Crystal Growth by the Electrolysis of Molten Salts - Advances in

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Crystal Growth by the Electrolysis of Molten Salts R . S.

FEIGELSON

Center for Materials Research, Stanford University, Stanford, CA 94305

One

of the unique

an electrically Applied

features

driven

to the growth

potentially crystal

significant

growth

of electrodeposition

process

it would

advantage

more

to the

growth

films of various the principles and

over

which

utilize

crystallization.

some recent work concerning sition

materials.

Included

experimental

crystal

synthesis

logically

useful

growth

semiconductors

driving is

electrodepoand

epitaxial

is a brief discussion growth,

studies,

and preparation

of

crystals

crystal

a

conventional

in this paper

the application

of large single

control. provide

a thermal

Discussed

of electrochemical

chemical

is that it is

of precise

of single crystals

techniques,

force to achieve

capable

of

theoretical

and the

electro-

of thin films of

such as Si, GaP,

techno-

InP,

and

GaAs.

i n l e c t r o d e p o s i t i o n is a n o l d t e c h n o l o g y t h a t dates b a c k to the e a r l y p a r t -

L /

of the 19th c e n t u r y .

Two

b r a n c h e s soon e m e r g e d ,

namely,

low-

t e m p e r a t u r e aqueous e l e c t r o c h e m i s t r y a n d m o l t e n salt e l e c t r o c h e m i s t r y (MSE).

B y f a r the largest c o n c e n t r a t i o n of effort w a s d e v o t e d

to t h e

l o w - t e m p e r a t u r e process f o r the o b v i o u s reasons t h a t these are s i m p l e systems to construct a n d operate a n d that aqueous s o l u t i o n c h e m i s t r y is m u c h better u n d e r s t o o d t h a n that for c o m p l e x m o l t e n salts. I n spite of the fact that m o l t e n salt t e c h n o l o g y

lagged far behind,

b o t h w i t h r e g a r d to t h e o r e t i c a l u n d e r s t a n d i n g a n d t e c h n o l o g i c a l s o p h i s t i c a t i o n , t w o processes of significant c o m m e r c i a l i m p o r t a n c e w e r e oped: (2)

(1)

devel

the H a l l process for r e f i n i n g a l u m i n u m f r o m b a u x i t e ore a n d

the a l k a l i m e t a l s e p a r a t i o n process.

A b r i e f h i s t o r y of M S E is g i v e n

i n T a b l e I. 0-8412-0472-l/80/33-186-243$08.25/l © 1980 American Chemical Society

244

SOLID S T A T E

Table I.

A CONTEMPORARY

OVERVIEW

Historical Highlights of the Field of M S E

Year

Investigator

Event

Davy

1807

potassium from K O H

1833

d e c o m p o s i t i o n of K I , P b C l , P b l , A g C l , S n l , P b O , S b 0 , S b S and borax

Faraday

1852

a l k a l i n e e a r t h m e t a l s f r o m fused salts

Bunsen

1855

d e c o m p o s i t i o n of M g C l

Bunsen and Matthisen

1861

synthesis of s o d i u m - t u n g s t e n bronze (Na,W0 )

Scheibler

1898

b e r y l l i u m f r o m fused salts

Lebeau

1929

synthesis of borides

Andrieaux

1936

a l u m i n u m f r o m fused salts

Hall

1936 to present

v a r i o u s a n t i m o n i d e s , arsenides, borides, i n t e r m e t a l l i c s , oxides, phosphides, r e f r a c t o r y m e t a l s , s i l i c i d e s , a n d sulfides

2

2

Solid State Chemistry: A Contemporary Overview Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/24/16. For personal use only.

CHEMISTRY:

2

3

2

2

3

2

3

I n a d d i t i o n to the d e v e l o p m e n t

of the H a l l process i n t h e

1930s,

w o r k e r s , p r i n c i p a l l y i n F r a n c e , started to explore the use of M S E for the synthesis of n o v e l c o m p o u n d s w h o s e p r e p a r a t i o n b y other t e c h n i q u e s w a s c o m p l i c a t e d b y h i g h m e l t i n g t e m p e r a t u r e s a n d h i g h d i s s o c i a t i o n pressures. T h e s e i n c l u d e d m a n y of the t r a n s i t i o n , r e f r a c t o r y , a n d r a r e e a r t h m e t a l

Table II.

Examples of Materials Produced by Molten

BaB CaB« CeB LaB NdB GdB YB BrB YbB SrB ThB CrB Cr B CrB Cr B Cr B 6

6

0

8

6

6

e

6

6

6

2

2

3

2

3

4

SILICIDES

PHOSPHIDES

BORIDES

MnB

M11B4

MnB NbB TaB TiB VB VB Zr B ZrB Mo B MoB WB SmB PrB

1 2

2

2

2

2

4

3

4

2

2

6

6

Fe P FeP FeP Fe P Ni P Ni P Ni P Ni P Co P CoP CoP Mo P MoP WoP WP Mn P 2

M113B4

2

3

3

5

2

2

6

5

2

2

3

2

MnP CrP V P VP Cu P Cu P NbP TaP Zn P Cd P GaP InP

2

2

2

2

2

2

3

2

3

3

TiSi ZrSi CrSi Mn SI Mn Si Cr Si Fe Si TiSi Li Si CaSi CeSi LaSi 3

2

2

2

2

6

2

2

2

2

13.

FEIGELSON

Crystal

245

Growth

b o r i d e s , c a r b i d e s , a n d s i l i c i d e s , as s h o w n i n T a b l e I I . E x c e l l e n t r e v i e w articles o n the subject of M S E w e r e w r i t t e n b y K u n n m a n n ( I ) , and Bellavance (2), and E l w e l l

Wold

(3).

S i n c e a v a r i e t y of p o t e n t i a l l y u s e f u l materials c a n be s y n t h e s i z e d t y M S E , the d e v e l o p m e n t of t e c h n i q u e s to p r o d u c e l a r g e h i g h - q u a l i t y single crystals w o u l d b e v e r y d e s i r a b l e . T h e a p p l i c a t i o n of this t e c h n o l o g y

to


c r y s t a l g r o w t h , h o w e v e r , has so f a r b e e n n e g l i g i b l e . T h i s is i n d e e d sur p r i s i n g since i t is r e l a t i v e l y easy to p r o d u c e electrodeposits w i t h s m a l l , w e l l - f o r m e d single crystals. F e w studies, h o w e v e r , h a v e b e e n

directed

t o w a r d c o n t r o l l i n g the e l e c t r o c h e m i c a l parameters necessary to i m p r o v e c r y s t a l size a n d q u a l i t y . A c c o r d i n g to K u n n m a n n ( I ) , " M a t e r i a l s electroc h e m i c a l l y p r e c i p i t a t e d f r o m f u s e d melts c a n almost a l w a y s b e o b t a i n e d i n the f o r m of r e a s o n a b l y l a r g e crystals w h e n sufficiently l o w c u r r e n t densities are e m p l o y e d . " W h i l e this is c l e a r l y a n o v e r s i m p l i f i c a t i o n , i t does suggest, as i m p l i e d b y l o w c u r r e n t densities, that crystals c a n

be

g r o w n i f c o n d i t i o n s of g r o w t h are c a r e f u l l y c o n t r o l l e d . It has o n l y b e e n i n the last f e w years, h o w e v e r , t h a t c r y s t a l g r o w t h technologists

have

r e c o g n i z e d that e l e c t r o d e p o s i t i o n offers several u n i q u e advantages

over

c o n v e n t i o n a l c r y s t a l p r o c e s s i n g t e c h n i q u e s a n d h a v e b e g u n to s t u d y the c o n d i t i o n s necessary to g r o w l a r g e single crystals a n d e p i t a x i a l layers w i t h useful properties. T h e p o t e n t i a l advantages c r y s t a l g r o w t h are as f o l l o w s :

of

m o l t e n salt e l e c t r o c r y s t a l l i z a t i o n f o r

(1)

it is a r e l a t i v e l y l o w - t e m p e r a t u r e

t e c h n i q u e that avoids t h e r m a l d e c o m p o s i t i o n sure;

(2)

i t is a n i s o t h e r m a l p r o c e s s — t e m p e r a t u r e

Electrocrystallization in Salt Systems Carbides Fe C MoC Mo C WC W C 3

2

2

a n d excessive v a p o r p r e s gradients are

not

Elevated-Temperature

Arsenides MoAs WAs FeAs FeAs GaAs

2

Sulfides M0S2 WS 2

Other

Groups

antimonides oxides refractory metals intermetallic compounds

246

SOLID S T A T E

CHEMISTRY: A

CONTEMPORARY OVERVIEW

n e e d e d for g r o w t h ; ( 3 ) i t is r e l a t i v e l y t e m p e r a t u r e - i n s e n s i t i v e a n d s m a l l changes i n t e m p e r a t u r e do not affect g r o w t h r a t e ; ( 4 ) m e l t i n g system is not r e q u i r e d ; ( 5 )

a congruently

deposition can be controlled very

a c c u r a t e l y b y c o n t r o l l i n g e l e c t r o c h e m i c a l p a r a m e t e r s alone; ( 6 )

solutions

c a n be purified b y u n i q u e electrochemical techniques; and (7)

growth

features c a n be s t u d i e d q u a n t i t a t i v e l y b y c a r e f u l l y v a r y i n g e l e c t r o c h e m i c a l Solid State Chemistry: A Contemporary Overview Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/24/16. For personal use only.

p a r a m e t e r s [for e x a m p l e , see B o s t a n o v

(4)].

F r o m the s t a n d p o i n t of c r y s t a l g r o w t h the most i m p o r t a n t p o t e n t i a l a d v a n t a g e is t h a t the c o n t r o l of the g r o w t h process c a n be a c h i e v e d b y c o n t r o l l i n g c e l l p o t e n t i a l or c u r r e n t density.

