- Email: [email protected]
Studies of electron structure of C60 thin films by surface photovoltage spectroscopy
Solid State Communications, Vol. 102, No. 6, pp. 489-492, 1997 0 1997 Published by Elsevier Science Ud Printed in Great Britain. All rights reserved 0038-1098197 $17.00+.00
Pergamon
PII: SOO38-1098(97)00026-4
STUDIES
OF ELECTRON
STRUCTURE
OF &,, THIN FILMS BY SURFACE SPECTROSCOPY
PHOTOVOLTAGE
B. Mishori,” E.A. Katz,b D. Faimanb’” and Yoram Shapira” ‘Department
of Electrical
Engineering
- Physical Electronics, Faculty of Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel bThe National Solar Energy Center, The Jacob Blaustein Institute for Desert Research, Sede Boqer, 84990 Israel ‘Department of Physics, The Ben-Gurion University of the Negev, Beersheba, 84105 Israel (Received and accepted 21 November
1996 by S. Alexander)
Polycrystalline Cm films have been studied before and after annealing using surface photovoltage spectroscopy. The results indicate the possibility of the existence of band tails, extending into the optical gap of these films, as well as other deep gap states. Annealing at moderate temperatures is shown to reduce the density of these states. A possible energy band scheme is proposed. 0 1997 Published by Elsevier Science Ltd Keywords: A. fullerenes, D. electronic states (localized), D. photoconductivity and photovoltaics.
1. INTRODUCTION The optical and electrical properties of C6a thin films [l-7] have recently attracted substantial interest, yet their electronic structure is still a matter of debate. For example, the bandgap energy is attributed a wide range of values by the various authors who have studied it. Saito and Oshiyama [B] using a pseudopotential local density formalism derived a theoretical value of Eg = 1.5 eV for f.c.c. Ceo crystals. Other calculations using a quasiparticle approach [9] find E, = 2.15 eV. Skumanich [l] studied the optical absorption spectrum of C& films and obtained a value of 1.6 eV for the optical bandgap. By using photoemission measurements, other groups obtained values of 2.1-2.2 eV [2] and 2.3 eV [3]. The photoconductivity edge is observed at 1.6-1.8 eV [4, 51. According to one of the current models [3] the band-gap energy has a value of 2.3 V, but light absorption in the region 1.6-2.3 eV is due to Frenkel exciton formation. However, light absorption [l], photoconductivity spectra [6] and total yield electron spectroscopy [7] of crystalline Cm have indicated the presence of exponential Urbach tails at the band edges (similar to those in amorphous silicon [lo]). Yet, another model [6, 111 suggested the evidence of a mobility edge at (2.3 eV) and exponential tails of states determining the photoconductivity edge at 1.6-1.8 eV [5].
We have used surface photovoltage spectroscopy, a well-known method for studying the electronic structure of semiconductor materials [ 12, 131 to shed more light on this issue. The surface photovoltage (SPV) spectrum contains information about the optical and electronic properties of semiconductor surfaces and interfaces, such as the bandgap energy and characteristics of gap states. The energy of the latter may be determined from the positions of slope changes in the SPV spectrum caused by photon absorption in the sub-bandgap region due to population or depopulation of localized states. SPV formation requires both photogeneration of charge carriers and their separation. In this paper we present a study of the electronic structure of Cbo thin films using SPV spectroscopy. The results seem to confirm the existence of tails of states close to the band edges and suggest, moreover, the existence of two deep gap states - one donor and one acceptor.
489
2. EXPERIMENTAL Several CM thin films were evaporated, using Cm (99.98%) powder (Hoechst AG), on optical glass substrates >predeposited with Ag electrodes. The base pressure of the evaporation chamber was 7 X lo-’ Torr. The deposition rate was about 0.2 nm s-’ and the thickness of the films
490
ELECTRON
STRUCTURE
nm. X-ray diffraction studies revealed that the structure of films is polycrystalline f.c.c. [14]. As-grown samples were kept for several days at ambient conditions before the SPV measurements and thermal annealing. The latter was performed at 200°C for 30 min in a III-IV system with a base pressure of 10m8 Torr. The SPV measurements were carried out in air at room temperature before and after thermal annealing of the samples using a commercial Kelvin probe (Besocke Delta Phi, Jiilich, Germany) with a sensitivity of -1 mV. Carrier generation was provided by light source and 0.25 m single monochromator (Oriel). The SPV signal was measured in the photon energy range of 0.62-3.1 eV. Light intensity was adjusted using neutral density filters.
