Potential of Sb2Se3 ®lms for photo-thermal phase change optical storage

P. Arun, A.G. Vedeshwar*

Department of Physics and Astrophysics, University of Delhi, Delhi 110 007, India

Received 22 January 1998; accepted 15 May 1998

Abstract

The effect of instantaneous heat treatment (of short duration ,60 s) is studied for thermally evaporated Sb2Se3 ®lms using chemical

compositional, structural and optical analyses. The results show an amorphous to crystalline phase change at 1408C and a constant

composition Sb0.6Se0.4 in the range 150±2108C. These results are compared with those of cw Ar1 laser irradiation in an attempt to understand

the photo-thermal phase changes induced by laser irradiation. The considerable optical contrast between amorphous and crystalline phases

and their stability indicate a good potential for WORM kind of storage applications. However, a good optical contrast between various phases

having constant chemical composition in the range 150±2108C hints at the possible reversible phase change kind of storage. q 1998 Elsevier

Science S.A. All rights reserved.

Keywords: Amorphous materials; Laser irradiation; Optical properties; Selenides

1. Introduction

There has been a growing interest in the materials suitable

for real time optical storage applications [1]. The main

requirement is to produce considerable optical contrast at

the laser-irradiated spots compared to unirradiated ®lm. In

other words, some of the optical properties of the ®lm like

shifting of band gap, changing of refractive index or the

altering of the ®lm thickness on laser irradiation can be

exploited for optical storage applications. In this way

many chemical compounds in the form of ®lm were exam-

ined for possible phase changes on irradiation. In the case of

phase changes in the type of storage, both amorphous to

crystalline and crystalline to amorphous phase changes

have been tested for this purpose.

The phase changes on laser irradiation are explained as

photo-induced processes which may be direct or indirect

type. In a direct process an electronic excitation takes

place which promotes atoms to non-bonding states causing

a bond breaking and charge transfer. In the indirect process

the excited atoms relax by converting the excess electronic

energy to vibrational energy inducing thermally activated

structural changes. Both are termed as photo-thermal

processes.

In Refs. [2±4] we have studied changes occurring in Sb2S3

®lms when heated at pre-determined temperatures for few

seconds (30±45 s). We proposed that the effect of instanta-

neous heating of the ®lms should be able to indicate the

process taking place during photothermal recording. Here,

in this case we supplied thermal energy straight away while

in photo-thermal recording the heating is generated via

photon absorption. We showed that this kind of study on

any thin ®lm sample would reveal the suitability of the

material for photo-thermal storage applications. Even

though both processes may not be exactly the same, the

comparison helped in understanding the process better.

We undertake a similar study on Sb2Se3 ®lms.

The chalcogenide Sb±Se system has attracted some atten-

tion in phase change type of storage due to its known photo-

sensitive nature [5,6]. In Ref. [7] write once read many

times (WORM) kind of storage was demonstrated on ther-

mally evaporated Sb2Se3 ®lms based on amorphous to crys-

talline phase change, while Ref. [8] discusses recording by

hole burning. Few alloys of Se (like In±Se±Tl) have also

been studied for reversible type of storage [9]. However, the

results are quite scattered and need detailed study to under-

stand this compound both stoichiometrically and non-stoi-

chiometrically, for its possible application in data storage.

Few basic studies regarding transport and optical properties

of SbxSe12x for varying x (0.1±0.9) can be found in the

literature [10,11]. Therefore, we extend our method of

analysis for Sb2S3 to this compound also.

Thin Solid Films 000 (1998) 1±20

TSF 11332

0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved.

PII S0040-6090(98)00882-7

* Corresponding author fax:100 91 1091116886427;; e-mail: ngaur@-

duocos.ernet.in.

2. Experimental

Sb2Se3 was prepared in the laboratory by fusing elemental

antimony and selenium in an evacuated quartz tube. Both

the elements were of 99.99% purity. The chemical compo-

sition and stoichiometry of the starting material was deter-

mined by electron spectroscopy for chemical analysis

(ESCA) analyses using Shimadzu model ESCA 750

employing MgKa X-rays. Films of Sb2Se3 were grown on

glass substrates at room temperature by thermal evaporation

using a molybdenum boat at a pressure greater than 1026 T.

