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Copper Oxides (Cu2O, CuO)
Copper Oxides (Cu20, CuO) CARL G. RIBBING and ARNE ROOS Institute of Technology Uppsala University Uppsala, Sweden
The established stoichometric copper oxides, cuprous oxide, Cu20 ("red") and cupric oxide, CuO ("black"), form cubic and monoclinic unit cells in their respective equilibrium crystalline forms [1]. Reports on the phase CuO.6v, described as a gross defect structure of Cu20, are found in the literature [2, 3], but will not be considered here. It has furthermore been claimed that Cu20 is actually not an equilibrium structure at temperatures below 375 ~ and therefore decomposes very slowly at room temperature into metallic copper and cupric oxide [4]. The Cu-O phase diagram is markedly pressure dependent, such that a CuO layer transforms into Cu20 and eventually into metallic copper when placed in vacuum. Cuprous oxide is a semiconductor with a direct band gap of 2.17 eV. It has been the object of several basic studies concerning optical-band-gap absorption, its temperature dependence, and the interpretation of the lowtemperature absorption line spectrum below the gap as transitions to excitonic levels [5-7]. The partly ionic character of the bonding causes structures in the optical functions in the infrared spectral region. The strongest IR-active mode, corresponding to the residual-ray region, was located at 609cm -~ (16.4~m) [8]. A secondary lattice mode at 146cm -~ (68~m) is reported for polycrystalline samples [9]. Several other lattice modes have been identified in later work and attributed to defects, impurities, and nonstoichiometry, and somewhat varying positions of the major lattice mode are given in the literature, which also include several Raman studies [9-11]. The recent discovery of high-temperature superconductors in which a key role seems to be played by the copper oxide in the formation of Cooper pairs has renewed the interest in the infrared properties of the copper oxides [12]. Considerably less has been reported about the black cupric oxide CuO. It is also a semiconductor, obviously with a smaller optical band gap than the red oxide. The values reported in the literature vary within the range 1.2-1.5 eV. A wet photoelectrochemical technique was used to obtain an indirect energy gap of 1.35eV [13]. Early measurements on a bulk, HANDBOOK OF OPTICAL CONSTANTS OF SOLIDS II
875
Copyright 9 1991 by Academic Press. All rights of reproduction in any form reserved. ISBN 0-12-544422-2
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Carl G. Ribbing and Arne Roos
polycrystalline sample, sintered in an oxygen atmosphere, resulted in an absorption edge too shallow to define an energy gap [14]. Simultaneous measurements on reactively sputtered films were also used to derive the optical constants without specifying an energy gap [15]. Recent transmittance measurements versus thickness on thin films prepared by rf, reactive sputtering gave an energy gap value of 1.21 _+ .05 eV [16]. Much of the work on the physical properties of the copper oxides has been motivated by various technical applications. Cuprous oxide was one of the very early model semiconductors, and the discovery of the CuzO-Cu rectifier dates back to 1920 [17]. The use of cupric oxide on copper as a photoelectric and/or photoconductive material was suggested even earlier [18]. It is still of technical interest [19] to utilize this metal-oxide system as a possible low-cost solar cell. Oxidizing copper has furthermore been one of the alternatives to prepare the selectively absorbing surface of a thermal solar collector [20]. In this application the black oxide results in higher efficiency owing to its higher absorption of solar radiation. Several methods have been used to oxidize a copper base giving optimum performance in terms of selectivity and stability [21], and the use of a warm mixture of acids has been commercialized [22]. It has been demonstrated that the wetoxidation techniques result in a rough interface between the oxide and the metal, which significantly increases the solar absorption [23, 24]. The ease with