Laser induced crystallization of amorphous Ge2Sb2Te5 films

Abstract

The crystallization behavior of Ge2Sb2Te5 thin films has been analyzed by atomic force microscopy and optical reflection measurements on various time scales in order to determine the crystallization kinetics including the crystallization mechanism, the corresponding activation barrier, and the Avrami coefficient. On the minute time scale, thin amorphous films were isothermally crystallized in a furnace under a protective Ar atmosphere. From these measurements the activation energy for crystallization was determined to be (2.0±0.2) eV, in close agreement with previous studies using different techniques. The isothermal measurements also revealed a temperature dependent incubation time for the formation of critical nuclei, which is compared with recent theories. On the nanosecond time scale, Ge2Sb2Te5 was locally crystallized with a focused laser. Either crystalline spots of submicron size were generated in an as deposited amorphous film or amorphous bits in an otherwise crystalline film were recrystallized. For the formation of crystalline spots in an as deposited amorphous film a minimum time of (100±10) ns was found, which is identified as the minimum incubation time for the formation of critical nuclei. In contrast, the complete crystallization of melt-quenched amorphous bits in a crystalline matrix was possible in 10 ns. This is attributed to the presence of quenched-in nuclei inside the amorphous bits. The combination of optical measurements with atomic force microscopy reveals the formation and growth of crystalline bits and shows that the crystal growth in vertical direction strongly affects the reflectivity changes.

Electron diffraction and high-resolution transmission electron microscopy of the high temperature crystal structures of GexSb2Te3+x (x=1,2,3) phase change material

“…[1][2][3][4][5][6][7][8][9] Amorphous spots in a crystalline surrounding act as bits of information. Both continuous laser light and short laser pulses at various higher power densities can be employed to read, write, and/or erase those local amorphous areas and thus the phase changes in the material are exploited.…”

“…Crystallization is considered the rate-limiting process, because amorphization is inherently a much faster process. 5,6 It has been argued that a possible disadvantage of the use of Ge 2 Sb 2 Te 5 is that the crystalline state used in phase change optical recording applications is metastable, 10,11 whereupon after an increased heat input ͑to higher temperatures͒ a stable structure develops. On the other hand, the demanded high crystallization rate of Ge 2 Sb 2 Te 5 is possibly a direct consequence of the metastability of the phase, which allows for a much easier crystallization process due to fewer constraints on the short range diffusion and ordering of the different atomic species.…”

Front propagation in a phase field model with phase-dependent heat absorption

“…At the same time, it is noteworthy that the value of E(0) increases with k = 0, emphasising the fact that the one-dimensional front does not solve the full problem with the transverse direction included. This fact can also be heuristically understood by inspecting the linearized stability problem (32) and observing that in this system as compared with (19), the eigenvalues µ have been substituted by µ + k 2 p 2 in the first equation and µ + dk 2 in the second equation. From here one can expect that if the eigenvalues µ are in the left half-plane in the longitudinal case, in the case of transverse perturbations they will have even more negative real part if k > 0.…”

“…It is also assumed that this process is uniform in time, so stationary nucleation and growth rates can be used for modelling the crystallisation. However, recent experimental evidence suggests that both these simplifications are somewhat unrealistic for not only does the nucleation takes place mainly on the boundary between the phase-change material and the surrounding substrates [21,25], but there are typically non-negligible incubation times preceding the onset of crystallisation [32]. In recognition of these difficulties we have proposed a model based on the so-called 'master equation' approach [5,29].…”

Influences of partial melting and overheating on amorphization of Ge2Sb2Te5 during the reset process

“…In contrast, the reset process consists of changing the hexagonal structure to the amorphous state by heating and rapid quenching within several tens of nanoseconds [3][4][5]. Many researchers have focused on the analysis of two crystalline structures of GST that appeared during the set process [6][7][8]: the metastable FCC and the stable hexagonal structure. However, little attention has been given to the atomic structure changing from the crystalline state to the amorphous state because there are several difficulties associated with this observation.…”