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Oxidation and reduction of zinc oxide surfaces: monitored by measuring the losses in guided optical modes
Applied Surface North-Holland
Science
72 (1993) 133-137
applied surface science
Oxidation and reduction of zinc oxide surfaces: monitored by measuring the losses in guided optical modes E.W. Koenig a, W.M.K.P. Wijekoon a~1and W.M. Hetherington a Department of Chemby, University of Arizona, Tucson, AZ 85721, USA
III b
b Department of Physics, Oregon State University, Corvallis, OR 97331, USA Received
3 May 1993; accepted
for publication
7 June
1993
Reduction and oxidation of the surface of a ZnO planar optical waveguide can be detected by measuring the losses in the guided waves. Reduction occurs when the film is heated to 100°C under UHV conditions or in a reducing atmosphere, and the optical losses increase by 6.0, 12.9 to 15.7 dB cm-’ for TE,, TE, and TE, modes, respectively. Subsequent oxidation under 10e2 to 0.1 Torr of 0, at 230°C completely eliminates the defects and yields losses lower than the original values. These effects are analyzed in terms of the change in absorption coefficient of only the surface layer as the stoichiometry changes.
1. Introduction The stoichiometry of the surface of an oxide is known to vary considerably as the chemical environment and the temperature vary. Generally, however, such phenomena have been observed under UHV conditions using techniques such as Auger electron spectroscopy &ES) and electron microscopy. Furthermore, high temperatures in the 400 to 1000°C range have been employed to study stoichiometric changes. One of the purposes of this work is to investigate stoichiometric changes, occurring at relatively low temperatures, using as a probe the optical loss experienced by a guided wave propagating in a planar optical waveguide constructed from a thin film of a metal oxide resting upon a fused silica substrate. Such an approach cannot determine exactly the nature of the chemical changes that occur, but it can set limits on the thickness of the affected region, and it provides a simple way to detect changes in real time under a wide variety of environments. From a different point of view, reducing the ’ Present University USA.
address: Photonics of New York
0169-4332/93/$06.00
Research Laboratory, at Buffalo, Buffalo,
0 1993 - Elsevier
Science
The State NY 14214,
Publishers
losses in planar and fiber optical waveguides is a critical concern in the design of waveguides and optical devices. Much research has been done in determining the optimal fabrication and postfabrication annealing conditions. However, studies of the chemistry of the surfaces of the materials have been sparse. In this work we demonstrate that a systematic study of the stoichiometric changes of the surface of a waveguide material under typical processing and operation conditions can provide insight into control of the quality of the waveguides.
2. Background ZnO waveguide films are composed of densely packed crystallites oriented with all c-axes perpendicular to the film [l]. No grain boundaries are visible in SEM pictures, and X-ray scattering measurements indicate that the crystallite size is approximately 30 nm. This (0001) waveguide surface has the Zn2+ on the surface, and the surface roughness is approximately f 1.0 nm rms. The surface morphology resulting from etching the waveguide with acid is similar to that seen for single crystals [2].
