Chemistry of High-Temperature Superconductors II - American

Chapter 19 Advances i n P r o c e s s i n g H i g h - T e m p e r a t u r e Superconducting T h i n Films with Lasers 1

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T. Venkatesan , X. D. Wu , A. Inam , M. S. Hegde, E. W. Chase , C. C. Chang , P. England , D. M. Hwang, R. Krchnavek , J. B. Wachtman, W. L. McLean , R. Levi-Setti , J. Chabala , and Y. L. Wang 1

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Downloaded by UNIV OF ROCHESTER on January 19, 2018 | http://pubs.acs.org Publication Date: September 26, 1988 | doi: 10.1021/bk-1988-0377.ch019

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1Bell Communication Research, Inc., 331 Newman Springs Road, Red Bank, NJ 07701-7020 Rutgers University, Piscataway, NJ 08854 Bell Communication Research, Inc., 435 South Street, Morristown, NJ 07906 University of Chicago, Chicago, IL 60637 2

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The stringent requirements for the preparation of high T superconducting thin films, based on the demands for device fabrication, pose a major challenge to the number of thin film deposition and processing techniques. In this paper we examine the generic problems of the various techniques and expand on the capabilities of a pulsed laser deposition process. We show that using suitable processing steps the laser deposition technique for preparation of high-T thin films is emerging as a strong contender among the various thin film deposition techniques. c

c

High temperature superconducting (FîTSC) materials are metal oxides and the metal oxide system has been very important for micro- and opto- electronics for properties other than superconductivity [1]. Properties such as ferroelectricity, optical nonlinearities, high optical transparency, relatively large controllable refractive indices, etc., have made metal oxides very useful for technological applications (table 1). The metal oxides could be doped with transition metal ions which significantly affect their optical properties; eg., Ti doping of LiNb0 to form waveguides and Cr doping of AI2O3 to form light emitters. Technologies such as ion implantation could be effectively utilized to modify the surface properties of metal oxides [2]. With the discovery of HTSC in metal oxide systems, the importance of these materials has significantly escalated. This is probably the only system where, by modifying the oxygen composition, the film could be tailored from a perfect dielectric to a superconductor. As a result, films of these materials have potential novel applications in advanced technologies such micro- and opto- electronics. 3

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0097-6156/88/0377-0234S08.75/D 1988 American Chemical Society

Nelson and George; Chemistry of High-Temperature Superconductors II ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

19.

VENKATESAN ET AL.

235

Processing Thin Films with Lasers

Table 1. A brief list of the properties and examples of metal oxides that have applications in micro- and opto-electronics Property

Example of metal oxide system

Application

High optical transparency

MgO, ZrO

Mirror Coatings


Low loss diectric with electro-op tic effect

LiNb0

3l

LiTa0

Optical wave guides and integrated optics

3

Piezoelectricity

BaTi0 , PbTi.4eZr.54O3

Transducers

Ferromagnetism

7-Fe 0

Magnetic tape memories

Optical nor linearity

Nb2 Og-SiOa-Naa O-Bag O - T i 0

High optical gain

Nd * doped Y A l e 0

3

2

3

s

2

3

3

13

All optical switching devices

Lasers

Tansparent conductors

InSnO.

Novel device coatings

Superconductivity

Y-Ba-Cu-0 Bi-Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-0

SQUID Superconducting electronics

Since one of the p o t e n t i a l l y important applications i s microe l e c t r o n i c s i t may be a worthwhile digression t o speculate on how e l e c t r o n i c s , p h o t o n i c s and HTSC d e v i c e s c o u l d c o e x i s t i n a s i n g l e device or system. The electron-electron i n t e r a c t i o n i s strong, and t h i s q u a l i f i e s e l e c t r o n i c s f o r e f f i c i e n t switching devices, whereas the photon-photon i n t e r a c t i o n i s weak, which q u a l i f i e s photonics f o r i n f o r m a t i o n t r a n s m i s s i o n w i t h minimum c r o s s - t a l k ( f u r t h e r low a b s o r p t i o n and d i s p e r s i o n media f o r photons e x i s t as w e l l ) . However, superconductors e x h i b i t p r o p e r t i e s o f b o t h e l e c t r o n s and photons; there i s a strong i n t e r a c t i o n between the b a s i c quanta, and s u p e r c o n d u c t i n g m a t e r i a l s e x h i b i t low absorption losses and d i s p e r s i o n i n p r o p a g a t i n g s i g n a l s . Hence, w h i l e superconductors w i l l not replace photons i n terms of t h e i r attractiveness f o r data t r a n s m i s s i o n w i t h low c r o s s - t a l k , a h y b r i d e v o l u t i o n o f superconductors coexisting with e l e c t r o n i c s and p h o t o n i c s seems a h i g h likelyhood f o r eventual high bandwidth applications. An example of a f u t u r i s t i c h i g h bandwidth h y b r i d system i s shown i n f i g . 1, where h i g h b i t r a t e i n f o r m a t i o n a r r i v e s a t a compound semiconductor i n t e r f a c e where photons get converted t o electrons; the e l e c t r o n i c s i g n a l i s p r o c e s s e d by VLSI S i based c h i p s as w e l l as s p e c i a l i z e d HTSC c h i p s b e f o r e b e i n g r e t r a n s m i t t e d as photons v i a a compound semiconductor interface.


