SPECTRALLY SELECTIVE ABSORPTION IN NICKEL CARBIDE AND NICKEL NITRIDE FILMS

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SPECTRALLY SELECTIVE ABSORPTION IN

NICKEL CARBIDE AND NICKEL NITRIDE FILMS

M. Sikkens

To cite this version:

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JOURNAL DE PHYSIQUE

CoZZoque Cl, suppzdment au nO1, Tome 42, janvier 1982 _page Cl-465

SPECTRALLY SELECTIVE ABSORPTION I N NICKEL CARBIDE AND NICKEL N I T R I D E

F1 LMS

M. Sikkens

Department of A p p l i e d P h y s i c s , M a t e r i a l s Sciefice C e n t r e , U n i v e r s i t y o f Groningen, N i jenborgh 1 8 , 9 74 7 AG Groningen, t h e Netherlands.

~BsumB.- Les propri6t6s optiques des electrons libres peuvent Btre m s e de deux cat6gories de surfaces sglectives. L'une d'elles ndcessite une densit6 d'6lectrons relativement Blevde ainsi qu'un petit temps de relaxation, ce qui peut se prdsenter dans des allia- ges concentres. Nous avons produit des couches minces de Ni-C et Ni-N par pulverisation cathodique rgactive d'une cible de nickel dans un melange (Ar

+

CH) ou (Ar

+

NZ).

Les propri6t6s de ces couches ont Bt6 examin6es par la diffraction de rayons X, la spectroscopie de photo-electrons aux rayons X (E9=A), la mesure de la r6sistance Blectrique et des mesures optiques. En accord avec la thBorie, nous avons observB la sQlectivitB spectrale dans des couches de Ni-C, pulv6ris6es dans des circonstances appso- priges. Nous avons trouv6 gue les absorptions de rgsonance donnent une contribution importante aux propriBtBs optiques.

Le comportement optique des couches Ni-N semble Btre similaire Ei celui des couches de Ni-C. Des r6sultats preliminaires sont presen- tes.

Abstract.- The optical properties of free (conduction) electrons can give rise to two different kinds of spectrally selective behaviour. One of these requires a relatively high effective electron density and a small relaxation time. This can be expected to occur in concentrated alloys. We have produced Ni-C and Ni-N films by reactive sputtering of Ni in the presence of methane and nitrogen, respectively. The properties of these films have been investigated using X-ray diffraction, X-ray photoelectron spectsoscopy (ESCA), DC resistivity measurements and optical measurements. In agreement with the theory, spectrally selective behaviour has been observed in Ni-C filmssputtered under appropriate conditions. Resonance absorptions have been found to give a significant contribution to the optical properties. The optical behaviour of the Ni-N films seems to be similar to that of the Ni-C films. Preliminary results are presented.

1. Introduction.- Many spectrally selective coatings are available to- day for the photothermal conversion of solar energy. Amongst the most promising selective absorbers are the composite materials, made up from conducting and insulating particles, and the interstitial transition- metal compounds. The former have drawn considerable attention /l-2/ but

the latter have been scarcely investigated in connection with photothermal solar energy conversion / 3 / . However, these compounds could be inte-

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resting as selective surfaces : they show a metallic behaviour and their electronic and optical properties depend strongly on deviations from stoichiometry. By controlling the composition the properties could be optimized for use as a selective absorber.

In this paper we will show that the behaviour of the free electrons in a metallic conductor can give rise to spectral selectivity suitable for solar energy conversion. More specifically, we present an investigation of the properties of reactively sputtered Ni-C alloys. Also, preliminary results on Ni-N alloys will be given. We will show that the optical and electrical properties depend strongly on the composition and that the free-electron behaviour is dominant in these properties.

2. Model for the optical properties of interstitial Ni alloys.- A model for the optical properties of the interstitial Ni-C and Ni-N alloys in the solar and thermal spectral regions has to incorporate both the effect of the free (conduction) electrons and the effect of the interband

f

transitions of bound electrons. The dielectric constant E (W) due to the effect of the free electrons can be described by the Drude equation /4/

where T is the relaxation time of the free electrons and W is the plas- P

ma frequency given by

In this equation n is the free electron density, so is the permittivity

of free space, e is the electron charge and is the effective electron mass.

