing
degree
of
markedly
orientation
of
temperature
with the
de-
oxide
in all cases,
the amount of the increase being greater for oxidation at atmospheric
pressure and large oxide thicknesses.
Oxide films formed
at atmospheric
pressure were
highly oriented for the first 500 to 600 A, but became disoriented
with
Oxide films formed
increasing
thickness
above
this.
at pressures from 0.8 to 25 mm up to
3000 to 5000 A. Contaminants in general caused a marked decrease
SUMMARY
crystals
but below this, the
of Hg were highly oriented for all thicknesses
and the (111) for the (122) and (133) faces.
The composition
of orientation
increased
of steps of (111) and (113) faces, the (122)
and (133) of (111) and (011) faces.
single
degree
200 mm of Hg;
(for a given
with
contaminants
prepared
were
in the amount of orientation Four
different
classes
of the oxide.
of
epitaxial
relationships
were found for Cu,O on the eight crystal faces studied. The controlling
factors in determining
tions can be summarized
electropolished
these orienta-
as follows:
1. At least one pair of [llO] directions packed
Studies between 170” and 450°C showed that C&O was the major oxide which was formed on all
parallel. This gives 18% misfit in one direction. 2. For low-index planes, a good fit (small misfit)
faces initially.
between
CuO formed above a certain minimum
directions
for both
(the closest-
specimens.
closely-packed
Cu and Cu,O)
rows
of
must be
atoms
is
more
TABLE 3.
Crystal plane cu
Parallel plane C%O
(001)
(111)
(111)
(111)
(011)
(011)
(012)
(012)
(012)
(012)
(113)
(110)
! I I I I I
(47%)
____ Cu,O (B)
1 (A, B, C, c’)
2.55
2.61
II (I)), III (E)
2.21
2.61
3.61
4.25
4.03
4.75
11 CD) 11 CD)
._ ._
III (E’, E”)
3.29
IV (F)
4.23
III
I
parallel plane
Cu (A)
IV (F)
(112)
(122)
Distance between closest-packed rows of Cu atoms in boundary (in A)
Orientation class
I
Misfit
B-A A
(%)
IL 18 -__
18
3.88
I
4.25
6.25 12.60 (2A) 18.75 (3A)
17.27
15.31 7.65 (2A)
18.78
18
18 0.5
38 -8
(E) ~.__
1
23 _. --
LAWLESS
important
than
densely-packed
the
requirement
planes.
This
not
important.
of parallelism
is illustrated
planes
such as the
The
orientation
mining
plane,
face.
by
i.e., by the If there is a
(110) and (111) planes as the deterthe (111) seems most
the two orientations non-parallel
directed
orientation
likely;
and of
by the (111) plane, the
seems
to be the preferred
one. In general, CuO, when present, was polycrystalline and completely Consideration
disoriented. of these results in terms of the rates
of oxidation
on different crystal faces reveals one interesting correlation. Although the rates of oxidation of the (001) and (111) faces are considerably
different,
both faces have (111) planes of the oxide parallel to them. lent
There are, however, orientations
on the
only two on the (111).
four energetically (OOl), whereas
diffusion
oxidizing
(011)
through
and
equiva-
there
are
Oxide on the (001) therefore
shows more grain boundaries enhanced
orientation
OF
OXIDE
of the oxide.
FILMS
163
This may partially
account
for the large rate differences between different crystal
and
plane is
is determined
faces near the high-index
choice between
of the
(122)
of misfit in the interface
the steps of which the face is composed, major
by
STRUCTURE
faces.
(11o)cu,o//(ll3)cu. 3. For high-index (133), the amount
THE
AND GWATHMEY:
(113)
and the possibility the faces
oxide. show
The only
of
slowone
ACKNOWLEDGMENT
This work
was supported
in part
by the Office
of Naval Research. REFERENCES 1. C. A. MURISON, Phil. Msg., 17,96 (1934). 2. G. D. PRESTON and L. L. BIRCUMSHAW. Phil. Mao.. ” I 20. ---, 706 (1935). 3. H. DUNHOLTER and H. KERSTEN, J. Applied Phye., 10, 523 (1939). 4. C. G. CRUZAN and H. A. MILEY, J. Applied Phys., 11, 631 (1940). 5. E. A, GULBRANSEN and W. R. MCMILLAN, J. Electrochm. sot., 10,393 (1952). 6. H. FRISBY, Compt. rend., 228, 1291 (1949). 7. G. P. THOMSON, Proc. Roy.Soc. (London), A133.1 (1931). 8. R. F. MEHL, E. L. M&AND&S, and. F. N: R&NE& Nature, 184,1009 (1934). 9. P. A. THIESSEN and H. SCH~~TZA, 2. anoTg. U. allgem. Chem., 233, 35 (1937). 10. T. YAMACWTI, Proc. Phys. Math. Sot. Japan, 20, 230 (1938). 11. C. F. ELAM, Trans. Furuday Sot., 32, 1604 (1936). 12. E. MENZEL, Ann. Phys. [Leipzig] 5, 163 (1949). 13. F. W. YOUNG, Jr., J. V. CATHCART, and A. T. GWATRMEY; Submitted for publication this journal. 14. A. B. WINTERBOTTOM, Trans. FaradaySoc., 42,487 (1946). 15. A. T. GWATHMEY and F. W. You~a, Jr., Rev. Mdtallurgie, 48, 434 (1951). 16. H. E. SWANSON and R. K. FUYAT, National Bureau of Standards Circular 539, Vol. II, 23 (1953).