OF C6a THIN FILMS
Vol. 102, No. 6
was varied in the range of 200-400
3. RESULTS
AND DISCUSSION
The data presented below are for a representative sample with a film thickness of about 300 nm though qualitatively similar results were obtained for the other films grown. Figure 1 illustrates the SPV spectra obtained at several light intensities. The negative sign of the SPV indicates the n-type of conductivity of the ChOfilms. To start with we will concentrate on the spectra obtained at high light intensities. It is possible to distinguish three particular regions in all these spectra. Region A starts with an increase of the SPV at about 1.0-1.1 eV. Region A contains also a slope change at 1.3 eV. The sharp increase of the SPV signal at 1.65 eV (for the maximum light intensity) is designated as the beginning of region B that is characterized by a “knee” followed by some fine structure. The clearly defined increasingly negative signal at 2.25-2.3 eV determines region C. Based on the published values of the mobility gap of about 2.3 eV [3, 51 we can attribute our observed SPV signal of region C to photo-induced transition from the valence band to the conduction band. Region A of Fig. 1 indicates the gap states contribution that will be discussed later. The most interesting part of these spectra is region B. In order to understand the origin of the SPV signal in region B and to obtain more information about the electronic structure of C60, a light-intensity-resolved experiment has been conducted. The results of these measurements depicted in Fig. 1 show that with decreasing light intensity, region B progressively shifts to higher photon energies. At the same time, the signal in region A as well as the structure in region B disappear, while no changes are observed regarding region C. As was noted earlier, light absorption in the energy range of 1.62.3 eV can be attributed to Frenkel exciton formation [3]. This is presumably the “knee” we observe below 2.3 eV and the associated fine structure which extends to
-10 t -20 c
F g > a
A
-30 -40 -50
/
-6Oi_
tc
s
I
1.o
0.5
1.5
I
I
2.0
2.5
.
/
,I
3.0
Energy WI
Fig. 1. The SPV spectra of Cm film before vacuum annealing at different illumination intensities (illumination intensities are indicated). lower energies at higher light intensities. However, in addition to Frenkel excitons our SPV spectra exhibit strong evidence for the existence of band tails. This is seen as a slope change between regions A and B which shifts from 1.6 eV to 1.7 eV as light intensity decreases. In other words, as the photon flux decreases, the transitions can only occur where the density of states in the tails is higher, i.e. at higher energies or closer to the band edges. The SPV spectra of an annealed CeO sample are presented in Fig. 2. The SPV curves are qualitatively similar to those of unannealed samples, i.e. all the regions described above are present. The decrease of the signal magnitude in region A and its increase in region C (as compared with the unannealed sample) may be caused by annealing-out of gap states that in turn increases the life-time charge carriers. We believe that this behavior originates from the improvement in film crystallinity due to the annealing process [15, 161. 0
.-1.6pW
_.._.._.._. ,6 @!#J -.--.--40 PW ------8'3N -80
10
0.5
.
4
1.0
.
I
I
1.5
2.0
.
1
2.5
1
30
Energy (eV)
Fig. 2. The SPV spectra of Cm film after vacuum annealing at different illumination intensities (illumination intensities are indicated).
