Research status of phase change memory and its materials

Phase change memory technology is a new technology in non-volatile memory technology. Phase change memory technology has many advantages, such as non-volatility, high reading and writing speeds, better data retention, and strong compatibility with CMOS technology, and has been paid attention to by many researchers. Phase change materials are mainly chalcogenide compound materials. Researchers have done a lot of research on Ge-Sb-Te, Ge-Te, and Sb-Te-based phase change materials and developed some new phase change material systems. In addition, researchers have carried out in-depth research on the phase transition mechanism, but the existing atomic umbrella jump theory, multiple ring theory, resonance bond theory, octahedral structure theory, etc. have not formed a unified understanding of the phase transition mechanism. At present, phase change memory is mainly prepared by magnetron sputtering of phase change material targets, which results in fast deposition and high purity of the prepared film. In this paper, based on the development of phase change storage materials, the system of phase change materials and its phase change mechanism, the phase transition mechanism of phase change memory, the preparation and characterization methods of phase change films, the industrialization progress, and other research work are reviewed.

separation, the feature size of MOS devices is further scaled down with Moore's law, and mainstream memory has exposed the shortcomings of the "memory bottleneck".A non-volatile memristor can provide both data storage and memory computation, thereby fundamentally solving the memory bottleneck.In the current state of non-volatile memory technology, phase change random-access memory (PCRAM), ferroelectric random-access memory (FeRAM), resistance random-access memory (RRAM) and magnetoresistive random-access memory (MRAM) are the main types [2][3][4][5] .Table 1 summarizes the physical performance of PCRAM compared to other non-volatile memory (e.g., Flash, MRAM, FeRAM, and RRAM) [6][7][8][9] .
The PCRAM technology is the most mature and considered to be one of the most promising next-generation non-volatile memory technologies.PCRAM is a new type of non-volatile random-access memory based on thin films of chalcogenide compounds.By changing the temperature, the phase change material can be converted between the low-resistance crystalline (conductive) state and the high-resistance amorphous (non-conductive) state [10] .Based on the Ovshinsky electronic effect [11] , phase change materials are reversible phase change materials that use the high resistance characteristics of semiconductors in amorphous states and the low resistance characteristics of semiconductors in polycrystalline states for storage.PCRAM is also known as chalcogenide random access memory (C-RAM) or Ovshinsky Effect Unified Memory (OUM).Phase change memory (PCRAM) is a new type of memory that has the advantages of being non-volatile, fast operation speed, low power consumption, long service life, strong data retention, good miniaturability, and being compatible with the CMOS process [12] .At present, the mainstream phase change materials are chalcogenide compound materials (such as GeSbTe), and phase change memory has finally entered a period of rapid development after decades of invention.Phase change memory is likely to replace the current dynamic random-access memory (DRAM) and FLASH memory (NAND FLASH) and become one of the mainstream semiconductor memory devices in the future [13] .

Working principle of phase change memory
PCRAM utilizes Joule heating generated by electrical pulses in the order of nanoseconds to realize the amorphous to polycrystalline reversible phase transition of sulfur compounds.The resolution of amorphous high resistance and polycrystalline low resistance is up to more than 10 2 , which is used to store data state 0 and 1 [14] .
PCRAM relies on electrical pulses to induce a reversible transformation between amorphous (high resistance) and crystalline (low resistance) phase change materials to achieve the purpose of writing and erasing information and then reading data by measuring resistance value changes [15] .Its schematic diagram is shown in Figure 1.
PCRAM has the following three operations: (1) SET process: an electrical pulse with a long-acting time and moderate intensity is applied to the phase change material, and the material in the amorphous state within the pulse acting region is heated to the crystallization temperature (Tg) above and the melting point (Tc) below, and the material changes from the amorphous state to the crystalline state to achieve the writing (SET) operation.
(2) RESET process: apply a strong, but short duration electrical pulse on the phase change material, heat the phase change material to the melting point (Tc) or higher temperature in the pulse area, produce a large temperature gradient between the melting area and the surrounding material, produce a very large cooling rate (about 10 10 K/S).Due to the short pulse falling edge, the melting area can be cooled down quickly to below the crystallization temperature (Tg), thus avoiding the crystallization process, and directly transitioning from the molten state to the high resistance amorphous state, realizing the RESET operation.
(3) READ process: read the data between the crystal state and the amorphous state, and then control the phase change material below the crystallization temperature by adding an extremely weak electric pulse, in order to avoid unnecessary phase change of the material, causing damage to the data state, because the resistance difference between the crystal state and the amorphous state of the phase change material is up to several orders of magnitude, so, the obtained current or voltage signal strength can be used to determine the crystalline state of the material.
Phase change materials have amorphous semiconductor properties and high resistance values.In the crystal state, it has the characteristics of semi-metal and small resistance value.The phase change material changes from metastable amorphous phase to stable crystalline phase and is heated for a long time at a temperature higher than the crystallization temperature of the material.In contrast, amorphous materials can be obtained by heating the crystalline state above the melting point to melt and quenching cooling to condense [15][16][17] .

