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Variation of conductivity and activation energy in metal-doped and undoped C60 films under oxygen exposure
~ ) Pergamon
Solid State Communications, Vol. 8% No. 5, pp. 437-440, 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098]9456.00+.00 0038-1098(93)E0128-K
V A R I A T I O N OF C O N D U C T I V I T Y A N D A C T I V A T I O N E N E R G Y IN M E T A L - D O P E D A N D U N D O P E D C6o FILMS UNDER OXYGEN EXPOSURE Shigeo Fujimori, Katsunori Hoshimono, Shizuo Fujita and Shigeo Fujita
Department of Electrical Engineering, Kyoto University, Kyoto 606-01, Japan (Received 26 August 1993 by H. Kamimura) Influence of oxygen on electronic conduction in metal (In or Sb)-doped and undoped C60 films is investigated. The conductivity decreases by several orders of magnitude and the activation energy in semiconductor-like conduction highly increases due to oxygen absorption, and vice versa due to desorption. These variations are attributed to the oxygen-related electron trap states formed at about 0.7 eV below the conduction band.
1. I n t r o d u c t i o n
(Sb) with 5N purity. Each source was evaporated from a quartz crucible. In order to measure the conductivity in vacuum successively to the deposition, we prepared the quartz substrate (15×15 mm) with interdigitated gold electrodes (50 #m spacing, 10 mm width, and about 500/~ thickness) which were made by vacuum evaporation, photolithography, and etching. The substrate was set in a vacuum chamber and the electrodes were wired to the external circuits through current terminals, as shown in Fig. 1. It was confirmed that the electrode resistance is low and the leakage current of the quartz substrate is small enough to be neglected in the measurements. Then the substrate was prebaked at 120 °C for 30 minutes to remove residual impurities on the surface, afterwards cooled to room temperature. Film deposition was carried out by simultaneously heating both C6o (about 550 °C) and dopant (either In: 850 °C or Sb: 520 °C) crucibles under 10-s Torr. The deposition rate was about 1 A/s, which was monitored by a quartz-crystal thickness meter, and the flux ratio between C60 and dopant was about 1 : 1. At present, we have not measured the quantity of dopant metal in the C6o thin films. The typical thickness of the film was about 2000 ~. The conductivity was determined from the gradient of linear region of current-voltage (I-V) characteristics at relatively low applied voltage. After the measurement in vacuum (10 -6 Torr) successively to the deposition, pure oxygen (02) gas or air was introduced into the chamber to the pressure of 0.2 or 1 atm, respectively. It takes about 5 minutes to fill the vacuum chamber with 02 from vacuum of 10-8 Torr to 0.2 atm. The variation of conductivity during the introduction of 02 was continuously monitored by measuring the current at a constant voltage in the linear I-V region. The temperature dependence of conductivity was measured by cool-
Buckminsterfullerene C60, according to the theoretical band calculation, possesses direct band gap of 1.5 eV, 1 which suggests that C60 would be available as a novel semiconductor material. However, the conductivity of Cso films ever reported 2'3 was so low (e.g., 10-s-10 T M S.cm -1) that their semiconducting properties could not be successfully utilized. Recently, we reported that the conductivity of amorphous Cs0 films was highly enhanced by metal (In or Sb) doping and the films exhibited n-type conduction,a The obtained carrier concentration and mobility values, e.g., 10TM cm -3 and 0.03 cm2/V.s, respectively, are nearly comparable to those of amorphous silicon, and this result opens potential applications of C60 films to new and unique semiconductor devices. It is, however, recognized that electronic properties of C6o are seriously influenced by oxygen. ArM et al.s showed that resistivity of undoped Cs0 single crystals became higher by 4 orders of magnitude due to oxygen . exposure at room temperature. In this paper, investigations are focused on the variation of conductivity and activation energy in metal-doped and undoped C6o amorphous films due to oxygen absorption and desorption, in order to understand the electronic properties, the conduction scheme, and the stability of these films. Discussion will be also given on energy levels of trap states formed by oxygen. 2. E x p e r i m e n t s Commercially available C60 powders with the purity higher than 99.9 % from Texas Fullerenes Co. were used as a deposition source after baking in vacuum of 10-6 Torr at 300-400 °C for 10 minutes and then at 150 °C for a few hours to remove solvent. As dopant sources, we chose elemental metals of indium (In) and antimony 437
METAL-DOPED AND UNDOPED c6O FILMS
43x
,
to pure
finallv
in about
Sb-doped
r
02 (0.2
posure
magnitude
Current Terminal
Vol. Xc), No. 5 atm)
2 hours,
values for In- or
the conductivity
film by factor reported.5
of 4, similarly
The
decrease
similar
der nitrogen conductivity small (within
of experimental
surement
conditions
were selected
after
Here, mea-
several
experi-
the effect of nitrogen
to that of oxygen.