236

CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS Π Compound Semiconductor itegrated ^

OUT

Special Function High T


c

Photonic High Bit Rate Information

Integrated Optics

Figure 1 . A f u t u r i s t i c chip combining conductor e l e c t r o n i c technologies.

opto-, micro-and

super

CHALLENGES

Table 2 i l l u s t r a t e s the requirements f o r t h i n f i l m HTSC materials i n o r d e r t o i n t e g r a t e t h e superconductors w i t h m i c r o - and o p t o electronics. In f i g . 2 i s shown the molecular and complex c r y s t a l s t r u c t u r e f o r t h e v a r i o u s HTSC m a t e r i a l s and t h e i n c r e a s i n g complexity of the m a t e r i a l s with increasing T i l l u s t r a t e s the d i f f i c u l t i e s inherent i n the process f o r synthesizing these f i l m s i n o r d e r t o a c c o m p l i s h t h e requirements l i s t e d i n t a b l e 2. With a viable f i l m f a b r i c a t i o n technique: ç

(a) (b) (c)

one must be able t o produce smooth films with the appropriate composition, get the r i g h t c r y s t a l phase, and produce the r i g h t oxygen stoichiometry i n the f i l m .

Table 2. Needs for device fabrication 1. H i g h T ( R = 0 ) 2. Small Δ Τ (transition width) 3. H i g h J (critical current density) c

c

4. Surface smoothness 5. Stable film on substrates such as S i 6. Sharp interfaces The number o f t h i n f i l m deposition techniques demonstrated t o date v a r y i n terms o f t h e i r ease i n meeting t h e above c r i t e r i a i n p r o d u c i n g good q u a l i t y f i l m s . The v a r i o u s d e p o s i t i o n t e c h n i q u e s could be g e n e r i c a l l y divided i n t o two classes: multiple sources o r a Nelson and George; Chemistry of High-Temperature Superconductors II ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

19.

VENKATESANETAL.



m

S

m

> r

La Cu0 2

La,

YBa Cu30 2

4

Sr«

Cir

Υβ

0"

7

Βα·

Cu«0°

ί > r 7*

^

Bi Sr CaCu 0 2

2

Bi Ca · Sr Cu-

2

5

TI Ba Ca Cu 0 2

Cu-0

e

e

8 + (

2

2

3

bonds are

1 0

shown

0°

Figure 2. Approximate c r y s t a l structures of the various high T superconductors (courtesy of J. M. Tarascon). c


237

238

CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS Π

single source f o r depositing the d i f f e r e n t elements. In f i g . 3a i s shown a t y p i c a l geometry f o r deposition from d i f f e r e n t sources. The elements are e j e c t e d from these sources u s i n g heat, e l e c t r o n s o r ions. The generic problems with these systems are:


1. the r e l a t i v e e j e c t i o n rate of the elements must be monitored ( i n mTorr oxygen ambients) and kept a constant, 2. the non-overlap o f the atomic t r a j e c t o r i e s from the three sources must be overcome e i t h e r with planetary m a n i p u l a t i o n o f the s u b s t r a t e h o l d e r o r mounting the sources i n a r a d i a l c o n f i g u r a t i o n . On the o t h e r hand, the use o f a s i n g l e t a r g e t ( f i g . 3b) has i t s associated problems: 1. the e j e c t i o n rate of the three elements i s not the same (eg. i o n sputtering y i e l d s are d i f f e r e n t f o r the three species), 2. the s t i c k i n g c o e f f i c i e n t s of these elements on the substrate are a l s o d i f f e r e n t . As a r e s u l t , the t a r g e t w i l l not t h a t o f the d e p o s i t e d f i l m , and composition w i l l also depend upon the sputtering, e f f e c t s such as negative must be overcome.

have the same c o m p o s i t i o n as f u r t h e r the optimum t a r g e t deposition parameters. In ion ion bombardment of the f i l m s

While the above problems have been overcome and good f i l m s have been demonstrated by e-beam and thermal evaporation from m u l t i p l e sources [3-5], s p u t t e r i n g from m u l t i p l e and s i n g l e targets [6-8], MBE d e p o s i t i o n [9,10], s o l - g e l techniques [11], one o f the most v e r s a t i l e techniques has been laser deposition [12]. LASER DEPOSITION The t e c h n i q u e c o n s i s t s o f f i r i n g a p u l s e d excimer l a s e r a t a s t o i c h i o m e t r i c p e l l e t o f the m a t e r i a l t o be d e p o s i t e d and under s u i t a b l e c o n d i t i o n s o f l a s e r energy d e n s i t y , oxygen p a r t i a l pressure, substrate temperature and deposition angle, high q u a l i t y f i l m s are d e p o s i t e d . What i s remarkable about the p r o c e s s i s the stoichiometric deposition o f f i l m s achieved by t h i s technique. The c o m p o s i t i o n o f the p e l l e t i s c l o s e l y reproduced i n the f i l m s . A schematic o f the d e p o s i t i o n system i s shown i n f i g . 4. The deposition and annealing parameters are shown i n table 3. In f i g . 5a i s shown a R u t h e r f o r d b a c k s c a t t e r i n g spectrum o f a f i l m deposited from a p e l l e t of Υ ι ^ ο ^ ^ ^ - χ · 9 with a s i m u l a t i o n o f Y j B a j C ^ O g 1 In t i g . 5b and 5c are shown the s p e c t r a f o r f i l m s d e p o s i t e d from p e l l e t s where Y was s u b s t i t u t e d with Eu and Gd, and the composition i s s t i l l preserved. The message i s v e r y c l e a r : the l a s e r d e p o s i t i o n p r o c e s s i s a s t r a i g h t f o r w a r d t e c h n i q u e t o produce a complex f i l m s t a r t i n g from a p e l l e t o f the d e s i r e d f i l m composition. S i n c e the f a b r i c a t i o n o f p e l l e t s o f complex materials i s r e l a t i v e l y easy, the l a s e r deposition technique becomes increasingly a t t r a c t i v e f o r the deposition of complex films. T

n

e

d

a

t

a

a

r e e s


19. VENKATESANETAL.


239

(a) Multiple targets Substrate


Targets

Substrate

Target Figure 3. Generic deposition systems: (a) multiple sources and (b) single source.

