• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.35 no.1
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• pp.44-49
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• 2022

In this paper, we investigated current (I)- and voltage (V)-sweeping properties in a double-stack structure, Ge₂Sb₂Te₅/Ti/W-doped Ge₈Sb₂Te₁₁, a candidate medium for applications to multilevel phase-change memory. 200-nm-thick and W-doped Ge₂Sb₂Te₅ and W-doped Ge₈Sb₂Te₁₁ films were deposited on p-type Si(100) substrate using magnetron sputtering system, and the sheet resistance was measured using 4 point-probe method. The sheet resistance of amorphous-phase W-doped Ge₈Sb₂Te₁₁ film was about 1 order larger than that of Ge₂Sb₂Te₅ film. The I- and V-sweeping properties were measured using sourcemeter, pulse generator, and digital multimeter. The speed of amorphous-to-multilevel crystallization was evaluated from a graph of resistance vs. pulse duration (t) at a fixed applied voltage (12 V). All the double-stack cells exhibited a two-step phase change process with the multilevel memory states of high-middle-low resistance (HR-MR-LR). In particular, the stable MR state is required to guarantee the reliability of the multilevel phase-change memory. For the Ge₂Sb₂Te₅ (150 nm)/Ti (20 nm)/W-Ge₈Sb₂Te₁₁ (50 nm), the phase transformations of HR→MR and MR→LR were observed at t<30ns and t<65ns, respectively. We believe that a high speed and stable multilevel phase-change memory can be optimized by the double-stack structure of proper Ge-Sb-Te films separated by a barrier metal (Ti).

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.37 no.1
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• pp.88-93
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• 2024

To evaluate the possibility as a multi-level memory medium for the Ge₂Sb₂Te₅/TiN/W-doped Ge₂Sb₂Te₅ cell structure, the crystallization rate and stabilization characteristics according to voltage (V)- and current (I)- pulse sweeping were investigated. In the cell structures prepared by a magnetron sputtering system on a p-type Si (100) substrate, the Ge₂Sb₂Te₅ and W-doped Ge₂Sb₂Te₅ thin films were separated by a barrier metal, TiN, and the individual thicknesses were varied, but the total thickness was fixed at 200 nm. All cell structures exhibited relatively stable multi-level states of high-middle-low resistance (HR-MR-LR), which guarantee the reliability of the multilevel phase-change random access memory (PRAM). The amorphousto-multilevel crystallization rate was evaluated from a graph of resistance (R) vs. pulse duration (T) obtained by the nanoscaled pulse sweeping at a fixed applied voltage (12 V). For all structures, the phase-change rates of HR→MR and MR→LR were estimated to be approximately t<20 ns and t<40 ns, respectively, and the states were relatively stable. We believe that the doublestack structure of an appropriate Ge-Sb-Te film separated by barrier metal (TiN) can be optimized for high-speed and stable multilevel PRAM.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.06a
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• pp.134-134
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• 2009

In this work, we report that crystallization speed as well as the electrical and optical properties about the N-doped $Ge_2Sb_2Te_5$ thin films. The 200-nm-thick N-doped $Ge_2Sb_2Te_5$ thin film was deposited on p-type (100) Si and glass substrate by RF reactive sputtering at room temperature. The amorphous-to-crystalline phase transformation of N-doped $Ge_2Sb_2Te_5$ thin films investigated by X-ray diffraction (XRD). Changes in the optical transmittance of as-deposited and annealed films were measured using a UV-VIS-IR spectrophotometer and four-point probe was used to measure the sheet resistance of N-doped $Ge_2Sb_2Te_5$ thin films annealed at different temperature. In addition, the surface morphology and roughness of the films were observed by Atomic Force Microscope (AFM). The crystalline speed of amorphous N-doped $Ge_2Sb_2Te_5$ films were measured by using nano-pulse scanner with 658 nm laser diode (power : 1~17 mW, pulse duration: 10~460 ns). It was found that the crystalline speed of thin films are decreased by adding N and the crystalline temperature is higher. This means that N-dopant in $Ge_2Sb_2Te_5$ thin film plays a role to suppress amorphous-to-crystalline phase transformation.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.4
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• pp.261-266
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• 2010

In this paper, we report the changes of electrical, structural and optical characteristics in $Ge_2Sb_2Te_5$ thin films according to an increase of Si content. The Si-doped $Ge_2Sb_2Te_5$ thin films were prepared by rf-magnetron co-sputtering method. Isothermal annealing was carried out at $N_2$ atmosphere. The crystallization speed (v) of amorphous thin films was evaluated by detecting the reflection response signals using a nano-pulse scanner (wavelength = 658 nm) with illumination power of 1~17 mW and pulse duration of 10~460 ns. Structural phase changes were evaluated by XRD, and the optical transmittance was measured in the wavelength range of 300~3000 nm using UV-vis-NIR spectrophotometer. The sheet resistance (RS) of the thin films was measured using 4 point probe. Conclusivlely, the v-value decreased with an increase of Si content, while the RS-values of both crystalline and amorphous phases were increased. In particular, fcc-to-hexagonal transition was suppressed by the added Si atoms.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.21-22
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• 2008

In the paper, we report several experimental data capable of evaluating the phase transformation characteristics of $Ag_x(Ge_2Sb_2Te_5)_{1-x}$ (x =0, 0.05, 0.1) thin films. The $Ag_x(Ge_2Sb_2Te_5)_{1-x}$ phase change thin films have been prepared by thermal evaporation. The crystallization characteristics of amorphous$Ag_x(Ge_2Sb_2Te_5)_{1-x}$ thin films were investigated by using nano-pulse scanner with 658 nm laser diode (power; 1~17 mW, pulse duration; 10~460 ns) and XRD measurement. It was found that the more Ag is doped, the more crystallization speed was 50 improved. In comparision with $Ge_2Sb_2Te_5$ thin film, the sheet resistance$(R_{amor})$ of the amorphous $Ag_x(Ge_2Sb_2Te_5)_{1-x}$ thin films were found to be lager than that of $Ge_2Sb_2Te_5$ film($R_{amor}$ $\sim10^7\Omega/\square$ and $R_{cryst}$ 10 $\Omega/\square$). That is, the ratio of $R_{amor}/R_{cryst}$ was evaluates to be $\sim10^6$ This is very helpful to writing current reduction of phase-change random acess memory.