- Email: [email protected]
Ion beam modification of transparent conducting indium-tin-oxide thin films
Nuclear Instruments and Methods in Physics Research B 12 1 ( 1997) 22 l-225
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Beam InteractIons Materials&Atoms
with
!!fJ
ELSEMER
Ion beam modification of transparent conducting indium-tin-oxide thin films T.E. Haynes av*, Y. Shigesato b, I. Yasui b, N. Taga ‘, H. Odaka ’ a Solid Stute Division, Oak Ridge National Laboratory, P.O.Box 2008, Building 3003, MS-6048, Oak Ridge, TN 37831.6048. b Institute of Industrial Science, University of Tokyo, Minato-ku, Tokyo 106, Japan ’ Asahi Glass Co., Ltd., Kanagawa-ku. Yokohamu 221. Japun
USA
Abstract We have examined the effects of ion implantation of various chemical species on the electrical properties of transparent, conducting indium-tin-oxide (ITO) polycrystalline films with resistivities less than 200 ~0 cm and optical transmission greater than 90%. We report on implantations of Nf, O’, Ff, Ne+ and In+ under a variety of conditions. At low to moderate doses, damage effects dominate and reduce the conductivity slightly before saturating at doses of - 1014/cm2. At higher doses, when the implanted concentration becomes comparable to the free-carrier concentration ( - lO”/cm’), some species (e.g., In’> can improve the conduction slightly, while other species (e.g., O+) can reduce the conduction, in some cases by several orders of magnitude. We also describe preliminary results of some new experiments begun with single-crystal IT0 films to permit better characterization of the damage effects.
1. Introduction Indium-tin-oxide (ITO) has the best combination of any known material of both electrical conductivity and optical transmission in the visible range to meet the requirements for a number of applications for transparent conducting films [ 11. Essentially, this material consists of In,O, doped with several atomic percent of Sn, which substitutes for In in the crystal lattice. In most applications, thin films of the order of 100-200 nm are deposited by sputtering. Various substrates are employed, including typically silicate glasses, as in the fabrication of flat-panel displays, or silicon, in either amorphous or polycrystalline form, as in solar cells. Films may be deposited at slightly elevated temperatures, say lOO-200°C. in which case they are immediately polycrystalline, or alternatively amorphous layers may be deposited at room temperature and then postannealed to induce crystallization. In either case, the final product is a polycrystalline layer, with a resistivity normally in the range of 100-200 pLn cm, and optical transmission exceeding 90% throughout the visible spectrum. IT0 can be classified as a highly degenerate semiconductor, n-type with n - 102’/cm3, and a band gap of - 3.7 eV. It is generally accepted that the source of the conduction electrons includes both substitutional Sn atoms and also oxygen vacancies [2]. Quite remarkably, IT0 films may be
* Comesponding [email protected].
author.
Fax:
+ l-423-576-6720;
email:
deposited under a variety of conditions, often by different methods, have different microstructures (characterized by grain size, texturing, etc.), and yet have nearly identical electrical properties. Recently, researchers have produced single-crystal, epitaxial IT0 films that also have very similar conductivity to the polycrystalline form [3,4]. It is highly desirable to further improve the conductivity of IT0 films, because this would enable simpler and cheaper device designs, particularly in flat-panel displays, mainly by reducing signal loss and delay. However, it is not yet clear what feature of the IT0 films limits their conductivity, and no firm estimate of the ultimate limit on the conductivity has been made. In particular, the significance of specific types of structural defects is not clear, such as Sn-0 precipitates, line defects or stacking faults, or the grain boundaries themselves, although in the latter case, as noted above, epitaxial films have comparable resistivities to polycrystalline films. While the defect concentrations must be controlled to attack this question experimentally, it is difficult to separately control such potential factors as crystallization, grain size, film stoichiometry and doping, and precipitation during deposition since all these properties are coupled by the deposition conditions. Ion implantation, on the other hand, can inject defects of various types and in controlled quantities into predeposited films, and so may provide a convenient method to vary concentrations of certain defects (including impurities) independently of these other factors, thus enabling researchers to decouple effects on conductivity in films that
0168-583X/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SO1 68-583X(96)00373-4
IV. ION IMPLANTATION
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Instr. and Meth. in
are otherwise prepared identically. Additionally, there is some hope that ion implantation may be a viable method for improving the conductivity in IT0 films (by co-doping), or at least for correcting or controlling conductivity variations during manufacturing. For these reasons, we have undertaken studies of implantation of various species into poly-IT0 films, and more recently into single-crystal films. This paper describes experimental results which suggest that ion implantation can be used to modify the oxygen vacancy concentration in various ways. In addition, we describe some preliminary results from the first implantation experiments in single-crystal ITO, which are aimed at better quantifying the relationship between point-defect concentration and conductivity.
