CVD MOLYBDENUM FILMS OF HIGH INFRARED REFLECTANCE AND SIGNIFICANT SOLAR ABSORPTANCE

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CVD MOLYBDENUM FILMS OF HIGH INFRARED REFLECTANCE AND SIGNIFICANT SOLAR

ABSORPTANCE

G. Carver, E. Chain

To cite this version:

G. Carver, E. Chain. CVD MOLYBDENUM FILMS OF HIGH INFRARED REFLECTANCE AND

SIGNIFICANT SOLAR ABSORPTANCE. Journal de Physique Colloques, 1981, 42 (C1), pp.C1-203-

C1-211. �10.1051/jphyscol:1981115�. �jpa-00220665�

CVD MOLYBDENUM F I L M S O F H I G H INFRARED REFLECTANCE AND S I G N I F I C A N T SOLAR ABSORPTANCE ⁽⁺⁾

G . E . Carver (U) and E.E. Chain

O p t i c a l S c i e n c e s C e n t e r , U n i v e r s i t y o f A r i z o n a , T u c s o n , A r i z o n a 8 5 7 2 1 , U . S . A .

A b s t r a c t . - Chemical Vapor D e p o s i t i o n (CVD) of t h i n f i l m s from mo- lybdenum c a r b o n y l (Mo(C0) 6 ) a t 300°C under atmospheric p r e s s u r e h a s r e s u l t e d i n e i t h e r r e f l e c t i v e o r b l a c k molybdenum, depending on t h e absence o r p r e s e n c e o f an oxygen b l e e d d u r i n g t h e d e p o s i - t i o n . A f t e r p o s t - d e p o s i t i o n a n n e a l , t h e r e f l e c t i v e f i l m e x h i b i t s an i n f r a r e d r e f l e c t a n c e o f 98.7 % a t 10 microns w h i l e t h e b l a c k f i l m e x h i b i t s a s o l a r a b s o r p t a n c e o f a = 0.74 a n d a t h e r m a l ( 5 0 0 ° ~ ) e m i t t a n c e o f e = 0.08, (e r e f e r r i n q t o t h e normal e m i t t a n c e )

.

The r e f l e c t a n d o f b o t h f i f m s , b e f o r e and a f t e r a n n e a l , h a s been r e l a t e d t o t h e i r composition, q r a i n s i z e , and c r y s t a l s t r u c t u r e . I n p a r t i c u l a r t h i s h a s b e e n done i n t h e i n f r a r e d f o r t h e r e f l e c t i v e f i l m s and i n t h e v i s i b l e f o r t h e b l a c k f i l m s . The s o l a r absorp- t a n c e o f b o t h f i l m t y p e s can be enhanced by one o r t w o a n t i r e f l e c t i o n l a y e r s w i t h o n l y s l i g h t i n c r e a s e s i n t h e e m i t t a n c e .

A b i l a y e r of CVD s i l i c o n (700 &) and s i l i c o n n i t r i d e (700

8)

on t h e a n n e a l e d r e f l e c t i v e f i l m r e s u l t s i n a a = 0.75 and e(500°C)

= 0.05. A l a y e r o f CVD s i l i c o n n i t r i d e (750 A ) on t h e b l a c k f i l m r e s u l t s i n a = 0.91 and e (500°C) = 0.11. Both c o n f i g u r a t i o n s ha- ve been t e s t e d f o r o v e r

1800

h o u r s a t 500°C i n a roughing pump vacuum w i t h o u t measured d e t e r i o r a t i o n .