The precision w i t h w h i c h

this c a n be a c c o m p l i s h e d is orders of m a g n i t u d e s better t h a n t h a t possible w i t h t e m p e r a t u r e c o n t r o l , w h e r e 0.1 ° C is c o n s i d e r e d excellent t e m p e r a t u r e s t a b i l i t y . A l s o , e l e c t r o c h e m i c a l p u r i f i c a t i o n , w h i c h w i l l b e discussed later, c a n b e u s e d to enhance the p u r i t y of the electrodeposits

a n d i n some

cases w o u l d p e r m i t the use of m o r e e c o n o m i c a l s t a r t i n g m a t e r i a l s . A m o n g the several disadvantages to the M S E m e t h o d are t h a t

(1)

the d e p o s i t i n g m a t e r i a l has to be e l e c t r i c a l l y c o n d u c t i n g ,

(2)

growth

rates t e n d to b e l o w since g r o w t h is f r o m s o l u t i o n , a n d ( 3 )

selection of

s u i t a b l e s o l v e n t - s o l u t e systems is difficult. I n this p a p e r some recent w o r k at the C e n t e r f o r M a t e r i a l s R e s e a r c h , S t a n f o r d U n i v e r s i t y , c o n c e r n i n g the extension of e l e c t r o d e p o s i t i o n to the g r o w t h of l a r g e single crystals of v a r i o u s types is d i s c u s s e d . A l s o d i s c u s s e d is the p r e p a r a t i o n of t e c h n o l o g i c a l l y u s e f u l s e m i c o n d u c t o r s u n d e r c o n d i tions u n i q u e l y different f r o m c o n v e n t i o n a l t e c h n i q u e s .

The

following

topics w i l l b e c o v e r e d : 1.

P r i n c i p l e s of e l e c t r o c h e m i c a l c r y s t a l g r o w t h

2.

C r y s t a l g r o w t h studies a. C o m p a r i s o n of e l e c t r o c r y s t a l l i z a t i o n w i t h t h e r m a l c r y s t a l l i z a t i o n processes b. Seeded g r o w t h (static growth) c. E l e c t r o c h e m i c a l C z o c h r a l s k i t e c h n i q u e ( d y n a m i c g r o w t h )

3.

Material preparation a. S i b. G a P a n d I n P c. G a A s

It s h o u l d b e r e c o g n i z e d at the outset that a l t h o u g h M S E is a v e r y o l d t e c h n o l o g y t h e state of the art f o r g r o w i n g u s e f u l crystals f o r d e v i c e a p p l i c a t i o n s is s t i l l v e r y p r i m i t i v e , a n d its competitiveness w i t h

other,

m o r e c l a s s i c a l approaches is s t i l l u n c e r t a i n . Principles

of Electrochemical

Crystal

Growth

I n M S E the passage of c u r r e n t t h r o u g h a m o l t e n salt electrolyte p r o v i d e s the d r i v i n g force for the d e p o s i t i o n of a d e s i r e d m a t e r i a l o n a n

13.

Crystal

FEIGELSON

247

Growth

electrode o f a n e l e c t r o l y t i c c e l l . T h e passage o f c u r r e n t i s the r e s u l t o f a n o x i d a t i o n - r e d u c t i o n r e a c t i o n i n w h i c h e l e c t r o c h e m i c a l l y a c t i v e species are r e d u c e d at the c a t h o d e a n d o x i d i z e d at the anode. A n o d e r e a c t i o n : A " - » A + V*

(1)

C a t h o d e r e a c t i o n : C * + ze~->C

(2)

y


+

w h e r e the o v e r a l l r e a c t i o n c a n b e w r i t t e n as zk-y + yC

+e

->zk

(3)

+ yC

T h e s e reactions o c c u r w h e n the c e l l p o t e n t i a l exceeds t h e d e c o m p o s i t i o n p o t e n t i a l ( E ) f o r t h e c r y s t a l l i z i n g species.

T h e equilibrium potential

d

E c a n b e d e s c r i b e d b y the N e r n s t e q u a t i o n :

»--(!£)'»([») w h e r e E ° is t h e s t a n d a r d c e l l p o t e n t i a l , R is t h e gas constant, T is t h e t e m p e r a t u r e ( K ) , F is F a r a d a y ' s constant, a n d [ ] i n d i c a t e s c o n c e n t r a t i o n o r a c t i v i t y of t h e r e a c t i n g species. equilibrium potential is E

d

T h e p o t e n t i a l just i n excess o f t h i s

a n d i s u s e d t o d r i v e the k i n e t i c a n d d i f f u s i o n

processes i n the m e l t a n d at the electrodes. W h e n v o l t a g e is a p p l i e d t o i n e r t ( n o n d i s s o l v i n g )

electrodes of a n

e l e c t r o l y t i c c e l l , t h e c u r r e n t b e h a v i o r i s as s h o w n i n F i g u r e 1. I n t h i s i d e a l i z e d case E

d

represents the d e c o m p o s i t i o n p o t e n t i a l for the d e p o s i t i o n

of t h e d e s i r e d species.

I n r e a l systems there is m o r e t h a n o n e set of

cr

dy d

V

CELL

Figure

I . Idealized

V

X

POTENTIAL—>

current vs. voltage

curves for deposition

Chemical Society Library 1155 16th St. N. W. Washington, 0. C. 2003$ hmmm

of two

248

SOLID S T A T E

CHEMISTRY: A


r e a c t i n g species t h a t c a n p l a t e out at the electrodes. E v e n i n a v e r y p u r e s i m p l e m o l t e n salt system, other species are present t h a t c a n d e p o s i t u n d e r nonideal conditions.

F o r e x a m p l e , the solvents themselves are c a p a b l e

of b e i n g d i s s o c i a t e d . I f i m p u r i t i e s are present, t h e y also m a y deposit i n a n u m b e r of f o r m s , d e p e n d i n g u p o n the o p e r a t i n g c o n d i t i o n s . Species w h o s e d e c o m p o s i t i o n Solid State Chemistry: A Contemporary Overview Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/24/16. For personal use only.

p o t e n t i a l s are l o w e r t h a n those of t h e d e s i r e d species e l e c t r o d e p o s i t ait c e l l p o t e n t i a l s less t h a n E

d

(E < E )

will

d

a n d w i l l codeposit w h e n the

c e l l p o t e n t i a l is e q u a l to o r greater t h a n E . I f t h e d e c o m p o s i t i o n p o t e n t i a l d

for a species is s i g n i f i c a n t l y greater t h a n t h a t of the d e s i r e d species, i t s h o u l d stay i n s o l u t i o n a n d n o t electrodeposit. I t is p a r t i c u l a r l y i m p o r t a n t , therefore, to c h o o s e a solvent system t h a t has a h i g h

decomposition

p o t e n t i a l , s u c h as a l k a l i m e t a l fluorides. T h o s e i m p u r i t i e s i n t h e system t h a t deposit at p o t e n t i a l s less t h a n E

d

can be removed f r o m the melt b y a preelectrolysis technique.

d e p o s i t i o n f o r some p e r i o d of t i m e at E

E . d

W h e n the i m p u r i t y

species has a d e c o m p o s i t i o n p o t e n t i a l e q u a l to t h a t of the species d e s i r e d , then codeposition w i l l occur. T h e segregation p h e n o m e n o n o b s e r v e d i n e l e c t r o d e p o s i t i o n is s i m i l a r to t h e n o r m a l segregation of i m p u r i t i e s d u r i n g c o n v e n t i o n a l r e c r y s t a l l i z a t i o n f r o m s o l u t i o n . I n these t e c h n i q u e s t h e d i s t r i b u t i o n coefficient (k =

C / C , where C S

L

s

(k)

is t h e c o n c e n t r a t i o n of solute i n the s o l i d a n d C

L

is t h e c o n c e n t r a t i o n of solute i n the l i q u i d ) describes the

segregation

b e h a v i o r of the solute o r a n i m p u r i t y i n s o l u t i o n . W h e n k =

1, t h e r e is

n o i m p u r i t y segregation, a n d this corresponds to t h e case i n e l e c t r o d e position where E to h a v i n g k
E

d ( c m p d )

, t h e n i t is s i m i l a r

1, w h e r e the i m p u r i t y is rejected f r o m the c r y s t a l a n d

segregates i n t h e m e l t

(or

solution).

When E ( d

i m p

)

1. T h e d e p e n d e n c e of i m p u r i t y i n c o r p o r a t i o n o n c e l l p o t e n t i a l a n d d e c o m p o s i t i o n p o t e n t i a l is the basis for b o t h e l e c t r o c h e m i c a l p u r i f i c a t i o n a n d d o p i n g c o n t r o l . N e i t h e r process, h o w e v e r , has b e e n t h o r o u g h l y s t u d i e d yet.

T h e d e c o m p o s i t i o n p o t e n t i a l is, as c a n b e seen f r o m t h e

Nernst equation, a function of b o t h temperature a n d concentration.

Of

p a r t i c u l a r i m p o r t a n c e is t h e solute or i m p u r i t y c o n c e n t r a t i o n i n s o l u t i o n . Crystal

Growth

Studies

Comparison of Electrocrystallization with Thermal Crystallization Techniques.

E l e c t r o d e p o s i t i o n has i n c o m m o n w i t h o t h e r c r y s t a l l i z a t i o n

t e c h n i q u e s , s u c h as t h e s o l i d i f i c a t i o n of m e l t s , s u b l i m a t i o n , r e c r y s t a l l i z a t i o n f r o m s o l u t i o n , a n d c h e m i c a l v a p o r reactions, that b o t h n u c l e a t i o n a n d

13.

Crystal

FEIGELSON

249

Growth

g r o w t h p h e n o m e n a are i n v o l v e d i n t h e d e p o s i t i o n process. T o use M S E for c r y s t a l g r o w t h , therefore, i t is v e r y i m p o r t a n t to c o n t r o l b o t h n u c l e a t i o n process a n d t h e g r o w t h rate. T h e process of

the

electrochemical

c r y s t a l l i z a t i o n c a n b e t h o u g h t of as analogous to n o r m a l c r y s t a l g r o w t h , w h e r e the s u p e r s a t u r a t i o n ( o r t e m p e r a t u r e g r a d i e n t ) is r e p l a c e d b y t h e e l e c t r i c a l p o t e n t i a l i n excess of E ( A E ) as the d r i v i n g force. d


E l w e l l et a l . (5)

h a v e suggested a n a n a l o g y b e t w e e n n o r m a l c r y s t a l

g r o w t h f r o m s o l u t i o n a n d the flow of c u r r e n t i n a n e l e c t r o l y t e c e l l .

