Effects of mass, energy and temperature on amorphization in ion implanted Ge 2 Sb 2 Te 5 thin films
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0997-I11-09
Effects of mass, energy and temperature on amorphization in ion implanted Ge2Sb2Te5 thin films Riccardo De Bastiani1, Alberto Maria Piro1, Salvatore Lombardo2, Maria Grazia Grimaldi1, and Emanuele Rimini2,3 1 MATIS CNR-INFM, Dipartimento di Fisica ed Astronomia, Universit‡ di Catania, 64 via S.Sofia, Catania, 95123, Italy 2 IMM-CNR, Catania, Italy 3 Dipartimento di Fisica ed Astronomia, Universit‡ di Catania, 64 via S.Sofia, Catania, 95123, Italy ABSTRACT The paper reports on the amorphization kinetics of chalcogenides ternary alloy induced by Ar+ or Sb+ ion irradiation. The reflectivity data, obtained ìin situî during irradiation, allow a description of the amorphization process in terms of a threshold fluence. The results demonstrate that amorphization is caused by the elastic collisions of the projectiles with target nuclei. The influence of the ion mass and energy, of the target temperature in the amorphization is also reported. INTRODUCTION Chalcogenides ternary alloy shows reversible phase transition between crystalline and amorphous phase induced by laser or electrical pulses. These switching properties are, nowadays, deeply investigated since they allow fast and stable data storing applications (PCRAM, Phase Change Random Access Memory). A considerable number of studies have focused on the crystal-amorphous (c-a) transition process of phase change materials [1,2,3]. It has been demonstrated that at room temperature the hexagonal Ge2Sb2Te5 is the equilibrium phase [4] while the metastable phase is a rocksalt-type structure with anion site occupied by Te atoms and cation site randomly occupied by Ge, Sb, with about 20% of vacancies [5,6]. The amorphous phase consists primarily of Ge atom switching between octahedral and tetrahedral symmetry positions within the Te fcc sub-lattice [7]. The aim of this work is to investigate the amorphization of Ge2Sb2Te5 under ion implantation. In fact, such a technique allows either a precise control of the energy density deposited into the film and to discriminate the effect of the anelastic end elastic energy loss. Moreover, by changing the ion mass, diluted or dense damage cascade can be generated and the amorphization threshold due to the formation of isolated point defects or to amorphous pocket can be determined. The rate of damage accumulation is caused by the competition between defects generation and recombination and the relative interplay can be changed by varying the substrate temperature. We report here a systematic study of the amorphization of Ge2Sb2Te5 films during irradiation by energetic heavy ions (Ar+, Sb+) at several energies and substrate temperature. The kinetics of the phase change have been investigated using in situ time-resolved reflectance (TRR) measurements, taking advantage of the different optical properties [2] of the two phases. The implantation damage distribution was estimated by Monte Carlo simulations of collision cascades with the assumption of a displacement energy threshold of the target atoms TRIM [8].
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