The structural studies of the as grown ®lms were done using

a Philips PW1840 X-ray diffractometer and were found to

be amorphous in nature without exception. The ®lms were

also found to be stoichiometric in nature. The morphology

of the ®lms was studied by SEM using JEOL-840. The

optical properties of the ®lms were studied in the visible

range using a photospectrometer model Shimadzu UV-

260. The thickness and uniformity of ®lms was measured

using a Dektek IIA surface pro®ler. Films grown on micro-

scope slides of size 2:5 £ 6 £ 0:08 cm were quite uniform in

thickness.

Small pieces of the size 0:5 £ 0:5 cm were cut for various

measurements. Each starting amorphous ®lm was placed for

a few seconds (,50±60 s) on a copper block maintained at

the desired temperature. Thus, many samples were gener-

ated which were heated instantaneously in air at different

temperatures for only a few seconds. The temperature at the

top surface of the empty substrate reaches the temperature

of the copper block in about 15±20 s. Therefore, the actual

heating time of the ®lm is about 30±40 s. In all cases ®lms

were facing air. Structural, compositional, morphological

and optical studies were carried out on the same samples

for the purpose of comparison.

3. Results and discussion

In Section 3.1we discuss the results of structural, compo-

sitional, morphological and optical studies carried out on

heat-treated Sb2Se3 ®lms. The temperature mentioned

throughout this paper indicates the temperature at which

the ®lm was subjected to the instantaneous heating for a

short duration of about 30±40 s. In Section 3.2 the results

of laser irradiation on Sb2Se3 ®lms will be discussed and

compared with results of heat-treatment.

3.1. Results of heat-treatment

3.1.1. Compositional analysis

The ESCA analysis shows a change in ®lm composition

with heat-treatment and the subsequent oxidation of Se (at

1008C) and Sb (at 2108C) as the ®lms were treated in air. A

maximum chemical shift of 3.6 eV for the 3d peak is

observed for selenium in bonding with antimony and 1.2

eV for those in bonding with oxygen, compared with its

elemental form. The areas of both peaks corresponding to

free and bound selenium decrease with heat-treatment. The

ratio of areas of 3d5/2 and 3d3/2 peaks of Sb shows an increase

apart from a shift of 2 eV as the function of treatment

temperature. The increase in the ratio of areas of Sb peaks

can be understood as due to oxygen 1s peak (531 eV) over-

lapping with 3d5/2 peak of Sb [12], which con®rms the incor-

poration of oxygen during heat-treatment.

The ratio of Se (which is in bonding with Sb) and Sb

decreases with treatment temperatures as shown in Fig. 1.

This behavior is very much reproducible as con®rmed with

many samples. Therefore, the phases formed at various

temperatures by heat treatment are stable. This is possible

because dichalcogenide compounds can exist in stable

phase with formula A1002xBx [1]. A systematic study of

the chemical compositions of the ®lms treated at different

temperatures reveals the breaking away of selenium atom

from antimony and its subsequent oxidation above 1008C.

The oxide of selenium is gaseous at room temperature and

hence sublimates from the ®lm surface [13]. The free anti-

mony atom starts reacting with atmospheric oxygen at

temperatures above 2108C. The complete oxidation of the

®lm takes place at 2408C. This fact is justi®ed by the

absence of Se peaks in ESCA for ®lms treated above

2408C. As can be seen in Fig. 1, at 2408C and above,

there is no selenium atom in bonding with antimony atom

and hence, Se/Sb (the ratio of Se in bonding with Sb) is

equal to zero. This was also con®rmed in X-ray diffraction

analysis which is discussed next.

