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Sputtered metal carbide and metal silicide solar absorbing surfaces
Thin Solid Films, 57 ( 1979) 3 15-320 $3 Elsevier Sequoia S.A.. Lausanne
SPUTTERED ABSORBING I. T. RITCHIE?
315
Printed In 11x Netlwlands
METAL CARBIDE SURFACES*
AND METAL
SILICIDE
SOLAR
AND G. L. HARDING
School of Physics, University of Sydney, Sydney, N.S. W. 2006 (Ausrralia) (Received
August
7, 1978 ; accepted
September
14, 1978)
Thin transition metal films reactively sputtered in Ar + CH, or Ar + SiH, show promise as solar selective absorbers that are stable at high temperatures in vacuum. An examination of the structure of these films shows that they consist of closely packed metallic particles that are electrically isolated from each other as in cermets. The optical properties of reactively sputtered Fe-C films are discussed. Multilayer and graded films produced by reactive sputtering can show considerably better absorptances than equivalent uniform films without their thermal emittances being seriously affected. Solar absorptances of 0.90-0.95 are routinely achieved, films with these absorptances having room temperature emittances of 0.03-0.04. However, degradation of the films may occur at high temperatures and may lead to changes in their solar absorptances.
1. INTRODUCTION
At present there is considerable interest in the production and properties of solar selectively absorbing surfaces for use in all-glass vacuum-insulated solar energy collectors which deliver heat in the 100-200 “C range. The refractory nature and low vapour pressure of metal-carbon and metalof high silicon systems’ 32 suggest the suitability of these systems in the construction temperature stable selective surfaces. Previous work has been reported on the preparation of uniform metal carbide3 and metal silicide4 films on copper substrates by high pressure d.c. reactive sputtering in CH, or SiH,. These uniform films show excellent stability in vacuum at high temperatures. It has also been shown4-’ that the grading of these absorber films so that they are metallic near the substrate and dielectric at the air-film interface may increase their solar absorptances substantially without seriously affecting their thermal emittances. In this paper the structure and optical properties of high pressure d.c. reactively sputtered Fe-C films are discussed. The solar selective properties and the thermal
*Paper presented at the Fourth International Congress on Thin Films, Loughborough, Cit. Britain, September 1 l-15,1978; Paper 2B10. TPresent address: Laboratoire d’Etudes des Materiaux Minces, Centre d’Etudes Nucleaires de Grenoble, 85X, 38041 Grenoble Ctdex, France.
316
I.T. RITCHIE,G.L. HARDING
stability of graded metal-carbon and metal-silicon films produced in a high pressure dc. discharge and of graded metal-carbon films produced in a low pressure d.c. magnetron discharge are also described. 2. PROPERTIESOFUNIFORM
Fe-C FILMS
Several Fe-C films were prepared by the reactive sputtering of iron in an Ar + CH, gas mixture in a standard d.c. diode’sputtering system which has been described previously’. Typically films were sputtered at a pressure of 45 Pa with an accelerating voltage of 1 kV. Various CH4 flow rates were used to produce a series of Fe-C films, each about 100 nm thick, on glass and evaporated gold substrates. The electrical resistance of the films varied from 640 Q/o (with CH, at 0.5% of the argon flow rate) to 4.2 MQ/U (with CH, at 2% of the argon flow rate). The thickness of each film was determined to within 3 nm by a Talystep measurement of a scratch in the film deposited on glass. The optical properties of the films were determined by measuring the reflectance of each film on glass and on gold and the transmittance of the film on glass as functions of wavelength. These data and the film thicknesses were used to perform a regression on the relevant Fresnel equations to obtain the refractive index n and the extinction coefficient k as functions of wavelength. Figure 1 shows the wavelength dependences of n and k for three Fe-C films.
