The chalcogen elements belong to the VI-A subgroup of the Periodic Table. These elements are: sulphur, selenium and tellurium. The chalcogens are the basic elements of the chalcogenides compounds. The chalcogenides are compounds of sulphur, sele-nium or tellurium with electropositive elements or with organic radicals. The name chalcogenide originates from Greek ϕαλκoε (copper) γǫνναω (born) ǫιδoε (type) and being given initially to the chalcogenide minerals that contain copper in combination with sulphur, selenium and tellurium [15]. Phase-change materials belong to the fam-ily of chalcogenides. These materials at room temperature can present the amorphous or the crystalline phase, being the phase transition a reversible process. Hence, the crystallization mechanism becomes fundamental, and its speed impacts the final per-formance of the device in which the phase-change material is integrated. As we will see later, the crystallization is the result of the combination of two different mecha-nisms: the nucleation, and the growth. These mechanisms vary dependently on the phase-change material considered. We present here the Ge2Sb2Te5 and GeTe, seen as the main phase-change materials used in PCM technology, and starting point of our work.
1.4.1 GeTe
Studied since 1968 [18] for its optical properties, the GeTe represents one of the first example of chalcogen compound that demonstrated a reproducible and controllable phase transition from a low resistive, crystalline and highly reflective state, to a high resistive, amorphous state. At room temperature the crystalline phase has been de-scribed as a rhombohedrally distorted Te sublattice phase with some of the Ge atoms misaligned, and a subsequent rupture of certain resonant bonds in the lattice (the bonding is supposed to be resonant with shorter, essentially covalent, bonds and longer resonance bonds formed through the back lobes of the same p orbitals that are used to form the shorter bonds [19]). When we increase the temperature, at around 400 ◦C, the rhombohedral angle approaches 90◦, giving rise to a cubic structure. GeTe is a degenerated p-type semiconductor with the top of the valence band formed by p elec-trons. In the near-perfect cubic phase all atoms and consequently their p orbitals are aligned, and the resonant-bonding network extends throughout the crystal, while in
1.4 The phase-change materials 13
Fig. 1.7. Thermal conductivity vs temper-ature for amorphous and crystalline GeTe films. In the table are reported the values of the resistivity for both the phases [16].
Fig. 1.8. A melt-quenched amorphous spot of GeTe (a), is recrystallized by multiple laser pulses procedure. First pulses (b) demonstrate the growth of the crystalline boundaries. Then, only after 8 laser pulses (c and d), the nucleation takes place in the central part of the spot [17].
the amorphous phase resonance bonding is localized to within fewer interatomic dis-tances. Consequently, the two phases have very strong different electrical and thermal conductivity, as shown in Fig. 1.7.
As confirmed by recent works on GeTe [17, 20], this material is still largely in-vestigated because of its high crystallization speed. Recrystallization provided with fs laser pulses has been demonstrated. Most of the pre-amorphized surface, has been observed recrystallizing starting from the interfaces between the amorphous region and the surrounding crystalline phase. Only in really long crystallization procedures, the nucleation has been observed coupled to the growth process, as observed in Fig. 1.8.
The melting temperature of this compound is around 725 ◦C [21] while its crys-tallization temperature is around 180 ◦C [22]. In the experimental results to evaluate the crystallization temperature, the transition from the amorphous to the crystalline phase is sharp: indeed, once a nucleus is generated during the heating process, it grows really fast. Both in optical and in resistivity measurements, it appears as an abrupt transition of the measured quantities.
1.4.2 Ge
2Sb
2Te
5Ge2Sb2Te5 or GST, thanks to its long-term stability at ambient temperatures and its relatively fast crystallization under laser irradiation (50 ns), has been considered, since the beginning of 1990s, as a great material for optical recording. Moreover, its properties made this material the first phase-change material considered candidate for PCM applications. Its crystallization temperature is around 150 ◦C, while its melting temperature is around 660 ◦C [23].
The resistivity measurement as a function of the temperature (Fig. 1.9), reveals 2-3 orders of magnitude drop in resistivity that coincides with the amorphous-fcc transfor-mation which is near 150 ◦C (first transition). This metastable fcc phase transforms
Fig. 1.9. Resistivity of full sheet material of GST, measured during increasing temperature [24]. The two transition amorphous-fcc (T1) and fcc-hcp (T2) are evidenced.
Fig. 1.10. AFM scans of a Ge2Sb2Te5 sam-ple surface, during isothermal annealing at 115◦C for 15 hours [26]. The continuous nu-cleation observed is reflected in the nunu-cleation- nucleation-dominated recrystallization.
into the stable hexagonal phase at a higher temperature of 375 ◦C, which varies de-pending on the sample characteristics (second transition). The resistivity of amorphous Ge2Sb2Te5 exponentially decreases as more carriers are excited at higher temperature, consistently with its semiconductive nature [24]. If it is cooled again, the resistivity essentially returns to its original value. However, the resistivity decrease of a crys-talline phase is mainly due to the increase of mobility rather than carrier concentration.
Crystal grains grow during heating and the scattering by grain boundaries decreases, increasing the mobility. This decrease in resistivity is irreversible, and the lower resis-tance persists once the sample is cooled back at room temperature.
The thermal conductivity of GST increases abruptly at 150 ◦C (amorphous-fcc transition), while it increases more gradually near ∼ 340 ◦C (fcc-hcp transition). The thermal conductivity, during the crystallization process, encompasses the entire range of values from 0.45 WK−1 m−1 up to almost 1.53 WK−1 m−1 [25]. This increase is likely due to the decreased defects density in the crystalline matrix.
Studies on the recrystallization mechanism of amorphous marks upon laser irradi-ation revealed that Ge2Sb2Te5 recrystallizes by nucleation and subsequent growth of crystals inside the amorphous mark [23], being the heterogeneous crystal nucleation the fundamental mechanism that controls the crystallization [26] (Fig. 1.10). The nu-cleation rate can represent a limit for the amorphisation. In standard Ge2Sb2Te5 the highest attainable experimental cooling rate is on the order of 10−10 Ks−1 (the highest cooling rate depends on the material parameters, and in particular on the thermal con-ductivity, on specific heat per volume, and on material dimensions). If now we suppose a range of temperatures of 100 K in which the nucleation is favorable, the range of time of the temperature decrease would be on the order of∼10 ns. For active volumes typical of a PCM device on the order of ∼ 10−23 m3, the nucleation rate represents a problem if is higher than 1031 m−3s−1, that is the case of GST. What makes possible