They

c o n s i d e r e d t h e v a r i o u s stages of g r o w t h as h a v i n g t h e c h a r a c t e r of a n i m p e d a n c e to the flow of c r y s t a l l i z i n g m a t e r i a l a n d , b y m e a s u r i n g t h e resistance i n t h e c i r c u i t a n d its d e p e n d e n c e o n e x p e r i m e n t a l p a r a m e t e r s , w e r e a b l e to s t u d y c r y s t a l g r o w t h m e c h a n i s m s

a n d the n a t u r e of

the

r a t e - c o n t r o l l i n g process. T h e l i n e a r g r o w t h rate (v)

e q u a t i o n for a t h e r m a l l y d r i v e n s o l u t i o n

g r o w t h process has b e e n d e r i v e d b y G i l m e r et a l . (6)

v

=

crD^n

£ A +

8 +

AA ZA" 8

2

+

A

(J^-

as f o l l o w s :

coth J L

_

i^J1 (5)

w h e r e o- is t h e r e l a t i v e s u p e r s a t u r a t i o n , D is the solute d i f f u s i o n coefficient, 7) is the e q u i l i b r i u m c o n c e n t r a t i o n of solute, O is the m o l a r v o l u m e , A is e

a n a d s o r p t i o n p a r a m e t e r , D / A is the d r i f t v e l o c i t y of m o l e c u l e s e n t e r i n g the a d s o r p t i o n l a y e r a n d b o u n d a r y l a y e r w i d t h , A

S

is a surface d i f f u s i o n

p a r a m e t e r , I is the step s p a c i n g , a n d A is the m e a n d i s t a n c e t r a v e l e d b y an adsorbed molecule.

T h e i m p e d a n c e s r e p r e s e n t e d b y t h e terms i n t h e

b r a c k e t relate to the v a r i o u s c r y s t a l g r o w t h steps, t h a t is, v o l u m e d i f f u s i o n a n d interface attachment kinetics incorporation).

(adsorption,

surface

diffusion,

and

T h e r e f o r e , t h e l i n e a r g r o w t h rate is p r o p o r t i o n a l to the

d r i v i n g f o r c e (o-) a n d i n v e r s e l y p r o p o r t i o n a l to t h e b r a c k e t e d i m p e d a n c e terms

(R). v

oc

At /

/ 11

-10-

1

-15

J

Journal of Crystal Growth

Figure 2. (a) Current vs. overpotential for tungsten bronzes: (O), Na W O ; (X), Na WO . Cathodic curves calculated using R = 0.612 + 0.164I ' fi and R = 0.458 + 0.09311" Q, respectively, (b) Current vs. overpotential for LaB (5). 061

0

a

1 2

7S

s

,/2

6

13.

Crystal

FEIGELSON

253

Growth

to t h e species to b e d e p o s i t e d .

M a n y s u c h s o l u t e - s o l v e n t systems are

l i s t e d i n the l i t e r a t u r e a n d c a n f o r m the b a s i c s t a r t i n g p o i n t for investigation.

an

O f p a r t i c u l a r i m p o r t a n c e i n M S E h a v e b e e n the m i x e d

a l k a l i m e t a l fluorides a n d m i x e d

fluoride-oxide

solvents.

H a v i n g chosen a n a p p r o p r i a t e solvent i n w h i c h t h e species to

be

d e p o s i t e d c a n be d i s s o l v e d i n reasonable c o n c e n t r a t i o n a n d h a v i n g c h o s e n Solid State Chemistry: A Contemporary Overview Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/24/16. For personal use only.

a s u i t a b l e solute t h a t has a l o w v o l a t i l i t y a n d p r o v i d e s a l a r g e f r a c t i o n of e l e c t r i c a l l y a c t i v e species, one m u s t next solve t h e p r o b l e m of c o m p a t i b i l i t y b e t w e e n this m o l t e n salt s o l u t i o n a n d t h e c r u c i b l e a n d

electrode

materials. A l s o , one m u s t r e m e m b e r that the c r u c i b l e a n d electrodes m u s t b e n o n r e a c t i v e w i t h the electrodeposit a n d m u s t b e c a p a b l e of w i t h s t a n d i n g t h e necessary o p e r a t i n g t e m p e r a t u r e of the system. A w i d e r a n g e of e l e c t r o d e a n d c r u c i b l e m a t e r i a l s h a v e b e e n u s e d i n v a r i o u s M S E cells. S i n c e i t is u s u a l l y necessary to use t w o different solute species electrodeposit a b i n a r y c o m p o u n d , t h e r e l a t i v e c o n c e n t r a t i o n of

to

solute

species i n s o l u t i o n is i m p o r t a n t . U s i n g s t o i c h i o m e t r i c ratios is not c o m m o n since i t is m o r e i m p o r t a n t to m a t c h the d e c o m p o s i t i o n p o t e n t i a l of the t w o species.

If the a p p r o p r i a t e c o m p o s i t i o n is not u s e d , t h e n one or the

other species m a y p l a t e out p r e f e r e n t i a l l y a n d a n excess of one

component

w i l l result. T h e a n o d e b y p r o d u c t c a n also present some p r o b l e m s . I n some cases (usually w h e n 0 away from

2

is f o r m e d at the a n o d e ) t h i s p r o d u c t m u s t be k e p t

the d e p o s i t e d

m a t e r i a l s since

a chemical reaction might

take p l a c e . Z u b e c k et a l . ( 7 ) chose l a n t h a n u m h e x a b o r i d e ( L a B ) , a m e m b e r of 6

the r e f r a c t o r y rare e a r t h b o r i d e s , as a m o d e l m a t e r i a l to s t u d y the p r o b l e m of c o n t r o l l e d n u c l e a t i o n a n d c r y s t a l g r o w t h .

L a n t h a n u m h e x a b o r i d e is

a g o o d e l e c t r o n e m i t t e r a n d is c u r r e n t l y b e i n g u s e d as a r e p l a c e m e n t for c o n v e n t i o n a l electron m i c r o s c o p e

filaments.

I t has a c u b i c m e t a l l i c s t r u c

t u r e a n d exists o v e r a w i d e s t o i c h i o m e t r y r a n g e . electrodeposited L a B

6

Andrieux (8)

f r o m a m i x t u r e of o x i d e a n d

fluoride

salts.

first The

d e p o s i t i o n p r o d u c t w a s i n t h e f o r m of s u b m i l l i m e t e r - s i z e d c r y s t a l l i t e s . It is a n e x a m p l e i n w h i c h the s o l u t e - s o l v e n t r a t i o is v e r y l o w , m a k i n g g r o w t h p a r t i c u l a r l y difficult b o t h b y e l e c t r o c h e m i c a l t e c h n i q u e s a n d other solution growth techniques. T h e emphasis of the L a B

6

w o r k b y Z u b e c k et a l . ( 7 ) w a s c o n t r o l l e d

e l e c t r o d e p o s i t i o n l e a d i n g to the p r o d u c t i o n of l a r g e single crystals. T h e electrolysis c e l l u s e d is s h o w n i n F i g u r e 3. u n d e r a n i n e r t H e atmosphere. c o n t a i n e d 2.2 m o l %

L a 0 , 33.5 m o l % 2

33.1 m o l % L i F . T h e B 0 2

source of b o r o n .

3

3

G r o w t h was

T h e b a t h used for

accomplished

electrodeposition

B 0 , 31.2 m o l % 2

3

L i 0 , and 2

w a s u s e d b o t h as a fluxing agent a n d as t h e

L i F w a s u s e d to dissolve the oxides a n d l o w e r m e l t

v i s c o s i t y a n d the L i 0 w a s p a r t of t h e l o w - t e m p e r a t u r e solvent. 2

254

SOLID S T A T E

CHEMISTRY: A CONTEMPORARY OVERVIEW

Valve for cathode removal

Viewing port

He Out


Vacuum port

He In ( B u b b l e r )

Light bulb Anode Thermocouple

Furnace windings

Crucible

Inconel atmosphere tube Journal of Crystal Growth

Figure 3.

Molten salt electrolysis system (7)

T h e c e l l reactions are as f o l l o w s : 2La 0 2

3

+

1 2 B 0 -> 4 L a B

A n o d e : 4 2 0 " -H> 2 1 0 2

Cathode: 4 L a

3 +

+

2

3

2

+

24B

3 +

N o t e that for e a c h m o l e c u l e of L a B

6

6

+

210

(17)

2

84e"

(18)

+ 84e" -> 4 L a B

(19)

6

e l e c t r o d e p o s i t e d , 21 electrons h a v e

to be t r a n s f e r r e d . A l a r g e n u m b e r of electrode m a t e r i a l s w e r e i n v e s t i g a t e d , f e w of w h i c h w e r e c o m p a t i b l e w i t h the m o l t e n salt solutions. G o l d w a s f o u n d to be the most suitable b o t h for the a n o d e a n d c a t h o d e .

Cell

c u r r e n t c o u l d be adjusted b y c h o o s i n g a n a p p r o p r i a t e electrode area ( 2 0 5 0 - m i l - d i a m e t e r w i r e for the c a t h o d e , 1.1-cm-wide

f o i l for the

anode).

A l l c o m p o u n d s u s e d w e r e i n the f o r m of reagent-grade c h e m i c a l s , b u t t h e baths w e r e p u r i f i e d b y preelectrolysis at a c e l l p o t e n t i a l near £ a n d the cathodes start of g r o w t h .

d

for L a B

s u b s e q u e n t l y r e p l a c e d w i t h n e w ones p r i o r to

6

the

13.

Crystal

FEIGELSON

255

Growth

U n d e r the c e l l c o n d i t i o n s d e s c r i b e d , E

d

for L a B

6

w a s 1.85 V . D e p o s i

t i o n , therefore, was a c c o m p l i s h e d w i t h c e l l potentials i n t h e r a n g e 2.1 V .