In concentrated alloys where T is relatively small as compared to pure materials, we can take w ~ < < l in most of the spectral region of interest. Introducing the DC conductivity "(=co w ~ T ) , equation (1) can be rewrit-

P ten in this approximation as :

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b

The contribution E ( W ) due to the interband transitions of bound electrons can be described as an assembly of damped harmonic oscillators frequency w oj ' b N E (W) = l + i

A .

j=l w2

-

w2

+

~ W / T 0 j j

-

1

Here, W * represents the oscillator strength and T the damping cons-

pj j

tant. The Drude expression (1) can be included in this expression by taking woj= o in one of the terms, in which case equation ( 3 ) describes the complete dielectric constant.

Although this description places restrictions on the interband absorption profiles and assumes no frequency dependence of the parameters, there is no other more satisfactory model which can be applied over such a large wavelength range.

Equation ( 3 ) can be rewritten in a different form which will be used

in this paper :

where

X = 2m/w : the plasma wavelength,

P

j P j

and A c j = 2 r c / ( w i j T ~ = ) 2 n c ~ ~ / o : the "characteristic" wavelength.

This is a convenient expression because the long-wavelength limits of the interband terms depend only on their E while the DC conductivity

j

'

contribution of the intraband term depends only on itsLj.

3. Spectral selectivity of a Drude-type metallic conductor.- The optical properties of a metallic conductor described by the Drude expression

(1) give rise to two distinct classes of spectrally selective behaviour. Using the notation of equation ( 4 ) , these classes are apparently charac- terized by the ratio A / A (=U T ) . This becomes clear when equation (1)

P C P

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cl-468 JOURNAL DE PHYSIQUE

'L

and next the complex refractive index n is calculated from the relation:

'L

n ~ n - i k lh = ~

First, we will consider the situation X /Ac>> 1. In that case we have P when

X 2 X

P E(X)

2

1

-

X2/Xi which gives and

Because the normal reflectance p is given by

It is clear that p will sharply increase with increasing

X

in the neighbourhood of X = h From the behaviour of the absorption coeffi-

P'

cient a = 4nk/X arounf X we can conclude that the material will be P

transparent below

X

(if the thickness is not too large) and opaque P

above A Selective absorption is obtained when an absorbing substrate P'

is coated with a film of this material. Selective transmission is obtained when this material is applied as a thin film onto a transparant substrate, for instance a solar collector glass cover.

The calculated reflectance for different values of X /h is given in

P c

figure 1. The steepness of the reflectance increase at X-X is determi- P

ned by the value of the ratio X

/Xc.

For values of X /Xc

1

5 the steep-

P P

ness is almost ideal. Unfortunately, up to X

/A

-

30, the reflectance

P C

reaches a value which is substantially below unlty and increases only slowly at longer wavelengths. This kind of behaviour is observed in In20j and Sn02. The maximum values of the ratio

X /Xc

which are obtai-

P

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This limits the normal thermal emittance of the material at 100°C to values between 0.10 and 0.15.

The second class of selectivity is obtained for

X

/X < 1. When P c -

A >> X2/X we obtain from equation (5) the equivalent of equation (2)

P

c

When h>,lc we have n

-

k

-

(h/2hc)1h and

When h<<Xcwe have n

-

1 and k

-

X/2X << 1. The absorption coefficient C

becomes

Fig.1.- Reflectance of a Drude-type conductor as a function of A/A for different values of

Xp/Xc. P

Comparison of equations (10) and (11) shows that the absorption coefficient is constant for X<ihc and decreases with increasing wavelength for ?,>>Xc. When the thickness of the material is appropriately chosen, absorption of radiation will occur below Xc while above

Xc

the material becomes more and more transparent. A thin film deposited onto a highly reflecting substrate will therefore cause selective absorption, as is shown in figure 2 for the case X /X = 0. The oscillatory behaviour at

P c

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tivity is obtained for a thickness of approximately 0.3 Xc. Only a weak selectivity is obtained with very thick films. For a practical selective surface we would require

X,

= 0.5

-

0.7 pm and a thickness t = 0.15- 0.20 urn. The DC resistivity a-' corresponding to the desired value of Xc is approximately 3 X 1 0 - ~ to 4 X 1 0 - ~ f2m which is intermediate between metals and doped semiconductors.