1. I n t r o d u c t i o n . - The e f f i c i e n c y of photothermal c o n v e r s i o n depends on t h e c o n v e r t e r s u r f a c e h a v i n g h i g h s o l a r a b s o r p t a n c e ( a ) and low thermal.

e m i t t a n c e ( e ) f o r c o n v e r s i o n schemes u s i n g c o n c e n t r a t i o n v a l u e s of one t o two hundred and t e m p e r a t u r e s n e a r 500°C / I / . The m a t e r i a l s which p r o v i d e t h e s e o p t i c a l p r o p e r t i e s must be d u r a b l e a t e l e v a t e d tempera- t u r e s f o r l o n g p e r i o d s of time and must be produced i n a c o s t e f f e c t i - ve manner /2/. We r e p o r t on two t y p e s of molybdenum f i l m s , r e f l e c t i v e and b l a c k , which can be used t o meet t h e s e r e q u i r e m e n t s .

A c o n v e r t e r w i t h h i g h a and low e l o r s p e c t r a l s e l e c t i v i t y , i s u s u a l l y p r o v i d e d by t a k i n g advantage of two o r more o f t h e f o l l o w i n g approaches : i n t r i n s i c s e l e c t i v i t y i n a s i n g l e m a t e r i a l , semiconductor

(+) T h i s work h a s been s u p p o r t e d by t h e U.S. Department o f Energy, Ad- vanced Technology Branch, under c o n t r a c t number EY-76-S-04-3709.

(++) V i s i t i n g S c i e n t i s t a t U.S.T.L., L a b o r a t o i r e d e S p e c t r o s c o p i e 11, F-34060, M o n t p e l l i e r , F r a n c e , May-October 1980, funded by CNRS-NSF Ex- changeof S c i e n t i s t s Program.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981115

JOURNAL DE PHYSIQUE

on metal tandems, i n t e r f e r e n c e , composite m a t e r i a l s , and s u r f a c e t e x t u - r e /2/. We have i n v e s t i g a t e d t h e use o f molybdenum (Mo) i n s o l a r con- v e r s i o n f o r two reasons. F i r s t , f l o h a s a high m e l t i n g p o i n t and i s s t a - b l e a t 500°C. Second, Mo e x h i b i t s both high i n f r a r e d r e f l e c t a n c e and moderate s o l a r absorptance. We show how t h e absorptance of !lo f i l m s can be i n c r e a s e d by combinations of some o f t h e above approaches f o r g a i - ning s p e c t r a l s e l e c t i v i t y .

Our Mo f i l m s have been d e p o s i t e d by Chemical Vapor Deposition (CVD) f o r two reasons. F i r s t , c o n t r o l o f CVD p r o c e s s parameters can a l - t e r t h e composition and s t r u c t u r e of t h e f i l m s . This c o n t r o l has a l l o - wed f o r t h e d i v e r s e compositions and s t r u c t u r e s of r e f l e c t i v e and black $40. Second, CVD can occur a t one atmosphere and t h e r e f o r e h a s po-

t e n t i a l f o r l a r g e through-puts i n a c o s t - e f f e c t i v e manner /I/.

2. Experimental procedure.- R e f l e c t i v e and black Mo f i l m s were deposi- t e d on fused s i l i c a s u b s t r a t e s by t h e p y r o l y t i c decomposition of molyb- denum carbonyl (Mo (CO) 6 ) a t 300°C under one atmosphere of argon. The r e a c t i o n f o r r e f l e c t i v e Mo i s a s f o l l o w s :

A s we have mentioned p r e v i o u s l y /3/, r e a c t i o n (1) does n o t go t o comple- t i o n a t 300°C. T h i s f a c t c a u s e s t h e i n c o r p o r a t i o n of c o n s i d e r a b l e c a r - bon and oxygen i n t h e growing f i l m . We d i s c u s s t h e i n f l u e n c e of t h e C and 0 on t h e i n f r a r e d r e f l e c t a n c e of t h e f i l m s i n t h e n e x t s e c t i o n .

Black !.lo i s formed by some combination of r e a c t i o n (1) and t h e following r e a c t i o n :

The oxygen flow i s c o n t r o l l e d with a simple r o t a m e t e r and r e s u l t s i n f i l m s c o n t a i n i n g l e s s carbon and more oxygen than e x i s t i n as-deposited r e f l e c t i v e Mo. D e t a i l s of t h e v a r i o u s d e p o s i t i o n parameters can be found elsewhere / 4 / .