B.V. All rights
reserved
134
E. W. Koenig et al. / Oxidation and reduction of zinc oxide
ZnO waveguides have been used for spectroscopic studies of molecular adsorption, surface chemistry and photochemistry [3-91. Knowledge and control of the stoichiometry of the waveguide surface is, of course, very important in such studies. The optical loss of ZnO waveguides has been found to be reduced from 16 to 9 db cm-’ by post-fabrication, rapid-heating annealing in 0, at 100°C for 30 s [lo]. Longer time intervals at temperatures above 450°C result in an increase in losses arising from recrystallization and grain growth throughout the film. Restructuring of ZnO films by alternate reduction and oxidation cycles has not been attempted. ZnO waveguides always develop increased loss, especially in the higher modes, over long periods of exposure to atmospheric conditions. The regeneration of the lowloss characteristic by chemical processes has not been investigated. ZnO can serve both as a catalyst for a variety of chemical reactions and as a reactant. In the presence of H, at elevated temperatures, ZnO reacts to form metallic Zn and H,O, for which the first step is through the reversible chemisorption reaction ZnO ti ZnH + OH [ 111. Under ultraviolet (one-photon absorption) or visible (twophoton absorption) illumination, the photo-generated electrons and holes can be trapped at the surface to create surface states, the nature of which depend upon the surface structure and the presence of adsorbates [ 111. These surface charges can lead to reduction or oxidation processes. Oxidation and reduction of the surfaces of ZnO powders and single crystals has been studied extensively by classical techniques which are not sufficiently sensitive to allow the determination of the chemistry of only the surface layer. A drastic 0, treatment (1 atm of 0, at 993 K for 24 h) leads to the formation of O,-rich ZnO layers on all ZnO single-crystal surfaces 1121. Those O,-rich ZnO films exhibited smaller thermal and photochemical stabilities and a higher reactivity toward H, and CO than stoichiometric or Zn-rich ZnO films. The reduction behavior of various ZnO samples depended upon the origin of the sample, e.g., storage in air or nitrogen or an 0, pretreatment of the sample. Furthermore, the reaction rate changed considerably as the reaction pro-
ceeded. After reduction of the surface layer, subsequent chemistry is controlled by diffusion through the underlying layers. The activation energy for the reduction has been shown to be independent of the nature of the reductant gas [12]. At higher temperatures (608-769 K) and at pressures below 10-l Torr, the activation energy had a value of 54-59 kcal mall’, and as the temperature decreased the activation energy increased to 121-130 kcal mall’. At higher temperatures direct interaction of 0, impinging from the gas phase onto surface defects takes place [13]. In the presence of point defects such as 02- vacancies or interstitial zinc atoms, chemisorbed 0; may dissociatively react to form ZnO [14]. Many of the physical and chemical properties of ZnO can be described by the existence of Zn interstitials as well as 02vacancies. Diffusion measurements indicate that the diffusion of Zn is distinctly faster than that of oxygen. Oxygen seems to diffuse through vacancies, whereas Zn diffuses via interstitials. The oxidation of the Zn single-crystal (0001) surface at an 0, pressure of 10-6-10-8 Torr and temperatures between 77 and 425 K, indicate a linear dependence of the growth of the ZnO film on the 0, partial pressure. But the growth of the film does not depend on the temperature [15]. UV irradiation enhances the decomposition rates of ZnO at a temperature of 595 K. Unless UHV conditions are employed, 0, photoadsorption and photodesorption processes overcome the photolysis. The hole capture cross section for chemisorbed 02- is much higher than that of the surface lattice 02- ion [16,17]. With all of these previous studies in mind, evidence will be presented for the relatively low temperature reduction and subsequent re-oxidation of ZnO films. The observations will be interpreted in terms of the absorption coefficient of only the surface layer.
3. Experimental
aspects
The ZnO optical waveguide, with a thickness of 0.60 k 0.03 pm, was made in an RF magnetron sputtering plant [18]. The thin ZnO film rested
E. W. Koenig et al. / Oxidation and reduction of zinc oxide
on a polished fused silica substrate. An aluminum structure was employed to press the input and output SrTiO, coupling prisms against the surface of the waveguide such that good optical tontact could be maintained. The polarized 6328 A beam from a helium-neon laser was coupled into the transverse electric (TE) film modes through the prisms. The films used in this investigation supported only three guided modes with typical losses of 1.4, 3.4 and 5.6 dB cm-’ for the TE,, TE, and TE, modes, respectively. The optical scattering losses were determined by measuring the intensity of scattered light as a function of propagation distance for each guided mode. The detection limit was 1.0 dB cm-‘. When baked under a pressure of 1 x lo-* Torr at 160°C for 48 h, a notable degradation in propagation distance for the waveguides was noted. The degradation was especially severe when the UHV chamber was heated to pump out hydrocarbons, such as benzene, that had been introduced at mTorr levels in previous experiments. The ZnO films were regenerated (oxidized) and even improved beyond their initial state by the simple procedures of heating them under pure oxygen. This was accomplished by placing the waveguide into a Pyrex glass tube fitted to a high-vacuum system capable of 1 X 1O-6 Torr. Once the tube had been sufficiently evacuated, clean and dry 0, was flushed twice through the system before filling the tube to between 10 and 100 mTorr. A thermocouple monitored the temperature which was kept above 200°C but did not exceed 230°C. Approximately every 40 min, the tube was flushed and refilled with fresh 0,.
