Simultaneous crystallization of both elements in amorphous GeSb and GeAl eutectic alloys

222

Journal of Non-Crystalline Solids 117/118 (1990) 222-225 North-Holland

SIMULTANEOUS CRYSTALLIZATION OF BOTH ELEMENTS IN AMORPHOUS GE-SB AND GE-AL EUTECTIC ALLOYS Toshio OKABE, Sakaru ENDO and Shigeru SAITO Department of Physics, Toyama U n i v e r s i t y , Toyama 930, Japan Electron microscopic i n v e s t i g a t i o n s have been c a r r i e d out on the e a r l y stages of c r y s t a l l i z a t i o n of amorphous e u t e c t i c Ge-Sb and Ge-AI a l l o y s with no intermediate compound. The Ge-Sb and Ge-AI f i l m s , prepared by vacuum coevaporation on the substrates at room temperature, are amorphous f o r Ge concentrations l a r g e r than 0.I and 0,5, r e s p e c t i v e l y , The c r y s t a l l i z a t i o n of these f i l m s are characterized by simultaneous formation of both c o n s t i t u e n t s in t h e i r c r y s t a l l i n e forms, adjacent each other with s p e c i f i c o r i e n t a t i o n r e l a t i o n s h i p s : (246)Ge / / (O03)Sb and [032]Ge / / [ l l O ] S b f o r Ge-Sb, and ( l l l ) G e / / (224)AI and [121]Ge / / [ITO]AI f o r Ge-AI, The c r y s t a l l i z a t i o n temperature decreases monotoneously with increasing Sb or A1 content, and is well f i t t e d to 2/3 values of the l i q u i d u s temperature in Kelvin in the case of Ge-Sb, I,

INTRODUCTION

of Ge-Sb and Ge-AI a l l o y systems, in which con-

Researches on what would happen in a system

s t i t u e n t s have considerably d i f f e r e n t melting

in which the two components cannot form a s o l i d

points, by using high r e s o l u t i o n electron micro-

s o l u t i o n when r a p i d l y quenched from the l i q u i d

scopy (HREM),

state led to a f i n d i n g of the nonequilibrium

o p e r a t i v e motion of c o n s t i t u e n t s during phase

c r y s t a l l i n e phase in Ag-Ge a l l o y system I and

separation over a wide composition range,

the f i r s t system, 2'3 solubility

In p a r t i c u l a r we wish to f i n d co-

m e t a l l i c glass in Au-Si a l l o y In these a l l o y s complete s o l i d

2, Experimental

is precluded because of d i s s i m i l a r

crystal structures,

The e q u i l i b r i u m phase d i a -

Amorphous f i l m s were prepared by vacuum coevaporation of the c o n s t i t u e n t s from two sources

grams of the a l l o y s are the e u t e c t i c type with

onto mica substrates previously and afterward

a small s o l u b i l i t y of germanium or s i l i c o n e in

coverd with carbon f i l m s .

noble metal and n e g l i g i b l e s o l u b i l i t y

arrangement such t h a t two tungsten boats were

of noble

metal in germanium or s i l i c o n e ,

By the geometrical

separated by 8 cm and the substrates were placed

Ge-AI and Ge-Sb a l l o y s belong to the same

at 12 cm above the boats, a number of t h i n f i l m s

a l l o y system whose e q u i l i b r i u m phase diagram

with d i f f e r e n t composition were prepared at a

is of the e u t e c t i c type with no i n t e r m i d i a t e

time in a vacuum system evacuated to < 1 xlO-4

compound and the metastable c r y s t a l l i n e phase

and maintained at < 7 x 10-4 Pa, on monitouring

is formed by rapid s o l i d i f i c a t i o n , 4'5

t h e i r thickness by three c r y s t a l o s c i l l a t i n g

While

t h e i r amorphous t h i n f i l m s are e a s i l y prepared

microbalances,

over a wide composition range by vacuum evapo-

tes were held at or near room temperature,

r a t i o n or s p u t t e r i n g ,

f i l m s were about 30 nm t h i c k ,

Recently the c r y s t a l l i -

During deposition, the substraThe

sandwiched with

zation process of amorphous AI-Ge a l l o y t h i n

carbon layers of about 7 nm t h i c k on both sides.

f i l m s is characterized by the phase separation

The f i l m s were w e t - s t r i p p e d from the substrates

and the growth of colonies t h a t contain both

and mounted on molybdenum electron microscope

constituents in t h e i r c r y s t a l l i n e form, 6

grids.

I t is

Carbon f i l m s were used to p r o t e c t the

the o b j e c t i v e of t h i s paper to show what would

amorphous a l l o y s from the i m p u r i t y contamination

happen on the e a r l y stages of c r y s t a l l i z a t i o n

during the observation made in the hot stage of

0022-3093/90/$03.50 (~) Elsevier Science Publishers B.V. (North-Holland)

T. Okabe et al. / CrystaJlization of both elements in amorphous Ge-Sb and Ge-AI

223

an e l e c t r o n microscope. The composition of the f i l m s was determined by energy d i s p e r s i v e x - r a y a n a l y s i s w i t h an ORTEC EEDS-II system attached to the e l e c t r o n microscope.