Fig.2.- Reflectance of a Drude-type conducting film on a perfect reflecting substrate ( E =

-

i a) as a function of

A/Ac, for different values of the film thickness.

Calculations performed with A /hc # 0 show an increased reflectance P

with respect to figure 2 for A/Ac < Ap/Ac. No significant degradation of the selectivity occurs as long as A < 0.50 um. This being only

P %

slightly larger khan in most metals, the high resistivity value has to be obtained by a scattering mechanism like impurity- and vacancy-scattering in alloys.

It will be shown that this latter type of selectivity occurs in sputtered Ni-C alloys with a high carbon concentration.

4. Physical properties of Ni-C and Ni-N alloys.- At room temperature, under conditions of thermodynamic equilibrium, only very small quanti- ties of carbon or nitrogen ( +- 1 0 - ~ at. % ) can be dissolved. By quen- ching from high temperatures solid solubilities of 1.5 at .% can be obtained /5/. In the fcc Ni lattice the carbon or nitrogen atoms occupy the octahedral interstices. At high concentrations of interstitial atoms metastable intermediate phases are formed. In the N1-C system a carbide Ni C exists which is found to have a hcp Ni sublattice with the carbon

3

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exhibit metallic conduction.

The optical properties of pure Ni have been studied extensively /9-10- 11/. In a strong ferromagnetic material like Ni, it is difficult to separate the interband and intraband (conduction electron)contributions to the dielectric constant. This is a consequence of the intersection of the d-like bands with the Fermi level, giving rise to a series of interband transitions extending into the infrared region of the spectrum. Al- though no detailed optical data are available, it can be expected that in dilute interstitial Ni alloys the separation of interband and intraband contributions will also be difficult. Considering the intraband contribution, we expect a strong decrease in the relaxation time of the conduction electrons due to impurity scattering. Also, the effective number of these electrons will be reduced due to the sp-d hybridi- zation in bond formation. The situation in more concentrated alloys and in the intermediate phases is not clear. Neither optical nor electrical properties have been reported for these cases.

In this paper, we will use an empirical approach to the description of the optical properties of concentrated Ni-C and Ni-N alloys.

5. Techniques and instrumentation.-The Ni-C and NI-N films are deposited by reactive sputtering of Ni in an argon-methane or an argon-nitrogen gas mixture, respectively. The reactive gas pressure can be set by means of a needle valve between the sputtering chamber and the supply tank which is kept approximately at atmospheric pressure. The argon pressure

is electronically controlled to keep the sputtering current at a constant value, while a constant sputtering voltage is maintained by a stabilized power supply. The sputtering conditions are summarized in table I. Glass microscope slides and fused quartz are used as substrate materials. The sheet resistance of the deposited films is measured using

the four-point probe technique. The film thickness is determined by the Tolansky method /12/. Reflectance and transmittance measurements are performed in the spectral range 0.4

-

25 pm using conventional spectro- photometers equipped with specular reflectance units. Structural properties are analyzed by means of X-ray diffraction and X-ray photoelectron spectroscopy (ESCA : Electron Spectroscopy for Chemical Analysis). 6. Experimental.-

6.1.

gC-g&epgyjcal

resist,ixi.sy.- Measurements of the DC resistivity have been performed on 0.2

-

0.8 pm thick films on glass substrates by means of the four-point probe technique. The results for the Ni-C and Ni-N films are given in figures 3 and 4, respectively.