A lpml
Fig. 1. The wavelength dependences kQ/O (0) and 4.2 MQ/O (#.
of n and k for Fe-C films with sheet resistances
of 640 Q/O (B). 14.9
Transmission electron micrographs of the Fe-C films scraped off glass substrates showed the particulate nature of the films; there appeared to be voids between the particles’. Further evidence of the particulate nature of the films was
METAL
CARBIDE AND METALSILICIDE SOLAR ABSORBINGSURFACES
317
given by measurements of the electrical resistance as a function of temperature. Abeles’ has shown that films composed of metallic particles which are electrically isolated from each other should have negative temperature coefficients of resistance due to electron tunnelling between the particles and that they should obey an equation of the form R = R,exp(~/T”~). The three films mentioned above all showed this behaviour’. If it is assumed that the films are composed of isolated metallic particles in a dielectric matrix, then the behaviour of their optical constants can be explained by assuming that as the films become more dielectric the volume fraction of metal in the composite decreases. Since the low resistance films are metallic, the volume fraction ofmetal in these films should be quite high. Consequently an extension developed by McKenzie and McPhedran’ of the Maxwell-Garnett theory for the optical properties of regular lattices of composite materials was used to model these films. This theory takes into account the effects of the proximity of particles which become important for high volume fractions of metalg.
k
Fig. 2. The wavelength dependences of n and k for simple cubic iron spheres in an air dielectric for volume fractions of iron of 0.55.0.45 and 0.35.
Figure 2 shows the wavelength dependences of n and k for simple cubic-packed iron spheres embedded in an air dielectric for volume fractions of iron of 0.55,0.45 and 0.35. (For computational simplicity only poles up to order 2’ were considered in these calculations; this shifts the divergence for touching spheres from 0.52 to 0.59 for a simple cubic lattice.) The refractive indices of iron given by Johnson and Christy” were used. There is good agreement between the experimental and the calculated wavelength dependences of n and k for the films and the agreement improves as the
318
I. T. RITCHIE, G. L. HARDING
films become more dielectric. The magnitudes of n and k in the modelled composites are in general lower than the experimental values since an air dielectric was used in the model. The use of a dielectric other than air should not alter the spectral agreement between the model and the experimental films. 3. GRADED REFRACTIVE INDEX FILMS Graded refractive index films can be readily deposited in a sputtering process by altering the amount of reactive gas (CH,, SiH, etc.) present in the discharge as a function of time4*6*7. To produce films that will be suitable as solar absorbers the reactive gas flow should be increased during the deposition so that the absorber will be metallic near the highly reflective substrate and dielectric towards the top of the film5. Films of this type were produced by sputtering transition metals in Ar + CH, and Ar + SiH, atmospheres :In the high pressure d.c. sputtering system used to make the Fe-C films described in Section 2 and by sputtering these metals in a low pressure d.c. magnetron discharge in an Ar + C,H, atmosphere. The details of these procedures are described elsewhere43 ‘. ’ ‘. The best films produced by each of these three processes had similar optical properties: solar absorptances were typically 0.92-0.94 and room temperature emittances were 0.03-0.04. Figure 3 shows the reflectance as a function of wavelength for a graded Fe-C film deposited at high pressure on a copper substrate. Graded metal-silicon films and metal-carbon films deposited at low pressure all had similar reflectance spectra. Also shown in Fig. 3 is the predicted reflectance spectrum of a cermet of iron spheres in an air dielectric on a copper substrate with a volume fraction of iron decreasing from 0.59 at the substrate to zero at the air-film interface. The agreement between this model and the experimental spectrum is good. The wavelength dependence of the reflectance for a uniform Fe-C film on copper3 is also shown.
06-
R OL -
OZ--.
o-
3
5
10 X
1.5
2.0
3.0
(pm)
Fig. 3. The reflectance vs. wavelength spectrum for a graded Fe-C film on copper (---_) compared with that of a graded particulate iron film on copper (- -) and that of a uniform Fe-C film on copper (- - -).
Graded films produced by the three sputtering processes actively pumped vacuum ( z 10m3 Pa) at various temperatures
were annealed to investigate
in an their
METAL CARBIIIE
AN11 METAI. SILICIDE
SOLAR ABSORBING
thermal stability. The results of this annealing quite different behaviours for the three films.
319
SURFACES
are summarized
in Table I and show
TABLE I Method offilm production
Initial values of
Final values of’ e300
80hat3OO”C
0.83
0.024
0.032
lOOhat2OO”C; 2OOhat3OO”C; lOOhat4OO”C; lOOhat5OO”C
0.935 0.93 0.93 0.915
0.032 0.030 0.030 0.030
0.035
1250 h at 400 “C
0.89
0.040
eJoo
Fe in Ar + CH,, high pressure d.c. diode’
0.93
0.025
Ti in Ar + SiH,, high pressure dc. diode4
0.935
316 stainless steel in Ar + C,H,, low pressure dc. magnetron”
0.92
4.