The i n f r a r e d r e f l e c t a n c e of both r e f l e c t i v e and black Mo a r e i n - c r e a s e d by p o s t - d e p o s i t i o n a n n e a l s a t temperatures above 700°C. A s ex- p l a i n e d i n a l a t e r s e c t i o n , a n n e a l s of r e f l e c t i v e M o i n e i t h e r He o r H2 r e s u l t i n i n c r e a s e d r e f l e c t a n c e , while t h e r e f l e c t a n c e o f black Mo does n o t change u n l e s s H2 i s p r e s e n t i n t h e a n n e a l i n g atmosphere.

A second approach f o r d e p o s i t i n g black Plo i s t h e hydrogen reduc- t i o n of molybdenum d i o x y d i c h l o r i d e :

ve black &lo f i l m s w i t h o u t need f o r p o s t - d e p o s i t i o n anneal. Keeping pro- d u c t i o n temperatures below 750°C i s important f o r avoiding film-substra- t e i n t e r a c t i o n s when m e t a l l i c s u b s t r a t e s a r e needed f o r a c t u a l s o l a r a p p l i c a t i o n s of black Blo.

The composition, c r y s t a l s t r u c t u r e and g r a i n s i z e o f ourCVDMowere determined by Auger Spectroscopy, X-ray d i f f r a c t i o n , and e l e c t r o n m i - croscopy r e s p e c t i v e l y / 4 / . Specular r e f l e c t a n c e t o w i t h i n

+

l % was mea- s u r e d from 3-15 microns r e l a t i v e t o aluminium s t a n d a r d s on a Perkin-El- mer 137 spectrophotometer. Measurements of a b s o l u t e i n f r a r e d r e f l e c t a n - c e t o w i t h i n i 0 - 1 % were done by D r . J. Bennett on t h e Bennett Abso- l u t e Reflectometer a t China Lake, C a l i f o r n i a / 5 and 6/. Normal emittan- c e v a l u e s ( e n ) were determined from t h e product of t h e 500°C black bo- dy spectrum and t h e near-normal s p e c u l a r r e f l e c t a n c e of our samples, measured a t 20°C on t h e i n s t r u m e n t s d e s c r i b e d above. Absorptances were c a l c u l a t e d from t h e p r o d u c t of t h e a i r mass 2 s o l a r spectrum and t h e h e m i s p h e r i c a l - d i r e c t i o n a l ( l g O ) r e f l e c t a n c e of our samples i n t h e v i s i - b l e and n e a r i n f r a r e d . The v i s i b l e and n e a r i n f r a r e d r e f l e c t a n c e s were measured a t 20°c on an instrument developed a t O p t i c a l S c i e n c e s anddes- c r i b e d elsewhere / 7 / . The c a l c u l a t i o n s of a and en were f a c i l i t a t e d by using "distorted-A p l o t s " . These p l o t s graph r e f l e c t a n c e a g a i n s t an ab- s c i s s a which i s l i n e a r i n e i t h e r f r a c t i o n s of t h e t o t a l s o l a r f l u x o r of t h e r e r a d i a t i v e l o s s /8/.

3 . Experimental o b s e r v a t i o n s and d i s c u s s i o n . - 3.1. g e f & e c t i v e Mo.- I n i t s a s - d e p o s i t e d c o n d i t i o n , r e f l e c t i v e Mo c o n t a i n s a t o t a l of 36

+

5

0

atomic % carbon and oxygen and i s composed of 300 A wide, face-centered- cubic columnar g r a i n s . P o s t - d e p o s i t i o n a n n e a l i n g above 750°C i n e i t h e r pure He ( l e s s than 0.1 ppm O 2 p l u s H20) o r a He-H2 mixture r a i s e s t h e r e f l e c t a n c e of r e f l e c t i v e Mo from 85 % t o t h e r e f l e c t a n c e of an alumi- nium s t a n d a r d on o u r commercial spectrophotometer a t 10 microns /9/.