Phase change material and mechanism
Phase change materials are mostly chalcogenide compounds, that is, alloy materials containing at least one chalcogenide (main group Ⅵ) element, mainly including Ge-Sb-Te, Sb-Te and Ge-Te three systems.
Researchers continue to explore the phase transition principle of these phase change materials systems, such as atomic umbrella jump theory, multiple ring theory, resonance bond theory, octahedral structure primitive theory, etc., can explain the phase transition mechanism of phase change materials from a certain angle.

Phase change material system GST system
Figure 2 shows a ternary phase diagram of the Ge-Sb-Te system, where some common phase change alloys are highlighted, with the red arrow indicating the trend to add Ge to the Ge2Sb1Te2 alloy.Sb-Te is a rapid phase change material system that has been studied and developed more frequently.Among them, Sb2Te3 has the characteristics of good performance and low melting point.Because its crystallization mechanism is growth oriented, Sb2Te3 has relatively high crystallization rate, but it has the problem of low crystallization temperature and poor thermal stability.Ge-Te alloy, as one of the early materials used in PCRAM, still has great application potential.It has a high crystallization temperature and can meet the requirements of high temperature working environment.However, Ge-Te material also has defects such as high melting point and low crystal resistance, and the power consumption of memory devices is relatively large.
As a phase change material system, Ge-Sb-Te has been the most mature research, mainly including Ge2Sb2Te5 (hereinafter referred to as GST), Ge1Sb4Te7, Ge1Sb2Te4 and so on.Among them, GST is a typical pseudo-binary phase change material, inheriting the advantages of GeTe excellent thermal stability and Sb2Te3 rapid crystallization, and has a relatively outstanding overall performance.

New phase change materials
The choice of raw materials for new phase change materials will determine the development space of consumer electronics in the future, and environmentally friendly non-Te rich Sb-type phase change materials are conducive to the sustainable development of new phase change materials, such as GeSb, SiSb, ZnSb, GaSb, AlSb, etc.In addition, the application of new materials such as Sb materials, SnSe/SnSe2 superlattice materials, V2O5 materials in phase-change memory has also been deeply studied.
(1) GeSb phase change material GeSb materials have the advantages of rapid phase transition and high crystallization temperature.More importantly, compared with traditional phase change materials such as GST, there is no intrinsic vacancy defect in the crystal state, so the density changes before and after phase transition is small.It has been found that C doping can further reduce the density change before and after the GeSb alloy phase transition, and the doped C atoms can increase the atom packing efficiency in amorphous GeSbC by forming a tetrahedral configuration with shorter bond length and more dense packing, thus reducing the volume difference between amorphous and crystalline states, and reducing the density change of phase transition [19] .N and O doped GeSb films reduce the operating current, retain the rapid phase transition characteristics, and the amorphous phase has good thermal stability [20] .
(2) SiSb phase change material With the increase of Si content in SixSb100-x binary phase change material, the crystallization temperature of the film can be increased.When the Si content is 19%, the crystallization temperature can reach 203℃.The thermal stability of SixSb100-x series PCRAM is obviously better than that of GST materials, but the crystallization mechanism of the two materials is the same, and both are nucleated crystals [21] .
(3) ZnSb phase change material ZnSb has the advantages of higher crystallization temperature (~257 ℃), lower melting point (~500 ℃), better 10-year data retention (~201 ℃) and greater crystallization resistance, so it is considered as an alternative to GST [22] .Despite its many advantages, the extremely large amorphous resistance still limits practical applications, where large voltages or long pulses (tens of nanoseconds) are required to provide sufficient power to induce phase transitions [23][24][25] .
(4) GaSb phase change material GaSb phase change material has attracted much attention due to their unusual crystallization behavior.Upon crystallization, GaSb demonstrates a volumetric expansion rather than contraction, resulting in a negative optical contrast in stoichiometric GaSb films [26] .The change of Sb content in Ga1-xSbx will affect the electrical and structural properties of the material.It was found that for Ga30Sb70 material with specific Sb content, the density changes before and after phase transition disappeared, while no density change before and after phase transition would be beneficial to increase the durability of PCRAM and reduce the resistance drift [27,28] . (

