Download Structural, optical, and electronic properties of magnetron

Structural, optical, and electronic properties of magnetron-sputtered
platinum oxide films
H. Neff
TZN Forschungs- und Entwicklungszentrum Unterlüss GmbH, Neuensothriether Strasse 20,
D-29345 Unterlüss
S. Henkel
Physikalisches Institut der Rheinisch-Westfälischen Technischen Hochschule (RWTH), D-52056 Aachen
E. Hartmannsgruber
Institut für Siliziumtechnologie (IsiT), Fraunhoferstrasse 1, D-25524 Itzehoe
E. Steinbeiss, W. Michalke, K. Steenbeck, and H. G. Schmidt
Institut für Physikalische Hochtechnologie (IPHT) Jena, Helmholtzweg 4, D-07743 Jena
~Received 23 October 1995; accepted for publication 16 January 1996!
Stable platinum oxide films have been prepared through magnetron sputtering and have been
analyzed on the bases of energy-sensitive microanalyses, x-ray diffraction, resistivity, and optical
reflectance measurements. The complex dielectric function has been determined for various oxygen
contents in the film covering the wave-number regime 50 cm21 –l21 –50 000 cm21. The vibrational
properties are dominated through a strong band, centered at 765 cm21, associated with a asymmetric
stretching mode of the Pt—O bond. The films are amorphous, with chemical composition PtOx ,
where 1,x,2.1, and are considered as a homogeneous solid solution of PtO and PtO2 . The
materials system displays a conductor–insulator transition at x>2, in connection with an optical
band gap E g of ;1.2 eV in the fully oxidized state. The conduction mechanism over the whole
range of compositions is thermally activated and is determined through a large density of localized
states extending into the band gap. At x,2 the optical gap disappears, consistent with the
semimetallic behavior of the materials system for this range of composition. © 1996 American
Institute of Physics. @S0021-8979~96!01509-2#
I. INTRODUCTION
Under certain conditions, noble metals form stable oxidic compounds. For platinum, the phases PtO, a-PtO2 ,
b-PtO2 , Pt3O4 , and possibly Pt3O8 have been reported.1–3
Although the noble metals, in connection with their stable
oxides, play a crucial role for numerous catalytic processes,
their fundamental physical properties still are poorly understood. The strong catalytic activity is related, in part, to the
high density of states at the Fermi energy E F . This establishes high charge transfer coefficients over the interface, and
therefore is particularly revealing for electrochemical
reactions.4 Metallic platinum is oxidizing at very high pressure and moderate temperature,3 at anodic potentials in aqueous solution, and also under Ar/O2 plasma conditions.5
The various known noble-metal oxides display remarkably different properties. For example, anodic oxidation of
iridium is leading to a pronounced electrochromic effect.6,7
The oxide is switching upon electrical polarization and associated change of the oxidation state from metallic behavior
with dark blue color to a clear, optically transparent and
electrically insulating state. This metal–insulater ~M–I! transition is not observed for electrochemically produced gold
and platinum oxides, where in the anodic regime only a reversible oxidation-reduction cycle of only a few monolayers
thick oxide film is observed. These films thus are of limited
practical use. Sputtered gold and platinum oxide films, in
contrast, remain stable at ambient temperature, but decompose upon heating into the pure metals at T.500 °C. The
materials system therefore has attracted renewed attention.
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J. Appl. Phys. 79 (10), 15 May 1996
The deposition of thin PtO films with metallic behavior
by means of reactive sputtering already has been reported
elsewhere.8 Both, a crystalline and an amorphous phase were
formed, depending on the actual sputter conditions, i.e., pressure, substrate temperature, and voltage applied. The Pt—O
system forms a metal–insulator composite, also known as a
cermet.