the oxygen
exposure,
and then
the
Sb-doped
l/3
oxygen
film,
desorption.
heating,
the
because
The
desorption
the conductivity
of the as-deposited
(10e6 Torr, treatment
value,
heating
of metal-
field at about
160 “C; hence
more recovery
of conductivity.
measurements
X-ray
diffraction
any diffraction
peaks,
indicating
that
did not show these films were
Results
and
morphology
A heat
of the samples tends to with applying
electric
we could not confirm Under the re-exposure
again
by the second
The
is recovered
As-deposited
decreases
Heat treatment
similar
varia-
marized.
and undoped
undoped
under
the ex-
deposited
results
In-doped
the variations
of conductivity
for metal-doped
conductivity
for both
Discussions
2 shows the hysteresis
tion at room temperature
have
been
and undoped
of conductivity
It should
the of
decreases heat treat-
generally
Css films.
that
the conductivity
completely
recovered
value by the heat treatment,
I ’ ’
’
’
I,
350
’
of
to the as-
whereas
that of
(K )
Temperature
Heat treatment
obtained In Table
for various films are sum-
be noted
film is almost
400 _
the sample.
the conductivity
The 3.
Css films.
by
to only
ment.
amorphous.
Figure
was promoted
0s after the heat treatment, and then
to
due to
may desorb more oxy-
on the flat surfaces
scopes.
probably
was recovered
at higher temperature
be rough during the measurement
micro-
at
value only by the evacuation
1 hour) without
gen, but the surface
electron
chamber
were heated
conductivity
of the as-deposited
No metal
and scanning
the vacuum
recovers
the
of films or oxygen desorption
doped Cm films by optical
to
For
can be eliminated.
were observed
it is con-
is attributed
samples
ments so that degradation particles
is negli-
Therefore,
of conductivity
during
the measurements
Un-
treatment).
l/100 ing down from 120 “C to room temperature.
has been ob-
to air (1 atm).
120 “C for 1 or 2 hours in 1O-6 Torr (heat about setup.
ever
absorption.
After
illustration
undoped
to the single crystals
of conductivity
10 %) that
was evacuated
Fig. 1 Schematic
exposure
of the amorphous
when exposed
cluded that the decrease
Vacuum Chamber
The oxygen
exposure (0.8 atm), on the other hand, the slightly decreases, but the decrease is so
gible compared
Metal
and
lower than the as-deposited Css films, respectively.
served for all samples
c60
the deposition,
7 or 3 orders *in
also reduces
oxygen
after
it becomes
1 ’
1
1
300
IT
10-k-+-+
10-‘4~W 0
50 Oxygen
100 150 exposure
0 time
50
100
( min. )
Fig. 2 Hysteresis of conductivity variation under oxygen exposure, heat treatment, re-exposure, and second heat treatment.
2.6
2.8
3
1000 / Temperature
3.2
( K-’ )
Fig. 3 Examples of temperature dependence of conductivity. This figure is for In-doped Cm film, but similar results have been obtained for Sb-doped and undoped films.
