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Preparation and characterization of copper(II) oxide thin films grown by a novel spray pyrolysis method
M a t . R e s . B u l l . , V o l . 24, p p . 7 5 3 - 7 6 0 , 1989. Printed in the 0025-5408/89 $3.00 + .00 Copyright ( c ) 1989 P e r g a m o n P r e s s
USA. ple.
PREPARATION AND CHARACTERIZATION OF COPPER(II) OXIDE THIN FILMS GROWN BY A NOVEL SPRAY PYROLYSIS METHOD
W. DeSisto, M. Sosnowski, F. Smith, J. Deluca*, R. Kershaw, K. Dwight and A. Wold' Department of Chemistry, Brown University Providence, RI 02912 *General Electric, Schenectady, NY (Received April 6, 1989; Refereed)
ABSTRACT A novel spray pyrolysis reactor was used to prepare thin f i l m s of CuO o n s i l i c a s u b s t r a t e s . The resulting films were characterized by x-ray diffraction, electron microscopy, optical and electrical measurements. The films were single phase, homogeneous, and uniform. MATERIALS INDEX: c o p p e r , o x i d e s Introduction Copper(II) oxide thin films have been grown by reactive-ion sputtering (1), o x i d a t i o n of e v a p o r a t e d c o p p e r m e t a l (2), a n d m e t a l - o r g a n i c chemical vapor d e p o s i t i o n (3,4). S p r a y p y r o l y s i s h a s b e e n s h o w n to be a n e f f e c t i v e m e t h o d for t h e p r e p a r a t i o n of t h i n f i l m s of o x i d e s i n c l u d i n g SnO 2, I n 2 0 3 , F e 2 0 3 , C r 2 0 3 , PbO, a n d ZnO (5,6); h o w e v e r , t h i s t e c h n i q u e h a s n o t b e e n a p p l i e d t o t h e p r e p a r a t i o n of CuO t h i n films. It w a s t h e p u r p o s e o f t h i s s t u d y t o i n v e s t i g a t e t h e p r e p a r a t i o n of CuO t h i n f i l m s b y d e c o m p o s i n g u l t r a s o n i c a l l y n e b u l i z e d aqueous copper(II) salts on silica substrates using a novel two-zone horizontal reactor. T h e CuO t h i n f i l m s w e r e c h a r a c t e r i z e d b y e l e c t r o n m i c r o s c o p y , x - r a y diffraction, and optical and electrical measurements.
Experimental T h e s p r a y p y r o l y s i s r e a c t o r u s e d i n t h i s i n v e s t i g a t i o n is s h o w n i n Fig. 1. A H o l m e s c o m m e r c i a l u l t r a s o n i c n e b u l i z e r (Fig. 1, No. 14) w a s u s e d to n e b u l i z e the copper(II) acetate solution. A n o x y g e n s w e e p flow c a r r i e d t h e m i s t t o t h e s u b s t r a t e a n d a n a d d i t i o n a l c o n s t a n t o x y g e n p u r g e flow w a s p a s s e d t h r o u g h t h e r e a c t o r a s l a b e l e d i n Fig. 1. T h e r e a c t o r w a s h e a t e d b y a t w o - z o n e m i r r o r f u r n a c e ( T r a n s t e m p Co., C h e l s e a , MA). T e m p e r a t u r e w a s m a i n t a i n e d u s i n g two Theall TC-1000 temperature controllers. L a y e r s of t h i n film w e r e d e p o s i t e d on t h e r o t a t i n g s u b s t r a t e i n t h e l o w e r t e m p e r a t u r e z o n e a n d w e r e t r a n s l a t e d to t h e h i g h e r t e m p e r a t u r e z o n e w h i c h d e n s i f i e d t h e film o n t o t h e s u b s t r a t e . This c y c l e w a s t h e n r e p e a t e d m a n y t i m e s t o p r o d u c e t h e f i n a l film. E l e c t r o n i c *Address all
correspondence 753
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e t al.
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No.
