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Spectroscopic investigation on phase transitions for Ge2Sb2Te5 in a wide photon energy and high temperature region
Thin Solid Films 520 (2012) 3458–3463
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Spectroscopic investigation on phase transitions for Ge2Sb2Te5 in a wide photon energy and high temperature region Y.K. Seo a, J.-S. Chung a, Y.S. Lee a,⁎, E.J. Choi b, B. Cheong c a b c
Department of Physics, Soongsil University, Seoul 156-743, Republic of Korea Department of Physics, University of Seoul, Seoul 130-743, Republic of Korea Electronic Materials Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
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Article history: Received 16 May 2011 Received in revised form 15 December 2011 Accepted 15 December 2011 Available online 22 December 2011 Keywords: Ge2Sb2Te5 (GST) Optical constants Ellipsometry Amorphous phase Crystalline phase
a b s t r a c t We investigated the electronic properties of phase-change material Ge2Sb2Te5 (GST) films using spectroscopic ellipsometry in a wide photon energy and high temperature region. Apart from the charge carrier response, the totality of optical conductivity spectra for three phases of GST films, i.e., amorphous (AM), face-centeredcubic (FCC), and hexagonal (HEX), is quite similar, composed of two interband transitions in visible and UV regions. From optical analysis in a wide photon energy region up to 8.7 eV, we found that the intensity as well as the position of the interband transition in the visible region changes significantly as the phase of GST films turns from the amorphous to the crystalline phase, which is consistent with previous theoretical studies. In high temperature measurements above room temperature for the three phases of GST films, we found that the change of optical response for the AM phase of GST film occurs abruptly through two successive phase transitions near 150 °C and 270 °C, while the optical spectra of the FCC phase shows a change only near 270 °C. In contrast to the two above-mentioned cases, a slight change in optical spectra is observed for the HEX phase with the increasing temperature. From the measured optical spectra, we derived the temperature dependence of optical bandgap for the three phases, which are closely correlated to the change of the transport property for the GST films. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The chalcogenide Ge–Sb–Te alloys materials have attracted a great deal of attention because of their potential application to data storage media such as digital versatile disk random access memory, compact disk rewritable, and phase-change random access memory (PRAM) [1–3]. These applications are based on a fast and reversible phase transition between the crystalline and amorphous phases. The difference in electronic response between the amorphous and crystalline phases leads to a sizable difference in electric resistance and optical constants. Among this class of materials, the Ge2Sb2Te5 (GST) is the most widely used composition [4–6]. This material has three phases, i.e., amorphous (AM), face-centered-cubic (FCC), and hexagonal (HEX) phases, with two successive structural phase transitions near 150 °C and 270 °C. In usual cases, the as-grown phase of GST film is amorphous. When it is heated, the as-prepared amorphous GST film is thermally crystallized into a FCC structure near 150 °C. At a higher temperature, i.e., 270 °C, the FCC phase turns into a hexagonal phase [7–9]. When the crystalline phase is heated above the melting point and then cooled rapidly, one can obtain the amorphous phase again.
⁎ Corresponding author. Tel./fax: + 82 2 820 0404. E-mail address: [email protected] (Y.S. Lee). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.12.043
The amorphous phase of GST is insulating with low reflectance in the visible region, while two crystalline phases have high conductivity with relatively high reflectance in the visible region. There have been many optical spectroscopic studies on the electronic structure of each phase in GST [10–15]. While most of the previous publications dealt with comparison of room temperature optical properties among the AM, FCC, and HEX phases, there have been few reports on the change in optical response during the two phase transitions, i.e., AM-to-FCC, and FCC-to-HEX, which may enable us to understand the electronic properties of the GST phases and the related phase-switching mechanism in depth. Actually, the origin of the metal-insulator transition for GST is still unclear: electronic vs. structural [16]. The high temperature spectroscopic investigation may provide useful information for solving the current issues on the phase transition in GST. Another notable point is that the spectral range in the previous studies was limited to below 5.5 eV [10–15]. Since the spectral change near 5.5 eV during the phase transition is still sizable, it is required to extend the spectral range of measurement in order to completely check out the spectral change during the phase transition. To overcome the limitation of the previous spectroscopic works on GST, we performed optical measurements on the AM, FCC, and HEX phases of GST films in a wide photon-energy and high temperature region. Using a high photon-energy ellipsometry up to 8.7 eV, we identified two interband transitions in the visible
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and UV region. While the peak near 7.5 in the UV region is insusceptible to the phase transition, the intensity as well as the position of the peak in the visible region changes significantly through the phase transitions. This optical change is well consistent with the recent theoretical studies. We also performed high temperature ellipsometry measurement on three phase of GST films in the course of heating up to 350 °C. In the AM phase we found abrupt changes of the optical constants during both structural phase transitions. Whereas, the FCC-GST shows a change during only a transition near 270 °C. In the HEX phase where no phase transition occurs, we found a slight change of the interband transition in visible region with the increasing temperature. The temperature dependence of the band-gap was obtained from the optical spectra for the GST films.
