Dedicated to Stanford R. Ovshinsky on the occasion of his 90th birthday
Discovery of the GeSbTe phase-change alloy in particular along the GeTe–Sb2Te3 tie-line took place in the mid-1980s. The amorphous alloys showed ideal properties, for example, high thermal stability at r.t. and laser-induced rapid crystallization with large optical changes. Thereafter, GeSbTe was successively applied to various optical disks such as DVDs and BDs. Through DSC and XRD analyses, the appearance of the metastable phase having a NaCl-type structure was observed over a wide compositional region. This was the “key” to realizing the ideal phase-change properties. During this year, the role of the constituent elements of Ge and Sb became clear by RMC modeling using AXS data at SPring-8, where the “nucleation dominant crystallization process” was well explained.
The aspect of the latest Blu-ray Disc (BD) product of Panasonic: GeSbTe phase-change films are utilized in every recording layer. It is seen that the front-side recording layers, L1 and L2, are highly transparent.
The phase-change technologies today originated from the proposal of the chalcogenide amorphous semiconductor materials by Dr. Ovshinsky at the end of the 1960s 1. Naturally, the innovative proposal greatly excited the R&D activities aiming for a recordable video disc all over the world. The development speed at the early stage were not so high during the 1970s and 1980s, and the activities sometimes stagnated. Nevertheless, the discovery of the GeSbTe phase-change materials by the Yamada et al. 2 drastically turned the tide.
It was important that we not only obtained the practical phase-change alloy but also obtained new insights into “what the superior phase-change material is” through a series of material studies. Soon after, the first product of an overwritable optical disk was commercialized in 1990; the way was opened to the “optical disk culture.” The digital versatile disc (DVD) and the Blu-ray Disc (BD) have completely replaced video tape recorders. Furthermore, the superior potentiality of the GeSbTe has greatly encouraged the R&D activities for the nonvolatile solid-state memory devices (PCRAM) 3.
This year is a memorial for the 90th birthday of Dr. Ovshinsky, while this year corresponds to just a quarter century from the first report of GeSbTe. I think it is significant to look back again “what is the GeSbTe material?” in the phase-change R&D history at this occasion. In this review paper, I will try to describe “what characteristics it has,” “why it works so well,” and for what applications it has been applied as briefly and comprehensively as possible especially for newcomers to this field.
2 What is the GeSbTe material?
2.1 GeTe–Sb2Te3 pseudobinary system
The GeTe–Sb2Te3 pseudobinary phase diagram was first reported by Abrikosov and Danilova-Dobryakova 4. The phase diagram looks unique. The liquidus curve is very flat lying near 600°C covering a wide compositional range. There exist three intermediate ternary compounds, Ge2Sb2Te5 (A), GeSb2Te4 (B), and GeSb4Te7 (C), between the two binary compounds of GeTe and Sb2Te3 as shown in Fig. 1. The structures of the three compounds were rather complicated; that is each of them has similarly long periodic hexagonal structure with the stacking rule of the face-centered cubic lattices, abc/abc/− 6, 7.
Subsequently, many other intermediate compounds aside from the above three have been found by Shelimova et al. 5. They are Ge7Sb2Te10 (7:1), Ge6Sb2Te9 (6:1), Ge5Sb2Te8, (5:1), Ge4Sb2Te7,(4:1), and Ge3b2Te6 (3:1) at the GeTe side of 〈A〉 and GeSb6Te10 (1:3), GeSb8Te13 (1:4) at the Sb2Te3 side of 〈C〉. As described later, among them, the Ge2Sb2Te5 (2:1) composition was applied for the first rewritable phase-change optical disk product in 1990. Since then, it has been a standard material for R&D in the phase-change field. This is because the material film possesses not only superior phase-change characteristics but also reveals various interesting physical properties.
2.2 Metastable cubic phase
It is to be noted here that the above-described crystalline phases in the stable state have never been used in any optical disk and electrical memory devices. Through analytical studies using differential scanning calorimetry (DSC) and X-ray diffraction (XRD), it was found that the phase-change of every GeSbTe alloy takes place not between the amorphous and the stable hexagonal phase but between the amorphous and the metastable cubic phase over a wide compositional range. Even when the composition deviates to the Ge-rich (or Sb-rich) side from the pseudobinary line, the initial crystalline phases are cubic 8.
The right-hand figure in Fig. 2 shows the DSC measurement results for GeTe–Sb2Te3 pseudobinary amorphous films. Every film initially crystallizes into cubic phase and at higher temperature it changes to hexagonal phase. From the XRD analyses, the initial phase is identified as NaCl-like structure as depicted in the left hand figure in Fig. 2. The structure is characterized by the isotropic atomic distribution and many vacant sites in 4b sites. The number of vacant sites becomes large toward Sb2Te3 on the GeTe–Sb2Te3 pseudobinary line. The ratio of the vacancy in 4b sites, Rv, is approximately estimated by
where x is the concentration of GeTe when the composition is expressed by (GeTe)x–(Sb2Te3)1−x. Typically, in the case of Ge2Sb2Te5, where the x equals 1/3, Rv becomes 20% of 4b sites.
