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1990 crystallization kinetics of thin amorphous in sb films
 

1990 crystallization kinetics of thin amorphous in sb films

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    1990 crystallization kinetics of thin amorphous in sb films 1990 crystallization kinetics of thin amorphous in sb films Document Transcript

    • Materials' Scienee and Engineering, B5 (199(I) 233-237 233 Crystallization Kinetics of Thin Amorphous InSb Films P. M. L. O. SCHOLTE Philips Research Laboratories, PO Box 80.000, NL-5(#)O JA Eindhoven (The Netherlands) (Received May 30, 1989) Abstract DSC only of limited use for the study of thin films for optical recording applications, in which high The kinetic parameters governing the crystal- transition rates are involved. lization of thin films of amorphous InSb have been In a recent paper van der Poel [2[ described a determined using an optical technique. Films set-up designed to study the crystallization pro- prepared by sputtering and by evaporation have •cess in thin films. The method uses the absorption different crystallization properties. The activation of light and its subsequent conversion into heat to energy of an evaporated film is 1.39(5) eV atom i, drive the transformation. During the crystalliza- while for a sputtered film it is 2. 7(1) e V atom t. tion process the reflectivity and transmittivity of From the measurement of the Avrami exponent the film are measured. The method also has the and from transmission electron microscopy experi- advantage that layer stacks can be measured ments', it is concluded that the sputtered film which are similar to those in an optical disk. crystallizes by nucleation and subsequent growth In this paper we have used this method to on these nuclei. The evaporated films crystallize by study the crystallization process of thin amor- three-dimensional growth on existing nuclei. phous InSb films. A comparison is made between films prepared by evaporation and by sputtering. The results are compared with experiments on a 1. Introduction transmission electron microscope (TEM). The laser-induced crystallization of thin amor- phous films is of increasing importance for opti- 2. Experimental details cal recording applications [1]. To write in an amorphous layer, crystalline spots are formed by The experimental technique has been heating a small area with a short laser pulse up to described in detail by van der Poel [2]. Therefore a temperature just below the melting point. A in this paper we will give only a short outline. A typical laser pulse has a duration of 100 ns and collimated beam of light is focused on the sample. the crystalline area formed has a diameter of The beam issues from a krypton ion laser with approximately 1 /~m. To retain data integrity for 2 = 752 nm. On the sample a light spot is produced long enough periods, the amorphous phase must with a gaussian spatial intensity distribution and a be stable at ambient temperatures. Therefore a full width at half-maximum (FWHM) of 15.2(8) material must fulfil stringent demands on its /,m. Part of the incident light is absorbed in the crystallization properties to be suitable for optical film and converted into heat, thus driving the recording applications. At high temperatures it crystallization. The temperature rise and conse- must crystallize rapidly, while at ambient temper- quently the crystallization rate can be controlled atures the crystallization rate must be negligible. by adjusting the power in the laser beam. At the The conventional technique used to obtain centre of the heating spot a second light beam is information about the crystallization properties focused in a small spot with FWHM = 1/~m. This of a material is differential scanning calorimetry light beam emerges from a solid state laser with (DSC). This technique permits the study of trans- 2 = 820 nm. The power in this spot is kept suffi- formations at heating rates of 0.01-100 K s -L ciently low that its temperature rise can be and with transformation times larger than 1 s. To neglected compared to the temperature rise due obtain sufficient signal-to-noise ratio, a substan- to the heating beam. The second spot is used to tial volume of material is needed. This makes probe the reflectivity and transmittivity during 0921-5107/90/$3.50 © Elsevier Sequoia/Printed in The Netherlands