4. Results and discussion Table 1 summarizes the typical guided-mode losses observed. One should particularly note that the optical losses before oxidation annealing are much higher for the higher modes. Though the loss for a guided wave was large, the visual appearance of the film and the absorption spectrum did not differ from that of a low-loss film. Since the relative electric-field amplitude at the surface
Table 1 Optical loss measurements oxidation
135
in dB cm-’
before
and
Mode
Before reduction
Before oxidation
After oxidation
Change
0 1 2
1.4 3.4 5.6
7.5 16.7 20.0
1.5 3.8 4.3
6.0 12.9 15.7
after
increases with the mode number, the mode dependence of the losses indicates that the modified region is limited to the surface region. After oxidation, the surface layer is brought into near uniformity with the bulk of the film and even some improvement occurs over the original, prereduction loss levels. Under the conditions of this investigation, the improvement in the waveguide performance seems to require only approximately one hour of treatment. This improvement is a result of the insertion of 02- into the surface lattice region and the elimination of the excess Zn/Zn2 i at the surface to yield a more stoichiometric structure. With the surface now more uniform with fewer defects to serve as absorption or scattering centers, the guided wave propagates further. Table 2 represents the data in table 1 converted to power-attenuation coefficients, y, in units of cm - ‘, to fit the equation P/P,
= epyz,
(1)
where PO is the initial power the power after propagating attenuation coefficient can in an absorption coefficient by
in the film and P a distance z. The turn be related to y = adN, with d
Table 2 Measured values of the attenuation coefficient, y (cm-‘), effective absorption coefficient, (Y (cm-‘), before and heating the waveguide in 0, Mode
Before
y
Before
a
After
y
and after
After u (d = 5.2 A,
Cd = 5.2 A, 0 1 2
1.73 3.47 4.62
18400 17600 13700
0.69 0.87 0.99
7200 4400 2900
Average
-
16600
_
4800
136
E. W Koenig et al. / Oxidation and reduction of zinc oxide
being the thickness of the absorbing layer and N = (1/2/r) cot 0 the number of reflections with the superstrate per cm in the z direction as the beam propagates through the waveguide. The film thickness is h, and the internal reflection angle is 8, which is different for each mode. cy is an effective absorption coefficient. The diffusion coefficient of Zn defects in ZnO is on the order of 6 X 1O-32 cm2 s-’ at 500 K and 8 x lo-l6 cm2 s-l at 1000 K. There is high degree of variation since the diffusion data are highly dependent on the measurement method and on sample pretreatment [ll]. The diffusion data for oxygen defects is much less clear than that for Zn, but the general consensus seems to be that the rate of oxygen diffusion is several orders of magnitude higher than for Zn diffusion. Since the rates of diffusion for both 02- and Zn are very small at 200°C it is reasonable to state that the reduction/oxidation was confined to within a monolayer or two of the surface. The evanescent fields for the TE,, TE, and TE, modes extend 500, 600 and 700 A above the surface. The effective absorption coefficient, (Y, is calculated by dividing the actual coefficient by 500, 600 and 700, respectively. Extrapolating from the bulk lattice constants, the surface lattice layer of ZnO is about 5.2 A in thickness. If the absorbing layer is assumed to have a molar absorptivity of 150 ! mol-’ cm-‘, then the effective absorption coefficients become 20, 25 and 30 cm-’ (taking the absorber concentration to be that of Znp at 68.8 mol P-l). For an absorbing layer of 5 A, y = 0.92, 1.9 and 3.2 cm-’ for mode TE,, TE, and TE,, respectively. These values compare quite favorably with those calculated from the experimental data and listed in table 2 for the surface before oxidation treatment. Both (Y and d are variables, so they cannot be individually specified. Under high-vacuum conditions, the reduction probably proceeds via the direct loss of O,, since there is not a sufficient concentration of reductant gas present. The most likely species on the surface of a reduced waveguide are then assumed to be ZnO-Zn complexes, Zn, or higher aggregates. Zn atoms absorb in the UV, and, even though the transition of a surface atom would be
broadened considerably, it is not likely that Zn species contribute to the absorptive loss at 6328 A. Zn, surface species are likely to exhibit an electronic transition in the visible region although the spectrum of such dimers has not been reported. If the inherent absorption coefficient of these dimers is a reasonable value such as 1.5 x lo5 cm-‘, then the density needs to be only 10% of a monolayer in order to account for the observed y values. Clearly, very small chemical perturbations of the surface layer have a profound effect on the propagation of a guided mode.