A s shown i n f i g u r e 1, t h e i n c r e a s e i n r e f l e c t a n c e d u r i n g anneal h a s been c o r r e l a t e d with t h e o u t g a s s i n g of C and 0 , t h e conversion t o t h e body-centered-cubic s t r u c t u r e c h a r a c t e r i s t i c of t h e bulk, and t h e

~ r o w t h of l a r g e r g r a i n s .

JOURNAL DE PHYSIQUE

ANNEAL OF CVD MOLYBDENUM

I I I I I 1

I I I I I

500 600 700 800 900 1000 T~r;~er;!~re of Anneal for 5 rninutesCC)

--

fcc-

-

hcp---

-

bee --

Fig. 1.- R e l a t i v e r e f l e c t a n c e a t 10 microns o f r e f l e c t i v e Mo p l o t t e d a g a i n s t t h e t e m p e r a t u r e o f a 5 minute a n n e a l . Changes i n oxygen and carbon c o n t e n t , c r y s t a l s t r u c t u r e , and g r a i n s i z e d u r i n g a n n e a l a r e a l - s o shown

The a s - d e p o s i t e d f c c phase i s a p p a r e n t l y s t a b i l i z e d by t h e p r e s e n c e o f t h e C and 0. An o p t i c a l a n a l y s i s p r e s e n t e d e l s e w h e r e /4/ h a s shown t h a t i m p u r i t i e s l o c a l i z e m e t a l l i c f r e e e l e c t r o n s i n 140-C and Mo-0 bonds. The r e s u l t a n t f r e e c a r r i e r d e p l e t i o n and t h e i n f l u e n c e o f i n t e r b a n d t r a n s i - t i o n s on t h e d i e l e c t r i c f u n c t i o n r e d u c e s t h e r e f l e c t a n c e t o 85 %. Impu- r i t y and g r a i n boundary s c a t t e r i n g do n o t r e d u c e t h e r e f l e c t a n c e a t 10 microns o f t h e a s - d e p o s i t e d f i l m s by more t h a n 1-2 %.

A f t e r a n n e a l i n g f o r 15 m i n u t e s a t 1000°C, t h e i n f r a r e d r e f l e c t a n - c e o f r e f l e c t i v e Mo o b t a i n s t h e v a l u e s shown i n f i g u r e 2 . A f t e r a n n e a l , t h e CVD f i l m i s more r e f l e c t i v e t h a n supersmooth b u l k Mo (98.7 v e r s u s 98.2 % a t 10 m i c r o n s )

/ l o / .

We a t t r i b u t e t h i s t o t h e p r e s e n c e o f a po- l i s h - i n d u c e d damage l a y e r a t t h e s u r f a c e of t h e b u l k sample. A s t a t e - o f - t h e - a r t Ilo f i l m s p u t t e r e d i n an u l t r a c l e a n vacuum system i s shown f o r comparison. The r e f l e c t a n c e measurements t o

+

0.1 %, and p r e p a r a - t i o n o f t h e b u l k and s p u t t e r e d Mo samples were done a t t h e Naval Wea- pons C e n t e r , China Lake, C a l i f o r n i a /9/.

3 . 2 . C o n v e r t e r s t a c k s u s i n g - r e f l e c t i v e Mo.- A f t e r a n n e a l , r e f l e c t i v e ilo e x h i b i t s a = 0.37 and en = 0.03. The low t h e r m a l e m i t t a n c e o f t h i s

f i l m can be u t i l i z e d i n p h o t o t h e r m a l c o n v e r s i o n i f i t s a c a n be i n c r e a - sed.

