Nonvolatile Phase-change Memory Research Articles

China's Wuhan National Laboratory for Optoelectronics

Wuhan National Laboratory for optoelectronics (WNLO), coestablished by the Ministry of Education, Hubei Provincial Government, and Wuhan Municipal Government, is one of the first five national laboratories authorized by the Ministry of Science and Technology of the People's Republic of China. WNLO is not only a major component of the state scientific and technological innovation system in China but also the innovation and research base of Wuhan Optics Valley of China (OVC). The outstanding achievements that WNLO has made are due to the 41 pioneering research teams with character and spirit, whose research disciplines cover nine cutting edge and strategic research areas, including fundamental photonics, integrated optoelectronics and micronanofabrication, laser science and technology, optical network and communication, optoelectro measurement and instrument, optoelectrical information storage, biomedical photonics and optoelectronic medical devices, organic optoelectronics, and advanced optoelectronic materials and nanotechnology. In this article, we give four examples of the major research activities currently at the WNLO including: nonvolatile phase-change random access memory, micro/nanometrology and surface biomimetics, silicon nanophotonics, and nanobiophotonics.

Nanosecond switching in GeTe phase change memory cells

The electrical switching behavior of GeTe-based phase change memory devices is characterized by time resolved experiments. SET pulses with a duration of less than 16 ns are shown to crystallize the material. Depending on the resistance of the RESET state, the minimum SET pulse duration can even be reduced down to 1 ns. This finding is attributed to the increasing impact of crystal growth upon decreasing switchable volume. Using GeTe or materials with similar crystal growth velocities, hence promises nonvolatile phase change memories with dynamic random access memorylike switching speeds.

Two generations of phase-change memory devices: Differences and common problems

Two generations of phase-change memory (PCM) devices are analyzed from the point of view of material requirements. The first generation of PCM devices was based on chalcogenide glasses. The lack of sufficient information on the physical and chemical processes within the material that occur during operation of the devices led to poor reliability of the first-generation PCM integrated circuits (ICs). The second generation of PCM devices uses memory switches based on crystalline chalcogenide alloys. The basic operating mechanisms of the first- and second-generation nonvolatile PCM devices are largely the same. The PCM ICs designers of either generation face the same problems: contradictory requirements toward the phase-change material, where both low SET and RESET power and high reliability of cells retention are important, the problem of the dispersions of cell characteristics in the SET and RESET states, the problem of the forming process of PCM elements and others. The analysis of the results allows the problems that need solving to be looked at to avoid making the same mistake twice and to create highly reliable PCM devices with acceptable power and geometrical characteristics.

Fabrication and electrical characterization of phase-change memory devices with nanoscale self-heating-channel structures

We proposed a material composition and an optimized patterning process for the phase-change memory devices with a nanoscale self-heating channel (NSC) structures. As a suitable composition, Ge 18Sb 39Te 43 was employed, which is the 22% Sb-excessive phase compared with the conventional Ge 2Sb 2Te 5. For fabricating the NSC memory devices, Ge 18Sb 39Te 43 layer was patterned into a thin channel having enlarged pad areas at both sides end by the developed two-step dry etching technique using a TiN hard mask. The NSC memory devices showed such good behaviors as lower power operations without any degradation of switching speed and better endurance for cyclic rewritings even in the scaling regime of tens-of-nanometer size. It can be concluded from the obtained results that the proposed NSC memory devices promise the feasibility for realizing both aggressive scaling with a simpler process and enhanced memory performances for the phase-change nonvolatile memory applications.

Cellular automata modelling of phase-change memories

A novel approach to modelling phase-transition processes in phase change materials used for optical and electrical data storage applications is presented. The model is based on a cellular automaton (CA) approach to predict crystallization behaviour that is linked to thermal and electrical simulations to enable the study of the data writing and erasing processes. The CA approach is shown to be able to predict the evolution of the microstructure during the rapid heating and cooling cycles pertinent to data storage technology, and maps crystallization behaviour on the nanoscale. A simple example based on possible future nonvolatile phase-change random access solid-state memory is presented.

Laser and electrical current induced phase transformation of In2Se3 semiconductor thin film on Si(111)

Phase transformation of thin film (∼30 nm)In2Se3/Si(111) (amorphous→crystalline) was performed by resistive annealing and the reverse transformation (crystalline→amorphous) was performed by nanosecond laser annealing. As an intrinsic-vacancy, binary chalcogenide semiconductor, In2Se3 is of interest for non-volatile phase-change memory. Amorphous InxSey was deposited at room temperature on Si(111) after pre-deposition of a crystalline In2Se3 buffer layer (0.64 nm). Upon resistive annealing to 380°C, the film was transformed into a γ-In2Se3 single crystal with its {0001} planes parallel to the Si(111) substrate and \((11\bar{2}0)\) parallel to Si \((1\bar{1}0)\) , as evidenced by scanning tunneling microscopy, low energy electron diffraction, and X-ray diffraction. Laser annealing with 20-ns pulses (0.1 millijoules/pulse, fluence≤50 mJ/cm2) re-amorphized the region exposed to the laser beam, as observed with photoemission electron microscopy (PEEM). The amorphous phase in PEEM appears dark, likely due to abundant defect levels inhibiting electron emission from the amorphous InxSey film.