which oxidation of copper can take place is partly a corrosion problem, but it is also one of the reasons for the technical interest in this system. Even relatively thick layers of copper can be transformed into either one of the stoichiometric oxides, merely by heating in air. The green hue of old copper roofs is often (but incorrectly) said to be due to copper oxide. Instead, this surface is formed in the presence of water together with some other substances. It consists mainly of copper hydroxysulfate or, especially in coastal areas, copper hydroxychloride. From a basic point of view, the fact that copper is an easily oxidized noble metal with the typical strong tendency for diffusion creates a strong incentive for careful studies of the oxidation mechanism, rate laws, temperature dependence, and so on. The literature on the oxidation of copper is therefore very rich [25-28]. Copper is also a suitable metal for the study of laser-oxidation techniques [29, 30]. The thermal oxidation of copper, in contrast to wet oxidation, produces a rough oxide-metal interface that is n o t absorbing, but causes strong light scattering in the visible and near-infrared regions. The amount of scattering and its conspicous spectral dependence was used to determine the RMS roughness amplitude of both the air-oxide and oxide-metal uncorrelated interfaces [31, 32]. Here the CuzO-Cu served as a model system because of the exceptional magnitude of the light scattering. For reasons made obvious above, there is a wealth of published optical constants for the cuprous oxide in the visible and near-infrared spectral
Copper Oxides (Cu20, CuO)
877
regions and considerably less data for cupric oxide in the same spectral regions. It has not been possible to find published optical constants for either one of the copper oxides in the vacuum UV or soft X-ray regions, however. Tables I and II and Figs. 1 and 2 therefore cover only the near UV, down to 300 nm. The valence bands and the core-level positions have been determined by electron spectroscopy as part of basic investigations about satellite formation in photoelectron spectra [33, 34]. Without knowledge of the numerical values of the optical functions, we can predict that they will exhibit oscillator structures, in the case of CuO around the core levels: O(ls)-530 eV (2=2.34 nm), Cu(2p3/2)-933 eV (1.33 nm) and Cu(2pl/2)- 954 eV (1.30 nm). In Cu20 there is a 1.2 eV chemical shift in the copper levels toward lower binding energy whereas the O(ls) is unchanged. For the cuprous oxide in the visible region, data have been selected for thermally oxidized copper films, which have been prepared in different thicknesses to allow combined R and T measurements. The optical thickness nd was calculated from the position of the interference fringes, and the refractive index was determined from their envelope function in the region of low absorption. The values from Ref. [35] cover the broadest region and are somewhat higher than the data from Refs. [36] (electrodeposited), [37], and [14]. The values given are also marginally higher than those obtained from the dispersion relation [36] n2(2) = 1 + A22/(22- B), which can be used for wavelengths longer than 0.550~m; here, A =4.81 (ktm) -2 and B=0.125 (/tm) 2. At shorter wavelengths this relation gives values that are too high, however. The k values are significantly lower than those obtained for sintered powder [14] and slightly higher than those of Ref. [371. The optical constants given here for cupric oxide have also been determined for thermally oxidized copper films [35]. In the visible part of the spectrum, the n and k values are in good agreement with those of Ref. [3]. In contrast, both functions are significantly lower than those of Refs. [2] and [14] as well as those for rf reactively sputtered films in Ref. [16]. For the cuprous oxide in the infrared, optical reflectance measurements on single crystals [8, 11] or polycrystalline films [9] were performed. Kramers-Kronig analysis of these spectra [11] or additional transmittance measurements on polycrystalline films [8, 9] were used to derive the optical constants in the vicinity of the resonance frequencies. It was found that the Lyddane-Sachs-Teller relation was valid for cuprous oxide, and the dielectric constant e0 is equal to 7.4 in the low-frequency region [11]. The e0 value given by O'Keeffe [8] (e0=6.46) does not take the low-frequency