timers controlled substrate travel between zones, substrate rotation, nebulization time, and oxygen sweep time. A t y p i c a l s e t o f r e a c t i o n p a r a m e t e r s i n v o l v e d in t h e g r o w t h o f CuO f i l m s a r e g i v e n in T a b l e I. E a c h l a y e r o f t h e film w a s d e p o s i t e d a t 375%" a n d f i r e d a t 700°C. T h e s u b s t r a t e - t o - n o z z l e d i s t a n c e a n d t h e o x y g e n s w e e p flow r a t e were c r i t i c a l for o b t a i n i n g uniform films. T h e a q u e o u s c o p p e r ( I I ) a c e t a t e s o l u t i o n w a s p r e p a r e d by d i s s o l v i n g c o p p e r m e t a l i n a 1:1 m i x t u r e o f n i t r i c a c i d : d i s t i l l e d w a t e r . The r e s u l t i n g s o l u t i o n w a s e v a p o r a t e d to n e a r d r y n e s s a n d a 1:3 m i x t u r e o f a c e t i c acid:distilled water was added. T h e r e s u l t i n g s o l u t i o n w a s a g a i n e v a p o r a t e d to n e a r d r y n e s s a n d r e d i s s o l v e d in t h e a c e t i c a c i d : d i s t i l l e d w a t e r s o l u t i o n . This s o l u t i o n w a s e v a p o r a t e d to a p p r o x i m a t e l y 25 ml a n d was b r o u g h t to t h e f i n a l desired volume with the acetic acid:distilled water solution. T h e s i l i c a s u b s t r a t e s u s e d in t h i s s t u d y w e r e 0.5 in. s q u a r e s c l e a n e d t h o r o u g h l y in b o i l i n g s u l f u r i c a c i d . J u s t p r i o r to d e p o s i t i o n , t h e s u b s t r a t e s w e r e s o a k e d in c o n c e n t r a t e d s u l f u r i c a c i d f o r 5 rain., f o l l o w e d by a t h o r o u g h r i n s e in d i s t i l l e d w a t e r . T h e s u b s t r a t e s w e r e t h e n i m m e r s e d in m e t h a n o l for 1 min. a n d a l l o w e d t o d r y . T h e c l e a n e d s u b s t r a t e w a s s e c u r e d on t h e s u b s t r a t e h o l d e r a n d p l a c e d in the reactor. F i f t e e n ml of 0.1M Cu(II) s o l u t i o n w a s p l a c e d in t h e s o l u t i o n chamber and deposition started when the furnace attained equilibrium. Ultrasonic Nebulization
Spray Pyrolysis Reactor
m
10
m
1
I f 1 Ultrasonic N e b u l i z a t i o n
Spray Pyrolysis Reactor 1. Two zone furnace 2. Low temperature zone 3. High temperature zone 4. Substrate rotating motor (Synchron 4 rpm) 5. Reversible motor 6. Substrate holder (Hurst 120 rpm) 7. Substrate 8. Injection tube 9. Oxygen purge flow Fig. 1.
Apparatus
10: 11. 12. 13. 14.
Oxygen sweep flow Solution c h a m b e r Polyethylene film Solution C o m m e r c i a l ultrasonic humidifier
f o r s p r a y p y r o l y s i s o f CuO f i l m s .
14
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Table I Reaction Parameters Firing zone temperature Deposition zone temperature
700 375
Furnace diameter P u r g e f l o w 02 Sweep flow 02
38 mm 200 c c / m i n 3 l/rain
Nebulization time Oxygen sweep time Complete cycle time
5 sec 20 s e c 160 s e c
Solution concentration
0.1
Substrate to nozzle distance Nozzle diameter
60 mm 9.3 mm
°C °C
M
Film C h a r a c t e r i z a t i o n Film t h i c k n e s s w a s m e a s u r e d u s i n g a S l o a n D e k t a k s u r f a c e p r o f i l e m e a s u r i n g s y s t e m (Part n u m b e r 900050). Cu/Si r a t i o s d e t e r m i n e d b y EDAX a n a l y s i s u s i n g a n AMR 1000A s c a n n i n g e l e c t r o n m i c r o s c o p e g a v e t h e u n i f o r m i t y of film t h i c k n e s s . T h e SEM o p e r a t e d a t 20 kV a c c e l e r a t i n g v o l t a g e , 16.5 kX m a g n i f i c a t i o n , a n d a 3x3 cm w i n d o w . P h o t o m i c r o g r a p h s o f t h e film s u r f a c e w e r e t a k e n scanning electron microscope.
on a n AMRAY 1830I
X - r a y d i f f r a c t i o n p a t t e r n s of t h i n film and p o w d e r s a m p l e s were o b t a i n e d u s i n g a P h i l i p s d i f f r a c t o m e t e r a n d m o n o c h r o m a t e d h i g h i n t e n s i t y CUKal, r a d i a t i o n (k = 1.5405A). T h e d i f f r a c t i o n p a t t e r n s w e r e t a k e n i n t h e r a n g e 12 ° < 2{} < 72" w i t h a s c a n r a t e o f 1 ° 2 e / r a i n a n d a c h a r t s p e e d o f 30 i n / h r . O p t i c a l t r a n s m i s s i o n s p e c t r a of t h e f i l m s on s i l i c a s u b s t r a t e s w e r e o b t a i n e d u s i n g a C a r y m o d e l 17 d u a l b e a m r a t i o r e c o r d i n g s p e c t r o p h o t o m e t e r in t h e r a n g e o f 500 nm t o 1500 nm. T h e o p t i c a l b a n d g a p w a s d e d u c e d f r o m t h e transmittance near the absorption edge. Resistivity at room temperature was measured in the center portion of the fllm on the substrate using the van der Pauw technique ( 7 ) . Contacts to the film were made by painting a colloidal mixture of graphite and isopropanol (Electrodag 154, Port Huron, MI) to the edge of the film. Ohmic behavior was established by measuring current-voltage characteristics. A qualitative Seebeck voltage measurement was made to determine the carrier type.
