Over the past few decades, chalcogenide phase change materials have received increased attention for next-generation non-volatile memory [1-3] and high density optical recording [4][5][6]. Typically, a chalcogenide material has two or more discrete states at which it exhibits distinguishable material properties. The change in the state is driven by thermal excitation, usually via an electrical or optical pulse. The significant difference between these states in electrical and optical properties upon the reversible switching allows storing the rewritable digital bit information. The most ubiquitous phase change material, GeSbTe (germanium-antimony-tellurium or GST), is a ternary compound consisting of germanium, antimony, and tellurium that is capable of reversibly switching at high speeds between its amorphous and crystalline states in response to thermal excitation. The crystallization temperature of the alloy is between 100 • C and 150 • C and the melting point is about 600 • C (873 K). Due to the non-volatility and high stability of both states, chalcogenide phase change materials have been used in rewritable optical recording media for years [7][8][9]. In the optical recording media application, a laser with controllable intensity and pulse duration is used to interact with the material, namely, heat a small volume to switch the material between crystalline and amorphous states. The information is then stored in the reflectivity of the phase change material layer. For electronic memories, even though Flash memory is the currently leading technology for non-volatile memory devices, the next generation of memory requires even higher speeds for write and erase processes, while maintaining high endurance, good scalability, low cost, and high power efficiency. With developments in lithography and discoveries in chalcogenide compounds, recently GST emerged as an important candidate for the electronic nonvolatile memory devices [10][11][12][13]. Within longer-term prospects, new In 1979, Phillips proposed a theory explaining the effect of the number of bonds per atom (coordination number) in chalcogenide alloys [19]. When the average coordination number for a material is between 2 and 3 (preferably 2.45), the ability of this material to form an amorphous state is high. By applying this theory into the phase-changing material GST, based on the assumption that the average coordination number can be calculated from the maximum number of bonds for the atoms, the Sb 2 Te 3 easily tends to form an amorphous state due to the coordination number being around 2.4, while is close to the ideal value. GeTe is less apt to form into an amorphous state because the coordination number is 3, which is relatively away from the ideal value [20]. Therefore, as a mixture of GeTe and Sb 2 Te 3 , the Ge-Sb-Te material becomes less apt to form an amorphous state as the proportion of GeTe increases, because of more GeTe content in the material, the greater difference between the average coordination number to the ideal number. In GeTe and GeTe-rich compounds, as indicated in the top region of the pseudobinary line, the number of excess vacancies move the Fermi energy to the region of extended states, which results in a metallic behavior. For the vacancy-rich GST compounds in the middle part of the line, the Fermi level lies in the region of localized states and the system exhibits insulating behavior. At the bottom region of the line, it becomes increasingly difficult to obtain a cubic phase experimentally. It has been found that Sb 2 Te 3 -rich compounds exhibit stable hexagonal structures with very elongated primitive cells [21].
In rewritable optical recording media and other PCM devices, the information storage utilizes either the large electrical or optical contrast between the two states for binary representation. As the operation for the typical memory devices shown in Figure 2, SET (conventionally linked to writing a logic '1') and RESET (writing a logic '0') states are controlled by a Joule heating process and are associated with amorphous-to-crystalline and crystalline-to-amorphous transitions, respectively. Since crystallization is a slower process than amorphization, the SET process in a device requires heating the material above its crystallization temperature for a sufficient length of time (tens to 100s of ns) so that the atoms rearrange themselves in crystalline order. A medium level laser or current (blue curve) for fairly long pulse times is used to re-crystallize the phase change material to its crystalline state. On the other hand, the RESET or amorphization process needs a higher temperature with a shorter duration to melt and convert the material to a liquid (amorphous) state then quickly quench the material such that the atoms do not have time to arrange in a crystalline fashion. The typical time required for RESET switching is shorter than a few tens of nanoseconds [22,23]. In this process, the temperature of the material needs to drop down quickly, i.e., the fast quench should occur faster than the timescale for thermal diffusion from cell to neighboring cell, preventing reorganization into a crystalline structure. A much lower level laser or current with essentially no Joule heating is used for reading the state, differentiating between amorphous (low reflectivity, high resistivity) and crystalline (high reflectivity, low resistivity) states.
These two approaches for switching the phase of a phase-change device, electrical and optical control, generate two formats of devices, driven by either electricity or light. For example, for PCRAM devices as in Refs. [24][25][26][27], the phase change material, and a heater element are connected in series, with the heater inducing localized Joule heating aimed at switching the volume near the interface region (between the heater and phase change material elements). We will discuss the details of PCRAMs in Section 3.1. Optical control is another effective approach that generally uses an optical pump beam that is focused on the PCM film, leading to Joule heating by absorption, which induces the phase transition. The rewritable optical disk is one of the well known optical control PCM applications [1,2].
Metasurfaces or two-dimensional metamaterials where light does not typically penetrate through the surfaces have been demonstrated to have usefulness in lenses, hologram and polarization control [92,93]. However, optical loss is a common theme for all those devices, due to the metal elements in conventional metasurface devices. Another limitation is that conventional metasurfaces-based devices are not cost-effective since once a device is made, it only works for a certain functionality and wavelength. Therefore, there has been considerable interest recently in developing tunable all-dielectric metasurfaces for light modulation applications by using the phase change materials [94][95][96][97] instead of metallic elements used in earlier works.