The variation of Se/Sb ratio with treatment temperature in

Fig. 1, shows two plateaus, the ®rst up to 908C and the

second in the temperature range 140±2008C. The second

plateau shown in a box is of interest. It can be noted that

in the temperature range bounded by the box, the change in

composition is negligible and we can assume a constant ®lm

composition in this temperature range. The mean composi-

P. Arun, A.G. Vedeshwar / Thin Solid Films 000 (1998) 1±202

Fig. 1. Selenium content in the ®lm as a function of treatment temperature

as determined from X-ray photo-electron spectra. The portion of the curve

marked by the box shows the region of constant composition (nominally

Sb0.6Se0.4).

tion of the ®lm here is Sb0.6Se0.4. Interestingly, a reversible

amorphous to crystalline transition under pulsed laser exci-

tation in Sb0.65Se0.35 ®lm is reported [5]. This ®lm composi-

tion is almost the same as of our constant ®lm composition

in the range 140±2008C. Therefore, we strongly believe that

Sb0.6Se0.4 could also prove useful as a reversible storage

medium. We now investigate the structural and optical

properties of treated ®lms.

3.1.2. Structural analysis

Fig. 2 shows X-ray diffractograms of heat-treated 1500 AÊ ,

thick Sb2Se3 ®lm at various temperatures. Fig. 2a is the

diffractogram of as grown ®lm which shows no sharp

peaks characterizing it as amorphous in agreement with

previous reports [7]. This is further supported by electron

diffraction (transmission electron microscopy, TEM) of a

grown ®lm, as shown in Fig. 3, which reveals no sharp

rings con®rming the amorphous nature of the ®lms. The

nature of diffractograms remains the same until a treatment

temperature of 1408C is reached, beyond which a lone

intense peak appears. Thus a phase transition is taking

place above 1408C. This is in agreement with ours as well

as reported differential thermal analyses (DTA) results (Tc

of Sb2Se3 is 1458C) [11,14]. Although the X-ray diffracto-

grams of ®lms treated between 50 and 1408C show the ®lm

to be amorphous, the electron diffraction shows faint spots

on halo rings for ®lms treated above 808C (as can be seen in

Fig. 3b). This is due to the free elemental Se forming crys-

talline phase on the amorphous Sb12xSex ®lm. The amor-

phous to crystalline transition temperature of pure Se is

808C [7] which was also con®rmed by our DTA on pure

selenium. Diffractograms (d) and (e) represent samples

heat-treated at 156 and 2108C, respectively, which are iden-

tical in nature. These diffractograms represent the crystal-

line phases of Sb0.6Se0.4 and are similar to diffractogram

given in Ref. [10], marked by few and very low intensity

peaks compared for the various x in Sb1002xSex.

Since the X-ray diffractogram showed only a few peaks

Miller indexing was done by taking ring diameters seen in

the electron diffractograms (Fig. 3c).The crystalline phase

was hexagonal with cell dimensions (a � 4:32 AÊ , and c �10:6 AÊ ) in agreement with the reported data for Sb0.6Se0.4

[10]. The peaks in sample (f) treated at 2408C matched with

data for Sb2O3 as given in ASTM card 5-534 con®rming the

complete oxidation of the ®lm, with SeO2 completely

evaporating from ®lm. We have investigated the morpholo-

gical changes with treatment temperatures using SEM as

shown in Fig. 4. The micrograph seen in Fig. 4a is of a

sample heat-treated at 708C which shows no distinct

features con®rming the amorphous nature of ®lm. The

micrograph in Fig. 4b depicts a ®lm treated at 1208C.

Here, we can see improved features in the morphology

depicting the grain developmental stage. However, we can

still believe that the ®lm is not crystallized. At Tc,1408Cthe ®lms get crystallized and the well developed grain struc-

P. Arun, A.G. Vedeshwar / Thin Solid Films 000 (1998) 1±20 3

Fig. 2. X-ray diffraction patterns of 1500 AÊ -thick Sb2Se3 ®lm heat-treated

at various temperatures: (a) as grown, (b) 1108C, (c) 1408C, (d) 1568C, (e)

2108C and (f) 2408C.

Fig. 3. Electron diffractogram of 1500 AÊ , thick Sb2Se3 ®lm heat-treated at various temperatures: (a) as grown, (b) 1108C and (c) 1768C.

ture appears as shown in Fig. 4c. The grains are just devel-

oped and small. A well developed grain structure and

improvement in grain size can be seen in ®lms treated

above 1508C as seen in Fig. 4d,e. However, the morphology

of the ®lms remains same in the range 140±1908C.