878
Carl G. Ribbing and Arne Roos
resonance at 146cm -1 into account, but is in excellent agreement with the e= value (eoo- 6.5) for this resonance. It has not been possible to find published values of n and k in the infrared region for cupric oxide. Poling [38] reports absorbance spectra for CuO without giving the n and k values. The oxide exhibits an intense single band around 5 1 0 c m -1. This is the only significant band in the 2 3 0 - 4 0 0 0 c m -1 region. REFERENCES
1. M. Hansen, "Constitution of Binary Alloys," 2nd ed., McGraw Hill, New York, 604 (1958). 2. H. Wieder and A. W. Czanderna, J. Appl. Phys. 37, 184 (1966). 3. P. C. Ladelfe, A. W. Czanderna, and J. R. Biegen, Thin Solid Films 10, 403 (1972). 4. R. Vogel and W. Pocher, Z. Metallkunde 21,333 and 368 (1929). 5. Y. F. Gross, Izvest, Akad. Nauk. S.S.S.R. Ser. 84, 261 and 471 (1952). Translated in Bull. Acad. Sc. (USSR) 20, 79 (1956). 6. J. H. Apfel and L. N. Hadley, Phys. Rev. 100, 1689 (1955). 7. P. W. Baumeister, Phys. Rev. 121,359 (1961). 8. M. O'Keeffe, J. Chem. Phys. 39, 1789 (1963). 9. E. C. Heltemes, Phys. Rev. 141,803 (1966). 10. J. Reydellet, M. Balkanski, and D. Trivich, Phys. Stat. Solidi (b) 52, 175 (1972). 11. P. Dawson, M. M. Hargreave, and G. R. Wilkinson, J. Phys. Chem. Solids 34, 2201 (1973). 12. G. Wendin, Physica Scripta T27, 31 (1989). 13. F. P. Koffyberg and F. A. Benko, J. Appl. Phys. 53, 1173 (1982). 14. T. Tanaka, Jap. J. Appl. Phys. 18, 1043 (1979). 15. V. F. Drobny and D. L. Pulfrey, Thin Solid Films 61, 89 (1979). 16. A. E. Rakhshani and F. K. Barakat, Materials Lett. 6, 37 (1987). 17. L. O. Grondahl, Rev. Mod. Phys. 5, 141 (1933). 18. A. H. Pfund, Phys. Rev. 3, 289 (1916). 19. A. E. Rakhshani, Solid-State Electr. 29, 7 (1986). 20. B. O. Seraphin, in "Solar Energy Conversion~Solid State Physics Aspects," (B. O. Seraphin, ed.), Springer-Verlag, Berlin, (1979). 21. D. J. Close, "Commonwealth Scientific and Industrial Research Organization," Australia, Engineering section, Report E. D. 7, (1962). 22. "Ebonol" of Enthone, Inc., New Haven, Conn. 23. A. Roos, T. Chibuye, and B. Karlsson, Solar Energy Mat. 7,453 (1983). 24. A. Roos and B. Karlsson, Solar Energy Mat. 7,467 (1983). 25. A. R6nnquist and H. Fischmeister, J. Inst. Metals 89, 65 (1960-1961). 26. J. V. Cathcart, G. F. Petersen, and C. J. Sparks, in "Surfaces and Interfaces," Vol. 1, (J. J. Burke, ed.) Syracuse University Press, New York, 333 (1967). 27. C. Tsiranovits, J. G. Antonopoulos, and J. Stoemenos, Thin Solid Films 71,133 (1980). 28. S. K. Roy and C. Sircar, Oxidation of Metals 15, 9 (1981). 29. L. Baufay, F. A. Houle, and R. J. Wilson, J. Appl. Phys. 61, 4640 (1987). 30. M. Wautelet and R. Andrew, Phil. Mag. B 55, 261 (1987). 31. A. Roos, M. Bergkvist, and C. G. Ribbing, Appl. Opt. 28, 1360 (1989). 32. M. Bergkvist, A. Roos, C. G. Ribbing, J. M. Bennett, and L. Mattsson Appl. Opt., 28, 3902 (1989). 33. A. Rosencwaig and G. K. Wertheim, J. Electron Spectrosc. Relat. Phenom. 1,493 (1972/ 73).
879
C o p p e r O x i d e s ( C u 2 0 , CuO)
34. H. Siegbahn and L. Karlsson, in "Handbuch der Physik," (W. Mehlhorn, ed), Vol. xxxi, Springer-Verlag, Berlin, 215 (1982). 35. B. Karlsson, C. G. Ribbing, A. Roos, E. Valkonen, and T. Karlsson, Physica Scripta 25, 826 (1982). 36. A. E. Rakhshani, J. Appl. Phys. 62, 1528 (1987). 37. K. B~rwinkel and H. J. Schmidt, Thin Solid Films 59, 373 (1979). 38. G. W. Poling, J. Electrochem. Soc. 116, 958 (1969). 39. Interpolated values of n.
,
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i
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Fig. 1. Log-log plot of n (solid line) and k (dashed line) versus wavelength in micrometers for cuprous oxide (Cu20).
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Fig. 2. Log-log plot of n (solid line) and k (dashed line) versus wavelength in micrometers for cupric oxide (CuO).