1 ^/

^{1 CVD Mo}

'35 2 Bulk MO

I 3 Mo sputtered in uhv

I

! . ' - ' 1 ' . . ' 7 -

5 10 15

WAVELENGTH ( pm

Fig. 2.- The a b s o l u t e r e f l e c t a n c e versus wavelength between 3 a n d l 5 microns of (curve 1) f u l l y annealed CVD r e f l e c t i v e Mo, (curve 2 ) super- smooth bulk No, and (curve 3 ) Mo sput- teredinanultracleanvacuumsystem

We have enhanced t h e a t o 0.75 by c o a t i n g t h e PIIo f i l m w i t h a b i l a y e r of 700

H

^{of C W} s i l i c o n and 700

1

of CVD Si3N4 The absorptance of t h i s s t a c k , with en = 0.05, b e n e f i t s from both i n t r i n s i c a b s o r p t i o n i n t h e s i l i c o n and d e s t r u c t i v e i n t e r f e r e n c e i n both non-metallic l a y e r s , l e a - ding t o a b s o r p t i o n i n t h e !lo. Though only e x h i b i t i n g moderate absorp- t a n c e , t h e s e s t a c k s have p o t e n t i a l use i n evacuated c o l l e c t o r s of t h e Winston d e s i g n which depend on low thermal e m i t t a n c e /11/. The s t a c k s have been exposed t o 500°C under a roughing pump vacuum f o r 1000 hours without r e f l e c t a n c e changes observable with o u r commercial spectropho- tometers. This degree of s t a b i l i t y i s due i n p a r t t o o u r use of a s i l i - con l a y e r which has been s t a b i l i z e d a g a i n s t c r y s t a l l i z a t i o n /12/.

3.3.g&_ac&-g~.- I n i t s as-deposited c o n d i t i o n , black Mo c o n t a i n s 68 2 5 atomic % oxygen and i s composed of 300 0 A wide columnar g r a i n s which e x h i b i t t h e monoclinic c r y s t a l s t r u c t u r e of Moo2. Auger p r o f i l e s i n d i - c a t e t h a t t h e composition i s uniform with depth. F j g u r e 3 shows t h e low v i s i b l e ( a = 0 . 7 7 ) and low i n f r a r e d ( e n = 0.31) r e f l e c t a n c e o f as-depo- s i t e d black I4o. P o s t - d e p o s i t i o n a n n e a l i n g i n a mixture o f flowing He

(90 % ) and H2 (10 % ) can remove most of t h e oxygen and c o n v e r t black Mo t o r e f l e c t i v e Mo, V J t h e following r e a c t i o n :

when x = 2 . C o n t r o l l i n g both

x

and T allows f o r d i f f e r e n t r a t i o s of mo- lybdenum t o molybdenum oxide i n t h e post-anneal f i l m .

JOURNAL DE PHYSIQUE

FRACTION OF 500 'C BLACKBODY

THERMAL EMITTANCE

---

BLACK MOLYBDENUM

SOLAR ABSORPTANCE

- -

1

^* WAVELENGTH

I

3 4 5 6 7 8 9 1 0 1 5 p m

09 ^{FRACTW OF} A M 2 SPECTRUM

07.

05

03, -9-9

01.

0 4 0 6 0 8 10 IS 2Oum

Fig. 3.- Distorted-X p l o t s g i v i n g hemispherical r e f l e c t a n c e , r (20°C), v e r s u s f r a c t i o n s of t h e a i r mass 2 s o l a r spectrum and s p e c u l a r h @ f l e c - t a n c e , ^1: (20°C), v e r s u s f r a c t i o n s of t h e 500°C blackbody spectrum f o r black ~ o ~ x f t e r : (curve 1) d e p o s i t i o n , (curve 2 ) p a r t i a l anneal a t 7 7 0 ' ~ f o r 7 minutes, and (curve 3) a n t i - r e f l e c t i o n w i t h Si3N4.