This may be due to the constant ®lm composition and

same structure in this range as discussed above. Fig. 4f

shows ®lm heat-treated at 2108C. As can be seen there are

two grain morphologies. The background of the grains is

similar to the morphology of grains seen in Fig. 4d,e, except

for a few whitish grains (as can be seen in Fig. 4f) of differ-

ent structure on this background which are of Sb2O3.

Apart from oxidation of Sb, the reduction of grain size of

the background as compared to Fig. 4d,e can be noticed

clearly with increasing temperature. Therefore, morpholo-

gical analysis reveals the same facts as expected from ESCA

and X-ray diffraction results.

3.2. Optical studies

The optical properties of Sb2Se3 show thickness depen-

dence. The as grown ®lms were reddish-brown in color.

However, ®lms of thickness greater than 2000 AÊ , showed

a sudden increase in re¯ectivity. We have limited our study

on ®lms with thickness less than 2000 AÊ , since the ®lms

must be absorbing in our case. Fig. 5 shows the recorded

absorbance spectra for ®lms treated at various temperatures.

As can be seen by visual inspection the absorption edge

P. Arun, A.G. Vedeshwar / Thin Solid Films 000 (1998) 1±204

Fig. 4. Morphological changes with treatment temperatures as depicted by SEM for 1500 AÊ , thick Sb2Se3 ®lms treated at (a) 708C, (b) 1208C, (c) 1408C, (d)

1608C, (e) 1808C and (f) 2108C.

decreases with increasing treatment temperature. The opti-

cal change due to heat-treatment is even visible to the naked

eye as improved re¯ectivity of the ®lm. We have shown

explicitly the absorbance determined from Fig. 5 as a func-

tion of treatment temperature at 520 and 700 nm in Fig. 6.

The region marking constant chemical composition

shows the maximum variation in absorbance. There is

more than a 50% change in absorbance between the limits

of the range. Since the optical contrast required for storage

applications can be achieved in this temperature range, the

®lm with composition Sb60Se40 (or Sb3Se2) may offer good

potential for reversible phase change type of storage appli-

cations. Referring to all the analyses together, we can say

that the transition between various crystalline phases of

different re¯ectivity in the range 140±2108C inducible by

a suitable laser, may be exploited for erasable kind of

storage applications. However, the as grown ®lm can be

used for a WORM kind of application also, if it is heated

in the irreversible region (2108C) to obtain the desired opti-

cal contrast. The optical gap of as grown and treated ®lms is

determined using absorbance data. In case of amorphous

materials the relationship between a and photon energy

hn near the band edge best ®ts the relationship [15,16]:

ahn � A�hn 2 Eo�n (1)

for n � 2, where Eo is the optical energy gap of the material.

The calculated ahn for samples heat-treated above 1408C®tted best to

ahn � A�hn 2 Eg�1=2 (2)

where Eg is the optical gap of the material, with n � 1=2 in

Eq. (1) indicating allowed direct transition for crystalline

material.

Fig. 7 and Fig. 8 show linear plots of (ahn )n as a function

of hn for n � 2 and 1/2, respectively. The optical edge (Eo)

for amorphous ®lms and energy band gap (Eg) for crystalline

®lms are shown in Fig. 9 as a function of treatment tempera-

ture. For the as grown ®lms we obtained an Eo value of 1.35

eV. This is in good agreement with values (1.2±1.35 eV)

reported for amorphous Sb2Se3 [17±20]. Transition from

amorphous to crystalline state (1408C, shown by broken

line) marks Eg varying from near IR range to visible range.