Quartz window

Laser beam (248 nm, 30 ns)

Vacuum chamber

Figure 4 .

Schematic o f the laser deposition system.


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Table 3. Parameters for depositing and processing the films

248 n m

Laser wavelength

30 ns

Pulse length Energy density

1 - 2 J/cm

2

RE Ba Cu O .

Target

1

Target-substrate distance

High Temperature Processing ( H T P ) T^MubstraU ~ 400 C Background pressure ~ 1 0 ~ Torr e

2

3

a

3 cm

Low Temperature Processing ( L T P ) ~ 600 C ~ 5 mTorr 0

2

Post Anneal i n Oxygen

Post Anneal i n Oxygen

800-900 C ~ 1 h

450 C ~ 3 h

and slow cool

and slow cool


19.


VENKATESAN ET AL.

Energy (MeV) 2.5 3.0

2.0


241

Energy (MeV) 0.5 80 • τ

1.0 !

Λ

(b) Ξ

6

[1

° Γ

*>-

1! Bo

40 x10

ίΖ

M

D

Ό

S σ Ε

2.0 1

1.5 1

Ο 20

Π

100

Il

Eu

Cu /'H

i 200

i

Τ" 400

300

500

Channel Energy (MeV) 1.0

1.5

500

Figure 5. Rutherford from various p e l l e t s . l i n e i s a simulation Eu i n (b) and Gd i n

backscattering spectra o f f i l m s deposited The s o l i d l i n e i s the data and the dashed f o r RE Ba CU30 _ where RE i s Y i n (a). (c). 1

2

7

x


242

CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS II

F i m CRYSTALLIZATION


There are two more obstacles t o be overcome before one can have good superconducting f i l m s . The atoms i n the f i l m must be i n the r i g h t c r y s t a l l o g r a p h i c phase and the r i g h t amount o f oxygen needs t o be incorporated i n t o the structure. There are two approaches t o accomplish t h i s as shown i n t a b l e 3: one i s t o d e p o s i t the f i l m i n some random phase a t a r e l a t i v e l y low temperature and follow t h i s up with a high temperature (800-900 C) anneal i n oxygen [12]. However, a more elegant way t o accomplish t h i s i s t o deposit the f i l m s d i r e c t l y i n the orthorhombic phase at a temperature of 600-700 C i n an oxygen ambient (a few mTorrs) [13]. From a t e c h n o l o g i c a l p o i n t o f view, i n o r d e r t o produce sharp interfaces, and minimize film-substrate i n t e r a c t i o n and stresses i n the f i l m , a low temperature p r o c e s s i s a b s o l u t e l y e s s e n t i a l . To t h i s extent, the l a t t e r p r o c e s s i s the p r e f e r r e d one. We w i l l i l l u s t r a t e the advantages o f the l a t t e r process with experimental data. In f i g . 6 are shown the SEM p i c t u r e s o f two HTSC s u r f a c e s prepared by the two processes [14]. Figs. 6a and 6b i l l u s t r a t e the d e l e t e r i o u s e f f e c t s o f h i g h e r temperature p r o c e s s i n g (HTP). The surface i s extremely grainy and one can even see cracks on the f i l m surface. The s u b s t r a t e i s s a p p h i r e and t h i s i s a t y p i c a l consequence of the high temperature anneal on such a substrate. The thermal expansion mismatch between the f i l m and substrate i s minimum f o r S r T i 0 and the l e a s t amount o f s u r f a c e c r a c k s are observed on t h i s s u r f a c e . On the o t h e r hand, i n f i g s . 6c and 6d we show the surface morphology of the low temperature processed (LTP) f i l m on sapphire which seems t o show no surface roughness what so ever. The s m a l l d e f e c t a t the c e n t e r was used t o focus the e l e c t r o n beam. Using cross s e c t i o n a l specimens the surface roughness was estimated t o be 100 A over a dimension of 1 um length of the specimen. 3

This r e s u l t i s further i l l u s t r a t e d i n the TEM cross sections i n f i g . 7 where i n 7a one sees the s u r f a c e o f the h i g h temperature processed f i l m [15] t o be rough, on the order of > 1000 A. Further, the o r i e n t a t i o n o f the c r y s t a l l i t e s seems t o be quite random, though the c r y s t a l i t e s i n close proximity t o the substrate do show a c axis o r i e n t a t i o n normal t o the s u r f a c e ( f i g . 7b). However, t h e low t e m p e r a t u r e p r o c e s s e d f i l m s , shown i n f i g . 7c, e x h i b i t a crystallographic structure with a smooth surface, where close t o 90% o f the c r y s t a l l i t e s have a c a x i s o r i e n t a t i o n normal t o the s u r f a c e , t h o u g h t h e r e i s some m o s a i c i t y w i t h a few d e g r e e d i s t r i b u t i o n of the c axis o r i e n t a t i o n with respect t o the surface normal ( f i g . 7d). In t h i s sense the c r y s t a l layers are analogous t o those found i n h i g h l y oriented p y r o l y t i c graphite. The r e s u l t s of TEM cross s e c t i o n a l analysis c l e a r l y show that f o r the HTP f i l m s the g r a i n boundaries develop w i t h the excess elements segregated at the g r a i n boundaries. Even though the f i l m s do s t a r t with the r i g h t stoichiometry the high temperature annealing causes some i n t e r d i f f u s i o n a t the i n t e r f a c e r e s u l t i n g i n the f o r m a t i o n o f o f f - s t o i c h i o m e t r i c phase boundaries. In f i g . 8a one



VENKATESAN ET AL.