5. Conclusion The fact that the surface of ZnO can be reduced and re-oxidized at relatively low temperatures is an important chemical observation. The sensitivity of the guided-wave losses to slight changes in the surface stoichiometry can be exploited for real-time monitoring of the surface to yield a better understanding of the surface chemistry. Much work needs to be done in identifying the surface structures and measuring rates of reduction and oxidation, and such work can be performed using AES and WSCARS spectroscopies as well as simple absorption spectroscopy with guided waves. Another conclusion is that the loss of a ZnO waveguide can be improved and recovered by a low-temperature oxidation. It is likely that similar procedures will prove to be effective for other waveguide materials.
Acknowledgements We thank G.I. Stegeman and R.M. Fotenberry for providing us with ZnO waveguides and many helpful discussions. This research was supported by The National Science Foundation.
References [l] F.S. Hickernell, in: Proc. IEEE 1984 Ultrasonics Symp. (1984) p. 309. [2] M. Grunze, W. Hirschwald and D. Hoffman, J. Cryst. Growth 52 (1981) 241.
E. Cc: Koenig et al. / Oxidation and reduction of zinc oxide [3] G.I. Stegeman, R. Fortenberry, R. Moshrefzadeh, W.M. Hetherington III, N.E. Van Wyck and J.E. Sipe, Opt. Lett. 8 (1983) 295. [4] W.M. Hetherington III, Z.Z. Ho, E.W. Koenig, G.I. Stegeman and R.M. Fortenberry, Chem. Phys. Lett. 128 (1986) 150. [5] W.M.K.P. Wijekoon, Z.Z. Ho and W.M. Hetherington III, J. Chem. Sot. Faraday Trans. 89 (1993) 1067. 161 Z.Z. Ho, W.M.K.P. Wijekoon, E.W. Koenig and W.M. Hetherington III, J. Phys. Chem. 91 (1987) 757. [7] W.M.K.P. Wijekoon, Z.Z. Ho and W.M. Hetherington III, J. Chem. Phys. 88 (1987) 4384. [8] W.M.K.P. Wijekoon, E.W. Koenig and W.M. Hetherington III, J. Phys. Chem. 97 (1993) 1065. [9] W.M. Hetherington III, E.W. Koenig and W.M.K.P. Wijekoon, Chem. Phys. Lett. 134 (1987) 203. [lo] F.S. Hickernell, in: Proc. IEEE 1981 Ultrasonics Symp. (1981) p. 489.
[ll]
[I21 [13] [14] [15] [16] [17] [18] [19]
137
W. Hirschwald et al., in: Current Topics in Materials Science, Vol. 7, Ed. E. Kaldis (North-Holland, Amsterdam, 1981) p. 143. M. Grunze, W. Hirshwald and E. Thull, Thin Solid Films 37 (1976) 315. W. Gopel, J. Vat. Sci. Technol. 15 (1978) 1298. A.J. Tenth and V.T. Lowson, Chem. Phys. Lett. 8 177 (1971) 177. W.N. Unertl and J.M. Blakely, Surf. Sci. 69 (1977) 33. VS. Zakharenko, A.E. Cherkashin and N.P. Keiev, Kinet. Katal. 16 (1975) 174. S.R. Morrision, J. Vat. Sci. Technol. 7 (1970) 84. R.M. Fortenberry, PhD Dissertation, University of Arizona (1986). S.C. Abrahams and J.C. Beinstein, Acta Cryst. 25 (1969) 1233.
