Xilin Zhou

Singapore University of Technology and Design · Division of Engineering Product Development

Chalcogenide based phase change random access memory (PCRAM) holds great promise for high speed and large data storage applications. This memory is scalable, requires a low switching energy, has a high endurance, has fast switching speed, and is nonvolatile. However, decreasing the switching time whilst increasing the cycle endurance is a key chall...

Light strongly interacts with structures that are of a similar scale to its wavelength; typically nanoscale features for light in the visible spectrum. However, the optical response of these nanostructures is usually fixed during the fabrication. Phase change materials offer a way to tune the properties of these structures in nanoseconds. Until now...

The objective of this work is to demonstrate the usefulness of fractional factorial design for optimising the crystal quality of chalcogenide van der Waals (vdW) crystals. We statistically analyse the growth parameters of highly c-axis oriented Sb2Te3 crystals and Sb2Te3-GeTe phase change vdW heterostructure superlattices. The statistical significa...

The intense light–matter interaction of plasmonic metasurfaces provides an appealing platform for optical sensing. To date, most metasurface sensors are not spectrally tuned. Moreover, they operate in a total reflection mode, which complicates the optical setup. Transmissive tunable filters are therefore desirable. A mid‐infrared (M‐IR) tunable met...

The objective of this paper is to review the characterisation methods and procedures used to laser switch phase change materials, and then assess their applicability for characterising phase change materials for active photonics devices. Specifically we characterise the performance of our pump-probe laser system and compare it with other ‘static’ a...

By confining phase transitions to the nanoscale interface between two different crystals, interfacial phase change memory heterostructures represent the state of the art for energy efficient data storage. We present the effect of strain engineering on the electrical switching performance of the –GeTe superlattice van der Waals devices. Multiple Ge...

We use an evolutionary algorithm to explore the design space of hexagonal Ge2Sb2Te5; a van der Waals layered two dimensional crystal heterostructure. The Ge2Sb2Te5 structure is more complicated than previously thought. Predominant features include layers of Ge3Sb2Te6 and Ge1Sb2Te4 two dimensional crystals that interact through Te-Te van der Waals b...

Strain engineering is an emerging route for tuning the bandgap, carrier mobility, chemical reactivity and diffusivity of materials. Here we show how strain can be used to control atomic diffusion in van der Waals heterostructures of two-dimensional (2D) crystals. We use strain to increase the diffusivity of Ge and Te atoms that are confined to 5 Å...

Supplementary figures 1-9, Supplementary table 1, Supplementary notes 1-6 and Supplementary references.

Van der Waals heterostructure superlattices of Sb2 Te1 and GeTe are strain engineered to promote switchable atomic disordering, which is confined to the GeTe layer. Careful control of the strain in the structures presents a new degree of freedom to design the properties of functional superlattice structures for data storage and photonics applicatio...

Herein a genetic algorithm for optimising the design of layered 2D heterostructure is proposed. As a proof-of-concept it is applied to Sb$_2$Te$_3$-GeTe phase-change material superlattices, and the resulting lowest energy structure is grown experimentally. The similarity of the computational and experimental structures is verified with the comparis...

The phase-change performance of WxSb3Te material were systemically investigated by in situ resistance-temperature measurement, X-ray diffraction (XRD), Raman scattering, adhesive strength test and transmission electron microscope (TEM) in this paper. Experimental results show that the thermal stability of Sb3Te was increased significantly with W do...

Oxygen-doped germanium telluride phase change materials are proposed for high temperature applications. Up to 8 at.% oxygen is readily incorporated into GeTe, causing an increased crystallisation temperature and activation energy. The rhombohedral structure of the GeTe crystal is preserved in the oxygen doped films. For higher oxygen concentrations...

Phase change material rules the basic scientific issues in research of phase change memory. As an important member in phase change material system, GeTe is competitive for both technological applications and fundamental studies. However, relatively poor thermal stability in amorphous state and serious grain clustering are needed to be overcome for...

Carbon-doped Ge 2Sb2Te5 (GSTC) film has been experimentally studied as a thermal stable material for high temperature applications. The 10-yr data retention temperature is remarkably increased through C doping. Furthermore, GSTC films have better interface properties after annealing at 410 °C for 30 min. The density variation of GSTC film is signif...

Phase-change materials are highly promising for next-generation non-volatile data storage technology. The pronounced effects of C doping on structural and electrical phase-change behaviors of Ge2Sb2Te5 material are investigated at the atomic level by combining experiments and ab initio molecular dynamics. C dopants are found to fundamentally affect...

Investigation of atomic migration behavior in nanoscale phase-change material is very valuable for phase-change memory applications. In this work, Ti0.5Sb2Te3-based phase-change nanobridges were fabricated and mass transport by atomic migration was studied. A 3-D finite-element simulation on the electrothermal field was introduced to describe the e...