In this work an attempt was made to characterize the
optical and electronic properties of the Pt—O system as a
function of oxygen content in the films, covering the regime
1,Ox ,2.1. The films have been analyzed by means of
energy-sensitive microprobe analyses ~EDX!, primarily to
determine the oxygen content, x-ray diffraction ~XRD!, electrical resistance, and optical reflectance measurements. Future applications of these films are ranging from metallization due to laser patterning of electrical interconnects and
contacts in microelectronic circuits and devices, to optical
switches, chemical sensors, and, finally, to effective surface
passivation layers of high-T c superconductors.9
II. EXPERIMENT
Briefly, platinum oxide films with composition PtOx and
1,x,2.1 were deposited by reactive sputtering onto ~100!oriented, single-crystalline polished ZrO2 and silicon substrates at a temperature of 80 °C. The oxygen/argon concentration ratio within the chamber was varied to adjust for
stoichiometry and oxygen content in the films. Gas pressure
in the sputter chamber was in the 1 Pa range. The deposited
films were semitransparent with a reddish-brownish appear-
0021-8979/96/79(10)/7672/4/$10.00
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FIG. 1. Film resistance as a function of oxygen content x of the films in
logarithmic scaling. Inset shows a set of Arrhenius plots, recorded at x51,
x51.8, and x52.1. The solid line is the best fit of the data to a third-order
exponential polynomial. Note the change of slope at x.1.8.
ance. Film thickness was ;5000 Å, as determined through a
Zeiss type-ME 10 D step profiler. The quality of the films
has been assessed through four-point resistance measurements. XRD measurements revealed a fully amorphous
structure of the sputtered films. Metallic platinum was not
detected. The exact oxygen content in the films was varying
continuously within the range 1,x,2.1, as indicated by
EDX measurements. The accuracy of the method is 65%.
Optical reflectance data from the films were recorded
with a Bruker 113 v Fourier spectrometer for wave numbers
,5000 cm21, while a Lambda 2 spectrometer was used
within the spectral region 5000 cm21,l21,50 000 cm21.
The dielectric function and absorption coefficients of the
films, eventually, were obtained from the optical measurements in connection with a multilayer Fresnel calculation
and an automated fitting routine to establish best agreement.
III. RESULTS AND DISCUSSION
Figure 1 displays the film resistance R at ambient temperature as a function of oxygen content in logarithmic scaling. The resistance increases monotonically, indicating an
exponential behavior with a somewhat larger slope at x.1.8.
Inset of Fig. 1 shows a set of Arrhenius plots, with resistance
r5r0 exp(DE/kT), recorded for compositions of x51,
x51.8, and x52.1. The data account for thermally activated
transport over the whole range of compositions. For the
highest oxidation state, a constant slope was observed, according to an activation energy DE'320 meV.
The optical properties of the materials system under
study are illustrated in Figs. 2 and 3, where the spectral range
from the far infrared to the UV region has been covered.
Figs. 2~a!–2~c! show both the complex dielectric function e8,
e9 and the absorption coefficient a as a function of wave
number up to 50 000 cm21 ~6.2 eV!. A recently published
fitting procedure10 to model the dielectric function of amorphous solids was employed to determine the dielectric function of the films from the optical reflectance data.11 For compositions with x51 and x51.6 the dielectric function only
J. Appl. Phys., Vol. 79, No. 10, 15 May 1996
FIG. 2. Dielectric function e8 ~solid line! and e9 ~broken line! and absorption coefficient a as a function of wave number with 5000
cm21,l21,50 000 cm21 for composition x51, x51.6, and x52.1. The
position of the optical gap is denoted by an arrow.
weakly depends on photon energy, but reveals a rather large
absorption coefficient. The absorption coefficient a was calculated from the dielectric function, using the relation
a 54 p /l A 12 @ 2 e 1 1 A~ e 21 1 e 211!# ,
where e5e11i e 11 is the complex dielectric function and l
the wavelength. The data reveal a steady increase of absorptivity of the film with photon energy, and clear absence of an
optical band gap for these compositions upon extrapolating
of a to lower wave numbers. This rather metallic behavior
applies to all oxide compositions within the oxidation state
regime 1,x,2. It clearly contradicts the observed thermally
activated transport, and associated negative thermal resistance coefficient, scaling as 2E 0 /kT 2 . The present Pt–O system at x,2, hence, is understood as semimetallic, where the
conduction- and valence-band edges are inverted and slightly
overlap. Under these circumstances, and in connection with
the disordered structure, concentrations of free carriers, their
mobility, and thus electrical transport typically are poor.