1.85-

It is p o s s i b l e to d r i v e the g r o w t h process i n e i t h e r a constant

c u r r e n t or v o l t a g e m o d e or b y v a r y i n g e i t h e r p a r a m e t e r i n a c o n t r o l l e d m a n n e r . F i g u r e 4 shows the c u r r e n t b e h a v i o r as a f u n c t i o n of t i m e for a constant c e l l p o t e n t i a l . T h e i n i t i a l c u r r e n t rise is d u e to the n u c l e a t i o n Solid State Chemistry: A Contemporary Overview Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/24/16. For personal use only.

of s m a l l crystallites that f o r m o n the c a t h o d e , i n c r e a s i n g s u b s t a n t i a l l y the surface area of the electrode a n d t h e r e b y l o w e r i n g t h e c e l l p o t e n t i a l . T h e c u r r e n t levels off as the crystals b e c o m e l a r g e r a n d the rate of surface area c h a n g e decreases s u b s t a n t i a l l y . N o r m a l l y i n a constant c u r r e n t m o d e the surface area increase d u e to d e p o s i t i o n w i l l result i n a d r o p i n c e l l p o t e n t i a l , w h i c h c a n t h e r e b y f a l l b e l o w E . U n d e r those c o n d i t i o n s the d

deposit w i l l start to dissolve or dissociate, as e v i d e n c e d b y e t c h p i t t i n g . I n fact, the range of c e l l p o t e n t i a l s possible f o r d e p o s i t i o n is v e r y n a r r o w a n d is a m a j o r f a c t o r to b e c o n t r o l l e d d u r i n g d e p o s i t i o n , a l o n g w i t h t h e c u r r e n t d e n s i t y ( w h i c h , as stated before, is e q u i v a l e n t to the g r o w t h rate).

The

best results, therefore,

were

obtained

with

constant

programmed cell potential.

t

Km A) 10-

642-

0

20

40

60

80 t(hr)-*

100 Journal of Crystal Growth

Figure 4.

Current vs. time for LaB

6

growth (7)

or


256

SOLID S T A T E

Figure 5.

LaB

6

CHEMISTRY: A

C O N T E M P O R A R Y OVERVIEW

electrodeposit after 100 hr of unseeded growth (7)

T h e stable g r o w t h rate range for L a B

6

crystals w a s f o u n d to

be

b e t w e e n 20 a n d 40 m A • ( c m ) " . A f t e r 300 h r , clusters of crystallites u p 2

1

to 4 m m i n d i a m e t e r w e r e p r o d u c e d , as s h o w n i n F i g u r e 5.

Observations

of t h e g r o w t h m o r p h o l o g y , r e p o r t e d b y E l w e l l et a l . ( 9 ) , s h o w e d t h a t at less t h a n 25 m A • ( c m ) " 2

1

crystals g r o w as layers f o r m e d at p y r a m i d a l

a c t i v e sites at the center of c r y s t a l faces, w i t h p r o p a g a t i o n o u t w a r d i n a l l d i r e c t i o n s . A s the c u r r e n t d e n s i t y is i n c r e a s e d to 3 0 - 5 0 m A • ( c m ) " , 2

1

t h e a c t i v e sites b e c o m e l o c a t e d at the corners a n d edges of the c r y s t a l l i t e s , w h i c h are closest to the source of n u t r i e n t , a n d the layers i n w a r d across the c r y s t a l face.

propagate

T h i s c o n d i t i o n is the p r e c u r s o r of h o p p e r

g r o w t h often seen i n s o l u t i o n g r o w t h . A t c u r r e n t densities greater t h a n 100 m A • ( c m ) " , d e n d r i t e s g r o w f r o m the corners a l o n g t h e [111]. 2

1

T h e electrodeposit s h o w n i n F i g u r e 5 c l e a r l y illustrates t h a t n u c l e a t i o n of L a B

6

is difficult to c o n t r o l e v e n at l o w c u r r e n t densities. N o t o n l y

is p r i m a r y n u c l e a t i o n a p r o b l e m b u t so, too, is s e c o n d a r y n u c l e a t i o n w h i c h results f r o m p e r t u r b a t i o n s i n t h e g r o w t h rate. To

a v o i d or m i n i m i z e p r i m a r y n u c l e a t i o n p r o b l e m s ,

use a seed c r y s t a l . I n the s t u d y b y Z u b e c k et a l . ( 7 )

LaB

6

one

should

seed crystals

13.

FEIGELSON

( 2 X 3 X 3

Crystal

257

Growth

m m ) w e r e o b t a i n e d f r o m a zone-refined b o u l e a n d u s e d i n

p l a c e of t h e g o l d w i r e cathode. o t h e r w i s e , the exposed

T h e seed h a d to b e e n t i r e l y s u b m e r g e d ;

p o r t i o n d e t e r i o r a t e d r a p i d l y . A f t e r 200 h r the

c r y s t a l size h a d i n c r e a s e d to 6 X 6 X 5 m m , as s h o w n i n F i g u r e 6.

By

e s t i m a t i n g the surface area of t h e seed c r y s t a l , the c e l l v o l t a g e c o u l d b e a d j u s t e d t o g i v e a c u r r e n t d e n s i t y of 20 m A • ( c m ) " . 2

1

Small periodic


adjustments i n c e l l p o t e n t i a l w e r e m a d e to c o n t r o l t h e shape of

the

current-versus-time curve. Secondary nucleation problems were eliminated through the mainte n a n c e of stable o p e r a t i n g c o n d i t i o n s ( c e l l v o l t a g e , c u r r e n t , a n d t e m p e r a -

Figure

6. Seeded growth of LaB suspended on gold wire cathode: (top) after 87 hr of growth; (bottom) after 200 hr of growth (7). 6


258

SOLID S T A T E


7. Crystals of Na WO. grown by standard MSE techniques use as seeds in electrochemical Czochralski technique (10)

Figure

ture).

CHEMISTRY: A

for

x

After

possible.

longer

growth

periods

even

l a r g e r crystals s h o u l d

be

F o r the g r o w t h of v e r y l a r g e crystals, t h e size of t h e c r u c i b l e

a n d t h e q u a n t i t y of the s o l u t i o n w o u l d h a v e to b e i n c r e a s e d to m i n i m i z e t h e effects of solute d e p l e t i o n as the crystals g r o w .

Also, E

d

changes as

a result of the c h a n g e i n s o l u t i o n c o m p o s i t i o n as the c r y s t a l grows

(unless

a d i s s o l v i n g a n o d e is u s e d ) , a n d i n t h e g r o w t h of v e r y l a r g e crystals, the c e l l p o t e n t i a l m i g h t h a v e to be adjusted a c c o r d i n g l y . i Seeded Growth Czochralski

(Dynamic):

The

Electrochemical

Technique

O n e of t h e m o s t i m p o r t a n t c r y s t a l g r o w t h t e c h n i q u e s is t h e C z o c h r a l s k i m e t h o d ( c r y s t a l p u l l i n g ) . It is w i d e l y u s e d c o m m e r c i a l l y to g r o w l a r g e , n e a r - p e r f e c t crystals s u c h as s i l i c o n a n d g a d o l i n i u m g a l l i u m garnet. D e M a t t e i et a l . (10)

s t u d i e d t h e p o s s i b i l i t y of a d a p t i n g t h e c o n c e p t of

c r y s t a l p u l l i n g to e l e c t r o d e p o s i t i o n .

Sodium-tungsten bronze was

used

13.

Crystal

FEIGELSON

259

Growth

as a m o d e l g r o w t h system because of the ease w i t h w h i c h l a r g e single crystals c a n be g r o w n b y c o n v e n t i o n a l static g r o w t h t e c h n i q u e s , u s i n g a Na W0 -W0 2

4

3

m o l t e n salt s o l u t i o n ( F i g u r e 7 ) .

T h e N a ^ W O a system is

a n e x a m p l e of a system w i t h a h i g h solute content a n d a l o w z. a p p a r a t u s u s e d is s h o w n i n F i g u r e 8. s i m i l a r to the L a B

It contains a g r o w t h

The

chamber

e l e c t r o l y t i c c e l l except that n o w t h e c a t h o d e r o d is

6


a t t a c h e d to a C z o c h r a l s k i p u l l i n g system. T h e cathode c o u l d be r o t a t e d

MOTOR AND ^

SPEED REDUCER

ROTATING r*-l MERCURY -» L CONTACT

PULLER

CATHODE v CONNECTION^"

ANODE REFERENCE

He OUTLET = £ 3 : He INLET=g3=f| FURNACEWINDINGS QUARTZCONTAINMENT VESSEL

TT

I WATER COOLED FLANGE

C"

C" CRUCIBLE

ppEfczAUXILARY INLET/OUTLET

THREE FUNCTION TEMPERATURE CONTROLLER

I

ZERO CROSSING SCR CONTROLLER

— A

A

208 VAC


Figure 8. Schematic of electrochemical Czochralski crystal-pulling and rotation mechanism, growth furnace, and temperature controller (10)


260

SOLID S T A T E


Figure 9. Electrochemical Czochralski crystals grown in [111] direction at two different current-^pull rate combinations: (top) constant pull rate, constant current; (bottom) constant pull rate, current increase of four times after initial growth period (10). as i n s t a n d a r d C z o c h r a l s k i g r o w t h .

S i n c e t h e c r y s t a l has to b e p u l l e d

a b o v e the surface of the m e l t d u r i n g g r o w t h , i t w a s f o u n d necessary to k e e p the o x y g e n generated a t t h e a n o d e f r o m t r a v e l i n g e i t h e r t h r o u g h t h e gas p h a s e or across t h e m e l t surface to the c r y s t a l , w h i c h results i n the chemical reaction Na*W0 Na W0 2

4

3

+

0

2

-> N a W 0 2

(20)

4

melts at t h e o p e r a t i n g . c e l l t e m p e r a t u r e ( 7 5 0 ° C ) .

To accomplish

this the a n o d e w a s p l a c e d i n a separate c o m p a r t m e n t s u c h t h a t H e gas flushed

the 0

2

g e n e r a t e d a w a y f r o m t h e c a t h o d e a n d o u t of the system.

T h e m e l t c o m p o s i t i o n u s e d w a s 25 m o l %

W0

3

a n d 75 m o l

N a W 0 , a n d at this c o m p o s i t i o n c u b i c N a a . W 0 is p r o d u c e d . 2

4

3

experiments were performed using [111]-oriented

The

% first

seeds o b t a i n e d f r o m

s t a t i c a l l y g r o w n crystals. C r y s t a l s of u p to 11 c m i n l e n g t h a n d 2.5 c m i n diameter were grown b y the electrochemical C z o c h r a l s k i technique, a t y p i c a l e x a m p l e of w h i c h is s h o w n i n F i g u r e 9.