5) AlSb phase change material
AlSb is an important indirect bandgap semiconductor with a wide range of uses, including as a semi-insulating substrate or buffer layer for epitaxial growth of GaSb, and as a barrier layer material for confining electrons in aluminum antimonide heterostructured devices [29] .The crystallization temperature and optical band gap of AlSb amorphous materials increase with the increase of Al content.The thermal stability and the randomness of the atomic configuration of the film are also enhanced.At the same time, the Sb-rich crystal region coexists with the Al-rich amorphous matrix, which indicates that most of the Al components are amorphous.What's more, Zhou et al. [30] obtained three different resistance levels in a phase-change memory device for Al50Sb50, suggesting that this material has potential for multistage data storage applications.
(6) Two-dimensional elemental Sb phase change material It can be found from Ge-Sb-Te ternary phase diagram that Sb is an important phase change material.
However, thick Sb films have the property of explosive crystallization, and when the film thickness is greater than 10nm, it will spontaneously crystallize at room temperature.However, when the thickness of Sb film is reduced to 3-5nm, its amorphous phase can remain stable at low temperatures for a period of time.In addition, SB-based elemental PCRAM has the advantages of anti-component segregation and reduced structural changes [31] .The good amorphous stability and lower drift coefficient of Sb films indicate that Sb films can not only be used as storage materials for PCRAM, but also have the possibility to accurately control their resistance states.
Two-dimensional Sb phase change films have good stability in amorphous state.The crystallization temperature, crystallization activation energy and 10-year data retention of Sb films increase with the decrease of film thickness.In particular, 3nm and 4nm Sb films have very good amorphous stability at room temperature [32] .
( decreases.By adjusting the thickness ratio, the transformation rate can be accelerated while maintaining high amorphous stability [33] .During crystallization, the resistance contrast of SnSe2 alloys exceeds five orders of magnitude [34] .In addition, the superlattice structure has a higher crystalline resistivity, while the lower phase transition temperature can reduce the heat absorbed by the phase transition, which can effectively reduce the operating current in practical applications.These advantages make superlattice structures have significant V2O5 have a good application prospect in phase change memory [35,36] .

Properties of phase change materials
Phase change material is the storage medium of phase change memory, and its performance requirements depend on the use of the device, so the development of phase change materials with excellent performance is the focus of the development of phase change memory technology.In order to adapt to the phase change memory write, erase, read and other functions, and stable storage information, recyclability and other performance requirements, phase change materials to consider many aspects, Table 2 is the material in the phase change before and after the necessary properties.
Table 2. Corresponding relationship between properties of phase change memory devices and material properties [13] .

Atomic Umbrella-flip theory
The atomic umbrella jump theory was first proposed by Kolobov et al. [37] using the Extended X-ray absorption Fine structure (EXAFS) method, and its model is shown in Figure 3.The Ge atoms of the GST crystal are distributed in the center of an octahedron composed of six Te atoms.On the amorphous GST, the tetrahedral part of the Ge atom is surrounded by four Te atoms, and the long Ge-Te bond breaks during the transition from GST to the amorphous state.In the direction <111>, the Ge atom jumps from the octahedral position into the tetrahedron.The crystallization process is reverse transformation, the Ge-Te bond in the tetrahedron is broken, the Ge atom is turned from the tetrahedron to the octahedron, and the Ge atom umbrella jump theory can explain the reversible phase transition process of GST.

Multiple ring theory
Kohara et al. [38] used synchrotron radiation X-ray diffraction and inverse Monte Carlo simulation to obtain three-dimensional atomic configurations of amorphous Ge2Sb2Te5 and GeTe.It was found that many 8-membered rings appeared in the amorphous GST material during the crystallization process, and the transformation of the 10-membered ring to the 4-membered ring structure and the 6-membered ring structure produced the crystalline GST.Therefore, in the amorphous GeTe material, even rings and odd rings coexist, and many bond breaks will occur during the crystallization process, as shown in Figure 4, the red keys represent Ge-Ge bonds, the first and second stages are writing processes, and the third stage is erasing processes.

Resonance bonding theory
Because of the significant difference in optical reflectance between amorphous phase change materials and crystalline phase change materials, umbrella jump theory and multiple ring theory can not explain them.
Shportko et al. [39] proposed the resonant bond theory.By measuring the post-infrared reflectance spectra of non-phase change materials and phase change materials before crystallization, they found that the post-infrared reflectance spectra of non-phase change materials did not change much before crystallization, and the post-infrared reflectance spectra of phase change materials before crystallization changed significantly.It is proposed that this phenomenon is due to the generation of saturated resonance bonds in crystalline materials, as shown in Figure 5.This theory is a good explanation for the change of properties of phase change materials before and after phase change.