A striking difference of the optical properties has been
observed in the fully oxidized state, at x>2. The complex
dielectric function and the absorption coefficient strongly
vary with photon energy, where the latter displays much
smaller values. The variation of e 11 and a indicate a clear
absorption edge and optical band gap E g , located at a photon
energy around 9500 cm21 ~1.17 eV!. The tail region extends
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FIG. 3. Dielectric function e8 ~solid line! and e9 ~broken line! and absorption coefficient a in the low-wave-number regime 50 cm21,l21,1000
cm21 for composition x51, x51.6, and x52.1.
down to 4000 cm21 ~0.5 eV!, far into the gap region. The
oscillations at higher photon energies are a result of the fitting procedure.
The set of dielectric functions and absorption coefficients a for compositions x51, x51.6, and at x'2.1, calculated for the far infrared regime, are shown in Fig. 3. Two
prominent features have been resolved in the absorptivity
data at x52.1. The spectra typically display a rather broad
vibrational band A, composed of two vibrational modes centered at 765 and 580 cm21, and a further single feature B,
located at 130 cm21. Band A also has been resolved at 600
cm21 for x51.6. A clear Drude part indicating free-carrier
absorption has been identified through the increase of e 11
toward the dc wave-number limit at x51 and, with reduced
magnitude, also at x51.6. This, again, optically confirms the
more metallic character of the film for the given compositions.
As already mentioned, the infrared properties of the materials system under investigation are characterized through
bands A and B, centered at 765 and 130 cm21, respectively.
The former rather broad band was already observed by
means of electron loss spectroscopy ~EELS! of electrochemically treated platinum surfaces,12 but no assignment regarding its structural microscopic origin has been given so far.
The vibrational properties of the present amorphous platinumoxide films are compared with the dominating vibrational
band, pertaining to amorphous SiO2 films. In this materials
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J. Appl. Phys., Vol. 79, No. 10, 15 May 1996
system, usually a very strong vibrational mode is observed
near 1100 cm21. It has been attributed to a transversaloptical ~TO! Si–O asymmetric stretching mode of the Si–O
tetrahedral.13 In view of the much larger mass of the platinum atom, the weaker chemical bond—as indicated though
the PtO2 decomposition temperature of 500 °C—and associated lower force constant between the platinum and oxygen
atoms, the frequency of the vibration would shift to lower
values. In fact, a rough estimate taking into account the appropriate masses and 50% of the force constant of the
silicon–oxygen bond would resemble the position the Pt–O
vibration around 700 cm21. Its splitting off might be assigned to longitudinal optical-transversal optical ~LO–TO!
mode coupling, which also is applying to SiO2 .13 The assignment of band B in the PtOx system remains rather difficult.
Due to its position at very low frequency, one might speculate about the possibility of a Pt–Pt vibration mode. The
required dipole selection rule would hold if the platinum
atoms were in different oxidation states. Since, in fact, the
films even in the highest oxidation state contain significant
concentrations of defect states with different oxygen stoichiometry, this possibility is not ruled out completely.
Both electrical and optical characterization of the oxide
material in the fully oxidized state at x>2 suggest rather a
Mott-type insulator,14 than a semiconductor-type behavior.
Charge carrier transport thus would be related to a hoppingtype conduction mechanism, where the large activation energy of ;0.3 eV makes the presence of electrically active
defects, i.e., donor or acceptor levels, within the band gap
unlikely.
In view of recent x-ray photoemission ~XPS! and ultraviolet photoemission spectroscopic ~UPS! investigations,5 reported for sputtered b-PtO2 films, the Fermi level E F would
be placed near the valence-band edge ~VBE!; i.e., the fully
oxidized PtO2 would behave as a p-type insulator. Taking
into account the optical data from this work, and the UPS
results of Ref. 5, a tentative band diagram of the fully oxidized platinum oxide is drawn. The Fermi level E F is placed
0.3 eV above VBE, within the center of the tail. Under this
assumption, electrons during the hopping process would be
thermally excited from the VBE of the host material into
localized states within the tail section above E F , acting as
hopping centers. Majority carrier transport thus would be
hole type. Hall experiments of the films to confirm this
model are currently under preparation.