Results and Discussion In s p r a y p y r o l y s i s a s o l u t i o n is a t o m i z e d , s p r a y e d o n t o a h o t s u b s t r a t e , a n d d e c o m p o s e d , r e s u l t i n g in t h i n film g r o w t h . T h e s o l u t i o n c a n be n e b u l i z e d u l t r a s o n i c a l l y , a n d s p r a y e d c o n t i n u o u s l y or in p u l s e s . V i g u i e a n d S p i t z (8) s u g g e s t e d four p o s s i b l e g r o w t h m e c h a n i s m s for t h e s p r a y p y r o l y s i s p r o c e s s as a
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f u n c t i o n of s u b s t r a t e t e m p e r a t u r e . T h e s e a r e shown in Fig. 2 and can be r e l a t e d to t h e g r o w t h of CuO films in t h i s s t u d y . In t h e f i r s t process, t h e s p r a y d r o p l e t impinges d i r e c t l y on t h e s u b s t r a t e , followed by e v a p o r a t i o n of t h e s o l v e n t , and d e c o m p o s i t i o n of t h e m e t a l s a l t to t h e oxide. In t h e s e c o n d process, t h e s o l v e n t is e v a p o r a t e d j u s t p r i or to c o n t a c t i n g t h e s u b s t r a t e , followed by d e c o m p o s i t i o n of t h e oxide. The t h i r d m ech an i sm i n v o l v e s v o l a t i l i z a t i o n of t h e dried m e t a l s a l t , d i f f u s i o n of t h e v a p o r to t h e s u b s t r a t e , followed by d e c o m p o s i t i o n to t h e oxide. This p r o c e s s is r e f e r r e d to as low t e m p e r a t u r e chemical v a p o r d e p o s i t i o n (LTCVD). The f o u r t h p r o cess is a ho m o g en eo u s n u c l e a t i o n of t h e v a p o r p h a s e forming t h e oxide p a r t i c l e which t h e n d e p o s i t s on t h e s u b s t r a t e . The f o r m a t i o n of CuO films in t h i s s t u d y p r o b a b l y p r o c e e d s by t h e t h i r d me ch an i s m s i n ce t h e d e p o s i t e d films a r e smooth, homogeneous, and h a v e a mirror-like appearance. Albin et al. (5), h a v e s u g g e s t e d t h e d e p o s i t e d film o b t a i n e d by t h e l a s t p r o c e s s would h a v e a powdery a p p e a r a n c e . Viguie and Spitz (6) h a v e shown t h a t films grown by p r o c e s s e s A and B h a v e a r o u g h e r m i c r o s t r u c t u r e t h a n films grown by p r o c e s s C. EDAX r a t i o s of c o p p e r - t o - s i l i c o n d e t e r m i n e d t h e r e l a t i v e u n i f o r m i t y of t h e CuO films and a Sloan Dektak s u r f a c e p r o f i l e m e a s u r i n g s y s t e m d e t e r m i n e d t h e a b s o l u t e t h i c k n e s s . The t h i c k n e s s and EDAX r a t i o s of t h e films v a r i e d with t h e number of c y c l e s i n v o l v e d in film p r e p a r a t i o n , and t h i s is shown in
Mechanisms for film growth in spray pyrolysis processing L
¸
substrate 2..;. 2:,z:
t solid
t t
eo o
000
vapor
•
•
precipitate
,,,.o,,o,
0
0
0
0
A
B
C
D
Increasing substrate temperature
Fig. 2.
Mechanisms of film growth.
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T a b l e II. A f t e r p r e p a r a t i o n , film t h i c k n e s s w a s d e t e r m i n e d from a c a l i b r a t i o n o f EDAX Cu/Si r a t i o v s . t h i c k n e s s o b t a i n e d from t h e d a t a in T a b l e II. T h e d e s i r e d t h i c k n e s s c o u l d b e a p p r o x i m a t e d by t h e n u m b e r o f c y c l e s in t h e preparation. EDAX r a t i o s w e r e t a k e n a t f i v e p o i n t s from e d g e t o e d g e on t h e f i l m s a n d t h e s e r a t i o s c l e a r l y s h o w t h a t CuO f i l m s p r e p a r e d b y t h i s m e t h o d a r e uniform.
T a b l e II EDAX Cu/Si R a t i o s v s T h i c k n e s s Position #
Film A
1 2 3 4 5
and Number of Cycles
B
C
D
0.38(I) 0.39 0.39 0.39 0.38
0.08(i) 0.08 0.08 0.08 0.08
0.18(1) 0.18 0.17 0.18 0.17
0.16(1) 0.16 0.16 0.17 0.17
(A)
2700
1200
1900
1850
Number Cycles
150
75
110
110
Thickness
An SEM p h o t o m i c r o g r a p h t a k e n a t low m a g n i f i c a t i o n o f a 2 0 0 0 A CuO film p r e p a r e d in t h i s s t u d y is s h o w n in Fig. 3. T h e n a t u r e o f t h e p a r t i c l e s in t h e s e CuO f i l m s a r e v e r y d i f f e r e n t f r o m CuO f i l m s p r e p a r e d u s i n g MOCVD by L a u r i e a n d N o r t o n (4), w h e r e t h e f i l m s a r e c o m p r i s e d o f n e e d l e - l i k e p a r t i c l e s .
Fig. 3.
Photomicrograph of a typical C u O film prepared from an acetate solution.
758
W. D e S I S T O ,
et a l .