Resonant bonding in crystalline phase-change materials

The identification of materials suitable for non-volatile phase-change memory applications is driven by the need to find materials with tailored properties for different technological applications and the desire to understand the scientific basis for their unique properties. Here, we report the observation of a distinctive and characteristic feature of phase-change materials. Measurements of the dielectric function in the energy range from 0.025 to 3 eV reveal that the optical dielectric constant is 70-200% larger for the crystalline than the amorphous phases. This difference is attributed to a significant change in bonding between the two phases. The optical dielectric constant of the amorphous phases is that expected of a covalent semiconductor, whereas that of the crystalline phases is strongly enhanced by resonant bonding effects. The quantification of these is enabled by measurements of the electronic polarizability. As this bonding in the crystalline state is a unique fingerprint for phase-change materials, a simple scheme to identify and characterize potential phase-change materials emerges.

Threshold switching mechanism by high-field energy gain in the hopping transport of chalcogenide glasses

Chalcogenide glasses are widely used in phase-change nonvolatile memories and in optical recording media for their ability to rapidly change their structure to crystalline, thus obtaining different electrical resistance and optical reflectivity. Chalcogenide glasses universally display threshold switching, that is a sudden, reversible transition from a high-resistivity state to a low-resistivity state observed in the current-voltage $(I\text{\ensuremath{-}}V)$ characteristic. Since threshold switching controls the operating voltage and speed of phase-change memories, the predictability of the switching voltage, current, and speed is of critical importance for selecting the proper chalcogenide material for memory applications. Although threshold switching has long been recognized to be an electronic process with an intimate relation to localized states, its detailed physical mechanism is still not clear. In this work, threshold switching is explained by the field-induced energy increase in electrons in their hopping transport, moderated by the energy relaxation due to phonon-electron interaction. The energy increase leads to an enhancement of conductivity and a collapse of the electric field within the amorphous chalcogenide layer, accounting for the observed negative differential resistance at switching. Threshold switching is found to obey to a constant electrical-power condition. The proposed model generally applies to low-mobility semiconductors featuring a deep Fermi level and hopping-type conduction, and can predict the thickness, temperature, and material dependence of threshold voltage and current.

Ultramultiple-level storage in TiN∕SbTeN double-layer cell for high-density nonvolatile memory

We report a phase-change nonvolatile memory (NVM) concept based on a TiN∕SbTeN (N-doped Sb2Te3) double-layer structure, which can be used for ultramultiple-level storage (UMLS). SbTeN shows a gradual resistivity drop and good phase stability with increasing annealing temperature, a characteristic which makes it suitable for UMLS applications. We demonstrate that the number of distinguishable resistance levels can readily reach 16 and even higher. These levels in this study result from the initial threshold switching and the subsequent current-controlled crystallization induced by Joule heating. The latter allows the creation of many distinct levels, thus, enabling the low-cost ultrahigh-density NVM.

Overview of the Current Status of Technical Development for a Highly Scalable, High-Speed, Non-Volatile Phase-Change Memory

The present status of technical development of a highly scalable, high-speed non-volatile PCM is overviewed. Major technical challenges are described along with solutions that are being pursued in terms of innovative device structures and fabrication technologies, new phase change materials, and new memory schemes.

Luminescence readout of nanoparticle phase state

We report that the phase state of bistable gallium nanoparticles, controlled by optical or electron beam excitations, can be identified via measurements of their cathodoluminescent emission, thus offering an innovative conceptual basis for the development of high density nonvolatile phase-change memories. Changes of up to 20% in visible emission intensity are observed following low-fluence optical or electron beam induced phase switching in a monolayer of 60nm particles.

TIME DEPENDENT RESISTANCE CHANGE OF AMORPHOUS PHASE IN PHASE-CHANGE NONVOLATILE MEMORIES

ABSTRACT For the realization of reliable nonvolatile phase-change memory operations, it is very important to understand the long-term behaviors of memory states with different resistance values. We investigated the time-dependent behaviors of RESET resistance states (RR) for the fabricated memory devices using Sn-doped GST, in which Sn amounts were varied to control the initial values of RR. It was found that the transient variations of RR showed an increasing trend with time, and that their behaviors were closely related to the microstructure of amorphous phase in the phase-change material.