Octahedral structure primitive theory
Song Zhitang et al. [14] found that the fast operation speed of the phase change memory is closely related to the octahedral structure of the primitive and the internal vacancy of the phase change material.The research shows that the phase change memory with different properties can be realized by rational design of phase change memory array.During the phase transition process, octahedral structure elements are recognized as the most basic units.During the amorphous phase transition, defects are relatively serious.Through local rearrangement, these configurations become more and more ordered, and then the halite phase grows to form a nucleation center.There are many vacancies in both the amorphous state and the halite phase, which shorten the time required for phase transformation.When the temperature rises, these vacancies break down and release electrons or holes.Driven by further thermodynamic driving forces, the vacancies move in a certain direction and are stratified, which makes the metastable rock salt structure transition to a stable hexagonal structure.

Preparation and characterization of phase change memory 4.1 Preparation method of thin film material for phase change memory
The existing film preparation technology is mainly physical vapor deposition (PVD), such as thermal evaporation, magnetron sputtering, pulsed laser deposition and so on.The common disadvantage of these methods is the difficulty of film thickness control and high cost.In addition, chemical vapor deposition or electrodeposition, hot atomic layer deposition and other methods can also be used to prepare composite phase change storage materials.However, physical vapor deposition is still the most important method in the preparation of phase change materials.Due to many defects and impurities in the film, it is necessary to modify its surface during the preparation process to obtain a high-quality film layer.Magnetron sputtering has better uniformity and is easier to control, so this method is mainly used to prepare thin film materials at this stage.
Chen et al. [40] prepared sulfide films by thermal evaporation and magnetron sputtering respectively and analyzed their chemical compositions.They found that the concentration of Sb in the films prepared by thermal evaporation increased by 3 at% compared with Sb2Te3, and the films obtained by magnetron sputtering had better stoichiometric number.Magnetron sputtering method is more advantageous for the preparation of films with specific stoichiometric ratios.In addition, after annealing at different temperatures, the film samples with higher crystal quality were obtained.Therefore, for the preparation of phase change storage thin film materials, magnetron sputtering is a very practical preparation method.

Performance characterization method of phase change memory
In order to test the properties of phase change materials, the characteristics are mainly from the aspects of film composition, thickness, structural morphology, thermodynamic properties and electrical properties.Rutherford backscattering spectroscopy (RBS) and energy dispersive X-ray spectroscopy (EDX) were used to determine the composition of the film.RBS can determine whether the composition of the film is close to the alloy material, and energy dispersive X-ray spectroscopy (EDX) can determine the proportion of atoms in the film.In addition, inductively coupled plasma spectrometer (ICP) or X-ray photoelectron spectrometer (XPS) can also be used to determine its composition.The surface morphology of the films was observed by scanning electron microscope (SEM).
Step meter, X-ray diffraction (XRD) and other instruments are used to measure the thickness of the film.
Transmission electron microscopy (TEM), Raman spectroscopy is used to analyze the structure of thin films.Hall effect tester was used to measure the relationship between conductivity, Hall mobility and carrier concentration before and after the film phase transition [10] .

Industrialization development of phase change memory
In memory is integrated with DRAM, the computational efficiency of big data centers and large servers will increase by about two orders of magnitude.In 2019, Micron finally released its first 3D XPoint SSD.The Micron X100 NVMe SSD, built for high-end data centers with superior performance and low latency, claims to be the world's fastest SSD, and is a counter to Intel's second-generation Optane SSDS to help customers cope with ultra-intensive data applications [19,[41][42][43][44][45] .
Overall, PCRAM has great potential for commercial development, but it still needs to solve the problems of high cost and low yield, and it is necessary to strengthen the upstream and downstream cooperation of the industrial chain to promote its industrial development.
) SnSe/SnSe2 superlattice phase change materials Due to the problem of high energy consumption, the further development of phase-change storage technology is limited.Superlattice structure can effectively reduce the power consumption of phase-change storage materials, which provides a new possibility for the development of this technology.Because of their low thermal conductivity and crystalline resistivity, SnSe and SnSe2 are made into periodic superlattice structures, which is beneficial to reduce power consumption in practical applications.With the increase of the thickness ratio of SnSe/SnSe2 components, the monotonicity of phase transition temperature and amorphous activation energy

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advantages in phase-change storage technology, and provide new possibilities for the further development of this technology.V2O5 phase change material V2O5 will undergo phase transition at 320℃ and 345℃, showing two obvious resistance mutations, and its 10-year data maintain temperature at 200℃.The primary reason for the rapid phase transition is the crystallization mechanism of one-dimension growth.Multi-level storage is realized in a V2O5-based phase-change memory with low SET and RESET voltages, storage speeds up to 100ns, and the power required to complete the SET→RESET process is 0.3mW, much less than the 1.22mW of GST.V2O5 phase change materials have high thermal stability, fast phase change speed and multistage storage function, which makes For GST phase transition mechanism of phase change materials has not yet formed a unified view, to explore the mechanism of phase change, however, the research and development, modification and application of phase change materials will have certain help.In recent years, researchers have carried out a lot of research on chalcogenide phase change materials through simulation and experiment and proposed a series of phase change mechanisms and theoretical models.