The resistivity at zero reactive gas pressure is substantially higher than the bulk Ni value, 6.84 X 10-8 Qm, which can be attributed to

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tivity with increasing reactive gas pressure, a steeper increase followed by a decrease in the resistivity can be observed in both cases. This seems to be due to structural changes in the films, as will be discussed in 6.2. Similar results have been obtained by Gerstenberg and Calbick

/13/ for Ta films sputtered in the presence of methane and nitrogen.

In figure 3 the resistivity reaches very high values at high methane pressures. This can be explained by the formation of amorphous carbon in these films. It has been shown by Anderson /14/ that such amorphous carbon films, deposited by the glow-discharge technique, can have resistivities in the order of 1 0 l O i h n .

Table I. ~eposikion conditions

Sputtering target Electrode diameter Electrode separation Max. substrate dimensions Base pressure of vacuum system Total pressure, Ar

+

reactive gas Partial reactive gas pressure

Gas flow rate Voltage Current density Deposition rate Substrate temperature Ni (99.99%) 100 mm 35 mm 30 by 30 mm 10-4 Pa 8

-

12 Pa 0

-

0.25 Pa (CHq) 0

-

10 Pa (N2) 130 Pa l/s 1.8 kV 5 ~ / m ~ 1

-

3 A / s (depending on reactive gas pressure) ca. 200°C

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Fig.3.- DC electrical resistivity of the Ni-C films as a function of the partial methane pressure present in the sputtering process. 16' 1 6

1

4 -

C

E

2

loL v , . U) W . 0 E1oSr

-

U W -P V) 1 8

;

-

0

.

*ee -

*$.

*

,

d a

Fig.4.- DC electrical resistivity of the Ni-N films as

a function of the partial nitrogen pressure present in the sputtering process.

1 0 0 002 7 004 r ~OD6 ~008 m MOs 012 ~ s014 a 016 ' 018 ~ ~ ~ ~ c n ~ c .

PARTIAL METHANE PRESSURE [ P a l -+

10

d

1

;

,

-

E c: -

-

> E

z

-

F- E

g

ld- -

PARTIAL NITROGEN PRESSURE I PO ) ---t

,-\

/

d

k

---A' d' / 4' / d , /

P

"

//' , X'

-+r.snt'wiy m 2.10 nitrogen pressure

?U'

2 ,

, , s ; o ~ l I - ; o - j l a - I S I

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DISTANCE FROM SUBSTRATE (pm l

-

Fig.5.- Local resistivity as a function of the distance from the glass substrate, for Ni-N films sputtered at a nitrogen pressure of 0.13 Pa.

6.2.

XZ'~y-d&ff~"Pt&"-"~-gS_~$-~e_~g&ts.-

In order to investigate the structure of the films, a number of 1 pm thick films on glass substrates have been analyzed with X-ray diffraction using a powder diffractometer. For the Ni-C fillms sputtered at methane pressures below 0.02 Pa, a fcc Ni sublattice is found which is considerably distorted due to the interstitial carbon atoms. The diffraction patterns indicate a preferred orientation with the <110> direction normal to the film surface. Both the distortion and the degree of orientation are found to increase with increasing methane pressure.

At 0.021 Pa methane pressure both the £cc and the hcp Ni phases exist

together, while between 0.03 and 0.07 Pa only the hcp Ni phase is found. At higher methane pressures the intensity of the diffracted lines becomes weaker and above 0.13 Pa the Nisublattice cannot be detected any- more. It appears that, at these pressures, the Ni atoms are randomly dispersed in a carbon film. Diffraction measurements on the Ni-N films show only the presence of a fcc Xi sublattice. No hcp lattice, connected with Ni3N, has been found up to a nitrogen pressure of 10 Pa. At nitrogen pressures above a pressure of 0.4 Pa, approximately corresponding to the resistivity maximum, there are indications that two types of fcc lattices are present, differing only in dimensions. This suggests that

the film contains a mixture of Ni4N and Ni crystallites, the latter with a considerable amount of dissolved nitrogen.

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suggest a large metastable extension of the solid solubility range up to concentrations of about 0 . 1

-

0 . 2 . This is confirmed by the large lattice expansions found in this pressure range. Such extensions of the solubility range are often observed in thin films deposited with sputtering methods /12/.

