Phase Change Memory Applications Research Articles

Carbon-doped Ge2Sb2Te5 phase change material: A candidate for high-density phase change memory application

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 Ge2Sb2Te5 by increasing the degree of disorder of the amorphous phase. The reversible electrical switching capability of the phase-change memory cells is improved in terms of power consumption with carbon addition. The endurance of ∼2.1 × 104 cycles suggests that C-doped Ge2Sb2Te5 film will be a potential phase-change material for high-density storage application.

Germanium Nitride as a Buffer Layer for Phase Change Memory

The results of adhesion improvement and SET-RESET operation voltage reduction for the GeN buffer layer are presented. It is found that the adhesive strength between the Ge2Sb2Te5(GST) layer and the layer below could be increased at least 20 times, which is beneficial for solving the phase change material peeling issue in the fabrication process of phase change memory (PCM). Meanwhile, the RESET voltage of the PCM cell with a 3-nm-thick GeN buffer layer can be reduced from 3.5 V to 2.2 V. The GeN buffer layer will play an important role in high density and low power consumption PCM applications.

Electrical and heat conduction mechanisms of GeTe alloy for phase change memory application

GeTe alloy has drawn much attention as one of the promising candidates for phase change memory application. In this work, the electrical resistivities and thermal conductivities of GeTe alloy have been determined as functions of temperature by the four-terminal method and hot strip method, respectively. The electrical resistivity increases and the thermal conductivity decreases monotonically with increasing temperature, and thus it is likely that free electron dominates the thermal conduction. The electrical resistivity increases slowly with time during holding at 773 K, and the thermal conductivity decreases corresponding to the change of the electrical resistivity, which suggests that small amount of high temperature phase might exist in the samples.

Improved thermal and electrical properties of Al-doped Ge2Sb2Te5 films for phase-change random access memory

Alx(Ge2Sb2Te5)100−x materials with different Al contents are systemically studied for applications in phase-change random access memory (PRAM) devices. Al-doped Ge2Sb2Te5 (GST) films show better thermal stability than GST because they do not have phase transformation from face-centred cubic (fcc) to hexagonal at high annealing temperatures. As the Al content increases, the resistance in both amorphous and crystalline phases improves and there is four to five orders of magnitude difference in the resistance between the amorphous and crystalline phases, all of which are helpful in achieving a higher On/OFF ratio for PRAM. In addition, the introduction of Al into the GST films can increase the optical band gap that is favourable to decrease the threshold current of PRAM devices. Raman spectra show that a significant change in the local bonding arrangement around Sb atoms has occurred due to the phase transformation from fcc to hexagonal in the GST film but this can be suppressed by Al addition during the crystallization process. All these results confirm that Al-doped GST films are suitable for use in PRAM.

Advantages of Zn1.25Sb2Te3 material for phase change memory

The thermal and electrical properties of Zn1.25Sb2Te3 film have been investigated for phase change memory (PCM) applications. Compared with the conventional Ge2Sb2Te5, Zn1.25Sb2Te3 film exhibits a higher crystallization temperature (∼200°C), greater activation energy (2.71eV), and better data retention of 10 years at 105°C. In addition, higher crystalline resistance and better resistance contrast are helpful to reduce RESET current and achieve a higher On/OFF ratio of PCM, respectively. Raman spectra of crystalline thin films suggests that the local bonding arrangement around Sb atoms changes; Sb2Te3 component is thus mainly responsible for the phase transition in Zn1.25Sb2Te3 alloys.

Synthesis and properties of phase-change Ge-Sb nanoparticles

We report on the synthesis and characterization of Ge15Sb85 phase-change nanoparticles by magnetron plasma sputtering and inert-gas condensation inside an ultra-high vacuum compatible system. Electrical and optical properties of Sb-rich nanoparticles for phase-change memory applications have been examined. The results show that phase-change properties of Ge15Sb85 material still exist for nanoparticles of 8.0 nm size. The amorphous to crystalline transformation proceeds at moderately elevated temperature (∼473 K) which shows that Ge15Sb85 nanoparticles are a good candidate for phase-change memory applications in terms of long data retention time. The observed phase transition in Ge15Sb85 nanoparticles is promising for down scaling the size of phase change solid-state memory devices.

Phase change behaviors of Zn-doped Ge2Sb2Te5 films

Zn-doped Ge2Sb2Te5 phase-change materials have been investigated for phase change memory applications. Zn15.16(Ge2Sb2Te5)84.84 phase change film exhibits a higher crystallization temperature (∼258 °C), wider band gap (∼0.78 eV), better data retention of 10 years at 167.5 °C, higher crystalline resistance, and faster crystallization speed compared with the conventional Ge2Sb2Te5. The proper Zn atom added into Ge2Sb2Te5 serves as a center for suppression of the face-centered-cubic (fcc) phase to hexagonal close-packed (hcp) phase transition, and fcc phase has high thermal stability partially due to the bond recombination among Zn, Sb, and Te atoms.

