Change Random Access Memory Applications Research Articles

SixSb2Te materials with stable phase for phase change random access memory applications

The physical and electrical properties of SixSb2Te system materials with various Si contents have been systemically studied with the aim of finding the most suitable composition for the phase change random access memory (PCRAM) applications. SixSb2Te shows better thermal stability than Ge2Sb2Te5 due to no Te separation under high annealing temperatures. The increase of Si content can enhance the data retention ability of SixSb2Te materials. When the value of x is larger than 0.44, the 10-year data retention temperature for SixSb2Te will exceed 110 °C, which meets the long-term data retention requirement. Furthermore, Si-rich SixSb2Te materials exhibit the improvement on thickness change after annealing compared with Ge2Sb2Te5. In addition, the PCRAM devices based on SixSb2Te (x = 0.31, 0.44) were fabricated and the electrical operations were carried out. Both of them show the outstanding performances with long-term operations.

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.

Thermal Stability of SiO2Doped Ge2Sb2Te5for Application in Phase Change Random Access Memory

Thermal stability of Ge<SUB>2</SUB>Sb<SUB>2</SUB>Te<SUB>5</SUB> (GST) and SiO<SUB>2</SUB> doped GST (SGST) films for phase change random access memory applications was investigated by observing the change of surface roughness, layer density and composition of both films after isothermal annealing. After both GST and SGST films were annealed at 325°C for 20 min, root mean square (RMS) surface roughness of GST was increased from 1.9 to 35.9 nm but that of SGST was almost unchanged. Layer density of GST also steeply decreased from 72.48 to 68.98 g/cm<SUP>2</SUP> and composition was largely varied from Ge : Sb : Te = 22.3 : 22.1 : 55.6 to 24.2 : 22.7 : 53.1, while those of SGST were almost unchanged. It was confirmed that the addition of a small amount of SiO<SUB>2</SUB> into GST film restricted the deterioration of physical and chemical properties of GST film, resulting in the better thermal stability after isothermal annealing.

Study on Interface Adhesion between Phase Change Material Film and SiO2 Layer by Nanoscratch Test

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 micrographs of the two cases. Critical load obtained from the nanoscratch tests was introduced to quantitative characterize the interfacial adhesion strength of the samples. Scanning electron microscope and energy dispersive spectrometer were utilized to further analysis the adhesive properties of the interfaces. Results show that Si2Sb2Te6 has better adhesive performance than Ge2Sb2Te5 with SiO2 due to its higher activation energy and weaker thickness variation upon crystallization as well as its smaller crystal grain size in the crystalline state. Considering the adhesive strength with SiO2, Si2Sb2Te6 is a preferable candidate over Ge2Sb2Te5 for future high density phase change random access memory application.

Study on Interface Adhesion between Phase Change Material Film and SiO2Layer by Nanoscratch Test

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 micrographs of the two cases. Critical load obtained from the nanoscratch tests was introduced to quantitative characterize the interfacial adhesion strength of the samples. Scanning electron microscope and energy dispersive spectrometer were utilized to further analysis the adhesive properties of the interfaces. Results show that Si2Sb2Te6 has better adhesive performance than Ge2Sb2Te5 with SiO2 due to its higher activation energy and weaker thickness variation upon crystallization as well as its smaller crystal grain size in the crystalline state. Considering the adhesive strength with SiO2, Si2Sb2Te6 is a preferable candidate over Ge2Sb2Te5 for future high density phase change random access memory application.

SiO2 doped Ge2Sb2Te5 thin films with high thermal efficiency for applications in phase change random accessmemory

This study examined the various physical, structural and electrical properties ofSiO2 dopedGe2Sb2Te5 (SGST) films for phase change random access memory applications. Interestingly, SGSThad a layered structure (LS) resulting from the inhomogeneous distribution ofSiO2 after annealing. The physical parameters able to affect the reset current of phase change memory(Ires) were predicted from the Joule heating and heat conservation equations. WhenSiO2 was doped into GST, thermal conductivity largely decreased by ∼ 55%. Theinfluence of SiO2-doping on Ires was examined using the test phase change memory cell.Ires wasreduced by ∼ 45%. An electro-thermal simulation showed that the reduced thermal conductivitycontributes to the improvement of cell efficiency as well as the reduction ofIres, while the increased dynamic resistance contributes only to the latter. The formation andpresence of the LS thermal conductivity in the set state test cell after repeated switchingwas confirmed.