Figure 6. SEM micrographs o f f i l m s d e p o s i t e d by (a) and (c) high temperature processing (ΗΓΡ), (b) and (d) low temperature p r o c e s s i n g (LTP) (ref. 14).



CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS H

Figure 7. TEM cross s e c t i o n a l view o f specimens prepared on S r T i 0 by (a) and (b) HTP, (c) and (d) LTP, a t two d i f f e r e n t magnifications. 3


VENKATESAN ET AL.



19.

Figure 8. SIMS images generated on a HTP f i l m using a focussed ion beam o f gallium; the beam s i z e i s about 30 nm and the scales are shown i n the figure. The images are f o r (a) secondary electrons, (b) oxygen, (c) barium and (d) carbon ions.


245

246


sees a f o c u s s e d i o n beam induced secondary e l e c t r o n image and the s u r f a c e roughness i s q u i t e e v i d e n t . The secondary i o n images i n f i g s . 8b and 8c i l l u s t r a t e the Ο and Ba s e g r e g a t i o n a t g r a i n boundaries which show a good c o r r e l a t i o n . The C image ( f i g . 8d) on the other hand shows l i m i t e d c o r r e l a t i o n with the Ο and Ba signals. T h i s r u l e s out BaC0 as the p o s s i b l e segregant i n these g r a i n boundaries. The most l i k e l y form i n which Ba i s segregated i s l i k e l y t o be Barium hydroxide. On the o t h e r hand the LTP f i l m s showed no phase boundaries a t a l l and the elemental composition was very homogeneous. The g r a i n boundaries i n the LTP f i l m s consisted of c r y s t a l defects such as dislocations or stacking f a u l t s .


3

The reduction of the process temperature r e s u l t s i n very l i t t l e r e a c t i o n between the f i l m and the s u b s t r a t e as shown by the Auger e l e c t r o n s p e c t r o s c o p y (AES) r e s u l t s o f f i g s . 9a and 9b. I n f i g . 9a the AES r e s u l t s indicate a reacted layer on the order o f 100 nm i n a OTP f i l m of the order o f 350 nm. However, i n the LTP f i l m shown i n f i g . 9b the interface reaction extends over a region o f less than 12 nm, close t o the instrument resolution o f the technique, ttiis r e s u l t i s v e r y i m p o r t a n t f o r the f a b r i c a t i o n o f s t r u c t u r e s c o n t a i n i n g abrupt junction. The r e s i s t a n c e v e r s u s temperature curve f o r the LTP f i l m on S r T i 0 3 shown i n f i g . 10 i n d i c a t e s a Τ o f < 1 Κ and the c r i t i c a l current densities i n the LTP f i l m s are quite reasonable, about 1x10 A/cm at 82 Κ [16]. The advantages of the process c l e a r l y point t o the need f o r f u r t h e r l o w e r i n g o f the p r o c e s s temperature. As a r e s u l t o f u s i n g low temperature p r o c e s s i n g , good f i l m s have been deposited d i r e c t l y ( T (R=0) o f 67 K) as w e l l as w i t h a v e r y t h i n Z r 0 b u f f e r l a y e r on s i l i c o n [17]. In f i g s . 11a and l i b a r e shown the transport data on a Υ ^ Β ^ α ^ Ο ^ f i l m deposited on S i with only a 500 A ZrOo b u f f e r l a y e r . The z e r o r e s i s t a n c e t r a n s i t i o n temperature of 80 Κ i s quite respectable though the voltage versus c u r r e n t c h a r a c t e r i s t i c s shown i n f i g . l i b i n d i c a t e a temperature dependence c h a r a c t e r i s t i c o f g r a n u l a r s u p e r c o n d u c t i v i t y [18-19]. The c r i t i c a l c u r r e n t i n t h i s f i l m was about 5 x l 0 A/cm* a t 10 K, which may be adequate f o r some n o v e l d e v i c e s such as d e t e c t o r s , w h i l e f o r h i g h c u r r e n t a p p l i c a t i o n s the c r i t i c a l c u r r e n t d e n s i t y needs t o be a couple of orders of magnitude larger. 2

c p

2

4

2

The v e r s a t i l i t y o f the technique i s i l l u s t r a t e d i n f i g s . 12a and 12b showing stoichiometric deposition of Bi^Sr3Ca3CUcO f i l m s , and the resistance versus temperature curve o f an annealed OTP f i l m showing a s l i g h t drop i n r e s i s t i v i t y at 110 K. x

RECENT PROGRESS Low temperature deposition. Recently i n addition t o those i n refs. 13, 20 and 21 other groups have developed low temperature processes using deposition techniques such as activated and microwave-assisted reactive evaporation [22-23], electron-enchanced l a s e r e v a p o r a t i o n [24] t o fabricate superconducting t h i n f i l m s at temperatures below 600 C. I t i s important at t h i s stage t o point out an inconsistency i n the l i t e r a t u r e w i t h regards t o the d e f i n i t i o n o f d e p o s i t i o n temperature. Most groups i n c l u d i n g us, now r e p o r t the s u b s t r a t e


19.