880
Carl G. Ribbing and Arne Roos
TABLE I Values of n and k for Cuprous Oxide Obtained from Various References a
eV 4.133 3.542 3.100 2.756 2.480 2.255 2.067 1.908 1.771 1.653 1.550 1.459 1.378 1.305 1.240 1.127 1.033 0.954 0.886 0.827 0.620 0.496 0.413 0.310 0.248 0.207 0.177 0.155 0.138 0.124 0.113 0.103 0.0954 0.0886 0.0827 0.0800 0.0785 0.0775 0.0761 0.0752 0.0729
cm- 1 33,330 28,570 25,000 22,220 20,000 18,180 16,670 15,380 14,290 13,330 12,500 11,770 11,110 10,530 10,000 9,090 8,330 7,690 7,110 6,670 5,000 4,000 3,333 2,500 2,000 1,667 1,429 1,250 1,111 1,000 909.1 833.3 769.2 714.3 666.7 645.2 632.9 625.0 613.5 606.1 588.2
a References are indicated in brackets
btm 0.3 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.10 1.20 1.30 1.40 1.50 2.00 2.5 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 15.5 15.8 16.0 16.3 16.5 17.0
n 2.0 [35] 2.40 2.80 3.06 3.12 3.10 3.02 2.90 2.83 2.77 2.70 2.66 2.63 2.61 2.60 2.59 2.58 2.57 2.57 2.57 2.56 2.56 2.55 [39] 2.54 2.53 2.52 2.51 2.50 [8] 2.48 2.46 2.42 2.37 2.29 2.15 1.79 1.13 0.92 1.09 2.94 4.71 3.78
k
1.85 [35] 1.44
0.99 0.60 0.35 0.19 0.13 0.10 0.083 0.070 0.060 0.053 0.048 0.043 0.040 0.033 0.027 0.021 0.017 0.013 0.002
0.04 [8] 0.13 0.46 1.68
2.52 3.49 2.27 0.42
(continued)
Copper Oxides (Cu20, CuO)
881
TABLE I (Continued)
Cuprous Oxide (CuzO) eV
0.0709 0.0689 0.0653 0.0620 0.0590 0.0564 0.0539 0.0517 0.0496 0.0413 0.0354 0.0310 0.0273 0.0248 0.0223 0.0211 0.0198 0.0192 0.0188 0.0186 0.01844 0.01823 0.01819 0.01810 0.01786 0.01761 0.01736 0.0161 0.0149 0.0136 0.0124 0.00992
cm
-1
571.4 555.6 526.3 500.0 476.2 454.5 434.7 416.7 400.0 333 286 250 220 200 180 170 160 155 152 150 148.7 147 146.7 146 144 142 140 130 120 110 100 80
a References are indicated in brackets.
~tm
17.5 18.0 19.0 20.0 21.0 22.0 23.0 24.0 25.0 30.0 35.0 40.0 45.5 50.0 55.56 58.82 62.50 64.52 65.79 66.67 67.25 68.03 68.17 68.49 69.44 70.42 71.43 76.9 83.3 90.9 100.0 125.0
3.42 3.21 2.98 2.89 2.82 2.79 2.77 2.76 2.75 2.75 [39] 2.75 2.75 2.75 2.74 2.72 [111 2.66 2.42 2.18 1.81 1.19 0.89 3.37 4.56 5.56 3.97 3.47 3.21 2.98 2.92 2.84
0.17 0.08 0.04 0
0.005 [9] 0.006 0.006 0.009 0.017 0.05 [11] 0.10 0.59
1.88 3.97 4.37 2.78 0.20 0.08 0.04 0.013 [9] 0.010 0.008 0.007 0.005
882
Carl G. Ribbing and Arne Roos
TABLE II Values of n and k for Cupric Oxide Obtained from Various References a eV
4.133 3.542 3.100 2.756 2.480 2.255 2.067 1.908 1.771 1.653 1.550 1.459 1.378 1.305 1.240 1.127 1.033 0.954 0.886 0.827 0.620 0.496
cm- 1
33,330 28,570 25,000 22,220 20,000 18,180 16,670 15,380 14,290 13,330 12,500 11,770 11,110 10,530 10,000 9,090 8,330 7,690 7,110 6,670 5,000 4,000
a References are indicated in brackets.