Science
72 (1993) 133-137
applied surface science
Oxidation and reduction of zinc oxide surfaces: monitored by measuring the losses in guided optical modes E.W. Koenig a, W.M.K.P. Wijekoon a~1and W.M. Hetherington a Department of Chemby, University of Arizona, Tucson, AZ 85721, USA
III b
b Department of Physics, Oregon State University, Corvallis, OR 97331, USA Received
3 May 1993; accepted
for publication
7 June
1993
Reduction and oxidation of the surface of a ZnO planar optical waveguide can be detected by measuring the losses in the guided waves. Reduction occurs when the film is heated to 100°C under UHV conditions or in a reducing atmosphere, and the optical losses increase by 6.0, 12.9 to 15.7 dB cm-’ for TE,, TE, and TE, modes, respectively. Subsequent oxidation under 10e2 to 0.1 Torr of 0, at 230°C completely eliminates the defects and yields losses lower than the original values. These effects are analyzed in terms of the change in absorption coefficient of only the surface layer as the stoichiometry changes.
1. Introduction The stoichiometry of the surface of an oxide is known to vary considerably as the chemical environment and the temperature vary. Generally, however, such phenomena have been observed under UHV conditions using techniques such as Auger electron spectroscopy &ES) and electron microscopy. Furthermore, high temperatures in the 400 to 1000°C range have been employed to study stoichiometric changes. One of the purposes of this work is to investigate stoichiometric changes, occurring at relatively low temperatures, using as a probe the optical loss experienced by a guided wave propagating in a planar optical waveguide constructed from a thin film of a metal oxide resting upon a fused silica substrate. Such an approach cannot determine exactly the nature of the chemical changes that occur, but it can set limits on the thickness of the affected region, and it provides a simple way to detect changes in real time under a wide variety of environments. From a different point of view, reducing the ’ Present University USA.
address: Photonics of New York
0169-4332/93/$06.00
Research Laboratory, at Buffalo, Buffalo,
0 1993 - Elsevier
Science
The State NY 14214,
Publishers
losses in planar and fiber optical waveguides is a critical concern in the design of waveguides and optical devices. Much research has been done in determining the optimal fabrication and postfabrication annealing conditions. However, studies of the chemistry of the surfaces of the materials have been sparse. In this work we demonstrate that a systematic study of the stoichiometric changes of the surface of a waveguide material under typical processing and operation conditions can provide insight into control of the quality of the waveguides.
2. Background ZnO waveguide films are composed of densely packed crystallites oriented with all c-axes perpendicular to the film [l]. No grain boundaries are visible in SEM pictures, and X-ray scattering measurements indicate that the crystallite size is approximately 30 nm. This (0001) waveguide surface has the Zn2+ on the surface, and the surface roughness is approximately f 1.0 nm rms. The surface morphology resulting from etching the waveguide with acid is similar to that seen for single crystals [2].
