The Roles of the Ge‐Te Core Network and the Sb‐Te Pseudo Network During Rapid Nucleation‐Dominated Crystallization of Amorphous Ge₂Sb₂Te₅

Abstract

Ge2Sb2Te5 (GST) has demonstrated its outstanding importance among rapid phase‐change (PC) materials, being applied for optical and electrical data storage for over three decades. The mechanism of nanosecond phase change in GST, which is vital for its application, has long been disputed: various, quite diverse scenarios have been proposed on the basis of various experimental and theoretical approaches. Nevertheless, one central question still remains unanswered: why is amorphous GST stable at room temperature for long time while it can rapidly transform to the crystalline phase at high temperature? Here it is revealed for the first time, by modelling the amorphous structure based on synchrotron radiation anomalous X‐ray scattering data, that germanium and tellurium atoms form a “core” Ge‐Te network with ring formation. It is also suggested that the Ge‐Te network can stabilize the amorphous phase at room temperature and can persist in the crystalline phase. On the other hand, antimony does not contribute to ring formation but constitutes a “pseudo” network with tellurium, in which the characteristic Sb–Te distance is somewhat longer than the covalent Sb–Te bond distance. This suggests that the Sb‐Te pseudo network may act as a precursor to forming critical nuclei during the crystallization process. The findings conclude that the Ge‐Te core network is responsible for the outstanding stability and rapid phase change of the amorphous phase while the Sb‐Te pseudo network is responsible for triggering critical nucleation.

Origin, secret, and application of the ideal phase‐change material GeSbTe

“…By comparing amorphous (A) and crystalline (B) structures, curious features were found; that is (i) basically, both the amorphous and the crystalline structures are constructed by -Ge(Sb)-Te-alternative rings, (ii) the bonding angles of -Ge(Sb)-Te-Ge(Sb)-in the amorphous state is centring at 908 resembling that in the crystalline state, (iii) many small fragments of NaCl structure exist in the amorphous phase, and (iv) large fractions of four-and sixfold rings are mainly formed chiefly by Ge-Te bonds forming core networks, while the contribution of Sb-Te bonds to the ring distribution is very small, where 60% of Ge-Te bond length are within 3.2 Å , while 70% of the Sb-Te form pseudonetworks beyond 3.2 Å in the amorphous phase [18].…”

“…By the environmental acceleration tests on the real optical disk device using the Arrhenius method, the estimated lifetime of the amorphous marks formed in the crystalline GeSbTe was more than several tens of years at room conditions (308C, 80% RH) [10]. It is amazing that the ratio between the estimated lifetime (30 years) and the laser crystallization time (30 ns) reaches 10 [17][18] .…”

Beyond von‐Neumann Computing with Nanoscale Phase‐Change Memory Devices

“…Indeed, it is only very recently that a detailed understanding has been achieved of what makes a true phase‐change material, and why some seemingly similar materials display technologically useful phase‐change behavior but others do not, thus allowing the recent development of a set of design rules for what constitutes a phase‐change material 14. One of the most extensively studied phase‐change materials, and the one that we use in this work, is the ternary alloy Ge 2 Sb 2 Te 5 which has been used for optical disk memories,15 scanning‐probe based storage,16–18 the fabrication of synaptic mimics19–21 and, perhaps its most widely known recent application, the development of binary non‐volatile electrical phase‐change memories (PCMs) 22–26. For binary storage, a PCM cell is switched between amorphous and crystalline phases (and back again) using single pulses.…”

Atomistic and electronic structures of functional disordered materials revealed by a combination of quantum-beam measurements and computer simulations

“…Journal of the Ceramic Society of Japan 125 [11] 799-807 2017 JCS-Japan factors for X-rays, the absence of an FSDP in S NN (Q) for l-ZrO 2 is a signature of a non-glass-forming liquid. Another important feature in S NN (Q) is that both l-SiO 2 and l-Al 2 O 3 exhibit a second principal peak (PP) at Qr AX = 3 while the PP is not distinct in l-ZrO 2 data.…”