2. Experimental details The Ge2Sb2Te5 (GST) thin films are deposited on the SiO2 (300 nm)/Si substrates at room temperature by using the radio frequency magneton sputtering technique. We used the stoichiometric GST target. The base pressure in the sputtering chamber was 1.3 × 10 − 5 Pa. The sputtering deposition rate was 1.1 Å/s, and the deposition time for our films was 15 min. The film thickness is about 100 nm. To identify the concentrations of Ge, Sb, and Te for our films, we performed the Rutherford backscattering spectroscopy (RBS) on our films. The collimated He + ion beams with 2 MeV were used. The samples were positioned on a three-axis goniometer in vacuum. The elastic recoil detection technique was employed. We simulated the RBS patterns predicted for the stoichiometric GST composition, and compared them with the experimental RBS data. We found two data quite consistent, which indicates that our films are stoichiometric. We also performed the X-ray fluorescence spectroscopy (Rigaku system 3272, Japan) and determined the composition of our films more accurately: the concentrations of Ge, Sb, and Te for our films are 23.1%, 24.1%, and 52.6%, respectively. The asgrown phase of GST film in our deposition condition is AM. To obtain the FCC and HEX phases of the crystalline GST films, we annealed the AM-GST at 200 °C and at 400 °C for 30 min in vacuum, respectively. For structural analysis X-ray diffraction (XRD) patterns (θ-2θ scan) were measured with a Bruker-AXS Discover D8 system with a Cu target X-ray tube. The X-ray beam was focused to a parallel beam with a Gobel mirror to enhance the intensity for thin film measurements. The XRD measurements were performed at room temperature. In order to investigate the electric properties of the GST films, the dc resistivity measurement was measured using the standard fourprobe method. The temperature range is from 30 K to 623 K (350 °C), covering both structural phase transitions occurring near 150 °C and 270 °C. We used the different experimental setups for low and high temperature region. A closed-circulated refrigerator was used for low temperature measurement below room temperature. For high temperature region up to 370 °C, we used a homemade vacuum chamber with continuous temperature change by the heating/cooling rate, 1.7 °C/min. The optical measurements for GST films were measured in the photon energy range 0.7–8.7 eV using spectroscopic ellipsometry (VASE model, J.A. Woolam Inc.). The angles of the incidence light are 65°, 70°, and 75°. Below 0.7 eV we measured the infrared reflectivity spectra of films using the infrared spectrometer (Bruker IFS 66v/S). To look into the change of optical constants through the phase transitions, we measured the three phases of GST films while varying temperature up to 400 °C. To avoid oxidation damage of the GST films at high temperatures, the samples are positioned in a chamber with an N2 atmosphere. The heating rate was 10 °C/min. After temperature reached a target temperature, we waited for 10 min more. The measurement time was about 20 min at a given
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temperature. In this case the measurement photon energy is limited to a range from 0.7 eV to 5.0 eV. 3. Results and discussion 3.1. Structural analysis Fig. 1 shows XRD θ-2θ scan results of the three phases of GST films, i.e., AM, FCC, and HEX phases. The XRD measurements were performed at room temperature. The sharp peaks near 2θ = 33°, 62°, and 70° come from the Si substrates. Apart from the Si peaks, no distinct XRD peak is detected for as-grown AM-GST film, and instead, broad peaks near 28° and 48° are resolved. In contrast to the case of the AM-GST, the XRD patterns of FCC-GST and HEX-GST show welldefined diffraction peaks. The observed patterns are quite consistent with the previous reports on polycrystalline samples [13,17,18]. 3.2. Electrical property of AM-, FCC-, and HEX-phases We discuss the electric properties of GST films from the dc resistivity data. The resistivity curves are measured by using the fourprobe method. Fig. 2(a) shows the low temperature dc resistivity curves ρ(T)of three phases of GST films as a function of temperature. It is clearly found that the electric phase of AM-GST is insulating with very high resistivity values. On the other hand, the FCC-GST exhibits a barely metallic behavior, just like a semi-metal [11]. While absolute values of the resistivity in the FCC-phase are much smaller by an order of 5 than those in the AM-phase, the temperature dependence of the resistivity is rather semiconducting-like, showing the upturn behavior at low temperatures. In contrast to the case of FCC-GST, the resistivity curve of the HEX-GST shows a typical metallic behavior. From these findings, it can be concluded that the electric phases of the AM-GST, FCC-GST, and HEX-GST films are insulating, barely semi-metallic, and typically metallic, respectively. We also performed a high temperature measurement of ρ(T) for the AM-GST film. The film was positioned in a vacuum chamber with continuous temperature change by the heating/cooling rate, 1.7 °C/min. As shown in Fig. 2(b), the dc resistivity curve of the AMGST exhibits anomalous drops with two onset temperatures, 150 °C and 270 °C, which indicate the structural phase transitions, i.e., AMFCC and FCC-HEX, respectively. While the drop of the resistivity for the AM-FCC transition is quite sharp with a broadness of 5 °C, the change of the resistivity for the FCC-HEX transition occurs in a broad temperature range of 270–300 °C. When it is cooled down
Fig. 1. The θ-2θ scan spectra of X-ray diffraction for AM, FCC, and HEX phases of GST.
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Fig. 2. (a) Low temperature dependent resistivity curves of the AM, FCC, and HEX phases of GST films. (b) High temperature resistivity curve of the AM-GST phase. The arrows indicate a thermal process.
from the highest temperature, the resistivity curve shows a metallic behavior which is almost the same with that of the HEX GST film prepared in a post-annealing process. 3.3. A wide photon energy range of electronic structures for three phases of GST Now we discuss the electronic structures of the AM-GST, FCC-GST, and HEX-GST films in a wide photon energy range up to 8.7 eV. We used the spectroscopic ellipsometry to obtain the optical constants which possess basic information on the electronic structure of materials. With a four phase model of surface roughness, GST layer, SiO2 layer, and Si substrate [13], we estimated the dielectric functions of the GST layer which is assumed to be composed of one Drude component and two Lorentz oscillators. The dielectric functions obtained with the mode fitting are quite consistent with those obtained in a point-by-point fitting method. We extrapolated the optical constants to the lower energy region, and found that the dc conductivity values estimated with the ellipsometric fitting coincide well to those obtained independently by using the four-probe transport measurement. We also performed a reflectivity measurement in a photon energy range of 5 meV–5 eV at room temperature, and found that the reflectivity data are quite consistent with those extracted from the ellipsometry fitting results. From the obtained dielectric functions, we calculated complex optical constants, refractive index n and extinction coefficient k. From the obtained n and k, we derived the absorption coefficient α and reflectivity spectra R(ω) using the 2 2 relation,α = 2k ⋅ ω, and RðωÞ ¼ ðn−1Þ2 þk2 [19,20]. The results are disðnþ1Þ þk played in Fig. 3. It is clearly found that the optical properties of the three phases of GST films show clear differences [18,21]. The
Fig. 3. (a) Refractive index n, (b) absorption coefficient α, and (c) reflectivity spectra R(ω) of AM, FCC, and HEX phases of GST films in a wide photon energy region up to 8.7 eV.