Vol.
24,
No.
CuO
PolxcryGtol Standard
(lid C-liD
>-, 4J @ C
O
C l-t 0
>
Thin Film on SIO~ 0 01
(11D (-lid
20
30
40
50
60
70
80
Oi??roction Angle 28 (de9) Fig. 4.
X - r a y d i f f r a c t i o n p a t t e r n o f a CuO film on a s i l i c a s u b s t r a t e a s c o m p a r e d w i t h t h a t o f a p o l y c r y s t a l l i n e CuO standard. The differences in relative peak intensities d e m o n s t r a t e t h e e x i s t e n c e of a p r e f e r r e d o r i e n t a t i o n in t h e film.
CuO ....
I00
i ....
?ilm
on
i ....
i
silico
....
i ....
i
....
/11a ~- ....
80
/ ;t
E 0
/
60
/ / /
0} O)
/ /
40
o= & 20
0 400
500
B00
700
Wovelength Fig. 5.
Optical transmission
800
000
I000
(nm)
s p e c t r u m of a CuO film on a s i l i c a s u b s t r a t e .
6
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X - r a y d i f f r a c t i o n d a t a is s h o w n in Fig. 4. A CuO s t a n d a r d w a s p r e p a r e d by h e a t i n g Cu m e t a l (JM 5 0 2 5 0 A ) in a i r a t 600°C. By c o m p a r i n g t h e x - r a y o f t h e f i l m to t h e s t a n d a r d , i t c a n be s e e n t h a t t h e i n t e n s i t y o f t h e (020) p e a k o f t h e film is i n c r e a s e d a n d t h e ( - 2 0 2 ) a n d (202) p e a k s a r e a b s e n t . This can be a t t r i b u t e d t o p r e f e r r e d o r i e n t a t i o n o f t h e p a r t i c l e s m a k i n g up t h e film, growing with t h e b - a x i s p e r p e n d i c u l a r to t h e s u b s t r a t e . The transmission spectra for a copper(If) oxide film prepared from copper(II) acetate is shown in Fig. 5, and is in agreement with copper(If) oxide films prepared by ,Qayi et al. ( 3 ) . The optical transmission was used to generate plots of (ahu) Ij2 vs hu. It was found that the indirect band gap of the prepared copper(II) oxide films was 1.36(i) eV. All CuO thin films prepared in this study were p - t y p e semiconductors with room temperature resistivity of 102 ohm-cm. Beensh-Marchwicka et al. (1) have reported resistivities of i ohm-cm at room temperature for CuO films grown by reactive-ion sputtering; however, carrier type was not measured. The low resistivity is consistent with the presence of mixed formal valence of copper. If Cu(II) and Cu(III) were present in the film, then anneal~ng ~he film in argon would increase the resistivity by reducing the Cu(III) to Cu(II). In contrast, if the low resistivity of the film was due to the presence of Cu(1) and Cu(II), then an argon anneal would further decrease the resistivity by" increasing the amount of Cu(1) in the sample. Typical films of CuO were annealed in argon at 650°C for 15 hrs and the resulting resistivity and carrier type were measured. The resistivity of the films increased to 103 ohm-era and remained p-type. R e a n n e a l i n g t h e s e f i l m s in o x y g e n a t 650°C for ]6 h r s r e s u l t e d in r e s i s t i v i t y d e c r e a s i n g t o ].02 o h m - c m . T h e r e s i s t i v i t y o f t h e f i l m s a n n e a l e d in a r g o n is h i g h e r t h a n t h e r e s i s t i v i t y o f t h e f i l m s p r e p a r e d in o x y g e n w i t h n o c h a n g e in c a r r i e r t y p e . This h i g h e r r e s i s t i v i t y of films a n n e a l e d in a r g o n is c o n s i s t e n t w i t h a d e c r e a s e in Cu(III) a s a r e s u l t o f t h e annealing process.
Conclusion U n i f o r m , h o m o g e n e o u s c o p p e r ( I I ) o x i d e t h i n f i l m s c a n be p r e p a r e d in a n o v e l t w o - z o n e h o r i z o n t a l s p r a y p y r o l y s i s r e a c t o r by d e c o m p o s i n g a q u e o u s copper(II) salts. The s p r a y in t h e form of an a e r o s o l was p r o d u c e d by a c o m m e r c i a l u l t r a s o n i c h u m i d i f i e r . T h e film w a s g r o w n b y s p r a y i n g p u l s e s o f t h e aerosol and heating the deposited thin layer at a higher temperature after each spray pulse. T h e f i l m s g r e w w i t h p r e f e r r e d o r i e n t a t i o n a s i n d i c a t e d by x-ray diffraction. In a d d i t i o n , t h e f i l m s w e r e p - t y p e s e m i c o n d u c t o r s w i t h room t e m p e r a t u r e r e s i s t i v i t y o f 102 o h m - c m . T h e p - t y p e n a t u r e o f t h e f i l m s w a s d e t e r m i n e d to be d u e t o t h e p r e s e n c e o f Cu(III).