btm
n
0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.10 1.20 1.30 1.40 1.50 2.00 2.5
2.18 [35] 2.24 2.3 4 2.45 2.54 2.58 2.65 2.72 2.88 2.97 2.94 2.81 2.74 2.69 2.65 2.61 2.58 2.57 2.56 2.56 2.55 2.55
k
1.50 [35] 1.03 0.87 0.77 0.68 0.59 0.50 0.40 0.31 0.22 0.11 0.04 0.03 0.02 0.01
The established stoichometric copper oxides, cuprous oxide, Cu20 ("red") and cupric oxide, CuO ("black"), form cubic and monoclinic unit cells in their respective equilibrium crystalline forms [1]. Reports on the phase CuO.6v, described as a gross defect structure of Cu20, are found in the literature [2, 3], but will not be considered here. It has furthermore been claimed that Cu20 is actually not an equilibrium structure at temperatures below 375 ~ and therefore decomposes very slowly at room temperature into metallic copper and cupric oxide [4]. The Cu-O phase diagram is markedly pressure dependent, such that a CuO layer transforms into Cu20 and eventually into metallic copper when placed in vacuum. Cuprous oxide is a semiconductor with a direct band gap of 2.17 eV. It has been the object of several basic studies concerning optical-band-gap absorption, its temperature dependence, and the interpretation of the lowtemperature absorption line spectrum below the gap as transitions to excitonic levels [5-7]. The partly ionic character of the bonding causes structures in the optical functions in the infrared spectral region. The strongest IR-active mode, corresponding to the residual-ray region, was located at 609cm -~ (16.4~m) [8]. A secondary lattice mode at 146cm -~ (68~m) is reported for polycrystalline samples [9]. Several other lattice modes have been identified in later work and attributed to defects, impurities, and nonstoichiometry, and somewhat varying positions of the major lattice mode are given in the literature, which also include several Raman studies [9-11]. The recent discovery of high-temperature superconductors in which a key role seems to be played by the copper oxide in the formation of Cooper pairs has renewed the interest in the infrared properties of the copper oxides [12]. Considerably less has been reported about the black cupric oxide CuO. It is also a semiconductor, obviously with a smaller optical band gap than the red oxide. The values reported in the literature vary within the range 1.2-1.5 eV. A wet photoelectrochemical technique was used to obtain an indirect energy gap of 1.35eV [13]. Early measurements on a bulk, HANDBOOK OF OPTICAL CONSTANTS OF SOLIDS II
875
Copyright 9 1991 by Academic Press. All rights of reproduction in any form reserved. ISBN 0-12-544422-2
876
Carl G. Ribbing and Arne Roos
polycrystalline sample, sintered in an oxygen atmosphere, resulted in an absorption edge too shallow to define an energy gap [14]. Simultaneous measurements on reactively sputtered films were also used to derive the optical constants without specifying an energy gap [15]. Recent transmittance measurements versus thickness on thin films prepared by rf, reactive sputtering gave an energy gap value of 1.21 _+ .05 eV [16]. Much of the work on the physical properties of the copper oxides has been motivated by various technical applications. Cuprous oxide was one of the very early model semiconductors, and the discovery of the CuzO-Cu rectifier dates back to 1920 [17]. The use of cupric oxide on copper as a photoelectric and/or photoconductive material was suggested even earlier [18]. It is still of technical interest [19] to utilize this metal-oxide system as a possible low-cost solar cell. Oxidizing copper has furthermore been one of the alternatives to prepare the selectively absorbing surface of a thermal solar collector [20]. In this application the black oxide results in higher efficiency owing to its higher absorption of solar radiation. Several methods have been used to oxidize a copper base giving optimum performance in terms of selectivity and stability [21], and the use of a warm mixture of acids has been commercialized [22]. It has been demonstrated that the wetoxidation techniques result in a rough interface between the oxide and the metal, which significantly increases the solar absorption [23, 24]. The ease with which oxidation of copper can take place is partly a corrosion problem, but it is also one of the reasons for the technical interest in this system. Even relatively thick