S i n c e t h e r e are n o

t h e r m a l constraints to fix t h e shape of t h e i n t e r f a c e or d i a m e t e r of t h e c r y s t a l , b o t h t h e i n t e r f a c e a n d sides of t h e crystals w e r e h i g h l y f a c e t e d .

13.

FEIGELSON

Crystal

261

Growth

T h e crystal a n d interface morphology

are a f u n c t i o n of

the

growth

direction. I n F i g u r e 10 the v a r i a t i o n of m a x i m u m stable p u l l i n g rate w i t h seed r o t a t i o n rate is s h o w n .

F o r a n y g i v e n r o t a t i o n rate a p u l l rate greater

t h a n the m a x i m u m gave rise to dendrites g r o w i n g at the g r o w t h interface. B a s e d i n L e v i c h ' s e q u a t i o n for the l i m i t i n g c u r r e n t d e n s i t y f o r a r o t a t i n g Solid State Chemistry: A Contemporary Overview Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/24/16. For personal use only.

electrode

(11),

the m a x i m u m a l l o w a b l e p u l l rate dy/dt

was calculated

to be

4^ —2.25 + fc't* 1

17

(21)

dt

w h e r e k! is a constant a n d w is the r o t a t i o n rate. 6|-

cj(rpm) Journal of Crystal Growth

Figure 10. Maximum stable pull rate vs. crystal rotation rate for electrochemical Czochralski growth of Na WO : (Q), experimental points; ( calculated; a = ± 0.06 (10). x

s

262

SOLID S T A T E


T h e s o l i d c u r v e i n F i g u r e 10 w a s d e r i v e d f r o m values c a l c u l a t e d b y u s i n g E q u a t i o n 18 a n d the e x p e r i m e n t a l d a t a p o i n t s , u p to a seed r o t a t i o n rate of 30 r e v o l u t i o n s p e r m i n u t e ( r p m ) , w e r e i n excellent

agreement

w i t h the c a l c u l a t e d values. A b o v e 30 r p m the g r o w n crystals e x h i b i t e d a d e p l e t e d z o n e at the g r o w t h facets. T h i s w a s b e l i e v e d to b e c a u s e d b y f l o w s e p a r a t i o n at the apex of the i n t e r f a c e . Solid State Chemistry: A Contemporary Overview Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/24/16. For personal use only.

I n n o r m a l C z o c h r a l s k i g r o w t h a u t o m a t e d t e c h n i q u e s for control have been recently developed.

diameter

T h e s e t e c h n i q u e s are b a s e d

on

either m e a s u r i n g the w e i g h t c h a n g e of the c r y s t a l or m e l t d u r i n g g r o w t h or m o n i t o r i n g the meniscus at the g r o w i n g interface, either w i t h respect to t e m p e r a t u r e or p o s i t i o n . I n the e l e c t r o c h e m i c a l

Czochralski technique, however,

c o n t r o l is a n i n t r i n s i c feature.

The volume

(V)

diameter

of m a t e r i a l d e p o s i t e d

e l e c t r o c h e m i c a l l y is

(22)

w h e r e M is the m o l e c u l a r w e i g h t , Q is the t o t a l c h a r g e i n c o u l o m b s , a n d P is t h e d e n s i t y of the m a t e r i a l a n d n is the n u m b e r of electrons t r a n s f e r r e d p e r u n i t of m a t e r i a l d e p o s i t e d . D i f f e r e n t i a t i n g w i t h respect to t i m e t gives

(23)

where K =

(M/pnF)

is the g r o w t h rate constant.

m a t e r i a l g r o w n is e q u a l to A(dy/dt),

S i n c e the v o l u m e of

i t w a s s h o w n that

(24)

It is, therefore, p o s s i b l e to c o n t r o l c r y s t a l d i a m e t e r d i n the [111] d i r e c t i o n b y c o n t r o l l i n g o n l y the p u l l rate dy/dt a n d the c u r r e n t I . T h i s c a n be a c c o m p l i s h e d i n p r i n c i p l e w i t h great a c c u r a c y . T h e t u n g s t e n bronzes e x h i b i t a n i s o t r o p i c g r o w t h b e h a v i o r , w i t h the [111] d i r e c t i o n the fastest g r o w t h d i r e c t i o n . T h e l e n g t h a n d d i a m e t e r of [111]-oriented Growth

on

crystals, therefore,

other

axes, h o w e v e r ,

were

found

proved

to b e

easy to

m o r e difficult.

control.

During

the

g r o w t h of either [ 1 0 0 ] - or [110]-oriented crystals, the crystals p u l l e d out of the m e l t after several centimeters h a d b e e n g r o w n .

I n the

[111]

d i r e c t i o n there is no t e n d e n c y for l a t e r a l g r o w t h , b u t i n the [100] [110]

or

g r o w t h the f a s t - g r o w t h d i r e c t i o n is i n c l i n e d to the p u l l d i r e c t i o n ,

13.

Crystal

FEIGELSON

263

Growth

a n d i n the absence of t h e r m a l constraints t h e c r y s t a l d i a m e t e r w i d e n s out, as s h o w n i n F i g u r e 11. S i n c e m a t e r i a l is d e p o s i t e d at a constant rate, the i n c r e a s e d area of the g r o w t h i n t e r f a c e causes the a x i a l g r o w t h r a t e to d r o p b e l o w the p u l l rate a n d the c r y s t a l w i l l not stay i n the m e l t . I n the n o r m a l C z o c h r a l s k i m e t h o d the c r y s t a l d i a m e t e r is c o n t r o l l e d b y t h e f r e e z i n g i s o t h e r m at the m e l t surface, w h i c h is c r e a t e d b y t h e Solid State Chemistry: A Contemporary Overview Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/24/16. For personal use only.

r a d i a l t e m p e r a t u r e gradient.

I n the e l e c t r o c h e m i c a l

Czochralski tech

n i q u e , since no s u c h t h e r m a l constraint exists, a m e c h a n i c a l constraint s u c h as a c y l i n d r i c a l d i e or r i n g o n the m e l t surface w o u l d h e l p r e s t r a i n u n l i m i t e d g r o w t h of the c r y s t a l d i a m e t e r i n the f a s t - g r o w t h d i r e c t i o n s . D e M a t t e i a n d F e i g e l s o n (12)

l o o k e d at the e l e c t r o c h e m i c a l C z o c h

r a l s k i t e c h n i q u e f o r g r o w t h of m a t e r i a l s t h a t e x h i b i t i s o t r o p i c

growth

b e h a v i o r . F o r t h e t w o crystals c h o s e n for g r o w t h , m e t a l l i c N b a n d F e , t h e m a x i m u m a l l o w a b l e p u l l rate w a s e x c e e d i n g l y s l o w . p r o p o r t i o n a l to Kd,

S i n c e p u l l rate is

i t w a s i m p o r t a n t , therefore, to s t u d y the significance

of the g r o w t h rate constant K o n e l e c t r o c h e m i c a l C z o c h r a l s k i g r o w t h . F o r N a ^ - W C ^ , K equals 2.136 c m

3

• (A • hr)'

1

a n d g r o w t h is r e l a t i v e l y

fast. F o r the case of m o s t m e t a l s , h o w e v e r , K lies b e t w e e n 0.08 a n d 0.38 cm

3

• ( A • h r ) " . I n general, D e M a t t e i a n d Feigelson concluded that the 1

g r o w t h rate constant K h a d a c r i t i c a l v a l u e n e a r K =

1. I f K is greater

t h a n 1.0, t h e n g r o w t h rates greater t h a n 0.5 m m • ( h r ) "

1

are p o s s i b l e .

If

the v a l u e is less t h a n 1.0, t h e n either a s l o w g r o w t h m u s t b e t o l e r a t e d or t h e g r o w t h rate m u s t b e o p t i m i z e d b y v a r y i n g process p a r a m e t e r s s u c h as c o m p o s i t i o n , s t i r r i n g , a n d c e l l p o t e n t i a l ( a p p l i c a t i o n of e l e c t r o p o l i s h i n g techniques).

Material

Preparation

O n c e the t e c h n i q u e s discussed a b o v e f o r g r o w i n g l a r g e single crystals of s o m e m o d e l m a t e r i a l s w e r e d e v e l o p e d , t h e l o g i c a l next step w a s t o a p p l y this t e c h n o l o g y to the g r o w t h of materials that are i n the m a i n stream of d e v i c e interest a n d w h o s e p r o p e r t i e s are not y e t w e l l c o n t r o l l e d by conventional techniques.

T h e area of I I I - V e p i t a x i a l layers a n d b u l k

single crystals w a s c h o s e n for this p u r p o s e . T h e m a i n advantages of M S E i n this case are the precise

electronic

c o n t r o l of t h e g r o w t h process

c o m p a r e d to t h e r m a l l y d r i v e n systems, the l o w g r o w t h t e m p e r a t u r e s u s e d , the

i n s e n s i t i v i t y to

temperature

fluctuations,

a n d the

possibility

for

electrochemically controlled purification and doping. O n e i m p o r t a n t p r o b l e m area i n v o l v e s finding a s u i t a b l e substrate m a t e r i a l ( c a t h o d e ) t h a t is c o m p a t i b l e w i t h the m o l t e n salt b a t h a n d has the a p p r o p r i a t e c r y s t a l l o g r a p h i c , c h e m i c a l , a n d electronic p r o p e r t i e s f o r the electrodeposit.

[100]

[no]

[ioo]«

L

J

OIRECTION

1

AXIAL GROWTH COMPONENT

J

LATERAL GROWTH — • COMPONENT

COMPONENT

G&WTH

LATERAL GROWTH r - ^ COMPONENT


PARALLEL TO GROWTH DIRECTION

Figure 11. Diagrams of interface morphologies found for electrochemical Czochralski crystals grown in the [111], [110], and [100] directions and their relationship to the fast-growth [111] direction (10)

ORIENTATION

PERPENDICULAR TO GROWTH DIRECTION


13.

