Large potential of Sb100−xTex films for optical storage

LARGE POTENTIAL OF Sb 1002xTex FILMS FOR OPTICAL STORAGE

P. Arun and A.G. Vedeshwar*Department of Physics and Astrophysics, University of Delhi, Delhi 110 007, India

(Refereed)(Received January 9, 1998; Accepted June 5, 1998)

ABSTRACTAs-grown microcrystalline stoichiometric Sb40Te60 films were examined byheat-treatment and laser-irradiation experiments for their potential use inoptical storage. Compositional, structural, and optical studies were carried outfor films heat-treated at various temperatures in the range 0–260°C. Theyrevealed a stable phase (Sb60Te40) in the range 110–185°C and a monotonicdecrease of Te below and above this range. The irradiation experimentshowed the possibility of both reversible and irreversible phase changesdepending on the laser power. Comparing the two experiments, we believe thephase responsible for reversible changes is Sb1002xTex (x 5 38–45). Theirreversible changes were found to be due to large deviation in this stoichi-ometry. © 1999 Elsevier Science Ltd

KEYWORDS: A. chalcogenides, A. thin films, B. vapor deposition, C. pho-toelectron spectroscopy, D. optical properties

INTRODUCTION

The search for materials suitable for optical data storage has increased over the past decadeor so. The initial effort was to store information in binary form by photo-thermally createdholes in metal or alloy films, but the focus was soon shifted to laser-induced phase changeprocess. Chalcogenide compound films have been found to be a source for such purposes.The write-once-and-read-many-times (WORM) as well as the erasable kind of storage hasbeen demonstrated [1]. Most of the erasable storage media developed are chalcogenide filmscontaining Te. Among these, the Sb–Te system has been investigated to a lesser extent [2,3].

*To whom correspondence should be addressed.

Materials Research Bulletin, Vol. 34, No. 2, pp. 203–216, 1999Copyright © 1999 Elsevier Science LtdPrinted in the USA. All rights reserved

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In ref. 2, the WORM kind of storage was demonstrated on thermally evaporated Sb2Te3 filmsbased on amorphous-to-crystalline phase change. Two other works have demonstratederasing based on amorphous-to-crystalline transformation on sputtered Sb2Te3 [4] and Sb–Tealloy [3] films. Therefore, we have investigated thermally evaporated Sb2Te3 films by twodifferent approaches, viz., heat treatment and irradiation using cw Ar1 laser, to have betterinsight into the basic process responsible for their potential as a storage medium.

EXPERIMENTAL

Sb2Te3 films were grown on glass substrates at room temperature, by thermal evaporation at10–6 Torr by placing crystalline starting material in a molybdenum boat. The starting materialwas 99.99% pure and was supplied by Aldrich (USA). The thickness and uniformity of thefilms were measured using a Dektek IIA surface profiler. The glass substrates used were theusual microscope slides, which were cut into small pieces (53 5 mm2 in size) for variousmeasurements. The pieces were placed on a copper plate maintained at the desired temper-ature for a few seconds (30–45 s). Such instantaneously heat-treated films were analyzed,and in the following passages we report the results of structural, compositional, and opticalstudies carried out on these samples. In the final section we compare these heat-treated filmswith laser-irradiated films, in an effort to understand photo-thermal changes induced inSb2Te3 films upon irradiation.

RESULTS AND DISCUSSION

The films were instantaneously heat-treated in air at various temperatures ranging from 60 to260°C. The results are divided into two sections, namely, (a) results of heat treatment and (b)results of laser irradiation. The temperatures mentioned in the first section below are those atwhich the films were subjected to instantaneous heat treatment.

Results of Heat Treatment

Compositional analysis.The chemical compositions of the heat-treated films were analyzedby electron spectroscopy for chemical analysis (ESCA) (Shimadzu ESCA 750). The as-grown film was confirmed to be stoichiometric. Since heat treatment was done in air, wesuspect the gradual decomposition and subsequent oxidation of the film. Figure 1 shows Te3d3/2 and 3d5/2 peaks [5] at two different temperatures. A shift of 3 eV is evident between thepeaks of as-grown film and that treated at 220°C. This is due to change from Sb to oxygenenvironment around Te [6]. Also, it was observed that the ratio of areas of the 3d3/2 and 3d5/2

peaks of Sb increased, accompanied by a shift of 2 eV as the function of treatmenttemperature. The increase in the ratio of areas of these two peaks can be understood to be dueto the O1s peak (531 eV) overlapping with 3d5/2 peak of Sb [7]. Thus we can confirm theincorporation of oxygen during treatment.

