Phase-change Probe Memory Research Articles

Applications of Phase Change Materials in Electrical Regime From Conventional Storage Memory to Novel Neuromorphic Computing

Phase-change materials, also well known as the Chalcogenide alloy, have received considerable attention during last two decades owing to its widespread applications in the field of the electrical storage market such as phase-change random access memory and phase-change probe memory. In addition to the storage devices, its unique electrical properties that can be dynamically tunable with respect to the electrical excitations lead to numerous novel applications represented by memristor and memristor-based neuromorphic electrical circuits. These emerging applications undoubtedly allows for a further exploitation of the potential of phase-change materials, and thus makes it advantageous over other storage medium like ferroelectric and magnetic materials. In order to help researchers understand the role of phase-change materials in these novel applications as well as their importance for citizen’s daily life, a comprehensive review that not only covers the traditional storage applications, but also the applications on these exotic devices becomes imperative. In this review, the chemical structure of phase-change materials and their remarkable electrical properties are first reviewed, followed by an introduction of their applications on the storage fields. The physical principles of various emerging electrical devices using phase-change materials are subsequently overviewed in association with their state-of-the-art progress. The prospect of phase-change materials for the future non-volatile electrical applications that are yet to be unraveled is finally envisaged.

Reading Contrast of Phase-Change Electrical Probe Memory in Multiple Bit Array

Reading contrast of phase-change electrical probe memory is investigated using a newly developed electrical switching model that accurately mimics the threshold switching of the phase-change media. Multiple bit array in either crystalline or amorphous phase is introduced into the model to take into account the influence of the readout cross-talk arising from inter-bit and inter-track interferences. Based on this configuration, the reading contrast is calculated by properly tailoring the electrical and geometrical properties of the capping, phase-change, and bottom layers as well as varying the length of the bit and track pitch. We found that the reading contrast can be significantly enhanced using an ultra-thin phase-change layer sandwiched by an ultra-thin capping layer with an intermediate electrical conductivity, and a relatively thick bottom layer with ultra-high electrical conductivity, as well as maintaining the length of the bit and track pitches similar to the size of the electrical probe. An optimized media stack that is also experimentally achievable from the state-of-the-art film deposition technologies is proposed, enabling a reading contrast of 0.85 for crystalline bit array and of 0.68 for amorphous bit array, close to those induced from isolated bit configuration immune to readout cross-talk. In order to thoroughly eliminate the adverse effect caused by readout cross-talk, the concept of patterned probe phase-change memory is introduced and its practicality for further alleviating the reading contrast is briefly discussed.

Electro-Optical Operation of Electrical Probe Phase-Change Memory With Ultra-High Electrically Conductive Capping Layer

An electrical probe phase-change memory using an ultra-high electrically conductive capping layer allows for a successful write of the amorphous bit without thermally damaging the device. Due to the severe current spreading effect, the recorded bit, however, cannot be sensed based on the conventional electrical means. We, therefore, developed a numerical optical model to calculate the device transmission resulting from a focused spot of a mobile scanning near-field optical microscopy. The feasibility of using this novel approach to distinguish bits from their background is also demonstrated.

Nanoscale thermal cross-talk effect on phase-change probe memory

Phase-change probe memory is considered as one of the most promising means for next-generation mass storage devices. However, the achievable storage density of phase-change probe memory is drastically affected by the resulting thermal cross-talk effect while previously lacking detailed study. Therefore, a three dimensional model that couples electrical, thermal, and phase-change processes of the Ge2Sb2Te5 media is developed, and subsequently deployed to assess the thermal cross-talk effect based on a Si/TiN/ Ge2Sb2Te5/diamond-like carbon (DLC) structure by appropriately tailoring the electro-thermal and geometrical properties of the storage media stack for a variety of external excitations. The modeling results show that the DLC capping with a thin thickness, a high electrical conductivity, and a low thermal conductivity is desired to minimize the thermal cross-talk, while the TiN underlayer has a slight impact on the thermal cross-talk. Combining the modeling findings with the previous film deposition experience, an optimized phase-change probe memory architecture is presented, and its capability of providing ultra-high recording density simultaneously with a sufficiently low thermal cross-talk is demonstrated.

Ultrahigh Storage Densities via the Scaling of Patterned Probe Phase-Change Memories

The scaling potential of patterned probe phase-change memory (PP-PCM) cells is investigated, down to single-nanometer dimensions, using physically realistic simulations that combine electrothermal modeling with a Gillespie Cellular Automata (GCA) phase-change model. For this study, a trilayer TiN/Ge2Sb2 Te5/TiN cell structure (isolated by an SiO2 insulator) was preferred, due to its good performance and practicability, over previously investigated probe-based structures such as those that used diamond-like carbon capping layers or immersion in an inert liquid to protect the phase-change layer (while still allowing for electrical contact). We found that PP-PCM cells with dimensions as small as 5 nm could be successfully amorphized and recrystallized (RESET and SET) using moderate voltage pulses. The resistance window between the RESET/SET states decreased with a reduction in cell dimensions, but it was still more than the order of magnitude even for the smallest cells, predicting that PP-PCM cells are indeed scalable and operable in the sub10-nm region. Most importantly, it was found that the storage density could be increased by cell size scaling with storage densities as high as 10 Tb/in2 being achieved, which is significantly higher than the storage densities previously reported in the phase-change probe storage, and other probe-based technologies such as thermomechanical, magnetic, and ferroelectric probe storage.

