Laser Synthesis of Sn-Ge-Sb-Te Phase Change Materials
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0918-H08-03
Laser Synthesis of Sn-Ge-Sb-Te Phase Change Materials W.D Song, L.P. Shi, X.S. Miao, and T.C. Chong Data Storage Institute, DSI Building 5, Engineering Drive 1, Singapore, 117608, Singapore ABSTRACT Sn-doped Ge-Sb-Te films on Si substrates were prepared by laser synthesis at the different growth temperatures. The compositions of Sn-doped Ge-Sb-Te films were analysized by X-ray photoelectron spectroscopy. The crystal structures of Sn-doped Ge-Sb-Te thin films with a Sn content of less than 30 at% are close to Ge2Sb2Te5. The crystallization behaviors of Sn-doped Ge-Sb-Te films were analyzed by self-developed phase change temperature tester. The crystallization temperatures of Sn4.3Ge32.9Sb28.1Te34.6, Sn9.8Ge20.3Sb28.4Te41.5 and o o Sn18.8Ge19.5Sb25.3Te36.4 are 141.5, 137.3 and 135.0 C at a ramp rate of 20 C/min, respectively. Doping Sn into Ge-Sb-Te will result in a decrease of crystallization temperature. It was also found that crystallization temperature increases with an increase of ramp rate for a phase change material. The crystallization speed of Sn-doped Ge-Sb-Te is estimated to be faster than Ge2Sb2Te5. INTRODUCTION Phase change material has a strong dependence of optical and electric properties upon its structure. Due to a difference in the reflectivity between the states, and a fast phase change induced by a pulsed laser, phase change materials can be used for rewriteable optical data storage. With a high resistance ratio between the states and a reversible phase change induced by an electric current, phase change materials are also very favorable for nonvolatile memory. Typical phase change materials for rewriteable optical data storage and nonvolatile memory are Ge-Sb-Te (GST) or Ag-In-Sb-Te alloys. In order to improve phase change material properties, an element or compound doped into phase change material was reported [1, 2]. In this paper, we report Sn-doped Ge-Sb-Te phase change material and its properties.
EXPERIMENTAL DETAILS The schematic diagram of laser deposition system is shown in Fig. 1. A KrF excimer laser was split into two beams and focused onto two rotating targets, Ge2Sb2Te5 and Sn, with two focus lenses. Two plumes were produced simultaneously. The laser fluence on each target is between 0.5 and 6 J/cm2. The targets were mounted at 45° with respect to the laser beams. Facing the targets at a distance of 4 to 6 cm, Si substrates were mounted on a two-inch stainless steel holder by silver paste. A background pressure of 2×10-6 Torr was achieved with a turbomolecular pump. The growth temperature was between room temperature and 300 °C. The synthesized materials on the substrates were typically grown for 12000 pulses at a repetition rate of 10 Hz. After laser synthesis, the materials were cooled to room temperature. The crystal structures of thin films were characterized by x-ray diffractometry (XRD). The composition was
studied by using X-ray photoelectron spectroscopy (XPS) while crystallized behaviors were analyzed by a phase change temperature tester. Laser
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