Vol. 89, No. 5
METAL-DOPED AND UNDOPED C60 FILMS
439
Table I. Conductivity a and activation energy E, of metal-doped and undoped C6o films. Dopant material In
Sb Undoped
As-deposited l x l 0 -2
After oxygen exposure 2x10 -9
After heat treatment 7x10 -s
E~ (eV) a (S.cm-1)
0.10 5x10 -4
0.68 6x10 -7
0.25 2x10 -4
Ea (eV) a (S.cm-1)
0.18 3x10 -1°
0.54 l x l 0 -13
0.25 3x10 -1°
a
E~
(S.cm-1)
(eV)
0.51
In-doped film after the heat treatment is as low as 1/100 of the as-deposited value. These differences will be discussed later together with the variation of activation energy. For the In-doped and undoped films, the conductivity rapidly decreases immediately after the first exposure to oxygen. The conductivity of Sb-doped films, on the other hand, increases slightly for a few minutes after the exposure (oxygen pressure is less than about 0.02 atm), and then slowly decreases. Compared to the first exposure, this phenomenon was not observed under the re-exposure after the heat treatment. This seems to indicate that the surface of Sb-doped as-deposited C60 film is somewhat unstable, but we cannot further discuss about this problem at present. The temperature dependence of conductivity for the In-doped film is shown in Fig. 3. The conductivity linearly increases in the Arrhenius plots as the temperature is raised, indicating that the C60 film maintains its semiconductor phase before and after oxygen exposure. The obtained values of activation energies of metal-doped and undoped Cs0 films are listed in Table I, together with conductivities. For all films, the activation energy increases under oxygen exposure and decreases after the heat treatment. The degree of recovery after the heat treatment is identical to that of conductivity. We have already reported by field-effect measurements4 that the conduction of both metal-doped and undoped C60 films is n-type, and that in metal-doped films, impurity states are formed by the dopant at an energy level closer to the conduction band edge and supply mobile electrons. In the view of these results, it seems likely that oxygen absorbed in either undoped or metaldoped Cs0 films forms electron trap states in the band gap, which captures electrons of majority carrier and decreases the conductivity. The formation of deep levels by incorporation of oxygen, therefore, increases the activation energy. For a C60 single crystal, the carrier compensation effect due to oxygen incorporation has been proposed by Arai e t al. s It is considered that the similar mechanism can be applicable to the amorphous films in this work. Assuming that the conduction is dominated by the oxygen-related states in undoped films after 02 exposure, we can deduce from Table I that the energy level of electron trap states formed by oxygen is located at
0.72
0.51
about 0.7 eV below the conduction band edge. Desorption of oxygen due to a heat treatment, on the other hand, decreases the number of oxygen-related states; hence the conductivity and activation energy are recovered to the as-deposited values. However, it should be noted that the activation energy of as-deposited undoped film without oxygen exposure is 0.51 eV, which is bigger than that of undoped single crystal, 0.26 eV. s There seems to be a possibility that this difference is due to residual impurities and the amorphous state. For the metal-doped films, the activation energy after oxygen exposure, shown in Table I, is not simply attributed to either oxygen-related states or dopantrelated states, because both states should be responsible for the conduction phenomena. It is, therefore, difficult to estimate the energy level of oxygen-related states. The energy level, however, does not seem so different from that of undoped film. We have pointed out that the recovery of conductivity and activation energy is not complete for metaldoped, especially for In-doped C80 films. This indicates that some of oxygen atoms oI molecules remain in the films after the heat treatment, which may be attributed to oxidation of dopant metals. The irreversible effects of oxygen on the symmetry of C60 molecules and their structure in solid state have been reported. 6-9 These effects may influence the electronic properties of C60 films fabricated in this work. In our present experiments of field-effect measurements, we have obtained preliminary data that the decrease of conductivity under oxygen exposure is attributed to the reduction of mobility as well as carrier concentration. From these results, we could draw a plausible mechanism for conductivity reduction by oxygen exposure that some of oxygen atoms or molecules in C6o film distort the highly symmetric wavefunctions of Cs0 molecules and at a certain place around a C60 molecule, the overlapping of wavefunctions between molecules is reduced, which results in the decrease of mobility. 4. C o n c l u s i o n s The conductivity of metal (In or Sb)-doped and undoped C¢o films decreases by several orders of magnitude and the activation energy increases, under oxygen exposure. They tend to approach to the as-deposited values
440
METAL-DOPED AND UNDOPED C60 FILMS
by a heat treatment in vacuum, but do not completely recover in metal-doped films. The decrease of conductivity and the increase of activation energy due to the exposure are explained by the formation of deep electron trap states located at about 0.7 eV below the conduction band edge, originating from oxygen absorbed in the
Vol. 89, No. 5
C60 films. The heat treatment in vacuum results in the desorption of oxygen, but the incomplete recovery ]n metal-doped films seems to be attributed to oxidation of metal dopants. It is clearly suggested that the effect of oxygen in C60 film must be fully taken into account in the discussion of electrical properties of C~0 films.