2. Experimental description
200 nm thick, polycrystalline IT0 films (5 wt.% SnO,) were prepared by electron-beam evaporation onto SiO,coated soda-lime glass substrates at 350°C. These films had a resistivity, p = 190 p,a cm, with carrier concentmtion, n = 1 X 102’/cm3, and mobility, p = 37 cm’/V s. The epitaxial IT0 film was also 200 nm thick, and was grown by molecular-beam epitaxy at 5OO”C, onto an optically polished (lOO)-oriented yttria-stabilized zirconia substrate. This film had p 5 190 l.& cm (n = 6.4 X 102’/cm3 and p = 50 cm*/V s). Detailed properties of these films will be reported elsewhere [5]. Implantations were performed on a 200 kV Varian Exuion implanter with a research endstation. In all cases, the reported ion energies were selected to give a projected range of 100 nm, centered within the IT0 film thickness. Initial resistivity measurements were made by a four-point probe, and in some cases these were followed by van der Pauw-Hall-effect measurements in a 6.4 kG dc magnetic field. Rutherford backscattering and ion channeling measurements were made on the epitaxial sample using a 2.3 MeV incident He+ beam and a scattering angle of 160”.
Phys. Res. 6 121 (1997) 221-225
dominated the conductivity change up to doses of - 2 X 10’5/cm2. For both of these species, the resistivity increased, and the size of the increase scaled directly with the nuclear component of energy loss [6]. Annealing to 2OO”C, in either vacuum or air, recovered most, but not all, of the conductivity. Subsequently, it was determined that (a) implantation at elevated temperature reduces the resistivity increase to precisely the same level as that of postimplant annealing at the same temperature, and (b) that for implantation above - 200°C the carrier concentration stopped decreasing as a function of dose when n reached a [8]. More recently, we have IeveI of - 6.5 X lO”/cm* found that the loss of carriers during implantation of In+ at elevated temperatures also saturates at the same value of n, even though In+ is a much heavier ion [9]. On the basis that this characteristic value of n = 6.5 X 102’/cm3 is precisely the number of free carriers associated with oxygen vacancies in these particular films, it was argued that the oxygen vacancy concentration must be stable against implantation damage at 200°C and above, such that the damage can only deactivate the Sn donors. Upon continued implantation of In+ well beyond this saturation, to doses 0.5-2 X 10t5/cm2, the carrier concentration recovers slightly. In this range, the rate of recovery is consistent with the generation of oxygen-vacancy donors expected if the In is incorporated substitutionally on the lattice [9]. To the best of our knowledge, this was the first evidence for chemical modification (doping) of low-resistivity IT0 films by ion implantation. It provides the basis for the further studies reported here. One point to note is that doping of this material requires much larger doses than conventional semiconductors, consistent with the large initial carrier concentration - the implanted “dopant” concentrations must be on the order of at least 1 a/o in order to have any measurable effect when n _ 10z1/cm3. For implantation of In+ and, as will be shown below, for O+, it appears that the “doping” results from changing the stoichiometry, such that ion implantation is really modifying the chemistry of the film. 3.2. High-dose oxygen implantation
3. Results and discussion
3.1. Moderate dose implantations: A reuiew of some earlier results To provide some background and perspective for the present experiments, it is useful at this point to include a brief review of the results and interpretation of some earlier experiments on identical polycrystalline IT0 films [6-91 using implantation doses between lOI and 10tS/cm2, i.e., in the range customarily used for doping semiconductors such as Si or GaAs. A comparison of the implantation of Hi (33 keV) and Of (80 keV) at room temperature indicated that implantation-induced damage
The implantation of In at high doses resulted in only a small reduction in resistivity relative to the lower doses and only over a limited dose range before a rapid degradation occurred. However, this small increase implied that damage did not continue to dominate the conductivity effects at such high doses. If chemistry is really becoming dominant in this regime, then we should expect high-dose implantation of 0+ to have the complementary effect to that of In+, namely to increase the resistance, in spite of the fact that we had earlier noted a saturation of the resistance increase for moderate dose O+ implantation. Furthermore, since the damage is believed to deactivate the Sn dopants at even lower doses, we may expect orders of magnitude increases in resistivity to be possible. Fig. 1
T.E. Haynes et al./Nucl. An = 2*[0+1,
lolS
lo’*
In&.
and Meth. in Phys. Rex B 121 (1997) 221-225
(cmJ>
10” 1o’S Ion Dose (cm-‘)
10”
Fig. 1. Comparison of conductivity changes observed in poly-IT0 films following room temperature implantation of 0 versus that for N and F ions.