Acknowledgements This r e s e a r c h was p a r t i a l l y s u p p o r t e d by t h e N a t i o n a l Science F o u n d a t i o n u n d e r G r a n t No. D M R - 8 8 0 3 1 8 4 a n d b y t h e E a s t m a n K o d a k C o m p a n y o f R o c h e s t e r , NY.
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PREPARATION AND CHARACTERIZATION OF COPPER(II) OXIDE THIN FILMS GROWN BY A NOVEL SPRAY PYROLYSIS METHOD
W. DeSisto, M. Sosnowski, F. Smith, J. Deluca*, R. Kershaw, K. Dwight and A. Wold' Department of Chemistry, Brown University Providence, RI 02912 *General Electric, Schenectady, NY (Received April 6, 1989; Refereed)
ABSTRACT A novel spray pyrolysis reactor was used to prepare thin f i l m s of CuO o n s i l i c a s u b s t r a t e s . The resulting films were characterized by x-ray diffraction, electron microscopy, optical and electrical measurements. The films were single phase, homogeneous, and uniform. MATERIALS INDEX: c o p p e r , o x i d e s Introduction Copper(II) oxide thin films have been grown by reactive-ion sputtering (1), o x i d a t i o n of e v a p o r a t e d c o p p e r m e t a l (2), a n d m e t a l - o r g a n i c chemical vapor d e p o s i t i o n (3,4). S p r a y p y r o l y s i s h a s b e e n s h o w n to be a n e f f e c t i v e m e t h o d for t h e p r e p a r a t i o n of t h i n f i l m s of o x i d e s i n c l u d i n g SnO 2, I n 2 0 3 , F e 2 0 3 , C r 2 0 3 , PbO, a n d ZnO (5,6); h o w e v e r , t h i s t e c h n i q u e h a s n o t b e e n a p p l i e d t o t h e p r e p a r a t i o n of CuO t h i n films. It w a s t h e p u r p o s e o f t h i s s t u d y t o i n v e s t i g a t e t h e p r e p a r a t i o n of CuO t h i n f i l m s b y d e c o m p o s i n g u l t r a s o n i c a l l y n e b u l i z e d aqueous copper(II) salts on silica substrates using a novel two-zone horizontal reactor. T h e CuO t h i n f i l m s w e r e c h a r a c t e r i z e d b y e l e c t r o n m i c r o s c o p y , x - r a y diffraction, and optical and electrical measurements.
Experimental T h e s p r a y p y r o l y s i s r e a c t o r u s e d i n t h i s i n v e s t i g a t i o n is s h o w n i n Fig. 1. A H o l m e s c o m m e r c i a l u l t r a s o n i c n e b u l i z e r (Fig. 1, No. 14) w a s u s e d to n e b u l i z e the copper(II) acetate solution. A n o x y g e n s w e e p flow c a r r i e d t h e m i s t t o t h e s u b s t r a t e a n d a n a d d i t i o n a l c o n s t a n t o x y g e n p u r g e flow w a s p a s s e d t h r o u g h t h e r e a c t o r a s l a b e l e d i n Fig. 1. T h e r e a c t o r w a s h e a t e d b y a t w o - z o n e m i r r o r f u r n a c e ( T r a n s t e m p Co., C h e l s e a , MA). T e m p e r a t u r e w a s m a i n t a i n e d u s i n g two Theall TC-1000 temperature controllers. L a y e r s of t h i n film w e r e d e p o s i t e d on t h e r o t a t i n g s u b s t r a t e i n t h e l o w e r t e m p e r a t u r e z o n e a n d w e r e t r a n s l a t e d to t h e h i g h e r t e m p e r a t u r e z o n e w h i c h d e n s i f i e d t h e film o n t o t h e s u b s t r a t e . This c y c l e w a s t h e n r e p e a t e d m a n y t i m e s t o p r o d u c e t h e f i n a l film. E l e c t r o n i c *Address all
correspondence 753
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e t al.