Compatibility study of Ti and Ge2Sb2Te5 for phase-change memory applications

In this study, the kinetics of titanium–Ge2Sb2Te5 chalcogenide compound formation has been investigated by transmission electron microscopy, MeV helium-ion backscattering spectrometry, X-ray diffraction and secondary ion mass spectrometry. Two sets of samples have been used, one with titanium on a Ge2Sb2Te5 film and the other with a reverse structure. To induce the compound formation, isothermal experiments were carried out at various temperatures. It has been found that titanium induces the dissociation of the chalcogenide layer and the formation of TiTe2 compound. In both the samples, the growth of TiTe2 is kinetically controlled with an activation energy of 0.9±0.1 eV. In excess of titanium atoms with respect to tellurium, the dissociation of what is left in the original chalcogenide proceeds by the formation of TiGe x compound, leaving antimony and eventually germanium grains.

Effect of selenium addition on the GeTe phase change memory alloys

Compositional dependent investigations of the bulk GeTe chalcogenides alloys added with different selenium concentrations are carried out by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), electron probe micro-analyzer (EPMA) and differential scanning calorimetry (DSC). The measurements reveal that GeTe crystals are predominant in alloys up to 0.20at.% of Se content indicating interstitial occupancy of Se in the Ge vacancies. Raman modes in the GeTe alloys changes to GeSe modes with the addition of Se. Amorphousness in the alloy increases with increase of Se and 0.50at.% Se alloy forms a homogeneous amorphous phase with a mixture of Ge–Se and Te–Se bonds. Structural changes are explained with the help of bond theory of solids. Crystallization temperature is found to be increasing with increase of Se, which will enable the amorphous stability. For the optimum 0.50at.% Se alloy, the melting temperature has reduced which will reduce the RESET current requirement for the phase change memory applications.

Te-based chalcogenide films with high thermal stability for phase change memory

This study reports on the synthesis of tellurium-based chalcogenide films that have high thermal stability for phase change memory application. Several Te-based chalcogenide alloys of In-Bi-Te, Ag-Bi-Te, In-Sb-Te, Sn-Sb-Te, Zn-Ge-Te, and Ga-Ge-Te are reported. Their thermal, optical, and electrical properties are investigated. The results show that Bi-Te-based films have a higher crystallization temperature and greater activation energy compared with the other Sb-Te-based and Ge-Te-based films. Especially, In2.8Bi36.6Te60.6 film exhibits high crystallization temperature (252 °C) and great activation energy (5.16 eV), showing much improved amorphous thermal stability. A relatively wider optical band gap (0.674 eV) of thermal annealed In2.8Bi36.6Te60.6 film is obtained. In addition, it also has a higher amorphous/crystalline resistance ratio of about 105, implying that current consumption could be low in the phase-change memory operation.

Deposition and Characterization of CVD-Grown Ge-Sb Thin Film Device for Phase-Change Memory Application

Germanium antimony (Ge-Sb) thin films with tuneable compositions have been fabricated on SiO2/Si, borosilicate glass, and quartz glass substrates by chemical vapour deposition (CVD). Deposition takes place at atmospheric pressure using metal chloride precursors at reaction temperatures between 750 and 875°C. The compositions and structures of these thin films have been characterized by micro-Raman, scanning electron microscope (SEM) with energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) techniques. A prototype Ge-Sb thin film phase-change memory device has been fabricated and reversible threshold and phase-change switching demonstrated electrically, with a threshold voltage of 2.2–2.5 V. These CVD-grown Ge-Sb films show promise for applications such as phase-change memory and optical, electronic, and plasmonic switching.

Phase change and optical band gap behavior of Ge–Te–Ga thin films prepared by thermal evaporation

The amorphous Ge11.4Te86.4Ga2.2 chalcogenide thin films were prepared by thermal evaporation onto chemically cleaned glass substrates. Properties measurements include X-ray diffraction (XRD), Scanning electron microscopy (SEM), Differential scanning calorimetry (DSC), Four-point probe and VIS–NIR transmission spectra. The allowed indirect transition optical band gap and activation energy of samples were calculated according to the classical Tauc equation and Kissinger's equation, respectively. The results show that there is an amorphous-to-crystalline phase transition of Ge11.4Te86.4Ga2.2 thin film. The investigated film has high crystallization temperature (∼200 °C) and activation energy (2.48 eV), indicating the film has good amorphous stability. The sheet resistance of the crystalline state is ∼10 Ω/and the amorphous/crystalline resistance ratio is about 105. Besides, a wide optical band gap (0.653 eV) of Ge11.4Te86.4Ga2.2 is obtained, indicating that the material possesses a low threshold current from amorphous-to-crystalline state for phase-change memory application.