difference between the amorphous and crystalline GST films is quite distinct in the near-infrared/visible region, which is the reason that these compounds are used for optical/memory devices. It is noted that there is a discernible difference between the two crystalline phases. Our data in the spectral region of below 5.5 eV are consistent with the previous literature [11–15]. In particular, the absorption coefficient spectra presented in Fig. 3(b) are quite similar to Lee et al.'s work [11]. The dielectric functions which Shportko et al. reported are also reproduced in our spectra (The detailed spectra are shown in Figs. 5, 6, and 7, including their temperature dependences.) [12]. The very recent literatures by Němec et al. are in good accord with our current results [14,15]. It is worthwhile to note that apart from this consistency, our spectra spanning in a wider photon energy region give significant information on the electronic property of GST, which is discussed below. To obtain further information on the electronic structure, we calculated the real optical conductivity spectra σ1(ω)(≡ 1/30 ⋅ n ⋅ k ⋅ ω) for GST films, which are associated with the joint density of state between the occupied and unoccupied bands. As shown in Fig. 4(a), the σ1(ω) of the AM-GST films exhibit the dominant peak structure near 2.5 eV. The fundamental gap is estimated to be 0.81 eV which is determined by the linear extrapolation of the lower energy edge of the 2.5 eV structure. For the FCC-GST film, the 2.5 eV peak shifts to a lower energy, 1.5 eV, with the peak width much narrower. Due to
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(a)
(b)
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experimental finding of the large enhancement of dielectric constants, which is also confirmed by our dielectric functions shown in Figs. 5, 6, and 7. We would like to emphasize that the increase of the spectral weight as well as the peak softening in the crystalline phase are directly identified by our results obtained by employing the wide photo-energy ellipsometry, as shown in Fig. 5(b). It is also worthwhile to note that in GeTe and GeSb2Te4 Welnic et al. suggested that the optical contrast between the amorphous and the crystalline phases could be explained by significant changes in the transition matrix elements, which corresponds to the intensity of the interband transition [23]. This idea could be applicable to the GST material which shows the increase of the spectral weight in the crystalline phase, as revealed by our work. Another interesting point is that there is no difference in spectral weight distribution between two crystalline phases, FCC and HEX. It has been known that compared with the case of the HEX phase, the ordering of the vacancies is insufficient in the FCC phase [13]. We speculate that the resonant bonding character and/or the optical matrix elements do not depend on the vacancy ordering significantly in the crystalline GST. 3.4. Temperature dependent behavior of optical response We now turn to the temperature dependence of optical constants for GST films. We performed a spectroscopic ellipsometry on three phases of GST films with increasing temperature up to 350 °C. Figs. 5, 6, and 7 show the real dielectric constants, ε1(ω) and the σ1(ω) of the AM-, FCC-, and HEX-GST films with temperature variation, respectively. First we start with the AM phase. The optical spectra of the AM-GST films do not show any significant change up to 130 °C, but near T = 150 °C, which corresponds to the transition
Fig. 4. (a) Optical conductivity spectra and (b) electronic spectral weight for the three phases of GST films at room temperature.
relatively high conductivity, compared with the AM phase a Drude mode is observed with a low intensity [18]. For the HEX-GST, the 1.5 eV peak is much sharper, but without a detectable peak shift. Simultaneously the Drude mode develops strongly. The interband structure in the visible region is assigned with the transition between the bonding and anti-bonding states in Ge–Te and Sb–Te [12,22]. The shift of the lowest energy of peak is mainly responsible for the change of the optical constants among the phases for GST. At higher energies, another transition with much smaller intensity is observed near 7 eV. The position and intensity of this transition are nearly constant for all phases of GST. It appears that this transition is rather insensitive to the phase transitions for GST. To get more insight, we estimated the electronic spectral weight Sel(ω) by integrating σ1(ω). The results are displayed in Fig. 4(b). We observed that Sel(ω) at 8,7 eV is not the same between the amorphous and crystalline phases, indicating that the sum rule is not satisfied. The Sel(ω) of the FCC and HEX phase at 8.7 eV are larger than that of the AM-GST. The change in the spectral weight means that the spectral change in the structural transition does not originate from the softening of the interband transition and the shift of the chemical potential, but instead, from the intensity as well as the position of the interband transition in the visible region. Considering that the intensity of the 7.5 eV peak does not change, the interband transition in the visible region is responsible for the spectral weight change during the phase transition from the amorphous to the crystalline phase. According to Shporko et al. [12], the bonding character between elements is changed from the covalent bonding in the AM phase to the resonant bonding in the FCC phase. The resonance bonding gives smaller average bandgap (bonding-antibonding splitting) and larger optical matrix elements than electron pair bonding. Shporko et al. supported their argument rather indirectly with the
Fig. 5. Change of (a) ε1(ω) and (b) σ1(ω) when the AM-GST film is heated.