ELECTRON
Vol. 102, No. 6
STRUCTURE
E. (ev) I/-
1
/
. G 4
2 i
1.05
8 -_-Q&.---__--3
L
‘G 0”
Acceptor-like stale
/ Fermi level \ ______----/
@a 2
Conduction band bollom
7
Donor-like state
1.3 Valence band lop Valence mobility edge
Fig. 3. Suggested band diagram for Cm thin films. Taking into account all our results a detailed electronic structure of Cm thin films may be suggested. The structure is illustrated in Fig. 3. A mobility gap of about 2.3 eV can be determined from the behavior of the SPV signal in region C. The signal in region B may be due to the contribution of tail-to-tail electron excitation, the distance between the valence band tail and the conduction band tail being 1.6 eV. This value also may be defined as the optical gap value [l]. In region A the signs of the slope changes may be interpreted as orogonating from the electron transitions to the bottom of the conduction band tail (E,) from a gap state at E, - (1.0-1.1) eV: and the electron excitation from the top of the valence band tail (Ey) to a gap state at E, + 1.3 eV. We note that the existence of such states are consistent with the Fermi level for CGOlying close to the midgap as required by the low intrinsic conductivity of the material. As noted earlier, the SPV signal requires also separation of the charge carriers. Therefore, the photoconductivity at photon energies in the range of 1.652.3 eV may be realized through a hopping mechanism of transport in the band tails [16], while electrons, that are photo-excited at energies higher than 2.3 eV, move by the conventional band-to-band activated transport. Solids with a significant degree of disorder develop tails of localized electronic states that extend into the gap from the conduction and/or valence bands [lo]. We observed the same SPV dependence of light intensity in the region B for the annealed and the unannealed sample. Hence, annealing, while probably improving crystallinity, does not change the density of states in band tails. Consequently, it is possible to suggest that not only structural defects are responsible for the disorder and the existence of the tails in our samples. This is consistent with reported results obtained by other methods. For example, the results of total yield spectroscopy [7]
OF &,, THIN FILMS
491
indicate that the density of gap states in crystalline &a films is very high (even higher than that in a a-Si : H). It is also important to note that measurements of the absorption coefficient of &,, single crystals [17] reveal band tails above 1.5 eV. Measurements of photoconductivity spectra of Cm films before and after exposure to oxygen [18] have shown that oxygen dramatically reduces the value of the photocurrent at all photon energies. However, both oxygen-free and oxygenated films exhibit similar features above 1.65 eV, ruling out oxygen as the main origin of the band tails. According to [5], exponential tails in the density of gap states for Cm originate from thermal disorder (related to dynamics of Cm molecular motion) rather than from static structural, topological or compositional disorder. It was also been suggested [19] that the Urbach tail observed in the optical absorption spectrum of semiconductor may originate from excitons transitions in the presence of electrical microfields. Further experiments are needed for detailed understanding of this question. The SPS measurements of CM films with different degree of crystallinity (reflecting a variation of static disorder) as well as similar measurements of crystalline films at different temperatures (reflecting the variation of dynamic disorder) are in progress. In summary, we have measured surface photovoltage spectra of polycrystalline Cm thin films exposed to air. The existence of band-tails was inferred from an observed energy shift of the SPV signal as a function of light intensity. The SPS results are discussed on the basis of a model of the electronic structure of C& thin film which includes a mobility gap of 2.2-2.3 eV, a photoconduction gap of about 1.65 eV and two gap states at 1.0-1.1 eV below conduction band tail and 1.3 eV above valence band tail. Reduction in the density of deep gap state as a result of annealing has been also reported. Acknowledgements-This work was funded in part by the Israel Ministry of Energy and Infrastructure. We would like to thank Prof. I. Balberg for useful discussions. REFERENCES 1. 2.
3.
4. 5.
Skumanich, A., Chem. Phys. Lett., 182, 1991, 486. Takahashi, T., Suzuki, S., Morikawa, T., KatayamYoshida, H., Hasegawa, S., Inokuchi, H., Seki, K., Kikuchi, K., Suzuki, S., Ikemoto, I. and Achiba, Y., Phys. Rev. Lett., 68, 1992, 1232. Lof, R.W., van Veenendaal, M.A., Koopmans, B., .Ionkman, H.T. and Sawatzky, G.A., Phys. Rev. Lett., 68, 1992, 3924. Kazaoui, S., Ross, R. and Minami, N., Solid State Commun., 90, 1994, 623. Hamed, A., in Organic Conductive Molecules and
492
6.
7.
8. 9. 10.
11.