Ultraviolet-visible spectrophotometer
or transmission electron microscope are used to study the optical properties of thin films.The high temperature vacuum four probe test platform is used to test the resistance change with temperature of phase change material thin films.Atomic force microscopy (AFM) was used to observe the surface morphology and roughness of amorphous and crystalline films.Fourier infrared spectrum (FTIR) was used to analyze the chemical composition of the film during annealing, and to study the effect of heat treatment time and temperature on the properties of the film.Differential scanning calorimeter (DSC) was used to measure the crystallization temperature and melting temperature of the phase change thermal storage materials.The electric storage properties of thin films were studied by electrochemical method under different conditions.
the past 10 years, due to the continuous progress of semiconductor technology and the continuous development of nanomachine technology, the industrialization research and development of phase change storage is changing with each passing day, and has a good effect at home and abroad.In the 1990s, reversible reflectance changes in chalcogenide alloy films were first successfully applied to phase change rewritable optical discs, which became the mainstream of optical discs.The first batch of PCRAM products such as 650-MB rewritable disc (CD-RW), 4.7-GB rewritable digital multi-function disc (DVD±RW), digital universal disk random access memory (DVD-RAM), 20GB-27GB high-density digital multi-function disc (HD-DVD) and Blu-ray Disc (BD) were produced.In May 2005, IBM, Infineon and Wanhong announced a joint development study for PCRAM.In December 2006, IBM unveiled a bridge structure memory cell based on GeSb new phase change material with an erase current reduced to 0.1mA.In 2010, Numonyx released a 128Mbit phase change memory chip.In 2011, the Shanghai Institute of Microsystems and Information Technology of the Chinese Academy of Sciences and SMIC International Integrated Circuit Manufacturing Co., Ltd. and other units developed China's first 8Mbit PCRAM chip with independent intellectual property rights, which can realize all functions of reading, writing and erasing memory.In 2012, Samsung introduced the 8GB PCRAM chip at the ISSCC conference, which has a 40Mbps data transfer capacity and performance close to DRAM.In 2015, Intel and Micron announced the launch of 3D X-Point called Optane Memory, which consists of 3D cross-array structures with selectors and memory cells, all based on phase change materials, and the technology is more than 1,000 times faster than existing NAND-based SSDS.And entered the market as Optane solid-state drives (SSDS) and storage-like memory (SCM).In 2016, IBM realized the development of phase change storage technology in each unit to store 3bit technology, to help reduce the cost of phase change memory, and is expected to accelerate the pace of its industrialization, and finally provide a simple and fast storage method for the doubled data in the era of the Internet of things.In 2017, Shanghai Institute of Microsystems and Information Technology in China developed a 128Mb embedded PCRAM chip that meets market needs, and has reached mass production with SMIC, and has been successfully used in printers.In 2018, Intel announced a new Optane memory called Optane DC Persistent memory, whose capabilities will bridge the gap between SRAM/DRAM and non-volatile storage while expanding the amount of available memory per CPU slot to 3 TB.If Optane DC

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Summary and prospectPCRAM is arguably the most mature new semiconductor memory technology to date.Ge-Sb-Te based phase change materials have been extensively studied and produced and have appeared on the market as digital memory products.Its attractive advantages of multilevel storage, fast read/write, non-volatility, long cycle life, small component size, low power consumption, radiation resistance and good scalability make it ideal for new storage material applications.However, the phase change principle, doping mechanism, the development of new phase change materials, and the matching relationship between phase change materials and memory structure design still need researchers to carry out in-depth research.With the addition of first-principle computing, machine learning, artificial intelligence and other research tools to the study of the microstructure of phase change materials and its phase change mechanism, the phase change mechanism and doping mechanism of phase change materials will be further clarified, the phase change memory structure will be further improved and optimized, and the future development mode of phase change materials will definitely develop in the direction of design and verification.Optimizing the rapid development path is bound to shorten the development cycle of phase change materials, and phase change memory with better performance will certainly appear in people's lives in the future.