B.V. All rights
reserved
134
E. W. Koenig et al. / Oxidation and reduction of zinc oxide
ZnO waveguides have been used for spectroscopic studies of molecular adsorption, surface chemistry and photochemistry [3-91. Knowledge and control of the stoichiometry of the waveguide surface is, of course, very important in such studies. The optical loss of ZnO waveguides has been found to be reduced from 16 to 9 db cm-’ by post-fabrication, rapid-heating annealing in 0, at 100°C for 30 s [lo]. Longer time intervals at temperatures above 450°C result in an increase in losses arising from recrystallization and grain growth throughout the film. Restructuring of ZnO films by alternate reduction and oxidation cycles has not been attempted. ZnO waveguides always develop increased loss, especially in the higher modes, over long periods of exposure to atmospheric conditions. The regeneration of the lowloss characteristic by chemical processes has not been investigated. ZnO can serve both as a catalyst for a variety of chemical reactions and as a reactant. In the presence of H, at elevated temperatures, ZnO reacts to form metallic Zn and H,O, for which the first step is through the reversible chemisorption reaction ZnO ti ZnH + OH [ 111. Under ultraviolet (one-photon absorption) or visible (twophoton absorption) illumination, the photo-generated electrons and holes can be trapped at the surface to create surface states, the nature of which depend upon the surface structure and the presence of adsorbates [ 111. These surface charges can lead to reduction or oxidation processes. Oxidation and reduction of the surfaces of ZnO powders and single crystals has been studied extensively by classical techniques which are not sufficiently sensitive to allow the determination of the chemistry of only the surface layer. A drastic 0, treatment (1 atm of 0, at 993 K for 24 h) leads to the formation of O,-rich ZnO layers on all ZnO single-crystal surfaces 1121. Those O,-rich ZnO films exhibited smaller thermal and photochemical stabilities and a higher reactivity toward H, and CO than stoichiometric or Zn-rich ZnO films. The reduction behavior of various ZnO samples depended upon the origin of the sample, e.g., storage in air or nitrogen or an 0, pretreatment of the sample. Furthermore, the reaction rate changed considerably as the reaction pro-
ceeded. After reduction of the surface layer, subsequent chemistry is controlled by diffusion through the underlying layers. The activation energy for the reduction has been shown to be independent of the nature of the reductant gas [12]. At higher temperatures (608-769 K) and at pressures below 10-l Torr, the activation energy had a value of 54-59 kcal mall’, and as the temperature decreased the activation energy increased to 121-130 kcal mall’. At higher temperatures direct interaction of 0, impinging from the gas phase onto surface defects takes place [13]. In the presence of point defects such as 02- vacancies or interstitial zinc atoms, chemisorbed 0; may dissociatively react to form ZnO [14]. Many of the physical and chemical properties of ZnO can be described by the existence of Zn interstitials as well as 02vacancies. Diffusion measurements indicate that the diffusion of Zn is distinctly faster than that of oxygen. Oxygen seems to diffuse through vacancies, whereas Zn diffuses via interstitials. The oxidation of the Zn single-crystal (0001) surface at an 0, pressure of 10-6-10-8 Torr and temperatures between 77 and 425 K, indicate a linear dependence of the growth of the ZnO film on the 0, partial pressure. But the growth of the film does not depend on the temperature [15]. UV irradiation enhances the decomposition rates of ZnO at a temperature of 595 K. Unless UHV conditions are employed, 0, photoadsorption and photodesorption processes overcome the photolysis. The hole capture cross section for chemisorbed 02- is much higher than that of the surface lattice 02- ion [16,17]. With all of these previous studies in mind, evidence will be presented for the relatively low temperature reduction and subsequent re-oxidation of ZnO films. The observations will be interpreted in terms of the absorption coefficient of only the surface layer.
3. Experimental
aspects
The ZnO optical waveguide, with a thickness of 0.60 k 0.03 pm, was made in an RF magnetron sputtering plant [18]. The thin ZnO film rested
E. W. Koenig et al. / Oxidation and reduction of zinc oxide
on a polished fused silica substrate. An aluminum structure was employed to press the input and output SrTiO, coupling prisms against the surface of the waveguide such that good optical tontact could be maintained. The polarized 6328 A beam from a helium-neon laser was coupled into the transverse electric (TE) film modes through the prisms. The films used in this investigation supported only three guided modes with typical losses of 1.4, 3.4 and 5.6 dB cm-’ for the TE,, TE, and TE, modes, respectively. The optical scattering losses were determined by measuring the intensity of scattered light as a function of propagation distance for each guided mode. The detection limit was 1.0 dB cm-‘. When baked under a pressure of 1 x lo-* Torr at 160°C for 48 h, a notable degradation in propagation distance for the waveguides was noted. The degradation was especially severe when the UHV chamber was heated to pump out hydrocarbons, such as benzene, that had been introduced at mTorr levels in previous experiments. The ZnO films were regenerated (oxidized) and even improved beyond their initial state by the simple procedures of heating them under pure oxygen. This was accomplished by placing the waveguide into a Pyrex glass tube fitted to a high-vacuum system capable of 1 X 1O-6 Torr. Once the tube had been sufficiently evacuated, clean and dry 0, was flushed twice through the system before filling the tube to between 10 and 100 mTorr. A thermocouple monitored the temperature which was kept above 200°C but did not exceed 230°C. Approximately every 40 min, the tube was flushed and refilled with fresh 0,.