layers of copper can be transformed into either one of the stoichiometric oxides, merely by heating in air. The green hue of old copper roofs is often (but incorrectly) said to be due to copper oxide. Instead, this surface is formed in the presence of water together with some other substances. It consists mainly of copper hydroxysulfate or, especially in coastal areas, copper hydroxychloride. From a basic point of view, the fact that copper is an easily oxidized noble metal with the typical strong tendency for diffusion creates a strong incentive for careful studies of the oxidation mechanism, rate laws, temperature dependence, and so on. The literature on the oxidation of copper is therefore very rich [25-28]. Copper is also a suitable metal for the study of laser-oxidation techniques [29, 30]. The thermal oxidation of copper, in contrast to wet oxidation, produces a rough oxide-metal interface that is n o t absorbing, but causes strong light scattering in the visible and near-infrared regions. The amount of scattering and its conspicous spectral dependence was used to determine the RMS roughness amplitude of both the air-oxide and oxide-metal uncorrelated interfaces [31, 32]. Here the CuzO-Cu served as a model system because of the exceptional magnitude of the light scattering. For reasons made obvious above, there is a wealth of published optical constants for the cuprous oxide in the visible and near-infrared spectral
Copper Oxides (Cu20, CuO)
877
regions and considerably less data for cupric oxide in the same spectral regions. It has not been possible to find published optical constants for either one of the copper oxides in the vacuum UV or soft X-ray regions, however. Tables I and II and Figs. 1 and 2 therefore cover only the near UV, down to 300 nm. The valence bands and the core-level positions have been determined by electron spectroscopy as part of basic investigations about satellite formation in photoelectron spectra [33, 34]. Without knowledge of the numerical values of the optical functions, we can predict that they will exhibit oscillator structures, in the case of CuO around the core levels: O(ls)-530 eV (2=2.34 nm), Cu(2p3/2)-933 eV (1.33 nm) and Cu(2pl/2)- 954 eV (1.30 nm). In Cu20 there is a 1.2 eV chemical shift in the copper levels toward lower binding energy whereas the O(ls) is unchanged. For the cuprous oxide in the visible region, data have been selected for thermally oxidized copper films, which have been prepared in different thicknesses to allow combined R and T measurements. The optical thickness nd was calculated from the position of the interference fringes, and the refractive index was determined from their envelope function in the region of low absorption. The values from Ref. [35] cover the broadest region and are somewhat higher than the data from Refs. [36] (electrodeposited), [37], and [14]. The values given are also marginally higher than those obtained from the dispersion relation [36] n2(2) = 1 + A22/(22- B), which can be used for wavelengths longer than 0.550~m; here, A =4.81 (ktm) -2 and B=0.125 (/tm) 2. At shorter wavelengths this relation gives values that are too high, however. The k values are significantly lower than those obtained for sintered powder [14] and slightly higher than those of Ref. [371. The optical constants given here for cupric oxide have also been determined for thermally oxidized copper films [35]. In the visible part of the spectrum, the n and k values are in good agreement with those of Ref. [3]. In contrast, both functions are significantly lower than those of Refs. [2] and [14] as well as those for rf reactively sputtered films in Ref. [16]. For the cuprous oxide in the infrared, optical reflectance measurements on single crystals [8, 11] or polycrystalline films [9] were performed. Kramers-Kronig analysis of these spectra [11] or additional transmittance measurements on polycrystalline films [8, 9] were used to derive the optical constants in the vicinity of the resonance frequencies. It was found that the Lyddane-Sachs-Teller relation was valid for cuprous oxide, and the dielectric constant e0 is equal to 7.4 in the low-frequency region [11]. The e0 value given by O'Keeffe [8] (e0=6.46) does not take the low-frequency
878
Carl G. Ribbing and Arne Roos