in deposited IT0 films. For instance, if the resistivity can be made large enough, by an appropriate recipe of Of implantation and annealing (the latter to increase the effectiveness of incorporation onto vacant sites), then nonconducting areas could be defined by standard lithography and implantation, leaving a pattern of the conducting signal lines in unimplanted regions as necessary for a device, while at the same time maintaining a planar surface. In certain applications, such a process may have significant advantages over the alternative present practice of etching to remove the unwanted conductor areas. Such an implantation process is not unrealistic, given ( 1f that the oxygen vacancy concentration integrated over a typical film thickness corresponds to an areal density of < 10’6/cm2. which is an easily achievable dose, and (2) that the other source of carriers in ITO, substitutional Sn, appears to be deactivated as well by the damage during O+ implantation. 3.3. Implantation
shows results for room-temperature implantation of O+ (80 keV) at such higher doses. This figure shows first the saturation of p over more than one order of magnitude of dose up to about 5 X 10”/cm2. But this saturation is followed by a rapid increase in resistivity, at a rate of approximately 3 orders of magnitude in p per 1 order of magnitude in dose. The onset of this resistivity increase has also been observed recently by Vink et al., up to a dose of I X 1016/cm2 [IO]. We note that the onset of this rapid increase occurs when 2 times the implanted oxygen concentration averaged over the layer thickness is approximately equal to the init@ carrier concentration, i.e., - 1 x 10*‘/cm3 (see upper scale in Fig. I, where the factor of 2 is the number of carriers per oxygen vacancy). So this effect is also consistent with chemical modification rather than with lattice damage. As further confirmation, Fig. 1 includes results for similar implants of N+ and F+, elements which lie on either side of 0 in the periodic table. For both F+ and N+, the resistivity increases, but the rate of increase is much smaller than for O+. This comparison conclusively rules out a damage effect. So it appears that onto lattice implanted O+ may also be accommodated sites, similarly to In+, and eliminate the oxygen-vacancy source of carriers. If this is the mechanism of the resistivity increase, then 99.97% of the oxygen vacancies have been removed by our highest dose of 1 X 10’7/cm2. However, at this dose, the number of 0 atoms added to the film by implantation is already approximately i5 times larger than the initial vacancy concentration, which was - 3 X 102’/cm3 (= f X 6.5 X 102’/cm3), so the process of incorporation in the as-implanted state is not completely efficient under these conditions. The results in Fig. 1 suggest an important prospect for the application of ion implantation in IT0 - namely, the use of ion implantation for patterning conducting pathways
223
into single-crystal
jlms
Single-crystal films are attractive for basic studies of defects in IT0 for several reasons. First, single-crystal films inherently provide a “cleaner” system for study. For example, the complication of grain boundaries is obviously eliminated, along with concomitant complications such as segregated impurities. Secondly, characterization of the defects by such standard structural techniques as X-ray diffraction, electron microscopy, and ion channeling becomes much more sensitive and quantitative. Such capability would provide for the first time a realistic possibility for a quantitative assessment of the relationship between defects and conduction in ITO, which will be necessary to confirm any proposed mechanisms. Suitable films have recently been produced by heteroepitaxial growth [3,4,11,12], so that a study of implantation damage in crystal IT0 may now be feasible. Here we describe our first experiments using such single-crystal films. For the single-crystal experiments, which are focused on damage effects, the implantation species was chosen to be Ne+ to eliminate the possibility of chemical effects while producing comparable damage as O+. In this sense, we regard the system of Ne-on-epitaxial IT0 to be a model system to help separate out the damage effects observed for 0-on-poly-ITO. Fig. 2 shows a set of ion channeling spectra from one single-crystal IT0 film after a series of sequential implantations of Ne+ at room temperature. The minimum yield in the unimplanted sample was - 7.5%. For a dose of 3 X 10’3/cm2, the channeling yield has clearly increased above the unimplanted spectrum, and this increase continued as the cumulative dose was increased to 1 x 10’4/cm2. Additional implantation up to a dose of 4 x 10’4/cm2 did not produce any additional increase in the channeling yield. So ion channeling now gives the first direct evidence that the implantation damage saturates as a function of dose. Furthermore. the damage saturation oc-
IV. ION IMPLANTATION
T.E. Haynes et al./Nucl.
