Glassy transformation and structural change in ...

Proc. VII European Society of Glass Science and Technology Conf., Athens, Greece, 25–28 April 2004 Phys. Chem. Glasses, 2005, 46 (2), 224–226

Glassy transformation and structural change in oxygen doped GeSb2Te4 films C. Rivera-Rodríguez, E.Prokhorov,1 G. Trapaga, E. Morales-Sánchez, M. Hernández-Landaverde, Yu. Kovalenko & J. González-Hernández CINVESTAV del IPN, Unidad Querétaro, Juriquilla, Querétaro 76230, México. Manuscript received 4 June 2004 Revision received 13 July 2004 Accepted 4 August 2004

The aim of this work is to study the amorphous to crystalline transition in pure and oxygen doped (in the range of 0–15 at%) GeSb2Te4 films in the process of isothermal heating. The transition was followed by x-ray diffraction and impedance measurements. The results show that in the material without oxygen, the initial nuclei have the GeSb4Te7 composition, which could appear due to local fluctuations in the film composition. With increasing annealing time these nuclei transform into the crystalline GeSb2Te4 phase. In films with an oxygen concentration in the range of 2–10%, the oxygen is located on tetrahedral interstitial sites and acts as the nucleation centre, producing nuclei with a single GeSb2Te4 composition, which grow until crystallisation of the films is completed. In films with oxygen concentration above 10 at%, germanium is partially oxidised. Upon isothermal annealing, the amorphous films first crystallises into the fcc GeSb2Te4 phase. Once this phase is completed, a new Sb2Te3 crystalline phase starts to form until crystallisation is completed. These phenomena explain anomalous behaviours of the Avrami plot in films with high oxygen content. Glassy chalcogenide semiconductors have been studied extensively due to their applications in optoelectronics and electronics. The most frequently used materials for erasing optical memory devices are Ge2Sb2Te5 and GeSb2Te4 alloys because of their fast crystallisation rate. In recent years, it has been reported that oxygen doping in Ge:Sb:Te recording layers changes the structure and therefore the crystallisation parameters, such as crystallisation temperature and the activation energy of the amorphous to crystalline transformation.(1–5) This material exhibits enhanced properties as a recording layer in phase change optical disks such as: better overwrite cyclability, improved signal to noise ratio and faster crystallisation rate.(1,4) However, the structure and the crystallisation mechanism of oxygen doped Ge:Sb:Te materials are so far not well known. In previous reports, two scenarios of how oxygen incorporates into the Ge:Sb:Te structure have been proposed. One of them proposes that oxygen is located at the tetrahedral inter1

Corresponding author. E-mail address: [email protected]

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stitial site of the Ge:Sb:Te structure.(2,3) The other proposes that the oxygen forms oxides such as GeO2, Sb2O3 or TeO2, which then condense at grain boundaries.(1,2,4) The aim of this article is to study the amorphous to crystalline transformation of oxygen doped GeSb2Te4 films during isothermal heating. X-ray diffraction and impedance measurements were used to analyse the structure and the kinetics of the transformation. Experimental The Ge:Sb:Te:O thin films (with thickness from 120– 150 nm) were prepared using a reactive dc magnetron in an oxygen and argon atmosphere. A target with the Ge14·2Sb28Te57·8 composition (close to the stoichiometric GeSb2Te4 alloy) was used to deposit all films on crystalline silicon and glass substrates. Films with oxygen content in the range of 0–15 at% were obtained by changing the ratio of O2 and Ar gas flow rates. The oxygen content was determined in films on crystalline silicon using energy dispersive spectroscopy. X-ray measurements were carried out in the air using a diffractometer (Rigaku Dmax/2100) equipped with a heating device to follow, in situ, the isothermal transformations. The temperature was controlled with a Watlow’s Series 982 microprocessor-based, with ramping controller. The temperature controller was programmed to increase the temperature at a constant heating rate of 5 K/min until reaching the desired temperature for isothermal annealing. These experiments allowed measuring the evolution of the nuclei volume fraction through the diffraction intensities generated by the amorphous (IA) and crystalline (IC) phases with background and noise removed. The volume fraction of crystallinity was then given by IC/(IC+IA).(6) The impedance measurements were carried out in the frequency range of 1 MHz–3 GHz with an Agilent Impedance/Material Analyser E4991A. The sample was heated in the air at the desired temperature and time, and then cooled down to room temperature for the impedance measurements using evaporated gold contacts. The amplitude of the measuring signal was 100 mV. The high frequency range allowed us to measure the electrical properties of low resistivity materials after the amorphous–crystalline phase transaction.