FEIGELSON

Crystal

265

Growth

Silicon S i l i c o n w a s first d e p o s i t e d e l e c t r o c h e m i c a l l y i n 1854 f r o m a N a A l C U S i m o l t e n salt b a t h b y D e v i l l e (13)

a n d later b y U l l i k (14)

from K S i F 2

i n K F . S i n c e t h e n o n l y a f e w a d d i t i o n a l studies w e r e u n d e r t a k e n .

basic e l e c t r o d e p o s i t i o n process for S i d e p o s i t i o n i n v o l v e s c o n v e r t i n g S i ions i n a m o l t e n salt b a t h to e l e m e n t a l S i o n a s u i t a b l e cathode. Solid State Chemistry: A Contemporary Overview Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/24/16. For personal use only.

6

The 4 +

The

o v e r a l l r e a c t i o n c a n b e w r i t t e n as Si0

-> S i +

2

G r o j t h e i m a n d cG-workers (15,16) at a b o u t 1 0 0 0 ° C w i t h 5 w t %

Oat

( ) 2 5

p r e f e r r e d t h e Use of c r y o l i t e solutions

Si0

2

as the solute. C o o k (17)

deposited

S i at 8 0 0 ° C onto several r e f r a c t o r y metals f r o m a s o l u t i o n of K S i F 2

i n an

6

a l k a l i f l u o r i d e m i x t u r e ( p a r t i c u l a r l y the L i F / N a F / K F eutectic k n o w n as F l i n a k ) . F l i n a k c a n also be u s e d as a solvent f o r S i 0 , a n d e l e c t r o d e p o s i 2

t i o n b e l o w 1 0 0 0 ° C is possible.

M o s t of t h e S i p r o d u c e d i n these e a r l y

experiments was p o w d e r y or d e n d r i t i c i n character. C o h e n a n d H u g g i n s (18)

p r o d u c e d c o h e r e n t e p i t a x i a l a n d p o l y c r y s t a l l i n e layers b y u s i n g a

salt m i x t u r e c o n t a i n i n g 5 m o l % K S i F , 10 m o l % K H F , a n d 85 m o l % 2

6

2

L i F - K F (47.5-37.5 m o l % ) , u s i n g a n a p p a r a t u s s i m i l a r to t h a t s h o w n i n F i g u r e 3. T h e K H F

2

dissociates to K F a n d H F , a n d the H F reacts w i t h

a n y o x y g e n i n the system to p r o d u c e H 0 v a p o r , w h i c h p r e s u m a b l y is 2

flushed

out of the system w h e n e v a c u a t e d .

T h e i r baths w e r e p u r i f i e d b y

p r e e l e c t r o l y s i s , u s i n g a s a c r i f i c i a l m o l y b d e n u m strip c a t h o d e .

T h e anode

u s e d w a s h i g h - p u r i t y S i , w h i c h dissolves a n d is t h e n t r a n s p o r t e d to the cathode.

T h e cathodes

were

either

[111]

S i substrates f o r e p i t a x i a l

g r o w t h or W , A g , M o , N b , a n d a A g / N i a l l o y for p o l y c r y s t a l l i n e layers. U n i n t e n t i o n a l l y d o p e d films wer'e p - t y p e , w i t h resistivities i n the r a n g e 0.05-0.1 n

• cm.

I n p o l y c r y s t a l l i n e films g r a i n size w a s 4 0 - 5 0

/im i n

d i a m e t e r . G r o w t h rates, a n d therefore l a y e r m o r p h o l o g y , w e r e i m p r o v e d by

u s i n g a n a l t e r n a t i n g square

wave

pulse

technique.

Coherent

Si

deposits w e r e o b t a i n e d at c u r r e n t pulses u p to 40 m A • ( c m ) " . W i t h o u t 2

1

this t e c h n i q u e the a l l o w a b l e r a n g e was 1-10 m A • ( c m ) " . 2

Ill—V

1

Semiconductors

T h e gallium phosphide ( G a P ) investigation by C u o m o and G a m b i n o (19)

w a s the p i o n e e r i n g effort o n t h e e p i t a x i a l g r o w t h of I I I - V

com

p o u n d s b y m o l t e n salt electrolysis. T h e layers of G a P a n d I n P p r o d u c e d (details on I n P electrodeposition were very sketchy)

were on S i , Ge,

a n d C substrates f r o m melts c o n t a i n i n g N a P 0 , G a 0 , or l n 0 , 3

either m i x e d c h l o r i d e or

fluoride

fluxes.

2

3

2

3

and

T h e e l e c t r i c a l characteristics of

a G a P p - n junction diode were measured. T h e i r w o r k strongly indicated


266

SOLID S T A T E C H E M I S T R Y :

Figure

12.

A


A GaP electrodeposit on a silicon substrate: (bottom) cross-sectional view (21).

(top)

surface;

that t h e G a P e p i t a x i a l layers w e r e r e l a t i v e l y easy to p r e p a r e b u t f e l l short of p r o v i d i n g a sufficiently refined process to a l l o w a d e q u a t e

comparison

of t h e properties of e l e c t r o c h e m i c a l l y p r o d u c e d m a t e r i a l to t h a t p r o d u c e d i n c o n v e n t i o n a l processes. S e v e r a l years later Y a m a m o t o a n d Y a m a g u c h i (20)

d e v e l o p e d a t e c h n i q u e for e l e c t r o d e p o s i t i n g

salt b a t h c o n t a i n i n g N a S e 0 D e M a t t e i et a l . (21)

ZnSe from a molten

and Z n O .

3

r e c e n t l y i d e n t i f i e d the v a r i a b l e s that c r i t i c a l l y

d e t e r m i n e the m o r p h o l o g y a n d u n i f o r m i t y of electrodeposited U s i n g a m o l t e n salt c o m p o s i t i o n phosphate

( N a P 0 ) , 7.7% 3

oxide ( G a 0 ) , 2

3

c o n t a i n i n g 75.1 w t

sodium

fluoride

G a P was electrodeposited

%

G a P layers.

sodium

( N a F ) , and 17.2%

metagallium

i n t h e 750°—900°C r a n g e

by

the f o l l o w i n g r e a c t i o n s : C a t h o d e : 16e~ + G a 0 2

3

+ 8 P ( V -> 2 G a P + 3 0 " + 2

16e" + G a a O s + 2 P 0 ' -> 2 G a P + 3

90 " 2

6P0

4

3

" (26)

13.

Crystal

FEIGELSON

Anode: 2 0 " - > 0 2

267

Growth + 4e"

2

2P0 ~ 4

3

2P0 " + 0 3

2

+ 4e"

(27)

T h e a p p a r a t u s u s e d w a s s i m i l a r to t h a t s h o w n i n F i g u r e 3. T h e c r u c i b l e u s e d w a s g r a p h i t e , w h i c h also s e r v e d as t h e a n o d e to h e l p scavenge t h e


0

2

i n t h e system b y t h e r e a c t i o n C + 0

Growths were

2

-> C 0 t

(28)

2

a t t e m p t e d o n three different substrate m a t e r i a l s :

(1)

g r a p h i t e , ( 2 ) p h o s p h o r u s - d o p e d n - t y p e ( 1 0 0 ) s i l i c o n (0.3 O • c m ) , a n d (3) sulfur-doped n-type (111) G a P single-crystal wafers. F i g u r e 12 shows a G a P l a y e r e l e c t r o d e p o s i t e d o n a n S i substrate at 900°C a n d 20 m A • ( c m ) " . T h e m i n i m u m decomposition potential was 2

1

0.5 V . T h e l a y e r is s i m i l a r i n a p p e a r a n c e to G a P layers p r o d u c e d o n s i l i c o n b y o r g a n o m e t a l l i c c h e m i c a l v a p o r d e p o s i t i o n (22).

Silicon was

chosen because o f its a v a i l a b i l i t y a n d close l a t t i c e m a t c h w i t h G a P . S i , h o w e v e r , a p p e a r e d to react s l i g h t l y w i t h t h e m e l t t o f o r m S i 0 , a n d since 2

there is also a l a r g e t h e r m a l e x p a n s i o n m i s m a t c h b e t w e e n S i a n d G a P , the use of S i as a u s e f u l substrate m a t e r i a l is l i m i t e d . G a P substrates w e r e also f o u n d to react w i t h t h e m e l t at t e m p e r a t u r e s of 9 0 0 ° C o r greater. A t 8 0 0 ° C , h o w e v e r , t h e r e a c t i o n is r e d u c e d to a

Figure 13. GaP electrodeposits on GaP substrates for a deposition time of 20 min at current densities of 48 mA • (cm )' (a and c); 24 mA • (cm ) (b and f); 12 mA • (cm )' (c and g); and 6 mA - (cm )' (d and h) (21). 2

2

1

1

2

2

1

1


268

SOLID S T A T E

CHEMISTRY: A


77 (VOLTS) (SUPERSATURATION) Journal of Crystal Growth

Figure 14.

Plot of current (I) versus overpotential (rj) for the deposition GaP on GaP (21)

of

p o i n t w h e r e e t c h i n g is n o t serious. T h e m i n i m u m d e c o m p o s i t i o n p o t e n t i a l w a s 1.16 V . T o increase t h e c o n d u c t i v i t y of t h e electrodeposit a n d a v o i d d e n d r i t i c g r o w t h , t h e researchers a d d e d 0 . 1 % Z n O to t h e m e l t , g i v i n g a Z n - d o p e d l a y e r . F i g u r e 13 shows a series of experiments r u n w i t h a systematic v a r i a t i o n i n c a t h o d i c c u r r e n t d e n s i t y [ 6 - 4 8 m A • ( c m ) " ] . 2

1

T h e cross sections of t h e deposits p r e p a r e d at t h e l o w e r c u r r e n t d e n s i t y s h o w t h e f o r m a t i o n of coherent layers of u n i f o r m thickness. T h e r e w a s no e v i d e n c e of d e n d r i t e s o r p o l y c r y s t a l s . A t 12 m A • ( c m ) " 2

1

a n d greater,

craters a p p e a r e d at t h e surface, a n d t h e y increase i n size a n d d e p t h as the c u r r e n t d e n s i t y increases. of excess p h o s p h o r u s

T h e s e craters are r e l a t e d to t h e f o r m a t i o n

(gas b u b b l e s )

at t h e cathode.

T h e f o r m a t i o n of

s m a l l G a P d e n d r i t e s adjacent to t h e b u b b l e s is q u i t e v i s i b l e . I t w a s clear f r o m this s t u d y that c u r r e n t densities b e l o w

10-20 m A • ( c m ) " 2

1

are

necessary f o r stable g r o w t h c o n d i t i o n s a n d t h e p r e p a r a t i o n of u n i f o r m layers of c o n t r o l l e d thickness.