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Fig. 6. Change of (a) ε1(ω) and (b) σ1(ω) when the FCC-GST film is heated.
temperature from AM to FCC phases, an abrupt change is observed in both ε1(ω) and σ1(ω). The transition temperature identified in the optical measurement is slightly lower than that in the resistivity measurement [Fig. 2(b)]. This difference may originate from the different heating techniques between two measurements. In σ1(ω), the 2.5 eV interband transition shifts to a lower energy, 1.5 eV, with its width narrower. At the same time, a small intensity of Drude mode in σ1(ω) shows up and a significant drop below 0.8 eV is detected in ε1(ω). At higher temperatures up to 250 °C the spectra do not change significantly, but a little broadening in the 1.5 eV peak and the development of the Drude mode is identified in σ1(ω). These changes are more clearly observed in ε1(ω). Interestingly, a sizable change in σ1(ω) is observed near T = 270 °C, where the FCC-HEX phase transition occurs. The 1.5 eV peak becomes sharper without any distinct peak shift. A strong development of the Drude mode occurs, which is clearly identified with the negative value in ε1(ω) in the lowest frequency region. The change in σ1(ω) for the AM-GST film reflects two successive phase transitions clearly. The similar behavior was reported by Li et al., while their work did not address the correlation between the optical change and the two phase transitions clearly [24]. It is noted that a clear electronic change through the AM-FCC phase transitions, which is revealed from our optical data, implies that this transition should be of the first order type. This behavior can be compared with the case of VO2 which experiences the metalinsulator transition accompanied by a structural phase change. In contrast to the abrupt change in electric and structural properties, the character of the electronic phase transition in VO2 is percolative with the coexistence of the insulating and metallic phases near the phase transition temperature [25,26]. There has been no report on
Fig. 7. (a) ε1(ω) and (b) σ1(ω) for the HEX-GST films in a high temperature region.
the percolative feature in the AM-FCC phase transition for GST. This might be related to the fast switching behavior in electric/optical properties. Next, we turn to the FCC-GST films. As shown in Fig. 6, there is no significant change in optical spectra with T increasing up to 270 °C. Then, a sizable change in σ1(ω) is detected near 270 °C: the 1.5 eV peak is sharper. This change is quite comparable to that observed near 270 °C for the AM-GST film. In contrast to the two aforementioned cases, as shown in Fig. 7, the σ1(ω) of the HEX-GST does not show any significant change with temperature increasing to the highest level, and instead a slight broadening of the 1.5 eV structure is observed, which is responsible for the decrease of band-gap, as displayed in Fig. 8 [27]. This behavior is consistent with the fact that the HEX phase does not exhibit any phase transition in the temperature range in our measurement. In the high temperature spectra two crystalline phases of GST show different optical response in the lowest energy region. As the temperature increase up to 270 °C, where the FCC phase of GST holds its original phase, the spectral weight of the Drude mode in FCC increases, as shown in Fig. 6(b). The Drude mode corresponds to the optical response of charge carriers. The development of the Drude mode indicates the increase of the density of thermally activated charge carriers, which is characteristic of a semiconductor. On the other hand, in the HEX phase, the Drude mode shows a significant broadening at higher temperatures. The width of the Drude mode is equivalent to the scattering rate of the charge carriers. Whereas, the zero-crossing point in ε1(ω), which corresponds to the screened plasma frequency, is rather insensitive to the variation of temperature. A slight red shift of the zero crossing point in ε1(ω) originates from the broadening of the 1.5 eV peak. Our finding of the increase in the scattering rate with the constant plasma frequency indicates that the
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transition occurs, we found a gradual broadening for the 1.5 eV interband transitions as well as the Drude mode with increasing temperature, which may be related to the decrease of the band-gap. On the other hand, the Drude mode increases in the FCC-GST with temperature increasing to 270 °C, which may indicate that its electronic property is semiconducting. The knowledge on high temperature optical properties of the GST films obtained in the current experiment should be quite valuable for designing GST-base devices and improving their performance [31]. Acknowledgments The authors thank Namjung Hur, and C. J. Won for technical assistance. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011-0018032). B. Cheong also would like to acknowledge a grant from the Fundamental R&D Program for Core Technology of Materials by the Ministry of Knowledge Economy, Republic of Korea. Fig. 8. Temperature dependence of the optical band-gap of GST films for three phases of GST films.
HEX-GST is of a typical metal. From the screened plasma frequency value with dielectric constant ε∞ ~ 20 [12], we estimate n/m * to be 4 × 10 20/cm 3. Here, n and m* are carrier density and mass enhancement, respectively. With the previous result of n ~ 2.7 × 10 20/cm 3 [11], m* is estimated to be 1.5, which is consistent with that of the hole carrier for GST [11]. 3.5. Change of optical bandgap at high temperatures It is interesting to see the temperature dependence of the optical bandgap in the GST films. As shown in Fig. 8, for the AM-GST the optical gap shows strong temperature dependence in close relation to the AM-FCC phase transition. Near 150 °C the optical gap decreases abruptly. In contrast, the change of the optical gap is negligible near the FCC-HEX phase transition. This behavior is related to the fact that the band-gap for FCC phase is a little smaller than that in the HEX phase [13,21,28]. On the other hand, the optical gap for the HEX-GST decreases gradually with increasing temperature. This change is comparable for that observed in typical semiconductors [29,30]. 4. Summary We performed optical measurements on the AM, FCC, and HEX phases of GST films in a wide photon energy and high temperature region. Using a high photon-energy ellipsometry up to 8.7 eV, we identified that the optical change occurring during the phase transitions is confined to the interband transition in the visible region, which is assigned to the bonding–antibonding transition. Especially, it turns out that the intensity of the peak in the visible region is increased in the crystalline phases, which may be indicative of the resonant bonding character and/or increase of the Ge\Te bonding number. In high temperature ellipsometry, in the AM phase we found an abrupt change of the optical constants during both structural phase transitions. In the FCC phase, we found abrupt changes of the optical constant during the FCC-HEX phase transition. This change for the FCC phase is quite similar to that shown near the second transition temperature for the AM phase. Sizable changes in optical constants in both phase transitions indicate that the characters of two phase transitions are of the first-order type. In HEX phase where no phase