224
Instr. andbfeth.
in Phys. Res. B 121 (1997) 221-22.5
tl....l....‘....‘....‘....i
O0 1.5
1.6
1.7
1.6
Energy
(MeV)
1.9
lmlplanted ilose
2.0
Fig. 2. ion channeling spectra from a heteroepitaxial IT0 film on a YSZ substrate following increments of implantation of 95 keV Ne+ ions at room temperature.
curs at the same dose as the resistivity saturation observed for 0+ implantation. This suggests that the resistivity saturation may be linked to a saturation of the damage growth in implanted ITO, consistent with our interpretation that for these doses, the damage effects are dominant. The channeling measurements are summarized as a function of implant dose in Fig. 3, along with the resistivity measured after each implant increment. p initially increased along with the minimum yield, as expected. However, p continued to increase, albeit more slowly, after x reached saturation and, moreover, for doses beyond those for which O+ produced saturation of p. From this result, we infer that some part of the increase in p must be caused by a component of the implantation damage to which ion channeling is not sensitive. One possibility is that the damage has evolved from isolated point defects, or small clusters, into some sort of larger, correlated defect structures, such as dislocations or platelets, or
20 , . . . . , . .1 . , . . I I , . . . . , . . . 1 20
-
. t - 10 s
95 keV Ne+ on epi-IT0
,....1....I....I....I.... 1 2 O 0 Ne*
3
4
Fig. 4. Differences in effect on resistivity between Ne and 0 implanted into poly-IT0 and between single-crystal and poly-IT0 implanted with Ne.
perhaps even to gas bubbles, since ion channeling would be less sensitive to any of these. Finally, Fig. 4 assesses the validity of our model system for damage studies. In poly-IT0 samples, we observe that implantation of either Ne+ or O+ shows a resistivity saturation at a dose of about 1 X 10L4/cm2. However, for Ne+, the saturated value of p in poly-IT0 is somewhat larger. According to the size of this difference, an explanation on the basis of displacements alone does not seem likely. The difference will probably ultimately be linked to chemical effects, and if so, then further similar studies should be able to give us an estimate on the size of the chemical effects in this regime, and help to sort out the chemical mechanisms. A second feature that becomes apparent from Fig. 4 is that Ne affects p differently in single-crystal IT0 than in poly-crystal IT0 - no saturation is observed in the single crystal. There may be a number of possible explanations for this difference, including the possibility of segregation of defects or impurities to the gram boundaries in poly-ITO.
4. Summary and prospects
15 f P
-
5
(1 S4/cm2;
P .$
15
*g
*
A?
so
Dose (1 0”/cm2)
Fig. 3. Correlation of the channeling minimum yield, x, and the resistivity, p. as a function of Ne implant dose.
We have reported on several experiments aimed at untangling the relationship between defects, chemistry, and conductivity in ITO. In this work, ion implantation is used as a tool to controllably inject defects or chemical constituents. The damage effects and chemical effects dominate in different temperature and dose regimes, with the crossover from damage-dominated to chemistry-dominated conductivity changes occurring at doses of about 10”/cm2. Since each ion generates many defects, we should expect damage effects may become manifest at lower doses than chemistry effects, as observed. In these materials, the relatively large crossover dose is consistent with the large initial carrier concentration characteristic of these degenerate semiconductors. There is evidence that
T.E. Haynes et ol./Nucl.
Instr.
nnd Meth.
both implanted O+ and In+ can occupy lattice sites to some degree after implantation, raising the prospect for patterning of IT0 conductive films by using ion implantation to selectively control the oxygen vacancy concentration. The current status of the single-crystal experiments can be summarized as follows. The model system we have chosen may not be ideal - qualitative differences were observed between the same implant species implanted in the two different types of ITO, and quantitative differences found between two different species (producing approximately the same number of displacements) in the poly-ITO. However, quantitative damage-conduction relationships are worth pursuing in single-crystal IT0 because such studies have great potential to more firmly establish the mechanisms contributing to the conductivity changes observed in ion-implanted ITO. We believe the differences we observe with our model system will prove to be very instructive. Viue la difference!
Acknowledgements The authors gratefully acknowledge stimulating discussions with C.C. Wong and S.A. Carter at Bell Laboratories. Research performed at Oak Ridge National Laboratory was sponsored by the U.S. Department of Energy, Division of Materials Sciences, under contract DE-ACOS-
225
in Phys. Res. B 121 (1997) 221-225
96OR22464 poration.