13.

FEIGELSON

Crystal

269

Growth

I n o r d e r to u n d e r s t a n d the influence of g r o w t h rate ( c u r r e n t d e n s i t y ) o n the g r o w t h process, the researchers p l o t t e d I against -q ( u s i n g a G a P reference e l e c t r o d e ) , as s h o w n i n F i g u r e 14. T h e c u r v e consisted of

(1)

a n i n i t i a l l i n e a r r e g i o n , w h i c h is p o s t u l a t e d to represent a r e g i o n w h e r e g r o w t h is c o n t r o l l e d b y v o l u m e d i f f u s i o n of solute ions; ( 2 )

a transition

r e g i o n ; a n d ( 3 ) a second l i n e a r r e g i o n at h i g h c u r r e n t densities, w h i c h is Solid State Chemistry: A Contemporary Overview Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/24/16. For personal use only.

also v o l u m e - d i f f u s i o n - c o n t r o l l e d a n d w h e r e the g r o w t h is h i g h l y d e n d r i t i c . F i g u r e 15 shows t w o G a P layers g r o w n at 8 0 0 ° C a n d at 10 m A • (cm )" . 2

1

T h e l a y e r o n the left w a s d e p o s i t e d i n 20 m i n , o n the r i g h t i n

3 h r . N o t e that the surface features are r e l a t i v e l y smooth. N o p r o p e r t y measurements w e r e m a d e i n this s t u d y , n o r w a s there a n y a t t e m p t to i m p r o v e the p u r i t y of the d e p o s i t b y s t a r t i n g w i t h u l t r a h i g h p u r i t y c h e m i c a l s or b y preelectrolysis of the m o l t e n salt b a t h . InP I n P has b e c o m e , i n recent years, a p o t e n t i a l l y i m p o r t a n t m a t e r i a l for e l e c t r o n i c d e v i c e a p p l i c a t i o n s . It has b e e n t r a d i t i o n a l l y difficult to

grow

h i g h - q u a l i t y single crystals of this m a t e r i a l because the h i g h v a p o r p r e s sure of P makes s t o i c h i o m e t r y , a n d therefore t h e electronic difficult t o c o n t r o l .

properties,

I n P is subject t o t h e r m a l d e c o m p o s i t i o n

by

loss

of P . C o n v e n t i o n a l m e t h o d s u t i l i z e temperatures near the m e l t i n g p o i n t (1062°C),

Figure

w h e r e the e q u i l i b r i u m pressure is 27.5 a t m . T h e a p p a r a t u s

15.

GaP layers electrodeposited at 800°C and 10 mA • (cm )' for 20 min (left) and 3 hr (right) (21) 2

1

270

SOLID S T A T E

Table III.

CHEMISTRY: A


Solvent Systems Studied for InP Electrodeposition Major Advantages

Solvent LiCl-KCl

Major Disadvantages

L o w melting point eu- H i g h volatility. Reactectic ( 3 2 8 ° C ) . H i g h tivity with nickel, s o l u b i l i t y for l n 0 N o I n P deposits, and I n F 2

3


3

NaPOa-NaF

U s e d for electrodepo s i t i o n of G a P

H i g h viscosity. Rather highmp (490°C). L o w s o l u b i l i t y of l n 0 at ~ 600°C. 2

3

Oxygen-free. L o w mp L o w solubility for l n 0 and I n F . eutectic ( 4 5 4 ° C ) . K P F as source of L o w viscosity. H a s phosphorus is v e r y g i v e n I n P deposits. hygroscopic.

LiF-NaF-KF

2

3

3

6

V e r y l o w s o l u b i l i t y for U s e d w i d e l y i n electrocrystallization ln 0 e.g. for L a B . Q u i t e low viscosity and volatility.

Li 0-B 0 -LiF 2

2

3

2

3

6

NaP0 -NaF-KP0 -KF

L o w e r m p eutectic (~ 439°C). Gives good I n P deposits. Low volatility

Rather high viscosity

LiP0 -LiF-KP0 -KF

Lower viscosity than NaP0 -NaFKP0 -KF

L o w e r s o l u b i l i t y for l n 0 than N a P 0 NaF-KP0 -KF ( £ l m / o ) . L i F is insoluble i n water.

V e r y low melting point (300°C)

H a s not given I n P de posits. L i F is i n s o l u ble i n w a t e r .

3

3

3

3

3

3

LiP0 -LiF-NaP0 -NaF 3

3

2

3

3

3

u s e d is expensive a n d u n d e r a h i g h a m b i e n t pressure, w h i c h is p o t e n t i a l l y dangerous. E l w e l l a n d F e i g e l s o n ( 2 3 ) h a v e r e c e n t l y started a p r o g r a m to i n v e s t i gate the c o n d i t i o n s necessary to p r e p a r e h i g h - q u a l i t y I n P e p i t a x i a l layers a n d crystals b y M S E a n d to s t u d y t h e i r electronic p r o p e r t i e s .

Electro

c h e m i c a l synthesis c a n p r o v i d e c o n t r o l over s t o i c h i o m e t r y a n d i m p u r i t i e s , as stated before.

U s e of a l o w m e l t i n g solvent a n d o p e r a t i o n u n d e r a

s l i g h t l y p o s i t i v e pressure s h o u l d h e l p to c o n t r o l P e v a p o r a t i o n . G e n e r a l l y , t h e defect c o n c e n t r a t i o n w o u l d b e e x p e c t e d

to b e l o w e r i n m a t e r i a l s

p r e p a r e d u n d e r these c o n d i t i o n s , a n d the e l e c t r o c h e m i c a l e q u i p m e n t is i n e x p e n s i v e a n d easy to assemble.

13.

Crystal

FEIGELSON

271

Growth

W h i l e i t w a s e x p e c t e d that I n P w o u l d electroplate as r e a d i l y as G a P from a similar bath composition by simply substituting l n 0 2

as suggested b y C u o m o a n d G a m b i n o ( 1 9 ) ,

for G a 0 ,

3

2

this w a s f o u n d n o t to

3

be

the case because of significant c h e m i c a l d i s s i m i l a r i t i e s b e t w e e n I n a n d G a salts. I n c o m p o u n d s

w e r e less soluble i n the melts s t u d i e d a n d h a d

h i g h e r v a p o r pressures t h a n s i m i l a r G a

compounds.


T a b l e I I I gives a n i d e a of the v a r i o u s m o l t e n salts s t u d i e d for the e l e c t r o d e p o s i t i o n of the I n P . T h e attractiveness of a c o m p l e t e l y system s u c h as t h e F l i n a k ( L i F , N a F , K F e u t e c t i c )

+

InF

nonoxide

-f KPF

3

o b v i o u s , b u t so f a r a s u i t a b l e , stable m e l t c o m p o s i t i o n has n o t

6

is

been

f o u n d f r o m w h i c h I n P c o u l d be d e p o s i t e d . T h e most s u i t a b l e b a t h f o u n d to date for I n P e l e c t r o d e p o s i t i o n is a q u a t e r n a r y solvent c o m p o s i t i o n c o n t a i n i n g N a P 0 / K P 0 / N a F / K F 3

nak) and l n 0 2

as t h e source of I n . T h e l n 0

3

2

3

3

(Pof-

s o l u b i l i t y w a s greater i n

this m e l t t h a n i n a c o m p a r a b l e m e l t of L i P 0 / K P 0 / L i F / K F , w h i c h h a d 3

3

a l o w e r v i s c o s i t y a n d m e l t i n g t e m p e r a t u r e . T h e I n P deposits f r o m P o f n a k w e r e of better q u a l i t y . T h e P o f n a k m e l t , therefore, has r e c e i v e d the most a t t e n t i o n to date.

T a b l e I V shows the s u i t a b i l i t y of v a r i o u s

electrode

m a t e r i a l s f o r I n P d e p o s i t i o n i n this m e l t . O f t h e m e t a l cathodes s t u d i e d , o n l y n i c k e l w a s r e a s o n a b l y c o m p a t i b l e w i t h t h e m e l t a n d t h e I n P deposit. T w o other c a t h o d e m a t e r i a l s w e r e f o u n d to b e v e r y g o o d substrate m a t e rials, I n P a n d C d S single crystals. T h e C d S I n P h e t e r o j u n c t i o n is a p o t e n 3

t i a l l y i m p o r t a n t s t r u c t u r e f o r solar c e l l a p p l i c a t i o n , a n d t h e

electrodepo

s i t i o n m e t h o d p r o v i d e s a l o w - c o s t process for p r o d u c i n g s u c h a m a t e r i a l .

Table I V .

Cathode Materials Studied for InP Electrodeposition

Material

Adhesion

of InP

Deposit

G r a p h i t e (rod)

P o o r ; adheres i n s m a l l , i s o l a t e d regions.

Germanium (single c r y s t a l )

P o o r ; adheres i n i s o l a t e d patches o n l y .

P y r o l y t i c graphite

F a i r l y p o o r ; o n l y l i t t l e better t h a n n o r m a l graphite. V e r y p o o r ; no t r a c e of I n P s h o w n b y microscope.

T u n g s t e n (sheet) I n d i u m phosphide (single c r y s t a l )

V e r y good

M o l y b d e n u m (sheet)

V e r y poor

G o l d (foil)

A t t a c k e d b y phosphate m e l t s

N i o b i u m (sheet)

Poor A t t a c k e d b y p h o s p h a t e melts

P l a t i n u m (foil) C a d m i u m sulfide (single c r y s t a l )

V e r y good

N i c k e l (sheet) T a n t a l u m (sheet)

Good V e r y poor


272

SOLID S T A T E

Figure 16.