4. Results and discussion Table 1 summarizes the typical guided-mode losses observed. One should particularly note that the optical losses before oxidation annealing are much higher for the higher modes. Though the loss for a guided wave was large, the visual appearance of the film and the absorption spectrum did not differ from that of a low-loss film. Since the relative electric-field amplitude at the surface
Table 1 Optical loss measurements oxidation
135
in dB cm-’
before
and
Mode
Before reduction
Before oxidation
After oxidation
Change
0 1 2
1.4 3.4 5.6
7.5 16.7 20.0
1.5 3.8 4.3
6.0 12.9 15.7
after
increases with the mode number, the mode dependence of the losses indicates that the modified region is limited to the surface region. After oxidation, the surface layer is brought into near uniformity with the bulk of the film and even some improvement occurs over the original, prereduction loss levels. Under the conditions of this investigation, the improvement in the waveguide performance seems to require only approximately one hour of treatment. This improvement is a result of the insertion of 02- into the surface lattice region and the elimination of the excess Zn/Zn2 i at the surface to yield a more stoichiometric structure. With the surface now more uniform with fewer defects to serve as absorption or scattering centers, the guided wave propagates further. Table 2 represents the data in table 1 converted to power-attenuation coefficients, y, in units of cm - ‘, to fit the equation P/P,
= epyz,
(1)
where PO is the initial power the power after propagating attenuation coefficient can in an absorption coefficient by
in the film and P a distance z. The turn be related to y = adN, with d
Table 2 Measured values of the attenuation coefficient, y (cm-‘), effective absorption coefficient, (Y (cm-‘), before and heating the waveguide in 0, Mode
Before
y
Before
a
After
y
and after
After u (d = 5.2 A,
Cd = 5.2 A, 0 1 2
1.73 3.47 4.62
18400 17600 13700
0.69 0.87 0.99
7200 4400 2900
Average
-
16600
_
4800
136
E. W Koenig et al. / Oxidation and reduction of zinc oxide
being the thickness of the absorbing layer and N = (1/2/r) cot 0 the number of reflections with the superstrate per cm in the z direction as the beam propagates through the waveguide. The film thickness is h, and the internal reflection angle is 8, which is different for each mode. cy is an effective absorption coefficient. The diffusion coefficient of Zn defects in ZnO is on the order of 6 X 1O-32 cm2 s-’ at 500 K and 8 x lo-l6 cm2 s-l at 1000 K. There is high degree of variation since the diffusion data are highly dependent on the measurement method and on sample pretreatment [ll]. The diffusion data for oxygen defects is much less clear than that for Zn, but the general consensus seems to be that the rate of oxygen diffusion is several orders of magnitude higher than for Zn diffusion. Since the rates of diffusion for both 02- and Zn are very small at 200°C it is reasonable to state that the reduction/oxidation was confined to within a monolayer or two of the surface. The evanescent fields for the TE,, TE, and TE, modes extend 500, 600 and 700 A above the surface. The effective absorption coefficient, (Y, is calculated by dividing the actual coefficient by 500, 600 and 700, respectively. Extrapolating from the bulk lattice constants, the surface lattice layer of ZnO is about 5.2 A in thickness. If the absorbing layer is assumed to have a molar absorptivity of 150 ! mol-’ cm-‘, then the effective absorption coefficients become 20, 25 and 30 cm-’ (taking the absorber concentration to be that of Znp at 68.8 mol P-l). For an absorbing layer of 5 A, y = 0.92, 1.9 and 3.2 cm-’ for mode TE,, TE, and TE,, respectively. These values compare quite favorably with those calculated from the experimental data and listed in table 2 for the surface before oxidation treatment. Both (Y and d are variables, so they cannot be individually specified. Under high-vacuum conditions, the reduction probably proceeds via the direct loss of O,, since there is not a sufficient concentration of reductant gas present. The most likely species on the surface of a reduced waveguide are then assumed to be ZnO-Zn complexes, Zn, or higher aggregates. Zn atoms absorb in the UV, and, even though the transition of a surface atom would be
broadened considerably, it is not likely that Zn species contribute to the absorptive loss at 6328 A. Zn, surface species are likely to exhibit an electronic transition in the visible region although the spectrum of such dimers has not been reported. If the inherent absorption coefficient of these dimers is a reasonable value such as 1.5 x lo5 cm-‘, then the density needs to be only 10% of a monolayer in order to account for the observed y values. Clearly, very small chemical perturbations of the surface layer have a profound effect on the propagation of a guided mode.