VENKATESAN ET AL.

0


247


2

4

β

8

10

12

14

Sputter Time (min.)

Sputter Time (min.) Figure 9. Auger depth p r o f i l i n g o f (a) HTP and (b) LTP f i l m ( r e f . 13).

lr

0

50

100

150

Temperature

200

250

300

(K)

Figure 10. Resistance versus temperature c h a r a c t e r i s t i c o f a LTP f i l m on SrTiC^ ( r e f . 16).

American Chemical Society Nelson and George; Chemistry of High-Temperature Superconductors II ACS Symposium Series; American Chemical Society: Washington, DC, 1988. Library


248

CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS II

20

Figure 11. (a) R e s i s t a n c e versus temperature c h a r a c t e r i s t i c o f an LTP f i l m p r e p a r e d on S i w i t h a 50 nm Z r 0 b u f f e r layer; (b) Voltage versus current c h a r a c t e r i s t i c o f the same f i l m as a function o f d i f f e r e n t temperatures (ref. 17). 2




VENKATESAN ET AL.

Figure 12. (a) RBS spectrum o f a f i l m d e p o s i t e d a t room temperature with the composition Bi^S^C^CugC^ from a p e l l e t o f s i m i l a r composition; (b) R e s i s t a n c e versus temperature c h a r a c t e r i s t i c of the f i l m subsequent t o annealing i n oxygen a t 830 C f o r 5 hours ( r e f . 16).


250


holder temperature because of the d i f f i c u l t y of measuring the actual s u b s t r a t e s u r f a c e temperature d u r i n g each d e p o s i t i o n run. The holder temperature i s higher than the surface temperature by 50 t o 150 C i n our system but we prefer t o report the holder temperature rather than an estimated surface temperature that i s less accurate. Using the pulsed l a s e r deposition technique (PUD), we have already demonstrated that stable t h i n f i l m s with surface smoothness of l e s s t h a n 10 nm and i n t e r f a c e s sharper than 15 nm can be f a b r i c a t e d a t substrate holder temperatures below 650 C [13,16]. In t h i s section, we report on an improved low temperature process that y i e l d s high J (~10 A/cm at 77 K) and high T (R=0 at -89 K) superconducting t h i n f i l m s without any post-deposition anneal. c

6

2


c

D e t a i l s o f the PLD p r o c e s s have been p u b l i s h e d elsewhere [12,13,16]; b r i e f l y , p u l s e s from a 248 nm KrF excimer l a s e r are f i r e d a t a r a t e o f one h e r t z onto a r o t a t i n g Y Ba2Cu30-7_ t a r g e t i n s i d e the d e p o s i t i o n chamber. The ensuing plume o f e j e c t e d material from the target i s c o l l e c t e d onto a substrate mounted on a r e s i s t i v e l y h e a t e d h o l d e r (the s u b s t r a t e h o l d e r temperature, as measured by a thermocouple, may be varied from 25 C t o 650 C), which i s h e l d at a distance of ~6 cm from the target. Equilibrium oxygen p r e s s u r e o f a few m i l l i t o r r s i s m a i n t a i n e d i n the chamber d u r i n g deposition. I t was shown e a r l i e r that as-deposited superconducting f i l m s , w i t h the c o r r e c t "123" s t o i c h i o m e t r y and a z e r o r e s i s t a n c e t r a n s i t i o n temperature T ~ 3 0 Κ are produced u s i n g t h i s process. Subsequent t o annealing at 450 C i n flowing oxygen f o r three hours, 1

x

c o

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o

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u

p

t

o

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6

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a

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d

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s

a

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f i l m s on S r T i 0 had improved ço'§ c' [16] of greater than 1.5 χ 10 A/cm . X-ray d i f f r a c t i o n analysis o f as-deposited f i l m s on (100) SrTiOo showed a h i g h l y c-axis oriented orthorhombic phase, but the f i l m s p r o b a b l y c o n t a i n e d oxygen d e f i c i e n t material possibly explaining the i n i t i a l l y low T's. Post d e p o s i t i o n a n n e a l i n g a t 450 C i n f l o w i n g oxygen improved the T 's s i g n i f i c a n t l y , but the maximum was 86 Κ and often lower. Since b u l k i 2 3 ° 7 - m a t e r i a l can have T ' s over 90 K, these f i l m s appear t o c o n t a i n d e f e c t s t h a t cannot be e l i m i n a t e d by low temperature (450 C) post deposition anneal i n oxygen. 3

5

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c

v

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C u

x

c o

I t has been c l e a r from our understanding o f the h i g h T t h i n f i l m formation process, that incorporation of s u f f i c i e n t oxygen was important f o r formation o f the c o r r e c t o r t h o r h o m b i c phase. Therefore, we introduced oxygen d i r e c t l y i n t o the plume emanating from the target by surrounding the plume with an open ended box-like copper e n c l o s u r e measuring about 2 cm , and i n j e c t i n g a j e t o f oxygen gas d i r e c t l y i n t o the center of t h i s box. The oxygen was fed i n at a rate of 5 seem while the background pressure i n the chamber, as measured by a pressure guage mounted about 25 cm from the nozzle, was maintained at 8 mTorr. We found that the plume passing through the oxygen column changed i t s c o l o r from b l u i s h - w h i t e t o red, i n d i c a t i n g a d i r e c t r e a c t i o n between the i o n s i n the plume and molecular oxygen. We speculate that metal ions react with oxygen f o r m i n g m e t a l - o x i d e i o n s t h u s c o m p e n s a t i n g f o r the oxygen d e f i c i e n c y . We are p r e s e n t l y p u r s u i n g a s p e c t r o s c o p i c study t o understand the r e a c t i o n s between the l a s e r induced plume and the oxygen. Apart from these m o d i f i c a t i o n s , the d e p o s i t i o n procedure was the same as outlined i n the preceding paragraph. Substrates of c