    • 234 the crystallization. The dimensions of the probe beam and the film thickness are small enough to 550 500 450 400 consider the temperature uniform over the 2 InSb volume probed. 20 ~ The samples that we have investigated consist of a single InSb layer on a thick (1.2 mm) glass oo / substrate. The heat diffusivity D and heat con- ductivity K of the substrate are D = 4.90 x 10 ~ / m 2 s 1 and K = 1.1 W K t, respectively. From / / these thermal properties it can be deduced that / the rise time of the temperature is approximately -2 / 200 /~s [2]. The temperature distribution has / -3 / been calculated using the expression of Pittaway for a surface heat source on a semi-finite sub- 1.8 2.0 2.2 2.4 2.6 strate [3]. The InSb layers were prepared by IO00/T [K1] ...... sputtering or by flash evaporation. The composi- tion of the film was determined by X-ray fluores- Fig. I. Arrhenius plot of the transformation time r', as cence (XRF) to be InSb within 2 at.%. The deduced from the transmittivity, vs. the temperature. The slope of the line gives the activation energy. The sample was a non-crystallinity of the samples was checked with 20 nm InSb film prepared by evaporation. X-ray diffraction (XRD) by the absence of sharp peaks. It appeared that evaporated films thicker than 100 nm were at least partially microcrystal- line. In our experiments we used evaporated films 30 t • oo with thicknesses between 20 and 90 nm, and I 2.5 • sputtered films between 20 and 160 nm. 2.0F > i 3. Results uZ 1.5~ :: s o At constant temperature T the crystalline frac- tion x(t) during the transformation is usually described by the Avrami equation [4] k k 0 4~0 8~0 120 160 200 x(t) = 1 -exp{ -(t/r)'"} d (nm) ---,,. where m is the Avrami exponent. For an activated Fig. 2. Activation energy vs. film thickness for a number of lnSb films. The full circles represent samples prepared by process the temperature dependence of the sputtering, the open circles films prepared by flash evapora- characteristic transformation time r can be tion. written as r = r 0 exp(EacJkT ) Transformation times longer than 1 ms are where East is the activation energy. included only, since for shorter times the non- This activation energy can be deduced directly isothermal part of the process cannot be neglec- from measurements of the transmittivity. In our ted. The crystallization time varies rapidly over a samples the transmittivity increases during the temperature range less than 200 K. Also, the amorphous-to-crystalline transition. For this experimental points can be fitted with a straight purpose let us define the transformation time r' line. This indicates that the crystallization process as the time needed for the transmittivity to cross a can be described with one activation energy only. level that is 1.15 times the start level. This r' is The same conclusions can be made for the other proportional to the transformation time r. How- samples that have been measured. ever, it should be noted that the proportionality From the slope of the line in Fig. 1 the activa- constant is different for different samples. From a tion energy can be calculated. In Fig. 2 the activa- plot of log r' against l/T, the activation energy tion energies for a number of evaporated and Eac t c a n be calculated. In Fig. 1 the result for an sputtered samples are shown. The activation evaporated sample of 20 nm of InSb is shown. energy is independent of the film tl-fickness. There
    • 235 is a large difference, however, between sputtered I and evaporated films. The average activation energy for an evaporated film is 1.39(5) eV atom-~, while for sputtered films it is 2.7(1) eV g120 140 u~ 100 / / atom ~. To investigate this difference further, we have 80 /" 60 determined the Avrami exponent m for one sput- d c = 1.08 d a 40 tered and one evaporated sample. From the 20 Avrami exponent it can be deduced whether the i i crystallization starts on existing nuclei or not and 0 2'0 4'0 6'0 dO 100 120 140 what the dimensionality of the growth process of da (nm) -- the crystallites is (see e.g. ref. 5). However, the Fig. 3. Film thickness after crystallization, d~, vs. film thick- measurement of rn is less trivial than that of E~,,t. ness of the amorphous phase, d,, for films prepared by To obtain accurate values of m, the crystalline evaporation. The solid line corresponds to d = 1.08(4)d.. .fraction x(t) has to be calculated from the experi- mental reflectivity and transmittivity. For this we need to know the values of the optical constants of the amorphous and crystalline phases at the temperature at which the transmittivity and reflectivity have been measured. Since accurate values for thin films are available only at room 0 1 2 3 4 5 6 temperature [6], the following procedure was In(t/tp) applied. The sample is heated with a short laser pulse from the heating laser. After the sample has Fig. 4. Avrami plot of the cryslallinc fraction x(t) for an evaporated sample of 70 nm; tr is the pulse time of the cooled down, the transmittivity and reflectivity heating pulse. are measured. This is repeated a large number of times on the same area of the sample. In this way the sample is crystallized gradually while the denced by XRF and Rutherford backscattering transmittivity and reflectivity are measured at measurements. room temperature. This allows us to use the opti- In Fig. 4 an Avrami plot is shown of the crys- cal constants that have been determined for sput- talline fraction x(t) resulting from an experiment tered InSb at room temperature: for amorphous on a 70 nm InSb layer produced by flash evapor- InSb n = 4 . 