References
1. S. Saito and A. Oshiyama, Phys. Rev. Lett. 66, 2637 (1991) 2. J. Mort, R. Ziolo, M. Maehonkin, D. R. Huffman and M. I. Ferguson, Chem. Phys. Lett. 186, 284 (1991) 3. C. Wen, J. Li, K. Kitazawa, T. Aida, I. Honma, H. Komiyama and K. Yamada, Appl. Phys. Lett. 61, 2162 (1992) 4. K. Hoshimono, S. Fujimori, Sz. Fujita and Sg. Fujita, Jpn. J. Appl. Phys. 32, L1070 (1993) 5. T. Arai, Y. Murakami, H. Suematsu, K. Kikuehi, Y. Achiba and I. Ikemoto, Solid State Communications
84, 827 (1992) 6. S. J. Duclos, R. C. ttaddon, S. H. Glarum, A. F. ttebard
and K. B. Lyons, Solid State Communications 80, 481 (1991) 7. K. M. Creegan, J. L. Robbins, W. K. Robbins, J. M. Millar, R. D. Sherwood, P. J. Tindal and D. M. Cox, J. Am. Chem. Soc. 114, 1103 (1992) 8. P. Zhou, A. M. Rao, K.-A. Wang, J. D. Robertson, C. Eloi, M. S. Meier, S. L. Ren, X.-X. Bi and P. C. Eklund, Appl. Phys. Lett. 60, 2871 (1992) 9. P. H. M. van Loosdrecht, P. J. M. van Bentum, M. A. Verheijen and G. Meijer, Chem. Phys. Lett. 198, 587 (1992).
Solid State Communications, Vol. 8% No. 5, pp. 437-440, 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098]9456.00+.00 0038-1098(93)E0128-K
V A R I A T I O N OF C O N D U C T I V I T Y A N D A C T I V A T I O N E N E R G Y IN M E T A L - D O P E D A N D U N D O P E D C6o FILMS UNDER OXYGEN EXPOSURE Shigeo Fujimori, Katsunori Hoshimono, Shizuo Fujita and Shigeo Fujita
Department of Electrical Engineering, Kyoto University, Kyoto 606-01, Japan (Received 26 August 1993 by H. Kamimura) Influence of oxygen on electronic conduction in metal (In or Sb)-doped and undoped C60 films is investigated. The conductivity decreases by several orders of magnitude and the activation energy in semiconductor-like conduction highly increases due to oxygen absorption, and vice versa due to desorption. These variations are attributed to the oxygen-related electron trap states formed at about 0.7 eV below the conduction band.