Annealing treatment
a,
0s
DISCUSSION
The spectra of the initial solar absorptances, emittances and reflectances as functions of wavelength are similar for the three films and it is likely that the mode1 developed for the Fe-C films, i.e. electrically isolated granular metal particles, is applicable to the other types of film. However, the behaviours of the three films on annealing are quite different. The annealing of graded selective absorbers has two important effects: interdiffusion takes place between the absorber and the reflector layers; the grading profile of the absorber film can alter owing to phase transitions and grain growth in this layer. Both these effects may change the solar absorptance and emittance of a graded selective surface and it is the rate at which the effects occur which determines the stability of the surface. From the annealing results it is not clear which is the most satisfactory combination of transition metal, reactive gas and sputtering regime for producing stable graded films. Further research is necessary to determine the degradation modes of graded films and the most stable grading profiles for these films. ACKNOWLEDGMENT
Financial support for the work described here was given by the New South Wales State Government through the Science. Foundation for Physics in the University of Sydney. REFERENCES
1 2 3 4
A. Goldsmith, T. Waterman and H. Materials, Vol. IV, Macmillan, New G. V. Samsonov, Handbook of High York, 1964. G. L. Harding, J. Vat. Sci. Technol., G. L. Harding, J. Vuc. Sci. Technol.,
Hirschhorn, Handbook of Thermophysical Properties ofSolid York, 1961. Temperature Materials No. 2, Properties Index, Plenum, New 13 (1976) 1070. I5 (1978) 65.
320
5 6 7 8 9 10 11
1.T. RITCHIE, G. L. HARDING
I. T. Ritchie and B. Window, Appl. Opr., 16 (1977) 1438. G. L. Harding and 1. T. Ritchie, Proc. In,. Solur Energy Congr., New Delhi, January 16-21, 197% Pergamon. Oxford, to be published. I. T. Ritchie, submitted to J. Vuc. Sci. Technol. B. Abeles, Appl. Solid Stare Sci., 6 (1976) 46. D. R. McKedzie and R. C. McPhedran, Nature (London), 265 (1977) 128. P. B. Johnson and R. W. Christy, Phys. Rev., Sect. E, 9 (1974) 5056. G. L. Harding and S. Craig, J. VW. Sci. Techno/., to be published.
SPUTTERED ABSORBING I. T. RITCHIE?
315
Printed In 11x Netlwlands
METAL CARBIDE SURFACES*
AND METAL
SILICIDE
SOLAR
AND G. L. HARDING
School of Physics, University of Sydney, Sydney, N.S. W. 2006 (Ausrralia) (Received
August
7, 1978 ; accepted
September
14, 1978)
Thin transition metal films reactively sputtered in Ar + CH, or Ar + SiH, show promise as solar selective absorbers that are stable at high temperatures in vacuum. An examination of the structure of these films shows that they consist of closely packed metallic particles that are electrically isolated from each other as in cermets. The optical properties of reactively sputtered Fe-C films are discussed. Multilayer and graded films produced by reactive sputtering can show considerably better absorptances than equivalent uniform films without their thermal emittances being seriously affected. Solar absorptances of 0.90-0.95 are routinely achieved, films with these absorptances having room temperature emittances of 0.03-0.04. However, degradation of the films may occur at high temperatures and may lead to changes in their solar absorptances.
1. INTRODUCTION
At present there is considerable interest in the production and properties of solar selectively absorbing surfaces for use in all-glass vacuum-insulated solar energy collectors which deliver heat in the 100-200 “C range. The refractory nature and low vapour pressure of metal-carbon and metalof high silicon systems’ 32 suggest the suitability of these systems in the construction temperature stable selective surfaces. Previous work has been reported on the preparation of uniform metal carbide3 and metal silicide4 films on copper substrates by high pressure d.c. reactive sputtering in CH, or SiH,. These uniform films show excellent stability in vacuum at high temperatures. It has also been shown4-’ that the grading of these absorber films so that they are metallic near the substrate and dielectric at the air-film interface may increase their solar absorptances substantially without seriously affecting their thermal emittances. In this paper the structure and optical properties of high pressure d.c. reactively sputtered Fe-C films are discussed. The solar selective properties and the thermal
*Paper presented at the Fourth International Congress on Thin Films, Loughborough, Cit. Britain, September 1 l-15,1978; Paper 2B10. TPresent address: Laboratoire d’Etudes des Materiaux Minces, Centre d’Etudes Nucleaires de Grenoble, 85X, 38041 Grenoble Ctdex, France.