To our knowledge, a single study on the optical properties of sputter-deposited platinum oxide film has been reported previously.15 The sputtered films have been identified
by XPS as semiconducting a-PtO2 . The rather sharp optical
absorption edge, set at 1.3 eV, has been attributed to an
allowed direct transition, and a second optical gap at an energy of 1.47 eV has been assigned to an allowed indirect
transition. Similar results have been reported in Ref. 18 for
very thin anodic platinum oxide films, where the position of
the absorption edge varied from 0.9 to 1.3 eV. It should be
noted, however, that these oxides are hydroxidic forms12
with composition Pt~OH!4 . Our fully oxidized films consistently revealed a slightly higher oxidation state with x52.1.
This may explain the somewhat smaller optical gap. The
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origin of excess oxygen in our films yet has not been fully
identified, but could be related to the different sputter conditions. Moreover, XPS data taken in the work of Ref. 15 do
not permit sufficient information on the exact oxygen content
as does EDX used in this work. The various crystalline
phases of the Pt–O system still seem to be a controversial
issue. There are reports on the existence of Pt3O8 , crystallizing in the bcc structure,1 that would establish x values .2, if
present in an amorphous phase in the film.
We fairly exclude the possibility of a percolation-type
materials system. Electrical transport behavior in a percolative conductor–insulator composite commonly is described
through the following set of equations:16,17
r 5 r 0 ~ V s 2V c ! 2t
with V s .V c ,
~1!
r 8 5 r 0 ~ V c 2V s ! u
with V s ,V c .
~2!
V c is the critical volume fraction, or percolation threshold, at
which the change of the resistance occurs. V s is the fraction
of the conducting PtO phase, which is 100% at x51, t and u
are critical components, ascribed to a percolative system.
The insulating phase thus would be attributed to PtO2 and,
possibly, some amount of Pt3O8 . From the present data the
exact concentration of both phases is difficult to estimate.
For an idealized percolative insulator–conductor system,16
V c is calculated to 17%, t and u are 1.7 and 0.7, respectively.
Whether the above stated set of equations and related theory
applies to a material where the dominating phases are amorphous and probably homogeneously mixed15 remains questionable. Also, there possibly are more than two phases
present, i.e., metallic platinum at x<1, Pt3O4 semiconducting
PtO2 at a reduced band gap, and also Pt3O8 at x>2. The best
fit of the experimental data to Eq. ~1! yields t'3.4, a value
much larger than expected through theory. The range V s ,V c
has not been addressed, since the percolation transition is
expected at x>2, and x values .2.1 yet have not been
achieved under the given sputter conditions.
J. Appl. Phys., Vol. 79, No. 10, 15 May 1996
In summary, amorphous platinum oxide films, forming
solid solutions mainly of PtO and PtO2 have been investigated on the bases of EDX, XRD, resistance, and optical
measurements. Both electrical transport and the dielectric
function sensitively depend on stoichiometry and oxygen
content of the films. The transition from Pt to PtO and
PtO21d is associated with a gradual change from metallic to
semimetallic and, eventually, insulating behavior. The Pt–O
system displays a semimetal–insulator transition in the fully
oxidized state, where the optical band gap increases to 1.2
eV.
O. Müller and R. Roy, J. Less Common Met. 16, 129 ~1968!, and references cited therein.
2
W. I. Moore, Jr. and L. Pauling, J. Am. Chem. Soc. 63, 1392 ~1941!.
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A. K. Vigh, U. Electrochem. Soc. 119, 1498 ~1972!.
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M. Hecq, A. Hecq, and I. P. Delrue, J. Less Common Met. 64, 25 ~1979!.
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R. Brendel and D. Bormann, J. Appl. Phys. 71, 1 ~1992!.
11
The experimental reflectance data have been fitted using a total of 12
Gaussian functions, in connection with a fully automated computer routine
to obtain the best fit.
12
M. Peuckert and H. Ibach, Surf. Sci. 136, 319 ~1984!.
13
See, for example, P. Lange, J. Appl. Phys. 66, 201 ~1989!, and references
cited therein.
14
Electronic Processes in Non-Crystalline Materials, edited by N. F. Mott
and E. A. Davis ~Clarendon, Oxford, 1979!.
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C. R. Aita, J. Appl. Phys. 58, 3169 ~1985!.
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J. P. Straley, in Percolation Structures and Processes, edited by G. Deutscher ~AIP, New York, 1984!, Vol. 5, p. 353.
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1
Neff et al.
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