Erasable laser recording characteristics in a GaSeTe optical disk medium

Journal of Non-Crystalline Solids 97&98 (1987) 1351-1354 North-Holland, Amsterdam

E R A S A B L E L A S E R RECORDING OPTICAL DISK MEDIUM M. OKUDA,

F.S.

JIANG,

1351

CHARACTERISTICS

J.C.

IN A Ga-Se-Te

RHEE and T. MATSUSHITA*

Department of Electronics, College of Engineering, University of O s a k a Prefecture Mozu, Sakai, Osaka, 591 Japan *Department of Electronics, College of Engineering, O s a k a Industrial University Nakagaito, Daito, Osaka, 574 Japan Reversible optical change in a thin film system of parylene/GaSe-Te film/parylene structure has been studied by measuring the time required to crystallize from the amorphous state (the erasing time) and the numbers of reversible reflectance changes occurring between amorphous (written) and crystalline (erased) states. From the discussion of Ga-Se-Te phase diagram, it becomes clear that the optimum composition showing write-erase characteristics is obtained at an eutectic point, in which the segregation does not occur and many cycle o p e r a t i o n is possible. i. Introduction Optical

data storage

on a disk using ity.

Write-once

duced

and a greater

ible optical alloy

products

research

state phase

transition

havior

to an optical

reflectivity Regarding

with

cycles,

transitions, were most

optimum

of Te- or Se-based media

to the crystalline

When we apply

films,

phase good

this bein the

transition.

sensitivity

amorphous-to-crystalline

as high quality

stability

In this work,

we prepared

T e x ( G a y S e l _ y ) l _ x films

owing

In this case,

we make use of a difference

for recording

as well

to revers-

recording

characteristics.

are amorphous

high-speed

important.

of c h a l c o g e n i d e s

intro-

has been devoted

an amorphous-crystalline

capability

write-erase

have been commercially

and vice versa.

disk,

spots activ-

as optical

laser-writing

characteristics

area with

Thin films

studied

write-erase

of micro-sized on-going

effort

materials.

widely

to their excellent

on the making

is a research

storage

storage

are being

based

a laser

of

phase

and r e v e r s i b i l i t y

in order

some compositions to learn

the

composition.

2. Experimental In this experiment, a crystalline

photo-induced

to an amorphous

phase

out by the use of a s e m i c o n d u c t o r

phase

transitions,

and vice versa,

laser

0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

(wavelength

i.e.

were

from

carrled

830 nm).

1352

M. Okuda et aL

As

the initial

area,

"write"

a short long

interval

In o r d e r

used

and

were

of the

laser

1.0-15

irradiated

transitions

intensity

(P=O.I

crystalline

in the w h o l e

of a s t r o n g

by a r e l a t i v e l y

weak

light

light

for

for a

pulse

or cycle

power

was

m W / c m 2. through

were

of a laser

controlled

In this a glass

detected beam

of w r i t e - e r a s e

pulses

substrate.

with

Hz)

in the range

experiment,

of

the G a - S e - T e The b e h a v i o r

by m e a s u r e m e n t s

biased

tran-

(f=O.l-l.O

of the

a constant

reflec-

low p o w e r

mW).

From

the

width,

relation

it is c l e a r

is d e s i r a b l e

w i t h x=0.7, pulse

lization than

was

illumination

the c h a r a c t e r i s t i c s

light

the

films

imum

by the

"erase"

to e x a m i n e

intensity

(Ga)

and

one-shot

light

tion

of G a - S e - T e

was made

interval.

sitions, were

film

/ Erasable laser recording characteristics

width

(the

8 % was

of m o r e

erasing).

than

2000

been

that m e m o r y

largely

time

depend

The m i n -

the

crystalmore

ratio the

of

above

characteristics

and

upon

in c o m p a r i s o n

cycling,

From

pulse

and y = O . I 5

for

a contrast

of y=O.15.

found

laser

in Fig.l.

attained

and

crystallization

times

(Te)

memory

reversible

be o b t a i n e d

we have

the

of x = O . 7

as shown

i ~s has

in G a c o m p o s i t i o n

results

and

optical

y=O.l,

During

could

the m i n i m u m

power

a composition

and x=0.7,

cycles

experimental

the erase

the r e v e r s i b l e

of about

observed

requiring

that for

y=O.O5

2000

between

reversible

the

cycles

additional

Ga

concentration.