ELECTRON
STRUCTURE
Polymers (Edited by H.S. Nalwa), New York (to be published). Kaiser, M., Maser, WK., Byrne, H.J., Reichenbach, J., Anders, J., Mittelbach, A. and Roth, S., in Electronic Properties of Fullerenes (Edited by H. Kuzmany, J. Fink, M. Mehring and S. Roth), Vol. 117, p. 418. Springer-Verlag, Berlin-Heidelberg, 1993. M., Arakane, T. and Gonda, S., Kawasaki, Koinuma, H., in Novel Forms of Carbon II (Edited by C.L. Renschler, D.M. Cox, J.J. Pouch and Y. Achiba), Vol. 349, p. 325. Materials Research Society, Pittsburgh, 1994. Saito, S. and Oshiyama, A., Phys. Rev. Lett., 66, 1991, 2637. Shirley, W.L. and Louie, S.G., Phys. Rev. Let& 71, 1993, 133. Semiconductors and Semimetals (Edited by J.J. Pankove), Vol. 21B. Academic Press, New York, 1984. Mishori, B., Shapira, Yoram, Belu-Marian, A.,
OF Cm THIN FILMS
12. 13. 14.
15. 16. 17.
18. 19.
Vol. 102, No. 6
Manciu, M. and Devenyi, A., Chem. Phys. Lett. (to be published). Balestra, C.L., Lagowski, J. and Gatos, H.C., Surf: Sci., 26, 1971, 317. Leibovitch, L., Kronik, L., Fefer, E. and Shapira, Y., Phys. Rev., BSO, 1994,1739. Faiman, D., Goren, S., Katz, E.A., Koltun, M., Melnik, N., Shames, A. and Shtutina, S., Thin Solid Films (in press). Rusakova, I., Hamed, A. and Hor, P.H., Mat. Res. Sot. Symp. Proc., 349, 1994, 102. Mott, N.F. and Davis, E.A., Electronic Processes in Non-Crystalline Materials Clarendon, Oxford, 1979. Wen, C., Aida, T., Homna, I., Komiyama, H. and Yamada, K., J. Phys. Condens. Matter, 6, 1994, 1603. Kaiser, M., Maser, WK., Byrne, H.J., Mittelbach, A. and Roth, S., Solid State Commun., 87, 1993, 281. Dow, J.D. and Redfield, D., Phys. Rev., B5, 1972, 594.
Pergamon
PII: SOO38-1098(97)00026-4
STUDIES
OF ELECTRON
STRUCTURE
OF &,, THIN FILMS BY SURFACE SPECTROSCOPY
PHOTOVOLTAGE
B. Mishori,” E.A. Katz,b D. Faimanb’” and Yoram Shapira” ‘Department
of Electrical
Engineering
- Physical Electronics, Faculty of Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel bThe National Solar Energy Center, The Jacob Blaustein Institute for Desert Research, Sede Boqer, 84990 Israel ‘Department of Physics, The Ben-Gurion University of the Negev, Beersheba, 84105 Israel (Received and accepted 21 November
1996 by S. Alexander)
Polycrystalline Cm films have been studied before and after annealing using surface photovoltage spectroscopy. The results indicate the possibility of the existence of band tails, extending into the optical gap of these films, as well as other deep gap states. Annealing at moderate temperatures is shown to reduce the density of these states. A possible energy band scheme is proposed. 0 1997 Published by Elsevier Science Ltd Keywords: A. fullerenes, D. electronic states (localized), D. photoconductivity and photovoltaics.
1. INTRODUCTION The optical and electrical properties of C6a thin films [l-7] have recently attracted substantial interest, yet their electronic structure is still a matter of debate. For example, the bandgap energy is attributed a wide range of values by the various authors who have studied it. Saito and Oshiyama [B] using a pseudopotential local density formalism derived a theoretical value of Eg = 1.5 eV for f.c.c. Ceo crystals. Other calculations using a quasiparticle approach [9] find E, = 2.15 eV. Skumanich [l] studied the optical absorption spectrum of C& films and obtained a value of 1.6 eV for the optical bandgap. By using photoemission measurements, other groups obtained values of 2.1-2.2 eV [2] and 2.3 eV [3]. The photoconductivity edge is observed at 1.6-1.8 eV [4, 51. According to one of the current models [3] the band-gap energy has a value of 2.3 V, but light absorption in the region 1.6-2.3 eV is due to Frenkel exciton formation. However, light absorption [l], photoconductivity spectra [6] and total yield electron spectroscopy [7] of crystalline Cm have indicated the presence of exponential Urbach tails at the band edges (similar to those in amorphous silicon [lo]). Yet, another model [6, 111 suggested the evidence of a mobility edge at (2.3 eV) and exponential tails of states determining the photoconductivity edge at 1.6-1.8 eV [5].