316
I.T. RITCHIE,G.L. HARDING
stability of graded metal-carbon and metal-silicon films produced in a high pressure dc. discharge and of graded metal-carbon films produced in a low pressure d.c. magnetron discharge are also described. 2. PROPERTIESOFUNIFORM
Fe-C FILMS
Several Fe-C films were prepared by the reactive sputtering of iron in an Ar + CH, gas mixture in a standard d.c. diode’sputtering system which has been described previously’. Typically films were sputtered at a pressure of 45 Pa with an accelerating voltage of 1 kV. Various CH4 flow rates were used to produce a series of Fe-C films, each about 100 nm thick, on glass and evaporated gold substrates. The electrical resistance of the films varied from 640 Q/o (with CH, at 0.5% of the argon flow rate) to 4.2 MQ/U (with CH, at 2% of the argon flow rate). The thickness of each film was determined to within 3 nm by a Talystep measurement of a scratch in the film deposited on glass. The optical properties of the films were determined by measuring the reflectance of each film on glass and on gold and the transmittance of the film on glass as functions of wavelength. These data and the film thicknesses were used to perform a regression on the relevant Fresnel equations to obtain the refractive index n and the extinction coefficient k as functions of wavelength. Figure 1 shows the wavelength dependences of n and k for three Fe-C films.
A lpml
Fig. 1. The wavelength dependences kQ/O (0) and 4.2 MQ/O (#.
of n and k for Fe-C films with sheet resistances
of 640 Q/O (B). 14.9
Transmission electron micrographs of the Fe-C films scraped off glass substrates showed the particulate nature of the films; there appeared to be voids between the particles’. Further evidence of the particulate nature of the films was
METAL
CARBIDE AND METALSILICIDE SOLAR ABSORBINGSURFACES
317
given by measurements of the electrical resistance as a function of temperature. Abeles’ has shown that films composed of metallic particles which are electrically isolated from each other should have negative temperature coefficients of resistance due to electron tunnelling between the particles and that they should obey an equation of the form R = R,exp(~/T”~). The three films mentioned above all showed this behaviour’. If it is assumed that the films are composed of isolated metallic particles in a dielectric matrix, then the behaviour of their optical constants can be explained by assuming that as the films become more dielectric the volume fraction of metal in the composite decreases. Since the low resistance films are metallic, the volume fraction ofmetal in these films should be quite high. Consequently an extension developed by McKenzie and McPhedran’ of the Maxwell-Garnett theory for the optical properties of regular lattices of composite materials was used to model these films. This theory takes into account the effects of the proximity of particles which become important for high volume fractions of metalg.
k
Fig. 2. The wavelength dependences of n and k for simple cubic iron spheres in an air dielectric for volume fractions of iron of 0.55.0.45 and 0.35.
Figure 2 shows the wavelength dependences of n and k for simple cubic-packed iron spheres embedded in an air dielectric for volume fractions of iron of 0.55,0.45 and 0.35. (For computational simplicity only poles up to order 2’ were considered in these calculations; this shifts the divergence for touching spheres from 0.52 to 0.59 for a simple cubic lattice.) The refractive indices of iron given by Johnson and Christy” were used. There is good agreement between the experimental and the calculated wavelength dependences of n and k for the films and the agreement improves as the
318
I. T. RITCHIE, G. L. HARDING
films become more dielectric. The magnitudes of n and k in the modelled composites are in general lower than the experimental values since an air dielectric was used in the model. The use of a dielectric other than air should not alter the spectral agreement between the model and the experimental films. 3. GRADED REFRACTIVE INDEX FILMS Graded refractive index films can be readily deposited in a sputtering process by altering the amount of reactive gas (CH,, SiH, etc.) present in the discharge as a function of time4*6*7. To produce films that will be suitable as solar absorbers the reactive gas flow should be increased during the deposition so that the absorber will be metallic near the highly reflective substrate and dielectric towards the top of the film5. Films of this type were produced by sputtering transition metals in Ar + CH, and Ar + SiH, atmospheres :In the high pressure d.c. sputtering system used to make the Fe-C films described in Section 2 and by sputtering these metals in a low pressure d.c. magnetron discharge in an Ar + C,H, atmosphere. The details of these procedures are described elsewhere43 ‘. ’ ‘. The best films produced by each of these three processes had similar optical properties: solar absorptances were typically 0.92-0.94 and room temperature emittances were 0.03-0.04. Figure 3 shows the reflectance as a function of wavelength for a graded Fe-C film deposited at high pressure on a copper substrate. Graded metal-silicon films and metal-carbon films deposited at low pressure all had similar reflectance spectra. Also shown in Fig. 3 is the predicted reflectance spectrum of a cermet of iron spheres in an air dielectric on a copper substrate with a volume fraction of iron decreasing from 0.59 at the substrate to zero at the air-film interface. The agreement between this model and the experimental spectrum is good. The wavelength dependence of the reflectance for a uniform Fe-C film on copper3 is also shown.