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251


single c r y s t a l (100) S r T i 0 and A 1 0 were used. Substrate holder temperatures, as measured v i a a thermocouple, were varied from 500 C t o 650 C. Typically, f i l m s with a thickness o f 2000 t o 5000 A were deposited i n about an hour. Immediately f o l l o w i n g t h e d e p o s i t i o n , the substrate holder was allowed t o cool down t o room temperature i n 250 Torr o f oxygen. 3

2

3

T r a n s p o r t measurements. The as-deposited f i l m s were black and had a smooth m i r r o r - l i k e s u r f a c e . Four probe DC r e s i s t i v i t y measurements were made using wires attached to the f i l m s with s i l v e r ink. Figure 13 shows the r e s i s t i v i t y versus temperature curves f o r as-deposited 2000 Â f i l m s on (a) A 1 0 and on (b) (100) S r T i 0 . The f i l m on sapphire was deposited a t a substrate holder temperature o f 580 C and has a zero resistance temperature o f 78 K. The f i l m grown on S r T i 0 was deposited a t a substrate holder temperature o f 650 C and has z e r o r e s i s t a n c e below 88.6 K. The r e s i s t i v i t y o f the a s d e p o s i t e d f i l m on S r T i O ^ j u s t b e f o r e t h e onset o f t r a n s i t i o n i s below 30 uohms-cm, which we b e l i e v e t o be among t h e lowest r e s i s t i v i t i e s r e p o r t e d i n t h e l i t e r a t u r e f o r a Y-Ba-Cu oxide superconducting t h i n f i l m . The normal t o superconducting t r a n s i t i o n f o r t h i s sample i s very sharp, with a width Δ T_( 10-90 % ) , less than IK.


2

3

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A p o r t i o n o f t h e f i l m on S r T i 0 shown i n f i g . 13(b) was patterned i n t o a 16 urn wide l i n e by reactive ion etching t o measure the c r i t i c a l c u r r e n t d e n s i t y and f o r o t h e r magnetotransport measurements. F i g u r e 14 shows the temperature dependence o f t h e c r i t i c a l c u r r e n t d e n s i t y , J . I t can been seen t h a t a t 77 Κ and i n z e r o f i e l d , a c u r r e n t d e n s i t y o f 0.69 χ 1 0 A/cm i s measured. The f i l m c r i t i c a l current density i s greater than 4 χ 10 A/cm a t 50 Κ and i n a f i e l d o f 14 Tesla. More d e t a i l e d magnetotransport r e s u l t s w i l l be mentioned later. 3

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Film crystallinity. A Scintag four c i r c l e diffractometer using Cu-K r a d i a t i o n was used t o perform x-ray d i f f r a c t i o n studies on the f i l m s . F i g u r e 15 shows the d i f f r a c t i o n data from an a s - d e p o s i t e d f i l m on (100) S r T i 0 grown a t a substrate holder temperature o f 650 C. The very strong (00L) r e f l e c t i o n s (where Lr=l,2,4,5,7,10; L=3,6,9 are obscured by t h e (100), (200), and (300) s u b s t r a t e peaks) i n d i c a t e t h e f i l m t o be o r i e n t e d w i t h i t s c - a x i s normal t o t h e substrate surface, and the absence o f any impurity peaks shows that the f i l m i s p r e d o m i n a n t l y s i n g l e phase. The h i g h degree o f o r i e n t a t i o n o f the f i l m with respect t o the substrate i s confirmed by a sharp peak i n a scan t r a n s v e r s e t o t h e (100) s u b s t r a t e d i r e c t i o n , a c r o s s t h e Y ^ B a C u 3 0 _ (005) r e f l e c t i o n ( f i g u r e 15 inset), where the peak width (FWHM) i s only 0.22 degrees. A x i a l i o n c h a n n e l i n g measurements, t o be r e p o r t e d below, i n d i c a t e a v e r y s i m i l a r v a l u e and r e v e a l t h a t these f i l m s are h i g h l y c r y s t a l l i n e . Raman measurements [25] f u r t h e r c o n f i r m t h e s i n g l e c r y s t a l l i n e nature o f t h e f i l m by t h e conspicuous absence o f t h e 500 cm" v i b r a t i o n mode i n the spectra. This would be expected i f the c-axis of the c r y s t a l l i t e s were oriented i n the d i r e c t i o n o f the laser beam with the Raman s i g n a l detected i n a backscattering mode. a

3

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Surface perfection.

