Nitrogen-doped Ge2Sb2Te5 Film Research Articles

Investigation of the evolution of free carriers during femtosecond laser-induced ultrafast amorphization in nitrogen doped Ge2Sb2Te5 films

The femtosecond laser-driven ultrafast evolution of free carriers during the phase-change process of N-doped Ge2Sb2Te5 (N-GST) film was studied in this paper. And mechanism of initial absorption peak was calculated by two-temperature model. We found that the formation of Ge-N weaken the initial optical absorbance and decreased the concentration of free carriers, then improved the temperature of crystallization in N-GST. The doping(N) can reduce the grain size because of the easing of optical energy absorbance. At last, we inferred that, if the incident laser fluence is enough for phase transformation point, the transformation velocity of N-doped GST is faster than GST.

The improvement of nitrogen doped Ge2Sb2Te5 on the phase change memory resistance distributions

In this paper, the performance of Ge2Sb2Te5 (GST) and nitrogen doped Ge2Sb2Te5 (NGST) have been investigated based on standard 40nm complementary metal–oxide–semiconductor (CMOS) technology. It shows a larger margin (∼2 orders) between SET and RESET resistance distributions in NGST cells. TEM and nano scratch test were implemented to find out that NGST film shows good contact with substrate which effectively increase the convergence of device resistance distributions. NGST cells embrace a higher RESET resistance and this is benefit for widening the sensing margin. Furthermore, TEM and three-dimension finite element model are developed to explain the reason for higher RESET resistance distribution of NGST cells.

Long-Term Structural Instabilities in Undoped and Nitrogen-Doped Ge2Sb2Te5 Films

We investigated the compositional, microstructural, and electrical properties of undoped and nitrogen-doped Ge2Sb2Te5 films subjected to long-term thermal annealing under air atmosphere. Considering the absence of chemical and structural changes, the sheet resistances of samples annealed at 200°C may potentially be related to changes in the lattice parameters. The disappearance of Ge–N bonds and decrease of Ge and N concentrations in samples treated at 300°C were found to depend on the cubic to hexagonal phase transition.

Effects of Nitrogen on Endurance of N-doped Ge2Sb2Te5 Films During Laser-Induced Reversible Switching

Void formation and compositional change in nitrogen doped Ge2Sb2Te5 (NGST) films during repeated phase transformations were examined. Stable reversible switching behavior between the crystalline and melt-quenched phase of NGST films containing 5% nitrogen was examined using laser irradiation on a nanosecond time scale. Transmission electron micrographs indicated that with repeated phase transformation, void formation and subsequent coalescing and movement to the top and bottom interfaces with SiO2 layers in NGST occurred in both amorphous and crystalline states. Moreover, void formation at the grain boundaries of crystalline NGST was clearly observed. Compositional change around voids also occurred, which was confirmed using energy dispersive spectroscopy. Void formation and compositional change during repeated reversible switching can cause serious issues related to the reliability of NGST materials during actual phase-change random access memory device operations.

Fermi-level pinning and charge neutrality level in nitrogen-doped Ge2Sb2Te5: Characterization and application in phase change memory devices

We study the dependence of the hole barrier height at the metal/α-Ge2Sb2Te5 interface as a function of nitrogen doping in Ge2Sb2Te5 as well as the vacuum work function of the metal. Materials parameters such as the band gap, dielectric constant, and electron affinity values of these nitrogen-doped films were also determined. All Ge2Sb2Te5 films studied in this work are amorphous. Following further physical analysis, the effective work functions of metals on nitrogen-doped Ge2Sb2Te5 films were obtained and found to differ from that of their values in vacuum. This led to the extraction of the slope parameter Sx and charge neutrality level ΦCNL which characterizes Ge2Sb2Te5. Appreciable metal Fermi-level pinning to the charge neutrality level of Ge2Sb2Te5, which is located near the valence band edge, was observed. We then demonstrate application of the extracted parameters to obtain the band alignment of α-Ge2Sb2Te5 with various other materials such as SiO2, giving good agreement with experimental results.