1. I n t r o d u c t i o n
(Sb) with 5N purity. Each source was evaporated from a quartz crucible. In order to measure the conductivity in vacuum successively to the deposition, we prepared the quartz substrate (15×15 mm) with interdigitated gold electrodes (50 #m spacing, 10 mm width, and about 500/~ thickness) which were made by vacuum evaporation, photolithography, and etching. The substrate was set in a vacuum chamber and the electrodes were wired to the external circuits through current terminals, as shown in Fig. 1. It was confirmed that the electrode resistance is low and the leakage current of the quartz substrate is small enough to be neglected in the measurements. Then the substrate was prebaked at 120 °C for 30 minutes to remove residual impurities on the surface, afterwards cooled to room temperature. Film deposition was carried out by simultaneously heating both C6o (about 550 °C) and dopant (either In: 850 °C or Sb: 520 °C) crucibles under 10-s Torr. The deposition rate was about 1 A/s, which was monitored by a quartz-crystal thickness meter, and the flux ratio between C60 and dopant was about 1 : 1. At present, we have not measured the quantity of dopant metal in the C6o thin films. The typical thickness of the film was about 2000 ~. The conductivity was determined from the gradient of linear region of current-voltage (I-V) characteristics at relatively low applied voltage. After the measurement in vacuum (10 -6 Torr) successively to the deposition, pure oxygen (02) gas or air was introduced into the chamber to the pressure of 0.2 or 1 atm, respectively. It takes about 5 minutes to fill the vacuum chamber with 02 from vacuum of 10-8 Torr to 0.2 atm. The variation of conductivity during the introduction of 02 was continuously monitored by measuring the current at a constant voltage in the linear I-V region. The temperature dependence of conductivity was measured by cool-
Buckminsterfullerene C60, according to the theoretical band calculation, possesses direct band gap of 1.5 eV, 1 which suggests that C60 would be available as a novel semiconductor material. However, the conductivity of Cso films ever reported 2'3 was so low (e.g., 10-s-10 T M S.cm -1) that their semiconducting properties could not be successfully utilized. Recently, we reported that the conductivity of amorphous Cs0 films was highly enhanced by metal (In or Sb) doping and the films exhibited n-type conduction,a The obtained carrier concentration and mobility values, e.g., 10TM cm -3 and 0.03 cm2/V.s, respectively, are nearly comparable to those of amorphous silicon, and this result opens potential applications of C60 films to new and unique semiconductor devices. It is, however, recognized that electronic properties of C6o are seriously influenced by oxygen. ArM et al.s showed that resistivity of undoped Cs0 single crystals became higher by 4 orders of magnitude due to oxygen . exposure at room temperature. In this paper, investigations are focused on the variation of conductivity and activation energy in metal-doped and undoped C6o amorphous films due to oxygen absorption and desorption, in order to understand the electronic properties, the conduction scheme, and the stability of these films. Discussion will be also given on energy levels of trap states formed by oxygen. 2. E x p e r i m e n t s Commercially available C60 powders with the purity higher than 99.9 % from Texas Fullerenes Co. were used as a deposition source after baking in vacuum of 10-6 Torr at 300-400 °C for 10 minutes and then at 150 °C for a few hours to remove solvent. As dopant sources, we chose elemental metals of indium (In) and antimony 437
METAL-DOPED AND UNDOPED c6O FILMS
43x
,
to pure
finallv
in about
Sb-doped
r
02 (0.2
posure
magnitude
Current Terminal
Vol. Xc), No. 5 atm)
2 hours,
values for In- or
the conductivity
film by factor reported.5
of 4, similarly
The
decrease
similar
der nitrogen conductivity small (within
of experimental
surement
conditions
were selected
after
Here, mea-
several
experi-
the effect of nitrogen
to that of oxygen.
the oxygen
exposure,
and then
the
Sb-doped
l/3
oxygen
film,
desorption.
heating,
the
because
The
desorption
the conductivity
of the as-deposited
(10e6 Torr, treatment
value,
heating
of metal-
field at about
160 “C; hence
more recovery
of conductivity.
measurements
X-ray
diffraction
any diffraction
peaks,
indicating
that
did not show these films were
Results
and
morphology
A heat
of the samples tends to with applying
electric
we could not confirm Under the re-exposure
again
by the second
The
is recovered
As-deposited
decreases
Heat treatment
similar
varia-
marized.
and undoped
undoped
under
the ex-
deposited
results
In-doped
the variations
of conductivity
for metal-doped
conductivity
for both
Discussions
2 shows the hysteresis
tion at room temperature
have
been
and undoped
of conductivity
It should
the of
decreases heat treat-
generally
Css films.
that
the conductivity
completely
recovered
value by the heat treatment,
I ’ ’
’
’
I,
350
’
of
to the as-
whereas
that of
(K )
Temperature
Heat treatment
obtained In Table
for various films are sum-
be noted
film is almost
400 _
the sample.
the conductivity
The 3.