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Spectroscopic investigation on phase transitions for Ge2Sb2Te5 in a wide photon energy and high temperature region Y.K. Seo a, J.-S. Chung a, Y.S. Lee a,⁎, E.J. Choi b, B. Cheong c a b c
Department of Physics, Soongsil University, Seoul 156-743, Republic of Korea Department of Physics, University of Seoul, Seoul 130-743, Republic of Korea Electronic Materials Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
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Article history: Received 16 May 2011 Received in revised form 15 December 2011 Accepted 15 December 2011 Available online 22 December 2011 Keywords: Ge2Sb2Te5 (GST) Optical constants Ellipsometry Amorphous phase Crystalline phase
a b s t r a c t We investigated the electronic properties of phase-change material Ge2Sb2Te5 (GST) films using spectroscopic ellipsometry in a wide photon energy and high temperature region. Apart from the charge carrier response, the totality of optical conductivity spectra for three phases of GST films, i.e., amorphous (AM), face-centeredcubic (FCC), and hexagonal (HEX), is quite similar, composed of two interband transitions in visible and UV regions. From optical analysis in a wide photon energy region up to 8.7 eV, we found that the intensity as well as the position of the interband transition in the visible region changes significantly as the phase of GST films turns from the amorphous to the crystalline phase, which is consistent with previous theoretical studies. In high temperature measurements above room temperature for the three phases of GST films, we found that the change of optical response for the AM phase of GST film occurs abruptly through two successive phase transitions near 150 °C and 270 °C, while the optical spectra of the FCC phase shows a change only near 270 °C. In contrast to the two above-mentioned cases, a slight change in optical spectra is observed for the HEX phase with the increasing temperature. From the measured optical spectra, we derived the temperature dependence of optical bandgap for the three phases, which are closely correlated to the change of the transport property for the GST films. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The chalcogenide Ge–Sb–Te alloys materials have attracted a great deal of attention because of their potential application to data storage media such as digital versatile disk random access memory, compact disk rewritable, and phase-change random access memory (PRAM) [1–3]. These applications are based on a fast and reversible phase transition between the crystalline and amorphous phases. The difference in electronic response between the amorphous and crystalline phases leads to a sizable difference in electric resistance and optical constants. Among this class of materials, the Ge2Sb2Te5 (GST) is the most widely used composition [4–6]. This material has three phases, i.e., amorphous (AM), face-centered-cubic (FCC), and hexagonal (HEX) phases, with two successive structural phase transitions near 150 °C and 270 °C. In usual cases, the as-grown phase of GST film is amorphous. When it is heated, the as-prepared amorphous GST film is thermally crystallized into a FCC structure near 150 °C. At a higher temperature, i.e., 270 °C, the FCC phase turns into a hexagonal phase [7–9]. When the crystalline phase is heated above the melting point and then cooled rapidly, one can obtain the amorphous phase again.
⁎ Corresponding author. Tel./fax: + 82 2 820 0404. E-mail address: [email protected] (Y.S. Lee). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.12.043
The amorphous phase of GST is insulating with low reflectance in the visible region, while two crystalline phases have high conductivity with relatively high reflectance in the visible region. There have been many optical spectroscopic studies on the electronic structure of each phase in GST [10–15]. While most of the previous publications dealt with comparison of room temperature optical properties among the AM, FCC, and HEX phases, there have been few reports on the change in optical response during the two phase transitions, i.e., AM-to-FCC, and FCC-to-HEX, which may enable us to understand the electronic properties of the GST phases and the related phase-switching mechanism in depth. Actually, the origin of the metal-insulator transition for GST is still unclear: electronic vs. structural [16]. The high temperature spectroscopic investigation may provide useful information for solving the current issues on the phase transition in GST. Another notable point is that the spectral range in the previous studies was limited to below 5.5 eV [10–15]. Since the spectral change near 5.5 eV during the phase transition is still sizable, it is required to extend the spectral range of measurement in order to completely check out the spectral change during the phase transition. To overcome the limitation of the previous spectroscopic works on GST, we performed optical measurements on the AM, FCC, and HEX phases of GST films in a wide photon-energy and high temperature region. Using a high photon-energy ellipsometry up to 8.7 eV, we identified two interband transitions in the visible
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and UV region. While the peak near 7.5 in the UV region is insusceptible to the phase transition, the intensity as well as the position of the peak in the visible region changes significantly through the phase transitions. This optical change is well consistent with the recent theoretical studies. We also performed high temperature ellipsometry measurement on three phase of GST films in the course of heating up to 350 °C. In the AM phase we found abrupt changes of the optical constants during both structural phase transitions. Whereas, the FCC-GST shows a change during only a transition near 270 °C. In the HEX phase where no phase transition occurs, we found a slight change of the interband transition in visible region with the increasing temperature. The temperature dependence of the band-gap was obtained from the optical spectra for the GST films.