We have used surface photovoltage spectroscopy, a well-known method for studying the electronic structure of semiconductor materials [ 12, 131 to shed more light on this issue. The surface photovoltage (SPV) spectrum contains information about the optical and electronic properties of semiconductor surfaces and interfaces, such as the bandgap energy and characteristics of gap states. The energy of the latter may be determined from the positions of slope changes in the SPV spectrum caused by photon absorption in the sub-bandgap region due to population or depopulation of localized states. SPV formation requires both photogeneration of charge carriers and their separation. In this paper we present a study of the electronic structure of Cbo thin films using SPV spectroscopy. The results seem to confirm the existence of tails of states close to the band edges and suggest, moreover, the existence of two deep gap states - one donor and one acceptor.
489
2. EXPERIMENTAL Several CM thin films were evaporated, using Cm (99.98%) powder (Hoechst AG), on optical glass substrates >predeposited with Ag electrodes. The base pressure of the evaporation chamber was 7 X lo-’ Torr. The deposition rate was about 0.2 nm s-’ and the thickness of the films
490
ELECTRON
STRUCTURE
nm. X-ray diffraction studies revealed that the structure of films is polycrystalline f.c.c. [14]. As-grown samples were kept for several days at ambient conditions before the SPV measurements and thermal annealing. The latter was performed at 200°C for 30 min in a III-IV system with a base pressure of 10m8 Torr. The SPV measurements were carried out in air at room temperature before and after thermal annealing of the samples using a commercial Kelvin probe (Besocke Delta Phi, Jiilich, Germany) with a sensitivity of -1 mV. Carrier generation was provided by light source and 0.25 m single monochromator (Oriel). The SPV signal was measured in the photon energy range of 0.62-3.1 eV. Light intensity was adjusted using neutral density filters.
OF C6a THIN FILMS
Vol. 102, No. 6
was varied in the range of 200-400
3. RESULTS
AND DISCUSSION
The data presented below are for a representative sample with a film thickness of about 300 nm though qualitatively similar results were obtained for the other films grown. Figure 1 illustrates the SPV spectra obtained at several light intensities. The negative sign of the SPV indicates the n-type of conductivity of the ChOfilms. To start with we will concentrate on the spectra obtained at high light intensities. It is possible to distinguish three particular regions in all these spectra. Region A starts with an increase of the SPV at about 1.0-1.1 eV. Region A contains also a slope change at 1.3 eV. The sharp increase of the SPV signal at 1.65 eV (for the maximum light intensity) is designated as the beginning of region B that is characterized by a “knee” followed by some fine structure. The clearly defined increasingly negative signal at 2.25-2.3 eV determines region C. Based on the published values of the mobility gap of about 2.3 eV [3, 51 we can attribute our observed SPV signal of region C to photo-induced transition from the valence band to the conduction band. Region A of Fig. 1 indicates the gap states contribution that will be discussed later. The most interesting part of these spectra is region B. In order to understand the origin of the SPV signal in region B and to obtain more information about the electronic structure of C60, a light-intensity-resolved experiment has been conducted. The results of these measurements depicted in Fig. 1 show that with decreasing light intensity, region B progressively shifts to higher photon energies. At the same time, the signal in region A as well as the structure in region B disappear, while no changes are observed regarding region C. As was noted earlier, light absorption in the energy range of 1.62.3 eV can be attributed to Frenkel exciton formation [3]. This is presumably the “knee” we observe below 2.3 eV and the associated fine structure which extends to
-10 t -20 c
F g > a
A
-30 -40 -50
/
-6Oi_
tc
s
I
1.o
0.5
1.5
I
I
2.0
2.5
.