An empirical relation exists giving the compositional

dependence of the optical energy gap in amorphous semi-

conductors like Sb1002xSex. The energy gap of a amorphous

semiconducting compound or alloy was described by the

relation [21,22],

EAB�Y� � YEA 1 �1 2 Y�EB (3)

where Y is the volume fraction of element A and EA and EB

are the optical gaps of elements A and B, respectively. The

experimentally determined energy gaps of amorphous phase

of varying stoichiometry upto 1408C of treatment tempera-

ture agrees very well with the above equation by taking the

volume fractions of Sb and Se from ESCA results of Section

3.1.1 and their energy gaps from literature. Also, the energy

gaps determined here for varying x in Sb1002xSex tallys well

with those reported in the literature for the corresponding

stoichiometry [23]. The constant composition above Tc

shown by dashed line is marked by a constant band gap.

4. Results of laser irradiation

The various analyses of heat-treated Sb2Se3 ®lms show

amorphous to crystalline transition at about 1408C and a

constant ®lm composition Sb0.6Se0.4 in the range 140±

2008C. Optical properties also show a considerable change

P. Arun, A.G. Vedeshwar / Thin Solid Films 000 (1998) 1±20 5

Fig. 5. The optical absorbance as a function of wavelength for ®lms heat-

treated at (a) as grown, (b) 1008C, (c) 1408C and (d) 1708C.

Fig. 6. The optical absorbance as a function of heat-treatment temperatures

at l � 520 and 700 nm for Sb2Se3 ®lm.

in this temperature range. Therefore, it is quite necessary to

analyze the effect of laser irradiation as a function of laser

power, ®lm thickness, etc., in light of heat-treatment results

to know about its potential for data storage.

The results of irradiation by cw Ar1 laser on as grown

1500 AÊ thick Sb2Se3 ®lms are shown in Fig. 10 for various

laser powers. The distinct spots in (a), (c) and (e) show the

transformed area at the irradiated spots at different laser

powers, and (b), (d) and (f) show the enlarged central

regions of the corresponding spots. In (a) we see more or

less uniform contrast within the spot which is still amor-

phous as can be seen even at the central region in (b) where

the temperature rise is maximum on irradiation. This means

temperature rise at the center is not more than 1408C for the

laser power of 80 mW. We can see radially varying contrast

in spots (c) and (e). However, the central region of both the

spots show similar morphology as seen in (d) and (f). There-

fore, in these two cases temperature rise at the center must

be in the range 140±2008C if we compare with the results of

heat-treatment. The radially varying contrast is due to the

radially decreasing grain size due to the radial distribution

of temperature rise during irradiation. The development of

grain structure with laser power can be seen explicitly in

Fig. 11 which shows the enlarged central regions of the

spots. The micrographs Fig. 10b, Fig. 11a,b, Fig. 10d,f

and Fig 11c±f show the systematic development of grains

on irradiation with laser power 80, 140, 170, 200, 240, 260,

390 and 410 mW, respectively. For low powers (up to 260

mW) the grain size increases linearly with irradiation time

up to 60 s. We see a growth in grain size up to a laser power

of 260 mW and a decrease above this as can be seen from

the two ®gures. Decrease in grain size with temperature is

also seen above 1808C in heat-treatment experiments as in

Fig. 4f. One striking feature is the identical morphology in

the range 140±260 mW of laser power which may be just

producing a temperature in the range 140±2008C at the

center of the irradiated spot. This result is as expected by

the results of heat-treatment which shows constant ®lm

composition and structure. However, the varying optical

properties may be due to the varying grain size. We could

not demonstrate the reversible nature of Sb0.6Se0.4 in this

study due to non-availability of suitable pulsed laser as

used in Ref. [5] where low power and large pulses were

used to write data (amorphous to crystalline) and high

power small pulses were used to erase the data (crystalline

to amorphous). This easy change (a±c and c±a) is due to the

fact that short range order in amorphous and crystalline

phases of Sb1002xSex is very similar [1]. Such measurements

on reversible amorphous to crystalline transition under

pulsed laser excitation in Sb0.65Se0.35 phase is reported [5].

P. Arun, A.G. Vedeshwar / Thin Solid Films 000 (1998) 1±206

Fig. 7. Plot of (ahn)2 vs. hn , for amorphous Sb2Se3 ®lms treated at the

temperatures identi®ed in the ®gure. The straight lines show the least square

®tting used for determining Eo by extrapolation.