GST is promising in both electrical and optical fields as discussed in the previous chapters, however, there are several issues that come to play in real applications, such as the stability of the amorphous state, power consumption and resistivity contrast in RF devices. One method to solve the problems is through doping other elements inside the GST material. A variety of elements such as C [98,99], O [100], N [100], Ni [101], Si [102], Al [103,104], Ti [105], W [52] and Cu [106] have been used to improve the device performance.
GST has a quite large resistivity at amorphous state-several hundred Ω•m-which can limit the potential applications in the electrical devices, especially for high speeds applications. The large resistivity requires a high voltage or long wait time to dissipate sufficient power in the devices to induce the transition, as well as resulting in a large impedance difference between the amorphous and crystalline phases. In the past few years, different dopant elements have been studied to modify the electrical properties of the host GST materials. The dopant can exist in the grain boundaries to suppress the grain growth, which results in a higher transition temperature.
One of the main issues faced by PCRAMs and other GST-based devices is the instability of the amorphous state after repeated cycling of the phase transition. One of the methods to improve the stability is by incorporating dopant in the GST structure, such as Sn [107]. The reason for the improved stability of the amorphous state is that the material effectively becomes a multi-element material consisting of four or more elements, which can effectively suppress the movement of atoms due to different atomic radiuses and hence, increase the activation energy [20].
Another challenge remaining in PCRAM technology, especially for high-speed cache-memory, is the operation speed. Generally, as the film goes thinner, the crystallization speed drops and phase transition temperature increases [29]. To increase the crystallization speed for very thin films, Yamada et al. [8] reported the Sn dopant in Ge 2 Sb 2 Te 5 which could increase the crystallization speed and even a 5 nm thick film showed a crystallization time less than 50 ns. Since SnTe has a lower crystallization temperature and a higher melting point than GeTe, the probability of nucleation will be higher in amorphous SnTe than GeTe. Which means the crystallization process in GST can be accelerated by substituting Sn for Ge. Furthermore, since both SnTe and Ge 2 Sb 2 Te 5 have a stable NaCl structure after crystallization, they tend to form a single-phase crystal after crystallization. It is strongly expected that Sn replaces Ge substitutionally, which is supported by the fact that both Sn and Ge belong to Group IV in the periodic table and possess relatively close atomic radii.
In 2017, Rao et al. introduced ScSbTe alloy to speed up the crystallization kinetics from tens of nanoseconds of GST down to 700 picoseconds, which comes from the reduced stochasticity of nucleation through geometrically matched and robust Sc-Te chemical bonds that stabilize crystal precursors in the amorphous state [108]. In other words, this compound geometrically matched very well to the base-alloy rock-salt crystalline product Sb 2 Te 3 , and the Sc 2 Te 3 bond is more robust as compared with Sb 2 Te 3 .
Different deposition methods affect the film composition, density, and stress hence the electrical and optical properties. Therefore, it is necessary to review the fabrication methods for GST. Sputtering (Section 5.1) and Pulsed Laser Deposition (PLD) (Section 5.2) tend to provide films with good density. A good conformal step coverage in fabrication means the created thin films have the same vertical and horizontal thickness. The non-conformal step coverage is good for lift-off to create GST optical device, while the conformal step coverage is critical in creating confined cell structures. Therefore, it is necessary to introduce the deposition methods for GST films that can provide a non-conformal step coverage or a good conformal step coverage. to have access to both conformal and nonconformal deposition methods. Evaporation (Section 5.3) is a typical directional deposition technique to create non-conformal step coverage which is preferred by the lift-off process. The techniques can provide good conformal step coverage include chemical vapor deposition (CVD) (Section 5.4), atomic layer deposition (ALD) (Section 5.5), and plasma enhanced chemical vapor deposition (PECVD). Evaporation is a typical directional deposition which is preferred by the lift-off process.
Sputtering is a widely used deposition method for a wide variety of thin films. It is particularly suited for GST because of its multi-element composite nature. Sputtering can produce high-quality dense films with preservation of the stoichiometry upon deposition. In sputtering, a target with the correct GST composition is typically used as the material source, see Figure 18a. Ar + atoms are accelerated by an electrical field towards the target where the atomic species are sputtered out. These atoms and clusters land on the substrate to form the thin films. One of the strengths in sputtering is the preservation of the target stoichiometry, which occurs only after a certain length of time (known as target conditioning) to account for the different sputter yields of the constituent elements of the target. The sputtered films properties can be controlled by adjusting the working pressure, power, and the plasma gas. During DC sputtering, a higher argon pressure leads to a lower phase transition temperature [109], see Figure 18b. The argon gas also has an effect on the sputtered thin film surface morphology, Bakan et al. [110] reported the sputtered GST film tends to form cracks with a higher Ar flow rate (higher pressure), presumably due to the accumulation of stress. Therefore, the higher quality GST can be deposited by sputtering at a lower argon pressure.
Appl. Sci. 2019, 9, 530; doi:10.3390/app9030530 www.mdpi.com/journal/applsci