PRAM (Phase Change Random access memory) is a promising candidate for next generation memory because of many advantages such as high speed, low power, non-volatility, high density and low cost.
The $Ge_2Sb_2Te_5$ ternary alloy is the active material most frequently used in phase-change memory technology due to its fast crystallization. It is known that upon annealing, amorphous layers of this material first undergo the amorphous - to - crystalline (fcc) transition and at higher temperature the crystalline (fcc) - to - crystalline (hcp) transition. Relative to the amorphous state, the crystalline state shows a dramatic increase in free electron density (similar to a metal). This difference in free electron density gives rise to a difference in resistivity. Since phase change is also accompanied by a dramatic resistivity variation, $Ge_2Sb_2Te_5$ film has been proposed for applications in memories.
In this research, we reported the Ag adding effects on the crystallization behavior of $Ge_2Sb_2Te_5$ film.
AgX($Ge_2Sb_2Te_5$)1-X films (where x = 0 ~ 20 at.%) with 100 nm thickness were deposited on natural-oxidized silicon wafer and glass substrates by RF magnetron co-sputtering of $Ge_2Sb_2Te_5$ and Ag targets. The effects of Ag on the structural, thermal and electrical properties of the Agx $(Ge_2Sb_2Te_5)_{1-X}$ films were examined by X-ray diffraction, X-ray reflectivity, AFM, DSC and 4-point probe analysis.
It was found that the crystallization temperature could be increased by doping a small amount of Ag in the $Ge_2Sb_2Te_5$ film. But the surface of Agx $(Ge_2Sb_2Te_5)_{1-X}$ films were rough when annealing temperature was increased and Ag content was more than 9 at.%. There were many protrusions composed of Ag and Te, when $Ag_{0.2}(Ge_2Sb_2Te_5)_{0.8}$ film was annealed at 500℃. We thought such protrusions came from segregation or volatilization and very harmful at surface roughness. So we decided optimal Ag adding content is 6 at.%.
Applying the Kissinger method, we calculated the crystallization activation energy. The crystallization activation energy of the film was increased as the Ag content was increased. The activation energy for the $Ge_2Sb_2Te_5$ is determined to be 2.41467 eV and for the Agx $(Ge_2Sb_2Te_5)_{1-X}$ to be 2.56466 eV.
It is thought that Ag atoms in $Ge_2Sb_2Te_5$ act as an amorphous stabilizer and they make it hard to transfer from amorphous phase to crystalline phase. So crystallization activation energy and crystallization temperature of Ag added $Ge_2Sb_2Te_5$ films may be increased compare with $Ge_2Sb_2Te_5$ film.
본 연구에서는 $Ge_2Sb_2Te_5$ 단일 타겟을 이용하여 RF magnetron sputtering법으로 증착하였으며, 균일하고 밀도 높은 $Ge_2Sb_2Te_5$ 박막을 얻기 위한 조건을 잡기위해 다양한 RF Power 와 Ar 압력 조건을 변수로 하여 실험을 하였다. 이렇게 증착된 박막의 미세구조와 전기적 특성을 비교하였다. 비정질로 증착 된 $Ge_2Sb_2Te_5$ 박막의 결정화를 조사하기 위하여 여러 온도에서 RTA 로 열처리를 한 뒤 XRD 분석을 하였다. Four point probe 로 각 어닐링 온도에 따른 비저항을 비교하였고, SEM 분석을 통해 미세구조 관찰과 두께를 측정하였으며 열처리 전ㆍ후의 두께 변화 및 밀도 변화를 측정하기 위해 XRR을 이용하였다. 위의 방법으로 최적의 $Ge_2Sb_2Te_5$ 박막의 증착 조건을 잡은 뒤, co-sputtering 법으로 $Ge_2Sb_2Te_5$ 에 소량의 Ag을 도핑하여 증착된 박막의 결정성과 전기적 특성을 평가하였다.