Phase-change characteristics of nitrogen-doped Ge2Sb2Te5 films during annealing process

The microstructures and electrical properties of nitrogen-doped Ge2Sb2Te5 thermally annealed in an N2 atmosphere were investigated. The 5.4% nitrogen-doped Ge2Sb2Te5 thin films showed discontinuous changes in resistance with annealing temperature, and corresponding changes in crystal structure. The phase transitions went through three states, amorphous→cubic→hexagonal, after annealing at 200 and 375 °C. No chemical compositional change occurred after 400 °C annealing. But the 20.1% nitrogen-doped Ge2Sb2Te5 thin films showed continuous changes in microstructure and resistance. According to XRD and TEM analyses, the hexagonal-type Ge–Sb–Te phase should be directly crystallized from the amorphous phase. Also, the SIMS and XPS spectra indicate that the oxygen in-diffusion and Sb and Te out-diffusion should have occurred simultaneously.

Investigation of crystallization behaviors of nitrogen-doped Ge2Sb2Te5 films by thermomechanical characteristics

It has been demonstrated that the crystallization behaviors of undoped and N-doped Ge2Sb2Te5 (GST) films can be evaluated by studying the thermomechanical behavior of the films. The crystallization temperatures (Tc) for undoped and 15 at. % N-doped GST films were determined to be about 150 and 250 °C, respectively. The activation energies for crystallization (Ea) were calculated to be 2.30, 3.29, and 3.72 eV for undoped, 10 at. % N-doped, and 15 at. % N-doped GST, respectively, by the Kissinger plot. For 15 at. % N-doped film, both the glass transition temperature (Tg) and Tc were determined to be about 150 °C, as determined by the thermomechanical characteristics.

Effect of abrasive material properties on polishing rate selectivity of nitrogen-doped Ge2Sb2Te5to SiO2film in chemical mechanical polishing

We investigated the polishing rate and selectivity of nitrogen-doped Ge2Sb2Te5(NGST) to SiO2film for different abrasive materials (colloidal silica, fumed silica, and ceria abrasives). They both were strongly dependant on abrasive material properties. The polishing rate of nitrogen-doped NGST decreased in the order ceria, fumed silica, and colloidal silica abrasives, which was determined by abrasive material properties, such as abrasive hardness, crystal structure, and primary and secondary abrasive sizes. In addition, the polishing rate slope of NGST film was not significantly different for different abrasive materials, indicating that the polishing of NGST film is mechanical dominant polishing. In contrast, the polishing rate slope of SiO2film decreased in the order ceria, fumed silica, and colloidal silica abrasives, indicating that the polishing of SiO2film is chemical dominant polishing. Furthermore, the difference in polishing rate slopes between NGST and SiO2film gave a polishing rate selectivity of NGST to SiO2film higher than 100:1 with colloidal silica abrasive.

Multiple-state storage capability of nitrogen-doped Ge2Sb2Te5 film for phase change memory

Nitrogen-doped Ge2Sb2Te5 (N-GST) film for phase change memory was prepared by reactive sputtering. Testing results show that doped nitrogen combines with Ge to form GeN, which not only restrains crystal grain growth but also increases the crystallization temperature and phase transformation temperature of Ge2Sb2Te5 (GST). Phase change memory (PCM) with N-GST can realize three-state storage in one PCM cell by using the resistivity difference between the amorphous state, the face centered cubic phase and the hexagonal phase of GST.