/
,I
3.0
Energy WI
Fig. 1. The SPV spectra of Cm film before vacuum annealing at different illumination intensities (illumination intensities are indicated). lower energies at higher light intensities. However, in addition to Frenkel excitons our SPV spectra exhibit strong evidence for the existence of band tails. This is seen as a slope change between regions A and B which shifts from 1.6 eV to 1.7 eV as light intensity decreases. In other words, as the photon flux decreases, the transitions can only occur where the density of states in the tails is higher, i.e. at higher energies or closer to the band edges. The SPV spectra of an annealed CeO sample are presented in Fig. 2. The SPV curves are qualitatively similar to those of unannealed samples, i.e. all the regions described above are present. The decrease of the signal magnitude in region A and its increase in region C (as compared with the unannealed sample) may be caused by annealing-out of gap states that in turn increases the life-time charge carriers. We believe that this behavior originates from the improvement in film crystallinity due to the annealing process [15, 161. 0
.-1.6pW
_.._.._.._. ,6 @!#J -.--.--40 PW ------8'3N -80
10
0.5
.
4
1.0
.
I
I
1.5
2.0
.
1
2.5
1
30
Energy (eV)
Fig. 2. The SPV spectra of Cm film after vacuum annealing at different illumination intensities (illumination intensities are indicated).
ELECTRON
Vol. 102, No. 6
STRUCTURE
E. (ev) I/-
1
/
. G 4
2 i
1.05
8 -_-Q&.---__--3
L
‘G 0”
Acceptor-like stale
/ Fermi level \ ______----/
@a 2
Conduction band bollom
7
Donor-like state
1.3 Valence band lop Valence mobility edge
Fig. 3. Suggested band diagram for Cm thin films. Taking into account all our results a detailed electronic structure of Cm thin films may be suggested. The structure is illustrated in Fig. 3. A mobility gap of about 2.3 eV can be determined from the behavior of the SPV signal in region C. The signal in region B may be due to the contribution of tail-to-tail electron excitation, the distance between the valence band tail and the conduction band tail being 1.6 eV. This value also may be defined as the optical gap value [l]. In region A the signs of the slope changes may be interpreted as orogonating from the electron transitions to the bottom of the conduction band tail (E,) from a gap state at E, - (1.0-1.1) eV: and the electron excitation from the top of the valence band tail (Ey) to a gap state at E, + 1.3 eV. We note that the existence of such states are consistent with the Fermi level for CGOlying close to the midgap as required by the low intrinsic conductivity of the material. As noted earlier, the SPV signal requires also separation of the charge carriers. Therefore, the photoconductivity at photon energies in the range of 1.652.3 eV may be realized through a hopping mechanism of transport in the band tails [16], while electrons, that are photo-excited at energies higher than 2.3 eV, move by the conventional band-to-band activated transport. Solids with a significant degree of disorder develop tails of localized electronic states that extend into the gap from the conduction and/or valence bands [lo]. We observed the same SPV dependence of light intensity in the region B for the annealed and the unannealed sample. Hence, annealing, while probably improving crystallinity, does not change the density of states in band tails. Consequently, it is possible to suggest that not only structural defects are responsible for the disorder and the existence of the tails in our samples. This is consistent with reported results obtained by other methods. For example, the results of total yield spectroscopy [7]
OF &,, THIN FILMS
491
indicate that the density of gap states in crystalline &a films is very high (even higher than that in a a-Si : H). It is also important to note that measurements of the absorption coefficient of &,, single crystals [17] reveal band tails above 1.5 eV. Measurements of photoconductivity spectra of Cm films before and after exposure to oxygen [18] have shown that oxygen dramatically reduces the value of the photocurrent at all photon energies. However, both oxygen-free and oxygenated films exhibit similar features above 1.65 eV, ruling out oxygen as the main origin of the band tails. According to [5], exponential tails in the density of gap states for Cm originate from thermal disorder (related to dynamics of Cm molecular motion) rather than from static structural, topological or compositional disorder. It was also been suggested [19] that the Urbach tail observed in the optical absorption spectrum of semiconductor may originate from excitons transitions in the presence of electrical microfields. Further experiments are needed for detailed understanding of this question. The SPS measurements of CM films with different degree of crystallinity (reflecting a variation of static disorder) as well as similar measurements of crystalline films at different temperatures (reflecting the variation of dynamic disorder) are in progress. In summary, we have measured surface photovoltage spectra of polycrystalline Cm thin films exposed to air. The existence of band-tails was inferred from an observed energy shift of the SPV signal as a function of light intensity. The SPS results are discussed on the basis of a model of the electronic structure of C& thin film which includes a mobility gap of 2.2-2.3 eV, a photoconduction gap of about 1.65 eV and two gap states at 1.0-1.1 eV below conduction band tail and 1.3 eV above valence band tail. Reduction in the density of deep gap state as a result of annealing has been also reported. Acknowledgements-This work was funded in part by the Israel Ministry of Energy and Infrastructure. We would like to thank Prof. I. Balberg for useful discussions. REFERENCES 1. 2.