5. Conclusion The fact that the surface of ZnO can be reduced and re-oxidized at relatively low temperatures is an important chemical observation. The sensitivity of the guided-wave losses to slight changes in the surface stoichiometry can be exploited for real-time monitoring of the surface to yield a better understanding of the surface chemistry. Much work needs to be done in identifying the surface structures and measuring rates of reduction and oxidation, and such work can be performed using AES and WSCARS spectroscopies as well as simple absorption spectroscopy with guided waves. Another conclusion is that the loss of a ZnO waveguide can be improved and recovered by a low-temperature oxidation. It is likely that similar procedures will prove to be effective for other waveguide materials.
Acknowledgements We thank G.I. Stegeman and R.M. Fotenberry for providing us with ZnO waveguides and many helpful discussions. This research was supported by The National Science Foundation.
References [l] F.S. Hickernell, in: Proc. IEEE 1984 Ultrasonics Symp. (1984) p. 309. [2] M. Grunze, W. Hirschwald and D. Hoffman, J. Cryst. Growth 52 (1981) 241.
E. Cc: Koenig et al. / Oxidation and reduction of zinc oxide [3] G.I. Stegeman, R. Fortenberry, R. Moshrefzadeh, W.M. Hetherington III, N.E. Van Wyck and J.E. Sipe, Opt. Lett. 8 (1983) 295. [4] W.M. Hetherington III, Z.Z. Ho, E.W. Koenig, G.I. Stegeman and R.M. Fortenberry, Chem. Phys. Lett. 128 (1986) 150. [5] W.M.K.P. Wijekoon, Z.Z. Ho and W.M. Hetherington III, J. Chem. Sot. Faraday Trans. 89 (1993) 1067. 161 Z.Z. Ho, W.M.K.P. Wijekoon, E.W. Koenig and W.M. Hetherington III, J. Phys. Chem. 91 (1987) 757. [7] W.M.K.P. Wijekoon, Z.Z. Ho and W.M. Hetherington III, J. Chem. Phys. 88 (1987) 4384. [8] W.M.K.P. Wijekoon, E.W. Koenig and W.M. Hetherington III, J. Phys. Chem. 97 (1993) 1065. [9] W.M. Hetherington III, E.W. Koenig and W.M.K.P. Wijekoon, Chem. Phys. Lett. 134 (1987) 203. [lo] F.S. Hickernell, in: Proc. IEEE 1981 Ultrasonics Symp. (1981) p. 489.
[ll]
[I21 [13] [14] [15] [16] [17] [18] [19]
137
W. Hirschwald et al., in: Current Topics in Materials Science, Vol. 7, Ed. E. Kaldis (North-Holland, Amsterdam, 1981) p. 143. M. Grunze, W. Hirshwald and E. Thull, Thin Solid Films 37 (1976) 315. W. Gopel, J. Vat. Sci. Technol. 15 (1978) 1298. A.J. Tenth and V.T. Lowson, Chem. Phys. Lett. 8 177 (1971) 177. W.N. Unertl and J.M. Blakely, Surf. Sci. 69 (1977) 33. VS. Zakharenko, A.E. Cherkashin and N.P. Keiev, Kinet. Katal. 16 (1975) 174. S.R. Morrision, J. Vat. Sci. Technol. 7 (1970) 84. R.M. Fortenberry, PhD Dissertation, University of Arizona (1986). S.C. Abrahams and J.C. Beinstein, Acta Cryst. 25 (1969) 1233.