2. Experimental details The Ge2Sb2Te5 (GST) thin films are deposited on the SiO2 (300 nm)/Si substrates at room temperature by using the radio frequency magneton sputtering technique. We used the stoichiometric GST target. The base pressure in the sputtering chamber was 1.3 × 10 − 5 Pa. The sputtering deposition rate was 1.1 Å/s, and the deposition time for our films was 15 min. The film thickness is about 100 nm. To identify the concentrations of Ge, Sb, and Te for our films, we performed the Rutherford backscattering spectroscopy (RBS) on our films. The collimated He + ion beams with 2 MeV were used. The samples were positioned on a three-axis goniometer in vacuum. The elastic recoil detection technique was employed. We simulated the RBS patterns predicted for the stoichiometric GST composition, and compared them with the experimental RBS data. We found two data quite consistent, which indicates that our films are stoichiometric. We also performed the X-ray fluorescence spectroscopy (Rigaku system 3272, Japan) and determined the composition of our films more accurately: the concentrations of Ge, Sb, and Te for our films are 23.1%, 24.1%, and 52.6%, respectively. The asgrown phase of GST film in our deposition condition is AM. To obtain the FCC and HEX phases of the crystalline GST films, we annealed the AM-GST at 200 °C and at 400 °C for 30 min in vacuum, respectively. For structural analysis X-ray diffraction (XRD) patterns (θ-2θ scan) were measured with a Bruker-AXS Discover D8 system with a Cu target X-ray tube. The X-ray beam was focused to a parallel beam with a Gobel mirror to enhance the intensity for thin film measurements. The XRD measurements were performed at room temperature. In order to investigate the electric properties of the GST films, the dc resistivity measurement was measured using the standard fourprobe method. The temperature range is from 30 K to 623 K (350 °C), covering both structural phase transitions occurring near 150 °C and 270 °C. We used the different experimental setups for low and high temperature region. A closed-circulated refrigerator was used for low temperature measurement below room temperature. For high temperature region up to 370 °C, we used a homemade vacuum chamber with continuous temperature change by the heating/cooling rate, 1.7 °C/min. The optical measurements for GST films were measured in the photon energy range 0.7–8.7 eV using spectroscopic ellipsometry (VASE model, J.A. Woolam Inc.). The angles of the incidence light are 65°, 70°, and 75°. Below 0.7 eV we measured the infrared reflectivity spectra of films using the infrared spectrometer (Bruker IFS 66v/S). To look into the change of optical constants through the phase transitions, we measured the three phases of GST films while varying temperature up to 400 °C. To avoid oxidation damage of the GST films at high temperatures, the samples are positioned in a chamber with an N2 atmosphere. The heating rate was 10 °C/min. After temperature reached a target temperature, we waited for 10 min more. The measurement time was about 20 min at a given
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temperature. In this case the measurement photon energy is limited to a range from 0.7 eV to 5.0 eV. 3. Results and discussion 3.1. Structural analysis Fig. 1 shows XRD θ-2θ scan results of the three phases of GST films, i.e., AM, FCC, and HEX phases. The XRD measurements were performed at room temperature. The sharp peaks near 2θ = 33°, 62°, and 70° come from the Si substrates. Apart from the Si peaks, no distinct XRD peak is detected for as-grown AM-GST film, and instead, broad peaks near 28° and 48° are resolved. In contrast to the case of the AM-GST, the XRD patterns of FCC-GST and HEX-GST show welldefined diffraction peaks. The observed patterns are quite consistent with the previous reports on polycrystalline samples [13,17,18]. 3.2. Electrical property of AM-, FCC-, and HEX-phases We discuss the electric properties of GST films from the dc resistivity data. The resistivity curves are measured by using the fourprobe method. Fig. 2(a) shows the low temperature dc resistivity curves ρ(T)of three phases of GST films as a function of temperature. It is clearly found that the electric phase of AM-GST is insulating with very high resistivity values. On the other hand, the FCC-GST exhibits a barely metallic behavior, just like a semi-metal [11]. While absolute values of the resistivity in the FCC-phase are much smaller by an order of 5 than those in the AM-phase, the temperature dependence of the resistivity is rather semiconducting-like, showing the upturn behavior at low temperatures. In contrast to the case of FCC-GST, the resistivity curve of the HEX-GST shows a typical metallic behavior. From these findings, it can be concluded that the electric phases of the AM-GST, FCC-GST, and HEX-GST films are insulating, barely semi-metallic, and typically metallic, respectively. We also performed a high temperature measurement of ρ(T) for the AM-GST film. The film was positioned in a vacuum chamber with continuous temperature change by the heating/cooling rate, 1.7 °C/min. As shown in Fig. 2(b), the dc resistivity curve of the AMGST exhibits anomalous drops with two onset temperatures, 150 °C and 270 °C, which indicate the structural phase transitions, i.e., AMFCC and FCC-HEX, respectively. While the drop of the resistivity for the AM-FCC transition is quite sharp with a broadness of 5 °C, the change of the resistivity for the FCC-HEX transition occurs in a broad temperature range of 270–300 °C. When it is cooled down
Fig. 1. The θ-2θ scan spectra of X-ray diffraction for AM, FCC, and HEX phases of GST.
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Fig. 2. (a) Low temperature dependent resistivity curves of the AM, FCC, and HEX phases of GST films. (b) High temperature resistivity curve of the AM-GST phase. The arrows indicate a thermal process.
from the highest temperature, the resistivity curve shows a metallic behavior which is almost the same with that of the HEX GST film prepared in a post-annealing process. 3.3. A wide photon energy range of electronic structures for three phases of GST Now we discuss the electronic structures of the AM-GST, FCC-GST, and HEX-GST films in a wide photon energy range up to 8.7 eV. We used the spectroscopic ellipsometry to obtain the optical constants which possess basic information on the electronic structure of materials. With a four phase model of surface roughness, GST layer, SiO2 layer, and Si substrate [13], we estimated the dielectric functions of the GST layer which is assumed to be composed of one Drude component and two Lorentz oscillators. The dielectric functions obtained with the mode fitting are quite consistent with those obtained in a point-by-point fitting method. We extrapolated the optical constants to the lower energy region, and found that the dc conductivity values estimated with the ellipsometric fitting coincide well to those obtained independently by using the four-probe transport measurement. We also performed a reflectivity measurement in a photon energy range of 5 meV–5 eV at room temperature, and found that the reflectivity data are quite consistent with those extracted from the ellipsometry fitting results. From the obtained dielectric functions, we calculated complex optical constants, refractive index n and extinction coefficient k. From the obtained n and k, we derived the absorption coefficient α and reflectivity spectra R(ω) using the 2 2 relation,α = 2k ⋅ ω, and RðωÞ ¼ ðn−1Þ2 þk2 [19,20]. The results are disðnþ1Þ þk played in Fig. 3. It is clearly found that the optical properties of the three phases of GST films show clear differences [18,21]. The
Fig. 3. (a) Refractive index n, (b) absorption coefficient α, and (c) reflectivity spectra R(ω) of AM, FCC, and HEX phases of GST films in a wide photon energy region up to 8.7 eV.