The ratio of Te (in bonding with Sb) and Sb decreased with treatment temperature, asshown in Figure 2. A systematic study of the chemical compositions of the films treated atdifferent temperatures revealed the existence of different phases of TeOx (x 5 1, 2, 3)between treatment temperatures 180 and 220°C. This fact is further supported by thebroadening of Te peaks in the given treatment temperature range. Estimates of TeO, TeO2,and TeO3 were made by deconvolution of these broadened peaks. However, films treated

204 P. ARUN and A.G. VEDESHWAR Vol. 34, No. 2

beyond 254°C showed a sharp Te peak, suggesting the existence of TeO2 only. As can beseen, at 260°C and above, Te/Sb was equal to zero. This suggests a complete oxidation of thefilms by the formation of oxides of both Sb and Te. This was confirmed by X-ray diffractionanalysis, which is discussed next. The nature of the curve in Figure 2 is like that reported ina similar study of Sb2S3 [8]. This suggests that the nature of the curve is dependent on thekinetics of the antimony compounds’ reaction with oxygen. The variation of the Te/Sb ratiowith treatment temperature shows two plateaus: one up to 80°C and the other in thetemperature range 110–185°C. The first plateau suggests the gradual loss of Te from bondingwith Sb above 80°C and formation of a non-stoichiometric compound. The second plateaushown is of interest. The mean composition of the films heat-treated between 110 and 185°Cis Sb60Te40 and, from the boxed area in Figure 2, seems to be stable.

Energy dispersive X-ray analysis (EDAX) was performed (ISIS200 EDS, Oxford) on allthese samples, to confirm the chemical composition of the various films. Unlike ESCA,which is essentially a surface analysis method, EDAX gives the composition of the bulk, dueto greater penetration depth. Figure 3 shows the spectra of (a) the as-grown film and (b) thefilm heat-treated at 120°C. In this figure, for clarity, we have shown only two spectra. Thespectrum in (b) is the same for all of the samples heat-treated in the range 110–185°C,

FIG. 1X-ray photoelectron spectra for Te of 2300 Å thick Sb2Te3 film: (a) as-grown (at roomtemperature) and (b) heat-treated at 220°C.

205ANTIMONY TELLURIUM FILMSVol. 34, No. 2

confirming the constant composition shown in Figure 2. Since film thickness was less than1 mm (d5 2350 Å), the spectra have various peaks (2–2.4 KeV) of Si (silicon) and S (sulfur)contributed from the substrate. The composition of the films was computed by accompanyingsoftware, based on the technique explained in ref. 9. The results of the EDAX analysis gavethe composition of the as-grown film as Sb2Te3. The composition of the heat-treated filmswas given as Sb0.62Te0.38 (Sb3Te2). This is essentially the constant composition of the filmstreated in the range 110–185°C; hence, the ESCA and EDAX results are in good agreement.

In the temperature range bounded by the box in Figure 2, the change in composition wasnegligible and, therefore, we can assume a constant film composition in this temperaturerange. Similar observations were reported in ref. 4 for an even larger variation of Sb contentin the film. Therefore, we can expect a reversible transformation of either amorphous-to-crystalline or crystalline-to-crystalline phases in this temperature range.

Structural analysis. The structural studies were carried out using a Philips PW1840 X-raydiffractometer. Figures 4 and 5 show diffractograms of heat-treated Sb2Te3 film thickness of

FIG. 2Tellurium content in the film as a function of treatment temperature as determined fromX-ray photoelectron spectra. The portion of the curve marked by the box shows the regionof constant composition (nominally Sb60Te40).