06-
R OL -
OZ--.
o-
3
5
10 X
1.5
2.0
3.0
(pm)
Fig. 3. The reflectance vs. wavelength spectrum for a graded Fe-C film on copper (---_) compared with that of a graded particulate iron film on copper (- -) and that of a uniform Fe-C film on copper (- - -).
Graded films produced by the three sputtering processes actively pumped vacuum ( z 10m3 Pa) at various temperatures
were annealed to investigate
in an their
METAL CARBIIIE
AN11 METAI. SILICIDE
SOLAR ABSORBING
thermal stability. The results of this annealing quite different behaviours for the three films.
319
SURFACES
are summarized
in Table I and show
TABLE I Method offilm production
Initial values of
Final values of’ e300
80hat3OO”C
0.83
0.024
0.032
lOOhat2OO”C; 2OOhat3OO”C; lOOhat4OO”C; lOOhat5OO”C
0.935 0.93 0.93 0.915
0.032 0.030 0.030 0.030
0.035
1250 h at 400 “C
0.89
0.040
eJoo
Fe in Ar + CH,, high pressure d.c. diode’
0.93
0.025
Ti in Ar + SiH,, high pressure dc. diode4
0.935
316 stainless steel in Ar + C,H,, low pressure dc. magnetron”
0.92
4.
Annealing treatment
a,
0s
DISCUSSION
The spectra of the initial solar absorptances, emittances and reflectances as functions of wavelength are similar for the three films and it is likely that the mode1 developed for the Fe-C films, i.e. electrically isolated granular metal particles, is applicable to the other types of film. However, the behaviours of the three films on annealing are quite different. The annealing of graded selective absorbers has two important effects: interdiffusion takes place between the absorber and the reflector layers; the grading profile of the absorber film can alter owing to phase transitions and grain growth in this layer. Both these effects may change the solar absorptance and emittance of a graded selective surface and it is the rate at which the effects occur which determines the stability of the surface. From the annealing results it is not clear which is the most satisfactory combination of transition metal, reactive gas and sputtering regime for producing stable graded films. Further research is necessary to determine the degradation modes of graded films and the most stable grading profiles for these films. ACKNOWLEDGMENT
Financial support for the work described here was given by the New South Wales State Government through the Science. Foundation for Physics in the University of Sydney. REFERENCES
1 2 3 4
A. Goldsmith, T. Waterman and H. Materials, Vol. IV, Macmillan, New G. V. Samsonov, Handbook of High York, 1964. G. L. Harding, J. Vat. Sci. Technol., G. L. Harding, J. Vuc. Sci. Technol.,
Hirschhorn, Handbook of Thermophysical Properties ofSolid York, 1961. Temperature Materials No. 2, Properties Index, Plenum, New 13 (1976) 1070. I5 (1978) 65.
320
5 6 7 8 9 10 11
1.T. RITCHIE, G. L. HARDING
I. T. Ritchie and B. Window, Appl. Opr., 16 (1977) 1438. G. L. Harding and 1. T. Ritchie, Proc. In,. Solur Energy Congr., New Delhi, January 16-21, 197% Pergamon. Oxford, to be published. I. T. Ritchie, submitted to J. Vuc. Sci. Technol. B. Abeles, Appl. Solid Stare Sci., 6 (1976) 46. D. R. McKedzie and R. C. McPhedran, Nature (London), 265 (1977) 128. P. B. Johnson and R. W. Christy, Phys. Rev., Sect. E, 9 (1974) 5056. G. L. Harding and S. Craig, J. VW. Sci. Techno/., to be published.