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timers controlled substrate travel between zones, substrate rotation, nebulization time, and oxygen sweep time. A t y p i c a l s e t o f r e a c t i o n p a r a m e t e r s i n v o l v e d in t h e g r o w t h o f CuO f i l m s a r e g i v e n in T a b l e I. E a c h l a y e r o f t h e film w a s d e p o s i t e d a t 375%" a n d f i r e d a t 700°C. T h e s u b s t r a t e - t o - n o z z l e d i s t a n c e a n d t h e o x y g e n s w e e p flow r a t e were c r i t i c a l for o b t a i n i n g uniform films. T h e a q u e o u s c o p p e r ( I I ) a c e t a t e s o l u t i o n w a s p r e p a r e d by d i s s o l v i n g c o p p e r m e t a l i n a 1:1 m i x t u r e o f n i t r i c a c i d : d i s t i l l e d w a t e r . The r e s u l t i n g s o l u t i o n w a s e v a p o r a t e d to n e a r d r y n e s s a n d a 1:3 m i x t u r e o f a c e t i c acid:distilled water was added. T h e r e s u l t i n g s o l u t i o n w a s a g a i n e v a p o r a t e d to n e a r d r y n e s s a n d r e d i s s o l v e d in t h e a c e t i c a c i d : d i s t i l l e d w a t e r s o l u t i o n . This s o l u t i o n w a s e v a p o r a t e d to a p p r o x i m a t e l y 25 ml a n d was b r o u g h t to t h e f i n a l desired volume with the acetic acid:distilled water solution. T h e s i l i c a s u b s t r a t e s u s e d in t h i s s t u d y w e r e 0.5 in. s q u a r e s c l e a n e d t h o r o u g h l y in b o i l i n g s u l f u r i c a c i d . J u s t p r i o r to d e p o s i t i o n , t h e s u b s t r a t e s w e r e s o a k e d in c o n c e n t r a t e d s u l f u r i c a c i d f o r 5 rain., f o l l o w e d by a t h o r o u g h r i n s e in d i s t i l l e d w a t e r . T h e s u b s t r a t e s w e r e t h e n i m m e r s e d in m e t h a n o l for 1 min. a n d a l l o w e d t o d r y . T h e c l e a n e d s u b s t r a t e w a s s e c u r e d on t h e s u b s t r a t e h o l d e r a n d p l a c e d in the reactor. F i f t e e n ml of 0.1M Cu(II) s o l u t i o n w a s p l a c e d in t h e s o l u t i o n chamber and deposition started when the furnace attained equilibrium. Ultrasonic Nebulization
Spray Pyrolysis Reactor
m
10
m
1
I f 1 Ultrasonic N e b u l i z a t i o n
Spray Pyrolysis Reactor 1. Two zone furnace 2. Low temperature zone 3. High temperature zone 4. Substrate rotating motor (Synchron 4 rpm) 5. Reversible motor 6. Substrate holder (Hurst 120 rpm) 7. Substrate 8. Injection tube 9. Oxygen purge flow Fig. 1.
Apparatus
10: 11. 12. 13. 14.
Oxygen sweep flow Solution c h a m b e r Polyethylene film Solution C o m m e r c i a l ultrasonic humidifier
f o r s p r a y p y r o l y s i s o f CuO f i l m s .
14
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Table I Reaction Parameters Firing zone temperature Deposition zone temperature
700 375
Furnace diameter P u r g e f l o w 02 Sweep flow 02
38 mm 200 c c / m i n 3 l/rain
Nebulization time Oxygen sweep time Complete cycle time
5 sec 20 s e c 160 s e c
Solution concentration
0.1
Substrate to nozzle distance Nozzle diameter
60 mm 9.3 mm
°C °C
M
Film C h a r a c t e r i z a t i o n Film t h i c k n e s s w a s m e a s u r e d u s i n g a S l o a n D e k t a k s u r f a c e p r o f i l e m e a s u r i n g s y s t e m (Part n u m b e r 900050). Cu/Si r a t i o s d e t e r m i n e d b y EDAX a n a l y s i s u s i n g a n AMR 1000A s c a n n i n g e l e c t r o n m i c r o s c o p e g a v e t h e u n i f o r m i t y of film t h i c k n e s s . T h e SEM o p e r a t e d a t 20 kV a c c e l e r a t i n g v o l t a g e , 16.5 kX m a g n i f i c a t i o n , a n d a 3x3 cm w i n d o w . P h o t o m i c r o g r a p h s o f t h e film s u r f a c e w e r e t a k e n scanning electron microscope.
on a n AMRAY 1830I
X - r a y d i f f r a c t i o n p a t t e r n s of t h i n film and p o w d e r s a m p l e s were o b t a i n e d u s i n g a P h i l i p s d i f f r a c t o m e t e r a n d m o n o c h r o m a t e d h i g h i n t e n s i t y CUKal, r a d i a t i o n (k = 1.5405A). T h e d i f f r a c t i o n p a t t e r n s w e r e t a k e n i n t h e r a n g e 12 ° < 2{} < 72" w i t h a s c a n r a t e o f 1 ° 2 e / r a i n a n d a c h a r t s p e e d o f 30 i n / h r . O p t i c a l t r a n s m i s s i o n s p e c t r a of t h e f i l m s on s i l i c a s u b s t r a t e s w e r e o b t a i n e d u s i n g a C a r y m o d e l 17 d u a l b e a m r a t i o r e c o r d i n g s p e c t r o p h o t o m e t e r in t h e r a n g e o f 500 nm t o 1500 nm. T h e o p t i c a l b a n d g a p w a s d e d u c e d f r o m t h e transmittance near the absorption edge. Resistivity at room temperature was measured in the center portion of the fllm on the substrate using the van der Pauw technique ( 7 ) . Contacts to the film were made by painting a colloidal mixture of graphite and isopropanol (Electrodag 154, Port Huron, MI) to the edge of the film. Ohmic behavior was established by measuring current-voltage characteristics. A qualitative Seebeck voltage measurement was made to determine the carrier type.