2350 Å. The diffractogram in Figure 4a of the as-grown film shows no sharp peaks,characterizing the film as amorphous, in agreement with previous reports [2]. TEM analysisof the as-grown film shows the existence of microcrystallinity (see further discussion under“Optical Properties”). Therefore, we refer to the as-grown film as being amorphous. Thenature of diffractograms remains the same until the treatment temperature 108°C, at whicha single peak appears. However, well-resolved peaks appear only at 130°C. Diffractograms(b) to (f) correspond to crystalline phases of Sb1002xTex, which were indexed using aprogram in Turbo Basic. All of the phases showed orthorhombic structures. The celldimensions (a 5 8.8,b 5 8.0, andc 5 4.6 Å) determined from diffractograms (b) to (f) arealmost the same, with a maximum variation of 12% at the extreme temperatures in the range130–190°C. This supports the constant film composition in this temperature range, asdiscussed earlier. The crystallization temperature Tc (temperature at which the peaks firstappear in the X-ray diffractogram) shows film thickness dependence, as shown in Figure 6.The crystallization temperatures are in good agreement with previous reports [4,10].

FIG. 3EDAX spectra of (a) as-grown Sb2Te3 film and (b) film heat-treated at 120°C.


We tried to identify the phases present in our samples by comparing their peaks as well astheir relative intensities with those given in standard ASTM cards. Diffractograms (g) and (h)of Figure 5 are of interest. The majority of the peaks for sample (g), which was treated at192°C, matched with data for Sb2O3, TeO3, and Sb as given in ASTM 11-689, 20-1240, and

FIG. 4X-ray diffraction patterns of (a) as-grown 2300 Å thick Sb2Te3 film and 2300 Å thick Sb2Te3

film heat-treated at (b) 130, (c) 146, (d) 158, and (e) 170°C.


35-732, respectively. The remaining peaks belong to the crystalline phase of Sb1002xTex; forclarity, we did not include their Miller indices in the figure. The relative concentrations,which were determined by the relative intensities of the four phases, suggest that the amountof Sb present was minute. Diffractogram (h) corresponds to the sample heat-treated at 254°Cand confirms the presence of Sb2O3 and TeO2, as given in ASTM 11-693 (TeO2). Thus it

FIG. 5X-ray diffraction patterns of 2300 Å thick Sb2Te3 film heat-treated at (f) 180, (g) 192, and(h) 254°C.


seems that the oxidation process leads to the formation of TeO at lower temperatures,followed by the formation of TeO3 and finally that of TeO2 at higher temperatures. Peaks ofTeO are, however, not seen in any of the diffractograms between (b) to (f). This suggests theabsence of crystalline phase of TeO at these temperatures, as was observed by Takenaya etal. [11].

We used SEM to investigate the morphological changes with treatment temperatures. Theresults are shown in Figure 7. Even though the grain structure is not seen in (a) or (b) of this figure,X-ray diffraction showed well-resolved peaks, confirming the crystalline phase. A well-developedgrain structure can be seen in films treated above 150°C as seen in (c) and (d). However, themorphology of the films remains the same in the range 150–190°C. This may be due to theconstant film composition and the same structure in this range, as discussed above.

Optical studies. We have studied the absorption spectra of various heat-treated films in theUV-vis and near-IR range, using a Shimadzu 260 photospectrometer. The optical propertiesshow thickness dependence, in agreement with previous work [2]. The as-grown filmsshowed increasing reflectivity with film thickness. The variation of optical absorption withtreatment temperatures for three different thickness is illustrated in Figure 8. The regionmarked by the broken lines refers to the temperature range 110–185°C discussed in earlieranalyses. There is more than 25% change in absorbance between the limits of this range. Thecontrast is better for films of thickness greater than 1000 Å. It should be noted that the changein reflectivity appears more pronounced in treated samples, which can even be seen by thenaked eye. One can easily distinguish the sample treated at different temperatures at aninterval of 20°C in this range, by visual examination. Since the optical contrast required for

FIG. 6Temperature at which peaks first appear in X-ray diffractogram (Tc) as a function of filmthickness.