Annealing a t 770°C f o r 7 minutes does, i n f a c t produce a two phase f i l m /13/ with a = 0.74 and e n = 0.08. The manner i n which t h e two phases a r e d i s p e r s e d i s under i n v e s t i g a t i o n . Due t o t h i s degree of s p e c t r a l s e l e c t i v i t y , a p a r t i a l l y annealed b l a c k Mo f i l m i s t h e r e f o r e a promi- s i n g s i n g l e Layer s o l a r c o n v e r t e r .

We now a d d r e s s t h e q u e s t i o n of why black !lo i s b l a c k , s t a r t i n g with t h e as-deposited film. Scanning-electron microscopy shows t h a t black Mo does n o t e x h i b i t s u f f i c i e n t t e x t u r e t o e x p l a i n t h e s e l e c t i v i - t y by wavelength d i s c r i m i n a t i o n . Figure 4 shows, however, t h a t t h e r e - f l e c t a n c e of as-deposited black M o i s s i m i l a r t o t h e r e f l e c t a n c e of Moo2 / 1 4 / . I n view of t h i s f a c t , and s i n c e as-deposited black Mo con- t a i n s g r a i n s of MooZ and n e a r l y enough oxygen t o be s t o i c h i o m e t r i c Moo2, it i s l i k e l y t h a t t h e s e l e c t i v i t y o f t h e as-deposited f i l m s i s caused by t h e i n t r i n s i c o p t i c a l p r o p e r t i e s of Moo2 /15/. D i f f e r e n c e s i n r e f l e c t a n c e between t h e two curves i n f i g u r e 4 a t e n e r g i e s above 3 eV a r e l a r - g e l y due t o non-specular s c a t t e r i n g from t h e black Mo.

F i g . 4.- S p e c u l a r r e f l e c t a n c e o f s i n g l e c r y s t a l l i n e flo02 and as-deposi- t e d b l a c k Mo v e r s u s e n e r g y

D i f f e r e n c e s between t h e c u r v e s below 3 eV a r e t o o s m a l l t o be d i s c u s s e d s i n c e we a r e comparing r e f l e c t a n c e s measured on s e v e r a l s p e c t r o p h o t o - m e t e r s . The o p t i c a l p r o p e r t i e s of p a r t i a l l y a n n e a l e d b l a c k Mo a r e more

i n v o l v e d . The enhanced s p e c t r a l s e l e c t i v i t y o f p a r t i a l l y a n n e a l e d b l a c k Elo i s due t o t h e f a c t t h a t t h e i n f r a r e d r e f l e c t a n c e r i s e s d u r i n g annea- l i n g f a s t e r t h a n does t h e v i s i b l e r e f l e c t a n c e . Reasons f o r t h e s e o b s e r - v a t i o n s may r e l a t e t o t h e two p h a s e s we have mentioned and a r e under i n v e s t i g a t i o n .

3 . 4 . C o n v e r t e r s t a c & g - u g ~ ~ g - b & a g k - ~ ~ . - F i g u r e 4 a l s o shows t h e r e f l e c - t a n c e o f p a r t i a l l y a n n e a l e d b l a c k Mo a f t e r it h a s been a n t i - r e f l e c t e d

0

w i t h 750 A of CVD Si3N4. The r e s u l t i n g a e q u a l s 0.91 and t h e e n e q u a l s 0.11 /16/. The s u b s t a n t i a l s e l e c t i v i t y o f t h i s s t a c k i s due t o t h e com- b i n e d a c t i o n o f t h e s e l e c t i v i t y o f t h e b l a c k MO and t h e a n t i - p e f l e c t i o n l a y e r . We e x p e c t e d t h i s s t a c k t o e x h i b i t s t a b i l i t y a t 500°C f o r two r e a s o n s . F i r s t , t h e f i l m s a r e produced a t t e m p e r a t u r e s up t o 770°C and s h o u l d t h e r e f o r e s u r v i v e e x p o s u r e s t o 500°C. Second, oxygen w i l l n o t o u t g a s from b l a c k 140 w i t h o u t t h e p r e s e n c e o f H z . Thus, p o s s i b l e l o n g term l o s s e s o f a b s o r p t a n c e t h r o u g h an oxygen o u t g a s s h o u l d n o t o c c u r . T h i s i s i n c o n t r a s t t o o u r r e f l e c t i v e Mo samples where C and 0 can be o u t g a s s e d a s CO w i t h o u t t h e p r e s e n c e o f H2.