The carbon concentration in the Ni-C films has been determined with ESCA at a pressure of 4 X 1om8 Pa and a sample temperature of 60°C. The

0 . 2 pm thick films have been sputtered onto quartz substrates. The carbon concentration has been calculated from the relative areas of the C 1s- and Ni 3p-lines. Before recording these lines the samples are bombarded with 1 keV Ar-ions to keep hydrocarbon contamination at a sufficiently low level. This ion etching can give rise to a difference in composition between the surface layer and the bulk when the sputtering yields of the constitueots are unequal. Therefore, the quantitative data presented in figure 6 should be interpreted with care. Experi- ments performed at higher temperatures indicate a slight carbon enrich- ment of the surface layer after ion etching which agrees with the re-

sults obtained by Wehner /16/ with ion etching of Tic samples.

Despite of the uncertainties in the quantitative data, it is clear that large carbon concentrations are obtained in the reactive sputtering pro- process.

The ESCA experiments on Ni-N samples are in an early stage at this mo- ment and no data are available yet.

PARTIAL METHANE PRESSURE (Pal -+

Z 0 C

3

06 &- z U1 U Z 0 o a4 z 0 m tY $ 0 2 -

6.3. gp&i~g&-megsurement~- Spectral reflectance and transmittance measurements have been performed on Ni-C films sputtered onto glass slides. The results are depicted in the figures 7, 8 and 9. As can be observed

0 002 OOL 0 0 6 008 010 - 0 - - - 0 O 8 o - 0 0 D ,F o g

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cl-476 JOURNAT., DE PHYSIQUE

in figure 7 and figure 8, the reflectance decreases with increasing methane pressure. At methane pressures above 0.1 Pa the films become in- creasingly transparent in the infrared, which can be concluded from the reflectance humps corresponding to the Si02 absorptions of the glass substrate. Also, the short-wavelength transmittance in figure 9 increases with increasing methane pressure, which is partly due to the decrease in deposition rate.

A very crude check of the applicability of the simple Drude model is obtained assuming the dielectric constant to be described by a single

free electron term. In the notation of equation (4) we have N=l and E=O while Ac can be calculated from the experimental resistivity. The plasma wavelength X is unknown but can be estimated, from equation (la)

P

and reported values of the effective conduction electron density nm/ms for transition metal carbides /17/, to be withinthe range 0.2-1 pm. The results of the reflectance calculations are not very sensitive to

A

P

especially at longer wavelengths. This is due to the fact that COT<< 1

(A>>XZ/A ) in most of the spectral region of interest, so equation (9)

P

C

can be applied. For simplicity we have taken X =O. Proceeding from these

P

assumptions, the reflectance has been calculated. The optical constants of the glass substrate have been approximated by linear interpolation of the data reported by Hsieh and Su /18/. No attempt has been made to calculate the short-wavelength transmittance because this simple model can not be expected to hold in this region.

Fig.7.- Spectral reflectance of Ni-C films sputtered for 1 hour at different methane pressures p :

a) p=O.O21Pa ; hc (calculated from the resitivity)=0.018 pm; thickness t=0.8 pm.

b) p=0.043 Pa ; Xc=0.133 ym ; t=0.8 um.

C) p=0.064 Pa ; X =0.58 pm ; t=0.8 vm. C

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Fig.8.- Spectral reflectance of Ni-C films sputtered for 1

hour at different methane pressures p :

d) p=0.107 Pa ; Ac=1.24 vm ; t+.71 vm. e) p=0.17 Pa ; A =960 pm ; t=0.45 vm.

C

~ l s o indicated are the results of calculations based on the Drude model.

Fig.9.- Short-wavelength spectral reflectance and transmittance of Ni-C films sputtered for 1 hour at different methane pressures.