difference between the amorphous and crystalline GST films is quite distinct in the near-infrared/visible region, which is the reason that these compounds are used for optical/memory devices. It is noted that there is a discernible difference between the two crystalline phases. Our data in the spectral region of below 5.5 eV are consistent with the previous literature [11–15]. In particular, the absorption coefficient spectra presented in Fig. 3(b) are quite similar to Lee et al.'s work [11]. The dielectric functions which Shportko et al. reported are also reproduced in our spectra (The detailed spectra are shown in Figs. 5, 6, and 7, including their temperature dependences.) [12]. The very recent literatures by Němec et al. are in good accord with our current results [14,15]. It is worthwhile to note that apart from this consistency, our spectra spanning in a wider photon energy region give significant information on the electronic property of GST, which is discussed below. To obtain further information on the electronic structure, we calculated the real optical conductivity spectra σ1(ω)(≡ 1/30 ⋅ n ⋅ k ⋅ ω) for GST films, which are associated with the joint density of state between the occupied and unoccupied bands. As shown in Fig. 4(a), the σ1(ω) of the AM-GST films exhibit the dominant peak structure near 2.5 eV. The fundamental gap is estimated to be 0.81 eV which is determined by the linear extrapolation of the lower energy edge of the 2.5 eV structure. For the FCC-GST film, the 2.5 eV peak shifts to a lower energy, 1.5 eV, with the peak width much narrower. Due to
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(a)
(b)
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experimental finding of the large enhancement of dielectric constants, which is also confirmed by our dielectric functions shown in Figs. 5, 6, and 7. We would like to emphasize that the increase of the spectral weight as well as the peak softening in the crystalline phase are directly identified by our results obtained by employing the wide photo-energy ellipsometry, as shown in Fig. 5(b). It is also worthwhile to note that in GeTe and GeSb2Te4 Welnic et al. suggested that the optical contrast between the amorphous and the crystalline phases could be explained by significant changes in the transition matrix elements, which corresponds to the intensity of the interband transition [23]. This idea could be applicable to the GST material which shows the increase of the spectral weight in the crystalline phase, as revealed by our work. Another interesting point is that there is no difference in spectral weight distribution between two crystalline phases, FCC and HEX. It has been known that compared with the case of the HEX phase, the ordering of the vacancies is insufficient in the FCC phase [13]. We speculate that the resonant bonding character and/or the optical matrix elements do not depend on the vacancy ordering significantly in the crystalline GST. 3.4. Temperature dependent behavior of optical response We now turn to the temperature dependence of optical constants for GST films. We performed a spectroscopic ellipsometry on three phases of GST films with increasing temperature up to 350 °C. Figs. 5, 6, and 7 show the real dielectric constants, ε1(ω) and the σ1(ω) of the AM-, FCC-, and HEX-GST films with temperature variation, respectively. First we start with the AM phase. The optical spectra of the AM-GST films do not show any significant change up to 130 °C, but near T = 150 °C, which corresponds to the transition
Fig. 4. (a) Optical conductivity spectra and (b) electronic spectral weight for the three phases of GST films at room temperature.
relatively high conductivity, compared with the AM phase a Drude mode is observed with a low intensity [18]. For the HEX-GST, the 1.5 eV peak is much sharper, but without a detectable peak shift. Simultaneously the Drude mode develops strongly. The interband structure in the visible region is assigned with the transition between the bonding and anti-bonding states in Ge–Te and Sb–Te [12,22]. The shift of the lowest energy of peak is mainly responsible for the change of the optical constants among the phases for GST. At higher energies, another transition with much smaller intensity is observed near 7 eV. The position and intensity of this transition are nearly constant for all phases of GST. It appears that this transition is rather insensitive to the phase transitions for GST. To get more insight, we estimated the electronic spectral weight Sel(ω) by integrating σ1(ω). The results are displayed in Fig. 4(b). We observed that Sel(ω) at 8,7 eV is not the same between the amorphous and crystalline phases, indicating that the sum rule is not satisfied. The Sel(ω) of the FCC and HEX phase at 8.7 eV are larger than that of the AM-GST. The change in the spectral weight means that the spectral change in the structural transition does not originate from the softening of the interband transition and the shift of the chemical potential, but instead, from the intensity as well as the position of the interband transition in the visible region. Considering that the intensity of the 7.5 eV peak does not change, the interband transition in the visible region is responsible for the spectral weight change during the phase transition from the amorphous to the crystalline phase. According to Shporko et al. [12], the bonding character between elements is changed from the covalent bonding in the AM phase to the resonant bonding in the FCC phase. The resonance bonding gives smaller average bandgap (bonding-antibonding splitting) and larger optical matrix elements than electron pair bonding. Shporko et al. supported their argument rather indirectly with the
Fig. 5. Change of (a) ε1(ω) and (b) σ1(ω) when the AM-GST film is heated.