For e l e c t r o n i c applications o f the new high


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Figure 13. R e s i s t i v i t y vs. Τ curve f o r an as-deposited 0.2 urn Y^Ba9Cu30 f i l m on (a) A I 9 O 3 made a t 580 C,and (b) on (ΓΟΟ) S r T i 0 deposited a t 6S0 C. R=0 i s achieved a t 78 Κ and 88.6 Κ respectively. 7-x

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Figure 14. C r i t i c a l current density as a function o f temperature for an as-deposited f i l m on (100) SrTi03. J a t 77 Κ i s 0.69 χ 1 0 A/cm i n zero f i e l d . c

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20 (deg.) Figure 15. Radial x-ray d i f f r a c t i o n pattern o f as-deposited YjBajCuoOy f i l m on (100) SrTiO-y T r a n s v e r s e scan a c r o s s (005) r e f l e c t i o n has a width (FWHM) o f 0.22 degrees.


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T superconductors i t i s c r u c i a l t o have f i l m s w i t h smooth super c o n d u c t i n g s u r f a c e s , i n o r d e r t o d e l i n e a t e micron and submicron f e a t u r e s and t o f a b r i c a t e j u n c t i o n d e v i c e s c o n s i s t i n g o f super conducting l a y e r s sandwiching an i n s u l a t o r o r a normal metal. I n these j u n c t i o n s , t h e superconducting m a t e r i a l must have a h i g h T l a y e r r i g h t up t o t h e i n t e r f a c e t o w i t h i n t h e superconducting coherence l e n g t h o f the m a t e r i a l , which i s ~ 0.43 nm a l o n g t h e c a x i s and ~ 3.1 nm i n t h e a-b plane [26]. By u t i l i z i n g s u r f a c e s e n s i t i v e techniques such as X-ray Photoelectron Spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS) i n the channeling mode, i t i s possible t o obtain information about the f i l m surfaces and i n t e r f a c e s . The c h a n n e l i n g technique has been w i d e l y used t o c h a r a c t e r i z e v a r i o u s c r y s t a l l i n e m a t e r i a l s , s i n g l e c r y s t a l s with some d i s o r d e r e d r e g i o n s , p o l y c r y s t a l l i n e f i l m s on s i n g l e c r y s t a l substrates and so on [27]. Recently, S t o f f e l et. al.[28] showed that s i n g l e c r y s t a l s o f Y B a C u O have e x c e l l e n t c r y s t a l l i n i t y and stoichiometry t o w i t h i n about 1 nm o f the surfaces by using RBS i n t h e c h a n n e l i n g mode. XPS h a s a l s o b e e n u t i l i z e d t o o b t a i n i n f o r m a t i o n on t h e c h e m i c a l s t a t e o f the s u r f a c e l a y e r o f t h e superconductors [29]. Here, we r e p o r t t h e r e s u l t s o f a x i a l i o n c h a n n e l i n g and XPS s t u d i e s on a s - d e p o s i t e d h i g h T Y-Ba-Cu o x i d e superconducting t h i n f i l m s on (100) S r T i 0 , and show t h a t t h e c r y s t a l l i n i t y and c o m p o s i t i o n o f t h e m a t e r i a l i s good up t o the surface t o w i t h i n 1 nm, which i s comparable t o the superconducting coherence length. c


c

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RBS and c h a n n e l i n g measurements were made on a d u a l - a x i s goniometer using 3 MeV He""" ions with a 1 mm beam size. XPS spectra were taken on a s - d e p o s i t e d f i l m s w i t h o u t any c l e a n i n g steps. The spectra were recorded i n a KRATOS XSAM800 instrument equipped with a multichannel detector. The r e s o l u t i o n o f the XPS system was s e t t o y i e l d a peak width o f 0.85 eV f o r the Ag(3d / ) l i n e . 1 4

5

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In F i g 16. we show a random RBS spectrum, and an a l i g n e d c h a n n e l i n g spectrum f o r a ~ 4100 A a s - d e p o s i t e d Y-Ba-Cu oxide superconducting t h i n f i l m on (100) S r T i 0 . The s o l i d l i n e i n t h e figure i s a simulation o f Y B a C u O / S r T i 0 using the RUMP program [30]. The r e s u l t shows t h a t t h e f i l m has a c o m p o s i t i o n c l o s e t o i d e a l s t o i c h i o m e t r y through t h e e n t i r e t h i c k n e s s . The a l i g n e d c h a n n e l i n g spectrum shows a l a r g e reduction i n the backscattering y i e l d i n the f i l m and about 50% reduction (which depends on the f i l m t h i c k n e s s ) i n t h e y i e l d from t h e s u b s t r a t e . The minimum y i e l d f o r Ba, measured near t h e s u r f a c e , i s ~7% compared t o 3.5% f o r channeling o f 1.66 Mev H e on s i n g l e c r y s t a l s [28]. Although the i o n energy f o r c h a n n e l i n g i s d i f f e r e n t from t h a t o f r e f . 28, t h e comparison shows that the as-deposited f i l m s have good long-range c r y s t a l l i n e order. The minimum y i e l d o f 7% i s t h e s m a l l e s t v a l u e reported f o r the high Τ superconducting t h i n f i l m s on any substrate. As mentioned e a r l i e r t h e X-ray d i f f r a c t i o n study shows that the as-deposited f i l m s on (100)SrTiO are oriented with the ca x i s p e r p e n d i c u l a r t o t h e s u b s t r a t e s u r f a c e . The 7% minimum y i e l d suggests t h a t a t l e a s t 95% o f t h e c - a x i s i n t h e f i l m i s o r i e n t e d . The rapid increase i n the channeling y i e l d from the f i l m indicates t h a t i n t h e f i l m t h e r e a r e a l a r g e number o f d e f e c t s , which cause dechanneling o f t h e He i o n s . P r e l i m i n a r y t r a n s m i s s i o n e l e c t r o n 3

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Figure 16. Random and aligned RBS (3Mev He""") spectra f o r an as-deposited Y-Ba-Cu oxide superconducting t h i n f i l m on S r T i 0 . The s o l i d l i n e i s a simulation o f 4100 k 3

Y BeL Oi 0 _ /Si?Ti0 1

2

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( r e f . 31).