Some nonvolatile phase change memory (PCM) cells with 80-nm heating electrodes are found very difficult to RESET at 3 mA, which directly affects the RESET distribution of the PCM. The large crystal grains with hexagonal structure in the active phase change area, discovered by transmission electron microscope, are the major reason. One preprogrammin...

The effects of nitrogen doping on the phase-change performance of Sb-rich Si–Sb–Te materials are systemically investigated, focusing on the chemical state and the role of nitrogen upon crystallization. The tendency of N atoms to bond with Si (SiNx) in the crystalline film is analyzed by X-ray photoelectron spectroscopy. The microstructures of the m...

The crystallization behavior of Al-Sb thin films is investigated for phase change memory application. The crystallization temperature and optical band gap of the amorphous material increase with Al content. The thermal stability and randomness in atomic configuration of the films are enhanced considerably. The shift of Raman modes associated mainly...

A nanoscale phase-change line cell based on focused-ion beam (FIB) technique has been proposed to evaluate the electrical property of the phase-change material. Thanks to the FIB-deposited SiO2 hardmask, only one etching step has been used during the fabrication process of the cell. Reversible phase-change behaviors are observed in the line cells b...

Nitrogen-doped Ge2Sb2Te5 thin film is proposed to present the feasibility for electrical phase-change memory application. The effect of nitrogen doping on the structure is studied by in situ sheet resistance measurement and X-ray diffraction method. The temperature upon crystallization from amorphous to cubic structure is increased by nitrogen addi...

Carbon-doped Ge2Sb2Te5 material is proposed for high-density phase-change memories. The carbon doping effects on electrical and structural properties of Ge2Sb2Te5 are studied by in situ resistance and x-ray diffraction measurements as well as optical spectroscopy. C atoms are found to significantly enhance the thermal stability of amorphous Ge2Sb2T...

The dry etching characteristic of Al1.3Sb3Te film was investigated by using a CF4/Ar gas mixture. The experimental control parameters were gas flow rate into the chamber, CF4/Ar ratio, the O2 addition, the chamber background pressure, and the incident RF power applied to the lower electrode. The total flow rate was 50 sccm and the behavior of etch...

W-Sb-Te phase-change material has been proposed to improve the performance of phase-change memory (PCM). Crystallization temperature, crystalline resistance, and 10-year data retention of Sb2Te increase markedly by W doping. The Wx(Sb2Te)1−x films crystallize quickly into a stable hexagonal phase with W uniformly distributing in the crystal lattice...

The nitrogen doped GeTe (GeTeN) thin film sputtered at a flow rate ratio (N2/Ar) of 0.04, which proves to be outstanding in various properties comparing with GeTe, was investigated for the application of phase-change memory. The GeTeN film crystallizes into a rhombohedral structure at ~372°C with no separated Te or Ge. A relatively wider optical ba...

Sb-rich Sb65Se6Te29 film was investigated for phase change random access memory (PCRAM) application. The crystallization temperature of the Sb65Se6Te29 film is 174 degrees C and the crystalline activation energy is about 2.7 eV. The 10-years' failure temperature for the Sb65Se6Te29 film is about 87 degrees C. sufficient for most consumer applicatio...

Phase-change alloys AlSbTe were investigated for material and electrical properties. A peak crystallization temperature of 236 degrees C was found with 16.7 at.% Al doping Sb3Te (AST-16.7%), which showed a 10-year data retention temperature up to 131 degrees C and a melting point as low as 542 degrees C. The phase-change memory device based on AST-...

With a high crystallization temperature of 211 ºC, Ti10Sb60Te30 phase change material exhibits a data retention of 10-year @ 137 ºC, which is much better than that of usual Ge2Sb2Te5. No other phase is formed in Ti10Sb60Te30 film except hexagonal Sb2Te phase. For Ti10Sb60Te30-based phase change memory cell, as short as 6 ns electric pulse can fulfi...

Al2Sb2Te6 is a pseudobinary material constructed by Sb2Te3 (fast crystallization speed but thermally unstable) and Al2Te3 (thermally stable but without memory switching ability). Al2Sb2Te6 material possesses advantages of these two binary compounds showing good memory switching ability with fast switching speed and good thermal stability. These imp...

Nitrogen incorporated Si0.6Sb2Te3 film shows higher crystallization temperature (similar to 185 degrees C) than Ge2Sb2Te5 (similar to 150 degrees C). No separated Si or Te phase is observed within crystalline nitrogen-doped Si0.6Sb2Te3 material (SST-N). Nitrogen incorporated in Si0.6Sb2Te3 material prefers to bind with Si atoms instead of Sb or Te...

An Sb-rich (48 at.%) Si-Sb-Te phase-change material with moderate Si content (24 at.%) is proposed for phase-change random access memory (PCRAM) applications. The real time amorphous to crystalline transformation was studied by in situ transmission electron microscopy observations and in situ resistance measurements. The results of time-dependent r...

The phase transformation properties of the nitrogen-doped Sb-rich Si–Sb–Te films were investigated in detail. It was found that the addition of N atoms into the Si–Sb–Te films increases the temperature for phase transition from the amorphous phase to a stable hexagonal structure and enhances the sheet resistance of the films following grain refinem...

