Study on nitrogen doped Ge2Sb2Te5 films for phase change memory

Abstract

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-