Material systems having a series of compounds with similar structures and properties are generically named “homologous systems.” It has been experimentally shown that metastable “cubic” phase often appears in the homologous systems such as GeTe–Bi2Te3 and Sb–Te-α besides GeTe–Sb2Te3.
3 Potential as a phase-change material
The essential requirements for a phase-change material are (i) good thermal stability of the amorphous state, (ii) rapid phase changes especially from amorphous to crystalline state, (iii) large optical/electrical changes accompanying the phase-changes, and (iv) large cycle number between the amorphous and crystalline states. Through a series of systematic experiments, the superior characteristics of the GeSbTe were experimentally clarified.
3.1 Crystallization properties
The essential experimental proofs for the requirements (i) and (ii) are shown in Fig. 3, where the left- and right-hand figures show the compositional dependences of the crystallization temperature, Tx, and that of the laser heating time required for crystallization τcry, respectively. It is apparent respectively in the left- and right-hand triangles that Tx is far higher than room temperature and τcry is sufficiently short, <50 ns. By the environmental acceleration tests on the real optical disk device using the Arrhenius method, the estimated lifetime of the amorphous marks formed in the crystalline GeSbTe was more than several tens of years at room conditions (30°C, 80% RH) 10. It is amazing that the ratio between the estimated lifetime (30 years) and the laser crystallization time (30 ns) reaches 1017–18.
It looks curious that every property is gently continuous to the compositional change and no special feature is observed in Fig. 2. The reason for this mystery has been assigned to the appearance of the metastable phase over the wide compositional range 8. From the industrial standpoint, the largest merit of the GeSbTe material system is the high degree of freedom for fine tuning of the phase-change properties, as shown in Figs. 2 and 5. Furthermore, it was reported that phase-change properties could be finely controlled by substituting each constituent element by the other. For example, Sn substitution for Ge, similarly Bi substitution for Sb, successfully accelerated the crystallization speed without destroying the structure 11, 12. Currently, we can see that the superior phase-change properties of the GeSbTe alloys are largely due to the highly inclusive crystalline structure. Concerning the details of this NaCl-like phase, several new aspects have been proposed mainly relating to the dislocation of Ge atoms from the lattice site 13–15. It will be important for clarifying the rapid phase-change mechanism of this material system.
3.2 Repetitious cycle property
For confirming the requirement of (iii) cyclability, also repetitious data rewriting test using a real device is indispensable, since various deviations will be observed as the degradation of the signal quality. In Fig. 4, the top graph shows the dependence of the jitter value on the overwrite cycle number when random signals are repetitiously rewritten on a same recording track. The jitter is a tool for evaluating the accuracy of the recorded mark's location and it is required to be <8.5% in the DVD-RAM system. The bottom photos indicate the eye-pattern signals for evaluating the resolution power of digital signals, where the diamond-shaped spaces surrounded by several signal curves named 〈eye〉 becomes the standard for evaluation. It should be clearly open after many times of data rewritings (crystallization–amorphization). As can be seen in the photos, the eye pattern signal of the DVD-RAM is very clear and unchanged after 500,000 cycles 16.
3.3 Optical properties
The optical properties corresponding to the requirement (iv) is shown in Fig. 517. As seen in the upper figure, the variation of the refractive index, Δn, is very large in the infrared range (telecom range) and increasingly small around the visible-light range (BD/DVD range). On the other hand, the variation of the extinction coefficient, Δk, is sufficiently large around the BD/DVD range. Accordingly, we can understand that not Δn but Δk plays the main role for these optical disks.
Nevertheless, it is also seen that Δk tends to decrease when the wavelength shortens from red (DVD) to blue-violet (BD). Thus, industrially it will be important to examine the compositional dependences of Δn and Δk to fully understand the potential of the whole material system. The lower figure in Fig. 5 shows the degree of the optical change n,k(cry) − n,k(amo) on the GeTe–Sb2Te3 pseudobinary line. The degree of optical contrast increases with increasing Ge content along the line. Thus, it is known that the GeTe-rich composition is suitable for Blu-ray Discs from the viewpoint of optical properties.
4 Rapid crystallization mechanism
The laser-induced rapid crystallization mechanism of the GeSbTe film has been discussed actively from the view point of atomic distributions. This is because we understood that the long crystallization time for the historical materials, such as Te-based eutectic alloys, were due to their phase-separation process that required large atomic rearrangements. Accordingly, material studies thereafter were concentrated on “single-phase materials” as referred to at the end of Section 2.
As mentioned above, metastable GeSbTe has a NaCl-like structure with a highly symmetrical atomic distribution. On the other hand, it is very plausible that an atomic distribution in the amorphous phase is generally very isotropic since amorphous phase should strongly reflect the random atomic distribution in the liquid phase. These two things suggest that crystallization will proceed by only small atomic rearrangements. Thus, understanding of the amorphous phase is key for clarifying the rapid crystallization process.