The results of the calculations are indicated in figure 7 and figure 8. The quantitative agreement with the experimental results is generally 2oor. Only for the samples sputtered at not too high methane pressures

theresults agree at very long wavelengths. This conclusion is not sur-

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in the infrared are expected to occur in Ni-C alloys, at least at low carbon concentrations. Only at long wavelengths the effect of the free electrons, which increases with wavelength, is large enough to be dominant in the optical properties. At high methane pressures the resistivity increases strongly and the effect of the free electrons on the optical properties decreases. Therefore, the spectral region where the free electron description holds shifts towards longer wavelengths and the properties, in the region considered here, will be more and more determined by other mechanisms.

In these Ni-C films two or more phases may exist together, as has been discussed in 6.2. Although equation ( 2 ) has been derived for a homoge- neous, single-phase material, at low frequencies it will also hold when more phases are present if U and E are interpreted as effective (mean) values. However, at high frequencies, the absorption may be increased

by conduction-resonance effects due to the presence of the insulating amorphous carbon phase.

As has been discussed in 3, theresistivity for optimum selectivity has to be approximately 4 X 1 0 - ~ am. From figure 3 we find that this resistivity corresponds to a methane pressure of ca. 0.08 Pa In order to determine the short-wavelength optical properties in this methane pressure range, three samples with different thicknesses have been prepared at a pressure of 0.087 Pa. From the results of reflectance and transmittance measurements on these films, we have calculated the short-wavelength dielectric constant by iteration. The results are shown in figure 1 0 and figure 11.

12 -

10 -

, o t x p o l m m t

--

c ~ ~ ~ Fig. ~ 10. ~

-

Real part of the dielectric ~ ~ ~ ~ . ~ ~ J ~ ~

obrorptton term

- - e a t c u l a t ~ d

.

shmt / constant, Re (E)

,

as a function of

ro..lrnsth proc.r.cr wavelength for Ni-C films sputtered

2

'or d6 de

;

2 3 4 6 8 1 0

k(prn1-

- ,,X'

,L - - A - - -

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--- colculoled. major abrarpt~on lrrm calculated. tree electron term

-

L - 2 -

---,---

_'.

---

_,

_---

_{---- __}

O ~ L

&

tie

i

2 3~ 6 8 ~ ) A ( v r n )

-

Fig.11.- -Im(&)/h as a function of wavelength for the Bi-C films of figure 10. Indicated are the calculated curves, obtained from a three- term fit to these date (see text).

To represent these results in the form of equation ( 4 1 , a least squares fit to the data has been made using three terms : a free electron term, a harmonic oscillator term representing the resonance absorption in the wavelength range of interest and a constant term representing the effect of absorptions at shorter wavelengths. The free- electron parameter hc has been calculated from the experimental resistivity.

gain,

X

is ta-

P

ken to be zero for the free electrons. The numerical results of the fit- ting procedure are given in table 11.

Table 11. Numerical values of the model parameters for samples sputtered at a methane pressure of 0.087 Pa.

Free electron term El = 0 '1

X = Q ' 1

P1

Xcl

= 0.70 pm 2 f

1st absorption

2nd absorption

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To investigate the improvement over the simple Drude model in describing the experimental reflectance results of figure 7 and figure 8, the reflectance of the samples b, c and d has been calculated using the three- term model. The parameters of the resonance terms are taken from table 11, while the free- electron parameters are taken to be the same as in the previous calculations. The calculated curves, given in figure 12, show a significant improvement in describing the experimental data. As can be expected, the best agreement with the experimental data is obtained for sample c, its sputtering conditions being close to those of the sample used in the calculation of the model parameters.

1.0

hc

0 1 3 3 p m . 1.Q80 ~m

-

0.58 # > m . C480 P m

.--- >.I.( p m . 1.0.31 pm .H.'

Fig.12.- Calculated spectral reflectance of the sample b, c and d of figures 7 and 8, using the three-term model of table I1 with the indicated values of Xc for the free electrons.

The resonance parameters are found to depend on the methane pressure, which effect is subject to further study. Nevertheless, the parameters

in table I1 being calculated for a methane pressure close to the value for optimum selectivity, the spectrally selective behaviour can be pre- dicted from these parameters with a reasonable accuracy.