resonance at 146cm -1 into account, but is in excellent agreement with the e= value (eoo- 6.5) for this resonance. It has not been possible to find published values of n and k in the infrared region for cupric oxide. Poling [38] reports absorbance spectra for CuO without giving the n and k values. The oxide exhibits an intense single band around 5 1 0 c m -1. This is the only significant band in the 2 3 0 - 4 0 0 0 c m -1 region. REFERENCES
1. M. Hansen, "Constitution of Binary Alloys," 2nd ed., McGraw Hill, New York, 604 (1958). 2. H. Wieder and A. W. Czanderna, J. Appl. Phys. 37, 184 (1966). 3. P. C. Ladelfe, A. W. Czanderna, and J. R. Biegen, Thin Solid Films 10, 403 (1972). 4. R. Vogel and W. Pocher, Z. Metallkunde 21,333 and 368 (1929). 5. Y. F. Gross, Izvest, Akad. Nauk. S.S.S.R. Ser. 84, 261 and 471 (1952). Translated in Bull. Acad. Sc. (USSR) 20, 79 (1956). 6. J. H. Apfel and L. N. Hadley, Phys. Rev. 100, 1689 (1955). 7. P. W. Baumeister, Phys. Rev. 121,359 (1961). 8. M. O'Keeffe, J. Chem. Phys. 39, 1789 (1963). 9. E. C. Heltemes, Phys. Rev. 141,803 (1966). 10. J. Reydellet, M. Balkanski, and D. Trivich, Phys. Stat. Solidi (b) 52, 175 (1972). 11. P. Dawson, M. M. Hargreave, and G. R. Wilkinson, J. Phys. Chem. Solids 34, 2201 (1973). 12. G. Wendin, Physica Scripta T27, 31 (1989). 13. F. P. Koffyberg and F. A. Benko, J. Appl. Phys. 53, 1173 (1982). 14. T. Tanaka, Jap. J. Appl. Phys. 18, 1043 (1979). 15. V. F. Drobny and D. L. Pulfrey, Thin Solid Films 61, 89 (1979). 16. A. E. Rakhshani and F. K. Barakat, Materials Lett. 6, 37 (1987). 17. L. O. Grondahl, Rev. Mod. Phys. 5, 141 (1933). 18. A. H. Pfund, Phys. Rev. 3, 289 (1916). 19. A. E. Rakhshani, Solid-State Electr. 29, 7 (1986). 20. B. O. Seraphin, in "Solar Energy Conversion~Solid State Physics Aspects," (B. O. Seraphin, ed.), Springer-Verlag, Berlin, (1979). 21. D. J. Close, "Commonwealth Scientific and Industrial Research Organization," Australia, Engineering section, Report E. D. 7, (1962). 22. "Ebonol" of Enthone, Inc., New Haven, Conn. 23. A. Roos, T. Chibuye, and B. Karlsson, Solar Energy Mat. 7,453 (1983). 24. A. Roos and B. Karlsson, Solar Energy Mat. 7,467 (1983). 25. A. R6nnquist and H. Fischmeister, J. Inst. Metals 89, 65 (1960-1961). 26. J. V. Cathcart, G. F. Petersen, and C. J. Sparks, in "Surfaces and Interfaces," Vol. 1, (J. J. Burke, ed.) Syracuse University Press, New York, 333 (1967). 27. C. Tsiranovits, J. G. Antonopoulos, and J. Stoemenos, Thin Solid Films 71,133 (1980). 28. S. K. Roy and C. Sircar, Oxidation of Metals 15, 9 (1981). 29. L. Baufay, F. A. Houle, and R. J. Wilson, J. Appl. Phys. 61, 4640 (1987). 30. M. Wautelet and R. Andrew, Phil. Mag. B 55, 261 (1987). 31. A. Roos, M. Bergkvist, and C. G. Ribbing, Appl. Opt. 28, 1360 (1989). 32. M. Bergkvist, A. Roos, C. G. Ribbing, J. M. Bennett, and L. Mattsson Appl. Opt., 28, 3902 (1989). 33. A. Rosencwaig and G. K. Wertheim, J. Electron Spectrosc. Relat. Phenom. 1,493 (1972/ 73).
879
C o p p e r O x i d e s ( C u 2 0 , CuO)
34. H. Siegbahn and L. Karlsson, in "Handbuch der Physik," (W. Mehlhorn, ed), Vol. xxxi, Springer-Verlag, Berlin, 215 (1982). 35. B. Karlsson, C. G. Ribbing, A. Roos, E. Valkonen, and T. Karlsson, Physica Scripta 25, 826 (1982). 36. A. E. Rakhshani, J. Appl. Phys. 62, 1528 (1987). 37. K. B~rwinkel and H. J. Schmidt, Thin Solid Films 59, 373 (1979). 38. G. W. Poling, J. Electrochem. Soc. 116, 958 (1969). 39. Interpolated values of n.
,
,
~ i~,,
I
'
'
; I'"'I
'
'
' I .''+ -
m
\
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-:-
\ /
: -
/ \
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\
I
I II fl ii
_
/
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_ -
I
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\
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I
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ji /
_ _
_ _ _
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\
I I J
,
,l;,Jll
i
,
,l,,lll
,
I0 ~ I01 WAVELENGTH
i
illl,il
i
102
i
il,,
ii
103
(,um)
Fig. 1. Log-log plot of n (solid line) and k (dashed line) versus wavelength in micrometers for cuprous oxide (Cu20).
_ _
I0 o :_
~,,\ \
,,,r -
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\ \
/
-
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-
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~ ,,J,,, 10 I (/u. m)
Fig. 2. Log-log plot of n (solid line) and k (dashed line) versus wavelength in micrometers for cupric oxide (CuO).