Proc. VII ESG, Athens, Greece

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C. RIVERA-RODRÍGUEZ ET AL: STRUCTURAL CHANGE IN OXYGEN DOPED GeSb2 Te4 FILMS


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Results and discussion Figure 1 shows the x-ray diffraction spectra for a GeSb2Te4 films with different atomic percentage of oxygen: (a) 0; (b) 4·2 and (c) 12·6%. These data was taken in all three samples at 393 K after the indicated annealing times. All as prepared samples have an amorphous structure, with a characteristic broad diffraction band centred at about 28° in the 2q scale (not shown in Figure 1). The pattern corresponding to the GeSb2Te4 sample with 0 at% of oxygen annealed during 30 min (Figure 1(a)) shows weak peaks with positions corresponding to the fcc phase of the GeSb4Te7 composition.(7) The intensity of these peaks increases with the increase in the annealing time. After annealing for 90 min or more, the position of these peaks begin to shift toward the position of the fcc GeSb2Te4 phase. The position of a GeSb4Te7 peaks is marked with the vertical dashed lines. The patterns of Figure 1(b) correspond to a GeSb2Te4 with 4·2 at% of oxygen. For annealing times of 70 min, the fcc GeSb2Te4 phase starts to appear, for longer annealing times this phase increases until complete crystallisation. Other samples with oxygen content in the range of 4–10 at% show the same annealing behaviour. Patterns of Figure 1(c) correspond to a GeSb2Te4 film with 12·6 at% of oxygen. After 30 min of annealing time, the fcc GeSb2Te4 phase appears and increases with the increase in the annealing time. For annealing times of 180 min, the Sb2Te3 crystalline phase appears and increases for longer times (the position of Sb2Te3 peaks are marked with dash vertical lines). The results in Figure 1 indicate that the crystallisation process is different in films with different oxygen content. For the analysis of the crystallisation kinetics during isothermal annealing, the classical Johnson–Melh– Avrami–Kolmogorov (JMAK) model is often used. According to this model, one of the parameters commonly evaluated for analysing experimental data, is the Avrami exponent, which is used as a tracer of the mechanisms underlying the transformation. Inserts in Figure 1 shows plots of ln[-ln(1-x)] versus ln(t) (Avrami plots), were x is the volume fraction of crystallised material for an annealing time t. The value of x was calculated from the x-ray measurements in Figure 1. According to JMAK theory, in a crystallisation process with random nucleation and isotropic growth, the Avrami plot must be a straight line, this is not the case for two plots in the inserts in Figure 1. For samples with 0% of oxygen, the Avrami plot shows a crystallisation process in three different stages, with three well defined slopes. The initial stage or slope is related to the nucleation of the metastable GeSb4Te7 phase, the final stage, for longer annealing times corresponds to the nucleation and growth of the GeSb2Te4 phase, intermediate annealing times correspond to a transition stage between the metastable and stable phases. According to previous studies in Ge2Sb2Te5 films, the intermediate stage has a low slope because the stable phase grows in both the amorphous and the metastable phase simultaneously.(8) The GeSb4Te7 composition has the lowest crys-

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tallisation temperature; the formation of this metastable phase in our films could be related with local fluctuations in the film composition. For samples with an oxygen content in the range of 2–10 at%, the Avrami plot is a straight line which according to the x-ray results, is related with the nucleation and growth of a single fcc GeSb2Te4 phase. The crystallisation kinetics in these materials can be described by an Avrami type analysis with an exponent of n=1·2. This value of n is consistent with diffusioncontrolled growth of particles with an appreciable initial volume.(9) To check the possibility of oxide formation with segregation to grain boundaries we made additional impedance measurements in samples heated at temperature above the amorphous to crystalline transition (428–473 K). Using the brick model,(10) which assumes that cubic shaped grains are separated by flat grain boundaries, it is possible to estimate the volume fraction (xgb) and resistivity (rgb) of grain boundaries. Figure 2 shows the dependence of these two parameters as a function of temperature in materials with 0, 4·6 and 12·2 at% of oxygen. Curves 1 and 2 (Figure 2(a)) represent the xgb value for films with 0 and 4·2 at%, 225


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two crystalline phases are formed during isothermal annealing; first, the nucleation of one phase occurs, it grows until complete depletion of the elements forming it, and after that the second phase starts to crystallise. This model predicts the same dependence of the Avrami exponent on annealing time as that determined experimentally in our materials with oxygen content higher than 10 at% (insert on Figure 1(c)). A similar crystallisation process is observed in our samples with more than 10 at% of oxygen. The fcc GeSb2Te4 phase crystallises during the initial annealing stages, after this phase is completed, due to the formation of the amorphous germanium oxide, the excess Sb and Te form an additional Sb2Te3 crystalline phase.