Using the interband parameters of table I1 and assuming X = 0 for the P

free electrons, the reflectance of a thin film on a Ni substrate has been calculated for different values of the free-electron parameter Ac. The film thickness is chosen to be 0.12 Vm, which appears to be the optimum value. Figure 13 gives the results of the calculations, showing optimum selectivity for Xc lum. At these values of A c t the reflectance is determined by the resonance absorption which, apparently, also gives rise to spectrally selective absorption under these conditions.

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followed by a 0.12 pm thick Ni-C film at a methane pressure of 0.056, 0.084 and 0.112 Pa, respectively. The film resistivities, which can not be deterrhined experimentally because of the highly conductive Ni film, have been estimated from figure 13 to be 1 0 - ~ , 4 X 10'~ and 4 x 1 0 - ~ Gm,

respectively. The corresponding

X

-values are 0.17, 0.67 and 6.7 pm. C

In figure 14 the reflectance of these samples is plotted.

Fig.13.- Calculated spectral reflectance of 0.12 vm thick films on a Ni substrate, using the three-term model of table 11

0 1 ,

.

'

OL a6 08 1 2 3 4 6 8 1 0

i0i5

with the indicated values of Xc

k i p m l - for the free electrons.

Fiq.14.- Experimental spectral reflectance of 0.12 urn thick Ni-

C films on glass substrates co- vered with a 0 . 3

urn

thick sputtered Ni film. The me.thane pressure used in the sputtering process is indicated.

Comparing figures 13 and 14,, it can be observed that the overall behaviour is very much alike, although there are substantial quantitative differences at short wavelengths. These discrepancies are not surpri- sing, considering the difference in short-wavelength reflectance between sputtered and bulk Ni and the neglect of any dependence of the resonance absorptions on the methane pressure. It can be concluded, that these results confirm the applicability of the proposed model to the Ni-C

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Preliminary optical measurements on Ni-N films show that the reflectance at long wavelengths again approaches the value obtained from the simple Drude model as is the case for the Ni-C films. This is illustrated in figure 15 in which the reflectance and short-wavelength transmittance are given for a 0.16 pm thick Ni-N film sputtered at a nitrogen pressure of 0.13 Pa. Also, the results of calculations based on the simple Drude model are indicated. As for the Ni-C films, the agreement is poor at short wavelengths. Determination of the optical constants at these wavelengths is handicapped by the inhomogeneities in nitrogen concentration mentioned earlier. This effect as well as the behaviour as a selective surface, is subject to further study.

Fig.15.- Spectral reflectance and transmittance of a 0.16 pm thick Ni-N film, sputtered at nitrogen pressure-of 0.13 Pa. Indicated are the results of calculations based on the Drude model, using A = 0.110 Um.

C

7. Discussion.- By reactive sputtering of Ni, at an appropriate partial methane pressure, Ni-C films are obtained which exhibit spectrally se-

lective absorption if they are deposited in the form of a thin film on- to a highly reflecting substrate. From X-ray diffraction and ESCA experiments it can be concluded that these films contain approximately

7 0 at.% C. This large amount of carbon is present in the form of inters-

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study. At temperatures of ca. 400°C in vacuum, precipitation of carbon has been observed, which affects both electrical and optical properties of the films. Prolonged heat treatments will be necessary to predict a possible degradation of the spectral selectivity.

The results with Ni-N films show that the properties are similar to those of the Ni-C films. However, there are important differences. The existence of an amorphous carbon phase in the Ni-C films sputtered at high methane pressures, causes the resistivity to be much higher than

in the Ni-N films, where a maximum resistivity of ca. 1 0 - ~ Qm is reached Also, theresistivityof the Ni-N films depends strongly on the film thickness, in contrast to the Ni-C films in which this effect has not been observed. Although the maximumresistivityof the Ni-N films is

somewhat smaller than required for optimum selectivity due to the free electrons, interband absorptions may contribute significantly in the selective absorption process. Further study is necessary to investigate the applicability as a selective absorber.

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/3/ Harding G.L., J. Vac. Sci. Technol.

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(1976) 80 1

/6/ Nagakura S., J. Phys. Soc. Japan

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