Recently, the development of information technology(IT) increases the demands of memory devices. In all kinds of current memory devices, dynamic random access memory(DRAM) is widely used because of its high speed, high density, low cost, simple structure and so on. But, DRAM must be refreshed to maintain the charge that holds its content. The phase change random access memory(PCRAM) is a possible substitute for its non-volatile feature. PRAM is the random access memory that combines the fast read and write access of DRAM with the ability to retain data when power is turned off such as ROM and flash memory. The PRAM has a simple cell structure with high scalability ; it is non-volatile, has a relatively high read/write operation speed(≤50ns) and long cycle life $(>10^{14} operations)$. In this work, we fabricated the $Ge_2Sb_2Te_5$ thin film on $SiO_2/Si$ substrate using dc-magnetron sputtering and investigated the temperature dependence of structural change and electrical properties.
The chemical composition of deposited film is nominally $Ge_2Sb_2Te_5$, examined by induced-coupled plasma(ICP) spectroscopy. As-deposited film is amorphous. After heating to 170℃, the peak position identifies the FCC crystal structure with lattice parameter a≒5.99Å. It has also small RMS value, indicating that the crystallization mechanism is nucleation dominant system. Heating the sample to 260℃ leads to the same diffraction pattern. But interestingly, increasing the annealing temperature leads to the reduction of the lattice constant a≒5.97Å. This result agrees with the reduction of film thickness from the SEM results. Heating to 360℃ leads to transformation from FCC to HCP and increases RMS value, indicating grain growth(TEM). Also, the activation energies calculated from Kissinger's plot are 2.11eV and 2.50eV corresponding to the low and high crystallization temperature(DSC), respectively.
An interesting thing is the observation of evaporation or delamination during the quenching of 360℃ sample to RT. Especially, as recording corresponds to amorphorization through a liquid phase, protective layers are required to prevent fluidization and vaporization during the heating process.
The resistivity of GST films drops abruptly during phase transformation(Amorphous→FCC→HCP). In horizontal I-V curve, $10^4∼10^5$ resistance difference between amorphous and crystalline state is shown. Also, we determine the applying voltage and pulse duration of 25nm, 100nm, 200nm film thickness through vertical I-V characteristics.
DC magnetron sputtering 방법으로 증착한 $Ge_2Sb_2Te_5$ 박막의 결정화 거동 및 특성을 Four point probe, X-ray diffraction(XRD), Differential scanning calorimeter(DSC), Scanning electron microscopy(SEM), Atomic force microscope(AFM)을 이용하여 측정하였다. 온도 증가에 따른 비저항의 급격한 변화 (Four point probe)는 amorphous에서 crystalline(FCC and HCP)로의 상변화에 의한 것임을 XRD를 통해서 확인하였다. Kissing method를 적용하여 FCC로 결정화 될 때 activation energy는 약 2.11eV 였다 (DSC). 또한 상변화에 따른 두께 변화를 SEM 을 통하여 측정하였다. 두께는 amorphous에서 FCC로 amorphous에서 HCP로 변할 때 각각 약 5.5%, 7.5% 씩 감소하였다.
수평 방향의 current-voltage(I-V curve)은 transmission line model(TLM) 방법을 이용하여 측정하였다. As-deposited 박막과 170℃에서 annealing한 박막 사이의 면저항이 차이를 보였다. 또한, Picoammeter와 DC voltage source를 이용하여 시편 두께(200nm, 100nm, 25nm), electrical current의 크기(20V, 15V, 10V, 5V, 3V, 1.5), pulse time(10ms, 1ms, 100㎲, 10㎲, 100ns, 50ns)의 변화에 따른 상전이 전후 수직 방향의 I-V를 측정하여 저항 변화를 분석하였다.