3.
4. 5.
Skumanich, A., Chem. Phys. Lett., 182, 1991, 486. Takahashi, T., Suzuki, S., Morikawa, T., KatayamYoshida, H., Hasegawa, S., Inokuchi, H., Seki, K., Kikuchi, K., Suzuki, S., Ikemoto, I. and Achiba, Y., Phys. Rev. Lett., 68, 1992, 1232. Lof, R.W., van Veenendaal, M.A., Koopmans, B., .Ionkman, H.T. and Sawatzky, G.A., Phys. Rev. Lett., 68, 1992, 3924. Kazaoui, S., Ross, R. and Minami, N., Solid State Commun., 90, 1994, 623. Hamed, A., in Organic Conductive Molecules and
492
6.
7.
8. 9. 10.
11.
ELECTRON
STRUCTURE
Polymers (Edited by H.S. Nalwa), New York (to be published). Kaiser, M., Maser, WK., Byrne, H.J., Reichenbach, J., Anders, J., Mittelbach, A. and Roth, S., in Electronic Properties of Fullerenes (Edited by H. Kuzmany, J. Fink, M. Mehring and S. Roth), Vol. 117, p. 418. Springer-Verlag, Berlin-Heidelberg, 1993. M., Arakane, T. and Gonda, S., Kawasaki, Koinuma, H., in Novel Forms of Carbon II (Edited by C.L. Renschler, D.M. Cox, J.J. Pouch and Y. Achiba), Vol. 349, p. 325. Materials Research Society, Pittsburgh, 1994. Saito, S. and Oshiyama, A., Phys. Rev. Lett., 66, 1991, 2637. Shirley, W.L. and Louie, S.G., Phys. Rev. Let& 71, 1993, 133. Semiconductors and Semimetals (Edited by J.J. Pankove), Vol. 21B. Academic Press, New York, 1984. Mishori, B., Shapira, Yoram, Belu-Marian, A.,
OF Cm THIN FILMS
12. 13. 14.
15. 16. 17.
18. 19.
Vol. 102, No. 6
Manciu, M. and Devenyi, A., Chem. Phys. Lett. (to be published). Balestra, C.L., Lagowski, J. and Gatos, H.C., Surf: Sci., 26, 1971, 317. Leibovitch, L., Kronik, L., Fefer, E. and Shapira, Y., Phys. Rev., BSO, 1994,1739. Faiman, D., Goren, S., Katz, E.A., Koltun, M., Melnik, N., Shames, A. and Shtutina, S., Thin Solid Films (in press). Rusakova, I., Hamed, A. and Hor, P.H., Mat. Res. Sot. Symp. Proc., 349, 1994, 102. Mott, N.F. and Davis, E.A., Electronic Processes in Non-Crystalline Materials Clarendon, Oxford, 1979. Wen, C., Aida, T., Homna, I., Komiyama, H. and Yamada, K., J. Phys. Condens. Matter, 6, 1994, 1603. Kaiser, M., Maser, WK., Byrne, H.J., Mittelbach, A. and Roth, S., Solid State Commun., 87, 1993, 281. Dow, J.D. and Redfield, D., Phys. Rev., B5, 1972, 594.