Three l i f e t i m e t e s t s w i t h a n t i - r e f l e c t e d , p a r t i a l l y a n n e a l e d , b l a c k Mo have been performed. I n e a c h c a s e t h e Si3N4 a c t s a s b o t h a n t i - r e f l e c t o r and p a s s i v a t o r a g a i n s t o x i d a t i o n . F i r s t , c o n t i n u o u s exposure t o 500°C f o r 1000 h o u r s i n r o u g h i n g pump vacuum caused no o b s e r v a b l e

JOURNAL DE PHYSIQUE

changes i n r e f l e c t a n c e t o w i t h i n 1 8. Second, 300 a d d i t i o n a l hours o f c y c l i n g between n e a r room temperature and 500°C a l s o caused no changes.

Third, exposure t o a i r a t 500°C d e s t r o y e d t h e s t a c k s w i t h i n 24 hours.

I n c o n t r a s t , p r e v i o u s work u s i n g CVD Si3N4 t o p a s s i v a t e r e f l e c t i v e Mo a g a i n s t o x i d a t i o n i n a i r a t 500°C was much more s u c c e s s f u l

/ l o / .

S i n c e black Mo from r e a c t i o n ( 3 ) h a s s i m i l a r o p t i c a l p r o p e r t i e s y e t somewhat d i f f e r e n t s u r f a c e morphology a s compared t o black blo from r e a c t i o n ( 2 ) , we c o a t e d f i l m s from r e a c t i o n ( 3 ) with Si3N4 and t e s t e d them i n a i r a t 500°C. These samples s u r v i v e d about 4 times l o n g e r , e s t a b l i s h i n g t h e p o t e n t i a l f o r improving t h e s t a b i l i t y of black Mo i n a i r by a l t e r i n g s u r f a c e morphology.

4. Conclusions- The v e r s a t i l i t y of CVD and p o s t - d e p o s i t i o n a n n e a l i n g has been erl~ployed t o produce both r e f l e c t i v e and black Mo. The absorp- t a n c e of both samples can be enhanced by o v e r c o a t i n g with e i t h e r CVD S i a n d / o r CVD Si3N4. A f t e r o v e r c o a t i n g , t h e high i n f r a r e d r e f l e c t a n c e o f r e f l e c t i v e M o l e a d s t o a s t a c k w i t h low normal e m i t t a n c e (0.05 a t 500°C) and moderate absorptance (0.75) w h i l e t h e s e l e c t i v i t y of b l a c k No l e a d s t o a s t a c k with a = 0.91 and en = 0.11 a t 500°C. I n a d d i t i o n , t h e f l e x i b i l i t y and c o s t - e f f e c t i v e a s p e c t s o f CVD can be e a s i l y exten- ded t o a s i m i l a r s t u d y based on t h e p y r o l y s i s of t u n g s t e n carbonyl.

Acknowledgments.- The a u t h o r s wish t o thank B.O. Seraphin, D.C. Booth, D.D. A l l r e d , and t h e e n t i r e S o l a r Group a t t h e O p t i c a l Sciences Center f o r guidance a s s i s t a n c e , and s u g g e s t i o n s throughout t h i s work, J . M . Bennett, G.K. Wehner, and L. Dener a r e thanked f o r t h e a b s o l u t e r e f l e c - t a n c e , Auger spectroscopy, and e l e c t r o n microscopy measurements, r e s - p e c t i v e l y .

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