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Fig. 6. Change of (a) ε1(ω) and (b) σ1(ω) when the FCC-GST film is heated.
temperature from AM to FCC phases, an abrupt change is observed in both ε1(ω) and σ1(ω). The transition temperature identified in the optical measurement is slightly lower than that in the resistivity measurement [Fig. 2(b)]. This difference may originate from the different heating techniques between two measurements. In σ1(ω), the 2.5 eV interband transition shifts to a lower energy, 1.5 eV, with its width narrower. At the same time, a small intensity of Drude mode in σ1(ω) shows up and a significant drop below 0.8 eV is detected in ε1(ω). At higher temperatures up to 250 °C the spectra do not change significantly, but a little broadening in the 1.5 eV peak and the development of the Drude mode is identified in σ1(ω). These changes are more clearly observed in ε1(ω). Interestingly, a sizable change in σ1(ω) is observed near T = 270 °C, where the FCC-HEX phase transition occurs. The 1.5 eV peak becomes sharper without any distinct peak shift. A strong development of the Drude mode occurs, which is clearly identified with the negative value in ε1(ω) in the lowest frequency region. The change in σ1(ω) for the AM-GST film reflects two successive phase transitions clearly. The similar behavior was reported by Li et al., while their work did not address the correlation between the optical change and the two phase transitions clearly [24]. It is noted that a clear electronic change through the AM-FCC phase transitions, which is revealed from our optical data, implies that this transition should be of the first order type. This behavior can be compared with the case of VO2 which experiences the metalinsulator transition accompanied by a structural phase change. In contrast to the abrupt change in electric and structural properties, the character of the electronic phase transition in VO2 is percolative with the coexistence of the insulating and metallic phases near the phase transition temperature [25,26]. There has been no report on
Fig. 7. (a) ε1(ω) and (b) σ1(ω) for the HEX-GST films in a high temperature region.
the percolative feature in the AM-FCC phase transition for GST. This might be related to the fast switching behavior in electric/optical properties. Next, we turn to the FCC-GST films. As shown in Fig. 6, there is no significant change in optical spectra with T increasing up to 270 °C. Then, a sizable change in σ1(ω) is detected near 270 °C: the 1.5 eV peak is sharper. This change is quite comparable to that observed near 270 °C for the AM-GST film. In contrast to the two aforementioned cases, as shown in Fig. 7, the σ1(ω) of the HEX-GST does not show any significant change with temperature increasing to the highest level, and instead a slight broadening of the 1.5 eV structure is observed, which is responsible for the decrease of band-gap, as displayed in Fig. 8 [27]. This behavior is consistent with the fact that the HEX phase does not exhibit any phase transition in the temperature range in our measurement. In the high temperature spectra two crystalline phases of GST show different optical response in the lowest energy region. As the temperature increase up to 270 °C, where the FCC phase of GST holds its original phase, the spectral weight of the Drude mode in FCC increases, as shown in Fig. 6(b). The Drude mode corresponds to the optical response of charge carriers. The development of the Drude mode indicates the increase of the density of thermally activated charge carriers, which is characteristic of a semiconductor. On the other hand, in the HEX phase, the Drude mode shows a significant broadening at higher temperatures. The width of the Drude mode is equivalent to the scattering rate of the charge carriers. Whereas, the zero-crossing point in ε1(ω), which corresponds to the screened plasma frequency, is rather insensitive to the variation of temperature. A slight red shift of the zero crossing point in ε1(ω) originates from the broadening of the 1.5 eV peak. Our finding of the increase in the scattering rate with the constant plasma frequency indicates that the
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transition occurs, we found a gradual broadening for the 1.5 eV interband transitions as well as the Drude mode with increasing temperature, which may be related to the decrease of the band-gap. On the other hand, the Drude mode increases in the FCC-GST with temperature increasing to 270 °C, which may indicate that its electronic property is semiconducting. The knowledge on high temperature optical properties of the GST films obtained in the current experiment should be quite valuable for designing GST-base devices and improving their performance [31]. Acknowledgments The authors thank Namjung Hur, and C. J. Won for technical assistance. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011-0018032). B. Cheong also would like to acknowledge a grant from the Fundamental R&D Program for Core Technology of Materials by the Ministry of Knowledge Economy, Republic of Korea. Fig. 8. Temperature dependence of the optical band-gap of GST films for three phases of GST films.
HEX-GST is of a typical metal. From the screened plasma frequency value with dielectric constant ε∞ ~ 20 [12], we estimate n/m * to be 4 × 10 20/cm 3. Here, n and m* are carrier density and mass enhancement, respectively. With the previous result of n ~ 2.7 × 10 20/cm 3 [11], m* is estimated to be 1.5, which is consistent with that of the hole carrier for GST [11]. 3.5. Change of optical bandgap at high temperatures It is interesting to see the temperature dependence of the optical bandgap in the GST films. As shown in Fig. 8, for the AM-GST the optical gap shows strong temperature dependence in close relation to the AM-FCC phase transition. Near 150 °C the optical gap decreases abruptly. In contrast, the change of the optical gap is negligible near the FCC-HEX phase transition. This behavior is related to the fact that the band-gap for FCC phase is a little smaller than that in the HEX phase [13,21,28]. On the other hand, the optical gap for the HEX-GST decreases gradually with increasing temperature. This change is comparable for that observed in typical semiconductors [29,30]. 4. Summary We performed optical measurements on the AM, FCC, and HEX phases of GST films in a wide photon energy and high temperature region. Using a high photon-energy ellipsometry up to 8.7 eV, we identified that the optical change occurring during the phase transitions is confined to the interband transition in the visible region, which is assigned to the bonding–antibonding transition. Especially, it turns out that the intensity of the peak in the visible region is increased in the crystalline phases, which may be indicative of the resonant bonding character and/or increase of the Ge\Te bonding number. In high temperature ellipsometry, in the AM phase we found an abrupt change of the optical constants during both structural phase transitions. In the FCC phase, we found abrupt changes of the optical constant during the FCC-HEX phase transition. This change for the FCC phase is quite similar to that shown near the second transition temperature for the AM phase. Sizable changes in optical constants in both phase transitions indicate that the characters of two phase transitions are of the first-order type. In HEX phase where no phase
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