storage applications can be achieved in this temperature range, the film with compositionSb60Te40 (or Sb3Te2) shows great potentiality for reversible phase change type of storageapplications. On the basis of all of the analyses together, we can say that the transitionbetween various crystalline phases of different reflectivity in the range 110–185°C, inducibleby a suitable laser, can be exploited for the erasable kind of storage applications. However,the as-grown film can be used for the write-once-and-read-many-times kind of applicationalso, if it is heated to the irreversible region to obtain the desired optical contrast. The opticalband gap of the as-grown and treated films can be determined using absorbance data. Theabsorption coefficienta was calculated as a function of photon energy hn, following standardprocedure [12,13]. The calculatedahn was fitted to a general relation for optical transitions[12,14]:

ahn 5 (hn 2 Eg)n (1)

where Eg is the optical gap of the material. The value of n determines the nature of Eg of thematerial [14]. The best fit of our data to eq. 1 yields n5 1/2, indicating the allowed directtransition of the polycrystalline material. In the case of amorphous state, n5 2. It should benoted that microcrystallinity in the as-grown films was shown in TEM, but was not detectablein X-ray diffraction. Eg shows thickness dependence and decreases almost exponentially withthickness. Eg 5 2 eV for a 750 Å thick film decreases to 0.21 eV for a 3000 Å thick film.Eg decreases linearly with treatment temperature up to 110°C, following a relation

Eg(T) 5 Eg(0) 2 aT (2)

FIG. 7Morphological changes with treatment temperatures as depicted by SEM for 2300 Å thickSb2Te3 film treated at (a) 118, (b) 145, (c) 158, and (d) 170°C.


where Eg(0) (5 2 eV) is the value of as-grown film,a (5 7.5 3 1023 eV/°C) is thetemperature coefficient of Eg, and T is the treatment temperature for 750 Å thick film. Eg isconstant within error of 0.1 eV in the range 110–180°C. This trend is similar for thicker filmsup to 3000 Å.

Results of laser irradiation. As-grown Sb2Te3 films were irradiated using a cw Ar1laser (Coherent’s INNOVA 70-4) ofl 5 514 nm, having a beam diameter of 400mm.The effect of laser irradiation was studied as a function of film thickness, irradiation time,and laser output power. Figure 9 displays SEM micrographs of laser-irradiated (30 s)areas of as-grown 6700 Å thick films. Uniformly developed crystallites can be seen in aspot of 178mm diameter (Fig. 9a) where the laser power (p) used was 110 mW. Theenlarged view of the boundary region of the spot is shown in Figure 9b to highlight thecrystallization within the spot more clearly. On increasing the laser power, the spotdiameter increases, accompanied by radially varying grain size and morphology. This isan indication of laser-induced photo-thermal process in which the laser acts as a heatsource, increasing the temperature at the site of irradiation. Therefore, it is necessary toknow the temperature rise and its profile at these spots. A precise value of temperaturerise upon laser irradiation can be calculated by numerically solving the inhomogeneous

FIG. 8The optical absorbance as a function of heat-treatment temperaturesl 5 540 nm for (a) 750,(b) 1500, and (c) 2300 Å thick Sb2Te3 films.


partial-differential heat equation [15]. The main difficulty is to incorporate into thecalculation the functional dependence of film parameters, such as thermal conductivity(k), absorbance (A), or reflectance (R), on changing phases of the film as a function oftemperature, as realized in heat-treatment experiment. However, a qualitative estimate ofthe temperature rise can be made using the equation [16]

Tr,z,t 5AP(1 2 R)e2(r/ro)2

e6(az)

pro2dkD

(1 2 e6kDt/c) (3)

FIG. 9SEM pictures of laser-irradiated as-grown 6700 Å thick Sb2Te3 films at laser power of 110mW (a and b), 220 mW (c), and 330 mW (d and e). Micrographs (b) and (e) show theenlarged view of the boundary region of the spots shown in (a) and (d), respectively, while(c) shows the central region of a spot not shown in the figure.


where

D 55.784

ro2 1

1

d2 (4)

However, it is more accurate for films of low thermal conductivity. Eq. 3 shows temperaturerise T is directly proportional to p and A, while it is inversely proportional tok.