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Figure 2. Volume fraction (a) and resistivity (b) of grain boundaries as a function of temperature calculated for pure GeSb2Te4 (black circle), and for GeSb2Te4 with 4·6 at% (black triangular) and with 12·2 % of oxygen (black square)

respectively, notice that xgb is larger for the former. Curves 3, 4 and 5 (Figure 2(b)) are the rgb values for films with 0, 4·6 and 12·2 at% of oxygen, similar values are observed for the two former samples. The measured rgb in samples with 0 and 4·6 at% of oxygen indicate that for these oxygen concentrations no oxides are formed on grain boundaries. The lower value of xgb in films with a content of oxygen lower than 10 at%, compared to oxygen free samples, is mostly due to an increase in the grain size as previously reported.(1) The results mentioned above, provide further evidence that in films with oxygen concentrations lower than about 10 at%, most of the oxygen is probably located at the tetrahedral interstitial sites. This conclusion is also supported by other results, which show an increasing unit cell distortion with the increase of oxygen.(1) It is possible that oxygen acts as the nucleation centre and this is the reason why the metastable GeSb4Te7 phase is not formed. In films with oxygen concentration above 10 at% it is possible that during film formation the Ge, being the most reactive of the three elements, forms some amorphous germanium oxide, not detected by x-ray diffraction. The formation of this oxide and its segregation at grain boundaries has been previously reported for similar films.(1,2,4) In our films with oxygen above 10 at%, the formation of the crystalline Sb2Te3 phase and larger rgb value support the formation and grain boundary segregation of the germanium oxide. Bakaiy et al(11) proposes that the crystallisation of heterogeneous glasses is limited by atomic diffusion, which results in phase segregation. In these materials, 226

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Conclusions The analysis of the crystallisation processes in GeSb2Te4 thin films with different contents of oxygen allow us to conclude that their structure and crystallization kinetics depend on their oxygen concentration. In the material without oxygen, nuclei with the GeSb4Te7 composition appear at the initial crystallisation stages, the formation of this phase could be related to local fluctuations in the film composition. In films with oxygen in the range of 2–10 at%, oxygen is probably located at tetrahedral interstitial sites and acts as the nucleation centre for the crystallisation of a single fcc GeSb2Te4 phase. In films with oxygen concentration above 10 at%, amorphous germanium oxide is formed. In these films, the fcc GeSb2Te4 phase crystallises first, once this stage is completed and since some Ge was taken to form oxide, and additional Sb2Te3 crystalline phase is formed. This annealing behaviour has been observed in heterogeneous glasses,(11) and it explains the anomalous behaviours in the Avrami plot (insert of Figure1(c)). Acknowledgement This work was partially supported by CONACyT of Mexico. References 1. Ebina, A., Hirasaka, M. & Nakatani, K. Oxygen doping effect on Ge–Sb–Te phase change optical disks. J. Vac. Sci. Technol. A, 1999, 17 (6), 3463–6. 2. Jeong, T. H., Seo, H., Lee, K. L., Choi, S. M., Kim, S. J. & Kim, S. Y. Study of oxygen-doped GeSbTe films and its effect as an interface layer on the recording properties in the blue wavelength. Jpn. J. Appl. Phys. B, 2001, 40 (3), 1609–12. 3. Jeong, T. H., Kim, M. R., Seo, H., Yeon, C. J., Park, J. W. & Yeon, C. Suppression of jitter bump GeSbTe phase change optical disk. Jpn. J. Appl. Phys. B, 2000, 39 (2), 741–4. 4. Takase, A., Fujinawa, G., Ebina, A., Hirasaka, M. & Sugiyama, I. Crystal structure of oxygen/nitrogen doped GeSbTe phase change media: investigation using grazing incidence x-ray diffraction. Jpn. J. Appl. Phys. A, 2002, 41 (4) 2189–90. 5. Men, L., Tominaga, J., Fuji, H. & Atoda, N. Oxygen doping effects on super resolution scattering mode near field optical data storage. Jpn. J. Appl. Phys. A, 2000, 39 (5), 2639–42. 6. Kratos. Analytical applications note ‘Percent Crystallinity Determination By X-ray Diffraction’, NY Office, September 1999. 7. Morales-Sánchez, E., Prokhorov, E., Vorobiev, Yu. & GonzálezHernández, J. The nature of the crystallisation nuclei in Ge 2Sb 2Te5 alloys. Solid State Commun., 2002, 122, 185–8. 8. Laine, B., Trápaga, G., Prokhorov, E., Morales-Sanchez, E. & GonzálezHernández, J. Model for isothermal crystallisation kinetics with metastable phase formation. Appl. Phys. Lett., 2003, 83 (24), 4969–71. 9. Christian, J. W. Transformations in Metals and Alloys, Pergamon, 1975. 10. Macdonald, J. R. Impedance spectroscopy. John Wiley & Sons. Inc., 1977. 11. Bakaiy, A. S., Hermannz, H. & Lazarevy, N. P. Diffusion-limited crystallisation of heterogeneous glasses. Philos. Mag. A, 2002, 82 (8), 1521–39.


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