Investigation on Sb-rich Sb–Se–Te phase-change material for phase change memory application

Sb-rich Sb65Se6Te29 film was investigated for phase change random access memory (PCRAM) application. The crystallization temperature of the Sb65Se6Te29 film is 174°C and the crystalline activation energy is about 2.7eV. The 10-years' failure temperature for the Sb65Se6Te29 film is about 87°C, sufficient for most consumer applications. The results of UV/visible/NIR spectrophotometer measurements show that the optical gap of the Sb65Se6Te29 film decreases as it transforms from amorphous phase to crystalline phase. Compared with the Ge2Sb2Te5 based PCRAM cell, the Sb65Se6Te29 based PCRAM cell has the advantages of lower threshold voltage and larger resistance contrast. Furthermore, as short as 10ns electrical pulse can achieve Reset operation with a Reset voltage of 2.9V.

Improved thermal and electrical properties of nitrogen-doped Ge-rich Ge 3Sb 2Te 5 for phase-change memory application

The thermal and electrical-property changes of 2.02 at.% nitrogen-doped Ge-rich Ge 3Sb 2Te 5 (NGST) were investigated. The crystallization temperature was 180 °C. The corresponding activation energy was 2.96 eV. The maximum temperature for a 10 year data lifetime was estimated to be 97 °C. NGST films showed retarded crystallization to a higher temperature, and higher resistivity in the crystalline state, compared to pure Ge 2Sb 2Te 5 (GST) films. An NGST-based cell showed relatively low power consumption and better contrast resistance than obtained with GST. The addition of N and Ge to the Ge 2Sb 2Te 5 film improves both the thermal stability and electrical properties. This work demonstrated the great promise of NGST composite films for application in phase-change memory (PCM).

Crystallization study of “melt quenched” amorphous GeTe by transmission electron microscopy for phase change memory applications

In situ transmission electron microscopy (TEM) observations were performed for a better understanding of the “melt quenched” GeTe crystallization mechanism. The evolution of the crystallite morphology observed during annealing shows a growth-dominated crystallization behavior. Scanning transmission electron microscopy—electron dispersive x-ray spectroscopy and high resolution electron microscopy experiments were also performed on cycled GeTe devices, showing that void formation is responsible for the cell failure after 107 cycles.

Sb-rich Si–Sb–Te Phase-Change Material for Phase-Change Random Access Memory Applications

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 resistance measurements show that the Si24Sb48Te28 phase-change material has data retention of 10 years at about 382 K, suggesting a more stable amorphous state than the usual Ge2Sb2Te5 (GST) phase-change material. The reversible set-reset ability of the PCRAM cell based on the Si24Sb48Te28 phase-change material is much better than that of the device employing GST. The programming cycles can reach 2.2 × 104 under a set pulse of 1.5 V/1000 ns with a 30-ns falling edge and a reset pulse of 3.5 V/400 ns, whereas the resistance contrast retains a value of as large as two orders of magnitude.

Advantages of GeTeN material for phase change memory applications

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 band gap of GeTeN results in a lower threshold current. GeTeN material also has a better surface contact with surrounding materials than that of GeTe according to surface roughness and adhesive strength results, which further leads to more stable SET and RESET states for phase-change memory device.

Instability of nitrogen doped Sb2Te3 for phase change memory application

By means of experimental methods and ab initio total energy calculations, we have studied the stability and properties of nitrogen doped Sb2Te3 (NST). The NST film displayed a higher crystallization temperature and sheet resistance than undoped Sb2Te3 (ST) film. Nevertheless, the sheet resistance of the crystalline NST film unexpectedly increased as the temperature increased when the temperature was above 260 °C. The X-ray photoelectron spectroscopy (XPS) showed that the nitrogen concentration and the Sb-N bonds were decreasing as the annealing temperature increased, and no nitrogen existed in the NST when annealed at 300 °C for 5 min. Our theoretical calculations showed that the incorporation of nitrogen into crystalline Sb2Te3 was not energetically favorable, and the nitrogen atoms preferred forming chemical bonds with Sb atoms to Te atoms.

Thermodynamic, kinetic and electrical switching studies on Si 15Te 85− xIn x glasses: Observation of Boolchand intermediate phase

An interesting topic for quite some time is an intermediate phase observed in chalcogenide glasses, which is related to network connectivity and rigidity. This phenomenon is exhibited by Si–Te–In glasses also. It has been addressed here by carrying out detailed thermal investigations by using Alternating Differential Scanning Calorimetry technique. An effort has also been made to determine the stability of these glasses using the data obtained from different thermodynamic quantities and crystallization kinetics of these glasses. Electrical switching behavior by recording I–V characteristics and variation of switching voltages with indium composition have been studied in these glasses for phase change memory applications.

Investigation of phase transition behaviors of the nitrogen-doped Sb-rich Si–Sb–Te films for phase-change memory

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 refinement. The surface topography of the crystalline films was improved by doping nitrogen atoms. The activation energy for crystallization of the films was increased from 1.84 to 2.89 eV with the increased nitrogen content from 0 to 21 at.%, which promises an improved thermal stability. A prolonged data lifetime up to 10 years at 149.4 °C was realized. From the device performance point of view, the N-doped Si–Sb–Te film with a moderate nitrogen content was preferable for the phase-change memory applications due to its advantage of higher reliability.