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microscopy (TEM) s t u d i e s show e x i s t a n c e o f s t a c k i n g f a u l t s i n the films. From the c h a n n e l i n g measurements [31], the m o s a i c i t y i s e s t i m a t e d t o be l e s s than 0.24 degrees ( c o n s i s t e n t w i t h the X-ray data) showing t h a t the f i l m has grown e p i t a x i a l l y on the S r T i C ^ substrate during deposition. The most i n t e r e s t i n g feature f o r us i n the c h a n n e l i n g spectrum i s the Ba s u r f a c e peak. The a r e a under the surface peak i s proportional t o the density of atoms on the surface and i s a measure o f the c r y s t a l l i n e o r d e r a t the s u r f a c e . A d i s o r d e r e d s u r f a c e l a y e r o f ~ 1 nm was e s t i m a t e d a t the s u r f a c e based on the surface peak assuming the "123" composition. Downloaded by UNIV OF ROCHESTER on January 19, 2018 | http://pubs.acs.org Publication Date: September 26, 1988 | doi: 10.1021/bk-1988-0377.ch019

5

The s u r f a c e l a y e r c o m p o s i t i o n and t h i c k n e s s can a l s o be determined u s i n g XPS. We found t h a t the s u r f a c e l a y e r o f a s d e p o s i t e d f i l m s i s barium e n r i c h e d . In f i g u r e 17a, MgKa XPS o f B a ( 3 d y ) r e g i o n f o r "123" f i l m a t two d i f f e r e n t t a k e - o f f angles, 15° (curve A) and 85° (curve B) are given. For camparison, Β^Οά^^) r e g i o n from an i n - s i t u , f r e s h l y scraped h i g h T superconducting Y^Ba2Cu3(>7_ p e l l e t i s a l s o shown (curve C). The Ba(3d) region shows two peaks, one a t 778 eV and the o t h e r a t 780 eV. These two peaks are resolved i n t o two Gaussian d i s t r i b u t i o n s of equal width (1.7 eV) shown i n f i g . 17b. The peak a t 778 eV i s due t o Ba i n the "123" phase [32]. A t a low angle o f c o l l e c t i o n when o n l y the s u r f a c e region i s examined, the peak at 780 eV i s the largest (curve A) and, t h e r e f o r e , t h e 780 eV peak i s due t o Ba a t t h e s u r f a c e . Corresponding 0 ( l s ) r e g i o n i s shown i n f i g . 18. C l e a r l y , we see t h r e e 0 ( l s ) peaks a t 528.5, 531, and 532.7 eV i n the "123" p e l l e t as w e l l as "123" f i l m a t a c o l l e c t i o n angle o f 85°. These t h r e e peaks are a s s i g n e d t o Ο , Ο , and Ο types o f oxygen. However, i n the f i l m , the i n t e n s i t y o f the 531 eV peak i s h i g h e r which c o r r e l a t e s w i t h the s u r f a c e Ba. A t 15°, the 531 eV peak i n t e n s i t y r e l a t i v e l y increases (curve A) and t h i s further confirms i t s association with s u r f a c e Ba i o n s . Even a t the low c o l l e c t i o n angle, s i g n i f i c a n t i n t e n s i t i e s at 528.5 and 532.7 eV are seen because the mean escape depth f o r the 0(ls) photoelectron i s larger than that f o r the Ba(3d) one. The XPS spectrum f o r C ( l s ) shows a s m a l l peak a t 288.5 eV a t t r i b u t a b l e t o carbonate ions. The i n t e n s i t y of oxygen due t o GO3 calculated from the C(ls) s i g n a l does not account f o r more than 5% o f the t o t a l i n t e n s i t y i n the 531 eV r e g i o n . F u r t h e r , B a ( 3 d / 2 ) peaks i n the case o f BaCO^ and Ba(0H) appear a t 782 eV and we do not see a peak i n t h i s r e g i o n . T h e r e f o r e , the 780 eV B a O d g / Q ) peak i s associated with the 531 eV 0(ls) peak and represents some form of Ba oxide. I t i s known that Cu (which i s a l s o present on the surface layer) with Ba gives 0(ls) and Ba(3d) at 531 and 780 eV respectively [33]. I t should be noted t h a t i n f i l m s w i t h Y:Ba:Cu i n the 1:2:3 r a t i o but i n which the "123" superconducting phase i s not formed, m a i n l y the 531 eV and 780 eV peaks i n 0 ( l s ) and B a ( 3 d / ) and the 528.5 and 778 eV peaks develop only a f t e r an oxygen anneal [34]. We therefore conclude that these two peaks are associated with the superconducting "123" phase. The s u r f a c e c o m p o s i t i o n o f the f i l m s was e s t i m a t e d form the i n t e n s i t i e s o f Ba(3d), Cu(2p), and Y(3d) peaks. At the 85° angle the c o m p o s i t i o n was Y r j . 8 8 2 . 0 5 3 x w h i l e a t the low c o l l e c t i o n angle, 15°, the c o m p o s i t i o n was 5

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Yo^^.œ ^Nelson and George; Chemistry of High-Temperature Superconductors II ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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