with Lockheed
Martin Energy Research
Cor-
References [l] 1. Hamberg and C.G. Granqvist, J. Appl. Phys. 60 (1986) R123. [2] G. Frank and H. Kosthn, Appt. Phys. A 27 (1982) 197. [3] M. Kamei, T. Yagami, S. Takai and Y. Shigesato, Appl. Phys. Lett. 64 (1994) 2712. [4] N. Taga, H. Odaka, Y. Shigesato, I. Yasui and T.E. Haynes, J. Appl. Phys. 80 (19%) 978. [5] N. Taga, M. Maekawa, Y. Shigesato, I. Yasui and T.E. Haynes, Transactions of the Materials Research Society of Japan 1996: Transparent Conductive MaterialsPrinciples and Applications, to be published. [6] Y. Shigesato, D.C. Paine and T.E. Haynes. .I. Appl. Phys. 73 (1993) 3805. [7] Y. Shigesato, D.C. Paine and T.E. Haynes, Jpn. J. Appl. Phys. 32 (1993) 1352. [8] Y. Shigesato. D.C. Paine and T.E. Haynes, Trans. Mat. Res. Sot. Jpn. 17 (1994) 503. [9] T.E. Haynes and Y. Shigesato, J. Appl. Phys. 77 (1995) 2572. [IO] T.J. Vink, M.H.F. Overwijk and W. Walrave, J. Appl. Phys., to be published. [l I] E.J. Tarsa, J.H. English and J.S. Speck, Appl. Phys. Lett. 62 (1993) 2332. [I21 M. Kamei, Y. Shigesato, S. Takaki, Y. Hayashi, M. Sasaki and T.E. Haynes, Appl. Phys. Lett. 65 (1994) 546.
IV. JON IMPLANTATION
__
Beam InteractIons Materials&Atoms
with
!!fJ
ELSEMER
Ion beam modification of transparent conducting indium-tin-oxide thin films T.E. Haynes av*, Y. Shigesato b, I. Yasui b, N. Taga ‘, H. Odaka ’ a Solid Stute Division, Oak Ridge National Laboratory, P.O.Box 2008, Building 3003, MS-6048, Oak Ridge, TN 37831.6048. b Institute of Industrial Science, University of Tokyo, Minato-ku, Tokyo 106, Japan ’ Asahi Glass Co., Ltd., Kanagawa-ku. Yokohamu 221. Japun
USA
Abstract We have examined the effects of ion implantation of various chemical species on the electrical properties of transparent, conducting indium-tin-oxide (ITO) polycrystalline films with resistivities less than 200 ~0 cm and optical transmission greater than 90%. We report on implantations of Nf, O’, Ff, Ne+ and In+ under a variety of conditions. At low to moderate doses, damage effects dominate and reduce the conductivity slightly before saturating at doses of - 1014/cm2. At higher doses, when the implanted concentration becomes comparable to the free-carrier concentration ( - lO”/cm’), some species (e.g., In’> can improve the conduction slightly, while other species (e.g., O+) can reduce the conduction, in some cases by several orders of magnitude. We also describe preliminary results of some new experiments begun with single-crystal IT0 films to permit better characterization of the damage effects.
1. Introduction Indium-tin-oxide (ITO) has the best combination of any known material of both electrical conductivity and optical transmission in the visible range to meet the requirements for a number of applications for transparent conducting films [ 11. Essentially, this material consists of In,O, doped with several atomic percent of Sn, which substitutes for In in the crystal lattice. In most applications, thin films of the order of 100-200 nm are deposited by sputtering. Various substrates are employed, including typically silicate glasses, as in the fabrication of flat-panel displays, or silicon, in either amorphous or polycrystalline form, as in solar cells. Films may be deposited at slightly elevated temperatures, say lOO-200°C. in which case they are immediately polycrystalline, or alternatively amorphous layers may be deposited at room temperature and then postannealed to induce crystallization. In either case, the final product is a polycrystalline layer, with a resistivity normally in the range of 100-200 pLn cm, and optical transmission exceeding 90% throughout the visible spectrum. IT0 can be classified as a highly degenerate semiconductor, n-type with n - 102’/cm3, and a band gap of - 3.7 eV. It is generally accepted that the source of the conduction electrons includes both substitutional Sn atoms and also oxygen vacancies [2]. Quite remarkably, IT0 films may be
* Comesponding [email protected].
author.
Fax:
+ l-423-576-6720;
email:
deposited under a variety of conditions, often by different methods, have different microstructures (characterized by grain size, texturing, etc.), and yet have nearly identical electrical properties. Recently, researchers have produced single-crystal, epitaxial IT0 films that also have very similar conductivity to the polycrystalline form [3,4]. It is highly desirable to further improve the conductivity of IT0 films, because this would enable simpler and cheaper device designs, particularly in flat-panel displays, mainly by reducing signal loss and delay. However, it is not yet clear what feature of the IT0 films limits their conductivity, and no firm estimate of the ultimate limit on the conductivity has been made. In particular, the significance of specific types of structural defects is not clear, such as Sn-0 precipitates, line defects or stacking faults, or the grain boundaries themselves, although in the latter case, as noted above, epitaxial films have comparable resistivities to polycrystalline films. While the defect concentrations must be controlled to attack this question experimentally, it is difficult to separately control such potential factors as crystallization, grain size, film stoichiometry and doping, and precipitation during deposition since all these properties are coupled by the deposition conditions. Ion implantation, on the other hand, can inject defects of various types and in controlled quantities into predeposited films, and so may provide a convenient method to vary concentrations of certain defects (including impurities) independently of these other factors, thus enabling researchers to decouple effects on conductivity in films that
0168-583X/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SO1 68-583X(96)00373-4
IV. ION IMPLANTATION
222
T.E. Huynes et ul./Nucl.