8 2 - 1.95i and for crystalline InSb ation. If the crystallization process could be n = 4 . 0 7 - 0 . 7 5 5 i [6]. The dielectric constant of described completely by the Avrami equation, the mixture of the amorphous and crystalline this would be a straight line with slope m. How- phase is calculated using an effective medium ever, the curve deviates strongly from the simple theory [2, 7]. This is valid if the crystalline phase Avrami behaviour. This is typical for all samples appears homogeneously distributed throughout considered, both sputtered and evaporated. This the volume of the film. deviation is not unexpected for a thin film. In the The effective optical constants are used to derivation of the Avrami expression the im- extract the crystalline fraction x(t). The calcula- pingement of crystallites is taken into account. tion includes volume changes during the transi- However, it is assumed that the material extends tion. For sputtered films Holtslag and Scholte [6] infinitely in all directions. This is obviously not observed that the film thickness d changes slightly true for a thin film. Therefore one may expect the during crystallization: d c = 1.015 d a. In flash- crystallization to become lower dimensional evaporated films the volume change is much when the crystallites reach the interfaces of the larger (see Fig. 3): d~=l.08d~. The volume film [9]. Also, one cannot neglect the effect of the change upon fusion of lnSb is reported to be stress induced by the volume expansion during 11.4%-13.7% [8]. Therefore the density of the the crystallization. This may become especially evaporated sample is comparable to the density important when the crystals start to impinge. of the liquid. The sputtered sample is less dense. From the slope at the beginning of the curve, This can be attributed to the incorporation the Avrami exponent at the start of the crystalliza- of a considerable amount of argon, as evi- tion process can be calculated. For an evaporated
    • 236 film of 70 nm m = 1.5(2) was found. For a sput- Combined with the high vaiues ot the Avrami tered film of 92 nm rn = 3(1). The value for the exponent and the activation energy, this indicates sputtered film is rather inaccurate owing to the that crystalline nuclei first have to be formed high activation energy of the film. To obtain before crystallization can start. The value of the enough points at the start of the crystallization, a Avrami exponent is in accordance with t ; ; - 2.5 short pulse time has to be used: 0.1 ms vs. 10 ms This value corresponds to a crystallization pro- for an evaporated film. Consequently, the non- cess that is similar to that in the evaporated layer isothermal part of the crystallization may have plus an extra nucleation step. Also, as one might influenced the experimental value of the Avrami expect, the value of the activation energy in the exponent. However, within the given limits the experimental value for the Avrami exponent is correct. 4. Discussion and conclusions In Table 1 the results for sputtered and evap- orated InSb are summarized. It is clear that the crystallization properties depend strongly on the preparation method. Unfortunately, there is no unique relationship between the value of the Avrami exponent and the characteristics of the crystallization process. Different processes may give the same value for the Avrami exponent [5]. To understand these differences we have to com- bine the results of the optical measurements with the results of T E M experiments on a sputtered and an evaporated layer of InSb on Si3N 4 sub- strates [10]. First consider the evaporated layer. From the Fig. 5. TEM pholograph of an amorphous lnSb layer low value of the Avrami exponent m = 1.5(2) it is prepared by evaporation. The number of crystalline nuclei highly unlikely that the crystallization starts with visible corresponds to approximately I() ~ crystatlites m ~' the formation of crystalline nuclei. This is evi- denced by Fig. 5, in which a section of an evap- orated layer is shown. The number of crystallites visible corresponds to 1 0 ~~ crystalhtes m - .- - " Therefore it can be concluded that in evaporated InSb the crystallization proceeds by three-dimen- sional diffusion-limited growth on already exist- ! ing nuclei. In sputtered InSb the situation is different. As can be seen from Fig. 6, no visible crystallites are present in the as-deposited layer. TABLE 1 Comparison between some parameters deter- mining the crystallization process in thin amorphous InSb films prepared by sputtering and by evaporation. E~t is the activation energy for crystallization, m is the Avrami exponent and d~ and d~ are the film thicknesses of the crystalline and amorphous layers respectively t~rameter E vapora~d Aput~red E~,~, 1.39(5)eV atom t 2.7(l)eV atom m 1.5(2) 3(1) Fig. 6. TEM photograph of an amorphous InSb laver d~/d. 1.08(4) 1.015(5) prepared by sputtering.