Fig. 8. Plot of (ahn)1/2 for amorphous Sb2Se3 ®lms treated at the tempera-

tures identi®ed in the ®gure. The straight lines show the least square ®tting

used for determining Eo by extrapolation.

Fig. 9. Variation of the energy gap of Sb2Se3 ®lm with treatment tempera-

tures. The broken line separates amorphous and crystalline phases.

We would have a better insight about the results of laser

irradiation if we could calculate the temperature at the irra-

diated spot. A precise value of temperature rise on laser

irradiation can be calculated by numerically solving the

inhomogeneous partial-differential heat [24]. The main

dif®culty is to incorporate in the calculation the functional

dependence of ®lm parameters like thermal conductivity (k )

absorbance (A) or re¯ectance (R) on changing phases of ®lm

as a function of temperature as realized in heat-treatment

experiment. However, a qualitative estimate of the tempera-

ture rise can be made using Eqs. (4) and Eqs. (5) [25]:

Tr;z;t � AP�1 2 R�e2�r=ro�2e2az

pr2odkD

�1 2 e2kDt=c� (4)

D � 5:784

r2o

11

d2(5)

though it is more accurate for ®lms of low thermal conduc-

tivity. Eq. (4) shows that the rise in temperature (T) is

proportional to laser power (P) and A, while it is inversely

proportional to k .

However, Sb2Se3 has moderate thermal conductivity

(k,1024 W/m 8C) and the calculation shows a smaller

temperature rise. The other point is that the overall observed

transformed spot on irradiation is larger than the laser beam

size for higher laser power. This is only possible when

suf®cient heat ¯ows outside the spot due to better thermal

conductivity. Of course, this will not limit the application

anyway because of the tracks in the actual disc. The present

P. Arun, A.G. Vedeshwar / Thin Solid Films 000 (1998) 1±20 7

Fig. 10. SEM pictures of laser irradiated as grown 1500 AÊ , thick Sb2Se3 ®lms at laser power of 80 mW (a,b), 200 mW (c,d) and 240 mW (e,f). The micrographs

(b), (e) and (f) show the enlarged view of the center region of the spots shown in (a), (c) and (d), respectively.

study shows the potential of Sb2Se3 ®lms for WORM kind of

applications clearly.

Finally, we show a representative energy dispersive x-ray

analysis (EDAX) spectra taken on laser irradiated spots at

80 and 170 mW of the Sb2Se3 ®lm in Fig. 12 to compare the

stoichiometries of heat-treatment and laser irradiation

experiments. We have calculated the temperature rise as

60±1208C for the above powers. The stoichiometry at

these two temperatures obtained from ESCA on heat-treated

samples agrees well within an error of 8% with the EDAX

result. The agreement in ESCA and EDAX results further

brings out the similarity between our heat-treatment experi-

ment and that of laser irradiation. Even though we have used

a continuous wave, laser results will be still valid even for a

pulse laser of pulse width greater than 50 ns as chemical

decomposition can take place for a heating time greater than

10 ns [26].

5. Conclusions

The results of compositional, structural and optical

analyses carried out on very short duration heat-treated

Sb2Se3 ®lms manifest changing phases of different optical

constants with temperature. The indirect way of locally

heating the ®lm by a cw laser irradiation reveals the same

features as shown by directly heat treated samples. This

shows clearly the good potential of Sb2Se3 ®lms for

WORM kind of storage. However, the constant composition

Sb0.6Se0.4 having considerable optical contrast in the range

140±2108C could be an interesting material for reversible

P. Arun, A.G. Vedeshwar / Thin Solid Films 000 (1998) 1±208

Fig. 11. SEM pictures of the center of laser irradiated as grown 1500 AÊ , thick Sb2Se3 ®lms at laser power of (a) 140 mW, (b) 170 mW, (c) 260 mW, (d) 300

mW, (e) 390 mW and (f) 409 mW.

storage medium which should be precisely investigated

using pulsed laser.

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Fig. 12. EDAX spectra of laser irradiated area in a 1450 AÊ , thick Sb2Se3

®lm.