The exploration of role of tip-sample contact on scanning probe phase-change memory

Scanning probe phase-change memory has recently been under intensive research due to its promising potential for future archival storage device. However, majority of efforts are devoted to optimization of phase-change stack, while ignoring the influence of the tip-sample contact on the write performance of the device. As phase transition mainly occurs in a region bounded by tip-sample interface, the necessity to correctly understand the role of tip-sample contact on scanning probe phase-change memory becomes increasingly imperative. Therefore, the effects of tip diameter and tip force on the recorded mark size are investigated here, associated with measurements of surface roughness for different storage samples. Results show that mark size is linearly proportional to tip diameter and tip force, closely matching the previous speculation. Additionally, the optimized media stack previously designed exhibits a lower surface roughness than other samples, further demonstrating its performance superiority.

Overview of Probe-based Storage Technologies.

The current world is in the age of big data where the total amount of global digital data is growing up at an incredible rate. This indeed necessitates a drastic enhancement on the capacity of conventional data storage devices that are, however, suffering from their respective physical drawbacks. Under this circumstance, it is essential to aggressively explore and develop alternative promising mass storage devices, leading to the presence of probe-based storage devices. In this paper, the physical principles and the current status of several different probe storage devices, including thermo-mechanical probe memory, magnetic probe memory, ferroelectric probe memory, and phase-change probe memory, are reviewed in details, as well as their respective merits and weakness. This paper provides an overview of the emerging probe memories potentially for next generation storage device so as to motivate the exploration of more innovative technologies to push forward the development of the probe storage devices.

Towards low energy consumption data storage era using phase-change probe memory with TiN bottom electrode

Abstract Phase-change probe memory has been extensively regarded as one of the most prospective candidates to satisfy the recording density requirement from the incoming age of big data. However, in spite of recent advances, the energy consumption of phase-change probe memory still remains fairly high due to the use of the diamond-like carbon bottom electrode usually having a relatively high electric resistivity. In this case, the possibility of using titanium nitride to replace the diamond-like carbon as the electrode materials is investigated in this paper. The thickness and time-dependent resistivity of titanium nitride film is measured, allowing for a more conductive characteristic and a better stability than diamond-like carbon film at the same condition. Consequently, the writing of crystalline bit using the previously designed phase-change probe memory architecture but with titanium nitride bottom electrode is performed experimentally, and results show that using titanium nitride as bottom electrode would enable an achievement of ultra-high recording density with lower energy consumption than the phase-change stack with diamond-like carbon electrode.

An Improved Electrical Switching and Phase-Transition Model for Scanning Probe Phase-Change Memory

Scanning probe phase-change memory (SPPCM) has been widely considered as one of the most promising candidates for next-generation data storage devices due to its fast switching time, low power consumption, and potential for ultra-high density. Development of a comprehensive model able to accurately describe all the physical processes involved in SPPCM operations is therefore vital to provide researchers with an effective route for device optimization. In this paper, we introduce a pseudo-three-dimensional model to simulate the electrothermal and phase-transition phenomena observed during the SPPCM writing process by simultaneously solving Laplace’s equation to model the electrical process, the classical heat transfer equation, and a rate equation to model phase transitions. The crystalline bit region of a typical probe system and the resulting current-voltage curve obtained from simulations of the writing process showed good agreement with experimental results obtained under an equivalent configuration, demonstrating the validity of the proposed model.

The Route for Ultra-High Recording Density Using Probe-Based Data Storage Device

Phase-change probe memory using Ge2Sb2Te5 has been considered as one of the promising candidates as next-generation data storage device due to its ultra-high density, low energy consumption, short access time and long retention time. In order to utmostly mimic the practical setup, and thus fully explore the potential of phase-change probe memory for 10 Tbit/in2 target, some advanced modeling techniques that include threshold-switching, electrical contact resistance, thermal boundary resistance and crystal nucleation-growth, are introduced into the already-established electrothermal model to simulate the write and read performance of phase-change probe memory using an optimal media stack design. The resulting predictions clearly demonstrate the capability of phase-change probe memory to record 10 Tbit/in2 density under pico Joule energy within micro second period.

The Oxidation Behaviour of Diamond Like Carbon for Phase-Change Probe Memory Application.

Phase-change probe memory, as a promising candidate for next-generation storage device, usually requires a capping layer to protect phase-change media from wear and corrosion. Diamond-like carbon film has been commonly used for capping layer due to its high mechanical hardness and easiness for tailoring physical properties. However, the possibility for such carbon thin film to react to surrounding oxygen when subjected to Joule heating during the recording process of phase-change probe memory is rarely investigated before from both experimental and simulation point of view. Therefore, a novel carbon oxidation model was developed to mimic the chemical reaction of carbon film to the surrounding oxygen in terms of the degradation of layer thickness. Results obtained from this model are in a good agreement with the experimental counterpart, indicating the physical reality of this proposed model.