Figure 6 shows the latest results of RMC (reverse Monte Carlo) modeling, where AXS (anomalous X-ray scattering) at SPring-8 was first applied to discriminate Te and Sb that are neighbors in the periodic table. In the amorphous structure of figure A, it is defined that atomic distances up to 3.2 Å are bonded and only atoms that belong to closed loops (rings) are shown.
By comparing amorphous (A) and crystalline (B) structures, curious features were found; that is (i) basically, both the amorphous and the crystalline structures are constructed by –Ge(Sb)–Te– alternative rings, (ii) the bonding angles of –Ge(Sb)–Te–Ge(Sb)– in the amorphous state is centring at 90° resembling that in the crystalline state, (iii) many small fragments of NaCl structure exist in the amorphous phase, and (iv) large fractions of four- and sixfold rings are mainly formed chiefly by Ge–Te bonds forming core networks, while the contribution of Sb–Te bonds to the ring distribution is very small, where 60% of Ge–Te bond length are within 3.2 Å, while 70% of the Sb–Te form pseudonetworks beyond 3.2 Å in the amorphous phase 18.
A crystallization process considering all these observations is shown in Fig. 7. It is possible in this model to explain the cause of the superior phase-change properties of GeSbTe: (i) high thermal stability at room temperature, (ii) rapid crystallization at high temperature, and (iii) the characteristic “nucleation-dominant crystallization process.”
5 Optical disk applications
5.1 Application to optical disks
The GeSbTe alloys and its variations have been applied to many rewritable-type optical disks since 1990, as shown in Fig. 8. It is wonderful that one class of materials have been responding to our expectations for the long term. This is because their phase-change properties can be rather easily modified, as mentioned above.
The progresses of optical disk performance since 1990 till today are shown in the bottom figure in Fig. 8. In these 20 years, the storage capacity has increased approximately 200-fold (100 GB), while the data rate has increased tenfold (133 Mbps). For optical disks, the effective laser-exposure duration during recording that will correspond to τcry, can be calculated approximately by the simple equation,
where d is the effective laser spot diameter (1/e) and v is the linear velocity showing the relative speed between the laser spot and the revolving disk. Thus, τcry for 2× BD-RE comes to approximately 30 ns (d is 0.29 µm, v is 9.84 m/s). The value is rather near the reported τcry that was obtained statically, suggesting the limit of the crystallization rate for GeSbTe. In the case of 12× DVD-RAM, the τcry value is as short as 12 ns (d is 0.66 µm, v is 53.0 m/s), where a GeBiTe-based alloy was applied as the recording material.
5.2 Triple-layer Blu-ray Disc
The increase of recording density of an optical disk is increasingly close to the limitation determined by the laser wavelength and the numerical aperture (NA) of the objective lens. As a method to overcome the limitation, a lamination method of the recording layers has been developed.
Figure 9 shows the layer stacks of the latest product: the triple-layer Blu-ray Disc 19. To smoothly record on and read from the deep recording layer, the front side recording layers must be highly transparent. Thinning of the phase-change films is very effective for that; however, it leads to a lowering of the reflectivity contrast (Rc − Ra)/(Rc + Ra) and the crystallization speed. In this triple layer optical disk, various novel technologies are used. The technologies are new dielectric material ZrSiO for the protection film 20, GeTe-rich GeSbTe phase-change films, and TiO2-based film with high refractive index (2.75) working to increase the total transmittance 21. Consequently, the difficulties were solved, as listed in Table 1.
Table 1. Reflectivity contrast and transmittance in the crystalline and amorphous area for each layer of the triple layer Blu-ray Disc.
(Rc − Ra)/(Rc + Ra)
The GeTe–Sb2Te3 pseudobinary amorphous alloys (GeSbTe) are the first practical phase-change materials and still today remain the “standard material.” In this review article, the author has described the material system from the viewpoints of: (i) the origin of the material research, (ii) the basic characteristics as the phase-change material, (iii) the latest structural analysis result that is well coincident with the “nucleation-dominant crystallization process,” and (iv) the historical applications including the latest 100 GB Blu-ray Disc. What the author emphasized here is the importance of the metastable NaCl-like structure in the crystallization process. It is the secret “key” giving the convincing answers to the gently continuous phase-change properties, the fine cyclability and the generous allowance for compositional modification. Hopefully, this review article will be useful for researchers and technologists in this field.
This work was achieved chiefly at Panasonic Corporation over many years with many colleagues. The analytical studies were achieved collaborating with SPring-8 group. The author would like to thank all of them, again.
Noboru Yamada graduated from Kyoto University with a B.S. degree (Electronics) and joined the Corporate R/D Section of Panasonic in 1974. His main area of study for >30 years has been concentrated in the phase-change field: materials studies for new classes of phase-change alloys such as GeSbTe and GeBiTe, characterization of them, and development of optical disk products, typically DVD and BD, by utilizing them. He obtained his Ph.D. under Hiroyuki Matsunami at Kyoto University in 2001. He retired from Panasonic at the end of August in 2012 and now he is a visiting researcher of AIST (National Institute of Advanced Industrial Science & Technology).