15

15

Teo.7(Go r 5%_y)0.3

o

Teoy (ra o ~ S e I_~ )0.3

v 0

y =o.o5

y=o.m

o rr

10

tO

.o 5

o

0.~

5

I.,LJ

0

~

I , 10 Pulse Widt h (las)

I

I 100

[

,

10

t

100

Pulse W i d t h ( ~ s )

Fig.l. C r y s t a l l i z e d r e g i o n s of some T e 0 . 7 ( G a y S e l _ y ) 0 . 3 films

Fig.2. C o n t r a s t ratio of some T e 0 . 7 ( G a y S e l _ y ) 0 . 3 films as a

as a f u n c t i o n of the erase p o w e r and the pulse width.

function

of the erase

pulse

width

1353

M. Okuda et aL / Erasable laser recording characteristics

In order

to determine

the crystalline be expressed

the change

and the amorphous

of reflection phase,

intensity

the contrast

at

ratio can

as ratio = (R c - Ra)/R c

Contrast

(1)

Here,

R is the reflection intensity at the crystalline phase c and R a is the reflection intensity at the amorphous phase. Figure 2 shows the contrast ratio at 6.5 mW erase power as a function of the erase pulse width. a contrast

width

of above

obtained we have trast

ratio

of under

50 ~s,

while

a contrast

for a Ga composition found

ratio

In the Ga composition

5 % could be obtained

ratio of 5-10 % could be

of y=O.l

and 0.15.

that as the Ga c o n c e n t r a t i o n

also gradually

of y=O.05,

at an erase pulse

From

increases,

the above, the con-

increases.

3. D i s c u s s i o n In order a Ga-Se-Te of Ga-Te,

to investigate phase Se-Te

diagram

and Ga-Se,

Te rich composition.

composition,

diagram

the melting

T A is the melting

position

diagram

points

for

temperature

can be approximately

as

T O = (TA/0A + TB/0B + T c / 0 C ) / ( I / O A Here,

phase

the melting

in Fig.3,

phase

we constructed

to the binary

and calculated

As shown

of point 0 in the ternary calculated

the optimum in relation

temperature

A, T B is the melting

B, and T C is the melting

+ I/OB + I/OC).

of the Se-Te

temperature

temperature

0A, OB and OC are the distances

system

of the Ga-Se

of the Ga-Te

at comsystem

system

from point 0 to points

C, respectively.

(2)

at

at C.

A, B and

I Te

5~

Tew(Ga?~'-Y~-x

u

~

02

501

OJ5

Se

B

Go 4s(

..................

oJ

Fig.3. A schematic picture used in the calculation of Ga-Se-Te ternary phase diagram. ~00

J

1.0

09

I

OB

J

O]

I

X

O~

Fig.4. Melting temperature of some Tex(GaySel_y)l_ x alloys function of the Te compositions.

as a

M. Okuda et al. / Erasable laser recording characteristics

1354

4 shows the melting

Figure

tions as a function eutectic

points

correspond

for Ga compositions

to the Te compositions

It was shown

that

disk materials,

segregation

While,

in order

segregation

point

the region

tions

These

of y=O.1,

O.15

Ga composi-

systems

have

and 0.2,

and

diagram

system

thus,

of reversible

unsuitable

the stable

is n e c e s s a r y

optical

such as the Se-Te for cyclic

operation, because

alloy operation.

the system hav-

at an eutectic

point,

has not occurred.

As can be seen

in Fig.4,

These

it is known

of Ga compositions

of x=O.7-0.8

erty.

and is,

to obtain

ing an eutectic

of various

of x=O.7-O.8.

in the phase

an isomorphous

causes

tem,

temperatures

of the Te compositions.

is suitable

results

agreed

that

in the Ga-Se-Te

of y=O.i-O.2

for obtaining

well with

sys-

and Te composi-

the eutectic

the experimental

prop-

results

in Fig.1. 4. Conclusion The experimental films

results

obtained

for erasable

of T e x ( G a y S e l _ y ) l _ x and the suitable

ing the reversible

chalcogenide

films

model

optical

are as follows;

i) In the Te composition

of T e x ( G a o . i S e o . 9 ) l _ x systems,

has good c h a r a c t e r i s t i c s

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

the contrast times)

ratio

(8-9 %) and the reversible

in comparison

with x=0.6

2) For the c r y s t a l l i z a t i o n leads

to a shorter

crystallization mum

time

time

and

time,

time

x=O.7

(8 ~s),

cycling

(2000

0.8.

the G a composition

than that of y = O . 0 5

of about

disk

for understand-

i us was

of y = O . 1 5

or y=O.1

obtained

and the

under

the opti-

condition.

3) From

the discussion

composition

on the G a - S e - T e

of the reversible

to the eutectic and many cycle

point.

Therefore,

operations

phase

chalcogenide

diagram,

materials

segregation

the optimum corresponds

has not occured

are possible.

References i) T.Matsushita, A.Suzuki, M.0kuda, J.C.Rhee and H.Naito: Jpn. J. Appl. Phys. 2__~4(1985) L504. 2) J.C.Rhee, M . 0 k u d a and T.Matsushita: Jpn. J. Appl. Phys. 2~6 (1987) 102. 3) M.Okuda, J.C.Rhee and T.Matsushita: Jpn. J. Appl. Phys.

26 ( 1 9 8 7 )

718.