Pulse number control of electrical resistance for multi-level storage based on phase change

Phase change nonvolatile memory devices composed of SeSbTe chalcogenide semiconductor thin film were fabricated. The resistivity of the SeSbTe system was investigated to apply to multi-level data storage. The chalcogenide semiconductor acts as a programmable resistor that has a large dynamic range. The resistance of the chalcogenide semiconductor can be set to intermediate resistances between the amorphous and crystalline states using electric pulses of a specified power, and it can be controlled by repetition of the electric pulses. The size of the memory cell used in this work is 200 nm thick with a contact area of 1 µm diameter. The resistance of the chalcogenide semiconductor gradually varies from 41 kΩ to 840 Ω within octal steps. The resistance of the chalcogenide semiconductor decreases with increasing number of applied pulses. The step-down characteristic of the resistance can be explained as the crystalline region of the active phase change region increases with increasing number of applied pulses. The extent of crystallization was also estimated by the overall resistivity of the active region of the memory cell.

GROWTH OF Ge-DOPED Sb2Te3 THIN FILMS BY METAL-ORGANIC CHEMICAL VAPOR DEPOSITION

ABSTRACT Thin films based on Ge–Sb–Te are the most popular material for non-volatile phase change memory, because this material is more stable in the over-write than other chalcogenide alloy. We fabricated the Ge-doped Sb2Te3 rhombohedral thin films by metal-organic chemical vapor deposition (MOCVD) with bubbler type systems for the nonvolatile PCRAM applications and also expected the effects of ratio of respective precusors pressure. In spite of the different Ge bubbler temperature, all thin films have same crystallinity of Sb2Te3 phase, which has c-axis orientation perpendicular to the substrates. Even though ratio of relative Ge vapor pressure (%) increases up to 63%, amount of Ge in the films is only about 0.4%. We concluded that reaction between Sb and Te are dominant than other reaction such as Ge–Te or Ge–Sb in current conditions. Also, to improve the reaction of Ge elements in the chamber, controlling amount of Ge precursors may be needed delicately. These results suggest that Ge–Sb–Te thin films can be deposited by metal organic CVD for phase change memory applications.

Electrical Switching Studies of Amorphous Ge1Se1Te2 Thin Film for a High-Performance Nonvolatile Phase-Change Memory

In the present work, we studied the changes in the electrical conductivity occurring in a new composition of amorphous Ge1Se1Te thin film for a high-performance nonvolatile phase-change memory. A phase-change random access memory (PRAM) device without an access transistor is successfully fabricated with the Ge1Se1Te2 phase-change resistor, which has a much higher electrical resistivity than that of conventional Ge2Sb2Te5, and its electric resistivity can be varied by the factor of 105, related to the degree of crystallization. Chalcogenide Ge1Se1Te2 thin film of 100 nm thickness was formed by vacuum deposition at 1.5×10-5 Torr. The static mode switching characteristics are tested for the 100 µm-sized Ge1Se1Te2 PRAM device. In the first sweep, the amorphous Ge1Se1Te2 thin film showed a high resistance state in the low voltage region. However, when it reached the threshold voltage, Vth, the electrical resistance of the device was drastically reduced through the formation of an electrically conducting path. The results of pulsed mode switching of the 20 µm-sized Ge1Se1Te2 PRAM devices show that the reset process of the device was accomplished with an 80 ns–8.6 V pulse and the set process of the device was accomplished with a 200 ns–4.3 V pulse.

Sb-Se-based phase-change memory device with lower power and higher speed operations

A phase-change material of Sb/sub 65/Se/sub 35/ was newly proposed for the nonvolatile memory applications. The fabricated phase-change memory device using Sb/sub 65/Se/sub 35/ showed a good electrical threshold switching characteristic in the dc current-voltage (I-V) measurement. The programming time for set operation of the memory device decreased from 1 /spl mu/s to 250 ns when Sb/sub 65/Se/sub 35/ was introduced in place of the conventionally employed Ge/sub 2/Sb/sub 2/Te/sub 5/ (GST). The reset current of Sb/sub 65/Se/sub 35/ device also dramatically reduced from 15 mA to 1.6 mA, compared with that of GST device. These results are attributed to the low melting temperature and high crystallization speed of Sb/sub 65/Se/sub 35/ and will contribute to lower power and higher speed operations of a phase-change nonvolatile memory.