Instr. and Meth. in
are otherwise prepared identically. Additionally, there is some hope that ion implantation may be a viable method for improving the conductivity in IT0 films (by co-doping), or at least for correcting or controlling conductivity variations during manufacturing. For these reasons, we have undertaken studies of implantation of various species into poly-IT0 films, and more recently into single-crystal films. This paper describes experimental results which suggest that ion implantation can be used to modify the oxygen vacancy concentration in various ways. In addition, we describe some preliminary results from the first implantation experiments in single-crystal ITO, which are aimed at better quantifying the relationship between point-defect concentration and conductivity.
2. Experimental description
200 nm thick, polycrystalline IT0 films (5 wt.% SnO,) were prepared by electron-beam evaporation onto SiO,coated soda-lime glass substrates at 350°C. These films had a resistivity, p = 190 p,a cm, with carrier concentmtion, n = 1 X 102’/cm3, and mobility, p = 37 cm’/V s. The epitaxial IT0 film was also 200 nm thick, and was grown by molecular-beam epitaxy at 5OO”C, onto an optically polished (lOO)-oriented yttria-stabilized zirconia substrate. This film had p 5 190 l.& cm (n = 6.4 X 102’/cm3 and p = 50 cm*/V s). Detailed properties of these films will be reported elsewhere [5]. Implantations were performed on a 200 kV Varian Exuion implanter with a research endstation. In all cases, the reported ion energies were selected to give a projected range of 100 nm, centered within the IT0 film thickness. Initial resistivity measurements were made by a four-point probe, and in some cases these were followed by van der Pauw-Hall-effect measurements in a 6.4 kG dc magnetic field. Rutherford backscattering and ion channeling measurements were made on the epitaxial sample using a 2.3 MeV incident He+ beam and a scattering angle of 160”.
Phys. Res. 6 121 (1997) 221-225
dominated the conductivity change up to doses of - 2 X 10’5/cm2. For both of these species, the resistivity increased, and the size of the increase scaled directly with the nuclear component of energy loss [6]. Annealing to 2OO”C, in either vacuum or air, recovered most, but not all, of the conductivity. Subsequently, it was determined that (a) implantation at elevated temperature reduces the resistivity increase to precisely the same level as that of postimplant annealing at the same temperature, and (b) that for implantation above - 200°C the carrier concentration stopped decreasing as a function of dose when n reached a [8]. More recently, we have IeveI of - 6.5 X lO”/cm* found that the loss of carriers during implantation of In+ at elevated temperatures also saturates at the same value of n, even though In+ is a much heavier ion [9]. On the basis that this characteristic value of n = 6.5 X 102’/cm3 is precisely the number of free carriers associated with oxygen vacancies in these particular films, it was argued that the oxygen vacancy concentration must be stable against implantation damage at 200°C and above, such that the damage can only deactivate the Sn donors. Upon continued implantation of In+ well beyond this saturation, to doses 0.5-2 X 10t5/cm2, the carrier concentration recovers slightly. In this range, the rate of recovery is consistent with the generation of oxygen-vacancy donors expected if the In is incorporated substitutionally on the lattice [9]. To the best of our knowledge, this was the first evidence for chemical modification (doping) of low-resistivity IT0 films by ion implantation. It provides the basis for the further studies reported here. One point to note is that doping of this material requires much larger doses than conventional semiconductors, consistent with the large initial carrier concentration - the implanted “dopant” concentrations must be on the order of at least 1 a/o in order to have any measurable effect when n _ 10z1/cm3. For implantation of In+ and, as will be shown below, for O+, it appears that the “doping” results from changing the stoichiometry, such that ion implantation is really modifying the chemistry of the film. 3.2. High-dose oxygen implantation
3. Results and discussion
3.1. Moderate dose implantations: A reuiew of some earlier results To provide some background and perspective for the present experiments, it is useful at this point to include a brief review of the results and interpretation of some earlier experiments on identical polycrystalline IT0 films [6-91 using implantation doses between lOI and 10tS/cm2, i.e., in the range customarily used for doping semiconductors such as Si or GaAs. A comparison of the implantation of Hi (33 keV) and Of (80 keV) at room temperature indicated that implantation-induced damage
The implantation of In at high doses resulted in only a small reduction in resistivity relative to the lower doses and only over a limited dose range before a rapid degradation occurred. However, this small increase implied that damage did not continue to dominate the conductivity effects at such high doses. If chemistry is really becoming dominant in this regime, then we should expect high-dose implantation of 0+ to have the complementary effect to that of In+, namely to increase the resistance, in spite of the fact that we had earlier noted a saturation of the resistance increase for moderate dose O+ implantation. Furthermore, since the damage is believed to deactivate the Sn dopants at even lower doses, we may expect orders of magnitude increases in resistivity to be possible. Fig. 1
T.E. Haynes et al./Nucl. An = 2*[0+1,
lolS
lo’*
In&.