Css films.
by
to only
ment.
amorphous.
Figure
was promoted
0s after the heat treatment, and then
to
due to
may desorb more oxy-
on the flat surfaces
scopes.
probably
was recovered
at higher temperature
be rough during the measurement
micro-
at
value only by the evacuation
1 hour) without
gen, but the surface
electron
chamber
were heated
conductivity
of the as-deposited
No metal
and scanning
the vacuum
recovers
the
of films or oxygen desorption
doped Cm films by optical
to
For
can be eliminated.
were observed
it is con-
is attributed
samples
ments so that degradation particles
is negli-
Therefore,
of conductivity
during
the measurements
Un-
treatment).
l/100 ing down from 120 “C to room temperature.
has been ob-
to air (1 atm).
120 “C for 1 or 2 hours in 1O-6 Torr (heat about setup.
ever
absorption.
After
illustration
undoped
to the single crystals
of conductivity
10 %) that
was evacuated
Fig. 1 Schematic
exposure
of the amorphous
when exposed
cluded that the decrease
Vacuum Chamber
The oxygen
exposure (0.8 atm), on the other hand, the slightly decreases, but the decrease is so
gible compared
Metal
and
lower than the as-deposited Css films, respectively.
served for all samples
c60
the deposition,
7 or 3 orders *in
also reduces
oxygen
after
it becomes
1 ’
1
1
300
IT
10-k-+-+
10-‘4~W 0
50 Oxygen
100 150 exposure
0 time
50
100
( min. )
Fig. 2 Hysteresis of conductivity variation under oxygen exposure, heat treatment, re-exposure, and second heat treatment.
2.6
2.8
3
1000 / Temperature
3.2
( K-’ )
Fig. 3 Examples of temperature dependence of conductivity. This figure is for In-doped Cm film, but similar results have been obtained for Sb-doped and undoped films.
Vol. 89, No. 5
METAL-DOPED AND UNDOPED C60 FILMS
439
Table I. Conductivity a and activation energy E, of metal-doped and undoped C6o films. Dopant material In
Sb Undoped
As-deposited l x l 0 -2
After oxygen exposure 2x10 -9
After heat treatment 7x10 -s
E~ (eV) a (S.cm-1)
0.10 5x10 -4
0.68 6x10 -7
0.25 2x10 -4
Ea (eV) a (S.cm-1)
0.18 3x10 -1°
0.54 l x l 0 -13
0.25 3x10 -1°
a
E~
(S.cm-1)
(eV)
0.51
In-doped film after the heat treatment is as low as 1/100 of the as-deposited value. These differences will be discussed later together with the variation of activation energy. For the In-doped and undoped films, the conductivity rapidly decreases immediately after the first exposure to oxygen. The conductivity of Sb-doped films, on the other hand, increases slightly for a few minutes after the exposure (oxygen pressure is less than about 0.02 atm), and then slowly decreases. Compared to the first exposure, this phenomenon was not observed under the re-exposure after the heat treatment. This seems to indicate that the surface of Sb-doped as-deposited C60 film is somewhat unstable, but we cannot further discuss about this problem at present. The temperature dependence of conductivity for the In-doped film is shown in Fig. 3. The conductivity linearly increases in the Arrhenius plots as the temperature is raised, indicating that the C60 film maintains its semiconductor phase before and after oxygen exposure. The obtained values of activation energies of metal-doped and undoped Cs0 films are listed in Table I, together with conductivities. For all films, the activation energy increases under oxygen exposure and decreases after the heat treatment. The degree of recovery after the heat treatment is identical to that of conductivity. We have already reported by field-effect measurements4 that the conduction of both metal-doped and undoped C60 films is n-type, and that in metal-doped films, impurity states are formed by the dopant at an energy level closer to the conduction band edge and supply mobile electrons. In the view of these results, it seems likely that oxygen absorbed in either undoped or metaldoped Cs0 films forms electron trap states in the band gap, which captures electrons of majority carrier and decreases the conductivity. The formation of deep levels by incorporation of oxygen, therefore, increases the activation energy. For a C60 single crystal, the carrier compensation effect due to oxygen incorporation has been proposed by Arai e t al. s It is considered that the similar mechanism can be applicable to the amorphous films in this work. Assuming that the conduction is dominated by the oxygen-related states in undoped films after 02 exposure, we can deduce from Table I that the energy level of electron trap states formed by oxygen is located at