Low electric field, easily reversible electrical set and reset processes in a Ge15Te83Si2 glass for phase change memory applications

We report here an easily reversible set–reset process in a new Ge15Te83Si2 glass that could be a promising candidate for phase change random access memory applications. The I-V characteristics of the studied sample show a comparatively low threshold electric field (Eth) of 7.3 kV/cm. Distinct differences in the type of switching behavior are achieved by means of controlling the on state current. It enables the observation of a threshold type for less than 0.7 mA beyond memory type (set) switching. The set and reset processes have been achieved with a similar magnitude of 1 mA, and with a triangular current pulse for the set process and a short duration rectangular pulse of 10 msec width for the reset operation. Further, a self-resetting effect is seen in this material upon excitation with a saw-tooth/square pulse, and their response of leading and trailing edges are discussed. About 6.5 × 104 set–reset cycles have been undertaken without any damage to the device.

Investigation of Sb-rich Si2Sb2+x Te6 material for phase change random access memory application

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 about 506, 502, and 450 K for Si2Sb2Te6, Si2Sb3.4Te6, and Si2Sb12Te6 films, respectively, and the corresponding activation energy is in the range from 2.70 to 1.69 eV, which is expected for a low-power and high-speed SET operation. In addition, maximum temperature for a 10 year data lifetime is estimated to be 133, 127, and 98°C, respectively. The memory devices are successfully fabricated employing these films, promising that the stability of the low-resistance crystalline state is improved by adding Sb into the stoichiometric Si2Sb2Te6 material, and the reversibility of the device is also realized for the Si2Sb12Te6-based cell.

Formation of Ge[sub 2]Sb[sub 2]Te[sub 5]–TiO[sub x] Nanostructures for Phase Change Random Access Memory Applications

Amorphous Ge 2 Sb 2 Te 5 clusters with a size of 20 nm, self-enclosed by a thin layer of TiO x , were obtained by cosputtering Ge 2 Sb 2 Te 5 and TiO 2 targets at room temperature with the aim of reducing the reset current for phase change random access memory applications. Eutectic decomposition during the deposition caused a phase separation of Ge 2 Sb 2 Te 5 and TiO x . The temperature-dependent resistance change results showed that the activation energy for crystallization increased from 2.44 ± 0.76 to 3.84 ± 1.43 eV in the Ge 2 Sb 2 Te 5 film. The set resistance can be tuned within an acceptable range, and the reliability of this microstructure during repetitive laser melt-quenching cycles was tested.

Ge 2Sb 2Te 5 and PbZr 0.30Ti 0.70O 3 composite films for application in phase change random access memory

The improvement in the phase change characteristics of Ge 2Sb 2Te 5 (GST) films for phase change random access memory (PCM) applications was investigated by doping the GST films with PbZr 0.30Ti 0.70O 3 (PZT) using cosputtering at room temperature. The doped films showed a retarded crystallization to a higher temperature and higher resistivity in the crystalline state compared to pure GST films. Phase separation has been observed in annealed GST-PZT films and the segregated domains exhibited relatively uniform size. The reduced reset voltage of GST-PZT based cell was due to the reduced programming volume by incorporating PZT into GST. This work clearly reveals the highly promising potential of GST-PZT composite films for application in PCM.

The reason for the increased threshold switching voltage of SiO2 doped Ge2Sb2Te5 thin films for phase change random access memory

This study examined the threshold switching voltage (VT) of 150 nm thick SiO2 doped Ge2Sb2Te5 (SGST) films for phase change random access memory applications. The VT of the SGST films increased from ∼0.9 V (for GST) to ∼1.5 V with increasing SiO2 content. The optical band gap and Urbach edge of the SGST films were similar regardless of the SiO2 concentration. The dielectric constant decreased by ∼37% and the electrical resistivity increased by ∼19%. The increase in VT of SGST films is associated with an effective increase in electric field and the decreased generation rate caused by impact ionization.

Dependency of threshold switching on density of localized states of Ge2Sb2Te5 thin films for phase change random access memory

The threshold switching of Ge2Sb2Te5 (GST) films for phase change random access memory applications was investigated by measuring the variation in the threshold voltage (VT) with the crystallinity of the GST films and photon energy absorption spectra. As the GST film was amorphized, VT increased to approximately 1 V and its electrical resistance increased. The optical band gap and Urbach edge of the GST increased from 0.66 to 0.97 eV and from 12 to 65 meV, respectively, upon its amorphization. It was experimentally confirmed that the threshold switching is associated with the density of localized states of the GST.

Phase transformation behaviors of SiO2 doped Ge2Sb2Te5 films for application in phase change random access memory

The improvement in the phase change characteristics of Ge2Sb2Te5 (GST) films for phase change random access memory applications was investigated by doping the GST films with SiO2 using cosputtering at room temperature. As the sputtering power of SiO2 increased from 0to150W, the activation energy for crystallization increased from 2.1±0.2to3.1±0.15eV. SiO2 inhibited the crystallization of the amorphous GST films, which improved the long term stability of the metastable amorphous phase. The melting point decreased with increasing concentration of SiO2, which reduced the power consumption as well as the reset current.