    • 237 sputtered layer is higher than in the evaporated during sputtering. Before crystallization can start, layer. argon first has to be removed. Until now, transient nucleation effects have In conclusion, it has been shown that the been neglected, since the Avrami analysis crystallization process in evaporated InSb pro- neglects the incubation time that is needed to ceeds by three-dimensional diffusion-limited reach a steady state nucleation rate. This is justi- growth on existing nuclei. In sputtered InSb fied for evaporated InSb, since nuclei are already the crystallization proceeds by nucleation and present. For sputtered InSb one has to take care. subsequent three-dimensional diffusion-limited However, Gravesteijn has shown that it is pos- growth. The high activation energy in sputtered sible to crystallize a thin InSb film near the melt- films is mainly due to the high barrier against ing point in 15 ns [1]. The incubation time at that nucleation in these layers. temperature will be even shorter. Also, a non- negligible incubation time would have turned up in the Arrhenius plot of the crystallization time as Acknowledgments a deviation from linear behaviour. No such devia- The author gratefully acknowledges Mr. P. van tion has been observed for crystallization times der Werf and Mr. N. Dreesen for the preparation larger than 1 ms. Therefore it is not unreasonable of the samples. Dr. J. Coombs, Dr. A. Holtslag to neglect the incubation time due to transient and Dr. G. Thomas are acknowledged for nucleation in sputtered InSb also. critically reading the manuscript. Now we can estimate the activation energies for the nucleation step and the growth separately. It can be deduced straightforwardly that the acti- References vation energy for a sputtered film can be written as [3, 1 1] D. J. Gravesteijn, Appl. Opt., 27(1988) 736. C. J. van der Poel, J. Mater. Res., 3 (1988) 126. Eac, spur= ( E n ~- E~)/rn~p~ L. G. Pittaway, Br. J. AppL Phys., 15 ( 19641967. M. Avrami, J. ('hem. Phys., 9 (1941 ) 177. and for an evaporated film as J. W. Christian, in R. W. Cahn (ed.), l~hysical Metallur~w, North-Holland, Amsterdam, 1971t, p. 47 I. E act evap = Eg/t~lcv.lp A. H. M. Holtslag and P. M. L. O. Scholte, to be published. where E n and Eg are the activation energies for J. C. Maxwell Garnett, l'hil. 7)ans. R. 3oc. Lond., 203 nucleation and growth respectively and rn~p~ and 119114) 385. rn~v,~ are the Avrami exponents of the sputtered p N. A. Goryunova, The ('hemist O, ~1 Diamond-like Semi- and evaporated films respectively. From this we conductors, Chapman and Halk London. 1965, p. 114. 9 M. C. Weinberg, J. Non-('ryst. Solids, 70 1985) 253. find Eg = 2.113) eV atom i and E n = 612) eV 10 F. J. A. M. Greidanus, B. A. J. Jacobs. F. J. A. den atom 1. The barrier against nucleation in the Broeder, J. H. M. Spruit and M. Rosenkranz, Appl. f'hvs. sputtered film is very high. This is most likely due Lett., 54 (1989) 963. to the argon that is incorporated in the film 11 E. A. Marseglia, .I. Non-(rvst. Solids, 41 1981)) 3 I.