Electrical properties and crystallization behavior of Sb x Se100−x thin films

The structural transformation and transformation kinetics of Sb x Se100−x films (60 ≤ x ≤ 70) were studied to investigate the feasibility of applying Sb x Se100−x alloys in phase-change nonvolatile memories. The temperature-dependent van der Pauw measurements, Hall measurements, X-ray diffraction and a static tester were used to investigate the electrical properties and crystallization behavior of the Sb x Se100−x films. The sheet resistance difference between amorphous and crystalline state was higher than 104 Ω per square According to Hall measurement, Sb x Se100−x films have p-type conduction and the Hall mobility and carrier concentration increases with the increase in Sb content. The crystalline structure of the metastable phase of Sb x Se100−x alloys, which plays a major roll in fast crystallization, is similar to that of Sb2Te (rhombohedral structure). The transition temperature, sheet resistance and activation energy for transformation decrease as the amount of Sb increases in the Sb x Se100−x film. Applying the Kissinger method, the activation energies for crystallization were in the range from 1.90 ± 0.15 to 4.16 ± 0.28 eV. The desired crystallization speed can be obtained by a systematic change of the composition owing to the variation of the activation barrier with stoichiometry.

Phase-change magnetic memory effect in cation-deficient iron sulfide Fe1−xS

We report the discovery of magnetic memory effect in cation-deficient iron sulfide Fe1−xS. This nonstoichiometric sulfide shows a transition from a ferrimagnet to an antiferromagnet induced by an order-disorder transition of cation defects. The magnetic properties of Fe0.92S, at the critical composition for the order-disorder transition, were reliably controlled by the use of appropriate heat treatments; the slow-cooled sample showed ferrimagnetism with a spontaneous magnetization, while the quenched samples displayed superparamagnetic behavior without hysteresis. We propose that this material can be used as part of a nonvolatile phase-change memory device.

Dielectric Constants and Endurance of Chalcogenide Phase-Change Non-Volatile Memory

AbstractInitial attempts to create memory from chalcogenide glasses (g-Ch) had limited success particularly because the first generation of these materials (labeled as CG1) has inferior endurance (about 106SET-RESET cycles). Recent progress in phase-change non-volatile memory (PC-RAM) related to superior properties of Ge2Sb2Te5(GST) alloy [1,2]. The paper answers the vital for PC-RAM development question: “Why is endurance of GST memory cells (about 1011cycles) much higher than of CG1 cells?” We show that superior endurance is related to features of –U centers [3] creation in GST during RESET of PC-RAM cells. The native –U centers exist in g-Ch due to the softness of atomic potentials [3]. They play a significant role in SET process of CG1 and GST [2,4]. The –U centers behavior in GST [5] is different compared with CG1 while other properties (threshold voltage, resistivity, etc.) are basically the same [1-4]. We found that dielectric permittivities e of CG1 and GST are also different.The e values in Ge-Sb-Te alloys films have been determined from impedance measurements in sandwich samples using method described in [6] and reported in the paper. Amorphous GST has relatively high and distinct static and optical dielectric permittivities eo=16.5 and e'=15.3 to compare with CG1 where e practically independent on frequency. Hence the second term in the effective polarization potential Ep = q^2[(1-1/e')/r + (1/e' - 1/eo)/L] is strong (here r and L are the average atomic radius and interatomic bond distance, q is the electron charge). It allows to screen the Coulomb repulsion at an –U center quite effectively in GST. Therefore polarization helps –U centers creation in amorphous GST and impedes these centers formation in crystalline hexagonal GST films where eo=38 and e'=61. In contrast to CG1 [3], the creation and destruction of –U centers during RESET and SET processes in GST [4] are not accompanied by strong plastic mechanical stresses in a memory cell. This feature (higher barrier between elastic and plastic deformations) predetermines possibility of numerous SET-RESET cycles in this alloy. Therefore, the deformation mechanism of –U centers formation in CG1 leads to inferior endurance while the polarization mechanism of their creation in GST ensures decent endurance. Second factor is hybritization of –U centers with extended states can also play role in good memory alloys. Obtained experimental e values in non-stochiometric films allow to conclude about expected endurance in the framework of the proposed model.

Nonvolatile phase change memory nanocell fabrication by femtosecond laser writing assisted with near-field optical microscopy

The phase change memory cells were developed by using a combination system of a femtosecond laser with near-field scanning optical microscopy. The memory cells with feature size varying from 800nm down to 90nm were achieved. The cell functional performances were tested, and the scalability of the programming current as a function of the memory cell features was investigated. The optical near-field distance which is one of the critical factors to achieve high resolution nanostructures was studied experimentally with the consideration of the whole fabrication process for functional devices. The Bethe-Bouwkamp model was employed to study the effects of the optical near-field distance to the nanostructure geometry. The programming current of 0.8mA was observed for the memory nanocell at a feature size of 90nm.