The f i l m s were heated up u n t i l

beginning o f the c r y s t a l l i z a t i o n nized by " i n s i t u "

the

could be recog-

o b s e r v a t i o n in the e l e c t r o n

microscope using heating stage and cooled q u i c k l y down t o room temperature f o r the successive high r e s o l u t i o n e l e c t r o n microscopy.

High r e s o -

l u t i o n e l e c t r o n micrographs were taken using a I0 nm- I

r a d i u s o b j e c t i v e a p e r t u r e under a x i a l

illumination

w i t h a JEM-2OOCX e l e c t r o n microscope

equipped w i t h a high r e s o l u t i o n s i d e e n t r y g o n i o meter stage and an o b j e c t i v e lens w i t h C : 1.9 s mm, operated at 200 kV. 3. Results and Discussion 3.1. HREM of e a r l y stages of c r y s t a l l i z a t i o n Transmission e l e c t r o n micrographs f o r the asdeposited f i l m s show a homogeneous and f e a t u r e less s t r u c t u r e on a m i c r o s c o p i c scale and the corresponding d i f f r a c t i o n halo-type, typical

p a t t e r n s are broad,

of amorphous s t r u c t u r e ,

with

Ge c o n c e n t r a t i o n s l a r g e r than 0. I and 0.5 f o r Ge-Sb and Ge-AI, r e s p e c t i v e l y .

On heating in

FIGURE 1 HREM image o f the c r y s t a l l i t e s of Ge and Sb nuc l e a t e d a d j a c e n t each o t h e r in the amorphous m a t r i x of GexSbl_ x w i t h x = 0.5.

the heating stage o f e l e c t r o n microscope, the Ge-Sb f i l m s show the c r y s t a l l i z a t i o n

taking

formed a d j a c e n t to a Ge c r y s t a l l i t e .

Owing to

place by homogeneous n u c l e a t i o n and growth p r o c -

the l i m i t a t i o n

ess w i t h a numerous n u c l e a t i o n s i t e s .

microscope, only the (003), the (011) and the

While the

o f r e s o l u t i o n o f the e l e c t r o n

Ge-AI f i l m s show a few nucleated c r y s t a l l i t e s

(102) l a t t i c e

growing r a p i d l y w i t h c r y s t a l l i t e s

0.35 and 0.31 nm, r e s p e c t i v e l y ,

ing in new s i t e s .

s c a r c e l y form-

By the e l e c t r o n d i f f r a c t i o n ,

planes o f Sb w i t h spacings o f 0.38, and the ( I I I )

plane of Ge can be analyzed on the micrographs.

i t confirms t h a t two c o n s t i t u e n t s c r y s t a l l i z e

Measurements on the angle between the l a t t i c e

s i m u l t a n e o u s l y , a d j a c e n t each o t h e r ,

planes of Ge and Sb reveal t h a t the phase sepa-

in amorphous

m a t r i x in both cases o f Ge-Sb and Ge-AI. Figure 1 shows an example of HREM images of crystallites

formed in the amorphous m a t r i x in

the e a r l y stages of c r y s t a l l i z a t i o n films.

of Ge-Sb

The image are taken near the boundary

r a t i o n r e s u l t s in the growth of c o n s t i t u e n t crystallites

with specific orientations;

Figure 2 shows the l a t t i c e

image t h a t the

(III)

c o n s i s t s o f Ge and Sb p o l y c r y s t a l s .

a spacing of 0.23 nm make an angle 90 ° .

crystallites

w i t h s i z e s less than I0 nm are

and

(102)Sb making angles 15 ° . 35 ° , 45 ° and 72 ° .

between amorphous m a t r i x and a m a c r o c r y s t a l which Small Sb

(III)G e

and (O03)Sb making an angle 90 ° and ( l l l ) G e

plane o f Ge and the ( I I I )

plane o f A1 w i t h This

micrograph is one of the t h r o u g h - f o c a l s e r i e s

T. Okabe et al. / Crystallization of both elements in amorphous Ge-Sb and Ge-AI

224

[l~O]Ge / /

[I~O]A 1 f o r Ge-AI with the same plane

orientations.

I t is noteworthy, however, t h a t

the present c r y s t a l l i z a t i o n

is governed by the

cooperative motion of the c o n s t i t u e n t atoms, e x p e l l i n g m u t u a l l y to form the respective c r y s t allites

simultaneously, instead of the e p i t a x i a l

growth in which atoms are deposited on the l a t t i c e s t r u c t u r e of the substrate. 3.3. C r y s t a l l i z a t i o n temperatures Figure 3 shows the v a r i a t i o n of c r y s t a l l i z a t i o n temperature, as well as c r y s t a l l i t e s

formed

in amorphous matrix, versus composition f o r GeSb system.