The crystalline grains develop at Tc, which is film thickness dependent, as shown in Figure5. From this figure, Tc 5 60°C for a film of 6700 Å thickness and, hence, we assume T560°C at the edge of the spot in Figures 9a and b. Therefore, no crystalline grains are seenbeyond the edge. A Gaussian profile of temperature rise gives a temperature at the centerT(0,z,t)5 70°C. A temperature rise directly proportional to P would mean a temperature of140 and 210°C at the center of spots irradiated at 220 mW (Fig. 9c) and 330 mW (Fig. 9d),respectively. However, the temperature would be quite lower than that estimated above dueto a largek. Temperature rise will increase with laser power and therefore the temperatureoutside the spot will also increase, due to largek, because of which crystalline grains developup to the region where T5 60°C, as can be seen in Figures 9d and 9e. Large thermalconductivity leads to melt-quenching when the laser is switched off [16] and might have ledto a crystalline-to-microcrystalline transition within an area of 150 m radius. Figure 9e showsthe enlarged boundary of this region and the grains present outside the boundary. Thesegrains have a morphology similar to that of those at the center of the spot irradiated with p5220 mW (Fig. 9c).

We varied the irradiation time for various laser output powers. For powers below p5 280mW, there was no irradiation time dependence and the results were similar to those shownin Figure 9. Well-developed grains similar to those shown in Figure 7c or d were seen in filmsirradiated for 15 s at laser power ranging from 280 to 415 mW, as shown in Figure 10a. Whenwe increased the irradiation time to greater than 15 s, the crystalline phase was transformedto microcrystalline phase irrespective of laser power in the range 280–415 mW. Thisindicates the possible photo-induced transformation of a certain photosensitive phase devel-oped during the initial stages of irradiation (t#15 s). The similarity between the grainstructure shown in Figures 7c, 7d, and 10a leads us to believe that the phase developedinitially and transformed to that shown in Figures 6b and 9b–d is Sb60Te40. The observedtransformation of the as-grown microcrystalline phase to a crystalline phase of bigger grainsand back to microcrystalline phase suggests the possibility of reversibility. However, a filmof Sb60Te40 composition has to be examined to resolve such a possibility and prove it to bea useful erasable storage medium. Such new phases (i.e., SbxSe12x) suitable for reversiblestorage have been reported for the Sb–Se system [17]. However, Sb2Se has been found [18]to be the most suitable composition, because of its short crystallization time [18].

The essential requirements of reversible phase change optical storage is that over a largetemperature range there is no chemical compositional change, as represented by a plateauregion in a graph such as that shown in Figure 2. The phase transition, which should bringabout an appreciable optical contrast, should lie on this plateau. Of the investigated andreported Sb–C systems (C5 S, Se, and Te), only Se and Te fulfill these requirements.However, the Sb–Te system allows for more control, since the transition temperature isthickness dependent. In actual practice, for optical storage, a laser diameter of a fewmicrometers and a time of irradiation of a few nanoseconds pulse are used. Although in ourexperiment, we maintained a laser diameter and irradiation time far greater than those used


in actual practice, the changes occurring in both cases would be similar. By decreasing thediameter for a given laser power, one can achieve higher temperature rise at the irradiatedsite. Oxidation of the film will take place even for short pulses, such as 10–100 ns [19].

CONCLUSIONS

On the basis of both experiments, we find that Sb1002xTex films may be used for reversible(for x 5 38–45) and irreversible (remaining nonstoichiometric phases) phase change opticalstorage applications.

ACKNOWLEDGMENTS

We are thankful to Drs. N.C. Mehra and S. Shukla, USIC, Delhi University, for theircooperation in the use of analytical techniques. The financial assistance of UGC, India, in theform of SRF to P. Arun is also gratefully acknowledged.

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FIG. 10Surface morphology dependence on irradiation time for a 6700 Å thick Sb2Te3 film. SEMpictures of the central region of the spots created on the film irradiated at (a) 326 mW for 15 s,(b) 326 mW for 60 s, (c) 380 mW for 60 s, and (d) 412 mW for 60 s.


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Large potential of Sb100−xTex films for optical storage