A Physics-Based Three Dimensional Model for Write and Read Performances of Phase-Change Probe Memory.

The write and read performances of phase-change probe memory were investigated for the first time by a physics-based pure three dimensional model. The written crystalline bit possessed from this developed model demonstrates the potential of phase-change probe memory for ultra-high density, low energy consumption, high data rate, and good readability. The cross-talk effect on the write and read performances of phase-change probe memory, which can not be modelled by previous two dimensional models, is also evaluated. The findings showed that the bit and track pitches should be remained sufficiently long so as to eliminate the undesired interferences. The simulated results exhibited a good agreement with the experimental observations, thus demonstrating the physical reality of the designed model.

Optimisation of readout performance of phase-change probe memory in terms of capping layer and probe tip

The capping layer and the probe tip that serve as the protective layer and the recording tool, respectively, for phase-change probe memory play an important role on the writing performance of phase-change probe memory, thus receiving considerable attention. On the other hand, their influence on the readout performance of phasechange probe memory has rarely been reported before. A three-dimensional parametric study based on the Laplace equation was therefore conducted to investigate the effect of the capping layer and the probe tip on the resulting reading contrast for the two cases of reading a crystalline bit from an amorphous matrix and reading an amorphous bit from a crystalline matrix. The results indicated that a capping layer with a thickness of 2 nm and an electrical conductivity of 50 Ω−1m−1 is able to provide an appropriate reading contrast for both the cases, while satisfying the previous writing requirement, particularly with the assistance of a platinum silicide probe tip.

A physics-based three dimensional readout model for phase-change probe memory

A physics-based three dimensional model is developed for the first time to assess the readout performance of phase-change probe memory. The isolated bit responses for reading a crystalline bit with an amorphous background and an amorphous bit with a crystalline background are investigated using this model under a calculated safe readout potential, resulting in a practicable readout current. The readout performances of multiple bit arrays for both cases are also evaluated to establish the influence of noise sources on the readout signal in terms of the inter-track interference and the inter-symbol interference. The results reveal that the configuration having an amorphous bit with a crystalline background exhibits a better anti-interference characteristic than the crystalline counterpart.

A contact resistance model for scanning probe phase-change memory

A novel mechanical model was proposed to calculate the contact resistance at tip and capping layer interface for scanning probe phase-change memory applications. The resulting I–V curve calculated from this model that combines Hertzian contact theory with the Schottky diode effect has exhibited a good agreement with the experimental measurements under the same system architecture. The role of contact resistance on the write efficacy of scanning probe phase-change memory was also evaluated by introducing the calculated contact resistance into the previous electrothermal simulations for cases of writing crystalline bits in amorphous starting phase and writing amorphous bits in crystalline starting phase. The consequent written marks and I–V curve show a closer match with the experimental observation compared to the case without including contact resistance.

Design of an optimised readout architecture for phase-change probe memory using Ge2Sb2Te5 media

Phase-change probe memory has recently received considerable attention on its writing performance, while its readout performance is rarely evaluated. Therefore, a three-dimensional readout model has been developed for the first time to calculate the reading contrast by varying the electrical conductivities and the thickness of the capping and under layers as well as the thickness of the Ge2Sb2Te5 layer. It is found that a phase-change probe architecture, consisting of a 10 nm Ge2Sb2Te5 layer sandwiched by a 2 nm, 50 Ω−1 m−1 capping layer and a 40 nm, 5 × 106 Ω−1 m−1 under layer, has the capability of providing the optimal readout performance.

Optimisation of scanning probe phase-change memory in terms of the thermal conductivities of capping and under layer

The optimisation of scanning probe phase-change memory using Ge2Sb2Te5 materials was investigated in terms of the thermal conductivities of the capping and under layer. It was revealed that the thermal conductivities of capping and under layer need to be determined carefully in order to mitigate the writing efficacy of the scanning probe phase-change memory. An optimal media stack stemming from the incorporation of such thermal conductivity optimisation with previous findings is designed, and the ability to write small bit with regular shape under low-energy condition using this storage stack was also demonstrated.

Terabit-per-square-inch scanning probe phase-change memory model based on nucleation-growth theory

Scanning electrical probe memory using Ge2Sb2Te5 has been considered as one of the promising contenders for next-generation data storage device, due to its ultra-high capacity, short access time, and low power consumption. Therefore, a numerical model based on classical nucleation-growth theory was developed to simulate the crystallisation behaviour of Ge2Sb2Te5 material in order to help understanding the physics mechanism of scanning electrical probe memory and thus provide the performance improvement. The crystallisation of Ge2Sb2Te5 in this model is first induced by the advent of the nuclei determined by nucleation probability, followed by the subsequent growth relying on the growth velocity. Results obtained from this model are in a good agreement with the experimental counterpart, revealing a potential of storage density of terabit per square inch.