Results and Discussion In s p r a y p y r o l y s i s a s o l u t i o n is a t o m i z e d , s p r a y e d o n t o a h o t s u b s t r a t e , a n d d e c o m p o s e d , r e s u l t i n g in t h i n film g r o w t h . T h e s o l u t i o n c a n be n e b u l i z e d u l t r a s o n i c a l l y , a n d s p r a y e d c o n t i n u o u s l y or in p u l s e s . V i g u i e a n d S p i t z (8) s u g g e s t e d four p o s s i b l e g r o w t h m e c h a n i s m s for t h e s p r a y p y r o l y s i s p r o c e s s as a
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f u n c t i o n of s u b s t r a t e t e m p e r a t u r e . T h e s e a r e shown in Fig. 2 and can be r e l a t e d to t h e g r o w t h of CuO films in t h i s s t u d y . In t h e f i r s t process, t h e s p r a y d r o p l e t impinges d i r e c t l y on t h e s u b s t r a t e , followed by e v a p o r a t i o n of t h e s o l v e n t , and d e c o m p o s i t i o n of t h e m e t a l s a l t to t h e oxide. In t h e s e c o n d process, t h e s o l v e n t is e v a p o r a t e d j u s t p r i or to c o n t a c t i n g t h e s u b s t r a t e , followed by d e c o m p o s i t i o n of t h e oxide. The t h i r d m ech an i sm i n v o l v e s v o l a t i l i z a t i o n of t h e dried m e t a l s a l t , d i f f u s i o n of t h e v a p o r to t h e s u b s t r a t e , followed by d e c o m p o s i t i o n to t h e oxide. This p r o c e s s is r e f e r r e d to as low t e m p e r a t u r e chemical v a p o r d e p o s i t i o n (LTCVD). The f o u r t h p r o cess is a ho m o g en eo u s n u c l e a t i o n of t h e v a p o r p h a s e forming t h e oxide p a r t i c l e which t h e n d e p o s i t s on t h e s u b s t r a t e . The f o r m a t i o n of CuO films in t h i s s t u d y p r o b a b l y p r o c e e d s by t h e t h i r d me ch an i s m s i n ce t h e d e p o s i t e d films a r e smooth, homogeneous, and h a v e a mirror-like appearance. Albin et al. (5), h a v e s u g g e s t e d t h e d e p o s i t e d film o b t a i n e d by t h e l a s t p r o c e s s would h a v e a powdery a p p e a r a n c e . Viguie and Spitz (6) h a v e shown t h a t films grown by p r o c e s s e s A and B h a v e a r o u g h e r m i c r o s t r u c t u r e t h a n films grown by p r o c e s s C. EDAX r a t i o s of c o p p e r - t o - s i l i c o n d e t e r m i n e d t h e r e l a t i v e u n i f o r m i t y of t h e CuO films and a Sloan Dektak s u r f a c e p r o f i l e m e a s u r i n g s y s t e m d e t e r m i n e d t h e a b s o l u t e t h i c k n e s s . The t h i c k n e s s and EDAX r a t i o s of t h e films v a r i e d with t h e number of c y c l e s i n v o l v e d in film p r e p a r a t i o n , and t h i s is shown in
Mechanisms for film growth in spray pyrolysis processing L
¸
substrate 2..;. 2:,z:
t solid
t t
eo o
000
vapor
•
•
precipitate
,,,.o,,o,
0
0
0
0
A
B
C
D
Increasing substrate temperature
Fig. 2.
Mechanisms of film growth.
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T a b l e II. A f t e r p r e p a r a t i o n , film t h i c k n e s s w a s d e t e r m i n e d from a c a l i b r a t i o n o f EDAX Cu/Si r a t i o v s . t h i c k n e s s o b t a i n e d from t h e d a t a in T a b l e II. T h e d e s i r e d t h i c k n e s s c o u l d b e a p p r o x i m a t e d by t h e n u m b e r o f c y c l e s in t h e preparation. EDAX r a t i o s w e r e t a k e n a t f i v e p o i n t s from e d g e t o e d g e on t h e f i l m s a n d t h e s e r a t i o s c l e a r l y s h o w t h a t CuO f i l m s p r e p a r e d b y t h i s m e t h o d a r e uniform.
T a b l e II EDAX Cu/Si R a t i o s v s T h i c k n e s s Position #
Film A
1 2 3 4 5
and Number of Cycles
B
C
D
0.38(I) 0.39 0.39 0.39 0.38
0.08(i) 0.08 0.08 0.08 0.08
0.18(1) 0.18 0.17 0.18 0.17
0.16(1) 0.16 0.16 0.17 0.17
(A)
2700
1200
1900
1850
Number Cycles
150
75
110
110
Thickness
An SEM p h o t o m i c r o g r a p h t a k e n a t low m a g n i f i c a t i o n o f a 2 0 0 0 A CuO film p r e p a r e d in t h i s s t u d y is s h o w n in Fig. 3. T h e n a t u r e o f t h e p a r t i c l e s in t h e s e CuO f i l m s a r e v e r y d i f f e r e n t f r o m CuO f i l m s p r e p a r e d u s i n g MOCVD by L a u r i e a n d N o r t o n (4), w h e r e t h e f i l m s a r e c o m p r i s e d o f n e e d l e - l i k e p a r t i c l e s .
Fig. 3.
Photomicrograph of a typical C u O film prepared from an acetate solution.
758
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et a l .
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24,
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CuO
PolxcryGtol Standard
(lid C-liD
>-, 4J @ C
O
C l-t 0
>
Thin Film on SIO~ 0 01
(11D (-lid
20
30
40
50
60
70
80
Oi??roction Angle 28 (de9) Fig. 4.