0.72
0.51
about 0.7 eV below the conduction band edge. Desorption of oxygen due to a heat treatment, on the other hand, decreases the number of oxygen-related states; hence the conductivity and activation energy are recovered to the as-deposited values. However, it should be noted that the activation energy of as-deposited undoped film without oxygen exposure is 0.51 eV, which is bigger than that of undoped single crystal, 0.26 eV. s There seems to be a possibility that this difference is due to residual impurities and the amorphous state. For the metal-doped films, the activation energy after oxygen exposure, shown in Table I, is not simply attributed to either oxygen-related states or dopantrelated states, because both states should be responsible for the conduction phenomena. It is, therefore, difficult to estimate the energy level of oxygen-related states. The energy level, however, does not seem so different from that of undoped film. We have pointed out that the recovery of conductivity and activation energy is not complete for metaldoped, especially for In-doped C80 films. This indicates that some of oxygen atoms oI molecules remain in the films after the heat treatment, which may be attributed to oxidation of dopant metals. The irreversible effects of oxygen on the symmetry of C60 molecules and their structure in solid state have been reported. 6-9 These effects may influence the electronic properties of C60 films fabricated in this work. In our present experiments of field-effect measurements, we have obtained preliminary data that the decrease of conductivity under oxygen exposure is attributed to the reduction of mobility as well as carrier concentration. From these results, we could draw a plausible mechanism for conductivity reduction by oxygen exposure that some of oxygen atoms or molecules in C6o film distort the highly symmetric wavefunctions of Cs0 molecules and at a certain place around a C60 molecule, the overlapping of wavefunctions between molecules is reduced, which results in the decrease of mobility. 4. C o n c l u s i o n s The conductivity of metal (In or Sb)-doped and undoped C¢o films decreases by several orders of magnitude and the activation energy increases, under oxygen exposure. They tend to approach to the as-deposited values
440
METAL-DOPED AND UNDOPED C60 FILMS
by a heat treatment in vacuum, but do not completely recover in metal-doped films. The decrease of conductivity and the increase of activation energy due to the exposure are explained by the formation of deep electron trap states located at about 0.7 eV below the conduction band edge, originating from oxygen absorbed in the
Vol. 89, No. 5
C60 films. The heat treatment in vacuum results in the desorption of oxygen, but the incomplete recovery ]n metal-doped films seems to be attributed to oxidation of metal dopants. It is clearly suggested that the effect of oxygen in C60 film must be fully taken into account in the discussion of electrical properties of C~0 films.
References
1. S. Saito and A. Oshiyama, Phys. Rev. Lett. 66, 2637 (1991) 2. J. Mort, R. Ziolo, M. Maehonkin, D. R. Huffman and M. I. Ferguson, Chem. Phys. Lett. 186, 284 (1991) 3. C. Wen, J. Li, K. Kitazawa, T. Aida, I. Honma, H. Komiyama and K. Yamada, Appl. Phys. Lett. 61, 2162 (1992) 4. K. Hoshimono, S. Fujimori, Sz. Fujita and Sg. Fujita, Jpn. J. Appl. Phys. 32, L1070 (1993) 5. T. Arai, Y. Murakami, H. Suematsu, K. Kikuehi, Y. Achiba and I. Ikemoto, Solid State Communications
84, 827 (1992) 6. S. J. Duclos, R. C. ttaddon, S. H. Glarum, A. F. ttebard
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