CHEMISTRY: A

InP layer electrodeposited


on a CdS substrate

M a n y I n P deposits h a v e b e e n p u t d o w n o n C d S substrates, one of w h i c h is s h o w n i n F i g u r e 16. T h i c k n e s s e s r a n g e f r o m 1-10 /*m d e p e n d i n g u p o n d e p o s i t i o n t i m e . A t present, a n analysis of surface

morphology,

c h e m i c a l c o m p o s i t i o n , i m p u r i t y content, a n d e l e c t r i c a l p r o p e r t i e s is u n d e r w a y . T h e effectiveness of e l e c t r o c h e m i c a l p u r i f i c a t i o n t e c h n i q u e s is also b e i n g s t u d i e d i n these experiments. T h e g e n e r a l c o n d i t i o n s u s e d to electrodeposit I n P o n C d S f r o m the P o f n a k m e l t are as f o l l o w s : ( 1 ) m e l t c o m p o s i t i o n 6 0 . 9 % N a P 0 , 1 4 . 0 % 3

K P 0 , 20.5% N a F , 4.7% K F , 3 % l n 0

3

6 0 0 ° C ; ( 3 ) c e l l p o t e n t i a l 0.90 V ( E

0.80 V ) ; ( 4 c u r r e n t d e n s i t y a b o u t

3

2

d

«

(mol % ) ;

(2) cell temperature

5 m A • ( c m ) " ; (5) deposition time 1 hr. 2

1

GaAs A f e w arsenides (see D e M a t t e i et a l . (24)

T a b l e I I ) h a d been previously electrodeposited.

i n v e s t i g a t e d the p o s s i b i l i t y of e l e c t r o d e p o s i t i n g the

most i m p o r t a n t of I I I - V s e m i c o n d u c t o r s , G a A s . A s a s t a r t i n g p o i n t , t h e y a t t e m p t e d s i m p l y to substitute N a A s 0 NaP0

3

3

(sodium

m e t a arsenate)

for

i n the p r e v i o u s l y d e s c r i b e d G a P g r o w t h s o l u t i o n . S i n c e the m e t a

arsenate is easily r e d u c e d , w i t h c o n v e r s i o n of A s

5 +

to A s

3 +

or e l e m e n t a l A s

i n the p r e s e n c e of c a r b o n , metals, or G a A s , t h e y f o u n d N a A s 0

2

(sodium

arsenite) to be a m o r e s u i t a b l e source of A s for G a A s e l e c t r o d e p o s i t i o n . T h e m e l t c o m p o s i t i o n t h a t gave the best results c o n s i s t e d of 6 7 . 4 % B 0 , 2

20.3%

N a F , 4.2%

G a 0 , and 8.1% 2

c o n c e n t r a t i o n s of N a A s 0

3

2

N a A s 0 , by weight. 2

a n d G a Q w e r e 4 . 1 % a n d 1.4%, 2

3

3

T h e molar respectively.

13.

FEIGELSON

The B 0 2

Crystal

w a s u s e d to r e d u c e

3

v o l a t i l i t y of N a A s 0

2

273

Growth the m e l t t e m p e r a t u r e a n d t h e r e b y

the

at the e l e c t r o d e p o s i t i o n t e m p e r a t u r e s , w h i c h w e r e

i n the r a n g e 7 2 0 ° - 7 6 0 ° C . T h e d e c o m p o s i t i o n p o t e n t i a l f o r d e p o s i t i o n o n a G a A s substrate w a s 1.7 V , a n d i t w a s 2.4 V o n n i c k e l . A n e p i t a x i a l


10-/xm-thick l a y e r of G a A s w a s d e p o s i t e d o n a G a A s substrate.

Conclusions Molten

salt e l e c t r o c h e m i s t r y

is a n o l d t e c h n o l o g y

t h a t has

just

r e c e n t l y b e g u n to b e c o n s i d e r e d seriously f o r use i n c r y s t a l g r o w t h . W h i l e c o n v e n t i o n a l t e c h n i q u e s u t i l i z e a t h e r m a l d r i v i n g force, e l e c t r o d e p o s i t i o n is a n e l e c t r o n i c a l l y d r i v e n process, a n d , as s u c h , t h e g r o w t h process i n p r i n c i p l e c a n b e v e r y p r e c i s e l y c o n t r o l l e d . R e c e n t w o r k has c o n c e n t r a t e d o n u n d e r s t a n d i n g the e l e c t r o d e p o s i t i o n processes w i t h respect to

both

n u c l e a t i o n a n d g r o w t h p h e n o m e n a , to p e r m i t the c o n t r o l l e d g r o w t h of l a r g e h i g h - q u a l i t y single crystals. T h e a p p l i c a t i o n of this k n o w l e d g e t h e p r e p a r a t i o n of i m p o r t a n t s e m i c o n d u c t o r

to

m a t e r i a l s s u c h as S i , I n P ,

G a A s , a n d S i C has just started, a n d the competitiveness of m o l t e n salt e l e c t r o c h e m i c a l c r y s t a l g r o w t h w i t h c o n v e n t i o n a l processes w i l l b e e v a l u a t e d o v e r the next f e w years.

Glossary of Symbols A c

=

area of electrode c o n c e n t r a t i o n of solute i n l i q u i d

L

C D

s

=

c o n c e n t r a t i o n of solute i n s o l i d

=

solute d i f f u s i o n coefficient

D/A

drift velocity crystal diameter

d

m a x i m u m a l l o w a b l e p u l l rate

dy/dt E°

=

standard cell potential

E = decomposition potential F == F a r a d a y ' s constant d

i= I = K = k

=

current density

(I/A)

cell current g r o w t h rate constant d i s t r i b u t i o n coefficient constant

K I=

step s p a c i n g

M — molecular weight = m o l t e n salt e l e c t r o c h e m i s t r y

MSE

t o t a l charge

Q R

= =

gas constant or i m p e d a n c e

Ret

=

i m p e d a n c e d u e to c h a r g e transfe

274

SOLID S T A T E

R R

CHEMISTRY: A


i k

=

i m p e d a n c e d u e to i n t e r f a c e a t t a c h m e n t k i n e t i c s

v d

=

i m p e d a n c e d u e to v o l u m e d i f f u s i o n

T =

temperature ( K )

V =

volume

v =

l i n e a r g r o w t h rate

w = r o t a t i o n rate Solid State Chemistry: A Contemporary Overview Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/24/16. For personal use only.

z =

n u m b e r of electrons t r a n s f e r r e d

[ ] =

c o n c e n t r a t i o n or a c t i v i t y of r e a c t i n g species

A =

a n o d e species ( e l e c t r o c h e m i c a l l y o x i d i z e d )

C = c a t h o d e species ( e l e c t r o c h e m i c a l l y r e d u c e d ) C =

constant

E =

equilibrium potential

^d(imp) = Ed(cmpd) =

d e c o m p o s i t i o n p o t e n t i a l of i m p u r i t y d e c o m p o s i t i o n p o t e n t i a l of c o m p o u n d

n = n u m b e r of electrons t r a n s f e r r e d u e r u n i t of m a t e r i a l d e p o s i t e d y =

i o n i z a t i o n state for a n o d e species

z = i o n i z a t i o n state f o r cathods species A = =

c

y = rj = e

relative supersaturation d e p o s i t i o n efficiency overpotential e q u i l i b r i u m c o n c e n t r a t i o n of solute

A = adsorption parameter A = s

surface d i f f u s i o n p a r a m e t e r

X=

m e a n distance t r a v e l e d b y a n a d s o r b e d m o l e c u l e

p=

density

Q = molar volume 8 = boundary layer w i d t h o- = r e l a t i v e s u p e r s a t u r a t i o n en =

constant

Literature Cited 1. Kunnmann, W. "Preparation and Properties of Solid State Materials"; Lefever, R. A., Ed.; Dekker: New York, 1971; p. 1. 2. Wold, A.; Bellavance, D. "Preparative Methods in Solid State Chemistry"; Hagenmuller, P., Ed.; Academic: New York, 1972; p. 279. 3. Elwell, D. "Crystal Growth and Materials"; Kaldis, E.; Scheel, H. J., Eds.; North Holland: Amsterdam, 1976; p. 606. 4. Bostanov, V. J. Cryst. Growth 1977, 42, 194. 5. Elwell, D.; De Mattei, R. C.; Zubeck, I. V.; Feigelson, R. S.; Huggins, R. A. J. Cryst. Growth 1976, 33, 232. 6. Gilmer, G. H.; Ghez, R.; Cabrera, N. J. Cryst. Growth 1971, 8, 79. 7. Zubeck, I. V.; Feigelson, R. S.; Huggins, R. A.; Pettit, P. A. J. Cryst. Growth 1976, 34, 85. 8. Andrieux, L. Ann. Chim. (Paris) 1929, 12, 423. 9. Elwell, D.; Zubeck, I. V.; Feigelson, R. S.; Huggins, R. A. J. Cryst. Growth 1975, 29, 65.


13.

FEIGELSON

Crystal

Growth

275

10. De Mattei, R. C.; Huggins, R. A . ; Feigelson, R. S. J. Cryst. Growth 1976, 34, 1. 11. Levich, V . C. "Physiochemical Hydrodynamics"; Prentice-Hall: Englewood Cliffs, NJ, 1962. 12. De Mattei, R. C.; Feigelson, R. S. J. Cryst. Growth, in press. 13. Deville, H. St. C. Compt. Rend. 1854, 39, 323. 14. Ullik, F . Ber. Akad. Wien 1865, 52, 115. 15. Grjotheim, K.; Matiasovsky, K.; Fellner, P. Can. Met. 1971, Q10, 19. 16. Boe, G.; Grjotheim, K.; Matiasovsky, K.; Fellner, P. Can. Met. 1971, Q10, 179. 17. Cook, N. C. U.S. Patent Re. 25 630 1964; Sci. Am. 1969, 38. 18. Cohen, U.; Huggins, R. A . J. Electrochem. Soc. 1976, 123, 381. 19. Cuomo, J. J.; Gambino, R. J. J. Electrochem. Soc. 1968, 115, 755. 20. Yamamoto, A.; Yamaguchi, M . Jpn. J. Appl. Phys. 1975, 14, 561. 21. De Mattei, R. C.; Elwell, D . ; Feigelson, R. S., submitted for publication in J. Cryst. Growth. 22. André, J. P.; Hallais, J.; Schiller, C. J. Cryst. Growth 1975, 31, 147. 23. Elwell, D . ; Feigelson, R. S., private communication. 24. De Mattei, R. C.; Elwell, D . ; Feigelson, R. S. J. Cryst. Growth 1978, 43, 643. RECEIVED September 29,

1978.

Crystal Growth by the Electrolysis of Molten Salts - Advances in

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