and Meth. in Phys. Rex B 121 (1997) 221-225
(cmJ>
10” 1o’S Ion Dose (cm-‘)
10”
Fig. 1. Comparison of conductivity changes observed in poly-IT0 films following room temperature implantation of 0 versus that for N and F ions.
in deposited IT0 films. For instance, if the resistivity can be made large enough, by an appropriate recipe of Of implantation and annealing (the latter to increase the effectiveness of incorporation onto vacant sites), then nonconducting areas could be defined by standard lithography and implantation, leaving a pattern of the conducting signal lines in unimplanted regions as necessary for a device, while at the same time maintaining a planar surface. In certain applications, such a process may have significant advantages over the alternative present practice of etching to remove the unwanted conductor areas. Such an implantation process is not unrealistic, given ( 1f that the oxygen vacancy concentration integrated over a typical film thickness corresponds to an areal density of < 10’6/cm2. which is an easily achievable dose, and (2) that the other source of carriers in ITO, substitutional Sn, appears to be deactivated as well by the damage during O+ implantation. 3.3. Implantation
shows results for room-temperature implantation of O+ (80 keV) at such higher doses. This figure shows first the saturation of p over more than one order of magnitude of dose up to about 5 X 10”/cm2. But this saturation is followed by a rapid increase in resistivity, at a rate of approximately 3 orders of magnitude in p per 1 order of magnitude in dose. The onset of this resistivity increase has also been observed recently by Vink et al., up to a dose of I X 1016/cm2 [IO]. We note that the onset of this rapid increase occurs when 2 times the implanted oxygen concentration averaged over the layer thickness is approximately equal to the init@ carrier concentration, i.e., - 1 x 10*‘/cm3 (see upper scale in Fig. I, where the factor of 2 is the number of carriers per oxygen vacancy). So this effect is also consistent with chemical modification rather than with lattice damage. As further confirmation, Fig. 1 includes results for similar implants of N+ and F+, elements which lie on either side of 0 in the periodic table. For both F+ and N+, the resistivity increases, but the rate of increase is much smaller than for O+. This comparison conclusively rules out a damage effect. So it appears that onto lattice implanted O+ may also be accommodated sites, similarly to In+, and eliminate the oxygen-vacancy source of carriers. If this is the mechanism of the resistivity increase, then 99.97% of the oxygen vacancies have been removed by our highest dose of 1 X 10’7/cm2. However, at this dose, the number of 0 atoms added to the film by implantation is already approximately i5 times larger than the initial vacancy concentration, which was - 3 X 102’/cm3 (= f X 6.5 X 102’/cm3), so the process of incorporation in the as-implanted state is not completely efficient under these conditions. The results in Fig. 1 suggest an important prospect for the application of ion implantation in IT0 - namely, the use of ion implantation for patterning conducting pathways
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jlms
Single-crystal films are attractive for basic studies of defects in IT0 for several reasons. First, single-crystal films inherently provide a “cleaner” system for study. For example, the complication of grain boundaries is obviously eliminated, along with concomitant complications such as segregated impurities. Secondly, characterization of the defects by such standard structural techniques as X-ray diffraction, electron microscopy, and ion channeling becomes much more sensitive and quantitative. Such capability would provide for the first time a realistic possibility for a quantitative assessment of the relationship between defects and conduction in ITO, which will be necessary to confirm any proposed mechanisms. Suitable films have recently been produced by heteroepitaxial growth [3,4,11,12], so that a study of implantation damage in crystal IT0 may now be feasible. Here we describe our first experiments using such single-crystal films. For the single-crystal experiments, which are focused on damage effects, the implantation species was chosen to be Ne+ to eliminate the possibility of chemical effects while producing comparable damage as O+. In this sense, we regard the system of Ne-on-epitaxial IT0 to be a model system to help separate out the damage effects observed for 0-on-poly-ITO. Fig. 2 shows a set of ion channeling spectra from one single-crystal IT0 film after a series of sequential implantations of Ne+ at room temperature. The minimum yield in the unimplanted sample was - 7.5%. For a dose of 3 X 10’3/cm2, the channeling yield has clearly increased above the unimplanted spectrum, and this increase continued as the cumulative dose was increased to 1 x 10’4/cm2. Additional implantation up to a dose of 4 x 10’4/cm2 did not produce any additional increase in the channeling yield. So ion channeling now gives the first direct evidence that the implantation damage saturates as a function of dose. Furthermore. the damage saturation oc-
IV. ION IMPLANTATION
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tl....l....‘....‘....‘....i
O0 1.5
1.6
1.7
1.6
Energy
(MeV)
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Fig. 2. ion channeling spectra from a heteroepitaxial IT0 film on a YSZ substrate following increments of implantation of 95 keV Ne+ ions at room temperature.