Over a wide composition range, s i m u l

taneous c r y s t a l l i z a t i o n

of the both c o n s t i t u e n t s

has been recognized with decreasing the temperat u r e monotoneously with increasing Sb content. At considerably low temperature ~300°C, Ge can form c r y s t a l

lattices

in the a l l o y f i l m s with

high Sb content, as compared with pure Ge f i l m s . The r a t i o of the c r y s t a l l i z a t i o n FIGURE 2 HREM image of the c r y s t a l l i t e s of Ge and A1 nucleated one above another in the amorphous matrix of GexAll_ x with x = 0.78.

temperature

coo I -., 5

of images t h a t resolves the l a t t i c e The angles between the l a t t i c e

plane of AI.

planes, f r e q u e n t -

l y observed, are 20 ° , 35 °, 60 °, 75 °, 90 ° and

ot .

400

OI E

120 ° . 3.2. O r i e n t a t i o n r e l a t i o n s h i p s For the e x p l a n a t i o n of the angles between the observed planes, only a r e l a t i v e o r i e n t a t i o n between the c o n s t i t u e n t c r y s t a l l i t e s

C 0 .f..i

1.0 0.8

...,4

~@~OOoOOO0

is enough

O

•I

O0

O~

OO00

0.6

to assume in each case:

0.4

(246)Ge / / (O03)Sb and [032]Ge / / ( I I I ) G e / / (224)A 1 and [121]G e / /

[llO]sb for Ge-Sb, [ITO]A 1 f o r Ge-AI.

0.2

q.0 l

Ge

!

!

I

I

0.8 0.6 0.4 0.2 x, Atomic £roction Ge

0.00"0 Sb

From the standpoint of geometrical arrangement of atoms at the i n t e r f a c e s ,

these o r i e n t a t i o n s

may not be the best one, since the l a t t i c e mismatch is smaller f o r the cases with the a x i a l o r i e n t a t i o n s of rl21lGe / / [lO0]sb f o r Ge-Sb and

FIGURE 3 Compositional dependence of the c r y s t a l l i z a t i o n temperature of Ge-Sb system. Also i n d i c a t e d in the f i g u r e are the c r y s t a l l i n e phases: Ge (0) and Sb (A). The l e f t axis i n d i c a t e s the r a t i o of the c r y s t a l l i z a t i o n temperature to the l i q uidus temperature expressed in K e l v i n .

T. Okabe et al. / Crystallization of both elements in amorphous Ge-Sb and Ge-A1

t o the l i q u i d u s temperature expressed in degrees a b s o l u t e is also p l o t t e d in f i g u r e 3. the c r y s t a l l i z a t i o n

I t shows

temperatures are well f i t t e d

s h o r t - r a n g e o r d e r w i t h composition.

225

So as to

p r o v i d e a continuous change on the h i g h t o f the potential

barriers,

the d i s t r i b u t i o n

of bonds

to 2/3 values of the l i q u i d u s temperatures over

is expected t o be p u r l y s t a t i s t i c a l

a wide composition range.

is no preference f o r one kind of bond over ano-

S o - c a l l e d "two t h i r d s

and t h e r e

r u l e " is a well-known e m p i r i c a l r e l a t i o n f o r 7 supercooled l i q u i d s , polymers and i n o r g a n i c

t h e r , which i s in accord w i t h random network,

glasses. 8 where the glass t r a n s i t i o n

the d i s c o n t i n u o u s change, i n t e r p r e t e d by a chemi-

temperature

in c o n t r a s t t o the Ge-Te system I0 which shows

o r the m e l t i n g temperature is a p p r o x i m a t e l y two

cal preference f o r the Ge-Te bond over Ge-Ge o r

t h i r d s of the m e l t i n g temperature.

Te-Te bonds, in accord w i t h the 4-2 c o o r d i n a t i o n .

The same c o m p o s i t i o n a l dependence of the c r y s t allization

temperature is found in the case of

the Ge-AI system.

The amorphous f i l m GexAll_ x

w i t h x = 0.2 shows the simultaneous c r y s t a l l i z a t i o n at 500 °C. w h i l e the f i l m s w i t h x : 0.4 at 300 °C. Based on the Lindeman's i n t e r p r e t a t i o n melting,

on

the same argument has been a p p l i e d to

the glass t r a n s i t i o n . 9

In a glassy s t a t e at

low temperatures, each atom. l i k e e.g. in a c r y s tal.

stays at or near a p o t e n t i a l energy minimum.

As the temperature is r a i s e d , a p o i n t of glass transition

will

be reached where s u f f i c i e n t

thermal energy w i l l

become a v a i l a b l e t o a l l o w

atoms in p o t e n t i a l w e l l s to make t r a n s l a t i o n a l motions over the p o t e n t i a l Since the p o t e n t i a l

barriers.

b a r r i e r s depend on t h e i r

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