880
Carl G. Ribbing and Arne Roos
TABLE I Values of n and k for Cuprous Oxide Obtained from Various References a
eV 4.133 3.542 3.100 2.756 2.480 2.255 2.067 1.908 1.771 1.653 1.550 1.459 1.378 1.305 1.240 1.127 1.033 0.954 0.886 0.827 0.620 0.496 0.413 0.310 0.248 0.207 0.177 0.155 0.138 0.124 0.113 0.103 0.0954 0.0886 0.0827 0.0800 0.0785 0.0775 0.0761 0.0752 0.0729
cm- 1 33,330 28,570 25,000 22,220 20,000 18,180 16,670 15,380 14,290 13,330 12,500 11,770 11,110 10,530 10,000 9,090 8,330 7,690 7,110 6,670 5,000 4,000 3,333 2,500 2,000 1,667 1,429 1,250 1,111 1,000 909.1 833.3 769.2 714.3 666.7 645.2 632.9 625.0 613.5 606.1 588.2
a References are indicated in brackets
btm 0.3 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.10 1.20 1.30 1.40 1.50 2.00 2.5 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 15.5 15.8 16.0 16.3 16.5 17.0
n 2.0 [35] 2.40 2.80 3.06 3.12 3.10 3.02 2.90 2.83 2.77 2.70 2.66 2.63 2.61 2.60 2.59 2.58 2.57 2.57 2.57 2.56 2.56 2.55 [39] 2.54 2.53 2.52 2.51 2.50 [8] 2.48 2.46 2.42 2.37 2.29 2.15 1.79 1.13 0.92 1.09 2.94 4.71 3.78
k
1.85 [35] 1.44
0.99 0.60 0.35 0.19 0.13 0.10 0.083 0.070 0.060 0.053 0.048 0.043 0.040 0.033 0.027 0.021 0.017 0.013 0.002
0.04 [8] 0.13 0.46 1.68
2.52 3.49 2.27 0.42
(continued)
Copper Oxides (Cu20, CuO)
881
TABLE I (Continued)
Cuprous Oxide (CuzO) eV
0.0709 0.0689 0.0653 0.0620 0.0590 0.0564 0.0539 0.0517 0.0496 0.0413 0.0354 0.0310 0.0273 0.0248 0.0223 0.0211 0.0198 0.0192 0.0188 0.0186 0.01844 0.01823 0.01819 0.01810 0.01786 0.01761 0.01736 0.0161 0.0149 0.0136 0.0124 0.00992
cm
-1
571.4 555.6 526.3 500.0 476.2 454.5 434.7 416.7 400.0 333 286 250 220 200 180 170 160 155 152 150 148.7 147 146.7 146 144 142 140 130 120 110 100 80
a References are indicated in brackets.
~tm
17.5 18.0 19.0 20.0 21.0 22.0 23.0 24.0 25.0 30.0 35.0 40.0 45.5 50.0 55.56 58.82 62.50 64.52 65.79 66.67 67.25 68.03 68.17 68.49 69.44 70.42 71.43 76.9 83.3 90.9 100.0 125.0
3.42 3.21 2.98 2.89 2.82 2.79 2.77 2.76 2.75 2.75 [39] 2.75 2.75 2.75 2.74 2.72 [111 2.66 2.42 2.18 1.81 1.19 0.89 3.37 4.56 5.56 3.97 3.47 3.21 2.98 2.92 2.84
0.17 0.08 0.04 0
0.005 [9] 0.006 0.006 0.009 0.017 0.05 [11] 0.10 0.59
1.88 3.97 4.37 2.78 0.20 0.08 0.04 0.013 [9] 0.010 0.008 0.007 0.005
882
Carl G. Ribbing and Arne Roos
TABLE II Values of n and k for Cupric Oxide Obtained from Various References a eV
4.133 3.542 3.100 2.756 2.480 2.255 2.067 1.908 1.771 1.653 1.550 1.459 1.378 1.305 1.240 1.127 1.033 0.954 0.886 0.827 0.620 0.496
cm- 1
33,330 28,570 25,000 22,220 20,000 18,180 16,670 15,380 14,290 13,330 12,500 11,770 11,110 10,530 10,000 9,090 8,330 7,690 7,110 6,670 5,000 4,000
a References are indicated in brackets.
btm
n
0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.10 1.20 1.30 1.40 1.50 2.00 2.5
2.18 [35] 2.24 2.3 4 2.45 2.54 2.58 2.65 2.72 2.88 2.97 2.94 2.81 2.74 2.69 2.65 2.61 2.58 2.57 2.56 2.56 2.55 2.55
k
1.50 [35] 1.03 0.87 0.77 0.68 0.59 0.50 0.40 0.31 0.22 0.11 0.04 0.03 0.02 0.01