Study on nitrogen doped Ge2Sb2Te5 films for phase change memory

The demand for non-volatile memories has increased as the market for mobile instruments, such as Personal Digital Assistants (PDAs), cellular phones, electronic digital cameras, and others has expanded. With its many advantages over other existing memories, Phase Change Memory (PCM) is one of the candidates for unified memory technology [1]. PCM is a technology that uses fast and reversible phase transition of materials between crystalline and amorphous. Compositions of the GeSbTe system are used for the phase change material. The phase of GeSbTe can be changed by the electrical currents. When using a lower electrical current, high resistance in the crystalline state is necessary to change the material from a crystalline state to an amorphous state. Thus, the nitrogen-doping method has been employed in an attempt to solve this problem by increasing the electrical resistance of the phase change material [2]. In the present study, we have investigated the crystalline structure and sheet resistance of nitrogen-doped Ge2Sb2Te5 films. Ge2Sb2Te5 films and nitrogen-doped GeSbTe films were deposited on p-type (100) Si (MEMC-Korea, KOREA) and SiO2 substrates by DC magnetron sputtering using a composite target, Ge2Sb2Te5, at room temperature. The Si wafers were cleaned in trichloroethylene (TCE), acetone, and methyl alcohol and rinsed in deionized water at room temperature for 3 min, respectively. Then, to remove the native oxide, the wafers were treated with a 10% hydrofluoric (HF) solution. The SiO2 substrates were cleaned in the same way as Si except for HF dipping. The background pressure was 4.0×10−6 torr, and the process pressure was 7.0 × 10−3 torr. A system power of 14 W was used for sputtering, and the deposition rate was 50 A/min. Nitrogen was doped in the Ge2Sb2Te5 films during sputtering by the introduction of N2/Ar mixed gases. The gas flow rate was 10 sccm for N2 and 30 sccm for Ar. The deposited films were annealed in a vacuum, ∼10−5 torr, for 20 min. The heat treatments were carried out at specific temperatures between 150 and 380. We used X-ray diffraction (D/MAX-2500H, Rigaku, Japan) to examine the structure of the crystallized films. Atomic force microscopy (AFM) and scan-

Nitrogen-doped Ge2Sb2Te5 films for nonvolatile memory

Nitrogen-doped Ge2Sb2Te5 (GST) films for nonvolatile memories were prepared by reactive sputtering with a GST alloy target. Doped nitrogen content was determined by using x-ray photoelectron spectroscopy (XPS). The crystallization behavior of the films was investigated by analyzing x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Results show that nitrogen doping increases crystallization temperature, crystallization-activation energy, and phase transformation temperature from fcc to hexagonal (hex) structure. Doped nitrogen probably exists in the grain vacancies or grain boundaries and suppresses grain growth. The electrical properties of the films were studied by analyzing the optical band gap and the dependence of the resistivity on the annealing temperature. The optical band gap of the nitrogen-doped GST film is slightly larger than that of the pure GST film. Energy band theory is used to analyze the effect of doped nitrogen on electrical properties of GST films. Studies reveal that nitrogen doping increases resistivity and produces three relatively stable resistivity states in the plot of resistivity versus annealing temperature, which makes GST-based multilevel storage possible. Current-voltage (I-V) characteristics of the devices show that nitrogen doping increases the memory’s dynamic resistance, which reduces writing current from milliampere to microampere.

Effect of N-implantation on the structural and electrical characteristics of Ge2Sb2Te5 phase change film

In this paper, the effect of nitrogen implantation on the structure and sheet resistance of Ge2Sb2Te5 film was studied in details. It is indicated that the phase transition of Ge2Sb2Te5 film from face-centered-cubic (FCC) to hexagonal (HEX) structure was suppressed by nitrogen implantation. From the results of X-ray diffraction, Raman spectra, and X-ray photoelectron spectroscopy measurements, more Te atom bonds to Sb–Sb bond and N atom bonds to Ge atom forming Ge3N4 in the nitrogen-doped Ge2Sb2Te5 film with the increase of implant dose. The sheet resistance of Ge2Sb2Te5–N film decreases with the increase of implant dose at low annealing temperature because the unit cell of crystalline Ge2Sb2Te5 film with FCC structure was distorted due to nitrogen atom occupying the tetrahedral interstitial site and the defects in film increased, which results in larger number of current carrier. On the contrary, the sheet resistance of Ge2Sb2Te5–N film increases slightly with the increase of implant dose at high annealing temperature. The two major reasons may be that the grain refinement, which results in being scattered of current carrier by grain boundary, and the suppression of phase transition from FCC to HEX structure.