X - r a y d i f f r a c t i o n p a t t e r n o f a CuO film on a s i l i c a s u b s t r a t e a s c o m p a r e d w i t h t h a t o f a p o l y c r y s t a l l i n e CuO standard. The differences in relative peak intensities d e m o n s t r a t e t h e e x i s t e n c e of a p r e f e r r e d o r i e n t a t i o n in t h e film.
CuO ....
I00
i ....
?ilm
on
i ....
i
silico
....
i ....
i
....
/11a ~- ....
80
/ ;t
E 0
/
60
/ / /
0} O)
/ /
40
o= & 20
0 400
500
B00
700
Wovelength Fig. 5.
Optical transmission
800
000
I000
(nm)
s p e c t r u m of a CuO film on a s i l i c a s u b s t r a t e .
6
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X - r a y d i f f r a c t i o n d a t a is s h o w n in Fig. 4. A CuO s t a n d a r d w a s p r e p a r e d by h e a t i n g Cu m e t a l (JM 5 0 2 5 0 A ) in a i r a t 600°C. By c o m p a r i n g t h e x - r a y o f t h e f i l m to t h e s t a n d a r d , i t c a n be s e e n t h a t t h e i n t e n s i t y o f t h e (020) p e a k o f t h e film is i n c r e a s e d a n d t h e ( - 2 0 2 ) a n d (202) p e a k s a r e a b s e n t . This can be a t t r i b u t e d t o p r e f e r r e d o r i e n t a t i o n o f t h e p a r t i c l e s m a k i n g up t h e film, growing with t h e b - a x i s p e r p e n d i c u l a r to t h e s u b s t r a t e . The transmission spectra for a copper(If) oxide film prepared from copper(II) acetate is shown in Fig. 5, and is in agreement with copper(If) oxide films prepared by ,Qayi et al. ( 3 ) . The optical transmission was used to generate plots of (ahu) Ij2 vs hu. It was found that the indirect band gap of the prepared copper(II) oxide films was 1.36(i) eV. All CuO thin films prepared in this study were p - t y p e semiconductors with room temperature resistivity of 102 ohm-cm. Beensh-Marchwicka et al. (1) have reported resistivities of i ohm-cm at room temperature for CuO films grown by reactive-ion sputtering; however, carrier type was not measured. The low resistivity is consistent with the presence of mixed formal valence of copper. If Cu(II) and Cu(III) were present in the film, then anneal~ng ~he film in argon would increase the resistivity by reducing the Cu(III) to Cu(II). In contrast, if the low resistivity of the film was due to the presence of Cu(1) and Cu(II), then an argon anneal would further decrease the resistivity by" increasing the amount of Cu(1) in the sample. Typical films of CuO were annealed in argon at 650°C for 15 hrs and the resulting resistivity and carrier type were measured. The resistivity of the films increased to 103 ohm-era and remained p-type. R e a n n e a l i n g t h e s e f i l m s in o x y g e n a t 650°C for ]6 h r s r e s u l t e d in r e s i s t i v i t y d e c r e a s i n g t o ].02 o h m - c m . T h e r e s i s t i v i t y o f t h e f i l m s a n n e a l e d in a r g o n is h i g h e r t h a n t h e r e s i s t i v i t y o f t h e f i l m s p r e p a r e d in o x y g e n w i t h n o c h a n g e in c a r r i e r t y p e . This h i g h e r r e s i s t i v i t y of films a n n e a l e d in a r g o n is c o n s i s t e n t w i t h a d e c r e a s e in Cu(III) a s a r e s u l t o f t h e annealing process.
Conclusion U n i f o r m , h o m o g e n e o u s c o p p e r ( I I ) o x i d e t h i n f i l m s c a n be p r e p a r e d in a n o v e l t w o - z o n e h o r i z o n t a l s p r a y p y r o l y s i s r e a c t o r by d e c o m p o s i n g a q u e o u s copper(II) salts. The s p r a y in t h e form of an a e r o s o l was p r o d u c e d by a c o m m e r c i a l u l t r a s o n i c h u m i d i f i e r . T h e film w a s g r o w n b y s p r a y i n g p u l s e s o f t h e aerosol and heating the deposited thin layer at a higher temperature after each spray pulse. T h e f i l m s g r e w w i t h p r e f e r r e d o r i e n t a t i o n a s i n d i c a t e d by x-ray diffraction. In a d d i t i o n , t h e f i l m s w e r e p - t y p e s e m i c o n d u c t o r s w i t h room t e m p e r a t u r e r e s i s t i v i t y o f 102 o h m - c m . T h e p - t y p e n a t u r e o f t h e f i l m s w a s d e t e r m i n e d to be d u e t o t h e p r e s e n c e o f Cu(III).
Acknowledgements This r e s e a r c h was p a r t i a l l y s u p p o r t e d by t h e N a t i o n a l Science F o u n d a t i o n u n d e r G r a n t No. D M R - 8 8 0 3 1 8 4 a n d b y t h e E a s t m a n K o d a k C o m p a n y o f R o c h e s t e r , NY.
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