Crystallization temperatures of the Sb2Te films increase remarkably from 139.4°C to 223.0°C as the N2 flow rates increasing from 0sccm to 1.5sccm. Electrical conduction activation energies for amorphous and crystalline states increase by doping nitrogen. A small amount of nitrogen atoms can locate at interstitial sites in the hexagonal structure, g...

Temperature dependent interfacial adhesion strength between phase change material film and a SiO2 layer was investigated employing Nano Indenter®. Phase change materials of Ge2Sb2Te5 and Si2Sb2Te6 were adopted for a comparative study. The decrease of adhesive strength with an increased annealing temperature can be deduced from the optical micrograp...

The crystallization temperature of GeTe film increases markedly from 187 to 372 °C as a result of 9.81 at.% nitrogen doping, and a rhombohedral–rocksalt phase transition is observed in both GeTe and nitrogen-doped GeTe (GeTeN) films. Up to 105 cycles of endurance for phase change memory (PCM) cells based on GeTeN have been achieved. Extrafine data...

Comparing with Ge2Sb2Te5, Al1.3Sb3Te is proved to be a promising candidate for phase-change memory use because of its higher crystallization temperature (~210 °C), larger crystallization activation energy (3.32 eV), and better data retention ability (124 °C for 10 yr). Furthermore, Al1.3Sb3Te shows fast phase change speed and crystallizes into a un...

The phase change memory with monolayer chalcogenide film (Si18Sb52Te30) is investigated for the feasibility of multilevel data storage. During the annealing of the film, a relatively stable intermediate resistance can be obtained at an appropriate heating rate. The transmission electron microscopy in situ analysis reveals a conversion of crystalliz...

The crystallization behaviors and mechanisms of Si2Sb 2Tex (x = 1, 3 and 6) were investigated. The crystallization process in Si2Sb2Tex films were proved to be layer by layer, which is the same as that of GeSbTe material. Among the three compositions, Si2Sb2Te3 has the best phase stability, the best data retention and a high crystallization speed....

Si-Sb-Te materials including Te-rich Si₂Sb₂Te₆ and Si(x)Sb₂Te₃ with different Si contents have been systemically studied with the aim of finding the most suitable Si-Sb-Te composition for phase change random access memory (PCRAM) use. Si(x)Sb₂Te₃ shows better thermal stability than Ge₂Sb₂Te₅ or Si₂Sb₂Te₆ in that Si(x)Sb₂Te₃ does not have serious Te...

In this study, novel Si2Sb2Te6 phase change material is investigated in detail for the phase change memory application using transmission electron microscopy and X-ray photoelectron spectroscopy. The phenomenon that Te diffuses to the film surface during phase switching and successively evaporates out has been confirmed. The phase change memory cel...

Sb-rich Si-Sb-Te phase change materials with different Si contents were proposed and fabricated, and the role of Si and Sb in the Si-Sb-Te alloys was discussed. The resistance-temperature and retention properties of the Sb-rich Si-Sb-Te alloys were studied. Devices based on the Sb-rich Si-Sb-Te alloys were fabricated by a 0.18 μm CMOS technology an...

A 197Au ion source was used to irradiate a Ge2Sb2Te5-alloy-based phase-change memory (PCM) cell to study the ion-irradiation effect on the properties of the cell. The PCM devices with the tungsten (W) heating electrode of 260 nm diameter were fabricated by 0.18 µm complementary metal-oxide-semiconductor (CMOS) technology. Four different doses (1010...

Phase memory cell based on Si2Sb2Te3 film shows reversible phase change ability between low and high resistance states. Crystalline Si2Sb2Te3 is a nano composite material consisted of amorphous Si and crystalline Sb2Te3, without separated Te phase. The thermally stable Si2Sb2Te3 material has a better data retention ability (10 years at 407 K) than...

Sb-rich Si2Sb2+x Te6 (x=0, 1.4, 10) thin films are proposed to present the feasibility for electronic phase change memory application. The crystallization behavior is improved by adding Sb into the material. The crystallization temperature is about506, 502, and 450K for Si2Sb2Te6, Si2Sb3.4Te6, and Si2Sb12Te6 films, respectively, and the correspondi...

TiO2 and HfO2 with different electrical conductivity were used as buffer layer for phase change memory (PCRAM) cell, where Si-Sb-Te alloy was used as storage medium in the PCRAM cell. The electrical properties of the cells with and without buffer layer were compared through resistance-voltage measurements. It is found that the PCRAM cell with both...

Novel Si2Sb2Te6 phase change material for low-power chalcogenide random access memory is prepared by sputtering of Si2Sb2Te6 alloy target. The motion of Te atom was confirmed from the X-ray diffraction patterns. Phase change random access memory device based on the Si2Sb2Te6 alloy was successfully fabricated. Threshold current (Ith) of the device i...