curs at the same dose as the resistivity saturation observed for 0+ implantation. This suggests that the resistivity saturation may be linked to a saturation of the damage growth in implanted ITO, consistent with our interpretation that for these doses, the damage effects are dominant. The channeling measurements are summarized as a function of implant dose in Fig. 3, along with the resistivity measured after each implant increment. p initially increased along with the minimum yield, as expected. However, p continued to increase, albeit more slowly, after x reached saturation and, moreover, for doses beyond those for which O+ produced saturation of p. From this result, we infer that some part of the increase in p must be caused by a component of the implantation damage to which ion channeling is not sensitive. One possibility is that the damage has evolved from isolated point defects, or small clusters, into some sort of larger, correlated defect structures, such as dislocations or platelets, or
20 , . . . . , . .1 . , . . I I , . . . . , . . . 1 20
-
. t - 10 s
95 keV Ne+ on epi-IT0
,....1....I....I....I.... 1 2 O 0 Ne*
3
4
Fig. 4. Differences in effect on resistivity between Ne and 0 implanted into poly-IT0 and between single-crystal and poly-IT0 implanted with Ne.
perhaps even to gas bubbles, since ion channeling would be less sensitive to any of these. Finally, Fig. 4 assesses the validity of our model system for damage studies. In poly-IT0 samples, we observe that implantation of either Ne+ or O+ shows a resistivity saturation at a dose of about 1 X 10L4/cm2. However, for Ne+, the saturated value of p in poly-IT0 is somewhat larger. According to the size of this difference, an explanation on the basis of displacements alone does not seem likely. The difference will probably ultimately be linked to chemical effects, and if so, then further similar studies should be able to give us an estimate on the size of the chemical effects in this regime, and help to sort out the chemical mechanisms. A second feature that becomes apparent from Fig. 4 is that Ne affects p differently in single-crystal IT0 than in poly-crystal IT0 - no saturation is observed in the single crystal. There may be a number of possible explanations for this difference, including the possibility of segregation of defects or impurities to the gram boundaries in poly-ITO.
4. Summary and prospects
15 f P
-
5
(1 S4/cm2;
P .$
15
*g
*
A?
so
Dose (1 0”/cm2)
Fig. 3. Correlation of the channeling minimum yield, x, and the resistivity, p. as a function of Ne implant dose.
We have reported on several experiments aimed at untangling the relationship between defects, chemistry, and conductivity in ITO. In this work, ion implantation is used as a tool to controllably inject defects or chemical constituents. The damage effects and chemical effects dominate in different temperature and dose regimes, with the crossover from damage-dominated to chemistry-dominated conductivity changes occurring at doses of about 10”/cm2. Since each ion generates many defects, we should expect damage effects may become manifest at lower doses than chemistry effects, as observed. In these materials, the relatively large crossover dose is consistent with the large initial carrier concentration characteristic of these degenerate semiconductors. There is evidence that
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both implanted O+ and In+ can occupy lattice sites to some degree after implantation, raising the prospect for patterning of IT0 conductive films by using ion implantation to selectively control the oxygen vacancy concentration. The current status of the single-crystal experiments can be summarized as follows. The model system we have chosen may not be ideal - qualitative differences were observed between the same implant species implanted in the two different types of ITO, and quantitative differences found between two different species (producing approximately the same number of displacements) in the poly-ITO. However, quantitative damage-conduction relationships are worth pursuing in single-crystal IT0 because such studies have great potential to more firmly establish the mechanisms contributing to the conductivity changes observed in ion-implanted ITO. We believe the differences we observe with our model system will prove to be very instructive. Viue la difference!
Acknowledgements The authors gratefully acknowledge stimulating discussions with C.C. Wong and S.A. Carter at Bell Laboratories. Research performed at Oak Ridge National Laboratory was sponsored by the U.S. Department of Energy, Division of Materials Sciences, under contract DE-ACOS-
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Martin Energy Research
Cor-
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IV. JON IMPLANTATION